Fluorenyl Based Macrocyclic Polyradicaloids - Journal of the American

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Fluorenyl Based Macrocyclic Polyradicaloids Xuefeng Lu,† Sangsu Lee,‡ Yongseok Hong,‡ Hoa Phan,† Tullimilli Y. Gopalakrishna,† Tun Seng Herng,§ Takayuki Tanaka,∥ María Eugenia Sandoval-Salinas,∇,⊥ Wangdong Zeng,† Jun Ding,§ David Casanova,⊥,# Atsuhiro Osuka,∥ Dongho Kim,*,‡ and Jishan Wu*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 03722, Korea § Department of Materials Science & Engineering, National University of Singapore, 119260 Singapore ∥ Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ∇ Departament de Ciència de Materials i Química Física, Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, Barcelona 08028, Spain ⊥ Kimika Fakultatea, Euskal Herriko Unibertsitatea & Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, 20018 Donostia-San Sebastián, Euskadi, Spain # IKERBASQUE - Basque Foundation for Science, Bilbao 48013, Euskadi, Spain ‡

S Supporting Information *

ABSTRACT: Synthesis of stable open-shell polyradicaloids including control of intramolecular spin−spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4−6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3−5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin−spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3−6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, onephoton absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

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INTRODUCTION The past few years have witnessed a rapid progress in the development of stable open-shell singlet diradicaloids, which exhibit unique electronic, optical and magnetic properties and have promising applications in organic electronics, spintronics, nonlinear optics and energy storage devices.1 The success in various diradicaloids2−8 motivated us to access stable open-shell polyradicaloids with significant polyradical characters, which is very challenging because molecules with a polyradical character are usually even more reactive than the diradicaloids. To achieve organic molecules with polyradical character, it is also critical to maintain a moderate intramolecular antiferromagnetic (AFM) exchange coupling.9 Stable polyradicaloids are of importance to understand the multiple spin−spin coupling and they are also practically useful for spintronic devices and molecular magnets. © 2017 American Chemical Society

We previously reported two fully fused carbazole macrocycles with moderate tetraradical and hexaradical characters, but the compounds are not very stable under ambient conditions and they decomposed during the silica gel column chromatography.10 On the other hand, a series of 3,6-linked fluorenyl radical oligomers up to hexamer (FR-n, n = 1−6, Figure 1a) synthesized by us demonstrated high stability due to the kinetic blocking of the 9-positions of the fluorenyl radicals by bulky anthryl groups.11 Moderate AFM exchange coupling between the fluorenyl radicals was observed and the higher oligomers display large diradical characters and moderate polyradical characters. The observed clear chain-length dependence of the physical properties provoked us to prepare the corresponding Received: July 18, 2017 Published: August 25, 2017 13173

DOI: 10.1021/jacs.7b07335 J. Am. Chem. Soc. 2017, 139, 13173−13183

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Journal of the American Chemical Society

Figure 1. Structures of 3,6-linked fluorenyl radical linear oligomers and macrocycles showing multiple spin−spin interactions.

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RESULTS AND DISCUSSION Synthesis. By using the optimal Suzuki coupling condition (using Pd2(dba)3-[(t-Bu)3PH]BF4 as catalyst), the macrocyclic precursors 3 and 4 containing methoxy groups were synthesized and isolated by preparative gel permeation chromatography (GPC) in 20% and 2% yield, respectively (Scheme 1a). Compounds 3 and 4 were then converted to the corresponding macrocyclic polyradicals FR-MC4 and FR-MC6 with SnCl2 in dichloromethane (DCM) in nearly quantitative yields (75% and 52% separation yield, respectively, due to sample loss during the chromatography). Importantly, both FR-MC4 and FR-MC6 are stable compounds and can be purified by routine silica gel column chromatography. However, this method can only generate macrocycles with even number of fluorenyl units. Therefore, alternative method was used for the synthesis of macrocycles with odd number of fluorenyl units. It was found that Ni(COD)2 mediated Yamamoto coupling reaction of the dibromo- monomer 1 in dilute condition (∼2 mM) worked well and gave the macrocyclic tetramer (3), pentamer (5) and hexamer (4) in 32%, 13% and 5% yield (after separation by GPC), respectively (Scheme 1b). Similarly, these precursors were converted into the corresponding fluorenyl radical macrocycles by reduction with SnCl2 in DCM. By this way, the pentamer FR-MC5 was also obtained. To synthesize the ethynylene bridged fluorenyl radical macrocycles, Pd-catalyzed Stille coupling reaction between 1 and bis(tributylstannyl)acetylene (6) was conducted in dilute condition (∼4 mM), which gave the macrocyclic trimer (7), tetramer (8) and pentamer (9) in 6%, 8% and 4% GPC separation yield, respectively (Scheme 1c). It seems that the reduced steric repulsion between the fluorene units after insertion of ethynylene unit makes the smaller size macrocycles

