university of groningen tuning of the luminescence in poly ... · studied in order to gain insight...

University of Groningen

Tuning of the luminescence in poly((silanylene)thiophene)sHerrema, Jan Karst

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Publication date:1996

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Citation for published version (APA):Herrema, J. K. (1996). Tuning of the luminescence in poly((silanylene)thiophene)s. s.n.

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Synthesis and characterization of some

oligo[(silanylene)thiophene]s and thiophene

oligomers having terminal oligosil(an)yl groups

ABSTRACT

Synthetic routes and spectroscopic data for a series of oligosil(an)yl-substituted oligothiophenes (Tx; x = 1, 2, or 3 rings) and some oligo-[(silanylene)thiophene]s are presented. The compounds have beensynthesized and studied as an aid in the interpretation of NMR,absorption and fluorescence spectra of the corresponding copolymers.With regard to the electronic configuration of the oligothiophenes, NMRspectra show clear differences between plain oligothiophenes andsil(an)yl-end-substituted oligothiophenes, indicative of - interactionsbetween the oligothiophene and sil(an)yl parts. Red-shifts in opticalspectra show a parallel trend across the various compounds based on thesame oligothiophene unit, related to the stabilization of photo-excitedstates of the oligothiophene by the oligosil(an)yl substituents. These effectsare strong in thiophene and in bithiophene-based compounds and becomeweaker for longer oligothiophenes. Fluorescence quantum efficiencies insolution are found to be remarkably high in some T2-based compounds.

Synthesis and characterization of oligomers

12

S2.1 INTRODUCTION

tudies on naturally occurring oligothiophenes and related compounds startedalready in the late fifties because of their nematicidal activity.1 Especially

α-terthiophene exhibits various biological properties.α-Terthiophene acts as aphotodynamic sensitizer which generates singlet oxygen and shows oxygen-dependentphototoxicity against nematodes, fish and plants as well as eggs and larvae of insects.2

More recently well-defined oligothiophenes (designated Tx , x = 2,3 ....) have beenstudied in order to gain insight into the structural and electronic properties of thecorresponding polymers.3,4 Because oligomers have a well-defined structure andconjugation length and are soluble and processable it makes them almost ideal modelcompounds. Interest has been focussed mainly on their application as semiconductingand nonlinear optical materials. A typical example of an oligomer for which theproperties exceed that of the polymer is the all-organic transistor havingα-sexithiophene as the active semiconducting material.5

Oligo- and polysilanes are another class of materials which have been subjected toa large number of investigations. Because of so calledσ-conjugation silanes haveintriguing electronic properties. An interesting question in the studies of organosiliconoligomers and copolymers which consist of a regular alternation of aπ-conjugated andan sil(an)ylene unit, is the interaction of the aromatic 2pπ orbital and the silicon 3dorbital. Several mechanisms have been proposed in relation to conjugation of some sorton silicon atoms:π-dSi interaction,6 π-σSi-Si hyperconjugation,7 and the "geminal"σ-orbital overlap on silicon atoms8 which leads to electronic band structure inpolysilanes. UV absorption spectra of phenylsilanes have been interpreted on a basisof π*-d orbital interaction by various authors.9 All authors concluded that the Si-Sibond is capable of conjugating with phenyl groups.

To our knowledge no systematic studies on synthesis and electronic properties ofthiophene and oligothiophenes having terminal sil(an)yl groups have been reported.Only recently Tour and coworkers reported the synthesis and UV properties of a seriesof solubleα-thiophene oligomers having terminal trimethylsilyl groups.10 Their studyis focussed on the properties of the oligothiophene; the terminal trimethylsilyl groupsare used because they improve solubility and control of the target molecules. In thischapter the studies of a series of oligothiophenes having terminal methylatedoligosil(an)yl groups and a series of oligo[(silanylene)thiophene]s is presented. Thelength of the oligothiophene unit of the compounds is limited to three rings, in the firstplace for reasons of solubility and secondly because studies of their properties arefairly conclusive in this range. Theπ-σ conjugation appears to decrease with increasinglength of the oligothiophene block. The decrease of theπ-σ conjugation is probablycaused by the lowering of the energy of theπ-system of the oligothiophene block withrespect to the available electronic levels on the sil(an)ylene moiety. The varioussynthetic routes that were followed to obtain these oligomers, as well as theirmolecular and spectroscopic characterization, are presented.

Chapter 2

13

T

2.1

2.2a-c: y = 1,2 and 4 2.3a-c: y = 1,2 and 4

2.1'

- Li+.TMEDAS - Li+

S-Li+

2.1''

2 eq.

y y

y yy

n-BuLi / TMEDA

pentane

n-BuLi

pentane

A B

S

Si

Me

Me

MeS

Si

Me

Me

MeSi

Me

Me

MeS

Si

Me

Me

MeCl Si

Me

Me

MeCl

Scheme 2.1 Reaction scheme for the synthesis of mono- and disubstituted permethyl-sil(an)ylthiophenes.

2.2 SYNTHETIC PROCEDURES

hiophene is an electron-rich aromatic compound. The ring system exerts aπ-electron donating and aσ-electron withdrawing effect on substituents in the 2-

and 5-positions. The chemistry of thiophenes can be compared with that of pyrrolesand furans. Although thiophene is somewhat less reactive than furan towardelectrophiles, and much less reactive than pyrrole, it is much more reactive thanbenzene, by a factor 103 - 105. The electrophilic substitution of metallated thiophenesis a particularly useful reaction; thiophenes are easily and selectively metallated to giveintermediates which react with a wide range of electrophiles. Thienyllithiumcompounds are often used for this type of reactions.11 Since the early sixties it has beenknown that thiophene can be converted into the mono- and dilithiumanion throughproton abstraction with an alkyllithium. It was not until the work by Chadwick andWilbe12 that reaction conditions were optimized and that the synthetic potential wasfully realized. Lithiation of thiophene withn-butyllithium (n-BuLi) occurs specificallyat the 2- and 5-positions. Formation of the mono- and/or dianion can be controlled bythe appropriate use of the solvent in combination with the addition ofN,N,N',N'-tetramethylethylenediamine (TMEDA).

Scheme 2.1 outlines the synthetic method used for the synthesis of the mono- anddisubstituted permethyloligosil(an)yl-end-capped thiophene compounds analogous tothe literature procedures.13

Thus, mono- and disubstituted thiophenes were obtained in reasonable yields (35-77%).The route A for the monosubstituted thiophenes2.2 gave, apart from the desiredproducts, also the disubstituted compounds2.3. In the case of the reaction of 1equivalent ofn-BuLi/TMEDA with thiophene followed by the addition of an excessof 1-chloro-nonamethyltetrasilane, a mixture of the nonamethyltetrasilanyl compounds


14

2.1

2.4a-c: p = 1; y = 1,2 and 42.5a-c: p = 2; y = 1,2 and 42.6: p = 3; y = 1

2.1'

-Li+.TMEDA

S -Li+

S-Li+

2.1''

+n-BuLi / TMEDA

pentane

Si

Me

MeSS

H

yp

Si

Me

Me

ClCl

y

S

Scheme 2.2 Reaction scheme for the synthesis of a series of oligo[(sil(an)ylene)thiophene]s.

2.5a

+1.) n-BuLi / TMEDASi

Me

MeSS

H

2 2.) Si

Me

MeSS

H

8

2.7

2.8

Si

Me

MeSS

H

5

Si

Me

Me

ClCl

Scheme 2.3 Reaction scheme for the synthesis of a series of oligo[(silylene)thiophene]s.

2.2c(44%) and2.3c(7%) was obtained, which could be easily separated by distillationand subsequent purification by means of column chromatography. For this reason thesynthesis of2.3cwas not optimized further.

The α,ω-dithienylpermethyloligosilanes and some oligo[(silanylene)thiophene]shave been synthesized by the reaction of a mixture of the thiophene monoanion andminor quantities of the dianion with two equivalents of the dichloropermethyloligo-silanes (Scheme 2.2). The main products are the expected dithienylpermethyl-oligosilanes2.4a-c, which were obtained in high yields (67-91%). By an appropriatechoice of the reaction conditions followed by fractional distillation, several dimeric andtrimeric compounds could be isolated. Compounds2.6 and 2.7 had already beenreported by Kauffmann and Kniese14 in 1973. In the case of dimethylsilylene blocksa pentamer and octamer were isolated (Scheme 2.3) starting from the trimer2.5a.

The permethylsil(an)ylbithiophenes were prepared in a similar way to the thiopheneanalogues as is outlined in Scheme 2.4. Trimethylsilyl mono- and disubstitutedbithiophenes could be prepared in good yields. The yield of pentamethyldisilanyl-bithiophene2.10b was surprisingly low (14%). The explanation for the low yield of

Chapter 2

15

S

S

2.10a: y = 12.10b: y = 2

2.11a: y = 12.11b: y = 2

i

ii

2.9

yS

Me

Me

MeS

Si

yyS

Si

Me

Me

Me

Me

Me

MeS

Si

Scheme 2.4 Reaction scheme for the synthesis of sil(an)yl-end-capped 2,2'–bithiophenes:(i) 1 eq. n-BuLi/TMEDA + 1 eq. Cl[SiMe2] yMe; (ii) 2 eq. n-BuLi/TMEDA + 2eq. Cl[SiMe2] yMe.

2.10b is probably a fast exchange of the bithiophene lithium-anion with themonosubstituted pentamethyldisilanylbithiophene to the lithium-anion of2.10b andbithiophene. Compound2.11bhas recently been reported by Ishikawa and coworkers15

who used a nickel-catalysed Grignard coupling reaction of 2-bromo-5-pentamethyldisilanylthiophene and its magnesium derivative.

Like the bithiophenes the disubstituted oligosil(an)yl-end-capped terthiophenes couldbe prepared in reasonable to good yields. Using the standard reaction conditions, nomonosubstituted trimethylsilyl- and pentamethyldisilanylterthiophene could be isolated.NMR spectra of reaction mixtures showed the presence of the starting material and thedisubstituted compound, even if less than one equivalent of alkyllithium was added.The explanation is similar to that for the synthesis of pentamethyldisilanylbithiophene.The trimethylsilylterthiophene (2.13a) has been prepared in a yield of 28% using oneequivalent of n-BuLi/diisopropylamine and subsequent addition of an excesstrimethylchlorosilane (Scheme 2.5). The synthesis of2.13bwas achieved by a nickel-catalysed cross-coupling of the monoiodobithiophene with the Grignard reagent of2-bromo-5-pentamethyldisilanylthiophene. This method was reported for the synthesisof 2.13aby Mac Eachern and coworkers.16


16

S

S

S2.13a

2.14a: y = 12.14b: y = 2

i

ii2.12

S

S

S

Me

Me

MeSi

2

S

S

SSi

Me

Me

MeS

SI

2

SBrMg Si

Me

Me

Me+

iii

2.13b

y

S

S

SSi

Me

Me

Me

Me

Me

Me Si

y

2.15

Scheme 2.5 Reaction scheme for the synthesis of sil(an)yl-end-capped -terthiophenes:(i) 1 eq. n-BuLi/diisopropylamine + 1.5 eq. Cl[SiMe2] yMe; (ii) 2 eq. n-BuLi +3 eq. Cl[SiMe2] yMe, y = 1 and 2;(iii) ether / NiCl2.dppp.

