Solid‐State ¹³C NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion‐like Structures

Abstract

The principal values of the 13C chemical shift tensor for the β and δ polymorphs of π‐[TTF⋅⋅⋅TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra‐dimer bonding of singlet π‐[TTFδ+⋅⋅⋅TCNEδ−]. These structures possess 12 intradimer contacts <3.40 Å, with the shortest intra π‐[TTF⋅⋅⋅TCNE] separations involving 2‐center (2c) C−S and 3c C−C−C orbital overlap contributions between the [TTF]δ+ and [TCNE]δ−. This solid‐state NMR study compares the [TTF⋅⋅⋅TCNE] 13C tensor data against previously reported π‐[TTF]22+ and π‐[TCNE]22− homo‐dimers to determine how the tensor principal values change as a function of electronic structure for both TTF and TCNE moieties. In the β and δ phases of [TTF⋅⋅⋅TCNE], the TCNE ethylenic 13C shift tensors predict TCNE oxidation states of −0.46 and −0.73, respectively. The TTF sites are less similar to benchmark 13C data with the β‐phase differing primarily in the ethylenic π‐electrons. The δ form differs significantly from the homo‐dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the π‐electrons and σ‐bonds. In both hetero‐dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo‐dimers.

Organic Donor‐Acceptor Complexes As Potential Semiconducting Materials

“…Recently, solid-state 13 C NMR studies gave the first experimental support for a long, multicenter bond in the TTF-TCNE complex. [183] Chemical shift tensor analysis of the β and δ polymorphs showed significant changes compared to TTF and TCNE homodimers, indicating alterations to both πelectrons and σ-bonds upon hetero-dimer formation. This agreed with the predictions of a long bond with mixed covalent/ionic character.…”

“…(Figure 27) Local contact potential difference measurements revealed that both conformations had similar charge, demonstrating that Kondo effect depended on the TCNE configuration. Recently, solid‐state 13 C NMR studies gave the first experimental support for a long, multicenter bond in the TTF‐TCNE complex [183] . Chemical shift tensor analysis of the β and δ polymorphs showed significant changes compared to TTF and TCNE homodimers, indicating alterations to both π‐electrons and σ‐bonds upon hetero‐dimer formation.…”