In contrast to our findings on thiophenol and its substituted derivatives, thionaphthol triplet formation was found after UV photo-excitation in various solvents. The T-1-T-n absorption spectra were characterized by a pronounced absorption band with lambda(max) = 430-470 nm. The triplet states reactivity with triplet quenchers and their energy levels were determined. The triplet quantum yields were estimated to be 0.15 and 0.65 for 1-NpSH and 2-NpSH, respectively. The experimental results were interpreted by quantum calculations using the DFT B3LYP/6-311+G(d,p) method, which indicated that the small S-1 -T-n=2,T-3 energy gap and different orbital configurations account for the direct triplet formation. (C) 2009 Elsevier B. V. All rights reserved.