Cd2+ aqueous solution interfaces

Materials Letters 57 (2003) 2156 – 2161 www.elsevier.com/locate/matlet

Supramolecular assemblies of Eu(TPyP)Pc at the air/water and air/Cd2+ aqueous solution interfaces Hong-Guo Liu a,b, Xu-Sheng Feng a,*, Li-Juan Zhang a, Jianzhuang Jiang a, Yong-Ill Lee b, Ki-Wan Jang b, Dong-Jin Qian a, Kong-Zhang Yang a a

Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Department of Chemistry, Shandong University, Jinan, Shandong 250100, PR China b College of Natural Science, Changwon National University, Changwon 641-773, South Korea Received 14 May 2002; received in revised form 15 July 2002; accepted 9 September 2002

Abstract The supramolecular assemblies of sandwich-type complex Eu(TPyP)Pc (TPyP: tetrapyridylporphyrin; Pc: phthalocyanine) were fabricated at the air/water and air/Cd2 + aqueous solution interfaces. One-dimensional columns and two-dimensional coordinate framework were formed, respectively, and characterized by surface pressure – area (p – A) isotherms, UV – vis spectra and polarized UV – vis spectra. The orientation angle of macrocycles with respect to the substrate surface is about 38j and 32j for the Langmuir – Blodgett films from pure water and Cd2 + subphase surfaces, respectively. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Supramolecule; Langmuir monolayer; Europium complex; Coordinate polymer; Langmuir – Blodgett films

1. Introduction Sandwich-type complexes, such as metal bisphthalocyanines, metal bisporphyrins, and metal porphyrin/ phthalocyanine (Pc) heterocomplexes, have attracted much attention since 1960s [1] due to their properties of electrochromism, gas sensing, and electric conductivity [2]. These molecules have two adjacent rigid macrocycles, which make them as good building blocks for supramolecular assembly. Supramolecular systems of

* Corresponding authors. Tel.: +86-531-856-4750; fax: +86531-856-5167. E-mail address: [email protected] (X.-S. Feng).

these compounds have been assembled in solutions [3 – 5] or at the air/water interface [6 – 8]. It was reported that these complexes formed one-dimensional, columnar and supramolecular structures at the air/water interface due to the stacking interactions between the macrocycles, as revealed by transmission electron microscopy (TEM) [9] and atomic force microscopy (AFM) [10,11]. Recently, supramolecular assemblies of low dimension formed at the air/water interface through coordination between ligand and metal ion have attracted considerable interest [12], because the direction of the coordinate bond is favorable for fabricating desirable supramolecular structure. Tetrapyridylporphyrin (TPyP) is an important porphyrin derivative, which can form various coordinated supramolecular assemblies [13]. Qian et al. [14] fab-

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01166-7

H.-G. Liu et al. / Materials Letters 57 (2003) 2156–2161

N NH

N

N

HN

N

N

Pc

N N

N N H N

TPyP

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10 h and then compressed. The experimental temperature was 20 F 2 jC. Langmuir – Blodgett (LB) films of Eu(TPyP)Pc/AA were deposited onto hydrophobic quartz plates at 20 mN/m by vertical-dipping method on a NIMA 2000 alternate trough. UV – vis spectra and polarized UV – vis spectra were measured by means of a HP8453E UV – vis spectrophotometer. For the measurement of the orientation angle of the macrocycles, the plate coated with the LB films was mounted at an incident angle of 0j or 45j, using s- or p-polarized light as incident beam. The calculations of porphyrin tilt angles with respect to the surface of the quartz plates were carried out according to literature method [16].

3. Results and discussion

N

H N

3.1. p –A isotherms N

N

Eu Fig. 1. The structure of Eu(TPyP)Pc molecule.

ricated the supramolecular network of TPyP with Cd2 + at the air/liquid interface. In this letter, we report the creation of the columnar supramolecular assemblies and the infinite framework of Eu(TPyP)Pc (Fig. 1) at the air/water and air/Cd2 + aqueous solution interface through the stacking interaction and the coordinate interaction.

2. Experimental Eu(TPyP)Pc was synthesized and characterized [15]. Surface pressure – area (p – A) isotherms were recorded on a NIMA 601M trough (Great Britain). The monolayers were formed by spreading the chloroform solution of Eu(TPyP)Pc onto a pure water surface and a 0.1 M Cd2 + aqueous solution surface or by spreading the chloroform solution of Eu(TPyP)Pc/ arachidic acid (AA) (molar ratio: 1:10) on a 0.01 M Cd2 + aqueous solution surface by use of a microsyringe. After spreading, the monolayers were allowed to equilibrate for some time from 15 min to

