Concerted Interplay of Excimer and Dipole Coupling Governs the Exciton Relaxation Dynamics in Crystalline Anthracenes

Article abstract of DOI:10.1002/chem.201804139

A combined theoretical and experimental investigation into the role of concerted long- (dipole coupling) and short-range (orbital overlap mediated excimer) electronic interactions in modulating the emission of six crystalline acetylanthracenes (1–3) is reported. Friedel–Crafts acylation of anthracene rendered crystalline acetylanthracenes with discrete close packing, varied orbital overlap, and resultant distinct emission (blue–green–yellow) from cooperative excimer and dipole coupling. Time-resolved emission spectroscopy (TRES) studies and the Kasha's exciton theory based quantitative estimation of dipole coupling (mean-field approximation) substantiates the exciton dynamics in crystalline 1–3. Extension of the Kasha's exciton model beyond the traditional nearest-neighbor approach, and consistent agreement among the computed spectral shifts and TRES temporal components, corroborate a holistic approach to decipher the exciton relaxation dynamics in the molecular assembly of novel photonic materials.

Full text of DOI:10.1002/chem.201804139

DOI: 10.1002/chem.201804139
Full Paper
&
Exciton Dynamics |Hot Paper|
Concerted Interplay of Excimer and Dipole Coupling Governs the
Exciton Relaxation Dynamics in Crystalline Anthracenes
Abbey M. Philip,[a] Sreenath K. Manikandan,[b, c] Anil Shaji,*[b] and Mahesh Hariharan*[a]
Abstract: A combined theoretical and experimental investi-
gation into the role of concerted long- (dipole coupling) and
short-range (orbital overlap mediated excimer) electronic in-
teractions in modulating the emission of six crystalline acety-
lanthracenes (1–3) is reported. Friedel–Crafts acylation of an-
thracene rendered crystalline acetylanthracenes with discrete
close packing, varied orbital overlap, and resultant distinct
emission (blue–green–yellow) from cooperative excimer and
dipole coupling. Time-resolved emission spectroscopy (TRES)
studies and the Kasha’s exciton theory based quantitative es-
timation of dipole coupling (mean-field approximation) sub-
stantiates the exciton dynamics in crystalline 1–3. Extension
of the Kasha’s exciton model beyond the traditional nearest-
neighbor approach, and consistent agreement among the
computed spectral shifts and TRES temporal components,
corroborate a holistic approach to decipher the exciton re-
laxation dynamics in the molecular assembly of novel pho-
tonic materials.
Introduction
neighbor approach is highly pursued, it is implausible to ne-
glect the contribution of the non-nearest neighbors. A com-
prehensive assessment of exciton coupling in molecular aggre-
gates, accounting for the influence of all nearest and non-near-
est neighbors, remains elusive.
Unraveling the fate of excitons in p-chromophoric aggregates
is imperative for the evolution of novel organic electronic ma-
terials.[1] Photonic properties of organic chromophoric assem-
blies archetypally rely on the relative orientation and 3D close
packing of constituent chromophoric units.[2] Congruently, in-
vestigations into the structure–packing–optical property rela-
tionship in molecular aggregates have received great attention
from theoretical[3] and experimental[2a,4] perspectives. Hitherto,
exciton dynamics[5] in the bulk of supramolecular ensembles
have predominantly been rationalized through the Kasha’s ex-
citon model (nearest-neighbor/periodic dimer model).[3a,6] De-
pending on the relative dipole orientation of the adjacent
chromophores, the excited state of the monomer splits to facil-
itate the transition to either upper (H-aggregate; cofacial,
head-to-head dipoles) or the lower (J-aggregate; staggered,
head-to-tail dipoles) excited states.[3a,c] Although the nearest-
Alternatively, cofacial molecular assemblies with prominent
p-orbital overlap can render excited di-/oligomers (excimer)[7]
and multimers[8] at short distances. Concurrently, the fluores-
cence emission can be readily tuned, depending on the extent
of p-orbital overlap between the vicinal arene multimers.[8,9] Al-
though excimers are undesired due to dramatic fluorescence
quenching,[10] a comprehensive account of exciton relaxation
dynamics between the Franck–Condon, Frenkel exciton, and
excimer trap states is yet to be fully understood.[11] Conse-
quently, the precise impact of short- (orbital overlap mediated
excimer) and long-range (Coulomb/dipole) couplings on the
optical properties of a molecular ensemble demands utmost
attention from a fundamental and technological overview.[12]
Herein, we attempt to deconvolute the structure–packing–op-
tical property relationship and the concurrent exciton relaxa-
tion dynamics through combined experimental/theoretical esti-
mations of excimer/dipole shifts in crystalline anthracenes.
