---
title: "What an Excited Molecule Does Next: Fluorescence and Phosphorescence"
description: "Absorption is only the first step. The excited molecule loses energy by several competing routes, and which one wins decides whether it glows at once, glows slowly, or heats its surroundings."
canonical: https://lightmysky.com/learn/science/what-an-excited-molecule-does-next-fluorescence-and-phosphorescence-mt_5AVF6uWed_
source: https://lightmysky.com/learn/science/what-an-excited-molecule-does-next-fluorescence-and-phosphorescence-mt_5AVF6uWed_.md
retrieved: 2026-09-12
---

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# What an Excited Molecule Does Next: Fluorescence and Phosphorescence

Absorption is only the first step. The excited molecule loses energy by several competing routes, and which one wins decides whether it glows at once, glows slowly, or heats its surroundings.

Subject: Science · Area: Chemistry · Ages 19 to 21
Page: https://lightmysky.com/learn/science/what-an-excited-molecule-does-next-fluorescence-and-phosphorescence-mt_5AVF6uWed_

## Ready when they can

- Draws the states and arrows of a Jablonski diagram and labels radiative against non-radiative steps
- Explains why fluorescence is emitted at longer wavelength than the light absorbed
- Says why phosphorescence is slow and why a heavy atom in the molecule speeds it up
- Predicts which of two compounds fluoresces more strongly from their rigidity and conjugation

## Lesson: What excited molecules do next

Absorption only starts the story. The excited molecule must shed energy, and spitting out a photon is just one route. It absorbs from S0 to S1, relaxes vibrationally, and then either fluoresces from S1 to S0, crosses silently to T1, or phosphoresces from T1 to S0. Absorption, fluorescence and phosphorescence trade photons. Internal conversion, vibrational relaxation and crossing to the triplet are silent heat steps.

Fluorescence always glows at longer wavelength than the lamp that drives it. Some energy drains away as vibrational heat before emission, so the outgoing photon carries less than the pumping put in. That energy account is called the Stokes shift. Fast drops give prompt bright lines, while blocked drops wait and leak slowly.

**Example.** A dye that spins freely in warm solvent barely glows, but the same dye frozen in a film shines. Free rotation opens heat loss paths that beat the photon route. Freezing blocks those silent paths, so emission wins. The rigid, flat, conjugated dye always wins over the floppy one, because stiffness blocks heat loss and conjugation favours the photon.

**Tip.** Phosphorescence is slow because the T1 to S0 step flips spin, which the photon step forbids. The molecule waits instead of glowing at once. A heavy atom inside the molecule boosts spin orbit coupling, which loosens the spin ban and lets the triplet emit faster and brighter. Slow light is the signature of a forbidden step.

**Recap.** Competing silent and glowing routes decide whether an excited molecule flashes at once, glows slowly, or just warms its surroundings.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Electronic Spectra, Franck-Condon and the Origin of Colour](https://lightmysky.com/learn/science/electronic-spectra-franck-condon-and-the-origin-of-colour-mt_y1hKy9nvAY)

## Opens up

- [The Boltzmann Distribution over Molecular Levels](https://lightmysky.com/learn/science/the-boltzmann-distribution-over-molecular-levels-mt_-DHIox6GBm)
- [Ultrafast Spectroscopy and Watching a Reaction Happen](https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5)
- [Photoredox and Electrochemical Routes to Radical Intermediates](https://lightmysky.com/learn/science/photoredox-and-electrochemical-routes-to-radical-intermediates-mt_YhqzHGcx31)