polymer. However, our attempted synthesis of the polymer by Suzuki coupling reaction between the dibromide 1 and diboronic ester 2 under various catalytic conditions (Scheme 1a) did not give the desired polymer, but mainly macrocyclic oligomers. This interesting finding suggests that we can obtain the macrocyclic fluorenyl radical oligomers by this method. Accordingly, 3,6-linked fluorenyl macrocycles FR-MC4 and FR-MC6, containing four and six kinetically blocked fluorenyl units, respectively, were isolated (Figure 1b). The macrocyclic pentamer FR-MC5 was also prepared but using an alternative synthetic approach (Scheme 1b). By comparison with their linear counterparts FR-4−FR-6, it was found that the geometric change from a flexible linear to a more rigid macrocyclic structure significantly influenced their electronic structure as well as physical properties. In particular, the distortional angle between the neighboring fluorenyl units plays a major role in the π-conjugation and intramolecular AFM coupling. To further tune the spin−spin interactions, an ethynylene moiety was inserted between the neighboring fluorenyl units, which can minimize the distortional angle between the fluorenyl units. Accordingly, three ethynylenebridged fluorenyl macrocycles MC-FnAn (n = 3−5) were synthesized (Figure 1c). In addition, macrocyclization of radicals may induce global (anti)aromaticity via cyclic conjugation like [n]annulenes,12 which is also of great interest to understand their unique electronic structures. In this article, we will report the successful synthesis and comprehensive physical characterization of these two new series of fluorenyl based macrocyclic polyradicaloids. A comparison among these three series of oligomers will give some new insights into the role of geometry and cyclic conjugation in their intrinsic polyradical character and physical properties. 13174


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Scheme 1. Three Strategies for the Synthesis of Fluorenyl Radical Macrocycles and Their Representative Resonance forms

Ground-State Geometries. Single crystals of FR-MC4 were successfully grown by slow diffusion of acetonitrile into the toluene solutions under ambient conditions and its crystallographic structure is shown in Figure 2a,b.13 The dihedral angle between the fluorenyl and anthryl moiety in FRMC4 is 72° (mean value), which is larger than that of the linear dimer FR-2 (59°),11 weakening the intramolecular spin−spin AFM coupling. The fluorenyl backbone adopts a saddle shape conformation due to macrocyclization induced strain, with torsion angles of 26.5° and 28.9°, which are larger than that in the single crystals of FR-2 (5.2°). The four C−C bonds linking the neighboring fluorenyl units are almost of the same length (1.468(6)/1.463(6) Å), which is longer than that of FR-2

as more favorable products. These precursors were then converted into the final ethynylene-bridged fluorenyl radical macrocycles MC-FnAn (n = 3−5), which are stable and can also be purified by silica gel column chromatography. All of the intermediates were well characterized by 1H/13C NMR and high-resolution mass spectra (HR MS). Because of their paramagnetic nature, the NMR spectra of the final products became broadened even at low temperatures, but their MALDITOF mass spectra and HR MS are consistent with the target structures (see Supporting Information (SI)). In addition, the structure of radical FR-MC4 was further confirmed by X-ray crystallographic analysis. 13175


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Figure 2. X-ray crystallographic data of FR-MC4: (a) top view; (b) side view; (c) selected bond lengths of the backbone and the calculated NICS(0) value in the center of the macrocycle. Optmised geometry of MC-F4A4: (e) top view; (f) side view; (g) selected bond lengths of the backbone and the calculated NICS(0) value in the center of the macrocycle. Calculated ACID plots of FR-MC4 (d) and MC-F4A4 (h), and the red arrows indicate the counterclockwise paratropic ring current flow.