Our continuing efforts to explore the emergent optical[9,13]
and photoexcited-state[14] properties of molecular aggregates
encouraged us to investigate the structure versus optical prop-
erty relation in a series of crystalline acetylanthracenes (1–3).
Classical Friedel–Crafts acylation[15] of anthracene afforded crys-
talline acetylanthracenes with discrete molecular structures,
varied close-packing motifs, and diverse solid-state emission
(blue–green–yellow; Scheme 1). To deconvolute the excimer/
dipole coupling mediated exciton relaxation dynamics,[12]
quantitative evaluation of Coulomb interactions between the
nearest and non-nearest neighbors were carried out through
the Kasha’s exciton coupling (mean-field approximation) for-
[a] A. M. Philip, Dr. M. Hariharan
School of Chemistry, Indian Institute of Science Education
and Research Thiruvananthapuram (IISER-TVM)
Maruthamala P. O., Vithura
Thiruvananthapuram, Kerala, 695551 (India)
E-mail: mahesh@iisertvm.ac.in
[b] S. K. Manikandan, Dr. A. Shaji
School of Physics, Indian Institute of Science Education
and Research Thiruvananthapuram (IISER-TVM)
Maruthamala P. O., Vithura
Thiruvananthapuram, Kerala, 695551 (India)
E-mail: shaji@iisertvm.ac.in
[c] S. K. Manikandan
Present Address: Department of Physics and Astronomy
University of Rochester, Rochester, NY 14627 (USA)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201804139.
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Scheme 1. Chemical and molecular structures, structural formulas, stacking, percentage overlap, and the distance (d) between the nearest p-stacked anthra-
cenes in a) 1A, b) 1B, c) 2A, d) 2B, e) 2C, and f) 3 with crystal images of the derivatives under ambient (left) and under UV illumination (l=354 nm, right).
d=distance between the nearest-neighbors; n.d. =not determinable due to oblique stacking.
malism. Estimated mean values of the dipole couplings gave a
quantitative description of dipole and excimer shifts in crystal-
line 1–3. Additionally, the time-resolved emission spectroscopy
(TRES) temporal components of the fluorescence emission
complement the numerically computed excimer shifts. Good
agreement between the theoretically computed spectral shifts
and the experimentally evaluated TRES temporal components
reveal the importance of an all-inclusive approach for narrating
the spectral characteristics of novel organic electronic materi-
als.
and Tables S1–S2 in the Supporting Information). Derivatives
1A, 1B, and 2C were crystallized by slow evaporation of a mix-
ture of CHCl3/n-hexane, whereas 2A, 2B, and 3 crystallized
from a mixture of ethyl acetate/n-hexane. To gain insights into
the structure–property relationship, close-packing analyses of
crystalline 1–3 were performed. Derivatives 1–3 displayed four
distinct packing arrangements of the anthracene units in the
crystal lattice (Figure 1). Compounds 1A and 1B display a her-
ringbone packing, which is typically observed in polyaromatic
hydrocarbons, wherein the anthracene units are arranged in an
edge-to-face manner aided by CꢀH···p interactions (Figure 1a,b
and Figures S1–S2a,b in the Supporting Information). The
close packing in 2A reveal a 1D lamellar/slip-stacked arrange-
ment, in which the slip stacks of anthracene propagate in an
end-to-face manner with p overlap of 24.4% between the
Results and Discussion
The syntheses, experimental details, and crystallographic data
for 1–3 are provided in the Supporting Information (Scheme S1
Figure 1. Close-packing arrangement in the acetylanthracene derivatives a) 1A, b) 1B, c) 2A, d) 2B, e) 2C, and f) 3, indicating the values of 1[(%C···H)/
(%C···C)]. Herringbone (1>4.5), sandwich herringbone (3.2<1<4.0), g (1.2<1<2.7), and b (0.46<1<1.0).
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nearest anthracenes units (Scheme 1c, Figure 1c, and Figur-
es S1–S2c and Table S3 in the Supporting Information). The
packing motif in 2B resembles a pile of bricks, a 2D lamellar/
brickwork arrangement, with an average interplanar distance
of 3.65 ꢁ and a p overlap of 15.4% among the stacked anthra-
cene units (Scheme 1d, Figure 1d, and Figures S1–S2d in the
Supporting Information). Derivative 2C exhibits a 2D columnar
arrangement in which the anthracene units are stacked in an
antiparallel fashion, with an interplanar distance and p overlap
of 3.52 ꢁ and 49.6%, respectively (Scheme 1e, Figure 1e, and
Figures S1–S2e in the Supporting Information). The triacetylan-
thracene derivative (3) forms a 2D-lamellar arrangement in
which the p-stacked (71.8% overlap) antiparallel sandwich
dimers propagate in a slip-stacked array (Scheme 1 f, Figure 1 f,
and Figures S1–S2 f in the Supporting Information).
