Fluorescence quenching features in non-conjugated diacetylene oligomers

Article abstract of DOI:10.1134/S1070427213110062

It was found that, in diacetylene oligomers constisred of monomer units with an non-conjugated system of π-electrons, the efficiency of fluorescence quenching depends on the chain length within the range from 2 to 40 repeating units. This effect is consistent with the model of propagation of delocalized excitons in a noncovalent aggregate of several oligomer chains. The result obtained in the study confirms the assumption that a collective effect due to excitons can exist in a supramolecular aggregate of non-conjugated molecules. Pleiades Publishing, Ltd., 2013.

Full text of DOI:10.1134/S1070427213110062

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 11, pp. 1663−1669. © Pleiades Publishing, Ltd., 2013.
Original English Text © Yu.G. Vlasov, A.A. Kruchinin, D.S. Ryabukhin, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 11, pp. 1711−1717.
PHYSICOCHEMICAL INVESTIGATION
OF SYSTEMS AND PROCESSES
Fluorescence Quenching Features in Non-Conjugated
Diacetylene Oligomers
Yu. G. Vlasov, A. A. Kruchinin, and D. S. Ryabukhin
St. Petersburg State University, St. Petersburg, Russia
e-mail: akr@pobox.spbu.ru
Received November 15, 2013
Abstract—It was found that, in diacetylene oligomers constisred of monomer units with an non-conjugated system
of π-electrons, the efficiency of fluorescence quenching depends on the chain length within the range from 2 to 40
repeating units. This effect is consistent with the model of propagation of delocalized excitons in a noncovalent
aggregate of several oligomer chains. The result obtained in the study confirms the assumption that a collective
effect due to excitons can exist in a supramolecular aggregate of non-conjugated molecules.
DOI: 10.1134/S1070427213110062
In recent years, considerable attention was paid
to syntheses and studies of organic semiconductors
because they are considered aspromising materials
for field-effect transistors [1], light-emitting diodes
[2], chemical sensors [3], and solar cells [4]. Most of
materials of this kind are polymers with conjugated
aromatic rrings and multiple bonds, which have a
delocalized system of π-electrons. The delocalization of
electrons is due to the migration of excitons [5], which
is of particular interest for chemical sensors and solar
cells. In a polymeric sensor material, the diffusion of
long-lived excitons along the polymer chain provides a
multifold amplification of the analytical signal, which
is a desirable positive effect [3]. By contrast, a long
exciton lifetime is undesirable in solar cells because of
reducing the charge separation efficiency [4].
aggregates in which repeating units are linked to each
other by a noncovalent interaction [9]. Materials of this
kind are are known as supramolecular. They can find
use in optoelectronic devices, and, therefore, are studied
now together with the classical polymers in which
repeating units are linked by covalent bonds [10].
At present, supramolecular materials are constructed
on the basis of conjugated molecules. This approach
looks like generally accepted rather than based on
fundamental limitations of physicochemical nature.
An interesting task in this situation is to study the
fluorescence quenching, which depends on the
exciton migration, in organic polymers with btoken
intramolecular conjugation and aggregation capability.
To solve this problem, we used in the present study
oligomers based on diacetylene compounds, with a
chain length of 2 to 40 repeating units.
Thus, it is of interest to reveal factors affecting the
lifetime and diffusion length of excitons in organic
materials.
EXPERIMENTAL
One of factors of this kind is the conjugation length
of the system of π-electrons. This length is reduced by
introduction into a repeating unit a hexane ring [6], an
ether linkage [7, 8] or CH2 group [8]. Polymer materials
with broken conjugation of π-electrons are of interest
because they can be implemented as self-organizing
Synthesis of diacetylene oligomers. The compounds
under study were synthesized by the Glaser’s reaction
[11] by polycondensation of DEPmonomer (see scheme,
c). Divalent copper was used as a catalyst [12], which
simplifies the reaction as compared with its conventional
version [11]. The general formula of the synthesized
1663

1664
VLASOV et al.
Scheme. Synthesis of diacetylene oligomers and their monomer compound
N
N
N
N
N
N
I2/HlO3, H2SO4/AcOH
N
N
N
a
I
I
B1I2
B1
N
N
N
OH
b
+ HC
I
I
N
N
N
N
[Pd(PPh3)2Cl2], CuI
K2CO3, C5H5N
OH
HO
N
N
N
N
KOH
_Me2CO
HC
CH
c
N
N
N
N
N
N
N
N
Cu(CH3COO)2
H
H
H
H
Solvent
Temperature
Time
n
DEP
ODAn
(a) Iodination of the ligand, (b) synthesis of the monomer, and (c) polycondensation of the monomer.
oligomers can be written as ODAn, where n is the chain
length. Monomer compound DEP was synthesized
in accordance with parts a and b of the scheme from
1,2-bis[-(3,5-dimethylpyrazol-1-yl)dimethyl]benzene
(B1) and 2-methyl-3-butyn-2-ol (Favorsky carbinol).
