Rational Design of Organic Magnets with Switchable Two-Way

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Rational Design of Organic Magnets with Switchable Two-Way-Couplers: Magnetic Modulation or Switching through Lactam-Lactim Tautomerization Dongxiao Chen, Tiange Deng, Lu Yang, and Yuxiang Bu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00944 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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The Journal of Physical Chemistry

Rational Design of Organic Magnets with Switchable Two-Way-Couplers: Magnetic Modulation or Switching through Lactam-Lactim Tautomerization Dongxiao Chen, Tiange Deng, Lu Yang, Yuxiang Bu* School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, People’s Republic of China ABSTRACT ： Lactam-lactim tautomerism which features a typical prototropic process has been widely applied to various functional processes. magnetism-tunable

organic

magnets.

Here, we use it to design the

Taking

experimentally

reported

phenazine-N,N’-dioxide (PDO) as template in which two nitroxide groups act as spin sources and two ortho-phendiyl groups act as two couplers, we further modify one coupler through replacing a –C=C– unit of an ortho-phendiyl by an amide (–CONH–) at three different positions, forming three switchable molecular magnets.

Calculations reveal intriguing

magnetic modulation or switching phenomena through tautomerization.

That is, all lactim

structures and PDO exhibit extremely strong antiferromagnetic spin couplings with magnetic coupling constants J ranging from -2485.7 cm-1 to -2732.4 cm-1, while the lactam structures are diamagnetic or moderately strong antiferromagnetic with J of -1703.6 cm-1, depending on different locations of the introduced amide/imide units.

Strong spin couplings (including

excessive couplings in diamagnetic cases) in all six molecules and PDO can be interpreted by proximity of two nitroxide groups, good planarity and conjugation, while the reason for the differences in magnetic characteristics of three lactam forms is that the destruction of aromaticity induces two types of rearrangement of chemical bonds, which leads to opposite changes of the spin coupling between two radical units.

Besides, the role of each coupler, ring

aromaticity, Mulliken spin densities and molecular orbitals are analyzed for the understanding of magnetic regulation.

Double lactam-lactim tautomerization through modifying two

couplers with amide/imide units, different orientations of hydroxyl H in lactim structures and tautomerization energetics assisted by solvent molecules are also discussed.

This work

provides a promising strategy for rational design of organic molecular magnetic switches with two switchable spin communication channels. 1

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INTRODUCTION In recent years, organic magnetic molecular switches have been widely investigated

because of its application prospects in the field of magnetic materials, such as magnetic devices and memory elements.1-3

In most cases, magnetic organic molecules (organic magnets) which

exhibit diradical characters are ideal magnetic materials.4

To our knowledge, two basic

components must be mentioned while studying the organic diradicaloids: one is the radical group or spin source and the other is the coupler.

As for the radicals, according to the

delocalization ability of spin unpaired electrons, they can be divided into the localized and delocalized ones.5

The former is well localized and is poorly interacted with other same type

of radicals, while the latter is easily delocalized through π-conjugation in a conjugated system. Among all of these diverse radical groups, the delocalized nitroxide group is commonly applied to the design of diradicaloids because of its unique advantages, such as high stability and low difficulty to be synthesized.

Meanwhile, the unpaired electron of it mainly distributes both on

the nitrogen and oxygen atom,6 which can diffuse out of the domain and delocalize to other parts of the molecule, making the molecule with nitroxide group have more stable spintronics properties.

Up till now, a lot of nitroxide-containing molecules have successfully been

synthesized by oxidizing the aromatic nitrogen compounds.

For example, Greer et al.7

successfully prepared phenazine-N,N’-dioxide (PDO), the derivatives of which have widely been used in tumour therapy and synthesis of hypoxic selective cytotoxins,8-10 by using mCPBA for the oxidation of phenazine in the solvent of CH2Cl2.

It should be noted that in

PDO, as shown in Scheme 1, the N-O unit actually has a pair of resonant structures: >N-O• versus >N•+→O- and features a three-electron π-bond with a larger spin density distribution over oxygen than over nitrogen atom.

On the other hand, a suitable coupler is also quite

crucial for the design of magnetic molecular switches.

For example, PDO shows a pattern

that two nitroxide radical groups connect with two ortho-phendiyls and the latter can be viewed as two ortho-site couplers through which the electronic communication between two radical units is realized.

Inspired by the PDO structure, we use the double couplers (i.e. a two-way

coupler) to design our targeted magnets and magnetic switches, which are widely used in the design of magnetic coordination polymers with various types of magnetic motifs, and turn out 2


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to be more functional and diverse than traditional one-way coupler because the simutaneous pathways may interplay with each other to modulate magnetic coupling.11

Clearly, the

coupler with good conjugacy and short coupling pathway is needed for better delocalization and strong coupling interaction among spin sources, as well reflected in PDO. To enable the tuning function of magnetic switches, a tunable method is also necessary. In most cases, the regulation usually takes places on a tunable coupler.

According to related

studies, there have been several approaches that could make the regulation of magnetism achievable, including using photo-,12 proton-,13 redox-,14 electrical-15 induced methods. Among all of these, prototropic tautomerism has a great potential to control magnetic transformation because it is associated with a π-bond shifting, which significantly affects electronic delocalization and coupling interaction.16

Up till now, diverse novel functional

materials have been designed through introducing an amide unit into the system, e.g. the amide-containing building blocks of nanowires.17-20

The hydrogen migration between the

nitrogen and oxygen atom in such a group leads to two tautomers (amide versus imide form), which has been used in determining the mechanism of enzymatic or photochemical reactions in proteins.21,22

In particular, the prototropic tautomerism in a cyclized amide group can lead to

its cyclized imide counterpart which is usually called the lactam-lactim tautomerism, and, as demonstrated, the optimal conformations of two lactam-lactim tautomers may be changed in different states23,24 and different polar solvents.25

Up till now, diverse works have

investigated this tautomerism both theoretically and experimentally and found various intriguing properties and promising applications.

For example, Nakane et al.26 designed a

quinoxalinone derivative capable of the lactam-lactim tautomerization that can be used as a fluorescence probe for sensing of both cation and anion.

Peng et al.27,28 directly observed

lactam-lactim tautomers using 2D IR spectroscopy and explored the kinetic characteristics of tautomerization combined with the density functional theory (DFT) calculations. al.29

theoretically

revisited

the

thermodynamic

and

kinetic

Hejazi et

properties

of

2-hydroxypyridine/2-pyridone tautomerization and obtained the data of energy gaps, activation energies between two tautomers using different computational methods.

All of these studies

motivate us to extend its applications to the design of molecular magnets and the tuning of magnetic spin coupling magnitude and characteristics. 3



Page 4 of 34

Allowing for all of these factors, we introduce an amide unit in one of two ortho-phendiyls (as a whole, the two-way coupler) of PDO to form a lactam-lactim tautomerizable coupler, and the other is maintained unchanged as an auxiliary coupler, in order to better control the spin coupling interaction between two >NO• groups.

That is, the two-way coupler (two

ortho-phendiyls) is modified as a conductivity-tunable one.

Owing to different replacing

positions or modes of an amide unit in the modified ortho-phendiyl, three pairs of magnetic molecular switches are designed as topic molecules in this work.

Besides, several model

molecules are also designed for further exploration of modulation mechanism.

The DFT

calculations were used to determine the magnetic spin coupling properties of these magnetic switches as well as PDO by estimating their magnetic exchange coupling constants (J).

Our

main findings are that all three pairs of tautomers have strong antiferromagnetic (AFM) lactim structures with considerably large coupling constants (J = -2485.7 cm-1, -2692.4 cm-1 and -2732.4 cm-1, respectively), similar to their parent PDO (J = -2699.8 cm-1), which is mainly because of good planarity, conjugation and proximity of two nitroxide groups.

