Room temperature phosphorescence from a benzothiophene-based N–B–N multi-resonance core

Abstract

We report a design strategy that enables room temperature phosphorescence from an N–B–N type heterocycle, reminiscent of a multi-resonance thermally activated delayed fluorescence (MR-TADF) compound, by disturbing the alternating frontier molecular orbital distribution pattern through five-membered ring fusion with the boron atom. The 2 wt% doped film in mCP host showed a long phosphorescence lifetime of 51.5 ms at 571 nm at room temperature, with an afterglow visible for up to 1 s.

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Introduction

Room temperature phosphorescence (RTP) is a term that describes long-lived luminescence originating from the triplet excited state of organic compounds.^1–3 This radiative relaxation from the T₁ excited state to the S₀ ground state is formally a spin-forbidden transition. As spin–orbit coupling is generally weak in organic materials, the emission lifetime for this process is usually slow, ranging from microseconds to several seconds.^1,4–7 For RTP, population of the T₁ state must also occur following photoexcitation, and this too is only possible due to intersystem crossing from the singlet manifold being mediated by spin–orbit coupling (SOC) that is competitive with fast radiative decay in the form of fluorescence from the S₁ state. Unlike thermally activated delayed fluorescent materials, typically in RTP emitters, the energy gap between S₁ and T₁, ΔE_ST, is large, so reverse intersystem crossing is not competitive with phosphorescence.

Over the past decade, significant efforts have been made to increase the efficiency of RTP by enhancing SOC through either the incorporation of heavy p-block atoms such as sulfur^8–10 and/or the introduction of moieties that engineer the triplet excited state to have a different orbital type to both S₁ and S₀, such as the use of carbonyl groups.¹¹ Non-radiative decay of the triplet excitons competes with RTP. A number of strategies have been used to mitigate the non-radiative decay and thus enhance the efficiency of the RTP. These include crystallization,^12,13 host–guest interaction,^14,15 matrix rigidification,^16,17 molecular aggregation,^18–21 and co-crystallization.^22,23 However, these strategies are not conducive to employing these materials in an amorphous state, such as in thin films. So, engineering RTP from organic compounds in amorphous films remains a challenge.^24–26

RTP can be turned on as a function of host medium. For instance, Yao et al. reported the narrowband RTP compound, 7MQ (Fig. 1a). When 7MQ is a dopant in benzophenone (BP) host, RTP is observed, whereas TADF is detected as a doped film in 9,9-(1,3-phenylene) bis-9H-carbazole (mCP) or bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) hosts. The ΔE_ST of 7MQ is 0.18 eV, which is sufficiently small to expect TADF.²⁷ However, the BP host acts as a triplet sensitizer, and triplet exciton transfer to 7MQ occurs via Dexter energy transfer, resulting in RTP. In contrast, mCP and DPEPO act only as high triplet energy hosts, enabling FRET to 7MQ, which then emits TADF. Another example comes from Du et al. who reported a series of cycloborylated thiabenzothiophene RTP emitters.²⁶ For example, the ΔE_ST of α-DThBSS-Cz is 0.48 eV, rendering reverse intersystem crossing (RISC) not thermally accessible; however, the four sulfur atoms enhance SOC (T₁ → S₀ = 0.51 cm⁻¹) and are responsible for turning on RTP in these compounds, which the authors exploited as dyes for anti-counterfeiting technology. Recently, our group reported a series of nitrogen-containing indolocarbazoles, some of which (6NICz and 6,10NICz) showed unusual biluminescence originating from RTP and TADF.²⁸ Very recently, Jang et al. reported a series of unsymmetrical narrowband fluorescent emitters, BFD, BFD-Ph, and BFD-DTB, featuring benzo[b]thiophene as a triplet exciton managing unit.²⁹ Despite having a structure that is reminiscent of an MR-TADF emitter skeleton, the BFD series exhibits neither TADF nor RTP; rather, these compounds are narrowband fluorescent. This is because the sulfur atom of the benzothiophene contributes to a stabilized T₁ level, resulting in large ΔE_ST of 0.38, 0.41, and 0.38 eV for BFD, BFD-Ph, and BFD-DTB, respectively, which renders TADF inaccessible at ambient temperatures. These molecules were employed as emitters in OLEDs, which exhibited a maximum external quantum efficiency, EQE_max, of 11.3%. Although these emitters are not exhibited TADF, the increase in EQE above 5% was ascribed by the authors as due to triplet–triplet annihilation (TTA), given the 3 wt% doping used in the device. All these results indicate that triplet excitons can be harvested either through their conversion to singlet excited states via TADF/TTA, or directly radiatively decaying via RTP. However, the molecular design strategy required to achieve pure RTP emission from molecules resembling an MR-TADF motif is not yet well understood.

