Effect of the Media on the Quantum Yield of Singlet Oxygen (O 2 ( 1 Δ g )) Production by 9 H ‐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 (O 2 ( 1 Δ g ), denoted as 1 O 2 ) by 9 H ‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from E T (30) has been chosen as a measure of solvent polarity using Reichardt 's betaine dyes. Quantum yields of 1 O 2 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 1 O 2 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.