Quadrupolar Molecules Research Articles

Effect of locally excited state on fluorescence transition dipole moment in quadrupolar molecules subjected to symmetry breaking charge transfer.

In excited centrosymmetric donor-acceptor triads of type A-D-A or D-A-D, symmetry breaking charge transfer (SBCT) in polar media has been explored for a few decades. SBCT is accompanied by significant reorganization of the electronic structure of the molecule, which leads to a change in the fluorescence transition dipole moment (TDM). Previously, experiments revealed a 20%-30% reduction in TDM, which occurs on the timescale of SBCT. Simple SBCT models explain this reduction. Here, the effect of the interaction of a locally excited state with zwitterionic states on TDM is investigated. This interaction is shown to have a drastic impact on the TDM and its dependence on the solvent polarity. The magnitude of TDM can decrease monotonically, increase monotonically, and also pass through a maximum with an increase in the SBCT degree due to the locally excited state effect. The scale of changes in TDM in the course of SBCT increases greatly. The conditions for the implementation of a particular scenario have been determined. This work clearly demonstrates the observable influence of upper excited states on the photochemistry and photophysics of molecules. Methods for controlling the fluorescent characteristics of quadrupolar molecules are proposed.

Excited-State Symmetry Breaking and Localization in a Noncentrosymmetric Electron Donor-Acceptor-Donor Molecule.

Electronic excitation in quadrupolar conjugated molecules rapidly localizes on a single electron donor-acceptor (DA) branch when in polar environments. The loss of center of inversion upon this excited-state symmetry breaking (ES-SB) can be monitored by exploiting the relaxation of the exclusion rules for IR and Raman vibrational transitions. Here, we compare ES-SB in a right-angled (1) and a centrosymmetric (2) DAD dyes using time-resolved IR spectroscopy. We show that the localization of the excitation can also be identified with the bent molecule 1. We find that contrary to dye 2, subpopulations with localized and delocalized excitation coexist for 1 in weak to medium polar solvents. This difference originates from the torsional disorder present in the excited state of 1 but not of 2. Additionally, irreversible localization in a bent molecule is shown to require higher solvent polarity than in a centrosymmetric one.

Controlling the symmetry breaking charge transfer extent in excited quadrupolar molecules by tuning the locally excited state.

The effect of a locally excited state on charge transfer symmetry breaking (SBCT) in excited quadrupolar molecules in solutions has been studied. The interaction of a locally excited state and two zwitterionic states is found to either increase or decrease the degree of SBCT depending on the molecular parameters. A strategy on how to adjust the molecular parameters to control the extent of SBCT is presented. The influence of level degeneracy on SBCT is identified and discussed in detail. The level degeneracy is shown to lead to the existence of a hidden dipole moment in excited quadrupolar molecules. Its manifestations in SBCT are analyzed. The main conclusions are consistent with the available experimental data.

Tunable Non-Kasha Behaviors and Excited-State Dynamics of Quadrupolar Squaraine Aggregates.

Versatile coupling theories have been developed for rationalizing unusual aggregation phenomena of multipolar chromophores. Here, diverse excitonic couplings of a quadrupolar squaraine dye protonated by trifluoroacetic acid could be achieved and tuned unprecedentedly in different solvation media. Subtle changes of the solvent and ion pair influenced the aggregation of the donor-acceptor-donor (D-A-D)-type SQC6 and led to significant variations in optical properties. In contrast to conventional H/J aggregates, strong spectroscopic evidence of nonfluorescent and red-shifted hJ aggregation was obtained. Assumptions of the excitonic interplay with variable strength stabilized by the synergic contributions of π-π stacking and electronic interaction were addressed. Comparative excited-state dynamics in the aggregates clarified the distinctive excitonic coupling of adjacent quadrupolar molecules and the nature of the excited state beyond the dimers. Meanwhile, dominant two-photon absorption transitions could be elucidated by a resonance-enhanced mechanism. The present unusual molecular interplay provides a strategy to fine tune the optical properties of multipolar aggregates.

Electric dipole flip in a quadrupolar molecule with broken symmetry upon excited state absorption.

