Yongjun Li

University of Florida, Gainesville, Florida, United States

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Publications (3)18.11 Total impact

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    ABSTRACT: A series of platinum tetrayne oligomers, all-trans-Cl-Pt(P2)-[(CC)4-Pt(P2)]n-Cl, where P = tri(p-tolyl)phosphine and n = 1-3 was subjected to a detailed photophysical investigation. The photoluminescence of each oligomer at low temperature (T < 140 K) in a 2-methyltetrahydrofuran (Me-THF) glass features an intense and narrow 0-0 phosphorescence band accompanied by a vibronic progression of sub-bands separated by ca. 2100 cm-1. The emission arises from a 3,* triplet state concentrated on the (CC)4 carbon chain and the vibronic progression originates from coupling of the excitation to the  (CC) stretch. All of experimental data including ambient temperature absorption, low temperature photoluminescence and ambient temperature transient absorption spectroscopy provide clear evidence that the triplet state is localized on a chromophore consisting approximately of two -[(CC)4-Pt(P2)]- repeat units. Density functional theory calculations support the hypothesis that the triplet-triplet absorption arises from transitions that are delocalized over two repeats.
    No preview · Article · May 2014 · The Journal of Physical Chemistry A
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    ABSTRACT: Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet-triplet energy transfer. The objective of this work is to understand the mechanisms of energy transfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of Pt4An give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet-triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet-triplet energy transfer to the anthracene segment.
    No preview · Article · Jul 2013 · The Journal of Physical Chemistry B
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    ABSTRACT: The series of platinum acetylide oligomers (PAOs) with the general structure trans,trans-[(RO)3Ph-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-(Ar)-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-Ph(OR)3], where Ar = 1,4-phenylene, 2,5-thienylene, or bis-2,5-(S-2-methylbutoxy)-1,4-phenylene and R = n-C12H25 gel hydrocarbon solvents at concentrations above 1 mM. Gelation is thermally reversible (T(gel-sol) approximately 40-50 degrees C), and it occurs due to aggregation of the PAOs resulting in the formation of a fibrous network that is observed for dried gels imaged by TEM. The influence of aggregation/gelation on the photophysical properties of the PAOs is explored in detail. Aggregation induces a significant blue shift in the oligomers' absorption spectra, and the shift is attributed to exciton interactions arising from H-aggregation of the chromophores. Strong circular dichroism (CD) is observed for gelled solutions of a PAO substituted with homochiral S-2-methylbutoxy side chains on the central phenylene unit. The CD is attributed to formation of a chiral supramolecular aggregate structure. The PAOs are phosphorescent at ambient temperature in solution and in the aggregate/gel state. The phosphorescence band is blue-shifted ca. 20 nm in the aggregate/gel, and the shift is assigned to emission from an unrelaxed conformation of the triplet excited state. Phosphorescence spectroscopy of mixed aggregate/gels consisting of a triplet donor/host oligomer (Ar = 1,4-phenylene) doped with low concentrations of an acceptor/trap oligomer (Ar = 2,5-thienylene) indicates that energy transfer occurs efficiently in the aggregates. Triplet energy transfer involves exciton diffusion among the host chromophores followed by Dexter exchange energy transfer to the trap chromophore.
    No preview · Article · Mar 2008 · Journal of the American Chemical Society