HX-Linear and Nonlinear Optical Responsiveness of Rationally Designed Heteroleptic d8-Metallo-dithiolene Complexes

TL;DR

This paper studies metal complexes that change color and optical properties when exposed to HCl and ammonia, showing potential for use as chemical sensors.

Contribution

The study introduces heteroleptic d8-metallo-dithiolene complexes with reversible optical responsiveness to HCl and ammonia.

Findings

Exposure to HCl vapors causes a visible color change in cellulose films of complex 1.

Complexes embedded in a polymer matrix show reversible SHG switching with HCl and ammonia.

DFT calculations reveal structure–property relationships in the complexes.

Abstract

This work presents the HX-responsiveness of the following heteroleptic donor–M–acceptor dithiolene complexes: Bu4N[MII(L1)(L2)] [M = Ni(1), Pd(2), Pt(3)], where L1 is the chiral acceptor ligand [(R)-α-MBAdto = chiral (R)-(+)α-methylbenzyldithio-oxamidate] and L2 is the donor ligand (tdas = 1,2,5-thiadiazole-3,4-dithiolato). Addition of hydrohalic acids induces a strong bathochromic shift and visible color change, which is fully reversed by ammonia (NH3). Moreover, the sensing capability of 1 was further evaluated by deposition on a cellulose substrate. Exposure to HCl vapors induces an evident color change from purple to green, whereas successive exposure to NH3 vapors fully restores the purple color. Remarkably, cellulose films of 1 were revealed to be excellent optical sensors against the response to triethylamine, which is a toxic volatile amine. Moreover, the HCl-responsiveness of the nonlinear optical properties of complexes 1, 2, and 3 embedded into a poly(methyl methacrylate) poled matrix was demonstrated. Reversible chemical second harmonic generation (SHG) switching is achieved by exposing the poled films to HCl vapors and then to NH3 vapors. The SHG response ratio HCl–adduct/complex is significant (around 1.5). Remarkably, the coefficients of the susceptibility tensor for the HCl–adduct films are always larger than those of the respective free-complex films. Density Functional Theory (DFT) and time-dependent DFT calculations help in highlighting the structure–properties relationship.

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