Photochemical Synthesis of 4H-Thieno[3,2-c]chromene and Their Optical Properties

4-{[(2-Iodoaryl)oxy]methyl}thiophene-2-carbaldehydes and 5-iodo-4-(aryloxymethyl)thiophene-2-carbaldehydes were obtained by the reaction of phenols with 4-(chloromethyl)thiophene-2-carbaldehyde or its 5-iodo analogue, respectively. These products underwent ring closure upon irradiation with UV light (254 nm) to form the corresponding 4H-thieno[3,2-c]chromene-2-carbaldehydes in high yield. The formation of intermediate radical species was detected by EPR spectroscopy. Comparative analysis of ring-closure methods showed that photochemical cyclization of 5-iodo-4-(aryloxymethyl)thiophene-2-carbaldehyde is superior to Pd-catalyzed intramolecular arylation. A series of substituted 4H-thieno[3,2-c]chromene-2-carbaldehydes were synthesized by the photochemical cyclization of the corresponding precursors, and the photophysical properties of the products were studied. The 4H-thieno[3,2-c]chromene-2-carbaldehydes can be used as covert marking pigments.