Combination effect pigments with enhanced fluorescence

Sevinç Sevim Kahya (Kimya Mühendisliği Bölümü, Orta Dogu Teknik Universitesi, Ankara, Turkey)
Yasemin Sönmez (Kimya Mühendisliği Bölümü, Orta Dogu Teknik Universitesi, Ankara, Turkey)
Güngör Gündüz (Kimya Mühendisliği Bölümü, Orta Dogu Teknik Universitesi, Ankara, Turkey)
Bora Mavis (Makina Mühendisliği Bölümü, Hacettepe Universitesi, Ankara, Turkey)

Pigment & Resin Technology

ISSN: 0369-9420

Publication date: 1 July 2019

Abstract

Purpose

The purpose of this paper is to enhance the limited fluorescence of mica titania (MT) effect pigments by coating them with peripherally substituted zinc phthalocyanines (ZnPc).

Design/methodology/approach

The effects of deposition medium, temperature, time, initial organic pigment/MT ratio on deposition behaviour and efficiencies were investigated separately for ZnPc, nitro (TNZnPc) and amino (TAZnPc) substituted ZnPc’s.

Findings

TNZnPc could be deposited in the form of well-defined crystals on MT with per cent 64 ± 5 efficiency in chloroform at 50°C within 5 h and the amount deposited was linearly dependent on the initial TNZnPc concentration. TNZnPc fluorescence emissions, which appear mainly at 460 and 685 nm in pure pigments, could be observed in combination with MT (MT-TNZnPc) as well. A spectral analysis on the non-overlapping region of the spectrum with two different excitations (385 and 630 nm) revealed that, respectively, up to threefold to sixfold increase is possible to attain, depending on the excitation wavelengths.

Research limitations/implications

Efficiency of deposition could not be taken above per cent 11 ± 4 with TAZnPc. Although ZnPc could be deposited with per cent 57 ± 3 efficiency, the pigment was not effective in imposing its fluorescence characteristics over MT.

Originality/value

Combining inorganic effect pigments with organic dye molecules is an idea that has been elaborated mainly for producing different colour effects and stabilization of dye molecules against agglomeration. Here, for the first time, it is used to enhance the fluorescence of the effect pigments.

Keywords

Citation

Kahya, S.S., Sönmez, Y., Gündüz, G. and Mavis, B. (2019), "Combination effect pigments with enhanced fluorescence", Pigment & Resin Technology, Vol. 48 No. 4, pp. 277-292. https://doi.org/10.1108/PRT-07-2018-0069

Publisher

:

Emerald Publishing Limited

Copyright © 2019, Emerald Publishing Limited


Introduction

Pigments that exhibit lustrous, iridescent and angle-dependent optical effects are known as “(special) effect pigments” or “pearlescent pigments”. They find applications in cosmetics, automotive top coatings, optical filters, printing inks and security printing (Lewis, 1988; Pfaff, 2008; Tenório Cavalcante et al., 2007; Pfaff, 2003; Challener, 2016; Topp et al., 2014). They induce optical depth, eye-catching effects, angle-dependent interference colours and pearl and lustre. Angle-dependent optical effects cannot be copied by machines; therefore, they own a kind of inherent security advantage in their applications. They are used on banknotes in many countries.

Effect pigments come out as substrate-free or substrate-based pigments, and among, these titania coated titania-mica pigments (where mica is substrate) finds the widest applications. Titania has a very high refractive index that induces the maximum interference, which, in turn, produces highly intense pearlescence (Gündüz, 2015). The manufacturing procedures and the effects of process parameters were discussed in the literature (Gündüz, 2015; Çağlar, 2004; Topuz et al., 2011; Song et al., 2003; Hildenbrand et al., 1997; Song et al., 2005; Ren et al., 2007; Bayat et al., 2008; Ryu et al., 2008; Damm and Israel, 2007). The introduction of very small amounts of tin dioxide promotes the formation of the rutile phase, which has a higher refractive index than the anatase phase of titania (Topuz et al., 2011).

The thickness of the titania coat induces different colours; and the colour changes in the order of “silver → gold → red → blue → green” with the increase of the thickness of titania. If magnetite is used instead of titania the colour changes as “bronze → copper → red → red-violet → red-green” (Pfaff, 2008). It is also possible to produce more enhanced colours by co-precipitating some colouring agents. Another method to improve colour intensity is to precipitate another coloured layer onto titania. These type of pigments are called combination pigments (Junru et al., 2003; Stafslien et al., 2012; Jing and Hanbing, 2007; Junru et al., 2002; Štengl et al., 2003). Metal oxides such as Cr2O3 (green), CoAl2O4 (blue), NiAl2O4 + Al2O3 (green), CoAl2O4 + Al2O3 (aquamarine blue), TiO2 + FeTiO5 (gold), TiO2 + CoTiO3 (dark green) and TiO2 + NiTiO3 (yellow) can be imparted to produce different colours (Štengl et al., 2003). Combination pigments can be synthesized also for some other special purposes such as forming graphene layer at the top to produce an antistatic effect (Wang et al., 2017a; Wang et al., 2017b; Cramer, 2016; Malm et al., 2014).

