By integrating the results of MS-CASPT2/CASSCF and TD-PBEO calculations, we propose a mechanism for the decay of the excited dark state in pyrimidine, fully consistent with all the available experimental results. An effective conical intersection (CI-nπ) exists between the spectroscopic π/π* excited state (Sπ) and a dark n/π* state (Sn), and a fraction of the population decays to the minimum of Sn (Sn-min). The conical intersection between Sn and the ground-state is not involved in the decay mechanism, because of its high energy gap with respect to Sn-min. On the other hand, especially in hydrogen bonding solvents, the energy gap between Sn-min and Cl-nπ is rather small. After thermalization in Sn-min, the system can thus recross Cl-nπ and then quickly proceed on the Sπ barrierless path toward the conical intersection with the ground state. © 2008 American Chemical Society.

*** IBB - CNR ***

Department de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel.lí Domingo, s/n-Campus Sescelades, 43007 Tarragona, Spain

Istituto per i Processi Chimico-Fisici, CNR, Area della Ricerca Del CNR, Via Moruzzi 1, I-56124 Pisa, Italy

Dipartimento di Chimica, Università Federico II, Via Cintia, I-80126 Napoli, Italy

Department de Qu mica F sica i Inorg nica, Universitat Rovira i Virgili, Marcel. l Domingo, s/n-Campus Sescelades, 43007 Tarragona, Spain

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