modeling colorimetric characteristics of on–off behavior of photochromic dyes based on...

Accepted Manuscript

Modeling Colorimetric Characteristics of ON-OFF Behavior of Photochromic

Dyes based on Bis-Azospiropyrans

Saeideh Gorji Kandi, Farahnaz Nourmohammadian

PII: S0022-2860(13)00656-X

DOI: http://dx.doi.org/10.1016/j.molstruc.2013.07.040

Reference: MOLSTR 19915

To appear in: Journal of Molecular Structure

Received Date: 25 June 2013

Revised Date: 21 July 2013

Accepted Date: 22 July 2013

Please cite this article as: S.G. Kandi, F. Nourmohammadian, Modeling Colorimetric Characteristics of ON-OFF

Behavior of Photochromic Dyes based on Bis-Azospiropyrans, Journal of Molecular Structure (2013), doi: http://

dx.doi.org/10.1016/j.molstruc.2013.07.040

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http://dx.doi.org/http://dx.doi.org/10.1016/j.molstruc.2013.07.040

1

Modeling Colorimetric Characteristics of ON-OFF Behavior of

Photochromic Dyes based on Bis-Azospiropyrans

Saeideh Gorji Kandi1*, Farahnaz Nourmohammadian2,3

1 Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box: 15875-441

2Department of Organic Colorants, Institute for Color Science and Technology P.O. Box 16765-654, Tehran, Iran; 3Center of Excellence for Color Science and Technology, Tehran, Iran

*Corresponding Author:

Email: [email protected]

Tel: +98 21 64542431

2

Abstract

In the present study, colorimetric characteristics of six photochromic bis-azospiropyrans based

dyes are investigated. The colorimetric axes of these dyes in terms of hue, chroma and lightness

are considered via UV exposure time. Results showed that, the hue angle of these dyes changes

rapidly at the first times of UV irradiation, and then remains almost constant. However, chroma

and lightness continuously change till about 80s to 240s for different dyes. In addition, it is

shown that variations of lightness and chroma by UV exposure time have an exponential trend,

which can be modeled mathematically.

Keywords: Colorimetric properties, Photochromic Dyes, bis-azospiropyrans, UV exposure time,

Transition spectra, CIELAB color space

1. Introduction

Molecular-level devices with controllable photoresponse molecules and signal transduction

capability are of great scientific interest and intense activity in the fields of chemistry as sensors

[1-5], biology [6], and photo-switching [7-9]. Photoresponse systems, with controllable light

induced color, have a vast variety of applications due to their potential use in high level

technologies such as data recording, optical storage, optical switching, displays, and non-linear

optics [10-13]. In these molecules, spiropyrans are an important class with interesting unique

molecular binding abilities and signal transduction functions [8, 9, 14]. Fundamentally, these

molecules are of significant interest since they are the key compounds for time-resolved studies

in organic photochemistry [7]. The exploitation of such characteristics in these molecules has led

to the development of molecular switches in material chemistry [8].

In our previous study [15], novel photochemically bifunctional compounds were synthesized

based on symmetric bis-azospiropyrans, in which two spiropyrans are linked by a bis-azo

extended aromatic system with great properties such as stability, and activity. Astonishingly,

they produce more color strength (large molar absorption coefficient in mero form up to 2.3-3.8

× 104 M-1.cm-1, which means high sensitivity to the light and more color intensity in their mero

3

form). In practice, it means that we have novel compounds for photoswitches that possess

improved light sensitivity and superior discrimination power between spiro (OFF) and mero

(ON) forms.

These results persuade us to find a model that mathematically fits the colorimetric data in terms

of hue, Chroma and lightness for these dyes during UV exposure time. Chroma by UV exposure

time has an exponential trend, which can be modeled mathematically.

2. Experimental

The colorimetric properties of the six photochromic dyes synthesized in the previous work [15]

are discussed in this paper. The changes of color characteristics via exposing to UV light are

investigated. There is also an attempt to find ways to model it mathematically.

2.1 Samples

Chemical structure of the six synthesized dyes named A, B, C, D, E and F are shown in Figure 1.

