Conjugated anthracene dendrimers with monomer-like fluorescence

Article abstract of DOI:10.1039/c4ra02341b

Two generations of highly emissive conjugated anthracene dendrimers containing up to 9 anthracene units are presented. In these dendrimers, anthracene-like absorption and emission properties are preserved due to the relatively weak electronic coupling between the anthracene units, while evidence of fast crosstalk within the molecular framework is still observed.

Full text of DOI:10.1039/c4ra02341b

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Conjugated anthracene dendrimers with
monomer-like fluorescence†
Cite this: RSC Adv., 2014, 4, 19846
Karl Borjesson,a Melina Gilbert,b Damir Dzebo,b Bo Albinssonb and Kasper Moth-
¨
´
a
*
Poulsen
Received 17th March 2014
Accepted 8th April 2014
DOI: 10.1039/c4ra02341b
www.rsc.org/advances
Two generations of highly emissive conjugated anthracene den- temporally and spatially energy transfer processes between
drimers containing up to 9 anthracene units are presented. In these peryleneimide chromophores in rigid polyphenylene den-
dendrimers, anthracene-like absorption and emission properties are drimers.18 Other examples of dendrimers showing intra-
preserved due to the relatively weak electronic coupling between the molecular energy transfer between dendrons have been
anthracene units, while evidence of fast crosstalk within the molecular reported by the groups of Meijer19 and Goodson.17,20,21
framework is still observed.
Recently dendrimers containing up to three anthracenes
have been prepared displaying interesting photophysical prop-
erties such as blue, green and white emission.22–29 In this
communication, two generations of highly emissive conjugated
anthracene dendrimers, containing up to nine anthracene
units, are reported (Scheme 1). The dendrimers only consist of
sp2 hybridized carbons in the form of diphenylanthracene,
forming a conjugated and rigid structure that without the need
Developing materials containing photoactive units in
a
dendritic fashion has gained much interest in the last decade
due to their attractive physical properties for molecular elec-
tronics.1–6 Compared to linear conjugated polymers, den-
drimers offer several advantages. They generally have higher
solubility and hence are more solution-processable. Their three-
dimensional structure prevents aromatic stacking interactions
to occur, and thus decrease tendencies of aggregate forma-
tion.7–12 For solid-state devices, incorporating emitter molecules
in dendritic structures, apart from increasing the solution
processability, also replaces the need of strong intermolecular
coupling with an intramolecular one.
From a photophysical point of view, multichromophoric
dendrimers, due to their well-dened structure, are also excel-
lent model systems for investigating electron and energy
transfer processes. Several examples of dendrimers functional-
ized with chromophoric groups showing intramolecular energy
and charge migration have been reported.13–17 For instance,
Hoens and co-workers have examined electronic communi-
cation between structurally identical chromophores. In partic-
ular by combining both time-resolved uorescence anisotropy
and single-molecule spectroscopy, they resolved both
aDepartment of Chemical and Biological Engineering/Applied Chemistry, Chalmers
¨
University of Technology, 412 96 Goteborg, Sweden. E-mail: kasper.moth-poulsen@
chalmers.se
bDepartment of Chemical and Biological Engineering/Physical Chemistry, Chalmers
Scheme 1 (i) Br2, CCl4, 0 ꢀC; (ii) (a) tert-butyllithium, THF, À78 ꢀC; (b)
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, À78 ꢀC
¨
University of Technology, 412 96 Goteborg, Sweden
† Electronic supplementary information (ESI) available: Experimental details of
synthesis and spectroscopy; analysis of exciton model, quantum mechanical
calculations and estimation of FRET rates. See DOI: 10.1039/c4ra02341b
to 0
ꢀC; (iii) Pd2(dba)3, TEAOH, tri(o-tolyl)phosphine, 1,3,5-tri-
bromobenzene, toluene, reflux; (iv) Pd2(dba)3, TEAOH, tri(o-tolyl)
phosphine, 1-anthracene-3,5-dibromobenzene, toluene, reflux.
