Luminescence quenching of an ultrathin tetraphenylporphyrin film on a conductive SnO2 substrate

Absorption and luminescence spectra of ultrathin tetraphenylporphyrin (H2TPP) films evaporated on SnO2 substrates have been measured in situ in a vacuum of 2×10-6 Pa at 100 K. The luminescence intensity from a H2TPP film on a SnO2 substrate having a conductivity of 77 -1 cm-1 is found to be 10 times smaller than that from a H2TPP film on a substrate of conductivity 4.5×10-3 -1 cm-1. This quenching of the luminescence intensity occurs in films thinner than 10 A, where most of the H2TPP molecules are directly attached to the SnO2 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 SnO2 substrate.