Preparation, Properties, and Structures of the Radical Anions and

Article Cite This: J. Am. Chem. Soc. 2017, 139, 15968-15976

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Preparation, Properties, and Structures of the Radical Anions and Dianions of Azapentacenes Lei Ji,† Alexandra Friedrich,† Ivo Krummenacher,† Antonius Eichhorn,† Holger Braunschweig,† Michael Moos,§ Sebastian Hahn,‡ Florian L. Geyer,‡ Olena Tverskoy,‡ Jie Han,∥ Christoph Lambert,*,§ Andreas Dreuw,*,∥ Todd B. Marder,*,† and Uwe H. F. Bunz*,‡ †

Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡ Organisch-Chemisches Institut and CAM, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270 and 225, 69120 Heidelberg, Germany § Institut für Organische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ∥ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen and Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: A series of diazapentacenes (5,14-diethynyldibenzo[b,i]phenazine, 6,13-diethynylnaphtho[2,3-b]phenazine) and tetraazapentacenes (7,12-diethynylbenzo[g]quinoxalino[2,3b]quinoxaline, 6,13-diethynylquinoxalino[2,3-b]phenazine) were reduced to their radical anions and dianions, employing either potassium anthracenide or lithium naphthalenide in THF. The anionic species formed were investigated by UV−vis−NIR, fluorescence and EPR spectroscopy, spectroelectrochemistry, and quantum chemical calculations. Single crystal X-ray structures of three of their radical anions and of three of their dianions were obtained. In contrast to the acenes, the anions of the azapentacenes are persistent and, in some cases, even moderately stable toward air, and were characterized.

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INTRODUCTION We describe the preparation of a series of radical anions and dianions of several oligoazaacene derivatives and discuss their properties in the solid state and in solution. Azaacenes,1−5 particularly the symmetrical tetraazapentacene,6 are attractive n-channel charge transport materials7,8 that have been employed in thin-film transistors with mobilities of up to several cm2 V s−1.9 The charge transporting moiety is thought to be the radical anion of the azaacenes. As this species is only fleetingly present in low concentrations in thin film devices, it is of fundamental interest to investigate such species in bulk. An important question is that regarding the influence of the negative charge upon the molecular structure of the azaacene framework. If it is distorted, the charge might be localized on a particular site in the moleculemost probably the nitrogen atoms. Such behavior could indicate that nchannel charge transport might be hindered, due to a trapping of the charge on the molecule. In a more fundamental way, the anions and dianions of different azapentacenes are fascinating objects for study, as the azaacenes are relatively easily reduced by standard reducing agents such as potassium anthracenide or lithium naphthalenide; they give reasonably stable anionic species due to their © 2017 American Chemical Society

low reduction potential. As the number and the position of the nitrogen atoms are easily permuted, a consequence of recent advances in synthetic methodology, one has an exquisitely tuned tool to examine the variation of structures, properties and spectra, depending upon position and number of nitrogen atoms. As a bonus, recent progress in azaacene chemistry allows the introduction of TIPS-acetylene groups as solubilizing and stabilizing substituents. These simplify the handling and crystallization of the radical anions and dianions. Interestingly enough, radical anions and dianions of larger aromatic hydrocarbons have been prepared, but the number of crystal structures of such species is limited. A significant number of those have been obtained by Bock and co-workers.10−12 Particularly interesting is that Bock et al. also prepared the radical anions, (Na+ and K+) salts of 2,3-diphenylquinoxaline.10 In their seminal paper, Bock et al. observed that both X-ray crystal structures (of the neutral and reduced compounds) show very similar bond lengths and bond angles; they also noted that the structural changes upon going from the neutral diphenylquinoxaline to the radical anions are relatively minor, Received: September 12, 2017 Published: October 7, 2017 15968

