Molecular orientation and optical luminescence properties of soluble star shaped oligothiophene molecules for organic electronic applications

Angle resolved near-edge absorption fine structure measurements of the C 1s edge of a film of the star-shaped molecule 4(HPBT) show a high degree of order, with the molecules maintaining a largely planar geometry in an upright configuration. X-ray excited optical luminescence measurements showed a dependence of the luminescence spectrum on the excitation energy, with (0-0), (0-1), and (0-2) vibrational modes being clearly visible when exciting off-resonance; the appearance of these features under off-resonance is attributed to a scattering process that is quenched when the excitation approaches a core absorption resonance. The HOMO-LUMO gap is estimated from the luminescence measurements (2.28 eV), and this value is confirmed by the comparison of the C 1s absorption and C Kα emission spectra.