Multistate_multifunctional switches based on photochromic Schiff base

Multistate_multifunctional switches based on photochromic Schiff base
Multistate_multifunctional switches based on photochromic Schiff base

Spectrochimica Acta Part A 67 (2007) 1120–1125

Multistate/multifunctional switches based on photochromic Schiff base Liyan Zhao, Qiufei Hou, Dan Sui, Yue Wang, Shimei Jiang

Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin

University, 2699 Qianjin Street, Changchun 130012, PR China

Received 17 July 2006; received in revised form 22 September 2006; accepted 30 September 2006

Abstract

The salicylidene Schiff base derivative, namely, N-salicylidene-(S)-a-naphthylethylamine (SNEA) has been synthesized to study the characteri-zation of multistate/multifunctional switches. Upon the stimulations of optical inputs (UV light and visible light) and chemical inputs (pH and Zn2+), SNEA undergoes reversible photochromism, deprotonation and complexation reactions. In this case, four chemical species are involved. These interconversions of SNEA between four states have been systematically investigated by the absorption and the emission spectra. Spectroscopic studies indicate that the photochromic switch, pH switch and fluorescent switch can be realized using the single molecular entity of SNEA.

? 2006 Elsevier B.V. All rights reserved.

Keywords: Deprotonation; Complexation; Photochromism; Molecular switch; Schiff base

1. Introduction

The rapid progress of information technology continues to stimulate the exploration of innovative materials for future data-elaboration, -storage, and -communication devices. Many organic molecules which exhibit reversible conversion between two-states when stimulated by an external input have been pro-posed and investigated. Based on them, monomolecular pho-tochromic switches [1–3], fluorescence switches [4–6], chiral switches [7–9], redox switches [10–12]and pH switches [13–15] have been successfully reported. But the information processed by these two-state molecular switches is limitary. In this context, the chemical systems which integrate several switchable func-tions into one single molecule have recently become the focus of research and will meet the need in the field of high density information process. To reach this multi-switchable goal, it is essential that the organic molecules can reversibly transfer its structures between more states in response to the combination of the external signals such as photonic, chemical, electrochem-ical, or magnetic stimuli and generate readable outputs such as electronic or optical signals those reflect the molecular states. To the best of our knowledge, only several molecules, behaving as a multiple switches, have been described until now [16].

Corresponding author. Tel.: +86 431 5168474; fax: +86 431

5193421. E-mail address: smjiang@https://www.360docs.net/doc/1c8099136.html,(S.Jiang).

1386-1425/$ – see front matter ? 2006 Elsevier B.V. All rights

reserved. doi:10.1016/j.saa.2006.09.033

Herein we report on a new single molecular system which is capable of integrating the effects of multiple stimuli and produc-ing three switchable functions. The target molecule is the sali-cylidene Schiff base N-salicylidene-(S)-a-naphthylethylamine (SNEA). Salicylidene Schiff bases are well known for their properties of photochromism [17]. As a member of them, SNEA can perform the tautomerism of enol-form and keto-form by the alternant action of UV light and visible light. In this paper, besides photic signals, pH change and transition metal ion (Zn

2+

) are also introduced to the target molecule. Upon these stimulations, SNEA undergoes reversible photochromism, deprotonation and complexation reactions. In this case, four chemical species (SNEA, SNEA-1, SNEA-2, and SNEA-3) are involved. The transformations between them are schematized in Fig. 1. Detailed studies show that all the observed processes are reversible and accompanied by obvious changes in absorp-tion and emission spectra. Based on these reversible reactions, SNEA can be used to mimic the ternary switch. The ―ON‖ or ―OFF‖ of this ternary switch can be judged through the obvious spectral changes.

