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.

A mathematical stability analysis is presented of the two-beam interactions in quadratic optical nonlinear media that are now attracting such a lot of attention. The averaged Lagrangian is used, within a variational method, and the analysis is based upon a Gaussian trial function. The stability is governed by parameters that can be classified into two groups. One describes spatial solitonlike beam positions and propagation directions and the other describes beam sizes and phases. It is shown that the evolution of these parameters is determined by ten, coupled, ordinary differential equations. The stationary states are proved, mathematically, to be stable for all linear phase mismatch parameter values provided the perturbations are symmetric, i.e., perturbations to beam positions and directions. However, for perturbations to beam sizes or phases, it is proved that a number of stability regimes exist, together with forbidden parameter ranges. The analytical conclusions are completely borne out by computer simulations, and some typical examples are reported here.