HX-linear and nonlinear optical responsiveness of rationally designed heteroleptic d8-metallo-dithiolene complexes

This work presents the HX responsiveness of the heteroleptic Donor-M-Acceptor dithiolene complexes: Bu4N[MII(L1)(L2)] [M=Ni(1), Pd(2), Pt(3)], where L1 is a chiral acceptor ligand [(R)-α-MBAdto] and L2 is a donor ligand (tdas). 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 has been 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 reveal to be excellent optical sensors against the response the triethylamine, which is a toxic volatile amine. Moreover, HCl responsiveness of the non-linear optical properties of complexes 1, 2, and 3 embedded into a PMMA poled matrix has been pointed out. Reversible chemical 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-adducts films are always larger than those of the respective free-complexes films. Density Functional Theory (DFT) and time-dependent DFT calculations help highlighting the structure-properties relationship.