Furthermore, Hirshfeld surface (HS) analyses[16] (Figures S3–
S4 in the Supporting Information) were carried out to probe
the percentage of weak intermolecular interactions present in
crystalline 1–3.[9a,17] Evaluation of the packing motif of anthra-
cene units in the crystalline state was carried out by calculating
the 1 value (1=(%C···H)/(%C···C); Figure 1, Table S4 in the Sup-
porting Information) for crystalline derivatives 1–3, as reported
earlier.[9a,17,18] To estimate the accurate longitudinal and trans-
verse slips among the nearest and non-nearest neighbors,
pitch and roll displacement analyses were carried out in crys-
talline 1–3 (Figures S5–S10 and Table S5 in the Supporting In-
formation).[19] Compared with a perfect cofacial motif, pitch ex-
presses the displacement of the molecule along the long mo-
lecular axis, whereas the roll outlines the displacement along
the short molecular axis. A large roll translation can considera-
bly impede the p overlap, as seen in herringbone motifs 1A
and 1B (Figures S6–S7 in the Supporting Information). Alterna-
tively, larger pitch displacements depict slip along the long
molecular axis, creating a staircase and/or brickwork-type chro-
mophoric assembly, while maintaining p overlap, as observed
in lamellar motifs 2A and 2B (Figures S5 and S8 in the Sup-
porting Information). Derivatives 2C and 3 exhibit small pitch
and roll displacements/inclinations, thereby facilitating a much
larger electronic coupling and p-orbital overlap among the
nearest anthracene units in the packing motif (Figures S9–S10
and Table S5 in the Supporting Information).
Figure 2. Normalized fluorescence excitation (solid lines) and emission
(dashed lines) spectra of a) 1A, b) 1B, c) 2A, d) 2B, e) 2C, and f) 3 in CHCl3
(shaded blue, lex =350 nm) and crystalline states (shaded red, lex =360 nm).
Excitation spectra were monitored at l=474 and 490 (1A), 450 and 513
(1B), 450 and 555 (2A), 461 and 487 (2B), 451 and 538 (2C), and 460 and
550 nm (3) for solution and crystalline states, respectively.
display a similar trend (Figure S11b in the Supporting Informa-
tion), and the excitation spectra monitored at individual emis-
sion wavelengths reveal no perturbation in the ground-state
properties (Figure S12 in the Supporting Information). Picosec-
ond time-resolved fluorescence (lex =377 nm) measurements
indicate a decrease in the lifetimes of 1–3 with the progressive
addition of acetyl group(s) (Figure S13 and Table S6 in the Sup-
To investigate the preliminary photophysical characteristics
of derivatives 1–3, steady-state and time-resolved experiments
were carried out in isotropic solutions (16–35 mm; Figure 2 and
Table S6 in the Supporting Information). Derivatives 1–3 exhibit
pale yellow or yellow colors in dilute solutions in chloroform
with blue/blue–green emission (Figure 2 and Figure S11 in the
Supporting Information). Depending on the position of acetyl
substitution, derivatives 1–3 display a redshifted absorption
with characteristic vibronic features as compared to the bare
anthracene chromophore (Figure S11a in the Supporting Infor-
mation). Although a change in the vibronic character was ob-
served, successive addition of acetyl group(s) revealed insignifi-
cant bathochromic shift in the ground-state absorption, plausi-
bly due to limited conjugation arising from rotational motion
of the flanking acetyl group(s) in solution. Upon excitation at
l=350 nm, the fluorescence emission of 1–3 in chloroform
porting Information). The fluorescence quantum yield (f) of
f
1–3 also exhibits a similar trend, which indicates the accessibil-
ity of an alternative decay pathway (knr ꢁkisc @kr) with the suc-
cessive addition of acetyl group(s) (Table S6 in the Supporting
Information).[9a,20]
Interestingly, the excited-state features of crystalline 1–3
reveal specific trends, depending on the molecular close pack-
ing and p overlap among the nearest and non-nearest neigh-
bors (Scheme 1 and Figure 1). Crystalline derivatives 1–3 exhib-
it pale yellow–yellow color under ambient conditions, as
shown in Scheme 1. Diffuse reflectance and excitation spectra
of crystalline 1–3 exhibit redshifted, broad absorption as com-
pared to the isotropic solutions (Figure 2 and Figure S14 in the
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Supporting Information). The redshifted, broad absorbance
and excitation spectra in crystalline arenes can be a conse-
quence of interplay among the nonresonant and excited-state
resonant electronic interactions.[3b,c,21] Upon excitation at l=
360 nm, crystalline 1–3 reveal distinct fluorescence (blue–
green–yellow) signatures that stem from long-range Coulomb
interactions among the discrete anthracene motifs in the crys-
talline ensemble (Figure 2a–f and Figure S15 in the Supporting
Information). Additionally, the cofacial di-/oligomers can cata-
lyze excimer formation at short stacking distances,[22] thereby
entailing an independent investigation of concerted long-/
short-range electronic interactions in crystalline 1–3.
