Transient Absorption Dynamics Research Articles

Optical properties and applications of dendrimer–metal nanocomposites

The use of novel nanostructured materials for optical applications continues to be an important issue for the creation of new devices. New materials including metal nanoparticles have played an important role for applications in photonics, biology, as well as medicine. This review primarily concerns the use of one particular metal nanoparticle topology, dendrimer–metal nanocomposites. The focus of this review is to describe the optical properties of dendrimer–metal nanocomposites as well as functionalized dendrimer–metal nanocomposites. The description of various synthetic methodologies to produce transition metal (Au, Ag, Pd, Pt, and Cu) dendrimer nanocomposites as well as lanthanide ion-cored dendrimers are presented in this review, with further details regarding the basic characterization of these systems. The experimental procedures of the optical measurements used to probe the steady-state and time-resolved dynamics in these novel nanoparticle architectures are provided. Analysis of optical properties of dendrimer nanocomposites (DNCs) includes a description of the characterization of the metal nanoparticles as well as the size and distribution of metal nanoparticles formed by use of organic dendrimer template synthetic procedures (such as PAMAM). The non-linear transmission properties of certain dendrimer–metal nanocomposites show promising behaviour, which may be useful for applications involving eye and sensor protection. Reports of non-linear transmission properties of both Au and Ag dendrimer nanocomposites are discussed. Metal nanoparticles have also been suggested as useful materials for biological fluorescence imaging and sensing applications. However, it is well known that the efficiency of metal emission is very small. Recent measurements using ultra-fast spectroscopic techniques (fluorescence upconversion) have shed new light on this matter in metal nanoparticles and in dendrimer–metal nanocomposites. Dendrimer–metal nanocomposites have been used to investigate the dynamics of both metal particle emission and the interaction of metal particles with dipolar chromophores. The measurement and mechanism of ultra-fast transient absorption and emission dynamics in metal topologies including dendrimer nanocomposites are also discussed in the review. Future directions regarding the optical properties of dendrimer nanocomposites in the areas of photonics, biology, and medicine are also discussed in this review.

Ultrafast excited state dynamics of the bi- and termolecular stilbene-viologen charge-transfer complexes assembled via host–guest interactions

Excited state dynamics of the highly stable 1:1 and 2:1 charge-transfer (CT) complexes assembled via host–guest interactions between a biscrown stilbene and a viologen vinylog was studied using transient pump-supercontinuum probe spectroscopy. In acetonitrile, both complexes showed ultrafast two-component transient absorption dynamics after excitation in the CT absorption band by a 616 nm, 70 fs laser pulse. The faster component (τ<200 fs) is assigned to relaxation processes in the lowest CT excited state. The second component is due to the back electron transfer (ET) reaction leading to the ground state. The measured ET time constants for the 1:1 and 2:1 CT complexes are about 540 fs and 1.08 ps, respectively. Excitation of the bimolecular complex by a 308 nm laser pulse gave rise to three-component transient absorption dynamics. The fastest transient (τ∼150 fs) is assigned to relaxation processes in the high-lying excited states of the complex. The high-amplitude rise component with a time constant of about 300 fs is due to the internal conversion from the high-lying excited states to the lowest CT excited state. The latter decayed to the ground state via the back ET with a time constant very close to that measured when the complex was excited in the CT absorption band.

Near-Field Optical Imaging of Transient Absorption Dynamics in Organic Nanocrystals

Collection-mode near-field scanning optical microscopy is combined with femtosecond two-color pump−probe spectroscopy to study the transient absorption dynamics in organic crystals. The temporal ev...

Visualization of transient absorption dynamics – towards a qualitative view of complex reaction kinetics

Photochemical reactions may involve complex reaction schemes and their investigation requires spectroscopic techniques extending over a broad spectral range and over several orders of magnitude in time. Two commonly used numerical procedures to evaluate such data sets – global fitting and singular value decomposition – are discussed and a method for the qualitative visualization is proposed: differentiation of the transient spectra on a logarithmic scale allows to extract special kinetic components and to obtain reasonable starting information for subsequent fitting procedures. The proposed method should be well adapted to situations where e.g. due to unstable samples the data cannot be recorded at the precision required for unambiguous analysis by standard data handling procedures. The method is applied to synthetic data sets as well as to experimental data taken from femtosecond absorption experiments on the laser dye DCM in DMSO.

