Lone Pair‐Electrons‐ and Aromaticity‐Dependent Optical Nonlinearity Responses of (ƞ5‐Cp)Fe(η5‐P5), Fe(ƞ5‐P5)2, and [Fe(η4‐P4)2]2− Ferrocene Analogs

Abstract

AbstractDriven by their unique electronic structures and geometries, quantum chemistry and wavefunction analyses are conducted to explore the effects of aromaticity and lone pair‐electrons on the linear and nonlinear optical (NLO) responses of four ferrocene analogs. Aromaticity indicators reveal that the stability of (η5‐Cp)Fe(η5‐P5) and [Fe(η4‐P4)2]2− is primarily due to their σ‐aromaticity. In contrast, Fe(η5‐P5)2 exhibits π‐aromaticity, characterized by significant diamagnetic ring currents and electron delocalization facilitated by both out‐of‐plane and in‐plane π‐conjugation, distinguishing it from planar systems like C18. Fe(η5‐P5)2, with the largest surface area (234.60 Å2), displays the strongest van der Waals (vdW) attraction in its central region (−0.95 kcal/mol), surpassing that of [Fe(η4‐P4)2]2−. Further analysis of second‐order NLO responses underscores the critical role of cyclo P4 and cyclo P5 lone pair‐electrons in enhancing polarization anisotropy and optical nonlinearity. Fe(η5‐P5)2 achieves maximum NLO dispersion at γxxxx(λ = 588 nm), showing a 12‐fold increase over Fe(ƞ5‐Cp)2 in the static regime. Real‐space function analyses, hyperpolarizability density, and tensor maps further support these findings, emphasizing the potential of cyclo P5 lone pair‐electrons for the development of high‐performance NLO materials.