Probing the Spectral Heterogeneity in Thin Films of Supramolecular Nanostructures Based on Heterotriangulenes

Well-defined supramolecular architectures based on organic chromophores exhibit unique photophysical characteristics that depend critically on the molecular packing, intermolecular electronic coupling, and presence of disorder. However, the optical properties of such systems are largely characterized in dilute solution, while spatial heterogeneity within and between supramolecular nanostructures in thin films is rarely addressed. Here, we combine optical spectroscopy with microscopy to spatially resolve photoluminescence (PL) spectra and PL lifetime decays on thin films of supramolecular nanostructures based on a carbonyl-bridged triarylamine (CBT) trisamide. CBT was self-assembled in n -dodecane and anisole, resulting in dispersions of single supramolecular nanofibers and bundles of nanofibers, respectively, which were subsequently deposited on glass substrates. For deposited nanofibers from n -dodecane, we consistently find broad, structureless, and strongly red-shifted PL spectra that indicate emission exclusively from delocalized exciton states. Films prepared from anisole feature more complex PL spectra that suggest emission from both delocalized excitons on single nanofibers and strongly localized excitons on nanofibers within bundles. In contrast to dispersions, in films we find broad distributions of spectral positions and line widths of PL spectra as well as of lifetimes, demonstrating heterogeneity in the photophysics within and between deposited nanostructures. Numerical simulations based on a Holstein Hamiltonian show that in nanofibers excitons are delocalized due to correlated site energies, while in bundles localization of excitons is induced by electronic disorder, destroying such correlations. Our data show that the interplay between solvent-driven self-assembly, electronic disorder, and excitonic properties influences the photophysics of (deposited) supramolecular systems.