Multiexponential Dynamics Research Articles

A Photoswitchable Metallocycle Based on Azobenzene: Synthesis, Characterization, and Ultrafast Dynamics.

The novel photoswitchable ligand 3,3'-Azobenz(metPA)2 (1) is used to prepare a [Cu2(1)2](BF4)2 metallocycle (2), whose photoisomerization was characterized using static and time-resolved spectroscopic methods. Optical studies demonstrate the highly quantitative and reproducible photoinduced cyclic E/Z switching without decay of the complex. Accordingly and best to our knowledge, [Cu2(1)2](BF4)2 constitutes the first reversibly photoswitchable (3d)-metallocycle based on azobenzene. The photoinduced multiexponential dynamics in the sub-picosecond to few picosecond time domain of 1 and 2 have been assessed. These ultrafast dynamics as well as the yield of the respective photostationary state (PSSZ = 65 %) resemble the behavior of archetypical azobenzene. Also, the innovative pump-probe laser technique of gas phase transient photodissociation (τ-PD) in a mass spectrometric ion trap was used to determine the intrinsic relaxation dynamics for the isolated complex. These results are consistent with the results from femtosecond UV/Vis transient absorption (fs-TA) in solution, emphasizing the azobenzene-like dynamics of 2. This unique combination of fs-TA and τ-PD enables valuable insights into the prevailing interplay of dynamics and solvation. Both analyses (in solution and gas phase) and quantum chemical calculations reveal a negligible effect of the metal coordination on the switching mechanism and electronic pathway, which suggests a non-cooperative isomerization process.

High-resolution spectroscopy of Holmium Perchlorate: Establishment of wavelength standard for spectrophotometer

We have investigated the excited state dynamics of Holmium Oxide (Ho2O3) dissolved in perchloric acid using ultrafast optical pump-probe spectroscopy by exciting the samples with laser light at 350 nm and 550 nm and detecting in the visible region. Using an ultrafast spectroscopic approach, the stability of Ho2O3 solution has been examined. UV–Visible spectroscopy revealed 14 bands in the Ho2O3 absorption spectrum and by using ultrafast transient absorption spectroscopy, we have probed the dynamics of the excited state and studied transient absorption lifetime in the visible range. We obtained multi-exponential dynamics of carrier relaxation for Ho2O3, with time constants spanning from 0.07 to >500 ps. The lifetime of molecules has been shown to be on the order of picoseconds (ps), proving the stability of Ho2O3 solution and making it suitable for use as a wavelength standard for spectrophotometer calibration.

Multiexponential dynamics of Mn2+(3d5) excitation in manganese doped ZnCdS quantum dots: Stimulated emission band in femtosecond transient spectra reveals ultrafast nonradiative energy transfer to Mn2+(3d5)

Stimulated emission (SE) 4T1 → 6A1 bands of Mn2+(3d5) in ZnxCd1−xS or ZnxCd1−xS/ZnS quantum dots were detected in the femtosecond transient absorption spectra at a time delay of 120 fs. Multiexponential rise of SE(Mn2+) amplitude due to nonradiative energy transfer to Mn2+(3d5) occurs both in the femtosecond and picosecond time domain. Transient kinetics of SE(Mn2+) band reveals the dynamics of energy transfer to Mn2+(3d5) with time constants τ1 ~ 400–600 fs, τ2 ~ 4–6 ps and τ3 ~ 87–112 ps. A possible mechanism of ultrafast energy transfer to the Mn2+(3d5) center accompanied by spin-flip is discussed.

