Influence of chlorine atoms in bay positions of perylene-tetracarboxylic acids on their spectral properties in Langmuir-Blodgett films

Article abstract of DOI:10.1016/j.saa.2017.08.043

The influence of chlorine atoms in the bay positions of the perylene-3,4,9,10-tetracarboxylic acids with the different alkyl chains length on their spectral properties in monomolecular films has been studied. The chlorinated (PCln) and for comparison non-chlorinated (Pn) perylene derivatives were deposited onto quartz plates using a Langmuir-Blodgett (LB) technique. The absorption spectra showed that the PCln and Pn dyes form in monolayers the I- and J-type aggregates, respectively. In turn, their steady-state and time-resolved emission spectra revealed presence of two emitter types, which we assigned to monomers and excimers. The luminescence lifetimes of the PCln monomers and excimers determined with a time-correlated single photon counting method (TCSPC) are significantly shorter than these obtained for the same emitter types in the Pn monolayers. In the case of the chlorinated dyes, the contribution of the monomer emission dominates over the excimer emission and is almost independent from the alkyl chain length. By contrast, the share of the Pn monomer emission increases strongly with a number of carbon atoms in their hydrocarbon chains. The luminescence quantum yields (LQY) of the Pn and PCln monolayers measured in an integrating sphere are in the range of 0.06–0.11. The presented results reveal that the PCln dyes exhibit lower tendency for aggregation than the non-chlorinated derivatives. It can be explained by limited intermolecular interaction between neighbouring PCln molecules caused by deformation of the perylene core as a result of strongly electronegative chlorine atoms in the bay positions of these dyes. Moreover, the strong influence of the alkyl chain length on the Pn aggregation contrary to the case of the PCln derivatives was observed.

Full text of DOI:10.1016/j.saa.2017.08.043

Influence of chlorine atoms in bay positions of perylene-tetracarboxylic acids
on their spectral properties in Langmuir-Blodgett films
Emilia Piosik, Anna Synak, Tomasz Marty n´ ski
PII:
DOI:
Reference:
S1386-1425(17)30667-4
doi:10.1016/j.saa.2017.08.043
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28 April 2017
1 August 2017
13 August 2017
Please cite this article as: Emilia Piosik, Anna Synak, Tomasz Marty n´ ski, Influence
of chlorine atoms in bay positions of perylene-tetracarboxylic acids on their spectral
properties in Langmuir-Blodgett films, (2017), doi:10.1016/j.saa.2017.08.043
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ACCEPTED MANUSCRIPT
Influence of chlorine atoms in bay positions of
perylene-tetracarboxylic acids on their spectral
properties in Langmuir-Blodgett films
Emilia Piosika,∗, Anna Synak , Tomasz Marty n´ ski
b
a
a
b
Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60-965
Pozna n´ , Poland
Department of Mathematics, Physiscs and Informatics, University of Gda n´ sk, Wita
Stwosza 57, 80-952 Gda n´ sk, Poland
Abstract
The influence of chlorine atoms in the bay positions of the perylene-3,4,9,10-
tetracarboxylic acids with the different alkyl chains length on their spectral
properties in monomolecular films has been studied. The chlorinated (PCln)
and for comparison non-chlorinated (Pn) perylene derivatives were deposited
onto quartz plates using a Langmuir-Blodgett (LB) technique. The absorption
spectra showed that the PCln and Pn dyes form in monolayers the I- and J-type
aggregates, respectively. In turn, their steady-state and time-resolved emission
spectra revealed presence of two emitter types, which we assigned to monomers
and excimers. The luminescence lifetimes of the PCln monomers and excimers
determined with a time-correlated single photon counting method (TCSPC) are
significantly shorter than these obtained for the same emitter types in the Pn
monolayers. In the case of the chlorinated dyes, the contribution of the monomer
emission dominates over the excimer emission and is almost independent from
the alkyl chain length. By contrast, the share of the Pn monomer emission
increases strongly with a number of carbon atoms in their hydrocarbon chains.
The luminescence quantum yields (LQY) of the Pn and PCln monolayers mea-
sured in an integrating sphere are in the range of 0.06-0.11. The presented
∗
Corresponding author.
Email addresses: emilia.z.piosik@doctorate.put.poznan.pl (Emilia Piosik),
a.synak@ug.edu.pl (Anna Synak), tomasz.martynski@put.poznan.pl (Tomasz Marty n´ ski)
Preprint submitted to Spectrochimica Acta Part A
August 14, 2017

