Influence of bridgehead substitution and ring annelation on the photophysical properties of polycyclic DBO-type azoalkanes

The photophysical data for the polycyclic, bridgehead-substituted derivativ es 1-10 of the photo-reluctant diazabicyclo[2.2.2]oct-2-ene (DBO) are prese nted. Substitution on the bridgehead positions with radical-stabilizing sub stituents enhances the photoreactivity (Phi(r)) and decreases the fluoresce nce quantum yields (Phi(f)) and lifetimes (tau) compared to the parent DBO. The annelated rings have no influence on the photoreactivity, except when steric interactions with an a substituent hinder the optimal radical-stabil izing conformation. The fused rings and some of the bridgehead substituents reduce the solvent-induced quenching of the singlet-excited azo chromophor e by steric shielding of the azo group and, thus, increase the fluorescence quantum yields and lifetimes.