A Platform for Blue-Luminescent Carbon-Centered Radicals

doi:10.21203/rs.3.rs-1499758/v1

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A Platform for Blue-Luminescent Carbon-Centered Radicals

https://doi.org/10.21203/rs.3.rs-1499758/v1

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Organic radicals, which have unique doublet spin-configuration, provide an alternative method to overcome the efficiency limitation of organic light-emitting diodes (OLEDs) based on conventional fluorescent organic mole-cules. Further, they have made great breakthroughs in deep-red and near-infrared OLEDs. However, it is diffi-cult to extend their fluorescence into a short-wavelength region because of the natural narrow bandgap of the organic radicals. Herein, we significantly expand the scope of luminescent radicals by showing a new platform of carbon-centered radicals derived from N-heterocyclic carbenes that produce blue to green emissions (444–529 nm). Time-dependent density functional theory calculations and experimental investigations disclose that the fluorescence originates from the high-energy excited states to the ground state, demonstrating an anti-Kasha behavior. The present work provides an efficient and modular approach toward a library of carbon-centered radicals that feature anti-Kasha's rule emission, rendering them as potential new emitters in the short-wavelength region.

Organic radicals are compounds that feature an unpaired electron with high reactivity. They have been recognized as reactive intermediates and play important roles in many chemical and biological systems¹. This field has undergone a profound revolution and has been extensively investigated as building blocks for magnetic, conductive, and optoelectronic functional materials^2-5. Unfortunately, most organic radicals are weakly luminescent or non-luminescent because of their strong non-radiative energy relaxation pathways⁶. Since the first doublet-emission organic light-emitting diodes (OLEDs) was developed⁷, the field has attracted considerable attention in recent years. Luminescent organic radicals as emitters for OLEDs can theoretically achieve 100% internal quantum efficiency because of their characteristic doublet emission; thus, the external quantum efficiency of photoelectric devices can be improved^8-10. Thus far, the reported organic radicals with inherent fluorescent are mainly derived from triaryl methyl fragments (Fig. 1a)^11-18. In these radicals, fluorescence produces radiative transition from the lowest excited state (D₁) to the ground state (D₀). According to Kasha's rule, such radicals have advantages in red and near-infrared (NIR) emissions because of their natural narrow bandgap (Fig. 1b)⁸. New categories of organic radicals with short-wavelength emission (particularly blue emission) should be researched, they can be applied to further develop luminescent radicals and radical-based OLEDs.

In search of a new platform for isolating functionalized carbon-centered radicals, we report here the synthesis and structural study of a series of luminescent carbon-centered radicals by incorporating diphenylaminophenyl (TPA) and carbazolylphenyl (CBP) to the core of N-heterocyclic carbenes (NHCs) (Fig. 1c). The targeted NHC-based radicals show fluorescence with short-wavelength emission (444–529 nm). Theoretical calculations and experimental results suggest that the short-wavelength emission in the current radicals can be attributed to anti-Kasha's rule behavior, in which the emission originates from the relatively high excited states (D₃/D₂) to D₀ (Fig. 1b)^19-24. This kind of radical is advantageous because they have a wide bandgap, and they exhibit blue-shifted absorption and emission. Thus, they are intrinsic candidates for potential applications for blue absorption and emission.

Synthesis of the NHC-based precursors. NHCs featuring bulky substituents and an empty p orbital offer excellent platforms to stabilize radical species^25–33. Moreover, the electronic properties of the organic radicals can be effectively tuned by varying the carbene units^34–36. The target molecules were chosen according to the ring backbone of the NHCs and the carbon center-attached units (Fig. 1c). The radical precursors (1a–c and 2a–c) were readily synthesized by conducting the C2-arylation of the corresponding free NHCs (IPr^Me = 4,5-dimethyl-N,N´-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, a; IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene, b; SIPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolidin-2-ylidene, c) with 4-iodo-N,N-diphenylaniline, as well as disubstituted TPA derivatives (–F, –CF₃, –CN) and 9-(4-iodophenyl)carbazole (CBP) in dioxane with Pd₂(dba)₃/Ni(COD)₂ as a catalyst (see Supplementary Pages 4–8 for more synthesis details)³⁷. The precursor salts 1 and 2 were fully characterized by nuclear magnetic resonance (NMR) spectroscopy (Supplementary Figs. 1–24), high-resolution electrospray ionization (HR-ESI) mass spectrometry (Supplementary Figs. 25–36), and X-ray diffraction (XRD) studies (Fig. 2, Fig. 3, and Supplementary Figs. 37–41). The cyclic voltammograms of all the precursor salts showed a reversible redox-wave (E_1/2 = − 2.07 to − 1.61 V, against Fc⁺/Fc) indicating the synthetic accessibility of the corresponding neutral radical species (Supplementary Figs. 42–44). The E_1/2 values were anodically shifted as the π-accepting properties of carbene ligands increased (E_1/2 = − 2.07, − 1.98, and − 1.75 V for 1a^I, 1b^I, and 1c^I; −1.92, − 1.85, and − 1.61 V for 2a, 2b, and 2c, respectively). This shows that the electrochemical potential can be tuned by changing the carbene ligand. Moreover, the electron-withdrawing groups on the TPA increased the reduction potential of the E_1/2. Specifically, the E_1/2 values were anodically shifted by 150 mV for the CN-substitutional precursors 1a^IV (− 1.90 V) and 1b^IV (− 1.83 V) relative to the counterparts 1a^I (− 2.17 V) and 1b^I (− 1.98 V).