The synthesis of the B1 ligand and its ability to form
aggregates and also complexes with seven cations
were described in [13]. Monomer compound DEP is
its diethynyl derivative synthesized by the previously
described procedure [14]. All the reagents were
purchased from Sigma-Aldrich. The number of repeating
units in the oligomer chain was adjusted by choosing the
Reaction conditions for synthesis of diacetylene oligomers with different number of repeating units in the backbone
Number of repeating units in
Compound
Т, °С
τ, min
Solvent
Yield, %
the backbone
ODA2
ODA5
ODA12
ODA40
2
0
20
20
50
15
5
Pyridine
85
81
98
68
5
12
40
Diethylamine
15
60
»
»
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FLUORESCENCE QUENCHING FEATURES
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(1)
Q = I0/I – 1,
reaction temperature and duration. Reaction conditions
for synthesis of diacetylene oligomers with the backbone
length of 2, 5, 12 and 40 repeating units are listed in the
table.
where I0 is the initial fluorescence intensity and I that
upon interaction with the quenching agent.
1H NMR spectra of the synthesized compounds
were measured with a Bruker 3500 instrument in a
CDCl3 solution. For ODA2 oligomer, the spectrum
demonstrated the following set of signals δ (ppm):
2.19 s (9H, Pz–CH3), 2.22 s (3H, Pz–CH3), 2.29 s
(9H, pZ–CH3), 2.31 s (3H, Pz-CH3), 3.19 s (2H, ≡C),
5.27 s (8H, CH2), 6.72 s (4H, Ph), and 7.21 s (4H,
Ph). The mass spectrum of this compound, measured
with a Bruker micrOTOF instrument, has a peak with
m/z = 683, which corresponds to a singly protonated
compound with formula C44H42N8. Thus, analysis of
the 1H NMR spectrum and mass spectrum confirms that
the compound we synthesized has the structural formula
shown in scheme c.
This parameter represents a product of Stern–
Volmer constant multiplied by quencher concentration
of the quenching agent [15]. This way to evaluate the
fluorescence quenching efficiency was chosen because
the dependence of the ratio between the fluorescence
intensities before and after the quenching on the
concentration of the quenching agent may be nonlinear
as, e.g., that described in [16]. In this case, the Stern–
Volmer “constant,” calculated as the derivative of
a curvilinear function, becomes dependent on the
concentration of the quenching agent and will contain
an error of a graphical or numerical differentiation.
The molar concentrations of the quenching agent
and of the ligand in the oligomer chain, used in the
present study, always had the same absolute value, and,
therefore, Eq. (1) makes it possible to exclude from the
estimate of the fluorescence quenching efficiency the
error associated with the differentiation of a nonlinear
Stern–Volmer dependence. The Cu(II) cation selected
as the fluorescence quencher was always added as
the CuCl2 salt. All fluorescence measurements of the
compounds under study were made with a PerkinElmer
LS-55 spectrofluorometer.
For ODA5, ODA12, and ODA40 oligomers, the
1H NMR spectra have no significant difference, except
the intensity of the signal of protons from terminal
CH groups (3.19 ppm), which decreases as the chain
becomes longer. The ratio of the signal area from CH2
groups (5.27 ppm) to the signal area from terminal
CH groups (3.19 ppm) is proportional to the number
of repeating units in the oligomer chain. It was used
to calculate this number. The ratio was 4.2, 9.9, 23.4,
and 73.7 for ODA2, ODA5, ODA12, and ODA40
compounds, respectively.
The conjugation length of the π-electrons system
was estimated by the method based on measuring the
optical absorption spectrum in the visible and UV
spectral ranges [17]. The essential of this method is the
fact that optical absorption spectrum undergoes specific
changes with increasing of oligomer chain length while
the oligomer length does not exceed the conjugation
length. When the conjugation length is reached and the
oligomer chain is lengthened further, the absorption
spectrum ceases to change. All the optical absorption
spectra were measured with a Shimadzu UV-2600
spectrophotometer. The size of aggregates into which
molecules of the compounds under study may combine
was estimated using a Malvern Zetasizer nano dynamic
light scattering (DLS) meter.
The B1 ligand incorporated into oligomer backbone
is expected to retain its features described in [13] for
free standing ligand. These are complexation with cation
Cu(II) and non-covalent aggregation at the concentration
above 10–4 М. In this case, the copper cation must be a
quenching agent for the fluorescence of the oligomer.