ACCEPTED MANUSCRIPT
results reveal that the PCln dyes exhibit lower tendency for aggregation than
the non-chlorinated derivatives. It can be explained by limited intermolecular
interaction between neighbouring PCln molecules caused by deformation of the
perylene core as a result of strongly electronegative chlorine atoms in the bay
positions of these dyes. Moreover, the strong influence of the alkyl chain length
on the Pn aggregation contrary to the case of the PCln derivatives was observed.
Keywords: Perylene dyes, Langmuir-Blodgett films, Time-resolved emission
spectra, Luminescence lifetime, Absolute quantum yield of luminescence
1
. Introduction
In recent years, semiconducting organic compounds have gained particu-
lar interest due to their intrinsic properties that make them promising can-
didates for optoelectronic applications, namely: solar cells [1, 2, 3, 4], light
emitting diodes [5, 6, 7, 8, 9, 10, 11], field effect transistors [12, 13, 14, 15], dye
lasers [16, 17] as well as sensors of different types [18, 19, 20, 21, 22, 23]. They
show particular advantages in comparison to inorganic semiconductors. Thanks
to their solubility in common solvents, some organic compounds can be fabri-
cated into active layers of large-area, thin and flexible devices by low-cost so-
lution technology, i.e. ink-jet printing, spin-coating, drop-casting or Langmuir-
Blodgett (LB) technique [3, 24, 25, 26]. Moreover, their molecular structure
can be easily modified to obtain desired spectroscopic, electronic and process-
ing properties. Organic semiconductors are classified as hole (p-type) and elec-
tron (n-type) transporting materials depending on a carrier type, which is the
most effectively transported through an active layer. The chemically stable p-
type semiconducting organic materials possess properties meeting application
requirements. However, there is still a lack of the electron transporting organic
semiconductors due to their low chemical stability under ambient conditions,
low carrier mobility and difficulties in synthesis.
One of the most important groups of organic semiconductors are perylene
derivatives. They have been initially applied as industrial pigments, but recently
2

