Key words
excimers - multichromophoric assembly - circularly polarized luminescence - exciton migration - singlet fission - symmetry-breaking charge separation
Introduction
In 1941, Szent Györgyi wrote a thought-provoking article titled “Towards a New
Biochemistry”,[1] in which he argued that a greater
number of molecules may join to form such energy continua, along which energy, viz.,
excited electrons, may travel a certain distance. It was one of the earliest
propositions in biochemistry that a collection of molecules could behave differently
from its constituents, and give rise to a new function. This very idea has also been
at the heart of organic semiconductors and molecular electronics research. Over the
years, the scope of organic semiconductors has considerably widened to include a
variety of materials, from molecular crystals,[2] thin
films,[3] conjugated polymers,[4] dendrimers[5] and more recently to
multichromophoric assemblies.[6] From what once was a
niche research area in physics, the field now attracts organic chemists, materials
scientists, device physicists, computational chemists and theoretical physicists
alike, all driven by the prospect of discovering new phenomena and designing better
materials. The commercialization of first organic light-emitting diode-based
displays at the turn of 21st century heralded a triumph for this versatile class of
materials. Since then organic semiconductors have steadily increased its footprint
in photovoltaics, printed electronics, battery technology etc.
The nature of light–matter interaction and low-lying excitations is of great
significance in organic semiconductors.[7] Processes such
as light emission[8] and light harvesting[9] depend on the nature of excited state and its dynamics.
Its understanding is therefore critical to improving material performance in various
optical and optoelectronic applications. While our understanding of light–matter
interaction is largely shaped by traditional inorganic semiconductors, there are key
distinctions between inorganic and organic systems. The latter is usually
characterized by a strong intramolecular and a much weaker intermolecular electronic
coupling.[10] This along with the fact that organic
solids also have low dielectric constants ensures that the photoexcited
electron–hole pair remains strongly correlated and fairly localized, spread over
only a few molecular units. Such excited states are referred to as the Frenkel
excitons. A consequence of this localized molecular nature of the excited state is
reflected in the optical absorption spectrum of an organic semiconductor, which
retains a strong resemblance to that of the free molecule. Localization of Frenkel
exciton has one other important consequence: electronic transitions can couple
strongly with molecular or lattice vibrations.[11] Since
the lowest energy transitions in organic semiconductors are mostly π–π* or
n–π* in nature, electronic reorganization following an optical transition is
likely to affect the π bond order and therefore the underlying structure. Such
strong coupling of an electronic transition to a vibrational mode increases the
effective mass of the exciton, and in turn affects its mobility through the
semiconductor layer.[12]
A localized photoexcitation can also influence the nature of van der Waals and other
noncovalent interactions between molecules. A common manifestation of this is the
formation of excimers.[13] As a transient dimeric
species that exists only in the excited state but is dissociative in the ground
state, excimers are often characterized by a structureless photoluminescence (PL)
spectrum that is strongly red-shifted compared to that of the isolated molecule. In
organic systems, excimers were first reported by Förster in concentrated solutions
of pyrene.[14] Over the years, excimers have been
observed in thin films, molecular crystals, and more recently in multichromophoric
systems like molecular clusters,[15] conjugated
polymers,[16] and dye assemblies. In the early days,
seminal works of Förster, Birks, McGlynn, Stevens and others contributed
significantly towards understanding the nature of molecular interactions in
excimers,[17] and their formation and dissociation
kinetics.[18] However, a few major developments in
the last 20 years have rekindled the interest in excimers. The advent of ultrafast
lasers made it possible to study details of exciton dynamics that were previously
inaccessible to experiments. Simultaneously, development of theoretical tools and
computational power has enabled accurate modelling of excited state behaviour in
complex systems. These combined with the discovery of fascinating material systems,
such as conjugated polymers and dye assemblies, have expanded the scope of
investigation immensely. In this review, we aim to chronicle the important
developments from the last decade that present a fresh perspective on excimers, and
discuss their role in light emission and light harvesting. In doing so, we restrict
ourselves to a specific class of multichromophoric systems, namely the dye
assemblies and related model systems.
Excimers in Multichromophoric Assemblies
Excimers in Multichromophoric Assemblies
The term excimer was introduced by Stevens and Hutton,[19] who described it as a dimer that is associated in the excited state,
but dissociates in the ground state. This definition was subsequently revised by
Birks to redefine the ground state as dissociative,[13] thus indicating the tendency of the bound excited pair to dissociate
upon recombination, in absence of any external restraint. Different electronic
interactions are possible between a pair of closely interacting excited- (M*)
and ground-state (M) molecules. Excimers are predominantly characterized by
two interactions: exciton coupling and charge resonance. For the purpose of this
review, we restrict ourselves to only singlet excimers. An exciton-coupled state,
|1(S
1
S
0)⟩ represents the exchange
of excitation energy between two identical chromophores, such that the probability
of being excited is equal for both chromophores.