units are predicted to have an open-shell singlet ground state, whereas those with an odd number of fluorenyl units are calculated to have a doublet state, all with substantially small vertical excitation energies from the low-spin (LS) ground state to the lowest high-spin (HS) excited state (Figure 3). The

(1.456(7) Å), again indicating that the bonding interaction between the four radicals are substantially weak. For all other macrocycles, their ground-state geometries are optimized at UCAM-B3LYP/6-31G** level of theory and compared with those of linear oligomers FR-n (n = 3−6), (Figures S1−S4 in SI). For all linear oligomers in all-trans configuration, the distortional angles between the fluorenyl units are substantially large (∼32°). However, macrocyclic trimer MC-F3A3 (Figure S1 in SI) and tetramer MC-F4A4 (Figure 2e,f) both have a nearly planar π-conjugated backbone. In MC-F3A3, the three ethynylene units are slightly bent due to strain. The distortional angles between the fluorenyl units in FR-MC5 and FR-MC6 (34°−56°) are even larger than those in linear oligomers, and due to steric strain, both have a distorted nonplanar structure. In particular, FR-MC6 has figure-of-eight geometry.14 The ethynylene-bridged pentamer MC-F5A5 is much more planar than FR-MC5. Therefore, incorporation of ethynylene unit indeed diminishes the steric repulsion between the neighboring fluorenyl units and leads to a more planar geometry compared with the corresponding directly linked fluorenyl macrocycles. Ground-State Electronic Strucstures, Polyradical Characters and Excitation Energies. Like the linear analogues FR-n, macrocycles FR-MCn and MC-FnAn can be drawn in several resonance forms. All of them are supposed to contribute significantly to the ground-state electronic structures because each variation involves the gain or loss of two aromatic sextet rings (the hexagons shaded in blue color) (Scheme 1). Their polyradical characters and vertical excitation energies were then calculated by the restricted active space spin-flip (RAS-SF/6-31G*) method,15a which has been demonstrated to be a better method than the spin-unrestricted density functional theory (DFT) method to deal with strongly correlated electron systems.10,11 The macrocycles with even numbers of fluorenyl

Figure 3. Calculated (RAS-SF) excitation energies from the LS ground state and the HS excited states of FR-MCn (n = 4−6) and MC-FnAn (n = 3−5).

calculated natural orbitals and the occupancy numbers in their respective ground states are shown in Figure 4. In all cases, the unpaired electron density is delocalized throughout the fluorenyl backbone, with the highest density localized at the carbon centers that link the anthryl units, indicating moderate AFM exchange coupling and large radical character. The radical 13176


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Figure 4. Calculated (RAS-SF/6-31G*) natural orbitals and occupation numbers of all macrocycles in their respective ground states.

2d) and MC-F4A4 (Figure 2h) both show a counterclockwise ring current delocalized along the periphery containing 36 and 44 (4n) π-electrons, respectively, implying a global antiaromatic character. In addition, both macrocycles have a positive NICS(0) value at the center of the macrocycle, +11.7 ppm for FR-MC4 and +14.26 ppm for MC-F4A4, further supporting their moderate antiaromatic character. Therefore, macrocyclization of linear tetramer leads to an antiaromatic cyclic π-conjugated system, which has an irresistible wish to become diradical. Accordingly, large diradical characters and moderate tetraradical characters were calculated for FR-MC4 and MC-F4A4. In addition, bond length analyses show that each fluorenyl unit is almost identical to each other (Figure 2c,g). The larger radical character of MCF4A4 compared to FR-MC4 may be ascribed to the longer

character of their ground-state structures was evaluated by the number of unpaired electrons (NU)15b and Yamaguchi’s polyradical characters indices yi (i = 0, 1, 2)16 (Table 1 and see SI for more details). MC-F3A3 (NU = 1.82) shows slightly smaller radical character than that of FR-3 (NU = 1.95) presumably due to the rigidified planar geometry which facilitates more efficient intramolecular AFM exchange coupling. Interestingly, from FR-4 (NU = 3.32) to FR-MC4 (NU = 3.46) and to MC-F4A4 (NU = 3.71), the radical character increases despite of the fact that the distortional angle between the fluorenyl units decreases. Such unusual observation could be due to cyclic π-conjugation effect in the macrocycles. Therefore, anisotropy of the induced currentdensity (ACID)17 plots and nucleus independent chemical shift (NICS)18 were calculated. The ACID plots of FR-MC4 (Figure 13177


