Electric dipole moment of excited octupolar molecules: Potential qubit implementation.

The first excited state of conjugated donor-acceptor molecules of C3 symmetry (octupolar molecules) is doubly degenerate. Such a doublet is known to be isomorphic to a spin 1/2. It is shown that a large electric dipole moment is associated with this spin.

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.

Symmetry breaking charge transfer and control of the transition dipole moment in excited octupolar molecules.

The structure of the energy levels of excited symmetric donor-acceptor octupolar molecules suggests a completely symmetric state and a degenerate doublet. For most molecules, the doublet is the first excited state, which is called the normal level order, but there are molecules with the reverse level order. Symmetry breaking charge transfer (SBCT) and its effect on the transient dipole moment in these structures are studied.

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.

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 effect of energy level degeneracy on symmetry-breaking charge transfer: Excited octupolar dyes.

A three-level model of symmetry-breaking charge transfer (SBCT) in excited octupolar molecules is developed. The model describes the joint dynamics of the solvent and the dye in the excited state. For this, a distribution function in the space of two reaction coordinates is introduced.

Minimal model of excited-state symmetry breaking in symmetric dimers and covalently linked dyads.

A model of symmetry breaking (SB) charge separation in symmetric excited dyads and dimers is presented. The minimal model should include at least four basis electronic states due to a small energy gap between the locally excited and charge separated (zwitterionic) states of the chromophores. There are electronic couplings between all these states.

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.

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.

Symmetry Breaking in an Excited Quadrupolar Acridine-Dione Derivative Driven by Hydrogen Bonding.

An acridine-dione derivative (3,3,11,11-tetramethyl-8,16-diphenyl-3,4,8,10,11,12,13,16-octahydroacridino[4,3-]acridine-1.9(2,5)dion) with quadrupolar motif has been synthesized and its stationary and transient spectra have been measured. Stationary absorption and fluorescence spectra as well as nonstationary spectra show no signs of symmetry breaking (SB) in aprotic solvents, even of high polarity.

Nonstationary Theory of Excited State Charge Transfer Symmetry Breaking Driven by Polar Solvent.

A consistent theory of electron transfer symmetry breaking (SB) dynamics in excited quadrupolar molecules in polar solvents is developed. The interaction of the electronic subsystem of the molecule with intramolecular degrees of freedom and solvent polarization is taken into account and is divided into interaction with inertial and inertialess degrees of freedom. A strong influence of the inertialess polarization of the solvent on the extent of symmetry breaking is revealed.

The Efficiency of Photoinduced Intramolecular Charge Separation from the Second Excited State: What Factors Can Control It?

The efficiency of photoinduced charge separation (CS) in electron donor-acceptor compounds is commonly limited due to fast deactivation processes, such as the excited-state internal conversion and ultrafast hot reverse electron transfer to the acceptor, charge recombination (CR). A traditional way to avoid undesired energy losses due to CR is to put the reverse electron transfer into the Marcus inverted region, thus effectively suppressing it. This method, however, is not generally applicable when considering CS from the second locally excited state because the driving force of CR to the first excited state is small, and thus charge recombination is ultrafast and efficient.

Quantum yield and energy efficiency of photoinduced intramolecular charge separation.

Kinetics of photoinduced intramolecular charge separation (CS) and the ensuing ultrafast charge recombination (CR) in electron-donor-acceptor dyads are studied numerically, taking into account the excitation of charge-transfer active intramolecular vibrations and multiple relaxation time scales of the surrounding polar solvent. Both energetic and dynamic properties of intramolecular and solvent reorganization are considered, and their influence on the CS/CR kinetics and quantum yield of ultrafast CS is explored. Particular attention is paid to the energy efficiency of CS, as one of the most important parameters indicating the promise of using a molecular compound as a basis for emerging optoelectronic devices.

Exact solution of three-level model of excited state electron transfer symmetry breaking in quadrupolar molecules.

An analytical solution of a three-level model of symmetry breaking in excited A-D-A quadrupolar triads with an electron donor D and identical electron acceptors A and A is derived, in particular, an analytical expression for the dissymmetry parameter (difference in charges, in electron charge units, on the left and right arms of the molecule) is obtained. The model predicts the threshold dependence of the symmetry breaking degree on the parameters of the molecule and its interaction with the solvent. It is shown that for typical molecular parameters, symmetry breaking occurs as a charge transfer from one arm of the molecule to the other with nearly invariable donor charge.

Spectral Dynamics of Nitro Derivatives of Xanthione in Solutions.

Nitro derivatives of xanthione, 2,7-dinitro-9 H-xanthene-9-thione and 2,4,7-trinitro-9 H-xanthene-9-thione, have been first synthesized and their stationary and transient spectra have been measured. The stationary spectra show that the attachment of the nitro groups to the xanthione scaffold leads to strong quenching of S → S fluorescence and the decrease of the oscillator strength of the S ← S electronic transition. Analysis of the transient absorption spectra uncovers the ultrafast stimulated emission quenching from the second excited state, S, in the both derivatives.

Magnetic field effect on ion pair dynamics upon bimolecular photoinduced electron transfer in solution.

