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Article

Homoconjugation Mediated Spin-Spin Coupling in Triptycene Nitronyl Nitroxide Diradicals

by
Chengfang Shi
1,
Laiwei Gao
2,
Martin Baumgarten
3,
Dongdong Wei
2,
Zhipeng Xu
2,
Wenping Wang
1,* and
Di Wang
2,*
1
School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China
2
Anhui Key Laboratory of Advanced Building Materials, School of Materials Science and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China
3
Max Planck Institute for Polymer Research, 55128 Mainz, Germany
*
Authors to whom correspondence should be addressed.
Magnetochemistry 2023, 9(7), 178; https://doi.org/10.3390/magnetochemistry9070178
Submission received: 23 May 2023 / Revised: 29 June 2023 / Accepted: 6 July 2023 / Published: 9 July 2023
(This article belongs to the Special Issue Magnetic Effects—a Tool for Studying the Mechanism of Chemical and Biochemical Processes)
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Abstract

:
In contrast to diradical linked by π-conjugation, there have been only a limited number of studies reported for those linked by homoconjugation systems. Bis(nitronyl nitroxide) diradicals and monoradical connected by a core non-rigid triptycene unit were synthesized. EPR spectroscopy and SQUID were employed to investigate the magnetic exchange interactions. The results demonstrate that the values of ΔEST are 0.19 kcal/mol (J = 34.4 cm−1) for 2,6-TP-NN and −0.21 kcal/mol (J = −36.9 cm−1) for 2,7-TP-NN, indicating ferromagnetic interaction and antiferromagnetic interaction, respectively. The spin polarization rule is not a precise predictor of the behavior of triptycene diradicals, and therefore, we improve the model. The experimental findings indicate that homoconjugation can function directly as a coupling pathway between the two spin centers, which is in qualitative agreement with the DFT theoretical calculations and the Borden rule. This research has found a special means of achieving spin coupling in non-rigid aromatics by means of homoconjugation.

1. Introduction

Recently, researchers have developed and studied numerous magnetic materials, especially in the realm of organic radicals [1,2,3,4,5,6,7,8,9,10]. These materials have a variety of applications, including organic radical-based batteries [4,5], magnetically switched [11,12,13,14], magneto-biology [15,16] and magnetic conductivity materials [17,18,19], in which the magnetic exchange interaction (J) between the spin centers plays a critical role in determining the bulk magnetic properties [20]. By tuning the interaction between two spin centers, it is possible to achieve different properties and degrees of coupling of diradicals, typically categorized as intermolecular [21,22,23,24,25] or intramolecular interactions [26,27]. Intermolecular coupling is commonly achieved through H-bonding [21], π-stacking [22,23,24,25,28], or other supramolecular interactions [29] between two molecules. Conversely, intramolecular coupling is primarily through chemical bonds [23,30]. The backbone that regulates the coupling of two spin units is called a bridge or coupler, such as π-olefin [31] aromatic conjugation [32,33] and aromatic heterocyclic systems [34]. To data, various diradicals connected by π-conjugated bonds have been extensively studied experimentally and theoretically. The ferromagnetic (FM) interaction and antiferromagnetic (AFM) interaction of these diradicals are readily predicted using the spin polarization [35] or the Borden rule [36,37].
However, predicting intramolecular magnetic exchange interaction for the σ-frame diradical systems [38,39] remains challenging. Examples of conjugated systems containing saturated atoms bridging diradicals have been reported [23,34,40,41,42,43,44]. Izuoka et al. [34] studied the A and B structures with neutral thianthrene diradicals in Scheme 1 consistent with the spin polarization of the connecting unit. Akpinar et al. [23] prepared C and D structures, in addition calculations showed weak ferromagnetic coupling and antiferromagnetic coupling, respectively. Nagata et al. [40] demonstrated that structure E possesses a ferromagnetic coupling that can be transformed into an electronic oxide species in the quartet ground state. The first spirocyclic conjugated diradical was synthesized by Frank et al. [41]. Subsequently, Yue et al. [42] investigated three spirobifluorene diradical isomers and further demonstrated that magnetic coupling interaction occurred through spiro conjugation. In 1942, Bartlett et al. [45] firstly synthesized aromatic hydrocarbon triptycene (TP), composed of three benzene rings joined by two sp3 hybrid carbons. Triptycene has become more popular due to its unique properties [46,47]. Tang et al. [48] synthesized triptycene-bridged tris(thianthrene) compound and discovered cationic main-group triradicals.
In this study, we prepared 2-TP-NN, 2,6-TP-NN and 2,7-TP-NN by using TP as a coupler to bridge nitronyl nitroxide (NN). The analysis focuses on the magnetic exchange coupling of the triptycene diradicals and the effect of homoconjugation on the overall intramolecular magnetic exchange coupling by linking two spin centers at different substitution positions. Before starting the experiments. We make a tentative qualitative evaluation (Figure 1) of the ground states of 2,6-TP-NN and 2,7-TP-NN and predicted 2,6-TP-NN as a singlet state and 2,7-NN-TP as a triplet state based on the spin polarization rule [35]. On the contrary, the nonbonding molecular orbitals (NBMOs) of 2,6-TP-NN (or 2,7-TP-NN) are expressed as nondisjoint (or disjoint) diradicals (Figure 1), suggesting that 2,6-TP-NN (or 2,7-TP-NN) favors FM coupling (or AFM coupling) via Hund’s first rule [37,49]. The two prediction methods give opposite results, which further stimulates our exploration of this topic. UV-visible spectra show that the three radicals NN moieties absorb in the region around 600 nm. Electron paramagnetic resonance (EPR) spectra clearly show different hyperfine splitting between the monoradical and diradicals and indicate that the two diradicals have a certain degree of coupling. Further, superconducting quantum interferometry (SQUID) data indicate that 2,6-TP-NN has a moderate FM coupling with a fitted parameter J = 34.4 cm−1, while 2,7-TP-NN has an AFM coupling with a fitted parameter J = −36.9 cm−1. Density functional theory (DFT) calculations show similar results to the SQUID study, however for the calculated magnetic exchange interaction J values are relatively small for the diradicals. The above experimental results demonstrate that the “classical” spin polarization rule does not fit triptycene system. So, our study reconsiders the coupling pathway using homoconjugation [50,51,52,53,54] and proposes a special coupling pathway. Apparently, the special pathway fits better with the experimental results as well as the DFT calculations.

