Name: how dyes absorb visible light
Uploaded: 2020-03-26T07:22:29.000Z
Duration: 4 min 55 s
Channel: JoVE

当光照射到物质上时，一部分被其吸收，其余部分则被反射或透射。我们所感知到的物质颜色取决于其最可能反射的波长。例如，我们看到的一块蓝色织物，其中含有一种染料，该染料强烈反射蓝光，同时强烈吸收橙光和红光。
染料通常是共轭化合物，这意味着它们具有交替的双键和单键。电子可以在共轭体系内自由移动。不同颜色的染料在吸收光的波长上必然存在差异。当我们观察一些实例时，会发现吸收波长随着共轭程度的增加而增大。
那么，波长与共轭程度之间有何关系呢？让我们从分子能级的角度来考虑。我们可以将离域电子视为占据分子轨道（MOs）。当分子吸收具有特定能量的光时，该能量恰好足以将一个电子激发到更高能级的分子轨道。最可能发生的跃迁是从最高占据分子轨道（称为HOMO）跃迁至最低未占据分子轨道（LUMO）。因此，我们预期被吸收最多的光的波长应与HOMO-LUMO之间的能隙相对应。
共轭程度较低或无共轭的分子通常具有较大的HOMO-LUMO能隙。它们吸收紫外光并反射所有可见光，因此呈现白色或无色。共轭键通过降低分子的能级（尤其是高能级）使其更稳定。共轭程度越高，HOMO-LUMO能隙越小，吸收的光波长越长。金属元素和取代基也会影响该能隙的大小。
让我们来看一个例子。视黄醇具有较小的共轭体系，而叶绿素a则具有含氮和镁的大型共轭体系。视黄醇在325 nm处吸收光，而叶绿素a在430 nm和662 nm处均有吸收。正如预期的那样，视黄醇的能量间隙更大。
我们可以使用紫外-可见光分光光度计（UV-Vis spectrophotometer）来研究吸收现象。分光光度计由光源、控制样品接收波长的装置以及光检测器组成。样品通常为透明溶液。由于化合物往往在多个波长处都有吸收，因此可以在特定波长下测量吸光度，也可以在一定波长范围内进行扫描测量。此外，由于分子处于不同的取向和振动态，每个能级跃迁都会表现出一段波长范围。
在测量过程中，光可能被吸收、穿过而不与任何分子接触，或从溶剂或化合物分子上发生散射。我们忽略向后散射的少量光。有时，本可能被分子吸收的光反而从其表面发生散射。我们使用独特的摩尔衰减系数来描述一种物质对特定波长光的透射能力。吸光度随浓度变化，而摩尔衰减系数则不随浓度改变。
测量完成后，分光光度计将接收到的光与原始光进行比较，其比值称为透射率。吸光度是透射率的负十进制对数。如果分光光度计已记录溶剂的吸光度，它会将其扣除，从而仅显示待测化合物的吸光度。结果通常以吸光度对波长作图的形式呈现。化合物吸收最强的波长称为最大吸收波长（lambda max）。如果我们计算每个波长下的摩尔衰减系数，则该系数在最大吸收波长处达到最高值。
摩尔衰减系数、吸光度、样品浓度以及光程长度（即光在样品中传播的距离）之间的关系由比尔-朗伯定律描述。如果我们已知其中任意三个变量，就可以计算出第四个变量。
在本实验中，您将使用紫外-可见分光光度计分析荧光素、β-胡萝卜素和靛蓝染料的吸收特性。随后，您将利用朗伯-比尔定律绘制β-胡萝卜素的标准曲线，并据此测定β-胡萝卜素溶液的浓度。

