Aromatic ketones as standards for singlet molecular oxygen O₂(¹Δ_g) photosensitization. Time-resolved photoacoustic and near-IR emission studies

Time-resolved near-IR emission and optoacoustic calorimetry studies were carried out on the aromatic ketones phenalenone, benzanthrone, 4-phenylbenzophenone and the benzophenone-naphthalene (0.1 M) system in order to assess their adoption as solvent-independent standards for singlet molecular oxygen O₂(¹Δ_g) photosensitization. All compounds show quantum yields of O₂(¹Δ_g) production (φ_Δ) in the range 0.9-1 in cyclohexane. Increasing solvent polarity or protic character reduces the φ_Δ values for all sensitizers except phenalenone. Laser-induced optoacoustic calorimetry was used to obtain the absolute φ_Δ values for the latter compound by applying both maximum amplitude and deconvolution methods. The former yields highly precise results (3%-5% uncertainty) and has been chosen for standardization purposes. The deconvolution method yields both kinetic and quantum yield data, albeit with lower precision ( 10%-15% uncertainty ).