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The synthetic methodology to covalently link donors to form cyclophane-based thermally activated delayed fluorescence (TADF) molecules is presented. These are the first reported examples of TADF cyclophanes with “electronically innocent” bridges between the donor units. Using a phenothiazine-dibenzothiophene-S,S-dioxide donor–acceptor–donor (D–A–D) system, the two phenothiazine (PTZ) donor units were linked by three different strategies: (i) ester condensation, (ii) ether synthesis, and (iii) ring closing metathesis. Detailed X-ray crystallographic, photophysical and computational analyses show that the cyclophane molecular architecture alters the conformational distribution of the PTZ units, while retaining a certain degree of rotational freedom of the intersegmental D–A axes that is crucial for efficient TADF. Despite their different structures, the cyclophanes and their nonbridged precursors have similar photophysical properties since they emit through similar excited states resulting from the presence of the equatorial conformation of their PTZ donor segments. In particular, the axial–axial conformations, known to be detrimental to the TADF process, are suppressed by linking the PTZ units to form a cyclophane. The work establishes a versatile linking strategy that could be used in further functionalization while retaining the excellent photophysical properties of the parent D–A–D system.

This work shows that trifluoromethyl (CF3) substituents can be used to increase the rate of thermally activated delayed fluorescence (TADF) in conjugated organic molecules by tuning the excitonic character of the singlet and triplet excited states.

In this work we successfully developed a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron donor (D) unit, and dibenzothiophene-S,S-dioxide as the acceptor (A), linked in D–A and D–A–D structures. In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted molecules 6 and 7 in particular show extremely high TADF efficiency in solution and solid film, even with a singlet–triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.

Biotinylated pharmaceuticals are of great interest due to the strong interactions between biotinyl-functionality and streptavidin/avidin, which opens up avenues for efficient targeting and localisation. Three new carbon monoxide-releasing molecules (CO-RMs) have been synthesised and characterised using chemical and biological analysis. An alkyne-containing CO-RM 2 was found to be toxic to RAW 264.7 murine macrophages; and thus therapeutically viable CO-RM 1 was employed as the alkyne precursor for [3 + 2] cycloaddition chemistry enabling a new acid-containing CO-RM 4 and biotin-bioconugate-CO-RM (BiotinCORM 5) to be prepared. CO-RM 4 showed significantly improved solubility and BiotinCORM 5 acts as a photo-CO-RM. We have found that an avidin–CORM adduct of 5 is a CO-releasing protein, releasing CO on irradiation with light (400 nm). The avidin–biotinCORM adduct of 5 was found to have a binding energy of 10 kcal mol−1.

By inverting the common structural motif of thermally activated delayed fluorescence materials to a rigid donor core and multiple peripheral acceptors, reverse intersystem crossing (rISC) rates are demonstrated in an organic material that enables utilization of triplet excited states at faster rates than Ir-based phosphorescent materials. A combination of the inverted structure and multiple donor–acceptor interactions yields up to 30 vibronically coupled singlet and triplet states within 0.2 eV that are involved in rISC. This gives a significant enhancement to the rISC rate, leading to delayed fluorescence decay times as low as 103.9 ns. This new material also has an emission quantum yield ≈1 and a very small singlet–triplet gap. This work shows that it is possible to achieve both high photoluminescence quantum yield and fast rISC in the same molecule. Green organic light-emitting diode devices with external quantum efficiency >30% are demonstrated at 76 cd m−2.

This study reports two novel D–A–D molecules, 2,7-bis(phenothiazin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DPT-TXO2) and 2,7-bis(1-methylphenothiazin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DMePT-TXO2), where the latter differs by only a methyl group incorporated on each of the donor units. DMePT-TXO2 in solution and in solid state shows dual charge-transfer (CT) emission. The CT states come from two distinctive conformations between the D and A units. Experiments show that the emission contribution of each state can be controlled by the polarity of the environment and by the excitation energy. Also, how the different conformers can be used to control the TADF mechanism is analyzed in detail. These results are important as they give a more in-depth understanding about the relation between molecular conformation and the TADF mechanism, thereby facilitating the design of new TADF molecules.

