Tens Of Picoseconds Time Scale Research Articles

Measuring the Ultrafast Correlation Dynamics of a Multilevel System Using the Center Line Slope Analysis in Two-Dimensional Electronic Spectroscopy.

In a two-dimensional (2D) optical spectrum of a multilevel system, there are diagonal peaks and off-diagonal cross-peaks that correlate the different levels. The time-dependent properties of these diagonal peaks and cross-peaks contain much information about the dynamics of the multilevel system. The time-dependent diagonal peakshape that depends on the spectral diffusion dynamics of the associated transition and characterized by the frequency-fluctuation correlation function (FFCF) is well studied. However, the time-dependent peakshape of a cross-peak that provides the correlation dynamics between different transitions is much less studied or understood. We derived the third-order nonlinear response functions that describe the cross-peaks in a 2D electronic spectrum of a multilevel system that arise from processes sharing a common ground state and/or from internal conversion and population transfer. We can use the center line slope (CLS) analysis to characterize the cross-peaks in conjunction with the diagonal peaks. This allows us to recover the frequency-fluctuation cross-correlation functions (FXCFs) between two transitions. The FXCF and its subsidiary quantities such as the initial correlation and the initial covariance between different transitions are important for studying the correlation effects between states in complex systems, such as energy-transfer processes. Furthermore, knowledge of how various molecular processes over different timescales affect simultaneously different transitions can also be obtained from the measured FXCF. We validated and tested our derived equations and analysis process by studying, as an example, the 2D electronic spectra of metal-free phthalocyanine in solution. We measured and analyzed the diagonal peaks of the Qx and Qy transitions and the cross-peaks between these two transitions of this multilevel electronic system and obtained the associated FFCFs and FXCFs. In this model system, we measured negative components of FXCF over the tens of picosecond timescale. This suggests that in phthalocyanine, the Qx and Qy transitions coupling with the solvent molecule motion are anticorrelated to each other.

Symmetry Breaking Charge Transfer in DNA-Templated Perylene Dimer Aggregates.

Molecular aggregates are of interest to a broad range of fields including light harvesting, organic optoelectronics, and nanoscale computing. In molecular aggregates, nonradiative decay pathways may emerge that were not present in the constituent molecules. Such nonradiative decay pathways may include singlet fission, excimer relaxation, and symmetry-breaking charge transfer. Singlet fission, sometimes referred to as excitation multiplication, is of great interest to the fields of energy conversion and quantum information. For example, endothermic singlet fission, which avoids energy loss, has been observed in covalently bound, linear perylene trimers and tetramers. In this work, the electronic structure and excited-state dynamics of dimers of a perylene derivative templated using DNA were investigated. Specifically, DNA Holliday junctions were used to template the aggregation of two perylene molecules covalently linked to a modified uracil nucleobase through an ethynyl group. The perylenes were templated in the form of monomer, transverse dimer, and adjacent dimer configurations. The electronic structure of the perylene monomers and dimers were characterized via steady-state absorption and fluorescence spectroscopy. Initial insights into their excited-state dynamics were gleaned from relative fluorescence intensity measurements, which indicated that a new nonradiative decay pathway emerges in the dimers. Femtosecond visible transient absorption spectroscopy was subsequently used to elucidate the excited-state dynamics. A new excited-state absorption feature grows in on the tens of picosecond timescale in the dimers, which is attributed to the formation of perylene anions and cations resulting from symmetry-breaking charge transfer. Given the close proximity required for symmetry-breaking charge transfer, the results shed promising light on the prospect of singlet fission in DNA-templated molecular aggregates.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Time-Resolved Structural Dynamics of Extended π-Electron Porphyrin Nanoring

Molecular structure design inspired by naturally occurring light harvesting systems has been intensely pursued over the last couple of decades. Interesting new structures include the π- conjugated porphyrin nanorings, which show promising features such as ultrafast excited-state delocalization, leading to suppressed radiative rates, superradiance with increasing temperature and energy transfer times comparable to their natural counterparts. An mportant question to be addressed in such systems is the role and time scale of structural motions and how they affect excited-state delocalization. Here it is shown that porphyrin nanorings which are not rigidified by a template are structurally heterogeneous in the ground state and evolve dynamically on a tens of picoseconds timescale. In the excited state a structural relaxation of the porphyrin nanorings is observed, on a picosecond timescale. Furthermore ultrafast excitation delocalization is observed, by anisotropy measurements, being insensitive to structural motions of the nanorings.

