Exploring the role of H-migration in the aromaticity, spectroscopic, photovoltaic and optical properties of planar heterocyclic compounds: a DFT study

In this study, five tautomer's IB1-IB5 are designed and studied, containing donor (-NH₂)-π(THDP)-acceptor (-NO₂) frameworks, formed through the migration of hydrogen atoms from/to the pyrazine rings. The designed and theoretically explored compounds IB2 and IB5 originate from double-hydrogen migration, while IB3 and IB4 are derived by single-hydrogen migration from adjacent pyrazine rings of IB1. Extensive DFT studies were conducted to explore the aromatic behavior, stability, reactivity, natural bond orbitals (NBOs), polarizabilities, and absorption behavior to investigate the effect of electron localization. Remarkably, the migration of hydrogen on the π(THDP) bridge modulates aromaticity <i>via</i> single and double hydrogen shifting, enhancing spectroscopic behavior and polarizability amplitudes. The migration of a hydrogen atom successfully lowers the H-L energy gap from 1.87 eV to 0.96 eV in the two identical conformers IB3 and IB4. IB5 exhibits higher aromaticity, reactivity, and the lowest transition energy, providing the highest first hyperpolarizability (<i>β</i>_tot) value of 70 671 a.u. at the M06-2X level of theory. The vibrational spectrum of IB5 shows a sharp N-H stretching peak at 3589 cm^-1 and a weaker NO stretching at 1377 cm^-1, indicating optimal donor-acceptor interactions. TD-DFT analysis reveals that the absorption wavelength exhibits a far-UV red shift upon H-migration in IB5. Similarly, π-π stacking interactions in IB5 facilitate efficient charge transfer pathways, significantly enhancing its hyperpolarizability and establishing it as the most promising compound for optoelectronic applications. Furthermore, charge carrier capacity was analyzed through light-harvesting efficiency (LHE) and radiative lifetime studies. This work provides insights into the development of efficient materials with enhanced aromaticity and charge transport efficiency, which could be beneficial for optical, photovoltaic, and data storage applications.