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Molecular internal conversion is a radiationless transition between electronic states of the same spin multiplicity. Electronic and vibrational energy are redistributed without emission of a photon in the transition itself. IUPAC describes the step as isoenergetic: a lower electronic state can be reached with increased vibrational excitation, followed by subsequent relaxation.^[ref]

This is distinct from intersystem crossing, which connects states of different multiplicity, and from vibrational relaxation within an electronic state. It is also distinct from nuclear internal conversion. A molecular singlet-to-singlet transition such as $S_2\to S_1$ fits the molecular definition; the energy need not be immediately transferred as heat to a surrounding solvent.

An original population model separates internal conversion from later decay. Suppose an impulsive excitation places population $N_0$ in $S_2$, transfer to $S_1$ occurs at rate $k_{21}$, and $S_1$ loses population at total rate $k_1$:For $k_{21}\ne k_1$,The lower-state population first rises and later decays. If only part of $k_1$ is radiative, fluorescence intensity is proportional to that radiative rate times $N_1$, not simply to the loss rate of $S_2$.

For stipulated $k_{21}=10^{12}{\mathrm{s}}^{-1}$ and $k_1=10^8{\mathrm{s}}^{-1}$, the transfer time scale is 1 ps while the lower-state lifetime is 10 ns. A fast population transfer can therefore precede much slower emission. These are model constants, not measured values assigned to every molecule.

An informative experiment resolves the time dependence of upper- and lower-state signals after a pulse and tests whether the proposed transfer accounts for both. A decay trace alone can have several explanations; spectral assignment and competing channels matter. Sequential excited-state population kinetics is one model to test against those measurements.

Michael Kasha's 1950 analysis linked radiationless transitions among like-multiplicity states to emission from the lowest excited level of a given multiplicity.^[ref] The rate competition supplies a physical motivation for Kasha's rule, while leaving room for molecules whose competing pathways behave differently.

A Jablonski diagram distinguishes electronic excitation from subsequent relaxation. Photon absorption takes a molecule from the electronic ground state $S_0$ to a vibrationally excited level of $S_1$ or a higher singlet state. Rapid vibrational relaxation and internal conversion usually populate the lowest vibrational levels of $S_1$. Fluorescence then emits a photon on a transition to $S_0$, often ending in one of its excited vibrational levels. Nonradiative relaxation competes with emission; intersystem crossing can also populate triplet states. Energy lost to relaxation and solvent reorganization commonly makes the fluorescence maximum occur at lower photon energy, hence longer wavelength, than the photon absorption maximum: the Stokes shift.

The intrinsic fluorescence quantum yield is the number of photons emitted by fluorescence per absorbed excitation. For a simple excited state with radiative rate $k_r$ and other decay rate $k_{nr}$,A nearby acceptor adds a nonradiative energy-transfer channel, not an emitted photon flying from donor to acceptor. The electric dipole-dipole interaction transfers the donor excitation while returning the donor toward its ground state and exciting the acceptor. The acceptor then relaxes and may fluoresce. The donor yield becomesFor the acceptor, distinguish its intrinsic yield per acceptor excitation, $Q_A=k_{r,A}/(k_{r,A}+k_{nr,A})$, from the number of acceptor photons per donor excitation. Transfer increases the supply of acceptor excitations, giving sensitized emission $E{Q}_A$ per donor excitation in the ideal model. It does not by itself change $Q_A$ if the acceptor's own decay rates remain unchanged. Environmental changes, saturation or additional quenching can change those rates, but increased acceptor intensity alone does not establish an increased intrinsic fluorescence quantum yield.

A quencher-independent intersystem-crossing rate adds to the unquenched decay rate:The normalized Stern-Volmer equation retains the same form, so intersystem crossing does not bias $k_Q$ if the actual unquenched lifetime is used. It reduces ${\varphi }_0$ below one and changes the slope through ${\tau }_0$; assuming ${\tau }_0=1/k_F$ would then give the wrong result. A quencher that itself changes intersystem crossing or returns triplet population to the singlet requires a larger kinetic scheme.

In the electronic state correlation diagram, the singly occupied reactant orbitals for triplet oxygen have symmetries $B_1$ and $A_2$, so their spatial product is $B_2$. The reactant state is therefore a triplet ${}^3{B}_2$, whereas the closed-shell product is ${}^1{A}_1$. A strictly spin-conserving path cannot correlate triplet reactants directly to the ground-state product. It correlates to an excited triplet product configuration or requires intersystem crossing; the symmetric direct reaction consequently has a substantial electronic obstacle.

Simply changing the oxygen spin does not repair the occupied carbon-orbital correlation. The lowest oxygen singlet manifold has spatial components of $A_1$ and $B_2$ symmetry in this subgroup. The $A_1$ singlet can have an avoided crossing with another $A_1$ configuration and reach the product singlet; the $B_2$ singlet cannot mix directly with the product ${}^1{A}_1$ while $C_{2v}$ is maintained. Following either closed-shell oxygen singlet component in the orbital diagram still leaves the carbon-pi electron pair in the rising ${\sigma }_{+}^{\ast }$ orbital. Its redistribution into the final occupied orbitals requires configuration mixing at an avoided crossing. A barrier is therefore expected even for the symmetry-constrained concerted singlet addition; singlet does not imply barrierless. The state curves in the original diagram are schematic, illustrating this avoided-crossing argument rather than computed energies.^[ref]

The real singlet reaction can lower this obstacle through asymmetric or stepwise pathways that are absent from the stipulated direct approach. A primary multireference study explicitly distinguishes singlet and triplet addition through biradical intermediates, supporting that limitation of a one-coordinate symmetry argument.^[ref] Neither correlation alone nor a spin label supplies a numerical activation energy.

A radiationless transition changes a system's electronic state without absorption or emission of a photon in that transition. Energy is redistributed among coupled degrees of freedom; it does not disappear. In molecular photophysics, the electronic change can populate vibrational motion before subsequent energy transfer to the surroundings.^[ref]

Molecular internal conversion connects electronic states of the same spin multiplicity, while intersystem crossing connects states of different multiplicity. These must be distinguished from vibrational relaxation within an electronic state and from nuclear internal conversion. A radiationless step need not instantaneously deliver all the electronic energy as heat to a solvent.

A simple competing-rate experiment illustrates why a dark channel can be inferred from light measurements. Suppose an emitting excited population loses members by fluorescence at rate $k_r$ and by nonfluorescent channels at total rate $k_{nr}$. With no population returning,These are the stated one-population assumptions of the fluorescence quantum yield model. For chosen rates $k_r=10^7{\mathrm{s}}^{-1}$ and $k_{nr}=4\times 10^7{\mathrm{s}}^{-1}$, the lifetime is $20\mathrm{ns}$ and the yield is $0.20$. A shorter lifetime here does not mean a larger radiative rate.

With a measured lifetime and yield, the model gives $k_r={\Phi }_f/\tau$ and $k_{nr}=(1-{\Phi }_f)/\tau$. Measuring only an emission intensity is less specific because it also depends on absorbed excitation and detection efficiency. Additional intermediate states, energy transfer or population return require a more complete model; the inferred missing fluorescence does not uniquely identify one radiationless mechanism.

Time-resolved emission paired with transient absorption can therefore test a proposed pathway: the first observes photons from an emitting state, while the second can reveal population in a state that does not emit in the monitored band. A mechanism must account for both signals and their time dependence.