Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene

Aug 1, 2017 - Yamaguchi , K.; Jensen , F.; Dorigo , A.; Houk , K. N. A Spin Correction Procedure for Unrestricted Hartree-Fock and Møller-Plesset ...
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Protonation-Enhanced Antiferromagnetic Couplings in AzobenzeneBridged Diradicals Fengying Zhang,† Xinyu Song,† and Yuxiang Bu*,†,‡ †

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Proton-induced magnetic enhancement in an organic diradical is an appealing phenomenon. Here, taking two nitroxide groups as spin sources, we predict the magnetic properties of the trans and cis forms of azobenzene (AB)-bridged diradicals in which the central −N N− unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably increase after protonation from −716.4 to −1787.1 cm−1 for the trans form and from −388.1 to −1227.9 cm−1 for the cis form, respectively. Such noticeable magnetic enhancements induced by protonation are mainly attributed to the strong mediating role of the coupler AB between two radical groups through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation. The planar structure for the protonated trans diradical and two reduced CCNN torsional angles due to protonation for the cis one are responsible for the significant magnetic enhancements. Protonation not only supports the development of π conjugation among the spin groups and coupler but also creates a very favorable condition for spin transmission through the coupler AB LUMO by lowering the LUMO energy level and improving spin polarization and charge delocalization and thus enhances the spin coupling effectively. In addition, different spin sources and linking modes of the radical groups are also considered to confirm our conclusions, and the possibilities of protonation of such diradical systems are further discussed. The studied diradicals could be the promising candidates for the rational design of magnetic molecular switches.

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INTRODUCTION Azobenzene (AB) can employ the cis and trans configurations, and the trans isomer is more stable than the cis one. When irradiated with appropriate light, it can transform from the trans to cis isomer, while the reverse reaction occurs easily in the dark.1 Based on this photochromic character, AB and its derivatives find a widespread technological application in optical control,1 molecular switches,2 glassy materials,3 protein probes,4 DNA modification,5 and so on. Apart from continuing exploration of the pre-existing usages in depth, development of their potential applications has been the subject of much interest. Excitingly, it has been found that AB and its derivatives can be modified by stable radical groups, and the modified molecules have been proved to be attractive candidates for the design of molecular magnets, which opens an opportunity for studies of the magnetic properties of the radicalized molecules.6−8 This is mainly attributed to the fact that AB and its derivatives can serve as the photochromic spin coupler to modulate the magnetic interaction of the AB-bridged two radical groups or units. In addition to the photoinduced photochromics,6−11 magnetic regulation (including magnetic switching, enhancement or weakening, etc.) can be also realized by means of other © XXXX American Chemical Society

ways. For example, investigation of trimethylenemethane-type (TMM-type) diradicals suggested that their magnetic behaviors can be largely weakened or even convert from ferromagnetism (FM) to antiferromagnetism (AFM) with the increase of the torsional angle between the coupler and radical units,12 and a reversible magnetic switching can be driven by temperature in the thermomagnetic molecular system.13 Recently, our group has noticed that the redox reaction can effectively modulate the magnetic properties of diradicals in which the meta-/parapyrazinyl act as the redox active units attached by two nitroxide radical groups.14 More interestingly, Ali and co-workers15 revealed that noncovalent cation/anion-π interactions have great effect on the magnetic couplings and found that anions (including F−, Cl−, and Br−) can dramatically strengthen the magnetic coupling below the equilibrium distance. Overall, magnetic regulations of organic molecules could be realized through photo-, geometrical distortion-, temperature-, and redox-induced methods or noncovalent cation/anion−π interactions. To our knowledge, the protonation-based Received: June 30, 2017 Revised: July 31, 2017 Published: August 1, 2017 A

DOI: 10.1021/acs.jpcc.7b06429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Diagram of the Unprotonated and Protonated Structures for the trans and cis Forms of AB-Based Diradicals with Two Nitroxide (NO), Verdazyl (VER), or Nitronyl Nitroxide (NN) Radical Groups at the para/para (pp), meta/ para (mp), or meta/meta (mm) Sites

