Delayed Blue Fluorescence via Upper-Triplet State Crossing from C–C Bonded Donor–Acceptor Charge Transfer Molecules with Azatriangulene Cores

We report the synthesis and structural and photophysical characterization of two series of molecules with functionalized azatriangulene electron donor cores and three pendant electron acceptor units. The presented donor and acceptor units are joined by C–C bonds, instead of the usual C–heteroatom bonds often found in thermally activated delayed fluorescence (TADF) emitters. The effects of the donor–acceptor strength and donor–acceptor dihedral angle on the emission properties are assessed. The data establish that the singlet–triplet energy gap is >0.3 eV and that delayed emission is present in only specific host matrices, irrespective of host polarity. Specific host behavior is atypical of many TADF materials, and we suggest the delayed emission in this work does not occur by a conventional vibronically coupled TADF mechanism, as the ΔEST value is too large. Detailed photophysical analysis and supporting density functional theory calculations suggest that some presented azatriangulene molecules emit via an upper-triplet state crossing mechanism. This work highlights that several different mechanisms can be responsible for delayed emission, often with highly similar photophysics. Detailed photophysical analysis is required to establish which delayed emission mechanism is occurring. Our results also highlight a clear future direction toward vibronically coupled C–C bonded TADF materials.

Melting points were carried out on a Stuart SMP40 machine under air with a ramping rate of 4 °C min −1 . Videos were replayed manually to determine the melting point.
TGA analysis was carried out on a Perkin Elmer Pyris 1 machine under nitrogen gas at 20 mL min −1 . Measurements were carried out from 25 °C -700 °C ramping at 10 °C min −1 .
High resolution mass spectrometry was carried out on a Waters LCT Premier XE using ASAP ionization with TOF detection. Samples were analyzed directly as solids. All references to Br within characterization data refer to 79 Br isotope.
Use of hexane refers to a mix isomers grade unless otherwise stated.

Photophysics
Thin films in zeonex were prepared by drop-casting with a guest concentration of 5-10% weight in zeonex matrix from toluene solutions. CBP films were prepared using 10wt.% guest in this host. Thin films in mCBP and TSPO1 hosts with a guest concentration of 10% weight were prepared by dropcasting from THF solutions and were allowed to thermally anneal at 60 °C for 2 h. Absorption and emission spectra were collected using a UV-3600 double beam spectrophotometer (Shimadzu), and a Fluorolog fluorescence spectrometer (Jobin Yvon). Phosphorescence, prompt fluorescence (PF), and delayed emission (DF) spectra and time-resolved emission decays were recorded using nanosecond gated luminescence with either a high energy pulsed Nd:YAG laser emitting at 355 nm (EKSPLA) or a N2 laser emitting at 337 nm with pulse width 170 ps. Emission was focused onto a spectrograph and detected on a sensitive gated iCCD camera (Stanford Computer Optics) between 350 and 700 nm with sub-nanosecond resolution. For initial development of these methods, see previously published literature. 2 PLQY measurements were performed using a Fluorolog 3 spectrometer using compounds in mCBP films at 10 wt% in air using an integration sphere. The PLQY values reported are an average of measurements on two different films for each compound.

Electrochemistry
For the oxidation processes cyclic voltammetry experiments were recorded using a BAS CV50W electrochemical analyzer fitted with a three-electrode system consisting of a glassy carbon (ϕ = 3 mm) working electrode, and Pt wire counter and quasi reference electrodes. Cyclic voltammetry

Figure S2b
Left) Time-resolved emission spectra for molecule 1c in zeonex. Prompt fluorescence emission at 300 K and phosphorescence emission at 80 K collected at given delay and integration times. Right) Time-resolved emission decay curve for 1c at 300 K and 80 K.

Supplementary discussion of HMAT-TRZ calculations
In order to gain an insight into the nature of singlet and triplet excitations, the set of natural transition orbitals (NTOs) was accessed. 12 NTOs are used to describe the superposition of transitions occurring during the certain transition excitation by means of hole and particle. The hole and particle are moderately delocalized in the case of the S0→S1 transition pointing at the mixed CT/LE character, this being due to the almost planar molecular skeleton of HMAT-TRZ. Strongly overlapping hole/particle distributions in both S0→T1 and S0→S1 transitions explain the low T1 energy and therefore the large ∆EST. The observed difference between the HOMO/LUMO and HONTO1-3/LUNTO1-3 for HMAT-TRZ clearly emphasizes the importance of the detailed investigation of optical transitions. The nature of S0→T2 and S0→T6, encompassing the S1 state, have a clear acceptor-localized character, and are important for rISC involving 3 LE. 13 S15 S4 X-ray Crystallography 1a 1b 1c Figure S4a. X-ray molecular structures of 1a·CH2Cl2, 1b and 1c. Here and below, thermal ellipsoids are drawn at the 50% probability level.

