Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-Breaking Charge Separation

Article abstract of DOI:10.1021/jacs.1c05793

A comprehensive understanding of the structure-property relationships in multichromophoric architectures has pushed the limits for developing robust photosynthetic mimics and molecular photovoltaics. The elusive phenomenon of null exciton splitting has gathered immense attention in recent years owing to the occurrence in unique chromophoric architectures and consequent emergent properties. Herein, we unveil the hitherto unobserved null exciton coupling assisted highly efficient photoinduced symmetry-breaking charge separation (SB-CS) in a Greek cross (+)-oriented spiro-conjugated perylenediimide dimer (Sp-PDI2). Quantum chemical calculations have rationalized the infrequent manifestation of null exciton coupling behavior in Sp-PDI2. Negligible contribution of long-range Coulombic and short-range charge-transfer mediated coupling renders a monomer-like spectroscopic signature for Sp-PDI2 in toluene. The Greek cross (+)-arranged Sp-PDI2 possesses a selective hole-transfer coupling, facilitating the ultrafast dissociation of null excitons and evolution of the charge-separated state in polar solvents. Radical cationic and anionic spectroscopic signatures were characterized by employing femtosecond transient absorption spectroscopy. The substantial hole transfer electronic coupling and lower activation energy barrier of Sp-PDI2 accelerated the charge separation rate. The rate of charge recombination (CR) markedly decelerated due to falling into the inverted region of the Marcus parabola, where the driving force of CR is larger than the total reorganization energy for CR. Hence, the ratio of the rates for SB-CS over CR of Sp-PDI2 exhibited an unprecedently high value of 2647 in acetonitrile. The current study provides impeccable evidence for the role of selective charge filtering in governing efficient SB-CS and thereby novel insights towards the design of biomimics and advanced functional materials.

Full text of DOI:10.1021/jacs.1c05793

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Article
Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-
Breaking Charge Separation
Ebin Sebastian and Mahesh Hariharan*
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ABSTRACT: A comprehensive understanding of the structure−
property relationships in multichromophoric architectures has
pushed the limits for developing robust photosynthetic mimics
and molecular photovoltaics. The elusive phenomenon of null
exciton splitting has gathered immense attention in recent years
owing to the occurrence in unique chromophoric architectures and
consequent emergent properties. Herein, we unveil the hitherto
unobserved null exciton coupling assisted highly efficient photo-
induced symmetry-breaking charge separation (SB-CS) in a Greek
cross (+)-oriented spiro-conjugated perylenediimide dimer (Sp-
PDI2). Quantum chemical calculations have rationalized the
infrequent manifestation of null exciton coupling behavior in Sp-
PDI2. Negligible contribution of long-range Coulombic and short-
range charge-transfer mediated coupling renders a monomer-like spectroscopic signature for Sp-PDI2 in toluene. The Greek cross
(+)-arranged Sp-PDI2 possesses a selective hole-transfer coupling, facilitating the ultrafast dissociation of null excitons and evolution
of the charge-separated state in polar solvents. Radical cationic and anionic spectroscopic signatures were characterized by employing
femtosecond transient absorption spectroscopy. The substantial hole transfer electronic coupling and lower activation energy barrier
of Sp-PDI2 accelerated the charge separation rate. The rate of charge recombination (CR) markedly decelerated due to falling into
the inverted region of the Marcus parabola, where the driving force of CR is larger than the total reorganization energy for CR.
Hence, the ratio of the rates for SB-CS over CR of Sp-PDI2 exhibited an unprecedently high value of 2647 in acetonitrile. The
current study provides impeccable evidence for the role of selective charge filtering in governing efficient SB-CS and thereby novel
insights towards the design of biomimics and advanced functional materials.
configuration). H-aggregates exhibit blue-shifted absorption
bands, high charge transport character, and quenched
fluorescence emission. Conversely, exciton coupling among
the staggered transition dipoles (J-aggregate) exhibits a red-
shifted absorption peak, weak charge transport behavior, and
superradiant fluorescence character.16,17 However, in close-
packed molecular aggregates, wave functions overlap among
the neighboring molecules prompting an additional short-
range charge-transfer-mediated exciton coupling (JCT).18 The
magnitude and the sign of CT coupling are governed by the
electron and hole transfer integral derived from the extent of
orbital overlap between the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs).19−21 Consequently, the interference between long-
INTRODUCTION
■
The three-dimensional (3D) coalition of π-conjugated organic
materials in photosynthesis, solar energy conversion, and
optoelectronic devices play an imperative role in ensuring a
high degree of solar energy harvesting, transport, and
conversion.1−6 Specifically, in the photosynthetic complexes,
impinging solar photons are transferred into the reaction
center through coherent exciton transport. The conversion of
excitation energy to chemical energy at the reaction center is
initiated by the symmetry-breaking charge separation process
(SB-CS), which trigger the sequential electron transfer.7,8 The
comprehensive understanding of the photophysical processes
(exciton formation, energy transfer, and charge separation) in
molecular dimers and its precise dependence on the spatial
arrangement, π-overlap, and intermolecular distances are
essential for the design of efficient and robust artificial solar
energy conversion systems.9−11
Received: June 4, 2021
Published: August 9, 2021
Davydov12 and Kasha13−15 independently worked out the
molecular exciton theory in the 1960s based on intermolecular
Coulombic interactions (point−dipole approximation), even-
tually leading to the primary classification of J-aggregates
(“head-to-tail” configuration) and H-aggregates (“sandwich”
https://doi.org/10.1021/jacs.1c05793
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range Coulombic coupling and short-range CT coupling
defines the effective excitonic coupling in interacting
chromophores. A substantial theoretical and experimental
effort has been invested in understanding CT coupling and
Coulombic coupling (JCoul) interferences.22−25
foldamer by the large charge-transfer coupling between the
chromophores.54 Our long-standing efforts to achieve
chromophoric aggregates in Greek cross (+)-arrangement
(face-to-face stack) and understand the diverse photoexcited
state processes in self-assembled donor−acceptor systems55,56
and bichromophoric systems57−59 motivated us to dwell into
the realm of null exciton and its concomitant effect on ultrafast
excited-state dynamics. Herein, we report the null exciton
coupling in spiro-conjugated perylenediimide dimer (Sp-
PDI2), wherein two planar π-conjugated PDI chromophores
are connected perpendicularly via a quaternary sp3 carbon
center (edge-to-edge arrangement), exhibiting ultrafast SB-CS.
