Time-resolved resonance Raman investigation and ab initio calculations of the T1-state structure of thiocoumarin

Journal of Molecular Structure 735–736 (2005) 115–122 www.elsevier.com/locate/molstruc

Time-resolved resonance Raman investigation and ab initio calculations of the T1-state structure of thiocoumarin G. Burdzinskia,*, G. Buntinxb, O. Poizatb, C. Lapougeb b

a Faculty of Physics, Quantum Electronics Laboratory, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland LASIR, CNRS, Centre d’Etude et de Recherches Lasers et Applications, baˆt. C5, Universite´ de Lille I, 59655 Villeneuve d’Ascq Cedex, France

Received 30 June 2004; revised 3 September 2004; accepted 9 October 2004 Available online 7 December 2004

Abstract The T1 state of thiocoumarin has been investigated by transient absorption, resonance Raman scattering, and ab initio calculations. For comparison, the ground state molecule has also been examined by Raman spectroscopy and ab initio calculations. This analysis shows that the benzo moiety is only slightly modified in the T1 state compared to the ground state and keeps its aromatic character. In contrast, stronger distortions arise in the pyranthione moiety, resulting essentially from a lowering of the p electron density on the CC bond in position b to the CS bond, concomitant with an increase of the p density on the CC bond adjacent to the CS bond. However, the CS bond appears not significantly modified on going from S0 to T1, demonstrating definitely the pp* nature of the T1 state. The planar conformation of the ground state is preserved in the triplet state. q 2004 Elsevier B.V. All rights reserved. Keywords: Thioketone; Thiocoumarin; Transient resonance Raman; Ab initio calculations

1. Introduction The photophysics and photochemistry of aromatic thioketones show many interesting and unusual features, which makes this class of molecules quite different from the corresponding aromatic ketones. Among others, direct S0/T1 absorption, fluorescence from a long-lived excited S2 state, phosphorescence from the T1 state, and thermally activated S1-fluorescence have been observed in solution at room temperature [1–3]. A large energy gap between the S2 and S1 states (DE(S2KS1)z7000– 11,000 cmK1) accounts for the long S2-state lifetime, whereas an ultrafast intersystem crossing from S1 to T1 (tS1z0.5!10K12 s) [4] explains the insignificant S1-state emission. Finally, the S2 state of thioketones is efficiently quenched by most solvents except perfluorohydrocarbons [1,5–9]. Despite the importance of these properties on a fundamental point of view, very little is known on the

* Corresponding author. Fax: C48 61 829 5155. E-mail address: [email protected] (G. Burdzinski). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.10.079

excited state structure and electronic configuration of thioketones. Structural information on excited-state molecules can be obtained via the analysis of vibrational data available either directly via time-resolved vibrational spectroscopy or from the vibronic structure observed in the electronic spectra of jet-cooled molecules. Up to now, very few vibrational data of aromatic thioketones in an excited state and even in the ground state have been reported. From infrared and Raman spectra, ab initio and semiempirical calculations, Steer and co-authors discussed shortly the vibrational modes involving the CaS group of xanthione (XT) in the ground state [10]. An assignment of the fundamental modes of pyranthione (PT) in the ground state has also been proposed by Somogyi et al. [11] from an analysis of the infrared spectrum based on ab initio calculations. On the other hand, vibrationally resolved emission excitation spectra have been obtained for XT, PT, and benzopyranthione (BPT) in the excited triplet T1 and singlet S2 states in molecular supersonic jet [12–16]. Similarly, the S0/T1 absorption spectrum of jet-cooled BPT and PT using cavity ring-down spectroscopy has