Synthesis of the NHC-based radicals. The chemical reduction of precursor salts 1a–c and 2a–c with one equivalent KC₈ afforded neutral radicals 3a–c and 4a–c in good to excellent yields after workup. The paramagnetic character of 3a–c and 4a–c was first indicated by NMR spectra; all of them were NMR silent. The radical nature of 3a–c and 4a–c was further confirmed by X-band electron paramagnetic resonance, which was successfully simulated based on the calculated coupling constants (Supplementary Figs. 45–56). All radicals were stable in solution and in the solid state under an inert gas atmosphere.

Solid-state structural characterization. The structures of the precursor salts 1a^I, 1b^I, 1c^I, and 1b^III; 2a–c and the radicals 3a^I, 3b^I, 3c^I, 3b^III; and 4a–c were determined by XRD. Each of the radicals showed that the carbene carbon atom (C1) was sp² hybridized. All the radicals showed shorter C1–C2 bond (1.397(3)–1.416(2) Å) length than the precursors (1.457(5)–1.472(3) Å); this suggests the presence of a C–C double bond character in all the radicals (Fig. 2 and Fig. 3). The C1–N1/N2 bond lengths of the TPA-radicals 3a^I (av. 1.401 Å), 3b^I (av. 1.400 Å), and 3c^I (av. 1.399 Å) were considerably longer than those of 1a^I (av. 1.350 Å), 1b^I (av. 1.351 Å), and 1c^I (av. 1.336 Å), while the N1–C1–N2 bond angle of 3a^I, 3b^I, and 3c^I was slightly smaller than that of 1a^I, 1b^I, and 1c^I. Further, the similar phenomenon was observed in CBP-radicals 4a, 4b, and 4c. Additionally, after the one-electron reduction of the precursors to radicals, the twisted angle between the N1–C1–N2 (carbene) plane and C3–C2–C4 (the central p-phenylene bridge) significantly decreased, particularly in radical 4a: the twisted angle was only 4.73° (Fig. 3). Furthermore, the C–C bond length of the central p-phenylene bridge was observed. The changes in the structural parameter indicate the enhanced conjugation degree of the carbene ligand and the central p-phenylene bridge in the radicals and the partial delocalization of the unpaired electron over the aryl substituent at the carbene carbon atom.

DFT calculations. The electronic structures of NHC-based radicals 3a–c and 4a–c were further investigated by DFT calculations at the UB3LYP/6-31G(d) level of theory. The results indicate that the carbene ligands negligibly affect the electron density distributions of the frontier molecular orbitals in 3a–c and 4a–c (Supplementary Figs. 57–68). The electron cloud of the singly occupied molecular orbital (SOMO) and the lowest singly unoccupied molecular orbital (SUMO) in 3a–c and 4a–c was mainly distributed on the carbene carbon atom and extended partially to the para and ortho positions of the central p-phenylene bridge; this is consistent with the single crystal structure. However, the energy of the frontier molecule orbital (SOMO and SUMO) decreased as the π-accepting capability of the carbene ligands increased from IPr^Me to SIPr^38–40 (Fig. 4a). The energy of the frontier molecule orbital was further reduced by increasing the electron-withdrawing character of the substituent group on the TPA (Fig. 4a). The Mulliken spin densities on the carbene carbon atom gradually increased with the increase in the π-accepting capability of the carbene moiety, whereas it decreased with the increase in the electron-withdrawing capabilities of the substituent (Fig. 4b). These results suggest that the electronic structure of the NHC-based radicals can be easily regulated by changing the ring backbone of the NHCs or the substituent group on TPA.

