Solvent-Assisted Enhanced Emission of Cationic Perylene Diimide Supramolecular Assembly in Water: A Perspective from Experiment and Simulation

We report a solvent-assisted increase in the emission of a cationic perylene diimide derivative, which is water-soluble in nature. Favorably, all the emission-assisting solvents used in all the experiments are water-miscible such as ethanol, tetrahydrofuran (THF), or dimethyl sulfoxide (DMSO). The preliminary assumption of associated fluorescent assembly of perylene stacks (via spectroscopic analysis) has been further explored with dynamic light scattering (DLS), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and time-correlated single photon counting (TCSPC) techniques. Spectroscopically observed drastic enhancement in the emission intensity seems to be born out of a major change in the assembly pattern of the interacting hydrophobic surfaces of perylene derivatives. These changes further have a macroscopic impact on the DLS or powder XRD outcomes. Furthermore, molecular dynamics simulations have been performed to interpret the nature of aggregation of perylene oligomers in water and in an ethanol/water binary mixture. The aggregation propensity (H-type dimer) of the dye molecules is found to decrease with increase in the ethanol concentration. It has been revealed that in water, the association of perylene-based oligomers is entropic in nature. In contrast, the association is driven via enthalpy in ethanol. Moreover, if we extend the oligomer size, the enthalpic stabilization for this unfamiliar supramolecular assembly of perylene moiety seems to be enhanced greatly. These results along with the calculated molar extinction coefficient of monomer and dimer provide the molecular rationale behind the observed solvent-assisted increase of emission intensity