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Optical properties of polyimines: UV-vis and photoluminescence study of undoped and doped polymers in aprotic and protic solvents.

INTRODUCTION

Most of the polymers currently being developed for electronic, optoelectronic, and photonic applications are based on [pi]-conjugated polymers [1-9]. This progress can be rationalized by analyzing the strategies used for improving the properties (external quantum efficiency, color purity, durability ...) of LEDs based on [pi]-conjugated macromolecular architectures [10]. Poly(p-phenylene vinylene)s (PPVs) and poly(fluorene)s (PFs) derivatives are the archetypes of solution processible polymeric semiconductors whose band-gap can be easily tuned using molecular, macromolecular, and supramolecular engineering approaches [10, 11]. While efficient "plastic electronic" devices based on PPVs and PFs have been reported, color purity and durability of LEDs are still issues of concern.

Our interest in combining the optoelectronic properties of [pi]-conjugated polymers with the self-assembly features of poly-ketanils-dopant supramolecules recently let to create photoactive aromatic and aromatic-aliphatic polymers with self-assembling features and supramolecular morphology [12-16].

We report herein observation of a novel self-assembly of [pi]-conjugated polymers having amphiphilicity induced by protonation of nonamphiphilic polymers in solution. In addition to the self-organization of [pi]-conjugated polymers demonstrated in this paper, the concept of change luminescent properties and amphiphilicity by used special kind of dopant with "C-3 symmetry" may also have applications in supramolecular chemistry.

In this study, we used the luminescent, thermostable polyimines (PAs) (see Fig. 1), which constitute a new class of [pi]-conjugated polymers for organic optoelectronics due to their close similarity to the widely-used poly(p-phenylenevinylene)s-based system. The synthesis, characterization and optical properties of PAs have previously been reported [17, 18]. As a protonating agent we used amphiphilic compound 1,3,5-benzenetricarboxamide sulfonic acid namely "dopant I", belonging to the family of functionalized 1,3,5-benzenetricarboxamide (see Fig. 1). Structure of the "dopant I" creates new type of supramolecular architectures in which the lateral groups-anions of sulfophthalic acid residue are ionically bonded to the main chain via protonation of imine nitrogen atoms. This specific interaction of the dopant with the host polymer influences the PAs properties and the following changes were observed: (i) bathochromic shift of photoluminescence (PL) emission band in DMA solution, (ii) blue shift of absorption bands in DMA solution, (iii) hypsochromic or bathochromic shift of PL emission band in m-cresol solution, (iv) blue shift of absorption bands in MC solution, (v) solubilization of the polymers in organic solvent, by protonation of the polymers.

EXPERIMENTAL

Materials

4,4'-methylenedianiline (Aldrich), dibenzoyl (Aldrich) as well as the following solvent dimethylacetamide (DMA from Aldrich) was used as received. 4,4'-diaminobiphenyl (Fluka), was recrystallized from boiling water in the presence of charcoal. m-Cresol (MC from Aldrich) was distilled prior to its use. p-Dibenzoylbenzene or 1,8-disebacoylbenzene were prepared using Friedel-Crafts acylation following the procedure reported earlier [16]. Polymers PA1, PA2, PA3, and PA4 (see Fig 1) were prepared and characterized as described in [17, 18]. Special protonating agent 1,3,5-benzenetricarboxamide sulfonic acid ("dopant I") was prepared and characterized as described in [19].

[FIGURE 1 OMITTED]

Protonation of PAs

Protonation of PAs with "dopant I" was carried out at room temperature using DMA or m-cresol as a solvent. Protonating agent was added to the solution of PAs studied in the 1:1, 2:1, or 3:1 molar ratio (It is the molar ratio of sulfonic acid groups (S[O.sub.3]H) to imine units (C=N)).

Characterization Techniques

UV-vis solution absorption spectra were recorded using a Hewlett-Packard 8452A spectrophotometer whereas the PL solution spectra were registered on a Fluorolog 3.12 Spex spectrometer with a 400 nm excitation line (450 W xenon lamp as the light source).

RESULTS AND DISCUSSION

Chemical structures of the PAs and the protonating agent ("dopant I") used in this research are presented in Fig. 1.

