Highly efficient light-harvesting system based on a phosphorescent acceptor coupled with dendrimer donors via singlet-singlet and triplet-triplet energy transfer

Abstract

A series of efficient light-harvesting systems (FIr3mGx, x = 1-3) based on dendrimer donor (mCP: N,N'-dicarbazolyl-3,5-benzene) derivatives coupled with a sky blue phosphorescent acceptor (FIrpic: iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C-2']-3-hydroxypicolinate derivative) were prepared, and their photophysical properties were investigated. The light-harvesting ability of FIr3mGx (x = 1-3) increases with the number of energy donor mCP units. The acceptor PL intensity of the donor-acceptor conjugate dendrimers (FIr3mGx, x = 1-3) increases over 6 times with the number of mCP units via sensitization from the light-harvesting antenna after excitation at the donor absorption (lambda(ex) = 310 nm), as compared to that of only an acceptor without a donor (FIrpic) upon excitation at the MLCT region (lambda(ex) = 380 nm) because of efficient intramolecular singlet-singlet and triplet-triplet energy transfer from the donor to the acceptor and less luminance quenching induced by the bulky dendrimer structure. The singlet-singlet intramolecular energy transfer in FIr3mGx from the dendrimer donor to the acceptor exhibits a high efficiency of greater than 90% via the steady-state PL method and 94% via the transient PL method. Additionally, the triplet-triplet energy transfer efficiency via the transient PL method exhibits an efficiency greater than 99%. This indicates that FIr3mGx exhibits good light-harvesting abilities because the mCP unit is an efficient light absorber and because the core FIrpic functions as a spatial energy concentrator that drains energy from the dendrimer donor.