Multiple relaxation pathways in push-pull polyenes

Damien Laage,a Pascal Plaza,a Mireille Blanchard-Desce a,b and Monique M. Martin*a

a Département de Chimie, Ecole Normale Supérieure (UMR CNRS 8640 PASTEUR),24 rue Lhomond, 75231 Paris Cedex 05, France. E-mail: monique.martin@ens.fr;Fax: �33 1 44 32 33 25; Tel: �33 1 44 32 24 12

b Synthèse et Electrosynthèse Organique (UMR CNRS 6510), Université de Rennes 1,Campus de Beaulieu, Bât. 10A, 35042 Rennes Cedex, France.E-mail: mblancha@univ-rennes1.fr; Fax: �33 2 23 23 67 38; Tel: �33 2 23 23 62 77

Received 2nd April 2002, Accepted 2nd May 2002First published as an Advance Article on the web 23rd May 2002

Subpicosecond absorption and gain spectroscopy are used to investigate the excited-state behavior of push-pullpolyenes made of a diethylthiobarbituric acid electron-acceptor group and a dibutylaniline electron-donor grouplinked by a π-conjugated chain. Four polyenes of increasing length, ranging from n = 2 to 5 double bonds, arecompared. The relaxation path and relaxation kinetics are studied in dioxane and in cyclohexane, a polar and anonpolar solvent, respectively. In dioxane, the results provide evidence for the formation of an emissive transient stateon an ultrashort time scale (2–3 ps) attributed to a charge transfer (CT) state. The regular shift of the gain peak ofthis transient state with increase in the chain length (ca. 100 nm per added double bond) indicates that its structure issimilar to that of a cyanine, i.e. with a fully conjugated polyenic chain. Its lifetime ranges from a few tens to a fewhundreds of picoseconds depending on the chain length. When the number of double bonds increases from n = 2 to 3,the lifetime increases, then decreases continuously for longer chains. In cyclohexane, where the transient CT state isnot formed, the decay of the initial excited state follows the same trend when the chain length increases but thelifetimes are shorter than that of the CT state in dioxane. In both solvents, the characterization of long-livedphotoproducts by synchronizing two low repetition-rate subpicosecond laser systems demonstrates a change in therelaxation route as the chain length increases. Isomerization occurs for n = 2, whereas intersystem crossing to thetriplet state occurs for n = 4. The change in the relaxation channel is observed for n = 3 in both solvents with howevera solvent-dependent behavior. In dioxane, relaxation to the triplet state is already observed for n = 3, while anintermediate regime with a relaxation directly to the ground state is observed in cyclohexane. The photophysics of thestudied push-pull polyenes is tentatively compared to that of polymethine cyanines and substituted carotenoids.

IntroductionConjugated hydrocarbon chains have a versatile structure,the properties of which play a central role in various opticalphenomena. Two major biological photoreceptors rely onstructural changes in polyenic systems: rhodopsin,1 which isthe retina photoreceptor in the mechanism of vision, and bac-teriorhodopsin,1 a natural photosynthetic system. Phototaxiswas also recently shown to rely on structural changes withinchromoproteins having an ethylenic bond, such as the Photo-active Yellow Protein.2,3 A large number of theoretical andexperimental studies were devoted to the spectroscopy andexcited-state dynamics of linear polyenes,4–9 carotenoids 10,11

Schiff bases and polymethine cyanines dyes.5,12–16 One of themost fundamental questions raised for polyenes and caro-tenoids is the role of the chain length on the relative location ofthe 11Bu and 21Ag lowest excited states, on their coupling andon their deactivation path and decay kinetics. In addition, thephotochemical behavior of substituted carotenoids was shownto depend strongly on the electron donor-acceptor characterof the terminal groups and on the length of the polyenic chainconnecting them.11,17 An ultrafast process involving a two-modeintramolecular motion toward a conical intersection has beenrecently proposed for the cis–trans isomerization of the retinalprotonated Schiff base involved in the vision process andof some polymethine cyanines dyes.15 Implication of the firsttriplet state as a photoproduct in carotenoids 10 and as anintermediate in the photoisomerization of some cyanines 18 hasalso been discussed. In addition to their fundamental interest

for photophysics and as model compounds for understanding anumber of natural photoprocesses, polyenic compounds arewidely studied for applications such as nonlinear optics, opticallimiting, photodynamic therapy and biochemical sensing,see for example refs. 12–14,19–22. These applications rely onthe detailed understanding of the photodynamics of thesecompounds, their excited-state spectroscopy and deactivationpath as a function of their chemical structure and environment.

In particular, promising perspectives in optical data process-ing have been opened by novel push-pull polyenic systems –characterized by electron donor and acceptor groups bridgedby a conjugated polyenic chain – which exhibit large opticalnonlinearities (e.g. large quadratic and cubic hyperpolariz-abilities).23–25 We previously initiated a study by subpicosecondspectroscopy of the excited-state dynamics of such push-pullpolyenes,26 drawn in Fig. 1. These polyenes, that we will callPAn, possess a diethylthiobarbituric acid electron-withdrawinggroup and a dibutylaniline electron-donating group linked by aπ-conjugated chain of n = 2 to 5 double bonds. They have

Fig. 1 PAn push-pull polyenes.

