The simplest structure of a stable radical showing high fluorescence efficiency in solution: benzene donors with triarylmethyl radicals

Abstract

Donor–radical acceptor systems have recently attracted much attention as efficient doublet emitters that offer significant advantages for applications such as OLEDs. We employed an alkylbenzene (mesityl group) as the simplest donor to date and added it to a diphenylpyridylmethyl radical acceptor. The (3,5-difluoro-4-pyridyl)bis[2,6-dichloro-4-(2,4,6-trimethylphenyl)phenyl]methyl radical (Mes₂F₂PyBTM) was prepared in only three steps from commercially available reagents. A stable radical composed of only one pyridine ring, four benzene rings, methyl groups, halogens, and hydrogens showed fluorescence of over 60% photoluminescence quantum yield (PLQY) in chloroform, dichloromethane, and PMMA. The key to high fluorescence efficiency was benzene rings perpendicular to the diphenylpyridylmethyl radical in the doublet ground (D₀) state. The relatively low energy of the β-HOMO and the electron-accepting character of the radical enabled the use of benzenes as electron donors. Furthermore, the structural relaxation of the doublet lowest excited (D₁) state was minimized by steric hindrance of the methyl groups. The reasons for this high efficiency include the relatively fast fluorescence transition and the slow internal conversion, both of which were explained by the overlap density between the D₁ and D₀ states.

By adding mesityl donors to a diphenylpyridyl radical acceptor, a highly luminescent stable radical was prepared in three steps from commercially available reagents. The photoluminescence quantum yield was as much as 69% in chloroform.

Introduction

Recently, stable radicals have attracted much interest as luminescent materials.^1,2 The key to their luminescence is the fluorescence from the D₁ state to the D₀ state. In contrast to the singlet lowest excited (S₁) state of closed-shell molecules, which is higher in energy than the triplet lowest excited state (T₁), it is normal for the energy of the D₁ state to be lower than that of the quartet lowest excited (Q₁) state. Therefore, the fluorescence of radicals can avoid unfavorable quenching from higher multiplicity excited states produced by the recombination of electrons and holes or intersystem crossing from the S₁ state. Stable luminescent radicals are one of the most promising substance groups of emitters suitable for highly efficient electroluminescent (EL) devices.^3,4 A relatively basic application would be use in a heavy atom environment.^5,6

Problems due to thermal stability of the radicals had been resolved with the invention of polychlorotriphenylmethyl radicals such as perchlorotriphenylmethyl radical (PTM)⁷ and tris(2,4,6-trichlorophenyl)methyl radical (TTM).⁸ For a luminescent material, higher stability, that is, stability under photoexcitation conditions (photostability), is necessary; however, the photodecomposition of PTM and TTM had been reported.^9,10 We have reported that the introduction of a pyridyl group instead of a phenyl group has greatly improved the stability of the radical under photoirradiation.¹¹ Compared with TTM, the (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM) showed ca. 70 times higher photostability in dichloromethane.

Dilute PyBTM doped in (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methane (αH-PyBTM) crystal had an excellent feature of high fluorescence efficiency (Φ_f = 89%).¹² In molecular solids of stable radicals, spin-derived properties such as magnetism are important properties,¹³ and PyBTM has been used to investigate interesting photophysical phenomena such as the coherent coupling between spin ensembles,¹⁴ a magnetic field effect on luminescence (magnetoluminescence),^12,15,16 and photoluminescence anisotropy amplified by exciton funneling.¹⁷ However, the fluorescence efficiency of PyBTM was low in liquid solutions, similar to other simple triarylmethyl radicals. The PLQYs of PyBTM and TTM were both 2% in dichloromethane and 3% in chloroform.¹¹ Introduction of fluorine atoms on the pyridine ring slightly improved the PLQY in solution, and the (2,5-difluoro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (F₂PyBTM) showed PLQY of 4% in dichloromethane and 6% in chloroform.¹⁸ Utilizing the coordination ability of a nitrogen atom on the pyridine ring, we have developed Au^I complexes of PyBTM derivatives and improved the fluorescence efficiency to 36%.^19–21

