Tweaking a BODIPY Spherical Self-Assembly to 2D Supramolecular Polymers Facilitates Excited-State Cascade Energy Transfer

Article abstract of DOI:10.1002/anie.202015390

Excited state properties such as emission, exciton transport, electron transfer, etc., are strongly dependent on the shape, size and molecular arrangement of chromophore based supramolecular architectures. Herein, we demonstrate creation and control of distinct supramolecular energy landscapes for the reversible control of the excited-state emission processes through cascade energy transfer in chromophore assemblies, facilitated by an unprecedented solvent effect. In methylcyclohexane, a tailor-made Y-shaped BODIPY derivative self-assembles to form an unusual spherical architecture of 400–1200 nm size, which exhibits a single emission at 540 nm upon 475 nm excitation through a normal excitation deactivation process. However, in n-decane, the same BODIPY derivative forms two-dimensional supramolecular sheets, exhibiting multiple emission peaks at 540, 610, 650, 725 and 790 nm with 475 nm excitation due to cascade energy transfer. Further control on the morphology and excitation energy transfer is possible with variable solvent composition and ultrasound stimulation, resulting in enhanced near-infrared emission with an overall pseudo Stokes shift of 7105 cm^?1.

Full text of DOI:10.1002/anie.202015390

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7851–7859
International Edition: doi.org/10.1002/anie.202015390
German Edition: doi.org/10.1002/ange.202015390
Self-Assembly
Tweaking a BODIPY Spherical Self-Assembly to 2D Supramolecular
Polymers Facilitates Excited-State Cascade Energy Transfer
Gourab Das, Sandeep Cherumukkil, Akhil Padmakumar, Vijay B. Banakar,
Vakayil K. Praveen,* and Ayyappanpillai Ajayaghosh*
In memory of Professor M. V. George
[
6]
[7]
Abstract: Excited state properties such as emission, exciton
transport, electron transfer, etc., are strongly dependent on the
shape, size and molecular arrangement of chromophore based
supramolecular architectures. Herein, we demonstrate creation
and control of distinct supramolecular energy landscapes for
the reversible control of the excited-state emission processes
through cascade energy transfer in chromophore assemblies,
facilitated by an unprecedented solvent effect. In methylcyclo-
hexane, a tailor-made Y-shaped BODIPY derivative self-
assembles to form an unusual spherical architecture of 400–
self-assembly and seeded supramolecular polymerization
are examples that control the length and dimension of
supramolecular structures through kinetic and thermodynam-
ic control of self-assembly. While living supramolecular
polymerization is undoubtedly a powerful tool for the precise
control of molecular self-assembly, it cannot be applied as
a general strategy. Therefore, different kinds of external
stimuli such as light, pH, redox,
sound, and solvents, have been exploited for controlled
supramolecular polymerization of functional p-systems.
[
8]
[9]
[10]
[11]
enzyme,
ultra-
[12]
[13]
[14]
1
4
200 nm size, which exhibits a single emission at 540 nm upon
75 nm excitation through a normal excitation deactivation
Cascade energy transfer in supramolecular architectures
is a fundamentally important process applicable to the
[
15]
process. However, in n-decane, the same BODIPY derivative
forms two-dimensional supramolecular sheets, exhibiting mul-
tiple emission peaks at 540, 610, 650, 725 and 790 nm with
biological and materials world. For example, the phycobi-
lisomes, which is the major light-harvesting complex in
cyanobacteria and red algae, is considered one of the most
efficient light-harvesting systems with more than 95%
4
75 nm excitation due to cascade energy transfer. Further
[15a,e]
control on the morphology and excitation energy transfer is
possible with variable solvent composition and ultrasound
stimulation, resulting in enhanced near-infrared emission with
efficiency due to the cascade energy transfer.
Though
inferior to natural light-harvesting systems, the artificial
supramolecular systems, due to the presence of aggregates
of varying energy levels with large anisotropy, provides ample
opportunities to simulate the natural light-harvesting pro-
cesses. In this context, molecular assemblies of certain
chromophores having the mixture of aggregates with different
levels of electronic coupling have been shown to facilitate
À1
an overall pseudo Stokes shift of 7105 cm .