It was shown that organic pigments such as copper phthalocyanine can be deposited onto mica-titania (MT) effect pigment to produce blue shade. Copper phthalocyanine tends to crystallize in rod-like geometries on the surface of the titania layer. Another phthalocyanine, tetracarboxamide copper phthalocyanine, which produces green colour, covers the titania surface very smoothly without forming any crystallites. It was observed that both copper phthalocyanine and tetracarboxamide copper phthalocyanine increased the gloss when coated as thin layers (Topuz et al., 2013).

Materials that show fluorescence, absorb waves in Ultraviolet–visible spectroscopy (UV–VIS) region, re-radiate at longer visible wavelengths and this is observed either as colour or intensification of colour. By changing the luminescence properties with a fluorescent pigment, a certain market appeal can be imparted to a coated product (Bianchi, 2016; Gündüz, 2015). The objective of this current research is to produce combination effect pigments, which exhibit fluorescence. There is no such a work done in the literature to our knowledge, though the effect pigments of different kinds are in use (Sekar, 2013). As phthalocyanines can be successfully adhered on the surface of MT substrates (Topuz et al., 2013), zinc phthalocyanines (ZnPc), which exhibit fluorescence (Zhang et al., 2014; Nas et al., 2012; Yaşa et al., 2012; Tuncer et al., 2017; Demirbaş et al., 2017; Safari et al., 2004; Yenilmez et al., 2013; Shaabani et al., 2007; Gurol et al., 2007) were used in the experiments. The experiments were carried using three different agents, these are:

  1. ZnPc;

  2. tetra nitro zinc phthalocyanine (TNZnPc); and

  3. tetra amino zinc phthalocyanine (TAZnPc).

Peripheral tetra substitutions change the absorption spectrum of phthalocyanines. Nitro group withdraw electrons, and thus, the electronic configuration of the phthalocyanine ring is affected (Kobayashi et al., 2003). On the other hand, amine group donates electrons, and it affects the phthalocyanine ring in the opposite way than that of the nitro group. For instance, the peripheral amino groups of nickel (II) tetra-amino phthalocyanine increases the electrical conductivity. Amino groups may increase the interaction between the 3d orbitals with the p-orbitals of phthalocyanine units facilitating the charge transfer (Achar and Lokesh, 2004). The adhesion of tetra substituted ZnPc’s to the surface of the titania layer will also be affected accordingly. Therefore, the use of TNZnPc and TAZnPc besides ZnPc will be of interest in the synthesis of fluorescent MT pigments.

Experimental

Synthesis of zinc phthalocyanine pigments

The use of microwaves in the synthesis of fluorescent dyes decreases the time of reaction by transferring energy directly to the reactive species (Topuz et al., 2013; Elgemeie and Masoud, 2016). Previously a slightly modified version of the microwave based procedure developed by Shaabani et al. (2007) was reported. In this study, the synthesis of ZnPc was carried out according to this modified approach, which was applied in the synthesis of copper phthalocyanine (Topuz et al., 2013). Here, the starting chemicals were kept same with the exception that instead of copper chloride, zinc chloride was used. In a typical experiment, urea (184 mmol) and phthalic anhydride (36 mmol) (i.e. ∼5:1 mole ratio) were ground in an agate mortar for 10 min. Then, zinc chloride (15 mmol) and ammonium molybdate (0.13 mmol) were added and another grinding step was applied for 10 min. The mixture was wetted with 5 mL distilled water in a flask and heated in a microwave oven for 3 min at 600 W. For purification, an initial treatment in hot water (200 mL – 70°C) was followed by sequential washing steps with 6 M HCl (200 mL – 40°C), 1 M NaOH (200 mL – 20°C), hot water (200 mL – 70°C) and ethanol (100 mL – 20°C), which were finalized by filtration as in (Topuz et al., 2013). The products and their purity were confirmed by comparison of their fourier-transform infrared spectroscopy (FTIR) and UV–VIS spectra with the ones given in the literature (Gladkov et al., 2001; Gurol et al., 2007; Kobayashi et al., 2003; Murray et al., 2010; Shaabani et al., 2007; Tackley et al., 2000; Zhang et al., 2009). C32H16N8Zn: FTIR (KBr): ν (cm−1) 730, 746, 773, 835, 953, 1,022, 1,062, 1,114, 1,160, 1,175, 1,257, 1,276, 1,327, 1,412, 1,473 and 1,603. UV–VIS dimethyl formamide (DMF): λmax (nm) 670, 605, 515 and 383.

