Core-Modified Porphyrin Diradicals with a C C Unit: Redox-Driven

Article

Subscriber access provided by RYERSON UNIVERSITY

Core-Modified Porphyrin Diradicals with a C=C Unit: Redox-Driven Magnetic Switching Meiyu Song, Xinyu Song, and Yuxiang Bu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07326 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

The Journal of Physical Chemistry

Core-Modified Porphyrin Diradicals with a C=C Unit: Redox-Driven Magnetic Switching Meiyu Song †, 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:

We explore the intramolecular spin interactions of the core-modified

porphyrin diradicals with a C=C unit (R-(C=C) and R-(C=C)2+) featuring (C=C)porphyrin and (C=C)porphyrin2+ as the couplers and verdazyl, nitronyl nitroxide and imino nitroxide as spin sources (R) at the B3LYP/6-31G(d) level and the C=C effect through comparison with the porphine-coupled diradicals (R-(Null)).

Structurally, modifications of porphine through

introducing radical groups to the edge sites and a C=C unit to its core lead to a nonplanar diradical structure featuring a curved (C=C)porphyrin coupler and twist linkages of radical groups.

Although such nonplanar structures seem unfavorable to the spin coupling between

spin sources, our results suggest that the core modification with a C=C unit noticeably enhances the spin couplings in R-(C=C) and R-(C=C)2+ compared with R-(Null) with a planar porphine coupler, and R-(C=C) possess mild ferromagnetic couplings but R-(C=C)2+ present strong antiferromagnetic ones, indicating that two-electrons redox can switch the magnetisms. The differences in the magnetic properties and coupling magnitudes should be attributed to distinctly different spin-interacting pathways among R-(Null), R-(C=C) and R-(C=C)2+. Besides, the energies of the lowest unoccupied molecular orbitals of the couplers regulate the magnetic couplings, and the linking modes of the radical groups to the couplers also affect the magnetic coupling strengths especially for R-(C=C)2+.

The observed magnetic coupling

regularities are reasonably analyzed by the modified spin alternation rule.

This work provides

a promising strategy for rational designs of the porphyrin-based diradicaloids and new insights into the spin interaction mechanisms in such diradicaloids which are useful bases for further applications in the future.

1

ACS Paragon Plus Environment


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

Page 2 of 32

INTRODUCTION Porphyrins are macrocyclic 18π-conjugated molecules that have been actively studied in

various fields owing to their potential for diverse applications, and especially the transition metal centered metalloporphyrins with localized 3d states are one of the hugely explored systems.1-9 Recent experimental and theoretical studies have already revealed a number of fascinating electronic and spintronic properties.1,2,4,6,9-11 To our knowledge, the transition metals in metalloporphyrins usually serve as the spin sources in most spintronic or magnetic studies.2,9,12 In spintronics, there are many ways to control the spin states of magnetic materials such as light irradiation,13 temperature control,14 molecular adsorption,3,6 substrate interaction,4 etc.

However, it may be an efficient and direct way to design novel magnetic porphyrin

derivatives with ferromagnetism (FM) or antiferromagnetism (AFM) by introducing radical groups to porphyrins or their derivatives.

Usually, the efficient coupler and stable radicals are

indispensable in organic magnetic molecules.

Thus, the development of appropriate couplers

which can enhance the magnetic coupling interactions between the radicals and tune or switch the magnetic properties in a single organic molecule magnet would contribute to opening up a promising area of the magnetic materials.15 On the other hand, the intramolecular magnetic coupling constant as well as intermolecular interaction that depends on the structure and feature of a molecular crystal control the magnetic properties of a molecular magnetic material, and thus an estimate of the intramolecular exchange coupling constant is necessary prior to designing and synthesizing a successful magnetic material based on organic diradicals.

As

known, various porphyrin derivatives are attractive building blocks for magnetic complexes because of their extended π conjugation, their ability to bind a wide variety of metals or other non-metallic groups, and the ease with which they can be functionalized.

Undoubtedly, it has

been a formidable and meaningful task to find and design appropriate porphyrin-based diradicaloids through the comprehensive theoretical calculations and predictions. Porphyrin complexes of nearly every metal in the periodic table have been prepared over the past decades.16

Further, some transition metal porphyrins with magnetic or radical

properties have been widely studied by researchers.2,5,17,18 However, these porphyrin complexes usually exhibit very weak magnetic coupling characteristics.17

2


For example, Eaton et al.

Page 3 of 32

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


prepared and studied a face-to-face coplanar bis-CuII diporphyrin with a short center-to-center distance of around 4.1 Å, finding that the |J| value of it is only about 0.5 cm-1.18

Long-range

antiferromagnetic coupling in directly linked copper(II) and silver(II) diporphyrins was explored by Ikeue and co-workers, but the magnetic coupling constants (|J| < 4 cm-1) of these compounds are not large.2

In addition, the spin-spin interactions in a complex consisting of a

metalloporphyrin with a verdazyl (VER) radical group attached at one of the β positions of the porphyrin ring were investigated by Poddutoori et al., but magnetic susceptibility and continuous-wave electron paramagnetic resonance data suggest that in the ground state the magnetic coupling between VER and metal spins is weak ( CS > BS > T and that

of others is CS > Q > BS > T, while for R-(C=C)2+, the energy order of VER-(C=C)-ββ2+ is Q > CS > T > BS and that of others is CS > Q >T > BS.

That is, the BS and T states are always

energetically the two lowest spin states and neither the CS nor Q state is the most stable one in all R-(C=C) and R-(C=C)2+.

R-(C=C) possess the T ground states while R-(C=C)2+ possess

the BS ground states, and their only two unpaired electrons really come from the introduced radical groups. Generally, since the calculated results may be improved and even become more accurate by adding more polarization and diffuse functions, single-point calculations were also performed with a slightly large 6-311+G(d,p) basis set to obtain more accurate values.33,34

In

fact, our examination indicate that the basis set dependence of J is not critical to the trend and intensity of the magnetic interactions (Table S5 in the SI).

Thus, in the present work, we

calculated the J values only using a cheap and mild basis set (6-31G(d)) by considering the 7



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

energies at the full optimized geometries.

Page 8 of 32

The main results are analyzed in detail below.

Magnetic Coupling Characteristics.

As a starting, taking the parent porphine and

nonmagnetic metalloporphyrins ((Null)porphine, (Mg)porphyrin and (Zn)porphyrin) as the couplers, we first examine the magnetic coupling properties of their corresponding diradicals (R-(Null), R-(Mg) and R-(Zn)).

As predicted, such parent porphine-, (Mg)porphyrin-, and

(Zn)porphyrin-based diradicals have very weak magnetic characteristics or even do not present the magnetic coupling interactions (Tables S1 and S2 in the SI).

However, R-(C=C) and

R-(C=C)2+ are magnetic (FM for the former, and AFM for the latter) and their magnetic coupling strengths are much stronger than the R-(Null), R-(Mg) and R-(Zn) diradicals. Meanwhile, compared with R-(C=C), the |J| values of R-(C=C)2+ are considerably larger. Clearly, these observations indicate that the magnetic coupling strengths are considerably enhanced by introducing a C=C unit to the center of the parent porphine-based diradicals or by replacing the center metal (Mg or Zn) by a C=C unit but for the IN-(C=C) diradical in which the improvement of the magnetic coupling is not obvious.

In particular, the magnetic

coupling strengths of the R-(C=C)2+ series become much stronger than those of the R-(C=C) series, indicating that, more importantly, two-electrons redox can realize the magnetic conversions of the R-(C=C)/R-(C=C)2+ magnetic systems between FM and AFM. The calculated J values of each set of diradicals are listed in Table 1.

Interestingly, the

same trends in the J values through the (C=C)porphyrin or (C=C)porphyrin2+ couplers are observed for different spin sources (e.g. VER, NN and IN radical groups).

