dye - sensitized solar cells 
 ( dsscs ) offer a promising , low - cost 
 alternative to silicon solar cells . dyes adsorbed 
 on a mesoporous semiconductor surface , typically tio2 or 
 zno , absorb light and inject electrons from the excited state into 
 the conduction band of the semiconductor . the dyes are then regenerated 
 by an electrolyte containing a redox - couple . ru - based dyes have long 
 set the standard for highly efficient dsscs , but are increasingly 
 replaced by pure - organic , metal free dyes , or compounds containing 
 common transition metals such as zinc . recently , a zinc porphyrin 
 based , cosensitized dssc achieved a conversion efficiency of 12.3% . porphyrins and metal - free organic dyes such as 
 indoline derivatives offer several advantages to ru - based dyes . they 
 can be produced at lower cost at a large scale , can be used with zno , 
 which has shown to be incompatible with many ruthenium complexes and , importantly , have higher molar absorption coefficients . the 
 latter is of special importance for solid - state and ionic liquid dssc , 
 where stringent limits are put on the thickness of the semiconductor . thinner layers tend not to absorb enough of the incoming light unless 
 dyes with very high absorption coefficients are employed . indoline 
 dyes have emerged as a promising class of compounds for 
 dssc applications . they are synthetically straightforward to obtain 
 and show high photon - to - current efficiency as well as high molar absorption 
 coefficients . the central indoline group acts 
 as an electron donating group , stabilized by additional phenyl rings , 
 and is conjugated to an electron accepting group . cyanoacrylic acid 
 provides accepting properties and acts as a binding group that links 
 to the semiconductor surface . alternatively , a carboxylic acid coupled 
 to one or more rhodanines has been shown to give intense charge transfer 
 electronic transitions and also high injection yields . a time - dependent density functional theory ( td - dft ) study investigating three different candidates of this 
 class confirmed the charge - transfer nature of the s0  
 s1 transition , which possesses a very large oscillator 
 strength ( f = 2.06 ) and leads to a dipole moment 
 of > 30 d in the excited state . also , the frontier orbitals 
 showed the highest occupied molecular 
 orbital ( homo ) to be delocalized over the indoline unit , and the lowest 
 unoccupied molecular orbital ( lumo ) to be more localized around the 
 cyanoacrylic acid or rhodanine ring(s ) . d149 ( structure given in figure 1 ) , one of the most promising of the indoline dyes , 
 has achieved 9.0% light - to - electricity conversion efficiency . to make best use of the mesoporous surface , tight 
 packing ( a monolayer 
 of dye ) is desirable for efficient light absorption . this , however , 
 can lead to interaction between nearest neighbors and fundamentally 
 change the photophysical properties of the dye . aggregation of surface - adsorbed 
 dyes was already observed in the 1970s on cyanine dyes on sno2 , and later on a range of other 
 systems such as squaraines , phthalocyanines , porphyrins , and also on indole - based 
 donor  acceptor dyes . in nearly all cases replacement of the ethyl chain of the d149 dye by an octyl 
 chain , aimed at reducing surface aggregation , led to a new compound , 
 d205 , that set the record for organic dyes - based solar cells at the 
 time , giving 9.5% conversion efficiency . interestingly , a coadsorbent ( cheno - deoxycholic acid , cdca ) that 
 helps prevent aggregation , was still used to achieve maximum device 
 performance . electron injection from the excited state of the 
 surface absorbed 
 dye , a crucial step in solar cell operation , is partially controlled 
 by competing processes that lead to a reduction of the excited state 
 lifetime ( such as fast internal conversion or relaxation to lower 
 excitonic states , among others ) . a thorough knowledge of the dye s 
 photophysics can therefore help optimizing design and conditions of 
 operation . two previous articles have investigated the excited state 
 properties of d149 . combining pump  probe transient absorption 
 spectroscopy , single photon counting and steady state techniques , 
 the solvent dependence and excited state relaxation was investigated 
 in a wide range of solvents . surprisingly , the s1 lifetime was in the range of 100800 
 ps in all solvents investigated , much shorter than the few - nanosecond 
 lifetime expected for a typical organic dye . also , this 
 is considerably shorter than the calculated lifetime for d149 according 
 to td - dft calculations where , based on the oscillator strength , a 
 lifetime of  = 3.2 ns in meoh was predicted . another study used the fluorescence upconversion 
 technique to examine 
 d149 in solution ( toluene , acetonitrile ) , on mesoporous surface ( tio2 and noninjecting al2o3 ) and embedded 
 in a polymer matrix ( poly(methyl methacrylate ) , pmma ) . while the solution properties largely matched the reported 
 values , very fast deactivation electron injection is expected to be 
 fast and will give short lifetimes for tio2 . however , even 
 on al2o3 , the primary decay component ( with 
 60% amplitude ) was 2.3 ps , with other components of 16 and 120 ps . for the pmma embedded d149 , 11 ps ( 41% ) and 210 ps ( 59% ) lifetimes 
 were observed . while the fast deactivation on al2o3 is partially explained by aggregation , no reason is given 
 for the fast 210 ps s1  s0 decay observed 
 in pmma . to reconcile d149 s very high efficiency as 