The above recipe was essentially kept same for the synthesis of TNZnPc, except 3-nitro phthalic acid (36 mmol) was used instead of phthalic anhydride (Safari et al., 2004; Shaabani et al., 2003). For purification, the wet cake was first dissolved in DMF (50 mL), and then the solution was added dropwise to 100 mL ethyl acetate at 60°C to precipitate the pigment (Alzeer et al., 2009). Once this mixture cooled down to room temperature, the filtered pigment particles were dried in an oven at 120°C. TAZnPc was obtained by reducing TNZnPc in water (100 mL) using sodium sulphide as reported in (Achar and Lokesh, 2004; Zhang et al., 2009). The reduction reaction was carried out at 65°C for 4 h and followed up by the purification steps with HCl, NaOH and water as in the synthesis of ZnPc. The FTIR and UV–VIS spectra taken from the products were in good agreement with the literature (Zhang et al., 2009). TNZnPc: FTIR (KBr): ν (NO2) (cm−1) 1350, 1540. UV–VIS (DMF): λmax (nm) 670, 605 and 340. TAZnPc: FTIR (KBr): ν (NH2) (cm−1) 3,330 and 3,420. UV–VIS (DMF): λmax (nm) 765 and 336.

All chemicals used were supplied from Merck.

Combination of mica-titania and zinc phthalocyanine pigments

To produce the combination mica-titania zinc phthalocyanine (MT-ZnPc) pigment, ZnPc was dispersed in DMF and titania coated mica pigment produced by the previously reported method (Topuz et al., 2011) was added into the solution. Deposition was carried out at different ZnPc/MT weight ratios, temperatures (25°C-120°C), and time (0.5-6 h). The suspension is filtered out and washed with DMF. Washing is repeated until the supernatant liquid became colourless. Finally, the wet cake was dried in an oven at 120°C.

Similar process parameters were used for MT-TNZnPc and also for MT-TAZnPc pigments. The pH was changed in between 1-8 in MT-TNZnPc synthesis and 3-8 in MT-TAZnPc. In pH adjustment, different organic solvents and their mixtures were used. DMF, chloroform, chloroform-acetic acid (AA), chloroform-trifluoro acetic acid (TFA) were used for MT-TNZnPc and DMF, DMF-AA and DMF-TFA were used for MT-TAZnPc. All chemicals were supplied from Merck used without further purification.

Characterization

The characterization of pigments was performed by UV–VIS spectrometer (Cary 5000), FTIR (Thermo Scientific Nicolet 6700), XRD (RIGAKU-D/Max-2200/PC), scanning electron microscope (SEM) (JSM-6400 Electron Microscope (JEOL) and FEI SEM (Quanta 200 FEG), Energy-dispersive X-ray spectroscopy (EDX) (JEOL-NORAN System 6 X-ray Microanalysis System & Semafore Digitizer), XPS (Thermo K-Alpha), CHNS/O elemental analysis (Thermo Scientific Flash 2000 and LECO CHNS-932), fluorescence spectrophotometer (Cary, Eclipse Varian) and colour spectrometer (X-Rite 65). Details of fluorescence experiments and paint formulations used in determining the optical properties of pigments are described in the respective sections.

Results and discussions

Fluorescence of zinc phthalocyanine pigments

The fluorescence spectra of the pigments were taken at different concentrations after dispersion in DMF and are given in Figure 1. DMF alone exhibited no fluorescence. The ZnPc pigment was excited at 340 nm, and the major emission peak appeared at 680 nm. A smaller excitation peak showed up at around 740 nm. The TNZnPc pigment was excited at 385 nm and its emission peaks appeared at 460 and 685 nm. The TAZnPc pigment was excited at 400 nm and its emission peaks showed up at 465 and 800 nm.

The apparent difference in their spectra is that the intensity decreases with concentration for ZnPc while it increases for TNZnPc and TAZnPc. The peaks slightly shift to the right in ZnPc, TNZnPc and TAZnPc with increasing concentration. The change of the maximum intensity (i.e. the intensity of major peaks) with concentration are given in Figure 2 and could be mathematically described by a normal distribution curve in ZnPc (Figure 2a) and by power law curves in TNZnPc and TAZnPc (Figure 2b).

The decrease of intensity with a concentration in ZnPc implies that there is a kind of self-absorption of photons in the medium and the increase of concentration causes higher absorption. In other words, quenching increases with the increase of concentration. The fact that a normal distribution is observed implies that the absorption phenomenon occurs randomly as in Gaussian processes.

