Excited-State Hydrogen-Atom Transfer along Solvent Wires:  Water Molecules Stop the Transfer

Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ·(NH3) n ·(H2O) m , n + m = 3, using resonant two-photon ionization (R2PI) and UV−UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is ∼200 cm-1 for 7HQ·(NH3)3, ∼350 cm-1 for both isomers of the 7HQ·(NH3)2·H2O cluster, and ∼600 cm-1 for 7HQ·NH3·(H2O)2 but increases to ∼2000 cm-1 for the pure 7HQ·(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ*) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ* fluorescence.