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Table 1. Calculated (RAS-SF/6-31G*) Number of Unpaired Electron Numbers (NU), Polyradical Characters (yi, i = 0−2), Photophysical, Electrochemical and Magnetic Properties of FR-n (n = 3−6), FR-MCn (n = 4−6) and MC-FnAn (n = 3−5)a molecules

NU

y0

y1

y2

τ (ps)

σ(2)max (GM)

EHOMO (eV)

ELUMO (eV)

EEC g (eV)

Eopt g (eV)

ΔELS‑HS (kcal/mol)

FR-3 MC-F3A3 FR-4 FR-MC4 MC-F4A4 FR-5 FR-MC5 MC-F5A5 FR-6 FR-MC6

1.95 1.82 3.32 3.46 3.71 3.23 3.19 3.36 5.05 5.05

0.18 0.12 0.75 0.99 1.00 0.28 0.33 0.53 0.80 0.78

0 0 0.59 0.51 0.71 0.24 0.18 0.14 0.68 0.74

0 0 0 0 0 0 0 0 0.59 0.56

1.0 1.0 0.5 0.3 1.0 0.45 0.3 1.0 0.4 0.3

1100 1500 1500 1600 1600 1800 1500 2000 1800 1400

−4.77 −4.66 −4.83 −4.72 −4.65 −4.65 −4.70 −4.65 −4.74 −4.77

−3.87 −4.11 −3.82 −3.85 −4.02 −3.88 −3.95 −3.99 −3.82 −3.82

0.90 0.55 1.01 0.87 0.63 0.77 0.75 0.66 0.92 0.95

1.24 1.02 1.26 1.08 1.07 1.21 1.19 1.11 1.23 1.24

−2.19 −2.10 −2.94 −2.80 −2.86 −1.76 −1.66 −2.02 −2.65 −1.65

a τ is singlet excited state lifetime obtained from TA. σ(2)max is the maximum TPA cross section. HOMO and LUMO energy levels derived from opt electrochemical data. EEC g : electrochemical energy gap. Eg : optical energy gap. ΔELS‑HS: excitation energy from the low-spin (LS) ground state to the first high-spin (HS) excited state estimated from the SQUID measurements.

Figure 5. χMT−T curves in the SQUID measurements for the solids of FR-MCn (n = 4−6) and MC-FnAn (n = 3−5). The solid lines are the fitting curves according to different models (see details in Supporting Information); g-factor was taken to be 2.00.

resonances for the protons on the fluorenyl backbone are significantly broadened even at low temperature (193 K) (SI). At the same time, strong and broad ESR signals were observed in toluene at room temperature and even at low temperature (−90 °C) (Figure S6 in SI). SQUID measurements on the powders were conducted to investigate the temperature dependent magnetic susceptibility behavior and to estimate the excitation energies from the LS ground states to the lowest HS excited states (Figure 5). The obtained data are fitted by different models (see more details in SI). For MC-F3A3, considering its rigid and planar geometry, it should be a spinfrustrated system. Therefore, the isosceles triangle model19 (where one magnetic coupling J′ is different from two other couplings, J, see model in Figure 1c) was used and gave better fitting than the equilateral triangle model (Figure 4d). The doublet-quartet energy gap (ΔED‑Q = −2.10 kcal/mol) was determined to be slightly smaller than that of FR-3 (ΔED‑Q = −2.19 kcal/mol), which may be due to its spin-frustrated

