Laser-pulsed photolysis, transient absorption

Figure 2.19. (a) Transient absorption spectra observed at 0.5 (broken line a) and 20 (dotted line b) ns after the second laser irradiation during two-color two-laser photolysis (266 and 532 nm) and the spectrum observed during one-laser photolysis (266 nm, solid line c) of BP (1.0 x 10 4hl) in Ar-saturated cyclohexane. The second laser irradiation was at 1 ps after the first laser pulse. The transient absorption spectrum of BPH Dj) (b) was given by subtracting spectrum c from spectrum a. The blank around 532 nm in the spectra is due to the residual SHG of the Nd3+ YAG laser. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Laser Flash Photolysis at 248 nm of TDI-PU. MDI-PUE. and Model Compounds. Figures 1 and 2 show the transient absorption spectra of MDI-PUE (5.5 X lO-3 g/dL) and TDI-PU (2.3 X 10 3 g/dL) in THF at a 2.0 ns delay after pulsing with a krypton fluoride excimer laser (Xex=248 nm) in air and nitrogen saturated samples. Both spectra have common peaks in nitrogen saturated solutions (shown by arrows) at 310 nm, 330-360 nm (broad), and above 400 nm (broad, diffuse absorbance).. The MDI-PUE sample has an additional and quite distinctive peak at 370 nm. In the presence of air, the peak at 370 nm for MDI-PUE is completely extinguished, while the sharp peaks at 310 nm for TDI-PU and MDI-PUE and the broad band above 400 nm are only marginally quenched by oxygen. 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

Laser flash photolysis (266 nm) of phenyl azide in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 11.4). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of 33s was reinvestigated and an additional very weak, long wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Figure 11.4 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( 2). 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

Fig. 16 The temperature dependencies of the rate constant of decay of singlet 2-fluorophenyInitrene 15a (1) and the apparent rate constant of formation of triplet 2-fluorophenylnitrene (20a) and ketenimine (18a) (2). Solid lines (1) and (2) results of non-linear global fit of the data to analytical solutions. Insert transient absorption spectra produced by LFP at 295 K (1) of 2-fluorophenyl azide 15a in pentane, detected 500 ns after the laser pulse (2) 4-fluorophenyl azide 15b detected 50 ns after the laser pulse and (3) persistent spectrum detected after 20 s of photolysis of 2-fluorophenyl azide 15a in methylcyclohexane at 77 K.

The double-beam transient absorption spectrometer utilized in this work is described in detail elsewhere [3]. Briefly, the output from a 1 kHz Ti Sapphire laser is frequency quadrupled to generate the 200 nm photolysis pulses. The probe pulses are generated by frequency doubling the output of an optical parametric amplifier (OPA) pumped at 400 nm or by sum-frequency mixing of the OPA output with 400 nm and 800 nm pulses. The sample consisted of a 0.1 mm jet of aqueous KNO3 solution. The acidity of the solutions was adjusted by addition of HN03(aq). 

Figure 9. The transient absorption spectra observed in laser flash photolysis of AcrH+ (5 x 10 5 mol dm-3) in deaerated MeCN containing diphenylmethane (5 x 10"2 mol dm 3). Spectra were recorded in 1 /is (O), 7 /is ( ), and 17 /is (A) after the laser pulse [94],...

FIGURE 6.8 Typical side-on (top) and front-face (bottom) optical arrangements for laser flash photolysis to detect transient changes in the absorption spectrum. Abbreviations S = light source (probe) L = lens C = cell holder + cell (typical path lengths 1-0.5 cm (top) and 0.5-0.2cm (bottom)) M = mirror Mo — monochromator P = photomultiplier. The most commonly used lasers deliver powers equal to or larger than 0.5 MW per pulse. 

Nanosecond laser Flash Photolysis experiments were performed with 355 and 532 nm laser pulses from a Brilland-Quantel Nd YAG system (5 ns pulse width) in a front face (VIS) and side face (NIR) geometry using a pulsed 450 W XBO lamp as white light source. Similarly to the femtosecond transient absorption setup, a two beam arrangement was used. However, the pump and probe pulses were generated separately, namely the pump pulse stemming from the Nd YAG laser and the probe from the XBO lamp. A schematic representation of the setup is given below in Fig. 7.3. 0.5 cm quartz cuvettes were used for all measurements. 

