Analysis of Hydrogen Atom Transfer in Photoexcited Indole(NH₃)_n Clusters by Femtosecond Time-Resolved Photoelectron Spectroscopy

Abstract

The photoinduced H-atom-transfer reaction in indole(NH3) n clusters has been analyzed by femtosecond time-resolved photoelectron−photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1−3) and larger (n ≥ 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited ππ* state to the πσ* state. In contrast, for the larger clusters (n ≥ 4) such an initial ultrafast process is not observable probably for Franck−Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.

Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: Competitive reaction paths and vibrational mode selectivity

“…In the 7-hydroxyquinoline· ͑NH 3 ͒ 3 ͓7HQ· ͑NH 3 ͒ 3 ͔ cluster, the 7-hydroxyquinoline molecule acts as a scaffold, its O-H donor and N acceptor groups forming hydrogen bonds to the ends of a homodromic threemembered hydrogen-bonded ammonia wire. 7,8 Experimental spectroscopic studies of phenol· ͑NH 3 ͒ n clusters 9-12 and the indole· NH 3 complex 13 as well as femtosecond pump-probe kinetic studies of indole· ͑NH 3 ͒ n clusters [14][15][16][17][18] and of pyrrole· ͑NH 3 ͒ n clusters 19,20 have demonstrated the occurrence of H-atom transfer to solvent clusters. In the final step the keto tautomer 7-ketoquinoline· ͑NH 3 ͒ 3 ͓7KQ· ͑NH 3 ͒ 3 ͔ is formed: the overall H-atom transfer reaction along the wire amounts to an enol→ keto tautomerization reaction.…”

Rotational spectrum, structure and internal dynamics of the pyrrole·ammonia complex

“…They suggest that the photochemical dynamics is governed by an excited-state hydrogen transfer reaction. Experimental evidence for this proposed mechanism is clearly shown with investigations on clusters of small organic molecules like phenole, indole and pyrrole with water and ammonia [6][7][8][9][10]. In order to describe microsolvation, both experimental and theoretical approaches are thus used to understand the geometry of binary hydrogen-bonded complexes and the internal dynamics of both moieties with respect to each other, besides further properties like the electronic structure.…”

Femtosecond Time‐Resolved Hydrogen‐Atom Elimination from Photoexcited Pyrrole Molecules

“…[2] The H-atom transfer reactions in phenol(NH 3 ) n [3,4] or indole(NH 3 ) n clusters, [5,6] which were studied experimentally and theoretically, have also been interpreted on the basis of the ps * state. The transfer of the H atom (from the OH group in phenol or the NH group in indole) to the NH 3 molecules of the cluster proceeds on the ps * potential surface.…”