2. Materials and Methods

All radical molecules were synthesized using triptycene as a starting material (see Supplementary Material). The previous literature referred to the preparation of 2,3-bis-(hydroxylamino)-2,3-dimethylbutane (BHA) [55]. 2-Formyltriptycene 1, as well as diformyltriptycene isomers (2 and 3), were prepared from triptycene [56]. The precursors were subjected to Ullman condensation reaction with BHA, resulting in the production of three target products through oxidation (Scheme 2). 2-TP-NN was prepared from triptycene with TiCl4 and Cl2CHOMe in dry CH2Cl2, and then condensed with BHA and oxidized by NaIO4 to give 2-TP-NN in CH2Cl2/H2O at 0 °C. 2-TP-NN appears blue after purification. The precursor diformyltriptycene isomers were prepared by varying the equivalents of TiCl4 and Cl2CHOMe. And after column chromatography to separate regioisomers 2 and 3, the structural characterization by NMR (Supplementary Material), comparing the chemical shifts (δ = 5.6 ppm) of the hydrogen spectra of the two isomers, it is clear that 3 results in two sp3 hybrid carbon atoms with non-identical proton hydrogen. However, this phenomenon is not present for precursor 2. The synthetic pathways of 2,6-TP-NN and 2,7-TP-NN are identical to 2-TP-NN (Supplementary Material). The two diradicals also appear as blue powder after purification. Full synthetic details for all target diradicals are provided in the Supplementary Material.
The single crystal 2-TP-NN was prepared by slowly evaporating in dichloromethane and n-hexane. 2-TP-NN (monoradical), 2,6-TP-NN and 2,7-TP-NN (diradicals) were characterized by UV-vis absorption, electron paramagnetic resonance (EPR), superconducting quantum interferometry (SQUID) and X-ray diffraction. The three radicals were also subjected to density functional theory (DFT) calculations. Additional instrument parameters see the Supplementary Material.

