Andrew P. Monkman

Attaining precise control of emission within the blue spectral region remains inherently challenging for ladder‐type π‐extended multiresonant thermally activated delayed fluorescent (MR‐TADF) compounds, despite their many advantageous design features. Here, we aim to address the undesired redshifting of the emission in ladder‐type MR‐TADF compounds by incorporating terminal fused carbazoles. We report two MR‐TADF compounds, B2N3 and B3N3 , the latter of which is a p‐ and n‐doped heptacene polycyclic aromatic π‐hydrocarbon (PAH), both of which contain fused carbazoles at both ends of the molecule. Relative to the previously reported MR‐TADF compound α ‐3BNMes , the emission of B3N3 is blueshifted and narrower, emitting at λ _PL of 427 nm and having a full width at half maximum (FWHM) of 25 nm in toluene. In contrast, B2N3 , which is more conformationally flexible, emits at λ _PL of 441 nm and has a FWHM of 32 nm in toluene. Sensitized organic light‐emitting diodes (OLEDs) using B2N3 and B3N3 as terminal emitters showed EQE _max of 12.8% and 10.7% emitting with CIE coordinates of (0.15, 0.10) and (0.16, 0.09), respectively.

Detailed photophysical measurements of the non-symmetric heptazine, HzTFEX ₂ , previously reported as having inverted S ₁ -T ₁ levels, show complex behaviour, highly dependent on environment, concentration and aggregation. Here we show that, single crystals of HzTFEX ₂ show anti-Kasha S ₀ ←S ₂ and excimer emission. Solution state absorption shows a hypsochromic shift of the π-π* S ₀ →S ₁ , linearly dependent on solvent polarizability. In most environments, emission is dominated by excimer species, only in toluene and frozen 2-MeTHF do monomeric properties dominate. In frozen solutions, aggregation phenomena are solvent dependent. Given such complexity, time-dependent emission spectra are crowded, making it difficult to resolve monomeric phosphorescence. However, by exciting S ₀ →S ₂ at 77 K, we observe a consistent phosphorescence signal at 425 nm, 110 ± 20 meV above the S ₀ ←S ₁ emission, whereas exciting S ₀ →S ₁ does not give phosphorescence or delayed fluorescence (DF). We conclude that coupling between S ₂ and S ₁ is very weak, S ₂ -T _N intersystem crossing is effective, whereas S ₁ ↔T ₁ intersystem crossing is thermally activated, fulfilling the conditions required for the observation of inverted phosphorescence. At room temperature, no long-lived DF is observed because exothermic T ₁ →S ₁ ISC becomes allowed, such that in an OLED, high efficiency can be achieved.

Polycyclic aromatic hydrocarbons are molecular frameworks that enable the development of efficient emitters owing to their extensive conjugation and rigid structure. Here, we introduce monohomotruxene (MHTX), designed by reducing the symmetry of homotruxene (HTX) and truxene (TX). This partial planarization greatly enhances the complex photophysics over symmetric, triply twisted HTX, substantially increasing the photoluminescence quantum yield (PLQY), a result we attribute to its enhanced molecular rigidity and symmetry breaking. MHTX displays an unconventional violation of Kasha’s rule, yielding excitation-dependent dual emission. Excitation of the S ₂ state under anaerobic conditions produces intense, 455–480 nm, ultralong room-temperature phosphorescence (RTP), lasting over 12 s at room temperature. In contrast, excitation into the S ₁ state yields prompt fluorescence and weak delayed fluorescence (DF) by triplet-triplet annihilation (TTA) in the neat film. Aggregate species also present a complex emissive behavior with weak orange phosphorescence and TTA-DF. These findings establish MHTX as a promising emissive scaffold, highlighting the potential for accessing complex excited-state dynamics and advanced photophysical functionalities.

Systematic investigation of multi-substituted carbazole-dendronized radicals reveals the relationship between dendron architecture and luminescence.

We show that integrating acceptor-extended donor dendrons into MR-TADF emitters tunes their electroluminescence spectra while maintaining peak performance across key metrics, including quantum yields and reverse intersystem crossing rates.

We introduce monohomotruxene ( MHTX ), a non-symmetric polycyclic aromatic hydrocarbon (PAH) designed by reducing the symmetry of homotruxene ( HTX ) and truxene ( TX ). This partial planarization greatly enhances the complex photophysics over symmetric, triply twisted HTX , substantially increasing the photoluminescence quantum yield (PLQY) in both solution and solid state, a result we attribute to its enhanced molecular rigidity and symmetry breaking. MHTX displays an unconventional violation of Kasha’s rule, yielding excitation-dependent dual emission, giving a greater total PLQY than measured when exciting S ₁ . Excitation of the S ₂ state under anaerobic conditions produces intense, 455–480 nm, ultralong room-temperature phosphorescence (RTP), lasting over 12 s at room temperature, the longest phosphorescence reported for a hydrocarbon. In contrast, excitation to the S ₁ state yields prompt fluorescence and weak delayed fluorescence (DF) by triplet-triplet annihilation (TTA) in the neat film. Aggregate species also present a complex emissive behavior with weak orange phosphorescence and strong, blue TTA-DF. These findings establish MHTX as a highly promising new emissive scaffold, highlighting the remarkable potential of non-symmetric PAH architectures for accessing complex excited-state dynamics and advanced photophysical functionalities.

