Rotationally resolved high-resolution spectrum of the S1âS0 transition

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Rotationally resolved high-resolution spectrum of the S1–S0 transition of jet-cooled thioanisole Mariko Hoshino-Nagasaka,a Tadashi Suzuki,w*a Teijiro Ichimura,a Shunji Kasahara,b Masaaki Babac and Susumu Kawauchid

Downloaded by Ulyanovsk State University on 13 October 2010 Published on 02 September 2010 on http://pubs.rsc.org | doi:10.1039/C004454G

Received 18th March 2010, Accepted 30th July 2010 DOI: 10.1039/c004454g The rotationally resolved high-resolution fluorescence excitation spectrum of the 0–0 band in the S1 ’ S0 electronic excitation of thioanisole was observed using the techniques of a collimated supersonic jet and a single-mode ultraviolet laser for the first time. High accurate rotational constants for the S0 and the S1 states have been determined by precisely calibrated transition energies of about 1000 assigned rotational lines. The molecular structure of thioanisole has been estimated by high-level MO calculations. The planarity of thioanisole in the S0 and the S1 states was also demonstrated clearly. The lifetime of the S1 state was estimated to be 2.0 ns from the observed line width. This line shape did not change with the magnetic field of 1 Tesla, suggesting that the main radiationless process should be internal conversion to the S0 state.

1. Introduction The conformation of flexible molecules is of great interest and attracts many researchers.1–5 The stability of each conformer is strongly affected by the properties of the substituents: steric hindrance, orbital interaction, hydrogen bonding, etc.6–9 Thioanisole, consisting of a benzene ring and a thiomethyl (–SCH3) group, is one of the fundamental benzene derivatives having a flexible substituent (Fig. 1). The molecular structure of thioanisole has been extensively studied with several experimental methods and quantum chemical calculations.10–16 It was proposed that thioanisole would have a planar and/or a non-planar conformer with respect to the internal rotation of the thiomethyl group around the C(sp2)–S bond. Microwave spectroscopy11 and gas electron diffraction study12 showed that the planar conformer was the most stable one in the electronic ground (S0) state. Recently, we reported laser-induced fluorescence excitation and dispersed fluorescence spectra of thioanisole

Fig. 1 Molecular structure and rotational axes of thioanisole. a

Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152-8551, Japan b Molecular Photoscience Research Center, Kobe University, Nada-ku, Kobe 657-8501, Japan c Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan d Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo, 152-8551, Japan w Present address: Department of Chemistry and Biological Science, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan. [email protected]

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in a supersonic jet.10 By precise analysis of the low-frequency vibrational mode in the dispersed fluorescence spectrum obtained by the 0–0 band excitation with high-level MO calculations, it was concluded that the most stable structure was planar in the S0 state. The planarity of thioanisole in the S1 state was also mentioned. However, the accurate geometrical structure in the electronic excited (S1) state has not been clarified yet. The molecular structure in the S1 state of thioanisole is supposed to be changed from that in the S0 state if the spatial distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are different. The molecular structure is related to the excited state dynamics. It is of great interest to examine the effect of the structural change on the internal conversion (IC), which is the radiationless transition between the singlet states. There have been several spectroscopic studies of the dynamics on the benzene derivatives such as phenol (C6H5OH)17,18 and anisole (C6H5OCH3).19–22 The lifetime in the S1 state of phenol-h6 was reported to be 2.4 ns, while that of phenol-d6 was much longer (12.5 ns). The lifetime of anisole was 19.9 ns.19 In the case of thioanisole, intersystem crossing (ISC) from the singlet state to the triplet manifolds is expected to be enhanced by the heavy atom effect on spin–orbit coupling interaction. However, the mechanism of dynamical processes in the S1 state of thioanisole has not been sufficiently elucidated. It is necessary to study the molecular structure and the dynamics of the thioanisole molecule in the S1 state in more detail. In this study, we present the rotationally resolved ultrahighresolution fluorescence excitation spectrum of the 0–0 band in the S1 ’ S0 electronic excitation of thioanisole in a collimated supersonic jet for the first time. The molecular structure of thioanisole in both the S1 and the S0 states and the relaxation process are discussed with the aid of high-level MO calculations.

