Influence of an identified dimer vibration on the emission spectrum of [2,2]paracyclophane

Abstract

The emission spectrum of polycrystalline [2,2]paracylophane shows a resolved vibronic structure with a 241 cm−1 progression at He temperatures. The dependence of the energy of this mode upon selective deuteration in combination with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration. The emission spectra could be simulated assuming a linear coupling of the torsional mode to the electronic transitions with coupling strengths of S = 10 (fluorescence) and S = 13 (phosphorescence). This corresponds to an equilibrium displacement of the benzene rings under electronic excitation by a torsional angle of 10.6° (S1) and 12.1° (T1), in addition to the small torsion in the ground state S0 by about 3°.