A polyradical consisting of alternating triarylamine and perchlorotriphenylmethyl radical moieties was synthesized by Horner-Emmons reaction. This compound is the first polymeric neutral mixedvalence compound that shows an intervalence charge transfer (IV-CT) band in the NIR. Comparison of the absorption spectra of the polymer with those of a reference monomer shows that the IV-CT transition is confined to one repeating unit. HOMO and LUMO levels are at -5.5 and -4.5 eV vs vacuum, respectively, as estimated by cyclic voltammetry. A very low exciton binding energy is indicated by comparison with the optical band gap (1.2 eV). The electron transfer properties of the polymer were investigated in solution by fs-pump-probe transient absorption spectroscopy. After optical excitation, the polymer shows a biexponential decay in the ps time regime. The short-living, solvent-dependent component refers to the direct decay from the IV-CT state to the ground state and the long-living, solvent-independent component is tentatively attributed to an equilibrium formation of the IV-CT state and a completely charge separated state. The charge-transport properties were investigated in films in organic field-effect transistor (OFET) devices. Electron and hole mobilities are both about 3 Â 10 -5 cm 2 V -1 s -1 , demonstrating ambipolar transport behavior of the polymer.

Ultraviolet irradiation of a manganese-tricarbonyl CO-releasing molecule (CORM) in water eventually leads to the liberation of some of the carbon monoxide ligands. By ultraviolet pump/mid-infrared probe femtosecond transient absorption spectroscopy in combination with quantum chemical calculations, we could disclose for the exemplary compound [Mn(CO)3(tpm)](+) (tpm = tris(2-pyrazolyl)methane) that only one of the three carbonyl ligands is photochemically dissociated on an ultrafast time scale and that some molecules may undergo geminate recombination.

Shot-to-shot broadband detection is common in ultrafast pump-probe spectroscopy. Taking advantage of the intensity correlation of subsequent laser pulses improves the signal-to-noise ratio. Finite data readout times of CCD chips in the employed spectrometer and the maximum available speed of mechanical pump-beam choppers typically limit this approach to lasers with repetition rates of a few kHz. For high-repetition (≥ 100 kHz) systems, one typically averages over a larger number of laser shots leading to inferior signal-to-noise ratios or longer measurement times. Here we demonstrate broadband shot-to-shot detection in transient absorption spectroscopy with a 100-kHz femtosecond laser system. This is made possible using a home-built high-speed chopper with external laser synchronization and a fast CCD line camera. Shot-to-shot detection can reduce the data acquisition time by two orders of magnitude compared to few-kHz lasers while keeping the same signal-to-noise ratio.

Tetrazolium salts are exploited in various fields of research by virtue of their low reduction potentials. Increasingly, associated applications also attend to the photochemical and luminescence properties of these systems. Here, we investigate the photoinduced dynamics of phenyl-benzo[c]tetrazolo-cinnolinium chloride (PTC), one of the very few known fluorescent tetrazolium compounds, by using time-correlated single-photon counting, femtosecond fluorescence upconversion, and ultrafast transient absorption spectroscopy. PTC is generated photochemically by ultraviolet illumination of 2,3,5-triphenyl-tetrazolium chloride (TTC) in various alcohols. Time-resolved fluorescence measurements on PTC with different excitation wavelengths disclose biphasic solvation and vibrational relaxation dynamics. Depending on the solvent, the emission behavior of PTC is characterized by quantum yields on the order of several tens of percent and corresponding excited-state lifetimes of several hundreds of picoseconds. The radiative rate is basically constant for the studied alcohols, whereas the rate of the competing non-radiative process is sensitive to the solvent polarity. Hence, we discuss the possible involvement of intermediate radicals and further presumptive reaction pathways pursued after photoexcitation of PTC.

Radicals in solution are crucial for many chemical processes. In this work, we unveil the photoreaction sequence leading to radical formation from tetrazolium salts, which are extensively used in enzyme assays and also exhibit a rich photochemistry. Upon UV irradiation, the tetrazolium ion turns into the tetrazolinyl radical via two intermediates on a nanosecond timescale. The solvent's polarity governs the rate of formation, but the reaction pathway towards the tetrazolinyl radical is identical for aqueous and alcoholic solutions, although the final photoproduct distribution differs. These observations provide new insight into the versatile reactivity of tetrazolium salts and ultrafast radical formation in the liquid phase.

No abstract

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.

No abstract