Introduction
Creation of supramolecular architectures of different
shapes, sizes and properties is the fundamental step to realize
next generation complex multi-component systems with
specific functions and applications. In this context, chromo-
phore based supramolecular systems have been at the center
stage due to their reversible optoelectronic properties that
can be modulated through the molecular assembly. In recent
times, several approaches have been introduced for the
controlled assembly of p-conjugated molecules.
[16]
cascade energy transfer leading to red-shifted emission.
This red-shift can lead either to a broad emission, if
aggregates of closely matching energy levels are involved or
to multiple emission if aggregates of discrete energy levels are
present in the hierarchical self-assembly. However, the latter
type of molecular assembly resulting in multiple emission
covering the entire visible-NIR region upon a single excita-
tion remains a challenge.
[
1]
[2]
[3–5]
Living
[3,5a]
supramolecular polymerization,
crystallization-driven
Among different chromophores, 4,4-difluoro-4-bora-3a-
4
a-diaza-s-indacene (BODIPY) is an excellent choice for
creating supramolecular optoelectronic materials due to their
photostability, high molar absorptivity and sensitive fluores-
[
*] G. Das, Dr. S. Cherumukkil, A. Padmakumar, V. B. Banakar,
Dr. V. K. Praveen, Prof. Dr. A. Ajayaghosh
Photosciences and Photonics Section, Chemical Sciences and
Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology (CSIR-NIIST)
[17–20]
cence features.
Here, we demonstrate how distinct
energy landscapes can be obtained by creating different
morphological features that facilitate reversible modulation
of emission behavior. This is possible by tweaking the self-
assembly of meso p-extended BODIPY dyes 1–3 (Figure 1a)
with solvents as established in this work. The high ground
Thiruvananthapuram, Kerala 695019 (India)
E-mail: vkpraveen@niist.res.in
ajayaghosh@niist.res.in
G. Das, A. Padmakumar, Dr. V. K. Praveen, Prof. Dr. A. Ajayaghosh
Academy of Scientific and Innovative Research (AcSIR)
Ghaziabad, Uttar Pradesh 201002 (India)
state dipole moment (m = 7.53 D) of Y-shaped 1 and 2 drives
g
the self-assembly in cyclic nonpolar solvents such as methyl-
cyclohexane (MCH) or trans-decalin, initially resulting in 10–
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
2
0 nm spheres, which transform into large spheres of 600–
1
002/anie.202015390.
800 nm with time as confirmed by dynamic light scattering
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Figure 1. a) Molecular structures of the Y-shaped BODIPY derivatives 1–3. b) Absorption (between 420–580 nm corresponding to the BODIPY
À4
À4
chromophore) and c) normalized emission spectra (l =475 nm) of 1 in chloroform (1”10 M), MCH, trans-decalin (3”10 M) and n-decane,
ex
À4
n-heptane (3”10 M). Deconvoluted d) absorption and e) emission spectra of 1 in n-decane at 506 and 610 nm, respectively. f) The plot of aagg
versus temperature at different concentrations of 1 in n-decane; aagg was calculated by monitoring the absorbance changes at 530 nm. Inset
À4
shows the van’t Hoff plot obtained through the controlled cooling experiments of 1 in n-decane. DLS size distribution of 1 in MCH (3”10 M):
À1
g) immediately after fast cooling, h) after aging the sample for 24 h, and i) after cooling at a controlled rate of 1 Kmin
.
(
2
DLS) experiments. Evaporation of the MCH solution of 1 or
on a substrate resulted in an assembly of spherical
as established by time-resolved emission and anisotropy
studies.
structures, wherein each large sphere is surrounded by a layer
of self-assembled nanospheres, appearing like a core–shell
structure with an average size of 400–1200 nm. The spherical Results and Discussion
particles exhibited a normal emission spectrum with a single
maximum at 540 nm. On the other hand, in linear nonpolar
After synthesizing 1–3 (Scheme S1–S3), we have studied
solvents such as n-heptane or n-decane, two-dimensional
their optical behavior and self-assembly processes (Fig-
À4
(
2D) supramolecular sheets having broad emission compris-
ure 1b,c and Figure S1). In CHCl (1 ” 10 M) at 295 K, the
3
ing of 540, 610, 650, 725 and 790 nm bands are formed.
