Optical absorption and emission properties of end-capped oligothienoacenes: A joint theoretical and experimental study

The electron–vibration coupling in a family of silyl end-capped oligothienoacenes is investigated on the basis of a joint experimental and theoretical study using UV–vis absorption and emission spectroscopies and density functional theory calculations. Well-resolved vibronic progressions are found in the low-temperature absorption and emission profiles of these silyl-functionalized organic molecules. As the size of the oligomer lengthens a bathochromic shift is observed in the near-UV–vis range, indicative of the extension of the effective π-conjugation. The absorption and emission bands are practically mirror-symmetric. The combination of two normal modes with frequencies of ∼1500 cm−1 and ∼500 cm−1 determines the main vibronic progression in absorption and emission for all the series, although for larger oligomers (n = 6, 7, and 8) the presence of low-frequency normal modes (∼100 cm−1) is also evident. The spacing of the vibrational features is slightly larger in absorption than in emission; this agrees with the predicted shifting of the C−C stretching modes of the inner-most ring toward the high-frequency region as a result of the reversal of the single–double C−C pattern in the electronic excited-state. Our calculations indicate that the contributions of the end-capping groups to the total relaxation energy of the S0 → S1 and S1 → S0 transitions are almost negligible. This result suggest that the vibronic structure and to a large extent the spectral profiles of the silyl end-capped oligothienoacenes are mainly determined by their respective oligothienyl core.