Excited-state dynamics of thiophene substituted betaine pyridinium compounds.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 1460

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Excited-state dynamics of thiophene substituted betaine pyridinium compounds† a a a a Ste ´phane Aloı¨se,* Zuzanna Pawlowska, Olivier Poizat, Guy Buntinx, b b c c Aure ´lie Perrier, François Maurel, Kazuhiro Ohkawa, Atsushi Kimoto and Jiro Abec

This work deals with the photophysics of novel pyridinium betaine based on 2-pyridin-1-yl-1H-benzimidazole (SBPa) substituted symmetrically by mono- (Th2SBPa) and bi-thiophene fragments (Th4SBPa). The study is based on a combination of steady-state, femtosecond transient absorption spectroscopic measurements Received 29th August 2013, Accepted 6th November 2013 DOI: 10.1039/c3cp53614a

supported by PCM–(TD)DFT calculations. It is found that the two step ICT process (S0 - S2 excitation followed by S2(CT) - S1(CT) internal conversion) occurring for the parent molecule remains unaffected for Th2SBPa while the situation is less clear for Th4SBPa. Actually, for both molecules, a new decay route involving the p-electron system localized in thiophenic groups is responsible for the production of triplet

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states. Involvement of this new route in the parallel production of S1(CT) is strongly suspected.

I. Introduction Pyridinium betaine dyes are well-known for their remarkable photophysical properties related to the presence of low-lying intramolecular charge-transfer (CT) excited states.1–5 In particular, these molecules exhibit large negative solvatochromism due to a dramatic reduction of the dipole moment on going from the ground state to the excited state, which has been used for polarity measurements of solvents.1,6 Also, a notable dipole moment change induces large first-order hyperpolarizability and thus large non-linear optical (NLO) response.7–10 In this regard, the strongly solvatochromic 2-pyridin-1-yl-1Hbenzimidazole molecule (SBPa, see Chart 1),11 predicted from semi-empirical and ab initio calculations to experience a dipole moment inversion in the excited-state, has been expected from hyper-Rayleigh scattering measurements to present an outstanding first-order hyperpolarizability, which makes it a very attractive compound for NLO applications.7,8,12,13 These conclusions were nicely supported by the excited-state dipole moment and

a

Laboratoire de Spectrochimie Infrarouge et Raman CNRS UMR 8516, Universite´ de Lille1 Sciences et Technologies, Universite´ Lille Nord de France, Bat C5, 59655 Villeneuve d’Ascq Cedex, France. E-mail: [email protected] b Universite´ Paris Diderot, Sorbonne Paris Cite´, ITODYS, CNRS UMR 7086, 15 rue Jean Antoine de Baif, 75205 Paris Cedex 13, France c Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan † Electronic supplementary information (ESI) available: (I) Synthesis protocols of Th2SBPa and Th4SBPa; (II) overall TDDFT results with absorption band assignments for Th2SBPa (Fig. S5) and Th2SBPa (Fig. S6); (III) additional time-resolved spectroscopic results for Th4SBPa (390 nm excitation case in Fig. S7) and discussion of solvent effects for Th2SBPa. See DOI: 10.1039/c3cp53614a

1460 | Phys. Chem. Chem. Phys., 2014, 16, 1460--1468

Chart 1 Structure of the SBPa, Th2SBPa, and Th4SBPa molecules.

hyperpolarizability values determined from an extensive quantitative analysis of the solvatochromism of SBPa in solution and TDDFT calculations.14 On the other hand, in order to get a real understanding of the photoinduced ICT process in SBPa at the molecular level, femtosecond photoionization (gas phase) and transient absorption (solution) spectroscopic experiments have been recently performed15 and complemented by PCM–TDDFT calculations.15,16 From this ensemble of data, a complex photophysical relaxation scheme has been derived, which involves two distinct ICT excited states of planar geometries, S1(CT) and S2(CT). Excitation within the lowest energy absorption band populates the S2(CT) state, which leads to partial charge transfer from the betaine phenylene to the pyridinium ring. The S2(CT) state undergoes ultrafast relaxation to an emissive S1(E) state in competition with the S1(CT) state, with a time constant ranging from 300 fs to 20 ps depending on the solvent. The S2(CT) - S1(CT) transition involves a charge transfer from the imidazole bridge to the pyridinium ring and thus leads to a further increase of the ICT character. This process was assumed to be mainly controlled by

