DETAILED DESCRIPTION OF THE INVENTION In recent years, photochromic plastic materials, particularly plastic materials for optical applications, have been the subject of considerable attention.
In particular, photochromic ophthalmic plastic lenses have been investigated because of the weight advantage they offer, vis-a-vis, glass lenses.
Moreover, photochromic transparencies for vehicles, such as cars and airplanes have been of interest because of the potential safety features that such transparencies offer.
Ideal photochromic compounds for use in optical applications, such as conventional ophthalmic lenses, are those which possess (a) a high quantum efficiency for coloring, (b) a relatively fast change in optical density over time, (c) a high optical density at saturation, (d) a low quantum yield for bleaching with visible light and (e) a relatively fast thermal fade at ambient temperature but not so rapid a thermal fade rate that the combination of visible light bleaching and thermal fade prevent coloring by the ultraviolet component of strong sunlight.
The aforesaid properties are desirably retained when the photochromic compound is applied to or incorporated within conventional rigid synthetic plastic materials customarily used for ophthalmic and plano lenses.
A naphthopyran such as 3,3-diphenyl-3H-naphtho[2,1-b]pyran, changes color on exposure to ultraviolet; but, at room temperature and above, this compound changes optical density too slowly, has too low an optical density at saturation, and bleaches too rapidly for use in an ophthalmic lens.
In accordance with the present invention, there has been discovered certain novel reversible photochromic naphthopyran compounds with unexpected properties.
These compounds are substituted at the number eight carbon atom on the naphtho portion of the naphthopyran, and exhibit a dramatic bathochromic shift of their absorption maximum in both the visible spectrum of the activated form and the UV spectrum of the unactivated form.
The shift in the UV spectrum has contributed to an increase in sensitivity as measured by how fast the optical density of the compounds change with time, and to an increase in the compounds optical density, as measured by how dark they become, vis a vis, naphthopyrans substituted at the number five, seven or nine carbon atom of the naphtho portion of the naphthopyran
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