Photoactivation Properties of Self-n-Doped Perylene Diimides: Concentration-dependent Radical Anion and Dianion Formation

Abstract

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R •– ) and dianions (D ••2– ). We observe R •– formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D ••2– formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R •– to the strongly reducing doublet excited state 2[R •– ]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state 1[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R •– and D ••2– formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

Establishing Self‐Dopant Design Principles from Structure–Function Relationships in Self‐n‐Doped Perylene Diimide Organic Semiconductors

“…Previous cyclic voltammetry experiments on A2I, B2I, C2I, and D2I reveal the first iodide oxidation wave (3I − ⇌ I 3 − + 2e − ) shifts and becomes more electronegative in C2I and D2I, which we have attributed to weak ion‐pairing between ammonium and iodide making them more soluble and stronger electron donors. [ 42 ] As discussed later, X‐ray photoelectron spectroscopy (XPS) measurements of the nitrogen 1s region also supports this assertion, as the binding energy of the NMe 3 + region shifts to lower binding energy with increasing steric encumbrance (vide infra) due to decreased stability of nitrogen. This leads us to postulate that the high degree of instability and reactivity imparted to the iodide in D2I, taken together with a higher degree of solubility from weaker pairing with its counter cation, results in triiodide formation that quenches the enhancive doping effect.…”

“…For example, samples excited at lower energies bleaches the S 0 → S 1 transition of N accompanied by stimulated emission and a S 1 → S n absorption transient of 1 [ N ]* while samples excited at higher energies bleaches the D 0 → D 1 transition of R •‐ and is accompanied by the transient absorption of the ground‐state excitation. [ 42 ]…”

“…For example, samples excited at lower energies bleaches the S 0 → S 1 transition of N accompanied by stimulated emission and a S 1 → S n absorption transient of 1 [N]* while samples excited at higher energies bleaches the D 0 → D 1 transition of R •− and is accompanied by the transient absorption of the ground-state excitation. [42] Doping alters the work function (Φ) of semiconductors and provides a fingerprint of electron injection into PDI from the counterions. Changes in Φ were measured by performing UPS measurements to understand how counterion selection impacts doping.…”

“…It is known that doping can proceed via a photoinitiated transfer mechanism wherein the neutral ground state 0 N of PDI is photoexcited to the singlet state 1 [N]*, a strong oxidant (+1.97 V vs SCE), that is then reductively quenched by the dopant to yield the radical anion R •− . [42,54] However, direct thermal excitation (≈25-40 meV) is insufficient to overcome the energy barrier of charge transfer (ΔG CT ≈1.4 eV), and it was proposed by Russ et al that thermal annealing promotes demethylation of the quaternary ammonium to yield the tertiary amine that is then able to dope the PDI backbone through a photoinitiated electron transfer process. [53] However, the absorption spectra of C6OH and D6OH in Figure 4B show dramatic increases in R •− in response to annealing, yet these dopants do not demethylate.…”

Concepts and principles of self-n-doping in perylene diimide chromophores for applications in biochemistry, energy harvesting, energy storage, and catalysis

“…The dopant with the lowest degree of steric encumbrance had the highest concentration of dianions. 142 We hypothesize that dianions favor these conditions due to Coulombic stabilization of the charge between adjacent chromophores in a manner similar to that of exciton stabilization in PDI aggregates. Additionally, bulky dopants would inhibit interactions between neighboring chromophores and impair charge delocalization.…”