Pyridinium-Functionalized Pyromellitic Diimides with Stabilized Radical Anion States

Abstract

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.

Figure 1

Syntheses of (a) 1·PF₆ and (b) 2·2PF₆.

Figure 2

Stick representation of the asymmetric unit of the solid-state structure of 1·PF₆ obtained through X-ray diffraction analysis. C = gray, H = white, F = green, N = blue, O = red, and P = orange.

Figure 3

(Top) Cyclic voltammograms of P, 1⁺, and 2²⁺ recorded (1 mM compound in DMF, 0.1 M Bu₄NPF₆, 100 mV/s, Fc standard). (Bottom) Summary of E_1/2 and ΔE_1/2 values.

Figure 4

Depictions of the HOMO (red/blue) and LUMO (green/yellow) of P, 1⁺, and 2²⁺ 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.

Figure 5

(Top) UV–vis absorption spectra of electrochemically generated singly and doubly reduced states of 1⁺ and 2²⁺ 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. are provided for reference.