Letter Cite This: Org. Lett. 2018, 20, 445−448
pubs.acs.org/OrgLett
Stable meso-Fluorenyl Smaragdyrin Monoradical Hemanta Kalita, Tullimilli Y. Gopalakrishna, and Jishan Wu* Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore S Supporting Information *
ABSTRACT: The facile synthesis and physical characterization of a meso-fluorenyl smaragdyrin monoradical 4, which is stable due to efficient spin delocalization and kinetic blocking, is reported. It has a small energy gap and can be oxidized and reduced into the respective cation and anion, showing different charge distribution and electronic absorption properties.
Scheme 1. Synthesis of meso-Fluorenyl Smaragdyrin Monoradical 4
I
n recent years, researchers have devoted their focus toward stable π-conjugated monoradical to polyradical systems, as they are prospective materials for organic electronics, nonlinear optics, spintronics, and energy storage devices.1,2 These intriguing properties, viz. molecular and electronic properties, of radical systems encourage exploring stable novel molecular motifs. However, due to high reactivity, the stability of such an open-shell radical system is a critical issue, as it impedes isolation and characterization. It is envisaged that these organic materials should be easily synthesized and stable enough to carry out their desired functions from a practical perspective. Expanded porphyrinoids attained wide interest lately due to their structural and tunable electronic properties, and recently, it has been demonstrated that expanded porphyrinoid based diradicaloids could exhibit good stability.3 Smargdyrin is a congener of the expanded porphyrinoid family containing five heterocyclic rings, having a 22 π electron count, and three meso positions.4 It was discovered at the same time when sapphyrins, the other pentapyrrolic 22 π electron macrocycles, were discovered by Woodward and co-workers during their pioneering work on vitamin B12.5 The chemistry of the smaragdyrin macrocycle is unexplored mainly due to its inherent unstable nature unlike other pentapyrrolic macrocycles such as sapphyrins.6 The first stable meso aryl oxasmaragdyrin was reported by Chandrashekar et al. following a McDonald type [3 + 2] condensation approach.7 It has been studied extensively over the past two decades in light of its anion sensing properties in its protonated states, coordination chemistry, and good absorption and electrochemical properties exploited in molecular dyads and triads.8 The meso positions of smaragdyrin can be modified to synthesize smaragdyrin molecules with various aryl groups. Herein, we report the facile synthesis of a stable meso-fluorenyl smaragdyrin monoradical 4 from the corresponding meso-fluorenyl smaragdyrin 3 (Scheme 1). It is stable due to efficient spin delocalization onto the smaragdyrin framework and kinetic blocking. First, the meso-fluorenyl smaragdyrin 3 was synthesized in an overall 30% yield by acid-catalyzed condensation7 of 5-fluorenyl dipyrromethane 19 and 5,10-diphenyl-16-oxa-15,17-dihydrotripyrrane 210 (Scheme 1), followed by oxidation with 2,3© 2018 American Chemical Society
dichloro-5,6-dicyano-p-benzoquinone (DDQ). The precursor 1 was synthesized9 from pyrrole and fluorene-9-carbaldehyde and 2 from phenyl diol and pyrrole by acid catalyzed condensation. Compound 3 was purified by column chromatography with basic alumina using CH2Cl2/n-Hexane as the eluent, and it was collected as a green solid. The identity of compound 3 was confirmed by MALDI-TOF and high-resolution ESI mass spectrometry (Figures S1−S2 in Supporting Information (SI)). In the 1H NMR spectrum of compound 3, seven doublets in the 8.03−9.46 ppm region corresponding to eight β-pyrrole protons and one singlet at 9.50 ppm corresponding to two βfuran protons are observed, together with the fluorenyl and Received: December 5, 2017 Published: January 10, 2018 445
DOI: 10.1021/acs.orglett.7b03784 Org. Lett. 2018, 20, 445−448
Letter
Organic Letters
