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The total spin of the exciton can either be S=0 or S=1. The associated spin multiplicity (using 2S+1) of S=0 is 1 and of S=1 is 3 hence these states are called singlets and triplets. With a total of 4 states and an equal chance of forming each state the probability of forming a singlet or triplet exciton is 25% and 75%, respectively. The singlet to triplet ratio has been measured experimentally and found to be within experimental limits of the expected 25:75. Though more recent work suggests that in some polymeric materials a higher ratio of singlets can be generated. Electrons can quickly and easily decay from singlet excitons to the ground state (HOMO) which …show more content…
Alternatively, the electroncan decay slowly from the triplet exciton to the ground state with the release of a photon- this is called phosphorescence. If the molecule has been excited electronically these terms are referred to as electrofluorescence and electrophosphorescence. The lifetimes of fluorescence and phosphorescence are very different with fluorescence being a much faster process relative to phosphorescence. This is because of the allowed and forbidden nature of fluorescence and phosphorescence, respectively. In fluorescence the decay of an electron from the exciton to the ground state is 'allowed' by Pauli's exclusion principle which states that no two identical fermions can occupy the same quantum state simultaneously. Hence the decay of an electron from a …show more content…
When the absorption spectrum of the guest overlaps with the emission spectrum of the host efficient transfer of excitons between the two can occur. An efficient transfer of singlet and triplet excitons should occur between host and guest as it is the latter that should exclusively emit within the device. Forster transfer (also called induced dipole transfer) can be used to explain the transfer of singlet excitons between the host and guest. However, if Forster transfer occurs alongwith the electron exchange between the host and guest (Marcus/Dexter transfer, Figure 5) then triplet excitons can also be transferred as the total spin of the system is conserved. Forster transfer occurs over relatively large distances of ~100 A, whereas Marcus transfer occurs over

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