Likewise, a charge-resonance state, |CR⟩, indicates the possibility of a symmetric
charge transfer between the molecules.
Individually, neither of these pure states can fully describe the excimer. Instead,
its mixed character is best captured by a coherent superposition of these
states:
For the superposition or mixing to be efficient, the pure states must bear structural
and energetic similarities, and a reasonably strong coupling that is comparable to
their energy difference. The superposition state, |ψ
Ex⟩ described
above represents a molecular association in the excited state. Upon relaxation, it
dissociates into a pair of monomers in the ground state |S
0⟩. One
realizes that the above description of excimers needs a revision before it can be
applied to multichromophoric systems, like dye assemblies ([Figure 1]). In these systems, reasonably strong electronic coupling can
exist between chromophores even in the ground state. Therefore, for the description
of a dissociative ground state to be valid, the nuclear coordinates in the ground
and the excited states of the multichromophoric system must be significantly
different. The revised definition must account for the possibility that excimers in
a system of multiple coupled chromophores may not be a strictly dimeric species.
Further, as the size and complexity of a multichromophoric system increase, one may
encounter more than one unique excimer that could interconvert during the lifetime
of the excited state.
Figure 1 Excimers in multichromophoric assemblies and their role in
various photophysical and photochemical processes.
In recent years, ultrafast transient absorption spectroscopy has been particularly
useful in characterizing excimers. One can identify distinct spectroscopic
signatures corresponding to the exciton-coupled and charge-transfer (CT) characters,
with relative contributions given by their time-dependent probabilities,
|c
i
2|. By monitoring how the coupling between the
pure states evolves over time, one can investigate relaxation pathways in excimers,
and its involvement in other excited-state processes. Every molecule is different.
And so are the structural changes that accompany excimer formation, and the dominant
interactions that operate within such a species. While the term excimer is broadly
used to indicate a transient, excited-state association, the way it manifests in
various photophysical and photochemical processes can be very different in different
multichromophoric systems.
Excimer Luminescence
A few notable exceptions aside,[20] excimers usually
suffer from low PL efficiency and a poor colour purity. These factors limit their
utility as emissive materials in applications that require a bright light output.
Yet, one of the more successful applications of excimers in the past relied on the
appearance of its characteristic red-shifted PL to report specific molecular
interactions. In particular, this approach has shown great promise in biomedical
science, where the ability to detect a point mutation or a single nucleotide
mismatch in a genome holds enormous significance in the early detection of genetic
diseases. The strategy involves the use of synthetically modified nucleotides with
carefully incorporated chromophores that upon hybridization with the complementary
strand gives rise to an excimer PL. The use of excimer luminescence in DNA
hybridization has been extensively reviewed by Kool et al.,[21] and will not be discussed here any further.
In recent times, a considerable amount of research has focussed on the possibility of
getting circularly polarized luminescence (CPL) from excimers.[22] CPL refers to the differential PL emission of the left
(I
L) and right (I
R) circularly polarized
light, and is quantified in terms of the luminescence dissymmetry factor
(g
lum) which is given by
g
lum = 2(I
R−I
L)/(I
R+I
L).
A high dissymmetry in CPL correlates directly with the chiral arrangement in the
excited state. Since excimers exist only in the excited state, a dissymmetric or
chiral arrangement of chromophores in the excimer state could be the key to
achieving a high CPL anisotropy. The possibility of excimer CPL was first
demonstrated by Brittain and Fendler using enantiomeric pyrene derivatives.[23] Despite a pronounced circular dichroism, the
molecular state showed no dissymmetry in the PL. The excimeric state on the other
hand exhibited CPL with a large g
lum. From a practical standpoint,
achieving CPL from excimers in concentrated solutions is not very useful. In a
significant development, Inouye et al. demonstrated the possibility of getting
excimer CPL from achiral chromophores.[24] A pair of
stacked alkynyl-substituted pyrene molecules was first trapped within a
γ-cyclodextrin cavity to form a 2 : 2 inclusion complex,[24a] which was subsequently converted into a threaded rotaxane using a
capping moiety ([Figure 2]c). Spatial restriction of
pyrene chromophores encouraged the formation of excimers that showed a high PL
quantum yield (φ = 3.7). Further, the chiral environment of the cyclodextrin
cavity imposed a twisted stacking of pyrene chromophores with a well-defined
chirality. This resulted in a CPL with a high g
lum value of
− 1.5 × 10−2 at 480 nm. The generality of this strategy was extended
to obtain excimeric CPL from a pair of perylene chromophores with
g
lum = − 2.1 × 10−2 at 573 nm.[24b] A unique advantage of excimer-based CPL materials
lies in the possibility of achieving CPL in the near-infrared (NIR) region, without
necessarily using NIR dyes. This was demonstrated using an anthracene-based double
helicate,[25] in which two helical
oligo(p‐phenyleneethynylene) units were hinged by flexible, chiral binaphthyl
moieties ([Figure 2]b). Structural change following the
photoexcitation led to the formation of an intramolecular anthracene excimer in
CHCl3 that exhibited a CPL emission with 15% quantum yield and a
g
lum = 1.1 × 10−2 at 690 nm. Interestingly, the
dissymmetry factor showed a 2-fold increase in methanol.