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the solvent dynamics in polar aprotic solvents. Finally the S1(CT) state decays essentially through efficient internal conversion (IC) to the ground state in less than 40 ps whereas the emissive S1(E) state decays via IC, ISC and fluorescence in the 130–260 ps time range depending on the solvent. The present paper reports an investigation by femtosecond transient absorption of the excited-state dynamics and photophysical processes of two SBPa molecules substituted symmetrically on the betaine phenylene ring by two thiophene (Th2SBPa) or bi-thiophene (Th4SBPa) units (see Chart 1). The originality of these novel compounds is the presence of thiophenic oligomer groups, which constitute a first and very important step toward the elaboration of conducting thiophenic polymers including a photoactivable switch. Indeed, their investigation is motivated by the eventual possibility to insert SBPa molecules within a polythiophenic chain as photoactive units allowing to control by light the conductivity through the polymer chain.17 More precisely, one desires to examine how the electronic conjugation between the two (bi-)thiophene units through the meta-linked phenylene is affected upon photoexcitation of the SBPa ICT transition. The aim of the time-resolved spectroscopic study described here is to understand to what extent the substituted thiophene groups modify the photophysics of the parent SPBa molecule and, in particular, check whether the photoinduced ICT properties are still present.

II. Experimental and theoretical methods The synthesis of SBPa has been performed according to a previously published procedure.18 Th2SBPa and Th4SBPa synthesis are given with details in ESI.† Solvents toluene, acetonitrile (MeCN), butyronitrile (BuCN), octanenitrile (OcCN), tetrahydrofuran (THF), ethylacetate (EtOAc), and methanol (MeOH) are from Aldrich and were used without any further purification. Steady-state absorption spectra were recorded using a double beam spectrometer (CARY 100bio). Measurements were performed at ambient temperature using 10 10 mm quartz cells with 10 5 M solutions. The spectral resolution was typically 2 nm. The femtosecond transient absorption set up has already been described.19a Briefly, a 1 kHz Ti:sapphire laser system (Coherent oscillator and a BM Industries regenerative amplifier) delivered 100 fs (0.8 mJ) pulses at 800 nm. Pump pulses were set at 390 nm or 266 nm by frequency doubling or tripling the fundamental beam, respectively, while probe pulses (white light continuum) were generated by focusing the fundamental beam on a CaF2 rotating plate. The transient absorption measurements covered a 400–750 nm spectral range and a 0–3 ns temporal range. Sample solutions (about 10 3 M, absorbance of 0.2 at the pump wavelength) were circulating in a flow cell equipped with 1 mm thick CaF2 windows and characterized by a 2 mm optical path length. The pump–probe cross-correlation time has been evaluated to be about 200 fs and 300 fs at 390 nm and 266 nm, respectively.19 The characteristic times were deduced by global fitting the uncorrected kinetics with multiexponential functions convolved


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with a Gaussian pulse (which approximates the pump–probe correlation function) taking into account corrections of the GVD and optical dispersion phenomena originating from the sample thickness.20 Within this approach, global fitting was systematically performed taking into account simultaneously four relevant wavelengths. Note that for graphs presentation, transient data were GVD corrected. The nanosecond laser-flash photolysis apparatus based on the Nd:YAG laser system (335 nm laser excitation is obtained by frequency tripling in BBO crystal) is fully described elsewhere.19b PCM–TDDFT calculations of the vertical absorption transition energy, oscillator strength, and contour plots of the involved molecular orbitals (MO) of Th2SBPa and Th4SBPa in MeCN, toluene, and THF solvents were done at the PCM–TD-PBE0/6-311++G(2d,p)// PBE0/6-311G(d) level using the Gaussian 09 package21 as previously calculated for SBPa.14,16 Solvent contributions were introduced by using the Polarizable Continuum Model (PCM).22

