Electronic Deactivation and Reaction Dynamics of H-bonded Molecular Systems Studied by Femtosecond Time-Resolved Vibrational Absorption Spectroscopy

The goal of this Thesis was the investigation of ultrafast electronic deactivation pathways in both intra- and intermolecularly H-bonded systems in a time-resolved and structure-sensitive manner by a combination of spectroscopic pump-probe techniques and quantum chemical calculations. Towards these ends, a transient vibrational absorption spectrometer was built, which allows for the investigation of the temporal evolution of characteristic vibrational marker bands in the mid-IR spectral region after UV excitation. Using this setup, 2-aminopurine (2AP) was investigated in its monomeric form and upon H-bonding with thymine (T) in Watson-Crick (WC) and Hoogsteen (HS) conformations as a model system for intermolecular H-bonding. An exploratory study on free 2AP revealed the involvement of an intermediate state in the deactivation dynamics towards concurrent efficient ground-state recovery and triplet state formation within ~740 ps. Upon H-bonding of 2AP with T in the WC conformation, internal conversion towards the intermediate state is hindered by an increased potential energy barrier. Thus, the excited-state lifetime of the 2AP chromophore in the H-bonded 2AP-T network is increased to ~1.6 ns. In the HS form, however, the 2AP moiety features a distorted excited-state minimum structure. Induced by H-bonds with T, this bent structure of 2AP opens up novel deactivation pathways for efficient ground-state recovery with ~68 ps. The observed kinetics are discussed in terms of excited-state interbase double-proton transfer. Moreover, in another case study, the influence of excited-state intramolecular proton transfer (ESIPT) on the electronic deactivation and reaction dynamics of the photoswitch N-(3-pyridinyl)-2-pyridinecarboxamide (NPPCA) was investigated. Unambiguous indications for ESIPT-induced switching dynamics on the ultrafast timescale were found in the reaction pathways towards transient formation of a novel photochromic proton-transferred species in ~20 ps.