Effect on Solvent Polarity and Concentration on the Photo Transformation of 6-Nitro BIPS

Shamsul Azrolsani Abdul Aziz Nazri a*, Mohamad Zahid Abdul Malek a, Mat Tamizi Zainuddin a, Noor Zalikha

Mohamed Islam a, Zaharah Wahid b Photonic & Electronic Material Program, Advanced Materials Research Centre (AMREC),

SIRIM Berhad, Kulim, Kedah, Malaysia a Kulliyyah of Engineering, International Islamic University of Malaysia,

Kuala Lumpur, Malaysia b.

Abstract— Polar and non-polar solvents; ethanol and toluene respectively were used to dissolve and investigate the photochromism behavior of 6-nitro BIPS at different solvent concentration. Absorption spectra of 6-nitro BIPS in low concentration solvent were arising to absorption peak of 530nm and 600nm after UV irradiation for ethanol and toluene respectively. At high concentration solvent of 6-nitro BIPS, the absorption spectra did not present any changes on absorption peak after UV irradiation whereas the absorption peak was remains at 530nm for ethanol and no absorption peak appeared for toluene. Shifting of 6-nitro BIPS wavelength spectrum from 530nm for dilution in ethanol to 600nm for dilution in toluene in low concentration solvent was observed and investigated.

Keywords—6-nitro BIPS, solvent polarity, solvent concentration, UV irradiation

I. INTRODUCTION Photochromism has gained much interest mainly due to

many promising application such optical memory [1],[2],[3], display communication systems [4], photoregulation of various physical and chemical properties of polymers [5], signal tranduser receptor [6], adaptive filter for optical image processing [7] and ophthalmic lenses [8]. A number of investigations also have been carried out on the photochromism of 6-nitro-1’,3’,3’-trimethylspiro[2H-1-benzopyran-2,2’~indoline] (6-nitroBips) in solution with UV irradiation [9][10][11][12]. It is become necessary to all application of photochromic which use spiropyrans to observe through the details understanding of the kinetics and the transition of Spiropyran (SP) and Merocyanine (MC).

The transition from SP to MC takes place through a photo chemical route involving ultraviolet photons. The SP ring opened when irradiated with UV and colored MC form gradually reposition back to the SP form scheme (1). The color of MC form as well as rate of reposition back to the SP form is both dependent on the solvent polarity [9][12][13]. Because of the various technological application of this class of compound, we were attracted to report here about the investigation on some behavior of this spiropyrans.

In this paper the comparison of solvent polarity and dilution concentration of 6-nitro BIPS were examined. The work focuses mainly on the effect of solvent polarity to the photochromism behavior of 6-nitro BIPS spiropyran.

II. EXPRIMENTAL The 6-nitro-1’,3’,3’-trimethylspiro [2H-1-benzopyran-

2,2’-indoline] (6-nitro BIPS) dyes (Sigma Aldrich) have been synthesized by dilution in ethanol (EtOH) as polar solvent and toluene as non-polar solvent in 1.0x10-3M (10-3M) and 1.0x10-5M (10-5M) concentration. The solutions were examined in a rectangular quartz cell. The absorption spectra were measured on fiber optic based UV-Vis spectrometer (OceanOptics USB 4000) with 9.9 watts short arc xenon lamp (OceanOptic PX-2). The spectra of 6 nitro BIPS in polar and non-polar solvent with low and high concentration were recorded before and after five second of UV light irradiation. Comparison of spectra between 6-nitro BIPS in ethanol and toluene were recorded in high and low concentration separately.

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III. RESULTS & DISCUSSION Figure 1 shows the comparison of 6-nitro BIPS

absorption spectra in ethanol and toluene with low concentration 10-5M before and after five second UV irradiation. The absorption spectra of 6-nitro BIPS in ethanol and toluene before UV irradiation did not show any absorption band. While after UV irradiation, 6-nitro BIPS in ethanol shows absorption bands at 530nm and 600nm for dilution in toluene. From this figure, we also can observe that, the absorption value of 6-nitro BIPS in ethanol is 0.14, higher than 6-nitro BIPS in toluene with absorption value is 0.1. In visible light condition, 6-nitro BIPS in both EtOH and toluene are in the closed SP form at appeared to be transparent as in fig 2(a). Upon UV irradiation, both 6-nitro BIPS dilution in EtOH and toluene shows a color changes from transparent to pink and blue respectively. Pink color is assigned by open MC form whereas the blue color can be is attributed to one cisoid open from H-bonded to one close Spiro form [15].

