Geminate Recombination Process Research Articles

Correction of the calculation method of CT state energy in ITIC and Y6 acceptor systems

The charge transfer (CT) state at the donor (D)/acceptor (A) interface in organic solar cell (OSC) is the core of exciton dissociation and geminate recombination processes, which are affected by geometries and electron transition. Recently, the CT mechanism developed by fullerenes has produced some unsuitable phenomena in the application of high performance NFA materials, due to the transformation of the highly (three dimensional) symmetric structure to a linear structure. In order to improve the calculation accuracy and custom-matched calculation models for NFA, two benzo[1,2-b:4,5-b′]difuran (BDF) polymers (P-FT)/3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (m-ITIC) and P-FT/2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl) bis (methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) were selected as the research objects, the four levels with gradient variation were constructed, including identification methods (CDD or NTO), range-separated hybrid (RSH) functionals, polarizable continuum models (PCM) and related ω parameter. The results show that RSH functional is the dominant factor affecting CT state. The ωB97XD functional should be selected without distinguishing CT state types. However, when considering the types and high-energy of CT states, it should be noted that there is a strong system dependence. In addition, if a solvent model needs to be added, solvent optimization should be performed on the ω parameter. This work aims to define uniform ECT calculation models and standards to improve accuracy and provide suitable target selection for relevant research.

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Charge‐Transfer States at Organic–Organic Interfaces: Impact of Static and Dynamic Disorders

AbstractMolecular dynamics simulations are combined with density functional theory calculations to evaluate the impact of static and dynamic disorders on the energy distribution of charge‐transfer (CT) states at donor–acceptor heterojunctions, such as those found in the active layers of organic solar cells. It is shown that each of these two disorder components can be partitioned into contributions related to the energetic disorder of the transport states and to the disorder associated with the hole–electron electrostatic interaction energies. The methodology is applied to evaluate the energy distributions of the CT states in representative bulk heterojunctions based on poly‐3‐hexyl‐thiophene and phenyl‐C61‐butyric‐acid methyl ester. The results indicate that the torsional fluctuations of the polymer backbones are the main source of both static and dynamic disorders for the CT states as well as for the transport levels. The impact of static and dynamic disorders on radiative and nonradiative geminate recombination processes is also discussed.

Proton Transfer Reaction Dynamics of Pyranine in DMSO/Water Mixtures.

Photoinduced intermolecular excited-state proton transfer (ESPT) reactions are ubiquitous in chemistry and biology. ESPT reactions are extremely sensitive to the nature of water molecules in its microenvironment and thus serve as a sensitive reporter for the water structure and dynamics in a system. Herein, the photoinduced intermolecular ESPT reaction of 8-hydroxypyrene-1,3,5-trisulfonic acid (HPTS, also known as pyranine) has been investigated in various DMSO/water mixtures by using steady-state and time-resolved emission spectroscopy. The DMSO/water binary mixture yields an interesting and anomalous behavior for the proton transfer reaction dynamics of HPTS at a mole fraction of DMSO (XDMSO ) of 0.41-0.51, which has also been previously investigated and projected as an anomalous region by molecular dynamics simulation and other experimental techniques. The extreme slowdown of the proton transfer reaction observed at XDMSO =0.41-0.51 has been attributed to the slow solvation dynamics, as well as the non-availability of free water molecules at this composition, which are required to solvate the newly generated proton. These observations have been also corroborated by time-resolved area-normalized emission spectra. The dimensionality of the proton diffusion process has been investigated by analyzing the geminate recombination process, and is found to be significantly different in DMSO/water mixtures (XDMSO =0.41-0.51) compared with three-dimensional proton diffusion in neat water.

Limits on the Fill Factor in Organic Photovoltaics: Distinguishing Nongeminate and Geminate Recombination Mechanisms.

In this Letter, we present transient optoelectronic experimental studies of the recombination processes limiting the fill factor (FF) in three conjugated polymer:fullerene systems, poly(3-hexylthiophene) (P3HT) and two lower-band-gap polymers that exhibit lower FFs poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) and poly(2,7-(9,9-dioctylfluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) (APFO-3). Using transient absorption spectroscopy, charge extraction, and transient photovoltage experiments, we show that the lower FF observed for the PCPDTBT-based device results from enhanced nongeminate recombination even at short circuit, In contrast, we show that for APFO-3 devices, the FF is primarily limited by a voltage-dependent free charge generation, which we assign to a geminate recombination process.

