FLEXPOLE is a PRIN-2022-funded research project dedicated to the discovery of mechanically flexible organic crystals with embedded permanent electrical fields. By combining elasticity and polar molecular organization within a single crystalline material, FLEXPOLE explores a new class of functional materials with strong potential for low-power electronics, photonics, and energy-related technologies.

The project develops unconventional crystallization strategies, including high-throughput nano-crystallization, inkjet printing, field-assisted growth, and epitaxy on polar surfaces, to access new polar polymorphs of organic molecules. Advanced synchrotron-based structural characterization and in operando mechanical, electrical, and optical measurements are used to uncover structure–property relationships.

FLEXPOLE addresses key scientific challenges at the interface of crystallography, crystal engineering, and materials science, while contributing to sustainable and energy-efficient technological solutions and providing advanced training for early-career researchers.

Scientific Motivation

Organic molecular crystals are attracting increasing attention due to their low density, chemical tunability, and potential for low-energy, sustainable technologies. However, most organic crystals are mechanically brittle and centrosymmetric, which limits their application in devices requiring mechanical compliance or non-centrosymmetric properties.

FLEXPOLE explores an emerging and still largely unexplored research direction: flexible organic crystals with a net electric dipole. Such materials are promising candidates for applications including:

• Piezoelectric and ferroelectric devices
• Photovoltaic and photodetection technologies
• Nonlinear optical systems
• Low-power, flexible, and distributed electronics

By enabling the rational discovery of polar, flexible polymorphs, FLEXPOLE aims to contribute both to fundamental crystal engineering and to pressing societal challenges related to energy efficiency, renewable energy production, and distributed information technologies.

Objectives

The main scientific objectives of FLEXPOLE are:

1. To develop uncommon and innovative crystallization techniques for small organic molecules.
2. To discover new polymorphic phases characterized by mechanical flexibility and net electric dipoles.
3. To establish structure–property relationships linking molecular packing, polarity, and elasticity.
4. To characterize mechanical, optical, and electrical properties of flexible polar crystals, both at rest and under bending stress.
5. To assess the technological potential of the developed materials for electronic, photonic, and energy-related applications.

Team and Expertise

The FLEXPOLE team is composed of 2 Research Units (R-UniTS and R-UniCH) and 1 Research Subunit (RS-Elettra), bringing together a complementary set of skills and expertise, including:

• Crystallography and synchrotron-radiation-based structural analysis
• Crystal engineering and polymorphism control
• Supramolecular chemistry
• Functional materials science

This interdisciplinary approach ensures the effective integration of synthesis, characterization, and application-oriented research.

Work Packages

The research activity of FLEXPOLE is structured into three Work Packages (WP). Each WP is subdivided into Tasks (T), each dedicated to a specific aspect of the planned work. This modular structure ensures a systematic approach to crystal growth, characterization, and functional evaluation.

Crystallization Strategies

To access new polar polymorphs beyond conventional crystal growth approaches, FLEXPOLE employs a range of unconventional crystallization techniques, including:

• High-throughput robotic nano-crystallization, enabling the rapid exploration of crystallization conditions.
• Inkjet printing of precursor solutions, allowing spatially controlled crystal growth.
• Crystallization under external electric and/or magnetic fields, to bias molecular orientation and polarity.
• Epitaxial growth on polar substrates, promoting non-centrosymmetric packing arrangements.

These techniques are applied individually or in combination to maximize the probability of stabilizing polar and flexible crystalline phases.

Model Molecular Systems

The project focuses on three representative molecular systems, chosen for their complementary intermolecular interactions and structural versatility:

• 4HCB: an aromatic molecule capable of forming complementary hydrogen-bond networks.
• Glycine: a zwitterionic molecule that can assemble via salt-bridge interactions and is known for its rich polymorphism.
• Macrocyclic host molecules: systems forming supramolecular polymeric chains.

These systems provide a robust testing ground for exploring how different interaction motifs influence polarity, flexibility, and polymorphism.

Structural and Functional Characterization

Atomic-level structural characterization of the obtained crystals is performed using state-of-the-art synchrotron radiation techniques, enabling precise determination of crystal symmetry, molecular packing, and dipole alignment.

In parallel, FLEXPOLE investigates the functional properties of the developed materials, including:

• Mechanical properties, with a focus on elastic bending.
• Electrical properties, such as polarization, piezoelectric response, and ferroelectric behavior
• Optical properties, including birefringence, and chiroptical properties

Custom experimental setups are designed and built to perform in operando measurements, allowing structural and functional characterization while the crystals are subjected to an electrical field, mechanical deformation, or both.

Training and Capacity Building

FLEXPOLE has a strong commitment to training early-career researchers. The project includes dedicated activities for Ph.D. students and postdoctoral fellows, providing advanced training in:

• Experimental crystallization techniques
• Synchrotron-based structural methods
• Mechanical and electrical characterization of materials
• Interdisciplinary research at the interface of chemistry, physics, and materials science

Sustainability and Ethics

The FLEXPOLE project is fully compliant with the DNSH (Do No Significant Harm) principle and embraces a Technological Sustainability approach. By focusing on organic materials, low-energy fabrication strategies, and applications in energy-efficient technologies, FLEXPOLE aligns with European and national priorities for sustainable research and innovation.

Funding Acknowledgement

Financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 2.2.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title “Flexible molecular crystals with embedded permanent electrical fields (FLEXPOLE)” – CUP J53D23008560006-Grant Assignment Decree No. 1064 adopted on 18/07/2023 by the Italian Ministry of University and Research (MUR).