It is interesting that in the case of TNZnPc and TAZnPc the intensity increases with concentration and it follows a power law behaviour. The increase of intensity with concentration implies that there is no self-absorption by TNZnPc and TAZnPc molecules. The increase due to power law indicates the nonlinearity in the rate of increase of intensity with concentration. These molecules seem to function very efficiently. Probably, the peripheral polar groups (nitro or amine) affect the electrostatic fields around the molecules. It may influence the internal conversion efficiency so that the increased concentration enhances the intensity of fluorescence nonlinearly. This effect may be interpreted to be a kind of improved persistence in photostability.

Spectral and elemental analysis of combination pigments

Mica-titania-zinc phthalocyanine

As mentioned above in the experimental section the deposition of phthalocyanine pigments onto MT was carried out at different temperatures, times and pigment/MT weight ratios.

In MT-ZnPc case the temperature was changed between 25°C-120°C by keeping the weight ratio at 0.067 w/w (0.04 g of ZnPc per 0.6 g of MT). The deposition was carried out for 1/2 h and could be tracked from the increasing intensities of C-C stretching peak (1,332 cm−1) of isoindole with temperature (Figure SI.1) and was very poor at 25°C and 50°C. Nevertheless, it was improved at 90°C and 120°C. Therefore, the deposition temperature was fixed as 120°C in further experiments as it yields the highest deposition.

The effect of deposition time was investigated at 120°C and three different times, namely, 1/2, 1 and 2 h were used (Figure SI.2). It was found out that thickening of layers at longer times caused desorption from the surface, and the most efficient deposition was accomplished in 1/2 h.

The effect of initial ZnPc concentration in solution on deposition efficiency was checked by producing a third series at 120°C in 1/2 h. The amount of ZnPc deposited was found out by determining the nitrogen content via elemental analysis. In three different deposition solutions weight ratios of ZnPc to MT were adjusted to 0.067, 0.133 and 0.267 by keeping the amount of dispersed MT same at 0.6 g. It was found out that the amount of deposition linearly increased with the increase in initial ZnPc (Figure SI.3). The percentage of deposition from solution was per cent 57 ± 3. In the dried combination pigments, the ZnPc quantities were found to be 3.5, 6.5 and 12.7 per cent, respectively.

Mica-titania-zinc phthalocyanine and mica-titania-tetra amino zinc phthalocyanine

The deposition of MT-TNZnPc could not be accomplished when DMF was used as the dispersing medium. The surface potential of MT did not much match with the surface potential of TNZnPc, which has highly polar peripheral nitro groups. Different chloroform and organic acid combinations were used as alternative dispersing medium; (i) CHCl3 (Chloroform) (pH = 5), (ii) CHCl3 + CH3COOH (acetic acid – AA) (pH = 4) and (iii) CHCl3 + CF3COOH (TAA – TFA) (pH = 2). Deposition could be achieved at 50°C and within 5 h for the cases (i) and (iii). While deposition in (ii) was not observable under similar conditions, the deposition in (iii) was poor and the mixture somehow interacted with the pigment and decolourized the green colour of pigment. The peaks associated with nitro groups of the pigment could be seen at 1,540 and 1,350 cm−1 in the FTIR spectra (Figure SI.4).

TAZnPc could not be well dispersed in DMF. The dispersion was successfully achieved in two acidic conditions, namely, (i) DMF + CH3COOH (pH = 4) and (ii) DMF + CF3COOH (pH = 3). The deposition was accomplished at 120°C in 3 h. The characteristic peaks of TAZnPc could be observed around at 1,600-1,400 cm−1 only in condition (i) (Figure SI.5).

FTIR spectra, which are discussed above were taken from combination pigments that were produced by dissolving 0.01 g organic pigment and dispersing 0.15 g MT in reaction solution. The effect of initial pigment concentration on deposition efficiency was tested in different deposition solutions, in which, weight ratios of TNZnPc or TAZnPc to MT were adjusted to 0.07, 0.13, 0.27 and 0.40 by keeping the amount of dispersed MT same at 0.15 g. The change of the weight percentage of TNZnPc and TAZnPc deposited on MT combination pigments from solutions with different initial organic pigment contents with respect to MT are given in Figure 3.

It is clearly seen that a high deposition yield is achieved when CHCl3 is used as the sole dispersant in MT-TNZnPc production. The percentage of deposition from solution is per cent 64 ± 5 and it is linearly increasing with the initial TNZnPc concentration. The deposition keeps increasing linearly when CHCl3 + TFA mixture is used, but the amount of loading is significantly lower than the CHCl3-only case (per cent 29 ± 10).

