S(1)/S(2) excitonic splittings and vibronic coupling in the excited state of the jet-cooled 2-aminopyridine dimer.

We analyze the vibronic band structure of the excitonically coupled S(1)<--S(0)/S(2)<--S(0) excitations of the 2-aminopyridine (2AP) self-dimer (2AP)(2), using a linear vibronic coupling model [R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964)]. The vibronic spectra of supersonically cooled (2AP)(2) and its (13)C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C(2)-symmetric form of (2AP)(2), the S(1)<--S(0) ((1)A<--(1)A) transition is very weak, while the close-lying S(2)<--S(0) ((1)B<--(1)A) transition is fully allowed. A single (12)C/(13)C isotopic substitution breaks the symmetry of the dimer so that the (2AP)(2)-(13)C isotopologue exhibits both S(1) and S(2) electronic origins, which are split by 11 cm(-1). In Fulton-Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)(2), a major vibronic coupling contribution arises from the intramolecular 6a(') vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (chi(')) and stretching (sigma(')) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals V(AB) are 50%-80% of that of the intramolecular 6a(') vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S(1)/S(2) electronic energy gap calculated by the approximate second-order coupled-cluster method is approximately 340 cm(-1). This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck-Condon active intramolecular vibrations.