Photodissociation of 4-azidopyridine and 4-azidoquinoline in neutral and cationic forms

The quantum yields of azido group photodissociation were determined for 4-azidopyridine and 4-azidoquinoline and their protonated and methylated derivatives at endocyclic nitrogen atoms. The dissociation yields are 0.49-0.83 (in MeCN) for neutral azides and decrease to 0.22-0.37 for the positively charged derivatives. The semiempirical and nonempirical quantum-chemical calculations were performed to interpret experimental data. For all azides studied, in the ground (S0) state, the calculations predict rather large positive charge at two terminal nitrogen atoms of the azido group. In the lowest excited singlet (S1) state, the antibonding σNN* molecular orbital is filled that results in relatively high quantum yields of photodissociation (φ > 0.1) for all azides. The decrease of quantum yields on going from neutral to charged azides appears to be connected with increasing barrier for electron density transfer from positively charged aromatic nucleus to azido group.