External environment sensitive circularly polarized luminescence properties of a chiral boron difluoride complex

Abstract

A chiral Schiff-base boron difluoride complex bearing a diethylamino group was synthesized. Its photophysical properties were investigated and compared with those of its non-substituted analogue. The complex was found to exhibit solvatofluorochromism with bluish-white emission in moderately polar solvents and intense blue emission in nonpolar solvent. Circularly polarized luminescence (CPL) properties were also examined and it was found that the absolute value of the luminescence dissymmetry factor (g_lum) increases significantly in the KBr-dispersed pellet state compared to the solution state. Notably, CPL intensity of the complex enhanced approximately three times upon addition of CH₃SO₃H in CH₂Cl₂. Density functional theory (DFT) calculations were conducted to further understand the photophysical properties.

---

Introductions

Circularly polarized luminescence (CPL)^1–4 which is defined as differential emission of left- versus right-circularly polarized light have attracted increasing attentions over the past decade as an important phenomenon with potential applications in 3D optical displays,⁵ biological probes,⁶ asymmetric synthesis,⁷ as well as CPL lasers.⁸ Among the various CPL-active materials, small organic molecules (SOMs)⁹ have drawn growing interest owing to the potential application for circularly polarized organic light-emitting diodes (CP-OLEDs).¹⁰ Numerous examples of CPL-SOMs with chiral frameworks such as helicenes,¹¹ cyclophanes¹² and binaphthyls¹³ have been developed and several studies have been reported to establish guidelines for designing molecules that exhibit high CPL efficiency.^{11d–g,12g,13f} Controlling CPL characteristics of CPL-SOMs is an important subject in the development of advanced information technologies.¹⁴ One of the strategy to achieve CPL control of SOMs is to design molecules that are sensitive to conformational changes in response to the external environment. To date, a variety of SOMs have been developed in which CPL properties can be controlled depending on the external environment, such as solvent,¹⁵ dispersed matrix¹⁶ and pH.¹⁷

Organoboron complexes have received increasing attention in recent years due to their efficient and tunable luminescent properties.¹⁸ Such characteristics have led to their wide utilization in optical and optoelectronic devices, including organic light-emitting diodes (OLEDs).¹⁹ Boron difluoride complexes, one of the families of organoboron complexes, have been recognized as promising materials for CPL-SOMs due to their ease of preparation and modification.²⁰ Various CPL-active boron difluoride complexes²¹ containing conjugated π-systems including axial chirality,^21a,c,i helical chirality^21b,d,h and planar chirality^21e–g have been reported.

As part of our program aimed at the creation of novel functional materials with CPL-activities, we have developed organic and organometallic complexes bearing chiral Schiff-base ligands.^16e,21j,22 Previously, we reported that boron difluoride complexes with chiral Schiff-base ligands exhibit multi-colour CPL properties in dilute solution and in the drop cast film state.^21j In the present work, we aimed to develop novel boron difluoride complexes with controllable CPL properties depending on the external environment for further applications. To this purpose, complex 1a bearing a diethylamino group was newly designed and the photophysical properties of the complex were compared with the non-substituted analogue 1b (Fig. 1). As a result, it was found that intensity and colour of CPL can be controlled by varying solvent polarity, dispersed matrix and pH of the solution. Theoretical calculations revealed a relationship between their structures and the photophysical properties including CPL-activities. Herein we describe the synthesis, structure and photophysical properties of chiral Schiff-base boron difluoride complexes with a focus on its tunable CPL properties.

Fig. 1 Structures of the boron difluoride complexes studied in this work.

Results and discussion

Synthesis and structures

The chiral boron difluoride complexes (R)-1a and (S)-1a were successfully synthesized by the reaction of BF₃·OEt₂ with optically pure Schiff-base ligands (R)-2a and (S)-2a bearing a diethylamino group in dry 1,2-dichloroethane (DCE) according to the reported procedure (Scheme 1).^20d The non-substituted analogues (R)-1b and (S)-1b were also prepared as reference compounds from the corresponding optically pure Schiff-base ligands (R)-2b and (S)-2b by the same synthetic methods. The newly synthesized compounds 1a and 2a were successfully characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy (Fig. S1 and S3, ESI†), infra-red (IR) spectroscopy, high-resolution mass spectrometry (HRMS) and elemental analysis, respectively.

Scheme 1 Synthesis of chiral boron difluoride complexes (R)/(S)-1a and 1b.

