Revealing the ESIPT process in benzoxazole fluorophores within polymeric matrices through theory and experiment

The impact of the polymeric matrix on the photophysical characteristics of monomeric dyes responsive to excited-state intramolecular proton transfer (ESIPT) was investigated through UV-Vis absorption as well as steady-state and time-resolved emission spectroscopies. For this purpose, two benzoxazole monomers (M1 and M2) with acryloyl groups at different positions in their molecular structures were employed to facilitate covalent bonding within a styrene chain. Our findings reveal significant variations in their excited-state properties due to the proximity of the acryloyl groups, which affects the energy barrier of the ESIPT reaction, the emission wavelength, and the balance between the normal and tautomeric forms. The experimental results were corroborated through theoretical investigations at the DFT/TDDFT level, specifically using the B3LYP-D3/def2-TZVP methodology. Three notable observations emerged: donor/acceptor groups at the meta/para positions induced electron distribution changes, causing red-shifted emission for M2; in the polymer film, particularly in PM1, intramolecular hydrogen bond deactivation favored N* emission over T* emission; and the zwitterionic character of the T* species. This study underscores the advantages of functionalization in polymers, which can lead to colorless films and prevalent N* or T* emission, and contributes valuable insights into molecular design strategies for tailoring the photophysical properties of polymeric materials.