The most important advantage of the use of organic materials as active components of electronic devices is the possibility of a precise tuning of their band-gap and by consequence optical and electronic properties via chain engineering. Two strategies are usually used. In the first one, the molecules or macromolecules are functionalized with side-groups influencing the electronic density of the conjugated backbone. In the second approach, monomers of different chemical nature are copolymerized to give random, alternate, or block copolymers with differently conjugated units. A modification of this route involves the introduction of saturated, usually aliphatic, spacers between the conjugated segments of the main chain. All these strategies can be briefly outlined using PPV family as an example. Polyazomethines i.e. polyconjugated systems containing HC=N groups in the main chain, being isoelectronic with the corresponding PPV, also exhibit interesting electro-optical properties. The principal feature of polyazomethines, which distinguishes them from the majority of polyconjugated systems, is the presence of basic centers in the polymer main chain associated with imine nitrogens. This implies that in addition to the procedures described above, acid-base chemistry can be used for the modification of their properties. PAs investigated in present work can be considered as a sub-class of polyazomethines with phenyl group attached to the methane carbon atom.

UV-vis of Undoped PAs in DMA and in m-Cresol Solution

PAs chain contains two types of chromophore groups being in conjugation: the aromatic ring and the imine (C=N) group. Spectroscopic signatures of both groups are clearly seen (except PA3) in their UV-vis solution absorption spectra and consist in two bands. The band at shorter wavelengths, can be ascribed to the [pi]-[pi]* transition in the aromatic ring. The second absorption band is located in the spectral range of 348-410 nm and its position and intensity are strongly dependent on the chemical constitution of the polymer chain and kind of the solvent (see Table 1).

A systematic bathochromic shift in the position of both bands is observed if DMA is replaced by m-cresol (MC) as the solvent (see Table 1). This behavior clearly indicates that MC is not an inert solvent and strongly interacts with the polymer chain modifying its conformation towards higher planarity. This can occur via hydrogen bonds formation between imine-type nitrogens of PAs which are hydrogen bonding acceptors and the hydroxyl group of MC which is a good hydrogen bonding donor [16, 20, 21].

In particular, for PA1 the band attributed to imine chromophores is slightly hypsochromically-shifted as compared with the analogous band in polymer PA2 in m-cresol solution. A comparison of the [[lambda].sub.max.C=N] and E of PA1 to PA2 in m-cresol solution shows that incorporation of a methylene spacer between the two phenylene rings in the sub-unit originating from the diamine results in a 30 nm shift of [[lambda].sub.max.C=N] compared to the PA2. On the other hand PA4 exhibits a red shift in [[lambda].sub.max.C=N] compared to another polymers in DMA and also in m-cresol solution (see Table 1). Also [E.sub.C=N] of PA4 (3.02 eV) is smaller than for PA2 (3.18 eV).

The UV-vis spectra of aliphatic-aromatic polyimine PA3 show one absorption peak at around 300-320 nm and the tail at around 350-380 nm. The second absorption band is not well defined. The absorption differences between PA3 and the other PAs could be explained by the fact that incorporation of aliphatic chain in the aromatic backbone usually breaks the conjugation of polymers. PA3 as an aromatic-aliphatic PA has only "[pi]-conjugated part" in the each constitutional unit (monomeric unit of polymer) and for this reason the second absorption band is not clearly seen. Similar phenomena was observed for the another aliphatic-aromatic polyketimines [17].

Figure 2 shows the electronic absorption spectra of the polyimines PA2-PA4 in MC solution, as example.

Additionally, we have studied the concentration behavior of PAs using UV-vis spectroscopy, monitoring the changes in the [pi]-[pi]* and n-[pi]* transitions (see Fig. 3). UV-vis spectra of the PAs are exemplified by PA1 and shown the evolution of the absorption spectra for PA1 upon increasing the polymer concentration in MC and in DMA solution.

We found no changes in the UV-vis spectra of the polymer PA1 in DMA and in MC solution along with change the concentration from 1 x [10.sup.-4] to 4 x [10.sup.-3] mol/l (see Fig 3). All of the spectra shown in Fig. 3 were recorded also after 24 and 48 h. No time-dependent in UV-vis absorption spectra of polymer was observed.