Figure 1 represents

the correlations among the calculated J values for the diradicals modified by any two radical groups (R-(C=C)/R-(C=C)2+ versus R′-(C=C)/R′-(C=C)2+): (a) VER versus NN (the slop is 1.15), (b) NN versus IN (the slop is 6.67), and (c) VER versus IN (the slop is 7.75).

These

results show that the magnetic coupling strengths in the VER-modified diradicals are similar to those in the NN-modified ones, while those in the VER/NN-modified diradicals are 6-7 times stronger than those in the IN-modified ones.

Clearly, the main reason why the VER-modified

diradicals give larger J values than the IN-modified diradicals should be the structural effects, e.g. the twist angles (ϴ) of VER-(C=C) and VER-(C=C)2+ are much smaller than those of IN-(C=C) and IN-(C=C)2+, which will be discussed in the next section.

Further, the fact that

the NN-modified diradicals give larger J values than the IN-modified diradicals should be 8


Page 9 of 32

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


attributed to the different spin delocalization of the NN and IN radical groups, as will be mentioned below.

That is, the spin densities on carbon atoms in the O-N-C-N-O segments of

the NN-based diradicals are larger, whereas those in the N-C-N-O segments of the IN-based diradicals are very smaller.43

These observations indicate that the VER and NN radical groups

are better suited for inducing strong magnetic coupling interactions than the IN radical group in the (C=C)porphyrin-based diradical systems.

For R-(C=C)2+, it is worth pointing out that the

relative positions of two linked radical groups have an effect on the magnetic coupling interactions and the sizes of all |J| values follow the order R-(C=C)-mm2+ > R-(C=C)-ββ2+ > R-(C=C)-mβ2+, while the position effects are not obvious in R-(C=C).

The linear correlations

indicate that the trends in these magnetic molecules considered here are maintained for three different radical group series and the (C=C)porphyrin and (C=C)porphyrin2+ couplers play the same role in mediating the spin coupling between two radical groups (spin sources) in R-(C=C) and R-(C=C)2+, respectively. different

radical

groups

Overall, the magnetic properties can be tuned by introducing (VER,

NN

or

IN)

and

couplers

((C=C)porphyrin and

(C=C)porphyrin2+), or changing the connecting position (the meso- or β- sites) between the radical groups and the coupler.

The above observations also imply that the trend obtained

here can be utilized as a measure of the mediating ability of a porphyrin-derivative coupler for the magnetic coupling when it is coupled with different radical species. Clearly, these results also provide theoretical guidance for practical applications. Structural Characters and Effects.

Usually, the above mentioned magnetic coupling

properties are closely associated with the structural characters of the diradicals. diradicals considered here are nonplanar.

In fact, all the

Not only the (C=C)porphyrin coupler is nonplanar

(curved), but also the linking radical groups and their linked porphyrin edge are not coplanar. Thus, it is necessary to define the ruffling distortion in a quantitative manner.

We choose the

torsional angles (also the bending angles of the (C=C)porphyrin patch, the C-N…N-C dihedral angles between two opposite pyrrole rings, denoted by Φ1 and Φ2, Figure S2 in the SI) as a quantitative measure of ruffling (the curving magnitude) of the porphyrin ring,61 and the twist angles (also the dihedral angles denoted by ϴx1 and ϴy2, x, y = m, β, Figure S2 in the SI) between the couplers and radical groups to measure the twist magnitudes of two linking radical 9



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

groups with respect to their linked porphyrin edges.

Page 10 of 32

Clearly, the optimized molecular

geometries (Figure S3 of the SI) exhibit fairly significant ruffling of the porphyrin units of all designed diradicals (R-(C=C) and R-(C=C)2+).

Different from the parent porphine

((Null)porphine), the optimized (C=C)porphyrin (Φ1=53.0°, Φ2=-55.4°) and (C=C)porphyrin2+ (Φ1=53.3°, Φ2=-53.3°) isolated molecules present similarly ruffled structures (Figure S4 in the SI), indicating that the considerable structural bending of all R-(C=C) and R-(C=C)2+ is due to the introduction of a C=C unit instead of oxidization or the introduction of radical groups. The C=C unit is fairly small for an entity at the center of a porphyrin, even in comparison with the single metal atoms that are usually present in porphyrin complexes, and that small size is the origin of the ruffling in R-(C=C) and R-(C=C)2+.27,61

The magnetic interactions are

primarily transmitted through π-electron conjugation as observed by other authors.32,62

The

magnitude of J depends strongly on the planarity of the molecular structure of the coupler since a more effective molecular orbital overlap and better conjugation extension can be achieved. Comparing R-(C=C) and R-(C=C)2+ with R-(Null), the (C=C)porphyrin and (C=C)porphyrin2+ couplers in R-(C=C) and R-(C=C)2+ are not planar due to the introduction of a C=C unit, but the magnetic coupling interactions of R-(C=C) and R-(C=C)2+ are much stronger than those of R-(Null).

Clearly, there are other reasons (such as twist angles (ϴx1 and ϴy2) or conjugation

degree and bond lengths) that contribute to the strong magnetic couplings in R-(C=C) and R-(C=C)2+, especially in R-(C=C)2+.

In the followings, we mainly focus on the twist angles

(ϴx1 and ϴy2) and bond lengths to help understand why the magnetic coupling interactions in R-(C=C) and R-(C=C)2+ are much stronger than those of R-(Null). In the diradical systems, it is well-known that the magnetic interactions between two radical sites can be influenced by their distances and dihedral angles between the radical groups and the coupler.

For a conjugated diradical, the magnetic interaction strength increases with

the decrease in the coupler length and twist angles between the coupler and radical groups.29,32-34,50,63,64

As described above, the ruffling angles (Φ) in the (C=C)porphyrin and

(C=C)porphyrin2+ couplers are unfavorable to the spin couplings, but the twist angles (ϴ) of R-(C=C) and R-(C=C)2+ are greatly reduced precisely because the ruffling bending can overcome the steric hindrance between the radical groups and the coupler for conjugation.

In

other words, the twist angles of R-(C=C) and R-(C=C)2+ are much smaller (∆ϴ = 6.3 - 40.1°, 10


Page 11 of 32

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


∆ϴ2+ = 22.4 - 108.1°) than those of R-(Null) (Table S6 in the SI), which is one of the important factors to enhance the magnetic coupling strengths in R-(C=C) and R-(C=C)2+.

In addition,

comparing among the diradicals with NN, VER and IN, we can find that the twist angles (ϴ) in NN-(C=C) and NN-(C=C)2+ are similar to those in IN-(C=C) and IN-(C=C)2+, but those of VER-(C=C) and VER-(C=C)2+ are very small.

Especially, ϴm1 (-0.7°) and ϴm2 (-2.1°) are so

small in VER-(C=C)-mm2+ that the magnetic coupling constant (J) is quite larger compared with

NN-(C=C)-mm2+

and

IN-(C=C)-mm2+.

Similarly,

small

twist

angles

for

VER-(C=C)-mm, VER-(C=C)-ββ and VER-(C=C)-mβ also play an important role in yielding their large J values (89 - 96 cm-1). On the other hand, the bond length is one of the most direct factors to describe a molecular structure.

In general, standard lengths of a single bond C-C and a double bond C=C

are 1.540 Å and 1.330 Å, respectively, and however, all optimized bond lengths of R-(C=C) and R-(C=C)2+ are 1.35-1.44 Å (Figure S5 in the SI), showing that each of the (C=C)porphyrin and (C=C)porphyrin2+ couplers is a large conjugated system.

The C-C bonds (Cmeso-Cradical

and Cβ-Cradical) between the (C=C)porphyrin or (C=C)porphyrin2+ unit and the linked radical groups are about 1.4-1.5Å, indicating a weak conjugation between the (C=C)porphyrin coupler and the radical groups and a favorable condition for the magnetic coupling interaction between two radical groups.