Fig. 2a shows the p – A isotherms of Eu(TPyP)Pc monolayers on the pure water and 0.1 M Cd2 + aqueous solution surfaces. Stable monolayers were formed. The mean molecular area of Eu(TPyP)Pc is ˚ 2 at 20 mN/m on the pure water surface, a about 95 A value of which is very close to that of Ce(TPyP)Pc [8]. This similarity indicates that the TPyP and Pc dominate the monolayer properties of the sandwichtype complexes. It is very interesting that the mean molecular area of Eu(TPyP)Pc on the Cd2 + aqueous ˚ 2 at 20 mN/m, a value solution surface is about 224 A that is more than twice as large as that on the pure water surface. Moreover, this value and the p – A isotherms were found to be dependent on the equilibration time. During the experiments, we found that the isotherms are reproducible when the equilibration time reaches 8 h. These results may indicate that the close stacking of the molecules was broken and a new structure was formed. Since the distance between the diagonal ‘‘N’’ ˚ [17], and atoms of the TPyP macrocycles is 15.5 A ˚ the diagonal distance of the Pc macrocycles is 13 A 2 ˚ [9], the molecular area should be ca. 170 A if the macrocycles lie parallel to the water surface. Here, the ˚ 2 on the mean Eu(TPyP)Pc molecular area is only 95 A pure water surface; this smaller value may be attributed to the tilted orientation of the macrocycles, just like porphyrins without substituted chain [18] or

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(a) Eu(TPyP)Pc on pure water 10 min

-1

Surface pressure (mN m )

40 30

2+

on Cd solution 10 min 2h 5h 8h 13h

20 10 0 0

100

200

300

400

500

2

Area per molecule (Å )

Surface pressure (mN/m)

(b) 60

40

20

0

200

400

600

800

1000

2

Area per molecule (Å )

Fig. 2. p – A isotherms of Eu(TPyP)Pc monolayers at the air/water and air/Cd2 + aqueous solution interfaces (a) and the mixed monolayer of Eu(TPyP)Pc/AA (1:10) at the air/Cd2 + solution interface (b).

Ln(Pc)2 [19]. Although it is difficult to know the orientation angle based only on the molecular areas from the p – A isotherms [20], we can estimate that the orientation angle of the macrocycle may be around 30j according to the relations between the molecular structure and monolayer structure of tetra-substituted free base porphyrins summarized by us [21]. The polar pyridyl groups would prompt the TPyP to make contact with the water surface [8]. The columnar supramolecular systems may be anticipated for Eu(TPyP)Pc molecules at the air/water interface with slipped face-to-face configuration and the tilt orientation of porphyrin and phthalocyanine rings. The Eu(TPyP)Pc areas at the air/Cd2 + aqueous solution interface increase with the equilibration time, ˚ 2 at 20 mN/m and reach the maximum value of 224 A at 8 h after spreading. This value is larger than 170 ˚ 2. The difference may indicate that the distance A

between the centers of the macrocycles becomes larger and larger with increasing the time, which should be attributed to the coordinate interaction between the pyridyl groups and Cd2 +. The closely packed supramolecular structure is broken gradually, and 2-D coordination polymer is formed. The concentrations of the Cd2 + ions in the aqueous solutions have great influence on the coordinate reaction. During our experiments, we used 0.1, 0.01, 0.001 and 4  10 4 M Cd2 + solutions as the subphase. The results showed that the coordinate reaction took place on the former two subphases, while on the other two subphases, no distinct change of the molecular area could be observed no matter how long the time was. This suggests that there is equilibrium between the coordinate reaction and the stacking interaction. The stacking supramolecular system was formed immediately when Eu(TPyP)Pc was spread on the surface. However, the coordinate reaction is a slower process, which needs longer reaction time and higher concentration of Cd2 + to compete with the stronger stacking interaction and to transfer the columnar structure to the coordinate polymer. The Cd2 + concentration used in literature [8] was also higher. Weissbuch et al. [12] have studied the supramolecular assemblies of pyridyl derivative at the air/ 0.001 M Cd2 + aqueous solution interface, wherein the lower Cd2 + concentration was effective or showed ability to form the supramolecules just because the stacking interaction between the pyridyl derivatives is weaker than that between Eu(TPyP)Pc. It was difficult to transfer this monolayer onto solid substrate by vertical dipping method. In order to get the multilayer, the mixed monolayer of Eu(TPyP)Pc/ arachidic acid (AA) (molar ratio: 1:10) on 0.01 M Cd2 + aqueous solution was used. Fig. 2b shows the p – A isotherms of Eu(TPyP)Pc/AA (1:10) with the equilibrium time of 8 h. The molecular area of ˚ 2, close Eu(TPyP)Pc at 20 mN/m is found to be 218 A 2 ˚ , indicating that the AA molecules in the to 224 A mixed monolayer have no influence on the formation of the coordinate polymer. 3.2. UV – vis and polarized UV –vis spectra Fig. 3 shows the UV – vis spectra of Eu(TPyP)Pc chloroform solution, the mixed LB films of Eu(TPyP)Pc/AA deposited from pure water and 0.01

H.-G. Liu et al. / Materials Letters 57 (2003) 2156–2161

0.08

Films deposited from

Absorbance

0.01 M Cd2+ solution Pure water

0.06

Eu(TPyP)Pc in CHCl3 solution

0.04 0.02 0.00

400

600

800

1000

Wavelength (nm) Fig. 3. UV – vis spectra of Eu(TPyP)Pc chloroform solution, LB films of Eu(TPyP)Pc/AA (1:10) deposited from pure water surface and Cd2 + aqueous solution surface.

