Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives

A strong steric hindrance effect on ground state, excited state, and charge separated state properties of a CuI-diimine complex captured by X-ray transient absorption spectroscopy

“…Based on the energy distinction of the shoulder feature for Cu I species at 8.986 keV, the excited state spectrum (blue curve) was obtained by subtracting 55% of the ground state from the laser-on spectrum until the shoulder feature at 8.986 keV disappears. Similar to the previous studies on Cu I diimine complexes, 17,22,25,30,31,34 the following spectral changes in the MLCT state compared to that of the ground state were observed: (1) the transition edge energy shifts to 3 eV higher, suggesting the higher positive charges in the copper center; (2) the shoulder feature at 8.986 keV, representing a 1s-to-4p z transition disappears, typically observed for pond to the Cu-C distances associated with C atoms of the phenanthroline connected directly to the four Cu-ligating N atoms as well as the C atoms of the phenyl groups in the 2,9 positions of phenanthroline. The change of these Cu-ligands can be quantified by fitting the experimental data using the IFEFFIT program.…”

“…Also, the MLCT state of such complexes undergoes a transformation in the coordination geometry due to the different preferences of the electronic configurations of 3d 10 Cu(I) in the ground state and 3d 9 Cu(II) in the MLCT state. 16 As summarized in a recent review, 17 the MLCT state properties of a series of Cu I bis-phenanthroline based complexes can be altered via different steric hindrance exerted by the substituents at the 2,9 positions of the phenanthroline ligands that can modulate the angle between the two phenanthroline ligand planes from 90°, such as in the complex with 2,9-di-tert-butyl-1,10-phenanthroline ligands with an orthogonal tetrahedral coordinating geometry, to ∼68°, a "flattened" tetrahedral geometry, such as in less hindered species that are capable of undergoing the flattening. 4,8,13,[17][18][19][20][21][22][23][24] It has been observed that when the two phenanthroline ligand planes are forced to be orthogonal, the intersystem crossing (ISC) from the 1 MLCT to 3 MLCT state occurs on a timescale of a few picoseconds, while the same process takes 10-15 ps when the two ligand planes are not orthogonal, 19,[23][24][25] i.e.…”

“…An analogous method in the X-ray regime, X-ray transient absorption (XTA) spectroscopy, is used to track electron configurations and the corresponding molecular structural changes of [Cu I (dppS) 2 ] + by measuring the transient X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS). 17,19,22,23,25,[30][31][32][33][34] We would like to answer the following questions: (1) what are the structures of the MLCT state and charge separated state structures; (2) what are the structural changes of the complex when it binds to TiO 2 nanoparticles; and (3) what we can learn from our study on Cu I diimine complex design for its applications in solar energy conversion? Understanding the molecular structures and dynamics of both photoexcited states and charge separated intermediates in the current system will not only provide guidance by revealing their practical applications in dye sensitized solar cells but also an insight into the nature of photochemical reactions.…”

The beneficial effects of trifluoromethyl-substituents on the photoconversion efficiency of copper(i) dyes in dye-sensitized solar cells

“…The intense, high-energy transitions arise from ligand-based p* ) p transitions, and the broad band in the visible region from metal-toligand charge transfer (MLCT). 45,46 In [Cu (2) (Fig. 4) 6 ] within the solventaccessible window (Fig.…”

Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis

“…Flattening distortions can be partially blocked by steric constrains [80,81]. Alternatively, the ligand frame can be extended to make the access of the solvent to the Cu more difficult.…”