Date of Award:

5-1983

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Chemistry and Biochemistry

Advisor/Chair:

William M. Moore

Co-Advisor/Chair:

Karen W. Morse

Abstract

This research has focused on the sensitized photoisomerization step of a solar energy storage system based on the interconversion of norbornadiene (NBD) and quadricyclane (Q). Sensitizers studied in the photoisomerization were arylphosphine complexes of copper(I) halides [(Ph_3P)_3CuX, (MePh_ 2P)_3CuX, (DIPHOS)CuCl; X = Cl, Br, I and DIPHOS = Ph_ 2P(CH_2)_2PPh_2]. The lowest electronic excited state of the complexes can be an effective triplet energy sensitizer as evidenced by a maximum quantum yield of 1.0 with (HePh_2P)_3CuX. The sensitization mechanism of the L_3CuX complexes is proposed as a bimolecular energy transfer process in which energy transfer takes place through the metal centered orbitals. The efficiency of the (MePh_ 2P)_3CuX sensitizers is unprecedented by other reported inorganic or organic sensitizers; however, ligand dissociation of the complexes in dilute solutions detracts from their otherwise attractive features. The free ligands undergo rapid photodecomposition and are extremely poor sensitizers of the NBD/Q system. The ligand dissociation of the L_mCu_nCl_n (L = Ph_3P, MePh_2P; m:n = 3:1, 2:1, 4:2, 3:2, 2:2) complexes was modeled by use of vapor pressure osmometry and UV absorption. The equilibria and equilibrium constants of dissociation were determined for benzene solutions at 37 °c. The stability of the L_mCu_nCl_n complexes (Ph_3P<< MePh_2P) toward ligand dissociation is attributed to the larger steric requirements of Ph_3P in comparison to MePh_2P. Evidence for ground state complexation of NBD and the L_mCu_nCl_n compounds (m:n < 3:1) is shown. Photoexcitation of the (NBD) L_mCu_nCl_n (m:n < 3:1) complexes can produce Q but with quantum efficiencies < 1.0. Photophysical properties of the arylphosphines and their copper(I) halide complexes are reported. The lowest electronic transition of the phosphines associated with an obscured absorption band at ~300nm is classified as an l -> a_pi transition. Coordination to copper() halides brings about an enhancement of the l -> a_pi transition of the ligand. The term o,d ->a_pi is used to describe the lowest electronic transition of the complexes indicating that both the o bond of the phosphorus lone electron pair to the metal and the d-d orbital bonding of the metal to the phosphorus are involved in an electron excitation from the phosphorus to an antibonding phenyl-pi-system (a_pi ). The photophysical properties of Zn(II), Cu(I), and Ni(O) complexes of MePh_2P are shown to follow the concept of a o,d -> a_pi electronic transition.

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