Spectroscopy and photophysics of organic clusters

Abstract

A supersonic free jet expansion has been used to prepare small clusters of aromatic organic compounds, and their structure and energy-transfer properties have been studied by laser-induced fluorescence. The dimer of s-tetrazine was found to have two observable conformations. In the first, the two monomer units were side by side in a planar configuration. In the second, the two molecules were in a T-shaped configuration. In the T-shaped geometry the two rings are not equivalent, and spectra resulting from excitation of one or the other ring could be distinguished. A mixed-gas expansion of tetrazine and benzene in helium produced a mixed tetrazine–benzene dimer, a trimer consisting of two tetrazines and one benzene, and bands that were attributed to larger clusters. The benzene–tetrazine dimer had a stacked, parallel-plate geometry, while the T₂B trimer had a benzene stacked parallel over a planar tetrazine dimer. Dimers of dimethyl tetrazine were found to be more tightly bound in the excited electronic state. Excitation of the 6a¹ ring mode in the dimethyl tetrazine dimer led to intramolecular vibrational relaxation into the cluster modes. Dissociation was observed following excitation of the 6a²₀ transition of the tetrazine–benzene dimer.