Porphyrin-quinone molecules

Our current work is concentrated on the series of porphyrin-quinone molecules (V). These are very similar to I in that amide linkages have replaced the ester linkages. We chose to study these molecules not only because they provided a definite change... 

Distance-Dependent Rates of Photoinduced Charge Separation and Dark Charge Recombination in Fixed-Distance Porphyrin-Quinone Molecules... 

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

Wasielewski M. R., NiemczykM. P., Svec W. A. andPewittE. B. (1985), Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules , J. Am. Chem. Soc. 107, 1080-1082. 

McIntosh, A. R Siemiarezuk, A., Bolton. J. R.. Stillman, M. J.. Ho. T.-F Weedon, A. C. (1983). Intramolecular photochemical electron transfer. 1. EPR and optical absorption evidence for stabilized charge separation in linked porphyrin-quinone molecules, y. Am. Chem. Soc., 105 7215. 

We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study the dependence of the rate of electron transfer proceeding from the lowest excited singlet state of the porphyrin on the free energy of reaction.(3-4) We now report measurement... 

Another example of compounds with the fixed mutual location of porphyrin and quinone are the porphyrin-quinone compounds with a rigid bridge. Charge photoseparation in P-L-Q molecules in which L is the trip-ticene bridge, P is tetraphenylporphin, TPP, or its zinc complex, and Q is benzoquinone, BQ, naphthoquinone, NQ, or anthraquinone, AQ, has been studied [55]. The distance between the centres of P and Q fragments in these... 

A number of covalently bound porphyrin-quinone systems have recently been synthesized as models for carefully spaced donor-acceptor systems. These are designed to test the possibility of controlling electron transfer rates by spatial separation of donor-acceptor pairs. Among these exceedingly clever studies (30) are molecules designed to separate the donor and acceptor by rigid insulating molecules, for example, 2 (31),... 

We have synthesized molecules with regioisomeric variations in the placement of the nitro group and with quinone functionality,31 heterocyclic and porphyrin-based molecules,32 pyridine-based molecules,33 molecules with diazonium alligator clips,34 biphenyl- and fluorenyl-based molecules,35 nitrile terminated molecules,36 previously-published molecules with improved syntheses,37 and combinatorial methods of synthesizing tens to hundreds of molecule types at a time.38... 

In Ref. [317] the temperature independence of the intramolecular electron transfer reaction in a cofacial Zinc porphyrin-quinone cage molecule was observed in the range 80-300 K and interpreted in terms of non-adiabatic electron tunneling. 

Tien and co-workers [100, 238] observed a photopotential and photocurrent arising in the planar BLM containing bridging molecules with a porphyrin bound covalently to a quinone (see System 44 of Table 1). The size of this porphyrin-quinone complex was not large enough to span across the whole of the membrane, therefore the mechanism of the arising photoeffect is most likely to be similar to those discussed in Sect. 2.4 for other BLMs doped with photosensitizers. 

P-Q — porphyrin-quinone bridging molecule Car-P — carotene-porphyrin bridging molecule... 

Fig. 2. Transient states of a typical porphyrin-quinone dyad molecule and related electron transfer pathways...

Fig. 4. Photoinitiated electron transfer rate constant (natural logarithm) vs. edge-to-edge porphyrin-quinone separation for C-P-Q triads 4-8 and related P-Q molecules. The separations shown are derived from H-NMR measurements...

In these devices, the acceptor is joined to two donors which can in principle transfer electrons to it with comparable efficiencies. From the point of view of long-lived charge separation, molecules of this type offer no apparent advantages over simple D-A systems. However, as mentioned above, they may be useful in some other situations, especially as components of larger devices when A is a 2-electron acceptor. There are a few examples of porphyrin-quinone systems with this structure in the literature. 

The fluorescence decay has a major component of 1 ns which is likely to correspond to unquenched porphyrin, whereas a much shorter component of -100 ps can also be detected which is attributed to quenched porphyrin by quinone molecules which sit within a distance corresponding to the radius of an active-sphere of quenching (Perrin, 1924). 

The first covalently linked porphyrin-based systems for mimicry of photosynthetic electron transfer were reported in the late 1970s. These were porphyrin-quinone dyad molecules 6 and 7 [45-47]. Subsequent to these reports, a very large number of porphyrin-quinone dyads have been studied, and still work in this field continues. Connolly and Bolton reviewed the work in the 1970s and 1980s [48]. Since that... 

The basic photophysical behavior of porphyrin-quinone systems is illustrated by dyad 8, in which a naphthoquinone derivative is linked to a porphyrin via an amide bond [61], The photophysics of this molecule has been investigated using techniques... 

Structures 9 to 22 illustrate a few of the many porphyrin-quinone dyads in the literature. Most of these demonstrate photoinduced electron transfer behavior similar to that described for 8 [48]. A large number of such dyads have been reported since 1990 [11, 39, 40, 52, 55, 56, 62-105]. As will be illustrated below, some classes of these molecules have proven to be very useful in the study and application of photoinduced electron transfer phenomena. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

By building the quinone molecule into a macrocycle, a more efficient palladium-catalyzed aerobic 1,4-oxidation was developed [63], Thus, with catalytic amounts of 40 and Pd(OAc)2, 1,3-cyclohexadiene was oxidized to 1,4-diacetoxycyclohex-2-ene at a rate which was more than twice that with the system having quinone and porphyrin as separate molecules. The trans selectivity with tetrakis(hydroquinone)poiphyrin 40, however, was moderate (trans/cis = 70 30). 

C-P-Q Triad Molecules. As discussed above, the natural reaction center has solved the problem of energy loss due to rapid charge recombination by employing a multistep electron transfer strategy. The same strategy may be applied to the porphyrin-quinone type systems. As we pointed out in 1982 [28], this requires the addition of a secondary electron donor or acceptor moiety. This strategy came to fruition in 1983 when we reported the synthesis of carotenoid-porphyrin-quinone (C-P-Q) triad 2 [29, 30]. This molecule features porphyrin and quinone moieties similar to those found in 1, but a... 

Triads 4-6 are similar to 2, with the exception that the methylene chain joining the porphyrin and quinone moieties has been increased to two, three, and four CHj groups, respectively. Molecules 9-11 are the corresponding P-Q systems which lack the carotenoid. Triad 7 features a porphyrin-quinone linkage as in 2, but a methylene group in the carotenoid-porphyrin linkage. 

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