Positive Charge Carriers on Isolated Chains of MEH−PPV with Broken Conjugation: Optical Absorption and Mobility

Electron and hole transport to trap groups at the ends of conjugated polyfluorenes

Polyfluorenes (pF) were synthesized having anthraquinone (AQ) or naphtylimide (NI) end caps that trap electrons or di- p-tolylaminophenyl (APT2) caps that trap holes. The average lengths of the pF chains in these molecules varied from 7 to 30 nm. End capping was found not to be complete in these molecules so that some were without caps. Electrons or holes were injected into these polymers in solution by pulse radiolysis. Following attachment, the charges migrated to the end cap traps in times near 2 ns in pF12AQ or 5 ns in pF35NI. From these observations, electron mobilities for transport along single chains to the end caps in THF solution were determined to be smaller by a factor of 100 than those observed by microwave conductivity. Despite this, the mobilities were sufficiently large to provide encouragement to the use of such single chains in solar photovoltaics. Most charges were observed to transport over substantial distances in these polymers, but 23, 18, and 37% of the charges attached to pFNI, pFAQ, and pFAPT2, respectively, were trapped in the pF chains and decayed by slower bimolecular reactions. For pFAQ and pFAPT2, all of the trapped charges were accounted for by estimates of the fraction of molecules having no end cap traps. For pF35NI, 23% of the attached electrons were found to be trapped in the chains, but only 4% of chains were expected to have no end caps. This could indicate some trapping by kinks or other defects but may just reflect uncertainties in the capping of this long polymer. When the charges reach the trap groups, their spectra have no features of pF(*-) or pF(*+), nor do the principal bands of the trapped ions resemble spectra of the radical ions of isolated trap molecules. The optical absorption spectra are rather dominated by new bands identified as charge-transfer transitions, which probably reinject electrons or holes into the pF chains. The energies of those bands correlate well with measured redox potentials.

Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O(2)) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M(3)(+)) produced via the corresponding dimer radical cation (M(2)(+)) is clearly seen in the solution containing O(2) as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M(3)(+) CR absorption is observed; this is due to the large M(3)(+)/M(2)(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at approximately 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M(3)(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M(3)(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.

Efficient Charge Transport along Phenylene−Vinylene Molecular Wires

We have studied the motion of charge carriers along isolated phenylene-vinylene (PV) chains using a combination of experimental and theoretical methods. The conductive properties of positive charges along PV chains in dilute solution were studied by using the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique. This technique enables the measurement of high-frequency (tens of GHz) charge carrier mobilities along isolated PV chains without the use of electrodes. The charge carrier mobility along PV chains with finite and infinite length was studied theoretically by charge transport simulations with parameters from density functional theory (DFT) calculations. The high-frequency charge carrier mobility is found to depend strongly on the conjugation length of the PV chains and is found to increase both with increasing length of the PV chain and with increasing conjugation fraction. The experimental results are in good agreement with the calculated results. On the basis of this combined experimental and theoretical study an intrachain charge carrier mobility of a few tens of cm2/Vs is expected for an infinitely long PV chain without conjugation breaks.

The effect of broken conjugation on the excited state: Ether linkage in the cyano-substituted poly(p-phenylene vinylene) conjugated polymer poly(2,5,2′,5′-tetrahexyloxy-8,7′-dicyano-di-p-phenylene vinylene)

We investigate the effect of broken conjugation on the excited state dynamics of excimers in cyano-substituted phenylene-vinylene polymers. We compare previous studies on the well-characterized poly(2,5,2',5'-tetrahexyloxy-8,7'-dicyano-di-p-phenylene vinylene) (CN-PPV) with poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethenylene-1,4-phenylene] (CN-ether-PPV), in which the conjugation is disrupted by the insertion of an oxygen atom within the polymer backbone. Despite the broken conjugation, the spectroscopic behavior of the two materials is similar, indicating that the cyano group dominates the photophysics in these materials. The emission in CN-ether-PPV is due to a single-chain exciton in solution and due to an interchain excimer in thin film, as previously reported for CN-PPV; however, the excimer absorption and emission in thin film are blueshifted by approximately 0.2 eV relative to CN-PPV, implying that the excimer in CN-ether-PPV is less stable. Furthermore, substitution of an ether group along the chain results in decay times in both solution and film that are twice as long than in CN-PPV due to the broken conjugation which restricts the exciton within a conjugation segment and reduces its access to internal quenching sites. These properties result in a decay time of 14 ns for CN-ether-PPV film, one of the longest decay times observed in a conjugated polymer film. The long lifetime indicates a large exciton diffusion length, making these species particularly vulnerable to quenching by other materials. This work has implications for the design of conjugated polymers for efficient optoelectronic devices, such as photovoltaics.

