Excited State Energy Transfer Research Articles

Probing the Effects of Electronic-Vibrational Resonance on the Rate of Excited-State Energy Transfer in Bacteriochlorin Dyads.

The impact of vibrational-electronic resonances on the rate of excited-state energy transfer is examined in a set of bacteriochlorin dyads that employ the same phenylethyne linker. The donor/acceptor excited-state energy gap is tuned from ∼200 to ∼1100 cm-1 using peripheral substituents on the donor and acceptor bacteriochlorin macrocycles. Ultrafast energy transfer is observed with rate constants of (0.3 ps)-1 to (1.7 ps)-1, which agree with those predicted by Förster theory to within a factor of 2. Furthermore, the measured rates follow a trend-line with only small deviations that do not correlate with the density of vibrations at the donor/acceptor excited-state energy gap. Thus, if vibrational-electronic resonances occur in any of these dyads, which seems likely, the impact on the rate of energy transfer is small.

Cascade Synthesis of Luminescent Difluoroboron Diketonate Compounds forRoom‐TemperatureOrganic Afterglow Materials

Comprehensive SummaryDifluoroboron β‐diketonate (BF2bdk) compounds represent an important class of luminescent materials, whereas their synthesis requires multiple steps, which restrict their application in diverse fields. Here we report a cascade reaction to prepare BF2bdk from aromatic ketones, carboxylic acids, trifluoroacetic anhydride and boron trifluoride diethyl etherate. The cascade reaction is very simple and straightforward to produce BF2bdk with desired functional groups in reasonable isolation yields. Further, when some specific BF2bdk compounds are used as luminescent dopants, organic room‐temperature phosphorescence with lifetimes >1.0 s and intense afterglow under ambient conditions can be achieved with the assistance by suitable organic matrices. The dopant‐matrix materials exhibit excellent processability into desired patterns and large‐area panels, and function as efficient donors of excited state energy transfer for the fabrication of red afterglow materials. The materials can be processed into aqueous afterglow dispersion that displays unique property of escaping the interference from strong fluorescence background.

Boosting Organic Afterglow Performance via a Two-Component Design Strategy Extracted from Macromolecular Self-Assembly

Because intersystem crossing and phosphorescence decay are spin-forbidden in organic systems, it is challenging to obtain high-performance organic afterglow materials. Inspired by two-component design strategy from macromolecular self-assembly, here we report the utilization of synthetic polymers to control the excited state properties of difluoroboron β-diketonate (BF2bdk) and deuterated BF2bdk compounds for the fabrication of room-temperature organic afterglow materials. The polymer component can interact with BF2bdk excited states by dipole-dipole interactions, lower BF2bdk S1 levels with insignificant effect on T1 levels, reduce ΔEST, and thus enhance intersystem crossing of BF2bdk excited states. The polymer component can also suppress intramolecular motion of BF2bdk triplets and protect BF2bdk triplets from oxygen quenching. The obtained BF2bdk-polymer afterglow materials exhibit emission lifetimes up to 2.2 s and high photoluminescence quantum yields under ambient conditions, display excellent processability and flexibility, and can function as efficient donors for excited state energy transfer to construct red afterglow materials.

Tuning Excited-State Energy Transfer for Light Energy Conversion: A Virtual Issue

Tuning Excited-State Energy Transfer for Light Energy Conversion: A Virtual Issue

Förster Resonance Energy Transfer Assisted Enhancement in Optoelectronic Properties of Metal Halide Perovskite Nanocrystals.

