Geminate Recombination Rate Research Articles

Inertial dynamic effects on diffusion-influenced reactions: Approach based on the diffusive Cattaneo system.

We investigate the inertial dynamic effects on the kinetics of diffusion-influenced reactions by solving the linear diffusive Cattaneo system with the reaction sink term. Previous analytical studies on the inertial dynamic effects were limited to the bulk recombination reaction with infinite intrinsic reactivity. In the present work, we investigate the combined effects of inertial dynamics and finite reactivity on both bulk and geminate recombination rates. We obtain explicit analytical expressions for the rates, which show that both bulk and geminate recombination rates are retarded appreciably at short times due to the inertial dynamics. In particular, we find a distinctive feature of the inertial dynamic effect on the survival probability of a geminate pair at short times, which can be manifested in experimental observations.

Fast Recombination of Charge-Transfer State in Organic Photovoltaic Composite of P3HT and Semiconducting Carbon Nanotubes Is the Reason for Its Poor Photovoltaic Performance

Although the photovoltaic performance of the composite of poly-3-hexylthiophene (P3HT) with semiconducting single-walled carbon nanotubes (s-SWCNT) is promising, the short-circuit current density jSC is much lower than that for typical polymer/fullerene composites. Out-of-phase electron spin echo (ESE) technique with laser excitation of the P3HT/s-SWCNT composite was used to clarify the origin of the poor photogeneration of free charges. The appearance of out-of-phase ESE signal is a solid proof that the charge-transfer state of P3HT+/s-SWCNT- is formed upon photoexcitation and the electron spins of P3HT+ and s-SWCNT- are correlated. No out-of-phase ESE signal was detected in the same experiment with pristine P3HT film. The out-of-phase ESE envelope modulation trace for P3HT/s-SWCNT composite was close to that for the polymer/fullerene photovoltaic composite PCDTBT/PC70BM, which implies a similar distance of initial charge separation in the range 2-4 nm. However, out-of-phase ESE signal decay with delay after laser flash increase for P3HT/s-SWCNT composite was much faster, with a characteristic time of 10 µs at 30 K. This points to the higher geminate recombination rate for the P3HT/s-SWCNT composite, which may be one of the reasons for the relatively poor photovoltaic performance of this system.

Photodetachment and Electron Dynamics in 1-Butyl-1-methyl-pyrrolidinium Dicyanamide.

The ultrafast transient absorption spectrum of 1-butyl-1-methyl-pyrrolidinium dicyanamide, [Pyr1,4+][DCA-], was measured in the visible and near-infrared (IR) spectral regions. Excitation of the liquid at 4.6 eV created initially delocalized and highly reactive electrons that either geminately recombined (69%) or localized onto a cavity with a time constant of ∼300 fs. Electron localization was reflected in the evolution of the TA spectrum and the time-dependent loss of reactivity with a dichloromethane quencher. The delocalized initial state and spectrum of the free electrons were consistent with computational predictions by Xu and Margulis [ J. Phys. Chem. B, 2015, 119, 532-542] on excess electrons in [Pyr1,4+][DCA-]. The computational study considered two possible localization mechanisms for excess electrons, localization on ions, and localization on cavities. In the case of photogenerated electron-hole pairs, the results presented here demonstrate localization to cavities as the dominant channel. Following localization onto a cavity, the free electrons underwent solvation and loss of reactivity with the quencher with rates that slowed in time. The dynamics were similar to an analogous prior study on the related liquid [Pyr1,x+][NTf2-]. One significant difference was the larger yield of free electrons from photoexcitation of [Pyr1,4+][DCA-]. This was found to primarily reflect more efficient localization onto cavities rather than a slower geminate recombination rate.

Geminate recombination in organic photovoltaic blend PCDTBT/PC71BM studied by out-of-phase electron spin echo spectroscopy.

The key process in organic solar cell operation is charge separation under light illumination. Due to the low dielectric constant of organic materials, the Coulomb attraction energy within the interfacial charge-transfer state (CTS) is larger than the thermal energy. Understanding the mechanism of charge separation at the organic donor/acceptor interface still remains a challenge and requires knowledge of the CTS temporal evolution. To address this problem, the CTS in the benchmark photovoltaic blend PCDTBT/PC71BM was studied by the out-of-phase Electron Spin Echo (ESE). The protocol for determining the CTS geminate recombination rate for certain electron-hole distances was developed. Simulating the out-of-phase ESE trace for the CTS in the PCDTBT/PC71BM blend allows precise determination of the electron-hole distance distribution function and its evolution with the increase in the delay after the laser flash. Distances of charge separation up to 6 nm were detected upon thermalization at a temperature of 20 K. Assuming the exponential decay of the recombination rate, the attenuation factor β = 0.08 Å-1 is estimated for the PCDTBT/PC71BM blend. Such a low attenuation factor is probably caused by a high degree of hole delocalization along the PCDTBT chain.

