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There is a resurgence of interest in the electronic structure of perylene forits applications in molecular devices such as organic photovoltaics and organiclight emitting diodes. In this study, we have obtained the low-lying singletstates of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonianof this system with 20 sites and 20 electrons, in the VB basis wheredimensionality is $\sim$ 5.92 billion. The triplet states of perylene areobtained using a DMRG scheme with symmetry adaptation. The one and two photonstates are very close in energy $\sim$ 3.2 eV while the lowest triplet state isslightly below 1.6 eV indicating that perylene is a good candidate for singletfission. To explore the tunability of the electronic states, we have studieddonor-acceptor substituted perylenes. The two donors and two acceptors aresubstituted symmetrically either at the four bay sites or four peri sites. Inall 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 blueemitters. We have also reported bond orders in all the cases and perylene aswell as substituted perylenes can be viewed as two weakly coupled naphthalenein the singlet states but in triplets these bonds tend to be comparable toother bonds in strength. The charge densities in substituted perylenes aremostly localized around the substitution sites in the ground state. Thepositive spin densities in triplets are concentrated around the peri and baysites with the remaining sites having small spin densities of either sign.

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