DOI: 10.1021/jacs.7b09460 J. Am. Chem. Soc. 2017, 139, 15968−15976

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Journal of the American Chemical Society yet, in this small system, an increase of two of the four C−Nbond lengths from 1.32 to 1.37 Å is observed. On the basis of MNDO calculations, Bock et al. speculated that increased charge density accounted for the slightly elongated bonds. The distances between the sodium cations and the nitrogen centers are around 2.4−2.5 Å (2.8 Å for K+−N), typical for contact ion pairs in nitrogen-based resonance-stabilized anions. In addition to these CN-heterocycles, dianions/radicals of CB-heterocycles are known, persistent and have been structurally characterized.13−18 While Bock and coauthors looked at pyrazine features, they also prepared and structurally investigated tetracene and rubrene radical anions,11,12 which are structurally related to azaacenes. In the case of tetracene, it was not possible to obtain the radical anion, as it is thermodynamically unstable toward disproportionation, but the dianion of tetracene could be crystallized. Bock et al. noted that, with increasing size of the aromatic hydrocarbon frame, the disproportionation of the radical anion is thermodynamically advantageous. The tetracene dianion salt forms black-violet crystals with a structure exhibiting a planar tetracene frame. The bond lengths in the peripheral rings are equalized and, therefore, different from the bond lengths in tetracene itself, where the 1,2 bond is shorter than the 2,3 bond. This is entirely reasonable, as the dianion of tetracene is a 4n-π-system, in which the antiaromaticity has a strong delocalizing effect.19 Are the azapentacenes similar to tetracene and their relatives or do they show differences with respect to reduction?

Scheme 1. Preparation of the Anions and Dianions of Compounds 1−4a

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RESULTS AND DISCUSSION In a previous short communication, we disclosed the reduction of the symmetrical tetraazapentacene-derivative 4 to its radical anion and dianion,20 and herein, we provide a full account of the reduction of the azaacenes 1−4. Scheme 1 shows the reduction of the starting materials, the syntheses of which have been published recently (1, 221 3,22 46). The azaacenes were dissolved in THF, and a single equivalent of 5, [K·18-crown-6· THF2]-anthracenide, was added. For 2, 3 and 4 the radical anions were crystallized, and single-crystal X-ray structures (vide infra) were obtained. We observed that none of the azaacene-based radical anions disproportionate. Upon addition of a second equivalent of reducing agent 5, the azaacene-dianions form and, in the case of 2 and 4, single crystals of the dianions were obtained. Other compounds, 1 and 3, did form their dianions with K+ as counterions; however, from these solutions we could not grow any single crystalline specimens. As an alternative, 1 was reacted with lithium naphthalenide 6 to give a solution of the dianion lithium salt [12− 2Li+] from which suitable single crystals were grown. Optical and Electronic Properties. Figure 1 and Table 1 show the spectra and the optical data of solutions of the investigated azaacenes 1−4 in their three different redox states (neutral, radical anion, and dianion), generated by spectroelectrochemical experiments in THF/N(n-Bu)4PF6 (TBA-PF6) solution (for more details, see Figure S1 in the Supporting Information (SI)). In Table 1 we also included data for the emission spectra of the neutral and dianionic species (from the isolated crystals) in Et2O (see also Figure S3 in the SI). Generally, the spectra of the species with the same reduction state are similar to one another. Thus, the neutral species all absorb in the range between ca. 13 500−19 000 cm−1 (662− 727 nm) with a pronounced vibrational progression; the

a 5: [K·18-crown-6·THF2] anthracenide in THF; 6: lithium naphthalenide in THF.

exception is compound 3, for which the lowest energy absorption band is more diffuse (Table 1). In principle, and in an (over)simplified picture, the number and positions of the N atoms do not have much influence on the optical properties of the neutral azaacenes, and thus they are all very similar to that of TIPS-pentacene.23 The observed differences stem from the perturbation of the coefficients of the HOMO upon electronegative substitution and have been discussed in detail.24,25 Upon reduction (either chemically in Et2O or by spectroelectrochemistry in THF, see Figure 1) to the radical anion, all of the azaacenes 1−4 show a dramatic red-shift of the absorption into the NIR, in the range of 6500−11 000 cm−1 (900−1500 nm). This absorption band is less intense than that of the neutral compound but shows a similar vibrational progression. While the neutral compounds are somewhat fluorescent in Et2O, the radical anions are nonfluorescent but are quite stable. In particular, the radical anion of 4 is stable in Et2O solution under air for several hours. Owing to a strong redox potential difference between M/M•− and M•−/M2− (for the cyclic voltammograms see insets in Figure 1), the radical anion is stable to disproportionation into the dianion and the neutral compound. The well-defined vibrational structure and the high intensity of the NIR absorption band indicate a typical ππ* transition and thus 15969


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Journal of the American Chemical Society

Table 1. Photophysical Properties of Azapentacene 1−4 and Their Anions ν̃maxa/cm−1 (nm)

ν̃vibb/ cm−1

λflc,d/nm (cm−1)

