Doublet Spin State Mediated Photoluminescence Upconversion in Organic Radical Donor-Triplet Acceptor Dyads

Donor-acceptor dyads are promising materials for improving triplet-sensitized photon upconversion due to faster intramolecular energy transfer (ET), which unfortunately competes with charge transfer (CT) dynamics. To circumvent the issue associated with CT, we propose a novel purely organic donor-acceptor dyad, where the CT character is confined within the donor moiety. In this work, we report the synthesis and characterization of a stable organic radical donor-triplet acceptor dyad (<b>TTM-Cz-Per</b>) consisting of the acceptor perylene (<b>Per</b>) linked to the donor (4-<i>N</i>-carbazolyl-2,6-dichlorophenyl)-bis(2,4,6-trichlorophenyl)methyl radical (<b>TTM-Cz</b>). Upon red-excitation of <b>TTM-Cz-Per</b>, the doublet emission of the donor (<b>TTM-Cz</b>) is significantly quenched, and a recorded delayed emission centered at ca. 490 nm was attributed to the fluorescence emission from the <b>Per</b> acceptor. Time-resolved transient absorption spectroscopy suggests doublet-to-triplet energy transfer (DTET) dynamics from the donor to the acceptor as the time constant τ for the donor transient species decreases from 21.47 ns for the <b>TTM-Cz</b> sensitizer to 8.73 ns for <b>TTM-Cz-Per</b> dyad. This process is accompanied by the appearance of a long-lived component with τ = 97.06 ns, which we ascribe to the triplet transient of the acceptor <b>Per</b>. Furthermore, computational results indicate that the DTET is intramolecular as computed spin densities of the quartet state show unpaired electrons of ρ ≈ 1 on the <b>TTM-Cz</b> donor and of ρ ≈ 2 on the acceptor <b>Per</b>. The present study highlights the possibility to employ doublet chromophoric systems for light-harvesting and energy upconversion, which can be further tailored for several optoelectronic applications.