INVESTIGATIONS ON THE FLUORESCENCE CONCENTRATION QUENCHING OF FLAVOMONONUCLEOTIDE IN GLYCERINE-WATER SOLUTIONS

We measured concentration changes of the fluorescence quantum yield eta/eta 0 of flavomononucleotide (FMN) in glycerine-water solutions of various viscosities. The results obtained show that FMN dimers are traps for the exciting light energy as well as the energy transferred non-radiatively. Very good agreement between the experimental and theoretical eta/eta 0 results was obtained. It has been shown that in the investigated solutions non-radiative energy transfer from monomers to dimers takes place, preceded by the migration of energy. It has been stated that no monomer quenching occurs in these solutions. The contribution of the individual fluorescence concentration quenching mechanisms has been determined.     

Effect of donor-acceptor interaction strength on excitation energy migration and diffusion at high donor concentrations

The migration and diffusion modulated excitation energy transfer has been studied in a new dye pair 7-diethylamino-4-methylcoumarin (donor) to 3,3'-dimethyloxacarbocyanine iodide (acceptor) by steady-state and picosecond time-resolved spectroscopy. To reduce the artifact of self-absorption, at high donor concentrations, the time-resolved studies have been carried out in thin films of polyvinyl alcohol (solid matrix) and in methanol (liquid phase) at front-face geometry of excitation. The Forster-type (nonradiative) energy transfer [Discuss. Faraday Soc. 27, 7 (1959)] takes place directly from donor to acceptor in case of solid matrix, while Yokota-Tanimoto model [J. Phys. Soc. Jpn. 22, 779 (1967)] for diffusion has been found to be operating in the liquid phase. It has been found here that the high interaction strength between donor and acceptor molecules as compared to that among donors masks the effect of energy migration and diffusion at high donor concentrations. The rate and efficiency of energy transfer increase with increasing the acceptor concentration. This has been confirmed by the study of acceptor kinetics.     

Resolution of the excitation-emission spectra of FMN in rigid poly(vinyl alcohol) matrices

The excitation-emission spectra of flavin mononucleotide (FMN) were measured in rigid PVA films for concentrations ranging from 6.92 x 10(-4)M to 1.03 M. The theoretical three-linear decomposition of the excitation-emission spectra indicated the presence of two absorption and emission centers corresponding to FMN monomer and dimer, respectively. The component of the fluorescence profile corresponding to the FMN monomer has a large negative part which is the mirror image of the emission band profile of the dimer. The elimination of this part by taking a linear combination of the emission components of the monomer and of the dimer resulted in emission spectrum, which is in a very good agreement with the monomer spectrum measured directly. The appearance of a negative part of the monomer emission profile obtained by trilinear decomposition of the emission-absorption spectra of FMN can be explained in terms of the non-radiative reverse energy transfer from the FMN dimers to the FMN monomers. The presented results confirm that the FMN molecules in rigid PVA form dimers but not higher order aggregates. Moreover, they enable to obtain fluorescence spectra of dimers and suggest that FMN dimers may take part in the process of non-radiative energy transfer occurring in photoreception phenomena.     

Experimental and theoretical study on molecular aggregation and its effect on the photo-physical properties of the mesogenic bi-1,3,4-thiadiazole derivative

A bi-thiadiazole derivative, 5,5'-bis(4- tetradecyloxyphenyl)-2,2'-bi-1,3,4-thiadiazole (BTD-14), was revealed to exhibit an extremely stable thermotropic SmC phase and very interesting aggregation behavior in solutions. H- and J-aggregates could be formed simultaneously in chloroform solutions of BTD-14 with moderate concentration (10(-4) M), and the population of J-aggregates enlarges during further concentration increase. All monomers, H-aggregates and J-aggregates in solutions could be reserved in the drop-cast films, and both the presence of J-aggregates and the energy transfer path from H-aggregates to J-aggregates were considered to contribute to the relative high solid state fluorescence quantum yield (33%). The BTD-1 dimer potential energy surface (PES) was computed with M062x/6-31G** method, and the molecular packing pattern corresponding to the lowest minimum of the PES are in good agreement with the crystal structures. Exploring the effect of molecular packing on its electronic structure with the TD-M062x method revealed that J-aggregates could be formed by enlarging the intermolecular displacement along the molecular long axis by about 9.8 ?.     