2. Experimental The experimental setup has been reported in our previous work.23 Thioanisole (Wako Chemical) was used without Phys. Chem. Chem. Phys., 2010, 12, 13243–13247

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further purification. The liquid sample was heated to 420 K in a stainless-steel container, and the vapor was mixed with Ar gas (2 atm). The mixed gas was expanded into a vacuum chamber through a pulsed nozzle (automobile fuel injector). The supersonic jet was collimated by a skimmer (2 mm orifice diameter) which was set at a position of 10 cm apart from the nozzle. The residual Doppler width is estimated to be 0.0013 cm1 (40 MHz) under the present experimental condition. The collimated supersonic jet was crossed with a beam of ultraviolet single-mode laser light at right angles. As the light source, we used a single-mode tunable ring dye laser (Coherent CR699-29, Rhodamine 6G dye, DE = 0.0001 cm1) pumped by a Nd3+:YVO4 laser (Spectra Physics Millennia Xs, 532 nm, 10 W). The output was fed into an enhancement cavity for the second harmonic generation (Spectra-Physics, Wavetrain SC) equipped with a BaB2O4 crystal. The power of the UV light was 40 mW. The fluorescence from the excited molecule was collected by a lens to a photomultiplier (Hamamatsu R464) through a glass filter (Toshiba UV35) to block the scattered laser light. The output was processed by a gated photon counter (Stanford Research SR400). The fluorescence intensity was recorded by scanning the wavenumber of the laser light and we obtained a sub-Doppler ultrahigh-resolution fluorescence excitation spectrum. A part of the laser output was phase-modulated at 30 MHz by an electro-optic modulator, and was passed through a stabilized etalon (Burleigh CFT-500, FSR = 150 MHz, finesse = 30). The cavity length of the etalon was stabilized by using a single-mode Nd3+:YAG laser (Innolight Prometheus 20, 532 nm, 20 mW), whose frequency was locked to a hyperfine line of the iodine molecule. We recorded the transmitted light intensity as a frequency mark. We also recorded the Doppler-free saturation spectrum of the iodine molecule. The transition wavenumber of each hyperfine line was calibrated using the Doppler-free high-resolution spectral atlas of the iodine molecule.24 The accuracy of the absolute wavenumber was 0.0002 cm1 for the ultraviolet light. The rotational temperature was estimated to be ca. 50 K. Quantum chemical calculations were carried out using GAUSSIAN 03 suit of program.25 The geometry optimization was performed with the MP2 and the CASSCF methods for the S0 and the S1 states. The rotational constants were estimated using the optimized molecular structure.

Fig. 2 Rotationally resolved high-resolution spectrum of the S1 ’ S0 0–0 band of jet-cooled thioanisole.

levels in the S0 and the S1 states have been determined successfully and summarized in Table 1. The rotational temperature was estimated to be about 50 K. The accurate values including higher order terms were obtained. The rotational constants of thioanisole in the S0 state were reported by the microwave spectroscopy.11 The values obtained well agree with those of microwave spectroscopy (see Table 1). It should be noted that we determined the molecular constants by analyzing about 1000 transitions in this study and our results well reproduce the spectrum in the wider range of J and K values. Fig. 3 shows the observed spectrum and the calculated one by using the obtained values in the P branch region. The calculated one well reproduced the observed spectral feature, indicating that the obtained rotational constants are sufficiently accurate. The inertial defect D = Ic Ib Ia was very small in the S0 state. In the case of anisole, it was reported that the inertial defect was 0.5664 1046 kg m2.22 The value of the inertial defect for thioanisole was in good agreement with that for anisole. Thus, it is considered that the molecular structure of thioanisole is planar in the S0 state except the two methyl hydrogen atoms as well as anisole. This result is consistent with our previous report.10 The gas electron diffraction study shows the geometrical Table 1 Molecular constants of the S0 1A 0 (u00 = 0) and the S1 1A 0 (u 0 = 0) states. Rotational constants A, B, C, DJ, DJK, DK, dJ, dK, band origin v00, and standard deviation ddev are in units of cm1. Moments of inertia Ia, Ib, Ic and inertial defect D are in units of 1046 kg m2.

