Ultrashort Component Research Articles

Ultrafast FRET in Ionic Liquid-P123 Mixed Micelles: Region and Counterion Dependence

Ultrafast fluorescence resonance energy transfer (FRET) in a mixed micelle containing a room-temperature ionic liquid (RTIL) is studied by picosecond and femtosecond emission spectroscopy. The mixed micelle consists of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) (Pluronic P123), and a RTIL, 1-pentyl-3-methyl-imidazolium tetra-flouroborate, ([pmim][BF(4)]) or 1-pentyl-3-methyl-imidazolium bromide ([pmim][Br]). Coumarin 480 (C480) is used as the donor, and the acceptor is rhodamine 6G (R6G). Multiple time scales of FRET were detected-an ultrashort component of 1-3 ps and two relatively long components (300-400 ps and 2500-3500 ps). The different time scales are attributed to different donor-acceptor distances. It is proposed that the ionic acceptor (R6G) is localized in the polar corona region of the mixed micelle, while the neutral donor (C480) is distributed over both corona and hydrophobic core regions. The ultrafast (1-3 ps) components are assigned to FRET at a close contact of donor and acceptor. This occurs for the donor in the polar corona region in close proximity of the acceptor. The longer components (300-400 ps and 2500-3500 ps) arise from long-distance FRET from the donor at the core and the acceptor at the corona region. The relative contribution of the ultrafast component of FRET (∼3 ps) increases from 5% at λ(ex) = 375 nm to 30% at λ(ex) = 435 nm in the 0.3 M [pmim][BF(4)] mixed micelle and from 25 to 100% in the 0.9 M [pmim][BF(4)] mixed micelle. It is suggested that, at λ(ex) = 435 nm, mainly the donor molecules present at the corona are excited, causing ultrafast FRET due to a short donor-acceptor distance. At shorter λ(ex), the donor (C480) molecule at the core regions is excited, giving rise to a very long 3400 ps component (R(DA) ∼ 50 Å). Thus, λ(ex) variation leads to excellent spatial resolution. The counterion dependence (Br(-) vs BF(4)(-)) is attributed to the difference in the local polarity and size of the two mixed micelles.

Nonradiative Deexcitation Dynamics of 9H-Adenine: An OM2 Surface Hopping Study

The nonradiative relaxation of 9H-adenine was studied at the semiempirical OM2/MR-CI level using the surface-hopping approach. Geometry optimizations of energy minima and conical intersections as well as single-point calculations of excitation energies at critical points were performed to characterize the relevant potential energy surfaces of 9H-adenine and to assess the accuracy of the OM2 results. Surface-hopping calculations were performed to describe the nonradiative dynamics of 9H-adenine after vertical excitation into the optically active state. They showed that the deexcitation process is mainly governed by a two-step relaxation consisting of an ultrashort component and a longer component. These findings compare well with experimental results from time-resolved photoelectron spectroscopy.

Formation of an electron beam with a duration shorter than 100 fs during photoemission of electrons by femtosecond laser pulses

Irradiation of a thin metal target by 38-fs laser pulses at a wavelength of 800 nm is shown to generate a beam of photoelectrons that contains a component whose duration is shorter than 100 fs. The ensemble of photoelectrons is formed by photoemission of a gold film about 10 nm thick sputtered on the base of a prism made of fused silica. The laser beam irradiates a dielectric-metal interface and propagates inside the prism at an angle of 45° to a normal to the interface. The photoelectron beam is formed by accelerating photoelectrons in a spatially inhomogeneous electrostatic potential. The ultrashort component of the photoelectron beam is found to be formed under the action of a ponderomotive potential. It is shown that the ultrashort electron component can be separated from the remaining part of the photoelectron beam with the help of an inhomogeneous electrostatic field.

Femtosecond fluorescence upconversion study of rigid dendrimers containing peryleneimide chromophores at the rim

The kinetics of a newly synthesized series of a first generation of polyphenylene dendrimers in which one phenyl in a dendritic arm was para-substituted by a peryleneimide chromophore are reported. One such peryleneimide chromophore is attached to 1, 3 or 4 arms. The results are compared to a series of polyphenylene dendritic compounds, which are identical except for the substitution at a meta-position of a phenyl ring. The para-substitution yields a better spatial definition of the peryleneimide units relative to one another and the influence of this aspect on the kinetics is studied. Four different kinetic components were resolved for both groups of dendrimers. An ultra-short component varying from 500 fs to 2 ps and attributed to intramolecular vibrational redistribution (IVR) is identical for both series. The decay time of a second component, which is comprized of the vibrational relaxation and a singlet–singlet annihilation process observed in both substituted dendrimer series, is shorter in the para-substituted dendrimers compared to the meta-substituted ones. It is also shown that the annihilation process, which is only present in the multi-chromophoric compounds of both the series and resolved with an excitation energy dependent study, has definitely a larger contribution in the partial amplitudes for the para-substituted compounds. This is related to the relative orientation of the transition dipoles of the chromophores in both the series.

Primary processes and structure of the Photosystem II reaction center: II. Low-temperature picosecond fluorescence kinetics of a D 1-D 2-cyt- b-559 reaction center complex isolated by short Triton exposure

The fluorescence kinetics of a D 1-D 2-cyt- b-559 reaction center complex isolated by short Triton-exposure has been measured with picosecond resolution as a function of temperature below 150 K. The data were analyzed by combined global analysis of data measured with different time resolutions and are presented as decay-associated fluorescence spectra (DAS). Emphasis is given to the resolution and assignment of the fast components below 100 ps. Six lifetime components were generally necessary for a good fit of the data over a long time-range. The shortest lifetime of 13–24 ps (depending on temperature) is attributed to an energy transfer from (accessory) Chlorophyll to P680, presumably via pheophytin. A component in the range of 40–70 ps is attributed to energy transfer from pheophytin to P680. These components can only be properly resolved and assigned from measurements at temperatures below about 40 K. An ultrashort component of 1–6 ps, which had been resolved in our previous measurements (Biochim. Biophys. Acta (1991) 1060, 237–244) was not resolved in the particles studied here. We propose that the absence of the ultrashort component in this study is attributable to the larger chlorophyll content of the present short-term Triton-exposed D 1-D 2-cyt- b-559 preparation as compared to the previous long-term Triton-exposed RC particles. In the particles studied here the effective charge separation kinetics in the short time-range is rate-limited by relatively slow energy transfer components (lifetimes from 13 to 24 ps) upon preferential excitation of the accessory chlorophylls. In the long time-range a multicomponent charge recombination kinetics giving rise to four fluorescence lifetimes in the range of 1.56–49.7 ns are resolved. All these components show basically identical DAS with maxima near 682 nm.