Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of Well-Defined

Article pubs.acs.org/JPCC

Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of WellDefined Open-Shell Singlet Diradicals Hongfang Yang, Qisheng Song, Wenchao Li, Xinyu Song,* and Yuxiang Bu* The Center for Modeling & Simulation Chemistry, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, People’s Republic of China S Supporting Information *

ABSTRACT: Two classes of multi-Zn-expanded oligoacenes from benzene to pentacene are computationally designed through introducing a Zn array into acene rings in two ways: acene-chain axial versus single-ring quasi-transversal direction. Combined density functional theory, CASSCF, and CCSD calculations predict that all these multi-Zn-expanded oligoacenes have the open-shell singlet diradical ground states, in contrast with the common fact that their parent oligoacenes are closed-shell systems or may have a triplet ground state and only acenes larger than octacene have open-shell singlet diradical ground states. These results offer the first theoretical prediction that the multi-Zn introduction into the acene ring(s), forming the Zn-expanded oligoacenes, can lead them to diradical structures. The diradical character of the ground states of these molecules arises from the Zn-participation-induced disjoint nature of the nonbonding molecular orbitals that are singly occupied in the diradicals. This work provides a strategy to design perfect and stable singlet diradicals from oligoacenes or their derivatives.

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INTRODUCTION Linear oligoacenes are a unique series of polycyclic aromatic hydrocarbons, which consist of linearly fused benzene rings, as rationalized previously.1 Their extended π-conjugation through out the entire carbon backbone results in very fascinating electronic properties.1−5 Oligoacenes have become prototypical models for charge carrier energetics and dynamics in organic molecular crystals,6−9 and their potential in organic electronic applications such as electrically pumped organic solid state injection laser,10,11 field-effect transistors and light-emitting diodes,3,10,12,13 organic conductors,14 and solar cells15 has stimulated interest in their fundamental electronic and geometric properties. Over the past few years, the longknown π-extended substances have evoked intense attention of chemists, both experimentally and theoretically, which makes the nature of a diradical with a singlet ground state prominent. In particular, it has been verified that many fascinating electromagnetic properties of their molecular solids come from their singlet diradical character. However, studies have also indicated that almost all linear oligoacenes possess the closed-shell singlet ground state, and only those larger than hexacene are prospective to have diradical character in their ground states but which are still controversial and need further clarifying.5,16,17 Moreover, up to now, experimental syntheses of such acenes have been limited to heptacene, and detailed property studies have ended with pentacene because of high instability and extreme difficulty in experiment syntheses of the higher acenes possessing diradical character.2,5 Similarly, as a class of π-extended derivatives of acenes, although small nanosized graphene patches or ribbons have also proved to have diradical character, elongation of the zigzag edge results in increase of diradical character, but those with long arm chair © 2012 American Chemical Society

edge only have weak and even never have diradical character.18,19 Clearly, it has been a formidable task to find or design novel, stable diradical acenes or their analogues for the fundamental and practical reasons. As is known, species with diradical character possess a special electronic structure that two electrons on the radical centers are weakly coupled within the molecule, and the adequately weak coupling leads to the species’ singlet diradical character. Just because of this character, most singlet diradical species are very reactive and thus short-lived, which makes the identification of some characteristic properties obscured and difficult, and the experimental elucidation of their diradical character encounters much difficulty.20,21 Actually, for an acene, it can be also viewed as two parallel chains of polyacetylene bound together, and the interchain distance smoothly increases as elongation of the acene, and at the same time, the diradical character also gradually increases. Clearly, this implies a correlation between the diradical character of an acene and the separation between the two chains, as also indirectly demonstrated by small nanosized graphene pathches.19 This also offers the opportunity to diradicalize acenes by expanding the benzene ring(s) to weaken the interaction between two electrons on the two radical moieties. Although much efforts have been made to improve stability and electronic properties of acenes by introducing substituents,22−26 to our knowledge, no attempt has been made to diradicalize oligoacenes through modification such as enlarging the separation of two polyacetylene radical chains of an acene Received: November 27, 2011 Revised: January 23, 2012 Published: February 1, 2012 5900

