Temperature-Tuning of Optical Properties and Molecular Aggregation

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Temperature-Tuning of Optical Properties and Molecular Aggregation in AnE-PVstat Copolymer Solution Asma Saaidia, Mohamed Amine Saidani, Emna Hleli, Shahidul Alam, Christoph Ulbricht, Samir Romdhane, Amel Ben Fredj, Christian Kästner, Daniel Ayuk Mbi Egbe, Ulrich S. Schubert, Habib Bouchriha, and Harald Hoppe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10709 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Temperature-Tuning of Optical Properties and Molecular Aggregation in AnE-PVstat Copolymer Solution

A. Saaidia1*, M. A. Saidani1,2, E. Hleli1, S. Alam3,4, C. Ulbricht5, S. Romdhane1,2, A. Ben Fredj1, C. Kästner6, D. A. M. Egbe5, U. S. Schubert3,4, H. Bouchriha1, and H. Hoppe3,4 1

Laboratoire Matériaux Avancés et Phénomènes Quantiques, Université Tunis El Manar, Faculté des Sciences de Tunis, 2092 Campus Universitaire Tunis, Tunisia.

2

Université de Carthage, Faculté des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia. 3

Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany. 4

Laboratory of Organic and Macromolecular Chemistry, Institute of Organic and

Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr 10, 07743 Jena, Germany. 5

Institute of Polymeric Materials and Testing, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria.

6

Institute of Thermodynamics and Fluid Mechanics, Technical University Ilmenau, Am Helmholtzring 1, 98693 Ilmenau, Germany.

*Corresponding author: Tel: +216 26550446

Email address: [email protected]

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ABSTRACT

The correlation between the optical and morphological properties of an anthracenecontaining poly(phenylene ethynylene)-alt-(phenylene vinylene) statistical copolymer (AnEPVstat) in solution has been investigated. As a function of temperature, the molecular aggregation in chloroform:chlorobenzene (CF:CB) solution was examined by means of absorbance and photoluminescence measurements. The study of the evolution of the 0 − 0 to the 0 − 1 photoluminescence (PL) ratio with temperature in the framework of the HJaggregate model unveiled the presence of both H and J-like behaviors at room temperature. Upon increasing temperature, the interchain intermolecular coupling decreases, leading to a dissociation of H-aggregates. Hence, molecular torsions are facilitated, polymer planarity is reduced and the intrachain electronic coupling decreases.


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1. Introduction

In general, various electronic states are accessible in conjugated organic molecular systems through optical excitation, tightly bound electron-hole pairs located at the same molecular unit, i.e. Frenkel excitons, and electron-hole pairs located at neighbouring molecular sites i.e. charge transfer excitons. For both kinds of excitons, two spin states (S) are accessible, singlet (S=0) and triplet (S=1).

1-4

These excitons are distinguished by different

energies, lifetimes and diffusion lengths. A transition between these excited states can take place through multiple physical processes such as intersystem crossing, reverse intersystem crossing, singlet fission, energy and charge transfer.

5,6

This provides the possibility of

controlling exciton generation and harvesting and offers the possibility of enhancing the performance of organic-based optoelectronic devices. Indeed, implementing thermally activated reverse intersystem crossing has enabled the transformation of non-radiative triplet excitons into radiative singlet excitons, which has given rise to organic light emitting devices exceeding the maximum theoretical 25% internal efficiency up to nearly 100%.7,8 In the case of photovoltaics, triplet excitons are appealing due to their higher lifetime and diffusion length. Making use of singlet to triplet fission and energy transfer mechanisms, external quantum efficiency above 100% has been denoted in an organic photovoltaic cell.9 The performance of organic devices strongly depends on morphology and molecular packing. 10,11 Indeed, in the case of relatively small molecules, excitation triggers a distinct change in the equilibrium position of the atoms of the original ground state, in which the strong exciton-phonon coupling and localization are rooted. 12 Moreover, in the case of single crystals, the crystal field leads to a split in the first excited state into multiple components having different symmetries and lifetimes. 13 In case of conjugated polymers where molecules 3 ACS Paragon Plus Environment


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are longer, the intra- and interchain interactions govern the molecular packing, the effective conjugation length as well as the exciton lifetime and diffusion length.14 The nature of the molecular interaction determines the type of the formed aggregates. Interaction between the nearest neighbors chromophores oriented in a side-by-side manner gives rise to H-aggregate, while J-aggregates result from interactions between chromophores oriented in a head-to-tail fashion. Each kind of aggregate is recognizable through its typical optical properties.