In addition, the most C-C bond lengths in the (C=C)porphyrin or

(C=C)porphyrin2+ coupler are shorter than those in the (Null)porphine coupler, but the Cα-N bond lengths in the (C=C)porphyrin coupler are longer than those of the (Null)porphine coupler due to introduction of a core C=C unit.

The lengths of the linking bonds (Cmeso-Cradical and

Cβ-Cradical) between the (C=C)porphyrin coupler and the radical groups in R-(C=C) and R-(C=C)2+ are shorter than those in R-(Null).

In brief, the shorter bond lengths in R-(C=C)

and R-(C=C)2+ are more favorable to the magnetic couplings between two radical groups, which is one of the major reasons that lead to stronger magnetic coupling interactions in R-(C=C) and R-(C=C)2+, in contrast with the parent porphine-based diradicals (R-(Null)). On the other hand, the differences between the magnetic coupling strengths between R-(C=C) and R-(C=C)2+ can be also explained by the structural characters.

Mostly, the

ruffling bending angles (∆Φ1′ and ∆Φ2′) of the (C=C)porphyrin units in R-(C=C)2+ are slightly smaller than those in R-(C=C) (Table S7 in the SI), while the twist angles of R-(C=C)2+ are 11



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

Page 12 of 32

smaller (∆ϴ′=7-43°) than those of R-(C=C), leading to stronger magnetic coupling interactions in R-(C=C)2+. R-(C=C).

Therefore, the |J| values of R-(C=C)2+ are considerably larger than those of

Further, Figure S4 (in the SI) shows the bond lengths for the isolated

(C=C)porphyrin and (C=C)porphyrin2+ molecules.

There are alternating C-C single and

double bonds along the 20-atom periphery of the (C=C)porphyrin coupler.

The average

differences between the lengths of single and double bonds are 0.049 Å for Cβ-Cβ, 0.052 Å for Cα-Cβ, and 0.061 Å for Cmeso-Cα, respectively.

Similar bond-length alternations are also

observed in Si(TPP)(THF)2 (TPP = tetraphenylporphyrin),20 Ge(TPP)(pyridine)2,51 and (B-B)TTP24.

Clearly, the bond length alternation phenomenon in (C=C)porphyrin2+ is not

more obvious than that in (C=C)porphyrin (Figure S4 in the SI).

In other words, the

differences between the longest bond and the shortest bond in (C=C)porphyrin and (C=C)porphyrin2+ are 0.12 Å and 0.05 Å, respectively, indicating that the bond lengths of (C=C)porphyrin2+ are more averaged (Figure S4 in the SI).

Thus, the conjugation effect in

(C=C)porphyrin2+ are better than that in (C=C)porphyrin, providing a favorable condition for strong magnetic coupling when it is used in R-(C=C)2+ as a coupler.

Further, the behavior of

bond lengths in the R-(C=C) and R-(C=C)2+ diradicals is similar to that of the isolated coupler molecules mentioned above (Figure S5 in the SI).

In short, the structural factors (bending and

twist angles and bond lengths) in R-(C=C)2+ are more favorable to yielding strong magnetic coupling interactions than those in R-(C=C). Properties of the Radical Groups.

Different radical groups in the magnetic molecules

must produce different magnetic coupling interactions because of their different structures and properties.

In fact, the spin density distributions of different radical groups could be very

different, featuring different spin delocalization degrees. Figure 2 shows three selected radical groups (NN, IN, and VER) in this study with their Mulliken atomic spin density distributions and the red-marked atoms are connected to the coupler and are named as the “connecting atoms”.

Recently, Lee and co-workers

49, 63, 65

reported that spin densities of the connected

atoms are closely related to the determination of magnetic interactions and J values. It was found that the Mulliken atomic spin densities of the connected atoms of monoradicals may play an important role in expectation of the intramolecular magnetic coupling constants in the

12


Page 13 of 32

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


diradical systems.

Therefore, the Mulliken atomic spin density of the connected atom can be

used as an index of magnetic interactions of a radical.49

In other words, the absolute value of

Mulliken spin density of the connected atom of one radical is larger than another radical, which result in that the former radical generally give stronger intramolecular magnetic interactions than do the latter radical.

Obviously, as shown in Figure 2, the absolute values of Mulliken

spin densities of the connected atoms of these three radical groups in this work follow the order NN > VER > IN.

Likewise, in R-(C=C) and R-(C=C)2+, the spin density distributions also

present such a behavior (Figure S6 in the SI).

Therefore, the |J| values basically follow the

order NN-(C=C) > VER-(C=C) > IN-(C=C) or NN-(C=C)2+ > VER-(C=C)2+ > IN-(C=C)2+. But, some individual |J| values of the VER-modified diradicals (such as VER-(C=C)-mm2+, VER-(C=C)-mm, VER-(C=C)-ββ, VER-(C=C)-mβ) are larger than those of the corresponding NN-modified diradicals because of smaller twist angles (ϴ) of the radical groups in the former. That is, the structural characters influence the regularity about the |J| values, leading to large |J| values for the VER-modified diradicals.

In addition, Ali and co-workers observed that the NN

radical groups are much more strongly coupled to each other than the IN radical groups.

This

is due to the larger spin densities on carbon atoms in the O-N-C-N-O segments of the NN-based diradicals, and the spin densities on carbon atoms in the N-C-N-O segments of the IN-based diradicals are much less.43 Clearly, the Ali’s results are consistent with ours that the magnetic interaction of the NN-based diradicals are stronger than that of the IN-based diradicals.

These analyses indicate that not only the structures but also the spin density

distributions of the connecting atoms of radical groups are the governing factors of the magnetic coupling between two associated radical groups. Magnetic Coupling Mechanism.

Shil et al.66 revealed that the itinerant exchange

between two radical centers in diradicals occurs through the lowest unoccupied molecular orbital (LUMO) and LUMO takes part in the exchange mechanism as in a schematic representation in Figure 3.

In general, the coupler with low HOMO–LUMO gap is

appropriate for getting strong magnetic exchange couplings of a diradical.

We examine the

LUMO energies and HOMO-LUMO energy gaps of the (Null)porphine and (C=C)porphyrin couplers at the B3LYP/6-31G(d) level.

The results show that the HOMO–LUMO gap of the

13



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

Page 14 of 32

(C=C)porphyrin coupler is 1.48eV, which is 1.41eV lower than that of the (Null)porphyrin coupler (2.91eV).

Thus, the (C=C)porphyrin coupler is in favor of strong magnetic exchange

coupling interactions in the (C=C)porphyrin-bridged diradical systems.

We further examine

the differences between HOMO (SOMO) of the radical groups and LUMO of the couplers, as shown in Figure 4.

Clearly, LUMO of the (C=C)porphyrin coupler is 0.49 kcal/mol lower

than that of the (Null)porphine coupler, leading to a smaller HOMO-LUMO gap between the radical groups and (C=C)porphyrin.

This implies a more favorable energetic basis for the

itinerant exchange between two spin centers in a diradical through LUMO in R-(C=C) compared with R-(Null).

However, the HOMO-LUMO gap between the radical groups and

(C=C)porphyrin2+ are larger than the HOMO-LUMO gap between the radical groups and (Null)porphine due to very low LUMO for (C=C)porphyrin2+.

It is worth pointing out that

comparing the energies of these three couplers including (Null)porphine, (C=C)porphyrin and (C=C)porphyrin2+, we can find that the (C=C)porphyrin coupler is better for designing diradicals with strong magnetic coupling interaction.

In fact, R-(C=C)2+ with the

(C=C)porphyrin2+ coupler generate stronger magnetic coupling interaction compared with R-(C=C).

