Thermochromism of metallophthalein in acrylamide copolymers with pendant pyridine substituents

Reactive polymers ELSEVIER

Reactive Polymers 24 (1995) 139-143

Thermochromism of metallophthalein in acrylamide copolymers with pendant pyridine substituents M. N a n a s a w a * , Y. Min, M. Hirai Department of Applied Chemistry and Biotechnology, Yamanashi University, Takeda 4, Kofu 400, Japan

Received 14 June 1994; accepted in revised form 7 September 1994

Abstract

The thermochromism of Ni(II) complexes of cresolphthalein dye was studied in pyridine-containing acrylamide copolymers with N-vinyl-pyrrolidone (VP) or N-isopropylacrylamide (IPA). Pendant pyridine moieties in the copolymers affect the sensitivity of color development and stability of color species. A large temperature dependence was obtained in the copolymer 2-acrylamido-N-3-pyridylpropanoamide pyridine with VP and the developed color was stable in copolymer of N-3-pyridylacrylamide with IPA. Keywords: Thermochromism; Pyridine-containing acrylamide copolymer; Triphenylmethane dye; Phthalein-metal complex

I. Introduction

Recently, organic erasable-direct-read-afterwrite (EDRAW) media have received much attention for application in optical recording devices with high storage density and integrity. These include thermo-optical recording system employing the ablative recording of pigments [1] and phase conversion of liquid crystals [2,3]. Whereas many kinds of thermally reversible coloring (thermochromic) compounds have been synthesized [4-9], they have not yet been exploited as EDRAW media, which is due, in part, to a too rapid decolorization reaction. The thermochromism of metal complexes of triphenylmethane dyes have been observed in aqueous buffer solution at moderate * Corresponding author.

temperature [10,11] and the developed color were found to be relatively stable in the organic phase [12]. The thermochromic behavior depends distinctly on the class of 3,3'bis [bis(carboxymethyl)aminomethyl] phthalein dyes (PC), bivalent metal ions, and organic bases [13]. In order to evaluate the potential of thermochromism of metallophthalein, this paper describes the reversible color change of PC-dyes embedded in copolymers. Since color development depends on deprotonation of PC-dyes with bases at elevated temperature, polymerizable pyridine derivatives were synthesized as the organic base and copolymerized with acrylamido monomers; the effect of pendant pyridine in copolymers was investigated for the sensitivity of color development and stability of color species of PC-dyes.

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M. Nanasawa et aL / Reactive Polymers 24 (1995) 139-143

2. Experimental

All reagents employed were of the highest purity, unless otherwise noted. N-Vinylpyrrolidone (VP, Tokyo Kasei Co.) andN-isopropylacrylamide (IPA, Kouzine Co.) were purified by distillation and recrystallization before use. IR (KBr disks) and 1H-NMR were recorded on a Hitachi 215 spectrometer and a JNM-PMX60 spectrometer, respectively. The thermal behavior of the copolymer was performed by Rigaku TG-DSC system, and elemental analyses were conducted with a Carbo Erba EAl108. Visible spectra were recorded on a Hitachi 200-10 and a Shimazu UV160 spectrophotometer.

2.1. N-3-Pyridylacrylamide(I) A solution of acryloylchloride (3.5 ml, 40 mmol) in anhydrous tetrahydrofuran (THE 10 ml) was dropped into a mixture of 3aminopyridine (2 g, 21 mmol), triethylamine (5.9 ml, 21 mmol), and hydroquinone (0.1 g) in anhydrous THF (40 ml) below 5°C with vigorous stirring. After addition, the reaction mixture was stirred in an ice bath for 5 h. The insoluble portion was filtered and thoroughly washed with THF. The filtrate and washing was concentrated in vacuo; an oily residue was purified by silica gel-column chromatography (acetone). The product was recrystallized from ethyl ether (Et20). Yield 42%, m.p., 117-119°C. 1H-NMR (CDC13, 8): 5.75 (t, 1H, vinyl), 6.4 (d, 2H, vinyl), 7.3 (d, 1H, 5-PyH), 8.3 (d, 2H, 2,5-PyH), 8.7 (s, 1H, 6-PyH), 9.2 (s, 1H, NH) ppm. IR (KBr): 1680 cm -1 (amide). Anal. Calcd. for C8HsN20: C, 64.85; H, 5.44; N, 18.91%. Found: C, 64.77; H, 5.86; N, 18.57%.

2.2. 2-Acrylamido-N-3-pyridylpropanoamide (II) Dicyclohexylcarbodiimide (DCC, 9.1 g, 44 mmol) and fl-acryloylalanine (6.0 g, 42 mmol, prepared from acryloylchloride and fl-alanine by Schotten Baumann reaction [14]), were dissolved in a solution of acetone-dioxane (1:2, 90 ml).

