Pressure Dependence of Photoluminescence of Oligothiophenes
- PDF / 352,023 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 82 Downloads / 200 Views
ABSTRACT We report on the nature of emitting states in two different quaterthiophenes: the unsubstituted quaterthiophenes (T4) and a bridged T4 (T4B), where the bridging have been realized by introducing a second bond between the two central thiophene rings. The effect of the chemical bridging on the photophysics of these compounds have been studied via photoluminescence measurements under hydrostatic pressure, which permit to clarify the influence of interchain coupling, conformational effects and intrachain structural changes on the optical properties. INTRODUCTION The class of thiophene oligomers has been widely studied because they possess extensive 7telectron delocalization along the molecular backbone which makes them interesting for various optoelectronic applications as active layers in field effect transistors [1-3] and light emitting diodes [4]. Moreover, they can be used as model compounds for the parent polymer: polythiophene. While optical properties of oligothiophenes in solution have been studied in details, open problems still exist on the electronic properties of oligothiophenes in the solid phase such as single crystals and crystalline thin films, where optical properties strongly depend upon supramolecular organization.
T4
S ý\I
S
S
S
S
T4B,
Fig. 1 Chemical structure of T4 and T4B: X=CH 2 Due to the flexibility of molecular design the optical properties of the oligothiophenes can be tuned by changing the length of the conjugated chain or, for a given chain length, by substituting hydrogen atoms with proper chemical groups. With the intent of controlling the molecular conformations and also to induce a controlled supramolecular ordering, we have synthesized a series of quaterthiophenes in which one or two inter-ring torsional degree of freedom have been blocked by chemical bridging [5]. 897 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
Here we report on the photophysics of a new synthesized quaterthiophene, namely T4B, compared with the unsubstituted T4. The chemical structure of T4B is sketched in Fig. 1, where X=CH 2. The bridge between the two central rings in T4B inhibits the inter ring rotation, thus reducing the efficincy of the non radiative decay channels of the excited state as pointed out by the higher photoluminescence (PL) quantum yield value (DF=0.24) measured in T4B in solution with respect to T4 ( 1DF=0. 17). EXPERIMENT We have evaporated thin films (d&3500 A) of T4 and T4B under high vacuum condition (2x10-6 mbar) and we have dispersed T4 and T4B molecules in a polymethylmethacrylate (PMMA) matrix. Absorbance and X-ray diffraction measurements were realized by conventional intrumentations. PL measurements under pressure were obtained by exciting with the 351.1/363 nm lines of an Ar' laser. The detection was realized via a combination of an optical fibre and an Ocean Optics CCD spectrometer. The sample was held in a pressure cell. The pressure was created via a Nova Swiss pressure amplifier, using oil on the low pressure side and He on the high pres
Data Loading...
