# Mixed-vs-Segregated Stack Polymorphism in the N,N,N′,N′-Tetramethylbenzidine-TCNQF4 Charge Transfer Complex

## TL;DR

This paper reports a new charge transfer complex that can form both mixed and segregated molecular stacks, revealing shared structural and optical properties between the two forms.

## Contribution

The discovery of a charge transfer complex that crystallizes in both mixed and segregated stack motifs, a rare phenomenon in CT cocrystals.

## Key findings

- The N-TMB-TCNQF4 complex exhibits both mixed and segregated stack phases.
- Common structural and optical features were observed in both phases using IR, Raman, and X-ray data.
- Low-temperature data suggest stack distortion in the segregated phase.

## Abstract

Organic
charge transfer (CT) cocrystals are a wide class
of 1-D
materials based on stacks of planar π-electron donor (D) and
acceptor (A) molecules overlapping their frontier orbitals due to
the CT interaction. Most CT crystals have either a segregated or mixed
stack motif: in segregated stacks, D and A molecules form separate
columns, while in mixed stacks, D and A alternate along the same stack.
Although CT crystals often display polymorphism, very few D–A
pairs are known to crystallize with both stack motifs. Here, we present
a new CT complex presenting both mixed and segregated stack phases,
based on the strong donor N,N,N′,N′-Tetramethylbenzidine
(N-TMB) and the strong acceptor TCNQF4. The combination
of polarized IR and Raman spectroscopy with X-ray diffraction on single
crystals found common structural and optical features between the
two phases. The low temperature data also suggest that a stack distortion
occurs in the segregated stack phase.