Single crystals of (S)-1a were obtained by recrystallization from a CH₂Cl₂/EtOH solution and the molecular structure was unequivocally established by X-ray diffraction (XRD) analysis at 113 K. The details of the crystal data and the structure refinement are presented in Table S1 (ESI†). ORTEP²³ drawings of (S)-1a are presented in Fig. 2. The boron atoms in (S)-1a adopt a typical tetrahedral geometry to form a six-membered ring which is similar to the previously reported boron difluoride complexes.²⁰ The packing structure and major interactions in the lattice are shown in Fig. S5 (ESI†). (S)-1a crystallizes in the chiral monoclinic space group P2₁. In the lattice, complex (S)-1a was fixed three-dimensional intermolecular H–F bonds.

Fig. 2 ORTEP representations of (S)-1a. Left figures: overhead views. Right figures: side views.

Photophysical properties

Circular dichroism (CD) and UV-vis absorption spectra of (R)-1a and (S)-1a were recorded in CH₂Cl₂ solution at room temperature (Fig. 3a and b). The spectra of complexes (R)-1b and (S)-1b under the same conditions are also shown for comparison.^20d (R)-1a and (S)-1a showed mirror image CD spectra with their maxima matching the maxima of the UV-vis absorption spectra. The low energy band of 1a was increased and bathochromically shifted compared to that of 1b in both CD and UV-vis absorption spectra. This is attributed to the participation of n–π* transition character from the lone pair of the nitrogen atom of the diethylamino group. The |g_abs| (=Δε/ε) values around the absorption maxima in the low energy region are calculated to be 1.4 × 10⁻⁴ (380 nm) for 1a and 1.0 × 10⁻³ (348 nm) for 1b, respectively. These results indicate that the participation of n–π* increases ε value, resulting in a decrease in the |g_abs| value. Further consideration will be discussed in a later section with the results of the theoretical calculations.

Fig. 3 (a) CD and (b) UV-vis spectra of 2.0 × 10⁻⁴ M solutions of 1a and 1b in CH₂Cl₂ at 298 K.

Complex 1a exhibited blue to bluish-white luminescence with moderate emission quantum efficiencies (Φ) under UV excitation at room temperature in dilute CH₂Cl₂ solution, in the crystalline and in the KBr-dispersed pellet state, respectively (Fig. 4a). The photophysical data for complexes 1a and 1b are presented in Table 1. The emission spectra in each state are shown in Fig. 4b. In the solution state, complex 1a exhibited two emission bands around 420 and 540 nm, while 1b showed only one emission band around 430 nm under the same condition. The emission band around 540 nm in complex 1a is attributed to the para-quinoidal resonance structure in the excited state (Scheme 2).²⁴ The first emission band in 1a showed a clear bathochromic shift in the solid state (λ_max = 472 nm for crystal and λ_max = 463 nm for crystal) compared to that of in solution state (λ_max = 420 nm). The CIE colour coordinates plotted on the CIE1931 chromaticity chart²⁵ (Fig. 4b, inset) indicate that the emission colour of 1a and 1b varies between the blue to white region in each state.

Fig. 4 (a) Photographs under UV illumination at 365 nm and (b) normalized emission spectra of 1a and 1b in CH₂Cl₂ (2.0 × 10⁻⁴ M), crystalline and in the KBr-dispersed pellet state at 298 K (λ_ex = 350 nm). The insets in (b) show the CIE colour coordinates of the emissions (1, 1a in CH₂Cl₂; 2, 1b in CH₂Cl₂; 3, 1a (KBr); 4, 1a (crystal)).

Table 1 Photophysical data for complexes 1a and 1b^a

Compound	Medium	λabs [nm]	λmax^b [nm]	Φ^b^,^c	|glum|^d	CIE (x, y)^b
a Data were obtained from a 2.0 × 10^−4 M solution, crystals or KBr-dispersed pellets at 298 K.b λex = 350 nm.c Luminescent quantum efficiencies measured using the absolute method with an integrating sphere.d The |glum| values around emission peak maxima are listed.e The data for 1b (reported in ref. 20d) are provided for comparison.
1a	CH2Cl2	281, 381	420, 539	0.13	2.8 × 10^−4 (426 nm)	0.23, 0.21
	CH2Cl2 + CH3SO3H (10 eq.)	267, 346	423	0.12	9.0 × 10^−4 (426 nm)	0.17, 0.09
	Toluene	279, 379	414	0.59	2.3 × 10^−4 (423 nm)	0.17, 0.07
	CHCl3	282, 381	417, 534	0.49	2.6 × 10^−4 (418 nm)	0.18, 0.12
	DCE	281, 380	421, 546	0.07	2.0 × 10^−4 (423 nm)	0.25, 0.22
	THF	279, 375	417, 550	0.04	2.4 × 10^−4 (424 nm)	0.22, 0.19
	Crystal	—	472	0.33	—	0.16, 0.23
	KBr pellet	—	463	0.30	2.4 × 10^−3 (461 nm)	0.17, 0.20
1b	CH2Cl2	354^e	433^e	0.32^e	1.0 × 10^−3 (433 nm)	0.15, 0.08

