The Jablonski Diagram

Introduction

After a molecule absorbs light, it need not immediately react or emit the same photon. It can lose vibrational energy, change electronic state without light, cross from singlet to triplet, fluoresce, phosphoresce or transfer energy to another molecule. A Jablonski diagram organizes these competing paths as energy levels and arrows so that their sequence and spin changes can be read at a glance.

Core explanation

Place singlet states S0, S1 and perhaps S2 in one vertical column, ordered by electronic energy, and triplet states such as T1 in a neighboring column. Each electronic state contains several vibrational sublevels, drawn as closely spaced horizontal lines. The vertical position indicates relative energy; the horizontal separation between singlet and triplet columns is a drawing convention rather than a spatial distance inside the molecule. The IUPAC definition of a Jablonski diagram describes this state layout and distinguishes straight radiative arrows from wavy radiationless arrows.

Absorption is an upward radiative transition from a lower to a higher electronic state, commonly S0 to an excited singlet. The transition is very fast, so the molecule can be deposited in a vibrationally excited level of S1 or S2. Vibrational relaxation releases excess vibrational energy within the same electronic state, often to the surroundings. Internal conversion is a radiationless transition between electronic states of the same multiplicity, such as S2 to S1 or S1 to S0. Intersystem crossing changes multiplicity, commonly S1 to T1, without emitting a photon. Each process competes with the others; the diagram is not a single compulsory route.

Fluorescence is radiative emission from an excited state of the same multiplicity as the lower state, commonly S1 to S0. Phosphorescence commonly describes emission from T1 to S0, a change in multiplicity that is spin-disfavored and often slower. Some molecules emit little light because nonradiative deactivation is fast. Other molecules react from S1 or T1 before emission. Rates depend on structure, solvent, temperature and quenchers, so arrow thickness or the appearance of a line in a generic diagram does not supply numerical lifetimes.

The diagram helps explain the Stokes shift . A molecule often absorbs at a higher energy than it emits because vibrational relaxation and solvent reorganization lower excited-state energy before fluorescence. The emitted photon is then typically longer in wavelength. This is a common pattern, not a requirement for every spectral line or exceptional anti-Stokes process. An absorption maximum and emission maximum can be compared only for specified conditions and spectra.

To predict a photochemical reaction, add an arrow from the reactive state toward product or energy/electron transfer. A triplet sensitizer might absorb light, cross to T1 and transfer energy to a substrate. A carbonyl substrate might absorb directly and undergo cleavage from an excited state. Oxygen can quench T1. A Jablonski diagram makes these options visible, but product assignment needs kinetic and spectroscopic evidence beyond the drawing.

Step-by-step reasoning

Find S0 and identify the upward absorption arrow and its wavelength or energy. Follow relaxation downward within the same state, then identify any state-to-state radiationless arrows. Check whether an arrow crosses between singlet and triplet columns; if so it is intersystem crossing. Identify downward straight emission arrows as fluorescence or phosphorescence according to the initial and final states. Finally mark the competing chemical reaction from the state actually proposed to react.

Visual explanation

Draw S0 at the bottom left, S1 above it and S2 higher; draw T1 in a separate right column. Add small vibrational lines on each state. Use a straight upward arrow for absorption, wavy downward arrows for vibrational relaxation and internal conversion, a wavy sideways arrow S1→T1, and straight downward arrows S1→S0 and T1→S0 for the two emissions.

Real-world analogy

A traveler is lifted to a high floor of a building. They may walk down stairs on the same side, cross a corridor to another wing, take an elevator to the ground or exit through a workshop on that floor. The lift ride is absorption; internal conversion and intersystem crossing are different non-emitting routes; emission and chemistry are different exits.

Real-world example

Fluorescent dyes used in imaging absorb light, relax and then emit longer-wavelength light. Oxygen-sensitive phosphorescent probes often populate triplet states; oxygen collisions reduce their emission. The signal therefore reflects competition between radiative decay and quenching, not merely how strongly the probe absorbs.

Why?

Excited molecules have multiple potential-energy surfaces and vibrational levels. Energy can leave as a photon, heat or chemical work, and spin coupling permits transitions among some surfaces. A single energy-level chart captures these mutually competing fates and helps explain why absorption intensity alone does not determine photoproduct yield.

Common misconception

The Jablonski diagram is not a chronological rule that every molecule must follow through S2, S1 and T1. Some absorb directly into S1; some react before intersystem crossing; others return nonradiatively. Horizontal placement does not represent the molecule moving sideways in space.

Worked example

Question: A diagram shows S0→S2 by a straight upward arrow, S2→S1 by a wavy arrow, S1→T1 by another wavy arrow and T1→S0 by a straight downward arrow. Name each event. Reasoning: Upward photon absorption populates S2. Same-multiplicity nonradiative S2→S1 is internal conversion. Singlet-to-triplet nonradiative S1→T1 is intersystem crossing. Triplet-to-singlet emission is phosphorescence. Answer: Absorption, internal conversion, intersystem crossing and phosphorescence, respectively.

Quick check

1. Which Jablonski arrow commonly represents fluorescence? Answer: A downward radiative arrow from excited singlet S1 to singlet ground state S0.

Exam focus

Label electronic states, multiplicity and arrow type. Distinguish vibrational relaxation within one state from internal conversion between states. State which excited state a proposed reaction uses and which other arrows compete with it.

Advanced insight

The rule that fluorescence usually comes from S1 is often called Kasha's rule, but exceptions exist when higher excited states emit before internal conversion. Ultrafast spectroscopy can observe transitions on femtosecond to nanosecond scales and reveal whether chemistry starts before full relaxation. A simple Jablonski diagram is a map of possibilities, while experimental rate constants determine traffic along each arrow.

Summary

A Jablonski diagram places singlet and triplet electronic levels with their vibrational sublevels on an energy map. Straight arrows indicate absorption or emission; wavy arrows represent nonradiative relaxation, internal conversion or intersystem crossing. The competing paths determine fluorescence, phosphorescence, heat loss and photochemistry.

Practice questions

1. What is internal conversion? Answer: A nonradiative transition between electronic states of the same spin multiplicity.

2. What changes during intersystem crossing? Answer: The molecule changes between states of different spin multiplicity, commonly singlet to triplet, without emitting a photon.

3. Why is fluorescence often at longer wavelength than absorption? Answer: Energy is commonly lost through vibrational and environmental relaxation before emission.

4. Does a drawn T1 level guarantee phosphorescence is observed? Answer: No. Quenching, nonradiative decay or chemical reaction may outcompete emission.