Photoinduced Processes in Self-Assembled Porphyrin/Perylene Bisimide Metallosupramolecular Boxes

Two new supramolecular boxes, (ZnMC)(2)(rPBI)(2) and (ZnMC)(2)(gPBI)(2), have been obtained by axial coordination of N,N'-dipyridyl-functionalized perylene bisimide (PBI) dyes to the zinc ion centers of two 2+2 porphyrin metallacycles (ZnMC = [trans,cis,cis-RuCl2(CO)(2)(Zn 4'-cis-DPyP)](2)) The two molecular boxes involve PBI pillars with different substituents at the bay area the red PBI (rPBI = N,N'-di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4 9 10-tetracarboxylic acid bisimide) containing tert-butylphenoxy substituents and the green PBI (gPBI = N N'-di(4-pyridyl)-1,7-bis(pyrrolidin-l-yl)perylene-3,4 9,10-tetracarboxylic acid bisimide) bearing pyrrolidinyl substituents Due to the rigidity of the modules and the simultaneous formation of four pyridine-zinc bonds, these discrete adducts self-assemble quantitatively and are remarkably stable in dichloromethane solution The photophysical behavior of the new supramolecular boxes has been studied in dichloromethane by emission spectroscopy and ultrafast absorption techniques A different photophysical behavior is observed for the two systems In (ZnMC)(2)(rPBI)(2), efficient electron transfer quenching of both perylene bisimide and zinc porphyrin chromophores is observed leading to a charge separated state, PBI--Zn+ in which a perylene bisimide unit is reduced and zinc porphyrin is oxidized In the deactivation of the perylene bisimide localized excited state an intermediate zwitterionic charge transfer state of type PBI--PBI+ seems to play a relevant role In (ZnMC)(2)(gPBI)(2) singlet energy transfer from the Zn porphyrin chromophores to the perylene bisimide units occurs with an efficiency of 0 7 This lower than unity value is due to a competing electron transfer quenching leading to the charge separated state PBI--Zn+ The distinct photophysical behavior of these two supramolecular boxes is interpreted in terms of energy changes occurring upon replacement of the red rPBI by green gPBI