Figure 2 Achieving dissymmetry in the excimer state. Substituting
multiple chromophores on a chiral backbone: (a) pyrene chromophores on a
twisted quaternaphthyl scaffold interact in the excited state to generate a
chiral excimer. Adapted with permission from Ref. 26a. Copyright 2018 The
Royal Society of Chemistry. (b) A flexible double helicate backbone that
allows two π-conjugated segments to interact closely in the excited state to
create an excimer that emits CPL in the NIR region. Adapted with permission
from Ref. 25. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
The other approach makes use of noncovalent interactions to organize achiral
chromophores in a dissymmetric fashion: (c) a pair of pyrene molecules
encapsulated inside a pair of γ-cyclodextrins assumes a twisted stack.
Adapted with permission from Ref. 24a. Copyright 2014 Wiley-VCH Verlag GmbH
& Co. KGaA, Weinheim. (d) An amphiphilic diblock copolymer PBdEO
organizes helically in the presence of a chiral additive
(d/l-tartaric acid), which in turn acts as a scaffold
on which achiral dye molecules organize to form extended chiral assemblies.
Adapted with permission from Ref. 27. Copyright 2020 American Chemical
Society.
There have also been attempts to covalently attach multiple chromophores to a chiral
scaffold to obtain CPL. Takaishi and co-workers used a quarternaphthyl skeleton on
which naphthalene rings were arranged in way to define a helical axis.[26a] By functionalizing the naphthalene rings with an
optimal number of pyrene residues, they were able to create conformationally rigid
structures that supported a well-defined helical organization of chromophores ([Figure 2]a). These molecules displayed intense excimer
CPL in both solution and solid state. Subsequently, the same group extended this
approach to organize other fluorophores such as perylene and anthracene to create a
library of CPL dyes spanning the visible spectrum.[26b]
With the aid of time-dependent density functional theory calculations, the authors
were able to establish a correlation between the sign of the CPL and the twist
(chirality) in the excited-state structures. The above approach, though elegant, is
synthetically demanding. A much simpler and perhaps a more general strategy was
developed recently by Li et al.[27] An achiral
amphiphilic diblock copolymer was made to assume a helical structure in presence of
a chiral additive. The helical polymeric structure was subsequently employed as the
template for the organization of a variety of achiral fluorophores ([Figure 2]d). The polymer–fluorophore co-assembly
exhibited an excimer CPL with the highest known dissymmetry value in the solid
state, g
lum = 2.3 × 10−2. Interestingly, the group also
reported an antihelical effect, in which a right-handed polymer helix induced a
left-handed CPL, and vice versa.
An interesting odd–even effect on the handedness of excimer CPL was reported in a
series of chiral oligopeptides decorated with a pair of pyrene units.[28] Pyrene units separated by an even number of
intervening atoms gave rise to a negative CPL signal, and conversely for an odd
number of atoms, the CPL signal turned positive in water. The dependence of the
handedness of CPL emission on the arrangement of chromophores in the excimeric state
opens up an avenue for the development of stimuli-responsive CPL dyes. This was
first reported by Mimura et al. in a study on oligopeptides with a pair of pyrene
pendant groups.[29] The oligopeptides exhibited
solvent-dependent sign inversion of CPL, even when the CD spectra showed no such
change, thus highlighting a key difference between ground- and excited-state
solvation effects. More recently, a similar behaviour was also reported for an
axially chiral molecule containing a pyrene moiety flanked between a pair of chiral
binaphthyls ([Figure 3]).[30] Hydroxyl substitution on binaphthyl rings had a marked influence on
solvent-dependent intermolecular hydrogen bonding in the excited state.
Consequently, the molecule showed a pronounced inversion of excimer chirality, from
g
lum of − 0.012 in toluene to + 0.012 in DMSO.
Figure 3 Solvent-induced inversion of excimer CPL. (a) A pyrene
chromophore sandwiched between a pair of axially chiral binaphthyls forms
twist-stacked excimers of opposite chirality in polar and non-polar
solvents. (b) Difference between left and right circularly polarized PL
intensities (ΔI = I
L – I
R), and
(c) g
lum values in different solvents show the inversion
effect. Adapted with permission from Ref. 30. Copyright 2020 American
Chemical Society.
