Multipolar Molecules Research Articles

Ultrafast Photophysics of Regioisomeric Triphenylamine Dyes having Dipolar D-π-A-A and Octupolar D-(π-A-A)3 Character with a Benzothiazole Unit in Matched or Mismatched Orientation.

Benzothiazole is among prominent electron-withdrawing heteroarene moieties used in a variety of π-conjugated molecules. Its relative orientation with respect to the principal dipole vector(s) of chromophores derived thereof is crucial, affecting photophysical and nonlinear optical properties. Here we compare the photophysics and ultrafast dynamics of dipolar and octupolar molecules comprising a triphenylamine electron-donating core, ethynylene π-conjugated linker(s) and benzothiazole acceptor(s) having the matched or mismatched orientation (with respect to the direction of intramolecular charge transfer), while a carbaldehyde group is attached as an auxiliary acceptor. Among chromophores without the auxiliary acceptor, stronger fluorescence solvatochromism and faster excited state dynamics are exhibited for the derivatives with the mismatched geometry. On the contrary, introduction of the auxiliary acceptor to the benzothiazole unit enhances the intramolecular charge transfer ICT (featuring ultrafast dynamics of the excited state) for the matched geometry. The data confirm the crucial role of the relative orientation of asymmetric heteroaromatic unit (regioisomeric effect) in dipolar as well as in multipolar molecules in tuning linear and nonlinear optical properties as well as excited state dynamics.

Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration

AbstractIn the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor–acceptor phosphonium dyes (D−π−)nA+[X−] (π=−(C6H4)x−). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion‐paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration‐induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π‐spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor–acceptor arms.

Multipolar Porphyrin-Triazatruxene Arrays for Two-Photon Fluorescence Cell Imaging.

Two-photon excited fluorescent (TPEF) materials are highly desirable for bioimaging applications owing to their unique characteristics of deep-tissue penetration and high spatiotemporal resolution. Herein, by connecting one, two, or three electron-deficient zinc porphyrin units to an electron-rich triazatruxene core via ethynyl π-bridges, conjugated multipolar molecules TAT-(ZnP)n (n=1-3) were developed as TPEF materials for cell imaging. The three new dyes present high fluorescence quantum yields (0.40-0.47) and rationally improved two-photon absorption (TPA) properties. In particular, the peak TPA cross section of TAT-ZnP (436 GM) is significantly larger than that of the ZnP reference (59 GM). The δTPA values of TAT-(ZnP)2 and TAT-(ZnP)3 further increase to 1031 and up to 1496 GM, respectively, indicating the effect of incorporated ZnP units on the TPA properties. The substantial improvement of the TPEF properties is attributed to the formation of π-conjugated quadrapole/octupole molecules and the extension of D-π-A-D systems, which has been rationalized by density function theory (DFT) calculations. Moreover, all of the three new dyes display good biocompatibility and preferential targeting ability toward cytomembrane, thus can be superior candidates for TPEF imaging of living cells. Overall, this work demonstrated a promising strategy for the development of porphyrin-based TPEF materials by the construction and extension of D-π-A-D multipolar array.

Computational studies of dative bond containing heterocyclic ring structures

Ab initio calculations are reported for several related heterocyclic compounds, each of which contains two dative bonds when they self-dimerize. Thus, these molecules are nominally dimers that contain either a boron–carbon–nitrogen (BCN) or boron–carbon–phosphorous (BCP) segment. Molecules with this motiff have been found experimentally to have several unusual properties that may be related to a “multi-polar framework’’ that results from charge separation associated with the two dative bonds. Structures obtained from full geometry optimizations without symmetry constraints, dative bond energies and charge distributions for four multipolar molecules are reported, the BCN–BCN dimer and the BCN–BCP dimer with and without carboxylation of one boron atom. Comparisons to single dative bond, self-cyclized monomers and the role of ring strain in molecular stabilities are also discussed.