Hence, the energetic basis is not the only factor in determining the strength of the

magnetic exchange coupling interactions.

The structural factors mentioned above play a

greater role than energetic factor in enhancing the magnetic exchange coupling interactions in R-(C=C)2. Figure 5 is given to further verify the role of the center C=C unit of the (C=C)porphyrin coupler.

Taking NN-(C=C)-mm as an example, single-point calculations were done on the

structure of NN-(C=C)-mm after vertically removing the C=C center and hydrogen-saturating two N at the (U)B3LYP/6-31G(d) level.

The results indicate that: (1) comparing Figure 5(a)

and 5(b), the magnetic switching can be realized through introducing or removing the C=C center; (2) comparing Figure 5(b) and 5(c), larger ruffling bending angles (Φ) caused by the introduction of the C=C center are conducive to reduce the twist angles (ϴ), leading to stronger magnetic couplings for NN-(C=C)-mm.

In brief, the C=C center plays an important role in

enhancing magnetic coupling interactions and realizing magnetic switching. HOMO in a diradical usually plays an important role in realizing or enhancing the magnetic couplings.

Thus, we display HOMOs of the CS states for all diradicals (Figure S7 in 14


Page 15 of 32

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


the SI) to analyze the magnetic coupling mechanism with two different isovalues (0.02 and 0.005).

When the isovalue is 0.02, HOMO of R-(C=C)2+ covers the whole molecule but

HOMO of R-(C=C) mostly covers the two radical groups, while HOMO (isovalue=0.005) of R-(C=C) spreads over the periphery of the (C=C)porphyrin coupler, but there is very few or even not HOMO distribution on the center C=C (Figure S7 from the middle molecules in the SI).

That is, the main difference between HOMO of R-(C=C) and R-(C=C)2+ is at the center

C=C unit.

On the other hand, as shown for the spin density distributions of the ground states

of all diradicals (Figure S8 in the SI), the major distinction between R-(C=C) and R-(C=C)2+ is that the spin density distribution at the C=C in R-(C=C) is much less than that in R-(C=C)2+. Besides, it is worth noting that the spin polarization from the spin sources to the center C=C in R-(C=C) is blocked (red-circled).

For R-(C=C)2+, the spin polarization to the periphery of the

(C=C)porphyrin2+ coupler is blocked (red-circled).

From these observations, we deduce that

R-(C=C) show FM but R-(C=C)2+ present AFM due to their different spin-interacting pathways. That is, the spin-interacting pathways in R-(C=C) do not pass through the center C=C unit, while those in R-(C=C)2+ must go through the C=C unit. Further, the spin alternation rule31, 67 based on Hund’s rule can be a reliable guideline for understanding the preference for a certain state of a given diradical linked by a conjugated electronic system.

Although there are certain factors that make it hard to predict the sign of J

according to the spin alternation rule, such as the presence of heteroatoms, the coexistence of more than one competitive spin polarization pathways, or the non-coplanarity between the π-conjugated subsystems, the spin alternation rule can be still used to predict the magnetic properties.

For the present diradical systems, the calculated J values show that all R-(C=C)

and R-(C=C)2+ possess the magnetic characteristics, agreeing with the prediction from the spin alternation rule.

Clearly, there are more than one spin polarization pathways which are

competitive or supportive in R-(C=C) and R-(C=C)2+ with extensive π-conjugated molecular frameworks.

As mentioned above, R-(C=C) show the FM coupling while R-(C=C)2+ exhibit

the AFM coupling.

At the first glance, these diradicals do not seem to follow the spin

alternation rule because of coexistence of mixed 5-, 6-, and 7-membered rings in the (C=C)porphyrin bridges.

However, in fact, the spin alternation rule can explain their different

magnetic behavior perfectly by one or two major supportive spin-interacting pathways. 15


In


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

Page 16 of 32

Scheme 2, two different spin-interacting pathways are given which are extracted according to the spin density distributions of the diradicals. The pink pathway leads to the FM coupling interaction but the blue one gives rise to the AFM coupling.

That is, the spin-interacting

pathway in R-(C=C) goes through the periphery of the (C=C)porphyrin coupler, leading to the FM coupling, but that in R-(C=C)2+ passes through the N atoms and center C=C, leading to the AFM coupling.

For example, as shown in Figure 6, for NN-(C=C)-mm, the spin density

distributions of the marginal carbons of the (C=C)porphyrin coupler are much larger than those of the C=C core and the spin polarization to the C=C core is blocked, indicating the reasonability of the peripheral coupling pathway.

For NN-(C=C)-mm2+, the spin density

distributions of atoms marked in pink and blue including the C=C core and four N atoms are larger than those of Cα and Cmeso atoms in the periphery of the porphyrin backbone and the C atoms marked in black.

By means of this modified spin alternation rule, we can reasonably

predict that the FM and AFM couplings are favored in NN-(C=C)-mm and NN-(C=C)-mm2+, respectively, in good agreement with the computational results.

Besides, we also notice the

similarity in the magnetic behaviors of other diradicals (Figure S6 in the SI).

Therefore, we

can deduce that the major spin-interacting pathway of R-(C=C) goes through the periphery of the (C=C)porphyrin coupler but that in R-(C=C)2+ passes through the N atoms plus C=C core. In a word, the above analyses about predictions of the ground states of the diradicals using the modified spin alternation rule coincide to the conclusions from the corresponding HOMO and spin density distributions and the signs of the calculated J values. In addition, it should be noted that in a general diradical, the unpaired electrons that are responsible for the magnetic exchange interactions reside in the singly occupied molecular orbitals (SOMOs) which are the frontier occupied orbitals, while the redox processes are also associated with such frontier orbitals.

Thus, it is necessary to figure out exactly from which

orbitals the electrons are taken out for the two-electron redox process. We calculate the natural population

analysis

(NPA)

charges

for R-(C=C)

and R-(C=C)2+ at the

B3LYP/6-311+G(d,p) level and discuss the charge differences between the neutral and two-electron oxidized cases.

The NPA charges for (C=C)porphyrin2+ and (C=C)porphyrin as

well as the charge differences between (C=C)porphyrin2+ and (C=C)porphyrin are given in Figure S9.

Besides, the NPA charge differences between R-(C=C)2+ and R-(C=C) (taking the 16


Page 17 of 32

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


NN-modified diradicals and IN-(C=C)-ββ2+, IN-(C=C)-ββ, VER-(C=C)-ββ2+, VER-(C=C)-ββ as examples) are given in Figure S10.

It can be seen from Figure S9 that although two

positive charges in (C=C)porphyrin2+ distribute over the whole molecule, about eighty percent of the two positive charges in R-(C=C)2+ distribute over the (C=C)porphyrin2+ coupler.

That

is, for the two-electron oxidization, the leaving two electrons are mainly taken out from the (C=C)porphyrin coupler, forming the (C=C)porphyrin2+ unit, and the two-electron oxidization only has a little effect on the introduced radical groups.

Therefore, two-electron redox only

changes the coupler which can switch the magnetisms but have little effect on the introduced radical groups. Anisotropic Character of the Coupler Edges.

Since the introduction of a C=C unit to the

parent porphine ring center not only curves the (C=C)porphyrin but also modifies its symmetry, the four edges of (C=C)porphyrin become unequivalent due to the orientation of the C=C unit. Thus, it would be expected that the spin coupling should be different when the radical groups are attached to the (C=C)porphyrin coupler at different Janus edges.

To clarify the difference

of the coupler in mediating spin coupling interaction through different Janus edges, we further examine the magnetic coupling interactions in NN-(C=C)-m'm' and NN-(C=C)-m'm'2+ in which both of the radical groups are linked to the two meso'-sites of (C=C)porphyrin or (C=C)porphyrin2+, respectively (Figure S11 in the SI).

