Nonradiative Decay Of Excited States Research Articles

High-efficiency long-wavelength NIR iridium complexes constructed by extending rigid coordination and optimizing peripheral donor position to break cocoon into butterfly

Due to the non-radiative decay of excited states, it is extremely complicated for iridium (III) complexes to simultaneously harvest high-efficiency and long-wavelength emission in the near-infrared (NIR) region based on the energy-gap law. In order to address this issue, we designed and synthesized two butterfly-type iridium(III) complexes of (PyPz-TPA)2Ir(acac) and (PyQl-TPA)2Ir(acac), in which pyrene-pyrazine (PyPz) and pyrene-quinoxaline (PyQl) are respectively used as the rigid C^N bidentate coordinated cocoons, peripherally with two triphenylamine (TPA) units at 4, 5-positions. Their photophysical, electrochemical and electroluminescent properties are primarily investigated. It is found that the (PyQl-TPA)2Ir(acac) presents stronger spin-orbital coupling effect, a smaller ΔEST energy difference, and lower reorganization energy than (PyPz-TPA)2Ir(acac) by selecting rigid pyrene-quinoxaline coordination and optimizing peripheral TPA position. Furthermore, the (PyQl-TPA)2Ir(acac)-doped OLEDs exhibit a red-shifted NIR emission peaked at 784 nm with a record external quantum efficiency of 5.42% using CBP as host matrix by solution process, which is a rare representation to shrink the limitation of energy-gap law in NIR region for iridium(III) complexes. This study provides a feasible strategy to construct high-efficiency NIR-emitting iridium(III) complexes by an artfully motivated strategy of extending and breaking the C^N coordinate cocoon into butterfly.

Mechanochemistry toward Organic "Salt" via Integer-Charge-Transfer Cocrystal Strategy for Rapid, Efficient, and Scalable Near-Infrared Photothermal Conversion.

Inspired by the concept of ionic charge-transfer complexes for the Mott insulator, integer-charge-transfer (integer-CT) cocrystals are designed for NIR photo-thermal conversion (PTC). With amino-styryl-pyridinium dyes and F4TCNQ (7,7',8,8'-Tetracyano-2,3,5,6-tetrafluoroquinodimethane) serving as donor/acceptor (D/A) units, integer-CT cocrystals, including amorphous stacking "salt" and segregated stacking "ionic crystal", are synthesized by mechanochemistry and solution method, respectively. Surprisingly, the integer-CT cocrystals are self-assembled only through multiple D-A hydrogen bonds (C-H⋅⋅⋅X (X=N, F)). Strong charge-transfer interactions in cocrystals contribute to the strong light-harvesting ability at 200-1500 nm. Under 808 nm laser illumination, both the "salt" and "ionic crystal" display excellent PTC efficiency beneficial from ultrafast (∼2 ps) nonradiative decay of excited states. Thus integer-CT cocrystals are potential candidates for rapid, efficient, and scalable PTC platforms. Especially amorphous "salt" with good photo/thermal stability is highly desirable in practical large-scale solar-harvesting/conversion applications in water environment. This work verifies the validity of the integer-CT cocrystal strategy, and charts a promising path to synthesize amorphous PTC materials by mechanochemical method in one-step.

TiO2–rGO nanocomposites with high rGO content and luminescence quenching through green redox synthesis

We developed TiO2–rGO nanocomposites with high exfoliated rGO content upto 47.1% (TG4) by weight hitherto not reported, through green redox synthesis. Progressive reduction of TiO2 nanocrystal size and their in situ intercalation occurred on increasing rGO via one pot synthesis, not reported previously and forming highly stable Ti–O–C bonds. With increase in rGO content, the exfoliation in the nanocomposite was also limited to single-few layered graphene oxide sheets through use of ascorbic acid avoiding restacking, and thereby the recombination of photogenerated carriers. TG4 showed 50% photoluminescence (PL) quenching indicating long-lived electron transfer state with rGO and apparent rate of nonradiative excited state decay of 1.15 × 108 s−1. Details of lattice orientation, Raman spectra, electronic states and PL lifetime decay are discussed to elucidate effective charge carrier separation in the nanocomposite. Near complete photodegradation of Alizarin Yellow GG azo dye, an industrial pollutant, could be obtained under direct sunlight using TG4 as catalyst.

