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.

Figure 1

(A) Chemical structures of the self-doped PDIs investigated in this work. The naming scheme denotes the degree of steric encumbrance in alphabetical order, followed by the carbon chain length, and finally the counterion (e.g., A2I, A2OH, etc). (B) Absorption spectra of A2I measured in the dark and (C) after 90 min of irradiation with a 405 nm lamp. (D) Absorption spectra of A2OH measured in the dark and (E) after 90 min of irradiation with a 405 nm lamp. Sample concentrations are denoted in the legend.

Figure 2

(A) Percentage radical anion abundance in iodide samples of varying concentration after 90 min of irradiation with a 405 nm lamp. (B) Percentage dianion abundance in A2OH and B2OH both before and after irradiation with a 405 nm lamp.

Figure 3

Graphical depiction of the doping mechanism. First, the neutral perylene core N is photoexcited to ¹[N]*. The anion then transfers an electron to ¹[N]*, thereby reducing it to generate R^•–.

Figure 4

Graphical representations of concentration-dependent doping (A) in the low and (B) high concentration regimes.

Figure 5

Cyclic voltammograms of (A) A2I and (B) A2OH in the dark and after various photoirradiation timepoints indicated in the legend. Each solution was measured at 10 μM in DMF and 100 mM tetrabutylammonium hexafluorophosphate (TBAPF₆) as a supporting electrolyte with ferrocene as an external standard reference potential, since a Ag/Ag⁺ quasi-reference electrode was used. Scan rate: 100 mV/s.

Figure 6

(A) Femtosecond transient absorption maps and spectra for A2OH pumped at 520 nm and (B) 400 nm and were not exposed to any other light sources. (C) Energy level diagrams depicting the two pathways activated by the different excitation wavelengths. Pumping at 520 nm (left) bleaches the S₀ → S₁ transition of N accompanied by stimulated emission and a S₁ → S_n absorption transient of ¹[N]* centered at 700 nm. In contrast, pumping at 400 nm (right) bleaches the D₀ → D₁ transition of R^•– and is accompanied by the transient absorption of the ground-state excitation.