The wavelength peak of the absorption spectra depends on the solvent which it is dissolved. Different wavelength cause different MC isomer to be formed, each with different absorption spectra. The peak wavelength of emitted photon and the peak wavelength of excitation vary depending on the wavelength used in coloration of molecule [12]. The color dependence of the MC form (known as solvatochromism) arises from the difference in polarity between the photo-

exited MC form and the zwitterionic ground state MC form for the 6-nitro BIPS the excited state of the MC form is less polar than the zwitterionic ground state. While in polar solvents the ground state of the MC form is stabilized relative to the excited state of the MC form leading to a blue shift in the absorption maximum as shown in fig 3. So a solution of 6-nitro BIPS appears pink in ethanol after UV irradiation as the MC form has an absorption maximum at 530nm. In comparison a solution of 6-nitro BIPS in non-polar solvent (toluene) appears blue as shown in figure 2 after UV irradiation because the MC form has an absorption maximum at a longer wavelength (600 nm).

Figure 4 shows the comparison of 6-nitro BIPS absorption spectra in ethanol and toluene with high concentration 10-3M before and after 5 second UV irradiation. Without any changes, 6-nitro BIPS in Ethanol shows an absorption band arise at 530nm with 0.38, while dilution in toluene did not show any absorption band at that referral peak although after UV irradiation. It is believed that at high concentration level of 6-nitro BIPS dilution in EtOH, SP form will changes to MC form as early at initial stage before irradiation of UV due to high polarity contributed by hydrogen bond. It is different if compared to non-polar solvent like toluene that would not change even after UV irradiation related to its non-polar properties.

Fig. 1: Absorption Spectra of 6-Nitro BIPS in Etoh and Toluene Before and After UV Irradiation with 10-5M Concentration

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Ethanol (Transparent)

Toluene (Transparent)

Ethanol (pink)

Toluene (blue)

(a) (b)

Fig. 2: Photocoloration of 6-nitro BIPS before (a) and after UV irradiation (b) in ethanol and toluene

Fig. 3: Illustration of the solvatochromic shift of the MC form of 6-nitro BIPS

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

420 470 520 570 620 670 720

abso

rban

ce

wavelength (nm)

6NB in EtOH

6NB in Toluene

6NB in EtOH UV 5s

6NB in Toluene UV 5s

Fig. 4: Absorption Spectra of 6-Nitro BIPS in Etoh and Toluene Before and After UV Irradiation with 10-3M Concentration

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Observation from all UV-Vis spectra can be translated in

table 1. Dilutions in high concentration (10-3 M) of 6-nitro BIPS in ethanol provide a higher wavelength peaks and not effected by UV irradiation. Similar with dilutions in high concentration of toluene, there are no changes when irradiated by UV and did not display any wavelength peaks due to high concentration of dilution. The existing of wavelength peak of 6-nitro BIPS dilution in ethanol and toluene can be clearly

IV. CONCLUSIONS Comparisons between solvent polarity, concentration and

UV irradiation effect on 6-nitro BIPS spiropyran have been investigated. It was observed that there are obvious shifting on absorption spectra between polar and non-polar solvent occurred in low concentration level. Difference of wavelength due to different MC isomers will change the molecule color for example pink color in EtOH and blue color in toluene. High concentration of solvent shows unchanged absorption spectra on both polar and non-polar solvent after UV irradiation. We believed that the SP form has already change to MC form by high hydrogen bond in high polarity solvent cause the absorption peak appeared even before UV irradiation on high concentration level.

observed in low concentration (10-5 M) dilution after five second UV irradiation. The UV absorption by wavelength peak of dilution in ethanol was reduced if compared to high concentration dilution from 0.38 to 0.14 of absorption at 530nm. However the wavelength peak in toluene was changes to 600nm, there is a red shift in the absorption band as the polarity of the solvent is decreased and reduces to 0.10 absorption [14].

ACKNOWLEDGMENT This research was supported by Ministry of Science,

Technology and Inovation Malaysia (MOSTI) under e-Science Fund grant No. 03-03-02-SF0145.

Condition 530nm 600nm 6NB 10-5 M ethanol

0

6NB 10-5 M toluene

0

6NB 10-5 M ethanol UV

0.14

6NB 10-5 M toluene UV

0.10 6NB 10-3 M ethanol

0.38

6NB 10-3 M toluene

0

6NB 10-3 M ethanol UV

0.38

6NB 10-3 M toluene UV 0

TABLE I. UV –VIS ABSORPTION OF 10-5M AND 10-3M 6-NITRO BIPS IN ETHANOL AND TOLUENE

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