3. Results and discussion The LIF excitation spectrum of the S1 ’ S0 transition of jet-cooled thioanisole was reported previously.10 The rotationally resolved high-resolution spectrum of the 0–0 band was observed for the first time. The whole band is shown in Fig. 2. Two intense maxima of P and R transitions were clearly seen in both sides, and no strong lines of Q transitions were found at the band center. This feature is obviously of the b-type band, in which the electronic transition moment is parallel to the b axis and the selection rules are DJ = 0, 1, DKa = 1, DKc = 0. The thioanisole molecule is a near-prolate asymmetric top and the energy calculation by Watson A-reduced Hamiltonian was carried out.26 About 1000 rotational lines were assigned. The molecular constants of the zero-vibrational 13244

Phys. Chem. Chem. Phys., 2010, 12, 13243–13247

S0 1A 0 (u00 = 0)

A B C DJ DJK DK dJ dK n00 Ia Ib Ic D ddev

S1 1A 0 (u 0 = 0)

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0.141103(12) 0.0390166(31) 0.0307709(19) 6.4(30) 109 1.02(36) 107 1.61(60) 107 3.7(15) 109 6.2(43) 108 — 19.8385 71.7457 90.9715 0.6127 0.00122

0.1411037 0.03901543 0.0307726 1.4 109 o|1.3 1010| 2.13 108 3.3 109 4.07 109 —

0.620

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0.1371246(29) 0.03926301(93) 0.03075711(21) 6.29(44) 109 1.286(69) 107 2.27(20) 107 3.71(88) 109 6.0(11) 108 34502.8419(2) 20.4141 71.2955 91.0129 0.6973 0.00086



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Fig. 3 An expanded portion of the high-resolution spectrum of the S1 ’ S0 0–0 band in the P branch region (the upper trace) and the corresponding part of the calculated spectrum using the obtained rotational constants (the lower trace). The assignments are also shown.

parameters of planar conformer.12 By using the reported geometrical parameters the rotational constants were calculated to be A = 0.1383 cm1, B = 0.03944 cm1 and C = 0.03088 cm1. These values are reasonably corresponding with our values. The minor differences of rotational constant would be due to vibrational effects. To obtain the most stable structure, high-level MO calculations were also carried out. The geometry optimization was performed at MP2/cc-pVTZ and CASSCF(8,7)/cc-pVTZ levels for the S0 state. Both the calculations predicted that the most stable structure should be planar. Fig. 4a shows the optimized structure in the S0 state. The rotational constants were calculated for the optimized structure of the planar conformer to be A = 0.140989 cm1, B = 0.0393126 cm1 and C = 0.0309521 cm1 at the MP2/cc-pVTZ level, and A = 0.1434866 cm1, B = 0.0387318 cm1 and C = 0.0306749 cm1 at the CASSCF(8,7)/cc-pVTZ level.

These values are in sufficiently good coincidence with the ones we obtained experimentally. The optimized structure is approximately considered to be the real equilibrium structure of the thioanisole in the S0 state. The molecular structure of thioanisole in the S1 state has not been well understood. In the present work, the rotational constants for the S1 state were obtained for the first time. The determined values were not much different from those in the S0 state although the A value was slightly smaller (see Table 1), suggesting that there is not a great difference in the nuclear configuration of the molecule upon the electronic excitation. The inertial defect was very small in the S1 state. Therefore, it is concluded that the molecular structure of thioanisole should be planar in the zero-vibrational level of the S1 state. The geometry optimization for the S1 state was performed at CASSCF(8,8)/6-31G(d) and CASSCF(8,8)/cc-pVTZ levels. The CASSCF(8,8)/6-31G(d) level calculation predicted that the non-planar conformer was the stable one. On the other hand, the CASSCF(8,8)/cc-pVTZ level calculation showed that the most stable structure was planar. The rotational constants were calculated using the optimized structures. The values were A = 0.1374257 cm1, B = 0.0378621 cm1 and C = 0.0298506 cm1 for the planar conformer at the CASSCF(8,8)/cc-pVTZ level, and A = 0.1323639 cm1, B = 0.0365055 cm1 and C = 0.0313707 cm1 for the nonplanar conformer at the CASSCF(8,8)/6-31G(d) level. The CASSCF(8,8)/cc-pVTZ calculation for the planar conformer was in good agreement with the experimental results, whereas the CASSCF(8,8)/6-31G(d) calculation for the non-planar conformer was not. Therefore, the molecular structure should be planar in the S1 state and this result is consistent with our experimental results. Recently, we reported that one should use the higher basis set, at least, composed of (4s3p2d) basis sets for the first row atoms and (5s4p2d) for the sulfur atom to describe the electronic structure of thioanisole correctly.10 Thus, the CASSCF(8,8)/cc-pVTZ calculation could reproduce

