Negative role of high molar mass of polymer matrix in dye–polymer hydrogen bond formation, trans – cis – trans isomerization, and generation of diffraction gratings

The paper reports on the influence of high polymer molar mass on polymer–dye interactions and the photochromic behavior of azopyridine‐based chromophores. Photoinduced trans – cis and thermal cis – trans isomerizations of azopyridine dyes were investigated in solution and in high‐molar‐mass polymer matrices based on poly(4‐vinylphenol) and poly(4‐vinylpyridine) using UV–visible spectroscopy. The photochromic behavior and thermal relaxation kinetics depend on both the chemical structure of the azo dye and its environment. The results have demonstrated that a high polymer molar mass suppresses the formation of hydrogen bonds between the polymer matrix and the azopyridine dyes. Unlike the behavior commonly reported for low‐molar‐mass polymers, no polymer–dye hydrogen bonding was detected in any of the investigated systems. The absence of these interactions, together with the likely reduced free volume in high‐molar‐mass matrices, limits the photoisomerization process, which is essential for efficient diffraction grating recording. These findings identify the polymer molar mass as a key parameter governing the performance of photorefractive azo‐based polymer materials.