526 Photochem. Photobiol. Sci., 2002, 1, 526–535 DOI: 10.1039/b203201p

This journal is © The Royal Society of Chemistry and Owner Societies 2002

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been designed and characterized as efficient nonlinear opticalchromophores with giant quadratic hyperpolarizabilities (β(0)up to 1000 × 10�30 esu).23,24 The estimated permanent dipolemoment change ∆µ upon S0–S1 excitation is as large as 42 D forthe longest chain (n = 5). In our previous study, we focused onthe initial dynamics of the excited state in polar solvents. Weshowed that in polar solvents the initially accessed excited stateevolves rapidly toward a rigid, fully conjugated, cyanine-likestructure, with a reaction time of 2.6 ps in dioxane, 1.8 ps intetrahydrofuran and 1.1 ps in acetonitrile, independently of thechain length for n = 2 to 4. A reaction time of about 2 ps wasfound for the chain with 5 double bonds, in the three solvents.The fast formation of a cyanine-like photoproduct could neverbe expected from the two valence bond state models commonlyused in the design of push-pull polyenes with optimized non-linear optical properties.25,27–31 Our study brought support to theproposal that more than two valence bond states should beinvolved in a proper description of the excited-state dynamicsof these compounds.26

In the present study, we focus on the behavior of thesemolecules in a non-polar solvent, cyclohexane, and in a weaklypolar solvent, dioxane. We highlight the influence of the solventand of the polyenic chain length on the decay channel, deter-mine the kinetics on the short timescale, and characterize thelong timescale (nanosecond to millisecond) reaction productsby synchronizing two low repetition-rate subpicosecondlaser systems.We therefore show the existence of multiple relax-ation pathways (trans–cis isomerization, intramolecular chargetransfer, singlet–triplet intersystem crossing) the respectivecontribution of which strongly depends on the chain lengthand on the solvent polarity. The photoinduced paths aretentatively compared to these of carotenoids and polymethinecyanines.

ExperimentalDifferential absorption spectra were measured by the pump–probe technique using ca. 0.5 ps, 10–20 µJ pump pulses at 546nm, 573 nm or 610 nm, provided by a dye laser system accord-ing to unconventional methods described elsewhere.32 Thewhole subpicosecond system is driven by a single seeded 10 HzQ-switched Nd:YAG laser delivering smooth 6 ns pulses at 532nm and 355 nm. 500 fs pulses at 610 nm are produced and usedeither directly in the pump–probe set-up or to generate acontinuum of white light by focalization in a heavy water cell.This continuum is divided into two beams, respectively, filteredat 546 nm and at 573 nm and amplified in rhodamine 110and rhodamine 6G amplifier chains pumped by the third andsecond harmonics of the same Nd:YAG laser.

In the pump-continuum probe set-up, when the pump pulsewas tuned at 573 or 610 nm the white-light continuum wasproduced by focusing about 200 µJ of the pump beam in a 1 cmH2O or D2O cell. In experiments carried out with the 546 nmexcitation pulse, the white-light continuum probe was generatedwith the 573 nm beam. The pump–probe experiments werecarried out with a two-beam probe arrangement: one beam issent onto the sample and the other one is used as a reference.The path of the sample cell was 1 mm long. The pump andprobe beams had a diameter of about 1 mm on the sample andcrossed at an angle of ca. 10�. The transmitted probe beamswere then guided through optical fibers to the 64 µm entranceslit of a polychromator (Spex 270 M, 100 grooves mm�1). Thespectra of the two probe beams were simultaneously recordedby a computer-controlled double-diode array detector(Princeton Instrument Inc. DDA-512). The pump–probe delaytime was adjusted by means of a stepper motor translation(Microcontrole M-MTM250PP1) which allows delays up toabout 1.7 ns. Pump and probe beam polarizations were set atthe magic angle. Data were accumulated over 500 or 1000 lasershots.

Pump–probe experiments were also carried out with timedelays up to milliseconds by using a second subpicosecondsystem, similar to that described above; one was used as thepump, while the other, synchronized with the former, was usedfor continuum-probe generation.

The design, preparation and purification procedures of thepush-pull polyenes were reported elsewhere.24 UV-spectroscopygrade (Merck, Uvasol) cyclohexane and dioxane were usedas the solvents. The solutions were not deaerated and usedimmediately after preparation or kept in a freezer (�27 �C).Deaerated room temperature solutions did not show anyphotodegradation when exposed to a visible-light lamp. Theabsorption and fluorescence spectra were checked respectivelywith a Cary 210 spectrophotometer and a PTI (Quanta-Master1) spectrofluorimeter. The corrected fluorescence spectra werelimited to wavelengths below 800 nm. In pump–probe experi-ments, the solute concentration was fixed to have an absorbancebetween 0.3 and 1 at the excitation wavelength depending on itssolubility and the solutions were recirculated. The experimentswere carried out at room temperature.