Regulated excited state energy and charge transfer play a pivotal role in nanoscale semiconductor device performance for efficient energy harvesting and optoelectronic applications. Herein, we report the influence of Förster resonance energy transfer (FRET) on the excited-state dynamics and charge transport properties of metal halide perovskite nanocrystals (PNCs), CsPbBr3, and its anion-exchanged counterpart CsPbCl3 with CdSe/ZnS quantum dots (QDs). We report a drop in the FRET efficiency from ∼85% (CsPbBr3) to ∼5% (CsPbCl3) with QDs, inviting significant alteration in their charge transport properties. Using two-probe measurements we report substantial enhancement in the current for the blend structure of PNCs with QDs, originating from the reduced trap sites, compared to that of the pristine PNCs. The FRET-based upshot in the conduction mechanism with features of negative differential resistance and negligible hysteresis for CsPbBr3 PNCs can add new directions to high performance-based photovoltaics and optoelectronics.

Brighten Triplet Excitons of Carbon Nanodots for Multicolor Phosphorescence Films.

Triplet excitons usually do not emit light under ambient conditions due to the spin-forbidden transition rule, thus they are called dark excitons. Herein, triplet excitons in carbon nanodots (CNDs) are brightened by embedding the CNDs into poly(vinyl alcohol) (PVA) films; flexible multicolor phosphorescence films are thus demonstrated. PVA chains can isolate the CNDs, and excited state electron or energy transfer induced triplet exciton quenching is thus reduced; while the formed hydrogen bonds between the CNDs and PVA can restrict vibration/rotation of the CNDs, thus further protecting the triplet excitons from nonradiative recombination. The lifetimes of the flexible multicolor phosphorescence films can reach 567, 1387, 726, and 311 ms, and the longest-lasting phosphorescence film can be observed by naked eyes for nearly 15 s even after bending 5000 times. The phosphorescence films can be processed into various patterns, and a dynamic optical signature concept has been proposed and demonstrated based on the phosphorescence films.

Interplay of vibration- and environment-assisted energy transfer

We study the interplay between two environmental influences on excited state energy transfer in photosynthetic light harvesting complexes, namely, vibrationally assisted energy transfer (VAET) and environment-assisted quantum transport (ENAQT), considering a dimeric chromophore donor–acceptor model as a prototype for larger systems. We demonstrate how the basic features of the excitonic energy transfer are influenced by these two environments, both separately and together, with the environment being fully quantum in the case of VAET and treated in the Haken–Strobl–Reineker classical limit in the case of ENAQT. Our results reveal that in the weak noise regime, the presence of a classical noise source is detrimental to the energy transfer that is resonantly assisted by the exciton-vibration interactions intrinsic to VAET. In the strong noise regime we reproduce all the features of ENAQT, including the turnover into a Zeno regime where energy transfer is suppressed, and VAET is insignificant.

Effect of Diffusion on Photo-Induced Excited-State Energy Transfer between Fluorescent Semiconducting Molecules: Tris-(8-hydroxyquinoline) Aluminum and 6,13-Bis (Triisopropylsilylethynyl) Pentacene

In the present work, the effect of diffusion on photo-induced excitation energy transfer between fluorescent organic semiconducting molecules tris-(8-hydroxyquinoline) aluminum (AlQ3, n-type donor) and 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-P, p-type acceptor) at a concentration range of 10–4 to 10–6 M in chloroform solution was studied by steady-state and time-domain fluorescence measurements. The donor–donor interaction strength is significantly weaker than the donor–acceptor strength (the ratio of donor–donor to donor–acceptor interaction strengths is 1.55 × 103) in chloroform solution. Considerable overlap between donor emission and acceptor absorption and high interaction parameters favors direct Förster energy transfer and exciplex (D*A) formation. Excitation energy migration does not take place between donor molecules. However, material diffusion appears to influence the donor decay dynamics by forming charge-transfer exciplex complexes and modulating the excitation energy transfer rate from the metal-to-ligand charge-transfer (MLCT) state of the donor to vibronic states of the acceptor at a low acceptor concentration. At high acceptor concentrations, energy transfer from the excited donor to acceptor occurs through the exchange mechanism along with Förster long-range dipole–dipole interactions, and the corresponding critical transfer distance is found to be ∼42 Å. A blue-shift in AlQ3 emission, a red-shift in the 0–0 vibronic peak of TIPS-P, and quenching of exciplex decay following Stern–Volmer quenching are observed, with the increase in acceptor concentration. No evidence of excimer formation is observed in acceptor molecules.

