Unravelling the photobehavior of a 2,1,3-benzothiadiazole-based HOF and its molecular units: experimental and theoretical insights into their spectroscopic properties in solution and in the solid state

Abstract

2,1,3-Benzothiadiazole (BTD) acid derivatives have emerged as versatile new building blocks for the fabrication of smart porous materials. Herein, we report experimental and theoretical studies on a BTD-based H-bonded organic frameworks (HOFs) and its methylated ester and carboxylic acid molecular units in solutions and in solid state. BTIA-ester and BTIA-COOH in solutions display a large Stokes-shifted emission (∼6000 cm⁻¹) as a result of an intramolecular charge-transfer (ICT) reaction in the excited species, followed by a twisting of phenyl moieties. Femtosecond (fs) experiments on the ester and acid derivatives in N,N-dimethylformamide (DMF) reveal that an ICT reaction takes place in ∼300 fs while the phenyl twisting occurs in ∼6 ps. BTIA-ester shows a single emission lifetime of 7.52 ns, while BTIA-COOH displays lifetimes of 390 ps and 1.15 and 7.75 ns assigned to different emitting species. Theoretical calculations on the ester compound agree with experimental observations. Flash photolysis experiments on both samples in DMF and under ambient conditions show a triplet state population living for ∼500 ns and a charge-separated species living for ∼2 μs. In solid state, BTIA-ester shows an abnormally slow ICT event (80 ps) leading to ICT aggregates with lifetimes of 1.20 and 2.87 ns, whereas BTIA-HOF exhibits fast ICT and intermolecular proton-transfer (PT) reactions (<15 ps), producing ICT and ionic species with lifetimes of 0.52 and 1.23 ns, respectively. At the single-crystal level, BTIA-HOF displays fluorescence properties not dependent on the interrogated point under fluorescence microscopy, suggesting a homogeneous distribution of the molecular units of the HOF. These results provide new findings for a better understanding of the photobehavior of BTD derivatives and related HOFs and will help in the development of new HOFs for photonic applications.