R

P

YD

C

a

ARRAA

KPBNPMO

1

iaiiadtc

0d

Coordination Chemistry Reviews 256 (2012) 759– 770

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews

jo ur n al homepage: www.elsev ier .com/ locate /ccr

eview

hotochromic four-coordinate N,C-chelate boron compounds

ing-Li Rao, Hazem Amarne, Suning Wang ∗

epartment of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7592. Azobenzene organoboron compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7603. Phenyl-pyridyl (ppy) chelate compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 760

3.1. The role of the mesityl groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7623.2. Substitution effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7633.3. �-Conjugation effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7633.4. Alternative photoisomerization pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7643.5. Polyboryl systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7653.6. Impact of metal chelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 767

4. Heterocyclic chelate compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7685. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 769

r t i c l e i n f o

rticle history:eceived 14 August 2011eceived in revised form 9 November 2011ccepted 10 November 2011vailable online 9 December 2011

a b s t r a c t

Four-coordinate organoboron compounds with a N,C-chelate backbone have been found recently to dis-play an unusual photoisomerization phenomenon with a distinct change of color. The photoisomerizationprocess is thermally reversible, enabling the potential use of this class of compounds as a new class ofphoto-responsive materials. This review provides an account of our recent investigation on the effect ofsubstitution, �-conjugation and metal chelation on the photoisomerization process of the N,C-chelateorganoboron compounds. The photoisomerization phenomenon of azobenzene-based organoboron com-

eywords:hotochromic compoundsoron,C-chelatehotoisomerizationetal coordination

pounds will also be presented.Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

rganoboron

. Introduction

Photochromic compounds have attracted much recent researchnterest because of their rapid and often instant change of physicalnd electronic properties such as color, luminescence, conductiv-ty, refractive index etc., which enable their potential applicationsn optical memory devices, molecular switches, smart windowsnd ophthalmic glasses etc. [1,2]. Photochromic compounds are

efined as molecules that are capable of undergoing a reversibleransformation between two structural forms with a distinctolor or absorption spectral change when excited at least in one

∗ Corresponding author.E-mail address: wangs@chem.queensu.ca (S. Wang).

010-8545/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.ccr.2011.11.009

pathway by light [1,2], as shown by Fig. 1. Previously investigatedorganic photochromic compounds concern mostly azobenzenes,diarylethenes (DTE), spiropyrans, spirooxazines, naphthopyransetc. [1,2]. With the exception of azobenzenes, the majority oforganic photochromic compounds undergo photoisomerizationthat involves a ring closure and opening process. Recent investiga-tion has shown that the attachment of a metal ion to photochromicorganic compounds such as DTE or azobenzene derivatives can havea significant impact on the excited state properties, thus allowingthe tuning of the photoisomerization process [3,4]. In addition, theincorporation of metal ions introduces new structural control and

functions to the photochromic systems [3,4].

Although organoboryl groups have been used to tune theproperties of DTE compounds [5], photochromic compounds thatinvolve structural transformation around an organoboron core

ghts reserved.

760 Y.-L. Rao et al. / Coordination Chemistr

Excited States

ΔG*hν

hν’

A

Bhν

hν’/ Δ

Fi

rcNNBpogtftoaN

2

mTat

Si

ig. 1. A diagram showing the pathways of photochromic switching between twosomers A and B.

emain rare. Only two classes of photochromic organoboronompounds are known in the literature, namely azobenzene,C-chelate boron compounds [6,7] reported by Kawashima and,C-chelate photochromic boron compounds reported by us [8].oth classes of molecules have been shown to display reversiblehotoisomerization phenomena. The boron core in both classesf molecules has a four-coordinate tetrahedral geometry with theeneral formula of BAr2(N,C-chelate). Nonetheless, the structuralransformation of these two classes of molecules have very dif-erent origins: one relys on the dissociation of a B N bond andhe subsequent trans–cis isomerization of the azo group while thether involves the B C and C C bonds breaking and formation. Thisrticle will discuss both types of compounds with the focus on the,C-chelate boron compounds discovered by our group.