1 2 3 4 1•− 2•− 3•− 4•− 12−

14 900 (671) 14 500 (690) 13 900 (719) 14 800 (676) 7600 (1316) 7100 (1408) 7500 (1333) 7100 (1408) 15 800 (633)

1300 1300 1000 1300 1400 1300 1400 1300 1400

676 (14 800) 722 (13 900) not emissive 692 (14 400) − − − − 616 (16 200)

4.7 14

2.5 9.0

36 − − − − 87

22− 32−

16 500 (606) 16 200 (617)

1300 1400

605 (16 500) 585 (17 100)

61 7.3

42−

16 600 (602)

1400

598 (16 700)

95

13 − − − − 5.2(10%), 8.3(90%) 5.5 2.5(20%), 5.7(80%) 6.1

Φe/%

τf/ns

a

Spectroelectrochemistry in THF/0.2 M TBA-PF6. bMain vibrational progression. cAll emission measurements were performed in dilute Et2O solutions with absorbance at the absorption maximum less than 0.2, data of the dianions are from the isolated salts. dFluorescence maximum. eFluorescence quantum yield. fFluorescence lifetime.

suggest a delocalization of the unpaired electron over the whole molecular skeleton. Further chemical or (spectro)electrochemical reduction is possible, and leads invariably to the doubly reduced species 12−−42−, all of which form red crystalline materials. The spectra of these dianions, as determined by spectroelectrochemistry, are displayed in Figure 1. The dianions give rise to blue-shifted absorption spectra with a lowest energy band between 16 000 and 20 000 cm−1. The dianion band again shows a strong vibrational progression. For 32− we noticed a strong influence of the water content of the solvent on the position of the band. Upon addition of moist THF in the spectroelectrochemistry experiment, this band shifts by ca. 1500 cm−1 to higher energy. However, it is unknown how many protons are attached to the nitrogen atoms in this case. The interpretation of the lowest energy absorption of the neutral, anion and dianion species of all compounds can be understood in terms of HOMO−LUMO (M), HOMO-SOMO (M•−) and HOMO−LUMO (M2−) excitations.26 While most of the dianions are highly fluorescent, the most unsymmetrical dianion 32− shows only a low fluorescence quantum yield. The low quantum yield of 32− might be caused by partial protonation, but we note also that 3 itself is non-emissive. EPR Spectra of the Radical Anions. With the help of density functional theory (DFT UB3LYP/EPR-iii) calculations, the solution EPR spectra of the radical anions 1•−, 2•−, 3•− and 4•− could be satisfactorily simulated (Figure 2; best-fit parameters are given in the caption). The spectra show contributions from several partially resolved 1H and 14N hyperfine couplings, which are smaller than those in azaacenes with fewer fused benzene rings, such as phenazine (a(14N) = 14.4 MHz, a(1H) = 5.41 and 4.51 MHz),27,28 quinoxaline (a(14N) = 15.8 MHz, a(1H) = 9.30, 6.50, 2.80 MHz)27,28 or 5,6,11,12-tetraazanaphthacene (a(14N) = 8.35 MHz, a(1H) = 3.92 and 2.35 MHz),28 consistent with an increased electron delocalization. Single Crystal Structures of the Radical Anions of 2, 3 and 4. We have obtained single crystals from several of the reduced species, and their molecular structures are shown in

Figure 1. Absorption spectra of compounds 1−4 in their neutral (black), monoreduced (green) and dianionic form (red) determined by spectroelectrochemistry in THF/TBA-PF6. The insets display the cyclic voltammograms. The absorption spectra of the isolated salts (Figure S2) and the reduction potentials of 1−4 (Table S1) are shown in the SI. 15970


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Figures 3−5. Unfortunately, we were unable to grow suitable single crystals for the reduction products of 1 as potassium

Figure 3. Molecular structure of the radical anion 2•− with [K(18crown-6)(THF)2]+ as the counterion. The large heterocycle is rotationally disordered with respect to the alkyne axis, and the bond lengths are thus of limited value in discussing the detailed structure of the anion.

