Photochemical hole burning of porphin-cross-linked polymethylmethacrylate

Volume 2 15. number 5

CHEMICAL PHYSICS LETTBRS

10 December 1993

Photochemical hole burning of porphin-cross-linked polymethylmethacrylate Kazuaki Sakoda and Kazuhiko Kominami Electronicand Imaging MaterialsResearchLaboratories,TorayIndustries,Inc., I-I, Sonoyama l-chome, Otsu,Shiga 520, Japan Received 30 August 1993; in final form 28 September 1993

We synthesized polymetltylmethacrylate (PMMA) cross-linked with a tetraphenylporphin (TPP) derivative with four functional groups. The formation of the cross-linked polymer was confinned by means of the double detection technique of Se1permeation chromatography. The irreversible broadening, or the spectral diffusion, of the photochemical hole burned in this crosslinked PMMA is less than in TPP merely dispersed in PMMA. Moreover, the high molecular weight component of the crosslinked PMMA, in which the cross-linkages are expected to be densely formed, exhibits still less spectral diffusion. From these observations, we conclude that the covalent bonds between the TPP molecule and the host polymer suppress the changes of the microscopic stereostructure in the vicinity of the TPP molecule and decrease its spectral ditTusion at low temperatures.

1. Introduction Amorphous materials such as organic polymers are in metastable states in a thermodynamic sense and their structures are not at all invariant. If we embed a dye molecule in an amorphous polymer, then the microscopic environment of the former is expected to be gradually and irreversibly changing even at low temperatures. This kind of structural changes causes the variation of the interaction energy between the dye molecule and the host polymer, and this fact leads to the irreversible and random shift of the absorption band of the dye molecule, which is called spectral diffusion. This spectral diffusion can be observed as an irreversible broadening of a persistent spectral hole [ l-3 1. We previously reported the photochemical hole burning (PHB) of porphin derivatives with ionic substituents dispersed in polyvinylalcohol (PVA) [ 4-6 1, and in a layered silicate [ 7 1, where the photochemical holes were burned through tautomerization of the porphin ring [ 8 1. In these systems, the irreversible broadening of the hole under temperature cycles is small compared with a typical neutral porphin/polymer system, i.e. tetraphenylporphin (TPP ) dispersed in polymethylmethacrylate (PMMA) [ 61. We presume that this small spectral 488

diffusion is due to the rigidity of the microscopic stereostructure brought about by the hydrogen bonds formed among PVA molecules or the Coulomb attraction between the cationic porphins and the anionic silicate layers. In this Letter, we present a new approach toward the suppression of spectral diffusion. Namely, we will show that the spectral diffusion in PMMA crosslinked with a TPP derivative with four functional groups, which we call pcl-PMMA hereafter, is smaller than in PMMA doped with TPP. Moreover, we will show that the high molecular weight component of the as-grown pcl-PMMA, in which the cross-linkages are expected to be densely formed, exhibits still less spectral diffusion. The ideal chemical structure of pcl-PMMA studied in this Letter is shown in fig. 1. pcl-MA was synthesized by copolymerization of methylmethacrylate (MMA) and tetra(pmethacryloylaminophenyl)porphin and the formation of the cross-linked polymer was confirmed by means of the double detection technique of gel permeation chromatography (GPC) . All of the four functional groups of tetra(pmethacryloylaminophenyl)porphin did not necessarily react with MMA, and so, the copolymer, i.e. pclPMMA, is expected to contain TPP molecules with

0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

Then, we hydrolyzed tetra(pacetaminophenyl)porphin in cont. hydrochloric acid to obtain tetra (p aminophenyl)porphin. Finally, we esterified it with methacryloyl chloride in dry acetone under the presence of pyridine to obtain tetra(pmethacryloyl‘;““” aminophenyl)porphin. NHCYH2--PMMA The product of each reaction was purified by colCH3 umn chromatography. The formation of the aimed product at each stage of the synthesis was always confirmed by means of ‘H NMR, IR and W-VIS PMMA spectra.

Y”

‘;“”

PMMA-cYYONH CH3

CY !=MMA

=

10 December 1993

CHEMICAL PHYSICS LETTERS

Volume 2 15, number 5

FA

fCH;+

WIC07CH2bCH3

CH3

Fig. 1. Chemical structure of porphincross-linked polymethylmethacrylate (p&PI&IA). The TPP derivative with four functional groups, tetm(pmethacryloylaminophenyl)porphin, acts as a cross-linkage reagent.

one to four covalent bonds to the host PMMA.

2. Experimental 2.1. Sample preparation 2.1.1. Tetra(pmethacryloylaminophenyl)porphin tetra (p-methacryloylaminoWe synthesized phenyl)porphin according to the scheme shown in tig. 2. First, tetra(pacetaminophenyl)porphin was synthesized by means of the cyclixation of pyrrole and pacetaminobenxaldehyde in propionic acid [ 9 1.