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been featured [17,18]. In all cases the nature of the observed vibrations has been poorly described. Recently, Brint et al. [19] reported the phosphorescence excitation spectrum of jet-cooled XT and compared with those of PT, and BPT. They observed 20 fundamental T1-state vibrations for XT and concluded to a large vibrational activity of the CaS group in the S0/T1 transition on the basis of a comparison with the calculated ground state vibrational modes (Hartee–Fock level). Unexpectedly, a weaker number of modes are observed for BPT despite a lower symmetry (CS) compared to that of XT or PT (C2v). The authors suggested that the CaS motion is distributed over so many normal modes in BPT due to the low molecular symmetry that the number of modes with sufficient CaS activity to be observed in the spectrum is reduced [19]. In this paper, we propose a vibrational and structural analysis of the lowest excited triplet state (T1) of thiocoumarin (TC), an isomer of BPT, by means of timeresolved resonance Raman spectroscopy in the nanosecond time scale combined with ab initio calculations (Hartree– Fock Configuration Interaction Singles, HF CIS). The method of investigation consists of determining the optimised CIS geometry of the T1 state, then testing the reliability of this geometry by comparing the related theoretical vibrational frequencies with the experimental Raman spectra. In complement, a similar analysis of the TC ground state (S0) Raman spectra is made with the help of Density Functional Theory (DFT) calculations. Thiocoumarin has been chosen for this investigation as it exhibits very weak fluorescence and phosphorescence emissions [20] and is characterised by a large extinction K1 coefficient for triplet–triplet absorption (3max T1 Z14,000 M K1 cm ), thus allowing obtaining good quality timeresolved Raman spectra. The results lead for the first time to a detailed description of the triplet state structure of a thioketone.

In the nanosecond transient resonance Raman experiments, 10 ns (FWHM), 355 nm pulses generated at a repetition rate of 20 Hz by a diode pumped Nd:YAG laser (Diva II Thales) were used as pump excitation. About 6 ns probe pulses of 0.3 cmK1 spectral width and 20 Hz repetition rate, tunable in the 240–2200 nm spectral range, were produced by an optical parametric oscillator (OPO Vega BM Industries) pumped by the third harmonic of a single longitudinal mode Nd:YAG laser (5000 BM Industries). The scattered light was collected at 908 and detected in a home made polychromator (1200 g/mm grating) coupled with a thermoelectrically cooled intensified photodiode array PDA (DILOR). The Rayleigh light was removed by using a holographic Notch filter. The data were transferred to a PC computer for processing. Pumpprobe pulse sequences of about 5000 were used to obtain each time-resolved resonance Raman spectrum. The synchronisation of the pump pulse, probe pulse, and detector gate (30 ns) was controlled by a TTL generator (DG535 Stanford Research Sys.). The spectral and temporal resolutions of the spectrometer were about 5 cmK1 and 10 ns, respectively. Steady-state Raman measurements were carried out using near IR FT-Raman spectrometry (Bruker IFS 88 instrument). Nanosecond transient absorption measurements were achieved by using the same apparatus and under similar experimental conditions as described previously [2]. 3. Computational methods All calculations were performed with the GAUSSIAN 98 program [21]. The ground state geometry and vibrational characteristics (force constants and harmonic frequencies) of TC were obtained using the density functional theory with the B3LYP hybrid functional and the 6–31G* basis set [22]. We have also calculated the ground state of TC at Hartree–Fock (HF) level to have a starting point of comparison for the excited-state calculations. Configuration

2. Experimental TC was synthesised as previously described [20] and was purified by high-performance liquid chromatography (HPLC). Spectroscopic grade solvents (Aldrich) were used as received. All nanosecond time-resolved resonance Raman and transient absorption experiments were performed at room temperature. Solution samples were contained in quartz cells of section 1 cm!1 cm and deaerated by bubbling high purity nitrogen. An absorbance of about 1 at the laser excitation wavelength was generally used for transient absorption experiments, which corresponds to a thioketone concentration of approximately 1!10K4 M. For transient resonance Raman measurements, higher concentrations in thioketone were used (about 0.5!10K3 M) to ensure a sufficient scattered photon flux.

Fig. 1. Atom numbering and internal coordinates of thiocoumarin.

