Entropic Mixing Allows Monomeric-Like Absorption in Neat BODIPY Films

Article abstract of DOI:10.1002/chem.202002463

Intermolecular interactions play a crucial role in materials chemistry because they govern thin film morphology. The photophysical properties of films of organic dyes are highly sensitive to the local environment, and a considerable effort has therefore been dedicated to engineering the morphology of organic thin films. Solubilizing side chains can successfully spatially separate chromophores, reducing detrimental intermolecular interactions. However, this strategy is also significantly decreasing achievable dye concentration. Here, five BODIPY derivatives containing small alkyl chains in the α-position were synthesized and photophysically characterized. By blending two or more derivatives, the increase in entropy reduces aggregation and therefore produces films with extreme dye concentration and, at the same time almost solution like absorption properties. Such a film was placed inside an optical cavity and the achieved system was demonstrated to reach the strong exciton-photon coupling regime by virtue of the achieved dye concentration and sharp absorption features of the film.

Full text of DOI:10.1002/chem.202002463

Accepted Article
Accepted Article
Title: Entropic mixing allows monomeric like absorption in neat
BODIPY films
Authors: Clara Schäfer, Jürgen Mony, Thomas Olsson, and Karl
Börjesson
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002463
Link to VoR: https://doi.org/10.1002/chem.202002463
01/2020

10.1002/chem.202002463
Chemistry - A European Journal
COMMUNICATION
Entropic mixing allows monomeric like absorption in neat
BODIPY films
Clara Schäfer,[a] Jürgen Mony,[a] Thomas Olsson[a] and Karl Börjesson*[a]
[a]
M.Sc. C. Schäfer, M.Sc. J. Mony, Dr. T. Olsson, Dr. K. Börjesson
Department of Chemistry and Molecular Biology
University of Gothenburg
Kemigården 4, SE-412 96 Gothenburg, Sweden
E-mail: karl.borjesson@gu.se
Abstract: Intermolecular interactions play a crucial role in materials
chemistry because they govern thin film morphology. The
photophysical properties of films of organic dyes are highly sensitive
to the local environment, and a considerable effort has therefore
been dedicated to engineering the morphology of organic thin films.
Solubilizing side chains can successfully spatially separate
chromophores, reducing detrimental intermolecular interactions.
However this strategy also significantly decreasing achievable dye
concentration. Here, five BODIPY derivatives containing small alkyl
chains in the α-position were synthesized and photophysically
characterized. By blending two or more derivatives, the increase in
entropy reduces aggregation and therefore produces films with
extreme dye concentration and, at the same time almost solution like
absorption properties. Such a film was placed inside an optical cavity
and the achieved system was demonstrated to reach the strong
exciton-photon coupling regime by virtue of the achieved dye
concentration and sharp absorption features of the film.
strength of the exciton-photon coupling depends on the ability of
a molecular film to absorb light. The coupling strength is
proportional to the transition dipole moment of the coupled
transition and the square root of the concentration of molecules.
Furthermore, the absorption envelope should be sharp as to
minimize dissipation of energy from the system. Highly
absorbing and small molecules having the ability of being
processed into neat films are therefore a limiting factor within
this research field.
Being half
a
porphyrin in size, BODIPY (boron
dipyrromethene) dyes have been shown to have a broad
spectrum of applications. Their excellent photophysical
properties such as high fluorescence quantum yields, large
molar absorptivity, narrow absorption and emission bands, small
Stokes shifts, tuneable emission, and a high photo- and
chemical stability make them highly versatile.[13] They are used
as
biological
labels[14]
,
as
fluorescent
switches[15]
,
photosensitizers[16] and as laser-dyes[17], to just name a few of
their applications. Although BODIPY dyes perform excellent
when in solution, in neat films, their main absorption band
broadens considerable and emission yields diminish, limiting
their application range. The cause for this are strong
intermolecular interactions between the π-core of the molecule
which leads to aggregation and thus scattering as well as highly
perturbed photophysical properties.[18] BODIPY dyes in the
strong coupling regime have been studied by the group of
Lidzey. They used BODIPY derivatives doped at 10-20% in an
optically inert polymer matrix to avoid aggregation, and by doing
so explored the optical properties[19] and the energy transfer
Organic dyes have a high tendency to aggregate due to their flat
structure and large polarizability. The close dye-dye proximity
give rise to strong interchromophore interactions resulting in new
and often unwanted effects such as H- and J-aggregates[1],
excimers[2], charge transfer states[3], and traps for charges[4]. In
materials chemistry, molecules ability to aggregate plays a
crucial role and a considerable effort has therefore been
dedicated to engineering the morphology of organic thin films.[5]
Self-assembly of chromophores is highly dependent on
interactions between the single molecules, especially π-π
interactions between the chromophore cores π-unit. Strong
intermolecular interactions are extremely sensitive on relative
distances and orientations, and the most widely adopted method
of reducing these interactions are by adding large bulky
substituents to the chromophore core.[6] These substituents,
whose van der Waal interactions are known to interplay with the
π-π interactions of the chromophore cores, enclose the
chromophores, insulating it from the environment, and thereby
enable unperturbed photophysical properties in neat films.[6b, 7]
However, substituents are often much larger than the
chromophore itself, causing an effective concentration decrease,
limiting the method to applications where the concentration and
intermolecular communication is less important.[8]
between two derivatives[9b]. Furthermore, polariton lasing[20]
formation of polariton condensates[21] and anti-stokes
fluorescence[22] have been successfully shown.
