Femtosecond excitation energy transport in triarylamine dendrimers

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.     

Coherent effects in energy transport in model dendritic structures investigated by ultrafast fluorescence anisotropy spectroscopy

Measurements of ultrafast fluorescence anisotropy decay in model branched dendritic molecules of different symmetry are reported. These molecules contain the fundamental branching center units of larger dendrimer macromolecules with either three (C(3))- or four (T(d), tetrahedral)-fold symmetry. The anisotropy for a tetrahedral system is found to decay on a subpicosecond time scale (880 fs). This decay can be qualitatively explained by F?rster-type incoherent energy migration between chromophores. Alternatively, for a nitrogen-centered trimer system, the fluorescence anisotropy decay time (35 fs) is found to be much shorter than that of the tetramers, and the decay cannot be attributed to an incoherent hopping mechanism. In this case, a coherent interchromophore energy transport mechanism should be considered. The mechanism of the ultrafast energy migration process in the branched systems is interpreted by use of a phenomenological quantum mechanical model, which examines the two extreme cases of incoherent and coherent interactions.     

Energy migration study of random immobile anthracene derivatives by time-resolved fluorescence anisotropy decays

We describe time-resolved fluorescence anisotropy measurements for a simple 2-substituted anthracene derivative (2-An-M) as a function of concentration in poly(methyl methacrylate) (PMMA) films. The anisotropy decays via energy migration among 2-An-M molecules in the matrix. The data were interpreted in terms of the survival probability of the initially excited chromophores and are in good agreement with the plot predicted using R0 = 1.8 nm, the value of the F?rster radius determined independently by the spectral overlap method. For all concentrations, we found r0 = 0.20 +/- 0.01 for the initial anisotropy at time zero, whereas the residual anisotropy, r(infinity), was concentration-dependent and higher in magnitude than the theoretically predicted value, 4% of r0. The higher residual anisotropy values may originate from the possibility that not all of the anthracene molecules are involved in the energy migration process. Similar anthracene anisotropy experiments were performed on anthracene-labeled poly(isoprene-b-methyl methacrylate) (PI-PMMA). The results show an increased depolarization rate for samples containing a higher fraction of polymers labeled at the junction with anthracene chromophores.     

Temperature dependence of anisotropy decay and solvation dynamics of coumarin 153 in gamma-cyclodextrin aggregates

Effect of temperature on the fluorescence anisotropy decay and the ultraslow component of solvation dynamics of coumarin 153 (C153) in a gamma-cyclodextrin (gamma-CD) nanocavity are studied using a picosecond set up. The steady-state anisotropy (0.13 +/- 0.01) and residual anisotropy (0.14 +/- 0.01) in fluorescence anisotropy decay in an aqueous solution containing 7 microM C153 and 40 mM gamma-CD are found to be quite large. This indicates formation of large linear nanotube aggregates of gamma-CD linked by C153. It is estimated that >53 gamma-CD units are present in each aggregate. In these aggregates with rise in temperature, the average solvation time ((obs)) decreases markedly from 680 ps at 278 K to 160 ps at 318 K. The dynamic Stokes shift is found to decrease from 800 cm(-1) at 278 K to 250 cm(-1) at 318 K. The fraction of dynamic Stokes shift (f(d)) detected in a picosecond set up is calculated using the Fee-Maroncelli procedure. The corrected solvation time ((corr) = f(d)<(tau(s)>(obs)) displays an Arrhenius type temperature dependence. From the temperature variation, the activation energy and entropy of the solvation process are determined to be 12.5 kcal M(-1) and 28 cal M(-1) K(-1), respectively. The ultraslow component and its temperature dependence are ascribed to a dynamic exchange between bound and free water molecules.     

