Pyrrole Derivatives and Their Modern Applications

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Abstract

Pyrrole is a versatile molecule which has various applications. Pyrrole derivatives have found use in dyes, pigments, polymers and coatings, pharmaceuticals. Most of the applications covered in this report are post 2006. Applications in dye sensitized solar cells, commercial pigments, functional applications such as hydrogen gas sensor, fluorescent probes, radar absorbing material etc are discussed. This reports summarises all the relevant modern applications and track modern developments in some of the applications.

Introduction

Pyrrole is an aromatic, heterocyclic compound containing 6  electron electrons. It is weakly basic in nature since the lone pair of nitrogen gets engaged in conjugation within the ring. It is colour less liquid that readily undergoes polymerization on exposure to air to yield a dark coloration. Pyrrole undergoes electrophilic reactions at position 2 and 5. It is very widely present in nature in form of natural pigments, bacterial and fungal products. Synthetically pyrrole derivatives are exploited widely in pharmaceuticals, dyes, pigments, polymers, metallurgical applications etc1 .  It can be synthesized easily through Paal knorr synthesis 2 [pic 4]

Chemistry of pyrrole

Being aromatic in nature, pyrrole is difficult to hydrogenate. It readily undergoes electrophilic reactions at α position. The resonance structures of pyrrole in fact explain this observation

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The reactivity of pyrrole is similar to that of benzene and aniline. Since pyrrole polymerizes easily in acidic medium, many electrophilic reactions catalysed by acid does not work on pyrrole as it polymerizes. 2 

Pyrrole has the advantage of being small sized as well having high electron density .This property is useful in many polymer and dye sensitized solar cell applications. Also since it contains nitrogen, it has high levels of biological activity and hence we find many drugs based on pyrrole system. Pyrrole as di keto pyrrolo pyrrole pigment have excellent fastness properties. It gives excellent hue and perfect balance of opacity and transparency. It is because of this that we find pyrrole derivatives in many applications.3

Classification of pyrrole based applications

Applications of pyrrole can be classified on basis of various areas where they are used, viz Dyes and pigments , polymers and coatings and pharmaceuticals. Some classes of pyrrole derivatives are common for more than one field of application. Di keto pyrrolo pyrrole is one such example. DPP is used as a commercial pigment as well as dye sensitizer in dye sensitized solar cell.

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         Fig 2: Atorvastatin (Anti Cholesterol)                        Fig 3: BODIPY ( Laser Dye)

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                                       Fig 4: Polypyrrole (A conducting polymer)

Pyrrole derivatives in Dye Sensitized Solar Cells 5,6,7,8,9

Alternate energy sources are the way forward to tackle current energy crises. One such alternate source of energy is solar energy which is harnessed through the means of dye sensitized solar cells. These cells have a dye coated on a suitable metal oxide and requires presence of an electrolyte capable of providing electrons, preferable I3-/I- system. The dye absorbs solar radiations and goes into excited state. Instead of radiative de-excitation, these electrons in excited state gets transferred to the metal oxide which then supplies electrons to external circuit. The dye sensitizers are derivatives of pyrrole.

Working of Dye sensitized solar cells

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Fig 5: Working of DSSC [31] Computational modelling of organic dye sensitizers for the application of solar cells, Narges Mohammadi,

Loss in efficiency

Interactions with other molecules: Dye molecules tend to aggregate and therefore suffer from aggregation caused quenching. This does not allow the excited electrons to be passed on to metal oxide and hence we have a loss in efficiency. Competition with electrolyte: The I3-/I- electrolyte has tendency interact with the metal oxide instead of interacting with dye. Hence it results in a competitive reaction of electrolyte with metal oxide and electrolyte and dye molecule. Recombination with oxidized dye or electrolyte: The excited dye molecule can react with another oxidized dye or electrolyte and not with metal oxide. This results in loss in efficiency.

Pyrrole derivatives are used in DSSC because they are one of the strongest electron rich five member heterocyclic, they have abundant electron density beneficial for charge transfer from donor to acceptor, they have high Thermal stability, Non linear optics effects in case of pyrrole derivatives are upto 3.3 times stronger than thiophene and furan analogues.

Developments in DSSC relating to pyrrole derivatives

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Fig 6: General structure of a dye sensitizer

The above diagram is a schematic of general dye sensitizer. Pyrrole derivatives have been with and as spacer, isolation group and acceptor. Current developments have been focused on how to manipulate these groups to achieve maximum efficiency.

Tuning of isolating group: To suppress aggregation of dyes, a strategy of using isolation groups is devised. A long alkyl chain is added to spacer to suppress aggregation. This chain does not affect the Donor-Acceptor conjugation. Again the isolation groups can tuned to get desired results. Experimentation are done starting from isolation as hydrogen to methyl phenyl to tert butyl phenyl. Also ‘ H type’ dyes are synthesized their efficiencies are checked. These H type dyes show higher efficiencies as they supress aggregation to a higher extent.

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Fig 8: Molecules with single Isolation group                        Fig 9: H type molecules with shared isolation group

Di keto Pyrrolo Pyrrole pigments in dye sensitized solar cells

DPP pigments are commercialized on a large scale. They have excellent light fastness properties and are heat stable. The purpose of having DSSC based on  DPP  is improving the lifetime of solar cells. This is done to move a steep forward towards commercialization of solar cells. DPP along triphenyl amine are used to construct dye sensitizers. These moieties are chosen due to their low cost and ease of availability. DPP based dyes also have furan and thiophene spacers incorporated in them to increase efficiency.