Understanding and controlling supramolecular organization in polymeric semiconductor materials is central for their exploitation in electronic devices. For example, regioregular poly(3-hexylthiophene) (rr-P3HT) chains can self-assemble into lamellae, which results in effective field-effect mobilities of the order of 0.1 cm 2 V -1 s -1 compared to mobilities several orders of magnitude lower in more disordered morphologies. [1,2] Quasi-steady-state [1,3] and time-resolved [4,5] spectroscopic studies of rr-P3HT and related polythiophene derivatives firmly establish that the nature of photoexcitations is fundamentally different in regioregular versus regiorandom materials. These differences extend to the direct charge photogeneration efficiency and the 3D delocalization of charged excitations, both of which are suggested to be higher in rr-P3HT films. However, the extent of intermolecular electronic interaction in rr-P3HT still remains elusive.[6] Studies on oligothiophene crystals indicate that the intermolecular electronic interactions can be substantial compared to intramolecular reorganization energies (placing these systems in the so-called "intermediate" electronic coupling regime [7] ) and that important differences in the consequences of these interactions are expected for oligomers with odd and even numbers of monomer units. [8,9] Cofacial arrangement of conjugated molecules into stacked aggregates is expected to result in energetic splitting of the highest occupied and lowest unoccupied molecular levels, producing new possible optical transitions between the resulting highest occupied state and the split excited states.[10]If the supramolecular arrangement is perfectly cofacial, the lowest energy transition is predicted to be symmetry forbidden.[10] Importantly, semiempirical quantum-chemical calculations show that the intermolecular electronic coupling decreases with increasing length of the molecules that form the H aggregate. [11,12] This would predict a nearly vanishing electronic interaction term in well-ordered rr-P3HT films.Here we seek to make direct comparison with these theoretical predictions and to evaluate the consequences of supramolecular assembly on the photoluminescence (PL) decay rates of oligothiophene nanostructures. We investigate, by means of time-integrated and time-resolved PL spectroscopy, a series of three functionalized oligothiophenes that have been previously shown to self-assemble into chiral supramolecular stacks in semipolar solvents.[13] These further self-assemble into vesicular structures. [14] The functionalized oligothiophenes are model systems for unambiguously investigating intermolecular electronic coupling effects because the supramolecular assembly process in solution is thermotropically reversible with a well-defined transition temperature (28°C for the sexithiophene derivative at concentrations of ∼ 2.6 × 10 -5 M, with a width of approximately 20°C), allowing direct comparison of the photophysics of assembled and dissolved systems. We find excellent agreement bet...

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.