Macromol. Chem. Phys. 2005, 206, 1769-1777

High-Content Pendant Photochromic Copolymer with Dithienylethene/Fluorene2:1Mole RatioXiaochuan Li,He Tian*Lab for Advanced Materials and Institute of Fine Chemicals,East China University of Science&Technology,Shanghai200237,ChinaFax:(þ86)2164252288;E-mail:tianhe@Received:May11,2005;Revised:July10,2005;Accepted:July12,2005;DOI:10.1002/macp.200500190Keywords:dithienylethene;photochromic;polyfluorene luminescence;synthesisIntroductionCompounds that interconvert between different isomers having unique absorption spectra when stimulated with light are referred to as photochromic and the process is called photochromism.In this system,the changes in the electronic patterns responsible for the changes in color also result in variations in other practical physical properties such as luminescence,[1–8]electronic conductance,[9–11]refractive index,[12,13]optical rotation,[14,15]and viscosity.[16,17]The photomodulation of these properties has the potential to advance optoelectronic technologies such as information storage system and actuators.[18–20]Among all the organic photochromic compounds,those possessing the hexatriene framework provided by the1,2-dithienylethene(DTE)structure have attracted special attention.This attention is well earned because numerous physical and chemical properties required for the practical use in functional materials technologies can be satisfied by DTE.[19–24]The development of DTE-based polymers that contain a high mass content of the photoactive DTE component,that maintain their photochromic activity in the solid state,and that are easily processed into thinfilms under a variety of conditions is critical for the implementa-tion of the materials into useful applications.In recent years,extensive research and great development have been achieved in solving the problems associated with the photochromic polymer materials such as poor solubility, low DTE content,and poor photochromic activity.[9,25–28] Dispersing the dye in a polymer matrix is an easy strategy to prepare photochromicfilms;however,it has some serious limitations.With a high content of photochromic unit dispersed infilms,segregation of the components is diffi-cult to avoid upon long-term storage.However,incorporat-ing DTE into polymers has its particular advantages because the polymers with high content of DTE could beSummary:Two photochromic dithienylethene units were incorporated into onefluorene unit as a monomer and the mole ratio of photochromic unit/repeat unit reached to2:1 using this monomer.The content of dithienylethene compo-nent in the polymer could be improved effectively by poly-merization of this monomer,which contains more than one dithienylethene units.All the starting materials concerned in the synthesis are commercially available and so the synthesis could be performed on a large scale.The polymer obtained by typical palladium-catalyzed Suzuki coupling reaction is photochromic both in solution and in solid state and possesses good solubility.The photochromic and photomodulation luminescence properties of the polymer are alsopresented.Structure of the photochromic polymer based on dithienyl-ethene.Full Paper1769achieved while maintaining the optical homogeneity of the material.In this paper,a new synthetic method was introduced and two dithienylethene units were connected withfluorene conveniently by synthetically modification of the bridging unit.This linkage is obviously different from that of linkage through thiophene ring which will,more or less,affect the DTE backbone and the interconversion between the parallel and the productive antiparallel conformations.It is also different from the bridging unit of maleimide and maleic anhydride due to their sensitivity to acid conditions.More freedom could be obtained by functionalization of the bridging unit,at the same time maintaining the activation of photochromism.Two photochromic units connected with onefluorene is a good example for its application in design-ing multi-function photochromic materials. Experimental PartMaterials and MeasurementsAll the reagent-grade chemicals were purchased from Aldrich, Across,and Lancaster,and were used without further puri-fication.All solvents were carefully dried according to the standard procedure and stored over4A molecular sieve. Melting points were measured in