Delocalization of Free Electron Density through Phenylene-Ethynylene: Structural Changes Studied by Time-Resolved Infrared Spectroscopy 1 Dmitry E. Polyansky, Evgeny O. Danilov, Sergey V. Voskresensky, Michael A. J. Rodgers, and Douglas C. Neckers* Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403 Received May 12, 2005; E-mail: neckers@photo.bgsu.edu Organic compounds with the phenylene-ethynylene motif appear suited for engineering molecular electronic devices, in that electronic conduction through oligo phenylene-ethynylene molecular wires was demonstrated, 2 prominent electrical rectifying current-voltage dependencies and negative differential resistances were observed, 3,4 and approaches to conductance switching and current control have been proposed. 5,6 Such conductive properties have been directly attributed to extended π-conjugation along the linear backbone and the relative orientation of the phenylene rings. One electron reduction triggers conformational change, trans- forming the phenylene-ethynylene backbone into a conductive medium and at the same time providing a charge carrier for current flow through the system, 5 implying that in the singly reduced state the wave function is delocalized over the conjugated core of the planar molecule, whereas it has a localized character in alternating conformations. 7 Bridge-mediated charge transfer was proposed to explain the distance dependence of electronic coupling along the molecule, 2,8,9 although evidence suggested that stochastic switching of the molecular wire was imperceptibly changed from replacement of a central ring proton by a nitro group. 5 One of the alternative theoretical models posited to rationalize the conductivity of phenylene-ethynylenes and a weak dependence on the substitution involved resonant tunneling through the central ring as a barrier. 10 Another assumed an influence of molecular vibrations on the conductance. 11 Both models imply that conductance depends primarily on the internal conformational twist of the aromatic rings. Time-resolved picosecond resonance Raman spectra (λ ex ) 267 nm) of 1,4-bis(phenylethynyl)benzene examined conformational and structural details in the ground- and the excited states. 12 Since the phenyls of the phenylene-ethynylene chromophore have a low rotational potential barrier in the ground state and a higher barrier in the excited state, 13 photoexcitation was presumed to provide transient control over conformation. An observed red-shift in the -CtC- vibrations suggested a weakening of the triple bond. To test electron communication in conjugated phenylene- ethynylenes, we have synthesized compounds capable of producing “long-lived” 14 single-electron species, and studied these by transient absorption spectral methodologies (see Figure 1). Thus, 1,4-bis(2- [4-tert-butylperoxycarbonylphenyl]ethynyl)benzene (1) and tert- butyl-4-(2-{4-[2-(4-phenyloxycarbonylphenyl)-1-ethynyl]phenyl}- 1-ethynyl)peroxybenzoate (2) upon excitation by a 10-ns laser pulse at 355 nm, rapidly form aroyloxyl radicals 3 and 4, and these can be studied by time-resolved IR (TRIR) spectroscopy, 15 see Chart 1. An observed red-shift in their -CtC- vibrational frequencies and an intensity increase following excitation indicates substantial concomitant changes in molecular structure, proposed to result from changes in triple bond character due to the formation of a partial cumulene-like structure resulting through delocalization of the free electron. This allows comparison of the radical species observed to the excited singlet state of the phenylene-ethynylene chro- mophore. The only stable products of photolysis of 1 and 2 were 1,4-bis- (4-chlorophenylethynyl)benzene and carbon dioxide. TRIR spectra of chloroform solutions of 1 and 2 after the 355-nm laser pulse featured ground-state depletion around 1750 cm -1 , transient absorp- tion around 2112 cm -1 , and formation of a stationary absorption at 2335 cm -1 (see Supporting Information for details). Ground-state absorption bleached within the instrument response time (ca. 30 ns) giving rise to the 2112 cm -1 absorption. This transient band decayed concomitantly with the growth of carbon dioxide absorption at 2335 cm -1 , indicating species providing this absorption to be direct precursors of carbon dioxide. Purging oxygen through the solution did not decrease the lifetime of the 2112 cm -1 transient, a result consistent with the expected slow trapping of aroyloxyl by molecular oxygen. The lack of reactivity of the observed transient to oxygen excludes its assignment to the triplet excited state, while the lifetime of the singlet excited state of the Figure 1. Time evolution of transient FTIR spectra of 1 and 2 acquired after 355 nm laser pulse. Chart 1 Structures of Peroxyesters 1 and 2 Published on Web 09/08/2005 13452 9 J. AM. CHEM. SOC. 2005, 127, 13452-13453 10.1021/ja053120l CCC: $30.25 © 2005 American Chemical Society