Photoexcitations in Oligothiophenes: The Role of Conformation Mobility and Intermolecular Interactions.
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onjugated molecules [7] while, in standard matrix isolated systems, a complete molecular separation is achieved only for extremely diluted molecular concentrations. In this paper we report the optical properties of oligothiophenes for which the intermolecular interactions are strongly reduced both in the case of unsubstituted (T4) or α,ω-dihexyl-substituted (DHT4) quaterthiophenes included in PHTP, and in the case of polycrystalline powders possessing different phases of β- substituted tetrahexylsexithiophene (4HT6).
EXPERIMENTAL The preparation of PHTP inclusion compounds [9] and the synthesis of the conjugated molecules are reported elsewhere [10,11]. Phases A and B of 4HT6 have been obtained by proper thermal treatment, as reported elsewhere [12]. The solid samples are powders dispersed in KBr pellets. Solutions are in tetrahydrofuran. The optical absorption is measured with a Cary 2400 spectrometer and the PL with a SPEX 270M spectrometer equipped with a liquid N2 cooled CCD detector. B
4HT6
R S
R S
S S
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R
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4HT6 DHT4
Optical Absorption
Photoluminescence
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DHT4
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T4 1.6
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2.4 2.8 3.2 Energy (eV)
3.6
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Figure 1 Optical absorption and emission spectra of T4, DHT4 and 4HT6 powders (dotted lines), solution (dashed lines), and inclusion compounds (solid lines) at room temperature. On the right side, the chemical structure of the molecules is shown (R=C6H13).
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RESULTS In Figure 1 the optical properties, at room temperature, of T4 and DHT4 inclusion compounds are compared to those of the powders and the solution. Both the decrease of the conformational disorder of the solution (more structured and red-shifted IC vs solution spectra) and the reduction of the intermolecular interactions of the aggregate (lack of the broad high energy absorption of the powders in the IC spectrum) appear evident in the IC spectra. While the different absorption of the IC compound, with respect to the solution, is consistent with the lower conformational disorder of the molecules in PHTP host, the changes observed both in the absorption and in the PL with respect to the powders, depend on the fact that the intermolecular interactions, which modifies the excited state properties of the molecules in the polycrystalline samples, are suppressed in the PHTP martix. As shown in Figure 1, the more relevant changes, from the powders to the IC compounds, are observed for DHT4, in agreement with the ordered close chain packing arrangement of these α-substituted molecules [13]. In Figure 1 the optical spectra of the β-substituted T6 powder are also reported for two different molecular packing arrangements, which yield planar (phase B) and distorted (phase A) molecular conformations [12]. As it can be seen the spectra of both the phases do not display the high energy broad absorption of the quaterthiophene powders, showing spectral shapes close to those of the IC compounds. This observation is consistent with the fact that in both the A and B phases the intermolecular interactions are negligible d
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