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

Quando a luz atinge uma substância, uma parte é absorvida por ela, enquanto o resto é refletido ou transmitido através dela. A cor da substância, como a percebemos, depende de quais comprimentos de onda ela provavelmente refletirá. Por exemplo, um pedaço de tecido que vemos como azul contém um corante que reflete fortemente a luz azul e absorve fortemente a luz laranja e vermelha.
Os corantes são tipicamente compostos conjugados, o que significa que eles têm ligações duplas e simples alternadas. Os elétrons podem se mover livremente dentro do sistema conjugado. Corantes de cores diferentes devem variar nos comprimentos de onda da luz que absorvem. Quando olhamos para alguns exemplos, vemos que o comprimento de onda absorvido aumenta com a quantidade de conjugação.
Então, como o comprimento de onda está relacionado ao grau de conjugação? Vamos considerar os níveis de energia molecular. Podemos pensar em elétrons deslocalizados como orbitais moleculares ocupantes, ou MOs. Uma molécula absorve luz com a energia exata necessária para excitar um elétron para um orbital molecular de energia mais alta. A transição mais provável é do orbital molecular ocupado mais alto, chamado HOMO, para o orbital molecular desocupado mais baixo, ou LUMO. Portanto, esperamos que o comprimento de onda mais absorvido corresponda à lacuna de energia HOMO - LUMO.
Moléculas com pouca ou nenhuma conjugação normalmente têm uma grande lacuna HOMO - LUMO. Eles absorvem a luz ultravioleta e refletem toda a luz visível, de modo que parecem brancos ou incolores. As ligações conjugadas estabilizam as moléculas diminuindo seus níveis de energia, principalmente em altas energias. Quanto maior o grau de conjugação, menor a lacuna HOMO - LUMO e maior o comprimento de onda absorvido. Metais e substituições também afetam a lacuna.
Vejamos um exemplo. O retinol tem um pequeno sistema conjugado, enquanto a clorofila a tem um grande sistema com nitrogênio e magnésio. O retinol é absorvido a 325 nm, enquanto a clorofila a é absorvida a 430 e 662 nm. Como esperado, a lacuna de energia do retinol é maior.
Podemos estudar a absorção usando um espectrofotômetro UV e luz visível, ou UV-Vis. Um espectrofotômetro consiste em uma fonte de luz, uma maneira de controlar os comprimentos de onda que a amostra recebe e um detector de luz. A amostra é normalmente uma solução transparente. A absorbância pode ser medida em um comprimento de onda específico ou em uma faixa de comprimento de onda, uma vez que os compostos geralmente absorvem em mais de um comprimento de onda. Além disso, vemos uma faixa de comprimentos de onda para cada transição porque as moléculas estão em diferentes orientações e estados vibracionais.
Durante a medição, a luz é absorvida, passa sem entrar em contato com nenhuma molécula ou reflete em um solvente ou molécula composta. Ignoramos a pequena quantidade de luz que reflete para trás. Às vezes, a luz que poderia ser absorvida por uma molécula é refletida nela. Descrevemos o quão bem uma substância transmite um comprimento de onda específico com um coeficiente de atenuação molar único. Enquanto a absorbância muda com a concentração, o coeficiente de atenuação molar não.
Após a medição, o espectrofotômetro compara a luz recebida e a original em uma proporção chamada transmitância. A absorbância é o logaritmo negativo de base 10 da transmitância. Se o espectrofotômetro tiver a absorbância do solvente, ele a subtrai para mostrar apenas o composto. Os resultados são geralmente exibidos como absorbância versus comprimento de onda. O comprimento de onda no qual o composto absorve mais é chamado de lambda max. Se calculássemos o coeficiente de atenuação molar para cada comprimento de onda, seria mais alto em lambda max.
O coeficiente de atenuação molar, a absorbância, a concentração da amostra e o comprimento do caminho, que é a distância que a luz percorreu através da amostra, estão relacionados pela lei de Beer-Lambert. Se conhecermos quaisquer três variáveis, podemos calcular a quarta.
Neste laboratório, você analisará as características de absorção de fluoresceína, betacaroteno e corante índigo usando um espectrofotômetro UV-Vis. Em seguida, você usará a lei de Beer-Lambert para criar uma curva de calibração de β-caroteno e, em seguida, determinar a concentração da solução de β-caroteno.

