2,2,4,6-Tetraaryl-2H-benzo[h]chromenes: The influence of electronic communication between aryl substituents on their photochromism

A series of photochromic 2,2,4,6-tetraaryl-2H-benzo[h]chromenes have been efficiently synthesised by two complementary strategies employing the condensation of 1,1,3-triarylprop-2-yn-1-ols with 4-substituted 1-naphthols and Suzuki cross-coupling chemistry. The novel 1,1,3-triarylpropynols were readily obtained either by the addition of an arylacetylide to the requisite benzophenones or by a Sonogashira coupling of a halobenzene with a preformed 1,1-diarylprop-2-yn-1-ol. The tetraarylbenzochromenes which possessed a strong electron withdrawing aryl moiety at C-4 exhibited good room temperature photochromism. The combination of a strong electron donating aryl group at C-6 with a strong electron withdrawing aryl group at C-4 resulted in the generation of a photomerocyanine which exhibited two absorption bands in the visible region of the electromagnetic spectrum, rather than one broad band when the C-6 aryl group was only weakly electron donating. Remarkably inverting the electronic nature of the C-4 and C-6 aryl groups resulted in the inhibition of photochromism and instead led to fluorescence. TD-DFT calculations were used to rationalise the shift from photochromism to fluorescence.