Comparative Investigation of Energy Relaxation Dynamics of Gold Nanoparticles and Gold−Polypyrrole Encapsulated Nanoparticles

We report the femtosecond transient absorption dynamics of the gold and gold−polypyrrole nanoparticles by photoexcitation at various wavelengths. The bleach recovery dynamics of the surface plasmon band for the gold and gold−polypyrrole nanoparticles exhibit different responses to the pump beam energy and intensity in thermal energy transfer from the gold nanoparticles to the surrounding medium or embedding material. This indicates that directly attached polypyrroles provide fast thermal energy transfer pathways for the core gold nanoparticles.

Exciton−Vibrational Relaxation and Transient Absorption Dynamics in LH1 of Rhodopseudomonas viridis: A Redfield Theory Approach

We explain the ultrafast transient absorption (TA) dynamics in the core LH1 antenna of Rhodopseudomonas viridis at 77 K (Monshouwer, R.; Baltuška, A.; van Mourik, F.; van Grondelle, R. J. Phys. Chem. A 1998, 102, 4360) using a disordered exciton model with strong coupling to two vibrational modes and weak coupling of vibrational and electronic coordinates to the thermal bath. A quantitative fit of the excitation wavelength- dependent TA dynamics was obtained using the density matrix equation with the Redfield relaxation operator. The sequential and coherent contributions to the TA dynamics were analyzed separately. The time-dependent red shift of TA was explained in terms of exciton relaxation. The lifetimes of higher exciton states of 12−75 fs (depending on the state) were determined from the fit.

Nanophotonics: a new multidisciplinary frontier

Nanophotonics, defined as nanoscale optical science and technology, is a new multidisciplinary frontier. This article presents our studies on nanoscale matter–radiation interactions utilizing nanoscale confinement of radiation as well as on nanoscale photochemical transformations, particularly nanoscale nonlinear optical interactions. The selected examples of our studies include nanoscopic optical harmonic generation, multiphoton fluorescence, transient absorption dynamics, surface-plasmon-enhanced two-photon fluorescence, and nonlinear optical information storage.

Dynamics of excitation energy transfer in the LH1 and LH2 light-harvesting complexes of photosynthetic bacteria.

Photosynthetic light harvesting is a unique life process that occurs with amazing efficiency. Since the discovery of the structure of the bacterial peripheral light-harvesting complex (LH2), this process has been studied using a variety of advanced laser spectroscopic methods. We are now in a position to discuss the physical origins of excitation energy transfer and trapping in the LH2 and LH1 antennae of photosynthetic purple bacteria. We demonstrate that the time evolution of the state created by the light is determined by the combined action of excitonic pigment-pitment interactions, energetic disorder, and coupling to nuclear motion in a pigment-protein complex. A quantitative fit of experimental data using Redfield theory allowed us to determine the pathways and time scales of exciton and vibrational relaxation and analyze separately different contributions to the measured transient absorption dynamics. Furthermore, these dynamics were observed to be strongly dependent on the excitation wavelength. A numerical fit of this dependence turns out to be extremely critical to a variation of the structure and disorder parameters and, therefore, can be used as a test for different antenna models (disordered ring, elliptical deformations, correlated disorder, etc.). The calculated equilibration dynamics in the exciton basis allow a visualization of the exciton motion using a density matrix picture in real space.

Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na− in THF

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ∼2 μm, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ∼4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances ⩾40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na−, on the other hand, takes place on two distinct time scales of ⩽2 and ∼200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments.

