Ultrafast unequilibrated charge transfer: A new channel in the quenching of fluorescent biological probes

The dynamics of two biological fluorescent probes, 2-aminopurine (Ap) and daunomycin, were studied using both femtosecond transient absorption and fluorescence upconversion techniques. Various Ap-containing structures were investigated in solution: free Ap, non-covalently bonded (with guanine, adenine, and tryptophan) and covalently bonded in DNA constructs (with guanine, 7-deazaguanine, and adenine). The distinct difference of transient absorption and fluorescence dynamics on the ultrafast time scale, and their dependence on free energy change (ΔG), and the abrupt decrease of the initial fluorescence intensity suggest the efficient depopulation by charge transfer from the unequilibrated hot molecules. We provide a model for this possibly general mechanism and obtain the rate constants for charge separation, vibrational relaxation, and charge recombination.