G. Burdzinski et al. / Journal of Molecular Structure 735–736 (2005) 115–122 Table 1 In-plane local symmetry coordinates of TC, as recommended by Pulay [24] Definition

Description

R1, R2, R3, R4, R7, R8, R9, R10, R11 R5, R6 R12 R13, R14, R15, R16, R17, R18 pffiffiffi b6 Z ð1= 2Þðb6 K c6 Þ pffiffiffi bi Z ð1= 2Þðbi K ci Þ; iZ1, 2, 3, 4, 7, 8 S11Za2Ka3Ca4Ka5Ca9Ka1 S21Za0Ka6Ca7Ka8Ca 0 9Ka 0 5 S12Z2a2Ka3Ka4C2a5Ka9Ka1 S22Z2a0Ka6Ka7C2a8Ka 0 9Ka 0 5 S13Za3Ka4Ca9Ka1 S23Za6Ka7Ca 0 9Ka 0 5

CC stretching CO stretching CS stretching CH stretching CS in-plane bending CH in-plane bending Benzo moiety deformation Pyranthione moiety deformation Benzo moiety deformation Pyranthione moiety deformation Benzo moiety deformation Pyranthione moiety deformation

The atomic numbering and the notation of internal coordinates are given in Fig. 1.

Interaction Singles (CIS) level was used with the same basis set to determine the optimised geometry, force constants, and harmonic vibrational frequencies of the T1 state of TC. CIS calculations were carried out to identify the transitions of lowest energies having high transition dipole moments and determine the molecular orbitals (MOs) involved in these transitions. The force constants calculated for the Cartesian displacements of the ground state and the triplet state were transformed into the force constants expressed in local symmetry coordinates for each vibration by using the REDONG program [23]. These nonredundant symmetry coordinates were defined as recommended by Pulay [24] from the internal coordinates shown in Fig. 1. The in plane symmetry coordinates are given in Table 1. The potential energy distributions (PEDs) were calculated for each vibrational mode from the force constants expressed in local symmetry coordinates.

4. Results and discussion 4.1. Equilibrium geometries The optimised geometric parameters (bond lengths and angles) calculated for the ground state S0 and triplet state T1 of TC are listed in Table 2. The corresponding atom numbering is provided in Fig. 1. The ground state of TC is characterised by a planar structure as attested by the values of the dihedral angles. This planar structure is consistent with single crystal X-ray results [25]. The comparison of the structural parameters obtained for the S0 state with the HF and DFT methods reveals expected trends, i.e. similar angle values and bond lengths slightly longer in the DFT calculation. Note however that the DFT values for the CC and CO bond lengths in the pyranthione ring are closer to each others than the HF values. A more delocalised electronic configuration is thus predicted for this heterocyclic ring by DFT. A clear C6–S double bond character is

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Table 2 Optimised geometrical parameters of the S0 (DFT and HF) and T1 (CIS) states of TC Geometrical parameters

S0 state B3LYP/6-31G*

Bond lengths (A˚) 1.388 C1C2 C2C3 1.404 C3C4 1.391 C4C5 1.395 1.368 C5O OC6 1.373 C6C7 1.444 C7C8 1.358 1.438 C8C9 C5C9 1.408 C9C1 1.408 1.651 C6S C1H1 1.087 C2H2 1.086 C3H3 1.086 1.085 C4H4 C7H5 1.083 C8H6 1.087 Bond angles (deg) C2C3C4 120.8 C3C4C5 118.8 C4C5O 117.1 123.0 C5OC6 OC6C7 116.5 C6C7C8 121.8 C7C8C9 120.4 124.6 C8C9C1 C9C1H1 118.9 C3C2H2 120.0 C4C3H3 119.4 119.1 C5C4H4 C7C6S 125.1 C8C7H5 121.9 C9C8H6 119.0 Dihedral angles (deg) C1C2C3H3 180.0 H2C2C3C4 180.0 0.0 H3C3C4H4 C3C4C5O 180.0 C4C5C9C8 180.0 C5OC6S 180.0 0.0 OC6C7C8 H5C7C8H6 0.0 C5C9C1H1 180.0

T1 state HF/6-31G* 1.375 1.396 1.378 1.387 1.354 1.333 1.451 1.336 1.447 1.386 1.398 1.637 1.076 1.074 1.075 1.073 1.071 1.075

CIS/6-31G* 1.376 1.392 1.389 1.376 1.370 1.359 1.393 1.428 1.420 1.409 1.408 1.658 1.075 1.075 1.074 1.074 1.071 1.071

120.9 118.6 117.3 124.2 116.6 121.0 120.5 124.6 119.2 120.0 119.4 119.3 124.1 122.5 119.0