 a solar cell 
 sensitizer with the observed short lifetimes in solution and inert 
 media , we have opted to reinvestigate the fluorescence behavior under 
 a range of conditions . three solvents were chosen  benzene , 
 acetonitrile , and methanol  for their nonpolar , polar , and h - bonding 
 properties , respectively . d149 was also investigated in polar and 
 nonpolar polymer matrices , pmma and polystyrene ( ps ) . zro2 , a high band gap semiconductor , was used to model the photophysics 
 on a noninjecting mesoporous surface with binding properties similar 
 to that of tio2 . if the short 
 lifetimes observed previously were to be confirmed , we would look 
 to find a mechanistic understanding of the involved photophysical 
 processes . as we will show , double bond isomerization competes with 
 radiative relaxation in solution , reducing the observed lifetimes . alternatively , if the earlier results could be attributed to effects 
 originating from sample preparation , we would try to optimize the 
 experimental procedure to obtain results that reflect the dye s 
 inherent properties . for solid samples , we will show that aggregation 
 occurs at higher concentrations and leads to fast quenching of the 
 aggregated molecules , and also to quenching of neighboring monomeric 
 molecules by energy transfer . d149 was obtained as a 
 kind gift from masakazu takata , mitsubishi paper mills , and used as 
 received . identity and purity , including isomeric purity , were confirmed 
 by means of nmr spectroscopy . the solvents , benzene ( merck , p.a . ) , 
 acetonitrile and chcl3 ( both sigma - aldrich , spectrophotometric 
 grade ) , meoh ( sigma - aldrich , chromasolve ) , and tetrahydrofuran ( thf ) 
 ( riedel de han , p.a . ) were used without further purification . 15 mm in diameter ) were 
 dissolved in chcl3 and the solution used for doctor blading 
 of polymer films as described below . absorption 
 spectra were measured on a varian cary 5000 , and emission measurements 
 were performed using a horiba jobin yvon fluorolog and automatically 
 corrected for wavelength dependent instrument sensitivity . solution 
 measurements where carried out at right angle in a 1 cm cuvette , while 
 films ( polymer and zro2 ) were measured using front face 
 geometry ( ca . excitation of the sample was done with a picosecond diode laser 
 ( edinburgh instruments , epl405 ) at 404.6 nm ( 77.1 ps pulses ) . 15 pj and was attenuated ( often 
 more than an order of magnitude ) to the desired count rate of ca . specific measurements were 
 performed with 470 nm excitation , but no significant difference in 
 the lifetimes was observed . a cooled ( ca . 40 c ) hamamatsu 
 mcp - photomultiplier r3809u-51 was used for detection of single photons , 
 and the signal passed through a discriminator ( ortec 9307 ) and into 
 a tac ( ortec 566 , 50 ns range used ) . the electrical trigger signal 
 from the laser was also passed through a discriminator ( tennelec tc454 ) 
 and on to the tac ( ortec 566 ) . the tac output was read by a daq-1 
 mca computer card using 8096 channels and collected with horiba jobin 
 yvon datastation 2.3 . measurements where done in reverse mode at 5 
 mhz and under magic angle polarization . a cutoff filter , gg475 , was 
 used to block stray excitation light . a dilute solution of ludox was 
 used to record the instrument response function without any filter 
 for solution measurements , while a microscopy glass slide was used 
 to scatter a small part of the excitation for recording the irf for 
 film measurements . no monochromator was used , i.e. , all wavelengths 
 transmitted by the cutoff filter were collected . excitation 
 of the sample ( concentrations between 7 and 10 m ) with ultrafast 
 laser pulses was performed using a frequency - doubled ti : sa oscillator 
 ( coherent mira ) output ( 400 nm ) at 76 mhz . excitation energies at 
 the sample were on the order of 5 pj ( after a cc8 filter to remove 
 residual ir light ) . a 1 cm cuvette was used , and the laser beam was 
 directed into the cuvette close to the cuvette wall on the emission 
 side , thus reducing the efficient cuvette length to 12 mm . fluorescence at a right angle to the excitation was passed through 
 a bruker spec 250is spectrograph ( ca . 200 nm observation window ) and 
 onto the streak camera ( hamamatsu streak camera and blanking unit 
 c5680 in combination with a synchroscan unit m5675 ) . the charge - coupled 
 device ( ccd ) camera ( hamamatsu orca - er c474295 ) was used in 
 binning mode ( 2  2 pixels ) to give a 512  512 pixel matrix . the observed time - windows in different time - ranges were ( fwhm instrument 
 response function in parentheses ) ca . 160 ( 5 ) , 800 ( 20 ) , or 2000 ( 50 ) 
 ps . nmr spectroscopic 
 data were collected with a varian mercury+ operating at a proton frequency 
 of 300.03 mhz . h spectra of d149 ( approximately 1 mg / ml ) 
 were recorded in deuterated dimethyl sulfoxide ( dmso - d6 ) at 298 k and referenced internally to solvent residual 
 peaks . the isolated singlets between 7.8 and 8.3 ppm corresponding 
 to the olefinic protons exhibited a signal - to - noise ratio of more 
 than 50 , which allowed a good estimation of the relative ratios of 
 the two isomers . decay - curves obtained 
 by single photon counting were analyzed by iterative reconvolution 
 using an exponential decay model with 1 , 2 , or 3 components in the 
 spectrasolve program . for streak camera data , global fits analyzed 
 several wavelengths simultaneously , searching for a global minimum . 
