TICT and triplet states of triarylpyrylium cations

4 July 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 272 (1997) 496-500

TICT and triplet states of triarylpyrylium cations Isabelle Lampre

a,

Sylvie Marguet a, Dimitra Markovitsi Jean Michel Nunzi b

a,

Strphane Delysse

b,

a DRECAM, SCM, CNRS-URA 331, CEA/Saclay, 91191 Gif-sur-Yvette, France b LETI, DEIN, CEA/Saclay, 91191 Gif-sur-Yvette, France

Received 14 March 1997; in final form 25 April 1997

Abstract Triarylpyrylium cations bearing an electron donating group are studied in solution by two pump-probe techniques with picosecond and nanosecond resolution. The transient spectra recorded on the picosecond time-scale as a function of solvent viscosity are in agreement with the formation of a TICT (twisted intramolecular charge transfer) state, predicted by quantum chemistry calculations. The ground state relaxation from the twisted conformation is observed. It is shown that the triplet state is formed only in solid matrices where the non-radiative decay of the excited singlet to the ground state is slowed down. © 1997 Elsevier Science B.V.

1. Introduction The 2,6,4-triarylpyrylium tetrafluoroborate {P1+_12, B F4} shown below, forms ion-pair dimers in solvents o f low dielectric constant [1-4].

R~N/R tY :

~® BF4" y'

Pl.l+: R=R'=CH3 Pl.12+:R~CH3,R'=CI2H25

In order to obtain a better understanding of the dimer behaviour, the properties of the cationic chromophores have been experimentally studied in solvents of medium and high dielectric constant where dimerisation does not occur [5]. Moreover, quantum chemistry calculations have shown that (i) the S O S 1 transition, polarized along the yy' axis, induces an important variation of the atomic charge distribution and (ii) conformational relaxation in the S I state involving rotation of the dialkylaminophenyl group (angle 8 ) occurs without an energy barrier [5]. The fluorescence and second order non-linear optical properties of the compound corroborate the formation of a non-fluorescent TICT state [5]. However, a study by transient absorption spectroscopy, used to investigate TICT formation [6-10], has not been performed so far. Within this context, and knowing that triplet states o f various pyrylium derivatives are observed [11], we have undertaken an investigation o f the excited state

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L Lampre et al. / Chemical Physics Letters 272 (1997) 496-500

relaxation of Pl+, and P+_ 12. Our aim was to examine the various relaxation paths of the S 1 charge transfer (CT) state. For this purpose, we used two different pump-probe techniques, Kerr ellipsometry [12,13] and flash photolysis with picosecond and nanosecond resolution, respectively.

2. Experimental The synthesis of P1+_12 and P l + l is reported in [1]. P?-12 is not soluble in glycerol. Solutions were degassed by the freeze thaw technique. Polymer films were prepared as described in [5] and studied without degassing. All measurements were performed at room temperature except for methanolethanol glasses (from 4 / 1 to 1/4; v / v ) . The latter were obtained by immersion of a cylindrical quartz cell in a quartz Dewar filled with liquid nitrogen and studied in situ without temperature control. Steady-state absorption spectra were recorded with a Cary3E. Flash photolysis experiments were performed using a N d / Y A G laser (532 nm, 15 ns, 4 - 8 mJ). A Xenon Arc provided the probing light. The detector was a Hamamatsu R928T photomultiplier. The zero time was defined as the time corresponding to the maximum of the laser pulse signal. The apparatus set-up for Kerr ellipsometry is described in [12]. The pump was a N d / Y A G laser (532 nm, 33 ps, 10-30 IxJ). The probe was a continuum generated in deuterated water. The transmitted light was detected by an optical multichannel analyser sensitive from 450 to 900 nm. The delay between pump and probe could be adjusted from - 100 ps to 1.5 ns. The zero time was defined as the time corresponding to the maximum of the signal at 550 nm. The parameter measured by this technique is the dichroic angle: 64)= ( A A I I - A AT)(InI0)/4, where A AII and A A T denote the absorbance in directions parallel and perpendicular to the pump polarization. Therefore, the spectral dependence of ~th is directly related to that of the absorbency. The temporal dependence of 3th is due to excited state relaxation a n d / o r rotational diffusion of the chromophores.