S6 Synthetic procedures and characterisation
Final compounds 1a-c and 2a-c Compound 1a. Tribromoarene (9) (200 mg, 0.33 mmol, 1.00 eq.) and SPhos (13.6 mg, 33.0 µmol, 0.10 eq.) were combined in dioxane (3.5 mL). A solution of K3PO4 (493 mg, 2.31 mmol, 7.00 eq.) in deionized water (0.75 mL) was then added and the resulting mixture was deoxygenated by bubbling with argon for 10 min. Pd(OAc)2 (3.7 mg, 16.5 µmol, 0.05 eq.) was next added and the resulting orange mixture was degassed for a further 20 min, before the addition of 2-cyanobenzeneboronic acid MIDA ester (12) (300 mg, 1.16 mmol, 3.50 eq.). The mixture was bubbled with argon for a final 10 min and heated in a 70 °C oil bath overnight under argon. The mixture was cooled to room temperature and diluted with DCM (50 mL) and water (20 mL Compound 1b. Tribromoarene (9) (100 mg, 0.17 mmol, 1.00 eq.) and 2-trifluoromethyl-4cyanobenzeneboronic acid pinacol ester (14) (197 mg, 0.66 mmol, 3.99 eq.) were combined in toluene (3 mL). A solution of Na2CO3 (211 mg, 1.99 mmol, 12.00 eq.) in deionized water (1 mL) was then added and the resulting mixture was bubbled with argon for 20 min. Pd(PPh3)4 (12 mg, 10.4 µmol, 0.06 eq.) was next added and the resulting yellow mixture was bubbled with argon for a further 10 min before it was heated in a 115 °C oil bath overnight. The mixture was cooled to room temperature and diluted with DCM (50 mL) and water (20 mL). The layers were then separated and the aqueous layer was further extracted with DCM (2 × 20 mL). The organic extracts were combined, dried over MgSO4 and evaporated under reduced pressure. The residue was purified by flash chromatography on silica gel (eluent: gradient DCM/ n-hexane 1:4-1:0 v/v). Pentane (20 mL) was added to the obtained solid and the solvent was evaporated under reduced pressure. This was repeated twice more. Finally, the residue was dried under high vacuum to obtain compound (1b) as a faint yellow powder (125 mg, 0.14 mmol, 86%). Crystals suitable for X-ray diffraction were grown by vapor diffusion of methanol into a solution of 1b in DCM.

Synthesis of azatriangulene core precursors
Triester 3 was synthesised as reported in the literature. 15 Compound 5 has not been reported previously. It was synthesized using a modified literature procedure for a similar molecule. 16 PhLi (13.15 mL, 1.9 M in n-butyl ether, 24.99 mmol, 10.5 eq.) was added to dry Et2O (50 mL) in a 2neck 250 mL round-bottomed flask at −10 °C. To the PhLi solution was added dropwise a solution of triester 3 (1.00 g, 2.38 mmol, 1 eq.) in THF (50 mL). The reaction mixture was allowed to warm to ambient temperature and was stirred for 16 h. The reaction mixture was quenched with dropwise addition of water (0.6 mL), followed by addition of CH2Cl2 (150 mL) to aid dissolution of the intermediate. The solvent was removed under reduced pressure to yield crude intermediate 4 (not isolated). AcOH (35 mL) was then added to the crude mixture and following 20 min stirring to aid solubility, conc. HCl (5 mL) was added to the mixture. The reaction mixture was heated to 120 °C (DrySyn temp.) for 4 h. The reaction mixture was cooled to ambient temperature and was poured into ice-water (100 mL). The product was collected by filtration and was washed in the glass sinter with H2O (3 × 100 mL) and cold EtOH (2 × 50 mL). The slightly pink crude mixture was dried under vacuum, and was subsequently recrystallized from CHCl3 to give product 5 as a white solid (1.23 g, 70%).

