Photochemistry and Photophysics at Extended Seams of Conical Intersection

The role of extended seams of conical intersection in excited-state mechanisms is reviewed. Seams are crossings of the potential energy surface in many dimensions where the decay from the excited to the ground state can occur, and the extended seam is composed of different segments lying along a reaction coordinate. Every segment is associated with a different primary photoproduct, which gives rise to competing pathways. This idea is first illustrated for fulvene and ethylene, and then it is used to explain more complex cases such as the dependence of the isomerisation of retinal chromophore isomers on the protein environment, the dependence of the efficiency of the azobenzene photochemical switch on the wavelength of irradiation and the direction of the isomerisation, and the coexistence of different mechanisms in the photo-induced Wolff rearrangement of diazonaphthoquinone. The role of extended seams in the photophysics of the DNA nucleobases and the relationship between two-state seams and three-state crossings is also discussed. As an outlook, the design of optical control strategies based on the passage of the excited molecule through the seam is considered, and it is shown how the excited-state lifetime of fulvene can be modulated by shaping the energy of the seamI thank my co-workers at the University of Girona for their efforts in recent years: Quansong Li, Sergi Ruiz-Barragan, David Asturiol, Annapaola Migani and Ferran Feixas. I also thank Mike Robb and Mike Bearpark (Imperial College London) for a long-term collaboration, where many ideas shown in this review have been developed, and Fabien Gatti (University of Montpellier) and Hans-Dieter Meyer (University of Heidelberg) for support in the use of the MCTDH program. Financial support has been provided by grants CTQ2011-26573 from the Spanish Ministerio de Economia y Competividad (MINECO), SGR0528 from the Catalan Agencia de Gestio d'Ajuts Universitaris i de Recerca (AGAUR), UNGI08-4E-003 and UNGI10-4E-801 from MINECO and the European Fund for Regional Development (FEDER), and the Xarxa de Referencia en Quimica Teorica i Computacional de Catalunya from AGAU