Colour polymorphism in molecular crystals

 Colour polymorphism is an intriguing characteristic of chemical systems that entails the manifestation of crystal polymorphs in various hues. Within the realm of organic molecular crystals, this phenomenon remains relatively uncommon, with scant instances documented in literature thus far. However, the potential applications of systems displaying colour polymorphism span across diverse domains, including stimuli responsive pigments development, and sensors technology. Our endeavour involves the strategic design, synthesis, and detailed characterization of novel organic molecular crystals showcasing colour polymorphism. This research has been counting with the collaboration of scientists from the Polytechnical of Milan (Italy).

• “Colour Polymorphism in Organic Crystals”, B. A. Nogueira, C. Castiglioni and R. Fausto, Comm. Chem. - Nature, 3 (2020) 34.
• “Unveiling the Red and Brownish-Green Polymorphs of a Novel ROY Derivative: 2-(4-((3-Cyanothiophen-2-yl)amino)-3-nitrophenyl)acetic acid”, B. A. Nogueira, S. M. M. Lopes, A. C. B. Rodrigues, M. E. S. Eusébio, V. André, T. Duarte, J. A. Paixão, T. M. V. D. Pinho e Melo and R. Fausto, Cryst. Growth Des. 24 (2024) 947.
• “Colour polymorphs of ROY-ol”, B. A. Nogueira, S. M. M. Lopes, A. Milani, V. André, J. A. Paixão, M. E. S. Eusébio, T. M. V. D. Pinho e Melo, T. Duarte, C. Castiglioni and R. Fausto, Cryst. Growth Des., 22 (2022) 5375.
• “Portrayal of the Colour Polymorphism in the 5-Acetyl-derivative of ROY”, B. A. Nogueira, M. Carvalho, J. A. Paixão, M. E. S. Eusébio, S. M. M. Lopes, T. M. V. D. Pinho e Melo and R. Fausto, CrystEngCom, 24 (2022) 1459.

Photochromism, solvatochromism and thermochromism

Solvatochromism, photochromism, and thermo-chromism are fascinating phenomena in the realm of molecular chemistry, referring to the change in colour of a substance due to alterations in the solvent environment, reversible change of a material’s colour upon exposure to light, and changes of colour in response to fluctuations in temperature, respectively. These behaviours offer insights into molecular structure and electronic transitions. Solvatochromism finds applications in solvent polarity assessment and environmental sensing, while photochromic materials can be used in optical devices, such as photochromic lenses and light-sensitive switches, and thermochromic compounds are utilized in indicators for temperature monitoring and security inks. We have been investigating these phenomena in different chemical systems using different experimental and theoretical approaches, specifically UV/VIS absorption, fluorescence, infrared and Raman spectroscopies, together with quantum chemistry calculations. Part of these investigations received the contribution of scientists from the University of Bitlis (Turkey).

• “Photochromism of a Spiropyran in Low-Temperature Matrix. Unprecedented Bidirectional Switching Between a Merocyanine and an Allene Intermediate”, C. M. Nunes, N. A. M. Pereira and R. Fausto, J. Phys. Chem. A. 126 (2022) 2222.
• “Reversal in Solvatochromism, Photochromism and Thermochromism in a New bis-Azo Dye Based on Naphthalen-1-amine”, İ. Sıdır, Y. E. Kara, Y. G. Sıdır, H. Berber and R. Fausto, J. Photochem. Photobiol. A: Chemistry, 446 (2023) 115138.
• “Solvato-, Thermo- and Photochromism in a New Diazo Diaromatic Dye: 2-(p-Tolyldiazenyl)naphthalen-1-amine”, İ. Sıdır, Y. G. Sıdır, H. Berber and R. Fausto, J. Mol. Struct. 1267 (2022) 133595.