Results

Steady-state spectroscopy

Absorption spectra. The ground-state absorption spectra ofthe PAn in cyclohexane and dioxane are shown in Fig. 2, for

n = 2 to 5. The absorption bands are normalized to the sameamplitude in order to emphasize the conjugated chain lengtheffect on their shape and position. In both solvents the absorp-tion band shifts to the red and broadens when the chain lengthincreases, yet with a saturation of the shift. The wavelength of

Fig. 2 Steady-state absorption spectra (normalized) of the PAn push-pull polyenes in cyclohexane (top) and dioxane (bottom) solutions.Four polyenic chain-lengths ranging from n = 2 to 5 double bonds arecompared.

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Table 1 Maximum of ground state absorption (λabs) of push-pull polyenes PA2 to PA5 in cyclohexane and dioxane solutions (m: maximum,s: shoulder). Maximum of gain band (λgain) measured a few ps after excitation at wavelength λexc with a 500 fs laser pulse. Short time componentsτi obtained from fit of time resolved differential absorption ∆D(t) at various probe wavelengths (λp). * risetime. λp gain: gain is dominant. b: bleachingcontribution. w: weak signal, ∆D < 0.05

PAn λabs/nm λgain/nm λexc/nm λp/nm τi/ps λp/nm τi/ps λp gain/nm τi/ps

Cyclohexane (ε = 2.02 at 20 �C) 37

PA2 513s 546 447 69 ± 2 557b 58 ± 2 576 65 ± 3 545m PA3 559m 610 531b 6.1 ± 0.5 623b 6.6 ± 1.3 752 5.8 ± 1.2 597s 110 ± 10 106 ± 15 82 ± 12PA4 560s 546 548b 2.6 ± 1.3 742w 3.7 ± 1.5 788w 2.8 ± 1.0 597m 89 ± 5 80 ± 6 74 ± 4 646s PA5 596 546 566b 2.9 ± 0.9 630 3.8 ± 1.4 906 3 ± 2 16.5 ± 2.5 13.5 ± 2.5 16 ± 3

Dioxane (ε = 2.21 at 25 �C) 37

PA2 558 616 572 429 17* ± 6 586b 2.5 ± 0.15 614 2.9* ± 0.5 650 ± 35 430 ± 20 420 ± 5 484b 2.7 ± 1.0 558b 1.4 ± 20 660 3.3* ± 0.5 620 ± 40 490 ± 15 490 ± 20PA3 583 718 609 512b 10 ± 3 595b 1.7 ± 3 761 3.2* ± 1.2 670 ± 90 60 ± 30 60 ± 30 660 ± 140 650 ± 160 650b 2.4 ± 0.2 30 ± 16 660 ± 150 PA4 599 834 573 525b 2.4* ± 0.4 635b – 823 3.2* ± 0.5 330 ± 10 290 ± 25 220 ± 10 536b 1.1* ± 0.2 712b w 4.4 ± 0.4 845 2.3* ± 0.4 316 ± 7 – 220 ± 10PA5 608 955 573 583b 1.8* ± 0.2 770w 2.0* ± 0.3 925w 1.8* ± 0.2 39 ± 1 46 ± 8 32 ± 2 616b 3.1 ± 0.4 958w 1.3* ±0.2 32 ± 2 33 ± 2

the absorption maxima are given in Table 1. A ca. 1150 cm�1

vibrational structure is observed for PA2 and PA3 in cyclo-hexane. In contrast, for longer chains, the structure is blurred.However, the shoulder reminiscent of this vibrational structureseen in the lower energy side of the PA5 spectrum indicates thatthe 0–0 transition is 4000 cm�1 (150 nm) red-shifted withrespect to that of PA2. In dioxane, the vibrational structure isabsent for all polyenes, and the spectra are broad and slightlyred-shifted with respect to those in cyclohexane.

Fluorescence spectra. The corrected fluorescence spectrarecorded with samples of PA2, PA3 and PA4 in cyclohexaneand in dioxane having the same absorbance at the excitationwavelength (540 nm) are shown in Fig. 3. Under these con-ditions the fluorescence of PA5 is hardly detected in bothsolvents in the given wavelength range. In dioxane it is likely tolie further in the near infrared, out of our detection window,while in cyclohexane the signal is probably too weak to bedetected. In cyclohexane, it is seen that all spectra exhibit aca. 1150 cm�1 vibrational structure. For PA2, the spectrum isroughly the mirror image of the ground-state absorption with amain peak and a shoulder. For PA3 and PA4, two fluorescencepeaks of similar intensities are observed and the mirror image islost. The fluorescence maxima are roughly shifted by 50 to 60nm per added double bond. In dioxane, the fluorescence spectraare broad and unstructured like the absorption spectra. TheStokes shift and the red shift due to the increase of the chainlength are both larger than in cyclohexane. The fluorescencespectrum of PA3 is 83 nm red-shifted with respect to that ofPA2, and there is a further ca. 70 nm shift for PA4. Fig. 3shows that the fluorescence intensity depends on the chainlength. The change in the area of the spectra indicates that theintensity is maximum for n = 3. In addition, the fluorescenceintensity is larger in dioxane than in cyclohexane by two ordersof magnitude.

Fig. 3 Relative intensity of the steady-state fluorescence spectra of thePAn push-pull polyenes ( n = 2 to 5 double bonds) in cyclohexane (top)and dioxane (bottom) solutions excited at 540 nm under the sameconditions.

