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    Exciton Migration and Surface Trapping for a Photonic Crystal Displaying Charge-Recombination Fluorescence
    (Wiley-V C H Verlag Gmbh, 2016) Al-Aqar, Roza; Atahan, Alparslan; Benniston, Andrew C.; Perks, Thomas; Waddell, Paul G.; Harriman, Anthony
    A compact donor-acceptor molecular dyad has been synthesized by attaching an N,N-dimethylamino fragment to a naphthalic anhydride residue. The dyad shows fluorescence from an intramolecular charge-transfer state (i.e., charge-recombination fluorescence) in solution, with the photo-physical properties being strongly dependent on the solvent polarity. Similar emission is seen for single crystals of the target compound, the molecules being aligned head-to-head, although time-resolved emission profiles display dual-exponential kinetics. A second polymorph with the head-to-tail alignment also gives rise to two lifetimes that differ somewhat from those of the first structure, which are assigned to bulk and surface-bound molecules. Growing the crystal in the presence of Rhodamine B localizes the dye around the surface. Excitation of the crystal is followed by sub-ps exciton migration along the aligned stacks, with occasional crossing to adjacent stacks and trapping at the surface. Rhodamine B present at very low levels acts as the acceptor for excitons entering the surface layer. Crystals embedded in a polyester resin form an artificial light-harvesting antenna able to sensitize an amorphous silicon solar cell.
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    The Photophysical Properties of Triisopropylsilyl-ethynylpentacene-A Molecule with an Unusually Large Singlet-Triplet Energy Gap-In Solution and Solid Phases
    (Mdpi, 2020) Schaberle, Fabio A.; Serpa, Carlos; Arnaut, Luis G.; Ward, Andrew D.; Karlsson, Joshua K. G.; Atahan, Alparslan; Harriman, Anthony
    The process of singlet-exciton fission (SEF) has attracted much attention of late. One of the most popular SEF compounds is TIPS-pentacene (TIPS-P, where TIPS = triisopropylsilylethynyl) but, despite its extensive use as both a reference and building block, its photophysical properties are not so well established. In particular, the triplet state excitation energy remains uncertain. Here, we report quantitative data and spectral characterization for excited-singlet and -triplet states in dilute solution. The triplet energy is determined to be 7940 +/- 1200 cm(-1) on the basis of sensitization studies using time-resolved photoacoustic calorimetry. The triplet quantum yield at the limit of low concentration and low laser intensity is only ca. 1%. Self-quenching occurs at high solute concentration where the fluorescence yield and lifetime decrease markedly relative to dilute solution but we were unable to detect excimer emission by steady-state spectroscopy. Short-lived fluorescence, free from excimer emission or phosphorescence, occurs for crystals of TIPS-P, most likely from amorphous domains.
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    Pulse Radiolysis of TIPS-Pentacene and a Fluorene-bridged Bis(pentacene): Evidence for Intramolecular Singlet-Exciton Fission
    (Amer Chemical Soc, 2018) Karlsson, Joshua K. G.; Atahan, Alparslan; Harriman, Anthony; Tojo, Sachiko; Fujitsuka, Mamoru; Majima, Tetsuro
    Exposing TIPS-pentacene in deaerated benzene to ionizing radiation generates a mixture of singlet- and triplet-excited states of the solute. The singlet undergoes radiative decay without spin conversion whereas the triplet undergoes radiationless decay on the microsecond time scale. The concentration of each species was established by dosimetry. The excited-singlet state is not observed on the nanosecond-time scale for a related fluorene-bridged bis(pentacene), but the triplet is present in high concentration. Failure to detect the excited-singlet state is attributed to fast intramolecular singlet-exciton fission (iSEF) which is found to produce two triplet species. A short-lived intermediate (tau(T) = 145 ns) is identified as the species (T_T) having both pentacene units present as triplet states. The second transient is longer lived (tau(T) = 7.5 mu s) and is assigned to the corresponding species (T_G) with a single pentacene promoted to the triplet level. Dosimetry is used to conclude that iSEF partitions overwhelmingly in favor of T_G (70%) relative to T_T (25%). The total triplet yield from iSEF, therefore, is ca. 120% in this system, where the pentacene terminals are weakly coupled.