[FIGURE 2 OMITTED]

PL of Undoped PAs in DMA and in m-Cresol Solution

PL spectra of the PAs investigated are also solvent-dependent (see Table 2). Again, the replacement of DMA by MC results in a bathochromic shift of the PL band, the solvent induced spectral changes being however more pronounced than in the case of UV-vis absorption spectra.

Solution PL spectra of PAs in DMA solution clearly demonstrated the interest of our macromolecular engineering approach for the tuning of the photoluminescent properties of PA materials: an emission band located in the range of 480-509 nm (typical of a blue-green and green emitter) is obtained. In the DMA solution polymers are more nonplanar and may undergo some conformational changes. It is clear, from the above presented data (see Table 2), that optical properties of PAs can be tuned by chain engineering. So, the fluorescence of PA1 and PA3 in DMA solution is blue-green in color. While for PA4 in DMA solution green color was detected.

[FIGURE 3 OMITTED]

Moreover, it is necessary to emphasize that solvatochromism is manifested in the PL spectra of the PAs. As stated previously, the emission maxima in MC solution are red shifted relative to those in DMA solution (see Table 2). The Stoke shift in this system can be clearly seen. The MC-solution PL spectra of PAs have a broad emission band at 514-525 nm, which are significantly red shifted from the corresponding polymers in DMA-solution which have a peak at 480-509 nm (Table 2). In addition, the shape of the emission spectrum changes significantly, and the emission intensity is much lover in MC than in DMA solution (Fig. 4).

The character of spectral changes induced by MC indicates that they may be significantly amplified if stronger than MC protonating agents are added to the solution. The selection of an appropriate acid is of crucial importance in this case. It has been demonstrated that in polymers containing basic centers in their main chain several structural features can be modified via protonation. This involves not only the conformation of an individual chain but also protonation induced self-assembling which leads to ordered supramolecular aggregations [11b]. Several technologically important properties can be tuned in this manner such as processibility, electrical transport [22, 23], spectroscopic properties [10, 11] and others.

[FIGURE 4 OMITTED]

Optical Properties of Protonated PAs

The main aim of this paper is preparation new types of luminescent, amphiphilic, supramolecular polymers in which the lateral groups are bonded to the [pi]-conjugated main chain not by covalent bonds as it has been done to date but by ionic-type bonds. These ionically bonded lateral groups must serve as dopant anions which strongly influence not only the processibility but also the band structure and by consequence the spectroscopic properties, of the polymers. This approach requires a careful design of the dopant structure which promotes supramolecular ordering of the systems by molecular recognition phenomena and 3D steric matching between the constituents of the system.

The presence of an electron lone pair at the nitrogen atom of the imine group enables the protonation of PAs with Bronsted acids. In our research we have selected 1,3,5-benzenetricarboxamide sulfonic acid ("dopant I") as the protonating agent. Our interest of 1,3,5-benzenetricarboxamide sulfonic acid ("dopant I") is caused by special properties of this class of compounds. In particular, the detailed characterizations of this class of 1,3,5-benzenetricarboxamide derivatives revealed particularly appealing features (Lyotropic and thermotropic liquid crystalline behaviors [24a,b,d], gelation at low concentration in organic solvents [24c,d] and the obtention of "infinite" [pi]-stacked rods encased in triply-helical H-bonded amide strands [24e]).

In our opinion this "dopant I" should form [pi]-stacks as a results of Van der Waals interactions between the core (phenyl rings), which should be further stabilized by three H-bonding interactions originating from the association of the three amide bonds (see Fig. 5).

Moreover since "dopant I" contains three sulfonic groups, we can activate one, two or three protonic centers provided that an appropriate geometry of the supramolecular aggregation is achieved (see Fig. 6).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

UV-vis of PA/"dopant I" Supramolecules

PAs with "dopant I" at 1:1, 1:2, and 1:3 molar ratio were dissolved in DMA and in MC solution and UV-vis spectra after 24 and 48 h were recorded. UV-vis spectra of the doped polymers are exemplified by PA1 and shown spectral changes in the imine band after protonation with "dopant I" (see Table 1).

