Laser-induced optoacoustics combined with near-infrared emission. An alternative approach for the determination of intersystem crossing quantum yields applied to porphycenes

An approach was developed to determine triplet quantum yields, Φ_T, which overcomes most of the difficulties usually encountered in flash photolysis studies of triplet states. The combined application of near-IR emission and optoacoustics yields Φ_T values which are independent of the absorption properties of the triplets. Thus, the triplet energies derived from near-IR phosphorescence spectra together with the values of the heat stored by the triplets determined from optoacoustic experiments afforded Φ_T = 0.3 ± 0.1 for porphycene (PO) and 0.4 ± 0.1 for its tetra-n-propyl derivative (TPrPO). Several calorimetric references for optoacoustic measurements in the red region were tested. The Φ_T values, combined with flash photolysis data, yielded accurate triplet- minus ground-state absorption coefficients. The quantum yields for singlet molecular oxygen production determined by time-resolved phosphorescence were Φ_Δ = 0.34 ± 0.05 for PO and 0.36 ± 0.03 for TPrPO which implies an efficiency near unity for O₂(¹Δ₈) production upon collision of the triplet states of the porphycenes with ground-state O₂.