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

Driven 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 Ų), 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 γ_xxxx(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.