Extended First Hyperpolarizability of Quasi-Octupolar Molecules by Halogenated Methylation: Whether the Iodine Atom is the Best Choice

Inspired by a previous report, in which the quasi-octupolar molecules DPATSB, DPATSP, and (DPATSP-Me)I-+(-) were synthesized and the iodized salt was proposed as a suitable optical limiting chromophore, we performed linear and nonlinear optical analyses on the synthesis molecules and several extension halogenated dyes ((DPATSP-Me)X-+(-) (X = F, Cl, and Br)) by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The energy gap of frontier molecular orbitals (FMOs) was diminished by introducing the pyridinium ion into the system, and a charge-separated state was formed in halogenated salts. The N-methylation of the dye leads to a sharp increase in response characteristics, accompanied by a red-shift of the absorption spectrum from the ultraviolet region to the visible light region. Importantly, our studies show that there exists a significant halogen atomic-species dependence of the nonlinearity of the salts. Fluorination and bromination of the molecule seem to be more potential in improving the nonlinearity in zero-frequency and frequency-dependent incident light, respectively, without causing the undesirable red-shift of the absorption band relative to their halogenated analogues. The nonlinear optical (NLO) responses calculated by different DFTs are identical in trend.