magnetic regulation in organic systems is easy to accomplish,16 and protonation is a successful strategy to modify some organic molecules to understand the chemical16,17 and biochemical18 phenomena. Especially, as an experimental evidence for a theoretical prediction that either of the nitrogen atoms of the −NN− unit in AB is the most susceptible site to protonation, the protonated trans-AB (ABH+) were experimentally observed.19,20 Thus, it should be of extreme interest to explore the protonation modulation possibility of the ABbased magnet systems. Inspired by these, herein we theoretically design the trans-/ cis-AB-based diradicals in which two nitroxide radicals are linked to the para positions (the pp series) of the two phenylene units with respect to the −NN− unit, and the azo unit could undergo single protonation to convert to its protonated counterpart (−H+NN−). Interestingly, the computational results indicate that the AFM coupling constants (J) for these unprotonated pp-type diradicals are considerably large no matter for the trans or cis configurations, indicating the favorable spin-coupling-mediating ability of AB. In particular, noticeable magnetic enhancements are also observed in both the trans/cis forms due to protonation although the signs of J do not change. That is, protonation can significantly enhance the AFM coupling of them but does not cause spin crossover or magnetic conversion. Such a remarkable spin-coupling enhancement should be attributed to the strong mediating role of the coupler AB between two radical groups through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation. The planar structure for the trans-AB-protonated diradical and two reduced CCNN torsional angles due to protonation for the cis one can not only support the development of π conjugation between the spin groups and coupler but also create a very profitable condition for the spin transport through the AB LUMO (by lowering its energy level) and thus facilitate the spin coupling effectively. Moreover, different spin sources and linking modes of the radical groups are also considered to verify the conclusions of the magnetic enhancement due to protonation. Clearly, the

observed protonation-induced magnetic enhancement in the AB-based diradical molecules exhibits promising applications in the fields of magnetic molecular switches, optoelectronic devices, data storage devices, and so on, and this work also provides useful information for the further experimental studies and further exploration.

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MOLECULAR DESIGN AND COMPUTATIONAL DETAILS The choice of the coupler and radical functional groups is of great importance for a diradical. In this work, we select the relatively stable nitroxide (NO) radical as spin sources and attach them to the trans and cis forms of AB at the para positions, respectively, denoted as ON-tABpp-NO (t: the trans form) and ON-cABpp-NO (c: the cis form), as shown in Scheme 1. After protonation, ON-tABpp-NO and ON-cABpp-NO convert to ON-tABppH+-NO and ON-cABppH+-NO, respectively. Besides, to further verify the protonation effect on magnetic couplings, we also consider other two commonly used radical groups (RG), verdazyl (VER) and nitronyl nitroxide (NN), as the spin sources which are also attached to the para/para sites of AB or ABH+, generating the VER- and NN-based diradicals similar to the NO-based ones, denoted as RG-tABpp-RG/ RG-tABppH+-RG and RG-cABpp-RG/RG-cABppH+-RG, RG = VER and NN, respectively, whose magnetic properties in the unprotonated forms were previously reported7,8 and thus can be viewed as a reference. Moreover, we also discuss the protonation effect on the magnetic couplings using different linking modes of the radical groups and also examine the structural topological effects due to different linkages, taking the NO-based diradicals as examples. The associated diradicals include the meta-/para-linked (mp) or meta-/meta-linked (mm) AB/ABH+-mediated diradicals in the trans/cis forms, denoted as ON-tABmp-NO/ ON-tABmpH+-NO, ON-cABmpNO/ON-cABmpH+-NO, ON-tABmm-NO/ON-tABmmH+-NO, and ON-cABmm-NO/ON-cABmmH+-NO, respectively. All of these newly designed diradicals are illustrated in Scheme 1, and their magnetic properties are evaluated computationally. In B

DOI: 10.1021/acs.jpcc.7b06429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C addition, to find more practical applications of the protonation strategy, we also examine the proton effects using weak acid ions (e.g., NH4+ and H3O+) in ON-tABpp-NO. The molecular geometric optimizations, frequency analyses, and energy calculations of the closed-shell (CS) singlet, brokensymmetry (BS) open-shell singlet, and triplet (T) state of all diradicals were carried out at the (U)B3LYP/6-311++G(d,p) level of theory. An expression of the magnetic exchange coupling constant is given as J = (EBS − ET)/(⟨S2⟩T − ⟨S2⟩BS), where EBS and ET refer to the energies of the BS and T states and ⟨S2⟩BS and ⟨S2⟩T denote the average spin square values of these spin states, respectively. This expression was developed by Yamaguchi and co-workers21,22 and considered as the most appropriate one for estimating the J value. All of these DFT calculations were performed using the Gaussian 03 suite of program.23

