Turning on the Light: Polymorphism-Induced Photoluminescence in Cysteine Crystals

TL;DR

This study reveals how different crystal packing of L-Cysteine in heavy water enhances photoluminescence through constrained vibrational modes and intersystem crossing, offering new insights into non-aromatic photoluminescent systems.

Contribution

It demonstrates the structural origins of photoluminescence in cysteine crystals and elucidates the mechanisms behind light emission in non-aromatic supramolecular assemblies.

Findings

Cysteine crystals in D2O exhibit stronger photoluminescence than in H2O.

Electronic transition analysis shows vibrational modes involving S-H and C-N groups influence emission.

Intersystem crossing contributes to the emission mechanism in D2O-formed crystals.

Abstract

Photoluminescence of non-aromatic supramolecular chemical assemblies has attracted considerable attention in recent years due to its potential for use in molecular sensing and imaging technologies. The underlying structural origins, the mechanisms of light emission in these systems, and the generality of this phenomenon remain elusive. Here, we demonstrate that crystals of L-Cysteine (Cys) formed in heavy water ($\text{D}_2\text{O}$) exhibit distinct packing and hydrogen-bond networks, resulting in significantly enhanced photoluminescence compared to those prepared in $\text{H}_2\text{O}$. Using advanced excited-state simulations, we elucidate the nature of electronic transitions that activate vibrational modes of Cys in $\text{H}_2\text{O}$, particularly those involving thiol (S-H) and amine (C-N) groups, which lead to non-radiative decay. For the crystal formed in $\text{D}_2\text{O}$, these modes appear to be more constrained, and we also observe intersystem crossing from the singlet to the triplet state, indicating a potentially more complex light emission mechanism. Our findings provide new insights into this intriguing phenomenon and introduce innovative design principles for generating emergent fluorophores.