Donor engineering of diphenylamine-substituted tris(2,4,6-trichlorophenyl)methyl radicals for controlling the intramolecular charge transfer and near-infrared photothermal conversion

Abstract

Donor engineering plays an important role in tuning the intramolecular charge transfer (ICT) interactions of donor–acceptor (D–A) molecules and the related performance in the further applications. In comparison to the complicated and time-consuming molecular skeleton exploitation, optimization of the donor number and the substitution position is more facile and efficient. Herein, different quantities of diphenylamine (DPA) units are incorporated into the tris(2,4,6-trichlorophenyl)methyl (TTM) radicals from various directions. The effects of the donor number and the substitution position on the ICT interactions and the photothermal conversion performance are investigated. It is found that TTM-DPA (644 nm), TTM-2DPA (650 nm), and TTM-3DPA (637 nm) with various DPA units attached on different arms of the TTM core exhibit similar absorption maxima and high photoluminescence (PL) quantum yields (over 56%) in cyclohexane solutions. However, when multi-DPA units are introduced on the unilateral arm of the TTM unit, the absorption maximum red shifts to 675 nm for TTM-BDPA and 717 nm for TTM-TDPA. Meanwhile, no obvious PL can be detected using a spectrometer. The near-infrared (NIR) absorbance and the nonradiative transition of the TTM radicals contribute to their photothermal conversion. Consequently, remarkable photothermal conversion efficiencies of 41% and 50% are achieved for TTM-BDPA and TTM-TDPA, respectively. Our study provides an alternative design rule for controlling the ICT effect and shows that organic radicals have potential for application in photothermal therapy in the near future.