Quantum Yield for ClOO Formation following Photolysis of Aqueous OClO

The photochemistry of chlorine dioxide (OClO) in aqueous solution was investigated by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 400 nm, the transient absorption dynamics were probed in the spectral range from 400 to 220 nm. As expected from earlier studies, the main photolytic products ClO + O, formed with a quantum yield of ∼90%, disappear through fast geminate recombination producing OClO in the electronic ground state. The total quantum yield for chlorine atom production is (ΦCl) ∼10%, with the chlorine atom production occurring through two competing processes. The dominant channel for chlorine atom production involves the formation of a short-lived intermediate on a ∼6 ps time scale with a quantum yield of 8 ± 2%. The remaining 2 ± 1% is formed through the formation and decomposition of ClOO. The lifetime of ClOO was found to be ∼0.32 ns, in very good agreement with the result of a recent time-resolved resonance Raman study. Finally, the UV absorption spectrum for aqueous ClOO is reported and compared with previously reported spectra obtained in condensed media.

Dominant role of surface states in photoexcited carrier dynamics in CdSe nanocrystalline films prepared by chemical deposition

We report on ultrafast carrier dynamics in nanocrystalline CdSe thin films prepared by chemical solution deposition. The photoluminescence and transient absorption dynamics have spectrally dependent decay faster at shorter wavelengths. The spectral behavior of the decay rates in samples with different nanocrystal sizes and the sensitivity of the dynamics on the surface modification indicate the dominant role of the surface states in the photoluminescence.

Unified picture of the photoexcitations in phenylene-based conjugated polymers: Universal spectral and dynamical features in subpicosecond transient absorption

Using subpicosecond transient absorption spectroscopy, we investigate the primary photoexcitations in thin films and solutions of several phenylene-based conjugated polymers and an oligomer. We identify several features in the transient absorption spectra and dynamics that are common to all of the materials which we studied from this family. The first spectral feature is a photoinduced absorption (PA) band peaking near 1 eV that has intensity-dependent dynamics that match the stimulated emission dynamics exactly over two orders of magnitude in excitation density. This band is associated with singlet intrachain excitons. The second spectral feature (observed only in thin films and aggregated solutions) is a PA band peaking near 1.8 eV, that is longer lived than the 1 eV exciton PA band, and that has dynamics that are independent (or weakly dependent) on excitation density. This feature is attributed to polarons, generated through a mechanism that is sample dependent. In pristine samples, polarons are generated via a mechanism that is quadratic in exciton density, whereas in photodegraded samples or samples doped with electron acceptors, the generation mechanism becomes linear in exciton density.

Excited-state absorption in luminescent conjugated polymer thin films: ultrafast studies of processable polyindenofluorene derivatives

Femtosecond transient absorption experiments on thin films of poly-2,8-indenofluorene, a processable step-ladder analogue of poly- p-phenylene, are presented. Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped singlet excited-state and charged excitation absorption bands. Morphological effects on the dynamics are investigated by modification of the aliphatic side chains in the monomer unit. Steric effects, controlled by the nature of the alkyl side groups, result in a group-dependent three-dimensional structure. The transient absorption dynamics are similar in all samples, in spite of distinct morphologies, indicating limited importance in the excited-state analysis.

Ultrafast laser action dynamics in Dooppv thin films

We have investigated the femtosecond transient photoinduced absorption (PA) dynamics of excitons in DOO-PPV thin films as a function of the excitation intensity, studied simultaneously with stimulated emission measurements. We observed both fast and slow PA components, which depend on the excitation intensity. At low excitation intensity only the slow PA component was measured with a life time of240 ps. However, for intensities above the threshold for spectral-narrowing, a fast PA dynamics dominate the decay for times less then 10 ps.

Ultrafast carrier dynamics in wide gap hydrogenated amorphous silicon

We report on picosecond and femtosecond pump and probe measurements of the dynamics of photoexcited carriers in wide gap a-Si : H prepared by microwave electron–cyclotron resonance plasma-enhanced chemical-vapour-deposition. We interpret the picosecond dynamics of transient absorption under strong picosecond excitation in terms of a bimolecular recombination process with the rate constant B≈5×10 −10 cm 3 s −1, followed by a slower nanosecond decay. In the femtosecond measurements, we observed an initial decay with effective time constant ≈20 ps. We have not found any change in the picosecond dynamics when tuning the excitation wavelength through photoluminescence (PL) excitation spectrum profile. The ultrafast dynamics do not differ in the samples with PL efficiency differing in more than one order of magnitude, and they agree well with those in standard a-Si : H.

