Chapter 57 - The ultrafast dynamics of some photochromic naphthopyrans

Abstract

Photochromic materials find wide use in the ophthalmic industry and it is becoming clear that future ophthalmic products will increasingly incorporate naphthopyrans (especially naphtho[2,l- b]pyrans - structure 1) as their photochromic element. The attraction of the naphthopyrans as photochromic materials stems from their broad range of intense vibrant colors combined with control over the rate of fade and resistance to fatigue. The mechanism of photochromism is shown in Scheme 1 for 3, 3-diaryl-3H-naphtho[2,l-b]pyrans. It is conventionally accepted that the trans open isomers are preferred, with the CT form preferred to the TT form. The excited state responsible for the ring scission process was proposed as the excited singlet state on the basis of nanosecond flash photolysis measurements and this is borne out by the ultrafast work of Aubard et al who report a lifetime for ring scission of 450 fs from the first excited singlet state for 1 in acetonitrile. This chapter discusses similar measurements on three naphtho[2,l-b]pyran derivatives.