Computational design of singlet fission biradicaloid chromophores

We present three different computational designs of potential singlet fission chromophores. In the first two cases, we propose structural modifications of two biradicaloid compounds already known with regard to singlet fission, namely 1,3-diphenyl-isobenzofuran (DPBF) and 2,3-diamino-1,4-benzoquinone (DABQ). In the former case, the introduction of methylene bridges between the phenyl rings and the isobenzofuran core of DPBF leads to an improvement of the singlet fission energetics and confers a higher degree of rigidity to the resulting molecule. In the second case, structural modifications of DABQ are proposed in order to overcome its fast nonradiative excited state decay and to improve its singlet fission energetics. The resulting diamino-fluoroquinone compounds fulfill the desired energy criteria and represent new potential chromophores for singlet fission. Finally, in the third proposed design we exploit the captodative effect to tune the energy levels of small biradicaloid molecules, leading to propose several compounds presented for the first time as suitable chromophores for singlet fission.

Unprecedented Improvement of Near-Infrared Photothermal Conversion Efficiency to 87.2% by Ultrafast Non-radiative Decay of Excited States of Self-Assembly Cocrystal.

Near-infrared (NIR) photothermal conversion is of great interest in many fields. Here, a self-assembly organic cocrystal (N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and pyromellitic dianhydride (PMDA)) with strong absorption in NIR range is constructed, with widespread absorption (200-1500 nm) and very high NIR photothermal conversion efficiency (87.2%). Essentially, in this cocrystal, a small HOMO-LUMO gap of donor-acceptor pair boosts the absorption ability of this cocrystal in the NIR range. The mixed stacking structure significantly enhances the intermolecular interactions as well as the electron-hole delocalization, suppressing the emission processes, leading to nonradiative decay processes from excited states. Strong intermolecular interactions enable the cocrystal to have dense electronic energy levels, leading to a high proportion (94.4%) vibrational cooling and internal conversion processes with ultrafast excited-state relaxation (0.12 ps), which contributes to high NIR photothermal conversion efficiency. Furthermore, the cocrystal has exhibited capable ability for being an excellent candidate for a NIR photothermal therapy agent.

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures.

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (-COC-), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S1 ) state to the ground (S0 ) state via a conical intersection (CI), at which the energy difference between the S1 and S0 states is approximately zero. This CI is induced after breaking one C-O bond at the -COC- moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.

Boosting Non-Radiative Decay to Do Useful Work: Development of a Multi-Modality Theranostic System from an AIEgen.

The efficient utilization of energy dissipating from non-radiative excited-state decay of fluorophores was only rarely reported. Herein, we demonstrate how to boost the energy generation of non-radiative decay and use it for cancer theranostics. A novel compound (TFM) was synthesized which possesses a rotor-like twisted structure, strong absorption in the far red/near-infrared region, and it shows aggregation-induced emission (AIE). Molecular dynamics simulations reveal that the TFM aggregate is in an amorphous form consisting of disordered molecules in a loose packing state, which allows efficient intramolecular motions, and consequently elevates energy dissipation from the pathway of thermal deactivation. These intrinsic features enable TFM nanoparticles (NPs) to display a high photothermal conversion efficiency (51.2 %), an excellent photoacoustic (PA) effect, and effective reactive oxygen species (ROS) generation. In vivo evaluation shows that the TFM NPs are excellent candidates for PA imaging-guided phototherapy.

