Charge separated state studies

A similar supramolecular approach, in which both n-n stacking stacking of pyrene on the SWNT surface and alkyl ammonium-crown ether interactions were used in the self-assembly process of a fullerene derivative with SWNTs, was recently reported (Scheme 9.22).72 The nanohybrid integrity was probed with various spectroscopic techniques, , and electrochemical measurements. Nanosecond transient absorption studies confirmed electron transfer as the quenching mechanism of the singlet excited state of C60 in the nanohybrid resulting in the formation of SWNT"1"/ Pyr-NH3 + /crown- charge-separated state. 

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

Aida et al. synthesised a dendritic zinc-porphyrin heptamer (7Pz -C60) with a fullerene terminal entity as an electron-transfer system, which harvests visible light for electron transfer to the fullerene group (Section 6.3.3.5). Electron transfer from the porphyrin to the fullerene gives rise to a charge-separated state (Pzn-Qo)> whose lifetime was the subject of investigation. Comparative studies... 

The lifetimes of the charge separated state in degassed toluene for 1 through 4 are 5, 104, 150, and 300 ns, respectively, as determined by time-resolved transient absorption experiments [60-62], As Table 3 illustrates, this trend is preserved in the liquid crystalline environment, with the caveat that the ion pair lifetimes are increased by at least one order of magnitude. This variation allows for the study of photorefractivity as a function of the lifetime of the intramolecular charge separated state. 

A theory used to study and interpret the photo-induced electron transfer in solution was described by Marcus.19-25 In this theory, the electron transfer reaction can be treated by transition state theory where the reactant state is the excited donor and acceptor and the product state is the charge-separated state of the donor and acceptor (D+-A ), shown in Figure 15. 

The rate of energy and electron transfer in any of these systems is an ensemble average of all sites within the layered assembly, and thus there is no single rate for any of the processes in the assemblies. Nonetheless, in addition to the increased quantum yield, assemblies constructed with the HTiNbOs spacer exhibit a long-lived charge-separated state component not observed in the a-ZrP spaced assemblies, of x = 900 ps. While there is still much to be learned from the study of multi-chromophore arrays, this synthetic approach appears to hold much promise for the creation of organic-inorganic hybrid solid state devices. 

Quinone functionalities appear as components in organic switches, and the coupled redox chemistry of quinones with transition metals may provide the basis for an organotransition metal switch [164]. A system that may exhibit light-induced switching was studied in the example of the quionone-tethered form of Ru(bipy)3+ [242], but the charge-separated state that results from the Ru(II) —Q electron transfer is short-lived [164,242],... 

Transient absorption studies demonstrated that the yield of the final C+-P-Q T state for 10 is 0.30 in dichloromethane. This increase by a factor of 7 over that found for 4 must be due to a higher efficiency for step 4, as was hoped for. Indeed, the efficiency of this step has been increased from 0.04 in 4 to 0.73 in 10. Thus, fine-tuning of the energetics of the various electron transfer steps in the C-P-Q triads is indeed a powerful tool for controlling quantum yield without necessarily affecting the amount of energy stored in the final charge separated state (this is essentially the same for both 4 and 10). 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

A relative of 12 has been prepared by Sanders, van der Plas, and coworkers [64], Triad 13 features an N,A-dimethylaniline-type donor and an anthraquinone acceptor. These moieties are linked to the ortho positions of the porphyrin aryl groups, and this leads to a folded conformation for the molecule, as determined from NMR studies. Both the free base and zinc derivatives of 13 were prepared. The folded conformation might be expected to facilitate electron transfer among the components of the triad, and while this could enhance the quantum yield of the initial charge separated state, it might unfavorably affect the yield of the final D+-P-QT state and its lifetime. Unfortunately, photochemical or spectroscopic studies were not reported. 

The lifetimes of the final, presumably charge separated, states in 15 and 16 were both less than 100 ps, as determined from transient absorption studies [69]. These rather short lifetimes are likely due to the short, perhaps partially conjugated linkages between the three moieties which would be expected to facilitate charge recombination. 

An interesting variation on this theme has been reported by Lehn and coworkers. In 1986, they reported the synthesis of macrocycle 33, which consists of a zinc porphyrin bearing two linked cyclic binding subunits [87]. It was later found that addition of silver triflate to a solution of 33 in methanol resulted in the incorporation of a silver ion in each of the binding subunits [88], Thus, the complex may be represented as Ag+-P-Ag+. The porphyrin fluorescence of the silver complex was quenched, and transient absorption studies demonstrated that the porphyrin singlet state was quenched with a rate constant of 5.0 x 109 s 1 to yield a charge separated state Ag°-P+-Ag+. Some quenching of the porphyrin triplet... 

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