Infrared dielectric function of photochromic thiazolothiazole-embedded polymer

Optical technologies that offer dynamic spectral control have been becoming more prevalent in recent years. Driven by this increased demand for tunable optical devices, substantial research efforts have been dedicated to the development of novel photo-responsive materials. In recent years, photochromic thiazolo[5,4-d]thiazole (TTz)-embedded polymers has emerged as promising candidate for technologies ranging from optical recording systems, tunable metasurfaces, to tinted lenses and smart windows. To effectively design, fabricate, and optimize tunable optical devices incorporating photochromic thiazolothiazole-embedded polymers, an accurate understanding of their complex dielectric function is fundamental to contemporary research. In this work, the infrared dielectric function of photochromic dipyridinium thiazolo[5,4-d]thiazole embedded in polymer is reported. Bulk TTz-embedded polymer samples were prepared by drop casting and dehydration in room temperature. The samples were investigated using spectroscopic ellipsometry in the infrared spectral range from 500 cm⁻¹ to 1800 cm⁻¹ before and after photochromism induced by a 405 nm diode laser. The model dielectric functions of the thiazolothiazole-embedded polymer film for its TTz²⁺ (unirradiated) and TTz⁰ (irradiated) states are composed of a series of Lorentz oscillators in the measured spectral range. A comparison of the obtained complex dielectric functions for the TTz²⁺ and TTz⁰ states shows that the oscillators located in the spectral ranges 500 cm⁻¹–700 cm⁻¹, 1300 cm⁻¹–1400 cm⁻¹, and 1500 cm⁻¹–1700 cm⁻¹ change in both amplitude and resonant frequency upon transition between the states. Additionally, a resonance at approximately 1050 cm⁻¹ exhibited a change in oscillator amplitude but not resonant frequency due to the photochromic transition.