spectrophotometry of dynamic colorants

Viková, M., Vik, M. SPECTROPHOTOMETRY OF DYNAMIC COLORANTS

SPECTROPHOTOMETRY OF DYNAMIC COLORANTS Viková, M., Vik, M. Technical University of Liberec, Liberec, CZECH REPUBLIC [email protected]

Abstract Dynamic colours are sensitive on different stimuli such as UV-VIS radiation, temperature, humidity, etc. Potential application of these systems in industry, fashion and other branches brings problem of its measurement for purpose of control their quality. For example photochromic colours need to be exposed by external light source during measurement. Based on that the special device was developed allowing such kind of measurement This unique device allows the measure colorimetric and spectral characteristics of colour changeable materials as photochromic sensors and also the fatigue test for the control of colour change stability. This measurement of colorimetric and spectral parameters in comparison together with intensity of UV irradiation allows finding the dependence of colour change on intensity of irradiation and the development the scale for individual visual observation and to evaluate dangerousness of UV irradiation. In this article is described possibility to use different light sources (Xe 450 W and LED 395-410) for this kind of measurement and also there influence of selected light sources on photochromic effect is discussed. Is evident, there is a problem for future standardization of colour changeable materials measurement. Keywords: e.g. Colour changeable materials, colorimetry, fatigue resistance, light source

1 Introduction There is considerable interest in photochromic materials arising from the many potential applications, which are associated with their ability to undergo reversible, light-induced colour change. There are many conformational changes that can take place in the excitation process, which lead to changes in electronic absorption spectra, resulting in a visible colour change. If the changes are thermally reversible, after removal of the irradiation, which activates the changes, the system returns to the state before irradiation and the induced absorption or colour spontaneously disappears. This was previously referred to as phototropism (Brown, G.H. 1971), and now more correctly as photochromism. Two chemical species showing a reversible transformation differ from one another not only in their absorption spectra but also in their physical and chemical properties. Photochromic materials are used most widely in ophthalmic sun-screening applications, and also find applications in security printing, optical recording and switching, solar energy storage, nonlinear optics and biological systems (Crano & Guglielmetti 1999). The existing ranges of commercial products generally undergo positive photochromism, a light-induced transition from colourless to coloured due to a ring-opening reaction. Simply, the photochromic processes may be described as follows: Colourless

Coloured

The determination of the photochromic parameters, such as the number, nature, and kinetic and spectral properties of the transient species formed under irradiation is not a trivial task because the photoproducts are too labile to be isolated in many cases.

1.1 Shade Intensity description Considering that significant current and future research is directed towards the development of Smart sensors with photochromic pigments, which can react under UV irradiation, it is necessary to provide an objective description of visual colour change for purposes of their calibration. The Kubelka – Munk function bases one possible solution on spectrophotometric description of colour appearance (McDonald 1997). In the case of photochromic pigments we

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can take into consideration the changes of spectral characteristic before and after illumination and they may be expressed by integral equation (1): (1) where I K/S λ

is the shade intensity; is the Kubelku-Munk function; is the wavelength.

In practice it is obtained by integration using equation (1) expressed by the sum and the equation going to form (2): (2) Δλ depends on the band pass of spectrophotometer. In the case of photochromic substances I depend on both time and intensity of illumination respectively.

1.2 Kinetic description Kinetics of many chemical and physical processes is well described by the first-order equations, according to which the conversion rate is proportional to the number (concentration) of unreacted species. Thus the momentary concentration of the reactant (n R (t)) should follow the equation (Maafi 2008): (3) Where (n R (0)) is the initial concentration and k stands for the rate constant. In principle, the kinetics of a photo induced to switch from the colourless to the coloured form can be deduced from monitoring one of the absorption bands characteristic. To that purpose the absorbance A= -log (T) is determined from measurements of the transmission T = I (d) /I (0) of sample thickness d. Here I (0) is the light intensity measured at the input face (z = 0) of the sample and I (d) is the transmitted light intensity measured behind its output face (z = d). For ‘standard’ first-order kinetics, the following equation is fulfilled: (4) -1

Here the time-constant (t) is defined as t = k R , A(0), A(t) and A(∞) are the initial, momentary and final values of the absorbance respectively. Sometime it is difficult to determine A(∞), especially in the case of long time relaxation processes where an instability in the measurement could play an essential role and in the case of some UV degradation of the material. Equation (4) is frequently written in following form: (5) For translucent media it is possible replace absorption A by Kubelka-Munk function K/S. Viková and Vik (Viková & Vik 2007) showed that also the equations (1), resp. (2) are suitable for calculation of some kinetic data that means Colour Intensity I is possible to be used as absorption A or Kubelka-Munk function K/S:

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(6) From these equations it is possible to calculate halftime of colour change t 1/2 : (6) Fig. 1 shows experimental data incorporating a first order kinetic model according to equation (6) for the exposure and for the reversion. It is evident that the proposed model functions fit the experimental data well.

