Examination of the MLCT Excited States of Ditungsten Quadruply Bonded Paddlewheel Complexes

Around the world today, the utilization of fossil fuels to provide humankind with the requisite magnitude of energy to maintain a technologically advanced society is becoming antiquated. Renewable processes that do not contribute to the alteration of the climate can help to ensure that Earth will remain recognizable for generations to come. Photons from the sun represent an excellent starting point due to the endless, free supply available in most places of the world. An obstacle that inhibits the transition to greater solar energy use is the need for more reliable, efficient, and inexpensive photon harvesting systems to be discovered and implemented. Charge transfer complexes are of particular interest due to their high absorption intensities throughout the visible region and the efficiency that these complexes can provide when converting photon energy to usable electricity. Such complexes, when comprised of a metal center surrounded by organic ligands (metal-organic complexes), exhibit a metal to ligand charge transfer (MLCT) transition upon excitation by absorption of a photon. This process induces a spatial transfer of electron density towards the periphery of the complex, which allows the charge to be used in many applications. Two complexes composed of a quadruply bonded ditungsten center coordinated by four carboxylate ligands were investigated in order to examine their structure, as well as their two long-lasting, prevalent excited states. One of the complexes (Complex 1) has four identical 9-anthracene carboxylate (AnCOO-) ligands bonded to the bimetallic core. The other complex (Complex 2) has two AnCOO- ligands coordinated in a trans fashion with respect to each other, as well as a pair of pivalate ligands in the other coordinating positions. Based on electronic absorption data, computed molecular orbital electron densities, and previous studies of similar compounds, both of these complexes undergo excitation into an MLCT state upon absorption of radiation from the visible region of the electromagnetic spectrum. Complexes 1 and 2 have relatively long-lived 1MLCT excited state with lifetimes of 26 ps and 25 ps, respectively, when compared to previously reported coordination compounds. Additionally, 1 goes on to populate a 3MLCT excited state from the 1MLCT state, by means of intersystem crossing, with lifetimes between 3 and 10 ns. In contrast, it appears as though the triplet excited state for 2 may be a metal-centered 3δδ* state. The lack of functional group that strongly, and uniquely, absorbs infrared radiation makes it difficult to make a concrete conclusion on the localization of charge between the ligands in these excited states. Finally, the crystal structures of both complexes were obtained. In order to gain a better understanding of the electronic conjugation between the metal center and the ligands while in the solid state, the torsion of the dihedral angle between the carboxylate group and anthracene rings was determined from the crystal structures. Ligands that were trans to each other were twisted out of conjugation to the same extent. For 1, this dihedral angle is 40.92º (57.1% relative conjugation) and 81.49º (2.2% relative conjugation) for the two pairs of trans ligands. In Complex 2, the dihedral angle of the anthracenecarboxylate ligands was determined to be 52.73º, or 36.7% relative conjugation.No embargoAcademic Major: ChemistryAcademic Major: Mathematic