Picosecond luminescence approach to charge transfer in a tetraphenylporphyrin/SnO₂ interface

The decay time of the luminescence from mono-molecular and excimer states in very thin tetraphenylporphyrin (H₂TPP) films evaporated on SnO₂ substrates has been measured in situ in a pressure of 2 × 10^-6 Pa using a picosecond dye laser at 100 K. The decay time of the 665 nm luminescence in H ₂TPP on a highly conductive SnO₂ substrate (σ = 77 Ω^-1 cm^-1 is 800 ps in films with a thickness above 25 Å and decreases with decreasing thickness to 170 ps at 7.5 Å. Contrarily, the decay time for films on low conductive SnO₂ (σ = 4.5 × 10^-3 Ω^-1 cm^-1) is independent of film thickness. The luminescence quenching ratio calculated from the decay time coincides well with that from the relative luminescence intensity. This coincidence leads to the conclusion that luminescence quenching occurs through the mono-molecular excited state or the excimer state. We propose the following mechanism to explain these experimental results. First, a conduction electron of the highly conductive SnO₂ substrate recombines with a hole of the H₂TPP excimers resulting in a charge transfer state. Second, an electron is transferred from the excimer state of H₂TPP to the conduction band.