Efficient Excitation Energy Transfer Research Articles

Quantitative Evaluation of Site Energies and Their Fluctuations of Pigments in the Fenna-Matthews-Olson Complex with an Efficient Method for Generating a Potential Energy Surface.

We develop an efficient method to generate an accurate semiglobal potential energy surface of a molecule in condensed phases with low computational cost. We apply the method to the calculation of the site energies and their fluctuations of bacteriochlorophyll (BChl) a pigments in the Fenna-Matthews-Olson (FMO) complex using the density functional properly describing the ground and excited states of BChl a in solutions in our previous work (J. Phys. Chem. B 2014, 118, 10906-10918). The errors of the potential energies calculated from the present and QM/MM methods are small: ∼1 kcal/mol for both the ground and excited states. The calculated site energies are in good agreement with the experimentally fitted results. The calculated spectral density also agrees with the experimentally available data. The spectral densities of BChl 2 and BChl 5 are much larger than those of the other five sites. The present method is expected to provide new insights into the efficient excitation energy transfer in light-harvesting antennas.

Role of Yb(3+) ions on enhanced ~2.9 μm emission from Ho(3+) ions in low phonon oxide glass system.

The foremost limitation of an oxide based crystal or glass host to demonstrate mid- infrared emissions is its high phonon energy. It is very difficult to obtain radiative mid-infrared emissions from these hosts which normally relax non-radiatively between closely spaced energy levels of dopant rare earth ions. In this study, an intense mid-infrared emission around 2.9 μm has been perceived from Ho3+ ions in Yb3+/Ho3+ co-doped oxide based tellurite glass system. This emission intensity has increased many folds upon Yb3+: 985 nm excitation compared to direct Ho3+ excitations due to efficient excited state resonant energy transfer through Yb3+: 2F5/2 → Ho3+: 5I5 levels. The effective bandwidth (FWHM) and cross-section (σem) of measured emission at 2.9 μm are assessed to be 180 nm and 9.1 × 10−21 cm2 respectively which are comparable to other crystal/glass hosts and even better than ZBLAN fluoride glass host. Hence, this Ho3+/Yb3+ co-doped oxide glass system has immense potential for the development of solid state mid-infrared laser sources operating at 2.9 μm region.

Supramolecular Polymeric Fluorescent Nanoparticles Based on Quadruple Hydrogen Bonds

A facile approach for the preparation of supramolecular polymer‐based fluorescent nanoparticles (FNPs) is reported. FNPs with homogeneous shape and size distribution are fabricated from low‐molecular‐weight molecules, and thus, different compositional constituents can be efficiently incorporated via copolymerization. The emission color of the FNPs covers a wide region from blue to near infrared and can be easily tuned using efficient excitation energy transfer. The photoswitchable fluorescent nanoparticles with high on–off fluorescence contrast are also simply prepared by copolymerization of monomers containing a fluorophore and a photochromic unit. Our FNPs are successfully applied in living cell imaging and as fluorescent inks.

Protein-Framed Multi-Porphyrin Micelles for a Hybrid Natural-Artificial Light-Harvesting Nanosystem.

A micelle-like hybrid natural-artificial light-harvesting nanosystem was prepared through protein-framed electrostatic self-assembly of phycocyanin and a four-armed porphyrin star polymer. The nanosystem has a special structure of pomegranate-like unimolecular micelle aggregate with one phycocyanin acceptor in the center and multiple porphyrin donors in the shell. It can inhibit donor self-quenching effectively and display efficient transfer of excitation energy (about 80.1 %) in water. Furthermore, the number of donors contributing to a single acceptor could reach as high as about 179 in this nanosystem.

