Introduction

Azo dyes cover the single largest group of dyes with respect to number and production scales with many industrial applications to introduce new and efficient colours to the substrates [1]. They are most versatile and robust having high tinctorial strengths with a full-color range from yellow to blue-green shades [2]. The high molar extinction coefficient, structural diversity, good fastness properties and ease of synthesis have made these dyes very successful [3,4,5]. Economy and ease of preparation of azo dyes have made them highly applicable to many colour industries [6, 7]. But fastness improvements are vital in the textile industry since dyed fabrics have found wide applications and are exposed to direct sunlight [8].

Benzophenone based dyes have found applications in photoinitiators and photosensitizers [9], thermal stabilizers [10], UV absorbers [11], oxidants in photo-induced electron transfer (PET) [12], fluorescent chemosensors [13], biological probes [14] and Excites State Intramolecular Proton Transfer (ESIPT) based fluorescent chemosensors [15, 16]. In general, benzophenone unit is photostabilizing and UV blocking entity [17]. Stability is influenced by appropriate substitutions in the aromatic ring at various positions of benzophenone [18]. Substitution at meta-position to the carbonyl group of benzophenone derivatives show good intramolecular redox process [19]. Similarly, substitution at para-position remarkably modifies photochemistry of the molecule [20]. Such appropriately modified few promising benzophenone based mordant and acid azo dyes are available in the literature [21, 22]. Some of the reported benzophenone derivatives are emissive [23,24,25,26]. In the same line Barsotti et al. recently did extensive studies on fluorescence properties of 4-hydroxy benzophenone [23, 27].

Similarly, dyes containing imidazole unit have found wide applications in dye sensitizer solar cells (DSSC) [28], hole transporting materials [29], ESIPT based dyes [30] fluorescent sensor [31] and solid-state emissive dyes [32]. Dyes with imidazole unit are also used in singlet oxygen generation [33] and radical photopolymerization reactions [34]. Due to strong accepting capacity [35] and high molar extinction coefficients [36] of imidazole-based dyes, photostability gets enhanced [37].

An important use of fluorescent dyestuffs is in the coloration of synthetic fibres like polyesters, acrylics and polyamides. Fluorescent textiles not only increases visibility but also provides high design options [38]. Many dyes for acrylic fibres, particularly methines such as CI Basic Violet 7, CI Basic Red 13 and CI Basic Red 74 are fluorescent and very bright resulting into important fluorescent textile fibres [38]. The dominating structural classes for fluorescent textiles dyes include coumarins, perylene and methines [38, 39]. But the available dyes do not meet the simultaneous requirement of both good fluorescent properties and light fastness properties. So, in order to develop synthetic dyes for textile applications, it is advisable that modern colourants should have both photostabilities as well as improved spectroscopic characteristics [40].

Considering combined advantages of both benzophenone core and imidazole unit we developed in-built stabilising phenyl(1H-benzo[d]imidazol-5-yl)methanone group. Presence of donors (–N(Et)2, -OH groups), acceptors (imidazole, >C=O) and long π-conjugation [41] in the present phenyl(1H-benzo[d]imidazol-5-yl)methanone based azo dyes enable them to act as good candidates for attractive fluorescent compounds. However, very few reports are available on fluorescent monoazo disperse dyes and the strategies to make them emissive [42,43,44,45,46]. Recently reported fluorescent dyes contained phenyl(1H-benzo[d]imidazol-5-yl)methanone as a core moiety [47]. So, we are interested to study the effect of incorporation of phenyl(1H-benzo[d]imidazol-5-yl)methanone moiety into few selected azo dyes available commercially. Moreover, as benzophenone core is well photostabilizing unit [17], it is expected that the designed dyes not only become red emission in nature but can also show good fastness properties.

In the present work, we are reporting three red emitting disperse dyes (5a, 5b and 5c) containing in-built photostabilising and emission enhancing unit i.e. phenyl(1H-benzo[d]imidazol-5-yl)methanone. 5a, 5b and 5c disperse dyes exhibited red-shifted absorption and emission compared to corresponding parent analogues CI Solvent Yellow 14 (5a’) (CI 12055), 4-diethylamino-2-hydroxy-1-diazobenzene (5b’) and 7-(diethylamino)-4-hydroxy-3-(phenyldiazenyl)-2H-chromen-2-one) (5c’) dyes, respectively. Dye 5a and its parent analogue dye 5a’ showed negative solvatochromism. Solvatochromic properties are in good agreement with solvent polarity graphs. Dyes 5a, 5b and 5c on dyed polyester and nylon showed excellent light and sublimation fastness compared to parent dyes 5a’, 5b’ and 5c’ respectively.