spin−spin interaction distance (due to one additional ethynylene unit) although it has a more planar geometry. For similar reason, MC-F5A5 (NU = 3.36) shows a larger radical character than FR-MC5 (NU = 3.19). On the other hand, FRMC5 displays a slightly smaller radical character than FR-5 (NU = 3.23) presumably due to more rigid structure and efficient conjugation. FR-MC6 exhibits even larger radical character (NU = 5.05), which is almost the same as that of FR-6 (NU = 5.05). ACID plots of FR-MC5, MC-F5A5 and FR-MC6 reveal substantial effective π-conjugation between the fluorenyl units, but there is almost no ring current along the macrocyclic periphery (Figure S5 in SI). Magnetic Properties. The magnetic properties of all the macrocycles were investigated by variable temperature nuclear magnetic resonance (NMR), electron spin resonance (ESR), and superconducting quantum interference device (SQUID) measurements. Because of the existence of significant amount of thermally populated paramagnetic species, the 1H NMR 13178


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Figure 6. Comparison of the UV−vis-NIR absorption of the oligomers with the same number of fluorenyl unit: (a) FR-3/MC-F3A3, (b) FR-4/FRMC4/MC-F4A4, (c) FR-5/FR-MC5/MC-F5A5 and (d) FR-6/FR-MC6. Insets are the magnified onset absorption bands.

enhanced diradical character (Figure 6b). MC-F4A4 displays even longer absorption maximum (λmax = 779 nm) and absorption onset, with a slightly smaller optical energy gap (Eopt g = 1.07 eV) than that of FR-MC4 (Eopt g = 1.08 eV). A similar trend was observed for FR-5, FR-MC5 and MC-F5A5 (Figure 6c), but the red-shift from FR-MC5 to MC-F5A5 is much larger than that from FR-5 to FR-MC5, indicating that incorporation of an ethynylene unit between the neighboring fluorenyl units is a more efficient way to enhance the πconjugation. On the other hand, compared to FR-6, the absorption onset of FR-MC6 is blue-shifted, which can be explained by the larger torsion angles between the fluourenyl units and weaker π-conjugation. Open-shell singlet diradicaloids/polyradicaloids with moderate diradical/polyradical characters usually exhibit largely enhanced two-photon absorption (TPA) cross section.1,21 To understand how the geometric changes affect their third order nonlinear optical properties, the TPA measurements of all the macrocycles were conducted by using the open aperture Z-scan method in the wavelength range from 1200 to 2200 nm where one-photon absorption contribution is negligible (Figures S7 and S8 in SI and Table 1). It was found that the rigid and planar MC-F3A3 showed a maximum TPA cross section value (σ(2)max) of 1500 GM (at 1600 nm), which is larger than that of FR-3 (σ(2)max = 1100 GM at 1500 nm), indicating that more efficient π-conjugation is beneficial for the TPA cross section. Compounds FR-MC4 and MC-F4A4 displayed a similar TPA cross section maximum (σ(2)max = 1600 GM at 1400 and 1600 nm, respectively), which is slightly larger than that of FR-4 (σ(2)max = 1500 GM at 1400 nm), in accord with the enhanced radical character. However, FR-MC6 exhibited a σ(2)max of 1400 GM at 1600 nm, which is much smaller than that of FR-6

feature. For FR-MC4, the product of molar magnetic susceptibility and temperature (χM·T) increases after 200 K and fitting of the data using Van Vleck’s equation20 based on a cyclic tetrameric ring model gave a singlet−triplet energy gap (ΔES‑T) of −2.80 kcal/mol (Figure 4a). MC-F4A4 gave a slightly larger singlet−triplet energy gap (ΔES‑T = −2.86 kcal/ mol). Both compounds have a smaller singlet−triplet gap than that of FR-4, presumably due to their antiaromatic character and large radical character. MC-F5A5 exhibited a larger doublet-quartet energy gap (ΔED‑Q = −2.02 kcal/mol) than FR-MC5 (ΔED‑Q = −1.66 kcal/mol) and FR-5 (ΔED‑Q = −1.76 kcal/mol) likely due to its more planar geometry. For FR-MC6, the χM·T increases after 100 K and fitting of the data based on a cyclic hexameric ring model gave a ΔES‑T of −1.65 kcal/mol (Figure 4c), which is much smaller than FR-6 (ΔES‑T = −2.65 kcal/mol) due to the less effective AFM coupling between the fluorenyl radicals induced by the larger torsion angles. Therefore, both distortional angle and global antiaromaticity affect the radical character and excitation energies. Optical Properties. A comparison of the one-photon electronic absorption spectra of the oligomers in toluene with the same fluorenyl units is shown in Figure 6, and the key data are collected in Table 1. MC-F3A3 shows an obviously redshifted vibronic absorption spectrum compared to FR-3, with absorption maximum (λmax) at 814 nm and absorption onset beyond 1300 nm (Figure 6a). The optical energy gap (Eopt g = 1.02 eV) is significantly smaller than that of FR-3 (Eopt g = 1.24 eV). Therefore, macrocyclization induced rigid and planar geometry in MC-F3A3 leads to more efficient conjugation and a smaller energy gap. FR-MC4 exhibits an absorption maximum at 710 nm and a broader spectral response extending up to 1250 nm with a bathochromic shift of 150 nm compared to that of FR-4, presumably due to the antiaromaticity and 13179


