Synthesis of 1-vinylidene-naphthofurans: a thermally reversible photochromic system that colors only when adsorbed on silica gel.

A set of new 1-vinylidene-1,2-dihydro-naphtho[2,1-b]furans were unexpectedly obtained in the reaction of 2-naphthol with readily available 1,1,4,4-tetraarylbut-2-yne-1,4-diols. Surprisingly, when adsorbed in silica gel, these compounds exhibit photochromism at room temperature, whereas not in solution and in the solid state. UV or sunlight irradiation leads, in a few seconds, to the formation of intense pink/violet to green colors that bleach completely in a few minutes in the dark. These new compounds also exhibit reversible acidochromic properties in solution: addition of TFA leads to the opening of the furan ring and addition of a proton to the allene function, leading to a conjugated and violet tertiary carbocation that returned immediately to the uncolored allenic structure upon addition of a weak base.