LOW-TEMPERATURE OPTICAL-ABSORPTION OF POLYCRYSTALLINE THIN-FILMS OF ALPHA-QUATERTHIOPHENE, ALPHA-SEXITHIOPHENE AND ALPHA-OCTITHIOPHENE, 3 MODEL OLIGOMERS OF POLYTHIOPHENE

UV-Vis absorption spectra of polycrystalline thin films of three conjugated oligothiophenes (quaterthiophene (alpha-4T), sexithiophene (alpha-6T) and octithiophene (alpha-8T)) have been studied over the temperature range 14-300 K. The fundamental pi-pi* band exhibits a succession of equally spaced side peaks on the low energy side (DELTA-E1=0.194 eV). Lowering the temperature down to 14 K considerably improves the spectral resolution and reveals additional hyperfine structures (DELTA-E2 = 0.019 eV). These complex multiplets are interpreted in terms of vibrational couplings to the fundamental S0-->S1 electronic transition.