Excitonic Molecules Research Articles

High-performance circularly polarized electroluminescence UV-OLED based on hot exciton molecules with preferred horizontal dipole orientation

Emitting circularly polarized electroluminescence (CPEL) directly endows the CP organic light-emitting diodes (CP-OLEDs) with great advantages in 3D display. But the highly efficient CP-OLEDs are still in great need, especially for devices emitting short-wavelength light such as ultraviolet (UV) CP-OLEDs. Herein, we report the first case of CP UV-OLED, fabricated by two pairs of enantiomers, (R)-/(S)-BNP-o-CzPI and (R)-/(S)-BNP-p-CzPI, established by chiral perturbation. Devices using 10 wt% enantiomers as dopants show UV CPEL at 396 nm with external quantum efficiency, Commission Internationale de l’Eclairage y value, and full width at half maxima of 7.2/6.9 %, 0.046/0.044 and 50/50 nm for (R)-/(S)-BNP-o-CzPI, respectively. The horizontal dipole orientations of the film with 10 wt% emitters in mCPCN are measured as high as 84.1/85.2 % and 88.1/88.7 % for (R)-/(S)-BNP-o-CzPI and (R)-/(S)-BNP-p-CzPI, respectively. Experiments and theoretical calculations indicate that the high EQEs are due to the high triplet exciton utility and large horizontal dipole orientation. In addition, the enantiomers show signigicant circularly polarized luminescence properties both in photoluminescence and in electroluminescence, and the device using (S)-BNP-o-CzPI has a large dissymmetry factors of 3.10 × 10−3 at emission maximum.

A Hot Exciton Luminogen Constructed by an o‐Carborane Scaffold

AbstractEmitters with the combination of an electron donor (D) and an acceptor (A) have been widely studied and usually exhibited interesting photophysical properties. Hot exciton materials involving an ordinary D‐A structure typically undergo a hybridized local charge transfer (HLCT) process. However, the hot exciton molecules containing more D/A fragments are much less explored. In this study, a new hot exciton material based on an inorganic boron cluster is designed and synthesized, which possesses a V‐type D–A–A′–D′ molecular architecture. The emitter shows an impressive photoluminescence quantum yield (PLQY) of up to 90% and demonstrates multiple emissions, excitation‐wavelength‐, solvent‐, and temperature‐dependent emissions. These characteristics are attributed to the coexistence of HLCT and the through‐space charge transfer process (TSTC) triggered by an o‐carborane scaffold in the excited state. Besides, the interchangeable HLCT/TSCT/hRISC processes are demonstrated by temperature‐dependent spectra and femtosecond time‐resolved transient absorption spectra. These findings confirm a novel hot exciton mechanism, simultaneously induced by HLCT and TSCT. This is also the first example of a carborane‐based hot exciton molecule. The current study not only reports a new molecular architecture for a hot exciton material but also gives rise to a new insight into the hot exciton mechanism.

High Efficiency Triplet‐Triplet Fusion Blue Fluorescence OLEDs by introducing a “Hot Exciton” Efficiency Enhancement Layer

AbstractTriplet‐triplet fusion (TTF) can effectively transfer triplet excitons to singlet energy level and exhibit good stability, making it widely used in blue fluorescence organic light emitting diodes (OLEDs) for practical application. However, the efficiency of the resulting blue fluorescence OLEDs is still fundamentally limited. Herein, this work introduces a hot exciton efficiency enhancement layer (EEL) between TTF emissive layer and electron‐transporting layer for the first time. It can be found that the triplet excitons are greatly utilized by the efficient hot exciton EEL. As a result, a high efficiency blue fluorescence OLED is successfully developed, exhibiting a maximum external quantum efficiency of 15.2% at the luminance of 4068.0 cd m−2, achieving a breakthrough in the efficiency of blue TTF‐OLEDs. Transient electroluminescence (TrEL) measurements and detailed theoretical calculations confirm the key role of the effective hot exciton process from the high‐lying triplet energy level (Tn, n ≥ 2) to S1 via a reverse intersystem crossing (hRISC). Furthermore, the device lifetime is also greatly improved owing to the fast and efficient hRISC in the introduced hot exciton EEL. This work has important guiding significance for constructing high efficiency blue fluorescence OLEDs by introducing hot exciton molecules.