C

2.3 RESULTS AND DISCUSSION

2.3.1 NMR spectroscopy

hemical shifts are important parameters in high resolution NMR studies ofoligomers and polymers. Apart from being the fingerprint of the primary chemical

structure of the molecule under consideration, chemical shifts are also useful as ameans of probing the secondary and higher-order structure. The chemical shift is theparameter that characterizes the magnetic environment of the nucleus considered. Themagnetic environment of a nucleus is determined by the total electromagnetic structureof the molecule which includes nuclear magnetic moments and the distribution ofelectrons. Almost all spectra were recorded in CDCl3.

1H, 13C and29Si chemical shiftsare all relative to tetramethylsilane (TMS, 0 ppm) with a positive sign indicatingdeshielding relative to the reference nucleus (δ scale). Chemical shift assignments foreach of the compounds were made using a combination of techniques such as1HNMR, 13C-APT,1H-13C HETCOR, proton coupled-13C and the relative signal intensitiesof the aromatic protons from the1H NMR spectra.

The 1H NMR chemical shifts of a series of mono- and disubstitutedpermethylsil(an)ylthiophenes and bisthienyloligosilanes are compiled in Table 2.1. Thepatterns of the1H NMR spectra of the thiophene derivatives are determined by theelectronic effects of the silicon substituent. Organosilyl and silanyl groups exert anelectron-releasing inductive effect (+I effect) toward aromatic rings and an electron-attracting effect (-T effect) byπ-σ* backbonding.17 In addition silanyl substituents candonate electron density toward aromatic rings byπ-σ hyperconjugation7,18 from theSi-Si bonding orbitals.

Chapter 2

17

SR R

2

34

5 = R1TR2

21

The chemical shiftδ of the sil(an)ylthiophenes shows an increase with respect totheδ values of unsubstituted thiophene. This is in sharp contrast with alkylthiophenes,which show a sharp decrease as is clearly seen by comparison with theδ value of2,5-di-t-butylthiophene (6.44 ppm versus 7.32 ppm for H(3) and H(4) of the analogoussilyl compound2.3a). The strong acceptor effect of the trimethylsilyl group exceedsthe inductive effect andδ values of all ring protons are shifted toward low-fieldcompared to thiophene. There is a small difference between a trimethysilyl group anda pentamethyldisilanyl group, which is most clearly seen for the protons at the3 position for the compounds2.2and2.4. The deshielding observed slightly decreases(≈ 0.1 ppm) from SiMe3 to Si2Me5. Lengthening of the oligosilanyl from two to foursilicon atoms has no discernable effect. Theδ values of the bisthienyloligosilanes2.4a-chave comparable values (within the experimental error) to the correspondingmono-substituted thiophenes2.2a-c.

Table 2.1 1H NMR data of substituted thiophenes

1H NMR, δ, ppma,b

H(2) H(3) H(4) H(5)

Thiophene = T (2.1) 7.20 6.96 6.96 7.20tBuTBut (c) - 6.44 (-0.52) 6.44 (-0.52) -

TSiMe3 (2.2a) - 7.33 (+0.37) 7.24 (+0.28) 7.64 (+0.44)

TSi2Me5 (2.2b) - 7.22 (+0.26) 7.20 (+0.24) 7.60 (+0.40)

TSi4Me9 (2.2c) - 7.19 (+0.23) 7.19 (+0.23) 7.58 (+0.38)

Me3SiTSiMe3 (2.3a) - 7.32 (+0.36) 7.32 (+0.36) -

Me5Si2TSi2Me5 (2.3b) - 7.26 (+0.30) 7.26 (+0.30) -

Me9Si4TSi4Me9 (2.3c) - 7.27 (+0.31) 7.27 (+0.31) -

TSiMe2T (2.4a) - 7.35 (+0.39) 7.24 (+0.28) 7.66 (+0.46)

TSi2Me4T (2.4b) - 7.21d (+0.25) 7.20d (+0.24) 7.61 (+0.41)

TSi4Me8T (2.4c) - 7.18d (+0.22) 7.19d (+0.23) 7.58 (+0.38)

aIn parentheses: delta =δ(compound) -δ(thiophene).bRecorded in CDCl3, conc.≈ 0.05 M.cS.Gronowitz and A. Hörnfeldt, Physical Properties of Thiophene Derivatives inThe Chemistryof Heterocyclic Compounds, Volume 44: Thiophene and its Derivatives, Part Four, ed. S.Gronowitz.dIndistinguishable.


18

SR R

2

34

5 = R1TR2

21

Table 2.2 13C and 29Si NMR data

13C NMRa, δ, ppm

C(2) C(3) C(4) C(5)

T (2.1) 124.9 126.4 126.4 124.9

SiMe4 - - - -

Me(SiMe2)2Me - - - -

Me(SiMe2)4Me - - - -

TSiMe3 (2.2a) [140.0] (+15.1) 133.8 (+7.4) 128.0 (+1.6) 130.2 (+5.3)

TSi2Me5 (2.2b) [138.8] (+13.9) 133.9 (+7.5) 128.0 (+1.6) 130.2 (+5.3)

TSi4Me9 (2.2c) [139.3] (+14.4) 133.9 (+7.5) 128.0 (+1.6) 130.2 (+5.3)

Me3SiTSiMe3 (2.3a) [145.7] (+20.8) 135.0 (+8.6) 135.0 (+8.6) [145.7] (+20.8)

Me5Si2TSi2Me5 (2.3b) [144.6] (+19.7) 135.0 (+8.6) 135.0 (+8.6) [144.6] (+19.7)

Me9Si4TSi4Me9 (2.3c) [144.8] (+19.9) 135.0 (+8.6) 135.0 (+8.6) [144.8] (+19.9)

TSiMe2T (2.4a) [137.2] (+12.3) 135.3 (+8.4) 128.1 (+1.7) 131.2 (+6.3)

TSi2Me4T (2.4b) [137.7] (+12.8) 134.5 (+8.1) 128.2 (+1.8) 130.7 (+5.8)

TSi4Me8T (2.4c) [139.0] (+14.1) 133.9 (+7.5) 128.1 (+1.7) 130.3 (+5.4)

aIn parentheses: delta =δ(compound) -δ(reference), wherein reference is thiophene (13C) and oligosilane (29Si),

We will focus for the rest mainly on the13C chemical shifts because they aremedium and concentration independent in a certain range. The sum of the diamagnetic(σdia) and paramagnetic (σpara) terms contribute to the relative chemical shift.19 Themost important shielding contributions to13C chemical shifts of compounds studiedhere are arising from inductive effects, hyperconjugation, mesomeric interactions andneighbour anisotropy effects. The chemical shift value depends on the type offunctional group to which the atoms belong.

The 13C and 23Si chemical shifts and assignments of the sil(an)yl substitutedthiophenes are listed in Table 2.2. The signals of the carbon atoms in the thiophenering of all the compounds2.2-2.4are shifted to lower fields compared to thiophene.As expected, the largest effects are observed for the substituted carbons. The "meta"carbons, i.e., C(4) of the compounds2.2 and2.4 absorb over a small range of 1.6-1.8ppm. The chemical shifts of the C(4) positions of 2-substituted thiophenes are ingeneral insensitive to substituents. This has been attributed to the resonance effects ofthe substituents, as shown in Figure 2.1, in which no structure contributes chargedelocalization at the C(4) position.

Chapter 2

19

SSi

R

R-+ S

Si

R

R-

+

SSi

R

R

-

+

SSi

R

R

-

+RS

Si

R

RS

Si

R

Figure 2.1

of substituted thiophenes13C NMRa, δ, ppm 29Si NMRa, δ, ppm

Cα Cβ Cγ Cδ Si(1) Si(2) Si(3) Si(4)

- - - - - - - -

0.0 - - - 0.0 - - -

-2.4 -2.4 - - -19.6 - - -

-1.1 -5.7 -5.7 -1.1 -15.1 -44.7 - -

-0.1 (-0.1) - - - -2.8 (-2.8) - - -

-2.8 (-0.4) -2.6 (-0.2) - - -24.1 (-4.5) -19.3 (+0.3) - -

-1.3 (-0.2) -5.9 (-0.2) -5.6 (+0.1) -1.4 (-0.3) -20.3 (-5.2) -44.7 (+0) -44.6 (+0.1) -15.1 (+0.1)

0.0 (+0) - - - -6.9 (-6.9) - - -

-2.7 (-0.3) -2.5 (-0.1) - - -24.6 (-5.0) -19.3 (+0.3) - -

-1.4 (-0.3) -6.0 (-0.3) -5.7 (+0) -1.3 (-0.3) -20.7 (-5.2) -44.7 (+0) -44.6 (+0) -15.1 (+0)

-0.1 (-0.1) - - - -15.1 (-15.1) - - -

-2.7 (-0.3) -2.7 (-0.3) - - -24.4 (-4.8) -24.4 (-4.8) - -

-1.6 (-0.5) -5.7 (-0.0) -5.7 (-0.0) -1.6 (-0.5) -20.2 (-5.1) -44.5 (+0.2) 44.5 (+0.2) -20.2 (-5.1)

respectively. Square brackets denote substituted carbons.

The "ortho" and "para" carbons of the bisthienylsilanes2.4show a correlation withthe substituents and comparison with the1H data illustrates a similar relation found forthe H(3) and H(5) shieldings. The carbon nuclei of the disubstituted compounds2.3show a deshielding compared to the monosubstituted thiophenes2.2.


20

S

S

3 4

52

2'

3'4'

5'

S

SS

3 4

52

2'

3'4'

5'

4" 3"

5" 2"

Table 2.3 1H and 13C NMR data

1H NMRa, δ, ppm

H(5) H(4) H(3) H(3') H(4') H(5')

T2 (2.9) 7.22 7.03 7.19 7.19 7.03 7.22

tBuT2tBu (b) -

6.72(-0.31)

6.92(-0.27)

6.92(-0.27)

6.72(-0.31)

-

T2SiMe3 (2.10a) -7.14

(+0.11)7.24

(+0.05)7.20

(+0.01)7.02

(-0.01)7.20

(-0.02)

T2Si2Me5 (2.10b) -7.07

(+0.04)7.23

(+0.04)7.18

(-0.01)7.01

(-0.02)7.19

(-0.03)

Me3SiT2SiMe3 (2.11a) -7.13

(+0.10)7.23

(+0.04)7.23

(+0.04)7.13

(+0.10)-

Me5Si2T2Si2Me5 (2.11b) -7.07

(+0.04)7.24

(+0.05)7.24

(+0.05)7.07

(+0.04)-

aIn parentheses: delta =δ(compound)-δ(T2).bGenerous gift from W. ten Hoeve (Syncom).

Table 2.4 1H NMR and13C data

1H NMRa, δ, ppm

H(5) H(4) H(3) H(3') H(4') H(3") H(4") H(5")

T3 (2.12) 7.22 7.08 7.18 7.09 7.09 7.18 7.08 7.22

tBuT3tBu (b) -

6.74(-0.34)

6.98(-0.20)

7.00(-0.09)

7.00(-0.09)

6.98(-0.20)

6.74(-0.34)

-

T3SiMe3 (2.13a) -7.14

(+0.06)7.22

(+0.04)7.08

(-0.01)7.08

(-0.01)7.17

(-0.01)7.02

(-0.06)7.21

(-0.01)

T3Si2Me5 (2.13b) -7.07

(-0.01)7.23

(+0.05)7.08

(-0.01)7.08

(-0.01)7.17

(-0.01)7.02

(-0.06)7.21

(-0.01)

Me3SiT3SiMe3 (2.14a) -7.13

(+0.05)7.22

(+0.04)7.08

(-0.01)7.08

(-0.01)7.22

(+0.04)7.13

(+0.05)-

Me5Si2T3Si2Me5 (2.14b) -7.085

(0.005)7.23

(+0.05)7.08

(-0.01)7.08

(-0.01)7.23

(+0.05)7.085

(0.005)-

aIn parentheses: delta =δ(compound) -δ(T3).bGenerous gift from W. ten Hoeve.

