Excitation Energy Transfer Processes Research Articles

Hierarchical coarse-graining model for photosystem II including electron and excitation-energy transfer processes

We propose a hierarchical reduction scheme to cope with coupled rate equations that describe the dynamics of multi-time-scale photosynthetic reactions. To numerically solve nonlinear dynamical equations containing a wide temporal range of rate constants, we first study a prototypical three-variable model. Using a separation of the time scale of rate constants combined with identified slow variables as (quasi-)conserved quantities in the fast process, we achieve a coarse-graining of the dynamical equations reduced to those at a slower time scale. By iteratively employing this reduction method, the coarse-graining of broadly multi-scale dynamical equations can be performed in a hierarchical manner. We then apply this scheme to the reaction dynamics analysis of a simplified model for an illuminated photosystem II, which involves many processes of electron and excitation-energy transfers with a wide range of rate constants. We thus confirm a good agreement between the coarse-grained and fully (finely) integrated results for the population dynamics.

Unique ultrafast energy transfer in a series of phenylene-bridged subporphyrin-porphyrin hybrids.

A series of phenylene-bridged subporphyrin-Zn(II) porphyrin (SubP-ZnP) hybrid systems undergo extraordinarily fast excitation energy transfer (EET) processes from the SubP to the ZnP, aided by conjugative electronic elongation of the SubP to the phenylene-bridge.

Synthesis and Characterization of Sm$^{3+}$–Yb$^{3+}$ Codoped Y$_{2}$O$_{3}$ Phosphor

The Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phosphors doped/codoped with Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions at different concentrations have been prepared using combustion technique. The prepared phosphors are characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The average crystallite size of the phosphors calculated based on the XRD is observed to increase with annealing. The FTIR analysis suggested that the impurity is reduced on annealing the sample. NIR to visible frequency upconversion emission has been observed from synthesized phosphors upon a 980 nm diode laser excitation. The concentration dependence study shows a tuning in light color emitted from samples. The upconversion mechanism involved in the Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> doped/codoped Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phosphors has been explained on the basis of excited state absorption (ESA) and energy transfer (ET) process.

Efficient Energy Transfer under Two-Photon Excitation in a 3D, Supramolecular, Zn(II)-Coordinated, Self-Assembled Organic Network

Multiphoton excited fl uorescence of organic molecules is promising in the applications of effi cient nonlinear optical devices and bioimaging. However, they usually have disadvantages of poor photostability and serious fl uorescence quenching in aqueous media or solid state, which seriously limit their related applications. In this work, for the fi rst time, the two-photon excited Forster resonance energy transfer (FRET) process is used to enhance the solid-state fl uorescence of the supramolecular centre (acceptor) in an artifi cial 3D metal‐organic complex (MLC), in which a 3D Zn (II)-coordinated tetrahedral core is utilized as the donor. More interestingly, the two-photon light harvesting system, which can be pumped with an optical intensity as low as 1 MW/cm 2 , exhibits an ultrafast energy transfer rate ( ∼ 6.9 ◊ 10 8 s 1 ) and ultrahigh photostability. The underlying physical mechanisms are revealed through comprehensive steady-state and time-resolved spectroscopic analysis. This work demonstrates that the 3D MLC can be directly used in two-photon bioimaging and also sheds light on developing other multiphoton harvesting systems, such as metal‐organic frameworks.

Spectroscopy and visible frequency upconversion in Er3+–Yb3+: TeO2–ZnO glass

The UV–Vis–NIR absorption studies of the Er3+/Er3+–Yb3+ doped/codoped TeO2–ZnO (TZO) glasses fabricated by the melting and quenching method has been performed. The spectroscopic radiative parameters viz. radiative transition probabilities, branching ratios and lifetimes have been determined from the absorption spectrum by using Judd–Ofelt theory. The near infrared (NIR) to visible frequency upconversion (UC) have been monitored by using an excitation of 976nm wavelength radiation from a CW diode laser. The effect of codoping with Yb3+ ions on the intensity of the UC emission bands from the Er3+ ions throughout visible region has been studied. The mechanism responsible for the observed upconversion emissions in the prepared samples have been explained on the basis of excited state absorption and efficient energy transfer processes.