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Time-resolved spectroscopy

Picosecond dynamics. Solvent effect. In cyclohexane. Immedi-ately after excitation with a subpicosecond laser pulse anintense transient absorption band (∆D > 0) is seen for allcompounds in the short wavelength edge of the ground-stateabsorption band and negative ∆D bands are observed wherebleaching and gain signals are expected. The time-resolveddifferential absorption spectra (∆D(t)) are shown for PA2 andPA4 in Figs. 4 and 5. The transient bands decay time is found to

depend on the chain length and at long delays, i.e., after one toseveral hundreds picoseconds depending on the compound,small bleaching and transient absorption signals remain, exceptfor PA3. In Figs. 4 and 5, the transient spectra of PA2 and PA4exhibit a temporary isosbestic point (TIP), respectively at 516and 599 nm, in the bleaching range. The TIP observed for PA4is clear only for delays larger than 10 ps. For PA5 a TIP is seenaround 745 nm in the small transient absorption band. Thebest fit of the observed ∆D(t) was obtained by convolutingthe pump–probe cross-correlation function (assumed to be aGaussian) with a sum of exponentials, plus a step functionwhich describes the residual signals at long delays. For PA2, theobserved decays could be fitted with a 65 ps single exponentialbut two exponentials were needed for the other compounds,with one component in the sub-10 ps regime and the other onein the sub-100 ps, both decreasing as the polyene chain length

Fig. 4 Time-resolved differential absorption and gain spectra(absorbance changes, ∆D) observed for PA2 in cyclohexane, in the2–1000 ps time-scale, after excitation at 546 nm with a subpicosecondlaser pulse. The arrows indicate the evolution at increasing time.

Fig. 5 Time-resolved differential absorption and gain spectra(absorbance changes, ∆D) observed for PA4 in cyclohexane, in the2–500 ps time-scale, after excitation at 546 nm with a subpicosecondlaser pulse. The arrows indicate the evolution at increasing time.

increases. The time-constants are given in Table 1 at a fewwavelengths. A detailed analysis of the variation of thesetime components across the spectra indicates that they do notdepend much on the wavelength within experimental error,except for PA3 which exhibits shorter time components in thered side of the transient spectra.

In dioxane. Within a few picoseconds after excitation, wefound in all cases a delayed rise of the red side of the gain band,which was not observed in cyclohexane. The time-resolveddifferential absorption spectra (∆D(t)) are shown in Figs. 6and 7 for PA3 and PA5, respectively. For PA2, the delayed rise

is observed until 8 ps; it is accompanied by a decay of ∆D below608 nm, where a TIP is observed, and by small changes in thetransient absorption band. Similar features are found for PA3within 10 ps after excitation; a TIP is seen in the blue edgeof the gain band at 699 nm (Fig. 6) while above and belowthis wavelength respectively, the gain band rises and the ∆Dsignal decreases. The TIPs in the blue edge of the PA4 and PA5gain bands are less clear, likely due to smaller and noisiersignals. The initial increase of the gain bands is, however,accompanied by a decrease of the negative signals, below750 nm for PA4 and 825 nm for PA5 (Fig. 7). In addition, theintense transient absorption peaks, found around 550 for

Fig. 6 Time-resolved differential absorption and gain spectra(absorbance changes, ∆D) observed for PA3 in dioxane, in the 2–1700ps time-scale, after excitation at 609 nm (the spikes due to scatteredlight are masked) with a subpicosecond laser pulse. The arrow indicatesthe initial temporary isosbestic point in the gain spectra.

Fig. 7 Time-resolved differential absorption and gain spectra(absorbance changes, ∆D) observed for PA5 in dioxane, in the 2–150 pstime-scale, after excitation at 573 nm with a subpicosecond laser pulse.The arrow indicates the initial temporary isosbestic point in the gainspectra.

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PA4 and around 600 nm for PA5, rise and slightly shift to theblue.

At long delays, i.e., after one to several hundreds of pico-seconds depending on the compound, similarly to cyclohexane,another TIP is seen for PA2 at 519 nm and for PA3 at 567 nm(Fig. 6), in the region of the ground-state absorption. For PA4and PA5 (Fig. 7) residual bleaching and transient absorptionbands remain at the longest delays allowed by our pump–probeset-up.

The fit of the ∆D(t) signals was done as described abovefor cyclohexane. For PA2, PA4 and PA5, ∆D(t) could be fittedwith a two-exponential function, but three exponentials wererequired for PA3. The time components at a few wavelengthsare reported in Table 1. The short component does not dependmuch on the chain length 26 but the long component ismaximum for PA3 and then decreases with the chain length.