After the

lactimlactam tautomerization, two of the corresponding lactam structures exhibit diamagnetic (DM) spin couplings, while the other one exhibits the weakened AFM spin coupling (J=-1703.6 cm-1), indicating that the lactimlactam tautomerization can modulate or switch the magnetic spin couplings of the PDO-derived molecular magnets.

The origin of

switching or modulation magnitude can be accounted intuitively for by aromaticity change and rearrangement of chemical bonds.

Spin density and molecular orbital analyses are used for

deep understanding of the modulation or switching mechanism.

As an expansion, we also

investigate the effect of double lactam-lactim tautomerization through modifying two ortho-phendiyl couplers with amide units.

The orientation effect of hydroxyl H in the lactim

structures, and solvent-assisting effect on the lactimlactam tautomerization were also examined to give more complete explanations of the designed magnetic molecular switches. This work reveals a rational design strategy of magnetic molecular switches with a tunable two-way coupler functionalized by the lactam-lactim tautomerization.

We hope it could

provide a new idea for the design of organic magnets and magnetic molecular switches, as well as their further applications.

4


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

DESIGN STRATEGY AND COMPUTATIONAL DETAILS Design Strategy.

In our design for a target molecule, as mentioned above, two nitroxide

radical groups are chosen as the π-type spin sources, and two π-conjugated rings are chosen as the couplers each of which links the two nitroxide groups using its two adjacent sites, forming a two-way linked diradical, similar to its parent PDO.

Different modifications are considered

on the couplers through introducing one amide or imide unit into them, leading to the lactam or lactim isomers, respectively.

For the notation of these designed molecule magnets (Scheme 1),

according to the feature of the two-way-couplers, we name them using the abbreviations of two coupler rings.

Roughly, the ring modified by an amide/imide unit is named as Ring L, putting

on the left side in both the structure and its abbreviated name, and the ring remains unchanged for auxiliary coupling is named as Ring R, putting on the right.

For example, HPY, PY and

Ben are the abbreviations of 2-hydroxypyridine, 2-pyridone and benzene rings, respectively, and thus we use the way “Ring L-Ring R” for abbreviation to denote the corresponding magnet (e.g. HPY-Ben or PY-Ben).

We also use the notation “Ring C” to denote the central

ring, in which two nitroxide units can be viewed as the para-located.

To define different

locations of the amide unit, we number all atoms in the molecule and select the serial numbers of nitrogen atom and hydroxyl in order to mark the molecule in the upper left corner of the ring’s abbreviation.

The serial numbers of the lactim forms are the same with their

corresponding lactam structures, and we also use serial numbers to describe a pair of lactam-lactim tautomers, i.e. the 12, 21, or 23 series (e.g. 12HPY-Ben and 12PY-Ben).

Clearly,

the lactam-lactim tautomerism suggests a scheme to interconvert the dominant configurations, during which the spin coupling interaction of two radicals could be changed, as well as the spin coupling properties, thus making magnetic regulation achievable.

In addition, apart from six

topic molecules mentioned above, some model molecules are also designed for further investigation.

To explore the function of each coupler, Ring R is replaced by a nonconductive

–CH2-CH2– short chain (1B for abbr.) as a coupler, and we classify these molecules as the “Ring L-1B” type, although 1B does not mediate the spin couplings. To better understand the mechanism of magnetic transformation induced by the lactam-lactim tautomerization, we also consider the two-way modification scheme, i.e. 5



Page 6 of 34

introducing two amide units into both Ring L and Ring R to form the two-way controllable molecular magnets.

Three types of tautomers, HPY-HPY, HPY-PY and PY-PY, are

obtained for each position isomers.

The serial number of Ring R is also used to show the sites

of nitrogen atom and hydroxyl in order, referring to the standard serial numbers of anthracene in IUPAC.

To find the influence of different orientations of hydroxyl hydrogen in the lactim

forms, three orientation isomers are also considered, denoted as 12HPY’-Ben, 21HPY’-Ben and 23HPY’-Ben,

where the prime is used to denote the other orientation opposite to that in HPY.

We also examine the pure hydroxyl effect by replacing the imide (–N=C(OH)–) unit in all lactim forms with an enol unit (–C=C(OH)–), from which we can better understand the nature of the H-orientation effect.

Four model molecules featuring the phenol ring (Phe) as Ring L

are obtained, and the corresponding molecules are denoted by 1Phe-Ben, 1Phe’-Ben, 2Phe-Ben and 2Phe’-Ben, where 1 and 2 denotes the sites of hydroxyls, respectively, and the prime denotes the H-orientation outward the molecular skeleton.

All molecules with their notations

are given in relevant figures and Figures S1-S2 in the Supporting Information (SI). Computational Details.

Molecular geometric optimizations and energy calculations of all

considered magnets were carried out at the B3LYP/6-311++G(d,p) level, which is suggested to have a good performance for predicting the magnetic properties in many diradical systems,4-5 and the broken-symmetry approach at the DFT framework developed by Noodleman and his co-workers30,31 was used to determine the open-shell singlet states at the same level. Frequency analyses were also made to confirm that all optimized geometries were the minima on the global potential energy surfaces.

The magnetic coupling constants J were determined

by the formula proposed by Yamaguchi and his co-workers,32,33 which can be expressed as J = (EBS-ET)/(T-BS), where EBS and ET denote the energies of the broken-symmetry open-shell singlet (BS) and triplet (T) of a molecule, while BS and T represent the average spin square values of the BS and T states, respectively.

Then, to further verify the

accuracy of computational results, we also use different functionals or different basis sets for recalculation.

That is, for all six topic molecules and PDO, geometric optimizations and

energy calculations were also done by using a more modern M06-2X functional with a 6-311++G(d,p) basis set, and single point calculations using the B3LYP functional with two large basis sets (6-311++G(3df,3pd) and aug-cc-PVDZ) were also performed at the 6


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B3LYP/6-311++G(d,p) optimized geometries to confirm the results.

Moreover, diradical

characters of six topic molecules and PDO were also measured through evaluating the occupation numbers of their lowest unoccupied natural orbitals (LUNO) by using the CASSCF(10,10)/6-311++G(d,p) single point calculations on the B3LYP/6-311++G(d,p) geometries.

In addition, we calculated the energies of frontier orbitals and HOMO-LUMO

gaps for all six topic molecules and PDO also by using B3LYP/6-311++g(d,p) method, which verifies to provide more stable results than MO based HF method34 and shows a great agreement with UV-vis experimental results35 in the study of phenazine derivatives.

In order

to clarify the aromaticity effect on the spin coupling properties, we characterized the aromaticity of the ground states of six topic molecules and PDO by calculating the Nucleus-Independent Chemical Shifts (NICS), which were done by putting two ghost atoms Bq at the centroid (NICS(0))36 of three rings and 1 Å above the centroid (NICS(1))37 in each molecule, respectively.

Then, we calculated their magnetic shielding values, a physical

indicator equivalent to the opposite value of NICS, for which a larger positive value indicates a greater shielding of the magnetic field and a stronger aromaticity.

The NICS value is a

common measure of aromaticity or antiaromaticity, in which a negative value represents an aromatic molecule, while a positive one shows an antiaromatic molecule.

The calculations of

this parameter were operated by the GIAO method at the 6-311++G(d,p) basis set level. As for the kinetics of the proton transfer isomerization, the solvent-assisting effect was examined through choosing two solvent molecules, H2O and CH3COOH, to assist the lactam-lactim tautomerization and evaluating the activation barrier variations.