For example, protonation of PA1 in DMA and in m-cresol solution causes a blue shift of the imine band at about 10 nm in comparison to the undoped one (see Table 1). Additionally it should be stressed that the second absorption band of the protonated polymer PA1 in DMA is broader and exhibit lower intensity in comparison with undoped polyimine PA1 (compare Figs. 3a and 7a).

On the other hand in MC solution the UV-vis spectra of PA1/"dopant I" complex show one absorption peak at around 300 nm and the shoulder at about 350 nm. The second absorption band is not well defined (see Fig. 7b). Figure 7 presents UV-vis spectra of doped PA1 in DMA (Fig. 7a) and in MC (Fig. 7b) solutions with molar ratio of "dopant I" to PA1 1:1, as example.

Additionally, the new absorption band at about 420 nm in DMA solution of protonated PA1 was observed (see Table 1). Moreover it should be noted that intensity of the third band in UV-vis spectra of the doped PA1 is higher than the intensity of third band of "dopant I" in DMA solution (compare Figs. 7a and 8b).

In the goal to interpretation of the new band in the UV-vis spectra of doped PA1 first the absorption spectra of "dopant I" in DMA and in MC solution should be discussion.

Spectroscopic signatures of the "dopant I" are clearly seen in their UV-vis solution absorption spectra and consist of one (two) more or less overlapping bands (see Fig. 8).

The band at shorter wavelengths, usually in the range of 260-270 nm in DMA solution and 298-305 nm in MC solution, can ascribed to the [pi]-[pi]* transition in the aromatic ring. The second absorption band is located in the spectral range of 420 nm for DMA solutions (see Fig. 8b) and exhibit low intensity. Moreover it should be stressed that the second absorption band of "dopant I" in m-cresol solution is not well defined and is observed as a tail. Additionally, the third absorption band in UV-vis spectra of "dopant I" at about 480 nm in DMA solution was observed and also exhibited very low intensity (see Fig. 8b).

As it was say previously the "dopant I" should form [pi]-stacks as a results of Van der Waals interactions between the phenyl rings which should be further stabilized by three H-bonding interactions originating from the association of the three amide bonds (see Fig. 5). Additionally in m-cresol solution interactions between sulfonic groups of "dopant I" and OH group of MC are possible. This behavior influences on the absorption spectra of "dopant I," and is observed as a broader absorption bands, what confirmed H-bonding interactions between phenol and sulfonic acid.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Also the effect of dopant concentration on the UV-vis spectra of doped polymer PA1 was detected. The absorption spectra of doped PA1 in DMA solution are blue shifted along with change the doping level from 1 to 3 (see Table 1). Additionally, it should be stressed that we not observed changes in absorption intensity of the doped PA1 with an increase of dopant concentration in DMA solution. Also the protonation of polyimine PA1 in MC solution caused blue shift in comparison with undoped one. Moreover, in m-cresol (MC) solution the absorption bands of doped polymer, along with an increase of dopant concentration, exhibit similar shape, and changes in the position of bands and absorption intensity are not observed.

Changes in the shapes of the UV-vis spectra of the PA1 investigated after protonation with "dopant I" in DMA and in MC solution confirm that some delocalization of electrons in the polymer chains took place (see Fig. 7).

All of the spectra shown in Fig. 7 were recorded also after 24 and 48 h. No time-dependent in UV-vis absorption spectra of doped polymer was observed.

Summary this part of our work, we would like to emphasized that the addition of "dopant I" to PA1 in DMA and also in MC solutions lead to the appearance of a new electronic transition related to the "dopant I"-protonated PA1 chromophore units. One also can be noted that the addition of "dopant I" lead to changes in the wavelength location of the bands present in absence of "dopant I." Additionally, the protonation effect can be detected also as a change of color in direction to dipper tone. All these complex modifications can be rationalized by invoking synergetic effects brought by the different approaches used to tune to the solution absorption features of these PA materials.