Experimental Section

Materials and Methods

3,4-Diaminobenzophenone and p-nitrobenzaldehyde were procured by Spectrochem Pvt. Ltd. Mumbai.Naphthalen-2-oland were obtained from Sigma-Aldrich.3-(diethylamino)phenol, sodium nitrate, sodium carbonate, sodium hydroxide, conc. HCl, urea, metamol (dispersing agent), sodium chloride and all organic solvents were purchased from S. D. Fine Chemicals Ltd., Mumbai, India. All reagents and solvents were characterised by melting or boiling point and used without further purification. Readymade dyeing polyester (100%) substrate (weight70 gm/m2) was purchased from Piyush Syndicate, Mumbai, India. Melting points were recorded on the instrument from Sunder Industrial Product, Mumbai. 1H NMR and 13C NMR spectra were recordedat 25 °C on Agilent NMR vnmrs 500 MHz and 125 MHz respectively. Chemical shifts were expressed in ppm using TMS as an internal standard. IR spectra were recorded on Perkin Elmer spectrum-100 FTIR spectrometer. Mass spectra were recorded on FINNIGAN LCQ ADVANTAGE MAX instrument from Thermo Electron Corporation (USA). All dying were performed on Flexi dyer machine (RossariLabtech, Mumbai, India).

Spectroscopic Instruments

Absorption and emission spectra of the compounds were recorded on Perkin Elmer Lambda 25 UV-Visible spectrophotometer and Varian Inc. Cary Eclipse spectrofluorometer respectively. 5 μM solutions of the dyes were prepared by using 7 different polarity solvents, viz. toluene, 1,4-dioxane, CHCl3 (chloroform), EtOAc (ethyl acetate), MeOH (Methanol), acetonitrile and N,N-dimethylformamide (DMF).

Fastness and Color Assessment Instruments

Light fastness of dyed samples was tested on Q-Sun Xenon Test Chamber (Q-Lab Corporation, Ohio, USA) by the AATCC 16–2004 method. Sublimation fastness of the dyed samples were tested on Sublimation fastness tester (RBE Electronics Engg. Pvt. Ltd., Mumbai, India) by the standard method ISO 105-F04. Shade change and staining of adjacent fabrics were rated according to appropriate Society of Dyers & Colourists (SDC) grey scales. Colour properties of the dyed samples were measured using Spectra Scan 5100+ under the illuminant D65 using 10° standard observers.

Computational Study

All the DFT computations were performed using Gaussian 09 package [48] on an HP workstation XW 8600 with Xeon processor, 4 GB RAM and Windows Vista as the operating system. DFT and TD-DFT methods were employed for the ground state and excited state optimisations respectively. The hybrid functional B3LYP (Becke3-Lee-YangPar) as and 6-31G(d) basis set were used for all the atom [49]. Solvents used for Polarizable Continuum Model (PCM) [50] were toluene, 1,4-dioxane, chloroform (CHCl3), ethyl acetate (EtOAc), methanol (MeOH), acetonitrile and N,N-dimethylformamide (DMF).

Synthesis and Characterization

Methanone (2-(4-nitrophenyl)-1H-benzo[d]imidazol-5-yl)phenyl (2)

Mixture of 3, 4 diaminobenzophenone 1 (1.09 g, 5.2 mmol), 4-nitrobenzaldehyde (0.71 g, 4.7 mmol) and water (25 mL) was taken in a round bottom flask. Potassium ferro-cyanide (0.17 g, 10 mol%) was added to the mixture and stirred at 30 °C for 2 h. Reaction was monitored by using TLC (thin layer chromatographic technique). After completion of the reaction, solid obtained was filtered, washed with water, dried and recrystallized from ethanol to obtain the desired product (Yield: 89%). Above said procedure and characterizations of compound 2 is reported in the literature [51].

Methanone(2-(4-aminophenyl)-1H-benzo[d]imidazol-5-yl)phenyl (3)

Compound 2 (2 g, 5.8 mmol) and Fe powder (0.81 g, 14.5 mmol) were added in the reaction-flask containing methanol (50 mL). To this mixture AcOH (0.83 mL, 14.5 mmol) was added slowly to keep temperature below 30 °C. The reaction mixture was then further refluxed at 64 °C for 30 min. Reaction was monitored by using thin layer chromatographic technique. After completion of the reaction, reaction mass was cooled and neutralized by using NaHCO3. Precipitate formed was filtered, washed with water and dried. Recrystallization in EtOH afforded the desired product 3 (Yield: 70%). Above said procedure is reported in the literature [13]. Compound 3 is characterized and provided in supporting information.

Yield: 70%, Melting point: 196–199 °C.

1H-NMR δH (500 MHz, DMSO, TMS)(ppm): 7.76 (2H, d, J = 8.0 Hz, Ar-H), 7.35 (2H, d, J = 8.0 Hz, Ar-H), 7.23 (2H, d, J = 7.5 Hz, Ar-H), 7.15 (2H, d, J = 7.5 Hz, Ar-H), 7.13 (1H, s, Ar-H), 7.05 (2H, t, J = 7.5 Hz, Ar-H), 6.99 (1H, d, J = 8.0 Hz, Ar-H), 5.52 (1H, s, N-H), 4.50 (2H, s, -NH2).