Interestingly, the single and multiple emission could be
reversibly controlled by tweaking the morphology between
the spherical and 2D assemblies by changing the solvent
composition. Molecule 3 having short alkyl chains failed to
form either the spherical morphology or 2D sheets, revealing
the importance of the alkyl chains in the self-assembly
process. Ultrasound stimulation of the 2D sheets exhibited
better efficiency for the successive energy transfer as evident
by a 2.2-fold increase in the intensity of the near infrared
UV-vis absorption spectrum exhibited two maxima at 290 and
504 nm with a shoulder band around 475 nm. The broad
absorption observed at 290 nm (e = 5.6 ” 10 M cm ) corre-
4
À1
À1
sponds to the phenyleneethynylene moiety, whereas the
5
À1
À1
narrow absorption feature at 504 nm (e = 1.3 ” 10 M cm )
implies the strong S ! S electronic transition involving (0 !
0
1
0) vibrational states of the BODIPY chromophore. The band
at 475 nm can be considered as the vibronic shoulder of S -S
0
1
[20]
transition. On the other hand, the absorption spectrum of
1 (3 ” 10 M) in MCH or trans-decalin, when heated to 363 K
À4
À1
(
NIR) bands at 725 and 790 nm. The morphological features
followed by cooling to 283 K at a rate of 1 Kmin , exhibited
4
À1
À1
and the associated emission behavior described here are
unprecedented and the origin of multiple emission ascribed is
the manifestation of a cascade effect of the excitation energy
an absorption maximum at 288 nm (e = 6.75 ” 10 M cm ),
5
À1
À1
with a broad band (lmax = 504 nm, e = 1.04 ” 10 M cm )
between 420–550 nm. Furthermore, 1 exhibits an emission
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maximum at 520 nm (F = 0.39) in CHCl , whereas in MCH
other concentrations of 1 in n-decane (Figure S4b–d, S5a).
F
3
a red-shifted maximum at 540 nm (F = 0.20). These obser-
BODIPY derivatives are known to exhibit broad multiple
F
vations indicate aggregation of 1 in MCH as further con-
firmed by temperature-dependent H NMR, absorption and
emission in solid/semicrystalline powder form, however, not
1
[19f,20d,e]
in any solvents.
While MCH and n-decane have more
emission studies (Figures S2 and S3). Upon gradual reduction
in temperature from 363 to 283 K, the intensity of the
absorption maximum was reduced and the absorption spec-
trum corresponding to both BODIPY and phenyleneethyny-
lene part became broad (Figure S3a). In contrast, the temper-
ature-dependent emission studies showed a gradual increase
in the intensity when 1 in MCH was cooled from 363 to 283 K,
indicating aggregation-induced enhanced emission behavior
or less identical polarity, except for their difference in
structure and shape (cyclic and linear), the observed differ-
ence in the emission behavior in these solvents could be
associated with the hierarchical assembly of the BODIPYs.
Plots of aagg of 1 with respect to temperature revealed
non-sigmoidal cooling curves for a wide range of concen-
À4
À4
trations (1 ” 10 to 3 ” 10 M) in n-decane (Figure 1 f and
S4). The cooling curves thus obtained were fitted to an
equilibrium model, characteristics of a co-operative self-
[
2a,21a]
(
Figure S3b).
Since the phenyl ring attached at the meso
[
21]
position of BODIPY chromophore 1 is unsubstituted, rota-
tional and vibrational motions are facilitated at higher
temperature, leading to non-radiative deactivation of the
assembly process. Details of the thermodynamic parame-
ters related to the fitting are summarized in Table S2. The plot
of the natural logarithm of the reciprocal concentration of
[20a]
excited state and less emission.