Photophysical properties. The ultraviolet–visible (UV–vis) absorption spectrum of radicals 3a–c and 4a–c recorded in THF was characterized by a new absorption band in the range of 460–680 nm compared to their corresponding precursors 1a–c and 2a–c (Supplementary Figs. 69–80). The TD–DFT calculations suggested that these absorptions were attributed to SOMO→LUMO + 3 (for 3a^I–3a^IV, 4a, 4b), SOMO→LUMO + 2 (for 3b^I–3b^IV, 3c^I, 4c), and SOMO→LUMO + 4 (for 3b^III) transitions. The calculated lowest energy transition corresponded to the SOMO→LUMO transition with considerably low oscillator strength (f); therefore, the lowest energy transition did not contribute significantly to the absorption profile. The excitation of radicals 3a–c and 4a–c in THF solution at room temperature with visible light above 460 nm only produced weak photoluminescence, while the emission intensity was significantly enhanced when the excitation was performed with UV at 375 nm. However, radicals 3a–c and 4a–c was non-emissive in the solid states probably because of the aggregation-caused quenching, and this is consistent with previous literature results^[9]. Figure 5 shows the emission spectra of radicals 3a–c and 4a–c in THF solution at room temperature, and their photophysical properties are summarized in Table 1. Notably, the emission color of NHC-based radicals 3a–c and 4a–c, from blue to green, was significantly blue-shifted with respect to those of known triaryl methyl radical derivatives⁸. Furthermore, radicals 3a^III, 3a^IV, 3b^III, and 3b^IV with strong electron-withdrawing groups (–CF₃ and –CN) on TPA had significantly shorter emission wavelength than their radical counterparts 3a^I and 3b^I, while the emission wavelength of fluorine-substituted radical derivatives 3a^II and 3b^II remained nearly unchanged (Table 1). Additionally, the emission wavelength of CBP–NHC radicals 4a–c was blue-shifted compared with that of their TPA–NHC radical counterparts 3a^I–3c^I (Supplementary Figs. 81 and 82). Therefore, we can conclude that the emission wavelength of the NHC-based radicals blue shifted with the decrease in the SOMO energy. All IPr^Me/IPr-based radicals showed absolute fluorescence quantum yields (Φ_f) from 6.5–36.8%, which were superior to those of SIPr-based radicals 3c^I (1.5%) and 4c (0.1%). Notably, the CIE (Commission Internationale de L’Eclairage) chromaticity coordinates of 3a^I (0.30, 0.44), 3b^II (0.25, 0.37), and 3a^III (0.17, 0.21) corresponded to green, cyan, and blue, respectively, with high quantum yields, making them promising as short-wavelength emitters in OLEDs (Figs. 5b and 5d). The short excited-state luminescence lifetime of radicals 3a–c and 4a–c suggests that the emission was fluorescent in nature (Table S1). Owing to the existence of an unpaired electron, the ground states of 3a–c and 4a–c were doublet (D₀). The TD–DFT calculations suggested that the lowest excited state (D₁) can be mainly assigned to the transition from α-LUMO to SOMO. However, the oscillator strength (f) of D₁ in these radicals was almost zero, which is indicative of the dark state nature of D₁^41,42. The relatively high excited states (D₂ and D₃) of radicals 3a–c and 4a–c afforded high oscillator strength. Therefore, we infer that the efficient emission of 3a–c and 4a–c most likely originated from D₂ or D₃, which corresponds with a radiative relaxation of α-LUMO + 1 or α-LUMO + 2 to SOMO and does not obey Kasha’s rule. Compared to the general Kasha’s rule, the anti-Kasha’s rule from relatively high excited states (D_n→D₀) is beneficial for obtaining organic radicals with short-wavelength emission, as their energy gap is larger than that for D₁→D₀^20,21.