3. Results

3.1. Optical Properties

The optical absorption spectra of 2,6-TP-NN and 2,7-TP-NN, together with the S = 1/2 monoradical 2-TP-NN were recorded at room temperature in Figure 2. Absorption is present in some bands for the three radical molecules. Between 250–400 nm, which is the absorption region for π-π* transitions for triptycene moieties, and another weak absorption in the visible region around 500–700 nm, as found for many NN diradicals [22]. All three radical structures have characteristic absorption peaks of NN around 600 nm, mainly caused by the n-π* transitions of the NN moieties. The UV-vis absorption spectrum strongly indicates that the NN has been synthesized and is absent of imino-nitroxide (IN).
The experimental data of the three molecules (ε, λmax and optical gap Eg) are presented in Table 1. The absorption intensity of 2-TP-NN is weaker near 600 nm (blue) than the other diradicals. The optical gap (Eg) of 2,7-TP-NN was determined to be 1.80 eV based on the onset of the n-π* transition absorption edge, which is larger than the optical gap (Eg) of 1.72 eV for 2,6-TP-NN. Figure S1 illustrates the more pronounced differences between the precursor and each of the three radicals.

3.2. Electron Paramagnetic Resonance Spectroscopy

The EPR spectra of the three radicals were recorded in degassed toluene solution (∼10−4 M) at room temperature. 2-TP-NN shows well-resolved five lines with a spacing of 0.75 mT (peak intensities around 1:2:3:2:1), a typical monoradical NN (Figure S2a). Both 2,6-TP-NN and 2,7-TP-NN (Figure 3) are represented by well-resolved nine lines spectra due to splitting by hyperfine coupling deriving from the four equivalent 14N nuclei (peak intensities around 1:4:10:16:19:16:10:4:1), suggesting delocalization of the electron throughout the entire system. Because of the remarkable intramolecular spin exchange coupling interaction between the two spin centers (J >> aN), each hyperfine coupling constant (HFC) is also cut in half (aN/2 = 0.375 mT). Despite the rare occurrence of sp3 hybrid carbon within the conjugate skeleton [48,57], 2,6-TP-NN and 2,7-TP-NN maintained a strong coupling (J >> aN) interaction as compared to the spiro conjugation coupling effect [43]. In addition, we also successfully simulated the EPR of the three triptycene radicals at room temperature (Figure 3 and Figure S2a). The more details are found in Table 1.
To further understand the internal magnetic coupling of the three triptycene radicals, we performed research tests under low-temperature conditions (100 K, toluene), as illustrated in Figure 4, Figures S2b and S3. We find that both diradicals exhibit strong single peak in the g = 2 region (Figure 4a,b). 2-TP-NN (i.e., monoradical) appears as a regular EPR single peak in this region in line with a simulated spectrum (Figure S2b). The difference between the two diradicals is that 2,6-TP-NN has a sizable main peak with a shoulder, while 2,7-TP-NN has only one main peak, suggesting a smaller zero-field splitting (ZFS) parameter D. The g factor of 2,6-TP-NN is anisotropic with gxx = 2.0023, gyy = 2.0078, gzz = 2.0043 [58]. The zero-field splitting parameters were determined by spectral simulation as |D/hc| = 0.00346 cm−1, |E/hc| = 0.00131 cm−1. The g factor of 2,7-TP-NN is anisotropic with gxx = 2.0037, gyy = 2.0042, gzz = 2.0021. The slightly smaller the zero-field splitting parameters were determined by spectral simulation as |D/hc| = 0.00197 cm−1, |E/hc| = 0.00048 cm−1. ZFS phenomenon is caused by dipole-dipole interactions, and the axial system parameter (D) is correlated with the distance between the two unpaired electrons (r). The average distance between the two spin centers is determined by the point dipole approximation [6,59]. The D value is affected by the dipole-dipole interactions between the spin units and not directly by the exchange interactions. Furthermore, the D value is a reflection of the spatial distance between the spin centers. The diradicals 2,6-TP-NN and 2,7-TP-NN correspond to spin centers average distances of 9.09 Å and 10.9 Å, respectively. It is observed that the average distance between 2,6-TP-NN spin centers is slightly smaller than that of 2,7-TP-NN. The results of the DFT calculations for 2,6-TP-NN (10.7 Å) and 2,7-TP-NN (10.2 Å) were also close to the experimentally measured results.
Notably, the weak forbidden transitions (|ΔmS| = 2) of triplet species were observed in the g = 4 region for 2,6-TP-NN and 2,7-TP-NN (Figure 4a,b inset). The variable-temperature EPR (VT–EPR) experiment was next performed from 10 to 80 K for the diradicals, which showed stronger signal intensities at lower temperatures, indicating the absence of strong antiferromagnetic coupling between the two spin centers (Figure 4c and Figure S4). It is worth noting that the frozen solution spectra of the diradicals both display the EPR signal of the doublet impurity (Figure 4a,b), so the enhancement of the signal at lower temperatures may be contributed by the doublet impurity (S = 1/2). Unfortunately, we failed to fit the IT–T plot of the diradicals data with the modified Bleaney-Bowers [60], which is likely to be caused by the presence of the monoradical. The behavior of I versus 1/T appears to be very close to linear, suggesting the exchange coupling interaction of 2,6-TP-NN and 2,7-TP-NN are relatively weak (Figure S4b,c).

