Quantum Yield Of Energy Transfer Research Articles

Optical and photophysical properties of light-harvesting phenylacetylene monodendrons based on unsymmetrical branching.

The optical and photophysical properties of phenylacetylene dendritic macromolecules based on unsymmetrical branching are investigated using steady-state and time-dependent spectroscopy. Monodendrons, up to the fourth generation, are characterized with and without a fluorescent perylene trap at the core. The higher generation monodendrons without the perylene trap exhibit high molar extinction coefficients (>10(5) M(-1) cm(-1)) and high fluorescence quantum yields (65-81%). When a perylene trap is placed at the core, then the monodendrons typically exhibit high energy transfer quantum yields (approximately 90%), as well as subpicosecond time scale excited-state dynamics, as evidenced by ultrafast pump-probe measurements. The photophysical properties of the unsymmetrical monodendrons are compared to those of phenylacetylene monodendrons with symmetrical branching, which have been described recently. The high fluorescence quantum yields and large energy transfer quantum efficiencies exhibited by the unsymmetrical monodendrons suggest they have potential for applications in molecular-based photonics devices.

Rational syntheses of cyclic hexameric porphyrin arrays for studies of self-assembling light-harvesting systems.

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of approximately 35 A. One array is comprised of five zinc (Zn) porphyrins and one free base (Fb) porphyrin (cyclo-Zn(5)FbU) while the other is comprised of an alternating sequence of two Zn porphyrins and one Fb porphyrin (cyclo-Zn(2)FbZn(2)FbU). The prior synthesis employed a one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5 + 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn(2)FbZn(2)FbU employed a magnesium porphyrin, affording cyclo-Zn(2)FbZn(2)MgU from which magnesium was selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins to the Fb porphyrin(s) was 60% in cyclo-Zn(5)FbU and 90% in cyclo-Zn(2)FbZn(2)FbU. Two dipyridyl-substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies.

Influence of Entanglements on the Time Dependence of Mixing in Nonradiative Energy Transfer Studies of Polymer Diffusion in Latex Films

We report energy transfer experiments on poly(butyl methacrylate) (PBMA) latex films prepared from a 1:1 mixture of donor- and acceptor-labeled latex particles. In one set of samples, the particles contain low molecular weight polymer (Mw = 34 000) comparable to the entanglement molecular weight (Me) of PBMA. For this sample, the extent of mixing fm (defined as the fractional increase in the energy transfer quantum yield) increased with time as t1/2, consistent with diffusion that follows Fick's law. In the second set of experiments, involving polymer with Mw = 380 000 (Mw > 10Me), we observe a change in the time dependence of fm. At early times, for values of fm 0.2. The most likely explanation of the early-time behavior is Fickian diffusion of the lowest molecular weight components of the broadly disperse polymer (Mw/Mn = 3). The scaling of fm with t1/4 does not appear to be consistent with the predict...

Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad.

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (P(H2)) porphyrin. In the resulting dyads (P(Zn)-Sp(c) and P(H2)-Sp(c)), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp(o)) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn-Sp(o). The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Sp(o). The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1P(H2)-Sp(o) and P(H2)-1Sp(o) exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.

Static energy transfer for Mn 2+ : Pr 3+ system in Phosphate glasses

The process of energy transfer from divalent manganese to trivalent praseodymium ions is studied with the help of Forster–Dexter theory. Some of the important energy transfer parameters such as transfer probability ( P SA), quantum yield of energy transfer ( μ), critical radius ( R 0) and critical concentration of the acceptor ( C A) are evaluated for the energy transfer pairs Mn 2+ : Pr 3+, Pr 3+ : Pr 3+ in phosphate glasses. The transfer of energy from Mn 2+ to Pr 3+ was confirmed by recording the fluorescence decay curves of Mn 2+ emission corresponding to various activator concentrations which show a non-exponential behaviour. Large values of transfer probabilities and transfer efficiencies suggest that Mn 2+ ion can act as a good sensitizer to Pr 3+ ion in phosphate glass.

Morphology-Dependent Photochemical Events in Aryl Ether Dendrimer Porphyrins: Cooperation of Dendron Subunits for Singlet Energy Transduction

A series of dendrimer porphyrins ((L5)nP, n = 1−4), which bear different numbers (n) of five-layered dendron subunits (L5) at the meso-positions of a porphyrin, were synthesized. Upon excitation of the dendron subunits in CH2Cl2 at 20 °C, (L5)nP showed an intramolecular singlet energy transfer from the dendron subunits to the porphyrin core, where tetrasubstituted (L5)4P with a spherical morphology exhibited a much higher energy transfer quantum yield (ΦENT = 80.3%) than partially substituted (L5)1P−(L5)3P (10.1−31.6%). Fluorescence depolarization characteristics of (L5)4P indicated that the excitation energy migrates very efficiently over the continuous dendrimeric array of dialkoxybenzyl building units within the lifetime of the excited state. Consequently, the probability of energy transfer to the porphyrin core was enhanced.