 for streak camera data , a self - written procedure in igorpro6 was employed 
 assuming a gaussian instrument response function . to test a highly viscous environment , this allows for optical examination of 
 the excited state properties , but limits the large - scale motion of 
 molecules . to obtain a thin film of polymers , the doctor blading technique 
 was employed : a substrate of microscope slides was attached to a horizontal , 
 even surface with tape . the outer edges ( ca . 3 mm ) of the slides were 
 covered by tape , which served as a spacer of several tens of micrometers . approximately 100 l of viscous polymer ( pmma or ps ) solution 
 in chcl3 was added to the lower edge of the slide and quickly 
 and evenly spread over the complete surface with a pasteur glass pipet 
 and the film dried in vacuo . the films looked even and completely 
 translucent ( colored when higher amounts of dye where used ) . fluorescence 
 measurements on blank samples were performed to ensure that no background 
 signal due to the polymers was observed . zro2 paste was prepared according to the procedure described 
 in ref ( 22 ) . this preparation 
 method had the advantages of giving highly transparent films ( estimated 
 particle size 12 nm in comparable samples ) , which strongly 
 reduces scattered light during laser excitation . heating of the films was carried 
 out with the following gradient program : heating from 20 to 180 c 
 ( 15 min ) , constant at 180 c for 10 min , from 180 to 320 c 
 ( 15 min ) , 320 c for 10 min , heating from 320 to 390 c 
 ( 15 min ) , stable at 390 c for 10 min , raised from 390 to 450 
 c during 15 min , kept at 450 c for 30 min , and then cooled 
 to 90 c over 2 h 22 min . films were sensitized in a solution 
 of d149 ( 0.5 mm ) in thf with varying amounts of cdca and afterward 
 dried in vacuo . to investigate 
 the behavior of the dye d149 in different environments we started 
 by measuring the steady - state absorption and emission properties in 
 three different solvents : nonpolar benzene ( c6h6 , r = 2.2 ) , polar acetonitrile ( ch3cn , 
 acn r = 36 ) and protic methanol ( ch3oh , 
 r = 33 ) , which is also able to interact with solutes 
 through hydrogen bonds . the absorption spectra show a major band centered 
 around 525550 nm , which has previously been assigned to an 
 intense charge transfer transition based on dft calculations . 390 nm ) and has been assigned to a  * transition 
 ( mainly lumo1homo ) . absorption 
 ( solid line ) and fluorescence ( symbols ) spectra of d149 
 in benzene ( black ) , acetonitrile ( green ) , and methanol ( red ) . the 
 inset shows emission curves normalized to the absorption , i.e. , relative 
 quantum yields . the emission spectra in benzene show a near - mirror 
 image with a 
 weak vibronic shoulder . broader and less resolved spectra are observed 
 for the more polar solvents , with a larger stokes shift of 3140 and 
 4018 cm for acetonitrile and methanol , respectively . 
 the absorption and emission data are summarized in table 1 . as discussed previously , the 
 larger stokes shift in polar solvents points toward a large dipole 
 moment in the excited state . this is in line with the results from 
 dft calculations , where  = 36 d in the excited state compared 
 to  = 19 d in the electronic ground state . the inset in figure 2 shows 
 emission spectra 
 normalized to the absorption at the excitation wavelength , i.e. , the 
 areas under these curves are relative quantum yields . it is already 
 clear from the weak signal in methanol that deactivation of d149 is 
 much faster in methanol when compared to the other two solvents . it 
 was appropriate to investigate the kinetics with the help of time - resolved 
 methods . following excitation of d149 in different solvents with ultrashort 
 laser pulses (  = 400 nm , few pj / pulse ) , we measured time - resolved 
 emission using a streak camera . by analysis of the data with a global 
 fit procedure , we obtained several lifetimes , as none of the measured 
 kinetics could be fitted with a monoexponential model . a global 
 fit yielded a short lifetime ( 1.2 ps , below the time resolution of 
 our equipment ) and two medium - long lifetimes of 30 and 330 ps . the 
 longer of these is taken to be the lifetime of the electronic state . the 30 ps component can not be attributed to solvent relaxation , as 
 the slowest component for acetonitrile is sub - picosecond ( 630 fs ) 
 as determined in a seminal work by the group of maroncelli . lohse et al . also 
 found a similar decay component ( 19  10 ps ) for this solvent 
 and a 30 ps time for thf , tentatively assigning it to collisional 
 cooling or structural s1 relaxation . also fakis et al . reported a 23  5 ps decay time for acetonitrile . in benzene , fitting of the dye s fluorescence resulted in 
 two lifetime components : 25 and 310 ps . it is also practically identical with 
 the slowest of the solvent relaxation times ( 24.7 ps ) . however , the latter carries only a small amplitude ( 3.4% 
 of the relaxation amplitude ) , while the major amplitude is on the 
 femtosecond time scale and will not reliably show up in our streak 
 camera experiments . therefore , even for benzene , the component has 
 to be attributed to similar processes as in acetonitrile and thf . the 310 ps lifetime is attributed to the electronic relaxation of 
 d149 in benzene . the decay of d149 in toluene , which is chemically 
 similar to benzene , 
 had been measured with the upconversion method , and besides a 450 
