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 pi-systems are investigated by means of ab initio quantum chemical calculations, employing the second-order coupled-cluster method CC:2 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 T(1) 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 pi-systems, which, allows a strong admixture of charge-transfer configurations. Conformations without pi-pi-overlap show only a weak coupling and are legs stable. The predicted transient T(n) <- T(1) 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 pi-stacked naphthalene oligomers (up to the pentamer), which might be used as a model system for excimer formation and propagation in larger pi-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.