Protein‐Framed Multi‐Porphyrin Micelles for a Hybrid Natural–Artificial Light‐Harvesting Nanosystem

AbstractA micelle‐like hybrid natural–artificial light‐harvesting nanosystem was prepared through protein‐framed electrostatic self‐assembly of phycocyanin and a four‐armed porphyrin star polymer. The nanosystem has a special structure of pomegranate‐like unimolecular micelle aggregate with one phycocyanin acceptor in the center and multiple porphyrin donors in the shell. It can inhibit donor self‐quenching effectively and display efficient transfer of excitation energy (about 80.1 %) in water. Furthermore, the number of donors contributing to a single acceptor could reach as high as about 179 in this nanosystem.

Excitation dynamics and structural implication of the stress-related complex LHCSR3 from the green alga Chlamydomonas reinhardtii

LHCSR3 is a member of the Light-Harvesting Complexes (LHC) family, which is mainly composed of pigment-protein complexes responsible for collecting photons during the first steps of photosynthesis. Unlike related LHCs, LHCSR3 is expressed in stress conditions and has been shown to be essential for the fast component of photoprotection, non-photochemical quenching (NPQ), in the green alga Chlamydomonas reinhardtii. In plants, which do not possess LHCSR homologs, NPQ is triggered by the PSBS protein. Both PSBS and LHCSR3 possess the ability to sense pH changes but, unlike PSBS, LHCSR3 binds multiple pigments. In this work we have analyzed the properties of the pigments bound to LHCSR3 and their excited state dynamics. The data show efficient excitation energy transfer between pigments with rates similar to those observed for the other LHCs. Application of an exciton model based on a template of LHCII, the most abundant LHC, satisfactorily explains the collected steady state and time-resolved spectroscopic data, indicating that LHCSR3 has a LHC-like molecular architecture, although it probably binds less pigments. The model suggests that most of the chlorophylls have similar energy and interactions as in LHCII. The most striking difference is the localization of the lowest energy state, which is not on the Chlorophyll a (Chl a) 610-611-612 triplet as in all the LHCB antennas, but on Chl a613, which is located close to the lumen and to the pH-sensing region of the protein.

Highly efficient energy transfer from quantum dot to allophycocyanin in hybrid structures

Excitation energy transfer (EET) is observed in hybrid structures that composed of allophycocyanin and CdSe/ZnS core-shell quantum dot (QD). We demonstrate that the EET efficiency in such systems could be significantly increased under conditions inducing monomerization of allophycocyanin trimers. For these purposes, the EET efficiency was estimated under different experimental conditions (pH, high temperature or the presence of NaSCN) for self-assembled hybrid structures. Additionally, the hybrid structures were stabilized by covalent coupling which resulted in approximately 20-fold enhancement of allophycocyanin fluorescence upon excitation of QDs. The observed effect provides new opportunities for the practical implementation of hybrid systems as fluorescent markers.

(Invited) Application of Ethane-Bridged Bis-Porphyrin Structural Motif for Effective Supramolecular Chirogenesis, Sensor Development, and Light Harvesting Systems

