Utilizing formation of dye aggregates with aggregation-induced emission characteristics for enhancement of two-photon absorption

Article abstract of DOI:10.1039/c7tc05509a

Femtosecond Z-scan measurements of aggregates of a quadrupolar D-π-D dye, 9,10-bis(N-hexylcarbazol-2-yl-vinyl-2)anthracene (BHCVA), provide direct evidence that aggregation of this AIE (aggregation-induced emission) chromophore leads to a sizeable enhancement of the intrinsic two-photon absorption cross section. Aggregation-induced improvement of nonlinear response is explained by theoretical analysis of electronic structure and geometrical parameters of aggregates.

Full text of DOI:10.1039/c7tc05509a

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Utilizing formation of dye aggregates with aggregation-induced
emission characteristics for enhancement of two-photon
absorption
Received 00th January 20xx,
Accepted 00th January 20xx
Adrian Justyniarski,a,# Jan K. Zaręba,a,# Piotr Hańczyc,b Piotr Fita,c Marta Chołuj,d Robert Zaleśny,d
Marek Samoć,a,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Femtosecond Z-scan measurements of aggregates of
quadrupolar D-π-D dye, 9,10-bis(N-hexylcarbazol-2-yl-vinyl- The formation of aggregates of some specific classes of
2)anthracene (BHCVA), provide direct evidence that aggregation compounds (called AIE-gens derivatives of
a
such as Metal-Organic Frameworks (MOFs).8, 9
-
of this AIE (aggregation-induced emission) chromophore leads to a tetraphenylethene, anthracene, phenylsiloles to name only a
sizeable enhancement of the intrinsic two-photon absorption few of them10) leads to the interesting phenomenon of
cross section. Aggregation-induced improvement of nonlinear aggregation-induced emission (AIE)11-14 which consists in,
response is explained by theoretical analysis of electronic sometimes dramatic, increase of the quantum efficiency, ꢀ, of
structure and geometrical parameters of aggregates.
luminescence of a chromophore.15-17 From the point of view of
applications of TPA chromophores, especially related to
imaging, the AIE effect is very beneficial, as it enhances the
Over the past two decades numerous synthetic strategies for
designing nonlinear chromophores with high two-photon
absorption (TPA) efficiency have been developed, focusing on
boosting the electron density delocalization within a molecule
and creating dipolar or multipolar charge distribution.1 Beyond
the molecular level, it has also been demonstrated that the
TPA strength can be increased by effects that arise from
through-space interactions between π-electron densities of
molecules or molecular fragments, sometimes referred to as
„cooperative effects“.2
brightness of the two-photon induced luminescence (ꢀσ2)
which is of importance for bioimaging of cells or tissues where
molecular crowding often leads to
aggregation.18, 19
a
spontaneous
While the increase of fluorescence quantum yield of molecules
featuring the AIE effect is a well-proven matter and has been
20-
exploited in TPEF (two-photon excited fluorescence) studies
22
a nontrivial point is whether the observed enhancement of
the two-photon induced emission is due solely to the increase
of the quantum yield, or if the TPA process itself (that is,
increase of the intrinsic TPA cross section) may also contribute
to the observed effects.
The above issue has been noted,23 but still lacks direct
experimental evidence. We present here a set of femtosecond
Z-scan results, corroborated by computer simulations clearly
demonstrating that aggregation of an AIE compound
significantly boosts (over two-fold) the intrinsic TPA cross
section.
Indeed, increase of the TPA cross section (σ2) upon dendrimer
formation3 or molecule aggregation4 has been postulated in
several reports, including huge cross sections in protein
aggregates like amyloids5 and silk,6 as well as aggregates of
relatively small molecules (i.e. sulfonated porphyrins and
Thioflavine T).7 Notably, cooperative effects elicited by the
close proximity of ligand molecules are also considered to be
responsible for strong TPA response of solid-state materials
Fig. 1. The structure of BHCVA. Bonds in bold indicate the dihedral angle
affected by aggregation, as discussed in further sections.
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As a model compound for our studies we have designed and GM∙mol∙g-1, which falls within the range of good organic and
prepared a quadrupolar donor-π-donor AIE molecule (BHCVA
organometallic TPA chromophores.25-27
,
Fig. 1) and, using different ratios of solvent and non-solvent, We have also studied the wavelength region corresponding to
we have induced aggregates formation. Synthetic procedures, three-photon absorption of BHCVA (1100-1350 nm).