11,15,16-19

Thus the molecular interactions in organic semiconductor materials are described in terms of either H or J-aggregates though often exhibiting hybrid HJ-properties. Spano et al. have recently developed the HJ-aggregate model in order to take the aforementioned hybrid HJ-properties into account. Besides the usual J-aggregates mentioned above, the latter model allows for defining unconventional J-aggregates resulting from intrachain through-bondinteracting monomer repeat units.20 So far, several organic semiconductors have been studied in the framework of this model such as some terrylene derivatives (TAT) and red-phase polydiacetylene (PDA), as well as poly(3-alkylthiophene) (P3HT). 21,22 In the present work, the optical properties of an anthracene-containing PPE-PPV with statistical side chain configuration (AnE-PVstat) are investigated in solution with regards to its morphological and packing features, and the balance between H and J-aggregates.

23,24

Several studies with detailed analysis of the correlation between morphological and spectral properties of PPV derivatives in thin films have been already published. 24-26 However, the photophysical behaviour of PPV derivatives, such as AnE-PVstat, in solution is less studied and understood so far. The choice of solvent (mix) and processing temperature have a dominant impact on the fabrication of polymeric active layers for organic-electronic applications. A proper understanding and control of the processing parameters is essential for the realization of fully optimized systems. The polymeric active layers in organic-based devices incorporate the 4 ACS Paragon Plus Environment

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fingerprints of the solutions from which they are processed. Therefore, the understanding and the control of the organic semiconductor morphology goes unavoidably through the investigation of their solutions.27,28 In this work, a mixed-solvent system, chloroform:chlorobenzene (CF:CB, 1:1 by volume) has been used since earlier investigations (also on a similar polymer) have shown that films produced using such a solvent mixture lead to a well-ordered structure and improved crystallinity, enhancing the photovoltaic properties of bulk heterojunction solar cells significantly.28-32 Absorbance and photoluminescence measurements at different temperatures have been performed for AnE-PVstat dissolved in a CB:CF mixture. The temperature-dependent spectra permitted us to study the contributions of interchain and intrachain interactions. The obtained PL ratios allowed us to conclude on the nature of the aggregates that occurred in the material solution within a temperature range between 20 and 90 °C.

2. Experimental

The chemical structure of AnE-PVstat, which is used in this study, is displayed in Figure 1. The material was synthesized as described elsewhere.33,34 For the preparation of the AnE-PVstat solution a sample of the polymer (1.2 mg), chloroform (500 µL) and chlorobenzene (500 µL) were filled into a vial. The vial was properly sealed, and the mixture was stirred at room temperature until complete dissolution of the polymer. For the spectroscopic investigations, the solution was transferred into a cuvette. A tightly sealing cap ensured no evaporation of solvents during the experiments. UV-Vis absorbance spectra were recorded

with

a

SPECORD

250

ANALYTIC

JENA


AG

spectrophotometer.


Photoluminescence measurements were performed using a Jasco FP-6500 spectrofluorometer. The solution was excited in the absorption band of AnE-PVstat at about 480 nm (2.58 eV) using a halogen lamp in combination with a monochromator. The impact of temperature on polymer chain conformation was studied within a temperature range of 20 to 90 °C. A Huber Ministat CC thermostat was used to set the temperatures for the experiments. Starting at 20 °C the temperature was increased in steps of 2 °C and held for 4 minutes at each step before initiating the recording of the spectrum. For simplification, only the spectra in steps of 10 °C are displayed.

Figure 1: Schematic representation of the chemical structure of the AnE-PVstat (Mn = 41 500 g/mol, Mw = 82 400 g/mol, ÐM = 1.98).