RG = NO considerably differ by 1.6−9.3 times due to protonation. In particular, for ON-tABpp-NO, photoinduction isomerization leads to a considerable decrease (by ca. 328.3 cm−1) of the magnetic coupling magnitude from −716.4 to −388.1 cm−1 (ON-cABpp-NO), whereas protonation leads to a noticeable increase (by ca. 1070.7 cm−1) to −1787.1 cm−1 (ON-tABppH+-NO) and even if for the cis form (ON-cABppNO) the magnetic coupling enhancement is also remarkable, being about 839.8 cm−1 (Figure 1). Undoubtedly, it is necessary and important to explore the essence of such noticeable protonation effects. Here we discuss the protonation-induced magnetic enhancement phenomenon and regularity from the following aspects: geometric factor, spin polarization, and charge delocalization, role of the coupler LUMO with an emphasis on those of two pairs of diradicals: ON- t AB p p -NO/ON- t AB p p H + -NO and ON- c AB p p -NO/ ON-cABppH+-NO. Finally, possible applications of the protonation effect are introduced briefly. All of the calculated data including the energies of the CS, BS, and T states, ⟨S2⟩ values, as well as J values for the pp, mm, and mp series of diradicals are gathered in Table S1. Geometric Factors. For the trans diradicals, ON-tABpp-NO and ON-tABppH+-NO, their optimized structures are coplanar between the coupler (tAB) and two radical groups (NO) as shown in Figure S1, which clearly creates a favorable condition for spin polarization. The spin polarization plays a crucial role in determining the magnitude of magnetic exchange coupling, and the diradical with large spin polarization usually contributes to a strong magnetic interaction.24 As a result, both ON-tABppNO and ON-tABppH+-NO give rise to relatively large |J| values because of large spin polarization, corresponding to 716.4 and 1787.1 cm−1, respectively. In addition, protonation of AB in ON-tABppH+-NO shortens the key bond lengths (C−N and N−O) due to introduction of a positive charge with strong electron-withdrawing effect (see Figure S1). As a consequence, the shorter coupling pathway provides a more facile means of spin polarization and coupling for ON-tABppH+-NO. For the cis-form diradicals, ON-cABpp-NO and ON-cABppH+NO, geometric optimizations reveal that the attached two NO radical groups are coplanar with their linked phenylene rings but twisting out of the plane of the bridge −NN− unit because of the repulsion between two phenylene hydrogen atoms. After protonation, great changes take place for the CNNC dihedral angle (ϕCNNC) and two CCNN torsional angles (ϕ1 and ϕ2) as illustrated in Figure S1. That is, protonation of cis-AB causes an increase of ϕCNNC but a decrease both of ϕ1 and ϕ2. Specifically, after protonation for ON-cABpp-NO, the dihedral angle ϕCNNC increases from 18.5° to 47.8°, whereas the torsional angles ϕ1 and ϕ2 decrease from 39.8° to 2.2° and 39.4° to 14.2°, respectively. The difference between ϕ1 and ϕ2 in ON-cABppH+-NO can be attributed to different C−N bond lengths between the phenylene ring and −NN− unit due to single protonation only at one N, which leads to different rotation degrees of the radicalized phenylene (HNO-phenylene). Due to the slightly larger torsional angles, a possible π−π interaction (face-to-face) between two phenylene rings is developed in ON-cABpp-NO, leading to a weak throughspace coupling. In contrast, small torsional angles of ON-cABppH+-NO support the development of π conjugation between the phenylene ring and −NN− unit and thus facilitates effective spin polarization and coupling with a large |J| value (−1227.9 cm−1).

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RESULTS AND DISCUSSION According to the different connection ways among the coupler (AB or ABH+) and two radical groups, we consider three kinds of diradicals classified as the pp, mm, and mp series. Regardless of the unprotonated or protonated diradicals, the calculated results indicate that the pp- and mm-type ones all tend to support an AFM coupling, while the mp-type ones are in favor of a FM interaction as shown in Figure 1. Interestingly,

Figure 1. Magnetic coupling constants |J| of unprotonated (blue) and protonated (red) diradicals including the trans diradicals ON-tABppNO/ON- t AB pp H + -NO, VER- t AB pp -VER/ VER- t AB pp H + -VER, NN-tABpp-NN/NN-tABppH+-NN, ON-tABmp-NO/ON-tABmpH+-NO, and ON-tABmm-NO/ON-tABmmH+-NO and the corresponding cis diradicals ON- c AB pp -NO/ON- c AB pp H + -NO, VER- c AB pp -VER/ VER-cABppH+-VER, NN-cABpp-NN/NN-cABppH+-NN, ON-cABmpNO/ON-cABmpH+-NO, and ON-cABmm-NO/ON-cABmmH+-NO from left to right in order. The pp- and mm-type diradicals present antiferromagnetism (AFM), whereas the mp-type ones exhibit ferromagnetism (FM).

