Charge-separated states, fluorescence

Excitation of the porphyrin moiety of 2 in dichloromethane solution yields the first excited singlet state, which can decay according to the pathways detailed in Figure 4. As with P-Q dyad 1, photoinitiated electron transfer competes with other decay processes to yield a C-P -Q charge-separated state. Fluorescence decay studies yielded a fluorescence lifetime r of 0.10 ns for 2 [27]. The hydroquinone form of the triad, 3, in which such electron transfer is not possible, has a fluorescence lifetime of 3.4 ns in the same solvent (see Section 111.A.). Application of Eq. (1) yields an electron transfer rate constant fcj in Figure 4 of 9.7 x 10 s", and consequently a quantum yield for this step of essentially unity. Thus, the addition of the carotenoid moiety to the molecule has had little influence upon the initial photodriven electron transfer step. 

After the TICT minimum is reached, the transition moment between charge-separated state and the ground state represented by A-B and AB, respectively, is expected to be fairly small owing to almost no overlap between part A and B. Therefore, the fluorescence intensity will be small and significant contributions most probably stem from neighboring geometries (6 90°) for which the emission from admixed locally excited states can occur. The return from Sj or Tj minimum to S0, which can proceed in radiative or radiationless manner, usually does not lead to formation of cis-trans isomers as one would expect from the assumed energy surfaces. This is due most probably to rapid thermal cis-trans interconversion in the S() state. So far in this Section electronic properties of the free molecules have been addressed. 

In summary, if the good combination of donor-acceptor subunits can be found so that the charge-separated state does not lie substantially higher than the locally excited states and if the polar solvent is adequately chosen to lower the energy of the charge-separated state, TICT state will be responsible for the anomalous fluorescence emission. 

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. 

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

This exciplex has a large amount of charge transfer character, as shown by the solvent dependence of its fluorescence emission spectrum. The exciplex can then receive an electron from the secondary donor to form the final charge separated state ... 

Triad 25 is another example of this general type [75]. As was the case with the previously discussed triads 15—18, the absorption spectrum of 25 indicates some degree of excitonic interaction between the porphyrins. The fluorescence quantum yield of 25 is 5 5 x 10-6, which indicates efficient quenching of the porphyrin singlet states, presumably by electron transfer. No information concerning the lifetime of any charge separated state was presented, but one would predict that it would be extremely short. 

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... 

Fig. 1.34. Excited state energy levels. The singlet (Si) energy levels of OPVn and MP-C60 (solid bars) were determined from fluorescence data. The MP-C6o(Ti) level (solid bar and dashed line) was taken from phosphorescence data in the literature [107]. The levels of the charge-separated states for (a) intermolecular charge transfer in OPVra/M P-CV,o mixtures and (b) intramolecular charge transfer in OPVn-Cgo dyads were determined using (1.2) (see text and Table 1.3). Open squares are for toluene and solid squares for ODCB...

The CT fluorescence arises from the charge-separated state, (b) Values of Vel calculated from the optical electron transfer bands for the radical anions 21(h).111 The optical transition takes place from the radical anion 21(h), in which the unpaired electron is localised on the DCV moiety, to the radical anion 20(h), in which the unpaired electron is localised on the DMN group. 

Macrotetracyclic cryptate 87 containing a Zn-porphyrin complex as a photoactive subunit and two lateral [18]-N204 macrocyclic receptors was developed by Lehn and co-workers.146 In a CHC13/ CH3OH (9 1) solution, the two receptors were able to complex Ag+ to form a polymetallic cryptate, 87-Ag+. The Ag+ ion in the polymetallic cryptate quenches the typical Zn- porphyrin fluorescence by a 10-fold factor via a PET process and by forming a long-lived charge-separated state. 

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