Pyridinium-Functionalized Pyromellitic Diimides with Stabilized Radical Anion States.

In this work, we report the stabilization of the reduced states of pyromellitic diimide by charge-balancing the imide radical anions with cationic pyridinium groups attached to the aromatic core. This structural modification is confirmed by single-crystal X-ray diffraction analysis. Characterization by (spectro)electrochemical experiments and computations reveal that the addition of cationic groups to an already electron-deficient ring system results in up to +0.57 V shifts in reduction potentials, largely as a consequence of charge screening and lowest unoccupied molecular orbital-lowering effects. This formal charge-balancing approach to stabilizing the reduced states of electron-deficient pyromellitic diimides will facilitate their incorporation into spin-based optoelectronic materials and devices.

Introduction

Organic radical systems have been the subject of intense research because of their potential in spin-based applications.[1−8] Aromatic diimides are a class of organic compounds that undergo reversible reduction processes to reach radical anion and dianion states[1,9−11] with absorption properties that are attractive for energy storage[12−14] and light-harvesting applications.[15−18] The smallest of aromatic diimides, pyromellitic diimide (PMDI), has received significantly less attention in these areas than the larger naphthalene and perylene diimides despite exhibiting promising behavior in electronic settings[14,19] alongside interesting absorption and emission characteristics.[9,20] Explanations for this disparity often point to the challenge of incorporating functionalities at the PMDI core positions[21] and the more negative voltages required by PMDI-based redox systems to reach their reduced states.[11,22] With these perceived limitations in mind, we sought to demonstrate a molecular design that leads to significant shifts of PMDI reduction potentials toward more positive voltages. We hypothesized that utilizing cations to neutralize the formal negative charges that form upon PMDI reduction would be a viable approach for achieving the desired changes in reduction potentials.

Despite the wealth of literature on aromatic diimides, there are only two reports of aromatic diimides that have been core-modified with positively charged groups. In 2014, Mukhopadhyay et al. demonstrated[23] phosphonium-appended naphthalene diimide radical species with remarkable stability against oxidation because of bonding interactions between the phosphonium site and the neighboring imide carbonyl oxygen atom. Shortly afterward, Würthner et al. described[24] the serendipitous formation of a zwitterionic radical perylene diimide that is stable under ambient conditions.

With these precedents in mind, we envisioned that cationic groups could be introduced onto the smaller PMDI core by a nucleophilic aromatic substitution (NAS) reaction between an appropriately selected nucleophile and a mono- or dibrominated PMDI precursor, a motif that has seen a resurgence in research interest by us and others in recent years.[20,21,25−29] The PMDI core, however, is thought to be more difficult to substitute than the core positions of naphthalene and perylene diimides because it is flanked by sterically demanding imide carbonyl groups. We found that relatively nucleophilic compound 4-(dimethylamino)pyridine (DMAP) was suitable for the desired NAS reactions with PMDIs. Herein, we describe the synthesis and characterization of PMDIs that have been mono- and bis-functionalized with 4-(dimethylamino)pyridinium units.

Results and Discussion

The syntheses of the cationic PMDIs entailed four steps from commercially available compounds. Pyromellitic diimide precursors Br-PMDI and Br-PMDI were obtained from mono- and dibromodurene, respectively, by exhaustive KMnO4 oxidation, acetic anhydride-mediated dehydration,[20,28] and imidization with 2,6-diisopropylaniline. The control PMDI compound (P) with no core modifications was also synthesized by imidization of pyromellitic dianhydride (see the Supporting Information). Initial trials for the NAS reaction between the brominated PMDIs and DMAP were conducted in refluxing polar solvents such as tetrahydrofuran (THF), 1,4-dioxane, and PhCN. In all cases, however, reaction times over 48 h were required to achieve isolable yields. We found that syntheses could be completed much more quickly under microwave conditions (1 h at 135 °C in THF), most likely as a consequence of the higher temperatures and pressures accessible by microwave reactors (Figure ). The bromide salt of the monocationic DMAP+-functionalized PMDI (1·Br) could not be precipitated from the reaction mixture; thus, it was directly converted to its PF6 form 1·PF by anion exchange with KPF6 in MeOH/H2O (2:3, v/v) in 60% overall yield. Difunctionalized salt 2·2Br, however, was essentially insoluble in THF, which facilitated its separation from any partially reacted monocationic intermediates by trituration of the crude product with additional THF. Anion exchange was conducted in the same manner as described previously to obtain 2·2PF in 65% overall yield.

