Photoinduced Absorbance Research Articles

Bimolecular Generation of Excitonic Luminescence from Dark Photoexcitations in Ruddlesden-Popper Hybrid Metal-Halide Perovskites.

The nature of photoexcitations in Ruddlesden-Popper (RP) hybrid metal halide perovskites is still under debate. While the high exciton binding energy in the hundreds of millielectronvolts indicates excitons as the primary photoexcitations, recent reports found evidence for dark, Coulombically screened populations, which form via strong coupling of excitons and the atomic lattice. Here, we use time-resolved mid-infrared spectroscopy to gain insights into the nature and recombination of such dark excited states in (BA)2(MA)n-1PbnI3n+1 (n = 1,2,3) via their intraband electronic absorption. In stark contrast to results in the bulk perovskites, all samples exhibit a broad, unstructured mid-IR photoinduced absorbance with no infrared activated modes, independent of excitonic confinement. Further, the recombination dynamics are dominated by a bimolecular process. In combination with steady-state photoluminescence experiments, we conclude that screened, dark photoexcitations act as a population reservoir in the RP hybrid perovskites, from which nongeminate formation of bright excitons precedes generation of photoluminescence.

Resolution enhancement of pump–probe microscope with an inverse-annular filter

Optical pump–probe microscopy can provide images by detecting changes in probe light intensity induced by stimulated emission, photoinduced absorbance change, or photothermal-induced refractive index change in either transmission or reflection mode. Photothermal microscopy, which is one type of optical pump–probe microscopy, has intrinsically super resolution capability due to the bilinear dependence of signal intensity of pump and probe. We introduce new techniques for further resolution enhancement and fast imaging in photothermal microscope. First, we introduce a new pupil filter, an inverse-annular pupil filter in a pump–probe photothermal microscope, which provides resolution enhancement in three dimensions. The resolutions are proved to be improved in lateral and axial directions by imaging experiment using 20-nm gold nanoparticles. The improvement in X (perpendicular to the common pump and probe polarization direction), Y (parallel to the polarization direction), and Z (axial direction) are by 15 ± 6, 8 ± 8, and 21 ± 2% from the resolution without a pupil filter. The resolution enhancement is even better than the calculation using vector field, which predicts the corresponding enhancement of 11, 8, and 6%. The discussion is made to explain the unexpected results. We also demonstrate the photothermal imaging of thick biological samples (cells from rabbit intestine and kidney) stained with hematoxylin and eosin dye with the inverse-annular filter. Second, a fast, high-sensitivity photothermal microscope is developed by implementing a spatially segmented balanced detection scheme into a laser scanning microscope using a Galvano mirror. We confirm a 4.9 times improvement in signal-to-noise ratio in the spatially segmented balanced detection compared with that of conventional detection. The system demonstrates simultaneous bi-modal photothermal and confocal fluorescence imaging of transgenic mouse brain tissue with a pixel dwell time of 20 µs. The fluorescence image visualizes neurons expressing yellow fluorescence proteins, while the photothermal signal detected endogenous chromophores in the mouse brain, allowing 3D visualization of the distribution of various features such as blood cells and fine structures most probably due to lipids. This imaging modality was constructed using compact and cost-effective laser diodes, and will thus be widely useful in the life and medical sciences. Third, we have made further resolution improvement of high-sensitivity laser scanning photothermal microscopy by applying non-linear detection. By this, the new method has super resolution with 61 and 42% enhancement from the diffraction limit values of the probe and pump wavelengths, respectively, by a second-order non-linear scheme and a high-frame rate in a laser scanning microscope. The maximum resolution is determined to be 160 nm in the second-order non-linear detection mode and 270 nm in the linear detection mode by the PT signal of GNPs. The pixel rate and frame rate for 300 × 300 pixel image are 50 µs and 4.5 s, respectively. The pixel and frame rate are shorter than the rates, those are 1 ms and 100 s, using the piezo-driven stage system.