However, upon gradual
1 against reciprocal of T exhibited a linear relationship (vanꢀt
e
reduction in temperature, aggregation of 1 restricts the
rotation and vibrational motion and activates radiative
excited state decay to display enhanced emission. Aggrega-
Hoff plot, Figure 1 f inset). The thermodynamic parameters
À1
of
1
in n-decane were DH8 = À84.26 kJmol , DS8 =
À1 À1
À1
À168.43 Jmol
K
and DG8 = À34.07 kJmol , indicating
À4
tion of 1 in MCH at a concentration of 3 ” 10 M was further
that the self-assembly takes place through an enthalpically
confirmed from the plot of the fraction of aggregates (aagg) of
driven co-operative process. DLS experiments of 1 in MCH
(3 ” 10 M) under fast cooling revealed the existence of two
À4
1
with respect to temperature, revealing a non-sigmoidal
cooling curve, which is characteristic of a co-operative self-
different bands with size distributions of 10–20 nm (80–90%)
and 200–300 nm (10–20%) (Figure 1g). After aging for 24 h,
the size of the aggregates increased to 600–800 nm (70%)
while the population of the 10–20 nm size aggregates
decreased to 30%, indicating a consecutive process taking
place, where the initially formed smaller spherical aggregates
were directly converted to bigger size assemblies without
[
21]
assembly process (Figure S3c and Table S1).
À4
In n-decane or n-heptane (3 ” 10 M), the absorption
spectra of 1 within 230–420 nm was almost similar to those in
CHCl3 and MCH (Figure S1), however, the absorption
corresponding to the BODIPY chromophore appeared much
4
À1
À1
broader (lmax = 506 nm, e = 7.2 ” 10 M cm ), with predom-
inant shoulder bands in the lower and higher wavelength
region (470 and 570 nm), indicating the formation of multiple
aggregates (Figures 1b and S4). Surprisingly, the emission
[5b]
converting into the monomers (Figure 1h).
However,
À1
a slow cooled (1 Kmin ) solution exhibited almost similar
size of the assemblies as obtained by aging (Figure 1i). No
time-dependent changes were noticed in the case of aggre-
gates of 1 in n-decane, implying that these aggregates have
reached equilibrium when cooled at a controlled rate (Fig-
ure S6).
À4
spectrum of 1 (3 ” 10 M) in n-decane or n-heptane exhibited
an unusually broad band with multiple maxima at 540, 610,
6
50, 725 and 790 nm (F = 0.14). The peak at 650 nm is more
F
resolved in n-heptane (Figure 1c). Deconvolution of the
broad absorption peak (506 nm) obtained in n-decane shows
two peaks at 473 and 509 nm, confirming the formation of
aggregates (possibly mixture of both H- and J-type aggre-
Transmission electron microscopy (TEM) images of the
À4
self-assembly in MCH (3 ” 10 M) exhibited a spherical
[23]
morphology akin to a core–shell type structure
having
[
22]
gates) (Figure 1d). Deconvolution of the broad emission
broad size distributions of 400–1200 nm. The spheres from the
fast-cooled solution of 1 in MCH have diameters between 400
and 600 nm (Figure 2a,b), which upon aging for 24 h showed
a size increase of 600–1200 nm (Figure 2c,d). However,
band at 610 nm in n-decane revealed two peaks at 612 and
6
86 nm (Figure 1e).
These observations were further confirmed from the
À1
temperature-dependent absorption and emission studies of
in n-decane (Figure S4, S5). At 363 K, 1 in n-decane
a slowly cooled solution (1 Kmin ) resulted in spherical
1
assemblies of 400–1000 nm (Figure S7). Magnified images of
the spherical structures revealed a hyperdense core region
having an average diameter of 200–900 nm (Figure 2b,d and
S7b) and a less dense shell region of about 100–300 nm width.
While these types of structures are common in the case of
inorganic semiconductors, such structures are rare in the case
of organic single molecular self-assembly. The solvent evap-
oration on the TEM grid may result in a thick core, around
which excess nanospheres present in the mixture self-assem-
ble to form a shell. On the other hand, from n-decane solution
displayed a sharp absorption band at 504 nm, which gradually
reduced its intensity and became broad at 283 K (Figure S4a).
