Himanshu Chakraborty, Alok Shukla
Several years back Angliker et al. [Chem. Phys. Lett. 87, 208 (1982)] 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. 49, 4125 (2010)] demonstrated that nonacene has a singlet ground state. Motivated by this experimental finding, we decided to perform a systematic theoretical investigation of the triplet states of polyacenes, with an emphasis on singlet-triplet splitting, 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 the singlet-triplet gap decreases with acene size and octacene onwards, it reaches saturation. Thus no singlet-triplet crossover takes place in longer acenes and the ground state is of singlet multiplicity. 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 diradical ground state. Moreover, when we compare the experimental absorption spectra of octacene and nonacene with their calculated singlet and triplet absorption spectra, we observe very good agreement for the singlet case. Hence, the optical absorption results also confirm the singlet nature of the ground state for longer acenes.
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http://arxiv.org/abs/1304.0075
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