BODIPY triads triplet photosensitizers enhanced with intramolecular resonance energy transfer (RET): broadband visible light absorption and application in photooxidation

Abstract

Resonance energy transfer (RET) was used to enhance the light absorption in triad triplet photosensitizers to access strong and broadband absorption in visible region (from 450–750 nm). This strategy was demonstrated by preparation of (BODIPY)₂-diiodo-aza-BODIPY triad (B-2) and (carbazole-styryl BODIPY)₂-diiodo-aza-BODIPY triad (B-3), in which the energy donor (BODIPY or styryl-BODIPY) and the energy acceptor (aza-BODIPY, also as the spin converter) parts were connected by click chemistry. Both the energy donors and the energy acceptors show strong absorption in the visible spectral region, but at different wavelengths, therefore the triads show broadband absorption in visible spectra region, e.g. the two major absorption bands of B-3 are located at 593 nm and 683 nm, with ε up to 220 000 M⁻¹ cm⁻¹ and 81 000 M⁻¹ cm⁻¹, respectively. For comparison, a reference compound with only diiodo-aza-BODIPY as the light-harvesting unit was prepared (B-1), which shows only one major absorption band in visible spectral region. Fluorescence studies indicated intramolecular energy transfer for these BODIPY hybrids, a conclusion which is supported by the femtosecond time-resolved transient absorption spectroscopy. Nanosecond transient absorption spectra show that triplet excited states of the dyad and the triad are localized on the iodo-aza-BODIPY part. The compounds were used as triplet photosensitizers for singlet oxygen (¹O₂) mediated photooxidation of 1,5-dihydroxylnaphthalene and the photosensitizing ability of the new triplet photosensitizers are more efficient than the mono-chromophore based triplet photosensitizers. The molecular design rationale of these RET-enhanced multi-chromophore triplet photosensitizer is useful for development of efficient triplet photosensitizers and for their applications in photocatalysis, photodynamic therapy, photovoltaics and upconversion.