Two series of novel thermally stable second-order nonlinear optical (NLO) and photochromic chromophores have been designed and synthesized. The two series of compounds were based on different combinations of donor groups (pyrrole or thienylpyrrole) which act simultaneously as -conjugated bridges, together with diazoaryl or diazothiazolyl as acceptor moieties. Their photochromic and electrochemical behavior were characterized, while hyper-Rayleigh scattering (HRS) was employed to evaluate their second-order nonlinear optical properties. The results of these studies have been critically analyzed together with two other related compounds reported earlier from our laboratories in which the thienylpyrrole system was used as the donor group keeping the functionalized diazoaryl as acceptor moiety.The measured molecular first hyperpolarizabilities and the observed photochromic behavior showed strong variations for the different donor systems used (pyrrole or thienylpyrrole) and were also sensitive to the acceptor strength of the diazenehetero(aryl) moiety.The thienylpyrrole based compounds endowed with extended -conjugated bridges and stronger donor auxiliary effects in comparison to the pyrrole compounds, when coupled to the stronger acceptor diazo(hetero)aryl groups gave rise to significantly larger hyperpolarizabilities ( = 274 -415 x 10 -30 esu) for an incident wavelength of 1064nm). These compounds also displayed improved photochromic behavior with very fast response to the visible light stimulus (1.5 s) and fast thermal return to the original forms (2-3 s).

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.

The use of benzothiazole as an electron-withdrawing group allows obtaining the fastest thermal isomerisation kinetics reported heretofore for neutral azo dyes (70 μs at 298 K). These green light activatable molecules are valuable candidates as molecular photoswitches since they tolerate thousands of working cycles with no sign of fatigue.

A new type of photochromic naphthopyran with a fused structure was embedded in an organic-inorganic amine-alcohol-silicate hybrid gel producing colourless, transparent and flexible photochromic films. Under UV-vis light these materials developed, in less than 30 s, intense yellow-orange colours that faded completely in the dark in 3 min. This behaviour contrasts with the usual performance of common naphthopyrans that always generate two coloured species which fade with different kinetics, leading to the formation of a residual colour that persists for several minutes/hours. The structural design of this new naphthopyran prevents the formation of the long-lived coloured species and therefore fully reversible photo-switchable materials can be easily obtained. Scheme 1 Photochromic equilibrium for 2H-naphtho[1,2-b]pyran 1 and 3Hnaphtho[2,1-b]pyran 2.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.