Table 1

Photophysical parameters of NHC-based luminescent radicals 3a–c and 4a–c in THF.
Radical	λ_ex (nm)	λ_em (nm)	τ (ns)	Φ_f (%)
3a^I	376	529	6.9	33.1
3a^II	377	520	4.3	24.8
3a^III	364	463	4.4	32.6
3a^IV	372	460	3.6	23.8
3b^I	375	490	6.1	32.7
3b^II	376	499	6.5	36.8
3b^III	370	444	3.5	10.4
3b^IV	370	464	3.4	14.3
3c^I	370	484	5.1	1.5
4a	365	482	8.1	6.5
4b	372	470	7.2	13.4
4c	376	469	5.8	0.1

In conclusion, our NHC-based design offers a platform to seek a new class of organic radical materials that display tunable luminescence from blue to green. The choice of the ring backbone of the NHC ligands and the electronic properties of the substituents on TPA both strongly affect the energy levels of the SOMO and emission wavelength of the radicals. Compared to the SIPr-stabilized radical, the IPr and IPr^Me-stabilized radical exhibited a red-shifted emission and a relatively high quantum efficiency. Additionally, the strong electron-withdrawing substituents (–CF₃ and –CN) on TPA can further promote the blue shift of the emission spectrum. According to the experimental results and TD–DFT calculation, the short-wavelength emission of the NHC-based radicals originate from the relatively high excited states rather than the lowest excited state, and this violates Kasha’s rule compared to those of known triaryl methyl radical derivatives. Notably, we can construct blue, cyan, and green luminescent radicals with high quantum efficiency (> 20%) through the proper choice of carbene ligands and TPA units. Moreover, we illustrate that the anti-Kasha’s rule is expected to guide the synthesis of new organic luminescent radicals with high-energy emission. This finding paves the way for the discovery of new organic radicals with rare blue fluorescence, which opens the possibility of the generation of radical-based blue-emitting OLEDs.

1. Parsaee, F. et al. Radical philicity and its role in selective organic transformations. Nat. Rev. Chem. 5, 486–499 (2021).

2. Kato, K. & Osuka, A. Platforms for stable carbon-centered radicals. Angew. Chem. Int. Ed. 58, 8978–8986 (2019).

3. Thorarinsdottir, A. E. & Harris, T. D. Metal–organic framework magnets. Chem. Rev. 120, 8716–8789 (2020).

4. Chen, Z. X., Li, Y. & Huang, F. Persistent and stable organic radicals: Design, synthesis, and applications. Chem 7, 288–332 (2021).

5. Yuan, D., Liu, W. & Zhu, X. Design and applications of single-component radical conductors. Chem 7, 333–357 (2021).

6. Hansen, K. A. & Blinco, J. P. Nitroxide radical polymers – a versatile material class for high-tech applications. Polym. Chem. 9, 1479–1516 (2018).

7. Peng, Q., Obolda, A., Zhang, M. & Li, F. Organic light-emitting diodes using a neutral π radical as emitter: The emission from a doublet. Angew. Chem. Int. Ed. 54, 7091–7095 (2015).

8. Ai, X. et al. Efficient radical-based light-emitting diodes with doublet emission. Nature 563, 536–540 (2018).

9. Cui, Z., Abdurahman, A., Ai, X. & Li, F. Stable luminescent radicals and radical-based LEDs with doublet emission. CCS Chem. 2, 1129–1145 (2020).

10. Ren, A. et al. Emerging light-emitting diodes for next-generation data communications. Nat. Electron. 4, 559–572 (2021).

11. Gamero, V. et al. [4-(N-Carbazolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)methyl radical an efficient red light-emitting paramagnetic molecule. Tetrahedron Lett. 47, 2305–2309 (2006).

12. Hattori, Y., Kusamoto, T. & Nishihara, H. Luminescence, stability, and proton response of an open-shell (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical. Angew. Chem. Int. Ed. 53, 11845–11848 (2014).

13. Guo, H. et al. High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle. Nat. Mater. 18, 977–984 (2019).

14. Abdurahman, A. et al. Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes. Nat. Mater. 19, 1224–1229 (2020).

15. Kusamoto, T. & Kimura, S. Photostable luminescent triarylmethyl radicals and their metal complexes: Photofunctions unique to open-shell electronic states. Chem. Lett. 50, 1445–1459 (2021).

16. Burrezo, P. M. et al. Organic free radicals as circularly polarized luminescence emitters. Angew. Chem. Int. Ed. 58, 16282–16288 (2019).

17. Liu, C. H., Hamzehpoor, E., Sakai-Otsuka, Y., Jadhav, T. & Perepichka, D. F. A pure-red doublet emission with 90% quantum yield: Stable, colorless, iodinated triphenylmethane solid. Angew. Chem. Int. Ed. 51, 23030–23034 (2020).