3.3. Magnetic Studies

The molar magnetic susceptibility (χm) of the three triptycene radical samples were measured using a SQUID magnetometer at temperatures ranging from 2–300 K and at a magnetic field of 1 T, in order to investigate the nature and extent of magnetic exchange coupling present in the synthesized triptycene radicals. After subtracting the magnetic signal of the empty sample holder, the diamagnetic susceptibility of triptycene was deducted from the Pascal’s table [61], and the resulting data are presented in the χm vs. T curves in the Figure 5. To model the two-spin systems of both diradicals, we employed the Hamiltonian H= −2 JS1S2 (where S1 = S2 = 1/2). The experimental data of triptycene diradicals were fitted using the Bleaney-Bowers [62,63] Equation (1).
χ m T = ( 1 − F ) 2 N A g 2 μ B 2 k B 1 3 + exp ⁡ − 2 J ∕ k B T T T − θ + F N A g 2 μ B 2 k B
where, J represents the intramolecular exchange of diradicals; θ represents the generalized mean field of intermolecular exchange; NA represents the Avogadro number, g represents the isotropic g-factor, kB represents the Boltzmann constant, μB represents the Bohr magneton and F represents the paramagnetic impurity. J, θ and Lande’g factors are fitted parameters (Table 2 and Table S1).
Figure 5a shows the χmT–T curve for the monoradical. The χmT value is 0.37 emu K mol−1 at 270 K, consistent with the theoretical value of 0.375 emu K mol−1. In lower temperature, χmT decreases rapidly, indicating the presence of antiferromagnetic interaction. For the diradicals, the χmT values of 2,6-TP-NN (Figure 5b) and 2,7-TP-NN (Figure 5c) were found to be 0.68 emu K mol−1 and 0.72 emu K mol−1 at 300 K, respectively. The values are lower than expected value (0.75 emu K mol−1) for a magnetically independent S = 1/2 two spin units, which may be attributed to additional nonmagnetic material in power sample, with the presence of ~13% and ~4% of a nonmagnetic impurity. With the temperature decreases, χmT of 2,6-TP-NN gradually increases and reaches the maximum value of 0.77 emu K mol−1 at 14.2 K. On further lowering the temperature, χmT abruptly decreases until χmT = 0.67 emu K mol−1 at 2.0 K. This suggests that there are at least two kinds of magnetic coupling interactions in 2,6-TP-NN, a much weaker intermolecular antiferromagnetic interaction (AFM) as well as moderate intramolecular ferromagnetic interaction (FM). However, χmT of 2,7-TP-NN keeps falling as the temperature drops until χmT = 0.28 emu K mol−1 at 2 K. The temperature dependence of χmT was fitted by equation (1), giving J(2,6-TP-NN) = 34.4 cm−1 (ΔEST = 0.19 kcal/mol), consistent with weak FM coupling, giving J(2,7-TP-NN) = −36.9 cm−1 (ΔEST = −0.21 kcal/mol), consistent with weak AFM coupling. Obviously, the behaviors of 2,6-TP-NN and 2,7-TP-NN do not obey the spin polarization rule. The inverse of molar magnetic susceptibility versus temperature curve are shown in Figure S5.

3.4. X-ray Crystallography

The arrangement and geometry of the molecules in the lattice have a significant impact on magnetic interactions, making it essential to have a clear understanding of radical crystal molecules. By slowly evaporating hexane into a dichloromethane solution, we were able to obtain a needle-like single crystal of 2-TP-NN for X-ray analysis (Figure 6). Many methods of crystallization were tried only to obtain the diradicals crystals that were not large enough to test. The crystal of 2-TP-NN was found to be in the monoclinic crystal system with the P21/c space group. The 2-TP-NN molecular crystal structure revealed that the benzene ring connected with the NN unit exhibited torsions of 32.5° for N1−C21−C1−C2 (Figure 6b). Such a large torsion angle can prevent the delocalization of electrons in the benzene ring region, resulting in a small spin population of the benzene ring. Similar phenomena have been studied both theoretically and experimentally [24,64,65,66].
The molecular stacking diagram reveals that 2-TP-NN is arranged vertically along an axis primarily through the NN moieties (Figure 6c and Figure S6c). The oxygen in the NN group forms a short contact distance of 2.71 Å (O1···H25A) with the adjacent oxygen. Formed dimer is likely to play a role in intermolecular coupling and even in diradicals. The two-dimensional structure shows that the molecules are laterally arranged in a herringbone shape (Figure S6b), with further extension in two dimensions through C–H···O hydrogen bonds. The oxygen atoms form hydrogen bonds with two hydrogen atoms on the laterally aligned parallel molecule at a distance of 2.53 Å (O2···H7) and 2.69 Å (O2···H9), stretching further in two dimensions through C–H···O hydrogen bonds (Figure S6b). Additional crystallographic data for 2-TP-NN is presented in Table S2, S3 and S4.

