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Nitrogen is a common structural element in the acceptor moieties of organic donor–acceptor nonlinear optical (NLO) chromophores. In order to assess the effect of substitution with heavier group 15 elements, computational studies of the nonlinear optical properties of betaine derivatives, (CH3)3X+CH2CO2− (X = N, P, As, Sb, Bi), have been performed. First hyperpolarisabilities, βHRS(−2ω; ω, ω), corresponding to hyper-Rayleigh scattering susceptibilities have been estimated for this series using TDDFT quadratic response calculations including polarisable-continuum method water solvation and show a five-fold increase on going from N to Bi. Differential frequency dispersion effects lead to a gradual increase in this ratio as the wavelength of the incident radiation is increased from 1907 nm to 800 nm. The depolarisation ratio of the NLO response indicates that the change in β is accompanied by a change in the type of chromophore from octupolar (X = N) to a linear donor–acceptor type (X = Bi). The observed increase of the NLO response correlates with the changing electronic configurations of the group 15 elements and alterations in the character of the frontier molecular orbitals. Relativistic effects are found to play an important part in enhancement of the NLO response in the Sb and Bi betaine derivatives. These results suggest that derivatisation of organic molecules that otherwise display small β values with heavy group 15 elements is a useful method for creating enhanced NLO chromophores.
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