Synthesis and Photophysical and Photoelectrochemical Properties of Diazaporphyrin-Fullerene Linked Dyad

Over the last two decades much effort has been devoted to explore excellent donor-acceptor (D-A) couples for artificial photosynthesis and solar energy conversion.^1-6 In this regard fullerenes have been frequently employed as acceptors to exhibit outstanding electron transfer (ET) properties in D-A systems as well as organic photovoltaics. Meanwhile, various donors such as porphyrins, phthalocyanines, and ruthenium bipyridyl complexes have been involved in such systems and devices. However, the examples of superior donors are still limited in terms of light-harvesting and ET properties.

5,15-Diazaporphyrins (DAPs) are a class of porphyrins that bear two meso-nitrogen atoms linking two dipyrromethene units. DAPs have received continuing interest in relation to their analogues, porphyrins and phthalocyanines. It has been shown that 5,15-aza-substitution significantly changes the intrinsic properties of the porphyrin p systems that would be comparable to or even better than their analogues. Nevertheless, due to synthetic difficulties of the cyclization and peripheral modification of the core in DAPs, the opportunity of using DAPs as materials has poorly been addressed. As such, DAPs have yet to be used as donors in D-A linked systems to elucidate the intrinsic ET properties.^7-9

Here we report, for the first time, the synthesis and photophysical and photoelectrochemical properties of zinc 5,15-diazaporphyrin (ZnDAP)-C₆₀ linked dyad. To evaluate the intrinsic photophysical properties of ZnDAP as a donor accurately, the corresponding zinc porphyrin (ZnP)-C₆₀ linked dyad with the same spacer was also designed (Figure 1). Owing to the electron-withdrawing character of the 5,15-aza-substitution ZnDAP would be more difficult to be oxidized than ZnP. Accordingly, we expected that once a charge-separated state was generated via photoinduced ET, charge recombination (CR) from the charge-separated state to the ground state in ZnDAP-C₆₀ would be slowed down as compared with ZnP-C₆₀, because CR in ZnDAP-C₆₀ is shifted more deeply into the Marcus inverted region. To attain sufficient solubility of the dyads bulky long alkoxy chains were also introduced to the ortho-positions of the meso-phenyl group.

In addition to the better matching of ZnDAP with solar energy distribution in the visible region than ZnP, the lifetime of charge-separated state in ZnDAP-C₆₀ was 4 times longer than that in ZnP-C₆₀ due to the high oxidation potential of ZnDAP relative to ZnP. This difference also paralleled the enhanced photocurrent generation of ZnDAP-C₆₀–based device in the visible region. These results unambiguously exemplify the potential utility of DAPs as an excellent donor in artificial photosynthesis and solar energy conversion. In this talk, we will present the synthesis, optical, electrochemical, photophysical, and photoelectro-chemical properties of ZnDAP-C₆₀ in comparison with the corresponding ZnP-C₆₀.

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