although the signs of their J do not change before and after protonation, we notice that the magnitudes for the protonated diradicals are significantly enhanced compared with those of the unprotonated ones especially for the pp series (Figure 1). In general, the |J| values for all pairs of diradicals RG-tABpp-RG/ RG-tABppH+-RG and RG-cABpp-RG/RG-cABppH+-RG for RG = NO, VER, and NN and RG-tABmp-RG/RG-tABmpH+-RG, RG-cABmp-RG/RG-cABmpH+-RG, RG-tABmm-RG/ RG-tABmmH+-RG, and RG-cABmm-RG/RG-cABmmH+-RG for C

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Figure 2. Comparison of spin polarization and the Mulliken atomic spin densities of the corresponding monoradicals of ON-tABpp-NO and ON-tABppH+-NO (the upper row) and ON-cABpp-NO and ON-cABppH+-NO (the lower row) for which one nitroxide radical group is removed. For the spin sources, the red circle denotes larger density distribution and the blue circle denotes small one. Besides, larger spin density variations of the connecting carbon atoms marked with a green or magenta circle and the azo units before and after protonation clearly show the spin polarization.

coupling interaction for ON-cABppH+-NO. Consistently, the atomic spin density distribution at the spin source in ON-cABpp-NO is larger than that of ON-cABppH+-NO (see Figure S4). The observed decrease in |J| values for ON-cABppNO and ON-cABppH+-NO can be assigned to the reductions of spin polarization due to the bad planarity,26 in comparison with their respective trans isomers, ON-tABpp-NO and ON-tABppH+NO. To get a better understanding of the spin polarization effect on the magnetic coupling and magnetic enhancement, we investigate the correlation between the |J| values and dihedral angles (ONCC) of two trans diradicals, as shown in Figure 3. It

Spin Polarization and Charge Delocalization Effects. Generally, most spin densities mainly localize at the radical groups, and however, spin polarization can take place readily from the radical groups to the coupler with an extensively conjugated structure.14,24 The spin density plots of ON-tABppNO and ON-tABppH+-NO in Figure S2 reveal that spin density distributions of the NO groups spread out on the whole molecular skeleton, suggesting a considerable spin polarization through delocalization along with the π-conjugated trans-AB. To quantitatively elucidate the effect of protonation on spin polarization from the spin groups to the phenylene and further to the −NN− unit, we make a comparison of the Mulliken atomic spin density distributions between ON-tABpp-NO and ON-tABppH+-NO in each of which one NO group is removed to exclude the possible influence on the coupler due to spin interaction between two radical groups. As shown in Figure 2, atomic spin density distributions of the unprotonated monoradical indicate that 18.6% spin density is delocalized over the trans-AB coupler, while spin polarization is noticeably enhanced to be 36.4% for the protonated one. That is, only 63.6% spin density within blue circle is localized at the NO group, indicating a considerable spin polarization or transfer from the NO group to the AB coupler after protonation. The protonation of another nitrogen atom presents a similar situation (Figure S3). The large spin polarization can strongly promote the spin coupling between two NO groups and thus contributes to a larger |J| value for ON-tABppH+-NO. In contrast, atomic spin density distributions on the coupler in the unprotonated ON-tABpp-NO are distinctly smaller than that in ON-tABppH+-NO (Figure S4), which further confirms that protonation of trans-AB can strengthen the polarization effect and gives rise to the intense coupling interaction accordingly. In addition, it should be noted that large values of atomic spin densities (absolute values: 0.137 and 0.186) at the connecting carbon atoms within the magenta circle between the phenylene ring and the NO group in the protonated monoradical in Figure 2 also imply strong spin polarization from the NO group, supporting the larger |J| value.25 Analogously, with regard to the cis-form diradicals, ON-cABpp-NO and ON-cABppH+-NO, the atomic spin density distributions in the radical NO groups are 82.2% for the unprotonated monoradical and only 55.7% for the protonated one (Figure 2), demonstrating that protonation can considerably enlarge spin polarization once more and thus the spin

Figure 3. Relationship between the |J| values and torsional angles (ONCC) in ON-tABpp-NO and ON- tABpp H+-NO. The inset represents the corresponding torsional angles (ϕ1 and ϕ2) between the coupler trans-ABH+ and NO radical groups.

