Dyads absorption spectra

Fig. 1.27. Normalised UV/VIS absorption (solid lines) and fluorescence excitation spectra of the fullerene emission at 715 nm (dashed lines) of the OPVn-C6o dyads in toluene at 295 K. (a) n = 1, (b) n = 2, (c) n = 3, and (d) n = 4. In each case the fluorescence excitation spectrum shows a close correspondence to the absorption spectrum...

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). 

A more remarkable elongation of the CS lifetime was attained by complex formation of yttrium triflate [Y(OTf)3] with the CS state in photoinduced ET of a ferrocene-anthraquinone (Ec AQ) dyad (53). Photoexcitation of the AQ moiety in Ec AQ in deaerated PhCN with femtosecond (150 fs width) laser light results in appearance of the absorption bands 420 and 600 run at 500 fs, as shown in Eig. 14(a) (53). The absorption bands 420 and 600 nm, which are assigned to AQ by comparison with the absorption spectrum of AQ produced by the chemical reduction of AQ with naphthalene radical anion (53). The decay process obeys first-order kinetics with the lifetime of 12 ps [Eig. um. 

One of the clearest examples of the increased absorption cross section for a photochemical process provided by carotenoid pigments is that observed in carotenoid-buckminsterfullerene dyad 7 (Imahori et al., 1995). The carotenoid absorption spectrum is distinct and much stronger than that of the underlying Ceo bands. Upon excitation of the carotenoid moiety of 7 with a 150 fs pulse of 600 nm laser Ught, the... 

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]. 

A polymer based on EDOT containing a perylenetetracarboxylic diimide unit has been prepared by electropolymerization of 250 [443]. The related absorption spectrum covers the visible range and extends up to 850 nm. Similarly, PV devices based on dyads 251 in which oligo(3-hexylthiophene)s are covalently linked to perylenemonoimide have been recently reported [444]. Preliminary results based on bulk heterojunction devices consisting of ITO/PEDOT-PSS/251 PCBM (l 4)/LiF/Al showed an open circuit voltage of 0.94 V and efficiencies of 0.33% (251, n=l) and 0.48% (251, =3) under standard test conditions (AM 1.5G, 1000 W m ). 

The nature of the bridge, if any, between D and A will usually define the mechanism by which the energy transfer is more likely to proceed. It should be pointed out that in most cases detailed in this chapter, weakly coupled systems will be considered. A weakly coupled dyad corresponds to a D-A pair in which Coulombic interactions in the ground state are small compared to the energy of the electronic transitions, and thus, the absorption spectra of separated species are very similar to the absorption spectrum of the dyad. 

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. 

The 100 MHz 1H—NMR spectra in chloroform-d afforded the following information. Isotactic polymers prepared by zinc catalyst and atactic polymers by KOH catalyst had a common feature in having a sharp, symmetrical AB-type quartet at 6.43,6.53,6.60, and 6.70 ppm. Therefore, this quartet could be assigned to a head-to-tail dyad, because the latter polymer was known to contain no head-to-head and tail-to-tail linkage. Thus, in this spectrum the absorptions originating from isotactic and syndiotactic dyads overlap. 

In the example discussed above, the heterotriad consists of a photosensitizer and an electron donor. In the following example, a ruthenium polypyridyl sensitizer is combined with an electron acceptor, in this case a rhodium(lll) polypyridyl center [15]. The structure of this dyad is shown in Figure 6.21 above. The absorption characteristics of the dyad are such that only the ruthenium moiety absorbs in the visible part of the spectrum. Irradiation of a solution containing this ruthenium complex with visible light results in selective excitation of the Ru(ll) center and in an emission with a A.max of 620 nm. This emission occurs from the ruthenium-polypyridyl-based triplet MLCT level, the lifetime of which is about 30 ns. This lifetime is very short when compared with the value of 700 ns obtained for the model compound [Ru(dcbpy)2dmbpy)], which does not contain a rhodium center. Detailed solution studies have shown that this rather short lifetime can be explained by fast oxidative quenching by the Rh center as shown in the following equation ... 

This is considerably different from the recombination reaction with, for example, typical ruthenium dyes. This slow re-reduction of the dyad is explained by the low redox potential of the osmium center, the value of 0.66 V (vs. SCE) observed, points to a small driving force for the redox process. This observation is important for the design of dyes for solar cell applications. Osmium compounds have very attractive absorption features, which cover a large part of the solar spectrum. However, their much less positive metal-based oxidation potentials will result in a less effective re-reduction of the dyes based on that metal and this will seriously affect the efficiency of solar cells. In addition, for many ruthenium-based dyes, the presence of low energy absorptions, desirable for spectral coverage, is often connected with low metal-based redox potentials. This intrinsically hinders the search for dyes which have a more complete coverage of the solar spectrum. Since electronic and electrochemical properties are very much related, a lowering of the LUMO-HOMO distance also leads to a less positive oxidation potential. 

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].)...

The MP-CfioC i) state formed via the intramolecular singlet-energy transfer in the OPVn Cgo dyads is expected to decay predominantly via intersys-tem crossing to the MP-Cfjo(Ti) state, apart from some radiative decay. Consistent with this expectation, the PIA spectrum recorded for all four dyads in toluene solution shows the characteristic MP Cf,o T <— Tn absorption at 1.78 eV with a shoulder at 1.54 eV (Fig. 1.28a). The PIA bands increase in a near-linear fashion with the excitation intensity (—AT oc Ip, p = 0.80-1.00) consistent with a monomolecular decay mechanism. The lifetime of the triplet state lies in the range 140-280 ps. 

The 18A T and 18B T isomers form a dyad each consisting of an electron donor (fullerene), an electron acceptor ( r-exTTF of T) and a crown ether as a bridge between them. Their absorption spectra are given in Fig. 31.5. The spectrum... 

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