Hosted by: Mircea Cotlet

Lowest energy electronic excited states (LEES) in transition metal complexes are the states most relevant for practical photophysical and photochemical processes. We investigated relaxation dynamic of two systems – copper chloride dianion with strong Jahn-Teller effect and hexabromoiridate dianion with spin-spin coupling, utilizing 2000 nm near-IR femtosecond (100 fs) pump-probe spectroscopy. In both systems, the Franc- Condon excited states of the transition metal complexes undergo internal conversion to the ground electronic states, but with significantly different lifetimes (55 fs and 360 ps, respectively), despite the fact that the metal-centered states are separated by the same energy gap (~5000 wavenumbers) from the respective ground state. This difference is explained by presence of a conical intersection between the first excited electronic and the ground states in the Cu(II) system due to strong Jahn-Teller linear distortion whereas the involved potential energy surfaces for the Ir(IV) complex are nested directly one above another. Another project under consideration is the ultrafast mechanisms of polyhalomethanes on the example of diiodomethane. This molecule has a tractable number of degrees of freedom, and, therefore, has served in literature as a model system for bond dissociation processes in both gas and condensed phases. In this work we implemented the state-of-the-art ultrafast (~35 fs) transient absorption experiment (supported by the most accurate multireference quantum chemical methods) to understand the UV photodissociation mechanism of methylene iodide molecules. We discovered previously unsuspected photochemical pathway in the UV photochemistry of methylene iodide, in which electronically excited molecules, rather than simply dissociate, undergo direct ~50-fs isomerization through a conical intersection into isomeric species. Host: Mircea Cotlet