Synthesis of the First Phthalocyanine-Containing Dendrimer.

The primary event in photosynthesis is the capture of sunlight by the light-harvesting antennas. These antennas absorb the sunlight and transmit the resulting singlet electronic excitation to the photochemical reaction centers.1,2 Disciplines Chemistry | Environmental Chemistry | Inorganic Chemistry | Organic Chemistry | Other Chemistry | Polymer Chemistry Comments Reprinted (adapted) with permission from The Journal of Organic Chemistry. 63(21); 7520-7521. Doi: 10.1021/jo980244q. Copyright 1998 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/542 Synthesis of the First Phthalocyanine-Containing Dendrimer George A. Kraus* and Steven Vander Louw Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received February 10, 1998 The primary event in photosynthesis is the capture of sunlight by the light-harvesting antennas. These antennas absorb the sunlight and transmit the resulting singlet electronic excitation to the photochemical reaction centers.1,2 The antenna is an assembly of intensely absorbing pigments that are oriented in such a way as to permit efficient transfer of the singlet electron to the reaction center. Natural systems contain hundreds of pigments per reaction center. The time needed for the absorption of light and resultant excitation at the reaction center seldom exceeds the tens of picoseconds in green plants.3-5 For the purposes of designing durable antennas, biodegradable pigments such as chlorophyll and bacteriochlorophyll are unattractive, since they are unstable outside of their protein hosts. However, phthalocyanines are extremely stable to heat, air, and light and mimic the spectral characteristics of chlorophyll a. Metalated phthalocyanines containing copper, zinc, cobalt, titanium, and gallium have been synthesized. The use of metallophthalocyanines containing silicon as the central atom avoids intersystem quenching of the excited state of the pigment. We envisioned the phthalocyanines being held at defined distances by connection as part of a dendrimer6 framework. Phthalocyanine 1 was synthesized by the method of Marks.7 The reaction of phenyl trichlorosilane with 1,3diiminoisoindoline generated phthalocyanine 1 in 39% yield. Compound 1 was reacted with excess sodium hydroxide in boiling pyridine following the procedure of Davison and Wynne8 to generate the hydroxy compound 2 (abbreviated as PC-OH) in quantitative yield. The synthesis of the dendrimer began by reacting the mono benzyl ether of propane-1,3-diol9 with cyanuric chloride to produce 3 in 68% yield. With the monosubstituted triazine readily available, the next step was to attach the silicon phthalocyanines. Although the reaction of chlorotriazines with nucleophiles is well precedented, there was no precedent for the use of silyloxy phthalocyanines as nucleophiles.10 Indeed, the reactions of compounds such as 2 seemed to be limited to silylation and acylation reactions. Interestingly, the substitution using the sodium salt of phthalocyanine 2 proceeded smoothly in chloroform using one equivalent of 2 and sodium hydride as a base to give the disubstituted triazine 4 in 71% yield. If another equivalent of the phthalocyanine anion was added to 4 and allowed to react over a four-day period in boiling chloroform, bis phthalocyanine adduct 5 was produced in 54% yield. The cleavage of the benzyl protecting group was readily accomplished with iodotrimethylsilane (TMSI) in chloroform at 25 °C over 30 min to afford the alcohol 6 in quantitative yield.11 A small dendrimer molecule would serve as a model system for the spectroscopy studies. To form this model system, compound 6 was treated with EDCI (1-(3dimethylaminopropyl)-3-carbodiimide hydrochloride), (dimethylamino)pyridine (DMAP), and 1,3,5-benzenetricarboxylic acid (7) to form dendrimer 8 in 81% yield. Although some dendrimers containing porphyrins have recently been reported, phthalocyanine 8 is the first dendrimer with phthalocyanine units surrounding a central core.12 Compound 8 will be useful for exploring energy transfer in photosynthetic systems. Experimental Section Unless otherwise noted, materials were obtained from commercial suppliers and were used without purification. Acetonitrile was purified by distillation from calcium hydride. The purity of all title compounds was