M. Schott

Structural studies of parasexiphenyl thin films: Importance of the deposition parameters

Abstract Parasexiphenyl was synthesized by electroreduction of monobromoterphenyl in the presence of a Ni catalyst and purified by vacuum sublimation and condensation in a temperature gradient. Thin films can be processed by vacuum evaporation under controlled deposition conditions, which play a major role. A high substrate temperature (470 K), a very low deposition rate (10−2 nm s−1) and a film thickness around 350 nm allow one to obtain the best crystallinity. Two textured crystalline phases are found, differing mainly by the c parameter length, possibly due to a slightly different tilt angle of the molecule along the c axis of the monoclinic unit cell. A preferred orientation shows up under these experimental conditions, the molecular axis being perpendicular to the substrate surface. Consequences of the texture and of the molecular packing in the unit cells on the vibrational and electronic absorption spectra are presented.

On singlet exciton fission in anthracene and tetracene at 77°K

Abstract Relative variations of prompt flourescence yield, on application of a magnetic field are studied at 77°K for anthracene and tetracene crystals excited by light with wavenumbers up to 50,000 cm -1 . The results obtained for anthacene, as compared to those previously reported at 300°K, indicate a very small temperature dependence. The variations observed for tetracene at 77°K are comparable in magnitude to those for anthracene. The singlet exciton fission process, responsible for the experimental observations, does not involve a thermally relaxed lowest bound or charge transfer singlet exciton. The possible role of upper excited vibronic states is discussed.

Magnetic field effect on prompt fluorescence in anthracene: evidence for singlet exciton fission

Abstract Prompt fluorescence in anthracene is shown to be modified by the application of a magnetic field: characteristic variations of the relative fluorescence quantum yield with field intensity and orientation together with their dependence on the excitation wavelength are reported. The results indicate that the radiationless transitions from the initially excited upper molecular states to the fluorescing levels involve an intermediate singlet state which can dissociate into a triplet pair.

Effects of exciton interactions on the fluorescence yield of crystalline tetracene

Abstract The relative fluorescence yield of crystalline tetracene as a function of the intensity of weakly absorbed light has been measured. The yield increases, due to the contribution of delayed fluorescence, with increasing intensities by a factor of 2.90 from the prompt fluorescence value. The value inferred for the total triplet-triplet annihilation rate constant is γ TT = (2.2±0.5)×10 −9 cm 3 sec −1 , and for the ratio of the rate constant of triplet fusions generating singlets to γ TT is f = 0.66±0.06. This last value shows that the triplet channel is significantly less important than predicted by simple spin statistics. At higher intensities the yield decreases with increasing intensities. The effect is attributed to quenching of singlets by singlet-triplet annihilation yielding an estimate of γ ST ≈ 2 × 10 −7 cm 3 sec −1 for the rate constant of the interaction.

The lowest triplet state of a diacetylene

Abstract Excitation and emission spectra of single crystals of (C 6 H 5 ) 3 Xue5f8C ue5fcCue5f8Cue5fcCue5f8X (C 6 H 5 ) 3 where X = Sn or Pb have been investigated. The lowest triplet state is an excitation of the diacetylene moiety and lies at 25325 ± 5 cm −1 . Energy transfer occurs from the triplet of the benzene to that of the diacetylene.

Dielectric properties of single crystals of the substituted diacetylene pTS: effect of the solid-state polymerization and phase transitions

Abstract Dielectric permittivities of the polymerizable organic solid, p TS diacetylene have been measured between 115 and 330 K in the directions parallel and perpendicular to the direction of polymer chain growth. The upper phase transition in monomer, polymer and mixed crystals at various stages of the solid-state polymerization manifests itself as a maximum in the temperature dependence of e measured in the direction parallel to the molecular stacks, being particularly well pronounced in fully polymerized crystals. The transition was identified as an antiferroelectric one, the sublattice polarization being most probably the order parameter. The lower phase transition could be observed only in monomer and monomer-rich crystals as a shoulder on the e( T ) dependence. This transition could be detected only in the crystals containing less than ≈ 10–15% of polymer. The dielectric permittivity was found to be independent of frequency up to 3 GHz. The polymerization results in changes of the dielectric permittivity. In samples where the direction of measurements coincides with the b axis, these changes follow the monomer-polymer conversion curve.

SPECTROSCOPY OF LOW-ENERGY ELECTRONS BACKSCATTERED FROM AN ORGANIC SOLID SURFACE: PENTACENE

Abstract The energy spectrum of electrons backscattered from polycrystalline pentacene films has been measured for incident elecron energies Ei between 5 and 40 eV, with an energy resolution ≈150 meV and an angular resoluton ≈5°. Energy losses ΔE corresponding to the excitation in the film of intramolecular vibrations, triplet and singlet excitons, and σ electrons, have been found, as well as “true secondaries”. The dominant loss at small Ei corresponds to excitation of backbone vibrations; triplet excitons are readily excited at 5

Studies on a polymerizable crystal. II. Microcalorimetric study of polymerization and polymorphism of pTS

Abstract The thermal polymerization of single crystals of p TS [bis( p -toluene sulphonate of 2,4-hexadiyne 1,6-diol] has been studied by DSC. The total heat of polymerization is 37.1 ± 0.4 kcal/mole at 323 K. The specific heats of monomer and polymer p TS have been measured from 320 to 360 and 385 K respectively and used to calculate heats of polymerization at other temperatures in that range. Isothermal polymerization has been studied from 333 to 363 K. The activation energies are different in the autocatalytic region (22.5 kcal/mole) and at the beginning of the induction period (25.5 kcal/mole), so that the ratio of the corresponding rate constants decreases from over 300 at 333 K to 200 at 360 K. The pre-exponential factors, or activation entropies, in the two regimes are equal. The melting of p TS has also been studied by DSC. There are always two endotherms, with maxima at = 92 and 94°C, in the pure monomer. The 92°C endotherm is more important in microcrystals obtained by precipitation or grinding and decreases in intensity without shifting in T , on annealing; this suggests the existence of a second, metastable, phase, less thermally reactive than the normal one.

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

Abstract The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ tot = (5.5 ± 2.0) × 10 −12 cm 3 s −1 , for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ 1 tot = (1.0 ± 0.4) × 10 3 cm s −1 . The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.

Polydiacetylene single crystal thin films

A method for obtaining single crystal thin films of polydiacetylenes using electron irradiation is described. Film thickness can be controlled by adjusting the incident electron energy. Typical thicknesses between 100 and 2000 A for a sample surface of a few tenths of cm2 are easily obtained. Such films may find application in nonlinear optics.