Elucidating the impact of nodal and non-nodal chiral tagging on the chiroptical properties of naphthalimides

Abstract

We present a systematic investigation into the positional effects of point chirality on the chiroptical behaviour of naphthalimide-based chromophores. Specifically, we compare the effect of chiral tags at electronically distinct positions: the nodal N-imide site (electronically decoupled) and the non-nodal C₄ site (electronically coupled) on their circular dichroism (CD) and circularly polarized luminescence (CPL) responses. A series of chiral naphthalimide derivatives was synthesized to enable a comparative mapping of chiral induction efficiency at electronically coupled versus uncoupled positions on the same molecular scaffold. Imide-substituted derivatives, in which the chiral tag remains electronically decoupled from the chromophore's frontier molecular orbitals (FMOs), show negligible CD and CPL activity. In contrast, C₄-substituted analogues exhibit markedly enhanced chiroptical signals, attributed to their electronic coupling with FMOs and to their involvement in intramolecular charge transfer (ICT). Dual-tagged derivatives further emphasize the dominant role of C₄-chiral tags in dictating the handedness of chiroptical responses, irrespective of the configuration at the N-imide site. The differences in CPL observed between diastereomeric species in both solution and solid states were supported by chirality quantification using the continuous chirality measure (CCM) tool. Since the diastereomers showed similar photoluminescence quantum yields in these states, the quantified chirality was identified as the key factor positively correlated with the differences in CPL strength. These findings, including the approach to quantifying chirality in diastereomers, provide a general design principle for optimizing chiroptical performance in small-molecule emitters.