M Cd2 + aqueous solution surfaces. The Soret bands of porphyrin appear at 405, 417 and 409 nm, respectively. The Q bands of Pc are very weak because of the formation of PcS . [2,15]. The absorption band at ca. 476 nm corresponds to the k – k* transition of delocalized molecular orbits between the two rings. The Soret band of porphyrin shows a red shift in the LB films compared with that in solution, suggesting the electronic interactions between the porphyrin macrocycles. Moreover, this red shift in LB film deposited from Cd2 + solution surface is 4 nm, greatly smaller than that from pure water surface, 12 nm, indicating the weaker electronic interactions between the molecules on the Cd2 + solution surface. Fig. 4 shows the polarized UV –vis spectra of the Eu(TPyP)Pc/AA LB films from pure water and 0.01 M Cd2 + subphase surfaces. The orientation angles of porphyrin rings with respect to the substrate surface were calculated according to the literature method [16], and found to be about 38j and 32j in the LB films from pure water and Cd2 + aqueous solution surfaces, respectively. These values are in accordance with those estimated from p – A isotherms. Since the Pc and TPyP rings are parallel in one molecule of Eu(TPyP)Pc [8], they should have similar orientation angles in the LB film. The orientation angle of TPyP ring in the Eu(TPyP)Pc LB film is close to that of TPyP ring in the Ce(TPyP)Pc LB film [8] and pure TPyP ring [14],

indicating that the Pc ring has no significant influence on the orientation of Eu(TPyP)Pc. TPyP has four polar pyridyl groups, resulting in a group that is more hydrophilic than Pc. Hence, the TPyP ring would contact the water surface in the monolayer of Eu(TPyP)Pc. Furthermore, the orientation angle is close to the porphyrins with four polar groups, such as – NO2 [20] and – COOH [18]. It is now believed that the orientation of the porphyrin ring depends on the interaction between the ring and the subphase [18,20]. Our results support this conclusion. Moreover, the orientation angle of the macrocycles on the Cd2 + aqueous solution surface is slightly smaller than that on the pure water surface, which may be attributed to the coordinate interactions between the molecules and the ions in the subphase.

(a) 0.15 LiEu/AA(1/10) Pure water

0.12

0, non 0, p-

Absorbance

Eu (TPyP) Pc/AA (1/10) LB

0, s-

0.09

45, p45, s-

0.06 0.03 0.00 400

600

800

1000

Wavelength (nm)

(b) 0.08 LiEu(TPyP)Pc/AA (1/10) LB films deposited from

0.06

Absorbance

0.10

2159

Cd2+ aqueous solution non-, 0; p-, 0; p-, 45;

0.04

s-, 45; s-, 0

0.02

0.00 400

600

800

1000

Wavelength (nm) Fig. 4. Polarized UV – vis spectra of mixed LB films deposited from (a) the air/water interface; (b) the air/Cd2 + solution interface.

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4. Summary One-dimensional columns and two-dimensional coordinate framework of a double-decker complex Eu(TPyP)Pc were constructed by using Langmuir technique at the air/water interface. Both the stacking interaction of the macrocycles and the coordinate interaction of the macrocycle and the metal ions take an important role in the formation of the supramolecular assemblies. The method can be developed for the fabrication of more and more novel structures.

Acknowledgements

Fig. 5. Supramolecular structure of Eu(TPyP)Pc on the air/Cd2 + aqueous solution (a) and air/water (b) interfaces.

One Cd2 + ion coordinates with four nitrogen atoms on the subphase surface. Thus, we proposed a schematic model for the supramolecular structures of Eu(TPyP)Pc LB films, as shown in Fig. 5. By applying the exciton theory [22], we can give a further comparison for the results of isotherms and UV – vis spectra. The exciton interactions between the parallel transition moments depend on the angles (a) between the transition moment and the center-to-center line of the parallel transition moments and the center-to-center distance (R). The peak shift of the aggregate relative to the monomer is inversely proportional to R3. We have investigated the stacking model of this kind of porphyrin in the monolayers and suggested that the porphyrin rings stack in parallel to form the columnar supramolecules, and the orientation angle of rings (h) can be regarded as a for this kind of meso-tetra-substituted porphyrins with stronger polar groups [21]. According to the exciton theory, the distance between the adjacent Eu(TPyP)Pc in the monolayer at the air/water interface should be greater than that in the monolayer on the Cd2 + subphase surface. This is in good agreement with that of p – A isotherms.

We are grateful for the financial support of the NNSFC Grants (No. 29873027 and No. 29701002), the National Fundamental Research Key Project of China and the Korea Research Foundation Grant (No. 1999-015-DP0149).

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