Chapter 2

21

of substituted bithiophenes

13C NMRa, δ, ppm

C(5) C(4) C(3) C(2) C(2') C(3') C(4') C(5')

124.2 127.6 123.6 137.2 137.2 123.6 127.6 124.2

[156.1](+31.9)

121.7(-5.9)

122.3(-1.3)

134.8(-2.4)

134.8(-2.4)

122.3(-1.3)

121.7(-5.9)

[156.1](+31.9)

[139.7](+15.5)

134.6(+7.0)

124.9(+1.3)

142.3(+5.1)

137.3(+0.1)

123.7(+0.1)

127.7(+0.1)

124.3(+0.1)

[138.8](+14.6)

134.6(+7.0)

125.0(+1.4)

142.3(+5.1)

137.4(+0.2)

123.6(+0.0)

127.7(+0.1)

124.1(-0.1)

[139.6](+15.4)

134.6(+7.0)

125.0(+1.4)

142.4(+5.2)

142.4(+5.2)

125.0(+1.4)

134.6(+7.0)

[139.6](+15.4)

[138.5](+14.3)

134.7(+7.1)

124.9(+1.3)

142.2(+4.9)

142.2(+4.9)

124.9(+1.3)

134.7(+7.1)

[138.5](+14.3)

Square brackets denote substituted carbons.

of substituted terthiophenes

13C NMRa, δ, ppm

C(5) C(4) C(3) C(2) C(2') C(3') C(4') C(5') C(2") C(3") C(4") C(5")

124.3 127.7 123.5 136.9 136.0 124.1 124.1 136.0 136.9 123.5 127.7 124.1

[156.8](+32.5)

121.9(-5.8)

122.8(-0.7)

134.1(-1.8)

136.1(+0.1)

123.4(-0.7)

123.4(-0.7)

136.1(+0.1)

134.1(-1.8)

122.8(-0.7)

121.9(-5.8)

[156.8](+32.5)

[139.9](+15.6)

134.7(+7.0)

124.8(+1.3)

142.0(+5.1)

136.1(+0.1)

124.3(+0.2)

124.3(+0.2)

136.1(+0.1)

137.1(+0.2)

123.5(+0.0)

127.8(+0.1)

124.3(+0.2)

[139.0](+14.7)

133.8(+6.1)

124.9(+1.4)

141.9(+5.0)

136.0(+0.0)

124.3(+0.2)

124.3(+0.2)

136.2(+0.2)

137.1(+0.2)

123.5(+0.0)

127.8(+0.1)

124.1(+0.0)

[139.8](+15.5)

134.7(+7.0)

124.8(+1.3)

142.1(+5.2)

136.2(+0.2)

124.3(+0.2)

124.3(+0.2)

136.2(+0.2)

142.1(+5.2)

124.8(+1.3)

134.7(+7.0)

[139.8](+15.5)

[139.0](+14.7)

134.7(+7.0)

124.8(+1.3)

142.1(+5.2)

136.8(+0.8)

124.2(+0.2)

124.2(+0.2)

136.8(+0.8)

142.1(+5.2)

124.8(+1.3)

134.7(+7.0)

[139.0](+14.7)

Square brackets denote substituted carbons.


22

A

The bisthienylsilanes2.4show a shielding for the C(2) nuclei and a deshielding of theC(3),C(4) and C(5) nuclei compared to compounds2.2. The latter effect is notablysmall for the bisthienyltetrasilane (2.4c). Actually, theδ values of the thiophene ringsof compound TSi4Me9 (2.2c) and TSi4Me9T (2.4c) are almost identical and one mayconclude that there is no long-range effect through the silanylene group from onethiophene ring to the other.

Only a few1H and even fewer13C NMR data for substituted oligothiophenes havebeen published. In most of the publications the assignment of13C resonances inparticular is lacking. In this work assignments of the sil(an)yl substituted bi- andterthiophenes were made using a combination of techniques such as1H-13C HETCOR,proton coupled-13C and the relative signal intensities of the aromaticprotons and theircoupling constants. In Tables 2.3 and 2.4, the1H and 13C chemical shift data arecollected for the substituted bi- and terthiophenes. Trends already observed for theanalogous thiophene compounds are displayed here in a similar manner but there arealso obvious differences. The13C chemical shiftsδ of the silyloligothiophenes showan increase with respect to theδ values of the unsubstituted oligothiophenes. Thestrong acceptor effect of the sil(an)yl substituent exceeds the inductive effect and theδ values of all carbon nuclei are shifted toward low-field compared to the respectiveoligothiophene. There is a small difference between a trimethysilyl group and apentamethyldisilanyl group, which is most clearly seen for the (ipso-) carbons at the5/5' and 5/5" positions, respectively, which are strongly deshielded in comparison tothe plain oligomer. Disilanyl end-caps appear to be a somewhat weaker acceptor sincedeshielding is reduced in going from -SiMe3 to -Si2Me5, and an upfield shift of approx.0.9 ppm results. The other carbon nuclei are much less affected by the substitution, theorder of the shift with respect to the plain oligothiophene being 5/5' > 4/4' > 2/2' > 3/3'for T2 and 5/5" > 4/4" > 2/2" > 3/3" > 2'/5'≈ 3'/4' for T3. For disubstituted T3compounds, only the outer rings show appreciable changes upon substitution. Formonosubstituted T2 and T3 compounds the silyl substituents induce only minor tonegligible changes in the unsubstituted rings. Electronic ground state interactions dueto π-σ conjugation are apparently limited to the thiophene rings bearing a silylsubstituent.

2.3.2 Optical Properties

UV-vis absorption spectroscopy of sil(an)yl-substituted thiophene compounds

lthough thiophene is one of the simplest and most investigated heterocycles, theinterpretation of its UV spectrum has been subject to debate well into the late

1980's. Based on the vapour and condensed-phase spectra and CNDO/S calculationsthe near-UV spectrum of thiophene and most of its derivatives contains three singlettransitions. These are the following:20

π4* π3 (B2 A1), π4* π2 (A1 A1), Rs π3.

Chapter 2

23

The first singlet transition of thiophene (240 nm) and its substituted derivatives (withnon-chromophore groups) can be observed both in solution and gas phase spectra, andin the MCD spectra as well.21 The second transition is only observable for thiophene(222 nm) in the MCD, CD22 or polarized spectra. For substituted derivatives it isobservable in both vapour phase and solution spectra. The third band-system belongsto a Rydberg transition (190 nm) and can only be observed in vapour phaseexperiments.23

The UV-vis absorption spectra of the sil(an)yl-substituted thiophenes2.2-2.4areplotted in Figures 2.2 and 2.3 and the positions of the spectral maxima and the valuesof the molar extinction coefficient (ε) are listed in Table 2.5. The compounds havingone or two silicon atoms have a similar feature in their spectra, namely one broadpeak. The compounds having four silicon atoms,2.2c and2.4c display an additionalshoulder. The absorption spectrum of 2,5-bis(nonamethyl-tetrasilanyl)thiophene2.3c(Figure 2.2c) has two distinct maxima with almost similar intensities.

Table 2.5 Absorption characteristics of substituted thiophenes in hexanea

Compound λmax, nm Emax, eV ε, M-1 cm-1

Me3SiSiMe3 (a) 197 6.29 8500

Me3Si[SiMe2]2SiMe3 (a) 235 5.28 14700

T (2.1) 231 5.37 5600

TSiMe3 (2.2a) 234 5.30 9200

TSi2Me5 (2.2b) 242 5.12 13300

TSi4Me9 (2.2c) 239 / 258(sh) 5.19 / 4.81 18000

Me3SiTSiMe3 (2.3a) 243 5.10 8800

Me5Si2TSi2Me5 (2.3b) 260 4.77 14700

Me9Si4TSi4Me9 (2.3c) 239 / 272 5.19 / 4.56 20000 / 20000

TSiMe2T (2.4a) 236 5.25 9500

TSi2Me4T (2.4b) 244 5.08 15000

TSi4Me8T (2.4c) 248 5.00 25400

TSiMe2TSiMe2T (2.5a) 243 5.10 27500

TSi2Me4TSi2Me4T (2.5b) 249 4.98 31800

TSi4Me8TSi4Me8T (2.5c) 248 / 256 5.00 / 4.84 33300 / 32700aGilman, H.et al. J. Organomet. Chem. 1964,2, 369.

Introduction of one terminal trimethylsilyl group into thiophene results in a slight redshift, followed by another small shift when a second silicon atom is introduced in thecase of pentamethyldisilanyl substitution. This is due to the participation of empty


iAbsorbance here and later in arbitrary units on figures

24

silicon d-orbitals in theπ-system of the ring (back-bonding), which results in anincrease of conjugation length. This phenomenon has been analyzed for analogousbenzene derivatives.24 These bathochromic shifts are enhanced when twopermethylsil(an)yl groups are introduced at the 2,5-positions into thiophene, as isclearly demonstrated in Figure 2.2.

Figure 2.2i

Absorption spectra of:(a) TSiMe3 (2.2a) [—] vs

Me3SiTSiMe3 (2.3a) [.. ];(b) TSi2Me5 (2.2b) [—] vs

Me5Si2TSi2Me5 (2.3b) [.. ];(c) TSi4Me9 (2.2c) [—] vs

Me9Si4TSi4Me9 (2.3c) [.. ].

Figure 2.3Absorption spectra of:(a) TSiMe3 (2.2a) [—] vs

TSiMe2T (2.4a) [−−];(b) TSi2Me5 (2.2b) [—] vs

TSi2Me4T (2.4b) [−−];(c) TSi4Me9 (2.2c) [—] vs

TSi4Me8T (2.4c) [−−].

The bisthienyloligosilanes2.4 have only a minor red shift of the maxima compared tothe analogous permethylsil(an)ylthiophene compounds2.2 (Figure 2.3). The opticaldata for oligosil(an)yl-substituted thiophenes are summarized in Table 2.5. Data forthiophene and two permethyloligosilanes are included for comparison. Twoobservations can be clearly extracted from these data. Firstly, a shift of the absorptionλmax to longer wavelengths with increasing length of the sil(an)ylene substituent occurs

Chapter 2

25

A

within the same series of compounds and, secondly, an increasing absorption intensity,also in the order Si4 > Si2 > Si. Going from the compounds2.4 ("monomer") to2.5("dimer") the absorption intensity is even further increased. This trend of increasingabsorption intensity with increasing conjugation length is known forπ-conjugatedsystems such as polyenes25 and oligo- and polythiophenes.26

Analysis of UV-vis absorption spectra of selected substituted thiophenes

spectral transition corresponds to the energy difference between two well-definedstates of the absorbing molecule. Because molecules have vibrational and

rotational energy in addition to electronic energy, their absorption spectra have thefamiliar band spectra as displayed in Figures 2.2 and 2.3. In solution, in contrast to thevapour state, the energy bands are further broadened resulting in diffuse bands. Thepresence of these diffuse bands in most cases complicates an analysis of UV-visabsorption spectra into separate bands having different electronic origins.