salts, but obtained crystals of the lithium salt of its dianion (Figure 6). In the case of 2•−, we obtained single crystals, but the large azaacene is disordered with respect to the position of the nitrogen atoms, which occupy all of the four possible positions (50% occupation) as shown in Figure 3. Therefore, a meaningful bond length analysis is not possible, but we conclude that the potassium cation does not have a strong attraction to the large azaacene moiety, or some localized bonding would have been observed. Instead, the potassium ion prefers coordination to the crown ether and two THF molecules, suggesting that the charge distribution on the ligand is fairly uniform, without any charged “hotspots”. In the case of 3 we isolated the radical anion and obtained its single-crystal structure (Figure 4). All bond lengths in the radical anion 3•− are similar to those in 3.22 The bonds shown in red and blue for 3•− in Figure 4 are those whose lengths change by more than 3σ (here σ2 = σ32 + σ32), but the differences are only ca. 0.02 Å, distributed among all of the rings of the azapentacene skeleton. There is no noticeable bond length change in the TIPSacetylene group. The C−N bond lengths in two of the rings are slightly increased, but the effect is small. In essence, the geometry of the azaacene is preserved. The molecular structure of 4•− is depicted in Figure 5. In the crystal, the radical anion of 4 lies on an inversion center.20 The important bond lengths of the tetraazapentacene cores are summarized in Figure 5. In 4•−, the K+ ion is fully separated from the heteroacene; it is coordinated by two THF molecules and the 18-crown-6 belt around its equatorial coordination sites. Thus, the K+ ion does not interact at all with the aromatic unit, and the changes of the bond lengths are fairly small and not easily interpreted. Interestingly, the same is true for the dianion of 4 (vide infra).20 Single-Crystal Structures of the Dianions of 1, 2, and 4. In a formal sense, the dianions of 1, 2, and 4 are antiaromatic, as the dianions represent 4n-π-systems. In the dilithio salt of 1, there is an interaction between the lithium cations and the azaacene unit, as Li+ cations are considerably smaller and “harder” than K+ ions; their interaction with the

Figure 2. Experimental (black) and simulated (red) continuous-wave (CW) X-band EPR spectra of 1•−−4•− in a toluene/THF mixture. Experimental parameters: temperature = 298 K; microwave frequency = 9.38 GHz (1 and 3), 9.85 GHz (2 and 4); modulation amplitude = 0.1 G; conversion time = 60 ms; modulation frequency = 100 kHz. Simulation parameters: 1•−: g = 2.004, a(14N) = 9.67 MHz, a(1H) = 5.52, 3.56, 2.72, 1.75, 1.05 MHz; 2•−: g = 2.004, a(14N) = 6.80 MHz, a(1H) = 5.28, 3.44, 3.42, 1.04, 0.97 MHz; 3•−: g = 2.004, a(14N) = 5.79 and 4.63 MHz, a(1H) = 2.38, 2.28, 1.66, 1.21 MHz; 4•−: g = 2.004, a(14N) = 5.32 MHz, a(1H) = 1.54, 1.40, and 1.35 (methine protons) MHz. 15971


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Figure 4. Molecular structure of 3•− is depicted (top). One of the two THF molecules of the [K(18-crown-6)(THF)2]+ counterion shows positional disorder. The important bond lengths of the tetraazapentacene cores of 3, reported by Bunz and co-workers,22 and 3•− (this study) are summarized (bottom). Bond length labels of bonds shortened upon reduction are shown in blue, those lengthened upon reduction in red.

Figure 5. Molecular structure of 4•− in the solid state, recently reported by Marder, Bunz and co-workers (top).20 Hydrogen atoms and solvent molecules (THF) are omitted for clarity. Atomic displacement parameters correspond to 50% probability at 100 K. Visible is the moderate change of different bond lengths of the tetraazapentacene cores of 4 (Bunz et al. 2009),6 4•−, and 42− (Marder, Bunz and co-workers;20 bottom); however, the sum of all unique bond lengths is 1.81 nm for 4, its radical anion and its dianion. Bond length labels of bonds shortened upon reduction are shown in blue, those lengthened upon reduction in red.

azaacene is considerably greater. Interestingly, there are only two molecules of THF attached to the Li cation, and there is an interaction with the alkyne group (Figure 6). Looking at the changes in bond lengths of 121 when going from the neutral compound to the dianion, we find that, in the outer rings, there is a significant decrease of the bond length alternation (Figure 6, 7). The same is true (vide infra) for the dianion of 4 (Figure 7).20 This is in line with the observation by Bock et al. for the dianion of tetracene11 and is a testament to the formation of a larger antiaromatic system in which the antiaromaticity induces delocalization, according to Schleyer et al.19 In the solid-state structure of 22− (Figure 8), the azaacene is centrosymmetric, and disordered with respect to the position of the nitrogen atoms. Each of the four possible positions (shown as C/N in Figure 8) have 50% occupation of nitrogen and 50% occupation of carbon. Although the potassium is coordinated by the C−C/N bond, the bond length of the K−C bond (3.295(1) Å) is shorter than that of the K−C/N bond (3.460(1) Å). It is also significantly longer than the reported K−N bond length (2.8 Å) in the potassium salt of 2,3diphenylquinoxaline,10 indicating the negative charges are delocalized in 22− instead of pinned at the disordered position

