Static and dynamic structural memory in polyaniline thin films.

Steady-state UV-visible and FTIR spectroscopies were used to characterize the electronic and structural changes that occur in polyaniline (PANI) thin films over the course of a single deprotonation and reprotonation cycle. The dedoping from the emeraldine salt (PANI-ES) to the emeraldine base (PANI-EB) form was achieved by treatment with a weak base (ammonia gas), and the PANI-ES was recovered by exposure to humid air and then dry air. The spectroscopic changes were classified into two general categories: those in which the recovered sample features were intermediate to the initial PANI-ES and the deprotonated PANI-EB and those in which the recovered sample features changed monotonically from the starting PANI-ES toward a unique observable. Two-dimensional IR vibrational echo spectroscopy (2D-IR VES) was then used to demonstrate that ultrafast structural dynamics on the time scales of hundreds of femtoseconds to a few picoseconds could also be organized into these two categories. In contrast, it was found that the slower dynamics on the tens of ps time scale appear unperturbed by the dramatic structural changes of the dedoping-redoping cycle. We discuss the relevance of these dynamics to charge mobilities in the initial and final PANI-ES states and compare their behavior to the film electrical resistances over the course of the protonation cycle. We show that specific structural dynamics are correlated with changes in the film conductivities and that PANI films have a memory of not only the static molecular structures of the as-cast materials but also some of the dynamics that are inherent to those morphologies.

Ground-state structural dynamics in doped and undoped polyaniline films probed by two-dimensional infrared vibrational echo spectroscopy.

Two-dimensional infrared vibrational echo spectroscopy (2D-IR VES) provides information about the structural dynamics occurring on the ultrafast time scale, a temporal regime that is comparable to that of charge-hopping events in conducting polymer films. In this study, 2D-IR VES is used to study polyaniline (PANI) thin films in three states of varying conductivity: emeraldine base (PANI-EB, semiconducting), emeraldine salt (PANI-ES) doped with dinonylnaphthalene sulfonic acid (conductive), and PANI-ES doped with camphor sulfonic acid (highly conductive). UV-visible and FTIR spectroscopies were used to characterize the static electronic and structural differences between these materials, and then these results were compared to the dynamical results from 2D-IR VES. The electronic ground state ultrafast dynamics for the PANI-EB reveal very fast motions that are not present in either of the PANI-ES samples. Despite differences in conductivity, no significant dynamical differences are observed for the films prepared with the two dopants. We interpret these results in light of previous work on the structural ordering induced by doping with sulfonic acids and the possible correlations between charge carrier mobilities and low frequency structural dynamics.

Probing phase segregation in porphyrin-polymer blends with multidimensional IR spectroscopy.

Multidimensional IR spectroscopy is used to study blends of the regioregular semiconducting polymer, poly(3-hexylthiophene) with ruthenium porphyrins in solid state thin films. A ruthenium-bound carbon monoxide ligand is a sensitive reporter of the local electrostatic environment and indicates that the porphyrins localize into phase-segregated domains upon annealing in solvent vapor. The FTIR spectrum of this ligand reports on the state of aggregation, which we then correlate with transmission electron microscopy images of these films before and after annealing. We then use 2D-IR spectroscopy, an ultrafast IR technique, to measure the fast structural dynamics that are present in the vicinity of the CO ligand when it is homogeneously blended in the polymer film and phase segregated into purely porphyrinic domains.

Infrared spectroscopic signatures of phase segregation in P3HT-porphyrin blends.

Phase segregation of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine ruthenium(II)carbonyl (RuOEP) and regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) in thin films is investigated with infrared and UV-visible spectroscopies as well as transmission electron microscopy (TEM). The Fourier transform infrared (FTIR) spectrum of the ruthenium-bound CO symmetric stretching mode exhibits significant changes as these films are annealed in solvent vapors. The development of multiple inhomogeneously broadened microenvironments is observed, and UV-visible spectra and TEM support a model of homogeneous porphyrin distribution throughout the P3HT films that gradually becomes more heterogeneous as the P3HT and RuOEP molecules phase segregate. A complete model for the phase segregation process experienced by the embedded RuOEP is proposed to explain the collective experimental observations.

IR spectrum of CH3CN-BF3 in solid neon: matrix effects on the structure of a Lewis acid-base complex.

We have observed several IR bands of CH3CN-BF3 in neon and nitrogen matrices. For the 11B isotopomer in neon matrices, we observed the BF3 symmetric deformation band (nu7) as a doublet at 600 and 603 cm(-1), the BF3 symmetric stretching band (nu6) as a doublet at 833 and 838 cm(-1), the BF3 asymmetric stretching mode (nu13) at 1281 cm(-1) (partially obscured), and the C-N stretching mode (nu2) as a doublet at 2352 and 2356 cm(-1). The nitrogen matrix data are largely consistent with those reported recently, though we do propose a refinement of one band assignment. Comparisons of the frequencies of a few key, structurally sensitive vibrational modes either observed in various condensed-phase environments or calculated for two minimum-energy gas-phase structures indicate that inert matrix media significantly alter the structural properties of CH3CN-BF3. Specifically, the B-N dative bond compresses relative to the gas phase and other concomitant changes occur as well. Furthermore, the frequency shifts depict structural changes that occur across the various matrix hosts in a manner that largely parallels the degree of stabilization offered by these inert media.