Femtosecond pulse radiolysis study of geminate ion recombination in biphenyl–dodecane solution

The kinetics of geminate ion recombination was studied by the femtosecond pulse radiolysis of n-dodecane with biphenyl solution. The complicated biphenyl concentration-dependence behaviors of the transient absorptions at 800nm are observed and elucidated with a Monte Carlo simulation of geminate recombination and charge transfer processes in the biphenyl-dodecane solution. The experimental data suggested the existence of n-dodecane excited radical cation in the radiolysis. It plays very important roles especially in the high-concentration solution. The reaction rate constant of the excited radical cation with biphenyl molecule was 3.5×1011dm3mol−1s−1, which is comparable with the reaction rate constant of the electron with biphenyl molecule. The contributions of the geminate ion recombination, the charge transfer reaction and the relaxation of the excited radical cation were obtained as a function of the biphenyl concentration.

Ultrafast Proton Transfer of Three Novel Quinone Cyanine Photoacids

Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), k(PT), of the three dyes is roughly 1.5 × 10(12) s(-1), a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H(2)O and D(2)O. It is twice as fast as the proton transfer rate between two adjacent water molecules in liquid water. We found that, as expected, two of the sulfonated photoacids geminately recombines with the proton at an elevated rate. The accelerated geminate recombination process of the sulfonated derivatives is different from a simple diffusion process of protons. The ESPT rate coefficient of these molecules is the highest recorded thus far.

Multiphoton absorption of myoglobin-nitric oxide complex: relaxation by D-NEMD of a stationary state.

The photodissociation and geminate recombination of nitric oxide in myoglobin, under continuous illumination, is modeled computationally. The relaxation of the photon energy into the protein matrix is also considered in a single simulation scheme that mimics a complete experimental setup. The dynamic approach to non-equilibrium molecular dynamics is used, starting from a steady state, to compute its relaxation to equilibrium. Simulations are conducted for the native form of sperm whale myoglobin and for two other mutants, V68W and L29F, illustrating a fair diversity of spatial and temporal geminate recombination processes. Energy flow to the heme and immediate protein environment provide hints to allostery. In particular, a pathway of energy flow between the heme and the FG loop is illustrated. Although the simulations were conducted for myoglobin only, the thermal fluctuations of the FG corner are in agreement with the large structural shifts of FG during the allosteric transition of tetrameric hemoglobin.

Isotope effect on ultrafast charge-transfer-to-solvent reaction from I− to water in aqueous NaI solution

A charge-transfer-to-solvent (CTTS) reaction from a photoexcited iodine atomic anion, I- (aq), in bulk water (H2O and D2O) was studied by time-resolved photoelectron spectroscopy using a liquid beam (microjet) of aqueous NaI solution. The P-2(3/2) CTTS state of I- (aq) was excited by a 226 nm femtosecond laser pulse and the evolution of the nonstationary electronic state was probed using another ultraviolet femtosecond laser pulse. Global fitting of the observed time-dependent photoelectron kinetic energy distributions provided the time constants of individual reaction steps and the photoelectron spectra from the CTTS state, a contact pair, the solvent-separated state, and a hydrated electron. Most of the elementary reaction steps revealed a strong deuterium isotope effect, indicating coupling of the electron dynamics and the hydrogen atomic motion of solvent water. However, nondiffusive geminate recombination processes from the CTTS state and a contact pair were almost insensitive to deuteration. Consequently, geminate recombination processes from the CTTS state and a contact pair occurs more efficiently in D2O, because the response of water is decelerated in D2O. In contrast, the recombination process from the solvent-separated state in the final step of the CTTS reaction is less efficient in D2O, presumably due to the smaller zero point energy.

