Energy transfer between fluorescent dyes in photonic crystals

Energy transfer from fluorescein (Fl) to Rhodamine B (RhB) in the opal photonic crystals has been investigated by photoluminescence. The results show that the energy transfer can be enhanced effectively by photonic bandgaps. When the fluorescence emission wavelength of donor Fl overlaps the photonic bandgap the fluorescence intensity of the donor is suppressed, while the fluorescence intensity of acceptor RhB is obviously enhanced. This enhancement can be attributed to the inhibition of radiative emission of the donor in the photonic crystals.     

Radiationless intermolecular energy transfer with participation of acceptor excited triplet states

Radiationless energy transfer between like and unlike molecules has been experimentally studied under conditions where acceptor molecules have been excited to the triplet state Homogeneous singlet-triplet-triplet migration has been discovered in highlyconcentrated chlorophyll “a” and pheophytin “a” solutions in castor oil at 183 K by measuring the variation of pigment relative quantum yields of fluorescence and triplet state formation as a function of exciting pulse intensity. Heterogeneous single-triplet-triplet energy transfer has been observed in solid solutions of different complex organic molecules (perylene + phenanthrene, Na-fluorescein+chlorophyll “a”, pyrene+Mg-phthalocyanine) as the fluorescent donor state quenching in the presence of acceptor triplet-excited molecules. Primary emphasis is placed on a direct observation of the effect of energy transfer on the excited-state lifetime of the donor. The benzophenone phosphorescence quenching (shortening of phosphorescence lifetime) in the presence of Mg-mesoporphyrin triplet molecules has been found to be caused by the heterogeneous triplet-triplet-triplet energy transfer. Good agreement of the theoretical and experimental results permits us to conclude that all types of observed transfer processes are described by the Förster-Galanin theory for dipole-dipole radiationless energy transfer with no additional assumptions.     

表面增强的分子间能量转移效应

研究了粗糙金属银表面对分子间能量转移效应的影响，实验观察到吸附分子增强的敏化荧光，结合表面局域电磁理论分析表明，吸附于银表面的分子间非辐射能量转移率被增强１０＾２倍，证实了表面增强的分子间能量转移效应的存在。     

Effect of a protein on the light energy conversion in dual fluorophore-nitroxide probes studied by ESR and fluorescence spectroscopy

Probing biological environment with dual fluorophore-nitroxide (FN) molecules in which fluorophore is tethered with nitroxide, a fluorescence quencher, opens unique opportunities to study molecular dynamics and micropolarity of the medium which affect intramolecular fluorescence quenching (IFQ), electron transfer, photoreduction and light energy conversion. In such molecules, the excited fragment of the chromophore can serve as an electron donor, and the nitroxide fragment as an electron acceptor. The same groups allow monitoring of molecular dynamics and also make it possible to measure micropolarity of the medium in the vicinity of the donor (by fluorescence technique) and acceptor (by electron spin resonance [ESR]) moieties. In the present work, two dual dansyl-nitroxide probes were incorporated in a binding site of bovine serum albumin. Their interactions with the protein, mobility, and photoreduction, as well as micropolarity of the media, have been studied by ESR and fluorescence methods. IFQ and spectral relaxation shift of the dansyl fragment have been monitored by time-resolved fluorescence technique. In parallel, computational studies on intramolecular dynamics of the FN probe were performed. On the basis of the Marcus model of the electron transfer between the excited dansyl fluorophore (donor) and nitroxide group (acceptor) and our experimental data, the mechanism of the electron transfer in the dual molecules incorporated into bovine serum albumin was established. It was shown that dual FN molecules in the protein meet main requirements for an efficient light energy conversion system.     

The effect of orientation and distance between donor and acceptor molecules on the efficiency of singlet–singlet energy transfer in Langmuir–Blodgett films

Singlet–singlet energy transfer between molecules of fluorescein and oxazine dyes in Langmuir–Blodgett films is studied experimentally. The dependence of the energy-transfer efficiency on the distance shows that the quenching of the donor fluorescence is the most efficient when the layers of the donor and acceptor molecules are in a direct contact. An increase in the distance between the donor and acceptor layers leads to a decrease in the energy-transfer efficiency. To establish the mutual orientation of the donor and acceptor molecules, quantum-chemical calculations of the energy transfer process in the donor–acceptor pair are carried out. The calculations show that the best correlation of the experimental and calculated values of the energy-transfer efficiency is observed when the interacting particles are shifted relative to each other by about ~0.12 nm in parallel planes. The presented approach can be used to estimate the relative orientation of interacting particles in multimolecular ensembles.     

