Polyacene and Cyclacene Geometries and Electronic Structures:  Bond Equalization, Vanishing Band Gaps, and Triplet Ground States Contrast with Polyacetylene

Abstract

The ground-state geometries and excited singlet and lowest triplet energies of polyacenes from benzene through nonacene are predicted with B3LYP/6-31G* calculations and compared to experimental data where available. The results are compared to these data for cyclacenes and polyenes. The polyacenes and cyclacenes have geometries consisting of two fully delocalized nonalternating ribbons joined by relatively long bonds. Polyacenes are predicted to have smaller band gaps than the corresponding polyenes and triplet ground states for nine or more benzene rings. The fully delocalized nonalternating nature of polyacenes differs from the bond alternation resulting from Peierls distortion in polyenes. The differences are rationalized in terms of a simple MO model, and the results are compared to extensive prior theoretical work in the literature. Predictions about the electronic structure of analogues containing polyacene units are made.

The radical character of the acenes: A density matrix renormalization group study

“…Based on extrapolating the experimental singlet-triplet gap of the acenes up to pentacene, Angliker et al 7 predicted that the ground state of higher acenes from nonacene upwards would be a triplet. Density functional calculations by Houk et al 11 also predicted a singlet-triplet cross over. However, Bendikov et al 12 noted that the restricted singlet density functional ground state would become unstable to an open-shell singlet, or singlet diradical, configuration for acenes longer than hexacene.…”

“…1,2 Due to their technological potential [3][4][5] and their intrinsic value as models for more complex conjugated molecules, they have been the subject of many theoretical and experimental investigations. [6][7][8][9][10][11][12][13][14] In a number of recent studies, it has been proposed that longer acenes may possess an unusual electronic ground state that is not the simple closed-shell singlet suggested by molecular orbital arguments. Based on extrapolating the experimental singlet-triplet gap of the acenes up to pentacene, Angliker et al 7 predicted that the ground state of higher acenes from nonacene upwards would be a triplet.…”

Role of exact exchange in thermally-assisted-occupation density functional theory: A proposal of new hybrid schemes

“…Recently, linear n-acenes (C 4n+2 H 2n+4 ), containing n linearly fused benzene rings (see Figure 10), have attracted considerable interest in the research community owing to their promising electronic properties [20,21,[45][46][47][48][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123][124]. The electronic properties of nacenes have been found to be highly dependent on the chain lengths.…”

Electronic structure and excitations in oligoacenes from ab initio calculations

“…[11][12][13][14][15][16][17][18][19] The most detailed study of their static properties was presented by Wiberg,12 who examined structures and energies of oligoacenes n = 1 , . .…”