Gary Hastings

Ultrafast and long-lived photoinduced charge separation in MEH-PPV/nanoporous semiconductor thin film composites

Photoinduced charge separation and recombination dynamics were investigated in composite materials of conjugated polymer deposited on nanocrystalline metal oxide thin film. Using femtosecond IR transient absorption spectroscopy, the electron transfer from the excited state of poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV) to SnO2 and TiO2 nanoporous thin films is shown to occur with timescales of 800 and <100 fs, respectively. Negligible carrier recombination is observed for MEH-PPV/SnO2 within 1 ns. Microsecond charge recombination dynamics were measured using step-scan FTIR. Long-lived charge separation is observed in both systems, persisting for microseconds to seconds.

Photoinhibition of Photosystem I electron transfer activity in isolated Photosystem I preparations with different chlorophyll contents

Photoinhibition of the light-induced Photosystem I (PS I) electron transfer activity from the reduced dichlorophenol indophenol to methyl viologen was studied. PS I preparations with Chl/P700 ratios of about 180 (PS I-180), 100 (PS I-100) and 40 (PS I(HA)-40) were isolated from spinach thylakoid membranes by the treatments with Triton X-100, followed by sucrose density gradient centrifugation and hydroxylapatite column chromatography. White light irradiation (1.1 × 104μE m−2 s−1) of PS I-180 for 2 hours bleached 50% of the chlorophyll and caused a 58% decrease in the electron transfer activity with virtually no loss of the primary donor, P700. The flash-induced absorbance change showed the decay phase with a half time of about 10 μs that was attributed to the P700 triplet, suggesting that the photoinhibitory light treatment caused the destruction of the PS I acceptor(s), Fx and possibly A1. PS I-100 was similarly photobleached by the irradiation and the electron transfer activity decreased. There was, however, no apparent photoinhibition of the electron transport activity in PS I(HA)-40. Photoinhibition similar to that seen in PS I-180 also occurred in membrane fragments that were isolated without any detergent from a PS II-deficient mutant strain of the cyanobacterium Synechocystis sp. PCC 6803. PS I-180 was not photoinhibited under anaerobic conditions. The production of superoxide and fatty acid hydroperoxide during white light irradiation was significantly greater in PS I-180 than in PS I(HA)-40. The mechanism of photoinhibition in PS I preparations is discussed in relation to the formation of toxic oxygen molecules.

Excited state dynamics in photosystem I: effects of detergent and excitation wavelength.

Femtosecond transient absorption spectroscopy has been used to investigate the energy transfer and trapping processes in both intact membranes and purified detergent-isolated particles from a photosystem II deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803, which contains only the photosystem I reaction center. Processes with similar lifetimes and spectra are observed in both the membrane fragments and the detergent-isolated particles, suggesting little disruption of the core antenna resulting from the detergent treatment. For the detergent-isolated particles, three different excitation wavelengths were used to excite different distributions of pigments in the spectrally heterogeneous core antenna. Only two lifetimes of 2.7-4.3 ps and 24-28 ps, and a nondecaying component are required to describe all the data. The 24-28 ps component is associated with trapping. The trapping process gives rise to a nondecaying spectrum that is due to oxidation of the primary electron donor. The lifetimes and spectra associated with trapping and radical pair formation are independent of excitation wavelength, suggesting that trapping proceeds from an equilibrated excited state. The 2.7-4.3 ps component characterizes the evolution from the initially excited distribution of pigments to the equilibrated excited state distribution. The spectrum associated with the 2.7-4.3 ps component is therefore strongly excitation wavelength dependent. Comparison of the difference spectra associated with the spectrally equilibrated state and the radical pair state suggests that the pigments in the photosystem I core antenna display some degree of excitonic coupling.

Infrared microscopy for the study of biological cell monolayers. I. Spectral effects of acetone and formalin fixation