Characterization of long-lived intermediates. Chain lengtheffect. As we emphasized in the previous section, once thefluorescent excited state has vanished, a residual bleachingband and a small transient absorption band are observed for allpolyenes except for PA3 in cyclohexane. This indicates that theexcited-state relaxation did not restore totally the ground-statepopulation and that part of the initial excited population leadsto a long-lived phototransient, the absorption of which isdetected. In dioxane, only PA4 and PA5 exhibit such a residualbleaching and transient absorption, and their excited-statedecay is short enough (Table 1) to be completely followed byour pump–probe set-up. We attempted to characterize the long-lived phototransients of PA2, PA3 and PA4 in the nanosecondto millisecond time range by synchronizing two low repetition-rate subpicosecond laser systems, one used as the excitationsource and the other to generate the white-light continuumprobe. In the milli- or submillisecond regime, the experimentswere carried out with non-recirculating samples in order toavoid artefacts in the observed kinetics due to the movement ofthe excited volume. Two-types of long-lived intermediates weredetected and their nature was characterized through their life-times: i) an isomer-type with a lifetime longer than millisecondsand ii) a triplet state with a lifetime increasing from hundredsof nanoseconds in air saturated samples to microseconds insamples under a nitrogen flow. The results obtained for PA2and PA4 in cyclohexane are illustrated in Fig. 8. In this non-polar solvent, the probed state is an isomer for PA2 and a tripletstate for PA4. No long-lived intermediate was observed for PA3in the sub-nanosecond experiments. In dioxane, isomerizationis still the long-time process probed for PA2, whereas inter-system crossing is observed for PA3 (Fig. 9) and PA4. Thedifferent photoproducts are summarized in Table 2.

Discussion

Solvent and chain-length effects on the steady-state spectra.Prediction of the excited-state behavior

Absorption spectra. The solvent-polarity and chain-lengtheffects on the absorption spectra have been previously studiedfor a number of these polyenes with different terminal groups.24

In the present study of the PAs the major polyenic chain-lengtheffect, both in cyclohexane and dioxane, is a red-shift and abroadening of the absorption band as the length increases, yet

Table 2 Polyenic chain-length effect on the nature of the photo-product formed after excitation of the PAn push-pull polyenes incyclohexane and in dioxane

Cyclohexane Dioxane

PA2 Isomer IsomerPA3 – Triplet statePA4 Triplet state Triplet state

with a saturation of the peak shift, as seen in Fig. 1. Such asaturation in the decrease of the first transition energy peradded double-bond is known for unsubstituted polyenes 7 andhas already been observed for asymmetric cyanines.33 From theresults of a recent study on a julolidinyl analogue of the PAswith a short chain (n = 1) by resonance Raman spectroscopy,34

one may propose that the ca. 1150 cm�1 vibrational pattern seenfor PA2 and PA3 in cyclohexane is mainly due to modes of thethiobarbituric acid ring. The disappearance of this vibrationalpattern for longer chains may be explained by the increase inthe molecule flexibility, i.e., the coupling of the ≈1150 cm�1

mode to low frequency torsional modes. This is the opposite towhat is known for unsubstituted trans-diphenylpolyenes forwhich the vibrational pattern due to chain CC stretching modes

Fig. 8 Time-resolved differential absorption (absorbance changes,∆D) observed for PA2 (a) and PA4 (b and c) in cyclohexane, in the ns–ms time-scale, by synchronizing two subpicosecond lasers (see text).Excitation wavelengths are respectively at 546 nm (a) and around 610nm (b and c) (the spikes due to scattered light are masked). For PA2 (a)the photoproduct is still present at 1 ms. For PA4, the photoproductlifetime is nanosecond in aerated solution (b) and microsecond in asolution under a nitrogen flow (c).

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becomes more apparent for increasing lengths.6,35,36 Theobserved spectral change from PA2 to PA5 from a non-symmetric to a quasi-symmetric absorption band with aca. 150 nm red-shift of the 0–0 transition seems to indicate thatthe minima of the excited and ground state potentials areincreasingly displaced as the chain lengthens. This increasingdisplacement could also explain the band broadening withincreasing n. For the longest chains, one might thus expect alarge photoinduced change in equilibrium configuration. In theRaman spectroscopy study cited above,34 the geometricalchanges in the excited state were found to be distributed overa number of vibrational modes but the authors stress that ifthe ground and excited states are not planar, they certainly havedifferent torsional minima. Thus, one might expect that thephotoinduced change in equilibrium configuration that we

Fig. 9 Time-resolved differential absorption (absorbance changes,∆D) observed for PA2 (a) and PA3 (b and c) in dioxane, in the ns–mstime-scale, by synchronizing two subpicosecond lasers (see text).Excitation wavelengths are respectively at 546 nm (a) and around 600nm (b and c) (the spikes due to scattered light are masked). For PA2 (a)the photoproduct is still present at 1 ms. For PA3, the photoproductlifetime is nanosecond in aerated solution (b) and microsecond in asolution under a nitrogen flow (c).

predict from the analysis of the steady-state spectra residesmainly in the change in torsional configuration.

