Pyrrole Derivatives and Their Modern Applications

...

[pic 18][pic 19]

[pic 20]

Fig 11: Efficiencies with different substituents as spacers

Efficiency in case of benzene as a spacer is greater than furan and thiophene. DSSCs sensitized by the dyes with benzene and furan as p-spacer means that the furan linkers have higher electron transfer yield than the dye with the thiophene linker. Therefore, the introduction of benzene and furan rings into DPP-based dye structure has a positive effect on IPCE. DPP 2 has the highest photocurrent due to higher loading and more effective electron injection efficiency. Decrease of co planarity is one of the reason for less aggregation.

Thus with finer tuning we can achieve higher efficiency and go one step closer to mass production of DSSC.

Pyrrole based pigments

Pyrrole based DPP pigments are best in class. They have high fastness properties and therefore they are used commercially in various applications. Ferrari red is the best example of DPP based pigments used in paints.

[pic 21]

Fig 12: Famous Ferrari red pigment based on DPP

Functional applications of DPP pigments are in fields of LCD as color screens. Due to their high clarity of hue and high temperature tolerance, high contrast value and brilliance, they are used extensively as red pigment in color screens. The optimum properties required for such applications are obtained by using a mixture of DPP derivatives. Their structures are as given below.

[pic 22] [pic 23] [pic 24] [pic 25][pic 26][pic 27]

[pic 28]

Fig 16: Color filter structure

DPP derivatives as hydrogen gas probes 14,15

One unique application of DPP derivatives is a sensor for hydrogen gas. Pyrrole substituted DPP pigments have sensitivity towards hydrogen ions. The working principle is as follows: Hydrogen gas dissociates easily under electric field and this dissociation is further aided by the derivatives mentioned above. The dissociated hydrogen protonates the pyridine site and changes the color of pigment from red to violet. Also it changes the resistivity of the medium which can be detected and correlated with the concentration of hydrogen gas.

[pic 29]

Fig 17: Deprotonated form Fig 18: Protonated form

The application of these probes have huge potentials in fuel cell technology where hydrogen gas is employed. These probes have high sensitivity and when combined with visual color change becomes a highly suitable probe/sensor.

DPP derivatives as fluorescent probes 17

DPP derivatives are highly suited for applications as fluorescent probes. They can incorporate large number of different substituent which helps in tuning the probe for various ions. In fluorescent probes, they act as an acceptor system. They exhibit high fluorescence quantum yields along with possessing thermal and photo stability. Some features of DPP derivatives are that they can be used in analysis of anions, cations, reactive oxygen species, thiols, pH, CO2 and H2. They can be used in Molecular imaging. Structurally modified DPP based Near Infrared Region Dyes

Pyrrole in Laser dyes 18

BODIPYs (boron−dipyrromethene complexes) are the pyrrole derivatives which are very useful as Laser dyes. Extensive research is in field of BODIPY due to various advantages associated with it

[pic 30][pic 31]

Fig 19: BODIPY structure Fig 20: Laser dyes showing fluorescence

Main advantages of BODIPY structure are that it presents strong absorption and fluorescence bands in visible spectral region, it gives high fluorescent and lasing efficiencies, it has Versatile, Robust, Low triplet state population , Thermal and photochemical stable, it can be incorporated in solid host matrices which leads to development of solid state tuneable dye lasers. It has very low tendency to aggregate (upto 2 x 10-2 M). It has low intersystem crossing probability. Low population of triplet state is the most important advantage since it minimizes resonance cavity losses, BODIPY laser dyes are highly compatible with polymer matrix and can be doped or covalently linked to polymer chain (Poly Methyl Methylacrylate). Experimental results indicate that the absorption and fluorescence band does not shift in polymer matrix

[pic 32]

Fig 21: BODIPY incorporated in polymer matrix through covalent bond

BODIPY in Organic Light Emitting Diodes 19

Borondipyrromethene (Bodipy) dyes with strong solid-state photoluminescence are highly desirable for their applications in OLEDs. Aggregation caused quenching makes BODIPY weakly emissive in solid state. Adding bulky group is a solution but has several draw backs. Aggregation induced emission is the best solution to have emission even at high concentration. Adding oligo substituents on meso phenyl ring is intermolecular and directs molecule in a particular configuration. The major advantage of aggregation induced emission are that they can be used in solid state and therefore it takes away the disadvantages associated various flammable solvents used as media for BODIPY.[pic 33]

[pic 34]

Fig 22: fluorescence characteristics at various level of doping

Pyrrole in Polymers and coatings

Pyrrole can be electrochemically polymerized to give polypyrroles. This polymer is conductive and nature and hence has huge potential. The major forms of polypyrroles are quinoid, polaron and bipolaron. These can be obtained by adjusting the state of oxidation.

[pic 35]

Fig 23: structures of different poly pyrrole obtained on oxidation

Major properties of poly pyrroles are that it conducts relatively high current compared to other conducting polymers 20 . It is Environmentally stable 20,Compatibility with aqueous systems 22 .Depending on the counter ion used, it can retain its conductivity as well as mechanical properties 20 Polypyrrole’s conductivity can be varied