2. I. 2. pcl-PMMA First, MMA was successively washed with acidulous water solution of sodium sulfite, 5% water solution of sodium hydroxide and 20% water solution of sodium chloride. Then, it was dried with sodium sulfate and distilled at 80 mmHg, 40°C. 200 mm01 (20 g) of MMA purified as above, 0.05 mm01 of tetra(pmethacryloylaminophenyl)porphin, 30 ml of methylethylketone dried with calcium chloride and 100 mg of axobis( isobutyronitrile) were put into a round-bottom flash, degas& and polymerized at 60°C for 2 h. The polymer was dissolved in 100 ml of methylethylketone and precipitated with 750 ml of methanol twice. We obtained 10.9 g of pclPMMA. Moreover, we separated a high molecular weight component from the as-grown pcl-PMMA by

NtiCOCl$

Q CHO

+lJ

d

L

CH3CONH

WlCOCH3

-

HCI

d

W NHCOCH)

CH2.CKH3)COCI N”2

H2N

iH2

-

ClSyCKH3)CONH

NHcoccH$-cH2

iHCOCCCH~-CH2

Fig. 2 Scheme for the synthesis of tetra(pmethacryloylaminophenyl)porphin.

489

Volume 2 15, number 5

CHEMICAL PHYSICS LETTERS

GPC. Film-shape samples for optical measurements were made by casting. 2.1.3. TPP/PMMA TPP (Wako Pure Chemical Industries) and PMMA (Aldrich) were dissolved in methylethylketone, and the two solutions were mixed and stirred. Then the mixture was cast into a laboratory dish, dried over 1 week and we obtained a film-shape sample. The concentration was 10 mM. 2.2. Sample analysis The amount of the cross-linkage reagent, i.e. tetra(p-methacryloylaminophenyl)porphin, was so small that we could not confirm the formation of the cross-linkages by usual spectroscopy like ‘H NMR or IR. So, we adopted the double detection technique of GPC. The apparatus was WATERS, GPC-244. In this method, the porphin unit was detected by W absorption at its Soret band (423 nm), whereas the MMA unit was detected by a differential refractometer. Therefore, the ratio of porphin unit to MMA unit at each molecular weight could be determined. As is fully described in the next section, we found that the higher molecular weight component contains more porphin units, and this is a clear evidence for the cross-linkage formation. 2.3. Opticalmeasurements Optical absorption spectra were measured by means of conventional transmission spectroscopy with a single-monochromator with focal length of 1 m (Jobin Yvon, THR-1000). The resolution was 0.02 nm (0.5 cm-’ ). The output signal of a photomultiplier (Hamamatsu, R453) was lock-in amplified and collected with a microcomputer (NEC, 98Olvm). The samples which were sandwiched between two transparent sapphire plates for the sake of good heat conduction were attached on the cold finger and cooled down in a flow-type cryostat (Air Products and Chemicals, LT-3-110). The hole buming was performed with a dye laser of standing wave type (LEXEL, model 700 ) with a tri-birefringent tilter and no etalon pumped by a cw argon laser (LEXEL, model 295 ) . A mixture of DCM and kiton red (4: 1) was used as an active dye. The spectral 490

10 December 1993

width of the dye laser was less than 0.5 cm- I.

3. Results and discussion The molecular weight distribution of the as-grown pcl-PMMA is shown in fg 3a. The average molecular weight is about 10’. Besides, we obtained the monomer ratio curve (fig. 3c) by means of the double detection technique mentioned above. In fig. 3c, we understand that the higher the molecular weight is, the larger becomes the portion of the porphin units which are contained in the polymer chain. This fact is a clear evidence for the cross-linkage formation. As a matter of fact, if we think of the growing process of the polymer chains, it is quite unlikely to happen that the four-functional TPP derivative reacts seleo tively with a longer polymer chain. The proper way for understanding the monomer ratio curve is to reverse the cause and the effect. Namely, the polymer chains which contained many TPP derivatives became densely cross-linked and came to be of high molecular weight. Now, all of the four functional groups of the TPP derivative did not necessarily participate in the covalent bonds to the host PMMA and we presume that pcl-PMMA contains TPP molecules with one to four covalent bonds to the host. As for pendant molecules, which have only one covalent bond to the host, the rigidness of their microscopic environment is not expected to be high compared with those which have more than one bond. Besides, the molecular weight

3

4

5

6

Log 04

Fig. 3. Molecular weight distribution of (a) as-grownpcl-PMMA and (b) a high molecularweight component of pcl-PMMA which was separated from as-grown pcl-PMMA by GPC, and (c) monomer ratio (TF’P/MMA) curve of as-grown pcl-PMMA.

