Hybrid Conjugated Organic Oligomers Consisting of Oligodiacetylene and Thiophene Units: Synthesis and Optical Properties

Novel and highly soluble hybrid conjugated organic oligomers consisting of oligodiacetylene and thiophene units have been synthesized in high purity through iterative and divergent approaches based on a sequence of Sonogashira reactions. The series of thiophene‐containing oligodiacetylenes (ThODAs) and homocoupled ThODAs (HThODAs) show—both in solution and in the solid state—a strong optical absorption, which is progressively red shifted with increasing chain length. The linear correlation of the absorption maximum (λAmax) with the inverse of conjugation length (CL=number of double and triple bonds) shows that the effective conjugation length of this system is extended up to at least CL=20. Furthermore, absorption measurements of dropcast thin films display not only a bathochromic shift of the absorption maxima but also a higher wavelength absorption, which is attributed to increased π–π interactions. The wavelength of the maximum fluorescence emission (λEmax) also increases with CL, and emission is maximal for oligomers with CL=7–12 (fluorescence quantum yield ΦF=∼0.2). Both longer and shorter oligomers display marginal emission. The calculated Stokes shifts of these planar materials are relatively large (0.4 eV) for all oligomers, and likely due to excitation to the S2 state, thus suggesting that the presence of enyne moieties dominates the ordering of the lowest excited states. The fluorescence lifetimes (τF) are short (τF,max=≪1 ns) and closely follow the tendency obtained for the fluorescence quantum yield. The anisotropy lifetimes show a near‐linear increase with CL, in line with highly rigid oligomers. Bringing units together: Oligodiacetylenes that incorporate thiophene as either the central unit and/or an end cap of homocoupled diacetylene–thiophene compounds yield a series of highly soluble oligodiacetylenes (see picture). The optical properties of these extended monodisperse oligomers were investigated by using steady‐state and time‐resolved techniques.