3.5. Computational Studies

To gain a better understanding of the exchange mechanism in the triptycene radicals, theoretical calculations [66] were performed using Gaussian09 software package. The coupling constants between triptycene diradicals were calculated from the broken-symmetry (BS) by DFT at the UB3LYP/6-31G level of theory, and the computational details are given in the SI. For two exchange coupled unpaired electrons, the simplest Hamiltonian quantity of the molecule is given: H = −J12S1S2. And the magnetic exchange coupling constant (J) was calculated according to the following equation proposed by Yamaguchi et al. [67],
J = E B S − E T S 2 T − S 2 B S
where, EBS and ET represent the total energy of the calculated BS singlet state and triplet state, <S2>T and <S2>BS represent the total spin angular momentum of the calculated broken-symmetry triplet and singlet state. UB3LYP/6-31G method provides extremely weak positive coupling constant (Jcalc = 1.06 cm−1) and small negative exchange interactions (Jcalc = −3.50 cm−1) for 2,6-TP-NN and 2,7-TP-NN, respectively (Table 2 and Table S5), indicating that 2,6-TP-NN has a weak FM coupling, while 2,7-TP-NN has an AFM coupling, in agreement with the experimental results. It is clear that the spin polarization rule is not satisfied for triptycene system, which is likely that sp3 hybrid carbon atoms break the alternating rule of the coupling path. Such abnormal phenomenon is still observed in the spiro-conjugation system [43].
The spin density distributions of the radicals are shown in Figure 7 and Figure S7. The spin density distributions are mainly delocalized in the NN moieties and the attached benzene rings, while other regions have only small spin densities. The spin population of all heavy atoms within the same phenyl group are added together (Figure 7 and Figure S8). Spin population of diradicals are mainly concentrated in the spin units, with the adjacent benzene ring having only the partial spin population. In addition, the spin population of sp3 hybrid carbon is almost negligible. Figure 8 and Figure S9 depict the highest single occupied molecular orbitals (SOMOs), visualizing the disjoint and nondisjoint properties of the radicals respectively. The nondisjoint 2,6-TP-NN possesses considerable SOMOs overlap densities and thus have moderate exchange integrals. The calculations reveal that the SOMOs in the diradicals compounds have similar energies (Figures S10 and S11 and Table S6), mainly confined to NN group and exhibiting less domain to the benzene ring. Nevertheless, the disjoint 2,7-TP-NN diradical is distributed in different spatial regions and, as a result, possesses no SOMOs overlap densities, with almost negligible exchange integrals. Consequently, the former prefers triplet state (S = 1) and the latter prefers singlet state due to the simplicial SOMOs energy level being broken (S = 0).
To further explore the spin coupling within triptycene, we take homoconjugation into account. The influence of the associated 2pπ-orbital orientation on magnetic coupling phenomena has also been reported [44]. The 2pπ-orbital of the sp2 carbon atoms adjacent to the methylene may play important role. The spatial distribution of the three benzene rings of triptycene enable the sp2 carbon atom p-orbitals to overlap partially (Figure 9), which can provide a pathway for intramolecular coupling, although this conjugation effect is small. The through space homoconjugation has been confirmed by many studies [51,52,53,68,69]. We reconsidered the path for the spin polarization rule, as shown in Figure 9. According to the renew prediction, 2,6-TP-NN exhibits FM coupling, while 2,7-TP-NN presents AFM coupling, which is opposite to previous the spin polarization predictions (Figure 1). The interaction through space is more convincing than the interaction through bond. More importantly, this also provides strong confirmation that the triptycene molecule does contain a special homoconjugation effect.