Porphyrin and multi-porphyrin systems play a vital role in various natural processes and in artificial assemblies resulting in various practical applications in the numerous research and technoligical fields such as catalysis and nonlinear optics, polymer and materials science, light harvesting and different sensors, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination. Recently, it was found that a simple ethane-bridged bis-porphyrin motif, 1 (M = Zn, Mg, Cu, 2H), can be effectively used for various application purposes owing to the specific molecular and supramolecular functionality. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of ethane linkage resulting in the environmentally assisted syn-to-anti conformational switching (for monodentate guests) and tweezer structure formation (for bidentate guests), which can be effectively employed in different areas.1-10 For example, in the case of monodentate chiral guests there is the unidirectional screw formation in the anti form of 1 (M = Zn, Mg, 2H) upon the corresponding host-guest interaction. The direction of screw governs by the size difference of ligand’s substituents at the chirogenic center resulting in chirality transfer to the bis-porphyrin host and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin.1-6 This chirogenic phenomenon was efficiently utilized for determining the absolute configuration of various chiral guests, thus making it possible to apply 1 as effective and universal chirality sensors for different types of organic molecules as in solution and in solid state. To expand further the applicability of these supramolecular systems Langmuir–Schaefer thin-films on the basis of 1 have been prepared and employed as highly selective and sensitive sensors for aromatic amines.7-9 For example, the monometallated copper complex of 1 is able to detect the presence of aniline in aqueous solution at a concentration as low as 1 nM.9 The sensory mechanism includes the corresponding host-guest interaction resulting in the syn-to-anti conformational switching, which can be easily detected by various spectroscopic methods. Another type of functional materials on the basis of the syn-anti flexibility of 1 (M = Zn) was nanoparticles.10 It was found that upon encapsulation into the semiconducting poly (9-vinylcarbazole) (PVK) polymer the conformation is switched to the anti form and the subsequent efficient excited energy transfer (up to 96%) from the PVK host to the energy acceptor, 1, was observed, thus opening further prospects in designing a new class of porphyrin-based functional nanoparticles for the application in effective light harvesting system. Consideration of the 12 Principles of Green Chemistry for supramolecular and chiroptical devices will also be highlighted.11 The authors would like to acknowledge funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 621364 (TUTIC-Green). V. Borovkov, Symmetry 2014, 6, 256.V. Borovkov, Symmetry 2010, 2, 184.V. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189.G. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.G. Giancane, V. Borovkov, Y. Inoue, L. Valli, J. Colloid Interface Sci . 2012, 385, 282.G. Giancane, V. Borovkov, Y. Inoue, S. Conoci, L. Valli, Soft Matter 2013, 9, 2302.S. Bettini, E. Maglie, R. Pagano, V. Borovkov, Y. Inoue, L. Valli, G. Giancane, Beilstein J. Nanotechnol. 2015, in press.S. Mandal, S. Bhattacharyya, V. Borovkov, A. Patra, J. Phys. Chem. C 2011, 115, 24029.P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, ch. 1, p. 3.

Is There Excitation Energy Transfer between Different Layers of Stacked Photosystem-II-Containing Thylakoid Membranes?

We have compared picosecond fluorescence decay kinetics for stacked and unstacked photosystem II membranes in order to evaluate the efficiency of excitation energy transfer between the neighboring layers. The measured kinetics were analyzed in terms of a recently developed fluctuating antenna model that provides information about the dimensionality of the studied system. Independently of the stacking state, all preparations exhibited virtually the same value of the apparent dimensionality, d = 1.6. Thus, we conclude that membrane stacking does not affect the efficiency of the delivery of excitation energy toward the reaction centers but ensures a more compact organization of the thylakoid membranes within the chloroplast and separation of photosystems I and II.

Multichromophoric π-Conjugation: Modular Design for Gated and Cascade Energy Transfer.

Multichromophore arrays allow for cascade energy transfer. As an isoelectronic analogue of indacenyl, bis(triazolo)benzene features a fused tricyclic skeleton that rigidly places two π-extended triazoles in close proximity. Such triazole-based fluorophores behave as electronically independent modules in the ground states, but become tightly coupled upon photoexcitation for highly efficient excitation energy transfer (EET) that can be gated by external stimuli. Taking this donor-acceptor fluorophore system a step further, we have designed and implemented a cascade EET. Here, the initial excitation takes part in a circular relay to arrive at the longest-wavelength emitting site as the final destination. Modularly constructed triazoloarenes should serve as versatile platforms for chemically controlled optical signaling.