Structural, luminescent and upconversion characteristics of Er3+ doped titanium zinc tellurite glass

The TeO 2 –TiO 2 –ZnO glass was single doped with Er 3+ and its structural, luminescent, and upconversion properties were analyzed. The amorphous nature of the fabricated glass was confirmed by X-ray diffraction, and the characteristic vibrational modes of the tellurite-titanium-zinc glass network were identified by Raman spectroscopy. The photoluminescent and upconversion properties of Er 3+ doped glass was analyzed by time decay measurements at different excitation wavelengths. The spectra recorded upon excitations at 808 nm and 975 nm show an intense green upconversion luminescence corresponding to the 2 H 11/2 , 4 S 3/2 → 4 I 15/2 transitions and a very weak red emission associated to the 4 F 9/2 → 4 I 15/2 transition. The experimental results suggest that in Er 3+ doped glass the excited state absorption and/or energy transfer mechanisms are responsible for upconversion luminescence observed upon 808 nm and 975 nm excitations. • TeO–TiO 2 –ZnO glasses optically activated with Er 3+ single doping. • Up-conversion luminescence takes place as a double photon absorption process. • Up-conversion produced by excited-state absorption and/or energy transfer mechanism. • Intense green up-conversion emission upon 808 and 975 nm excitation.

Spectral Fingerprint of Excited-State Energy Transfer in Dendrimers through Polarization-Sensitive Transient-Absorption Pump–Probe Signals: On-the-Fly Nonadiabatic Dynamics Simulations

The time-resolved polarization-sensitive transient-absorption (TA) pump-probe (PP) spectra are simulated using on-the-fly surface-hopping nonadiabatic dynamics and the doorway-window representation of nonlinear spectroscopy. A dendrimer model system composed of two linear phenylene ethynylene units (2-ring and 3-ring) is taken as an example. The ground-state bleach (GSB), stimulated emission (SE), and excited-state absorption (ESA) contributions as well as the total TA PP signals are obtained and carefully analyzed. It is shown that intramolecular excited-state energy transfer from the 2-ring unit to the 3-ring unit can be conveniently identified by employing pump and probe pulses with different polarizations. Our results demonstrate that time-resolved polarization-sensitive TA PP signals provide a powerful tool for the elucidation of excited-state energy-transfer pathways, notably in molecular systems possessing several optically bright nonadiabatically coupled electronic states with different orientations of transition dipole moments.

Discovering of the L ligand impact on luminescence enhancement of Eu(Dibenzoylmethane)3.Lx complexes employing transient absorption spectroscopy

The effect of luminescent enhancement under exchange of the auxiliary ligand in Europium(III) tris(1,3-diphenyl-1,3-propanedionato) monohydrate was investigated by steady-state and time-resolved transient absorption spectroscopy. The excited state relaxation dynamics of this complex was analysed through a comparison of the experimental data obtained for several model compounds, namely Eu(DBM)3·NH3, Eu(DBM)3.EDA, Eu(DBM)3.Phen, Al(DBM)3 and dibenzoylmethane (DBM) in various solutions and polymer matrices. The results show there is no linear relationship between enhancement of the emission quantum yield and the luminescent lifetime, which suggests that the auxiliary ligand reduces the rate of nonradiative relaxation of the lanthanide ion, but also affects the excited state energy transfer from ligand to metal ion. Transient absorption data shows a clear correlation between the efficiency of the energy transfer and the degree of triplet state population expressed by an amplification of the signal for its excited state absorption band on going from Eu(DBM)3·H2O to the Eu(DBM) = .L complex. The results show that this auxiliary ligand exchange acts as a “switch” turning the intersystem crossing on or off as a competitive pathway for excited state relaxation of the europium(III) complexes.

