Coupling between Photochromism and Second-Harmonic Generation in Spiropyran- and Spirooxazine-Doped Polymer Films

Abstract

The photochemical ring opening of spiropyrans in polymethylmethacrylate (PMMA) host films in the presence of a high-voltage electric field (2-5 MV/cm) leads to a photo-orientation of photomerocyanines. Since photomerocyanines have rather high first-order hyperpolarizabilities, this photo-orientation leads to secondharmonic generation (SHG). The second-order susceptibility & of PMMA films doped with 6-nitro-1',3',3'-trimethylspiro[2H-l-benzopyran-2,2'-indoline] (25% w/w) has been measured at 10 p m N . If a PMMA film doped with spiropyran and preoriented by corona poling at 90 "C is irradiated in the UV, an SHG signal of photomerocyanine is also observed as a result of a memory effect of the orientation of spiropyran molecules. The photoswitching of this nonlinear optical property can be inverted by visible light irradiation. Only partially reversible switching is observed because of a gradual light-induced disorientation of photomerocyanine dipoles.Photochemical quantum yields of the spiropyran * photomerocyanine reaction have also been determined in PMMA for one system. Dipole moments and hyperpolarizabilities of closed and open forms have been calculated by semiempirical and finite field methods for an analogous system.

Linear and Hyperpolarizabilities of Spiropyran and Derivatives

“…(2), the maxima are found to occur at wavelengths 346 nm, 370 nm, and 485 nm for the closed, the quinonic and the zwitterionic forms, respectively. It is clear that the peak of the absorption of the closed form is around 350 nm, which is in good agreement with the experiment [8]. And there is a shift from ultraviolet to violet and to blue.…”

“…Atassi et al [8,9] reported experimentally two new classes of materials that exhibit the two distinct properties, photochromism and second order nonlinear optical properties; spiropyran (SP)-photomerocyanine (PM) in polymer systems. Spiropyran is one type of organic photochromic molecule that upon application of ultraviolet irradiation interconverts into colored photomerocyanine form.…”

“…Beside the above experimental work there was a little theoretical work to calculate the first order hyperpolarizability of SP by finite field methods in combination with the modified neglect of differential overlap (MNDO) molecular orbital method [8]. The frequency-dependent hyperpolarizabilities of SP and derivatives have not been reported yet.…”

Local coherence and the temporal development of second harmonic emission

“…Here the key factor is a photochemically induced interconversion between two molecular forms which have hyperpolarizabilities of opposite signs. A good example is afforded by spiropyran and photomerocyanine (β values of 1.9 × 10 −30 esu and −43.0 × 10 −30 esu, respectively [34]), which interconvert through the application of UV or UV/visible radiation. Clearly, optical coherence within locally ordered domains (for example, in a suspension of polymer particles) where both species are present during the relatively long timescale (10 2 s) for interconversion should lead to a dip in the SHG signal at a time corresponding to ∼ 4.4% photomerocyanine.…”

Preparation of the Inclusion Complex-Type Nonlinear Optical Polymer

“…Photomerocyanine (PMs) are comprised of strong electron donor and acceptor groups connected by a -conjugated bridge. ey possess delocalized -electron systems capable of exhibiting charge transfer resonances, which leads to a preferential orientation of the molecules along the applied electric �eld [31][32][33]. Experimental values of dipole moments and �rst-order hyperpolarizability were reported for various PMs [34][35][36][37].…”