Exciton Migration Dynamics Research Articles

Migration dynamics of excitons/biexcitons induced by a funnel-like nonuniform compression strain over organic polymers

As a promising strategy, strain engineering has been employed on different semiconductor materials, aiming to modulate their photoelectric properties, thus innovate device applications. An interesting experimental result is the directional exciton migration induced by creating some local nonuniform strains in materials, such as “exciton funneling.” In this Letter, to clarify the dynamical mechanism and the impacting factors, we theoretically investigate the migration dynamics of excitons/biexcitons along organic polymers induced by a funnel-like nonuniform compression strain. First, the migration dynamics of an exciton/biexciton are separately demonstrated right after the strain created. It is found that both of them will migrate toward the strain center with the speeds up to 7–10 nm/ps, comparable with the experimental observations in a bending ZnO microwire, where the difference between the exciton and biexciton migration dynamics is also emphasized. We attribute the present exciton/biexciton migration mechanism to the strain-induced gradient of the exciton/biexciton creation energy along polymers. Furthermore, some typical factors impacting the exciton migration dynamics are considered, such as the strain gradient, the initial ratio of the exciton located in the strain region, the electron-lattice interaction, etc. Finally, based on these findings, we briefly discuss the possible results about the strain-modulated luminescence of organic polymers.

Directional Exciton Migration in Benzoimidazole-Based Metal-Organic Frameworks.

Highly luminescent metal-organic frameworks (MOFs) have recently received great attention due to their potential applications as sensors and light-emitting devices. In these MOFs, the highly ordered fluorescent organic linkers positioning prevents excited-state self-quenching and rotational motion, enhancing their light-harvesting properties. Here, the exciton migration between the organic linkers with the same chemical structure but different protonation degrees in Zr-based MOFs was explored and deciphered using ultrafast laser spectroscopy and density functional theory calculations. First, we clearly demonstrate how hydrogen-bonding interactions between free linkers and solvents affect the twisting changes, internal conversion processes, and luminescent behavior of a benzoimidazole-based linker. Second, we provide clear evidence of an ultrafast energy transfer between well-aligned adjacent linkers with different protonation states inside the MOF. These findings provide a new fundamental photophysical insight into the exciton migration dynamics between linkers with different protonation states coexisting at different locations in MOFs and serve as a benchmark for improving light-harvesting MOF architectures.

Dynamical study of ultrafast exciton migration in coujugated polymers driven by nonuniform field

Due to the exciton migration dynamics playing an important role in the photovoltaic process of organic solar cells, which are usually composed of polymer donor and fullerene (or non-fullerene) acceptor, in this paper we propose a new strategy to achieve the ultrafast exciton migration in polymers. Here, the effects of some nonuniform fields on the exciton migration dynamics in polymers are emphasized, such as the nonuniform electric field and the nonuniform polymer packing configuration field. Both of the two kinds of nonuniform fields can be intrinsically existent or modulated in an actual photovoltaic system. In this work, the nonuniform electric field and the nonuniform configuration field are assumed to be separately created by a confined charge and a linear polymer packing, therefore, their model Hamiltonian is established. In dynamical simulations of the exciton migration dynamics in polymers, an extended version of one-dimensional Su-Schrieffer-Heeger tight-binding model combined with a nonadiabatic evolution method is employed. It is found that the nonuniform electric field and the nonuniform configuration field both can drive exciton to an ultrafast migration process. Compared with the exciton migration speed dominated by the traditional Förster or Dexter mechanism, the exciton migration speed dominated by the nonuniform electric field and that by the nonuniform configuration field can be increased by one and two orders of magnitude, respectively. In addition, the driving mechanisms of the two kinds of nonuniform fields for the exciton migration dynamics are separately clarified, where the corresponding driving forces are also quantitatively calculated. Finally, in view of the factors affecting the distributions of the two kinds of nonuniform fields (such as the distance <i>d</i> between confined charge and polymer, and the linear packing slope <i>k</i> between polymers), we discuss their effects on the exciton migration dynamics. It is found that the exciton migration in polymer can be apparently accelerated by shortening the distance <i>d</i> between confined charge and polymer, and there exists a critical value of <i>d</i>, beyond which the exciton will be dissociated into free charges in its migration process. For the linear packing slope <i>k</i> between polymers, we find that there exists an optimal value, at which the exciton has the highest migration speed in polymers.