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perylene derivatives have attracted attention also as functional compounds with
application potential in optoelectronic devices due to their excellent fluorescent
properties, high electron mobility and photoconductivity[27, 28, 29, 30]. The
most of them exhibit excellent thermal and chemical stability under ambient
conditions[27, 28]. Perylene derivatives possess also high molar absorption co-
efficients and quantum yields of luminescence (LQY) close to unity in diluted
solutions[27, 28, 31, 32, 33]. However, in the concentrated solutions and well-
packed thin films formed by the solution-based techniques, these dyes create
dimers or higher molecular aggregates due to intermolecular π-π interactions,
van der Waals forces and H-bonding[34, 35, 36]. The molecular aggregation
quenches the luminescence of perylene derivatives and decreases the LQY of
perylenes in thin films[37]. Although, it is a negative effect in relation to spec-
tral properties of optoelectronics, the charge transport through an active layer
benefits from the higher degree of organization[38]. Thanks to tendency of pery-
lene derivatives for coplanar stacking into long columns upon the π-π interaction
between their aromatic cores (e.g. in the case of perylene bisimides), the charge
mobility is emphasized[31, 39].
Perylene itself shows p-type semiconducting properties. Some of the perylene
derivatives, such as 3-4-9-10-perylenetetracarboxylic bisimides and perylenete-
tracarboxylic dianhydrides, are n-type semiconductors. The same type of the
conductivity is also observed for other perylene dyes with atoms or bigger sub-
stituent groups attached in the bay-positions of the perylene ring, e.g. F [40, 41],
Cl [42, 43] or CN [43, 44, 45] and phenoxy groups[46]. As a result of such a
substitution, the energy of the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) is decreased, what reduces
an electron injection barrier to an active layer of a dye[42]. Furthermore, the
substitution in the bay positions implies twisting of the central six-member ring
of the perylene core with the dihedral angle between two naphthalene units
dependent from the substituents type. In the case of the molecular crystal of
tetrachloro-substituted perylene bisimides, the X-ray diffraction shows the di-
◦
hedral angle of 36.7 [47]. High dihedral angle strongly reduces overlapping of π
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electrons of neighbour molecules. It limits tendency of perylenes for the molec-
ular aggregation in concentrated solutions and ultra-thin films, e.g. created by
the Langmuir-Blodgett or drop-casting techniques, and thus limits fluorescence
quenching. The steric hindrance and repulsion forces between strongly electron-
withdrawing atoms, e.g. F, Cl or O in phenoxy substituents play here a crucial
role. The twisting of the perylene ring and additional substituents attached
peripherally to the perylene-tetracarboxylic acides have a strong influence also
on the structure of molecular crystals.
In this paper, we present spectroscopic studies on chlorinated (PCln)and
non-chlorinated (Pn) esters of perylene tetracarboxylic acid in monomolecular
films formed using the LB technique. We compared the spectral properties of
three esters of tetrachloro-tertacarboxylic perylene acid and three correspond-
ing members of the tetracarboxylic acid esters series with the same number
of carbon atoms in the alkyl chains (n=3, 5, 7). Comparison in pairs of the
chlorinated and non-chlorinated perylenes allowed us to evaluate the effect of
four chlorine atoms in bay position of the perylene core eliminating the effect
of the alkyl chain length attached laterally to the carboxylic groups. All types
of substituents predominantly affect an organization of dye molecules in con-
densed films such as LB mono- and multilayers as well as structure of molecular
crystals. The monomolecular films were formed with the use of the LB tech-
nique to control their thickness, molecules arrangement and intermolecular dis-
tances. The information on types of the molecular aggregates created by the Pn
and PCln dyes in the LB films was obtained by optical spectroscopy methods:
steady-state absorption and fluorescence spectroscopy as well as time-resolved
emission spectroscopy (TRES) and time-correlated single photon counting (TC-
SPC). The absolute luminescence quantum yields (LQY) of the films formed by
the investigated dyes were determined using a integrating sphere method.
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2
. Experimental details
The molecular structures of the investigated compounds are presented in
Figure 1. The Pn and PCln dyes were synthesized and chromatographically
Pn dyes
H2n+1Cn
O
O
C H
n
2n+1
O
O
O
O
H2n+1Cn
O
O
C H
n
2n+1
PCln dyes
Cl Cl
H2n+1Cn
O
O
C H
n
2n+1
O
O
O
O
H2n+1Cn
O
O
C H
n
2n+1
Cl Cl
Figure 1: The molecular structure of the Pn and PCln dyes, n = 3, 5, 7.
purified in the Institute of Polymer and Dye Technology of the Lodz University
of Technology (Lodz, Poland) according to previously reported procedures [48,
4
9, 50, 51]. Their synthesis is briefly described in Supporting Information.
The compounds were dissolved in a spectroscopically pure chloroform (POCH,
Poland) at the concentration of 10 mol·dm−3 to prepare spreading solutions.
The LB films were formed on a commercially available polytetrafluoroethy-
lene (PTFE) trough equipped with two hydrophilic barriers for symmetrical
monolayer compression and a vertical lift for their transfer onto solid supports
−3
(Minitrough 2 from KSV NIMA, Finland). As a surface pressure sensor a plat-
inum Wilhelmy plate with highly porous surface hanged on an electronic balance
was used. Ultrapure deionized water with specific resistance of 18.2 MΩ·cm
from the Milli-Q system (Merck, Germany) system was used as a subphase.
5

