Excitation-dependent evolution of emissive states via interfacial electronic coupling in a carbon quantum dot/rhodamine 6G hybrid system.

Carbon quantum dots (CQDs) exhibit heterogeneous electronic structures that give rise to complex excitation-dependent photophysical behavior. Although interactions between CQDs and molecular fluorophores are commonly interpreted in terms of fluorescence quenching or Förster resonance energy transfer (FRET), their influence on the accessible excited-state landscape remains poorly understood. Here, we investigate the interaction between plasma-derived CQDs and Rhodamine 6G (R6G) using steady-state and excitation-dependent photoluminescence (PL), time-resolved photoluminescence (TRPL), and complementary DFT/TDDFT calculations. Increasing the R6G concentration progressively suppresses the intrinsic CQD emission while enhancing a distinct emission band near 550 nm. Excitation-dependent PL and TRPL collectively indicate a systematic change in the relative contributions of emissive states, accompanied by modified relaxation dynamics. Ground-state DFT calculations reveal interaction-induced redistribution of frontier orbitals consistent with interfacial electronic coupling. At the same time, TDDFT indicates interaction-induced modification of the accessible excited-state manifold, providing qualitative electronic support for the observed spectral evolution. Collectively, these complementary experimental and computational results are consistent with the hypothesis that the CQD-R6G interaction may modify the accessible excited-state landscape, giving rise to photophysical behavior not fully described by conventional quenching models alone. The combined spectroscopic and theoretical analyses provide a unified framework linking excitation-dependent spectral evolution with interaction-induced electronic perturbation in CQD-dye hybrid systems. These findings provide mechanistic insight into the photophysics of CQD-dye systems and may facilitate the rational design of fluorescence sensing, bioimaging, and hybrid optoelectronic materials.