Bulk organisation and alignment in Langmuir and Langmuir–Blodgett films of tetrachloroperylene tetracarboxylic acid esters

Accepted Manuscript Bulk organisation and alignment in Langmuir and Langmuir–Blodgett films of tetrachloroperylene tetracarboxylic acid esters Anna Modlińska, Marek Filipowicz, Tomasz Martyński PII:

S0022-2860(16)30735-9

DOI:

10.1016/j.molstruc.2016.07.054

Reference:

MOLSTR 22757

To appear in:

Journal of Molecular Structure

Received Date: 19 May 2016 Revised Date:

12 July 2016

Accepted Date: 14 July 2016

Please cite this article as: A. Modlińska, M. Filipowicz, T. Martyński, Bulk organisation and alignment in Langmuir and Langmuir–Blodgett films of tetrachloroperylene tetracarboxylic acid esters, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.07.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Bulk organisation and alignment in Langmuir and Langmuir–Blodgett films of tetrachloroperylene tetracarboxylic acid esters Anna Modli´ nskaa , Marek Filipowiczb , Tomasz Marty´ nskia,∗

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a Faculty of Technical Physics, Poznan University of Technology, ul. Piotrowo 3, 60-965, Pozna´ n, Poland, e–mail: [email protected], [email protected] b Faculty of Advanced Technologies and Chemistry, Military University of Technology, ul. Kaliskiego 2, 00-908, Warsaw, Poland, e–mail: [email protected]

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Abstract

Perylene derivatives with chlorine atoms attached at the bay position to the dye core are expected to affect organisation and tendency to aggregation in Langmuir and Langmuir–Blodgett (LB) films. Therefore, newly synthesized core–twisted homologous series of tetrachloroperylene tetracarboxylic acid esters with n = 1, 4, 5, 6, 9 carbon atoms in terminal alkyl chains were studied.

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Phase transitions and crystalline structures were specified by differential scanning calorimetry (DSC) and single crystal X–ray diffraction (XRD), respectively.

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Intermolecular interactions and organisation of the dyes in monomolecular films were investigated by means of Brewster angle microscope (BAM), UV–Vis absorption and emission spectroscopy, fluorescence microscopy and atomic force

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microscopy (AFM).

The dyes investigated do not form thermotropic mesogenic phases in bulk.

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The crystalline triclinic elementary cell with P–1 symmetry is revealed from X–ray experiments.

In Langmuir and Langmuir–Blodgett films molecular

tilted head–on alignment is postulated. Spectroscopic research confirmed by AFM texture images of the LB films show that in Langmuir and LB films dyes, depending on length of terminal chains, have a tendency to create H ∗ Corresponding

author Email address: [email protected], tel:+48-61-665-31-72, fax: +48-61-665-31-64 (Tomasz Marty´ nski)

Preprint submitted to Journal of LATEX Templates

July 14, 2016

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or I molecular aggregates. The impact of the twisted core on the molecular

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behavior in a bulk and thin films is discussed.

Keywords: Perylene–like dye, Langmuir–Blogdett film, Aggregate, Brewster angle microscope, Absorption spectrum, AFM image

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2010 MSC: 00-01, 99-00

1. Introduction

Perylene derivatives as the most valuable functional dyes have been a focus

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of intensive research. Ease synthetic modifications of their molecular structures allow to design dyes with specified physico–chemical properties desired by or5

ganic electronic devices. Therefore, most of the perylene derivatives exhibit outstanding thermal and chemical robustness and photostability [1]. Moreover, they show strong absorption in the visible region and high fluorescence quantum yield in isotropic solvents [2]. Nevertheless, the most meaningful property

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is photoconductivity which implies for application of these materials in photovoltaic devices [3, 4], organic field–effect transistors [5] or light emitting diodes

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[6].

A rapid progress in optoelectonics observed during last decades induce searching for new materials with strictly determined properties. Nowadays,

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number of organic compounds with good n–type semiconductivity is still very limited.

Incorporation of chlorine atoms as an electron–withdrawing sub-

stituents in the bay position of perylene aromatic core should allows to obtain

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such materials. However, introduction of chlorine atoms into molecular system has a dramatic effect on its properties. Due to steric interactions between substituents such molecules show twisted skeleton [7], which may disturb formation

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of typical sandwich–type π–π stacking [8]. On the other hand, such molecules

can more efficiently lock each other hindering their rotation along the long axis what allows to form even more ordered systems. Possible well ordered columnar liquid–crystalline properties arising from combination of large aromatic core

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and flexible alkyl chains are also relevant due to the improvement of molecules alignment in active layers of electronic devices [9]. Afterwards this may have positive influence on efficiency of electric charge transfer.