Fig. 1 (a) Molecular structures of selected reported RTP emitters and BT-DBIP, (b) frontier orbital distribution of distinct N–B–N cyclo-borylated skeletons with different electron density distribution patterns and the molecular design of this work, “P” indicates the phenyl ring involved in the cyclo-borylation.

It is interesting to cross-compare the HOMO and LUMO electron distribution arrangements in N–B–N MR-TADF motif, like in DABNA-1, with those in the BFD series of emitters (Fig. 1b). The cycloborylated benzothiophene ring exhibits a random HOMO and LUMO distribution, which disrupts the alternating pattern of the electron density distribution that is responsible for the short-range charge transfer (SRCT), producing compounds with much larger ΔE_ST and no TADF; notably, the two cycloborylated phenyl rings (P) display alternating HOMO and LUMO distributions. By considering these results, we envisaged that the formation of a stabilized triplet excited state, facilitated by a non-alternate HOMO and LUMO arrangement within an N–B–N MR motif would result in pure RTP emission.

Here, we report an organic RTP emitter, BT-DBIP, based on an unusual motif consisting of a cycloborylated benzothiophene (Fig. 1a). The structure is reminiscent of MR-TADF scaffolds but its emissive excited states are of locally excited character, not the required SRCT character that is characteristic of MR-TADF emitters. The cycloborylation to benzothiophene alters the HOMO and LUMO topologies, resulting in a large singlet–triplet gap, ΔE_ST, of 0.82 eV in 2 wt% doped films in mCP host, while the presence of two proximal sulfur atoms enhances, turning on RTP. We believe that this design offers valuable insight for the development of organic small-molecule RTP emitters.

Results and discussion

The target emitter BT-DBIP was synthesized in three steps as shown in Scheme 1. The synthesis began with a palladium-catalysed C–N coupling reaction between 2,6-dimethylaniline and 3-bromobenzothiophene to obtain intermediate 1 in 49% yield. Compound 1 was reacted with 3,5-dichloro-N,N-diphenylaniline in a second Buchwald–Hartwig cross-coupling to yield intermediate 2 in 84%. Electrophilic borylation of 2 with BBr₃ and diisopropylethylamine afforded BT-DBIP in 56% yield. The targeted emitter was characterized by ¹H and ¹³C NMR spectroscopy, high-resolution mass spectrometry (HRMS), elemental analysis (EA), melting point determination, and high-performance liquid chromatography (HPLC) analysis (Fig. S1–S11). Thermal stability of BT-DBIP was evaluated by thermogravimetric analysis, BT-DBIP possesses a high thermal decomposition temperature (T_d) of 444 °C at 5% weight loss (Fig. S12).

Scheme 1 Synthetic scheme of BT-DBIP.

Theoretical studies

We first modelled the optimized geometry and optoelectronic properties of BT-DBIP in the gas phase, starting from a structure obtained in Chem3D, using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the PBE0/6-31G(d,p) level.^30,31 The frontier molecular orbitals, energy levels, and low-lying excited-state energies of BT-DBIP are shown in Fig. 2a and the optimized geometry is depicted in Fig. S13. The HOMO and LUMO are each distributed across the benzothiophene and central aryl-substituted rings, while there is no electron density on any of the peripheral amine groups in the HOMO and limited electron density on the peripheral diphenylamine nitrogen atom in the LUMO. The electron density distributions of the HOMO and LUMO do not alternate like typical MR-TADF molecules such as PAB³² (Fig. S14). For instance, in BT-DBIP, there is a significant contribution to the HOMO from the ortho-sulfur atom relative to the boron atom. This is because the electron-rich sulfur atom influences the electron density distribution at this position, which decreases the LUMO contribution and increases the HOMO contribution. A typical MR-TADF material, such as PAB, has no contribution at the ortho-carbon position to boron in the HOMO, while in the LUMO, this contribution is large at this position.

