Photophysical study of 5-substituted benzofurazan compounds as fluorogenic probes

Abstract

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 Φ_f values. The Φ_f 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 Φ_f values in each solvent, thus enabling us to predict the fluorescence properties of the 5-substituted benzofurazans based on their chemical structures.