The intersystem crossing of a cyclic (alkyl)(amino) carbene gold (i) complex.

The intersystem crossing (ISC) mechanism of a cyclic (alkyl)(amino) carbene gold (I) complex (CMA1) is studied using quantum dynamics. A model spin-vibronic Hamiltonian is developed, which includes 10 excited states and two important nuclear degrees of freedom. The quantum dynamics reveals that ISC from S1 → T1 occurs on the tens of picosecond time scale, consistent with recent experiments. It is driven by motion along the torsional degree of freedom of the carbazole (Cz) ligand, which causes orthogonality between the donor and acceptor groups closing the gap between the initial (S1) and final (T1) states. The role of higher triplet states through spin-vibronic interactions is also discussed. Although previous calculations, evaluated in the Condon approximation, yield large ISC rates, our present dynamical treatment, taking into account the large amplitude torsional motion, increases the calculated rate by an order of magnitude improving the agreement with experiments. The model spin-vibronic Hamiltonian developed can also be used to understand the properties of related linear metal carbene compounds, facilitating molecular design.

Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study.

Excited state ultrafast conformational reorganization is recognized as an important phenomenon that facilitates light-induced functions of many molecular systems. This report describes the femtosecond and picosecond conformational relaxation dynamics of middle-ring and terminal ring twisted conformers of the acetylene π-conjugated system bis(phenylethynyl)benzene, a model system for molecular wires. Through excitation wavelength dependent, femtosecond-transient absorption measurements, we found that the middle-ring and terminal ring twisted conformers relax at femtosecond (400-600 fs) and picosecond (20-24 ps) time scales, respectively. Actinic pumping into the red flank of the absorption spectrum leads to excitation of primarily planar conformers, and results in very different excited state dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental findings, DFT and time-dependent DFT calculations were carried out. The Franck-Condon simulation indicated that the vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings relax on a few tens of picosecond time scale.

Time-Resolved Twisting Dynamics in a Porphyrin Dimer Characterized by Two-Dimensional Electronic Spectroscopy.

Molecular conformational changes in electronic excited states play a key role in numerous light-activated processes. In the case of porphyrin oligomers intramolecular twisting influences energy and charge transport dynamics. Here we address the twisting reaction in both ground and excited states in a model porphyrin dimer, employing two-dimensional electronic spectroscopy (2D ES). By spreading the information over excitation and detection frequencies, cross-peaks reveal the twisting reaction in both the ground and excited states unambiguously and distinctly from other dynamics. A quasi-barrierless planarization reaction is observed in the excited state on a tens of picoseconds time scale. This is accompanied by a spectral narrowing, indicative of a reduction in conformational disorder. The reverse reaction is suppressed in the excited state due to a steep activation energy barrier. However, in the ground state the barrier is within the thermal energy distribution, and therefore contributions from reverse and forward reactions could be observed on the subnanosecond time scale. Crucially 2D ES enables simultaneous assessment of ground and excited state reactions through analysis of different spectral regions on the 2D spectral maps.

Interligand electron transfer in heteroleptic ruthenium(II) complexes occurs on multiple time scales.

