Influence of Temperature on Photoinduced Reactions of Organic Molecules in the Gas Phase

The influence of temperature on the rate constants of photoinduced intermolecular electron transfer, representing the first stage of photoinduced reactions, has been investigated based on analysis of the quenching of fluorescence of carbazole vapor by halomethanes (CHCl₃, CH₂Br₂, CCl₄, CHBr₃) and delayed fluorescence of benzophenone and anthraquinone vapors by aliphatic amines (diethylamine, dibutylamine, cyclohexylamine, triethylamine) and pyridine. It has been established that the rate constants of photoinduced electron transfer in different donor-acceptor pairs in the gas phase can increase or decrease with increase in the temperature from 433 to 623 K. The energies of activation and enthalpy of the fluorescence-quenching process have been determined. The interrelation between the rate constants of fluorescence quenching k_q and the free energy of electron transfer G_ET has been analyzed with account for the mean vibrational energy <E_vib> of the interacting molecules. It is shown that positive and negative temperature dependences k_q(T) are characteristic, respectively, of the regions of normal (k_ET increases with decrease in G_ET) and inverted (k_ET decreases with decrease in G_ET) changes in the rate constants caused by an increase in the exothermicity of the photoinduced electron transfer process.