S-1/S-2 Exciton Splitting In The (2-Pyridone)(2) Dimer

The S-1<---->S-0 and S-2<---->S-0 vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)(2) and its C-13-, d(1)-, and d(2)-isotopomers were investigated 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 ((1)A(g)<---->(1)A(g)) transition is forbidden. A single H/D or C-12/C-13 isotopic substitution reduces the symmetry to C-s, so that (2PY)(2)-C-13 and (2PY)(2)-d(1) both exhibit S-1<---->S-0 and S-2<---->S-0 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 different, 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 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. (C) 2002 American Institute of Physics.