Ferroelectric nanofibers with an embedded optically nonlinear benzothiazole derivative

We report measurements of the molecular first hyperpolarizability, thermal stability, photophysical, piezoelectric and ferroelectric properties of a benzothiazole derivative bearing an arylthiophene π-conjugated bridge both in solution and when embedded into a poly (L-lactic acid) (PLLA) matrix in the form of electrospun fibers with an average diameter of roughly 500 nm. The embedded nanocrystalline phenylthienyl-benzothiazole derivative, with crystal sizes of about 1.4 nm resulted in a good piezoelectric response from these functionalized electrospun fibers, indicative of a polar crystalline structure.Fundação para a Ciência e a Tecnologia (FCT

Exact static polarizabilities of correlated finite model systems

A finite-field method for calculating exact polarizabilities of correlated conjugated model systems within the valence bond (VB) framework is presented. The correlations reduce the polarizabilities from their noninteracting values and extend the range of linearity to higher external fields. The large nonlinear polarizabilities observed in strongly correlated conjugated organic molecules cannot be directly attributed to electron correlations. The method described can be employed to calculate static polarizabilities for any desired state of a correlated system

Role of σ−π\sigma-\pi interactions on linear and nonlinear susceptibilities in polyenes

The validity of the $\sigma-\pi$ separability approximation has been well established for some properties of the low-lying states in long polyenes. However, properties such as polarizabilities and hyperpolarizabilitles apparently depend upon the entire excitation spectrum of the molecule and a detailed study of the $\sigma-\pi$ separability approximation is warranted. We find from model calculations of the exact dynamic and static polarizability, $\alpha$, and third-harmonic-generation (THG) coefficient,$\tau$, in butadiene that the contribution of $\sigma$-electrons to the total polarizability is $\approx30\%$ while their contribution to THG coefficients is $\approx10\%$. The $\sigma$ electrons make a positive contribution to $\alpha$ while their contribution to $\tau$ is small and negative. It is, therefore, likely that the excited states contributing to $\alpha$ are not the same as those contributmg to $\tau$. The $\sigma-\pi$ separability approximation improves with increase in conjugation length and our calculations, being restricted to butadiene, provide an upper limit on the contribution of the $\sigma$-electrons

Sudden polarization in interacting model π\pi systems: An exact study

The twisted excited state of polyenes has attracted much theoretical and experimental attention due to the postulate that it exhibits sudden polarization for twist angle $\theta = 90^°$. We examine this phenomenon in long chain polyenes using correlated model Hamiltonians. While the Hubbard model is employed to study this process as the correlations are gradually turned on, the Pariser-Parr-Pople (PPP) Hamiltonian is employed for a realistic study of long chain polyenes. Exact polarizabilities of the ground and excited states of polyenes with upto ten carbon atoms are calculated within the Hubbard and PPP models as a function of twist angle. We find that the dipole allowed excited states exhibit sudden polarization in all the cases. Besides, the $3^1A_g$ state in the covalent subspace which is a two-photon state also exhibits this phenomenon as it becomes degenerate with the dipole allowed excited state. It is suggested that photochemical processes involving zwitterionic intermediates should be possible through two-photon excitation to this covalent excited state

Model exact study of dc-electric-field-induced second-harmonic-generation coefficients in polyene systems

We report model exact electric-field-induced second-harmonic- (EFISH) generation coefficients of polyene systems, within the Pariser-Parr-Pople model. For a given unsubstituted polyene, we find that the EFISH generation coefficient is largest when the field is applied along the chain axis and depends nearly linearly on the field strength. The EFISH coefficients increase with increase in chain length according to a power law with an exponent of $\simeq3.5$ for $\parallel\hspace{1mm}E_x \parallel=0.05 V/ \AA$. The EFISH generation coefficients are sensitive to backbone stereochemistry and increase rapidly for twist angle $\theta$ up to about 75° but suddenly drop to very low values for$\theta$=90°. In push-pull polyenes, the effect of the electric field on the SHG coefficients depends upon the direction of the field relative to the push-pull groups. The size dependence of the SHG coefficients in the presence of the external fields is weaker in these systems, although the behavior of the SHG coefficients with respect to backbone stereochemistry is very similar to that of the EFISH coefficients as well as that of the SHG coefficients in the absence of external fields

A comparative theoretical study of higher harmonic generation and electro-optic effects in polyene systems

