Probing the Mechanism of Fluorescence Quenching of QDs by Co(III)-Complexes: Size of QD and Nature of the Complex Both Dictate Energy and Electron Transfer Processes

The decrease in photoluminescence (PL) of four different sized CdSe colloidal quantum dots (donors) has been investigated in the presence of three different Cobalt(III) complexes (acceptors). The steady-state and time-resolved PL (TRPL) spectroscopy have been used to investigate the mechanism of quenching. The complex concentration driven change in lifetimes of QDs and stronger PL quenching than predicted solely by TRPL data indicate that the quenching is neither purely static nor purely dynamic in nature. Further, the absence of any ground state complex absorption feature suggests that the static quenching contribution is due to the close proximity of the QDs fluorophores and deactivating sites of complexes. The dynamic quenching processes like diffusion mediated collisional quenching, Dexter energy transfer, and hole transfer have been methodically ruled out, leaving Forster resonant energy transfer (FRET) and the electron transfer (eT) between the QDs and complexes as the possible mechanisms. The Marcus model of eT has been successfully used to demonstrate the otherwise looking random trends of experimental eT rates. The apparent static contributions have been separated from the total quenching by normalization of steady state PL with TRPL data. Finally, FRET and eT mediated dynamic quenching in conjunction with the donor acceptor proximity driven static quenching was used to explain steady state PL quenching trends.