The time-dependent localization of the metal-to-ligand charge transfer (MLCT) excited states of ruthenium(II) complexes containing 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) ligands was studied by femtosecond transient absorption spectroscopy. Time-resolved anisotropy measurements indicate that the excited state hops randomly among the three ligands of each complex by subpicosecond interligand electron transfer (ILET). Although the bpy- and phen-localized (3)MLCT states have similar energies and steady-state emission spectra, pronounced differences in their excited-state absorption spectra make it possible to observe changes in excited state populations using magic angle transient absorption measurements. Analysis of the magic angle signals shows that the excited electron is equally likely to be found on any of the three ligands approximately 1 ps after excitation, but this statistical distribution subsequently evolves to a Boltzmann distribution with a time constant of approximately 10 ps. The apparent contradiction between ultrafast ILET revealed by time-dependent anisotropy measurements and the slower ILET seen in magic angle measurements on the tens of picoseconds time scale is explained by a model in which the underlying rates depend dynamically on excess vibrational energy. The insight that ILET can occur over multiple time scales reconciles contradictory literature observations and may lead to improved photosensitizer performance.

Time-Resolved Spectroscopic and Density Functional Theory Study of the Photochemistry of Irgacure-2959 in an Aqueous Solution

The photocleavage reaction mechanism of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure-2959) was investigated using femtosecond (fs) and nanosecond (ns) transient absorption (-TA) spectroscopy and also picosecond (ps) and nanosecond (ns) time-resolved resonance Raman (-TR(3)) spectroscopy experiments in a water-rich (volume ratio of acetonitrile/water = 3:7) solution. TA spectroscopy was used to study the dynamics of the benzoyl radical growth and decay as well as to investigate the radical quenching process by the radical scavenger methyl acrylate. ps- and ns-TR(3) spectroscopies were employed to monitor the formation of the benzoyl radical and also to characterize its electronic and structural properties. The fs-TA experiments results indicate that the Irgacure-2959 lowest lying excited singlet state S1 underwent efficient intersystem crossing (ISC) to convert into its triplet state with a time constant of 4 ps. Subsequently, this triplet species dissociated into the benzoyl and alkyl radicals with a corresponding maximum absorption band at 415 nm. The TR(3) results in conjunction with results from DFT calculations confirmed that Irgacure-2959 cleaved into the benzoyl and alkyl radicals at a fast rate on the tens of picosecond time scale.

Observation of slow charge redistribution preceding excited-state proton transfer

The photoacid 8-hydroxy-N,N,N',N',N',N'-hexamethylpyrene-1,3,6-trisulfonamide (HPTA) and related compounds are used to investigate the steps involved in excited-state deprotonation in polar solvents using pump-probe spectroscopy and time correlated single photon counting fluorescence spectroscopy. The dynamics show a clear two-step process leading to excited-state proton transfer. The first step after electronic excitation is charge redistribution occurring on a tens of picoseconds time scale followed by proton transfer on a nanosecond time scale. The three states observed in the experiments (initial excited state, charge redistributed state, and proton transfer state) are recognized by distinct features in the time dependence of the pump-probe spectrum and fluorescence spectra. In the charge redistributed state, charge density has transferred from the hydroxyl oxygen to the pyrene ring, but the OH sigma bond is still intact. The experiments indicate that the charge redistribution step is controlled by a specific hydrogen bond donation from HPTA to the accepting base molecule. The second step is the full deprotonation of the photoacid. The full deprotonation is clearly marked by the growth of stimulated emission spectral band in the pump-probe spectrum that is identical to the fluorescence spectrum of the anion.

Concentration−Temperature Dependencies of Structural Relaxation Time in Trehalose−Water Solutions by Brillouin Inelastic UV Scattering

A Brillouin scattering investigation has been carried out on trehalose-water solutions in a wide range of concentrations (0<phi<0.74) in the ultraviolet regime. A complete set of data as a function of temperature (-10 degrees C<or=T<or=100 degrees C) has been obtained for each concentration. The T-phi evolution of the system has been analyzed in terms of energy position and line width of inelastic peaks. These results have been used to derive the structural relaxation time, tau, of the system. This was found to be in the tens of picoseconds time scale, and its T dependence can be described with an activation (Arrhenius) law. Most importantly, a significant slowing down of the relaxation dynamics has been observed as trehalose concentration was increased. At low phi, the activation energy of the relaxation has been found to be consistent with literature data for pure water and comparable with intermolecular hydrogen bond (HB) energy. This evidence strongly supports the hypothesis that the main microscopic mechanism responsible for the relaxation process in trehalose solutions lies in the continuous rearrangement of the HB network. Finally, the results are discussed in terms of the evolution of the system upon increasing trehalose concentration, in order to provide a complete description of the viscoelastic stiffening in real biological conditions.