4. Conclusions

In summary, three triptycene radicals were synthesized successfully and characterized. The work confirms 2,6-TP-NN with FM coupling as well as 2,7-TP-NN with AFM coupling, consistent with DFT theory. The non-conjugated triptycene system can also provide a pathway for the coupling of spin centers. Unusually, this system did not obey the “classical” spin polarization rule. The coupling pathway does not pass through the sp3 hybrid carbon atom. To further explain these anomalies, we take the special homoconjugation of triptycene into account. and it was discovered that the unique homoconjugation perfectly accounts for the anomalous prediction by spin polarization This indicates orbital overlap deriving from the p-orbital of three sp2 carbon atoms through space can form homoconjugation, making exchange coupling interaction possible. Although the coupling effect of homoconjugation is modest, it provides a new insight into coupling paths for non-conjugation systems, with potential applications in spintronics and molecular electronics.

Supplementary Materials

The following supplementary Material can be downloaded at: https://www.mdpi.com/article/10.3390/magnetochemistry9070178/s1, Figure S1: Normalized UV−vis absorption spectra of (a) 2-TP-NN, 7 (b) 2,6-TP-NN and (c) 2,7-TP-NN in toluene; Figure S2: EPR spectra in degassed toluene (10−4 M) solution for 2-TP-NN at room temperature (bule) and simulated (red) (a). Experimental EPR spectra recorded in toluene solution for 2-TP-NN at 100 K (blue) and simulated (red) (b); Figure S3: EPR frozen state ZFS spectra for testing, and then rotated 90° for testing of diradicals 2,6-TP-NN (a) and 2,7-TP-NN (b) at 100 K; Figure S4: VT-EPR spectra of (a) 2,6-TP-NN and (b) IT−T plot of 2,6-TP-NN. The observed normalized I−T curve presented as a straight line for (c) 2,6-TP-NN and (d) 2,7-TP-NN; Figure S5: Curie–Weiss model straight line of (a) 2-TP-NN, (b) 2,6-TP-NN and (c) 2,7-TP-NN; Figure S6: Single-crystal structures of 2-TP-NN. (a) The benzene ring connected with NN unit with torsions of 32.5°, (b) The crystal packing of 2-TP-NN along axis b and (c) axis a; Figure S7: Spin density distributions and spin population of 2-TP-NN were calculated using UB3LYP/6-31G method (blue and green surfaces correspond to α and β spin densities, respectively, with an isovalue of 0.004.); Figure S8: Atomic spin populations calculated using Mullikens protocol at the UB3LYP/6-31G level. (a) 2-TP-NN (b) 2,6-TP-NN (c) 2,7-TP-NN; Figure S9: The 2-TP-NN spatial distribution of molecular orbitals; Figure S10: The 2,6-TP-NN spatial distribution of molecular orbitals; Figure S11: The 2,7-TP-NN spatial distribution of molecular orbitals; Table S1: Magnetic properties of 2-TP-NN. Table S2: Crystal data and structure refinement for 2-TP-NN; Table S3: Bond Lengths for 2-TP-NN; Table S4: Bond Angles for 2-TP-NN; Table S5: Summary of diradical calculation results. Table S6: HOMO, LUMO and HOMO−LUMO (ΔEHL) energy. Crystallographic data (CIF); CCDC 2261580 include crystallographic data can be obtained by https://www.ccdc.cam.ac.uk/data_request/cif (accessed on 9 May 2023).

Author Contributions

Conceptualization, W.W. and D.W. (Di Wang); methodology, C.S. and L.G.; software, C.S., D.W. (Dongdong Wei) and Z.X.; formal analysis, C.S., L.G, D.W. (Dongdong Wei ) and Z.X.; investigation, D.W. (Di Wang) and W.W.; resources, D.W. (Di Wang) and W.W.; data curation, C.S., D.W. (Dongdong Wei ) and Z.X.; writing—original draft preparation, C.S.; writing—review and editing, C.S., M.B., D.W. (Di Wang) and W.W.; supervision, C.S., D.W. (Di Wang) and W.W.; project administration, D.W. (Di Wang) and W.W.; funding acquisition, D.W. (Di Wang) and W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of China (No. 52003004 and 2013GJMS0526) for the financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available from the corresponding author on reasonable request.

Acknowledgments

The authors thank the Max Planck Institute for Polymer Research for providing computing resources. The authors also thank Electron paramagnetic resonance spectroscopy spectra were measured on CIQTEK EPR200-Plus with continues-wave X band frequency.

Conflicts of Interest

The authors declare no conflict of interest.

