Theoretical studies of the electronic structure, confomations, spectra, and hyperpolarisabilities of squarates and related molecules

Calculations are reported on the structure and conformational energies of substituted deltates, squarates, croconates and rhodizonates at the ab initio 3-21G level. The calculated hyperpolarisabilities of these structures using both the coupled Hartree–Fock and sum-over-states methods increase with increasing ring size. Both dimethyl croconate and dimethyl rhodizonate give values which are comparable to N,N-dimethyl-4-nitroaniline. These molecules are predicted to absorb in the ultraviolet region of the spectrum and have potential applications as second harmonic generators.