Laser-pulsed photolysis, transient absorption profiles

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... 

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 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... 

Porphyrin cation radical species as intemiediates in ET can be directly detected by laser-flash photolysis or ESR spectroscopy. In particular, transient absorption spectra of porphyrin-acceptor assembly systems excited by laser pulses often show the characteristic broad band assigned to porphyrin cation radical species at 500-800nm," so it is possible to determine the forward and/or back ET rate constants from the formation and decay profiles of the intermediate absorption. In noncovalent ET model systems, sufficient affinity between the porphyrin and the acceptor is required to obtain an appropriate inten,sity of transient absorption derived from porphyrin cation radical species. However, it is difficult to monitor the intermediate when charge recombination processes occur faster than the charge-separation process. 

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