Synthesis, Electronic and Optic Studies of Symmetric and Asymmetric Pyridine–Diarylamine Derivatives

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Abstract
In this work were made the synthesis of symmetric and asymmetric pyridine–diarylamine derivatives through Buchwald–Hartwig cross-coupling reactions using 2,6- and 2,5-dibromopyridine cores functionalized with bis(4-methoxyphenyl)amine (DPA) and iminodibenzyl (IMB) donor units. The influence of molecular symmetry and donor structure on the electronic and optical properties of the resulting compounds was systematically investigated through photophysical, electrochemical, and theoretical studies. UV–Vis spectroscopy revealed absorption bands associated with π–π* transitions and intramolecular charge-transfer (ICT) processes, with the asymmetric 2,5-substituted derivatives exhibiting red-shifted absorption relative to their symmetric 2,6 analogues, indicating enhanced donor–acceptor electronic communication. Fluorescence measurements showed blue emission with quantum yields strongly dependent on both substitution pattern and donor rigidity. Solvatochromic studies confirmed the presence of excited-state charge-transfer processes, while cyclic and square-wave voltammetry demonstrated that asymmetric substitution significantly modifies the frontier molecular orbital energies, leading to improved electron-donating character. Density functional theory (DFT) and time-dependent DFT calculations successfully reproduced the experimental optical behavior and revealed distinct ICT mechanisms depending on the substitution pattern. In the 2,5-substituted derivatives, electron transfer occurs from the diarylamine donor toward the pyridine core, whereas an opposite charge-transfer direction was observed for the 2,6-substituted systems. These findings provide valuable insights into the structure–property relationships governing pyridine–diarylamine conjugated systems and highlight molecular symmetry as a key parameter for tuning their optoelectronic behavior.

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