Photophysical behavior of polysubstituted hydrazones and Triazaborolo[4,5-a]pyridines: Insights into structure-property relationships

In this study, we present the synthesis through a prefunctionalization strategy of two series of fully polysubstituted (Z)-2-(2-((1H-pyrrol-2-yl)methylene)hydrazineyl)pyridine (HPY) and triazaborolo[4,5-a]pyridine (TBPY) bearing substituents with diverse electronic characters. The photophysical and electrochemical properties of HPY and the corresponding TBPY series were investigated using UV–Vis, static and dynamic fluorescence, and cyclic voltammetry. The experimental results point out that the central hydrazone and azine-boro coordinated groups interrupt the electronic conjugation between the pyrrole and its substituents (segment A), and between the =N-N=C- (B-complex in the TBPY)-pyridine and its substituents (segment B). The A and B segments act as separated chromophores and opto-electroactive moieties. No significant effect of the substituents R¹, R² (in pyrrole), and R³ (in pyridine) is observed on the absorption and fluorescence maxima or oxidation/reduction peaks because they are out of the plane of the molecule, as found by theoretical studies. All compounds are generally poorly fluorescent in solution due to a possible mutual rotation of segments A and B. Moreover, according to the DFT analysis, the molecules have push-pull character where the first excitation state has a charge transfer (HPY) or hybridized local charge transfer character (TBPY), which can be associated with intramolecular charge transfer that contributes to the low fluorescence emission in solution. Interestingly, the fluorescence increases in the solid state for many compounds, where rotation is more restricted, getting values from 1.4 % up to 87 %.