Electric Dipole Transition Moments Research Articles

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Photoluminescence Anisotropy in Eutectic Crystals of Polynuclear Lanthanide Complexes and Silver Clusters

AbstractAnisotropy is an intrinsic property of crystalline materials. However, the photoluminescence anisotropy in eutectic crystals of organometallic complexes has remained unexplored. Herein, the eutectic of polynuclear lanthanide complexes and Ag clusters was prepared, and the crystal shows significant photoluminescence anisotropy. The polarization anisotropy of emission δ and degree of excitation polarization P are 2.62 and 0.53, respectively. The rare excitation polarization properties have been proved to be related to the regular arrangement of electric transition dipole moments of luminescent molecules in the crystal. Our design provides a reference for developing new photoluminescence anisotropy materials and expanding their applications.

Photoluminescence Anisotropy in Eutectic Crystals of Polynuclear Lanthanide Complexes and Silver Clusters.

Anisotropy is an intrinsic property of crystalline materials. However, the photoluminescence anisotropy in eutectic crystals of organometallic complexes has remained unexplored. Herein, the eutectic of polynuclear lanthanide complexes and Ag clusters was prepared, and the crystal shows significant photoluminescence anisotropy. The polarization anisotropy of emission δ and degree of excitation polarization P are 2.62 and 0.53, respectively. The rare excitation polarization properties have been proved to be related to the regular arrangement of electric transition dipole moments of luminescent molecules in the crystal. Our design provides a reference for developing new photoluminescence anisotropy materials and expanding their applications.

Liquid Crystalline Magneto-Optically Active Peralkylated Azacoronene.

Organic Faraday rotators have gained significant attention in recent years as a promising alternative to traditional inorganic magneto-optical (MO) materials as a result of their lower cost, superior mechanical properties, and potential for large-scale deployment. This interest is peaked by the fact that a number of high symmetry, rigid, strongly optically absorbing organic chromophores display Verdet constants an order of magnitude higher than commercial inorganic Faraday rotators. Critical to the development of new generations of organic materials is the ability to organize them in optimal structures for optical coupling/measurements. We report herein the synthesis of a dodecyl-substituted hexapyrrolohexaazacoronene (C12-HPHAC) displaying discotic liquid crystalline (LC) properties and large Faraday rotation. Thin films with a redox mixed C12-HPHAC/C12-HPHAC+2 composition display a discotic columnar LC phase, are stable to air and moisture in the solid and solution states, and achieve a maximum Verdet constant of 3.36 × 105 deg T-1 m-1 at 700 nm. This result is consistent with Serber's model of magnetic circular birefringence and displays one of the largest reported Verdet constants for organic materials in the UV-Vis range. The LC phase aligns the molecules and leads to gains in Verdet constants of up to 105% through the favorable orientation of the molecules' magnetic and electric transition dipole moments with respect to the applied magnetic field.

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties.

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 × 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Explorations of pseudo-Dirac dark matter having keV splittings and interacting via transition electric and magnetic dipole moments

Explorations of pseudo-Dirac dark matter having keV splittings and interacting via transition electric and magnetic dipole moments

Highly Strained Oxygen‐Doped Chiral Molecular Belts of the Zigzag‐Type with Strong Circularly Polarized Luminescence

AbstractHerein we report a two‐directional cyclization strategy for the synthesis of highly strained depth‐expanded oxygen‐doped chiral molecular belts of the zigzag‐type. From the easily accessible resorcin[4]arenes, an unprecedented cyclization cascade generating fused 2,3‐dihydro‐1H‐phenalenes has been developed to access expanded molecular belts. Stitching up the fjords through intramolecular nucleophilic aromatic substitution and ring‐closing olefin metathesis reactions furnished a highly strained O‐doped C2‐symmetric belt. The enantiomers of the acquired compounds exhibited excellent chiroptical properties. The calculated parallelly aligned electric (μ) and magnetic (m) transition dipole moments are translated to the high dissymmetry factor (|glum| up to 0.022). This study provides not only an appealing and useful strategy for the synthesis of strained molecular belts but also a new paradigm for the fabrication of belt‐derived chiroptical materials with high CPL activities.