3. Results and discussion: Temperature-dependent chain conformation

Temperature-dependent PL spectra of AnE-PVstat in a mixture of CF:CB were recorded and are displayed in Figure 2. The emission spans from about 1.6 eV (750 nm) to about 2.2 eV (550 nm). In Figure 2a, at a first glance, the PL spectra exhibit two main bands spaced by about 0.15 eV, with a decrease of the band intensity on going from the high energy band down to the lowest energy band. In Figure 2b, the transitions in the PL spectra are assigned and fitted by three Gaussians curves using the measurement at 30 °C as an example. The highest energy band at about 2.10 6 ACS Paragon Plus Environment

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eV with the highest intensity can be attributed to the 0-0 transition, the second and the third band at about 1.97 and 1.83 eV are assigned to the 0-1 and the 0-2 vibronic transitions, respectively.

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Figure 2: (a) Temperature-dependent PL spectra of the polymer AnE-PVstat in CF:CB solution upon excitation at 480 nm (2.58 eV). (b) PL spectrum recorded at 30 °C (black circles) and its best fit (red line) using Gaussian fitting components (blue dashed lines).

We note that with increasing temperature (from 20 to 90 °C) the intensities of all transitions decrease and the spectra are blue shifted. Furthermore, the ratio of the first PL transitions, PL0-0/PL0-1, was found to be temperature-dependent. As reported by Yamagata et al., for J(H)-aggregates the PL ratio decreases (increases) with rising temperature and increasing disorder.20,35 In Figure 3 the correlation between temperature and PL peak ratio, which is derived from the integrated areas of the respective Gaussian fitting components, is depicted.



4.2 4.0 3.8 3.6 PL0-0/PL0-1

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3.4 3.2 3.0 2.8 2.6 2.4 2.2

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Temperature (°C)

Figure 3: Temperature dependence of the intensity ratio of the PL transitions (PL0-0/PL0-1) for AnE-PVstat in CF:CB solution. The values for PL0-0 and PL0-1 are derived from the weights of the respective Gaussian fitting components in Figure SI-1 of the supporting information.

Figure 3 shows on one hand that the PL ratio exceeds unity significantly, which is a typical indication of unconventional J-aggregates (intrachain through-bond-interactions). In addition, the PL ratio increases with rising solution temperature, which is the typical sign of H-aggregate behavior (interchain interactions).20 Upon increasing the temperature, the interchain coupling decreases down to a break up of H-aggregates. At high temperature, isolated chains exist with reduced conjugation lengths due to a greater prevalence of torsional defects.36,37 In addition, to provide more information about the interchain and intrachain interaction, temperature-dependent UV-Vis measurements have been carried out (Figure 4a). The absorbance spectra show vibronic structures with two main bands, the dominant one at about 2.2 eV is attributed to the 0-0 transition and the second band at about 2.4 eV is assigned to the 0-1 vibronic transition. 8 ACS Paragon Plus Environment

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Figure 4: Temperature-dependent (a) UV−Vis absorbance spectra, (b) absorbance peak ratio, A0-0/A0-1 (derived from the heights of Gaussian fitting components), of the polymer AnEPVstat in CF:CB solution. In Figure 4(a) it can be observed that the 0-0 band has a different temperature-dependence than the 0-1 band. Upon increasing temperature, the 0-0 band intensity decreases continuously, but remains dominant over the 0-1 band, its intensity appears to be practically not affected by the temperature variation. This temperature-dependent behaviour of absorption has been reported for other copolymers in chlorobenzene solution.38 Furthermore, we note that the decrease in the 0-0 band intensity is accompanied by a blue shift with increasing temperature while the 0-1 transition is less affected. These results show that the AnE-PVstat chains show distinct J-aggregate character in moderate temperature solution, exhibiting a high intensity A0−0 peak. Upon heating of the polymer solution, intrachain interactions decrease. On the other hand, as reported by Spano et al., absorption is more related to the interchain interactions originating from van-der-Waals and π-π stacking interactions between the polymer chains.29,39 This might explain the difference in the observed peak ratio as compared to the photoluminescence. 9 ACS Paragon Plus Environment