The nature of the second excited state in a quadrupolar molecule of the A-D-A structure, where A and D are an electron acceptor and an electron donor, respectively, has been studied. The orthogonality condition of the wave functions requires that the direction of the molecular dipole moment arising due to the charge transfer symmetry breaking be opposite in the first and second excited states. The dipole moment flip leads to large reorganization energy of the solvent upon excited state absorption. The manifestations of dipole flip are discussed. The dependence of the energy gap on the solvent polarity is found. The symmetry breaking effect on the transition dipole moment suppression is calculated. The available experimental data confirm the main conclusions.

Quadrupole versus dipole photomotors: Symmetry constraints and operating properties

AbstractThe operating properties of different‐type Brownian photomotors have been compared in relation to the symmetry interplay between their moving (particle/molecule) and immovable (substrate) units. As found, essentially different symmetry constraints and hence different motor behavior are characteristic of molecules if their electron density redistribution on excitation involves only odd‐order or only even‐order multipole moments. The two respective types of molecules give rise to dipole and quadrupole photomotors. Purely dipole photomotors can operate only on substrates with symmetric charge distribution and their ensemble with random molecular orientations undergoes equidirectional diffusion rather than directed motion. Contrary to this, purely quadrupole photomotors need substrates with antisymmetric charge distribution for directed motion to occur and they can move directionally in an ensemble. Also, quadrupole photomotors provide much smaller velocities of directed motion than their dipole counterparts (as a result of the much weaker interaction of quadrupoles than dipoles with an electric field). The generic distinctions between the two photomotor types are exemplified by donor‐acceptor‐substituted stilbenoids (dipole molecules) and symmetric squaraines (quadrupole molecules). The model used affords the rational design of photomotors by the selection of promising moving molecules and by the continuous wide‐range tuning of charge‐distribution symmetry in substrates.

Modeling the Effect of H-Bonding of Excited Quadrupolar Molecules with a Solvent on Charge Transfer Symmetry Breaking.

A model of the H-bonding effect on charge transfer symmetry breaking in excited quadrupolar dyes is proposed. A dye is assumed to have two symmetrically arranged H-bond acceptors. The effect of H-bonding is described in terms of two quantities: (i) the free energy of H-bond formation by an excited symmetric dye with a protic solvent and (ii) a parameter that determines the susceptibility of the H-bond strength to the charge of the H-bond acceptor. The model assumes that an increase in dye dissymmetry results in a charge shift from one acceptor to the other, making one acceptor more negative and the other less negative. As a result, the strength of the H-bond on one branch increases, while on the other branch, it decreases. An analytical solution of the mathematical model is obtained. Regardless of the strength of the H-bond, the effect of the H-bonding on the symmetry breaking degree is shown to be small, as long as the free energy of formation of the weaker H-bond is negative. A strong effect is expected only if this free energy becomes positive, that is, when the H-bond is formed on only one arm. An unexpected result of weakening of the dissymmetry degree caused by the strengthening of the H-bond is predicted and discussed. An approach for quantitative determination of the susceptibility of the H-bond strength to the charge of the H-bond acceptor is proposed.

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy.

We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz'-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz'-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck-Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz'-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents.

New insights into the excited state of an A-D-A quadrupolar molecule strongly hydrogen bonded to molecules of methanol and hexafluoro isopropanol

In this work, we carried out density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to study the hydrogen-bonding dynamics of 2,5-bis(4- cyanophenyl)-1,4-bis(4-methylphenyl)-1,4-dihydropyrrolo [3,2-b] pyrrole (ADA) in strongly H-bonding liquids. ADA, is a quadrupolar A–π–D–π–A type molecule (A-D-A), where A and D are electron-accepting and electron-donating moieties. Our theoretical results are consistent with previously reported experimental data. According to our computations, the CN···HO intermolecular hydrogen bonds formed between ADA and methanol, and between ADA and hexafluoro isopropanol are tremendously strengthened in the electronically excited-state upon the photoexcitation of their hydrogen-bonded complexes. The enhancement of the intermolecular hydrogen bond motives the dispersion of holes and electrons and the asymmetry of the ADA molecules in strongly H-bonded liquids, which promotes the red shift of the CN stretching vibration and the changes in the form of the solvent ADA around. The degree of asymmetry harm is associated to the solvent proton-donating ability and the phenomenon of increased asymmetry can be prolonged to the application of different dye molecules. The lookup will help experimenters develop different dyes with excellent performance.