Figure 1

Syntheses of (a) 1·PF and (b) 2·2PF.

Slow evaporation of solutions of 1·PF in MeCN yielded single crystals of sufficient quality to be analyzed by X-ray diffraction (Figure ). The dihedral angle between the central benzene ring of PMDI and the appended DMAP+ group (54.2(5)°) arises as a consequence of the steric demands of the flanking imide carbonyls. This angle closely matches that observed for the energy-minimized geometry of 1·PF obtained through gas phase density functional theory (DFT) calculations (see below). The C–NMe2 bond in 1·PF (1.339(5) Å) is shorter than the analogous bond in DMAP (1.363(1) Å, CCDC BUKJOG11), and its torsion angle is only 11.1(6)°, suggesting that conjugation between the pyridinium and the dimethylamino groups leads the pyridinium to exhibit some quinoidal character. The PF6– anion appears to be involved in weak anion−π interactions with the PMDI core, with average F-to-benzene ring plane distances of 3.03 Å.

Figure 2

Stick representation of the asymmetric unit of the solid-state structure of 1·PF obtained through X-ray diffraction analysis. C = gray, H = white, F = green, N = blue, O = red, and P = orange.

Cyclic voltammetry (CV) experiments (1 mM compound and 0.1 M Bu4NPF6 in dimethylformamide (DMF), 100 mV/s, internal ferrocene (Fc)/Fc+ reference) were conducted on 1 and 2 to assess how their reduction potentials differed in comparison to those of the analogous unfunctionalized PMDI (P), which is well-known to exhibit two reversible one-electron reduction processes (Figure , top). Two reduction processes were observed for both 1 and 2, with the first reduction of 2 taking place at a potential even more positive than that of the analogous processes in larger naphthalene and perylene diimides.[1] The reversibility of these processes was demonstrated by the clean reoxidation scans in the CV and the symmetry of reduction peaks observed using differential pulse voltammetry (DPV) (Figure S1). No oxidation processes were observed for either compound when scanning to more positive potentials.

Figure 3

(Top) Cyclic voltammograms of P, 1, and 2 recorded (1 mM compound in DMF, 0.1 M Bu4NPF6, 100 mV/s, Fc standard). (Bottom) Summary of E1/2 and ΔE1/2 values.

Substantial shifts toward more positive reduction potentials in 1 and 2 provided insight into the stabilization mechanisms at play in these cationic PMDIs. Compared to those of P, the first reduction processes of 1 and 2 were shifted toward more positive potentials by 146 and 355 mV, respectively, suggesting an approximately linear relationship between the amount of positive charge and the extent of radical anion stabilization. Furthermore, the difference between the first and second reduction potentials (ΔE = E11/2 – E21/2) became smaller when more positive charge was present at the PMDI core (Figure , bottom). In the most drastic case, there was a +566 mV shift in the second reduction potential of 2 relative to that of P. These data suggest that the radical anion stabilization observed in these cationic PMDI compounds can be attributed to (1) overall lowest unoccupied molecular orbital (LUMO)-lowering effects of electron-deficient groups (positive shifts in both E1/2 values) and (2) charge screening between redox sites (decreased ΔE values).