dx.doi.org/10.1021/jp2107654 | J. Phys. Chem. C 2012, 116, 5900−5907

The Journal of Physical Chemistry C

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stabilization for the two radical moieties. Qualitatively speaking, in a short acene, larger cross-link stabilization than the overchain delocalization stabilization can lead to a closedshell singlet ground state, whereas in a long acene, large overchain delocalization stabilization prospectively leads to a perfect open-shell singlet diradical ground state and cooperative weakening of the cross-linking interaction with an elongation of the cross-linked C−C bonds compared with the short acenes. Clearly, no matter if the long acenes could become diradicals, the overchain delocalization stabilization gradually increases, while the cross-linking interaction basically does not change or changes a little. Naturally, this observation about electronic state switches in acenes evokes people to wonder whether we can realize that an oligoacene diradicalized through weakening the cross-linking interaction between its two polyacetylene chains and which is its ground state, triplet or singlet, upon being diradicalized. Understanding the diradical character and its controllability is a fundamental step toward designing promising diradical-character-based molecular materials. Clearly, to this end, exploring novel and effective linkers becomes very important and also very necessary. We herein choose transition metal Zn as linkers to mediate two polyacetylene radical chains or two radical moieties of an acene according to the modification schemes shown in Chart 1 together with topological schematic structures of the Znmodified acenes although the routines for their syntheses may be completely different. The choice is based on the following considerations. (1) Providing that Zn is a good carrier of many functions, particularly in nanofields, it is hoped to be a suitable candidate dopant for modification of acenes. It is conceivable that introduction of Zn into acenes would intrinsically improve a variety of properties and functions of acenes. (2) Zn usually exhibits a divalent state in a Zn(II) ion form and readily forms organozinc compounds with more covalent character, featuring the stable linear X−Zn(II)−Y (X,Y = C, N, etc.) structures.27−29 Moreover, providing that X and Y are conjugated species, the fully filled d-orbitals (d10) and empty p-orbitals of Zn(II) may weakly mediate the interaction between X and Y, and thus modulate the π-extended degree between X and Y moieties. (3) Because of their d10 electronic structure, the adjacent Zn(II) does not form metal−metal multiple bonds, thus avoiding the formation of short Zn−Zn covalent bonds, which can distort the acene structures, but two adjacent Zn(II) ions can form the metallophilic interaction (d10···d10), which is due to the correlation effect. More importantly, this kind of van der Walls interaction can balance the repulsion among adjacent Zn(II) ions, leading to a Zn−Zn distance very suitable to generate the regular just-planarly expanded acene structures without other distortions. In particular, Zn(II), a closed-shell ion, does not introduce an extra radical center when forming X−Zn−Y organozinc species. (4) Most of the resembling organozinc compounds including those with the C−Zn−C structures could be prepared easily by some suitable procedures.39,40 Thus, it is rational to expect that the Zn-expanded acenes could be experimentally synthesized in view of the experimental realization of similarly structured X− Zn−Y or other X−M−Y species.27−29,41 To the best of our knowledge, modification of acenes and even nanosized graphene patches with Zn has not been reported. In the course of our studies we have found that, compared with the parent acenes,5,16,42−45 all of these multi-Znexpanded acenes starting from the smallest 2Zn-expanded

adequately or introducing regulating groups, which can stabilize a diradical. With the aim at the pursuit of stable singlet ground state diradicals, herein, we report a computational design with a ringexpansion strategy to convert the closed-shell singlet oligoacenes to the open-shell singlet diradicals. Recently, various organozinc(II) and other organometallic compounds featuring the X−M−Y (M = metal; X,Y = C, N, etc.) linkages have been successfully synthesized.27−29 Inspired by their special linking mode, X−M−Y, and electronic mediation effect of the metal between X and Y groups,30 transition metal zincs are utilized as the linkers to expand acenes in this scenario. As a most essential aspect, this work provides an executable strategy to diradicalize oligoacenes, which then have the enhanced electronic properties.

