S_1/S_2 exciton splitting in the (2-pyridone)_2 dimer

摘要

The S_1<->S_0 and S_2<->S_0 vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)_2 and its ~(13)C-, d_1~-, and d_2-isotopomers wer einvestigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C_(2h) symmetric (2PY)_2-h_2 and (2PY)_2-d_2 complexes, the 2PY moieties are equivalent and the S_1<->S_0 (~1A_g<->~1A_g) transition is forbidden. A single H/D or ~(12)C/~(13)C isotopic substitution reduces the symmetry to C_s, so that (2PY)_2-~(13)C and (2PY)_2-d_1 both exhibit S_1<->S_0 and S_2<->S_9 transitions. The S_1/S_2 state exciton splittings are 43.6 cm~(-1) and 52.4 cm~(-1), respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)_2-d_1, whose 2PY subunits are differnet, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S_1<->S_0 and S_2<->S_0 spectra are analyzed and analyzed and assigned. Several b_u intermolecular vibrations of S_1 appear via vibronic coupling to the S_2(B_u) state. Combination of the fluorescence data from excitation of the S_1 and S_2 origins and vibrational excitations of (2PY)_2-h_2, (2PY)_2-d_1,and (2PY)_2-d_2 allows the determination of the six S_0 state intermolecular vibrational frequencies.