. Azobenzene organoboron compounds

Kawashima and coworkers reported [6] in 2005 the photoiso-

erization phenomenon of compound (E)-1 shown in Scheme 1.

his molecule has an absorption maximum at 339 nm in benzene,ssigned to the � → �* transition of the azo group. Irradia-ion at 360 nm causes a decrease of the 339 nm band and the

N N

B O

O

N

B O

O

nm360

nm431C6D6

N

(E)-1 (Z)-1

N N

BO

O

N

N

B

O

O

N

N

nm431C6D6

nm360

(E)-1- py (Z)-1-p y

py py

cheme 1. Photoisomerization of compound 1 and the influence of pyridine on thesomerisation process.

y Reviews 256 (2012) 759– 770

appearance of a new band at 460 nm, attributed to the azo groupin the cis isomer (Z)-1. This isomerization process is believed tooccur by the dissociation of the B N bond, followed by the subse-quent trans (E) to cis (Z) isomerization of the azobenzene moiety.The Z isomer can be reversed back to the E-isomer by irradiationat 431 nm. The photoisomerization process has been found to beuseful for tuning/switching the Lewis acidity of the boron center inthis type of compounds, as shown by Scheme 1. For example, thepyridine binding constant to (E)-1 and its isomer (Z)-1 are found tobe (1.45 ± 0.08) × 10 and (5.3 ± 0.9) × 103 M−1, respectively, corre-sponding to an increase of Lewis acidity by a factor of ∼300, upontransformation from the trans isomer to the cis isomer.

Kawashima and coworkers established the impact of the B Nbond length on the photoisomerization process of the catecholbo-rane compounds by examining a series of derivatives of compound(E)-1 [7]. The representative examples are shown in Chart 1. Com-pounds such as (E)-1, (E)-2, (E)-4 and (E)-5 that have a verylong B N bond (1.823–1.895 A) undergo photoisomerization whilecompounds as such as (E)-3 and (E)-6 that have a relative shortB N bond (1.721 A and 1.773 A, respectively) do not undergo pho-toisomerization upon irradiation at 360 nm. The shortening of theB N bond in (E)-6 is attributed to the electron donating methoxygroup that strengthens the B N bond while the relatively shortB N bond of (E)-3 may be explained by the electron-withdrawingchloro groups of the catechol, that weaken the B O bonds, which inturn strength the B N bond. Thus, the photoisomerism displayedby this class of molecules has the same molecular origin as theparent azobenzene molecules. The role of the boron unit is simplyto inhibit or partially inhibit the isomerization by binding to theazo nitrogen atom, thus tuning the properties of the azobenzenechromophore.

The photoisomerization phenomenon of the catecholboranecompounds is in sharp contrast to the related diarylboryl azoben-zene compounds 7 and 8 shown in Chart 1, also reported byKawashima and coworkers [9,10]. Compounds 7 and 8 and theirderivatives do not undergo photoisomerization. Their stabilitytoward photolysis can be explained by the much shorter B N bondlengths (1.625(2)–1.638(3) A), compared to those of the catecholb-orane analogues. In fact the B N bond lengths in compounds 7–8are typical of dative B N single bonds for four-coordinate boroncompounds [11]. The unusually long B N bonds in the catecholb-orane compounds can be attributed to the �-conjugation of theboron center with the oxygen lone pairs that substantially weakensthe B N bond.

3. Phenyl-pyridyl (ppy) chelate compounds

Yamaguchi and coworkers reported the monoboryl compound9 and the three isomers of diboryl compounds 9a–9c (Chart 2)in 2006 [12]. These are the first examples of four-coordinate N,C-chelate boron compounds that have two bulky mesityl groups. Thisclass of compounds are very interesting and unusual because dime-sitylarylborane compounds are well known to have the tendencyto retain a three-coordinate trigonal planar geometry due to thesteric bulkiness of the mesityl groups, enabling their effective use asanion sensors or electron transport materials in OLEDs [13]. In addi-tion, this class of compounds display a high electron mobility (e.g.,compound 9b has an electron mobility of 1.5 × 10−4 cm2 V−1 s−1),which can be attributed to the extended �-conjugation and theboron chelation [12].

Intrigued by the highly congested four-coordinate boron com-

pounds reported by Yamaguchi and coworkers, we decided toinvestigate the related compound 10A shown in Scheme 2 [8].Despite the steric congestion, this molecule was found to retain itsstructural integrity in solution and the solid state. Compound 10A

Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770 761

Chart 1. Structures of compounds 1 to 8.

Comp

dtttttaetbAu

Chart 2. Structures of

oes not react with anions such as fluoride – a common reaction forriarylboranes [13b]. However, upon irradiation by light (365 nm),his compound was found to change color rapidly from colorlesso dark blue and lose its fluorescence (�em = 458 nm, = 0.10 inoluene) completely, as shown in Scheme 2. This process is fullyhermally reversible with an activation barrier of 110 kJ mol−1 and

t1/2 = ∼7.7 h at 323 K. The structure of the dark specie 10B wasstablished by NMR and computational studies. NMR studies showhat the transformation of 10A to 10B is quantitative with a C C

ond being formed and the de-aromatization of a mesityl group.lthough the mechanism of this transformation has not been fullynderstood, it may be considered as a Zimmerman rearrangement

Scheme 2. The isomerisation of compound

ound 9 and its dimers.