Distinctive Excited State Symmetry Breaking Dynamics in Typical Donor-Acceptor-Donor Fluorophore: Strong Photoluminescence and Ultrafast Charge Separation from a Partial Charge Transfer State.

Understanding the structure-property relationships in organic semiconductors is crucial for controlling their photophysical properties and developing new optoelectronic materials. Quadrupolar molecules, donor-acceptor-donor (DAD), have attracted extensive attention in various optoelectronic applications. However, the systematic studies on the differences on photophysical properties between DAD and simple donor-acceptor (DA) chromophores are rarely reported. Herein we present a comparative study on the excited state dynamics of DA and DAD fluorescence systems using theoretical calculation and transient absorption spectroscopy. Results show that DA and DAD molecules exhibit similar excited state dynamics, which are attributed to the distinctive excited-state symmetry breaking (ESSB) phenomenon observed in a DAD system. The strong photoluminescence (PL) and ultrafast charge separation (CS) from an ESSB-induced partial charge transfer (CT) state were clearly detected in different solvent environments. These results not only offer insight into the excited state dynamics of the DAD fluorescence system but also provide some basic guidelines for designing new optoelectronic materials.

AIEE-TICT quadrupolar push-pull quinoxaline derivatives displaying solvatochromism, acidofluorochromism and logic gate operation

A series of novel quadrupolar push-pull molecules based on quinoxaline moiety as an electron acceptor, biphenyl as a spacer and methoxy groups as electron donors was designed, synthesized and characterized. Their structural, photophysical, thermal, aggregation and acidofluorochromic properties were investigated. Photophysical studies showed that the introduction of an electron withdrawing group onto the quinoxaline moiety caused red shifts in absorption and emission maxima due to enhanced ICT. A positive solvatochromic behavior with large bathochromic shifts, up to 106 nm, was observed in the emission wavelength upon increasing solvent polarity from toluene to DMSO. All compounds displayed TICT-AIEE characteristics in DMSO-Water. Single crystal X-ray diffraction studies of QM2, QM4-QM6 revealed highly twisted structures and the absence of π-π stacking interactions. A visual color change from colorless to yellow accompanied by fluorescence quenching was observed in the molecules upon treatment of DCM solution with TFA and the changes were reversed upon addition of TEA. Reversible acidofluorochromism was also observed in the solid state. Filter paper strips coated with QM1 and QM4 were fabricated for detecting TFA. An IMPLICATION molecular logic gate operation can be performed on these molecules using acid and base as chemical inputs.

Probing effect of solvation on photoexcited quadrupolar donor-acceptor-donor molecule via ultrafast Raman spectroscopy

The symmetric and quadrupolar donor-acceptor-donor (D-A-D) molecules usually exhibit excited-state charge redistribution process from delocalized intramolecular charge transfer (ICT) state to localized ICT state. Direct observation of such charge redistribution process in real-time has been intensively studied via various ultrafast time-resolved spectroscopies. Femtosecond stimulated Raman spectroscopy (FSRS) is one of the powerful methods which can be used to determine the excited state dynamics by tracking vibrational mode evolution of the specific chemical bonds within molecules. Herein, a molecule, 4,4′-(buta-1,3-diyne-1,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline), that consists of two central adjacent alkyne (-C≡C-) groups as electron-acceptors and two separated, symmetric N,N-bis(4-methoxyphenyl)aniline at both branches as electron-donors, is chosen to investigate the excited-state photophysical properties. It is shown that the solvation induced excited-state charge redistribution in polar solvents can be probed by using femtosecond stimulated Raman spectroscopy. The results provide a fundamental understanding of photoexcitation induced charge delocalization/localization properties of the symmetric quadrupolar molecules with adjacent vibrational markers located at central position.

