The formation of cyclobutane pyrimidine dimers (CPDs) by a “dark” pathway in melanocytes has been attributed to chemisensitization by dioxetanes produced from peroxynitrite oxidation of melanin or melanin precursors. These dioxetanes are proposed to decompose to triplet state compounds which sensitize CPD formation by triplet–triplet energy transfer. To determine whether such compounds are capable of sensitizing CPD formation, the putative decomposition products of 2,3-dioxetanes of variously substituted indoles were synthesized and their triplet state energies determined at 77 K. Their ability to photosensitize CPD formation was determined by an enzyme-coupled gel electrophoresis assay in comparison with norfloxacin (NFX) which has the lowest triplet energy known to sensitize CPD formation. The decomposition products of 2,3-dioxetanes of 5-hydroxy and 5,6-dimethoxy indoles used as models for melanin precursors had lower triplet energies and were incapable of photosensitizing CPD formation. Theoretical calculations suggest that the decomposition products of the 2,3-dioxetanes of melanin precursors DHI and DHICA will have similarly low triplet energies. Decomposition products of the 2,3-dioxetanes of indoles lacking oxygen substituents had higher triplet energies than NFX and were capable of photosensitizing CPD formation, suggesting that peroxynitrite oxidation of tryptophan could play a hitherto unrecognized role in the dark pathway to CPDs.