---

Scheme 2 Plausible equilibrium of the resonance structures of complex 1a in the excited state.

While it has been reported that 1b exhibits identical fluorescent properties in any solvent,^20d 1a exhibits solvatofluorochromism depending on the polarity of the solvents. Fig. 5a shows photographs of 1a in various organic solvents under UV irradiation. Bluish-white emissions were observed in moderately polar solvents such as CHCl₃, DCE and THF as well as CH₂Cl₂. In nonpolar solvents like toluene, 1a showed intense blue emission with a high Φ value (0.59) (Fig. 5a). In highly polar solvents (CH₃CN, acetone, DMF and MeOH), 1a exhibited weak blue emission with a low Φ value (0.01) (Fig. S6 and Table S2, ESI†). The UV-vis absorption spectra of 1a in various organic solvents are shown in Fig. S7 (ESI†), where identical absorption spectra were observed in all solutions. From these results, we conclude that the polarity of the solvents affects the stability of the resonance structures (benzoid- and quinoid-form) in the excited state which is the key to the dual emission properties of 1a.

Fig. 5 (a) Photographs under UV illumination at 365 nm and (b) normalized emission spectra of 2.0 × 10⁻⁴ M solutions of (S)-1a in various organic solvents at 298 K (λ_ex = 350 nm). The insets in (b) show the CIE colour coordinates of the emissions (solvents: 1, toluene; 2, CHCl₃; 3, THF; 4, CH₂Cl₂; 5, DCE).

The CPL spectra of the enantiomeric samples 1a and 1b in dilute organic solutions show mirror image spectra (Fig. 6 and S8, ESI†), and the emission peak maxima of the CPL signals correspond well to the emission spectra taken under the same measurement conditions (Fig. 4b and 5b). In general, the efficiency of CPL is usually quantified by means of the luminescence dissymmetry factor (g_lum = 2ΔI/I = 2(I_L − I_R)/(I_L + I_R), in which I_L and I_R are the intensity of left- and right-circularly polarized luminescence).²⁶ The |g_lum| values around the maximum emission wavelength in CH₂Cl₂ solution are calculated to be 2.8 × 10⁻⁴ (426 nm) for 1a and 1.0 × 10⁻³ (433 nm) for 1b, respectively, which are typical values for small organic and organometallic molecules.²⁷ The CPL spectra of (R)-1a and (S)-1a in toluene, CHCl₃, DCE and THF also showed g_lum values of the 10⁻⁴ order (Table 1 and Fig. S8, ESI†). The decrease in the |g_lum| value caused by the introduction of a diethylamino group corresponds to the decrease in the |g_abs| value calculated from the CD spectra, which will be also discussed with the results of theoretical calculations described below. The CPL spectra of (R)-1a and (S)-1a were also recorded in the KBr-dispersed pellet state, showing clear mirror image signals. Their maximum emission |g_lum| values are calculated to be 2.4 × 10⁻³ (461 nm), which is approximately 9 times higher than the value measured in solution (Fig. 7). This improvement of CPL chirality in the KBr-dispersed pellet is considered to be due to the emergence of supramolecular chirality in the aggregated state.

Fig. 6 CPL spectra of (R)-and (S)- (a) 1a and (b) 1b in CH₂Cl₂ (2.0 × 10⁻⁴ M) at 298 K (λ_ex = 350 nm).

Fig. 7 CPL spectra of (R)-and (S)-1a in the KBr-dispersed pellet state at 298 K (λ_ex = 350 nm).