Infrared spectroscopy of biological cell monolayers grown on surfaces is a poorly developed field. This is unfortunate because these monolayers have potential as biological sensors. Here we have used infrared microscopy, in both transmission and transflection geometries, to study air-dried Vero cell monolayers. Using both methods allows one to distinguish sampling artefactual features from real sample spectral features. In transflection experiments, amide I/II absorption bands down-shift 9/4 cm(-1), respectively, relative to the corresponding bands in transmission experiments. In all other spectral regions no pronounced frequency differences in spectral bands in transmission and transflection experiments were observed. Transmission and transflection infrared microscopy were used to obtain infrared spectra for unfixed and acetone- or formalin-fixed Vero cell monolayers. Formalin-fixed monolayers display spectra that are very similar to that obtained using unfixed cells. However, acetone fixation leads to considerable spectral modifications. For unfixed and formalin-fixed monolayers, a distinct band is observed at 1740 cm(-1). This band is absent in spectra obtained using acetone-fixed monolayers. The 1740 cm(-1) band is associated with cellular ester lipids. In support of this hypothesis, two bands at 2925 and 2854 cm(-1) are also found to disappear upon acetone fixation. These bands are associated with C-H modes of the cellular lipids. Acetone fixation also leads to modification of protein amide I and II absorption bands. This may be expected as acetone causes coagulation of soluble cellular proteins. Other spectral changes associated with acetone or formalin fixation in the 1400-800 cm(-1) region are discussed.

Time-Resolved Step-Scan Fourier Transform Infrared and Visible Absorption Difference Spectroscopy for the Study of Photosystem I

Step-scan Fourier transform infrared and visible absorption difference spectroscopy, with nanosecond to millisecond time resolution, has been applied to the study of photosystem I particles from photosynthetic oxygen-evolving organisms. In particular, time-resolved infrared (1800–1200 cm−1) and visible (680–850 nm) difference spectra associated with flash induced oxidation of the primary electron donor in photosystem I particles from Synechocystis sp. 6803 and Acarychloris marina are presented. These spectra are compared directly to static, photoaccumulated Fourier transform infrared and visible difference spectra from both species. The detailed bonding interactions of P740, the primary donor in photosystem I particles from Acarychloris marina, are very similar to that found for P700, the primary donor in photosystem I particles from Synechocystis sp. 6803. P740 consists of at least two chlorophyll molecules, both of which are probably chlorophyll-d. Combined time-resolved step-scan Fourier transform infrared and visible absorption difference spectroscopy is shown to be a powerful tool for the study of single molecular bond dynamics in proteins as large as 0.4 mega-Daltons.

Near- and far-infrared p‐GaAs dual-band detector

A dual-band homojunction interfacial workfunction internal photoemission infrared detector that responds in both near- and far-infrared (NIR and FIR) regions is reported. In the p+‐i‐p+ detector structure, the emitter is carbon doped to 1.5×1019cm−3, and a 1μm thick GaAs layer acts as the barrier, followed by another highly p-doped GaAs contact layer. The NIR response is due to the interband transition in GaAs barrier layer and the threshold wavelength observed at 0.82μm is in good agreement with the 1.51eV band gap of GaAs at 4.2K. The intraband transition giving rise to FIR response is observed up to 70μm. Interband responsivity was (under 100mV reverse bias at 20K) ∼8A∕W at 0.8μm, while the intraband responsivity was ∼7A∕W. The detector has peak detectivities D*∼6×109 and 5×109cmHz1∕2∕W at 0.8 and 57μm wavelengths, respectively, under 100mV reverse bias at 20K.

Subpicosecond photoinduced electron transfer from a conjugated polymer to SnO2 semiconductor nanocrystals

Photoinduced electron transfer from the conjugated polymer poly[2-methoxy, 5-(2 � -ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) to a SnO2 nanoporous thin 5lm was studied using ultrafast IR spectroscopy. The absorption signals observed using this technique correspond to electron transfer, minimizing signal contamination from other processes. The electron transfer from polymer to nanocrystal was found to occur on an 800 fs timescale. The fast and e9cient charge separation is attributed to the large interface area and favorable energetics for transfer. Charge separation is long-lived, persisting for microseconds. ? 2002 Elsevier Science B.V. All rights reserved.

Time-Resolved FTIR Difference Spectroscopy in Combination with Specific Isotope Labeling for the Study of A1, the Secondary Electron Acceptor in Photosystem 1

