Intraligand Charge-Transfer Excited States in Re(I) Complexes with Donor-Substituted Dipyridophenazine Ligands

The donor–acceptor
ligands 11-(4-diphenylaminophenyl)­dipyrido­[3,2-a:2′,3′-c]­phenazine (dppz-PhNPh2) and 11-(4-dimethylaminophenyl)­dipyrido­[3,2-a:2′,3′-c]­phenazine (dppz-PhNMe2), and their rhenium complexes, [Re­(CO)3X] (X =
Cl–, py, 4-dimethylaminopyridine (dmap)), are reported.
Crystal structures of the two ligands were obtained. The optical properties
of the ligands and complexes are dominated by intraligand charge transfer
(ILCT) transitions from the amine to the dppz moieties with λabs = 463 nm (ε = 13 100 M–1 cm–1) for dppz-PhNMe2 and with λabs = 457 nm (ε = 16 900 M–1 cm–1) for dppz-PhNPh2. This assignment
is supported by CAM-B3LYP TD-DFT calculations. These ligands are strongly
emissive in organic solvents and, consistent with the ILCT character,
show strong solvatochromic behavior. Lippert–Mataga plots of
the data are linear and yield Δμ values of 22 D for dppz-PhNPh2 and 20 D for dppz-PhNMe2. The rhenium­(I) complexes
are less emissive, and it is possible to measure resonance Raman spectra.
These data show relative band intensities that are virtually unchanged
from λexc = 351 to 532 nm, consistent with a single
dominant transition in the visible region. Resonance Raman excitation
profiles are solvent sensitive; these data are modeled using wavepacket
theory yielding reorganization energies ranging from 1800 cm–1 in toluene to 6900 cm–1 in CH3CN. The
excited state electronic absorption and infrared spectroscopy reveal
the presence of dark excited states with nanosecond to microsecond
lifetimes that are sensitive to the ancillary ligand on the rhenium.
These dark states were assigned as phenazine-based 3ILCT
states by time-resolved infrared spectroscopy. Time-resolved infrared
spectroscopy shows transient features in which Δν­(CO)
is approximately −7 cm–1, consistent with
a ligand-centered excited state. Evidence for two such states is seen
in mid-infrared transient spectra.