*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 [30]. 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 [31]. 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 **43a-g** were synthesized by the condensation of quaternary salts of quinoline derivatives with 1*H*-indole-3-carbaldehydes in the presence of piperidine under solvent-free microwave irradiation at 126–329 W in 89–98% yields in only 2–5 min. The fluorescence spectra of the dyes showed absorption maxima (λabs) at 453–471 nm. However, in the presence of DNA, a

bathochromic shift (red shift) at 483–499 nm was observed. Further, the living cell imaging experiments of the dyes **43b**, **43e**, and **44** have shown preferable staining of the head of the human sperm containing the nuclear DNA. Also, the motility of the sperm didn't slow down which indicated low cell cytotoxicity. Hence, these dyes could be used as potential fluorescent probes for labeling DNA to measure human sperm viability [31]. Likewise, the condensation of benzothiazole with quarternary salts of quinoline which upon coupling with the tertiary diamine linkers gave tetracationic analogous (bis-intercalators) of monomethine cyanine dyes **45**. The steady-state fluorescence spectral studies of **45** revealed greater labeling affinity toward DNA and proved for singlet oxygen sensitization property, and found to be a potential candidate for photodynamic therapy [32].

#### *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 *via* the substitution reaction of naphthalimides with various nucleophiles. The aromatic nucleophilic substitution reaction of 4-bromo-*N*-alkylnaphthalimides with amines, alkoxides, and thiols in the presence of KF/Al2O3 under solvent-free microwave irradiation yielded corresponding fluorescent dyes **46–48** which exhibited increased fluorescent intensity in the polar solvents [33]. Similarly, the derivatives **49** and **50** were obtained by the substitution reaction of 1,8-naphthalimides with primary amines. These dyes were further evaluated for the free radical scavenging properties against 2,2-diphenyl-1-picrylhydrazyl (DPPH). The results showed IC50 values at lower concentrations than the common synthetic antioxidant 2,6-ditertiarybutyl-4-methylphenol (BHT) [34].

#### *2.2.3 Coumarin dyes*

Coumarin dyes have been found commercial significance due to their intense fluorescence and are widely employed as fluorescent brighteners [35]. A one-pot

**67**

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

microwave promoted synthesis of benzimidazol/benzoxazol functionalized coumarin dyes (**51a-e**) was developed which involved the reaction of 4-diethylam

ino-2-hydroxybenzaldehyde, diethylmalonate, and *o*-phenylenediamine/ *o*-hydroxyaniline in *n*-pentanol within 3 min. The synthesized dyes showed higher fluorescence emission intensity and the dyes **51a-c** were further investigated for the effects of ink media on the fluorescence properties. Dye **51b** has exhibited an intense green fluorescence at 531 nm for mixed very long alkyl resin and maleic varnish in 60:40 ratios at 1% (w/w) concentration, and the fluorescence emission intensity of the dye reduced by 11% after 30 h of exposure to light. As a result, the dye **51b** could potentially be used in security offset ink [36]. Similarly, one-pot three-component microwave-assisted reaction of 7-diethylamino-coumarin ethylidene malononitrile, aromatic aldehydes and malononitrile to get highly fluorescent 3,5-disubstituted-2,6-dicyanoaniline coumarin dyes **52a-d** at 80°C in good yields in 2 min are prepared. The optical and thermal screening studies of **52a-d** exhibited excellent photophysical and thermal stability properties [37]. A group of 8-aza-7-hydroxy-4-methylcoumarin dye **53** was synthesized by reacting 2,6-dihydroxypyridine hydrochloride with ethylacetoacetate in the presence of magnesium bromide as a Lewis acid catalyst. This fluorophore is adequately soluble in water and has a high fluorescent quantum yield and showed increased fluorescence in protic solvents at neutral pH, which could be useful in biosensors that are required for finding biologically active compounds [38]. Furthermore, a microwave-assisted Knoevenagel condensation of salicylaldehyde and cyano-N′-methyleneacetohydrazide in the presence of piperidine catalyst gave 3-carbohydrazide coumarin fluorescent dye (**54**) which could be

used to print polyester and polyamide fabrics [35].