Electronic Excited States in Amorphous MEH-PPV Polymers from Large-Scale First Principles Calculations

The electronic excited states of amorphous polymeric semiconductor MEH-PPV are investigated by first principles quantum chemical calculations based on trajectories from classical molecular dynamics simulations. We inferred an average conjugation length of ∼5-7 monomers for lowest vertical excitations of amorphous MEH-PPV at room temperature and verified that the normal definition of a chromophore in a polymer based on purely geometric "conjugation breaks" is not always valid in amorphous polymers and a rigorous definition can be only on the basis of the evaluation of the polymer excited state wave function. The charge transfer character is observed to be nearly invariant for all excited states in low energy window while the exciton delocalization extent is found to increase with energy. The interchain excitonic couplings for amorphous MEH-PPV are shown to be usually smaller than 10 meV suggesting that the transport mechanism across chain can be described by incoherent hopping. All these observations about the energetic and spatial distribution of the excitons in polymer as well as their couplings provide important qualitative insights and useful quantitative information for constructing a realistic model for exciton migration dynamics in amorphous polymer materials.

Exciton migration dynamics between phenylene moieties in the framework of periodic mesoporous organosilica powder

To understand the excitation migration process between the phenylene (Ph) moieties in the framework of periodic mesoporous organosilicas (PMOs), the exciton–exciton annihilation behavior between the Ph moieties is studied. The dependence of the excitation light intensity on the fluorescence decay curve of the Ph exciton was successfully analyzed with a two-dimensional exciton diffusion and collision model. The quenching distance and diffusion length of the Ph exciton are estimated to be 0.4–0.8 nm and at least 4 nm, respectively. It is suggested that the Ph exciton is delocalized between up to three Ph moieties and moved between approximately nine Ph moieties in the pore wall of Ph–PMO.

Exciton migration dynamics in a dendritic molecule: quantum master equation approach using ab initio molecular orbital configuration interaction method.

We investigate the exciton migration dynamics in a dendritic molecular model composed of pi-conjugation linear-leg units (acetylenes and diacetylene) and a benzene ring (branching point) using the quantum master equation approach with the ab initio molecular orbital (MO) configuration interaction (CI) method. The efficient migration of exciton from short-length linear legs (acetylenes) to long-length linear leg (diacetylene) via a benzene ring is observed. As predicted in previous studies, the exciton (electron and hole) distributions are relatively well localized in each generation segmented by the meta-branching point (meta-substituted benzene ring) though the electron and hole distributions are delocalized and are somewhat spatially different from each other within each generation. It is found that the excitons localized in the generation composed of short linear legs occupy in higher-lying exciton states, while those in the generation composed of long linear legs do in lower-lying ones. These features suggest the decoupling of pi-conjugation at the meta-branching point. On the other hand, the relaxation effect between exciton states is found to be caused by the exciton-phonon coupling, in which the existence of common configurations (electron-hole pairs) in CI wave functions between adjacent exciton states (having primary distributions on short and long linear-leg regions, respectively) is important for the relaxation between their exciton states. This feature indicates the importance of partial penetration of pi-conjugation through the meta-substituted benzene ring in excited states for such exciton migration.

EXCITON MIGRATION IN DENDRITIC AGGREGATE SYSTEMS USING THE QUANTUM MASTER EQUATION APPROACH INVOLVING WEAK EXCITON-PHONON COUPLING

The quantum master equation approach involving a weak exciton-phonon coupling is applied to the exciton migration dynamics of dendritic molecular aggregates modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of efficient exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton-phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by step-by-step transfer from the periphery to the core via multi-step exciton states. The same calculation method is applied to the exciton dynamics of a larger dendritic aggregate model, D127, modeled after the largest synthesized phenylacetylene dendrimer D127 in order to examine the dependencies of the exciton migration on the strength of the intermolecular interaction and the temperature of phonon bath. In the case of the relatively weak dipole-dipole coupling, exciton is not observed to migrate efficiently from the periphery to the core, while the largest exciton population is found to remain in the intermediate generations. This is ascribed to the fact that the thermal excitation to the higher exciton states significantly contributes to the exciton distribution in the equilibrium state when the weak intermolecular interaction reduces the energy difference between exciton states. This indicates that the intermolecular interaction is important not only for the overlaps of the exciton distributions between the exciton states, but also the relation between the exciton energy differences and thermal excitations, which spoil the distinct concentration of exciton distribution in the core generation. In the case of a low temperature, even if the intermolecular interaction is weak, the exciton population in the core region is found to be the largest in all the generations. This suggests that exciton tends to efficiently migrate from the periphery to the core when the temperature is sufficiently low. Implications of these theoretical results are discussed in relation to design of magneto-optical materials and other technological applications.

