Singlet Fission - Department of Chemistry

AF Chemical Reviews, XXXX, Vol. xxx, No. xx Smith and Michl
in transient absorption measurements performed under
identical conditions on a single crystal of 3, and the authors
attributed its absence to different morphology. 147,179 However,
since they did not specify the monitoring laser beam
polarization direction relative to crystal or molecular axes,
one cannot exclude the possibility that A was formed but its
transition moment was out of alignment with the electric field
of the monitoring laser beam.
The dynamics of the decay of A can be altered by the
presence of underlying structures. For 3 on glass, 149 there is
an initial fast (∼175 fs) component of absorption decay,
which is not observed for 3 on Ag nanohole films. This is
thought to be due to changes in the rate of vibrational cooling
or in the relative energies of the S1 and T1 states due to
interaction with surface plasmons.
In ultrafast transient absorption studies of films of 3, 147,179,151
the spectral signature of A is less clear. Upon cooling to 5
K, 151 the spectra change considerably, and the peak of A
becomes more prominent relative to ground-state bleach. It
is not obvious whether the results represent an observation
of an additional transient species (D) with a sharp peak at
1.9 eV that also lives for many ns and whose formation is
enhanced when the film is doped with C60.
Species B. This transient has only been detected in a thin
film of 3 observed at 65° incidence, as a weak absorption
band at 2.30 eV. 76,77 Its rise time of ∼1 ps and a lifetime of
a few hundred ps are clearly distinct from those of A and
demand the assignment of B as a different species. The 2.3
eV band is not seen at normal incidence under otherwise
identical conditions and is therefore evidently polarized along
the long axis of 3.
Species C. This transient, with a broad absorption peak
at 1.4 eV, a temperature-independent 700 fs rise time, and a
lifetime of many ns, was observed in a study 147,179 of a single
crystal and in thin films of 3, 147,179,151 where its rise time is
shorter than the 200 fs time resolution of the measurement. 151
In a thin film, the intensity of this absorption peak relative
to the ground-state bleach was much weaker than in the
single crystal and was reduced further when the film was
doped with C60. 147,179 The authors attribute the difference
between results for a single crystal and a film to different
morphology. Another possible explanation is that the 1.4 eV
transition in C is mostly long-axis polarized and therefore
hard to observe in their thin films at normal incidence (the
ground-state absorption is short-axis polarized).
Proposed Interpretations. Four different hypotheses have
been presented to account for the fast decay of the initially
excited S1 state to the species A observed in solid 3, and the
authors often differ in the interpretation of the nature of the
transients B and C as well. The proposed fates of S1 are (i)
fission to free triplet excitons, which have been assigned by
some authors 150 (and later accepted by certain others 149,207 )
as species A, in other publications as species C, 147,179,151 and
in one paper as species B 77 (which we consider the most
likely), (ii) formation of an excimer 76,77 (this also is one of
the two possibilities considered likely in ref 151), (iii) fission
to a strongly bound triplet exciton pair, an intermolecular
doubly excited state 1 (TT) 181 (this is the other possibility
considered likely in ref 151), and (iv) internal conversion to
a intramolecular doubly excited state (which we believe can
be safely excluded), followed by conversion to a pair of free
triplet excitons. 40 We find none of these four hypotheses
entirely satisfactory but are unable to propose a convincing
detailed alternative. We have considered the possibility that
A might be a quintet state of 3, formed by energy transfer
from a quintet state of the intermolecular triplet pair 5 (TT),
but a few simple density functional theory (DFT) calculations
convinced us that such a locally excited quintet is probably
too high in energy and that its absorption spectrum is unlikely
to fit A.
(i) An assignment of A as the T1 state of crystalline 3
was made based on its long lifetime and the vague similarity
of its absorption spectrum to that of the triplet of 3 in
solution. 150 It was felt that the large red shift of the bands of
A relative to the solution spectrum of the triplet of 3 would
be acceptable. This assignment of A as the triplet exciton of
3 was later convincingly criticized. 76,77 It was pointed out
that the red shift of the absorption peaks of A, which start
at 1.88 eV, relative to the solution spectrum of the triplet of
3, which starts at 2.46 eV, 193 actually is unreasonably large.
Moreover, the observed intensity of the bands of A is
comparable to that of the ground-state bleach, whereas in
solution, the absorption intensity is an order of magnitude
higher for T1 than for S0. Here, it could be argued that the
S1 state is delocalized and exciton-coupled (superradiant) and
the ground-state bleach intensity is thus enhanced, similarly
as has been recently proposed for solid 2. 120 However, most
importantly, the first intense absorption band of monomeric
3 is long-axis polarized, whereas the absorption of A is shortaxis
polarized. Others concurred with the criticism, 181 and it
now seems inevitable to us that the assignment of A as triplet
exciton must be rejected. The proposed attribution of A to
the radical cation of 3 147 can be rejected as well, based 151 on
the analysis given above.
Still assuming that singlet fission to triplet excitons is the
dominant fate of the initially excited singlet state, we need
to next consider the proposal that it actually is species C
that is the free triplet exciton. 147,151,179 This species was
observed in a single crystal of 3, and only much more weakly
in a solid film, 147,179 where the peaks of A also were observed
only weakly if at all, and species B was not observed. The
long decay time of C, its sub-ps rise time, and the temperature
independence of its formation, expected for exothermic
singlet fission, were taken to be sufficient evidence for the
assignment.
Because the location of the absorption peak of C (1.4 eV)
differs so dramatically from that of triplet 3 in the gas phase
(2.68 eV 189 ) or in solution (2.46 eV 193 ), it is hard to imagine
that the assignment of C to a triplet exciton could be correct.
It is true that calculations predict a long-axis polarized T1 to
T2 transition in this energy range (e.g., 1.24 40 and 1.41 182
eV), but its calculated intensity is 182 300 times lower than
that of the transition at 2.68 eV. Indeed, all efforts to observe
such a transition in the isolated molecule have failed, and it
has been established that in benzene solution the absorption
coefficient of triplet 3 in this region is