Observation of Excited-State Proton Transfer in Green Fluorescent Protein using Ultrafast Vibrational Spectroscopy

The photodynamics of wtGFP have been studied by ultrafast time-resolved infrared spectroscopy (TIR). In addition to the expected bleaching and transient infrared absorption of bands associated with the chromophore, we observe the dynamics of the proton relay reaction in the protein. Protonation of a protein carboxylate group occurs on the tens of picoseconds time scale following photoexcitation. Comparison with data for mutant GFPs, in which excited-state proton transfer has been disabled, supports the assignment of the carboxylate to the side chain of E222, a component of the hydrogen bonding network that links the two ends of the chromophore. The TIR data show that the rate-limiting step in the proton relay is deprotonation of the chromophore.

Hydrogen bond breaking probed with multidimensional stimulated vibrational echo correlation spectroscopy

Hydrogen bond population dynamics are extricated with exceptional detail using ultrafast (&amp;lt;50 fs) infrared multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved infrared pump–probe experiments performed on the hydroxyl stretch of methanol–OD oligomers in CCl4. Hydrogen bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calculations demonstrate that vibrational relaxation leads to hydrogen bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy (red) side of the hydroxyl stretch spectrum, the spectral region that is associated with the strongest hydrogen bonds. Frequency resolved pump–probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump–probe spectra, in conjunction with assignments made possible through the correlation spectra, it is demonstrated that the residual ground state and photoproduct of hydrogen bond breaking are prepared near their thermal equilibrium distribution. The spectrum of the hydrogen bond breaking photoproduct and the residual ground state approach the steady-state temperature difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale.

Photoinduced intramolecular charge separation of p-N,N-dimethylamino- p′-cyano-diphenylacetylene in polar solvents

By using picosecond transient absorption measurement techniques, we have investigated the intramolecular charge separation process and the dynamic spectral shift of p-N,N-dimethylamino- p′-cyano-diphenylacetylene in various polar solvents. In alcohols the spectral shift of the intramolecular charge-separated state was observed in the tens of picosecond time scale, while in aprotic polar solvents the spectral shift was too fast to measure with our time resolution. The dipole moment of the intramolecular charge-separated state, which gives a transient absorption spectrum in the red region, is about 36 D.

Ultrafast Study of the Photodissociation and Recombination of Aqueous O3-

The photodissociation of the strongly solvated radical O3- (generating O2 and O-) has been studied in aqueous solution by femtosecond pump−probe spectroscopy. The near-UV absorption band of O3- exhibits a prompt bleach due to photolysis (390 nm excitation), which is followed by a partial recovery (3.5 ps) that is assigned to delayed geminate recombination. The transient spectrum of O3- shows no evidence of excess vibrational energy content, indicating that vibrational relaxation of O3- occurs on a faster time scale than the recombination process (3.5 ps). This overall picture is in strong contrast to the standard prototype for solution recombination reactions, i.e. the widely investigated photodissociation of I2 in nonpolar solvents. For I2, photodissociation and prompt recombination (∼0.3 ps) lead to hot I2 molecules that relax on the tens of picoseconds time scale. The extremely fast vibrational relaxation and the slow recombination of O3- in water apparently stem from strong solute−solvent interactions for both O3- and O-. The O3- behavior may in fact be a common situation for polar reactions in polar solvents. In these studies, O3- was generated via a known reaction by photolysis of an oxygenated basic solution of hydrogen peroxide. The quantum yield of cage escape for O3- photodissociation has been measured to be 0.50 ± 0.02, assuming that the long-lived bleach is due entirely to cage escape. The long-lived bleach may, however, also be due to other photophysical pathways that involve long-lived, low-lying O3- excited states. Both the apparent cage escape yield and recombination dynamics are identical within experimental error in H2O and D2O. The rotational reorientation time of O3- has been measured to be 2.3 ps.

