Exciton Delocalization Length Research Articles

Efficient Near-Infrared Luminescence of Self-Assembled Platinum(II) Complexes: From Fundamentals to Applications.

ConspectusDesigning bright and efficient near-infrared (NIR) emitters has drawn much attention due to numerous applications ranging from biological imaging, medical therapy, optical communication, and night-vision devices. However, polyatomic organic and organometallic molecules with energy gaps close to the deep red and NIR regime are subject to dominant nonradiative internal conversion (IC) processes, which drastically reduces the emission intensity and exciton diffusion length of organic materials and hence hampers the optoelectronic performances. To suppress nonradiative IC rates, we suggested two complementary approaches to solve the issues: exciton delocalization and molecular deuteration. First, exciton delocalization efficiently suppresses the molecular reorganization energy through partitioning to all aggregated molecules. According to the IC theory together with the effect of exciton delocalization, the simulated nonradiative rates with the energy gap ΔE = 104 cm-1 decrease by around 104 fold when the exciton delocalization length equals 5 (promoting vibronic frequency ωl = 1500 cm-1). Second, molecular deuterations reduce Franck-Condon vibrational overlaps and vibrational frequencies of promoting modes, which decreases IC rates by 1 order of magnitude in comparison to the rates of nondeuterated molecules under ΔE of 104 cm-1. Although deuteration of molecules has long been attempted to increase emission intensity, the results have been mixed. Here, we provide a robust derivation of the IC theory to demonstrate its validity, especially to emission in the NIR region.The concepts are experimentally verified by the strategic design and synthesis of a class of square-planar Pt(II) complexes, which form crystalline aggregates in vapor deposited thin films. The packing geometries are well characterized by the grazing angle X-ray diffraction (GIXD), showing domino-like packing arrangements with the short ππ separation of 3.4-3.7 Å. Upon photoexcitation, such closely packed assemblies exhibit intense NIR emission maximized in the 740-970 nm region through metal-metal-to-ligand charge transfer (MMLCT) transition with unprecedented photoluminescent quantum yield (PLQY) of 8-82%. To validate the existence of exciton delocalization, we applied time-resolved step-scan Fourier transform UV-vis spectroscopy to probe the exciton delocalization length of Pt(II) aggregates, which is 5-9 molecules (2.1-4.5 nm) assuming that excitons mainly delocalized along the direction of ππ stacking. According to the dependence of delocalization length vs simulated IC rates, we verify that the observed delocalization lengths contribute to the high NIR PLQY of the aggregated Pt(II) complexes. To probe the isotope effect, both partially and completely deuterated Pt(II) complexes were synthesized. For the case of the 970 nm Pt(II) emitter, the vapor deposited films of per-deuterated Pt(II) complexes exhibit the same emission peak as that of the nondeuterated one, whereas PLQY increases ∼50%. To put the fundamental studies into practice, organic light-emitting diodes (OLEDs) were fabricated with a variety of NIR Pt(II) complexes as the emitting layer, showing the outstanding external quantum efficiencies (EQEs) of 2-25% and the remarkable radiances 10-40 W sr-1 m-2 at 740-1002 nm. The prominent device performances not only successfully prove our designed concept but also reach a new milestone for highly efficient NIR OLED devices.This Account thus summarizes our approaches about how to boost the efficiency of the NIR emission of organic molecules from an in-depth fundamental basis, i.e., molecular design, photophysical characterization, and device fabrication. The concept of the exciton delocalization and molecular deuteration may also be applicable to a single molecular system to achieve efficient NIR radiance, which is worth further investigation in the future.

Mode-Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers.

Exciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode-specific kinetics in the S1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Mode‐Specific Vibrational Analysis of Exciton Delocalization and Structural Dynamics in Conjugated Oligomers

AbstractExciton delocalization in organic semiconducting polymers, affected by structures at a molecular level, plays a crucial role in modulating relaxation pathways, such as charge generation and singlet fission, which can boost device efficiency. However, the structural diversity of polymers and broad signals from typical electronic spectroscopy have their limits when it comes to revealing the interplay between local structures and exciton delocalization. To tackle these problems, we apply femtosecond stimulated Raman spectroscopy in archetypical conjugated oligothiophenes with different chain lengths. We observed Raman frequency dispersions of symmetric bond stretching modes and mode‐specific kinetics in the S1 Raman spectra, which underpins the subtle and complex interplay between exciton delocalization and bond length alternation along the conjugation coordinate. Our results provide a more general picture of exciton delocalization in the context of molecular structures for conjugated materials.