 fs rise component , two lifetimes of 40  10 ps ( 54% ) and 630 
  60 ps ( 46% ) were reported . the 
 faster of these ( 40 ps ) is again comparable to the solvent relaxation 
 in benzene , but has a high amplitude and is likely connected to the 
 2030 ps component discussed earlier . the longer , however , 
 is more than twice as large as our result in benzene . to investigate 
 this aspect further , we measured the fluorescence decay in toluene 
 and benzene with single photon counting and streak camera , employing 
 both single wavelength analysis at 600 nm and global analysis , and 
 found lifetimes between 300 and 330 ps ( data not shown ) , very similar 
 to the ones in benzene mentioned above . although is not fully clear 
 what the difference can be attributed to , at least two aspects might 
 give an explanation : the data measured by upconversion were measured 
 only up to 700 ps . this is too short to reliably fit a 630 ps lifetime , 
 especially as the fit is multiexponential and the component in question 
 carries less than 50% of the amplitude . this aspect has already been 
 pointed out by lohse et al . for the measurements 
 in acetonitrile . a second rationalization of the different results 
 is that there may be intermolecular interactions between dye molecules 
 in solution that affect relaxation . this aspect has been partially 
 addressed in the work by fakis , but the lowest concentration investigated 
 was 10 m ( in acetonitrile ) . we are currently looking at a possible 
 concentration dependence of the lifetimes with single photon counting , 
 a method that allows considerably lower concentration regimes to be 
 investigated . it is noteworthy that for the relatively less polar 
 thf , lohse et al . found a lifetime of 720  20 ps ( besides the 
 30 ps component discussed earlier ) . therefore , for the nonpolar 
 c6h6 , a longer lifetime might be expected , based 
 on polarity . our results show that , besides polarity , additional effects 
 must come into play . fluorescence decay for d149 in acetonitrile ( left ) and 
 in methanol 
 ( right ) . intensities are normalized , i.e. , are not comparable between 
 solvents . due to the presence of several functional groups 
 ( such as n , cooh , 
 c = o , c = s ) , hydrogen bonding may affect the excited state 
 of d149 . to specifically test for these effects , we chose to study 
 d149 in methanol solvent , which has comparable dielectric properties 
 to acetonitrile but allows for specific hydrogen bonding . the observed 
 fluorescence decay is seen in figure 3 , where 
 a distinctly nonsymmetric shape is evident . fitting yields two major 
 lifetimes of 13 and 103 ps and a fast 2.5 ps component . the blue side of the emission shows significantly faster 
 decay , with amplitudes of the ca . 2.5 and 13 ps components dominating . 
 these can be attributed to fast solvent relaxation , including specific 
 h - bonding . a 15.3 ps relaxation was obtained previously for methanol 
 as the longest of the solvent relaxation times , while the faster components are not fully resolved in our 
 measurements . interestingly , however , the major component related 
 to the electronic excitation ( 103 ps ) is substantially shorter compared 
 to acetonitrile ( 330 ps ) , in spite of the similar solvent polarity . this effect has already been described by lohse et al . , who found a faster decay ( 178 ps ) for ethanol , 
 despite being less a polar solvent compared to acetonitrile . amplitudes are given for the 
 emission maximum . from the new time - resolved fluorescence data and previous work , 
 it can be concluded that the excited state lifetimes of d149 in a 
 wide range of solvents are on the order of 100s of ps and that lifetimes 
 are further reduced by protic solvents . while the specific effect 
 of hydrogen bonding deserves further attention and is currently under 
 investigation in our group , we note that even in aprotic solvents 
 ( polar and nonpolar ) , short lifetimes are observed . concerning 
 the mechanistic details of deactivation , it is important to note that 
 d149 contains three exocyclic double - bonds . twisting of double bonds 
 in the excited state is a very well - known mechanism for fast deactivation , 
 with great relevance in biological processes , such as light detection 
 by retinal in the visual protein rhodopsin or phototherapy of neonatal jaundice . many of the observed reactions happen on a very fast time - scale , 
 hundreds of femtoseconds to a few picoseconds . in the case of stilbene , 
 for example , a barrierless excited state reaction is observed with 
 time constants of ca . for the photoreaction of thermodynamically more 
 stable trans - stilbene , an excited state barrier is 
 assumed to slow down the reaction and increase the time constant to 
 ca . , a temperature dependence 
 is also observed following the arrhenius equation . in another example , 
 efficiencies of dsscs based on the triphenylamine dye nk7 were reduced 
 compared to those using amines with methyl side chains ( nk1 and nk2 ) . this was explained , at least in part , by the excited state rotation 
 of the phenyl groups competing with electron injection . the time scale for this isomerization - like behavior 
 was assigned to tens to hundreds of femtoseconds . rotation around 
 the double bond lowers the excited state energy 
 while at the same time increasing the ground state energy , often leading 
 to a touching of surfaces ( conical intersection ) , thereby allowing 
 the molecule to pass efficiently to the ground state . a conical intersection 
