The Dynamics of Electronic Energy Transfer in Novel Multiporphyrin Functionalized Dendrimers: A Time-Resolved Fluorescence Anisotropy Study (original) (raw)
Related papers
The dynamics of electronic energy transfer (EET) for a series of spherical porphyrin arrays based on different generations of poly(propylene imine) dendrimers have been investigated using time-resolved fluorescence anisotropy measurements (TRAMS) in a glass environment. The first, third, and fifth generation dendrimers consisting of 4, 16, and 64 porphyrin chromophores, respectively, are investigated in this study. We observe a depolarization of the fluorescence in all three dendrimers as compared to the monoporphyrin model compound, indicating that EET takes place between the chromophores within the dendrimers. The experimental TRAMS results were compared to computationally simulated data obtained from the Pauli master equation. For the first generation dendrimer, we find the rate of energy transfer is well described by Förster theory. Anomalous behavior is observed in the third generation dendrimer where the limiting anisotropy value suggests that energy transfer is confined to only the porphyrins contained within a dendron. Interdendron porphyrin EET is thus unfavorable due to dendron segregation. In the fifth generation dendrimer, the TRAMS data is best explained by a model which includes independent and simultaneous rapid EET between porphyrins contained on the surface of the dendrimer sphere and slow EET between porphyrins in adventitious dendrons found probably either outside or inside of the sphere.
Energy transfer and conformational dynamics in Zn–porphyrin dendrimers
Chemical Physics Letters, 2005
The energy transfer within a series of Zn-porphyrin appended dendrimers was studied by means of time-resolved fluorescence anisotropy. We show that the energy transfer process between the Zn-porphyrin units in the dendrimers is limited to a maximum of four porphyrin units. At 200 K, the energy transfer process takes place on a 100-ps time scale, and can be modeled by Fö rster theory. Our results at room temperature further show that the porphyrin units are very mobile within the dendrimer, exhibiting rotational dynamics similar to that of a monomeric building block.
Macromolecules, 2003
Anionic meso-tetrakis(4-sulfonatophenyl)porphine (TSPP) interacts with poly(amido) amine (PAMAM) dendrimers of generations 2.0 and 4.0 in aqueous solution to form several dendrimer-associated species, which depend on the relative concentrations of dendrimer to porphyrin (D/P ≡ [PAMAM]/[TSPP]). At D/P ratios above the isoelectric point of the charge balance between the dendrimer and porphyrin, only two spectroscopic species were detected. An equilibrium model assuming a dendrimer-induced TSPP H-dimer coexisting with dendrimer-associated TSPP monomer afforded a good description of the experimental data, giving equilibrium constants (K d) for the dimer dissociation of 0.78 and 2.67 for generations 2.0 and 4.0 dendrimers, respectively. The decomposition of the Soret band afforded TSPP H-aggregates' spectra with bandwidth values in the range 910-1210 cm-1 , clearly larger than the monomer bandwidth around 711-800 cm-1. The fluorescence decays are nearly exponential with a long decay time component, which varies from 10 to 12 ns depending on the D/P ratio. However, at pH 2, there are striking differences between generations 2.0 and 4.0, which should be related to a more hydrophobic environment provided by the latter one. Time-resolved fluorescence anisotropy gave rotational times for the porphyrindendrimer complex from which hydrodynamic radii of approximately 14 and 21 Å, similar to those of generations 2.0 and 4.0 PAMAM dendrimers, were retrieved.
