Cyclophane Molecules Exhibiting Thermally Activated Delayed Fluorescence: Linking Donor Units to Influence Molecular Conformation

Abstract

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 phenothiazinedibenzothiophene-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 analysis shows 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 non-bridged 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.

Intramolecular Hydrogen Bonding in Thermally Activated Delayed Fluorescence Emitters: Is There Evidence Beyond Reasonable Doubt?

“…Since this occurs for all acridine signals regardless of their proximity to the nitrogen atom of the bridging heterocycle, it is not reasonable to attribute this to a localized intramolecular H-bonding interaction. Rather, as previously observed for phenothiazine segments in D-A compounds, a comparable downfield shift of all donor proton signals can be observed when comparing planar to slightly folded conformers of the respective donor units . This downfield shift is therefore in line with reduced electronic conjugation within the donor segment for the more folded structures of 2 and 3 and also follows trends in the cyclic voltammetry data discussed in the Supporting Information (section 9).…”

“…Rather, as previously observed for phenothiazine segments in D-A compounds, a comparable downfield shift of all donor proton signals can be observed when comparing planar to slightly folded conformers of the respective donor units. 33 This downfield shift is therefore in line with reduced electronic conjugation within the donor segment for the more folded structures of 2 and 3 and also follows trends in the cyclic voltammetry data discussed in the Supporting Information (section 9). Alternatively or additionally, a reduction of the intersegmental dihedral angle in 2 and 3 due to alleviated steric hindrance could impact the intersegmental electronic conjugation in solution and result in a minor downfield shift of the acridine proton signals of 2 and 3 as compared to 1 .…”

“…Despite their ability to harvest triplet states, CT states with near-perpendicular D-A geometries frequently possess low oscillator strength, resulting in unsuitably low photoluminescence quantum yields (PLQYs) and/or long singlet exciton lifetimes. − Hence, efficient TADF molecules must balance these competing factors, with designs that allow for some torsional/vibrational flexibility (a perturbation that enables emission) centered about a near-perpendicular “compromise” average geometry (enabling rISC). Reports in which the D-A geometry is influenced by external groups to understand or control TADF properties are now commonplace. ,− Successful strategies can include rigidification of the overall D-A structure or major modification of the average molecular geometry (including D-A angle or axial/equatorial conformers) through molecular design. Incorporation of heteroatoms to induce attractive intramolecular interactions, most commonly N···H hydrogen bonds, is also now frequently reported (see Supporting Information section 2 for selected relevant examples). − , …”

Highly Efficient Blue Thermally Activated Delayed Fluorescence Emitters with a Triphenylamine‐Based Macrocyclic Donor

“…The macrocyclic donor is anticipated to restrict the donor conformation to localize the HOMO distribution so as to reduce the ΔE ST . [39][40][41] To shed light on the combined effects of introducing methyl substituents and macrocyclic donor, a compound c-NN-MeTRZ was synthesized for further studying. Through detailed theoretical calculations and experimental characterizations, all three new compounds show extremely high PLQYs of ≈100% in the blue emission region.…”

Intramolecular Through-Space Interactions Induced Emission of Pillar[4]arene[1]dicyanobenzene

“…Besides, the supramolecular polymerization pattern offers another effective method. , Although the photophysical process triggered by supramolecular self-assembled polymerization would impart the system with high emission efficiency and fascinating optical properties, this strategy cannot avoid complicating the component compositions, thus impeding further reutilization. As such, fabricating self-emissive macrocycles with intrinsic D/A structures is imperative. − …”