Microwave chemistry involves the application of microwave radiation to chemical reactions and has played an important role in organic synthesis. Functional dyes are those with hi-tech applications and this chapter attempts to provide an overview of the recent developments in microwave-assisted synthesis of functional dyes. Emphasis has been paid to the microwave-assisted synthesis of dye molecules which are useful in hi-tech applications such as optoelectronics (dye-sensitized solar cells), photochromic materials, liquid crystal displays, newer emissive displays (organic-light emitting devices), electronic materials (organic semiconductors), imaging technologies (electrophotography viz., photocopying and laser printing), biomedical applications (fluorescent sensors and anticancer treatment such as photodynamic therapy). In this chapter, the advantages of microwaves as a source of energy for heating synthesis reactions have been demonstrated. The use of microwaves to functional dyes is a paradigm shift in dye chemistry. Until recently most academic laboratories did not practice this technique in the synthesis of such functional dyes but many reports are being appeared in the journals of high repute.
- microwave-assisted organic synthesis
- functional dyes
- solar cells
- fluorescent sensors
- organic-light emitting diodes
- photochromic materials
Microwaves are the portion of the electromagnetic spectrum with the wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz. The frequencies used for cellular phones, radar, and television satellite communications are within this portion of the electromagnetic spectrum . Microwaves have been employed in a non-classical heating technique which is popularly known as “Bunsen Burner of the 21st century” and has attained enormous importance since many materials (solids or liquids) can transform electromagnetic energy into heat. The microwave-assisted organic synthesis (MAOS) has made revolutionary changes in the methodology since there is a dramatic enhancement in the yield of the reaction, modifications of selectivity, increased purity of products, simplified work-up procedure, and above all reduction in the reaction time. These are the primary benefits over conventional methods. The microwave technique has been applied efficiently in the organic synthesis, polymer chemistry, material sciences, nanotechnology, biochemical processes, thermal food processing, hydrothermal and solvothermal processing, etc. . The energy efficiency is higher in the case of microwave heating in comparison with the conventional heating as evidenced by one such Suzuki reaction in which there is an 85 fold reduction in energy demand when compared to a reaction on an oil bath and a microwave reactor .
During a chemical reaction under the conventional heating, the energy is introduced by convection, conduction, and radiation of heat from the surfaces of the reactants in the solution, and the energy transfer occurs due to thermal gradients. But in the case of the microwave irradiation, the energy is introduced through the electromagnetic field interaction into the molecules and the transfer of electromagnetic energy to thermal energy is energy conversion instead of heat transfer. This variation in the mode of introduction of energy leads to the advantages of using microwaves during chemical reactions. The microwaves penetrate easily into the bulk and, hence, heat evolves throughout the volume of the reaction mixture. As a result, fast and uniform heating of the reaction mixture can be advanced. In conventional heating, it is necessary to slow rates of heating to minimize the steep thermal gradients and obviate the process-induced stresses. As microwaves can transfer energy into all volumes of the reaction mixture, the potential exists to reduce the processing time and enhance the overall quality .
Although the use of microwaves for organic synthesis is widespread, the documentation of this technology to the synthesis of the functional dyes is a relatively new development. The use of microwave energy for their synthesis has the potential to offer similar advantages in reduced reaction times and energy savings for obtaining useful materials such as dyes possessing hi-tech applications.
2. Functional dyes
Color plays an important role in the world in which we are living. Color can sway thinking, change actions, and cause reactions. If properly used, color can even save on energy consumption. The colors are characterized by their ability to absorb light in the visible spectrum (from 380 to 750 nm). The dyeing industry is in existence since 2000 years BCE wherein dyes were obtained from natural sources
It was Yoshida and Kato who used the term “functional dye” for the first time in 1981 due to the advancements and growth of dye chemistry related to high-technology (hi-tech) applications that are divergent from the well known traditional applications . Hi-tech applications of dyes include the fields
Common dyes have been synthesized by applying mainly the conventional methods and also by microwave assistance. In the following sections the functional dyes used in solar cells, fluorescent sensors, fluorescent dyes to print on fibres, photochromic materials, O-LEDs, and dyes with advanced applications which were synthesized only under microwave irradiation are discussed.