4

N

N

ArN

NN

N

R

R

O

O

CH3

CH3CH3

CH3

CH3

CH3

4 (A-F)

Compound R Ar

A H

B H

C H

D N,N-Diethylamino

E N,N -Diethylamino

F N,N -Diethylamino

Figure 1: The chemical structure of the six synthesized photochemically bifunctional dyes

The structures of bis-azospiropyrans A-F were deduced from their MS, FT-IR, and1H-NMR spectroscopic data and CHN analysis, and are completely accordance to our latest results [15] as mentioned at below.

3,3’-[benzene-1,4-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethylspiro(2H-1-benzopyran-2,2'-indoline)-6-yl] (A) Yield (89%). White powder. M.p. 203-205˚. IR (KBr): 2966 ( CH= ), 1605 ( C(3)=C(4) ), 1311 ( C(2)-N ), - 1021 ( C(2)-O ). 1H-NMR (DMSO-d6 ): 1.25 ( s, 2 Me ); 1.28 ( s, 2 Me ); 2.80 ( s, 2 NMe ); 4.20 ( d, 3JHH =10.2, 2 H-C(3) ); 6.60-7.26 ( m, 18 arom.H, and 2 H-C(4) ). MS m/z (%) = 685 (M+1, 1), 655 (M- 2 Me, 1), 493 (15), 404 (2), 318 (88), 274 (25), 212 (100), 183 (18). Anal.calc. for C44H40N6O2 ( 684.84 ): C 77.17, H 5.89, N 12.27; found: C 77.20, H 5.85, N 12.30.

5

3,3’-[benzene-1,3-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethylspiro(2H-1-benzopyran-2,2'-indoline)-6-yl] (B). Yield (80%). White powder. M.p. 182-184˚.IR (KBr): 2966 ( CH= ), 1606 ( C(3)=C(4) ), 1313 ( C(2)-N ), 1020 ( C(2)-O ). 1H-NMR ( CDCl 3): 1.35 ( s, 2 Me ); 1.38 ( s, 2 Me ); 2.90 ( s, 2 NMe ); 4.27 ( d, 3JHH =10.2, 2 H-C(3) ); 6.57-7.37 ( m, 18 arom.H, and 2 H-C(4) ). MS m/z (%) = 685 (M+1, 1), 655 (M- 2 Me, 1), 493 (18), 404 (3), 318 (85), 274 (22), 212 (100), 183 (20). Anal. calc. for C44H40N6O2 ( 684.84 ): C 77.17, H 5.89, N 12.27; found: C, 77.11, H 5.93, N 12.24.

3,3’-[naphthalene-1,5-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethylspiro(2H-1-benzopyran-2,2'-indoline)-6-yl](C). Yield (85%). White powder. M.p. 264-267 ˚. IR (KBr): 2966 ( CH= ), 1606 ( C(3)=C(4) ), 1310 ( C(2)-N ), 1020 ( C(2)-O ). 1H-NMR ( CDCl3 ): 1.35 ( s, 2 Me ); 1.42 ( s, 2 Me ); 2.89 ( s, 2 NMe ); 4.26 ( d, 3JHH = 10.2, 2 H-C(3) ); 6.57-7.36 ( m, 20 arom.H, and 2 H-C(4) ). MS m/z (%) = 735 (M+1, 1), 705 (M- 2 Me, 1), 543 (17), 454 (2), 368 (84), 274 (30), 262 (100), 233 (22). Anal. calc. for C48H42N6O2 ( 734.90 ): C 78.45, H 5.76, N 11.43; found: C 78.48, H 5.71, N 11.46.

3,3’-[benzene-1,4-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethyl-6-diethylamino spiro(2H-1-benzopyran-2,2'-indoline)-6-yl] (D). yield (60%); White powder. m.p. 216-218 ˚C. IR ( KBr ): 2966 ( CH= ); 1605 ( C(3)=C(4) ); 1313 ( C(2)-N ), 1236, 1020 ( C(2)-O ). 1H NMR ( 500 MHz, CDCl3 ): δH

1.13 ( 6H, t, 3JHH 6.97 Hz, 2 Me ) 1.38 ( 6H, S, 2 Me ); 1.43 ( 6H, S, 2 Me ); 2.92 ( 6H, S, 2 N Me ); 3.28 ( 4H, q, 3JHH 6.97 Hz, 2 NCH2 ); 4.3 ( 2H, d, 3JHH 10.1 Hz, 2 H-C(3)); 6.37-7.28 (m, 16 arom.H, and 2 H-C(4)). m/z (EI) (%): 827 (M+1, 1), 797 (M- 2 CH3, 1), 493 (16), 404 (2), 318 (79), 274 (28), 212 (100), 183 (20). Anal. Calcd for C52H58N8O2 ( 826 ): C, 75.54; H, 7.02; N, 13.56%. Found C, 75.58; H, 7.05; N, 13.53%.