NTCDA–TTF first axial fusion: emergent panchromatic, NIR optical, multi-state redox and high optical contrast photooxidation

The first synthetic entry into axially fused NTCDA/PMDA-TTF multipolar molecules demonstrates a high optical contrast photooxidation, panchromism, low HOMO-LUMO gap, generation of a stable radical cation, NIR absorption/emission beyond 2150/800 nm and theoretically calculated NLO activity.

Two-photon-absorption properties of multipolar molecules studied using femtosecond z-scan method

Two-photon-absorption (TPA) properties of multipolar molecules were studied using femtosecond z-scan method. We measured the z-scan curves at 800 nm, 1 kHz using 150 fs-laser pulse. The z-scan curves were measured for two kinds of multipolar molecules: three different quadrupole molecules (Q1, Q2, Q3) and three different octupolar molecules (O1, O2, O3). Each multipolar molecule has same molecular skeleton but the lengths of the dipolar subunits or the donor strengths are different each other. Their TPA cross-sections were obtained from the magnitudes of the transmittance dip in the measured z-scan curves: 233 (Q1), 127 (Q2), 164 (Q3) and 381 (O1), 230 (O2), 85 (O3) cm4/(photon/s) for quadrupole and octupolar molecules, respectively. These results are ascribed to the fact that TPA cross-sections of the multipolar molecules are raised by increasing the number of the alternating double bonds in the dipolar subunits or by enlarging the electron-donating strength of the substituent.

Molecular Simulation of Vapour-Liquid Coexistence Curves for Hydrogen Sulfide-Alkane and Carbon Dioxide-Alkane Mixtures

Gibbs ensemble Monte Carlo simulations, combined with the configurational-bias technique applied to alkane chains, were performed to calculate hydrogen sulfide-alkane and carbon dioxide-alkane liquid-vapour phase equilibria for several n-alkane molecules (propane, pentane and decane). Recently proposed effective pair potentials that describe accurately the pure components coexistence curves were used in the simulations. It is shown that the use of these force fields together with the Lorentz-Berthelot mixing rules yield a description of the coexistence curves of these binary mixtures that is in good agreement with the experimental data is most cases. Some deviations from the experimental results were observed in the range of high H2S mole fraction and high pressure. Further work is needed in order to improve the level of accuracy of potential models for fluid mixtures of non polar and multipolar molecules.

Multipolar molecules and multipolar fields: probing and controlling the tensorial nature of nonlinear molecular media

A general class of multipolar molecules is introduced in the context of quadratic nonlinear optics by way of extension of the more specific cases of dipolar and octupolar molecules. An adequate irreducible tensor formalism permits us to define rotationally invariant molecular features that couple to corresponding field tensor components, thereby enabling us to account for a variety of coherent and noncoherent processes such as harmonic light (hyper-Rayleigh) scattering, coherent second-harmonic generation in electrically poled media, and the recently proposed optical poling scheme. Experiments in both harmonic light scattering in solution (for some multipolar molecules) and optical poling (in Disperse Red 1–methyl methacrylate thin films) are analyzed in light of this model. A general tensorial permutation lemma of broad validity allows nonlinear light–matter interactions to be condensed in a statistical medium in compact rotationally invariant expressions: The main tensorial symmetry features for both molecular susceptibility and read–write field polarization tensors that jointly drive these interactions are clearly revealed.

Ion-induced nucleation. II. Polarizable multipolar molecules

Density functional theory is applied to ion-induced nucleation of polarizable multipolar molecules. The asymmetric nature of the ion–molecule interaction is shown to cause the sign preference in ion-induced nucleation. When the ion–molecule interaction is weak, the observed sign preference is consistent with that of the bare ion–molecule interaction potential and decreases with increasing supersaturation. However, as the ion–molecule interaction becomes stronger, the sign preference in the reversible work exhibits some nontrivial behavior. For molecular parameters applicable for CS2 and CH4, the predicted values of the reversible work of nucleation depend on the sign of the ion charge, yielding a difference in the nucleation rate by factors of 10 to 102 and 10 to 105, respectively.