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Taking into account above, we studied spectral and bulk properties and

molecular alignment of newly synthesized perylene derivatives at air–water

and air–solid phase interfaces using Langmuir–Blodgett (LB) technique. Phase transitions and crystalline structures were specified by differential scanning

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calorimetry (DSC) and single crystal X–ray diffraction (XRD). Different intermolecular interactions and organisation of the dyes in Langmuir and LB

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films depending on length of terminal hydrocarbon chains was investigated by means of Brewster angle microscope (BAM), UV–Vis spectroscopy, fluorescence 35

microscopy and atomic force microscopy (AFM). Furthermore, we discuss the impact of the twisted core on the molecular arrangement, aggregation process and optical properties of the thin films.

The molecular structure of compounds under investigation is given in Figure 1.

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2. Materials and methods

The dyes tetrachloroperylene tetracarboxylic acid esters (PCn) for

n = 1, 4, 5, 6, 9 were synthesized and chromatographically purified at the Insti-

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tute of Polymer Technology and Dyes, Lodz University of Technology, Poland [patent RP no. P.403565]. 45

Differential scanning calorimetry (DSC) measurements were performed by

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using a 141 SETARAM calorimeter (France) with a heating and cooling rate of 3°/min. Optical textures of thin dye layers at cross polarizers were obtained with a polarized optical microscope — POM (Olympus, model BX51TF, Japan) equipped with a heating stage and temperature controller (Linkam Cl 94, Aus-

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tria). Single crystal X–ray diffraction (XRD) were obtained at 121 K with D8 Quest Kappa Diffractometer (Bruker, USA) using CuKa radiation from an Incoatec ImS microsource with Montel multi–layered mirror, a Photon100 CMOS

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detector and Apex2 software. The structure was solved using direct methods

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(SHELXS), expanded with Fourier techniques and refined with SHELXL. All non–hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the structure factor calculation on geometrically idealized positions.

A commercially available Minitrough 2 manufactured by KSV Instruments

Ltd., Finland, was used for the formation of Langmuir and Langmuir–Blodgett

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films. The water subphase was deionized and purified to a final resistivity of 18.2 MΩ·cm by a Milli–Q system (Millipore Corporation, Austria). A constant subphase temperature (21℃) was maintained by a cooling circulator (F12, Julabo,

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Germany). The compounds were dissolved in spectroscopically pure chloroform (POCH, Poland) at a concentration of 0.1 mM in order to obtain stock solutions. The sample material was spread on the clean air–water interface with a glass 65

microlitre syringe (Hamilton, Great Britain) and left for 10 min for the solvent evaporation. Next, the monolayer was compressed at a barrier motion speed of 5 mm·min−1 . Simultaneously, the surface pressure (Π) was monitored by a platinum Wilhelmy plate hanged on a balance with an accuracy of 0.1 mN·m−1 as

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a function of mean molecular area (A) giving Π–A isotherm. All measurements were repeated on the fresh subphase three times to confirm reproducibility.

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Standard trough cleaning procedure was adopted between measurements. From the Π–A isotherm some characteristic points were obtained such as ΠC

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and AC which are the values of the surface pressure and mean molecular area at the collapse point and Aext — area obtained from the tangent to the slope of 75

the Π–A curve extrapolated to Π = 0. The collapse point is recognised as the

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point in the Π–A isotherm where the ratio ∂Π/∂A begins to decrease due to phase transition and indicates the formation of the condensed monolayer. For better characterisation of compounds investigated we have calculated ϕ angle — average angle between the normal to the water surface and rigid molecular

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core of the dyes as well as compressional modulus, CS−1 , using the following equation: CS−1 = −A

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∂Π ∂A

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(1)

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The morphology of the Langmuir films was visualized by means of a Brew-

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ster angle microscope (BAM). The instrument we used is based on Hoenig and Moebius setup [10] and was built in our laboratory. The image features were 85

observed with a terminal resolution of ≈2 µm.

For LB film deposition hydrophilic polished quartz plates (35×15×1 mm3 )

and silicon wafers (30×10×0.5 mm3 ) were used. The floating monolayers were

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transferred at a constant surface pressure during the vertical upward movement of the solid slide with a dipping rate of 2 mm·min−1 and dipping stroke of 90

25 mm. The transfer ratios were near unity.

