Single molecule spectroscopic investigation on conformational heterogeneity of directly linked zinc(II) porphyrin arrays

We have comparatively investigated the photophysical properties of a series of meso-meso directly linked orthogonal porphyrin arrays (Zn, n = 1, 2, 3, 4, 6, 8, 9, 12, 16, 32, 48, 64, and 96) by ensemble average and single molecule fluorescence spectroscopy. In single molecule fluorescence study, we have recorded the fluorescence intensity trajectories of Zn arrays as the number of porphyrin molecules in the array increases. Up to Z8 in porphyrin arrays, each single array exhibits multiple stepwise photobleaching behaviors in fluorescence intensity trajectories, indicating that each porphyrin unit in the array acts as an individual fluorescent unit due to a maintenance of linear rigid structure of the array. On the other hand, porphyrin arrays longer than Z8 such as Z16, Z32, Z48, and Z64 show complicated photobleaching behaviors in fluorescence intensity trajectories. The origin of complex photobleaching behaviors is believed to be increasing nonradiative decay channels contributed by the enhanced structural nonlinearity in longer arrays. The fluorescence measurements of Zn arrays on single molecule level show a mismatch in the maximum fluorescence intensity level as compared to the solution measurements, which is attributable to the difference in local environment surrounding the porphyrin array. In this work, we have demonstrated the presence of conformational heterogeneity in longer porphyrin arrays by analyzing average survival times and fluorescence spectra of single arrays as the number of porphyrin molecules in the array increases. We believe that the fluorescence properties of porphyrin arrays on single molecule level will provide a platform for further applications as molecular photonic wires.