The deposition efficiency of TAZnPc first increases slightly with the amount of initial pigment concentration in the solution, but it soon keeps constant when DMF + AA mixture was used. A similar recessing trend is also observed when DMF + TFA mixture is used with even lower deposition efficiencies. The percentage of depositions achieved in DMF + AA and DMF + TFA are per cent 11 ± 4 and per cent 5 ± 4, respectively.

Morphological analysis

The SEM micrographs of uncoated MT flakes and those of coated with ZnPc are shown in Figures 4a and 4b, respectively. The ZnPc crystals appear as tiny grains on the surface (Figure 4b). On the other hand, the ZnPc pigment crystals can be seen both on and among the MT flakes in a sample with 2 h of deposition time in solution (Figure 4c). The FTIR results given above supports the fact that with increasing process time there is a possibility that ZnPc might be desorbing from the surface.

The representative surface morphologies of MT-TNZnPc are given in Figures 5a and 5b, and those of MT-TAZnPc in Figures 5c and 5d. The TNZnPc pigment particles appear somehow in well-defined geometrical shapes. Majority of these crystals are well integrated to the surface, while only some might be physically associated. TAZnPc pigment particles, on the other hand, are rather featureless and their association with the MT surface seems weaker. This shows a certain affinity difference between nitro and amino groups towards oxide surfaces, indications of which also exists in the literature (Pujari et al., 2014).

To be sure about the elemental identity of the particulates observed in SEM micrographs EDX analysis was carried out. The elemental analyses were performed both on the particulates and on the smooth areas. It was found out that zinc content of particulates was about 15 per cent, whereas that of the smooth regions was about 3.7 per cent (Figure SI.6). Due to the nature of EDX analysis, detection of zinc signals from the smooth regions is not surprising. It is either due to molecular level associated pigments on these seemingly smooth regions or random reflections from the observable particulates. In either case, the particulates giving a significantly higher zinc content is a proof that the particulates are zinc containing pigments.

Fluorescence of combination pigments

Mica-titania-zinc phthalocyanine

The dependence of fluorescence property of MT-ZnPc pigment on process parameters, namely, on the deposition temperature and initial pigment concentration in solution was studied. The fluorescence data for MT-ZnPc were collected from samples prepared using KBr pellets. Also, spectra from the uncoated MT was taken as reference. Samples were excited at 370 nm. The change of fluorescence spectrum with process temperature at three different ZnPc/MT ratios are given in Figure 6.

MT alone exhibits a significant fluorescence at 420 nm with a minor 505 nm emission when excited at 370 nm in the solid state. In all spectra collected from MT-ZnPc, these features of mica fluorescence remained the same, with changes being limited to the intensities of these emissions. In other words, the 680 and 740 nm emissions observed in pure ZnPc (Figure 1a) could not be independently observed in MT-ZnPc combination. Nevertheless, the changes in intensities may bear some importance with regards to the effects of processing conditions on the deposition behaviour of pigments.

In Figure 6a, fluorescence intensity exhibited by 50°C sample is higher than that of plain MT, but the ones measured for 90°C and 120°C samples are lower than that of plain MT. This seems to be a perplexing result, but can be explained from an adhesion standpoint. Deposition at low temperature was not effective at 50°C, because, the adhesion is poor at low temperature. Anyway, the elemental analysis shows that a very thin coating exists on the surface. Therefore, the fluorescence of ZnPc adds on to that of MT. At higher temperatures say at 90°C, possibly pigment thicknesses increase because of aggregation that, in turn, causes self-absorption. Signs of aggregation were observed in SEM micrographs. MT and ZnPc are quite incompatible materials, and the nucleation sites of ZnPc are not too many when its concentration is low. Thus, ZnPc particulates would rather aggregate vertically and absorb the photons emitted by MT particles, which causes a decrease in the overall emission. The increase of temperature to 120°C increases the thickness of particulates further and causes more self-absorption. When the ZnPc/MT ratio was increased to 0.133 (Figure 6b), it is likely that more nucleation sites develop on the surface and a better widespread coating is achieved. The adhesion at low temperatures is usually poor, and the thickened aggregates because of higher ZnPc/MT ratio in the solution can desorb at low temperatures such as at 50°C because, as the thickness increases the cohesive forces become larger than adhesive forces, and so, desorption can be facilitated. The same may be true also for 90°C but the increase of nucleation sites can cure some of the adverse effects. However, at 120°C, the surface is possibly covered more smoothly because of the increased number of nucleation sites both because of higher ZnPc/MT ratio and higher temperature. Therefore, the fluorescence is maximized when the deposition is carried at 120°C. Increasing the concentration of ZnPc in the deposition solution further does the two things mentioned above (Figure 6c). At low temperature such as 50°C desorption becomes easy as adhesion is poor at low temperatures and the increased cohesive forces in thickened particulates easily overcome adhesive forces on the surface. At high temperatures such as 120°C, the regions where we have smooth coating becomes quite thick and it possibly absorbs the fluorescence emitted by MT layer.