can be seen that the |J| values of ON-tABppH+-NO and ON-tABpp-NO decrease greatly with the increase of both the ONCC torsional angles. In particular, when both of the ONCC torsional angles are increased to be 90°, the coupler and radical groups are orthogonal to each other, completely inhibiting the spin polarization because of the destruction of the π conjugation. As a result, the J value becomes zero (Figure 3). As displayed in Figure S5, the spin density plots at a dihedral angle of 90° reveal that spin polarizations through the coupler trans-AB and ABH+ greatly decreases or even reaches to zero, not contributing to the coupling interaction between two spin D

DOI: 10.1021/acs.jpcc.7b06429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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UV−vis spectrum of ON-tABppH+-NO in contrast to that of ON-tABpp-NO with respect to the same transition is also a clear manifestation of charge delocalization due to protonation (Figure S8).20,28 Accordingly, the charge delocalization originating from protonation greatly enhances the spin interactions of the protonated diradicals ON-tABppH+-NO and ON-cABppH+-NO. Role of the Coupler LUMO. To further explore the more critical factors that can directly affect spin polarization or charge delocalization, we turn our discussion to the orbital properties of the coupler AB and ABH+ including the LUMO role and the orbital energies. As reported, the nature of the coupler can determine the magnetic interaction between two radical groups.24,29,30 So it is necessary to analyze the orbital characters and orbital energies of the coupler to gain further insight into the magnetic properties and protonation-induced magnetic enhancement in the studied diradicals. Particularly, as depicted in Figures 3 and S5, the magnetic couplings of ON-tABpp-NO and ON-tABppH+-NO are drastically reduced to zero when their SOMOs are mainly localized in the radical groups (NO) and are perpendicular to the LUMO of the coupler (tAB or tABH+). In theory, the magnetic coupling constants of them should not be changed if there is no mediating role of the coupler LUMO. In fact, the calculational results verify the participation of LUMO in the spin interaction between two NO radical groups. It can be seen from Figures S5 and S6 that two SOMOs of two radical groups and LUMO of the coupler cannot match well with the increase of torsional angle for ON-tABpp-NO and ON-tABppH+-NO, which can further influence the spin delocalization through the coupler as displayed from the spin density plots. It is evident in the fact that LUMOs of the coupler AB and ABH+ can act as the bridges of two NO groups to mediate their spin delocalization and coupling interaction. When the torsional angles are large, the conjugation is poor between SOMOs of the NO groups and LUMO of the coupler, leading to small |J| values due to weak spin delocalization, while a good conjugation is developed between SOMOs and LUMO if the torsional angles are small enough, thus giving rise to large |J| values. The observations reveal that the coupler plays a strong mediating role in realizing the exchange coupling between two radical groups through its LUMO,31 which can be further illustrated by virtue of the orbital interaction especially for the protonated diradicals in the following. As shown in Figure 5, both the energies of HOMO and LUMO of the coupler decrease upon protonation. That is, introduction of a proton can not only lower the HOMO energy but also reduces the LUMO π* energy level (Figure S2) to a large extent because of strong charge delocalization, regardless of the trans or cis forms. Specifically, through protonation the energies of HOMO and LUMO decrease by 4.19 and 5.12 eV for the trans-AB and by 4.92 and 5.38 eV for the cis-AB, respectively. It has been established that, if the orbital energies of the radical groups and coupler are very close, they can combine together to form a stable diradical with considerable spin−spin coupling, or vice versa.31 Interestingly, we notice that the energy difference of HOMO between the trans-/cis-AB coupler and NO radical group is very small and that between LUMO of the trans-/cis-ABH+ coupler and HOMO of the NO radical group is also much closer, implying that the coupler LUMO considerably participates in the formation of conjugation which facilitates the spin coupling after protonation (Figure 5). First, the strong mediating role of the protonated coupler through LUMO can promote the spin polarization.

