Time-resolved polarized spectra

Figure 19.11. Time-resolved polarized spectra for the reorientation dynamics of a ferroelectric liquid crystal after reversal of the external electric field. (Reproduced from [25], by permission of the Society for Applied Spectroscopy copyright 1993.)...

Fig. 1 left FTIR spectrum (a) and 2D-IR spectrum (b) of the ground state for perpendicular polarization of pump and probe pulse. A broadband IR probe pulse measures the spectral change as a function of delay and frequency of a narrowband IR pump pulse (c). right time resolved absorption spectrum (d, magic angle polarization) and transient 2D-IR spectrum (e) recorded 20 ps after UV excitation. The T2D-IR spectrum was recorded with magic angle between UV-pump (500 fs, 5 iJ) and IR-pump polarizations and perpendicularly polarised IR-pump and probe beams, pulse sequence (f). 

Figure 4. a, CIDEP spectrum observed during the photolysis of a flowing GAV solution (flow rate at 5 mlVmin). b, The time-resolved polarization profiles monitored at selected magnetic fields 1, ketyl radicals and 2, phenacyl radicals. 

Until recently, the mechanism of the inhibition of light-induced yellowing was subject to speculation. However, solid state ESR and CIDEP have provided insight into the mode of inhibition by thiols (Wan, J.K.S., et al, J. Wood Chem. Technol., in press). Near-uv irradiation of unbleached and peroxide bleached thermomechanical pulp impregnated with thiols caused a rapid increase of the thiol radicals. The time resolved CIDEP spectrum, however, shows a symmetric broad band characteristic of the polarized phenoxy radical. This result suggests that thiols quench triplet generated phenoxy radicals in a secondary thermal process. 

Experimentally, these effects are tested by fluorescence and absorption measurements. These directly probe solvent polarization dynamics on molecular time-scales [100, 101]. For instance, the time resolved fluorescence spectrum of a chromophore, whose excited state dipole moment is subject to interactions with the surrounding solvent molecules, will exhibit fluorescence spectra that are strongly solvent dependent. The solvent molecules attempt to compensate the changes of charge density in the chromophore and, in sum, the fluorescence... 

To experimentally probe the CO trajectory after dissociation, ultrafast time-resolved polarized mid-IR spectra of photolyzed h-MbCO in G/W were recorded (34), the results of which are plotted in Fig. 8A. This study was performed in G/W primarily because the flatness of the solvent absorbance spectrum near 2100 cm-1 minimizes temporal distortion of the transmitted femtosecond IR probe pulse, thereby maximizing the effective time resolution of the measurement. Two features are already apparent at 0.2 ps, the earliest time shown, and these features rapidly develop into the docked states denoted Bi and B2. The development of the docked CO spectrum is further quantified by the time dependence of the polarization anisotropy, as defined in Equation (2). The B and B2 polarization anisotropies, plotted in Fig. 8B, evolve exponentially with time constants of 0.20 0.05 ps and 0.52 0.10 ps, respectively, and converge to the same anisotropy of approximately 0.2. According to Fig. 8C, ligand translocation is accompanied by a 1.6 0.3 ps growth of the integrated isotropic B-state absorbance. 

Time-resolved emission spectra Although there have been several attempts to simplify the characterisation of the SR process, the determination of time-resolved emission spectra (TRES) is certainly the most general and most precise way to quantitatively describe the solvent response. The time-resolved emission spectra are usually determined by spectral reconstruction [96, 97, 106]. The time-resolved emission spectrum at a given time t is calculated from the wavelength dependent time-resolved decays by relative normalization to the steady-state spectrum [107]. By fitting the TRES at different times t by the empirical log-normal function, the emission maximum frequencies i (t) (or 2(t) see Fig. 6.26) and the total Stokes-shift Ac (or A2) are usually derived [106]. Since c(t) contains both information about the polarity (Ac) and the viscosity of the reported environment, the spectral shift c(t) may be normalized to the total shift Ac. The resulting correlation functions C(t) (Eq. (7)) describe the time course of the solvent response and allow for comparison of the SR-kinetic and, thus, of relative micro-viscosities, reported from environments of different polarities [96, 97, 106, 108, 109, 116, 117, 122]... 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

The pronounced shoulder in the emission spectrum of PRODAN in the binary supercritical fluid composed of C02 and CH3OH indicates that at least two distinct species are emitting. That is, because PRODAN is the only fluorescent species in this system, emission must result from PRODAN in at least two different environments. Further, one environment must be more polar than the other(s). Figure 1 shows that a blue-shifted spectrum is indicative of a less polar environment. Thus, the shoulder on the blue edge of the emission spectrum of PRODAN in the C02/CH30H system is due to PRODAN in a nonpolar environment. The red edge is then due to PRODAN in a more polar environment. Therefore, at some point during the emission process, PRODAN emits from more than one environment. Time-resolved... 

The time-resolved emission spectra were reconstructed from the fluorescence decay kinetics at a series of emission wavelengths, and the steady-state emission spectrum as described in the Theory section (37). Figure 4 shows a typical set of time-resolved emission spectra for PRODAN in a binary supercritical fluid composed of CO2 and 1.57 mol% CH3OH (T = 45 °C P = 81.4 bar). Clearly, the emission spectrum red shifts following excitation indicating that the local solvent environment is becoming more polar during the excited-state lifetime. We attribute this red shift to the reorientation of cosolvent molecules about excited-state PRODAN. 

Fig. 1.14. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A Ti02 (0.3M) modified with dopamine (0.08 M).The lower trace was obtained with a 550 nm laser (laser intensity 10 mJ per pulse, 10 ns pulse duration, 20 scans), 1 ps after the laser pulse. Both spectra were recorded at 8 K. Right section shows how triplet radical pair mechanism of CIDEP in addition to fast exchange can contribute to the observed polarized spectrum.

The decay of the carotenoid radical cation absorption of C +-P-C6o occurs on the micro second time scale in the frozen glass. It is accompanied by the rise of C-P-Ceo generated by charge recombination of the C -P-Ceo biradical, which is formed with a quantum yield of 0.07. The major component of the decay of the - C-P-Ceo transient has a time constant of 10 ps, which is a typical lifetime for a carotenoid triplet state. The absorption of C -P-Ceo " at 77 K does not decay exponentially, but an average decay rate of 7.5 x 10 s may be calculated from the data [155]. Time-resolved experiments have allowed detection of the EPR resonances of the C +-P-C6o biradical and C-P-Ceo- The spin-polarization of the carotenoid triplet spectrum verifies formation of this state by the radical pair... 

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