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Pariser–Parr–Pople Model Based Investigation of Ground and Low-Lying Excited States of Long Acenes
journal contribution
posted on 2013-12-27, 00:00 authored by Himanshu Chakraborty, Alok ShuklaSeveral years ago, Angliker et al.
[Chem. Phys. Lett. 1982, 87, 208] predicted nonacene to be the first linear acene with the triplet
state 13B2u as the ground state, instead of
the singlet 11Ag state. However, contrary to
that prediction, in a recent experimental work, Tönshoff and
Bettinger [Angew. Chem.
Int. Ed. 2010, 49, 4125] demonstrated that nonacene has a singlet ground state.
Motivated by this experimental finding, we decided to perform a systematic
theoretical investigation of the nature of the ground and the low-lying
excited states of long acenes, with an emphasis on the singlet–triplet
gap, starting from naphthalene, all the way up to decacene. The methodology
adopted in our work is based upon the Pariser–Parr–Pople
model (PPP) Hamiltonian, along with the large-scale multireference
singles-doubles configuration interaction (MRSDCI) approach. Our results
predict that even though the singlet–triplet gap decreases
with the increasing conjugation length, nevertheless, it remains finite
till decacene, thus providing no evidence of the predicted singlet–triplet
crossover. We also analyze the nature of many-particle wave function
of the correlated singlet ground state and find that the longer acenes
exhibit a tendency toward an open-shell singlet ground state. Moreover,
when we compare the experimental absorption spectra of octacene and
nonacene with their calculated singlet and triplet absorption spectra,
we observe excellent agreement for the singlet case. Hence, the optical
absorption results also confirm the singlet nature of the ground state
for longer acenes. Calculated triplet absorption spectra of acenes
predict two well-separated intense long-axis polarized absorptions,
against one such peak observed for the singlet case. This is an important
prediction regarding the triplet optics of acenes, which can be tested
in future experiments on oriented samples.