Highly Strained Oxygen-Doped Chiral Molecular Belts of the Zigzag-Type with Strong Circularly Polarized Luminescence.

Herein we report a two-directional cyclization strategy for the synthesis of highly strained depth-expanded oxygen-doped chiral molecular belts of the zigzag-type. From the easily accessible resorcin[4]arenes, an unprecedented cyclization cascade generating fused 2,3-dihydro-1H-phenalenes has been developed to access expanded molecular belts. Stitching up the fjords through intramolecular nucleophilic aromatic substitution and ring-closing olefin metathesis reactions furnished a highly strained O-doped C2 -symmetric belt. The enantiomers of the acquired compounds exhibited excellent chiroptical properties. The calculated parallelly aligned electric (μ) and magnetic (m) transition dipole moments are translated to the high dissymmetry factor (|glum | up to 0.022). This study provides not only an appealing and useful strategy for the synthesis of strained molecular belts but also a new paradigm for the fabrication of belt-derived chiroptical materials with high CPL activities.

Stepwise Amplification of Circularly Polarized Luminescence in Chiral Metal Cluster Ensembles.

Chiral metal-organic frameworks(MOFs)are usually endowed by chiral linkers and/or guests. The strategy using chiral secondary building units in MOFsfor solving the trade-off of circularly polarized luminescence (CPL)-active materials, high photoluminescence quantum yields (PLQYs) and high dissymmetry factors (|glum |) has not been demonstrated. This work directionally assembles predesigned chiral silver clusters with ACQ linkers through reticular chemistry. The nanoscale chirality of the cluster transmits through MOF's framework, where the linkers are arranged in a quasi-parallel manner and are efficiently isolated and rigidified. Consequently, this backbone of chiral cluster-based MOFs demonstrates superb CPL, high PLQYs of 50.3%, and |glum | of 1.2 × 10-2 . Crystallographic analyses and DFT calculations show the quasi-parallel arrangement manners of emitting linkers leading to a large angle between the electric and magnetic transition dipole moments, boosting CPL response. As compared, an ion-pair-direct assembly without interactions between linkers induces one-ninth |glum | and one-sixth PLQY values, further highlighting the merits of directional arrangement in reticular nets. In addition, a prototype CPL switching fabricated by a chiral framework is controlled through alternating ultraviolet and visible light. This work is expected to inspire the development of reticular chemistry for high-performance chiroptical materials.

Symmetry-breaking host–guest assembly in a hydrogen-bonded supramolecular system

Bio-inspired self-assembly is invaluable to create well-defined giant structures from small molecular units. Owing to a large entropy loss in the self-assembly process, highly symmetric structures are typically obtained as thermodynamic products while formation of low symmetric assemblies is still challenging. In this study, we report the symmetry-breaking self-assembly of a defined C1-symmetric supramolecular structure from an Oh-symmetric hydrogen-bonded resorcin[4]arene capsule and C2-symmetric cationic bis-cyclometalated Ir complexes, carrying sterically demanding tertiary butyl (tBu) groups, on the basis of synergistic effects of weak binding forces. The flexible capsule framework shows a large structural change upon guest binding to form a distorted resorcin[4]arene hexameric capsule, providing an asymmetric cavity. Location of the chiral guest inside the anisotropic environment leads to modulation of its Electric Dipole (ED) and Magnetic Dipole (MD) transition moments in the excited state, causing an increased emission quantum yield, longer emission lifetime, and enhancement of the dissymmetry factor (glum) in the circularly polarized luminescence.

Vibrational infrared and Raman spectra of HCOOH from variational computations.