Characterisation of tungstate and molybdate crystals ABO4 (A = Ca, Sr, Zn, Cd; B = W, Mo) for luminescence lifetime cryothermometry

Luminescence lifetime thermometry for remote temperature monitoring of cryogenic objects requires materials that exhibit a suitably large change of the luminescence kinetics at low temperatures. Results of systematic studies of the temperature-induced changes in the luminescence of tungstates and molybdates with the general formula ABO4 (A = Ca, Sr, Zn, Cd; B = W, Mo) over the 4.5–300 K temperature range are summarized. It is shown through analysing changes of the emission and excitation spectra, as well as the decay kinetics, that in these materials the luminescence is due to the emission of self-trapped excitons, a process that exhibits strong temperature dependence. The main emphasis of the study is on establishing the factors that determine the character of the temperature dependence of the luminescence decay time constant. We discuss our findings in terms of a model that analyses the dynamics of radiative and non-radiative transitions between the excited and ground states of the emission center. Two thermally activated processes drive the observed changes. The first is the non-radiative decay of excited states, resulting in a decrease of the luminescence decay time constant at high temperatures. Additionally it is demonstrated that in molybdates and tungstates the fine splitting of the excited state facilitates a second mechanism for thermally activated exchange of charged carriers between the two split levels. This has a noticeable effect on the dynamics of the radiative decay at low temperatures. We established that the sensitivity of the luminescence lifetime to temperature changes can be estimated by using information on the energy structure of materials. It is concluded that within tungstates and molybdates under study SrWO4 is the most promising material for application in luminescence lifetime cryothermometry.

AIE-active organic polymorphs displaying molecular conformation-dependent amplified spontaneous emissions (ASE)

The Schiff base (E)-4-chloro-2-(((4-(dimethylamino)phenyl)imino)methyl)phenol showing aggregation-induced emission has been found to form brightly yellow- and orange-emissive organic polymorphs. As indicated by the photophysical studies, the suppression of nonradiative excited-state decay and the improvement of radiative decay are responsible for the emission enhancement from solutions to crystalline polymorphs. Crystallographic studies disclose that the two polymorphs adopt quite similar molecular packing modes but different molecular conformations. Thus, the difference in emission colors of the two polymorphs is mainly attributed to the influence of molecular conformations. For the rod-like orange-emissive polymorph, ultralong single crystals can be prepared, and these crystals show the attractive property of low-loss optical waveguide. More importantly, both polymorphs exhibit amplified spontaneous emission properties. This study provides a good model system for understanding the influence of molecular conformations on solid-state luminescent properties by minimizing the intervention of molecular packing modes in luminescent performance. Moreover, the study further confirms that the utilization of organic polymorphs can be a facile strategy to realize multicolor crystal lasing.

Microscopic dissipative structuring and proliferation at the origin of life

Some fundamental molecules of life are suggested to have been formed, proliferated, and evolved through photochemical microscopic dissipative structuring and autocatalytic proliferation under the UV-C/UV-B solar environment prevalent at Earth’s surface throughout the Archean. Evidence is given in the numerous salient characteristics of these, including their strong absorption in this spectral region and their rapid non-radiative excited state decay through inherent conical intersections. The examples of the dissipative structuring and dissipative proliferation of the purines and of single strand DNA are given. UV-C and UV-B-induced stationary state isomerizations and tautomerizations are shown to be crucial to the formation of the purines from hydrogen cyanide in an aqueous environment under UV-C light, while UV-C induced phosphorylation of nucleosides and denaturing of double helix RNA and DNA are similarly important to the production and proliferation of single strand DNA. This thermodynamic dissipation perspective provides a physical-chemical foundation for understanding the origin and evolution of life.

Observation of Photovoltaic Action from Photoacid-Modified Nafion Due to Light-Driven Ion Transport.