120.0 119.7 116.6 121.0 119.7 121.6 117.2 123.9 118.5 120.0 119.7 118.9 122. 6 121.1 121.7

180.0 180.0 0.0 180.0 180.0 180.0 0.0 0.0 180.0

180.0 180.0 0.0 180.0 180.0 180.0 0.0 0.0 180.0

˚ , DFT reflected by a calculated bond length (HF 1.637 A ˚ 1.651 A) closer to the mean value expected for a double˚ ) than for a single bond (1.81 A ˚ ). bond (1.55 A The CIS structure predicted for the T1 state results from a relatively pure one-electron excitation. In fact, the S0/T1 transition involves a predominant contribution (81%) of the highest occupied (HOMO) and lowest empty (LUMO) molecular orbitals represented schematically in Fig. 2, plus five additional minor contributions (3–6% each). In order to present a consistent analysis of the structural modifications induced by the S0/T1 excitation, we now

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Fig. 2. CIS/6-31G* description of the HOMO and LUMO orbitals mainly involved in the electronic transition from the ground state to the lowest excited triplet state of TC.

compare the geometric parameters obtained for these states at the CIS and HF levels, respectively, because these values are obtained with the same calculation methodology. The corresponding changes in bond length between the S0 and T1 states of TC are calculated and reported in Fig. 3. Some important points can be underlined concerning the T1 structure relative to the S0 one: † The molecule keeps a planar conformation attested by the values of the dihedral angles listed in Table 2. † The benzo moiety appears not significantly modified and keeps its aromatic character. Only the C4–C9 bond shared with the pyranthione ring undergoes an appreciable though small increase in length upon excitation. † Stronger distortions arise in the pyranthione moiety. The ˚ of the major effects are an increase by about 0.09 A C7–C8 bond length on going from S0 to T1, indicating a notable reduction of double bond character of this bond, ˚ of the C6–C7 bond and a reduction by about 0.06 A length, which reflects a marked double bond character in the T1 state. Concomitant modifications by about 38 of the C5OC6, OC6C7 and C7C8C9 angle values are also observed. These specific changes correspond to the changes in the electronic density observed between the HOMO and LUMO orbitals dominantly involved in the electronic transition (Fig. 2). † Finally, the C6–S bond length shows only a minor variation upon excitation to the triplet T1 state (increase ˚ ), indicating that the initial double bond by about 0.02 A character of this bond is nearly unchanged in T1. This conclusion implies that the T1 state has pure pp* character, confirming definitely an assignment pre-

Fig. 3. Changes in the TC bond lengths (in angstroms) resulting from the S0/T1 transition. The corresponding optimised geometries have been calculated at the HF level for the ground state and at the CIS level for the triplet state.

Fig. 4. Transient absorption spectra of a deoxygenated solution of TC (0.9!10K4 M) in n-hexane after excitation at 355 nm (0.8 mJ per pulse). Inset: decay kinetics monitored at 470 nm (black trace).

viously suggested from the analysis of the photophysical properties of TC [20]. In fact, in case of np* excited state nature, a notably weaker C–S bond (almost a single bond) is expected. For instance, in the case of PT, for which the T1 state has dominant np* character, an increase by ˚ of the C–S bond length has been found on about 0.1 A going from the ground state to the triplet state [26]. 4.2. Nanosecond transient absorption and resonance Raman results It is known that the photoexcitation of TC to the S2 state leads to the formation of the T1 state with efficiency near to unity via fast S2/S1 internal conversion and S1/T1 intersystem crossing processes [20,27]. Fig. 4 presents the transient absorption spectra of TC in n-hexane recorded in the 320–670 nm spectral range at different times from 30 to 2250 ns following 355 nm excitation. This pump wavelength matches the S 0/S 2 (p,p*) transition lying in the 313–435 nm spectral range. The spectra are characterised by a negative band between 320–420 nm and a positive band in the 420–670 nm region (lmaxw470 nm). The signal in the 320–420 nm spectral range is unambiguously related to the ground state depletion signal since its spectral position corresponds to that of the S0/S2 absorption band of TC. The decay kinetics of both the negative and positive bands (inset in Fig. 4) can be fitted with the same single exponential function (tZ510G20 ns), in agreement with the presence of an isosbestic point at 420 nm. All kinetics being sensitive to the presence of oxygen in the solution, we assign the kinetics measured in the 420–670 nm spectral range to the decay of the triplet state absorption T1/Tn, while those in the 320–420 nm spectral range to the repopulation of the ground state due to decay of the triplet state (decay of bleaching). As a confirmation, the shape of the 420–670 nm absorption band is similar to that reported for the T1 absorption spectrum of

