Electronic Interactions and Photoinduced Electron Transfer in Covalently Linked Porphyrin-C 60 (pyridine) Diads and Supramolecular Triads Formed by Self-Assembling the Diads and Zinc Porphyrin Francis D’Souza,* ,† Gollapalli R. Deviprasad, † Melvin E. Zandler, † Mohamed E. El-Khouly, ‡ Mamoru Fujitsuka, ‡ and Osamu Ito* ,‡ Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai, 980-8577, Japan ReceiVed: February 14, 2002 Supramolecular triads of the type (donor-1)-acceptor:(donor-2) composed of free-base porphyrin, fullerene, and zinc porphyrin, have been formed by a “covalent-coordinate” approach. Toward this, two diads, namely, 5-(3′-(2′′-(3′′′ or 4′′′-pyridyl)fulleropyrrolidinyl-N)ethoxyphenyl)-10,15,20-triphenylporphyrin, bearing C 60 as acceptor and free-base porphyrin, H 2 P, as donor were first synthesized. The diads and self-assembled supramolecular triads, which were formed by coordinating the pyridine group located on the diads to zinc tetraphenylporphyrin, ZnP, have been characterized by semiempirical PM3, electrochemistry, and steady- state and time-revolved spectroscopic techniques. Subpicosecond and nanosecond transient absorption spectral studies of diads revealed the occurrence of the electron transfer from the H 2 P moiety to the C 60 entity via the excited singlet state of H 2 P. Clear evidence for the formation of triads in o-dichlorobenzene was obtained from the steady-state and time-resolved fluorescence measurements, which revealed quenching of the ZnP emission on addition of the pyridine bearing diads. Semiempirical PM3 energy optimized structures of the triads suggested substantial intramolecular interactions between the H 2 P and C 60 entities. Cyclic voltammetric studies on these triads exhibited a total of 12 one-electron redox processes involving the three redox active ZnP, H 2 P, and C 60 entities. Nanosecond transient absorption studies revealed additional charge separation in the triads as compared to that observed for the diads, suggesting that the coordinated ZnP accelerates the charge-separation process. However, in a coordinating solvent such as benzonitrile, intermolecular electron transfer from the 3 ZnP* to the diads is shown to take place. Introduction Studies of supramolecular systems bearing redox and pho- toactive entities are valuable for designing light energy harvest- ing systems as well as for developing redox and optoelectronic devices. 1,2 Covalent attachment of donor-acceptor entities with one or more spacer units to control the distance and orientation has been the popular choice 2 even though noncovalent linkage is considered nature’s choice of assembling the different redox and photoactive entities in the active centers of biological systems. 3 In this regard, a few elegantly designed self-assembled donor-acceptor type diads and supramolecular systems bearing one or more different type of the chromophores have been synthesized and studied. 2 These diads contain electron donor chromophore such as porphyrins, phthalocyanines, or ruthenium- (II) tris(bipyridine) and electron acceptors of the type quinone, nitroaromatic compounds, metal complexes, pyrromellitic an- hydride, etc. 1-4 Recently, fullerenes 5 have drawn special atten- tion as electron acceptors owing to their three-dimensional structure, 6 reduction potentials comparable to benzoquinone, 7 absorption spectrum extending over most of the visible spectral region 8 and a small re-organization energy in electron-transfer reactions. 9 Several covalently linked donor-acceptor systems 10-12 and a few self-assembled via metal-ligand coordination donor- acceptor systems 13,14 bearing fullerene have been reported. In the present study, we have utilized a “covalent-coordinate” bonding approach to form supramolecular triads consisting of covalently linked free-base porphyrin-C 60 diads, i.e., 5-(3′-(2′′- (3′′′ or 4′′′-pyridyl)fulleropyrrolidinyl-N)ethoxyphenyl)-10,15,- 20-triphenylporphyrin (compounds 1 and 2 in Scheme 1) and (tetraphenylporphyrinato)zinc (ZnP). In the diads, the fullero- pyrrolidine is functionalized to bear a pyridyl moiety that is subsequently utilized to coordinate ZnP to obtain the desired supramolecular triads composed of free-base porphyrin (H 2 P), C 60 , and ZnP entities (3 and 4 in Scheme 1). That is, (donor- 1)-acceptor:(donor-2) type triads, in which the (donor-1)- acceptor refers to the covalently linked entities and the acceptor: (donor-2) refers to the entities held by a coordinate bond. The electrochemical behavior of the redox active diads and su- pramolecular triads has been investigated. Efficient intramo- lecular electron transfer from the singlet excited free-base porphyrin to the covalently attached fullerene has been observed in the studied diads in o-dichlorobenzene and benzonitrile. Interestingly, upon coordination of the pyridine entity of 1 or 2 to ZnP to form the supramolecular triads, one would ex- pect a competition between electron-transfer from the singlet excited ZnP and/or H 2 P to C 60 and/or energy-transfer from the singlet excited ZnP to H 2 P. The present investigations show that the second donor, ZnP of the triads actually ac- celerates the charge-separation process and the energy-transfer quenching involving excited ZnP and H 2 P is not an efficient process. † Wichita State University. ‡ Tohoku University. 4952 J. Phys. Chem. B 2002, 106, 4952-4962 10.1021/jp0204307 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/18/2002