Fig. 2 (a) Frontier orbitals of BT-DBIP (isovalue of 0.02) and (b) natural transition orbitals (NTO) (isovalue of 0.02) of singlet (S₁) and triplet excited states (T₁ and T₂), hole (blue) and particle (red). All calculations at the PBE0/6-31G(d,p) level in the gas phase.

The excited-state properties were calculated by using time-dependent density functional theory (TD-DFT) within the Tamm–Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries at the same level of theory.^33,34 The calculated energy of the first singlet state (S₁) is 3.14 eV, and the first triplet (T₁) state is 2.43 eV, leading to a singlet–triplet energy gap, ΔE_ST, of 0.71 eV. This is too large for TADF to be operational. Natural transition orbital (NTO) analysis shows a significant hole (blue) density sited at the sulfur (Fig. 2b), while there is a much smaller particle (red) density contribution, which indicates that the sulfur atom makes the ortho-position electron richer (relative to boron) rather than electron-deficient. Thus, the hole and particle in the excited state do not show an alternating pattern that is consistent with an MR-TADF compound.³⁵ There is an intermediate T₂ state that lies between the S₁ and T₁ energy levels (0.71 eV), and so may contribute to any ISC/RISC processes. The spin–orbit coupling matrix element (SOCME) values between T₂ and S₁, T₂ and S₀, T₁ and S₁ and T₁ and S₀ were calculated at the same level of theory using the T₁ optimized geometry. The SOCME value of T₂–S₀ (〈S₀|H_SO|T₂〉) is 0.97 cm⁻¹, which is higher than that for T₂–S₁ (〈S₁|H_SO|T₂〉) of 0.41 cm⁻¹. There is a large SOCME value for T₁–S₀ (〈S₀|H_SO|T₁〉) of 1.36 cm⁻¹, whereas the SOCME value between T₁–S₁ (〈S₁|H_SO|T₁〉) of 0.08 cm⁻¹ is small. The difference density plot of two lowest lying singlet and triplet excited states of BT-DBIP shows no alternating pattern of increasing electron density (red) and decreasing electron density (blue) (Fig. S14b), which implies that this state does not possess SRCT character. Taken together, the calculations predict that this compound is likely to be room temperature phosphorescent; note that the S₀–S₁ transition is predicted to be moderately strong, with an oscillator strength, f, of 0.33.

Electrochemistry

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements in DCM with 0.1 M [ⁿBu₄N]PF₆ as the supporting electrolyte were used to infer the HOMO/LUMO levels of BT-DBIP (Fig. 3a). BT-DBIP possesses a reversible oxidation wave at E_ox at 0.83 V vs. SCE, which is assigned to the oxidation of the embedded amine units present in the cycloborylated motif.^29,32,36 The corresponding HOMO level is −5.15 eV. The HOMO level of BT-DBIP is destabilized compared to those of the BFD series (−5.42 eV to −5.46 eV),²⁹ the arylamine analogue PAB (−5.21 eV),³² and the carbazole analogue tDPA-DtCzB³³ (−5.24 eV), reflecting the significant contribution from the electron-rich benzothiophene units to increase the HOMO level. No reduction was detected within the solvent window, so the LUMO level was estimated from the HOMO level and the optical band gap (E_opt = 3.03 eV), itself determined from the intersection of the normalized absorption and emission spectra in toluene (Fig. S15). The LUMO of BT-DBIP is thus at −1.68 eV.

