  Several years back Angliker et al [Chem. Phys. Lett. 1982, 87, 208] predicted
nonacene to be the first linear acene with the triplet state $1^{3}B_{2u}$ as
the ground state, instead of the singlet $1^{1}A_{g}$ state. However, contrary
to that prediction, in a recent experimental work T\"onshoff 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.
Methodology adopted in our work is based upon Pariser-Parr-Pople model (PPP)
Hamiltonian, along with large-scale multi-reference 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 wavefunction of the correlated singlet ground state and find
that the longer acenes exhibit tendency towards a 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.