NMR characterization, and discussion of mechano-, vapo- and Aggregates in chloroform do not exhibit clean three-photon
termochromic properties are provided in the ESI, Figs. S1 – absorption signatures. Instead, open-aperture Z-scan traces
S12.
clearly showed presence
Confirmation of AIE nature of BHCVA was performed in
THF:water mixtures at low compound concentration (see ESI,
Fig. S14). As it was expected, in pure THF and in mixtures with
low contribution of water almost no fluorescence was
observed. For 50% (v/v) content of water BHCVA started to
crystallize, which was deduced from emission peak at 529 nm,
being the same as for the bulk BHCVA. Further experiments
were performed in highly concentrated BHCVA solutions in
THF and chloroform (2.74 mM), required for NLO studies (see
below). High concentration already promotes aggregate
formation, even in pure solvents, which is observed in
absorption spectrum. However, in order to obtain progress in
aggregation and at the same time avoiding bulk crystallization,
for those systems acetone was used as a poor solvent. The
determined thresholds above which bulk crystallization of
BHCVA occurs in chloroform/acetone and THF/acetone
mixtures are 20% and 50% (v/v) of acetone, respectively.
Subsequently, we explored the two-photon absorption using
the well-established Z-scan technique in wavelength range
625-900 nm, employing low repetition rate (1 kHz), ~130
femtosecond laser pulses to minimize contributions from
processes other than instantaneous nonlinear absorption. A
crucial advantage of this method (over TPEF measurement) is
that it directly provides information on the strength of two-
photon absorption properties, since the results do not depend
on the quantum yield of fluorescence.24 Details on the
experimental setup and sample preparation are given in the
ESI. Additional information about non-radiative processes was
provided by pump-probe experiments.
Fig. 2 presents two-photon absorption spectra of BHCVA in six
Fig. 2. Two-photon cross-section of BHCVA a) in solution of increasing volume
different solvent/non-solvent mixtures. We found the most ratio of acetone in chloroform plotted with one-photon absorption of diluted
BHCVA in chloroform (solid gray line); b) in solution of increasing volume ratio
of acetone in THF plotted with one-photon absorption of diluted BHCVA in THF
(solid gray line). Full lines, connecting Z-scan-derived experimental points are
significant effects near the 675 nm wavelength. Firstly,
sizeable enhancement of two-photon properties for BHCVA is
drawn just to guide the eyes.
observed when the solvent is changed from pure chloroform
to mixtures with acetone. Secondly, 2.5-fold enhancement is
observed (from 164 GM to 408 GM) in a solution with 10% of
acetone, and further 1.25-fold increase up to 503 GM, for 20%
acetone content. For the pure THF and THF solution with 40%
of acetone the two-photon spectra are almost the same
(peaking around 370 GM at 700 nm). However, nearly two-fold
enhancement of TPA response to 720 GM is observed when
the acetone content is equal to 50%.
of photochemical processes that occurred at the focal point of
the laser beam (ESI, Fig. S15). This was confirmed in a separate
experiment, in which we irradiated the chloroform solution of
BHCVA with 800 nm laser light for
3 hours. The
photodegradation was evident, as judged from 1H NMR
spectroscopy (ESI, Fig. S16). Photochemical reactions occurring
in the focal point seem to be main reason why no clear
comment on possibility of enhancement of the three-photon
response can be provided (ESI, Fig. S17).
It is a known fact that two-photon response can scale in
various ways with the size of the conjugated scaffold, thus
meaningful interpretation and comparison of the two-photon
cross sections, especially between different types of
compounds, necessitates the use of figures of merit such as
the cross section normalized by molar mass (σ2/M). Here, we
take as an example BHCVA aggregates in 50% THF - 50%
acetone mixture; σ2/M value at 700 nm is equal to 0.99
Since the formation of aggregates is clearly found to increase
the TPA cross section, one needs to identify the possible
structural and electronic origin of such an enhancement.
Two alternate mechanisms may be considered. The first notion
points to the fact that aggregation causes molecules
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Fig. 3. Schematic representation of changes in electronic structure upon type A dimer formation. Vertical bars represent relative changes in the two-photon
absorption cross section. Orbitals on the left (right) side correspond to 4Ag ← 1Ag (9Ag ← 1Ag) large-intensity two-photon transitions. Dashed lines on both
diagrams depict photon energy.
possessing normally rotatable fragments (such as single bond of the TPA cross section, we suggest that type A aggregation is
connected to vinylic carbons in BHCVA, Fig. 1) to be a much more probable scenario.28 Therefore, further comment
planarized, which results in better π-electron delocalization will be provided mainly for this model of aggregation.
throughout the aromatic skeleton; the second approach In single BHCVA molecule the dihedral angle between the
focuses on intermolecular interactions between neighbouring plane of vinylic bond and the plane of anthracene fragment,
molecules.
denoted as φ in Fig. 1, is equal to 127.4°. Centrosymmetric
To assess which of these factors are likely to be of importance, BHCVA dimer and trimer aggregation models of type A show
we performed electronic structure calculations employing the strong tendency for planarization of vinyl bond-anthracene
density functional theory for monomer, and set of aggregation region. Indeed, for dimer two distinct φ angles, equal to 128.4°
models (two dimers and two trimers), assuming Ci symmetry and 138.6°, can be found while for trimer the planarization is
point group. The details of these calculations are presented in even more pronounced since three φ angles were determined
the ESI, here we briefly discuss the most important findings.
