Persistent Photoconductivity and Photo-induced Morphology Changes of Porphyrin Nanorods

Tetrakis(4-sulfonatophenyl) porphine self assembles into well-defined nanorods with intriguing photoelectronic properties.$^{1}$ Recently, we have found that, over long time scales, they undergo a transition from non-persistent photoconductivity (NPPC) to a new mode, in which part of the conductivity persists after the light is blocked, decaying over hundreds of seconds. NPPC initially dominates, but its growth asymptotes within 2-3 hours of illumination, while the persistent current continues to grow, even after 8 hours of light exposure. The decay of persistent current after the light is blocked can be roughly modeled by a single exponential; a double exponential fits much better. The morphology of some nanorods changes as a result of long-term illumination---they become shorter, thinner and less well-formed. Other rods, however, appear unchanged. This and other structural changes may be related to the slow growth of persistent current. $^{1}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004).