Synthesis and Photochromism of Fused Pyran Systems

Two complementary approaches have been employed to synthesise an extensive series of photochromic 4,6-diaryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans. In the former approach 1,1,3-triarylprop-2-yn-1-ols, derived either from the addition of an arylacetylides to substituted benzophenones or from the Sonogashira cross-coupling between an aryl halide and 1-(2,4-dimethoxyphenyl)-1-(4-methoxyphenyl)prop-2-yn-1-ol, were subjected to an acid-catalysed condensation with4-bromo-1-naphtholto afford a series of 6-bromo-4-aryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans. Subsequent replacement of the 6-bromo substituent with an aryl group was achieved using a Suzuki-Miyaura cross-coupling reaction with a range of arylboronic acids to efficiently afford the 4,6-diaryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans.

The alternative approach relied upon the preparation of 4-aryl-1-naphtholsfrom 4-bromo-1-naphthol via a Suzuki-Miyaura cross-coupling reaction. Condensation of the foregoing 4-aryl-1-naphthols with 1,1,3-triarylprop-2-yn-1-ols afforded the 4,6-diaryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans directly in good yield. A limited number of 6-methoxy-4-aryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans were also obtained by the acid-catalysed reaction between 4-methoxy-1-naphthol and 1,1,3-triarylprop-2-yn-1-ols.

Whilst the majority of the novel 4,6-diaryl-2-(2,4-dimethoxyphenyl)-2-(4-methoxyphenyl)-2H-naphtho[1,2-b]pyrans displayed a photochromic response generating red hues upon irradiation, the best photochromic response was noted from the combination of a strongly electron-withdrawing 4-aryl moiety with an electron-donating 6-aryl unit; such a combination in the secondary chromophore results in opposition of the traditional donor-acceptor system in the photomerocyanine i.e. a cross-conjugated system is established upon irradiation. Remarkably inverting the donor-acceptor properties of the 4-and 6-aryl groups, establishing a secondary chromophore which supported that of the photomerocyanine, resulted in complete loss of photochromism and instead lead to fluorescence.