Benzimidazole dyes are known to exhibit photophysical, photovoltaic, and optical properties [39]. An approach has been made to synthesize benzimidazoquinolines **55a-c**, substituted with piperidine, pyrrolidine, and piperazine moieties by uncatalyzed amination protocol under microwave heating in relatively high yields (56–90%), which by conventional heating after several days gave **55a-c** only in low yields (<10%). The emission spectra of **55a-c** showed an increase in the fluorescence intensity when interacted with the calf thymus DNA (*ct*-DNA) [40]. The microwave promoted synthesis of bisbenzimidazolyl derivatives upon *N*-alkylation gave water-soluble fluorescent dyes **56a-b**. These dyes proved to be highly selective fluorescent probe toward Zn2+ in aqueous solution and the mixture of dye-Zn2+ could detect picric acid by fluorescence quenching [41]. Under solvent-free microwave irradiation, a series of 2-substituted styryl benzimidazole dyes **57a-g** and **58a-f** were prepared by the condensation of 2-alkyl benzimidazoles with aromatic aldehydes in the

*2.2.4 Benzimidazole dyes*

presence of acetic anhydride [42].

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

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

bathochromic shift (red shift) at 483–499 nm was observed. Further, the living cell imaging experiments of the dyes **43b**, **43e**, and **44** have shown preferable staining of the head of the human sperm containing the nuclear DNA. Also, the motility of the sperm didn't slow down which indicated low cell cytotoxicity. Hence, these dyes could be used as potential fluorescent probes for labeling DNA to measure human sperm viability [31]. Likewise, the condensation of benzothiazole with quarternary salts of quinoline which upon coupling with the tertiary diamine linkers gave tetracationic analogous (bis-intercalators) of monomethine cyanine dyes **45**. The steady-state fluorescence spectral studies of **45** revealed greater labeling affinity toward DNA and proved for singlet oxygen sensitization property, and found to be a potential candidate for photodynamic

1,8-Naphthalimide dyes are proved to be important fluorescent compounds due

Coumarin dyes have been found commercial significance due to their intense fluorescence and are widely employed as fluorescent brighteners [35]. A one-pot

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 *via* the substitution reaction of naphthalimides with various nucleophiles. The aromatic nucleophilic substitution reaction of 4-bromo-*N*-alkylnaphthalimides with amines, alkoxides, and thiols in the presence of KF/Al2O3 under solvent-free microwave irradiation yielded corresponding fluorescent dyes **46–48** which exhibited increased fluorescent intensity in the polar solvents [33]. Similarly, the derivatives **49** and **50** were obtained by the substitution reaction of 1,8-naphthalimides with primary amines. These dyes were further evaluated for the free radical scavenging properties against 2,2-diphenyl-1-picrylhydrazyl (DPPH). The results showed IC50 values at lower concentrations than the common synthetic antioxidant 2,6-ditertiarybutyl-4-methylphenol

**66**

therapy [32].

(BHT) [34].

*2.2.3 Coumarin dyes*

*2.2.2 Naphthalimide dyes*

microwave promoted synthesis of benzimidazol/benzoxazol functionalized coumarin dyes (**51a-e**) was developed which involved the reaction of 4-diethylam ino-2-hydroxybenzaldehyde, diethylmalonate, and *o*-phenylenediamine/ *o*-hydroxyaniline in *n*-pentanol within 3 min. The synthesized dyes showed higher fluorescence emission intensity and the dyes **51a-c** were further investigated for the effects of ink media on the fluorescence properties. Dye **51b** has exhibited an intense green fluorescence at 531 nm for mixed very long alkyl resin and maleic varnish in 60:40 ratios at 1% (w/w) concentration, and the fluorescence emission intensity of the dye reduced by 11% after 30 h of exposure to light. As a result, the dye **51b** could potentially be used in security offset ink [36]. Similarly, one-pot three-component microwave-assisted reaction of 7-diethylamino-coumarin ethylidene malononitrile, aromatic aldehydes and malononitrile to get highly fluorescent 3,5-disubstituted-2,6-dicyanoaniline coumarin dyes **52a-d** at 80°C in good yields in 2 min are prepared. The optical and thermal screening studies of **52a-d** exhibited excellent photophysical and thermal stability properties [37]. A group of 8-aza-7-hydroxy-4-methylcoumarin dye **53** was synthesized by reacting 2,6-dihydroxypyridine hydrochloride with ethylacetoacetate in the presence of magnesium bromide as a Lewis acid catalyst. This fluorophore is adequately soluble in water and has a high fluorescent quantum yield and showed increased fluorescence in protic solvents at neutral pH, which could be useful in biosensors that are required for finding biologically active compounds [38]. Furthermore, a microwave-assisted Knoevenagel condensation of salicylaldehyde and cyano-N′-methyleneacetohydrazide in the presence of piperidine catalyst gave 3-carbohydrazide coumarin fluorescent dye (**54**) which could be used to print polyester and polyamide fabrics [35].