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The steady–state absorption spectra of LB films were recorded in the UV–Vis spectral region by means of a spectrophotometer CARY 400 (Varian, Australia) with the incident light directed normally to the substrate surface. Furthermore, the steady–state fluorescence measurements of LB films were carried out with 95

PTI QM4–2003 fluorescence spectrometer and fluorescence quantum yield was estimated in the integrating sphere (C10027-01, Hamamatsu, Japan). The LB samples of PC1 were excited at 395 nm whereas of PC4–PC6 at 450 nm.

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Textures of Langmuir–Blodgett films deposited onto quartz plates were visualised by means of fluorescence microscope Nicon Eclipse LV100 equipped with halogen light source and high sensitivity camera. We used 10× objective and

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filter to get excitation wavelength 380–420 nm. For LB films deposited onto sil-

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icon wafers we used AFM. AFM measurements were performed under ambient conditions using a Bruker AXS Multimode 8 system operating in tapping mode in air. Silica cantilevers (OMCL–AC160TS, Olympus, Japan) with a resonance frequency of 300 kHz and a spring constant of 40 N·m−1 were used.

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3. Results and discussion 3.1. Thermotropic and structural properties Combination of aromatic cores with flexible alkyl chains often leads to for-

mation of liquid crystalline phases due to microsegregation of distinct parts of

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the molecules. Moreover, molecular structure of tetrachloroperylene tetracar-

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boxylic acid esters is typical for discotic liquid crystals. Therefore, to check bulk thermotropic properties of the compounds investigated they were studied by DSC and POM.

At room temperature all dyes possess yellow crystalline structure. The ex115

emplary DSC thermogram of PC4 is shown in Figure 2 whereas transition

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temperatures and transition enthalpies for all dyes investigated are gathered in Table 1. DSC and polarized optical microscopy studies reveal that all of

the perylene derivatives form only crystal phases. Moreover, with increase of

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length of hydrocarbon terminal chains transition to isotropic state occurs at lower temperature but this relation is not linear. For PC5, PC6 and PC9 upon cooling process to room temperature no subsequent crystallisation took place suggesting formation of an isotropic glass. Only for PC5 crystallisation occurred at room temperature after 24 hours. The most complex thermogram was observed for PC4 (Figure 2), which show crystal–crystal transition, prior 125

to the melting transition at 164.5 °C and crystallisation process. Nevertheless,

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the largest value of enthalpy was obtained for PC1 what indicates high crystallinity of this material. Comparison of our results with similar perylene dyes

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but without chlorine atoms [11] shows that substitution of these atoms in the bay positions causes shift of clearing temperatures to higher values and strongly prevent formation of liquid crystalline phases. None of the tetrachloroperylene

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tetracarboxylic acid esters investigated possess liquid crystalline phase whereas for unsubstituted perylene derivatives columnar phase is formed for wide range

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of length of hydrocarbon chains. i.e. for n ∈< 2, 9 > [11]. To shed more light on organisation of PCn in crystal phase, for PC6 XRD

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measurements were performed. Due to presence of strongly electro–negative chlorine atoms in the bay position of perylene core, the twisted skeleton of the molecule is expected. Accordingly, PC6 form triclinic system with two molecules in a cell of space group P–1 (Figure 3).

The twisted molecules

are shifted between each other in this way that one of the naphtalenes of 140

two adjacent molecules are parallel to each other with a distance of 4.7 ˚ A 6

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(Supplementary Figure 1). Therefore, the molecules are tilted with respect to

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column axis.

3.2. Langmuir films 145

To study organisation of PCn in Langmuir films, the Π–A isotherms were

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recorded (Figure 4). For almost all compounds (the only exception is PC9)

with a decrease of the available area, the surface pressure rises up to the collapse point at appropriate ΠC and AC values. These values as well as the values

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of Aext , CS−1 and ϕ angle are gathered in Table 2. From Figure 4 and Table 2 the change of the shape of the Π–A isotherms and different molecular organisation of the perylene dyes on the water is clearly seen. The highest surface pressure is observed for PC1 what clearly demonstrates that this dye forms the most stable and rigid monolayer. The increase of carbon atoms in terminal alkyl chains causes that the monolayers collapse at lower ΠC values. Simulta155

neously, the slope of the Π–A isotherms increases. It indicates that with the

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rise of alkyl chain length Langmuir films are less fragile. By contrast, values of the compressional modulus, CS−1 , changes without any tendency indicating only

