Triads absorption spectra

A UV-visible absorption spectrum of a 10 (J.M ethanol solution of a model compound (DAFc in Fig. 5) for the D moiety is shown together with that of the A-S-D triad in Fig. 16. The absorption bands between 200 and 300 nm can be clearly seen for DAFc. Qualitatively, absorption spectra for A-S-D triads in the LB films and in the 10 pM ethanol solution are similar. It is interesting to note, however, that the relative intensities of the acylated perylene band around 450 nm against the 200-300 nm UV absorption bands are different between the LB films and the ethanol solution. This difference can be attributed to orientation of the perylene moiety in the LB films in the same way as in the antenna LB films reported previously [38]. 

Figure 1 Differential absorption spectrum obtained upon nanosecond flash photolysis (532 nm) of 7.2 X 10M solutions of Fc-ZnP-H2P-C6o triad in nitrogen saturated benzonitrile with a time delay of 50 nsec at 298 K. (From Ref. 47.)...

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. 

Perylenediimides represent another class of photoactive dyes which are characterized by their strong fluorescence emission and facile electrochemical reduction. Recently, a supramolecular bis(phthalocyanine)-perylenediimide hetero-triad (compound 42) has been assembled through axial coordination [47]. Treatment of perylenediimide 43, which has two 4-pyridyl substituents at the imido positions, with 2.5 equiv. of ruthenium(II) phthalocyanine 44 in chloroform affords 42 in 68% yield (Scheme 3). This array shows remarkable stability in solution due to the robustness of the ruthenium-pyridyl bond. Its electronic absorption spectrum is essentially the sum of the spectra of its molecular components 43 and 44 in... 

The work of Baerends et al. using density functional theory (DFT) methods confirmed that the energies of the orbitally forbidden or spin-forbidden MLCT transitions are lower than the energies of the metal-centered (MC) transitions (Fig. 3, Table 1) and this placed the MC transitions further up the energy scale in the UV/ Vis absorption spectrum [10-12], The MC (2t7g—>6eg) transitions move to higher energy for the heavier metals of the triad. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

Comparing the ground-state absorption features of oPPE and oMPE triads (Fig. 9.28), the main difference can be found in the absorption of the linkers, i.e. in the 350-550 nm region. In the meta-isomers the absorptions exhibit a distinctive pattern with maxima that hardly shift to the red part of the spectrum. This implies weaker or even a lack of 7i-conjugation relative to the corresponding para systems. For that reason, C6o (380 and 434 nm) and exTTF (450 nm) absorption features are clearly distinguishable. 

The NMR spectrum of PVAc in carbon tetrachloride solution at 110°C shows absorptions at 4.865 (pentad) of the methine proton 1.785 (triad) of the methylene group and 1.985, 1.965, and 1.945, which are the resonances of the acetate methyls in isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The NMR spectra of ethylene vinyl acetate copolymers have also been obtained (35). The IR spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

For isotactic polypropylene, as shown in structure 9.2, six different placements are possible when considering triads. C-NMR spectroscopy can be used to determine isolated head-to-head and tail-to-tail units in polypropylene (PP). Polypropylenes produced using vanadyl catalysts possessed the normal head-to-tail structure [1]. More detailed examination, however shows that the amorphous fractions isolated from these PP do show (IR) absorption at 13.3 pm pointing to the presence of methylene sequences of two units, which can only mean tail-to-tail arrangement of propylene units does occur. A very small absorption peak at 13.3 pm was also found in the spectrum of the crystalline fractions. Polypropylenes prepared with catalysts based on VCI3 show only the normal head-to-tail arrangement in both amorphous and crystalline fractions, as do polymers prepared from TiClj catalyst. The amount of propylene units coupled tail-to-tail was estimated to range from 5 to 15% for the amorphous and from 1 to 5% for the crystalline fractions. 

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