Crystal Field Effects on Atomic and Functional-Group Distributed Polarizabilities of Molecular Materials

Abstract

To rationally design new molecular materials with desirable linear optical properties, such as refractive index or birefringence, we investigated how atomic and functional-group polarizability tensors of prototypical molecules respond to crystal field effects. By building finite aggregates of urea, succinic acid, p-nitroaniline, 4-mercaptopyridine, or methylbenzoate, and by partitioning the cluster electronic density using quantum theory of atoms in molecules, we could extract atoms and functional groups from the aggregates and estimate their polarizability enhancements with respect to values calculated for molecules in isolation. The isotropic polarizability and its anisotropy for the molecular building blocks are used to understand the functional-group sources of optical properties in these model systems, which could help the synthetic chemist to fabricate efficient materials. This analysis is complemented by benchmarking density functionals for atomic distributed polarizabilities in gas phase, by comparing the results with refractive-index calculations under periodic boundary conditions, and by estimating functional-group optical properties from a classical electrostatic atom–dipole interaction model.

Accurate Atom–Dipole Interaction Model for Prediction of Electro-optical Properties: From van der Waals Aggregates to Covalently Bonded Clusters

“…These interactions are similar in both 1,3- and 1,4-clusters, but properties of the 1,3-nitrophenol aggregate are slightly better estimated from ADIM than those of the 1,4-version. This fact has already been observed for larger aggregates of carboxylic acids and may have to do with the efficiency of the crystal packing. The periodic environment around a molecule of 1,3-nitrophenol could be such that electronic polarization and/or charge-transfer effects from the vicinity in one functional group are at least partially compensated by another, in a way that the central molecule retains more or less its density as one would have seen in isolation.…”

“…Molecular clusters can be seen in Figure , along with their distributed polarizabilities. It was verified that calculations on largely anisotropic clusters often result in unrealistic electro-optical properties mainly due to failures associated with the challenging process of determining QTAIM zero-flux boundaries. , On the other hand, we have demonstrated that environmental effects can be accurately recovered from linear clusters as they already feature the most important sources of polarization and charge transfer on the central molecule due to vicinity …”

“…Electronic structure calculations were carried out for isolated molecules and their aggregates using Gaussian 16 at the M06-HF/aug-cc-pVDZ level of theory. As demonstrated before, the family of hybrid functionals M06 is very accurate to estimate atomic and molecular distributed polarizabilities with respect to benchmarks at the CCSD correlation level. Electronic densities were partitioned using AIMAll by means of the quantum theory of atoms in molecules (QTAIM).…”

“…However, the limited amount of electronic correlation introduced often hampers accurate predictions. An alternative to account for the crystal field is to simulate finite molecular aggregates, as well as to extract the electronic density of the central molecule as an estimation of the entire crystal, even though results could be highly dependent on the size and shape of the chosen clusters …”

“…In particular, one of the most successful implementations is known as the atom–dipole interaction model (ADIM), which is characterized by a dipole field tensor whose components depend on each interacting pair of the unit cell . We have built a modified version of ADIM that uses a distributed atomic or functional-group implementation instead of the more traditional one in which an entire molecule is treated as a point dipole . While the correctness of both versions to reproduce the crystal field due to long-range interactions is doubtless, their ability to mimic polarization and charge-transfer effects due to the closest interaction sphere remained to be better understood, particularly for the cases where relatively strong intermolecular interactions are present.…”