a glass capillary by laboratory devices X-4equipped with a metal heating block.The1H NMR spectra were recorded on a Brucker AM-500spectrometer operating at frequencies of500MHz in DMSO-d6.Chemical shifts were referenced to internal Me4Si(TMS).The UV-vis absorption spectra andfluorescence spectra were obtained on a Varian Cary500spectrometer and a Varian Cary Eclipse, respectively.Mass spectra were obtained by a HITACHI-80 instrument.The solidfilms were spin-coated using KW-4A. Molecular weights of the polymer were determined by gel permeation chromatography(GPC)against the standard poly-styrene on a Perkin-Elmer model system and THF as the eluting solvent.DSC measurements were performed on Perkin-Elmer DSC7at a rate of108CÁminÀ1.Synthesis2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene(11)were prepared as described in the literature.[29]11:1H NMR:d¼7.83(d,2H,J¼7.6Hz),7.78(s,2H),7.75 (d,2H,J¼7.6Hz),2.03(m,4H,J¼3.4Hz),1.43(s,24H), 1.26–1.04(m,20H),0.83(t,6H,J¼6.6Hz),0.58(m,4H). ESI:665.5(MþþNa).Synthesis of Ethyl Benzal-bis-Acetoacetate(2)2was synthesized according to literature method with a little modification.A mixture of55.5g(0.3mol)4-bromobenzal-dehyde,78g(0.6mol)of ethyl acetoacetate,and12mL of piperidine in300mL of95%ethanol was placed in a one-neck flask and allowed to stand at room temperature until the mixture was solid(about4.5h),and heated to30–408C and stirred for6h.This solid wasfiltered,washed with95% ethanol,and recrystallized twice from95%ethanol to give a white powder115g(90%),m.p.117–1198C.Synthesis of b-Arylglutaric Acid(3)85g of substituted ethyl benzal-bis-acetoacetate(0.2mol)was dissolved in300mL of distilled water and300g sodium hydroxide.The resulting mixture was refluxed vigorously for 3h,and then cooled to room temperature and poured into a beaker containing about1500mL of water.The resulting mix-ture was extracted with ether(2Â250mL).The water phase was acidified with concentrated hydrochloric acid.The preci-pitated glutaric acid was removed byfiltration and washed with cold water to remove sodium chloride.The b-arylglutaric acid was recrystallized from water and dried in vacuum.49g white solid was obtained with85%yields,m.p.139–1418C. Synthesis of Diacid Chloride(4)The diacid chloride was prepared by one of the two methods. Thefirst method involved the addition of PCl5to a mixture of the b-arylglutaric acid in cold stirring chloroform.The mixture was slowly warmed to reflux temperature and gently refluxed until gaseous HCl ceased to be avoided.After removal of the chloroform and POCl3,the crude diacid chloride was purified by distillation under reduced pressure.In this paper the second method was adopted mainly.18mL of SOCl2(0.24mol) was added to a mixture of b-arylglutaric acid23g(0.08mol), 13.4mL pyridine(0.17mol),and100mL benzene at room temperature(r.t.).After3h,the benzene was removed by evaporation and pyridinium hydrochloride was removed by filtration.The diacid chloride was not purified prior to use. Synthesis of1,5-Diketone(5)25.6g AlCl3(0.192mol)and18.2mL2,5-dimethylthiophene (0.16mol)were added to300mL CS2.The mixture was heated to reflux,and the25.9g diacid(0.08mol)chloride prepared by the above-mentioned method in80mL CS2was added dropwise.After the addition of diacid chloride,the reaction mixture was refluxed for2h.After cooling to room temper-ature,120mL cold water was carefully added to the mixture and the water layer was extracted with diethyl ether(3Â75mL).The combined organic phase was washed with100mL saturated NaHCO3solution,and100mL water,and saturated NaCl solution(100mL),dried with MgSO4,filtered,and the solvent evaporated in vacuum to yield a white solid.The solid was recrystallized from petroleum ether to yield25.5g(67%) white powder.M.p.134–1358C.1H NMR:d¼7.39(d,2H,J¼8.3Hz),7.15(d,2H,J¼8.3Hz),6.98(s,2H),3.90(m,1H),3.23(dd,2H,J¼6.8, 6.8Hz),3.10(dd,2H,J¼7.3,7.3Hz),2.57(s,6H),2.39 (s,6H).MS(EI):m/z¼476,474(Mþ).Element analysis for C23H23BrO2S2:Calcd.C58.10,H4.88; Found C58.13,H 4.92%.1770X.Li,H.TianSynthesis of1,2-Bisthiophenecyclopentene(6)50mL THF and2.5g Zn powder were put in a three-neckflask under nitrogen.3.3mL TiCl4(30mmol)was added very cautiously by a glass syringe.The solution turned yellow and was refluxed for45min.After that it was cooled in an ice bath and5(7.13g,15mmol)was added in portions.Then the mixture was refluxed for