Photophysics and excited state electronic communication in quadruply bonded paddlewheel complexes of molybdenum and tungsten.

摘要

Molecule based electronics and devices are an increasingly popular area of research in chemistry. These molecular-based devices largely rely on the separation of charge from (solar cell, LED) or movement of charge through (wires) a molecular unit. Largely, it is desirable for these materials to be easily fabricated, absorb throughout the visible/NIR spectrum or emit certain wavelengths. Organic systems generally provide good fabrication properties while the incorporation of metals can provide more easily tunable physical properties. Metallo-organic paddlewheel compounds involving quadruple bonds have previously been made into soluble, linear polymers with tunable absorption and have been incorporated into an LED to show electroluminescence. In terms of device performance, it is important to know how well charge can be expected to flow through the material. In devices that rely upon photon absorption, charge transport ability is dependent on charge delocalization and rates of transport. As a first step in these regards a series of complexes which represent simple monomeric analogs to the individual repeating units of the polymer have been studied. They serve as model complexes to the polymeric and a better understanding of their fundamental properties should relate to better design of polymeric materials.;This dissertation uses both electronic and vibrational spectroscopies to characterize photoexcited states, determine their lifetimes, and evaluate the electronic delocalization within these states. Theoretical calculations also supported the results. Four molecules constitute limiting cases across a wide set of properties and are the focus of this work. Chapters 2 describes the molecules M2(O2CTiPB)2(O 2C-C6H4-C≡N)2 (2a and 2b) and 3 describes the molecules M2(O2CCH3) 2(NiPr)2C-C≡C-C6H5] 2 (3a and 3b), where M = Mo (a) or W (b), each focusing on results from electronic spectroscopy. In particular, assignments for the photophysical excited states were made as well as some elucidation of the electronic delocalization. Chapter 4 describes both ground state and time-resolved infrared (TRIR) experiments and directly compares the systems studied in chapters 2 and 3. The vibrational experiments confirm the excited state assignments and classify the electronic delocalization according to the Robin and Day Scheme.;In chapters 2 and 3, each compound was found to have S1, MLCT states. Irradiation of these states resulted in a bifurcation of the absorbed energy, giving both fluorescence and intersystem crossing to the T1 state. When the metal is Mo, the T1 state is metal centered and characterized as 3deltadelta* whereas when the metal is W the T1 state is MLCT. These conclusions were suggested by steady state absorption, emission, DFT calculations, and both nanosecond and femtosecond transient absorption methods. In chapter 4, each compound was investigated by time-resolved infrared spectroscopy. These results showed through nu(C≡C) and nu(C≡N) excited state absorptions characteristic of the MLCT states. Furthermore, by the presence of a single (2a and 2b) nu(C≡N) peak or a set of two (3a and 3b) nu(C≡C) peaks, the MLCT states were shown to be either delocalized or localized, respectively.