DOI: 10.1002/asia.201300804 Light Emission in Water-Containing Cocrystals: the Influence of Water Molecules on the Fluorescence Properties of a Schiff-Base Molecule Feng Zhou, [a] Pengli Tan, [b] Yong Ma, [a] Youyong Li, [b] Najun Li, [a] Hua Li, [a] Lihua Wang, [a] Hongwei Gu, [a] Qingfeng Xu,* [a] and Jianmei Lu* [a] Introduction Highly efficient organic solid-state emitters have attracted great recent interest because of their promising applications in the fields of light-emitting devices, [1] bio-labeling, [2] and chemosensors. [3] Optical and electronic properties of organic molecules are strongly dependent on their primary molecu- lar structure and their intermolecular interactions in con- densed states. [4] However, in the solid state, organic chromo- phores generally suffer from a concentration-quenching effect, owing to enhanced non-radiative decay, [5] which limits their use. In 2001, Tang and co-workers found that the fluorescence of some molecular systems could be turned “on” in the solid state to display an aggregation-induced emission (AIE) effect. [6] The luminescence mechanism of these AIE-active molecules was mainly attributed to the re- striction of intramolecular rotation in the aggregates. [7] The ability to switch on/off and tune organic solid-state lumines- cence through a supramolecular-assembly approach has also been an interesting topic, [8] because optical properties and supramolecular architectures could be modified through in- termolecular interactions and guest molecules. [9] Therefore, investigating the relationships between molecular assemblies and their luminescence properties is important for the design of new emissive materials. In addition, water has long been used to induce molecular aggregation and lumines- cence in AIE systems. [10] Water molecules must surely play specific roles in these previously reported AIE systems. However, so far, there have only been a few studies on the role of water molecules in these processes, likely because the architectures of nano- and microsized aggregates are dif- ficult to measure. Herein, we report the synthesis of a new compound, 4- amino-3-(2-(2-hydroxybenzylidene)hydrazinyl)-1H-1,2,4-tria- zole-5(4H)-thione (HATT, Scheme 1 a). HATT forms at least two types of crystal packing (HATT and HATT·2H 2 O), in which guest water molecules change the molecular stacking modes in the crystal lattice. More inter- estingly, these different crystalline states also exhibit distinct fluorescence behaviors (Figure 1). Solutions of HATT in or- ganic solvents and as a crystalline material are weakly emis- sive; however, the water-containing microcrystals (m- HATT·nH 2 O) and crystals (HATT·2H 2 O) exhibited remark- ably enhanced fluorescence emission. This compound repre- sents a new example of light emission in the crystalline state by using a supramolecular approach, which is caused by a water-triggered structural transformation. The mechanism of luminescence was assigned to a possible structural trans- formation of the metastable molecular conformation in the corresponding excited state, as well as rigidification of the molecules in the crystal lattice. [a] F. Zhou, Y. Ma, Dr. N. Li, Dr. H. Li, Dr. L. Wang, Prof.Dr. H. Gu, Prof. Dr. Q. Xu, Prof. Dr. J. Lu Key Laboratory of Organic Synthesis of Jiangsu Province School of Chemistry Chemical Engineering and Materials Soochow University Suzhou, 215123 (China) E-mail : lujm@suda.edu.cn xuqingfeng@suda.edu.cn [b] P. Tan, Prof.Dr. Y. Li Institute of Functional Nano and Soft Materials Laboratory (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices Soochow University Suzhou, 215123 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300804. Keywords: cocrystals · lumines- cence · Schiff bases · self-assembly · water Abstract: In the presence or absence of water, a Schiff-base compound, 4-amino- 3-(2-(2-hydroxybenzylidene)hydrazinyl)-1H-1,2,4-triazole-5(4H)-thione (HATT), forms different crystalline states (HATT, HATT·2H 2 O, and a lamellar structure, m-HATT·nH 2 O), which show different luminescence emission properties. Herein, we investigate the emission of HATT and the role of water molecules. A water molecule, which acts as both a hydrogen-bond acceptor and -donor, enlarges the distance between adjacent HATT molecules and hinders non-radiative decay path- ways. Chem. Asian J. 2014, 9, 223 – 228 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 223 FULL PAPER