1 Introduction

Azo dyes established as one of the leading dyestuff of industrial colorants [1]. Benzophenone is a well-known aromatic ketone obtained by organic synthesis or from natural products [2]. Benzophenone based compounds have wide applications as excellent UV absorbers [3] and thermal stabilizers [4]. Benzophenone based textile dyes are also well reported [5,6,7]. To attain the excellent photostability and hence light fastness property different approaches have been made [7,8,9]. In some cases, fabric dyed with commercially available disperse dyes disperse yellow 86 [10], Disperse Yellow GLFS [11], CI Disperse Yellow 42 [12], 2,4-dihydroxy benzophenone and 4-hydroxy benzophenone were directly applied as UV absorber in the dyeing liquid or by conducting treatment to the dyed fabrics after dyeing [13]. Freeman et al. was the first to investigate the consequence of incorporation of photo-stabilizer moiety into Disperse Yellow 42 in 1992 [14]. Further lot of efforts have been focussed to increase light fastness of dyes by incorporating photo stabilising unit [7, 15, 16]. Also enhancement of the light fastness value is considered to be an important criteria in the textile industry, since Polyethylene Terephthalate (PET) fabrics are extensively used in coloured automobile stuffing, which can be visible to direct sunlight above 50 °C [17]. In order to minimize the above said problems, textiles can be served to create sunblind sun umbrella and other shielding clothing. Compared to cosmetics, clothing can be considered as relatively appropriate barrier against UV radiations [18]. So, there is a need to design light fast fluorescent disperse dyes for light energy dissipations [19, 20].

Fluorescent dyes are having great significance in the colouring of synthetic fabrics (e.g. polyacrylonitrile, polyester and polyamides) [19]. Some of the important fluorescent textile dyes (usually disperse dyes) were applied on PET and thus can be considered as important disperse class of dyes [20]. Most of the commercial dyes are yellow, providing the very similar greenish-yellow fluorescent colour which allowed the dyed materials to be more bright, visible and simply noticeable and thus, very commonly encountered on garments worn for safety purposes [21]. The most important fluorescent disperse dyes are C.I. Disperse Yellows 82,184,186 and 232 and C.I. Disperse reds 277, 374. The commercially available fluorescent dyes for textile application are frequently objected for light fastness stability and most of them are incompatible with standards essential for the additional demanding applications [22].

It is reported in the literature that benzophenone and some of it’s derivatives are emissive in nature [23,24,25,26]. Recently, Barsotti et al. have done wide-ranging work on4-hydroxy benzophenone for it’s fluorescence properties [23, 27]. In general emission properties of azobenzene dyes were induced or altered by various ways which include restricted isomerisation and rotations across –N=N– group by N-B bond formation [28], by incorporation of benzothiazole fluorescent moiety to the azo dyes [29, 30], by cooling of dye sample to very low temperature or by embedding dyes in rigid solvents [31]. However, there are very few fluorescent azobenzenes reported so far [28, 32,33,34,35,36,37]. Considering decent π-conjugation [38] and restricted rotations across –N=N– group [28], these benzophenone dyes can perform as good applicants for preparation of attractive photo stable fluorescent compounds.

Recently, effect of viscosity on fluctuating emission properties of some of the organic dyes were provided and studied in the literature [39, 40], however, only few reports were based on viscosity effects on the properties of azo dyes [20, 41, 42]. Intramolecular rotation in azo dyes are restricted due to either chemical interaction or adhesion of dyes on fabrics during dyeing [43, 44]. So, it is important to consider intramolecular rotation contributed to the reduction mechanism for the dye in the solution [43]. Although azo-dyes are considered to be non-fluorescent specifically in solutions, but few of them became fluorescent when bound to the substrate [31]. Some of the monoazo dyes can act as molecular rotors, where fluorescence emission property is sensitive to solution [45]. Some of the azo dyes were reported with respect to emission intensity enhancement as viscosity of the solution increases [45]. This varying emission characteristics of azo dyes might be described on the basis of constrained rotational motion across the bonds by viscosity in the micro-environment around the molecule [45]. Briefly, it can be suggested that the either viscous micro-environment or the substrate (e.g. fibers)might be responsible for the generation of necessary restriction in azo dyes rotational motions which further contribute to overall emission of these dyes. So, it is of interest to study systematically viscous micro-environmental effects on emission properties of the benzophenone based mono azo disperse dyes.

To meet the requirements of fluorescent disperse dyes with excellent fastness properties, we used the key ingredient benzophenone which acts as a UV stabilizer and it has its own emission, which facilitate to prepare photo-stable fluorescent dyes. As azo (–N=N–) moiety in azo dyes reduce the fluorescent properties because of photo-isomerisation [46, 47] and other dynamics such as excimer or exciplex formation may be contribute to reduce fluorescence [39, 48]. So, to avoid these process and increase fluorescence one can use micro-viscosity as a strategy to make molecule more rigid and restrict its rotations.

Here, we has synthesized a series of benzophenone inbuilt mono azo disperse dyes. Moreover, parallel series of methylated analogues enabled to restricted azo-hydrazone tautomerization to ensure study of exclusive azo form systematically. Detailed photophysical characterizations were performed to study effect of viscosity, polarity and role of hydrogen bonding in the emission enhancement in these dyes. The dyes are fully characterized in terms of their dyeing properties including the color assessment, sublimation fastness, and light fastness was carried out. Theoretical calculation by DFT supports our experimental outcomes.

2 Experimental section

2.1 Materials and methods

Sodium nitrite, sodium carbonate, sodium hydroxide, aniline, p-chloro aniline, p-nitro aniline, conc. H2SO4, conc. HCl, urea, 4-hydroxy benzophenone, anhydrous methanol, acetone, dimethyl sulphate (DMS), N,N-dimethyl formamide (DMF), Saragen 50 (dispersing agent), sulphamic acid, sodium acetate, sodium chloride and acetic acid procured from Sigma Aldrich India. All A.R. mark organic solvents were procured from SD Fine Chemicals, Mumbai. All organic solvents after distillation as per the standard procedures were utilized further. Melting and boiling points of all the chemicals confirmed and used without any purifications. Ready for dyeing polyester (PET) (100%) fabric (weight 70 g/m2, Ends/105 in. and picks/94 in.), nylon (NYL) (70 d/25 f, plain weaved, 219 × 191/5 cm), acrylic (ACR) was purchased from Piyush Syndicate, Mumbai. For dyeing application Flexi dyer machine procured from Rossari Labtech Mumbai, India has been used.

Thin layer chromatography on 0.25 mm (silica gel 60F-254 pre-coated aluminum plate) used to check completion of the reactions. Purification of all synthesized compounds accomplished using re-crystallizations technique. Melting point instrument procured from Veego Instruments, Mumbai, India. 1H-NMR and 13C-NMR spectra were documented on Agilent NMR 500 MHz and 125 MHz respectively. Chemical shifts stated in δ-ppm and used CDCl3 as a solvent and TrimethylSilane (TMS) used as an internal standard. FT-IR spectra noted on JASCO-4100 FT-IR instrument with ATR accessories.