The intensity of the emission spectrum of 1 in n-decane was
initially found to increase with an appearance of a new peak
at 725 nm upon reducing the temperature 363 to 348 K
(
Figure S5b, upper panel). Between 343 to 313 K, the
emission intensity of the peak at 552 nm was found to
gradually decrease with the appearance of a new peak at
6
00 nm (Figure S5b, middle panel). Further reduction in
À4
temperature to 283 K resulted in an overall enhancement in
the intensity of emission profile with three distinct maxima at
(3 ” 10 M), 2D sheet-like structures were obtained (Fig-
ure 2e,f and Figure S8). Cross-sectional analysis indicates an
average height of 1.9 Æ 0.1 nm with marginal surface rough-
ness. Upon increasing the concentration of 1 in n-decane (8 ”
5
52, 600 and 725 nm (Figure S5b, lower panel). A similar
absorption and emission changes were also noticed at various
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À4
Figure 2. TEM images of 1 in MCH (3”10 M), a) immediately after fast cooling and c) after aging the sample for 24 h. b, d) Magnified images
À4
corresponding to Figure 2a and c, respectively. e) TEM and f) AFM images of 1 in n-decane (3”10 M). The height profile along the yellow line
À3
is shown in the inset of Figure 2 f. g) SEM image of the n-decane gel (8”10 M). Inset shows the photograph of the corresponding gel under the
illumination of 365 nm UV light. h) The plot of dynamic storage (G’, blue) and loss modulus (G’’, magenta) against shear stress (s) for n-decane
À3
À1
gel (8”10 M) at 298 K. Frequency (n) was kept fixed at 1 rads . i) Fluorescence microscopy image of the diluted n-decane gel.
À3
1
0
M), the nanosheets were grown into large interconnected
1 in n-decane (Figure 3g). Similarly, the enhancement in NIR
emission was also noticed when n-decane aggregates of 1 were
cast as a film (Figure S10a). Upon sonication, the absorption
intensity was slightly reduced (Figure S10b) and the forma-
tion of multilayered 2D structures was observed by TEM
analysis (Figure S10c,d). In contrast, when sonication was
applied to 1 aggregates in MCH, the emission profile
remained almost unaltered (Figure 3h). The results of these
studies indicate that sonication and film formation favor
multilayered 2D assemblies of 1, which enhance the NIR
emission.
structures leading to an orange-red emitting gel (Figure 2g-i).
However, 1 failed to form a gel in MCH (Figure S9). The
viscoelastic nature of the n-decane gel is evident from the
stress sweep experiments, which revealed the dependencies of
storage modulus (G’) and loss modulus (G’’) on the applied
shear stress. Both the G’ and G’’ were independent of applied
stress up to 5.75 Pa, indicating the substantial elastic nature of
the gel (Figure 2h). These observations underpin the ability
of 1 to undergo 2D supramolecular polymerization in n-
decane.
[24]
[4,25,26]
We hypothesized that, if the emission behavior is asso-
ciated with morphological features, tweaking the spherical
assembly to the 2D sheets may change the single emissive
state to the multi-emissive one. Addition of different volume
A deeper understanding of the origin of the multi-
emission feature was possible with time-resolved emission
spectral (TRES) studies of 1 in MCH and n-decane (3 ”
À4
10 M) (Figure 4a–f). The emission spectra at different time
À4
fractions (f = v/v) of n-decane to 1 in MCH (3 ” 10 M), we
scales in MCH (l = 375 nm) showed an increase in the
ex
found the spherical structure slowly changing to elongated
sheets and then to large 2D sheets (Figure 3a–d). Upon
increasing f, the intensity of the absorption of the BODIPY
moiety (lmax = 504 nm) gradually decreased with the appear-
ance of a shoulder band at a higher wavelength. Interestingly,
we could see multiple emission peaks at 600 and 725 nm in
addition to the peak at 540 nm (Figure 3e). Similarly, upon
addition of MCH to 1 in n-decane, the multiple emission
changed to a single emission (Figure 3 f). The reversibility in
the emission feature along with the change in morphology
intensity at 550 nm up to 280 ps followed by a gradual
decrease in the intensity while retaining the nature of the
emission profile up to 2.80 ns (Figure 4a). This observation
indicates a normal excitation-deactivation process (Fig-
ure 4d). However, upon excitation in n-decane at 375 nm,
a growth profile at 555 nm was observed within 32–210 ps
(Figure 4b). With further increase in timescale up to 520 ps,
the 555 nm emission intensity gradually decayed with a red-
shift of the emission peak to 600 nm within the time scale of
950 ps. Thereafter, the emission peak at 600 nm showed
a time-dependent decay up to a timescale of 2.40 ns. The time-
dependent decay of the emission at a lower wavelength and
the growth at the higher wavelength with dynamic red-shift of
the emission are characteristic of energy migration from the
excited states of lower-order aggregates (higher excited state
[27]
underpins the role of the solvent-induced self-assembly in
[28]
perturbing the excited state properties.