S27
To a stirring solution of 5 (1.00 g, 1.35 mmol, 1 eq.) in CHCl3 (80 mL) at 0 °C was added bromine (347 µL, 6.77 mmol, 5 eq.) dropwise over 5 min. The reaction mixture was allowed to warm to ambient temperature and the reaction mixture was stirred at ambient temperature for 16 h in the dark. The reaction mixture was quenched by addition of saturated sodium thiosulfate(aq) (10 mL) which was then diluted with H2O (50 mL). The mixture was extracted with CHCl3 (3 × 75 mL) and the organic extracts were combined and dried with MgSO4 and filtered. Removal of solvent under reduced pressure yielded pure product 6 as an off-white solid (1.20 g, 91%). To dry Et2O (50 mL) under argon atmosphere in a two-necked round-bottomed flask was added MeMgI (3.0 M in Et2O, 60 mL, 180 mmol 18.9 eq.). To this solution was added a solution of triester 3 (4.00 g, 9.5 mmol) in toluene (120 mL) over 30 min dropwise via cannula. The reaction mixture was heated to 60 °C (Drysyn kit temperature) for 40 h. The reaction was cooled to room temperature and the excess MeMgI was quenched carefully by adding water dropwise (100 mL). The reaction mixture was extracted with DCM (3× 150 mL). The organic extracts were washed with H2O (2× 100 mL), dried with MgSO4 and filtered. The solvent was removed under reduced pressure to yield crude triol 7, which was used directly in the next step without further purification. Crude triol 7 was dissolved in CHCl3 (50 mL) and Amberlyst-15 (17.11 g, 4.7 g mol −1 'SO3H') was added. The reaction mixture was refluxed for 2 h with vigorous stirring. Following cooling to ambient temperature, amberlyst-15 was filtered off in a glass sinter. Amberlyst-15 beads were washed copiously with CHCl3 (5 × 100 mL) to wash product off the Amberlyst. Removal of solvent from all filtrates under reduced pressure yielded 8 as an off-white solid (753 mg, 22% from triester 3).
Molecule 9 is known, but was synthesized using a slightly modified procedure. 15 The 1 H NMR spectrum is consistent with the literature. 15 To a solution of triarylamine core 8 (736 mg, 2.01 mmol, 1 eq.) in CHCl3 (25 mL) at 0 °C was added NBS (1.07 g, 6.03 mmol, 3.0 eq.) portion wise over 10 min. The reaction mixture was allowed to warm to ambient temperature and was stirred in the dark for 16 h. Water (50 mL) was added to the reaction mixture and the organic layer was separated. The aqueous layer was extracted with CHCl3 (1 × 50mL) and both organic layers were combined and washed with H2O (3 × 50mL). The organic extracts were dried with MgSO4 and filtered. Removal of solvent under reduced pressure yielded crude product which was recrystallized from ethanol to give 9 as a white solid (703 mg, 58%). (trifluoromethyl)benzonitrile (13) (3.00 g, 12 mmol, 1 eq.) were dried under vacuum for 15 min in a 250 mL two-necked round-bottom flask equipped with an air condenser. The setup was back filled with argon and dry 1,4-dioxane (54 mL) was added via syringe. The reaction mixture was deoxygenated by bubbling with stirring for 20 min. The reaction was heated to 105 °C (DrySyn temperature) for 19 h. The reaction was allowed to cool to ambient temperature and EtOAc (100 mL) was added to the reaction mixture. The reaction was filtered through Celite TM and H2O (100 mL) and brine (50 mL) was added. The organic layer was separated and extracted with EtOAc (2 × 100 mL). The organic layers were combined and washed with brine (100 mL), dried with MgSO4 and filtered. Removal of solvent under reduced pressure gave crude product, which was purified by silica gel column chromatography eluting with gradient ethyl acetate:hexane 6:1 to 1:0 (v/v). Removal of solvent under reduced pressure followed by drying overnight under high vacuum (10 −2 mbar) gave the product (14)  Synthesis of 16-18 was carried out under air. All molecules in the scheme are known 20 but were synthesized using a modified literature route 21 . Reported NMR data is consistent with the literature. 20 Acid Chloride 16: Commercially available 4-bromoisophthalic acid (15) (10.0 g, 40.81 mmol, 1 eq.) was dissolved in SOCl2 (100 mL) in a 500 mL round-bottomed flask equipped with a reflux condenser and was refluxed for 18 h. The reaction was allowed to cool to ambient temperature and the solvent was removed under reduced pressure to yield crude intermediate acid chloride which was used in the next step without further purification. The acid chloride 16 was dried under high vacuum to ensure complete SOCl2 removal. The next step was carried out in the same flask.
Amide 17: Crude 4-bromoisophthaloyl dichloride (16) in a round-bottomed flask was cooled to 0 °C in an ice bath. Ammonia solution (12 wt.%, 150 mL) was initially added dropwise very carefully and slowly for the first 10 mL solution. The remaining 140 mL of ammonia solution was then added portion wise over 30 min. The reaction flask was equipped with a reflux condenser and the mixture was refluxed for 4.5 h with vigorous stirring. Following cooling to ambient temperature, the reaction mixture was cooled again to 0 °C, resulting in significant precipitation of the product. The white precipitate was collected by filtration using a glass sinter, and was washed copiously with water with agitation in the sinter. The solid was dried at 70 °C in an oven overnight. The amide 17 was isolated as a white powder (8.02 g, 81% from diacid). Nitrile 18: 4-Bromoisophthalamide (17) (7.74 g, 31.84 mmol, 1 eq.) was refluxed in POCl3 (100 mL) for 2 h, during which time the mixture went from cloudy to clear. The mixture was added dropwise to lukewarm water (1000 mL) to safely degrade the POCl3. (Safety note: Do NOT quench in ice water! A violent exothermic reaction can occur several hours later on warming to room temperature!). Following quenching, the mixture was cooled to 0 °C for 30 min to allow for efficient precipitation of the product. The precipitate was collected by filtration and was washed copiously with water in the sinter. Drying of the product under high vacuum gave nitrile 18 as a white powder (5.23 g, 79%).