Fig. 3 (a) Cyclic and differential pulse voltammograms of BT-DBIP in degassed DCM with 0.1 M [ⁿBu₄N]PF₆ as the supporting electrolyte and Fc/Fc⁺ as the internal reference versus SCE (0.46 V vs. SCE)³⁷ (scan rate = 100 mV s⁻¹); (b) absorption and photoluminescence spectra of BT-DBIP in 1 × 10⁻⁵ M toluene (λ_exc = 374 nm), (inset: Picture of BT-DBIP in toluene solution under 364 nm excitation).

Photophysical studies

The photophysical properties of BT-DBIP were first measured in dilute toluene solution (Table 1 and Fig. 4b). There are two pronounced low-lying absorption bands at 385 and 404 nm, attributed to locally excited (LE) π–π* transitions based on TDA-DFT calculations (Table S2). The absorption spectrum of BT-DBIP is hypsochromically shifted by 55 nm compared to that of the structurally closely related compound, α-DThBSS-Cz (λ_abs = 459 nm),²⁶ indicating the absence of SRCT bands in the former. The molar absorptivity, ε, of the band at 404 nm in BT-DBIP (36.3 × 10³ M⁻¹ cm⁻¹) is larger than the lowest energy band at 459 nm in α-DThBSS-Cz (ca. 28 × 10³ M⁻¹ cm⁻¹), likely due to greater involvement of the diphenylamine unit, which increases the absorptivity for the LE transition in BT-DBIP compared to the more conformationally twisted ^tCz group in α-DThBSS-Cz. The emission spectrum is a mirror image of the lowest energy part of the absorption spectrum and is structured, reflecting emission from a LE state. The PL peaks at λ_PL 416 nm and shows a vibronic progression with λ_PL of 440, 467, and 500 nm (Table 1). The Stokes shift is 12 nm (0.09 eV or 726 cm⁻¹, Fig. S15), and the FWHM is 53 nm (0.37 eV or 2984 cm⁻¹), the magnitude of both reflecting the rigid structure of this emitter. We calculated the Huang–Rhys (HR) factor as a function of the normal vibrational frequencies (cm⁻¹) in the gas phase to quantify the coupling strength between structural displacement and vibrational modes.³⁸ The vibrational mode analysis indicates that the low-frequency modes below 50 cm⁻¹ (with HR factors of 0.24 at 37 cm⁻¹, 0.15 at 54 cm⁻¹, and 0.16 at 92 cm⁻¹), can be attributed to the twisting vibrations of the 2,6-dimethylaniline. In contrast, the high-frequency vibrational modes in the range of 1407–1675 cm⁻¹ make significant contributions to the reorganization energies of BT-DBIP. These modes are primarily dominated by the stretching and scissoring vibrations of the B–C–S bonds of the cyclo-borylated benzothiophene rings (Fig. S16). Fig. 4a shows the steady-state PL spectra in 2-MeTHF at RT and at 77 K, and the delayed emission spectrum (gated emission, 1–9 ms acquisition window) at 77 K. The steady-state PL spectrum closely aligns with that at 77 K, indicating emission from the same LE state. The phosphorescence spectrum is also structured, peaking at 562 and 616 nm. The onset of the steady-state PL spectrum at 77 K provides an estimate of the S₁ energy, whereas the onset of the gated emission spectrum at 77 K relates to the energy of the T₁ state. The S₁ and T₁ energies are 3.07 eV and 2.23 eV, respectively, and the ΔE_ST is 0.84 eV, which is much higher than the ΔE_ST (0.48 eV) of α-DThBSS-Cz (Table 1). The large ΔE_ST indicates that TADF is almost certainly not operational, and this aligns with the DFT study. The solvatochromism study (Fig. S17 and Table S1) corroborates the assignment of a LE character to the emissive excited state, as there is only a slight spectral red-shifting and broadening. Time-resolved PL measurements in toluene at room temperature under vacuum showed a monoexponential decay of the emission, with a PL lifetime, τ_PL, of 3.13 ns, there was no delayed emission detected (Fig. S18). The photoluminescence quantum yield (Φ_PL) of BT-DBIP in degassed toluene is 33%, which decreases to 27% upon exposure to air. The PL spectra of degassed and aerated solutions of BT-DBIP in toluene (Fig. S19) likewise reveal a quenching in the presence of O₂. These studies suggest that there are triplet excited states that are being populated upon photoexcitation. The phosphorescence quantum yield (Φ_Ph) is 6.3% in toluene.