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Magnetochemistry 09 00178 sch001 550
Scheme 1. Example polycyclic aromatic-linked nitronyl nitroxide diradicals. Two red arrows in the same direction represent FM coupling, and two red arrows in opposite direction represent AFM coupling.
Scheme 1. Example polycyclic aromatic-linked nitronyl nitroxide diradicals. Two red arrows in the same direction represent FM coupling, and two red arrows in opposite direction represent AFM coupling.
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Figure 1. Spin prediction model for triptycene diradicals (The red asterisks represent the same spin direction and the dots represent electrons). Preliminary prediction model is that 2,6-TP-NN is antiferromagnetic (AFM) coupling and 2,7-TP-NN is ferromagnetic (FM) coupling.
Figure 1. Spin prediction model for triptycene diradicals (The red asterisks represent the same spin direction and the dots represent electrons). Preliminary prediction model is that 2,6-TP-NN is antiferromagnetic (AFM) coupling and 2,7-TP-NN is ferromagnetic (FM) coupling.
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Scheme 2. Synthetic routes of the three targeted molecules. (a) 5 eq TiCl4, 5 eq Cl2CHOMe, DCM; (b) 10 eq TiCl4, 10 eq Cl2CHOMe, DCM; (c) BHA, DCM/MeOH, reflux, 48 h; (d) NaIO4, DCM/H2O, 0 °C.
Scheme 2. Synthetic routes of the three targeted molecules. (a) 5 eq TiCl4, 5 eq Cl2CHOMe, DCM; (b) 10 eq TiCl4, 10 eq Cl2CHOMe, DCM; (c) BHA, DCM/MeOH, reflux, 48 h; (d) NaIO4, DCM/H2O, 0 °C.
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Figure 2. UV-vis absorption spectra for 2-TP-NN (blue), 2,6-TP-NN (green) and 2,7-TP-NN (yellow) in toluene (∼10−5 M) solution at room temperature. Inset plot: amplification from 400–800 nm.
Figure 2. UV-vis absorption spectra for 2-TP-NN (blue), 2,6-TP-NN (green) and 2,7-TP-NN (yellow) in toluene (∼10−5 M) solution at room temperature. Inset plot: amplification from 400–800 nm.
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Figure 3. EPR spectra in toluene (10−4 M) solution for (a) 2,6-TP-NN and (b) 2,7-TP-NN at room temperature (bule) and simulated (red).
Figure 3. EPR spectra in toluene (10−4 M) solution for (a) 2,6-TP-NN and (b) 2,7-TP-NN at room temperature (bule) and simulated (red).
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Figure 4. Experimental EPR spectra recorded in toluene at 100 K (blue) and simulated (red) of (a) 2,6-TP-NN (2D = 74 G, Fitting results: gxx =2.0023, gyy = 2.0078, gzz = 2.0043, |D/hc| = 0.00346 cm−1, |E/hc| = 0.00131 cm−1) and (b) 2,7-TP-NN (2D = 42 G, Fitting results: gxx =2.0037, gyy = 2.0042, gzz = 2.0021, |D/hc| = 0.00197 cm−1, |E/hc| = 0.00048 cm−1). The insets show the forbidden transition at half-field (top right). (c) Variable-temperature EPR measurement, 2,7-TP-NN in toluene solution from 10 to 80 K. (d) The measured (solid blue ring) IT–T plot of 2,7-TP-NN. The central peak with a green asterisk shows the signal due to the doublet impurity (S = 1/2).
Figure 4. Experimental EPR spectra recorded in toluene at 100 K (blue) and simulated (red) of (a) 2,6-TP-NN (2D = 74 G, Fitting results: gxx =2.0023, gyy = 2.0078, gzz = 2.0043, |D/hc| = 0.00346 cm−1, |E/hc| = 0.00131 cm−1) and (b) 2,7-TP-NN (2D = 42 G, Fitting results: gxx =2.0037, gyy = 2.0042, gzz = 2.0021, |D/hc| = 0.00197 cm−1, |E/hc| = 0.00048 cm−1). The insets show the forbidden transition at half-field (top right). (c) Variable-temperature EPR measurement, 2,7-TP-NN in toluene solution from 10 to 80 K. (d) The measured (solid blue ring) IT–T plot of 2,7-TP-NN. The central peak with a green asterisk shows the signal due to the doublet impurity (S = 1/2).
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Figure 5. SQUID test (blue hollow circle) of the radicals: plots of χmT–T at a magnetic field of 1 T. Inset plots: χm–T (blue hollow circle) and fitting (red line) curves of (a) 2-TP-NN, (b) 2,6-TP-NN and (c) 2,7-TP-NN.