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GENERAL CONSIDERATION AND CALCULATIONS Design. Since diradicals are molecules featuring two unpaired electrons, each of which is occupying two degenerate or nearly degenerate molecular orbitals,15,16,18,31−33 the linkage of two radical moieties appears to be one of the most successful and extensively studied strategies to yield molecules of diradical character. Moreover, this procedure allows regulation of the interaction between two unpaired electrons and even switch of low-spin and high-spin states and further achievement of different electronic properties by modifying the nature of linker arbitrarily for practical purpose.20,21,34−38 Appearance of diradical character of the acenes has its interior origin. Actually, two polyacetylene chains in an acene may be viewed as two radical moieties, which are bound together by more cross-linking C−C bonds (see Chart 1). The Chart 1. Topological Representation of Acenes and Their Diradical Character and Multi-Zn-Expansion Schemes

electronic-state-switchable phenomena from the unambiguous closed-shell singlet ground states of benzene through pentacene to the perfect open-shell singlet diradicals (if available) for higher acenes with a transition zone for the intermediate acenes could be attributed to competition between delocalization stabilization along each polyacetylene chain and cross-linking 5901



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Figure 1. Optimized (UB3LYP/(6-31G** for C,H and SDD for Zn)) geometries of 6Zn-pentacene (a, open-shell singlet; b, triplet), 2Zn-pentacene (c, open-shell singlet; d, triplet), and their parent pentacene (e, open-shell singlet; f, triplet) with indicated bond lengths (Å).

the Zn-modified acenes possess the diradical ground states. Since this kind of calculation is much more expensive, only 2Zn-benzene was chosen as a representative for UCCSD(T)/ (6-31G* for C,H and SDD for Zn) single-point calculations at the UB3LYP/(6-31G** for C,H and SDD for Zn) optimized geometry to get its orbital occupation number and singly occupied molecular orbitals. To prove the correctness of the UB3LYP and CASSCF(10,10) results and effect of elongation of the cross-linked C−C bonds on the diradical character, a scan on some parent acenes was also done at the UB3LYP/6-31G(d,p) level with respect to the cross-linked C−C bond with relaxation of other geometrical parameters, and then, the relevant steady points were used for single-point calculations at the UCCSD(T)/6-31G(d,p) level. In addition, since the unrestricted UB3LYP singlet calculations can only obtain the results that appear to be very similar to those of RB3LYP for the singlet state49 and do not really reflect the diradical character and the lowest state energies for the diradical molecules, a mix keyword must be added to all optimizations in a guess = mix form, which can obtain an unrestricted broken spin-symmetry (UBS) solution by mixing the near-energy low-lying triplet and other higher multiplets into the singlet state to yield the correct ground states. Thus, the distributions of the molecular orbitals and the spin densities are displayed in all relevant figures to intuitively show the diradical character and our further analyses.

benzene possess an open-shell singlet ground state with more obvious diradical character. Calculations. In view of successful application of density functional theory in predicting various physical properties that would prove otherwise difficult to measure or determine experimentally, herein we conducted a detailed computational analysis to obtain a basic understanding of the changes of the structural and electronic properties of acenes that take place in the presence of Zn. To identify the diradical characters and to verify the structural stability of the Zn-expanded acene derivatives shown in Chart 1, we first optimized their geometries in the singlet and triplet states at the unrestricted broken symmetry UB3LYP/6-31G**(C,H)/SDD(Zn) level of theory,46,47 which has been successfully used to predict a diradical character of acenes5 and also verified them as local minima on the basis of vibrational frequency analyses, which showed no imaginary frequencies. Figure 1 provides two optimized representatives of the two series (see the Supporting Information for others). The relevant energy quantities including the adiabatic/vertical ionization potentials (IPs), electron affinities (EAs), the singlet−triplet gaps, and the HOMO−LUMO gaps were determined. Further, the magnetic coupling constants, J, were calculated using a simple formalism proposed by Yamaguchi.52 Usually, the amount of the diradical character can be theoretically measured directly by the occupation number of the LUMO.20,21 A perfect diradical is characterized by occupation numbers of 1.0 in HOMO and LUMO, whereas a perfect closed-shell molecule possesses occupation numbers of 2.0 and 0.0 in HOMO and LUMO, respectively. Thus, we also determined the LUMO occupation number by performing CASSCF calculations to measure diradical character of the multi-Zn expanded acenes.48 In view of the inconsistent results in producing the ground states of the acenes higher than hexacene16,17 and to give confirmative conclusions for the ground states of them, the CCSD(T) correlation method was also employed to clarify if

