Anilide Linker Group as a Participant in Intramolecular Electron Transfer Guilford Jones, II,* and Ding-Xue Yan Department of Chemistry and the Center for Photonics Research, Boston UniVersity, Boston, Massachusettes 02215 Scott R. Greenfield, ² David J. Gosztola, ² and Michael R. Wasielewski* ,²,‡ Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: February 10, 1997; In Final Form: April 28, 1997 X The photochemical properties of derivatives of 10-methylacridinium ion that have been modified by substitution with anilide moieties (e.g., -C 6 H 4 NHCO-) at the 9-position have been examined. Intramolecular electron transfer involving the anilide group as electron donor and the acridinium ring as electron acceptor was verified by observation of the quenching of fluorescence associated with the local acridinium chromophore and by the recording of electron transfer phototransient spectra in the picosecond time domain. The amidobiphenyl linkage was a superior electron donor and displayed rapid forward and reverse photoinduced electron transfer with rate constants for the latter reaching 5.2 × 10 11 s -1 (CH 3 CN solvent). The combination of amide bonds and benzene rings has provided a common linkage (e.g., an anilide spacer, -C 6 H 4 - NHCO-) for a variety of systems in which electron or energy transfer agents are tethered together. 1,2 This assembly proves to be versatile in terms of synthesis of complicated arrays, since donors and acceptors can each carry unique amine and car- boxylic acid functions that are reliably connected via condensa- tion. Linkages involving porphyrins and other chromophores or redox active groups have been prominent among the examples of linked donor-acceptor systems that display this synthetic strategy. These arrays are frequently prepared for the study of photoinduced charge separation that in most cases occurs across the anilide link. The capacity of the amide link to function as a redox auxiliary can be assessed in part in terms of single- parameter substituent constants, which reveal that the NHCOR group is moderately electron withdrawing [0.22 and 0.08 for Hammett σ (meta) and σ (para) constants and 1.68 for the Taft σ* parameter 3,4 ]. Dual parameter analyses that take into account the electrostatic and covalent contributions to substituent constants 5 reveal a dominant “covalent” constant (ΔC x ) 0.26), indicative of moderate electron donating capability for NH- COCH 3 . Consistent with this finding is the reduction of the ionization potential of benzene (9.2 eV) on introduction of the amide group in acetanilide (8.4 eV) 6 which shows a pronounced stabilizing influence for NHCOR in radical cation formation. As part of a photochemical study of linkages that involve the 10-alkyl acridinium ion as electron acceptor and a variety of substituent groups at the 9-position, 7,9 we have employed the benzanilide group (-C 6 H 4 NHCO-) as a molecular spacer. During these investigations, we have established that this common link can serve as a discrete electron donor. For a series of rigidly linked systems, we have determined the dependences of electron transfer rates on solvent and on changes in structure for the 9-aryl group, features that allow a better understanding of this linkage and its potential for photochemical reaction. The anilide link results in surprisingly fast nonradiative deactivation of excited species via reversible electron transfer. The results bear on the problem of the effectiveness of various bridges, particularly those carrying aromatic rings and π conjugation, in providing long-range communication for electron transfer via charge hopping or superexchange mechanisms. 10 Experimental Section Materials and General Procedures. Silica gel 40 μm (230- 400 mesh, Baker) was employed as stationary phase for flash column chromatography (typically 2.5 cm (ID) × 32 cm for a 0.5-1.0 g of sample of crude product). Solvents composed of 5-30% acetone in chloroform, which gave an R f for product of 0.2-0.3 on TLC (silica gel 60 F254), were used for elution of the acridinium salts. Half-wave potentials for acridinium PF 6 - salts were determined by cyclic voltammetry (nitrogen- purged acetonitrile solutions at 22 °C with tetrabutylammonium perchlorate serving as supporting electrolyte) using an EG&G Princeton Applied Research model 273 potentiostat/galvanostat and a platinum bead working electrode. Electron densities and optimum geometries were obtained by computation using the MOPAC algorithm (QCPE program no. 455, version 6.0) on a Silicon Graphics 4D/340 computer. A total charge of +1 was specified for calculations for ground (S o ) and first excited singlet (S 1 ) states for each acridinium species using the MNDO Hamiltonian (limited configuration interaction, four configura- tions, for S 1 calculations). For optimized geometries, the dihedral angles made by aromatic ring planes were found to be approximately 90° ((10°). Corrected emssion spectra were recorded on an SLM 48000 phase-shift spectrofluorimeter. Fluorescence quantum yields ² Chemistry Division. ‡ Department of Chemistry. X Abstract published in AdVance ACS Abstracts, June 15, 1997. 4939 J. Phys. Chem. A 1997, 101, 4939-4942 S1089-5639(97)00518-5 CCC: $14.00 © 1997 American Chemical Society