Unperturbed spectroscopic features of Greek cross (+)-ar-
ranged Sp-PDI2 in toluene evidence minimal excitonic
coupling. Monomer-like spectral signatures displayed by Sp-
PDI2 can be rationalized by the interference of negligible
contribution from both long-range Coulombic and short-range
charge-transfer mediated couplings. The orthogonally oriented
Sp-PDI2 are found to possess a selective charge-filtering
phenomenon (selectively hole-transfer coupling). Significant
wave function overlap (hole-transfer coupling) among
monomeric chromophores of Sp-PDI2 facilitates efficient
ultrafast SB-CS in polar solvents (THF and ACN). The rate
of charge recombination (CR) processes of Sp-PDI2 is
significantly decelerated due to the low molecular reorganiza-
tion energy when compared to the free energy change of CR
(ΔGCR > λ). The ratio of rates of SB-CS over CR of SP-PDI2
is estimated as 2647, which is the most long-lived charge-
separated state obtained so far among the multichromophoric
PDI derivatives (Table S1).
Photoinduced SB-CS is a process where a symmetrical pair
of identical chromophores forms a charge-separated excited
state with the hole and electron on different chromophores,
i.e., M−M + hν → M·+ − M·−, upon photoexcitation.26 SB-CS
has found wide applications in artificial photosynthesis and
optoelectronic devices owing to the minimal energy loss during
the charge-separated state formation (energy loss between the
exciton and charge-separated state is <100 mV for SB-CS, in
contrast to the value of ∼500 mV or more for charge
separation in the donor−acceptor systems).27−29 The rate of
SB-CS mainly depends on the strength of electronic coupling
and solvent environment.26,30 In a weak exciton coupling
regime, the electronic absorption spectra of the interacting
chromophores show minor variation when compared to the
noninteracting systems. Theoretically, the excited states of the
molecular dimer can be represented as locally excited singlet
(LE) states and charge-separated (CS) states.31,32 SB-CS
processes in weakly coupled dyad and the triad of
perylenediimide,30,33−35 perylene,26,36,37 BODIPY,28,38 metal-
lopyrrins,38−40 bianthryl,41 subphthalocyanine,42 and terryle-
nediimide43,44 are well explored and documented in the
literature. Moreover, probing SB-CS processes in film samples
and polymers is an emerging research topic.45 Strong exciton-
coupled aggregates show shifted or split absorption bands and
excimer formation.24,26,46 The excimer states can act as an
exciton trap and compete with the desired products. In a
strongly coupled system, the excitation energy is delocalized
over the dimer molecule, and the excited state may lack the
charge separation character.36
RESULTS AND DISCUSSION
■
Synthesis, Single-Crystal Structure, and Geometry
Optimizations. PDI, Ref-PDI, and Sp-PDI2 were synthesized
and characterized according to the previous reports (Figure 1
An exciting class of aggregates called “null aggregates”,
exhibiting monomeric optical properties in the condensed
state, was initially proposed by Kasha and co-workers and later
by Spano and co-workers.15,47 Besides the theoretical
predictions, innovative designs exhibiting minimal exciton
coupling have been reported in recent years.49 Xie and co-
workers reported minimal excitonic coupling in a perylenedii-
mide crystal possessing magic angle (slip angle = 54.7°)
stacking.50 Wu
̈
rthner and co-workers demonstrated very weak
coupling in a molecular foldamer of perylenediimide (PDI)
owing to the destructive interference between Coulombic and
CT coupling.25 Our group has devised an alternative strategy
for constructing molecular aggregates with minimal excitonic
interactions in a crystalline state through the orthogonal (face-
to-face) stacking of chromophores at a 90° rotational angle.
The first crystalline evidence for the Greek cross (+)-archi-
tecture with null exciton coupling mediated monomeric optical
properties was reported in 1,7-dibromoperylene-3,4,9,10-
tetracarboxylic tetrabutylester.51 Yet another example of null
exciton splitting was realized in a series of pentacene
derivatives existing in orthogonal molecular stacks.52 The
orthogonally cross-stacked linear and nonlinear acene dimers
were found to exhibit mutually exclusive hole and electron
transfer coupling.53
Though vital for optoelectronic application, a comprehen-
sive understanding of the excited-state dynamics of very weak/
null excitonic coupled chromophores remains rare. Recently,
Kim, Wurthner, and co-workers reported efficient multiexciton
̈
state generation in a null exciton-coupled perylenediimide
Figure 1. Molecular structure of (a) PDI, (b) Ref-PDI, and (c) Sp-
PDI2. (d and e) Single-crystal X-ray structure of Sp-PDI2. “R”
represents the 3-pentyl group.
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and Scheme S1).60,61 Ref-PDI is the monomer unit of Sp-
PDI2. Slow evaporation of a homogeneous solution of Sp-PDI2
(in a 1:1 chloroform/methanol solvent) at room temperature
led to good-quality crystals for single-crystal X-ray diffraction.
The X-ray crystallography technique unambiguously confirmed
the molecular structure of Sp-PDI2 (Figure 1).
of the perylene core (vc=c ∼ 1400 cm−1), resulting in
pronounced vibronic progression in the absorption spectrum
at 495 nm (λ = 20 202.02 cm−1), 464 nm (λ = 21 551.72
Abs
Abs
0−0
0−1
cm−1), and 435 nm (λ = 22 988.51 cm−1). The A0−0/A0−1
Abs
0−2
oscillator strength ratio, as determined by the ratio of the
intensities of the 0−0 and 0−1 absorption bands, amounts to
1.74 (Table 1).
An sp3-hybridized spiro-carbon atom connects the two
blades of PDI subunits to form a Greek cross (+)-architecture
(edge-to-edge arrangement) with a rotational angle (θ) of
87.43° between the chromophores (CCDC number 2086823
and Table S2). Geometry optimization of PDI, Ref-PDI, and
Sp-PDI2 was performed at the B97D3/def2-TZVP level of
theory.62,63 The geometrical features of Sp-PDI2 are essentially
the same in the vacuum, and θ was found to be 90° between
the spiro-fused PDI units (Figure S1). As a response to the
increased steric constraint of the five-membered cyclic carbon
ring in the PDI core’s bay region, a small inward bend along
the long axis of the perylenediimide chromophore of Ref-PDI
and Sp-PDI2 is observed (Figure S2).