III. Results and discussion III.1.

Ground-state spectroscopic properties

The steady-state absorption spectra of SBPa, Th2SBPa and Th4SBPa between 280 and 690 nm in MeCN, THF, and toluene are presented in Fig. 1(a), (b) and (c) respectively, including comparison of absorption and emission spectra in (MeCN case only). PCM–TDDFT transitions (oscillator strengths f ) in the three solvents are displayed at the top of each left-side spectrum. For the sake of simplicity, MO contour plots energy diagram from HOMO – 1 to LUMO + 1 are compared in Fig. 2 for only one solvent (MeCN). Note that all TDDFT results are the listed in Table S1 (ESI†) and a more complete energy diagram is displayed in Fig. S5 and S6 (ESI†) for Th2SBPa and Th4SBPa respectively. Th2SBPa: coexistence of two chromophores. As seen in Fig. 1(b), the absorption spectra show three absorption components: a strong and sharp band peaking around 350 nm and almost independent of the solvent, a smaller band in the 400–450 nm range with a peak position that undergoes a red-shift upon decreasing the solvent polarity, and a weak and broad absorption tail extending up to 600–650 nm characterized by a similar negative solvatochromism. As evidenced from Fig. 1(a), the 400–450 nm band can be safely assigned to the CT absorption band reported for the SBPa parent molecule14 that was concerning the shape, similar band position, and negative solvatochromic effect (B2700 cm 1 blue-shift for Th2SBPa and B3000 cm 1 for SBPa on going from toluene to MeCN). According to TDDFT calculations able to reproduce the experimental solvatochromism (see oscillator strengths in Fig. 1(b)), this characteristic CT band correlates with the calculated S0 - S2 transition, H-1 - L excitation, characterizing an instantaneous ICT from the betaine group to the pyridinium ring.15,16 This well localized excitation is clearly seen in Fig. 2 which also provides evidence that MO energetic order is not altered by the monothiophenic substitution of the parent molecule leading to an unaffected CT band position (and similar solvatochromism). The red absorption tail can be related to the calculated weakly-allowed S0 - S1 ICT

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Fig. 1 (Left) Stationary absorption spectra in MeCN, THF and toluene for SBPa (a), Th2SBPa (b) and Th4SBPa (c) together with the corresponding PCM-TD-PBE0/6-311++G(2d,p)//PBE0/6-311G(d) calculations (stick representation of oscillator strength). (right) Comparisons of stationary absorption (full lines) and emission (dashed lines) spectra for MeCN. Arrows indicate the laser excitations used for time-resolved experiments.

transition, H - L shown in Fig. 2, that mimics correctly the observed solvatochromism as seen in Fig. 1(b). This transition was too weak to be observed in the spectrum of the parent molecule SBPa (Fig. (1a)) in which S1(CT) has a null oscillator strength due to symmetry restrictions. In Th2SBPa, those symmetry restrictions are cancelled out by the presence of thiophene. For the latter, the HOMO being entirely delocalized on the thiophene– betaine–thiophene moieties (Fig. 2), the thiophene rings are strongly involved in this weak ICT transition. In the following, this kind of donor–acceptor ICT transition toward the pyridinium core will be systematically noted as Sn(CT) with n = 1 or 2 by analogy with our previous study on SBPa14,16 while the notation Sn(pp*) will be used to design the singlet states localized in the thiophenic chromophore. In contrast to S1(CT) and S2(CT), the 350 nm absorption band has no counterpart in the spectrum of SBPa, but presents an analogy with the strong absorption of terthiophene at lmax = 354 nm,23


suggesting its assignment to a (p,p*) transition localized in the thiophene–phenyl–thiophene moiety. This assumption is confirmed by the theoretical calculation which predicts a strongly allowed, solvent-insensitive S0 - S3 (S0 - S4 in toluene) transition corresponding mainly to a H - L + 1 excitation rather than the localized one in the thiophene–phenyl–thiophene fragment as seen in Fig. 2. This statement is confirmed by emission experiments. Whereas no fluorescence could be detected upon excitation of Th2SBPa at lexc > 390 nm, a strong emission spectrum peaking at 410 nm is induced at 266 nm excitation (MeCN solvent), which seems to be the mirror image of the 350 nm absorption band assigned to the S0 - S3(pp*) excitation. This emission is radically different from that observed from the emissive S1(E) state in SBPa reported in Fig. 1(a) but resembles notably the S1(pp*) terthiophene emission (lmax = 407 nm23), which is consistent with the notable localization of the S0 - S3 transition in the thiophene–phenylene– thiophene moieties of the molecule. These results agree with


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Fig. 2

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PCM–DFT energy diagram and representation of the orbitals involved in the four lowest energy transitions.