18. Cho, E., Coropceanu, V. & Brédas, J. L. Organic neutral radical emitters: Impact of chemical substitution and electronic-state hybridization on the luminescence properties. J. Am. Chem. Soc. 142, 17782–17786 (2020).

19. Qian, H. et al. Suppression of Kasha’s rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission. Nat. Chem. 9, 83–87 (2017).

20. Itoh, T. Fluorescence and phosphorescence from higher excited states of organic molecules. Chem. Rev. 112, 4541−4568 (2012).

21. Demchenko, A. P., Tomin, V. I. & Chou, P. T. Breaking the Kasha rule for more efficient photochemistry. Chem. Rev. 117, 13353–13381 (2017).

22. Xin, H., Hou, B. & Gao, X. Azulene-based π‑functional materials: Design, synthesis, and applications. Acc. Chem. Res. 54, 1737–1753 (2021).

23. Guo, J. et al. Mechanical insights into aggregation-induced delayed fluorescence materials with anti-Kasha behavior. Adv. Sci. 6, 1801629 (2019).

24. Li, Y. et al. Photoinduced radical emission in a coassembly system. Angew. Chem. Int. Ed. 60, 23842–23848 (2021).

25. Hopkinson, M. N., Richter, C., Schedler, M. & Glorius, F. An overview of N-heterocyclic carbenes. Nature 510, 485–496 (2014).

26. Mahoney, J. K. et al. Air-persistent monomeric (amino)(carboxy) radicals derived from cyclic (alkyl)(amino) carbenes. J. Am. Chem. Soc. 137, 7519–7525 (2015).

27. Hansmann, M. M., Melaimi, M. & Bertrand, G. Crystalline monomeric allenyl/propargyl radical. J. Am. Chem. Soc. 139, 15620–15623 (2017).

28. Li, Y. et al. C₄ Cumulene and the corresponding air-stable radical cation and dication. Angew. Chem. Int. Ed. 53, 4168–4172 (2014).

29. Antoni, P. W., Bruckhoff, T. & Hansmann, M. M. Organic redox systems based on pyridinium-carbene hybrids. J. Am. Chem. Soc. 141, 9701–9711 (2019).

30. Rottschäfer, D. et al. Crystalline radicals derived from classical N-heterocyclic carbenes. Angew. Chem. Int. Ed. 57, 4765–4768 (2018).

31. Zhao, J., Li, X. & Han, Y.-F. Air-/heat-stable crystalline carbon-centered radicals derived from an annelated N‑heterocyclic carbene. J. Am. Chem. Soc. 143, 14428–14432 (2021).

32. Maiti, A. et al. Anionic boron- and carbon-based hetero-diradicaloids spanned by a p‑phenylene bridge. J. Am. Chem. Soc.143, 3687–3692 (2021).

33. Kim, Y. et al. Highly stable 1,2-dicarbonyl radical cations derived from N‑heterocyclic carbenes. J. Am. Chem. Soc. 143, 8527–8532 (2021).

34. Yang, W. et al. Persistent borafluorene radicals. Angew. Chem. Int. Ed. 59, 3850–3854 (2020).

35. Böhnke, J. et al. Isolation of diborenes and their 90°-twisted diradical congeners. Nat. Commun. 9, 1197 (2018).

36. Welz, E., Böhnke, J., Dewhurst, R. D., Braunschweig, H. & Engels, B. Unravelling the dramatic electrostructural differences between N-heterocyclic carbene-and cyclic (Alkyl)(amino) carbene-stabilized low-valent main group species. J. Am. Chem. Soc. 140, 12580–12591 (2018).

37. Ghadwal, R. S., Reichmann, S. O. & Herbst-Irmer, R. Palladium-catalyzed direct C2-arylation of an N-heterocyclic carbene: An atom-economic route to mesoionic carbene ligands. Chem. Eur. J. 21, 4247–4251 (2015).

38. Dröge, T. & Glorius, F. The measure of all rings—N-heterocyclic carbenes. Angew. Chem. Int. Ed. 49, 6940–6952 (2010).

39. Nelson, D. J. & Nolan, S. P. Quantifying and understanding the electronic properties of N-heterocyclic carbenes. Chem. Soc. Rev. 42, 6723–6753 (2013).