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that all monolayers are in liquid–condensed phase [12]. Interestingly, for PC6 after collapse point wide plateau region is observed what strongly resembles 160

Π–A isotherms of liquid crystals e.g. 8CB, 8OCB or 8OCPFB [13]. Gener-

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ally, plateau region indicates a phase transition and is characteristic for many thermotropic liquid crystals of rod–like shaped (calamitic) molecules. Similar

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Π–A isotherm run was observed for previously investigated perylene like dyes without chlorine atoms in the bay position [14]. The only compound for which

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surface pressure did not rise during compression process was PC9. Apparently, for this dye hydrocarbon chains are long enough to screen interactions between perylene cores of the molecules whereby layer formation is impossible. This fact emphasise how profound influence on Langmuir films formation and properties has number of carbon atoms in terminal hydrocarbon chains of compounds in-

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vestigated. It also expressively shows that for molecules, possessing perylene 7

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core as a rigid core, the shorter hydrocarbon chains favour formation of more

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stable Langmuir film. The conceptual organisation of the compounds investigated can be considered by comparison of the values of the mean molecular area of the dye molecules 175

at the collapse point AC (Table 2) as well as the cross–sectional area of rigid

core of the perylene derivatives. The cross–sectional area of the perylene core

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with incorporated chlorine atoms in the bay positions and four ester groups obtained from single crystal XRD is approximately 0.87 nm2 . Far less values of AC imply that molecules do not lie flat at the air–water interface but are arranged in head–on configuration. Moreover, the average ϕ angle increases with rise of

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length of hydrocarbon terminal chains suggesting that molecules are more tilted with respect to normal to the water surface. Our observations are consistent with results obtained for other perylene like dyes without substituents in the bay position [15, 16, 17]. These dyes have a planar structure and due to strong π–π 185

interactions form Langmuir films with head–on arrangement. Furthermore, Liu el at. [18] also suggested that some molecules with perylene core can be tilted

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with respect to the water. This assumption was confirmed by Hertmanowski el at. [14]. However, in case of PCn the tilt angle is slightly bigger than for

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unsubstituted molecules. It results from fact that the electrostatic repulsion between the chlorine atoms in the perylene core of PCn highly twists the plane of

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the molecule with a torsion angle of 37° [19]. Therefore, molecules are expected to require bigger mean molecular area than unsubstituted molecules and a tilt angle should be also bigger. On the other hand, molecules with twisted skeleton

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are known to show increased ordering within the columnar liquid crystalline 195

phases [20]. This fact in our opinion have profound influence on the organisation of such molecules at air–water interface. Here, molecules are locked on the water surface and the move in the vertical direction is restricted. Therefore, they can more efficiently lock each other hindering the rotation of the molecules along their long axis. This enables molecules to create more condensed, ordered

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and stiff Langmuir films.

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3.3. Brewster Angle Microscopy Images

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Brewster angle microscopy (BAM) enables the visualization of Langmuir monolayers in situ. This technique provides information about two–dimensional

organisation, size and shape of domains and heterogeneity in layers at the air– 205

water interface. Imaging of the surface films was performed at different surface

pressures during a slow continuous compression of the monolayers formed on

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the water.

Figure 5 illustrates the change of the textures of PC1 monolayer during compression process. From the Figure 5a it is clearly seen that immediately after the start of the compression process, molecules strongly interact with each

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other forming large patches of condensed phase (bright area) which coexists with gaseous phase (dark background). What interesting, part of rafts already have linear boundaries what may suggest tendency of molecules to columnar organisation. With decreasing of mean molecular area patches start to coalesce 215

and create homogeneous film of high density (for A between 0.50 nm2 and 0.38 nm2 ) giving image of one shade of gray (Figure 5b). After reaching the

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collapse point of the Π–A isotherm when area for all molecules is too small BAM image (Figure 5c) reveals very bright wrinkles indicating that some of the

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molecules at high surface pressure are pushing out from the uniform film and create thicker film areas with irregular three dimensional organisation.