The

B3LYP/6-311++G(d,p) method was also used in the activation energy calculations with or without the help of the assisting molecules because a good agreement of the B3LYP/6-311++G(d,p) results with the high-precision CBS results was observed for tautomerization and protonation of triazoles and tetrazole.38

All the calculations were

performed on Gaussian 09.39 

RESULTS AND DISCUSSIONS According to the introduction positions of the amide/imide unit, three pairs of the

lactam/lactim structures were obtained for the targeted magnets. 7


All the energies of


Page 8 of 34

open-shell singlet (BS), closed-shell singlet (CS) and triplet (T) states of six topic molecules and PDO were calculated at the B3LYP/6-311++G(d,p) level, as well as their J values.

The

calculated results in Table 1 reveal that all the six topic molecules and PDO have low-spin singlet ground states.

In two lactam forms, 12PY-Ben and 21PY-Ben, the CS states are their

ground states, while the others possess the BS ground states with large negative J values.

In

addition, the J values estimated at the M06-2X/6-311++G(d,p) level (optimization), the B3LYP functional with the 6-311++G(3df,3pd) and aug-cc-PVDZ basis sets (only for single point calculations) also show good agreement with those at the B3LYP/6-311++G(d,p) level (Table S1 in the SI), except for the AFM

21PY-Ben

which is discussed in detail in the SI. B3LYP/6-311++G(d,p) results.

calculated at the M06-2X/6-311++G(d,p) level,

Thus, the following analyses are based on the

Moreover, as shown in Table S2 (the SI), for all AFM

magnets, the occupation numbers of LUNO calculated by the CASSCF(10,10)/6-311++G(d,p) method are in good agreement with the values of the BS state estimated by the unrestricted DFT method.

While for all DM molecules, the calculated occupation numbers of

LUNO are very small, suggesting weak diradical characters in them, which may result in unobservable BS states when applying the BS method.

Clearly, all of these discussions

further confirm the accuracy of our calculated results at the B3LYP/6-311++G(d,p) level. In addition, in the case of

12PY-Ben,

a close distance between O of nitroxide and H of

amide group may lead to the generation of the third tautomer with a hydroxylamine group and a “CON” radical.

The similar situation also exists in 21HPY’-Ben, which will be discussed in

the Supporting Information.

To clarify this, here we calculate the former molecule in which

the proton shift from N of amide group to O of nitroxide, and named it as Calculation results show a DM characteristic of

1(-h)2PY-Ben

energy barrier (9.33 kcal/mol) in the transition from

(same with

12PY-Ben

to

1(-h)2PY-Ben.

12PY-Ben),

1(-h)2PY-Ben,

and an

indicating O

atom of nitroxide is less favored to be the proton acceptor, and the magnetic switching function of the 12 series is unaffected even if the transition occurs.

Thus in the followings, we do not

discuss this structure too much for simplification. From now on, firstly, the structures of six topic molecules and their parent PDO are discussed for a preliminary cognition, and the mechanism of magnetic transformation between two tautomers in each pair is explored based on their structures. 8


The role of each single-way

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coupler of a two-way-coupler in supporting the spin coupling interaction of a magnet is studied by using the model magnet with a single-way coupler in which Ring R is replaced by a nonconductive –CH2-CH2– chain.

The NICS values of each coupler and Ring C are also

discussed to explore the aromaticity effect on the magnetic modulation.

Then, the

delocalization effect and spin density distributions are analyzed, and the spin alternation rule is also used for further clarification of the magnetic coupling characteristics.

Next, the

molecular orbitals of six topic molecules and PDO are checked for comprehensive understanding of diradical characters.

Finally, double lactam-lactim tautomerization in

two-way-couplers, different orientations of hydroxyl H in the lactim forms, as well as tautomerization mechanisms with the assistance of catalyst molecules are also investigated. In short, this work reveals a rational design of magnetic molecular switches with a tunable two-way coupler governed by the lactam-lactim tautomerization.

We hope it may provide a

new idea for the design of organic magnets and magnetic molecular switches. A) Structures, Magnetic Coupling Characteristics and Aromaticity Structural Characters and Relative Stability.

In all designed molecules, a main feature

is that two nitroxide radical units (>NO•) are linked to the ortho-position of each one of two six-membered ring couplers, forming a tricycle structure, which implies the proximity of two spin centers.

At the same time, the optimized configurations of all molecules also show

perfect planarity (Figure 1).

To quantitatively examine the changes of chemical bonds and

molecular skeleton, the bond lengths of all six topic molecules and PDO were also obtained (Table 2).

For six topic molecules, the introduction of amide/imide unit in Ring L does make

a little difference on the molecular skeleton compared with their template PDO, and the left side (C5-C6, C6-N7 and C5-N8) of Ring C is more variable than the right side because the former is adjacent to Ring L, the amide/imide-modified ring.

It can be also found that the

lactam forms are more variable than the lactim ones because the introduction of amide unit changes the bond types in Ring L and Ring C to some extent, whereas the lactim forms can remain their aromatic conjugation properties.

In addition, if an amide unit is introduced in an

aromatic ring, as mentioned above, the stability of the lactim form could be improved, making two lactam-lactim tautomers level-pegging.

Thus, it is also necessary to determine the 9



favorable configuration by theoretical calculations.

Page 10 of 34

All three pairs of molecules were

calculated for relevant energetic quantities at the B3LYP/6-311++G(d,p) level.

The

calculated results (Table 1) reveal that in the 12 and 21 series, the lactam forms are more stable than the lactim forms, while the contrary is observed in the 23 series.

Moreover, calculations

by using other methods (M06-2X optimization and B3LYP single point calculations, Table S3 in the SI) also show the identical favorable configurations and similar energy gaps, further confirming the reliability of our calculations.

It should be noted that the energy gaps between

two tautomers in each pair are small (8.81, 5.90 and 3.45 kcal/mol for the 12, 21, and 23 series, respectively), which also conforms the general characteristics of the lactam-lactim tautomerism. However, what should be noted is that this factor doesn’t completely show the possibility of tunable tautomerism, and further kinetic calculations are also needed for verification, which is discussed later. Magnetic Spin Coupling Characteristics and Molecular Structural Basis.

As for the

magnetic characteristics of the magnets, in general, the magnetic exchange coupling constant (J) can be viewed as a reflection of the spin coupling interaction between two radical units.

The

calculated results show large negative J values in three lactim forms (-2485.7 cm-1, -2692.2 cm-1, -2732.4 cm-1) and PDO (-2699.8 cm-1), suggesting that two radical units have very strong spin coupling interactions, which is also consistent with the proximity of two spin centers and perfect planarity of all these molecules that can generate great spin polarization of radicals to the couplers.

Among all of lactam forms,

23PY-Ben

has a relatively small negative J value

(-1703.6 cm-1), showing a relatively weak spin coupling interaction. 21PY-Ben,

In

12PY-Ben

and

the DM character and CS ground states are observed, which can be clearly

explained as the excessive coupling of two radical units.

That is to say, two spin single

electrons of two >NO• units are more willing to pair with each other, forming a CS state rather than to separate, forming a BS state.

Therefore, during the tautomerization from the lactim to

lactam form, in the 12 and 21 series, the spin coupling interactions are strengthened, leading to the magnetic conversion from strong AFM to DM coupling, while in the 23 series, the coupling interaction is weakened, resulting in the magnetic modulation from extremely strong AFM to moderately strong AFM.

Due to various factors that can influence the spin coupling

interactions, diverse explanations are given here to help us understand the mechanism of their 10


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magnetic spin couplings.

Herein, we firstly try to give an intuitive explanation from the bond

lengths as mentioned in the last section.

