Side-chains at the imidic position of naphthalimide rendered a firm control over (i) the degrees of π–π overlap and (ii) distances between the perylenimide units in a crystalline naphthalimide–perylenimide dyad as determined using single crystal XRD and Hirshfeld surface analyses. Steady-state and time-resolved electronic spectroscopy in addition to DFT calculations revealed a decline in intermolecular excitonic interaction due to interfering alkyl chains.