2.2 Photophysical study

Absorption Study of all synthesized dyes documented using Perkin Elmer UV–Vis spectrometer Lambda-25 instrument. Eight unlike polarity solvents used to make 1–5 μM concentration solutions of all dyes (e.g. toluene, 1,4 dioxane, chloroform (CHCl3),ethyl acetate(EtOAc), methanol (MeOH), tetrahydrofuran (THF), dimethylformamide (DMF) and dimethylsulphoxide (DMSO).

2.3 Viscosity study and calculations

Viscosity sensitivity study was carried out in various polarity viscous mixture such as DMSO—Glycerol, toluene—polyethyleneglycol400 (PEG-400) and toluene-paraffin oil mixture. Varian-Cary Eclipse fluorescence-spectrometer were used to document fluorescence emission spectra of all dyes for this constant concentration 1–5 μM solutions of dyes used. Viscosity (η) of three viscous mixtures at different percentage ratio were mathematically estimated by equation [49] (Table S1).

$$ \mathrm{In}\upeta =\mathrm{X}1\kern0.49em \mathrm{In}\upeta 1+\mathrm{X}2\kern0.37em \mathrm{In}\upeta 2\kern1.25em $$
(1)

Where,

ƞ = viscosity

X1, X2 = mole fractions of binary mixture (for each constituent separately)

ƞ1, ƞ2 = viscosities of the binary mixture (for each constituent separately)

Varying dynamic viscosity in mPas* against varying solvent ratios in volume % was plotted for three different binary mixtures (i.e. DMSO-Glycerol, toluene-PEG400 and toluene-paraffin oil) (Fig. S1).

2.4 Computational study

DFT computational experiments executed on a HP-workstation XW-8600 with4GB RAM, Xeon processor and Windows Vista as the operating systems. Gaussian 09 W used as the software package [50]. Geometry of ground state optimized by using B3LYP as a functional and 6-311G (d) as a basis set for all the molecules [51, 52]. Oscillator strengths, Vertical excitation energies and orbital contributions (lowest 10 singlet transitions) to the optimized geometry of the ground states of molecules were attained by TD-DFT computations. Polarizable Continuum Model (PCM) implemented for optimizations in various solvent phases in Gaussian 09 [53]. The solvents used in DFT study were toluene, 1,4-dioxane, CHCl3, EtOAc, MeOH, THF, DMF and DMSO.

2.5 Dyeing study

2.5.1 Color assessment

Color assessment the dyed substrate done on Spectra Scan-5100+ with 10° standard observer under D65illuminant.K/S value of dyed substrate determined by Kubelka-Munk equation.

2.5.2 Fastness property

Dyed samples were tested for light fastness by Q-Sun Xenon Test Chamber (Q-Lab Corporation, Ohio, USA)by AATCC 16/2004 method.

Sublimation fastness tester, RBE Electronics Engineering Private Limited Mumbai, India has been used to evaluate sublimation property of dyed sample by ISO 105-PO1 1993standard method [54].

The change in the shade along with stainings of the in-line fabrics, were evaluated according SDC (Society of Dyers & Colorists) grey scale.

2.5.3 Percentage exhaustion determination

Exhaustion (%) of the of the dye bath were estimated by method [55].

2.5.4 UPF Factor determination

UPF and UVA and UVB percentage transmission estimated by AATCC Test Method-183.Blocking or Transmittance of Erythemally Weighted UV (Ultra-violet)radiations through fabric by AATCC, 2002 (AATCC - Technical Manual, 2015). UV transmittance analyzer was used for the measurements for UPF testing. Measurements were performed using a UV transmittance UPF Tester V-2000F Fabric Analyzer, Labsphere, US.

2.6 Syntheses and characterization

The synthetic skeleton of synthesized azo dyes (4a–4c) and (5a–5c) is shown in Scheme 1. Dye 4c is reported in the literature [56].

Scheme 1
scheme 1

Synthetic scheme of mono azo disperse dyes (4a–4c and 5a–5c)

Full size image

2.6.1 Synthesis of azo dyes (4a–4c)

The mixture having aromatic amines (0.003 mol), conc. Hydrochloric acid (HCl) 3 ml (0.035 mol), and 15 ml (1.38 mol) water heated till solution become cleared and this mixture cooled down to 0 °C. The acidity range of the solution determined by using litmus paper. Sodium nitrite 0.25 g (0.003 mol) solution was little by little added in step wise manner with constant stirring at below 5 °C temperature. Further solution permitted to stir for 40 min at 5 °C, after complete addition. Excess nitrous acid quenched by Urea. Meanwhile, sodium hydroxide (0.025 N, 0.003 mol) was dissolved the solution containing 4-hydroxy benzophenone(0.5 g, 0.003 mol)as coupler resulting into clear solution (pH = 11–12). The above diazonium mixture slowly added to the coupler solution with continuous stirring at temperature 0–5 °C. While addition of diazonium salt to the coupler solution pH decreases, so Na2CO3 (10% w/v) solution was slowly added to the reaction medium to avoid further decrease in pH and was maintain in between 7.5–8.0 [6]. The further mixture was stirred for 3 h (Scheme 1). The precipitated resultant azo dye filtered and thoroughly washed with water. Recrystallization in ethanol gave desired azo compound. Yield of 4a–4c after recrystallization was then reported.

2.6.2 Characterizations

4a: Pale yellow powder, Yield–75%, melting point: 130–132 °C.

1 H-NMR δ H (500 MHz,CDCl3): 13.48 (1H, s, −OH), 8.50–8.45 (1H, m, Ar-H), 7.98–7.94 (1H, m, Ar-H), 7.92–7.88 (2H, m, Ar-H), 7.84 (2H, d, J = 8 Hz, Ar-H), 7.66–7.60 (1H, m, Ar-H), 7.55 (2H, d, J = 1 Hz, Ar-H), 7.53 (2H, s, Ar-H), 7.53–7.51 (1H, m, Ar-H), 7.15 (1H, dd, J = 8 Hz and 1 Hz, Ar-H) ppm.

13 C-NMR δ C (125 MHz, CDCl3): 194.7, 156.7, 150.0, 137.8, 136.4, 136.0, 134.9, 132.2, 131.8, 129.7, 129.6, 129.4, 128.4, 122.4, 118.6 ppm.

FT-IR: 3057 (phenolic OH), 1680 (C=O), 1644 (C=C), 1275 (C–N) cm−1.

HRMS: 303.1189 (M+H+) (Calculated for C19H15N2O2: 303.1165).

4b: Pale yellow powder, Yield– 85%, melting point: 204–206 °C.

1 H-NMR δ H (500 MHz,CDCl3): 13.23 (1H, s, −OH), 8.45 (1H, d, J = 2 Hz, Ar-H), 7.96 (1H, dd, J = 8.5 Hz and 2.0 Hz, Ar-H), 7.86 (1H, s, Ar-H), 7.84 (1H, s, Ar-H), 7.83 (1H, s, Ar-H), 7.65–7.60 (2H, m, Ar-H), 7.53 (1H, s, Ar-H), 7.53 (2H, d, J = 2 Hz, Ar-H), 7.51 (1H, s, Ar-H), 7.15 (1H, d, J = 8.5 Hz, Ar-H) ppm.