As shown in Figure 1 and Table 2, the introduction

of amide unit has a large impact on the bond types in Ring L and Ring C of all three lactam forms if we analyze their resonant Kekule structures. increases the double bond character of C5-C6 in single bond character of it in

23PY-Ben.

That is, the introduction of amide unit

12PY-Ben

and

21PY-Ben,

but increases the

Since three π electrons in the >NO• unit has great

delocalization propensity, a double-bond-like C5-C6 can effectively conjugate with two >NO• units, generating the N7-C6-C5-N8 conjugated channel.

In particular, the C6-N7 and C5-N8

bonds are extremely short, quite close to normal C=N double bonds.

These observations

indicate a very strong mediating role of C5-C6 for the spin coupling between two >NO• units which leads to very strong spin coupling (being DM) and thus a CS ground state. 23PY-Ben,

As for

all C6-N7, C5-C6 and C5-N8 bonds are considerably longer than those in the other

two cases, which undoubtedly inhibits the spin coupling between two >NO• units.

Due to

rearrangement of chemical bonds in the lactam-lactim tautomerization, some critical bond lengths are changed, resulting in the change of distance between two >NO• spin sources, and thus the coupling interaction is also changed.

In particular, for the amide-functionalized

six-membered ring coupler, it consists of an amide unit and a butadiene unit (C=C-C=C).

The

butadiene unit uses its one of the terminal double bond (C=C) parts in 12PY-Ben and 21PY-Ben or the middle single bond (C-C) part in 23PY-Ben to assist the spin coupling between two >NO• units, respectively.

As well known, the double bond character of terminal C-C is larger than

that of the middle C-C part for a butadiene molecule, and undoubtedly the mediating role of the former is larger than the latter in realizing the spin coupling, which leads to different spin coupling characteristics.

Therefore, through analyzing the critical bond lengths on the

coupling channel, we can preliminarily determine the extent of spin coupling between two >NO• spin sources and give a reasonable illustration for the magnetic spin coupling characteristics of the designed molecular magnets. Owing to particularity of the two-way coupler, it is necessary to understand different roles of two channels (sub-couplers) in supporting the spin coupling interaction between two >NO• radical units.

Besides, the function of C-C double bond and excessive coupling in the DM

molecules should be further confirmed.

Herein, we replace the Ring R coupler with a 11



Page 12 of 34

–CH2-CH2– unit (denoted by 1B), generating an insulating channel, and thus obtain three pairs of

the

lactam/lactim

23PY-1B/23HPY-1B)

tautomers

(12PY-1B/12HPY-1B,

21PY-1B/21HPY-1B,

featuring one-way conductive channel for each.

and

The ortho-phendiyl and

–CH=CH– double bond (denoted by 2B) are also further considered to replace the Ring L coupler, forming Ben-1B and 2B-1B.

That is, totally eight model molecules are examined for

the clarification of the spin coupling mechanisms.

All of their coupling constants, bond

lengths of Ring L are shown in Figure 2 and Table S4 of the SI.

Meanwhile, we also

examined the stereoisomers of these model molecules generated by –CH2-CH2– (1B), and the calculated results show a very little difference of J values (no more than 3.73 cm-1, Figure S1 in the SI) between two stereoisomers, and thus we only select one stereoisomer for investigation for each model molecule here.

Firstly, all of these model molecules have good planarity,

while none of them shows the DM coupling characteristics, reflecting the nonconductive property of 1B.

Among them 2B-1B exhibits the largest spin coupling interaction with a J

value of -2795.9 cm-1 although the 1B coupler is nonconductive, verifying that strong spin coupling occurs when choosing a double bond as a coupler. forms (12HPY-1B,

21HPY-1B

and

23HPY-1B)

The J values of all three lactim

are basically close to that of Ben-1B (from

-972.8 to -1364.6 versus -1287.9, cm-1), while for the lactam forms 12PY-1B and 21PY-1B have large |J| values (-2333.9 and -2057.4 cm-1) which are close to that of 2B-1B, and 23PY-1B has nearly unnoticeable magnetic characteristics with a J value of -181.3 cm-1.

Clearly, all of

these results confirm the analysis of magnetic transformation mechanism above. More interestingly, we also note that the J coupling constants of our designed two-way coupled AFM molecules have the additive property.

For example (Figure 2), J of 12HPY-Ben

is -2692.4 cm-1, and can be approximately viewed as the sum (-2652.5 cm-1) of J (12HPY-1B, -1364.6 cm-1) and J (Ben-1B, -1287.9 cm-1), again indicating that 1B is a nonconductive channel.

As for

12PY-Ben

and

21PY-Ben,

their DM characteristics can be explained as

excessive spin coupling interaction through two channels with strong electronic communicating ability.

Thus, the spin coupling interaction mediated by the two-way coupler can be

approximately viewed as the additive contributions of two spin coupling channels.

Clearly,

this property is helpful and practical for the understanding of overall spin coupling interaction in such two-way coupling magnets.

However, some facts such as |J (12HPY-1B)| > |J 12


Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21HPY-1B)| but |J (12HPY-Ben)| < |J (21HPY-Ben)| also show the cooperativity of two couplers in mediating the spin coupling interaction between the two >NO• spin sources in an additive way. Aromaticity.

According to the discussion of bond lengths above, we can envision that

the aromaticity of molecules should be different for different systems, which may be associated with different spin coupling phenomena.

That is, strong aromaticity produces great molecular

conjugation and average bond length, while weak aromaticity enhances single or double bond characters in the conjugated systems.

To examine possible associations of the observed spin

coupling phenomena with the aromaticity of three rings (Ring L, Ring R and the center ring, Ring C) in the designed molecules, the NICS values of the three rings in six topic molecules and PDO were calculated as shown in Table 3.

As for Ring L, PDO has the largest NICS

value (-9.18 for NICS(0) and -10.13 for NICS(1)), very close to those (-8.04, -10.22) of a normal benzene ring, while the introduction of amide/imide unit considerably weakens the aromaticity in different degrees.

In all of the lactim forms (the HPY coupler), Ring L has

large NICS negative values (-7.71 ~ -9.12 for NICS(0) and -8.24 ~ -9.57 for NICS(1)), while it has considerably small negative NICS values (-2.46 ~ -5.12 for NICS(0) and -3.06 ~ -5.17 for NICS(1)) for all lactam forms (the PY coupler).

But they all are smaller than those of a

benzene ring, indicating the aromaticity lowering in these systems. Comparing the aromaticity indices with the J values, we find that the stronger aromaticity the Ring L has, the weaker the spin coupling interaction is.

This is because HPY featuring

the lactim form is an aromatic ring (i.e. an aromatic pyridine with a hydroxyl substituent at the 2 site), and its aromatic structure has a relative weak ability of accepting spin electrons due to its stable electronic properties, and thus its mediating ability (of adjacent two C as a bridge) in realizing the spin coupling between two radical units is weakened compared with a nonaromatic C=C unit.

When the lactim form transforms to the lactam one, the NICS values

of Ring L greatly decrease (Table 3), verifying the destructive effect of the amide unit on the ring aromaticity.

The rearrangement of chemical bonds all cuts down the NICS values of

Ring L but generates two opposite situations about the coupling pathways.

One is to

strengthen the conjugation of the N7-C6-C5-N8 system with the C5’-C6’ unit to form an aromatic Ring C, which is demonstrated in 12PY-Ben and 21PY-Ben from both the analysis of 13



Page 14 of 34

bond lengths and slight increase of the NICS values of Ring C compared with those of the corresponding lactim forms (12HPY-Ben and 21HPY-Ben).

The other is to elongate the bonds

in the N7-C6-C5-N8 system and weaken the conjugation, as reflected by small NICS values of Ring C in 23PY-Ben.

