Chromenes, photochromism

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

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 photochromic mechanism for the chromenes is very similar to that for spiropyrans given in Figure 1.1. Under the influence of UV the C-0 bond in the pyran ring is broken to give either the zwitterionic form or, more likely, the cis- and fran.y-quinoidal forms (Figure 1.6). Studies suggest that formation of the cis-quinoidal species occurs in picoseconds, followed by generation of the trans-form in nanoseconds. 

Simple 2,2-dialkyl-2/f-benzopyrans, or chromenes as they are very often called in the photochromic hterature, can be synthesised by several well established routes, a selection of which are shown in Figure 1.7. ... 

Fulgides such as the diphenyl derivative (144), dithienylethylenes (145), certain 2H-pyrans (146) and 2H-chromenes (147) and their respective thio analogues (148) and (149) also exhibit photochromism by valence tautomerism. 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances.5 6,7-Chromenediols have been proposed as analytical reagents for the spectrophotometric assay of rare earth cations.279 Chromenes with structure and activity similar to those of hashish constituents have been prepared.280 A number of chromenes, mostly with aryl substituents in positions 2,3, and 4 have been patented as biologically active substances.126,280 290... 

The chromene ring of spirobenzopyrans such as (168) and (169) is opened by treatment with mineral add and closed again by alkali — a reaction of potential importance in the thermochromic or photochromic application of such compounds (69T5995,74JOU1516). 

This method has found particular application in the synthesis of spiro-annelated chromenes, which are of interest because of their photochromic behaviour. The alkene is commonly a 2-methylenedihydroindole, which leads to spiro 2//-l-benzopyranoindolines, or the corresponding benzothiazole (67BSF2824). Provided a base is present to liberate the free heterocyclic base, indolinium or thiazolinium salts may be used in the synthesis. Enamine attack at the carbonyl group is followed by dehydration and cyclization (Scheme 9). 

H-chromene synthesis from, 3, 767 Indenobenzazepines pharmacological properties, 7, 546 Indenone oxide, 2,3-diphenyl-photochromic compound, 1, 385 Indeno[ 1,2-e][ 1,2,4]triazines synthesis, 3, 434 Indicated hydrogen nomenclature, 1, 33 Indigo, I, 317, 318-319, 4, 370 Baeyer synthesis, 1, 319 colour and constitution, 1, 344-345 molecular structure, 4, 162 photochromic compound, 1, 386 synthesis, 4, 247 Indigoid dyes... 

The complex formation and photochromic behaviour of the azacrown-containing chromene 15b and its crown-free analogue 15a were studied in detail [38,39], The addition of Ca(C104)2 to a solution of 15b led to the shift of the band at 400 nm, indicating the binding of Ca2+ by the macrocyclic unit of 15b. In contrast, the presence of Ca(C104)2 in a solution of 15a even at a high concentration of 0.1 mol dm-3 had no effect on the absorption spectrum of 15a. 

Fedorova, O.A., Maurel, Fr., Ushakov, E.N., Nazarov, V.B., Gromov, S.P., Chebun kova, A.V., Feofanov, A.V., Alaverdian, I.S., Alfimov, M.V., Barigelletti, Fr. (2003) Synthesis, photochromic behaviour and light-controlled complexation of 3,3-diphenyl-3//-benzo[/]chromenes containing a dimethylamino group or an aza-15-crown-5 ether unit, New J. Chem., 27, 1155-1167. 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

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,... 

A recent review discusses the synthesis, spectral properties, and photochemical behavior of novel spirobenzopyrans, Spiro-1,4-oxazines, and 2//-chromenes. It emphasizes the structural features needed for infrared-absorbing photoisomers, luminescent stable forms, improved fatigue resistance, and spiropyrans exhibiting photochromism in vacuum-deposited films.147... 

M. A. Gal bershtam, N. M. Przhiyalgovskaya, O. R. Khrolova, I. B. Lazarenko, G. K. Bobyleva, and N. N. Suvorov, Photochromic properties of some N-substituted 3,3-dimethyl-6 -nitro-indoline-2-Spiro-2 -2/7-chromenes, Chem. Heterocycl. Cpds., 1977, 1309-1313. 

M. Uchida and M. Irie, Photochromic chromene derivatives with a thiophene moiety, Chem. Lett., 1992, 2257-2260. 

R. Guglielmetti, J. L. Pozzo, and A. Samat, Preparation of photochromic heterocyclic-fused chromenes and their use in ophthalmic optics, Eur. Pat. Appl. 0,562,915,A1 (1993). 

The chromenes are important photochromic compounds that are structurally related to their parent compound, 2-//-chromene, whose photochromic behavior is... 

No quantum mechanical calculation or similar theoretical treatment seems to have been dedicated to chrom-3-ene or to its simple derivatives. Interatomic distances and angles of two bromo derivatives of natural chromenes have been obtained by X-ray analysis, The UV absorption, emission,fluorescence, and fluorescence excitation spectra of some 2,2-dialkylchromenes have been studied in connection with their photochromic behaviour. 

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