Synthesis, luminescence and excited state absorption properties of conjugated D-π-A and D-π-D phenothiazine compounds

Four D-π-A or D-π-D phenothiazine compounds bearing different substituents on central π-bridge (R = -H (PTZ-1), -Br (PTZ-2), -CN (PTZ-3) and -OC₁₂H₂₅ (PTZ-4)) were synthesized and characterized. The influence of the substituents on the photophysics of these compounds is systematically investigated by spectroscopic methods and simulated by density functional theory (DFT) calculations. All compounds exhibit intense absorption bands in UV and visible region, which are influenced by the different substituents on the central π-bridge. Both electron-withdrawing and electron-donating substituents cause bathochromic shifts of the low energy ¹π-π*/¹ICT (intramolecular charge transfer) absorption bands. Meanwhile, all compounds are emissive in solution at room temperature (λ_em = 400–800 nm, Φ_em = 0.01–0.74). Their fluorescent quantum yields decrease when the electron-withdrawing substituents attached on the central π-bridge, but increase when electron-donating substituents were introduced. In addition, PTZ-1–PTZ-4 all exhibit strong triplet transient absorption (TA) in visible region. When the electron-donating substituents attached, the triplet excited-state molar extinction coefficient (190200 L mol⁻¹ cm⁻¹) and triplet excited-state lifetime (53.2 μs) of PTZ-4 were significantly enhanced. Their luminescent and excited state absorption properties could provide us a basis for elucidating the structure-property correlations of phenothiazine compounds and developing novel optical functional materials.