Excimers in Light Harvesting
Excimers in Light Harvesting
In the context of light harvesting, long-lived excitons are highly desirable, because
of their ability to transfer excitation energy over large distances.[31] However in a multichromophoric system, long-lived
excitons are also vulnerable to other relaxation processes that can occur on
timescales faster than its lifetime. Self-trapping of Frenkel excitons into excimers
is one such process that plagues exciton migration in multichromophoric systems.
This is particularly true for perylene bisimide (PBI),[b] an exceptional molecular dye with a very high molar extinction
coefficient, an impressive chemical and photochemical stability, and an inherent
tendency to form cofacially stacked H-aggregates.[32c]
One of the earlier attempts to explain the self-trapping mechanism in PBI
H-aggregates was undertaken by Engels and coworkers.[33]
They calculated the potential energy surfaces of ground and excited states of
cofacially stacked PBI dimers as a function of different structural parameters, such
as the interplanar distance, longitudinal and transverse shifts, and the twist
angle. Their calculations showed that the excimer formation process in PBI involved
a considerable lowering of the twist angle between the stacked chromophores. Though
the authors stopped short of calling the self-trapped state as an excimer, in a
subsequent work they modeled the process leading up to the geometry distortion in
the excited state using time-dependent wavepacket calculations ([Figure 4]a).[34] According
to this model, the initial delocalized exciton undergoes an ultrafast localization
onto a dimeric unit, which is followed by a crucial energy dissipation step. This
stabilization of the Frenkel exciton is mediated through a transient gateway state
with large CT character. Using femtosecond transient absorption (fsTA) spectroscopy,
the timescale of this process was estimated to be of the order of 200 fs. Formation
of a lower energy Frenkel exciton is followed by a much slower (picosecond)
relaxation along the interstack torsional coordinate, which concludes the
self-trapping process. The importance of configurational mixing between Frenkel and
CT states was also proposed by Gao et al. to explain the broad, red-shifted
excimer-like PL from PBI oligomers.[35] The
excited-state relaxation effects that aid the formation of excimers are not
exclusively intermolecular in nature. In a recent work, Engels and co-workers
investigated intra-monomer distortions associated with excimer formation, and its
dependence on the extent of exciton delocalization.[36]
Figure 4 Excimer formation in PBI H-aggregates. a) Frenkel exciton
rapidly (sub-picosecond) transitions into a lower energy exciton state
through a transiently populated CT state. This is followed by a much slower
(picoseconds) geometric rearrangement that reduces the twist angle. Adapted
with permission from Ref. 34a. Copyright 2013 American Chemical Society.
Femtosecond upconversion PL spectroscopy of dimeric (b, c, d) and extended
(e, f, g) PBI H-aggregates reveals vibronically resolved PL from the Frenkel
exciton in early stages (within 200 fs) after photoexcitation. Adapted from
Ref. 41a published under a creative commons license (CC BY).
One of the more successful approaches to investigate the excimer formation process
has been based on ultrafast spectroscopy on structurally rigid, covalently bridged
dimers.[37] A precise arrangement of chromophores in
such model dimeric systems allowed one to systematically investigate the role of
different structural aspects. Wasielewski and coworkers investigated the formation
kinetics of excimers in a series of rigid, covalently bridged PBI dimers using fsTA
spectroscopy. Their results established a direct correlation between excimer
formation timescales and the strength of interchromophoric coupling.[38] This work also presented an unambiguous way to probe
the PBI excimer by monitoring its transition to higher lying CT states in the NIR, a
spectral region that is usually free of interference from other excited-state
absorption processes. Kim, Würthner and coworkers took the challenge further, and
presented an interesting comparison of exciton dynamics and excimer formation in
model PBI dimers vis-à-vis extended helical H-aggregates. With the help of transient
absorption anisotropy decay measurements, they found excimer formation in large
aggregates to be considerably slower (40 – 50 ps) than that in dimers (~ 20 ps). A
longer formation time was also found consistent with a distinctly slower rise
(~ 300 ps) in the time-resolved PL. These results presented a compelling proof in
favour of multimeric excimers in extended dye assemblies.[39] The same group further quantified the effect of excimers on exciton
mobility using exciton–exciton annihilation experiments. Their studies showed that
Frenkel excitons migrate incoherently along an aggregated stack before getting
trapped into the excimer state. The diffusion length was estimated to be in the
range of 3 – 5 PBI units.[40] Sung et al. employed
femtosecond broadband fluorescence upconversion spectroscopy to investigate Frenkel
exciton dynamics in helical PBI aggregates.[41a] An
