Boundaries of Anion/Naphthalenediimide Interactions: From Anion−π Interactions to Anion-Induced Charge-Transfer and Electron-Transfer Phenomena

Abstract

The recent emergence of anion-π interactions has added a new dimension to supramolecular chemistry of anions. Yet, after a decade since its inception, actual mechanisms of anion-π interactions remain highly debated. To elicit a complete and accurate understanding of how different anions interact with π-electron-deficient 1,4,5,8-naphthalenediimides (NDIs) under different conditions, we have extensively studied these interactions using powerful experimental techniques. Herein, we demonstrate that, depending on the electron-donating abilities (Lewis basicity) of anions and electron-accepting abilities (π-acidity) of NDIs, modes of anion-NDI interactions vary from extremely weak non-chromogenic anion-π interactions to chromogenic anion-induced charge-transfer (CT) and electron-transfer (ET) phenomena. In aprotic solvents, electron-donating abilities of anions generally follow their Lewis basicity order, whereas π-acidity of NDIs can be fine-tuned by installing different electron-rich and electron-deficient substituents. While strongly Lewis basic anions (OH(-) and F(-)) undergo thermal ET with most NDIs, generating NDI(•-) radical anions and NDI(2-) dianions in aprotic solvents, weaker Lewis bases (AcO(-), H(2)PO(4)(-), Cl(-), etc.) often require the photoexcitation of moderately π-acidic NDIs to generate the corresponding NDI(•-) radical anions via photoinduced ET (PET). Poorly Lewis basic I(-) does not participate in thermal ET or PET with most NDIs (except with strongly π-acidic core-substituted dicyano-NDI) but forms anion/NDI CT or anion-π complexes. We have looked for experimental evidence that could indicate alternative mechanisms, such as a Meisenheimer complex or CH···anion hydrogen-bond formation, but none was found to support these possibilities.

Anion-induced N-doping of naphthalenediimide polymer semiconductor in organic thin-film transistors

“…41 Since the Lewis acidity of the NDIs was not the reported limitation for the electron transfer in this process, we predicted that thermally induced electron-transfer might also occur with the NDI units of P(NDI2OD-T2), leading to a n-doping effect.…”

“…Irrespective of the electron depletion mode of the anion, the corresponding NDI molecule was reduced to the same radical anion product that displayed identical spectroscopic signals. 41 We note that the exact mechanism of radical anion formation is under debate, particularly with highly electronegative fluoride anions in which direct electron transfer seems thermodynamically unlikely. Nevertheless, Saha's results suggested that soluble anions might be suitable as readily available n-type dopants of polymeric NDI derivatives.…”

“…41 In addition the longer wavelength peaks shift and change in intensity. The changes are similar to those observed upon electrochemical reduction of P(NDI2OD-T2).…”

“…[39][40][41] Various functionalised NDI-based molecules were examined in which the electron-deficient nature of the core was modified to tune the strength of the π-acidity. Saha and co-workers reported that for some NDI-anion pairs, the interaction went beyond the typical non-chromogenic anion-π interactions, and that when the energy level of the HOMO of the anion was greater than the LUMO level of the NDI, a thermally induced anion-to-NDI electron transfer was suggested to afford the NDI radical anion in solution.…”

Anion-Controlled Architecture and Photochromism of Naphthalene Diimide-Based Coordination Polymers

“…The organic ligand DPNDI was synthesized according to the same procedures reported in the literature [35]. The infrared spectra were obtained in the range of 400-4000 cm -1 from a Perkin-Elmer FT-IR spectrophotometer (Waltham, MA, USA), and powder X-ray diffraction (PXRD) were recorded on a Rigaku MiniFlex-II X-Ray diffractometer (Tokyo, Japan) using graphite-monochromated Cu Kα radiation (λ = 1.5406 Å) in the range of 5-50 • .…”