Mecanismo de transferência protônica em compostos benzotiazóicos: um estudo teórico-experimental

Abstract

This is an investigation of benzothiazoic compounds in the solid state with emphasis on fluorescence by excited state intramolecular proton transfer (ESIPT), a mechanism that can cause large Stokes displacements and intense emission when compared to traditional fluorophores. Have been characterized theoretically and experimentally derivatives of the benzazole family, specifically the species of photoluminescent heterocycles 2- (4'- aminophenyl) benzothiazole - 4HBSN and 2- (4'-amino-2'-hydroxyphenyl) benzothiazole - 4HBS. To characterize the photophysical processes, experimental techniques of absorption in the ultraviolet and fluorescence emission were used, while the vibrational studies were based on analyzes of Raman and infrared spectroscopy (FT-IR). At the same time, were used theoretical calculation methods based on the density functional theory (DFT), theory level B3LYP / TZVP, in order to assist in the interpretation of experimental results and to obtain additional information through the construction of a potential energy surface (PES) of the systems, inspection of frontier orbitals and distribution of electronic charge, in addition to the verification of changes in strength of intramolecular hydrogen bonds during the ESIPT process. Stokes' displacement from 92 nm to 4HBS (structure with probable ESIPT) and from 31 nm to 4HBSN (does not have ESIPT) was verified, which is in agreement with the literature. Exclusive contribution of OH vibration (hydroxyl present in 4HBS), involved in proton transfer, was observed at 3047 cm-1 (Raman) and 3224 cm-1 (FT-IR). Theoretically, a conformational analysis of the species was performed and obtained optimized molecular structures. The PES of the 4HBS molecule reported an activation barrier of 10.5 kcal / mol. The frontier orbitals indicated that the HOMO to LUMO transitions in both species occur from ligand (π) to anti-ligand (π *), so that 4HBS is more reactive than 4HBSN, which presented an energy gap of 4.00 and 4.12 eV, respectively. The set of analyzes pointed to the existence of ESIPT in the 4HBS molecule, while 4HBSN may have its photoluminescence influenced by intermolecular bonds.