Exciton Research Articles

Strong Coupling of Chiral Frenkel Exciton for Intense, Bisignate Circularly Polarized Luminescence.

We show that chiral Frenkel excitons yield intense circularly polarized luminescence with an intrinsic dissymmetry factor in emission glum as high as 0.08. This outstanding value is measured through thin films of cyanine J-aggregates that form twisted bundles. Our measurements, obtained by a Mueller polarization analysis, are artifact-free and reveal a quasi-perfect correlation between the dissymmetry factors in absorption, gabs , and in emission glum . We interpret the bisignate dissymmetry factors as the signature of a strong coupling between chiral Frenkel excitons longitudinally excited along the bundles. We further resolve by polarimetry analysis the split in energy between the excited states with a Davydov splitting as small as 28 meV. We finally show the anti-Kasha nature of the chiral emission bands with opposite optical chirality. These mirror-imaged emissive chiroptical features emerge from the structural rigidity of the bundles that preserves the ground- and excited-state chirality.

Strong Coupling of Chiral Frenkel Exciton for Intense, Bisignate Circularly Polarized Luminescence

AbstractWe show that chiral Frenkel excitons yield intense circularly polarized luminescence with an intrinsic dissymmetry factor in emission glum as high as 0.08. This outstanding value is measured through thin films of cyanine J‐aggregates that form twisted bundles. Our measurements, obtained by a Mueller polarization analysis, are artifact‐free and reveal a quasi‐perfect correlation between the dissymmetry factors in absorption, gabs, and in emission glum. We interpret the bisignate dissymmetry factors as the signature of a strong coupling between chiral Frenkel excitons longitudinally excited along the bundles. We further resolve by polarimetry analysis the split in energy between the excited states with a Davydov splitting as small as 28 meV. We finally show the anti‐Kasha nature of the chiral emission bands with opposite optical chirality. These mirror‐imaged emissive chiroptical features emerge from the structural rigidity of the bundles that preserves the ground‐ and excited‐state chirality.

Flexible Miniaturized Multispectral Detector Derived from Blade-Coated Organic Narrowband Response Unit Array.

Multispectral sensing is extremely desired in intelligent systems, e.g., autonomous vehicles, encrypted information communication, and health biometric monitoring, due to its highly sensitive spectral discrimination ability. Nevertheless, rigid bulky optics and delicate optical paths in devices significantly increase their complexity and size, which subsequently impede their integration in smart optoelectronic chips for universal applications. In this work, a filterless miniaturized multispectral photodetector is realized with an organic narrowband response unit array. With the manipulation of Frenkel exciton dissociation in active layers, a series of narrowband organic sensing units with full-width-at-half-maximum (fwhm) narrowing to ∼50 nm are achieved from 700 to 1050 nm with a laudable performance of responsivity of over 60 mA/W, -3 dB bandwidth over 10 kHz, linear dynamic range (LDR) reaching ∼120 dB, and a low noise current of less than 4 × 10-14 A·Hz-0.5. Furthermore, a 6 × 8 multispectral sensing array on a flexible substrate was fabricated with blade-coating. Assisted by a computational process, we successfully demonstrate the spectral recognition with a resolution of ∼50 nm and a mismatch of ∼10 nm. Finally, the function of matter identification is successfully achieved with our multispectral detector array.

Mesoporous Carbon Nitride with π-Electron-Rich Domains and Polarizable Hydroxyls Fabricated via Solution Thermal Shock for Visible-Light Photocatalysis.

Carbon nitride semiconductors are competitive candidates for visible-light-responsive photocatalysts, but encounter weakened exciton dissociation arising from the elevated Coulomb force of singlet Frenkel excitons with narrowing bandgaps. We overcome this contradiction by co-infusing π-electron-rich domains and polarizable hydroxyl units into mesoporous carbon nitride, realized by solution thermal shock. The embedded delocalized π-conjugated aromatic domains derived from nonconjugated macromolecules downshift the conduction band edge and contribute to spatial separation of photogenerated electrons in the lowest unoccupied molecular orbital and holes in the highest occupied molecular orbital. Meanwhile, polarizable hydroxyls induce distinct electron flow from heptazine-based skeletons to periphery sites and enhance water adsorption as well as proton reduction capacity. Consequently, the polymeric carbon nitride delivers an enhanced hydrogen evolution rate that is 17.5 times larger than thermally treated counterparts derived from urea fabricated via conventional strategies. These results show that our strategy can infuse different functional motifs into carbon nitride and thus improve photocatalytic activity.

Near-Unity Superradiant Emission from Delocalized Frenkel Excitons in a Two-Dimensional Supramolecular Assembly.