Investigation of electronic energy transfer in a BODIPY-decorated calix[4]arene

The photophysics of a donor-acceptor system in which efficient electronic energy transfer occurs is analyzed and discussed by the combined use of steady-state and time-resolved spectroscopy and DFT/TD-DFT computations. The donor and acceptor units belong to the class of BODIPY chromophores, and are conveniently linked through a calixarene scaffold, which allows the control of the mutual orientation and distance between chromophores. Our results highlight that the energy transfer process occurs with multiexponential dynamics strongly influenced by the solvent. Although the conformation adopted by the system is very similar in all the analyzed solvents, highly polar media favour fast and efficient energy transfer. On the contrary, in non-polar media, the concomitant occurrence of backward energy transfer causes a significant slowdown of the process. The inverse of the energy transfer rates calculated at the TDDFT level are in very good agreement with the experimental kinetics measured with transient absorption spectroscopy.

Ultrafast excited-state relaxation of a binuclear Ag(i) phosphine complex in gas phase and solution.

The binuclear complex [Ag2(dcpm)2](PF6)2 (dcpm = bis(dicyclohexylphosphino)methane) exhibits a structure with a close silver-silver contact mediated by the bridging ligand and thus a weak argentophilic interaction. Upon electronic excitation this cooperative effect is strongly increased and determines the optical and luminescence properties of the compound. We have studied here the ultrafast electronic dynamics in parallel in gas phase by transient photodissociation and in solution by transient absorption. In particular, we report the diverse photofragmentation pathways of isolated [Ag2(dcpm)2]2+ in an ion trap and its gas phase UV photodissociation spectrum. By pump-probe fragmentation action spectroscopy (λex = 260 nm) in the gas phase, we have obtained fragment-specific transients which exhibit a common ultrafast multiexponential decay. This is fitted to four time constants (0.6/5.8/100/>1000 ps), highlighting complex intrinsic photophysical processes. Remarkably, multiexponential dynamics (0.9/8.5/73/604 ps) are as well found for the relaxation dynamics in acetonitrile solution. Ab initio calculations at the level of approximate coupled-cluster singles-doubles (CC2) theory of ground and electronically excited states of the reduced model system [Ag2(dmpm)2]2+ (dmpm = bis(dimethylphosphino)methane) indicate a shortening of the Ag-Ag distance upon excitation by 0.3-0.4 Å. In C2 geometry two close-lying singlet states S1 (1MC(dσ*-pπ), 1B, 4.13 eV) and S2 (1MC(dσ*-pσ), 1A, 4.45 eV) are found. The nearly dark S1 state has not been reported so far. The excitation of the S2 state carries a large oscillator strength for the calculated vertical transition (266 nm). Two related triplets are calculated at T1 (3.87 eV) and T2 (3.90 eV). From these findings we suggest possible relaxation pathways with the two short time constants ascribed to ISC/IVR and propose from the obtained similar values in gas phase that the fast solution dynamics is dominated by intramolecular processes. A further relaxation by IC/IVR in the triplet manifold is likely to account for the observed intermediate time constants. For the acetonitrile relaxation dynamics additional modifications are invoked based on solvent-induced shifts of the energy levels and the possible formation of solvent and counterion exciplexes on a longer timescale.

Bacteriophytochrome Photoisomerization Proceeds Homogeneously Despite Heterogeneity in Ground State

Phytochromes are red/far-red photoreceptors that are widely distributed in plants and prokaryotes. Ultrafast photoisomerization of a double bond in a biliverdin cofactor or other linear tetrapyrrole drives their photoactivity, but their photodynamics are only partially understood. Multiexponential dynamics were observed in previous ultrafast spectroscopic studies and were attributed to heterogeneous populations of the pigment-protein complex. In this work, two-dimensional photon echo spectroscopy was applied to study dynamics of the bacteriophytochromes RpBphP2 and PaBphP. Two-dimensional photon echo spectroscopy can simultaneously resolve inhomogeneity in ensembles and fast dynamics by correlating pump wavelength with the emitted signal wavelength. The distribution of absorption and emission energies within the same state indicates an ensemble of heterogeneous protein environments that are spectroscopically distinct. However, the lifetimes of the dynamics are uniform across the ensemble, suggesting a homogeneous model involving sequential intermediates for the initial photodynamics of isomerization.

Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications.