13 C-NMR δ C (125 MHz, CDCl3): 194.6, 156.6, 148.5, 137.8, 137.7, 136.4, 136.0, 135.1, 132.2, 129.8, 129.8, 129.7, 128.4, 123.6, 118.6 ppm.

FT-IR: 3063 (phenolic OH), 1691 (C=O), 1652 (C=C), 1284 (C–N), 620 (C–Cl) cm−1.

HRMS: 335.0622 (M−H+) (Calculated for C19H12ClN2O2: 335.0619).

4c: Pale yellow powder, Yield- 85%, melting point: 194–195 °C.

1 H-NMR δ H (500 MHz, CDCl3): 13.04 (1H, s, −OH), 8.51 (1H, s, Ar-H), 8.41 (2H, d, J = 8.5 Hz, Ar-H), 8.05 (1H, s, Ar-H), 8.02 (2H, d, J = 9 Hz, Ar-H), 7.84 (2H, d, J = 7.5 Hz, Ar-H), 7.64 (1H, t, J = 7.5 Hz, Ar-H), 7.54 (2H, t, J = 7.5 Hz, Ar-H), 7.18 (1H, d, J = 8.5 Hz, Ar-H) ppm.

13 C-NMR δ C (125 MHz, CDCl3): 194.3, 156.5, 153.5, 149.0, 137.5, 137.1, 136.4, 132.8, 132.4, 130.2, 129.7, 128.4, 125.0, 123.0, 118.9 ppm.

FT-IR: 3049 (phenolic OH), 1673 (C=O), 1638 (C=C), 1272 (C–N) cm−1.

HRMS: 346.0848 (M−H+) (Calculated for C19H12N3O2: 346.0868).

2.6.3 Preparation of methylated forms of azo dyes (5a, 5b, 5c)

General procedure for preparation of azo dyes (5a–5c)

Dye 4a, 4b, or 4c was dissolved in acetone along with potassium carbonate (3 eq.) and DMS (0.5 eq.) in 250 ml round bottom flask. The reaction mass was allowed to stirred at 25 °C for 30 min. Reaction was monitored by silica gel TLC. After the starting was consumed, acetone was concentrated under reduce pressure on rota evaporator and solid form thoroughly washed with water and dried. The obtained dyes was pure and single spotted on TLC further no purification is carried out. Therefore, reported yield of 5a–c is in its pure form with very good yields.

2.6.4 Characterizations

5a: Yellow powder, Yield- 96%, melting point: 204 °C.

1 H-NMR δ H (500 MHz,CDCl3): 8.12 (1H, d, J = 2 Hz, Ar-H), 7.99 (1H, dd, J = 8.5, 2 Hz, Ar-H), 7.92 (1H, s, Ar-H), 7.91 (1H, s, Ar-H), 7.82 (1H, s, Ar-H), 7.80 (1H, s, Ar-H), 7.60 (1H, t, J = 7.5 Hz, Ar-H), 7.54–7.48 (5H, m, Ar-H), 7.19 (1H, d, J = 8.5 Hz, Ar-H), 4.12 (3H, s, −OCH3) ppm.

13 C-NMR δ C (125 MHz, CDCl3): 195.2, 159.9, 152.9, 141.6, 137.8, 134.2, 132.2, 131.2, 130.1, 129.8, 129.1, 128.3, 123.1, 119.4, 112.1, 56.5 ppm.

FT-IR: 1688 (C=O), 1651 (C=C), 1274 (C–N), 1013 (O–CH3) cm−1.

HRMS: 317.1353 (M + H+) (Calculated for C20H17N2O2: 317.1374).

5b: Yellow powder, Yield −92%, melting point: 163–165 °C.

1 H-NMR δ H (500 MHz, CDCl3): 8.12 (1H, s, Ar-H), 8.00 (1H, d, J = 9 Hz, Ar-H), 7.87 (2H, d, J = 8.5 Hz, Ar-H), 7.80 (2H, d, J = 8 Hz, Ar-H), 7.61 (1H, t, J = 8 Hz, Ar-H), 7.51 (2H, d, J = 8 Hz, Ar-H), 7.48 (2H, d, J = 8.5 Hz, Ar-H), 7.19 (1H, d, J = 8.5 Hz, Ar-H), 4.12 (3H, s, −OCH3) ppm.

13 C-NMR δ C (125 MHz, CDCl3): 195.2, 16.1, 151.2, 141.4, 137.7, 137.1, 134.5, 132.2, 130.1, 129.8, 129.3, 128.3, 124.3, 119.3, 112.2, 56.6 ppm.

FT-IR: 1694 (C=O), 1661 (C=C), 1283 (C–N), 1008 (O-CH3) cm−1.

HRMS: 351.0955 (M + H+) (Calculated for C20H16ClN2O2: 351.0965).

5c: Yellow powder, Yield −88%, melting point: 225 °C.

1 H-NMR δ H (500 MHz,CDCl3): 8.39 (1H, s, Ar-H), 8.37 (1H, s, Ar-H), 8.18 (1H, d, J = 2 Hz, Ar-H), 8.06 (1H, dd, J = 8.5 Hz and 2 Hz, Ar-H), 8.04 (1H, s, Ar-H), 8.02 (1H, s, Ar-H), 7.81 (2H, d, J = 8.5 Hz, Ar-H), 7.62 (1H, t, J = 7.5 Hz, Ar-H), 7.52 (2H, t, J = 7.5 Hz, Ar-H), 7.23 (1H, d, J = 8.5 Hz, Ar-H), 4.15 (3H, s, −OCH3) ppm.

13 C-NMR δ C (125 MHz, CDCl3):195.0, 160.6, 155.9, 148.8, 141.3, 137.6, 135.6, 132.4, 131.1, 129.9, 128.4, 124.7, 123.6, 119.2, 112.4, 56.7 ppm.

FT-IR: 1685 (C=O), 1649 (C=C), 1273 (C–N), 1105 (O-CH3) cm−1.

HRMS: 362.1137 (M + H+) (Calculated for C20H16N3O2: 362.1161).

2.6.5 Procedure of dyeing (PET, Nylon, Acrylic)

PET, nylon and acrylic fabrics dyeing were performed on a Flexi Dyer dyeing machine with 1:30 MLR (material to liquor ratio) and percentage shade 2% (calculated on the weight of fabric). Firstly, the azo dye was pasted using dispersing agent Saragen-50 and further diluted using 15 ml of buffered solution with acetic acid in water (pH 4–5) [57]. Complete dispersion of the dye in water was acquired by ultra-sonication for 30 min. PET, NYL and ACR substrates dyed using above solutions. Dyeing was initiated at room temperature and raised to 130 °C (PET) and 90 °C (nylon), 90 °C (acrylic) respectively, maintain this raised temperature for 1 h and cool down to 60 °C. The dyed substrate thoroughly washed with warm and cold water and further for PET dyed fabric, reduction clearing treatment is given with 2gpl soda ash (Na2CO3), 1gpl soap solution and 2gpl Sodium hydrosulphite at 70 °C for 15 min, then treated fabric was washed with water and dried.

3 Results and discussion

3.1 Photo-physical properties

Dyes (4a–4c) showed dual absorption peaks while their corresponding methoxy derivatives (5a–5c) showed only single absorption peak (Fig. S2). Significant effect of substituents (–Cl and –NO2 group) was observed on absorption maxima of dyes (4b and 4c) on comparing with unsubstitued dye (4a) in various solvents. But on comparing 4b (–Cl) with 4c (–NO2), a very small difference was detected in the absorption maxima of the dyes. Only 4b dye is slightly red shifted (1–3 nm) compared to 4c in different solvents. Effect of –NO2 and –Cl substituents on absorption spectra of the dyes are provided in the literature [58, 59]. The first absorption maxima of 4a–4c corresponds to the azo form, while the second red shifted absorption maxima corresponds to the hydrazone form [60]. Methoxy group is better donating group as compared to the hydroxyl group and significant effect has been observed in absorption spectra. 5a, 5b and 5c (exclusive azo forms) are absorbing at red shifted region as compared to corresponding azo form of hydroxyl derivatives 4a, 4b and 4c, respectively. Additionally, distinct second absorption peak (red shifted) of hydrazone form at around 369–382 nm was observed for compounds 4a, 4b and 4c which was disappeared in methoxy series (5a, 5b and 5c) due to the absence of hydrazone formation. Methoxy substituted parallel series (5a, 5b and 5c) showed similar trend solvatochromism as compared to their corresponding hydroxyl analogues (4a, 4b and 4c). The absorption maxima trend of dyes is as follows: 4a < 4c ≤ 4b < 5a ≤ 5b < 5c (Table 1). It is clearly observed that dyes 4a, 4b and 4c has very good molar absorptivity as matched to methoxy analogues 5a, 5b and 5c, respectively (Table 1).

Table 1 Photophysical properties of all dyes in various solvent
Full size table

3.2 Viscosity influenced spectroscopic characterization

Synthesized azo dyes are not emissive in solvents. The possible for non-radiative relaxation during de-excitation may be isomerizations and rotational motions through single bonds beside to azo group [61]. Substituent effects, solvent viscosity, solvent polarity and hydrogen bonding are also possible factors affects non-radiative rate [62,63,64,65]. So, viscosity helps to restrict rotations through the single bonds adjacent to azo. Azo-hydrazone tautomerism can also has contribution in restriction of the rotations in these dyes additionally. With the aim of consider polarity effects different polarity-viscosity binary mixtures (DMSO-Glycerol), (toluene-PEG400) and (toluene-paraffin oil) have been used. In this study we have chosen three different viscous binary mixtures i.e. DMSO-Glycerol (polar protic),toluene-PEG 400 (polar aprotic) and toluene–paraffin oil (non-polar aprotic). Quinine sulphate (in 0.5 M H2SO4, Φfl = 0.546) is used as a standard to calculate relative fluorescence quantum yield (Φfl).

3.2.1 DMSO- glycerol mixture

  1. (a)

    Spectral data


    Absorption maxima of dyes were documented in varying extremely viscous medium Glycerol-DMSO at constant concentration (Fig. 1). Dyes (4a–4c) exhibited broad and dual absorption peaks (azo form and hydrazone form) while only single absorption peak with slight red shift was observed for the corresponding methoxy derivatives (5a–5c) (Fig. 1).

    Azo forms of dyes (4aA, 4bA and 4cA) did not exhibit noteworthy emission intensity progressing Glycerol-DMSO mixtures (Fig. 2). Calculated relative fluorescence quantum yields and fold of increase and at Glycerol(9.5):DMSO(0.5) v/v compared to blank DMSO for the dyes are 8.8 fold(Φfl = 4%) for 4aA, 8.3 fold(Φfl = 5%)for 4bA and 6.3 fold (Φfl = 3%) for 4cA. Similar trends were observed for the corresponding methoxy analogues, i.e. 5a showed 17.5 fold (Φfl = 3%), 5b showed 15.7 fold (Φfl = 3%) and 5c showed 12.5fold (Φfl = 4%) at Glycerol (9.5): DMSO(0.5) v/v binary mixture (Fig. 2). On the other hand, hydrazone form of dyes 4aH, 4bH and 4cH in Glycerol-DMSO binary mixture exhibited significant emission intensity enhancement with increasing Glycerol content. Hydrazone forms 4aH showed 96 fold (Φfl = 30%), 4bH showed 78 fold (Φfl = 27%) and 4cH showed 75 fold (Φfl = 29%) at Glycerol (9.5):DMSO (0.5) v/v (Fig. 2).

    This result suggest hydrogen bonding involved in azo-hydrazone tautomerizm along with viscosity has a significant part in emission enhancement. Methoxy analogues of these dyes 5a, 5b and 5c did not show significant emission intensity enhancement may be due to absence of hydrogen bonding center. Alternately, this can be explained and supported by more planarity attainment of hydrazone forms than the corresponding azo-methoxy forms of the dyes in viscous environment. Hence, both azo form and methoxy forms are less emissive as compared to hydrazone forms of the dyes.

  2. (b)

    Viscosity sensitivity result


    Forster-Hoffman equation is used to quantify viscosity sensitivity of synthesized azo dyes

    $$ \mathrm{Log}\ I=C+x\ \log $$
    (2)

    Where,

    η = viscosity (mPa s).

    X = viscosity sensitivity parameter,

    I = maximum emission intensity and.

    C = constant.

    Viscosity sensitivity against emission intensity plotted. Viscosity rises from 1.99 mPa s (DMSO) to 688.51 mPa s for (DMSO:Gly: 5:95).It is clearly observed that all the hydrazone forms of the dyes have significant viscosity sensitivity 4aH(x = 0.676), 4bH (x = 0.690) and 4cH(x = 0.673) with good regression coefficients on excitation @ 370–381 nm (Fig. 3). On the other hand azo forms 4aA, 4bA and 4cA have exhibited viscosity sensitivity (x) 0.349, 0.337 and 0.292, respectively. Similarly, methoxy analogues 5a, 5b and 5c have exhibited viscosity sensitivity (x) 0.499, 0.456 and 0.470, respectively. Comparative viscosity trend for all the dyes is found to be 4bH ≥ 4aH ≥ 4cH > 5a ≥ 5c ≥ 5b > 4aA ≥ 4bA ≥ 4cA (Fig. 3).