It is important to note that these conclusions were reached under the assumption that the pyridinium groups themselves do not undergo reduction processes at these potentials. This assumption is backed by ample literature evidence that, unlike easily reduced 4,4′-bipyridinium derivatives,[5,30,31] DMAP+-polysubstituted arenes and quinones undergo reductions at far more negative voltages and that these reductions are often irreversible in nature.[32,33]

To further probe the validity of this assumption and investigate the electronic structure of the cationic PMDIs, DFT calculations (Figure ) with the M06-2X functional at the 6-31G(d) level of theory were performed on the N,N′-dimethyl analogues of P, 1, and 2 in the gas phase. Much like the solid-state structures obtained through single-crystal X-ray diffraction, the sterically demanding imide groups force the DMAP+ units to twist at 52.5 and 55.8° dihedral angles to the PMDI benzene ring in 1 and 2, respectively. In both molecules, the LUMO is localized exclusively on the PMDI core, whereas the highest occupied molecular orbital (HOMO) is distributed largely on the DMAP+ units, an observation which shows that the pyridiniums are unlikely to be participating in the reduction chemistry. The HOMO and LUMO energies in 1 and 2 are both lower than they are in P, with the overall narrowing of the HOMO–LUMO gap being driven by the more significant decrease in LUMO energies.

Figure 4

Depictions of the HOMO (red/blue) and LUMO (green/yellow) of P, 1, and 2 obtained through DFT calculations on their N,N′-dimethyl analogues (M06-2X/6-31G(d), isovalue = 0.03). Relative energies of the HOMOs and LUMOs are provided in electronvolts.

The absorption features of the cationic PMDIs and their reduced states were investigated to identify further implications of pyridinium incorporation near imide redox sites. In their unreduced forms, 1 and 2 have absorption maxima at 317 and 336 nm, respectively, that can be attributed to the extended delocalization of the dimethylamine lone pair in the DMAP+ units (Figure S2). The red shift that is observed with increasing DMAP+ substitution corresponds well with the decreasing HOMO–LUMO gaps predicted by computation (Figure ). A slight red shift in absorption was observed for 2 in going from CH2Cl2 to DMF as the solvent, with absorption maxima observed at 326 and 336 nm, respectively (Figure S3). The singly and doubly reduced states of 1 and 2 could be separately accessed by bulk electrochemical reduction in DMF solution with 0.1 M Bu4NPF6 as the electrolyte (Figure ). The absorption profiles of singly reduced states 1• and 2•+ are similar to those of the radical anion form of unfunctionalized N,N′-dioctyl-PMDI (Q, λmax = 718 nm),[9] with strong absorption bands centered at around 720 nm arising from π*−π* transitions that are commonly observed for carbonyl anion radicals.[34] The higher-energy but weakly absorbing bands at 525 nm observed for 1• and 2•+ are likely related to π–π* transitions that were predicted but not observed for Q.[9]

Figure 5

(Top) UV–vis absorption spectra of electrochemically generated singly and doubly reduced states of 1 and 2 recorded in DMF. (Bottom) Summary of absorption maxima and their molar absorptivities. Spectral features of compound Q (N,N′-dioctyl-PMDI) reported by Gosztola et al.[9] are provided for reference.

The doubly reduced states of the DMAP+-functionalized PMDIs (1 and 2) differ from those of Q, which absorbs significantly more intensely at 552 nm. In 1, there are two distinct absorption bands of similar intensity at 487 and 531 nm in addition to a weaker broad absorption band centered at 852 nm. For neutral 2 on the other hand, only a single absorption band is observed at 492 nm. This band is blue-shifted by 60 nm compared with the analogous feature found for Q.[9] These data show that unlike those of the singly reduced states the photophysical properties of the doubly reduced states are perturbed by the DMAP+ functionalization. This perturbation likely occurs in 1 and 2 because the lower-energy molecular orbital involved in these electronic transitions is localized to the DMAP+ unit rather than the PMDI core, much like the HOMOs of their oxidized forms (Figure ).

Conclusions

In summary, we have demonstrated the successful mono- and bis-functionalization of the pyromellitic diimide core with 4-(dimethylamino)pyridinium groups through a facile nucleophilic aromatic substitution approach. By appending these cationic groups to the electron-deficient aromatic ring, the redox activity of the imide functional groups can be shifted by over +0.5 V. The various redox states of the cationic compounds all display strong absorption characteristics that correlate well with the features observed in the parent compound. Taken together, these results provide concrete evidence that the addition of formal charge is an effective strategy for modulating the electronic and optical properties of aromatic diimides.

