J. Phys. Chem. B 2009, 113, 16483–16493
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Linker Dependence of Energy and Hole Transfer in Neutral and Oxidized Multiporphyrin Arrays Hee-eun Song,† Masahiko Taniguchi,‡ James R. Diers,§ Christine Kirmaier,† David F. Bocian,*,§ Jonathan S. Lindsey,*,‡ and Dewey Holten*,† Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889, Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, and Department of Chemistry, UniVersity of California RiVerside, RiVerside, California 92521-0403 ReceiVed: July 29, 2009; ReVised Manuscript ReceiVed: October 19, 2009
The excited-state photodynamics of the neutral and one-electron-oxidized forms of five porphyrin dyads were studied in benzonitrile containing tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Each dyad contains a zinc porphyrin (Zn) and a free base porphyrin (Fb) joined by a linear biphenylene (Φ2), terphenylene (Φ3), quaterphenylene (Φ4), diphenylbutadiyne (L), or phenylethyne (E) linker (ZnFbΦ2, ZnFbΦ3, ZnFbΦ4, ZnFbL, ZnFbE). The findings along with recent results on the neutral and oxidized forms of ZnFb dyads containing a diphenylethyne or phenylene linker (ZnFbU, ZnFbΦ) and steric hindrance to porphyrin-linker internal rotation at one or both ends of a diarylethyne linker (ZnFbD, ZnFbP, ZnFbB) give insights into the effects of linker characteristics (length, orbital energies, orbital overlap with the porphyrins) on the rate constants for excited-state energy transfer, excited-state hole transfer, and ground-state hole transfer. Analysis of the results is aided by density functional theory molecular orbital calculations and Fo¨rster energytransfer calculations. Although the rate constants for linker-mediated through-bond excited-state energy transfer can be modulated significantly using a number of molecular design criteria (e.g., linker characteristics, interplay between porphyrin orbital characteristics, and linker attachment site), ground-state hole transfer, which also occurs via a linker-mediated through-bond electron-exchange mechanism, is primarily affected by the freeenergy driving force for the process as dictated by the redox characteristics of the interacting porphyrins. The insights gained from this study should aid in the design of next-generation multichromophore arrays for solar energy applications. I. Introduction Photophysical studies of covalently linked arrays composed of tetrapyrrole pigments have been primarily motivated by the desire to model the energy- and charge-transfer processes that occur in natural photosynthetic systems.1-11 These studies have largely focused on either excited-state charge separation or energy transfer following photoexcitation of one component. More limited measurements have also been made on the rates of ground-state hole transfer between inequivalent tetrapyrroles, such as between a free base porphyrin (Fb) and a zinc porphyrin (Zn) in molecular triads and tetrads. These latter cases have typically involved a hole shift that occurs down a thermodynamic gradient (owing to the redox properties of the reactants) and against a Coulomb potential (arising from the initial photoinduced charge-separation event). A generic example involves electron transfer from a photoexcited free base porphyrin (Fb*) to an acceptor (X) followed by the hole shift to the more easily oxidized zinc porphyrin in the sequence ZnFb*X f ZnFb+X- f Zn+FbX-. Note that the term “hole transfer” is used herein even though the net transport of a hole could occur via either direct hole migration or electron migration in the reverse direction. * Corresponding authors. E-mail:
[email protected] (D.H.); jlindsey@ ncsu.edu (J.S.L.);
[email protected] (D.F.B.). † Washington University. ‡ North Carolina State University. § University of California, Riverside.