Assessment of thermodynamic models via Joule–Thomson inversion

The performance of thermodynamic models is assessed with respect to Joule–Thomson inversion on the basis of 29 fluids with varying molecular structure. Three thermodynamic model types are considered: classical force fields, the molecular-based PC-SAFT equation of state (EOS) and empirical multi-parameter EOS. The force fields consist of Lennard–Jones and multi-polar interaction sites and were evaluated by molecular dynamics simulation and integration of Mayer’s f function. The force fields and PC-SAFT were parameterized solely to vapor-liquid equilibrium data, while basically all available experimental thermodynamic property data have been used in the development of the empirical EOS. The fluids considered in this comparison comprise noble gases, triangular polar molecules, linear quadrupolar molecules, hydrogen boding molecules, cyclic alkanes, aromatics and siloxanes. It is found that the physically motivated models (force fields and PC-SAFT) provide throughout realistic predictions for the Joule–Thomson inversion curve. Upon extrapolation, about half of the empirical EOS yield reasonable results, the remainder show deficiencies. The three model types are in good mutual agreement at low and moderate temperatures, deviations are observed at high temperatures. In most cases, PC-SAFT systematically predicts a higher ideal gas limit temperature of the Joule–Thomson inversion curve than the force fields. By expressing the characteristic points of the Joule–Thomson inversion curve in terms of the critical temperature and pressure, the adequacy of the classical corresponding states principle is discussed.

Effect of Symmetry Breaking in Excited Quadrupole Molecules on Transition Dipole Moment.

Manifestations of charge transfer symmetry breaking in excited quadrupolar molecules in optical spectra are theoretically studied. The molecules are supposed to have π-conjugated structures of A-π-D-π-A or D-π-A-π-D character, where electron acceptors (A) or electron donors (D) are identical. A theory describing the effect of symmetry breaking and solvent fluctuations on the dipole moments of optical transitions associated with absorption by a quadrupolar dye in the ground and excited states, as well as fluorescence, is developed. Simple equations describing the influence of the symmetry breaking extent on the transition dipole moments are found. The orientational solvent fluctuations are predicted to decrease the transition dipole moment of the ground state absorption. The decrease does not exceed 10%. A considerably larger effect of symmetry breaking and the solvent fluctuations on the emission dipole moment is found. Equations describing dependencies of the transition dipole moment associated with excited state absorption on the solvent polarity and the parameters of the dye are derived. The scale of the changes in the transition dipole moments due to symmetry breaking in the excited state are determined. The influence of the polar solvent fluctuations is also taken into account. The theoretical findings are shown to be consistent with the available experimental data.

Two-dimensional three-body quadrupole–quadrupole interactions

The interactions between dipoles are called dipole–dipole interactions. One type of dipole–dipole interaction is van der Waals interaction. Similar to van der Waals interactions, the interactions between quadrupoles are called quadrupole–quadrupole interactions. Atoms can be treated not only as dipoles but also as quadrupoles. In this article, we study the quadrupole interactions between highly excited atoms or Rydberg atoms. In addition, unlike many other calculations, in which the primary focus was on the one-dimensional two-body quadrupole–quadrupole interactions, the primary aim of this article is to study the two-dimensional few-body quadrupole–quadrupole interactions. Specifically, the two-dimensional three-body interactions are investigated. This research has many applications, such as quadrupole-blockade for quantum computing, creating molecules based on quadrupole interactions, or quadrupole–quadrupole molecules.

Features of the Electronic Structure of Excited Quadrupolar Molecules in Non-Polar Solvents

Interactions of the electronic subsystem of the quadrupolar fluorophore with intramolecular highfrequency antisymmetric vibrations and solvent polarization are responsible for charge transfer symmetry breaking (SB), which is observed after optical excitation of such molecules in polar solvents. It is known that although these two interactions are mathematically described in similar ways, only the interaction of the fluorophore with solvent orientational polarization can create a state with broken symmetry if this interaction is strong enough. Nevertheless, the interaction of a quadrupolar fluorophore with intramolecular highfrequency antisymmetric vibrations in nonpolar solvents leads to a considerable reconstruction of the electronic subsystem. The analysis of the excited state of quadrupolar molecules in nonpolar solvents performed in this study reveals that such molecules can behave like quantum twostate systems, that is, as a quasispin s = 1/2, having an electric dipole moment instead of a magnetic one. This feature of excited quadrupolar molecules may be of interest to emerging technologies of molecular electronics.