The reaction mixture was stirred at room temperature for 15 h, and then added to a solution of 3-aminopyridine (3.9 g, 41 retool) and hydroquinone (0.1 g) in 1 : 2 acetone-dioxane (45 ml). The reaction mixture was heated to reflux for 2 h with stirring; DCC-urea thus precipitated was separated by filtration and washed with hot dioxane. The filtrate and washing was concentrated to 50 ml, and cooled in a refrigerator overnight. The product thus precipitated was isolated and recrystallized from ethanol. Yield, 62%, m.p., 169-170°C. 1H-NMR (DMSO, d-6, 8): 2.6 (t, 2H, CH2C=O), 2.5 (t, 2H, CH2--N), 5.5 (t, 1H, vinyl), 6.3 (t, 2H, vinyl), 7.3 (t, 1H, 5-PyH), 8.2 (t, 3H, 2,3-Pyn, NH), 8.8 (d, 1H, 6-Pyn), 10.1 (s, 1H, NH) ppm. IR: 1660 cm -1 (amide). Anal. Calcd. for Cli H13N302; C, 60.27; H, 5.94; N, 19.18%. Found: C, 60.86; H, 6.54; N, 19.19%.

2.3. 3-(2-Acrylamidoethylcarbamoyl)pyridine(III) 3-(2-Aminoethylcarbamoyl)pyridinewas prepared by aminolysis of methyl nicotinate with ethylenediamine [15], and recrystallized from acetone, yield 85%. Compound IH was synthesized from 3-(2-aminoethylcarbamoyl)pyridine and acrylchloride by the similar method to that of I. Yield, 45%, 1H-NMR (CDC13, 8), 3.7 (m, 5H, CH2, N--H), 5.8 (t, 1H, vinyl), 6.2 (d, 2H, vinyl), 7.3 (d, 1H, 5-PyH), 8.1 (d, 2H, 2,5-PyH), 8.7 (s, 1H, 6-PyH), 9.1 (s, 1H, N--H) ppm. IR: 3400 (N--H), 1730 (ester), 1670 (amide) cm -1. Anal. Calcd. for C11H13N302: C, 60.27; H, 5.94; N, 19.18%. Found: C, 59.55; H, 5.73; N, 19.41%.

2.4. Copolymerization A 15-ml polymerization tube was charged with I (0.3 g, 2 mmol), VP (6 mmol), AIBN (4 wt% to monomer), and a solution of acetonitrile-dioxane (1 : 1, 10 ml). The tube was degassed by three freeze-thaw cycles and sealed at 0.5 mm Hg. The polymerization was carded out at 60°C for 60 h; the solution was then poured into vigorously stirred ether (100 ml). The suspension was filtered, the resulting copoly-

M. Nanasawa et al./ Reactive Polymers 24 (1995) 139-143

mer was washed with methanol. The amount of pendant pyridine unit was determined by titration of an aqueous copolymer solution with N/10 HCI solution.

amido- and 3-carbamoyl-pyridine were chosen as polymerizable organic bases 1, which were prepared by acylation of 3-aminopyridine with acryloylchloride or with N-acryloylalanine by means of DCC. Basicity of amino group at the 3position is lower than that of nitrogen atoms in the pyridine ring, therefore pyridine acylnium ion is predominantly formed, thus the acylating agent must be used in excess. Co-monomers VP and IPA were used in order to afford a compatible solid phase with P C metal and to elucidate the effect of the Tg of the copolymers. Water soluble monomers were polymerized with AIBN in organic solvent, because an aqueous polymerization with potassium persulfate in water produced gel-polymer insoluble in conventional volatile solvents, presumably due to occurrence of hydrogen transfer polymerization [16]. The results were summarized in Table 1. The copolymerization conversions were satisfactory and the content of pendant pyridines in copolymer was almost consistent with the molar ratio employed. Transparent films were prepared from solutions containing 2 wt% PC-Ni in copolymer with VP by casting from aqueous solution. In contrast the films of copolymer with IPA became opaque as water evaporated due to low solubil-

2. 5. Films and measurement Films were prepared by spreading a solution of a copolymer (30 mg), PC (0.6 mg) and nickel(II) nitrate (2.2-fold molar ratio to dye) in water 0.3 ml) over a glass plate of 1.2 cm width so as to afford a 1.2 x 2.0 cm2 area, followed by drying for 2 days at room temperature. The film, approximately 20-/zm thick, was then stored in a desiccator of saturated sodium bromide solution (58% RH) at least overnight before use. The film was heated at 90°C for 1 min on a hot plate, and the resulting color development was determined by transmission. 3. Results and discussion

3.1. Syntheses of copolymer and preparation of films Thermochromism takes place by reversible neutralization between the 2:1 nickel(II) complex of phthalein dye (PC-Ni) and base as shown in Eq. 1 (Scheme 1). The optimum pKa value of pyridine derivates affording a large temperature dependence is between 3.5 and 4 in organic solvent, so that 3-

H

1The dissociation constants were calculated from Hammett equation: pKa = 5.25 - 5.90E~.

