EXCITONIC INTERACTIONS AND EXCIMER FORMATION IN PURE AND MIXED CLUSTER ISOTOPOMERS OF NAPHTHALENE

The nature of electronic excitation and subsequent excited-state dynam ics in small naphthalene clusters has been elucidated using isotope la beling techniques. The S-1 <-- S-0 spectra of tetramer isotopomers are characterized by structurally inequivalent site splitting which is in duced by an exciton interaction in the S-2 State through vibronic coup ling. It is suggested that the tetramer involves a pair of nearly over lapped chromophores, which is responsible for the excitonic interactio n, and other two chromophores acting as solvents. An excitonic state o riginating in this pair is proposed to correlate with the lowest excim er state of the cluster as the interplanar separation and dihedral ang le are reduced. In contrast, the spectra of trimer isotopomers are ana lyzed by invoking a symmetric geometry in which three chromophores are less overlapped and thus give rise to weak excitonic effects. The iso merization dynamics of these clusters is discussed in terms of a vibra tionally activated process which makes the cluster framework wobble, a llowing for stronger exciton interaction. (C) 1995 American Institute of Physics.