Inverse correlation between efficiency of singlet oxygen production and rate constant for oxygen quenching in the S1 state of anthracene derivatives

Abstract

The quantum yield of singlet oxygen (¹Δ_g) production (Φ_Δ) in the quenching by oxygen of photoexcited anthracene derivatives was measured with a time-resolved thermal lensing (TRTL), and the efficiency of singlet oxygen production (f^S_Δ) in the quenching of the lowest excited singlet state (S₁) was estimated in several solvents. In methylcyclohexane, f^S_Δ was approximately zero for anthracene and its derivatives possessing electron-donating groups of methyl and methoxy, and their rate constants of oxygen quenching (k^S_q = (26–31) × 10⁹ M⁻¹ s⁻¹) are nearly in a diffusion-controlled limit. On the other hand, values of f^S_Δ for 9-cyanoanthracene (CNA) and 9,10-dichloroanthracene (DCLA) possessing electron-withdrawing groups were 0.42 and 0.45, respectively, and values of k^S_q were (9.0 and 11) × 10⁹ M⁻¹ s⁻¹, respectively, which are definitely smaller than the diffusion-controlled rate constant. The values of f^S_Δ for CNA and DCLA depended significantly on the solvent, and these values were 0.50, 0.46, 0.45, 0.16 and 0.03 ∼ 0.18 for CNA and 0.68, 0.54, 0.56, 0.12 and 0.23 for DCLA in hexane, dodecane, cyclohexane, benzene and acetonitrile, respectively. The corresponding values of k^S_q (M⁻¹ s⁻¹) are (7.7, 8.3, 8.6, 12 and 15) × 10⁹ for CNA and (13, 13, 11, 21, 31) × 10⁹ for DCLA, respectively. These results on the solvent effect indicate the decrease of f^S_Δ and the increase of k^S_q in benzene and acetonitrile when compared to those values in nonpolar solvents except benzene. The results obtained from both substituent and solvent effects represent the inverse correlation between f^S_Δ and k^S_q, suggesting the involvement of charge transfer interaction in the oxygen quenching.