Recently, we developed an approach for probing ground-state hole transfer between inequivalent porphyrins (e.g., Zn and Fb) in the absence of a Coulomb potential.12 These studies focused on ZnFb dyads joined via phenylene or diarylethyne linkers (Chart 1). The approach involves electrochemical oxidation of the zinc porphyrin (to produce Zn+Fb), excitation of the free base porphyrin (to produce Zn+Fb*), and monitoring of the subsequent dynamics using time-resolved optical spectroscopy. One of the excited-state decay pathways produces the metastable state ZnFb+, which decays to the pre-excitation state Zn+Fb by ground-state hole transfer. Although this hole transfer is downhill by ∼0.2 eV (the difference in Zn and Fb oxidation potentials), the process is a ground-state event because each entity (Zn, Zn+, Fb, Fb+) is in its electronic ground state. The rate constants determined were (17 ps)-1 for the oxidized phenylene-linked dyad ZnFbΦ and (20 ps)-1 for the oxidized diphenylethyne-linked dyad ZnFbU. In addition to providing rate constants for ZnFb+ f Zn+Fb hole transfer (absent a Coulomb potential), the above-mentioned studies also afforded a detailed understanding of the photodynamics involving adjacent zinc and free base porphyrins in arrays in which the former is oxidized prior to excitation of the latter constituent. This knowledge underpins another new strategy that we recently developed to assess the rates of groundstate hole transfer between redox-equivalent sites (ZnZn+Fb f Zn+ZnFb and ZnFbFb+ f ZnFb+Fb) in oxidized ZnZnFb and ZnFbFb triads (Chart 2).13,14 These studies provided rate constants for Fb/Fb and Zn/Zn ground-state hole transfer of
10.1021/jp9072558 2009 American Chemical Society Published on Web 12/04/2009
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J. Phys. Chem. B, Vol. 113, No. 52, 2009
CHART 1
(0.5 ns)-1 and (0.6 ns)-1, respectively, across a phenylene linker and (0.6 ns)-1 and (0.8 ns)-1 across a diphenylethyne linker. Prior to these studies, only an upper limit of (50 ns)-1 had been determined for the rate of ground-state hole transfer between equivalent porphyrins in diarylethyne-linked arrays.15-17 In the present study, we extend our prior work on the photodynamics of oxidized dyads to five additional arrays that differ in the nature of the covalent linker (Chart 3). One array is joined with a biphenylene linker (ZnFbΦ2), which is intermediate in length between the phenylene and diphenylethyne linkers. Another array is joined with a phenylethyne linker (ZnFbE), which is also intermediate in length between phenylene and diphenylethyne linkers, but provides stronger interporphyrin electronic coupling due to direct attachment of the ethyne to the free base porphyrin. The remaining arrays are joined with linkers longer than any of those mentioned above, including diphenylbutadiyne (ZnFbL), terphenylene (ZnFbΦ3), and quaterphenylene (ZnFbΦ4). The studies of the photodynamics of the oxidized ZnFbΦ2, ZnFbΦ3, ZnFbΦ4, ZnFbE, and ZnFbL dyads are accompanied by parallel studies of the neutral forms of the five arrays. Density functional theory (DFT) calculations were also performed on the various linkers to elucidate energies and electron-density distributions of the frontier molecular orbitals. The combined results give a broad assessment of the effect of linker properties on the rate constants for excited-state energy transfer in both the oxidized (Zn+Fb* f Zn+*Fb) and neutral (Zn*Fb f ZnFb*) arrays as well as excited-state hole transfer (Zn+Fb* f ZnFb+*) and groundstate hole transfer (ZnFb+ f Zn+Fb) in the oxidized arrays.
Song et al. Collectively, the results should aid in the design of multiporphyrin architectures for light-harvesting and charge-transfer applications. II. Experimental Methods A. Synthesis. Dyads ZnFbE18 and ZnFbL19 were prepared as described previously. The synthesis of the oligophenylenelinked dyads ZnFbΦ2, ZnFbΦ3, and ZnFbΦ4 will be described elsewhere. B. Physical Studies. The methods for the electrochemical studies and the static and time-resolved optical measurements are the same as those described for our previous studies of other oxidized porphyrin dyads.12 All studies were carried out at room temperature on samples in benzonitrile (PhCN) containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as the supporting electrolyte. The oxidized ZnFbΦ2, ZnFbΦ3, ZnFbΦ4, ZnFbE, and ZnFbL arrays were prepared in a three-compartment spectro-electrochemical cell that contains an appended optical cell through which the electrolyzed solution (held at constant potential and under N2) could be flowed during the transient-absorption measurements. The oxidized dyad showed little or no (