One of the most important photophysical properties of complex 1a is the acid-induced CPL enhancement, observed in CH₂Cl₂ upon addition of an excess CH₃SO₃H. As shown in Fig. 8a, the addition of excess amounts of CH₃SO₃H (10 equiv.) to a bluish-white emissive solution of complex 1a in CH₂Cl₂ typically causes the solution to exhibit blue fluorescence at 298 K. ¹H NMR spectrum of complex 1a with CH₃SO₃H in CDCl₃ suggests that 1a is stable under low concentrated acidic solution (Fig. S10†). Fig. 8b and c show changes in the CPL and total emission spectra of 2.0 × 10⁻⁴ M solutions of 1a in CH₂Cl₂. The addition of CH₃SO₃H resulted in an increase in CPL intensity around 420 nm (Fig. 8b). In the total emission spectrum, the emission band around 540 nm disappeared (Fig. 8c). The maximum emission |g_lum| values in the CPL spectra were calculated to be 9.0 × 10⁻⁴ (426 nm), which is approximately 4 times higher than the value measured in the original solution. The UV-vis spectra of 1a in CH₂Cl₂ with varying equivalents of CH₃SO₃H are shown in Fig. S9 (ESI†), where the decrease of n–π* transition band around 380 nm was observed. Hence, the increase in CPL upon acid addition can be attributed to the decrease of n–π* character in the luminescence process.

Fig. 8 (a) Photographs under UV illumination at 365 nm, (b) CPL and (c) normalized total emission spectra of (R)- and (S)-1a in CH₂Cl₂ (2.0 × 10⁻⁴ M) with and without CH₃SO₃H at 298 K (λ_ex = 350 nm).

Theoretical calculations

To get more insight into the photophysical properties of the present chiral boron complexes, we performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the B3LYP/6-31+G(d,p) level, using the Gaussian 16 program. The optimized structures and frontier orbitals of (S)-1a and (S)-1b in the S₀ (ground state) and S₁ (excited state) states were estimated using DFT calculations on the basis of the X-ray structures (Fig. 9). The HOMOs are principally π orbitals of the ligand, including the non-bonding orbital of the diethylamino group of (S)-1a, whereas the LUMOs are in the ligand (π*). The energy levels and electronic configurations of the singlet states of these complexes were estimated from TD-DFT calculations (B3LYP/6-31+G(d,p)) (Tables S3 and S4, ESI†). The major contribution of the electronic configuration of the S₁ states is the HOMO-to-LUMO transition, which implies that the present fluorescence is principally attributable to a mixture of n–π* and π–π* transitions of (S)-1a and the π–π* transition of (S)-1b. The S₁-to-S₀ transition energies for S₁ states were calculated to be 2.98 eV (416 nm) for (S)-1a and 2.96 eV (419 nm) for (S)-1b, which is consistent with the emission peak maxima of the first emission band of the experimental spectra (λ_max = 420 nm for (S)-1a and λ_max = 433 nm for (S)-1b).

Fig. 9 Molecular orbitals (overhead views) and eigenvalues [eV] for the frontier orbitals of (S)-1a and (S)-1b estimated from DFT calculations (B3LYP/6-31+G(d,p)) on the basis of the optimized geometries in the (a) S₀ ground and (b) S₁ excited states. Hydrogen atoms are omitted for clarity.

We considered the variation of chiroptical properties of (S)-1a and (S)-1b from the viewpoint of transition dipole moments using TD-DFT calculations. The dissymmetry factors g_abs for CD and g_lum for CPL are calculated with the following equation g = 4(|μ_e||μ_m|cos θ_e,m)/(|μ_e|² + |μ_m|²), where |μ_e|, |μ_m| and θ_e,m are the electric transition dipole moments, magnetic transition dipole moments and the angles between the two vectors μ_e and μ_m, respectively.²⁸ In the case of CPL-SOMs, |μ_m| is basically much smaller compared with |μ_e| and can be neglected. Thus, the equation of g can be replaced as follows: g = 4(|μ_m|cos θ_e,m)/|μ_e|, in which the g value is directly proportional to |μ_m| and inversely proportional to |μ_e|. Fig. 10 shows the electric and magnetic dipole moments calculated for the upward S₀-to-S₁ and the downward S₁-to-S₀ transitions of (S)-1a and (S)-1b in the optimized geometries. For the upward S₀-to-S₁ transition, g_abs values were calculated to be −2.8 × 10⁻⁴ for (S)-1a and −1.0 × 10⁻³ for (S)-1b (Fig. 10a), which is consistent with the results from experimental CD spectra (−1.4 × 10⁻⁴ for (S)-1a and −1.0 × 10⁻³ for (S)-1b, Table 1). The scalar values |μ_e| of (S)-1a is more than 2 times higher than that of (S)-1b, whereas |μ_m| was less than half compared to that of (S)-1b. Following the equation of the g value, the decrease of the g_abs value for (S)-1a compared to that of (S)-1b is attributed to the changes in the scalar values |μ_e| and |μ_m|. For the downward S₁-to-S₀ transition, g_lum values were calculated to be −2.8 × 10⁻⁴ for (S)-1a and −1.1 × 10⁻³ for (S)-1b (Fig. 10b) which is consistent with the result from experimental CPL spectra (−2.8 × 10⁻⁴ for (S)-1a and −1.0 × 10⁻³ for (S)-1b, Table 1). The small g_lum for (S)-1a can be traced back to the nearly orthogonal electric and magnetic dipole moments: θ_e,m = 91°. The angle θ_e,m for the downward transition of (S)-1b is θ_e,m = 94°. This apparently small change in θ_e,m alone would influence the g_lum by a factor of 4 (as cos 94°/cos 91° = 0.07/0.0017). Given the calculation results, we can be certain that the orientation of dipole moments in the downward S₁-to-S₀ transitions is a key for the decrease in g_lum for (S)-1a compare to that of (S)-1b. The enhancement of the CPL intensity with the addition of CH₃SO₃H was also attributed to the changes in orientation of dipole moments, as the protonation of the diethylamino group reduces the contribution of the n–π* transition character.

