Synthesis and Photochromism of Naphthopyrans Bearing Naphthalimide Chromophore: Predominant Thermal Reversibility in Color-Fading and Fluorescence Switch

Two novel photochromic naphthopyrans containing naphthalimide moieties (Nip1 and Nip2) were studied in solution under flash photolysis conditions, exhibiting highly photochromic response, rapid thermal bleaching rate and good fatigue-resistance. Owing to the different N-substituted imide groups at the naphthalimide units, the thermal bleaching rate of Nip2 bearing phenyl on the naphthalimide unit is found to be approximately 2 times that of Nip1 bearing n-butyl, indicating that the photochromic properties can be modulated with introduction of different functional groups on the naphthalimide unit. In Nip1 and Nip2, the strong electron-withdrawing effect of the imide group incorporated at the naphthalimide moiety maintains several merits: (i) shifting absorption bands to longer wavelength, (ii) beneficial to an enhancement in the ratio of transoid-cis (TC) isomer and an increase in the transformation rate from transoid-trans (TT) to TC with respect to reference compound NP, and (iii) resulting in a preferable color bleaching rate and fading absolutely to their colorless state with thermal reversibility. As demonstrated in the system of NP, the slow transformation process from TT to TC might be the predominant dynamic step in thermal back process, leading to the residual color of NP being only faded to its original colorless state by visible light irradiation. The optical densities of colored forms for Nip1 and Nip2 are dependent upon the intensity of incident light, ensuring a possible application in the manufacture of ophthalmic lenses and smart windows. Moreover, the fluorescence of Nip1 and Nip2 can be switched on and off by photoinduced conversion between the closed and open forms.