The results indicate that

NN-(C=C)-m'm' exhibits FM interaction (J = 53.6 cm-1) but NN-(C=C)-m'm'2+ presents AFM coupling (J = -106.1 cm-1), which are in magnitude close to or weaker than the corresponding mm-sites linking cases (36.7 cm-1 for NN-(C=C)-mm and -371.7 cm-1 for NN-(C=C)-mm2+), respectively. pathways.

In fact, the differences should be attributed to their different spin coupling As confirmed in Figure S11 by the spin density maps, the spin polarization to the

center C=C unit in NN-(C=C)-m'm' is blocked, leading to the spin-interacting pathway going only along the periphery of the (C=C)porphyrin coupler.

In particular, the spin density

distribution on the center C=C unit of NN-(C=C)-m'm'2+ is basically neglectable, indicating that the center C=C unit does not contribute to the strong AFM coupling interactions when the radical groups are attached to the (C=C)porphyrin2+ coupler at the meso'-sites.

In short, the

magnetic coupling interactions of the diradicals with two meso'-site-linked radical groups to (C=C)porphyrin (the R-(C=C)-m'm' case) are basically close to those of the meso-site-linked 17



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

Page 18 of 32

one (the R-(C=C)-mm case) but for the R-(C=C)2+ case the magnetic couplings in the meso'-site-linked diradicals are remarkably weaker than those of the meso-site-linked ones, indicating an inhibiting role in the meso' direction of the (C=C)porphyrin coupler.

In other

words, introduction of the center C=C unit leads to anisotropy of the (C=C)porphyrin coupler in transmitting the spin information or mediating the spin couplings between the linked spin sources.

CONCLUSION In summary, we theoretically design two series of intriguing porphyrin-based diradicals

(R-(C=C) and R-(C=C)2+) by using the (C=C)-centered porphyrins ((C=C)porphyrin and (C=C)porphyrin2+) as the couplers and the VER, NN and IN radical groups as the spin sources. The structures and electronic properties especially the magnetic coupling properties of these diradicals are explored and analyzed systematically and comprehensively.

In structure,

introduction of anyone kind of three radical groups (VER, NN and IN) does not produce the coplanar structures of the radicals with their linking porphyrin edges due to the steric repulsion, while core-modification of the porphyrin coupler by a C=C unit also curves the molecular backbone.

Although such a modification seems unfavorable to the π-conjugation and thus

mediating spin interaction, it really enhances the spin couplings in R-(C=C) and R-(C=C)2+ compared with R-(Null) without the C=C unit in the center of the porphyrin coupler. R-(C=C) possess mild FM couplings while two-electrons oxidized ones (R-(C=C)2+) present considerably strong AFM ones, indicating that two-electrons redox can switch their magnetisms. Clearly, such differences in the magnetic properties and coupling magnitudes should be attributed to the distinctly different spin-interacting pathways among R-(Null), R-(C=C) and R-(C=C)2+.

Introduction of the C=C unit not only lowers the LUMO energies of the

porphyrin-based couplers and thus regulates the magnetic couplings, while two-electrons redox changes the spin density distributions of the couplers and thus also change the spin coupling pathways which can be predicted by the modified spin alternation rule.

In addition, the

linking modes of the radical groups to the couplers also considerably affect the magnetic coupling strengths especially for R-(C=C)2+.

18


Page 19 of 32

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


This work provides new insights into the spin interaction mechanisms in such diradicaloids which are useful bases for further applications in the future.

More importantly,

there is also tremendous potential for the syntheses of the diradicals designed here because (C=C)porphyrin and (C=C)porphyrin2+ (the couplers) have been prepared and the radical groups (VER, NN and IN) have also been extensively applied to prepare the diradicals with various structures.

We believe that this work would open a new prospective for the rational

design of such porphyrin-based diradicaloids.

ASSOCIATED CONTENT

(S) Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/******. All data and figures calculated at the (U)B3LYP/6-31G (d) level including the energies for the close-shell singlet, broken-symmetry open-shell singlet and triplet, the corresponding values; related data about verification of the results; optimized geometries of all diradicals and those with ruffling torsional angles and twist angles; figures of major bond lengths; scheme of spin density distributions and spin alternation rule; HOMOs and spin density maps (PDF).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

Phone: +86-531-88363670 (X. S.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by NSFC (21373123, 21573128 and 21773137).

All the

calculations were carried out at the National Supercomputer Center in Jinan and High-Performance Supercomputer Center at SDU-Chem. 19



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

REFERENCES

(1) Auwärter, W.; Écija, D.; Klappenberger, F.; Barth, J. V. Porphyrins at Interfaces. Nat. Chem. 2015, 7, 105−120. (2)

Ikeue, T.; Furukawa, K.; Hata, H.; Aratani, N.; Shinokubo, H.; Kato, T.; Atsuhiro Osuka, A. The Importance of a β–β Bond for Long-Range Antiferromagnetic Coupling in Directly Linked Copper(II) and Silver(II) Diporphyrins. Angew. Chem. Int. Ed. 2005, 44, 6899 –6901.

(3) Wäckerlin, C.; Tarafder, K.; Siewert, D.; Girovsky, J.; Hählen, T.; Iacovita, C.; Kleibert, A.; Nolting, F.; Jung, T. A.; Oppeneer, P. M.; et al. On-Surface Coordination Chemistry of Planar Molecular Spin Systems: Novel Magnetochemical Effects Induced by Axial Ligands. Chem. Sci. 2012, 3, 3154−3160. (4)

Wende, H.; Bernien, M.; Luo, J.; Sorg, C.; Ponpandian, N.; Kurde, J.; Miguel, J.; Piantek, M.; Xu, X.; Eckhold, P.; et al. Substrate-Induced Magnetic Ordering and Switching of Iron Porphyrin Molecules. Nat. Mater. 2007, 6, 516−520.

(5)

Poddutoori, P. K.; Pilkington, M.; Alberola, A.; Polo, V.; Warren, J. E.; van der Est, A. Spin-Spin Interactions in Porphyrin-Based Monoverdazyl Radical Hybrid Spin Systems. Inorg. Chem. 2010, 49, 3516–3524.

(6) Wäckerlin, C.; Chylarecka, D.; Kleibert, A.; Müller, K.; Iacovita, C.; Nolting, F.; Jung, T. A.; Ballav, N. Controlling Spins in Adsorbed Molecules by a Chemical Switch. Nat. Commun. 2010, 1, 61. (7)

Kim, H.; Chang, Y. H.; Lee, S.-H.; Kim, Y.-H.; Kahng, S.-J. Switching and Sensing Spin States of Co-Porphyrin in Bimolecular Reactions on Au (111) Using Scanning Tunneling Microscopy. ACS Nano 2013, 7, 9312−9317.

(8) Girovsky, J.; Tarafder, K.; Wäckerlin, C.; Nowakowski, J.; Siewert, D.; Hählen, T.; Wäckerlin, A.; Kleibert, A.; Ballav, N.; Jung, T. A.; Oppeneer, P. M.; et al. Antiferromagnetic Coupling of Crporphyrin to a Bare Co Substrate. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 220404. (9)

Ghosh, D.; Parida, P.; Pati, S. K. Spin-State Switching of Manganese Porphyrin by Conformational Modification. J. Phys. Chem. C 2016, 120, 3625−3634.

