Photophysical properties of molecular aggregates are largely determined by exciton coherence size: a spatial extension of exciton delocalization. Increase in exciton coherence size can lead to fast energy transport as well as efficient charge separation. Here, we demonstrate that introducing alkyl chains to organic molecules can enhance the exciton coherence size significantly. Focusing on the thin films of excellent hole transport materials, dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) and its alkyl-substituted derivative, we analyze the steady-state and picosecond time-resolved photoluminescence spectra of the films to estimate exciton coherence sizes. The alkyl substitution enhances the coherence size by a factor of 2-3, indicating that a long-range ordering in the molecular aggregates is achieved with the additional van der Waals interaction between saturated alkyl chains. The coherence sizes of both the films decrease with increasing temperature owing to thermal populations within the vibronic exciton manifolds.
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