As an example, scheme i) gives a new transient Raman spectrum in which all the observed vibrational bands have the same rise time and the same enhancement profile. In scheme ii) all the new bands should have the same rise time but the relative band intensity of the new spectrum should change upon changing the probe laser frequency (if B and C have different optical absorption profiles.). Scheme iii) predicts changes in the relative intensity of the new bands with both the laser probe frequency as well as the time of delay between the photolysis and probe laser pulses. The difference between scheme iii) and iv) is that in iii) the bands of C and D could have different rise and decay times while in iv) they all should have similar rise times. Schemes iii) and v) are similar except that A in iii) disappears permanently upon laser exposure while in v) A regains its concentration and no permanent photochemical damage takes place. In scheme vi) the rise time of the vibrational bands of the (AB) transient (an excimer or an exciplex) should depend on the concentration of B. 

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]. 

In fact, the addition of 1,4-dimethoxybenzene (DMB) (0.13 equiv.), easier to oxidize than 21, appreciably quenches the reaction. The laser flash photolysis with a nitrogen-pulsed laser (337 nm) of a TPP+BF4 (1.6 x 10-4 M) dichloromethane solution, in the presence of 21 (6.2 x 10 2 M), conditions in which nearly 80% of the singlet excited sensitizer (Ered = 0.29 V vs SCE) is quenched by 21, gives a transient absorption (A 550 nm) assigned to a pyranil radical TPF. 

The author s group [12] tried to find saturation behavior of the MFEs due to the AgM in fluid solutions with our pulsed magnet and found that the MFEs on the escape radical yield (1e(B)) observed for the photoreduction of 4-methoxybenzophenone with thiophenol (Reaction S-5 in Table 7-2) were almost saturated by the fields of -30 T. The isotropic g-values of the thiyl and ketyl radicals have been determined to 2.0082 and 2.0027 so Ag=0.0055 [12]. From ns-laser photolysis measurements with our electromagnet, superconducting magnet, and pulsed magnet, we observed the time profiles of the transient absorption (A(f) curves) of the ketyl radical and obtained the MFEs (A(B)=Ye(B)/1e(0 T)) on the )4eld. The R(B) values obtained at room temperature in 2-methyl-1-propanol are plotted... 

Reactions of OH, 0 and SO " radicals with cresols were studied in detail by pulse radiolysis and laser flash photolysis techniques combined with product analysis.The rate constants of the OH reaction with cresols are very high (fe 1 x 10 ° dm mol s ) whereas 0 was found to be less reactive k 2.4 x 10 dm mol s ). The second-order rate constants of the reaction for SO " reaction with cresols are in the range of (3 to 6) x 10 dm mol s" The transient absorption spectra obtained in the reaction of OH with isomers of cresols have peaks in the region 295-325 nm. Merga et al and Choure et carried out a detailed product analysis by HPLC on radiolysis of chlorotoluenes and cresols. Table 2 lists the products obtained in deoxygenated and oxygenated solutions of these systems. 

Figure 2.6. (a) Transient absorption spectra recorded before the laser flash (open circles) and immediately after (filled circles) and 200 ns (open triangles) and 1 ps (filled triangles) after the laser flash during pulse radiolysis-laser flash photolysis of c-St (5 x 10-3 M) in Ar-saturated 1,2-dichloroethane. (b, c) Kinetic traces of AO.D.480 and AO.D.515, respectively, as a function of time after the electron pulse. 

Figure 2.7. Transient absorption spectra observed at Ons, 100 ns, and 1 ps after the 8-ns electron pulse, and transient fluorescence spectrum of TMB + observed at 300 ns after the electron pulse during pulse radiolysis-laser flash photolysis of TMB (lx 10-2M) in Ar-saturated 1,2-dichloroethane. Excitation wavelength, 532 nm. Laser pulse energy, 140 mJ pulse-1.

Figure 2.18. (a) Transient absorption spectra observed at 40 ps before and 20 and 150 ps after the laser flash during two-color two-laser flash photolysis of 4T in toluene employing a nanosecond YAG laser (355 nm, FWHM 5 ns, 7 mJ pulse-1) and a picosecond YAG laser (532 nm, FWHM 30 ps, 21 mJ pulse-1), (b) Difference spectra of transient absorption spectra at 20 and 150 ps. (c) Kinetic traces of AO.D. at 650 and 600 nm. Thick lines are fitted curves. 

Figure 2.23. Formation and decay of NDI - and the effect of the delay time between two laser pulses on the consumption of G during the laser flash photolysis of NDI-ODN (NDI-TTTCGCGCTT/AAAGCGCGAA).The transient absorption of NDI - was monitored at 495 nm following the 355-nm excitation (left axis). The consumption of G is plotted as a function of the delay of the 532-nm pulse with respect to the 355-nm pulse ( , right axis). The dashed line shows the consumption of G in the absence of the 532-nm pulse.

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. 

---