  3. (c)

    Viscosity polarity effect


    As polarity of viscous media increases not only emission intensity enhancement but also red shifting in emission wavelength has been observed. Therefore, we have related solvent polarity functions with stokes shift and emission wavelength of dyes. For this purpose Lippert-Mataga and Weller equation has been used and solvatochromic behavior of synthesized dyes estimated. Lippert-Mataga plot indicated that azo forms of the dyes 4aA, 4bA and 4cA exhibited large Stokes shifts 6646–7649 cm−1 and positive solvatochromism (Fig. 4). On the other hand small Stokes shifts and positive solvatochromism were observed for corresponding methoxy analogues 5a, 5b and 5c (3828–5126 cm−1). Similarly, hydrazone forms of the dyes showed small Stokes shift and positive solvatochromism (4053–5015 cm−1) and are in good correlation to fluorescence quantum yields.

    Maximum Stokes shifts were observed for 4a–4c and found to have very good correlation of having low fluorescence quantum yields. Weller plot showed that azo forms of the dyes (4aA, 4bA and 4cA) exhibited emission wavelengths 16,000–23,980 cm−1. Similar trend was observed for the corresponding methoxy analogues 5a, 5b and 5c (23,255–24,154 cm−1). On the other hand hydrazone forms of the dyes showed longest emission wavelengths (21,231–22,831 cm−1) among all dyes. Hydrazone forms of dyes show more red shifting emission and lowest Stokes shifts as compared to corresponding azo and methoxy forms of dyes and are in a good agreement with quantum yields. Positive solvatochromism observed for all the dyes presented in (Fig. 4).

Fig. 1
figure 1

Absorption spectra of 4a, 4b, 4c and 5a, 5b, 5c dyes in DMSO–Glycerol mixture ratio

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Fig. 2
figure 2

Emission spectra of 4aA (excited @ 326 nm), 4bA (excited @ 333 nm) and 4cA (excited @ 332 nm), 5a (excited @ 351 nm), 5b (excited @ 365 nm), 5c(excited @ 367 nm), 4aH (excited @ 372 nm), 4bH (excited @ 384 nm) and 4cH (excited @ 381 nm) in DMSO–Glycerol mixture ratio

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Fig. 3
figure 3

Comparative viscosity against intensity (logarithmic) spectrum of dyes in different DMSO-Glycerol mixture ratio

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Fig. 4
figure 4

Weller and Lippert-Mataga plots in different DMSO-Glycerol mixture ratio

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3.2.2 Toluene-PEG 400 mixture

  1. (a)

    Spectral data


    Absorption spectra of (4a–4c) and (5a–5c) dyes having same concentration were recorded in varying toluene-PEG400 mixture ratio. Dyes (4a–4c) showed dual and broad absorption peaks (azo form and hydrazone form) while only single absorption peak with slight red shift was observed for the corresponding methoxy derivatives (5a–5c) (Fig. S3). Azo forms of dyes (4aA, 4bA and 4cA) did not exhibit significant emission intensity enhancement in varying ratio of toluene-PEG400 binary mixture (Fig. 5). Calculated relative fluorescence quantum yields and fold of increase at toluene(0.5):PEG400(9.5) v/v compared to toluene for the dyes are 8.6 fold (Φfl = 3%) for 4aA, 7.6 fold (Φfl = 4%) for 4bA and 7 fold (Φfl = 3%) for 4cA. Similar trends were observed for methoxy analogues, i.e. 5a showed 7 fold (Φfl = 2%), 5b showed 6.5 fold (Φfl = 3%) and 5c showed 7.3 fold (Φfl = 4%) at toluene-PEG400 0.5:9.5 (v/v) binary mixture. On the other hand, hydrazone form of dyes 4aH, 4bH and 4cH in toluene-PEG400 binary mixture exhibited significant emission intensity enhancement with increasing PEG 400 content. Hydrazone forms 4aH showed 47.5 fold (Φfl = 24%), 4bH showed 55.6 fold (Φfl = 22%) and 4cH showed 47.5 fold (Φfl = 25%) at toluene-PEG400 (0.5:9.5 v/v) (Fig. 5). In toluene-PEG 400 mixture polarity is changing drastically as compared to DMSO-Glycerol mixture, but hydrazone forms of the dyes (4aH, 4bH and 4cH) showed similar emission enhancement in both the viscous medium.

  2. (b)

    Viscosity sensitivity effect


    In toluene-PEG400binary mixtures viscosity increases from 0.55 mPa s (toluene) to 42.21 mPa s (toluene: PEG400: 5:95). Hydrazone forms of the dyes showed excellent viscosity sensitivity 4aH(X = 0.646), 4bH(x = 0.697) and 4cH(x = 0.675) with moderately good regression coefficients on excitation @ 372–378 nm (Fig. 6). On the other hand azo forms 4aA, 4bA and 4cA have exhibited viscosity sensitivity (x) 0.407, 0.417 and 0.413 respectively. Similarly, methoxy analogues 5a, 5b and 5c have exhibited viscosity sensitivity (x) 0.399, 0.366 and 0.387 respectively. Comparative viscosity trend for all the dyes is found to be 4bH ≥ 4cH ≥ 4aH > 4bA ≥ 4cA ≥ 4aA > 5a ≥ 5c ≥ 5b.

  3. (c)

    Viscosity polarity effect


    Lippert-Mataga plot showed that azo forms of the dyes 4aA, 4bA and 4cA exhibited positive solvatochromism and large Stokes shifts 5148 cm−1 to 7246 cm−1 (Fig. 7). On the other hand small Stokes shifts and positive solvatochromism were observed for corresponding methoxy analogues 5a, 5b and 5c (2498–5343 cm−1). Similarly, hydrazone forms of the dyes showed small Stokes shift and positive solvatochromism (2668–4810 cm−1) and are in good correlation to fluorescence quantum yields. Maximum Stokes shifts were observed for 4aA–4cA and found to have very good correlation of having low fluorescence quantum yields. Weller plot showed that azo forms of the dyes (4aA, 4bA and 4cA) exhibited emission wavelengths 22,831–25,188 cm−1. Similar trend was observed for corresponding methoxy analogues 5a, 5b and 5c (23,148–25,125 cm−1). On the other hand hydrazone forms of the dyes showed longest emission wavelengths (21,645–24,213 cm−1) among all dyes. Hydrazone forms of dyes exhibited more red shifted emissions and lowest Stokes shifts paralleled to analogous azo and methoxy dyes. Positive solvatochromism has been observed in all the dyes as shown in Fig. 7.