Electrochemiluminescence of hot exciton nanomaterial with boosted efficiency for visual bioanalysis

The electrochemiluminescence (ECL) efficiency (ΦECL) of organic ECL emitters is greatly influenced by the excited state property and component. Here we design a hot exciton molecule (BCzP-BT) with hybridized local and charge-transfer (HLCT) excited state property to prepare the first hot exciton organic nanorods (BB NRs). The BB NRs exhibit both sensitive annihilation and intense coreactant ECL emissions with a band gap emission model. Due to the reverse intersystem crossing among high-lying excited states (hRISC), the HLCT excited state property leads to highly efficient utilization of triplet excitons and thus high photoluminescence quantum yields (ΦPL) of more than 80% at 554 nm, which is demonstrated by solvatochromic experiments and quantum chemical calculations. The high ΦPL endows BB NRs with superior ΦECL over other nanoemitters, even the thermally activated delayed fluorescence materials, and thus offers highly-efficient nanoemitters for the design of ECL imaging strategy. As a proof of concept, an ECL probe is constructed by assembling BHQ2-ssDNA on BB NRs to integrate CRISPR/Cas12a system for target recognition, which produces a rapid and high-throughput ECL imaging platform. The proposed imaging method for HPV16 DNA detection shows excellent performance with a detection limit of 0.6 fM. This work broadens the application of hot exciton materials in ECL field, and opens a new avenue for developing next-generation ECL devices.

Effect of molecular concentration on excitonic nanostructure based refractive index sensing and near-field enhanced spectroscopy

Organic thin film based excitonic nanostructures are of great interest in modern resonant nanophotonics as a promising alternative for plasmonic systems. Such nanostructures sustain propagating and localized surface exciton modes that can be exploited in refractive index sensing and near-field enhanced spectroscopy. To realize these surface excitonic modes and to enhance their optical performance, the concentration of the excitonic molecules present in the organic thin film has to be quite high so that a large oscillator strength can be achieved. Unfortunately, this often results in a broadening of the material response, which might prevent achieving the very goal. Therefore, systematic and in-depth studies are needed on the molecular concentration dependence of the surface excitonic modes to acquire optimal performance from them. Here, we study the effect of molecular concentration in terms of oscillator strength and Lorentzian broadening on various surface excitonic modes when employed in sensing and spectroscopy. The optical performance of the modes is evaluated in terms of sensing, like sensitivity and figure of merit, as well as near-field enhancement, like enhancement factor and field confinement. Our numerical investigation reveals that, in general, an increase in oscillator strength enhances the performance of the surface excitonic modes while a broadening degrades that as a counteracting effect. Most of all, this demonstrates that the optical performance of an excitonic system is tunable via molecular concentration unlike the plasmonic systems. Moreover, different surface excitonic modes show different degrees of tunability and equivalency in performance when compared to plasmons in metals (silver and gold). Our findings provide crucial information for developing and optimizing novel excitonic nanodevices for contemporary organic nanophotonics.

Reconstituted LH2 in multilayer membranes induced by poly-l-lysine: Structure of supramolecular and electronic states

To explore the effect of cell membrane stacking on the function of light-harvesting complex 2 (LH2) in purple nonsulfur photosynthetic bacteria, LH2 from Rhodobacter sphaeroides 2.4.1 (R. sph 2.4.1) was reconstituted into lipid bilayer vesicles (LH2@liposome) and further formed multi-layer structure by electrostatic interaction with poly-l-lysine (LH2@liposome/PLL), which was characterized by cryo-electron microscopy (cryo-EM) and TEM. When embedded in liposomes and additionally in multi-layer liposomes, the absorption band, zero-crossing point of CD signals and fluorescence emission of LH2 B850 excitons were observed to uniformly have 1–2 nm red-shifting. Combining with the corresponding fluorescence quench and the generation of shorter-living fluorescence species, a new excitonic species generated through B850 structural splitting was proposed. By FT-Raman and triplet carotenoid dynamics, the structural mechanism was deduced and discussed. Briefly, all environmental changes, including LH2 aggregating and multi-layer membrane stacking, eventually applied forces on B850 exciton molecules mainly through the hydrogen bonding between the C3-acetyl carbonyl groups of B850 BChls and Tyr44 and 45 residues at C-terminus of α-polypeptides. The effect of multi-layer structure on LH2 could be assigned as a kind of photoprotection.