Electrical properties of adsorbed or spread films: The effective value of permittivities in the Helmholtz equation (plane distribution of point dipoles)

In contrast to the Helmholtz formula (HF), surface potential data obtained with spread or adsorbed films formed with multipolar molecules were recently analyzed assuming different dielectric constants for the potential components due to the various dipoles involved with each molecule. It is shown that HF is valid in special cases only (isolated dipoles at a metal-vacuum or air interface). Imaging effects generally should be taken into account. A calculation for foliated plane systems based on the Fourier-Bessel transform is reported. It provides a better theoretical basis than HF for analyzing data with mono- or multipolar molecules.

The arrangement of multipolar molecules on the surface of graphite

Grand ensemble and canonical ensemble Monte Carlo methods have been used to calculate the full distribution functions for a multipolar molecule adsorbed onto a graphite surface.

Lattice summation of multipole interaction energies in crystals. Calculation of the outer-sphere contribution to the crystal-field splitting in nickel(II) oxide

The Bertaut method for calculating the lattice energy of an ionic crystal is extended to include multipole–multipole interactions. Equations are obtained for the electrostatic-interaction energy of compact multipolar ions or molecules of any symmetry in a crystal of arbitrary structure, and their application is illustrated using α-N2, K[HF2], and Pb[NO3]2 as examples. The method is used to estimate the contribution to the ligand-field splitting Δ in NiO from outside the first co-ordination sphere; the interaction between cation multipoles is too small to account forobserved differences between spectroscopic and thermochemical estimates of Δ.

Pairwise additive third virial coefficients for multipolar molecules : Application to water vapour

Pairwise additive third virial coefficients for axially symmetric multipolar molecules have been calculated using a non-product multidimensional integration formula. Results for the Stockmayer potential agree with the corrected results of Rowlinson. It is found that the inclusion of the dipole-quadrupole and quadrupole-quadrupole terms in the potential function has a marked qualitative effect on the shape of the C-T curve. Values of the third virial coefficient for water vapour calculated using a potential function derived from gaseous viscosity and second virial coefficient data are in good agreement with the experimental values.

DC electric field-induced optical second harmonic generation by interacting multipolar molecules

From classical molecular-statistical theory, second harmonic generation by multipolar substances placed in a DC or AC electric field is shown to be related not only with nonlinear electronic polarisation of molecules but essentially with orientation of the electric molecular multipoles and their interaction in the condensed state.

The equation of state of multipolar gases

The equation of state in the approximation of the second virial coefficient B is discussed theoretically for gases consisting of unlike multipolar molecules. A molecule of species i is assumed to posses a permanent 2n-pole moment M(n)i, as well as a 2n-pole moment P(n)i induced in it by the electric field F(n)i of order n of a neighbouring molecule of species j having the permanent 2m-pole electric moment M(m)j. Tensor formalism is used for deriving general expressions for the potential energy of electrostatic and inductional interaction of two multipolar molecules of species i and j. These energies then serve for computing two contributions to B: the one, of inductional type, is obtained in the first approximation and contains, in addition to terms with dipolar polarizability, terms accounting for the quadrupolar polarizability induced in the molecule by the molecular electric field gradient; the other, which is purely electrostatic, is obtained in the second approximation of the theory. Moreover, cross contributions to B between the electrostatic and inductional energies as well as contributions from dipolar anisotropy of dispersional-type interactions are computed. In computing the mean values of the respective powers of rij (the distance between the two molecules), a general Lennard-Jones (s-t)-type potential is applied. The theoretically derived non-central contributions to B are valid in general for molecules of arbitrary symmetry and arbitrary electrical structure; the expressions are applied to cases of axially symmetric, tetrahedral and octahedral molecules. For methane, the theoretical formulas allowed to determine numerically the octopole moment from experimental data on B to be ΩCH4 = 5 × 10−34 e.s.u. cm3.