Ultrafast Energy Transfer in Orthogonal Heptamethine Cyanine-Naphthalimide Systems: A Pathway toward High-Energy...

We report the synthesis and photophysical characterization of a family of bichromophoric systems, in which substituted naphthalimides (NMI) are covalently attached at the meso position of heptamethine cyanines (Cy7). Single crystal X-ray diffraction and DFT calculations reveal a near-perpendicular arrangement of the NMI and Cy7 moieties, resulting in minimal electronic communication. Complementary variable-temperature NMR measurements reveal potential fluctuations in the dihedral angle between the two moieties. Steady-state spectroscopy and femtosecond transient absorption measurements reveal a non-negligible excited state coupling, linked to ultrafast internal conversion/energy transfer from an NMI localized high-lying excited state to the cyanine S₁ state. This process occurs on a subpicosecond time scale for all dyads, regardless of NMI substitution, leading to a large excitation-emission wavelength difference and an effective monochromophoric fluorescence behavior. Comparative studies with the NMI-pyridine precursors further highlight the role of intramolecular charge transfer in shaping the ultrafast dynamics in these systems, prior to cyanine attachment. Altogether, our results establish clear structure-property relationships for cyanine-based antenna constructs, thus providing design principles for multichromophoric systems that broaden cyanine absorption while preserving the integrity of the polymethine core.