Charge recombination in excited donor-acceptor complexes

The influence of the excitation pulse carrier frequency on the dynamics of ultrafast charge recombination in donor-acceptor complexes was studied in the limit of strong electron coupling. An increase in the carrier frequency of excitation pulses invariably decreased the effective rate constant. The dependence of the degree to which the decay of the excited state deviated from the exponential law on reaction exothermicity and the dynamic characteristics of the medium was revealed.     

Excitation Frequency Dependence of Ultrafast Photoinduced Charge Transfer Dynamics

The dependence of the photoinduced charge transfer rate constant on the pump pulse carrier frequency is shown to be strong, and it is considerably affected by the value of the reorganization energy of low‐frequency modes at the stage of excitation. In the area of small values of the reorganization energy, the dependence of the charge transfer rate constant on the pump pulse carrier frequency is strongly nonmonotonic that is caused by vibrational resonances and variation of the initial position of the wave packet on the term of the locally excited state. Increasing the reorganization energy smoothes the dependence. The smoothing is caused by the broadening of the vibrational resonances and their overlapping. The high‐frequency vibrational mode excitation typically accelerates the charge transfer in both areas of high and low exergonicity and decelerates it in the vicinity of the Marcus barrierless region.     

Charge recombination in excited donor-acceptor complexes with two absorption bands

The dynamics of charge recombination in a photoexcited donor-acceptor complex comprising 1,2,4-trimethoxybenzene (electron donor) and tetracyanoethylene (electron acceptor) in several polar solvents (acetonitrile, valeronitrile, and octanonitrile) was studied in terms of the stochastic approach. The Gibbs energy of charge recombination and the reorganization energies of the medium and quantum and vibrational degrees of freedom were found by fitting the stationary absorption spectrum. The electronic couplings were determined by analyzing the time dependences of the population of the ionic state in acetonitrile. A comparison of the numerical simulation results with the experimental data showed that the nonstationary model under consideration quantitatively described the dynamics of charge recombination and its dependence on the carrier frequency of excitation pulses and the relaxation properties of solvents. Original Russian Text ? V.N. Ionkin, A.I. Ivanov, E. Vauthey, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 4, pp. 791–797.     

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent substituted quinolines with donor-acceptor character

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes and singlet excitation energies of PnQ. The formation of the intramolecular charge transfer state is assumed for PnQ in the singlet excited state with a strong electron donating substituent. The emission from PnQ is suggested to originate from PnQ in the singlet excited state formed from the charge recombination between the PnQ radical cation and the PnQ radical anion during the pulse radiolysis. This is strong evidence for the efficient electrogenerated chemiluminescence of PnQ.     

Effect of the excitation pulse frequency on the ultrafast photoinduced electron transfer dynamics

The dependence of the ultrafast photoinduced electron transfer dynamics in donor-acceptor complexes on the excitation pulse carrier frequency (spectral effect) has been investigated in the framework of a model involving three electronic state. The spectral effect has been shown to strongly depend on the angle theta between the reaction coordinate directions corresponding to optical and charge transfer transitions. Describing the solvent as a linear homogenous polar medium and accounting for Coulombic interaction of the transferred charge with the medium polarization fluctuations, the angle theta has been found out to be typically in the area 40 degrees -85 degrees. Exactly in this area of theta the spectral effect is predicted to be most pronounced.     

Time-resolved studies of charge recombination in the pyrene/TCNQ charge-transfer crystal: evidence for tunneling

Previous studies of solid-state tetracyanobenzene-based donor-acceptor complexes showed that these materials were highly susceptible to both laser and mechanical damage that complicated the analysis of their electron-transfer kinetics. In this paper, we characterize the optical properties of a pyrene/tetracyanoquinodimethane charge-transfer crystal that is much more robust than the tetracyanobenzene compounds. This donor-acceptor complex has a charge-transfer absorption that extends into the near-infrared, rendering the crystal black. We use time-resolved fluorescence and diffuse reflectance transient absorption to study its dynamics after photoexcitation. We show that the initially excited charge-transfer state undergoes a rapid, monoexponential decay with a lifetime of 290 ps at room temperature. There is no evidence for any long-lived intermediate or dark states; therefore, this decay is attributed to charge recombination back to the ground state. Fluorescence lifetime measurements demonstrate that this process becomes temperature-independent below 60 K, indicative of a thermally activated tunneling mechanism. The subnanosecond charge recombination makes this low-band-gap donor-acceptor material a poor candidate for generating long-lived electron-hole pairs.     

Excited states of the high-frequency vibrational modes and kinetics of ultrafast photoinduced electron transfer

The effect of the carrier frequency of the exciting laser pulse on the kinetics of intramolecular photoinduced charge transfer in the multi-channel stochastic model is studied. It is shown that the population of different states of high-frequency intramolecular modes upon varying the frequency of the excitation pulse can considerably alter the rate constant of ultrafast charge transfer. It is found that a negative vibrational spectral effect is expected in the vicinity of a barrier-free area (the rate constant of photoinduced charge transfer decreases along with the carrier frequency of the excitation pulse), while a positive effect is predicted in areas of high and low exergonicity (an inverse dependence). It is concluded that the value of the spectral effect falls along with the time of vibrational relaxation. For ultrafast photo-induced charge transfer, however, it remains considerable up to relaxation times of 100 fs.     