(10) Friesen, B. A.; Wiggins, B.; McHale, J. L.; Mazur, U.; Hipps, K. Differing HOMO and 20


Page 20 of 32

Page 21 of 32

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


LUMO Mediated Conduction in a Porphyrin Nanorod. J. Am. Chem. Soc. 2010, 132, 8554−8556. (11) Kondo, H.; Nara, J.; Ohno, T. Possibility of Gas Sensor Using Electronic Transport Properties of Iron- Porphyrin Molecular Junction System. J. Phys. Chem. C 2011, 115, 6886−6892. (12) Singh, H. K.; Kumar, P.; Waghmare, U.V. Theoretical Prediction of a Stable 2D Crystal of Vanadium Porphyrin: A Half-Metallic Ferromagnet. J. Phys. Chem. C 2015, 119, 25657−25662. (13) Dediu, V. A.; Hueso, L. E.; Bergenti, I.; Taliani, C. Spin Routes in Organic Semiconductors. Nat. Mater. 2009, 8, 707−716. (14) Miyamachi, T.; Gruber, M.; Davesne, V.; Bowen, M.; Boukari, S.; Joly, L.; Scheurer, F.; Rogez, G.; Yamada, T. K.; Ohresser, P.; et al. Robust Spin Crossover and Memristance Across a Single Molecule. Nat. Commun. 2012, 3, 938. (15) Ali, Md. E.; Staemmler, V.; Illas, F.; Oppeneer, P. M. Designing the Redox-Driven Switching of Ferro- to Antiferromagnetic Couplings in Organic Diradicals. J. Chem. Theory Comput. 2013, 9, 5216−5220. (16) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vols. 1-10.

Handbook of Porphyrin Science; Kadish, K. M.,

Smith, K. M., Guilard, R., Eds.; World Scientific: Singapore, 2010; Vols. 1-20. (17) Matano, Y.; Fujii, D.; Shibano, T.; Furukawa, K.; Higashino, T.; Nakano, H.; Imahori, H. Covalently Linked 5,15-Diazaporphyrin Dimers: Promising Scaffolds for a Highly Conjugated Azaporphyrin π System. Chem. Eur. J. 2014, 20, 3342–3349.

Scholz, W. F.;

Reed, C. A.; Ja Lee, Y.; Scheidt, W. R.; Lang, G. Magnetic Interactions in Metalloporphyrin π-Radical Cations of Copper and Iron. J. Am. Chem. Soc. 1982, 104, 6791-6793.

Erler, B.

S.; Scholz, W. F.; Ja Lee, Y.; Scheidt, W. R.; Reed, W. A. Spin Coupling in Metalloporphyrin π-Cation Radicals. J. Am. Chem. Soc. 1987, 109, 2644-2652. (18) Eaton, S. S.; Eaton, G. R.; Chang, C. K. Synthesis and Geometry Determination of Cofacial Diporphyrins. EPR Spectroscopy of Dicopper Diporphyrins in Frozen Solution. J. Am. Chem. Soc. 1985, 107, 3177-3184. (19) Kane, K.M.; Lemke, F. R.; Petersent, J. L. 21



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

Bis(trifluoromethanesulfonato)(tetra-p-tolylporphyrinato)silicon(IV), (TTP)Si(OTf)2: The First Structurally Characterized (Porphyrinato)silicon(IV) Complex. Inorg. Chem. 1995, 34, 4085-4091. (20) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. An Antiaromatic Porphyrin Complex: Tetraphenylporphyrinato(silicon)(L)2 (L = THF or Pyridine). J. Am. Chem. Soc. 2005, 127, 12212-12213. (21) Zheng, J.-Y.; Konishi, K.; Aida, T. Crystallographic Studies of Organosilicon Porphyrins: Stereoelectronic Effects of Axial Groups on the Nonplanarity of the Porphyrin Ring. Inorg. Chem. 1998, 37, 2591-2594. (22) Yamamoto, Y.; Nadano, R.; Itagaki, M.; Akiba, K.-Y. Synthesis and Structure of Phosphorus(V) Octaethylporphyrins That Contain a σ-Bonded Element-Carbon Bond: Characterization of a Porphyrin Bearing an R-P=O Bond and Relation of the Ruffling of the Porphyrin Core with the Electronegativity of the Axial Ligands. J. Am. Chem. Soc. 1995, 117, 8287-8288. (23) Akiba, K.-y.; Nadano, R.; Satoh, W.; Yamamoto, Y.; Nagase, S.; Ou, Z.; Tan, X.; Kadish, K. M. Synthesis, Structure, Electrochemistry, and Spectroelectrochemistry of Hypervalent Phosphorus(V) Octaethylporphyrins and Theoretical Analysis of the Nature of the PO Bond in P(OEP)(CH2CH3)(O). Inorg. Chem. 2001, 40, 5553-5567. (24) Weiss, A.; Hodgson, M. C.; Boyd, P. D. W.; Siebert, W.; Brothers, P. J. Diboryl and Diboranyl Porphyrin Complexes: Synthesis, Structural Motifs, and Redox Chemistry: Diborenyl Porphyrin or Diboranyl Isophlorin? Chem. Eur. J. 2007, 13, 5982 –5993. (25) Toganoh, M.; Hihara, T.; Furuta, H. N-Heterocyclic Carbene Embedded in an N-Confused Porphyrin Framework. Inorg. Chem. 2010, 49, 8182-8184. (26) Setsune, J.; Dolphin, D. Reaction of (Octaethylporphyrinato)cobalt Complexes with Diazoacetaldehyde. N- (Formylmet hyl)- and N- (2-Hydroxyethyl)porphyrins. Organometallics 1984, 3, 440-443. (27) Vaid, T. P. A Porphyrin with a C=C Unit at Its Center. J. Am. Chem. Soc., 2011, 133, 15838–15841. (28) Wautelet, P.; Le Moigne, J.; Videva, V.; Turek, P. Spin Exchange Interaction through Phenylene-Ethynylene Bridge in Diradicals Based on Iminonitroxide and 22


Page 22 of 32

Page 23 of 32

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


Nitronylnitroxide Radical Derivatives. 1. Experimental Investigation of the through-Bond Spin Exchange Coupling. J. Org. Chem. 2003, 68, 8025−8036. (29) Vyas, S.; Ali, M. E.; Hossain, E.; Patwardhan, S.; Datta, S. N. Theoretical Investigation of Intramolecular Magnetic Interaction through an Ethylenic Coupler. J. Phys. Chem. A 2005, 109, 4213−4215. (30) Barone, V.; Bencini, A.; Matteo, A. di. Intrinsic and Environmental Effects in the Structure and Magnetic Properties of Organic Molecular Magnets: Bis (imino) Nitroxide. J. Am. Chem. Soc. 1997, 119, 10831−10837. (31) Trindle, C.; Datta, S. N.; Mallik, B. Phenylene Coupling of Methylene Sites. The Spin States of Bis (X-Methylene)-p-Phenylenes and Bis(chloromethylene)-m-Phenylene. J. Am. Chem. Soc. 1997, 119, 12947−12951. (32) Ali, M. E.; Datta, S. N. Broken-Symmetry Density Functional Theory Investigation on Bis-Nitronyl Nitroxide Diradicals: Influence of Length and Aromaticity of Couplers. J. Phys. Chem. A 2006, 110, 2776−2784. (33) Latif, I. A.; Panda, A.; Datta, S. N. Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers: Nitronyl Nitroxide Coupled to Oxoverdazyl via Polyene Spacers. J. Phys. Chem. A 2009, 113, 1595−1600. (34) Latif, I. A.; Singh, V. P.; Bhattacharjee, U.; Panda, A.; Datta, S. N. Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers. II. Nitronyl Nitroxide Coupled to Tetrathiafulvalene via Spacers. J. Phys. Chem. A 2010, 114, 6648−6656. (35) Bhattacharya, D.; Shil, S.; Misra, A.; Klein, D. J. Intramolecular Ferromagnetic Coupling in Bis-Oxoverdazyl and Bis-Thioxoverdazyl Diradicals with Polyacene Spacers. Theor. Chem. Acc. 2009, 127, 57−67. (36) Lescope, C.; Luneau, D.; Rey, P.; Bussiere, G.; Reber, C. Synthesis, Structures, and Magnetic and Optical Properties of a Series of Europium(III) and Gadolinium(III) Complexes with Chelating Nitronyl and Imino Nitroxide Free Radicals. Inorg. Chem. 2002, 41, 5566-5574. (37) Oshio, H.; Watanabe, T.; Ohto, A.; Ito, T. A One-Dimensional Helical Copper(II) Imino Nitroxide. Inorg. Chem. 1997, 36, 1608-1610. (38) Romero, F. M.; Ziessel, R.; Bonnet, M.; Pontillon, Y.; Ressouche, E.; Schweizer, J.; Delley, 23



