Dynamics of optically excited tetraphenylporphyrin films evaporated on a conductive SnO 2 Substrate

Abstract

The spectrum and the decay time of the excimer luminescence in very thin tetraphenylporphyrin (H 2TPP) films evaporated on SnO 2 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 H 2TPP on a highly conductive SnO 2 (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 SnO 2 (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 H 2TPP to the conduction band of the highly conductive SnO 2 substrate.