Table 1. UV-vis and Fluorescence Spectroscopic Data of
Ref-PDI and Sp-PDI2 Measured in Toluene at Room
Temperature
Ref-PDI
Sp-PDI2
λabs(A0−0), nm/cm−1
λabs(A0−1), nm/cm−1
λem, nm/cm−1
533/18761.73
495/20202.02
542/18450.18
95690
549/18214.94
509/19646.37
558/17921.15
123590
εmax, M−1/cm−1
A
Δv
ϕFl, %
0−0/A0−1
̃
1.74
9/311.50
97.7
1.60
9/293.79
90.20
Stokes, nm/cm−1
Optical Properties. The ground-state electronic properties
of Sp-PDI2 along with the reference compounds (Ref-PDI and
PDI) were explored by means of steady-state absorption
spectroscopy in toluene (c0 = 0.1−0.5 μM) at room
temperature (Figure 2 and Figure S3). The UV−visible
τFl, ns
4.17
6.62
In concurrence with the theoretical predictions, the UV−
visible absorption spectrum of Sp-PDI2, a Greek cross-shaped
molecule with two π-conjugated perylenediimide rings
embraced by a spiro-linker, reveals virtually unperturbed
spectral features as compared to the Ref-PDI. Though Sp-
PDI2 exhibits a monomeric spectral signature, the absorption
Abs
maximum is bathochromically shifted to 549 nm (λ
=
max
18 214.94 cm−1) with an extinction coefficient of 123 590
M−1cm−1. The observed redshift (546.79 cm−1), though not
insignificant, arises from the self-energy correction (ΔCT, vide
infra).48 A minor decrease in the ratio of the absorption band
intensities (A0−0/A0−1 = 1.60 (Sp-PDI2) and 1.74 (Ref-PDI))
and minimal absorption band broadening (Δv1/2
̃
= 721.35
cm−1 for Ref-PDI and Δv1/2
̃
= 746.17 cm−1 for Sp-PDI2)
suggest negligible exciton coupling among the orthogonally
connected PDI units in Sp-PDI2.
To get additional insights into the excitonic coupling of Sp-
PDI2, steady-state fluorescence spectroscopy was performed in
toluene (c0 = 0.1−0.5 μM) at room temperature (Figure 2 and
Figure S3). Photoexcitation of Ref-PDI shows a characteristic
fluorescence emission spectrum with a well-resolved vibronic
fine structure of PDI. The emission maximum of Ref-PDI
(λem) is bathochromically shifted to 542 nm (18 450.18 cm−1),
and a Stokes shift of 9 nm was observed. The fluorescence
quantum yield of Ref-PDI is quantified as 97.7%, which is
comparable to the reported value of PDI.33,64 Fascinatingly,
the fluorescence emission spectrum of Sp-PDI2 reveals a
monomer-like spectral signature with well-resolved vibronic
bands (Figure 2) with the peak maximum (λem) at 558 nm.
Furthermore, high fluorescence quantum yield (ϕFl = 90.20%)
Figure 2. Normalized absorption (solid black line) and emission
spectra (solid red line) of Ref-PDI (top) and Sp-PDI2 (bottom) in
toluene at room temperature.
and identical Stokes shift (ΔvStokes = 9 nm) reconfirm the
̃
exiguous exciton communication between the PDI monomers
and rule out the other possible deactivation channels (Table
1).
absorption spectrum of Ref-PDI shows characteristic spectral
signatures of monomeric PDI dye with a 10 nm bathochromic
shift (Figure S3). The absorption maximum of Ref-PDI
Theoretical Investigation. Having rationalized the
presence of null exciton coupling of Sp-PDI2 in toluene, we
explored the nature of excitonic interactions arising from
relative interchromophoric orientations by employing theoreti-
cal methods. The excitonic coupling (J) of the Sp-PDI2 was
scrutinized by evaluating the contributions from both long-
range and short-range effects20,48 (during the theoretical
observed at 533 nm (λ = 18 761.73 cm−1) with a molar
Abs
max
extinction coefficient (εmax) of 95 690 M−1 cm−1 corresponds
to the S0−S1 electronic transition (f = 0.72, Table S3). The
dipole-allowed electronic transition of Ref-PDI (S0−S1
transition) is strongly coupled to the vinyl stretching mode
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calculation, the spiro-carbon atom of the DFT-optimized Sp-
PDI2 molecule was replaced by four H atoms).
energy of the local Frenkel exciton state. Equation 2 holds
under the assumption that diabatic Frenkel and CT states are
energetically well separated.
The net exciton coupling (J) between neighboring molecules
derived from the interference between the long-range
Coulombic coupling (Jcoul) and the short-range charge-transfer
mediated coupling (JCT) is given by:68,69
The long-range Coulombic coupling between any two
chromophores arises from the interaction between their
transition charge distribution which is often calculated using
point-dipole approximation.65,66 Herein, we have employed the
transition charge from electrostatic potential (TrEsp) method
for calculating Coulombic coupling energy (Jcoul) between the
interacting molecules.67 The method provides a good result
when the interchromophore distance is shorter than the
chromophores size, where dipole−dipole approximation over-
estimates the exciton coupling energy. In the TrESP method,
the transition electrostatic potential is fitted to atomic partial
charges. The Coulomb interaction between the transition
charges of the different chromophores is equal to long-range
Coulombic coupling (Jcoul) and can be efficiently calculated
using eq 1:
J = J + JCT
(3)
coul
The hole and electron-transfer integral were calculated using
the energy-splitting dimer method (Table 2, Figure 3a, and
Figure 3b).70,71 Sp-PDI2 imparts a selective hole-transfer
coupling (th = −1185.63 cm−1, Figure 3b) owing to the
constructive interference between the HOMO of the
monomeric PDI molecule, whereas the destructive LUMO−
LUMO orbital interaction of the PDI unit resulted in the
minimal value of electron transfer coupling (te = −8.06 cm−1,
Figure 3a). Constructive and destructive interference of the pz
orbital around the spiro-linkage is depicted in Figure 3a and
Figure 3b (pz orbitals of these four carbon atoms are close
enough to interact through space). We further proceeded to
calculate the JCT value using eq 2. The energy difference
between the charge-transfer state (ECT) and the local Frenkel
exciton state (ES ) for Sp-PDI2 (ECT − ES = 0.875 eV) was
qi(1)qj(2)
|ri(1) − r(j2)
1
4πε0
J
=
∑ ∑
coul
|
i
j
(1)
Here, q(i m) represents the transition charge on the ith atom of
chromophore m; r(i m) corresponds to the position vector of the
respective transition charge; and ε0 is the vacuum permittivity
(Supporting Information, Section 1.2). Sp-PDI2 shows a
negligible long-range Coulombic coupling (Jcoul) value of 3.34
cm−1, reinforcing the hypothesis of minimum Coulombic
coupling between orthogonally (edge-to-edge) arranged
transition dipole vectors of PDI molecules (Table 2 and
Figure S4).
1
1
calculated using TheoDORE (vide infra) which is in good
agreement with the recent literature.72 The minimal magnitude
of CT coupling (JCT = −2.79 cm−1) obtained upholds the
insignificant nature of the charge-transfer coupling between the
orthogonally arranged PDI units (θ = 90ο) of Sp-PDI2 (Table
2). Thus, the spiro-conjugated perylenediimide dimer exhibits
null exciton coupling (J = −0.55 cm−1) due to negligible
contributions from Coulombic and CT-mediated couplings.