3,3’-[benzene-1,3-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethyl-6-diethylamino spiro(2H-1-benzopyran-2,2'-indoline)-6-yl] (E). Yield (68%). Greenish White powder. M.p. 178-180˚. IR (KBr): 2968 ( CH= ), 1607 ( C(3)=C(4) ), 1310 ( C(2)-N ), 1022 ( C(2)-O ). 1H-NMR ( CDCl3 ): 1.15 ( t, 3JHH = 6.9, 2 Me ); 1.32 ( s, 2 Me ); 1.39 ( s, 2 Me ); 2.90 ( s, 2 NMe ); 3.25 ( q, 3JHH = 6.9, 2 NCH2 ); 4.29 ( d, 3JHH = 10.3, 2 H-C(3) ); 6.29-7.28 ( m, 16 arom.H, and 2 H-C(4) ). MS m/z (%) = 827 (M+1, 1), 797 (M- 2 Me, 1), 493 (16), 404 (2), 318 (84), 274 (22), 212 (100), 183 (17). Anal. calc. for C52H58N8O2 ( 827.09 ): C 75.51, H 7.07, N 13.55; found: C 75. 55, H 7.04, N 13.60.

3,3’-[naphthalene-1,5-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethyl-6-diethyl aminospiro(2H-1-benzopyran-2,2'-indoline)-6-yl] (F). Yield (79%). Pinkish White powder. M.p. 191-193˚. IR (KBr): 3007 ( CH= ), 1601 ( C(3)=C(4) ), 1284 ( C(2)-N ), 1078 ( C(2)-O ). 1H-NMR ( CDCl3 ): 1.12 ( t, 3JHH = 6.7, 2 Me ); 1.33 ( s, 2 Me ); 1.38 ( s, 2 Me ); 2.91 ( s, 2 NMe ); 3.26 ( q, 3JHH = 6.7, 2 NCH2 ); 4.27 ( d, 3JHH = 10.2, 2 H-C(3) ); 6.59-7.3 ( m, 18 arom.H, and 2 H-C(4) ). MS m/z (%) = 877 (M+1, 1), 847 (M- 2 Me, 1), 543 (16), 454 (2), 368 (80), 274 (23), 262 (100), 233 (18). Anal.calc. for C56H60N8O2 ( 877.15 ): C 76.68, H 6.89, N 12.77; found C: 76.65; H 6.93; N 12.79.

Furthermore, Figure 2 displays the samples color changes after exposing to UV light.

6

A, B, C, D, E, F

Figure 2: Color of the six synthesized photochemical dyes before (left) and after (right) exposing

to 365 nm UV light

2.2 Instruments

Chemicals were purchased from Merck and used without further purification. UV/Vis Spectra

was measured with a Multispec-1501-Shimadzu UV/Vis spectrophotometer. A 365 nm UV

hand-held lamp (8 watt cm-2) were used as the excitation light sources for the photochromic ring-

opening reactions.

M.p.: Büchi B-545 melting-point apparatus; uncorrected. IR Spectra: Perkin-Elmer-Spectrum-One-BX FT-IR spectrometer; in KBr; ν in cm-1. Fluorescence spectra: Perkin-Elmer-LS-55 spectrometer. 1H- NMR Spectra: Bruker-500-Avance Fourier-transform (FT) NMR instrument, at 500 and 125.7 MHz, resp., in dimethyl sulfoxide ((D6)DMSO) and CDCl3 ; δ in ppm rel. to SiMe4 , J in Hz. MS: Finnigan-Mat-8430 mass spectrometer; ionization potential 20 eV, electrospray ionization (ESI), pos. made; in m/z (rel. %). Elemental analyses for C, H, and N: Heraeus-CHN-O-Rapid analyzer.

2.3 Colorimetric Computations

The spectral absorptions of the photochromic dyes during the 365 nm UV exposure were

measured until no more changes occurred. To compute the colorimetric values, conversion of

absorbance to transmittance is necessary, which can be done using equation 1:

Aλ = log(Tλ) or Tλ = exp(Aλ) (1)

Wherein, Aλ is absorbance at a defined wavelength (λ) and Tλ indicates the corresponding

transmittance.