497

3. Results

3.1. Nanosecond flash photolysis The steady-state absorption spectrum of P1+_12 in PMMA is characterized by an intense band peaking at 550 nm and a weaker one at 385 nm (Fig. la). The transient signal consists of a rapid component, decaying within the laser pulse duration, followed by a slower one. The ensemble of the slowly decaying signals recorded at wavelengths ranging from 350 to 650 nm can be fitted by a single exponential, yielding a lifetime of 55 _ 10 Ixs. The spectra recorded at zero time show an absorption between 350 and 470 nm (Fig. lb); because of laser scattering and intense fluorescence [5], it was not possible to perform reliable measurements at longer wavelengths. On the spectrum corresponding to the slow component, the bleaching of both ground state bands and absorption in the 410-490 and 590-650 nm intervals are observed. It is worth noticing that the positive absorbance found for the rapid component is at least one order of magnitude higher than that correspond-

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I. Lampre et aL / Chemical Physics Letters 272 (1997) 496-500

498

ing to the slow component extrapolated to zero time. The results obtained for Pl+] are practically the same as those found for P1+12. It is not possible to detect any transient absorption for the compounds in fluid solvents (ethanol, dichloromethane, ethyleneglycol, glycerol) under the same experimental conditions used for the study of P M M A films. If much larger slits are used in the monochromator and the laser pulse energy is doubled, a weak signal is observed only in the case of glycerol solutions. This signal is composed of a rapid decay and a slow component decaying within a few hundreds of microseconds. A similar transient signal is also obtained for methanol-ethanol glasses at ca. 77 K. In this case, the slow component corresponds to a lifetime of 150 _ 50 ms. When the organic glass is molten, no transient signal is detected.

3.2. Kerr ellipsometry The dichroic spectrum obtained for Pl+12 in P M M A at t = 0 is characterized by the bleaching of ,

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the visible absorption band distorted on the red side by the stimulated emission (Fig. 2a); the spectra recorded at longer times have the same profile• In contrast, for P;-12 in ethanol (Fig. 2b) and dichloromethane, the spectra recorded have a timedependent profile• On the spectra recorded at t = 0, besides the bleaching and the stimulated emission, an absorption band peaking at 600 nm is present. The latter band and the stimulated emission disappear more rapidly than the bleaching. Thus, on the spectrum recorded at t = 90 ps (Fig. 2b), only the bleaching is visible, the corresponding 6~b values being five times smaller than the initial ones. The spectrum profile of PI+12 ethyleneglycol solutions at t -- 0 (Fig. 3, inset) is intermediate between those found for P~+_lz in P M M A and ethanol solutions in the sense that the dichroic angle at 600 nm is practically zero. If we focus on this part of the spectrum and consider its time evolution (Fig. 3), we can see that a band around 600 nm rises, reaching its maximum intensity within 50 ps. At 400 ps both the band peaking at 600 nm and the stimulated emission have disappeared while the bleaching is still present. Fig. 4 shows that the time evolution of lt&ll is wavelength dependent. The fluorescence lifetime, estimated from the decay at 725 nm and taking into account the laser pulse, is 20 + 10 ps. At 590 nm, the 184~1 decay can be described by a time constant of 70 ps, while the l a4,1 recorded at 500 nm decreases by 90% within 250 ps; the 10% left decays

1. Lampre et al. / Chemical Physics Letters 272 (1997) 496-500 1 0 "1

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with a time constant of ca. 1 ns. The latter value is only indicative since it is not measured over a sufficiently long time scale. When the same experiment is performed for P|+-l, the rise time corresponding to the peak at 600 nm is slightly shortened (40 instead of 50 ps). The temporal evolution of the dichroic spectra recorded for PI+_I in glycerol are similar to those obtained for Pl+_l ethyleneglycol solutions. The same band growing at 600 nm appears but the time scale at which the change occurs differs by more than one order of magnitude. Typically, the spectra obtained for P i ~ in glycerol at t = 450 and t = 800 ps, are identical to those recorded for ethyleneglycol solutions at t = 0 and t = 50 ps, respectively.

4. Discussion

The rapid decay of the flash photolysis signal obtained in PMMA is attributed to the absorption of the fluorescent S~ state. The differential spectrum corresponding to the slow component of the signals, characterized by an absorption at wavelengths longer than the main ground state absorption band, resembles the spectrum reported for the triplet state of 2,4,6-triphenylpyrylium [11]. Moreover, the lifetime of this transient at 77 K is longer by three orders of magnitude than that determined at room temperature. Therefore, it is assigned to the triplet state of the triarylpyrylium cations. The fluorescence lifetime and quantum yield of PI+_J2 in PMMA being 3.2 ns and 0.8 [5], it is not surprising that triplet formation is