We demonstrate three general effective strategies to mitigate non-radiative losses in the superradiant emission from supramolecular assemblies. We focus on J-aggregates of 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and elucidate the nature of their nonradiative processes. We show that self-annealing at room temperature, photo-brightening, and the purification of the dye monomers all lead to substantial increases in emission quantum yields (QYs) and a concomitant lengthening of the emission lifetime, with purification of the monomers having the largest effect. We use structural and optical measurements to support a microscopic model that emphasizes the deleterious effects of a small number of impurity and defect sites that serve as non-radiative recombination centers. This understanding has yielded a room temperature molecular fluorophore in solution with an unprecedented combination of fast emissive lifetime and high QY. We obtain superradiant emission from J-aggregates of TDBC in solution at room temperature with a QY of 82% coupled with an emissive lifetime of 174 ps. This combination of high QY and fast lifetime at room temperature makes supramolecular assemblies of purified TDBC a model system for the study of fundamental superradiance phenomena. High QY J-aggregates are uniquely suited for the development of applications that require high speed and high brightness fluorophores such as devices for high speed optical communication.

Effects of Heterogeneous Protein Environment on Excitation Energy Transfer Dynamics in the Fenna-Matthews-Olson Complex.

The Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria has been serving as a prototypical light-harvesting protein for studying excitation energy transfer (EET) dynamics in photosynthesis. The most widely used Frenkel exciton model for FMO complex assumes that each excited bacteriochlorophyll site couples to an identical and isolated harmonic bath, which does not account for the heterogeneous local protein environment. To better describe the realistic environment, we propose to use the recently developed multistate harmonic (MSH) model, which contains a globally shared bath that couples to the different pigment sites according to the atomistic quantum mechanics/molecular mechanics simulations with explicit protein scaffold and solvent. In this work, the effects of heterogeneous protein environment on EET in FMO complexes from Prosthecochloris aestuarii and Chlorobium tepidum, specifically including realistic spectral density, site-dependent reorganization energies, and system-bath couplings are investigated. Semiclassical and mixed quantum-classical mapping dynamics were applied to obtain the nonadiabatic EET dynamics in several models ranging from the Frenkel exciton model to the MSH model and their variants. The MSH model with realistic spectral density and site-dependent system-bath couplings displays slower EET dynamics than the Frenkel exciton model. Our comparative study shows that larger average reorganization energy, heterogeneity in spectral densities, and low-frequency modes could facilitate energy dissipation, which is insensitive to the static disorder in reorganization energies. The effects of the spectral densities and system-bath couplings along with the MSH model can be used to optimize EET dynamics for artificial light-harvesting systems.

Time-Evolving Quantum Superpositions in Open Systems and the Rich Content of Coherence Maps.

We discuss the general features of the time-evolving reduced density matrix (RDM) of multistate systems coupled to dissipative environments and show that many important aspects of the dynamics are visualized effectively and transparently through coherence maps, defined as snapshots of the real and imaginary components of the RDM on the square grid of system sites. In particular, the spread, signs, and shapes of the coherence maps collectively characterize the state of the system and the nature of the dynamics, as well as the equilibrium state. The topology of the system is readily reflected in its coherence map. Rows and columns show the composition of quantum superpositions, and their filling indicates the extent of the surviving coherence. Linear combinations of imaginary RDM elements specify instantaneous population derivatives. The main diagonal comprises the incoherent component of the dynamics, while the upper/lower triangular areas give rise to coherent contributions that increase the purity of the RDM. In open systems, the coherence map evolves to a band surrounding the principal diagonal whose width decreases with increasing temperature and dissipation strength. We illustrate these behaviors with examples of 10-site model molecular aggregates with Frenkel exciton couplings, where the electronic states of each monomer are coupled to harmonic vibrational baths.

Facile synthesis and photodetection characteristics of new pyrrolo[2,3-b]pyrrole-based metal-free organic dyes containing phenols as the potential candidates towards energy conversion