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theoretical vibrational spectra derived from the corresponding ab initio geometries calculated above. 4.3. Vibrational assignments

Fig. 5. (A) Time-resolved resonance Raman spectrum of the triplet state (T1) of TC in n-hexane (0.5!10K3 M) probed at 465 nm (1 mJ pulses) at a time delay of 20 ns after pump excitation at 355 nm (1 mJ pulses) and (B) Raman spectrum of the ground state (S0) of TC in acetonitrile (1! 10K1 M). Solvent bands are subtracted in all cases (the symbol * indicates an artefact due to the imperfect subtraction of a solvent band). The main transient band frequencies are indicated.

TC in benzene [27]. Assuming, a triplet formation quantum yield of 1, the comparison of the absorbance variation between the ground state depletion signal at 370 nm and the triplet state absorption signal at 470 nm provides a lower K1 cmK1 for the triplet–triplet limit value of 3max T1 Z14,000 M absorption coefficient. Fig. 5a shows the resonance Raman spectrum of the TC triplet T1 state in n-hexane recorded in the 1650–200 cmK1 spectral range at a time delay of 20 ns following pump excitation at 355 nm. This spectrum is obtained by setting the probe wavelength at 465 nm, i.e. in resonance with the T1/Tn electronic transition characterised above (near the absorption maximum to ensure a strong resonance effect) and outside the strong S0/S2 absorption band. For comparison, the steady-state Raman spectrum recorded for TC in acetonitrile is presented in Fig. 5b. The assignment of the transient Raman spectrum in Fig. 5a to the T1 state species is confirmed by the observation that all Raman lines decay with a single exponential kinetics consistent with that characterising the T1/Tn absorption and equally sensitive to the presence of oxygen in the solution. Very similar resonance Raman spectra were obtained for solutions of TC in methanol and acetonitrile. They show many vibrational bands, the most intense being situated at 1541, 1509 and 1026 cmK1. We discuss now the vibrational assignments of the S0 and T1 states Raman spectra on the basis of

Forty-five internal modes of vibration of TC are distributed between 31 in-plane modes and 14 out-ofplane modes. The experimental Raman spectrum of the triplet state being obtained in resonance with a strongly allowed electronic transition, only the totally symmetric modes are expected to be active according to the Franck– Condon principle [28]. Since the only element of symmetry of the molecule is the molecular plane, the totally symmetric modes are all the in-plane vibrations. Accordingly, we take into consideration only these in-plane modes in the following discussion. Moreover, the six C–H stretching in-plane vibrations, lying above 3000 cmK1, are not expected to provide important information about the molecular structure. They are thus also excluded from the discussion. The vibrational analysis presented below for the ground and triplet states of TC is thus restricted to the 25 inplane modes expected in the 200–1650 cmK1 spectral range. Table 3 presents the experimental Raman frequencies observed for the S0 and T1 states of TC (Fig. 5) and the scaled theoretical frequency values calculated from the optimised geometries. The ground state vibrational values are obtained from DFT-B3LYP calculations (scaling factors of 0.98) and the triplet state values from CIS calculations (scaling factor 0.91). The scaling factors, determined from the mean deviation between experimental and calculated frequencies, are comparable to those usually encountered for this type of molecule. A description of all vibrations by means of their potential energy distribution (PED) is also provided in Table 3. A comparison of the time-resolved resonance Raman spectrum of the T1 state of TC with the theoretical scaled frequencies calculated for the totally symmetric modes (vertical bars) is shown in Fig. 6. A high consistency between the experimental and calculated frequencies is found, in such a way that the assignment of the observed Raman bands proposed in Table 3 is unambiguous. We note that all the observed Raman bands of S0 TC (Fig. 5b) and resonance Raman bands of T1 TC (Fig. 5a) can be correlated to calculated modes and thus correspond to in-plane (totally symmetric) vibrations. A very good agreement is found between the computed and experimental values. The root-mean square deviation values are 11 and 10 cmK1 for the S0 and T1 states, respectively. This agreement definitively validates the structures and electronic configurations calculated for TC in the S0 and T1 states. In particular, the pp* character of T1 is confirmed. The nature of the vibrational modes can be determined from their PED and by viewing the corresponding atomic displacements. In most cases, in both the S0 and T1 states, the TC modes are described by complex PEDs and cannot be correlated to specific internal coordinates. For instance,