Photothermal melting and energy migration in conjugated oligomer films doped with CdSe quantum dots

The effect of thin film morphology on energy transfer and migration in host-guest systems involving a phenylene-ethynylene oligomer matrix doped with colloidally prepared CdSe quantum dots is studied. Using correlated spectroscopy techniques including DSC, Raman, and temperature-dependent photoluminescence, we find that annealing the film produces continuous domain structures that enhance excitation migration by extending the excitation diffusion length. Under optical excitation, the thin films exhibit rapid melting of the host lattice, followed by resonant energy transfer to the CdSe QD guests. The ability to optically manipulate the structure and subsequently optically detect this change makes this material an important candidate for an all-optical read-write memory system.     

Effect of polymer microenvironment on excitation energy migration and transfer

Excitation energy transfer between the dye pair acriflavine (donor) to rhodamine-6G (acceptor) in various polymers [polyvinyl alcohol (PVA), cellulose acetate, and polymethyl methacrylate (PMMA)] was studied using steady-state and time-resolved fluorescence spectroscopy at room temperature. In all these polymers, at higher acceptor concentrations, direct energy transfer from acriflavine to rhodamine-6G followed the F?rster theory, which is indicated by the agreement in the values of the observed critical transfer distance with that calculated from spectral overlap. On the other hand, at low acceptor concentrations, the excitation energy migration influences the kinetics, resulting in a significantly higher value of the observed critical transfer distance, which is explained on the basis of Loring et al. (Loring, R. F.; Anderson, H. C.; Fayer, M. D. J. Chem. Phys. 1984, 80, 5731-5744) and Huber (Huber, D. L. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 20 2307-2314) theories. It was observed that the spectral overlap for donor-donor transport (excitation migration) and donor-acceptor transfer (energy transfer) and thereby other energy transfer parameters were influenced by the microenvironment of the polymers. The efficiency of energy transfer (eta) was the highest in PMMA and the lowest in PVA. Further, the study of acceptor dynamics under energy transfer showed that the rise time of the acceptor also depends on the nature of the polymer microenvironment.     

Long distance energy transfer in a polymer matrix doped with a perylene dye

Exciton migration over long distances is a key issue for various applications in organic electronics. We investigate a disordered material system which has the potential for long exciton diffusion lengths in combination with a high versatility. The perylene bisimide dye Perylene Red is incorporated in a polymer matrix with a high concentration. The dye molecules represent active sites with a narrow energy distribution for the electronically excited states. Excitons can be efficiently exchanged between them by F?rster resonance energy transfer (FRET). The narrow energy distribution reduces drastically the trapping probability of the excitons compared to polymers and allows for long transfer distances. To characterize the mobility of the excitons and their diffusion length the dye Oxazine 1 is added as an acceptor in low concentration and the transfer probability to the acceptor is determined by measuring the reduction of Perylene Red fluorescence. The quenched quantum yield is measured for dye concentrations varying from 0.05?M to 0.15 M for Perylene Red and from 0.3 mM to 3 mM for Oxazine 1. The experimental results are compared to a model which assumes that excitons can diffuse through the material by FRET between Perylene Red sites and are trapped at an acceptor with a final hetero FRET step. We find a quite good match between theory and experiment though the observed diffusion constant is about two times smaller than the calculated one. The exciton diffusion length extracted from the data is 30 nm for a Perylene Red concentration of 0.1 M and demonstrates that long distance energy transfer is possible in this disordered material system.     

Role of diffusion in excitation energy transfer and migration

Effect of diffusion on excitation energy transfer and migration in a dye pair sodium fluorescein (donor) and Rhodamine-6G (acceptor) has been studied for different viscosities by both steady state and time domain fluorescence spectroscopic measurements. The donor-donor interaction appears to be weaker as compared to donor-acceptor interaction and thus favors direct Forster-type energy transfer. Interestingly, at low viscosity (water in this case) transfer appears to be controlled by material diffusion/energy migration. Further, acceptor dynamics reveals the fact that direct Forster transfer dominates in viscous media.     

