ADVANCED MATERIALS FOR OPTICS AND ELECTRONICS

Adv[ Mater[ Opt[ Electron[ 09\ 134Ð138 "1999#

Received 04 September 1999Copyright Þ 1999 John Wiley + Sons\ Ltd[ Accepted 10 September 1999

Limited Photochromism in CovalentlyLinked Double 1,2-Dithienylethenes

Andrea Peters and Neil R. Branda*Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2

Only one photo-induced cyclization reaction occurs when two 1,2-dithienylethene photochromes arecovalently joined. Attempts made to ring-close the second photochrome by prolonged irradiationquantitatively produced a rearranged product that could not be photobleached. Copyright 2000John Wiley & Sons, Ltd.

Structure 0

KEYWORDS data storage; dithienylethenes; molecular switches; photochromism

The increasing demand for high!density datastorage devices has led to the development of mol!ecule!based optical read:write systems employingphotochromic compounds[0 The thermally irre!versible photo!induced ring!closing and ring!open!ing reactions of 0\1!dithienylethene derivatives areparticularly e}ective for this technical applicationas the photochromes are thermally stable and exhi!bit excellent resistance to photofatigue[1

Recently\ the 0\1!dithienylethene backbone hasbeen integrated into the main chain of short poly!mers for use as potential conducting materials[2 Ourinterests are focused on examining the electroniccommunication between only two intimately con!nected 0\1!dithienylethene photochromes[ Know!

� Correspondence to] N[ R[ Branda\ Department of Chemistry\University of Alberta\ Edmonton\ Alberta\ Canada\ T5G 1G1[E!mail] neil[brandaÝualberta[ca

Contract:grant sponsor] Natural Sciences and EngineeringResearch Council of Canada[Contract:grant sponsor] Petro!Canada[

ing that as the length of the p!conjugation pathwaythrough the molecular backbone increases\ thephotochrome absorbs light lying towards the red!end of the visible spectrum3 and that 0\1!dithi!enylethenes convert from being cross to linearly p!conjugated upon ring!closure\ we can assume thatthe photochromic state of one photochrome willin~uence the reactivity of another when they arecovalently joined[ Also\ the fact that the reactivityof the 0\1!dithienylethene photochrome decreasesupon extending the p!conjugation onto pendantgroups3a\b suggests that the _rst photo!inducedcyclization will occur faster than the second[ Theoutcome is the realization of multi!addressible pho!tochromes that can potentially express di}erent col!ours depending on their states "openÐopen\ closedÐopen and closedÐclosed in Fig[ 0#[ Such systemshave obvious applications as high!density digitaldata storage devices[

Double photochromes 0a and 0b were preparedas outlined in Scheme 0[ Our synthesis began withFeringa|s 0\1!bis"4!chlorothiophen!2!yl#cyclopen!

A[ PETERS AND N[ R[ BRANDA

Copyright Þ 1999 John Wiley + Sons\ Ltd[ Adv[ Mater[ Opt[ Electron[ 09\ 134Ð138 "1999#

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Fig[ 0[

Scheme 0[ Rea`ents and conditions] "a# t!BuLi\ Et1O\ −67 >C^"b# ZnCl1\ Et1O\ −67 >C to room temperature^ "c# Pd"PPh2#3\THF\ re~ux\ 49)^ "d# t!BuLi\ THF\ −67 >C\ then CH2OH\ 81)

tene derivative "1#[4 We chose this photochromeinstead of its more commonly used per~uoro!cyclopentene counterpart because it can be con!veniently prepared in multigram scale frominexpensive starting materials\ although\ as willbecome apparent\ the absence of the ~uorine atomshas a dramatic e}ect on the photochrome|s behav!iour[ Monolithiation of dichloride 1\ conversion tothe alkylzinc compound\ and treatment with cata!lytic palladium generated the homo!coupled double

Fig[ 1[ Molecular structure of 0a in the crystal[ The thermalellipsoids are drawn at the 19) probability level

photochrome 0a[ Dichloride 0a was also readilydechlorinated to a}ord 0b[ All new compoundswere characterized using NMR\ IR and UV:VISspectroscopy and mass spectrometry[

X!ray quality single crystals of the openÐopenform of 0a were isolated from hexane by slow evap!oration[ The crystal structure "Fig[ 1#� highlightsthe near coplanarity of the two central thiophenerings "the S0!C00!C00?!S0? torsional angle is

� Crystal data for 0a"oo#] C29H17Cl1S3\ Mr � 476[55\ monoclinicspace group P1:c\ a � 7[2763"4#\ b � 01[2980"6#\ c � 02[6938"7#A� \ b � 090[4782"09#>\ V � 0275[96"03# A� 2\ Z � 1\ rcalcd � 0[397gcm−2\ m � 9[444 mm−0\ crystal dimensions � 9[33×9[12×9[19mm\ T � 082 K\ 1732 unique re~ections\ 1368 observed re~ec!tions ðF1

o−1s"F1o#Ł\ R0 � 9[9228\ wR1 � 9[9873\ S � 0[951\ larg!