,
Entropic mixing uses entropy as a mean to reduce a
systems ability to aggregate.[23] The method utilize the entropy
increase of blends of photophysically indistinguishable but
physically different chromophores to reduce the Gibbs free
energy of the amorphous state.[24] This effectively counteracts
intermolecular interaction and therefore reduces the system’s
ability to aggregate. In theory, the method therefore offers the
possibility to make thin organic films having very high molecular
concentrations but still monomeric like photophysical properties.
In this work, five BODIPY dyes with linear as well as
branched alkyl chains in the α-position were synthesized with
the discovery of a previously unknown substitution effect that
enabled a fourfold increase in yield of the boron complexation.
The photophysical properties of the BODIPY dyes in solution
were assessed to be indistinguishable using steady state and
Strong light-matter coupling using organic molecules is a
relatively new research field, but has already harvested
considerable attention.[9] Landmarks such as selective chemical
reactions[10], room temperature Bose-Einstein condensation[11]
,
and modification of singlet and triplet energy levels[12], has for
instance been achieved in the strong coupling regime. The
1
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time resolved optical spectroscopy. Neat films showed a high
tendency to aggregate, considerable light scattering, and the
sharp BODIPY absorption features were considerable
broadened. Films consisting of mixed derivatives having
branched substituents showed reduced light scattering and
monomeric like absorption features. However, when derivatives
having branched and linear substituents were mixed together,
phase separation became evident, highlighting the importance of
using very similar substituents in entropic mixing schemes.
Finally, to demonstrate on the applicability of the concept, a film
consisting of mixed BODIPY derivatives where encapsulated
within an optical cavity, and the attained system was determined
to reach the strong coupling regime, due to the strong and
narrow absorption band of the BODIPY film.
Et3N and BF3·OEt2 to perform the complexation to the desired
BODIPY dyes.
This last two steps produced yields in the range of 10 to
50 %. The compounds with linear sidechains only gave yields of
~10 %, whereas adding a small branched sidechain like
isopropyl increased the yields substantially. The α-isopropyl-
pyrrole yielded 44 % and the α-secbutyl-pyrrole yielded 49 % of
the desired product. Adding a more bulky sidechain though, like
a tert-butyl group, only gave the product in 15 % yield, which
agrees well with earlier reported values.[27c] This leads to the
conclusion that steric bulk in the α-position either prevent side
reactions or increase the stability of the dipyrromethene-
intermediate (based on the proposed mechanism of this reaction,
there is no reason why steric bulk should affect the rate of the
reaction8). However, adding a too bulky substituent seems to
create a sterical hindrance, most probably in the complexation
step, lowering the reaction yield. In summary, five BODIPY
derivatives with ethyl, isopropyl, n-butyl, sec-butyl or tert-butyl
substituents were synthesised, with the surprising discovery that
the right amount of bulkiness on the α-position significantly
increases the yield of the complexation reaction.
BODIPY is a strongly absorbing dye in solution, but in the
solid state,
a
variety of aggregates result in strong
interchromophore interactions and hence a change in the
photophysical properties. To enable highly concentrated
BODIPY films with monomeric like photophysical properties, the
method of entropic mixing was employed. The synthesis of five
BODIPY derivatives, having ethyl (Et-BODIPY), n-butyl (nBu-
BODIPY), iso-propyl (IP-BODIPY), sec-butyl (sBu-BODIPY) and
Entropic mixing requires the blended molecules to exhibit
indistinguishable photophysical properties.[24] To verify that the
prepared BODIPY derivatives satisfy this criterion, steady-state
and time resolved optical spectroscopy was performed. The five
synthesized alkylated derivatives display almost identical
absorption and emission envelopes when in solution (Figure 1).