Guided by predictive discovery framework, we investigate bismuth triiodide (BiI3) as a candidate thin-film photovoltaic (PV) absorber. BiI3 was chosen for its optical properties and the potential for "defect-tolerant" charge transport properties, which we test experimentally by measuring optical absorption and recombination lifetimes. We synthesize phase-pure BiI3 thin films by physical vapor transport and solution processing and single-crystals by an electrodynamic gradient vertical Bridgman method. The bandgap of these materials is ∼1.8 eV, and they demonstrate room-temperature band-edge photoluminescence. We measure monoexponential recombination lifetimes in the range of 180-240 ps for thin films, and longer, multiexponential dynamics for single crystals, with time constants up to 1.3 to 1.5 ns. We discuss the outstanding challenges to developing BiI3 PVs, including mechanical and electrical properties, which can also inform future selection of candidate PV absorbers.

Ultrafast solvation response in room temperature ionic liquids: Possible origin and importance of the collective and the nearest neighbour solvent modes

Recent three-pulse photon echo peak shift (3PEPS) measurements [M. Muramatsu, Y. Nagasawa, and H. Miyasaka, J. Phys. Chem. A 115, 3886 (2011)] with several room temperature ionic liquids (RTILs) have revealed multi-exponential dynamics with ultrafast solvation timescale in the range, 20 < τ(1)∕fs < 250, for both imidazolium and phosphonium RTILs. This is striking for two reasons: (i) the timescale is much faster than those reported by the dynamic Stokes shift (DSS) experiments [S. Arzhantsev, H. Jin, G. A. Baker, and M. Maroncelli, J. Phys. Chem. B 111, 4978 (2007)] and (ii) sub-hundered femtosecond solvation response in phosphonium ionic liquids is reported for the first time. Here, we present a mode coupling theory based calculation where such ultrafast solvation in 3PEPS measurements has been visualized to originate from the nearest neighbour solute-solvent interaction. Consideration of Lennard-Jones interaction for the nearest neighbour solute-solvent non-dipolar interaction leads to biphasic dynamics with a predicted ultrafast time constant in the ∼100-250 fs range, followed by a slower one similar to that reported by the 3PEPS measurements. In addition, the calculated fast time constants and amplitudes are found to be in general agreement with those from computer simulations. Different microscopic mechanisms for ultrafast solvation response measured by the 3PEPS and DSS experiments have been proposed and relative contributions of the collective and nearest neighbour solvent modes investigated. Relation between the single particle rotation and ultrafast polar solvation in these RTILs has been explored. Our analyses suggest 3PEPS and DSS experiments are probably sensitive to different components of the total solvation energy relaxation of a laser-excited dye in a given ionic liquid. Several predictions have also been made, which may be re-examined via suitable experiments.

Does Bridging Geometry Influence Interfacial Electron Transfer Dynamics? Case of the Enediol-TiO2 System

We have employed femtosecond transient absorption spectroscopy in enediol-TiO2 systems (catechol, resorcinol, and quinol) to understand localized vs delocalized interfacial electron transfer dynamics in dye-nanoparticle systems. Optical absorption studies confirmed the formation of a charge transfer (CT) complex between the enediols and TiO2 nanoparticles. CT interaction between enediols and TiO2 was found to be decreased from catechol to resorcinol to quinol. The decrease in interaction strength from catechol to quinol was explained on the basis of a reduced overlap between the HOMO localized on the enediol and the conduction band of TiO2. Femtosecond transient absorption studies confirmed an ultrafast electron injection (<50 fs) into the conduction band of TiO2 in all enediol-TiO2 systems. Interestingly back electron transfer (BET), which follows multiexponential dynamics, is faster in the catechol-TiO2 system as compared to the other two enediol-TiO2 systems. As we increase the distance between the bridging ligand from catechol to quinol, we find that decay time increases proving the influence of bridging distance between enediols and TiO2. Our theoretical studies indicate an increase in the delocalization of the injected electron over several Ti atoms as the distance between the bridge linkers increases. Analysis of BET results in the framework of the Marcus theory indicated a significant influence of electronic coupling on BET in the enediol-TiO2 systems.

