The photophysical properties of a series of laser-dye-labeled poly(aryl ether) dendrimers, generations 1-4, have been determined. The dendrimers act as extremely efficient light-harvesting antennae capable of transferring light energy through space from their periphery to their core. The light-harvesting ability of these molecules increases with generation due to an increase in the number of peripheral chromophores. The energytransfer efficiency was found to be quantitative for generations 1-3, with only a slight decrease observed for the fourth generation (∼93%). Due to the high extinction coefficients and fluorescence quantum yields of the chromophores and the efficient intramolecular energy transfer of the dendritic assemblies, these macromolecules have the potential to become integral components of molecular photonic devices.

An enzyme from Treponema denticola that hydrolyzes a synthetic trypsin substrate, N-a-benzoyl-L-argininep-nitroanilide (BAPNA), was purified to near homogeneity, as judged by gel electrophoresis. The molecular weight of the enzyme was estimated to be ca. 69,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and ca. 50,000 by gel filtration on Sephadex G-100. The pH optimum for the hydrolysis of BAPNA was around 8.5. The enzyme was heat labile and irreversibly inactivated at low pH values. Enzyme activity was enhanced by Ca2+, Mg2+, and Ba2+ but inhibited by Mn2+, Hg2+, Co2+, and Zn2+. Metal chelators and sulfhydryl reagents had no effect on this activity. The enzyme was inhibited by certain protease inhibitors such as diisopropyl fluorophosphate, N-a-p-tosyl-L-lysine chloromethyl ketone, phenylmethylsulfonyl fluoride, L-1-tosylamide-2-phenylethylchloromethyl ketone, a-1-antitrypsin, and soybean trypsin inhibitor. The Km values for BAPNA and N-x-benzoyl-L-arginine ethyl ester were 0.05 and 0.12 mM, respectively, and the V.max values were higher than those observed with trypsin. Although the purified enzyme hydrolyzed some low-molecular-weight synthetic trypsin substrates, it did not hydrolyze casein, hemoglobin, azocasein, azocoll, bovine serum albumin, or gelatin. Thus, this enzyme is probably not a protease but is capable of hydrolyzing ester, amide, and peptide bonds involving the carboxyl group of arginine and lysine.

Bacteriorhodopsin (bR) is an efficient light-driven proton pump which shows a trans-cis isomerization reaction of its retinal chromophore after light absorption. BR exhibits a large reorganization energy λ of 2520 cm-1 on optical excitation. In this paper, we have studied the nature, origin, and dynamical aspects of this extensive reorganization. We report the results of a femtosecond three-pulse echo peak shift (3PEPS), transient grating (TG) and transient absorption (TA) study, complemented with those of steady-state absorption and fluorescence spectroscopy in wild-type bR and the D85S mutant in its blue and purple, halide-pumping forms. We have simulated the results in the context of the multimode Brownian oscillator (MBO) formalism. A simple model that incorporates retinal's known intramolecular vibrations, which represent 1094 cm-1 or reorganization energy, and a single Gaussian protein relaxation with a decay of 50 fs representing 1430 cm-1 of reorganization energy, yielded satisfactory results for all linear and nonlinear experimental results on wild-type bR. For the D85S mutant in its blue form, the same model could be applied with a Gaussian relaxation of 1050 cm-1 amplitude. It is concluded that the protein environment of the retinal chromophore only exhibits an inertial response, and does not show any diffusive-type motions on a sub-ps to ps time scale, which is probably a consequence of the covalently constrained, polymeric nature of the protein. Our results are in close agreement with earlier molecular dynamics simulations on bR (Xu, D.; Martin, C. H.; Schulten, K. Biophys. J. 1996, 70, 453−460), which indicated that after retinal excitation, which is accompanied by a significant charge relocation along the polyene backbone, the protein exhibits an extensive dielectric relaxation on a 100 fs time scale representing an energy change of ∼1700 cm-1. We conclude that on the sub-ps to ps timescale, the protein's major influence is electrostatic via a large number of small-amplitude motions of charges and dipoles. Major structural rearrangements of the protein do not occur on the timescale of isomerization. Polarized transient absorption measurements on bR and the D85S mutant indicated a time-independent anisotropy of the stimulated emission of 0.35, indicating that in the excited state, no change of the direction of the transition dipole moment of retinal takes place during the excited-state lifetime.

The excited-state dynamics of the carotenoids (Car) in light-harvesting complex II (LHC II) of Chlamydomonas reinhardtii were studied by transient absorption measurements. The decay of the Car S 1 population ranges from ∼200 fs to over 7 ps, depending on the excitation and detection wavelengths. In contrast, a 200 fs Car S 1 fChlorophyll (Chl) energy transfer component was the dominant time constant for our earlier two-photon fluorescence up-conversion measurements (Walla, P. J.; et al. J. Phys. Chem. B 2000, 104, 4799-4806). We also present the two-photon excitation (TPE) spectra of lutein and β-carotene in solution and compare them with the TPE spectrum of LHC II. The TPE-spectrum of LHC II has an onset much further to the blue and a width that is narrower than expected from comparison to the S 1 fluorescence of lutein and β-carotene in solution. Different environments may affect the shape of the S 1 spectrum significantly. To explain the blue shift of the TPE spectrum and the difference in the time constants obtained from two-photon vs one-photon methods, we suggest that a major part of the Car S 1 fChl electronic energy transfer (EET) is due to efficient EET from hot vibronic states of the Cars. We also suggest that the subpicosecond kinetics has a very broad distribution of EET time scales due to EET from hot states. † Part of the special issue "Noboru Mataga Festschrift".