PL Properties of PA/"dopant I" Supramolecules

PL spectra provide a facile method of elucidating the effects of Bronstedt acid protonation on the molecular structures of the polymers. PAs with "dopant I" at 1:1, 1:2, and 1:3 molar ratio were dissolved in DMA or MC and PL spectra were recorded. [[lambda].sub.max] of PL emission bands of PAs after protonation in DMA solution are collected in Table 3.

For example, protonation of PA1 shifts bathochromically the PL to 506 nm whereas by protonation of PA3 it is possible to obtain a luminescence band at 531 nm (Table 3). Moreover it should be noted that in DMA solution the PL bands of doped polymers, along with an increase of dopant concentration, exhibit similar shape, and small changes in the maximum of emission bands were found (Table 3). Additionally, it should be stressed that with an increase of dopant concentration decrease of the relative intensity of the PL of the PAs was detected (see Fig. 9).

[FIGURE 9 OMITTED]

Additionally, the PL properties of doped-PA1 and PA3 in MC solution were investigated (see Table 4).

The MC-solution PL spectra of doped PA3 have a broad emission band at 555-573 nm, which are significantly red shifted from the corresponding undoped polymer PA3 which has a peak at about 525 nm (Table 4). On the other hand the emission spectra of doped PA1 in MC solution are blue shifted in comparison with undoped one (see Table 4).

Spectroscopic data discussed above clearly indicate that MC and "dopant I" are not an inert and strongly interacts with the polymer chain modifying its conformation towards higher planarity. As mentioned early while quite efficient tuning of the UV-vis absorption and PL features of [pi]-conjugated materials can be obtained through the branching of the [pi]-conjugated backbone by lateral groups bearing appropriate functionality (solubilizing, plasticizing).

Additionally, we have studied the emission spectrum of free transparent foils obtained by casting chloroform solution of polyimine PA3 with a nonemissive polymer, poly(methyl methacrylate) (PMMA). The concentration of PA3 in PMMA in foil was [10.sup.-4] mol/g.

Our preliminary data on the luminescence spectra of blended PA in the solid-state show that PA3 exhibits the PL band maximum at 435 nm and is hypsochromically shifted not only with respect to the spectrum measured for MC solution but also as compared to the band registered in DMA solution. For example, PA3 in solid-state causes a blue shift of the maximum of emission band at about 50 nm in comparison to the polyimine PA3 in DMA solution. While PL spectrum of PA3 in solid-state is 90 nm hypsochromically shifted in comparison to PA3 in MC solution.

The observed PL shifts when going from DMA solution to MC solution and finally to the solid-state in blend clearly indicate that this property is principally governed by local conformation of the polymer chain and interaction between the polymer chain and environment.

CONCLUSIONS

In summary, we have demonstrated that amphiphilicity activated by protonation can facilitate the self-assembly of electroactive and photoactive structure from nonamphiphilic [pi]-conjugated polymers exemplified by the PAs. The PAs constitute a new family of photoluminescent materials whose emission spectra can be conveniently tuned via protonation of the imine nitrogens with an appropriate, amphiphilic protonating agent. Another possibility of PL spectra alteration involves the selection of an appropriate solvent. Aprotic and protic solvents interact differently with the polymer chain, the latter induce better planarity of the PAs chains which results in a bathochromic shift of the PL band maximum. When different solvents along with "dopant I" contain three sulfonic groups, (we can activate one, two or three protonic centers), capable of protonation the free electrons pair of nitrogen atom in imine groups in the polymer chains, was applied. The concept of amphiphilicity could facilitate exploration of novel organizations in supramolecular chemistry and may find applications in sensors, photonic and optical materials and other photonic devices.

ACKNOWLEDGMENTS

Authors thank Prof. A. Pron and Dr P. Rannou from CEA in Grenoble, France for fruitfull discussions and gift sample of 1,3,5-benzenetricarboxamide sulfonic acid ("dopant I"). The authors thank Department of Earth Sciences, Silesian University in Sosnowiec for made it possible to do the fluorescence spectra measurements. The research was carried out within the (PAN)/CNRS-Direction des Relations Internationals collaboration program No. 14494.

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Agnieszka Iwan, Zbigniew Mazurak, Danuta Sek

Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland

Correspondence to: Agnieszka Iwan: e-mail: aiwan@cchp-pan.zabrze.pl
TABLE 1. UV-vis bands of polyimines in DMA and m-cresol solutions.