The tetrasil(an)yl-substituted thiophene compounds presented in the formerparagraph clearly display in the absorption spectrum a shoulder or two separate bands(see Figure 2.2 and 2.3). For this reason an attempt was made to unravel the absorptionspectra of selected thiophene compounds with di- and tetrasilanyl(ene) substituents. Acurve-fitting procedure was applied to the experimental spectra, based in part on theinformation known for thiophene and its derivatives as described in the previousparagraph. The results of this curve-fitting of tetrasilanylene-substituted thiophenecompounds are reproduced in Figures 2.4a-d.

Table 2.6 Absorption characteristics of "analyzed" thiophenes in solutiona

Absorption,δ, nma

Compound T Sin

localT electron

transfer

TSi2Me5 (2.2b) 224[9.2] - 234[39.3] 248[51.5]

TSi4Me9 (2.2c) 223[13.1] 235[18.9] 239[18.4] 255[49.5]

Me5Si2TSi2Me5 (2.3b) 221[21.6] - 250[28.0] 266[50.4]

Me9Si4TSi4Me9 (2.3c) 225[21.4] 240[24.6] 256[9.3] 274[44.7]

TSi2Me4T (2.4b) 227[24.2] - 241[29.1] 254[46.7]

TSi4Me8T (2.4c) 221[14.3] 235[22.9] 245[19.4] 261[43.4]

TSi2Me4TSi2Me4T (2.5b) 223[18.2] - 248[48.1] 271[33.7]

TSi4Me8TSi4Me8T (2.5c) 222[28.2] 244[25.3] 259[12.5] 274[33.9]aIn square brackets the normalized area of the peak.


26

Figure 2.4 Analysis of absorption spectra of compounds: (a) TSi4Me9 (2.2c); (b)Me9Si4TSi4Me9 (2.3c); (c) TSi4Me8T (2.4c); (d) TSi4Me8TSi4Me8T (2.5c); markersare experimental data and dashed lines are the sum of the separate contributions.Peak around 200 nm is due to solvent.

Chapter 2

27

F

In each figure the dotted lines stand for the curves obtained from the curve-fittingprocedure. The dashed line is the sum of these curves and the 'line' represented bymarkers is the experimental curve. Details of the curves obtained from the curve-fittingprocedure, such as peak maxima and areas, are compiled in Table 2.6. For thedisilanyl(ene) compounds a satisfactory fit has been obtained by introducing threebands. For the tetrasilanyl(ene) compounds a fourth band was neccessary for obtaininga good fit. We assigned this band to the tetrasilanylene chromophore (Si4 local).Firstly, because this band was lacking in the disilanyl(ene) compounds and secondlybecause the wavelength of this band (235-245 nm) is comparable to that of theabsorption maximum of dodecamethyltetrasilane. The most intense band has beenassigned to an 'electron-transfer' excitation due toπ-σ conjugation of the silane unitwith thiophene. The other two bands are assigned to the thiophene chromophore. Some(obvious) results and trends can be extracted from this curve-fitting analysis: (1)Comparison of e.g. theλmax values or the molar extinction coefficient of thesecompounds, as presented in Table 2.5, within a series should be done with great care.(2) For the 'electron-transfer' band there is a small but systematic red-shift of 3-7 nmgoing from a disilanyl(ene) to a tetrasilanyl(ene) substituent. (3) The largest red-shift(18-19 nm) for the 'electron-transfer' band is observed going from the monosubstitutedthiophenes2.2 to the disubstituted ones (2.3). (4) The dimers2.5 show a red-shift of7-13 nm compared to the monomers2.4, but show only a minor red-shift comparedto the compounds2.3; the series2.3 and2.5 both comprise disubstituted thiophenes.

UV-vis absorption spectroscopy of sil(an)yl end-capped bi- and terthiophenes.

igures 2.5 and 2.6 show the UV-vis and fluorescence spectra of a series ofsil(an)yl-end-capped bi- and terthiophenes in solution. The absorption spectra of

all compounds have one broad main peak and a shoulder at the long wavelength sidebut are devoid of fine structure. Additionally they all possess a second peak atapproximately 235-240 nm of which the position is hardly affected by sil(an)ylsubstitution. The absorption maxima shift to lower energies with increasing length ofthe oligothiophene unit. The terthiophenes are 40-50 nm red-shifted compared to theirbithiophene analogues. Theλmax values of the solutions of all T2 and T3 compounds aregiven in Table 2.7. For comparison purposes theλmax values of the oligothiophenes Tx

with x = 1, 4 and 6 areincluded. The comparison of a sil(an)yl substituted mono-, bi-or terthiophene compound and its corresponding oligothiophene shows that there is adifference in the onset of the absorption band as well as in the position of themaximum (Figure 2.5 and 2.6); the spectra of substituted compounds are all red-shifted. We attribute this toπ-σ hyperconjugation. The replacement of the hydrogensat the terminalα-carbon atoms by permethylsil(an)yl groups is expected to affect theenergy of the excited state, since the electronic configuration at the terminal carbonsis characteristic of this state. The excited state wavefunction imposes the character ofthe LUMO on the oligothiophene block, which results in a quinoid bonding patternextending over a few rings.27 For short oligomers (T2, T3) this state is resonance-


28

stabilized by charge-separated and biradical configurations and lends reactivity to theterminalα-carbons, which acquire some radical character. Peralkylsil(an)yl groups areclaimed to be both electron-donating (through the Si-CAr σ-bond and, in the case ofsilanyl groups, through hyperconjugation between theπ-system and theσSi-Si bond) andelectron-accepting (through interaction of theπ-system with emptyd-levels on Si andespecially with the antibondingσ*-level in silanyl groups), and may stabilize theexcited state through (hyper)conjugation or through limited charge transfer.

Table 2.7 Absorption and fluorescence characteristics of oligosil(an)yl-end-cappedbi- and terthiophenes in solutiona

Absorption Fluorescence

Compound λmax

nmEmax

eVε

M-1cm-1λmax

nmEmax

eVφF Stokes

shift, eVE0-0

b,c

eV

T (2.1) 231 5.37 5600 - - - - 5.02

T2 (2.9) 301 4.12 12200 360 3.44 0.01 0.68 3.68/3.68d,e

T2SiMe3 (2.10a) 309 4.01 13200 368 3.37 0.03 0.64 3.62 (0.06)

T2Si2Me5 (2.10b) 314 3.95 15300 374 3.31 0.05 0.64 3.57 (0.11)

Me3SiT2SiMe3 (2.11a) 316 3.92 11800 374 3.31 0.06 0.61 3.55 (0.13)

Me5Si2T2Si2Me5 (2.11b) 325 3.82 21300 390 3.18 0.23 0.64 3.46 (0.22)

T3 (2.12) 351 3.53 23100 423 2.93 0.07 0.60 3.17/3.11d,e

T3SiMe3 (2.13a) 358 3.46 11300 413/433 3.00/2.86 0.10 0.46/0.60 3.12 (0.05)

T3Si2Me5 (2.13b) 360 3.44 22900 416/438 2.98/2.83 0.10 0.46/0.61 3.09 (0.08)

Me3SiT3SiMe3 (2.14a) 362 3.42 12200 418/440 2.97/2.82 0.10 0.45/0.60 3.07 (0.10)

Me5Si2T3Si2Me5 (2.14b) 368 3.37 29000 424/450 2.92/2.76 0.12 0.45/0.61 3.02 (0.15)

T4 (e) 385 3.22 32000 444/472 2.79/2.62 0.20 0.43/0.60 2.84/2.82d

T6 (d,e) 432 2.87 23100 508/541 2.44/2.29 0.32 0.43/0.58 2.52

a n-Hexane unless stated otherwise;φF = Fluorescence quantum yield;b E0-0 was taken as thecrossing point of the absorption and the fluorescence specta.c In parentheses the difference∆(E0-0) of the compound with respect to the corresponding oligothiophene.d Garciaet al., J.Phys. Chem.1993, 97, 513. e CH2Cl2.

Our own MO-calculations, performed at the non-correlated level, indicate that theaddition of a silyl group to a thiophene or bithiophene system results in a lowering ofthe energies of both HOMO and LUMO, but that stabilization is stronger for theLUMO level. The spectra of the model compounds demonstrate that disilanyl endgroups have a stronger stabilizing interaction than silyl groups, but silyl-substitutionat both ends of the oligothiophene block is slightly more effective than a singledisilanyl group at one end.

Chapter 2

29

T

Figure 2.5Absorption (—) and fluorescence (--) spectraof sil(an)yl end-capped 2,2'-bithiophenes:(a) T2 (2.9) ; (b) T2SiMe3 (2.10a); (c)T2Si2Me5 (2.10b); (d) Me3SiT2SiMe3 (2.11a);(e) Me5Si2T2Si2Me5 (2.11b).

Figure 2.6Absorption (—) and fluorescence (--) spectraof sil(an)yl end-capped -terthiophenes:(a) T3 (2.12); (b) T3SiMe3 (2.13a); (c)T3Si2Me5 (2.13b); (d) Me3SiT3SiMe3 (2.14a);(e) Me5Si2T3Si2Me5 (2.14b).

Fluorescence spectroscopy of sil(an)yl-end-capped bi- and terthiophenes

hiophene itself is non-fluorescent, probably because radiationless transitions to atriplet state compete successfully with fluorescence.28 (This is probably also related

to the relative energy levels of1A and 1B states,29 about which there is controversy.30)All compounds having two or more thiophene rings show fluorescence, but the


30

quantum efficiencies vary strongly. While bithiophene shows only a broad band, thefluorescence spectra of the longer oligothiophenes T3 and T4 are characterized by thepresence of two distinct peaks and a shoulder. The appearance of multiple peaks hasbeen attributed to the coexistence of distinct conformations (rotamers) of the molecules.They arise because planar conformations are favoured by the excitation, and in these,the inter-ring configurations can be eithersynor anti with regard to the position of thesulfur atoms.31,3 These details are not discernable in the spectrum of T2. This may resultpartly because the signal is weak and noisy, but is probably mainly determined by thelifetime of the planar excited states.

All sil(an)yl-end-capped oligothiophenes longer than thiophene itself show amoderate to intense fluorescence in solution. In a fashion similar to the absorptionspectra, the fluorescence bands of terthiophenes are shifted to longer wavelengthscompared to bithiophenes. As a result, bithiophenes emit light in the UV to blue regionand terthiophenes in the blue region. Figures 2.5 and 2.6 show the fluorescence of aseries of permethylsil(an)yl-end-capped bi- and terthiophenes. For comparison thespectra of 2,2'-bithiophene and 2,2':5',2"-terthiophene are also included. Thefluorescence spectra of the T2 compounds having sil(an)yl substituents show some finestructure where bithiophene has only one band. All the T3 compounds, includingterthiophene, show some degree of resolution. Fluorescence maxima, fluorescencequantum yields and the E0-0 (chosen as the crossing of the absorption and fluorescencespectra) are given in Table 2.7. The E0-0 values and fluorescence maxima of the T2 andT3 compounds have a systematic bathochromic shift depending upon sil(an)ylsubstitution. The net bathochromic shift is largest for the terminal pentamethyldisilanyl-disubstituted compounds,2.11band2.14b, respectively.

The very low quantum efficiency of bithiophene in solution, which isapproximately 1%, is raised to 6% by end-capping T2 with trimethylsilyl groups andfurther increased to 23% by end-capping with pentamethyldisilanyl groups. Longeroligothiophenes have a higher quantum efficiency by themselves, and the silyl-end-capping is found to have a much weaker effect. It must be concluded that the levelordering in T2 is considerably altered by the substitution, with the effect that excitedstates become more stable towards the non-radiative processes that compete withfluorescent decay, i.e. internal conversion and intersystem crossing (singlet-to-tripletconversion).31b,32 Clearly, the stabilization is increasingly less effective, but also lessimportant, for longer oligomers. The trend in the transition energies is the same as thatfound in absorption: disilanyl(ene) groups provide a red shift with respect tomonosilyl(ene) groups.

