# When Reality Defies Prediction: Polymorphism, Twinning, and Accordion Crystals

## TL;DR

This paper explores the unusual crystallization behavior of terephthalic dihydrazide, revealing how real-world conditions can defy computational predictions.

## Contribution

The study reports the first observation of two concomitant polymorphs with unique morphologies and stability discrepancies between experiments and computations.

## Key findings

- Form I (FI) of terephthalic dihydrazide forms accordion-like stacks with twin domains and remains stable in solution for years.
- Form II (FII) exists as large needles but disappears in solution after 20 hours.
- Computational methods predict FII as the most stable polymorph, while experiments show FI is more stable.

## Abstract

The ability to understand
crystallization and predict the resulting
solid form of a system is not always easily achieved, but it is critical,
particularly in the field of materials science. Intriguing (and previously
unreported) crystallization behavior is observed with terephthalic
dihydrazide (TeDi) as it rapidly forms two concomitant crystalline
polymorphs upon cooling in solution. The crystal morphology of Form
I (FI) has not been seen before in organic systems and involves impressive,
accordion-like stacks, composed of numerous twin domains and remains
stable in solution for years. Form II (FII) exists as large needles
that disappear in solution after 20 h. All experimental methods employed
reveal that FI is the most stable polymorph. Conversely, all computational
methods utilized (conformational analyses, lattice energy calculations,
and crystal structure prediction) suggest that FII is the most stable
polymorph. Isolation of FII was achieved by the crystallization of
TeDi powder with a supramolecular mimetic gelator, as the gel fibers
act as a template for the preferential crystallization of FII, due
to the comparable crystal packing of FII and the gelator. This work
highlights the impact of crystallization behavior in a real laboratory
and the defects, disorder, and twinning that lead to remarkable crystal
morphologies that may not be accounted for with idealized calculations,
and also explores approaches for controlling and directing crystallization
outcomes.