Photoinduced Electron Transfer within Porphyrin-Containing Poly(amide) Dendrimers
Organic Letters, 2001
Porphyrin-or zinc-porphyrin-cyanuric acid conjugants (TPP-CA or ZnTPP-CA) and anthraquinone connected with the known "Hamilton receptor" (AQ-H) were synthesized. The supramolecular dyads constructed via the AQ-H and TPP-CA/ZnTPP-CA exhibit six hydrogen bonds, which provide the association constant K APP approximately (2.8±0.3)×10 3 mol-1 •L in toluene. Selective excitation of the porphyrin/zinc porphyrin chromophores results in an electron transfer between TPP-CA/ZnTPP-CA and AQ-H within the supramolecular assemblies, leading to an efficient quenching of the TPP-CA/ZnTPP-CA fluorescence. The singlet electron transfer from porphyrin/ zinc-porphyrin to anthraquinone proceeds mainly via a 'through space' mechanism with efficiencies of 43%, 58%, and rate constants of 7.6×10 7 , 7.0×10 8 s-1 , respectively.
The Journal of Physical Chemistry B, 2008
We have performed computational simulations of porphyrin-dendrimer systemssa cationic porphyrin electrostatically associated to a negatively charged dendrimersusing the method of classical molecular dynamics (MD) with an atomistic force field. Previous experimental studies have shown a strong quenching effect of the porphyrin fluorescence that was assigned to electron transfer (ET) from the dendrimer's tertiary amines (Paulo, P. M. R.; Costa, S. M. B. J. Phys. Chem. B 2005, 109, 13928). In the present contribution, we evaluate computationally the role of the porphyrin-dendrimer conformation in the development of a statistical distribution of ET rates through its dependence on the donor-acceptor distance. We started from simulations without explicit solvent to obtain trajectories of the donor-acceptor distance and the respective time-averaged distributions for two dendrimer sizes and diffferent initial configurations of the porphyrin-dendrimer pair. By introducing explicit solvent (water) in our simulations, we were able to estimate the reorganization energy of the medium for the systems with the dendrimer of smaller size. The values obtained are in the range 0.6-1.5 eV and show a linear dependence with the inverse of the donor-acceptor distance, which can be explained by a two-phase dielectric continuum model taking into account the medium heterogeneity provided by the dendrimer organic core. Dielectric relaxation accompanying ET was evaluated from the simulations with explicit solvent showing fast decay times of some tens of femtoseconds and slow decay times in the range of hundreds of femtoseconds to a few picoseconds. The variations of the slow relaxation times reflect the heterogeneity of the dendrimer donor sites which add to the complexity of ET kinetics as inferred from the experimental fluorescence decays.
Ultrafast Energy Transport in a First-Generation Coumarin-Tetraphenylporphyrin Dendrimer
Journal of Physical Chemistry B, 2004
Energy transfer in a newly synthesized coumarin-tetraphenylporphyrin donor-acceptor system was studied by time-and frequency-resolved fluorescence spectroscopy. The energy transfer kinetics was shown to be fast (transfer time ca. 500 fs) and efficient (quantum yield ca. 97%). The influence of interactions between excitations on the energy transfer dynamics was studied by intensity-dependent experiments. Although annihilation of excitations occurs for high irradiation doses, this does not affect the observed fluorescence transients. A kinetic model was constructed to explain these findings. Both the difference between the rate constants of energy transfer to singly and doubly excited acceptor states and the rate of radiationless decay from such doubly excited states were shown to be key parameters in the explanation of the intensity-dependent effects.