Characterization of iso-CF2I2 in frequency and ultrafast time domains

The photolysis of diiododifluoromethane (CF(2)I(2)) in condensed phases was studied by a combination of matrix isolation and ultrafast time-resolved spectroscopy, in concert with ab initio calculations. Photolysis at wavelengths of 355 or 266 nm of CF(2)I(2):Ar samples (1:5000) held at approximately 8 K yielded iso-CF(2)I(2) (F(2)C-I-I), a metastable isomer of CF(2)I(2), characterized here for the first time. The infrared (IR) spectra of this isomer were recorded in matrix experiments, and the derived positions of the C-F stretching modes are in very good agreement with the predictions of high level ab initio calculations, which show that the iso-form is a minimum on the CF(2)I(2) ground state potential energy surface. The formation of this isomer following 350 nm excitation of CF(2)I(2) in room temperature CCl(4) solutions was monitored through its intense C-F stretching mode by means of ultrafast time-resolved IR absorption. Together, matrix isolation and ultrafast IR absorption experiments suggest that the formation of iso-CF(2)I(2) occurs via recombination of CF(2)I radical and I atom. Ultrafast IR experiments detect a delayed rise of iso-CF(2)I-I absorption, placing an upper limit of 400 fs for the C-I bond dissociation and primary geminate recombination processes. The product absorption spectrum recorded 1 ns after 350 nm excitation of CF(2)I(2) in solution is virtually identical to the visible absorption spectrum of iso-CF(2)I(2) trapped in matrix isolation experiments [with subtracted I(2)(X) absorption]. The formation of this isomer in solution at room temperature has direct dynamic implications for the ultrafast production of molecular iodine from electronically excited CF(2)I(2).

Effect of Charge Trapping on Geminate Recombination and Polymer Solar Cell Performance

In this letter, we examine the effect of charge trapping on geminate recombination and organic photovoltaic performance using a Monte Carlo model. We alter the degree of charge trapping by considering energetic disorder to be spatially uncorrelated or correlated. On correlating energetic disorder, and so reducing the degree of trapping, it is found that power conversion efficiency of blend and bilayer devices improves by factors of 3.1 and 2.6, respectively. These results are related to the experimental data and quantum chemical calculations for poly[9,9-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine] (PFB)/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as well as poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl-C(61)-butyric acid methyl ester (PCBM) solar cell systems. The minimization of traps at the heterojunction between electron- and hole-accepting materials, perhaps by molecular design, appears to be a promising strategy to achieve large gains in PV performance. It is also shown that macroscopically measurable quantities such as mobility and energetic disorder are not necessarily good predictors of nanoscale geminate recombination process.

Ultrafast Dynamics of Diatomic Ligand Binding to Nitrophorin 4

Nitrophorin 4 (NP4) is a heme protein that stores and delivers nitric oxide (NO) through pH-sensitive conformational change. This protein uses the ferric state of a highly ruffled heme to bind NO tightly at low pH and release it at high pH. In this work, the rebinding kinetics of NO and CO to NP4 are investigated as a function of iron oxidation state and the acidity of the environment. The geminate recombination process of NO to ferrous NP4 at both pH 5 and pH 7 is dominated by a single approximately 7 ps kinetic phase that we attribute to the rebinding of NO directly from the distal pocket. The lack of pH dependence explains in part why NP4 cannot use the ferrous state to fulfill its function. The kinetic response of ferric NP4NO shows two distinct phases. The relative geminate amplitude of the slower phase increases dramatically as the pH is raised from 5 to 8. We assign the fast phase of NO rebinding to a conformation of the ferric protein with a closed hydrophobic pocket. The slow phase is assigned to the protein in an open conformation with a more hydrophilic heme pocket environment. Analysis of the ultrafast kinetics finds the equilibrium off-rate of NO to be proportional to the open state population as well as the pH-dependent amplitude of escape from the open pocket. When both factors are considered, the off-rate increases by more than an order of magnitude as the pH changes from 5 to 8. The recombination of CO to ferrous NP4 is observed to have a large nonexponential geminate amplitude with rebinding time scales of approximately 10(-11)-10(-9) s at pH 5 and approximately 10(-10)-10(-8) s at pH 7. The nonexponential CO rebinding kinetics at both pH 5 and pH 7 are accounted for using a simple model that has proven effective for understanding CO binding in a variety of other heme systems (Ye, X.; et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 14682).

Probing the Inner Cavities of Hydrogels by Proton Diffusion

The reversible proton dissociation and geminate recombination of a photoacid was studied in peptide-based hydrogels over a large concentration range of the self-assembled peptide nanotube (PNT) network. 8-Hydroxypyrene-1,3,6-trisulfonate trisodium salt (HPTS) was used as the photoacid. We found that the proton transfer to the solvent rate is not affected by the presence of the fibrous structures. However, the proton geminate recombination process is affected by the PNT walls. We used the photoprotolytic reversible geminate recombination model to analyze the time-resolved emission data in the hydrogel samples. Our assumptions lead to the suggestion that the hydrogels form water nanopools of the order of 100 A.

