“…Similar intensity-dependent relaxation in large TPPS 4 aggregates has also been attributed to the exciton annihilation . Annihilation studies have been used to analyze energy transfer for small aggregates of three self-assembled porphyrin dimers. , …”
Section: Resultsmentioning
confidence: 85%
“…21 Annihilation studies have been used to analyze energy transfer for small aggregates of three self-assembled porphyrin dimers. 24,25 Before analyzing the results, we will briefly discuss the annihilation model. When multiple excitations are present in an aggregate, energy transfer from one excited monomer to a second excited monomer becomes possible.…”
Section: Resultsmentioning
confidence: 99%
“…Some estimates of the energy transfer time between porphyrins, τ hopping , could be obtained using models developed to analyze similar questions in photosynthetic antennas and self-assembled aggregates. A following simple relationship is based on a migration-limited annihilation model ,,, τannihilation=N2−124τhopping where N is the number of hopping sites (number of TPPS 4 molecules) and τ annihilation is two-exciton annihilation time. For peptide−TPPS 4 aggregates studied here N = 3 and τ hopping = 3τ annihilation .…”
We used picosecond transient absorption and fluorescence lifetime spectroscopy to study singlet exciton annihilation and depolarization in self-assembled aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS(4)) and a synthetic 22-residue polypeptide. The polypeptide was designed and previously shown to bind three TPPS(4) monomers via electrostatic interactions between the sulfonate groups and cationic lysine residues. Additionally, the peptide induces formation of TPPS(4) J-aggregates in acidic solutions when the peptide secondary structure is disordered. In neutral solutions, the peptide adopts an α-helical secondary structure that can bind TPPS(4) with high affinity but J-aggregate formation is inhibited. Detailed analysis of excitation-power dependent transient absorption kinetics was used to obtain rate constants describing the energy transfer between TPPS(4) molecules in an aggregate under acidic and neutral conditions. Independently, such analysis was confirmed by picosecond fluorescence emission depolarization measurements. We find that energy transfer between TPPS(4) monomers in a peptide-TPPS(4) complex is more than 30 times faster in acidic aqueous solution than in neutral solutions (9 vs 279 ps). This result was attributed to a conformational change of the peptide backbone from disordered at low pH to α-helical at neutral pH and suggests a new approach to control intermolecular energy transfer with possible applications in fluorescent sensors or biomimetic light harvesting antennas.
“…Similar intensity-dependent relaxation in large TPPS 4 aggregates has also been attributed to the exciton annihilation . Annihilation studies have been used to analyze energy transfer for small aggregates of three self-assembled porphyrin dimers. , …”
Section: Resultsmentioning
confidence: 85%
“…21 Annihilation studies have been used to analyze energy transfer for small aggregates of three self-assembled porphyrin dimers. 24,25 Before analyzing the results, we will briefly discuss the annihilation model. When multiple excitations are present in an aggregate, energy transfer from one excited monomer to a second excited monomer becomes possible.…”
Section: Resultsmentioning
confidence: 99%
“…Some estimates of the energy transfer time between porphyrins, τ hopping , could be obtained using models developed to analyze similar questions in photosynthetic antennas and self-assembled aggregates. A following simple relationship is based on a migration-limited annihilation model ,,, τannihilation=N2−124τhopping where N is the number of hopping sites (number of TPPS 4 molecules) and τ annihilation is two-exciton annihilation time. For peptide−TPPS 4 aggregates studied here N = 3 and τ hopping = 3τ annihilation .…”
We used picosecond transient absorption and fluorescence lifetime spectroscopy to study singlet exciton annihilation and depolarization in self-assembled aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS(4)) and a synthetic 22-residue polypeptide. The polypeptide was designed and previously shown to bind three TPPS(4) monomers via electrostatic interactions between the sulfonate groups and cationic lysine residues. Additionally, the peptide induces formation of TPPS(4) J-aggregates in acidic solutions when the peptide secondary structure is disordered. In neutral solutions, the peptide adopts an α-helical secondary structure that can bind TPPS(4) with high affinity but J-aggregate formation is inhibited. Detailed analysis of excitation-power dependent transient absorption kinetics was used to obtain rate constants describing the energy transfer between TPPS(4) molecules in an aggregate under acidic and neutral conditions. Independently, such analysis was confirmed by picosecond fluorescence emission depolarization measurements. We find that energy transfer between TPPS(4) monomers in a peptide-TPPS(4) complex is more than 30 times faster in acidic aqueous solution than in neutral solutions (9 vs 279 ps). This result was attributed to a conformational change of the peptide backbone from disordered at low pH to α-helical at neutral pH and suggests a new approach to control intermolecular energy transfer with possible applications in fluorescent sensors or biomimetic light harvesting antennas.
“…The synthesis of the cyclic hexamers shown in Chart was facilitated by the extensive prior work of Anderson and Sanders, who described the synthesis and host−guest properties of diverse cyclic arrays composed of two, three, or four porphyrins . In the past two years, remarkably large cyclic arrays (containing as many as 24 porphyrins) have been constructed. − However, to date only a handful of cyclic multiporphyrin arrays has been shown to bind a guest porphyrin, as is desired for energy-funneling studies. − 1 …”
Static and time-resolved optical measurements are reported for two cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0 or 3 free base (Fb) porphyrins (denoted Zn(6) or Zn(3)Fb(3), respectively). The guests are a tripyridyl arene (TP) and a dipyridyl-substituted free base porphyrin (DPFb), each of which coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have an overall gradient of excited-state energies that affords excitation funneling within the host and ultimately to the guest. Collectively, the studies delineate the various pathways, mechanisms, and rate constants of energy flow among the weakly coupled constituents of the host-guest complexes. The pathways include downhill unidirectional energy transfer between adjacent chromophores, bidirectional energy migration between identical chromophores, and energy transfer between nonadjacent chromophores. The energy transfer to the lowest-energy chromophore(s) within the backbone of a hexameric host (Fb porphyrins in Zn(3)Fb(3) or pyridyl-coordinated zinc porphyrins in Zn(6)*TP and Zn(6)*DPFb) proceeds primarily via a through-bond mechanism; the transfer is rapid (approximately 40 ps depending on the array) and essentially quantitative (>or=98%). The energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the Fb porphyrin guest in the Zn(6)*DPFb complex is almost exclusively Förster through-space in nature; this process is much slower ( approximately 1 ns) and has a lower yield (65%). These studies highlight the utility of cyclic architectures for efficient light harvesting and energy transfer to a designated trapping site.
“…As in the case of D1, the relatively broad fluorescence spectra of D2 and D3 became sharpened in B2 and B3, probably due to the structural rigidification particularly with regard to the dihedral angle of the meso-meso linked diporphyrin upon box formation. 14 In line with this interpretation, the Stokes shifts in B2 and B3 are much less than those in D2 and D3. These characteristic fluorescence spectra of B2 and B3 are rather concentration independent up to 1.0 Â 10 À8 M, from which the association constants have been estimated to be at least larger than 10 24 M À3 .…”
It has been demonstrated that the direction and magnitude of transition dipole moments, and hence rates in the excitation energy hopping in the self-assembled porphyrin boxes can be tuned by insertion of ethynyl groups as well as the dielectric constant of solvent.
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