Replacing passive ion-exchange membranes, like Nafion, with membranes that use light to drive ion transport would allow membranes in photoelectrochemical technologies to serve in an active role. Toward this, we modified perfluorosulfonic acid ionomer membranes with organic pyrenol-based photoacid dyes to sensitize the membranes to visible light and initiate proton transport. Covalent modification of the membranes was achieved by reacting Nafion sulfonyl fluoride poly(perfluorosulfonyl fluoride) membranes with the photoacid 8-hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide). The modified membranes were strongly colored and maintained a high selectivity for cations over anions. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ion-exchange measurements together provided strong evidence of covalent bond formation between the photoacids and the polymer membranes. Visible-light illumination of the photoacid-modified membranes resulted in a maximum power-producing ionic photoresponse of ∼100 μA/cm2 and ∼1 mV under 40 Suns equivalent excitation with 405 nm light. In comparison, membranes that did not contain photoacids and instead contained ionically associated RuII-polypyridyl coordination compound dyes, which are not photoacids, exhibited little-to-no photoeffects (∼1 μA/cm2). These disparate photocurrents, yet similar yields for nonradiative excited-state decay from the photoacids and the RuII dyes, suggest temperature gradients were not likely the cause of the observed photovoltaic action from photoacid-modified membranes. Moreover, spectral response measurements supported that light absorption by the covalently bound photoacids was required in order to observe photoeffects. These results represent the first demonstration of photovoltaic action from an ion-exchange membrane and offer promise for supplementing the power demands of electrochemical processes with renewable sunlight-driven ion transport.

Electrical Detection of Quantum Dot Hot Electrons Generated via a Mn2+-Enhanced Auger Process.

An all-solid-state quantum-dot-based photon-to-current conversion device is demonstrated that selectively detects the generation of hot electrons. Photoexcitation of Mn2+-doped CdS quantum dots embedded in the device is followed by efficient picosecond energy transfer to Mn2+ with a long-lived (millisecond) excited-state lifetime. Electrons injected into the QDs under applied bias then capture this energy via Auger de-excitation, generating hot electrons that possess sufficient energy to escape over a ZnS blocking layer, thereby producing current. This electrically detected hot-electron generation is correlated with a quench in the steady-state Mn2+ luminescence and the introduction of a new nonradiative excited-state decay process, consistent with electron-dopant Auger cross-relaxation. The device's efficiency at detecting hot-electron generation provides a model platform for the study of hot-electron ionization relevant to the development of novel photodetectors and alternative energy-conversion devices.

On the Origins of Nonradiative Excited State Relaxation in Aryl Sulfoxides Relevant to Fluorescent Chemosensing.

We provide herein a mechanistic analysis of aryl sulfoxide excited state processes, inspired by our recent report of aryl sulfoxide based fluorescent chemosensors. The use of aryl sulfoxides as reporting elements in chemosensor development is a significant deviation from previous approaches, and thus warrants closer examination. We demonstrate that metal ion binding suppresses nonradiative excited state decay by blocking formation of a previously unrecognized charge transfer excited state, leading to fluorescence enhancement. This charge transfer state derives from the initially formed locally excited state followed by intramolecular charge transfer to form a sulfoxide radical cation/aryl radical anion pair. With the aid of computational studies, we map out ground and excited state potential energy surface details for aryl sulfoxides, and conclude that fluorescence enhancement is almost entirely the result of excited state effects. This work expands previous proposals that excited state pyramidal inversion is the major nonradiative decay pathway for aryl sulfoxides. We show that pyramidal inversion is indeed relevant, but that an additional and dominant nonradiative pathway must also exist. These conclusions have implications for the design of next generation sulfoxide based fluorescent chemosensors.

ChemInform Abstract: A ′Bottom Up′, ab initio Computational Approach to Understanding Fundamental Photophysical Processes in Nitrogen Containing Heterocycles, DNA Bases and Base Pairs

AbstractReview: 207 refs.

The role of local and remote amino acid substitutions for optimizing fluorescence in bacteriophytochromes: A case study on iRFP.