Mica-titania-tetra nitro zinc phthalocyanine and mica-titania-tetra amino zinc phthalocyanine

Among the two peripherally substituted ZnPc’s, TNZnPc proved to have a significant affinity towards the oxide surfaces compared to TAZnPc. In addition, the TNZnPc crystallites were coated on MT with better integrity. Therefore, further fluorescence studies were carried out only with MT-TNZnPc.

In the case of MT-TNZnPc, the fluorescence spectra were collected from DMF solutions in which different amounts of the same MT-TNZnPc pigment was dispersed. The dispersions were stable against sedimentation for about 20 min after agitation and the measurements were taken at most within 2 min upon agitation while all flakes were in dispersion. The amounts dispersed were same as the ones used in constructing the pure pigment calibrations, namely, 0.1, 0.05, 0.033, 0.025 and 0.0125 mg/mL (i.e. fluorescence spectra given in Figure 1b for pure TN-ZnPc). However, as it is MT-TNZnPc that is being dispersed at this level, the effective TN-ZnPc for which fluorescence measurements should ascribed to is considerably less. As it was stated earlier, the combination pigment from which the fluorescence data were collected is marked in Table SI.I. This is the pigment which was deposited in chloroform with a TNZnPc/MT ratio of 0.27 in the initial solution. The amount of TNZnPc with respect to the whole combination pigment is 14.5 Wt.%. Therefore, it is possible to find the amount of effective TNZnPc pigment that is being carried to the measurement solution on MT by multiplying the dispersed MT-TNZnPc quantities by 0.145.

The pure pigment fluorescence values follow a power law with respect to the pigment concentrations if the maximum intensities at 460 nm are considered (Figure 2b). From Figure 6, it can be seen that MT in KBr gives a considerable amount of fluorescence in the 400-500 nm region. MT alone still gives a certain fluorescence in the stated region also when dispersed in DMF at the two extremes of the concentrations (i.e. 0.1 and 0.0125 mg/mL) and excited at 385 nm. This can be seen in Figure 7.

Also, certain shifts in the maximum intensities of the combination pigments with increases in concentration is observable in the spectra that will be presented shortly. Due to these reasons, it was decided that it would be better to have both the calibration and the fluorescence measurements made from the less disturbed regions of the spectra. 680 nm peak of the TN-ZnPc is a suitable emission for analyses as the MT spectra is smooth around this region. Another noticeable feature in Figure 7 is the second-order transmission through the emission monochromator, which results in a peak at twice the excitation wavelength (i.e. 770 nm) (Lakowicz, 2006). However, as a concentration in measurement vial is increased the right shifts in the 680 nm peak starts to combine partially with this second-order peak. Therefore, three measures were taken to proceed with more precision in fluorescence data analyses:

  • all calibrations were repeated with 680 nm peak;

  • to take into account the wavelength shifts and overlaps, the area underneath the emissions were considered instead of single point intensities. The peak deconvolutions were made by Fityk using Pearson7A function embedded in the software (Wojdyr, 2010). The 770 nm peak was included in the peak fitting process, but excluded from the cumulative area;

  • another set of the calibration was carried out with an excitation at 630 nm (Figure 8) and measurement with MT-TNZnPc pigments were also repeated with this excitation wavelength.

This would not only prevent the second-order peak from coinciding with 680 nm emission but also form the basis of a check on Kasha (1950) rule, which basically states that; if there is a single fluorophore, the emission spectrum would not depend on the excitation wavelength.

The results of peak deconvolutions from 680 nm emission of TNZnPc pigment made after 385 (Figure 1b) and 630 nm (Figure 8) excitations were found to obey a power law, and given in Figure 9. The representation with power law is more accurate for the 630 nm excitation, probably due to the elimination of overlap with the second-order peaks in this case.

The fluorescence spectra collected from DMF solutions with different concentrations of MT-TNZnPc excited at 385 and 630 nm are given in Figures 10a and 10b, respectively.