In addition, we find that the bond lengths of Ring R in six molecules

are nearly the same, but their NICS values are slightly different, and are higher than those (-8.04/NICS(0), -10.22/NICS(1)) of normal benzene ring, indicating that the aromaticity of them except for 23PY-Ben.

This can be explained by different increases of circulation electron

densities that two radicals contribute in Ring R of these molecules.

Delocalized electrons

from nitrogen atoms participate in the circulation of Ring R, generating a strong circulation and large NICS negative values.

Thus, the NICS value can reflect the delocalization degree of

spin electrons from two >NO•, and thus is directly associated with the spin coupling interaction. Our calculated results show that the NICS values of Ring R also reflect the J couplings in all six molecules, which suggests that besides the auxiliary coupling the benzene ring (i.e. Ring R) also has the function of coupling detector. B) Delocalization Effect and Spin Density Distributions In fact, the spin coupling interaction between two >NO• radical units is closely associated with the π-conjugation-based spin delocalization of unpaired single electrons.

That is, if two

radical units are connected with extensively conjugated structures, the spin distributions are more likely to move from the >NO• spin centers to the couplers.

The powerful delocalization

effect can lead to a strong spin polarization, which is helpful to enhance the spin coupling of two >NO• radicals.40,41

In other words, the delocalization effect can be regarded as expanded

distributions of two >NO• radicals.

For the multiple π-electron radical groups such as >NO•,

this is particularly noticeable when they are connected with the π-conjugated structures because of the excellent conductivity of the latter.

To have a better understanding of the spin coupling

characteristics, the spin delocalization effect as well as spin density distributions are analyzed below.

The spin density maps of PDO and four AFM magnets of six topic molecules are

illustrated in Figure 3 and S4 (in the SI), respectively.

They reveal that the spin density

distributions are uniform and well matched, and correspond to strong magnetic spin coupling interactions.

In order to describe it more quantitatively, the Mulliken atomic spin densities are 14


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


also displayed in Figure S5 in the SI.

Taking

12HPY-Ben

as an example, the results reveal

that atomic spin density distributions of two >NO• units are 0.279 and -0.270, respectively.

In

other words, only 27.9% and 27.0% spin densities are localized at two >NO• groups, indicating large spin polarization or delocalization.

While for

23PY-Ben,

58.1% and 55.7% spin

densities are localized at two >NO• groups, quite larger than those in other three molecules, suggesting weak delocalization effect and weak coupling interactions.

However, in the DM

molecules, the CS ground state suggests the maximum spin delocalization and full spin coupling (i.e. spin paired) of two >NO• radicals, and thus the spin density distributions can’t be obtained.

Clearly, from atomic spin density distributions, the spin delocalization effect of the

>NO• radicals can be quantitatively evaluated, and the strength of spin coupling can be judged more accurately. In addition, as a widely used method to predict the ground state of a molecule and sign of J conveniently, the spin alternation rule is also used here for further examination.

It indicates

that the sign of J depends on the number of bonds in the spin-interacting channel through the coupler.

The odd number denotes the AFM spin coupling and a negative J value, while the

even number denotes the FM spin coupling and a positive J value.

As an expansion, if there is

the atom that provides two π-electrons for conjugation in the coupler, such as N, O, S atoms, the amount of these atoms should be also counted because each one of these atoms is equivalent to a chemical bond.

The spin alternation rule is convenient and accurate, especially

for judging the magnetic spin coupling properties of a conjugated molecule.42

In each of our

designed molecules, two >NO• radical groups can be viewed as para-located on Ring C or ortho-located on Ring L and Ring R, and the numbers of chemical bonds for any channels are odd, suggesting an AFM spin coupling characteristics, as shown in Figure 3.

The same

conclusions can be obtained when choosing Ring L and R as the spin-interacting channels for analyses.

While in all DM spin coupling molecules, strong spin coupling interactions make

the molecules lose their radical characters, and thus the spin alternation rule is not applicable. C) Molecular Orbital Analysis Analyses of molecular orbitals can help us to understand the diradical characters of the systems, and make preliminary judgments on their ground states. 15


It is believed that the


Page 16 of 34

ground state of a molecule can be easily inferred by the method of graphic analysis or numerical analysis of frontier molecular orbitals.

The former is to judge whether two SOMOs

in the BS state are disjoint or not, and then to predict the magnetic spin coupling characteristics based on the SOMOs.

The latter is developed by analyzing the HOMO-LUMO energy gap in

the CS state and the SOMO-SOMO energy gap in the T state in turn to make a right judgement. The relevant calculated results are listed in Table 4. SOMO Effect.

Borden and Davidson43 pointed out that if two SOMOs of a molecule are

nondisjoint, a triplet is stable and is likely to be the ground state, while disjoint shape in two SOMOs would make the singlet be the ground state, which can be explained by the mutual interaction of two SOMOs.

The nondisjoint shapes of SOMOs enhance the repulsion between

two electrons because of the extended overlap region of their original orbitals, and thus spin parallel orientation is favorable, leading to a stable triplet with a large singlet and triplet energy gap.44

While in the situation of disjoint character of SOMOs, the overlap of original orbitals

occupied by two electrons is small, and the orbital splitting energy exceeds the electron pairing energy, contributing to the nearly degenerate ground states.

In Figure 3 and S4 (in the SI), the

types of SOMOs for all AFM molecules are disjoint (atoms are not common), bringing about BS ground states.

Besides, two SOMOs in 23PY-Ben show a little nondisjoint character at an

atomic site (Figure S4 in the SI), which is in good agreement with the fact that 23PY-Ben has relatively weak AFM spin coupling compared with three lactim forms.

Clearly, our calculated

J results are in good agreement with the above analyses of the SOMO effect. HOMO-LUMO Energy Gaps and SOMO-SOMO Energy Level Splitting.

As shown in

Table 4, the HOMO-LUMO energy gaps of the DM molecules (12PY-Ben and 21PY-Ben) are 2.71 eV and 2.68 eV, respectively, larger than other five AFM diradicals (from 1.86 to 2.43 eV), suggesting that a large HOMO-LUMO energy gap is not conducive to the HOMO-LUMO electron transition and could produce a close-shell ground state, while in four AFM diradicals the electrons in HOMO are more easily excited to LUMO, forming a low-lying triplet excited state with a small SOMO-SOMO energy gap, and further CS-T configuration interaction generates an open-shell ground state with two antiparallel spin electrons in two SOMO. Clearly, for a molecule with an open-shell ground state, the SOMO-SOMO energy level splitting is useful to predict the spin multiplicity of ground state. 16


Hoffmann et al.45 believed

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that a triplet ground state exists if two successive SOMOs have a small energy gap (usually called ΔESS) less than 1.5 eV, while Constantinides et al.44 suggested that molecules that has a ΔESS value more than 1.3 eV may result in a BS ground state based on the research of some linear and angular polyheteroacene compounds.

Actually, it is hard to find a critical value of

ΔESS because it varies from system to system, which has been proved by Zhang et al.46.

Table

4 show that all the molecules except for 23PY-Ben have large ΔESS values (1.49~2.11, eV), and thus a singlet state is more favorable to be the ground state.

As for 23PY-Ben, relatively small

ΔESS (1.04 eV) indicates a relatively stable triplet state compared with other unstable triplets of strong AFM magnets, but a BS state is still stable enough to be the ground state.

Meanwhile,

our research group41 also designed several molecules with the >NO• groups previously, getting two AFM molecules with the spin coupling constants of -1645.4 cm-1 and -1550.9 cm-1 and ΔESS values of 1.06 eV and 0.92 eV, respectively.