groups. This observation indicates that spin polarization plays a critical role in the control of magnetic coupling. Furthermore, a comparison of spin density plots of ON-tABpp-NO and ON-tABppH+-NO at a dihedral angle of 80° (Figure S6) definitely suggests that strong spin polarization in ON-tABppH+NO gives rise to the considerably enhanced magnetic coupling, which confirms the rationality of the aforementioned explanation. By analogy, it is understandable that the |J| values of ON-tABppH+-NO are always higher than those of ON-tABppNO as their ONCC torsional angles change in Figure 3. Interestingly, introduction of a proton to the coupler can not only promote spin polarization but also facilitate charge delocalization. The positive charge carried by the proton can delocalize to the phenylene rings further to the NO groups. As evidenced,27 the aromatic bridge phenylene can efficiently regulate the intramolecular electron transfer and thus also could tune the spin coupling interaction between the two spin sources through further modification. Charge delocalization due to protonation can be quantitatively elaborated by means of atomic NBO charges. As displayed in Figure S7, the positive charge can mainly transfer to the carbon atoms at each of the para and ortho sites relative to the NO groups and further delocalize to the NO groups along with the π-conjugated phenylene rings including the linked hydrogen atom for ON-tABppH+-NO and ON-cABppH+-NO. Specifically, charge delocalization into the HNO-phenylene unit is calculated to be about 37.6% (L: left) and 24.7% (R: right) through protonation for ON-tABpp-NO, respectively, and 37.6% positive charge is localized at the hydrogen atom of the unit −H+NN−. As for ON-cABppH+-NO, the charge delocalization is slightly smaller, and 40.7% positive charge is localized at the hydrogen atom of the −H+NN− unit. That is, the charge delocalization into the left HNO-phenylene unit (close to the protonating H+) is estimated to be 32.8% and 26.5% into the right one. All of these detailed data are presented in Figure 4. Besides, a red shift of

Figure 4. Protonation-induced charge delocalization percentages into the phenylene rings and NO groups marked by R (right) and L (left) for ON-tABppH+-NO and ON-cABppH+-NO, respectively, by comparing the differences of atomic NBO charge distributions between ON-tABpp-NO and ON-tABppH+-NO and between ON-cABpp-NO and ON-cABppH+-NO. E

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and the calculated results (Table S2) also confirm that single protonation of the −NN− unit can enhance the magnetic coupling interactions in both the pp-type and mp-type diradicals (IN-trans-/cis-azothiophene-IN) as well. Linking Site Effect. Our group recently revealed that the spin couplings of the mm- or mp-type NO-based diradicals which are linked through m-phenylene-phenylene-m or mphenylene-phenylene-p (−27.8 versus +99.0 cm−1) decrease considerably in contrast with that (−408.9 cm−1) of the pp-type one connected by a p-phenylene-phenylene-p coupler.32 Herein, we take the NO-based diradicals as examples to discuss the protonation effects on the magnetic couplings by changing the connection modes of the radical groups (including the pp, mm, and mp series) and also examine the structural topological effects because of the different linkages. For these three kinds of NO-based diradical series, it is found that that the |J| values of the pp series are much larger than those of the mp and mm ones before or after protonation, which are in the order pp > mp > mm (Figure 1). This observation is well consistent with the calculated results in ref 32, and the differences in |J| values of these three series are also attributed to different atomic spin density distributions due to different linking sites of the NO groups. The enhanced coupling interactions for the protonated diradicals ON-tABmpH+-NO, ON-cABmpH+-NO, ON-tABmmH+NO, and ON-cABmmH+-NO are also attributed to stronger spin delocalization (Figure S4), analogous to ON-tABppH+-NO and ON-cABppH+-NO. Besides, we notice that the linking site effect can not only regulate the magnitudes of J values but also change the magnetic behaviors from AFM (for the pp and mm series) to FM (for the mp series), as shown in Table S1. It is worth mentioning that the magnetic behaviors of all of these studied diradicals can be predicted by means of the SOMO effect33 and spin alternation rule34,35 (see Figures S2 and S10). In addition, as for the relative sizes of the effects of protonation in the three diradical series (pp, mp, and mm types), they can be also explained from the differences of the spin density distributions among three different linking modes. As shown in Figure 2 for the model systems, the spin density distribution at the para-site C of the right phenylene is considerably larger than that at the meta site no matter before or after protonation, which is more favorable to the spin coupling when another radical group is linked to the para site than that of linking to the meta site. Furthermore, protonation considerably increases the spin density distribution at the para site much more than that at the meta site, which is more favorable to the spin coupling through the para-site channel. As a result, the effect of protonation is more pronounced for the pp-type diradicals (with larger increments of the |J| values) than that for the mp-type ones. In other words, although protonation at the −NN− can enlarge the spin polarization, it cannot noticeably improve the spin density distributions at the meta site and thus cannot considerably improve the spin communication through the meta-site channel. Thus, the effect of protonation is less pronounced in the mp-type FM systems compared to that of the pp-type AFM ones. However, it should be noted that the relative effects of protonation are actually comparable for such diradical systems. For example, taking the ON-tABpp-NO/ON-tABppH+-NO, ON-cABpp-NO/ON-cABppH+-NO, ON-tABmp-NO/ ON-tABmpH+-NO, and ON-cABmp-NO/ON-cABmpH+-NO diradicals as representatives, their |J| values after protonation are 2.5, 3.2, 1.8, and 5.5 times those before protonation, respectively. This indicates that, although the |J| values of the

Figure 5. HOMO and LUMO energy levels of the couplers (trans-AB, trans-ABH+, cis-AB, and cis-ABH+) as well as the NO, VER, and NN radical groups.