Moreover, the absorption ratio, A0-0/A0-1 progressively decreases with increasing temperature but remains approximately unity or slightly greater (Figure 4b). Increasing temperature leads to a decrease of the absorption ratio indicating a decrease of the intrachain coupling. In addition, upon heating the pure electronic transition (0-0) decreases in intensity and is accompanied by an increased energy (E0-0) (blue shift). The course of the blue shift is depicted in Figure 5. We ascribe the temperature-induced energy-increase of the 0-0 absorption band to the increase of molecular distortion in the ground and excited states.36,37

2.15 2.14 2.13 E0-0(eV)

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2.12 2.11 2.10 2.09 20

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Temperature (°C)

Figure 5. Illustration of the temperature-dependent blue shift of the purely electronic transition energy (E0-0).

From all those figures, we may interpret the temperature dependence of the recorded PL spectra in terms of increased molecular torsions, which consequently reduce the effective conjugation length and the intrachain electronic coupling, respectively. In fact, the electronic transition energy is related to the effective conjugation length by an inverse proportionality to the temperature.25,40,41 10 ACS Paragon Plus Environment

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Referring to the works of Marcus et al., one can also consider explaining the behavior of the absorption spectra upon increasing temperature in terms of an increased inhomogeneous broadening. Since the oscillator strength is preserved, the area underneath the absorption spectrum has to remain constant, which results in broadened bands in the high-energy side and a reduced intensity. However, as it can be seen from Figure 4(a) the blueshift is not accompanied by an important band broadening. This suggests a relatively weak contribution of the homogeneous broadening in the aforementioned behavior.36,37 In the light of the previously mentioned information, our results can be interpreted as follows. Upon heating the polymer solution, the intermolecular interactions decrease, copolymer chains are allowed to separate, AnE-PVstat loses its planarity and molecular torsions are expected to take place. The temperature affects the molecule itself, by decreasing the effective conjugation length31 which gives rise to a displacement of the excitonic band edge toward higher energy and leads in turn to the herein observed spectral blue shift in the PL spectra. The emission of the aggregates differs in their vibrational structure. These differences arise from the involved vibrational mode and strength of the excitonic coupling between chromophores.31,42,43

4. Conclusion

We studied the molecular aggregation of AnE-PVstat in CF:CB solution at various temperatures. Through spectroscopy analysis, we have shown that, on one hand, emission spectra are dominated by the 0-0 transition band which is characteristic for J-aggregate type. On the other hand, the PL ratio is strongly temperature-dependent, and increasing temperature leads to increasing disorder, weakens the intermolecular interactions between the polymer chains and the photoluminescence spectra appear to be more J-aggregate type. For high



temperatures the AnE-PVstat polymer behaves as an isolated-molecule referred to a single chain. Upon heating CF:CB solutions, and according to absorbance measurements AnEPVstat presents a phase transition from ordered planarized chains in an aggregated phase to disordered chains in an amorphous phase, and AnE-PVstat loses planarity. In summary, the investigation of AnE-PVstat in solution presents an interesting example for the delicate interplay between intrachain and interchain coupling, this interplay is affected and be addressed by the temperature settings. Increasing temperature disables the formation of aggregates and decreases the intermolecular interactions. This study contributes to the understanding of the photophysics of AnE-PVstat in solution and might provide another piece of jigsaw towards the optimization of organic electronic devices. Supporting information The Gaussian decomposition and the corresponding parameters for each PL spectrum for each temperature are given in Supporting Information. Notes The authors declare no competing financial interest. Acknowledgements C. Ulbricht and D. A.M. Egbe thank FWF for research funding through grant No: I 1703-N20. D. A. M. Egbe also acknowledges funding from the SOLPOL project (www.solpol.at). S. Alam, H. Hoppe are grateful for financial support through DFG in the framework of “PhotoGenOrder”.


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Authors gratefully acknowledge the use of the Jasco FP spectrofluorometer kindly provided by Dr Dieter Weiss from the Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University Jena.

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Temperature-Tuning of Optical Properties and Molecular Aggregation

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