Origin of the Photoinduced Geometrical Change of Copper(I) Complexes from the Quantum Chemical Topology View

Abstract

Copper(I) complexes (CICs) are of great interest due to their applications as redox mediators and molecular switches. CICs present drastic geometrical change in their excited states, which interferes with their luminescence properties. The photophysical process has been extensively studied by several time‐resolved methods to gain an understanding of the dynamics and mechanism of the torsion, which has been explained in terms of a Jahn–Teller effect. Here, we propose an alternative explanation for the photoinduced structural change of CICs, based on electron density redistribution. After photoexcitation of a CIC (S0→S1), a metal‐to‐ligand charge transfer stabilizes the ligand and destabilizes the metal. A subsequent electron transfer, through an intersystem crossing process, followed by an internal conversion (S1→T2→T1), intensifies the energetic differences between the metal and ligand within the complex. The energy profile of each state is the result of the balance between metal and ligand energy changes. The loss of electrons originates an increase in the attractive potential energy within the copper basin, which is not compensated by the associated reduction of the repulsive atomic potential. To counterbalance the atomic destabilization, the valence shell of the copper center is polarized (defined by ∇2ρ(r) and ∇2Vne(r)) during the deactivation path. This polarization increases the magnitude of the intra‐atomic nuclear–electron interactions within the copper atom and provokes the flattening of the structure to obtain the geometry with the maximum interaction between the charge depletions of the metal and the charge concentrations of the ligand.

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes

“…Malmström, Vallee, and Williams have all proposed that conformational rigidity in blue copper proteins facilitates rapid electron transfer (ET) through the enforcement of an entatic rack-induced state. − Cu I is a “soft” d 10 metal ion with no ligand field stabilization and primarily adopts tetrahedral/trigonal geometries, while Cu II is a “hard” d 9 metal ion and principally adopts square planar or tetragonal coordination geometries. In blue copper proteins, the peptide framework holds the Cu ion in an entatic state: a coordination geometry and environment that is intermediate between those favored by Cu I and Cu II , lowering the reorganization energy associated with ET and hence, speeding it up. , An example of these principles comes from the photochemistry of Cu I diimine complexes, Cu­(phen) 2 + (phen = 1,10-phenanthroline). − In these systems, metal-to-ligand charge transfer (MLCT) leads to a tetrahedral Cu II excited state that undergoes Jahn–Teller relaxation, flattening to a more tetragonal geometry, which can be trapped in coordinating solvents. − This flattening is inhibited for 2,9-functionalized phenanthroline derivatives, where bulky substituents both lock the complex into a tetrahedral coordination geometry and prevent excited-state flattening. 3 MLCT lifetimes in such systems are elongated .…”

Computer-Aided Design of Luminescent Linear N-Heterocyclic Carbene Copper(I) Pyridine Complexes

“…In many cases, the photophysical properties of these complexes were investigated by a combination of spectroscopy and density functional theory (DFT)/time-dependent DFT (TDDFT) calculations of energies and orbitals at the equilibrium geometries of the S 0 , S 1 , and T 1 states. ,− ,− More advanced theoretical studies deal with structure–property relationships by scanning interligand dihedral angles ,,− and P–Cu–P bite angles . Dynamics simulations on the [Cu­(dmp) 2 ] + complex cover the ultrafast decay of the initially excited singlet into the S 1 state after photoexcitation. , Some studies also shed light on temperature and/or structural effects on the emission properties by calculating radiative and (r)­ISC rate constants. , …”

Intermediate Detection in the Casiopeina–Cysteine Interaction Ending in the Disulfide Bond Formation and Copper Reduction

“…In this process, the copper valence shell changes, and then the geometry of the complex becomes tetrahedral. Our group has previously reported the spin change effect on the metal valence shell and thus on the complex structure [ 44 , 45 ]. From this point, the thiyl radical is free to participate in the following reaction to form the disulfide bond.…”