The shift between the absolute maxima of absorption and fluorescence is generallyin the range 0.4-0.7 eV (Stokes shift). In organic molecules, part of the shift wouldcorrespond to the stabilization of the excited state through a small change in themolecular geometry. In the jargon of conjugated polymers, this is the electron-phononcoupling that gives rise to the exciton state.27a,33 It is understood that in the case ofcharged excitations the extent of the lattice deformation characteristic of a fully relaxed(bi)polaron cannot be realized in a unit as short as bithiophene or even terthiophene,

Chapter 2

31

W

and a longer oligothiophene block is required to accommodate it;34 the delocalizationof a neutral, bound exciton (photoexcitation) is expected to be more limited.

2.4 CONCLUSIONS

e have demonstrated the chemical synthesis of model compounds consisting ofwell-defined oligothiophenes having sil(an)yl substituents. Their spectroscopic

properties have been explored and were found to show a systematic dependence on thesize of the oligothiophene and/or the sil(an)yl substituent.

From a scientific perspective, the model compounds are interesting, in a broadsense, because of the obvious effects of electronic interaction between theoligothiophene and sil(an)yl(ene), a phenomenon commonly referred to asπ-σconjugation. This effect is clearly manifested by end-capped oligothiophenes. Disilanylsubstituents are found to have a stronger overlap with theπ-system than monosilylgroups. The difference between disilanyl and silyl groups is also significant withrespect to fluorescence efficiency, especially for T2-based compounds. Solutionfluorescence efficiencies of the substituted model compounds based on T2 amount upto 23%, and thus are much higher than that of bithiophene itself (1%).The influence of the sil(an)yl blocks on the spectroscopic signature of theoligothiophenes becomes smaller as the number of rings in the latter increases, i.e. asthe HOMO-LUMO gap of the oligothiophene block decreases.

2.5 EXPERIMENTAL

General ProceduresNMR-data were recorded at 299.95 MHz (1H), 75.42 MHz (13C) and 59.59 MHz (29Si) on

a Varian (VXR 300) spectrometer operating in the Fourier transform mode. The1H-NMRchemical shifts are determined relative to the solvent (CDCl3) and converted to the TMS scaleusingδ(CDCl3 )= 7.26 ppm. The coupling constants are denoted in Hz. The13C-NMR chemicalshifts are denoted inδ units (ppm) relative toδ(CDCl3)=76.91 ppm.29Si-NMR spectra wererecorded using a DEPT pulse sequence. The29Si-NMR chemical shifts are externally referencedto TMS (0 ppm). Splitting patterns are designed as follows: s (singlet), d (doublet), dd (doubledoublet), t (triplet), q (quartet), m (multiplet) and br(broad). Full assignment of13C-NMRchemical shifts based on13C, 1H-coupled and 2D HETCOR spectra are included for somerepresentative compounds. IR-spectra of solids in KBr pellets were recorded on a MattsonInstruments FT-IR spectrometer. Spectra of liquids were taken as neat films on NaCl plates.The absorptions are denoted as s (strong), m (medium), w (weak), br (broad) and sh (shoulder).UV-spectra of oligomers and monomers were recorded on a Philips PU 8740 UV/VIS scanningspectrophotometer or a Milton Roy Spectronic 3000 ARRAY spectrometer in spectral gradehexane. Luminescence spectra were recorded on a SLM Aminco 500 spectrophotometer.Quantum yields of fluorescence were determined using quinine sulfate (φfl(1 N H2SO4) = 0.55)as standard.35 Solutions were purged with argon. Mass spectra were obtained with an AEI MS9mass spectrometer (Mr. A. Kiewiet, Department of Organic Chemistry, University ofGroningen). Elemental analysis were carried out at the Microanalytical Department of theUniversity of Groningen (Mr. H. Draaijer and Mr. J. Ebels).


32

Materials and ProceduresAll reactions were performed under a dry argon atmosphere unless stated otherwise.

Diethylether was distilled subsequently from P2O5 and LiAlH4. THF was distilled subsequentlyfrom potassium and LiAlH4. Pentane was distilled from P2O5. Thiophene,n-BuLi (1.6 M or 2.5M solution in hexane) and diisopropylamine (all Janssen) were used as received (Janssen).2,2'-Bithiophene was synthesized according to Tamao and coworkers.36 2,2':5',2"-Terthiophenewas prepared from 1,4-di-2-thienyl-1,4-butanedione.37 N,N,N',N'-tetramethylethylenediamine(TMEDA, Janssen) was dried on KOH before use. Dichlorosilanes (Petrarch) were distilledfrom CaH2 before use. 1,2-Dichlorotetramethyldisilane was synthesized according to Hu andWeber.38 1,4-Dichlorooctamethyltetrasilane and chloropentamethyldisilane were prepared from1,4-diphenyloctamethyltetrasilane.39 1,4-Diphenyloctamethyltetrasilane was prepared from1-chloro-2-phenyltetramethyldisilane in 90% yield using an excess sodium in refluxing tolueneinstead of a sodium-potassium alloy in xylene at 80°C. The catalyst dichloro[1,3-bis-(diphenylphosphino)propane]nickel(II) (Ni[dppp]Cl2) was synthesized according to van Heckeand Horrocks.40

2-Trimethylsilylthiophene 2.2a (TSiMe3).TMEDA (6.12 g, 50 mmol) was added to a solution ofn-BuLi (20 mL, 50 mmol, 2.5M inn-hexane). After 5 min the clear warm solution was cooled to -10°C and thiophene (8.41 g,50 mmol) was added dropwise, while the temperature was kept between 0 and 10°C. After 5min pentane (10 mL) was added and stirring was continued for 1 hour. The reaction mixturewas cooled to 0°C and chlorotrimethylsilane (6.25 g, 58 mmol) was added. The mixture wasstirred for 1 hour before being poured into water. The aqueous layer was extracted with ether(2 x 75 mL) and the combined organic layers were washed with 5% NaCl-solution (25 mL),dried (MgSO4), filtered, and the solvents were removed by evaporation under reduced pressure.The residue was vacuum distilled (74°C, 40 mbar) to provide 6.73 g (43 mmol, 74 %) of thetitle product as a colourless liquid.1H-NMR δ: 0.40 (s, 9H, SiMe3), 7.23 (dd, 1H,3J= 4.6 and3.3 Hz, H-4), 7.32 (dd, 1H,3J= 3.3 Hz,4J= 0.9 Hz, H-3), 7.63 (dd, 1H,3J= 4.6 Hz,4J= 0.9 Hz,H-2); 13C-NMR δ: -0.1 (SiMe3), 128.0 (1J= 167.5 Hz,2J= 4.0 Hz,3J= 6.4 Hz, C-4), 130.3 (1J=186.7 Hz,2J= 7.2 Hz,3J= 9.6 Hz, C-5), 133.9 (1J= 165.1 Hz,2J= 6.0 Hz,3J=10.7 Hz, C-3),140.0 (C-2);29Si-NMR δ: -6.5 ppm. IR 2958(m), 1407(w), 1251(m), 1214(w), 1083(w), 993(m),858(m), 840(s), 757(m), 707(w) cm-1. MS, m/e (rel intensity) 156 (M+, 21), 141 (M+-Me, 100);HRMS calcd for C7H12SSi 156.043, found 156.044.

5-Pentamethyldisilanylthiophene 2.2b (TSi2Me5).The reaction was performed on a 10 mmol scale, following the same procedure as applied for2.2a, using chloropentamethyldisilane (2.0 g, 12 mmol) instead of chlorotrimethylsilane.Kugelrohr distillation (85°C, 17 mbar) provided 0.75 g (3.5 mmol, 35 %) of the title productas a colourless liquid.1H-NMR δ: 0.12 (s, 9H, SiMe3), 0.40 (s, 6H, SiMe2), 7.20 (dd, 1H,3J=4.5 and 3.3 Hz, H-4), 7.22 (dd, 1H,3J= 3.3 Hz,4J= 1.1 Hz, H-3), 7.60 (dd, 1H,3J=4.6 Hz,4J=1.1 Hz, H-2);13C-NMR δ: -2.8/-2.6 (Me), 128.0 (C-4), 130.2 (C-5), 133.9 (C-3), 138.9(C-2); 29Si-NMR δ: -24.1 (SiMe2), (SiMe2), -19.3 (SiMe3) ppm. IR 2953(s), 1405(m), 1247(s),1212(m), 1082(w), 986(m), 833(s), 796(s), 762(m), 703(m) cm-1. MS , m/e (rel intensity) 214(M+, 46), 199 (M+-Me, 62), 141 (M+-SiMe3, 100), 73 (SiMe3, 62), 28 (Si, 22); HRMS calcd forC9H18SSi2 214.067, found 214.067.

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2-Nonamethyltetrasilanylthiophene 2.2c (TSi4Me9).The reaction was performed on a 10 mmol scale following the same procedure as applied aboveusing 1-chlorononamethyltetrasilane (4.52 g, 16 mmol) instead of chlorotrimethylsilane.Kugelrohr distillation (125°C, 0.5 mbar) provided 2.51 g (7.6 mmol, 76 %) of the title productas a colourless liquid.1H-NMR δ: 0.059 (s, 6H, SiMe2), 0.063 (s, 9H, SiMe3), 0.15 (s, 6H,SiMe2), 0.42 (s, 6H, SiMe2), 7.17 (1H), 7.19 (1H), 7.58 (dd, 1H,);1H-NMR (C6D6) δ: 0.10 (s,9H, SiMe3), 0.14 (s, 6H, SiMe2), 0.22 (s, 6H, SiMe2), 0.44 (s, 6H, SiMe2), 7.00 (dd, 1H,3J=4.6 and 3.2 Hz, H-4), 7.16 (dd, 1H,2J= 3.2 Hz,3J=1.0 Hz, H-3), 7.28 (dd, 1H,2J= 4.6 Hz,3J=1.0 Hz, H-5)13C-NMR δ: -5.9/-5.6/-1.4/-1.3 (Me), 128.1 (C-4), 130.2 (C-5), 133.9 (C-3), 139.2(C-2); 29Si-NMR δ: -44.7 -44.6 -20.3 (T-SiMe2), -15.1 (SiMe3) ppm. IR 2951(s), 1404(m),1246(s), 1212(m), 1082(w), 986(m), 832(s), 775(s), 727(m), 707(w) cm-1. MS m/e, (relintensity) 330 (M+, 10), 257 (M+-SiMe3, 100), 141 (Si2Me5, 23), 73 (SiMe3, 65); HRMS calcdfor C13H30SSi4 330.115, found 330.115.A second fraction was obtained (155°C, 0.5 mbar), providing after column purification (silica-gel-60, pentane) 0.43 g (0.7 mmol, 0.75%) of2,5-bis(nonamethyltetrasilanyl)thiophene 2.3c(Me9Si4TSi4Me9) as a thick colourless liquid which solidified upon standing, mp 28-30°C. 1H-NMR δ: 0.09 (s, 15H), 0.16 (s, 6H), 0.44 (s, 6H), 7.27 (s, 2H);13C-NMR δ: -6.0/-5.7/-1.4/-1.3(Me), 135.0 (CH), 144.9 (C);29Si NMR δ: -44.7/-44.6/-20.7/-15.1 ppm. IR 2950(m), 1401(w),1246(m), 1200(w), 1001(m), 832(s), 801(m), 774(s) cm-1. MS, m/e (rel. intensity) 576 (M+,1)561 (M+-CH3, 4), 503 (M+-SiMe3, 89), 189 (Si3Me7, 38) 131 (Si2Me5, 20), 73 (SiMe3,100);HRMS calcd for C22H56SSi8 576.226, found 576.226.