Et-, nBu-, sBu- and IP-BODIPY show sharp absorption and
emission bands as well as very small Stokes shifts, large
fluorescence quantum yields, single exponential excited state
lifetimes and high molar absorptivities (Table 1, Figure S2).
However, tBu-BODIPY has a slightly broader absorption and
emission profile as well as a larger stokes shift as compared to
the others. This difference in the photophysical behaviour is
probably due to a distortion of the fluorines, caused by the
sterically demanding tertbutyl groups at the α-positions of the
BODIPY core. In conclusion, the photophysical properties of the
prepared BODIPY-derivatives are indistinguishable from each
other (perhaps with the exception of tBu-BODIPY). The
BODIPY-derivatives can therefore be mixed together in order to
achieve high entropy blends with monomeric like photophysical
properties.
tert-butyl
(tBu-BODIPY) substituents was successfully
conducted via alkylation of pyrrole, followed by condensation
and complexation (Scheme 1).
Scheme 1. Synthesis of α-alkyl BODIPY dyes. I.) 1. POCl3, (CH3)2NOOR,
DCE 2. NaOAc, H2O II.) LiAlH4, THF or NaBH4, IPA III.) 1. MgI, Et2O 2. BrR',
Et2O 3. NH4Claq IV.) 1. POCl3, DMF, DCE 2. NaOAc, H2O V.) 1. POCl3, DCM
2. Et3N, BF3·OEt2.
Alkylation of pyrrole with linear sidechains was achieved
by performing a Vielsmeier-Haack type reaction using N,N-
dimethylalkylamides[25] and POCl3 to form the corresponding
pyrrylketones, and a following reduction of the carbonyl moiety
to the alkane, using either LiAlH4 or NaBH4. The alkylated
pyrroles were subsequently submitted to a Vielsmeier-Haack
formylation, using DMF and POCl3, which afforded the 2-alkyl-5-
formylpyrroles that were used for the BODIPY-condensation.[26]
Substitution of branched alkylchains was accomplished by a
Grignard reaction via the formation of a pyrrolegrignard, which
was treated with the designated alkylbromide to afford a mix of
α- and β-substituted alkylpyrroles.[27] As the formed pyrroles
could not be separated properly via distillation or column
chromatography, the regioisomer mixtures were subjected to the
next step, a Vielsmeier-Haack formylation, and the obtained 2-
and 3-alkyl-5-formylpyrroles could be separated using column
chromatography. The prepared 2-alkyl-5-formylpyrroles were
treated with POCl3 to undergo condensation to the
dipyrromethene-intermediate, which were directly treated with
Table 1. Photophysical data of BODIPY-derivatives in DCM solution.
Et
nBu
513
518
188
sBu
512
516
151
IP
tBu
509
515
229
λmax_abs (nm)
λmax_em (nm)
512
517
189
511
515
152
Stokes shift
(cm-1)
Φf
0.90
0.85
1.0
0.93
1.0
τ (ns)
5.42
5.26
5.09
98 000
5.15
5.35
ε (M-1cm-1)
97 000
98 000
101 000
100 000
The BODIPY core is flat, providing the possibility for molecular
stacking. Furthermore, its low energy high intensity electronic
transition indicates a large polarizability, and thus large London
2
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10.1002/chem.202002463
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dispersive forces, further reinforcing the molecules ability to
aggregate. The physical and photophysical properties of pristine
films of the BODIPY-derivatives were first studied, and these
were later compared to those of blended systems. All five
derivatives produce pristine films having a significant degree of
aggregated domains as revealed by polarized microscopy
(Fig. S3 and S4). Bulky- and linear substituents all seem to give
rise to similar aggregation, although n-Bu shows the largest
aggregated domain sizes, and IP more rod like aggregates than
the others. When chromophores are tightly packed together,
interactions between their transition dipole moments can be so
strong that the energetics are perturbed. The best-known
examples of intermolecular strong coupling is H- and J-
aggregates[1]. All pristine films of the derivatives scatter light to a
large degree and show broad absorption bands (Fig. S5). This is
quite unlike the sharp transitions in dilute solution, and it indicate
an inhomogeneous environment for the molecules in the films.