2. Experimental method

2.1. Material and preparation

Salicylaldehyde and (S)-a-naphthylethylamine are pur-chased from Aldrich. The salicylidene compound SNEA is

L. Zhao et al. / Spectrochimica Acta Part A 67 (2007) 1120–1125

1121

Fig. 1. (a) Structural transformations between the four states for the multistate and multifunction molecular switches and (b) the switchable function based on the SNEA system.

synthesized by the condensation of the (S )-a -naphthylethylamine with salicylaldehyde according to the standard procedures as previously reported [18]. Pure SNEA is obtained after recrystallization twice from an absolute ethanol. The analysis data are as follows. SNEA . 1H NMR (500 MHz, CDCl 3, 25 ?

C, TMS): δ = 13.68 (s,

1H), 8.43 (s, 1H), 7.90–6.85 (11H; Ar –H), 5.43 (q, 3

J(H, H) = 6.63

Hz), 1.80 (d, 3

J(H, H) = 6.72 Hz, 3H). FT-IR trans-

mission (KBr pellet): 3045 cm ?1

(νOH ), 2976 cm ?1

(νCH 3 ), 2776 cm ?1

(νOH ),

1628 cm ?1 (νC N + n C C ), 1596 cm ?1

(νC C ), 1494 cm ?1 (νC –O + νC C ), 1459 cm ?1 (νC –N ), 1369 cm ?1

(δCH in CH –N), 1354 cm ?1 (νC C ), 1209 cm ?1 (νCO + νOH ),

1173 cm ?1 (νC –H in phenyl), 1128 cm ?1 (νC C ), 1030 cm ?1

(νC C

), 1010 cm ?1 (ν C C ). MS: m /z = 275 (M +

). 2.2. Experimental details for spectral studies

The concentration of SNEA solution for all spectral analysis is

6.5 × 10?5

M. The photochromism process is accomplished in spectrograde chloroform and the time-dependent spectroscopy is used. Before the spectral measurement, all the solutions are stored in the dark for more than 72 h to prevent any effect caused by room light. A low-pressure mercury lamp (16 W) and sodium lamp (100 W) are used as ultraviolet and visible light sources, respectively. After each irradiation, the absorption cell is placed quickly into the spectrophotometer to get the absorp-tion spectrum. The deprotonation –protonation is accomplished in spectrograde ethanol. The titration is treated directly in 1cm absorption cell by successive additions of 2 m l 0.01 M NaOH solution or 2 m l 0.01 M HCl solution using a microliter syringe. After each addition, the cell is capped and mixed by inversion and the spectrum is taken. The complexation is accomplished in spectrograde ethanol. The mole ratios of zinc(II) acetate/SNEA are quantified from 0.00 to 0.74.

2.3. Instrumentation

The UV –vis absorption spectra are taken on a Shimadzu 3100 UV-VIS-NIR recording spectrophotometer using a 3 nm slit width. The fluorescence spectra are determined with a Shi-madzu RF-5301 PC spectrofluorophotometer using 3 nm input and 3 nm

output width (excitation at 370 nm). 1

H NMR (TMS) are recorded on a Bruker UltraShield 500 MHZ spectrometer. FT-IR transmission spectrum is performed on a Bruker IFS66V FT-IR spectrometer equipped with a DTGS detector for KBr pellet. Sample chamber and optics are vacuumed in order to eliminate carbon dioxide and water in air. The positive EI mass spectrum is recorded on a Finnigam spectrometer on chloroform solutions with an ionizing voltage of 70 eV.

2.4. Quantum yield measurement

The quantum yield is measured at room temperature by a single excitation wavelength (370 nm) referencing to quinine sulfate in sulfuric acid aqueous solution (φfr = 0.546), and cal-culated according to Eq. (1) [19]: where φfs is the radiative quantum yield of the sample; φfr the radiative quantum yield of the standard; A s and A r are the absorbance of the sample and standard at the excitation wavelength, respectively; D s and D r are the integrated area of the sample and standard, respec-tively; L s and L r are the length of absorption cells for the sample and standard test; N s and N r are the indexes of refraction of the sample and standard solutions (pure solvents were assumed), respectively:

1 ? 10?A r L r N 2

D s

φf s = φfr ×

× s

× (1)

1 ? 10?A s L s

N 2 D r

r

2.5. pK a measurement

The pH control is achieved by using the modified universal

buffer solutions [20]. In order to increase the solubility of SNEA

1122

L. Zhao et al. / Spectrochimica Acta Part A 67 (2007) 1120–1125

and to rule out its hydrolysis, ethanol –water mixture solvent with higher mass fraction (ethanol:water = 94.3%) is used for our p K a measurement. The pH values are corrected by using Eq.

(2) reported by Douheret [21], where pH *

is the corrected value and pH(R) is the pH read value obtained by meter. Value of δ for ethanol –water mixture is recommended by Douheret at the same ionic strength. The solutions are

thermostated at (20 ± 0.05) ?

C before measuring their spectra.

The absorption spectra are recorded on Shimadzu 3100 UV-VIS-NIR spectrophotometer containing a thermoelectrically temperature controlled cell holder. The pH measurements are carried out using PHS-2F digital pH meter accurate to ±0.01 pH unit and the pH meter is standardized using aqueous buffers at 20 ?C. All measurements are carried out at 20 ?

C, and tem-perature control is achieved using an ultrathermostat of accuracy

±0.05 ?C:

pH = pH(R ) ? δ (2)

3. Results and discussion

3.1. Spectral studies of SNEA under the actions of light irradiation, pH and zinc ion

Before discussing the switchable function of the single molecule SNEA, it is necessary to describe the reversible trans-formations between the four different states. Their chemical structures have been presented in Fig. 1a. The first one is due to a proton transfer reaction involving the target molecule SNEA and its photochromic product SNEA-1. The second one is attained by pH change, which leads SNEA to its deprotonated form SNEA-2. The last one is reached by the complexation with transition metal

ion Zn 2+

and the decomposition of this complex under the UV light irradiation. Two chemical species SNEA and SNEA-3 are included in this step. The important characteristics of struc-tural changes of above are the presence of pronounced spectral variations observed upon continuous stimulation by the inputs. The spectral results will be established and accounted for as follows.

3.1.1. Reversible photochromism process

Numerous spectroscopic and theoretical studies [22–26] on photochromic reaction of N-salicylideneaniline Schiff base enlighten people to utilize the optical signal as an input to drive their isomerization. So, we primarily introduce the UV light onto the target molecule. The irradiation time-dependent absorption spectra of the compound SNEA are represented in Fig. 2. The absorption spectra show that the original absorption bands at 258 and 315 nm decrease along with the increase of the absorbance at 286 nm, moreover, a new red-shifted absorption band at 370 nm appears and intensifies with the increasing irradiation time. The new generated absorption band at 370 nm is attributed to the keto-form [27]. This photoisomerization reaction involves an intramolecular proton transfer from the O -hydroxy group to the imino nitrogen atom. The well isosbestic points in Fig. 2 mean that an equilibrium system presents in the solution. These results clearly indicate that the SNEA has undergone the pho-

Fig. 2. UV –vis absorption spectral changes for SNEA solution with 254 nm UV light irradiation. From 1 to 9, each irradiation time is 2 min; total, 16 min. The first (has not irradiated by UV light) is given in dash.

tochromism and changed from the enol-form SNEA to keto-form SNEA-1 (Fig. 1a) under UV light irradiation.

The absorption spectra revealed in Fig. 3 demonstrate that the keto-form SNEA-1 will switch back to SNEA under visible light. This complete decoloration taking place in the absorption spectra indicates that the photochromism of SNEA is reversible.

3.1.2. Deprotonation –protonation

The phenolic group is quite sensitive to base and acid. So, the phenolic group in SNEA endows it with the quick respondence to the acid/base. We have measured the synchronized absorption along with titrating SNEA continuously with NaOH which pro-duces striking absorption spectral changes (Fig. 4): the original absorption bands around 270–330 nm gradually decrease, at the same time, a new absorption at 360 nm emerges and intensifies by degrees through the isosbestic points of λ = 330 nm. This new absorption is assigned to the deprotonated product SNEA-2 [28], which implies that SNEA has been deprotonated and SNEA-2 has been formed (as shown in Fig. 1a).

Introducing HCl into the SNEA-2, the reverse absorp-tion spectral change is obtained (Fig. 5): absorbance around

Fig. 3. UV –vis absorption spectral changes for SNEA-1 solution with visible light irradiation. From 1 to 10, each irradiation time is 5 min; total, 45 min. The first (has not irradiated by visible light) is given in dash.

L. Zhao et al. / Spectrochimica Acta Part A 67 (2007) 1120–11251123 Fig. 4. UV–vis absorption spectral changes for SNEA solution with titration

by NaOH. From 1 to 9, the quantity of NaOH increases gradually. The first

(has not titrated by NaOH) is given in dash.

270–330 nm increase, at the same time, 360 nm gradually dis-

appears. This circle is repeated and the absorption intensity

changes are monitored when the sample is exposed to alter-

nating solution of base and acid. Under the alternating action

of base and acid, it can perform the reversible deprotonation–

protonation process.

In order to quantificationally reveal the capability of SNEA

responding to acid/base, the absorption spectra of SNEA depen-

dent on pH

*

changes in buffer solutions have been measured

(Fig. 6). At lower pH

*

values, the band at 315 nm appears, which

represents the absorption of the nonionized species. While at

higher pH

*

values, the band at 378 nm belonging to the absorp-

tion of ionized species shows up. On increasing the pH value of

the medium, the absorbance of 315 nm decreases while that of

378 nm increases, where a fine isosbestic point is achieved,

denoting the existence of the acid/base equilibrium.

Based on these absorption spectra shown in Fig. 6, the p K a of

SNEA can be determined. There are three different spec-

trophotometric methods to determine the p K a according to the

literatures [29,30]. In the half-curve height method, p K a= pH

*

at

A1/2; A1/2 = (A max?A min)/2 + A min, where A max and A min are

the maximum and minimum absorbance on the absorbance–pH

*

Fig. 6. UV–vis absorption spectra of SNEA solution with continuous increas-

ing the pH

*

values. From 1 to 20, pH

*

values are 6.20, 6.59, 6.79, 7.17, 7.56,

7.75, 8.14, 8.33, 8.52, 8.72, 9.11, 9.30, 9.49, 9.69, 10.07, 10.27, 10.46, 10.65,

10.85 and 10.98, respectively. Inset (a): absorbance at 378 nm depen-dent on

pH

*

curve of 6.5 × 10

?5

M SNEA in buffer solution. Inset (b): plot of log[(A

?A min)/(A max?A)] against pH

*

of 6.5 × 10

?5

M SNEA in buffer solution.

curve (shown in inset (a) of Fig. 6). In the isosbestic point

method, the pH

*

corresponding to the isosbestic point equals the

p K a value. In the limiting absorbance method, p K a can be

determined from the plot of log(A?A min)/(A max?A) against

the pH

*

(shown in inset (b) of Fig. 6), where p K a = pH

*

when

log(A?A min)/(A max?A) = 0. According to the above three

methods, the dissociation constant p K a is 8.94, 8.69 and 8.83,

respectively. So the average value of p K a is 8.82.

3.1.3. Complexation–decomposition

Schiff bases are an important class of ligands in complexation

chemistry and have been studied extensively [31,32] for they are

selective and sensitive toward various metal ions. In our present

work we report that the complexation reaction of Schiff base

ligand SNEA with zinc ion which causes obvious changes in the

absorption spectra, as shown in Fig. 7. The spectrum obtained at

the end of titration comparing with the spectrum recorded

Fig. 5. UV–vis absorption spectral changes for SNEA-2 solution with titration

by HCl. From 1 to 7, the quantity of HCl increases gradually. The first (has

not titrated by HCl) is given in dash.