13C NMR δC (125 MHz, DMSO, TMS)(ppm): 196.51 (>C=O), 160.47 (-C=N), 153.63 (Ar-C-NH2), 151.73 (Ar-C-NH), 139.09 (Ar-C-N=), 136.50 (Ar-C-CO), 134.00 (Ar-C), 133.05 (Ar-C-CO), 131.79 (Ar-C), 130.42 (Ar-C), 129.39 (Ar-C), 128.86 (Ar-C), 121.61 (Ar-C), 112.41 (Ar-C), 107.97 (Ar-C), 97.92 (Ar-C).

FT-IR: 3458 (N-H stretching), 1687 (C=O stretching), 1635 (Imine C=N stretch), 1281 (C-N stretching).

Mass (m/z): Calculated 314.12, [M + H] + for C20H16N3O+ found 314.1, [M + H] +.

Elemental analysis (%) - Found: C, 76.6; H, 4.8; N, 13.4%; molecular formula C20H15N3O calculated: C, 76.66; H, 4.82; N, 13.41%.

General Procedure for Diazotization-Coupling Reactions

Mixture of amine 3 (3.3 mmol), conc. HCl (3 ml, 35.0 mmol), and water (15 mL) was boiled in reaction flask to get clear solution. Solution was then cooled to 0 °C followed by gradual addition of sodium nitrite (0.25 g, 3.6 mmol) with constant stirring. Solution was allowed to stir below 5 °C for 30 min and starch iodide paper was used as a process control test. Reaction was monitored by spot test. After completion of reaction urea was added to quench excess nitrous acid. Meanwhile, the couplers (3.3 mmol) were dissolved in 10% sodium hydroxide solution to get clear solution. Diazonium salt solution was gradually added to the respective coupler solutions with continuous stirring and maintaining the temperature below 5 °C. During addition of diazonium salt pH was maintained in between 7.5–8 using NaCO3 (10% w/v) solution.

After completion of reaction precipitated was formed which was then filtered using a nutsche and washed thoroughly with water further. Recrystallized in ethanol afforded desired final dyes 5a, 5b and 5c in good yields. Above said procedure is reported in literature [1].

Couplers used for coupling reactions were Naphthalen-2-ol, 3-(diethylamino)phenol and 7-(diethylamino)-4-hydroxy-2H-chromen-2-one. Synthetic scheme for the preparations of monoazo disperse dyes (5a-5c) is shown in Scheme 1. Structural analogues of 5a is available in the literature [52]. 7-(diethylamino)-4-hydroxy-2H-chromen-2-one was prepared by reported procedure [45]. Dyes 5a’ (CI Solvent Yellow 14), 5b’ (4-diethylamino-2-hydroxy-1-diazobenzene) and 5c’ (7-(Diethylamino)-4-hydroxy-3-(phenyldiazenyl)-2H-chromen-2-one)) are available in the literature [45, 53,54,55]. Structural variations of present synthesised phenyl(1H-benzo[d]imidazol-5-yl)methanone based dyes are compared with previously reported parent dyes (Scheme 2).

Scheme 1
scheme 1

Synthesis of monoazo disperse 5a, 5b and 5c dyes

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Scheme 2
scheme 2

Comparative structural variations of previously reported and present dyes

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Characterizations (Supporting Information)

5a: Phenyl(2-phenyl 4-(2-hydroxynaphthalene-1-diazene)1H-benzo[d]imidazol-5-yl)methanone.

Yield: 85%, Melting point: 129–131 °C.

1H-NMR δH (500 MHz, DMSO, TMS) (ppm): δ13.40 (1H, s, Hydrogen bonding), 8.57 (1H, d, J = 8.5 Hz, Ar-H), 8.34 (2H, d, J = 8.5 Hz, Ar-H), 8.06–8.01 (2H, m, Ar-H), 7.97 (1H, d, J = 9.5 Hz, Ar-H), 7.90 (1H, d, J = 4.0 Hz, Ar-H), 7.81 (1H, d, J = 8.5 Hz, Ar-H), 7.79–7.75 (2H, m, Ar-H), 7.70 (2H, dd, J = 12.0, 7.5 Hz, Ar-H), 7.64 (2H, dd, J = 15.5, 7.5 Hz, Ar-H), 7.58 (2H, t, J = 7.5 Hz, Ar-H), 7.48 (1H, t, J = 7.5 Hz, Ar-H), 6.88 (1H, dd, J = 9.5, 4.0 Hz, Ar-H), 5.48 (1H, s, N-H).

13C NMR δC (125 MHz, DMSO, TMS)(ppm): 195.88 (>C=O), 156.02 (Ar-C-OH), 154.23 (-C=N), 153.34 (Ar-C-N=N), 147.75 (Ar-C-N=N), 146.02 (Ar-C-NH), 143.73 (Ar-C), 138.57 (Ar-C-NH), 135.14 (Ar-C), 134.77 (Ar-C), 133.50 (Ar-C), 133.15 (Ar-C), 132.53 (Ar-C), 131.65 (Ar-C), 131.50 (Ar-C), 130.39 (Ar-C), 129.88 (Ar-C), 128.88 (Ar-C), 128.78 (Ar-C), 128.66 (Ar-C), 128.45 (Ar-C), 126.91 (Ar-C), 125.16 (Ar-C), 122.16 (Ar-C), 119.34 (Ar-C), 114.60 (Ar-C), 111.96 (Ar-C), 109.98 (Ar-C), 106.98 (Ar-C), 100.13 (Ar-C).