the design of the S3(pp*) state as the emissive state of Th2SBPa in MeCN. They also indicate that the S3(pp*) state is long-lived and thus does not relax efficiently to the S2(CT) and S1(CT) states. As a partial conclusion, it is worth recalling that thiophenic substitution has induced the coexistence of two chromophores with or without ICT properties. Th4SBPa: lake of efficient ICT transitions? In Fig. 1(c), the absorption spectra show essentially one strong absorption band maximizing at ca. 410 nm in all solvents. A weak absorption tail extending up to 600–700 nm is also present, which exhibits some red shift upon decreasing the solvent polarity. Based on PCM–TDDFT results, the assignment of the latter is straightforward while it corresponds to the same S1(CT) state already present for the other molecules. It is worth noting that in Th4SBPa as in Th2SBPa the electron donor group includes the thiophenic substituents (see the HOMO representation in Fig. 2). In addition, concerning the 410 nm absorption band, those theoretical results predict only one transition of large oscillator strength (f B 1.5) in the blue region – S0 - S2 in MeCN and THF solvents or S0 - S3 in toluene – mainly arising from a H - L + 1 (pp*) excitation (see Fig. 2). Furthermore, this absorption band is lying almost at the same position as the lowest energy absorption of the pentathiophene molecule (416 nm23), suggesting a localization of the excitation in the bithiophene–phenylene–bithiophene chain. This localization excludes any ICT character, which is consistent with the absence of apparent solvatochromism. It is confirmed by the PCM–DFT contour plots of the MOs involved in the transition, which are indeed mainly restricted to the bithiophene–phenylene– bithiophene chain (Fig. 2). This statement is doubly confirmed


by the notable fluorescence signal (Fig. 1(c)) characterized by a vibronic structure with two maxima at 470 and 490 nm and a shoulder at ca. 530 nm. This band presents a clear resemblance to the emission of the pentathiophene molecule concerning the position, vibronic structure (lmax = 482 and 511 nm, shoulder at 550 nm23), and overall band shape. Similarly to the case of Th2SBPa, this analogy suggests that the emission of Th4SBPa arises from the S3(pp*) state localized in the bithiophene– phenylene–bithiophene chain. At this point, one wonders if the CT solvatochromic band in Fig. 1(c), present in previous cases, is really missing or if it is just hidden by the intense S0 - S3(pp*) absorption. Indeed, in the analysis of the Th2SBPa data, we observed that the main S0 - S2(CT) band was insensitive to the substitution by thiophenic groups. The presence of this CT transition can thus also be expected on the one hand around 400 nm in Th4SBPa. On the other hand, the strong pp* absorption band is located exactly in this region. Comparing now the energy diagrams in Fig. 2 reveals that the H-1 orbital related to the solvatochromic band in the case of SBPa and Th2SBPa is correlated with the H-2 orbital in the case of Th4SBPa. As a consequence it suggests that the characteristic CT transition corresponds to a S0 - S4(CT) rather than S0 - S2(CT) transition (see Table S1 (ESI†) as well) and is thus somewhat shifted to higher energy. As we will see later, femtosecond experiments will be decisive to determine if the S4(CT) state excitation is feasible despite the presence of strong pp* resonance. In summary, upon substituting the SBPa phenylene by thiophene or bithiophene groups, the strongly allowed solvatochromic S0 - S2(CT) transition typical of the SBPa skeleton keeps approximately the same nature and energy but decreases


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markedly in intensity as the p-electron delocalization between the betaine phenylene and the thiophenic entities becomes important (Th4SBPa). Simultaneously, a new pp* transition characteristic of this p-conjugated thiophenic-phenylene fragment appears with an intensity increasing with the conjugation length. On the other hand, the S0 - S1 ICT transition, not detected for SBPa, gains some intensity in the thiophenic compounds but remains very weak. Finally, the emission arising from the S1(E) state in the parent molecule SBPa is not observed for the substituted compounds Th2SBPa and Th4SBPa (Fig. 1(c)). Instead, strong emission from the pp* state characteristic of this p-conjugated thiophenic-phenylene fragment is detected although this excited state is not the lowest energy one. This ensemble of results suggests that the Th2SBPa and Th4SBPa compounds could have quite different photophysical properties and excited-state dynamics compared to those of the unsubstituted molecule SBPa. To examine this aspect, we present now the results of a time-resolved absorption investigation of Th2SBPa and Th4SBPa and a comparative analysis of these results with those previously reported for SBPa.15 III.2.