A new characteristic with inexpensive preparation tools and best application performance is needed. These new properties have been achieved by using novel heterocyclic dyes by attaching different electron-rich aromatics to the pyrrolo[2,3-b]pyrrole system. A new derivatives of diethyl 3,4-diamino-1,6-diphenyl-1,6-dihydropyrrolo [2,3-b]pyrrole-2,5-dicarboxylate (PPy) (compound 1) was easily and efficiently synthesized using catalyzed condensation reaction. The final push-pull conjugated dyes were yielded by azo coupling with resorcinol, and 2-naphthol, denoted as compounds 2 and 3, respectively. Very good adhesive and homogenous thin films of these compounds are successfully prepared by thermal evaporation technique. On the basis of its corrected analytical and spectral data, the structure of these compounds was determined. The relation between dye structures, photophysical, and molecular structures had been discussed. The unique simulation characteristics of the prepared compounds 1–3 were achieved by density functional theory, DFT/B3LYP, using 6–311 ++ G (d,p) basis set. Optical spectra have revealed a large direct band gap at 3.99, 3.15, and 2.85 eV for compounds 1, 2, and 3 respectively, corresponding to π−π* transitions. Finally, an experimental approach was used to determine the exciton binding energy to depict Frenkel exciton binding energy. The properties of these compounds proved that it is more suitable as a potential application for energy conversion.

Cross Determination of Exciton Coherence Length in J-Aggregates.

The coherence length of the Frenkel excitons (Ncoh) is one of the most critical parameters governing many key features of supramolecular J-aggregates. Determining experimentally the value of Ncoh is a nontrivial task since it is sensitive to the technique/method applied, causing discrepancies in the literature data even for the same chemical compound and aggregation conditions. By using a combination of different experimental techniques including UV-vis-NIR, fluorescence emission, time-resolved photoluminescence, and transient absorption spectroscopies, we determined Ncoh values for J-aggregates of a cyanine dye. We found that the absorption spectroscopy alone - a widely used technique- fails in determining right value for Ncoh. The correct approach is based on the modification of photoluminescence lifetime and nonlinear response upon aggregation and careful analysis of the Stokes shift and electron-phonon coupling strength. This approach revealed that Ncoh of JC-1 J-aggregates ranges from 3 to 6.

Frenkel exciton photodynamics of self-assembled monolayers of azobiphenyls.

We performed computational simulations of the photodynamics of a self-assembled monolayer (SAM) of an azobenzene derivative (azobiphenyl, ABPT) on a gold surface. An excitonic approach was adopted in a semiempirical framework, which allowed us to consider explicitly the electronic degrees of freedom of 12 azobenzene chromophores. The surface hopping scheme was used for nonadiabatic molecular dynamics simulations. According to our results for an all trans-ABPT SAM, the excitation energy transfer between different chromophores, very fast in the ππ∗ manifold, does not occur between nπ∗ states. As a consequence, the excitation transfer does not play an important role in the quenching of the azobenzene photoisomerization in the SAM (experimentally observed and reproduced by our calculations) which, instead, has to be attributed to steric effects.

Excitonic switching across a Z2 topological phase transition: From Mott-Wannier to Frenkel excitons in organic materials

Controllable topological phase transitions are appealing as they allow for tunable single particle electronic properties. Here, by using state-of-the-art manybody perturbation theory techniques, we show that the topological $Z_2$ phase transition occurring in the single particle spectrum of the recently synthetized ethynylene bridged polyacene polymers is accompanied by a topological excitonic phase transition: the band inversion in the non-trivial phase yields real-space exciton wave functions in which electrons and holes exchange orbital characters with respect to the trivial phase. The topological excitonic phase transition results in a broad tunability of the singlet-triplet splittings, opening appealing perspectives for the occurrence of singlet fission. Finally, the flatness of the single-particle electronic structure in the topological non trivial phase leads to negatively dispersing triplet excitons in a large portion of the Brillouin zone, opening a route for spontaneously coherent energy transport at room temperature.

(Invited) Ultrafast Symmetry-Breaking Charge Separation in Organic Semiconductors

Understanding the photophysics and photochemistry of molecular π-stacked chromophores is important for utilizing them as functional photonic materials. However, these investigations have been mostly limited to covalent molecular dimers, which can only approximate the electronic and vibronic interactions present in higher oligomers typical of functional organic materials. In this work, we show that a comparison of the excited-state dynamics of a covalent slip-stacked perylenediimide (PDI) dimer(1)and trimer (2) provides fundamental insights into electronic state mixing and symmetry-breaking charge separation (SB-CS) beyond the dimer limit. We find that coherent vibronic coupling to high-frequency modes facilitates ultrafast state mixing between the Frenkel exciton (FE) and charge transfer (CT) states. Subsequently, solvent fluctuations and interchromophore low-frequency vibrations promote CT character in the coherent FE/CT mixed state. The coherent FE/CT mixed state persists in 1, while in 2, low-frequency vibronic coupling collapses the coherence resulting in ultrafast SB-CS between the distal PDI units. Figure 1

