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

Abstract

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 (3)ILCT 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.

Circular Photoinduced Electron Transfer in a Donor‐Acceptor‐Acceptor Triad

“…TAA-th-BTD,i nw hich the TAA donor and BTD acceptor are free to adopt ac oplanar arrangement, exhibits an intense low-energy TAA !BTD CT band at l = 447 nm (black trace, e = 22,300 m À1 cm À1 ), consistent with previously reported D-A systems. [7] Thes econd lowest band (l = 351 nm, e = 25,800 m À1 cm À1 )a ppears to be consistent with ath!BTD CT transition. In the symmetrical TAA-th-BTD-th-TAA reference compound, the ortho substitution pattern around the central BTD acceptor sterically inhibits adoption of ac oplanar arrangement, and consequently the TAA !BTD CT band is hypsochromically shifted to about l = 415 nm (green trace) such that it becomes merely visible as ashoulder on the higher energy th!BTD CT band (l = 355 nm).…”

Probing the excited state nature of coordination complexes with blended organic and inorganic chromophores using vibrational spectroscopy

“…[48,49] This is expressed by Tsuboi's rule, which states that if the excitation wavelength is chosen to coincide with an electronic transition then vibrational bands will be enhanced in intensity if the vibrations mimic the structural distortion upon photoexcitation. [50,51] As structural distortion is related to the movement of electrons, empirical use of this relationship can be made to determine the donor/acceptor regions involved in an excited state.…”

Stretching the phenazine MO in dppz: the effect of phenyl and phenyl–ethynyl groups on the photophysics of Re(i) dppz complexes

“…3). Whilst Suzuki couplings on dppz have been previously studied, 18,34 this study represents the first attempt at directly functionalizing dppz through other crosscoupling reactions. [ReCl(CO) 3 (dppz)] complexes were prepared utilizing literature methods for related complexes.…”

“…Solvatochromic behaviour has been observed in dppz-type ligands in which the emission has charge-transfer character. 34 For the complexes the data are less clear cut. Each complex shows two emission bands in room temperature solution, see Fig.…”

“…All ligands showed μs lifetimes of the transient absorbing states, see 10,20,34 Lifetimes can vary considerably with solvent and temperature, due to the environmental sensitivity of dppz, 8 and also depend on the nature of the probed state. 15 For example, Kuimova et al measured a 10 μs lifetime for [Re(CO) 3 ( py)(dppz-(CO 2 Et) 2 )] + in MeCN, while Dyer et al measured 3.5 μs for [Re(CO) 3 ( py)(dppz)] + in MeCN; in both cases this state is assigned as π,π* in nature.…”