Detailed photophysical measurements of the non-symmetric heptazine, HzTFEX2, previously reportedly as having inverted S1-T1 levels, show complex behaviour, highly dependent on environment, concentration and aggregation. Single crystals of HzTFEX2 show anti-Kasha S0S2 and excimer emission. Solution state absorption shows hypsochromic shifts of the π-π* S0•S1, linearly dependent on solvent polarizability. In most environments, emission is dominated by excimer species, only in toluene and frozen 2-MeTHF do monomeric properties dominate. In frozen solutions, aggregation phenomena are solvent dependent. Given such complexity, time-dependent emission spectra are crowded, making it difficult to resolve monomeric phosphorescence. However, by exciting S0•S2, we observe a consistent phosphorescence signal at 425 nm, lying 110±20 meV above the S1 singlet emission. Importantly, exciting S0•S1 does not give phosphorescence or delayed emission (DF). We conclude that coupling between S2 and S1 is very weak, S2-TN intersystem crossing (ISC) is effective, whereas S1 T1 intersystem crossing is highly ineffective because vibronic coupling is negligible at 77 K, fulfilling the conditions required for inverted phosphorescence to be observable. At room temperature, no long-lived DF is observed because vibrational coupling is now strong enough to drive exothermic T1•S1 ISC, such that in an OLED high efficiency can be achieved.

The relief of excited‐state antiaromaticity (ESAA) is a powerful driving force that underpins the photochemical behavior of photoexcited aromatic molecules. Photo‐induced isomerization, cycloaddition, or proton transfer pathways alleviate the large energetic destabilization caused by ESAA. However, the excited‐state dynamics of annulenes displaying only weak ESAA remain largely unexplored. Here, we fine‐tune the electronic character of a series of annulenes to minimize their ESAA. Excited‐state proton transfer (ESPT) converts hydroxytropyliums (HTs) and hydroxybenzotropyliums (HBTs) to their nonaromatic tautomers, which are only slightly lower in energy than the excited‐state antiaromatic forms. As a result of this near‐degeneracy, we find that HBTs exhibit weak photoacidity (pKa drop of 0.3–1.2 units) and unusually slow, reversible ESPT, proceeding on the ns timescale rather than the typical ps timescale. The fluorescence spectra of these HBTs display the hallmarks of ESPT emitters: (i) dual fluorescence; (ii) excitation‐dependent emission; and (iii) large Stokes shifts. The driving force for this ESPT is increased by the addition of electron‐donating groups, which promote intramolecular charge transfer. Our results demonstrate that judicious structural changes can moderate the ESAA of nonbenzenoid annulenes, minimizing the driving force for ESPT, which leads to slow proton transfer kinetics and weak photoacidity.

The relief of excited‐state antiaromaticity (ESAA) is a powerful driving force that underpins the photochemical behavior of photoexcited aromatic molecules. Photo‐induced isomerization, cycloaddition, or proton transfer pathways alleviate the large energetic destabilization caused by ESAA. However, the excited‐state dynamics of annulenes displaying only weak ESAA remain largely unexplored. Here, we fine‐tune the electronic character of a series of annulenes to minimize their ESAA. Excited‐state proton transfer (ESPT) converts hydroxytropyliums (HTs) and hydroxybenzotropyliums (HBTs) to their nonaromatic tautomers, which are only slightly lower in energy than the excited‐state antiaromatic forms. As a result of this near‐degeneracy, we find that HBTs exhibit weak photoacidity (pKa drop of 0.3–1.2 units) and unusually slow, reversible ESPT, proceeding on the ns timescale rather than the typical ps timescale. The fluorescence spectra of these HBTs display the hallmarks of ESPT emitters: (i) dual fluorescence; (ii) excitation‐dependent emission; and (iii) large Stokes shifts. The driving force for this ESPT is increased by the addition of electron‐donating groups, which promote intramolecular charge transfer. Our results demonstrate that judicious structural changes can moderate the ESAA of nonbenzenoid annulenes, minimizing the driving force for ESPT, which leads to slow proton transfer kinetics and weak photoacidity.

We report thermally activated delayed fluorescence (TADF) emitters, namely, PTZ-Dipp-SO2B, and PTZ-Dipp-(Bu)SO2B, featuring an additional n-butyl group on the 10H-dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B) core enhancing its solid-state luminescence through an internal solvation effect. In dilute low-polarity solutions, both molecules show a yellow-orange photoluminescence with λ_em = 561–588 nm and red to near-infrared (NIR) PL in neat films, λ_em = 680–706 nm. The PL of PTZ-Dipp-(Bu)SO2B is systematically blue-shifted with respect to PTZ-Dipp-SO2B thanks to a ∼0.1 eV difference in LUMO energy caused by the σ-donor properties of the n-butyl group. These molecules were successfully applied as luminescent dopants to organic light-emitting diodes (OLEDs). Although both compounds have similar electronic structures and exhibit comparable Φ_PL in solution, the PTZ-Dipp-(Bu)SO2B is a significantly more efficient luminophore in the solid state than its counterpart. The OLED external quantum efficiency (EQE) at 7% load is 12.2% and in the neat film is 1.0% for PTZ-Dipp-(Bu)SO2B, while those for PTZ-Dipp-SO2B are 5.4% and 0.4%, respectively. The difference in λ_em or λ_EL between the two emitters is too small to explain the differences in EQE exclusively with the effects of the energy gap law. Instead, we believe that the n-butyl group disrupts the environment around the molecule through locally lowering the polarity and disturbing the formation of aggregates in the solid state. From previous observations of solvation of the emitter with hexane molecules, we term this phenomenon as the internal solvation effect.