Ultrafast Transient Absorption Anisotropy to Monitor Relaxation in Carbon Dots

In recent years, carbon dots (CDs) have emerged as a new class of carbonaceous materials that are easy to synthesize and characterize and show promising optical properties. One of the interesting aspects is their excitation wavelength dependent photoluminescence (PL). Although several CDs with different precursors are synthesized and characterized. Corresponding excited state optical properties on these materials are sparse. In this study, ultrafast excited state relaxation dynamics of CDs based on Pyrene are presented. Steady state, femtosecond transient absorption, and anisotropy data measurements are carried out to understand the relaxation pathways in these CDs. Pyrene-based aminated CDs are synthesized by hydrothermal method. The CDs were transferred to organic solvents (e.g., toluene, THF, and ACN) using surfactants via the phase transfer method. It is interesting that the CDs in different solvents exhibit both solvent dependent and excitation wavelength dependent PL. Excited state absorption decay of these CDs has shown ultrafast excited state decay that can be assigned to inter chromophore or chromophore solvent bath interactions. Transient absorption anisotropy measurements have shown faster picosecond time scale relaxation that cannot be assigned to the rotational reorientation of the CDs as the volume is greater. Transient absorption measurements have also shown differences in relaxation for bare CDs in water and CDs passivated with surfactants as in the case of CDs in non-aqueous solvents. The decay of anisotropy was assigned to energy transfer between the chromophores of different conjugation lengths in the CD. This is ascribed to HOMO fluorescence resonance energy transfer (HOMO-FRET) with time constants ranging from 50 to 90 ps. In THF, long-lived anisotropy decay is observed which is assigned to the rotational relaxation of the entire CD. Interesting ultrafast growth of anisotropy was observed in non-aqueous solvents, which is assigned to faster localization of the excitation from the core to a chromophore.