[14], followed by a sigmatropic shift. Currently the mechanisticpathways for the unusual isomerization process are being inves-tigated and modeled by computational methods in collaborationwith our colleague in computational chemistry and the results willbe published in due course.

The photoisomerization from 10A to 10B is very efficient witha quantum yield of ∼0.85 (�ex = 365 nm) in toluene. This photoiso-merization was also found to occur readily in polymer matricessuch as PMMA (poly(methyl methacrylate)), polystyrene, EVOH

(poly[ethylene-co-(vinyl alcohol)]), or PDMS (polydimethylsilox-ane) doped by compound 10A with a similar color and absorptionspectral change as shown in Fig. 2. Polymers such as polystyrene

10A and the reaction of 10B with O2.

762 Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770

V ligh

aoaebmt

tftmaoscHittt∼laot

Bodt

Fo

Fig. 2. UV–vis spectral change of 10A upon irradiation by U

nd EVOH shield the photochromic molecules from exposure toxygen such that photochromic switching becomes possible undermbient conditions. In contrast, with PDMS films that have anxcellent permeability toward oxygen, the dark color of 10B cane quenched rapidly by oxygen, thus enabling the use of theseolecules as potential solid state oxygen indicators that can be

urned on by light [15].TD-DFT calculations and experimental studies established that

he lowest energy electronic transition of 10A is a charge trans-er transition from the mesityl to the phenylpyridine chelate, withhe HOMO and LUMO levels being located predominately on one

esityl group and the chelate backbone, respectively (Fig. 3). Ingreement with this is the red shift of the fluorescence spectrumf 10A with increasing solvent polarity. TD-DFT computations alsohowed that the electron density and the HOMO level of 10B areoncentrated on the BC2 ring and the cyclohexadienyl ring and theOMO energy is destabilized by ∼1 eV, compared to 10A, which

s responsible for its sensitivity toward oxygen. Upon exposureo oxygen, the dark isomer 10B loses its color rapidly, generatinghe C C coupled product 10C quantitatively (Scheme 2). Consis-ent with this is the observation of a distinct low oxidation peak at−0.54 V in the CV diagram of 10B (relative to FeCp2

0/+). The LUMOevel of 10B is localized on the BC2 ring – the cyclohexadienyl ringnd the ppy chelate, respectively. Thus the lowest energy transitionf 10B can also be ascribed to a charge transfer from the BC2 unito the ppy ring.

Although C C coupling reactions between two aryl groups in

Ar4

− anions under UV (250 nm) irradiation were reported previ-usly [16,17], these reactions are not reversible. Furthermore, if twoifferent aryl groups are present in the BAr4

− molecule, no selec-ivity was observed for the C C coupling reactions. For compound

ig. 3. HOMO and LUMO diagrams of 10A and 10B with a surface isocontour valuef 0.03.

t (365 nm) in toluene (left) and in a PMMA matrix (right).

10A, the C C coupling occurs exclusively between a mesityl andthe benzene ring of the ppy ligand. The low energy charge transfertransition from the mesityl to the ppy chelate in 10A is believed toplay a key role in promoting its highly selective C C coupling. TheB N bond of the ppy chelate ligand in 10A is believed to play a keyanchoring role to enable the reversible structural transformation.

3.1. The role of the mesityl groups

To determine the role of the steric bulky mesityl groups in thephotoisomerization of 10A, we examined compounds B(ppy)Ph2(11) and B(ppy)(C6F5)2 (12) (Chart 3) [18]. In contrast to compound10A, both 11 and 12 do not undergo photoisomerization when irra-diated by UV light. For the fluorinated compound 12, irradiationcauses decomposition of the molecule. Compound 11 is howeverstable toward photolysis but no photoisomerization was observedat all, even when irradiated at its absorption maximum, 310 nm.DFT computational results show that the HOMO and LUMO levels of11 resemble those of 10A, while the HOMO level of 12 is dominatedby the � orbital of the ppy chelate. Thus the instability of com-pound 12 toward UV irradiation may have an electronic origin, butthe inertness of 11 toward photoisomerization is unlikely causedby electronic effects. We believe that in addition to the mesityl tothe chelate charge transfer, the key driving force for the photoi-somerization process of 10A is the steric congestion imposed bythe two mesityl groups. The B N and B Cppy bond length of 10Aare 1.653(2) A and 1.625(2) A, respectively while the correspondingones for 11 are 1.618(3) A and 1.628(3) A, respectively. The averageB CMes bond length is 1.649(2) A for 10A, much longer than thatof B CPh of 11 (1.613(3) A). Furthermore, the arrangement of thetwo mesityls in 10A is much less symmetric than the two phenylgroups in 11 with respect to the ppy chelate. As shown in Chart3, for 10A, one mesityl is closer to the ppy carbon atom (2.60 A)

than the phenyl in 11 (2.64 A). This, along with the relatively weakB CMes bonds of 10A is believed to be most likely the key drivingforce for its facile photoisomerization.

