= Radiationless transition
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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[radiationless transition, R05057]{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,
$$
N(t)=N_0e^{-(k_r+k_{nr})t},\qquad
\tau=\frac1{k_r+k_{nr}},\qquad
\Phi_f=\frac{k_r}{k_r+k_{nr}}.
$$
These are the stated one-population assumptions of the <fluorescence quantum yield> model. For chosen rates $k_r=10^7\,\mathrm{s^{-1}}$ and $k_{nr}=4\times10^7\,\mathrm{s^{-1}}$, 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.
Back to article page