Fig. 10 Electric (μ_e, orange) and magnetic (μ_m, purple) dipole moments of the (a) S₀ → S₁ transition and (b) S₁ → S₀ transition for (S)-1a (left) and (S)-1b (right) calculated at the B3LYP/6-31+G(d,p) level. Calculated values of transition dipole moments (|μ_e|, |μ_m| and θ_e,m) and g_lum are given under each structure.

Conclusions

In summary, we have demonstrated external environment sensitive circularly polarized luminescence based on Schiff-base difluoride boron complexes. These chiral compounds exhibited solvatofluorochromism and acid-induced CPL enhancement in the solution state. The CPL intensity was also enhanced in the KBr-dispersed pellet state with a g_lum value of 2.4 × 10⁻³. DFT and TD-DFT calculations of the structures and electronic configurations of (S)-1a and (S)-1b revealed a relationship between molecular structure and photophysical properties. Theoretical consideration of the effect of solvent polarity for solvatofluorochromic behavior observed in 1a is now in progress.

Author contributions

The project was conceived by Masahiro Ikeshita, who also directed all experiment work, theoretical calculation and wrote the manuscript. Hongxi He performed experimental works except for CPL measurements. Maho Kitahara measured CPL spectra. Yoshitane Imai and Takashi Tsuno gave constructive guidance for this study.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (Grant Numbers JP21K20541 (M. I.), JP21K18940 (Y. I.), and JP21K05234 (T. T.)), JST, CREST (Grant Number JPMJCR2001 (Y. I.)), JGC-S Scholarship Foundation (M. I.), and Nihon University Industrial Technology Fund for Supporting Young Scholars (M. I.). We gratefully acknowledge Prof. Dr Henri Brunner (Universität Regensburg) for helpful discussion and comments. We also acknowledge Prof. Dr Takayoshi Fujii (Nihon University) for measurements of emission properties, and Prof. Dr Takeshi Naota (Osaka University) and Assoc. Prof. Dr Shuichi Suzuki (Osaka University) for measurements of HRMS spectrometry.

