DOE OSTI2018
The overall goal of this project is to provide a molecular-level understanding of the mechanisms and requirements for efficient exciton multiplication through singlet fission processes. Such understanding would open the paths for improving alternative sources of energy including an improved efficiency of dye-sensitized solar cells. Using high-resolution anion photoelectron spectroscopy, electronic structures of chromophores are explored by probing directly the singlet and triplet states involved in singlet fission, determining precisely their energetics, vibrational structures and couplings. To carry out these experiments, we built a flexible-source instrument that allows for cryogenic cooling of precursor anions, providing improved spectral resolutions. Moreover, our velocity map imaging spectrometer includes novel design considerations that provide better focus with less stringent parameters. Using this instrument, we studied the three-ring polycyclic aromatic hydrocarbon molecules, anthracene and fluoranthene, the latter can be considered as a covalently-bonded benzyl and naphthyl system. Starting from the radical anion ground state, we accessed the singlet and triplet states at the same time, and the results revealed intriguing vibronic coupling signatures in both electronic states, which may possibly be related to singlet fission mechanisms. We further studied aromatic oxide molecules, phenoxy, 1-naphthoxy, and 2-naphthoxy, whose large molecular dipole moments supported highly delocalized dipole bound states. An intrinsic property of singlet fission is the delocalization of excitation to nearby chromophore molecules. In an effort to extend our studies to solvated chromophore clusters, we development new instrumental capabilities that allowed for formation of weakly bound clusters around any ion by utilizing liquid-nitrogen cooled octupole ion traps. In characterizing this new approach for gas phase cluster formation, we studied the structures of a variety of solvated clusters, from solvated ionic liquid cation, where intermolecular interactions are dominated by electrostatic interactions, to solvated peptides, where hydrogen-bonding with solvent molecules can directly alter the peptide structure. Overall, we found that the formation of weakly-bound clusters in a cryogenic ion trap can be a viable approach for accessing structures that better represent those found in condensed phase. In the process, we also implemented a straightforward double-resonance setup and established a protocol for isotopic substitution using cryogenic ion traps, both of which aided in decongesting crowded experimental spectra that often accompany these larger solvated systems.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