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 S2 ← S0 absorption band is located close to the S1 ← S0 absorption band. These results suggest that the reactive state might be the S2 state, and that the interaction between the S1 and S2 states promotes the photoreaction. The separation of the S1 state from the S2 state decreases the rate of photoreaction, resulting in an increase in the Φf values. The Φf values of the 5-substituted benzofurazans in cyclohexane and acetonitrile were compared with their ΔE(S1, S2) (energy gap between the S1 and S2 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 ΔE(S1, S2) values correlated well with the Φf values in each solvent, thus enabling us to predict the fluorescence properties of the 5-substituted benzofurazans based on their chemical structures.