An Unequivocal Demonstration of the Importance of Nonbonded Contacts in the Electronic Coupling between Electron Donor and Acceptor Units of Donor-Bridge-Acceptor Molecules A. M. Napper, I. Read, and D. H.Waldeck* Chemistry Department, UniVersity of Pittsburgh Pittsburgh PennsylVania, 15260 Nicholas J. Head, Anna M. Oliver, and M. N. Paddon-Row* School of Chemistry, UniVersity of New South Wales Sydney, 2052 Australia ReceiVed February 21, 2000 ReVised Manuscript ReceiVed April 11, 2000 Because of their ubiquity, electron transfer (ET) reactions have received considerable attention over the past few decades. The current view of a superexchange mechanism to treat the electronic interaction for electron-transfer processes in the nonadiabatic limit has been quite successful. Although it is widely believed that covalent linkages between donor and acceptor units provide the dominant pathway for this mechanism, 1 recent work suggests that other pathways involving hydrogen-bonded linkages 2,3 and non- bonded interactions 4,5 can be important. This work assesses the importance of nonbonded contacts by comparing three different unimolecular ET systems that differ by the juxtaposition of a pendant group between the electron donor and acceptor units. This design provides an avenue to quantify the importance of an aromatic moiety’s placement on the electron-transfer rate. The work presents unequivocal evidence that electronic coupling through nonbonded moieties can compete effectively with cova- lent linkages, when the mediating moiety lies between the electron donor and acceptor groups. This study utilizes a U-shaped donor-bridge-acceptor (DBA) dyad in which a pendant moiety (P) is placed between the electron donor and acceptor units by a covalent linkage to the bridge (see the cartoon in Chart 1). Through systematic change of the pendant molecular unit it is possible to demonstrate its importance to the ET and the role of its placement on the efficiency of ET. This approach has several advantages over earlier approaches. First, the moiety that mediates the superexchange interaction (solvent molecule in earlier studies 4,5 ) is clearly located between the donor and acceptor groups. Second, the nature of P can be changed, and a homologous series of DBA molecules can be studied in a single solvent, thereby minimizing any differences in the reaction free energy and outer sphere reorganization energy that may result from solvation changes. These systems also promise an ability to change the geometry of the mediating unit and to investigate how its nuclear dynamics impact the ET. The ET rates of 1-3 in Chart 1 were studied in three different solvents (acetonitrile, dichloromethane, and tetrahydrofuran) as a function of temperature. The general synthetic strategy for these molecules and the specific synthesis of 3 has been reported elsewhere. 6 (See Supporting Information for NMR data.) The molecules in Chart 1 have the same electron donor unit, 1,4- dimethoxy-5,8-diphenylnaphthalene. Molecules 1, 2, and 3 have a 1,1-dicyanovinyl (DCV) acceptor unit, and ET occurs when the naphthalene moiety is electronically excited by 375 nm light. These donor and acceptor units have been used for intramolecular ET studies in the past. 1c Molecules 4 and 5 have a 1,3-dioxolane unit in place of the DCV acceptor. These molecules do not undergo ET and are used as experimental controls. A comparison of the ET rate constant for 1, 2, and 3 provides information on the effectiveness of an aromatic ring for mediating the electronic coupling in the ET, as compared to that of an alkyl unit, and addresses the importance of its placement. The ET rate constant was determined by subtracting the excited-state relaxation rate of the control molecules (4 and 5) from that of the ET molecules (1,2, and 3) (see Supporting Information for more details). The ET rate constants as a function of temperature are shown in Figure 1 for compounds 1, 2, and 3. In each solvent studied the ET rate for 2 is significantly faster than that found for the other compounds. The larger ET rate constant for 2 compared to 3 demonstrates the benefit of placing an aromatic unit between the electron donor and acceptor rather than an alkyl unit. The larger ET rate constant for 2 compared to that for 1 demonstrates the importance of the aromatic unit’s placement between the donor and acceptor groups. Molecular modeling calculations of the molecular geometries of 1 and 2 show that the phenyl ring in compound 2 is in the “line-of-sight” between the donor and acceptor groups (see Figure 2), whereas the phenyl ring in compound 1 is shifted down from the line-of-sight position. 7 The very similar rates for 3 and 1 corroborate this conclusion. In short, the propyl 3 and 2-phenylethyl 1 pendant units are similar with respect to their influence on the ET, but the p-ethylphenyl unit (1) (a) Oevering, H.; Paddon-Row, M. N.; Heppener, H.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. Soc. 1987, 109, 3258. (b) Closs, G. L.; Miller, J. R. Science 1988, 240, 440-447. (c) Paddon- Row, M. N. Acc. Chem. Res. 1994, 27, 18. (2) (a) Berman, A.; Izraeli, E. S.; Levanon, H.; Wang B.; Sessler, J. L. J. Am. Chem. Soc. 1995, 117, 8252. (b) Roberts, J. A.; Kirby, J. P.; Nocera, D. G. J. Am. Chem. Soc. 1995, 117, 8051. (c) de Rege, P. J. F.; Williams, S. A.; Therien, M. J. 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Ed. 1999, 38, 3219. (7) The images in Figure 2 were calculated at the MM2 level. More sophisticated geometry calculations are underway. Preliminary calculations on 2 at the HF3-21G level indicate that the phenyl ring is located on a line of sight between the donor and acceptor, but it is twisted ( ∼70°) from the plane of the imide ring. Chart 1 5220 J. Am. Chem. Soc. 2000, 122, 5220-5221 10.1021/ja000611r CCC: $19.00 © 2000 American Chemical Society Published on Web 05/16/2000