**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 [52]. 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 **69a** has been prepared under microwave conditions, starting from 1-nitroso-2-naphthol in presence of triethylamine as a catalyst in a CEM focused microwave reactor provided with temperature control [53]. Electric power 25–35 W, temperature 80–180°C were found to be the optimized conditions to get yields comparable to the traditional thermal method. Indolinespironaphthooxazine **69b-d** have been prepared from 1-nitroso-2,7 dihydroxy naphthalene by a microwave irradiation technique [54]. The reaction was carried out in microwave synthesizer (MAS-I). Microwave irradiation was done at 600 W. The products were obtained in very good yield within a few minutes of reaction time.

Fulgides **70a-b** and their derivatives fulgimides **71a-b** are an important class of photochromatic materials used mainly in optical memory devices and optical switches. Fulgides are intense colored compounds which are good in resisting the photodegredation in comparison to fulgimides. However, fulgimides have better resistance to acid or base hydrolysis further that their N-substituent can be used as a link to prepare photochromic films. Both these classes of compounds have been thoroughly researched. A successful attempt to synthesize fulgimides using domestic microwave ovens has been made [55]. As compared to classical thermal method microwave-assisted synthesis has led to 3 fold times reduction in duration of synthesis, an increase in the yield up to 2 times, and minimization of the use of organic solvents. The efficient synthesis of N-functionalized fulgimides **72** was achieved under microwave irradiation [56]. Fulgides were converted to fulgimides in two steps in the presence of DMAP and DCC by microwave irradiation in presence of pyridine and xylene as the solvent. They have attained from 50 to 84% increase in

**71**

*Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

yield by benign microwave method in very short reaction time. Oxazole and indole based heterocyclic fulgides **73**, **74** were synthesized by microwave method using clay as a catalyst from fulgenic acids [57]. Their synthesis involved stirring of the blended mixture of fulgenic acid and montmorillonite KSF along with isopropyl acetate in a flask. The yield was improved to 72–84% by MWAS as compared to the conventional Stobbe condensation method. One-pot three-component microwave-assisted synthesis of novel azo-imidazoles **75a-h** is reported which exhibited photochromatic property with UV–Visible light [58]. Azo dye, ammoniumacetate, and benzil were

At optimal power 230 W microwave irradiation for 2 min duration 87% yield of the dye **75a**-**h** was obtained. It did not involve any thermal degradation by-products and economical use of organic solvents makes this protocol a green synthetic method. The microwave synthetic method was applied to successfully prepare photochromic spiropyran **76** [59]. Spiropyrans are spiro-fused indolochromenes. Due to their photochromic isomerization property, they are used in optical switches and sensors. The synthesis involves one-pot two-step reaction. Initially, watermediated reaction was carried out between 1, 2, 3-trimethylindole, and benzyl bromide under microwave environment. Microwave irradiation was done at De Rosa and Soriente's conditions (i.e. 200 MW power and 150°C temperature for 8 minutes). Then, the resulting reaction mixture after a simple workup procedure was treated with 5-nitrosalicylaldehyde under microwave irradiation using ethanol as the solvent. They have obtained product **76** in excellent yield after the flash chromatographic workup procedure. It is an environmentally benign synthetic

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. *O*-LED are

reacted under microwave irradiation using acetic acid as solvent.

method using a minimum amount of solvent.

**2.4 Organic-light emitting diodes (***O***-LEDs)**

#### *Microwave Synthesized Functional Dyes DOI: http://dx.doi.org/10.5772/intechopen.94946*

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

methods to the total synthesis or in one or two intermediate steps.

very good yield within a few minutes of reaction time.

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 **69a** has been prepared under microwave conditions, starting from 1-nitroso-2-naphthol in presence of triethylamine as a catalyst in a CEM focused microwave reactor provided with temperature control [53]. Electric power 25–35 W, temperature 80–180°C were found to be the optimized conditions to get yields comparable to the traditional thermal method. Indolinespironaphthooxazine **69b-d** have been prepared from 1-nitroso-2,7 dihydroxy naphthalene by a microwave irradiation technique [54]. The reaction was carried out in microwave synthesizer (MAS-I). Microwave irradiation was done at 600 W. The products were obtained in

Fulgides **70a-b** and their derivatives fulgimides **71a-b** are an important class of

photochromatic materials used mainly in optical memory devices and optical switches. Fulgides are intense colored compounds which are good in resisting the photodegredation in comparison to fulgimides. However, fulgimides have better resistance to acid or base hydrolysis further that their N-substituent can be used as a link to prepare photochromic films. Both these classes of compounds have been thoroughly researched. A successful attempt to synthesize fulgimides using domestic microwave ovens has been made [55]. As compared to classical thermal method microwave-assisted synthesis has led to 3 fold times reduction in duration of synthesis, an increase in the yield up to 2 times, and minimization of the use of organic solvents. The efficient synthesis of N-functionalized fulgimides **72** was achieved under microwave irradiation [56]. Fulgides were converted to fulgimides in two steps in the presence of DMAP and DCC by microwave irradiation in presence of pyridine and xylene as the solvent. They have attained from 50 to 84% increase in

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 [52]. In the recent past attempts have been made to apply microwave-assisted synthetic

**70**

yield by benign microwave method in very short reaction time. Oxazole and indole based heterocyclic fulgides **73**, **74** were synthesized by microwave method using clay as a catalyst from fulgenic acids [57]. Their synthesis involved stirring of the blended mixture of fulgenic acid and montmorillonite KSF along with isopropyl acetate in a flask. The yield was improved to 72–84% by MWAS as compared to the conventional Stobbe condensation method. One-pot three-component microwave-assisted synthesis of novel azo-imidazoles **75a-h** is reported which exhibited photochromatic property with UV–Visible light [58]. Azo dye, ammoniumacetate, and benzil were reacted under microwave irradiation using acetic acid as solvent.

At optimal power 230 W microwave irradiation for 2 min duration 87% yield of the dye **75a**-**h** was obtained. It did not involve any thermal degradation by-products and economical use of organic solvents makes this protocol a green synthetic method. The microwave synthetic method was applied to successfully prepare photochromic spiropyran **76** [59]. Spiropyrans are spiro-fused indolochromenes. Due to their photochromic isomerization property, they are used in optical switches and sensors. The synthesis involves one-pot two-step reaction. Initially, watermediated reaction was carried out between 1, 2, 3-trimethylindole, and benzyl bromide under microwave environment. Microwave irradiation was done at De Rosa and Soriente's conditions (i.e. 200 MW power and 150°C temperature for 8 minutes). Then, the resulting reaction mixture after a simple workup procedure was treated with 5-nitrosalicylaldehyde under microwave irradiation using ethanol as the solvent. They have obtained product **76** in excellent yield after the flash chromatographic workup procedure. It is an environmentally benign synthetic method using a minimum amount of solvent.