Direct Detection of the Oxygen Rebound Intermediates, Ferryl Mb and NO2, in the Reaction of metMyoglobin with Peroxynitrite

Oxygenated hemoproteins are known to react rapidly with nitric oxide (NO) to produce peroxynitrite (PN) at the heme site. This process could lead either to attenuation of the effects of NO or to nitrosative protein damage. Peroxynitrite is a powerful nitrating and oxidizing agent that has been implicated in a variety of cell injuries. Accordingly, it is important to delineate the nature and variety of reaction mechanisms of PN reactions with heme proteins. Here, we present direct evidence that ferrylMb and NO(2) are both produced during the reaction of PN and metmyoglobin (metMb). Kinetic evidence indicates that these products evolve from initial formation of a caged radical intermediate [Fe(IV)=O *NO(2)]. This caged pair reacts mainly via internal return with a rate constant k(r) to form metMb and nitrate in an oxygen rebound scenario. Detectable amounts of ferrylMb are observed by stopped-flow spectrophotometry, appearing at a rate consistent with the rate, k(obs), of heme-mediated PN decomposition. Freely diffusing NO(2), which is liberated concomitantly from the radical pair (k(e)), preferentially nitrates Tyr103 in horse heart myoglobin. The ratio of the rates of in-cage rebound and cage escape, k(r)/k(e), was found to be approximately 10 by examining the nitration yields of fluorescein, an external NO(2) trap. This rebound/escape model for the metMb/PN interaction is analogous to the behavior of alkyl hyponitrites and the well-studied geminate recombination processes of deoxymyoglobin with O(2), CO, and NO. The scenario is also similar to the stepwise events of substrate hydroxylation by cytochrome P450 and other oxygenases. It is likely, therefore, that the reaction of metMb with ONOO(-) and that of oxyMb with NO proceed through the same [Fe(IV)=O *NO(2)] caged radical intermediate and lead to similar outcomes. The results indicate that while oxyMb may reduce the concentration of intracellular NO, it would not eliminate the formation of NO(2) as a decomposition product of peroxynitrite.

Photodetachment of Ferrocyanide in Reverse Micelles

Solvated electrons have been generated in reverse micelles (RMs) through photodetachment of ferrocyanide (Fe(CN)(6)(4-)) in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) RMs. We have measured both bleach recovery of the parent ferrocyanide CN stretch in the infrared and the decay of the solvated electron absorption at 800 nm. The bleach recovery has been fit to a diffusion model for the geminate recombination process. The fit parameters suggest a narrowing of the spatial distribution of ejected electrons due to confinement in the RMs when compared to bulk water. The diffusion coefficient of the solvated electron does not appear to be significantly affected by RM confinement. The decay of the solvated electron absorption exhibits an additional decay component that is not observed in bulk water and is smaller for larger RMs. No corresponding additional component is seen in the parent ferrocyanide IR bleach recovery, which supports our interpretation that the confinement-induced new decay process in RMs is due to electrons reacting with AOT headgroups.

The Position 68(E11) Side Chain in Myoglobin Regulates Ligand Capture, Bond Formation with Heme Iron, and Internal Movement into the Xenon Cavities

After photodissociation, ligand rebinding to myoglobin exhibits complex kinetic patterns associated with multiple first-order geminate recombination processes occurring within the protein and a simpler bimolecular phase representing second-order ligand rebinding from the solvent. A smooth transition from cryogenic-like to solution phase properties can be obtained by using a combination of sol-gel encapsulation, addition of glycerol as a bathing medium, and temperature tuning (-15 --> 65 degrees C). This approach was applied to a series of double mutants, myoglobin CO (H64L/V68X, where X = Ala, Val, Leu, Asn, and Phe), which were designed to examine the contributions of the position 68(E11) side chain to the appearance and disappearance of internal rebinding phases in the absence of steric and polar interactions with the distal histidine. Based on the effects of viscosity, temperature, and the stereochemistry of the E11 side chain, the three major phases, B --> A, C --> A, and D --> A, can be assigned, respectively, to ligand rebinding from the following: (i) the distal heme pocket, (ii) the xenon cavities prior to large amplitude side chain conformational relaxation, and (iii) the xenon cavities after significant conformational relaxation of the position 68(E11) side chain. The relative amplitudes of the B --> A and C --> A phases depend markedly on the size and shape of the E11 side chain, which regulates sterically both ligand return to the heme iron atom and ligand migration to the xenon cavities. The internal xenon cavities provide a transient docking site that allows side chain relaxations and the entry of water into the vacated distal pocket, which in turn slows ligand recombination markedly.