550−900 nm. However, the Q-bands of 3 are slightly redshifted by 5−10 nm compared to those of meso-triphenyl-25oxasmaragdyrin.8a On the other hand, the meso-fluorenyl smaragdyrin radical 4 exhibits one high energy band at 420 nm and two broad bands with peaks at 524 and 824 nm in the region expanding from 500 to 1100 nm (Figure 2). A comparison of the absorption spectra of compounds 3 and 4 shows that the Soret band of smaragdyrin radical 4 is blueshifted by ∼15 nm while the second Q-band is red-shifted to the near-infrared (NIR) region, implying a very different electronic structure. The optical energy gap of 4 is estimated to be 1.15 eV from the onset of the lowest energy absorption whereas this value is 1.65 eV for 3. A photostability test in toluene under ambient air and light conditions reveals that compound 4 has a half-life time of about 226 h (Figures S9 in SI). The electrochemical properties of the meso-fluoreneyl radical 4 along with compound 3 were investigated with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2 using tetrabutylammonium hexafluorophosphate (nBu4NPF6) as the supporting electrolyte, and the electrode potentials were externally calibrated with the ferrocene/ ferrocenium couple (Fc/Fc+). As shown in Figure 3, compound
phenyl signals resonating in the region 7.02−8.52 ppm (Figure S3 in SI). The spectrum was further assigned by 2D COSY/ NOESY measurements (Figures S4−S5 in SI). The mesofluorenyl smaragdyrin monoradical compound 4 was then obtained from compound 3, by treating it with 1.1 equiv of DDQ at room temperature for a period of 1 h. Compound 4 is stable enough under ambient conditions which facilitates purification on silica gel using CH2Cl2/ethyl acetate and is collected as a maroon colored solid. The identity of the compound was confirmed from MALDI-TOF mass and a molecular ion peak at 679.2466 in high resolution mass spectrum (Figures S7−S8 in SI). ESR measurements were performed for compound 4 in the solid and solution state which displays a broad signal with ge = 2.0027. Variable-temperature (VT) ESR studies performed for 4 in the solid state show that the ESR intensity (I) increases gradually with lowering the temperature (T) (Figure 1a). A linear relationship between I and 1/T was observed (Figure 1b), in accordance with its monoradical character.
Figure 1. (a) ESR spectra of compound 4 measured in solid state at various temperatures and (b) the variation of ESR intensity (I) versus the inverse of temperature (1/T).
The electronic absorption spectrum of smaragdyrin 3 in CH2Cl2 displays one intense high energy Soret band at 446 nm along with four low energy Q-bands in the region 500−800 nm (Figure 2). These absorption characteristics are similar to those of meso-triphenyl-25-oxasmaragdyrin.8a Compound 3 shows a split Soret band at 444 nm and four Q-bands in the region
Figure 3. Cyclic voltammograms (CV) and differential voltammograms (DPV) compounds 3 and 4 recorded in dry CH2Cl2 containing 0.1 M TBAPF6 as supporting electrolyte.
3 showed two reversible oxidation waves with the half-wave potential E1/2ox at 0.27 and 0.60 V, and no reversible reduction wave was detected in the electrochemical window, implying the electron-rich character of the smaragdyrin framework. The HOMO energy level was determined to be −5.0 eV from the onset potential of the first oxidation wave (Eoxonset) according to HOMO = −(4.8 + Eoxonset) eV. Compound 4 exhibited one oxidation with E1/2ox at 0.46 V and two reversible reduction waves with the half-wave potential E1/2red at −0.01 and −0.24 V, indicating that it can easily lose or accept electrons and become cationic/anionic species. Therefore, chemical oxidation and reduction of the monoradical 4 were investigated by titration and monitored by UV−vis−NIR absorption spectroscopy. Compound 4 can be chemically oxidized to its monocationic species by using 1.1 equiv of nitrosonium hexafluoroantimonate (NO·SbF6) in dry CH2Cl2 (Figure S10 in SI). The cation 4+ shows three split Soret bands at 425/450/486 nm and a redshifted Q-band at 837 nm (Figure 2). Further addition of oxidant does not yield any change in the absorption spectra.