bands at l=485, 527, and 592 nm (Figure 2d). The fluores-
cence excitation spectra recorded at different wavelengths
reveal the presence of a marginal ground-state association
(Figure S16d in the Supporting Information). Picosecond time-
resolved fluorescence decays monitored at the emission
maxima display lifetimes similar to those in isotropic solution
with an additional long-lived species (Figure S17 and Table S6
in the Supporting Information). Slow fluorescence decay in
crystalline 2A and 2B, relative to that in monomeric solution
and upon monitoring at the red edge of the emission, evokes
an excimer-like character in the molecular ensemble (Figur-
es S17c,d and S18c,d in the Supporting Information).
The herringbone assembly in crystalline 1A exhibits a vi-
bronically resolved cyan emission centered at l=466 nm and
diffuse bands at l=492, 536, and 568 nm (Figure 2a). Compa-
rable fluorescence emissions and quantum yields in isotropic
solution and crystalline states suggest the monomer-like be-
havior of 1A in the interpenetrated zigzag herringbone assem-
bly (Table S6 in the Supporting Information). Additionally, the
fluorescence excitation spectra of crystalline 1A display negli-
gible perturbation, relative to that of the ground-state absorp-
tion (Figure S16a in the Supporting Information). Picosecond
time-resolved fluorescence measurements monitored at the
emission maxima (l=468 nm) reveal a biexponential decay
with an average lifetime comparable to that in monomeric so-
lution (Figures S17–S18 and Table S6 in the Supporting Infor-
mation). Examination of the fluorescence decay towards the
red edge (l=525 nm) of the emission spectra reveals a minor
long-lived component, which suggests an excimer-like state in
crystalline 1A (Figure S17a in the Supporting Information).
Contrastingly, herringbone-packed 1B exhibits a broad, green
fluorescence spanning l=420–680 nm with an emission maxi-
mum centered at l=512 nm (Figure 2b). The broad, feature-
less emission and lack of a match with the emission in CHCl3
signify the apparent role of preassociated di-/oligomers in
modulating the optical properties of 1B through long-/short-
range electronic interactions. The fluorescence excitation spec-
tra of 1B monitored at long wavelength reveal the emergence
of a new, redshifted band, which suggests ground-state inter-
actions in 1B (Figure S16b in the Supporting Information).
Time-resolved fluorescence decay of crystalline 1B reveals a
triexponential decay with lifetime components of 0.66, 4.8, and
16.2 ns (Figure S17b in the Supporting Information). Quenched
fluorescence, triexponential fluorescence decay, and a long-
lived species (16.2 ns), compared with that in the solution, sug-
gests the presence of an excimer-like species in crystalline 1B.