Bicarbonate stimulates the electron donation from Mn2+ to [formula omitted] in isolated D1/D2/cytochrome b559 complex

Influence of bicarbonate on the efficiency of the electron donation from Mn2+ to P680+ in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO3-/CO2. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (−ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn2+ under anaerobic conditions were measured. Addition of bicarbonate (1mM) increased the amplitude of these ΔA and −ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P680, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05mM Mn2+ alone or jointly with 1mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680nm. The effect of Mn2+ (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12mM EDTA. All these bicarbonate effects were not observed if MgCl2 or formate were used instead of MnCl2 and bicarbonate, respectively. In the absence of Mn2+, bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P680). The presented data suggest that bicarbonate stimulates the electron donation from Mn2+ to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes.

Magnetic field and methylene chain length effects on photochemistry of bichromophoric chain species containing 7-nitro-2-fluorenyloxy and anilino chromophores

The magnetic field effects (MFEs) and methylene chain length effects on photochemistry of [(7-nitro-2-fluorenyloxy)alkyl]aniline have been investigated by means of electronic absorption spectroscopy and high performance liquid chromatography (HPLC). When the number of methylene groups was 12, appreciable MFEs were detected on photoredox reaction yields, while when the chain length was short (three or six methylenes) almost no MFEs on photoproduct yield were observed. Photoinduced absorbance changes suggest that the photochemical reaction mechanism for the long-chain species is totally different from that for the short-chain compounds. MFEs on photoproduct yields observed for the long-chain species can be explained in terms of competition between cage and escape processes, spin-lattice relaxations and methylene chain dynamics which governs the interconversion between folded and extended conformers.

Predominant protection of D2-protein against photodestruction in isolated D1/D2/cytochrome b559 by K15, a phenolic-type inhibitor of electron transfer in photosystem 2.

A protective action of K15 (4-[methoxy-bis(trifluoromethyl)methyl]-2,6-dinitrophenylhydrazone methyl ketone), an inhibitor of electron transport in photosystem 2 (PS 2), against photoinactivation of the PS 2 reaction center (RC) D1/D2/cytochrome b(559) complex, isolated from pea chloroplasts, by red light (0.7 mmol photons/sec per m(2)) has been investigated under aerobic conditions. The inhibitor K15 causing cyclic electron transfer around PS 2 and thus prohibiting stabilization of separated charges has been shown to effectively protect RC both against the loss of photochemical activity (measured as reversible photoinduced absorbance changes related to photoreduction of pheophytin) and aggregation and degradation of the proteins D2 and D1 during photoinactivation. Comparison of the protective action of K15 and of another inhibitor of electron transfer in PS 2, diuron, against light-induced destruction of proteins D1 and D2 shows that diuron stabilizes protein D1 and K15 stabilizes protein D2. The preferential protection of D2 against photoinduced destruction revealed in our work is in accord with the concept of a specific binding of K15 with this protein. It is proposed that this binding site may be that of the primary quinone electron acceptor Q(A) located on the D2 protein (in contrast to diuron, which is known to replace the secondary electron acceptor Q(B) from its binding site on D1).

A new pathway of photoinactivation of photosystem II. Irreversible photoreduction of pheophytin causes loss of photochemical activity of isolated D1-D2-cytochrome b559 complex.

A new pathway of photoinactivation of photosystem II (PS II) connected with irreversible photoaccumulation of reduced pheophytin (Ph) in isolated D1-D2-cytochrome b559 complexes of reaction center (RC) of PS II was discovered. The inhibitory effects of white light illumination on photochemical activity of D1-D2-cytochrome b559 complexes of RCs of photosystem II, isolated from pea chloroplasts, have been compared under anaerobic conditions in the absence and in the presence of sodium dithionite, electron transfer from which to the oxidized primary electron donor P680+ results in the photoaccumulation of anion-radical of the primary electron acceptor, Ph(-.). In both cases, prolonged illumination (1-5 min, 120 W/m(2)) led to a pronounced loss of the photochemical activity as it was monitored by measuring the amplitude of the reversible photoinduced absorbance changes at 682 nm related to the photoreduction of Ph. The extent of the photoinactivation depended on the illumination time and pH of the medium. At pH 8.0, the presence of dithionite during photoinactivation brought about a protective effect compared to that in a control sample. In contrast, lowering pH to 6.0 increased the sensitivity to photoinactivation in the dithionite-containing samples. For 5 min irradiation, the photochemical activity in the absence and in the presence of dithionite decreased by 35 and 72%, respectively (this was accompanied by an irreversible bleaching of the pheophytin Q(x) absorption band at 542 nm). Degradation of the D1 and D2 proteins was not observed under these conditions. A subsequent addition of an electron acceptor, potassium ferricyanide, to the illuminated samples restored neither the amplitude of the signal at 682 nm nor absorption at 542 nm. It is suggested that at pH. 7.0 the photoaccumulated Ph(-.) is irreversibly converted into a secondary, most probably protonated form, that does not lead to destruction of the RCs but prevents the photoformation of the primary radical pair [P680+Ph(-.)]. A possible application of this effect to photoinactivation of PS II in vivo is discussed.

Photoinduced Transient Absorbance Spectra of P840/P840 + and the FMO Protein in Reaction Centers of Chlorobium vibrioforme

The kinetics of photoinduced absorbance changes in the 400-ns to 100-ms time range were studied between 770 and 1025 nm in reaction center core (RCC) complexes isolated from the green sulfur bacterium Chlorobium vibrioforme. A global, multiple stretched-exponential analysis shows the presence of two distinct but strongly overlapping spectra. The spectrum of the 70- μs component consists of a broad bleaching with two minima at 810 and 825 nm and a broad positive band at wavelengths greater than 865 nm and is assigned to the decay of 3Bchl a of the Fenna-Matthews-Olson (FMO) protein. The contribution of the 70- μs component correlates with the amount of FMO protein in the isolated RCC complex. The spectrum of the 1.6- μs component has a sharp bleaching at 835 nm, a maximum at 805 nm, a broad positive band at wavelengths higher than 865 nm, and a broad negative band at wavelengths higher than 960 nm. When the RCC is incubated with inorganic iron and sulfur, the 1.6- μs component is replaced by a component with a lifetime of ∼40 μs, consistent with the reconstruction of the F X cluster. We propose that the 1.6- μs component results from charge recombination between P840 + and an intermediate electron acceptor operating between A 0 and F X. Our studies in Chlorobium RCCs show that approaches that employ a single wavelength in the measurement of absorption changes have inherent limitations and that a global kinetic analysis at multiple wavelengths in the near-infrared is required to reliably separate absorption changes due to P840/P840 + from the decay of 3Bchl a in the FMO protein.

A Single Arginyl Residue in Plastocyanin and in Cytochrome c6 from the Cyanobacterium Anabaenasp. PCC 7119 Is Required for Efficient Reduction of Photosystem I

Positively charged plastocyanin from Anabaena sp. PCC 7119 was investigated by site-directed mutagenesis. The reactivity of its mutants toward photosystem I was analyzed by laser flash spectroscopy. Replacement of arginine at position 88, which is adjacent to the copper ligand His-87, by glutamine and, in particular, by glutamate makes plastocyanin reduce its availability for transferring electrons to photosystem I. Such a residue in the copper protein thus appears to be isofunctional with Arg-64 (which is close to the heme group) in cytochrome c(6) from Anabaena (Molina-Heredia, F. P., Diaz-Quintana, A., Hervás, M., Navarro, J. A., and De la Rosa, M. A. (1999) J. Biol. Chem. 274, 33565-33570) and Synechocystis (De la Cerda, B., Diaz-Quintana, A., Navarro, J. A. , Hervás, M., and De la Rosa, M. A. (1999) J. Biol. Chem. 274, 13292-13297). Other mutations concern specific residues of plastocyanin either at its positively charged east face (D49K, H57A, H57E, K58A, K58E, Y83A, and Y83F) or at its north hydrophobic pole (L12A, K33A, and K33E). Mutations altering the surface electrostatic potential distribution allow the copper protein to modulate its kinetic efficiency: the more positively charged the interaction site, the higher the rate constant. Whereas replacement of Tyr-83 by either alanine or phenylalanine has no effect on the kinetics of photosystem I reduction, Leu-12 and Lys-33 are essential for the reactivity of plastocyanin.

Theoretical analysis of the flash photolysis kinetics of purple membrane

The laser flash excitation and thermal relaxation kinetics of purple membrane is theoretically analyzed using the rate equations of multiple states system. The derived equation indicates that the photoinduced absorbance change is a complex multi-exponential process. This is proved by a flash photolysis experiment using 532 nm, 10 ns laser pulse as pumping source and 632.8 nm CW laser as probing light. Comparison between the experimental fitting result and the theoretical analysis reveals the possibility of the existence of the KL state.

Absorption of pulsed CO 2 laser in silicon and resulting laser ablation and surface damage formation

The fluence dependence of the absorption of TEA CO 2 laser in silicon wafers at various dopant concentrations has been studied. The transmittance decreased with increasing fluence, and the effect of the dopant concentration was found to be small. The time profile of the pulse intensity after transmitting through a sample showed a narrowing of the width. The results indicate a buildup of photo-induced absorbance in the middle of a pulse, leading to the absorption of the later part of the pulse, and also suggest that this effect is insensitive to the ground state absorbance of the silicon samples.

An Azulenic Bacteriorhodopsin Analog Has Photoinduced Activity

Abstract— The photoinduced changes of absorbance as well as proton release and uptake have been observed for an azulenic analog of wild‐type bacteriorhodopsin. The release and uptake of protons have been measured using a highly sensitive electrochemical technique. Bacteriorhodopsin membrane patches on a tin‐oxide electrode produce a transient photocurrent that is negative for proton release and positive for proton uptake. For azulenic bacteriorhodopsin the photocurrent is approximately 20% of the transient observed with native bacteriorhodopsin. The existence of the photoinduced absorbance changes and the transient photocurrent are important results for gaining further insight into the photoinduced function of bacteriorhodopsin.

Picosecond absorbance difference spectra of the antenna of photosynthetic purple bacteria The influence of exciton interactions and librations

The influence of exciton interactions and pseudolocal librational modes on the relaxation of photo-induced absorption changes of the light-harvesting antenna of the photosynthetic purple bacterium, Rhodospirillum rubrum, was studied in the ps time domain. A hypothesis is put forward that the long-lived librational modes can occur in the circular aggregate of light-harvesting bacteriochlorophyll molecules of the core antenna. The model spectra obtained quantitatively fit the shape of the photo-induced picosecond absorbance difference spectra. The model predicts the decay of photo-induced spectra with the time constant within tens of picoseconds.

Identification and separation of spectral change due to temperature increase in nanosecond transient absorption spectra of polydiacetylene thin films

Time dependence of a photoinduced absorbance change in a thin film of polydiacetylene PDA-3BCMU (poly-4,6-decadiyne-1, 10-diol-bis( n-butoxycarbonyl-methylurethane)) was separated from the effect of increasing sample temperature by a newly designed method. The resultant nonthermal contribution relaxes via a geminate recombination process, probably due to polarons.

Ultrafast non-linear optical properties and photoinduced relaxation dynamics of poly(2,5-thienylenevinylene)

The transient absorption spectra of a thin film of poly(2,5-thienylenevinylene) at photon energies between 1.15 and 2.55 eV are measured by femtosecond pump-probe spectroscopy, where the pump pulse (≈ 100 fs) is resonant with the π ∗ → π transition. Several ultrafast non-linear optical processes are observed during the temporal overlap of the pump and probe beams, including bleaching, Raman gain and induced phase modulation. The photoinduced absorbance change signals are found to shift spectrally to higher photon energies with an estimated time constant of 100–200 fs in the spectral region from 1.15 to 1.90 eV, indicating the geometrical relaxation to a self-trapped state. The transient decay curves are not described well by single exponential decays but can be fitted to a biexponential decay function. The fast and slow decay time constants are both found to increase from 0.16 to 0.86 ps and 4 to 14 ps, respectively, with higher probe photon energies from 1.44 to 1.82 eV. The fast decay component included both the relaxation of the hot self-trapped exciton to the adiabatic ground-state potential-energy surface and a phonon emission process. The slow decay component corresponds to both the decay of a thermalized self-trapped state via tunnelling to the ground state and a thermalization process. The observed spectral shift and decay kinetics are similar to photoexcitation dynamics previously observed in polydiacetylenes and polythiophenes.

