Spectra, Raman time-resolved resonance

Time-resolved resonance Raman (TR ) spectroscopy experiments were first reported in 1976 and used a 30 ns pulse radiolytic source to generate the intermediates that were then probed on the microsecond time-scale by a laser source to generate the TR spectrum. TR spectroscopy was then extended to study intermediates... 

Figure 3.30. (a) UV-vis absorption spectra of the HPAA product (solid line) and the HPDP substrate (dash line) in a H20/MeCN (1 1) mixed solvent, (b) Picosecond time-resolved resonance Raman (ps-TR ) spectra of HPDP obtained with a 267 nm pump and 200 nm prohe wavelengths in a HjO/MeCN (1 1) mixed solvent. Resonance Raman spectrum of an authentic sample of HPAA recorded with 200 nm excitation is displayed at the top. (Reprinted with permission from reference [49]. Copyright (2006) American Chemical Society.)... 

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

Figure 3. Time-resolved resonance Raman spectrum of the phenoxyl radical observed 1 ls after pulse irradiation of a 2 mM phenolate solution (N20 saturated) at pH 11 (excitation at399nm). [Adapted from (18b).]...

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... 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

The use of picosecond pulses to minimize the Interference of fluorescence with the Raman spectrum was also demonstrated (5) at about that time. The use of vldicon detection in Raman spectroscopy was demonstrated (6) in 1976. The first resonance Raman spectrum taken for a photobiologlcal system (bacteriorhodopsin) in the nanosecond time scale was (7) in 1977. The resonance Raman spectra of bacteriorhodopsin have also been measured in the microsecond (8,9,10) and in the millisecond (11) time domain. Recently the time resolved resonance Raman spectra of photolyzed hemoglobin derivatives have been reported (12). 

Figure 3-26 Resonance Raman (RR) and time-resolved resonance Raman (TR3) spectra of [Ru(bpy)3]2+. (a) RR spectrum using 350.7nm excitation (Ar+ laser) (b) TR3 spectrum using 354.7 nm excitation (c) RR spectrum of Li(bpy) using 350.7 nm excitation (Ar+ laser). (Reproduced with permission from Ref. 84. Copyright 1990 American Chemical Society.)...

The time-resolved resonance Raman spectrum of the colored species produced during a 30-ns laser pulse indicated that a short-lived intermediate, almost certainly the cis form, is a precursor to the stable trans form of the merocyanine.163... 

FIGURE 3. Time-resolved resonance Raman difference spectra in Fe J2 d stretching frequency region of cytochrome c oxidase 0.1 msec after the initiation of the reaction of fully reduced enzyme with Q. Observed spectra and calculated spectra are given in left side and right side, respectively. (a) 02— Oj (b) 0 0—" O, (c) 02— 0" 0 (d) < 0 0—(" O, + 02)/2. (e)Fe— 02(1), Ev—" 0" 0(2), Fc— O OCS), and Fc— 02(4) stretching Raman bands assumed in the simulation. In the calculation for thei 0 O spectrum, (Spectrum(2) + Spectrum(3))/2 was used. The difference between the observed and calculated spectra are depicted along the same ordinate scale as that of the observed spectra under each individual calculated spectra. 

Figure 5. Resonance Raman spectra (1400-1700 cm ) of transient intermediates produced in the pulse radiolysis of a 2 mM aqueous solution of p-methoxyphenol (N20 saturated pH 11). Key a-d, the spectra obtained at different times (At) after the electron pulse d, the spectrum of p-benzosemiquinone anion (marked A) and e, the spectrum of tp-methoxyphenoxy radical (marked B) obtained after subtracting dfrom a. This figure illustrates the use of time-resolved resonance Raman spectroscopy to time-resolve the spectral overlap between transients decaying at different time scales.

Transient Resonance Raman and Time-Resolved Resonance Raman Studies. Before describing the first experiment it is useful to point out that most of the TR data obtained are presented as difference spectra. The reason for this is that the system is quite complex, both structurally and from the standpoint of the large number of modes which are RR active (the enzyme complex contains two heme a groups, and in some experiments the redox partner, cytochrome c, is also present in solution). Inasmuch as the TR studies are focused on the detection and characterization of the O2 reduction products, it is most efficient to obtain spectra during the reaction with 02 and also during the reaction with 02 and to generate the difference spectrum by subtraction. In this way all other features will cancel and even very weak spectral features associated with the bound O2 and its reduction products can be more readily observed. 

The different schemes above can also be distinguished by using TRRR techniques. At the moment this technique might take more effort than the optical methods. However, it can be done with more accuracy since vibrational Raman bands are better resolved than optical absorption bands. A detailed study of the observed change of the resonance Raman spectrum with time and with probe laser frequency should, in principle, enable one to distinguish between the different schemes given above. This will be possible if the photoproducts in a certain scheme are produced with different rates or have different optical absorption maxima (and thus different resonance Raman enhancement profiles). 

Brouwer and Wilbrandt have applied resonance Raman spectroscopy and calculations to questions of structure of amine radical cations [73]. Well-resolved Raman spectra of trialkylamine radical cations that are so short-lived that their electrochemical oxidation waves are irreversible may be obtained at room temperature in solution by photoionization and time-resolved detection. Comparison of the observed spectrum with calculations for various isomers provides a powerful method of answering structural questions. Density-functional calculations prove much easier to apply to open-shell species than Hartree-Fock calculations, which require cumbersome and expensive corrections to introduce suffieient electron correlation to eonsider questions like the charge distribution of disubstituted piperazine (1,4-diazacyclohexane) radical cations. The dimethyl- and diphenyl-substituted piperazine radical cations are delocalized, but charge is localized on one ArN unit of the dianisyl-substituted compound [73dj. 

When the vibronic structure in the electronic emission or absorption spectra are not well enough resolved to analyze as above, Raman data must be used in combination with the electronic spectra. The most efficient method of obtaining the distortions will be to use a pre-resonance Raman spectrum in conjunction with the electronic spectrum. (In the pre-resonance Raman experiment the exciting light is detuned just off resonance.) The pre-resonance Raman spectrum corresponds to short time dynamics and a big damping factor (Section III.F.l.c). In the short time limit, the intensities in the Raman spectrum are related to the displacements by Eq. (12). In the short time limit, the absorption spectrum becomes [42]... 

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