Fig. 5
figure 5

Emission spectra of 4aA (excited @ 323–327 nm), 4bA (excited @ 329–333 nm) and 4cA (excited @ 328–332 nm), 5a (excited @ 346–349 nm), 5b (excited @ 361–364 nm) and 5c (excited @ 362–365 nm), 4aH (excited @ 369–373 nm), 4bH (excited @ 380–383 nm) and 4cH (excited @ 377–380 nm) in various toluene-PEG400 mixture ratio

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Fig. 6
figure 6

Comparative viscosity against intensity (logarithmic) spectrum of dyes in differenttoluene-PEG400 mixture ratio

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Fig. 7
figure 7

Lippert-Mataga and Weller plots in toluene-PEG400 mixture ratio

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3.2.3 Toluene–paraffin oil mixture

  1. (a)

    Spectral data


    Absorption spectra of dyes in varying toluene–paraffin oil (non-polar viscous medium)were recorded at constant concentration. Dyes (4a–4c) showed dual and broad absorption peaks (azo form and hydrazone form) while only single absorption peak with slight red shift was observed for the corresponding methoxy derivatives (5a–5c) (Fig. S4). Azo forms of dyes (4aA, 4bA and 4cA) did not showed noticeable emission intensity enhancement in varying ratio of toluene-parafin oil binary mixture (Fig. 8). Calculated relative fluorescence quantum yields and fold of increase at toluene(0.5):parafin oil(9.5) v/vin comparison with blank toluene for the azo forms of dyes 4aA, 4bA and 4cA are7.9 fold (Φfl = 3%), 6.7fold (Φfl = 5%)and 7.3fold (Φfl = 4%) respectively at toluene-parafin oil0.5:9.5 (v/v) (Fig. 8). Methoxy analogues, i.e. 5a showed 6.5fold (Φfl = 3%), 5b showed 7.5 fold (Φfl = 4%) and 5c showed 6.6 fold (Φfl = 5%) increase at toluene–paraffin oil at 0.5:9.5 (v/v) binary mixture compared to toluene. On the other hand, hydrazone forms 4aH showed 50.5 fold (Φfl = 27%), 4bH showed 58.9 fold (Φfl = 26%) and 4cH showed 52.1 fold (Φfl = 28%) at toluen-paraffin oil 0.5:9.5 (v/v) (Fig. 8). In non-polar viscous mixture also hydrazone forms have found viscosity influenced emission enhancements compared to their respective azo and methoxy forms confirms that hydrogen bonding along with the viscosity played an important role in the emission.

  2. (b)

    Viscosity sensitivity effect


    In toluene-paraffin oil binary mixture viscosity increases from 0.55 mPa s (toluene) to 220.93 mPa s (toluene:paraffin: 5:95). The hydrazone forms of the dyes show high viscosity sensitivity and are found to be 4aH (x = 0.448), 4bH (x = 0.479) and 4cH (x = 0.458)with regression coefficients on excitation @ 372–378 nm. On the other hand azo forms 4aA, 4bA and 4cA have exhibited viscosity sensitivity (x) 0.294, 0.264 and 0.269 respectively. Similarly, methoxy analogues 5a, 5b and 5c have exhibited viscosity sensitivity (x) 0.252, 0.270 and 0.256 respectively. Comparative viscosity trend for all the dyes is found to be in the order 4bH ≥ 4cH ≥ 4aH > 4aA ≥ 5b ≥ 4cA > 4bA > 5c ≥ 5a (Fig. 9).

  3. (c)

    Viscosity polarity effect


    Lippert-Mataga plot showed that azo forms of the dyes 4aA, 4bA and 4cA showed positive solvatochromism and large Stokes shifts from 4841 to 5612 cm−1 (Fig. 10). On the other hand small Stokes shifts and positive solvatochromism were observed for corresponding methoxy analogues 5a, 5b and 5c (2307–3755 cm−1). Similarly, hydrazone forms of the dyes showed small Stokes shift and positive solvatochromism (2335–2609 cm−1) and is in good correlation to fluorescence quantum yields. Maximum Stokes shifts were observed for 5a–5c. Azo dye and found to have very good correlation of having low fluorescence quantum yields. Weller plot showed that azo forms of the dyes (4aA, 4bA and 4cA) exhibited emission wavelengths in between 24,691 and 25,316 cm−1. Similar trend was observed for corresponding methoxy analogues 5a, 5b and 5c (24,813–25,380 cm−1). On the other hand hydrazone forms of the dyes showed emission wavelengths (23,866–24,509 cm−1) among all dyes. Hydrazone forms of dyes showed red shifting emissions and lowest Stokes shifts related to analogous azo and methoxy forms and are in good agreement with quantum yields. Positive solvatochromism has been observed for all as shown in Fig. 10.

Fig. 8
figure 8

Emission spectra of 4aA (excited @ 323 nm), 4bA (excited @ 329 nm) and 4cA (excited @ 328 nm), 5a (excited @ 346 nm), 5b (excited @ 361 nm) and 5c (excited @ 362 nm), 4aH (excited @ 369 nm), 4bH (excited @ 380 nm) and 4cH (excited @ 377 nm) in various toluene–paraffin oil mixture ratio

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Fig. 9
figure 9

Comparative viscosity against intensity (logarithmic) spectrum of dyes in different toluene–paraffin oil mixture ratio

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Fig. 10
figure 10

Lippert-Mataga and Weller plots in toluene–paraffin oil mixture ratio

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In summary hydrazone forms of dyes (4aH–4cH) have shown maximum and significant enhancement in emission intensity as compared to azo forms of dyes (4aA–4cA) and parallel methoxy derivatives (5a–5c) in varying polarity viscous mixtures i.e. DMSO-Glycerol (polar protic), toluene-PEG 400 (mid polar) and toluene–paraffin oil (non-polar aprotic). This suggested that viscosity and hydrogen bonding (involved in aza-hydrazone tautomerism) are playing crucial role in the enhancement while polarity of the viscous mixture has a significant role in bathochromic shift in the absorption maxima and emission wavelength. We observed good correlation of Stokes shifts (Δῡ) and fluorescence quantum yields (Фf) of all dyes in the viscous mixture. Hydrazone forms may be attaining more planarity due to hydrogen bonding along with viscosity.

3.3 Spectral characteristics

Influence of viscosity and its variable polarity of three viscous binary mixtures on spectral characteristics of dyes has been assessed. It has been identified that with increasing viscous Glycerol, PEG 400 and paraffin oil medium in non-viscous DMSO and toluene medium caused increase in quantum yield (Фfl), radiative rate constant (Kr) and life time(τ) (Table 2).

Table 2 Estimated photophysical properties of all dyes in varies viscous mixtures
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All the dyes exhibited positive solvatochromism. Hydrazone forms showed highest fluorescence quantum yields and small Stokes shifts than corresponding azo and methoxy analogues in all the viscous mixtures and are in a good agreement with the radiative rate constant (Kr). Non-radiative rate constant (Knr) decreases with the increasing viscosity. Knr > > Kr clearly indicates that these azo dyes de-excited favorably in non-radiative manner even in the viscous medium (Table 2). Estimated spectral characteristics at the lowest and the highest viscous media of three binary mixtures are presented in Table 2.

3.4 Aggregation induced emission enhancement

Aggregation induced emission enhancement study carried out for synthesized azo dyes with different H2O fractions in THF. Dyes (4a–4c) showed two absorptions in the range of 325,331,331 nm and second absorption at 373, 382,375 nm respectively. Excitation at shorter wavelength did not exhibit any AIEE characteristics for (4a–4c) dyes but, at longer wavelength the dyes exhibited steady change in emission intensity. In THF and THF/H2O mixtures with different H2O fractions lower than 50% dyes exhibited weak fluorescence and fluorescence increased with water fractions from 60 to 90% in THF/H2O mixture (Fig. 11 and Fig. S2). We noticed that emission intensity also increased at concentration from 20 to 40% in THF/H2O mixture but compared to the 50% THF/H2O mixture the emission intensity observed was less. This emission intensity enhancement could be attributed to the restricted rotation of N=N bond in solvent-water mixture prominent to formation of molecular aggregates.