In English

In review, deals with the theory of exciton quasimolecules (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric colloidal quantum dots (QDs) synthesized in a dielectric and semiconductor matrixs. It has been shown that the exciton quasimolecule formation is of the threshold character and possible in a nanosystem, where the distance D between the surfaces of QD is given by the condition (where and are some critical distance). We have shown that in such a nanoheterostructures acting as “exciton molecules” are the QDs with excitons localizing over their surfaces. The position of the quasimolecule state energy band depends both on the mean radius of the QDs, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure. It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor and dielectric QDs is a significant large values, larger than the binding energy of the biexciton in a semiconductor and dielectric single crystals almost two orders of magnitude. It is shown that the major contribution to tue binding energy of singlet ground state of exciton quasimolecule is made by the energy of the exchange interaction of electrons with holes and this contribution is much more substantial than the contribution of the energy of the Coulomb interaction between the electrons and holes. It is established that the position of the exciton quasimolecule energy band depends both on the mean radius of the QDs and the distance between their surfaces. It is shown that with increase in temperature above the threshold (), a transition can occur from the exciton quasimolecule to exciton state. It has been found that at a constant concentration of excitons (i.e. constant concentration of QD) and temperatures Т below , one can expect a new luminescence band shifted from the exciton band by the value of the exciton quasimolecule binding energy. This new band disappears at higher temperatures (). At a constant temperature below , an increase in exciton concentration (i.e. in QD concentration) brings about weakening of the exciton luminescence band and strengthening of the exciton quasimolecule. These exciton quasimolecules are of fundamental interest as new quasi-atomic colloidal nanostructures; they may also have practical value as new nanomaterials for nanooptoelectronics. The fact that the energy of the ground state singlet exciton quasimolecule is in the infrared range of the spectrum, presumably, allow the use of a quasimolecule to create new infrared sensors in biomedical research.

Effects of a long-short axis skeleton on the excited-state properties of ultraviolet hot exciton molecules: luminescence mechanism and molecular design

Wise design strategies for efficient ultraviolet (UV) hot exciton molecules are highly desired. In this work, inspired by the long-short axis skeleton strategy, a theoretical study on the substituent effect of the long-axis on the photophysical properties of UV hot exciton molecules is performed. A multiscale simulation is performed to study the photophysical properties of the reported compound 2BuCz-CNCz and theoretically designed promising compounds 2Cz-CNCz, 2TPA-CNCz2TPA-CNCz, 2Na-CNCz and 2An-CNCz, which all possess unique features of UV emission and hot exciton properties. The packing modes of the five molecules in a film are obtained by molecular dynamics (MD) simulations, and then the photophysical properties with the consideration of the SSE (solid-state effect) are studied by using the combined quantum mechanics and molecular mechanics (QM/MM) method. Finally, the exciton evolution process is revealed by the rate equations. The results show that different substituents in the long axis have relatively little effect on the larger twist angle in the short axis. The tert-butyl and triphenylamine groups increase the vibration of the molecule, enhance the non-radiative rate of the molecule and intensify the energy dissipation, but the vibration of tert-butyl can be greatly restrained in the solid state. Furthermore, our designed 2Na-CNCz compound possesses the maximum reverse intersystem crossing rate and radiative decay rate. Therefore, when studying the effect of the long-axis substituents on the properties of hot excitons, 2Na-CNCz could be a profitable candidate molecule. This work should enrich the theoretical calculation methods to investigate the luminescence properties of organic molecules in OLEDs.

Charged Bosons Made of Fermions in Bilayer Structures with Strong Metallic Screening.

Two-dimensional monolayer structures of transition metal dichalogenides (TMDs) have been shown to allow many higher-order excitonic bound states, including trions (charged excitons), biexcitons (excitonic molecules), and charged biexcitons. We report here experimental evidence and the theoretical basis for a new bound excitonic complex, consisting two free carriers bound to an exciton in a bilayer structure. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy with two different TMD materials (MoSe2 and WSe2) with both n- and p-doping if and only if all the required theoretical conditions for this complex are fulfilled, in particular, only in the presence of a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers. Because these four-carrier bound states are charged bosons, they could eventually be the basis for a new path to superconductivity without Cooper pairing.

Multiphoton-excited exciton molecules in diamond

In this paper we report on observation of exciton molecules in the low-temperature photoluminescence spectra of diamond. The charge carriers are excited using multiphoton absorption of few-cycle near-infrared laser pulses. We observe that peaks corresponding to exciton molecules are well resolved and the formation of electron hole droplets is suppressed due to the initial carrier dynamics and low excited carrier densities.

