Zinc Donor–Acceptor Schiff Base Complexes as Thermally Activated Delayed Fluorescence Emitters

Abstract

Four new zinc(II) Schiff base complexes with carbazole electron donor units and either a 2,3-pyrazinedicarbonitrile or a phthalonitrile acceptor unit were synthesized. The donor units are equipped with two bulky 2-ethylhexyl alkyl chains to increase the solubility of the complexes in organic solvents. Furthermore, the effect of an additional phenyl linker between donor and acceptor unit on the photophysical properties was investigated. Apart from prompt fluorescence, the Schiff base complexes show thermally activated delayed fluorescence (TADF) with quantum yields up to 47%. The dyes bearing a phthalonitrile acceptor emit in the green–yellow part of the electromagnetic spectrum and those with the stronger 2,3-pyrazinedicarbonitrile acceptor—in the orange–red part of the spectrum. The emission quantum yields decrease upon substitution of phthalonitrile with 2,3-pyrazinedicarbonitrile and upon introduction of the phenyl spacer. The TADF decay times vary between 130 µs and 3.5 ms at ambient temperature. The weaker phthalonitrile acceptor and the additional phenyl linker favor longer TADF decay times. All the complexes show highly temperature-dependent TADF decay time (temperature coefficients above −3%/K at ambient conditions) which makes them potentially suitable for application as molecular thermometers. Immobilized into cell-penetrating RL-100 nanoparticles, the best representative shows temperature coefficients of −5.4%/K at 25 °C that makes the material interesting for further application in intracellular imaging.

Bright and Photostable TADF‐Emitting Zirconium(IV) Pyridinedipyrrolide Complexes: Efficient Dyes for Decay Time‐Based Temperature Sensing and Imaging

“…All immobilized dyes show mono-exponential TADF decays (Figure 5A for Zr( Mes BrPDP C6F5 ) 2 as an example) that are beneficial for sensing applications. This is in contrast to previously reported TADF emitters based on anthraquinones, [60] dicyanobenzenes, [60] and Zn(II) Schiff base complexes [64,65] which all showed bi-or tri-exponential decays in the immobilized form. Such behavior of the donor-acceptor dyes is attributed to existence of at least two different rotamers in the immobilized dyes since the rigid matrix prevents the dyes from rotation.…”

“…More recently, several optical thermometers have been reported using organic dyes derived from anthraquinone and dicyanobenzene, [60] platinum(II), palladium(II), and zinc benzoporphyrins, [61,62] polymers based on 1,8-naphthalimide, [63] carbon dot-based nanocomposite [43] and zinc complexes with Schiff bases. [64,65] Although the luminescence decay was shown to be strongly affected by temperature at ambient conditions (>2% K −1 ), practical usability of the state-ofthe-art systems is still limited. The challenges include moderate luminescence brightness, significant contribution of prompt fluorescence to the overall emission, bi-exponential decay in immobilized form, and often very long TADF decay times (>1 ms) which imply significant cross-talk to molecular oxygen even in case of polymer matrices with low gas permeability.…”

Multi‐Responsive Thermally Activated Delayed Fluorescence Materials: Optical ZnCl2 Sensors and Efficient Green to Deep‐Red OLEDs

“…[22][23][24] However, very few reports exist documenting the use of organic TADF materials as sensors. [25][26][27] The first reported example employed a TADF compound, acridine yellow (Figure 1a), as a temperature sensor. [28] Steinegger et al subsequently reported a series of carbazole-substituted dicyanobenzene and diphenylaminesubstituted anthraquinone donor-acceptor (D-A) TADF emitters, such as compound 3 (Figure 1a), for use as oxygen and temperature sensors.…”

Multi-Responsive Thermally Activated Delayed Fluorescence Materials: Optical ZnCl2 Sensors and Efficient Green to Deep-red OLEDs

“…[22][23][24] However, very few reports exist documenting the use of organic TADF materials as sensors. [25][26][27] The first reported example employed a TADF compound, acridine yellow (Figure 1a), as a temperature sensor. [28] Steinegger et al subsequently reported a series of carbazolesubstituted dicyanobenzene and diphenylamine-substituted anthraquinone donor-acceptor (D-A) TADF emitters, such as compound a3 (Figure 1a), for use as oxygen and temperature sensors.…”