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

Abstract

AbstractNovel 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‐β were also observed. Copyright © 2006 John Wiley & Sons, Ltd.