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A Jablonski diagram, named after the Polish physicist AleksanderJabłoński, is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Radiative transitions are indicated by straight arrows and nonradiative transitions by squiggly arrows. The vibrational ground states of each electronic state are indicated with thick lines, the higher rotational states with thinner lines.
Physical Processes Undergone by Excited Molecules
S o + hv --- S 1 Excitation
S 1 v -- S 1 + heat Vibrational Relaxation
S 1 ----- S o + hv Fluorescence
S 1 ---- S o + heat Internal Conversion
S 1 --- T 1 Intersystem Crossing
T 1 v -- T 1 + heat Vibrational Relaxation
T 1 v -- S o + hv Phosphorescence
T 1 --- S o + heat Intersystem Crossing
S 1 + A (S o ) --- S o + A (S 1 ) Singlet-Singlet Energy Transfer
T 1 + A (S o ) -- S o + A (T 1 ) Triplet-Triplet Energy Transfer
Why Use Photochemistry
Overcome large kinetic barriers in a short amount of time
Produce immense molecular complexity in a single step
Form thermodynamically disfavored products
Allows reactivity that would otherwise be inaccessible by almost any other synthetic method
The reagent (light) is cheap, easily accessible, and renewable
Reactivity is often unpredictable
Many substrates are not compatible
Selectivity and conversion are sometimes low
Chemical Processes undergone by Excited Molecules
1) α-Cleavage (Norrish type I reaction). In solution the radicals undergo further reactions to give products.
2) Hydrogen Abstraction followed by cleavage = Norrishtype II cleavage. The radicals can abstract a Hydrogen atom from a donor. The resulting radicals can then undergo further reactions.