Temperature dependence of solvation dynamics and anisotropy decay in a protein: ANS in bovine serum albumin

Temperature dependence of solvation dynamics and fluorescence anisotropy decay of 8-anilino-1-naphthalenesulfonate (ANS) bound to a protein, bovine serum albumin (BSA), are studied. Solvation dynamics of ANS bound to BSA displays a component (300 ps) which is independent of temperature in the range of 278-318 K and a long component which decreases from 5800 ps at 278 K to 3600 ps at 318 K. The temperature independent part is ascribed to a dynamic exchange of bound to free water with a low barrier. The temperature variation of the long component of solvation dynamics corresponds to an activation energy of 2.1 kcal mol(-1). The activation energy is ascribed to local segmental motion of the protein along with the associated water molecules and polar residues. The time scale of solvation dynamics is found to be very different from the time scale of anisotropy decay. The anisotropy decays are analyzed in terms of the wobbling motion of the probe (ANS) and the overall tumbling of the protein.     

Intrachain energy migration to weak charge-transfer state in polyfluorene end-capped with naphthalimide derivative

Polyfluorene end-capped with N-(2-benzothiazole)-1,8-naphthalimide (PF-BNI) is a highly fluorescent material with fluorescence emission modulated by solvent polarity. Its low energy excited state is assigned as a mixed configuration state between the singlet S(1) of the fluorene backbone (F) with the charge transfer (CT) of the end group BNI. The triexponential fluorescence decays of PF-BNI were associated with fast energy migration to form an intrachain charge-transfer (ICCT) state, polyfluorene backbone decay, and ICCT deactivation. Time-resolved fluorescence anisotropy exhibited biexponential relaxation with a fast component of 12-16 ps in addition to a slow one in the range 0.8-1.4 ns depending on the solvent, showing that depolarization occurs from two different processes: energy migration to form the ICCT state and slow rotational diffusion motion of end segments at a longer time. Results from femtosecond transient absorption measurements agreed with anisotropy decay and showed a decay component of about 16 ps at 605 nm in PF-BNI ascribed to the conversion of S(1) to the ICCT excited state. From the ratio of asymptotic and initial amplitudes of the transient absorption measurement, the efficiency of intrachain ICCT formation is estimated in 0.5, which means that, on average, half of the excited state formed in a BNI-(F)(n)-BNI chain with n = 32 is converted to its low energy intrachain charge-transfer (ICCT) state.     

Energy migration in novel pH-triggered self-assembled beta-sheet ribbons

Energy migration between tryptophan residues has been experimentally demonstrated in self-assembled peptide tapes. Each peptide contains 11 amino acids with a Trp at position 6. The peptide self-assembly is pH-sensitive and forms amphiphilic tapes, which further stack in ribbons (double tapes) and fibrils in water depending on the concentration. Fluorescence spectra, quenching, and anisotropy experiments showed that when the pH is lowered from 9 to 2, the peptide self-assembly buries the tryptophan in a hydrophobic and restricted environment in the interior of stable ribbons as expected on the basis of the peptide design. These fluorescence data support directly and for the first time the presence of such ribbons which are characterized by a highly packed and stable hydrophobic interior. In common with Trp in many proteins, fluorescence lifetimes are nonexponential, but the average lifetime is shorter at low pH, possibly due to quenching with neighboring Phe residues. Unexpectedly, time-resolved fluorescence anisotropy does not change significantly with self-assembly when in water. In highly viscous sucrose-water mixtures, the anisotropy decay at low pH was largely unchanged compared to that in water, whereas at high pH, the anisotropy decay increased significantly. We concluded that depolarization at low pH was not due to rotational diffusion but mainly due to energy migration between adjacent tryptophan residues. This was supported by a master equation kinetic model of Trp-Trp energy migration, which showed that the simulated and experimental results are in good agreement, although on average only three Trp residues were visited before emission.     