unprecedented temporal resolution allowed them to observe for the first time a
vibronically resolved PL spectrum from the short-lived Frenkel exciton state ([Figure 4]b – g). From the vibronic peak ratio
(I
0 – 0/I
0 – 1), it was estimated that the
nascent exciton is coherently delocalized over at least 3 PBI units.[41] A subsequent decrease in exciton coherence over a few
hundred femtoseconds indicated the excimer formation process in PBI aggregates to be
much faster than previously thought. This ultrafast self-trapping was attributed to
an efficient mixing between Frenkel and CT exciton states. The detrimental effect of
excimers is not solely limited to a reduced exciton mobility. Using model
donor–acceptor foldameric systems, Fimmel et al. showed that excimers also impede
photoinduced electron transfer across intramolecular p/n heterojunctions.[42]
The studies reviewed so far stressed on the need to prevent excimer traps in order
for H-aggregates to support a long-range exciton transfer. A large family of
PBI-based H-aggregates, where excimer formation is notoriously unavoidable,
overwhelmingly supports this view. In this context, a notable exception was reported
by Chaudhuri et al. H-aggregated nanowires of cyclohexyl-appended PBI used in their
study were uncharacteristically free of excimers.[43]
The low-temperature PL spectrum featured a pronounced vibronic progression, and a
lifetime of 46 ns that is consistent with the long-lived Frenkel excitons of an
H-type aggregate. While the factors contributing to this unprecedented stability of
long-lived Frenkel excitons were not investigated, its implication in exciton
migration was immediately clear. Local quenching sites (diameter ~ 15 nm) created on
the nanowire surface using an Atomic force microscopy (AFM) tip could quench
excitonic PL from nearly a micron-sized area ([Figure
5]a). This indicated the ability of excitons to migrate over hundreds of
nanometers, with an estimated exciton diffusion constant of
1.4 × 10−2 cm2/s that is nearly 1 – 2 orders of magnitude
higher than that for conjugated polymers. The possibility of suppressing excimer
formation was also demonstrated by our group in extended H-aggregates of a PBI
folda-dimer.[44] The dimer which can exist in two
distinct conformations: a non-interacting open and an intramolecularly stacked
folded form, displayed a competition between two self-assembly pathways in solution.
The faster pathway led to aggregates of the folded dimer with pronounced excimeric
PL. These reorganize to form the more-stable excimer-free aggregates of the open
conformer. A detailed investigation into possible factors that can prevent excimer
formation in PBI H-aggregates was recently carried out by our group. We reported
ambient-stable, bright, long-lived excitonic PL from an H-aggregated PBI, in both
carefully grown single crystals and a relatively disordered solution
self-assemblies.[45] From the polarization
dependence of absorption and PL spectra of single microcrystals, and correlating the
results with the crystal structure, it was possible to identify two major factors
that contributed to the stability of long-lived excitons against excimer formation.
An unusually large exciton splitting (~ 1265 cm−1), resulting from the
combination of H-type Coulombic and J-type CT interaction (Hj coupling), stabilized
the lowest-energy Frenkel exciton, thus inhibiting its crossover to the excimer
state ([Figure 5]b). A recent theoretical work on
covalent PBI dimers also suggested an upper threshold of CT contribution (~ 58%),
above which excimer formation in cofacial PBI aggregates could be suppressed.[46] In addition to the large exciton splitting, the
exciton wavefunction also experiences an efficient self-localization that further
safeguards it from potential trap sites in the vicinity. In view of these findings,
it is prudent to take special note of two recent studies. A very large exciton
splitting of 1230 cm−1 was also reported for an H-aggregated PBI crystal
by Austin et al.[47] Though PL characteristics of the
crystals were not investigated, these crystals could make an interesting system to
study the influence of large exciton splitting on the fate of excimers. In another
work, Würthnerʼs group reported a supramolecular polymorph of a PBI dye that
featured a broad absorption spectrum and a vibronically resolved steady-state PL
spectrum.[48] Both these systems and the one we
studied shared certain similarities in molecular packing: a twisted stacking of PBI
units with a longitudinal shift. Such chromophoric arrangement can promote a strong
coupling between Coulombic and CT interactions,[49]
which can be crucial towards stabilizing the exciton against excimeric trapping.
Figure 5 Escaping excimeric traps in PBI H-aggregates. a) Correlated
AFM topography and far-field PL microscopy suggest nearly micron-scale
exciton migration in excimer-free H-aggregates of PBI. Adapted with
permission from Ref. 43. Copyright 2011 American Chemical Society. b) A
combination of very large exciton splitting (2J) and an efficient exciton
localization safeguards Frenkel excitons from excimer traps, resulting in a
bright, vibronically resolved PL. Adapted from Ref. 45 published under a
creative commons license (CC BY-NC).
