Excitation energy migration processes in various multi-porphyrin assemblies

Source abstract

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.