Dyads transient absorption

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

The recombination behavior of this immobilized dyad is illustrated in Figure 6.30, which shows the time-resolved transient absorption spectra both without and in the presence of I—/13. This figure shows clearly that recombination in the absence of the redox relay is slow and takes place on the microsecond time-scale. 

Figure 3 Transient absorption difference spectra obtained 40 ns following 355-nm pulsed excitation (10-ns pulse width). (A) Model complex 11 (B) metal-organic dyad 10a. Reprinted with permission from Ref. 83.

The Mallouk study stands apart from all other studies of type 1 dyads in that the charge separated state (e.g., 18 in Scheme 9) is detected by picosecond transient absorption spectroscopy. Analysis of the back ET rate data for complexes 20 indicate that JtBET increases as AGBET becomes less negative, consistent with Marcus inverted region behavior (see Fig 2). A more detailed discussion of the back ET rate data is provided below. 

Figure 9 Transient absorption difference spectrum of metal-organic dyad 40 obtained 20 ns after 417-nm excitation pulse (10-ns pulse width). (Figure provided by Professor G. J. Meyer, Johns Hopkins University [110].)...

Fig. 15 Rate data for photoinduced charge separation and subsequent charge recombination in the dyads 18(h)- Charge separation rates, cs, in THF at 20°C were determined both from fluorescence lifetimes92,102 and by pump-probe (time-resolved transient absorption) spectroscopic measurements.105 The mean lifetimes towards charge recombination, rcn were obtained from time-resolved conductivity measurements in 1,4-dioxane.99 101,103,104...

Significant effects of metal ions on photoinduced FT in a zinc porphyrin-naphthalenediimide (ZnP NIm) dyad were reported to attain the long-lived CS state (50). A transient absorption spectrum observed at 0.1 Xs after the laser pulse excitation of a PhCN solution of ZnP NIm is shown in Fig. 10(a) (50). The transient absorption bands at479,531,583,620,685 (sh), and 763 nm are assigned to NIm" by comparison with those of NIm" produced independently by the one-electron reduction of ZnP NIm with tetramethylsemiquinone radical anion [Fig. 10(Z))] (50). The absorption band due to ZnP" + is also observed at 410 nm. Thus, the photoexcitation of ZnP NIm results in FT from ZnP to NIm to give the CS state (ZnP —NIm" ). Fach absorption band decays at the same rate, obeying first-order kinetics [inset of Fig. 10(a)]. The rate constant (Atcr) of the CR process of the CS state is obtained from the first-order plot ZnP —NIm as 7.7 x 10 s (the lifetime x= 1.3 xs) (50). 

In 1990, Osuka, Maruyama, Mataga, and coworkers examined porphyrin dyad 4 and other molecules closely related to dyad 3 with different aromatic linkers joining the macrocycles [19], When the zinc porphyrin of 4 was excited in dimethylforma-mide solution, Pzn-PFe(m) was produced. It decayed with a time constant of 52 ps, and the Pzn -PFe ii) state was observed by transient absorption techniques. The charge-shifted state decayed in 1.6 ns. By studying photoinduced electron transfer in the entire series of molecules with different linkages, the dependence of rate constant on the separation of the porphyrins was determined. A value for f in Eq. 2 of 0.4A- was obtained. 

The rate constant for photoinduced electron transfer k4 and the charge recombination rate constant ks are directly observed experimentally. The reciprocal of the 3-ps time constant detected in the transient absorption experiments, equals kn, 3 X 10 s. This assignment is verified by the results for a model P-C6o dyad, where the same value was obtained for the rate constant for photoinduced electron transfer. The charge recombination of (Pzp)3-Pzc-P -C6o is associated with the 1330-ps decay component observed in transient absorption, as demonstrated by the spectral signature of the fullerene radical anion with absorption in the 1000-nm region. This lifetime is within a factor of 2.5 of the lifetime observed for the P" -C6o in a model dyad (480 ps). 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

Figure 3. Rise of the transient absorption at 660-662 nm of dyad 5 in henzonitrile solution following excitation at 590 nm with -250-fs laser flashes. The fit shown yields rise times of 350fs and 3.8 ps.

Figure 4. Transient absorption of dyad 6 under the conditions described in Figure 3. The solid line is the fit to the data with a 250-fs rise-time constant. The decay-time constant presented in Table II was taken from data over a longer time window.

Photophysical properties of porphyrin-Cjo dyads (9) have also been smd-ied systematically by Imahori, et al. [110]. In addition, a picosecond transient absorption study of a series of ferrocene-CgQ dyads for distance dependent photoinduced electron transfer has been carried out by Guldi et al. [111]. 

Sariciftci et al. prepared a supramolecular dyad consisting of rutheniumfll) tris(bipyridine) functionalized Cgo (10) [112], While the supramolecule shows no interaction between donor and acceptor moieties in the ground state with no charge transfer band in the observed absorption spectrum, there is clearly photoinduced electron transfer in 10 according to results from transient absorption and time-resolved luminescence measurements [112]. 

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