We describe the quantitative [2 + 2] photocycloaddition of crystalline trans-2,4-dichloro-6-styrylpyrimidine to produce the corresponding htt r-ctt cyclobutane dimer, and we present (1)H NMR analysis of the photolysis of this and six other mono-, di-, and triazastilbenes in solid and solution states. Density functional (M06-2X) and correlated ab initio (MP2) calculations were used to obtain interaction energies between two monomers of each azastilbene. These energies mirror the relative polarization of the stilbene moieties and can be quantitatively correlated with the rate of reaction and selective formation of the htt r-ctt dimers. In the solid state, poor correlation is observed between interaction energy and reactivity/selectivity. This lack of correlation is explained through X-ray analysis of the azastilbene monomers and is shown to be in accordance with the principles of Schmidt's topochemical postulate. Conversely, in solution there is a strong positive correlation (R(2) = 0.96) between interaction energies and formation of the htt r-ctt dimer. These results are the first to show this correlation and to demonstrate the utility of calculated interaction energies as a tool for the prediction of stereo- and regioselectivity in solution-state stilbene-type photocycloadditions.