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 (λmax = 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.

Dibenzodipyridophenazines with Dendritic Electron Donors Exhibiting Deep-Red Emission and Thermally Activated Delayed Fluorescence

“…Materials exhibiting thermally activated delayed fluorescence (TADF) exhibit unique photophysics − that has recently unlocked applications spanning photocatalysis, , bioimaging, − photodynamic therapy, − and organic light-emitting diodes (OLEDs). − TADF can be observed when the energy gap between the lowest energy singlet (S 1 ) and triplet (T 1 ) excited states in a material is small enough to enable conversion of triplet excitons to singlets, by a process termed reverse intersystem crossing (RISC). This process can enable the harvesting of triplet excitons in purely organic materials, giving compounds that exhibit both prompt (ns) and delayed (μs–ms) emission lifetimes.…”

Highly Effective Generation of Singlet Oxygen by an Imidazole-Linked Robust Photosensitizing Covalent Organic Framework

“…The SEM image of sonicated PyPor-COF showed the nanoparticles with an average size around 180 nm (Figure 5a), which was also confirmed by the dynamic light scattering (DLS) results (Figure S13). 73 As PyPor-COF can easily aggregate into larger particles and settle out from water (Figures S13 and S14), liposome reagents were then prepared by mixing PyPor-COF with soybean lecithin and cholesterol to enhance its dispersion in water for physiological tests. To determine the optimal ratio of the COF to liposome reagents, we compared the sample dispersion capabilities in water with different ratios of soybean phosphate/cholesterol/COF and tested the change of their average particle size by DLS (Figures 5b and S14−S16).…”

Excited-state intramolecular proton transfer (ESIPT)-based fluorescent probes for biomarker detection: design, mechanism, and application

“…Additionally, ESIPT probes with a long lifetime might be another rising field, including ESIPTbased chemiluminescence and Thermal Activation Delayed Fluorescence (TADF) probes, showing advantages in observing long-term biological processes. [190][191][192][193] It is definite that the ESIPT-based fluorescent probes for biomarker detection will still be a challenging and flourishing area with developing measurement techniques and pluralistic biomarker probes, assisted by improved targeting properties, efficient recognizing groups, and excellent photophysical properties. We hope this feature article will inspire researchers to construct more novel, powerful and not-yet-involved probes.…”