Exciton Delocalization in Amino-Functionalized Inorganic MoS2 Quantum Disks: Giant Davydov Splitting and Exchange Narrowing

Exciton delocalization, a phenomenon in which spatial extension of the excitons spreads over aggregates, has attracted significant attention in both academia and industry. Studies on exciton delocalization focus mainly on dye molecules and organic conjugated solids because of their high binding energies. Here, we report on exciton delocalization from inorganic ${\mathrm{Mo}\mathrm{S}}_{2}$ quantum disks (QDs) functionalized with diethylenetriamine (DETA). By increasing the QD concentration, a giant Davydov splitting with an energy of up to 850 meV and an exchange narrowing with an exciton delocalization length up to several tens of QDs are observed. Notably, exciton delocalization in amino-functionalized ${\mathrm{Mo}\mathrm{S}}_{2}$ QDs can be controlled by the DETA concentration. Amino-related bonding is suggested to modulate the Davydov splitting. Understanding the mechanism of exciton delocalization in two-dimensional QDs is expected to be useful and open up a route for the discovery of unique physical phenomena and the development of as-yet unrealized devices.

Exciton delocalization length in chlorosomes investigated by lineshape dynamics of two-dimensional electronic spectra.

A chlorosome, a photosynthetic light-harvesting complex found in green sulfur bacteria, is an aggregate of self-assembled pigments and is optimized for efficient light harvesting and energy transfer under dim-light conditions. In this highly-disordered aggregate, the absorption and transfer of photoexcitation energy are governed by the degree of disorder. To describe the disorder, the number of molecules forming excitons, which is termed exciton delocalization length (EDL), is a relevant parameter because the EDL sensitively changes with the disorder of the constituent molecules. In this work, we determined the EDL in chlorosomes using two-dimensional electronic spectroscopy (2D-ES). Since spectral features correlated with EDL are spread out in the two-dimensional (2D) electronic spectra, we were able to determine the EDL accurately without the effects of homogeneous and inhomogeneous line broadening. In particular, by taking advantage of the multi-dimensionality and the time evolution of 2D spectra, we not only determined the excitation frequency dependence of EDL but also monitored the temporal change of EDL. We found that the EDL is ∼7 at 77 K and ∼6 at 298 K and increases with the excitation frequency, with the maximum located well above the maximum of the absorption spectrum of chlorosomes. The spectral profile of EDL changes rapidly within 100 fs and becomes flat over time due to dephasing of initial exciton coherence. From the coherent oscillations superimposed on the decay of EDL, it was learned that high-frequency phonons are more activated at 298 K than at 77 K.

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

Auger Recombination and Multiple Exciton Generation in Colloidal Two-Dimensional Perovskite Nanoplatelets: Implications for Light-Emitting Devices

Improving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10–2 cm2/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured “intrinsic” AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of “geminate biexcitons”, upon excitation at low fluence with high-energy photons.

Postsynthesis Spontaneous Coalescence of Mixed-Halide Perovskite Nanocubes into Phase-Stable Single-Crystalline Uniform Luminescent Nanowires.

All inorganic mixed-halide perovskite, CsPb(Br xI1- x)3 (0 ≤ x ≤ 1), nanocrystals possess tunable photoluminescence with high quantum yield in the visible window. However, the photoluminescence degrades rapidly with postsynthetic aging due to the spontaneous ion separation and phase instability. Here we show that the postsynthetic aging of CsPb(Br xI1- x)3 nanocubes spontaneously forms highly uniform single-crystalline nanowires with a diameter of 9 ± 0.5 nm and length of up to several micrometers. The nanowires show bright photoluminescence with an absolute photoluminescence quantum yield of 41%. Rietveld refinement identifies the stable orthorhombic phase of the nanowires, implying a phase transition from the cubic crystallographic phase of the nanocubes during the morphology evolution. Transient absorption spectroscopy reveals a faster excited-state decay dynamic with a large exciton delocalization length in 1D nanowires. Our findings elucidate the insights into the postsynthesis morphology evolution of mixed-halide perovskite nanocrystals leading to luminescent nanowires with excellent crystal phase stability for potential optoelectronic applications.

Exciton-Exciton Annihilation as a Mechanism for Uphill Transfer in a Molecular Excitonic System

Exciton dynamics in a HJ-aggregate of cyanine dye TTBC are investigated by transient absorption with a time resolution of <60 fs and power-dependent emission spectroscopies. Both measurements are consistent with an exciton delocalization length of ~28 monomers. A model assuming diffusive exciton motion reveals that the exciton mobility is at least bimodal and restricted to one spatial dimension. J-band diffusion rates of 2.69 and 2.79e-3 cm2s-1 are found, leading to maximal diffusion lengths of 449 and 14.5 nm. The findings indicate that exciton-exciton annihilation is the origin of effective uphill transfer. This mechanism, if present under solar radiation, maybe useful for organic photovoltaic systems.

