Journal of the American Chemical Society
Article
our four-state model, the third species, as a key intermediate
triplets have long-lived lifetimes at the μs time scale. Besides
1
1
5
for triplet formation, has some features of both 3 A and free
(TT), the high-spin triplet-pair (TT) state is also involved in
g
20−22,46−48
T states and then is assigned to an intermolecular triplet-pair
intermolecular SF dynamics.
For instance, Guldi and
1
1
state, labled as (TT). That is, after photoexcitation, the initial
bright 1 B (S ) state populates 3 A via conical intersection at
an ultrafast rate of 0.2 ps. Then the 3 A state as a key ME
co-workers recently explored the SF dynamics of carbene-
1
1
1
5
u
2
g
based diradical dimers and found both (TT) and (TT) are
close to each other in energy because of the weak binding
1
g
46
intermediate can undergo an intramolecular to intermolecular
evolution (1.7 ps) that occurs in adjacent BDPP molecules and
energy of intertriplets. This is also consistent with the recent
47
theoretical work by the Scholes group. However, the further
1
1
5
loads the (TT) state. Within τ = 22.6 ps, the formed (TT)
exploration on the role of (TT) in SF dynamics of our studied
can further decorrelate to dissociate into free triplets, which
system in solid films is beyond the scope of the present study
and will be studied in the near future. Finally, Sandoval-Salinas
and Casanova recently proposed a dark-state-mediated SF
mechanism for a simple ethylene-dimer model in which the
locally doubly excited HOMO-to-LUMO singlet state has a
significant impact on SF dynamics at the eclipsed conforma-
can persist over several microseconds. In the current system,
1
the quasi-degeneracy and intersection of the 3 A dark and
g
1
1
Bu bright states result in an ultrafast and quantitative SF
process that outperforms other radiative and nonradiative
3
1,43
processes, such as to the excimer trap state
and to the low-
1
17,30,44,45
48
lying 2 A dark state.
In addition, CT and excimer-like
states are frequently involved in SF systems.
g
tion. However, this dark state is the lowest excited singlet
state of an intermolecular SF system, which is different from
our proposed higher-lying dark-state-mediated SF mechanism
1
,9,11,17,20,22
Mauck
et al. presented that an excimer intermediate with distinct CT
configurations can promote a fast SF process in diketopyrro-
lopyrrole (DPP), a similar molecule to the current BDPP
for intramolecular SF systems.
The proposed model highlights that the high-lying 3 A dark
1
9
1
g
system. Given the slightly broader absorption band of 3 A in
g
state serves as a key intermediate to promote efficient ME
TA spectra of films relative to solution and the partial
generation in solution and the triplet-separation process in
1
1
overlapping ESA bands of 3 A or (TT) with radical species,
1
g
solid films (Scheme 2). The quasi-degeneracy of the 3 A and
1
1
g
the 3 A or (TT) state might have partial CT or excimer
1
g
1
B states renders the minimum energy loss from the bright
u
character to mediate the fast SF process of the current system
1
state, while the low-lying 2 A state acts as a potential IC
1
,9,20,22
g
as observed in DPP derivatives.
channel competing with the SF process but is at a disadvantage
relative to the latter. Therefore, such a high-lying dark-state-
mediated SF process not only features ultrafast and highly
efficient merits of ME generation but also could reserve the
absorbed photon energy to the maximum extent conducive to
Proposed Model for Dark-State-Related Photophy-
sics. Based on the results of transient spectra and theoretical
calculations, we proposed a new model for dark-state-related
photophysics: a distinct high-lying dark-state-mediated SF
process. As shown in Scheme 2, the ultrafast IC process from
1−3
practical photovoltaic conversion. Moreover, the results also
1
1
indicate that more electronic states, not just 1 B and 2 A
u
g
Scheme 2. Proposed Model for the High-Lying Dark-State-
Mediated SF Process
states, need to be taken into consideration to supply more
realistic photophysics. The generality of this model is
underway in our lab.
CONCLUSION
■
Combining transient spectroscopy and theoretical calculations,
we provided a clear picture of the high-lying 3 A dark-state-
1
g
mediated efficient SF and subsequent triplet pair separation
process in the benzodipyrrolidone skeleton. Such a dark state
features distinctive double excitation character and could be
described as the ME state composed of two strongly coupled
triplet states. Moreover, this high-lying dark state, which is
1
quasi-degenerate in energy with the 1 B bright state, could be
u
1
1
the initially populated 1 Bu to 3 A ME state occurs within 1
populated from the latter at an ultrafast rate via the intersection
of two states. Furthermore, the reported SF system features
strong absorption in the blue-green region, suitable triplet
energy, an ultrafast and efficient SF process, and high
resistance against air and light, which could provide great
potential for integration into practical photovoltaic devices.
Our work not only enriches the pool of SF materials and
provides robust candidates for practical applications but also
contributes new insights to dark-state-related photophysics,
which could be widely applied to the development of new SF-
active materials.
g
ps. Such an ultrafast excited-state event fails the sequential IC
processes to the lower 2 A and 1 A states. However, the
further separation of the 3 A ME state is heavily suppressed in
1
1
g
g
1
g
dilute solution because of the much stronger binding
interaction of intertriplets. This is consistent with the
5
calculated high-energy (TT) state, which is 1.5 eV more
the TTA process takes place. Differently, in solid films, the
unimolecular 3 A ME state could first evolve into an
intermolecular (TT) state with a time costant of 1.7 ps.
1
g
1
Due to the much smaller binding energy of intertriplets in the
BDPP dimer, the (TT) state can easily dissociate into two free
triplets in aggregetes (22.6 ps). This is seconded by the
ASSOCIATED CONTENT
1
■
*
sı Supporting Information
calculated smaller energy difference between the intermolec-
ular (TT) and (TT) states (0.1 eV, Table S3). These free
1
5
5
695
J. Am. Chem. Soc. 2021, 143, 5691−5697