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

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 (τT = 199.7 μs) upon visible light photoexcitation. The compounds were used as triplet photosensitizers for singlet oxygen ((1)O2)-mediated photooxidation of 1,5-dihydroxylnaphthalene (DHN) to produce juglone. (1)O2 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.