Picosecond lasers laser flash photolysis

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

The mechanisms proposed by both of these groups to explain the details of the chemistry of esters that give rise to short-lived nitrenium ions are not the only mechanisms that could fit these data. Further developments in this area will require application of picosecond spectroscopic methods to ion pairs generated by laser flash photolysis. 

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis.100 Excitation was carried out into the MLCT absorption band of the Ru11 -> 7t (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

Picosecond and nanosecond transient spectroscopic studies to elucidate the nature of various intermediates formed in the photoinduced electron-transfer reaction of amines have been most extensive for the ketone-amine systems. These studies have been described in detail in a recent review by Yoon et al. [10]. Some aspects of these studies are briefly described here. In the earlier studies of Cohen and coworkers, the photochemical reactions of benzophenone with aliphatic amines were probed by fluorescence quenching, determination of product quantum yields, and nanosecond laser-flash photolysis [143-147]. They proposed that the reactions of amines with... 

The nanosecond-picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D, state. We applied nanosecond-picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV-visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. 

FIGURE 4 - Schematic diagram of the optics and detection system employed in picosecond diffuse reflectance laser flash photolysis... 

Formation of iodine is detected tens of picoseconds after the irradiation of CH2ICH2I or CF2ICF2I in acetonitrile solution. In a continuation of their work on 1,1-dihalo compounds, Phillips and his group have investigated the photochemical behaviour of 1,1-dibromoacetic acid. Irradiation provides evidence for the formation of the iso-dibromoacetic acid (120). This is akin to the observations with 1,1-diodomethane. The authors propose that this species (120) could exhibit carbenoid character. Laser flash photolysis of 1,2-dibro-moethane at 248 nm in water provides a source of bromine atoms. ""... 

Kobayashi et al. [86] studied (4-dimethylamino)phenyl azide by means of laser flash photolysis on a picosecond time scale. They found that the triplet nitrene is formed from an unobserved precursor that has a lifetime of approximately 120ps. By consideration of model compounds, these workers suggested that the precursor is the singlet nitrene, but this conclusion must await confirmation by alternative experiments. However, in this work Kobayashi was able to show that triplet (4-dimethylamino)phenyl nitrene reacts to form the (4-dimethylamino)azobenzene by a kinetically second-order process. 

The homolysis product, Phl"+, has been observed directly by laser-flash photolysis probed with transient absorption spectroscopy on both the nanosecond [23,69] and picosecond [22] time scales. Klemm et al. [23] and Hacker and Dektar [70] suggest the possibility of electron transfer between the homolysis fragments leading to products identical with those of heterolysis. DeVoe et al. found no evidence for this process [22], Electron transfer... 

The properties of the excited states and the reduced and oxidized forms of bis[4-(dimethylamino)phenyl]squaraine (3), bis[(4-(dimethylamino-2-hydroxy-phenyl)]squaraine (21), and aza-crown ether squaraine derivatives 22-24 (Structure 6) have been investigated by picosecond and nanosecond laser flash photolysis [67,68], and the results are summarized in Table 1. 

Laser flash photolysis studies of the well characterised soil humic substance, Armadale Fulvic Acid (P.E.I., Canada), have been carried out, with excitation at 355 nm on picosecond and nanosecond time scales. Three principal transient absorption sijtnals have been observed in aqueous solution a component with a maximum absorption at 675 nm and a lifetime of about J microsecond (at pH = 7.0), a second component with a maximum absorption at <175 nm and a lifetime of I -10 microseconds, and a third component with a broad, featureless transient absorption spectrum and a lifetime in excess of LUO microseconds. The 675 nm signal is believed to be a solvated electron on the basis of lifetime and quenchinjt data, and is observed 20 ps after excitation. The 475 nm signal is believed to he a radical cation on the basis of its concurrent appearance with the electron at 20 ps. The third featureless component emerges nanoseconds after excitation and is believed to correspond to the triplet states of the humic material. 

In this work, we have carried out laser flash photolysis studies on a well characterized sample of fulvic acid (Armadale, P.B.I., Canada), for which the physicochemical data are well established (X> 8i ) Our studies have extended from picosecond to microsecond domains and include studies of pH effects, O2 and metal ion quenching. Our objective has been to link the photophysical events of the nanosecond time scale where transients decay, with those of the earlier scale, where they appear, and to study the evolution of the observed transient species over the entire tine scale. On this basis we hope to identify the multiple pathways of the photophysics of this complex natural mixture which can account for the diversity of consequent photochemistry. 

Moore, Gust, and coworkers synthesized the quinone-porphyrin-carotenoid (Figme 5) triad molecule. Upon excitation of the porphyrin moiety, initial charge separation occurred between porphyrin and quinone. Hole shift from porphyrin to carotenoid formed the final charge-separated state, that is, quinone radical anion and carotene radical cation, with a lifetime of 170 ns. These processes were confirmed by means of the picosecond and nanosecond laser flash photolysis. Their covalent bonding system was extended to tetrad and pentad using similar chromophores. 

The development of highly correlated methods of ab initio theory has provided us with insight into the properties of singlet aryl nitrenes. Time-resolved laser flash photolysis (LFP) with picosecond time-... 

The most important common features in the photodecarboxylation of ketoprofen, suprofen, tiaprofenic acid, tohnetin, and benoxaprofen in neutral aqueous solutions are the following (1) All of them produce carbanion intermediates, which upon protonation give rise to the decarboxylated photoproducts (Scheme 4), and (2) the photodecarboxylation quantum yields are much higher from the carboxylate forms than from the non-dissociated acids. However, there are also some differences. Thus, while photodecarboxylation of benoxaprofen is produced from its singlet excited state, in the case of the benzophenone derivatives (ketoprofen, suprofen, tiaprofenic acid, tohnetin), the process seems to take place from the triplet excited states.Clear evidence for the participation of these states has been provided by laser flash photolysis studies. Thus, picosecond laser excitation of ketoprofen in aqueous... 

TRUV-VIS studies have also provided evidence to support the role of conformation in the rearrangement reaction. For example, a series of simple diazoketones RCONjH, where R = H, Me, i-Pr, and t-Bu, has been examined by laser flash photolysis methods and also by picosecond transient grating spectroscopy. Within this series, as the steric bulk of R increases, the extent of the anti diazoketone conformer present at equilibrium decreases relative to the syn conformer. Consistent with a direct rearrangement pathway leading to ketene via only the syn conformer, the amount of trappable carbene intermediate decreases as R becomes more bulky. 

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