of the C/N atoms. The long K−C and K−C/N bond also reveals that the interaction between the large azaacene and the potassium cation is weak. The bond lengths of the C−C/N bonds that coordinate to the potassium cations are similar to those of the C−C/N bonds uncoordinated to potassium. The molecular structure of 42− is depicted in Figure 9. In the case of the dianion, the two potassium ions are not fully screened from the aromatic face; however, inspection of the single crystal structure shows that the weak coordination of the potassium ion does not seem to influence the bond lengths significantly. The two K−C bond lengths (3.240(2) and 3.264(2) Å) are fairly similar, which are similar to the K−C bond length in 22−. Notably, the potassium ions do not interact with the pyrazinic nitrogen atoms at all; the charge is essentially 15972


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Figure 7. Changes of bond lengths upon going from the neutral compound to the dianions of 1 and 4. Bond length labels of bonds shortened upon reduction are shown in blue, those lengthened upon reduction in red.

Figure 8. Molecular structure of the dianion 22− with [K(18-crown6)(THF)2]+ as the counterion. The large heterocycle is rotationally disordered with respect to the alkyne axis, and the bond lengths are thus of limited value in discussing the detailed structure of the anion.

Figure 6. Molecular structure of the dilithio salt of 12− (top). Each lithium atom (labeled in green) is chelated by one of the nitrogen atoms and the CC triple bond, as well as coordinated to two THF molecules. This coordination persists in the presence of a large excess of 12-crown-4 or TMEDA. One lithium atom, Li1, lies almost in the plane of the pentacene unit and the angle between N1−Li1 and the pentacene is 2.81(16)°, while the other lithium atom has an angle of 27.04(17)° between N2−Li2 and the pentacene. The decrease of the bond length alternation between 1 and the dianion 12− is shown in the bottom figures. Bond length labels of bonds shortened upon reduction are shown in blue, those lengthened upon reduction in red.

fully delocalized in this dianion and not pinned at the positions of the pyrazinic nitrogens (see Figures 5 and 9). Quantum Chemical Calculations. The geometries of the neutral, anionic and dianionic species of 1−4 have been optimized using density functional theory (DFT) in combination with the long-range corrected exchange-correlation functional CAM-B3LYP and the 6-311G** basis set, employing a polarizable continuum model for THF solvation. The changes in the bond lengths follow the same trend as the experimentally determined X-ray structures (see Figure S7 in the SI). While the anions are open-shell radicals with a doublet ground state,

Figure 9. Molecular structure of the dianion of 4.20 The molecule lies on an inversion center. 15973


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Journal of the American Chemical Society the neutral and dianions are closed-shell molecules with a stable electronic ground state, and no triplet instabilities exist. As a first step, the relative stabilities of the anions and dianions of 1−4 in THF with respect to their neutral forms have been investigated. Therefore, based on the optimized geometries, the energy differences between the neutral species and the anion or dianion have been computed at DFT/CAMB3LYP/6-311++G** level. In Table 2, it can be seen that the

Table 3. Computed Vertical Absorption Energies, Wavelengths and Oscillator Strengths of the Lowest Excited State of 1−4 and Their Anions and Dianions, as well as the Main Orbital Contributionsa 1 2 3 4 1•− 2•− 3•− 4•− 12− 22− 32− 42−

Table 2. Computed Energies of the Anions E[M•−] and Dianions E[M2−] of 1−4 Relative to the Neutral Parent Molecule and Corresponding Disproportionation Reaction Energies ΔEdisa 1 2 3 4

E[M•−]b kJ/mol

E[M2−]b kJ/mol

ΔEdisc kJ/mol

−361 −357 −404 −391

−616 −612 −700 −684

105 101 107 98

νmaxa/cm−1

λabs/nm

fosc

excitation characterb

16 050 15 320 15 160 16 130 8950 7980 9030 8310 18 790 20 160 19 360 20 240

623 653 660 620 1117 1253 1107 1203 532 496 517 494

0.26 0.27 0.21 0.32 0.10 0.11 0.08 0.12 1.00 0.98 1.06 1.11

H → L (98.9%) H → L (99.4%) H → L (98.4%) H → L (99.2%) Hβ → Lβ (93.7%) Hβ → Lβ (95.2%) Hβ → Lβ (88.8%) Hβ → Lβ (96.6%) H → L (95.8%) H → L (93.9%) H → L (97.2%) H → L (96.7%)

a Computed using TDDFT/CAM-B3LYP/6-311++G** using a PCM model for THF solution. bFor the shape of the orbitals see Figure 10.

