Singlet molecular oxygen photosensitized by Rhodamine dyes: correlation with photophysical properties of the sensitizers

By measuring its IR phosphorescence the formation of singlet molecular oxygen O2 photosensitized by rhodamine dyes is directly proved. The O2 formation rate is compared with that expected from the low probability (≈1%) of intersystem crossing of the photosensitizers. The quantum yield for triplet population and the triplet lifetime of the investigated dyes is measured by using a laser-scanning-microscopy technique. The influence of quenching agents (nitrobenzene and COT) is discussed. It results that the formation of O2 can be prevented effectively by quenching of the S1 or T state of the photosensitizer. The influence of the molecular ground-state oxygen O2 concentration [O2] is investigated. The presence of the paramagnetic O2 leads to an increased S1→T intersystem crossing rate of the photosensitizers and therefore to a reinforced formation of singlet molecular oxygen. It is found for rhodamine 6G as well as for rose bengal that in air-saturated acetonitrile nearly the half of the excited dye triplets are quenched by molecular oxygen. The O2 concentration can be significantly reduced by decreasing the O2 concentration below its air saturated level. ©1999 Elsevier Science S.A. All rights reserved.