S[sub 1]/S[sub 2] exciton splitting in the (2-pyridone)[sub 2] dimer.

The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)[sub 2] and its [sup 13]C-, d[sub 1]-, and d[sub 2]-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C[sub 2h] symmetric (2PY)[sub 2]–h[sub 2] and (2PY)[sub 2]–d[sub 2] complexes, the 2PY moieties are equivalent and the S[sub 1]↔S[sub 0] ([sup 1]A[sub g]↔[sup 1]A[sub g]) transition is forbidden. A single H/D or [sup 12]C/[sup 13]C isotopic substitution reduces the symmetry to C[sub s], so that (2PY)[sub 2]–[sup 13]C and (2PY)[sub 2]–d[sub 1] both exhibit S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] transitions. The S[sub 1]/S[sub 2] state exciton splittings are 43.6 cm[sup -1] and 52.4 cm[sup -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)[sub 2]–d[sub 1], whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] spectra are analyzed and assigned. Several b[sub u] intermolecular vibrations of S[sub 1] appear via vibronic coupling to the S[sub 2](B[sub u]) state. Combination of the fluorescence data from excitation of the S[sub 1] and S[sub 2] origins and vibrational excitations of (2PY)[sub 2]–h[sub 2], (2PY)[sub 2]–d[sub 1], and (2PY)[sub 2]–d[sub 2] allows the determination of the six S[sub 0] state intermolecular vibrational frequencies.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]