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

Abstract

Several years ago, Angliker et al. [ Chem. Phys. Lett. 1982 , 87 , 208 ] predicted nonacene to be the first linear acene with the triplet state 1(3)B2u as the ground state, instead of the singlet 1(1)Ag state. However, contrary to that prediction, in a recent experimental work, Tönshoff 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. The methodology adopted in our work is based upon the Pariser-Parr-Pople model (PPP) Hamiltonian, along with the large-scale multireference 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 wave function of the correlated singlet ground state and find that the longer acenes exhibit a tendency toward an 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. Calculated triplet absorption spectra of acenes predict two well-separated intense long-axis polarized absorptions, against one such peak observed for the singlet case. This is an important prediction regarding the triplet optics of acenes, which can be tested in future experiments on oriented samples.

The Longest Acenes

“…A linear correlation of the energies of the SOMO-1 → SOMO transition was found for the radical cations of the acene series from pentacene to nonacene. Finally, semiempirical Pariser-Parr-Pople multireference CI based computations [18] are in agreement with the transition energies obtained in the matrix isolation study.…”

“…[7,8] Various conceivable mechanisms of symmetry breaking to create a band gap have been put forward. [8][9][10] Large acenes are strongly correlated systems that have received considerable attention from the theoretically [11][12][13][14][15][16][17][18] and experimentally [19][20][21][22][23] oriented chemical community.…”

A Practical General Method for the Preparation of Long Acenes

“…This topology, with only one Clar aromatic sextet spread over the entire molecule, leads to a decrease of the HOMO–LUMO gap and an increase in the chemical reactivity with increasing number of benzene rings. The nature of the electronic structure of long acenes is still actively discussed, in particular with respect to the partial contribution of the open‐shell singlet state to the electronic ground‐state configuration beyond decacene . On the other hand, these compounds are predicted to show attractive electronic and magnetic properties .…”

Theoretical Investigation of the Intermolecular Charge‐Transfer Interactions of Poly‐p‐Phenylene Oligomers

“…43 This model has been used extensively in the studies of π-conjugated systems and proven to give reliable results for spectroscopic and other linear or nonlinear optical properties with respect to the experimental results and computationally highly extensive ab initio calculations. [44][45][46][47][48] The Pariser-Parr-Pople model has been shown to well reproduce the excitation energies as well as excited-state properties compared to the all-valence INDO/S model 33,39,40 which provides reliable data in comparison with the UV-vis and UPS spectra of many conjugated molecules. 8,49 To obtain the contribution of the CT characters to the lowest two excitation energies, four truncated models were used to compare with the full CM Hamiltonian (CTE(f ) model).…”

“…The excited state wave functions of the monomers and dimers of PPP n were obtained by the single configuration‐interaction (SCI) scheme with all occupied and unoccupied π‐levels included within the Pariser–Parr–Pople Hamiltonian . This model has been used extensively in the studies of π‐conjugated systems and proven to give reliable results for spectroscopic and other linear or nonlinear optical properties with respect to the experimental results and computationally highly extensive ab initio calculations . The Pariser–Parr–Pople model has been shown to well reproduce the excitation energies as well as excited‐state properties compared to the all‐valence INDO/S model which provides reliable data in comparison with the UV–vis and UPS spectra of many conjugated molecules .…”