DFT schemes based on conventional exchange-correlation (XC) functionals have been employed to determine
the dipole moment (μ), polarizability (α), and first (β) and second (γ) hyperpolarizabilities of push−pull
π-conjugated systems. In addition to the failures already pointed out for α and γ in a recent study on
polyacetylene chains [J.
Chem. Phys.
1998, 109, 10489; Phys. Rev. Lett.
1999, 83, 694], these functionals are
also unsuitable for the evaluation of μ and β. In the case of β, in particular, an almost catastrophic behavior
with respect to increasing chain length is found. We show that the C functional has a negligible effect on the
calculated μ, α, β, and γ whereas the X-part is responsible for the large property overestimations when the
size of the system increases. The overly large μ values are associated with an overestimation of the charge
transfer between the donor and the acceptor whereas for α, β, and γ, incomplete screening of the external
electric field is responsible for the large discrepancies with respect to accurate values. Our results show that
current XC functionals incorrectly describe the polarization of conjugated systems when the polarization is
due to donor/acceptor substitution or an external field or both.
Assessment of conventional density functional schemes for computing the polarizabilities and hyperpolarizabilities of conjugated oligomers: An ab initio investigation of polyacetylene chains.
Ab initio restricted Hartree–Fock 6-31G calculations are reported for the static longitudinal hyperpolarizability of the linear polyenes C4H6 through C44H46. Using a new extrapolation technique the infinite chain value in polyacetylene is determined with an accuracy similar to that achieved for small molecules. This is the first in a series of articles leading to a comprehensive ab initio treatment for the nonlinear optical properties of conjugated polymers.
This paper reports novel self-assembling T-shaped conjugated molecules based on asymmetric bisphenazines in which hexadecyloxy substituted bisphenazine is orthogonally fused with 1,4-bis(2-phenylethynyl)benzene. Tunability of one-dimensional (1D) self-assembly and electronic properties of the system was demonstrated by peripheral substitution with small functional groups. These functional groups (OCH3, H, CN) progressively reduced LUMO levels as predicted by theoretical calculations and experimentally verified by cyclic voltammetry (CV). Furthermore, the morphologies of 1D assembly were greatly influenced by the substituents. While conformational flexibility of methoxy hampered successful assembly, hydrogen and cyano substitution induced the formation of rigid microstrands and flexible nanofibers, respectively, using phase transfer methods. Detailed instrumental analyses of the 1D assembled clusters including scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) are presented. The design strategy of the new T-π-core and peripheral substitution provides a tool to control the morphology of 1D clusters with minimal structural modification of the π-core while allowing modulation of electronic properties.
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