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

Abstract

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 (TR) 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 TR 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.

Modulation of Donor-Acceptor Distance in a Series of Carbazole Push-Pull Dyes; A Spectroscopic and Computational Study

“…To further probe the observed electronic transitions and attempt to better understand how the implemented structural changes result in the observed optical changes, resonance Raman spectroscopy (RRS) was carried out at a number of excitation wavelengths across the absorption bands of the compounds. Resonance Raman spectroscopy shows band enhancements for modes that are associated with the resonant chromophore [ 16 , 20 , 41 , 50 , 51 ], thus it is possible to provide experimental evidence for the nature of the observed chromophores to support the calculated electronic transitions data.…”

“…While a wide range of different donor (carbazole [ 11 , 12 , 13 ], triphenylamine [ 14 , 15 , 16 ], dimethylaniline [ 17 , 18 ], fluorene [ 13 ] and coumarin [ 17 ]) and acceptor (benzothiadiazole [ 13 , 15 ], hexaazatrinaphthalene [ 16 ], napthlenediimides [ 19 ], dipyridophenazine [ 20 ] and cyanoacrylic acid [ 21 ]) moieties have been studied this work focuses on the carbazole donor, cyanoacrylic acid acceptor combination. The use of this donor acceptor pairing has been used in a number of studies in the literature [ 12 , 22 , 23 , 24 , 25 ].…”

Breaking the barrier: an osmium photosensitizer with unprecedented hypoxic phototoxicity for real world photodynamic therapy

“…59,60 The prolonged excited state lifetimes characteristic of 3 ILCT states result in sensitivity to trace oxygen. 61 Finally, we have previously shown that certain Os(II) complexes are tolerated in vivo, can be panchromatically activated with visible and near-infrared (NIR) wavelengths of light, and give rise to phototoxic effects both in vitro and in vivo. 62,63 Here we introduce an Os(II)-based PS (Os-4T) that is extremely potent, can be activated with NIR light (733 nm), and exhibits photocytotoxic effects in hypoxia (1% O 2 ).…”

Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes

“… 14 It is implicit that an electron potentially hops from one ligand to another and, in turn, is directed to the energetically most favorable orbital. 15 – 19 Once the electron reaches the energetically lowest orbital it becomes accessible as a reductive equivalent for catalysis or for injection into electrodes. 20 …”