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RESULTS AND DISCUSSION Two representatives (6Zn-pentacene and 2Zn-pentacene) of two series in both singlet and triplet states are shown in Figure 1, together with the corresponding pentacene for comparison. Both singlet and the corresponding triplet state of each structure have very similar geometries with a maximal bond length difference of 0.005 Å, and in addition, two resembling polyacetylene chains or two small conjugated fragments are connected by a mediating Zn array instead of direct C(sp2)− C(sp2) bonds, respectively. Like their parent pentacene, 6Zn5902



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pentacene and 2Zn-pentacene are both planar within chemical accuracy. Similarly, all of the other multi-Zn-expanded acene derivatives are also planar. Overall, multi-Zn insertion does not induce any significant structural distortions for those acene derivatives except for elongation of the cross-linking C···C distances. As is shown in Figure 1, only the originally crosslinked C(sp2)−C(sp2) bonds1,5 are elongated significantly due to the Zn introduction. That is, the two parallel polyacetylene chains in the multi-Zn-expanded acenes are further separated away from each other than those in the parent ones. To illustrate the expansion sizes quantitatively, Figure 1 lists the interatomic distances of the cross-ring C−Zn−C and other main bond lengths associated with the polyacetylene chains. As expected, the Zn···Zn distances are about 2.51−2.56 Å (the interiors) and 2.66−2.89 Å (the outers) for all cases considered here, indicating that no covalent bonds between two adjacent Zn formed, and the Zn···Zn contacts could attributed to the d10···d10 metallophilic van der Waals interaction. These character distances are very close to those (2.43−2.50 Å) between two parallel opposite edges in each benzene ring unit. Thus, introduction of Zn atoms into the elongated C···C bonds can still maintain the parallel orientations of all the C−Zn−C units, as characterized by the fact that all C−Zn−C units are basically linear for the interiors and slightly bent (or quasilinear) for the outers and mutually parallel. Compared with the parent acenes, the cross-linked C···C distances in the Zn-modified derivatives are largely elongated by 2.4 Å from about 1.5 Å (in their parent acenes) to about 3.9 Å due to the introduction of Zn atoms. Clearly, from chemical tuition, it is rational to envision that, when these cross-linked C−C units are replaced by the C−Zn−C units with such an elongation (about 3.9 Å) for the C···C distances, two radical polyacetylene chains become relatively separated and their interaction should become very weak. Consequently, two unpaired electrons on two polyacetylene radical centers become weakly coupled. Conceivably, such adequately weak coupling more possibly leads to a triplet diradical ground state for the Zn-expanded acenes. Surprisingly, however, our DFT calculations lead to an openshell singlet ground state with the significantly large amount of diradical character for all oligoacene derivatives (derived from benzene to pentacene, respectively), implying that their openshell singlet ground states are energetically more favorable than the corresponding closed-shell singlet and triplet states (see Figure 2 and Table 1). At the B3LYP level of theory, for all nZn-acenes, their singlet−triplet gaps are 2.9−3.0 kcal/mol (triplet over singlet), basically being a constant. The spin contaminations for their singlet states are also very stable with an almost constant value close to 1.0 (⟨S2⟩ = 0.93−0.96), which clearly indicates that the singlet state is the mixture of pure singlet (⟨S2⟩ = 0.0) and pure triplet state (⟨S2⟩ = 2.0). For 2Znbenzene, the closed-shell RB3LYP solution is 10.67 kcal/mol above the open-shell singlet diradical state (Table 1). From 2Zn-benzene to 6Zn-pentacene, the RB3LYP solutions still remain higher in energy relative to their corresponding openshell singlet diradical states with the increase of the number of benzenoid units. Figure 2 combined with Table 1 clearly shows that nZn-acenes possess the open-shell singlet states with diradical character as their ground states, but for the 2Zn-acene series, although spin contamination (⟨S2⟩ = 0.93−1.07) for their singlet states is also close to 1.0, the gaps rapidly decreases from 3.0 (2Zn-benzene) to 0.3 kcal/mol (2Zn-pentacene). From this trend, we can predict that the gap would be gradually