Nanomembrane transfer process for intricate photonic device applications

We demonstrate a process for the fabrication and transfer of silicon nanomembranes (Si-NMs) that have been released from their host substrates and redeposited on foreign flexible or flat substrates. The transfer process developed allows intricate photonic devices to be transferred via NMs to a variety of new substrate materials. This allows the transferred devices to benefit from the material properties of both substrate and NM. Our process is designed to transfer and stack large-area photonic devices without compromising their optical performance. The process has been used to transfer large-area unpatterned silicon NMs, in excess of 2.5 cm(2), and photonic devices with intricate device designs containing various fill factors. We have also demonstrated transferred photonic crystal devices that have maintained structural integrity and functionality.     

Langmuir-Blodgett单分子膜功能体系的研究——(Ⅰ)膜系间敏化荧光的Förster机制

利用L-B技术制作L-B功能单分子膜。以两种花菁染料作给体和受体,用硬脂酸单分子膜作隔离夹层。在这种体系中的能量转移是Förster机制,有效距离为～100。     

FRET Studies of the Interaction of Dimeric Cyanine Dyes with DNA

Fluorescence Resonance Energy Transfer (FRET) is a powerful tool to determine distances between chromophores bound to macromolecules, since the efficiency of the energy transfer from an initially excited donor to an acceptor strongly depends on the distance between the two dye molecules. The structure of the noncovalent complex of double-strand DNA (dsDNA) with thiazol orange dimers (TOTO) allows FRET analysis of two intercalated chromophores. By intercalation of two different TOTO dyes we observe an energy transfer from TOTO-1 as donor and TOTO-3 as acceptor. In this manner we are able to determine the mean distance between two proximate TOTO molecules bound to dsDNA. Thus the maximum number of binding positions for this type of intercalation dyes in the dsDNA can be obtained. Furthermore the dependency of the acceptor emission on the donor concentration is analysed. The emission of TOTO-3 reaches a maximum when the acceptor-to-donor ratio is 1:10.     

Analysis of ultra-small photonic large scale integrated circuits

A detailed study of a platform of ultra-small photonic large-scale integrated circuits was conducted. Bandgap structure calculations of silicon-on-insulator (SOI) based photonic crystals have been investigated. The photonic crystal consists of dielectric cylinders in air. Using the band structure calculations we obtained design parameters for the proposed structures. The coupling between the photonic crystal and a waveguide fabricated from SOI system has been analysed. It is shown that the optical coupling is improved by interfacing different types of spot-size converters (SSCs) between the SOI waveguide and the photonic crystal. Also, the possibility and limitations of silicon doped germanium and SOI photonic crystals to analyse the light guiding in the third dimension is discussed.     

Nonradiative and radiative Förster energy transfer between quantum dots

We theoretically study nonradiative and radiative energy transfer between two localized quantum emitters, a donor (initially excited) and an acceptor (receiving the excitation). The rates of nonradiative and radiative processes are calculated depending on the spatial and spectral separation between the donor and acceptor states and for different donor and acceptor lifetimes for typical parameters of semiconductor quantum dots. We find that the donor lifetime can be significantly modified only due to the nonradiative Förster energy transfer process at donor–acceptor separations of approximately 10 nm (depending on the acceptor radiative lifetime) and for the energy detuning not larger than 1–2 meV. The efficiency of the nonradiative Förster energy transfer process under these conditions is close to unity and decreases rapidly with an increase in the donor–acceptor distance or energy detuning. At large donor–acceptor separations greater than 40 nm, the radiative corrections to the donor lifetime are comparable with nonradiative ones but are relatively weak.     

