Focal Point Analysis of the Singlet-Triplet Energy Gap of Octacene and Larger Acenes

A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet triplet (ST) excitation energies of n-acenes (C4n+2H2n+4) ranging from octacene (n = 8) to undecacene (n = 11) is presented. The T1 diagnostics of coupled cluster theory and further energy-based criteria demonstrate that all investigated systems exhibit predominantly a (1)A(g) singlet closed-shell electronic ground state. Singlet-triplet (S-0-T-1) energy gaps can therefore be very accurately determined by applying the principle of a focal point analysis (FPA) onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ basis sets (X = D, T, Q 5) and single-reference methods [HF, MP2, MP3, MP4SDQ CCSD, and CCSD (T)] of improving quality. According to our best estimates, which amount to a dual extrapolation of energy differences to the level of coupled cluster theory including single, double, and perturbative estimates of connected triple excitations [CCSD(T)] in the limit of an asymptotically complete basis set (cc-pV infinity Z), the S-0-T-1 vertical (adiabatic) excitation energies of these compounds amount to 13.40 (8.21), 10.72 (6.05), 8.05 (3.67), and 7.10 (2.58) kcal/mol, respectively. In line with the absence of Peierls distortions (bond length alternations), extrapolations of results obtained at this level for benzene (n = 1) and all studied n-acenes so far (n = 2-11) indicate a vanishing S-0-T-1 energy gap, in the limit of an infinitely large polyacene, within an uncertainty of 1.5 kcal/mol (0.06 eV). Lacking experimental values for the S-0-T-1 energy gaps of n-acenes larger than hexacene, comparison is made with recent optical and electrochemical determinations of the HOMO-LUMO band gap. Further issues such as scalar relativistic, core correlation, and diagonal Born-Oppenheimer corrections (DBOCs) are tentatively examined.