Reversible Persistence and Effects of Oxygen on the Photoconductivity of Porphyrin Nanorods

Tetrakis(4-sulfonatophenyl) porphine (TPPS$_{4})$ self assembles$^{1}$ into well-defined nanorods with intriguing photoelectronic properties.$^{2}$ New experiments show that illumination under Ar for several hours induces a change to persistent behavior, i.e. conductivity decays slowly when light is removed, rather than dropping to zero. After resting 24 hours, the sample recovers non-persistent behavior. The dark conductivity of TPPS$_{4}$ aggregates formed by a different technique is sensitive to O$_{2}$.$^{3}$ We find that the conductivity under illumination of nanorod aggregates decreases when 0.2{\%} O$_{2}$ is added, but this change is reversible. By contrast, if the sample is exposed to 21{\%} O$_{2}$ shortly after light is removed, the photoconductivity is permanently lowered. These effects may be due to a combination of O$_{2}$-mediated quenching of excited state porphyrin and oxidation. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{3}$Y. Otsuka \textit{et al.}, Nanotechnology \textbf{15}, 1639 (2004).