Article pubs.acs.org/JPCA
Theoretical Prediction of the Structures and Energies of Olympicene and its Isomers Andrew J. S. Valentine and David A. Mazziotti* Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States ABSTRACT: Pentacene, a linear five-ringed polyaromatic hydrocarbon, has recently been used as an organic semiconductor in field-effect transistors. The recently synthesized olympicene molecule, so named because of its resemblance to the olympic rings, is a more compact five-ringed structure. This paper offers the first theoretical study of the kinetic stability of olympicene and its isomers. We use the parametric two-electron reduced density matrix (2-RDM) method, which takes the 2-RDM as the basic variable in lieu of the traditional wave function in calculations [Mazziotti, D. A. Phys. Rev. Lett. 2008, 101, 253002]. Our calculations demonstrate that olympicene’s isomers may be separated into aromatic and diradical isomers, the latter of which require accurate treatment of strong electron correlation to detect multireference character. Albeit formally a single-reference method, the parametric 2-RDM captures the multireference correlation of the diradical isomers; relative to olympicene, the 2-RDM predicts five diradical isomers that are 16−22 kcal/mol lower in energy than those from coupled cluster with single and double excitationsa significant change that causes these isomers to be stable to dissociation by 2−20 kcal/mol. We characterize the transition states between olympicene’s isomers, observe differences in aromaticity among the different isomers, and compare the electronic properties of olympicene to those of pentacene. The olympicene molecule has the potential to complement pentacene as an organic semiconductor. below the carbon plane, and the central carbon’s sp3hybridization precludes its participation in the π-bond network. Acenes have previously been studied in linear7,8 and twodimensional configurations. Hachmann et al. showed that polyradical character increases as a function of system size in linear acenes,9 and Pelzer et al. demonstrated a similar trend in two-dimensional acene sheets,10 while also discovering greater electron correlation in linear geometries than in nonlinear geometries. A similar four-ringed structure, benzo[c]phenanthrene, has been known for many years.11 Lacking the central −CH2− group, the carbon structure of benzo[c]phenanthrene is nonplanar due to steric repulsion between hydrogen atoms. In contrast, olympicene’s carbon network is indeed planar. One of the out-of-plane -H functional groups in olympicene may migrate about the molecule via π-bond rearrangement, moving from the central carbon to a site located two carbons away. Instead of passing through a single transition state during this migration, the hydrogen moves along the C−C bonds, passing through two transition states and a diradical intermediate state centered above the intermediate carbon. This diradical
I. INTRODUCTION The olympicene molecule, a five-ringed polyaromatic hydrocarbon (PAH), was recently synthesized.1 PAHs have long been studied in astrophysics,2 and they are now used as molecular semiconductors.3 Olympicene is related to a class of PAHs known as acenes and two-dimensional acene sheets, which represent finite approximations to graphene.4 Of these, the linear pentacene molecule, which commonly serves as a semiconductor in field-effect transistors,5,6 is the least compact five-ringed structure, whereas olympicene represents the most compact five-ringed structure (Figure 1). As with all acenes, olympicene has delocalized orbitals extending over the entire πbond network. However, unlike other acenes, olympicene is neither fully aromatic nor fully planar due to the central −CH2− group. The two hydrogen atoms extend above and
Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 16, 2012 Revised: March 1, 2013 Published: March 19, 2013
Figure 1. Olympicene. © 2013 American Chemical Society
9746
dx.doi.org/10.1021/jp312384b | J. Phys. Chem. A 2013, 117, 9746−9752
The Journal of Physical Chemistry A
Article
intermediate is weakly bound, nonaromatic, and strongly correlated. Two-electron reduced density matrix (2-RDM) methods have proven to be computationally efficient in treating strongly correlated systems.12 Although the active-space variational 2RDM method was previously used to study nonlinear acene sheets,10 in this paper we use the parametric 2-RDM method13 due to its success in treating multireferenced scenarios such as bond-breaking and transition-state geometries.14,15 Olympicene was only recently synthesized, and these calculations provide predictions of its energy and properties, along with those of its isomers. We demonstrate that there exists a stable structure of olympicene, and we examine olympicene, its isomers, and the transition states connecting them. In addition, we provide information about the spin and band gaps in olympicene and compare them to those of pentacene.
2 ab Δij
(2)
1
(0)
D ≈ 1D
(2)
ab 2
= + ∑ 2T ij a