Excited-state transient absorption properties

We present first the results of measurements performed for Th2SBPa upon excitation of the S0 - S2 ICT transition at 390 nm, i.e., under the same excitation conditions as the previous study made for SBPa.15 The influence of the solvent on the excited-state relaxation processes will be only partially examined (see ESI† for a more detailed discussion). Then additional data upon 266 nm excitation are described with the aim of characterizing the specific photophysical processes arising in the S3(pp*) state localized in the thiophene–phenylene–thiophene fragment. In a third part, results obtained for the Th4SBPa molecule for which 390 and 266 nm excitations gave similar transient data sets will be presented. The goal of this part is dual: (i) depicting a realistic photochemical pathway for thiophenes substituted betainepyridinium taking into account the coexistence of several excited states (S1(CT), S2(CT) and Sn(pp*). . .) with distinct natures; (ii) understanding the lack of apparent CT transitions for Th4SBPa. Th2SBPa excited at 390 nm: evidence for ICT transitions. Subpicosecond transient absorption spectra of Th2SBPa have been recorded in MeCN, BuCN, OcCN, THF, EtOAc, and MeOH in the 0–100 ps time domain following pump excitation at 390 nm, in the 400–750 nm range. Typical spectra obtained in THF at different pump–probe time delays are shown in Fig. 3. In the early time domain (0–0.5 ps, in Fig. 3a), one can see the concomitant growth of a negative band at 412 nm and a transient absorption (TA) band at 490 nm. The 412 nm negative band is ascribed with confidence to the ground state bleach (GSB) as it matches closely the steady-state absorption spectrum (see dashed lines). The 490 nm TA band is assigned to the S2(CT) excited state initially populated by the pump excitation. As expected, the 0.5 ps transient spectrum is comparable to that observed under the same conditions for the S2 ICT state of SBPa.15 However a crucial difference is the absence of any stimulated emission (SE) signal, whereas a strong negative SE band was


Fig. 3 Femtosecond transient absorption spectra following 390 nm excitation of Th2SBPa in THF in three temporal windows: (a) 1–0.6 ps; (b) 0.65–1.5 ps and (c) 1.5–100 ps. All characteristic times indicated are obtained with the global fitting method using a pump–probe correlation time: t = 190 fs. The sharp negative peak at 447 nm seen at the shortest pump–probe delay times is due to stimulated Raman scattering of the solvent. The dashed line corresponds to the ground-state absorption spectrum.

observed in the red region for SBPa. This result is consistent with the fact that Th2SBPa is nonemissive upon excitation of the ICT transition. The t1 = 110 fs observed rise of the transient GSB and TA bands corresponds (instrumental response being deconvoluted) possibly to an ultrafast excited-state evolution from the initial Franck–Condon geometry to a relaxed geometry of the S2(CT) state. In the following 0.6–1.5 ps time domain (in Fig. 3b), the TA band shape evolves with some intensity decrease at 490 nm and the rise of a new band at 545 nm. Isosbestic points are present at 518 and 597 nm. This spectral evolution is comparable to that observed for SBPa and can be assigned similarly to the S2(CT) - S1(CT) transition, the ICT process depending on the solvent.15 Indeed, this assignment is supported by similar characteristic times for Th2SBPa (t2 = 0.6 ps) and SBPa (0.4 ps) in a given solvent (THF here). As evidenced in Table 1, focusing on the alkylnitrile family for example, this fundamental ICT process depends as expected on solvent