Fig. 11
figure 11

Emission spectra of 4a, 4b, 4c and 5a, 5b, 5c dyes in THF–H2O mixtures with different water fractions (5 μM)

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3.5 Color co-ordinates

K/S values of dyes estimated using equation [66].

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

Where,

R = reflectance

K and S = absorption and scattering coefficients, respectively.

Synthesized disperse azo dyes applied on PET, NYL and ACR fabric at 2% shade, 1:30 MLR (Material to liquor ratio). The CIELAB system (L*, a* and b*) used for evaluation of dyed substrate. Dyes on applying to PET dyeing showed that hue (h0) of methylated form is less than compare to hydroxyl form of benzophenone based azo dyes. 4a show highest hue (yellowish) and 4c show lowest hue (Reddish) (Table 3a). Table 3b showed color coordinates of nylon dyeing where no specific observation observed. All the dyes show hues (h0) in between 77.25–89.26 (Yellowish) but K/S values of hydroxyl dyes (4a, 4b, 4c) is slightly greater than methylated dyes (5a, 5b, 5c). Dyes on applying to acrylic dyeing showed similar observations as that on nylon (Table 3c).

Table 3a Color coordinates (CIELAB) for PET dyeing
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Table 3b Color coordinates (CIELAB) for nylon dyeing
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Table 3c Color coordinates (CIELAB) for acrylic dyeing
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On the other hand, dyes bond strongly to PET and are slightly affected by UV radiations [67, 68]. Hence, PET is more suitable than nylon for the dyes under investigations.

3.5.1 Fastness properties

After heat setting at 180 °C for 60 s the dyed substrates were tested in a sublimation testing machine at 177 °C for 30 s. The tested fabrics were rated against the standard Grey scales (1–5 grade) where 1 is poor and 5 is excellent grade for sublimation fastness. Sublimation fastness test result values range from moderate to excellent (Table 4). On comparison between all dyed fabrics PET samples show little better sublimation fastness than acrylic and nylon dyed fabric. The sublimation fastness results in (Table 4) observed that the dyes containing substituent groups such as –NO2, –Cl (4b, 4c, 5b, 5c) has better sublimation fastness than unsubstitued (4a, 5a) dyes. Substituted group tend to increase the and hence their sublimation fastness were increased [69]

Table 4 Light and sublimation fastness ratings for dyed PET, nylon and acrylic samples
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Comparative light fastness of hydroxyl dyes and corresponding methylated forms of these benzophenone based azo dyes has performed to get insight into role of H-bonding and methylation simultaneously. From Table 4 it is observed that light fastness of all the dyes varies from very good to excellent. On PET fabric dyes show excellent light fastness. On nylon fabric dyes show very good to excellent light fastness. On acrylic fabric dyes show less light fastness rating as compared to PET and nylon.

It is well known that nitro-substituted dyes give poor light fastness on nylon but, in our case dye 4c and 5c exhibited very high photostability on nylon. Most commercial disperse dyes are aniline or N,N-dialkylaniline substituted. On such dyes some light fastness—structure correlations indicate that the fastness increases as the substituent in the para position is made more electronegative [70] and second substituent in the ortho position have a profound effect on light fastness. Light fastness increases with increasing electron-donating power of the ortho substituents to the azo linkage [71]. The dye 4c and 5c is an identical example for this criteria (Scheme 1). Therefore, p-nitro dye (4c and 5c) being most light fast on nylon.

Comparative light and sublimation fastness of present benzophenone based disperse azo dyes and previously reported disperse azo dyes are tabulated in Table 5. Both hydroxyl and methylated forms of benzophenone based azo dyes show much better sublimation and light fastness as compared to the earlier reported dyes (Table 5).

Table 5 Comparison of light fastness and sublimation fastness with previous published work
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3.5.2 Percentage exhaustion determination

The % exhaustion of dye bath achieved by collecting sample before and after dyeing. Optical density of each sample were recorded using UV–Vis spectrometer (Perkin Elmer Lambda 25 at λmax) of the corresponding dye to get concentration of dye (g/l). The % exhaustion (E%) of dyes on PET, nylon and acrylic fabrics was calculated using equation [72].

$$ \%\mathrm{Exhaustion}=\frac{\mathrm{Initial}\ \mathrm{OD}_{0}-\mathrm{Final}\ \mathrm{OD}_{1}}{\mathrm{Iniltial}\ \mathrm{OD}_{0}}\times 100 $$
(3)

Where,

OD = optical density.

OD0 = optical density before the start of dyeing.

OD1 = optical density at the end of dyeing.

Table 6 represents percentage exhaustion of dyes. All dyes showed exhaustion better on PET, nylon than acrylic substrate.

Table 6 Percentage exhaustion and fixation of all dyes on PET, nylon, acrylic
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3.5.3 Ultraviolet protection factor

Ultraviolet protection factor (UPF) is the term used to quantify the amount of ultraviolet (UV) protection provided to skin by fabric [73]. Ultra-Protection Factor (UPF), UV-A blocking [400–320 nm] and UV-B blocking [320–280 nm] were tested to check the UV stability of dyed substrate. UVA range has major effects on skin such as wrinkles, drying, discoloration, skin cornification and photo-toxic reactions with drugs and cosmetics [74].

UPF value measured by calculating mean of percentage transmission in UVA and UVB region using the following equation [75].

$$ UPF=\frac{\sum_{\lambda =280}^{400} E\lambda S\lambda \Delta \lambda }{\sum_{\lambda =280}^{400}\mathrm{E}\uplambda\ \mathrm{S}\uplambda\ \mathrm{T}\uplambda \Delta \lambda } $$

Where,

Eλ = relative erythemal spectral effectiveness

Sλ = solar spectral irradiance

Tλ = average spectral transmission of the specimen

Δλ = measured wavelength interval (nm)

Table 7 shows UPF ratings and UVA and UVB blocking values of dyed substrate. A molecules contains benzophenone as core moiety and it known for its photo stabilizer property.

Table 7 UPF rating and UVA/UVB blocking in comparison with blank dyed fabric
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3.6 DFT study

In order to evaluate and correlated experimental observations with theoretical calculations DFT computations were performed. We have evaluated azo-hydrazone tautomerization [76], stable tautomeric forms and correlated photophysical interpretations with the DFT calculations.

3.6.1 Geometry optimization

To know the insight details of the azo and hydrazone forms geometries, computational calculations were performed by using DFT (B3LYP /6-31G(d)). Azo and hydrazone forms optimized geometries at Ground state indicated alteration in bond lengths. Optimized geometries of dye 4b azo-hydrazone forms at ground state in chloroform confirms tautomerization in the dye (Fig. 12).

