Conjugation-modulated optoelectronic and anti-corrosion properties of pyrrole oligomer: Insights from DFT/TD-DFT analysis

To investigate the influence of n-conjugation on optical properties, models of Pyrrole oligomers with varying n-bridge lengths, ranging from 1 to 5, were constructed. The n-bridge plays a crucial role in tuning and modu-lating the optoelectronic characteristics of these molecules. By employing FMO (Frontier Molecular Orbital) analysis, the band gap values were determined. Notably, the pyrrole oligomer with a 1n-bridge exhibited a higher band gap (2.68 eV) compared to the 5n-bridge variant (1.53 eV). Moreover, the calculated hyperpolarizability (beta degrees) values exhibited a substantial increase from 37 esu to 1876 esu as the extent of n-conjugation was augmented. This suggests that enhanced n-conjugation promotes greater nonlinear optical properties. Further analysis revealed that the pyrrole oligomer with 5n-bridges displayed maximal absorption at a longer wave-length, specifically 1024 nm. This information provides valuable insights into the absorption characteristics of these molecules. Additionally, the NBO (Natural Bond Orbital) analysis successfully captured the stable donor-acceptor interactions responsible for the ground state stabilization observed in pyrrole oligomers. On the other hand, the QTAIM (Quantum Theory of Atoms in Molecules) analysis provided detailed information regarding weak interactions, such as hydrogen bonding, which contribute to the stabilization of the modeled monomers. In summary, the creation of Pyrrole oligomer models with varying n-bridge lengths facilitated the evaluation of the impact of n-conjugation on the optical properties. The findings indicate that increasing the n-conjugation leads to a decrease in the band gap, an increase in hyperpolarizability, and absorption at longer wavelengths. Additionally, the stability of the ground state and the nature of weak interactions were elucidated through NBO and QTAIM analyses, respectively.