Photoinduced Electron-Transfer within Free Base and Zinc Porphyrin Containing Poly(Amide) Dendrimers
Journal of Physical Chemistry B, 2001
The synthesis and photophysical characterization of a series of free base and zinc porphyrin containing, Newkome-type dendrimers terminated with anthraquinone groups (FbP-G a-AQ n and ZnP-G a-AQ n) and ethyl groups (FbP-G a-Et n and ZnP-G a-Et n) are described. These dendrimers were designed for use as mimics of the photosynthetic reaction center. Red-shifts in the absorption spectra, particularly in the anthraquinoneterminated series, were interpreted as resulting from backfolding of the dendrimer branches. Dendrimers FbP-G a-AQ n were shown to exhibit substantial quenching (58-75%) of the porphyrin fluorescence as measured against the analogous ethyl-terminated dendrimers (FbP-G a-Et n) in steady-state fluorescence experiments. The zinc porphyrin containing dendrimers ZnP-G a-AQ n exhibited nearly complete quenching (96-99.5%) of the porphyrin fluorescence. An intramolecular electron-transfer mechanism is proposed for the substantial decrease in fluorescence in both series of dendrimers. Time-resolved fluorescence experiments for FbP-G a-AQ n were fit to 2-3 exponentials and indicated that multiple orientations of the porphyrin and anthraquinone groups contribute to the electron-transfer event. These results were in good agreement with the steady-state fluorescence results. From the time-resolved fluorescence data, the electron-transfer rate constants were calculated, indicating k ET values in the range of 3.77 × 10 7 s-1 to 2.28 × 10 8 s-1 that were dependent upon both dendrimer generation number and solvent. Similar experiments on ZnP-G a-AQ n also indicated that multiple zinc porphyrin anthraquinone conformations were likely responsible for the electron-transfer. Dramatic differences between the steady-state and time-resolved fluorescence data in the zinc porphyrin dendrimers were interpreted in terms of ligation of the terminal anthraquinone groups with the zinc porphyrin that results in either a nonemissive state or an ultrafast electron-transfer.
Journal of the American Chemical Society, 2002
The influence of macromolecular architecture on the physical properties of polymeric materials has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results clearly confirm the anticipation that dendrimers are unique when compared to other architectures. Physical properties, from hydrodynamic volume to crystallinity, were shown to be different, and in a comparative study of core encapsulation in macromolecules of different architecture, energy transduction from the polymer backbone to a porphyrin core was shown to be different for dendrimers as compared to that of isomeric four-or eight-arm star polymers. Fluorescence excitation revealed strong, morphology dependent intramolecular energy transfer in the three macromolecular isomers investigated. Even at high generations, the dendrimers exhibited the most efficient energy transfer, thereby indicating that the dendritic architecture affords superior site isolation to the central porphyrin it surrounds.
Investigations of Electronic Energy Transfer Dynamics in Multiporphyrin Arrays
The Journal of Physical Chemistry A, 1999
A study of the dynamics of electronic energy transfer (EET) in arrays containing three, four, and six tetraphenylporphine units connected with phenylethynyl spacers is reported. For arrays containing the same chromophores, the EET rate constant was determined from the reorientational dynamics of the transition dipole using the crossed grating technique. EET time constants ranging from 150 ps up to 33 ns were measured, depending on the distance between the chromophores and on the metal ion complexed in the porphyrins. For the trimeric planar arrays, the interchromophoric distance varies by a factor of 2, while the ratio of the through space to through bond distances is constant. By comparing the measured EET rate constants with those calculated using Förster theory, the contributions of the Coulombic, through space, mechanism and of the exchange, through bond, mechanism could be estimated. For the arrays with the shortest spacer (through space distance of 23 Å), EET occurs through both exchange and Coulombic interactions with a ratio of about 3:1. This ratio increases up to about 10 as the distance is increased to 34.5 Å. At 46.5 Å, the ratio decreases and it appears that the Coulombic interaction becomes the dominant mechanism at longer distances. In the tetrahedral compound, the presence of a central saturated carbon strongly alters the electronic conducting properties of the spacer and makes the exchange mechanism inoperative.
Fluorescence and Intramolecular Energy Transfer in Polyphenylene Dendrimers
Macromolecules, 2003
The fluorescence of polyphenylene dendrimers and the intramolecular energy transfer in polyphenylene dendrimers containing a perylenediimide core have been investigated in this paper. Polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units exhibit strong fluorescence, with quantum yields ranging from 0.2 to 0.5 depending on the dendrimer generation and its degree of functionality. The fluorescence of polyphenylene dendrimers can be efficiently quenched by the incorporated perylenediimide core, and consequently, a predominant emission from the core has been observed, indicating a very efficient intramolecular energy transfer.