The nature of electronic excitation and subsequent excited-state dynamics in small naphthalene clusters has been elucidated using isotope labeling techniques. The S-1 <-- S-0 spectra of tetramer isotopomers are characterized by structurally inequivalent site splitting which is induced by an exciton interaction in the S-2 State through vibronic coupling. It is suggested that the tetramer involves a pair of nearly overlapped chromophores, which is responsible for the excitonic interaction, and other two chromophores acting as solvents. An excitonic state originating in this pair is proposed to correlate with the lowest excimer state of the cluster as the interplanar separation and dihedral angle are reduced. In contrast, the spectra of trimer isotopomers are analyzed by invoking a symmetric geometry in which three chromophores are less overlapped and thus give rise to weak excitonic effects. The isomerization dynamics of these clusters is discussed in terms of a vibrationally activated process which makes the cluster framework wobble, allowing for stronger exciton interaction.