Diarylethenes with two different side groups (phenyl and chloro) were appended to both alpha-ends of a sexithiophene unit. The temperature dependent aggregation properties for both compounds were characterized by steady state and transient absorption spectroscopy. The peripheral side groups show an unexpectedly significant influence on the electronic properties of the sexithiophene core. Furthermore, the relative influence of the phenyl and chloro substituents on the aggregation behavior observed is remarkable. The phenyl compound exhibits formation of H-aggregates over a narrow temperature range, between 240 and 200 K, typical of strong intermolecular interactions. In contrast, the chloro compound shows gradual aggregation over a wide temperature range, forming H-aggregates albeit with weaker intermolecular interactions. The results demonstrate that minor changes in the structure lead to tunability of the aggregation and corresponding luminescence properties of sexithiophenes in solution and hold particular relevance to supramolecular and polymer systems based on sexithiophene units.