Spectral and Physical Properties of Diarylnaphthopyrans

As mentioned in sections 1.2.2.2 and 1.2.3.2, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzo and naphthopyrans. Consequently, spectral data collected from the literature is only comparable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any other one listed in this and related sections of this chapter. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (2. ) of the coloured state, the change in optical density (absorbance) on exposure to the xenon light source (AOD) and the fade rate which is the time in seconds for the AOD to return to half of its equilibrium value. 

Two other measurements, often quoted in the literature, are the lOD (initial optical density) at and the lODTj value already described in section I.2.3.2. 

The influences on the absorption spectra and the other photochromic properties of compounds with substituents in the 3//-naphtho[2,l-h]pyran ring and on the 3,3 -aryl groups have been stndied in detail. Electron-donating gronps in one or both of the 3-phenyl gronps, especially in the p-position, show a marked bathochromic shift in the absorption maxima of the coloured state, whilst electron-withdrawing groups have the opposite effect (Table 1.4). Substitutions in the a-position have little effect on the absorption maxima but have a very marked effect on the rate of return back to the colourless state, presumably due to stabilisation of the open chain form (Table 1.4). 

A = toluene solution. B = imbibed into diethyleneglycol bis(allyl carbonate) polymer. 

As mentioned above, the main problem with the 2H-naphtho[l,2-fc]pyran ring system (1.20) is the very slow rate of fading back to the colourless state (1.20 R,  

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