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◦
The subphase temperature was kept constant at 20 C by a cooling circulator
(F12-ED from Julabo, Germany). The Pn and PCln spreading solutions were
deposited at the clean air/water interface with a glass microliter syringe (Hamil-
ton, Switzerland). The monolayers were compressed symetrically at the barrier
−
motion speed of 5 mm·min after the time of 10 min for chloroform evapo-
1
3
ration. The films transfer onto hydrophilic quartz plates (35×15×1 mm ) for
spectroscopic characterization took place at the particular stage of compression
(at two-thirds of surface pressure increase) by the LB dipping method. The
monolayers were deposited during upstroke of a substrate initially located un-
der the water surface at the lift motion speed of 2 mm·min−1. Transfer ratios
were close to unity.
The absorption of Pn and PCln LB films was measured with the use of a
two-beam spectrophotometer Cary 400 (Varian, Australia). Their steady-state
emission spectra were measured by a high-sensitive in-house spectrofluorometer
with a single photon detection system based on H6240-02 photon counting head
from Hamamatsu (Japan) [52]. The PTFE integrating sphere (90 mm in diam-
eter) was coupled to the spectrofluorometer with a 3 mm optical fiber for the
measurement of the Pn and PCln monolayers LQY in an absolute manner. The
developed earlier measurement procedure – described in detail in [37] – allowed
to overcome such experimental difficulties as scattering, anisotropy of angular
emission and wave-guide effect as well as to determine this parameter with a
high accuracy (0.02). A laser diode (300 mW) with the emission maximum at
4
43 nm was used for the front face excitation of the samples.
The TRES spectra were measured using a pulsed spectrofluorometer of pi-
cosecond resolution described elsewhere [53]. A EKSPLA (Lithuania) laser sys-
tem composed of a pulsed Nd:YAG laser (PL 2143A/SS) and optical parametric
generator system (PG 401/SH) constitute the excitation source of the spectroflu-
orometer, which produces short, spectrally tunable light pulses with a full width
at half maximum (FWHM) of about 30 ps. The detection part consists of a spec-
trograph (2501S, Bruker Optics, USA) and streak camera (C4334-01, Hamma-
matsu, Japan). During TRES measurements the samples were excited with the
6

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wavelength of 450 nm. The fluorescence intensity decay profiles were measured
in the front face mode upon the excitation at 440 nm with the use of a laser
head LDH-D-C-440 with a controller PDL 800-D and PCI-board for TCSPC
TimeHarp 200 (Pico-Quant, Germany). The fluorescence light was recorded
by a H10721P-01 photomultiplier (Hamamatsu Photonics K.K., Japan) com-
bined with the slit of a Czerny-Tuner spectrograph Shamrock 303i-B (Andor
Technology, UK). The analysis of the fluorescence intensity decay profiles were
performed using FluoFit Pro version (PicoQuant, Germany).
3
. Results and discussion
The aggregation process of perylene derivatives in LB films can be analysed
by the Kasha exciton theory [54], which is a quantum mechanical method based
on a dipole-dipole interaction of monomeric units. The molecular aggregates
constitute quantum-mechanical systems featured by common eigenfunctions.
Their presence in a monolayer leads to formation of a new more energetically
stable state with lower or higher energy than the monomer excited state depend-
ing on the aggregates geometry. Therefore, the aggregation process of perylene
derivatives in the ground state causes a change in positions and shapes of ab-
sorption as well as luminescence bands with respect to the monomer spectrum.
To identify types of the aggregates formed by perylenes in LB films, absorption
and luminescence spectra of their strongly diluted solutions, in which molecules
exist in the monomeric state, are used as a reference.
Thin films formed by perylene dyes show emission similar to that of the α-
perylene crystal. It is associated with the tendency of excited molecules packed
within ordered monolayer to form Y- and E-type excimers. The Y-emission
originates from a partially relaxed excited pair of molecules (excimer precursor),
while the E–type emission is related to the presence of fully relaxed excimers in
a LB monolayer. The existence of these two emission types results from different
overlapping degrees of the perylene cores in excimers (in the case of the E-type
excimers overlapping of the aromatic rings is much stronger). Their presence
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in a LB film is accompanied by appearance of a broad and structureless band
in the luminescence spectrum. The Y-type emission is observed at 530-550 nm
for the α–perylene crystal [55, 56] as well as for the LB films of the perylene
derivatives [52, 57, 58, 59]. In turn, the E-type excimer emission occurs at
6
6
30 nm for the α–perylene crystal and at ≈600 nm in the case of monolayers [60,
1].
The absorption and fluorescence spectra of the P5 (green lines) and PCl5
−
6
(
blue lines) dyes diluted in the chloroform to the concentration of 3 ·10 M are
presented in Figure 2. The spectra recorded for the P5 solution are similar to
A
F
1
1
0
0
350
450
550
650
λ [nm]
Figure 2: The absorption and fluorescence spectra of the P5 (green lines) and PCl5 (blue
−
6
lines) dyes dissolved in chloroform solution at the concentration of 3 · 10
M.
those of the pure perylene monomer [62]. The absorption band at 375–500 nm
with the vibronic structure is related to the S0–S1 transition with the dipole
moment directed parallel to the long axis of the molecule. The fluorescence
spectrum is a mirror image of the absorption spectrum, what discloses simi-
larity of the vibrational energy levels of the S0 and S1 states. Corresponding
absorption and fluorescence bands of the PCl5 solution are broader and posses
less visible vibronic structure than in the case of the P5 dye. This effect is
associated with the loss of the PCl5 core planarity and lower symetry of its
8

