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 (TTM-Cz-Per) consisting of the acceptor perylene (Per) linked to the donor (4-N-carbazolyl-2,6-dichlorophenyl)-bis(2,4,6-trichlorophenyl)methyl radical (TTM-Cz). Upon red-excitation of TTM-Cz-Per, the doublet emission of the donor (TTM-Cz) is significantly quenched, and a recorded delayed emission centered at ca. 490 nm was attributed to the fluorescence emission from the Per 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 TTM-Cz sensitizer to 8.73 ns for TTM-Cz-Per 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 Per. 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 TTM-Cz donor and of ρ ≈ 2 on the acceptor Per. 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.