Luminescence quenching of an ultrathin tetraphenylporphyrin film on a conductiveSnO2substrate

Absorption and luminescence spectra of ultrathin tetraphenylporphyrin (${\mathrm{H}}_2$TPP) films evaporated on ${\mathrm{SnO}}_2$ substrates have been measured in situ in a vacuum of 2×${10}^{\mathrm{-}6}$ Pa at 100 K. The luminescence intensity from a ${\mathrm{H}}_2$TPP film on a ${\mathrm{SnO}}_2$ substrate having a conductivity of 77 ${\Omega }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$ is found to be 10 times smaller than that from a ${\mathrm{H}}_2$TPP film on a substrate of conductivity 4.5×${10}^{\mathrm{-}3}$ ${\Omega }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$. This quenching of the luminescence intensity occurs in films thinner than 10 Å, where most of the ${\mathrm{H}}_2$TPP molecules are directly attached to the ${\mathrm{SnO}}_2$ substrate. We propose a modified charge-transfer model for the quenching mechanism, in which the transfer efficiency depends on the charge density of conduction electrons in the ${\mathrm{SnO}}_2$ substrate.