The lamellar motif in 2A exhibits emission ranging from l=
450 to 600 nm with a maximum centered at l=505 nm. The
fluorescence excitation spectrum of 2A depicts a sharp, red-
shifted band upon monitoring at longer wavelength (Fig-
ure S16c in the Supporting Information). Fluorescence lifetime
analysis of crystalline 2A depicts a triexponential decay with
fast decay components, similar to that in monomeric solution
(tf =0.88 and 3.35 ns), and a slow decay species (tf =12.54 ns;
Table S6 in the Supporting Information). In contrast, the brick-
work/2D lamellar motif in crystalline 2B corroborates a mono-
mer-like cyan emission centered at l=462 nm, with diffuse
Interestingly, the p-stacked columnar motif in 2C exhibits a
broad, featureless, yellow emission centered at l=538 nm
with a large Stokes shift (Figure 2e). Large degree of orbital
overlap among the p-stacked columnar anthracenes and close-
stacking distances can catalyze long-range dipole interactions,
while simultaneously instigating plausible excimer formation at
short-range. The redshifted fluorescence excitation spectrum
of crystalline 2C, relative to that of the monomeric solution,
suggests an apparent ground-state interaction among the p-
stacked anthracenes in 2C. Concurrently, the structureless,
broad emission and large Stokes shift in the cofacial assembly
of columnar anthracenes propose an apparent excimer-like
character. A longer fluorescence lifetime component of 32.8 ns
further supports the excimer-like character in crystalline 2C
(Table S6 in the Supporting Information). Analogously, the 2D
lamellar stack in crystalline 3 displays 1) a broad, structureless
emission with a large Stokes shift; 2) a redshifted excitation
spectrum compared to that in solution; and 3) a longer fluo-
rescence lifetime (32.43 ns), which substantiates the influence
of excimer and dipole interactions in corroborating a yellow
emission in 3 (Figure 2 f and Table S6 in the Supporting Infor-
mation). Observed progressive redshift in the fluorescence
emission of crystalline mono- to triacetylanthracene can be a
consequence of 1) an increase in the number of acetyl groups,
and 2) p-overlap regulated cooperation of short- (excimer) and
long-range (Coulomb/dipole) electronic interactions. Discrete
molecular structures and the packing motifs in crystalline 1–3,
and the consequent intricate spectroscopic signatures, necessi-
tate a partition of the cooperative excimer and dipole interac-
tions in the molecular ensemble.
The Kasha’s exciton model provides critical insights into the
long-range electronic interactions between spatially proximate
chromophoric units, which can lend resonance splitting of the
excited state into two nondegenerate levels through Coulomb
interactions (Figure 3a).[6] Transition to either the upper or
lower states exclusively depends on stacking (cofacial/stag-
gered) and relative orientation (head-to-head/head-to-tail) of
chromophore transition dipoles.[6] Exciton coupling among co-
facial, head-to-head (staggered, head-to-tail) dipoles lends a
net nonzero transition moment, which allows the transition to
the upper (lower) state, whereas the transition to the lower
(upper) state is forbidden due to the vanishing transition
moment of opposing dipoles.[2a,3a,23] In contrast, transition di-
poles with oblique (a¼0) orientation lend a Davydov splitting
of the excited state, which allows transitions to both the upper
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^
tween the centers of molecules A and B; R is the unit vector in
that direction; a denotes the angle between the polarization
axes for the component absorbing units; and qA/qB are the
angles made by the polarization axes of molecules A/B, respec-
tively, with the line joining the molecular centers.
Equations (1)–(3) can quantitatively account for the long-
range Coulomb interaction mediated resonance splitting in
nearest chromophore neighbors.[6] The nearest-neighbor ap-
proach fails if a molecular ensemble possesses more than two
molecules in vicinity, each with a difference in their relative ori-
entations with respect to a central molecule, akin to a periodic
crystal with more than one molecule/unit cell (Figure 3c).[23,24]
Given that the exact diagonalization of the Hamiltonian be-
comes difficult with an increasing number of constituent mole-
cules in unit cell, a simpler approximation is to add up all near-
est and non-nearest neighbor interactions in a mean-field ap-
proximation.[27] Herein, a central molecule is assumed to be
electronically affected by a mean field created by all nearest
and non-nearest neighbors (Figure 3c). By using this approxi-
mation, the exciton coupling to the central molecules due to
the presence of all the neighbors can be approximated as
Equation (4):
X
N
dꢀd
dꢀd
Figure 3. Simplified schematic depiction of a) the exciton band energy dia-
gram for molecular H-type, J-type, and oblique dimers; b) characteristic ex-
cited-state transitions, illustrating concerted excimer and dipole couplings in
monomer versus crystalline states; and c) monomer, dimer (nearest-neigh-
bor), and molecular ensembles (mean-field approximation) used for exciton
coupling calculations in crystalline 1–3.[26] For clarity, the central molecule of
the assembly is depicted in purple in c).
ð4Þ
V
¼
V
total
Ai
i¼1
in which A is the central molecule and the exciton coupling of
A with i (i=1, 2, …, N) of its neighbors are summed up.
The interaction of the transition densities employing the for-
malism depicted in Equations (1)–(4) gives the mean-field
dipole coupling energies (2DeDip) for a molecular ensemble
(Table 1 and Figures S19–S20 in the Supporting Information).