Notwithstanding the undesirable influence of excimers on exciton migration, there
also exists an alternate perspective that highlights the positive role of excimers
in singlet fission (SF). SF refers to a spin-allowed process in which a photoexcited
singlet exciton interacts with a neighboring molecule in the ground state to
generate two lower energy triplet excitons.[50] In the
context of photovoltaics, generating multiple excitons (electron–hole pairs) through
SF can push the efficiency of a solar cell well above the Shockley–Queisser limit of
~ 33%. SF proceeds through an excited state that is best described as a coherent
superposition of exciton-coupled singlet excited
|1(S
1
S
0)⟩, charge-transfer |CT⟩
and correlated triplet pair
|1(T
1 T
1)⟩ states.[37e] With two out of these three interactions also
operating in excimers, it is only natural that excimers could influence the fate of
SF.
The role of a short-lived excimer intermediate was first proposed theoretically by
Zimmerman et al. to explain SF in pentacene.[51] An
interaction between excited- and ground-state pentacene molecules was proposed to
facilitate a state crossing from the molecular singlet excited state to a dimeric
multiexcitonic dark state, with the characteristics of a correlated triplet pair.
This key step is believed to be mediated through a transient excimer-like species
that holds the two molecular units together. Once the multiexcitonic state is
formed, the repulsive interactions lead to the separation of two pentacene monomers,
each with a triplet exciton localized on it. Friend and coworkers experimentally
confirmed excimer-mediated SF in concentrated solution of TIPS-pentacene
molecules,[52] which showed a near-unity quantum
yield: two triplet excitons for every absorbed photon. By comparing the rate of
growth of triplet states vis-à-vis the decay of excimer PL, they proposed a two-step
phenomenological model for SF: a fast, diffusion-limited formation of excimers,
followed by its dissociation to generate two triplet excitons on a timescale of
400 – 530 ps. The importance of CT interactions in SF involving donor–acceptor
chromophores was investigated in a series of thiophene-diketopyrrolopyrrole-based
molecular crystals.[53] Here too, a robust correlation
between intermolecular donor–acceptor interactions, excimer decay rates and SF
efficiencies confirmed the involvement of an intermediate excimer state with a large
CT character ([Figure 6]a). The intermediary role of
excimers in SF is however not universally true. A notable exception was reported by
Kolata et. al., who observed contrasting SF efficiencies along two different
crystallographic axes of perfluoropentacene single crystals.[54] SF happened exclusively along the b-axis, where molecules
arranged in a slip-stacked (J-type) fashion, but was suppressed along the
a-axis that supported a face-to-edge (herringbone) packing. Interestingly,
fsTA spectroscopy results showed that the correlated triplet pair,
|1(T
1 T
1)⟩, is formed directly
from the exciton-coupled singlet
|1(S
1
S
0)⟩ state, through a
process that precedes excimer formation by ~ 300 fs ([Figure
6]b). This clearly showed that excimers in perfluoropentacene crystals
neither compete with nor mediate in SF.
Figure 6 Excimers in SF. a) Excimer-mediated exothermic SF in
polycrystalline thin films of diketopyrrolopyrrole derivatives with
slip-stacked packing. Globally fit species-associated spectra show strongly
correlated excimer (τ1) and triplet formation (τ2)
processes. Adapted with permission from Ref. 53. Copyright 2016 American
Chemical Society. b) SF in perfluoropentacene single-crystals happens
exclusively along the b-axis. False color plots showing evolution of
various differential absorption features, corresponding to disappearance of
Frenkel exciton (1.7 – 1.8 eV), appearances of excimer (1.5 – 1.7 eV) and
correlated triplet pair (2.1 – 2.5 eV). Kinetic analysis shows excimers
neither compete nor assist SF. Adapted with permission from Ref. 54.
Copyright 2014 American Chemical Society. c) Time-resolved PL of a tetracene
derivative shows appearance of excimers at intermediate timescales
(~ 25 ns). Decay profile of excimers to singlet excitons suggests a
competition between excimer formation and SF processes. Adapted with
permission from Ref. 58. Copyright 2018 Macmillan Publishers Limited, part
of Springer Nature.
The examples discussed so far were limited to materials that exhibit an exothermic
SF, satisfying the energy criterion of E(S
1)
> 2E(T
1). While exothermic SF can show very high
triplet yields, the energy of the resultant triplet state is often too low to be
practically useful. In this context, endothermic SF offers a useful alternative with
higher energy conversion efficiencies. The role of excimers in endothermic SF
remains intensely debated, with more results suggesting their detrimental influence.
Friend and co-workers used TA and time-resolved PL to monitor the process in
concentrated solution of TIPS-tetracene, a chromophore in which S
1
and T
1 energies satisfy the criterion for endothermic SF.[55] An ultrafast conversion of the photoexcited singlet
state into an excimer state was observed on a timescale of < 100 ps.