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Similar pictures were observed for PC4, PC5 and other perylene like dyes [21], whereas for PC6 (Figure 6) images resemble textures observed for typical nematic liquid crystals [22] than for perylene dyes even if this compound

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does not show any liquid crystalline phase in a bulk. At the beginning of the 225

formation of Langmuir film, condensed monolayer islands in equilibrium with a foam–like structure is observed (Figure 6a). This is indicative of weaker interactions between molecules and lower packing density. As the surface pressure increases the islands pack together into a completely compressed monolayer, giving a homogeneous image (Figure 6b). Subsequently, at the plateau region,

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small very bright domains constituent of molecules pushed out from layer being in contact with water are seen (Figure 6c). Their number increases with 9

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further compression (enhancing the contrast of the image) but they do not co-

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alesce (Figure 6d). However, some long parallel lines are observed what may suggests that for PC6 dye π–π interactions are strong enough to determine the 235

organisation of molecules in the upper layer as a regularly stacked molecules in columns.

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3.4. Spectroscopic properties of Langmuir–Blodgett films

To give further insight in molecular interactions and packing of perylene derivatives investigated, Langmuir films of PC1, PC4–PC6 were transferred onto hydrophilic quartz plates forming LB films and their absorption and flu-

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orescence spectra were compared to monomeric form — diluted in chloroform at low concentration of 2 · 10−6 mol·dm−3 . Figure 7 shows steady–state normalised absorption and emission spectra of PC5 in chloroform whereas Figure 7 presents spectra of PCn in the LB deposited at Π = 15 mN·m−1 for 245

PC1–PC5 and Π = 4 mN·m−1 for PC6. Characteristic spectral parameters of dyes in chloroform as well as in LB films are gathered in Table 3.

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Absorption band of monomeric form at 325–525 nm and maximum at λmax = 456 nm shows vibronic structure which can be ascribed to the vibration

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of perylene core being strongly coupled with S1 –S0 transition, the dipole moment of which is directed parallel to the long axis of the molecule [23]. Whereas, emission spectrum is structureless with maximum at λmax = 508 nm and long

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tail up to 700 nm. Comparison of spectra of PC5 in chloroform with similar molecule without chlorine atoms in the bay positions [14] reveals that presence

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of the substituents causes that spectra are broader and show less vibronic struc255

ture. It can be attributed to the loss of the planarity of the perylene core and the lower symmetry of the molecule. Moreover, the Stockes shift for PC5 is

almost three times higher than for unsubstituted molecule. Due to this fact one can notice almost 3–fold decrease of fluorescence quantum field of PCn dyes (QY= 0.35 [24]) in comparison with the same dyes without chlorine atoms in

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the bay positions (QY= 0.87 [25]). For all LB films absorption spectra are relatively weaker with respect to the 10

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monomeric form. Moreover, with increase of n change of the intensity ratio of

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the absorption peaks (A3 /A2 and A3 /A1 in Table 3) is observed causing decrease of the half–bandwidth of absorption spectra for dyes showing vibronic structure. 265

For all the LB films these ratios are smaller than for solution in chloroform. It results from different orientation of PCn molecules in solution and LB films. In

strongly diluted chloroform solution molecules are randomly oriented whereas in

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Langmuir films they are densely packed. Such organisation favours aggregation

of the molecules what is confirmed by the broadening of the all LB spactra with 270

respect to chloroform solution.

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The biggest difference between steady–state absorption spectra in LB film with respect to chloroform is observed for PC1. In absorption band information about vibronic structure disappears — it possesses only one maximum at λ1 = 396 nm with half–bandwidth δ = 3650 cm−1 . In case of LB films of PC4– 275

PC6 absorption spectra show vibronic structure which is more clearly seen with increase of n and for PC6 is the most similar to monomeric form. Changes in the absorption spectra of the PCn are related to different orientation of the

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molecules in LB films. For PC1, only the band at 396 nm is seen what suggests the strongest interactions between molecules. It appears to be reasonable due to the shortest terminal alkyl chains and leads to almost vertical orientation of

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PC1 in LB films. Consequently, only the dipole moment directed parallel to the

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short molecular axis may interact with the electronic vector of incident light. Additionally, formation of H–aggregates of PC1 in LB film is also possible in the ground state. For PC4–PC6 with rise of n increase of the intensity of the absorption spectra is observed suggesting that molecules are progressively

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tilted with respect to the normal to the solid substrate. Accordingly, bigger contribution from the dipole moment directed parallel to the long molecular axis is seen. However, due to the torsion of the perylene core the band at λ1 = 396 nm is also present in absorption spectra. The lack of shift of the

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absorption spectra with respect to monomeric form as well as their broadening may indicate creation of I–aggregates. Even taking into account that during transfer some reorientation of the molecules takes place, the orientation of the 11

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dyes in LB films is consistent with the changes of ϕ angle in Langmuir films and

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indicate the influence of the length of alcoxy chains on the spectral properties of the dyes.