In English

In review, deals with the theory of exciton quasimolecules in a nanoheterostructures. It has been found that the formation of a exciton quasimolecule in a nanoheterostructures made up of aluminum oxide quantum dots synthesized in a dielectric matrix is of threshold character and can occur in a nanosystem where the distance D between the surfaces of quantum dots is given by the condition  . The existence of such distance arises from quantum size effects in which the decrease in the energies of interaction of the electrons and holes entering into the Hamiltonian of the “exciton molecule” with decrease of the distance D between the surfaces of the QD cannot compensate for the increase in the kinetic energy of the electrons and holes. At larger distances D between the surfaces of quantum dots: the biexciton breaks down into two excitons (consisting of spatially separated electrons and holes), localized over QD surfaces.          It was shown that the convergence of two quantum dots up to a certain critical value   between surfaces of quantum dot lead to overlapping of electron orbitals of superatoms and the emergence of exchange interactions. In this case the overlap integral of the electron wave functions takes a significant value. As a result, the conditions for the formation of quasi-molecules from quantum dots can be created.          We have shown that in such a nanoheterostructures acting as “exciton molecules” (biexcitons consisting of spatially separated electrons and holes) are the quantum dots of aluminum oxide with excitons localizing over their surfaces. The position of the biexciton state energy band depends both on the mean radius of the quantum dots, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure.          As our variational calculations show, the interaction of the excitons with the surfaces of quantum dots (“intramolecular” interaction) is much stronger than that between quantum dots (“intermolecular” interaction). Due to the translational symmetry of such a nanoheterostructures of quantum dots, it permits propagation of electronic excitation in the form of biexcitons.          As follows form the results of the variational calculations, the major contribution to the biexciton binding energy is from the energy of exchange interaction of electrons and holes, which by far surpasses that from their Coulomb interaction.          It is established that at constant concentrations of biexcitons at temperatures T below a certain critical temperature Tc due to the radiative annihilation of one of the excitons forming a biexciton one can expect a new spectral band of luminescence shifted relative to the exciton band by the biexciton binding energy . This new luminescence band disappears at temperatures above Tc. At a constant temperature Т < Tc   the growth of exciton concentration brings about weakening of the exciton band and strengthening of the biexciton band of luminescence.

Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells.

We show that optical gain in 2D CdSe colloidal quantum wells (CQWs) shows little saturation and coexists with exciton absorption over a broad range of excitation densities, in stark contrast with 0D CdSe colloidal quantum dots (CQDs). In addition, we demonstrate that photoexcited CQWs can absorb or emit light through the thermodynamically driven formation or radiative recombination of singlet excitonic molecules. Invoking stimulated emission through the molecule-exciton transition, we can quantify all of the remarkable gain characteristics of CQWs using only experimentally determined parameters, an advance that highlights a fundamental difference between multiexcitons in CQWs and CQDs. While strong confinement prohibits the dissociation of multiexcitons into separate excitons in 0D CQDs, excitons and excitonic molecules coexist in a 2D CQW at room temperature, with densities governed by an association/dissociation equilibrium, not by state-filling. Our finding points out future directions to optimize stimulated emission by excitonic 2D materials in general.

Compounding Plasmon⁻Exciton Strong Coupling System with Gold Nanofilm to Boost Rabi Splitting.

Various plasmonic nanocavities possessing an extremely small mode volume have been developed and applied successfully in the study of strong light-matter coupling. Driven by the desire of constructing quantum networks and other functional quantum devices, a growing trend of strong coupling research is to explore the possibility of fabricating simple strong coupling nanosystems as the building blocks to construct complex systems or devices. Herein, we investigate such a nanocube-exciton building block (i.e. AuNC@J-agg), which is fabricated by coating Au nanocubes with excitonic J-aggregate molecules. The extinction spectra of AuNC@J-agg assembly, as well as the dark field scattering spectra of the individual nanocube-exciton, exhibit Rabi splitting of 100–140 meV, which signifies strong plasmon–exciton coupling. We further demonstrate the feasibility of constructing a more complex system of AuNC@J-agg on Au film, which achieves a much stronger coupling, with Rabi splitting of 377 meV. This work provides a practical pathway of building complex systems from building blocks, which are simple strong coupling systems, which lays the foundation for exploring further fundamental studies or inventing novel quantum devices.

Charge Carrier Cooling Bottleneck Opens Up Nonexcitonic Gain Mechanisms in Colloidal CdSe Quantum Wells

Ultrathin two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications because of their pronounced room-temperature excitonic features, leading to unique properties in terms of light–matter interaction. However, only a few studies focus on light amplification and the complex photophysics at high excitation density. The beneficial nature of strong excitonic effects on optical gain remain hence unquantified, and despite the increased binding energies of the excitonic species, it remains unclear what the involvement of 2D excitons would be in optical gain. Here, we use colloidal CdSe nanoplatelets as a model system for colloidal 2D materials and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several excitation density-dependent optical gain regimes exist. At low density, optical gain originates from excitonic molecules delivering large material gains up to 20 000 cm–1 with an Auger limited lifetime of a few hund...

Nonlinear Emission of Molecular Ensembles Strongly Coupled to Plasmonic Lattices with Structural Imperfections.