FT-IR: 3058 (O-H stretching phenolic), 1685 (C=O stretching), 1643 (C=C stretching), 1528 (N=N stretching), 1274 (C-N stretching) cm−1.

Mass (m/z): Calculated 469.17, [M + H] + for C30H21N4O2+ found 469.1, [M + H] +.

Elemental analysis (%) - Found: C, 76.9; H, 4.3; N, 11.9%; molecular formula C30H20N4O2 calculated: C, 76.91; H, 4.30; N, 11.96%.

5b: Phenyl(2-phenyl 4-(3-(diethylamino)phenol)1H-benzo[d]imidazol-5-yl)methanone.

Yield: 71%, Melting point: 120–122 °C.

1H-NMR δH (500 MHz, DMSO, TMS) (ppm): 13.33 (1H, s, Hydrogen bonding), 8.29 (2H, d, J = 8.5 Hz, Ar-H), 7.87 (3H, d, J = 8.5 Hz, Ar-H), 7.76 (2H, d, J = 7.5 Hz, Ar-H), 7.68 (2H, d, J = 7.5 Hz, Ar-H), 7.65 (1H, s, Ar-H), 7.58 (2H, d, J = 7.5 Hz, Ar-H), 7.56 (1H, s, Ar-H), 7.51 (1H, d, J = 9.0 Hz, Ar-H), 6.53 (1H, d, J = 9.0 Hz, Ar-H), 6.05 (1H, s, N-H), 3.46 (4H, q, J = 7.0 Hz, N-(CH2)2), 1.15 (6H, t, J = 7.0 Hz, N-(C-CH3)2).

13C NMR δC (125 MHz, DMSO, TMS)(ppm): 195.98 (>C=O), 159.94 (Ar-C-N(Et)2), 153.10 (Ar-C-OH), 151.20 23 (-C=N), 148.12 (Ar-C-N=N), 143.76 (Ar-C-N=N), 138.57 (Ar-C-NH), 135.97 (Ar-C), 133.47 (Ar-C-NH), 132.52 (Ar-C), 131.50 (Ar-C), 131.27 (Ar-C), 130.72 (Ar-C), 129.89 (Ar-C), 128.86 (Ar-C), 128.33 (Ar-C), 126.84 (Ar-C), 125.12 (Ar-C), 124.27 (Ar-C), 121.97 (Ar-C), 121.08 (Ar-C), 119.05 (Ar-C), 114.55 (Ar-C), 111.44 (Ar-C), 107.44 (Ar-C), 97.39 (Ar-C), 44.87 (N,N diethyl-CH2), 13.18 (N,N diethyl-CH3).

FT-IR: 3286 (N-H stretch imidazole), 2973 (O-H stretching phenolic).

1688 (C=O stretching), 1640 (C=C stretching), 1521 (N=N stretching), 1269 (C-N stretch) cm−1.

Mass (m/z): Calculated 490.22, [M + H] + for C30H28N5O2+ found 490.2, [M + H] +.

Elemental analysis (%) - Found: C, 73.6; H, 5.5; N, 14.3%; molecular formula C30H27N5O2 calculated:C, 73.60; H, 5.56; N, 14.31%.

5c:Phenyl(2-phenyl4-(7-(diethylamino)-4-hydroxy-2H-chromen-2-one)1H-enzo[d]imidazol-5-yl)methanone.

Yield: 69%, Melting point: 139–142 °C.

1H-NMR δH (500 MHz, DMSO, TMS) (ppm): 13.02 (1H, s, Hydrogen bonding), 7.98 (2H, d, J = 8.5 Hz, Ar-H), 7.56 (3H, d, J = 8.5 Hz, Ar-H), 7.45 (2H, d, J = 7.5 Hz, Ar-H), 7.36 (2H, d, J = 7.5 Hz, Ar-H), 7.34 (1H, s, Ar-H), 7.27 (2H, d, J = 7.5 Hz, Ar-H), 7.24 (1H, s, Ar-H), 7.20 (1H, d, J = 9.0 Hz, Ar-H), 6.22 (1H, d, J = 9.0 Hz, Ar-H), 5.74 (1H, s, N-H), 3.15 (4H, q, J = 7.0 Hz, N-(CH2)2), 0.84 (6H, t, J = 7.0 Hz, N-(C-CH3)2).

