W. Adam et al., Influence of bridgehead substitution and ring annelation on the photophysical properties of polycyclic DBO-type azoalkanes, J ORG CHEM, 64(10), 1999, pp. 3695-3698
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.