Triazenes are a unique class of polyazo compounds containing three consecutive nitrogen atoms in an acyclic arrangement and are promising NLO candidates. In the present work, a series of 15 donor-π-acceptor type vinyl coupled triazene derivatives (VCTDs) with different acceptors (-NO(2), -CN, and -COOH) have been designed, and their structure, nonlinear response, and optoelectronic properties have been studied using density functional theory and time-dependent density functional theory methods. B3LYP/6-311g(d,p) optimized geometries of the designed candidates show delocalization from the acceptor to donor through a π-bridge. Molecular orbital composition analysis reveals that HOMO is stabilized by the π-bridge, whereas acceptors play a major role in the stabilization of LUMO. Among the three acceptors, nitro derivatives are found to be efficient NLO candidates, and tri- and di-substituted cyano and carboxylic acid derivatives also show reasonably good NLO response. The effect of solvation on these properties has been studied using PCM calculations. From TDDFT calculations, the computed absorption spectra of these candidates lie in the range of 350-480 nm in the gas phase and have positive solvatochromism. The ground-state stabilization interactions are accounted from NBO calculations. In an effort to substantiate the thermal stability of the designed candidates, computations have been done to identify the weak interactions in the systems through NCI and AIM analysis. In summary, 10 out of 15 designed candidates are found to have excellent NLO and optoelectronic properties.

Hypercoordination in silicon has long been reviewed. Dihalogenated perhalocyclohexasilane inverse sandwich complexes (ISCs) are the only group of hypercoordinate Si complexes with anion donors that contact six neutral silicon atoms; opening prospective applications in Si self-assembled nanostructures. Hypercoordinate bonds in 16 such ISCs were studied and their anion ring interactions have been understood with respect to halides. μ(6) mode of coordination was confirmed by the presence of 6 equivalent (3,-1) bond critical points through Bader's QTAIM perspective. The presence of Lewis acid sites above and below the flat Si rings were examined through a reduced density gradient (RDG) analysis, and the ability of halide anions (X' = F, Cl, Br, I) to hypercoordinate has been understood. Role of the ring halides (X) in tuning size and acidity of Lewis sites has been addressed. While the total interaction between the two anions and the ring is quantified through EDA, each SiX' hypercoordinate bond was identified as either purely ionic or transient through QTAIM computations. CDA shows that these complexes are of donor-acceptor type with significant back-donation. The analysis shows that BrF' and IF' were found to reach maximum covalency within the group. Hence in future, tuning these ISCs for construction of nanocrystalline Si structures for optoelectronic properties can essentially utilize the collective, weak yet hypercoordinate Si in these complexes.

Like siloles, benzosiloles have low lying LUMOs due to σ*-π* conjugation between Si and the butadiene moiety but are more amenable for structural tuning. In total, 27 benzosiloles, 12 of them already synthesized and another 15 newly reported here, have been investigated using DFT and TDDFT calculations with an aim to check their suitability for optoelectronic applications. Our results show that all these molecules have excellent π-conjugation throughout. Frontier molecular orbital analysis gives an estimate of the band gap of these benzosilole derivatives and further reveals that the LUMOs are highly localized on the benzosilole moiety whereas HOMOs are localized on both the benzosilole moiety and the substituents. TDDFT calculations have been performed to understand the absorption properties in gas and solvent phases. PCM calculations show that solvation has a minimum effect on absorption maxima. Among the different functionals, PBE0 was found to perform well compared to other functionals and the computed absorption spectra are in good agreement with experiments. Among the designed candidates, styryl substituted benzosiloles are the most promising, showing higher wavelength of absorption and would make better OLED materials. NBO and AIM analysis provide evidence for complete delocalization in these systems. It is interesting to note that eleven out of the fifteen newly designed candidates have lower band gaps than the best known benzosilole derivatives synthesized so far.

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