Efficient charge separation in C60-based dyads: triazolino

Triazoline[4,5][60]fullerenes are strong electron acceptors that form with tetrathiafulvalene (TTF), a novel type of donor-acceptor dyad exhibiting efficient improved electron-transfer dynamics. In particular, a rapid photoinduced intramolecular electron transfer, forming a charge-separated state, is followed by a slow charge recombination to generate the fullerene triplet excited state in moderate quantum yields.     

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.     

The influence of changes in the dipole moment of reagents on the rate of photoinduced electron transfer

The influence of spatial charge redistribution modeled by a change in the dipole moment of the reagent that experiences excitation on the dynamics of ultrafast photoinduced electron transfer was studied. A two-center model based on the geometry of real molecules was suggested. The model described photoexcitation and subsequent electron transfer in a donor-acceptor pair. The rate of electron transfer was shown to depend substantially on the dipole moment of the donor at the photoexcitation stage and the direction of subsequent electron transfer. These parameters also determined the most important characteristic of ultrafast photoinduced electron transfer, the angle ? between the reaction coordinates corresponding to these reaction stages. The regions of model parameters corresponding to the strongest influence of the carrier frequency of the exciting pulse on the rate of electron transfer were established.     

Effect of high-frequency modes and hot transitions on free energy gap dependence of charge recombination rate

The charge recombination (CR) dynamics of geminate ion pairs formed by excitation of the ground-state donor-acceptor complexes in polar solvent have been investigated within the framework of stochastic approach. It is shown that for low exergonic reactions these dynamics critically depend on the reorganization energy of intramolecular high-frequency mode. Even moderate reorganization energies (0.1-0.2 eV) significantly accelerate the excited-state population decay making it nearly exponential. In the solvent-controlled regime, the majority of the excited donor-acceptor complexes recombine at nonthermal (hot) stage when the nonequilibrium initial wave packet passes through a number of term crossings corresponding to the transitions with creation of several vibrational quanta. Analysis of this mechanism allows to conclude (i) the CR in viscous solvents proceeds much faster than the diffusive relaxation of solvent, (ii) under certain conditions, the CR rate becomes practically independent of the diffusive component of solvent relaxation which is determined by solvent viscosity, (iii) in contrast to predictions of Marcus theory, the CR rate decreases monotonically with the rise of reaction exergonicity even at small free energy gaps, in accordance with experimental results. Two semiquantitative approaches providing rather simple analytical expressions for the hot charge recombination dynamics are suggested. These approximations give a good reproduction of the excited-state decay in the wide area of model parameters.     

A theoretical investigation of the photo-induced intramolecular charge transfer excitation of cuprous (I) bis-phenanthroline by density functional theory

This work reported an investigation on the excited state and electronic transfer excitation of cuprous (I) bis-phenanthrouline complex by density functional theory. The intramolecular charge transfer from central metal to ligand (MLCT) during the excitation was observed. The transfer direction and degree were discussed on the basis of analyzing the Mulliken charge. The structural distortion caused by the charge transfer in the excited state was confirmed. The excited state was found having the characters similar with Cu(II) complex both in electronic and geometrical properties. The large structural distortion found between ground state and excited state could lead to a decrease in the lifetime of excited state as well as a non-radiative decay. The excitation energies and oscillator strengths of cuprous (I) bis-phenanthrouline were derived using time-dependent density functional method. The values of excitation energies are good agreement with the results of the experimental measuring.     

Coherent control of the population transfer in complex solvated molecules at weak excitation. An experimental study

We performed a series of successful experiments for the optimization of the population transfer from the ground to the first excited state in a complex solvated molecule (rhodamine 101 in methanol) using shaped excitation pulses at very low intensities (1 absorbed photon per 100-500 molecules per pulse). We found that the population transfer can be controlled and significantly enhanced by applying excitation laser pulses with crafted pulse shapes. The optimal shape was found in feedback-controlled experiments using a genetic search algorithm. The temporal profile of the optimal excitation pulse corresponds to a comb of subpulses regularly spaced by approximately 150 fs, whereas its spectrum consists of a series of well-resolved peaks spaced apart by approximately 6.5 nm corresponding to a frequency of 220 cm(-1). This frequency matches very well with the frequency modulation of the population kinetics (period of approximately 150 fs), observed by excitation with a short (approximately 20 fs) transform-limited laser pulse directly after excitation. In addition, an antioptimization experiment was performed under the same conditions. The difference in the population of the excited state for the optimal and antioptimal pulses reaches approximately 30% even at very weak excitation. The results of optimization are reproducible and have clear physical meaning.     

Ab initio description of photoabsorption and electron transfer in a doubly-linked porphyrin-fullerene dyad

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.     

Temperature dependence of charge separation and recombination in porphyrin oligomer-fullerene donor-acceptor systems

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor-acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1-4, 6) appended to C(60) were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5-25 ps) electron transfer to C(60) followed by slower (200-650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor-acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C(60)-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.