All vibrational energies of the formic acid molecule in different forms (trans-, cis-, delocalized-) were converged up to 4500 cm-1 beyond the zero-point vibrational energy with the GENIUSH-Smolyak variational approach and using an ab initio potential energy surface [D. P. Tew and W. Mizukami, J. Phys. Chem. A, 120, 9815-9828 (2016)]. The full-dimensional dipole and polarizability surfaces were fitted to points computed at the CCSD/aug-cc-pVTZ level of theory. Then, body-fixed vibrational dipole and polarizability transition moments were evaluated and used to simulate jet-cooled infrared and Raman spectra of HCOOH. The benchmark-quality vibrational energy, transition moment, and wave function data will be used in further work for comparison with vibrational experiments, and in further rovibrational computations.

The fundamental relation between electrohelicity and molecular optical activity.

Electrohelicity arises in molecules such as allene and spiropentadiene when their symmetry is reduced and helical frontier molecular orbitals (MOs) appear. Such molecules are optically active and electrohelicity has been suggested as a possible design principle for increasing the chiroptical response. Here we examine the fundamental link between electrohelicity and optical activity by studying the origin of the electric and magnetic transition dipole moments of the π-π* transitions. We show that the helical character of the MOs drives the optical activity in allene, and we use this knowledge to design allenic molecules with increased chiroptical response. We further examine longer carbyne-like molecules. While the MO helicity also contributes to the optical activity in non-planar butatriene, the simplest cumulene, we show there is no relation between the chiroptical response and the helical π-MOs of tolane, a simple polyyne. Finally, we demonstrate that the optical activity of spiropentadiene is inherently linked to mixing of its two π-systems rather than the helical shape of its occupied π-MOs. We thus find that the fundamental connection between electrohelicity and optical activity is very molecule dependent. Although electrohelicity is not the underlying principle, we show that the chiroptical response can be enhanced through insight into the helical nature of electronic transitions.

Common Trends of Chiral Induced Spin Selectivity and Optical Dichroism with Varying Helix Pitch: A First‐Principles Study

AbstractElectrons ransported through a chiral molecule become spin−polarized; this phenomenon is known as chiral induced spin selectivity (CISS). It has implications for spintronics, for electrochemical and enantioselective reactions, and for electron transfer in biological systems. The CISS‐induced spin polarization in simulations and in experiment differs by orders of magnitude, and the detailed underlying mechanism is still an open question. Structure−property relationships can help elucidate this question. For this purpose, the effect of helix pitch is studied for a model helix of 20 carbon atoms for two quantities which have been found to correlate in some experiments: spin‐polarization in transmitted electrons and electronic circular dichroism (ECD). We find that even though the chirality of these model helices goes down with increased pitch, ECD and CISS go up, along with ultraviolet−visible (UV/Vis) spectra and magnetic and electric transition dipole moments. Orbital contributions to the most intense UV/Vis transition do not show a consistent qualitative picture. Tentatively, we can assign the increase in these properties to an increase of the electric polarizability with spatial extension of these helices by changing pitch.

Probing van der Waals interactions and detecting polar molecules by Förster-resonance energy transfer with Rydberg atoms at temperatures below 100 mK

Electric-field-controlled F\"orster resonance energy transfer (FRET) between Rydberg helium (He) atoms and ground-state ammonia (NH$_3$) molecules has been studied at translational temperatures below 100 mK. The experiments were performed in an intrabeam collision apparatus with pulsed supersonic beams of NH$_3$ seeded in He. A range of F\"orster resonances, between triplet Rydberg states in He with principal quantum numbers of 38, 39 and 40, and the inversion intervals in NH$_3$ were investigated. Resonance widths as low as $100\pm20$ MHz were observed for Rydberg-Rydberg transitions with electric dipole transition moments of 3270 D. These widths result from binary collisions at a mean center-of-mass speed of $19.3\pm2.6$ m/s. For transitions in which the initially prepared Rydberg states were strongly polarized, with large induced static electric dipole moments, van der Waals interactions between the collision partners increased the resonance widths to $\sim750$ MHz. From measurements of the rate of FRET for the narrowest resonances observed, a density of NH$_3$ of $(9.4\pm0.3)\times10^{9}$ cm$^{-3}$ in the upper ground-state inversion sublevel in the interaction region of the apparatus was determined non-destructively.

