Kathleen A. Robins

Assessment of conventional density functional schemes for computing the polarizabilities and hyperpolarizabilities of conjugated oligomers: An ab initio investigation of polyacetylene chains

DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of π-conjugated polyacetylene chains. These functionals fail in one or more of several ways: (i) the correlation correction to α is either much too small or in the wrong direction, leading to an overestimate; (ii) γ is significantly overestimated; (iii) the chain length dependence is excessively large, particularly for γ and for the more alternant system; and (iv) the bond length alternation effects on γ are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen–Baerends XC potential show that the overestimations are not related to the asymptotic behavior of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends.

Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Hartree–Fock static longitudinal hyperpolarizability of polyacetylene

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.

Hartree–Fock static longitudinal (hyper)polarizability of polyyne

The evolution in the static longitudinal polarizability and second hyperpolarizability of the C4H2 through C54H2 oligomers of polyyne is studied at the Hartree–Fock level of theory. We find that the calculated values are highly sensitive to the adopted geometry. Using improved geometries, new extrapolation procedures and careful finite field determinations we extend earlier work by others so as to provide reliable estimates for the above properties in the infinite polymer limit.

Substituent effect on the electronic properties and morphologies of self-assembling bisphenazine derivatives.

Tuning electronic properties and morphologies: We report a unique design platform of n-type organic semiconductors based on asymmetrically substituted bisphenazines that enable tuning of both electronic properties and morphologies of 1D nanostructures (see figure) by using small substituents with various sizes and electronic demands.This paper reports the synthesis and characterization of novel self-assembling n-type organic semiconductors based on asymmetrically substituted bisphenazines with various functional groups of different size, electron-withdrawing ability, and conjugation length. The overarching objective of this research is to tune electronic properties and morphologies of self-assembled structures of this system simultaneously, which offers a potentially useful platform for future optoelectronic applications. The thermal, optical, and electrochemical properties associated with different substituents were studied by differential scanning calorimetry (DSC), UV-visible and fluorescence spectroscopy, and cyclic voltammetry (CV). Electronic properties were calculated using density functional theory, and results were compared to experimental HOMO, LUMO, and energy gaps. The one-dimensional (1D) self-assembly properties of these new n-type molecules are discussed in terms of the type of peripheral substituents, alkyl side group length, and assembly conditions. This study includes extensive investigations by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Abinitio polarizability study of polypyrrole

The longitudinal polarizability, αzz, and second order hyperpolarizability, γzzzz, of the (C4H3N)RH2 oligopyrroles, where R=1−9, are determined at the Hartree–Fock (HF) level of theory using a 6‐31G basis set. The effect of electron correlation is taken into account using Moller–Plesset perturbation theory through second order for the R=1−5 oligopyrroles. Methods for extrapolation to the infinite polymer values are explored. The convergence behavior of the correlated results as a ratio with the corresponding HF values may indicate that a scaled HF procedure could be appropriate for finding correlated estimates to the infinite polymer property values. We also propose a generalized finite field recipe which can easily be used in conjunction with popular black‐box electronic structure programs for determining molecular polarizabilities through second order. The αzz and γzzzz values obtained from using this procedure on a variety of polyacetylene and polypyrrole oligomers are in excellent agreement with those...

AB INITIO STUDIES OF THE POLARIZABILITIES OF RETINAL ANALOGS

Axial components of the polarizability, hyperpolarizability, and second order hyperpolarizability of retinal model molecules are calculated using the ab initio coupled‐perturbed Hartree–Fock theory. Conformational rearrangements are shown to affect some components in a systematic fashion. We also find that the general trend in the hyperpolarizability behavior as a function of protonation and charge transfer is in agreement with available experimental data. The second hyperpolarizability of the protonated Schiff base is found to be highly dependent on the adopted geometry optimization. We explore a possible explanation for this as well as present a brief discussion with regard to future experimental and theoretical developments towards the construction of molecularly based nonlinear optical devices.

Ab initio study of nitroxide, HNO−

The first ab initio theoretical study has been performed on the ground state of nitroxide, HNO−, in combination with a comparison study of the ground state of two chemically similar systems, peroxy radical, HO2, and the neutral nitroxyl radical, HNO. An optimum geometry and vibrational frequencies are reported for all species, and results are compared to experimental and theoretical values. A bent geometry is obtained for HNO−, with an equilibrium bond angle of 106.2°, and bond lengths of RNO=1.333 A and RNH=1.045 A. The calculated fundamental vibrational frequencies (in cm−1) are: ν1 (N–H stretch), 3029; ν2 (bend), 1474; and ν3 (N–O stretch), 1183. The bending frequency and the N–O stretching frequency in the ion are similar to the bending frequency and the O–O stretching frequency of HO2, while the N–H stretching frequency of HNO− is similar to the N–H stretching frequency of HNO. In addition, the GAUSSIAN 1 and GAUSSIAN 2 methods are used for the determination of an electron affinity for HNO. The calcu...

Theory guided systematic molecular design of benzothiadiazole–phenazine based self-assembling electron-acceptors

We report two new self-assembling n-type materials in which the design process was systematically analyzed using theoretical calculations prior to experimental synthesis. Benzothiadiazole (A′) and alkoxyphenazine (A) serve as our basic electron-deficient building blocks to construct two candidate molecules with an acceptor (A) – acceptor (A′) – acceptor (A) configuration. We conducted a computational characterization (B3LYP/6-31G*) of the electronic properties for structural subunits linked together and analyzed in a step-by-step fashion, thereby culminating in the target molecules of interest. We found that ELUMO was controlled primarily by benzothiadiazole as was evident by orbital localization on this subunit. Meanwhile, EHOMO was influenced by the dihedral angle between A and A′. The molecule with A and A′ coupled with a C–C triple bond (BTD-P-T) was found to be planar with a more stabilized ELUMO and a reduced Egap when compared to its C–C single bond counterpart (BTD-P-S). The two molecules were synthesized and characterized to verify the theoretical findings. Optical, electrochemical, and thermal properties were characterized with UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, and differential scanning calorimetry, respectively. In addition, the molecular packing was examined by X-ray powder diffraction. As predicted by theory, BTD-P-T exhibited a lower Egap based on the systems lower ELUMO in comparison to BTD-P-S. BTD-P-T exhibited a higher Tm and crystallinity due to its planarity. Both BTD-P-S and BTD-P-T exhibited excellent fibrillation ability upon solvent casting. Bulk heterojunction (BHJ) organic solar cell (OSC) devices were fabricated using BTD-P-S or BTD-P-T as an acceptor and P3HT as a donor at different weight ratios. For both P3HT:BTD-P-S and P3HT:BTD-P-T BHJs, the weight ratio of 6 : 1 produced the highest power conversion efficiency (PCE) of 0.17% and 0.44%, respectively. These results are consistent with fluorescence quenching experiments in which 90% of P3HT fluorescence was quenched at that ratio. Nearly two times higher PCE was observed for the BTD-P-T based device compared to that of the BTD-P-S based system, mainly due to the higher Jsc. Presumably, the flat geometry of BTD-P-T allows for more efficient intermolecular π-orbital overlap, enhancing charge transport.