another2h,subsequently quenched with10%K2CO3(80mL),and extracted with diethyl ether (3Â100mL).The combined organic phase were washed with H2O(100mL),saturated NaCl solution(100mL),and dried (MgSO4).The solvent removed in vacuo.The product was purified by column chromatography using petroleum ether afforded7.19g(96%).1H NMR:d¼7.42(d,2H,J¼8.4),7.21(d,2H,J¼8.4Hz), 6.43(s,2H),3.57(m,1H),3.19(dd,2H,J¼8.6,8.6Hz),2.82 (dd,2H,J¼6.5,6.5Hz),2.35(s,6H),1.88(s,6H).MS(EI): 444,442(Mþ).Element analysis for C23H23BrS2:Calcd.C62.29,H5.23; Found C62.31,H5.25%.Synthesis of7Under nitrogen6(6.2g,14mmol)in anhydrous THF(50mL) was placed in aflame dried Schlenk tube.The mixture was cooled toÀ808C and butyllithium(9.65mL of1.6M solution in hexane,15.4mmol)was added slowly.One hour after the addition the reaction mixture was quenched with anhydrous dimethylformamide(1mL,14mmol).The mixture was then stirred for another2h,before it was poured into HCl(2M, 50mL).The mixture was extracted with diethyl ether(3Â50mL).The combined organic phases were washed with satu-rated sodium bicarbonate solution(2Â30mL),H2O(1Â50mL),and saturated NaCl solution(50mL)in succession, and was dehydrated over anhydrous MgSO4.Afterfiltered and evaporated in vacuo,colorless oil was obtained.The crude product was purified by column chromatography using EtOAc/ petroleum ether(6:100)afforded2.8g(51%).1H NMR:d¼9.99(s,1H),7.84(d,2H,J¼8.1Hz),7.50(d, 2H,J¼8.1Hz),6.44(s,2H),3.70(m,1H),3.27(dd,2H, J¼8.7,8.7Hz),2.91(dd,2H,J¼6.4,6.4Hz),2.36(s,6H), 1.90(s,6H).MS(EI):392(Mþ).Element analysis for C24H24OS2:Calcd.C73.43,H6.16; Found C73.44,H6.18%.Synthesis of82.8g(7.1mmol)of7was dissolved in ethanol(15mL)and anhydrous THF(15mL)and the temperature lowered to08C. NaBH4(0.33g,8.6mmol)was added in portions and the temperature was maintained for1h.After evaporation of most of the solvent,H2O(50mL)was poured into the mixture.The mixture was extracted with diethyl ether(3Â20mL).The combined organic phase was washed with saturated NaCl solution(30mL),and was dehydrated over anhydrous MgSO4. The residue after rotary evaporation of the solvent was obtained as a white solid.The crude product was purified by column chromatography using petroleum ether/EtOAc(4:1) afforded2.56g(92%).1H NMR:7.30(d,2H,J¼8.2Hz),7.26(d,2H,J¼8.2Hz), 7.25(s,2H),5.09(t,1H),4.47(d,2H,J¼5.7Hz),3.60(m,1H), 3.16(dd,2H,J¼8.6,8.6Hz),2.79(dd,2H,J¼6.9,6.9Hz), 2.31(s,6H),1.83(s,6H).MS(EI):m/z¼394(Mþ).Element analysis for C24H26OS2:Calcd.C73.05,H6.64; Found:C73.02,H6.66%.Synthesis of9A mixture of8(1.58g,4mmol),pyridine(0.35g,4.4mmol), and acetone(20mL)was lowered to08C in an ice bath.PBr3 (1.1g,2.6mmol)dissolved in acetone(5mL)was added to the reaction mixture dropwise.After stirring for1h,most solvent was evaporated in vacuo.The residue was purified by column chromatography using petroleum ether/EtOAc(3:1)afforded the compound as a white solid1.57g(86%).1H NMR:d¼7.35(d,2H,J¼8.2Hz),7.31(d,2H,J¼8.2 Hz),6.44(s,2H),4.51(s,1H),3.62(m,1H),3.20(dd,2H, J¼8.6,8.6Hz),2.88(dd,2H,J¼6.7,6.7Hz),2.35(s,6H), 1.89(s,6H).MS(EI):458,456(Mþ).Element analysis for C24H25BrS2:Calcd.C63.01,H5.51; Found C62.98,H5.49%.Synthesis of Dithienylethene Dimer(10)To a solution offluorene(66mg,0.4mmol)in dimethyl sulfoxide(DMSO,30mL)was added50%NaOH(2mL)and TBABr(0.02g)dissolved in5mL DMSO.The system was kept under an argon atmosphere.After5min,0.37g(0.8mmol) dissolved in10mL DMSO was added to the reaction mixture by syringe.The mixture was heated to35–408C and stirred until the reaction completed monitored by TLC(about19h). The reaction mixture was poured into water(50mL)and extracted with ether(3Â30mL).The organic extracts were washed with brine(50mL)and dried over magnesium sulfate. After removing most of the solvent under reduce pressure,the residue was purified by column chromatography using petro-leum ether/EtOAc(20:1)afforded the compound as a white solid0.33g(88%).1H NMR:d¼7.44(s,2H),7.36(d,2H,J¼1),7.26(d,2H, J¼1Hz),6.96(d,4H,J¼8.0Hz),6.67(d,4H,J¼8.0Hz), 6.39(s,4H),3.49(m,2H),3.27(s,4H),3.09(dd,4H,J¼8.6, 8.6Hz),2.75(dd,4H,J¼6.6,6.6Hz),2.34(s,6H),1.85(s, 6H).ESI:1115.2(MþþK).Element analysis for C61H56Br2S4:Calcd.C68.02,H5.24; Found C68.05,H5.26%.Synthesis of Dithienylethene PolymerTo a mixture of11(141mg,0.22mmol),14(240mg, 0.22mmol),and Pd(PPh3)4(5.1mg,1.0mol-%)was added toluene(5mL)and aqueous2M potassium carbonate(3mL) under an