Excitonic coupling strength and coherence length in the singlet and triplet excited states of meso-meso directly linked Zn(II)porphyrin arrays

Excitation-energy transport (EET) phenomena in meso - meso directly linked Zn(II)porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady-state and time-resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II)porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm^-1, and that of the Q bands is about 570 cm^-1. The coherence length in the S₁ state of the Zn(II)porphyrin arrays was determined to be 4-5 porphyrin units, which is comparable to that of the well-ordered two-dimensional circular structure B850 in the peripheral light-harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II)porphyrin arrays are well suited for mimicking natural light-harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 μs^-1, and the very weak coupling between the triplet states (ca. 0.003 cm^-1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.