Double Exciton State Research Articles

Raman Activities of Cyano-Ester Quinoidal Oligothiophenes Reveal Their Diradical Character and the Proximity of the Low-Lying Double Exciton State

Quinoidal oligothiophenes have received considerable attention as interesting platforms with remarkable amphoteric redox behavior associated with their diradical character increasing with the conjugation lengths. In this work, we considered a family of quinoidal oligothiophenes bearing cyano-ester terminal groups and characterized them by UV-Vis-NIR absorption and Raman spectroscopy measurements at different excitation wavelengths. The experimental investigation is complemented by quantum-chemical studies to assess the quality of computed density functional theory (DFT) ground state structures and their influence on predicted Raman intensities. In addition, resonance conditions with the optically active HOMO→LUMO transition as well as with the more elusive state dominated by the doubly excited HOMO,HOMO→LUMO,LUMO configuration, are determined with DFT-MRCI calculations and their contributions to Raman activity enhancement are discussed in terms of computed vibrational Huang–Rhys (HR) factors.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

The Low Lying Double-Exciton State of Conjugated Diradicals: Assessment of TDUDFT and Spin-Flip TDDFT Predictions

Conjugated singlet ground state diradicals have received remarkable attention owing to their potential applications in optoelectronic devices. A distinctive character of these systems is the location of the double-exciton state, a low lying excited state dominated by the doubly excited HOMO,HOMOLUMO,LUMO configuration, (where HOMO=highest occupied molecular orbital, LUMO=lowest unoccupied molecular orbital) which may influence optical and other photophysical properties. In this contribution we investigate this specific excited state, for a series of recently synthesized conjugated diradicals, employing time dependent density functional theory (TDDFT) based on the unrestricted parallel spin reference configuration in the spin-flip formulation (SF-TDDFT) and standard TD calculations based on the unrestricted antiparallel spin reference configuration (TDUDFT). The quality of computed results is assessed considering diradical and multiradical descriptors, and the excited state wavefunction composition.

The double exciton state of conjugated chromophores with strong diradical character: insights from TDDFT calculations.

A peculiar characteristic of open-shell singlet diradical molecules is the presence of a double exciton state (H,H → L,L) among low lying excited states. Recent high-level quantum-chemical investigations including a static and dynamic electron correlation have demonstrated that this state can become the lowest singlet excited state, a diagnostic fingerprint of the diradical system. Here we investigate the performance of less computationally demanding TDDFT calculations by employing two approaches: the spin-flip TDDFT scheme and TD calculations based on unrestricted broken symmetry antiparallel-spin reference configuration (TDUDFT). The calculations are tested on a number of recently synthesized, large conjugated systems displaying from moderate to large diradical character and showing experimental trace of the double exciton state. We show that spin-flip (SF) TDB3LYP calculations in the collinear approximation generally underestimate the excitation energy of the double exciton state. When the molecule displays a strong diradical character, the unrestricted antiparallel-spin reference configuration of TDUDFT calculations is characterized by strongly localized frontier molecular orbitals. We show that under these conditions the double exciton state is captured by TDUB3LYP calculations since it is described by singly excited configurations and its excitation energy can be accurately predicted. Owing to the improved description of the ground state, also the excitation energy of the single exciton H → L state generally improves at the TDUB3LYP level. With regard to the double exciton state, SF TDB3LYP performs slightly better for small to medium diradical character while a large diradical character (and strong orbital localization) is a prerequisite for the success of TDUB3LYP calculations whose quality otherwise deteriorates.

An Ab Initio Exciton Model Including Charge-Transfer Excited States.

The Frenkel exciton model is a useful tool for theoretical studies of multichromophore systems. We recently showed that the exciton model could be used to coarse-grain electronic structure in multichromophoric systems, focusing on singly excited exciton states [ Acc. Chem. Res. 2014 , 47 , 2857 - 2866 ]. However, our previous implementation excluded charge-transfer excited states, which can play an important role in light-harvesting systems and near-infrared optoelectronic materials. Recent studies have also emphasized the significance of charge-transfer in singlet fission, which mediates the coupling between the locally excited states and the multiexcitonic states. In this work, we report on an ab initio exciton model that incorporates charge-transfer excited states and demonstrate that the model provides correct charge-transfer excitation energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2 calculations shows that our exciton model is robust with respect to system size, screening parameter, and different density functionals. Inclusion of charge-transfer excited states makes the exciton model more useful for studies of singly excited states and provides a starting point for future construction of a model that also includes double-exciton states.

Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots

We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton ESX in resonance to a double exciton EDX. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a 4 nm PbS quantum dot.

Charge-Transfer Excitations Steer the Davydov Splitting and Mediate Singlet Exciton Fission in Pentacene

Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments.

Manipulation of two-photon-induced fluorescence spectra of chromophore aggregates with entangled photons: A simulation study

The non-classical spectral and temporal features of entangled photons offer new possibilities to investigate the interactions of excitons in photosynthetic complexes, and to target the excitation of specific states. Simulations of fluorescence in the bacterial reaction center induced by entangled light demonstrate a selectivity of double-exciton states which is not possible using classical stochastic light with the same power spectrum.

Biradicaloid and Polyenic Character of Quinoidal Oligothiophenes Revealed by the Presence of a Low-Lying Double-Exciton State

Evidence of the biradicaloid and polyenic character of quinoidal oligothiophenes is reported by proving at the CASSCF//CASPT2 computational level the presence of a low-lying double exciton state responsible for the weak features observed in the NIR absorption region of the longest members of this class of molecules. The energy lowering of this state, accompanying the length increase in the oligomers, causes a displacement of the ground-state equilibrium geometry toward more biradicaloid structures because of the more efficient S0-S1 state mixing. Furthermore, it is shown that the doubly excited state is strongly coupled to the ground electronic state, and the coupling is mediated by a collective mode dominated by the out-of-phase stretching of adjacent CC bonds, recently shown to govern the Raman activity. All together, this evidence offers a unified view of the low-lying electronic states for quinoidal oligothiophenes and polyenes.

Ultrafast exciton-exciton coherent transfer in molecular aggregates and its application to light-harvesting systems

Effects of the exciton-exciton coherence transfer (EECT) in strongly coupled molecular aggregates are investigated from the reduced time-evolution equation which we have developed to describe EECT. Starting with the nonlinear response function, we obtained explicit contributions from EECT to four-wave-mixing spectrum such as photon echo, taking into account double exciton states, static disorder, and heat-bath coupling represented by arbitrary spectral densities. By using the doorway-window picture and the projection operator technique, the transfer rates between two different electronic coherent states are obtained within a framework of cumulant expansion at high temperature. Applications of the present theory to strongly coupled B850 chlorophylls in the photosynthetic light harvesting system II (LH2) are discussed. It is shown that EECT is indispensable in properly describing ultrafast phenomena of strongly coupled molecular aggregates such as LH2 and that the EECT contribution to the two-dimensional optical spectroscopy is not negligible.

W STATE GENERATION AND EFFECT OF CAVITY PHOTONS ON THE PURIFICATION OF DOT-LIKE SINGLE QUANTUM WELL EXCITONS

A scheme of three-particle entanglement purification is presented in this work. The physical system undertaken for investigation is dot-like single quantum well excitons independently coupled through a single microcavity mode. The theoretical framework for the proposed scheme is based on the quantum jump approach for analyzing the progress of the trible-exciton entanglement as a series of conditional measurement has been taken on the cavity field state. We first investigate how cavity photon affects the purity of the double-exciton state and the purification efficiency in two-particle protocol. Then we extend the two-particle case and conclude that the three-exciton state can be purified into W state, which involves the one-photon-trapping phenomenon, with a high yield. Finally, an achievable setup for purification using only modest and presently feasible technologies is also proposed.

Cooperative Exciton States in Molecular Crystals

In this paper we calculate the transition probability for excitation of double-exciton states in absorption and emission processes in molecular crystals. The formalism is based on the tight-binding approximation, treating the intermolecular electrostatic and exchange interactions and the interaction with the radiation field by perturbation theory. It is found that: (1) A major contribution to the transition probability for cooperative excitation to electronic states arises from intermolecular Coulomb interactions, while the contribution of intermolecular electron-exchange interactions is relatively small. (2) Contributions to the transition probability from high-order-transition multipole interactions are of considerable importance. (3) Approximate selection rules for cooperative electronic excitation imply that for one component the initial and final states are of the same parity, while for the second component the transition is symmetry allowed. (4) Theoretical evidence is obtained for the appearance of double-excitation bands in the infrared spectra of solids in the overtone region. (5) A major contribution to the intensity of the double-vibrational-exciton states arises from an intermolecular Fermi resonance effect. (6) The direct detection of the radiative annihilation of a double-exciton state is not likely to be experimentally feasible.