Radical-ion-pair state

By means of time-resolved fluorescence studies we were able to determine the C60 fluorescence deactivation rates, as 2.1 x 1010 s-1 in 9a, 6.6 x 109 s 1 in 9b and 1.3 x 109 s-1 in 9c. Importantly, the indulging trend resembles the relationship between the quantum yields of the conjugates (9a-d) and reference (1). In short, an intensified excited-state deactivation emerges with decreasing bridge length. However, no measurable decay rates were found for the trimer 9d. Conclusively, the indirect or direct population of Cgo possibly leads to an exothermic electron-transfer reaction, resulting in the radical-ion-pair state ... 

Conclusively, in the presence of a strong electron donor, namely exTTl, the resulting product of the photoexcitation is not the triplet excited state of C60 but the energetically lower-lying radical-ion-pair state, exTTF +-oPPE -C6o Here, it should be stressed that the spectral identification of the radical ion pair holds for... 

Fig. 9.7 Time-absorption profile of the spectra shown above at 1010 nm to monitor the formation of the radical-ion-pair state (blue = monomer 9b, light blue = dimer 9c)...

Indisputably, photoexcitation is followed by a rapid deactivation of the singlet-excited state of the oPPE moiety resulting in the generation of a charge-separated species, i.e. the radical-ion-pair state exTTF +-oPPE -C o, which is apparently lower in energy than the corresponding triplet state of C o- The radical ion pairs decay on the ps time-scale with charge-recombination rates that prove wire-like... 

In fact, it was possible to prove the formation of the radical ion pair state by transient absorption spectroscopy. Particularly, at the expense of the vanishing H2P/ZnP singlet absorption new features with maxima in the 600-700 nm range as well as at 480 nm grow in. These maxima correspond to the one-electron oxidized 7t-radical cations of H2P (H2P +) and ZnP (ZnP +). Additionally, in the near-infrared region the spectral signatures of the one-electron reduced anion of Ceo are discernible at 1000 nm (Fig. 9.23). 

Kinetic analyses of the formation of the radical ion pair state—formed through bond in 17a and through space in 17b, 17c, and 17d—revealed that the latter are meta-stable on the femto-/picosecond time scale. Hence, charge recombination... 

Fig. 9.31 a Differential absorption spectra (visible and near-infrared) obtained upon nanosecond flash photolysis (355 nm) of 17c (2.0 x 10 6 M) in nitrogen-saturated oDCB solutions with a time delay of 100 ns at room temperature, indicating the radical ion pair state features at 680 and 1010 nm. b Time-absorption profiles of the spectra shown above at 1010 nm to monitor the decay of the radical ion pair state... 

Additionally, in order to examine the charge-recombination dynamics we turned to complementary nanosecond transient absorption measurements. Once more, the spectral fingerprints of the radical ion pair state emerged immediately after the laser pulse and their decays yielded charge-recombination lifetimes in the order of 4.0 ps (Fig. 9.38). 

Since the radical ion pair states are stable on the time-scale of the femtosecond experiments, the charge-recombination rates were analyzed in complementary nanosecond experiments (Fig. 9.46). Therein, the decays of the C o and exTTF + features result in the refurbishment of the singlet ground state of 18a,b lacking any detectable triplet features. The corresponding rate constants for the charge-recombination process are listed in Table 9.5. 

In contrast to the references, 23a,b, the singlet-excited state deactivation in the Fc-oFL -C60 conjugates, 24a,b, occurs with rate constants of around 1010 s-1 (Fig. 9.64). The values are in good agreement with the observed quantitative quenching of the oFL fluorescence (Fig. 9.62a). At the conclusion of the singlet-excited state decay, two important transient maxima resemble the successful formation of the radical ion pair state, namely a weak shoulder of the transient... 

Apart from zinc(II) (na)phthalocyanines, the ruthenium(II) analogs have also been used to complex with pyridyl fullerenes. Arrays 33-35 show electronic coupling between the two electroactive components in the ground state as shown by UV-Vis spectroscopy and electrochemical measurements [45], The use of ruthe-nium(II) instead of zinc(II) phthalocyanines reduces the undesired charge recombination, increasing the lifetime of the radical ion pair state (on the order of hundreds of nanoseconds). 

Lukas A.S. Bushard P.J. Wasielewski M.R. Ultrafast molecular logic gate based on optical switching between two long-lived radical ion pair states. J. Am. Chem. Soc. 2001, 123, 2440-2441. 

The synthesis and properties of ruthenium phthalocyanines, RuPcs, have been well studied over the last 30 years, but only recently they have been rediscovered for their potential application in metallosupramolecular chemistry. Basically, RuPcs are different from ZnPcs in their tendency to form stronger complexes with basic sp nitrogen atoms (pyridine and imidazole) and in the possibility to form complexes on one side or on both sides of the macrocycle. Another interesting point of ruthenium phthalocyanines resides in the longer lifetime of their radical-ion-pair state when compared to that of zinc phthalocyanines. Thus, all things being equal, RuPcs display a richer potential for supramolecular chemistry than ZnPcs. 

Fullerene-phthalocyanine ensembles 30-32 (Figure 20) were self-assembled in good to excellent yields from the bisbenzonitrile Ru(II) Pc and the corresponding fullerene derivatives." These dyads and triads gave rise to longer charge-separated lifetime than their zinc-based counterparts—see Section 4.1.1—the lifetime of radical-ion-pair state being several hundreds of nanoseconds. 

Similarly, dispersible SWNTs grafted with poly(4-vinylpyridine) (PVPy) have been coordinated with ZnP (Figure 12.11b). Support for the grafting of the polymers to the sidewalls of SWNTs came from Raman and near-infrared spectra, and the composition of the resulting SWNT-PVPy was estimated as 61 /39. Kinetic and spectroscopic evidence corroborates the successful formation of SWNT-PVPy/ZnP nanohybrids in solution, which upon photoexcitation generate a ps-lived radical ion pair state [107]. 

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