Alkylthieno[2,3- d ][1,3]thiazole as a Dual-Mode Design Principle for Single-Beam Unidirectional Spontaneous Surface-Relief Gratings in Push–Pull Azobenzenes

Abstract

Pseudostilbene (PS) azobenzene-functionalized polymethacrylate (PMAzo) films are known to spontaneously form surface-relief gratings (SP-SRGs) under single-beam irradiation, driven by reversible photoisomerization-induced mass transport. Yet achieving macroscopic single-beam unidirectional SP-SRG in covalently bonded systems with a value of maximum absorption wavelength (λabsmax) above 500 nm remains elusive due to the trade-off between red-shifted absorption and chain mobility. Here, the alkylthieno[2,3-d][1,3]thiazole (TTz) heterocycle resolves this trade-off through a dual-mode design principle: its electron-withdrawing nature extends absorption range from 400 to 650 nm and the value of λabsmax to 536 nm (film spin-coated from THF), while systematic modulation of alkyl substituent bulkiness (i.e., methyl, n-butyl, 2-ethylhexyl) on TTz simultaneously controls chromophore aggregation and decreases the polymer’s glass transition temperature (Tg) from 144 to 75 °C. A family of polymethacrylate azobenzene homo- and copolymers (PMAzo-TTz, P1a-P4c) was synthesized and characterized, demonstrating that this Tg tuning operates independently of the chromophore’s electronic structure, a key feature enabling decoupled material optimization. As a result, the optimized copolymer P2c (Tg = 94 °C), spin-coated from DCM, exhibits reduced chromophore aggregation (λabsmax = 505 nm) and achieves, to the best of our knowledge, the first highly ordered unidirectional single-beam SP-SRG in a covalently bonded PS–PMAzo system absorbing beyond 500 nm (periodicity 380–400 nm, amplitude ∼12 nm, order parameter S = 0.73, corresponding to SP-SRG directional spread of 22.9°). This work establishes clear structure-property relationships linking chromophore aggregation, Tg behavior, and light-induced mass transport. The dual-mode design principle of the TTz heterocycle provides a general strategy for developing next-generation red-shifted optomechanically active polymers for photonic and light-responsive applications.