Bacteriophytochromes are promising tools for tissue microscopy and imaging due to their fluorescence in the near-infrared region. These applications require optimization of the originally low fluorescence quantum yields via genetic engineering. Factors that favour fluorescence over other non-radiative excited state decay channels are yet poorly understood. In this work we employed resonance Raman and fluorescence spectroscopy to analyse the consequences of multiple amino acid substitutions on fluorescence of the iRFP713 benchmark protein. Two groups of mutations distinguishing iRFP from its precursor, the PAS-GAF domain of the bacteriophytochrome P2 from Rhodopseudomonas palustris, have qualitatively different effects on the biliverdin cofactor, which exists in a fluorescent (state II) and a non-fluorescent conformer (state I). Substitution of three critical amino acids in the chromophore binding pocket increases the intrinsic fluorescence quantum yield of state II from 1.7 to 5.0% due to slight structural changes of the tetrapyrrole chromophore. Whereas these changes are accompanied by an enrichment of state II from ~40 to ~50%, a major shift to ~88% is achieved by remote amino acid substitutions. Additionally, an increase of the intrinsic fluorescence quantum yield of this conformer by ~34% is achieved. The present results have important implications for future design strategies of biofluorophores.

Bond Fission and Non-Radiative Decay in Iridium(III) Complexes.

We investigate the role of metal-ligand bond fission in the nonradiative decay of excited states in iridium(III) complexes with applications in blue organic light-emitting diodes (OLEDs). We report density functional theory (DFT) calculations of the potential energy surfaces upon lengthening an iridium-nitrogen (Ir-N) bond. In all cases we find that for bond lengths comparable to those of the ground state the lowest energy state is a triplet with significant metal-to-ligand change transfer character ((3)MLCT). But, as the Ir-N bond is lengthened there is a sudden transition to a regime where the lowest excited state is a triplet with significant metal centered character ((3)MC). Time-dependent DFT relativistic calculations including spin-orbit coupling perturbatively show that the radiative decay rate from the (3)MC state is orders of magnitude slower than that from the (3)MLCT state. The calculated barrier height between the (3)MLCT and (3)MC regimes is clearly correlated with previously measured nonradiative decay rates, suggesting that thermal population of the (3)MC state is the dominant nonradiative decay process at ambient temperature. In particular, fluorination both drives the emission of these complexes to a deeper blue color and lowers the (3)MLCT-(3)MC barrier. If the Ir-N bond is shortened in the (3)MC state another N atom is pushed away from the Ir, resulting in the breaking of this bond, suggesting that once the Ir-N bond breaks the damage to the complex is permanent-this will have important implications for the lifetimes of devices using this type of complex as the active material. The consequences of these results for the design of more efficient blue phosphors for OLED applications are discussed.

Unusual Aggregation-Induced Emission of a Coumarin Derivative as a Result of the Restriction of an Intramolecular Twisting Motion.

Aggregation-induced emission (AIE) is commonly observed for propeller-like luminogens with aromatic rotors and stators. Herein, we report that a coumarin derivative containing a seven-membered aliphatic ring (CD-7) but no rotors showed typical AIE characteristics, whereas its analogue with a five-membered aliphatic ring (CD-5) exhibited an opposite aggregation-caused quenching (ACQ) effect. Experimental and theoretical results revealed that a large aliphatic ring in CD-7 weakens structural rigidity and promotes out-of-plane twisting of the molecular backbone to drastically accelerate nonradiative excited-state decay, thus resulting in poor emission in solution. The restriction of twisting motion in aggregates blocks the nonradiative decay channels and enables CD-7 to fluoresce strongly. The results also show that AIE is a general phenomenon and not peculiar to propeller-like molecules. The AIE and ACQ effects can be switched readily by the modulation of molecular rigidity.

