Singlet Fission - Department of Chemistry

AD Chemical Reviews, XXXX, Vol. xxx, No. xx Smith and Michl
excited states are not of much relevance in the present
context. In the low-resolution absorption spectrum of the S0
state of the isolated molecule of 3, the La band is located at
2.1-2.7 eV and its vibrational structure consists of four peaks
separated by ∼1500 cm -1 .
Although really accurate calculations for a molecule of
the size of 3 are difficult, many more or less approximate
calculations have been published and reproduce the main
spectral features of the isolated molecule within 0.2-0.3 eV.
Recent examples are single-reference TD-DFT 30,181 and
CC2 182 and multireference CAS-MP2 40 calculations. Multireference
calculations were used to obtain useful results for
doubly excited states.
Properties of Gas Phase and Rare-Gas Matrix Isolated
3. In isolated molecules cooled in a supersonic jet, the La
band origin occurs at E(S1) ) 2.31 eV. 183-185 Fluorescence
originates in the same state and is a fairly close mirror image
of the absorption. Low-temperature matrix-isolation absorption
and emission spectra 186,187 are similar to the jet-cooled
spectra, but somewhat red-shifted. In a neon (krypton)
matrix, 188 the shift of the 0-0 transition is ∼0.025 (∼0.125)
eV relative to the gas phase. There are no indications of a
presence of additional electronic transitions nearby. These
results leave no doubt that in an isolated molecule of 3 the
optically allowed La state is the lowest excited singlet and
that the probably present recently calculated doubly excited
singlet state must lie above La, not below as proposed. 40
Flash photolysis of the vapor 189 yields a transient whose
absorption spectrum lasts for tens of µs and which was
assigned to the T1 state. It contains four intense peaks starting
at 2.68 eV, separated by ∼0.17 eV. If the T1 state also
absorbs at lower energies, it does so only very weakly.
For a hydrocarbon, the gas-phase ionization potential of
3 is unusually low, 6.61 eV, 190 and electron affinity is
unusually high, 1.35 eV. 191 Gas-phase spectra of the radical
cation and the radical anion do not appear to have been
reported, but rare-gas matrix absorption spectra are known
and resemble each other closely, 188 as expected from the
alternant pairing theorem. 192 In both cases, the lowest-energy
absorption peaks occur near 1.3-1.4 eV and are due to two
nearly degenerate transitions. The radical cation has a 2 B3g
ground state, and the two transitions are to states of
symmetries 2 B1u (y-polarized) and 2 Au (stronger, x-polarized).
In a Ne matrix, they lie at 1.26 and 1.31 eV, respectively.
They are red-shifted by ∼0.01 and ∼0.025 eV, respectively,
in a Kr matrix. The radical anion has a 2 B1u ground state,
and the two transitions are to states of symmetries 2 B3g
(y-polarized) and 2 B2g (stronger, x-polarized). In a Ne matrix,
they occur at 1.37 and 1.41 eV, respectively. They are again
red-shifted, by ∼0.01 and ∼0.025 eV, respectively, in a Kr
matrix.
Properties of 3 in Solutions. Solution absorption spectra
closely resemble the gas-phase and rare-gas matrix spectra
but are broadened and considerably red-shifted. In benzene,
the red shift from the gas phase amounts to ∼0.2 eV and
E(S1) equals 2.15 193 (2.13 194 ) eV. In cyclohexane, the value
reported as an average of the energies of the first peaks in
absorption and in fluorescence is 2.10 eV. 128 The Stokes shift
between these peaks is very small, e.g., in 2-methyltetrahydrofuran
at 77 K, only 2 nm even without correction for
self-absorption (in this solvent, E(S1) is 2.13 194 ).
From phosphorescence in frozen cyclohexane, E(T1) is
0.95 eV. 128 Early flash photolysis measurements in hexane 166,195
revealed a transient with three absorption peaks separated
by ∼1400 cm -1 and with the first peak at 2.51 eV in a
spectrum that lasted for tens of µs. More recent work in
benzene produced an essentially identical spectrum with the
fourth peak now visible as an indistinct shoulder, and the
first peak shifted to 2.46 eV. 193 The red shift of this fairly
strong transition is thus again about 0.2 eV between gas phase
and benzene solution. A search for weak absorption at
energies as low as 1.1 eV did not reveal any, down to a
sensitivity limit of 50 M -1 cm -1 (numerous calculations
suggest that there is a very weakly allowed transition in the
low-energy region, variously predicted for instance at 1.24 40
or 1.41 182 eV). Several bands are present at higher energies,
with the most intense one at 4.04 eV. A comprehensive
determination of photophysical parameters in cyclohexane 128
yielded the quantum yields of fluorescence, intersystem
crossing, and internal conversion as 8, 76, and 16%, and a
fluorescence lifetime as 7.0 ns, which corresponds to an
intersystem crossing rate constant of ∼1.1 × 10 -8 s -1 .
All the authors agree on the assignment of the ∼2.5 eV
transient to T1 based on its long lifetime. The transient
spectrum measured in benzene upon sensitization with triplet
1 (54 µs single exponential lifetime) is identical with the
spectrum obtained upon direct excitation, 194 proving the
identity of this transient as triplet with certainty. There is
no doubt that the T1 state is of HOMO-LUMO nature and
that its symmetry is B2u. All computations since the pioneering
semiempirical effort in 1956 47 have agreed that the
intense transition at 2.4-2.7 eV is to a B1g state and is
polarized along the long axis (x). In solution, the triplet reacts
with ground-state 3 to yield a transient that absorbs in the
UV and has a lifetime in the ms range. This was assigned to
a thermally unstable photochemical dimer. 193
Properties of Solid 3. The situation is complicated
because 3 is known to crystallize in at least four polymorphs
characterized by different layer periodicity. 196 The four have
been grown as thin films and one also as a single crystal,
whose structure has been determined. 196,197 The structure of
one form, obtained by vapor deposition of a fiber-structured
very thin film on a suitable substrate, is known as well 198
and is complicated in that the molecular arrangement within
the unit cell depends on the substrate used. Unfortunately,
in most optical studies of films, little attention was paid to
the structural characterization of the film used.
Nevertheless, all authors find similar general features of
single-crystal and polycrystalline film absorption. The first
absorption band shows fairly broad peaks reminiscent of
those found in isolated molecules, but the first peak exhibits
a 0.14 eV Davydov splitting 199,200 into a pair of peaks at
1.83 106,201 and 1.97 201 eV, polarized parallel and perpendicular
to the crystal b axis, respectively. 201 The others occur at 2.12
and 2.3 eV 149,202 and have been assigned to intermolecular
charge-transfer transitions. 106 The red shift of the average
of the Davydov pair relative to the gas phase is 0.4 eV, twice
the shift observed upon going from the gas phase to benzene
solution.
It was reported a long time ago that that solid 3 does not
fluoresce detectably, 158 and there is general agreement on
the subject. The E(T1) energy is 0.86 177 eV from the
activation energy of heterofission with 3 as a guest in host
crystals of 2 and 0.85 eV from direct measurement on a
film. 178 If the red shift from the gas phase to the solid were
to be again the same for the strong absorption band of the
T1 state as it is for the La transition from the S0 state (0.4
eV), as is the case in benzene solution (0.2 eV), the average