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polarity (because solvent effects are dominated by specific interactions for betaine pyridinium,14 we reported this discussion in ESI†). Note that during this relaxation, the GSB signal keeps constant intensity, indicating that there is no significant deactivation from the S2 state to the ground state. Finally, in the 1.5–100 ps time window (in Fig. 3c), both the GSB signal and S1 state TA band decay totally with a common single-exponential kinetics (t3 = 7.6 ps), due to the efficient decay of S1(CT) by internal conversion to the ground state (intramolecular back charge transfer). Note that, in the case of SBPa, biexponential excited-state decay was observed with time constants t3 = 2.6 ps and t4 = 134 ps. The fast component was assigned to the decay of the S1(CT) state and the longer one to the decay of the emissive S1(E) state. The absence of any slow decay component in the case of Th2SBPa is consistent with the lack of SE signal and steady-state emission. It means that the photophysics of Th2SBPa, unlike that of SBPa, does not involve the population of an emissive S1(E) state. Apart from this difference, the above results indicate that the same excited-state relaxation pathway S2(CT) - S1(CT) occurs in the two molecules with comparable rate constants when their similar characteristic solvatochromic band is excited. Th2SBPa excited at 266 nm: additional photochemistry localized on thiophene chromophores. Excitation at 266 nm falls at the low energy edge of a strong absorption lying above 250 nm, at slightly higher energy than the S0 - S3(pp*) absorption (see arrows in Fig. 1). A series of transient absorption spectra recorded for Th2SBPa in MeCN in the 0–500 ps time domain are shown in Fig. 4 together with stationary absorption and emission spectra. The overall spectral evolution is notably more complex than upon excitation at 390 nm and the global fitting analysis leads to at least four different time constants (see Table 1). In the 0–1.5 ps time domain (Fig. 4a), a transient spectrum showing negative components below 425 nm and a positive absorption peak at 540 nm appears with a t1 = 160 fs kinetics. The negative signal has a spectral shape clearly opposite to that of the steady-state absorption spectrum (B350 nm) and emission spectrum (B410 nm), which allows us to assign the negative band to a combination of GSB and SE components. As seen in Fig. 5, the 1.5 ps spectrum upon 266 nm excitation looks like the 390 nm excitation counterpart but notably broader. By analogy with the results above, we Table 1 Main time constants ti (ps) found for the excited-state dynamics of Th2SBPa and Th4SBPa in different solvents upon 390 or 266 nm excitation. Error on t: 30% (t o 1 ps), 10% (t > 1 ps)

Solvent

t1 growth

t2 ICT

t3 decay

t4 decay

t5 decay

266

MeCN BuCN OcCN THF EtOAc MeOH MeCN

0.080 0.100 0.110 0.110 0.110 0.090 0.160

0.3 0.8 1.1 0.6 1.0 0.4 —

3.1 5.9 13.8 7.6 7.3 4.9 1.9

— — — — — — 28

450

266 390

MeCN MeCN

0.250 0.110

—

2.1 2.5

16 18

660 nm

lexc (nm) Th2SBPa

Th4SBPa

390


assign this 1.5 ps TA spectrum not only to the S1(CT) state but also to additional transient species. In the 1.7–10 ps time domain (Fig. 4b), the positive TA band and negative GSB signal decay with a time constant t3 = 1.9 ps, which is of the same order of magnitude as the t3 = 3.1 ps decay kinetics measured in the 390 nm excitation experiment (note that t2 notation is exclusively reserved for the ICT process). We assign this kinetics by analogy to the internal conversion from the S1(CT) to S0 state. However, whereas upon 390 nm excitation all transient signals disappear completely during this process, at 266 nm excitation a residual transient spectrum is still present at 10 ps, which decays much more slowly. It can be concluded that the initial 1.5 ps spectrum is in fact the superposition of two spectral contributions: the S1(CT) state spectrum, also present upon excitation at 390 nm, and the spectrum subsisting at 10 ps, corresponding to a new transient state(s) not populated upon excitation at 390 nm. Both states are probably populated in parallel from the initial Franck–Condon excited-state level with a kinetics that might correspond to time t1 or even faster. The 10 ps spectrum presents a TA band with two maxima at 525 and 560 nm, persistent negative components in the GSB/SE region, and a sharp peak of absorption at 380 nm, inserted among the latter. It evolves with two consecutive steps of time constants t4 = 28 ps (in Fig. 4c) and t5 = 450 ps (in Fig. 4d), respectively. The former affects mainly the spectral range below 400 nm probably due to vibrational cooling of the hot ground state molecules arising from the S1(CT) state depopulation.24 The 450 ps evolution shows the decay of the visible TA band and the rise of a shoulder peaking at B480 nm. Complementary flash photolysis measurements (266 nm excitation) reveal the existence of a long-lived transient species characterized by a TA band peaking at 480 nm and a GSB signal at 340 nm. The clear analogy of the band maximum of the spectrum displayed in Fig. 4d (230 ns trace) and of the final transient spectrum rising with 450 ps kinetics suggests that they both characterize the same transient species. Being strongly quenched in the presence of oxygen, the 230 ns spectrum can be assigned with confidence to the T1 state of Th2SBPa. Furthermore, the 480 nm TA band resembles the main T1 - Tn absorption band of triaryl compounds, reported at ca. 470 nm for terthiophene25,26 and terphenyl.27 We thus tentatively propose that the 450 ps kinetics corresponds to intersystem crossing (ISC) relaxation S3(pp*) T1(pp*), the T1 state being localized in the thiophene–phenylene– thiophene fragment. On this basis, the strong ISC quantum yield reported for terthiophene (FISC = 0.90 in MeCN23) is consistent with the efficient triplet population and small ground state recovery observed in Th2SBPa. Th4SBPa cases: no excitation wavelength effect? As discussed above, Th4SBPa seems to not show solvatochromic properties, apart from the weak S0 - S1 transition. It was found that the S2 state, which for SBPa and Th2SBPa molecules was identified as a CT state, became for Th4SBPa a pp* state with strong intensity. In fact, as discussed earlier, the missing CT state is correlated with the S4 electronic state rather than the S2 state, the former having energy just above the latter. Because the solvatochromic band may be overlapped by the strong pp* absorption band,