Luminescent solar concentrators. I. Concentrators based on mixtures of dyes in PMMA. Spectral and luminescent properties, reabsorption and energy transfer

Spectral and luminescent properties of efficient and stable commercial organic dyes in solutions and polymethylmetacrilate (PMMA), absorbing and emitting in wavelength ranges 300–680 nm and 390–750 nm, respectively, are investigated. Using different excitation and registration schemes, it is demonstrated that spectral shifts of initial radiation caused by reabsorption are observed at wavelengths around 1.5 cm. The process of excitation energy transfer from the UV range to the red range of the spectrum is investigated in mixtures of 4–6 dyes of different compositions. The maximum efficiency of radiation transfer from 300 nm to 560–580 nm, exceeding the efficiency of individual fluorophores by a factor of >1.6 is obtained for mixtures of four dyes POPOP(bis-MSB)–Coumarin 30–DCM–Pyrromethene 580 (Rhodamine 11B) in PMMA.

Energy-Transfer Mechanisms in IrIII -EuIII Bimetallic Complexes.

The bridging ligands in d-f bimetallic complexes play an important role in the excitation energy transfer (EET) process. To elaborate on the effect of the ligand on the EET process, a series of bridging ligands (μ-L), 1-(1',10'-phenanthrolin-2'-yl)-4,4,4-trifluorobutane-1,3-dione (phen3f), 1-(1',10'-phenanthrolin-2'-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (phen5f), 1-(2,2'-bipyridine-6-yl)-4,4,4-trifluorobutane-1,3-dione (bpy3f), and 1-(2,2'-bipyridine-6-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (bpy5f), and their corresponding iridium complexes, [(dfppy)2 Ir(μ-L)] (dfppy=2-(4',6'-difluorophenyl)pyridinato-N,C2'), as well as their corresponding heteroleptic IrIII -EuIII complexes [{(dfppy)2 Ir(μ-L)}3 EuCl]Cl2 were synthesized and characterized. Photophysical and kinetic results revealed that the alternation of the bridge ligand resulted in a systemic difference in the lowest triplet-state energy (T1 ) of the iridium complexes, the EET efficiency from iridium complexes to the EuIII ion, and a significant difference in the total luminescence quantum yields. Based on the nanosecond time-resolved phosphorescence spectra, a model for the energy-transfer mechanism was proposed for d-f bimetallic complexes, which indicated that the nonradiative relaxation of the excited energy of EuIII , especially energy dissipation by means of the T1 state, was the main reason for the discrepancy in the quantum yields of the four IrIII -EuIII complexes.

Influence of Yb3+ doping concentration on upconversion luminescence of Er3+ in Lu2O3 phosphor

Cubic phase Lu2O3:Er3+/Yb3+ nanocrystal phosphors were prepared by sol–gel method. Fourier transform infrared (FT-IR) spectra were measured to evaluate the vibrational feature of the samples. Green and red radiations were observed upon 980nm diode laser excitation. Laser power and Er3+ or Yb3+ doping concentration dependence of upconversion luminescence were studied to understand upconversion mechanisms. Excited state absorption, cross relaxation and energy transfer processes are the possible mechanisms for the visible emissions.

Photo-induced energy transfer in ruthenium-centred polymers prepared by a RAFT approach

Triplet and singlet excitation energy transfer processes are investigated in two novel di- and hexa-functionalised ruthenium centred polymers prepared by RAFT methods. The polymers consist of a ruthenium trisbipyridyl ligand complex (Ru(bpy)3) core and pendant polymer ‘arms’ of 9,10-diphenylanthracene (DPA). Steady state and transient emission and absorption experiments are used to investigate intramolecular energy transfer processes following photoexcitation of Ru(bpy)3 and DPA. Triplet energy transfer from the Ru(bpy)3 core to pendant DPA chromophores in the polymer arms is found to be more efficient than the reverse singlet energy transfer process, attributed in part to the long triplet lifetime of Ru(bpy)3 and the polymer structure. A consequence is that intra-polymer chain triplet energy transfer from Ru(bpy)3 to DPA chromophores followed by triplet–triplet annihilation and delayed fluorescence from the DPA can be observed.

Photoluminescence study of Y2O3:Er3+-Eu3+-Yb3+ phosphor for lighting and sensing applications

The Er3+, Eu3+, and Yb3+ codoped Y2O3 phosphors have been synthesized by combustion synthesis process. For the structural information, the XRD analysis of the developed phosphor has been done. The frequency upconversion (UC) emissions in the codoped Y2O3 phosphor on excitation with 980 nm diode laser in the visible region have been performed and explained on the basis of excited state absorption and energy transfer process. The mechanism responsible in UC emissions was observed to involve two photon absorption and efficiency of the UC luminescence is significantly enhanced by introducing the Yb3+ ions. The tunability in colour of emitted radiation has been visualized by chromaticity diagram on increasing power of excitation source. The temperature sensing behaviour of developed phosphor material has been investigated using fluorescence intensity ratio technique.