2.84N

B

2.60

10A

N

B2.74

11

2.64N

F

FF

FF

2.78B

F

F

F

FF

2.61

12

Chart 3. Comparison of structures 10A. 11 and 12.

Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770 763

NB

SiMe3 NB

O

HNB

BMes

Mes NB

F F

F

F

s of co

3

wpsdeeegltblgttaehHtsf

mastgibtgeg

F

13 14

Chart 4. Structure

.2. Substitution effects

To gain further insight on the photoisomerization phenomenon,e investigated a series of substituted ppy chelate boron com-ounds [8,18]. First we examined a number of compoundsubstituted by either an electron donating or an electron with-rawing group shown in Chart 4. Consistent with the lowestlectronic transition being charge transfer from �mesityl to �∗

ppy,lectron donating groups such as SiMe3 were found to blue-shift themission energy (�em = 418 nm for 13) while electron-withdrawingroups such as C(O)H and BMes2 shift the emission energy toonger wavelengths (�em = ∼525 nm for 14 and 15). In the absorp-ion spectra of compounds 13–15, a low energy shoulder absorptionand ascribed to the mesityl to ppy charge transfer shifts toward

onger wavelengths with substitution by electron-withdrawingroups. Compounds 13–15 have bond lengths and angles aroundhe B center similar to those of 10A. DFT calculation results confirmhat the HOMO and LUMO levels of 13–15 resemble those of 10And that the substitution has a much greater impact on the LUMOnergy level than the HOMO level. The fluorinated compound 16as an absorption spectrum resembling that of 10A, with bothOMO and LUMO levels being stabilized significantly, compared to

hose of 10A. The average B CMes bond length of 16 (1.633(5) A) ishorter than that of 10A. Its fluorescence is about 10 nm red shiftedrom that of 10A.

Compounds 13–15 all undergo photoisomerization in the sameanner as 10A does with a distinct color change. The UV–vis

bsorption spectra of the dark isomers of these compounds arehown in Fig. 4 along with that of 10A for comparison. The absorp-ion maximum of the low energy peak of the dark isomers for thisroup of compounds follows a similar trend as the light coloredsomers, i.e., red-shifted with an electron withdrawing group andlue shifted with an electron donating group. This illustrates the

unability of the color of the dark isomers by varying a substituentroup. The photoisomerization of 13–15 are thermally reversiblexcept compound 14 – its dark isomer undergoes decompositionradually even at ambient temperature. Compound 16 does not

ig. 4. Absorption spectra of the dark isomers of 10A and 13–15 in toluene.

S

15 16

mpounds 13–16.

undergo isomerization at all, instead it decomposes when irradi-ated at 365 nm. The consistent poor stability toward photolysis bythe fluorinated compounds 12 and 16 has not been understood yet.Under 365 nm excitation, compound 10A has the highest photoi-somerization quantum efficiency among this group of compounds(0.85). Under ambient light irradiation, 15 has a photoisomerizationrate constant that is about 5 times greater than that of 10A, due toits much greater absorbance at the 400–480 nm region, comparedto 10A.

Another important observation is that the triarylboron unitin compound 15 remains intact through the photoisomerizationprocess and the C C coupled product 15C can be isolated quan-titatively when 15 is photolyzed under air [8]. The presence of a3-coordinate boron center in the photochromic molecule such as 15allows further tuning of the system by anions such as fluoride [15].From coordination chemistry point of view, coordinatively satu-rated four-coordinate boron compounds are in general expected tohave a greater thermodynamic stability than their three-coordinatecounter parts. The unusual and facile photoisomerization phe-nomenon by compounds 10A, and 13–15 demonstrated that stericcongestion in coordinatively saturated boron compounds can be akey driving force in structural transformation via the excited state.

3.3. �-Conjugation effect

Substitution that leads to the extension of the �-conjugation ofthe chelate backbone has been found to have a significant impacton the quantum efficiency of the photoisomerization process andthe color of the compounds. Examples of molecules that have anextended �-conjugation on the backbone are shown in Chart 5.

