Academy of Scientific and Innovative Research (AcSIR) , CSIR-CLRI Campus, Adyar, Chennai 600020, India.
Department of Chemical Engineering, Indian Institute of Technology Madras , Adyar, Chennai 600036, India.
Langmuir. 2017 Nov 28;33(47):13504-13514. doi: 10.1021/acs.langmuir.7b03453. Epub 2017 Nov 14.
Charge-transfer (CT) gel materials obtained from low-molecular-weight (LMW) compounds through a supramolecular self-assembly approach have received fascinating attention by many researchers because of their interesting material property and potential applications. However, most of the CT gel materials constructed were of organogels while the construction of CT gels in the form of a hydrogel is a challenge because of the solubility issue in water, which considerably limits the use of CT hydrogels. Herein, for the first time, we report a new LMW gelator [N-(fluorenylmethoxycarbonyl)-N-(δ-butyric-1-pyrenyl)-l-lysine, (FmKPy)], composed of two functional moieties such as fluorenylmethoxycarbonyl and pyrene, which not only parade both hydro and organo (ambidextrous) supramolecular gel formation but also exhibit CT ambidextrous gels when mixed with an electron acceptor such as 2,4,7-trinitro-9-fluorenone (TNF). This finding is significant as the established CT organogelator in the literature did not form an organogel in the absence of an electron acceptor or lose their gelation property upon the addition of the acceptor. CT between pyrene and TNF was confirmed by the color change as well as the appearance of the CT band in the visible region of the absorption spectrum. CT between FmKPy and TNF was supported by the solvent dilution method using tetrahydrofuran as the gel breaker and pyrene fluorescence quenching in the case compound containing pyrene and TNF. The morphology of FmKPy ambidextrous gels indicates the fibrous nature while the self-assembled structure is primarily stabilized by π-π stacking among fluorenyl and pyrenyl moieties and hydrogen bonding between amide groups. The FmKPy-TNF CT ambidextrous gel retains the fibrous nature; however, the size of the fibers changed. In FmKPy-TNF CT gels, TNF is intercalated between pyrene moieties in the self-assembled structure as confirmed by fluorescence quenching and powder X-ray diffraction. The FmKPy ambidextrous gel exhibits significant properties such as low minimum gelation concentration (MGC), thixotropic nature, pH stimuli response, and high thermal stability. Upon the addition of TNF, the FmKPy-TNF CT ambidextrous gel maintains all these properties except MGC which increased for FmKPy-TNF. Because pyrene-based LMW organogels have been developed widely for many applications while their hydrogels were limited, the current finding of the pyrene-based ambidextrous fluorescent gel with the CT property provides a wide opportunity to use FmKPy as a soft material maker and also for potential applications in fields like surface coating, three-dimensional printing, and so forth.
通过超分子自组装方法从低分子量（LMW）化合物获得的电荷转移（CT）凝胶材料因其有趣的材料性质和潜在应用而受到许多研究人员的关注。然而，大多数 CT 凝胶材料都是有机凝胶，而在水中形成 CT 水凝胶是一个挑战，因为水溶性问题，这极大地限制了 CT 水凝胶的使用。在这里，我们首次报道了一种新的 LMW 凝胶剂[N-（芴甲氧羰基）-N-（δ-丁酸-1-基）-l-赖氨酸，（FmKPy）]，由两个功能部分组成，如芴甲氧羰基和芘，不仅展示了水凝胶和有机凝胶（左右开弓）的超分子凝胶形成，而且当与电子受体如 2,4,7-三硝基-9-芴酮（TNF）混合时，也表现出 CT 左右开弓凝胶。这一发现意义重大，因为文献中建立的 CT 有机凝胶剂在没有电子受体的情况下不会形成有机凝胶，或者在加入受体后失去其凝胶性质。通过颜色变化以及可见区域吸收光谱中 CT 带的出现，证实了芘和 TNF 之间的 CT。FmKPy 和 TNF 之间的 CT 通过使用四氢呋喃作为凝胶破坏剂的溶剂稀释方法以及含有芘和 TNF 的化合物中芘荧光猝灭得到支持。FmKPy 左右开弓凝胶的形态表明其具有纤维性质，而自组装结构主要通过芴基和芘基部分之间的π-π堆积和酰胺基团之间的氢键稳定。FmKPy-TNF CT 左右开弓凝胶保留了纤维性质；然而，纤维的尺寸发生了变化。在 FmKPy-TNF CT 凝胶中，TNF 被插入到自组装结构中的芘基之间，这一点通过荧光猝灭和粉末 X 射线衍射得到证实。FmKPy 左右开弓凝胶具有低的最低凝胶浓度（MGC）、触变性、pH 刺激响应和高热稳定性等显著性质。加入 TNF 后，FmKPy-TNF CT 左右开弓凝胶保持了除 MGC 增加外的所有这些性质。由于基于芘的 LMW 有机凝胶已被广泛开发用于许多应用，而其水凝胶受到限制，因此当前基于具有 CT 性质的芘的左右开弓荧光凝胶的发现为 FmKPy 作为软材料制造商提供了广泛的机会，并且也为表面涂层、三维打印等领域的潜在应用提供了机会。