argon atmosphere.The mixture was vigorously stirred at85–908C in dark for2d.At the end of polymerization,2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-fluorene(11)was added to remove bromine end groups,and bromobenzene was added as a monofunctional end-capping reagent to remove boracic ester end group because boron andHigh-Content Pendant Photochromic Copolymer with Dithienylethene/Fluorene2:1Mole Ratio1771bromine units could quench emission and contribute to exci-mer formation in LED application.After the mixture was cooled to room temperature,it was poured into100mL methanol.The precipitated material wasfiltered and extracted by Soxhlet apparatus with methanol for2d to remove oligo-mers and catalyst residues.Final product was given by a second reprecipitate into methanol as yellowish powder(232mg,80% yield).1H NMR:d¼7.66(m,12H),7.07(s,4H),6.88(s,4H),6.42 (s,4H),3.56(m,2H),3.49(s,4H),3.13(m,4H),2.86(m,4H), 2.33(s,6H),2.1(m,4H),1.23–1.12(m,20H),0.78(m,6H), 0.70(m,4H).Results and DiscussionThe method for the preparing the monomers and the poly-mer is outlined in Scheme1and2.The monomer11,2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dio-ctylfluorene,was prepared using2,7-dibromofluorene following a literature procedure.The diaryl cyclopentene switch9was prepared by a new synthetic approach.In this route,the central cyclopentene is formed by a ring-closure reaction of a1,5-diketone via McMurry reaction.[30]More-over,by following this approach the undesired and often troubling formation of the trans isomer of the1,2-diaryl-ethene could be avoided.[31,32]It should be noted that the application of b-arylglutaric acid(3)is innovative.Another important intermediates(4,5,6)were readily synthesized derived from the succeeded synthesis of b-arylglutaric acid. For thefirst time,functional group(bromo-)was introduced into the bridging unit of dithienylethene.It makes the synthesis of intermediate9easy by functional group transformation.It also provided a method about how to introduce a functional group on the bridging unit of DTE and how to carry out the functional group transformation. The monomer10was synthesized in the same way as described in the literature.[29]To improve the solubility of the copolymer in organic solvents,alkyl chains are intro-duced into thefluorene unit at the9-position.It has been reported that polyfluorenes with substituted alkyl chains undergo changes in solubility,glass temperature,and processability of the resulting polymers.[33,34]In fact,the resulted copolymer is readily soluble in common organic solvents,such as CHCl3,THF,and toluene.In this paper,the polymerization reaction was conducted by the well-known palladium-catalyzed Suzuki coupling reaction between compounds10and11in toluene yielded a yellowish solid.[35]The advantages associated with this reaction include reactivity being less sensitive to steric hindrance, mild reaction conditions,less side reactions,and higher conversions.[36]Moreover,a well-defined alternation copo-lymer can be produced by this reaction.Suzuki coupling is among the most popular synthetic routes for the preparation of conjugated copolymers.They can be used directly in the copolymerization reaction.[37–39]A special feature of Suzuki coupling is that it allows an alternating type of copolymer to be obtained in the case of A-B comonomers, while typical cross-coupling of dibromides provides a multiblock copolymer.8910Scheme1.Synthetic route of the photochromic monomer10.1772X.Li,H.TianThe molecular weight of the polymer was evaluated by GPC using polystyrene standards.In the pristine colorless form,the number-average molecular weight M n and the weight-average molecular weight M w were estimated to be 6967and 13326g Ámol À1,respectively,with a polydisper-sity index of 1.8.The average number of the dithienylethene units in one chain is about 10–20.The T g was estimated to be 83.68C.The photochromic reactions (Scheme 3)of this polymer can be monitored using UV-vis spectroscopy.A THF solution of the polymer was prepared in which the concen-tration of dithienylethene subunit is maintained at 5Â10À5M .The results of the photoinduced isomerization studies,carried out by irradiating the THF solutions at 254nm with a standard lamp used for visualizing TLC plates,are shown in Figure 1.Within the first 1min of irra-diation,absorption bands appear