On the other hand, the PLQYs of PTM or TTM derivatives have been improved by constructing a donor–acceptor system using nitrogen-containing electron donors such as carbazoles^22–26 or triphenylamines.^25,27 Highly fluorescent radicals showing PLQY above 50% in solution were first reported as a carbazole donor–TTM radical acceptor system showing 53% PLQY in cyclohexane.²² However, its fluorescence was quenched by the polarity of solvent molecules to 2% in chloroform. By adding electron-withdrawing groups on carbazole, strong fluorescence was maintained in chloroform.²⁴ It is noteworthy that efficient fluorescence in more polar solvents has recently been reported in completely different systems: pyrene-dithiadiazolyl radical (Φ_f = 50% in acetonitrile)²⁸ and the π-radical stabilized with boron (Φ_f = 67% in DMF).²⁹

Here, we report that simple aromatic hydrocarbons, mesityl groups, work as donors, and significantly enhance the fluorescence efficiencies of PyBTM and F₂PyBTM in dichloromethane and chloroform solutions. We explain the reasons for the high efficiencies of these nonplanar π-electron systems by photophysical theories and calculations using DFT and TD-DFT.

Results and discussion

From commercially available reagents, αH-PyBTM¹¹ and (3,5-difluoro-4-pyridyl)bis(2,4,6-trichlorophenyl)methane (αH-F₂PyBTM)¹⁸ were prepared in one step (Scheme 1). In the second step, the Suzuki–Miyaura coupling reaction with a micellar catalysis³⁰ and 2,4,6-trimethylphenylboronic acid selectively yielded mesityl substituents at the para position. The stable radicals MesPyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM were synthesized by deprotonation and oxidation processes. Thus, these radicals were all prepared in three steps from commercially available reagents. ESR spectra revealed that these radicals have S = 1/2 spin on one molecule (Fig. S1†).

Scheme 1. Synthesis of MesPyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM.

These radicals were stable under ambient conditions similar to other triarylmethyl radicals protected by halogen atoms.^7,8 Actually, these radicals were purified by chromatography on silica gel under ambient conditions, and their melting points could be determined. All the spectroscopic measurements were conducted under ambient conditions as no effect due to oxygen was observed similar to the other PyBTM derivatives,^5,11,18–21 probably due to short fluorescence lifetimes. As with PyBTM,¹¹ no change was observed in the solutions stored in the dark.

Absorption and emission spectra of MesPyBTM and Mes₂PyBTM in dichloromethane are compared with those of PyBTM and (3,5-dichloro-4-pyridyl)bis(2,6-dichloro-4-phenylphenyl)methyl radical (PyPBTM)³⁰ in Fig. 1a. The absorption spectrum of PyPBTM was clearly different from that of PyBTM, and both the α-HOMO–α-LUMO band (λ_max = 370 → 401 nm) and the β-HOMO–β-LUMO band (541 → 564 nm) were significantly redshifted. On the other hand, the shapes of the absorption spectra of MesPyBTM and Mes₂PyBTM were rather similar to that of PyBTM with the same absorption maxima at λ_max = 370 and 541 (±1) nm. This result is attributed to the only slightly changed energy levels of the frontier orbitals.

Fig. 1. (a) Absorption (solid line) and emission (broken line, λ_ex = 450 nm) spectra of MesPyBTM (orange), Mes₂PyBTM (red), PyPBTM (light green), and PyBTM (pink) in dichloromethane. Enlarged portions of the absorption spectra (10 fold) are shown from λ = 450 to 650 nm (dotted lines). The concentration of MesPyBTM, Mes₂PyBTM, PyPBTM, and PyBTM was 7.2 × 10⁻⁵, 7.7 × 10⁻⁵, 2.0 × 10⁻⁵, and 2.5 × 10⁻⁵ M, respectively. (b) Absorption (solid line) and emission (broken line, λ_ex = 450 nm) spectra of Mes₂F₂PyBTM (purple) and F₂PyBTM (light blue) in dichloromethane. Enlarged portions of the absorption spectra (10 fold) are shown from λ = 450 to 650 nm (dotted lines). The concentration of Mes₂F₂PyBTM and F₂PyBTM was 6.2 × 10⁻⁵ and 2.5 × 10⁻⁵ M, respectively.

This occurs because the mesityl group has bulky methyl groups at ortho positions and can barely conjugate with π-orbitals on the neighboring phenyl group. In order to estimate structures using DFT, we adopted the UB3LYP level of theory with 6-31G(d, p) basis sets, since they closely reproduced the experimental absorption and emission spectra from previous studies.^{11,19–21,31} The solvent effect of dichloromethane was taken into account by using a polarizable continuum model (PCM).^32,33 The dihedral angle between the mesityl and dichlorophenyl groups was 83° for MesPyBTM and 84° for Mes₂PyBTM in the DFT optimized D₀ state model (Table S1†). These are nearly perpendicular in contrast to the rather flat angles (34°) between the phenyl and dichlorophenyl groups in PyPBTM.

Redshifts of emission from PyBTM (λ_em = 585 nm) were seen in MesPyBTM (λ_em = 645 nm) and Mes₂PyBTM (λ_em = 628 nm), although these shifts were smaller than that of PyPBTM (λ_em = 654 nm). In the TD-DFT (UB3LYP/6-31G(d, p)) optimized D₁ structure, the mesityl groups under the β-HOMO are electron-deficient, and the dihedral angle decreased to 50° to conjugate to the relatively electron-rich dichlorophenyl group (Table S1†). This structural relaxation was the cause of this redshift. The dihedral angle became even smaller in PyPBTM, decreasing to 25°.

Absorption and emission spectra of Mes₂F₂PyBTM and F₂PyBTM are shown in Fig. 1b. The shape of the absorption spectrum of Mes₂F₂PyBTM (λ_max = 352 nm) resembles that of F₂PyBTM (λ_max = 351 nm). The dihedral angle between the mesityl and dichlorophenyl groups was 87° in the DFT optimized D₀ state (Table S1†). The redshift of emission from F₂PyBTM (λ_em = 566 nm) was seen in Mes₂F₂PyBTM (λ_em = 623 nm), similar to the case of Mes₂PyBTM. The optimized dihedral angle between the mesityl group under the β-HOMO and the neighboring dichlorophenyl group was 49° in the DFT optimized D₁ state.

Bright reddish-orange fluorescence was observed when the solutions of new radicals were irradiated with a UV lamp. The PLQYs in dichloromethane are shown in Table 1, and the PLQYs in chloroform are shown in Table S2.† As observed, the addition of mesityl groups dramatically increased the fluorescence efficiency of the radical, and the effect was much larger than that by the phenyl groups. The PLQY of 2% for PyBTM was elevated to 30% by the addition of a mesityl group and to 47% by double substitution of mesityl groups. In particular, Mes₂F₂PyBTM displayed as much as 66% PLQY in dichloromethane, and 69% in chloroform (Fig. S2†). As far as we know, a higher PLQY of the fluorescent radical in a liquid solution has only been reported for pyridoindole donor–TTM acceptor systems in 2020³⁴ and 2022³⁵ and the π-radical stabilized with boron in 2022.²⁹

PLQYs and photophysical parameters of radicals in dichloromethane.

	λ em (nm)	Φ f (%)	τ/ns	k f/107 s−1	k nr/107 s−1
PyBTMa	585	2	6.4	0.3	14
F2PyBTMb	566	4	12.5	0.3	7.7
PyPBTM	654	9.5	12	0.8	7.5
MesPyBTM	645	30	26	1.2	2.7
Mes2PyBTM	628	47	38	1.2	1.4
Mes2F2PyBTM	623	66	44	1.5	0.8

^aCited from ref. 11.

^bCited from ref. 18. All Φ_fs were obtained by absolute PLQY measurement.

The major structural difference between pyridoindole donor–TTM acceptor systems^34,35 and Mes₂F₂PyBTM is that the former has two nitrogen atoms in the donor and the latter has one nitrogen atom in the radical. In contrast, the TTM radical is made of a carbon skeleton and the mesityl group is a hydrocarbon. In terms of organic chemistry of nitrogen-containing aromatics, an indole ring is electron-rich and a pyridine ring is electron-deficient. A pyridoindole is thought to cancel out the two effects internally. The use of mesityl groups simplifies the situation and provides important scientific or economic insights.

