Structural Changes upon Photoexcitation into the Metal-to-Ligand Charge-Transfer State of [Cu(pqx)(PPh₃)₂]⁺ Probed by Resonance Raman Spectroscopy and Density Functional Theory

The structural changes that occur when [Cu(pqx)(PPh3)2]+ (pqx is 2-(2‘-pyridyl)quinoxaline) undergoes excitation through a metal-to-ligand charge-transfer (MLCT) transition are investigated using resonance Raman excitation profiles coupled with density functional theory (DFT). The DFT calculations predict bond lengths to within 3 pm and absolute deviations of 7 cm-1 for the vibrational frequencies of [Cu(pqx)(PPh3)2]+. TD-DFT calculations of oscillator strengths (f = 0.089) and band positions (419 nm) showed close agreement with experiment (f = 0.07, 431 nm). Resonance Raman spectra show the 527 cm-1 (ν29) and 1476 cm-1 (ν75) modes undergo the largest dimensionless displacement (Δ = 1.5 and 1.1, respectively) following photoexcitation into the MLCT Franck−Condon region. The solvent couples strongly to the MLCT transition and resonance Raman intensity analysis (RRIA) gives a solvent reorganization energy of 3400 cm-1 for dichloromethane and 2800 cm-1 for chloroform solutions. A large inner-sphere reorganization of 3430 cm-1 in dichloromethane solution (3520 cm-1 in chloroform solution) was found for [Cu(pqx)(PPh3)2]+, indicating that the molecule as a whole undergoes significant distortion following MLCT excitation.