499

not detected on the dichroic spectra recorded at times up to 1.5 ns. Regarding the results obtained for these compounds in fluid solvents, the band of the dichroic spectra around 600 nm is not due to a triplet state since no triplet is detected by flash photolysis in these solutions. In glycerol, where the triplet is formed with low yield ( < 0.001), it decays within a few hundreds of microseconds while the peak at 600 nm has significantly decreased at 1.5 ns; the latter decay time is not due to rotational diffusion since the rotational diffusion time of P~+-~2 in glycerol is longer than 170 ns. Moreover, the decay of this peak is slower than that of the stimulated emission. For P]+-12 in ethyleneglycol, it vanishes within 350 ps (Fig. 4). This time is comparable to that of the disappearance of the second harmonic signal generated by the six wave mixing technique in the same solutions (fig. 9 in [5]). Consequently, this band is due to the absorption of a non-fluorescent CT singlet state formed upon relaxation of the initial Franck-Condon state. Although Kerr ellipsometry does not provide information about conformational changes, the fact that the formation of the relaxed CT state is delayed when the solvent viscosity or the size of the rotating group increases, allows us to assign it to the TICT state, predicted theoretically. The bleaching remaining after the disappearance of the band attributed to the TICT absorption proves that a transient species not absorbing in the probed spectral range is formed. We are tempted to correlate it to the ground-state relaxation, from the conformation corresponding to the TICT state (O = 90 °) to that of the most stable conformation (O = 30°). Indeed, when the dialkylaminophenyl group is perpendicular to the plane formed by the pyrylium core, the S O--+ S 1 transition is symmetry forbidden and, consequently, its absorption should not be observed. Moreover, the absorption corresponding to S O-+ S 2, close in energy to S O--+ S 1 [5], should be masked by the bleaching. The results obtained for P1+_12 in ethyleneglycol, where the various processes related to the excited state relaxation are distinguishable (Fig. 4), allows an estimation of the rate constants involved (Fig. 5). These values are estimated without taking into account the rotational diffusion of the chromophores and must be considered only as orders of magnitude.

500

1. Lampre et al. / Chemical Physics Letters 272 (1997) 496-500

28560cm" ~ _. k 3 o - ~ s o .

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0 c m "1

Fig. 5. Energy levels and dihedral angles determined theoretically for the most stable conformation of P+_] in the ground and first excited singlet state [5]; a value O = 32 ° is found experimentally for the ground state [2]. The rate constants are those deduced for P+- ] 2 in ethyleneglycol. +

However, the rotational diffusion time of Pl-12 in ethyleneglycol being longer than 2.4 ns ], the signals decaying at t < 1.5 ns correspond approximately to the rate constant of the photophysical process. The fluorescence decay constant of Sl.30o (k n) is 50 × 109 S - 1. The rate constant of the TICT deactivation (kvlcr) is 14 × 10 9 S- I while that associated to the ground state relaxation (k90o-~ 30o) is 10 9 S 1 The rate constant for the TICT formation (k30o_ , 90o) is determined as follows. The ground state relaxation rate being much slower than kT]CT, the intensity of the bleaching present in the dichroic spectra after the TICT decay corresponds to the concentration of the ground state twisted conformation [S0,90o]. Fig. 4 shows that 3% of the initial bleaching intensity remains at 350 ps and, consequently, [S0,90o]350ps//[Sl.3OO]0ps = 0.03. Then, considering that each transient species disappears via firstorder kinetics, we deduce the equation: [S0.90o]350ps//[Sl,30O]0ps = 0 . 4 4 k30o_ , 9 0 o / ( k f l - kTICT).

Thus, k30o_,90 o is found to be 2.5 × 109 S - 1 , the same order of magnitude a s k90o__, 30o. Finally, knowing that the radiative decay constant of $1,30o (k r) is 0 . 1 5 × l 0 9 S - ! [5], t h e n o n r a d i a t i v e decay constant ( k ~ ) , given by the equation: k.r = k n - - k r k30o ~ 90o, is f o u n d to b e 4 7 × 10 9 s - 1 .

] The rotational difusion time r D of a spherical molecule is related to the solvent viscosity (r/), the molecular volume (V) and the temperature (T) through the Debye-Stokes-Einstein equation, ~.o = ~ ? V / k T where k is the Boltzmann constant. Pt+12 (V = 500 A3 [5]) is not spherical and, therefore, the rotational diffusion time is expected to be longer than that determined by the above equation; thus, at room temperature r D in ethyleneglycol (or glycerol) should be longer than 2.4 ns (or 170 ns).

The rate constants on Fig. 5 show that the dominant process in ethyleneglycol is the non radiative decay of Sl.30o. In low viscosity solvents k30o~90 o increases and the TICT yield, [Sl,90O]tot//[Sl,30O]tot , becomes higher than 20% (Fig. 2b). Furthermore, fast non radiative decay of S 1 (O = 30 ° or 90 °) to the ground state is responsible for thelack of triplet formation in fluid solvents. In conclusion, we have found evidence for TICT formation and ground state relaxation of these compounds. We have shown that the triplet formation is mainly affected by the fast non-radiative decay of the excited singlet to the ground state. This process is slowed down in solid matrices where TICT formation is inhibited and the triplet state is observed.

Acknowledgements We thank Dr H. Strzelecka who provided us with the triarylpyrylium salts.

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