Femtosecond depolarization dynamics of tris(8-hydroxyquinoline) aluminum films

For vapor-deposited tris(8-hydroxyquinoline) aluminum thin films, steady-state and subpicosecond transient optical anisotropy are investigated. It is found that the transient absorption anisotropy decays within tens of picoseconds. With a simple model calculation, the excited state population and anisotropy decay dynamics are disentangled, and the latter signal, the depolarization of the excited state, is explained by the energy transfer between the non-orthogonally-coordinated quinolate ligands. It is also shown that there are two pathways for this fast interligand energy transfer.     

Evidence for rigid binding of rhodamine 6G to silica surfaces in aqueous solution based on fluorescence anisotropy decay analysis

Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.     

Dendrimers with a Pentaphenylene Core: A Photophysical Study

Novel dendrimers G2PC and G4PC consisting of a p‐pentaphenylene core ( PC ) appended in the para position with two second‐generation ( G2 ) or two fourth‐generation ( G4 ) sulfonimide branches and two n‐octyl chains, as well as a model compound of the pentaphenylene core ( G0PC ), are prepared. The photophysical properties (absorption, emission, and excitation spectra; fluorescence decay lifetime; and fluorescence anisotropy spectra) of the three compounds are investigated under different experimental conditions (dichloromethane solution and solid state at 293 K, dichloromethane/methanol rigid matrix at 77 K). In the absorption spectra contributions from both the branches and the core can be clearly identified. The fluorescence spectra show only the characteristic fluorescence of the pentaphenylene unit with λ_max around 410 nm in fluid solution and 420 nm in the solid state. In solution the fluorescence quantum yields are 0.78, 0.76, and 0.72 for G0PC , G2PC , and G4PC , respectively, and the fluorescence lifetime is about 0.7 ns in all cases. Energy transfer from the chromophoric groups of the dendrimer branches to the core does not occur. The three compounds show the same, high steady‐state anisotropy value (0.35) in dilute rigid‐matrix solution at 77 K. In dichloromethane at 293 K, the increasing anisotropy values along the series G0PC (0.17), G2PC (0.27), and G4PC (0.32), with increasing molecular volume of the three compounds, show that depolarization takes place by molecular rotation. In the solid state the anisotropy is very low (0.015, 0.017, and 0.035 for G0PC , G2PC , and G4PC , respectively), probably because of fast depolarization via energy migration.     

Trapping time statistics and efficiency of transport of optical excitations in dendrimers

We theoretically study the trapping time distribution and the efficiency of the excitation energy transport in dendritic systems. Trapping of excitations, created at the periphery of the dendrimer, on a trap located at its core, is used as a probe of the efficiency of the energy transport across the dendrimer. The transport process is treated as incoherent hopping of excitations between nearest-neighbor dendrimer units and is described using a rate equation. We account for radiative and nonradiative decay of the excitations while diffusing across the dendrimer. We derive exact expressions for the Laplace transform of the trapping time distribution and the efficiency of trapping, and analyze those for various realizations of the energy bias, number of dendrimer generations, and relative rates for decay and hopping. We show that the essential parameter that governs the trapping efficiency is the product of the on-site excitation decay rate and the trapping time (mean first passage time) in the absence of decay.     

Observation of a free rotation transient in hot stilbene vapor

The fluorescence anisotropy decay of collision-free trans-stilbene vapor at 463 K has been measured with 5 ps time resolution using fluorescence up-conversion. The anisotropy measured with 302 nm excitation shows a pulsewidth-limited decay attributed to free rotation followed by a constant value at longer times. The long-time anisotropy of 0.069 is close to the theoretical regular rotor value of 0.074, indicating minimal vibration-rotation energy transfer on a time scale of ≈ 50 ps. Comparison with the longtime anisotropies previously observed using shorter excitation wavelengths indicates that the rotational motion becomes more nearly statistical with increasing excess vibrational energy.     

