Thomas J. Barton

Two-photon absorption spectra of luminescent conducting polymers

Abstract We report the two-photon absorption (TPA) spectra of poly(2,5-dibutoxy-p-phenylene acetylene) (PPA-DBO), poly(2,5-dioctyloxy-p-phenylene vinylene) (PPV-DOO), and poly(3-hexylthiophene) (PAT6) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution, we measured a strong two-photon allowed transition in all three materials which we attribute to the mAg essential state. The mAg in all three polymers occurs at, approximately, 1.2 times the optical bandgap, Eg [E(1Bu)]. The excitonic nature of the Ag-character states is indicated by the lack of a TPA band at or near Eg. Calculated χ(3) spectra, using the mAg state, two additional states above the 1Bu exciton, and a lineshape function to account for chain length distribution, exhibit the same spectral features observed in the TPA spectra.

Photoluminescence, electroluminescence, and optically detected magnetic resonance study of 2,5-dialkoxy derivatives of poly(p-phenyleneacetylene) (PPA) and PPA-based light-emitting diodes

Abstract The photoluminescence (PL) (i.e., optically)-detected magnetic resonance (ODMR) of the 2,5-dialkoxy derivatives of poly(p-phenyleneacetylene) (PPA) films and blends, and the electroluminescence (EL), conductivity (σ)-, and EL-detected magnetic resonance of PPA-based LEDs is described. The weak vibronic structure of the intense PL peaking at ∼1.95 eV and the broad tail extending beyond 800 nm are consistent with a relatively high degree of disorder. The ODMR spectra are similar to those of poly(3-alkylthiophenes) (P3ATs) and poly(p-phenylenevinylenes (PPVs), and attributable to magnetic resonance enhancement of singlet exciton generation by either polaron-polaron or triplet-triplet fusion. The broad, nearly symmetric half-field triplet ODMR is also consistent with considerably greater disorder than in P3ATs or PPVs. The EL spectrum of PPA-based diodes also reveals considerable disorder. They yeild intense EL-quenching but σ-enhancing resonances at g ≈ 2.002, which are discussed in terms of polaron-to-bipolaron decay, and half-field EL- and σ-detected resonances, assigned to triplet-triplet exciton fusion.

Synthesis and study of a polysilole

Abstract The thermal polymerization of Ph 2 Si(C≡CH) 2 reported in 1968 by Luneva et al. has been reexamined and spectroscopic analysis reveals that it does not produce the claimed structure. Different synthetic procedures yeild either a deep red polymer of molecular weight (MW ∼ 55,100 or a violet polymer of MW ∼ 105,300. Structural models and spectral examination of both systems strongly suggest that they are a polysilole of the form: If this structure is correct, this is the first example of a polymetallole with the heteroatom devoid of lone-pair electrons. The red polymer remains insulating even after doping with AsF 5 or I 2 . The conductivity of the intially semiconducting violet polymer ( σ ∼ 10 −7 S/cm), however, increases by four orders of magnitude upon doping with I 2 . Photoabsorption, photoluminescence, and ESR measurements are also described and discussed. The results suggest that this polysilole is intrinsically an insulator.

Generation and trapping of 6,6-dimethyl-6-silafulvene

Abstract Flow pyrolysis of allylcyclopentadienyldimethylsilane and of 1-dimethylmethoxysilyl-1-trimethylsilylcyclopentadiene afforded 6, 6-dimethyl-6-silafulvene which was either trapped or allowed to dimerize.

Pyrolysis of 1,1,2-trimethyl-1-silacyclobutane. Site of initial ring cleavage

Abstract The gas-phase, flow pyrolysis of 1,1,2-trimethyl-1-silacyclobutane (I) is described. A total of six products, containing two silicon atoms, have been analyzed with respect to relative yields (at 520°, 570°, 620° and 680°) and mechanistic origin. It is concluded that thermolysis of I occurs with predominant initial cleavage of the carboncarbon bond rather than the siliconcarbon bond and that further cleavage affords a silaalkene, Me 2 Si=CHCH 3 . The pyrolysis of 1,1,3,3-tetramethyl-1,3-disilacyclobutane at 700° leads to silaalkene production as shown through trapping with benzaldehyde.

Femtosecond dynamics of the nonlinear optical response in polydiethynylsilane

We have measured the femtosecond dynamics of the nonlinear optical response χ(3) in polydiethynylsilane (C4H2SiBu2)x, a novel class of π‐conjugated polymers incorporating Si, using degenerate four‐wave mixing and photoinduced absorption techniques. In resonance conditions at 620 nm we found χ(3)=3×10−9 esu, which decays in a record time of 135 fs, followed by a slower decay component of 750 fs, without a long tail even at high laser intensities. This material is an excellent candidate for nonlinear optical devices in the sub‐THz frequency range.

Unexpected chemistry in an attempted generation of a sterically hindered silene

Abstract In an attempt to synthesis a silene, stabilizer both electronically and by extreme steric bulk, two things of considerable interest were discovered: (1) perchlorate can function as a uniquely effective leaving group for alkylation on silicon, and (2) stable αhalosilyl carbanions can be generated through the aid of steric bulk.

Synthesis and thermochemistry of 4-diazo-1,1-dimethyl-1-silacyclohexa-2,5-diene

Abstract A vastly improved synthesis of 1,1-dimethyl-1-silacyclohexa-2,5-diene-4-one is described. Direct conversion of the ketone to the diazo derivative is easily accomplished, but a simpler method, not involving the ketone, is presented for synthesis of the title compound. In an attempt to observe rearrangement of the diazo system, pyrolysis under a variety of techniques was performed. No evidence of formation of a silabenzene was obtained.

Electrically Symmetric Poly(Phenylene Acetylene) Diodes

Abstract A diode has been fabricated with poly(phenylene acetylene) [PPA] as the electroluminescent polymer. The device exhibited an unusual symmetric (positive and negative bias) I-V characteristic and electroluminescent output. These experimental results are discussed in terms of tunneling of electrons and holes via localized states.

Conductivity, optical absorption, photoluminescence, and X-band optically detected magnetic resonance of novel poly(2,5-dibutoxyparaphenyleneacetylene) (PDBOPA)

Abstract The optical properties of novel PDBOPA (average molecular weight ≈ 48 000) are described and discussed. Exposure to I2 vapor at ≈80 °C increases the conductivity to ≈5 × 10−3 S/cm, but the I2 rapidly evolves from the film at room temperature. The absorption onsets at 2.53 eV and peaks at 2.81 eV. The 2.71 eV-excited photoluminescence (PL) peaks at ≈2.0 eV in films and solutions. In all cases, it is very intense and largely structureless. This contrasts with the clear vibronic structure of the PL of poly(3-alkylthiophene) (P3AT) and poly(2,5-dialkoxyparaphenylenevinylene) (PDOPV) films and blends. As in P3AT and PDOPV, three PL-enhancing optically detected magnetic resonance (ODMR) features are observed: (i) a strong narrow (≈15 G wide) resonance at g = 2.0025, attributed to intrachain polaron recombination; (ii) a ≈1.0 kG wide triplet powder pattern around g ∼ 2, and (iii) the Δms = 2 transitions of these excitons at g≊4.07 . The results are discussed in relation to conformational defects resulting from the near cylindrical symmetry of the backbone acetylene units.