Chromophores in UV-Visible Spectroscopy

1. Introduction

When ultraviolet (UV) or visible radiation interacts with a molecule, the molecule may absorb radiation if the energy of the radiation matches the energy difference between two allowed molecular electronic states.

The structural part of a molecule associated with a particular electronic absorption band is called a chromophore.

Chromophores are therefore fundamental to understanding why different compounds absorb UV or visible radiation at different wavelengths.

In UV-Visible spectroscopy, identifying chromophores helps us understand:

• why a compound absorbs UV or visible radiation,

• which type of electronic transition occurs,

• why different compounds have different absorption maxima,

• how conjugation affects absorption,

• why some compounds are coloured,

• and how structural changes influence the UV-Visible spectrum.

The IUPAC Gold Book defines a chromophore as the part of a molecular entity in which the electronic transition responsible for a particular spectral band is approximately localized.

2. Definition of Chromophore

Definition

A chromophore is an atom or group of atoms within a molecule that is associated with an electronic transition responsible for absorption of electromagnetic radiation in a particular spectral region.

In simpler words:

A chromophore is the light-absorbing part of a molecule responsible for a characteristic absorption band.

The term originally came from the study of dyes and was associated with molecular groups responsible for colour.

Important Point

A chromophore does not necessarily mean that the entire molecule absorbs visible light or appears coloured.

Many chromophores absorb in the ultraviolet region, so the compound may remain colourless to the human eye.

3. Why Do Chromophores Absorb UV-Visible Radiation?

Chromophores contain electrons that can be promoted from a lower-energy molecular orbital to a higher-energy molecular orbital when the molecule absorbs radiation of suitable energy.

The energy of electromagnetic radiation is:

ΔE = hν = hc/λ

Where:

• ΔE = energy difference between electronic states

• h = Planck's constant

• ν = frequency of radiation

• c = speed of light

• λ = wavelength

Therefore:

• Higher energy transition → shorter wavelength

• Lower energy transition → longer wavelength

A chromophore absorbs radiation when the photon energy corresponds to an available electronic transition.

4. Molecular Orbitals Involved in Chromophores

The electrons responsible for UV-Visible absorption generally belong to molecular orbitals such as:

• σ orbitals

• π orbitals

• non-bonding (n) orbitals

The corresponding antibonding orbitals are:

• σ*

• π*

Important electronic transitions include:

σ→σ∗\sigma \rightarrow \sigma^*n→σ∗n \rightarrow \sigma^*π→π∗\pi \rightarrow \pi^*n→π∗n \rightarrow \pi^*

For many organic chromophores, π-electron systems and non-bonding electrons are particularly important in UV-Visible spectroscopy.

5. Common Types of Chromophores

Several functional groups can behave as chromophores.

Common examples include:

ChromophoreExampleImportant transitionC=CAlkeneπ → π*C≡CAlkyneπ → π*C=OCarbonyl compoundsn → π*, π → π*C=NIminesπ → π*, n → π*N=NAzo compoundsπ-related transitionsN=ONitro/nitroso-related systemsn → π*, π → π*Aromatic/conjugated systemsBenzene and conjugated compoundsMainly π → π*

The exact absorption wavelength and intensity depend on the molecular structure and surrounding environment. Representative chromophore data show that C=C, C≡C, C=O and related groups can produce characteristic UV absorptions.

6. Chromophores Containing π Electrons

One of the most important classes of chromophores contains π bonds.

Examples include:

• C=C

• C≡C

• C=O

• C=N

• N=N

The electrons in these π systems can participate in electronic transitions.

For example, an alkene contains a π bond. Absorption of suitable UV radiation can promote a π electron to a π* antibonding orbital:

π→π∗\pi \rightarrow \pi^*

Example: Ethene

Ethene contains a C=C bond and therefore has a π-electron system.

Its principal UV absorption is associated with a:

π→π∗\pi \rightarrow \pi^*

transition.

7. Chromophores Containing Non-Bonding Electrons

Some chromophores contain atoms such as:

• Oxygen

• Nitrogen

• Sulfur

• Halogens

These atoms may possess non-bonding electron pairs, commonly represented as n electrons.

These electrons can participate in transitions such as:

n→π∗n \rightarrow \pi^*

or

n→σ∗n \rightarrow \sigma^*

Example: Carbonyl Group

A carbonyl group:

C=OC=O

contains:

• a π bonding system,

• and non-bonding electrons on oxygen.

Therefore, carbonyl compounds can show both:

n→π∗n \rightarrow \pi^*

and

π→π∗\pi \rightarrow \pi^*

transitions.

8. Conjugated Chromophores

A particularly important type of chromophore is a conjugated chromophore.

Conjugation occurs when alternating π bonds are separated by single bonds, allowing the π-electron system to become delocalized.

Example

C=C−C=CC=C-C=C

is a conjugated diene system.

The presence of conjugation generally reduces the energy difference between relevant molecular orbitals.

Therefore:

More conjugation→smaller energy gap→longer wavelength absorption\text{More conjugation} \rightarrow \text{smaller energy gap} \rightarrow \text{longer wavelength absorption}

This is called a bathochromic shift or red shift when the absorption maximum moves toward a longer wavelength.

Extended π systems can therefore move absorption from the UV region toward the visible region.

9. Why Does Conjugation Affect Absorption?

Consider two systems:

System A

C=CC=C

System B

C=C−C=C−C=CC=C-C=C-C=C

System B has a more extended conjugated π system.

As conjugation increases:

1. π electrons become more delocalized.

2. The energy separation between relevant molecular orbitals decreases.

3. Lower-energy radiation can produce the electronic transition.

4. The absorption therefore occurs at a longer wavelength.

Relationship

Increasing conjugation⇒ΔE↓⇒λmax⁡↑\text{Increasing conjugation} \Rightarrow \Delta E \downarrow \Rightarrow \lambda_{\max} \uparrow

This principle is extremely important in understanding the UV-Visible spectra of conjugated organic compounds.

10. Chromophore and Colour

A substance appears coloured when it interacts selectively with visible light.

If a chromophore absorbs radiation in the visible region, certain wavelengths are removed from the incident light.

The colour observed is associated with the light transmitted or reflected, rather than simply being the wavelength that was absorbed.

Highly conjugated molecular systems can have absorption shifted into the visible region and may therefore appear coloured.

Example

Highly conjugated compounds such as carotenoid pigments absorb strongly in parts of the visible spectrum and therefore appear coloured.

11. Chromophore and λmax

In a UV-Visible spectrum, the wavelength at which maximum absorption occurs is called:

λmax⁡\lambda_{\max}

Chromophores contribute characteristic absorption bands, and the position of these bands depends on molecular structure.

For example, representative data include:

• C=C chromophore → π → π* transition

• C=O chromophore → n → π* and π → π* transitions

• C≡C chromophore → π → π* transition

Exact λmax⁡\lambda_{\max} values should not be treated as fixed universal numbers because molecular environment, conjugation and substitution can alter the spectrum.

12. Chromophore and Absorption Intensity

UV-Visible absorption is described not only by its wavelength but also by its intensity.

The intensity can be expressed using molar absorptivity (ε\varepsilon).

Beer-Lambert law gives:

A=εbcA = \varepsilon bc

Where:

• AA = absorbance

• ε\varepsilon = molar absorptivity

• bb = path length

• cc = concentration

The magnitude of ε\varepsilon depends on factors including the nature of the transition and the probability of absorption. Strongly absorbing chromophores can have much larger molar absorptivities than weakly absorbing transitions.

13. Chromophore vs Auxochrome

A chromophore is associated with an electronic absorption band, whereas an auxochrome is a substituent that can modify the absorption characteristics of a chromophore.

Commonly discussed auxochromic groups include:

• –OH

• –NH₂

• –OR

• –SH

They can influence:

• wavelength of maximum absorption,

• absorption intensity,

• and electronic distribution.

Important terminology note

The term auxochrome is still widely used in traditional UV-Visible spectroscopy teaching, although IUPAC currently marks the term as obsolete in its Gold Book definition.

For B.Pharm examination preparation, however, the traditional chromophore–auxochrome terminology remains useful because it appears in conventional spectroscopy curricula.

14. Effect of Substitution on a Chromophore

The absorption characteristics of a chromophore can change when substituents are introduced into the molecule.

Substituents may:

• alter electron density,

• extend conjugation,

• change the HOMO-LUMO energy gap,

• shift λmax⁡\lambda_{\max},

• and change absorption intensity.

Electron-donating and electron-withdrawing groups can have significant effects when incorporated into conjugated systems.

Therefore, the UV-Visible spectrum provides useful information about the electronic structure of a molecule.

15. Important Examples of Chromophores

Example 1: Ethene

Structure:

CH2=CH2CH_2=CH_2

Chromophore: C=C

Main transition:

π→π∗\pi \rightarrow \pi^*

Example 2: Carbonyl Compound

Chromophore:

C=OC=O

Possible transitions include:

n→π∗n \rightarrow \pi^*

and

π→π∗\pi \rightarrow \pi^*

Example 3: Conjugated Diene

Structure:

C=C−C=CC=C-C=C

The conjugated π system acts as an extended chromophore.

Compared with an isolated double bond, conjugation generally shifts absorption toward a longer wavelength.

Example 4: Carotenoid

Carotenoids contain extended conjugated π systems.

The extensive conjugation causes absorption in the visible region, contributing to their characteristic colours.

16. Chromophore and UV-Visible Spectroscopy

Chromophores are important in UV-Visible spectroscopy because they provide the structural basis for molecular absorption.

UV-Visible spectroscopy can therefore be used to study:

• presence of absorbing functional groups,

• extent of conjugation,

• changes in electronic structure,

• absorption maximum (λmax⁡\lambda_{\max}),

• absorption intensity,

• and quantitative concentration measurements.

However, UV-Visible spectroscopy is generally more useful for studying electronic and conjugation-related features than for determining the complete structure of an unknown molecule by itself.

17. Important Relationships

Energy and wavelength

ΔE=hcλ\Delta E = \frac{hc}{\lambda}

Therefore:

ΔE∝1λ\Delta E \propto \frac{1}{\lambda}

So:

Higher energy → shorter wavelength

Lower energy → longer wavelength

Conjugation

Increased conjugation→smaller orbital energy gap→longer wavelength\text{Increased conjugation} \rightarrow \text{smaller orbital energy gap} \rightarrow \text{longer wavelength}

18. Key Points to Remember

• A chromophore is the part of a molecule associated with an electronic transition responsible for a spectral band.

• Chromophores are fundamental to UV-Visible absorption.

• Many organic chromophores contain π-electron systems.

• Atoms containing non-bonding electrons can also contribute to UV-Visible absorption.

• Important transitions include π → π* and n → π*.

• C=C, C≡C, C=O and C=N are common examples of UV-absorbing groups.

• Conjugation generally shifts absorption toward longer wavelengths.

• Extended conjugated systems may absorb visible radiation and produce colour.

• λmax⁡\lambda_{\max} represents the wavelength of maximum absorption.

• Absorption intensity can be described using molar absorptivity (ε\varepsilon).

• Molecular environment and substitution can change the absorption characteristics of a chromophore.