Observation of a free rotation transient in hot stilbene vapor

The fluorescence anisotropy decay of collision-free trans-stilbene vapor at 463 K has been measured with 5 ps time resolution using fluorescence up-conversion. The anisotropy measured with 302 nm excitation shows a pulsewidth-limited decay attributed to free rotation followed by a constant value at longer times. The long-time anisotropy of 0.069 is close to the theoretical regular rotor value of 0.074, indicating minimal vibration-rotation energy transfer on a time scale of ≈ 50 ps. Comparison with the longtime anisotropies previously observed using shorter excitation wavelengths indicates that the rotational motion becomes more nearly statistical with increasing excess vibrational energy.     

Triplet excitation transfer in glassy systems: spatial and spectral diffusion

Triplet excitation transfer among benzophenone molecules dissolved in glassy 2-methyltetrahydrofuran is studied by recording the emission and the optical depolarization as a function of wavelength and time. The transport mechanism is based upon exchange interaction and subject to the random character of both jump distances and site energies. Optical anisotropy data are used to gauge the probability of an excitation to remain on its original site. The anisotropy is observed to decrease by a factor of 2 from high to low energies within the inhomogeneously broadened emission band, clearly indicating hopping-mediated thermalization within the density of states. Within their excited-state lifetime the excitons do not reach the steady-state energies, but solvation allows the observation of that energy level. Unexpectedly, we find that the transfers at very short times do not contribute as much to spectral diffusion as the subsequent transport. Because the short-time hops target sites as close as approximately 1 nm, this observation suggests spatially correlated site energies for these short distances.     

Thermal switching of the optical anisotropy of a macroscopically aligned film of a discotic liquid crystal

We have studied the temperature dependence of anisotropy in the optical absorption and charge transport properties of an aligned film of hexakis-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) formed by zone-casting on a quartz substrate. At room temperature the film displays a large anisotropy in (photo)conductivity, as determined using the flash photolysis time-resolved microwave conductivity technique, with charge transport in the casting direction favoured by a factor of at least 10. The anisotropy in the optical absorption is however negligible. At the temperature corresponding to the transition from the crystalline solid to the liquid crystalline mesophase (c. 110°C), the optical anisotropy increases abruptly, with absorption of light polarized in the direction perpendicular to the alignment direction favoured by a factor of c. 3. On cooling, the dichroism reverts to its initial very low value with a hysteresis of c. 30°C. The results are explained in terms of a reversible change in the orientation of the molecules with respect to the axis of the aligned columnar stacks from tilted (at c. 45°) in the crystalline phase to close to orthogonal in the liquid crystalline phase.     

Vibrational energy transfer and anisotropy decay in liquid water: is the Förster model valid?

Ultrafast pump-probe anisotropy experiments have been performed on liquid H(2)O and D(2)O. In both cases, the anisotropy decay is extremely fast (on the order of 100 or 200 fs) and is presumed due to resonant vibrational energy transfer. The experiments have been interpreted in terms of the Fo?rster theory, wherein the rate constant for intermolecular hopping transport is proportional to the inverse sixth power of the distance between the vibrational chromophores. In particular, the anisotropy decay is assumed to be simply related to the survival probability as calculated with the Fo?rster theory. While the theory fits the data well, and is a reasonable model for these systems, there are several assumptions in the theory that might be suspect for water. Using our mixed quantum/classical model for vibrational spectroscopy and dynamics in liquid water, which agrees well with anisotropy decay experiments on the pure liquids as well as H(2)O/D(2)O mixtures, we critically analyze both the survival probability and anisotropy decay, in order to assess the applicability of the Fo?rster theory.     