Figure 2. Dependences of singlet−triplet state energy gaps of acenes and their corresponding multi-Zn expanded derivatives. The data in the parentheses are the ⟨S2⟩ values of the corresponding species.

close to zero if the number of benzenoid units increases. Although the 2Zn-acene series presents a decay trend similar to their parent acene series in the singlet−triplet gap, the decay rate of the former is noticeably slower than the latter. What’s more, large values of ΔE(OS−CS) for the 2Zn-acene series reflect the advantage of the open-shell singlet over the corresponding closed-shell ones. All in all, the ground states of all multi-Znmodified acenes considered here are open-shell singlet, as a result of disjoint diradical nature. In addition, in order to confirm the accuracy of our (U)B3LYP results, we have also chosen another method named (U)BHandHLYP with the same basis set 6-31G** (for C,H) and SDD (for Zn), and similar results were obtained, as shown in the Supporting Information. Further, as an interesting finding, the singlet−triplet gaps for two Zn-modified series are considerably smaller than those of their parent acene series especially of the shorter acenes (before hexacene). They are close to and even smaller than those (about 5 kcal/mol) of higher acenes, which are diradicalcharacterized perfectly. This observation indicates that multi-Zn introduction into some C−C bonds of acenes can produce the Zn-modified acene derivatives, which possess considerably small singlet−triplet gaps, and also implies that multi-Zn modification of acenes can provide a favorable energetic basis for the appearance of diradical character in the Zn-modified acene derivatives. Clearly, difference in the variation trend of the singlet−triplet gaps of the two series could be due to their different Zn-doping modes, which lead to different radical delocalization. That is, although density of diradical decreases along with increase of the number of the fused benzene rings for both cases, the distance between two radicals maintains unchanged for nZnacenes but becomes gradually large for 2Zn-acenes. As a result, the coupling interaction between two radicals in the former also basically maintains a constant, while that in the latter gradually decreases toward zero. Although the singlet−triplet gaps are so small that the triplet state is merely several kcal/mol above the corresponding singlet state and might be thermally realizable, we still predict that multi-Zn-expanded acenes will maintain singlet as their ground state. When a species is in its singlet state, its structure has the potential to offer a possibility for the two unpaired electrons to occupy different parts of space, so its diradical character appears.50 Further, the species has partially filled orbitals, which 5903



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Table 1. Energies (kcal/mol) for the Singlet-Triplet Gaps (ΔE(T−OS)), Spin Contamination for the Open-Shell Singlet (⟨S2⟩), Differences between Open-Shell Broken-Symmetry Singlet and Closed-Shell Singlet RB3LYP Solution, ΔE(OS−CS), of nZnacenes and 2Zn-acenes Calculated at the (U)B3LYP/(6-31G** for C,H and SDD for Zn) Level of Theorya

a

nZn-acenes

ΔE(T−OS)

⟨S2⟩

ΔE(OS−CS)

2Zn-acenes

ΔE(T−OS)

⟨S2⟩

ΔE(OS−CS)

2Zn-benzene 3Zn-naphthalene 4Zn-anthracene 5Zn-tetracene 6Zn-pentacene

2.98 2.91 2.89 2.90 2.92

0.93 0.94 0.94 0.95 0.96

−10.67 −7.87 −6.33 −5.33 −4.65

2Zn-benzene 2Zn-naphthalene 2Zn-anthracene 2Zn-tetracene 2Zn-pentacene

2.98 2.08 0.89 0.50 0.25

0.93 0.97 1.02 1.05 1.07

−10.67 −11.35 −13.54 −15.23 −17.11

Note that ΔE(T−OS) = E(T) − E(OS) and that ΔE(OS−CS) = E(OS) − E(CS).

