Excitation energy migration in a dodecameric porphyrin box

The excitation energy hopping (EEH) in a dodecameric porphyrin box (TB2) prepared by self-assembly of four meso–meso linked zinc(II) triporphyrin (T2)s is explored by picosecond fluorescence and femtosecond transient absorption (TA) spectroscopy. The exciton–exciton annihilation and anisotropy depolarization observed in both fluorescence and TA provide a consistent EEH rate, i.e. 300 ± 20 ps−1, assuming Förster energy hopping among four T2 units in TB2. The observed energy hopping time 300 ± 20 ps−1 is slower than the previous 98 ± 3 ps−1 of B2 (an octameric porphyrin box), even though the size of hopping unit is larger in TB2.