Abstract
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 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.
Highlights
-conjugated oligomers, such as oligoacenes and oligothiophenes are amongst the most widely studied molecular materials involved in the development of the generation of optoelectronic devices such organic solar cells and organic field-effect transistors.[1,2,3,4] Among the many different molecular materials for organic electronic applications, oligothienoacenes are of particular interest because they combine the rigid planarity and extended conjugation of acenes with the chemical stability of oligothiophenes.[5, 6] the structural rigidity of these materials limits its solubility in common organic solvents, which in turn, restricts their use in the development of solution-processed organic semiconductors
For the smaller molecules, we found that the vibronic progression of their optical spectra are mainly determined by the presence of two normal modes with frequencies of ~ 1500 cm-1 and 500 cm-1
The larger molecules are characterized by those frequencies and by the presence of a low-frequency normal mode at around 100 cm-1. We note that this particular vibration is responsible for the experimentally observed peak-broadening of the vibrational absorption and emission features of these compounds
Summary
-conjugated oligomers, such as oligoacenes and oligothiophenes are amongst the most widely studied molecular materials involved in the development of the generation of optoelectronic devices such organic solar cells and organic field-effect transistors.[1,2,3,4] Among the many different molecular materials for organic electronic applications, oligothienoacenes (linearly fused thiophenes) are of particular interest because they combine the rigid planarity and extended conjugation of acenes with the chemical stability of oligothiophenes.[5, 6] the structural rigidity of these materials limits its solubility in common organic solvents, which in turn, restricts their use in the development of solution-processed organic semiconductors. Oligothienoacenes are frequently functionalized with solubilizing trimethylsilyl (TMS) and triisopropylsilyl (TIPS) substituents, which aid in purification and solution processability.[5,6,7,8] despite the important role of the silyl groups in the solubility properties of molecular materials for organic electronic applications, only a few studies up to date have investigated the electronic and optical properties and electron transfer characteristics of silyl-functionalized -conjugated oligomers.[9,10,11] On the other hand, even though qualitative interpretations of the well-resolved absorption and emission spectra of oligoacenes can be found in the literature,[12, 13] to the best of our knowledge, a detailed analysis of the vibronic features in silyl end-capped oligothienoacenes has not yet been conducted
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