THE INFLUENCE OF FLUORESCENCE CONCENTRATION QUENCHING ON THE EMISSION ANISOTROPY OF FLAVINS IN GLYCERINE-WATER SOLUTIONS

The measurements of the emission anisotropy r/r0 of flavomononucleotide (FMN) within a range of concentrations of 10(-5)-10(-1) mol/L in glycerine-water solutions of different viscosities--0.056 Pa/s (system I) and 0.256 Pa/s (system II) have been carried out. In the range of high concentrations the repolarization effect due to the sharp drop of the quantum yield has been observed. The experimental results have been compared with theoretical expressions evaluated by taking into account both concentration and rotation depolarization. A good agreement on the values of the theoretical parameters obtained from independent measurements has been found. It has been stated that in the investigated systems the excitation energy transfer may be treated as a Markov process.     

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

Flavin-mediated photooxidations have been described for applications in synthetic organic chemistry for some time and are claimed to be a route to the use of solar energy. We present a detailed investigation of the involved photophysical and photochemical steps in methoxybenzyl alcohol oxidation on a timescale ranging from sub-picoseconds to tens of microseconds. The results establish the flavin triplet state as the key intermediate for the photooxidation. The initial step is an electron transfer from the alcohol to the triplet state of the flavin catalyst with (3)k(ET)≈ 2 × 10(7) M(-1) s(-1), followed by a proton transfer in ～6 μs. In contrast, the electron transfer involving the singlet state of flavin is a loss channel. It is followed by rapid charge recombination (τ = 50 ps) without significant product formation as seen when flavin is dissolved in pure benzylic alcohol. In dilute acetonitrile/water solutions of flavin and alcohol the electron transfer is mostly controlled by diffusion, though at high substrate concentrations >100 mM we also find a considerable contribution from preassociated flavin-alcohol-aggregates. The model including a productive triplet channel and a competing singlet loss channel is confirmed by the course of the photooxidation quantum yield as a function of substrate concentration: We find a maximum quantum yield of 3% at 25 mM of benzylic alcohol and significantly smaller values for both higher and lower alcohol concentrations. The observations indicate the importance to perform flavin photooxidations at optimized substrate concentrations to achieve high quantum efficiencies and provide directions for the design of flavin photocatalysts with improved performance.     

Lattice-Boltzmann simulations of the dynamics of polymer solutions in periodic and confined geometries

A numerical method to simulate the dynamics of polymer solutions in confined geometries has been implemented and tested. The method combines a fluctuating lattice-Boltzmann model of the solvent [Ladd, Phys. Rev. Lett. 70, 1339 (1993)] with a point-particle model of the polymer chains. A friction term couples the monomers to the fluid [Ahlrichs and Dunweg, J. Chem. Phys. 111, 8225 (1999)], providing both the hydrodynamic interactions between the monomers and the correlated random forces. The coupled equations for particles and fluid are solved on an inertial time scale, which proves to be surprisingly simple and efficient, avoiding the costly linear algebra associated with Brownian dynamics. Complex confined geometries can be represented by a straightforward mapping of the boundary surfaces onto a regular three-dimensional grid. The hydrodynamic interactions between monomers are shown to compare well with solutions of the Stokes equations down to distances of the order of the grid spacing. Numerical results are presented for the radius of gyration, end-to-end distance, and diffusion coefficient of an isolated polymer chain, ranging from 16 to 1024 monomers in length. The simulations are in excellent agreement with renormalization group calculations for an excluded volume chain. We show that hydrodynamic interactions in large polymers can be systematically coarse-grained to substantially reduce the computational cost of the simulation. Finally, we examine the effects of confinement and flow on the polymer distribution and diffusion constant in a narrow channel. Our results support the qualitative conclusions of recent Brownian dynamics simulations of confined polymers [Jendrejack et al., J. Chem. Phys. 119, 1165 (2003) and Jendrejack et al., J. Chem. Phys. 120, 2513 (2004)].     

