Nature of ground and electronic excited states of higher acenes

Abstract

Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particleparticle random-phase approximation calculation. The 1 A g ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state 3 B 2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state 1 B 2u is a zwitterionic state to the short axis. The excited 1 A g state gradually switches from a doubleexcitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the 1 B 2u and excited 1 A g states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved.higher acenes | diradical | double excitation | particle-particle random-phase approximation | charge-transfer excitation

Incommensurate quantum-size oscillations in acene-based molecular wires—Effects of quantum fluctuations

“…Very recent quantum-chemistry calculations on the level of density -functional multi-reference theory (DFT/MRCI) for oligoacenes (up to N R = 9) and pp-RPA (up to N R = 12) may give an indication that this indeed could be the case. 7,8 The authors of Ref. 7 confirm earlier findings 29 that best agreement between DFT/MRCI results and experimental IRexcitation energies is achieved for closed-shell reference states (with B3LYP functional) even though spin-unrestricted calculations may produce a lower total energy.…”

“…8 propose an exponential decay of the lowest lying spin-triplet gap with increasing N R saturating near N R = 11 at roughly 0.13 eV without a trace of IO. If indeed correct, this finding could still be interpreted within the short-range model.…”

“…Due to their peculiar electronic a) Electronic mail: peter.schmitteckert@physik.uni-wuerzburg.de structure, oligo-acenes have received a considerable amount of attention in organic chemistry over the past 50 years. [6][7][8] A recent technological interest is based on suggestions to use them as light harvesters in organic photo-voltaic devices. 9 Besides being a simple realization of nano-wires, acenes can be viewed as the narrowest graphene nano-ribbons with zigzag terminated edges.…”

How to efficiently tune the biradicaloid nature of acenes by chemical doping with boron and nitrogen

“…These are continuously elongated when going from the end rings to the central bond. Because of the lack of experimental data for the structures of octacene, the MP2 bond lengths are compared with the values derived from both restricted (RB3LYP) and unrestricted (UB3LYP) DFT geometry optimizations performed by Yang et al 40 (see Table S1, ESI †). The C-C bond distances predicted by MP2 are shorter by less than 0.02 Å in comparison with those obtained using RB3LYP.…”

Orbital optimisation in the perfect pairing hierarchy: applications to full-valence calculations on linear polyacenes

“…Famously, DMRG calculations have been used to show that the larger linear polyacenes exhibit strong correlation for their π electrons [55]. Other approaches used have included density functional theory [56][57][58][59][60][61], multiconfiguration pair-density functional theory [62], projected Hartree-Fock theory [63], the random phase approximation [64], configuration interaction [60] (CI), adaptive CI [27], GW theory [65], variational two-electron reduced density matrix (VRDM) theory [66][67][68][69], Møller-Plesset perturbation theory [61,[70][71][72][73], spin-flip methods [74,75], CAS-SCF [53,59,71,72,76] as well as restricted-active space self-consistent field theory [77], valence bond [78] and CC valence bond (CCVB) theory [79,80], CC theory [70][71][72]80] and multireference averaged quadratic CC theory [81,82], an algebraic diagrammatic construction scheme [83], as well as PPH methods with approximate orbitals [50].…”