Journal Article

Nature of ground and electronic excited states of higher acenes

Yang, Yang, Yang, Weitao, Davidson, Ernest R.

2016

Overview

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 ¹Ag 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 ³B2u 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 ¹B2u is a zwitterionic state to the short axis. The excited ¹Ag 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 ¹B2u and excited ¹Ag 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.

• Publisher: National Academy of Sciences,Proceedings of the National Academy of Sciences
• Subject: Approximation / catalysis (heterogeneous) / Chemical reactions / Chemistry / defects / Energy efficiency / energy storage (including batteries and capacitors) / hydrogen and fuel cells / INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY / materials and chemistry by design / mechanical behavior / Photovoltaic cells / Photovoltaics / Physical Sciences / PNAS Plus / Semiconductors / solar (photovoltaic) / synthesis (novel materials)