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Iminoboronate polymers as dynamic smart materials

Periodic Reporting for period 1 - IminoBoron (Iminoboronate polymers as dynamic smart materials)

Reporting period: 2016-03-01 to 2018-02-28

The aim of this project is the formation of a new class of dynamic, conjugated polymers based on a reversible conjugated polyimine backbone with pendant boronic acids. Monomers incorporating aldehyde, amines and boronic acid functionalities will condense together to construct the polymer backbone. Further functionalization by formation of boronate esters with diols will constitute the key tool to tuning the properties of the polymer. The presence of the two independently interchangeable dynamic covalent bonds (imine and boronate ester) opens the door to new functional material able to respond to two orthogonal external stimuli.

In regards of the applications, our objectives are dual: firstly, we will prepare highly tunable materials with promising electronic properties, structural rigidity, which comes from the dative N→B stabilisation, and electrochemical stability, as a consequence of the non-conjugated boron functionalization, which renders them strong candidates for use in light-emitting devices. We aim to succeed in the construction of new conjugated and easily processable blue-light emitting polymers that will take a leading role on the next generation of multifunctional OLED devices, leading to a more sustainable energy consumption.

Secondly, conjugated polymers with chemical receptors directly attached to the polymer backbone can act as highly sensitive fluorescent chemosensors. The dynamism of the iminoboronate polymers also renders them as promising candidates for sensing applications. Their projected sensitivity is ascribed to electronic communication between receptors, via the conjugated backbone. Moreover, the reversible formation of cyclic boronates between boronic acid functional polymers and diols has been extensively used in saccharide and anion recognition. We will focus our attention on glucose, which is directly related to diabetes, a disease that is becoming alarmingly common in our society.

All the synthetic effort put into this project has led us to understand and establish the methods and procedures by which smart iminoboronate-based polymers can be accessible. Therefore, we have gained the necessary knowledge to successfully synthesise iminoboronate polymers and we are now in the position of effectively designing new structures to overcome the solubility problems that we have faced in the past, and therefore, construct the desired devices.
Firstly, we focused our attention on the synthesis of the target monomers to obtain the iminoboronate polymers. The target homocomplementary (AB-type) and heterocomplementary (AA- and BB-type) monomers’ structures are shown in Figure 1.

The synthetic route to the monomers 1, 2 and 3 (Figure 1) consisted of successive protection of the anilines, Palladium cross coupling reactions (Suzuki-Miyaura, Sonogashira and Heck, respectively), triflation, borylation and final selective deprotection. Following this route, protected derivatives were obtained for the three AB-type monomers. Several deprotection reactions were tested, employing a wide range of conditions and/or altering the order of the selective deprotections (starting by either the amine or the boronate ester). These include among others:

After thoroughly studying this process, we found that these deprotections always result in an insoluble red solid that, unfortunately, we were unable to characterize. A possible explanation for this unexpected solubility is that uncontrolled oligomers are being formed during the deprotection step by a fast imine condensation reaction.
We envisaged different approaches to overcome this problem:

A) Pre-formation of the aryl-boronate ester employing a catechol bearing solubilizing alkyl chains to increase the solubility of the monomer.

B) Employing a photo-responsive masked aldehyde to avoid the uncontrolled reaction between the amine and the aldehyde.

C) Preformation of the iminoboronate moiety and synthesis of the polymer by a Palladium crossed coupling reaction.

Next, we moved to AA- and BB- heterocomplementary monomers, where two different monomers are employed in the polymerization reaction. In this particular case, shorter co-polymers are expected. On the other hand, systems of increasing complexity can be easily obtained by simply combining two species with different electronic properties. An optimization of the reaction conditions for the formation iminoboronate oligomers was carried out by 1H-NMR by reacting monomers and terminating groups in different ratios, aiming to monitor the changes in the NMR spectrum upon increasing the length of the polymeric species. We analysed the impact of different factors in the reaction such as the concentration, solvent, temperature and the nature of the catechol. Chlorinated and aromatic solvents were designated as best solvents to stabilize the iminoboronate moiety. After a thorough study, we were able to obtain polymeric red films by imine condensation reaction of AA- BB- type monomers in tetachloroethane at 70 °C for 24h. However, the characterization of these polymeric films was exceptionally challenging due to its solubility features. Solvents such as chlorobenzene or dichlorobenzene led to the formation of shorter polymers/oligomers in the same reaction conditions. The presence of these shorter units was confirmed by 1H-NMR in deuterated tetrachloroethane, where the disappearance of the aldehyde proton signal and appearance of the imine signal were observed.
Light is a key resource in our everyday lives, and hence, the construction of a new generation of multifunctional materials, such as blue-light emitting polymers, is necessary for realizing LED-based full-colour displays and general lighting applications. Blue OLEDs with high efficiencies have attracted considerable scientific interest during the last decades for their potential applications, because performance corresponding to that of red and green emitters has not been reached. We have gained the necessary knowledge and experimental experience to construct new soluble, conjugated and easily processable blue-light emitting polymers that will take a leading role on the next generation of multifunctional OLED devices with high quantum efficiencies and photochemical stabilities. The successful fabrication of these new devices will constitute a major step towards the smart policies that the European Commission is pursuing to develop sustainable urban areas. Furthermore, iminoboronate blue OLEDs will make a valuable contribution to take action to promote environmental protection and resource efficiency having a great impact on society.

On the other hand, the development of efficient and highly sensitive polyreceptors to surmount critical social problems is of paramount importance, since current reports of the World Health Organization (WHO) indicate that diabetes affects 9% of the global population and projects that diabetes will be the seventh leading cause of death by 2030. The new dynamic iminoboronate polymers are able to reversibly exchange the diol attached to the boronic acid moiety in organic media, which renders them as extremely interesting materials for biomedical applications. The conjugated polymer will act as an antenna to enhance the chemosensitivity of the reception sites. The development iminoboronate polymers will improve the capacity to detect and monitor diabetes by facilitating the construction of low cost devices.
Iminoboronate-based polymeric films
Chemical structure of the target monomers