Excited state characterization of a polymeric indigo

A comprehensive investigation of the solution photophysics of a 5,5'-methylene-bridged polymeric indigo, a statistical copolymer consisting of indigo and N-acetylindigo units, was performed in organic solvents at room temperature and further compared with indigo. A complete spectral and photophysical characterization based on photoacoustic calorimetry, steady-state and time-resolved fluorescence data was undertaken. A fluorescence quantum yield of 0.00037 and an intersystem crossing singlet-to-triplet quantum yield of 0.006 (close to the value for indigo) were obtained, leading to a value of 0.9936 for the S(1) → S(0) internal conversion (IC) quantum yield. Spectral and photophysical characteristics similar to indigo were obtained with, however, a special signature: it (mainly) decays single exponentially (in contrast with indigo, found to decay bi-exponentially), with a decay time value of 40-50 ps and an even more efficient S(1) → S(0) IC deactivation channel, related to an efficient energy migration within an energetic ladder of the polymer chromophoric segments. The photochemistry of this polymer, namely the degradation under light excitation, was also investigated and the obtained photoreaction quantum yield (?(R)) in DMF was found to be 0.003, which is lower than the previously determined value for indigo in the same solvent (?(R) = 0.0078). The overall data indicate that although the polymer and indigo have a close finger-print, the former is more stable which is suggested to be due to the additional intramolecular energy transfer process (within different chromophoric units) found with the polymer.     

Fluorescence correlation spectroscopy of flavins and flavoenzymes: photochemical and photophysical aspects

Fluorescence Correlation Spectroscopy (FCS) was used to investigate the excited-state properties of flavins and flavoproteins in solution at the single molecule level. Flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) and lipoamide dehydrogenase served as model systems in which the flavin cofactor is either free in solution (FMN, FAD) or enclosed in a protein environment as prosthetic group (lipoamide dehydrogenase). Parameters such as excitation light intensity, detection time and chromophore concentration were varied in order to optimize the autocorrelation traces. Only in experiments with very low light intensity ( < 10 kW/cm2), FMN and FAD displayed fluorescence properties equivalent to those found with conventional fluorescence detection methods. Due to the high triplet quantum yield of FMN, the system very soon starts to build up a population of non-fluorescent molecules, which is reflected in an apparent particle number far too low for the concentration used. Intramolecular photoreduction and subsequent photobleaching may well explain these observations. The effect of photoreduction was clearly shown by titration of FMN with ascorbic acid. While titration of FMN with the quenching agent potassium iodide at higher concentrations ( > 50 mM of I-) resulted in quenched flavin fluorescence as expected, low concentrations of potassium iodide led to a net enhancement of the de-excitation rate from the triplet state, thereby improving the fluorescence signal. FCS experiments on FAD exhibited an improved photostability of FAD as compared to FMN: As a result of stacking of the adenine and flavin moieties, FAD has a considerably lower triplet quantum yield. Correlation curves of lipoamide dehydrogenase yielded correct values for the diffusion time and number of molecules at low excitation intensities. However, experiments at higher light intensities revealed a process which can be explained by photophysical relaxation or photochemical destruction of the enzyme. As the time constant of the process induced at higher light intensities resembles the diffusion time constant of free flavin, photodestruction with the concomitant release of the cofactor offers a reasonable explanation.     

Quantum dynamics of H + LiH reaction and its isotopic variants

Time-dependent quantum wave packet dynamics study is carried out to investigate the initial state selected channel specific reactivity of H + LiH collisional system on a new and more accurate ab initio potential energy surface developed by Wernli et al. [J. Phys. Chem. A 113, 1121 (2009)]. The H + LiH reaction proceeds through LiH depletion and H-exchange paths. While the former path is highly exoergic (by ～2.258 eV), the latter path is thermoneutral. State selected and energy resolved integral reaction cross sections and thermal rate constants are reported and compared with the literature data. The reactivity of the LiH depletion channel is found to be greater than the H-exchange channel. Rotational excitation of the reagent LiH molecule causes a decrease of reactivity of both the channels. On the other hand, the vibrational excitation of the reagent LiH decreases the reactivity of the LiH depletion channel and increases the reactivity of the H-exchange channel. The effect of isotopic substitution (H by D) on the reaction dynamics is also examined.     