Table 1 Photophysical properties of BT-DBIP in solution and thin film

Compound	Medium	λ abs /nm	λ PL /nm	FWHM^e/nm	E S1 /eV	E T1 /eV	ΔEST^g/eV	Φ PL /%	Φ Ph /%	τ p /ns	τ d /ms
a In toluene solutions (10^−5 M). b Spin-coated thin films consisting of 2 wt% BT-DBIP in mCP under vacuum, λexc = 340 nm. c Lowest energy absorbance band. d Steady-state PL maximum at 300 K, λexc = 374 nm for solution and 340 nm for thin film. The values in parentheses are when the sample is under vacuum. e Full-width at half maximum of PL peak. f S1 and T1 energies obtained from the onsets of the respective steady-state PL at 77 K (prompt) and delayed emission spectra (delay: 1 ms; gate time: 9 ms) at 77 K (λexc = 374 nm for glassy 2-MeTHF and 340 nm for thin film). g in 2-MeTHF (10^−6 M), ΔEST = E(S1) – E(T1). h Photoluminescence quantum yields (ΦPL) of BT-DBIP measured by a relative method using quinine sulfate as the reference (ΦPL = 54.6% in 1 N H2SO4) in optically dilute solutions in toluene under degassed/aerated conditions.^39 Absolute ΦPL of the thin films measured using an integrating sphere under degassed/aerated conditions. i The ΦPh is calculated by subtracting the ΦPL in air from the ΦPL under N2. j Prompt and delayed PL lifetimes were obtained by TCSPC (λexc = 375 nm) and MCS (at λPL of 571/627 nm, λexc = 340 nm), respectively.
BT-DBIP	Sol.^a	404	416, 440, 467	53	3.07	2.23	0.84	32.8/26.5	6.3	3.13	—
	Film^b	—	420, 445, 473 (550, 571, 627)	—	3.03	2.21	0.82	8.7/5.0	3.7	2.87	51.5/46.3

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Fig. 4 (a) Steady-state fluorescence PL (SS PL) spectra at room temperature and 77 K and phosphorescence spectrum (acquisition time range of 1–9 ms) at 77 K of BT-DBIP in 2-MeTHF (λ_exc = 374 nm); (b) steady-state PL spectra SS PL at 300 K under vacuum and air and phosphorescence spectrum (acquisition time range of 1–9 ms) at 300 K of 2 wt% doped film of BT-DBIP in mCP (λ_exc = 340 nm); (c) steady-state PL spectrum SS PL at 77 K and phosphorescence spectra (acquisition time range of 1–9 ms) at 300, 200, and 77 K of 2 wt% doped film of BT-DBIP in mCP (λ_exc = 340 nm).

We next evaluated the photophysical properties of BT-DBIP as a 2 wt% doped film in 1,3-bis(N-carbazolyl)benzene (mCP). The steady-state PL spectrum under air is similar to that measured in toluene, peaking at λ_PL of 420 nm, with major vibrational bands at 445, 473, and 506 nm (Table 1 and Fig. 4b). When the sample is placed under vacuum, a second lower-energy structured emission appears at λ_PL 550 nm (with bands also at 571 and 627 nm) that we assign to RTP. The room temperature gated emission spectrum (1–9 ms acquisition) under vacuum corroborates the assignment of the low-energy emission to phosphorescence. The Φ_PL of BT-DBIP is 8.7%, which decreased to 5.0% upon exposure to air (Table 1). Thus, the calculated Φ_Ph of BT-DBIP is 3.7%.