2,5-Bis(trimethylsilyl)thiophene 2.3a (Me3SiTSiMe3).TMEDA (6.12 g, 50 mmol) was added to a solution ofn-BuLi (20 mL, 50 mmol). After 5minutes the clear warm solution was cooled to -10°C and thiophene (8.41 g, 50 mmol) wasadded dropwise, while the temperature was kept between 0 and 10°C. After 5 min. pentane (10mL) was added and stirring was continued during another 5 min. The reaction mixture wascooled to 0°C after which a second equivalent ofn-BuLi (20 mL, 50 mmol) was added. Thewhite suspension dissolved between 20 and 25°C. Upon slowly heating to 30°C a whitesuspension was formed again, which was heated under reflux for 30 min. and subsequentlycooled down to 0°C. Ether (50 mL) was added, followed by an excess of chlorotrimethylsilane(12.5 g, 115 mmol). The temperature was raised to 30°C and stirring was continued for 1 hour.The reaction mixture was poured into 50 mL of cold water and extracted with ether (2 x 75mL). The combined organic layers were washed with 5% NaCl solution (25mL), dried(MgSO4), filtered, and the solvents were removed by evaporation under reduced pressure.Theresidue was vacuum distilled (95°C, 10 mbar) to provide 8.9 g (39 mmol, 78 %) of the titleproduct as a white solid, mp 31-32°C. 1H-NMR δ: 0.33 (s, 18H, SiMe3), 7.33 (s, 2H);13C-NMR δ: 0.0 (Me), 135.0 (C-3), 145.8 (C-2);29Si-NMR δ: -6.9 ppm. IR 2959(m), 1487(w),1413(w), 1266(w), 1247(m), 1199(w), 1014(m), 839(s), 806(m), 757(m) cm-1. MS, m/e (rel.intensity) 228 (M+, 18), 213 (M+-Me, 100), 73 (SiMe3, 21); HRMS calcd for C10H20SSi2228.082, found 228.082.

2,5-Bis(pentamethyldisilanyl)thiophene 2.3b (Me5Si2TSi2Me5).The reaction was performed on a 10 mmol scale, following the same procedure as applied for(SiTSi). Chloropentamethyldisilane (4.0 g, 24 mmol) was used instead of chlorotrimethylsilane.Kugelrohr distillation (150°C, 18 mbar) provided 1.20 g (3.5 mmol, 35 %) of the title product


34

as a colourless liquid.1H-NMR δ: 0.11 (s, 18H, SiMe3), 0.39 (s, 12H, SiMe2), 7.26 (s, 2H, H-3);13C-NMR δ: -2.7/-2.5 (Me), 135.0 (C-3), 144.6 (C-2);29Si-NMR δ: -24.6/-19.3 ppm. IR2952(m), 1403(w), 1246(m), 1201(w), 1141(w), 1001(m), 833(s), 794(s), 762(m), 724(w) cm-1.MS , m/e (rel. intensity) 344 (M+, 30), 329 (M+-Me, 26), 241 (100), 73 (SiMe3, 100); HRMScalcd for C14H32SSi4 344.130, found 344.129.

Bis(2'-thienyl)dimethylsilane 2.4a (TSiMe2T).TMEDA (58.0 g, 0.5 mol) and thiophene (42.0 g, 0.5 mol) were added in 15 minutes to 1.1 eq.of n-BuLi. Dichlorodimethylsilane (36.0 g, 0,28 mol) was added at 0°C. The reaction mixturewas poured into 1 L. of an aqueous 5% NH4Cl solution and extracted with pentane (2x250 mL).The concentrated residue was distilled using a 15 cm vacuum-jacked Vigreux column. NMR-pure fractions of2.4a, 2.5a and2.6a were obtained:Bis(2'-thienyl)dimethylsilane2.4a, 37.5 g (167 mmol, 67 %), b.p. 78-80°C / 0.1 mm Hg.1HNMR δ 0.69 (s, 6H), 7.24 (dd, H-4), 7.35 (dd, 2H, H-3), 7.66 (dd, 2H, H-5) ;13C NMR δ -0.1(Me), 128.2/131.3/135.4/136.4 (CH), 137.1/144.0 (C);29Si NMR δ -15.4 ppm. Spectral datawere similar to those reported in literature;2,5-Bis(2'-thienyldimethylsilylene)thiophene 2.5a (T[SiMe2T]2). 8.4 g, (23 mmol, 14 %), b.p.207-210°C / 1.0 mm Hg.1H NMR δ 0.66 (s, 12H), 7.21 (dd,3JH-5'= 4.5 Hz,3JH-3'= 3.0 Hz, 2H,H-4'), 7.35 (dd,4JH-5'= 1.0 Hz,3JH-4'= 3.0 Hz, 2H, H-3'), 7.40 (s, 2H, H-3 and H-4), 7.65 (dd,3JH-4'= 4.5 Hz,4JH-3'= 1.0 Hz, 2H, H-5') ;13C NMR δ -0.1 (Me), 128.2/131.3/135.4/136.4 (CH),137.1/144.0 (C);29Si NMR δ -15.4 ppm. UV:λmax = 243.5 nm,ε = 27500. MS, m/e (rel.intensity) 364 (M+, 43), 349 (M+-CH3, 100) 281 (M+-C4H3S, 64), 141 (C4H3SSiMe2, 60); HRMScalcd for C16H20S3Si2 364.026, found 364.027;Bis[5-(2'-thienyl)dimethylsilylene)-2-thienyl]dimethylsilane 2.6 (T[SiMe2T]3), 4.0 g (7 %),b.p. 248-250°C / 0.1 mm Hg. Elemental analysis calcd for C22H28S4Si3 : C, 52.32; H, 5.59; S,25.39. Found: C, 52.59; H, 5.61; S, 25.42.1H NMR δ 0.68 (s,18H), 7.24 (dd, 2H, H-4'), 7.37(dd, 2H, H-3'), 7.42 (s, 2H, H-2), 7.42 (s, 2H, H-3), 7.66 (dd, 2H, H-5') ;13C NMR δ -0.1/0.15(Me), 128.2/131.2/135.4/136.4 (CH), 137.2/143.9 (C);29Si NMR δ -15.7/-15.4 ppm. UV:λmax

= 245 nm, ε = 22700. MS, m/e (rel. intensity) 504 (M+,43), 489 (M+-CH3,80) 349 (M+-C4H2SSiMe3,100); HRMS calcd for C22H28S4Si3 504.038, found 504.038;

1,2-Bis(2'-thienyl)-1,1,2,2-tetramethyldisilane 2.4b (TSi2Me4T).The reaction was performed on a 0.1 mol scale, analogously to2.4a from 1,2-dichlorotetramethyldisilane (9.4 g, 50 mmol). The concentrated crude reaction mixture waskugelrohr distilled (b.p. 135°C / 0.5 mm Hg), yielding 12.2 g (43 mmol, 86%) of NMR pureproduct, which solidified on standing. Melting point (38-40°C) and spectral properties were inaccordance with the literature values.A second fraction (b.p. 250°C / 0.5 mm Hg) was obtained yielding 0.4 g (8.3 mmol, 3%) of2,5-bis(2'-thienyl-1,1,2,2-tetramethyldisilanylene)thiophene 2.5b (T[Si2Me4T]2) which couldbe purified by crystallization from ether/methanol (4:1), mp 70-74°C. 1H NMR δ 0.42 (s, 24H),7.17 (d, 2H), 7.19 (d, 2H), 7.23 (d, 2H), 7.60 (dd, 2H);13C NMR δ -2.6 (Me), 128.2/130.6/-134.4/135.6 (CH), 137.8/143.8 (C);29Si NMR δ -24.8/-24.5 ppm. UV:λmax = 249 nm,ε = 31800. MS, m/e (rel. intensity) 480 (M+, 15), 141 (C4H3SSiMe2, 100); HRMS calcd forC20H32S3Si4 480.074, found 480.074.

1,4-Bis(2'-thienyl)-1,1,2,2,3,3,4,4-octamethyltetrasilane 2.4c (TSi4Me8T).TMEDA (2.32 g, 20 mmol) was added to a solution ofn-BuLi (8.2 mL, 20.5 mmol). After 5

Chapter 2

35

minutes the clear warm solution was cooled to -20°C, and thiophene (1.68 g, 20 mmol) wasadded within 5 minutes, while the temperature was kept between 0 and 5°C. After 5 min. 8mL of ether was added and stirring was continued during another 5 min. The reaction mixturewas further cooled to -70°C after which a solution of 1,4-dichlorooctamethyltetrasilane (3.03g, 10 mmol) in 5 mL of ether was added within 5 min. The temperature was raised slowly toroom temperature during 2 hours. The reaction mixture was poured into 200 mL of a cold 5%aqueous NH4Cl solution and extracted with pentane (2 x 50 mL). The combined organic layerswere washed with 5% NaCl solution (25 mL), dried (MgSO4), filtered, and the solvents wereremoved by evaporation under reduced pressure. The solidified residue was kugelrohr distilled(b.p. 185°C / 0.5 mm Hg), yielding 3.6 g (91%). Crystallization from ether/methanol resultedin an analytically pure product, mp 62-63°C. Elemental analysis calcd for C16H30Si4S2 (398.9):C, 48.18; H, 7.58; S, 16.08. Found: C, 48.36; H, 7.66; S, 15.93.1H NMR δ 0.09 (s, 12H), 0.39(s, 12H), 7.18 (d, 2H), 7.19 (d, 2H), 7.58 (dd, 2H);13C NMR δ -5.8/-1.6 (Me),128.2/130.3/134.0 (CH), 139.0 (C);29Si NMR δ -44.5/-20.2 ppm. UV:λmax = 248.6 nm,ε = 25400. MS, m/e (rel. intensity) 398 (M+,7), 257 (M+-C4H3SSiMe2,100), 141(C4H3SSiMe2,36); HRMS calcd for C16H30S2Si4 398.086 ,found 398.087.From the residue of the distillation, 0.1 g (0.14 mmol, 2 %) of2,5-bis(2'-thienyl-1,1,2,2,3,3,4,4-octamethyltetrasilanylene)thiophene 2.5c (T[Si4Me8T]2) mp 75-79°C, was obtained aftercolumn purification (silicagel-60, solvent CH2Cl2 / pentane) and crystallization from methanol.1H NMR δ 0.06 (s,12H), 0.07 (s,12H), 0.38 (s,12H), 0.38 (s,12H), 7.18 (d,2H), 7.19 (d,2H),7.21 (bs,2H), 7.58 (dd,2H) ;13C NMR δ -5.8/-1.6 (Me), 128.2/130.3/134.0/135.1 (CH),139.1/144.8 (C);29Si NMR δ -44.5/-44.5/-20.7/-20.2 ppm. UV:λmax = 248 nm,ε = 33300 andλmax = 256 nm,ε = 32700. MS, m/e (rel. intensity) 712 (M+,0.3) 571 (M+-C4H3SSiMe2,88 ), 141(C4H3SSiMe2,49), 73 (SiMe3,100); HRMS calcd for C28H56S3Si8 712.171, found 712.170.