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molecules due to the presence of chlorine atoms in the bay positions [47, 42, 63].
The PCln core deformation caused also over twofold decrease of their LQY in
chloroform solution [50] comparing to the non-chlorinated perylenes [48]. The
electronic structure and energy levels of the isolated perylene chromophore does
not alter with the alkyl chains length attached to the molecule aromatic core.
Therefore, the absorption and fluorescence spectra as well as the LQY of other
members of the Pn and PCln series in chloroform are identical with those of
the P5 and PCl5 dyes and are presented in the Supporting Information (Figure
S2 and S3).
The absorption and fluorescence spectra of the Pn and PCln dyes in the
LB films are shown in Figure 3a and 3b, respectively. Their characteristic pa-
rameters are given in Table 1. The absorption bands of the Pn monolayers are
broader and bathochromically shifted with respect to the corresponding bands
in the absorption spectra of their chloroform solutions. This suggests, on the
basis of the Kasha exciton theory [54], that the molecules of these dyes form the
J–type aggregates in the ground state. In the case of the PCln films, the signif-
icant broadening and change in the absorption bands shape were also observed.
However, the absorption bands position is almost unchanged in comparison to
the spectrum of the dye in the monomeric form, what is characteristic for the
I-type aggregates. Furthermore, one can notice that the absorption spectra
recorded for the films of the both dye types retain the vibrational structure.
This fact indicates the formation of dimers by the Pn and PCln molecules
rather than higher order aggregates.
The fluorescence spectra of the Pn LB films are not mirror images of their
absorption spectra. The spectrum of the P3 monolayer possess two distinguish-
able fluorescence bands with the maxima at 535 and 610 nm. The unstructured
peak with the maximum at 610 nm and high fluorescence intensity can be as-
cribed to the E-type excimer emission, while the much less intense peak with the
maximum at 535 nm is related to the P3 monomer emission [65, 66]. The con-
tribution of the monomer emission increases with a number of carbon atoms in
the alkyl chains, what is reflected in a change in the fluorescence bands shape.
9

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Figure 3: The absorption and fluorescence spectra of the Pn (a) and PCln (b) dyes in the
Langmuir-Blodgett films (P3, PCl3 - red lines, P5, PCl5 - green lines , P7, PCl7 - blue lines).
10