The nitrogen atom in the radical was introduced to improve the photostability of the radical as described in the Introduction. The photostabilities of the new radicals were measured in dichloromethane under UV light (370 nm) irradiation. The decay of fluorescence is plotted in Fig. S3,† and the stabilities of the new radicals were of about the same order as that in PyBTM (Table S3†). The photostability of Mes₂F₂PyBTM was slightly higher than that of PyBTM. The photostability of PyBTM was ca. 70 times that of TTM and similar to that of TIPS pentacene.³⁶ Therefore, it was found that a reasonable degree of photostability of PyBTM could be maintained.

The availability of the radicals as fluorophores in polar solvents means that they are also useful in polymers having polar substituents. Poly(methyl methacrylate) (PMMA) is one of the most useful polymers in optical applications, and it has polar carboxyl groups that can quench the fluorescence of some donor–acceptor type fluorophores. Mes₂F₂PyBTM in a PMMA film displayed PLQY of 62%, proving its usefulness in this polymer.

As mentioned in the Introduction, the Q₁ state is higher in energy, and all of the excited states are thought to converge to the D₁ state in a short time. The PLQYs of the radicals are determined by competition between the rate of fluorescence (k_f) and the rate of nonradiative decay (k_nr) from the D₁ state (Fig. 2). Here, k_f and k_nr were calculated from the PLQYs and the fluorescence lifetimes (τ, Fig. S4†) of the radicals, and they are shown in Table 1.

Fig. 2. Energy diagram for the D₁–D₀ fluorescence of the radicals and the β-HOMO and β-LUMO of MesPyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM at the D₁ optimized structure calculated using UB3LYP/6-31G(d, p).

One cause of the low PLQY of triarylmethyl radicals was the small k_f. The small transition dipole moment between D₁ and D₀ is interpreted as being caused by the C₃ symmetry of the molecule³⁷ or cancellation of the HOMO–SOMO and SOMO–LUMO transition dipole moments.³⁴ This barrier for emission is partially eliminated by constructing a donor–acceptor system. The β-HOMOs and β-LUMOs calculated using the TD-DFT (UB3LYP/6-31G(d, p)) at the optimized D₁ structure are shown in Fig. 2 (see also Fig. S5†). Since the D₁ states are mainly generated via the β-HOMO–β-LUMO transition, the radicals are regarded as a mesityl donor–PyBTM acceptor system. Experimentally, k_f in MesPyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM was enhanced to a level 4 or 5 times that of PyBTM.

In 2020, Abdurahman et al. suggested that the large oscillator strength of a donor–acceptor system is due to intensity borrowing from the intense high-lying transition of the radical,³⁴ while Cho et al. showed that the k_f values are dominated by the coupling between the CT and ground state, and nearly independent of the donor strength.³⁸ We propose another view of the increase in k_f from visualization of the overlap density distribution by TD-DFT. Consideration of interactions with local excitations is no longer necessary for intuitive understanding.

Within the crude adiabatic approximation,^39,40 we consider transitions from the initial vibronic state |Φ_mν〉 = |Ψ_m〉|χ_mν〉 to the final one |Φ_nν′〉 = |Ψ_n〉|χ_nν′〉, where |Ψ_m〉 (|Ψ_n〉) and |χ_mν〉 (|χ_nν′〉) are the initial (final) electronic and vibrational states, respectively. The initial and final vibronic energies are denoted as E_mν and E_nν′, respectively. According to Fermi's golden rule, the rate constant of fluorescence from |Ψ_m〉 to |Ψ_n〉 is given by eqn (1).⁴¹

1

where ω is the angular frequency of the photon, c is the speed of light, P_mν(T) is the Boltzmann distribution function of |χ_mν〉 at temperature T, μ_nm is the transition dipole moment between |Ψ_m〉 and |Ψ_n〉, and ℏ is the Dirac constant. The radiative constant of fluorescence actually observed, k_f = k^f_0←1 is proportional to the square of the transition dipole moment between the D₁ and D₀ states (μ₁₀).