 has , for instance , been shown for a bilirubin model , explaining the 
 fast lifetimes observed in the processes relevant for phototherapy 
 of neonatal jaundice . the exact potential 
 energy landscape determines the quantum yield , in some cases allowing 
 molecules to deactivate without forming a photoproduct ( isomer ) . to 
 test whether large - scale intramolecular motions are involved in the 
 deactivation , we sought to restrict rotational freedom of d149 and 
 chose to embed the dye in solid plastic matrices . pmma , containing 
 ester groups , and aromatic ps were chosen as transparent media with 
 polar and nonpolar properties , respectively . the results for time - resolved fluorescence measured 
 by the tc - spc 
 method are given in figure 4 , with the obtained 
 fitting parameters given in table 3 . laser 
 pulses of 405 nm with pulse energy less than 4 pj were used to excite 
 the sample . clearly , lifetimes increase considerably compared to solution 
 and are not very sensitive to the polar versus nonpolar environment . of the two lifetime components , the longer one is likely to be close 
 to the natural lifetime . have estimated the expected lifetime based on the oscillator strength 
 and calculated fluorescence energy , according to the einstein transition 
 probability formula . the obtained value of 3.23 ns is longer than 
 our longer value , but it is not clear whether the difference is due 
 to other remaining deactivation processes in the experimental value 
 or due to approximations for obtaining the theoretical value . in any 
 case , a nearly 10-fold increase in fluorescence lifetimes compared 
 to solution shows that the major deactivation path acting in solution 
 is blocked in the matrix for the longer component , while a second 
 decay channel still exists and keeps lifetimes of 1.2 ns for a subpopulation 
 of molecules . fluorescence decay of d149 in ps ( black ) 
 and pmma ( red ) after excitation 
 at 405 nm . 400 m , corresponding to od = 0.05 ) . 
 increasing the concentration of d149 by a factor of 4 led to an important 
 change : a short - lived component visible on the blue side of the emission 
 spectrum appears ( figure 5 ) . the new fast ( few - ps ) 
 decay is likely to occur due to aggregation of d149 within the matrix . it is probable that this species was observed by fakis et al . , explaining their fast decay times of 11 and 
 210 ps . the precise lifetimes will then be dependent on the exact 
 degree of aggregation and are expected to vary from sample to sample . fluorescence 
 decay in a concentrated ( left ) and dilute ( right ) 
 d149/pmma matrix . seeing a large increase 
 in the excited state lifetimes of d149 in a rigid environment points 
 toward a large - scale twisting motion as the relaxation pathway . thus 
 it would be of value to identify any possible photoproducts that may 
 have formed as a result of isomerization . isomers of similar dyes 
 have been calculated by dft previously . upon irradiation of a solution of d149 in acetonitrile with uv light 
 ( 387 nm , maximum of the s2 band ) , the absorption spectrum 
 changes in two ways : a shift and broadening of the s1 band , 
 and a decrease in intensity of the s2 band ( figure 6 ) . a change of the absorption spectrum after irradiation 
 with uv light and a photostationary state was previously observed , 
 but not examined in detail . the light - induced 
 changes can be reversed upon illumination with visible light ( 590 
 nm ) , proving that a reversible photoisomerisation is taking place , 
 and that the molecule is not chemically transformed by , e.g. , photo - oxidation . absorption 
 spectra of d149 in ch3cn after irradiation 
 with uv light ( left ) and with visible light after uv irradiation ( right ) . as d149 contains three exocyclic double bonds , 
 it is not obvious 
 which of these is involved in the formation of a photoproduct . from 
 a symmetry argument , isomerization of the diphenyl - vinyl double bond 
 will not lead to a photoproduct different from the thermodynamic isomer 
 and can be excluded . to further differentiate between the two remaining 
 double bonds , we have used d102 for comparison . here , only one rhodanine 
 group is present ( see figure 1 for structure ) . irradiation into the s2 band was performed in a similar 
 way as for d149 , and a similar , but smaller red shift in the absorption 
 spectrum could be seen ( data not shown ) . also in this case , a recovery 
 of the initial isomeric form was observed when irradiated with visible 
 light . as d102 only has one double bond that can lead to a photoisomer , 
 we can already conclude that rotation around the c = c linkage 
 connecting the rhodanine and phenyl ring is involved in the isomerization . d149 was dissolved in dmso - d6 and 
 split in two equal samples : one was irradiated at 400 nm under stirring 
 until a photostationary state was reached , while the other sample 
 was kept in the dark . the region from 6.8 ppm to 8.4 ppm of the recorded 
 spectra is depicted in figure 7 . as reported 
 earlier , d149 shows a large number of 
 unresolved aromatic and olefinic signals between 7.0 and 7.5 ppm . two resolved singlets of approximately 1h integral at 7.73 and 8.31 
 ppm can be attributed due to their chemical shift and multiplicity 
 ( no coupling ) to the proton ( h in figure 1 ) in the vinyl group bridging the rhodanine and the phenyl 
 unit and the acidic proton ( h ) , respectively . exchange 
 with residual water slightly lowers the acidic proton integral signal . 