Energy transfer from Mn 2+ to Nd 3+ in phosphate glasses

Energy transfer between Mn 2+ and Nd 3+ ion is studied in four different phosphate glassy matrices. In the Mn 2+ : Nd 3+ system energy is found to transfer from the a 4 Γ emitting state of Mn 2+ ion to 4G 5 2 absorbing state of Nd 3+ ion. From the overlap of the emission spectrum of Mn 2+ ion and the absorption spectrum of the Nd 3+ ion, the overlap integral between this transition is evaluated and is found to be a measure of the energy transfer. This in turn is utilized in evaluating the energy transfer probability using the well known Forster-Dexter theory. Some of the important energy transfer parameters such as quantum yield of energy transfer (η), critical radius ( R 0) and critical concentration ( C 0) are evaluated in the light of this theory. Also the energy transfer is found to occur through a dipole-dipole interaction.

Kinetics of radiationless energy transfer from upper excited states

A model of the kinetics of radiationless energy transfer from upper excited states is presented, where both the possibility of back energy transfer from the excited acceptor to a lower lying level of the donor and the possibility of energy migration over acceptors is considered. The model leads to a complex variation of the energy transfer quantum yield with the acceptor concentration. This dependence results from the interplay of direct energy transfer, back energy transfer and energy migration. Back energy transfer reduces the efficiency of donor-acceptor overall transfer efficiency. On the other hand, energy migration over the acceptors decreases the efficiency of back energy transfer and results in an increase in the energy transfer quantum yield. The known experimental results of energy transfer from upper excited states are in qualitative agreement with the present treatment.

Molecular Dynamics and FEP Monte Carlo Studies of Calix[4]arene-Derived Complexes of Eu3+: The Role of the Counterions Investigated

The structural properties of the Eu3+ complexes of three lower-rim functionalized calix[4]arenes have been investigated by means of molecular dynamics (MD) and free energy perturbation Monte Carlo (FEP-MC) simulations in OPLS acetonitrile. Ligand 1, having four 2,2‘-bipyridine-5-methyl groups on the lower rim, complexes Eu3+ tightly with all four bipy (2,2‘-bipyridine) moieties as bidentate ligands. Additionally strong support is raised for the binding of one counterion (nitrate) in the first coordination shell of Eu3+. Ligand 2, having four 2,2‘-bipyridine-6-methyl groups on the lower rim, shows less strong binding of Eu3+. In fact, only three of the four bipy moieties are coordinated to the Eu3+, which are likely to be in dynamic equilibrium. It is predicted that two nitrate ions are also coordinated and maybe one acetonitrile. These calculations support the explanation that this complex has a lower quantum yield of sensitized emission compared to system 1·Eu3+ due to the fact that the bipy ligands interact less strongly with the Eu3+ and therefore will have a reduced “antenna” effect. The interaction between the bipy ligands and the Eu3+ determines, among others, the quantum yield of energy transfer. A factor also relevant to the quantum yield is the symmetry of the crystal field, which can be assessed by simulations but is not easily quantified. Calculations on the complex of 3, having three 2,2‘-bipyridine-5-methyl groups on the lower rim, with Eu3+ showed an artifact of the MD protocol, which was revealed by FEP-MC calculations and confirmed by some additional MD calculations. The structure of 3·Eu3+ most likely has one monodentate and one bidentate nitrate coordinated to the Eu3+ besides the three tightly bound bipy moieties. This study shows that in general long MD simulations are needed.

Effects of central metal ion (Mg, Zn) and solvent on singlet excited-state energy flow in porphyrin-based nanostructures

F. Li, S. Gentemann, William A. Kalsbeck, J. Seth, Jonathan S. Lindsey, D. Holten and David F. Bocian, J. Mater. Chem., 1997, 7, 1245 DOI: 10.1039/A700146K

Time-resolved two-step excitation thermal grating method. Measurement of the quantum yield of energy transfer from a higher excited triplet state

Triplet-triplet absorption is sensitively detected by the time-resolved two-step excitation thermal grating method. This method provides zero-background detection of the transient absorption. As an example of the application, measurement of the quantum yield of energy transfer from the hisher excited triplet state is reported. The improvement of this method from other methods such as the time-resolved transient thermal lens method is discussed.

Roles of FAD and 8-hydroxy-5-deazaflavin chromophores in photoreactivation by Anacystis nidulans DNA photolyase.