Experimental Section

Materials and Synthetic Methods

Reagents were purchased from TCI Chemicals or Sigma-Aldrich and used as supplied. Anhydrous THF was obtained from a Vacuum Atmospheres 103991 Solvent Purification System. Microwave reactions were performed and monitored using a Biotage Initiator+ Microwave Reactor equipped with a Robot Eight automation system. 1H NMR and 13C NMR spectra were collected on Bruker Avance III 400 MHz and referenced to the residual solvent as the internal standard. High-resolution electrospray ionization mass spectrometry (ESI-MS) data were collected on a Bruker BioTOF II ESI/time-of-flight mass spectrometer. Internal calibration was performed using poly(ethylene glycol) or poly(propylene glycol) standards for ESI+ samples and NaHCOO for ESI– samples.

Crystallographic Methods

A yellow prism crystal of 1·PF was obtained by slow evaporation of a CH3CN solution. All measurements were made on a Rigaku XtaLAB mini diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71075 Å). The diffractometer was equipped with an Oxford Cryosystems desktop cooler (Oxford Cryosystems Ltd., Oxford) for low-temperature data collection. The crystals were mounted on a MiTeGen micromount (MiTeGen, LLC, Ithaca, NY) using STP oil. The positions of all nonhydrogen atoms were freely refined with anisotropic thermal parameters. All hydrogen atoms were placed in calculated positions (with riding thermal parameters) using C–H distances of 0.95 Å for the aryl rings, 0.98 Å for the methyl groups, and 1.00 Å for the methine groups. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre. All calculations were performed using the SHELXL97 and CrystalStructure crystallographic software packages. The crystallographic data are summarized in Table S1.

Electrochemistry and UV–Vis Spectroscopy

All electrochemical samples were thoroughly deoxygenated by bubbling Ar through the solutions. Cyclic and differential pulse voltammetry experiments were performed using a Pine Research WaveNow potentiostat equipped with a Pt working electrode (1.6 mm diameter disk, polychlorotrifluoroethylene (PCTFE) shroud), Pt coil counter electrode (PCTFE shroud), and nonaqueous reference electrode (Ag wire, 0.01 M AgNO3, 0.1 M Bu4PF6, MeCN). Ferrocene (Fc) was used as an internal standard, and each sample was evaluated both with and without Fc to ascertain that there were no significant interactions between Fc and the analyte. Spectroelectrochemistry was achieved using a Pine Research Spectroelectrochemical Cell Kit equipped with a Pt screen-printed ceramic honeycomb electrode (1.7 mm path length) and Ag wire pseudoreference electrode. All UV–vis spectra were collected on an Avantes fiber optic spectrometer (AvaSpec-ULS2048-USB2-50).

Computational Methods

Computational calculations were performed using the Gaussian 09 software package, employing the M06-2X functional at the 6-31G(d) level of theory. Energy-minimized geometries were confirmed by frequency calculations. Structures and orbital isosurfaces were visualized with ChemCraft and Visual Molecular Dynamics (VMD) and rendered using POV-Ray.

