Spectroscopy picosecond transient absorption

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The Mallouk study stands apart from all other studies of type 1 dyads in that the charge separated state (e.g., 18 in Scheme 9) is detected by picosecond transient absorption spectroscopy. Analysis of the back ET rate data for complexes 20 indicate that JtBET increases as AGBET becomes less negative, consistent with Marcus inverted region behavior (see Fig 2). A more detailed discussion of the back ET rate data is provided below. 

Not only has binding of imidazoles and pyridines to Fe protoporphyrin IX been studied, as discussed in Section 4.1.2, but also photodissociation of axial ligands such as pyridines, imidazoles, or piperidines from six-coordinate, low-spin Fe porphyrins, in which the porphyrin is derived from protoporphyrin IX, or proto- or deuteroporphyrin IX dimethyl ester, has been investigated in nonaqueous solvents using picosecond transient absorption spectroscopy (see Photochemistry of Transition Metal Complexes). It has been shown that photodissociation leads to the formation of five-coordinate complexes, that is, only one ligand appears to be released upon excitation of the six-coordinate complex. ... 

Enescu and Lindqvist [44] have investigated the deactivation pathways for the complexes of several purine nucleotides with MB+ by means of picosecond transient absorption spectroscopy. They observe rapid biexponential decay of complexes of singlet MB+ with GMP and XMP. The fast decay components (ti < 10 ps) were attributed to solvent relaxation and the slower decay components (tj 50 ps for GMP and 10 ps for XMP) to charge transfer (Eq. 14),... 

The primary intermediates formed from the benzophenone triplet state in the presence of tertiary amines such as dimethylaniline have been observed by picosecond transient absorption spectroscopy [99]. The initial step of the process involves one-electron transfer from amine (dimethylaniline, DMA) to the triplet state... 

In a picosecond transient absorption spectroscopy study of 6-nitro-BIPS derivatives, Krysanov and Alfimov8 identified a transient absorbing at 440 nm and proposed its formation within 8 ps from the excited singlet state SPNo2 of the... 

Fig. 8. Transient absorbance change at 700 nm (left panel), 810 nm (center) and 690 nm (right) in a PS-I particle induced by 1.5-ps, 610-nm pulses (after changes due to excited antenna chlorophylls had been subtracted). Figure source Wasielewski, Fenton and Govindjee (1987) The rate of formation of [PTOCT Aq ] in photosystem I particles from spinach as measured by picosecond transient absorption spectroscopy. Photosynthesis Res 12 184-186.

Fig. 11. (A) Decay-associated difference spectra (DADS) of the 3-, 28-ps and the non-decaying components of Synechocystis PS-I core complex in the 380-500 nm region under reducing conditions and at room temperature. (B) The absorbance-difference spectrum AA [Ao"-Ao] (solid line) the same spectrum [see Fig. 9 (A), left panel] obtained from spinach is included for comparison. Figure source Mi, Lin and Blankenship (1999) Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I. Biochemistry 38 15234. 15235.

MR Wasielewski, JM Fenton and Govindjee (1987) The rate of formation ofP700 -A in photosystem I par-tides from spinach as measured by picosecond transient absorption spectroscopy. Photosynthesis Res 12 181-190... 

D Mi, S Lin and RE Blankenship (1999) Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I. Biochemistry 38 15231-15237... 

Techniques and equipment used to obtain data in this type of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American Scientist . An apparatus for carrying out pump-probe broad band spectroscopy by transient absorption in the subpicosecond region has been described in detail. This equipment has been used to observe the photodissociadon of bis-(p-aminophenyl) disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers . Okamoto and Yoshihara ... 

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

Hashimoto H, Koyama Y, Ichimura K and Kobayashi T (1989) Time-resolved absorption spectroscopy of the triplet state produced from the all-trans, 1-cis, 9-cis, 13-cis, and 15-dv isomers of /3-caroteiie. Chem Phys Lett 162 517-522 Hashimoto H, Koyama Y, Hirata Y and MalagaN (1991) S and TI species of j3-car0tene generated by direct photoexcitation from the all-lrans, 9-cis, 13-ds, and 15-dv isomers as revealed by picosecond transient absorption and transient Raman spectroscopies. J Phys Chem 95 3072-3076 Hashimoto H, Miki Y, Kuki M, Shimamura T, Utsumi H and Koyama Y (1993) Isolation by high-pressure liquid chromatography of the cis-trans isomers of )3-apo-8 -carotenal. Determination of their So-state configurations by NMR spectroscopy and prediction of their S - and Ti-state configurations by transient Raman spectroscopy. J Am Chem Soc 115 9216-9225... 

Kleima FJ, Gradinaru CC, Calkoen F, Van Stokkum IHM, Van Grondelle Rand Van Amerongen H (1997) Energy transfer in LHCH monomers at 77K studied by sub-picosecond transient absorption spectroscopy. Biochemistry 36 15262-15268 Kleinherenbrink FAM, Hastings G, Wittmershaus BP and Blankenship RE (1994) Delayed fluorescence from Fe-S type photosynthetic reaction centers at low redox potential. Biochemistry 33 3096-3105... 

HashimotoH, KoyamaY, HirataYandMatagaN (1991)Si and TI species of -carotene generated by direct photoexcitation from the dA -trans, 9-cis, 13-cA and 15-cA isomers as revealed by picosecond transient absorption and transient Raman spectroscopies. J Phys Chem 95 3072-3076... 

Experimentally, the singlet states of free-base and zinc-porphyrins can be conveniently monitored not only by fluorescence spectroscopy but also by picosecond transient absorption (Fig. 24). By comparison with the absorp-tion/emission spectra of Fig. 22, it can be seen that the transient spectra consist of a broad featureless positive absorption throughout the visible region, with superimposed bleaching of the groimd-state Q-bands and additional apparent bleaching corresponding to stimulated fluorescent emission. As such. 

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes (stilbene-4,4 -dicarboxylic acid, bis(3-hydroxypropyl)amide of stilbene-4,4 -dicarboxylic acid, bis(2-hydroxyethyl)stilbene 4,4 -diether) (Figure 11.1) using femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements [11]. Nanosecond time-resolved spectra of stilbenes attached to DNA are shown in Figure 11.4. 

Linkage isomerism of DMSO in Ru(II) poly-pyridyl complexes has been used as a synthetic tool to access enantio-pure complexes such as [Ru(bpy)2(DMSO)Cl] [97]. The mechanism of the isomerisation and the nature of the excited states involved continue to be investigated by a variety of means, including picosecond transient absorption spectroscopy and modem crystallographic methods. 

The chemistry of the unsubstituted ortho-biphenyl system is complicated by the fact that the key intermediate of this reaction, 67a, undergoes two cyclization processes at competitive rates, and that azepine formation is reversible. To simplify the chemistry of singlet nitrene and allow straightforward study of the isocarbazole formation, 2-azido-3,5-dichlorobiphenyl (65b) was synthesized (Scheme 11.41) and its photochemistry was studied using nano- and picosecond transient absorption spectroscopy. 

Ujj, L, Devanathan, S., Meyer, T.E., Cusanovich, M.A., Tollin, G., and Atkinson, G.H., New photocycle intermediates in the photoactive yellow protein from Ectothiorhodospira halophila picosecond transient absorption spectroscopy, Biophys. J., 75,1, 406-412,1998. 

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