posted on 2013-02-14, 00:00authored byMichael
W. Mara, Nicholas E. Jackson, Jier Huang, Andrew
B. Stickrath, Xiaoyi Zhang, Nosheen A. Gothard, Mark A. Ratner, Lin X. Chen
The effects of structural constraints on the metal-to-ligand
charge
transfer (MLCT) excited state structural dynamics of cuprous bis-2,9-diphenyl-phenanthroline
([Cu(I)(dpp)2]+) in both coordinating acetonitrile
and noncoordinating toluene were studied using X-ray transient absorption
(XTA) spectroscopy and density functional theory (DFT) calculations.
The phenyl groups attached to the phenanthroline ligands not only
effectively shield the Cu(I) center from solvent molecules, but also
force a flattened tetrahedral coordination geometry of the Cu(I) center.
Consequently, the MLCT state lifetime in [Cu(I)(dpp)2]+ is solvent-independent, unlike the previously studied 2,9-methyl
substituted bis-phenanthroline Cu(I) complex. The MLCT state of [Cu(I)(dpp)2]+ still undergoes a “pseudo Jahn-Teller
distortion,” with the angle between the two phenanthroline
ligand planes decreased further by 7°. The XTA results indicate
that, in the MLCT excited state of [Cu(I)(dpp)2]+, the phenyls at the 2, 9 positions of the phenanthroline rotate,
breaking the π–π interaction with the phenanthroline
ligands without ever rotating in-plane with the phenanthroline ligands.
Hence, the transferred electron density from the Cu(I) center is localized
on the phenanthroline moiety with no charge density present on the
phenyl rings. The insight about the effect of the structural constraints
on the MLCT state properties will guide the design of Cu(I) diimine
complexes with suitable excited-state properties to function as earth-abundant
dye sensitizers for solar electricity generation.