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Figure 7. Cyclic voltammograms and differential pulse voltammograms of FR-MCn (n = 4−6) and MC-FnAn (n = 3−5) in DCM.

Figure 8. UV−vis-NIR spectra of neutral and oxidized species of FR-MCn (n = 4−6) and MC-FnAn (n = 3−5) in DCM during the spectroelectrochemical measurements. The applied potential is vs E(Fc+/Fc).

(σ(2)max = 1800 GM at 1400 nm), which again can be explained by the less effective π-conjugation. Femtosecond transient absorption (TA) measurements were conducted for all the macrocycles in toluene to understand their excited state dynamics (Figures S9 and S10 in SI and Table 1). Generally, a ground state bleaching (GSB) band in accordance with the one-photon absorption spectra and a broad excited state absorption (ESA) band were observed in the TA spectra. Because of the large radical characters, all these compounds exhibited ultrafast excited states lifetimes. All the ethynylene-bridged macrocycles MC-FnAn (n = 3−5) showed a similar excited state lifetime (τ) of 1 ps, whereas the directly

linked macrocycles FR-MCn (n = 4−6) exhibited nearly the same but shorter excited state lifetimes of 300 fs, which is also slightly shorter than that of FR-n (n = 4−6). Electrochemical Properties. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) of FR-MCn (n = 4−6) and MC-FnAn (n = 3−5) were recorded in dry DCM by using 0.1 M tetra-n-butylammoniumhexfluorophosphate as the supporting electrolyte (Figure 7 and Table 1). FR-MC4 displayed two reversible oxidation waves with Eox 1/2 = 0.01 and 0.55 V (vs Fc+/Fc) and three quasi-reversible reduction waves with Ered 1/2 = −1.07, −1.50 and −1.63 V. FR-MC5 exhibited four reversible oxidation waves (Eox 1/2 = −0.04, 0.04, 0.26, 0.46 V) 13180



