CAS: Collaborative Research: Mapping Excited State Trajectories of Multi-metal Centered Complexes by Two-Dimensional Electronic Spectroscopy

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With support of the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Lin X. Chen of the Department of Chemistry at Northwestern University and Professor Felix N. Castellano of the Department of Chemistry at North Carolina State University seek to understand the effects of vibronic coherence in excited state photochemical and photophysical processes for energy and electron transfer reactions. This project aims to gain new insights into light-matter interactions at the levels of electrons and atoms and use this knowledge to leverage controlled reactivity in condensed phases, molecular photonics, and photocatalysis via both chemical structural tuning and light modulation. The knowledge obtained through the proposed studies has the potential to significantly enhance our ability to rationally design chemical materials/devices for catalysis, optoelectronics, and energy sustainability. The proposed research engages University graduate students in advanced laser spectroscopy for characterization, incorporates state-of-the-art computational chemistry, and utilizes advanced chemistry laboratory facilities for molecular syntheses and structural characterization, engendering multidisciplinary scientific expertise. The proposed research and education activities are integrated with K-12 public outreach events at both institutions. The collaborative team will investigate the effects of vibronic coherence in excited state electron transfer in three classes of multi-chromophore synthetic platforms using two-dimensional electronic spectroscopy (2DES) and broadband transient absorption (BBTA) spectroscopy. The three specific molecular platforms are (1) new generation Pt(II) co-facial dimers featuring terminal tridentate cyclometalating ligands bridged by a single substituted pyrazolate, hydroxypyridine, mercaptopyridine, or diphosphine featuring metal-metal-to-ligand charge transfer (MMLCT) excited states where a metal-metal bond is transiently formed following light excitation; (2) Pt(II) dimers from (1) whose terminal cyclometalating ligands are decorated with naphthalenediimide (NDI)-based electron acceptors; and (3) co-facial self-assembled polynuclear aggregates formed from Pt(II) monomer building blocks with and without electron acceptors. These experimental systems are engineered to tune electronic coupling in their excited states, which are either responsible for or can be correlated to electronic and vibronic coherence originating from in-phase nuclear motions induced by photon excitation. The detection of electronic and vibronic coherences in these designed molecules will be assessed using broadband 2DES featuring ~7 femtosecond time resolution and sufficiently broad spectral bandwidth. The measured coherences will be correlated with photophysical processes and photoinduced intramolecular electron transfer chemistry, rationalized using continuously developing theory to assess the role of electronic and/or vibronic coherence in these fundamental primary processes related to light activation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.