Transient absorption data for

Hypomycin B is unique in that it has only one intramolecular hydrogen bond as opposed to the two in hypocrellin A and the four in hypericin (Fig. 1.1). Picosecond transient absorption data for hypomycin B fail to reveal any stimulated... 

Figure 6 Transient absorption data for free 9AC and 9AC-Ti02 recorded at magic angle polarization. The pump wavelength was 390 nm and the probe laser wavelength was 780 nm for 9AC and either 780 nm or 650 nm for 9AC-Ti02. (From Ref. 201.)...

Figure 4. Fulse radiolysis transient absorption data for Fe-Pj2-Vy illustrating the prompt reaction of CO with the viologen moieties in the maquette. Note that there is a slow (ca. 40 decay of the viologen absorption, effectively putting...

Figure 5. Pulse radiolysis transient absorption data for Co-Piq-V, showing the decay at 600 nm due to oxidation of the viologen radical cation moiety and the bleach at 308 nm due to the reduction of the Co -tris-bipep species. The initial rapid increase at 600 nm is due to the reaction of CO2 with viologen.

Fig,l Transient absorption data for reaction centers from Rb,sphaeroides (a,b) and Rps. viridis (c,d). The filled circles represent the experimental data, the solid lines correspond to model calculations with time constants given in the text. The broken lines are calculated without the fast (0,9 ps or 0.65 ps, respectively) kinetic. The excitation wavelengths are 860 run and 955 run for Rb. sphaeroides and Rps. viridis, respectively. 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

No evidence was found from the picosecond absorption data for an excited state intermediate of the EDA complex. This formulation represente a confirmation of Mulliken s theory, in which CT band excitation of the quite nonpolar ground state produces an ion pair. Accordingly, indene and TCNE form ground state complexes which undergo fast electron transfer on irradition. However, back electron transfer occurs after relatively long time (ca. 500 ps) via a transient... 

Figure 1.10. Transient absorption kinetics for hypericin as a function of pump wavelength. In each panel, the first value corresponds to the pump wavelength the second, to the probe wavelength. The significant feature of these data is that they can be uniformly fit to two time constants, of which one corresponds to the characteristic H-atom-transfer time of 10 ps. The results of a global fit to these data are compiled in Table 1.1.

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

Figure 12-32. Concentration dependence of the hole mobility for BDAD in polysulfone obtained from transient absorption data. The abscissa scale shows the average distance between the BDAD molecules (Ref. [96]).

Pulse-probe transient absorption data on the rise time of prompt species such as the aqueous electron can be used to measure the instrument response of the system and deduce the electron pulse width. Figure 7 shows the rise time of aqueous electron absorbance measured with the LEAF system at 800 nm in a 5 mm pathlength cell. Differentiation of the absorbance rise results in a Gaussian response function of 7.8 ps FWHM. Correcting for pathlength, the electron pulse width is 7.0 ps in this example. 

Fig. 7 Transient absorption data showing the decay of the cation state of Ru(dcbpy>2(NCS)2 adsorbed on a nanocrystalline Ti02 electrode with an ethanol/D-l M tetrabutylammonium triilate electrolyte for different applied potentials 0, 100, 200, 300, and 400 mV (circles, right to left) [33]. Lines were calculated using the simulatirai procedure described in [32] assuming one catirai pCT nanoparticle. The parameters obtained fiorn fitting are a = 0.37, 0.4, 0.47, 0.58, 0.81 and quasi Fermi energy Ep as a ratio of k T of 25.1, 21.1, 17.9, 15.5, 13.1 (right to left). Reproduced with permission from [32] 2002 American Chemical Society...

The DIFFERENCE CROSS-SECTION SPECTRA are due to the absorption difference of the respective intermediates and the unexcited ground-state RC. Note that the spectra are calculated from transient absorption data according reaction model I. In Fig. 3 the features of the respective intermediates show up. For example the spectrum of I shows all features expected for an intermediate P B where one monomeric bacteriochlorophyll is reduced. The spectrum of I shows the properties of intermediate with the anion absorption band of the pheophythin H around 665 nm. 

In conclusion we have discussed several aspects of transient absorption spectroscopy related to the complicated case where several intermediate states with similar time constants occur and where the reaction model is not known in advance. We have shown for the primary electron transfer of the RC s from Rhodobacter sphaeroides that four time constants and four intermediate states are required to explain the transient absorption data. 

Fig. 4. Transient absorption data measured at 920 nm for the wildtype (top), FM210 (middle) and LM210 (bottom) RC. The solid curves are calculated using a rate equation system while the time constants are given in the text.

We have discussed here the relative merits of each of the three models. None of them can be definitely excluded at present on the basis of our data and those in the literature. In any case it is obvious that the electron transfer process in RCs of Rb. sphaeroides are more complex than assumed so far on the basis of transient absorption data alone. The 12 ps component has a substantial relative amplitude and can not be ignored. Most likely the 100 ps component must also be included in a model. The observation of these new components poses important new questions as to the nature of the primary processes in purple bacterial RCs. It will be necessary to include these components in a coherent mechanistic framework which is consistent with both fluorescence and transient absorption data. We suspect that these components will also be present in transient absorption data but have possibly not been resolved so far in most data sets in view of the generally smaller S/N ratio of transient absorption data. However, reports on heterogeneity" in the rates seen in transient absorption may be taken as evidence for the presence of one or both fluorescence components. 

Figure 2. Transient absorption data (points) for RC from Rps. viridis recorded in the Qx (left) and Qy (right) absorption band of the accessory bacteriochlorophyll. The solid curves are calculated for a four component (0.65 ps, 3.5 ps, 200 ps, co) the broken curve for a three component model (3.5 ps, 200 ps, co).

However, most transient absorption data also fit to the two models Bi and B2 where the subpicosecond reaction is assumed to precede the 3.5 ps process Here the intermediate I2 is formed very fast. It decays with 3.5 ps in a second step. Calculating the absorption spectrum of I2 for model Bi and B2 leads to the following characteristics I2 is similar to the electronically excited state P. It also exhibits gain thus it should be another excited electronic state of the special pair - we call it P. Its further absorption properties differ only slightly from those of P. The most straightforward interpretation of P would be that P is a vibra-tionally relaxed P state (Model Bi). Here the eleetron will be transferred directly in a super-... 

The observation of an additional time constant requires an extension of the reaction models Trivial is the assumption of a functional heterogeneity of the sample. In this case one would deal with two components having a different speed of the primary ET reaction. For a homogeneous sample one has to assume that the longer emission decay time is related to a new intermediate state (we call it N). The experimental observation of N in emission indicates that it is coupled directly to P. There are several possibilities to introduce the new state in a reaction model. We only want to discuss here the simplified situation where N is a not emitting state coupled only to P while model A applies for the further reaction. In this case one can calculate the reaction rates to and from N (via the emission experiment) and the spectral properties of intermediate N from previously measured transient absorption data. This evaluation yields The reaction from P to N is slow with a rate of 1/13 ps while the reactive rate from P" to F Ba" is four times faster. The back reaction from N to P is fast with a rate of 1/4.8 ps. The difference spectrum of state N shows spectral properties which are similar to those of P+Ba As a consequence one could speculate that N is the radical pair state F Bb" where the electron is transiently brought to the B branch. The further evaluation of the transient data shows that the spectra of the other intermediate states remain very similar to those obtained with the simplified reaction mcxiel A. 

---