Tunable Absorption and Photochromism in Polyheptazine Imides Via a Solvent Mediated Synthetic Pathway
Authors: Das Saunak Kumar , Repp Stefan , Bette Sebastian , Samanta Manisha , Tesi Lorenzo , Lotsch Bettina Valeska 
Published: December 2024

Layered, two dimensional metal containing polyheptazine imides (M-PHI) offer a complex playground for structure variation resulting in fascinating optoelectronic properties for several energy conversation and storage applications and beyond.1,2,3 Ionothermal or high temperature synthesis using both bottom-up molecular precursor routes and top-down approaches from melon have been used to synthesize M-PHI.4,5 A pressing problem for M-PHIs and infact for most polymeric carbon nitrides (PCN) is their insufficient visible light absorption arising from their symmetry forbidden n-π* electronic transition and fast charge recombination.4 Varying precursor ratios during thermal condensation of urea and oxamide, low pressure-high temperature melamine polymerization, urea pyrolysis, salt-melt synthesis using amino tetrazoles and many other methods have been successfully applied to synthesize red carbon nitrides with extended vis-absorption. These methods are majorly cumbersome and since each synthetic pathway is unique to one final material, structural relations between red carbon nitrides vs. their poor light-absorbing counterparts does not exist.7

In this work, we show the first solvent mediated route for red carbon nitride synthesis by a post-synthetic modification of yellow K-PHI via polymeric self-assembly. A competition between the solvent mediator bulkiness and its basicity, reaction temperature and time is shown to tailor KPHI absorption spectra, according to need. A combination of solid-state NMR, p-XRD, thermogravimetric analysis, with ground and excited state photophysical characterizations offer structural insights into red PHI. Variation in the long-ranged stacking order and pore content seem to have an impact on light absorption but does not limit their charge storage behavior as seen from electron paramagnetic resonance data upon illumination and time-delayed H2 production from water-splitting, experiments. In fact, this spectrally modified red PHI acts as a photochromic switch by a complete disappearance of the n-π* electronic transition during the formation of a stable photo-reduced state.

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