Australian Journal of Chemistry Australian Journal of Chemistry Society
An international journal for chemical science

Polymorphs of Neutral Red, a Redox-Mediating Phenazine in Biological Systems

Mackenzie Labine-Romain A , Sabrina Beckmann A , Mohan Bhadbhade B , Saroj Bhattacharyya B , Michael Manefield A , Christopher E. Marjo B C and Anne M. Rich B
+ Author Affiliations
- Author Affiliations

A School of Chemical Engineering, University of New South Wales, Kensington, NSW 2052, Australia.

B Mark Wainwright Analytical Centre, Room G61, Chemical Sciences Building (F10), University of New South Wales, Kensington, NSW 2052, Australia.

C Corresponding author. Email:

Australian Journal of Chemistry -
Submitted: 6 March 2017  Accepted: 10 April 2017   Published online: 1 May 2017


Neutral red 1 is a heterocyclic phenazine that, as a crystalline solid, has been observed to accelerate microbial methane generation from coal. Scale-up to an industrial process will require large quantities of neutral red crystals, hence an understanding of any polymorphic behaviour is essential for careful control of this process. A room-temperature structure of 1 (Form I) has been reported previously, and this study describes a new polymorph (Form II) crystallising from aqueous solution at 50°C, or transforming from Form I over an incubation time of one week at 70°C. Single-crystal X-ray diffraction has been used to study the molecular arrangements and intermolecular interactions in the new polymorph, and compared with those found in the room temperature form. Both polymorphs have been characterised using Raman and infrared spectroscopy, and a synthetic mixture of polymorphs successfully imaged using Raman spectroscopy. Raman imaging is proposed as a quality control method for small quantities of sample to ensure the correct polymorph is produced as a feedstock for this new methanogenesis process.


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