The Control of Photochromism of [3 H ]-Naphthopyran Derivatives with Intramolecular CH−π Bonds

International audienceThe photochromism of [3H]-naphthopyran derivatives can be switched from T-type to inverse-or P-type through the manipulation of relative thermodynamic stabilities of open isomers with intramolecular CH- bonds. Photoswitchable molecules such as azo compounds and dithienylethenes have been extensively investigated over the last years, due to their potential in technological advanced applications ranging from molecular electronics to bioimaging. 1 These applications rely on the capacity of the photochromic compounds to switch reversibly between two states thermally (meta)stable having different properties by light (P-type photochromism). 2 In contrast, photochromic organic naphthopyrans have attracted much attention due to their industrial application in ophthalmic technology. 3 The reversible change of color under solar exposition that ceases after the removal of the illumination constitutes the first required property for the use of naphthopyrans in plastic photochromic lenses (T-type photochromism). Changing the photochromism of naphthopyrans from T-to P-type is very challenging and would offer a new family of photochromes that could be critical of use in advanced applications and constitute an interesting alternative to azocompounds and dithienylethenes. The photochromism of [3H]-naphthopyrans is believed to start with CO bond cleavage of the closed pyran ring (CF) induced by UV absorption that leads after some internal rearrangement of double bond to an open colored isomer transoid-cis (TC) which returns thermally to the initial state (Scheme 1). 4 Under continuous irradiation, TC can eventually produce a transoid-trans open isomer (TT) following an isomerization of one double bond. 4 The reversible change of color comes from the fact that the TC isomer is the major product under solar or UV irradiation. 5 In fact, TC and TT isomers differ from their thermal and thermodynamic stabilities. In the case of the reference compound namely the 3,3-diphenyl-[3H]-naphtho[2-1b]pyran at room temperature in toluene solution, TC is less thermally stable by about 3 to 4 orders of magnitude lower compared to TT while its thermodynamic stability is about 8.9 kJ higher than that of TT. 6 Indeed, a hydrogen-bond is established between proton H2 and the oxygen of the ketone group stabilizing TC while the same proton in TT is in a repulsive interaction with the naphthalene ring, consequently destabilizing TT. The isomerization of the double bond involved in the formation of TT from TC is rendered more difficult due to the presence of this hydrogen-bond that should be broken in this transformation. Consequently, the photochromism of naphthopyran is controlled by