Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Abstract

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (dCdSe = 2.8 nm) and cationic porphyrins (H2TMPyrP4+ and ZnTMPyrP4+) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS⊕Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the Forster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H2TMPyrP4+ and its metal complex ZnTMPyrP4+. It has been revealed that, as the QDs ↔ Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H2TMPyrP4+ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP4+. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S−(CH2)nCOO− and its dependence on the number of CH2 groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ⇐ S−(CH2)nCOO− hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)⊕ZnTMPyrP4+, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP4+ molecule (QD* ⇒ ZnTMPyrP4+).