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Fig. 4 Femtosecond transient absorption spectra of Th2SBPa (left) and Th4SBPa in MeCN after excitation with a 266 nm pulse, divided into four temporal windows (parts a–d). All characteristic times are obtained with the global fitting method using a pump–probe correlation time: t = 290 fs. Stationary absorption (full lines) and emission (dashed lines) spectra are indicated as well (part e).

Fig. 5 Comparison of transient absorption signals at 1.5 ps for Th2SBPa in MeCN at two different wavelengths of excitation: 390 nm (in pink) and 266 nm (in blue).

femtosecond transient absorption experiments become decisive. With anticipation, it is fundamental to note here that in the case of Th4SBPa both 390 nm and 266 nm excitations are likely to lie above S2(pp*) and S4(CT) states. Indeed, it is worthy to note that we obtain almost the same transient data for 390 nm and 266 nm excitations and for this reason we presented the former in ESI†


(Fig. S7) while the latter is presented in Fig. 4. In this figure, the time transient absorption spectra are divided into similar temporal windows (parts a to d) to facilitate the comparison with Th2SBPa. Stationary absorption and fluorescence are also shown (part e in Fig. 4). Clearly, all transient events are similar to the previous case, with four characteristic times gathered in Table 1, and we will briefly review each process. The spectrum appearing at early times (1.6 ps spectrum in Fig. 4) is dominated by a broad TA band in the red region of negative components corresponding to GSB and SE components. The rising time of 250 fs (deconvoluted) is assigned as FC states relaxation toward S2(pp*) and CT states as demonstrated below. A partial decay of the GSB and TA bands is observed in the 1.6–4.0 ps time range (t3 = 2.0 ps) with an isosbestic point at 495 nm (see Fig. 4b). By analogy with the Th2SBPa molecule which presented a similar evolution (time constant 1.9 ps and isosbestic point at 420 nm), we propose to ascribe this first event to the S1(CT) - S0 internal conversion responsible for the ground state recovery. This fundamental result brings the evidence that the S1(CT) state is well populated with a rate higher than 250 fs 1. Note that with the present data we are not able to determine exactly whether the S1(CT) precursor is the S4(CT) (analogue of S2(CT) for Th2SBPa), or the S2(pp*), or both; this point will be commented later.