Two-Photon Excited Fluorescence Energy Transfer: A Study Based on Oligonucleotide Rulers

The use of two-photon excitation of fluorescence for detection of fluorescence resonance energy transfer (FRET) was studied for a selected fluorescent donor–acceptor pair. A method based on labeled DNA was developed for controlling the distance between the donor and the acceptor molecules. The method consists of hybridization of fluorescent oligonucleotides to a complementary single-stranded target DNA. As the efficiency of FRET is strongly distance dependent, energy transfer does not occur unless the fluorescent oligonucleotides and the target DNA are hybridized. A high degree of DNA hybridization and an excellent FRET efficiency were verified with one-photon excited fluorescence studies. Excitation spectra of fluorophores are usually wider in case of two-photon excitation than in the case of one-photon excitation [1]. This makes the selective excitation of donor difficult and might cause errors in detection of FRET with two-photon excited fluorescence. Different techniques to analyze the FRET efficiency from two-photon excited fluorescence data are discussed. The quenching of the donor fluorescence intensity turned to be the most consistent way to detect the FRET efficiency. The two-photon excited FRET is shown to give a good response to the distance between the donor and the acceptor molecules.     

Nature of the Electron Excited States and Energy Transfer in Bichromophore Coumarin Molecules

Results of experimental and theoretical research for three bichromophore molecules, trans-stilbene-CH₂-coumarin 120 (I), 4-methylumbelliferone-CH₂-UC 17, and 4-(3-fluoro)-methylumbelliferone-CH₂-UC 17 (II, III), are presented. Schemes of photophysical processes in the bichromophore molecules based on quantum chemical calculations by the INDO method and theory of radiationless transitions in polyatomic organic molecules are suggested. After optical excitation to the strong donor absorption band, the fast internal conversion processes develop there. As a result, the molecule is found in the S ₁ ^* -state localized on the acceptor moiety. It is shown that a mechanism of intramolecular transfer energy in bichromophores different from that proposed by Förster may be realized. Excitation energy, initially located on D, will be transferred from the donor moiety to the acceptor chromophore in convenience of the internal conversion process. The intramolecular electronic energy transfer from energy donor to energy acceptor may be interpreted as the internal conversion process. The rate constants of internal conversion are calculated.     

Förster-type energy transfer mechanism in PF2/6 to MEH-PPV conjugated polymers

Energy transfer mechanism between Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF_2/6) as a donor and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an acceptor in different solvents has been studied using steady-state emission measurements. Four different solvents namely, tetrahydrofuran (THF), toluene, chlorobenzene (C.B) and benzene have been used in this study. The absorption and luminescence behaviors of the samples are measured at a fixed concentration of donor (0.1 μM) while the concentrations for acceptor are kept in the range of 0.1–1.0 μM. Based on these measurements, the energy transfer properties namely quenching rate constant (k_SV), energy transfer rate constant (k_ET), energy transfer probability (P_DA), transfer efficiency (η) and critical distance of energy transfer (R_o) are calculated. The use of THF resulted in the highest energy transfer. Long range dipole–dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer as proven by the large values of R_o.     

A Theory of Electronic Energy Transfer in Complex Molecular Systems

The Förster theory for energy transfer is critically reviewed in the context of the present-day theory of nonradiative transitions, and the fundamental tenets of the Förster theory are shown to be erroneous. A new theory of electronic energy transfer is constructed taking into account the electronic transition theory. The intermolecular interaction between donor and acceptor molecules is assumed to perturb electron states of isolated molecules before the donor-molecule excitation. A distinguishing characteristic of the intermolecular interaction is spatial delocalization of the wave functions of electron states of the interacting molecules. It is this fact that has made it possible to realize ordinary photophysical processes between electron states of various molecules in a bimolecular system. In the experiments under study, the result of the intermolecular nonradiative photoprocess is given as evidence of electronic energy transfer from the donor molecule to the acceptor molecule.     

Isotope effect,energy gap law and temperature effect in resonance energy transfer

The purpose of this paper is to derive an expression for the energy transfer probability that differs from that of Förster and Dexter in that it can be used explicitly for discussing the effects of deuteration and temperature on the rate of energy transfer, and the relation between the energy gap of the donor and acceptor molecules and the rate of energy transfer. It is found that, as in radiationless transitions, the temperature dependence of the rate of energy transfer can be expressed as

and effects of isotope substitution, and the energy gap between the excited donor and unexcited acceptor molecules on the energy transfer probability, are determined mainly by the Franck-Condon factor.     