est di}erence peak and hole 9[376 and −9[108 e A� −2\ BrukerP3:RA:SMART 0999 CCD di}ractometer\ graphite!mono!chromated Mo Ka radiation "l � 9[60962 A� #[

Crystallographic data "excluding structure factors# for thestructures reported in this paper have been deposited with theCambridge Crystallographic Data Centre as supplementarypublication no[ CCDC!033302[ Copies of the data can beobtained free of charge on application to CCDC\ 01 UnionRoad\ Cambridge\ CB1 0EZ\ UK[ "Fax] ¦33!0112!225!922^e!mail] depositÝccdc[cam[ac[uk

LIMITED PHOTOCHROMISM

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0[3"2#># which is necessary for through!bond com!munication between the photochromes[

The changes in the UV:VIS absorption spectrathat accompany the photo!induced isomerizationof photochrome 0a in deoxygenated cyclohexane"0×09−4 M# upon irradiation with a hand!held "254nm# lamp� are shown in Fig[ 2[ In the _rst 09seconds of irradiation\ an absorption band centredat 492 nm appears as the photochrome is convertedfrom the colourless!open to the linearly p!con!jugated red!closed form "Fig[ 2"a##[ After 09seconds at this concentration\ the increase in theabsorbance levels o}[ The presence of the isobesticpoint at 189 nm clearly indicates that 0a"oo# is cle!anly converted to a second unique photocyclizedproduct within these short irradiation times[Irradiation at wavelengths 143 and 202 nm pro!duced analogous trends\ although the ring!closingreaction occurs faster when light centred at 202 nmis used[

The photo!induced changes in the 0H NMR spec!tra in ðD01Łcyclohexane shed light on the identityof the _rst cyclization product "Table 0#[ Uponirradiation "0×09−2 M\ 1 minutes\ 254 nm#\ thetwo signals for the heterocyclic protons in 0a"oo#disappear\ and a new set of four singlets appears\two of which resonate in a similar region as thosefor 0a"oo# "see Table 0#[$ The two new resonancesare signi_cantly up_eld as would be expected forthe ring!closed product[ The dissymetric nature ofthis product indicates that only one of the photo!chromes has cyclized to form 0a"co#[ Further evi!dence for this is provided by the fact that the closedform of control compound 2a displays similar chan!ges in the absorption and 0H NMR spectra "seeTable 0#[

Further irradiation of 0a"co# with light in therange of 143Ð254 nm did not a}ord the anticipatedred!shift of the absorption maximum that wouldhave resulted from the second ring!closing reaction[Instead\ only a decrease in intensity of the absorb!ance for 0a"co# and an accompanying slight blue!shift was observed "Fig[ 2"b##[ The presence of four

� Standard lamps used for visualizing TLC plates were used"Spectroline E!Series\ 369 mW:cm1#[ The emission at 254 nm iscomprised of a broad emission band with a peak at 254 nm[$ The spectral characteristics of both photocyclization products"0a"co# and 3# generated in situ are identical to those of the twospecies isolated by HPLC "Porasil\ 4) CH1Cl1:hexane#[ Noother peaks were observed in the chromatograph[

Fig[ 2[ UV:VIS spectral changes of 0a "0[9×09−4 M# in deoxy!genated cyclohexane upon irradiation "l � 254 nm#[ Irradiationintervals are "a# every second for the _rst 09 seconds\ and "b#every 29 seconds for 3 minutes

aromatic signals in the 0H NMR spectrum of thissecond photogenerated product "Table 0# arguesthat it cannot be the doubly!cyclized species 0a"cc#but must be the outcome of another photo!induced

A[ PETERS AND N[ R[ BRANDA

Copyright Þ 1999 John Wiley + Sons\ Ltd[ Adv[ Mater[ Opt[ Electron[ 09\ 134Ð138 "1999#

