Dynamics of optically excited tetraphenylporphyrin films evaporated on a conductive SnO₂ Substrate

The spectrum and the decay time of the excimer luminescence in very thin tetraphenylporphyrin (H₂TPP) films evaporated on SnO₂ substrates have been measured in situ at a pressure of 2 × 10^-6Pa at 100K using a picosecond dye laser. The decay time of the 665 nm luminescence in H₂TPP on a highly conductive SnO₂ (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. On the other hand, the decay time in the films on a low conductive SnO₂ (σ = 4.5 x 10^-3 Ω^-1 cm^-1) is dependent of film thickness. It is found that luminenscence quenching occurs through the excimer state because of the coincidence in the quenching ratios calculated from the decay time and the luminiscence intensity. We propose a mechanism where an excited electron is transferred from the excimer state of H₂TPP to the conduction band of the highly conductive SnO₂ substrate.