40. Back, O., Henry-Ellinger, M., Martin, C. D., Martin, D. & Bertrand, G. ³¹P NMR chemical shifts of carbene phosphinidene adducts as an indicator of the π-accepting properties of carbenes. Angew. Chem. Int. Ed. 52, 2939–2943 (2013).

41. Imran, M., Wehrmann, C. M. & Chen. M. S. Open-shell effects on optoelectronic properties: Antiambipolar charge transport and anti-Kasha doublet emission from a N‑substituted bisphenaleny. J. Am. Chem. Soc. 142, 38–43 (2020).

42. Feng, Z. et al. Stable boron-containing blue-photoluminescent radicals. Chin. J. Chem. 39, 1297–1302 (2021).

Experiments were performed under an inert gas (Ar or N₂) atmosphere using standard Schlenk techniques and glovebox. THF, dioxane and hexane were dried by standard methods and distilled under nitrogen. ¹H, ¹³C{¹H} were recorded on Bruker AVANCE III 400. Chemical shifts (δ) are expressed in ppm downfield from tetramethylsilane using the residual protonated solvent as an internal standard. Mass spectra were obtained with a Bruker microTOF-Q II mass spectrometer (Bruker Daltonics Corp., USA) in the electrospray ionization (ESI) mode. IR spectra were recorded in Nujol oil using KBr plates as the infrared transmission window on a Nicolet AVATAR 360 FT-IR spectrometer. Elemental analyses were carried out on an Elementar VarioEL III instrument. UV−visible spectra were recorded using an Agilent Technologies Evolution 300 spectrophotometer. The continuous wave (CW) EPR spectra were obtained using an X-band Bruker E500 spectrometer at room temperature. Diffraction data of compound were collected with a Bruker APEX-II CCD diffractometer.

Cyclic voltammetry (CV) experiments were carried out using a CHI660E electrochemical workstation (CH Instruments, Inc). All experiments were carried out under an atmosphere of nitrogen in degassed and anhydrous acetonitrile solution containing Bu₄NPF₆ (0.1 M) at a scan rate of 100 mV s⁻¹. The setup consisted of a glassy carbon working electrode, a platinum wire counter electrode, and a silver wire inserted in a small glass tube fitted with a porous Vycor frit and filled with a AgNO₃ solution in acetonitrile (0.01 M). Ferrocene was used as a standard, and all reduction potentials are reported with respect to the E_1/2 of the Fc⁺/Fc redox couple.

The fluorescence experiments were performed on a Horiba Fluorolog-3 spectrometer. Fluorescence decay profiles were recorded on a FLS920 instrument. The experimental quantum yields were determined by recording the emission signals within an integrating light sphere on a FLS980 Photoluminescence Spectrometer (Edinburgh Instruments) equipped with an ozone free Xenon Arc Lamp (450 W), photomultiplier R928P and double grating excitation and emission monochromators (Czerny-Turner type).

DFT calculations were executed using the Gaussian 09 program package. The geometries of the compounds were optimized without symmetry constraints using the crystal structure coordinate as the starting structure. Calculations were performed using the unrestricted Becke three-parameter hybrid functional with Lee–Yang–Parr correlation functional (B3LYP) with the 6-31G(d) basis set. Frequency calculations were carried out to ensure that the optimized geometries were minima on the potential energy surface, in which no imaginary frequencies were observed in any of the compounds. TD-DFT calculations were performed using wB97XD function and 6-31G(d,p) basis.

Data availability

All data generated and analysed during this study are included in this Article and its Supplementary Information. Crystal structure data for the solid-state structures are available free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/ under reference numbers CCDC-2107665 (1a^I), CCDC-2160168 (1a^II), CCDC-2160169 (1a^III), CCDC-2160170 (1a^IV), CCDC-2107667 (1b^I), CCDC-2160163 (1b^II), CCDC-2160165 (1b^III), CCDC-2160166 (1b^IV), CCDC-2107668 (1c^I), CCDC-2107672 (2a), CCDC-2107673 (2b), CCDC-2107674 (2c), CCDC-2107669 (3a^I), CCDC-2107670 (3b^I), CCDC-2160172 (3b^III), CCDC-2107671 (3c^I), CCDC-2107675 (4a), CCDC-2107676 (4b), CCDC-2107677 (4c), respectively. The supplementary materials for this paper include synthetic and characterization data for all reported compounds as well as computational details.

There is NO Competing Interest.

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