Study of the Long-Time Fluorescence Tail of the Green Fluorescent Protein

The time-resolved optical response of the wild-type green fluorescent protein (WT-GFP) in water and D2O was measured at room temperature by two optical techniques. The pump probe technique, with about 150 fs resolution, was used to measure the short-time response up to 150 ps. The short-time signals are similar to previously reported measurements. Time-correlated single photon counting (TCSPC) was used to measure the fluorescence of both the protonated and deprotonated emission bands of WT-GFP. The long-time fluorescence decay of the protonated form of WT-GFP decays nonexponentially. When this decay curve is multiplied by exp(t/τf) where τf is the lifetime of deprotonated form, the long-time tail decays as a power law of about t-3/2. Such a long-time fluorescence decay behavior represents the general emission decay pattern of excited photoacids in solutions and microemulsions and adds additional previously unrevealed information on the dynamics of this reaction. We attribute the long-time behavior to a diffusion-assisted geminate recombination process of the proton with the deprotonated species to reform the protonated chromophore.

Structural and functional properties of the CO-sensing transcriptional activator CooA

The CO-sensing transcriptional activator CooA contains a protoheme that acts as a CO sensor. Mutagenesis and spectroscopic studies revealed the unique coordination structure of the heme in CooA, the Pro 2/Cys 75, Pro 2/His 77, and CO/His 77 coordinations for the ferric, ferrous, and CO-bound hemes, respectively. The ligand exchange between Cys 75 and His 77 took place during the change in the oxidation state of the heme, which caused the hysteresis in redox titration of CooA. Furthermore, CO replaced Pro 2 to activate CooA under physiological conditions. A truncated mutant, CooAΔN5, in which the first five residues in the N-terminal were deleted, showed similar spectroscopic and functional properties to those of the wild-type, indicating the flexibility of the polypeptide chain of CooA in its N-terminal region. In flash photolysis of CO-bound CooA, a biphasic geminate recombination process with the time constants of 78 and 386 ps was observed. The yield of the geminate recombination of CO was 90% for the CO-bound CooA.

Dissociation and Recombination between Ligands and Heme in a CO-sensing Transcriptional Activator CooA: A FLASH PHOTOLYSIS STUDY

CooA from Rhodospirillum rubrum is a transcriptional activator in which a heme prosthetic group acts as a CO sensor and regulates the activity of the protein. In this study, the electronic relaxation of the heme, and the concurrent recombination between ligands and the heme at approximately 280 K were examined in an effort to understand the environment around the heme and the dynamics of the ligands. Upon photoexcitation of the reduced CooA at 400 nm, electronic relaxation of the heme occurred with time constants of 0.8 and 1.7 ps. The ligand rebinding was substantially completed with a time constant of 6.5 ps, followed by a slow relaxation process with a time constant of 173 ps. In the case of CO-bound CooA, relaxation of the excited heme occurred with two time constants, 1.1 and 2.4 ps, which were largely similar to those with reduced CooA. The subsequent CO recombination process was remarkably fast compared with that of other CO-bound heme proteins. It was well described as a biphasic geminate recombination process with time constants of 78 ps (60%) and 386 ps (30%). About 10% of the excited heme remained unligated at 1.9 ns. The dynamics of rebinding of CO thus will help us to understand how the physiologically relevant diatomic molecule approaches the heme binding site in CooA with picosecond resolution.

Charge separation and geminate recombination in sensitized photoconducting polymers

Dynamics of charge pair photogeneration and recombination in sensitized photoconducting polymers was analysed by means of Monte Carlo simulations of the charge migration in the cubic lattice framework by taking into account the diagonal and off-diagonal disorder. On the basis of the experimental data, which indicate that the mean time of initial charge separation is on the subpicosecond time scale while the random hopping of charges in the geminate recombination process is of a few orders of magnitude slower, the mechanism, by suggesting correlated oscillations of the environment polarisation as a positive feedback in the unrelaxed charge transfer state, is suggested. By assuming wide initial hole distribution over the separation distance it has been demonstrated that faster recombination in the films with higher concentration of the sensitizer can be related to the dynamic scattering of the unrelaxed charge carrier by the sensitizer molecules in the course of the initial separation process.