of a polymer chain which has many pendant-type molecules is likely to be low because the chance of cross-linkage is small. Therefore, we postulated that the high molecular weight component of the as-grown pcl-PMMA, in which the cross-linkages are expected to be densely formed, exhibits less spectral diffusion. So, we separated a high molecular weight component by GPC, whose molecular weight distribution is shown in fig. 3b. Its average molecular weight was 6x 105. The absorption spectrum of the as-grown pclPMMA at 4.2 K is shown in fig. 4, which is quite similar to that of TPP simply dispersed in PMMA [ 51. Therefore, we understand that the covalent bonds were introduced between the four-functional TPP derivative and the host PMMA without large influence on the electronic state of the rt conjugated system of the porphin ring. The O-O band of the So-S1 transition is observed around 645 nm. A photochemical hole was burned in each sample at 5 K by the irradiation at 650 nm with the power density of 0.6 mW/cm2 for 5 min. Fig. 5 shows the irreversible increase of the half width, U, of the photochemical hole under temperature cycles. The samples were kept at each cycle temperature for 15 min. The hole widths were always measured at 5 K. For all samples, it was expected that the formed holes were broadened by saturation [ lo]. Moreover, the resolution of the monochromator and the spectral width of the dye laser were not sufficient for the accurate measurements of the absolute hole widths. Here, we confine our arguments within the relative changes of the hole widths. In comparison with the case of the simple dispersion of a porphin derivative into an amorphous

w 0

450

500

10 December 1993

CnEMxcAL PHYSICS LETrERs

Volume 2 15, number 5

550 Wavelength

600

*L-

650

700

(nm)

Fig. 4. Absorption spectrum of as-grown pcl-PMMA at 4.2 K.

Cycle Temperature(K)

Fig. 5 The irreversible increase of the half width, U, of phot+ chemical holes under temperature cycles, (a) TPP/PMMA, (b) as-grown pcl-PMMA, and (c) the high molecular weight component of pcl-PMMA. The photochemical holes were burned at 5 K. The samples were kept at each cycle temperate for 15 min. The hole widths were always measured at 5 K.

polymer, i.e. TPP/PMMA, the irreversible broadening of the photochemical hole, Lv: is small in the two cross-linked polymers. Especially, the high molecular weight component exhibits the least broadening as expected. From these observations, we can conclude that the reason for the small irreversible broadening in the cross-linked polymers is that the covalent bonds between the TPP molecule and the host polymer suppress the changes of the microscopic stereostructure in the vicinity of the TPP molecule.

4. Conclusion We synthesized the cross-linked PMMA by copolymerization of MMA and the TPP derivative with four functional groups. The formation of the crosslinked polymer, whose average molecular weight was 105, was confirmed by means of GPC double detection technique. The irreversible broadening of the photochemical hole under temperature cycles was less in this cross-linked PMMA than in TPP merely dispersed in PMMA. Moreover, the high molecular weight component with the average molecular weight of 6x lo’, in which the cross-linkages are expected to be densely formed, exhibits still less spectral dif491

Volume215,number5

CHEMICAL PHYSICSLETTRRS

fusion. Therefore, we can conclude that the covalent bonds between the TPP molecule and the host polymer suppress the changes of the microscopic stereostructure in the vicinity of the TPP molecule and decrease its spectral diffusion.

Acknowledgement

The authors are grateful to Dr. T. Goto of Toray Industries for his technical advice concerned with the double detection technique of GPC. They also would like to express their sincere thanks to Dr. Y. Ishimuro of Toray Research Center for GPC measurements. This work was performed under the management of Toray Industries as a part of the “R&D Project of Basic Technology for Future Industries” supported by NED0 (New Energy and Industrial Technology Development Organization ) .

492

10 December 1993

References [ 1] D.W.Pack, L.R. Narasimlm

and M.D. Fayer, J. Chem. Phys. 92 (1990) 4125. [2] KA. Littau, Y.S. Bai and M.D. Fayer, J. Chem. Phys. 92 (1990) 4145. [3] W. Koehler, J. Zollfrank and J. Friedrich, Phys. Rev. B39 (1989) 5414. [ 41 K. Sakoda, K. Kominami and M. Iwamoto, Japan J. Appl. Phys. 27 (1988) L1304. [ 51 K. Sakoda, K. Kominami and M. Iwamoto, Japan J. Appl. Phys. Suppl. 28-3 (1989) 229. [ 61 K Sakoda, K. Kominami and M. Iwamoto, Proceedings of the MRS International Meeting on Advanced Mathematics 12 (1989) 123. [7] K Sakoda and K. Kominami, Chcm. Phys. Letters, submitted for publication. [ 81 K Sakoda and M. Maeda, Chem. Phys. Letters, submitted for publication. [ 9 ] A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff, J. Org. Chem. 32 (1966) 476. [lo] S. Voelker, R.M. Macfarlane, A.Z. Genack, H.P. Trommsdorffand J.H. van der Walls, J. Chem. Phys. 67 (1977) 1759.