Spectral characteristics and energy transfer from Ce3+ to Tb3+ in compounds Lu1 – x – y Ce x Tb y BO3

The structure, IR absorption spectra, morphology, and spectral characteristics of compounds Lu1 – x – y Ce x Tb y BO3 have been investigated. It has been shown that the Tb3+ luminescence excitation spectrum of the Lu1 – x – y Ce x Tb y BO3 compounds is dominated by a broad band coinciding with the excitation band of Ce3+ ions, which clearly indicates energy transfer from the Ce3+ ions to the Tb3+ ions. The spectral position of this band depends on the structural state of the sample: in the structures of calcite and vaterite, the band has maxima at ~339 and ~367 nm, respectively. By varying the ratio between the calcite and vaterite phases in the sample, it is possible to purposefully change the Tb3+ luminescence excitation spectrum, which is important for the optimization of the spectral characteristics of Lu1 – x – y Ce x Tb y BO3 when it is used in light-emitting diode sources. An estimate has been obtained for the maximum distance between Ce3+ and Tb3+ ions, which corresponds to electronic excitation energy transfer. It has been shown that the high intensity of Tb3+ luminescence in these compounds is due to the high efficiency of electronic excitation energy transfer from the Ce3+ ions to the Tb3+ ions as a result of the dipole–dipole interaction.

An artificial photosynthetic model based on a molecular triad of boron dipyrromethene and phthalocyanine.

A boron dipyrromethene (BDP) unit and its monostyryl derivative (MSBDP) were introduced at the axial positions of a silicon(iv) phthalocyanine (SiPc) core. The absorption spectrum of this compound virtually covered the entire visible region (300-700 nm) and could be interpreted as a superposition of the spectra of individual components. The intramolecular photoinduced energy and charge transfer processes of this triad were studied using steady-state and time-resolved spectroscopic methods in polar and nonpolar solvents. Upon BDP-part excitation, a fast and highly efficient excitation energy transfer (EET) occurred resulting in strong quenching of its fluorescence and the formation of the first excited singlet state of SiPc or MSBDP. It was found that both EET and charge transfer (CT) processes competed with each other in the depopulation of the first excited singlet state of the MSBDP moiety. The former strongly superseded CT in nonpolar toluene, whereas the latter was dominant in a polar environment. Direct or indirect (via EET) excitation of the SiPc-part of the triad was followed by CT yielding the charge-separated (CS) species BDP-SiPc(˙-)-MSBDP(˙+). The energy gap between the CS state and the S1-state of the SiPc moiety was found to be only 0.06 eV in toluene, which facilitated the back CT process and resulted in the appearance of thermally activated delayed fluorescence. With increasing solvent polarity, the energy of the CS state reduced resulting in the disappearance of the delayed fluorescence in CHCl3, tetrahydrofuran or N,N-dimethylformamide. The charge recombination rate, k(CR), was very fast in polar DMF (3.3 × 10(10) s(-1)), whereas this process was two-orders of magnitude slower in nonpolar toluene (k(CR) = 4.0 × 10(8) s(-1)).

Rapid method for evaluating the efficiency of excitation energy transfer between ytterbium and erbium ions in an active fiber preform

A rapid method is proposed to evaluate the efficiency of excitation energy transfer between ytterbium and erbium ions from measured relative luminescence intensity in different spectral regions in an active fiber preform, taking into account the concentration of active ions and their spectroscopic constants.

Photon Harvesting in Sunscreen‐Based Functional Nanoparticles

The ultraviolet light component in the solar spectrum is known to cause several harmful effects, such as allergy, skin ageing, and skin cancer. Thus, current research attention has been paid to the design and fundamental understanding of sunscreen-based materials. One of the most abundantly used sunscreen molecules is Avobenzone (AB), which exhibits two tautomers. Here, we highlight the preparation of spherically shaped nanoparticles from the sunscreen molecule AB as well as from sunscreen-molecule-encapsulated polymer nanoparticles in aqueous media and study their fundamental photophysical properties by steady-state and time-resolved spectroscopy. Steady-state studies confirm that the AB molecule is in the keto and enol forms in tetrahydrofuran, whereas the enol form is stable in the case of both AB nanoparticles and AB-encapsulated poly(methyl methacrylate) (PMMA) nanoparticles. Thus, the keto-enol transformation of AB molecules is restricted to a nanoenvironment. An enhancement of photostability in both the nanoparticle and PMMA-encapsulated forms under UV light irradiation is observed. The efficient excited energy transfer (60 %) from AB to porphyrin molecules opens up further prospects in potential applications as light-harvesting systems.