 in the irradiated sample , due to the large shift ( h = 0.2 ppm ) 
 from 7.73 to ca . 7.53 ppm , this signal can be attributed to the hydrogen 
 attached to the isomerising bond ( h ) , whereas the carboxylic 
 acid is hardly effected ( h = 0.02 ppm ) . two 
 additional doublets at 6.91 and 7.80 ( jhh 8.5 hz ) can be further discriminated in the aromatic 
 region , which are assigned to the meta ( h ) and ortho ( h ) protons of the phenyl 
 ring adjacent to the isomerising double bond , respectively . these 
 newly appearing doublets were previously hidden in the unresolved 
 aromatic region between ca . 7.00 and 7.50 ppm , and their large change 
 in chemical shift originates from the significant change of environment 
 in the isomerized species . these findings confirm that the isomerization 
 can be assigned to the c = c double bond linking the rhodanine 
 moiety and the phenyl group . integration of the isolated singlets 
 of both the isomerized and 
 unisomerised form gives an estimate of the ratio of the two species 
 in the irradiated sample , yielding a ratio of ca . this in turn allows for the calculation 
 of the absorption spectrum of the pure photoisomer . we assume isomerically 
 pure starting material ( as shown by nmr ) with an absorption a( ) = ad149 . as indicated 
 by the isosbestic points , irradiation leads to exactly one photoproduct , 
 isomd149 . the mixed sample s absorption consists thus of two 
 contributions : knowing the mole fraction x and y corresponding to the amount of photoconversion 
 from the nmr data 
 ( x + y = 1 ) , we can solve for the absorption of the 
 pure isomerized form : accordingly , from the absorption sample 
 of the mixture , the fraction 
 of nonisomerized absorption was subtracted and the spectrum normalized . the result gives an approximated uv  vis absorption spectrum 
 of the pure photoisomer , as can be seen in figure 8 . the absorption maximum in the s1-band is located 
 at 537 nm , a 7 nm ( 246 cm ) shift compared to d149 . the large difference in excited state lifetimes in different solvents 
 ranging from 100 ps in methanol to > 700 ps in thf can not be explained 
 by a simple excited state reaction depending only on , e.g. , solvent 
 viscosity or polarity . protic media show a reduction of lifetime that 
 is separate from the effects of polarity . also , as mentioned above , 
 a concentration dependence of lifetimes is observed in some solvents . 
 from this it becomes clear that several processes besides isomerization 
 are contributing to the nonradiative rates . for the samples in pmma 
 and ps , however , all of the competing reactions can be excluded for 
 the major part of the population , giving the natural lifetime with 
 a large amplitude . it is thus at the moment not possible to attribute 
 a single time constant to the isomerization process , but it is becoming 
 clear that a thermal barrier is involved , as a barrierless reaction 
 would happen on a femtosecond  picosecond time scale and outcompete 
 other side reactions . we have tested for the possibility of a barrier 
 in the excited state by measuring lifetimes of d149 in acetonitrile 
 at different temperatures ( 5 to 60 c ) and see a clear increase 
 in rates with higher temperature ( data not shown ) . h - nmr spectrum of irradiated (  
 = 400 nm , top ) and unirradiated ( bottom ) d149 in dmso - d6 ; h  h refer to the protons given in 
 figure 1 , their primed form h - h to the isomerized form . absorption spectra of unirradiated d149 ( black curve ) , 
 d149 in 
 the photostationary state ( red ) and the calculated , normalized absorption 
 spectrum of the pure photoisomer ( blue ) . d149 has been developed for solar cell applications . in order to 
 find out if any of the above - mentioned deactivation processes are 
 relevant on the surface of a semiconductor , we chose to investigate 
 the photophysical behavior of d149 adsorbed to a film of mesoporous 
 zirconium oxide . zro2 is a high - band gap semiconductor 
 that allows similar anchoring of sensitizers as tio2 , but 
 prevents electron injection due to the high - lying conduction band . this enabled us to study purely photophysical processes independent 
 of electron injection that could be taking place when using tio2 . the previous upconversion study of d149 had shown very short lifetimes on al2o3 ( another high - band gap semiconductor ) , but these may have been affected 
 by aggregation , similar to the polymer matrix sample . we used different 
 concentrations of the additive cdca as coadsorbent to minimize dye 
 molecule aggregation . at the same time , however , the amount of dye 
 that can be loaded onto a given surface is reduced and therefore leads 
 to lowered light - harvesting efficiencies . steady - state fluorescence 
 spectra of d149 on zro2 with 
 different amounts of cdca as coadsorbent . the inset shows spectra 
 normalized to the number of absorbed photons , i.e. relative quantum 
 yield . time - resolved fluorescence ( tc - spc ) of d149 on zro2 with 
 various amounts of cdca . steady - state fluorescence spectra of d149 adsorbed 
 to zro2 are seen in figure 9 . while 
 showing a roughly 
 similar shape , they are systematically shifted to shorter wavelengths 
 with higher concentration of coadsorbent . put differently , increasing 
 the concentration of ( and thus reducing the distance between ) d149 
 molecules leads to a red - shift . first , relaxation of the environment can lower the 
 energy of the excited state , leading to a red - shifted emission as 
 is observed in meoh and acetonitrile . the surrounding cdca would in 
 this case behave like a nonpolar environment , with d149 itself being 
 more polar and allowing for relaxation to lower energy . a second and 
 more plausible reason is long - range energy transfer between d149 molecules 
 with different microenvironments , which leads to trapping of energy 
 at the lowest site within the transfer radius . as can be seen in the 
 inset of figure 9 , the emission of d149 without 
 cdca corresponds to the red - most part of the emission band of d149 
 in the presence of cdca . also apparent is the reduction in intensity , 
 which is in line with nonquantitative energy transfer , i.e. , the loss 
 of energy at each transfer step . because quantum yields are difficult 
 to measure on films that are mesoporous ( and thus highly scattering ) 
 and often also macroscopically heterogeneous , we performed tc - spc 
 experiments on each sample . . the decay curves 
 of four samples with varying cdca content , together with the instrument 
 response function , are seen in figure 10 . the 
 lifetimes obtained from biexponential fits are also given in figure 10 and in addition summarized in table 3 . here , we have also calculated quantum yields relative to 
 the 40 mm cdca sample , based on the lifetimes and amplitude . as expected 
 from the steady - state emission measurements , using higher concentrations 
 of cdca leads to longer lifetimes and thus higher quantum yields . 
