A simple
model is presented for interpreting the presence of substantial
second harmonic generation (SHG) activity from assemblies of centrosymmetric
molecular building blocks. Using butadiene as a computationally tractable
centrosymmetric model system, time-dependent Hartree–Fock calculations
of the nonlinear polarizability of butadiene dimer were well-described
through exciton coupling arguments based on the electronic structure
of the monomer and the relative orientation between the monomers within
the dimer. Experimental studies of the centrosymmetric molecule 2,6-di-tert-butylanthraquinone suggest the formation of a combination
of SHG-active and SHG-inactive crystal forms. The structure for the
centrosymmetric form is known, serving as a negative control for the
model, while the presence of an additional SHG-active metastable form
is consistent with predictions of the model for alternative molecular
packing configurations.