A density form of the transition dipole moment (τ₁₀(x)) is written as a product of a three-dimensional Cartesian coordinate (x = (x, y, z)) and an overlap density between the D₁ and D₀ states (ρ₁₀(x)) as in eqn (2) and (3).^41,42

μ₁₀ = ∫dxτ₁₀(x) 2

τ₁₀(x) = −exρ₁₀(x) 3

Thus, an overlap density having widespread distribution generally gives a large transition dipole moment. The calculated transition dipole moments of Mes₂PyBTM and Mes₂F₂PyBTM were larger than that of PyBTM (Table 2). This result was attributed to the overlap densities of Mes₂PyBTM and Mes₂F₂PyBTM delocalizing on both the PyBTM moiety and the mesityl group (Fig. 3).

Calculated transition dipole moments between the D₁ and D₀ states in x, y, and z components of PyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM at the D₁ optimized structure calculated using UB3LYP/6-31G(d, p).

	|μ10|/a.u.
	x	y	z
PyBTM	0.7012	0.3654	0.0725
Mes2PyBTM	1.3525	0.7567	0.0451
Mes2F2PyBTM	1.2884	0.7722	0.0132

Fig. 3. Overlap densities between the D₁ and D₀ states of PyBTM, Mes₂PyBTM, and Mes₂F₂PyBTM. These were approximately given by the product of the β-HOMO and β-LUMO at the D₁ optimized structure.

The suppressed k_nr contributed to the high PLQY no less than the enhanced k_f. Since the only state lower in energy than the D₁ state is the ground state (D₀), the rate of intersystem crossing (k^ISC) to the other spin multiplet states is substantially zero, and 100% of k_nr is the rate constant of internal conversion (k^IC) to the D₀ state. Compared to the phenyl groups (PyPBTM), the relaxation of the D₁ state with the mesityl groups is sterically hindered by the methyl groups at the ortho positions. As a result, the energy of the D₁ state was raised and the internal conversion from D₁ to D₀ was slowed by the energy-gap law.⁴³ The fluorine atoms on the pyridine ring also have the effect of widening the D₁–D₀ gap,^18,20 and the record-small k_nr among diphenylpyridyl radicals was achieved in Mes₂F₂PyBTM.

In addition, the k^IC can also be discussed from the overlap density distribution. The rate constant of internal conversion from |Ψ_m〉 to |Ψ_n〉 is given by eqn (4).⁴¹

4

where V_nm,α is an off-diagonal vibronic coupling constant between |Ψ_m〉 and |Ψ_n〉 for the vibrational mode α, and Q_α is a mass-weighted normal coordinate. Here, k^IC is proportional to the square of the off-diagonal vibronic coupling constant (VCC) between the D₁ and D₀ states (V_10,α with α being a vibrational mode).

A density form of the off-diagonal VCC (η_10,α) is written as a product of the overlap density and potential derivative (v_α(x)) as in eqn (5) and (6).^41,42

V_10,α = ∫dxη_10,α(x) 5

η_10,α(x) = ρ₁₀(x) × v_α(x) 6

Note that the density forms of both the transition dipole moment and the off-diagonal VCC are expressed using the overlap density. Mes₂PyBTM and Mes₂F₂PyBTM have smaller off-diagonal VCCs than PyBTM (Fig. S6†) because their overlap densities are delocalized on the PyBTM moiety and on the mesityl group, which couple weakly to potential derivatives.

Conclusions

Enhancement of fluorescence by the addition of mesityl groups to PyBTM is explained from the viewpoint of the overlap density between the D₁ and D₀ states. In particular, Mes₂F₂PyBTM shows high fluorescence efficiency above 60% in dichloromethane, chloroform, and PMMA at room temperature under ambient conditions. Although the PLQY of 69% in chloroform is not the highest among those of luminescent radicals today,^29,34,35 this value is comparable to those of famous fluorescent dyes such as rhodamine B (Φ_f = 0.70 in methanol,⁴⁴ Fig. S7†). This paper has raised many important points: Simple benzene rings can behave as an electron-donor for electron-accepting PyBTM radicals, their fluorescence is not quenched in polar solvents due to their non-excessive electron-donating nature, and steric hindrance of the ortho methyl groups plays an important role in reducing internal conversion. Introducing a nitrogen atom into a radical without introducing a nitrogen atom into the donor is a reversal of the previous highly efficient stable luminescent radicals. This study will provide important clues about donor selection in donor–acceptor systems or non-planar fluorescent π-systems. We also expect that easily synthesized and highly fluorescent Mes₂PyBTM and Mes₂F₂PyBTM will be used in a variety of applications.