 in the case of high cdca concentration ( 40 mm ) , the longer component 
 ( > 2 ns ) is similar to the longest component observed in pmma and 
 ps 
 matrix . for the other samples containing less cdca , it is noteworthy that 
 the maximum amount of cdca used corresponds to an 80-fold excess in 
 concentration relative to the sensitizer . this also leads to a severe 
 decrease of adsorbed dye and thus strongly reduced light - harvesting 
 efficiency . still , such high concentrations of coadsorbent are needed 
 to reach lifetimes comparable to the natural lifetime . while electron 
 injection is much faster than fluorescence and can compete more efficiently 
 with other processes such as energy transfer , it is obvious that coadsorbers 
 are not the best way of preventing aggregation . instead of wasting 
 precious surface area with inactive molecules , the long - term strategy 
 should be to design molecules that are less prone to aggregation . in all zro2 samples , a second lifetime component is 
 present that could be attributed to isomerization of surface - bound 
 molecules that have enough steric freedom to allow large scale rotation . it should be noted that a reduction of excited state lifetime does 
 not require for the isomerization mechanism , i.e. , double bond twisting , 
 to form a photoproduct . a partial twisting , as can occur in somewhat 
 restricted environment , dsscs rely on efficient electron injection 
 from the excited state of the absorbing dye into the semiconductor 
 layer . in order for this crucial step to be efficient electron injection has frequently 
 been assumed to be happening on the time - scale of ca . these measurements were often performed in the absence of electrolyte 
 and using laser intensities that far exceed normal operating conditions 
 of ca . one sun ( am = 1.5 ) . on the basis of an injection rate of ( 100 
 fs ) and using 300 ps as the lifetime of d149 in 
 fluid solution taken as the sum of competitive processes , the corresponding 
 efficiencies would exceed 99.96% and could be safely ignored . however , 
 recent experiments show hundreds of femtoseconds to a few picoseconds 
 injection times for surface - adsorbed n3 dye in the absence of electrolyte , 
 increasing to hundreds of picoseconds when an electrolyte is present 
 with 50% of injection occurring with ca . we are not aware of any comparable measurement for metal - free 
 dyes in the presence of electrolyte and with low excitation energy . the behavior can possibly be attributed to ru - dyes specifically , although 
 aspects such as excitation laser power and repetition rate are still 
 likely to be relevant for pure organic dyes . for d149 , the maximum 
 incident photon - to - current efficiency ( ipce ) is around 95% . as the ipce includes injection and regeneration 
 efficiency , isomerization as a competitive process can in this case 
 account for at most 5% , ( or less in the case of regeneration efficiency 
 < 100% ) . however , isomerization can have larger importance for other 
 dyes and has hitherto not been taken into consideration . in 
 order to achieve the highest possible open circuit voltage , the design 
 of the dye - semiconductor couple should be such that the conduction 
 band is relatively close to the lumo level of the injecting dye . as 
 a consequence , the driving force will be lowered and electron injection 
 slowed down . the effects of photoisomerisation on the 
 performance of the dssc 
 are not limited to the reaction itself . also the reaction product , 
 the photoisomerised dye , needs to be taken into account . at the moment , 
 it is unclear whether the photoproduct has comparable properties as 
 sensitizer and will inject and regenerate with similar efficiency . report a reduced efficiency of a dye ( s1 ) in its z - form 
 ( 3.72% vs 5.77% in the e - form ) . this 
 is explained by the geometric arrangement of the dye on the surface : 
 the electron donating part of the molecule is close to the tio2 in the z - form , whereas the larger distance maintained in 
 the e - form prevents recombination between the electron in the tio2 and the positive donor group of the molecule . while 
 it is not clear at the moment to what extent isomerization 
 processes occur on the semiconductor surfaces , it is not the only 
 competitive pathway for excited molecules . large - scale motion under 
 isomerization of d149 moieties can also lead to a disruption of molecular 
 order of the surface - adsorbed dyes . this could allow for increased 
 contact between tio2 and electrolyte and open a channel 
 for recombination of injected electrons with the electrolyte . here , 
 even a small quantum yield of isomerization could have a detrimental 
 effect due to the lost surface coverage . in one of the few studies 
 investigating the effects of isomerization 
 of solar cell dyes , lin et al . have looked 
 at a triphenylamine - stilbene system that isomerizes under the influence 
 of light . in a comparative molecule , the stilbene double bond was 
 incorporated into a five - membered ring , preventing isomerization . 13% in efficiency was observed , ranging from 
 4.52 to 4.98% for different isomerizing systems , compared to 5.14 