染料的紫外-可见光谱分析

UV-Vis Spectroscopy of Dyes

吸光度
当光与物质相互作用时，部分光被吸收，其余部分则被反射或透射。我们所感知到的有色物质会反射可见光范围内的光。我们能够看到的物质颜色取决于其反射的光的波长。例如，我们 perceived 为蓝色的物质会反射可见光谱中的蓝色范围（430 – 480 nm）的光。根据色轮原理，同一物质会吸收与其反射光互补的光。因此，该蓝色物质吸收可见光谱中橙色区域（590 – 630 nm）的光。并非所有化合物都会吸收可见光区域的光，因此它们对人眼而言呈现无色。
光由其能量 E 和波长 λ 定义。其中，h 为普朗克常数，c 为光速。
<img src="/CDNSource/lm/labs/70/70_Concepts_1.jpg" alt="能量公式 \(E = \frac{hc}{\lambda}\)，公式，波长，光子能量计算" data-altupdatedat="1754904545000" data-alt="能量-波长关系公式 \(E = \frac{hc}{\lambda}\)，物理方程。" />
光的波长与其能量成反比。因此，能量较高的光具有较短的波长。
<h4>染料</h4>
不同颜色的染料在吸收光的波长上有所差异。大多数染料是具有交替双键和单键的共轭化合物，通常在可见光区域吸收光。
染料分子的共轭部分可以很短，即共轭程度较低，双键和单键交替较少；也可以很长，即共轭程度较高，具有大量交替的双键和单键。这些交替的双键不一定仅存在于两个碳原子之间，共轭键还可包括羰基以及碳和氧之间的双键。共轭程度决定了化合物吸收光的波长。例如，共轭程度高的化合物比共轭程度低的化合物吸收更长波长的光。
根据分子轨道理论，离域电子占据分子轨道。最高占据分子轨道（HOMO）是指具有电子的最高能量轨道，最低未占据分子轨道（LUMO）是指不含电子的最低能量轨道。缺乏或没有共轭的分子通常在HOMO与LUMO之间具有较大的能隙；然而，共轭分子的HOMO与LUMO之间的能隙较小。
要将电子从较低能级激发到较高能级，或从HOMO激发到LUMO，分子必须吸收能量等于这两个轨道之间能隙的光。因此，具有较大能隙的分子需要更高能量的光（如紫外光）来激发电子。然而，染料的能隙较小，需要较低能量的光（如可见光）来激发电子。
<img src="/CDNSource/lm/labs/70/70_Concepts_2.jpg" alt="能隙方程 ΔE(HOMO-LUMO) = hc/λ；分子光谱学，量子化学。" data-altupdatedat="1754904545000" data-alt="跃迁能公式 ΔE(HOMO-LUMO)=hc/λ，表示分子轨道能隙。" />
因此，具有较大能隙的分子需要更高能量的光（如紫外光）来激发电子。然而，染料的能隙较小，需要较低能量的光（如可见光）即可激发电子。
请记住，光的能量与波长成反比。因此，高能光的波长比低能光的波长短，低能光具有较长的波长。
<h4>分光光度计</h4>
实验上，使用紫外-可见（UV-Vis）分光光度计测量光吸收。该仪器利用光源，通过单色器将其转化为特定波长的光，光穿过样品后进入另一端的检测器。样品必须呈液态，因此若有机化合物为固体，则需要使用溶剂。该溶液被盛放在一种称为比色皿的样品容器中。根据样品的不同，比色皿可由石英晶体、玻璃或塑料制成，并具有特定的光程长度。该光程长度是指光穿过样品所经过的距离。由于溶剂本身也会吸收光，因此需要使用仅含溶剂的空白样品。这样，当仪器采集样品化合物的吸收光谱时，可扣除溶剂的背景光谱，从而仅显示由样品引起的吸光度。透射率（T）是指透过样品的原始光所占的分数。
<img src="/CDNSource/lm/labs/70/70_Concepts_3.jpg" alt="透射率公式 T=P/P₀，用于光谱分析和光学研究的关键公式。" data-altupdatedat="1754904545000" data-alt="透射比公式，\( T = \frac{P}{P_0} \)；用于光学分析研究的方程。" />
此处，P0 表示入射到样品前的光束辐照度，即单位时间单位面积上的能量；P 表示照射到探测器上的光束辐照度。通常 P 小于 P0，因为部分光被样品吸收。
吸光度（A）定义为透射率的负对数。
<img src="/CDNSource/lm/labs/70/70_Concepts_4.jpg" alt="吸光度公式：A=log(P₀/P)=-logT；光谱分析中的光学原理。" data-altupdatedat="1754904545000" data-alt="用于光学分析的吸光度方程 A=log(P₀/P)=-logT，示意图，光谱学概念。" />
吸光度的取值范围在 0（无吸收）到 2（99% 吸收）之间。当没有光被吸收时，P0 等于 P，透射率为 1，因此吸光度为 0。如果 90% 的光被吸收，则 10% 的光被透射，T 等于 0.1，此时吸光度为 1。如果 99% 的光被吸收，则 1% 的光被透射（T = 0.01），吸光度等于 2。
获得的光谱是吸光度与波长的关系图。对于紫外-可见分光光度计，该范围在 200 至 800 nm 之间。
<h4>比尔-朗伯定律</h4>
特定化合物的透射率和吸光度与其在溶液中的浓度相关。这种关系由比尔-朗伯定律描述。
<img src="/CDNSource/lm/labs/70/70_Concepts_5.jpg" alt="比尔-朗伯定律方程；A=εcl；光谱学中的吸收示意图；光学研究。" data-altupdatedat="1754904545000" data-alt="比尔-朗伯定律方程，A=εcl，吸收测量，光学光谱学概念。" />
样品的吸光度等于化合物浓度、光程长度和摩尔衰减系数三者的乘积。该系数是每种化合物特有的，且随波长不同而变化。然而，若保持波长恒定，则摩尔衰减系数不随浓度变化而改变。对应于样品最大吸光度的波长，即 λmax，也具有最大的摩尔衰减系数。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Silberberg, M.S. (2012). 化学：物质的分子本质与变化。波士顿，马萨诸塞州：麦格劳希尔出版社。</li>
	<li style="font-size:17px">Harris, D.C. (2015). 定量化学分析。纽约，纽约州：W.H. 弗里曼公司。</li>
</ol>