Second, the itinerant exchange between two spin sources is more likely to occur through LUMO with a lower energy level for the protonated coupler. Therefore, it is comprehensible that the spin couplings of the protonated diradical ON-tABppH+-NO and ON-cABppH+-NO are larger than those of the corresponding unprotonated ON-tABpp-NO and ON-cABpp-NO, respectively. In short, the protonation-induced magnetic enhancement can be attributed to the strong mediating role of the coupler between two NO radical groups through its LUMO with a lower energy level (Figure S9). VER and NN-Based Diradicals. To further prove the protonation effect on magnetic interactions, we also consider the other two commonly used radical groups, verdazyl (VER) and nitronyl nitroxide (NN), as spin sources to modify the coupler AB or ABH+ by connecting to the para/para sites of AB or ABH+ (Scheme 1). As shown in Figure 1, protonation can also distinctly enhance the magnetic couplings of the VER and NN-based diradicals, VER- t AB pp -VER, VER- c AB pp -VER, NN-tABpp-NN, and NN-cABpp-NN, but their |J| values are considerably smaller compared with those of the NO-based ones. The dominating reason should be that the NO radical group belongs to a more localized one, being advantageous for spin delocalization to the conjugated coupler and its small size favors its conjugation with the coupler, while both VER and NN radical groups are attributable to the slightly delocalized ones with larger sizes, which are not favorable to the spin delocalization to the coupler. As evidenced by the Mulliken atomic spin density distributions of the VER or NN-based monoradicals (Figure S3), spin polarization to the coupler AB or ABH+ is very small, and the spin densities are mainly localized at the VER or NN radical groups. In addition, the poor matching in the orbital energy levels between the coupler and VER or NN radical group is also not favorable to the magnetic interaction (Figure 5). Similar to the NO-based diradicals, slightly stronger spin polarization to the coupler still leads to larger spin coupling for the protonated VER or NNbased diradical, and it is especially obvious for VER-cABppH+VER and NN-cABppH+-NN as shown in Figure S4. Fortunately, the calculated J values of the unprotonated VER and NN-based diradicals are in good accord with the estimated results in ref 8. In addition, we further select the trans-/cis-azothiophene as the spin coupler and the bis-imino nitroxide (IN) radical groups as the spin sources in the pp-/mp-type linking modes to examine the effect of protonation on the magnetic couplings, F

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The Journal of Physical Chemistry C mp-type diradicals are relatively small, the relative protonationenhanced effect is also noticeable especially for the cis-form diradicals ON-cABmp-NO/ON-cABmpH+-NO (5.5 times). Possible Applications of Protonation Effect. Considering the practical applications of the studied diradicals, some simple comparisons are made including the cis/trans-isomerization barriers along with the protonation energies of the coupler and NO-based diradicals in the pp series. It is found that the isomerization barrier of trans-AB to cis-AB is 0.67 eV, which is lowered to 0.42 eV through protonation. Similarly, the isomerization barrier of ON-tABpp-NO to ON-cABpp-NO decreases from 0.77 to 0.49 eV by protonation. This observation suggests that a much more rapid trans-to-cis transition for the AB-based molecules could take place through acid catalysis.19 Nevertheless, we notice that the protonation energies of ON-tABpp-NO/ON-cABpp-NO and the corresponding couplers (trans-/cis-AB) are all large, manifesting that protonation of such AB-based molecules can occur spontaneously, but the deprotonation process is relatively difficult in the gas phase. All of these detailed data are summarized in Table S3. To find more extensive applications of the protonation strategy, we also consider the weak acid ions, NH4+ and H3O+, as the protonation agents to examine the protonation effect on the magnetic properties in ON-tABppNO. However, optimizations directly lead to the protontransferred structures featuring NH3···ON-tABppH+-NO and H2O···ON-tABppH+-NO. The calculated J values of NH3··· ON-tABppH+-NO and H2O···ON-tABppH+-NO are −1620.2 and −1676.0 cm−1, respectively, slightly smaller than that (−1787.1 cm−1) of ON-tABppH+-NO due to the influence of an extra hydrogen bond (ABH+···NH3 or ABH+···OH2) which slightly lowers the Lewis acidity of the protonating proton (Table S1). These observations confirm that single protonation of the −NN− unit in the trans-/cis-AB-based diradicals are thermodynamically favorable and can enhance the magnetic coupling indeed. However, we also find that ON-tABpp-NO and ON-cABpp-NO would exhibit nonmagnetic characters when the −NN− unit in each is doubly protonated, indicating that overprotonation could lead to a CS ground state of the system, losing the magnetism. However, it should be noted that double protonation at the azo unit is thermodynamically unfavorable. It has been reported that molecular switches can transform from one state to another and change their physical or chemical properties through external stimuli.36 Thus, each conjugated pair (protonated or not) of such diradical molecules can be utilized to serve molecular switches on the basis of their different magnetic properties before and after protonation. The magnetic molecular switch induced by protonation is easy to operate and definitely distinct from conventional switches in which it is necessary to overcome the energy barrier between different states or properties, such as light, temperaturemodulated magnetic switches. Clearly, such a magnetic regulation in these diradicals is also intriguing in the fields of data storage devices, molecular electronics, etc.