Interestingly, the excimer displayed a triplet absorption and a singlet emission,
which is consistent with the characteristic of a bound pair of correlated triplets.
The excimer would eventually dissociate generating a pair of separated triplet
excitons. A contrasting picture emerged from the work of Korovina et al., who
investigated SF in a pair of covalently bridged tetracene dimers with a varying
degree of interchromophoric interaction.[56] In the
strongly coupled dimer, the excimer state formed rapidly (~ 180 fs) that promptly
decayed to the ground state without forming triplet excitons. The weakly coupled
dimer, on the other hand, showed an efficient conversion of the singlet exciton to a
correlated triplet pair, through a process that completely bypassed the excimer
intermediate. That excimers could be detrimental to SF also found support in
theoretical studies.[57] Dover et al. carried out
solution-phase investigations of endothermic SF and its exact reverse process, that
is triplet–triplet annihilation (TTA) for a tetracene derivative.[58] The upconverted PL spectrum generated by sensitized
TTA did not show any appreciable excimer signature. Further, by fitting the
time-dependence of molecular (S
1) to excimer PL intensities to
different kinetic models ([Figure 6]c), it was found
that excimers were 14 times more likely to decay to the ground state than undergo
SF. A recent study by Kim and co-workers on covalent PBI dimers, also an endothermic
SF material,[59] further highlighted the deleterious
effect of excimers in multiexciton generation, a key step in the SF process.[60] All three dimers used in the study supported a strong
H-type Coulombic and varying degrees of J-type CT coupling. A weaker CT coupling was
found to encourage the formation of an excimer trap (~ 200 fs) that inhibited its
further transition to the multiexcitonic state. Conversely, dimers with strong CT
coupling readily relaxed into the multiexcitonic state, completely bypassing the
excimer. The rate of CT-mediated transition to the multiexcitonic state further
showed an order of magnitude increase in polar solvents, thus highlighting the
importance of CT interactions. A similar CT-mediated transfer between singlet and
correlated triplet pair states was also reported in slip-stacked dimers of
terrylenediimide (TDI), which is incidentally an exothermic SF material.[61a] Here too, excimer formation was shown to inhibit
SF. A competition between CT-mediated SF and excimer formation was also studied by
the same group in different polycrystalline thin films of TDI.[61b] They investigated the role of molecular packing to
show that a twisted stacking between TDI chromophores led to 190% triplet yield,
while a slip-stacked packing predominantly favoured excimer formation. The instances
discussed above allow one to draw a few conclusions about the contrasting role of
excimers in SF. Excimer-mediated SF requires a strong coupling between the excimer
and the correlated triplet state. Thus, a fast structural relaxation that stabilizes
the excimer state and lowers its energy significantly is likely to weaken its
coupling with the correlated triplet state, and negatively impact the SF process.
The problem is expected to be more severe in case of endothermic SF, where the
energy of the correlated triplet state is comparatively higher than that of the
excimer.
Power conversion efficiency of a photovoltaic cell is directly proportional to its
open circuit voltage, a quantity that is usually very low for most organic
photovoltaic systems. Bartynski et al. showed that symmetry-breaking charge
separation (SBCS) between closely interacting pair of identical chromophores can
significantly increase the open circuit voltage, and lower recombination
losses.[62] SBCS is a process by which one excited
chromophore interacts with the other in ground state to create a charge-separated
radical ion-pair, wherein the electron and the hole separately reside on two
different molecules.[63] SBCS also plays a key role in
certain natural photosynthetic reaction centers.[64]
Since the process requires an interaction between excited- and ground-state
molecules, the possibility of excimers interfering with SBCS looms large. In
polycyclic aromatic chromophores, excimers typically involve molecular stacking,
which makes it possible to dissuade excimer formation and promotes SBCS by bringing
multiple chromophores in a closely interacting, but a non-stacked, arrangement. This
was successfully demonstrated in a couple of cyclic PBI dimers and trimers, where
chromophores were either widely separated using appropriate spacers, or arranged in
a rigid triangular geometry. In both instances, it was possible to discourage
structural relaxation that could lead to deep excimer traps, resulting in SBCS with
a high quantum yield.[65] A notable contribution was
made by Würthner and coworkers, who attempted to distinguish between “shallow”
excimer-like states and structurally relaxed deep excimer traps in the context of
SBCS. The PBI cyclophane used in their study exhibited interesting solvent-dependent
behaviour ([Figure 7]a). In CHCl3, a strong
correlation was observed between the rise time of PBI•− radical anion and
the decay of excimer PL, suggesting that the excimer could be an intermediary in
SBCS.[66] In THF however, a tighter stacking between
the two PBI units resulted in the formation of a structurally relaxed deep excimer
state that fully suppressed SBCS. A recent report on polycrystalline thin films of
PBI presented a new perspective on the role of interchromophoric coupling in excimer
formation.[67] Subtle differences in the
slip-stacked packing pattern manifested itself in varying degrees of Coulombic and
CT coupling, which in turn modulated the CT rate between PBI chromophores.
