Darius Kuciauskas

Mimicry of carotenoid photoprotection in artificial photosynthetic reaction centers: Triplet-triplet energy transfer by a relay mechanism

Abstract Two artificial photosynthetic reaction centers consisting of a porphyrin (P) covalently linked to both a carotenoid polyene (C) and a fullerene derivative (C 60 ) have been prepared and found to transfer triplet excitation energy from the fullerene moiety of C-P- 3 C 60 to the carotenoid polyene, yielding 3 C-P-C 6 . The transfer has been studied both in toluene at ambient temperatures and in 2-methyltetrahydrofuran at lower temperatures. The energy transfer is an activated process, with E a =0.17 eV. This is consistent with transfer by a triplet energy transfer relay, whereby energy first migrates from C-P- 3 C 60 to the porphyrin, yielding C- 3 P-60 60 in a slow, theramally activated step. Rapid Rapid energy transfer from the porphyrin triplet to the carotenoid gives the final state. Triplet relays of this sort have been observed in photosynthetic reaction centers, and are part of the system that protects the organism from damage by singlet oxygen, whose production is sensitized by chlorophyll triplet states. The fullerene-containing triads can also demonstrate stepwise photoinduced electron transfer to yield long-lived C .+ -P-C 60 -− charge-separted states. Electron transfer occurs even at 8 K. Charge recombination of C .+ -P-C 60 .− yeilds 3 C-P-C 60 , rather than the molecular ground state. These protochemical events are reminiscent of photoinduced electron transfer in photosynthetic reaction centers.

Self-assembly of peptide-porphyrin complexes leads to pH-dependent excitonic coupling.

Using absorbance, fluorescence, resonance light scattering, and circular dichroism spectroscopy, we studied the self-assembly of the anionic meso-tetra(4-sulfonatophenyl)porphine (TPPS(4)(2-/4-)) and a cationic 22-residue polypeptide. We found that three TPPS(4)(2-/4-) molecules bind to the peptide, which contains nine lysine residues in the primary sequence. In acidic solutions, when the peptide is in the random-coil conformation, TPPS(4)(2-) bound to the peptide forms excitonically coupled J-aggregates. In pH 7.6 solutions, when the peptide secondary structure is partially alpha-helical, the porphyrin-to-peptide binding constants are approximately the same as in acidic solutions (approximately 3 x 10(6) M(-1)), but excitonic interactions between the porphyrins are insignificant. The binding of TPPS(4)(2-/4-) to lysine-containing peptides is cooperative and can be described by the Hill model. Our results show that porphyrin binding can be used to change the secondary structure of peptide-based biomaterials. In addition, binding to peptides could be used to optimize porphyrin intermolecular electronic interactions (exciton coupling), which is relevant for the design of light-harvesting antennas for artificial photosynthesis.

Driving Force and Electronic Coupling Effects on Photoinduced Electron Transfer in a Fullerene‐based Molecular Triad¶

Abstract Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (≥96%). Excitation of the porphyrin moiety of C–P–C60 is followed by a combination of photoinduced electron transfer to give C–P·+–C60·− and singlet–singlet energy transfer to yield C–P–1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C–P·+–C60·−. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C·+–P–C60·− is at least 60 times faster than recombination of C–P·+–C60·−, leading to the overall quantum yield near unity for the final state. Formation of a similar charge-separated species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C·+–P–C60·− in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.

Exciton Annihilation and Energy Transfer in Self-Assembled Peptide-Porphyrin Complexes Depends on Peptide Secondary Structure

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.

Photoinduced electron transfer in a symmetrical diporphyrin–fullerene triad

Two triad molecules consisting of either two zinc, or two free-base porphyrins symmetrically joined to a fullerene via phenyleneethynylene-containing linkages have been synthesized, and their photochemistry investigated. In the zinc form of the triad, PZn–C60–PZn, excitation of a zinc porphyrin in 2-methyltetrahydrofuran solution is followed by photoinduced electron transfer to the fullerene with a time constant of 20 ps. The resulting PZn˙+–C60˙−–PZn charge-separated state is formed with a quantum yield of 98% and has a lifetime of 820 ps. The first excited singlet state of the free-base analog gives the P2H˙+–C60˙−–P2H charge-separated state with a time constant of 200 ps and a yield of 98%. The charge-separated state decays with a lifetime of 2.8 ns. The difference in the rates of photoinduced electron transfer is consistent with reaction in the normal region of the Marcus–Hush relationship of transfer rate and driving force, and charge recombination is consistent with Marcus–Hush inverted behavior. The presence of the two porphyrin electron donors in these triads enhances the absorption cross section for light collection, and the molecular framework employed could be used to prepare molecules with enhanced energy conversion or optoelectronic properties.

Optical susceptibilities of supported indium tin oxide thin films

The third-order nonlinear optical susceptibility of indium tin oxide (ITO) thin films on glass substrates, χ(3)ITO, was determined in the near-IR spectral region using degenerate four wave mixing (DFWM) spectroscopy with 100fs laser pulses. A DFWM method for measuring thin films on thick substrates was refined for the characterization of films less than 100nm thick and applied to ∼25nm thick ITO films. It was found that χ(3)ITO is purely electronic at 900–1300nm (11000–7700cm−1) and has a value of (2.16±0.18)×10−18m2V−2. The χ(3)ITO value reaches (3.36±0.28)×10−18m2V−2 at 1500nm (6700cm−1) due to two-photon absorption by free carriers (electrons). Ultrafast electron relaxation was also observed. The ∼100fs lifetime of this process could reflect electron scattering in the conduction band.

Scanning tunneling microscopy of CdSe single crystal cleaved and “real” surface

Ultrahigh vacuum-cleaved and as-grown surfaces of CdSe single crystals were investigated by scanning tunneling microscopy. The single crystals were grown by Reynolds-Green method. Striations and terrace-step structure have been found. The surface atomic geometry was found and investigated. The (1120) face geometry (structure formed by elementary cell of 0.75 × 0.7 nm2) as well as other type structures (e.g., 2.1 × 0.75 nm2 elementary cell) have been determined. The variations of the band gap at the surface have been found. The band values in the range 2.0–2.6 eV on a cleaved surface and 1.1–2.0 eV on an as-grown surface were measured and explained as being the influence of surface relaxation and gas adsorption.