Rate constants anthracene dimerization

The quantum yield of fluorescence of many fluorescent substances in solution decreases with increasing concentration. In some cases, e.g., with aqueous solutions of thionine and Methylene Blue, this selfquenching is due to formation of stable dimers in equilibrium with the monomer.78 In other cases, e.g., with solutions of anthracene, perylene, and coronene in solvents such as benzene, chloroform, and kerosene, Bowen and co-workers have shown that quenching takes place by collision17 the self-quenching rate constants obtained are very close to those given in Table I (see also ref. 19). 

Reversibility of the dimerization step on a voltammetric time scale has been verified for several of the 9-substituted anthracenes in the absence of added acids [259]. For 91a and 91e the reversibility of the dimerization step was shown to be prevented by addition of AcOH, which protonates the dimer dianion in a fast follow-up reaction (the radical anions are not protonated by AcOH) [251]. Since the rate and equilibrium constants for dimerization, A dim dim for 91e are of magnitudes that conveniently can be measured using CV and DPSC [247,248,250-253,257,259,260], 91e has been used as a model compound for mechanistic studies of systems undergoing reversible dimerization, and values of A dim and Adim have been determined under a number of different conditions (Table 21). 

At constant initial concentration of the dimer, the initial rate of decomposition is given by const. 2p(l -p) where p = [An]0/2[K+, aa, K+]0. This expression requires equal probability of anthracene addition to either end of the dimer, independent of the fate of the other end - whether associated or not with another anthracene molecule. A similar behavior was observed in a system with pyrene replacing anthracene as the electron acceptor. 

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