a

Computed using DFT/CAM-B3LYP/6-311++G** using a PCM model for THF solution. bA negative sign indicates the anion/dianion to be more stable than the neutral compound. cA positive energy means the disproportionation reaction is endothermic.

coefficients of the dianions, in comparison to those of the neutral compounds measured experimentally. The interpretation of the transitions in the open-shell anions is slightly more involved, as these anions possess two sets of orbitals, one for the alpha and one for the beta electrons. As an example, the molecular frontier orbitals and their energies of 3, 3•−, and 32− are shown in Figure 10 (for those of other compounds, see Figure S4−S6 in the SI). In the azapentacenes 1•−−4•−, however, the lowest excited state can be understood as an electronic transition from the HOMO of the neutral into the now half-filled LUMO of the anion. This is in accord with the fact that the experimentally determined molar extinction coefficient of the D0 → D1 transition of the monoanion is only half that of the corresponding neutral compound. The D0 → D2 transition of the monoanion is from the half-filled LUMO to the LUMO+1 of the neutral compound, which is the same as the S0 → S1 transition of the dianion. This again explains why the molar extinction coefficient of D0 → D2 transition of the monoanion is smaller than that of the S0 → S1 transition of the dianion. Overall, our calculations fully support the interpretation of the measured absorption spectra as those of the free neutral, anionic and dianionic species of 1−4 and explain the observed trends nicely.

anions are strongly stabilized in THF by 300−400 kJ/mol. The same is true for all dianions, which are more stable than the neutral compounds by 600−700 kJ/mol and more stable than the anions. Hence, the dianions are also electronically stable entities in THF solution. To investigate whether the anionic species disproportionate into neutral and dianionic species, the reaction energies of the disproportionation reactions were calculated for 1•−-4•− (Table 2). This energy is given as ΔEdis = E[M2−] − 2E[M•−] and it can be seen that the disproportionation is endothermic by approximately 100 kJ/mol for all anions. Even though the dianion is strongly stabilized by THF solvation, it is not sufficient to provide a large enough driving force to make the disproportionation of the anions energetically feasible. For this to happen, an additional stabilization of the dianions (by the counterions for instance) would be necessary. These results confirm the experimental observation that 1−4 form stable isolated anions and dianions in THF solution. To further corroborate the existence of free anionic and dianionic species of 1−4 in THF, the vertical excited states of the parent neutral, anionic and dianionic molecules have been computed at the theoretical level of TDDFT/CAM-B3LYP/6311++G**, again in combination with a PCM for THF solution. The results for the first excited state of all molecules are in excellent agreement with the observed experimental absorption spectra (Table 3; for more details, see Table S3− S10 in the SI). While the neutral species exhibit a first excited S1 state at excitation energies between 15 150 and 16 130 cm−1, the corresponding open-shell anions have a strongly red-shifted first excited state between 7980 and 9030 cm−1 at the level of TDDFT/CAM-B3LYP. The first excited states of the dianions are then strongly blue-shifted, even with respect to the neutral compounds, and occur between 18 790 and 20 240 cm−1. All lowest excited states correspond to typical ππ* excited states, as can be seen from the main orbital contributions to the electronic transitions. The S0 → S1 transition of the closed-shell neutral and the dianions are typical HOMO → LUMO transitions,26 but the latter have much larger oscillator strengths. This is in agreement with the larger molar extinction

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CONCLUSIONS In this work, we have investigated the reduction of four different azapentacenes containing two or four nitrogen atoms in the ring systems and found that the radical anions are stable with respect to disproportionation into the dianion and the neutral compound. All of the respective dianions were prepared and were sufficiently stable when air and water are excluded. The charged species were investigated by UV−vis−NIR spectroscopy, spectroelectrochemistry, by EPR (radical anions) and quantum chemical methods. The radical anions show a strongly red-shifted absorption (into the NIR), while the dianions all have blue-shifted absorptions and are highly fluorescent, with the exception of 32−, which is weakly fluorescent. The one- and two-electron reductions do not lead to a dramatic change in the overall geometry of the investigated azapentacenes, even though for the dianions, the loss of bond length alternation is in line with 15974