Heterogeneity of water at the phospholipid membrane interface

Femtosecond infrared (IR) two-color pump-probe experiments were used to investigate the nonlinear response of the D2O stretching vibration in weakly hydrated dimyristoyl-phosphatidylcholine (DMPC) membrane fragments. The vibrational lifetime is comparable to or longer than that in bulk D2O and is frequency dependent, as it decreases with increasing probe frequency. Also, the lifetime increases when the water content of the sample is lowered. The measured lifetimes range between 903 and 390 fs. A long-lived spectral feature grows in following the excitation and is attributed to photoinduced D-bond breaking. The photoproduct spectrum differs from the steady state difference Fourier transform infrared (FTIR) spectrum, showing that the full thermalization of the excitation energy happens on a much longer time scale than the time interval considered (12 ps). Further evidence of the inhomogeneous character of the water residing in the polar region of the bilayer comes from the spectral anisotropy. The water molecules absorbing on the low frequency side of the absorption band show no decay at all of the anisotropy, while an ultrafast partial decay appears when the high frequency side of the spectrum is probed. The overall behavior differs remarkably from that observed with similar experiments in bulk water and in water segregated in inverse micelles. In weakly hydrated phospholipid membranes, water molecules are present mostly as isolated species, prevalently involved in strong, rigid, and persistent hydrogen bonds with the polar groups of the bilayer molecules. This specific character appears to have a direct effect on the structural stability and thermal properties of the membrane.     

Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units.     

Temperature effect on the fluorescence anisotropy decay dynamics of Coumarin-153 dye in Triton-X-100 and Brij-35 micellar solutions

The fluorescence anisotropy decay dynamics of the fluorescent probe Coumarin-153 (C153) have been investigated in two neutral micelles, Triton-X-100 (TX-100) and Brij-35 (BJ-35), at different temperatures and analyzed on the basis of the well-known two-step model. Because steady-state fluorescence spectra of the above probe do not show any noticeable changes with respect to temperature, for either of the studied micelles, suggests a similar polarity in the microenvironment around the probe at all the temperatures studied. The anisotropy results indicated that, for both the micelles, the fluidity inside the Palisade layer increases with temperature. However, the temperature effect on the anisotropy decay is relatively more pronounced in TX-100 than in BJ-35. It is inferred that the temperature effect on the anisotropy decay in the BJ-35 micelle is mainly due to the thermal effect on the microviscosity in the micellar phase. In the case of TX-100, the results indicate that, along with the above thermal effect, an additional effect is observed due to the increased size and hydration of the micelle with temperature, with the result being that the fluorescence anisotropy decay in TX-100 is more sensitive to temperature than in BJ-35. In the TX-100 micelle, our studies show that with an increase in temperature, even though the micellar size increases substantially, the distance of the probe from the micellar core does not increase that significantly. Thus, with increasing temperature, the probe undergoes a relative migration toward the micellar core to avoid the increased hydration in the micellar Palisade layer.     

Time-resolved emission of flavin adenine dinucleotide in water and water-methanol mixtures

Time-resolved fluorescence decay of flavin adenine dinucleotide (FAD) was studied at room temperature in water and water-methanol mixtures by a fluorescence upconversion technique. The observations were focused on the most initial decay phase (200 ps), before the residual fluorescence assumes a single exponential decay, typical for an extended conformation of the fluorophore. Within the first few picoseconds, where most of the electron transfer coupled quenching takes place, the emission decay curves could be fitted by a stretched exponent, compatible with the inhomogeneous distance dependent electron transfer model. This implies that the population of the excited FAD molecules exhibits a large number of non-identical states, each with its own separation between the donor (adenine) and acceptor (isoalloxazine) moieties, having its own rate of electron transfer. To evaluate the distribution of the separation between the donor-acceptor pair, we carried out molecular dynamics simulations of closed conformation of the FAD in water and water-methanol mixtures, sampling the structure at 10 fs intervals. The analysis of the dynamics reveals that within the 4 ps time frame, where most of the nonexponential fluorescence relaxation takes place, the relative motion of the donor-acceptor pair is consistent with a one-dimensional Brownian motion, where the diffusion coefficient and the shape of the confining potential well are solvent dependent. The presence of methanol enhances the diffusion constant and widens the width of the potential well. On the basis of these parameters, the relaxation dynamics was accurately reconstructed as an electron transfer reaction in an inhomogeneous system where the reactants are diffusing within the time frame of the observation.