A new cyanoaromatic photosensitizer vs. 9,10-dicyanoanthracene: systematic comparison of the photophysical properties

The cyanoanthracene derivative, benzo[b]triphenylene-9,14-dicarbonitrile (1) can be prepared readily with a graftable function while maintaining O-1(2) photosensitizing properties comparable to those of the standard compound 9,10-dicyanoanthracene (DCA). In view of the high potential of the derivatives of 1 for photooxidation reactions under heterogeneous conditions, we compared the photophysical properties of 1 in solution with those of DCA. In pursuing the comparison of 1 and DCA, we observed small but significant changes of the vibronic bands in the electronic absorption spectra of DCA in different solvents, which were well correlated with solvent polarity, similar to the pyrene polarity scale. The main difference between 1 and DCA is in the emission properties: we observed a much stronger sensitivity of the fluorescence emission spectrum to the electron-donating ability of the solvent than for DCA. The emission spectrum of 1 is in general structureless with a large Stokes shift. The ability of the singlet state of 1 to participate in charge transfer interactions with electron-donating solvents is proposed to account for these results. It makes 1 a highly sensitive probe to the surrounding medium. Reversible reduction was observed for both photosensitizers, with a small shift to more negative potentials for 1 compared to DCA. The reduction potential of the first singlet excited state is of the same order of magnitude in both cases. Several photo-oxidation reactions sensitized by 1 and DCA are compared in homogeneous solution and at the gas-solid interface by embedding 1 and DCA in silica monoliths. Our results confirmed the dual character of both cyanoanthracene derivatives as electron transfer and energy transfer sensitizers, highly efficient for singlet oxygen production.