Radiation-Induced Fluorescence from Doped Polyolefins on a Nanosecond Time Scale: Kinetics of the Processes Involving Geminate Radical Ions.

The delayed radiation-induced fluorescence from polyethylene and its alkyl- and fluorine-substituted analogues doped with aromatic luminophores was studied in the time range of 1-1000 ns. Qualitative analysis of the effects of a magnetic field on the fluorescence decay indicated that, in all polyolefins studied, the main portion of the fluorescence observed arose from the recombination of geminate spin-correlated radical ion pairs (RIPs). In the case of polyethylene, this conclusion was supported by observing the effect of an external electric field on the fluorescence decay. It was shown by comparison with the computer simulation of intratrack recombination that the tunneling character of the RIP recombination, which had an asymptotic time dependence of the geminate recombination rate close to t-1, was typical of most studied polyolefins at temperatures below 273 K in the time range studied. The increase to room temperature and above caused a gradual transition to a regime where the geminate recombination rate was mainly determined by the migration of RIP partners with time dependence close to t-3/2. The low estimate of the electron transfer distance upon the ion recombination in this regime was about 2 nm. In polyethylenes, exposed to an irradiation of 0.3-0.4 MGy, the role of charge carrier diffusion became hardly noticeable because of the cross-linking of polyethylene chains and the increase in polymer matrix stiffness. Oxygen, dissolved in a polymer doped with aromatic molecules, caused quenching of the recombination luminescence due to electron transfer from the dopant radical anion to the oxygen molecules. At room temperature, typical distances for such electron transfer were estimated to be ∼1.5 nm.

How does the mutation in the cap domain of methylcobalamin-dependent methionine synthase influence the photoactivation of the Co-C bond?

Methionine synthase (MetH) is a methylcobalamin (MeCbl)-dependent mammalian enzyme which plays a critical role in carrying out the transfer of a methyl group from methyl tetrahydrofolate to homocysteine to generate methionine and tetrahydrofolate. This catalytic cycle proceeds via cleavage of a Co-C bond which is formally heterolytic. This cleavage results in a structural change in the MeCbl cofactor bound to an enzyme. Unlike the native catalysis, upon photoexcitation, the Co-C bond in MeCbl-bound MetH generates the Co(ii)/CH3 radical pairs (RPs). Protein residues of the cap domain, particularly phenylalanine708 (F708) and leucine 715 (L715), which surrounds the upper face of the MeCbl cofactor, inhibit the photolysis of MeCbl by caging the CH3 radical and inducing the geminate recombination of the Co(ii)/CH3 RP. A molecular-level understanding of these effects requires a detailed investigation of the low-lying electronic states. Toward this, we have mutated the F708 residue with alanine (A708) and constructed the potential energy surfaces (PESs) for the low-lying S1 electronic state using a combined quantum mechanics/molecular mechanics (QM/MM) approach. The S1 PESs for the wild-type (WT) and mutant enzymes are the result of crossing of two electronic states, namely metal-to-ligand charge transfer (MLCT) and ligand field (LF) states, indicated by a seam. It is shown that the topologies of the S1 PESs are significantly modulated by introducing a mutation at the F708 position. Specifically, for the WT enzyme, the energy barrier of photoreaction and the energy difference between MLCT and LF minima are markedly higher than those of its mutant counterpart. Moreover, mutation influences the photoactivation of the Co-C bond in enzyme-bound MeCbl by decreasing the rate of geminate recombination and altering the rate of radical pair formation. This theoretical insight was also compared with transient absorption spectroscopic (TAS) studies which are in good agreement with the present findings.

Minimizing geminate recombination losses in small-molecule-based organic solar cells

Geminate recombination rates are successfully predicted for series of small-molecule bulk heterojunction solar cells applying the Marcus–Levich–Jortner equation.