Characterizing the excited states of large photoactive systems by exciton models

AbstractThe theoretical investigation of excited state for large photoactive systems plays the fundamental role in understanding various optical processes in material and biological system. Frenkel exciton (FE) model describing the excitation of the whole system as a collective effect of quasi‐particles of excitons, that is, bound electron–hole pairs, is well‐known as a simple but powerful theoretical scheme to present a clear and insightful physical picture for complicated excited state problems. In this mini‐review, we summarize our recent developments of quantum chemical methods based on exciton models and their related applications for large photoactive systems. It is shown that our developed ab initio renormalized exciton model (REM) and block interaction product state (BIPS) schemes provide new efficient and automatic low‐scaling excited state methods for both localized and delocalized excited states in large systems. Illustrative examples including simulations of both absorption and emission spectrum in large sized molecular aggregates, indicate the exciton model based methods provide promising computational tools for unravel the mechanism of photophysical and photochemical processes in large photoactive systems.

A 1D helical eco-friendly Mn(II) halide coordination polymer: Luminescent properties involving resonant energy transfer and magnetic characterization

In the present work, we discuss the synthesis of nontoxic Mn(II) halide coordination polymer and explore the potential light emission properties for solar-cell devises through experimental and computational studies. The crystal structure was unveiled through Single Crystal Diffraction, whereas physical properties were investigated by means of thermal, spectroscopic, magnetic, optical and photoluminescence measurements. The observed left-right handed helical structure is built up from an infinite 1-D chains of Mn(II) octahedra. The polymeric network is stabilized through inter-molecular hydrogen bonding interactions and π···π face-to-face interactions. Furthermore, ferro-antiferromagnetic ordering has been proven through magnetic measurements. A deep investigation of the photoluminescence study reveals the presence of a bright bluish light emission with corresponding CIE color coordinates of (0.27, 0.35), CCT value of 8092 K and a very high CRI value of 96. The origin of this intense light emission is mainly attributed to the occurrence of the resonant energy and charge transfer processes between Triazolate ligand and inorganic Mn–Cl sub-lattices. Such behavior leads to the conversion of Frenkel excitons localized within the organic part to Mott-Wannier excitons localized in inorganic clusters. Based on these results, we strongly believe that the elaborated material can be a promising candidate for eco-friendly electronic applications.

Optoelectronic properties of a self-assembling rigidly-linked BF2-curcuminoid bichromophore

We describe the synthesis and spectroscopic study of the bichromophoric molecule DA-meso consisting of two boron difluoride curcuminoid (BF2-curcuminoid) units tethered by the rigid diacetylenic bridge. This structural feature imparts the quadrupolar-like DA-meso with a strong ability to self-assemble in solution, which induces a profound change in the optical properties. The UV–vis absorption spectrum of the aggregate is characterized by the appearance of a redshifted band at low energy and its fluorescence emission occurs in the near infrared (λmax = 700 nm) with a low quantum yield (5%). This is in contradiction with the strong emission expected for J-aggregates considering the Frenkel exciton model. From concentration-dependent UV–vis absorption spectroscopy we demonstrate the formation of a dimer composed of four BF2-curcuminoid units. Single-crystal X-ray diffraction structure analysis and in-depth quantum chemical calculations based on density functional theory (DFT) and subsequent decomposition of the electronic excited states on a diabatic basis enable to discuss the geometry of the DA-meso dimer, and to establish the presence of excited states close in energy to that of the unperturbed monochromophore, which are not predicted by the classical Frenkel exciton model. Different dimer structures characterized by π-stacks of two and up to four chromophores are investigated, in which low-lying excited states with vanishing oscillator strength are found to be redshifted with respect to the lowest-energy allowed state of the aggregate. From these experimental and theoretical data, the spectral features of the DA-meso dimer are rationalized by considering the formation of a “non-fluorescent J-aggregate”, illustrating the trend in non-Kasha behavior observed for quadrupolar-like dyes.

Spotlight on Charge-Transfer Excitons in Crystalline Textured n-Alkyl Anilino Squaraine Thin Films

Prototypical n-alkyl terminated anilino squaraines for photovoltaic applications show characteristic double-hump absorption features peaking in the green and deep-red spectral range. These signatures result from coupling of an intramolecular Frenkel exciton and an intermolecular charge transfer exciton. Crystalline, textured thin films suitable for polarized spectro-microscopy have been obtained for compounds with n-hexyl (nHSQ) and n-octyl (nOSQ) terminal alkyl chains. The here released triclinic crystal structure of nOSQ is similar to the known nHSQ crystal structure. Consequently, crystallites from both compounds show equal pronounced linear dichroism with two distinct polarization directions. The difference in polarization angle between the two absorbance maxima cannot be derived by spatial considerations from the crystal structure alone but requires theoretical modeling. Using an essential state model, the observed polarization behavior was discovered to depend on the relative contributions of the intramolecular Frenkel exciton and the intermolecular charge transfer exciton to the total transition dipole moment. For both nHSQ and nOSQ, the contribution of the charge transfer exciton to the total transition dipole moment was found to be small compared to the intramolecular Frenkel exciton. Therefore, the net transition dipole moment is largely determined by the intramolecular component resulting in a relatively small mutual difference between the polarization angles. Ultimately, the molecular alignment within the micro-textured crystallites can be deduced and, with that, the excited state transitions can be spotted.