In case of fluorescence spectra, all dyes in LB films show broad and structureless band with long tail to about 650 nm with intensity increasing with increase of length of terminal hydrocarbon chains. For PC1 and PC4 fluorescence band

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is strongly red–shifted what gives large Stockes shift of about 150 nm. Accord-

ing to [26, 27] this band is strongly related to Y–type excimers interpreted as the metastable parallel perylene dimer. The position of emission maximum for

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α–perylene crystal and for some perylene derivatives in LB films occurs between 535 and 550 nm [26, 28]. Presence of excimer band results from formation of well organised structures with overlapping perylene cores of dye molecules in 305

LB films. In contrast, for PC5 and PC6 emission spectra strongly resemble this obtained in chloroform. Moreover, only slight changes of the absorption spectra were observed upon aggregation indicating weak excitonic couplings in the dimers. This may be related to much bigger tilt angle of the molecules

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with respect to the solid substrate in LB films resulting in displacement of the perylene cores. Usually, in perylene derivatives π–π interactions yield to pres-

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ence of the excimer emission in fluorescence, as it is observed for LB films of planar unsubstituted perylene derivatives [14] as well as for PC1 and PC4.

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Notwithstanding, Sluch et al [28] shown that long alkyl chains prevent excimer formation. This observation is consistent with our results and confirms that 315

for tetrachloroperylene tetracarboxylic acid esters with twisted perylene core

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alkyl chains containing ≥ 5 carbon atoms are long enough to perturb interactions between main core of the molecules. Therefore, these dyes can not create configuration giving excimer emission. The fluorescence quantum yield obtained in LB films (Table 3) reveals for

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PC1 very strong quenching to value close to zero what is expected for H– aggregates. For the rest of the dyes strong almost 3–fold decrease with respect to the monomeric form is observed. Despite the fact that values of QY for PC4– PC6 are the same within experimental uncertainty, the intensity of steady–state 12

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fluorescence spectra decreases with decrease of n. This may suggests that integrating sphere used in our experiment is not enough sensitive instrumentation

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to study the slight differences in orientation and/or aggregation of PC4–PC6 in LB monolayer films. 3.5. Morphology of Langmuir–Blodgett films

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The morphology of LB films visualised by fluorescence microscope shows that for dyes with the shortest alkyl chains (PC1, PC4) the film cracks during transition from water to the solid substrate (Figure 9a). However, the gaps

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within the film are relatively small and generally the fluorescence is quite homogeneous (without brighter or darker areas). This observation is consistent with spectroscopic results because only PC1 and PC4 are forming excimers 335

due to the biggest overlapping region of perylene cores. As a consequence of the strongest interactions between molecules, the film is rigid and breaks during transfer. For PC5 a significant improvement in homogeneity of the layer was observed whereas for PC6 the film was completely uniform (Figure 9b). It re-

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hydrocarbon chains.

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sults from increase of the flexibility of the Langmuir film due to longer terminal

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The AFM examination gives better insight in arrangement of the molecules in LB films. Figure 10 illustrates the AFM image and the height analysis of the LB film formed of PC1 (a) and PC6 (b). It can be seen that even for dyes

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with shorter terminal chains LB film within cracked islands is homogeneous. However, the thickness of the LB film is changing as a function of length of hy-

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drocarbon terminal chains. For PC1 the thickness of the LB film was measured to be 3.0 ± 0.3 nm whereas for PC4 and PC5 — 1.4 ± 0.2 nm and for PC6 — 1.3 ± 0.2 nm. In case of PC1 the thickness of the LB film and height analysis suggest that we deal with double layer. However, the transfer ratio for all sam-

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ples T R = 1. Therefore, we believe that during transfer of PC1 onto silicon wafer some reorganisation of the molecules took place. Still, the thickness of PC1 if higher than double value of thickness of PC4, PC5 and PC6. Hence, these results are in agreement with our previous observations indicating that 13

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with increase of n molecules are more tilted at interfaces.