Figure 3. Singly occupied molecular orbitals of open-shell singlet (left column) 6Zn-mediated pentacene and (right column) 2Zn-mediated pentacene (UB3LYP/(6-31G** for C,H and SDD for Zn); isovalue = 0.02).

may contribute to its important electronic properties because of the open-shell character.1 According to these results, we could conclude that the existence of open-shell singlet diradical ground state of the multi-Zn-expanded linear acenes originates from multi-Zn participation instead of the mere geometrical expansion of them. To further explore the geometric properties and diradical character of those multi-Zn-expanded acenes, we have performed a UB3LYP/(6-31G** for C,H and SDD for Zn) calculation for polyacetylene monoradicals, which can be viewed as semistructures of parent acenes and multi-Znexpanded acenes without Zn atoms. As can be seen in Table S14, Supporting Information, our DFT calculations clearly indicate that polyacetylene monoradicals tend to become delocalized forms as their most stable states, which could be concluded from the variation trend in the C−C bond lengths while the number of acetylene units increases. That is, bond length alternation,51 defined as the difference between the long and short carbon bonds in such conjugated molecule, becomes smaller and smaller. However, for nZn-polyacene with large amount of diradical character, their single polyacetylene chains would rather be of a mixed aromatic/radical character even from the smallest 2Zn-expanded benzene. Thus, we predict that the mixed aromatic/radical character still remains with increase of the number of the fused benzene rings. Actually, nZnpolyacene could be viewed as polyacetylene monoradicals linked by one Zn atom chain. The presence of zinc atoms makes the distance between the two corresponding polyacetylene monoradicals large enough so that coupling and electronic communication between both sides become very weak, and this helps polyacetylene monoradical to maintain its own inherent delocalized character as much as possible. These

analyses indicate that the solution with a mixed aromatic/ radical character is the most stable state of those nZnpolyacenes. In order to depict the diradical character quantitatively, we performed CASSCF(10,10) calculations using a mixed basis set (6-31G(d) for C,H and SDD for Zn) (Supporting Information, additional calculational details) to get occupation numbers of LUMO as a measure of the amount of the diradical character for both series of multi-Zn-expanded acenes. The results confirmed the DFT prediction in that a pronounced diradical character appears in the orbital occupation even for the smallest 2Zn-benzene. The number of electrons outside closed-shell bonding orbitals and the occupation number of LUMO are shown in Table S1 and the percentages of the diradical character are given in Table S3 (Supporting Information). For example, LUMO occupation numbers of 6Zn-pentacene and 2Zn-pentacene are 0.734 (⟨S2⟩ = 0.96) and 0.932 (⟨S2⟩ = 1.07), and consequently, the amounts of the diradical character are estimated to be 73.4% and 93.2%, respectively.20,21 Overall, for all multi-Zn-expanded acenes, the diradical character percentages are 65−73% (nZn-acene series) and 65−93% (2Zn-acene series), respectively, far larger than those (6−21%) of their parent acenes (the well-defined closed-shell singlet species). In view of the sensitivity of radical distribution to the electron correlation effect, we also calculated the LUMO occupation number of 2Zn-benzene at the CCSD(T) level as a verification of the above results. The LUMO occupation number and the amount of the diradical character are 0.897 and 89.7%, respectively, slightly larger than those (0.656 and 65.6%) at the UB3LYP level. To confirm the accuracy of our previous CASSCF results, we have repeated a preliminary (10,10) CASSCF investigation but using the RB3LYP-optimized 5904