13C NMR δC (125 MHz, DMSO, TMS)(ppm): 196.30 (>C=O), 160.26 (Ar-C-N(Et)2), 157.37 (-CO-O), 153.42 (Ar-C-OH), 151.52 (-C=N), 148.44 (Ar-C-N=N), 144.08 (Ar-C-N=N), 138.89 (Ar-C-NH), 136.29 (Ar-C), 133.79 (Ar-C-NH), 132.84 (Ar-C), 131.81(Ar-C), 131.59 (Ar-C), 131.03 (Ar-C), 130.21 (Ar-C), 129.18 (Ar-C), 128.65 (Ar-C), 127.15 (Ar-C), 125.44 (Ar-C), 124.59 (Ar-C), 122.29 (Ar-C), 121.40 (Ar-C), 119.37 (Ar-C), 114.87 (Ar-C), 112.15 (Ar-C), 107.76 (Ar-C), 102.83 (Ar-C), 97.71 (Ar-C), 92.78 (Ar-C), 45.19 (N,N diethyl-CH2), 13.49 (N,N diethyl-CH3).

FT-IR: 3549 (N-H stretch imidazole), 3064 (O-H stretching phenolic), 1694 (C=O stretching), 1642 (C=C stretching), 1526 (N=N stretching), 1274 (C-N stretch) cm−1.

Mass (m/z): Calculated 558.21, [M + H] + for C33H28N5O4+ found 558.2, [M + H] +.

Elemental analysis (%) - Found: C, 71.1; H, 4.8; N, 12.5%; molecular formula C33H27N5O4 calculated: C, 71.08; H, 4.88; N, 12.56%.

General Procedure of Dyeing

Nylon and polyester fabrics dyeing were carried out using 2% shade depth and material to liquor ratio of 1:30. Total dye solution calculated on the weight of fabric. As azo disperse dyes were insoluble in water, hence dissolved in 5 ml of N,N-dimethylformamide followed by dilution with 15 ml of buffered solution of pH 4 to 5 by using acetic acid in water. Ultrasonication for 30 min resulted in thefine dispersion of the dye in water. Saragen 50 was used as a dispersing agent. Nylon and polyester fabrics were dyed using the above dye solution. Dyeing was started at room temperature and raises to 130 °C (PET), 95 °C (Nylon) respectively temperatures for 50 min, and cooled to 60 °C. The dyed fabrics were rinsed with warm & cold water. Reduction clearing treatment was given only Polyester fabric using 2 g/l soda ash (Na2CO3), 2 g/l Sodium hydrosulphite and 1 g/l soap solution at 70 °C for 30 min (1:50) then treated fabrics were rinsed with cold water and allowed to dry in the open air.

Results and Discussion

Spectroscopic Characteristics

Absorption spectra of synthesised azo dyes were recorded in different polarity solvents (Fig. 1). Tabulated results in Table 1 suggested that dye 5a (λ max  = 504 nm to 516 nm) and its parent analogue 5a’ (CI Solvent Yellow 14) (λ max  = 472 nm to 476 nm) exhibited negative solvatochromism in absorption properties. On the other hand, dyes 5b (λ max  = 498 nm to 513 nm) and 5c (λ max  = 497 nm to 511 nm) and their respective parent analogues dyes 5b’ (λ max  = 364 nm to 376 nm) and 5c’ (λ max  = 456 nm to 460 nm) showed positive solvatochromism in absorption properties. Similarly, 5a, 5b and 5c on excitation from 497 nm to 516 nm exhibited significant solvatochromism in emission properties (Fig. 2). Dye 5a and is emitting with negative solvatochromism from 569 nm to 602 nm, while dyes 5b and 5c is emitting with positive solvatochromism from 565 nm to 627 nm in varying polarity solvents. 5a, 5b and 5c showed very high molar extinction coefficients (ε) as compared to parent dye 5a’, 5b’ and 5c’, respectively suggested increased in conjugation and the chromophore area.

Fig. 1
figure 1

Normalized Absorption spectra of 5a’, 5a, 5b and 5c dyes in different solvent

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Table 1 Spectroscopic characteristics of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes in different solvents
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Fig. 2
figure 2

Normalised Emission spectra of dyes 5a (excited at 504–516 nm), 5b (excited at 498–513 nm) and 5c (excited at 497–511 nm) in different solvents

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Dyes 5a, 5b and 5c exhibited a small peak at around 340 nm which corresponds to the benzophenone unit. Stokes shift (Δϑ) of 5a decreases from 2769 cm−1 (toluene) to 2267 cm−1 (DMF). On the other hand, Stokes shift (Δϑ) of 5b and 5c increases from 2444 cm−1 (toluene) to 2937 cm−1 (DMF) and from 2422 cm−1 (toluene) to 3621 cm−1 (DMF), respectively (Table 1). All the synthesised dyes have a significant effect of solvent polarity on Stokes shifts (Δϑ). 5a exhibited minimum Δϑ in DMF (2267 cm−1) while maximum in toluene (2769 cm−1). On contrary, 5b and 5c exhibited minimum Δϑ in toluene (2444 cm−1) and (2422 cm−1) while maximum Δϑ in DMF (2927 cm−1) and (3621 cm−1) respectively.

Among all the parent dyes, only 5c’ is weakly emissive in nature and show positive solvatochromism. The dye 5c’ exhibited significantly larger Stokes shifts having minimum Δϑ in toluene (3377 cm−1) while maximum in acetonitrile (5662 cm−1) (Table 1). Hence, it can be concluded that significant effect of substituents on spectroscopic characteristics of the dyes have been observed.

