Correlating Structural Changes with the Photophysics of Terrylenediimide Films during Spontaneous Annealing

Interchromophore distance and geometry determine the nature of intermolecular coupling and therefore the photophysical behavior of the system. Here we demonstrate the close relationship between molecular orientation and coupling by characterizing the initial and final states of a N,N-bis(tricosan-12-yl)terrylene-3,4:11,12-bis(dicarboximide) (C23TDI) film undergoing spontaneous annealing at room temperature. Appending long aliphatic tails to the large TDI π-surface produces an initially liquid crystal-like structure that reorganizes to the thermodynamically stable structure at room temperature. Monitoring the structural changes using UV-vis absorption and X-ray diffraction techniques reveals metastable intermediates we can identify and characterize, and the slow timescale of this change allows for a clear comparison between interchromophore structures and photophysical behavior at various points along the reaction coordinate, providing important insights into the relationship between chromophore packing and function. Initially upon film preparation, C23TDI exhibits no long-range order but shows strong charge-transfer (CT) interactions, as observed through steady-state and transient absorption measurements. Within 60 hours under ambient conditions, the TDI units in the film reorganize to an orientation with higher crystallinity and diminished CT interaction, highlighting the dominance of long alkyl chain interactions over π-stacking interactions.