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

B.; Grand, A.; Paulsen, C. Evidence for Transmission of Ferromagnetic Interactions through Hydrogen Bonds in Alkyne-Substituted Nitroxide Radicals: Magnetostructural Correlations and Polarized Neutron Diffraction Studies. J. Am. Chem. Soc. 2000, 122, 1298-1309. (39) Caneschi, A.; Ferraro, F.; Gatteschi, D.; Lirzin, A. L.; Rentschler, E. Ferromagnetic Intermolecular Coupling in the Nitronyl Nitroxide Radical 2-(4-Thiomethylphenyl)-4,4,5,5-Tetramethylimidazoline-1-Oxyl-3-Oxide, NIT(SMe)Ph. Inorg. Chim. Acta 1995, 235, 159-164. (40) Gilroy, J. B.; McKinnon, S. D. J.; Kennepohl, P.; Zsombor, M. S.; Ferguson, M. J.; Thompson, L. K.; Hicks, R. G. Probing Electronic Communication in Stable Benzene-Bridged Verdazyl Diradicals. J. Org. Chem. 2007, 72, 8062-8069. (41) Lemaire, M. T.; Barclay, T. M.; Thompson, L. K.; Hicks, R. G. Synthesis, Structure, and Magnetism of a Binuclear Co(II) Complex of a Potentially Bis-tridentate Verdazyl Radical Ligand. Inorg. Chim. Acta 2006, 359, 2616-2621. (42) Kuhn, R.; Trischmann, H. Surprisingly Stable Nitrogenous Free Radicals. Angew. Chem. Int. Ed. 1963, 2, 155. (43) Ali, M. E.; Datta, S. N. Density Functional Theory Prediction of Enhanced Photomagnetic Properties of Nitronyl Nitroxide and Imino Nitroxide Diradicals with Substituted Dihydropyrene Couplers. J. Phys. Chem. A 2006, 110, 10525−10527. (44) Ess, D. H.; Johnson, E. R.; Hu, X. Q.; Yang, W. T. Singlet-Triplet Energy Gaps for Diradicals from Fractional-Spin Density-Functional Theory. J. Phys. Chem. A 2011, 115, 76−83. (45) Noodleman, L. Valence Bond Description of Antiferromagnetic Coupling in Transition Metal Dimmers. J. Chem. Phys. 1981, 74, 5737-5743. (46) Noodleman, L.; Baerends, E. J. Electronic Structure, Magnetic Properties, ESR, and Optical Spectra for 2-Fe Ferredoxin Models by LCAO-Xα Valence Bond Theory. J. Am. Chem. Soc. 1984, 106, 2316-2327. (47) Yamaguchi, K.; Takahara, Y.; Fueno, T.; Nasu, K. Ab Initio MO Calculations of Effective Exchange Integrals between Transition-Metal Ions via Oxygen Dianions: Nature of the Copper-Oxygen Bonds and Superconductivity. Jpn. J. Appl. Phys. 1987, 26, 24


Page 24 of 32

Page 25 of 32

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


L1362−L1364. (48) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. A Spin Correction Procedure for Unrestricted Hartree-Fock and Møller-Plesset Wavefunctions for Singlet Diradicals and Polyradicals. Chem. Phys. Lett. 1988, 149, 537−542. (49) Ko, K. C.; Cho, D.; Lee, J. Y. Systematic Approach To Design Organic Magnetic Molecules: Strongly Coupled Diradicals with Ethylene Coupler. J. Phys. Chem. A 2012, 116, 6837−6844. (50) Ko, K. C.; Cho, D.; Lee, J. Y. Scaling Approach for Intramolecular Magnetic Coupling Constants of Organic Diradicals. J. Phys. Chem. A 2013, 117, 3561−3568. (51) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. Reversible Oxidation State Change in Germanium(tetraphenylporphyrin) Induced by a Dative Ligand: Aromatic GeII(TPP) and Antiaromatic GeIV(TPP)(pyridine)2. J. Am. Chem. Soc. 2007, 129, 7841-7847. (52) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. Aluminum Tetraphenylporphyrin and Aluminum Phthalocyanine Neutral Radicals. Inorg. Chem. 2006, 45, 2367-2369. (53) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. The Doubly Oxidized, Antiaromatic Tetraphenylporphyrin Complex [Li(TPP)][BF4]. Org. Lett. 2006, 8, 2401-2404 (54) Yao, Y.; Han, S.; Zhang, Y.; Zhang, X.; Jiang, J. Structures and Spectroscopic Properties of Meso-tetrasubstituted Porphyrin Complexes: Meso-Substitutional and Central Metallic Effect Study Based on Density Functional Theory Calculations. Vibrat. Spectrosc. 2009, 50, 169–177. (55) Frisch, M. J.; Trucks, G. W.; et al. Gaussian 09, revision B.01.; Gaussian, Inc.: Wallingford, CT, 2010. (56) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Acc. Chem. Res. 2008, 41, 157−167. (57) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (58) Sadhukhan, T.; Hansda, S.; Latif, I. A.; Datta, S. N. Metaphenylene-Based Nitroxide Diradicals: A Protocol to Calculate Intermolecular Coupling Constant in a 25



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

One-Dimensional Chain. J. Phys. Chem. A 2013, 117, 13151−13160. (59) Zhang, F. Y.; Song, X. Y.; Bu, Y. X. Redox-Modulated Magnetic Transformations between Ferro- and Antiferromagnetism in Organic Systems: Rational Design of Magnetic Organic Molecular Switches. J. Phys. Chem. C 2015, 119, 27930−27937. (60) Ali, M. E.; Oppeneer, P. M. Influence of Noncovalent Cation/Anion-π Interactions on the Magnetic Exchange Phenomenon. J. Phys. Chem. Lett. 2011, 2, 939–943. (61) Vangberg, T.; Ghosh, A. A First-Principles Quantum Chemical Analysis of the Factors Controlling Ruffling Deformations of Porphyrins: Insights from the Molecular Structures and Potential Energy Surfaces of Silicon, Phosphorus, Germanium, and Arsenic Porphyrins and of a Peroxidase Compound I Model. J. Am. Chem. Soc. 1999, 121, 12154-12160. (62) Polo, V.; Alberola, A.; Andres, J.; Anthony, J.; Pilkington, M. Towards Understanding of Magnetic Interactions within a Series of Tetrathiafulvalene-π Conjugated-Verdazyl Diradical Cation System: A Density Functional Theory Study. Phys. Chem. Chem. Phys. 2008, 10, 857-864. (63) Ko, K. C.; Park, Y. G.; Cho, D.; Lee, J. Y. Simple but Useful Scheme toward Understanding of Intramolecular Magnetic Interactions: Benzene-Bridged Oxoverdazyl Diradicals. J. Phys. Chem. A 2014, 118, 9596–9606. (64) Ali, M. E.; Roy, A. S.; Datta, S. N. Molecular Tailoring and Prediction of Strongly Ferromagnetically Coupled Trimethylenemethane-Based Nitroxide Diradicals. J. Phys. Chem. A 2007, 111, 5523-5527. (65) Cho, D.; Ko, K. C.; Lee, J. Y. Organic Magnetic Diradicals (Radical-Coupler-Radical): Standardization of Couplers for Strong Ferromagnetism. J. Phys. Chem. A 2014, 118, 5112–5121. (66) Shil, S.; Roy, M.; Misra, A. Role of the Coupler to Design Organic Magnetic Molecules: LUMO Plays an Important Role in Magnetic Exchange. RSC Adv. 2015, 5, 105574–105582. (67) Bhattacharya, D.; Misra, A. Density Functional Theory Based Study of Magnetic Interaction in Bis-Oxoverdazyl Diradicals Connected by Different Aromatic Couplers. J. Phys. Chem. A 2009, 113, 5470–5475. 26


Page 26 of 32

Page 27 of 32

Scheme 1. The Design Scheme and Schematic Structures of the Parent Porphine Diradicals (R-(Null)) and R-(C=C) Diradicals and R-(C=C)2+ Diradicals with Two Positive Charges. |J| Decrease

R N

+ C=C C=C

HN

N R

C C

R

N

+ C=C

N

N

dielectron oxidation

N

dielectron reduction

C C

C=C HN

N R

R

N

N

HN

R-(Null)-m

Table 1.

