C. P. An

Spectroscopic studies of photoexcitations in regioregular and regiorandom polythiophene films

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.

Delocalized polarons in self-assembled poly(3-hexyl thiophene) nanocrystals

Abstract We have studied the characteristic properties of charged excitations (polarons) in thin films of regioregular poly(3-hexylthiophene) (RRPHT) by optical spectroscopies. We found that the increased interchain coupling that exists in self-assembled lamellae in these films drastically changes the properties of the polaron excitations. The traditional self-localized polaron in one-dimension delocalizes in two-dimensions, resulting in a much-reduced relaxation energy and multiple absorption bands, the most dominant being in the mid-infrared (IR) spectral range. Also it has associated IR and Raman active vibrations with reverse absorption bands [anti-resonances (AR)] compared with the continuum electronic band. The AR are attributed to the electron–phonon interaction.

Raman scattering study of electrochemically doped single wall nanotubes

Sheets of single wall carbon nanotubes (SWNT) bundles were studied by Raman scattering spectroscopy when doped electrochemically in an aqueous NaCl solution. At non-resonant conditions, the radial breathing mode (RBM) and tangential displacement modes (TDM) change very little intensity and frequency with electrochemical doping. Resonant Raman scattering for the TDM band, however, show large changes when applying or negative voltages. We conclude that the electronic resonance condition disappears due to larger change of the electronic occupancy with electrochemical doping.

Optical studies of photoexcitations of poly(9,9-dioctyl fluorene)

We have studied the interplay between the morphology and photophysics of poly(9,9-dioctyl fluorene) (PFO) films. Spin-casting from solution produces a glassy film with a broad absorption band and a large Stokes shift due to exciton migration. Upon slowly warming a sample from 80 to 300 K, a fraction of the sample converts into a new phase with extended conjugation, which we refer to as the β-phase. Its absorption band exhibits a clear vibronic structure, and the fluorescence red-shifts by approximately 0.1 eV. The photoinduced absorption (PA) spectrum of the glassy phase is dominated by triplet excitons, whereas, both charged polarons and triplet excitons are seen in the PA spectrum of a sample containing both phases.

FTIR studies of charged photoexcitations in regio-regular and regio-random poly(3-alkylthiophene) films

Abstract We have studied the low energy photo-induced absorption (PA) band of photo-generated polarons in a variety of poly(3-alkylthiophene) (P3AT) films which including regio-regular and regio-random stereo orders, different length of alkyl side group, and various film preparation techniques. In addition to the traditional one-dimensional (1D) polaron PA bands, we observed quasi two-dimensional excitations (DP) in regio-regular P3AT films, where the polymer chains self-organize to form 2D lamella structures. Due to increased interlayer and interchain coupling, the two plaron energy levels split by 2 Δ (≈2 t ⊥ ), resulting in two allowed optical transitions where the lower energy band (DP 1 ) dominates and red-shifts into the mid infrared range. The DP 1 band blue-shifts when the length of the alkyl side group increases, or when using lower boiling point solvent, or when preparing films at higher temperature.