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

Cuando la luz llega a una sustancia, una parte es absorbida por ella, mientras que el resto se refleja o se transmite a través de ella. El color de la sustancia, tal como lo percibimos, depende de las longitudes de onda que es más probable que refleje. Por ejemplo, una pieza de tela que vemos como azul contiene un tinte que refleja fuertemente la luz azul y absorbe fuertemente la luz naranja y roja.
Los colorantes suelen ser compuestos conjugados, lo que significa que tienen enlaces dobles y simples alternados. Los electrones pueden moverse libremente dentro del sistema conjugado. Los tintes de diferentes colores deben variar en las longitudes de onda de la luz que absorben. Cuando observamos algunos ejemplos, vemos que la longitud de onda absorbida aumenta con la cantidad de conjugación.
Entonces, ¿cómo se relaciona la longitud de onda con el grado de conjugación? Consideremos los niveles de energía molecular. Podemos pensar en los electrones deslocalizados como orbitales moleculares ocupantes, o MO. Una molécula absorbe luz con la energía exacta necesaria para excitar un electrón a un orbital molecular de mayor energía. La transición más probable es desde el orbital molecular ocupado más alto, llamado HOMO, hasta el orbital molecular desocupado más bajo, o LUMO. Por lo tanto, esperamos que la longitud de onda más absorbida coincida con la brecha de energía HOMO - LUMO.
Las moléculas con poca o ninguna conjugación suelen tener una gran brecha HOMO - LUMO. Absorben la luz ultravioleta y reflejan toda la luz visible, por lo que parecen blancos o incoloros. Los enlaces conjugados estabilizan las moléculas al reducir sus niveles de energía, particularmente a altas energías. Cuanto mayor sea el grado de conjugación, menor será la brecha HOMO - LUMO y mayor será la longitud de onda absorbida. Los metales y las sustituciones también afectan a la brecha.
Veamos un ejemplo. El retinol tiene un pequeño sistema conjugado, mientras que la clorofila a tiene un gran sistema con nitrógeno y magnesio. El retinol se absorbe a 325 nm, mientras que la clorofila a se absorbe tanto a 430 como a 662 nm. Como era de esperar, la brecha energética del retinol es mayor.
Podemos estudiar la absorción utilizando un espectrofotómetro UV y luz visible, o UV-Vis. Un espectrofotómetro consta de una fuente de luz, una forma de controlar las longitudes de onda que recibe la muestra y un detector de luz. La muestra suele ser una solución transparente. La absorbancia se puede medir a una longitud de onda específica o en un rango de longitud de onda, ya que los compuestos a menudo absorben a más de una longitud de onda. Además, vemos un rango de longitudes de onda para cada transición porque las moléculas están en diferentes orientaciones y estados vibratorios.
Durante la medición, la luz se absorbe, pasa sin entrar en contacto con ninguna molécula o rebota en una molécula de disolvente o compuesto. Ignoramos la pequeña cantidad de luz que rebota hacia atrás. A veces, la luz que podría ser absorbida por una molécula rebota en ella. Describimos qué tan bien una sustancia transmite una longitud de onda específica con un coeficiente de atenuación molar único. Mientras que la absorbancia cambia con la concentración, el coeficiente de atenuación molar no lo hace.
Después de la medición, el espectrofotómetro compara la luz recibida y la original en una relación llamada transmitancia. La absorbancia es el logaritmo de base negativa 10 de la transmitancia. Si el espectrofotómetro tiene la absorbancia del solvente, la resta para mostrar solo el compuesto. Los resultados generalmente se muestran como absorbancia frente a longitud de onda. La longitud de onda a la que el compuesto absorbe más se denomina lambda max. Si calculáramos el coeficiente de atenuación molar para cada longitud de onda, sería más alto en lambda máx.
El coeficiente de atenuación molar, la absorbancia, la concentración de la muestra y la longitud del camino, que es la distancia que recorrió la luz a través de la muestra, están relacionados por la ley de Beer-Lambert. Si conocemos tres variables, podemos calcular la cuarta.
En este laboratorio, analizará las características de absorción de la fluoresceína, el betacaroteno y el tinte índigo utilizando un espectrofotómetro UV-Vis. A continuación, utilizará la ley de Beer-Lambert para crear una curva de calibración de β caroteno y, a continuación, determinará la concentración de la solución de β caroteno.

espectroscopía uv-vis de colorantes

absorbancia
Cuando la luz interactúa con una sustancia, una parte de la luz se absorbe mientras que el resto se refleja o se transmite a través de ella. Las sustancias que percibimos como de color reflejan la luz en el rango visible. El color de la sustancia que podemos ver depende de la longitud de onda de la luz que se refleja. Una sustancia que percibimos como azul refleja la luz en el rango azul (430 – 480 nm) del espectro visible. Según la rueda de colores, la misma sustancia absorbe la luz que es complementaria a la luz reflejada. Por lo tanto, la sustancia azul absorbe la luz en la región naranja (590 – 630 nm) del espectro visible. No todos los compuestos se absorben en la región visible y, como resultado, parecen incoloros para el ojo humano.
La luz se define por su energía, E, y su longitud de onda, λ. Aquí, h es la constante de Planck y c es la velocidad de la luz.
<img src="/CDNSource/lm/labs/70/70_Concepts_1.jpg" alt="Ecuación de energía \(E = \frac{hc}{\lambda}\), fórmula, longitud de onda, cálculo de energía de fotones" data-altupdatedat="1754907201000">
La longitud de onda de la luz es inversamente proporcional a su energía. Por lo tanto, la luz de mayor energía tiene una longitud de onda más corta.
<h4>tintes</h4>
Los tintes de diferentes colores varían en la longitud de onda de la luz que absorben. La mayoría de los tintes son compuestos conjugados con enlaces dobles y simples alternados y, por lo general, absorben la luz en la región visible.
La parte conjugada de la molécula de colorante puede ser muy corta, lo que significa que hay un bajo grado de conjugación y pocos enlaces dobles y simples alternados, o larga, lo que significa que hay un alto grado de conjugación con muchos enlaces dobles y simples alternados. Estos dobles enlaces alternos no necesariamente tienen que ser solo entre dos carbonos. Estos enlaces conjugados pueden incluir los grupos carbonilo y los dobles enlaces entre el carbono y el oxígeno. El grado de conjugación determina la longitud de onda de la luz que absorbe el compuesto. Por ejemplo, los compuestos con un alto grado de conjugación absorben una longitud de onda más larga que los compuestos con un menor grado de conjugación.
Según la teoría de orbitales moleculares, los electrones deslocalizados ocupan orbitales moleculares. El orbital molecular ocupado más alto, u HOMO, es el orbital de mayor energía con un electrón. El orbital molecular desocupado más bajo, o LUMO, es el orbital de menor energía sin electrón. Las moléculas con poca o ninguna conjugación suelen tener una gran brecha de energía entre el HOMO y el LUMO. Sin embargo, las moléculas conjugadas tienen una brecha de energía más pequeña entre el HOMO y el LUMO.
Para excitar un electrón de un nivel de energía más bajo a un nivel de energía más alto, o de HOMO a LUMO, la molécula debe absorber luz con una energía igual a la brecha de energía entre los dos orbitales. Por esta razón, las moléculas con una gran brecha de energía requieren luz de mayor energía, como la luz ultravioleta, para excitar un electrón. Los tintes, sin embargo, tienen una brecha de energía más pequeña y requieren luz de menor energía, como la luz visible, para excitar un electrón.
<img src="/CDNSource/lm/labs/70/70_Concepts_2.jpg" alt="Ecuación de brecha de energía ΔE(HOMO-LUMO) = hc/λ; espectroscopia molecular, química cuántica." data-altupdatedat="1754907201000">
Por esta razón, las moléculas con una gran brecha de energía requieren luz de mayor energía, como la luz ultravioleta, para excitar un electrón. Los tintes, sin embargo, tienen una brecha de energía más pequeña y requieren luz de menor energía, como la luz visible, para excitar un electrón.
Recordemos que la energía de la luz es inversamente proporcional a la longitud de onda. Por lo tanto, la luz de mayor energía tiene longitudes de onda más cortas que la luz de menor energía que tiene longitudes de onda más largas.
<h4>espectrofotómetro</h4>
Experimentalmente, la absorbancia de la luz se mide utilizando un espectrofotómetro UV-Visible (UV-Vis). Este instrumento utiliza una fuente de luz que es transformada por un monocromador en longitudes de onda específicas de luz que pasarán a través de una muestra y a un detector en el otro extremo. Las muestras deben estar en un líquido, por lo que se requiere un solvente si el compuesto orgánico es un sólido. Esta solución se mantiene en un portamuestras conocido como cubeta. Dependiendo de la muestra, la cubeta puede estar hecha de cristal de cuarzo, vidrio o plástico, y tiene una longitud de paso particular. Esta longitud de camino es la distancia que la luz debe recorrer a través de la muestra. Dado que el solvente también absorberá la luz, se requiere una muestra en blanco del solvente solo. Por lo tanto, cuando el instrumento captura el espectro de absorbancia del compuesto de la muestra, puede restar el espectro de fondo del solvente para mostrar la absorbancia causada solo por la muestra. La transmitancia, T, es la fracción de la luz original que pasa a través de la muestra.
<img src="/CDNSource/lm/labs/70/70_Concepts_3.jpg" alt="Fórmula de la ecuación de transmitancia T=P/P₀, clave en análisis espectroscópicos y estudios ópticos." data-altupdatedat="1754907201000">
Aquí, P0 es la irradiancia, o la energía por segundo por unidad de área, del haz de luz antes de incidir en la muestra. P es la irradiancia del haz de luz que incide en el detector. P suele ser menor que P0, ya que parte de la luz es absorbida por la muestra.
La absorbancia, A, se define como el logaritmo negativo de la transmitancia.
<img src="/CDNSource/lm/labs/70/70_Concepts_4.jpg" alt="Fórmula de absorbancia: A = log (P ₀ / P) = -logT; Principios ópticos en el análisis de espectroscopia." data-altupdatedat="1754907201000">
La absorbancia tiene un rango de valores entre 0 (sin absorción) y 2 (99% de absorción). Cuando no se absorbe luz, P0 es igual a P y la transmitancia es igual a uno. Por lo tanto, la absorbancia es cero. Si se absorbe el 90% de la luz, se transmite el 10% y T es igual a 0,1. Esto da como resultado una absorbancia igual a 1. Si se absorbe el 99% de la luz, se transmite el 1% (T = 0,01) y la absorbancia es igual a 2.
El espectro obtenido es un gráfico de la absorbancia frente a la longitud de onda. Para un espectrofotómetro UV-Vis, este rango está entre 200 y 800 nm.
<h4>Ley Beer-Lambert</h4>
La transmitancia y la absorbancia de un compuesto en particular están relacionadas con la concentración del compuesto en solución. Esta relación está descrita por la ley de Beer-Lambert.
<img src="/CDNSource/lm/labs/70/70_Concepts_5.jpg" alt="Ecuación de la ley de Beer-Lambert; A = εcl; diagrama de absorción en espectroscopia; estudio óptico." data-altupdatedat="1754907201000">
La absorbancia de la muestra es igual al producto de la concentración del compuesto, la longitud del camino y el coeficiente de atenuación molar. Este coeficiente es único para cada compuesto y variará según la longitud de onda. Sin embargo, si la longitud de onda se mantiene constante, el coeficiente de atenuación molar será el mismo independientemente de los cambios en la concentración. La longitud de onda que corresponde a la absorbancia más alta de la muestra, conocida como λmax, también tendrá el coeficiente de atenuación molar más grande.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Silberberg, M.S. (2012). Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análisis químico cuantitativo. Nueva York, NY: W.H. Freeman and Company.</li>
</ol>

Espectroscopía UV-Vis de colorantes

Absorbancia
Cuando la luz interactúa con una sustancia, una parte de la luz se absorbe mientras que el resto se refleja o se transmite a través de ella. ...

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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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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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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

Prepárese para enseñar Espectroscopía UV-Vis de colorantes con esta guía para instructores de 4 minutos. Cubre los pasos clave de preparación para el laboratorio Chemistry en JoVE Lab Manual.

Preparación del instructor para Espectroscopía UV-Vis de colorantes en Chemistry - Más de 8.8K visualizaciones | JoVE Lab Manual

Siga los procedimientos paso a paso de Espectroscopía UV-Vis de colorantes en este video de protocolo estudiantil de 10 minutos. Parte de la colección Chemistry en JoVE Lab Manual.

Protocolo estudiantil de Espectroscopía UV-Vis de colorantes en Chemistry - Más de 37K visualizaciones | JoVE Lab Manual

How Dyes Absorb Visible Light

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