Surprisingly, the
intensities of the 540 and 600 nm bands were decreased while
the intensity of the NIR emission band at 725 and 790 nm
[12,24a,29]
exhibited a 2.2 times enhancement upon sonication
of
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Figure 3. TEM images of aggregates of 1 upon increasing the volume fractions of n-decane in MCH (f): a) f=0.02–0.05, b) f=0.25, c) f=0.75 and
d) f=0.95–0.98. Figure 3c inset shows the zoomed image. Changes in emission (l =475 nm) upon varying the solvent compositions:
ex
e) addition of various fractions of n-decane to aggregates of 1 in MCH and f) addition of various fractions of MCH to aggregates of 1 in n-decane
À4
À4
(
3”10 M). Emission spectra of 1 (l =475 nm) in g) n-decane and h) MCH (3”10 M), before (blue) and after (magenta) sonication.
ex
energy) to the higher-order aggregates (lower excited state
reached their maximum within 0.49 and 0.56 ns, respectively
and then decayed. A negative pre-exponential factor ob-
served at 650 and 725 nm emission decays confirms the step-
wise energy transfer from 555 to 600 nm, 600 to 650 nm, 650 to
725 nm and then a weak energy transfer from the 725 to
790 nm, suggesting multiple long-range cascade energy trans-
[16]
energy) leading to the multiple emission. Figure 4e reveals
that within 0.17 ns, the peak intensity at 555 nm reaches
a maximum and then undergoes a rapid tri-exponential decay
with time constants of 1.49 ns (44.76%), 5.82 ns (35.63%) and
0
.25 ns (19.61%). The emission at 600 nm reaches the
[6a,16b,c,20d,e]
maximum at 0.25 ns with a negative pre-exponential. The
fitted rise time value of 0.34 ns at 600 nm correlates with the
fer.
The BODIPY 2, albeit having a chiral side
chain, did not show any supramolecular chirality and ex-
hibited the same behavior as of 1 with respect to the optical
and morphological properties (Figures S11–S13).
0
.25 ns decay component observed at 555 nm emission, thus
validating the energy migration process. Similarly, the 650 nm
emission peak also reached the maximum at 0.35 ns, followed
by a successive decay.
Further insights on the cascade energy transfer could be
obtained from time-resolved anisotropy studies (Figure 4g–i).
In general, the loss of anisotropy of organized donors within
an assembly occurs either through rotational motion or
Since the multi-emissive nature is found to be strongly
influenced by ultrasound, TRES analysis of the sonicated
samples in n-decane was performed (Figure 4c). The initial
increase in the emission profile at 555 nm within 30–160 ps
exhibited a subsequent decrease in the intensity within a time
scale of 390 ps, with the formation of a new red-shifted peak
at 600 nm. The peak at 600 nm exhibited an increase in the
emission intensity within 260–590 ps followed by a successive
time-dependent decay. Within 650 ps, another peak at 725 nm
[16,30]
energy migration.
In n-decane, depolarization through
rotational motion is less favored as spontaneous nucleation
assists the formation of 2D assemblies and hence energy
[
16,30]
migration can be the possible pathway for depolarization.
