Yasuyuki Kowaka

Vibrational and rotational structure and excited-state dynamics of pyrene

Vibrational level structure in the S(0) (1)A(g) and S(1) (1)B(3u) states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm(-1). This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S(1) (1)B(3u) state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0(0) (0) band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S(0) and S(1) states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of M(J) levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S(0) state is also slow. We conclude that the similarity of pyrenes molecular structure and potential energy curve in its S(0) and S(1) states is the main cause of the slow radiationless transitions.

Geometrical structure of benzene and naphthalene: ultrahigh-resolution laser spectroscopy and ab initio calculation.

Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D(6h)) and the structure has been determined with accuracies of 2 × 10(-14) m (0.2 mÅ; 1 Å = 1 × 10(-10) m) for the C-C bond length and 1.0 × 10(-13) m (1.0 mÅ) for the C-H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D(2h)) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.

Mode-selective internal conversion of perylene

We observed fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation of jet-cooled perylene-h 12 and perylene-d 12, and carefully examined the vibrational structures of the S0 1 A g and S1 1 B 2u states. We performed vibronic assignments on the basis of the results of ab initio calculation, and found that the vibrational energies in the S1 state are very similar to those in the S0 state, indicating that the potential energy curves are not changed much upon electronic excitation. We conclude that the small structural change is the main cause of its slow radiationless transition and high fluorescence quantum yield at the zero-vibrational level in the S1 state. It has been already reported that the lifetime of perylene is remarkably short at specific vibrational levels in the S1 state. Here, we show that the mode-selective nonradiative process is internal conversion (IC) to the S0 state, and the ν16(a g ) in-plane ring deforming vibration is the promoting (doorway) mode in the S1 state which enhances vibronic coupling with the high-vibrational level (b 2u ) of the S0 state.

CH3 internal rotation in the S0 and S1 states of 9-methylanthracene.

Fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled 9-methylanthracene-h12 and -d12 (9MA-h12 and 9MA-d12) have been observed, and the energy levels of methyl internal rotation (CH3 torsion) in the S0 and S1 states have been analyzed. The molecular symmetry of 9MA is the same as that of toluene (G12). Because of two-fold symmetry in the pi system, the potential curve has six-fold barriers to CH3 rotation. In toluene, the barrier height to CH3 rotation V6 is very small, nearly free rotation. As for 9MA-h12, we could fit the level energies by potential curves with the barrier heights of V6(S0) = 118 cm(-1) and V6(S1) = 33 cm(-1). These barrier heights are remarkably larger than those of toluene and are attributed to hyperconjugation between the pi orbitals and methyl group. The dispersed fluorescence spectrum showed broad emission for the excitation of 0(0)(0) + 386 cm(-1) band, indicating that intramolecular vibrational redistribution efficiently occurs, even in the vibronic level of low excess energy of the isolated 9MA molecule.

Electronic, vibrational, and rotational structures in the S0 1A1 and S1 1A1 states of phenanthrene

Electronic and vibrational structures in the S(0) (1)A(1) and S(1) (1)A(1) states of jet-cooled phenanthrene-h(10) and phenanthrene-d(10) were analyzed by high-resolution spectroscopy using a tunable nanosecond pulsed laser. The normal vibrational energies and molecular structures were estimated by ab initio calculations with geometry optimization in order to carry out a normal-mode analysis of observed vibronic bands. The rotational structure was analyzed by ultrahigh-resolution spectroscopy using a continuous-wave single-mode laser. It has been demonstrated that the stable geometrical structure is markedly changed upon the S(1) ← S(0) electronic excitation. Nonradiative internal conversion in the S(1) state is expected to be enhanced by this structural change. The observed fluorescence lifetime has been found to be much shorter than the calculated radiative lifetime, indicating that the fluorescence quantum yield is low. The lifetime of phenanthrene-d(10) is longer than that of phenanthrene-h(10) (normal deuterium effect). This fact is in contrast with anthracene, which is a structural isomer of phenanthrene. The lifetime at the S(1) zero-vibrational level of anthracene-d(10) is much shorter than that of anthracene-h(10) (inverse deuterium effect). In phenanthrene, the lifetime becomes monotonically shorter as the vibrational energy increases for both isotopical molecules without marked vibrational dependence. The vibrational structure of the S(0) state is considered to be homogeneous and quasi-continuous (statistical limit) in the S(1) energy region.