We further calculated the excitonic coupling of Sp-PDI2 by
employing a method developed by Spano and co-workers
based on the 0−0 and 0−1 absorption band intensities that
show exciton−vibrational coupling (Supporting Information,
Section 1.3).73,74 Excitonic coupling energy obtained by the
method is comparable with the one obtained by eq 3 (Table
2). The marginal disparity in excitonic coupling energy could
be a consequence of exciton coupling stemming from the small
rotational degree of freedom around the spiro-linkage of Greek
cross (+)-arranged Sp-PDI2 under the experimental conditions
(Figure S5).41,75,76 Therefore, the exciton coupling calculated
by the ratio of the intensity of the 0−0 and 0−1 absorption
band observed in the UV−visible absorption spectra and
theoretical methods restate the null excitonic coupling between
the orthogonally (edge-to-edge) arranged PDI chromophores.
To obtain further insight into the nature of different excited
states of Sp-PDI2, fragment-based excited-state analysis
developed by Plasser, implemented in TheoDORE, was
performed (Supporting Information, Section 1.4).77,78 Sp-
PDI2 has two nearly degenerate localized Frenkel states S1 and
S2 with equal oscillator strengths (f = 0.79), while the
degenerate S3 and S4 states (f = 0.06) possess pure CT state
character (S3 and S4 states with CT = 0.82), where the hole is
localized on one fragment and the electron is localized on the
other. The higher-energy virtual CT state (S3 and S4) and local
Frenkel exciton state (S1 and S2) are energetically well-
Table 2. Experimentally and Theoretically Calculated
Excitonic Coupling and Hole/Electron Transfer Integrals of
Sp-PDI2
(eV)
(cm−1
)
a
b
J
J
1.72 × 10−2
0.64 × 10−4
4.10 × 10−4
−1.03 × 10−3
−1.47 × 10−1
−3.46 × 10−4
139.13
0.55
3.34
−8.06
−1185.63
−2.79
c
Jcoul
te
th
d
JCT
a
Estimated from the intensity ratio of the A0−0 and A0−1 absorption
b
c
bands in the UV−vis spectra. Determined based on eq 3. Calculated
by the transition charge method based on eq 1. Determined based
d
on eq 2.
The charge-mediated short-range coupling stemming from
the wave function overlap between neighboring molecules has
a significant role in defining the effective excitonic
interaction.17,20,48 The LUMO−LUMO and HOMO−
HOMO orbital overlap, which can be substantial in the short
interchromophoric distance, enables a virtual high-energy
charge-transfer state between the molecules. The short-range
exciton coupling energy can be calculated by eq 268,69
teth
JCT = −2
ECT − E
S1
(2)
separated, and ECT − ES = 0.875 eV is calculated for Sp-PDI2
where te and th represent the electron and hole-transfer
integrals which depend on the LUMO−LUMO and HOMO−
HOMO orbital overlap of the monomer, respectively; ECT is
1
in toluene. The description is pictorially represented using an
electron−hole correlation plot (Table S4 and Figure S6).
Further, natural transition orbitals (NTO)79 and hole−
the energy of the virtual charge-transfer state; and ES is the
1
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Figure 3. Schematic of the Sp-PDI2 molecule with the pz orbitals around the spiro-link showing (a) destructive interference between LUMOs and
(b) constructive interference between HOMOs of the PDI molecule. (c) Top: schematic of the LUMO−LUMO orbital interaction of the PDI
dimer responsible for the electron transfer integral in different rotational angles. Middle: the electron and hole transfer integrals as a function of the
rotational angle between two orthogonally arranged PDI molecules. Bottom: schematic of the HOMO−HOMO orbital interaction of the PDI
dimer responsible for hole transfer integral in different rotational angles (representation of HOMO and LUMO molecular orbitals of PDI molecule
is restricted to the pz orbitals that are close enough to interact through space).
Figure 4. Energy level diagram of different singlet excited states of Sp-PDI2, depicting hole and electron transfer between the neighboring PDI
molecule and corresponding isosurface of hole (blue) and electron (yellow) distribution.
electron isosurface analyses80 of the S0 to Sn transition (n = 1,
2, 3, and 4) of Sp-PDI2 are portrayed in Figures S6, S7, and
Figure 4. Localization of hole−electron distribution in one of
the PDI monomers of Sp-PDI2 is observed for S0 → S1 and S0
→ S2 transitions. In contrast, S0 → S3 and S0 → S4 electronic
transitions hole−electron distribution isosurfaces show that the
hole and electron are localized on different PDI units of Sp-
PDI2. The excited-state characteristics of Sp-PDI2 in its
ground-state geometry resembles the bianthryl molecules
reported in the literature, where all excited states reflect the
symmetry of the molecular structure. However, transition
dipole vectors are arranged parallelly in bianthryl and
orthogonally in Sp-PDI2 (Figure S4).81,82
The short-range CT-mediated exciton coupling, or super-
exchange coupling, JCT, represent an energy-transfer process
that proceeds through a high-energy virtual CT state, as
depicted in Figure 4.20 In Sp-PDI2, the exciton localized
initially on a single PDI molecule dissociates by transferring a
hole (th = −1185.63 cm−1) to its neighbor PDI molecule,
resulting in creating the higher energy CT state. Electron
transfer is forbidden due to negligible electron transfer
coupling te = −8.06 cm−1. For energy transfer to the
neighboring molecule (JCT), the created CT state has to
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decay by transferring an electron to the neighboring molecule.
In contrast, because of the minimal magnitude of te, the CT
state decay to the initial local Frenkel exciton state by the back-
transfer of hole resulted in self-energy correction (ΔCT):20,48
depicts inverted-parabolic behavior as a function of intra-
molecular rotational angle (θ). As the θ decreases from 90°,
the electron-transfer coupling increases from te = −8.06 cm−1
to reach a maximum value of te = 924.21 cm−1 at 60°
(destructive interference between LUMOs of PDI changes to
constructive interference). Similarly, the rotational angle
dependence of short-range CT coupling, JCT, follows
inverted-parabola behavior and attains a maximum at ∼65°
(Figure 5). At the angle of ∼41°, both th and te become zero,
effectively shutting off charge-transfer coupling and creating an
aggregate driven solely by Coulombic couplings, i.e., an ideal
Kasha aggregate.
To shed light on the solvent polarity modulated photo-
physical properties of null exciton-coupled Sp-PDI2, we carried
out solvent-dependent absorption and fluorescence measure-
ments in solvents of different dielectric constants, i.e., toluene
(ε = 2.4), tetrahydrofuran (THF, ε = 7.6), and acetonitrile
(ACN, ε = 37.50) (Figures S8, S9, S10, and Table S7).83 The
line shapes of the absorption spectra of Sp-PDI2 in different
solvents have a strong resemblance. However, fluorescence
quantum yield decreases substantially, and the fluorescence
lifetime decay exhibits biexponential lifetimes as the solvent
polarity increases (Figure S8 and Table S7), which indicates
the existence of additional decay pathways such as SB-CS. In
high-polar solvents, such as ACN, the fluorescence is almost
quenched (ΦFl < 1%), and biexponential emission lifetimes are
observed (τ1Fl = 1.63 ns (15.75%) and τF2l = 6.87 ns (84.25%)).
