Symmetric vs. asymmetric: Which one is the better molecular configuration for achieving robust NLO response?

The current study reports a comparative investigation for the nonlinear optical (NLO) response for some symmetric and asymmetric molecular configurations. The designed compounds are featuring a similar 2,7-bis(phenyl ethynyl)cyclopentaaceanthrylene core connected to different terminal donor and acceptor moieties such as dimethylamine, methoxy, nitro and ethene-tricarbonitrile groups based on three topologies, "D-π-D" (1-2), "A-π-A" (3-4) and "D-π-A" (5-8). A comparative analysis of these symmetrically and asymmetrically substituted chromophores is done to investigate the effect of different terminal groups on their NLO response, optical and charge transfer properties. The asymmetric compounds display larger first hyperpolarizability amplitudes (β_//) than those of the symmetric compounds where a β_// amplitude as large as 1851 × 10^-30 esu is calculated for asymmetric compound 6. Interestingly, the average second hyperpolarizabilities (γ) for symmetric compound 4 have been calculated to be 16, 096 × 10^-36 esu and for asymmetric counterpart compound 6 it is found to be 23, 162 × 10^-36 esu using M06/6-311G* methodology. Our findings were also inline with experimental observations of Mathews et al. (Mathews, S.J., et al., 2007. Optics Communications, 280, 206-212), where they also experimentally determined the larger third-order susiptibility for asymmetric phthalocyanine (Pc3) than their symmetric counterparts. Additionally, the NLO polarizabilities of all the compounds have also been compared with a typical donor-acceptor molecule of p-NA to specify the comparison of designed compounds. TD-DFT studies have shown that the higher β and γ values were found for asymmetric compounds owing to their lower transition energies and larger trasition dipole moments. The depiction of molecular orbitals and electrostatic potentials have shown more efficient intramolecular charge transfer (ICT) for asymmetric compounds leading to the robust NLO response. Thus, a systematic comparison of the NLO polarizabilities and other electronic properties of our designed symmetric and asymmetric compounds shows that symmetry based designing of NLO compounds are very crucial to get robust NLO response properties.