Yoriko Sonoda

Interface formation between poly(2,5-diheptyl-p-phenylenevinylene) and calcium: implications for light-emitting diodes

The early stages of metal/polymer interface formation between calcium and poly(2,5-diheptyl-p-phenylenevinylene) (PDHPV) have been studied using both X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Charge transfer is observed from the metal atoms to the polymer; as a result the calcium atoms at the interface are ionic, and negative bipolarons appear as the charge-carrying species on the polymer chains. This n-type doping of PDHPV by calcium leads to the appearance of new electronic states in the polymer bandgap. The calcium atoms appear to diffuse into the near surface region of the polymer, rather than forming a well-defined overlayer on the organic films.

Fluorescence Spectroscopic Properties of Nitro-Substituted Diphenylpolyenes: Effects of Intramolecular Planarization and Intermolecular Interactions in Crystals

The steady-state absorption and fluorescence properties of (E,E,E)-1,6-diaryl-1,3,5-hexatrienes (2, aryl = 2-nitrophenyl; 3, aryl = 3-nitrophenyl; 4, aryl = 4-nitrophenyl) have been investigated in solution and in the crystalline state. The solid-state absorption spectra of 2-4 shifted to longer wavelengths than those in solution. A combination of theoretical calculations and single-crystal X-ray structure analyses shows considerable planarization of molecules in the solid state, which is mainly responsible for the spectral red shifts. The effects of intermolecular interactions on the absorption spectra appeared to be relatively small in these crystals. This is consistent with the monomeric origin of the solid-state emission. Molecule 2 was nonfluorescent in all solvents studied, probably due to the efficient nonradiative deactivation from ionic species produced by excited-state intramolecular proton transfer (ESIPT) along the C-H...O-type hydrogen bonds. The fluorescence of 3, observed only in medium polar solvents, originated from an intramolecular charge transfer (ICT*) state, while that of 4 derived from locally excited (LE*) and/or ICT* states depending on the solvent polarity. All three molecules exhibited LE* fluorescence in the solid state. No observation of ICT* emission in crystals strongly suggests the twisted geometries for ICT* (TICT) of 3 and 4 in solution. The measurable fluorescence from crystal 2 can be attributed to the restricted torsional motions in the solid excited state.

Preparation and properties of poly(p-phenylene-1,3,5-hexatrienylene)

Abstract Poly(p-phenylene-1,3,5-hexatrienylene) (1P) was prepared by the Wittig reaction of terephthalaldehyde with bis-ylide derived from trans-1,4-bis(triphenylphosphonium)-2-butene dichloride. The polymer was an orange-red powder with maximum absorption at 407 nm, and its degree of polymerization was found to be 4–9 by g.p.c. analysis. The 1H n.m.r. and i.r. spectral data indicated that 1P predominantly has an all-trans structure in its triene unit. The correlation between the u.v.-vis. absorption maximum and the conjugation length of 1P was investigated, and the photochemical reactivity of 1P was studied by means of u.v.-vis. absorption spectroscopy. The d.s.c. thermogram of 1P showed an exothermic peak at 118°C, which was assignable to crystallization. A pellet of 1P could be doped with iodine and its maximum electrical conductivity was 0.50 S cm−1.

Fluorescence properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatriene (n = 1, 2): effects of internal rotation.

The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λ(a)) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λ(f)) (1, 545 nm; 2, 453 nm) and quantum yields (φ(f)) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λ(ex)), whereas the spectrum of 2 was weakly λ(ex)-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λ(a) = 437-438 nm, λ(f) = 496-505 nm, φ(f) = 0.04-0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck-Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)-CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar-CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.

Experimental and theoretical studies of the electronic structure of substituted and unsubstituted poly(para-phenylenevinylene) (PPV)

Abstract The electronic structure of poly(p-phenylenevinylene) and that of its ring-substituted derivatives, poly(2,5-diheptyl-1,4-phenylenevinylene), poly(2,5-dimethoxy-1,4-phenylenevinylene), and poly(2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene), are studied by Ultraviolet Photoelectron Spectroscopy, UPS, and X-ray Photoelectron Spectroscopy, XPS. It is observed by UPS that the π-bands closest to the valence band edge are strongly affected by the presence of the substituents. The influence of the side groups on the experimental spectra is studied theoretically using the Valence Effective Hamiltonian, VEH, model. Calculations are carried out on isolated polymer chains, including full treatment of the aliphatic side groups. Particular attention is paid to the effect of chain torsion angles on the π-band edge. For the diheptyl derivative, the experimental results can be explained on the basis of side-group-induced torsions of the phenylene rings along the backbone, which influence the π-band widths and contribute to differences in both optical absorption threshold and binding energy of the valence band edge. For the alkoxy derivatives, the side groups cause strong modifications in the shape of the upper two occupied π-bands, which results in significant changes in the electronic density of states.