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Table 0[ Selected UV:VIS and 0H NMR data "499 MHz# of photochromes 0a and 2a in cyclohexane

lmax ðnmŁ d for heterocyclic CÐH protonsa ðppmŁCompound"o ðLcm−0 mol−0Ł#

Ha Hb Hc Hd

0a"oo# 213 "07 499# 5[42 5[56 Ð Ð0a"co#b 494 "07 079# 4[70 4[85 5[65 5[493c 490 "09 419# 5[04 5[11 5[73 5[382a"o# 219 "12 199# 5[43 5[69 Ð Ð2a"c#d 388 "19 899# 4[70 4[87 Ð Ð2a"3#e\f 378 "01 799# 5[04 5[14 Ð Ð

a For the assignment of protons\ see Fig[ 0 and structure 2a[b Samples for UV:VIS "09−4 M# and for NMR analysis "09−2 M# were obtained by irradiating "l � 254nm# a benzene solution of 0a until the open form was 89) consumed as monitored by 0H NMRand isolating by HPLC[c Samples for UV:VIS "09−4 M# and for NMR analysis "09−2 M# were obtained by irradiating "l � 254nm# a benzene solution of 0a until 89) conversion to 3 as monitored by 0H NMR and isolating byHPLC[d Samples for UV:VIS studies "09−4 M# were irradiated for 09 seconds "l � 254 nm#[ Samples forNMR studies "09−2 M# were irradiated for 1 minutes[e Samples for UV:VIS studies "09−4 M# were irradiated for 4 minutes "l � 254 nm#[ Samples forNMR studies "09−2 M# were irradiated for 29 minutes[f We are assigning the structure for this product as one analogous to 3[

reaction "see footnote $ on previous page#[ We haveassigned this second product as 3 which appears toresult from a diatropic rearrangement of the ring!closed photochrome[ A similar rearrangement hasbeen reported by Irie et al[5 although they report itas only a minor side!product formed after severalhundred ring!closingÐopening cycles[ In our case\however\ 3 is quantitatively generated after only 29minutes at 09−2 M[ The similarity in the length ofand electronic distribution within the conjugatedbackbone of 0a"co# and 3 account for the insig!ni_cant changes in the absorption maxima[ Modelcompound 2a underwent analogous photo!inducedreactions[

Despite the formation of 3\ further irradiation"as long as 89 minutes# with 143Ð254 nm light didnot result in the cyclization of the second photo!chrome "for another example of a double photo!chrome showing limited reactivity see Ref[ 6#[Irradiation periods of this length were more than

Structure 1[

adequate to a}ect photochromism in similarly con!jugated 0\1!dithienylethenes[3a Varying the solventalso had minimal e}ect\ and samples irradiated inDMSO\ acetonitrile\ benzene and dichloromethanea}orded only minor changes in the absorptionmaximum for 0a"co# "lmax at 492\ 385\ 495 and 492nm\ respectively#[ The terminal chlorine atoms alsoplayed no observable role in the photochromism\and UV:VIS and 0H NMR studies showed identicaltrends for 0b and 2b[�

The only factor that had an e}ect on the photo!chemical behaviour was the nature of the cyclo!alkene joining the heterocycles within eachphotochrome[ The changes in UV:VIS and 0HNMR spectra of per~uorocyclopentene analogue4$ are analogous to those for 0a except for onesigni_cant di}erence[ The photogenerated purpleclosedÐopen form of 4 "generated by irradiation of0×09−4 M cyclohexane solutions for 09 seconds at254 nm# was considerably more stable than 0a"co#\and a decrease in the intensity of the absorbance at447 nm was evident only after longer irradiationtimes "3 minutes# whereas 0a began to convert to 3

� Model compound 2b was prepared in an analogous manner to0b[$ Double!photochrome 4 was prepared from 0\1!bis"1?!methyl!thiophene!2?!yl#!per~uorocyclopentene "see Ref[ 1"e## asdescribed for 0a[

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Structure 2[

after only 09 seconds[ Although the ~uorine atomsobviously have a signi_cant in~uence on the diatro!pic rearrangement\ they have none on the secondcyclization and 0H NMR spectroscopic studies wereanalogous to those for 0a "for another example ofwhere the presence of a low energetic excited!statesubunit inhibits the photochemical 0\1!dithi!enylethene isomerization\ see Ref[ 7#[

The closedÐopen isomer 0a"co# exhibited highthermal stability\ and 0H NMR spectra of ðD01Łcy!clohexane solutions remained unchanged afterthree months when stored in the dark[ Ring!open!ing and decolourizing solutions of 0a and 0b can beachieved after a 09 minute irradiation period usinga lamp and an appropriate 323 nm cut!o} _lter[Compound 3\ however\ could not be photobleachedand remained unchanged[

In light of our results\ it is interesting to note thatin the polymeric form reported by Zerbi et al[2b\c

each 0\1!dithienylethene photochrome acts inde!pendently\ and all undergo ring!closing reactions[It appears that there is an alternative photochemicalpathway to account for this observation[ The pre!cise reason for the photochromic inhibition\ the

Structure 3[

e}ect the linker between the photochromes has onthis inhibition\ and the precise mechanism of thediatropic rearrangement reaction are currentlyunder investigation[

ACKNOWLEDGEMENTS

This work was supported in part by a grant fromthe Natural Sciences and Engineering ResearchCouncil of Canada and by a grant from Petro!Canada[

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