Modulation of Triplet-State Reactivity and Enhanced Singlet Oxygen Generation in Tricyclic Thiopurine Analogues

Thiopurines are efficient triplet-state photosensitisers; however, the practical application of canonical derivatives such as 6-thioguanine (6TG) and 6-thioguanosine (6TGuo) is limited by competing deactivation pathways that reduce the fraction of triplet states available for productive interaction with molecular oxygen. In this work, we investigated how structural modification of the thiopurine scaffold through introducing of an additional five-membered etheno ring affects triplet-state energetics, deactivation pathways, and singlet oxygen sensitisation. The photophysical properties of four tricyclic thiopurine analogues—9-thio-1,N2-ethenoguanine (TEGua), 9-thio-1,N2-ethenoguanosine (TEGuo), 6-methyl-9-thio-1,N2-ethenoguanine (6MeTEGua), and 6-methyl-9-thio-1,N2-ethenoguanosine (6MeTEGuo)—were investigated using steady-state spectroscopy, low-temperature phosphorescence, nanosecond transient absorption spectroscopy, and direct detection of singlet oxygen phosphorescence. All investigated compounds exhibited efficient intersystem crossing and microsecond-lived triplet states. Compared with canonical thiopurines, the tricyclic analogues displayed lower triplet-state energies and significantly enhanced singlet oxygen generation. Quantum yields of singlet oxygen sensitisation reached ~0.56 in acetonitrile, approximately twofold higher than those observed for 6TG and 6TGuo under identical conditions. Analysis of triplet-state deactivation pathways showed that the enhanced photosensitising efficiency does not result from increased triplet formation, but from more effective use of the triplet-state population for energy transfer to molecular oxygen leading to singlet oxygen formation. These findings demonstrate that structural modification of the thiopurine scaffold enables control over triplet-state reactivity and provides a strategy for designing improved thiopurine-based photosensitisers for photodynamic therapy applications (PDT).