High intrachain hole mobility on molecular wires of ladder-type poly(p-phenylenes)

We have studied the high-frequency (34 GHz) mobility of positive charge carriers on isolated ladder-type polymer chains in dilute solution. We find that the high-frequency mobility is limited by the chain ends on chains as long as 35 monomers. The intrachain motion of charge carriers can be described by one-dimensional diffusion between infinitely high reflecting barriers, representing the chain ends. Our data indicate that the intrachain mobility for ladder-type polymer chains is close to 600 cm(2)/V s. With this high mobility the ladder-type polymer is a promising candidate for future use as an interconnecting wire in molecular electronics.

Products of the Triplet Excited State Produced in the Radiolysis of Liquid Benzene

The radiation chemical yields of the products derived from the triplet excited state produced in the radiolysis of liquid benzene with gamma-rays, 10 MeV 4He ions, and 10 MeV 12C ions have been determined. Iodine scavenging techniques have been used to examine the formation and role of radicals, especially the H atom and phenyl radical. For all irradiation types examined here, the increase in hydrogen iodide yields with increasing iodine concentration matches the increase in iodobenzene yields. This agreement suggests that the benzene triplet excited state is the common precursor for the H atom and the phenyl radical. Pulse radiolysis studies in liquid benzene have determined the rate coefficients for the reactions of phenyl radicals with iodine and with the solvent benzene to be 9.3 x 10(9) M(-1) s(-1) and 3.1 x 10(5) M(-1) s(-1), respectively. Direct measurements of polymer formation, which refers to trimers (C18) and higher order compounds (>C18), in liquid benzene radiolysis using gamma-rays, 4He ions, and 12C ions at relatively high doses have been performed using gel permeation chromatography. The yields of trimers increase from gamma-rays to 12C ions due to the increased importance of intratrack radical-radical reactions that can be scavenged by the radical scavenging reactions of iodine. On the other hand, the >C18 product yields decrease from gamma-rays to 12C ions. The structure of the polymer consists of a partly saturated ring as determined by infrared and gas chromatography/mass spectrometry studies. A schematic representation for the radiolytic decomposition of the benzene triplet excited state is presented.

Intrachain Triplet Energy Transfer in Platinum−Acetylide Copolymers

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.

Efficient Emission from Charge Recombination during the Pulse Radiolysis of Electrochemical Luminescent Donor−Acceptor Molecules with an Ethynyl Linkage

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.

Efficient Emission from Charge Recombination during the Pulse Radiolysis of Electrochemical Luminescent Substituted Quinolines with Donor−Acceptor Character

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes and singlet excitation energies of PnQ. The formation of the intramolecular charge transfer state is assumed for PnQ in the singlet excited state with a strong electron donating substituent. The emission from PnQ is suggested to originate from PnQ in the singlet excited state formed from the charge recombination between the PnQ radical cation and the PnQ radical anion during the pulse radiolysis. This is strong evidence for the efficient electrogenerated chemiluminescence of PnQ.

One- and Two-Photon Photochemistry and Photophysics of Poly(arylenevinylene)s Containing a Biphenyl Moiety

Photochemical and photophysical properties were investigated for poly(arylenevinylene)s containing a flexible biphenyl "hinge" unit by applying one-photon (OP) and two-photon (TP) excitation to explore excited-state properties. The poly(arylenevinylene)s were poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biphenylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biphenylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylenevinylene)] (3). Effective emission quantum yields and related photonic properties were evaluated on a realistic per-chromophore basis using effective conjugation lengths based on the Strickler-Berg relationship. Intramolecular photocyclization was deduced to occur in the one case where the biphenyl molecular connectivity permitted the reaction, based on matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF), heteronuclear multiple-quantum coherence (HMQC)-NMR, and gel-permeation chromatography (GPC) results. The various photoprocesses could be induced by either OP or TP excitation, though the first excited singlet state is the photoactive state. The higher excitation energy of the TP excited state favors indirect population of the S, state by electronic coupling between the TP and OP excited states [lambda(max)TPE (nm): 726; delta (GM): 1=229, 2=215, 3= 109). Photochemical processes occurring from the lowest OP excited state (S1) could therefore also be indirectly induced by TP excitation.