If we now compare the absorption spectra in the twosolvents, the spectra of the PAs in dioxane are slightly red-shifted with respect to those in cyclohexane and the vibrationalstructure is absent for all polyenes, which indicates additionalsolvent interaction effects. Here we should stress that dioxaneis non polar on a macroscopic scale, as indicated by its lowdielectric constant,37 because of the zero dipole moment ofthe molecule; however, microscopically, the partial chargeson the molecule produce a locally polar environment, and onempirical scales its polarity is close to that of tetrahydrofuran.Thus, solvation phenomena are expected. The disappearanceof such vibrational structures when going from a nonpolarto a polar solvent was also reported, for example, for somecarotenoids with a CHO end group and was alternativelyattributed to an increase of the CT character of the excitedstate.17

Fluorescence spectra. Considering the fluorescence spectra inthese two solvents, it is seen in Fig. 3 that they are quite dif-ferent in spectral position, shape and intensity. In dioxane, theStokes shift and red shift induced by the chain length are largerthan in cyclohexane. In addition, the intensity is larger by twoto three orders of magnitude. This is a strong indication that thenature of the emitting state is different in these two solvents.

In our previous fast-spectroscopy study in a few polarsolvents, we reported a gain-peak shift of ca. 100 nm per addeddouble bond that we attributed to an emissive state with acyanine-like fully conjugated structure.26 The fluorescence peakshift per added double bond observed here in dioxane is20% smaller and is not strictly linear. A decrease of thevinylenic shifts with increasing length is known for the absorp-tion of polyenes and of unsymmetrical polymethine dyessuch as merocyanines in nonpolar solvents,38 which may offera better comparison basis for the cyanine-like structure ofpush-pull polyenes. We will further discuss this point in the nextsection.

The analysis of the fluorescence spectra of the differentPAs in a given solvent shows that the chain-length affects thephotoinduced phenomena. In cyclohexane, one observes amirror image for the fluorescence and absorption spectra ofPA2 but not for PA3 and PA4. The fluorescence spectra of thelatter two exhibit a clear ca. 1150 cm�1 vibrational structurewhile their respective absorption spectra exhibit, at most,shoulders. This difference may indicate that PA3 and PA4have a more rigid structure in their emitting state than in theirground state. Another striking observation is that thefluorescence intensity is maximum for PA3. The fluorescencequantum yield of cyanine dyes was also reported to passthrough a maximum value when going from short to longchains.12

A change in the chain length, the terminal groups or theenvironment undoubtedly affects the intrinsic nature of thetwo lowest excited singlet states and their energy separation,which in turn modifies the photophysical behavior. This is atpresent much discussed in the literature in the case of polyenes,carotenoids and polymethine cyanines.17,33,39–51

The ordering and energy separation of the two lowest excitedsinglet states of the unsubstituted polyenes, the opticallyallowed 1Bu and optically-forbidden 2Ag states, are known todepend on the chain length.4,9,52 The 1Bu state is the loweststate for short polyenes whereas the 2Ag becomes the lowestfor longer polyenes; the crossover is estimated to occur forhexatriene (n = 3), but this determination is still the object oftheoretical research.53 Among α,ω-diphenylpolyenes, diphenyl-hexatriene is the first chromophore in the series for which thelowest state is the 2Ag state.6,36 A universal curve for the 1Bu and2Ag energies of polyenes versus the length of the conjugatedchain was proposed including the data of substituted dialkyl

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and diphenyl polyenes.9 The energy, vibronic coupling andrelaxation of these states in polyenes and carotenoids arethe object of both current theoretical and experimentalstudies.36,43–45,51 It seems to us difficult to discuss the changein the fluorescence yield observed for the present push-pullpolyenes as a function of the chain length in terms of inversionof these optically-forbidden and optically-allowed states, sincethe C2h symmetry is no longer obeyed due to the presenceof electron donor-acceptor asymmetric end groups. It must benoted, however, that highly substituted carotenoids were foundto exhibit spectroscopic and photochemical characteristics ofthe short unsubstituted polyenes that do have the C2h sym-metry.11 The nonradiative relaxation pathways and possiblechanges in the nature of the relaxation path as a function of thechain length will be further discussed in the next section wherethe time-resolved spectra are analyzed.

Solvent and chain length effects on the photoinduced reactionpath and kinetics

The time-resolved transient absorption spectra recorded inthe present study give direct evidence that the excited-staterelaxation path and dynamics are determined by both thesolvent polarity and the chain length. The results demonstratethat the relaxation path is first determined by the solventpolarity for ultrashort times (2–3 ps), then by the polyenic chainlength for longer times with a change of path for n = 3.

Solvent-polarity controlled initial relaxation path. The trans-ient behavior found in nonpolar cyclohexane is quite differentfrom that in polar dioxane. In dioxane, the presence of a TIP inthe early time-resolved spectra indicates that a fast reactionoccurs in the excited state, with a precursor-successor relation-ship between the initially prepared excited-state population andthat of a product species. In addition, the presence of a TIP inthe gain band (Figs. 6 and 7) means that both the initiallyexcited state and the product state are emissive.54 We previouslyattributed this emissive product state to an excited state with acyanine-like fully conjugated structure.26 Our attribution wasbased on the regular shift of the gain peak while the chainlength increases. It can indeed be checked from Table 1 that indioxane the gain peak (λgain) shifts by ca. 110 nm per addeddouble bond in the conjugated chain. The apparent gain peakdoes not coincide exactly with the fluorescence peak due to itsoverlap with the excited-state absorption band so that theobserved shift per bond is overestimated. However, the regularred shift is confirmed, including PA5 for which the IR steady-state fluorescence could not be measured.

