偶氮苯基型离子液体溶液对空气中湿度的变色响应（英文）
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摘要
室温离子液体对空气湿度发生比色响应,在现有的文献中鲜有报道。本论文主要报道偶氮苯酚型离子液体溶液可自发地发生明显的颜色变化,这主要是由于偶氮苯酚阴离子与水分子形成氢键的缘故。该工作通过借助核磁共振技术、紫外-可见吸收光谱、实验结果及理论计算对其中的机理进行了深入的分析。具体地说,由紫外-可见吸收光谱可知,随着时间的推移,离子液体溶液在455 nm左右的吸收峰强度逐渐降低,同时在343 nm左右的吸收峰强度逐渐增强,并伴有由橙红色向浅黄色的颜色转变。这一自响应的现象也可以从核磁共振光谱中观测到。当溶液放置时间足够长时,偶氮苯酚阴离子的氢谱出峰全部向低场发生位移,且在高场处没有新峰产生。所以,很容易将刺激源锁定在空气中的气体比如弱酸性的二氧化碳以及湿度上。由此,我们向溶液中通入二氧化碳气体,溶液可从橙红色变为浅橙红色,但却不能进一步变为浅黄色,从而排除了二氧化碳的可能性。反之,我们却发现,向溶液(乙腈作溶剂)中逐渐加入少量的水,在474nm的吸收峰强度逐渐减弱,且在347 nm处的吸收峰强度逐渐增强,并伴随由橙红色向浅黄色的颜色变化,这与氯仿、四氯化碳溶液自发过程中产生的颜色变化几乎一致。并且,将两只装有离子液体溶液的比色皿分别放置在相对湿度为28%和100%的条件下,发现在较低的相对湿度下,溶液需要比在高湿度下长得多的时间实现整个的颜色转变,这表明湿度是引起溶液发生自发颜色变化最可能的刺激源。由高斯09软件计算(在B3LYP/6-31++G(p,d)水平)可知,偶氮苯酚阴离子的氧原子和水分子的氢原子之间的距离为0.174nm,相应的键角为171.12°;同时,偶氮苯酚阴离子中的氧原子与水发生作用后,氧原子的ADCH电荷由原来的-0.52变为-0.62。进一步地,由约化密度梯度分析可知,在-0.04 a.u.左右出现尖头可归属于O···H―O氢键。所有以上数据表明,空气中湿度是通过以与离子液体的阴离子形成氢键的方式,诱使离子液体溶液对其发生响应并伴随着肉眼可见的颜色变化。据我们所知,这是首次发现离子液体溶液可以对空气中湿度发生变色响应。我们希望这个工作可以加深对一些貌似反常现象背后科学道理的理解。
Room temperature ionic liquids(ILs) that can exhibit a colorimetric response to moisture in the air are rarely reported in the literature. In this study, an azophenolic IL solution exhibited a spontaneous a colorimetric response, driven by the formation of hydrogen bonding between the [PhN =NPhO ] anion and moisture in the air. This phenomenon was clearly understood using ultraviolet-visible(UV-Vis) absorption spectroscopy, nuclear magnetic resonance(NMR) spectra, experimental data, and theoretical calculations. Specifically, in the UV-Vis absorption spectra, absorption around 455 nm decreased, while the band around 343 nm increased in the IL CHCl_3 solution as time progressed; this was accompanied by a color change from orange to faint yellow. This spontaneous, self-responsive process was further observed using ~1H NMR data. When the IL solution was placed with sufficient time, all the ~1H NMR peaks of the azophenolic anion shifted downfield, but no new signals appeared in the upfield region. The reason for this was easily identified as the stimuli in the air, such as CO_2 and moisture. When pure CO_2 was bubbled through the IL CHCl_3 solution, the solution color changed from its original orange to light orange, but could not change further to faint yellow, which ruled out CO_2 gas as a stimulus. When a small amount of water was gradually added to the IL solution(MeCN solvent), the absorption band around 474 nm decreased, coupled with an increase in the absorption band around 347 nm. This was accompanied by a color change from orange to faint yellow, which was almost identical to the self-responsive process in CHCl_3 and CCl_4. Moreover, two cuvettes of IL CHCl_3 solution were placed under relative humidities of 28% and 100%, respectively; the IL CHCl_3 solution required a much longer time to exhibit a complete color change from orange to faint yellow under a lower relative humidity, demonstrating that moisture is the most likely stimulus triggering the self-responsive color change of the IL solution. As revealed by the Gaussian 09 program at the B3 LYP/6-31++G(p,d) level, the distance between the oxygen atom on the azophenolic anion and the hydrogen atom on the H_2O molecule was 0.174 nm, and the corresponding angle was 171.12°. Furthermore, the atomic dipole moment corrected Hirshfeld(ADCH) charge of the oxygen atom on the azophenolic anion was -0.52, and it increased to -0.62 after the azophenolic anion interacted with the H_2O. Reduced density gradient analysis revealed that the spike corresponding to O···H―O for the IL-H_2O complex was located at around -0.04 a.u.. All the above data indicate that the presence of hydrogen bonding rendered the IL solution responsive to the moisture stimulus, and this response was accompanied by a color change that was visible to the naked eye. To the best of our knowledge, this is the first demonstration of a colorimetric change in an IL solution in response to moisture. We hope this work can help us to gain insight into some seemingly abnormal phenomena that occur during the research process.