Experimental

Materials and methods

All reactions were carried out under an argon atmosphere. The starting materials, αH-PyBTM¹¹ and αH-F₂PyBTM,¹⁸ were prepared according to the reference. Commercially available compounds were used as received without further purification. Preparative recycling gel permeation chromatography was performed with a recycling preparative HPLC, LaboACE LC-5060, Japan Analytical Industry Co., Ltd. ¹H (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded on a JEOL JNM-ECS 400 spectrometer using CDCl₃. The residual solvent signals (¹H NMR: δ 7.26, ¹³C NMR: δ 77.16) were used as the internal standards. Elemental analysis was conducted at the Center for Organic Elemental Microanalysis, Graduate School of Pharmaceutical Sciences, Kyoto University. ESR spectra were recorded with a JEOL JES-FR30EX spectrometer with X-band microwave. Sample solutions were charged in a 2.5 mmϕ sample tube. Magnetic field was calibrated with the Mn²⁺/MgO standard. Mass spectrometry was performed with a JEOL-JMS-S3000 (MALDI-Spiral-TOF MS) mass spectrometer with DCTB (20 mg mL⁻¹ in CHCl₃) as a matrix and TFANa (1 mg mL⁻¹ in THF) as a cationization agent. Melting points were measured on a Yanaco MP-500D. Absorption and emission spectra were monitored on a Hitachi U-4150 spectrophotometer and a Hitachi F-7100 fluorescence spectrophotometer, respectively. Photostability under 370 nm light was recorded with a JASCO FP-8600KS spectrofluorometer. Absolute luminescence quantum yields were measured using a Hamamatsu Photonics Quantaurus QY. Photoluminescence decay curves were measured using a measurement system with a picosecond diode laser with the emission wavelength of 375 nm (Advanced Laser Diode Systems PIL037X) as the light source, a single grating spectrometer (Andor Kymera193i-B1), and a photon counting detector (MPD SPD-050-CTE) operated using a time-correlated single photon counting (TCSPC) technique.

Preparation of emulsion

An emulsion of toluene in aqueous 2 wt% Kolliphor EL (K-EL) is prepared by mixing a 2 wt% aqueous dispersion of K-EL (1.8 g of K-EL in 88.2 mL of deionized water) with 10 mL of toluene until a stable, milky dispersion is obtained. The emulsion and triethylamine were deoxygenated by bubbling argon before use.

Synthesis of αH-MesPyBTM and αH-Mes₂PyBTM

In a Schlenk tube, αH-PyBTM (783 mg, 1.50 mmol), 2,4,6-trimethylphenylboronic acid (746 mg, 4.55 mmol), and Pd(dtbpf)Cl₂ (79.3 mg, 0.122 mmol), were put under an argon atmosphere. The degassed K-EL 2 wt%: toluene (9 : 1 v/v) emulsion (3.5 mL) was added, and the mixture was heated at 70 °C. Degassed triethylamine (1.3 mL, 9.3 mmol) was finally added, and the reaction mixture was stirred at 70 °C overnight. The reaction mixture was cooled down to room temperature, dichloromethane was added, and filtered on a celite pad. The solvent was evaporated and purified by silica gel column chromatography (CHCl₃ : hexane = 1 : 1). The crude product (742 mg) was separated by GPC (CHCl₃) to obtain pure αH-MesPyBTM (351 mg, 0.581 mmol, 39%, mp 192–195 °C) and αH-Mes₂PyBTM (252 mg, 0.367 mmol, 24%, mp 122–124 °C).

