A mechanistic understanding of the structure and dynamics of a chemically tunable N-methylfulleropyrrolidine (8-NMFP)-assisted gold nanocomposite and its aggregation via a controllable interparticle interaction is reported as a function of the molar ratio and pH of the medium. Electronic structure calculations adopting density functional theory methods implied electrostatic interactions to play a dominant role between 8-NMFP and citrate-capped gold nanoparticles. MM+ molecular mechanics force field computations revealed intermolecular gold-gold interactions, contributing toward the formation of spherical composite aggregates. Corroborating these, optical absorption spectra showed the usual surface plasmon band along with a higher-wavelength feature at ∼600-650 nm, indicative of the aggregated nanocomposite. pH-controlled reversible tuning of the plasmonic features in the composite was evident in a pH interval ∼5-6.8, revealing prevalent interparticle electrostatic interactions. In addition, photoluminescence (PL) and time-correlated single-photon counting studies revealed a strong nanocomposite interaction with a pure fluorescent dye, Rhodamine B, indicating excitation energy transfer from the dye to the composite. The dye upon interaction with the nanocomposite showed a significant quenching of its PL intensity and shortening of lifetime. Energy coupling between the metal nanoparticle composite and the emitting molecular dipole resulted in a long-range surface energy transfer (SET) from the donor dye to the surface plasmon modes of the nanoparticle following a donor-acceptor distance dependence of 1/r4. This molecular beacon with correlation between the nanoscale structure and the nonradiative nanometal SET can be used as a spectroscopic/molecular ruler in probing advanced functional materials. © 2017 American Chemical Society.