Fig. 12
figure 12

Optimized geometry of 4b (a) Azo isomer (b) Hydrazone isomer using B3LYP/6-31G(d) level

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Further evaluation of aza-hydrazone tautomerism based on bond distances between the selected atoms of the dyes were done (Table 8). –N=N– bond distances have been increased from 1.2695–1.2715 Å in azo form to 1.29317–1.30214 Å in hydrazone form. –OH bond lengths also increased from 0.99926–1.00099 Å in azo forms to 1.70376–1.72029 Å in hydrazone form. Similarly, –NH bond length get reduced from 1.71397–1.72326 Å of azo forms to 1.03922–1.04188 Å of hydrazone form suggesting clearly the existence of two forms.

Table 8 Bond lengths of between selected atoms of tautomeric forms (4aA–4cA and 4aH–4cH)
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3.6.2 Electronic energies of tautomeric forms

Two divergent peaks of absorption spectra for these dyes indicate two tautomeric forms, so one can study comparative stabilities of the tautomer forms. Electronic energies of optimized azo forms and respective hydrazone forms in chloroform have been matched using B3LYP functional 6-31G(d) basis set. It is clearly noticeable that all the azo forms of dyes are comparatively more stable than their respective hydrazone forms by 4.60–7.06 kJ mol−1 (Table 9).

Table 9 Calculated energies of tautomeric forms at B3LYP/6-31G(d) in CHCl3
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3.7 TD-DFT computations

The optimized ground state geometries of the dyes in various polarities solvents were exposed to TD-DFT with B3LYP functional and 6-31G (d) basis set level. As minimum as ten excited states considered for each dye. Absorption maxima, oscillator strengths, vertical excitations, and corresponding orbital contributions in chloroform were recorded (Table 10). DFT and TD-DFT outcomes advise that there is noticeable impact of donating moieties on the absorption of the dyes. Computationally obtained values for the dyes are in decent agreement with experimental interpretations.

Table 10 Observed UV-visible absorption and computed absorption spectral data at B3LYP/6-31G (d) for all dyes in chloroform
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3.7.1 Frontier Molecular orbitals

Electronic density distributions at highest occupied molecular orbital level (HOMO) and at lowest unoccupied molecular orbital level (LUMO) were assessed through Frontier Molecular Orbitals during excitations. Electron density distribution for azo forms (4aA, 4bA and 4cA) and corresponding methoxy analogues (5a, 5b and 5c) are situated mostly on –N=N– core which is slightly shifted to hydroxyaryl core resulting into less charge transfer and are in good agreement with the spectral data (Fig. 13).

Fig. 13
figure 13

Frontier molecular orbitals diagrams in CHCl3 optimized at B3LYP/ 6-31G(d) level. Optimized structure of 4b at B3LYP/6-31G(d) level (a) Azo form (b) Hydrazone form

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On the other hand, electronic density at HOMO of hydrazone forms (4aH, 4bH and 4cH) is situated mostly on–N=N– core and adjacent phenyl moieties which get transferred to the carbonyl of benzophenone core resulting into more red shift in the photophysical properties.

3.7.2 HOMO–LUMO energy gap

HOMO and LUMO energy band gaps of dyes 4aH, 4bH and 4cH hydrazone are lowered than the corresponding optimized azo forms (4aA, 4bA and 4cA) and methoxy forms (5a, 5b and 5c) supported red shifted absorption maxima. Moreover, energy separation between HOMO and LUMO levels follow the same trends suggested large Stokes shift in the hydrazone-form of dyes in comparison to their corresponding azo analogues.–NO2 substituted dyes (4cA, 4cH and 5c) are having more red shift than –Cl substituted (4bA, 4bH and 5b) dyes than unsubstitued (4aA, 4aH and 5a) dyes (Fig. 14).

Fig. 14
figure 14

HOMO–LUMO energy gap in CHCl3 optimized at B3LYP/6-31G(d) level

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3.8 Electrophilicity index

Stability of dyes were estimated mathematically by means of computationally gathered electronic energies at B3LYP/6-31G (d) level. So, ω (electrophilicity index) was utilized. Parr et al. defined the electrophilicity-index as the measurement of tendency of the group to absorb electrons and is mathematically expressed as [77, 78]:

$$ \upomega =\frac{\upmu^2}{2\upeta} $$
(4)

Where,

ω = electrophilicity-index

μ = chemical-potential

η = chemical-hardness

Chemical-hardness and chemical-potential determined by using the following equations:

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

Where,

IP = ionization potential, (energy change to the system when an electron is get removed)

EA = electron affinity, (energy change to the system when an electron get added)

But then for finding EA and IP, one can determine three energy values (EN, EN − 1 and EN + 1)

Where,

E symbolises for total energy of a system and N symbolises total electron counts.

Above said methods make the whole system time consuming and monotonous. So alternatively values of chemical hardness and chemical potential can be determined by using Koopmans theorem:

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

Where,

ELUMO = energy associated with LUMO

EHOMO = energy associated with HOMO

Also, the Net electrophilicity is expressed as,

$$ {\upomega}^{\pm }={\upomega}^{+}+{\upomega}^{-} $$

Where,

$$ {\omega}^{+}=\frac{{\left({E}_{LUMO}\right)}^2}{2\left({E}_{LUMO}-{E}_{HOMO}\right)} $$
(5)

and

$$ {\omega}^{-}=\frac{{\left({E}_{HOMO}\right)}^2}{2\left({E}_{LUMO}-{E}_{HOMO}\right)} $$
(6)

Hence, among possible tautomeric structure’s, stable confirmation can be predicted by calculating net elerophilicity index (ω±) (Table 11).

Table 11 Eletrophilicity index of azo and hydrazone forms in chloroform
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4 Conclusion

Dyes 4a, 4b and 4c exhibited dual absorption peaks (azo form and hydrazone forms) and exhibited elivated molar-extinction coefficients as related to corresponding methoxy analogues 5a–chaving singal absorption. Azo forms (4aA, 4bA and 4cA) and their methoxy analogues (5a–c) are less sensitive to the viscosity in varying polarity viscous media (glycerol-dimethylsulfoxide, toluene–polyethylene glycol 400 and toluene-paraffin oil). However, hydrazone forms (4aH, 4bH and 4cH) exhibited highly viscosity sensitive emission enhancement of (47–96 fold) in varying polarities viscous mixtures as a combined effect of viscosity and H-bonding. Systematic substitutional variations along with comprehensive photophysical characterization of 4-hydroxy benzophenone and corresponding parallel methoxy analogues (4-methoxy benzophenone) dyes allowed to evaluate the role of H-bonding in the emission enhancement. Forster-Hoffman plots showed viscosity sensitivity (x = 0.448–0.697) for hydrazone forms of the dyes (4aH, 4bH and 4cH). Solvent polarity graphs showed positive solvatochromism for all the dyes. Theoretical calculations by DFT revealed azo-hydrazone tautomerization in 4-hydroxy benzophenone series of dyes. Electrophilicity index calculations and HOMO–LUMO energy levels suggested that hydrazone forms are most stable. TD-DFT, Frontier molecular orbitals and HOMO–LUMO energy gap calculations are in good agreement with the experimentally noticed data.