αH-MesPyBTM (1 : 1 mixture of two conformers)

¹H NMR (400 MHz, CDCl₃, ppm): δ 8.50 (s, 0.5H), 8.49 (s, 0.5H), 8.38 (s, 0.5H), 8.37 (s, 0.5H), 7.41 (d, J = 2.2 Hz, 0.5H), 7.39 (d, J = 2.2 Hz, 0.5H), 7.29 (d, J = 2.2 Hz, 0.5H), 7.25 (d, 0.5H), 7.16 (d, J = 1.7 Hz, 0.5H), 7.15 (d, J = 1.7 Hz, 0.5H), 7.04 (d, J = 1.7 Hz, 0.5H), 7.01 (d, J = 1.7 Hz, 0.5H), 6.93 (s, 2H), 6.80 (s, 1H), 2.32 (s, 3H), 2.02–2.00 (m, 6H).

¹³C NMR (100 MHz, CDCl₃, ppm): δ 149.7, 149.5, 147.9, 147.8, 144.2, 144.2, 143.1, 143.1, 138.1, 138.0, 137.8, 137.5, 137.4, 137.4, 137.4, 136.8, 135.8, 135.7, 135.6, 134.6, 134.6, 134.2, 134.2, 133.7, 133.6, 133.3, 132.2, 132.0, 131.4, 131.1, 130.5, 130.0, 129.7, 129.5, 128.8, 128.5, 128.4, 50.0, 21.2, 20.7.

Elemental analysis calcd for C₂₇H₁₈Cl₇N: C 53.64, H 3.00, N 2.32; found: C 53.92, H 3.04, N 2.25.

HRMS (MALDI-TOF MS positive mode) m/z: [MH]⁺ calcd for C₂₇H₁₉Cl₇N⁺ 601.93317; found 601.93316.

αH-Mes₂PyBTM

¹H NMR (400 MHz, CDCl₃, ppm): δ 8.51 (s, 1H), 8.38 (s, 1H), 7.19 (d, J = 1.7 Hz, 1H), 7.16 (d, J = 1.7 Hz, 1H), 7.06 (d, J = 1.7 Hz, 1H), 7.02 (d, J = 1.7 Hz, 1H), 6.94 (s, 4H), 6.91 (s, 1H), 2.32 (s, 6H), 2.07–2.02 (m, 12H)

¹³C NMR (100 MHz, CDCl₃, ppm): δ 149.6, 147.8, 144.8, 142.9, 142.8, 137.7, 137.7, 137.6, 137.4, 137.0, 136.9, 135.9, 135.7, 134.7, 133.8, 132.9, 132.5, 131.5, 131.0, 129.6, 129.5, 128.4, 50.3, 21.2, 20.7.

Elemental analysis calcd for C₃₆H₂₉Cl₆N: C 62.82, H 4.25, N 2.03; found: C 63.06, H 4.32, N 1.98.

HRMS (MALDI-TOF MS positive mode) m/z: [MH]⁺ calcd for C₃₆H₃₀Cl₆N⁺ 686.05039; found 686.05075.

Synthesis of MesPyBTM

Under an argon atmosphere, αH-MesPyBTM (76.2 mg, 0.126 mmol) was dissolved in dry THF (∼4 mL), and tBuOK in THF (1M solution, 0.2 mL, 1.6 eq.) was added. The reaction mixture was stirred overnight in the dark. Silver nitrate (61.3 mg, 0.361 mmol) in acetonitrile (1.5 mL) was added and stirred for 2.5 h. The reaction mixture was filtered, evaporated, and purified by flash chromatography on silica gel (CHCl₃ : hexane = 1 : 1) and dried in vacuo to afford MesPyBTM (74.1 mg, 0.123 mmol, 97%) as a red solid (mp 99–100 °C).

HRMS (MALDI-TOF MS negative mode) m/z: [M]⁻ calcd for C₂₇H₁₇Cl₇N⁻ 599.91862; found 599.91850.

