Excitation energy migration processes in various multi-porphyrin assemblies

The electronic interactions and excitation energy transfer (EET) processes of a variety of multi-porphyrin arrays with linear, cyclic and box architectures have been explored. Directly meso-meso linked linear arrays (Z(N)) exhibit strong excitonic coupling with an exciton coherence length of approximately 6 porphyrin units, while fused linear arrays (T(N)) exhibit extensive π-conjugation over the whole array. The excitonic coherence length in directly linked cyclic porphyrin rings (CZ(N)) was determined to be approximately 2.7 porphyrin units by simultaneous analysis of fluorescence intensities and lifetimes at the single-molecule level. By performing transient absorption (TA) and TA anisotropy decay measurements, the EET rates in m-phenylene linked cyclic porphyrin wheels C12ZA and C24ZB were determined to be 4 and 36 ps(-1), respectively. With increasing the size of C(N)ZA, the EET efficiencies decrease owing to the structural distortions that produce considerable non-radiative decay pathways. Finally, the EET rates of self-assembled porphyrin boxes consisting of directly linked diporphyrins, B1A, B2A and B3A, are 48, 98 and 361 ps(-1), respectively. The EET rates of porphyrin boxes consisting of alkynylene-bridged diporphyrins, B2B and B4B, depend on the conformation of building blocks (planar or orthogonal) rather than the length of alkynylene linkers.