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

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.


Introduction
Recently, stable radicals have attracted much interest as luminescent materials. 1,2 The key to their luminescence is the uorescence from the D 1 state to the D 0 state. In contrast to the singlet lowest excited (S 1 ) state of closed-shell molecules, which is higher in energy than the triplet lowest excited state (T 1 ), it is normal for the energy of the D 1 state to be lower than that of the quartet lowest excited (Q 1 ) state. Therefore, the uorescence of radicals can avoid unfavorable quenching from higher multiplicity excited states produced by the recombination of electrons and holes or intersystem crossing from the S 1 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(2,4,6trichlorophenyl)methane (aH-PyBTM) crystal had an excellent feature of high uorescence efficiency (F 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 eld effect on luminescence (magnetoluminescence), 12,15,16 and photoluminescence anisotropy amplied by exciton funneling. 17 However, the uorescence 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 uorine atoms on the pyridine ring slightly improved the PLQY in solution, and the (2,5-diuoro-4-pyridyl)bis(2,4,6trichlorophenyl)methyl radical (F 2 PyBTM) 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 uorescence efficiency to 36%. [19][20][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][23][24][25][26] or triphenylamines. 25,27 Highly uorescent radicals showing PLQY above 50% in solution were rst reported as a carbazole donor-TTM radical acceptor system showing 53% PLQY in cyclohexane. 22 However, its uorescence was quenched by the polarity of solvent molecules to 2% in chloroform. By adding electron-withdrawing groups on carbazole, strong uorescence was maintained in chloroform. 24 It is noteworthy that efficient uorescence in more polar solvents has recently been reported in completely different systems: pyrene-dithiadiazolyl radical (F f = 50% in acetonitrile) 28 and the p-radical stabilized with boron (F f = 67% in DMF). 29 Here, we report that simple aromatic hydrocarbons, mesityl groups, work as donors, and signicantly enhance the uorescence efficiencies of PyBTM and F 2 PyBTM in dichloromethane and chloroform solutions. We explain the reasons for the high efficiencies of these nonplanar p-electron systems by photophysical theories and calculations using DFT and TD-DFT.
These radicals were stable under ambient conditions similar to other triarylmethyl radicals protected by halogen atoms. 7,8 Actually, these radicals were puried 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][19][20][21] probably due to short uorescence lifetimes. As with PyBTM, 11 no change was observed in the solutions stored in the dark.
Absorption and emission spectra of MesPyBTM and Mes 2 -PyBTM 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 a-HOMO-a-LUMO band (l max = 370 / 401 nm) and the b-HOMO-b-LUMO band (541 / 564 nm) were signicantly redshied. On the other hand, the shapes of the absorption spectra of MesPyBTM and Mes 2 PyBTM were rather similar to that of PyBTM with the same absorption maxima at l max = 370 and 541 (AE1) nm. This result is attributed to the only slightly changed energy levels of the frontier orbitals.
This occurs because the mesityl group has bulky methyl groups at ortho positions and can barely conjugate with porbitals 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][20][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 Mes 2 PyBTM in the DFT optimized D 0 state model (Table S1 †). These are nearly perpendicular in contrast to the rather at angles (34°) between the phenyl and dichlorophenyl groups in PyPBTM.
Redshis of emission from PyBTM (l em = 585 nm) were seen in MesPyBTM (l em = 645 nm) and Mes 2 PyBTM (l em = 628 nm), although these shis were smaller than that of PyPBTM (l em = 654 nm). In the TD-DFT (UB3LYP/6-31G(d, p)) optimized D 1 structure, the mesityl groups under the b-HOMO are electron-decient, 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 redshi. The dihedral angle became even smaller in PyPBTM, decreasing to 25°.
Absorption and emission spectra of Mes 2 F 2 PyBTM and F 2 PyBTM are shown in Fig. 1b. The shape of the absorption spectrum of Mes 2 F 2 PyBTM (l max = 352 nm) resembles that of F 2 PyBTM (l max = 351 nm). The dihedral angle between the mesityl and dichlorophenyl groups was 87°in the DFT optimized D 0 state (Table S1 †). The redshi of emission from F 2 PyBTM (l em = 566 nm) was seen in Mes 2 F 2 PyBTM (l em = 623 nm), similar to the case of Mes 2 PyBTM. The optimized dihedral angle between the mesityl group under the b-HOMO and the neighboring dichlorophenyl group was 49°in the DFT optimized D 1 state.