The explicit effect of the exciton splitting energies on the emis-
sion character of crystalline 1–3 was further assessed. The sign
of the exciton coupling, that is, positive (H-type) or negative
(J-type), in Table 1 signifies a global effect of the exciton
(mean-field approximated) coupling on the spectral properties
of the molecular ensemble. Negative exciton splitting energies
in the molecular motifs of 1A (ꢀ1.9 meV), 1B (ꢀ39 meV), 2A
(ꢀ0.013 meV), and 2B (ꢀ18.9 meV) suggest a low-to-moderate
J-type coupling, whereas 2C (241 meV) and 3 (110.7 meV) with
and lower states.[3a,24] To deconvolute the influence of long-
(dipole) and short-range (excimer) electronic interactions, and
unravel the impact of 1) diverse geometric stacking, and
2) more than one molecule/unit cell, quantification of dipole
interactions among the nearest and non-nearest anthracenes
was carried out by extending the Kasha’s exciton coupling for-
malism to a mean-field approximation.[6] The Kasha’s exciton
model approximates the interacting chromophores in a dimer
as point dipoles, wherein the Coulomb coupling among two
transition dipole moments results in resonance splitting
(2DeDip) of the excited state.[6] Coulomb coupling among the
transition densities (M) of two molecules, A and B, can be
given by Equations (1), (2), and (3):[6,25]
Table 1. Excimer and dipole coupling (mean-field approximation) mediat-
ed spectral shifts, and the optical properties of acetylanthracene deriva-
tives 1–3 in solution/crystalline state.[a]
Z Z
e2
4 pe0
MAMB
dꢀd
ð1Þ
ð2Þ
ð3Þ
V
¼ 2DeDip
¼
dr1dr2
1A
1B
2A
2B
2C
3
AB
jr1 ꢀ r2j
lfCry [nm]
lfSol [nm]
Dlf [nm]
DeDip [meV]
lfDip [nm]
468
466
2
ꢀ1.9
465.5
ꢀ0.5
2.5
512
451
61
ꢀ39
456.6
5.6
505
460
45
ꢀ0.01
459.1
ꢀ0.9
45.9
464
462
2
ꢀ18.9
464.4
2.4
539
453
86
241
495.8
42.8
43.2
549
459
ꢂ
ꢀ
ꢁꢀ
ꢁꢃ
1
^
^
ꢁ
PAPB ꢀ 3 PA ꢂ R PB ꢂ R
90
4 pe0jR3j
110.7
477.7
18.7
71.3
27
*
jR3j
57:17 10 PAPB
dipole shift [nm]
excimer shift [nm]
*
ꢁ ꢀ
ðcos a ꢀ 3 cos qA cos qBÞeV
55.4
ꢀ0.4
[a] lfCry and lfSol are the fluorescence emission maxima in crystalline and
solution states, respectively; Dlf =lfCryꢀlfSol; DeDip =1/2 (mean-field ap-
proximated dipole–dipole interaction energy); lfDip =1240/(DeSolꢀj
DeDip j); dipole shift=(lfDipꢀlfSol); excimer shift=Dlfꢀdipole shift.
in which MA and MB are the transition densities of molecules A
and B, respectively. PA/PB can be viewed as the dipole moment
of the transition density MA/MB; jRj denotes the distance be-
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positive exciton interaction energy suggest a strong H-type
coupling. The computed exciton splitting (mean-field approxi-
mated) energies suggest a definite role of the dipole coupling
in modulating the fluorescence emission characteristics of crys-
talline 1–3.
coupling exist,[12] the energy difference between the excited
states in the crystalline state and monomeric solution can be
rationalized by Equation (5) (Figure 3b).[7b,12]
Def ¼ DevdW þ DeDip þ DeExc
ð5Þ
Having quantified the influence of the long-range (dipole)
interactions on the molecular ensemble, the role of the short-
range interactions in modulating the spectral characteristics of
crystalline 1–3 were explored. Long lifetime components in
the picosecond time-resolved fluorescence decay of crystalline
1–3 provide a preliminary indication of the presence of an ex-
cimer-like species (Table S6 in the Supporting Information). To
probe the origin of the long fluorescence lifetimes, TRES meas-
urements, followed by global analysis (GA) of the kinetic traces
(at all wavelengths), were performed (Figure 4 and Figure S21
and Table S7 in the Supporting Information). The GA of time
versus wavelength plots gave an EAS-based deconvolution of
the temporal components in fluorescence emission (Table S7 in
the Supporting Information). Depending on the discrete struc-
ture and stacking in the molecular ensemble, the EAS of crys-
talline 1–3 reveal an initial short-lived species evolving into an
excimer-like emission. The evolution (lifetime) and spectral
character of the long-lived species irrefutably defines the role
of the excited di-/oligomer in moderating the optical proper-
ties of crystalline 1–3.