All the ODAn oligomers are soluble in tetrahydrofuran
(THF). This solvent was used in optical absorption and
fluorescence measurements of the compounds under
study. The molar concentration of all solutions is given
per single repeating unit.
Methods of study. The most interesting properties
of the compounds we synthesized are the efficiency of
fluorescence quenching due to the interaction with a
quencher and the conjugation length of the π-electrons
system. The fluorescence quenching efficiency was
evaluated by the parameter Q, which is expressed by the
equation:
Properties of diacetylene oligomers. The optical
absorption spectra of diacetylene oligomers with 2 and
40 repeating units and of the monomer are shown in
Fig. 1. It can be seen that the absorption spectra of both
oligomers contain the same, but higher intensity bands as
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VLASOV et al.
those in the spectrum of the monomer. The characteristic
fundamental change of the absorption spectrum,
with respect to that of the monomer, observed in [17]
with increasing length of the conjugated diacetylene
oligomer chain is not observed for the diacetylene
oligomers synthesized. The “red” (bathochromic) shift
of the absorption bands is also not observed neither for
increase in oligomer length from 2 to 40, nor for the
transition from monomer to dimer. In other words, the
system of π-electrons of each monomer unit is localized
in these compounds. This result corresponds to that
expected because the pyrazole rings and benzene ring
in the B1 ligand are linked by CH2 groups breaking the
electron conjugation.
important in working with high concentration solutions
whose optical density in the UV spectral range may
exceed a value of 2 (Fig. 1).
The fluorescence spectra of ODA40 compound
at various concentrations are shown in Fig. 2. Three
broad bands with peak intensities of about 420, 445,
and 490 nm can be distinguished in these spectra. With
increasing concentration, the intensity of the band at
490 nm grows and this bands becomes dominant at a
concentration of 9 × 10–4 M. This behavior indicates
that, with increasing solution concentration, aggregates
are formed due to the noncovalent interaction upon a
decrease in the average distance between molecules to a
rather small value.
The excitation and emission spectra for the
fluorescence of all the compounds ODAn are similar
to each other. The maximum of the excitation spectrum
lies within the range 370–385 nm. However, we used the
excitationwavelengthof400nminfurthermeasurements
of the fluorescence from ODAn compounds. The reason
of such a choice is the possibility to decrease the optical
density of a solution at the excitation wavelength
with acceptable loss of the emission intensity. This is
The aggregation of the compounds under study is
confirmed by DLS measurements. Unfortunately, the
positions and widths of the DLS peaks on the particle
size axis are unstable and give no way of unambiguously
relating the aggregate size to the oligomer length.
Nevertheless, it can be stated that the size of the
aggregates being formed is about 20 nm, and the “tail”
of the widest DLS peaks demonstrates the presence of
particles up to 300 nm in size.
λ, nm
λ, nm
Fig. 2. Fluorescence spectra of the diacetylene oligomer
ODA40 at various concentrations. Excitation wavelength
390 nm. (I) Intensity and (λ) wavelength; the same for Fig. 3.
Concentration (M): (1) 1.3 × 10–4, (2) 3.8 × 10–4, (3) 6.4 ×
10–4, and (4) 9 × 10–4.
Fig. 1. Optical absorption spectra of (1) DEP monomer and
(2–3) diacetylene oligomers (2) ODA2 and (3) ODA40.
Concentration 9 × 10–4 M. (D) Optical density and (λ)
wavelength.
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FLUORESCENCE QUENCHING FEATURES
1667
At present, three types of aggregates formed
by molecules in the ground (non-excited) state are
distinguished. These are aggregates of H and J types [18]
and also на joint type aggregates [19]. All of these yield
characteristic bands in the absorption spectrum, which
study the efficiency of fluorescence quenching in
ODAn compounds by Cu(II) cations. The variation
of the intensity of this band upon interaction with an
equimolar amount of Cu(II) with respect to the ligand is
shown in Fig. 3 for ODA12 oligomer. The copper cation
can be used to identified the type of an aggregate. A also quenches the emission in the 490-nm band for the
particular type of aggregates is constituted by excimers
[20], which are formed only from excited molecules
and, therefore, are only seen in the fluorescence spectra
and give no contribution to the absorption spectrum.
rest of the oligomers under study, but the quenching
efficiencies calculated by Eq. (1) are different. The
dependence of the quenching efficiency on the length
of the oligomer chain is shown in Fig. 4. It can be seen
that the maximum fluorescence quenching efficiency
is reached at an oligomer chain length of 12 repeating
units.