However, in THF, moderately polar solvent, Sp-PDI2, exhibits
considerable fluorescence emission (ΦFl = 24%) and the
biexponential decay profile (τ1Fl = 14.2 ns (90.20%) and τF2l =
6.80 ns (9.80%)). The normalized emission spectrum of Sp-
PDI2 in polar solvents (Figure S9) is observed as broad and
distinct from that in toluene and not exhibiting any Stokes
shift, implying that the emission may have originated from
newly formed states. By virtue of the orthogonal arrangement
of PDI molecules and restricted rotation around the spiro-
linkage, the presence of emission from low-lying excimer and
relaxed singlet states can be ruled out.33,35 The nature of the
emissive states in THF and ACN has to be further explored,
employing transient absorption and emission measurement
(vide infra).
(te2 + th2)
ΔCT = −2
ECT − E
S1
(4)
ΔCT is universally negative whenever the virtual CT state is
higher in energy than the local Frenkel exciton. Calculated self-
energy correction (ΔCT = −396.34 cm−1) successfully
accounts for the spectral absorption shift of 546.94 cm−1
observed for the Sp-PDI2 in toluene. Interestingly, the spectral
absorption shift stemmed from ΔCT complicates Kasha’s
conventional classification of H- and J-aggregates.
To assess the effect of relative spatial orientation on the
Coulombic and charge-transfer coupling, rotational angle-
dependent Jcoul, te, th, and JCT were probed for orthogonally
arranged PDI monomers of Sp-PDI2 (spiro-carbon of the
optimized structure was replaced by four H atoms, Figure 3c,
Figure 5, Table S5, and Table S6). The PDI dimer with a
Figure 5. Variation of the modulus of Coulombic coupling |Jcoul| and
charge-transfer coupling (JCT) as a function of rotation angle (θ)
The rate constants (k) for SB-CS and charge recombination
(CR) were calculated through the classical expression of
Marcus electron transfer theory:84,85
between the perylenediimide monomers (for evaluating JCT; ECT − ES
1
= 0.875 eV was used as a constant).
VD2A
(ΔG + λ)2
4λkbT
i
y
2π
ℏ
j
z
rotational angle of 90° has a Jcoul value of 3.34 cm−1. Jcoul
drastically increased to 76.90 cm−1 for rotation of a PDI
monomer with respect to the other by about an angle of 5°
from the initial 90° cross-architecture. A linear increase of Jcoul
with the decrease of rotational angle is observed, and a
maximum value of 798.59 cm−1 reached at the rotational angle
of 40°, indicating the relatively more robust Coulomb-coupled
regime. On the other hand, the behavior of te, th, and JCT will
depend on the nodal pattern of the molecular orbitals involved.
j
z
z
k =
expj−
j
j
z
z
4πλkbT
(5)
k
{
According to eq 5, the nonadiabatic electron transfer rate
depends on the three variables that can be computed or
determined experimentally. The first Marcus parameter is the
Gibbs free energy change (ΔG) between the equilibrium
reactant and product states; the other Marcus parameters are
total reorganization energy (λ), which is the total energy
required to distort the equilibrium geometry of the reactant
state to the product state, and electronic coupling (VDA). The
magnitude of the electronic coupling matrix element indicates
the coupling strength between the initial and the final state.
The region in which the VDA is small is commonly referred to
as nonadiabatic electron transfer. The most profound feature
of the Marcus theory for charge separation is the so-called
inverted region effect; that is, the charge recombination
The hole transfer integral shows a significant value (th
=
−1185.63 cm−1) at the rotational angle of 90°, while the
electron transfer integral exhibits minimal magnitude (te =
−8.06 cm−1), reflecting the charge filtering effect in Sp-PDI2.
A linear decrease in th as a function of θ was observed and
resulted in th = 0 at ∼41°. The constructive interference
between HOMOs of PDI at 90° is gradually transforming to
destructive interference as θ reduces (Figure 3c). In contrast, te
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Figure 6. (Top) Femtosecond transient absorption spectra of Sp-PDI2 in (a) toluene, (b) THF, and (c) ACN showing the excited-state dynamics
after photoexcitation. (Middle) Species associated spectra reconstructed from target analysis of the A → GS model for toluene, the A ⇋ B → GS
model for THF, and the A → B → GS model for ACN, respectively, where A is 1*Sp-PDI2, B is the SB-CS state, and GS is the ground state (λex
440 nm). (Bottom) Relative population profiles of the excited state are fitted by kinetic models.
=
kinetics gets slowed down, given that the free energy changes
are more significant than the reorganization energy (λ).30,86
Energetics. The feasibility of the photoinduced symmetry-
breaking charge-transfer is determined with the Rehm−Weller
formulation87 based on the Born dielectric continuum model,
which estimates the energy gap between the optically excited
gives ΔGCS ≈ +0.37, −0.16, and −0.30 eV for Sp-PDI2 in
toluene, THF, and ACN, respectively. The estimated positive
free energy change of charge separation (ΔGCS ≈ +0.37) in
toluene would render SB-CS infeasible in nonpolar solvents
(vide infra). Calculated ΔGCS in toluene seems to be a bit
higher than expected which could be due to the overestimation
of polarity of nonpolar solvents by the Born dielectric
continuum model.88
state (E00) and the ion pair using the redox potentials (Eredox
)
and the Coulomb penalty for holding the charges at a fixed
distance. The optical bandgap (E00) was determined by the
crossing points of the normalized absorption and emission
spectra. The electrochemical bandgap (Eg) of Sp-PDI2 was
calculated by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) with the ferrocene/ferrocenium (Fc/Fc+)
couples as a reference in dry CH2Cl2 under a nitrogen
atmosphere (Figure S11 and Table S8). As previously
reported,60,61 the Ref-PDI exhibits two distinct reversible
reduction waves (E1(red) = −1.15 and E2(red) = −1.35 V) and
one quasi-reversible oxidation wave (E1(ox) = 1.20 V), like
PDI,33 whereas Sp-PDI2 shows four consecutive redox waves
SB-CS Dynamics. To unravel the excited-state dynamics
responsible for its efficient nonradiative decay, we carried out
solvent-dependent femtosecond transient absorption (fsTA)
measurement of Sp-PDI2 in toluene, THF, and ACN solvents.
