Design, Synthesis, Structural and Nonlinear Optical Properties of Photochromic Crystals:  Toward Reversible Molecular Switches

Abstract

Two new anil molecules exhibiting photochromism in the crystalline state, N -(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) ( 2 ) and N -(3,5-di- tert -butylsalicylidene)-4-aminopyridine ( 3 ), are obtained. Upon irradiation in the UV, the yellow crystals change color to red, owing to enol-keto intramolecular tautomerism. The red color disappears, when crystals are left in the dark or irradiated with visible light. 3 has the most stable keto form among all anil-type photochromic compounds (τ = 460 days at room temperature). Both exhibit nonlinear optical (NLO) properties and show powder second harmonic generation (SHG) of respectively 10 and 3 times vs urea. X-ray diffraction shows acentric structures where molecules line up “head-to-tail” through hydrogen bonds for 2 (space group Pc ), or form a chiral helix 3 (space group P 3 2 ). Evidence of reversible structural change is given for 3, and we demonstrate the functionality of this crystal as an NLO switching material, as SHG can be photomodulated by about 30%.