The Low-Lying Electronic States of Pentacene and Their
Roles in Singlet Fission
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Abstract
We present a detailed study of pentacene
monomer and dimer that
serves to reconcile extant views of its singlet fission. We obtain
the correct ordering of singlet excited-state energy levels in a pentacene
molecule (<i>E</i> (<i>S</i><sub>1</sub>) < <i>E</i> (<i>D</i>)) from multireference calculations
with an appropriate active orbital space and dynamical correlation
being incorporated. In order to understand the mechanism of singlet
fission in pentacene, we use a well-developed diabatization scheme
to characterize the six low-lying singlet states of a pentacene dimer
that approximates the unit cell structure of crystalline pentacene.
The local, single-excitonic diabats are not directly coupled with
the important multiexcitonic state but rather mix through their mutual
couplings with one of the charge-transfer configurations. We analyze
the mixing of diabats as a function of monomer separation and pentacene
rotation. By defining an oscillator strength measure of the coherent
population of the multiexcitonic diabat, essential to singlet fission,
we find this population can, in principle, be increased by small compression
along a specific crystal direction