Organic Super‐Acceptors with Efficient Intramolecular Charge‐Transfer Interactions by [2+2] Cycloadditions of TCNE, TCNQ, and F4‐TCNQ to Donor‐Substituted Cyanoalkynes
Authors: Kivala Milan , Boudon Corinne , Gisselbrecht Jean‐Paul , Enko Barbara , Seiler Paul , Müller Imke B. , Langer Nicolle , Jarowski Peter D. , Gescheidt Georg , Diederich François 
Published: March 2009

Rivaling the best one: Thermal [2+2] cycloadditions of TCNE, TCNQ, and F4‐TCNQ to N,N‐dimethylanilino‐substituted cyanoalkynes afforded a new class of organic super‐acceptors featuring efficient intramolecular charge‐transfer interactions. These acceptors rival the acceptor F4‐TCNQ in the propensity for reversible electron uptake as well as in electron affinity (see figure), which makes them interesting as p‐type dopants for potential application in optoelectronic devices.

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Thermal [2+2] cycloadditions of tetracyanoethene (TCNE), 7,7,8,8‐tetracyanoquinodimethane (TCNQ), and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) to N,N‐dimethylanilino‐substituted (DMA‐substituted) alkynes bearing either nitrile, dicyanovinyl (DCV; ‐CHC(CN)2), or tricyanovinyl (TCV; ‐C(CN)C(CN)2) functionalities, followed by retro‐electrocyclization, afforded a new class of stable organic super‐acceptors. Despite the nonplanarity of these acceptors, as revealed by X‐ray crystallographic analysis and theoretical calculations, efficient intramolecular charge‐transfer (CT) interactions between the DMA donors and the CN‐containing acceptor moieties are established. The corresponding CT bands appear strongly bathochromically shifted with maxima up to 1120 nm (1.11 eV) accompanied by an end‐absorption in the near infrared around 1600 nm (0.78 eV) for F4‐TCNQ adducts. Electronic absorption spectra of selected acceptors were nicely reproduced by applying the spectroscopy oriented configuration interaction (SORCI) procedure. The electrochemical investigations of these acceptors by cyclic voltammetry (CV) and rotating disc voltammetry (RDV) in CH2Cl2 identified their remarkable propensity for reversible electron uptake rivaling the benchmark compounds TCNQ (Ered,1=−0.25 V in CH2Cl2 vs. Fc+/Fc) and F4‐TCNQ (Ered,1=+0.16 V in CH2Cl2 vs. Fc+/Fc). Furthermore, the electron‐accepting power of these new compounds expressed as adiabatic electron affinity (EA) has been estimated by theoretical calculations and compared to the reference acceptor F4‐TCNQ, which is used as a p‐type dopant in the fabrication of organic light‐emitting diodes (OLEDs) and solar cells. A good linear correlation exists between the calculated EAs and the first reduction potentials Ered,1. Despite the substitution with strong DMA donors, the predicted EAs reach the value calculated for F4‐TCNQ (4.96 eV) in many cases, which makes the new acceptors interesting for potential applications as dopants in organic optoelectronic devices. The first example of a charge‐transfer salt between the DMA‐substituted TCNQ adduct (Ered,1=−0.27 V vs. Fc+/Fc) and the strong electron donor decamethylferrocene ([FeCp*2]; Cp*=pentamethylcyclopentadienide; Eox,1=−0.59 V vs. Fc+/Fc) is described. Interestingly, the X‐ray crystal structure showed that in the solid state the TCNQ moiety in the acceptor underwent reductive σ‐dimerization upon reaction with the donor.