Fig. 4 Optimized geometries of thioanisole in (a) the S0 at the MP2/cc-pVTZ level and (b) the S1 states at the CASSCF(8,8)/cc-pVTZ level. Bond lengths are in A˚ and angles are in degrees. The CCphSC dihedral angles in the S0 and the S1 states were obtained to be 01.

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Fig. 5 (a) The highest occupied molecular orbital (HOMO) and (b) the lowest unoccupied molecular orbital (LUMO) obtained by the MP2/cc-pVTZ level calculation for the optimized molecular configuration in the S0 state.

the experimentally determined rotational constants of thioanisole for the S1 state due to the higher basis set. The observed high-resolution spectrum of the S1 ’ S0 0–0 band was purely of the b-type. It indicates that the transition moment is parallel to the b axis. The rotational principal axis a is slightly tilted from the central CC axis of the phenyl ring. This angle is about 101 when we assume the geometrical structure obtained by the MO calculations as shown in Fig. 4. Therefore, the a-type component is considered to be negligibly small. The observed results also indicated that the S1 state should be represented by the configuration of electronic excitation from HOMO to LUMO. Fig. 5 shows the HOMO and the LUMO obtained by the MP2/cc-pVTZ level calculation for the optimized molecular configuration in the S0 state. It seems that a transfer of electron density from the sulfur atom to the benzene ring occurs with the electronic excitation. It is expected that this transfer leads to the structural change of the SCH3 moiety. The CphSC angle increases from 104.21 to 105.11 with the electronic excitation (Fig. 4). It is reasonable that the electronic repulsion between the non-bonding sulfur orbital and the methyl group decreases due to the electron density change with the electronic excitation. Such an interaction was reported for anisole.22 This geometrical change is supported by the fact that the CphSC bending mode 15 was observed in the fluorescence excitation spectrum in a supersonic jet.10 The Cph–S bond length slightly decreases from 1.779 A˚ to 1.756 A˚. It is suggested that the order of the Cph–S bond changes with the electronic excitation. The CC bond length of the benzene ring also increases with the electronic excitation. This structural change of aromatic ring such as a breathing mode is reasonable for the benzene derivatives.19 The A value for the S1 state was remarkably smaller than that for the S0 state in the Table 1. As mentioned above, the Cph–S bond length and the CphSC angle changed with the electronic excitation. This geometrical change is the main cause of the difference in the A values. It is assumed to be a key factor of radiationless transitions in the zero-vibrational level of the S1 state, because the excited state dynamics of analogous molecules such as phenol and anisole have been shown to be sensitive to the structure in the CphOH and the 13246


Fig. 6 A part of the rotationally resolved fluorescence excitation spectrum of the S1 ’ S0 0–0 band with the external magnetic field of 0 (the upper trace) and 1 (the lower trace) Tesla.

CphOCH3 parts.17–22 Deuteration or methyl substitution remarkably changes the fluorescence lifetime in the S1 state. There are two candidates for the radiationless process on the zero-vibrational level of the S1 state, that is, the ISC to the triplet state and the IC to the ground state. Generally, ISC is expected to be promoted by heavy atom substitution such as a sulfur atom. To confirm the ISC process, the high-resolution spectrum with an external magnetic field of 1 Tesla was obtained. The magnetic field of 1 Tesla is usually satisfactory to observe changes of the transition energy and the line shape. The high-resolution spectra with external magnetic fields of 0 and 1 Tesla are shown in Fig. 6. However, no change was found in the transition energy and the line shape. Therefore, a mixing with the triplet state is considered to be negligibly small. It is concluded that the main radiationless process is not the ISC but the IC in the zero-vibrational level of the S1 state for thioanisole. The S1 state couples with the S0 state by the vibronic interaction through an in-plane vibration. The interaction strength is proportional to the matrix element27 0

