Photochemical dihydroindolizine

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

The photochromism of dihydroindolizines (DHIs) - a new class of photo-chromic molecules - was discovered only in 1979.1,2 These molecules are among the few new photochromes discovered in the past 20 years.3 Their synthetic access has been exploited in detail and their photochemical and photophysical parameters have been studied in a broad sense.3 7 The potential applications are vast and compare favorably with those of the known photochromes.8 The chemistry and photochemistry of the DHIs have been described previously in several surveys.3-7 This review will serve as a short introduction to the field. It will as well include typical preparation modes for the class of dihydroindolizines as well as their aza-homologs, typical five-membered heterocycles. A new class of photochromies derived from these photochromic heterocycles (DHIs) that allows for supramolecular interactions is presented. Some typical applications that are close to commercially useful systems are discussed. 

The characteristic structural feature of the dihydroindolizines-based molecules is a five-membered ring, typically a cyclopentene anion. The substitution of the carbanion in this molecule by a heteroatom generates heterocyclic five-membered rings. 9-15 These can undergo a ring opening to zwitterionic species or neutral heteropentadienes. The process can be induced photochemically and is reversed either thermally or photochemically. Thus a new photochromic system based on a five-atom, six-electron (4n + 2)k system is created. This system can in principle be... 

N. A. Garcia, G. Rossbroich, S. E. Braslavsky, H. Durr, and C. Dorweiler, Photoacoustic measurements and MINDO/3 calculations of energy storage by short-lived species the spiro[l,8-a]dihydroindolizine-betaine system, J. Photochem. 37,297-305 (1985). 

The UV spectra of the dihydroindolizines 25, X = CH, and dihydropyrrolo[l,2-b]pyridazine 25, X = N, usually have absorption maxima in the near-UV between 360 and 410 nm. The betaines 35 produced by photochemical ring opening absorb at about 505 to 726 nm. 

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