Preparation of ketocoumarins as heavy atom-free triplet photosensitizers for triplet–triplet annihilation upconversion

Abstract

A series of ketocoumarin compounds were prepared as heavy atom-free triplet photosensitizers. The photophysical properties of the compounds were studied with steady state and time-resolved spectroscopy. The compounds show absorption in the visible spectral region (molar absorption coefficients are up to ε = 136 000 M⁻¹ cm⁻¹ at 448 nm) and long-lived triplet excited states (τ_T = 199.7 μs) upon visible light photoexcitation. The compounds were used as triplet photosensitizers for singlet oxygen (¹O₂)-mediated photooxidation of 1,5-dihydroxylnaphthalene (DHN) to produce juglone. ¹O₂ quantum yields of these compounds were determined in the range of 0.28–0.48. The ketocoumarins were also used as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion, and upconversion quantum yields up to 11.3% were observed. The results are useful for preparation of heavy atom-free triplet photosensitizers and for their application in photocatalysis and TTA upconversion.