Energy transfer from CdSe quantum dots to porphyrin via two-photon excitation.

CdSe quantum dots have been used as energy donors to activate meso-tetra (4-sulfonatophenyl) porphine dihydrochloride. Pulses of 130 fs duration at a wavelength of 800 nm are used as the two-photon excitation light source. After excitation, steady-state and time-resolved fluorescence spectra are collected for samples with different ratio between the amount of porphyrins and quantum dots. Decay kinetic curves of CdSe quantum dots with and without porphyrins are well fitted by the biexponential decay curve, which indicates a combination of two components (excitonic and trapping state) in the luminescence behavior of CdSe quantum dots. Relative intensity weights of the excitonic and trapping state, total and nonradiative energy transfer efficiency, average luminescence lifetimes of donors are calculated. The nonradiative transfer mechanism becomes the leading factor as the concentration of acceptors gets higher. It is considered to take place through the channel of trapping states of CdSe quantum dots, which is expressed by the weight change between the fast and slow luminescence components. This deduction presents a new way of raising the energy transfer efficiency by increasing the trapping state proportion of the quantum dots, which can be easily realized by surface modification.