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Scheme 1.

M. Nanasawa et al. / Reactive Polymers 24 (1995) 139-143

142

Table 1 Copolymerization of vinyl-pyridine derivatives

1

Polymer Pyridine CoYield [T/]b Tg Pyridine no. deriv, monomer a (%) (°C) unit e VP-I IPA-I VP-II IPA-II VP-III

I I II II III

VP IPA VP IPA VP

95 79 83 81 90

0.28 0.21 0.29 0.24 0.29

82 113 78 96 71

0.25 0.27 0.24 0.26 0.24

a 3-fold molar to pyridine derivatives. b In N,N-dimethylacetamide at 25°C. e mol% in copolymer.

Ah

o.4.

16

20

.~

48

0.2"

ity of Copoly-IPA in water. To circumvent this, a casting solvent was employed: an azeotropic solution of water and propanol (3 : 7). i

3.2. Thermochromic behavior in copolymer Color development takes place by dissociation of phenolic proton from PC-Ni chelates with pyridine derivatives (see Scheme 1), its absorbance depends on the stability of a combination of metalloquinonoid anion and protonated pendant pyridine. In protic matrices such as poly(vinylalcohol), both ions are strongly solvated with protic media [17], therefore the absorption at 600 nm attributable to quinonoid structure appeared even at room temperature. However, the absorption of copolymers was small in spite of the fact that 3-aminopyridine derivatives possess relatively high pKa-values. Aprotic matrices tend to solvate only pyridinium cations which do not form at room temperature, because thermal molecular motion is highly restricted in the glassy aprotic state, so that bimolecular reaction might be hindered to a considerable extent. Upon heating the films, the absorbance at 600 nm increased by the multiplication of thermochromic behavior of PC-metal chelates and by promotion of bimolecular reaction in softened solid matrices (Fig. 1). The rate constant in solutions was estimated approximately a s 1 0 - 3 dm 3 mo1-1 s -~ for the reverse reaction, which is 100-fold faster than that for the forward reaction [13], while the developed color in polymer matri-

t

50O 6~)0 7O0 Wave 1 e,,tt h (~m) Fig. 1. Spectral change of PC-Ni(II) in PV-H copolymer. Ao, before heating and after recovery; Ah, immediately after heating; 1, 6, 20 and 48 h indicate time after heating.

ces was stable for a long time. Table 2 indicates absorption difference before and after heating. The color development in the copolymer bearing pendant 3-nicotinamide derivatives was small due to low stability of protonated pyridine cations. The largest difference was obtained in VP-copolymer bearing flexible 3-amidopyridine. PV-copolymer possesses relatively low Tg, and is susceptible to absorb atmospheric moisture [18]. Moisture-included film acts as a plasticizer to induce the bimolecular reaction between metallophthalein and the flexible pyridine ring. Half Table 2 Absorbance of PC-Ni(II) chelate in copolymer Copolymer

Film appearance

Ao a

Ah a

tl/2 b (day)

VP-I IPA-II VP-II IPA-H VP-IH

Stiff Brittle Tough Brittle Tough

0.02 0.01 0.03 0.01 0

0.48 0.29 0.47 0.36 0.07

2.4 10< 1.2 ~10 0.4

a Ao, Ah, absorbance before and after heating (baseline

method). b Half recovery time.

M. Nanasawa et al. / Reactive Polymers 24 (1995) 139-143

the plasticizer effect of moisture and the initial state absorption was reached within 10 h. The cycle (color development, fixing and bleaching processes) was controllable and repeatable, thus the thermochromism of metallophthalein in copoly(vinylpryidine derivative)s has potential in thermo-optical recording.

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lo

480

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K

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Fig. 2. Stability and color bleaching of metalloquinonoid in copolymers. RH, relative humidity; 58%, over saturated NaBr solution; 84%, saturated KBr solution.

recovery time (tl/2) and absorption change of the color developed films were exemplified in Table 2 and Fig. 2. The hi2 of VP-I and -II were less than 60 h, while the developed color of IPA-I was stable for more than half a year in a 58% RH desiccator. Although poly (IPA) is a water soluble polymer, the hygroscopity in the solid state is low and its amide-hydrogen is weakly hydrogen-bonded due to steric hindrance of bulky isopropyl groups [19]. This results in the pyridinium cation formed at elevated temperature being tightly solvated by neighboring isopropylcarbamoyl groups so that the back reaction is highly restricted in glassy solid state. When the colored films were stored in 84% RH desiccator, the back reaction proceeded by

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