Robustness, efficiency, and optimality in the Fenna-Matthews-Olson photosynthetic pigment-protein complex.

Pigment-protein complexes (PPCs) play a central role in facilitating excitation energy transfer (EET) from light-harvesting antenna complexes to reaction centres in photosynthetic systems; understanding molecular organisation in these biological networks is key to developing better artificial light-harvesting systems. In this article, we combine quantum-mechanical simulations and a network-based picture of transport to investigate how chromophore organization and protein environment in PPCs impacts on EET efficiency and robustness. In a prototypical PPC model, the Fenna-Matthews-Olson (FMO) complex, we consider the impact on EET efficiency of both disrupting the chromophore network and changing the influence of (local and global) environmental dephasing. Surprisingly, we find a large degree of resilience to changes in both chromophore network and protein environmental dephasing, the extent of which is greater than previously observed; for example, FMO maintains EET when 50% of the constituent chromophores are removed, or when environmental dephasing fluctuations vary over two orders-of-magnitude relative to the in vivo system. We also highlight the fact that the influence of local dephasing can be strongly dependent on the characteristics of the EET network and the initial excitation; for example, initial excitations resulting in rapid coherent decay are generally insensitive to the environment, whereas the incoherent population decay observed following excitation at weakly coupled chromophores demonstrates a more pronounced dependence on dephasing rate as a result of the greater possibility of local exciton trapping. Finally, we show that the FMO electronic Hamiltonian is not particularly optimised for EET; instead, it is just one of many possible chromophore organisations which demonstrate a good level of EET transport efficiency following excitation at different chromophores. Overall, these robustness and efficiency characteristics are attributed to the highly connected nature of the chromophore network and the presence of multiple EET pathways, features which might easily be built into artificial photosynthetic systems.

Strong nonlinear optical phosphorescence from water-soluble polymer dots: Towards the application of two-photon bioimaging

Multiphoton fluorescence imaging of tissues offers an essential tool for studying biological systems. However, autofluorescence becomes a serious issue in complicated tissue imaging where biological molecules compete with the fluorophore. Such a critical issue can be resolved through the design of phosphorescent molecules with long emission lifetime. In this work, the nonlinear optical properties of small phosphorescent polymer dots (Pdots) with strong red-emitting phosphorescence (620 nm) have been disclosed. An analysis of decay data from fs pump-probe experiment yields a value of 164 ps for the singlet excited state lifetime of the Pdots. Notably, thanks to efficient two-photon excited energy transfer and moderate phosphorescence quantum yield (η = 1.36%), the Pdots exhibit large two-photon absorption action cross-section (ηδmax = 15 GM) in aqueous solution. Results of living cell imaging experiments show the potential of these Pdots in two-photon time-resolved phosphorescent microscopy.

Protective effect of UV-A radiation during acclimation of the photosynthetic apparatus to UV-B treatment

We examined the acclimation response of the photosynthetic apparatus of barley (Hordeum vulgare L.) to a combination of UV-A and UV-B radiation (UVAB) and to UV-B radiation alone. Our aim was to evaluate whether UV-A radiation prevents UV-B-induced damage to the photosynthetic apparatus and whether UV-A pre-acclimation is required to mitigate the negative influence of UV-B radiation. Barley plants were grown from seeds under low photosynthetically active radiation (50 μmol m−2 s−1) either in the absence or presence of UV-A radiation (UVA− and UVA+ plants, respectively). After 8 days of development, plants were exposed simultaneously to UV-A and UV-B radiation for the next 6 days. Additionally, UVA− plants were exposed to UV-B radiation alone. The UVA+ plants had a higher CO2 assimilation rate near the light-saturation region (AN) and a higher content of both total chlorophylls (Chls) and total carotenoids than the UVA− plants. Chls content, AN, the potential quantum yield of photosystem II (PSII) photochemistry (FV/FM), the capacity of light-induced thermal energy dissipation and the efficiency of excitation energy transfer within PSII remained the same or even increased in both UVA+ and UVA− plants after UVAB treatment. On the contrary, exposure of UVA− plants to UV-B radiation itself led to a reduction in all these characteristics. We revealed that the presence of UV-A radiation during UVAB treatment not only mitigated but completely eliminated the negative effect of UV-B radiation on the functioning of the photosynthetic apparatus and that UV-A pre-acclimation was not crucial for development of this UV-A-induced resistance against UV-B irradiation.

