Photochromism of dihydroindolizines Part VI: Synthesis and photochromic behavior of a novel type of IR-absorbing photochromic compounds based on highly conjugated dihydroindolizines

Novel photochromic bis-styryl dihydroindolizines (DHIs) 5a-β bearing different cycloalkanone ring sizes were prepared in 11-42% yield via nucleophilic addition of bis-styryl cycloalkanones 2a-k to substituted spirocyclopropenes 1a-g. The absorption maxima (λmax) of the colored betaines 4a-β (formed after UV-irradiation) were detected by millisecond flash photolysis, laser flash photolysis as well as FT-UV/VIS measurements. All betaines 4a-β showed two absorption maxima, one in the visible region (500-575 nm) and another one in the IR-region (800-878 nm). The kinetics of the fast cyclizing process of betaines 4a-β to DHIs 5a-β were studied in dichloromethane solution by millisecond flash photolysis and were found to take place in the millisecond range (76-450 msec). The strong colorability of betaines 4a-β may be attributed to the highly conjugated betaine structures as well as to the presence of the carbonyl chromophore. Interestingly, a remarkable photo-fatigue resistance of some studied betaines 4 showed t30- values higher than the standard one. Large solvatochromic effects on the absorption maxima (λmax) as well as a substantial increase of the half-lives (t1/2) with solvent polarity of betaines 3a-α were also observed. Copyright © 2006 John Wiley & Sons, Ltd.