The involvement of rotational processes in the intramolecular proton‐transfer cycle

AbstractEncapsulation processes and picosecond time‐resolved transient absorption spectra of 2‐(2î‐hydroxyphenyl) imidazo[1,2‐a]pyridine have been used to elucidate the fluorescence band shift and the transient absorption dynamics (in the picosecond time scale) of the proton transfer phototautomer in cyclodextrins and in cyclohexane, respectively. Restriction of the internal motion of the phototautomer by cyclodextrins leads to a 70 nm blue shift of the proton transfer fluorescence band. A shaping time (14 ps) of the transient absorption spectrum has been observed and assigned to a combination of this internal rotational motion and a vibrational‐cooling process of the phototautomers in the excited state. Both studies suggest the involvement of rotational processes in the intramolecular proton‐transfer cycle.

Intensity-dependent relaxation dynamics and the nature of the excited-state species in solid-state conducting polymers

Femtosecond transient-absorption dynamics are presented for an oligomer and a number of polymers from the poly(arylene vinylene) family for excitation densities in the range from ${10}^{18}$--${10}^{21}$ cm${}^{\ensuremath{-}3}$. We report careful studies of the wavelength and pump-intensity dependence of the photoinduced absorption in a model five-ring oligomer, 2-methoxy-5-(2${}^{\ensuremath{'}}$-ethylhexyloxy)-distyryl benzene. We show that a single photoexcited species, the intrachain exciton, is responsible for all observable spectral features at low excitation densities. At higher excitation densities, nonlinear relaxation processes are observed in both polymers and the oligomer, consistent with exciton-exciton annihilation as a nonlinear-decay mechanism.

Observation of stimulated emission and ultrafast transient absorption dynamics from a novel alkyl-substituted PPV

Ultrafast transient absorption, stimulated emission (SE), and photoluminescence (PL) measurements were performed on solid films of the conducting polymer: poly[2-butyl-5-(2′-ethyl-hexyl)-1,4-phenylenevinylene](BuEH-PPV). The ground state absorption and luminescence of BuEH-PPV are similar to those of unsubstituted PPV, indicating that alkyl substitution does not significantly alter the electronic structure of the ground state. The PL lifetime and PL quantum yield of BuEH-PPV, however, are ≥ 900 ps and over 0.6, respectively. These numbers are nearly 3 times longer and higher than those for unsubstituted PPV. Moreover, BuEH-PPV exhibits a strong, relatively long-lived SE (≥ 60 ps). The SE decays more quickly than the PL due to an interfering photoinduced absorption which undergoes a dynamic blue-shift to mask the SE at later times. These observations taken together offer a prescription for controlling the excited state electronic structure of conjugated polymers, and open the possible production of optically or electrically pumped solid polymer lasers.

Ultrafast Dynamics of Chlorine Dioxide Photochemistry in Water Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast dynamics of chlorine dioxide (OClO) photochemistry in aqueous solution was studied by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 395 nm, the transient absorption dynamics were probed at 12 different wavelengths ranging from 350 to 700 nm. The transient absorption features observed in the visible wavelengths are assigned to correspond to the vibrationally hot photoisomer ClOO*. The spectral dynamics reveal the vibrational relaxation of this molecule in its ground electronic state. The total vibrational energy relaxation occurs within ∼9 ps. The dynamics of the formation of chlorine atom was examined by measuring the absorption dynamics in the 350−390 nm range. The time constant for Cl formation is calculated to be ∼200 ps. The data show that the dominant pathway for Cl formation is via the isomerized ClOO molecule. No fast component for Cl is detected, indicating that the ClOO molecule reaches thermal equilibrium before dissociating into Cl and O2.

Dynamics of transient absorption in the active media of Penning Ne I lasers

The method of absorption probing was used to determine the temporal behaviour of transient absorption in the active media of Penning lasers based on the 3p—3s transitions in the Ne I atom. The pumping was provided by an electron beam and the current density was 1.7 A cm-2. The efficiency of coupling out of radiation from a cavity containing an optimised laser mixture could exceed 80% for the transitions resulting in the emission at the wavelengths λ = 585 and 725 nm, and 50% for the λ = 703 nm transition.