2.1 Dyes (sensitizers) used in solar cells
2.1.1 Dye-sensitized solar cells (DSSCs)
To prevent harmful impact on the environment by conventional energy sources it is necessary to use the alternative energy sources, specially, the solar cells. The conversion of sunlight into electricity is a clean, abundant, and renewable energy source. The amount of energy available from the sun to the earth is of the order of 3 × 1024 joules/year thus making it the best among sustainable energies. Photovoltaic devices have been fabricated using inorganic materials of high purity and energy-intensive processing techniques. The fabrication using these inorganic materials is not economical and often used scarce toxic materials. Therefore, such solid-state junction devices have been challenged by the 3rd generation dye-sensitized solar cells (DSSCs) which are based on interpenetrating network structures containing metal-free organic dyes as sensitizers .
In the conventional systems, the semiconductor does the task of light absorption as well as charge carrier transport. However, these two functions are separated in DSSCs by the metal-free organic dye and TiO2 in presence of an electrolyte. Hence, new ways of manufacturing the solar cells that can be scaled economically up to large volumes are essential. In this regard, a new generation of DSSCs also known as “Grätzel cells” has been fabricated by O’Regan and Grätzel . A Grätzel cell consists of nanoporous titanium dioxide applied on transparent conducting oxide which is further made to absorb the dye from its solution. This film loaded with dye/sensitizer is immersed in an electrolyte containing a redox couple and placed on a platinum counter electrode. After irradiation, the excited electron from the dye (sensitizer) is transferred to the conduction band of TiO2 and diffuses through its porous network to the contact. Thus oxidized dye is further reduced to the original state by the supply of electrons through a liquid electrolyte redox couple within the pores .
The organic dye sensitizers consist of three important components
2.1.2 Microwave synthesized dyes/sensitizers in DSSCs
Novel donor-π-acceptor (D-π–A) dyes bearing the pyrimidine unit as an electron acceptor appended to thiophene and carbazole unit
The devices obtained using these sensitizers
Three push-pull Donor-π-Acceptor structured dyes
Due to inefficient electron injection from HOMO to TiO2 conduction band or dye aggregation leading to a potential barrier the dye
In view of the importance of thiophene as the significant moiety in the design of polymer-based sensitizers, narrow band gap conjugated polymer
Triphenylamine based dye sensitizers
The general design of organic dye sensitizers is usually in the order D-π-A. However, molecular conjugated chromophores combining only electron donor (D) and acceptor (A) blocks have also been designed and synthesized as active materials for organic solar cells . In view of this, D-A-D dyes
A new series of oxindole sensitizers (
The substitution of fluoro substituent with other halo substituents showed further enhancement in the DSSC performance. Among all the halogen substituted sensitizers, the bromo substituted sensitizer
Computationally designed thiazolo [5,4-
Two isoindigo-based conjugated polymers
Fabrication of the solar cells was produced using
Novel dye sensitizers
2.2 Fluorescent dyes
Fluorescence is a photophysical process which involves the emission of light by the substance as a consequence of the absorption of electromagnetic radiation. In most of the cases, the emitted light radiation has a longer wavelength (
2.2.1 Cyanine dyes
Cyanine dyes are found to be important functional dyes due to their typical optical properties, and act as sensitizers in solar cells, photography, and laser discs . A significant property of cyanine dyes is the affinity for biological structures, specifically for DNA, and possesses wide color change, high photostability and increased fluorescent intensity when bound to biological structures . Due to high fluorescence quantum yields and high molar extinction coefficients, they have been extensively used in cell imaging and gel staining techniques. Typically, cyanine dyes are obtained by heating a mixture of substituted quaternary salts with bisaldehyde or bis-imine. Accordingly, a series of cyanine dyes
2.2.2 Naphthalimide dyes
1,8-Naphthalimide dyes are proved to be important fluorescent compounds due to their greater photostability and high fluorescent quantum yield. The basic spectral properties of these dyes depend on the polarization of naphthalimide molecule as a result of electron donor-acceptor interaction occurring between the substituents at the C-4 position and the carbonyl groups of the imide ring. Generally, 1,8-naphthalimide dyes are prepared
2.2.3 Coumarin dyes
Coumarin dyes have been found commercial significance due to their intense fluorescence and are widely employed as fluorescent brighteners . A one-pot microwave promoted synthesis of benzimidazol/benzoxazol functionalized coumarin dyes (
2.2.4 Benzimidazole dyes
Benzimidazole dyes are known to exhibit photophysical, photovoltaic, and optical properties . An approach has been made to synthesize benzimidazo-quinolines
2.2.5 Imidazole dyes
The imidazole moiety is immensely employed in DSSC’s . Interestingly these dyes
