Synthesis of novel π-extended D–A–D-type dipyrido[3,2-a:2′,3′-c]phenazine derivatives and their photosensitized singlet oxygen generation

Electron donor–acceptor–donor (D–A–D) π-conjugated molecules based on dipyrido[3,2-a:2′,3′-c]phenazine (dppz) were developed as photosensitizers for singlet oxygen generation. These dppz derivatives showed electronic absorption assigned to an intramolecular charge transfer transition in the red region at a wavelength of around 600 nm. Evaluation of singlet oxygen generation quantum yield (ΦΔ) using the trapping agent 1,3-diphenylisobenzofuran showed relatively efficient singlet oxygen generation (ΦΔ; 0.173–0.255) for the dppz derivatives with selenophene-containing π-conjugated side-arms. On the other hand, 2,7-dibromo substitution and Pt(II)Cl2 complexation of the dppz derivative with thiophene-containing side-arms did not facilitate photosensitized singlet oxygen generation. Density functional theory (DFT) and time-dependent DFT calculations, as well as evaluation of the kinetics of the photophysical processes, revealed that appropriate placement of heavy atoms on the frontier orbitals facilitates intersystem crossing from the lowest singlet excited state to the triplet excited state, thus improving ΦΔ.