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The polycyclic p-quinodimethanes are proposed to be the novel candidates of the high-performance nonlinear optical (NLO) materials because of their large third order polarizabilities ( γ). We investigate the switchable NLO responses of a series of polycyclic p-quinodimethanes with redox properties by employing the density functional theory (DFT). The polycyclic p-quinodimethanes are forecasted to exhibit obvious pure diradical characters because of their large y0 index (the y0 index is a value between 0 [closed-shell state] and 1 [pure biradical state]). The γ values of these polycyclic p-quinodimethanes and their corresponding one-electron and two-electron reduced/oxidized species are calculated by the (U)BHandHLYP method. The γ values of polycyclic p-quinodimethanes and their corresponding one-electron reduced species are all positive and significantly different. The large differences of the γ values are due to a change in the transition energy and are related to the different delocalization of the spin density, which demonstrates that the NLO switching is more effective on one-electron reduction reactions. Therefore, the study on these polycyclic p-quinodimethanes provides a guideline for a molecular design of highly efficient NLO switching. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The static second-order nonlinear optical (NLO) propertieson a series of the two-dimensional (2D) pincer Ru(II) complexes withthe substituted Tpy and H2SCS tridentate ligands (Tpy =2,2′:6′,2″-terpyridyl and H2SCS =2,6-bis(benzylaminothiocarbonyl)phenyl) have been investigated bydensity functional theory (DFT). Introducing different donor/acceptorsubstituents to two ligands has an influence on the static first hyperpolarizabilities(βtot) of the 2D systems. Compared to the referencesystem 1[Ru(H2SCS)(Tpy)]+, introducingthe branches with strong electron acceptor group (p-NO2-phenylethynyl) to the Tpy ligand or the brancheswith strong electron donor group (p-NH2-phenylethynyl) to the H2SCS ligand can effectively improvethe βtotvalues. Time-dependent DFT (TDDFT) calculationsindicate that the enhanced βtotvalues of the substitutedsystems are dominated by the intraligand charge transfer (ILCT), metal-to-ligandcharge transfer (MLCT) and ligand-to-metal charge transfer (LMCT)transitions. Furthermore, the proton abstraction plays an importantrole in tuning the second-order NLO response. Particularly, for system 5bearing the branches with NO2groups on H2SCS ligand, there is a dramatic enhancement in the βtotvalues for its deprotonated forms. The βtotvalues of the monodeprotonated system 5-Hand the dideprotonatedsystem 5-2H(58.712 × 10–30and761.803 × 10–30esu) are about 7.58 times and36.4 times larger than their diprotonated system 5, respectively.The second-order NLO responses based on substituent effect and protonabstraction switch are two-dimensional in characteristic with thelarge off-diagonal tensor values. [ABSTRACT FROM AUTHOR]