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
Other samples, services and questions:
When you use PaperHelp, you save one valuable — TIME
You can spend it for more important things than paper writing.