Insights into the Mechanism of Enhanced Rhodamine 6G Dimer Fluorescence in Mesoscopic Pluronic-Silica Matrixes

Comparative study of the spectroscopic properties of Rhodamine 6G dimers in ethanol solutions and in mesoscopic silica-Pluronic sol–gel films has been performed by means of steady state and transient absorption and fluorescence methods. The dimers act as fluorescence quenchers in solutions, while their fluorescence yield is about 36 times higher in the mesoscopic films where they dominate in the fluorescence spectra. The difference is caused by about 6 times slower nonradiative excited state decay and about 6 times stronger oscillator strength of the fluorescence transition of the dimers in the films in comparison with solutions. We demonstrate that the dimer fluorescence originates from the oblique sandwich-type dimers both in solutions and in mesoscopic films. We suggest that the higher fluorescence yield of the dimers in the mesoscopic films is mainly caused by the stronger deviation from the planar geometrical arrangement of the sandwich-type dimer, thus causing opening of the forbidden low energy exc...

Energy dissipation channels affecting photoluminescence from resonantly excited Er3+ ions doped in epitaxial ZnO host films

We identified prerequisite conditions to obtain intense photoluminescence at 1.54 μm from Er3+ ions doped in ZnO host crystals. The epitaxial ZnO:Er films were grown on sapphire C-plane substrates by sputtering, and Er3+ ions were resonantly excited at a wavelength of 532 nm between energy levels of 4I15/2 and 2H11/2. There is a threshold deposition temperature between 500 and 550 °C, above which epitaxial ZnO films become free of miss-oriented domains. In this case, Er3+ ions are outside ZnO crystallites, having the same c-axis lattice parameters as those of undoped ZnO crystals. The improved crystallinity was correlated with enhanced emissions peaking at 1538 nm. Further elevating the deposition temperature up to 650 °C generated cracks in ZnO crystals to relax the lattice mismatch strains, and the emission intensities from cracked regions were three times as large as those from smooth regions. These results can be consistently explained if we assume that emission-active Er3+ ions are those existing at grain boundaries and bonded to single-crystalline ZnO crystallites. In contrast, ZnO:Er films deposited on a ZnO buffer layer exhibited very weak emissions because of their degraded crystallinity when most Er3+ ions were accommodated into ZnO crystals. Optimizing the degree of oxidization of ZnO crystals is another important factor because reduced films suffer from non-radiative decay of excited states. The optimum Er content to obtain intense emissions was between 2 and 4 at. %. When 4 at. % was exceeded, the emission intensity was severely attenuated because of concentration quenching as well as the degradation in crystallinity. Precipitation of Er2O3 crystals was clearly observed at 22 at. % for films deposited above 650 °C. Minimizing the number of defects and impurities in ZnO crystals prevents energy dissipation, thus exclusively utilizing the excitation energy to emissions from Er3+ ions.

Intramolecular vibrational redistribution in the non-radiative excited state decay of uracil in the gas phase: an ab initio molecular dynamics study.

We report a study of intramolecular vibrational distribution (IVR) occurring in the electronic ground state of uracil (S0) in the gas phase, following photoexcitation in the lowest energy bright excited state (Sπ) and decay through the ethylene-like Sπ/S0 Conical Intersection (CI-0π). To this aim we have performed 20 independent ab initio molecular dynamics simulations starting from CI-0π (ten of them with 1 eV kinetic energy randomly distributed over the different molecular degrees of freedom) and 10 starting from the ground state minimum (Franck-Condon, FC, point), with the excess kinetic energy equal to the energy gap between CI-0π and the FC point. The simulations, exploiting PBE0/6-31G(d) calculations, were performed over an overall period of 10 ps. A thorough statistical analysis of the variation of the geometrical parameters of uracil during the simulation time and of the distribution of the kinetic energy among the different vibrational degrees of freedom provides a consistent picture of the IVR process. In the first 0-200 fs the structural dynamics involve mainly the recovery of the average planarity. In the 200-600 fs time range, a substantial activation of CO and NH degrees of freedom is observed. After 500-600 fs most of the geometrical parameters reach average values similar to those found after 10 ps, though the system cannot be considered to be in equilibrium yet.