An inhomogeneous environment is also evident when observing
emission from the films. All exhibit broad and structured
emission, although two very distinct emission bands are clear in
sBu and IP films, indicating at least two distinct type of
aggregates. The emission lifetime of these two emissive states
differs substantially, where the low-energy transition always is
the long-lived one (Fig. S6). To summarize, pristine films of the
BODIPY derivatives have a substantial degree of aggregation
causing light scattering and a highly broadened absorption
profile.
blends containing linear alkyl chain derivatives, rod like
aggregate structures were observed, leading to broad
absorption and emission bands. Figure 2 display absorption and
emission spectra of selected blended films (see Fig. S11 for
other blend combinations). For the IP-sBu blend, very little
scattering is present and the linewidth of the main absorption
band is considerable narrower as compared to pristine films.
The absorption maximum is at 532 nm, a small redshift as
compared to the solution value of 512 nm. The emission show
two distinct bands, one sharp with a small Stokes shift, in shape
resembling the emission from dilute solution, and another
redshifted and slightly broader. Other combinations containing
sBu show similar photophysical properties (as for the sBu-IP
blend). To assess the effect of blending on the emission
quantum yield, the quantum efficiency of the sBu-IP blend as
well as the individual components, sBu and IP, were determined
to 15, 8, and 2%, respectively. This can be compared to earlier
reports of quantum efficiencies in the range of 7-13% for
BODIPYs doped at 15w/w% in polymers.[28] Blending thus
outperforms single components and allows for much higher
concentrations compared to dyes dispersed in polymers with
retained quantum efficiencies.
To examine if the low energy emission band is the result
from excimeric or J-aggregate like states, time resolved
emission spectroscopy was performed. The decay of emission
from the sBu-tBu blend film was recorded at the maximum of the
two emission bands (Fig. S14 and S15). The average lifetime of
the high energy monomeric like emission band is 1.0 ns, and the
average lifetime of the low energy emission band is 3.3 ns. A
global analysis of the decays from the two emission bands
reveals a build-up of low energy emission with the same time
constant as the high-energy emission decay. This indicates that
these two emissive states does not come from different locations
in the film, but rather that throughout the film the low energy
state acts as an energy relaxing route from the high energy
monomeric excited state. The absence of strong absorption in
the wavelength range of the low energy emission indicates that
either the aggregational state is present in low concentrations, or
it has a low transition dipole moment. Furthermore, absorbance
of the low energy transition is not seen in excitation spectra (Fig
S12). The relative long lifetime of the low energy state supports
excimer formation, whereas the relative narrow bandwidth of the
emission advocates J-aggregates. Previous studies have
pointed to the existence of J-aggregated dimers in BODIPY-
polymer mixes.[28] Other combinations of mixtures show broader
absorption features, indicating that the type of aggregates
present in the various films might differ.
1
1 1
1
0
1
0 0
1 1
0
1
0
0 0
0
400 450 500 550 600 650 400 450 500 550 600 650
1
1
0
0
400 450 500 550 600 650
Wavelength (nm)
Figure 1. Absorption (solid line) and emission (dotted line) of the BODIPY
derivatives in DCM solution.
Interestingly, we see emission around 500 nm in various
blends and in neat IP films. Considering the large emission
quantum yield, and that the feature is present in different blend
combinations, we assign this feature to an aggregational state
rather than an impurity. Excitation spectroscopy reveals that the
state being responsible for the emission only absorbs on the
high energy shoulder of the film (Fig S12). In summary, the
absorbance features from thin films consisting of blends of sBu
and IP/tBu is sharp and monomeric like. This indicates that for
entropic mixing to be successful, very similar substituents should
be used as to reduce possible phase segregation. The blending
furthermore reduces light scattering due to a reduced amount of
large aggregates.
Pristine films of the derivatives display a broad absorption profile
as well as significant light scattering. These undesirable features
are due to aggregated domains, being large enough to scatter
light, and providing a close packing that results in strong
interactions between the transition dipole moments of the
individual chromophores. To see if entropy can be used as a
means of disrupt packaging without diluting the concentration
compared to pristine films, the derivatives were blended together.