À4
At 3 ” 10 M, 1 in n-decane exhibited an initial anisotropy
value (r ) of 0.25 and reached the plateau region (r ) at 0.13
0
1
by losing the anisotropy with a decay time, t = 45 ps (Fig-
r
(
NIR region) was formed, which reached its maximum and
ure 4h). The r value of 0.25 at around zero time indicates that
0
then gradually decayed, showing a perfect correlation be-
tween 600 and 725 nm peaks, indicating energy transfer from
the 600 nm emitting species. Plots of the normalized emission
counts at different emission peaks and the time scale of the
successive energy transfer is clear from Figure 4 f. The peak
intensity at 555 nm reached a maximum within 0.19 ns
followed by decay, whereas the 600 nm emission reached its
maximum at 0.41 ns. The rise time value (0.97 ns) obtained at
the initial orientation of the transition dipole moments has an
angle of 308. On the other hand, at a similar concentration
[31]
in MCH, the initial anisotropy (r ) was found to be 0.15, which
0
reached the final anisotropy (r ) value of À0.04 (Figure 4g).
1
The difference in the initial anisotropy value indicates the
random independent orientation of the chromophores in
MCH, where a large angle (408) exists between the excitation
and emission dipole moments. Interestingly, the studies
carried out in n-decane after sonication showed a higher
6
00 nm can be correlated with the decay component (0.81 ns)
observed at 555 nm. In a similar fashion, 650 and 725 nm peak
initial anisotropy (r ) value of 0.32, where the angle (218)
0
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Figure 4. Time-resolved emission (TRES) of 1 in a) MCH, b) n-decane before sonication and c) after sonication. The normalized fluorescence
À4
decay curves in d) MCH, e) n-decane before sonication and f) after sonication; monitored at different emission wavelengths (3”10 M,
l =375 nm). Time-resolved fluorescence anisotropy decay in g) MCH (l =550 nm), h) n-decane (lem =555 nm) before sonication and i) after
ex
em
À4
sonication (3”10 M, l =375 nm).
ex
between the excitation and emission transition dipole mo-
ments was found to be much smaller in comparison to the
involve in the nucleation process dictated by the nature of
the solvent molecules (Figure 5). In the case of MCH, the
[
27]
other cases (Figure 4i). In addition, the final anisotropy (r )
nucleation process was sluggish (T = 321 K) in comparison to
1
e
value of 0.04 was reached within a short decay time (t ) of
that in n-decane (T = 358 K) due to the difference in the
r
e
2
8 ps. The anisotropy decay time t , which gives an estimation
activation energy barriers (Figure 5). The fast nucleation-
elongation process in n-decane resulted in 2D sheets stabi-
lized by intermolecular H-bonding and interdigitated alkyl
chains. Fourier transform-infrared (FT-IR) studies have
shown that the 2D sheets formed in n-decane are stabilized
by various non-covalent interactions (Figure S15). Film state
FT-IR spectrum of 1 obtained from n-decane exhibited bands
r
of the rate of energy migration (k ) from the higher to the
lower energy state,
1
The fast fluorescence depolarization and energy migration
rate obtained in sonicated n-decane sample signify a fast
interchromophore energy migration of a singlet exciton,
facilitating successive and partial energy transfer process
within the aggregates having different HOMO–LUMO
energy levels.
The solvent dependent origin of the distinct morpholog-
ical features and the associated excited state property could
be rationalized based on the difference in molecular packing
EM
[
16b,c]
10
was found to be 3.57 ” 10 and 2.22 ”
1
0
À1
0 s
for sonicated and non-sonicated samples, respectively.
[16,30]
À1
at 3288, 1640 and 1545 cm , suggesting intermolecular H-
[
12b,21a]
bonding.