DNA photolyase from the cyanobacterium Anacystis nidulans contains two chromophores, flavin adenine dinucleotide (FADH2) and 8-hydroxy-5-deazaflavin (8-HDF) (Eker, A. P. M., Kooiman, P., Hessels, J. K. C., and Yasui, A. (1990) J. Biol. Chem. 265, 8009-8015). While evidence exists that the flavin chromophore (in FADH2 form) can catalyze photorepair directly and that the 8-HDF chromophore is the major photosensitizer in photoreactivation it was not known whether 8-HDF splits pyrimidine dimer directly or indirectly through energy transfer to FADH2 at the catalytic center. We constructed a plasmid which over-produces the A. nidulans photolyase in Escherichia coli and purified the enzyme from this organism. Apoenzyme was prepared and enzyme containing stoichiometric amounts of either or both chromophores was reconstituted. The substrate binding and catalytic activities of the apoenzyme (apoE), E-FADH2, E-8-HDF, E-FAD(ox)-8-HDF, and E-FADH2-8-HDF were investigated. We found that FAD is required for substrate binding and catalysis and that 8-HDF is not essential for binding DNA, and participates in catalysis only through energy transfer to FADH2. The quantum yields of energy transfer from 8-HDF to FADH2 and of electron transfer from FADH2 to thymine dimer are near unity.

Energy Transfer between Localized Electronic States. II. Transition during Vibrational Relaxation

Energy transfer (ET) is greatly enhanced in a very short time after the initial electronic state was prepared by photon excitation. ET is assisted by violent lattice vibrations which have not been relaxed yet, when electrons interact strongly with phonons in the two electronic states (coupled by the matrix element J ). An expression of the quantum yield of ET, which bridges the nonadiabatic (small J ) and the adiabatic (large J ) limits, is given as a function of time elapsed after pulsed excitation at an energy E . In the case of steady excitation, transitions during vibrational relaxation manifest themselves in a strong E dependence of the quantum yield. Generalization of Forster's formula for ET in this dynamic regime is also mentioned. We use a formulation developed in the part I (J. Phys. Soc. Jpn. 49 (1980) 1701) which treat transitions after vibrational relaxation. The Landau-Zener formula plays an important role in describing ET.

Application of classical electromagnetic theory to an understanding of molecular vibrational energy transfer into metal surfaces

The quantitative results and qualitative understanding of the classical electromagnetic theory of energy transfer between an infrared dipole and a metal surface are explored. There are two limiting regimes: (1) a short range behavior where the rates decrease as D−3, and a long range behavior where rates decrease exponentially with D. For the analyzed surfaces, these regimes essentially represent transfer to bulk excitations and to surface plasmons, respectively. In the D−3 region, the normalized rates and energy transfer quantum yields can be extremely large as compared with electronic energy transfer in the visible and ultraviolet. A 1000 cm−1, perpendicularly oriented dipole 4000 Å from a Sn surface has a 69% energy transfer quantum yield. Extrapolation to molecules actually ’’on’’ metal surfaces suggests 10−9–10−11 sec lifetimes. Energy transfer to plasmons is explored for plasmons of varying lifetimes and electric field dependences.

Study of Excitons in SiO2. Luminescent Centres as Exciton Detectors

AbstractThe spectral and temperature dependences of the quantum yield of energy transfer to a luminescent detector by an exciton are studied in SiO2. The results obtained show, that in addition to the “hot” exciton migration there is a diffusion of the relaxed exciton. The exciton structure of SiO2, consists of three states, a self‐trapped exciton, a relaxed exciton, and a resonant exciton.

O мeчaниэмe paдиoлюминecceнии в aлифaтчecкич pacтвopитeляч

The radiochemical yields of excited states of luminescent additives with positive (diphenyl-oxazole, triphenylpyrazoline) and negative (benzol) affinities towards the electron, and quenching of radioluminescence of these systems in presence of CCl 4 were measured in cyclohexane and hexane solutions. In solutions containing luminescent additives with positive electron affinity the excited state yields are larger, while quenching by CCl 4 is weaker. The curves of radioluminescence decay were measured. The duration of energy transfer from the solvent to the luminescent additive is less than 5 × 10 −10 s. Quantum yields of energy transfer in solutions photoexcited by λ = 1608 A ̊ and λ = 1215 A ̊ were measured too. The results obtained can be interpreted assuming that a part of the excited states of the luminescent additives results from charge transfer to the luminescent additive and subsequent recombination, the rest being obtained as a result of excitation energy transfer to the luminescent additive. In the case of high concentrations of luminescent additives the main way of generating excited states is the recombination, while in the case of small concentrations it is due to the excitation transfer.