Effect of the media on the quantum yield of singlet oxygen (O₂(¹Δ_g)) production by 9H-fluoren-9-one: Solvents and solvent mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O₂(¹Δ_g), denoted as ¹O₂) by 9H-fluoren-9-one (FLU). The normalized empirical parameter E^N_T derived from E_T(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of ¹O₂ production ^Φ_Δ decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (^Φ_ISC). Values of ^Φ_Δ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, Φ_ISC and, therefore, ^Φ_Δ decrease due to solvent-induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor ¹O₂ sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing ^Φ_Δ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of ^Φ_Δ upon addition of increasing amounts of the latter.