5-Trimethylsilyl-2,2'-bithiophene 2.10a (T2SiMe3).The reaction was performed on a 10 mmol scale, following the same procedure as applied for2.2a using bithiophene instead of thiophene. Kugelrohr distillation (155°C, 20 mbar) of thecrude reaction product provided 1.63 g (6.8 mmol, 68 %) of the title product as a colourlessliquid. 1H-NMR δ: 0.34 (s, 9H, SiMe3), 7.01 (dd, 1H,3J= 5.0 and 3.6 Hz, H-4'), 7.13 (d, 1H,3J= 3.4 Hz, H-4), 7.18 (dd, 1H,3J= 3.6 Hz,4J= 1.2 Hz, H-3'), 7.205 (dd, 1H,3J= 5.1 Hz,4J=1.2 Hz, H-5'), 7.23 (d, 1H,3J= 3.4 Hz, H-3);13C-NMR δ: -0.2 (Me), 123.7 (1J= 166.7 Hz,2J=5.-5 Hz, 3J=9.1 Hz, C-3'), 124.3 (1J= 186.8 Hz,2J= 6.6 Hz,3J= 10.8 Hz, C-5'), 124.9 (1J= 165.9Hz, 2J= 6.6 Hz, C-3), 127.7 (1J= 168.5 Hz,2J= 3.8 Hz,3J= 5.0 Hz, C-4'), 134.6 (1J= 165.7 Hz,2J= 5.0 Hz, C-4), 137.3 (C-2'), 139.7 (C-5), 142.3 (C-2);29Si-NMR δ: -6.5 ppm. IR 2956(m),1420(m), 1253(m), 1202(w), 1076(w), 996(m), 887(w), 838(s), 804(s), 755(m) cm-1. MS , m/e(rel. intensity) 238 (M+, 66), 223 (M+-Me, 100); HRMS calcd for C11H14S2Si 238.031, found.238.031.

5-Pentamethyldisilanyl-2,2'-bithiophene 2.10b (T2Si2Me5).The reaction was performed on a 10 mmol scale, following the same procedure as applied for2.2b, using 2,2'-bithiophene (1.66 g, 10 mmol) in ether (10 mL) instead of thiophene. Kugelrohrdistillation (135 °C, 0.01 mbar) and subsequent column purification (silicagel-60, CH2Cl2/-pentane = 2:1) provided 0.41 g (13.8 mmol, 14 %) of the title product as a thick colourlessliquid. 1H-NMR δ: 0.11 (s, 9H, SiMe3), 0.37 (s, 6H, SiMe2), 7.01 (dd, 1H,3J= 5.1 and 3.7 Hz,H-4'), 7.07 (d, 1H,3J= 3.4 Hz, H-4), 7.18 (dd, 1H,3J= 3.7 Hz,4J= 1.1 Hz, H-3'), 7.20 (dd, 1H,


36

3J= 5.1 Hz,4J= 1.1 Hz, H-5'), 7.23 (d, 1H,3J= 3.4 Hz, H-3);13CNMR δ: -2.7/-2.3 (Me), 123.5(1J= 166.2 Hz,2J= 5.5 Hz,3J= 9.1 Hz, C-3'), 124.1 (1J= 186.8 Hz,2J= 7.0 Hz,3J= 11.1 Hz,C-5'), 125.0 (1J= 165.2 Hz,2J= 6.0 Hz, C-3), 127.7 (1J= 168.2 Hz,2J= 3.8 Hz,3J= 5.0 Hz, C-4'),134.6 (1J= 165.2 Hz,2J= 5.0 Hz, C-4), 137.3 (C-2'), 139.7 (C-5), 142.3 (C-2);29Si-NMR δ:-24.0/-19.2 ppm. IR 2951(m), 1419(w), 1246(m), 1068(w), 983(m), 835(s), 795(s), 762(w),725(w) cm-1. MS, m/e (rel. intensity) 296 (M+, 96), 281 (M+-Me, 48), 223 (M+-SiMe3, 100), 73(SiMe3, 36); HRMS calcd for C13H20S2Si2 296.054, found 296.054.

5,5'-Bis(trimethylsilyl)-2,2'-bithiophene 2.11a (Me3SiT2SiMe3).The reaction was performed on a 10 mmol scale, following the same procedure as applied for2.3a. Kugelrohr distillation (190°C, 1mm Hg) of the crude reaction product provided 2.11 g(6.8 mmol, 68 %) of the title product as a white solid, mp 91-92°C. 1H-NMR δ: 0.34 (s, 18H,SiMe3), 7.13 (d, 2H,3J= 3.5 Hz, H-4), 7.23 (d, 2H, H-3);13C-NMR δ: -0.2 (Me), 125.0 (1J=165.2 Hz,2J=7.2 Hz, C-3), 134.6 (1J= 166.0 Hz,2J= 4.8 Hz, C-4), 139.8 (C-5), 142.4 (C-2);29Si-NMR δ: -6.6 ppm. IR 2953(m), 1423(m), 1249(s), 1198(m), 1074(m), 988(s), 874(s),840(s), 798(s), 754(s) cm-1. MS, m/e (rel. intensity) 310 (M+, 100), 295 (M+-Me, 90), 73(SiMe3, 31); HRMS calcd for C14H22S2Si2 310.070, found 310.071.

5,5'-Bis(pentamethyldisilanyl)-2,2'-bithiophene 2.11b (Me5Si2T2Si2Me5).The reaction was performed on a 10 mmol scale, following the same procedure as applied for2.3b, using 2,2'-bithiophene (1.66 g, 10 mmol) instead of thiophene. Kugelrohr distillation (190°C, 0.01 mm Hg) and subsequent crystallization from chloroform/methanol provided 1.82 g (4.3mmol, 43%) of the title product as a white solid, mp 77-78°C. 1H-NMR δ: 0.12 (s, 18H,SiMe3), 0.38 (s, 12H, SiMe2), 7.07 (d, 2H,3J= 3.5 Hz, H-4), 7.24 (d, 2H, H-3);13C-NMR δ:-2.9/-2.5 (Me), 124.9 (C-3), 134.7 (C-4), 138.5, (C-5), 142.2 (C-2);29Si-NMR δ: -24.0/-19.2ppm. IR 2950(m), 1420(w), 1242(m), 1193(w), 1070(w), 986(m), 873(w), 832(m), 793(s),763(w), 725(w) cm-1. MS, m/e (rel. intensity) 426 (M+, 59), 353 (M+-SiMe3, 82), 73 (SiMe3,100) HRMS calcd for C18H34S2Si4 426.118, found 426.118.

2-Bromo-5-trimethylsilylthiophene BrTSiMe3.1H-NMR δ: 0.34 (s, 9H, SiMe3), 7.01 (d, 1H,3J= 3.4 Hz, H-4), 7.11 (d, 2H, H-3);13C-NMRδ: -0.3 (Me), 131.0/134.2 (CH), 116.6/143.1 (C);29Si-NMR δ: -6.1 ppm. NMR data were inaccordance with the literature values.

2-Bromo-5-pentamethyldisilanylthiophene BrTSi2Me5.To a solution of diisopropylamine (1.02 g, 10 mmol) in THF (10 mL) at -60°C n-BuLi (4.0mL, 10 mmol) was added dropwise. The mixture was warmed to 0°C for 5 min. and thenrecooled to -60°C. 2-Bromothiophene (1.65 g, 10 mmol ) was added dropwise without furthercooling. After 30 min. the clear solution was recooled to -50°C and chloropentamethyldisilane(2.0 g, 12 mmol) was added in one portion and the solution was allowed to warm to roomtemperature within 30 min. The mixture was poured into water (50 mL) and the aqueous layerwas extracted with ether (3 x 50 mL). The combined organic layers were washed with 5% NaClsolution (25 mL), dried (MgSO4), filtered, and the solvents were removed by evaporation underreduced pressure. Kugelrohr distillation (80°C, 1 mbar) provided 2.35 g (8 mmol, 80 %) of thetitle product as a colourless liquid.1H-NMR δ: 0.10 (s, 9H, SiMe3), 0.34 (s, 6H, SiMe2), 6.92(d, 1H, 3J= 3.5 Hz, H-4), 7.08 (d, 1H, H-3);13C-NMR δ: -3.0/-2.6 (Me), 131.2/134.3 (CH),116.2/151.4 (C);29Si-NMR δ: -23.6/-19.3 ppm.

Chapter 2

37

5-Trimethylsilyl-2,2':5',2''-terthiophene 2.13a (T3SiMe3).To a solution of diisopropylamine (1.53 g, 15 mmol) in THF (10 mL) at -60°C n-BuLi (6.0mL, 15 mmol) was added dropwise. The mixture was warmed to 0°C for 5 min. and thenrecooled to -60°C. 2,2':5',2''-Terthiophene (7.44 g, 30 mmol) in THF (50mL) was addeddropwise and the green suspension was warmed to 0°C for 5 min. After recooling to -60°C,chlorotrimethylsilane (2.7 g, 25 mmol) was added in one portion and the solution was allowedto warm to room temperature for 30 min. The mixture was poured into water (50 mL) andextracted with ether (3 x 100 mL). The combined organic layers were washed with 5% NaClsolution (50 mL), dried (MgSO4), filtered, and the solvents were removed by evaporation underreduced pressure. A pure fraction of2.13a (0.80 g, 2.5 mmol, 10 %) was obtained afterKugelrohr distillation (205°C, 0.04 mm Hg) as a green-yellow solid, mp 62-64°C. 1H-NMRδ: 0.34 (s, 9H, SiMe3), 7.02 (dd, 1H,3J= 5.0 and 3.6 Hz, H-4"), 7.08 (s, 2H, H-3'/4') 7.14 (d,1H, 3J= 3.4 Hz, H-4), 7.17 (dd, 1H,3J= 3.6 Hz,4J= 1.2 Hz, H-3"), 7.21 (dd, 1H,3J= 5.1 Hz,4J= 1.2 Hz, H-5"), 7.22 (d, 1H,3J= 3.4 Hz, H-3);13C-NMR δ: -0.2 (Me), 123.5 (1J= 166.7 Hz,2J= 5.5 Hz,3J= 9.1 Hz, C-3"), 124.25 (1J= 167.4 Hz,2J= 4.3 Hz, C-3'/4'), 124.32 (1J= 187.0 Hz,2J= 6.8 Hz,3J= 10.6 Hz, C-5"), 124.8 (1J= 165.2 Hz,2J= 6.6 Hz, C-3), 127.8 (1J= 168.7 Hz,2J=4.0 Hz,3J= 5.0 Hz, C-4'), 134.7 (1J= 166.0 Hz,2J= 5.2 Hz, C-4), 136.1 (C-2'/5'), 137.1 (C-2"),139.7 (C-5), 142.3 (C-2);29Si δ: -6.4 ppm. IR 2954(m), 1425(m), 1251(m), 1198(m), 1073(m),989(s), 866(m), 835(s), 794(s), 754(m) cm-1. MS , m/e (rel. intensity) 320 (M+, 100), 305(M+-Me, 61); HRMS calcd for C15H16S3Si 320.018, found 320.017.