Second- and third-order dynamic polarizabilities related to the Pockels and Kerr coefficients of polyenes and substituted polyenes have been computed exactly for a Pariser-Parr-Pople model. It is found that at any given frequency away from resonance the susceptibility relating to the Pockels effect is larger than the second-harmonic- generation (SHG) coefficient and that corresponding to the Kerr effect is larger than either the electric-field induced SHG or the third harmonic generation coefficients, although the size dependence is governed by the order of nonlinearity. The Pockels coefficient, unlike the Kerr coefficient, is sensitive to the push-pull strength

Optical and magnetic properties of the exact PPP states of biphenyl

The low-lying singlets and triplets of biphenyl are obtained exactly within the PPP model using the diagrammatic valence bond method. The energy gaps within the singlet manifold as well as the lowest singlet-triplet gap are found to be in good agreement with experimental results. The two weak absorptions between 4·1 and 4·2 eV reported experimentally are attributed to the two states lying below the optical gap that become weakly allowed on breaking electron-hole and inversion symmetries. The observed blue shift of the spectral lines, attributed to a change in dihedral angle, on going from crystalline to solution to vapour phase is also well reproduced within the PPP model. The bond orders show that the ground singlet state is benzenoidal while the dipole excited state as well as the lowest triplet state are quinonoidal and planar. Comparison with the experimental spin densities and the fine structure constants D and E in the triplet state point to slightly weaker correlations than assumed by the PPP model. The introduction of a 1-8 bond to mimic poly(paraphenylene)s gives an optical gap that is in good agreement with experiment

Nature of exchange interactions in stacked radicals/radical-ions of cyclic polyenes

From full configuration interaction calculations on stacked cyclic polyene radicals/radical-ions, within the Hubbard and the Pariser-Parr-Pople models, we show that the ground state is not always a triplet state as predicted by the McConnell mechanism for organic ferromagnetism. On the contrary, the (2n + 1) cyclic polyene radical stacks always possess a singlet ground state. In 2n cyclic polyene radical-ions, for small n (n < 3), the ground state is a singlet. For larger n values, the ground state appears to be a triplet. We rationalize these results based on topological arguments

Properties of some low-lying electronic states in polymethineimines and poly(2,3-diazabutadienes)

Polymethineimines (PMI's) and poly(2,3-diazabutadienes)(PBD's) are isoelectronic with polyacetylenes (PA's), but they do not possess the electron-hole or the spatial symmetries of PA. While PMI has been synthesized, PDB still remains to be prepared in the laboratory. In this paper, we have calculated the properties of some low-lying electronic states of PMI and PDB polymers, employing a Pariser-Parr-Pople model Hamiltonian. The nitrogen-atom parameters used in the model are obtained after extensively testing them in simple nitrogen-containing molecules. The electronic states in small polymers are obtained by exactly solving the finite model Hamiltonians employing a valence-bond procedure. The energy levels and other properties of the infinite system are then obtained from extrapolations. Our studies show that the optical gap in a single strand of PMI is2.6\pm 0.2 eV, which is very close to the single-strand PA value of 2.8\pm 0.2 eV. The gap in PDB is 3.8\pm 0.4 eV and, unlike the case for PA systems, there are no "in-gap" states in PMI or PDB. The transition dipoles to these states from the ground state as well as dipole moments in the ground state point to a nonlocal nature of the ground and excited states

Stability of the high-spin ground state in alternant π\pi-conjugated organic molecules

Alternant quantum cell models with unequal numbers of atoms on the two sublattices have been predicted to have a high-spin ground state. In this paper, we examine the stability of this high-spin ground state with respect to breaking the alternancy symmetry and distortion of the backbone conjugation. We find that in the Pariser-Parr-Pople (PPP) models and the Hubbard models with weak correlations, the ground state continues to be the high-spin state, even when alternancy symmetry is broken by introducing large site-energy differences. In the Hubbard model, for strong correlation strengths, the ground state switches from a high-spin to a low-spin state when large site-energy differences are introduced. The bond-order calculations in all these models shows that the low-spin state is susceptible to dimerization of the backbone. In the distorted chains, the low-spin state stabilizes to a greater extent leading to low-spin ground states at least in ‘‘soft’’ lattices. However, experience with one-dimensional systems suggests that the lattice distortion could occur unconditionally leading to low-spin ground state in infinitely long polymers. Thus, realization of organic ferromagnetics via high-spin polymers could be elusive