Hexacene

Abstract: In the CTOCD-DZ (continuous transformation of origin of current density-diamagnetic zero) formulation of coupled Hartree−Fock theory for magnetic response of closed-shell systems, induced current density at each point is calculated with the gauge origin at that point. In addition to its economy and accuracy for total current maps, CTOCD-DZ is shown to yield a unique and physically motivated definition of, and symmetry criteria for, orbital contributions to current density. This leads to a few-electron interpretation of ring currents. Only the four HOMO electrons of an aromatic (4n+2)-electron monocycle contribute significantly to the ring current, and in general only a small subset of the high-lying π electrons dominate the more complex patterns of current in polycyclic π systems. Benzene, naphthalene, hexacene, pyracylene, coronene, and corannulene are treated as examples.

Abstract: The successive replacement of CH moieties by nitrogen atoms in oligoacenes (benzene to hexacene) has been studied computationally at the B3LYP/6-311+G(d,p)//6-31G(d) level of theory, and the effects of different heteroatomic substitution patterns on structures, electron affinities, excitation, ionization, and reorganization energies are discussed. The calculated tendencies are rationalized on the basis of molecular orbital arguments. To achieve electron affinities of 3 eV, a value required to allow for efficient electron injection from common metal electrodes, at least seven nitrogen atoms have to be incorporated into tetracenes or pentacenes. The latter require rather small reorganization energies for electron transfer (<0.20 eV) making these compounds promising candidates for n-channel semiconducting materials. Particularly interesting are heptaazapentacenes 5 and 6 in which the nitrogen atoms are arranged to form self-complementary systems with a maximum number of intermolecular CH−N contacts in planar...

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 particle–particle 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 double-excitation 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.

Abstract: Substituent effects have been exploited to produce an unusually persistent heptacene derivative. In total, four new heptacene derivatives with varying levels of photooxidative resistance (1 < 2 < 3 < 4) have been synthesized. A combination of p-(t-butyl)thiophenyl substituents at positions 7 and 16 (i.e., arylthio substituents attached to the most reactive ring) and o-dimethylphenyl substituents at positions 5, 9, 14, and 18 (i.e., steric resistance on neighboring rings) make heptacene derivative 4 especially resistant to photooxidation. It persists for weeks as a solid, for 1−2 days in solution if shielded from light, and for several hours in solution when directly exposed to both light and air. Heptacene derivative 4 has been fully characterized. It possesses a small HOMO−LUMO gap of 1.37 eV.