Dielectric catastrophe at the Wigner-Mott transition in a moiré superlattice

The bandwidth-tuned Wigner-Mott transition is an interaction-driven phase transition from a generalized Wigner crystal to a Fermi liquid. Because the transition is generally accompanied by both magnetic and charge-order instabilities, it remains unclear if a continuous Wigner-Mott transition exists. Here, we demonstrate bandwidth-tuned metal-insulator transitions at fixed fractional fillings of a MoSe2/WS2 moiré superlattice. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe2 sensor layer. The exciton spectral weight is negligible for the metallic state with a large negative dielectric constant. It continuously vanishes when the transition is approached from the insulating side, corresponding to a diverging dielectric constant or a ‘dielectric catastrophe’ driven by the critical charge dynamics near the transition. Our results support the scenario of continuous Wigner-Mott transitions in two-dimensional triangular lattices and stimulate future explorations of exotic quantum phases in their vicinities.

π-Stacking-Dependent Vibronic Couplings Drive Excited-State Dynamics in Perylenediimide Assemblies.

Vibronic coupling, the interplay of electronic and nuclear vibrational motion, is considered a critical mechanism in photoinduced reactions such as energy transfer, charge transfer, and singlet fission. However, our understanding of how particular vibronic couplings impact excited-state dynamics is lacking due to the limited number of experimental studies of model molecular systems. Herein, we use two-dimensional electronic spectroscopy (2DES) to launch and interrogate a range of vibronic coherences in two distinct types of perylenediimide slip stacks─along the short and long molecular axes, which form either an excimer or a mixed state between the Frenkel exciton (FE) and charge transfer states. We explore the functionality of these vibronic coherences using quantum beatmaps, which display the Fourier amplitude signal oscillations as a function of pump and probe frequencies, along with knowledge of the characteristic signatures of the FE, ionic, and excimer species. We find that a low-frequency vibrational mode of the short-axis slip stack appears concomitantly with the formation of the excimer state, survives 2-fold longer than in the FE state in the reference monomer, and shows a phase shift compared to other modes. For the long-axis slip stacks, a pair of low-frequency modes coupled to a high-frequency coordinate of the FE state were found to play a critical role in mixed-state generation. Our findings thus experimentally reveal the complex and varying roles of vibronic couplings in tightly packed multimers undergoing a range of photoinduced processes.

Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals

High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure—unknown before—was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.

Real- and momentum-space description of the excitons in bulk and monolayer chromium tri-halides

Excitons with large binding energies ~2–3 eV in CrX3 have been characterized as being localized (Frenkel) excitons that emerge from the atomic d − d transitions between the Cr-3d-t2g and eg orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if a symmetry-breaking mechanism is present. While what can be classified as a purely Frenkel exciton is a matter of definition, we show using an advanced first principles parameter-free approach that these excitons in CrX3, in both its bulk and monolayer variants, have band origin and it is the dp hybridization between Cr and X that primarily acts as the symmetry-breaking mechanism that relaxes the Laporte rule. We show that the character of these excitons is mostly determined by the Cr-d orbital manifold, nevertheless, the fractions of the spectral weight shared with the ligand halogen states increases as the dp hybridization enhances. The hybridization enhances as the halogen atom becomes heavier, bringing the X-p states closer to the Cr-d states in the sequence Cl → Br → I, with an attendant increase in exciton intensity and a decrease in binding energy. By applying a range of different kinds of perturbations that qualitatively mimics the effects originating from the missing vertex in self-energy, we show that moderate changes to the two-particle Hamiltonian that essentially modifies the Cr-d-X-p hybridization, can alter both the intensities and positions of the exciton peaks. A detailed analysis of several deep-lying excitons, with and without strain, elucidates the fact that the exciton is most Frenkel-like in CrCl3 and CrBr3 and acquires mixed Frenkel–Wannier character in CrI3, making the excitons in CrI3 most susceptible to environmental screening and spin–orbit coupling.