The fsTA spectra of Sp-PDI2 (optical density = 0.2 to 0.3) are
shown in Figure 6. Upon photoexcitation at 440 nm, using an
∼100 fs laser pulse, the fsTA spectra of Sp-PDI2 in nitrogen-
purged toluene solution (Figure 6a) exhibit the characteristic
features of the singlet excited state of a perylenediimide
chromophore (Figure S14).33,35 The fsTA spectra display
negative ground-state bleach (GSB)/stimulated emission
bands (SE) at ∼465 to 626 nm, alongside the positive broad
excited-state absorption (ESA) features are observed, with
peaks at 651, 733, and 755 nm. The SB-CS process is switched
off in toluene because the charge-separated state has higher
energy than the excited singlet (ΔGCS ≈+0.37 eV). On the
other hand, Sp-PDI2 in THF (εs = 7.6, ΔGCS ≈ −0.16 eV)
shows remarkably different excited-state dynamics, as shown in
(E1(red) = −0.99, E2(red) = −1.02, E3(red) = −1.21, and E4(red)
=
−1.33 V) and a single oxidation wave at 1.23 V. The distinct
redox properties of Sp-PDI2 conforms the electronic
communication between the orthogonally arranged PDI
chromophores. The near degeneracy of first and second
reduction potentials (−0.99, −1.02 V) of Sp-PDI2 can be
attributed to the destructive interference between the LUMO
of the PDI monomer and is consistent with the calculated te
value (Figures S12 and S13). Thus, Rehm−Weller treatment
1
Figure 6b, characterized by fast decay of the Sp-PDI2* local
singlet state (GSB was seen as a negative band extending from
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Table 3. Time Constants (τ), Rate Constants (k), Driving forces (ΔG), Reorganization Energy (λ), and Activation Barrier
(ΔG#) for Symmetry-Breaking Charge Separation (SB-CS), Back Charge Transfer (BCT), and Charge Recombination (CR) in
a
Sp-PDI2
τCS
kCS × 1010
(s−1
ΔG
λCS
τBCT
kBCT × 108
(s−1
τCR
kCR × 108
ΔG
ΔGCR
c
λCR
ΔG#
CbS
CbR
solvent
(ps)
)
(eV)
(eV)
(ps)
)
(ns)
(s−1
-
)
(eV)
(eV)
(eV)
(eV)
toluene
THF
ACN
-
-
+0.37
−0.16
−0.30
0.24
0.57
0.96
-
-
-
−2.60
−2.08
−1.94
−2.70
−2.41
−2.30
0.48
0.81
1.20
0.38
0.18
0.11
2.96
0.627
33.78
159.48
132
-
75.75
-
12.20
1.66
0.82
6.02
a
b
τ and k = 1/τ from target fits of visible fsTA spectra. ΔG for CS and CR from Rehm−Weller analysis. Constrained density functional
c
computation. λ for CS and CR calculated based on eqs 6, 7, and 8.
∼465 to 585 nm, accompanied by a positive ESA band in the
∼608−770 nm range) with the evolution of a broad new
transient species characterized by positive features in the
visible region at ∼590, 699, and 731 nm. The newly emerging
ESA signature serves as a direct measure of the SB-CS in THF;
however, the characteristic signature of the PDI radical cation
peak at ∼730 nm is not evident in ESA as compared to the
radical anion peak of PDI at ∼587 nm.35,89 The intensity of the
ESA band at ∼650−750 nm is almost intact during the SB-CS,
which could be due to the presence of an equilibrium between
the singlet excited state and CS state. The solvation of Sp-PDI2
by moderately polar THF reduces the energy of the CS state
close to the local singlet excited state. A similar equilibrium
between the excited singlet state and the SB-CS state of zinc
dipyrrins, 9,9′-bianthryl, and PDI dimers was documented in
the literature in weakly polar solvents.35,40,41
Further, we performed fsTA measurements of Sp-PDI2 in
more polar ACN (εs = 37.50) to probe the effect of solvent
polarity on charge separation, where SB-CS is more energeti-
cally feasible, ΔGCS ≈ −0.30 eV (Figure 6c). The dynamics of
the photoexcited dimers in ACN was similar to that observed
in THF and displays a broad positive band (ESA) around
∼657 nm and pronounced negative bands after photo-
excitation at the initial time (∼465−567 nm). Subsequently,
these bands decay rapidly by a concomitant growth of two new
absorption bands at around ∼581 and ∼727 nm corresponding
to the spectral features of the PDI radical cation and radical
anion, respectively.35,89
→ GS) was employed. The forward and backward charge-
transfer rate constants (kA→B and kA←B) between the locally
excited state and CS states of Sp-PDI2 in THF are (2.96 ps)−1
and (132 ps)−1, respectively. In ACN, the excited singlet state
(A) of Sp-PDI2 decays within a rate constant of kA→B = (627
fs)−1, which initiates the ultrafast growth of a new species
characterized by positive features in the visible region (∼565 to
775 nm). The spectroscopic signature of the newly formed
species in ACN and THF persists across the entire time
window of the transient absorption measurement (3.5 ns).
Subsequently, these charge-separated states decay by charge
recombination (CR) to the ground state with rate constants of
kB→GS = (1.66 ns)−1 and (12.20 ns)−1 for ACN and THF,
respectively, with no evidence of triplet excited state. An
isosbestic point observed at ∼625 and 634 nm for ACN and
THF, respectively, indicates the transformation of a singlet
state to the prolonged charge-separated state. The observed
ultrafast transient dynamics show photoinduced intramolecular
SB-CS behavior of Sp-PDI2 in a polar solvent, consistent with
its quenched fluorescence in polar solvents (THF and ACN).
The CS and CR rate constants (k) and time constants (τ) for
Sp-PDI2 in different solvents are summarized in Table 3. The
target fitted time constants of charge separation and
recombination are τCS = ∼2.96 ps and τCR = ∼12.20 ns in
THF, respectively, while the corresponding time constants are
somewhat shorter in more polar ACN (τCS = 0.627 ps and τCR
= ∼1.66 ns).