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and four quasi-reversible reduction waves (Ered 1/2 = −1.01, −1.19, 1.33 and −1.52 V). FR-MC6 displayed four major quasireversible oxidation processes (Eox 1/2 = 0.03, 0.12, 0.25 and 0.44 V) and four quasi-reversible reduction processes (Ered 1/2 = −1.07, −1.23, 1.30 and −1.51 V). The HOMO and LUMO energy levels of FR-MC4, FR-MC5 and FR-MC6 were determined to be −4.72, −4.70, −4.77 and −3.85, −3.95, −3.82 eV, respectively, from the onset of the first oxidation/reduction wave. FR-MC5 has a higher lying HOMO compared to FRMC4 and FR-MC6 because it possesses odd number of electrons. The HOMO level of FR-MC4 is higher than FR-4 presumably due to its antiaromatic character. However, the HOMO energy levels of FR-MC5 and FR-MC6 are slightly lower in comparison to their respective linear analogous FR-5 and FR-6, respectively. The ethynylene-bridged macrocycles MC-FnAn (n = 3−5) showed less reversible redox waves, and DPV measurements revealed at least three oxidation and three reduction waves (Figure 7). Because of the electron-deficient nature of the ehtynylene unit, they exhibited lower lying LUMO energy levels compared with their respective linear oligomers FR-n (n = 3−5) and the directly linked macrocycles FR-MCn (n = 4−5) (Table 1). The measured electrochemical energy gaps (EEC g ) are all in consistence with the trend of the optical energy gaps (Table 1). Spectroelectrochemical measurements were conducted in dry DCM solutions and the absorption spectra of the major oxidized species at different electrode potentials (vs Fc+/Fc) are shown in Figure 8. FR-MC4 can be easily oxidized into diradical dications at 0.3 V, with three major intense absorption bands in the near-infrared (NIR) region (λmax = 1006, 1624, 2505 nm). The longest absorption maximum of the diradical dications is red-shifted compared to that of FR-4 (λmax = 2340 nm). Further oxidation to tetracations at 0.8 V led to two blueshifted NIR bands (λmax = 1021, 1625 nm). Similarly, FR-MC5 can be oxidized into triradical dications at 0.2 V (λmax = 1682 nm), diradical trications at 0.5 V (λmax = 1154, 1730, 2280 nm) and pentacations at 0.9 V (λmax = 1151 nm). FR-MC6 can be oxidized into triradical trications at 0.4 V (λmax = 1172, 2005 nm) and hexacations at 0.9 V (λmax = 1187, 2051 nm). For MCFnAn (n = 3−5), their monocations can be generated first, giving NIR absorption with maximum at 2380, 1840 and 2005 nm, respectively. Further oxidation to the respective dications resulted in slight shift of the longest absorption band (λmax = 2360, 1966, 2015 nm, respectively). The fully oxidized state can be reached by applying higher electrode potentials and they displayed an intense band at 1281, 1186 and 1243 nm, respectively. It was noted that the longest absorption band of the different cations of MC-F4A4 was always blue-shifted compared to the respective cations of the MC-F3A3 and MCF5A5, presumably because it possesses an even number of radicals, whereas the latter two have odd number of radicals. It turned out that spectroelectrochemical studies on the reduction process did not work well and thus chemical reduction by using strong reductant such as KO2 together with 18-crown-8 was conducted. Fully reduced multiple anions can be obtained by using excessive reductant, and all of FR-MCn (n = 4−6) gave a similar absorption spectrum with λmax at 386 nm, correlated to the absorption of the aromatic fluorenyl anion (Figure S11 in SI). On the other hand, the fully reduced anions of MC-FnAn (n = 3−5) showed a similar absorption spectrum with λmax at 380 nm, but with additional shoulder band at about 498 nm, presumably due to the extended π-conjugation with the ethynylene units.

CONCLUSIONS In summary, two series of stable fluorenyl based macrocyclic polyradicaloids were successfully synthesized by three different methods, mainly using Suzuki, Yamamoto and Stille coupling reactions, respectively. Because of the moderate intramolecular AFM exchange coupling, these macrocycles show moderate polyradical characters. Macrocyclization of the flexible linear oligomers results in more rigid structures with different distortional angles depending on the number of the fluorenyl units as well as the spacer. The incorporation of ethynylene units dramatically leads to an increase of the planarity and thus enhances the π-conjugation. Another important consequence of the cyclic conjugation is the observation of global antiaromaticity (although not strong) in the tetramers FR-MC4 and MC-F4A4, which leads to larger diradical characters and smaller energy gaps compared to the linear counterparts. Our systematic studies demonstrated how the geometry and cyclic conjugation influenced their radical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties, which is of importance for future design of stable polyradicaloids with tunable properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07335. Synthetic procedures and characterization data of all new compounds; details for all physical characterization and theoretical calculations; additional spectroscopic and Xray crystallographic data (PDF) Data for C41H35, (C2N) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Atsuhiro Osuka: 0000-0001-8697-8488 Dongho Kim: 0000-0001-8668-2644 Jishan Wu: 0000-0002-8231-0437 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.W. acknowledges financial support from the MOE Tier 3 programme (MOE2014-T3-1-004) and MOE Tier 2 grant (MOE2014-T2-1-080). The work at Yonsei University was supported by Global Research Laboratory (2013K1A1A2A02050183) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning. D.C. is thankful to Eusko Jaurlaritza and the Spanish Government MINECO/FEDER (projects IT58813 and CTQ2016-80955). M.E.S.-S. acknowledges CONACyT-México for a Ph.D. fellowship (ref 591700).

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