It is reported that tautomer forms of monoazo dyes in equilibrium can be shifted towards hydrazone form by treating these dyes with acid titrations [56]. Further it is also described that azo bridge get protonated in acidic medium resulted into formation of azonium cation [57]. So in order to get into the details of possible forms of the dyes, trifluoroacetic acid (acid) was added to dyes solutions (in methanol) resulting into a new strong red shifted absorption band in absorption spectra at around 582 nm to 592 nm region (Supporting Information Fig. S1 (a)). In the acidic medium highly expected protonated azonium cation is formed [57].

Stability study was performed under Ultraviolet irradiation (at 254 nm) for better understanding of stabilities of azo and protonated forms of the dyes. Stabilities studies of protonated forms carried out at pH = 3 (in methanol) suggested that protonated forms of 5b and 5c are stable under irradiations. Azo forms of 5a, 5b and 5c dyes are also found to be stable under ultraviolet irradiation (Supporting Information Fig. S1 (b)).

Estimations of Photo-Physical Properties

The role of solvent polarity and different substituents for altering spectroscopic processes have been estimated in terms of the radiative rate constant (k r ), then on-radiative rate constant (k nr ) and life time τ (ns) evaluations. Relative fluorescence quantum yields (Ф f ) of 5a, 5b and 5c were obtained by using Rhodamine 6G (Ф f  = 0.94 in Ethanol) as standard. k nr > > k r for all dyes indicate so much loss of energy during non-radiative processes. k r are found to be in good agreement with low fluorescence quantum yield (Ф f ). Life time (τ) has not shown significant variations with the varying polarity of the solvents (Table 2).

Table 2 Estimated spectroscopic (emission) characteristics of 5a, 5b, 5c and 5c’ in different solvent
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Solvent Polarity Graphs

Statistical evalution for solvatochromic properties of 5a, 5b and 5c is provided by solvent polarity functions like Lippert-Mataga, Weller and Bhakshiev. Lippert-Mataga plot [58] has shown very good linearity of Stokes shift vs. ƒLM(ε,η) functions with excellent regression coefficients (R2 ≥ 0.9834) suggested a significant effect of solvent polarity on Stokes shift. Stokes shift decreases for 5a, while increases for 5b and 5c with increasing solvent polarity (Fig. 3a). Weller’s equation allows estimation of the excited state dipole moments [59] and gave the plot of emission frequency (cm−1) versus Weller’s function [60]. Weller plot showed distinct positive slope with excellent regression coefficients (R2 = 0.9853) for 5a indicated negative solvatochromism. On the other hand, anegative slope with excellent regression coefficients of R2 = 0.9868 and R2 ≥ 0.9960 for 5b and 5c indicated positive solvatochromism (Fig. 3b). Not only Stokes shift and emission properties were affected by solvent polarity, but also absorption is also sensitive to the solvent polarity. So we utilised Bakhshiev plot which depends on additive frequencies of absorption and emission with solvent polarity functions [60]. Bakhshiev plot showed significant positive slope for dyes 5a (R2 = 0.9939), and significant negative slope for dyes 5b (R2 = 0.9940) and 5c (R2 = 0.9958) (Fig. 3c). Hence we can reveal that significant effect of naphthalene and N,N-diethylamine substituents were observed, resulting into negative solvatochromism for dye 5a in both absorption and emission while opposite trend of positive solvatochromism was observed for 5b and 5c in both absorption and emission.

Fig. 3
figure 3

Solvent polarity plots: a Lippert-Mataga, b Weller plot and c Bakhshiev plot of 5a, 5b and 5c dyes for different polarity solvents

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Colour Properties of the Dyes

Dyes 5a, 5b, 5c, 5a’, 5b’and 5c’were applied at 2% shade with MLR 1:30 on nylon and polyester respectively. Dyeing was evaluated using the CIELAB system in terms of L*, a* and b*.

K/S values were determined by using below eq. 1 [61].

$$ \frac{\mathrm{K}}{\mathrm{S}}=\frac{{\left(1-R\right)}^2}{2R} $$
(1)

where, R is the reflectance of coloured samples and K and S are the absorption and scattering coefficients respectively.

Comparative K/S values of Dyes all the dyes dyed on nylon are tabulated in Table 3. 5c showed highest K/S compared to other dyes due to presence of presence of extended conjugated fluorescent coumarin core. Phenyl(1H-benzo[d]imidazol-5-yl)methanone based 5a, 5b and 5c dyes have good colour strengths compared to respective parent analogues 5a’, 5b’ and 5c’. Good K/S strength of a dyeing and higher values of present dyes represent darker and more saturated colours.

Table 3 Color coordinates (CIELAB) of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes for Nylon dyeing
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Comparative K/S values of all the dyes dyed on polyester are tabulated in Table 4. 5a showed highest K/S compared to other dyes due to presence of presence of extended conjugated fluorescent coumarin core. Dyes 5a, 5b and 5c are having higher K/S values compared to their respective analogues 5a’, 5b’ and 5c’. Good K/S strength of a dyeing and higher values of present dyes represent darker and more saturated colours.