τCR obtained from target analysis can be employed to explain
the fluorescence emission profile and fluorescence lifetime
decay of Sp-PDI2 in THF and ACN. In polar solvents, Sp-
PDI2 displays biexponential decay (Figure S8 and Table S7),
and the fluorescence lifetime of the first component (τ1Fl = 1.63
and 14.2 ns for ACN and THF, respectively) is comparable
with the charge recombination lifetime of Sp-PDI2 (τCR = 1.66
and 12.20 ns for ACN and THF, respectively). Thus, the
fluorescence emission of the first component can be assigned
to emission originated from the SB-CS state → ground state
(GS) transition. Analogous radiative emissions from SB-CS to
GS transition are reported for DIPYR dimers and 9,9′-
bianthryl and 10,10′-dicyano-9,9′-bianthryl molecules.38,41
Since the locally excited state of Sp-PDI2 in polar solvents
decays within a few picoseconds (627 fs and 2.96 ps in ACN
and THF, respectively), the absence of a short lifetime
component in the TCSPC lifetime measurements suggests the
nonemissive nature of a locally excited state (Figure S8). The
symmetry-breaking charge separation of Sp-PDI2 in polar
solvents results from the asymmetric distribution of polar-
izabilities around the Greek cross-arranged molecule. The
origin of emissive second species having τ2Fl = 6.87 and 6.80 ns
for ACN and THF, respectively, can be explained by the
presence of different microenvironments around the photo-
To extract the kinetic components responsible for these
transformations in the total spectra, a singular value
decomposition (SVD) followed by target analysis (using A
→ GS or A → B → GS or A ⇋ B → GS kinetic models) of the
time versus wavelength-based three-dimensional map of fsTA
spectra was executed.90,91 Selected kinetic trace superimposed
target analysis fitted curves at different wavelengths are chosen
to display the fitting qualities as shown in Figures S15, S16,
and S17. In the case of Sp-PDI2 in toluene, species associated
spectra (SAS) show a single principal component correspond-
ing to the locally excited singlet state (A) (S1−Sn electronic
transitions). The lowest singlet excited state (1Sp-PDI2*) does
not decay entirely to the ground state (GS) within the given
experimental time scale, plausibly due to the long singlet
lifetime (fitted lifetime, τA = 6.95 ns), consistent with the
fluorescence decay of Sp-PDI2 in toluene (τFl = 6.62 ns).
However, in polar solvents, THF, and ACN, species associated
spectra (SAS) of Sp-PDI2 showed two principal components
that are ascribed to the locally excited singlet state (A) and the
SB-CS state (B, Figure S18). The fs-TA data of Sp-PDI2 in
THF were fitted with a target model (A ⇋ B → GS) to
account for an equilibrium process between the LE state and
CS state, whereas in ACN, the sequential target model (A → B
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excited molecules; i.e., a photoexcited donor/acceptor dyad
undergoes charge separation in some environments, and in
another microenvironment, it may not undergo the charge
separation.92,93
The first term corresponds to the energy calculated at the
optimized CS geometry in the GS state, and the second term is
the energy of the GS at their equilibrium geometries.
Calculated λi(CR) = 0.44 eV (Figure S21 and Table S9, and
Supporting Information, section 1.10). The solvent reorganiza-
tion energy is computed using the dielectric continuum theory
and depends on the radii of the donor (rD) and acceptor (rA),
the center-to-center distance (rDA), and the optical (εop) and
static (εs) dielectric constants of the solvent as given in eq 8:84
The emissive nature of the charge-separated state of Sp-
PDI2 in THF can further be substantiated by the picosecond
time-resolved emission spectra (TRES) measurement, fol-
lowed by global analysis (GA) of the kinetic traces (Figures
S19 and S20 and Supporting Information, section 1.9). The
GA of time versus wavelength plots gave a DAS-based
deconvolution of the temporal components in fluorescence
emission. The DAS and population profile of Sp-PDI2 in THF
reveal the presence of two independent emissive species (τA =
5.8 ns and τB = 13.9 ns). The spectral character of the long-
lived species (τB = 13.9 ns) is assigned to CS → GS emission
which is red-shifted to the LE → GS emission feature (τA = 5.8
ns) in THF where the microenvironmental effect does not
favor the charge separation.
Evaluation Using Electron Transfer Theory. Since the
nature of Sp-PDI2 excited states is different in the diverse
solvent environment (toluene, THF, and ACN), discussion of
SB-CS and CR dynamics in terms of classical Marcus theory is
not straightforward.81,82 Though SB-CS is a part of the general
electron transfer reaction and realizations of electron transfer
theory can be embraced to describe the dynamics of SB-CS
and CR.26,30,94 More quantitative analysis of CS and CR
kinetics requires estimation of Marcus parameters (ΔG, λ, and
V) and the activation barrier (ΔG#) of Sp-PDI2. Driving forces
(i.e., Gibbs free energy changes) of charge separation (ΔGCS)
and charge recombination (ΔGCR) were estimated by the
Weller approximation (Table 3 and Supporting Information,
section 1.7).87 The Gibbs free-energy change of charge
separation with negative values shows that SB-CS is
thermodynamically allowed for Sp-PDI2 in both THF and
ACN solvents, which agrees with the fluorescence quenching
behavior. For Sp-PDI2, in moderate polar aprotic solvent THF,
the value of ΔGCS is −0.16 eV, and in polar solvent
acetonitrile, Gibbs free energy changes of SB-CS become
more negative (ΔGCS = −0.30 eV); i.e., the driving forces for
charge separation are more prominent with the increasing
solvent polarity. In contrast, Gibbs free energy changes of CR
in ACN (ΔGCR = −1.94 eV) are less negative than THF
(ΔGCR = −2.08 eV); the driving forces for charge
recombination are smaller with the increasing solvent polarity.
From the Marcus eq 5, it is evident that the reorganization
energy (λ) plays a crucial role in charge separation kinetics.
Reorganization energy is composed of the solvent shell
reorganization energy (λs) and the solute molecular structure
e2
1
1
2rD
1
1
1
εs
i
j
j
j
j
j
j
y
z
z
z
z
z
z
i
j
j
j
j
j
y
z
z
z
z
z
λS =
+
−
−
4πε0 2rA
rDA εop
k
{
(8)
k
{
Sp-PDI2 is a rigid organic molecule for which theoretically
calculated low internal reorganization energy (λi(CS) = 0.20 eV
and λi(CR) = 0.44 eV) is reasonable.30 Thus, the total
reorganization energy for the charge recombination (λCR)
process would be 1.20 eV for ACN, which is approximately
0.74 eV lower than −ΔGCR. The charge recombination of Sp-
PDI2 in ACN falls under the inverted region (where −ΔGCR
λCR), whereas in THF, the total reorganization energy (λCR
>
=
0.81 eV) is 1.27 eV lower than −ΔGCR (2.08 eV), pushing the
system into the deeper inverted region compared to ACN
(Table 3). This could rationalize the slow charge recombina-
tion of Sp-PDI2 in moderately polar THF (τCR = 12.20 ns)
than more polar solvent ACN (τCR = 1.66 ns). Several research
groups unequivocally established the presence of an inverted
region in charge recombination kinetics.99−102 Theoretically
calculated ΔGCR for different solvents is in good agreement
with the magnitude estimated by Weller analysis (Table 3 and
Supporting Information, section 1.10).
