Infrared spectroscopy of electronically excited state molecular clusters of coumarins and aminophthalimides

This thesis presents exploratory work on the jet-cooled molecular clusters of coumarins and aminophthalimides, especially in the electronically excited state, using mainly the IR-UV double-resonance technique. While ground-state infrared measurements of these large clusters provide important reference information such as equilibrium structures, conformers and vibrational energies, the emphasis is placed upon identifying the structural, spectroscopic and dynamic consequences of electron density redistribution in the electronically excited state. Our data indicate that given the relative positions of the hydrogen-bonded key functional groups on the parent chromophores, sizable changes occur in their basicity/acidity as a result of large, directional electron density flow across the aromatic template. This in many cases leads to significantly altered hydrogen-bonding strengths and well-shifted IR frequencies up to several hundred wavenumbers. Some of the species display further spectral complexities with characteristic resonance patterns possibly associated with strong Fermi coupling. In addition, a lot of clusters with interesting intermolecular structures are made in the jet and spectroscopically characterized, including symmetric molecular dimers with anti-parallel double hydrogen-bonds, and a variety of solvated species with bridge or cyclic hydrogen-bonded structures. The effects of vibronic excitation in the excited state are also explored, revealing several cluster systems that undergo conformational relaxation subject to a small barrier energy. Infrared vibronic signatures of both the initial reactant and the final product are obtained, which illustrate the changing cluster structures and intermolecular interactions