Compounds 17–20 all undergo photoisomerization in the samemanner as 10A does while compound 21 does not undergophotoisomerization at all [18,19]. Since extending �-conjugationstabilizes the �* level (LUMO), both absorption and fluorescencespectra of these molecules experience substantial red shift, com-pared to those of 10A (e.g., �em = 490 nm for 18, 480 nm for 20).

The characteristic low energy absorption peak of the dark iso-mers of this group of compounds also shifts somewhat to lowerenergy, as shown in Fig. 5. One important feature for this groupof compounds is the great enhancement of fluorescence efficiency,

iMe3 NB

Ph

17

S

20

NB

18

NB

21

N

S

S

19

NB

NB

Chart 5. Structures of compounds 17–21.

764 Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770

Fi

c1ppccTcsTatefitptpe

simdc

�mwa

3

eoc[atr�tUts

Fig. 6. Photographs showing the colors of selected examples of ppy-chelate boroncompounds in toluene before and after irradiation at 365 nm.

NB

trans-

NB

23

NB

22

NB

N

cis-

hυ

Δ

NB

NB

24

22

B

Chart 6. Isomerization of compound 22 and the structures of 23 and 24.

ig. 5. A diagram showing the absorption spectra of dark isomers of 10A and 17–19,n toluene.

ompared to 10A (e.g., ˚FL = 0.28 for 17, 0.37 for 18 and 0.18 for9) which indicates that fluorescence becomes a highly effectiveathway for relaxation from the excited state that competes withhotoisomerization. Hence it is not surprising that this group ofompounds have a much lower photoisomerization quantum effi-iency than that of 10A (e.g., ˚photo = 0.33 for 18 and ∼0.03 for 19).he enhancement of fluorescence efficiency with �-conjugationan be explained by the increased contribution of the � → �* tran-ition localized on the chelate backbone to the lowest excited state.his is in fact confirmed by TD-DFT calculation results, which show

substantial increase of contributions of � → �* transitions fromhe chelate backbone to the first excited state (10–20%) in thextended �-conjugated compounds, compared to 10A where therst excited state is almost a pure charge transfer transition (HOMOo LUMO, 96%). The �-conjugation effect is manifested by com-ound 21, which does not undergo photoisomerization at all whilehe monothienyl compound 20 does [15]. Thus, to enhance thehotoisomerization efficiency of the ppy-chelate based systems,xtended conjugation should be avoided.

The photoisomerization for all compounds that have a sub-tituent group at the meta position of the pyridyl ring (except 14)s thermally reversible but at a much faster rate than the parent

olecule 10B does. This is attributed to the destabilization of theark isomer imposed by the substituent due to the increased stericongestion.

The impact of the aryl groups, the substituent groups and-conjugation on photoisomerization and the color of the dark iso-ers of B(ppy)Ar2 is illustrated by the photographs shown in Fig. 6,here the color change of representative examples after irradiation

t 365 nm is shown.

.4. Alternative photoisomerization pathways

A vinyl group or an olefinic bond is commonly used in buildingxtended �-conjugated materials [20]. To examine the impact of anlefinic bond on photochromic switching of the N,C-chelate boronompounds, we synthesized compounds 22–24 shown in Chart 621]. These molecules are all isolated as trans or all trans isomers,s supported by NMR and X-ray crystal structures. DFT computa-ion indicated that the HOMO and LUMO levels of these moleculesesemble those of 10A except that the LUMO spreads over the entire-backbone. These molecules are bright blue or blue-green emit-

ers with ˚FL = 0.30–0.59 in solution and ∼0.23 in the solid state.pon irradiation at 365 nm, no color change is observed for any of

he compounds. In the UV–vis spectrum of trans-22, a distinct bluehift of the main absorption band at 370 nm is observed (Fig. 7),

Fig. 7. The UV–vis spectral change of compound 22 in toluene with irradition at365 nm.

Y.-L. Rao et al. / Coordination Chemistr

Ft

titiTc

wapBifet

pui

stCoig

ig. 8. Absorption and luminescence spectra of 25 and its metal complexes inoluene.

hat is characteristic of olefin trans to cis isomierzation. NMR stud-es confirmed that instead of photoisomerization of the boron core,he olefinic bond undergoes a trans–cis isomerization with the cis-somer being the major isomer at the photostationary state for 22.he olefinic bond isomerization causes a significant loss of fluores-ence intensity in these compounds.

For the polyboryl compounds 23 and 24, only one olefinic bondas observed to undergo trans–cis isomerization. Nonetheless,

single olefinic bond isomerization is sufficient to shut off thehotoisomerization around the boron center, thus stabilizing the(ppy)Mes2 chromophore toward UV irradiation. The preferential

somerization of the olefinic bond is attributed to the relativelyaster kinetics of trans–cis isomerization in the excited state, thatffectively quenched the alternative isomerization pathway aroundhe boron core.