N

+ C=C C=C

O

Nitronyl nitroxide (NN) R

N

N C C N

N

R-(C=C)- -BS (AFM)

R

N


N


C C

R N

N

N

N

R-(C=C)-m -BS2+ (AFM)

R-(C=C)-m -T (FM)

NH

N

NH

O

N C C

R

N

Verdazyl (VER)

2+

R

R

N

N

R

R NH

O

N

R-(C=C)- -T (FM)

R-(Null)-

R

R-(C=C)-mm-BS 2+ (AFM) R

R N

N C C

N

R-(C=C)-mm-T (FM)

R-(Null)-mm

NH


N

N

N R

R

|J| Increase

N

N

R


=

NH

|J| Increase

|J| Decrease

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


N N

Imino nitroxide (IN)

The Singlet-Triplet Energy Gaps (∆E(BS-T), kcal/mol) and Intramolecular Magnetic

Coupling Constants (J, in cm-1) of Diradicals Calculated at the UB3LYP/6-31G(d) Level. The ∆E(BS-T) Values of VER-(C=C) Are Obtained From the Jcorrect Values NN-(C=C) NN-(C=C)-mm NN-(C=C)-ββ NN-(C=C)-mβ VER-(C=C) VER-(C=C)-mm VER-(C=C)-ββ VER-(C=C)-mβ IN-(C=C) IN-(C=C)-mm IN-(C=C)-ββ IN-(C=C)-mβ

∆E(BS-T) 0.113 0.120 0.076 ∆E(BS-T) 0.275 0.273 0.256 ∆E(BS-T) 0.026 0.054 0.054

NN-(C=C)2+ NN-(C=C)-mm2+ NN-(C=C)-ββ2+ NN-(C=C)-mβ2+ VER-(C=C)2+ VER-(C=C)-mm2+ VER-(C=C)-ββ2+ VER-(C=C)-mβ2+ IN-(C=C)2+ IN-(C=C)-mm2+ IN-(C=C)-ββ2+ IN-(C=C)-mβ2+

J 36.7 37.0 24.3 Jcorrect 96.1 95.7 89.4 J 8.8 18.4 18.3

27


∆E(BS-T) -0.812 -0.767 -0.345 ∆E(BS-T) -1.100 -0.367 -0.237 ∆E(BS-T) -0.125 -0.092 0.024

J -371.7 -297.5 -129.2 J -477.9 -138.5 -86.6 J -45.0 -32.5 -8.3


(b) R-(C=C)-ββ R-(C=C)-mβ R-(C=C)-mm

100 0

The J values of NN group (cm -1 )

The J values of VER group (cm -1)

(a)

2+

R-(C=C)-mβ

-100

2+

R-(C=C)-ββ

-200 -300 -400

R-(C=C)-mm2+

-500 -450 -400 -350 -300 -250 -200 -150 -100 -50

0

100

R-(C=C)-mm 0

R-(C=C)-ββ R-(C=C)-mβ

-100

R-(C=C)-mβ2+

-200

R-(C=C)-ββ2+

-300

R-(C=C)-mm2+

-400

50 100

-50

-40

-30

-20

-10

0

10

20

The J values of IN group (cm-1)

The J values of NN group (cm-1)

(c) The J values of VER group (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

Page 28 of 32

R-(C=C)-ββ R-(C=C)-mm R-(C=C)-mβ

100 0

2+

R-(C=C)-mβ

-100

R-(C=C)-ββ2+

-200 -300 -400

R-(C=C)-mm2+

-500 -50

-40

-30

-20

-10

0

10

20

The J values of IN group (cm-1)

Figure 1. Correlations among the calculated J values of three groups of diradicals. (a) The VER versus NN groups, (b) the NN versus IN groups, and (c) the VER versus IN groups. dots R-(C=C)-mm and R-(C=C)-ββ are overlapped in (a).

28


The

Page 29 of 32

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


Figure 2. Structures of the nitronyl nitroxide (NN) radicals, verdazyl (VER) radicals, imino nitroxide (IN) radical groups, and their Mulliken atomic spin density distributions, where the red-marked atoms denote the connecting atoms to the coupler.

Figure 3. Schematic representation of spin interaction via LUMO of the coupler in diradicals.

29



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

Figure 4. The energy levels of SOMOs of radicals (NN, VER and IN) and LUMOs of the couplers ((C=C)porphyrin2+, (C=C)porphyrin and (Null)porphine) and HOMO-LUMO gaps in kcal/mol.

Figure 5. Geometric and magnetic comparisons of R-(Null) and R-(C=C) to illustrate the role of the C=C core, where the structures in (a) and (c) are the optimized ones and that in (b) is obtained by vertically removing the C=C core from that in (a) and other parts keep unchanged and two N are saturated by two H.

30


Page 30 of 32

Page 31 of 32

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


Scheme 2. Schematic Representation of Different Spin-Interacting Pathways (Denoted by the Pink and Blue Arrows, Respectively) and Spin Alternation Analyses in the (C=C)Porphyrin Coupler of the R-(C=C) Diradicals and (C=C)Porphyrin2+ Coupler of the R-(C=C)2+ Diradicals

-0.035

-0.075 -0.041 0.052 0.027

0.052 O 0.365 0.375 O 0.019 -0.015 0.001 N N 0.265 -0.058 -0.047 N0.271 N -0.023 -0.023 0.084 -0.220 0.083 -0.004C C -0.003 -0.219 -0.023 -0.023 -0.052 N 0.269 -0.054 N N 0.271 N -0.014 0.001 0.339 0.330 O 0.053 0.025 O 0.029 0.053 -0.075 -0.042 -0.037

O

O N

N

N C

N

N

N

N

O

O

NN-(C=C)-mm-T

N

C

NN-(C=C)-mm-T

0 0.019 -0.018 0.001 -0.002 -0.021O -0.403 0.404 O 0.021 -0.020 0.020 N N 0.271 0.066 -0.066 N-0.270 N 0.023 -0.023 -0.155 0.156 0.217 -0.048C C 0.048 -0.216 0.022 -0.022 0.069 N -0.257 -0.068 0.257N N N -0.021 0.021 -0.377 0.377 O 0.025 -0.025O 0 0 0.024 -0.024 0

O

O N

N

N C

N

N

C N

N O

O

NN-(C=C)-mm-BS2+

N

NN-(C=C)-mm-BS2+

Figure 6. Spin density distribution and spin alternation analysis, where the pink numbers and arrows denote α spin, while the blue ones denote β spin, respectively.

The red bonds in each

backbone denotes the major spin-interacting pathway between two radical groups (NN here). All hydrogen atoms are omitted.

31



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

TOC Graphic

32


Page 32 of 32

Core-Modified Porphyrin Diradicals with a C C Unit: Redox-Driven

Recommend Documents