The Triplet Excimer of Naphthalene: A Model System for Triplet−Triplet Interactions and Its Spectral Properties

Basic concepts of triplet excimer formation and triplet−triplet interactions between molecules with conjugated π-systems are investigated by means of ab initio quantum chemical calculations, employing the second-order coupled-cluster method CC2 and the second-order propagator method ADC(2). The naphthalene dimer turns out to be a very fruitful model system for which weak and strong electronic coupling can be identified depending on the mutual arrangement of the monomer moieties. From geometry optimizations in the excited state, we determine binding energies, including solvent effects, and transient absorption spectra. The most stable T1 conformation turns out to be a face-to-face arrangement with a rather short intermolecular distance. Its stabilization can be rationalized by a very strong electronic coupling due to a maximum overlap of the π-systems, which allows a strong admixture of charge-transfer configurations. Conformations without π−π-overlap show only a weak coupling and are less stable. The predicted transient Tn ← T1 spectrum of the face-to-face dimer is in agreement with experimental observations for the triplet excimer, while all other considered conformations give rise to a transient spectrum, which closely resembles that of the monomer. In addition, we study the dependence of the triplet excitation manifold on the intermolecular distance, which yields valuable information on the nature of the triplet excited states in various conformations. The findings are likely to carry over to excimers of other aromatic molecules. We also investigate π-stacked naphthalene oligomers (up to the pentamer), which might be used as a model system for excimer formation and propagation in larger π-stacks. These can occur in amorphous layers of similar molecules, and the model may contribute to the understanding of the elementary processes in organic semiconductors.