The Processes of Energy Transformation in Activated Laser Materials with Ions Er3 (Review)

In paper the features of energy transformation processes in activated with erbium ions laser materials based on optical transitions in 4f shell of Er3+ ionwere discussed. Methods of excitation of the luminescence in chalcogenide glasses doped with Er3+ ions were described and found how its intensity depends on concentration of the ions. Up-conversion and cross-relaxation play an important role in the transformation of excitations in erbium-doped materials. In cross-relaxation the energy of one center can be nonradiatively transferred to another center or divided between the two centers, while in the up-conversion, however, energy of several centers summed up in one center, bringing it acts as an additional channel of luminescence quenching, or as a way of pumping of the higher energy levels. To improve the efficiency of optical excitation of many laser materials doped with erbium ions the sensitization with ytterbium ions that have intense absorption band in the range of ~0.9-1 µm with "effective" width of about 1000 cm-1 as well a channel of the efficient nonradiative excitation energy transfer to Er3+ ions is widely used.

Noise assisted excitation energy transfer in a linear model of a selectivity filter backbone strand

In this paper, we investigate the effect of noise and disorder on the efficiency of excitation energy transfer (EET) in a sites linear chain with ‘static’ dipole–dipole couplings. In fact, here, the disordered chain is a toy model for one strand of the selectivity filter backbone in ion channels. It has recently been discussed that the presence of quantum coherence in the selectivity filter is possible and can play a role in mediating ion-conduction and ion-selectivity in the selectivity filter. The question is ‘how a quantum coherence can be effective in such structures while the environment of the channel is dephasing (i.e. noisy)?’ Basically, we expect that the presence of the noise should have a destructive effect in the quantum transport. In fact, we show that such expectation is valid for ordered chains. However, our results indicate that introducing the dephasing in the disordered chains leads to the weakening of the localization effects, arising from the multiple back-scatterings due to the randomness, and then increases the efficiency of quantum energy transfer. Thus, the presence of noise is crucial for the enhancement of EET efficiency in disordered chains. We also show that the contribution of both classical and quantum mechanical effects are required to improve the speed of energy transfer along the chain. Our analysis may help for better understanding of fast and efficient functioning of the selectivity filters in ion channels.

Surfactant-Induced Modulation of Nanometal Surface Energy Transfer from Silicon Quantum Dots to Silver Nanoparticles

In the present study, we have demonstrated the excitation energy transfer (EET) from silicon quantum dots (Si QDs) to silver nanoparticles (Ag NPs) and its modulation in the presence of cetyltrimethylammonium bromide (CTAB) surfactant by means of steady-state and time-resolved photoluminescence (PL) spectroscopy. Significant spectral overlap between the emission spectrum of Si QDs and localized surface plasmon resonance of Ag NPs results in a substantial amount of PL quenching of Si QDs. In addition, the PL lifetime of Si QDs is shortened in the presence of Ag NPs. The origin of this PL quenching has been rationalized on the basis of increased nonradiative decay rate due to excitation energy transfer from Si QDs to Ag NPs surface. The observed energy-transfer efficiency correlates well with the nanometal surface energy transfer theory with a 1/d4 distance dependence rather than conventional Forster resonance energy transfer theory. It has also been observed that the EET efficiency drastically reduces in t...