Light-Harvesting in Porous Crystalline Compositions: Where We Stand toward Robust Metal–Organic Frameworks

Molecular assemblies in metal–organic frameworks (MOFs) have the potential to be considered as functional artificial light-harvesting (LH) system with features similar to the natural LH machinery. With large photon absorptivity, the frameworks provide novel energy-transfer pathways enabling long-distance energy migration both in singlet and triplet manifolds. Furthermore, considering the eventual energy conversion utility, various strategies have been explored to achieve long-lived excited states and integration of photochemical “reaction-centers”. Understanding the excited state properties, such as exciton size, delocalization length, and dynamics within MOF-based compositions as a function of their underlying topological-net can help to implement new strategies with improved LH utility. This paper summarizes the various unique photophysical process and elucidates how their structural parameters play a critical role in defining the photophysical processes within MOF structures with a particular emphasis on the robust Zr6IV-oxo node derived frameworks as future compositions.

Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

AbstractExciton delocalization in organic semiconductors, due to its direct relation to device efficiency, is of outstanding importance. Time‐resolved electron paramagnetic resonance spectroscopy of light‐induced triplet excitons gives access to the delocalization length in a unique way, connecting it to both, electronic structure and overall conformational flexibility. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens the understanding of the structure–function relationship in organic semiconductors. Applying this approach to the n‐type polymer poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (PNDIT2) known for its remarkable charge‐carrier mobility reveals the triplet exciton in this polymer to be strongly confined to no more than two repeat units. Furthermore, neither planarity nor extent of backbone conjugation seem to be good indicators for the exciton delocalization length.

DNA as UV light-harvesting antenna.

The ordered structure of UV chromophores in DNA resembles photosynthetic light-harvesting complexes in which quantum coherence effects play a major role in highly efficient directional energy transfer. The possible role of coherent excitons in energy transport in DNA remains debated. Meanwhile, energy transport properties are greatly important for understanding the mechanisms of photochemical reactions in cellular DNA and for DNA-based artificial nanostructures. Here, we studied energy transfer in DNA complexes formed with silver nanoclusters and with intercalating dye (acridine orange). Steady-state fluorescence measurements with two DNA templates (15-mer DNA duplex and calf thymus DNA) showed that excitation energy can be transferred to the clusters from 21 and 28 nucleobases, respectively. This differed from the DNA–acridine orange complex for which energy transfer took place from four neighboring bases only. Fluorescence up-conversion measurements showed that the energy transfer took place within 100 fs. The efficient energy transport in the Ag–DNA complexes suggests an excitonic mechanism for the transfer, such that the excitation is delocalized over at least four and seven stacked bases, respectively, in one strand of the duplexes stabilizing the clusters. This result demonstrates that the exciton delocalization length in some DNA structures may not be limited to just two bases.

The Role of Linkers in the Excited-State Dynamic Planarization Processes of Macrocyclic Oligothiophene 12-Mers.

Linkers adjoining chromophores play an important role in modulating the structure of conjugated systems, which is bound up with their photophysical properties. However, to date, the focus of works dealing with linker effects was limited only to linear π-conjugated materials, and there have been no detailed studies on cyclic counterparts. Herein we report the linker effects on the dynamic planarization processes of π-conjugated macrocyclic oligothiophene 12-mers, where the different ratio between ethynylene and vinylene linkers was chosen to control the backbone rigidity. By analyzing transient fluorescence spectra, we demonstrate that the connecting linkers play a crucial role in the excited-state dynamics of cyclic conjugated systems. Faster dynamic planarization, longer exciton delocalization length, and higher degree of planarity were observed in vinylene inserted cyclic oligothiophenes. Molecular dynamics simulations and density functional theory calculations also stress the importance of the role of linkers in modulating the structure of cyclic oligothiophenes.

A stochastic reorganizational bath model for electronic energy transfer.