Table 4 Color coordinates (CIELAB) of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes for Polyester dyeing
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Light Fastness Properties

Evaluations of light fastness properties of dyes were performed by comparative light fastness measurements of before and after exposure of dyed polyester and nylon samples to the xenon lamp. Same dyed sample was used for measurement in order to have more precision and better comparison of light fastness values. Half part of the sample was exposed to a xenon lamp for 48 h while the other half was covered during the light fastness determination. The samples were then compared with the Blue wool standard scale and fastness ratings were given. On comparing light fastness values of dyed polyester and dyed nylon for any dye, it can be concluded that dyed polyester sample have slightly better light fastness compared to dyed nylon sample. Moreover, light fastness of all the synthesized dyes (5a, 5b and 5c) varies from good to very good for dyed polyester and dyed nylon samples (Table 5). It can be further concluded that synthesized dyes showed better light fastness as compared to commercially available respective analogues 5a’, 5b’ and 5c’ dyes.

Table 5 Light and sublimation fastness ratings of 5a, 5b, 5c, 5a’, 5b’ and 5c’ dyes for dyed nylon and polyester
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Sublimation Fastness

In order to get the sublimation fastness, dyed samples were sandwiched between two undyed cloth pieces (cotton and polyester/nylon) and were subjected to 150 °C, 180 °C and 210 °C for 30 s in a sublimation fastness tester. Two parameters, i.e. colour change (cc) and colour staining (on undyed cloth) [cs] were rated. Synthesized dyes 5a, 5b and 5c showed better sublimation fastness as compared to previously available 5a’, 5b’ and 5c’ derivative at all the temperatures (Table 5) on polyester and nylon. Dyes 5a, 5b and 5c exhibited excellent sublimation fastness for (150 °C, 180 °C, 210 °C) on both polyester and nylon.

DFT Study

Geometry Optimisation and Calculated Energies of Tautomer Forms

There is a possibility of the existence of tautomer forms that is evident from shoulder peaks in the absorption spectra of these dyes (Fig. 1). Optimised structures of Azo and Hydrazone forms of dyes 5a at B3LYP/6-31G(d) in chloroform have shown in supporting information Fig. S2 . It is clearly observed that both the forms have a significant difference at the 47H atom. An azo form of CI Solvent Yellow 14 (5a’) and 5a are stable than the corresponding hydrazone forms. On the other hand hydrazone forms of 5b and 5c are stable than azo forms (Table 6).

Table 6 Comparative energies of azo and hydrazone forms of 5a’, 5a, 5b and 5c dyes in Chloroform at B3LYP/6-31G(d)
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Frontier Molecular Orbital Energies

Energy levels of frontier molecular orbitals i.e. HOMO, LUMO and their spatial distributions can give insight into excitation properties and the idea of the ability of hole or electron injection, which in turn allowed to understand photostability and spectroscopic characteristics of the dyes. In order to get into the stability details of azo and hydrazone forms of the dyes, comparative energy levels and electronic densities need to study. Pictorial diagram of FMO’s suggested that the electron densities at HOMOs of CI Solvent Yellow 14 (5a’) and 5a dyes were located on the N=N motif, while electron densities on the LUMOs were found to be localised on N=N motif periphery of the azo forms of the dyes (Fig. 4a). On the other hand, electron densities at HOMOs of 5b and 5c dyes were located on N,N-diethylamino, -N=N- and imidazole motif. Electron densities at the LUMOs were found to be localised mostly on the >C=O motif of the azo forms of the dyes. This suggested that there is good charge transfer was observed in 5b and 5c dyes resulting into significant positive solvatochromism. HOMO-LUMO energy gap is significantly reduced in 5a compared to CI Solvent Yellow 14. Moreover, there is lowering of HOMO and LUMO energy levels of 5a compared to CI Solvent Yellow 14 clearly suggested red-shifted absorption maxima in 5a. Dyes 5b and 5c are also having similar energy levels as compared to 5a. So, improved spectroscopic characteristics were observed for dyes 5a, 5b and 5c compared to 5a’ dye.

Fig. 4
figure 4

(a) Comparative molecular orbitals and energy gaps of azo forms of 5a’, 5a, 5b and 5c dyes optimized at B3LYP/6-31G(d) in chloroform (b) Comparative molecular orbitals and energy gaps of hydrazone forms of 5a’, 5a, 5b and 5c dyes optimised at B3LYP/6-31G(d) in chloroform

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HOMO-LUMO electronic densities of hydrazone forms of 5a’ and 5a showed truncated delocalization of electronic densities compared to corresponding azo forms of dyes (Fig. 4b). Moreover, HOMO energy levels of azo forms of 5a’ and 5a are more stabilized than hydrazone forms. On the other hand, HOMO and LUMO electronic densities of hydrazone forms of 5b and 5c dyes are comparatively same as that of their azo forms, but HOMO energy levels of hydrazone forms are more stabilized than their corresponding azo (Fig. 4b).

