Photochemical characteristics of cyanine dyes. Part 1.—3,3′-diethyloxadicarbocyanine iodide and 3,3′-diethylthiadicarbocyanine iodide

The dyes 3,3′-diethyloxadicarbocyanine iodide (DODI) and 3,3′-diethylthiadicarbocyanine iodide (DTDI) were investigated using laser and conventional flash photolysis techniques. In both cases direct excitation results in the formation of a photo-isomer having an absorption similar to and slightly to the red of the normal ground state absorption. The quantum efficiency for photo-isomer formation in both cases is less than 0.1. The quantum efficiency for intersystem crossing to the triplet state was observed to be < 0.01 in both cases and the triplet state extinction coefficients and spectra were therefore obtained by energy transfer techniques. The quantum efficiencies for fluorescence and radiative lifetimes obtained indicated that the fluorescence lifetimes were 1.24 and 1.20 ns for DODI and DTDI respectively.