A model Exact Study of the properties of low-lying electronic states of Perylene and Substituted Perylenes

TL;DR

This study precisely models the low-lying electronic states of perylene and substituted perylenes, revealing their potential for optoelectronic applications and tunability through substitution.

Contribution

It provides an exact solution of the Pariser-Parr-Pople Hamiltonian for perylene's electronic states and explores substitution effects on optical properties.

Findings

Perylene's singlet and triplet states are characterized with energies suitable for singlet fission.

Substitutions lower the optical gap to about 2.8 eV, enabling blue emission.

Bond and spin density analyses reveal weak coupling in singlets and localized spins in triplets.

Abstract

There is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons, in the VB basis where dimensionality is $\sim$ 5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one and two photon states are very close in energy $\sim$ 3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically either at the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalene in the singlet states but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign.