Dielectric tensor, effective polarizability and local electric field in pyrene crystals

The complete dielectric tensor of pyrene single crystals has been measured at 298 K for frequencies of 100 kHz and 1 MHz. The principal components of the tensor are ε{lunate}₁ = 2.70 ± 0.05, ε{lunate}₂ = 3.07 ± 0.03 and ε{lunate}₃ = 3.80 ± 0.08, the 3-axis making an angle of 70° ± 2° with the ⁺a-axis. The results are used to calculate the effective molecular polarizability and the local electric field, with the molecules treated either individually or as 'dimers' pairs. The theory is similar to that for naphthalene, with no unique solution, but a simple particular form of solution applicable to either case is chosen to allow comparisons. For a 'dimer' pair, the mean polarizability is close to twice that of a free molecule, but more isotropic. For an individual molecule, the mean polarizability is 50% larger than for a free molecule, but the anisotropy is little different. Similarly, the local field due to 'dimer' pairs is nearly isotropic whereas the due to individual molecules is markedly anisotropic. These effects stem from the different anisotropy of the interactions explicitly treated in each case. Two simplified local fields resemble that calculated for 'dimer' pairs.