Dual luminescence, i.e. intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor–acceptor–donor (D–A–D) molecules. This dual luminescence is stronger in the “angular” isomers, compared to their “linear” regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1–Tn, below the lowest singlet state, S1, in the “angular” regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin–vibronic mechanism. In the absence of this “triplet ladder”, due to the larger energy difference between T1 and Tn in the “linear” regioisomers, the ISC and rISC are not efficient. Remarkably, the enhancement of the ISC rate in the “angular” regioisomers is accompanied by an increase of the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to the harvest of triplet states in organic compounds, and cast the TADF and RTP mechanisms into a common conceptual framework.

This study describes the synthesis and characterization of a new class of ferrocene-containing carbon monoxide-releasing molecules (CORMs, 1–3). The ferrocenyl group is both a recognized therapeutically viable coligand and a handle for informative infrared spectroelectrochemistry. Deoxymyoglobin CO-release assays and in situ infrared spectroscopy confirm compounds 2 and 3 as photoCORMs and 1 as a thermal CORM, attributed to the increased sensitivity of the Mn–ferrocenyl bond to protonation in 1. Electrochemical and infrared spectroelectrochemical experiments confirm a single reversible redox couple associated with the ferrocenyl moiety with the Mn tetracarbonyl center showing no redox activity up to +590 mV vs Fc/Fc+, though no concomitant CO release was observed in association with the redox activity. The effects of linker length on communication between the Fe and Mn centers suggest that the incorporation of redox-active ligands into CORMs focuses on the first coordination sphere of the CORM. Redox-tagged CORMs could prove to be a useful mechanistic probe; our findings could be developed to use redox changes to trigger CO release.

The potential for carbon monoxide-releasing molecules (CO-RMs) as antimicrobials represents an exciting prospective in the fight against antibiotic resistance. Trypto-CORM, a tryptophan-containing manganese(I) carbonyl, is toxic against E. coli following photo-activation. Here, we demonstrate that Trypto-CORM is toxic against Neisseria gonorrhoeae in the absence of photoactivation. Trypto-CORM toxicity was reversed by the high CO affinity globin leg-haemoglobin (Leg-Hb), indicating that the toxicity is due to CO release. Release of CO from Trypto-CORM in the dark was also detected with Leg-Hb (but not myoglobin) in vitro. N. gonorrhoeae is more sensitive to CO-based toxicity than other model bacterial pathogens, and may serve as a viable candidate for antimicrobial therapy using CO-RMs.

A series of phenothiazine–dibenzothiophene-S,S-dioxide charge-transfer molecules have been synthesized. Increasing steric restriction around the donor–acceptor bond significantly alters contributions from TADF and phosphorescence. Bulky substituents on the 1-(and 9) position(s) of the phenothiazine result in no TADF in the solid state; instead strong phosphorescence is observed at ambient temperature.

The synthesis of a new pyrene-containing Fischer carbene complex is described. The complex has a broad absorbance spectrum between 300 and 400 nm and, on excitation at 345 nm in CH2Cl2 solution, emission is observed at 395 and 415 nm. Emission is also observed in PBS buffer, but in this case the resulting spectra are much broader. Confocal and fluorescence lifetime imaging indicate that emission occurs on treating HeLa cells with the complex and co-localisation studies demonstrate that this is from the mitochondria and lipid-rich regions of the cell.

The first visible-light-activated carbon-monoxide-releasing molecule (CO-RM) to exhibit a potent effect against Escherichia coli is described. The easily prepared tryptophan-derived manganese-containing complex (TryptoCORM) released 1.4 moles of CO at 465 nm, and 2 moles at 400 nm. A comprehensive synthetic, mechanistic and microbiological study into the behaviour of TryptoCORM is reported. The complex is thermally stable (i.e., does not release CO in solution in the absence of light), shows low toxicity against mammalian cells and releases tryptophan on photoinduced degradation, all of which point to TryptoCORM being therapeutically viable.

A new class of photochemically-activated CO-releasing molecule (photo-CO-RM), based on a Mn(CO)4(C^N) system, is reported in this study. Three CO molecules are released per CO-RM molecule. Complex 3 is a fast releaser, thermally stable in the dark and a viable therapeutic agent.