Excitation energy migration processes in self-assembled porphyrin boxes constructed by conjugated porphyrin dimers

meso-Pyridine-appended alkynylene-bridged zinc(II) porphyrin dimers D2 and D4 assemble spontaneously, in noncoordinating solvents such as toluene, into tetrameric porphyrin boxes B2 and B4, respectively. Interestingly, the formation of Bn from Dn leads to the two kinds of self-assembled porphyrin boxes constructed by planar and orthogonal conformers, respectively. Excitation energy migration processes within these assemblies have been investigated in detail by using both steady-state and time-resolved spectroscopic methods. The pump-power dependence on the femtosecond transient absorption decay profiles is directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time is well-described in terms of the Föster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates in porphyrin boxes constructed by planar and orthogonal conformers have been estimated to be (∼1.2 ps)^-1 and (∼1 ps)^-1, respectively. Furthermore, the porphyrin boxes constructed by orthogonal conformers show additional slow excitation energy hopping rates of (∼12 ps)^-1. Overall, the dihedral angle in the constituent dimers is a key factor to control the energy transfer efficiency for the fabrication of artificial light-harvesting complexes using conjugated porphyrin dimer systems.