magnetic coupling constants J do not change, the magnitudes remarkably increase after protonation from −716.4 to −1787.1 cm−1 for the trans form and from −388.1 to −1227.9 cm−1 for the cis form, respectively. That is, protonation at the azo unit can remarkably enhance the spin coupling between the two spin sources through the AB coupler but does not cause spin crossover or magnetic conversion. Such noticeable magnetic enhancements induced by protonation are mainly attributed to the strong mediating role of the coupler AB between two radical groups through its LUMO with a lower energy level after protonation. The planar structure for the protonated trans diradical and two reduced CCNN torsional angles due to protonation for the cis one are responsible for the significant magnetic enhancement. Protonation not only supports the development of π conjugation from the spin groups to the coupler but also creates a very favorable condition to spin transmission through the coupler AB LUMO by lowering its energy level and improving spin polarization and charge delocalization from the radical groups to the coupler and thus enhances the spin coupling effectively. For such diradicals with different spin sources and linking modes of the radical groups, the same spin coupling regularities are also observed. Besides, calculations also indicate that protonation of such diradical systems at their azo units is thermodynamically favorable, and certainly its corresponding deprotonation is also controllable. Clearly, such protonation-modulated AB-based diradicals could be the promising candidates for the rational design of magnetic molecular switches or devices.

CONCLUSIONS In summary, we primarily discuss the magnetic behavior and magnetic enhancement induced by protonation for several couples (the trans vs cis forms) of the AB-bridged diradicals containing the NO, VER, or NN radical groups as spin sources. The AB bridge can undergo single protonation at its −NN− unit to convert to its protonated counterpart or vice versa. Our more interesting findings are that, although the signs of their

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06429. All estimated data and figures calculated at the (U)B3LYP/6-311++G (d,p) level including the energies of the closed-shell singlet, broken-symmetry open-shell singlet, and triplet, ⟨S2⟩ values, as well as intramolecular magnetic exchange coupling constants for all designed diradical molecules; isomerization barriers and protonation energies of the coupler and NO-based diradical molecules of the pp series; the optimized geometries of all diradicals and key bond lengths between adjacent carbon and nitrogen atoms for the trans/cis forms as well as the CCNN and CNNC dihedral angles for the cis forms; SOMOs and spin density distributions of all diradicals as well as HOMO and LUMO plots of the coupler; Mulliken atomic spin density distributions of six monoradicals of the pp series; Mulliken atomic spin density distributions of all diradicals; SOMOs, LUMO, and spin density maps with different torsional angles; atomic NBO charge distributions; UV−vis spectrum; schematic diagram of exchange coupling between two radical groups through the coupler LUMO; and spin alternation plots. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinyu Song: 0000-0002-7486-9388 Yuxiang Bu: 0000-0002-6445-5069 G

DOI: 10.1021/acs.jpcc.7b06429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21373123, 21573128, and 20973101) and NSF (ZR2013BM027) of Shandong Province. A part of the calculations were carried out at National Supercomputer Center in Jinan and High-Performance Supercomputer Center at SDU-Chem.

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DOI: 10.1021/acs.jpcc.7b06429 J. Phys. Chem. C XXXX, XXX, XXX−XXX