Triplet state harvesting and search for forbidden transition intensity in the nitrogen molecule.

Triplet excited states of the N2 molecule play an important role in electric discharges through air or liquid nitrogen accompanied by various afterglows. In the rarefied upper atmosphere, they produce aurora borealis and participate in other energy-transfer processes connected with atmospheric photochemistry and nightglow. In this work, we present spin-orbit coupling calculations of the intensity of various forbidden transitions, including the prediction of the electric dipole transition moment of the new band, which is strongly prohibited by the (+|-) selection rule, the new spin-induced magnetic transition, magnetic and electric quadrupole transitions for the B3Πg Wilkinson band, and the Lyman-Birge-Hopfield a1Πg ← X1Σg transition. Also, two other far-UV singlet-singlet quadrupole transitions are calculated for the first time, namely, the Dressler-Lutz a"1Σg +-X1Σg + and the less studied z1Δg-X1Σg + weak transitions.

Visualization of UV and ECD spectra of E&Z isomers of N-(4′-Hydroxy-cinnamoyl)− 5- hydroxyanthranilic acid

Avenanthramides are secondary metabolites most prominently found in oat (Avena sativa.L). These are amides of cinnamoyl-anthranilic acids derivatives. Due to their fascinating biological potential, these compounds have attracted the curiosity of chemists. Herein, the theoretical background of the electronic transition in UV-Vis and ECD spectra of the (E&Z) isomers of N-(4′-Hydroxy-cinnamoyl)− 5- hydroxyanthranilic acid was studied. The physical mechanism of the electron transfer process during light excitation is more intuitively described with two-dimensional and three-dimensional visualization methods. The characteristics of the UV-Vis spectrum were extended to the electron-hole conceptualization and the charge transfer process was described with charge difference density (CDD) and transition density matrix (TDM). The interaction mechanism between light and molecule in ECD is quantitatively revealed by the transition electric dipole moment, transition magnetic dipole moment, and their tensor product. A well-defined UV –Vis spectrum was obtained for the E isomer whereas the ECD spectrum for the Z isomer.

Chiral Exciplex Acceptor Enables Circularly Polarized Electroluminescence with High Dissymmetry Factor Close to 10−2

AbstractChirality is a magical property that exists in nature universally, ranging from the spin of microscopic electrons to the galaxies. Furthermore, circularly polarized organic electroluminescent materials have promising prospects in improving the brightness of displays, but there are still some problems. At present, most of chiral thermally activated delayed fluorescence small molecules show relatively low circularly polarized luminescence (CPL) dissymmetry factors between 10−4 and 10−3 because of the electric dipole‐allowed but magnetic dipole‐forbidden transition of the singlet state. In this work, a promising strategy is reported for constructing chiral exciplex systems with a chiral electron acceptor and a regular achiral donor to achieve high CPL dissymmetry factors in both photoluminescence and electroluminescence. From theoretical calculation results, the exciplex systems possess a large separation of the highest occupied molecular orbital and the lowest unoccupied molecular orbital in donor and acceptor molecules, respectively, which results in comparable electric and magnetic transition dipole moments. The resultant CPL organic light‐emitting diodes based on the exciplex systems as emitters show low turn‐on voltage of 2.4 V, high external quantum efficiencies of 12.7% and 9.3%, and opposite circularly polarized electroluminescence with high dissymmetry factors (gEL) of +7.25 × 10−3 and −9.89 × 10−3.

Discriminating Chiral Supramolecular Motions by Circularly Polarized Luminescence.