Vis or Δ

UV

7

The samples tristimulus values, X, Y and Z can be calculated by CIE standard equations as

follows [16-17]:

)2(

..

..

..

∑

∑

∑

=

=

=

λλλλ

λλλλ

λλλλ

TezX

TeyX

TexX

Where x , y and z are the CIE color matching functions, eλ represents the spectral power

distribution of the illuminant and T is the spectral transmittance of the sample.

In the present study, the colorimetric values are computed under illuminant D65, which is a

simulation of the daylight, and for the CIE 1964 (10◦) standard observer.

The CIE 1976 ∗∗∗ baL color space, abbreviated as “CIELAB”, was intended for equal perceptual

differences for equal changes in the coordinates L*, a* and b*. It is used intensively in many

industries and provides a standard scale for comparison of color values. The 1976 CIELAB

coordinates (L*, a*, b*) can be calculated from the tristimulus values (CIEXYZ) with the

following formulas.

)3(

200

500

16116

⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−⎟⎠

⎞⎜⎝

⎛=

⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−⎟⎠

⎞⎜⎝

⎛=

−⎟⎠

⎞⎜⎝

⎛=

∗

∗

∗

nn

nn

n

ZZfY

Yfb

YYfX

Xfa

YYfL

Where nX , nY and nZ are the tristimulus values of the reference white. The L*, a* and b*

represent lightness, redness-greenness and yellowness-blueness respectively. Figure 3 shows a

schematic form of this color space.

8

Figure 3. A schematic form of CIELAB 1976 color axis

The CIELAB color coordinates can also be expressed in cylindrical coordinates with chroma ∗abC

and hue abh . The ∗abC axis represents Chroma or saturation which is obtained as a distance

between the origin (center or achromatic axis of CIELAB color space) and the point expressed

by the coordinate ∗a and ∗b according to equation 4 [17]. Higher saturated color represents

higher chroma value and is closer to the edge of the circle and vice versa.

( ) )4(2/122 ∗∗∗ += baCab

The CIE 1976 hue angle ( abh ) is obtained by the following equation.

(5)arctan **

⎟⎠

⎞⎜⎝

⎛=a

bhab

Where abh lies between 0°and 90° if ∗a and ∗b are both positive, between 90° and 180° if ∗b is

positive and ∗a is negative, between 180° and 270° if ∗b and ∗a are both negative and between

270° and 360° if ∗b is negative and ∗a is positive.

3. Results and discussion

Figure 4 shows the transmission spectra of the sample A. The CIEa*b* color coordinates and the

trend of lightness, hue and chroma variations by passing time are given in Figure 5. As illustrated

9

hue, lightness and chroma curves, A has a very light yellowish color at the first and be changed

to a red and continuously violet by exposing UV. It can be seen that after 15s or 20s, the hue has

no significant changes, which is also illustrated in transmission spectra (While the peak of

absorption wavelengths remains constant). However, lightness and chroma at first rapidly and

then slowly decreases and increases, respectively. After about 130s-140s, color of the sample is

practically steady.

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure 4: The transmission spectra of the sample A exposed to UV light by passing time

-10 0 10 20 30 40-10

-8

-6

-4

-2

0

2

4

6

8

10

a*

b*

0 20 40 60 80 100 120 1400

50

100

150

200

250

300

350

h*

Time, second

10

0 20 40 60 80 100 120 1400

5

10

15

20

25

30

35

40

45C

*

Time, second0 20 40 60 80 100 120 140

60

65

70

75

80

85

90

95

100

L*

Time, second

Figure 5: The colorimetric positions of sample A exposed to UV by passing time

From Figure 5 it seems that lightness and chroma changes with time of UV exposure might be

modeled mathematically. Equation 6 shows chroma variations with UV exposure time obtained

by curve fitting method. The R-square between the predicated values of the model and

experimental data is 0.998. Equation 7 with a R-square value of 0.993 is obtained for the

lightness values. Therefore, the fitted equations can be used to precisely predict the colorimetric

values of this photochromic dye based on the UV exposure time. As it is shown in Figure 6, both

lightness and chroma follows an exponential trend as a function of UV exposure time till

approximately 140s; after that, there is no more color changes.