Bright reddish-orange uorescence 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 uorescence 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, Mes 2 -F 2 PyBTM displayed as much as 66% PLQY in dichloromethane, and 69% in chloroform ( Fig. S2 †). As far as we know, a higher PLQY of the uorescent radical in a liquid solution has only been reported for pyridoindole donor-TTM acceptor systems in 2020 34 and 2022 35 and the p-radical stabilized with boron in 2022. 29 The major structural difference between pyridoindole donor-TTM acceptor systems 34,35 and Mes 2 F 2 PyBTM 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-decient. A pyridoindole is thought to cancel out the two effects internally. The use of mesityl groups simplies the situation and provides important scientic 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 uorescence 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 Mes 2 F 2 PyBTM 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 uorophores 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 uorescence of some donor-acceptor type uorophores. Mes 2 F 2 PyBTM in a PMMA lm displayed PLQY of 62%, proving its usefulness in this polymer.
As mentioned in the Introduction, the Q 1 state is higher in energy, and all of the excited states are thought to converge to  the D 1 state in a short time. The PLQYs of the radicals are determined by competition between the rate of uorescence (k f ) and the rate of nonradiative decay (k nr ) from the D 1 state (Fig. 2).
Here, k f and k nr were calculated from the PLQYs and the uorescence lifetimes (s, Fig. S4 †) of the radicals, and they are shown in Table 1.
One cause of the low PLQY of triarylmethyl radicals was the small k f . The small transition dipole moment between D 1 and D 0 is interpreted as being caused by the C 3 symmetry of the molecule 37 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 b-HOMOs and b-LUMOs calculated using the TD-DFT (UB3LYP/6-31G(d, p)) at the optimized D 1 structure are shown in Fig. 2 (see also Fig. S5 †). Since the D 1 states are mainly generated via the b-HOMO-b-LUMO transition, the radicals are regarded as a mesityl donor-PyBTM acceptor system. Experimentally, k f in MesPyBTM, Mes 2 PyBTM, and Mes 2 F 2 PyBTM 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 jF mn i = jJ m ijc mn i to the nal one jF nn ′i = jJ n ijc nn ′i, where jJ m i (jJ n i) and jc mn i (jc nn ′i) are the initial (nal) electronic and vibrational states, respectively. The initial and nal vibronic energies are denoted as E mn and E nn ′, respectively. According to Fermi's golden rule, the rate constant of uorescence from jJ m i to jJ n i is given by eqn (1). 41 where u is the angular frequency of the photon, c is the speed of light, P mn (T) is the Boltzmann distribution function of jc mn i at temperature T, m nm is the transition dipole moment between jJ m i and jJ n i, and ħ is the Dirac constant. The radiative constant of uorescence actually observed, k f = k f 0)1 is proportional to the square of the transition dipole moment between the D 1 and D 0 states (m 10 ).
A density form of the transition dipole moment (s 10 (x)) is written as a product of a three-dimensional Cartesian coordinate (x = (x, y, z)) and an overlap density between the D 1 and D 0 states (r 10 (x)) as in eqn (2) Thus, an overlap density having widespread distribution generally gives a large transition dipole moment. The calculated transition dipole moments of Mes 2 PyBTM and Mes 2 F 2 PyBTM were larger than that of PyBTM (Table 2). This result was attributed to the overlap densities of Mes 2 PyBTM and Mes 2 F 2 PyBTM delocalizing on both the PyBTM moiety and the mesityl group (Fig. 3).
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 D 1 state is the ground state (D 0 ), 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 D 0 state. Compared to the phenyl groups (PyPBTM), the relaxation of the D 1 state with the mesityl groups is sterically hindered by the methyl groups at the ortho positions. As  a result, the energy of the D 1 state was raised and the internal conversion from D 1 to D 0 was slowed by the energy-gap law. 43 The uorine atoms on the pyridine ring also have the effect of widening the D 1 -D 0 gap, 18,20 and the record-small k nr among diphenylpyridyl radicals was achieved in Mes 2 F 2 PyBTM. In addition, the k IC can also be discussed from the overlap density distribution. The rate constant of internal conversion from jJ m i to jJ n i is given by eqn (4). 41 where V nm,a is an off-diagonal vibronic coupling constant between jJ m i and jJ n i for the vibrational mode a, and Q a is a mass-weighted normal coordinate. Here, k IC is proportional to the square of the off-diagonal vibronic coupling constant (VCC) between the D 1 and D 0 states (V 10,a with a being a vibrational mode). A density form of the off-diagonal VCC (h 10,a ) is written as a product of the overlap density and potential derivative (v a (x)) as in eqn (5) and (6). 41,42 Note that the density forms of both the transition dipole moment and the off-diagonal VCC are expressed using the overlap density. Mes 2 PyBTM and Mes 2 F 2 PyBTM 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 uorescence by the addition of mesityl groups to PyBTM is explained from the viewpoint of the overlap density between the D 1 and D 0 states. In particular, Mes 2 F 2 PyBTM shows high uorescence 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 uorescent dyes such as rhodamine B (F 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 electronaccepting PyBTM radicals, their uorescence 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 nonplanar uorescent p-systems. We also expect that easily synthesized and highly uorescent Mes 2 PyBTM and Mes 2 F 2 PyBTM will be used in a variety of applications.