Room temperature ionic liquids(ILs) that can exhibit a colorimetric response to moisture in the air are rarely reported in the literature. In this study, an azophenolic IL solution exhibited a spontaneous a colorimetric response, driven by the formation of hydrogen bonding between the [PhN =NPhO ] anion and moisture in the air. This phenomenon was clearly understood using ultraviolet-visible(UV-Vis) absorption spectroscopy, nuclear magnetic resonance(NMR) spectra, experimental data, and theoretical calculations. Specifically, in the UV-Vis absorption spectra, absorption around 455 nm decreased, while the band around 343 nm increased in the IL CHCl_3 solution as time progressed; this was accompanied by a color change from orange to faint yellow. This spontaneous, self-responsive process was further observed using ~1H NMR data. When the IL solution was placed with sufficient time, all the ~1H NMR peaks of the azophenolic anion shifted downfield, but no new signals appeared in the upfield region. The reason for this was easily identified as the stimuli in the air, such as CO_2 and moisture. When pure CO_2 was bubbled through the IL CHCl_3 solution, the solution color changed from its original orange to light orange, but could not change further to faint yellow, which ruled out CO_2 gas as a stimulus. When a small amount of water was gradually added to the IL solution(MeCN solvent), the absorption band around 474 nm decreased, coupled with an increase in the absorption band around 347 nm. This was accompanied by a color change from orange to faint yellow, which was almost identical to the self-responsive process in CHCl_3 and CCl_4. Moreover, two cuvettes of IL CHCl_3 solution were placed under relative humidities of 28% and 100%, respectively; the IL CHCl_3 solution required a much longer time to exhibit a complete color change from orange to faint yellow under a lower relative humidity, demonstrating that moisture is the most likely stimulus triggering the self-responsive color change of the IL solution. As revealed by the Gaussian 09 program at the B3 LYP/6-31++G(p,d) level, the distance between the oxygen atom on the azophenolic anion and the hydrogen atom on the H_2O molecule was 0.174 nm, and the corresponding angle was 171.12°. Furthermore, the atomic dipole moment corrected Hirshfeld(ADCH) charge of the oxygen atom on the azophenolic anion was -0.52, and it increased to -0.62 after the azophenolic anion interacted with the H_2O. Reduced density gradient analysis revealed that the spike corresponding to O···H―O for the IL-H_2O complex was located at around -0.04 a.u.. All the above data indicate that the presence of hydrogen bonding rendered the IL solution responsive to the moisture stimulus, and this response was accompanied by a color change that was visible to the naked eye. To the best of our knowledge, this is the first demonstration of a colorimetric change in an IL solution in response to moisture. We hope this work can help us to gain insight into some seemingly abnormal phenomena that occur during the research process.
引文
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