Two different bands can be identified in Figure 10a, one at ∼475 nm and the other around at 680 nm with slight red shifts with increasing concentration. It is obvious that as the concentration of the combination pigment increased the specific emission character of TNZnPc can be more readily observed in the 400-500 nm range, as opposed to that of ZnPc on MT. However, the 420 nm emission from pure MT flakes still contributes to the low wavelength region of the emissions and prevent a monotonous change with the concentration of pigments. On the other hand, if the 770 nm peak could be integrated into the deconvolution procedure, an area analysis from the 680 nm peak would definitely bring accuracy in fluorescence evaluation. Being free from the second-order peak, the analysis on the same emission from 630 nm excitation (Figure 10b) would be even more representative.

To capture a possible synergistic fluorescence interaction between MT and TNZnPc due to the crystallization of TNZnPc on MT flakes, the analysis was carried out further as follows:

  • emission from plain MT flake at 680 nm was considered zero;

  • effective TNZnPc concentrations being delivered to measurement vial on MT-TNZnPc flakes can be used to calculate the anticipated fluorescence from a physical mixture of TNZnPc and MT by using the calibration equations given in Figure 9 for pure TNZnPc; and

  • actual fluorescence from MT-TNZnPc can be calculated from peak fitting procedures explained above and compared to fluorescence anticipated from the physical mixture of MT and TNZnPc.

The results are summarized in Table SI.II and presented in Figure 11. The actual fluorescence values calculated by analysis from areas underneath the 680 nm peaks (excluding the second-order peak in 385 nm excitation) is significantly higher than those anticipated from the effective pigment concentrations of TNZnPc. The difference being even more striking in the 630 nm excitation (Figures 11a and 11b). The increasing trend in “per cent increase” continues up to effective concentration of 0.0073 and then starts to wane, reaching a maximum of about 220 per cent with 385 nm excitation and 500 per cent with 630 nm excitation. One can achieve a fluorescence value, which is about 90 per cent of the fluorescence that can be obtained from pure TNZnPc (i.e. 0.1 mg/mL in Figure 11), but at a significantly lower effective TNZnPc concentration (around 15 per cent of 0.1 mg/mL). In other words, when TNZnPc is coated on MT surfaces, a synergistic photon interaction between the pigments arise and result in an increase in fluorescence, which is significantly higher than both of the components’ individually anticipated fluorescence. The nitro groups of MT-TNZnPc provide not only better dispersion in solvents and adhesion on oxide surfaces but also produce better florescence effects. A part of incident light that is transmitted through MT flakes, can be reflected back by the opposite surface of the flake. Therefore, the overall reflection from the MT flakes consists of photons that have travelled different paths and have different wavelengths (Tenório Cavalcante et al., 2007). It is very likely that these reflections are easily captured by the TNZnPc that are integral to the MT flake surfaces. In addition, these reflections with variety of wavelengths probably cause excitations in several wavelengths in TNZnPc. As it was confirmed that Kasha’s rule is followed in MT-TNZnPc, the emissions from these different wavelengths would be expected to coherently intensify at singular fluorescence wavelengths that are specific to TNZnPc (like the one that appear at 680 nm). As it was observed more readily in ZnPc, a similar self-absorbance is probably preventing the observation of a steady increase in the fluorescence based on this mechanism, as the combination pigments’ concentrations are increased in the measurement medium.

Optical properties

The overall effect of fluorescence on the colour shift was studied by measuring optical properties. MT-ZnPc was tested in a long oil alkyd resin (60 per cent) (Kar Kimya San. Ltd. Şti.) and the formulation included 89.52 per cent resin, 7.96 per cent white spirit, 0.55 per cent cobalt naphtanate and 2 per cent MT-ZnPc by weight. MT-TNZnPc and MT-TAZnPc were mixed with water based styrene-acrylic resin (50 per cent w/w – Betapol SA-5017B – BETEK Boya San. A.Ş.). White spirit was used as the solvent (10 Wt.%) and first mixed with pigments (2 Wt.%) to aid the dispersion of pigments. Then, the mixture was introduced into styrene-acrylic resin (88 Wt.%). Films were applied on glass plates by a paint applicator (50 µm) and dried at room temperature.

A standard paint was prepared with only MT and other combination pigment samples were measured and compared with this reference. Colour properties of the paints were characterized according to dL*, da*, db* and dE* values of the samples with respect to the reference. Numerical results of measurements are given in Table SI.III and colour properties of the paints based on different combination pigments are given in Figure 12.

With the increasing amount of ZnPc in the MT-ZnPc pigments, the yellow and green shifts are observed in all cases (Figure 12a). Deposition of ZnPc at 120°C led to the highest difference in the total colour. MT-TNZnPc and MT-TAZnPc-based samples appear mostly in the green and yellow area except for the MT-TNZnPc produced in CH3Cl+TFA mixture where there is a shift to blue and green area. MT-TAZnPc produced both in DMF+TFA and DMF+AA mixtures shifts to the green–yellow area, and with the increases in the initial concentration of pigment a red shift is added on. While MT-TNZnPc from CH3Cl solutions display colours in blue, green and yellow regions, the fluorescence measurements were taken from the yellow shifted one.