Structural and luminescence studies on Er3+/Yb3+ co-doped boro-tellurite glasses

The present work reports structural and optical behaviour of the Er3+/Yb3+ co-doped boro-tellurite glasses 50TeO2+15H3BO3+(5−x−y)La2O3+15MgCO3+15SrCO3+xEr2O3+yYb2O3 (where x=1, y=0, 0.5, 1 and 2 in wt%) studied through Fourier transform infrared (FTIR), Raman, absorption, luminescence, upconversion luminescence and lifetime measurements for green laser applications. The TeOTe, O3BOBO3 bond bending vibrations and the fundamental stretching vibration of OH groups are confirmed through FTIR and Raman spectra. Through the absorption spectra, bonding parameters, oscillator strengths and Judd–Ofelt (JO) parameters were calculated and reported. From the luminescence spectra, radiative parameters have been calculated for the 2H9/2→4I15/2,4S3/2→4I15/2 and 4I13/2→4I15/2 excited state transitions. Upconversion luminescence spectra of the prepared glasses have been recorded and the excited state absorption (ESA) and Energy transfer (ET) processes are discussed. The luminescence decay curves for the 4I13/2→4I15/2 transition of the title glasses under 980 excitation wavelength have also been studied and reported.

Energy and Electron Transfer of One-Dimensional Nanomaterials of Perylenediimides

This review describes the photodriven energy- and electron-transfer processes of the columnar perylenediimide (PDI) nanostructures for efficient light-energy conversion by employing the self-assembly models. The charge separation and the directional charge transport of the oligomers, aggregates, and one-dimensional (1D) nanostructures of PDIs have been clarified by using time-resolved transient absorption techniques. Excited states, exciton migration, and excitation energy-transfer processes were examined in the first part. The second part involves the electron hopping, photodriven electron transfer and transport events taking place within the various PDI nanostructures.

3P253 光化学系IIにおける励起エネルギーと電子の輸送過程に関する階層的粗視化運動論モデル(18B.光生物:光合成,ポスター,日本生物物理学会年会第51回(2013年度))

3P253 光化学系IIにおける励起エネルギーと電子の輸送過程に関する階層的粗視化運動論モデル(18B.光生物:光合成,ポスター,日本生物物理学会年会第51回(2013年度))

Excitation energy migration processes in various multi-porphyrin assemblies

The electronic interactions and excitation energy transfer (EET) processes of a variety of multi-porphyrin arrays with linear, cyclic and box architectures have been explored. Directly meso-meso linked linear arrays (Z(N)) exhibit strong excitonic coupling with an exciton coherence length of approximately 6 porphyrin units, while fused linear arrays (T(N)) exhibit extensive π-conjugation over the whole array. The excitonic coherence length in directly linked cyclic porphyrin rings (CZ(N)) was determined to be approximately 2.7 porphyrin units by simultaneous analysis of fluorescence intensities and lifetimes at the single-molecule level. By performing transient absorption (TA) and TA anisotropy decay measurements, the EET rates in m-phenylene linked cyclic porphyrin wheels C12ZA and C24ZB were determined to be 4 and 36 ps(-1), respectively. With increasing the size of C(N)ZA, the EET efficiencies decrease owing to the structural distortions that produce considerable non-radiative decay pathways. Finally, the EET rates of self-assembled porphyrin boxes consisting of directly linked diporphyrins, B1A, B2A and B3A, are 48, 98 and 361 ps(-1), respectively. The EET rates of porphyrin boxes consisting of alkynylene-bridged diporphyrins, B2B and B4B, depend on the conformation of building blocks (planar or orthogonal) rather than the length of alkynylene linkers.

Synthesis and luminescence properties of La2Ti2O7:Er3+ nanocrystals with pyrochlore structure

Er3+-doped La2Ti2O7 nanocrystals with pyrochlore structure are prepared by sol–gel method and examined by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV–vis spectrophotometer. An intense green emission along with a red emission is observed under the excitation at 980nm. La2Ti2O7:Er3+ exhibits the intense green emission bands around 525 and 549nm, and the weak red emission bands around 660nm. The optimal annealing temperature is 1000°C and the appropriate doping concentration of Er3+ is 13at.%. Power dependence analysis reveals that the green and red emissions originate from a two-photon absorption process. Excited state absorption and energy transfer processes are discussed as the possible upconversion mechanisms.