Charge Pair Separation Dynamics in Organic Bulk‐Heterojunction Solar Cells

AbstractCharge pair separation in organic bulk‐heterojunction (BHJ) solar cells is a complex interplay between numerous factors, such as the spatial geometry of the blend, the distribution of energetic disorder, the electric field, thermal fluctuations, and the mutual electron–hole Coulomb attraction. Insufficient separation from the interface and concomitant charge pair recombination is a main limitation in improving the PCE of organic BHJ solar cells and requires an in‐depth understanding of the timescales involved. Here, a 3D kinetic Monte Carlo model of a BHJ organic solar cell is set up and the time‐dependent evolution of mutual electron–hole pair distances separating from the heterojunction interface is investigated. Large fluctuations in separation times are found, in particular in dependence of the energetic disorder and the permittivity of the organic materials. At commonly observed values of energetic disorder, slight modifications of the permittivity can drastically influence the charge separation time and even outweigh orders of magnitude of geminate recombination rates, hence help to suppress geminate recombination. Thus, the results strongly support the recent trend of developing high‐permittivity organic materials for solar cell applications.

Charge Generation in Organic Solar Cells: Interplay of Quantum Dynamics, Decoherence, and Recombination

The dynamics of photoinduced charge generation is studied for donor–acceptor (D–A) organic interfaces, with focus on the interplay of quantum dynamics, decoherence effects, and recombination. A coarse-grained molecular envelope function model is developed to enable the investigation of large scale D–A heterojunctions, taking into account morphology and molecular orientation as well as the underlying quantum nature of the system. Simulations show that, upon photoexcitation, Frenkel excitons delocalize over several molecules in <300 fs. At the interface, they dissociate without dwelling in intermediate charge transfer states, evincing that exciton motion and dissociation cannot be describe by point particle models. Moreover, as decoherence suppresses the excitonic quantum coherence length, it also decreases the geminate recombination rate. Although ultrafast coherent charge separation is more efficient at early times and, particularly, for excitons created at the interface, diffusion becomes important for e...

Counterion effects on the ultrafast dynamics of charge-transfer-to-solvent electrons.

We performed femtosecond transient absorption (TA) experiments to monitor the solvation dynamics of charge-transfer-to-solvent (CTTS) electrons originating from UV photoexcitation of ammoniated iodide in close proximity to the counterions. Solutions of KI were prepared in liquid ammonia and TA experiments were carried out at different temperatures and densities, along the liquid-gas coexistence curve of the fluid. The results complement previous femtosecond TA work by P. Vöhringer's group in neat ammonia via multiphoton ionization. The dynamics of CTTS-detached electrons in ammonia was found to be strongly affected by ion pairing. Geminate recombination time constants as well as escape probabilities were determined from the measured temporal profiles and analysed as a function of the medium density. A fast unresolved (τ < 250 fs) increase of absorption related to the creation/thermalization of solvated electron species was followed by two decay components: one with a characteristic time around 10 ps, and a slower one that remains active for hundreds of picoseconds. While the first process is attributed to an early recombination of (I, e-) pairs, the second decay and its asymptote reflects the effect of the K+ counterion on the geminate recombination dynamics, rate and yield. The cation basically acts as an electron anchor that restricts the ejection distance, leading to solvent-separated counterion-electron species. The formation of (K+, NH3, e-) pairs close to the parent iodine atom brings the electron escape probability to very low values. Transient spectra of the electron species have also been estimated as a function of time by probing the temporal profiles at different wavelengths.

Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications.

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This “two-step” charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.

Modulating Distal Cavities in the α and β Subunits of Human HbA Reveals the Primary Ligand Migration Pathway

The free volume in the active site of human HbA plays a crucial role in governing the bimolecular rates of O(2), CO, and NO binding, the fraction of geminate ligand recombination, and the rate of NO dioxygenation by the oxygenated complex. We have decreased the size of the distal pocket by mutating Leu(B10), Val(E11), and Leu(G8) to Phe and Trp and that of other more internal cavities by filling them with Xe at high gas pressures. Increasing the size of the B10 side chain reduces bimolecular rates of ligand binding nearly 5000-fold and inhibits CO geminate recombination due to both reduction of the capture volume in the distal pocket and direct steric hindrance of Fe-ligand bond formation. Phe and Trp(E11) mutations also cause a decrease in distal pocket volume but, at the same time, increase access to the Fe atom because of the loss of the γ2 CH(3) group of the native Val(E11) side chain. The net result of these E11 substitutions is a dramatic increase in the rate of geminate recombination because dissociated CO is sequestered close to the Fe atom and can rapidly rebind without steric resistance. However, the bimolecular rate constants for binding of ligand to the Phe and Trp(E11) mutants are decreased 5-30-fold, because of a smaller capture volume. Geminate and bimolecular kinetic parameters for Phe and Trp(G8) mutants are similar to those for the native HbA subunits because the aromatic rings at this position cause little change in distal pocket volume and because ligands do not move past this position into the globin interior of wild-type HbA subunits. The latter conclusion is verified by the observation that Xe binding to the α and β Hb subunits has little effect on either geminate or bimolecular ligand rebinding. All of these experimental results argue strongly against alternative ligand migration pathways that involve movements through the protein interior in HbA. Instead, ligands appear to enter through the His(E7) gate and are captured directly in the distal cavity.