Effect of the Molecular Conformation on Excitation Energy Transfer in Conformationally Constrained Boryl-BODIPY Dyads

We studied the dual emission characteristics of a series of boryl-BODIPYs (1-6) comprised of triarylborane (TAB) as an energy donor and BODIPY as an energy acceptor. The molecular conformations of dyads 1-6 were systematically tuned by judiciously changing the spacer that bridged the boryl and BODIPY moieties. Frontier molecular orbitals (FMOs) are localized in 3, 4, and 6 with a twisted molecular conformation. In contrast, FMOs are significantly delocalized in 1, 2, and 5 with the least-twisted molecular conformation. Dyads 1-6 showed dual emission features when they were excited at the TAB-dominated absorption band. However, the ratio between the two emission bands in 1-6 significantly varied depending on the molecular conformations. Systematic photoluminescence (PL) studies (both steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads. These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle between these two moieties.

Excited State Energy Transfer in Metal-Organic Frameworks.

Excited state energy transfer in metal-organic frameworks (MOFs) is of great interest due to potential application of these materials in photocatalysis and fluorescence sensing. In photocatalysis, a light-harvesting antenna of MOFs can collect energy from a much larger area than a single reaction center and efficiently transport the energy to the active site to enhance photocatalytic efficiency, mimicking nature photosynthesis. In fluorescence sensing, excited state traveling on the framework can search for analyte quencher molecules to give amplified fluorescence quenching, so that one quencher turns off multiple excited states to enhance signal. Key to these designer performances is highly efficient energy transfer on these framework materials that are determined by types of excited states, dimension of the materials, and structure of the frameworks. Advancement of MOF synthetic chemistry provides new tools to control the rate and directionality of energy transfer in these materials, opening opportunities in manipulating excited states at an unprecedented level.

Analysis of bath motion in MM-SQC dynamics via dimensionality reduction approach: Principal component analysis.

The system-plus-bath model is an important tool to understand the nonadiabatic dynamics of large molecular systems. Understanding the collective motion of a large number of bath modes is essential for revealing their key roles in the overall dynamics. Here, we applied principal component analysis (PCA) to investigate the bath motion in the basis of a large dataset generated from the symmetrical quasi-classical dynamics method based on the Meyer-Miller mapping Hamiltonian nonadiabatic dynamics for the excited-state energy transfer in the Frenkel-exciton model. The PCA method clearly elucidated that two types of bath modes, which either display strong vibronic coupling or have frequencies close to that of the electronic transition, are important to the nonadiabatic dynamics. These observations were fully consistent with the physical insights. The conclusions were based on the PCA of the trajectory data and did not involve significant pre-defined physical knowledge. The results show that the PCA approach, which is one of the simplest unsupervised machine learning dimensionality reduction methods, is a powerful one for analyzing complicated nonadiabatic dynamics in the condensed phase with many degrees of freedom.

Ultrafast Spectroscopic Analysis of Pressure-Induced Variations of Excited-State Energy and Intramolecular Proton Transfer in Semi-Aliphatic Polyimide Films.

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm-1) through an excited-state intramolecular proton transfer (ESIPT) induced by keto-enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky -CH3 and -CF3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Förster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole-dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Spectroscopy and Near-Infrared to Visible Upconversion of Er3+ Ions in Aluminosilicate Glasses Manufactured with Controlled Optical Transmission

In this work we report on the spectroscopic properties and the near-infrared to visible upconversion of Er3+ ions in aluminosilicate glasses manufactured by directionally solidification with the laser floating zone technique. Glasses were manufactured in a controlled oxidizing atmosphere to provide them with high optical transmission in the visible spectral range. Absorption and emission spectra, and lifetimes were assessed in both the visible and the near infrared spectral range. Green upconversion emissions of the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions at 525 nm and 550 nm attributed to a two-photon process were observed under excitation at 800 nm. Mechanisms responsible for the upconversion luminescence were discussed in terms of excited state absorption and energy transfer upconversion processes. Excitation spectra of the upconverted emission suggest that energy transfer upconversion processes are responsible for the green upconversion luminescence.