Fig. 7. UV–vis absorption spectra of SNEA solution with continuous titration

by Zn

2+

. From 1 to 9, the mole ratio of Zn

2+

/SNEA increases from 0.00 to

0.74. The first is given in dash; inset: titration profiles at 315 and 365 nm.

1124

L. Zhao et al. / Spectrochimica Acta Part A 67 (2007) 1120–1125

Fig. 8. Fluorescence spectra (λex = 370 nm) of SNEA solution (dash line) with

continuous titration by Zn 2+. The mole ratio of Zn 2+

/SNEA increases from 0.00 to 0.74. The first is given in dash; inset: dependence of the quantum

yield of the titration reaction on the mole ratio of Zn 2+

/SNEA.

before Zn 2+

addition shows that the intensities of original peaks at 258 and 315 nm decrease and the increases take place at the shorter band 238 nm and 265–290 nm. Four isosbestic points are observed at λ = 248.5, 265, 290, and 335 nm, respectively. A new

band at 365 nm due to the N, O-complexation with Zn 2+

develops [33]. The mole ratio plot using the decrease in the absorbance at 315 nm and the increase in absorbance at 365 nm

reaches the saturation point when the ratio of Zn 2+

reaches the equivalent concentration clearly demonstrates the formation of a Zn 2+:SNEA = 1:2 complex. Titration of ligand SNEA with Zn 2+

induces the appearance of a strong fluorescence (Fig. 8): the fluorescence intensity of SNEA in ethanol is very weak, but a remarkable blue emis-sion

at 452 nm is observed by adding of Zn 2+

to the SNEA solution.

The fluorescence intensity gradually increases until the Zn 2+

amount reaches the complexation concentration. The plot of quantum yield (Fig. 8, inset) is linear and reaches a plateau after

the addition of 0.5 equiv. of Zn 2+

, showing that a 1:2 complex SNEA-3 is formed (Fig. 1a). This remarkable blue emission can be quenched by the irradiation of 254 nm UV light (Fig. 9), namely, the complex SNEA-3 has been decomposed. Placing the decomposed solution of the complex for several hours, bright blue fluorescence can be obtained again which indi-

Fig. 9. Fluorescence spectra of SNEA-3 (dash line) and after continuous irra-diation by 254 nm UV light (solid line). Inset —emission photographs in the ethanol solution: (1) the ligand SNEA in the absence of Zn 2+

; (2) the Zn 2+

complex SNEA-3; (3) the SNEA-3 solution after irradiated by 254 nm UV light.

Fig. 10. Electric circuit schemes representing for the ternary switches based on the SNEA system.

cates the switching between the ligand SNEA and its complex SNEA-3.

3.2. Ternary switch analysis based on

multistate/multifunctional Schiff base SNEA

In the spectral investigations we have shown that the reversible interconversions of SNEA between four states can be driven by the UV light, visible light, pH change and transition

metal ion (Zn 2+

). An interesting aspect of this system is that it can mimic the ternary switch at monomolecular level. The orig-inal molecule SNEA can be considered as the ―OFF ‖ (Fig. 1b) state. The stimulation of UV light will drive the intramolecular proton transfer from the O -hydroxy group to the imino nitrogen atom, in this case, SNEA converts to its isomer SNEA-1. Cor-respondingly, the absorption spectra give out the output: strong absorption at 370 nm. So the ―OFF ‖ is switched to ―ON ‖. Con -versely, the visible light will reversely drive ―ON ‖ to ―OFF ‖. So one photonic switch ―OFF –ON ‖ can be accomplished. Under the same consideration, the deprotonation –protonation cycle of phenolic group and the reversible complexation –decomposition have represented the pH switch and the fluorescence switch. Fig. 10 gives the more straightforward representation based on the electric circuit schemes. In this visual circuit the target molecule plays the role of one ternary switch to control the three lamps. The complicated ―OFF –ON ‖ or ―ON –OFF ‖ processes of three lamps are automatically executed by the external stimuli such as UV light, visible light, pH change and the transition metal ion.

4. Conclusions

In summary, we have demonstrated that a simple-structured molecule, SNEA, behaves as ternary molecular switch with

相关主题
相关文档
最新文档