Molecular motions are closely associated with the behaviors and properties of organic materials. However, monitoring molecular motions is challenging. Herein, a chiral supramolecular system consisting of L-/D-phenylalanine (LPF/DPF) as a chiral inducer and an achiral tetraphenylethene derivative (TPEF) as a molecular rotor has been proposed and explored for real-time discriminating the supramolecular motions by the visualization of circularly polarized luminescence (CPL) signal variations. Derived from the ordered molecular motions of TPEF induced by LPF/DPF, highly organized aggregates have been progressively assembled in a controlled manner with differentiated morphologies, including spherical particles, one-dimensional fibers, and floor-shaped supercrystals. Notably, increasing level of ordered aggregates, in turn, led to quenching emissions, while the CPL signals have been dramatically amplified accompanying by a sharp enhancement of luminescence dissymmetry factors (glum ) from nearly 0 to -0.1. The significant amplification of CPL is attributed to the ordered aggregates of supramolecules, leading to the decrease of electric transition dipole moments in supramolecular system. As a result of the chiral supramolecular motions powered by supramolecular crystallization, the supramolecular motions are conveniently discriminated by visual CPL signal variation with an enhancement of glum value from 0 to -0.1 in real time.

Diatomic-py: A Python module for calculating the rotational and hyperfine structure of 1Σ molecules

We present a computer program to calculate the quantised rotational and hyperfine energy levels of Σ1 diatomic molecules in the presence of dc electric, dc magnetic, and off-resonant optical fields. Our program is applicable to the bialkali molecules used in ongoing state-of-the-art experiments with ultracold molecular gases. We include functions for the calculation of space-fixed electric dipole moments, magnetic moments and transition dipole moments. Program summaryProgram Title: Diatomic-PyCPC Library link to program files:https://doi.org/10.17632/3yfxnh5bn5.1Developer's repository link:https://doi.org/10.5281/zenodo.6632148Licensing provisions: BSD 3-clauseProgramming language: Python ≥ 3.7Nature of problem: Calculation of the rotational and hyperfine structure of Σ1 molecules in the presence of dc magnetic, dc electric, and off-resonant laser fields.Solution method: A matrix representation of the Hamiltonian is constructed in the uncoupled basis set. Eigenstates and eigenenergies are calculated by numerical diagonalization of the Hamiltonian.Additional comments including restrictions and unusual features: Restricted to calculating the Stark and Zeeman shifts with co-axial electric and magnetic fields.

Effect of Microhydration on the Peptide Backbone of N-Acetyl-Phenylalaninylamide (NAPA) Using IR, Raman and Vibrational Chiroptical Spectrosocpies (VCD, ROA): A Computational Study

In this work, N-acetyl-phenylalaninylamide (NAPA) and microhydrated NAPA i.e. [NAPA-A(H2O)n (n = 1-4)] complexes were studied with the aid of density functional theory (DFT) calculation in the gas phase to determine the absolute configuration, hydration effect on amide modes (amide I-IV), the robustness of amide modes, and associated amide modes. IR, Raman, vibrational chiroptical (VCD and ROA) spectra, VCD rotational strength, VCD dipole strength and the angle between the electric transition dipole moment (ETDM) and the magnetic transition dipole moment (MTDM) were performed using DFT/wB97XD/cc-pVTZ level of the computational approach. The absolute configuration (D/L) of the most stable conformer of NAPA (NAPA-A) was confirmed by the analysis of the VCD spectra that show opposite sign, and intensity. Amide I (C=Os) VCD bands are non-robust and amide II-IV (C-Ns, N-Hb and N-Hs) bands are robust and reliable on the basis of the robustness concept. Consequently, the VCD stretching and scissoring modes of NH2 and H2O for [NAPA- A(H2O)n (n=1-4)] complexes are also robust and reliable. On the other hand, ROA calculation confirmed that the C5 interaction in the peptide backbone of NAPA-A is entirely lost in presence of a single configuration of an explicit water molecule. [NAPA-A(H2O)1] complex has a C7 interaction in the peptide backbone which is also lost in the presence of the 2nd and 3rd water molecules. DCPI (180) Raman and DCPI (180) ROA calculations were also performed in the solution phase (implicit water) using the integral equation formalism variant polarizable continuum model (IEF-PCM) to compare the gas phase results.