Characterization of Photoinduced Isomerization and Intersystem Crossing of the Cyanine Dye Cy3

Several important photophysical properties of the cyanine dye Cy3 have been determined by laser flash photolysis. The triplet-state absorption and photoisomerization of Cy3 are distinguished by using the heavy-atom effects and oxygen-induced triplet → triplet energy transfer. Furthermore, the triplet-state extinction coefficient and quantum yield of Cy3 are also measured via triplet−triplet energy-transfer method and comparative actinometry, respectively. It is found that the triplet → triplet (T1→Tn) absorptions of trans-Cy3 largely overlap the ground-state absorption of cis-Cy3. Unlike what occurred in Cy5, we have not observed the triplet-state T1→Tn absorption of cis-Cy3 and the phosphorescence from triplet state of cis-Cy3 following a singlet excitation (S0−S1) of trans-Cy3, indicating the absence of a lowest cis-triplet state as an isomerization intermediate upon excitation in Cy3. The detailed spectra of Cy3 reported in this paper could help us interpret the complicated photophysics of cyanine dyes.