Recently we have synthesized molecule 25 (Chart 7) that incor-orates a 2,2′-bipyridine chelating site for metal ions, and alloweds to examine the impact of metal ions on the olefinic trans–cis

somerization process of 25 [22].The free ligand 25 undergoes olefin photoisomerization in the

ame manner as compound 23 does, with the trans, cis-isomer beinghe dominant isomer, upon irradition at 365 nm. Binding of 25 to

u(PPh3)2

+ has no significant influence on the photoisomerizationf 25 except that the trans,trans-isomer becomes the dorminatingsomer at the photostationary state, which can be explained by thereater steric congestion of the complex 25-Cu. Binding of 25 to

B

25-Cu

N

BN

N NCu

(Ph)3P P(Ph)3

[Cu(PPh3)2

BN

N

N

]NO3

[NO3]

Chart 7. Structure of compound 25 and the

y Reviews 256 (2012) 759– 770 765

PtPh2, however, appears to completely inhibit the olefin bond pho-toisomerization of 25 (this observation is for the exposure timewindow ∼1 h, we used, for the free ligand and the metal complexes.Prolonged irradiation of 25-Pt causes irreversible decomposition ofthe PtPh2 unit), which has been attributed to the presence of a low-lying MLCT state (Fig. 8) that effectively intercepts the excited stateenergy, thus enhibiting the olefin isomerization. Despite the lack ofolefin isomerization in 25-Pt, the B(ppy)Mes2 units are inert towardphotoisomerization within the expsosure time we used. The pres-ence of olefin bonds and the low-lying MLCT state in this moleculeare likely both responsible for the photochemical stability of theboron chromophore.

3.5. Polyboryl systems

To elucidate the impact of multi-boryl units on photoisomer-ization of a conjugated molecule, we synthesized the polyborylcompounds 26–28 shown in Chart 8 where a benzene ring and analkyne bond are used as the linkers [23]. The crystal structure ofthe hexaboryl molecule 28 is shown in Fig. 9.

These compounds are bright blue-green emitters with�em = ∼500 nm and ˚FL = 0.20–0.39. In contrast to the olefiniccompounds, they all undergo photoisomerization upon irradiationat 365 nm in the same manner as the monoboryl 18, B(Ph- -ppy)Mes2, does, changing color from light yellow to dark green.This provides further support to the role of the olefin isomerizationin stabilizing compounds 22–25 by providing an alternative photoi-somerization pathway. Interestingly, however, only one boron unitin these polyboryl compounds 26–28 undergoes isomerization.This is in contrast to the extensively studied dithienylethene (DTE)photochromic systems, where simultaneous photoisomerizationof multiple chromophores in a single molecule has been frequentlyobserved [24]. We believe that the single-boryl isomerizationphenomenon is caused by the much slower kinetics of theB(ppy)Mes2 photoisomerization that could not compete with themuch faster energy transfer process that dissipates the excitedstate energy to the low energy absorption of the isomerizedboron unit. The presence of multi-boryl units does, however,appear to enhance the efficiency of the photoisomerization ofthe single boryl unit through the “antenna” effect. Furthermore,the multi-boryl units also accelerated the thermal reversal rate

of these compounds, compared to the monoboryl compound 18,although this phenomenon is not yet well understood. Anotherinteresting phenomenon of the polyboryl compounds is the com-plete quenching of fluorescence caused by the photoisomerization

B

[PtPh

N

BN

N NPt

2(SMe 2)] 2

25

BN

25-Pt

synthesis of its Cu and Pt complexes.

766 Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770

NB

BN

BN

26

27

NB

hυ

Δ

BN

B

NB

B

N

N

B N

BN

NB

BN

BN

comp

ot

occgyilf

Chart 8. The single boryl isomerisation of

f a single boryl unit as shown in Fig. 10. Intramolecular energyransfer is again believed to be responsible for this phenomenon.

To determine if �-conjugation was a key factor in the behaviorf compounds 26–28, we synthesized and investigated two non-onjugated molecules 29 and 30 shown in Chart 9 [25]. These twoompounds respond to light in the same manner as the conju-ated polyboryl compounds 26–28 do, changing color from lightellow to dark green with only a single boron unit undergoing

somerization, as evidenced by NMR and UV–vis data (Fig. 11). Thiseads us to suggest that extended �-conjugation is not requiredor the single-boryl isomerization phenomenon. For 29 and 30, the

Fig. 9. The crystal structu

28

ound 26 and the structures of 27 and 28.

photoisomerization quantum efficiency is 0.60 and 0.58, respec-tively, much higher than those of the corresponding monomers 13,B(Me3Si-ppy)Mes2 (0.51) and 18 (0.33). This again supports theantenna effect observed in molecules 26–28, with one boryl unitfacilitating the isomerization of the other boryl unit through fastintramolecular energy transfer. The thermal reversal rate constantof the dark isomer of 30 is similar to that of 18 while that of 29much faster than the monomer 13, which may be attributed to a

greater steric congestion in the dark isomer of 29.