In our case, we can reliably determine that there are
no bands associated with J-type aggregates and those
of a joint type in the absorption spectrum. The absence
of bands associated with H-type aggregates cannot be
considered reliably determined because these bands
may lie in the short-wavelength part of the spectrum,
which cannot be measured with sufficient accuracy
because of the strong absorption by THF. The excimer
nature of the band at 490 nm is also consistent with
all the available data. Thus, a type-H aggregate or an
excimer may be responsible for this band. It is also
possible that the excimer is formed from a preliminarily
formed H-aggregate.
RESULTS AND DISCUSSIONS
The unequal quenching efficiencies for oligomers
with different chain lengths point to the existence of a
collective effect which has an influence on the whole
chain. The electron delocalization cannot be the reason
for this effect because the electron conjugation lengths
of all the compounds studied is equal to single repeating
unit. In this situation, there appears a substantiated
assumption that the observed collective effect is
due to the propagation of delocalized excitons in a
supramolecular aggregate.
We further used the emission band at 490 nm to
At present, delocalized excitons have been observed
400
200
400
500
600 λ, nm
Fig. 3. Quenching of the fluorescence of the diacetylene
oligomer ODA12 by the Cu(II) cation. (1) Initial fluorescence
and (2) that upon interaction with an equimolar amount of
copper chloride.
Fig. 4. Fluorescence quenching efficiency vs. the length n of
the oligomer chain. (I0/I) Ratio between the initial fluorescence
intensity and that upon interaction with the quenching agent.
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VLASOV et al.
Thus, the results we obtained supplement those
published previously and confirm the possibility of
appearance of a collective effect in a supramolecular
aggregate of non-conjugated molecules. The
mechanism of a collective effect in the absence of an
electron conjugation can be based on the formation of
delocalized excitons, in which aggregated oligomer
chains are involved [21].
experimentally in noncovalent aggregates [10]. Their
propagation length is about 10 repeating units. However,
the aggregates studied in [10] were formed from
conjugated molecules having no oligomer structure.
For this reason, the term “repeating unit” is understood
by the authors as a constituent of a supramolecular
aggregate, which differs from its meaning in the present
study and complicates the comparison of results.
The theory of propagation of a delocalized exciton
in a type-H aggregate was considered in [21]. In this
study, its motion is compared to rolling of a solid ball
over a spring mattress in which each spring is an analog
of a molecule in an aggregate. The delocalization of an
exciton in this model is due to the resonance interaction
of neighboring molecules and is limited by the disorder
in their arrangement. If we assume that aggregate with
the most regular structure is formed from the oligomer
with a chain length of 12, then the highest fluorescence
quenching efficiency for this oligomer can be explained
in terms of the model [21]. However, the applicability of
this model to molecules with btoken conjugation remains
questionable because the model has been developed to
describe conjugated molecules.
CONCLUSIONS
(1) The system of π-electrons of each repeating unit
of the diacetylene compounds studied is localized and
cannot be responsible for the collective effect which has
an influence on the whole oligomer chain.
(2) The dependence of the fluorescence quenching
efficiency on the number of repeating units in the chain
is indicative of the existence of a collective effect
operative along the entire oligomer length.
(3) The highest fluorescence quenching efficiency at
a chain length of 12 repeating units is consistent with
the model in which the collective effect is due to the
formation of delocalized excitons in a noncovalent
supramolecular aggregate.
Nevertheless, the propagation of delocalized
excitons was also observed in [8] for a polyelectrolyte
in which the conjugation was btoken due to the ether
linkage between repeating units. According to [8], this
effect can be attributed to the helical conformation of
the polymer molecule, for which a resonance interaction
appears between neighboring coils of the spiral, as it
occurs in the H-aggregate.
(4) The results obtained in the study indicate that
supramolecular materials for electronic devices can
be constructed from non-conjugated molecules, which
extends the variety of materials suitable for this purpose
due to the use of chemical compounds not previously
considered as such.
ACKNOWLEDGMENTS
If we take into account that fluorescence quenching
efficiency was enhanced with respect to the repeating by
up to a factor of 25 and the excitons were propagated in
both direction from the place of their generation, then
the exciton delocalization range can be estimated as
approximately 12 coils of the molecular spiral [8]. This
is in good agreement with the results of the present study
and data of [10]. The difference of our results from those
obtained in [10, 21] consists in the fact that in our case
aggregate is formed from non-conjugated molecules. In
compounds P1 and P2 from [8], the conjugation of the
polymer chain is btoken, but neither the chain length nor
the molecular mass was specified for these compounds,
which impairs the informative value of this publication.
In addition, the repeating unit of compounds P1 and P2
has a structure different from that in the present study.
This work was supported by grant 12.37.128.2011 for
basic research according to the Program of advancement
of St. Petersburg State University.
Some equipment of the "Center for chemical analysis
of materials" at St. Petersburg State University was used
in this research.
The useful assistance of S.S. Levichev is greatly
appreciated by authors.
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