Light in Gold Catalysis.

Within the wide family of gold-catalyzed reactions, gold photocatalysis intrinsically features unique elementary steps. When gold catalysis meets photocatalysis, a valence change of the gold center can easily be achieved via electron transfer and radical addition, avoiding the use of stoichiometric sacrificial external oxidants. The excellent compatibility of radicals with gold catalysts opens the door to a series of important organic transformations, including redox-neutral C-C and C-X coupling, C-H activation, and formal radical-radical cross-coupling. The photocatalysis with gold complexes nicely complements the existing photoredox catalysis strategies and also opens a new avenue for gold chemistry. This review covers the achieved transformations for both mononuclear gold(I) catalysts (with and without a photosensitizer) and dinuclear gold(I) photocatalysts. Various fascinating methodologies, their value for organic chemists, and the current mechanistic understanding are discussed. The most recent examples also demonstrate the feasibility of both, mononuclear and dinuclear gold(I) complexes to participate in excited state energy transfer (EnT), rather than electron transfer. The rare applications of gold(III) photocatalysts, both homogeneous and heterogeneous, are also summarized.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes.

Nine-coordinate [ErN9] or [ErN3O6] chromophores found in triple helical [Er(L)3]3+ complexes (L corresponds to 2,2′,6′,2′′-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)3]3− (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)3]9+ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10−11 ≤ ϕuptot(ESA) ≤ 1.7(2) × 10−9 appear to be 1–3 orders of magnitude larger than those predicted by the accepted single-center excited-state absorption mechanism (ESA). Switching to the alternative energy transfer upconversion mechanism (ETU), which operates in multi-centers [CrErCr(bpb-bzimpy)3]9+, leads to an improved quantum yield of ϕuptot(ETU) = 5.8(6) × 10−8, but also to an even larger discrepancy by 4–6 orders of magnitude when compared with theoretical models. All photophysical studies point to Er(4I13/2) as being the only available ‘long-lived’ (1.8 ≤ τ ≤ 6.3 μs) and emissive excited state, which works as an intermediate relay for absorbing the second photon, but with an unexpected large cross-section for an intrashell 4f → 4f electronic transition. With this in mind, the ETU mechanism, thought to optimize upconversion via intermetallic Cr → Er communication in [CrErCr(bpb-bzimpy)3]9+, is indeed not crucial and the boosted associated upconversion quantum yield is indebted to the dominant contribution of the single-center erbium ESA process. This curious phenomenon is responsible for the successful implementation of light upconversion in molecular coordination complexes under reasonable light power intensities, which paves the way for applications in medicine and biology. Its origin could be linked with the presence of metal–ligand bonding.

Rationalization of excited state energy transfer in D–π–A porphyrin sensitizers enhancing efficiency in dye-sensitized solar cells

Donor–π–acceptor (D–π–A) porphyrin based photo-sensitizers are extensively utilized in dye sensitized solar cells (DSSCs).

Understanding the mechanism of direct visible-light-activated [2 + 2] cycloadditions mediated by Rh and Ir photocatalysts: combined computational and spectroscopic studies.

The mechanism of [2 + 2] cycloadditions activated by visible light and catalyzed by bis-cyclometalated Rh(iii) and Ir(iii) photocatalysts was investigated, combining density functional theory calculations and spectroscopic techniques. Experimental observations show that the Rh-based photocatalyst produces excellent yield and enantioselectivity whereas the Ir-photocatalyst yields racemates. Two different mechanistic features were found to compete with each other, namely the direct photoactivation of the catalyst–substrate complex and outer-sphere triplet energy transfer. Our integrated analysis suggests that the direct photocatalysis is the inner working of the Rh-catalyzed reaction, whereas the Ir catalyst serves as a triplet sensitizer that activates cycloaddition via an outer-sphere triplet excited state energy transfer mechanism.