Guest-induced photophysical property switching of artificial light-harvesting dendrimers

An artificial light-harvesting multiporphyrin dendrimer (8P _ZnP_FB) composed of a focal freebase porphyrin (P _FB) with eight zinc(II) porphyrin (P_Zn) wings exhibited unique photophysical property switching in response to specific guest molecule binding. UV/Vis titration studies indicated stable 1:2 host-guest complex formation between 8P_ZnP_FB and meso-tetrakis(4-pyridyl)- porphyrin (TPyP) for which the first and second association constants were estimated to be >10⁸ M^-1 and 3.0×10⁷ M^-1, respectively. 8P_ZnP_FB originally shows 94 % energy transfer efficiency from P_Zn to the focal P_FB. By the formation of the host-guest complex (8P_ZnP_FB2TPyP) the emission intensity of 8P_ZnP_FB is significantly decreased, and an ultrafast charge separation state is generated. The energy transfer process from P_Zn wings to the P_FB core in 8P _ZnP_FB is almost entirely switched to an electron transfer process by the formation of 8P_ZnP_FB2TPyP. Multiporphyrin dendrimers exhibit an excellent excitation energy transfer from the zinc porphyrin wings to the focal free-base porphyrin. The energy transfer is switched to an ultrafast electron transfer by the formation of a host-guest complex with tetrapyridyl porphyrin guests (green in picture). Thus, the multiporphyrin dendrimers serve as unique biomimetic models for both light-harvesting complexes LHC1 and LHC2.