Consequently, SBCS was favoured over excimer formation in films that supported a
stronger CT interaction ([Figure 7]b). The competition
between exciton-coupled and charge-separated states can also be tilted in favour of
the latter in more polar solvents, as was shown using different model bichromophoric
systems.[68] More recently, Kim and co-workers
reported that in highly polar solvents, the CT resonance character of deep excimer
trap of a bay-substituted PBI cyclophane could be sufficiently enhanced to achieve
an incomplete SBCS.[69]
Fig. 7 Competition between excimer formation and SBCS. a) The
competition between formation of deep excimer traps and SBCS in PBI
cyclophane is dependent on the strength of interchromophoric coupling. The
schematic energy level diagram highlights the important processes and their
respective timescales. Adapted with permission from Ref. 66. Copyright 2016
American Chemical Society. b) Nature of molecular packing in polycrystalline
thin films of two bay-substituted PBI derivatives controls the interactions
between PBI chromophores and the fate of the photoexcited state;
imide-hydrogenated derivative (top) supports SBCS, whereas strong mixing of
singlet exciton and CT states in the di-octyl derivative (bottom) favours
excimer formation. Adapted with permission from Ref. 67. Copyright 2020
American Chemical Society.
Finally, in the context of solar energy harvesting, under-utilization of the
low-energy part of the solar spectrum has been an outstanding concern. TTA-based
photon upconversion offers a potential solution to address this loss.[70] Here too, formation of excimers competes against
upconversion of triplet to singlet excitons, and contributes to a significant energy
loss. Börjessonʼs group investigated sensitized TTA upconversion in a perylene dye,
and elucidated the role of a triplet dimer
(T
1
S
0) precursor in excimer formation.[71] A significant increase in the upconversion yield was
reported by suppressing excimer formation in less polar solvents.
Conclusions and Outlook
This review started with the aim to showcase excimers in a new light, one that is
shaped by the recent developments in the area of organic multichromophoric systems.
In the first four decades after its discovery, excimers were often viewed in
isolation: as a transient excited-state species that resulted from, and therefore
reported on, closely interacting chromophores. This outlook changed completely in
the last decade, as the ability to probe the dynamics of excited states witnessed a
marked improvement. This widened our perception of what excimers are, and how they
might influence different photophysical and photochemical processes in
multichromophoric systems. When viewed in the larger context of the electronic
interactions that define it, the excimer appears to be an integral part of all such
excited-state phenomena and processes that involve exciton coupling, charge transfer
interactions and excited-state structural relaxations of chromophores. Nowhere is
this connection more obvious than in the area of light harvesting, where we are only
beginning to unravel the role of excimers, in processes such as exciton migration,
SF and SBCS. In judging whether excimers are useful or detrimental, there is clearly
a need to avoid generalizations. We have discussed systems where a strong mixing
between exciton-coupled and CT states, combined with geometric relaxations, can lead
to the formation of low-energy excimers. Such deep excimers can indeed trap mobile
excitons, and inhibit SF and SBCS. In contrast, a shallow excimer state is often
suitably poised to act as a key intermediate in the same processes. Therefore, going
forward, an ability to fine tune the characteristics of the excimer state through
molecular design appears to be a goal worth pursuing. Structurally well-defined
dimers have served as useful model systems in the systematic investigation of
factors controlling different aspects of excimer formation. However, there is a need
to extend that approach to large, supramolecular dye aggregates. An obvious
challenge lies in the form of large-scale structural heterogeneities and lack of
long-range order in these aggregates. To this end, investigation of well-defined
oligomeric systems and crystalline aggregates can play an important role. Recently,
Kim and co-workers studied the structural evolution during SBCS in a PBI-based
donor–acceptor–donor molecule, using time-resolved Raman spectroscopy.[72] This unique perspective into the structural changes
associated with excited-state processes will pave way for a better understanding of
structure–property relationships in complex multichromophoric systems.
Finally, the examples that we have reviewed do not explicitly account for temporal
fluctuations in interchromophoric interactions, which might result from molecular
dynamics on the timescale of seconds. It is well known that an exciton in extended
multichromophoric systems is not a static unit. Following every absorption event,
the exciton can randomly localize on different segments of the larger
multichromophoric system.[73] This can introduce
large-scale fluctuations in the nature of exciton coupling and CT interactions that
define an excimer. Recently, Lupton and coworkers confirmed such temporal
fluctuations in excimeric interactions using single-molecule PL spectroscopy on
rigid bichromophoric systems.[74] To what extent such
fluctuations may influence processes such as SF and SBCS is a question that could
stimulate future quests.
