Transient absorption spectroscopy detection

Optical Spectroscopy General principles and overview, 246, 13 absorption and circular dichroism spectroscopy of nucleic acid duplexes and triplexes, 246, 19 circular dichroism, 246, 34 bioinorganic spectroscopy, 246, 71 magnetic circular dichroism, 246, 110 low-temperature spectroscopy, 246, 131 rapid-scanning ultraviolet/visible spectroscopy applied in stopped-flow studies, 246, 168 transient absorption spectroscopy in the study of processes and dynamics in biology, 246, 201 hole burning spectroscopy and physics of proteins, 246, 226 ultraviolet/visible spectroelectrochemistry of redox proteins, 246, 701 diode array detection in liquid chromatography, 246, 749. 

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. 

The primary steps of the photolysis of aqueous monuron and diuron were investigated by Canle et al. by means of transient absorption spectroscopy using an ArF laser (X = 193 nm) for excitation [89]. Under these conditions, photoionization occurred with a quantum yield of about 10%. Radical cations were detected after the laser pulse and found to deprotonate to yield neutral radicals [89]. 

The absorption spectrum of 22 is nearly identical to the sum of the spectra of unlinked model compounds. The long wavelength band of the free base porphyrin is observed at 650nm in chloroform, whereas that of the zinc porphyrin is at 590nm. Excitation of a chloroform solution of 22 with a 15 ns pulse of 650 nm laser light leads to the formation of a carotenoid radical cation which can be detected by transient absorption spectroscopy (Figure 14). This ion arises from the charge separated state C -Pzh-P-Qa-Qb, and is formed with a quantum yield of 0.83. The lifetime of the species is 55/iS. 

Photolysis of 5-chloro-2-hydroxybenzonitrile (70) in aqueous solution gives the triplet carbene (71), which can be detected by transient absorption spectroscopy (X,max at 368 and 385 nm). The carbene was recognized by its reactions, e.g. with O2, to produce the corresponding carbonyl O-oxide (A ax at 470 nm), and with propan-2-ol to give 2-cyanophenoxyl radical. In deoxygenated solutions the main stable products are 2,5-dihydroxybenzonitriIe and two substituted biphenyls. 

Here, we briefly describe the laser ablation dynamics of phthalocyanine film where the primary species was confirmed, by femtosecond transient absorption spectroscopy [25], to be its electronically excited state. The exdton absorption band was replaced in 20 ps by a hot band of the ground electronic state. This rapid decay was ascribed to mutual interactions between densely formed excitons leading to sudden temperature elevation. The elevated temperature was estimated by comparing transient absorption spectra with the temperature difference ones. It is worth noting that quite normal dynamics of the excited states are detected, even under ablation conditions, which is the reason why we do not accept the plasma mechanism. Then we considered how to correlate these electronic processes of phthalocyanine films with their fragmentation. 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

The photodissociation of bromobenzene in solution has been investigated with ultrafast transient absorption spectroscopy, following excitation at 266 nm. The main kinetic feature in acetonitrile was a 9 ps decay that was assigned to predissociation similar decays were observed in hexane, dichloromethane and tetrachloromethane. Laser-aligned iodobenzene have been photodissociated into phenyl radicals and iodine atoms with a 1.5 ps laser pulse at 266 nm, and the yield of iodine photoproducts detected by resonant multiphoton ionization." Significant yield enhancements were observed when the dissociation laser was polarized parallel instead of perpendicular to the alignment laser polarization. 

The triplet-excited states of azides 31 and 32 were detected chemically and by transient absorption spectroscopy It was concluded that nitrogen extrusion, after near-UV irradiation of the azides, occurs exclusively from the excited triplet states of the azides. However, products formed under these conditions are consistent with reactions originating from the singlet state of the aroylnitrenes. 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

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