A Schiff base type polymer containing phenanthrene and an aliphatic spacer in the main chain
was prepared, and its photophysical behavior in diluted and concentrated solutions as well as in film form was
studied by steady-state fluorescence spectroscopy. These data indicated that ground-state fluorescent phenanthrene
aggregates were present in the films. Time-resolved measurements in diluted conditions showed biexponential
decay implying in different microenvironments around the chromophore. Theoretical simulations predicted
aggregation in π-stack arrangement, and calculation of optical properties for small aggregates shows the introduction
of new states at lower energies, in good agreement with the experimental results. Theoretical studies also forecast
the possibility of domains with highly ordered morphology, which was confirmed by DSC measurements providing
further support for the presence of the emitting aggregated form.
Semiempirical Hartree-Fock techniques are widely used to study properties of long ring-structured chains, although these types of systems were not included in the original parametrization ensembles. These techniques are very useful for an ample class of studies, and their predictive power should be tested. We present here a study of the applicability of some techniques from the NDDO family (MNDO, AM1, and PM3) to the calculation of the ground state geometries of a specific set of molecules with the ring-structure characteristic. For this we have chosen to compare results against ab initio Restricted Hartree-Fock 6-31G(d,p) calculations, extended to Møller-Plesset 2 perturbation theory for special cases. The systems investigated comprise the orthobenzoquinone (O(2)C(6)H(4)) molecule and dimers (O(2)C(6)H(4))(2), as well as trimers of polyaniline, which present characteristics that extend to several systems of interest in the field of conducting polymers, such as ring structure and heterosubstitution. We focus on the torsion between rings, because this angle is known to affect strongly the electronic and optical properties of conjugated polymers. We find that AM1 is always in qualitative agreement with the ab initio results, and is thus indicated for further studies of longer, more complicated chains.
We present a detailed analysis of the application of density functional theory (DFT) methods to the study of structural properties of molecular and supramolecular systems, using as a paradigmatic example three para-phenylene-based systems: isolated biphenyl, single chain poly-para-phenylene, and crystalline biphenyl. We use different functionals for the exchange correlation potential, the local density (LDA), and generalized gradient approximations (GGA), and also different basis sets expansions, localized, plane waves (PW), and mixed (localized plus PW), within the reciprocal space formulation for the hamiltonian. We find that regardless of the choice of basis functions, the GGA calculations yield larger interring distances and torsion angles than LDA. For the same XC approximation, the agreement between calculations with different basis functions lies within 1% (LDA) or 0.5% (GGA) for distances, and while PW and mixed basis calculations agree within 1 degrees for torsion angles, the localized basis results show larger angles by approximately 8 degrees and a nonmonotonic dependence on basis size, with differences within 6 degrees. The most prominent features, namely the torsion between rings for isolated molecule and infinite chain, and planarity for the molecule in crystalline environment, are well reproduced by all DFT calculations.
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