Twenty‐two Carbon Homologue of 11‐cis Retinal. Photophysical and Photochemical Properties

Results concerning absorption‐emission spectra and fluorescence quantum yields at 77 and 298K. triplet absorption spectra, and quantum yields of intersystem crossing and photoisomerization at 298 K, are presented for 11‐cisβ‐apo‐14′‐carotenal (C22‐Ald), the immediate higher homologue of 11‐cis retinal. The absorption spectra are characterized by two band‐systems with maxima at 390–400 and 270–280 nm, respectively. Upon cooling from 298 to 77 K, the intensities of these two band‐systems undergo changes in opposite directions indicating 12‐s‐cis: 12‐s‐trans conformational changes. No intermediate band‐system analogous to the one located at 270–310 nm in cis retinals is observed for 11‐cis C22‐Ald. In nonpolar hydrocarbon solvents (e. g. cyclohexane) at room temperature, quantum yields of fluorescence (0.01), intersystem crossing (0.6) and photoisomerization (0.4) are all quite pronounced. The photophysical and photochemical properties of 11‐cis C22‐Ald are discussed in the light of similarities and dissimilarities with those of all‐trans C22‐Ald and 11‐cis retinal under comparable conditions.