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of solubilizing aliphatic side chains, is soluble in common
organic solvents. Their photophysical properties are compared
with 9,10-diphenylanthracene (DPA) which emits blue light at a
close to unity quantum yield.30 In the vast majority of multi-
chromophoric conjugated polymers, chromophore–chromo-
phore interactions lead to red-shied absorption and lower
uorescence quantum yields, thus altering their photophysical
properties compared to the monomers. Our dendrimer system
demonstrates that, with proper electronic coupling between the
constituents, the excited state can be partially delocalized with
fast exciton diffusion within the molecule while the attractive
absorption and emission properties of the monomeric unit are
retained.
Fig. 1 Absorption and emission (excited at 354 nm) spectra of DPA (a),
The two nearly C3 symmetric dendrimers consist of 3 (G1)
and 9 (G2) anthracene units respectively. They are made by a
convergent synthetic approach, using 1,3,5-tribromobenzene as
the core unit to produce the symmetric dendrimers from den-
dron arms (Scheme 1, for synthetic procedures see ESI†). This
method enables separation of the dendron arms and not fully
functionalized cores from the product, producing dendrimers
of high purity.31–33 The dendrons are synthesized from benzenes
and anthracenes, with the key synthetic step being the intro-
duction of a bromide functionality at the 9 position of anthra-
cene. This reaction is surprisingly selective using 1 eq. of
G1 (b) and G2 (c) dissolved in toluene.
The uorescence quantum yields in toluene of G1 and G2 are
close to unity, which is similar to DPA (Table S1†). Fluorescence
emission of both dendrimers decays mono-exponentially and
faster than for DPA, indicative of a higher radiative rate
constants for G1 and G2 (Fig. S1 and Table S1†). Prolonged
photolysis also reveals that the dendrimers are, due to their
larger radiative rate constant, more photochemically stable
than DPA (Fig. S2 and S3†).
ꢀ
bromine at 0 C. The yield of the reaction is close to quantita-
One challenge in dendritic structures is to understand how
electronic communication between the different dendrons
occurs. Two extreme cases for electronic communication within
the dendrimers can be distinguished. In the rst case, the
excitation energy is initially localized on one DPA unit and
subsequently migrates by hopping to other DPA chromophores.
The second case is a fully delocalized model where excitation
energy is directly delocalized on the whole dendrimer upon
excitation (see Fig. S4–S6† for visualization). Here the similar
spectral properties of the dendrimers and the monomer suggest
a relatively weak electronic coupling among the DPA moieties in
G1 and G2, and favors an exciton located on individual
anthracene moieties. The existence of a weak coupling is also
supported by semi-empirical calculations (INDO/S) of the
oscillator strengths (Fig. S7†), which show a linear increase of
the oscillator strength from DPA to G1 to G2 as the chromo-
phore gets larger. However the faster radiative rate constants of
the dendrimers indicate a more cooperative excitation and
emission of the individual DPA units (radiative coupling) and
points towards at least a partially delocalized exciton.34
In order to investigate how localized or delocalized the
exciton is in the dendrimers, uorescence anisotropy experi-
ments were performed (Fig. 2 and S8†). At 100 K, DPA has, as
reported previously,35 an increasing anisotropy with increasing
excitation wavelength over the lowest absorption band reaching
a value close to 0.4 at the red-edge of the absorption, indicating
that the transition dipoles responsible for the absorption and
emission events are close to collinear.36 G1 and G2, however,
exhibit a much more structured (and interesting) anisotropy
prole. The anisotropy is lower and oscillates, with the maxima
of the peaks off resonance with the vibronic peaks seen in the
absorption spectra (Fig. 2). The amplitude of the oscillation is
tive, and it does not seem to be very concentration dependent
(vide infra). The brominated anthracene is transformed to the
corresponding boronic ester by lithiation followed by boronic
esterication. This reaction produces boronic esters in high
yields but works best at quite high concentrations (ꢁ90 mM).
