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

The quantum yield of singlet oxygen (1Δg) production (ΦΔ) 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 (fSΔ) in the quenching of the lowest excited singlet state (S1) was estimated in several solvents. In methylcyclohexane, fSΔ was approximately zero for anthracene and its derivatives possessing electron-donating groups of methyl and methoxy, and their rate constants of oxygen quenching (kSq = (26–31) × 109 M−1 s−1) are nearly in a diffusion-controlled limit. On the other hand, values of fSΔ for 9-cyanoanthracene (CNA) and 9,10-dichloroanthracene (DCLA) possessing electron-withdrawing groups were 0.42 and 0.45, respectively, and values of kSq were (9.0 and 11) × 109 M−1 s−1, respectively, which are definitely smaller than the diffusion-controlled rate constant. The values of fSΔ 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 kSq (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 fSΔ and the increase of kSq 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 fSΔ and kSq, suggesting the involvement of charge transfer interaction in the oxygen quenching.