title: Electronic and structural elements that regulate the excited-state dynamics in purine nucleobase derivatives.
abstract: The excited-state dynamics of the purine free base and 9-methylpurine are investigated using experimental and theoretical methods. Femtosecond broadband transient absorption experiments reveal that excitation of these purine derivatives in aqueous solution at 266 nm results primarily in ultrafast conversion of the S2(ππ*) state to the vibrationally excited (1)nπ* state. Following vibrational and conformational relaxation, the (1)nπ* state acts as a doorway state in the efficient population of the triplet manifold with an intersystem crossing lifetime of hundreds of picoseconds. Experiments show an almost 2-fold increase in the intersystem crossing rate on going from polar aprotic to nonpolar solvents, suggesting that a solvent-dependent energy barrier must be surmounted to access the singlet-to-triplet crossing region. Ab initio static and surface-hopping dynamics simulations lend strong support to the proposed relaxation mechanism. Collectively, the experimental and computational results demonstrate that the accessibility of the nπ* states and the topology of the potential energy surfaces in the vicinity of conical intersections are key elements in controlling the excited-state dynamics of the purine derivatives. From a structural perspective, it is shown that the purine chromophore is not responsible for the ultrafast internal conversion in the adenine and guanine monomers. Instead, C6 functionalization plays an important role in regulating the rates of radiative and nonradiative relaxation. C6 functionalization inhibits access to the (1)nπ* state while simultaneously facilitating access to the (1)ππ*(La)/S0 conical intersection, such that population of the (1)nπ* state cannot compete with the relaxation pathways to the ground state involving ring puckering at the C2 position.
authors.affiliation: †Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States.
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Crespo-Hernández
authors.affiliation: ‡Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Campus de Excelencia UAM-CSIC, Módulo 13, Cantoblanco, 28049 Madrid, Spain.
authors.forename: Lara
authors.initials: L
authors.lastname: Martínez-Fernández
authors.affiliation: §Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Clemens
authors.initials: C
authors.lastname: Rauer
authors.affiliation: †Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States.
authors.forename: Christian
authors.initials: C
authors.lastname: Reichardt
authors.affiliation: §Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Mai
authors.affiliation: †Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States.
authors.forename: Marvin
authors.initials: M
authors.lastname: Pollum
authors.affiliation: §Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Philipp
authors.initials: P
authors.lastname: Marquetand
authors.affiliation: §Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Leticia
authors.initials: L
authors.lastname: González
authors.affiliation: ‡Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Campus de Excelencia UAM-CSIC, Módulo 13, Cantoblanco, 28049 Madrid, Spain.
authors.forename: Inés
authors.initials: I
authors.lastname: Corral
mesh_terms: D065966:Absorption, Physicochemical; D004583:Electrons; D008958:Models, Molecular; D008968:Molecular Conformation; D011687:Purines / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics; D014732:Vibration
chemical_list: D011687:Purines; C030985:purine