A novel generation of hybrid photochromic vinylidene-naphthofuran silica nanoparticles through fine-tuning of surface chemistry

Hybrid vinylidene-naphthofuran nanosilicas with fast and reversible photochromism under UV/sunlight were fabricated through fine-tuning of surface chemistry and a grafting process.

Naphthopyran-Based Silica Nanoparticles as New High-Performance Photoresponsive Materials

Hybrid nanomaterials based on the covalent grafting of silylated naphthopyrans (NPTs) onto silica nanoparticles (SiO2 NPs) were successfully prepared and studied as new photochromic materials. They were prepared by a two-step protocol consisting of (i) NPTs (derivatives from 2H-naphtho[1,2-b]pyran (2H-NPT) and 3H-naphtho[2,1-b]pyran (3H-NPT)) silylation by a microwave-assisted reaction between hydroxyl-substituted NPTs and 3-(triethoxysilyl)propyl isocyanate, followed by (ii) covalent post-grafting onto SiO2 NPs. In order to study the role of the silylation step, the analogous non-silylated nanomaterials were also prepared by direct adsorption of NPTs. The characterization techniques confirmed the successful NPTs silylation and subsequent grafting to SiO2 NPs. All SiO2-based nanomaterials revealed photoswitching behavior, following a biexponential decay. The SiO2 NPs functionalized with silylated 3H-NPTs (SiO2@S3 and SiO2@S4) presented the most promising photochromic properties, showing fast coloration/decoloration kinetics (coloring in 1 min under UV irradiation and fading in only 2 min) and high values of total color difference (ΔE*ab = 30-50). Also, the 2H-NPTs-based SiO2 NPs (SiO2@S1 and SiO2@S2) presented fast coloration and good color contrasts (ΔE*ab = 54), but slower fading kinetic rates, taking more than 2 h to return to their initial color. In contrast, the SiO2 NPs functionalized with non-silylated NPTs (SiO2@1 and SiO2@3) showed weaker color contrasts (ΔE*ab = 6-10) and slower fading kinetics, proving that the NPT silylation step was crucial to enhance the photochromic behavior of SiO2 NPs based on NPTs. Furthermore, the silylated-based nanomaterials showed good photostability upon prolonged UV light exposure, keeping their photochromic performance unchanged for at least 12 successive UV/dark cycles, anticipating interesting technological applications in several areas.

Influence of electronic vs. steric factors on the solid-state photochromic performances of new polyoxometalate/spirooxazine and spiropyran hybrid materials

New supramolecular spirooxazine and spiropyran/polyoxotungstate materials have been designed. Their solid-state photochromic performances are strongly exalted compared to their isostructural Mo counterparts due to a more favorable electronic effect.

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.