Quasi-crystals from irradiated photochromic dyes in an applied electric field

Abstract

IN many photochromic systems, and certainly in the spiro-pyrans, the colourless (A) form is much less polar than the photocoloured (B) form¹. Our previous experiments^2,3 revealed unusual properties of spiropyrans in non-polar solvents at low temperatures (173–240 K). On irradiation of these solutions with suitable light there were formed dimers (AB) and charge transfer complexes (CTC≡A_n⁺B⁻ with n ≃ 2–3), and these in turn formed aggregates with degrees of association >10⁶. The CTC exhibit spectral red shift of about lOO nm as compared to the dimers. The relative populations of dirners and charge transfer complexes in the aggregates depended both on light intensity and temperature, because CTC formation involves an activation energy of about 5 Kcal mol⁻¹ associated with the interaction of dirners with further A molecules. At temperatures sufficiently low or at light intensities sufficiently high it was possible to obtain aggregates composed of dimers only. When the constant field was applied during irradiation, threads were formed which extended from one electrode to the other (along the electric lines of force). These threads are composed of dimers and CTC, and their absorption spectrum is identical to that of aggregates in solution. Linear dichroism measurements showed that the CTC are oriented along the thread axis, but the dimers are unoriented. These threads were formed only in the temperature range in which CTC arise in solution. Here we describe investigations of the structure of such threads, which led us to conclude that the threads consist of highly dipolar crystallites composed of the charge transfer complexes and coated by an amorphous phase composed of dimers. These crystallites in the amorphous envelope are oriented along the electric field and joined together to give the quasi-crystalline threads.