Photochromic system pyrans

Benzo-2//-pyrans and their thia analogs, e.g., heterocycles not treated in this chapter, are photochemically labile and form reversible photochromic systems with open-chain and colored photoisomers. This behavior was observed for 2-benzyl-2,4,6-triphenyl-2//-species 467a,b (Y = H)399,409 and naturally occurring 2//-pyran 92a452a but only at about 77K. At room temperature, these as well as other 2//-pyrans are photochemically stable. Only 2,2,4,6-tetramethyl-2//-pyran (176) was photochemically changed to its exocyclic double bond isomer 308a.404 An irreversible photolysis of 2-azido-2//-thiopyran derivative 212b (R = H) has been discussed in reference 254. 

Other photochromic systems are using the ring opening/ring closing of thienyl pyrans. Examples are 134 <20060L4931> as a biphotochromic system or 135. 

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

The valence isomerization of the 2H-pyran system is the basis for the commercially valuable photochromic properties exhibited by the benzologs. 

Of the three isomeric naphthopyrans 2-4, the linear isomer 2//-naphtho[2,3-6]pyran 4 displays no significant photochromic response at ambient temperature, a feature which may be rationalised by considering the extensive 7i-system reorganisation which must accompany an electrocyclic ring opening and which would disrupt the aromaticity of both rings of the naphthalene unit. 

A variety of heterocyclic rings have been incorporated into the benzopyran system either as substituents at the critical sp hybridised centre adjacent to the O heteroatom or around the periphery of the molecule. In a different vein, heterocycles have been fused onto both benzo-and naphtho- pyrans. Aspects of the synthesis of these compounds and the influences of the new heterocyclic moiety on the photochromic properties are discussed. 

An even less reliable system in terms of predictability was obtained for the polyamide dyed by 2,2,6-tris(4-ethoxyphenyl)-9-methoxy-2//-naphtho-[l,2-I ]pyran-5-carboxylat. The textile was more prone to photodegradation and fatigue, while the increasing trend of residual colour on the photochromic polyamide was far greater than in the case of polyamide dyed by 2,2-bis(4-dimethoxyphenyl)-6-acetoxy-2H-naphtho-[l,2-I>]pyran-5-carboxylat. Despite the fatigue, the sample was found to be photochromic even after the last (20th) cycle of testing. 

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