Oriented Thin Films of a Benzodithiophene Covalent Organic Framework

A mesoporous electron-donor covalent organic framework based on a benzodithiophene core, BDT-COF, was obtained through condensation of a benzodithiophene-containing diboronic acid and hexahydroxytriphenylene (HHTP). BDT-COF is a highly porous, crystalline, and thermally stable material, which can be handled in air. Highly porous, crystalline oriented thin BDT-COF films were synthesized from solution on different polycrystalline surfaces, indicating the generality of the synthetic strategy. The favorable orientation, crystallinity, porosity, and the growth mode of the thin BDT-COF films were studied by means of X-ray diffraction (XRD), 2D grazing incidence diffraction (GID), transmission and scanning electron microscopy (TEM, SEM), and krypton sorption. The highly porous thin BDT-COF films were infiltrated with soluble fullerene derivatives, such as [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), to obtain an interpenetrated electron-donor/acceptor host–guest system. Light-induced charge transfer from the BDT-framework to PCBM acceptor molecules was indicated by efficient photoluminescence quenching. Moreover, we monitored the dynamics of photogenerated hole-polarons <i>via</i> transient absorption spectroscopy. This work represents a combined study of the structural and optical properties of highly oriented mesoporous thin COF films serving as host for the generation of periodic interpenetrated electron-donor and electron-acceptor systems

Charge Photogeneration in Donor–Acceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

We investigate the role of excess excitation energy on the nature of photoexcitations in donor–acceptor π-conjugated materials. We compare the polymer poly­(2,6-(4,4-bis­(2-ethylhexyl)-4H-cyclopenta­[1,2-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-4,7-benzo­[2,1,3]­thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor–acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics