Abstract
Studies on the following were reviewed: (1) the structure of spiropyrans and spirooxazines (two kinds of spiro compounds) under external stimuli and (2) the construction and applications of composite systems based on fluorescence resonance energy transfer (FRET) with fluorescent materials. When treated with different stimuli (light, acids and bases, solvents, metal ions, temperature, redox potential, and so on), spiropyrans/spirooxazines undergo transformations between the ring-closed form (SP), the ring-opened merocyanine (MC) form, and the protonated ring-opened form (MCH). This is due to the breakage of the spiro C–O bond and the protonation of MC, along with a color change. Various novel, multifunctional materials based on photochromic spiropyrans and spirooxazines have been successfully developed because of the vastly differently physiochemical properties posssed by the SP, MC and MCH forms. Among the three different structural forms, the MC form has been studied most extensively. The MC form not only gives complexes with various inorganic particles, biological molecules, and organic chemicals but also acts as the energy acceptor (of energy from fluorescent molecules) during energy transfer processes that take place under proper conditions. Furthermore, spiropyran and spirooxazine compounds exhibit reversible physicochemical property changes under proper stimuli; this provides more advantages compared with other photochromic compounds. Additionally, the molecular structures of spiropyrans and spirooxazines can be easily modified and extended, so better compounds can be obtained to expand the scope of already known applications. Described in detail are: (1) the structural properties of spiropyrans and spirooxazines and related photochromic mechanisms; (2) composite systems based on spiropyrans and spirooxazines, and (3) fluorescent materials which have potential applications in sensing, probing, and a variety of optical elements.
Highlights
Introduction to Spiropyrans and SpirooxazinesSpiropyran and spirooxazine compounds can undergo reversible structural transformations under the influence external stimuli; this induces a color change, as well as changes in their physical and chemical properties [1,2]
Because characteristic absorption peaks for the electron transitions of naphthoxazines. When the hydrogen they posses benzopyrene and indoline ring,alkoxyl most ofand thehalogen, absorption peaks of spiropyrans atom onthe thesame indoline ring was substituted by alkyl, the absorption spectra varied greatly; for example, the two absorption peaks in the long wavelength region of 5-alkoylsubstituted spirooxazines changed to a single peak [48]
After UV light irradiation or treatment with acid, spirooxazines transformed from the SP form to MC, and fluorescence resonance energy transfer (FRET) occurred between MC and Bodipy
Summary
Spiropyran and spirooxazine compounds can undergo reversible structural transformations under the influence external stimuli; this induces a color change, as well as changes in their physical and chemical properties [1,2]. Thekind authors of molecular switch that occurs with spiropyrans and spirooxazines) endowed many composite introduced the structural transformation processes and aggregation of the ring-opened isomers systems with new properties that have potential applications in bioimaging, energy transfer, of spiropyrans in detail. The reversible structural transformations of spiropyrans were used to quench or restore the fluorescence used in species ranging from relatively small molecules and assemblies to nanostructured composite of fluorophores. These studies brought about the precise control of optical signals, which could be systems. Single molecule level [29]
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