We have recently completed our investigation on several newdiboryl compounds where the two N,C-chelate boron units are

re of compound 28.

Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770 767

Fq

lcOattgm

3

tvhccc

FI

NB

L DMSO, 19-Pt1 =

L 4- t-Bu-py, 19 -Pt2=

NPt

L

NB

O

19-Pt3

NPt

O

Chart 10. Structures of the Pt(II) compounds of 19.

ig. 10. A schematic illustration of the single boryl isomerization and its conse-uence on fluorescence of compound 28.

inked together by a bis-thienyl linker [15]. Preliminary study indi-ates that these molecules are inert toward photoisomerization.ne significant difference between these new diboryl moleculesnd the polyboryl compounds 26–28 is the large contribution ofhe first excited state by the �-orbitals of the bis-thienyl unit andhe chelate backbone. Thus, it appears that in addition to steric con-estion, the lowest energy transition being a charge transfer from aesityl to the chelate is critical in the photoisomerization process.

.6. Impact of metal chelation

Using metal ions via chelation or simply coordination to tunehe properties of photochromic systems have been found to beery effective for systems such as DTE or azobenzene [3,4]. We

ave recently initiated our investigation on the impact of metalhelation on the photoisomerization process of the B(ppy)Mes2hromophore. The results of our investigation on the Pt(II) chelatedompounds of molecule 19, shown in Chart 10 have been disclosed

NB Si

MeMe

29

N B hυ

Δ

NB

MesMes

NB

Me

Si

Me Me

30

NBSi

Me

NB

Mes Mes

Chart 9. Isomerization of compound 29 and the structure of 30.

ig. 11. The UV–vis spectral change of 29 in toluene upon irradiation at 365 nm.nset: photographs showing the color change of the solution.

Fig. 12. The crystal structure of compound 19-Pt3.

in a recent report [19]. The crystal structure of 19-Pt3 is shown inFig. 12.

The Pt(II) chelate compounds 19-Pt1 to 19-Pt3 are all brightlyphosphorescent with fairly high quantum efficiencies (0.13–0.45)at ambient temperature in toluene. The fluorescence spectrum of19 and the phosphorescence spectra of its Pt(II) compounds areshown in Fig. 13. The phosphorescence of the Pt(II) compoundsoriginates mostly from the 3LC state of the chelate ligand, due toits similarity to the phosphorescence spectrum of 19 at 77 K. Therole of the Pt(II) chelation appears to enhance the ligand centeredsinglet to triplet intersystem crossing, hence the phosphorescenceof the molecule. As a consequence, phosphorescence becomes ahighly efficient relaxation pathway from the excited state.

Although the B(ppy)Mes2 chromophore in all three Pt(II) com-

pounds undergoes photoisomerization in the same manner as 19does, the quantum efficiency is much lower as illustrated in Fig. 14.This is explained by the highly efficient phosphorescence of the

Fig. 13. The fluorescence spectrum of 19 and the phosphorescence spectra of itsPt(II) compounds in toluene.

768 Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770

) or phosphorescence (19-Pt3) and photoisomerization processes.

PmB

4

bN(britmbit�ehbccho

Ca

Fig. 14. A diagram showing the competing fluorescence (19

t(II) compounds, that competes effectively with the photoiso-erization process, thus inhibiting the photoisomerization of the

(ppy)Mes2 chromophore.

. Heterocyclic chelate compounds

To establish if the photoisomerization observed for the ppy-ased boron compounds is a general phenomenon for conjugated,C-chelate BMes2 compounds, we synthesized compounds 31–32

