Polyhedral oligomeric silsesquioxane cage integrated soluble and fluorescent poly(3,4-propylenedioxythiophene) dye

Highlights

• Functionalization of ProDOT monomer with covalently bonded alkyl substituted POSS unit.

• Synthesis of soluble and fluorescent ProDOT derivative.

• Potential use of ProDOT-POSS dye in electro-optical and bio-applications.

• Effective use of ProDOT-POSS polymer in dye removal applications.

Abstract

A new analog of poly (3,4-propylenedioxythiophene) conjugated polymers called PProDOT-POSS, where polyhedral oligomeric silsesquioxane (POSS) nanocage with alkyl substitutions was integrated on the bridge of 3,4-propylenedioxythiophene unit and its structure was confirmed by spectroscopic techniques, was reported. The polymers were synthesized via both chemical and electrochemical polymerization techniques. While chemical polymerization was carried out in the presence of anhydrous FeCl₃ as an oxidant, a solution of 0.1 M tetrabutylammonium hexafluoride electrolyte dissolved in a mixture of dichloromethane and acetonitrile (1/3: v/v) was used as an electrolyte solution for electrochemical polymerization. Electro-optical properties of the corresponding polymers were characterized by using electroanalytical techniques such as cyclic voltammetry and square wave potentiometry, and spectrophotometric methods like ultraviolet–visible and fluorescent spectrophotometry. Corresponding polymers obtained both electrochemically and chemically are soluble completely in common organic solvents such as chloroform, toluene, dichloromethane, tetrahydrofuran, etc. Polymer samples both in film and solution forms can be doped/dedoped reversibly by using a chemical oxidant or an external potential. The optical bandgap of the neutral polymer film with a maximum absorption band at 555 nm was calculated as 1.95 eV. Polymers have fluorescent property and excited polymers represented a red/orange light with an emission band centered at 605 nm in toluene. Also, PProDOT-POSS polymers have electrochromic properties under external potentials and they have an optical contrast of 55% at 555 nm between their neutral and oxidized states. Upon oxidation, they showed high transparency and they can switch between redox states in a short time (switching time = ~1.0 s) as well as high coloration efficiency (502 cm²/C for 95% switching). It can be concluded that POSS based PProDOT polymers can be good candidates for optoelectronic and bioelectronics applications.

Short Abstract for Paper Submission: A new analog of poly (3,4-propylenedioxythiophene) conjugated polymers called PProDOT-POSS, was reported with the integrated of alkyl-substituted polyhedral oligomeric silsesquioxane nanocage structure. The polymers were synthesized via both chemical and electrochemical polymerization techniques. Corresponding polymers obtained both electrochemically and chemically are soluble completely in common organic solvents such as chloroform, toluene, dichloromethane, tetrahydrofuran, etc. Polymer samples both in film and solution forms can be doped/dedoped reversibly by using a chemical oxidant or an external potential. The optical bandgap of the neutral polymer film with a maximum absorption band at 555 nm was calculated as 1.95 eV. Polymers have fluorescent property and excited polymers represented a red/orange light with an emission band centered at 605 nm in toluene.

Introduction

The interest shown by many scientists in the alkylenedioxythiophene based conjugated polymers has been still growing because of their optical and electrochemical properties [[1], [2], [3], [4], [5], [6]], which causes them to use in various applications like polymeric light-emitting diodes [7], photovoltaic/solar cells [8,9], electrochromic screens [2,9,10], camouflage materials [10], smart windows [10], mirrors [10], biosensors [10], etc. The popularity of these polymer results from the advantages that they provide such as simplicity of synthetic route, solubility in common solvents, processability, and easy film-forming, low price, and adjustable electro-optical properties. For instance, poly (3,4-ethylenedioxythiophene) (PEDOT), known as the most famous member of polyalkylenedioxythiophenes, has superior properties such as low oxidation potential, high optical contrast, short switching time between the neutral and oxidized states and high stability at oxidized state [1]. Unfortunately, all these properties can be overshadowed by insolubility properties, which hinders its use in various applications. The substitution on the bridge of the parent monomer structure via alkyl chains cannot be enough to get soluble PEDOT derivatives [1,[11], [12], [13]]. When the scientists introduced any problem like that, they can overcome this problem by functionalizing the monomer such as increasing the size of a bridge or the number of alkyl chains substituted on the bridge. For example, Reynolds's and his colleagues utilized propylenedioxy bridge instead of ethylenedioxy bridge to synthesize new derivatives called 3,4-propylenedioxythiophenes (ProDOTs). Alkyl substituted derivatives of this new member were found resulting in soluble polymers and exhibited better electrochromic properties like percent transmittance change between redox states, coloration efficiency and switching times [1,4,5,14]. On the other hand, in our previous work, polyhedral oligomeric silsesquioxane (POSS) containing EDOT analog was synthesized and polymerized successfully via both chemical and electrochemical methods to get a soluble PEDOT polymer [15]. When compared to PEDOT, the corresponding polymer (PEDOT-POSS) showed redox couple positioned at slightly higher positive potential, robust electrochemical and optical stability, faster switching time and higher optical contrast, etc. POSS units are inorganic nano-sized cages (sizes ranging from 1 to 3 nm) containing silicon-oxygen inner center, and they can be modified by organic functional side groups like various lengths of alkyl chains. It can be concluded that conjugated polymers bearing POSS units can represent better thermal [16,17], electrochemical [15,18] and mechanical properties [19,] as well as better solubility and processibility [15]. For example, the integration of POSS cages on polyfluorene prevented undesired green emission (keto effect) [20] by improving the thermal stability and therefore POSS based polyfluorenes can be amenable to use in PLED technology as a blue light-emitting layer [21]. Recently, the octa-ProDOT-functionalized POSS unit was synthesized and its PEDOT copolymer was characterized and studied to explore its use in bioelectronics devices. While a homopolymer could not be achieved electrochemically, a copolymer containing PEDOT and octa-ProDOT-POSS was synthesized successfully via electropolymerization. It was concluded that the addition of a limited amount of POSS-ProDOT enhanced the electrochemical stability of PEDOT [22]. Also, it was reported that POSS containing polyanilines shows improved optical contrast with 40% and increased ionic conductivity than the parent polyaniline [23].

Despite these reports and promising results, there are not many publications on POSS integrated conjugated polymers to enhance the optical and electrochemical properties. These findings in hand, we turned our attention to synthesize soluble and fluorescent POSS based conjugated polymeric dyes. To receive this goal, the bridge of ProDOT unit was functionalized with alkyl-substituted POSS cage and the corresponding monomer was polymerized chemically and electrochemically. The findings showed that the soluble corresponding polymers exhibited electrochromic and fluorescent properties. ProDOT-POSS polymers, to the best of our knowledge, can be stated as the first fluorescent PProDOT derivative. This property, besides electro-optical applications, makes ProDOT-POSS a promising dye material for bio-applications like cancer cell imaging.