We demonstrate nonlinear emission from molecular layers strongly coupled to extended light fields in arrays of plasmonic nanoparticles in the presence of structural imperfections. Hybrid light-matter states, known as plasmon-exciton polaritons (PEPs), are formed by the strong coupling of Frenkel excitons in molecules to surface lattice resonances. These resonances result from the radiative coupling of localized surface plasmon polaritons in silver nanoparticles enhanced by diffraction on the array. By designing arrays with different lattice constants, we show that the nonlinear emission frequency is solely determined by the relaxation of exciton polaritons through vibrational quanta in the molecules. We also observe long-range spatial coherence in the samples, which supports the explanation in terms of a nonlinear collective emission of strongly coupled PEPs. In contrast to recent observations of exciton-polariton lasing and condensation in organic systems, photonic modes play a minor role at the emission frequency in our system, and this emission has an undefined momentum because of the structural imperfections. This remarkable result reveals the rich and distinct physics of strongly coupled organic molecules to photonic cavities.

Biexciton states in nanoheterostructures of dielectric quantum dots

It has been found that the formation of a biexciton in a nanoheterostructures (NHS) made up of aluminum oxide quantum dots (QD) synthesized in a dielectric matrix is of threshold character and can occur in a nanosystem where the distance D between the surfaces of QD is given by the condition Dc(1)≤D≤Dc(2). We have shown that in such a NHS acting as “exciton molecules” (biexcitons consisting of spatially separated electrons and holes) are the quantum dots of aluminum oxide with excitons localizing over their surfaces. The position of the biexciton state energy band depends both on the mean radius of the quantum dots, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure.

Hybrid States of Biomolecules in Strong-Coupling Regime

The strong coupling of exciton and plasmon states is the result of the reversible energy exchange between the excited states of atomic exciton systems or molecules and the electromagnetic field. This leads to the formation of hybrid (mixed) states whose energies differ from those of the exciton and photon. To date, the implementation of strong-coupling hybrid states has been attracting great attention in terms of designing state-of-the-art emitting systems and quantum information technologies; controlling chemical reaction efficiency and targeted influence on biological systems; and applying the observed effects in medicine, microelectronics, robotics technologies, and other fields. This review deals with a model of strong light-matter interaction and its characteristics, ways to the practical implementation of hybrid states (including those in biological systems), and parameters affecting strong coupling. The recent advances in practical applications of strong coupling effects, prospects for their use, and the problems entailed are discussed as well.

Biexciton in nanoheterostructures of dielectric quantum dots

It has been found that the formation of a biexciton in a nanoheterostructures (NHS) made up of aluminum oxide quantum dots (QDs) synthesized in a dielectric matrix is of threshold character and can occur in a nanosystem, where the distance D between the surfaces of QD is given by the condition Dc(1)≤D≤Dc(2). We have shown that with such an NHS acting as “exciton molecules” (biexcitons consisting of spatially separated electrons and holes) are the QDs of aluminum oxide with excitons localizing over their surfaces. The position of the biexciton state energy band depends both on the mean radius of the QDs, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure.

Vertically coupled dipolar exciton molecules

While the interaction potential between two dipoles residing in a single plane is repulsive, in a system of two vertically adjacent layers of dipoles it changes from repulsive interaction in the long range to attractive interaction in the short range. Here we show that for dipolar excitons in semiconductor heterostructures, such a potential may give rise to bound states if two such excitons are excited in two separate layers, leading to the formation of vertically coupled dipolar exciton molecules. Our calculations prove the existence of such bound states and predict their binding energy as a function of the layers separation as well as their thermal distributions. We show that these molecules should be observed in realistic systems such as semiconductor coupled quantum well structures and the more recent van-der-Waals bound heterostructures. Formation of such molecules can lead to new effects such as a collective dipolar drag between layers and new forms of multi-particle correlations, as well as to the study of dipolar molecular dynamics in a controlled system.

Gain-assisted plasmonic metamaterials: mimicking nature to go across scales

Nature as a source of inspiration for designing and fabricating nanostructured materials with unconventional properties is an unparalleled driving force of this work leading to low-loss metamaterials. Here, we report about a multipronged approach to create optical metamaterials based on plasmonic nanostructures, hierarchical organization and interplay between plasmon elements and excitonic molecules. This work is focused on strategies and approaches to produce gain to metamaterials across scales with the aim of realizing low-loss optical materials and unlocking their unconvetional electromagnetic properties. Finally, we describe how a biomimetic approach based on gain-functionalized bionanoparticle can be harnessed for diagnostics and theranostics.