An Introduction to Fluorescence

As phosphorescing molecules can luminesce for a much longer time than fluorochromes, there must be a difference in the way they store the excitation energy. The basis for this discrepancy is found in the two forms of excitation levels, the singlet excited state and the triplet excited state, which are based on different spin alignments.

Spins are an attribute of electrons. In simplified terms, the spin describes the angular momentum of the electron caused by its rotation. The orientation of an electron’s spin can be positive (+1/2) or negative (–1/2). Spin pairs of higher energy levels can either be parallel or antiparallel in their orientation to each other. In antiparallel spin pairs the individual angular momentums compensate each other and the total spin gets a value of zero. This spin alignment is called singlet state. Two parallel spins do not compensate and get a value different from zero. In this case the spins are said to be in a triplet state.

Fluorescence occurs when electrons go back from a singlet excited state to the ground state.  But in some molecules the spins of the excited electrons can be switched to a triplet state in a process called inter system crossing. These electrons lose energy until they are in the triplet ground state. This state is of higher energy than the ground state but also of lower energy than the singlet excited state. The electrons can therefore not switch back to the singlet state, nor can they easily go back to the ground state, as only total spins with a value of zero are allowed due to quantum mechanics. The molecules are therefore trapped in their energy state. 

But a few changes from the triplet ground state to the ground state are possible at a time. These changes give rise to the emission of photons and the phosphorescence. As only a few events are possible at a time, the triplet ground state presents a kind of energy reservoir, making phosphorescence possible over a longer time period.

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