| Abstract
| - CASSCF computations show that the hydrogen-transfer-induced fluorescence quenching of the1(π,π*) excited state of zwitterionic tryptophan occurs in three steps: (1) formation of an intramolecularexcited-state complex, (2) hydrogen transfer from the amino acid side chain to the indole chromophore,and (3) radiationless decay through a conical intersection, where the reaction path bifurcates to aphotodecarboxylation and a phototautomerization route. We present a general model for fluorescencequenching by hydrogen donors, where the radiationless decay occurs at a conical intersection (real statecrossing). At the intersection, the reaction responsible for the quenching is aborted, because the reactionpath bifurcates and can proceed forward to the products or backward to the reactants. The position of theintersection along the quenching coordinate depends on the nature of the states and, in turn, affects theformation of photoproducts during the quenching. For a 1(n,π*) model system reported earlier (Sinicropi,A.; Pogni, R.; Basosi, R.; Robb, M. A.; Gramlich, G.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed.2001, 40, 4185−4189), the ground and the excited state of the chromophore are hydrogen acceptors, andthe excited-state hydrogen transfer is nonadiabatic and leads directly to the intersection point. There, thehydrogen transfer is aborted, and the reaction can return to the reactant pair or proceed further to thehydrogen-transfer products. In the tryptophan case, the ground state is not a hydrogen acceptor, and theexcited-state hydrogen transfer is an adiabatic, sequential proton and electron transfer. The decay to theground state occurs along a second reaction coordinate associated with decarboxylation of the amino acidside chain and the corresponding aborted conical intersection. The results show that, for 1(π,π*) states,the hydrogen transfer alone is not sufficient to induce the quenching, and explain why fluorescencequenching induced by hydrogen donors is less general for 1(π,π*) than for 1(n,π*) states.
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