( ) ( )TimedcTimebaC .exp..exp. +=∗ for sTimes 14010 ≤≤ (6)

Where:

a=34.1, b= 0.001845, c=-32.94, d=-0.03163

)140(* sTimeCC =≅ ∗ for sTime 140⟩

11

20 40 60 80 100 120 1405

10

15

20

25

30

35

40

45

UV exposure time

C*

experimental datafit

20 40 60 80 100 120 14060

65

70

75

80

85

90

95

UV exposure time

L*


Figure 6: The fitted functions for chroma and lightness changes with UV exposure time for

sample A

( ) ( )TimedcTimebaL .exp..exp. +=∗ for sTimes 14010 ≤≤ (7)

Where

a =79.2, b = -0.008659, c =17.56, d = 0.006193

)140(* sTimeLL =≅ ∗ for sTime 140⟩

Figure 7 shows the spectral transmission of sample B for different UV exposure times. The

CIEa*b* color coordinates, chroma (C*), and hue (hab) values of the sample as a function of UV

exposure time are shown in Figure 8.

12

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure 7: The transmission spectra of the sample B

0 5 10 15 20 25-10

-8

-6

-4

-2

0

2

4

6

8

a*

b*

0 10 20 30 40 50 60 70 800

50

100

150

200

250

300

350

h*

Time, second

0 10 20 30 40 50 60 70 800

5

10

15

20

25

UV exposure time

C*


0 10 20 30 40 50 60 70 8075

80

85

90

95

100

UV exposure time

L*


Figure 8: The colorimetric positions of sample B exposed to UV by passing time

13

As illustrated, at first the dye has a very slight yellowish hue, then its hue changes to red and

subsequently violet, after about 20 seconds the hue remains constant. Lightness decreases and

chroma increases till about 70s-80s of UV exposure, then they remain almost constant. Again, it

seems that it is possible to model the chroma and lightness variations as a function of UV

exposure time mathematically. Equation 8 is obtained for the chroma values versus the time of

exposing to UV. The R-square between experimental and computational data is 0.988.

Similarly, equation 9 is calculated for lightness with a R-square value of 0.996. These equations

are just like equations 6 and 7 only with different constant values; furthermore, Figure 8 shows

the fitted curves.

( ) ( )TimedcTimebaC .exp..exp.* += for sTimes 800 ≤≤ (8)

Where:

a=1.936e+006, b=-0.008602, c=-1.936e+006, d=-0.008602


Also:


Where

a =98.81, b = -0.00379, c =0.01746, d = 0.07384


Figure 9 shows the transmission spectra of the sample C for different UV exposure time. Similar

to A and B, the CIEa*b* and the trend of its hue, chroma and lightness values are shown in

Figure 10. This photochrimic dye has a slightly yellowish color at first, which is changed to red,

violet and again red.

14

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure 9: The transmission spectra of the sample C

0 2 4 6 8 10 12 14 16 18-2

0

2

4

6

8

10

a*

b*

0 20 40 60 80 10070

75

80

85

90

95

100

105

h*

Time, second

20 40 60 80 1006

8

10

12

14

16

18

20

UV exposure time

C*


20 40 60 80 10085

86

87

88

89

90

91

92

93

UV exposure time

L*


Fi

gure10: The colorimetric positions of sample C exposed to UV by passing time

15

Similar to the previous samples, the same exponential trends can be seen in chroma and lightness

curves (Figure 10). The obtained equations are as follows with the same R-square value of 0.987

for chroma and lightness. After 110s exposing to UV, no more color changes is observed for this

sample.


Where:

a =11.98, b =0.004139, c =-14.07, d =-0.08957


Also:


Where:

a =10.47, b = -0.03677, c =85.33, d = 1.006e-005


Figure 11 shows the spectral transmission of the sample D for different UV exposure times. As

illustrated in Figure 12, this sample has a yellow-green color, which is changed to yellow and

then orange by exposing to UV. The color coordinates of this dye in terms of hue, chroma and

lightness change rapidly till 60s, then it slowly changes and after about 80s UV exposing it

almost remains constant.

