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The rate constant (k,) and net electron-transfer yield (0,) in the quenching of an excited oxonine singlet by 24 aromatic amines and methoxy compounds were determined in methanol from stationary fluorescence measurements and flash spectroscopy. The systems investigated correspond to a variation of AGdo, the standard free reaction enthalpy of the electron transfer from the quencher to excited singlet oxonine, between -0.12 and -1.56 eV. Whereas the k, values observed are all close to the diffusion controlled limit, 9, increases when AGeto becomes more negative, whereby the following linear correlation is observed: In 0, = -5.7 -(3.2 eV-' )AG,,O.The implications of this relationship on the mechanism of free-radical formation and electronic deactivation are discussed. Mechanisms where either a solvent-shared radical pair or a relaxed exciplex are assumed as exclusive intermediates of the quenching reaction are shown to be incompatible with the experimental result. It is suggested that in the quenching reaction radical pairs and exciplexes are formed via a nonrelaxed charge-transfer state, the branching ratio between radical pair and exciplex formation depending on AG,,'.

Abstract—Rate constants, kq, for the reaction of cationic and neutral acridine orange and 10‐methylacridine orange triplet states (3AOH +, 3AO, 3MAO+) with a series of electron donors have been measured. Two different protolytic forms of the semireduced dye radical are produced by acridine orange triplet quenching at various pHM values in methanolic solution. It is found that k4 decreases with increasing oxidation potential of the reducing agent for all triplet states in a manner which is expected for electron transfer reactions. The different reactivities of the cationic and neutral triplet forms can, therefore, be attributed to the difference in reduction potentials of these species. The difference in reduction potentials is related to the pKM values of triplet state, pKTM, and semireduced dye radical, pKMS, by thermodynamic consideration. pKTM (3AOH+/3AO) is determined to be 11.2. From thispKSM (AOH./AO;) is estimated to be 17–18. This is in striking contrast to the protolytic equilibrium of the semireduced dye radicals found to be pKF= 4.1. We conclude that the last value represents the second protonation equilibrium (AOH+2./AOH). This conclusion is confirmed by spectroscopic data.

Abstract— The photoreduction of oxonine, thionine and selenine with the reducing agent allylthiourea was investigated by flash photolysis. The oxonine triplet state was produced by triplet‐triplet energy transfer with 9,10‐dibromoanthracene as donor. For all three dyes the rate constant of the electron transfer is considerably higher for the acid triplet form than that of the corresponding reaction of the basic triplet form. It is shown that the higher reactivity of the acid triplet can be related to its higher reduction potential which is available from the difference of the pK values of triplet and semiquinone of the dye.

Quenching of the excited states of lumiflavin and 3-methyl-5-deaza-lumiflavin by methyl-and methoxy-substituted benzenes and naphthalenes in methanol was investigated. The observed difference in the reactivity of acid and neutral lumiflavin triplets is explained thermodynamically by applying the Michaelis cycle, as being due to the higher reduction potential of the acid triplet. In this connection the pK values of lumiflavin triplet (pKM = 6.5) and semiquinone (pKM = 11.3) have also been determined in methanol. The difference in the reactivity between the singlet and triplet states of lumiflavin is found to be greater as predicted by the difference in excitation energy. The reactivities of the excited states of flavin and 5-deazaflavin differ only slightly in contrast to the marked difference in the ground state reactivities of electron transfer reactions. This is explained in terms of the model of Rehm and Weller. The pH dependence of the electron transfer quenching of 5-deazaflavin triplet was investigated in water, yielding a triplet pK of 2.5. In contrast to the flavin, this triplet pK does not significantly differ from the pK of the 5-deazaflavin ground state. From this, different sites of protonation are deduced for the photoexcited triplet states of flavin and 5-deazaflavin.