Photodissociation and geminate dynamics in solution phase: Picosecond transient absorption studies of tetraphenylhydrazines and diphenyl disulfides

Photodissociation of aromatic molecules and the following geminate dynamics of the photochemically produced fragment radical pair in liquid phase are discussed in relation to the results of ultrafast transient absorption measurements. Photodissociation of tetraphenylhydrazine occurs from the unequilibrated excited singlet state competing with the relaxation to the fluorescence state. In addition to the rapid fragmentation, the N−N bond rupture is clearly demonstrated to take place in the fluorescence state of tetraphenyl-and tetratolyl-hydrazine in tens of picoseconds time scale. In the case of di-p-aminophenyl disulfide in nonpolar solvents, geminate recombination of the p-aminophenylthiyl radical and the formation of the radical dimer were observed in tens of picoseconds. The observed slow geminate recombination can be explained by the repulsion between the large dipole of the fragment radical preventing the reformation of the S−S bond and by the restricted conformation required for the dimer formation at the encounter of radicals.

Ultrafast Measurements of Geminate Recombination of NO with Site-specific Mutants of Human Myoglobin

Flash photolysis studies of NO recombination to heme proteins offer a direct probe of protein structural changes on the tens of picoseconds timescale where they can be compared with molecular dynamics simulations. The geminate recombination of NO to site-specific mutants of human myoglobin (Mb) was studied following photodissociation of the MbNO form. Single amino acid changes were introduced at positions Val68, His64, Lys45 and Asp60 because motions of residues at these positions are generally regarded as important for the mechanism of ligand binding. In sharp contrast to the properties of simple porphyrin-NO complexes, the rebinding kinetics are found to be non-exponential for all mutants, even in aqueous solution at 298 K. The Val68 and His64 mutants substantially affect the NO rebinding rates but, surprisingly, so do changes on the protein surface that are further away from the iron. These changes in kinetics occur on a tens of picoseconds timescale, and therefore there is either a fast communication between protein residues over quite long distances or there are subtle differences in protein structure that exert great control over the reaction dynamics. Various models for the rebinding kinetics are evaluated. A model-free approach to data analysis using the maximum entropy method is found to be most useful. This analysis shows that the rate distributions are very different for the mutants, but are generally bimodal.

The dynamics of exciton tunneling and trapping in condensed xenon on ultrafast time scales

We report the use of picosecond transient absorption spectroscopy to directly observe the dynamics of formation, tunneling, and subsequent cooling of the n=1, Γ(3/2) exciton in condensed xenon over a density range of 0.10 to 1.8 g/ml. At lower densities, only Xe*2 excimers are formed, which undergo vibrational cooling on a tens of picoseconds time scale. At densities high enough to support exciton formation, tunneling from the free to the trapped exciton state takes place in ≤3 ps, and cooling of the localized exciton takes place in 5–10 ps; neither rate is strongly dependent on Xe density over the range investigated. The results are compared to theories that describe the formation and cooling rates of the trapped exciton state, and are consistent with a resonant energy transfer mechanism in which the excitation hops between neighboring Xe atoms during the trapping process.

Exciton transfer and dephasing in polysilanes at low temperature

We report on time-correlated single photon counting studies of the sigma conjugated polysilane, poly(di-n-hexylsilane), at 2 K. The emission processes of Raman and dephased flourescence are distinguished. It is shown that the risetime of flourescence and the decay time of the Raman polarization are similar and that energy transfer occurs on a tens of picosecond timescale. Simultaneous depolarization of the emission suggests energy transfer along a twisted chain. The energy transfer time is several times longer than the (T1) dephasing time derived from hole burning studies of the polymer.