2.2.6 Thiophene dyes
Thiophene oligomers and polymers have put forward extensive applications in organic electronics, owing to their remarkable performance as organic semiconductors . A series of thiophene oligomer based fluorophores appended with 4-sulfo-2,3,5,6-tetraflurophenyl ester
2.2.7 Inorganic dyes
Inorganic dyes are procured when the organic dyes are combined with appropriate metals. Typically monoazodyes containing additional groups such as amino, hydroxyl, and carboxyl groups which are capable of forming coordination complexes with metal ions are used. This organo-metallic combination could lead to enhanced optical properties. The synthesis of organo soluble 4-
2.3 Photochromatic dyes
Some materials at their molecular level exhibit a property of changing their absorption spectra on exposure to light radiation. This is usually a reversible change and is accompanied with alteration in the physical or chemical property. This kind of photo transformation is referred to as photochromism. The reverse change may be induced thermally (photochromism type T) or photochemically (phtochromism type P). The discovery of photochromic materials can be retraced to the middle of 19th century when Hirshberg and his team (1950) have contributed significantly towards the synthesis and mechanistic studies of photochromic materials. Hirshberg coined the term “Photochromism” from Greek words ‘photos’ meaning light and ‘chroma’ means color. Varieties of materials like minerals, nanoparticles, inorganic–organic compounds, organic dyes, polymers, and biomolecules have been explored to exhibit photochromic property. They have been in use in modern applications like erasable optical memory media, photo-optical switch components, sunscreen applications, contact lenses, security glasses, and thin films. Some of the organic photochomic compounds undergo reversible light-driven reaction hence these compounds are often incorporated into polymers, liquid crystals, and other such matrices. Although the decade 1950–1960 has remarked synthesis of photochromic materials with the advancement in newer supportive technologies such as spectroscopy the field has not gained acceleration. This is due to the sensibility of organic materials towards the light which makes them undergo degradation (they were not fatigue resistant). After the report of the synthesis of fatigue resistant spironapthoxazines many-fold increase in the applications of photochromic materials has been reported. Spiropyrans, spriooxazines, chromenes, fulgides, fulgimides, diarylethenes, spirodihydro indazolines, azocompounds, polyarenes, quinones, anils are the photochromic dyes in the industrial and general application field . In the recent past attempts have been made to apply microwave-assisted synthetic methods to the total synthesis or in one or two intermediate steps.
Spirooxazines are the important photochromic dyes being popularly seen in very common to high tech applications. Due to their brilliant light fatigue resistance nature, they are the dyes of bright prospects. The reports of the synthesis of spirooxazines by conventional methods are many. Successful efforts have also been made to obtain them by environment-friendly microwave-assisted synthetic methods. Spirooxazine
At optimal power 230 W microwave irradiation for 2 min duration 87% yield of the dye
2.4 Organic-light emitting diodes (
The light-emitting diode (LED) is a light-emitting semiconducting material when current flows through it. The current flow induced light emission was first observed by Captain Henry Joseph Round in 1907. Light emission takes place when electrons undergo a transition from the conduction band to the empty valence band. The band gap in semiconducting material decides the color of emitted light.
The amalgamation of organic moieties and inorganic matrices results in the synergetic effects by augmenting of the properties like flexibility and shape ability with stability . Poly (2-hydroxyethyl methacrylate) (PHEMA) silica-hybrids have been prepared by microwave irradiation . Organoboron dye diketonate BF2 complex
N,N-Diphenylamine (DPA) were transformed to form precursors for
Polyfluorene is regarded as an important source for the development of
Since from the centuries, dyes have played a very important role in human life. The functional dyes have changed the technologies drastically and have gained immense importance now a day. A specific property of the dye depends on the various factors such as the donor, electron acceptor/π-conjugation, linker, etc. present at appropriate positions. More effort has been established into searching for better dyes with expected properties. Microwave-assisted synthesis has changed the methodology of organic synthesis and hence is also efficiently applied in the synthesis of functional dyes. Therefore, a number of dyes synthesized under microwaves along with their applications were discussed. There is a possibility for further development in organic synthetic methodology under microwaves to obtain dyes having wider applications in organic photovoltaics, fluorescence sensors, photochromic materials, OLEDs, etc.
The authors thank the DST, New Delhi for the sanction of PURSE Phase -II to the Karnatak University, Dharwad.