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Two poly(arylene ethynylene)s (PAEs) that are end-capped with R-terthiophene (T 3 ) groups were prepared and structurally characterized by proton NMR, GPC, and optical spectroscopy. One of the polymers (T 3 PPE 13 ) features a backbone structure that alternates phenylene ethynylene and bis(alkoxy)phenylene ethynylene repeat units. The second T 3 end-capped polymer (T 3 PBpE 12 ) features an alternating structure with biphenylene ethynylene and bis(alkoxy)phenylene ethynylene repeats. The absorption spectra of the T 3 end-capped polymers are almost the same as those of the corresponding "parent polymers" (PPE 164 and PBpE 21 , respectively) that lack the T 3 end-groups. By contrast, whereas the fluorescence spectra of the parent polymers is dominated by a blue fluorescence with λ max ) 425 nm, the emission spectra of the end-capped polymers contains a significant contribution of a green fluorescence (λ ) 500-550 nm). This signals that the singlet exciton is efficiently trapped by the T 3 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 (λ ) 600 nm) and the second in the near-IR (λ ) 1600-2000 nm). The cation radicals of the T 3 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 T 3 end-capped polymers show important differences. Specifically, the cation radical spectra of T 3 BpE 12 and PBpE 21 are identical, which reveals that the hole is not trapped by the T 3 end-cap in the biphenylene polymer. By contrast, the cation radical absorption spectra of T 3 PPE 13 (λ max ) 640 and 1350 nm) and PPE 164 (λ max ) 600 and 1950 nm) are distinctly different. This difference suggests that the hole is localized on the T 3 end-group in T 3 PPE 13 . Bimolecular hole-transfer experiments using bithiophene (T 2 , E°o x ) 1.21 V), terthiophene (T 3 , E°o x ) 0.91 V), and quaterthiophene (T 4 , E°°o x ) 0.76 V) with PPE 164 and PBpE 21 allowed the determination of the oxidation potentials for the PAEs. The values are PPE 164 , E°o x ) 0.91 V; PBpE 21 , E°o x ) 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 T 3 end-groups in T 3 BpE 12 . The dynamics of intrachain hole transfer in T 3 PPE 13 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 k HT g 1 × 10 8 s -1 .

A series of platinum-acetylide homo- and copolymers was prepared and characterized by using photophysical methods. The polymers feature repeat units of the type [trans-Pt(PBu3)2(-CC-Ar-CC-)], where Ar = 1,4-phenylene (P) or 2,5-thienylene (T). The properties of homopolymers that contain only the 1,4-phenylene or 2,5-thienylene repeat units were compared with those of random copolymers having the structure -[-(Pt(PBu3)2(-CC-T-CC-))x-(Pt(PBu3)2(-CC-P-CC-))(1-x)-)] where x = 0.05, 0.15, and 0.25. Absorption and photoluminescence spectroscopy demonstrates that the singlet and triplet excitations localized on 1,4-phenylene units are higher in energy relative to those localized on the 2,5-thienylene units. The mechanism and dynamics of intrachain triplet energy transfer from 1,4-phenylene to the 2,5-thienylene repeats were explored in the copolymers. Photoluminescence and nanosecond transient absorption spectroscopy indicate that at room temperature P --> T energy transfer is efficient and rapid (k >> 10(8) s(-1)), even in the copolymer that contains only 5% 2,5-thienylene repeat units. At 77 K, steady-state and time-resolved photoluminescence spectroscopy reveals that triplet energy transfer is much less efficient and a fraction of the triplet excitations is "trapped" on the high-energy 1,4-phenylene units. Intrachain energy transfer is believed to occur by two mechanisms, one involving P --> T singlet energy transfer followed by intersystem crossing, whereas the other involves intersystem crossing prior to P --> T triplet energy transfer. The relationship between the observed energy transfer efficiencies and mechanisms in the copolymers is discussed.

Triplet states of poly(phenylene ethynylene), (3)PPE(*), not easily formed by direct photoexcitation, were produced by pulse radiolysis in toluene, along with triplet states of T(3)PPE having terthiophene end-caps. Intense triplet-triplet absorption maxima, epsilon(680)((3)PPE(*)) = 9.5 x 10(4) M(-1) cm(-1) and epsilon(780)((3)T(3)PPE(*)) = 2.8 x 10(4) M(-1) cm(-1) enable identification of these two species, which have triplet energies of 2.12 and 1.77 eV determined in bimolecular energy transfer equilibria. Bleaching of ground-state absorption measures (3)PPE(*) to be the delocalized over a 1.8-nm length. Triplet states formed in the PPE chains were transported to and trapped by the end caps in a time <<5 ns.