Articles you may be interested inAnalysis of cross peaks in two-dimensional electronic photon-echo spectroscopy for simple models with vibrations and dissipation J. Chem. Phys. 126, 074314 (2007); 10.1063/1.2435353Photoelectron spectroscopy of S 1 toluene: II. Intramolecular dynamics of selected vibrational levels in S 1 toluene studied by nanosecond and picosecond time-resolved photoelectron spectroscopies Site-specific vibrational dynamics of the CD3ζ membrane peptide using heterodyned two-dimensional infrared photon echo spectroscopyWe model recent experimental wavelength dependent Three Pulse Photon Echo Peak Shift ͑WD-3PEPS͒ and Transient Grating ͑WD-TG͒ signals considering both solvation dynamics and vibrational contributions. We present numerical simulations of WD-3PEPS and WD-TG signals of two probe molecules: Nile Blue and N,N-bisdimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide to investigate the influence of intramolecular vibrations in the signals. By varying the excitation wavelength, we show that the different initial conditions for the vibrational wave packets significantly affect the signals, especially through the contributions associated with high frequency modes, often neglected in experimental analyses. We show that the temporal properties of both WD-TG and WD-3PEPS signals display sensitivities to both the excitation wavelength and the vibronic structure of the specific probe molecule used. Several mechanisms for generating vibronic modulations in the signals are discussed and their effects on the signals are described. Quantitative agreement between experiment and simulated signals requires accurate characterization of the laser pulses, specifically the magnitude and sign of chirp has a significant effect on the initial temporal properties of the signals. We provide a description of the experimental considerations required for accurate determination of molecular dynamics from 3PEPS and TG experiments and conclude with a brief discussion of the implications of our results for previous analyses of such experiments.

This is the first in a two-paper series that investigates the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and Three Pulse Photon Echo Peak Shift (3PEPS) signals were collected from several probe molecules: Nile Blue, N,N-bis-dimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide, and Rhodamine 6G dissolved in different solvents: benzene, dimethylsulfoxide, and acetonitrile. The effects of excitation of different vibronic transitions on the electronically resonant signals were identified by comparing signals collected with laser pulses at different excitation wavelengths. In the 3PEPS profiles, we find that excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of the different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. In paper II we demonstrate that detailed account of the vibronic nature of the chromophore is required to describe the wavelength dependent signals.

We report a study of the exciton dynamics in 1,1Ј-diethyl-3,3Ј-bis͑sulforpropyl͒-5,5Ј,6,6Ј-tetrachlorobenzimidacarbocyanine ͑BIC͒ J-aggregates in water solution at room temperature by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. The temporal profiles of the transient grating signals depend strongly on the excitation intensity as a result of exciton-exciton annihilation. On the other hand, the peak shift measurement gives information on the fluctuations of the transition frequency of the system. The peak shift decays with time constants of 26 and 128 fs. There is no finite peak shift on a longer time scale. The electronic state of J-aggregates is described by a Frenkel exciton Hamiltonian, and the exciton population relaxation processes is described by Redfield equations. Based on the numerical simulations, the peak shift data can only be explained even qualitatively when both exchange narrowing and exciton relaxation process are included in the model. The 128-fs component is assigned to a ''hopping'' time between exciton units. We confirmed that while the static disorder within an exction state that is partially delocalized due to static disorder is exchange-narrowed, the exchange narrowing of the dynamical disorder is not complete but appears as lifetime broadening, which competes with the exchange narrowing of the fluctuations. The effect of the exciton relaxation on the absorption spectrum is discussed.

Three pulse stimulated photon echo peak shift ͑3PEPS͒ measurements were used to probe the solvation of a quadrupolar solute in three room temperature nondipolar solvents; benzene, CCl 4 , and CS 2 , and the results were compared with those for two polar solvents, methanol and acetonitrile, and one weakly polar solvent, toluene. Our data reveal three distinct solvent dynamical time scales; a sub-100 fs ultrafast component attributed to inertial motions, a slow ͑ϳ2-3 ps͒ component attributed to structural relaxation, and an intermediate time scale ͑ϳ600 fs͒ of uncertain origin. The six solvents were chosen to reflect a range of possible interactions, but exhibit similar dynamics, suggesting that similar mechanisms may be at work or that different mechanisms may exist, but occur on similar time scales. A viscoelastic continuum solvation model proposed to describe nonpolar solvation ͓J. Phys. Chem. A 102, 17 ͑1998͔͒ was used for a preliminary analysis of our data.