Dynamics of photoexcitations in cis-polyacetylene.

The photoinduced absorption of cis-(CH${)}_{\mathrm{x}}$ has been investigated by nanosecond time-resolved spectroscopy. Photoinduced absorption at 1.55 eV and bleaching of the interband transition at 2.05 eV are observed. The photoinduced absorbance change decays as exp[-(t/\ensuremath{\tau}${)}^{1/2}$] with \ensuremath{\tau}=2.3\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s at 80 K. The photoinduced absorption is considered to be due to exciton bipolarons or polarons. The decay kinetics is interpreted by the trapping model, i.e., an excited species disappears at traps after a one-dimensional random walk.

Electric-field dependence of the quantum yield in reaction centers of photosynthetic bacteria

Multilayer Langmuir-Blodgett films of reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides have been fabricated with partial net orientation. The films showed substantial electrical response under pulsed illumination. From measurements of the light-induced voltage generated across the Langmuir-Blodgett film, we have succeeded in quantitating the electric-field dependence of the quantum yield of charge separation in photosynthesis. The results presented here are compared with our previous determination of the field effect on quantum yield, in which flash-activated charge separation as a function of the applied field was assayed by the extent of bacteriochlorophyll dimer, (BChl)2, oxidation measured optically at 860 nm. The two methods provided consistent dependencies of quantum yield on applied electric field. Analysis of the data reveals that the quantum yield of (BChl)⨥2BPhQ⨪A formation decreases from a value of 0.96 at zero applied field to about 0.75 for a field of 120 mV/nm vectorially directed to hinder light-activated electron transfer. For oppositely applied fields, the quantum yield saturates at unity. The source of the effects is considered to reside in the electric field dependence of the free-energy difference between the energy levels that are involved in the initial charge separation between the (BChl)2 in the first singlet excited state, (BChl)∗2, through the bacteriopheophytin, BPh, to the primary ubiquinone, QA. Possible contributions to the field-induced loss of quantum yield of (BChl)⨥2BPhQ⨪A formation are: (1) a decrease in the free-energy gap between the states (BChl)∗2 and (BChl)⨥2BPh⨪QA, leading to an increased rate of decay via the excited singlet state back to the ground state; (2) a stimulated return from (BChl)⨥2BPh⨪QA directly or via the (BChl)2 triplet state to the ground state and (3) an impeded electron transfer from (BChl)⨥2BPh⨪QA to (BChl)⨥2BPhQ⨪A. These possibilities are discussed. Correlation of the electrical response with measurements of the photo-induced absorbance change allows determination of the projection of the electron-transfer distance on the normal to the plane of the film, which is in good agreement with previous measurements using different techniques.

Rotational mobility of the photoreaction center in chromatophore membranes of Rhodospirillum rubrum

We investigated the rotational mobility of the photoreaction center in chromatophores of Rhodospirillum rubrum by studying the photoinduced linear dichroism of absorption changes at 865 nm. The study was carried out in suspensions of chromatophores treated with ferricyanide in order to bleach their antenna bacteriochlorophyll and thus minimize depolarization by energy transfer. Very little depolarization of the photoinduced absorbance change at 865 nm was observed at room temperature for chromatophores immersed in a highly viscous medium over the time range 0–10 ms following an exciting light flash. In the light of independent evidence for transmembrane arrangement of the photoreaction center, we conclude that the photoreaction center protein is immobilized in the chromatophore membrane for at least 10 ms.