In our previous study we found that the formation rate ofthis cyanine-like state increases with the solvent polarity.26 Thepresent results confirm and stress that solvent polarity is adetermining factor since the reaction is not observed incyclohexane, a nonpolar solvent. One can thus emphasize ourprevious claim 26 that the cyanine-like intermediate state is morepolar than the initially excited state and propose that in cyclo-hexane its energy is raised so that it cannot be an intermediatein the relaxation process. Such a polarity-controlled photo-induced charge transfer (CT) reaction was observed for variousclasses of electron donor-acceptor compounds 55,56 includingethylenic or polyenic compounds such as the merocyanineDCM,54,57–60 push-pull stilbenes,61–64 styrils 16 and otherexamples of unsymmetrical cyanines,42 substituted diphenyl-polyenes 47,48 substituted carotenoids 11,17 and peridinin.41 Animportant and often raised question deals with the role ofintramolecular and solvent motions in such a CT reaction.In our previous study,26 we noted that the formation of thecyanine-like polar intermediate is slightly slower than solvation,indicating that the reaction is slightly activated and is likely toinvolve intramolecular coordinates. The cyanine character ofthe intermediate is indicative of a fully conjugated polyenic

chain with a planar and rigid structure whereas the initiallyexcited state is likely to have less conjugation (i.e., some singleand double bond alternation character) and thus, one endgroup free to rotate around a single bond (see Fig. 1). A drasticreduction in the π-bond order alternation in the (Franck–Condon) excited state is known for asymmetric cyanines.33

The photoinduced CT reaction toward such a fully conjugatedstate would thus involve not only geometrical changes along thepolyenic chain but also twisting of the terminal groups in orderto reach the expected flat configuration. Changes in torsionalminima between the ground and excited state was suggestedin the analysis of resonance Raman spectra of a similar push-pull polyene.34 The fast component of the anisotropy decay oftransient absorption of some polymethine dyes in polymericmatrix was interpreted as being due to rotation of molecularfragments in the excited state.65 Large amplitude relaxation inthe excited state of some unsymmetrical cyanines was demon-strated by comparison with selectively bridged analogues.16,42

Chain-length controlled relaxation path. We first consider thekinetics in polar dioxane. From Table 1, it can be seen that indioxane the formation kinetics of the cyanine-like intermediatedoes not depend much on the chain length for n = 2 to 4, with a2–3 ps time constant, but is slightly faster for n = 5. On thecontrary, its decay depends strongly on the chain length.Indeed, PA3 exhibits a nonexponential decay with two timeconstants of ca. 60 ps and 660 ps while PA2, PA4 and PA5decays could be fitted respectively with ca. 500 ps, 250 ps and 35ps single exponentials. We stressed above that the fluorescenceyield was maximum for PA3; this is likely due to the 660 ps longcomponent. Such an increase in the fluorescence lifetime fromPA2 to PA3 was also reported, for example, for diphenyl-polyenes when going from n = 2 to 3 36 or when going fromaryl-substituted ethenes (n = 1) to aryl-substituted butadienes(n = 2).49 For longer chains, one observes an acceleration of thecyanine-form decay rate by a factor of about 2 from PA3 to PA4and 7 from PA4 to PA5. Such a decrease in the excited-statelifetime is known for β-carotene and the spheroidene series witha linear decay of log 1/τ versus 1/(2n�1).10 A decrease of thefluorescence lifetime, from the nanosecond regime to the sub-100 ps, was observed for diphenylpolyenes having 3 to 7 doublebonds.36 Recent studies report a decrease in the S1 lifetime ofsome substituted carotenoids by a factor of 2 per added bondfor n = 8 to 10; this factor depends on the solvent-polarity andon the nature of the substituent.17 For carotenoids, a crossoverbetween the S1 (

1Ag) and S2 (1Bu) fluorescence for n ≥ 8 has been

much discussed in terms of S2–S1 energy gap as a function ofthe chain length.40 The comparison of our results to thoseobtained for unsubstituted polyenes and unsubstituted caro-tenoids is certainly not directly relevant since substitutionchanges the symmetry properties. On the other hand, thepresence and nature of terminal end groups were reported tohave a strong impact on carotenoids photophysics.11,17

One can also note in Table 1 that the time components arealmost systematically longer in the short wavelength range,i.e. with probes in the transient absorption and/or bleachingregion, rather than in the gain region. By studying excitationwavelength effects, we previously proposed that the long com-ponent such as the 17 ps component in the blue edge ofPA2 transient absorption band might be due to vibrationalcooling,26 but the origin of the difference between the long timeconstants remains unclear to us.

