Photophysical study of 5-substituted benzofurazan compounds as fluorogenic probes

Nine 5-substituted benzofurazans including the non-substituted benzofurazan were synthesized, and measurements of the fluorescence, photolysis, transient absorption, and time-resolved thermal lensing signal were performed in order to understand the relaxation processes of these compounds. The results indicated that the main relaxation process was a photoreaction from an excited singlet state and the rate of the primary bond cleavage in the excited state tended to increase in the compound in which the S-2 <-- S-0 absorption band is located close to the S-1 <-- S-0 absorption band. These results suggest that the reactive state might be the S-2 state, and that the interaction between the S-1 and S-2 states promotes the photoreaction. The separation of the S-1 state from the S-2 state decreases the rate of photoreaction, resulting in an increase in the Phi(f) values. The Phi(f) values of the 5-substituted benzofurazans in cyclohexane and acetonitrile were compared with their DeltaE(S-1, S-2) (energy gap between the S-1 and S-2 states) values calculated by using a combination of AM1(EXCITED) and AM1-CAS/CI(CI = 6) for geometric optimization and calculation of the energy levels, respectively. The DeltaE(S-1, S-2) values correlated well with the Phi(f) values in each solvent, thus enabling us to predict the fluorescence properties of the 5-substituted benzofurazans based on their chemical structures.