This observation could suggest a new

criterion for judging a triplet ground state molecule in the nitroxide systems instead of the previous critical value (1.5 eV). D) Double Lactam-Lactim Tautomerism Diradicals To further clarify the function of these two-way couplers, we also extend our work through introducing two amide units into two ortho-phendiyl couplers of PDO, forming the two-way tunable magnets and their tautomerization counterparts. spin coupling properties and controllability.

We further examine their

The considered systems are HPY-HPY,

HPY-PY, and PY-PY model molecules, and only the configurations with C2v and C2h symmetries and their proton transfer tautomers are examined.

For example, for the 12 series

(the series numbers are about Ring L), the C2v-symmetric 12PY-4’3’PY

12PY-1’2’PY,

C2h-symmetric

and their single and double lactam-lactim tautomerization products, 12HPY-1’2’PY,

12HPY-4’3’PY

and 12HPY-1’2’HPY, 12HPY-4’3’HPY, are considered.

Results (Figure S2 in the

SI) indicate that all C2v-symmetric HPY-HPY have strong AFM spin couplings, while the corresponding C2v-symmetric HPY-PY and PY-PY are DM in the 12 and 21 series and relatively weak AFM spin couplings in the 23 series.

That is, proton transfer isomerization in

anyone of two lactim rings in the C2v-symmetric HPY-HPY can tune the magnetic spin coupling magnitude or switch the characteristics.

This observation is easy to be understood

17



Page 18 of 34

because the generation of the PY ring can greatly enhance the spin coupling interaction and so that the molecular magnetism disappears.

However, the spin coupling constants in the 23

series noticeably decrease along with the increase of the generated PY rings (i.e. -2327.3, -1539.9 and -673.0 cm-1 for

23HPY-2’3’HPY, 23HPY-2’3’PY

23PY-2’3’PY,

and

respectively),

satisfying such a relationship: J23HPY-2’3’PY = (J23HPY-2’3’HPY + J23PY-2’3’PY)/2, the additive property of the J values.

Similar variation regularity of the J values are observed for the

C2h-symmetric case, and the additive property of J also exists in the 23 series, further confirming the universality of an empirical rule. E) Orientation Effect of Hydroxyl Hydrogen Actually, each lactim form has an orientation isomer of hydroxyl H.

In general, the

barrier of conformational transition is smaller than that of tautomerization, and thus it’s necessary to examine the H-orientation effect on the spin coupling properties.

The calculated

results of three H-orientation isomers in the lactim forms (Figure 4) indicate that H-orientation 12HPY-Ben/12HPY’-Ben

effect slightly cuts down the spin couplings in the 12 series (i.e.

in

this section, the same below) by 89.5 cm-1 and improves the spin couplings in the 23 series by 84.2 cm-1 but with a magnetic interconversion between AFM 21HPY-Ben and DM 21HPY’-Ben. This indicates that the H-orientation effect might have a remarkable influence on the spin coupling interactions, and can even be used as a new chemical tuning method for magnetic modulation or switching.

To look into the origin of H-orientation effect, we then replace the

N atom of the imide unit with a C atom to exclude the interference of N atom on the system, totally forming four model molecules as the Phe-Ben type.

The diagram as well as the

corresponding calculated results are put in Figure S3 (in the SI).

The results of Phe-Ben

model molecules reveal the similar difference of |J| values (81.3 cm-1) between 2Phe-Ben (relatively strong AFM, N-removed product of

12HPY-Ben

(relatively weak AFM, N-removed product of

12HPY’-Ben

and and

23HPY’-Ben)

and 2Phe’-Ben

23HPY-Ben)

compared with

the 12 and 23 series, and also a magnetic interconversion between DM 1Phe-Ben and AFM 1Phe’-Ben

(N-removed product of

21HPY’-Ben

and

21HPY-Ben,

respectively).

The above

indicates that orientation of hydroxyl H can directly lead to magnetic modulation, and the N-doping only affects the magnetic properties of all the N-doped molecules. 18


Clearly, these

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observations further illustrate the additive property of the magnetic spin coupling constants contributed from the H-orientation effect of hydroxyl H and the N-doping effect of the coupler ring. On the other hand, further examination of atomic NBO charge distributions and bond lengths of these pairs of orientation isomers (Figure S6 in the SI) reveals that the magnetic interconversion should originate from the charge transfer (i.e. spin delocalization effect) governed by intramolecular H-bonding.

The intramolecular H-bonding between the –OH and

>NO• not only stabilizes the system, but also causes the charge redistribution of the >NO• unit and, as a result, distorts the local molecular structure and changes the spin coupling mode.

In

fact, there exist two resonant structures (>N:-O•  >N•+-O:-) of a >NO• radical unit but with a preference to the former.

Due to larger electron-attracting ability of O than N, the latter can

take a certain proportion, as evidenced by the calculated charge distributions of relevant systems (Figure S6 in the SI), which can be also viewed as the spin delocalization.

In addition,

since the latter resonance structure is closer to ionic state, the calculated N-O bond length should be longer (Figure S7 in the SI), and the intramolecular H-bonding further increases the possibility or proportion of the ionic state.

For example, for

21HPY’-Ben

(intramolecular

H-bonded mode), the formation of intramolecular H-bonding increases the negative charge on the O9 atom (from -0.448 to -0.549), thus considerably elongating the N7-O9 bond by 0.018 Å compared with 21HPY-Ben (no intramolecular H-bond).

As a result, such an intramolecular

H-bonding greatly increases the spin coupling interaction and converts the molecule as a DM one.

The same is true in the 1Phe-Ben/1Phe’-Ben orientation isomers.

the hydroxyl H of

23HPY’-Ben

As for the 23 series,

repels the α-H atom on the C4 site and forces it to produce

weak H-bond like C4-HO10-N8 interaction as evidenced by the H(C4)...O10 distance change from 2.447 Å in 23HPY-Ben to 2.415 Å in 23HPY’-Ben (Figure S7 in the SI) which is in favor of the charge redistribution in the >NO• group (NBO charge of -0.492 for O10 of 23HPY-Ben versus -0.509 for that of

23HPY’-Ben,

Figure S6 in the SI).

While in the 12 series, the

hydroxyl H of 12HPY-Ben attracts the lone pair electrons on the N1 site, and thus the smaller repulsion between O9 and electrons of N1 is also favorable to the same charge redistribution (NBO charge of -0.452 for O9 of 12HPY’-Ben versus -0.473 for that of 12HPY-Ben).

Since

these two kinds of interaction are far weaker than the H-bonding, the magnetic coupling 19



constant |J| just has a slight increase.

Page 20 of 34

Clearly, all of the analyses can be also verified by the

calculated results of their corresponding N-removed products, i.e. four Phe-Ben model molecules. F) Tautomerization and Molecular Catalysis Mechanism Actually, the lactam-lactim tautomerization can be easily realized through molecular assisting although the calculated results show that tautomerization has a slightly large barrier of direct single proton transfer (45.94/37.13, 40.10/34.20 and 34.89/38.35 kcal/mol for the forward/backward reaction activation barriers of the reactions from PY to HPY in the 12, 21 and 23 series, respectively, Figure S8 in the SI).

The molecule assisted tautomerization can be

viewed as a concerted double proton transfer process in which the assisting molecule has two H-bonding site (one accepter and one donor) to generate two H-bonds with a tautomer, and tautomerism may occur by exchanging two H atoms.27

To realize efficient lactam-lactim

tautomerization for the regulation of magnetic spin coupling properties, two molecules for assisting double proton transfer are considered here, i.e. H2O and CH3COOH.

All the lactam

and lactim forms supported by these two catalyst molecules are calculated at the B3LYP/6-311++G(d,p) level.