Conclusions

Organic dye molecules which show fluorescence can be deposited on MT surfaces with different efficiencies depending on the type of peripheral substitutions, choice of reaction medium, deposition temperature and time. As, only reactants are MT and the dye molecules, the optimization of these reaction parameters forms the basis of a simple one-pot deposition method. The strong interaction of nitro groups with the oxide surfaces leads to the highest deposition efficiencies and improves the integrity between the pigment crystals and MT. This intimate combination of MT and pigments with well-defined geometries synergistically increases the fluorescence to values higher than those would be anticipated from the physical mixtures of the ingredients. Depending on the excitation wavelength, these increases can reach up to 220-500 per cent.

Figures

The fluorescence spectrum of

Figure 1

The fluorescence spectrum of

The change of maximum peak intensities with concentration

Figure 2

The change of maximum peak intensities with concentration

The change of weight per cent TNZnPc or per cent TAZnPc deposited on MT combination pigments from solutions with different initial organic pigment contents with respect to MT. Deposited amounts were determined from elemental nitrogen analysis on dried combination pigments. Data are also given in a tabular from in Table SI.I and the combination pigment from which the fluorescence data were collected is marked

Figure 3

The change of weight per cent TNZnPc or per cent TAZnPc deposited on MT combination pigments from solutions with different initial organic pigment contents with respect to MT. Deposited amounts were determined from elemental nitrogen analysis on dried combination pigments. Data are also given in a tabular from in Table SI.I and the combination pigment from which the fluorescence data were collected is marked

SEM micrographs of

Figure 4

SEM micrographs of

MT-TNZnPc flakes (50°C, 5 h, CHCl3)

Figure 5

MT-TNZnPc flakes (50°C, 5 h, CHCl3)

Fluorescence spectra of MT-ZnPc samples produced at different temperatures with initial ZnPc/MT ratios of

Figure 6

Fluorescence spectra of MT-ZnPc samples produced at different temperatures with initial ZnPc/MT ratios of

Fluorescence spectra of MT dispersed in DMF with two different concentrations (0.1 and 0.0125 mg/mL) and excited at 385 nm

Figure 7

Fluorescence spectra of MT dispersed in DMF with two different concentrations (0.1 and 0.0125 mg/mL) and excited at 385 nm

The fluorescence spectra of pure TNZnPc when excited at 630 nm

Figure 8

The fluorescence spectra of pure TNZnPc when excited at 630 nm

The change of area underneath the 680 nm emission peak with TNZnPc pigment concentration at two excitations (385 and 630 nm). The fit equations are given as figure insets

Figure 9

The change of area underneath the 680 nm emission peak with TNZnPc pigment concentration at two excitations (385 and 630 nm). The fit equations are given as figure insets

Fluorescence spectra of MT-TNZnPc with different concentrations in DMF excited at

Figure 10

Fluorescence spectra of MT-TNZnPc with different concentrations in DMF excited at

The anticipated fluorescence from equivalent concentrations of TNZnPc and actual fluorescence from MT-TNZnPc as a function of effective TNZnPc in measurement solution

Figure 11

The anticipated fluorescence from equivalent concentrations of TNZnPc and actual fluorescence from MT-TNZnPc as a function of effective TNZnPc in measurement solution

Color properties of the paints based on different combination pigments and processing conditions (temperature or solvent and initial pigment/MT ratio)

Figure 12

Color properties of the paints based on different combination pigments and processing conditions (temperature or solvent and initial pigment/MT ratio)

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Acknowledgements

Authors would like to thank; Professor Dr Ahmet Önal for his invaluable advice and help in fluorescence measurements of TNZnPc, Professor Dr Vefa Ahsen for his advices on phthalocyanine purification, and Kar Kimya San. Ltd. Şti. and Betek Boya San. A.Ş. for providing the resins of some paints produced in these studies. Authors also acknowledge the partial financial supports provided by Orta Doğu Teknik Üniversitesi (Grant No’s: BAP-07-02.2009.00.01, BAP-2011-03-04-10).

Erratum: It has been brought to the attention of the publisher that the article Sevinc Sevim Kahya, Yasemin Sonmez, Gungor Gunduz and Bora Mavis, “Combination effect pigments with enhanced fluorescence”, published in Pigment and Resin Technology, failed to include an acknowledgment and the appendix containing supplementary data. The publisher apologises for any confusion and inconvenience caused.

Corresponding author

Bora Mavis can be contacted at: bmavis@hacettepe.edu.tr