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Figure 4. Spin density maps of the open-shell singlet (a) 6Zn-pentacene and (b) 2Zn-pentacene. The calculations are performed at UB3LYP/(631G** for C,H and SDD for Zn) levels of theory. Blue and green surfaces of the spin density maps represent α and β spin densities, respectively (isovalue = 0.004).

than those obtained at the UB3LYP and CASSCF levels, proving again that the observed diradical character is real. We also calculated the HOMO−LUMO gaps of the Znexpanded and their parent acenes for comparison (Table S6, Supporting Information). For the nZn-acene series, the calculated HOMO−LUMO gaps decrease from 2.94 eV (2Zn-benzene) to 1.80 eV (6Zn-pentacene), while for the 2Zn-acene series, those decrease from 2.94 eV (2Zn-benzene) to 1.15 eV (2Zn-pentacene). In comparison to those (from 6.79 eV/benzene to 2.21 eV/pentacene) of their parent species, the HOMO−LUMO gaps of the Zn-expanded acenes are small enough so that they could allow the promotion of an electron to give a diradical more easily.50 To some extent, this could explain why diradical character in the multi-Zn-expanded acenes appears much earlier than their parent ones, and the diradical percentages are also much greater compared with those of their parent acenes (see Table S3, Supporting Information). Clearly, these results further confirm the above conclusion that the multi-Zn-expanded acenes have welldefined open-shell singlet diradical ground states. To scale the magnetic coupling interaction between two radical centers of the multi-Zn-expanded acene, we calculated the magnetic exchange constant, J, which denotes the ferromagnetic (positive) or antiferromagnetic (negative) coupling between two radical centers. For the diradicals considered here, the J values were calculated using a simple formalism proposed by Yamaguchi, which is based on the broken symmetry unrestricted results (energies and spin contaminations) of the singlet and triplet states and has been proved to be the most appropriate one for estimating the magnetic exchange constant of diradicals.52 The results are given in Table S9, Supporting Information. All J values fall in a range of −930 to −844 cm−1 for nZn-acenes or −930 to −88 cm−1 for 2Zn-acenes, respectively. Large negative values mean that these multi-Zn-expanded acenes possess a strong antiferromagnetic interaction except for 2Zn-tetracene (J = −173.6 cm−1) and 2Zn-pentacene (J = −88.1 cm−1) that possess weak ones. Clearly, anyone in the nZn-acene series can be utilized to assemble strong antiferromagnetic materials, while the 2Zn-acene series can be used to design magnetismtunable ones. Redox stability were also examined through comparing their ionization potentials (IPs) and electron affinities (EAs) and those of the corresponding acenes in view of possible practical applications of these multi-Zn-acenes. Comparison between both series reveals that IPs of two Zn-expanded acenes derived from the same parent acene are almost equivalent to each other. For example, IP of 6Zn-pentacene in the nZn-acene series is 5.30 eV, very close to that (5.28 eV) of 2Zn-pentacene in the 2Zn-acene series. For all Zn-expanded acenes, their IPs are