Preparation of 1·PF

Reaction setup: An oven-dried microwave vial (0.5–2 mL size, Biotage #352016) equipped with a magnetic stir bar was charged with Br-PMDI (50 mg, 81 μmol, 1 equiv) and 4-(dimethylamino)pyridine (9.9 mg, 81 μmol, 1 equiv) and then sealed with a septum cap. The vial was subjected to three vacuum and Ar backfill cycles, after which anhydrous THF (2 mL) was added to the vial. The reaction mixture was then heated in a Biotage Initiator+ microwave reactor for 1 h at 135 °C. Workup and purification: The reaction mixture was cooled to room temperature. The solids were collected by vacuum filtration and subsequently rinsed with MeOH to extract the product from unreacted starting material. Anion exchange was achieved by adding saturated KPF6 in MeOH/H2O (2:3, v/v) to the combined THF and MeOH filtrates until no further precipitation was observed. The precipitate was collected by vacuum filtration, rinsed with deionized (DI) water, and dried in a vacuum oven (60 °C, 230 mbar). The resultant yellow solids were found to be 1·PF (39 mg, 49 μmol, 60% yield). Characterization: 1H NMR (400 MHz, (CD3)2CO) δ 8.68 (s, 1H), 8.53 (d, 3J = 8.0 Hz, 2H), 7.53 (t, 3J = 7.8 Hz, 2H), 7.40 (d, 3J = 7.8 Hz, 4H), 7.30 (d, J = 8.0 Hz, 2H), 3.49 (s, 6H), 2.82 (hept, 3J = 6.9 Hz, 4H), 1.15 (d, 3J = 6.9 Hz, 12H), 1.13 (d, 3J = 6.9 Hz, 12H). 13C NMR (100 MHz, (CD3)2CO) δ 165.78, 165.23, 158.24, 148.19, 143.12, 139.87, 134.81, 132.34, 131.61, 127.15, 125.00, 121.31, 108.05, 41.00, 29.92, 24.27, 24.24. High-resolution mass spectrometry (HRMS) (ESI+/QTOF) m/z: [M – PF6]+ calcd for C41H45N4O4 657.3435; found 657.3440.

Preparation of 2·2PF

Reaction setup: An oven-dried microwave vial (10–20 mL size, Biotage #354833) equipped with a magnetic stir bar was charged with Br-PMDI (500 mg, 0.720 mmol, 1 equiv) and 4-(dimethylamino)pyridine (352 mg, 2.88 mmol, 4 equiv) and then sealed with a septum cap. The vial was subjected to three vacuum and Ar backfill cycles, after which anhydrous THF (20 mL) was added. The reaction mixture was heated in a Biotage Initiator+ microwave reactor for 1 h at 135 °C. Workup and purification: The reaction mixture was cooled to room temperature. The solids were collected by vacuum filtration and subsequently rinsed copiously with THF to remove monosubstituted byproducts, which are orange in color. The resultant yellow solids were found to be 2·2Br (521 mg, 0.555 mmol, 77.1% yield). Anion exchange was achieved by adding saturated KPF6 in MeOH/H2O (2:3, v/v) to a solution of 2·2Br (102 mg, 0.109 mmol) in MeOH (7 mL). The precipitate was collected by vacuum filtration, rinsed with DI water, and dried in a vacuum oven (60 °C, 230 mbar). The yellow solids were found to be 2·2PF (91 mg, 0.093 mmol, 85% yield for anion exchange). Characterization of 2·2Br: 1H NMR (400 MHz, CDCl3) δ 9.17 (d, 3J = 7.9 Hz, 4H), 7.40 (t, 3J = 7.8 Hz, 2H), 7.24 (d, 3JA = 7.8 Hz, 4H), 6.87 (d, 3J = 7.9 Hz, 4H), 3.32 (s, 12H), 3.16 (hept, 3J = 6.7 Hz, 4H), 1.16 (d, 3J = 6.7 Hz, 24H). 13C NMR (100 MHz, CDCl3) δ 163.79, 157.49, 148.13, 143.96, 134.23, 133.31, 130.60, 125.70, 124.15, 106.61, 40.69, 29.19, 24.50. HRMS (ESI+/QTOF) m/z: [M – Br]+ calcd for C36H46N6O4Br 705.2764; found 705.2763. Characterization of 2·2PF: 1H NMR (400 MHz, (CD3)2CO) δ 8.29 (d, 3JAX = 8.0 Hz, 4H), 7.51 (t, 3J = 7.8 Hz, 2H), 7.36 (d, 3JA = 7.8 Hz, 4H), 7.35 (d, 3JAX = 8.0 Hz, 4H), 3.52 (s, 12H), 2.87 (hept, 3J = 6.9 Hz, 4H), 1.11 (d, 3J = 6.9 Hz, 24H). 13C NMR (100 MHz, (CD3)2CO) δ 164.12, 158.28, 148.47, 142.47, 135.17, 134.38, 131.73, 126.69, 124.93, 108.39, 41.07, 28.77, 24.23. HRMS (ESI+/QTOF) m/z: [M – 2PF6]2+ calcd for C48H54N6O4 389.2103; found 389.2177.