5-Pentamethyldisilanyl-2,2':5',2''-terthiophene 2.13b (T3Si2Me5).2-Bromo-5-pentamethyldisilanyl-thiophene (2.20 g, 7.50 mmol) in ether (20 mL) was added toa flask containing Mg turnings (0.26 g, 15 mmol). After complete addition the mixture wasrefluxed for 1 hour. 5-Iodo-2,2'-bithiophene (2.2 g, 7.5 mmol) in ether (5 mL) was slowlyadded to the mixture while NiCl2.dppp (54 mg, 0.10 mmol) was added gradually. The mixturewas stirred for 2 hours, refluxed for 4 hours and stirred overnight at room temperature. Themixture was poured into water (50 mL), extracted with ether (3 x 50 mL) and the combinedorganic layers were washed with 5% NaCl solution (25 mL), dried (MgSO4), filtered, and thesolvents were removed by evaporation under reduced pressure. Kugelrohr distillation (95°C,0.01 mm Hg) provided 1.42 g (3.75 mmol, 50 %) of the title product as a light-yellow liquid.1H-NMR δ: 0.11 (s, 9H, SiMe3), 0.40 (s, 6H, SiMe2), 7.02 (1H, dd,3J= 5.0 en 3.6 Hz, H-4"),7.07 (m, 1H, H-4), 7.08 (s, 2H, H-3'/4'), 7.17 (dd, 1H,3J= 3.6 Hz,4J= 1.0 Hz, H-3"), 7.21 (dd,1H, 3J= 5.2 Hz,4J= 1.2 Hz, H-5"), 7.23 (d, 1H,3J= 3.4 Hz, H-3);13C-NMR δ: -2.8/-2.5 (Me),123.5 (1J= 166.7 Hz,2J= 5.5 and3J= 8.1 Hz, C-3"), 124.1 (1J= 186.8 Hz,2J= 7.1 and3J= 10.6Hz, C-5"), 124.3 (1J= 166.5 Hz,2J= 5.0 and3J= 9.6 Hz, C-3' and C-4'), 124.9 (1J= 165.7 Hz,2J= 6.5 Hz, C-3), 127.8 (1J= 168.7 Hz,2J= 3.5 and3J= 5.0 Hz, C-4"), 134.7 (1J= 165.7 Hz,2J=5.0 Hz, C-4), 136.0, 136.2, 137.1, 139.0 and 141.9 (C);29Si-NMR δ: -24.1 /-19.3 ppm. IR2951(m), 1425(w), 1246(m), 1069(w), 984(m), 868(w), 835(m), 794(s), 761(w), 724(w) cm-1.MS , m/e (rel. intensity) 378 (M+, 96), 305 (M+-SiMe3, 100), 73 (SiMe3, 30), 28 (Si, 35);HRMS calcd for C17H22S3Si2 378.042, found 378.042.

5,5''- Bis(trimethylsilyl)-2,2':5',2''-terthiophene 2.14a (Me3SiT3SiMe3).n-BuLi (2.0 mL, 3.2mmol) was added dropwise to a solution of 2,2':5',2''-terthiophene (0.80 g,3.2 mmol) in THF (10 mL) at -40°C, while keeping the temperature below -30°C. Aftercomplete addition the mixture was recooled to -40°C andn-BuLi (2.0 mL, 3.2mmol) wasadded dropwise. After recooling to -40°C, chlorotrimethylsilane (1.1 g, 10 mmol) was added


38

1. (a) Uhlenbroek, J.H.; Bijloo, J.D.Recl. Trav. Chim. Pays-Bas1958, 77, 1004. (b)Uhlenbroek, J.H.; Bijloo, J.D.Recl. Trav. Chim.Pays-Bas1960, 79, 1181.

2. Kagan, J.; Bazin, M.; Santus, R.J.J. Photochem. Photobiol.1989, B6, 165.3. Martinez, F.; Voelkel, R.; Naegele, D.; Naarmann, H.Mol. Cryst. Liq. Cryst.1989, 167,

227.4. Fichou, D.; Garnier, F.; Charra, F.; Kajzar, F.; Messier, J. InOrganic materials for

nonlinear optics,Hanh, R.; Bloor, D. (eds), Royal Soc. Chem., London, 1989; p176.5. Garnier, F.; Horowitz, G.; Peng, X.; Fichou, D.Adv. Mater.1990, 2, 592.6. (a) Shizuka, H.; Sato, Y.; Ueki, Y.; Ishikawa, M.; Kumada, M.J. Chem. Soc., Faraday

Trans. 11984, 80, 341. (b) Sakurai, H.; Sugiyama, H.; Kira, M.J. Phys. Chem.1990, 94,1837.

7. Egorochkin, A. N.Russ. Chem. Rev.1984, 53, 445.8. Miller, R. D.; Michl, J.Chem. Rev.1989, 89, 1359.9. (a) Gilman, H.; Atwell, W.H.; Schwebke, G.L.J. Organomet. Chem.1964, 2, 369. (b)

Hague, D.N.; Prince, R.H.Chem. Ind.1964, 1492.10. Tour, J. M.; Wu, R.Macromolecules1992, 25, 1901.11. Wakefield, B.J.Organolithium Methods, Academic Press, London, 1988.12. Chadwick, D.J.; Willbe, C.J. Chem. Soc., Perkin Trans. 11977, 887.13. Brandsma, L.; Verkruijsse, H D.Preparative Polar Organometallic Chemistry, Springer:

Berlin, 1987; Vol.1.14. Kauffmann, T.; Kniese, H.Tetrahedron Lett.1973, 41, 4043.

in one portion. The mixture was allowed to warm to room temperature for 2 hours before beingpoured into water (50 mL). The mixture was extracted with ether (3 x 100 mL) and thecombined organic layers were washed with 5% NaCl solution (50 mL), dried (MgSO4), filtered,and the solvents were removed by evaporation under reduced pressure. Column purification(silicagel-60, CH2Cl2/pentane = 1:10) of the crude reaction product provided 0.98 g (2.5 mmol,78 %) of the title product as a light-green solid, mp 121-123°C. 1H-NMR δ 0.32 (s, 18H,SiMe3), 7.08 (s, 2H, H-3'/4'), 7.13 (d, 2H,3J= 3.4 Hz, H-4), 7.22 (d, 2H, H-3);13C-NMR δ: -0.2(Me), 124.3 (1J= 167.6 Hz,2J= 4.8 Hz, C-3'/4'), 124.8 (1J= 166.0 Hz,2J= 6.4 Hz, C-3), 134.7(1J= 166.0 Hz,2J= 4.8 Hz, C-4), 136.2 (C-2'), 139.9 (C-5), 142.0 (C-2);29Si δ: -6.4 ppm. IR2953(m), 1430(m), 1247(m), 1200(w), 1070(m), 987(s), 912(w), 839(s), 795(s), 756(m) cm-1.MS , m/e (rel. intensity) 392 (M+, 100), 377 (M+-Me, 61); HRMS calcd for C18H24S3Si2392.057, found 392.058.

5,5''-Bis(pentamethyldisilanyl)-2,2':5',2''-terthiophene 2.14b (Me5Si2T3Si2Me5).The reaction was performed on a 5.0 mmol scale, following the same procedure as applied for2.14a. Kugelrohr distillation (220°C, 0.01 mm Hg) and subsequent crystallization fromchloroform/methanol provided 1.42 g (2.8 mmol, 56%) of the title product as a green solid, mp131-133°C. 1H-NMR δ: 0.11 (s, 18H, SiMe3), 0.39 (s, 12H, SiMe2) 7.08 (d, 2H,3J= 3.5 Hz,H-4), 7.08 (s, 2H, H-3'/ 4'), 7.23 (d, 2H,3J= 3.5 Hz, H-3);13C-NMR δ: -2.9/-2.5 (Me), 124.2(C-3'/4'), 124.8 (C-3), 134.7 (C-4), 136.1 (C-2'), 138.9 (C-5), 142.0 (C-2);29Si-NMR δ:-23.9/-19.2 ppm. IR 2949(m), 1424(w), 1245(s), 1190(w), 1068(m), 981(s), 891(w), 832(s),789(s), 726(w) cm-1. MS, m/e (rel. intensity) 508 (M+, 100), 435 (M+-SiMe3, 46), 378 (M+-Si2-

C5H14, 42), 305 (M+-Si3C8H23, 23), 73 (SiMe3, 54); HRMS calcd for C22H36S3Si4 508.106, found508.105.

2.6 REFERENCES

Chapter 2

39

15. Ohshita, J.; Kanaya, D.; Ishikawa, M.Appl. Organomet. Chem.1993, 269.16. Mc Eachern, A.; Soucy, M.; Leitch, L.; Arnason, J. T.; Morand, P.Tetrahedron1988, 44,

2403.17. Bischof, P. K.; Dewar, M. J. S.; Goodman, D. W.; Jones, T. B.J. Organomet. Chem.

1974, 82, 89.18. Sakurai, H.J. Organomet. Chem.1980, 200, 261.19. Wehrli, F.W.; Wirthlin, T.Interpretation of Carbon-13 NMR spectra, Wiley: New York,

1983.20. Nyulászu, L.; Veszprémi, T.Chemica Scripta1986, 26, 629.21. Igarashi, N.; Tajiri, A.; Hatano, M.Bull. Chem. Soc. Jpn.1981, 54, 1511.22. Svensson, A.; Håkansson, R.Chemica Scripta1982, 20, 188.23. Nyulászu, L.; Veszprémi, T.J. Mol. Struct.1986, 140, 253.24. Sakurai, H.J. Organomet. Chem.1980, 200, 261.25. Kuhn, H.J. Chem. Phys.1949, 17, 1198.26. Chosrovian, H.; Rentsch, S.; Grebner, D.; Dahm, D.U.; Birckner, E.; Naarmann, H.Synth.

Met. 1993, 60, 23.27. (a) Kiess, H. G. (ed.)Conjugated Conducting Polymers; Springer-Verlag: Berlin, New

York, 1992. (b) Brédas, J. L.; Street, G. B.Acc. Chem. Res.1985, 18, 309. (c) Friend, R.H.; Bradley, D. D. C.; Townsend, P. D.J. Phys. D: Appl. Phys.1987, 20, 1367. (d)Kanemitsu, Y.; Suzuki, K.; Masumoto, Y.; Tomiuchi, Y.; Shiraishi, Y.; Kuroda, M.Phys.Rev. B1994, 50, 2301.

28. Jaffé, H.H.; Orchin, M.Theory and Applications of Ultraviolet Spectroscopy; John Wiley:New York, 1962; p 545.

29. Serrano-Andrés, L.; Merchán, M; Fülscher, M.; Roos, B. O.Chem. Phys. Lett.1993, 211,125.

30. (a) Birnbaum, D.; Kohler, B. E.J. Chem. Phys.1992, 96, 2492. (b) Beljonne, D.; Shuai,Z.; Brédas, J. L.J. Chem. Phys.1993, 98, 8819. (c) Soos, Z. G.; Galvão, D. S.; Etemad,S. Adv. Mater.1994, 6, 280. (d) Becker, R. S.; Seixas de Melo, J.; Maçanita, A. L.;Elisei, F.Pure Appl. Chem.1995, 67, 9.

31. Birnbaum, D.; Fichou, D.; Kohler, B. E.J. Chem. Phys.1992, 96, 165.32. Wintgens, V.; Valat, P.; Garnier, F.J. Phys. Chem. 1994, 98, 228.33. Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W.-P.Rev. Mod. Phys. 1988, 60, 781.34. (a) Stafström, S; Brédas, J.-L.Phys. Rev. B1988, 38, 4180. (b) Ehrendorfer, Ch.;

Karpfen, A.J. Phys. Chem.1994, 98, 7492.35. Guilbault, G. G. (ed.),Practical Fluorescence, 2nd ed.; Marcel Dekker Inc.: New York,

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university of groningen tuning of the luminescence in poly ... · studied in order to gain insight...

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