  Using d. functional theory (DFT) computations, the authors demonstrated a
substantial skeletal relaxation when the structure of
2,5-bis-[4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is
optimized in the gas-phase comparing with the mol. structure detd. from
monocrystal x-ray diffraction. The origin of such a relaxation is explained by
a strong environmental strains induced by the presence of anthracene entities.
Also, the estn. of the frontier orbital levels showed that this structural
relaxation affects mainly the LUMO that is lowered of 190 meV in the gas phase.
To check if these theor. findings would be confirmed for thin films of BAS, the
authors turned to UV photoemission spectroscopy and/or inverse photoemission
spectroscopy and electrooptical measurements. The study of the c.d. or voltage
and luminance or voltage characteristics of an ITO/PEDOT/BAS/Au device clearly
demonstrated a very unusual temp.-dependent behavior. Using a thermally
assisted tunnel transfer model, this behavior likely originated from the
variation of the electronic affinity of the silole deriv. with the temp. The
thermal agitation relaxes the mol. strains in thin films as it is shown when
passing from the cryst. to the gas phase. The relaxation of the intramol. thus
induces an increase of the electronic affinity and, as a consequence, the more
efficient electron injection in org. light-emitting diodes.