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recorded using an Edinburgh Instruments FLSP920 spectrometer equipped with a double monochromator for both excitation and emission, operating in right angle geometry mode. The fluorescence quantum yields were measured using a calibrated integrating sphere (150 mm inner diameter). Lifetime measurements were conducted using the time-correlated single-photon counting method (TCSPC) on the FLSP920 spectrometer equipped with a high-speed photomultiplier tube positioned after a single emission monochromator. Decays were recorded to 10 000 counts in the peak channel with a record length of at least 1000 channels. The quality of all decay fits was judged to be satisfactory, based on the calculated values of the reduced χ2 and Durbin−Watson parameters and visual inspection of the weighted and autocorrelated residuals. All absorption and emission spectra were recorded in standard quartz cuvettes (1 cm × 1 cm) under argon. EPR Measurements. EPR measurements at X-band (9.38 GHz) were carried out at room temperature using a Bruker ELEXSYS E580 CW EPR spectrometer. CW EPR spectra were measured using 0.1 mW microwave power and 0.1 G field modulation at 100 kHz, with a conversion time of 120 ms. The spectral simulations were performed using MATLAB 8.6 and the EasySpin 5.0.18 toolbox.31 Single-Crystal X-ray Diffraction. Crystals suitable for singlecrystal X-ray diffraction were selected, coated in perfluoropolyether oil, and mounted on MiTeGen sample holders. Diffraction data were collected on Bruker X8 Apex II 4-circle diffractometers with CCD area detectors, using Mo Kα radiation monochromated by graphite or multilayer focusing mirrors. The crystals were cooled using an Oxford Cryostreams or Bruker Kryoflex II low-temperature device. Data were collected at 100 K. The images were processed and corrected for Lorentz-polarization effects and absorption as implemented in the Bruker software packages. The structures were solved using the intrinsic phasing method (SHELXT)32 and Fourier expansion technique. All non-hydrogen atoms were refined in anisotropic approximation, with hydrogen atoms “riding” in idealized positions, by full-matrix least-squares against F2 of all data, using SHELX33 software. Diamond34 software was used for graphical representation. Crystal data and experimental details are listed in Table S2 in the SI; full structural information has been deposited with Cambridge Crystallographic Data Centre. CCDC-1550098 (12−), 1550099 (2•−), 1550100 (22−), and 1550101 (3•−). Cyclic Voltammetry and Spectroelectrochemistry. All measurements were performed in 0.2 M TBA-PF6 in THF under an argon atmosphere. THF was distilled over sodium wire prior to use and stored under argon over molecular sieves (3 Å) overnight. The synthesis of TBA-PF6 is described elsewhere.35 Voltammograms were recorded using a Gamry Reference 600 Potentiostat and a three electrode cell (1 mm Pt-disc working electrode, Pt counter and pseudoreference electrode). All voltammograms were referenced to the Fc/Fc+ redox couple. Substance concentration was ∼1.5 × 10−3 M. Spectroelectrochemistry: Spectra of the anions were recorded in reflection mode in a three-electrode custom-made cell (6 mm Pt-disc working electrode, Pt counter and pseudo reference electrode) implemented in an Agilent Cary 5000 UV−vis−NIR spectrometer. The optical path was adjusted to 100 μm with a micrometer screw. Potentials were applied with a PAR 283 Potentiostat (Princeton Applied Research). Substance concentration was ∼2.5 × 10−4 M. Theoretical Studies. The hyperfine coupling constants of the radical anions were calculated by DFT using the UB3LYP/EPR-iii level of theory, based on the optimized geometry. The TIPS groups were replaced by protons because the basis set for EPR-iii does not include silicon. All calculations were performed with the Gaussian 09 (Revision D.01) program package.36

Figure 10. Molecular frontier orbitals and their energies of 3 (top, left), 32− (top, right) and 3•− (bottom).

the formation of an antiaromatic 4n π-system. Overall, our investigations have shed considerable light on the reduction of azapentacenes. We conclude that the presence of the electronegative nitrogen atoms in the rings is more important than their position to stabilize the radical anions, which do not disproportionate.