Calculated results in Figure 5(a) show that with the catalysis of

H2O and CH3COOH, the lactam-lactim energy gaps are reduced in the 12 and 23 series, and nearly remain unchanged in the 21 series.

As for the spin coupling constants, all the designed

molecules maintain their magnetic characteristics except for

12HPY-Ben

which becomes DM

mainly because the H-bond generates the same situation of excessive spin coupling with the H-orientation effect.

Figure 5(b) shows the J values of other three AFM molecules without or

after combining with those assisting molecules, which points out that different auxiliary molecules make same functions but different contributions.

In general, the AFM spin

couplings of 21HPY-Ben and 23PY-Ben are strengthened (by 15.5/33.7 cm-1 of 21HPY-Ben and 106.2/163.5 cm-1 of

23PY-Ben

with the assisting of H2O/CH3COOH, the same below), while

that of 23HPY-Ben is weakened (by 63.1/121.2 cm-1) for two catalyst molecules.

To further

explore the thermodynamic properties of tautomerization, we take the 23 series as examples to calculate the activation barriers of the lactam-lactim transformation.

As shown in Figure S8

(in the SI), the catalyzed forward/backward reaction activation barriers 13.67/15.30 (by H2O) 20


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or 6.34/6.54 (by CH3COOH) kcal/mol for the lactam-to-lactim processes, indicating that the catalyzed tautomerization is a low barrier process and is solvent-controlled. According to all aspects above, all three pairs of tautomers are reasonable to show magnetic modulation or switching phenomena during the process of lactam-lactim tautomerism, which are also confirmed by the calculation results.

In addition, all AFM molecules haves

large |J| values, suggesting a good stability in liquid or solid system,47 while in all DM molecules, the close-shell ground states also exhibit their inactive properties.

That is to say,

our designed molecular magnets have good practical application prospects in the designing of organic magnetic molecule switches.

For example, the detection of radical response signal

can be transformed as “ON” signal (such as

12PY-Ben

response signal can be converted to “OFF” signal (like

and

21PY-Ben),

12HPY-Ben

and

while no radical

21HPY-Ben).

The

strong-weak radical response signal can also be translated as “ON-OFF” switches, which provides a possible application avenue in magnetic-modulation process (23 series).

Therefore,

an application of these three pairs of tautomers in the field of magnetic information storing or switch controlling might be acceptable.

Moreover, recent studies48,49 have shown a possible

method to produce high-spin coupling molecular magnets with triplet ground state by introducing halide ions, which may expand the applications of our designs to low-to-high spin transformation (see Supporting Information in detail).

Of course, apart from all of the

discussions above, the specific way to make full use of them needs more in-depth studies.



CONCLUSIONS Using computational approach we designed three pairs of lactam-lactim tautomers with

AFM or DM spin coupling properties based on the experimentally synthesized phenazine-N,N’-dioxide (PDO), and their applications for magnetic molecular switches could be achieved by controlling the lactam-lactim tautomerism.

From the perspective of structural

characteristics, because of the good planarity, noticeable π-conjugated features and proximity of two radical groups, all of them have an extremely strong coupling interaction.

All the

lactim forms have large AFM spin coupling constants (J = -2485.7 cm-1, -2692.4 cm-1 and 21



Page 22 of 34

-2732.4 cm-1, respectively, similar to their parent PDO (J = -2699.8 cm-1) and have open-shell singlet ground states.

However, in the lactam structures, owing to the destruction of

aromaticity and rearrangement of chemical bonds in Ring L caused by introduction of the amide/imide unit, two of them (12PY-Ben and

21PY-Ben)

exhibit excessive spin couplings

between two radical groups and thus have the CS ground states.

The other one (23PY-Ben)

exhibits a weakened spin coupling with the J value of -1703.6 cm-1.

Together with the

examination on the “Ring L-1B” model molecules, we find that the spin coupling constants J of all AFM molecules present additive property.

Further the NICS calculations verify the spin

coupling changes and magnetic modulation mechanism and also indicate that Ring R has a function of coupling detector.

In addition, spin density distributions and molecular orbital

analyses also confirm the magnetic characteristics of these molecules and magnetic modulation or switching.

However, studies on double lactam-lactim tautomerism and H orientation effect

also show two application situations but which need further exploration.

Examination of

tautomerization assisted by H2O and CH3COOH catalyst molecules validates the accessibility and practical application prospects of modulating tautomerization and tuning magnetic properties.

Therefore, we hope this work can provide new strategy for the design of organic

magnetic molecular switches or magnets with switchable two-way couplers, and expect our designed molecules could be verified by experimental studies and applied in the future.



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS

This work is supported by NSFC (21873056, 21773137 and 21573128), and the calculations in this work were carried out on the HPC Cloud Platform of Shandong University. 22


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




ASSOCIATED CONTENT

(S) Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.******. The calculated relevant data for relevant molecules including energies, magnetic properties, magnetic coupling constants and diradical characters, critical bond lengths and NICS values, SOMO plots, spin density maps and distributions of Mulliken atomic spin densities for all antiferromagnetic diradicals of topic molecules, atomic NBO charge distributions, optimized molecular geometries of lactam-lactim tautomers and transition states of tautomerization as well as calculated activation energies with or without the assisting of catalyst molecules H2O and CH3COOH, explanation and expansion of an abnormal calculated result for some molecules when using the M06-2X functional and counterion effect of the magnets (PDF).



REFERENCES

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Page 28 of 34

Magnetic Exchange Phenomenon. J. Phys. Chem. Lett. 2011, 2, 939-943.

Table 1. The Calculated Energies (E, a.u.) of the Closed-Shell Singlet (CS), Broken-Symmetry Open-Shell Singlet (BS), Triplet (T) States as Well as the Corresponding Values and Magnetic Coupling Constants (J, cm-1) of All Six Topic Molecules and PDO at the B3LYP/6-311++G(d,p) Level Molecules

E(CS)/a.u.

E(BS) /a.u. ()

E(T) /a.u. ()

J

12PY-Ben

-813.4416291

-813.4416291 (0.000)

-813.4049145 (2.021)

─

12HPY-Ben

-813.4274699

-813.4275930 (0.143)

-813.4046452 (2.013)

-2692.4

21PY-Ben

-813.4250895

-813.4250895 (0.000)

-813.3932503 (2.011)

─

21HPY-Ben

-813.4154475

-813.4156948 (0.198)

-813.3931174 (2.011)

-2732.4

23PY-Ben

-813.4156347

-813.4189572 (0.637)

-813.4082283 (2.019)

-1703.6

23HPY-Ben

-813.4239561

-813.4244587 (0.278)

-813.4048133 (2.013)

-2485.7

PDO

-722.1283283

-722.1285098 (0.172)

-722.1058724 (2.012)

-2699.8

28


Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Some of the Optimized Bond Lengths (in Å) of the Ground States of All Six Topic Molecules and PDO at the B3LYP/6-311++G(d,p) Level Molecules N7-O9 N7-C6

C5-C6

C5-N8 N7-C6’ C5’-C6’ C5’-N8 N8-O10

12PY-Ben

1.292

1.353

1.408

1.365

1.390

1.416

1.399

1.273

21PY-Ben

1.261

1.371

1.424

1.365

1.409

1.409

1.391

1.284

23PY-Ben

1.277

1.398

1.436

1.405

1.387

1.420

1.393

1.275

1.271

1.387

1.418

1.377

1.394

1.418

1.387

1.279

1.268

1.386

1.422

1.380

1.398

1.414

1.386

1.280

1.275

1.385

1.416

1.386

1.388

1.420

1.388

1.278

1.278

1.385

1.419

1.385

1.385

1.419

1.385

1.278

12HPY-Be

n 21HPY-Be n 23HPY-Be n PDO

Table 3. The Nucleus-Independent Chemical Shifts (NICS) at the Center or at a Point 1 Å above Ring L, Ring R and Ring C of the Ground States of All Six Molecules and PDO Estimated at the B3LYP/6-311++G(d,p) Level Molecules