4. Conclusions

In summary, five members of the newly synthesized core–twisted homologous series of tetrachloroperylene tetracarboxylic acid esters (PCn) for

n = 1, 4, 5, 6, 9 have been investigated. From XRD data it was found that four

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chlorine atoms substituted in the bay position to the perylene core strongly

twist the perylene plane. Molecular unit triclinic cell has a symmetry P–1 and an average distance between neighbouring molecules is significantly longer

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(about 4.7 ˚ A) than in a case of non–substituted perylene derivatives crystals (3.5 ˚ A). DSC shows only crystalline form for all the dyes investigated without of any symptoms of mesogenic properties. 365

Most of the perylene derivatives investigated (n = 1, 4, 5, 6) are able to form stable and compressible Langmuir films. The tilted head–on orientation of the molecules with respect to normal to the water surface is postulated. All of the

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dyes which form Langmuir films can be transferred onto quartz slides and silicon wafers forming Langmuir–Blodgett films. Absorption and emission spectra with strong fluorescence quenching for PC1 suggest that this dye is forming

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H–aggregates. Whereas for PC4–PC6 LB films presence of I–aggregates is postulated. Furthermore, depending on the length of hydrocarbon chains the

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dye molecules have different orientation in LB films with respect to the normal to the solid substrate what was confirmed by absorption and emission spectra 375

as well as AFM measurements.

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The twisted plane of perylene core of the dyes under investigation and elon-

gated distance between neighbouring molecules into elementary cell of molecular crystal causes changes in the compression isotherm course, packing and orientation of the molecules in the monolayers. Types and fractions of aggregated

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molecules of the chlorinated derivatives in Langmuir and LB films are reflected in their absorption and emission spectra which are similar to the monomeric form of molecules. The absence of mesogenic phases in bulk causes less ho-

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mogeneity of the monolayers because of the lack of long–distance forces in the

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created two–dimensional system.

Acknowledgements

This article was financially supported within the project ”Engineer of the

Future. Improving the didactic potential of the Poznan University of Tech-

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nology” — POKL.04.03.00–00–259/12, implemented within the Human Capital Operational Programme, co–financed by the European Union within the Euro390

pean Social Fund.

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The authors would like to thank Prof. Frank W¨ urthner, Dr. Vladimir Stepanenko and Ana–Maria Krause for single crystal X–ray analysis as well as the chance to carry out UV/vis absorption and fluorescence spectroscopy experiments and AFM measurements at the Institute of Organic Chemistry at the 395

University of W¨ urzburg. We also thank to Prof. Matthias Lehmann from the Institute of Organic Chemistry at the University of W¨ urzburg for allowing to

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use fluorescence microscope.

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List of figures

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1. Molecular structure of perylene derivatives investigated.

2. Differential scanning calorimetry profiles (3°/min) of PC4. 3. Image of elementary cell of PC6 molecular crystal.

4. Surface pressure–mean molecular area isotherms of Langmuir films formed of PC1 (1), PC4 (2), PC5 (3), PC6 (4) and PC9 (5).

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5. BAM images of Langmuir film formed of PC1 at A = 0.70 nm2 (a) and 0.24 nm2 (b).

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6. BAM images of Langmuir film formed of PC6 at A = 0.70 nm2 (a), 0.66 nm2 (b), 0.47 nm2 (c) and 0.32 nm2 (d). 505

7. Normalised steady–state absorption (A) and emission (F) spectra of PC5 in chloroform at concentration of 2 · 10−6 mol·dm−3 .

8. Steady–state absorption (a) and emission (b) spectra of PC1 (1), PC4 (2), PC5 (3) and PC6 (4) in LB film deposited for PC1–PC5 at

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Π = 15 mN·m−1 and for PC6 at Π = 4 mN·m−1 .

9. Fluorescence microscope images of PC1 (a) and PC6 (b) in LB film

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deposited at Π = 15 mN·m−1 and Π = 4 mN·m−1 , respectively. 10. AFM images and height analysis of the LB film of PC1 (a) and PC6 (b)

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deposited at Π = 15 mN·m−1 and Π = 4 mN·m−1 , respectively.

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Figure 1: Molecular structure of perylene derivatives investigated.

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Figure 2: Differential scanning calorimetry profiles (3°/min) of PC4.

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Figure 3: Image of elementary cell of PC6 molecular crystal.

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Figure 4: Surface pressure–mean molecular area isotherms of Langmuir films formed of

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PC1 (1), PC4 (2), PC5 (3), PC6 (4) and PC9 (5).