geometries, and the results are in line with the previous ones. For instance, LUMO occupation numbers of 6Zn-pentacene and 2Zn-pentacene are 0.709 and 0.970, and the amounts of the diradical character are 70.9% and 97.0%, respectively (see Table S13 in the Supporting Information). These results clearly indicate much enhanced diradical character for the multi-Znexpanded acenes relative to their corresponding parent acenes. This finding might be explained with the fact that the parent acene is a whole π-conjugated aromatic system with strong coupling between the two polyacetylene chains where electron delocalization over two chains can be achieved,50 whereas in the modified one, the whole molecule is divided into two parts so that electronic communication between both sides becomes very weak. To further explore the nature of the open-shell singlet diradical ground state in these acene derivatives, it is natural to focus on their frontier molecular orbitals in the respect that they could express the diradical character qualitatively and vividly. We visualized the nonbonding singly occupied molecular orbitals (SOMOs) and spin density distributions of all designed species (as shown in the Supporting Information) in their higher multiplets-mixed singlet states, and only those of 6Zn-pentacene and 2Zn-pentacene are shown for an illustration in Figure 3 and 4, respectively. The unpaired electrons reside in the almost degenerate nonbonding SOMOs, which are basically localized on the two different zones in each structure with a small number of shared atoms, leading to a spin density distribution over the entire molecule but being spin-up and spin-down distinguishable.1,50 The calculated spin densities in 6Zn-pentacene and 2Zn-pentacene (Table S2, Supporting Information) are in perfect accord with the spin density maps, and they are strong evidence of their diradical character. Indeed, all of these multi-Zn-expanded linear acenes possess an open-shell singlet diradical ground state, which is different from their parent acenes well-characterized by a closed-shell singlet ground state.1 To further clarify that the diradical character of these Znexpanded acenes originates from the expansion between two polyacetylene chains induced by Zn-intercalation, we determined the diradical percentage of three representative acenes, benzene, and anthracene by scanning their relaxed potential energy surfaces with respect to the cross-linking C−C bonds and calculated the LUMO occupation numbers at the CCSD(T)/6-31G(d,p)//UB3LYP/6-31G(d,p), CASSCF(6,6)/6-31(d,p)//HF/6-31G(d), and UB3LYP/6-31(d,p) levels (see Table S11, Supporting Information). These results clearly indicate that elongation of the cross-linking C−C bonds can trigger diradicalization of the acenes. In addition, the CCSD(T)-calculated diradical percentages are slightly larger 5905



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Supercomputer Center at SDU, and High-Performance Computational Platform at SDU-Chem.

slightly smaller than those of their corresponding parent acenes, and the decrements decrease from Zn-expanded benzene (2.70 eV) to Zn-expanded pentacene (0.83 or 0.85 eV for the two multi-Zn-expanded acene series). More importantly, although Zn-introduction can cause a slight lowering of IP of an acene, the IP values (5.28−6.39 eV) of these multi-Zn-expanded oligoacene diradicals are still larger than those (e.g., IP = 5.19 eV for decacene) of higher acenes featuring diradical character and high redox reactivity, and at least they are comparable to that (6.13 eV) of nondiradical pentacene. Similar variation trends are also observed in EAs (Table S5, Supporting Information). Overall, multi-Zn-modification can slightly increase electron-accepting ability of the acenes and makes them comparable to pentacene. Together with the small HOMO−LUMO gaps, both IP and EA results indicate that the two classes of multi-Zn-expanded acenes possess not only enough redox-stability but also enhanced and tunable magnetic properties.

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CONCLUSIONS In summary, we theoretically designed two intriguing classes of multi-Zn-expanded oligoacenes (from benzene to pentacene) through introducing Zn-arrays into acene rings in two ways and explored their relevant electronic/magnetic properties. Combined unrestricted DFT and CASSCF calculations together with a part of CCSD(T) calculations predict that all these multi-zinc-expanded acenes have the open-shell singlet diradical ground states, in contrast with the common fact that their corresponding parent oligoacenes are the closed-shell singlet systems. Our research offered the first theoretical prediction that the multi-Zn introduction into the acene ring(s) can make them diradicalized with a diversity of magnetic coupling interaction. The diradical character of the ground states of these molecules arises from the Zn-expansion-induced disjoint nature of the nonbonding molecular orbitals that are singly occupied in the diradicals. This work also provided a strategy for the preparation of well-defined singlet diradicals from acenes or their derivatives. Further examination of these predictions and property exploration of these molecules, especially their assemblies, are currently underway in our laboratories.

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ASSOCIATED CONTENT

S Supporting Information *

Computational detail and calculated data and figures including geometrical parameters, IPs, EAs, singlet−triplet gaps, HOMO−LUMO gaps, magnetic coupling constants, SOMOs and spin density maps, and others. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.S.); [email protected] (Y.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (20633060 and 20973101), NCET, and Independent Innovation Foundation (2009JC020) of Shandong University. A part of the calculations were carried out at Shanghai Supercomputer Center, High-Performance 5906



Article

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Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of Well-Defined

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