Synthesis of Mes₂PyBTM

Under an argon atmosphere, αH-Mes₂PyBTM (60.7 mg, 0.0882 mmol) was dissolved in dry THF (∼3 mL), and tBuOK in THF (1M solution, 0.15 mL, 1.7 eq.) was added. The reaction mixture was stirred overnight in the dark. Silver nitrate (59.6 mg, 0.351 mmol) in acetonitrile (1.5 mL) was added and stirred for 2 h. Chloroform was added to the mixture and filtered on a celite pad. The solvent was evaporated and the reaction mixture was purified by silica gel column chromatography (CHCl₃ : hexane = 2 : 1) and dried in vacuo to afford Mes₂PyBTM (52.4 mg, 0.0762 mmol, 86%) as a red solid (mp 122–125 °C).

HRMS (MALDI-TOF MS negative mode) m/z: [M]⁻ calcd for C₃₆H₂₈Cl₆N⁻ 684.03584; found 684.03660.

Synthesis of αH-Mes₂F₂PyBTM

In a Schlenk tube, αH-F₂PyBTM (488 mg, 1.00 mmol), 2,4,6-trimethylphenylboronic acid (494 mg, 3.01 mmol), and Pd(dtbpf)Cl₂ (53.1 mg, 0.0815 mmol), were put under an argon atmosphere. The degassed K-EL 2 wt% : toluene (9 : 1 v/v) emulsion (3.6 mL) was added, and the mixture was heated at 70 °C. Degassed triethylamine (0.85 mL, 6.1 mmol) was finally added, and the reaction mixture was stirred at 70 °C overnight. The reaction mixture was cooled down to room temperature, dichloromethane was added, and filtered on a celite pad. The solvent was evaporated and purified by silica gel column chromatography (CHCl₃ : hexane = 1 : 1). The mixture (402 mg) was separated by GPC (CHCl₃), and recrystallization from dichloromethane–methanol gave pure αH-Mes₂F₂PyBTM (114 mg, 0.174 mmol, 17%, mp 219–221 °C).

¹H NMR (400 MHz, CDCl₃, ppm): δ 8.36 (s, 1H), 8.24 (s, 1H), 7.12 (s, 4H), 6.93 (s, 4H), 6.85 (s, 1H), 2.32 (s, 6H), 2.04 (s, 12H).

¹³C NMR (100 MHz, CDCl₃, ppm): δ 142.8, 137.7, 136.6, 135.9, 135.8, 135.4, 135.1, 133.7, 133.5, 132.2, 130.5, 128.4, 125.4, 125.3, 125.1, 42.3, 21.2, 20.7.

Elemental analysis calcd for C₃₆H₂₉Cl₄F₂N: C 65.97, H 4.46, N 2.14; found: C 66.00, H 4.44, N 2.10.

HRMS (MALDI-TOF MS positive mode) m/z:[MH]⁺ calcd for C₃₆H₃₀Cl₄F₂N⁺ 654.10949; found 654.10945.

Synthesis of Mes₂F₂PyBTM

Under an argon atmosphere, αH-Mes₂F₂PyBTM (28 mg, 0.043 mmol) was dissolved in dry THF (3 mL), and tBuOK in THF (1M solution, 0.1 mL, 2.3 eq.) was added. The reaction mixture was stirred overnight in the dark. Silver nitrate (35.5 mg, 0.209 mmol) in acetonitrile (0.7 mL) was added and stirred for 3 h. Chloroform was added to the mixture and filtered on a celite pad. The solvent was evaporated and the reaction mixture was purified by silica gel column chromatography (CHCl₃ : hexane = 1 : 1) and dried in vacuo to afford Mes₂F₂PyBTM (25.6 mg, 0.0391 mmol, 90%) as a red solid (mp 107–109 °C).

HRMS (MALDI-TOF MS positive mode) m/z: [M]⁺ calcd for C₃₆H₂₈Cl₆NF₂⁺ 652.09384; found 652.09347.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Author contributions

Y. H. conceived the project. Y. H. and R. K. prepared the compounds. W. O. and Y. H. conducted the DFT calculations. Y. H., R. M. and R. K. carried out the photophysical measurements. T. S. and W. O. contributed to the theoretical interpretation. Y. H. and W. O. wrote the original draft, and R. M., T. K., T. S. and K. U. reviewed and edited.

Conflicts of interest

There are no conflicts to declare.