The Role of Electronic Coupling in Linear Porphyrin Arrays Probed by Single-Molecule Fluorescence Spectroscopy

Single‐molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single‐molecule fluorescence detection techniques. Butadiyne‐linked arrays (ZNB) with extensive π‐conjugation perform as photostable one‐quantum systems. This demonstration has been suggested by the long‐lasting initial emissive state and subsequent discrete one‐step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one‐quantum system, ZNB shows anti‐bunching data in the coincidence measurements. On the other hand, in directly‐linked arrays (ZN) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of ZN do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron‐hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single‐molecule level. Effective communication: At a single‐molecule level, butadiyne‐linked linear porphyrin arrays with extensive π‐conjugation perform as photostable one‐quantum systems, whereas strong excitonic dipole coupling observed for directly‐linked linear porphyrin arrays favors strong communication between porphyrins, but allows each porphyrin to maintain its individual character (see figure).