A R T I C L E S
Scheme 1
Tanaka et al.
most important types of noncovalent binding because of its
important role in biological systems, such as for π-stacking of
double-strand DNA.22 For example, a bisporphyrin has been
shown to form a π-complex with fullerene, which is inserted
between the two porphyrin rings.23 A DNA intercalator such
as acridinium ion can also be inserted between the two porphyrin
rings of a water-soluble bisporphyrin to form a π-complex.24
Acridinium ion is suitable as a component of an artificial
photosynthetic reaction center because of the small reorganiza-
tion energy (λ) of electron transfer,25 which results in fast
photoinduced electron transfer but extremely slow back electron
transfer.26 However, there has been no report on the photody-
namics of π-complexes of acridinium ion with electron donors.
We report herein the formation of a π-complex of a free-
base bisporphyrin (H4DPOx) with acridinium perchlorate
(AcH+ClO4-) and the photodynamics in benzonitrile (PhCN),
as shown in Scheme 1. H4DPOx is regarded as a model
compound of [(BChl)2], which has two cofacial porphyrin rings
and a flexible spacer, diphenyl oxide. The formation of the
electron-transfer (ET) state is examined by laser flash photolysis
experiments. The decay of the ET state has been found to be
highly temperature dependent. As a result, a remarkably long-
lived ET state has been attained at low temperature, and virtually
no decay of the ET state was observed at 77 K (vide infra).
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Experimental Section
Materials. The synthesis details and analytical data for H4DPOx
are shown in the Supporting Information, S1-S3. Similar systems have
been reported in the literature.27 5,10,15,20-Tetraphenyl-21H,23H-
porphyrin (H2TPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-por-
phyrin (H2OEP) were obtained from Aldrich Chemical Co. Perchloric
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14626 J. AM. CHEM. SOC. VOL. 128, NO. 45, 2006