Development of phenazine-2,3-diol-based photosensitizers: effect of substitution of the cyano group for the nitro group on singlet oxygen generation

Abstract

Cyano-substituted phenazine-2,3-diol-based dyes, YC-1 and YC-2, which have n-hexyl and formyl groups, respectively, on thiophene units, have been newly developed as halogen-atom-free-heteroanthracene-based photosensitizers (PSs) possessing the ability to generate singlet oxygen (¹O₂). The photoabsorption edge (λ^abs_edge) of YC-1 reached 700 nm which is in a longer wavelength region by 50 nm in comparison with that (650 nm) of YC-2. However, the λ^abs_edge of YC-1 and YC-2 exhibited blue-shifts, compared to those (700 nm and 800 nm) of nitro-substituted phenazine-2,3-diol-based dyes KI-3 and KI-5 with formyl groups and the n-hexyl group, respectively, in our previous work. KI-3 and KI-5 show low fluorescence and ¹O₂ generation quantum yields (Φ_fl < 0.01 and Φ_Δ = 0.05 and 0.02, respectively), which are due to the rapid nonradiative decay of the excited states by the nitro group. YC-1 and YC-2 exhibited also feeble fluorescence with low Φ_fl values (<0.01 and 0.022, respectively), meanwhile YC-2 showed a moderate Φ_Δ value (0.29) but the Φ_Δ value (0.0081) of YC-1 was significantly lower than those of the other PSs. Time-dependent density functional theory (TDDFT) calculations revealed that YC-2 with the formyl groups has the smallest energy gap ΔE_ST (−0.05 eV) between the S₁ (ππ*) state and the T_n state with nπ* characteristics among these PSs (−0.12 eV for KI-3, −0.36 eV for KI-5, and −0.24 eV for YC-1), leading to efficient intersystem crossing (ISC) according to the El-Sayed rule. In addition, it was suggested that the low Φ_Δ value of YC-1 with n-hexyl groups is due to the relatively large ΔE_ST value as well as the flexible n-hexyl groups which accelerate internal conversion (IC) from the S₁ to S₀ state, as evidenced by the low Φ_fl values, resulting in inferior ISC. Consequently, we demonstrated that cyano-substituted phenazine-2,3-diol-based PSs reduce the rapid nonradiative decay of the excited states and the ΔE_ST values, leading to efficient ISC for ¹O₂ generation, compared to nitro-substituted phenazine-2,3-diol-based PSs.