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Table 1: The parameters of absorption and fluorescence spectra of Pn and PCln dyes in
chloroform solution and LB films.
Absorption
Fluorescence
Sample
λABS
δABS
λFLUOR
δFLUOR LQY
−
1
−1
[nm]
[cm
]
[nm]
[cm
]
P5 solution
P3 LB
419 443 472
2560
4350
4350
4250
492 520
535 610
2060
2550
3000
3550
0.871
0.08
0.06
0.10
-
-
-
461 495
457 490
456 485
P5 LB
-
-
611
596
P7 LB
PCl5 solution 394 438 456
4500
6300
6150
6200
-
501
2510
3300
3450
3350
0.352
0.11
0.09
0.11
PCl3 LB
PCl5 LB
PCl7 LB
394 426 451
395 440 459
396 443 459
537 570
544 581
535 564
−
1
−1
∆
λABS/FLUOR = ± 1 nm, ∆δABS/FLUOR = ± 10 cm
for solutions, ∆δABS/FLUOR = 50 cm
3
for LB films, ∆LQY =0.05 for solutions, ∆LQY =0.02 for LB films
1
2
3
Wolarz et al.[64], Modli n´ ska et al.[63], Makowiecki and Marty n´ ski[37]
11

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Therefore, it is clearly seen that long alkyl chains can constitute stearic hin-
drance and limit a number of the E-type excimers formed by the Pn molecules.
The fluorescence spectra of the PCln LB films possess wide bands with long
tails up to 750 nm red-shifted with respect to the perylene monomer band (see
Figures 2 3b). The positions of their maxima (see Table 1) indicate that we
deal in this case with the Y-type emission of parallel dimers with smaller degree
of the perylene cores overlapping than the E-type excimers formed by the non-
chlorinated dyes. On the short-wavelength side of these bands, there are visible
shoulders at 535-544 nm depending on the dye associated with the monomer
emission bands overlapped by these originating from the excimers. The shape
of the fluorescence bands of the PCln films do not change significantly with
the length of the alkyl chains attached in the perylene core side positions. It
means that the PCln skeleton deformation and electrostatic repulsion between
molecules with attached strongly electronegative chlorine atoms in the bay po-
sitions lead to formation of the less densely packed monolayers than those of
the non-chlorinated dyes, in which the π − π stacking is already limited and
the alkyl chain length play no longer a crucial role in terms of the aggregation
hindering.
The absorbance value as well as the fluorescence intensity of the monolayers
formed by the Pn dyes decrease with the length of the alkyl chains. In the case
of the PCln LB films exactly reverse effect was observed. The differences in the
changes in the absorbance and fluorescence intensities of the Pn and PCln films
can be explained by different arrangement of the molecules in the monolayers
transferred onto quartz substrates. The molecular organization of both dye
types in the Langmuir and LB films was discussed elsewhere [65, 59, 63, 66].
The deeper insight into the aggregation properties of the Pn and PCln dyes
in the monolayers gave the fluorescence lifetimes measurements. The fluores-
cence intensity decay profiles were best fitted with two-exponential functions,
which means that two types of the emitters are present in the Pn and PCln LB
2
films. The fitting of exponential functions was estimated based on a χ param-
2
eter. The determined lifetimes with the corresponding χ parameter values are
12

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gathered in Table 3. This table includes also the fractional contributions of the
particular exponential curves to the total decay profiles.
The fluorescence lifetimes reported in the literature for perylene monomer
are in the range of 3–5 ns depending on substituents attached to the molecule
main core [31, 62, 67, 39, 68]. Makowiecki et al. [65] has shown that the lifetime
−
of the Pn dyes diluted in chloroform at concentration of 3 · 10 M is equal to
.85 ns. In turn, the lifetimes measured for the monomers of the dyes from the
6
3
PCln series by Paluszkiewicz et al. [50] take the values between 1.69 and 1.86 ns.
The shorter fluorescence lifetimes (τ1) determined for the Pn and PCln LB films
are quite similar to these measured for the corresponding dyes in solutions by
Makowiecki et al.[65] and Paluszkiewicz et al.[50] (see Table 3). For this reason,
they can be ascribed to the the Pn and PCln monomer emission. Moreover, the
comparison of the shorter fluorescence lifetime values determined for the both
dye types leads to conclusion that the attachment of chlorine atoms to the PCln
main core caused over twofold shortening of the monomer lifetime.
The tendency of perylene derivatives for excimers formation in the LB films
causes emerging of an additional depopulation channel associated with the
longer fluorescence lifetime (τ2). The values of the longer fluorescence lifetimes
determined for the Pn films are in the range of 18.31–27.57 ns (see Table 3),
what is typical for the excimer emission of the perylene derivatives [31, 62, 67,
3
9, 68]. In turn, the fluorescence lifetimes of excimers formed by the chlorinated
perylenes are over three times shorter than in the case of the dyes from the Pn
series.
The fractional contribution of the exponential functions related to the partic-
ular lifetimes of the Pn and PCln dyes in LB films (τ1 and τ2) to the fluorescence
intensity decay profile gives indirect information on the share of emitters in the
total emission. In the case of the Pn monolayers, the fractional contribution
of the exponential function associated with the longer fluorescence lifetime (τ2)
ascribed to the excimers decreases with the increase of a carbon atoms num-
ber in the alkyl chains. This is consisted with the conclusions derived from
the steady-state emission spectroscopy that the long alkyl chains can constitute
13