On the other hand, the BODIPY 1 in CDCl
3
À1
displayed two bands at 3450 and 3348 cm , assigned as amide
A bands. The amide I and II bands appeared at 1654 and
À1
1543 cm , respectively. Intramolecular H-bonding was over-
ruled based on the negligible hysteresis observed between
cooling and heating curves for the n-decane aggregates and
also from the identical cooling curves obtained when the
(
Figure 5) as evident from various experimental results. The
antiparallel arrangement of the dipoles can lead to a centro-
symmetric dimer structure, as inferred from the d-spacing
value of 56.2 Š obtained from the wide-angle X-ray scattering
[32]
À1
cooling rate was varied from 1 to 5 Kmin (Figure S16). In
addition, the symmetric and asymmetric -CH vibrations of
2
À1
(
Figure S14). These centrosymmetric dimers can further
1 in the xerogel state were observed at 2852 and 2922 cm ,
7
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Figure 5. Schematic energy landscape diagram for the morphology dependent exciton deactivation in BODIPY assemblies. Self-assembly in MCH
(
(
left panel, pathway A) and n-decane (right panel, pathway B), forming spherical supramolecular structures and 2D sheets, leading to normal
single emission) and cascade energy transfer, respectively.
respectively and shifted towards the lower frequency region Conclusion
with respect to 1 in CDCl (Figure S17a), indicating that the
3
dodecyl chains of 1 remain in all trans configuration and
interdigitated in the 2D assembly forming aggregates.
These sheets possess anisotropically organized aggregates of
BODIPY chromophores with different energy band gaps
leading to multi-emissive feature. In addition, the sluggish
nucleation-elongation of 1 in MCH resulted in small particles
The spherical morphology formed through the multistep
[12b]
assembly of the BODIPY 1 in MCH behaves like a normal
chromophore assembly in terms of the excited state behavior;
however, the 2D sheets formed in n-decane exhibits an
unusual multi-emission associated with successive energy
transfer within the energetically anisotropic chromophore
assembly landscape. The long hydrocarbon chains in 1 and 2
are essential for forming 2D sheets, as illustrated with the
inability of 3 to form such assembly in linear non-polar
solvents. The transformation between the spherical assembly
in MCH and 2D sheets in n-decane along with the changes
from single to multiple emission underpin an assembly-
dependent excited state behavior. Our results show that it is
possible to reversibly control the excited state properties with
two different stimuli such as solvent composition and ultra-
sound. Moreover, the effect of ultrasound stimulation of the
2D sheets on successive energy transfer with one of the largest
(
upon fast cooling) and their subsequent coalescence to large
spheres with time, forming isotropically organized assembly
of the BODIPY chromophores leading to a single emission
peak. In this case, amide A, I and II bands were observed at
À1
3
289, 1637 and 1506 cm , respectively, suggesting the for-
mation of intermolecular H-bonding between the molecules
Figure S15 and S16). However, FT-IR bands corresponding
(
to symmetric and asymmetric -CH vibrations were almost
2
indistinguishable in the case of 1 in MCH and CDCl3,
indicating randomly arranged dodecyl chains and thus unable
to form gels (Figure S17b). The self-assembly of BODIPY
À4
À1
derivative 3 having short butyl chains in n-decane (3 ” 10 M)
observed pseudo Stokes-shift of 7105 cm may be useful for
resulted in a single emission maximum at 527 nm upon
designing self-assembly based light-harvesting and photonic
devices. Though there are many reports on cascade energy
transfer, the present system is the only example available on
intentionally modulated cascade energy transfer in a chromo-
phore based supramolecular system, which can be reversibly
controlled with solvents as a consequence of morphological
change.
excitation at 475 nm (Figure S18a). Furthermore, failure of 3
À3
to form a gel in n-decane (8 ” 10 M) and instead of forming
spherical particle-like precipitates (Figure S18b) underpin the
role of long hydrocarbon side chains in the observed 2D
morphology of 1 and 2 in n-decane.
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Acknowledgements
1
A.A. is grateful to the DST-SERB, Govt. of India, for a J. C.
Bose National Fellowship (SB/S2/JCB-11/2014). G.D. and
A.P. acknowledges CSIR and UGC, Govt. of India for
research fellowships, respectively. We acknowledge Dr. B.
Vedhanarayanan and Dr. V. Karunakaran (CSIR-NIIST) for
their support and scientific discussion at different stages of
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Conflict of interest
The authors declare no conflict of interest.
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