An extremely long-lived charge-separated state of zinc tetraphenylporphyrin coordinated with pyridylnaphthalene-diimide

The nitrogen of pyridylnaphthalenediimide (PyNIm) coordinates to the metal center of zinc tetraphenylporphyrin (ZnTPP) to form a donor-acceptor complex: ZnTPP-PyNIm. Formation of the ZnTPP-PyNIm complex was probed by UV-vis, fluorescence and NMR spectra. The fluorescence of ZnTPP is strongly quenched and the fluorescence lifetime is shortened significantly in the complex. The transient absorption spectrum of the charge-separated state (ZnTPP center dot+-PyNIm(center dot-)) is successfully detected by laser flash photolysis measurements of the ZnTPP-PyNIm system in benzonitrile. The charge-separated state of the complex produced by the photoinduced electron transfer has the longest lifetime, (450 mu s) at 288 K, ever reported for donor-acceptor systems linked covalently or non-covalently in solution. However, when benzonitrile is replaced by benzene, the triplet excited state ((ZnTPP)-Zn-3*), rather than the charge-separated state, is formed upon laser excitation of the ZnTPP-PyNIm complex, due to the lower energy of (3)ZiiTPP* compared to the charge-separated state in benzene. Copyright (c) 2006 Society of Porphyrins & Phthalocyanines.