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Figure 5: BAM images of Langmuir film formed of PC1 at A = 0.70 nm2 (a), 0.50 nm2 (b)

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and 0.24 nm2 (c).

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Figure 6: BAM images of Langmuir film formed of PC6 at A = 0.70 nm2 (a), 0.66 nm2 (b),

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0.47 nm2 (c) and 0.32 nm2 (d).

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Figure 7: Normalised steady–state absorption (A) and emission (F) spectra of PC5 in chlo-

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roform at concentration of 2 · 10−6 mol·dm−3 .

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Figure 8: Steady–state absorption (a) and emission (b) spectra of PC1 (1), PC4 (2), PC5

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Π = 4 mN·m−1 .

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(3) and PC6 (4) in LB film deposited for PC1–PC5 at Π = 15 mN·m−1 and for PC6 at

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Figure 9: Fluorescence microscope images of PC1 (a) and PC6 (b) in LB film deposited at

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Π = 15 mN·m−1 and Π = 4 mN·m−1 , respectively.

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Figure 10: AFM images and cross–sectional analysis of the LB film of PC1 (a) and PC6 (b)

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deposited at Π = 15 mN·m−1 and Π = 4 mN·m−1 , respectively.

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List of tables

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1. Transition temperatures and transition enthalpies of PCn.

2. Characteristic parameters of Π–A isotherm of Langmuir films formed of PCn.

3. The maximum positions (λ1 –λ3 ), the half–bandwidth (δ) and ratio of the

absorbance values of the maximums A3 /A2 and A3 /A1 of the absorption

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band as well as and maximum position, the half–bandwidth and fluorescence quantuum yield (QY) of the fluorescence band of a monomeric form

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of PC5 in chloroform (c = 2 · 10−6 mol·dm−3 ) and all dyes in LB films.

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Table 1: Transition temperatures and transition enthalpies of PCn.

Transition temperatures [°C] and enthalpies [J·g−1 ] in brackets

PC1

Cr1 299.9 (79.9) Iso

PC4

Cr1 139.0 (12.8) Cr2 164.5 (47.9) Iso

PC5

Cr1 125.0 (52.1) Iso

PC6

Cr1 64.5 (53.4) Iso

PC9

Cr1 34.6 (36.8) Iso

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∆T = ±0.1°C, ∆H = ±0.1 J·g−1

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Table 2: Characteristic parameters of Π–A isotherm of Langmuir films formed of PCn.

Aext [nm2 ]

AC [nm2 ]

ΠC [mN·m−1 ]

CS−1 [mN·m−1 ]

ϕ [°]

PC1

0.56

0.35

47.8

107.2

24

PC4

0.70

0.49

27.8

96.1

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PC5

0.67

0.57

22.1

126.6

41

PC6

0.69

0.65

7.4

156.9

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Dye

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PC9 is not able to form Langmuir film

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Table 3: The maximum positions (λ1 –λ3 ), the half–bandwidth (δ) and ratio of the absorbance

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values of the maximums A3 /A2 and A3 /A1 of the absorption band as well as and maximum position, the half–bandwidth and fluorescence quantuum yield (QY) of the fluorescence band

of a monomeric form of PC5 in chloroform (c = 2 · 10−6 mol·dm−3 ) and all dyes in LB films.

Absorbance Dye λ1 [nm]

λ2 [nm]

Fluorescence

δ [cm−1 ]

λ3 [nm]

λ1 [nm]

δ [cm−1 ]

QY

2.15

508

2680

0.350

—

—

540

3000

0.025

A3 /A2

PC5

395

437

456

3460

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Chloroform

A3 /A1

1.09

LB film 396

—

PC4

396

437

PC5

397

435

PC6

396

435

—

3650

457

5850

0.96

0.95

537

2400

0.133

456

5200

1.00

1.38

509

2450

0.128

457

4850

1.05

1.50

509

2450

0.132

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PC1

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∆λ1,2,3 = ±1 nm, ∆δ = ±10 cm−1 (for chloroform), ∆δ = ±50 cm−1 (for LB film), ∆QY = 0.001

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HIGHLIGHTS  Tetrachloroperylene tetracarboxylic acid esters can crystallize into polycrystalline powder  In Langmuir and LB films tilted head-on orientation of molecules is postulated  Different molecular aggregates are formed depending on molecular structure  Strong impact of twisted core of molecules on their arrangement is showed