Excited state vibrational spectroscopy of metal complexes of dipyrido[3,2-a:2′,3′-c]phenazine

Revealing the Photophysical Dynamics of Selected Rigid Donor-Acceptor Systems: From Ligands to Ruthenium(II) Complexes

Newly designed push-pull ligands (L1 and L2) with bithiophene (bth) as a donor and phenazine (phz) or quinoxalino[2,3-b]quinoxaline (qxq) as acceptors were synthesized and also incorporated with a bipyridyl Ru(II) complex to give Ru1 and Ru2, respectively. The ultrafast photophysical dynamics of the ligand and their respective Ru(II) complexes were well-characterized using time-resolved spectroscopies and quantum chemical calculations. Photoinduced charger transfer (CT) and intersystem crossing (ISC) processes were directly observed for L1 and L2. In addition, the interplay of three different triplet excited states was directly observed in the related Ru(II) complexes. The lowest-lying triplet excited states of the ligands and their respective Ru(II) complexes were both attributed to the CT transitions from donor (bth) to acceptor (phz or qxq) and result in ³ICT (intramolecular charge transfer) and ³ILCT (intraligand charge transfer) excited states, respectively. The lifetimes of the lowest-lying triplet excited states of L1, L2, Ru1, and Ru2 were measured to be 21.3, 50.4, 2.75, and 4.16 μs, respectively.

Two Emissive Long-Lived Excited States of an Imidazole-Functionalized Ruthenium Dipyridophenazine Complex

A ruthenium(II) polypyridine-type complex based on the dipyridophenazine ligand with a directly fused imidazole unit (L1, dipyrido[3,2-a:2',3'-c]phenazine-10,11-imidazole) has been synthesized, and its electrochemical and photophysical properties have been studied. The cyclic voltammogram of [Ru(tbbpy)₂(L1)]²⁺ (C1) (tbbpy is 4,4'-tert-butyl-2,2'-bipyridine) shows a cathodic shift of the phenazine-based reduction process compared to similar molecules, while the first detected reduction wave (-1.34 V vs Fc/Fc⁺) is assigned to the imidazole unit within the molecule. On the basis of the TD-DFT calculations, the strong visible absorption band exhibited by C1 is assigned to a metal-to-ligand charge transfer (MLCT) transition with a concurrent ligand-centered (LC) transition. At room-temperature, C1 features emission (Φ = 0.04) from its lowest excited states with time constants of 1.2 and 18.3 μs. These lifetimes are assigned to emission processes from the ³MLCT and ³LC state, respectively. This is the first time that a long-lived dual emission has been observed for a ruthenium(II) complex bearing a directly fused extended π-system. Furthermore, the emission of C1 is quenched upon water addition. In contrast to related compounds based on a dipyridophenazine ligand, the excited state energy is not shifted, and the lifetime is drastically decreased to 169 ns.

Di- and tetranuclear transition metal complexes of a tetrakisguanidino-substituted phenazine dye by stepwise coordination

Coordination of a tetrakisguanidino-substituted phenazine dye in two steps provides rational access to tetranuclear homo- and heterobimetallic complexes.

Substituent influence in phenanthroline-derived ancillary ligands on the excited state nature of novel cationic Ir(iii) complexes

Influence of ancillary ligands derived from phenanthroline on the nature of the deactivation pathways of novel cationic Ir(iii) cyclometalated complexes.

Effect of Bridge Alteration on Ground- and Excited-State Properties of Ruthenium(II) Complexes with Electron-Donor-Substituted Dipyrido[3,2-a:2',3'-c]phenazine Ligands

A series of Ru(II) 2,2'-bipyridine (bpy) complexes with an electron-accepting dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand coupled to an electron-donating triarylamine (TAA) group have been investigated. Systematic alteration of a bridging unit between the dppz and TAA allowed exploration into how communication between the donor and acceptor is perturbed by distance, as well as by steric and electronic effects. The effect of the bridging group on the electronic properties of the systems was characterized using a variety of spectroscopic methods, including Fourier transform-Raman (FT-Raman) spectroscopy, resonance Raman spectroscopy, and transient resonance Raman (TR²) spectroscopy. These methods were used in conjunction with ground- and excited-state absorption spectroscopy, electrochemical studies, and DFT calculations. The ground-state electronic absorption spectra show distinct variation with the bridging group, with the wavelength observed for the lowest energy electronic transition ranging from 449 nm to 522 nm, accompanied by large changes in the molar absorptivity. The lowest-energy Franck-Condon state was determined to be intra-ligand charge transfer (ILCT) in nature for most compounds. The presence of higher-energy metal-to-ligand charge transfer (MLCT) Ru(II) → bpy and Ru(II) → dppz transitions was also confirmed via resonance Raman spectroscopy. The TR² spectra showed characteristic dppz^• - and TAA^• + vibrations, indicating that the THEXI state formed was also ILCT in nature. Excited-state lifetime measurements reveal that the rate of decay is in accordance with the energy gap law and is not otherwise affected by the nature of the bridging unit.

Contrasting photophysical properties of rhenium(i) tricarbonyl complexes having carbazole groups attached to the polypyridine ligand

The fac-[Re(CO)₃(cbz₂phen)(L)]^0/+1 complexes showed a remarkable presence of the ILCT_cbz2phen fluorescence in addition to the usually observed ³MLCT_{Re→cbz2phen}. In PMMA films the emission is completely turned into a triplet excited state manifold.

The photophysics of fac-[Re(CO)3(NN)(bpa)]+ complexes: a theoretical/experimental study

The influence of the polypyridyl ligand on the photophysics of Re(i) tricarbonyl complexes was investigated by steady-state and time resolved spectroscopy as well as by theoretical calculations.

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

Dppz complexes with extended conjugation show π,π* and MLCT excited states in solution. Raman spectral features for the short-lived π,π* state are observed in single-colour transient experiments.

Heteroleptic Cu(I) bis-diimine complexes of 6,6'-dimesityl-2,2'-bipyridine: a structural, theoretical and spectroscopic study

A series of heteroleptic Cu(I) complexes containing 6,6'-dimesityl-2,2'-bipyridine and phenanthroline-, bipyridine-, and biquinoline-based ligands is studied. The HETPHEN strategy is utilized to synthesize the heteroleptic complexes, which are stable in solution. The X-ray crystal structures of the complexes are presented; the solid-state four-coordinate Cu(I) geometries are quantified by using the τ4 parameter. A feature of the crystal structures is the intramolecular π-stacking between the mesityl ring(s) and the diimine ligand; the phen-based complexes exhibit stacking between the phen ligand and one of the mesityl rings, creating a "Pac-Man" motif. On the other hand, the bpy-based complexes show different types of packing interaction, with both mesityl rings "clamping down" on the bpy based ligand to give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-dimethyl-1,10-phenanthroline and 6,6'-dibromo-2,2'-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced over bpy(Mes)2 bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-based LUMO compared to the bpy(Mes)2 LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)2 bands.