Chart 11) where a heterocycle group such as benzofuryl [15],enzothienyl [18] or N-phenyl-indolyl [26] replaces the benzeneing of ppy in 10A. The average B N, B Cheterocycle and B CMes

n compounds 31 are 1.67(1) A, 1.62(1) A, and 1.64(1) A, respec-ively [15,18,26], comparable to those of 10A, and 13–20. The two

esityls in 31 also have an asymmetric arrangement with oneeing much closer to the carbon atom of the heterocycle (2.60 A

n average) than the other (2.86 A in average) [18,26], similar tohose of 10A. Compounds 31-O and 31-S are blue emitters withem = ∼450 nm and ˚FL = 0.30–0.80 while 31-NPh is a blue-greenmitter with �em = ∼490 nm and ˚FL = 0.32. For 31, the HOMO levelas a very large contribution from the � orbital of the chelate back-one and a much reduced contribution from the mesityl group,

ompared to that of 10A, while the LUMO level is a �* orbital of thehelate backbone (Fig. 15) [18,26]. In addition, the first excited stateas a significant contribution from HOMO-1 (� orbital localizedn the chelate) to LUMO transition. Therefore, the lowest energy

N

X

B

O,=X 31-OS,=X 31-S

N

X

B

NB

31-NPh

hυ

Δ

N

Ph

=X=X

N

B

X

N

X

B

O, 32-OS, 32-S

hart 11. Isomerization of structures of heterocyclic N,C-chelate compounds 31nd 32.

Fig. 15. The HOMO and LUMO diagrams of 31-S with a surface isocontour value of0.02.

transition of 31 has a much less charge transfer character than thatof 10A.

Upon irradiation by light (365 nm), compounds 31 all isomer-ize to a dark isomer (dark blue for 31-O, dark green for 31-S, anddark blue-green for 31-NPh) in the same manner as that of 10A.The absorption spectra of the dark isomers of 31 are somewhatred shifted, compared to that of 10A (Fig. 16). The photoisomer-ization quantum efficiencies for compounds 31 are much lower(∼0.10) than that of 10A. The greater contributions of the � → �*transition of the chelate backbone to the first excited state and thegreater steric impediment for the C C bond formation imposedby the benzo group in 31 are believed to be responsible for thelow photoisomerization quantum efficiency of this group of com-pounds.

One noteworthy feature that is common for compounds 31 is the

high thermal stability of their dark isomers. For example, the darkisomer of 31-NPh has a thermal reversal half life of 9.5 h at 50 ◦C.The high thermal stability of the dark isomer of 31-NPh allowed

Fig. 16. The absorption spectra of the dark isomers of 31 and 32 in toluene.

Y.-L. Rao et al. / Coordination Chemistry Reviews 256 (2012) 759– 770 769

er (lef

uaucf

rzcmdustdi

5

dmawmwnoEittpBmmtkto

[[

Fig. 17. Crystal structures of yellow colored isom

s to crystallize it and determine its structure by X-ray diffractionnalysis [26], thus unambiguously establishing the structure of thenusual dark isomer (Fig. 17). Our study on the heterocyclic chelateompounds establishes that the photoisomerization phenomenonor N,C-chelate BMes2 compounds is quite general.

To further establish the generality of this phenomenon, weecently synthesized compounds [15] 32-O and 32-S where a ben-othiazolyl or a benzoxazolyl replaces the pyridyl in 10A. Theseompounds undergo photoisomerization and produce dark iso-ers similar to 10B, based on NMR data. However, thermally the

ark isomers of this class of compounds appear to be unstable andndergo further structural transformation, forming species thattill need to be identified. Efforts are being taken in our laboratoryo understand the unique and unusual structural transformationisplayed by this class of compounds. The details will be reported

n due course.

. Concluding remarks

The new photochromic N,C-chelate organoboron compoundsiscovered by our group have highly tunable colors and photoiso-erization kinetics, making them potentially useful and attractive

s a new class of photochromic materials. Based on our study,e suggest the following for considerations in future develop-ent of photochromic systems based on N,C-chelate moleculesith the general formual of B(N,C-chelate)Ar2. (1) The Ar groupseed to be sterically bulky groups such as mesityls. (2) Flu-rinated chelate ligands or Ar groups should be avoided. (3)xtended �-conjugation of the chelate backbone should be min-mized. (4) Electron-withdrawing groups on the chelate backbonehat enhance the Ar to chelate charge transfer contributions tohe lowest excited state may be a good strategy to enhance thehotoisomerization efficiency. (5) The influence of metal ions on(ppy)Mes2 systems needs to be further examined by varying theetal ion and the location of the binding site of the metal ion, whichay lead to new and interesting discoveries in photochemical reac-

ivities. (6) The long term stability and the photoisomerizationinetics of the photochormic compounds in polymer matrices needo be carefully examined to establish the viability of this new classf compounds in practical photochromic devices.

t) and the dark colored isomer (right) of 31-NPh.

Acknowledgments

We thank the Natural Sciences and Engineering Research Coun-cil of Canada for financial support. We also thank all the graduatestudents and postdoctoral fellows in Wang group for their originalresearch contributions for work presented in this article.

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