Orientation of the hemes of high potential cytochromes relative to photosynthetic membranes, as shown by the linear dichroism of oriented preparations

The orientations of high potential cytochromes with respect to photosynthetic membranes was investigated in spinach chloroplasts and in Rhodopseudomonas viridis. The general approach consists in detection with polarized light of photoinduced absorbance changes related to the oxidation of the cytochromes. The orientation of cytochrome c-558 was measured at room temperature in chromatophores and whole cells of Rps. viridis, oriented on glass slides and in a magnetic field, respectively. The orientation of cytochrome b-559 of green plants was detected at 77 K in magnetically oriented chloroplasts. In both cases the dichroic ratio for the α band shows that the heme plane makes an angle greater than 35°C with the membrane plane. Moreover, the dichroic ratio is not constant throughout the α and β bands, for both cytochrome c-558 and b-559. Linear dichroism spectra of oriented pure horse heart cytochrome c and cytochrome c 2 of Rhodopseudomonas sphaeroides in stretched polyvinyl alcohol films show that the variations of the dichroic ratio in the α and β bands can be explained by the occurrence of x- and y-polarized transitions absorbing at slightly different wavelengths.

Low temperature photo-induced reactions in green leaves and chloroplasts

1. Swiss chard leaves, spinach leaves and spinach chloroplasts at liquid nitrogen temperatures exhibit photo-induced absorbance changes with maxima at 556–557, 680–682 and 703–705 nm. Experiments reported indicate that the 556 nm peak is due to the α-band of cytochrome b 559. The peak at 680 nm is attributed to P 680, the photoactive center of photosystem II, and the peak at 703 nm is attributed to P 700. 2. Laser-induced oxidation of cytochrome b 559 has a half-time of 4.6 msec. Laser oxidized P 680 has two reductive phases, one having a half-time of 30 μsec and the other of 4.5 msec. P 700 has only one reductive phase with a half-time of approx. 30 μsec. The kinetics strongly suggest that P 680 is the primary oxidant of cytochrome b 559 at low temperatures. 3. The slow reductive phase of P 680 and oxidation of cytochrome b 559 tended to have equal and approximately constant rates between 80 and 220°K.

Fractionation of Chlorophyll Forms from Euglena and Measurement of Light-Induced Absorbance Changes

Since the realization that 2 or more light-reactions participate in photosynthesis and that plants contain several absorbing forms of chlorophyll, correlations have been sought between photochemical activity and particular pigment forms. Traditionally, action spectra have been used to identify functional pigments, btut this nmethod fails either when the activity being measured results from several reactions or when the absorption spectra of the pigments overlap strongly. The chlorophyll forms fall into this latter category. Therefore several investigators have been attempting to break-up the photochemical al)paratus of chloroplasts and bacterial chromatophores by the aid of detergents, to fractionate by centrifugation, and then to measure absorption, fluorescence, or some type of photochemical activity in the fractions. A beginning in this type of investigation has been achieved (2, 3, 5, 1 1 ). The alga, Euglena, when grown with dim light has 3 chlorophyll a absorption maxima at approximately 670, 680, and 695 m,u. Aqueous colloidal extracts of this alga have been extensively studied by us in attempts to isolate these forms and to learn about the nature of chlorophyll in vivo (6). In the whole extract chlorophyll a-695 is much more labile than the other 2 forms and always disappears first when the extract is heated, strongly irradiated, acidified, or differentially extracted with organic solvents. No separation of the different absorbing forms was accomplished by differential centrifugation of the aqueous extract. In new experiments we have observed that a low concentration of the detergent, sodium deoxycholate (DOC), does not change the position or height of the chlorophyll absorption bands. By centrifugation of the detergent extract a supernatanit fraction containing Ca670 can be separated from a sedinment with Ca680 and Ca695. We observed 2 types of photoinduced absorbance changes at 419, 518, and 591 mi.t in both fractions but the rate anld degree of the response differed in the 2 fractions. The different responses may be explained by differences in the forms of chlorophyll present and possibly also by a partial separation of photosynthetic electron transfer components.