Fluorescence dynamics of directly meso-meso linked porphyrin rings probed by single molecule spectroscopy

Porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins, immobilized in a thin PMMA film, have been investigated using single molecule fluorescence spectroscopy with a focus on the influences of the overall structural rigidity as the ring size of porphyrin array increases. Neighboring porphyrin moieties were linked directly to enhance through-bond electronic interactions and, as a consequence, efficient excitation energy migration processes like the natural LH2 complex. Unlike the ensemble study, the single molecule study using confocal microscopy could eliminate the averaging effect, and consequently provide detailed information on individual molecular behaviors. Indeed, in solution, as a dihedral angle between neighboring porphyrins decreases in the order of CZ6 > CZ8 > CZ4, red-shifted Q-absorption bands and faster excitation energy hopping rates were observed. However, at the single molecule level, we found that they show longer survival times, less frequent on-off behaviors, narrower fluorescence lifetime distributions, and high relative single molecular brightness in the order of CZ8 > CZ6 > CZ4, by recording fluorescence intensity trajectories. Especially, CZ4 reveals high photostability with its rigid structure, and about 3 porphyrin units among the 4 chromophores-constituted molecule behave as a collective coherent domain. Thus, our results single out CZ4 as a potential and promising candidate for application in artificial light harvesting solid-state devices.