00

Vvb / hCS1 j@=@Qi jCS0 ihwnS1¼0 j@=@Qi jwnS0 i;

ð1Þ

where CS0 and CS1 express the electronic wavefunctions of the S0 and the S1 states, respectively. Qi is a normal coordinate. 0 wnS1¼0 is the vibrational wavefunction of the zero-vibrational 00 level in the S1 state, and wnS0 is that of the high-vibrational level in the S0 state at the same energy. The matrix element of the vibrational part is roughly estimated by the overlap of vibrational wavefunctions between the S1 zero-vibrational level and the S0 high-vibrational level. When the equilibrium stable geometries of these levels are remarkably different, the matrix element becomes large, because the vibrational wavefunction of high-vibrational level is localized at the potential edge of which the coordinate is different from that of the zerovibrational level. For thioanisole, the specific in-plane vibration, the CphSC bending mode 15 was observed in the fluorescence spectra.10 High-level MO calculations predicted that the CphSC angle changes with the electronic excitation. Thus, it is considered that the S1 state couples with the S0 state by the vibronic interaction through the CphSC bending mode 15. This journal is

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The observed width of the rotational line was 120 MHz (0.004 cm1) and the line shape is approximated by a Lorentzian function. It was dominated by a lifetime broadening because residual Doppler width was estimated to be 40 MHz. The lifetime in the zero-vibrational level of the S1 state was evaluated to be 2.0 ns. It is indicated that the IC efficiently takes place in the zero-vibrational level of the S1 state. There would be another candidate for the radiationless process: predissociation through conical intersection. In the case of phenol, OH bond fission via a 1pp*/1ps* conical intersection was reported.28 The fluorescence intensity of thioanisole was rather weak, revealing that the fluorescence quantum yield should be remarkably low. The experiment detecting the photo-fragments will give us more information on the dissociative process. The lifetime of some benzene derivatives has been reported. In the case of anisole, it was reported that the lifetime in the zero-vibrational level of the S1 state was 19.9 ns.19 The lifetime of thioanisole was shorter than that of anisole. It is suggested that the sulfur substitution leads to promotion of the radiationless processes. The rotational lines are similarly broadened in phenol, and the lifetime is determined to be 3 ns17,18 and shorter than that of anisole. It should be noted that the methyl substitution suppresses the radiationless process. To elucidate the effect of methyl substitution for thioanisole, we attempted to observe the fluorescence spectrum of benzenethiol (C6H5SH). However, the experiment was not successful, indicating that the lifetime of benzenethiol should be much shorter than that of thioanisole.

4. Conclusion The rotationally resolved high-resolution electronic excitation spectrum of the 0–0 band of thioanisole in the S1 ’ S0 electronic excitation was observed using the techniques of a collimated supersonic jet and a single-mode ultraviolet laser for the first time. The accurate rotational constants for the S0 and the S1 states have been determined by precisely calibrated transition energies of about 1000 assigned rotational lines. The inertial defect was very small both in the S0 and the S1 states. It is concluded that the molecular structure of thioanisole is planar in the S0 and the S1 states. The geometrical structure of thioanisole was estimated by the obtained rotational constants and quantum chemical calculations. The structure in the CphSC part has been shown to remarkably change with electronic excitation. No change was found in the transition energy and the line shape of the high-resolution spectrum with an external magnetic field of 1 Tesla. It is indicated that the main radiationless process would be the IC to the S0 state. The structural change in the CphSC part was considered to be the main cause of mixing with the S0 state, and to enhance the IC. The observed lineshape was approximated by a Lorentzian function with a linewidth of 0.004 cm1. The lifetime in the zero-vibrational level of the S1 state was estimated to be 2.0 ns. It is suggested that the sulfur substitution leads to promotion of the radiationless processes. Further studies for the other vibrational bands will be necessary to elucidate the excited state dynamics in the S1 state of thioanisole. This journal is

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Rotationally resolved high-resolution spectrum of the S1âS0 transition

Rotationally resolved high-resolution spectrum of the S1âS0 transition

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