between 420and 480nm as the photochromic monomers are converted from their colorless ring-open to their colored ring-closed forms,with a concomitant decrease in the intensity of the peak at 254nm.The resulting colored solutions can be decolorized by irradiating the solution with broad-band light greater than 420nm resulting in the complete disappearance of absorption bands in the visible region.Obviously,there was an isobestic point each at 280nm in the process of cycli-zation and cycloreversion,and it indicated that the equili-brium of two isomers (ring-open and ring-closed forms)existed in isomerization upon irradiation.The cyclization and cycloreversion quantum yields in THF were deter-mined to be 0.25and 0.20,respectively.According to well-defined isobestic point,one can calculate the relative conversion of photocyclization reaction a ps following the expressions based on the absorbance of the two forms.The conversion of photocyclization reaction a ps at photo-stationary state (PSS)can be determined as 0.30.The photochemical coloration/decoloration could be repeated 80recycles without obvious degradation.The coloration/decoloration cycles confirmed the effect that the flexibility of the polymer allows for adequate separation of the fluorene and photochromic components to permit photochromism.The UV-vis spectrum of the polymer is dominated by dithienylethene subunits for the polymer as can be seen from Figure 1and 2.We can attribute the 375nm absor-bance to polyfluorene unit and it is the typical absorbance of polyfluorene as described in the literature.[40]The cycliza-tion and cycloreversion quantum yields of 8in THF were determined to be 0.30and 0.20,respectively.The cycliza-tion quantum yield of 8was higher than that of the polymer.This can attribute to the overlap of their UV absorption at 254nm of them (Figure 3),and result into lower cyclization quantum yield for the polymer.The polymer exhibited photoluminescence at 410nm in THF solution when excited at 375nm.The fluorescence quantum yield of the polymer in THF was measured using anthracene (F ¼0.46)as a reference and determined to be 0.71,which was quite high as compared to that of previously reported.[8,11,41]For the polymer,the photo-luminescence spectrum was dominated by fluorene emis-sion,while the emission of DTE unit at 410nm was very weak (Figure 3).The emission of 8appeared at around 310nm (excited at 285nm)and its fluorescence quantum yield of it was very small (0.06,using naphthalene as a reference).The fluorescence intensity of the polymer decreased along with the photochromism from the ring-open form to the ring-closed form irradiation at 254nm,as shown in Figure 4.The blue emission was suppressed at thePolymer1011Scheme 2.Synthetic route of the photochromic polymer.254nm >405nmSSArAr'SSArnScheme 3.Photochromic process of the polymer.High-Content Pendant Photochromic Copolymer with Dithienylethene/Fluorene 2:1Mole Ratio1773photostationary state (PSS)upon irradiation with 254nm light.The ring-closed form has the second absorption band ranging 375–500nm.The overlapping of the emission and the absorption bands suggests efficient energy transfer to the ring-closed form,resulting in quenching of the photo-luminescence.[42]The original photoluminescence was restored by irradiation with l >405nm light.The success of photochromism in polymer suggests that the dithienyl-ethene subunits are covalently linked and the length of the linker is great enough so that the two chromophores are sufficiently separated from each other within the polymer matrix.The fluorescence,as decays shown in Figure 5,with two exponential kinetics (excited at 375nm)with the lifetimes of 7.41ns (61.7%)and 0.64ns (38.3%).The polymer was dissolved in chloroform,and thin film was obtained by spin-coating the solution onto a quartz plate.The polymer film retains its photochromic behavior (Figure 6).Irradiation of the films with 254nm UV light results in the immediate change in color,indicating that the photochromic properties of the polymer is conserved in the processed state.The changes in the UV-vis absorption spectrum of the polymer was similar to that obtained in solution,with the exception that slightly longer irradiation times were required to reach the photostationary state (7min compared to 10min for the polymer),as shown in Figure 6.An isobestic point was observed at 280nm also,indicating a two-component photochromic reaction and low electronic interaction between the dithienylethene sites.