Alison M. Funston, Eric E. Silverman, John R. Miller, and Kirk S. Schanze
J. Phys. Chem. B, 108, 1544 - 1555 (2004). [Find paper at ACS Publications]
Abstract:
Two poly(arylene ethynylene)s (PAEs) that are end-capped with a-terthiophene (T3) groups were prepared and structurally characterized by proton NMR, GPC, and optical spectroscopy. One of the polymers (T3PPE13) features a backbone structure that alternates phenylene ethynylene and bis(alkoxy)phenylene ethynylene repeat units. The second T3 end-capped polymer (T3PBpE12) features an alternating structure with biphenylene ethynylene and bis(alkoxy)phenylene ethynylene repeats. The absorption spectra of the T3 end-capped polymers are almost the same as those of the corresponding "parent polymers" (PPE164 and PBpE21, respectively) that lack the T3 end-groups. By contrast, whereas the fluorescence spectra of the parent polymers is dominated by a blue fluorescence with lmax = 425 nm, the emission spectra of the end-capped polymers contains a significant contribution of a green fluorescence (l = 500-550 nm). This signals that the singlet exciton is efficiently trapped by the T3 end groups. Pulse radiolysis studies were carried out on all of the poly(arylene ethynylene)s in an effort to characterize the spectra and dynamics of the cation and anion radicals of the polymers. Pulse radiolytically generated solvent holes, and solvated electrons were transferred to the PAEs at nearly diffusion controlled rates. The absorption spectra of the anion radicals of the PAEs were similar and featured two strong absorption bands, one in the visible (l = 600 nm) and the second in the near-IR (l = 1600-2000 nm). The cation radicals of the T3 end-capped polymers also feature two absorption bands, one in the visible and the second in the near-IR. However, the spectra of the cation radicals of the T3 end-capped polymers show important differences. Specifically, the cation radical spectra of T3BpE12 and PBpE21 are identical, which reveals that the hole is not trapped by the T3 end-cap in the biphenylene polymer. By contrast, the cation radical absorption spectra of T3PPE13 (lmax = 640 and 1350 nm) and PPE164 (lmax = 600 and 1950 nm) are distinctly different. This difference suggests that the hole is localized on the T3 end-group in T3PPE13. Bimolecular hole-transfer experiments using bithiophene (T2, E°ox = 1.21 V), terthiophene (T3, E°ox = 0.91 V), and quaterthiophene (T4, E°°ox = 0.76 V) with PPE164 and PBpE21 allowed the determination of the oxidation potentials for the PAEs. The values are PPE164, E°ox = 0.91 V; PBpE21, E°ox = 0.85 V (all potentials vs SCE). The lower oxidation potential of the biphenylene based PAE explains why the hole is not trapped by the T3 end-groups in T3BpE12. The dynamics of intrachain hole transfer in T3PPE13 are much faster than the rate of hole transfer from the solvent, and on the basis of this result, the lower limit for intrachain hole transfer is determined to be kHT >= 1 × 108 s-1.