吸光度  
当光与物质相互作用时，部分光被吸收，其余部分则被反射或透过该物质。...

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染料的紫外-可见光谱

how dyes absorb visible light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and wavelength, and higher-energy light has a shorter wavelength.

Most dyes are conjugated compounds. This means they contain alternating double and single bonds, and in some cases carbonyl groups and carbon-oxygen double bonds also take part in the conjugated system. The length of the conjugated region affects the wavelength the dye absorbs. A dye with more conjugation absorbs a longer wavelength than a dye with less conjugation. This happens because delocalized electrons occupy molecular orbitals. The highest occupied molecular orbital, or HOMO, is the highest-energy orbital with an electron, and the lowest unoccupied molecular orbital, or LUMO, is the next available empty orbital.

Molecules with little or no conjugation usually have a large energy gap between the HOMO and LUMO. Conjugated molecules have a smaller gap. An electron must absorb light with the same energy as that gap to move from the HOMO to the LUMO. Because of this, molecules with a large gap need higher-energy UV light, while dyes with a smaller gap can be excited by lower-energy visible light. Since light energy and wavelength are inversely related, higher-energy light has a shorter wavelength.

Absorbance is measured experimentally with a UV-Visible, or UV-Vis, spectrophotometer. The instrument uses a light source and a monochromator to select specific wavelengths. That light passes through a liquid sample and then reaches a detector. If the compound is a solid, it must first be dissolved in a solvent. The solution is placed in a cuvette, which may be made of quartz crystal, glass, or plastic. The cuvette has a pathlength, which is the distance the light travels through the sample. Because the solvent also absorbs light, a blank made of solvent alone is used to subtract background absorbance.

The instrument measures transmittance, which is the fraction of the original light that passes through the sample. P0 is the irradiance before the sample, and P is the irradiance that reaches the detector. Absorbance is the negative log of transmittance. It ranges from 0, which means no absorption, to 2, which means 99% absorption. If 90% of the light is absorbed, transmittance is 0.1 and absorbance is 1. If 99% is absorbed, transmittance is 0.01 and absorbance is 2. The spectrum is a plot of absorbance versus wavelength, and for UV-Vis spectroscopy, the range is 200 to 800 nm.

The amount of light absorbed by a dye also depends on its concentration in solution. This relationship is described by the Beer-Lambert law. Absorbance equals the concentration of the compound multiplied by the path length and the molar attenuation coefficient. The molar attenuation coefficient is unique to each compound and changes with wavelength. If the wavelength is kept constant, it does not change with concentration. The wavelength with the highest absorbance is called lambda max, and it also has the largest molar attenuation coefficient.

How Dyes Absorb Visible Light

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Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how dye structure affects visible light absorption, how UV-Vis spectrophotometers measure absorbance, and how Beer-Lambert law links absorbance to concentration.

How Dyes Absorb Visible Light | JoVE K12

通过这段 4 分钟的教师指南，准备教授 染料的紫外-可见光谱分析。涵盖 JoVE Lab Manual 上 Chemistry 实验的关键准备步骤。

Chemistry 中的 染料的紫外-可见光谱分析 教师准备 - 1.1K+ 观看次数 | JoVE Lab Manual

在这段 10 分钟的学生实验流程视频中，逐步学习 染料的紫外-可见光谱分析 的操作步骤。属于 JoVE Lab Manual 上的 Chemistry 系列。

Chemistry 中的 染料的紫外-可见光谱分析 学生实验流程 - 3.9K+ 观看次数 | JoVE Lab Manual

How Dyes Absorb Visible Light

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