(134.6°, 144.4°, 145.3°). By contrast, aggregation models of
There is no straightforward experimental method that could type B reveal fairly changed φ angles with respect to monomer
provide the precise structure of nanoaggregates, which makes (127.1°, 134.8° for dimer, 125.3°, 127.2°, 133.3° for trimer). In
electronic structure calculations of those systems challenging. the ESI, Fig. S18, are drawn all BHCVA models with indicated φ
For this reason, we assume that due to the presence of angles. Those results suggest that molecular aggregation of
aromatic rings and significant dispersion interactions, the type A leads to flattening of the aromatic skeleton, which
studied molecule aggregates and the anthracene central appears to be directly connected to the elevated TPA
moieties form parallel stacks. Two scenarios of aggregation are response. Next, we focused on the electronic origin of
drawn on the above assumption. In the aggregation model of observed TPA enhancement. The aggregation leads to changes
type A the dimer and trimer ensembles tend to glide along in electronic structure, i.e. there is a 35 nm red shift in the
long axis of BHCVA molecule, while in type B aggregation the excitation wavelength corresponding to the large-intensity
BHCVA molecules form stacks, significantly glided along short one-photon transition to the 1Au state and this is in line with
axis (that is, along anthracene fragment) of BHCVA. Note that the experimental finding. The results of calculations also reveal
the latter type of aggregation model is proposed on the basis that the most intense two-photon transitions are 4Ag ← 1Ag
of analysis of crystallographic motifs of molecules structurally (578 nm, monomer) and 9Ag ← 1Ag (614 nm, dimer). The TPA
related to BHCVA (see ESI for discussion).
cross sections associated with these two transitions are at
In the first step, we have compared the TPA transition least one order of magnitude larger than the values for any
moments calculated for monomer and aggregate structures of other symmetry-allowed transitions (see ESI).
type A and B. A comparison of values collected in Tables S18- Fig. 3 shows the nature of these TPA transitions for monomer
S20, ESI, shows that type A aggregation affords ca. 50% and and dimer of type A. There is a symmetrical electron density
105% higher two-photon absorption cross section for dimer shift from carbazole moieties to anthracene central unit (and
and trimer, respectively, than that for the monomer. reverse). It is important to note that in the case of the dimer
Interestingly, type
B aggregation results in negligible one observes ca. 50% increase in TPA cross section in
enhancement of TPA cross section (maximally 10% for dimer, comparison with the monomer. This suggests that aggregation
see Tables S24 and S25, ESI). In light of experimental results, gives rise to very large TPA in the spectral region above 600
which clearly show that the aggregation yields even 250% rise nm. In order to gain an insight into aggregation-induced
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increase in TPA intensity we performed an analysis of leads to planarization of the aromatic vinyl bond-anthracene
electronic structure parameters (transition moments and dihedral angle, which enhances the electron delocalization
excitation energies). It turns out that the change in the across the molecule. This phenomenon is assisted by the
transition energy to the 1Au state upon dimer formation is the decrease of the detuning factor, which, according to quantum
most important factor responsible for increase in TPA mechanical sum-over-states expressions, leads to the increase
response.
of two-photon absorption probability.
We have also attempted to apply an aggregation model of
higher complexity. To this end, we calculated absorption We acknowledge the financial support from the Polish
features of aggregate consisting of three stacked BHCVA National Science Centre under “Maestro” DEC-
molecules. The maximum of one-photon excitation 2013/10/A/ST4/00114 grant and the Faculty of Chemistry,
wavelength of trimer is shifted by additional 16 nm (with Wrocław University of Science and Technology. We also
respect to the dimer). Moreover, calculation of two-photon gratefully
acknowledge
the
instrumental
grant
properties shows that there are a few intense two-photon 6221/IA/119/2012 from the Polish Ministry of Science and
transitions occurring in close proximity to the most intense Higher Education. The calculations were performed at the
one, which is 12Ag ← 1Ag. Fig. 3 shows the energy diagrams Wroclaw Center for Networking and Supercomputing.
corresponding to two-photon 4Ag ← 1Ag (monomer) and 9Ag
← 1Ag (dimer) transitions. As seen, in the case of the dimer the
Conflicts of interest
detuning factor (the difference between 1Au ← 1Ag transition
energy and the photon energy) is much smaller than that for
the monomer which, according to quantum mechanical sum-
over-states expressions, results in the increase of two-photon
absorption probability. Together they contribute to further
increase of the TPA cross section (see ESI).
The authors declare no conflict of interest.
Notes and references
1
2
3
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results show that the aggregation can significantly influence
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Observed lengthening of fluorescence lifetime is related with
progress of aggregation present in AIE compounds.
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and self-absorption of light of compound’s samples.
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S14, ESI, curves for 50% and 95% content of water,
respectively).
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