All-Atom Semiclassical Dynamics Study of Quantum Coherence in Photosynthetic Fenna–Matthews–Olson Complex

Although photosynthetic pigment-protein complexes are in noisy environments, recent experimental and theoretical results indicate that their excitation energy transfer (EET) can exhibit coherent characteristics for over hundreds of femtoseconds. Despite the almost universal observations of the coherence to some degree, questions still remain regarding the detailed role of the protein and the extent of high-temperature coherence. Here we adopt a theoretical method that incorporates an all-atom description of the photosynthetic complex within a semiclassical framework in order to study EET in the Fenna-Matthews-Olson complex. We observe that the vibrational modes of the chromophore tend to diminish the coherence at the ensemble level, yet much longer-lived coherences may be observed at the single-complex level. We also observe that coherent oscillations in the site populations also commence within tens of femtoseconds even when the system is initially prepared in a non-oscillatory stationary state. We show that the protein acts to maintain the electronic couplings among the system of embedded chromophores. We also investigate the extent to which the protein's electrostatic modulation that disperses the chromophore electronic energies may affect the coherence lifetime. Further, we observe that even though mutation-induced disruptions in the protein structure may change the coupling pattern, a relatively strong level of coupling and associated coherence in the dynamics still remain. Finally, we demonstrate that thermal fluctuations in the chromophore couplings induce some redundancy in the coherent energy-transfer pathway. Our results indicate that a description of both chromophore coupling strengths and their fluctuations is crucial to better understand coherent EET processes in photosynthetic systems.

Effects of Mo on phase structure and up-conversion emissions of Er:Al2O3 nanocrystals

A simple and efficient approach was presented to enhance up-conversion emissions significantly for the Er:Al2O3 nanocrystals by Mo support (Er-Mo:Al2O3) with a 976 nm laser diode excitation. Mo support had evident effects on the phase structure and up-conversion emissions for the Er:Al2O3 nanocrystals, which promoted the θ-(Al,Er)2O3 transformed to α-(Al,Er,Mo)2O3 phase. Compared with the Er:Al2O3, the maximal green and red up-conversion emissions intensities increased about 3×103 and 1.4×102 times for the Er-Mo:Al2O3 nanocrystals, respectively. It suggests that the enhancement of up-conversion emissions is caused by the high excited state energy transfer process from |4I15/2, 3T2> state of the Er3+-MoO42− dimer to the 4F7/2 level of Er3+.

Structural, thermal, and optical properties of Pr3+/Yb3+ co-doped oxyhalide tellurite glasses and its nano-crystalline parts

Nano-crystalized glass-ceramics and ceramics co-doped with Pr3+/Yb3+ have been successfully prepared by heat treatment of the precursor oxyhalide glass sample synthesized by melt-quench method. Thermal measurement of glass sample shows its glass transition temperature (Tg), and onset crystallization temperature (Tc). Formation of nano-crystals was verified by X-ray diffraction patterns and transmission electron microscopy (TEM) images. Values of optical band gap (Eopt) were found to be 2.85 eV and 1.54 eV for direct and indirect transitions, respectively. Up and downconversion emission spectra recorded with 976 and 532 nm laser wavelengths show emission from Pr3+ ions. Emission of Pr3+ enhances approximately thirty times when Yb3+ ion is added with the matrix. The Stark splitting and the intensity of different emission bands increase effectively when we approach to ceramic via glass-ceramic sample. Intense upconversion emission observed by naked eyes has been quantified in terms of standard chromaticity diagram (CIE). Power dependence study shows that the upconversion of NIR radiation to visible takes place mainly via photon avalanche (PA) process. Upconversion mechanisms have been discussed in terms of excited state absorption (ESA) and energy transfer (ET) processes.

PH Control of Intramolecular Energy Transfer and Oxygen Quenching in Ru(II) Complexes Having Coupled Electronic Excited States

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 μs at pH 0 to >10 μs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.

Resonant energy transfer assisted by off-diagonal coupling

Dynamics of resonant energy transfer of a single excitation in a molecular dimer system are studied in the simultaneous presence of diagonal and off-diagonal exciton-phonon coupling. It is found that, at given temperatures, the off-diagonal coupling can enhance both the coherence of the resonant energy transfer and the net quantity of energy transferred from an initially excited monomer to the other. Also studied is the dynamics of entanglement between the dimer system and the phonon bath as measured by the von Neumann entanglement entropy, and the inter-monomer entanglement dynamics for the excitonic system.