[43,44]The fluorescence emission spectrum of the film has one sharp peak (410nm)in the blue region.In addition,a broad and intense peak (535nm)appears in the blue region.This broad peak is probably from interchain excimer emis-sion.[45,46]In the dilute solution,the polymer chains are separated from each other,and excimer emission is sup-pressed.The color of the fluorescence emission from the film appears sky blue.Moreover,the two fluorescence emis-sion peaks have almost identical excited spectra,as shown in Figure 7.The fluorescence emission at 410nm,after excitation with a short laser pulse of 375nm,produced a two exponential decay with lifetime of 16.49and 3.07ns after deconvolution with the excitation-pulse profile.How-ever,the fluorescence emission at 540nm decays with a single exponential kinetics with the lifetime of 9.37ns.The fluorescence intensity decreased when irradiated with UV light (254nm),and increased again after visible-light irradiation (!405nm,Figure 8),similar to that in THF solution.That is,the fluorescence intensity could be modu-lated by the photochromic reaction,where thering-closedFigure 1.Absorption changes of the polymer in THF upon irradiation at 254nm (left)and its photochromic bleaching upon irradiation at !405nm (right).The concentration is 5Â10À5M with respect to the photochromicunit.Figure 2.Absorption changes of 8(5Â10À5M )in THF uponirradiation at 254nm.Figure 3.Absorbance and photoluminescence spectrum of 8(solid line)and fluorene (dash line)excited at 285and 375nm,respectively.The concentration of 8and fluorene is 5Â10À5M .1774X.Li,H.Tianform quenches the fluorescence of the open,as in solution.The decrease in the photoluminescence intensity upon photoisomerization was much remarkable in the polymer film than that in THF solution.The remarkable suppression in the polymer film indicates that the excited state migratesin the polymer film,and is efficiently quenched by the ring-closed form.Both intra-and inter-chain energy migrations are considered to take place in the film.For the solid film of the polymer,the photochromic conversion from the open-ring form to the closed-ring form under irradiation with UV light (254nm)was evaluated to be 26%,which is smaller than that obtained in the solution phase.This should be attributed to the stereo-chemical inhibition of the photocyclization reaction in the solid film.The photochemical ring-cyclization reaction of the dithi-enylethene should be hindered when the open-form dithi-enylethene are in the parallel conformation,or even in the antiparallel conformation when the distance between two reacting carbon atoms is larger than 0.4nm.[47–49]Some fractions of dithienylethene sites in the film of polymer seem to be fixed in an inactive conformation,such as the parallel conformation,and so the polymer showed slightly poor photochemical reactivity in the film state.In this polymer,the photochromic activity was only slightly diffe-rent from that of solution because dithienylethene subunit was perpendicular,which facilitated the thiophene hetero-cycles to rotate freely in the material.This allows theDTEFigure 4.Changes of fluorescence emission of the polymer excited at 375nm.The concentration is 5Â10À5M with respect to the photochromic unit.Total irradiation (254nm)periods are 0,0.5,1,2,3,5,and 7min.Figure 5.Time-resolved fluorescence decay curves of the polymer excited at 375nm.Figure 6.Absorbance changes of the polymer net film upon irradiation at 254nm.Irradiation periods are 0,1,2,3,5,8,and 10min.Figure 7.The fluorescence emission spectra (excited at 375nm)of the film (left);excited spectra corresponding to 410nm (right,broken line);excited spectra corresponding to 535nm (right,dot line).High-Content Pendant Photochromic Copolymer with Dithienylethene/Fluorene 2:1Mole Ratio1775units to interconvert between the parallel and antiparallel conformations.The content of the active photochromic component in polymer reached to 53wt.-%,which was higher than that in most side-chain polymers.[19]This is due to the perpendicular nature of two photochromic units in one polymeric unit.ConclusionA new copolymer was synthesized with the mole ratio of 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