Probabilistic sleep architecture models in patients with and without sleep apnea

Sleep fragmentation of any cause is disruptive to the rejuvenating value of sleep. However, methods to quantify sleep architecture remain limited. We have previously shown that human sleep-wake stage distributions exhibit multi-exponential dynamics, which are fragmented by obstructive sleep apnea (OSA), suggesting that Markov models may be a useful method to quantify architecture in health and disease. Sleep stage data were obtained from two subsets of the Sleep Heart Health Study database: control subjects with no medications, no OSA, no medical co-morbidities and no sleepiness (n = 374); and subjects with severe OSA (n = 338). Sleep architecture was simplified into three stages: wake after sleep onset (WASO); non-rapid eye movement (NREM) sleep; and rapid eye movement (REM) sleep. The connectivity and transition rates among eight 'generator' states of a first-order continuous-time Markov model were inferred from the observed ('phenotypic') distributions: three exponentials each of NREM sleep and WASO; and two exponentials of REM sleep. Ultradian REM cycling was accomplished by imposing time-variation to REM state entry rates. Fragmentation in subjects with severe OSA involved faster transition probabilities as well as additional state transition paths within the model. The Markov models exhibit two important features of human sleep architecture: multi-exponential stage dynamics (accounting for observed bout distributions); and probabilistic transitions (an inherent source of variability). In addition, the model quantifies the fragmentation associated with severe OSA. Markov sleep models may prove important for quantifying sleep disruption to provide objective metrics to correlate with endpoints ranging from sleepiness to cardiovascular morbidity.

Quantitative analysis of the dynamic behaviour of photochromic systems

The good understanding of a photochromic reaction mechanism requires the establishment of the list of all the transient species and the definition of their connecting processes. The purpose of kinetic studies is the determination of the main photochromic parameters, such as the quantum yields of photoisomerization, rate constants of thermal relaxation and spectra of transient species. These data allow the establishment of structure properties relationships in order to select the best substituents to improve photochromic performances within a given series. In this review, we describe the dynamic behaviour of various photochromic systems during thermal relaxation after irradiation, from the simplest mono-exponential decay to the more complicated multi-exponential dynamics. Then, we analyse the evolution of the long-lived isomers under continuous irradiation. Several pedagogical examples and tricks to perform easy kinetic analysis are given in the appendices.

Fast Excited-State Deactivation in N(5)-Ethyl-4a-hydroxyflavin Pseudobase

We present a study of the excited-state behavior of N(5)-ethyl-4a-hydroxyflavin (Et-FlOH), a model compound for bacterial bioluminescence. Using femtosecond pump-probe spectroscopy, we found that the Et-FlOH excited state exhibits multiexponential dynamics, with the dominant decay component having a 0.5 ps lifetime. Several possible mechanisms for fast excited-state decay in Et-FlOH were considered: (i) excited-state deprotonation of the -OH proton, (ii) thermal deactivation via (1)n,π* → (1)π,π* conical intersection, and (iii) excited-state release of OH(-) ion. These mechanisms were excluded based on transient absorption studies of two model compounds (N(5)-ethyl-4a-methoxyflavin, Et-FlOMe, and N(5)-ethyl-flavinium ion, Et-Fl(+)) and based on the results of time-dependent density functional theory (TD-DFT) calculations of Et-FlOH excited-states. Instead, we propose that the fast decay in Et-FlOH is caused by S(1) → S(0) internal conversion, initiated by the excited-state nitrogen planarization (sp(3) → sp(2) hybridization change at the N(5)-atom of Et-FlOH S(1) state) coupled with out-of-plane distortion of the pyrimidine moiety of flavin.

