Quantum chemical study of benzimidazole derivatives to tune the second-order nonlinear optical molecular switching by proton abstraction

A novel sequence for reversible second-order nonlinear optical (NLO) molecular switching with protonation/deprotonation has been achieved and tuned as well. The NLO switching with first hyperpolarizabilities (β₀) as low as 14 × 10^-30 esu (Off-phase) and as large as 1189 × 10^-30 esu (On-phase) have been computed by using density functional theory (DFT). Remarkably large differences between the β₀ values of benzimidazole containing chromophores and their deprotonated anions have shown their significant potential for a new type of NLO molecular switching, as (1-(4-methoxyphenyl)-2-(2-thienyl)pyrrolyl)-1,3- benzimidazole anion (1^-) has a β₀ value computed to be 61 × 10^-30 esu, which is 4 times larger than its neutral molecule 1. This β₀ value has been tuned up to 2028 × 10^-30 esu by effective substitutions in the derivatives of 1 ^- (1a^-, 1b^-, 1c^-, and 1d ^-). Interestingly, the substituted compounds have illustrated robustly large off-on NLO switching with a difference in β₀ values of 7, 63, 85 and 75 times larger than their neutral counterparts, respectively. TD-DFT calculations along with natural bond orbital (NBO), frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP) analyses show that the abstraction of an imido proton brings about a change in push-pull configurations resulting in a red shift for both absorption and emission spectra which subsequently leads to a high performance second-order NLO molecular switching. A similar trend of NLO switching in F^- compounds of these chromophores has also been observed with significantly large β₀ values having analogous electro-optical properties like deprotonated anions. Furthermore, gas-phase acidity (GPA) calculations for the neutral molecule 1 and its derivatives (1a, 1b, 1c, and 1d) have also revealed that these are rationally potent nitrogen acids and can easily be dissociated to produce stable deprotonated anions.