Evaluating the Extent of Intramolecular Charge Transfer in the Excited States of Rhenium(I) Donor-Acceptor Complexes with Time- Resolved Vibrational Spectroscopy Yuankai Yue, † Tod Grusenmeyer, † Zheng Ma, ‡ Peng Zhang, ‡ Tri Tat Pham, † Joel T. Mague, † James P. Donahue, † Russell H. Schmehl,* ,† David N. Beratan,* ,‡ and Igor V. Rubtsov* ,† † Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States ‡ Departments of Chemistry, Biochemistry, and Physics, Duke University, Durham, North Carolina 27708, United States * S Supporting Information ABSTRACT: Excited states in transition-metal complexes, even in those featuring ligands with strong electron donating and accepting properties, often involve only partial charge transfer between the donor and acceptor ligands. The excited-state properties of [Re(bpy)(CO) 3 L] + compounds were studied, where L is 4- dimethylaminobenzonitrile (Re4DMABN), 3-dimethylaminobenzonitrile (Re3D- MABN), and benzonitrile (ReBN) using time-resolved infrared (TRIR) and electronic spectroscopy methods as well as electronic structure computations. The DMABN complexes exhibit strongly solvent-dependent luminescence; the excited state lifetime decreases from microseconds in dichloromethane to several nanoseconds in mixed MeOH:DCM (1:1) solvent. Despite the similarities in the solvent dependence of the excited state dynamics and redox properties for Re3DMABN and Re4DMABN, the nature of the lowest energy excited states formed in these two compounds is drastically different. For example, the lowest energy excited state for Re4DMABN in the mixed solvent is assigned to the (4DMABN → bpy) ligand-to-ligand charge transfer (LLCT) state featuring partial charge transfer character. An equilibrium between a 3DMABN intraligand triplet ( 3 IL) and a metal-ligand-to-ligand charge transfer (MLLCT) state is found for Re3DMABN in the mixed solvent with the latter at ca. 400 cm -1 lower energy. The origin of such a drastic difference between the states involved in Re4DMABN and Re3DMABN is attributed to a difference in the energies of polarized quinoidal resonance structures in 4DMABN and 3DMABN ligands. ■ INTRODUCTION The mechanism of photoinduced intramolecular electron transfer (ET) has been the subject of intensive study for more than 30 years. 1-7 Since photoinduced ET begins with light absorption, the electron-density distribution and degree of charge transfer associated with the initially formed and relaxed excited states are critical but are often poorly characterized. For transition metal complexes with metal-to-ligand charge transfer (MLCT) excited states, the structural and charge polarization changes that follow from the initial excitation to the generation of the intramolecular photoredox products can be subtle. 3,8,9 This is especially true when the free energy of the charge- separated state formed upon electron transfer is structurally similar to that of the initial excited state. Formation of an intramolecular charge-separated state may occur in discrete steps (i.e., initial formation of a MLCT excited state may be followed by an intramolecular electron-transfer step leading to polarization of charge from D to A) or absorption may directly create an excited electronic state with a significant admixture of the charge-separated DA product that then further relaxes. Distinguishing between these two closely related pathways requires a spectroscopic approach that provides direct temporal information on the electron-density distribution. The ability to evaluate the degree of state mixing using a direct spectroscopic approach may allow the correlation of the donor-acceptor interaction with the full course of the reaction on the excited state manifold. Rhenium tricarbonyl-diimine complexes, [Re I (CO) 3 (N,N)- (L)] +/0 (+ when L is a neutral ligand with a substituent capable of electron donation to the excited complex), are appealing candidates to dissect the charge-transfer evolution as a function of time after excitation because infrared modes of the carbonyl and imine ligands provide direct probes of charge localization. Recent studies of Vlcek 10-14 and others provide insight into the vibrational relaxation of the MLCT states of [Re I (CO) 3 (N,N)- (L)] +/0 complexes, where L = pyridine or chloride. In these complexes, the lowest energy metal to ligand charge transfer (MLCT) state is directly or indirectly prepared by optical Special Issue: Michael D. Fayer Festschrift Received: September 26, 2013 Revised: October 12, 2013 Published: October 13, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 15903 dx.doi.org/10.1021/jp409628e | J. Phys. Chem. B 2013, 117, 15903-15916