Modification of the cyclopentene ring

The central cyclopentene ring can also be replaced with five-membered rings containing a carbonate, carbamate or sulfur equivalents and compounds have been prepared and studied. Compounds 62 [130], 63 [131] and 64 (Chart 21.15) [132] all underwent reversible photochromic reactions when alternatively irradiated with UV and visible light. However, only a few derivatives have been prepared and the photochromic 

When foiu thiophene rings are connected to all four carbons of a central alkene, as illustrated with compounds 67 and 68 [136], photochromic performance is sacrificed and only minor amounts of the ring-closed isomers are observed in the photostationary states when solutions of the compounds are irradiated with UV light. In fact, in the case of derivative 67, no new absorption peaks appear in the visible region of the 

Spectrum and the cyclization product is thermally converted back to its ring-closed counterpart. In either case, only one pair of thiophene rings can donate their C=C double bond to the hexatriene at any one time. As a result, the derivatives cannot really be considered to contain two different hexatriene structures. 

An interesting example of a compound that contains a different flve-membered central ring is derivative 69 (Chart 21.17) [137]. The compound proved to be photochromic and acidichromic (i.e. it changed its color when treated with protic acid) however, the authors failed to indicate whether the acidified compound maintains its photoreactivity. 

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