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EXPERIMENTAL SECTION

General Manipulations and Synthetic Techniques. Compounds 1−4,6,21,22 [K(18-crown-6)(THF)2] anthracenide,29 and lithium naphthalenide30 were synthesized according to literature procedures. THF was dried using an Innovative Technology Inc. solvent purification system (SPS) and stored over potassium. Pentane was distilled from Li[AlH4], Et2O used for absorption spectroscopy was dried on the SPS, and both were stored over NaK alloy in an argon-filled glovebox from Innovative Technology Inc. All of the reactions were performed at room temperature in an argon-filled glovebox. Photophysical Measurements. All solutions used in photophysical measurements had concentrations of ca. 10−6 to 10−5 M in Et2O, while those used for the emission spectra have a maximum absorbance less than 0.2 to avoid reabsorption. Electronic absorption measurements were performed on a Varian Cary 5E UV−vis−NIR spectrophotometer and an Agilent 8453 diode-array UV−vis spectrophotometer. The emission and excitation spectra were

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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.7b09460. Crystallographic data (CIF) 15975


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(17) Hubner, A.; Diehl, A. M.; Diefenbach, M.; Endeward, B.; Bolte, M.; Lerner, H. W.; Holthausen, M. C.; Wagner, M. Angew. Chem., Int. Ed. 2014, 53, 4832−4835. (18) Chiu, C. W.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2007, 46, 1723−1725. (19) Wu, J. I.; Wannere, C. S.; Mo, Y.; Schleyer, P. v. R.; Bunz, U. H. F. J. Org. Chem. 2009, 74, 4343−4349. (20) Ji, L.; Haehnel, M.; Krummenacher, I.; Biegger, P.; Geyer, F. L.; Tverskoy, O.; Schaffroth, M.; Han, J.; Dreuw, A.; Marder, T. B.; Bunz, U. H. F. Angew. Chem., Int. Ed. 2016, 55, 10498−14501. (21) Engelhart, J. U.; Lindner, B. D.; Tverskoy, O.; Rominger, F.; Bunz, U. H. F. Chem. - Eur. J. 2013, 19, 15089−15092. (22) Tverskoy, O.; Rominger, F.; Peters, A.; Himmel, H. J.; Bunz, U. H. F. Angew. Chem., Int. Ed. 2011, 50, 3557−3560. (23) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482−9483. (24) Appleton, A. L.; Brombosz, S. M.; Barlow, S.; Sears, J. S.; Bredas, J. L.; Marder, S. R.; Bunz, U. H. F. Nat. Commun. 2010, 1, 91. (25) Engelhart, J. U.; Lindner, B. D.; Schaffroth, M.; Schrempp, D.; Tverskoy, O.; Bunz, U. H. F. Chem. - Eur. J. 2015, 21, 8121−8129. (26) These excitations refer to the electron configuration of the specified anions; that is, the HOMO in the dianion corresponds to the LUMO in the neutral compound. (27) Carrington, A.; dos Santos-Veiga, J. Mol. Phys. 1962, 5, 21−29. (28) Gerson, F.; Huber, W. Electron Spin Resonance Spectroscopy of Organic Radicals; John Wiley & Sons: Weinheim, Germany, 2003. (29) Rosokha, S. V.; Kochi, J. K. J. Org. Chem. 2006, 71, 9357−9365. (30) Hilmey, D. G.; Paquette, L. A. Org. Synth. 2007, 156−162. (31) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55. (32) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, A71, 3−8. (33) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (34) Brandenburg, K. Diamond, Crystal and Molecular Structure Visualization, version 4.2.0; Crystal Impact: Bonn, Germany, 2016. (35) Dümmling, S.; Eichhorn, E.; Schneider, S.; Speiser, B.; Würde, M. Curr. Sep. 1996, 15, 53−56. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F. o.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, D. N.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.

Experimental procedures, additional data and spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] *[email protected] ORCID

Holger Braunschweig: 0000-0001-9264-1726 Christoph Lambert: 0000-0002-9652-9165 Todd B. Marder: 0000-0002-9990-0169 Uwe H. F. Bunz: 0000-0002-9369-5387 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS U.B. and A.D. thank the SFB 1249 (DFG) for financial support. T.B.M. thanks the Julius-Maximilians-Universität Würzburg for support. T.B.M. and C.L. thank the Bavarian State Ministry of Science, Research, and the Arts (Collaborative Research Network “Solar Technologies Go Hybrid”) and Deutsche Forschungsgemeinschaft (Grant Number: GRK 2112). F.G. thanks the Studienstiftung des Deutschen Volkes.

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REFERENCES

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Preparation, Properties, and Structures of the Radical Anions and

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