3 research outputs found
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-bisÂ(2-pyridyl)Âpyrimidine and
the metal ions Fe<sup>II</sup> and Co<sup>II</sup> are presented.
While <b>FeL1-MEPE</b> (<b>L1</b> = 1,4-bisÂ[2,6-bisÂ(2-pyridyl)Âpyrimidin-4-yl]Âbenzene)
is deep blue, <b>FeL2-</b> and <b>CoL2-MEPE</b> (<b>L2</b> = 5,5′-bisÂ[2,6-bisÂ(2-pyridyl)Âpyrimidin-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<sub>f</sub> = 82%). Viscosity and static light-scattering measurements
reveal that the molar masses of these MEPEs are in the range of 1
Ă— 10<sup>8</sup> 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<sup>II</sup>/Fe<sup>III</sup> couple at 0.86 and 0.82 V versus Fc<sup>+</sup>/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-diethynylÂdibenzoÂ[<i>b</i>,<i>i</i>]Âphenazine, 6,13-diethynylnaphthoÂ[2,3-<i>b</i>]Âphenazine) and tetraazapentacenes (7,12-diethynylbenzoÂ[<i>g</i>]ÂquinoxalinoÂ[2,3-<i>b</i>]Âquinoxaline, 6,13-diethynylquinoxalinoÂ[2,3-<i>b</i>]Âphenazine) 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