Fluorescence Excitation Spectrum, Lifetimes and Photoisomerization of Jet-Cooled Conformers of 1,1-bi(benzocyclobutylidene)

First measurements of fluorescence excitation spectra of the recently synthesized rigid stilbene analogue 1,1′-bi(benzocyclobutylidene) in a supersonic jet expansion show that, in contrast to the parent compound, both the trans- and the cis-conformer fluoresce under these conditions. The excitation energy dependence of fluorescence lifetimes indicates the onset of an efficient non-radiative decay channel above energy thresholds of 1340 cm^-1 and 990 cm^-1 for the trans- and cis-form, respectively, which is assigned to photoisomerization in the singlet state. From an RRKM analysis of the microcanonical rate coefficients an estimate of the high pressure limit of the thermal photoisomerization rate coefficient is obtained and compared with photoisomerization rate coefficients measured in low viscosity solution and in thermal vapor. There are strong indications that for this compound there are no dynamic or static solvent induced effects that lead to an anomalous acceleration of the reaction in solution.