Notes and references

1. J. P. Riehl and F. S. Richardson, Chem. Rev., 1986, 86, 1–16.
2. F. Zinna and L. Di Bari, Chirality, 2015, 27, 1–13.
3. J. P. Riehl and G. Muller, in Comprehensive Chiroptical Spectroscopy: Instrumentation, Methodologies, and Theoretical Simulations, John Wiley & Sons, Inc., 2012, vol. 1, pp. 65–90.
4. T. Mori, Circularly Polarized Luminescence of Isolated Small Organic Molecules, Springer, 2020.
5. D.-Y. Kim, J. Korean Phys. Soc., 2006, 49, S505–S508.
6. M. C. Heffern, L. M. Matosziuk and T. J. Meade, Chem. Rev., 2014, 114, 4496–4539.
7. Y. Inoue, Chem. Rev., 1992, 92, 741–770.
8. (a) J. Jiménez, L. Cerdán, F. Moreno, B. L. Maroto, I. García-Moreno, J. L. Lunkley, G. Muller and S. de la Moya, J. Phys. Chem. C, 2017, 121, 5287–5292; (b) D. Qu, M. Archimi, A. Camposeo, D. Pisignao and E. Zussman, ACS Nano, 2021, 15, 8753–8760.
9. (a) E. M. Sánchez-Carnerero, A. R. Agarrabeitia, F. Moreno, B. L. Maroto, G. Muller, M. J. Ortiz and S. de La Moya, Chem.–Eur. J., 2015, 21, 13488–13500; (b) N. Chen and B. Yan, Molecules, 2018, 23, 3376.
10. (a) J. R. Brandt, F. Salerno and M. J. Fuchter, Nat. Rev. Chem., 2017, 1, 0045; (b) M. Li, S.-H. Li, D. Zhang, M. Cai, L. Duan, M.-K. Fung and C.-F. Chen, Angew. Chem., Int. Ed., 2018, 57, 2889–2893; (c) P. Xue, X. Wang, W. Wang, J. Zhang, Z. Wang, J. Jin, C. Zheng, P. Li, G. Xie and R. Chen, ACS Appl. Mater. Interfaces, 2021, 13, 47826–47834.
11. Helicenes: (a) J. E. Field, G. Muller, J. P. Riehl and D. Venkataraman, J. Am. Chem. Soc., 2003, 125, 11808–11809; (b) T. Kaseyama, S. Furumi, X. Zhang and K. Tanaka, Angew. Chem., Int. Ed., 2011, 50, 3684–3687; (c) K. Nakamura, S. Furumi, M. Takeuchi, T. Shibuya and K. Tanaka, J. Am. Chem. Soc., 2014, 136, 5555–5558; (d) H. Tanaka, M. Ikenosako, Y. Kato, M. Fujiki, Y. Inoue and T. Mori, Commun. Chem., 2018, 1, 38; (e) H. Kubo, D. Shimizu, T. Hirose and K. Matsuda, Org. Lett., 2020, 22, 9276–9281; (f) H. Kubo and T. Hirose, Chem. Lett., 2021, 50, 804–807; (g) H. Kubo, T. Hirose, T. Nakashima, T. Kawai, J. Hasegawa and K. Matsuda, J. Phys. Chem. Lett., 2021, 12, 686–695; (h) T. Mori, Chem. Rev., 2021, 121, 2373–2412.
12. Cyclophanes: (a) Y. Morisaki, M. Gon, T. Sasamori, N. Tokitoh and Y. Chujo, J. Am. Chem. Soc., 2014, 136, 3350–3353; (b) M. Gon, Y. Morisaki and Y. Chujo, J. Mater. Chem. C, 2015, 3, 521–529; (c) S. P. Morcillo, D. Miguel, L. Alvarez de Cienfuegos, J. Justicia, S. Abbate, E. Castiglioni, C. Bour, M. Ribagorda, D. J. Cardenas, J. M. Paredes, L. Crovetto, D. Choquesillo-Lazarte, A. J. Mota, M. C. Carreno, G. Longhi and J. M. Cuerva, Chem. Sci., 2016, 7, 5663–5670; (d) S. Sato, A. Yoshii, S. Takahashi, S. Furumi, M. Takeuchi and H. Isobe, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 13097–13101; (e) P. Renie, A. G. Campaña, L. A. de Cienfuegos, V. Blanco, S. Abbate, A. J. Mota, G. Longhi, D. Miguel and J. M. Cuerva, Chem. Commun., 2019, 55, 10685–10688; (f) Y. Morisaki and Y. Chujo, Bull. Chem. Soc. Jpn., 2019, 92, 265–274; (g) N. Miki, R. Inoue and Y. Morisaki, Bull. Chem. Soc. Jpn., 2022, 95, 110–115.