Another factor controlling the kCS and kCR is the electronic
coupling matrix element (V). This term is used to quantify the
mixing of two states or, more intuitively, the electronic
communication between the electron/hole donor and accept-
or. Theoretically calculated charge-transfer integrals provide an
estimation of effective electronic coupling (V). The magnitude
of hole transfer coupling (th = −0.147 eV) is significantly
higher than that of electron transfer coupling (te = −0.001 eV),
facilitating the excited state SB-CS of Sp-PDI2 in polar solvents
via hole transfer (Figure S22). The effective hole transfer
electronic coupling (VCS = −0.147 eV) can explain the origin
of the significantly accelerated charge separation rate in THF
and ACN. Calculated electronic coupling of CR (VCR
=
−0.018 eV) is much lower than CS (VCS = −0.147 eV),
supporting the decelerated CR processes over the CS of Sp-
PDI2 in THF and ACN. Since the 2V < λ, SB-CS in Sp-PDI2
falls under a weak coupling regime.103 A remarkable feature of
Marcus theory is the relation between the kinetic barrier of
charge-transfer and the reaction thermodynamics. When the
reaction is more exergonic in nature, the barrier for electron
transfer reduces. The activation barrier for the SB-CS is
calculated by eq 9:
reorganization energy upon charge separation (λi(CS)) and
95−97
charge recombination λi(CR)
.
Internal reorganization
energy (λi) was calculated by employing constrained DFT
for the charge-separated state and DFT for the ground-state
calculation.98
(ΔGCS + λ)2
ΔG# =
CS
λi(CS) = ECS(ReGqS) − ECS(R
)
eq
(6)
(9)
4λ
The first term is the energy calculated at the optimized ground
state (GS) geometry in the CS redox state, while the second
term is the energies of the CS at their equilibrium geometries.
This calculation provides λi(CS) = 0.20 eV.
The reorganization energy has a strong effect on ΔG#. The
estimated activation barrier in different solvents is given in
Table 3.
The ultrafast charge separation of Sp-PDI2 in ACN (kCS
=
159.48 × 1010 s−1) compared to the THF (kCS = 33.78 × 1010
GS
eq
λi(CR) = EGS(ReCqS) − EGS(R
)
s−1) solvent stems from the low activation energy for a
(7)
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nonadiabatic reaction. ΔG# for the more polar ACN solvent
has amounted to 0.11 eV, and moderate polar THF is 0.18 eV.
Estimated Marcus parameters and activation barriers are given
in Table 3. The potential energy surfaces are shown
schematically in Figure 7.
analysis of Sp-PDI2 exhibited localized Frenkel exciton in the
first two degenerate excited states (S1 and S2) and charge-
transfer (CT) character in the degenerate S3 and S4 states. The
rotational angle-dependent Coulombic and charge-transfer
coupling calculations provided significant values for lower
rotational angles, manifesting impeccable evidence for the
ability of Greek cross (+)-architecture in lowering excitonic
coupling. The solvent-dependent fsTA measurement of Sp-
PDI2 demonstrated the SB-CS in polar solvents. The charge
separation reactions of Sp-PDI2 in polar solvents are in the
Marcus normal region (ΔGCS < λ), and kCS values follow the
expected quadratic dependence on ΔGCS; i.e., kCS increases
with increment in CS driving forces. The lower activation
barrier (ΔG#) for CS in ACN than THF further supported that
k
CS (ACN) > kCS (THF). The CR reactions of Sp-PDI2 lie far
into the Marcus inverted region (ΔGCS > λ). Solvent-
dependent studies demonstrate the rate of CR in polar
solvents following the order ACN > THF, with CR becoming
slower at higher driving forces. The current work integrates
two fundamental concepts of photophysics, null exciton
splitting and symmetry-breaking charge separation, as an
innovative strategy to generate an efficient long-lived charge-
separated state. Greek cross (+)-arrangement of Sp-PDI2 can
be considered as an ideal system that highlights the explicit
correlation between the relative orientation of the chromo-
phoric system and its orbital interaction in dictating complex
excited-state dynamics. Further efforts are underway to achieve
a more prolonged charge-separated state in near-orthogonal
arranged PDI dimers.
ASSOCIATED CONTENT
■
sı
* Supporting Information
Figure 7. Schematic potential energy diagrams for SB-CS of Sp-PDI2
in different solvents.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c05793.
Experimental and computational methods, CCDC
number, tables, and figures (PDF)
Chemical redox titration experiments using cobaltocene and
antimony pentachloride were performed to confirm the charge-
separated state features of Sp-PDI2 observed in the femto-
second transient absorption spectroscopy. Upon addition of
reducing agent cobaltocene (CoCp2), a decrease in the neutral-
state absorption band is accompanied by the appearance of
new bands at ∼715 nm (Figure S23). On the other hand,
partial oxidation of Sp-PDI2 with antimony pentachloride
(SbCl5) results in positive features at ∼574 nm (Figure S24).
Accession Codes
CCDC 2086823 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
AUTHOR INFORMATION
■
To conclude, we report the unequivocal evidence for the
prolonged SB-CS state (kCS/kCR = 2647 in ACN) (Table S1)
in a null exciton-coupled spiro-conjugated perylenediimide
dimer (Sp-PDI2). Single-crystal structure examination of Sp-
PDI2 confirms the Greek cross (+)-shaped molecule with two
π-conjugated perylenediimide chromophores linked by an sp3-
hybridized carbon atom. The monomer-like steady-state
absorption spectrum and photoexcited-state properties of Sp-
PDI2 in toluene advocate negligible exciton coupling among
the orthogonally arranged PDI molecules. The interference
between the theoretically calculated negligible contributions of
Jcoul and JCT resulted in null exciton coupling of an edge-to-
edge arranged perylenediimide dimer possessing orthogonal
orientation of transition dipoles. Sp-PDI2 imparts selective
hole-transfer coupling owing to the constructive and
destructive interaction between the HOMOs and LUMOs of
PDI molecules, respectively. The fragment-based excited-state
Corresponding Author
Mahesh Hariharan − School of Chemistry, Indian Institute of
Science Education and Research Thiruvananthapuram
(IISER TVM), Thiruvananthapuram 695551 Kerala,
India; orcid.org/0000-0002-3237-6235;
Email: mahesh@iisertvm.ac.in
Author
Ebin Sebastian − School of Chemistry, Indian Institute of
Science Education and Research Thiruvananthapuram
(IISER TVM), Thiruvananthapuram 695551 Kerala, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c05793
Notes
The authors declare no competing financial interest.
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and Polymers: A Molecular Picture. Chem. Rev. 2004, 104, 4971−
5003.
ACKNOWLEDGMENTS
■
M.H. acknowledges the Science and Engineering Research
Board, Department of Science and Technology, Govt. of India,
for the support of this work, CRG/2019/002119. E.S.
acknowledges UGC for financial assistance. We thank Mr.
Alex P. Andrews for the X-ray diffraction analyses.
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