Materials and methods
All reactions were carried out under an argon atmosphere. The starting materials, aH-PyBTM 11 and aH-F 2 PyBTM, 18 were prepared according to the reference. Commercially available compounds were used as received without further purication. Preparative recycling gel permeation chromatography was performed with a recycling preparative HPLC, LaboACE LC-5060, Japan Analytical Industry Co., Ltd. 1 H (400 MHz) and 13 C NMR (100 MHz) spectra were recorded on a JEOL JNM-ECS 400 spectrometer using CDCl 3 . The residual solvent signals ( 1 H NMR: d 7.26, 13 C NMR: d 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 mmf sample tube. Magnetic eld was calibrated with the Mn 2+ /MgO standard. Mass spectrometry was performed with a JEOL-JMS-S3000 (MALDI-Spiral-TOF MS) mass spectrometer with DCTB (20 mg mL −1 in CHCl 3 ) 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 uorescence spectrophotometer, respectively. Photostability under 370 nm light was recorded with a JASCO FP-8600KS spectrouorometer. 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 Mes 2 PyBTM
Under an argon atmosphere, aH-Mes 2 PyBTM (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 ltered on a celite pad. The solvent was evaporated and the reaction mixture was puried by silica gel column chromatography (CHCl 3 : hexane = 2 : 1) and dried in vacuo to afford Mes 2 PyBTM (52.4 mg, 0.0762 mmol, 86%) as a red solid (mp 122-125°C

Synthesis of Mes 2 F 2 PyBTM
Under an argon atmosphere, aH-Mes 2 F 2 PyBTM (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 ltered on a celite pad. The solvent was evaporated and the reaction mixture was puried by silica gel column chromatography (CHCl 3 : hexane = 1 : 1) and dried in vacuo to afford Mes 2 F 2 PyBTM (25.6 mg, 0.0391 mmol, 90%) as a red solid (mp 107-109°C).

Conflicts of interest
There are no conicts to declare.