Singlet Fission - Department of Chemistry

AJ Chemical Reviews, XXXX, Vol. xxx, No. xx Smith and Michl
high, as is the yield recently indirectly determined from
ground-state bleach for a carotenoid aggregate by ps timeresolved
resonance Raman spectroscopy 110 (section 4.1).
These results make 8 the first successful compound
purposely designed to exhibit efficient singlet fission, using
the design principles outlined in section 2, although it is
admittedly only accidental that the crystal structure of the
C2 conformer of 8 contains slip-stacked pairs of molecules
in an arrangement that appears to be nearly optimal for the
direct mechanism of singlet fission based on the arguments
made there (Figure 12). It is not presently certain that the
same crystal structure is present in the film sublimed on
sapphire.
Triplet formation was also observed in polycrystalline
solids of three dimers of 8 (30, 31, and 32), 82 presumably
again due to singlet fission, because in cyclohexane solution
these compounds exhibit no detectable triplet formation.
These materials have not yet been examined in detail.
3.5. Miscellaneous
Perylene (9)
The study of singlet fission in 9 is made particularly
interesting by the fact that this hydrocarbon is available in
two crystalline forms, R-9 and -9, the former of which
forms excimers. Optically induced fission is a suitable probe
for excimer formation because it provides information about
whether the excimer forms on a faster time scale than
vibrational relaxation and fission. 212,213 If it does, the excimerforming
R form should exhibit a higher threshold energy
for singlet fission due to the additional energy needed to
break the excimer. This has been found to be the case. 138,212,213
Using the magnetic field effect on prompt fluorescence as a
function of excitation energy for an indicator, the threshold
energy for R-9 is 3.51 eV, blue-shifted by 0.5 eV from the
-9 threshold of 3.01 eV, 212 and one can conclude that in
the R form excimer formation is faster than singlet fission.
A theoretical estimate puts the rate of excimer formation at
10 13 s -1 . 212 On the basis of the blue shift of the threshold
energy in R-9, the binding energy for two triplets on
neighboring molecules is 0.04 eV.
Singlet fission has been observed in single crystals of 9
(unspecified form, R or ) excited with vacuum UV radiation
at energies higher than twice the triplet energy, as evidenced
by magnetic field effects on the prompt fluorescence intensity.
80 The fluorescence enhancement increased with increasing
magnetic field strength before eventually saturating and
was anisotropic with respect to magnetic field orientation.
Tris-(8-hydroxyquinoline)aluminum (Alq3, 10)
This material is of interest for use in organic light-emitting
diodes (OLEDs), where it is often desirable to keep triplet
formation as low as possible to produce light by fluorescence
rather that phosphorescence. Because high current densities
are commonly used, it is of interest to study triplet formation
and decay under these conditions. At high excitation intensities,
transient absorption signals in thin films of 10 121 show
that excited singlets fuse to form higher excited singlets.
Because of the prevalence of annihilation as a relaxation
process for singlets under these conditions, the singlet
population is proportional to Iex 1/2 . Triplets formed by singlet
fission were also observed at high excitation intensities.
Triplets formed by intersystem crossing have a density
proportional to Iex 1/2 , whereas those formed by singlet fission
have a density proportional to Iex. Thin films were excited
with various intensities of 3.49 eV light. At Iex < 2 × 10 21
photons cm -2 s -1 , intersystem crossing accounted for all
triplet generation. At Iex > 2 × 10 22 photons cm -2 s -1 , the
density of triplets is proportional to the excitation intensity.
By subtracting the triplet density expected from intersystem
crossing, the triplet quantum yield from singlet fission was
determined to be 30%.
Benzophenone (11)
Singlet fission has been observed in single crystals of 11
under two different excitation intensities using ps timeresolved
absorption spectroscopy. 214 At lower excitation
intensities, the triplet rise time equals the decay time of
excited singlet absorption, indicating that intersystem crossing
is solely responsible for triplet formation. At four times
higher excitation intensities, the total triplet rise time was
faster than would be expected from intersystem crossing
alone. A two-photon absorption to a highly excited singlet,
followed by singlet fission, was consistent with the observed
triplet formation. A good fit of the triplet rise time under
high excitation intensity was obtained assuming that 40%
of triplets are formed by fission.
Rubrene (12)
In a recent study of exciton diffusion in rubrene single
crystals using photoconductivity measurements, long-lived
excitons assigned as triplets were observed. Singlet fission
was suggested as a possible source of these triplets, but no
attempts to obtain direct evidence for singlet fission were
made. 235
4. Aggregates
As crystal size diminishes, the relative importance of
surface molecules increases, and by the time nanocrystal size
is reached, order tends to decrease. Very small nanocrystals
are difficult to differentiate from molecular aggregates. The
exact structure of such species is frequently known only
approximately or not at all. The limiting cases are noncovalent
dimers.
With one exception, no singlet fission work seems to have
been done on nanocrystals and aggregates. This is unfortunate,
because one can easily imagine that they could be used
as sensitizers on semiconductor nanoparticles in photovoltaic
cells. The striking exception is the carotenoids, which occur
naturally in the photosynthetic apparatus in the form of
aggregates and have been almost exclusively investigated
for singlet fission in aggregates only.
4.1. Carotenoids
Carotenoids contain a series of linearly conjugated double
bonds and can be viewed as oligomers on the way from
ethylene and 1,3-butadiene to the simplest conjugated
polymer, polyacetylene. As mentioned in section 2.2.2 and
summarized in more detail in the introduction to section 5,
in the usual all-anti configuration of such structures the
lowest excited singlet electronic state S1 is the doubly excited
2Ag - state that can be viewed as a singlet-coupled combination
of triplet excitations localized in two different parts of
the chromophore. The strongly allowed 1Bu + HOMO-LUMO
state that represents the usual optical entry into the excited