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Table 2: The fluorescence lifetime and fractional contribution of the LB films.
2
χ
Sample
τ1 [ns] C1 [%] τ2 [ns] C2 [%]
P3 LB
P5 LB
P7 LB
5.72
3.57
4.10
37
59
71
27.57
20.48
18.31
63
41
29
1.222
1.729
1.488
PCl3 LB
PCl5 LB
PCl7 LB
1.67
1.05
1.60
76
87
83
6.38
5.38
6.69
24
13
17
1.203
1.312
1.303
stearic hindrance and efficiently limit intermolecular interaction in monolayers
and as a result reduce a number of the excimers formed by the Pn dyes. In
the case of the PCln LB films the monomer emission clearly dominates over
this originating from the excimers. The share of the Y-type excimers in to-
tal emission does not exceed 24% and remains almost independent from the
hydrocarbon chain length.
The emission spectrum of the excimers formed by the perylne dyes cannot be
experimentally recorded separately due to their coexistence with the monomers
in the LB films. However, it is possible to shed the light on this problem by
taking advantage of the huge difference between the fluorescence lifetimes of the
monomers and excimers and applying the time-resolved emission spectroscopy.
The TRES images were taken for the monolayers of all investigated dyes and cut
with the stripes with the width of 0.4 ns to obtain the time-resolved emission
spectra. The TRES spectra obtained for the P3 and P7 as well as PCl3 and
PCl7 monolayers are presented in Figure 3, while these recorded for the LB films
14

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Figure 4: Time resolved emission spectra of the LB films formed by the P3 (a), P7 (b), PCl3
(
c) and PCl7 (d) dyes – the original TRES image. The inserts in the right part of each chart
show the time evolution of the emission spectra. Numbers (in nanoseconds) represent time
delay of the spectrum relative to the excitation point.
15

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formed by the P5 and PCl5 dyes are shown in Supporting Information (Figure
S3).
Just after excitation (after 1.2 ns) of the P3 LB film the emission spectrum
possess two bands with the maxima at 563 and 588 nm, which means that we
deal hear with the monomer and E-type excimer emission. The time evolution
of the emission spectrum shows that the monomer emission vanishes gradually
and only the broad and structure-less band with the maximum at 620 nm char-
acteristic for the E-type emission remains visible. The TRES spectra recorded
for the P7 monolayer have the wide band at 560 nm, which is superposition
of the bands associated with the monomer and E-type excimers emission and
retains almost unchanged in time peak position and shape.
The time-resolved spectrum recorded for the monolayer formed by the PCl3
dye directly after the excitation is superposition of the bands related to the
monomer and Y-type emission with the corresponding maxima at 513 and 532
nm. After the timer greater than the fluorescence lifetime of the monomers,
only the emission originating from partially relaxed dimers with the maximum
at 547 nm was observed. Similar behaviour took place in the case of the PCl7
LB film.
The LQYs determined for the Pn monolayers are 9-15 times smaller than
these measured for the corresponding dyes in the diluted chloroform solution (0.87)
and do not exceed the value of 0.10 (see Table 1). The lower tendency for the
aggregation of PCln molecules seems to be not reflected in the LQYs of the LB
films formed by them, which take similar values (0.09-0.11) to these of the Pn
LB monolayers (0.06-0.10). However, it should be taken into account that the
LQY of the PCln dyes is over two times smaller than this of the non-chlorinated
perylenes already in the strongly diluted solution. As a result, the aggregation
process in the PCln monolayers lowered only about threefold the LQY value in
relation to the monomeric form. For this reason, a good alternative for chlorine
atoms in the bay positions of the perylene-3,4,9,10-tetracarboxylic acid deriva-
tives may be fluorine or bromine atoms, which cause twisting of the molecule
core, but simultaneously do not decrease the monomer LQY [40, 69].
16