Figure 1 – Typical growth and decay processes of shade intensity for a sample of pigment P1 in concentration 3g/30g UV-A irradiance = 714,6 µW.cm-2 power (979,3 lx) In case of photochromic effect measurement the big attention is given to exciting light sources and their spectral power distribution. There are many simulator of daylight and some are usable for photochromic effect excitation. The intensity of UV amount in spectrum is responsible for creation of colour effect and also shade intensity. Therefore this paper is directed to the effect of used light sources, in this case to the comparison of Xe 450 W and LED 395 - 410.

2 Materials and methods Main task of our research work is to develop a simple textile sensor, which is sensitive to UV light and kinetic study of behaviour and main attention was given to possibility use the technology of textile printing by screen-printing. For experiment were used five commercial photochromic pigment Photopia Blue from Matsui and five pigments P1- P5 from PPG. Typical problem with measurement of kinetic behaviour of photochromic pigments by commercial spectrophotometer is relative long time period between individual measurements (cca 5s) and impossibility of measurement whole colour change during exposure without interruption of illumination of sample during measurement. That means classical commercial spectrophotometers enable off-line measurement of kinetic behaviour during exposure period and quasi on-line measurement during reversion period. The difference of time delay between exposure and real measurement of photochromic change affects on validity of measured data. The types of this measurement are divided according to the use of device, equipment or measured object and according to the object’s interaction with light. Usual measurement is in cuvette, which allows either lateral or axial illumination of sample - see on the Figure 2a and Figure 2b.

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Figure 2a – Lateral exposure

Figure 2b – Axial exposure

The photomultiplier tube (PMT) monitors the intensity of the monochromatic through sample (S). A flash from the strobe source is focused by lenses (L) and the neutral density filter (F) before it falls on the sample. The sample is held in block, through which water of given temperature circulates. The temperature of measured directly with a thermocouple (not shown).

light passing attenuated by an aluminium the sample is

If is fulfil condition that sample in cuvette is not turbid medium, we can adapt abovementioned method the analytical spectrophotometer. The speed of sensor is only one limitation (usually photo multiplayer). This system allows to study the photochromic systems, theirs halftime of colour change is longer than 50 ms. If we want to study systems with faster photochromic changes it is necessary to use the other type of measurement for example by time resolved method via femtosecond laser using pumped probe technique (Cojocariu & Rochon 2004). The light source for exposure can be for example laser, discharge lamp, LED or the other light source. We can study also spectral sensitivity of the sample system in case of including the irradiation monochromator or system of band pass filters. If we need to measure the colour surfaces, the basic problem of photochromic system measurement becomes the controlled exposure by the selected irradiation. As follow the above-mentioned description of basic systems of illumination and observation for colorimetric devices, these systems are not usually designed for the addition of other light source. In case of the analytical spectrophotometer it is possible to use simple adaptation of Praying-Mantis accessory (Wehrle & Limbach 1989) that is presented in Figure 3. This system allows with the adding of light source for irradiation the measurement of photochromic surfaces. Disadvantage of this system is reality that only small area is measured and we can use this configuration only for homogenous smooth surfaces.

Figure 3 – Modification of Praying-Mantis optical accessory for measurement photochromic property In case of textured surfaces it is necessary to use adaptation of hemispherical illumination in integrating sphere so that we include other aperture for irradiation (Vik & Vikova 2007). This system allows the study of colour photochromic kinetic, the influence of exposure time, and

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the thermal sensitivity and in case of including of excitation monochromator spectral sensitivity (Vikova 2010) as shown on Figures. 4 and 5.

Figure 4 – Optical scheme off LCAM PHOTOCHROM measuring system Dual light source construction of spectrophotometer with shutter over exciting light source makes possible continuous measurement of photochromic colour change during reversion after switching off exciting light source. Obviously Xenon discharge lamp with continuous discharge is used. Pulsed discharge lamp allows using such system as fatigue tester for photochromic systems with halftime of photochromic colour change around 500 ms. For textile samples, which are frequently slowest, it is obvious to use electronic shutter and continuous discharge lamp for fatigue tests.

Figure 5 – Photochrom measuring system with LED 395-410 nm, which was developed at the Laboratory of Colour and Appearance Measurement (LCAM), Faculty of Textile Engineering, Technical University of Liberec

3 Results and discussion In previous presented articles (Viková, M. & Vik, M. 2007, Viková, M., Vik, M. 2014, Viková et all. 2014) we have used polychromatic illumination, i.e., for excitation illumination from 200400 nm. For spectral sensitivity tests, a special arrangement of the measuring system developed was used, which allowed selection of variable bandwidth and dominant wavelength. Therefore addition of excitation monochromator as shown in Figure 4 allows measurement of spectral sensitivity of photochromic materials.

