Electroabsorption study of ordered polyfluorene thin films: Origin of oscillatory structure near the bottom of the continuum state

Abstract

The excited-state structure of ordered ͑␤-phase͒ poly͑9,9-dioctylfluorene͒ ͑F8͒ thin films has been investigated by electroabsorption ͑EA͒ spectroscopy. The EA spectra of a number of conjugated polymers, including disordered ͑glassy͒ F8, are well explained by a theoretical essential-state model, which consists of three characteristic excited states and the ground state ͓Y. Kawabe et al., Phys. Rev. B 44, 6530 ͑1991͔͒. However, it is found that the essential-state model fails to explain the EA spectrum of ␤-phase F8 because of the appearance of an oscillatory EA signal near the bottom of the continuum state. From the applied electric-field dependence and the directional dependence of the field and polarization of the light, it has been revealed that the oscillation results from several intrachain excited states, some of which are not observed in glassy F8 because of considerable inhomogeneous broadening. To reproduce the EA spectrum of ␤-phase F8 by a sum-over-states calculation, at least seven excited states, in addition to the ground state, have to be taken into account.

Third-order optical susceptibility of ordered and disordered polyfluorene thin films

“…Interestingly, the crystalline thin film exhibits a considerably higher Im χ (3) value around 4.0 eV owing to interaction between several intrachain excited states lying close to one another [22,23]. Although at this spectral range, F8 has no large absorption band, it still has an absorbance of 0.15 (30,000 cm − 1 ).…”

“…Although its maximum absorbance appears to be small, oscillator strength in the displayed spectral range is conserved between all three phases. The EA spectra of the glassy, crystalline, and β-phase F8 thin films were reported elsewhere [13,22,23]. The χ (3) spectra calculated from the EA spectra are summarized in Fig.…”

Oscillatory Structure in the Electroabsorption Spectrum of π-Conjugated Polymer Thin Films: How to Identify the Franz–Keldysh Oscillation

“…2,3) For those fundamental studies, the electronic structure is of great importance and has been determined for a range of -conjugated polymers. [4][5][6][7][8][9][10][11][12] Electroabsorption (EA) spectroscopy, which monitors small absorption change due to an applied electric field, is the most suitable technique for the determination of the electronic structure because the signal is due not only to dipole-allowed states, but also dipoleforbidden states can be detected. This is in contrast to conventional absorption measurements, in which the latter states are silent.…”

“…Thus, whenever EA measurements are carried out, special attention has been paid to the spectral region near the bottom of the continuum state. [6][7][8][9][10][11][12] However, the FK oscillation has not yet been observed in -conjugated polymers except for single crystals of polydiacetylene derivatives, [15][16][17] which are prepared by solid-state polymerization from single crystals of diacetylene monomers.…”

“…12) The structure is not a FK oscillation, but is due to several higher-lying dipole-allowed and forbidden states that are strongly coupled with one another. Nevertheless, the oscillatory structure shows several features similar to the FK oscillation: (1) The oscillatory structure appears in the spectral region where no characteristic absorption band exists.…”