16

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure11: The transmission spectra of the sample D

-10 -5 0 5 10 1540

41

42

43

44

45

46

47

a*

b*

0 20 40 60 80 10070

75

80

85

90

95

100

105

h*

Time, second

10 20 30 40 50 60 70 8041

42

43

44

45

46

47

48

49

UV exposure time

C*


10 20 30 40 50 60 70 8089

90

91

92

93

94

95

96

97

UV exposure time

L*


Figure12: The colorimetric positions of sample D exposed to UV by passing time

17

Similar to previous samples, an exponential trend is observed. The estimated equations are as

follows. The R-square values for chroma and lightness fit equations are 0.996 and 0.991. These

equations are acceptable till about 80s and after this time the color of this sample shows no more

changes by exposing to UV. Figure 12 illustrates the fitted curves for chroma and lightness

values versus UV exposure time.


Where:

a= -0.01243, b= 0.07249, c= 40.29, d= 0.00317


Also:


Where:

a=96.54, b= -.001129, c= 2.978*10-6, d= 0.1327


The spectral transmission of sample E is shown in Figure 13. The CIE color coordinates versus

UV exposure time are shown in Figure 14. It can be seen that, this dye has a slightly yellowish

hue which continuously changes to violet by exposing to UV. Chroma and lightness variations

with an exponential trend are almost similar to the others, which can be seen in Figure 14. The

computed formulae with 0.985 and 0.989 R-square values are shown in equations 14 and 15 for

chroma and lightness, respectively. This sample shows no more color changes after about 100s.

18

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure 13: The transmission spectra of the sample E

-5 0 5 10 15 20-4

-2

0

2

4

6

8

a*

b*

0 20 40 60 80 1000

50

100

150

200

250

300

350

h*

Time, second

0 20 40 60 80 1000

2

4

6

8

10

12

14

16

18

20

UV exposure time

C*


0 20 40 60 80 10080

82

84

86

88

90

92

94

96

98

100

UV exposure time

L*


Fi

gure 14: The colorimetric positions of sample E exposed to UV by passing time

19


Where:

a=16.44, b= 0.001838, c=-14.24, d=-0.03596


Also:


Where:

a =41.02, b = -0.01318, c =57.73, d =0.002125


The spectral transmittance of the sample F and the corresponded color coordinates are shown in

Figure 15 and 16, respectively. As illustrated in Figure 16, it has a slight yellowish color, which

changes to orange and approximately brown by exposing to UV. As same the others, lightness

and chroma can be mathematically modeled with an exponential equation as a function of UV

exposing time (Equations 16 and 17). The R-square values of the predicted formulae for chroma

and lightness are 0.998 and 0.999, respectively. The color of this sample shows no more changes

after about 230s or 250s; the fitted curves are shown in Figure 16.

20

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

wavelength, nm

Tra

nsm

issi

on s

pect

ra

Figure 15: The transmission spectra of the sample F

0 1 2 3 4 5 6 72

4

6

8

10

12

14

16

a*

b*

0 50 100 150 200 25065

70

75

80

85

h*

Time, second

50 100 150 200 25010

11

12

13

14

15

16

17

UV exposure time

C*


50 100 150 200 25084

86

88

90

92

94

96

UV exposure time

L*


Figure 16: The colorimetric positions of sample F exposed to UV by passing time

21


Where:

a =17.83, b =-0.0004064, c =-12.82, d =-0.03791


Also:


Where:

a=11.89, b= -0.0335, c=88.81, d=-0.000146


4. Conclusion

In this study, the colorimetric properties of six synthesized photochromic dyes were investigated.

The color coordinates of these dyes for different time of exposing to UV light were calculated

from the measured transmission spectra. It was shown that, the hue angle is changed rapidly and

then remains constant. However, the other color properties included chroma and lightness can be

change by continuing UV exposure till about 80s to 240s for different dyes. Furthermore, the

changes of chroma and lightness via the time of UV irradiant have an exponential trend which

can be mathematically modeled. The obtained formulae for the six studied dyes are the same

with only difference in constant values. It is of interest to study the colorimetric properties of

other photochromic dyes with different chemical structures in future researches.

5. References

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24

Highlights

• Colorimetric properties of some Bis-Azospiropyrans based photochromic dyes were investigated.

• Variations of transmission spectra, hue, chroma and lightness via UV exposure time were studied.

• It was tried to find a model that mathematically fits the colorimetic data during UV exposure time.

modeling colorimetric characteristics of on–off behavior of photochromic dyes based on...

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