 UV-vis
 DMA
 [[lambda].sub.arom] [[lambda].sub.C=N]
Code (nm) E (eV) (nm) E (eV)

PA1 269 4.61 348 3.56
PA2 -- (a) -- (a) -- (a) -- (a)
PA3 300 4.13 tail --
PA4 284 4.37 362 3.42
PA1/dopant 290 4.27 345 3.59
 I (b)
PA1/dopant 290 4.27 340 3.65
 I (c)
PA1/dopant 290 4.27 335 3.70
 I (d)

 UV-vis
 DMA
Code [lambda] (nm) E (eV) [[lambda].sub.arom] (nm)

PA1 -- -- 300
PA2 -- (a) -- (a) 290
PA3 -- -- 320
PA4 -- -- 320
PA1/dopant 425 2.92 300
 I (b)
PA1/dopant 420 2.95 300
 I (c)
PA1/dopant 410 3.02 300
 I (d)

 UV-vis
 m-cresol
Code E (eV) [[lambda].sub.C=N] (nm) E (eV)

PA1 4.13 360 3.44
PA2 4.27 390 3.18
PA3 3.88 tail --
PA4 3.88 410 3.02
PA1/dopant 4.13 ~350 3.54
 I (b)
PA1/dopant 4.13 ~350 3.54
 I (c)
PA1/dopant 4.13 ~350 3.54
 I (d)

(a) Insoluble.
(b) Molar ratio of "dopant I" to PA1 (1:1).
(c) Molar ratio of "dopant I" to PA1 (2:1).
(d) Molar ratio of "dopant I" to PA1 (3:1).

TABLE 2. Fluorescence emission bands of polyimines in DMA and m-cresol
solutions.

 PL (exc. = 400 nm)
 DMA
Code [[lambda].sub.emis] (nm) E (eV)

PA1 480 2.58
PA2 -- (b) --
PA3 485 2.58
PA4 509 2.44

 PL (exc. = 400 nm)
 VIC
 [[lambda].sub.emis]
Code (nm) E (eV) [DELTA][[lambda].sub.emis] (a) (nm)

PA1 514 2.41 +34
PA2 521 2.38 --
PA3 525 2.36 +40
PA4 524 2.37 +15

(a) Stokes shift.
(b) Insoluble.

TABLE 3. Photoluminescence of undoped and doped polyimines in DMA
solution with different concentration of "dopant I" under 400 nm
excitation wavelength.

 Molar ratio
 of PAs to
 "dopant I" [[lambda].sub.emis.] [DELTA][[lambda].sub.emis] (a)
Code (mol:mol) (nm) (nm)

PA1 1:0 480 --
PA2 1:0 nd --
PA3 1:0 485 --
PA4 1:0 509 --
PA1 1:1 502 +22
PA2 1:1 484 --
PA3 1:1 525 +40
PA4 1:1 -- (b) --
PA1 1:2 499 +19
PA2 1:2 487 --
PA3 1:2 531 +46
PA4 1:2 519 +10
PA1 1:3 506 +26
PA2 1:3 490 --
PA3 1:3 529 +44
PA4 1:3 521 +12

(a) Stokes shift of undoped and doped PAs in DMA solution.
(b) Low intensity.

TABLE 4. Photoluminescence of undoped and doped polyimines PA1 and PA3
in MC solution with different concentration of "dopant I" under 400 nm
excitation wavelength.

 Molar ratio
 of PAs to
 "dopant I" [[lambda].sub.emis.] [DELTA][[lambda].sub.emis] (a)
Code (mol:mol) (nm) (nm)

PA1 0:1 514 --
PA1 1:1 508 -6
PA1 1:2 505 -9
PA1 1:3 503 -11
PA3 0:1 525 --
PA3 1:1 573 +48
PA3 1:2 565 +40
PA3 1:3 555 +30

(a) Stokes shift of undoped and doped PAs in MC solution.
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Author:Iwan, Agnieszka; Mazurak, Zbigniew; Sek, Danuta
Publication:Polymer Engineering and Science
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Date:Aug 1, 2007
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