Environmentally induced fluctuations of the optical gap play a crucial role in electronic energy transfer dynamics. One of the simplest approaches to incorporate such fluctuations in energy transfer dynamics is the well known Haken-Strobl-Reineker (HSR) model, in which the energy-gap fluctuation is approximated as white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to account for excitation energies' thermal fluctuations. On the other hand, since the original work of HSR, many groups have employed stochastic models to simulate the same transfer dynamics. Here, we discuss a rigorous connection between the stochastic and the atomistic bath models. If the phonon bath is treated classically, time evolution of the exciton-phonon system can be described by Ehrenfest dynamics. To establish the relationship between the stochastic and atomistic bath models, we employ a projection operator technique to derive the generalized Langevin equations for the energy-gap fluctuations. The stochastic bath model can be obtained as an approximation of the atomistic Ehrenfest equations via the generalized Langevin approach. Based on this connection, we propose a novel scheme to take account of reorganization effects within the framework of stochastic models. The proposed scheme provides a better description of the population dynamics especially in the regime of strong exciton-phonon coupling. Finally, we discuss the effect of the bath reorganization in the absorption and fluorescence spectra of ideal J-aggregates in terms of the Stokes shifts. We find a simple expression that relates the reorganization contribution to the Stokes shifts - the reorganization shift - to the ideal or non-ideal exciton delocalization in a J-aggregate. The reorganization shift can be described by three parameters: the monomer reorganization energy, the relaxation time of the optical gap, and the exciton delocalization length. This simple relationship allows one to understand the physical origin of the Stokes shifts in molecular aggregates.

Control of electron–lattice interaction in organic nanoclusters

In this study we demonstrate that the electron–lattice interaction (ELI) could be controlled by changing exciton delocalization length in ordered organic nanoclusters called J-aggregates. Particularly it could be done via the J-aggregates solvate shell manipulation using surfactants. The strong correlation between the J-aggregates luminescence quantum yield and the ELI strength has been reviled that allows us to consider the exciton self-trapping as the main mechanism of the J-aggregates luminescence losses.

Manipulation by optical properties of luminescent ordered organic nanoclusters via exciton-phonon coupling

The way to manipulate by spectral properties of cyanine dye luminescent ordered nanoclusters (J-aggregates) has been demonstrated. It has been observed a surfactant shell formation around the J-aggregates of four different dyes. This leads to exciton delocalization length increasing which causes an exciton self-trapping suppression. The latter results in spectral changes observed.

Manifestation of Exciton-Lattice Interaction in J-Aggregates

The inverse correlation between the exciton-lattice interaction strength and the exciton delocalization length is shown for dielectric organic nanoclusters – J-aggregates. Since the exciton delocalization length defines all optical properties of J-aggregates, it allows one to control them via the exciton-lattice interaction. We demonstrate the exciton self-trapping suppression in amphi-PIC J-aggregates as a result of the exciton-lattice interaction weakening due to the formation of a surfactant shell around J-aggregates. The control over optical properties via the exciton-lattice interaction for other types of J-aggregates is discussed.

One-Dimensional Exciton Diffusion in Perylene Bisimide Aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Förster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.

Encapsulating Light-Emitting Polymers in Block Copolymer Micelles

Conjugated polymers have excellent light-emitting properties that are useful for biological imaging and sensing applications. Here, we report a new way to form stable, water-soluble suspensions of conjugated polymers by encapsulating them in amphiphilic block copolymers. It was found that the folding property of conjugated polymers is a critical factor for solubilizing them into typical coil-coil block copolymer micelles. By introducing saturated bonds into conjugated polymers, they were readily encapsulated in block copolymer micelles, resulting in highly fluorescent polymer nanoparticles. The emission wavelength of fluorescent micelles can be tuned by controlling the number of encapsulated light-emitting polymers per micelle as well as changing the exciton delocalization length in conjugated polymers. This strategy has been extended to make multifunctional (i.e., fluorescent and magnetic) nanoparticles by encapsulating iron oxide nanoparticles in light-emitting polymers.

Control of Exciton Migration Efficiency in Disordered J-Aggregates

Using luminescent exciton traps, an efficiency of the exciton migration in J-aggregates of an amphiphilic analogue of pseudoisocyanine (amphi-PIC) dye in solutions, has been investigated. Applying a modified Stern−Volmer equation for analysis of the J-aggregate luminescence quenching by the trap, the quenching of 50% of amphi-PIC J-aggregate luminescence accessible for trapping at the ratio amphi-PIC/DiD = 120:1 has been revealed. To increase the exciton migration efficiency, the J-aggregate structure was improved by the formation of a “J-aggregate−surfactant” complex. The J-aggregate structure improvement is confirmed by the about 3 times increase in the exciton delocalization length that leads to the 1.3 times enhancement in the exciton migration efficiency in solutions with the surfactant. To the best of our knowledge, such a control of the exciton transport parameters in J-aggregates in solutions has not been demonstrated yet.