Heptazine‐Based TADF Materials for Nanoparticle‐Based Nonlinear Optical Bioimaging

Abstract s ‐Heptazines are emerging as strong electron acceptors for efficient thermally activated delayed fluorescent (TADF) materials, yet the difficulties in synthesizing them have limited their practical use. Here, three novel s ‐heptazine TADF materials with green to deep‐red emission (λ max = 525–664 nm) and high photoluminescence quantum yields, synthesized by either pseudoelectrophilic substitution or Negishi coupling routes, are described. These materials also demonstrate strong nonlinear optical absorption, with two‐photon cross sections up to 1260 GM. With deep‐red fluorescence, strong two‐photon absorption, high quantum yield, and delayed fluorescence, the emitter HAP‐3MeOTPA is ideally suited for use in nanoparticle‐based bioimaging experiments. The two kinds of luminescent nanoparticle are prepared, namely hostless, aggregate‐based organic dots ( a ‐Odots) and rigid, glassy Odots ( g ‐Odots) as biocompatible and water‐dispersible TADF probes. The g ‐Odots are shown to retain the nonlinear optical properties, high photoluminescence quantum yield, and TADF observed in the constituent heptazine dye. These g ‐Odots are then used as biological imaging probes with immortalized human kidney cancer (HEK293) cells, and single and multi‐photon‐excited microscopy coupled with time‐gated luminescence measurements are demonstrated. This work not only describes new routes to efficient heptazine‐based TADF materials, but also demonstrates their potential as nanoparticle‐based bioimaging probes combining several advanced optical functions.