The effect of halides and coordination mode of 4-amino-2,1,3-benzothiadiazole on the luminescence properties of its Zn complexes

Abstract

We report a systematic study of the structure–luminescence relationship of novel coordination compounds [ZnL₂Br₂] (1), [ZnLBr₂]_n (2), [ZnL₂I₂] (3) and [ZnLI₂]_n (4) with 4-amino-2,1,3-benzothiadiazole (L). Variation of the halide and reaction conditions resulted in the formation of four structural types that differ from each other by the coordination mode of L and spatial geometry of the complexes. Drastic differences in the photophysical properties of the compounds were considered in terms of the electronic and stereochemical effects of the halide co-ligands, as well as in terms of the coordination mode of L. The results are discussed by means of TD-DFT quantum chemical calculations in comparison with the previously reported chloride, [ZnL₂Cl₂]. To interpret electronic transitions in coordination polymer 4, we performed calculations of finite-fragment cropped models, which are quite a common approximation used for coordination polymers. However, the finite models of different length gave questionable results. To overcome this problem, TD-DFPT calculations using a periodic model were performed, which gave a more correct interpretation of the electronic transitions. Compound 1 features a dual-band emission spanning the entire visible region. The long wavelength band is attributed to free L as a dopant in crystals of 1 implying the energy transfer from the excited state of the complex to L. Iterative treatment of the powder with hexane can control the doping concentration of L and, as a consequence, the emission colour. The prepared film of 1 in a polystyrene matrix shows a similar emission, but with increased intensity of the long wavelength band, which results in a warmer luminescent colour.