Direct Tracking Excited-State Intramolecular Charge Redistribution of Acceptor-Donor-Acceptor Molecule by Means of Femtosecond Stimulated Raman Spectroscopy.

Symmetric quadrupolar molecules generally exhibit apolar ground states and dipolar excited states in a polar environment, which is explained by the excited state evolution from initial charge delocalization over all molecules to localization on one branch of the molecules after a femtosecond pulse excitation. However, direct observation of excited-state charge redistribution (delocalization/localization) is hardly accessible. Here, the intramolecular charge delocalization/localization character of a newly synthesized acceptor-donor-acceptor molecule (ADA) has been intensively investigated by femtosecond stimulated Raman scattering (FSRS) together with femtosecond transient absorption (fs-TA) spectroscopy. By tracking the excited state Raman spectra of the specific alkynyl (-C≡C-) bonds at each branch of ADA, we found that the nature of the relaxed S1 state is strongly governed by solvent polarity: symmetric delocalized intramolecular charge transfer (ICT) characters occurred in apolar solvent, whereas the asymmetric localized ICT characters appeared in polar solvent because of solvation. The solvation dynamics of ADA extracted from fs-TA is consistent with the time constants obtained by FSRS, but the FSRS clearly tracks the excited state intramolecular charge transfer delocalization/localization.

Quantum phase transition of a two-dimensional quadrupolar system

Ensembles with long-range interactions between particles are promising for revealing strong quantum collective effects and many-body phenomena. Here we study the ground-state phase diagram of a two-dimensional Bose system with quadrupolar interactions using a diffusion Monte Carlo technique. We predict a quantum phase transition from a gas to a solid phase. The Lindemann ratio and the condensate fraction at the transition point are $\gamma=0.269(4)$ and $n_0/n=0.031(4)$, correspondingly. We observe the strong rotonization of the collective excitation branch in the vicinity of the phase transition point. Our results can be probed using state-of-the-art experimental systems of various nature, such as quasi-two-dimensional systems of quadrupolar excitons in transition metal dichalcogenide (TMD) trilayers, quadrupolar molecules, and excitons or Rydberg atoms with quadrupole moments induced by strong magnetic fields.

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor-acceptor branch length.

Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor–acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.

CO2 Adsorption in Metal-Organic Framework Mg-MOF-74: Effects of Inter-Crystalline Space.

Metal-Organic Frameworks (MOF) have been identified as highly efficient nanoporous adsorbents for CO2 storage. In particular, Mg-MOF-74 has been shown to promise exceptionally high CO2 sorption. Although several studies have reported adsorption isotherms of CO2 in Mg-MOF-74, the effect of inter-crystalline spacing in Mg-MOF-74 on the sorption of CO2 has not been addressed. These effects have been shown to be profound for a quadrupolar molecule like CO2 in the case of silicalite (Phys. Chem. Chem. Phys. 22 (2020) 13951). Here, we report the effects of inter-crystalline spacing on the adsorption of CO2 in Mg-MOF-74, studied using grand canonical Monte Carlo (GCMC) simulations. The inter-crystalline spacing is found to enhance adsorption at the crystallite surfaces. Larger inter-crystalline spacing up to twice the kinetic diameter of CO2 results in higher adsorption and larger crystallite sizes suppress adsorption. Magnitudes of the inter-crystalline space relative to the kinetic diameter of the adsorbed fluid and the surface to volume ratio of the adsorbent crystallites are found to be important factors determining the adsorption amounts. The results of this study suggest that the ideal Mg-MOF-74 sample for CO2 storage applications should have smaller crystallites separated from each other with an inter-crystalline space of approximately twice the kinetic diameter of CO2.