Bridge rigidity affects the source and magnitude of line broadening in covalently linked squaraine dimers

Covalently linked squaraine dimers exhibit characteristic collective excitonic effects based on their packing geometries, which can be controlled by a bridge linking two monomers. A previous study showed that the bridge moiety strongly impacted the excitonic line broadening. Here, we use ultrabroadband two-dimensional electronic spectroscopy to quantify the homogeneous and inhomogeneous line-broadening components. Both rigid and flexible bridges reduce homogeneous broadening relative to the monomer, but the flexible bridge increased inhomogeneous broadening and thereby masked the exchange-narrowing effect. These results reveal that one can design a bridge structure of covalently linked dimers to control both line broadening and packing geometry. • Proposed uses of molecular aggregates are light harvesting & quantum information. • Squaraine dimers exhibit desirable collective excitonic effects. • Rigid (flexible) bridge leads to homogeneous (disordered) sample. • 2D spectroscopy can reveal exchange narrowing even in disordered samples. • All dimers exhibit exchange narrowing, even in presence of disorder.