Chapter 19 — Case Study 2: Conjugated Systems Beyond Dienes — UV Spectroscopy and Color

"Conjugated π systems absorb light. The longer the conjugation, the lower the energy of the absorbed photon, the more red-shifted the absorption. From colorless 1,3-butadiene (UV at 217 nm) to deeply colored β-carotene (visible at 450 nm), conjugation length determines the color of organic molecules." — paraphrase from a spectroscopy text

This case study explores how conjugation length determines the absorption wavelength of organic molecules — connecting Chapter 19's diene chemistry to the colors we see in nature, food, and dyes.

The chemistry of color

When light hits a molecule, photons can excite electrons from the HOMO to the LUMO. The energy of the absorbed photon (ΔE = h·ν = h·c/λ) corresponds to the HOMO-LUMO gap.

For most organic molecules with isolated π bonds, the HOMO-LUMO gap is ~6-7 eV (200-220 nm — UV). Such molecules don't absorb visible light → colorless.

For conjugated π systems, the HOMO-LUMO gap is smaller. The longer the conjugation, the smaller the gap, the lower the absorption energy, the longer the absorbed wavelength.

When the absorption reaches into the visible range (~400-700 nm), the molecule has color.

The progression: from colorless to deeply colored

The pattern: each additional C=C in conjugation shifts λ_max by ~30 nm to the red.

Tomatoes, carrots, and natural pigments

Many natural pigments are highly conjugated:

Carotenoids (yellow, orange, red)

• β-carotene (carrots): 11 conjugated C=C bonds; orange.
• Lycopene (tomatoes): 11 conjugated C=C; red.
• Lutein (yellow leaves, egg yolks): yellow.
• Astaxanthin (salmon flesh): pink.
• Bixin (annatto food coloring): red-orange.

These carotenoids are diterpenes and tetraterpenes (Ch 34) with extended conjugation. They absorb in the blue-green region; the transmitted light is in the red-orange range, hence their color.

Chlorophyll (green)

Chlorophyll has a tetrapyrrole macrocycle (a porphyrin) with extended conjugation around the ring + a Mg²⁺ in the center. Absorbs red and blue; transmits green.

Anthocyanins (red, blue, purple)

The pigments of berries, red cabbage, eggplant. The chromophore is a flavylium cation with extended conjugation. Color depends on pH (red at low pH; blue at high pH).

Hemoglobin (red)

Iron-porphyrin (heme); extended conjugation in the porphyrin ring + Fe-O₂ coordination. Red color depending on the oxidation/binding state.

Indigo (blue)

The classic blue dye: N-N indigo-type structure with extended π conjugation. Used for blue jeans for centuries. Originally extracted from plants; now synthesized.

UV-Vis spectroscopy

Conjugated systems are characterized by UV-Vis absorption spectroscopy: - Sample dissolved in a UV-transparent solvent (e.g., methanol, hexane). - Light of variable wavelength is shone through. - Absorbance is measured as a function of wavelength. - The spectrum shows λ_max (the wavelength of maximum absorbance) and ε (extinction coefficient).

For the Diels-Alder substrate identification: - Conjugated diene: λ_max ~215-235 nm. - Isolated alkene: λ_max ~190 nm. - α,β-Unsaturated carbonyl: λ_max ~220-260 nm (with shifts depending on substituents).

The Woodward-Fieser rules predict λ_max from substituents on a diene or enone. Each substituent contributes ~5-10 nm.

The chemistry of vision

Cis-retinal (vitamin A's aldehyde form) bound to opsin protein via a Schiff base is the chromophore in rhodopsin (the visual pigment in the eye).

When a photon hits, the C=N+ bond's π electrons absorb the photon; the chromophore relaxes by isomerization of the 11-cis double bond to all-trans. This conformational change cascades through the protein, eventually triggering a nerve impulse.

The chemistry: conjugation absorbs visible light; isomerization is the trigger. Without conjugation, no vision.

Dye chemistry

The synthetic dye industry was born when William Perkin (1856) synthesized mauveine — the first synthetic dye — accidentally during attempts to synthesize quinine.

Modern synthetic dyes: - Azo dyes (most common; ~70% of dyes): Ar-N=N-Ar' chromophore. Color tunable via substituents. - Anthraquinones: red/blue/violet. - Phthalocyanines: blue/green; used in printer inks. - Triphenylmethanes: brilliant red. - Indigo and indigoid dyes: blue.

Each dye class has a specific chromophore with extended conjugation. The substitution pattern fine-tunes the exact absorption wavelength.

Photophysics: fluorescence and phosphorescence

When a molecule absorbs a photon and reaches an excited state, it can: - Fluoresce: emit a photon of slightly longer wavelength (lower energy) by returning to the ground state. Fast (~ns). - Phosphoresce: emit slowly (~ms to s) due to spin-forbidden transition. - Photo-isomerize (e.g., retinal cis-trans). - Photochemistry: undergo a chemical reaction in the excited state.

Fluorescent dyes are widely used in biology (GFP, fluorescein, BODIPY) and materials science (OLEDs, solar cells).

Modern applications

OLEDs (organic light-emitting diodes)

Conjugated organic molecules are used as the light-emitting components in OLED displays. Different conjugated chromophores give red, green, and blue colors.

Organic photovoltaics

Conjugated polymers absorb solar photons → exciton (electron-hole pair) → split → current. Some commercial products use this; major research area.

Solar fuels

Photochemical generation of fuels (H₂ from water, hydrocarbons from CO₂) often relies on conjugated chromophore absorption.

Sensors

Fluorescent sensors with conjugated chromophores detect specific analytes (metals, biomolecules, etc.) by changes in fluorescence.

Take-home

• Conjugated π systems absorb light at wavelengths inversely proportional to the conjugation length.
• The longer the conjugation, the lower the absorption energy, the longer the wavelength, the more "red-shifted" the absorption.
• Carotenoids (β-carotene, lycopene) have 11 conjugated C=C bonds and absorb in the visible range, giving orange/red colors.
• Chlorophyll, anthocyanins, hemoglobin, indigo are other natural conjugated chromophores.
• UV-Vis spectroscopy characterizes conjugated systems; Woodward-Fieser rules predict λ_max.
• Vision uses retinal (a conjugated polyene) bound to opsin; light triggers cis-trans isomerization.
• Dyes (azo, anthraquinone, phthalocyanine) are designed conjugated chromophores.
• OLEDs, solar cells, fluorescent sensors are modern applications of conjugated systems.
• The chemistry of conjugation in Chapter 19 connects to color, vision, photochemistry, and modern materials science.