Calculated HOMO and LUMO energy levels for the azo forms of dyes in solvents of varying polarity suggested that there were an increase in the energy gap for dyes 5a (2.850 to 2.880 eV) and CI Solvent Yellow 14 (5a’) (3.723 to 3.752 eV) with increasing solvent polarity, correlated with negative solvatochromism. On the other hand, 5b (2.729 to 2.704 eV) and 5c (2.766 to 2.745 eV) showed decrease in the energy gap with increase in the solvent polarities correlated with the positive solvatochromism (Supporting Information Fig. S3 a). Similar trends were observed for hydrazone forms of the dyes. 5a’ and 5a dyes showed slight elevation of HOMO energy levels while 5b and 5c showed lowering of HOMO levels compared to their corresponding azo forms. LUMO energy levels and HOMO-LUMO band gap is lowered in all the hydrazone forms of the dyes compared to their corresponding azo forms (Supporting Information Fig. S3 b).

TD-DFT

Ground state optimised geometry of dyes at ground state in solvents of various polarities was subjected to TD using B3LYP/6-31G(d) function. At least 10 excited states were calculated for each molecule. Comparative absorption maxima, computed vertical excitations, oscillator strength and their orbital contributions in chloroform are listed in Table 7. The results of DFT and TD-DFT suggest that there was the observable influence of (1H-benzo[d]imidazol-5-yl)(phenyl)methanone group on the absorption spectra of the dyes. Computed values for these dyes are in good agreement with the experimental observations.

Table 7 Comparison of experimental data and computed data at B3LYP/6-31G(d) for 5a’, 5a, 5b and 5c dyes in chloroform
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Electrophilicity Index

We have calculated the stability of the dyes mathematically using computationally deduced energies at B3LYP/6-31G(d). For this reason, the electrophilicity index (ω) was utilised. As defined by Parr et al. [62, 63], this electrophilicity index measures the propensity of the moiety to absorb electrons and is mathematically formulated as:

$$ \upomega =\frac{\upmu^2}{2\upeta} $$
(2)

where,

ω:

electrophilicity index

μ:

chemical potential

η:

chemical hardness

$$ \mu =-\frac{\left( IP+ EA\right)}{2} $$
(3)
$$ \eta =\frac{\left( IP- EA\right)}{2} $$
(4)

where, IP = Ionization Potential, i.e., the change in the energy when an electron is removed from the system.

EA = Electron Affinity, i.e., the change in the energy when an electron is added to the system

$$ \mu =\frac{\left({E}_{LUMO}+{E}_{HOMO}\right)}{2} $$
(5)
$$ \eta =\frac{\left({E}_{LUMO}-{E}_{HOMO}\right)}{2} $$
(6)

where,ELUMO is the energy of the lowest unoccupied molecular orbital and EHOMO is the energy of the highest occupied molecular orbital.

$$ {\upomega}^{\pm }={\upomega}^{+}+{\upomega}^{-} $$
(7)

where,

$$ {\omega}^{+}=\frac{{\left({E}_{LUMO}\right)}^2}{2\left({E}_{LUMO}-{E}_{HOMO}\right)} $$
(8)

and

$$ {\omega}^{-}=\frac{{\left({E}_{HOMO}\right)}^2}{2\left({E}_{LUMO}-{E}_{HOMO}\right)} $$
(9)

Among azo and hydrazone forms of the dyes the stable conformation of dyes were justified by calculating net electrophilicity index (ω±) [1]. The net electrophilicity index (ω±) allowed us to predict comparative stabilities of azo and hydrazone tautomer forms of the dyes. Net electrophilicity index (ω±) suggested that azo form of 5a’ and 5a dye are slightly more stable than hydrazone forms. On the other hand, the exactly reverse trend was observed for 5b and 5c dyes with hydrazone forms were slightly more stable than azo forms (Table 8).

Table 8 Electrophilicity index of azo and hydrazone forms of 5a’, 5a, 5b and 5c dyes optimized at B3LYP/6-31G(d) in chloroform
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Conclusion

Three novel phenyl(1H-benzoimidazol-5-yl)methanone based fluorescent monoazo disperse dyes were successfully synthesised. The dyes 5a, 5b and 5c exhibited red-shifted absorption maxima from 497 nm to 516 nm and much higher molar extinction coefficient as compared to compared to parent dyes 5a’, 5b’ and 5c’ respectively. The dyes 5a, 5b and 5c are emitting in the far-red region (565–627 nm) while only 5c’ is weakly emitting (539 to 621 nm). Dye 5a and 5a’ showed negative solvatochromism, while dyes 5b, 5c, 5b’ and 5c’ showed positive solvatochromism. Solvent polarity graphs are in good agreement with solvatochromic data. Dyes 5a, 5b and 5c on dyed polyester and nylon showed very good light and sublimation fastness. DFT calculated energies, electrophilicity index and Frontier Molecular Orbitals calculations of 5a, 5b and 5c are in good agreements with the experimental observations. 5a, 5b and 5c are emission in nature and has good fastness properties on fabrics, so can find the potential applications in high-visibility colour dyeing of textile products [64].