Benzo and Naphthopyrans Chromenes

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 two naphthopyrans of interest (1.19) and (1.20) show quite different photochromic behaviour. Isomer (1.20 R, R = Ph) produces a more bathochromic coloured state than (1.19 R, R = Ph), (X. = 481 mn versus 432 nm), a large 

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Benzo - and naphthopyrans (chromenes) - also of interest for their resistance to fatigue ... 

Chapter 3 (Benzo and naphthopyrans or chromenes). This chapter reviews another important type of thermoreversible photochromic compounds that have found application in the variable optical transmission materials field, specifically,... 

All of the compounds discussed are based on three molecules 2/f-pyran (1), 4//-pyran (2) and the pyrylium cation (3). Names which have been used for the benzologue (4) of 2//-pyran include 2H- 1-benzopyran, benzo-a-pyran, chrom-3-ene and 2//-chromene. A similar situation exists for the corresponding derivative (5) of 4/f-pyran. The unambiguous and simplest name chromene is used in the present work. The benzologue (6) of pyrylium is known both as benzopyrylium and chromylium the former name is preferred here. Higher benzologues are referred to as naphthopyrans, such as 2H-naphtho[ 1,2-6 jpyran (7), but the names xanthene and xanthylium are used for (8) and (9). 

Almost every class of natural phenolic compounds contains examples of substances with a 2,2-dialkylchromene ring, and the number of those that are discovered increases every year. It would be difficult to give an exhaustive list of these compounds. Their chemistry will be discussed here only when it is related to some particular behavior of the benzo-pyran ring. Examples include simple chromenes substituted in the aromatic ring,3,4,35-44 benzodipyrans,3,39 dimers of chromenes,45 naphthopyrans,46,47 quinones,48 flavones,49 flavonols,49 chalcones,49 flavanones,49 isoflavonoids,50 rotenoids,50 pterocarpans,50 couma-rins,17,51-53 3-arvl-4-hydroxycouniarins,50 4-phenylcoumarins,54 chroma-... 

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