A phylloquinone molecule (2-methyl, 3-phytyl, 1, 4-naphthoquinone) occupies the A(1) binding site in photosystem 1 particles from Synechocystis sp. 6803. In menB mutant photosystem 1 particles from the same species, plastoquinone-9 occupies the A(1) binding site. By incubation of menB mutant photosystem 1 particles in the presence of phylloquinone, it was shown in another study that phylloquinone will displace plastoquinone-9 in the A(1) binding site. We describe the reconstitution of unlabeled ((16)O) and (18)O-labeled phylloquinone back into the A(1) binding site in menB photosystem 1 particles. We then produce time-resolved A(1)(-)/A(1) Fourier transform infrared (FTIR) difference spectra for these menB photosystem 1 particles that contain unlabeled and (18)O-labeled phylloquinone. By specifically labeling only the phylloquinone oxygen atoms we are able to identify bands in A(1)(-)/A(1) FTIR difference spectra that are due to the carbonyl (C=O) modes of neutral and reduced phylloquinone. A positive band at 1494 cm(-1) in the A(1)(-)/A(1) FTIR difference spectrum is found to downshift 14 cm(-1) and decreases in intensity on (18)O labeling. Vibrational mode frequency calculations predict that an antisymmetric vibration of both C=O groups of the phylloquinone anion should display exactly this behavior. In addition, phylloquinone that has asymmetrically hydrogen bonded carbonyl groups is also predicted to display this behavior. The positive band at 1494 cm(-1) in the A(1)(-)/A(1) FTIR difference spectrum is therefore due to the antisymmetric vibration of both C=O groups of one electron reduced phylloquinone. Part of a negative band at 1654 cm(-1) in the A(1)(-)/A(1) FTIR difference spectrum downshifts 28 cm(-1) on (18)O labeling. Again, vibrational mode frequency calculations predict this behavior for a C=O mode of neutral phylloquinone. The negative band at 1654 cm(-1) in the A(1)(-)/A(1) FTIR difference spectrum is therefore due to a C=O mode of neutral phylloquinone. More specifically, calculations on a phylloquinone model molecule with the C(4)=O group hydrogen bonded predict that the 1654 cm(-1) band is due to the non hydrogen bonded C(1)=O mode of neutral phylloquinone.

FTIR Difference Spectroscopy in Combination with Isotope Labeling for Identification of the Carbonyl Modes of P700 and P700+ in Photosystem I

Room temperature, light induced (P700(+)-P700) Fourier transform infrared (FTIR) difference spectra have been obtained using photosystem I (PS I) particles from Synechocystis sp. PCC 6803 that are unlabeled, uniformly (2)H labeled, and uniformly (15)N labeled. Spectra were also obtained for PS I particles that had been extensively washed and incubated in D(2)O. Previously, we have found that extensive washing and incubation of PS I samples in D(2)O does not alter the (P700(+)-P700) FTIR difference spectrum, even with approximately 50% proton exchange. This indicates that the P700 binding site is inaccessible to solvent water. Upon uniform (2)H labeling of PS I, however, the (P700(+)-P700) FTIR difference spectra are considerably altered. From spectra obtained using PS I particles grown in D(2)O and H(2)O, a ((1)H-(2)H) isotope edited double difference spectrum was constructed, and it is shown that all difference bands associated with ester/keto carbonyl modes of the chlorophylls of P700 and P700(+) downshift 4-5/1-3 cm(-1) upon (2)H labeling, respectively. It is also shown that the ester and keto carbonyl modes of the chlorophylls of P700 need not be heterogeneously distributed in frequency. Finally, we find no evidence for the presence of a cysteine mode in our difference spectra. The spectrum obtained using (2)H labeled PS I particles indicates that a negative difference band at 1698 cm(-1) is associated with at least two species. The observed (15)N and (2)H induced band shifts strongly support the idea that the two species are the 13(1) keto carbonyl modes of both chlorophylls of P700. We also show that a negative difference band at approximately 1639 cm(-1) is somewhat modified in intensity, but unaltered in frequency, upon (2)H labeling. This indicates that this band is not associated with a strongly hydrogen bonded keto carbonyl mode of one of the chlorophylls of P700.

Directionality of electron transfer in cyanobacterial photosystem I at 298 and 77 K

Electron transfer processes in cyanobacterial photosystem I particles from Synechocystis sp. PCC 6803 with a high potential naphthoquinone (2,3‐dichloro‐1,4‐naphthoquinone) incorporated into the A1 binding site have been studied at 298 and 77 K using time‐resolved visible and infrared difference spectroscopy. The high potential naphthoquinone inhibits electron transfer past A1, and biphasic P700+A1 − radical pair recombination is observed. The two phases are assigned to P700+A1B − and P700+A1A − recombination. Analyses of the transient absorption changes indicate that the ratio of A‐ and B‐branch electron transfer is 95:5 at 77 K and 77:23 at 298 K.