Electronically tunable cyanoethynylethenes and Tetracyanobuta-1, 3-di- enes based Push-pull organic chromophores for enhanced nonlinear optical Activity: A DFT perspective

Small organic molecules that have a few number of double and triple bonds will not show large optical excitation energies however, with a strategic design and substitution with donor and accepter group, enhance the second order nonlinear optical (NLO) properties. We have designed small organic molecules by modifying donor-acceptor (DA) substituent with compact pi-conjugated systems. We have performed density functional theory calculations by using CAM-B3LYP/6-311+G (d, p) level of theory. The calculated results reviled that the proper donor group substitution leads to absorption within the visible region. The maximum absorption wavelength of the designed molecules shows a red shift trend. The observed trend in absorption and first hyperpolarizability indicate that the crucial transition energy become smaller and dipole moment is larger. Among the designed molecules, the largest first order hyperpolarizability of compound 9 is showing multifold large first hyperpolrizability than that of p-nitroaniline. Additional new peak arises in LUMO region that directly affect the band gap. Further, to see the structure property relationship towards enhancement of NLO response, we have calculated the frontier molecular orbital (FMO), transition density matrix (TDM) and mo-lecular electrostatic potential (MESP) of the designed molecules.