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Table 3 Theoretical scaled frequencies (cmK1) and potential energy distributions (PEDs) calculated for the totally symmetric modes of TC in the ground state S0 (DFTB3LYP/6-31G*) and triplet state T1 (CIS/6-31G*) compared to the experimental Raman frequencies S0

T1

nexp

ncalc

PED (%)a

nexp

ncalc

PED (%)a

b6(66) S23(42), R12(20), S22(14) b6(44), S12(15), S22(12) S12(37), S23(28) S13(43), S22(34) R12(26), S21(20), R7(11), S12(11), S22(11) R11(19), R9(15), R10(11) S11(43), R5(10), R12(11) S21(35), R6(24)

219 368 417 507 591 620 743 837 911 –b 1026 1069

221 369 421 505 590 643 741 842 899 958 1019

b6(70) S23(39), S22(17), R12(14) b6(41), S22(15), S12(13) S12(34), S23(28), R9(10) S13(45), S22(33) R12(30), S21(18), S12(10), S22(10) R11(24), R5(12), R10(12) S11(33), R12(14), R9(13), S12(12) S21(36), R6(25), R8(10) R8(43), R7(15), b8(11) R2(40), R3(14), b4(15)

1105 1153

1103 1161

S11(19), R1(17), b2(12) R12(22), b7(16), b1(16)

1183 1206 –b –b –b

1174 1205 1229 1281 1300

R9(24), R10(13), b8(13) b2(20), b3(24), R11(10), b4(10) R5(35), S11(14) b4(24), b1(15), S21(13), b3(10) R6(24), b7(22)

230 384 429 519 592 654 758 868 –b

224 380 426 517 594 652 756 865 904

1026 1096 1129

1035 1082 1091 1131

R2(40), b4(17), R3(13), R1(11), b1(10) R3(22), R2(21), R4(13), R1(12), R10(10) R6(26), b7(16), S11(13), R12(11), b8(10) b3(19), b4(14)

1154 1190 –b –b

1165 1200 1205 1239

b2(37), b1(18), b3(14) R7(23), R12(23), b8(11) R6(17), R12(16), b8(14), R5(11) b7(18), R9(16), R10(11), b1(10)

1248 1277

1254 1281

R5(20), S11(15), b8(13), R7(12), R9(12) b4(26), b1(26), b3(11), S21(12)

1331 1416 1453

1355 1429 1467

R1(17), R3(16), R11(15), R2(13), R4(12), R10(12) b7(23), b8(22), b2(14), R7(11) b3(20), b4(13), R1(10)

–b 1509

1392 1484

–b

1506

b2(17), R9(13), b4(13), b1(13)

–b 1541

1451 1553

b8(46), R9(19), b7(18), b4(18), b3(16), b1(15), R1(13), R11(10) b1(27), b2(26), R9(10) R7(29), b7(20), R8(19)

1555

1575

R8(22), R2(15), b3(15), R3(11) –b –b

1576 1585

R2(20), R4(19), b2(14) R3(17), R1(13), R10(12)

1606 –b a b

1631 1635

R8(26), R10(12), b8(10) R1(21), R4(17)

Values less than 10% are neglected. Not observed.

