Strategic design of thiophene-fused nickel dithiolene derivatives for efficient NLO response.

The NLO properties of two synthesized aryl extended thiophene fused nickel dithiolenes have been studied theoretically. Based on these two systems, a systematic modification has been made by substituting neutral and zwitterionic donor-acceptor groups at the aryl (phenyl and thenyl) ring to enlarge their NLO responses. Among the four designed systems, the zwitterionic donor-acceptor group significantly reduces the HOMO-LUMO energy gap, resulting in an enormous increase in the first hyperpolarizability (β) values. To judge their high NLO response, transition dipole moment (TDM) density values have been plotted and it has been found that electron dissipation occurs from one donor part to the acceptor part with a high Δr index value. It should be noted that the high Δr index values are a quantitative measurement to understand the type of transitions, and we noticed that a charge transfer transition occurs in the case of zwitterionic systems. Hence, a relationship between the first hyperpolarizability and TDM has been established. In order to highlight the NLO active segment in a molecule, a density analysis has also been done. We anticipate that as our designed systems possess high β values, they should have significance in optical uses.