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Then, in the 4–100 ps window (see Fig. 4c), a new spectral evolution (t4 = 16 ps) shows the further decay of the GSB signal at 400 nm while the red TA band displays a slight intensity decrease. At the same time, the SE band vibronic structure (100 ps trace), due to the emissive pp* excited singlet state, becomes more pronounced. The apparent increase of the SE band is probably just the result from the decay of GSB. As this relaxation is comparable to the 28 ps dynamics observed for Th2SBPa it can be ascribed to a hot ground state vibrational relaxation consequence of previous S1(CT) decay. Note that the 100 ps spectrum has been assigned to the relaxed emissive S2(pp*)state, the latter being localized on the p-conjugated thiophenic-phenylene fragment of Th4SBPa. In agreement with the localized character of the excitation, one can see that the 450–660 nm region of the 100 ps spectrum, composed of the negative SE band and positive broad TA band peaking above 650 nm, resembles notably the S1 state spectrum of pentathiophene.28 After 100 ps, all positive and negative signals decay simultaneously with a time constant t4 of B660 ps (see Fig. 4d), which characterizes the depopulation of the emissive excited state. From flash photolysis measurements, a long-lived transient spectrum is measured (dotted-line spectrum in Fig. 4d) and assigned confidently (oxygen sensitivity) to the triplet T1 state (tT = 5.9 ms). The T1 state spectrum shows a main TA band peaking at 630 nm, exactly at the position of the T1 state spectrum of pentathiophene.23 Therefore, both the emissive singlet state and T1 state are likely pp* states confined in the thiophenic-phenylene 5-ring chain. In summary, if the two step ICT process, S0 - S2(CT) S1(CT) found for the parent SBPa molecule is one more time evidenced by transient spectroscopy for Th2SBPa toward excitation within the solvatochromic band, this is not the case for Th4SBPa (apparent lack of the solvatochromic band). As a new result, it has been found that similar double photophysical deactivation routes occur in both the Th2SBPa and Th4SBPa compounds. Indeed, excitation of Th4SBPa (390 nm and 266 nm) or excitation of Th2SBPa at 266 nm leads primarily to the population of the (pp*) state localized in the thiophenicphenylene fragment, which disappears via fluorescence and ISC to a T1(pp*) state also localized in this fragment. The second fast excited-state dynamics is tentatively assigned to an excited-state ICT process occurring from either CT or pp* upper state and leading to S1(CT). In this last hypothesis, the efficiency of the fast excited state quenching route via the S1(CT) state can be roughly estimated from the ratio of the GSB band intensity, measured at the early time (B1.5 ps spectrum) and at the end of the repopulation of the relaxed ground state (B100 ps spectrum). It is found that the fraction of the initial population of excited state molecules that yields back the S0 state via the ICT route decreases upon increasing the substituted oligothiophene chain, from about 100% in the parent SBPa molecule to B80% in Th2SBPa and B50% in Th4SBPa. This reduction in efficiency of the ICT process can be accounted for by the increasing stabilization of the pp* state localized in the thiophenic-phenylene chain as the length of this chain increases.


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IV. Conclusion The main goal of this article was to rationalize the influence of adding thiophenic groups of different lengths to the parent SBPa skeleton on the photophysical properties. First, we found that the overall photophysics of betaine pyridinium molecules bearing two thiophenic groups substituted in symmetric positions can be interpreted by considering that these entities involve two orthogonal systems: (i) a first ‘‘CT system’’ corresponding to the parent molecule SBPa clearly evidenced for Th2SBPa but not for Th4SBPa. (ii) A second oligothiophenic-like system including the two thiophenic groups and the central benzene ring. This partition is justified by several concordant results. The thiophenic system has been evidenced by the various similarities between Th2SBPa and 3T, on one hand, as well as between Th4SBPa and 5T, on the other hand, regarding the pp* absorption and emission band position and the triplet–triplet spectral shape. Concerning the ‘‘CT system’’, it has been found that, in SBPa and Th2SBPa, the solvatochromic transition shows almost the same absorption maximum as well as a similar solvatochromic behaviour. The femtosecond measurements have demonstrated unambiguously that the S2(CT) - S1(CT) process extensively studied in the previous articles in the case of SBPa still occurs for Th2SBPa. Concerning Th4SBPa the situation is more complex because the two systems arise at the same energy level impeding the possibility to detect either the solvatochromic band (stationary measurements) or the characteristic S2(CT)–S1(CT) transient signal with femtosecond measurements. At the present time, we obtained quite serious evidence for the production of S1(CT) but the hypothesis of an alternative mechanism like S(p,p*) - S1(CT) is likely to occur. In the future, because these compounds have been tailored for photoconductivity purposes, it appears highly desirable to explore the possibility to excite directly the S1(CT) state because the p-system is engaged in S0 - S1(FC) excitation despite the fact that the extinction coefficient of this transition is very low. Synthesis of new thiophene substituted SBPa molecules to resolve this issue (with better S1 state absorbance) is now in progress in our laboratories.

Acknowledgements The authors are glad to acknowledge the financial support provided by the CNRS through the GDRI 91 ‘‘PHENICS’’. The Centre de Ressources Informatiques de Lille1 is thankfully acknowledged for the CPU time allocation.

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