Figure 2. UV−vis−NIR absorption spectra of compounds 3, 4 and the cation 4+ and anion 4− in solution. 446
DOI: 10.1021/acs.orglett.7b03784 Org. Lett. 2018, 20, 445−448
Letter
Organic Letters
in SI) by both the smaragdyrin and fluorenyl unit explain the good stability of the monoradical 4. On the other hand, the dihedral angle between the fluorenyl and smaragdyrin units in the cation 4+ (ca. 42°) and anion 4− (42°) is smaller than that in the neutral compound, indicating a stronger interaction in the charged species. Anisotropy of the induced current density (ACID) plot12 of 4 displays a clockwise ring current along the smaragdyrin macrocycle, corroborating its aromatic character (Figure 4b). This is further supported by nucleus-independent chemical shift calculation13 showing that the center of the smaragdyrin has a negative NICS(0) value of −11.28 ppm. This suggests that the relatively large dihedral angle between the fluorenyl and smaragdyrin units leads to less disturbance of the aromaticity of the smaragdyrin ring. The electrostatic potential map calculations reveal that the positive charge in 4+ is mainly distributed through the smaragdyrin macrocycle (Figure 4c and Table S1 in SI), while the negative charge in 4− is mainly localized at the fluorenyl unit. This is reasonable considering that the fluorenyl cation is antiaromatic while its anion is aromatic. Such a difference can explain their different absorption spectra where 4+ shows intense absorption in the NIR region while 4− display typical absorption of the segregated smaragdyrin chromophore (Figure 2). ACID calculations disclose weak clockwise ring current for 4+ and 4− (Figure S14 in SI) and the NICS(0) value at the center of the smaragdyrin macrocycle was calculated to be −4.95 and −3.69 ppm, respectively, indicating that charging results in weakening of the aromaticity of the smaragdyrin ring. In addition, in the neutral 4 and cation 4+, the fluorenyl unit shows localized aromaticity on the two benzene rings, while, in the anion 4−, the fluorenyl anion exhibited global aromaticity due to its overall aromatic character (Figure S14 in SI). In summary, we have designed and synthesized a stable mesofluorenyl smaragdyrin monoradical 4. It is stable both in solid state and in solution due to the efficient spin delocalization as well as kinetic blocking of the high spin density site. It has a small energy gap and displays amphoteric redox behavior. Its closed-shell cation and anion can be facilely obtained by chemical oxidation/reduction and exhibited very different electronic structure and absorption spectra. Our study provides a good example of using expanded porphyrinoid to build up stable monoradicals. Such stable radicals may find material applications as both an NIR absorption dye and a paramagnetic magnetic probe.
We tried to record the 1H NMR spectrum of the cation (Figure S11 in SI), but overlap of the peaks in a narrow range made the assignment of individual peaks complex. Furthermore, compound 4 was subjected to one-electron reduction by using potassium superoxide (KO2) in the presence of 18-crown-6 (18C6) (Figure S12 in SI). There is a color change of compound 4 from red to green on addition of KO2 in DMSO. Electronic absorption studies show that the absorption profile of the reduced species is similar to that of compound 3 with slight red shifts in the peak positions (Figure 2). Compound 4− also shows similar absorption characteristics to those of a mesotriphenyl-25-oxasmaragdyrin with a split Soret band at around 446 nm and four Q-bands in the region 500−900 nm.8a However, the Q-bands of 4− are slightly red-shifted by 5−15 nm. This observation suggests that the negative charge of the anion 4− is not participating in extending the conjugation with the macrocyclic core. The formation of anion 4− was further corroborated by the 1H NMR spectrum recorded by in situ reduction of 4 with sodium anthracenide (Na+An−) in dry THF-d8, which shows four sets of β-pyrrole proton signals and one set of singlets corresponding to furan (Figure S13 in SI). Density functional theory (DFT) calculations ((U)B3LYP/ 6-31G(d,p)) were performed to further understand the fundamental electronic structure of 4 and its charged species (4+ and 4−). It is found that in the neutral radical 4 the smaragdyrin moiety has a nearly planar geometry, with a torsional angle of ca. 52° with respect to the fluorenyl unit. The spin density is mainly distributed at the fluorenyl unit, but also partially delocalized to the smaragdyrin framework (Figure 4a,
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03784. Synthetic procedures and characterization data, additional spectra, and DFT calculation details (PDF)
Figure 4. (a) Calculated (UB3LYP/6-31G(d,p)) spin density distribution of 4; (b) calculated (UB3LYP/6-31G(d,p)) ACID plot of 4, and calculated (RB3LYP/6-31G(d,p)) electrostatic potential map for 4+ (c) and 4− (d).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure S15 in SI). Such partial delocalization character of the spin was further corroborated with the observed broad ESR spectrum, in contrast to the normally split ESR spectrum for 9substituted fluorenyl radicals.11 The spin delocalization as well as the kinetic blocking of the 9-position of the fluorenyl radical (the position with the highest spin density of 0.54; see Table S1
ORCID
Jishan Wu: 0000-0002-8231-0437 Notes
The authors declare no competing financial interest. 447
DOI: 10.1021/acs.orglett.7b03784 Org. Lett. 2018, 20, 445−448
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Organic Letters
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(13) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. V. R. Chem. Rev. 2005, 105, 3842.
ACKNOWLEDGMENTS We acknowledge financial support from the MOE Tier 3 programme (MOE2014-T3-1-004).
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REFERENCES
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DOI: 10.1021/acs.orglett.7b03784 Org. Lett. 2018, 20, 445−448