in which DevdW, DeDip, and DeExc are the relative stabilization of
the excited state in the crystalline assembly through van der
Waals, dipole–dipole, and excimer interactions, respectively.[26]
The estimated dipole interaction energy (DeDip) and the appar-
ent difference between the emission in the crystalline state
and monomeric solution (Dlf/Def) can further provide the exci-
mer (DeExc) contribution towards the spectral shifts observed in
crystalline derivatives 1–3 (Table 1).[12,26] Equation (5) offers a
quantitative method to deconvolute the spectral shifts arising
from excimer and dipole couplings and narrate the exciton re-
laxation dynamics in crystalline 1–3 (Table 1).
The excited-state properties of crystalline 1–3 can be further
substantiated by correlating the computed excimer and dipole
shifts with the experimentally estimated temporal components
of the TRES data. TRES measurements of crystalline 1A (her-
ringbone) and 2B (brickwork) reveal the presence of an initial
short-lived species that transforms into a long-lived excimeric
emission, which substantiates the slow decaying components
in the fluorescence lifetimes (Figure S17 in the Supporting In-
formation). Although having a long lifetime, crystalline 1A and
2B seem to have a low excimer formation yield from 1) the
low relative amplitude of the long lifetime component, 2) sig-
nificant monomer-like vibronic, and 3) diffuse excimer charac-
ter. Concurrently, the computed excimer (2.55 (1A) and ꢁ0 nm
The computed DeDip and experimentally observed excimer
emission endorses a concerted role of excimer and dipole cou-
plings in crystalline 1–3 (Figure 3b and Table 1). Assuming that
the fluorescence always evolves from the lowest exciton state
(Kasha’s rule)[28] and a cooperation of the excimer and dipole
Figure 4. 2D TRES contour plots (a–c), normalized evolution-associated spectra (EAS; d–f), and the relative population profiles (g–i) of crystalline 2A (a, d, g),
2C (b, e, h), and 3 (c, f, i); the excitation wavelength is l=377 nm (pulse width ꢃ100 ps). The emission spectra of 2A, 2C, and 3 in CHCl3 (shaded gray) solu-
tion are depicted for comparison in d)–f).
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(2B)) and dipole (ꢁ0 (1A) and 2.4 nm (2B)) shifts also empha-
size the monomer-like character in crystalline 1A and 2B
(Table 1). Contrastingly, crystalline 1B (herringbone) and 2A
(lamellar) display a major excimer component, as observed
from the redshifted emission (centered at l=510–530 (1B)
and 505 nm (2A)) and high relative amplitudes of the long-
lived components (Figure 2b,c and Table S6 in the Supporting
Information). Edge-to-face geometry of the nearest neighbors
and an excimer band centered at l=510–530 nm in 1B sug-
gests a T-shaped excimer, as proposed earlier by Ma and co-
workers for substituted anthracenes.[29] The computed excimer
(55.4 nm) and dipole (5.6 nm) shifts agree with the experimen-
tally observed excimer character in 1B (Table 1). Derivative 2A
being a symmetric molecule (low transition dipole moment,
dipole shift ꢁ0 nm), the slip-stack di-/oligomers might play a
crucial role in moderating a major excimer-like (ꢁ45.9 nm)
emission (Table 1).
ing of the various factors that affect the excited-state charac-
teristics of a molecular ensemble. The insights gained from a
comprehensive experimental/theoretical investigation of exci-
mer/dipole coupling controlled emission can further shed light
on the fundamentals of manipulating molecular aggregates for
optoelectronic applications.
Conclusion
To summarize, we report an elegant approach to decipher the
subtlety of excimer and dipole interaction mediated exciton re-
laxation dynamics in the molecular ensembles of crystalline de-
rivatives 1–3. Classical Friedel–Crafts acylation of anthracene
rendered a series of acetylanthracene derivatives (1–3) with
1) discrete molecular structures; 2) varied orbital overlap/spa-
tial orientations, and 3) diverse packing motifs. Distinct molec-
ular orientation and packing arrangements in crystalline 1–3
ensured perturbed electronic communication among the near-
est and non-nearest anthracene units through an interplay of
excimer and dipole couplings. The manifestation of coopera-
tive dipole and excimer interactions was reflected in the excit-
ed-state characteristics of crystalline 1–3, resulting in distinct
blue–green–yellow emission. Independent assessment of the
excimer and dipole couplings through 1) TRES analyses, and
2) by extending the Kasha’s exciton coupling formalism
beyond the traditional nearest-neighbor (mean-field approxi-
mation) approach offered a quantitative estimation of the exci-
mer and dipole shifts in crystalline 1–3. Good agreement be-
tween the experimentally evaluated (long-lived EAS compo-
nents) spectral shifts and numerically computed excimer shifts
provide a comprehensive approach to unravel the excited-
state properties in crystalline molecular ensembles. We believe
that a tacit understanding of cooperative fundamental phe-
nomena (excimer and/or dipole interactions) that control the
excited-state characteristics of molecular ensembles can help
rationalize the design of novel photofunctional materials.