The kinetics obtained in the present study confirm ourdeduction from the steady-state fluorescence measurementsthat PA3 has a specific behavior. The characterization of thelong-lived photoproducts (Figs. 8 and 9, Table 2) demonstratesthat this is due to a change in the relaxation route of the polarcyanine-like intermediate state, which isomerizes at n = 2but undergoes intersystem crossing for n = 3 and above. Thisis definite evidence that the chain length affects the relative

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position of the different excited-state surfaces and thus theirintersection, leading to changes in the nature of the relaxationpath. It has been shown experimentally and theoretically thatthe photoisomerization of symmetrical cyanine dyes becomesan activated process as the chain length increases with anincreasing activation energy.15 The theoretical descriptionshows that for short symmetrical cyanines the initial relaxationoccurs along a barrierless path dominated by skeletal stretchingstrongly coupled with concurrent torsional deformation whilefor longer chains (n = 5–7), due to a different topology of the S1

energy surface, the torsional mode is populated after partialequilibration at a metastable untwisted intermediate. Allcyanines are found to relax down to S0 at a 90� twisted S1/S0

conical intersection, which is not located at the minimum of theS1 surface but near its minimum.15 The S1 minimum is found tohave CT character. The existence of a perp intermediate withsome CT character in the photoisomerization of polymethinecyanines had already been proposed by Momicchioli et al.66

The same group also showed that trans–cis isomerization inasymmetric cyanines 33 can be described by the same theoreticalmodel. In addition they found that in both cases, at the perpposition the singlet and triplet state are degenerate. WhileS1 T1 is not a competitive process for the examined dyes,33,66

intersystem crossing during the course of isomerization wasreported for thiacarbocyanine dyes with at least 2 doublebonds.18 Generation of T1 species was detected after photo-excitation of carotenoids in solution, with however a very lowquantum yield.10 High triplet yield is known for some nitro-substituted stilbenes with isomerization from the triplet state.67

For diphenylpolyenes, isomerization occurs mainly from thesinglet state 6 but intersystem crossing was found to occur for n= 3.68 For the present PAs, one may thus tentatively propose thatthe crossing to the triplet state becomes more favorable as thechain length increases as a result of the concomitant increase ofthe barrier to isomerization.

We now turn to the case of nonpolar cyclohexane, where thepolar cyanine-like intermediate is not formed and the initialexcited-state decay is also found to depend strongly on thechain length, with a change in route and in kinetics for n = 3. Inaddition, the lifetimes are shorter than that of the transient CTstate in dioxane, by a factor of about 7 for PA2 and PA3, and 3for PA4 and PA5. In cyclohexane, PA2 has a single exponentialbehavior with a ca. 65 ps lifetime (Table 1) while PA3–PA5exhibit a two-exponential decay with time-components ofroughly 6 and 100 ps for PA3, 3 and 80 ps for PA4 and 3 and 15ps for PA5. Although one is probing an excited state differentfrom that in dioxane, the longest decay component is stillobserved for PA3 and the decay rate is accelerated for larger n.Our characterization of the long-lived products demonstratesthat PA3 is in an intermediate regime between short and longlength. Like in dioxane, the relaxation route changes fromisomerization to intersystem crossing when n increases butno long-lived photoproduct could be found for PA3: this couldbe explained by a more favorable crossing of the excited statewith the trans ground state. Recent developments of quantumchemical methods demonstrated the influence of the chemicalstructure on the excited-state surface topologies and on thenature and site of the photochemical funnels involved in therelaxation path of cyanines and protonated Schiff bases.15

The present push-pull polyenes are likely to enter into verysimilar classes of photoinduced reactions.

ConclusionThe presently reported time-resolved absorption spectra pro-vide direct evidence that the excited-state relaxation path anddynamics of push-pull polyenes, with a diethylthiobarbituricacid and a dibutylaniline as the end groups, are determined byboth the solvent polarity and the chain length. The results con-firm that the solvent polarity is a determining factor in the

primary ultrafast process that we previously reported in a seriesof polar solvents.26 As a matter of fact, the cyanine-like inter-mediate (i.e., with a fully conjugated polyenic chain) whichis formed within a few picoseconds in polar solvents is notobserved in cyclohexane, a nonpolar solvent. The photo-induced charge-transfer state formed in polar solvents (here indioxane), is proposed to be raised above the initially excitedstate in cyclohexane and thus not reached in the course of thedeactivation process. The transient spectra recorded at longdelays (ns–ms) by synchronization of two subpicosecond lasers,demonstrate that subsequently to the initial solvent-controlledpath, the excited-state relaxation route depends strongly on thelength of the polyenic chain. In cyclohexane and dioxane,although the relaxing excited singlet state has a differentcharacter, the relaxation route changes from isomerization tointersystem crossing as the chain length increases. The polyenicchain with n = 3 double bonds is found to be at the frontierbetween these two regimes with however a solvent-dependentbehavior. In dioxane, the relaxation of the intermediate charge-transfer state changes from isomerization to intersystem cross-ing for n = 3 but in cyclohexane, the relaxation of the initiallyexcited state leads to an intermediate regime where direct relax-ation to the ground state seems to occur. A detailed knowledgeof the parameters influencing the probed reaction trajectories(i.e., static and dynamic properties of the solvent, chemicalnature of the end groups, chain length, chain flexibility) iscertainly of primary importance for the understanding of thephotophysics of polyenic compounds.

Acknowledgements

Part of this work was done while the ENSa group was still atthe Laboratoire de Photophysique Moléculaire of CNRSat Orsay University, France. Financial support from theGDR 1017, CNRS Department of Chemical Sciences, is alsoacknowledged.

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