Photophysics and Ultrafast Relaxation Dynamics of the Excited States of Dimethylaminobenzophenone

Steady-state as well as time-resolved absorption and emission studies in the femto-, pico-, and nanosecond time-domains have been performed to investigate the relaxation dynamics and the photophysical properties of dimethylaminobenzophenone (DMABP) by characterizing its excited singlet (S1) and triplet (T1) state properties in different kinds of solvents. Large bathochromic shifts of the steady-state absorption maximum in more polar solvents suggest an intramolecular charge transfer (ICT) character of the S1 state. DMABP is weakly fluorescent but shows the features of “dual-fluorescence”. Dynamics of very efficient nonradiative relaxation process in the S1 state of DMABP has been described by invoking a “three state model”, in which the involvement of the ground state and two adiabatically coupled S1 states, a locally excited (LE) ππ* or ICT state and a conformationally relaxed or twisted intramolecular charge transfer (TICT) state, have been considered. In aprotic solvents, the fluorescence maximum arises due to emission from the TICT state. On the other hand, in alcohols, the fluorescence maximum arises due to emission from the LE state, since the TICT state is weakly fluorescent due to strong intermolecular hydrogen-bonding interaction with the solvent. The excited-state relaxation process follows multiexponential dynamics. An ultrafast component, having lifetime τ1, which varies in the range 0.2−0.5 ps in different polar solvents, may possibly originate from either the relaxation of the solvent-perturbed low-frequency intramolecular modes or the inertial solvation of the ICT state. The lifetime of another slower component (τ2 ∼ 0.2−5 ps) increases linearly with an increase in the viscosity of the solvent. This component has been attributed to the diffusive twisting motion of the dimethylaminophenyl group with respect to the benzoyl group. This twisting process with a rate, which is comparable to or faster than the average solvation time of the solvent, is associated with the crossing of a low barrier (or quasi-barrierless) and responsible for the population relaxation in the S1 state. The third component (τ3 ∼ 5−900 ps), which is sensitive to the hydrogen bonding ability of the solvent, represents the decay of the S1 state to the ground state. The spectroscopic nature, energy, and yield of the T1 state are very sensitive to the characteristics of the solvent or medium.

Transient and steady state photorefractive responses in dye-doped nematic liquid crystal cells

The photorefractive response of several dye-doped nematic liquid crystal samples is studied experimentally. Particular attention is given to the observation and modeling of the multiexponential dynamics of the photorefractive grating, and of the photocurrent. Both involve at least two different mechanisms with largely different time constants. These phenomena occur on time scales on the order of seconds, for applied voltages about 1 V, and light intensities as low as milliwatts/cm2.

The relationship between excitation energy transfer, trapping and antenna size in Photosystem II

A systematic study of a range of particles, isolated from both higher plants and the cyanobacterium Synechocystis 6803, has investigated the effect of antenna size on energy transfer and trapping in Photosystem II. Time resolved fluorescence experiments have been employed to obtain the absolute singlet state concentrations as a function of time from each sample, rather than focusing on a multiexponential lifetime analysis of the data. Models that may be used to describe the multiexponential dynamics of energy and electron transfer in isolated reaction centre samples were investigated, to establish whether they could be extended to predict the experimentally observed trapping dynamics in open and closed core particles without the need to vary the reaction centre (`trapping engine?) parameters. It is demonstrated that macroscopic and microscopic reaction centre `trapping engine? models may be extended to describe the experimentally observed trapping dynamics in a range of particles with QA in a fully closed state. This suggests that the presence of the shallow trap and slow charge separation kinetics, observed in isolated D1/D2/cyt b559 reaction centres, are retained in larger particles. Fluorescence decays obtained from cores particles with QA in a fully open state were more difficult to reconcile using macroscopic models without changing the free energy between the excited singlet and primary radical pair states. A microscopic description of the trapping engine may however be used to provide an alternative representation of the dynamics in open core particles that does not require the original parameters employed in the reaction centre trapping engine model to be altered.