The boronic ester of the dendron is coupled using palladium
chemistry with 1,3,5-tribromobenzene or 1-anthracene-3,5-bro-
mobenzene to produce the dendrimer or one generation higher
dendron, respectively. This is also a reaction that gives the best
result when performed at high concentrations (ꢁ20 mM).
When the dendron grows the mass solubility is more or less
retained, however, the molar solubility is not. This forces the
reactions to be performed in more dilute solutions, with longer
reaction times and dryer solvent requirements. All three reac-
tion types retain their selectivity when performed in diluted
solutions, but their sensitivity towards oxygen increases. This is
seen as an increased fraction of dendrons having lost their
boronic ester in the reaction mixture (loss of ester functionality
is the only detected side product in this reaction). So even
though the limit of how large this family of dendrimers can be
has not been reached here, it is probably solubility rather than
selectivity issue that will determine the practical size limit. Still
the solubility of G2 is high enough (solubility > 5 mg mlÀ1 in
toluene) for solution processing in molecular electronics
applications.
Despite the different size of their conjugated system, the
three compared molecules (DPA, G1 and G2) show similar
spectral properties. Fig. 1 compares the absorption and emis-
sion spectra of the dendrimers and DPA. Only small changes in
the ratio of the intensities of the vibronic peaks, and a minor
red shi of the 0–0 transition energy level (E0–0 located at 401,
405 and 408 nm for DPA, G1 and G2 respectively) are observed.
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strength of 0.7) electronic transitions constituting the lowest
absorption band of G1. For G2, despite the increased number of
combinations of the transition dipole moments to be consid-
ered, explanation of the anisotropy oscillations by the presence
of several nearly degenerate transitions seems to prevail as well.
Further evidence on the electronic communication in the
dendrimers comes from time-resolved anisotropy measure-
ments. Both temperature-dependent and excitation wavelength-
dependent anisotropy decays of G1 and G2 were measured and
compared to the ones of DPA. At room temperature, the
anisotropies decay mono-exponentially with a gradual increase
of the correlation time from DPA via G1 to G2, which can be
attributed to the rotational motion of the chromophores. Also
no excitation wavelength dependency of the anisotropy decay is
observed at room temperature. When the temperature is low-
ered, the correlation time gets, as expected, longer due to the
increased viscosity of the solvent that slows down the rotation of
the molecules (Table S2 and Fig. S9†). More interestingly for G1
at intermediate temperatures (160 K and 140 K), when the
sample was excited at 385 nm and 395 nm, corresponding to
high and low fundamental anisotropy values respectively, two
correlation times were necessary to t the anisotropy decays
(Fig. 4).
While the anisotropy decay measured at 385 nm shows an
initial fast decay, the anisotropy decay measured at 395 nm
exhibits a corresponding rise time, and subsequently a slow
decay as for 385 nm (Table 1). The long correlation time agrees
well with our estimation of rotational correlation time (Table
S2†), while the short lifetime suggests the presence of two non-
degenerate emitting states. Excitation at 385 nm initially
populates the lowest excited state with high anisotropy while
the excited state with higher energy and low anisotropy is
mainly populated when exciting at 395 nm (Fig. 3). This agrees
well with our interpretation of the oscillating steady-state
anisotropy for G1 originating from the presence of two nearly
degenerate transitions. However, this short lifetime (z0.5 ns at
160 K) is relatively long compared to the times expected for
internal conversion (1 ps or less) or excitation energy transfer
Fig. 2 Absorption (dashed lines) and steady-state fluorescence exci-
tation anisotropy (solid lines) of DPA (a), G1 (b) and G2 (c) in 2-MTHF at
100 K.
larger in G1 than G2, which has the lowest anisotropy of the
three molecules.