 to 5.67% for corresponding locked variants . interestingly , lifetimes 
 reported for the systems undergoing photoisomerisation in an aprotic 
 solvent ( 2.122.52 ns ) were not much shorter than the ones 
 for the nonisomerising systems ( 2.432.95 ns ) . for d149 , lifetimes 
 in aprotic solvents are reduced more dramatically to hundreds of picoseconds , 
 compared to ca . 2.5 ns for d149 in polymer . preventing isomerization 
 in a structurally modified d149 derivative could thus have a larger , 
 positive impact on d149 efficiencies when compared to the stilbene 
 systems . the existence of an additional , short lifetime 
 component of concentrated samples of d149 in polymer matrices shows 
 that a new pathway , competing with electron injection , is opened if 
 molecules get in close contact . while the phenomenon is less pronounced 
 on a mesoporous surface ( as zro2 ) , a reduction of lifetimes 
 is still visible , with a further component of energy transfer emerging . the latter is of less concern as the associated rates of less than 
 1 ns are much smaller than the direct 
 quenching due to aggregation ( few ps ) . the most 
 efficient solar cell devices based on d149 and similar dyes have relied 
 on the addition of cdca as coadsorbent to separate molecules efficiently and an increase in efficiency of 7% ( for d149 ) 
 and 11% ( d205 ) was observed , showing that aggregate formation has 
 direct consequences on the performance of working solar cells , confirming 
 the trend seen in our fluorescence measurements . while the approach 
 of adding coadsorbent is feasible , it reduces the amount of dye that 
 can be placed onto the semiconductor , and thereby reduces precious 
 surface space and the light - harvesting efficiency . a promising approach to separate molecules while keeping tight coverage is the addition 
 of long - chain side groups . this approach has been chosen in the dyes 
 d205 and d358 and has let to improved efficiencies of up to 9.5% for 
 d205 . we are currently studying these 
 new molecules to see whether the modifications are reflected in longer 
 excited state lifetimes due to reduced aggregation . excited state life times 
 for the sensitizer d149 have been measured 
 and compared to previous results . the relatively fast deactivation 
 in solution ( 100 ps in h - bonding , polar solvents to ca . 700 ps in 
 nonpolar solvents ) is confirmed and explained by , among others , a 
 large - scale molecular motion of d149 . incorporation of d149 in a solid 
 polymer matrix leads to a large increase in lifetimes , approaching 
 the natural lifetime ( > 2 ns ) . very fast deactivation ( fs to few 
 ps 
 range ) can be explained by aggregation and is the likely reason for 
 fast decays observed earlier , with more dilute samples showing longer 
 lifetimes . the effect of aggregation is seen both in polymer matrix 
 and on zro2 , having severe consequences for the electron 
 injection by competitive deactivation . investigation of the 
 photoproduct of irradiation into the s2 band revealed that 
 isomerization around an exocyclic double 
 bond is a further deactivation mechanism . the position of the isomerising 
 double bond , and thus the structure of the photoisomer , is resolved 
 by nmr spectroscopy . structural modifications to avoid the detrimental 
 effects of aggregation and isomerization , for example competition 
 with electron injection and disruption of surface order , can hopefully 
 lead to a further improvement in efficiencies for this class of molecules .
d149 , a metal - free indoline dye , is one of the most promising 
 sensitizers 
 for dye - sensitized solar cells ( dsscs ) and has shown very high solar 
 energy conversion efficiencies of 9% . 
 effective electron injection 
 from the excited state is a prerequisite for high efficiencies and 
 is lowered by competing deactivation pathways . 
 previous investigations 
 have shown surprisingly short - lived excited states for this dye , with 
 maximum lifetime components of 100720 ps in different solvents 
 and less than 120 ps for surface - adsorbed d149 . using steady - state 
 and time - resolved fluorescence , we have investigated the photochemical 
 properties of d149 in nonpolar and polar solvents , polymer matrices , 
 and adsorbed on zro2 , partially including a coadsorbent . 
 in solution , excitation to the s2 state yields a product 
 that is identified as a photoisomer . 
 the reaction is reversible , and 
 the involved double - bond is identified by nmr spectroscopy . 
 our results 
 further show that lifetimes of 100330 ps in the solvents used 
 are increased to more than 2 ns for d149 in polymer matrices and on 
 zro2 . 
 this is in part attributed to blocked internal motion 
 due to steric constraint . 
 conversely , concentration - dependent aggregation 
 leads to a dramatic reduction in lifetimes that can affect solar cell 
 performance . 
 our results explain the unexpectedly short lifetimes 
 observed previously . 
 we also show that photochemical properties such 
 as lifetimes determined in solution are different from the ones determined 
 on semiconductor surfaces used in solar cells . 
 the obtained mechanistic 
 understanding should help develop design strategies for further improvement 
 of solar cell dyes .