Dramatic Alteration of ³ILCT Lifetimes Using Ancillary Ligands in [Re(L)(CO)₃(phen-TPA)]ⁿ⁺ Complexes: An Integrated Spectroscopic and Theoretical Study

Abstract

The ground and excited state photophysical properties of a series of fac-[Re(L)(CO)(α-diimine)] complexes, where L = Br, Cl, 4-dimethylaminopyridine (dmap) and pyridine (py) have been extensively studied utilizing numerous electronic and vibrational spectroscopic techniques in conjunction with a suite of quantum chemical methods. The α-diimine ligand consists of 1,10-phenanthroline with the highly electron donating triphenylamine (TPA) appended in the 5 position. This gives rise to intraligand charge transfer (ILCT) states lying lower in energy than the conventional metal-to-ligand charge transfer (MLCT) state, the energies of which are red and blue-shifted, respectively, as the ancillary ligand, L becomes more electron withdrawing. The emitting state is ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR), time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Systematic modulation of the ancillary ligand causes unanticipated variation in the ILCT lifetime by 2 orders of magnitude, ranging from 6.0 μs for L = Br to 27 ns for L = py, without altering the nature of the excited state formed or the relative order of the other CT states present. Temperature dependent lifetime measurements and quantum chemical calculations provide no clear indication of close lying deactivating states, MO switching, contributions from a halide-to-ligand charge transfer (XLCT) state or dramatic changes in spin-orbit coupling. It appears that the influence of the ancillary ligand on the excited state lifetime could be explained in terms of energy gap law, in which there is a correlation between ln( k) and E with a slope of -21.4 eV for the ILCT emission.

Spin Transition and Charge Transfer in Co2+/Co3+ Complexes of Meridional Ligands Holding Nearby Redox‐active Triarylamine

“…The electronic spectra are dominated by the intensive ligand‐centered π–π* transitions in the UV region. Furthermore, moderate‐intense absorptions assigned to an intraligand charger transfer (ILCT) from the diarylamine unit to the pyridine moieties in the adjacent Vis region can be observed . Absorption spectra of compounds 1 – 4 exhibit the same ILCT bands ( λ max = 368, 373, 374, 373 nm for 1 – 4 ) which are blue‐shifted with respect to their corresponding Co 2+ complexes (Figure S6, Supporting Information).…”

Tailoring of the Frontier Orbital Character in Co2+/3+ Complexes with Triarylamine Substituted Terpyridine Ligands

“…Additionally, neutral Taras exhibit absorptions around λ = 300 nm , . A second set of significant but less intense absorptions in the vis region at around λ = 415 nm ( ε max = 8000 L mol –1 cm –1 ) is attributed to an intraligand charge transfer (ILCT) from the diaryl amine unit to the pyridine moieties , . Cobalt complexes deriving from native tpy lack of intense transitions in this spectral region , .…”

Optically Controlled Electron Transfer in a ReI Complex

“…As a consequence, the interplay of two close‐lying 3 MLCT (metal‐to‐ligand charge transfer)states of different nature in fac ‐[Re(L)(CO) 3 (DPPZ)] n + complexes renders them promising for development of phosphorescent labels and probes of biomolecules such as DNA and proteins [8] . To modulate the energy levels of these two molecular orbitals, electron‐donating or ‐withdrawing substitutes have been appended to the DPPZ core, leading to the formation of 3 ILπ–π* or 3 ILCT (intra‐ligand charge transfer) along with two 3 MLCT excited states in the corresponding Re I complexes [6d, 8] …”