Exciton migration dynamics in a dendritic molecular aggregate

We investigate exciton migration dynamics in two types of dendritic molecular aggregate models, i.e., Bethe lattice, with mutually distinct intermolecular interaction between adjacent linear legs at the branching points. Each monomer molecule is assumed to be a dipole unit (a two-state model) coupled with each other by the dipole–dipole interaction. The generation of one exciton in the aggregates by the incident laser field and its subsequent dynamical behavior are investigated in a numerically exact manner. The two types of models show different exciton energy structures and distribution. It is also found that relaxation effects in the exciton states are essential for exciton migration from the periphery to the core.

Exciton percolation. II: Naphthalene 1B 2u supertransfer

The exciton percolation theory has been tested for the migration of the lowest singlet exciton in our model organic alloy system at low temperature (2 K): binary isotopic mixed crystals of naphthalene (C 10H 8—C 10D 8), with an added exciton sensor (supertrap) of betamethylnaphthalene (about 10 −3 mole fraction). At these relatively high sensor concentrations the system's migration dynamics are described quantitatively by the simple limiting case of supertransfer, i.e. by a dynamic percolation formulation of Hoshen and Kopelman that depends only on the concentrations and interaction topology of the ternary crystal. Experimentally, the exciton's migration dynamics is monitored by fluorescence spectra, taken under controlled conditions (crystal quality, purity, concentration, excitation and temperature). The effects of exciton tunneling (superexchange), exciton—phonon coupling, coherence, exciton delocalization and non-equilibrium chemical solubility are considered. We show that a complete concentration study of this simple energy transport model system reveals four kinds of exciton transfer regions, but only one (above the critical guest site percolation concentration) with long ranged, multistep, direct guest exciton transport. The same study, on the low concentration side, yields relative trapping efficiencies and thus eliminates the need for adjustable parameters. The substitutional randomness of the isotopic mixed crystal is confirmed, as well as the short range nature of the 1B 2u exciton interactions and the dominance of the interchange equivalent, nearest neighbor, pairwise (molecular) excitation ( 1B 2u) exchange integrals in the pure, perfect, low-temperature, naphthalene crystal. Our present exciton percolation study is also relevant to the primary energy transport process in heterogeneous photosynthetic units.

Long range exciton percolation and superexchange: Energy denominator study on 3B 1u naphthalene

The long-range exciton percolation model is found to describe the lowest triplet exciton superexchange (“tunneling”) migration at low temperature (2 K), in our model alloy system: Binary isotopic mixed naphthalene crystals with dispersed exciton sensors (supertraps) consisting of small concentration of betamethylnaphthalene (−10 −3 mole fraction) or isotopic substituted naphthalene molecules (with lower excitation energies than the partially deuterated naphthalene guest species). While the “host” is C 10D 8 throughout, the “guest” species in our five experimental systems are: C 10H 8, 2-DC 10H 7. 1-DC 10H 7, 1,2-D 2C 10H 6 and 1,4,5,8-D 4C 10H 4. The variation in guest—host (and supertrap—guest) energy denominator in the above systems enables a quantitative test of our physical exciton superechange (tunnelling) migration model. In conjunction with a mathematical long-range percolation model (J. Hoshen, E.M. Monberg and R. Kopelman, unpublished). The experimental monitoring of the exciton migration dynamics consists of refined phosphorescence measurements to our systems, under highly controlled conditions (crystal quality, purity, concentration, temperature and excitation). Using only the known nearest neighbor (interchange-equivalent) exciton exchange interaction, quantitative agreement with the experimental dynamic percolation concentration is achieved, without adjustable parameters, for four of the five investigated systems. The fifth one is known to involve a cooperative percolation—thermalization exciton migration, and is effective in qualitative agreement with the predicted upper limit for the exciton percolation concentration. The nearest-neighbor 3B 1u excitation exchange interactions, and their square lattice topology, play the dominant role in determining the guest triplet exciton energy transfer and migration. This energy conduction involves an extremely narrow “impurity band”, on the order of 10 to 10 3 Hz, formed by the superexchange (tunneling) exciton interactions resulting from the above mentioned exciton exchange interactions (integrals). The latter are thus confirmed as the major contributors to the 3B 1u exciton transfer, migration and energy bond (3 × 10 11 Hz) in the ordinary naphthalene crystal. Just below the percolation concentration the “impurity conduction band” further shrinks by one or two orders of magnitude, resulting in a bandwidth of about one hertz or less, and thus practically resulting in the “switching off” of the exciton transport. The tunneling radius is about 30 Å or larger, depending on the system, but essentially in the ab plane.