Photochemistry of oxygenated and deoxygenated solutions of the photosensitizer (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-cyclopentanone, a ketocyanine dye

Photochemical studies of (2E,5E)-2,5-bis(4-dimethylaminobenzylidene)-cyclopentanone (I) have been carried out. Compound I has been shown to sensitize the production of singlet oxygen (1O2) in deuterated toluene (C7D8), chloroform (CDCl3), and methanol (CD3OD). Tetramethylethylene (TME) was used as the indicator for 1O2 production. Photobleaching of solutions of the dye was observed, which led to an investigation of the chemical reactivity of I with 1O2. Based on 1H NMR, UV-Visible absorption spectroscopy, and HPLC/MS experimental data, it is proposed that the self-sensitized photooxidation of I with 1O2 yields 4-dimethylaminobenzaldehyde and (E)-3-(4-dimethylaminobenzylidene)-1,2-cyclopentadione as products. Photooxidation of I was not detected in CH3OH but was found in CD3OD. The absence of photooxidation in CH3OH is attributed to the short lifetime of singlet oxygen in this solvent (τΔ = 9.5 μs), compared to its lifetime in CD3OD (τΔ = 270 μs). UV-Visible absorption spectra of irradiated solutions of I in deoxygenated solvents demonstrate that (E,E) → (E,Z) photoisomerization of the dye takes place under these conditions.