Lamellar motifs 2C and 3, with considerable orbital overlap,
display a broad, featureless (H-type/excimer-like) emission with
a large Stokes shift and high fluorescence lifetimes (Figure 2e,f
and Table S6 in the Supporting Information). A large degree of
p-orbital overlap and close-stacking distances between the di-/
oligomer ensure strong electronic communication in the mo-
lecular ensemble, which enables the occurrence of cooperative
Coulomb and excimer-like interactions (Scheme 1).[22,30] The de-
convoluted spectral shift in 2C suggests equal contributions
from dipole (42.8 nm) and excimer (43.2 nm) interactions in
the crystalline state as compared to the monomeric solution
(Table 1). On the other hand, the lamellar motif in 3 displays a
marginal dipole (ꢁ18.7 nm) and major excimer (71.3 nm) char-
acter, owing to higher orbital overlap in 3 than that in 2C
(Scheme 1 and Table 1). Interestingly, the bathochromic shift of
the long-lived EAS temporal component in crystalline 1–3 can
be correlated to the computed excimer shifts (Figure 4 and
Figure S21 in the Supporting Information). The spectral shift of
the experimentally deconvoluted long-lived EAS temporal
component (63 nm), corresponding to the excimer emission in
3, almost agrees with the theoretically computed excimer shift
(71.3 nm; Table 1 and Figure 4, and Figure S21 in the Support-
ing Information). An analogous trend is also observed among
the experimental (EAS temporal component) and computed
excimer shifts in 1B, 2A, and 2C.
Marginal disparity among the experimentally observed EAS
temporal components and the theoretically computed excimer
shifts (Table 1) could be a consequence of 1) incomplete decay
of the long-lived EAS component in the given experimental
window (Tables S6–S7 in the Supporting Information), 2) stabili-
zation of the monomeric acetylanthracene excited state(s) in
chloroform (gas-to-solvent shift), and 3) self-absorption in crys-
talline derivatives 1–3.[3c] Thus, interplay among the dipole and
excimer interactions result in a monomer-like emission in 1A
and 2B and a major excimer contribution in 1B and 2A,
whereas 2C and 3 reveal the cooperation of both excimer and
dipole coupling in modulating the spectroscopic characteristics
of the aggregate. Consistent experimental and theoretical nar-
ration of the observed spectroscopic/excited-state character in
crystalline 1–3 reveal the significance of a global understand-
Acknowledgements
M.H. acknowledges the Department of Science and Technology
(DST) Nanomission (DST-SR/NM/NS-23/2016(C)), and A.S. ac-
knowledges SERB, DST, Government of India (Grant no. EMR/
2016/007221) for the support of this work. We thank Mr.
Alex P. Andrews for X-ray structure analyses, and Dr. Anoop
Thomas for valuable suggestions and help with the TRES data
acquisition of crystalline derivatives 1–3. A.M.P. and S.K.M. ac-
knowledge IISER-TVM and DST INSPIRE (DST/INSPIRE-SHE/IISER-
T/2008) for research fellowships.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aggregation · chromophores · crystal engineering ·
exciton interaction · fluorescence
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Manuscript received: August 14, 2018
Accepted manuscript online: September 27, 2018
Version of record online: && &&, 0000
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FULL PAPER
Balancing the force: A combined theo-
retical and experimental investigation
into the role of concerted long- (dipole
coupling) and short-range (orbital over-
lap mediated excimer) electronic inter-
actions in modulating the emission of
six crystalline acetylanthracenes is re-
ported. A subtle interplay between
these interactions dictate the optical
signatures of the six crystalline anthra-
cene assemblies.
&
Exciton Dynamics
A. M. Philip, S. K. Manikandan, A. Shaji,*
M. Hariharan*
&& – &&
Concerted Interplay of Excimer and
Dipole Coupling Governs the Exciton
Relaxation Dynamics in Crystalline
Anthracenes
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