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It is well known that shade depth of photochromic colour change is dependent also on irradiance. This is a major problem, because most light sources are not spectrally equienergy. The solution to this problem is based on a diaphragm. The diaphragm is placed in the light path of an excitation beam, and the size of the aperture regulates the amount of light. The centre of the diaphragm's aperture coincides with the optical axis of the optical irradiation system of the measured sample. By controlling the intensity of transmitted radiation, it was possible to obtain similar energy for each selected band pass of irradiation. Figure 6 presents the spectral distribution of irradiation, which was used for the spectral sensitivity experiment. The spectral characteristic of irradiance was measured by an Avantes USB2000 spectrometer after calibration by an AvaLight-DH-CAL light source.

Absolute Irradiance [µWatt/cm2/nm]

20 18 16 14 12 10 8 6 4 2 0 200

250

300

350

400

450

Wavelength [nm] M- f350

M- f375

M- f400

M- f325

M- f275

M- f300

Figure 6 – Spectral power distribution in selected band pass of irradiation For every exciting band pass a record of on-off kinetics of the photochromic colour change was made. The following graphs in figure 7 presents records of absolute values of the Kubelka Munk function for average energy of irradiation 800 µW.cm-2 for selected pigment P1 from PPG (3,3,5,6-tetramethyl-1-propylspiro [indoline-2,3'[3H] pyrido [3,2-f][1,4]benzoxazine]).

Figure 7 – Spectral sensitivity during on/off kinetics of P1 pigment PPG– concentration 1g/30g Figure 8 documents that except for pigment P5(3,3-diphenyl-3H-naphtho[2,1-b]pyran), the pigments have their main sensitivity at 375 nm. Both spiroindolinonaphthopyran structures with the same orientation of the naphthopyran system (pigment P2: methyl2,2,6-tris(4methoxyphenyl)-9-methoxy-2H-naphtho-[1,2-b]pyran-5-carboxylate and pigment P3: methyl

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2,2-bis(4-methoxyphenyl)-6-acetoxy-2H-naphtho-[1,2-b]pyran-5-carboxylate) narrowest bandwidth in comparison to spironaphthooxazines (pigment tetramethyl-1propylspiro[indoline2,3'[3H]pyrido[3,2f][1,4]benzoxazine] and pentamethyl(indoline-2,3’-[3H] naphtho [2,1-b][1,4]oxazine])).

show the P1: 3,3,5,6P4:1,3,3,5,6-

Figure 8 – Dependence of relative sensitivity on wavelength for pigment P1 – P5 from -2 PPG, average intensity of illumination = 900 µW.cm Only pigment P5: 3,3-diphenyl-3H-naphtho [2,1-b]pyran, with the opposite orientation of the naphthopyran structure, has a maximum of sensitivity shifted to lower wavelengths (to 350 nm) and a flat shape of the sensitivity curve. From these results we can see sensitivity of different photochromic substances and their colour reaction on different wavelengths. Also is evident from graphs on Figures 9 and 10 different results for used light sources Xe 450 W and LED 395 – 400nm. LED 395 – 410 nm light source evocates higher colouration effect than Xe 450 W light source. The K/S values are for LED light source approximately 3 times higher.

Figure 9 – Dependence of K/S value on drawing ratio – excitation Xe 450W with monochromator. Dominant wavelength 365 nm

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Figure 10 – Dependence of K/S value on drawing ratio – excitation UV LED. Dominant wavelength 400 nm For better understanding we can see on fig. 11, where we presenting spectral power distribution for both used light sources. It is visible, that ratio in absolute irradiance between light sources xenon with monochromator and LED is approximately 1:70, nevertheless due to spectral sensitivity of tested samples is effectiveness of xenon much higher and resulting ratio is decreased into 1:3.

Figure 11 – Spectral power distribution of tested light sources at level of measured photochromic samples

4 Conclusion We can from presented results see that effect of coloration depends on spectral power distribution and intensity of UV irradiation from used exciting light source. Many photochromic substances are sensitive on dominant wavelength in spectrum evocating coloration (selective absorption). Also time of excitation and relaxation of photochromic effect is important and plays a role in coloration of used photochromic dyestuff. The time depends on chemical structure of dyestuff and composition of applied medium. W e also presented in this article that the used light sources evokes the different response in measured reflectance spectrum of colour changeable materials. Due these reasons is necessary to standardise such kind of measurements and we could prepare common standards for the methodology of measurement

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including used light source for the excitation of photochromic colour effect, which improved traceability of measured data.

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