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 2. The photophysics of 1,4-diethynyl-2-fluorobenzene in solution and in crystals

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60--80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Phi(F) = 0.15) and the singlet lifetime short (tau(F) = 3.8 ns). A lower limit for the triplet yield of Phi(T) = 0.64 was determined indirectly in solution by (1)O(2) sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K.     

Conjugation enhancement of intramolecular exciton migration in poly(p-phenylene ethynylene)s

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper understanding of the nature of this process. We make use of specially designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition, we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The ability to improve the exciton migration efficiency through precise control of the polymer structure with liquid crystalline solutions demonstrates the importance of a polymer's conformation for energy transfer, and provides a way to improve the energy transporting performance of conjugated polymers.     

Anomalous green emission and energy transfer in the n,n-dimethylformamide/hydrochloric acid/europium chloride system

The unusual green photoluminescence (PL) of N,N-dimethylformamide (DMF)/hydrochloric acid (HCl)/europium chloride (EuCl3) solutions discovered earlier was investigated in more detail to clarify the emission mechanism. It was revealed that the DMF/HCl pair alone can yield a green PL band under UV excitation, and the emission has features of that of excimers. The addition of EuCl3 salt to the solution further stimulates the green emission. The quantum yield of the line emission of Eu3+ ions at 592 and 612 nm is also affected by the presence of HCl in the solution. Both the green emission band and Eu3+ emission lines possess a common channel of excitation at approximately 280 nm. This channel is the only source for the green emission band and an additional source for the Eu3+ emission lines, which can also be stimulated through a conventional Eu3+ excitation channel at 394 nm. The common excitation channel was found to be time-dependent, and its excitation maximum gradually shifts to longer wavelengths. Changes in the PL profiles of europium ions were also observed depending on the presence of HCl and the solution aging.     

ROOM-TEMPERATURE STEADY-STATE FLUORESCENCE PROPERTIES OF POLY(dG-dC).SPOLY(dG-dC)

We report the steady-state fluorescence properties of the alternating polynucleotide poly(dG-dC).poly(dG-dC) in low-salt solution at room temperature for excitation at the Hg lines 265, 280 and 297 nm. Its fluorescence spectrum peaks at about 325 nm and, within the experimental error, its shape does not change significantly with the excitation wavelength. The fluorescence anisotropy is found to decrease strongly for short-wavelength excitation, a behavior which is very similar to that exhibited by free guanine. In view of the fact that the anisotropy for free cytosine is virtually constant at the aforementioned three excitation wavelengths, the results suggest that in this polynucleotide the emission stems from guanine. The values of the fluorescence quantum yield for the three excitation wavelengths are found to be very low, 0.8 x 10(-5), 0.8 x 10(-5), and 2.8 x 10(-5), respectively; these are compatible with transfer of energy from the lower-energy electronic state of guanine, before vibronic relaxation is established, to cytosine. Upon denaturation, the fluorescence spectrum becomes very broad and the fluorescence quantum yield increases; these observations support the authenticity of the emission from the nondenatured polynucleotide.     

Photophysical properties of near-infrared-emitting Ln(III) complexes with 1-(9-Anthryl)-4,4,4-trifluoro-1,3-butandione (Ln = Nd and Er)

We report the synthesis and photophysical properties of Nd(III) and Er(III) complexes with 1-(9-anthryl)-4,4,4-trifluoro-1,3-butandione (9-ATFB). The complexes of [Nd(9-ATFB)4]- and [Er(9-ATFB)4]- produced sensitized near-infrared (NIR) luminescence via the excitation of anthracene. This suggests that the intramolecular energy transfer occurred from the singlet excited state of anthracene to the resonance levels of the metal ions, since the phosphorescence of anthracene is forbidden under normal conditions. The observed quantum yield of the visible luminescence showed that the energy transfer is more efficient for [Nd(9-ATFB)4]- than for [Er(9-ATFB)4]-. The lifetimes of the NIR luminescence of the complexes were in the microsecond range. The quantum yields of the sensitized NIR of the complexes were estimated using the lifetime and the energy-transfer quantum yield.