The dynamics of the ion pairs produced upon fluorescence quenching of the electron donor 9,10-dimethylanthracene (DMeA) by phthalonitrile have been investigated in acetonitrile and tetrahydrofuran using transient absorption spectroscopy. Charge recombination to both the neutral ground state and the triplet excited state of DMeA is observed in both solvents. The relative efficiency of the triplet recombination pathway decreases substantially in the presence of an external magnetic field.

The effect of solvent relaxation time constants on free energy gap law for ultrafast charge recombination following photoinduced charge separation.

To elucidate the regularities inherent in the kinetics of ultrafast charge recombination following photoinduced charge separation in donor-acceptor dyads in solutions, the simulations of the kinetics have been performed within the stochastic multichannel point-transition model. Increasing the solvent relaxation time scales has been shown to strongly vary the dependence of the charge recombination rate constant on the free energy gap. In slow relaxing solvents the non-equilibrium charge recombination occurring in parallel with solvent relaxation is very effective so that the charge recombination terminates at the non-equilibrium stage.

Solvent-assisted multistage nonequilibrium electron transfer in rigid supramolecular systems: Diabatic free energy surfaces and algorithms for numerical simulations.

An approach to the construction of diabatic free energy surfaces (FESs) for ultrafast electron transfer (ET) in a supramolecule with an arbitrary number of electron localization centers (redox sites) is developed, supposing that the reorganization energies for the charge transfers and shifts between all these centers are known. Dimensionality of the coordinate space required for the description of multistage ET in this supramolecular system is shown to be equal to N - 1, where N is the number of the molecular centers involved in the reaction. The proposed algorithm of FES construction employs metric properties of the coordinate space, namely, relation between the solvent reorganization energy and the distance between the two FES minima.

A simple model of solvent-induced symmetry-breaking charge transfer in excited quadrupolar molecules.

A simple model has been developed to describe the symmetry-breaking of the electronic distribution of A-D-A type molecules in the excited state, where D is an electron donor and A and A are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed.

Verification of Nonequilibrium Mechanism of Ultrafast Charge Recombination in Excited Donor-Acceptor Complexes.

Control of charge transfer requires knowledge of its detailed mechanism. Due to the large number of known mechanisms, the identification of the mechanism in specific systems is a challenge so far. In this article we propose the idea of how to distinguish between thermal and nonequilibrium modes of charge recombination in excited donor-acceptor complexes.

Free-Energy-Gap Law for Ultrafast Charge Recombination of Ion Pairs Formed by Intramolecular Photoinduced Electron Transfer.

In this article, regularities of ultrafast charge recombination (CR) kinetics in photoinduced intramolecular electron transfer in polar solvents are studied. The kinetics of charge separation and ensuing ultrafast CR are simulated within the framework of the multichannel stochastic model. This model accounts for the reorganization of both the solvent and a number of intramolecular high-frequency vibrational modes.

Simulations of the Ultrafast Transient Absorption Dynamics of a Donor-Acceptor Biaryl in Solution.

A model for simulating the transient electronic absorption spectra of donor-acceptor dyads undergoing ultrafast intramolecular charge transfer in solution has been developed. It is based on the stochastic multichannel point-transition approach and includes the reorganization of high-frequency intramolecular modes (treated quantum mechanically) and of low frequency intramolecular and solvent modes (described classically). The relaxation of the slow modes is assumed to be exponential with time constants taken from experiments.

Dynamic Solvent Effect on Ultrafast Charge Recombination Kinetics in Excited Donor-Acceptor Complexes.

Manifestation of the dynamic solvent effect (DSE) on the charge recombination (CR) kinetics of photoexcited donor-acceptor complexes in polar solvents has been investigated within the framework of the multichannel stochastic model. The model takes into account the reorganization of both the solvent and a number of intramolecular high-frequency vibration modes as well as their relaxation. The non-Markovian solvent dynamics is described in terms of two relaxation modes.

Effect of Intramolecular High-Frequency Vibrational Mode Excitation on Ultrafast Photoinduced Charge Transfer and Charge Recombination Kinetics.

A model of photoinduced ultrafast charge separation and ensuing charge recombination into the ground state has been developed. The model includes explicit description of the formation and evolution of nonequilibrium state of both the intramolecular vibrations and the surrounding medium. An effect of the high-frequency intramolecular vibrational mode excitation by a pumping pulse on ultrafast charge separation and charge recombination kinetics has been investigated.

Dynamics of ground state absorption spectra in donor-acceptor pairs with ultrafast charge recombination.

A theoretical approach to simulation of the transient spectra in molecular systems with ultrafast photoinduced nonradiative electronic transitions is developed. The evolution of the excited and ground state populations as well as the nonradiative transitions between them are calculated in the framework of the stochastic multichannel point-transition model involving the reorganization of the medium and the intramolecular high frequency vibrational modes. Simulations of transient spectra of donor-acceptor pairs excited in the charge-transfer band that are accompanied by ultrafast charge recombination into the ground state demonstrate a possibility of positive band appearance in the transient absorption spectrum caused by those systems in the ground state, which returned there from the excited state.