Tuning Molecular Electron Affinities against Atomic Electronegativities by Spatial Expansion of a π-System

2,1,3-Benzochalcogenadiazoles C₆ R₄ N₂ E (E/R; E=S, Se, Te; R=H, F, Cl, Br, I) and C₆ H₂ R₂ N₂ E (E/R'; E=S, Se, Te; R=Br, I) are 10π-electron hetarenes. By CV/EPR measurements, DFT calculations, and QTAIM and ELI-D analyses, it is shown that their molecular electron affinities (EAs) increase with decreasing Allen electronegativities and electron affinities of the E and non-hydrogen R (except Cl) atoms. DFT calculations for E/R+e⋅^- →[E/R]⋅^- electron capture reveal negative ΔG values numerically increasing with increasing atomic numbers of the E and R atoms; positive ΔS has a minor influence. It is suggested that the EA increase is caused by more effective charge/spin delocalization in the radical anions of heavier derivatives due to contributions from diffuse (a real-space expanded) p-AOs of the heavier E and R atoms; and that this counterintuitive effect might be of the general character.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (¹MLCT) and thiazole (tz)-induced intraligand charge-transfer (¹ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered ³MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ³ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ_Em,1 ≈ 40-300 ns and τ_Em,2 ≈ 100-200 ns, which are attributed to emission from the ³ILCT and ³MLCT manifolds, respectively.

A Naphthalene Diimide Based Macrocycle Containing Quaternary Ammonium Groups: An Electron-Deficient Host for Aromatic Carboxylate Derivatives

Naphthalene diimide (NDI) compounds are widely used as electron acceptors in various applications. Herein, we combine NDI with quaternary ammonium groups for the synthesis of a highly electron-deficient linear compound 2 and macrocycle 3. The complexation studies of the water-soluble macrocycle 3 with aromatic di- and tetra- carboxylate anions in water were done using absorption, emission, ¹ H NMR and NOESY spectroscopic titrations. The NDI incorporated macrocycle 3 showed high binding affinities towards linear aromatic tetracarboxylate anions owing to the size and charge complementarity of the host-guest complex. Macrocycle 3 binds tetracarboxylate anion much better than dicarboxylate anions. Furthermore, the macrocycle 3 is solvated differently in acetonitrile and in water or dimethyl sulfoxide, which induces changes in conformation and photophysical properties. Such electron-deficient optically active macrocycles are useful for developing useful sensor materials.