espectroscopia uv-vis de corantes

Absorvância
Quando a luz interage com uma substância, uma parte da luz é absorvida enquanto o restante é refletido ou transmitido através dela. Substâncias que percebemos como tendo uma cor refletem a luz na faixa visível. A cor da substância que podemos ver depende de qual comprimento de onda da luz é refletido. Uma substância que percebemos como azul reflete a luz na faixa azul (430 – 480 nm) do espectro visível. De acordo com a roda de cores, a mesma substância absorve luz complementar à luz refletida. Assim, a substância azul absorve luz na região laranja (590 – 630 nm) do espectro visível. Nem todos os compostos são absorvidos na região visível e, como resultado, parecem incolores ao olho humano.
A luz é definida por sua energia, E, e seu comprimento de onda, λ. Aqui, h é a constante de Planck e c é a velocidade da luz.
<img src="/CDNSource/lm/labs/70/70_Concepts_1.jpg" alt="Equação de energia \(E = \frac{hc}{\lambda}\), fórmula, comprimento de onda, cálculo da energia do fóton" data-altupdatedat="1754907201000">
O comprimento de onda da luz é inversamente proporcional à sua energia. Portanto, a luz de energia mais alta tem um comprimento de onda mais curto.
<h4>Corantes</h4>
Diferentes corantes coloridos variam no comprimento de onda da luz que absorvem. A maioria dos corantes são compostos conjugados com ligações duplas e simples alternadas e normalmente absorvem a luz na região visível.
A parte conjugada da molécula de corante pode ser muito curta, o que significa que há um baixo grau de conjugação e poucas ligações duplas e simples alternadas, ou longa, o que significa que há um alto grau de conjugação com muitas ligações duplas e simples alternadas. Essas ligações duplas alternadas não precisam necessariamente estar apenas entre dois carbonos. Essas ligações conjugadas podem incluir os grupos carbonila e as ligações duplas entre carbono e oxigênio. O grau de conjugação determina o comprimento de onda da luz que o composto absorve. Por exemplo, compostos com alto grau de conjugação absorvem um comprimento de onda mais longo do que compostos com menor grau de conjugação.
Com base na teoria dos orbitais moleculares, os elétrons deslocalizados ocupam orbitais moleculares. O orbital molecular ocupado mais alto, ou HOMO, é o orbital de energia mais alta com um elétron. O orbital molecular desocupado mais baixo, ou LUMO, é o orbital de menor energia sem elétrons. Moléculas com pouca ou nenhuma conjugação normalmente têm uma grande lacuna de energia entre o HOMO e o LUMO. No entanto, as moléculas conjugadas têm uma lacuna de energia menor entre o HOMO e o LUMO.
Para excitar um elétron de um nível de energia mais baixo para um nível de energia mais alto, ou do HOMO para o LUMO, a molécula deve absorver luz com energia igual à lacuna de energia entre os dois orbitais. Por esse motivo, moléculas com uma grande lacuna de energia requerem luz de energia mais alta, como a luz ultravioleta, para excitar um elétron. Os corantes, no entanto, têm uma lacuna de energia menor e requerem luz de menor energia, como a luz visível, para excitar um elétron.
<img src="/CDNSource/lm/labs/70/70_Concepts_2.jpg" alt="Equação de lacuna de energia ΔE(HOMO-LUMO) = hc/λ; espectroscopia molecular, química quântica." data-altupdatedat="1754907201000">
Por esse motivo, moléculas com uma grande lacuna de energia requerem luz de energia mais alta, como a luz ultravioleta, para excitar um elétron. Os corantes, no entanto, têm uma lacuna de energia menor e requerem luz de menor energia, como a luz visível, para excitar um elétron.
Lembre-se de que a energia da luz é inversamente proporcional ao comprimento de onda. Portanto, a luz de energia mais alta tem comprimentos de onda mais curtos do que a luz de energia mais baixa que tem comprimentos de onda mais longos.
<h4>espectrofotômetro</h4>
Experimentalmente, a absorbância de luz é medida usando um espectrofotômetro UV-Visível (UV-Vis). Este instrumento utiliza uma fonte de luz que é transformada por um monocromador em comprimentos de onda específicos de luz que passarão por uma amostra e por um detector na outra extremidade. As amostras devem estar em um líquido, portanto, um solvente é necessário se o composto orgânico for sólido. Esta solução é mantida em um porta-amostras conhecido como cubeta. Dependendo da amostra, a cubeta pode ser feita de cristal de quartzo, vidro ou plástico e tem um comprimento de caminho específico. Esse comprimento de caminho é a distância que a luz deve percorrer através da amostra. Como o solvente também absorve luz, é necessária uma amostra em branco apenas do solvente. Portanto, quando o instrumento captura o espectro de absorbância do composto da amostra, ele pode subtrair o espectro de fundo do solvente para exibir a absorbância causada apenas pela amostra. A transmitância, T, é a fração da luz original que passa pela amostra.
<img src="/CDNSource/lm/labs/70/70_Concepts_3.jpg" alt="Fórmula da equação de transmitância T=P/P₀, chave na análise espectroscópica e estudos ópticos." data-altupdatedat="1754907201000">
Aqui, P0 é a irradiância, ou a energia por segundo por unidade de área, do feixe de luz antes de atingir a amostra. P é a irradiância do feixe de luz que atinge o detector. P é tipicamente menor que P0, pois parte da luz é absorvida pela amostra.
A absorbância, A, é definida como o log negativo da transmitância.
<img src="/CDNSource/lm/labs/70/70_Concepts_4.jpg" alt="Fórmula de absorbância: A = log (P ₀ / P) = -logT; princípios ópticos em análise de espectroscopia." data-altupdatedat="1754907201000">
A absorbância tem uma faixa de valores entre 0 (sem absorção) e 2 (99% de absorção). Quando nenhuma luz é absorvida, P0 é igual a P e a transmitância é igual a um. Assim, a absorbância é zero. Se 90% da luz for absorvida, então 10% é transmitido e T é igual a 0,1. Isso resulta em uma absorbância igual a 1. Se 99% da luz for absorvida, então 1% é transmitido (T = 0,01) e a absorbância é igual a 2.
O espectro obtido é um gráfico da absorbância versus o comprimento de onda. Para um espectrofotómetro UV-Vis, este intervalo situa-se entre 200 e 800 nm.
<h4>Lei Beer-Lambert</h4>
A transmitância e a absorbância de um determinado composto estão relacionadas à concentração do composto em solução. Essa relação é descrita pela lei de Beer-Lambert.
<img src="/CDNSource/lm/labs/70/70_Concepts_5.jpg" alt="Equação da lei de Beer-Lambert; A = εcl; diagrama para absorção em espectroscopia; estudo óptico." data-altupdatedat="1754907201000">
A absorbância da amostra é igual ao produto da concentração do composto, do comprimento do caminho e do coeficiente de atenuação molar. Este coeficiente é único para cada composto e varia de acordo com o comprimento de onda. No entanto, se o comprimento de onda for mantido constante, o coeficiente de atenuação molar será o mesmo, independentemente das mudanças na concentração. O comprimento de onda que corresponde à maior absorbância da amostra, conhecido como λmax, também terá o maior coeficiente de atenuação molar.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Silberberg, M.S. (2012). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análise Química Quantitativa. Nova York, NY: W.H. Freeman and Company.</li>
</ol>

Espectroscopia UV-Vis de Corantes

Absorvância
Quando a luz interage com uma substância, uma parte da luz é absorvida enquanto o restante é refletido ou transmitido através dela. ...

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UV-Vis Spectroscopy of Dyes

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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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

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How Dyes Absorb Visible Light

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