To interpret these oscillations in anisotropy, a fully delo-
calized exciton model is considered.37 For G1, this would give
rise to three transitions, one forbidden at high energy, and the
other two at lower energy being degenerate and perpendicular
to each other (Fig. S5 and S6†). By considering these two major
perpendicular transitions and assuming for one of them a
limiting anisotropy of 0.3 (the maximum observed value of the
anisotropy for G1), the absorption spectra associated to these
two electronic transitions are resolved with the help of the
uorescence excitation anisotropy (Fig. 3; for details see ESI†).38
The degeneracy of the lowest electronic transitions is lied
probably due to static and dynamic distortions of the dendrimer
geometry. The energy splitting between the two transitions is
estimated from the resolved absorption spectra to 250 cmÀ1
.
Thus, anisotropy oscillations in G1 can be attributed to the
presence of two nearly degenerate transitions and point towards
a delocalization of the exciton. INDO/S calculations of AM1
optimized structures (Fig. S7†) of the dendrimers conrm the
presence of two nearly degenerate equally strong (oscillator
Fig. 3 Absorption (black) and resolved spectral components (blue and
red) of the lowest absorption band in G1. The dashed lines indicate the Fig. 4 Time-resolved fluorescence anisotropy decays of G1 at (a)
excitation wavelengths chosen for time-resolved anisotropy 160 K and (b) 140 K, excited at 385 nm (black) and at 395 nm (red). Solid
measurements (approximately at the local max and min of the exci- gray lines show fits to a bi-exponential model and the fitting param-
tation anisotropy (cf. Fig. 2b)).
eters are summarized in Table 1.
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Table 1 Fitted fluorescence anisotropy decay parameters of G1 in 2-
MTHF as function of excitation wavelength and temperature
These highly soluble systems show an absorption and emission
envelope very similar to 9,10-diphenylanthracene, as well as a
uorescence quantum yield close to unity. Based on the pho-
tophysical measurement and theoretical framework presented
here we conclude that the excited state in the dendrimers is at
least partially delocalized and the partially delocalized exciton
migrates within the conjugated structure faster than a few ps.
These dendrimers illustrate the fact that it is possible to make
highly soluble, conjugated molecules where the electronic
coupling between its sub units is low enough to retain mono-
mer-like absorption and emission properties, but still high
enough to exhibit an extremely fast crosstalk within the
molecular framework.
T (K)
160
lexc (nm)
r0,a
0.04
qa (ns)
r0,b
qb (ns)
c2
385
395
385
395
0.42
0.66
1.5
0.09
0.09
0.14
0.02
8.6
8.6
34
1.516
0.879
1.187
0.973
À0.04
140
0.02
À0.02
1.04
34
processes. A rough estimation of the minimum excitation
energy transfer (EET) rate in G1 using the FRET model gives a
transfer rate of 1013 sÀ1 and a EET time of 100 fs (more details in
the ESI†). Thus, the observed short correlation time of the order
0.5 ns is unlikely to reect excitation energy migration but
instead can be interpreted as the time to reach equilibrium
between the two emitting states. The fact that this equilibrium
is slow despite the small energy difference between the two
emitting states can be explained by one of the reactions being
uphill in energy. As we further decrease the temperature to
140 K, both the short correlation times (rise and decay) become
longer. The change with temperature is consistent with Arrhe-
nius activation energy of roughly 250 cmÀ1, supporting the
interpretation of these decay times as the time to reach equi-
librium between the two nearly degenerate lowest electronic
transitions. At 80 or 100 K (Fig. S10†), the upward rate is pre-
dicted to be too slow to be observed during the excited state
lifetime. This corresponds to a system that undergoes the
normal fast internal conversion to the lowest excited state and
emitting from this state alone, leading to the oscillatory
behavior of the steady state anisotropy when varying the exci-
tation wavelength (Fig. 2).
Acknowledgements
The authors would wish to acknowledge Victor Gray for deter-
mining melting points on the dendrimers.
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