The properties of blended films can be classified into two
categories, those consisting of binary blends of IP, sBu, and tBu
that do not show any large aggregation domains, and others that
show signs of large aggregates forming (Fig. S7 – S10). In
3
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1
1 1
1
sBu-IP-tBu
sBu-IP
0
0 0
1 1
0
1
1
sBu-tBu
nBu-sBu-IP
Figure 3. a) Energy diagram showing how the molecular transition couples to
a cavity mode to form the two polaritonic states, the upper (UP) and lower (LP)
polaritons. b) Angle resolved reflectivity of the optical cavity, red and black line
show a fit to a coupled oscillator model, which as used to extract the coupling
strength, where the black line shows the lower polariton, the red line the upper
polariton, the white dashed line the energy of the cavity mode, and the white
dotted line the transition energy of the molecule.
0
0 0
0
500
600
700
500
600
700
Wavelenght (nm)
Wavelength (nm)
Figure 2. Absorbance and emission of a selection of blended films (see Figure
S11 for more combinations). The mixed films were prepared from toluene
solutions of the BODIPY dyes (cdye = 0.003 M). The ratio of the components
used in the mixes is 1:1 and 1:1:1 respectively.
In conclusion, five different BODIPY derivatives, alkylated
in the α-position, were synthesized. The highest achieved yield
in the boron complexation step was 49%, which is considerable
higher than previously reported yields for this kind of compounds.
This is presumably due to a certain steric bulk of the introduced
alkyl sidechains, although support for this assumption cannot be
rationalized by the suggested mechanism of BODIPY
condensation using POCl3.[26] As expected, all five derivatives
show very similar photophysical properties when in solution.
Unlike the properties of the dyes in solution, neat films of the
BODIPY derivatives show broad absorption and emission bands.
Blending of two or more derivatives leads to a notable
improvement of thin film properties. Some blends exhibit almost
solution like absorption and emission features. Furthermore, the
increased entropy in mixed films reduce the molecules ability to
aggregate, which is observed as a reduction in light scattering
due to a lower amount of large aggregated domains. To
demonstrate on the importance of thin films having sharp
absorption features and extreme dye concentrations, a blended
film was placed in an optical cavity, and the achieved system
reached the strong coupling regime. Given the prospects shown
here, we envision that entropic mixing will become a standard
tool as to control thin film morphology within materials science.
To reach the strong light-matter coupling regime, strongly
absorbing dyes need to be processed at high concentrations in
an optical cavity. The optical cavity provides a mean to enhance
the electromagnetic field strength at the frequency of the
molecular electronic transition. This by having a cavity thickness
that supports a standing wave with the same energy as the
BODIPY absorption. Now energy is interchanged between the
cavity and the dye faster than energy dissipation, and the strong
coupling regime is reached. When this happens, the molecular
transition is replaced by two polaritonic states, the upper and
lower polaritons, located at higher and lower energy as
compared to the molecular transition, respectively (Fig. 3a). The
polaritonic states consist of contributions from both light and
matter, and have therefore exotic properties such as being
delocalized over the entire cavity. Figure 3b displays angular
resolved reflectivity of a Fabry-Pérot cavity containing a BODIPY
film (sBu-IP 1:1). The BODIPY film is sandwiched between
layers of polyvinylalcohol (Fig. S1), which insulates the film from
the Ag mirrors in order to remove possible disturbances from Ag
surface plasmons. The molecular transition is clearly replaced
by two polaritonic states. As the angle of reflectivity increases,
the energy of the lower polariton approaches the energy of the
bare molecule transition, but never cross it. The observed
avoided crossing is a typical feature of a strongly coupled
system. The Rabi splitting (the minimal splitting between the
upper and lower polariton) was calculated to 358 meV by fitting
the angular resolved reflectivity to a coupled oscillators’ model
(see methods section). This value is considerably larger than the
full width at half maximum of the molecular absorbance
(115 meV) of the film, and the condition for entering the strong
coupling regime is therefore met. Thus, the ability to process
BODIPY in small films at extreme concentrations (about 3.1 M,
Acknowledgements
We gratefully acknowledge financial support from the European
Research council (ERC-2017-StG-757733).
Keywords: Entropic mixing • BODIPY • strong exciton-photon
coupling • chromophores • morphology
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10.1002/chem.202002463
Chemistry - A European Journal
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Entropic mixing of two or more, highly similar dye molecules results in thin films showing almost solution like photophysical properties.
Mixed films of new, low molecular weight BODIPY dyes, with short alkyl chains, were fabricated and furthermore used in an optical
cavity, where the strong coupling regime was reached.
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