Ring L

Ring R

Ring C

NICS(0)

NICS(1)

NICS(0)

NICS(1)

NICS(0)

NICS(1)

12PY-Ben

-2.46

-3.70

-9.75

-10.63

-6.46

-7.00

12HPY-Ben

-7.85

-8.65

-9.20

-10.22

-5.79

-6.93

21PY-Ben

-2.48

-3.06

-9.86

-10.75

-5.37

-6.29

21HPY-Ben

-7.71

-8.24

-9.25

-10.23

-4.96

-6.15

23PY-Ben

-5.11

-5.17

-6.86

-8.07

-0.21

-2.20

23HPY-Ben

-9.12

-9.57

-8.63

-9.70

-4.68

-5.97

PDO

-9.18

-10.13

-9.18

-10.13

-5.80

-6.99

29



Page 30 of 34

Table 4. Energy Levels (1ES, 2ES, a.u.) of Two SOMOs for the Triplet States, Their Energy Gaps (∆ESS, eV) and HOMO-LUMO Energy Gaps (∆EHL, eV) for the Closed-shell States of All Six Topic Molecules and PDO Calculated at the B3LYP/6-311++G(d,p) Level Molecules (AFM or DM) 12PY-Ben (DM) 12HPY-Ben (AFM) 21PY-Ben (DM) 21HPY-Ben (AFM) 23PY-Ben (AFM) 23HPY-Ben (AFM) PDO (AFM)

1Es

2Es

ΔESS

∆EHL

-0.26902 -0.25255 -0.24888 -0.24679 -0.24078 -0.24981 -0.24393

-0.19160 -0.18777 -0.18485 -0.19006 -0.20240 -0.19513 -0.18527

2.11 1.76 1.74 1.54 1.04 1.49 1.60

2.71 2.42 2.68 2.43 1.86 2.30 2.42

Spin Sources H

O

O

N Ring L Ring C Ring R

N

12HPY-Ben

H

N

O

O

21HPY-Ben

O9 2 3 4

N7 6 6’ 5 5’ 8N 10O

N O

H N

O

N

N O

HN

21 series

O

1’ 2’ 3’ 4’

PDO

H

O

HN

N

23HPY-Ben

O

12PY-Ben

O N N O

21PY-Ben

O

N

N

N

O

O

O

O

N

12 series

N

N

Couplers 1

Lactam

Lactim

O

O

N N

23 series

O 23PY-Ben

Scheme 1. The schematic diagram of spin sources and two-way couplers, the structure of the experimentally synthesized phenazine-N,N’-dioxide (PDO), as well as the serial numbers and notations of all three pairs of tautomers (six topic molecules) designed by introducing amide/imide unit into Ring L of PDO at different positions.

Here HPY, PY and Ben denote

2-hydroxypyridine, 2-pyridone, and benzene ring units, respectively.

30


Page 31 of 34

Series

Lactam

Lactim

Resonant structure of lactam O

H N

O

H N

O

N

12

O N

N

N

O

O

Strengthen coupling O

O

21

O

N

HN

23

N

N

O

O

O

N

O

O N

HN

O HN

N

HN O

N O

N O

Weaken coupling

Figure 1. The optimized geometries of all six topic molecules and the resonant structures of three lactam forms.

The red boxes in resonant structures reflect the extent of conjugation

between the coupler and two nitroxide units which are derived from the calculated bond lengths.

A 1

-3000

2

-2333.9

-2000 -1500 2000 -1000 1500 -500 1000 0 500 500 0 1000 -500 1500 -1000 2000 -1500 -2000 -2500

3

4

5

6

7

8

-2795.9

-2500

J (cmJ-1)(cm-1J) (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ring I-1B series

-2057.4 -1364.6

-1319.3

-1287.9 -972.8 -181.3

2B-1B

12PY-1B

21PY-1B 12HPY-1B 21HPY-1B

Ben-1B

12PY-Ben 21PY-Ben 12HPY-Ben 21HPY-Ben

DM

PDO

23HPY-1B 23PY-1B 23HPY-Ben 23PY-Ben

DM

-1703.6

Ring I-Ring II series -2692.4

-3000 1

2

3

4

-2732.4

-2699.2

5

6

-2485.7 7

8

A

Figure 2. The calculated J values of all “Ring L-1B” type model molecules, as well as mainly designed six topic molecules and PDO for analyses.

The additive properties of J can be

obtained by comparing J(Ring L-Ring R) with the sum of J(Ring L-1B) and J(Ring R-1B). diamagnetism, reflecting an excessive coupling of two spin centers.

31


DM denotes


Page 32 of 34

O N N O Spin Alternation

Spin Density Map

SOMO(𝜶)

SOMO(𝜷)

Figure 3. Scheme of spin alternation, SOMOs (isovalue = 0.02) and spin density maps (isovalue = 0.0004) for the BS state of PDO calculated at the B3LYP/6-311++G(d,p) level and that of four AFM magnets of six topic molecules are displayed in Figure S4 (in the SI).

21HPY’-Ben

21HPY-Ben

21

1Phe’-Ben

DM

J=-2647.9cm-1

J=-2763.8cm-1

J=-2682.5cm-1

2Phe-Ben

2Phe’-Ben

Replace N with C J=-2732.4cm-1

DM 12HPY-Ben

1Phe-Ben

12

12HPY’-Ben

J=-2602.9cm-1 Replace

J=-2692.4cm-1

N with C 23 J=-2570.0cm-1

J=-2485.7cm-1

23HPY’-Ben

23HPY-Ben

Figure 4. Diagram of the hydroxyl H orientation effect and the generation of “Phe-Ben” type model molecules by replacing N atom of imide group (marked in blue circle) with C atom. Yellow circle denotes H orientation and accompanied H-bonding or H-bonding-like interaction in the stronger coupling one of two orientation isomers, while the opposite orientation of H in weaker one.

The optimized geometries and coupling constants J are determined at the

B3LYP/6-311++G(d,p) level.

32


Page 33 of 34

origin origin H2O H2O CH3COOH CH3COOH

-2700

Gaps (kcal/mol)

J (cm-1) origin H2O CH3COOH

-2700 3.10 -2700

-2400

3.1

-2400 0.00 -2400

origin

0.0

-2100

origin

origin

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2100 -3.1 -3.10 -2100

-1800 -6.2 -1800 -6.20

-1800

1

2

-150021 series 12 series A

(a)

3

-9.3 -1500 -1500 -9.30

23 series 1

1

21HPY-Ben

2

3

2

23PY-Ben A

A

3 23HPY-Ben

(b)

Figure 5. (a) The energy gaps between each pair of two lactam-lactim tautomers (Gaps = E(PY)-E(HPY), kcal/mol) for the 12, 21 and 23 series before (marked origin in legend) and after catalyst combination. 12HPY-Ben

(b) The J values for three AFM magnets (calculations show a DM

after catalyst combination) of six topic molecules before (marked origin in legend)

and after catalyst combination.

Specific values and optimized geometries are given in Figure

S8 (in the SI).

33



Page 34 of 34

Table of Contents Graphic

Lactim Forms

Tautomerization

Lactam Forms

Switching J = -2692.4cm-1

J = -2732.4cm-1

Magnetic Transformation

DM

DM

Modulation J = -1703.6cm-1

J = -2485.7cm-1

Tunable coupling

Two-way Couplers

Auxiliary coupling

34


Rational Design of Organic Magnets with Switchable Two-Way

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