Enhanced performance of P3HT/PCBM bulk heterojunction photovoltaic devices by adding spin ½ radicals

We have studied bulk heterojunction organic photovoltaic devices (solar cells) based on polymer/fullerene blends using both electrical and magneto-optical methods. We show that adding spin ½ galvinoxyl radicals to the device active layer, consisting of a regio-regular polythiophene/fullerene derivative [P3HT/PCBM] blend, significantly improves the device performance. Compared to pristine photovoltaic devices, the radical-rich devices show improved short-circuit current density, fill factor, and power conversion efficiency. The enhanced device performance is attributed to a reduced geminate recombination rate and improved carrier transport, both of which result from spin–spin interactions between the radical impurities and the photogenerated carriers. Optically detected magnetic resonance, a technique that is sensitive to spin-lattice relaxation rates, is used to verify the proposed mechanism.

Effect of spin ½ radicals on the ultrafast photoexcitation dynamics in RR-P3HT/PCBM blends for photovoltaic applications

We report the effect of spin ½ radical impurity, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of regio-regular poly(3-hexylthiophene) (RR-P3HT)/[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) blend. The addition of Galvinoxyl radical impurities to the blend reduces the geminate recombination rate of photogenerated polaron pairs. Consequently organic photovoltaic solar cells made from the radical/blend mixture as the active layer show increased short-circuit current ( J sc ), fill-factor, and power conversion efficiency. We speculate that the spin–spin interaction of the radical impurity with the electron and hole forming the polaron pairs is the main mechanism responsible for the obtained reduced recombination rate.

Simple relation for estimating the relative mobility of geminate ions using the influence of electric field on the geminate recombination

The influence of an external electric field on the geminate recombination of ion pairs in weakly polar solutions has been considered. Using an asymptotic dependence of the geminate recombination rate in low electric field [K.M. Hong, J. Noolandi, J. Chem. Phys. 69 (1978) 5026] the simple relation for estimating the relative mobility of the recombining ions has been proposed. The relation has been verified by the results of the computer simulation of ion recombination in solution of charge acceptors both for an isolated ion pair and for a spur containing four primary (solvent cation/electron) pairs.

The Role of Adenine in Fast Excited-State Deactivation of FAD: a Femtosecond Mid-IR Transient Absorption Study

We present a study of excited-state dynamics of two flavin cofactors: flavin-adenine dinucleotide (FAD) and flavin-mononucleotide (FMN). We used femtosecond mid-R transient absorption spectroscopy to study the effect of FAD conformation on its excited-state behavior. The conformation of FAD was modulated by changing the solvent polarity: in D(2)O, FAD is present predominantly in the "stacked" conformation, in which flavin and adenine moieties are in close proximity to each other, whereas the increased amount of DMSO led to an increased amount of the "open" conformer. FMN served as a model system which lacks adenine. We found that the "stacked" conformer undergoes an intramolecular photoinduced electron transfer from adenine to flavin with the forward electron transfer rate of k(f) = 1.9.10(11) s(-1) and the geminate recombination rate of k(b) = 1.1.10(11) s(-1). In the case of the "open" conformer, no intramolecular electron transfer was observed.

Linking conformation change to hemoglobin activation via chain-selective time-resolved resonance Raman spectroscopy of protoheme/mesoheme hybrids.

Time-resolved resonance Raman (RR) spectra are reported for hemoglobin (Hb) tetramers, in which the alpha and beta chains are selectively substituted with mesoheme. The Soret absorption band shift in mesoheme relative to protoheme permits chain-selective recording of heme RR spectra. The evolution of these spectra following HbCO photolysis shows that the geminate recombination rates and the yields are the same for the two chains, consistent with recent results on (15)N-heme isotopomer hybrids. The spectra also reveal systematic shifts in the deoxyheme nu (4) and nu (Fe-His) RR bands, which are anticorrelated. These shifts are resolved for the successive intermediates in the protein structure, which have previously been determined from time-resolved UV RR spectra. Both chains show Fe-His bond compression in the immediate photoproduct, which relaxes during the formation of the first intermediate, R(deoxy) (0.07 micros), in which the proximal F-helix is proposed to move away from the heme. Subsequently, the Fe-His bond weakens, more so for the alpha chains than for the beta chains. The weakening is gradual for the beta chains, but is abrupt for the alpha chains, coinciding with completion of the R-T quaternary transition, at 20 micros. Since the transition from fast- to slow-rebinding Hb also occurs at 20 micros, the drop in the alpha chain nu (Fe-His) supports the localization of ligation restraint to tension in the Fe-His bond, at least in the alpha chains. The mechanism is more complex in the beta chains.

