Pariser-Parr-Pople Model based Investigation of Ground and Low-Lying Excited States of Long Acenes

TL;DR

This study uses the Pariser-Parr-Pople model and MRSDCI approach to investigate the ground and excited states of long acenes, confirming their singlet ground state and analyzing the singlet-triplet gap.

Contribution

First systematic theoretical analysis of long acenes' electronic states using PPP model and MRSDCI, clarifying ground state nature and singlet-triplet gap behavior.

Findings

Singlet-triplet gap decreases with length but remains finite up to decacene.

Long acenes tend towards an open-shell singlet ground state.

Experimental absorption spectra agree with singlet state calculations.

Abstract

Several years back Angliker et al [Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet state $1^{3}B_{2u}$ as the ground state, instead of the singlet $1^{1}A_{g}$ state. However, contrary to that prediction, in a recent experimental work T\"onshoff and Bettinger [ Angew. Chem. Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the nature of the ground, and the low-lying excited states of long acenes, with an emphasis on the singlet-triplet gap, starting from naphthalene, all the way up to decacene. Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP) Hamiltonian, along with large-scale multi-reference singles-doubles configuration interaction (MRSDCI) approach. Our results predict that even though the singlet-triplet gap decreases with the increasing conjugation length, nevertheless, it remains finite till decacene, thus providing no evidence of the predicted singlet-triplet crossover. We also analyze the nature of many-particle wavefunction of the correlated singlet ground state and find that the longer acenes exhibit tendency towards a open-shell singlet ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe excellent agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes.