Dyes and chromophores
About this app
A polyene chain grows from 1 to 11 conjugated double bonds: the HOMO–LUMO gap shrinks, the absorption moves from the UV into the visible range, and the absorbed colour is missing from the screen. With the box model, predictions on a colour wheel, a pH slider for methyl orange and phenolphthalein and an azo series with auxochromes.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What it is about
A dye absorbs light when the photon energy matches the gap between HOMO and LUMO. In a conjugated chain the π electrons belong to the whole system, and this gap shrinks as the chain grows. The absorption moves from the UV into the visible range. What you then see is the light that is left over, roughly the complementary colour. A substance that absorbs blue therefore looks orange.
What the app shows
At the top lies a polyene chain in 3D with delocalised π electrons, below it a light path of lamp, solution, prism and screen. The absorbed colour is missing from the screen. A panel compares measured λmax values for 1 to 11 conjugated double bonds with the particle-in-a-box model and shows the energy levels. The model gets the trend right and greatly overestimates the wavelength for long chains. The colour of the glass is calculated from assumed bands.
What you try out
You pull the plus on the chain and see when the solution turns yellow and orange. For β-carotene, indigo and phenolphthalein you predict the colour on the wheel before the glass shows it. On the pH slider methyl orange and phenolphthalein change colour. In the azo series you compare azobenzene with the auxochromes –NH₂ and –N(CH₃)₂ and with methyl orange. Eight test questions with reasons round off the app.
Frequently asked questions
- Why does longer conjugation shift absorption to longer wavelengths?
- In a longer π system the energy levels lie closer together and the gap between HOMO and LUMO shrinks. Because ΔE = h·c/λ, a smaller ΔE goes with a longer wavelength. This is called a bathochromic shift. Butadiene absorbs at 217 nm, lycopene with 11 conjugated double bonds at about 474 nm.
- Does a substance have the colour it absorbs?
- No. Absorbed light is missing from the transmitted light. The eye sees the rest, roughly the complementary colour. β-Carotene absorbs at about 450 nm in the blue and looks orange. Because bands are broad, the colour often lies next to the exact opposite on the colour wheel.
- What is the difference between a chromophore and an auxochrome?
- The chromophore is the conjugated system that carries the absorption, for example the azo framework. An auxochrome such as –NH₂ or –N(CH₃)₂ adds a lone pair and shifts the absorption to longer wavelengths. 4-(Dimethylamino)azobenzene absorbs at about 405 nm, while the strong band of azobenzene lies at about 320 nm.
- What can I do in the app myself?
- You extend the chain, predict colours on the wheel, move the pH of indicators and choose auxochromes. An explain mode guides you from colourless to orange.
Subject: Chemistry
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