A cation-selective and anion-controlled benzothiazolyl-attached macrocycle for NLO-based cation detection: variational first hyperpolarizabilities

Abstract

Recently, a benzothiazolyl group bearing the NO₂S₂-macrocycle L has been the subject of great interest due to its unique advantages in binding metal cations. Here, we systematically investigate the second-order nonlinear optical (NLO) properties of L, its metal cation derivatives L*M (M = Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Cd²⁺, and Hg²⁺), and some anion-controlled complexes [Hg(L)(ClO₄)₂ and Hg(L)(Cl)₂] by density functional theory (DFT). The results show that the addition of the transition metal cation Hg²⁺ leads to the increasing transition energy of the S₀ → S₁ transition, which induces a significant decrement in the NLO response (β_tot = 8.2 × 10⁻³⁰ esu) with respect to the corresponding macrocycle L (β_tot = 125.8 × 10⁻³⁰ esu). The depolarization ratio (DR) and anisotropy parameter (ρ) render it possible to control the NLO contribution. Further analysis of the charge transfer (CT) parameters, transferred electrons (q^CT), CT distance (d^CT), and t index indicates that L*Hg²⁺ reaches the minimum CT and spatial charge separation. Thus, the transition metal cation Hg²⁺ is distinguishable and easier to detect by utilizing the variations in the NLO response. Furthermore, the addition of ClO₄⁻ to the complex L*Hg²⁺ forms a distorted pentagonal bipyramid configuration, which drives the significant reduction of the first hyperpolarizability. Therefore, the obvious NLO contrasts can be switched by different metal cations and anions, which will broaden the scope of optical molecular detectors and further inspire us to investigate it.