13. (a) T. Kawai, K. Kawamura, H. Tsumatori, M. Ishikawa, M. Naito, M. Fujiki and T. Nakashima, ChemPhysChem, 2007, 8, 1465–1468; (b) T. Kimoto, N. Tajima, M. Fujiki and Y. Imai, Chem.–Asian J., 2012, 7, 2836–2841; (c) K. Takaishi, R. Takehara and T. Ema, Chem. Commun., 2018, 54, 1449–1452; (d) K. Takaishi, K. Iwachido, R. Takehana, M. Uchiyama and T. Ema, J. Am. Chem. Soc., 2019, 141, 6185–6190; (e) Y. Nojima, M. Hasegawa, N. Hara, Y. Imai and Y. Mazaki, Chem. Commun., 2019, 55, 2749–2752; (f) Y. Nojima, M. Hasegawa, N. Hara, Y. Imai and Y. Mazaki, Chem.–Eur. J., 2021, 27, 5923–5929.
14. (a) J. Kumar, T. Nakashima and T. Kawai, J. Phys. Chem. Lett., 2015, 6, 3445–3452; (b) J.-L. Ma, Q. Peng and C.-H. Zhao, Chem.–Eur. J., 2019, 25, 15441–15454; (c) F. Song, Z. Zhao, Z. Liu, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem. C, 2020, 8, 3284–3301; (d) Y. Gao, C. Ren, X. Lin and T. He, Front. Chem., 2020, 8, 458; (e) Y. Imai, Chem. Lett., 2021, 50, 1131–1141.
15. (a) A. Statrijio, S. C. J. Meskers and T. M. Swager, J. Am. Chem. Soc., 2006, 128, 9030–9031; (b) Y. Nagata, T. Nishikawa and M. Suginome, J. Am. Chem. Soc., 2014, 136, 15901–15904; (c) Y. Mimura, S. Kitamura, M. Shizuma, M. Kitamatsu, M. Fujiki and Y. Imai, ChemistrySelect, 2017, 2, 7759–7764; (d) S. Nakanishi, N. Hara, N. Kuroda, N. Tajima, M. Fujiki and Y. Imai, Org. Biomol. Chem., 2018, 16, 1093–1100; (e) K. Takaishi, K. Iwachido and T. Ema, J. Am. Chem. Soc., 2020, 142, 1774–1779; (f) L. Ji, Y. Liu, Z. Li, G. Ouyang and M. Liu, Chem. Commun., 2021, 57, 11314–11317.
16. (a) T. Kimoto, T. Amako, N. Tajima, R. Kuroda, M. Fujiki and Y. Imai, Asian J. Org. Chem., 2013, 2, 404–410; (b) K. Nakabayashi, T. Amako, N. Tajima, M. Fujiki and Y. Imai, Chem. Commun., 2014, 50, 13228–13230; (c) M. Gon, R. Sawada, Y. Morisaki and Y. Chujo, Macromolecules, 2017, 50, 1790–1802; (d) G. Albano, G. Pescitelli and L. Di Bari, Chem. Rev., 2020, 120, 10145–10243; (e) M. Ikeshita, M. Mizugaki, T. Ishikawa, K. Matsudaira, M. Kitahara, Y. Imai and T. Tsuno, Chem. Commun., 2022, 58, 7503–7506.
17. (a) N. Saleh, B. Moore II, M. Srebro, N. Vanthuyne, L. Toupet, J. A. Gareth Williams, C. Roussel, K. K. Deol, G. Muller, J. Autschbach and J. Crassous, Chem.–Eur. J., 2015, 21, 1673–1681; (b) S. Pascal, C. Besnard, F. Zinna, L. D. Rari, B. Le Guennic, D. Jacquemin and J. Lacour, Org. Biomol. Chem., 2016, 14, 4590–4594; (c) H. Sakai, T. Kubota, J. Yuasa, Y. Araki, T. Sakanoue, T. Takenobu, T. Wada, T. Kawai and T. Hasobe, Org. Biomol. Chem., 2016, 14, 6738–6743; (d) T. Otani, A. Tsuyuki, T. Iwachi, S. Someya, K. Tateno, H. Kawai, T. Saito, K. S. Kanyiva and T. Shibata, Angew. Chem., Int. Ed., 2017, 56, 3906–3910; (e) K. Takaishi, M. Yasui and T. Ema, J. Am. Chem. Soc., 2018, 140, 5334–5338; (f) A. H. G. David, R. Casares, J. M. Cuerva, A. G. Campaña and V. Blanco, J. Am. Chem. Soc., 2019, 141, 18064–18074; (g) E. Yen-Pon, F. Buttard, L. Frédéric, P. Thuéry, F. Taran, G. Pieters, P. A. Champagne and D. Audisio, JACS Au, 2021, 1, 807–818.