Temperature-dependant gated emission spectra under vacuum (Fig. 4c) document that the phosphorescence intensity increases with decreasing temperature, reflecting the suppression of non-radiative decay pathways at lower temperatures. The measured S₁ and T₁ energies of the 2 wt% doped film of BT-DBIP are 3.03 and 2.21 eV, and the ΔE_ST is 0.82 eV, values that are very similar to those measured in glassy 2-MeTHF (Fig. 4c). The time-resolved PL decays were measured at each of λ_PL of 420, 445, 571, and 627 nm under vacuum at RT. At 420 and 445 nm, the emission decays with similar bi-exponential kinetics, with τ_PL,avg of 2.87 ns (Fig. S20). At 571 and 627 nm, the emission decays with similar monoexponential kinetics, with τ_PL of 51.5 and 46.3 ms, respectively. This long-lived emission is completely quenched upon exposure to air (Fig. 5a). The τ_PL increases to 269.74 ms at 200 K and to 309.1 ms at 77 K (detecting at λ_PL of 571 nm, Fig. 5b and Fig. S21), while similarly the τ_PL increased to 258.0 ms at 200 K and to 289.5 ms at 77 K (detecting at λ_PL of 627 nm, Fig. 5c). This behavior is consistent with the temperature-dependent steady-state PL data. Indeed, the RTP emission (λ_exc = 365 nm) from the 2 wt% doped film in mCP under vacuum was observed for up to one second after the UV irradiation (Fig. 6).

Fig. 5 (a) Time-resolved PL decays of 2 wt% doped film of BT-DBIP in mCP under air and vacuum at 300 K for λ_PL= 571 nm and λ_PL= 627 nm (λ_exc= 340 nm); (b) temperature-dependent time-resolved PL decays of 2 wt% doped film of BT-DBIP in mCP for λ_PL = 571 nm (λ_exc= 340 nm); (c) temperature-dependent time-resolved PL decays of 2 wt% doped film of BT-DBIP in mCP for λ_PL = 627 nm (λ_exc= 340 nm).

Fig. 6 Photos of 2.0 wt% doped film of BT-DBIP in mCP at room temperature under vacuum with and without UV irradiation (λ_exc = 365 nm). The times represent the delay after the excitation source was turned off.

Conclusions

A new RTP emitter BT-DBIP was developed by incorporating benzothiophene segment within a prototypical boron-based MR-TADF skeleton. Calculations showed that the alternating frontier molecular orbitals are disturbed by the five-membered ring at the cycloborylated benzothiophene core, resulting in LE emission rather than emission from a SRCT state. The presence of sulfur atoms in BT-DBIP increases SOC between S₁ and T₁ and T₁ and S₀, enhancing both ISC and phosphorescence rates. Because of the rigid molecular structure, the PL spectrum of BT-DBIP in toluene is structured, and the Stokes shift is small. The 2 wt% doped film of BT-DBIP in mCP likewise showed RTP in the absence of air, with a phosphorescence lifetime of 51.5 ms. This work demonstrates a design concept to develop RTP emitters based on perturbing the chemical structure of an N–B–N type MR-TADF skeleton.

Author contributions

D. K.: conceptualization, synthesis, investigation, writing–original draft preparation, review & editing. A. K. G.: data curation, formal analysis, investigation, theoretical and photophysical studies, writing – review & editing. E. Z-C.: project management, supervision, resources, writing–review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The research data supporting this publication can be accessed at https://doi.org/10.17630/35824801-2ed5-458d-a7d8-2fd3a1593f27.

Supplementary information (SI): ¹H NMR and ¹³C NMR spectra, HRMS and HPLC; supplementary computational data and coordinates; additional photophysical data are available. See DOI: https://doi.org/10.1039/d5qm00893j.

Acknowledgements

We thank Gavin Peters for performing TGA and DSC experiments. We thank Samsung Display Co., Ltd., the Leverhulme Trust (RPG-2022-032), and the Engineering and Physical Sciences Research Council for support (EP/R035164/1, EP/W007517/1, EP/Z535291/1).

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Footnotes

† Current address: Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur – 603203, Tamil Nadu, India.

‡ These authors contributed equally.

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