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 p-conjugation. The absorption and emission bands are practically mirror-symmetric. The combination of two normal modes with frequencies of similar to 1500 cm (1) and similar to 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 (similar to 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 S-0 -> S-1 and S-1 -> S-0 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. (C) 2010 Elsevier B.V. All rights reserved.