18. (a) F. Jäkle, Chem. Rev., 2010, 110, 3985–4022; (b) Y.-L. Rao and S. Wang, Inorg. Chem., 2011, 50, 12263–12274; (c) G. Wesela-Bauman, P. Ciećwierz, K. Durka, S. Luliński, J. Serwatowski and K. Woźniak, Inorg. Chem., 2013, 52, 10846–10859; (d) D. Frath, J. Massue, G. Ulrich and R. Ziessel, Angew. Chem., Int. Ed., 2014, 53, 2290–2310; (e) M. Gon, K. Tanaka and Y. Chujo, Bull. Chem. Soc. Jpn., 2019, 92, 7–18.
19. (a) H. Yersin, Highly Efficient OLEDs with Phosphorescent Materials, Wiley-VCH, Weinheim, 2008; (b) L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido, Adv. Mater., 2011, 23, 926–952; (c) D. Li, H. Zhang and Y. Wang, Chem. Soc. Rev., 2013, 42, 8416–8433.
20. (a) D. Frath, S. Azizi, G. Ulrich, P. Retailleau and R. Ziessel, Org. Lett., 2011, 13, 3414–3417; (b) Y.-W. Chen, Y.-C. Lin, H.-M. Kuo and C. K. Lai, J. Mater. Chem. C, 2017, 5, 5465−–5477; (c) M. Urban, K. Durka, P. Jankowski and S. Luliński, J. Org. Chem., 2017, 82, 8234–8241; (d) P. A. A. M. Vaz, J. Rocha, A. M. S. Silva and S. Guieu, New J. Chem., 2018, 42, 18166–18171.
21. (a) F. Zinna, T. Bruhn, C. A. Guido, J. Ahrens, M. Bröring, L. Di Bari and G. Pescitelli, Chem.–Eur. J., 2016, 22, 16089–16098; (b) Y. Gobo, M. Yamamura, T. Nakamura and T. Nabeshima, Org. Lett., 2016, 18, 2719–2721; (c) Z. Jiang, X. Wang, J. Ma and Z. Liu, Sci. China: Chem., 2019, 62, 355–362; (d) C. Maeda, K. Nagahata, T. Shirakawa and T. Ema, Angew. Chem., Int. Ed., 2020, 59, 7813–7817; (e) C.-H. Chen and W.-H. Zheng, Org. Chem. Front., 2021, 8, 6622–6627; (f) K. Li, H. Ji, Z. Yang, W. Duan, Y. Ma, H. Liu, H. Wang and S. Gong, J. Org. Chem., 2021, 86, 16707–16715; (g) W. Duan, H. Ji, Z. Yang, Q. Yao, Y. Huo, X. Ren, J. Zhao and S. Gong, Dalton Trans., 2021, 50, 12963–12969; (h) A. Macé, K. Hamrouni, E. S. Gauthier, M. Jean, N. Vanthuyne, L. Fréderic, G. Pieters, E. Caytan, T. Roisnel, F. Aloui, M. Srebro-Hooper, B. Carboni, F. Berrée and J. Crassous, Chem.–Eur. J., 2021, 27, 7959–7967; (i) L. Cui, H. Shinjo, T. Ichiki, K. Deyama, T. Harada, K. Ishibashi, T. Ehara, K. Miyata, K. Onda, Y. Hisaeda and T. Ono, Angew. Chem., Int. Ed., 2022, 61, e202204358; (j) M. Ikeshita, T. Suzuki, K. Matsudaira, M. Kitahara, Y. Imai and T. Tsuno, Phys. Chem. Chem. Phys., 2022, 24, 15502–15510.
22. (a) M. Ikeshita, S. Furukawa, T. Ishikawa, K. Matsudaira, Y. Imai and T. Tsuno, ChemistryOpen, 2022, 11, e202100277; (b) M. Ikeshita, T. Yamamoto, S. Watanabe, M. Kitahara, Y. Imai and T. Tsuno, Chem. Lett., 2022, 51, 832–835.
23. M. N. Burnett and C. K. Johnson, ORTEP-III: Oak Ridge Thermal Ellipsoid Plot-Program for Crystal Structure Illustrations, Report ORNL-6895, Oak Ridge National Laboratory, Oak Ridge, 1996.
24. H. H. T. Al-Sharif, R. Ziessel, P. G. Waddell, C. Dixon and A. Harriman, J. Phys. Chem. A, 2020, 124, 2160–2172.
25. T. Smith and J. Guild, Trans. Opt. Soc., 1931, 33, 73–134.
26. F. S. Richardson and J. P. Riehl, Chem. Rev., 1977, 77, 773–792.
27. H. Tanaka, Y. Inoue and T. Mori, ChemPhotoChem, 2018, 2, 386–402.
28. L. Rosenfeld, Z. Phys., 1929, 52, 161–174.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 2205119. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ra07386b

---