Linking Heme Activation to Conformation Change in Hemoglobin Via Chain Selective Time-resolved Resonance Raman Spectroscopy on Mesoheme Hybrids

Time-resolved Resonance Raman spectra are reported for Hb tetramers, in which the α and β chains are selectively substituted with mesoheme. Hb function is unaffected by the substitution, but the Soret absorbtion band shift in meso- relative to proto-heme permits chain-selective excitation of heme RR spectrum. The evolution of these spectra following HbCO photolysis show that geminate recombination rates and yields are the same for the two chains, consistent with recent results with 15N-heme isotopomer hybrids. The spectra also reveal systematic shifts in the deoxy-heme ν4 and νFe-His RR bands, which are anti-correlated. These shifts are resolved for the successive intermediates in the protein structure, as previously determined from time-resolved UVRR spectra. Both chains show Fe-His bond compression in the immediate photoproduct, which relaxes during the formation of the first intermediate, Rdeoxy (0.07 μs), in which the proximal F-helix is proposed to move away from the heme. Subsequently, the Fe-His bond weakens, more so for the α than the β chains. The weakening is gradual for the β chains, but abrupt for the α chains, coinciding with completion of the R-T quaternary transition, at 20 μs. Since the transition from fast- to slow-rebinding Hb also occurs at 20 μs, the drop in the α chain νFe-His supports the localization of ligation restraint to tension in the Fe-His bond, at least in the α-chains. The mechanism is more complex in the β chains.

The pH Dependence of Heme Pocket Hydration and Ligand Rebinding Kinetics in Photodissociated Carbonmonoxymyoglobin

We monitored the occupancy of a functionally important non-coordinated water molecule in the distal heme pocket of sperm whale myoglobin over the pH range 4.3-9.4. Water occupancy was assessed by using time-resolved spectroscopy to detect the perturbation of the heme visible band absorption spectrum caused by water entry after CO photodissociation ( Goldbeck, R. A., Bhaskaran, S., Ortega, C., Mendoza, J. L., Olson, J. S., Soman, J., Kliger, D. S., and Esquerra, R. M. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 1254-1259 ). We found that the water occupancy observed during the time interval between ligand photolysis and diffusive recombination decreased by nearly 20% as the pH was lowered below 6. This decrease accounted for most of the concomitant increase in the observed CO bimolecular recombination rate constant, as the lower water occupancy presented a smaller kinetic barrier to CO entry into the pocket at lower pH. These results were consistent with a model in which the distal histidine, which stabilizes the water molecule within the distal pocket by accepting a hydrogen bond, tends to swing out of the pocket upon protonation and destabilize the water occupancy at low pH. Extrapolation of this model to lower pH suggests that the additional increase in ligand association rate constant observed previously in stopped-flow studies at pH 3 may also be due in part to reduced distal water occupancy concomitant with further His64 protonation and coupled protein conformational change.

Effect of Electrolytes on the Excited-State Proton Transfer and Geminate Recombination

Time-resolved emission and steady-state fluorescence techniques are used to study the excited-state intermolecular proton transfer from 8-hydroxypyrene-1,3,6-trisulfonate (HPTS or pyranine) to water in the presence of inert salts, NaCl and MgCl(2). At low salt concentrations, up to about 0.5 M MgCl(2) or about 0.8 M NaCl, the time-resolved emission of both the photoacid and conjugate base can be quantitatively fitted by our diffusion-assisted geminate recombination model. In this concentration range, the proton transfer and geminate recombination rate constants are almost independent of the salt concentrations whereas the proton diffusion constant decreases as the salt concentration increases. At higher salt concentrations, the proton-transfer rate constant decreases while the recombination rate constant increases slightly. For the saturated solution of MgCl(2) (about 5 M at room temperature), the steady-state emission consists of only a single band of the protonated photoacid. Careful examination of the time-resolved emission of HPTS in the presence of a large concentration of MgCl(2) shows that the quality of the fit to the geminate recombination model is rather poor and we fail to find adjustable parameters for a good quality fitting. For this large concentration range of MgCl(2) we were able to get a good fit of the experimental data with a model based on a distribution of proton-transfer rates. The model is consistent with an inhomogeneous water environment next to the excited HPTS molecule in such concentrated solutions.