Preparation and properties of po y(1,4-phenylenevinylene) derivatives

Abstract Poly(2,5-diheptyl-1,4-phenylenevinylene) (Hp-PPV) was prepared by the sulfonium salt pyrolysis procedure. Due to flexible side chains, Hp-PPV was soluble in organic solvents. The molecular weights were Mn=3.8×10 4 and Mw=1.4×10 5 . The ratio of trans to cis olefinic units was found to be 93:7 from 1 H NMR spectrum. Although the peak energy was almost identical to that of PPV, the electrical conductivity was much lower than that of PPV. Poly(1,4-phenylene-1,3,5-hexatrienylene) (PPHT) was also prepared. The product obtained by the Wittig reaction was an orange-red powder and its degree of polymerization was 4–9. The 1 H NMR and IR spectra indicated that the hexatrienylene units in PPHT had predominantly all-trans structure. PPHT could be doped with iodine and the maximum electrical conductivity was 0.5 Scm −1 .

Optical pumped lasing in solution processed perovskite semiconducting materials: Self-assembled microdisk lasing

Optical pumped lasing has been observed in solution-processed perovskite semiconducting materials, such as CH3NH3PbBr3. Square or disk-shaped self-assembled microcavities have been easily obtained and their sizes distribute from 4–30 µm. The square and disk microcavities show clear multimode lasing under pulsed optical pumping. The mode intervals are spread in the case of disk cavities. The observed multimode patterns are compared with the spectra obtained by finite differential time domain calculations. The results can be explained by the whispering gallery mode (WGM) or quasi-WGM in microdisk cavities.

Solvent effects on the photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene

The photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene [(E,E,E)-1] have been studied in various solvents. The fluorescence emission maxima of (E,E,E)-1 show an increasing Stokes shift with increasing solvent polarity. Picosecond time-resolved fluorescence (TF) and transient absorption (TA) spectra do not show any significant time-dependent shifts in nonpolar solvent whereas, in more polar solvents, large red and blue shifts are observed in the TF and TA spectra, respectively. The fluorescence quantum yield reaches a maximum in moderately polar solvents and the quantum yield of intersystem crossing decreases strongly with increasing solvent polarity. Z–E-Isomerization of triene double bonds is inefficient in all solvents. In contrast, the absorption and fluorescence maxima for (E,E,E)-1,6-bis(4-cyanophenyl)hexa-1,3,5-triene [(E,E,E)-2] are practically solvent-independent. When the solvent polarity is increased, fluorescence quantum yield decreases monotonically and Z–E-isomerization quantum yield increases strongly. The results for (E,E,E)-1 can be understood in terms of an additional charge transfer excited state, which is absent for (E,E,E)-2.

Emission behavior of trifluoromethyl bis-styrylbenzene derivative

We measured detailed photophysical properties of 1,4-bis(4-trifluoromethylstyryl)benzene (3PV-CF3). 3PV-CF3 showed a high fluorescence quantum efficiency and a high radiative rate constant. Furthermore, we evaluated the emission behavior of 3PV-CF3 under excitation of femtosecond laser pulses. Microcrystals of 3PV-CF3 were prepared easily by spin-coating, and were confirmed for amplified spontaneous emission. It has a low threshold of 34 µJ cm−2.

(E, E, E)-1,6-bis(2,4-dichlorophenyl)-hexa-1,3,5-triene

The title molecule, C(18)H(12)Cl(4), lies about an inversion centre and the hexatriene chain is planar. The torsion angle of the single bond between the planes of the chain and the benzene ring is -8.6 (3) degrees. The dihedral angle between the planes defined by the chains of adjacent molecules is 50.0 (2) degrees. The shortest intermolecular distance between the Cl atoms is 3.514 (1) A. The molecules are joined through pi-pi-stacking and strong attractive Cl.Cl interactions.