Figure 5. SQUID test (blue hollow circle) of the radicals: plots of χmT–T at a magnetic field of 1 T. Inset plots: χm–T (blue hollow circle) and fitting (red line) curves of (a) 2-TP-NN, (b) 2,6-TP-NN and (c) 2,7-TP-NN.
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Figure 6. Single-crystal structure of 2-TP-NN. (a) ORTEP views, molecular packing along the crystallographic (b) axis a and (c) axis b.
Figure 6. Single-crystal structure of 2-TP-NN. (a) ORTEP views, molecular packing along the crystallographic (b) axis a and (c) axis b.
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Figure 7. Ground state spin density distributions and spin population of 2,6-TP-NN and 2,7-TP-NN (blue and green surfaces represent α and β spin densities, respectively) calculated at the UB3LYP/6-31G level of theory with an isovalue of 0.004 (The arrows represent the direction).
Figure 7. Ground state spin density distributions and spin population of 2,6-TP-NN and 2,7-TP-NN (blue and green surfaces represent α and β spin densities, respectively) calculated at the UB3LYP/6-31G level of theory with an isovalue of 0.004 (The arrows represent the direction).
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Figure 8. Side views SOMOs of 2,6-TP-NN and 2,7-TP-NN (red and blue color represents the different phase of the orbital coefficients), isovalue of 0.004.
Figure 8. Side views SOMOs of 2,6-TP-NN and 2,7-TP-NN (red and blue color represents the different phase of the orbital coefficients), isovalue of 0.004.
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Figure 9. Prediction of the spin polarization rule of the triptycene diradicals under homoconjugation (The arrows represent the direction).
Figure 9. Prediction of the spin polarization rule of the triptycene diradicals under homoconjugation (The arrows represent the direction).
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Table
Table 1. Optical and EPR properties with data of the three triptycene radicals.
Table 1. Optical and EPR properties with data of the three triptycene radicals.
Radicalsλmax (nm) 1ε (cm−1M−1) 1Egopt (eV) 2g 3aN/2 (G) 3
2-TP-NN6208661.752.00657.50
2,6-TP-NN6229741.722.00633.75
2,7-TP-NN61910941.802.00623.74
1 Obtained from UV-vis absorption spectra. 2 Optical energy gaps estimated from the onset of absorption spectra. 3 Calculated from experimental EPR spectra.
Table
Table 2. Magnetic Properties of radicals of 2-TP-NN (see Table S1), 2,6-TP-NN and 2,7-TP-NN.
Table 2. Magnetic Properties of radicals of 2-TP-NN (see Table S1), 2,6-TP-NN and 2,7-TP-NN.
RadicalsTmax (K) 1θ (K)Jexp (cm−1) 2Jexp (cm−1) 3Jcalc (cm−1) 4
2,6-TP-NN14.2−0.36-34.41.06
2,7-TP-NN-−4.7-−36.9 −3.50
1 Tmax is the temperature corresponding to when χmT is at the maximum. 2 Jexp was estimated by fitting IT-T curve from VT-EPR. 3 Estimated from SQUID. 4 Calculated at the UBLYP/6-31G level of DFT.
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Shi, C.; Gao, L.; Baumgarten, M.; Wei, D.; Xu, Z.; Wang, W.; Wang, D. Homoconjugation Mediated Spin-Spin Coupling in Triptycene Nitronyl Nitroxide Diradicals. Magnetochemistry 2023, 9, 178. https://doi.org/10.3390/magnetochemistry9070178

AMA Style

Shi C, Gao L, Baumgarten M, Wei D, Xu Z, Wang W, Wang D. Homoconjugation Mediated Spin-Spin Coupling in Triptycene Nitronyl Nitroxide Diradicals. Magnetochemistry. 2023; 9(7):178. https://doi.org/10.3390/magnetochemistry9070178

Chicago/Turabian Style

Shi, Chengfang, Laiwei Gao, Martin Baumgarten, Dongdong Wei, Zhipeng Xu, Wenping Wang, and Di Wang. 2023. "Homoconjugation Mediated Spin-Spin Coupling in Triptycene Nitronyl Nitroxide Diradicals" Magnetochemistry 9, no. 7: 178. https://doi.org/10.3390/magnetochemistry9070178

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