The study of electro-optical sensor based on slotted photonic crystal waveguide

A compact and sensitive electro-optical sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. The electro-optical sensor can be realized in photonic crystal (PhC) slabs of silicon in Silicon-on-Insulator (SOI). Nonlinear optical polymer is used as infiltration. By applying three-dimensional finite difference time domain (3D-FDTD), the sensitivity and quality factor of electro-optical sensor with different slotted waveguide width are calculated. In addition, sensitivity and the optical properties such as transmission spectrum and field distributions are compared between electro-optical sensor based on line defect photonic crystal waveguide (W1-PhCW) and that based on slotted photonic crystal waveguide (S-PhCW). Simulation results demonstrate that, compared with electro-optical sensor based on line defect photonic crystal waveguide, the sensitivity and quality factor is improved by 30 times and 6.6 times respectively in sensor based on slotted photonic crystal waveguide. Besides, the proposed PhC sensor devices have the advantage of a compact structure with the potential for monolithic integration with optical-to-electrical on-chip conversion and detection.     

Optical properties of one-dimensional photonic crystals obtained by micromatchining silicon (a review)

The theoretical and experimental investigations of photonic band gaps in one-dimensional photonic crystals created by micromatchining silicon, which have been performed by the author as part of his doctoral dissertation, are presented. The most important result of the work is the development of a method of modeling photonic crystals based on photonic band gap maps plotted in structure–property coordinates, which can be used with any optical materials and in any region of electromagnetic radiation, and also for nonperiodic structures. This method made it possible to realize the targeted control of the optical contrast of photonic crystals and to predict the optical properties of optical heterostructures and three-component and composite photonic crystals. The theoretical findings were experimentally implemented using methods of micromatchining silicon, which can be incorporated into modern technological lines for the production of microchips. In the IR spectra of a designed and a fabricated optical heterostructure (a composite photonic crystal), extended bands with high reflectivities were obtained. In a Si-based three-component photonic crystal, broad transmission bands and photonic band gaps in the middle IR region have been predicted and experimentally demonstrated for the first time. Si–liquid crystal periodic structures with electric-field tunable photonic band-gap edges have been investigated. The one-dimensional photonic crystals developed based on micromatchining silicon can serve as a basis for creating components of optical processors, as well as highly sensitive chemical and biological sensors in a wide region of the IR spectrum (from 1 to 20 μm) for lab-on-a-chip applications.     

Electronic States and Photoprocesses in Bichromophore Systems

We present the results of quantumchemical investigation of energy transfer in organic molecules and systems and the inferences drawn. The Förster theory has been subjected to a critical analysis in order that the energy transfer could be described in the context of the current theory of nonradiative transitions and the incorrectness of the basic premises of the Förster theory has been demonstrated. A new variant of the mechanism of electronic energy transfer on the basis of the theory of electron transitions and of the quantum mechanics of molecules has been suggested. It is shown that the interaction of the molecules of the donor and acceptor perturbs the electronic states of isolated molecules even before the excitation of the donor molecule. A characteristic feature of the manifestation of intermolecular interaction is the spatial delocalization of the wave functions of the electronic states of interacting molecules, leading to the possibility of occurrence of conventional photophysical processes with participation of the electronic states of various molecules of the bimolecular system. In experimental investigations, the result of the intermolecular nonradiative transition is recorded as evidence of the spatial transfer of the energy of electronic excitation from the donor molecule to the acceptor molecule.     

Evidence of resonance energy transfer in molecular bilayers on Al2O3 (0 0 0 1)

Energy transfer between aromatic molecules was observed on a dielectric surface. The donor and acceptor molecules were vacuum deposited as bilayers and the surface temperature was linearly ramped in a temperature programmed desorption experiment (TPD). During the TPD procedure, the luminescence of the surface molecules was monitored as a function of temperature. Close to the desorption temperature of the lower layer, mixing of the layers occurred that resulted in energy transfer.     

Spectral Photosensitization of Optical Anisotropy in Solid Poly(Vinyl Cinnamate) Films

The possibility and features of formation of sensitized photoinduced optical anisotropy in amorphous films of poly(vinyl cinnamate) and its derivative poly(vinyl-4-metoxicinnamate) under the action of polarized light (including light that is not absorbed by polymer macromolecules themselves) have been investigated. It is found that the effect of induced optical anisotropy is based on the transfer of electron excitation energy from donor (sensitizer) molecules to acceptor molecules and is observed in the course of phototopochemical biomolecular cyclization reaction of cinnamate fragments in polymer macromolecules. The detected photoinduced anisotropy in solid films of poly(vinyl cinnamate) and its derivative poly(vinyl-4-metoxicinnamate) ensures sensitized photo-orientation of low-molecular thermotropic liquid crystals.