In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.

The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Møller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.

The gas-phase internal elimination (E(i)) reaction of the sulfoxide (-SO-CH(3)) precursors of ethylene and model oligomers of PPV and PITN has been investigated by means of Hartree-Fock, Møller-Plesset (second and fourth order), and Density Functional Theory (B3LYP, MPW1K) calculations. Considerable differences between the obtained ground state and transition state geometries and the calculated activation energies are observed from one approach to the other, justifying first a careful calibration against the results of a benchmark CCSD(T) study of the E(i) reaction leading to ethylene. In comparison with the CCSD(T) results, as well as with available experimental data, DFT calculations along with the MPW1K functional are found to be a very appropriate choice for describing the E(i) pathway. The leading conformations of the precursors, the relevant transition state structures, and the energy barriers encountered along the lowest energy path to unsubstituted, alpha and beta chloro-, methoxy-, and cyano-substituted ethylene, styrene, stilbene in its cis and trans forms, and at last trans-biisothianaphthene have therefore been identified and characterized in detail employing DFT (MPW1K). Depending on the substituents attached to the C(alpha) and C(beta) atoms, different reaction mechanisms are observed.

The elimination and side reactions involved in the thermal conversion of sulfonyl precursor chains into poly(p-phenylene vinylene) (PPV) have been studied in detail, using Density Functional theory, along with the MPW1K functional. The performance of the MPW1K functional for describing radical dissociation and internal conversion reactions of sulfonyl precursors has been assessed against the results of benchmark CCSD(T) calculations. Enthalpies as well as entropies are calculated at different temperatures at the level of the rigid rotor-harmonic oscillator approximation. Entropy effects on internal elimination reactions are very limited. In sharp contrast, at the temperatures under which the conversion is usually performed (550 K), entropy contributions to the activation energies are found to be very significant and to strongly favor direct radical dissociations of the precursors. Further radical side reactions following an E(i) conversion through an alkyl substituent may also significantly contribute to the formation of sp(3) defects and/or cross-linked structures in the polymer-an advantageous feature for the making of materials with improved photoluminescence efficiencies.

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