as can be seen in Table 3, there is no normal mode dominantly localised on the CaS stretch since the CaS stretching coordinate (R12) is distributed over many normal modes (380, 652, 865, 1091, 1200, 1205 cmK1 for the ground state, 369, 643, 842, 1161 cmK1 for the triplet state) with individual contributions %30%. However, we observe that the nature of many vibrations of TC is not drastically modified on going from S0 to T1. This is particularly true in the 200–1050 cmK1 spectral range, where all the modes of the ground state can be directly correlated to modes of the T1 state. In the 1050–1650 cmK1 spectral range, some correspondences can also be established but several vibrations undergo a significant redistribution of their potential energy on going from S0 to T1. Those modes for which a clear correlation is found between the S0 and T1 states are written on the same line in Table 3. We observe that, as a general rule, the vibrational frequencies are slightly lower in the triplet state than in the ground state, in agreement with the expected reduction of the p-bonding electronic density in the molecule

on going from the ground state to a pp* state. In particular, the mean frequency value of the modes involving the CaS stretching motion (see above) appears slightly lower in T1 (w750 cmK1) than in S0 (w900 cmK1), which is consistent with the slightly longer CaS bond length calculated for the T1 state geometry compared to the ground state one (see Table 2). It is worthwhile to make a comparison with the situation encountered in ketones. The above approximate frequency shift of K150 cmK1 found for the CaS bond of TC is similar to the frequency downshift of the CaO bond stretch reported, for example, for the keto analogue coumarin (DnZK180 cmK1 [29]) or for 4-phenylbenzophenone (DnZK143 cmK1 [30]) on going to the T1 pp* state but is notably weaker than that determined for benzophenone (DnZK443 cmK1 [31]) on going to the T1 np* state. This is in accord with the assignment of the T1 state of TC to a pp* configuration. Another interesting point to consider is the variation between S0 and T1 of the distribution of the R7 and R8

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Acknowledgements The authors thank the Groupement de Recherche GDR 1017 from CNRS and the Centre d’e´tudes et de Recherches Lasers et Applications (CERLA) for their help in the development of this work. CERLA is supported by the Ministe`re charge´ de la Recherche, Re´gion Nord/Pas de Calais, and the Fonds Europe´en de De´veloppement Economique des Re´gions. The paper was also prepared under the financial support of KBN (Polish State Committee for Scientific Research) Grant No. 4T09A 166 24.

References Fig. 6. Comparison of the time-resolved resonance Raman spectrum of the T1 state of TC with the theoretical (CIS/6-31G*) scaled frequencies calculated for the totally symmetric modes (vertical bars). The modes which are not observed are indicated with a dotted bar. The experimental Raman frequency values are indicated.

internal coordinates associated to the C6–C7 and C7–C8 bonds, respectively, among the normal modes (Table 3). In the ground state, the R7 coordinate has contributions higher than 10% in the PEDs of four modes at 652 (11%), 1200 (23%), 1254 (12%), and 1429 cmK1 (11%). In the triplet state, this coordinate appears only in the PED of two modes at 958 and 1553 cmK1, with a dominant contribution to the latter (29%). This clear shift toward the high frequency region denotes a strengthening of the C6–C7 bond in the excited state. The opposite effect is remarked for the R8 coordinate, mainly localised in the high frequency normal modes lying at 1575 (22%) and 1631 cmK1 (26%) in the ground state but transferred in the triplet state essentially to the 958 cmK1 mode (43%) and, to a lesser extent, to the 1553 cmK1 mode (19%). This change reflects a lowering of the p electron density on the C 7–C8 bond. These conclusions concerning the concomitant reinforcement of the C6–C7 bond and reduction of the C7–C8 bond strength on going from the ground state to the triplet state are in agreement with the predicted major geometry changes discussed above (Fig. 3) and the schematic representation of the molecular orbitals mainly involved in the electronic transition (Fig. 2). It is worth noting that similar conclusions have been proposed for the keto-analogue coumarin (CM) [29]. An assignment of the pp* triplet state Raman spectrum of CM has been proposed on the basis of the frequency shifts observed upon isotopic (18O, 13C, deuterium) substitution [29]. The C7–C8 bond has been found to be drastically lengthened in the T1 state, whereas the C9–C8 and C7–C6 bonds are shortened. The analogy between TC and CM concerning the T1 state structure is not surprising since, on one hand, the T1 state is pp* in both cases and, on the other hand, these molecules differ only by the replacement of a CaS group by a CaO group, which is not expected to have much influence on a pp* transition.

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