Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2-pyridyl)pyrimidine Bithiophene

The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis­(2-pyridyl)­pyrimidine and the metal ions Fe^II and Co^II are presented. While <b>FeL1-MEPE</b> (<b>L1</b> = 1,4-bis­[2,6-bis­(2-pyridyl)­pyrimidin-4-yl]­benzene) is deep blue, <b>FeL2-</b> and <b>CoL2-MEPE</b> (<b>L2</b> = 5,5′-bis­[2,6-bis­(2-pyridyl)­pyrimidin-4-yl]-2,2′-bithiophene) are intense green and red in color, respectively. These novel MEPEs display a high extinction coefficient and solvatochromism. Ligand <b>L2</b> shows a high absolute fluorescence quantum yield (Φ_f = 82%). Viscosity and static light-scattering measurements reveal that the molar masses of these MEPEs are in the range of 1 × 10⁸ g/mol under the current experimental conditions. In water, <b>FeL1-MEPE</b> forms a viscous gel at 20 °C (<i>c</i> = 8 mM). Thin films of high optical quality are fabricated by dip coating on transparent conducting indium tin oxide (ITO) glass substrate. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. Green to red and blue to colorless electrochromism is observed for <b>FeL2-MEPE</b> and <b>FeL1-MEPE</b>, respectively. The results show that the electrochromic properties are affected by the ligand topology. The Fe-MEPEs show a reversible redox behavior of the Fe^II/Fe^III couple at 0.86 and 0.82 V versus Fc⁺/Fc and display an excellent redox cycle stability under switching conditions. <b>FeL2-MEPE</b> in its oxidized state exhibits a broad absorption band covering the near-IR region (ca. 1500 nm) due to the ligand-to-metal charge transfer transition originating due to charge delocalization in the bithiophene spacer

Measuring Charge-Separation Dynamics via Oligomer Length Variation

We study the optically induced charge-transfer dynamics in donor–acceptor oligomers of different chain lengths. The combination of systematic synthesis, electrochemical measurements, and ultrafast transient absorption spectroscopy allows us to determine the charge-transfer properties and dynamics in donor–acceptor systems of confined lengths. Calculations within Marcus and Jortner electron-transfer theory explain the different charge-recombination times. For compounds in which complete charge separation can occur we deduce fast equilibration between different charge-transfer configurations prior to charge recombination. Thus, monoexponential charge-recombination kinetics are observed, as only the smallest-barrier configuration leads to relaxation to the ground state. The systematic oligomer length variation along with time-resolved spectroscopy allows us to determine how far apart charges can be separated in multichromophore donor–acceptor systems. Such information is relevant for understanding on a microscopic level the charge carrier mobility in materials for organic electronics and photovoltaics

Preparation, Properties, and Structures of the Radical Anions and Dianions of Azapentacenes

A series of diazapentacenes (5,14-diethynyl­dibenzo­[<i>b</i>,<i>i</i>]­phenazine, 6,13-diethynylnaphtho­[2,3-<i>b</i>]­phenazine) and tetraazapentacenes (7,12-diethynylbenzo­[<i>g</i>]­quinoxalino­[2,3-<i>b</i>]­quinoxaline, 6,13-diethynylquinoxalino­[2,3-<i>b</i>]­phenazine) were reduced to their radical anions and dianions, employing either potassium anthracenide or lithium naphthalenide in THF. The anionic species formed were investigated by UV–vis–NIR, fluorescence and EPR spectroscopy, spectroelectrochemistry, and quantum chemical calculations. Single crystal X-ray structures of three of their radical anions and of three of their dianions were obtained. In contrast to the acenes, the anions of the azapentacenes are persistent and, in some cases, even moderately stable toward air, and were characterized