Molecular triad

The light-driven dethreading of a pseudorotaxane whose thread component is immobilized on the surface of a silica sol-gel film was investigated.94 A molecular triad for photoinduced charge separation was attached to a gold electrode and used to drive the dethreading of a pseudorotaxane in a photoelectrochemical setup.95... 

One of the favourite generic arrangement is the molecular triad, consisting of a photoactive centre (PC), an electron donor (D) and an electron acceptor (A). In systems such as D-PC-A, the charge separated state D+-PC-A is obtained in two consecutive-electron transfer processes after excitation of PC. Of course, several variants exist, depending on the electron transfer properties of PC and its excited state, PC, as well as on the precise arrangement of the various components (PC-Ai-A2 or D2-Di-PC, in particular, if PC is an electron donor or an electron acceptor, respectively). 

Ir as Electron Relay The Molecular Triads PH2-lir(lll)-PAu4+ and PZn-lr(lll)-PAu4+... 

Ir(terpy)2+ is reminiscent of Ru(bpy)2+ by some of its photochemical properties but, it is at the same time very different as far as its geometrical properties are concerned and for its electronic and photochemical characteristics. This complex has been used both as a chromophore and as an electron relay, in relation to charge separation. It is expected that, in the future, long-lived charge-separated states will be obtained by constructing carefully designed molecular triads with an Ir(terpy)2+ central core. 

In the next example, a mixed SAM is discussed which aims to utilize photoinduced energy and electron transfer processes to create a photocurrent in an approach which is reminiscent of the natural photosynthetic process. Figure 5.33 illustrates the molecular structures of the components of interest, i.e. the molecular triad ferrocene-porphyrin-fullerene (Fc-P-C6o) and a boron dipyrrin thiol (BoDy) [67]. Mixed monolayers were generated by coadsorption onto vacuum-deposited gold... 

The back reaction M+-A" M-A, which regenerates the initial state, often occurs in the inverted Marcus region, which makes it much slower than the forward electron transfer. In this situation, the charge-separated state can be utilized in follow-up reactions (energy conversion, catalysis) or as a bit of chemical information. A long-lived, long-distance charge separation can be produced in molecular triads in which an electron donor and acceptor are attached simultaneously to the photoactive center ... 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

Figure 25. (A) Structure of an artificial photosynthetic reaction center, the molecular triad C-P-Q, and the proton-shuttling quinone, Qsl (B) Schematic diagram showing orientation of the triad In the liposome and the sequence of events after photoexcitation (see table at right and text for details) (C) Fluorescence excitation spectra of the pH-indicator dye pyraninetrisulphonate as a measure of the concentration of the protonated form of the indicator dye (D) Fluorescence excitation-band intensity as a function of irradiation time in the absence and in the presence of FCCP. Figures adapted from Steinberg-Yfrach, Liddeii, Hung, (AL) Moore, Gust and (TA) Moore (1997) Conversion of light energy to proton potential in liposomes by artificial photosynthetic reaction centres. Natu re 385 239-241.

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

The photochemistry of a molecular triad consisting of a porphyrin covalently linked to a carotenoid polyene and a fullerene derivative has been studied at 20 K by time-resolved EMR spectroscopy following laser excitation (Carbonera et al., 1998). Excitation of the porphyrin yields a coupled radical pair with a carotenoid cation and a C o anion. The exchange interation in the pair has been determined to approx. 

In the early 1980s we designed an artificial photosynthetic reaction center that overcomes this problem by using a multistep electron transfer strategy such as that found in natural reaction centers (Moore et al, 1984). More recently, molecular triad 9 which consists of a porphyrin chromophore (P) bound covalently to a naphthoquinone derivative (NQ) and a carotenoid electron donor (C) was designed both to undergo multistep electron transfer and to organize... 

Photoinduced Electron and Proton Transfer in a Molecular Triad... 

Hung et al. Photoinduced Electron it Proton Tranter in a Molecular Triad 181... 

---