<p>Herein, we have studied the fluorescence resonance energy transfer (FRET) phenomena between deep eutectic solvents (DES), derived from methyltriphenylphosphonium chloride/bromide (MTPC/MTPB) and piperonyl alcohol (PPA), and acceptor dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). The DESs exhibit excellent intrinsic fluorescence emission, making them suitable for use as FRET donors. The fluorescent DES and DCM, meet the spectroscopic requirements necessary for a resonance energy transfer (RET) event to occur. Both steady-state and time-resolved fluorescence spectroscopy studies demonstrate significant FRET between the donor and acceptor. Furthermore, a decrease in the donor's fluorescence lifetime and the appearance of a rise component in the acceptor's fluorescence decay upon addition of the acceptor have been observed, which are definitive indicators of FRET. The estimated donor-acceptor distances for the MTPC-PPA and MTPB-PPA DESs have been found to be 27.41 Å and 26.93 Å, respectively. Since the donor-acceptor distances in both systems are quite similar, this suggests that their FRET efficiencies are also comparable, at approximately 85%. This finding suggests that the choice of anion in the DESs has minimal impact on its ability to transfer energy to the DCM dye. Therefore, the present study demonstrates a hitherto unobserved energy transfer phenomenon between fluorescent DESs and an acceptor dye.</p> Graphical abstract <p>This study reports a novel FRET phenomenon involving intrinsically fluorescent deep eutectic solvents (DESs), composed of methyltriphenylphosphonium halide (Cl- or Br-) and piperonyl alcohol, acting as donors, and the acceptor dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran. The findings demonstrate that these DESs serve as effective FRET donors due to their significant intrinsic fluorescence emission.</p> <p></p>