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4
. Conclusion
Here, we present the studies on the spectral properties of the Langmuir-
Blodgett (LB) films formed by the chlorinated (PCln) and non-chlorinated (Pn)
derivatives of perylene-3,4,9,10-tetracarboxylic acid with four lateral alkyl chains
composed of n number of carbon atoms (n=3, 5, 7). The absorption measure-
ments revealed that PCln molecules in the ground state form the I-type ag-
gregates in the monolayers, while the dyes from the Pn series exhibit tendency
to stack into the J-type aggregates. The steady-state and time-resolved emis-
sion spectroscopy allowed us to identify the types of the emitters present in
the LB films formed by these two dye types. In all cases the monomer and
excimer emission were observed. The dyes from the PCln series form the Y-
type excimers in the monolayers. In turn, the LB films of the Pn derivatives
show the E-type emission. The fluorescence lifetimes of the Pn monomers and
excimers are equal to 3.85 ns and tens of nanoseconds, respectively. The attach-
ment of chlorine atoms in the bay positions of the PCln dyes caused significant
shortening of the monomers as well as excimers lifetimes. The contribution
of the monomer and excimer emission to the total luminescence of the PCln
monolayers remains almost independent from the alkyl chain length with the
outbalance of the monomer emission. By contrast, in the case of the Pn LB
films, the share of the monomer emission significantly increases with the length
of the alkyl chains of the Pn derivatives. The both types of the investigated
perylene derivatives possess similar quantum yields of luminescence (LQY) in
the LB monolayers, which do not exceed the value of 0.11. The introduction of
the PCln dyes into highly ordered LB films caused only threefold decrease of
the LQY in comparision to the monomeric form. This decrease is much stronger
in the case of the Pn monolayers.
The spectral characterization of the PCln and Pn LB monolayers performed
by us allows for better understanding of a role of lateral substituents in the
aggregation process of the perylene derivatives, which is crucial for performance
of optoelectronic devices. Our results show that the PCln dyes exhibit lower
17

ACCEPTED MANUSCRIPT
tendency for the aggregation in the LB films than the non-chlorinated deriva-
tives. Chlorine atoms in their bay positions are responsible for the perylene
core deformation and electrostatic repulsion between molecules leading to the
formation of the less densely packed monolayers. As a result the intermolecular
interaction and thus molecular aggregation are limited almost independently
from the alkyl chain length. Nevertheless, in the case of the the Pn dyes with
the planar cores, the long enough hydrocarbon chains hinders their stacking into
the aggregates.
Acknowledgements
Authors thank Dr. Eng. M. Niedbalska and Dr. R. Stolarski for helpful
discussion in the presentation of the dyes synthesis.
Presented work has been financed by the Ministry of Science & Higher Ed-
ucation of Poland in 2017 year under Project No 06/65/DSMK/0004. TM is
thankful for financial support by the Ministry of Science and Higher Education
of Poland in the framework of the project 06/65/DSPB/5171. AS was financed
within the grant 2015/17/B/ST5/03143 by the National Science Centre.
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Graphical abstract

ACCEPTED MANUSCRIPT
Highlights
•
•
•
•
Spectral properties of chlorinated and non-chlorinated perylene derivatives
in Langmuir-Blodgett films have been compared.
Deformation of chlorinated perylenes core reduces intermolecular interac-
tions and limits an aggregation process.
Long alkyl chains in side positions of non-chlorinated perylenes hinder
stacking of their molecules into aggregates.
Determined fluorescence lifetimes and luminescence quantum yields strongly
support discussion about aggregate types in LB films of perylene deriva-
tives.