determined to be >95% by 300 MHz proton NMR and/or elemental analysis. PcSi(Ph)(OH) (2). To a solution of 1 (1.00 g, 1.5 mmol) in 15 mL of boiling pyridine and 2 mL of water was added sodium hydroxide (0.245 g, 4 equiv). After (1) van Grondelle, R. Biochim. Biophys. Acta 1984, 811, 147. (2) van Grondelle, R.; Dekker, J. P.; Gillbro, T.; Sundstom, V. Biochim. Biophys. Acta 1994, 1187, 1. (3) Owens, T. G.; Webb, S. P.; Mets, L.; Alberte, R. S.; Fleming, G. R. Biophys. J. 1989, 56, 95. Owens, T. G.; Webb, S. P.; Mets, L.; Alberte, R. S.; Fleming, G. R. Biophys. J. 1988, 53, 733. (4) Holzwarth, A. R. Excitation dynamics in antennae and reaction centers of photosystems I and II; Murata, N., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; p 187. (5) Hastings, G.; Durrant, J. R.; Barber, J.; Porter, G.; Klug, D. R. Biochemistry 1992, 31, 7638. (6) Junge, D. M.; McGrath, D. V. J. Chem. Soc., Chem. Commun. 1997, 857. Newkome, G. R. J. Heterocycl. Chem. 1996, 33, 1445. (7) Dirk, C. W.; Inabe, T.; Schoch, K. F.; Marks, T. J. J. Am. Chem. Soc. 1983, 105, 1539. (8) Davison, J. B.; Wynne, K. J. Macomolecules 1978, 11, 186. (9) For a preparation of the alcohol, see the following: Kotsuki, H.; Ushio, Y.; Yoshimura, N.; Ochi, M. J. Org. Chem. 1987, 52, 2594. (10) For the reaction of cyanuric chloride with alkoxides, see the following: Hirt, R.; Nidecker, H.; Berchtold, R. Helv. Chim. Acta 1950, 33, 1365. (11) Jung, M. E.; Lyster, M. A. J. Org. Chem. 1977, 42, 3761. (12) Bhyrappa, P.; Young, Y. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. 7520 J. Org. Chem. 1998, 63, 7520-7521 S0022-3263(98)00244-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/17/1998 heating for 4 h, the reaction mixture was cooled to room temperature and filtered. The remaining solid was washed with refluxing methanol in a Soxlet extractor for 24 h. The solid was then dried under reduced pressure at 110 °C for 3 h to give 0.950 g (99%) of product as a green powder. 1H NMR (CD3OD) δ 6.81 (s, 5H), 8.378.42 (m, 8H), 9.63-9.69 (m, 8H). IR (Nujol) 1077, 1072, 916, 830, 794 cm-1. Anal. calcd for C38H22N8OSi: C, 71.91; H, 3.49; N, 17.65. Found: C, 72.04; H, 3.58; N, 17.77. 2,4-Dichloro-6-(3-benzyloxy-1-propanoxy)-1,3,5triazine (3). To a solution of the monobenzyl ether of 1,3-propanediol (1.75 g, 11.0 mmol) in 22 mL of freshly distilled chloroform was added 0.463 g of sodium hydride (11.5 mmol, 1.1 equiv). In a separate flask, 1.93 g of cyanuric chloride (11.0 mmol, 1 equiv) was dissolved in 22 mL of freshly distilled chloroform and heated to 50 °C. The sodium hydride/alcohol solution was then added dropwise with stirring to the cyanuric chloride solution over a period of 1 h. After 4 h of stirring, 50 mL of water was added and the layers were separated. The aqueous layer was then extracted with methylene chloride and the combined organic layers dried over sodium sulfate. Removal of the solvent left 2.35 g of triazine ether 3 as a colorless oil. 1H NMR (CDCl3) δ 2.09-2.13 (m, 2H), 3.65 (t, J ) 5 Hz, 2H), 4.53 (s, 2H), 4.61 (t, J ) 5 Hz, 2H), 7.28 (s, 5H). IR (film) 3030, 2975, 1675, 1460, 1107 cm-1. MS (CI, m/z) 318 (M+ + 4), 316 (M+ + 2), 314 (M+), 166. HRMS m/z calculated for C13H13O2N3Cl2, 313.0385, measured, 313.0389. Triazine Phthalocyanine ether (4). To a solution of 2 (0.100 g, 0.16 mmol) in 20 mL of freshly distilled chloroform was added 6.8 mg of sodium hydride (0.17 mmol, 1.1 equiv). In a separate flask, 54.0 mg of 3 (0.17 mmol, 1.1 equiv) was dissolved in 10 mL of freshly distilled chloroform and heated to 50 °C. The sodium hydride/alcohol solution was then added dropwise with stirring to the triazine solution over a period of 1 h. After stirring overnight, 50 mL of water was added and the solution filtered. The solid was washed with hexane and methanol until the filtrate was colorless and then dried in vacuo to afford 0.103 g (71%) of 4 as a green powder. 1H NMR (CDCl3) δ 2.09-2.24 (m, 2H), 3.68 (t, J ) 5.0 Hz, 2H), 4.59 (s, 2H), 4.64 (t, J ) 5.0 Hz, 2H), 6.81 (s, 5H), 7.28 (s, 5H), 8.37-8.52 (m, 8H), 9.65-9.82 (m, 8H). Anal. calcd for C51H34N11ClO3Si: C, 67.14; H, 3.76; N, 16.88. Found: C, 67.01; H, 3.82; N, 16.96. Triazine Bisphthalocyanine Ether (5). To a solution of 2 (63.5 mg, 0.10 mmol) in 20 mL of freshly distilled chloroform was added 4.4 mg of sodium hydride (0.11 mmol, 1.1 equiv). In a separate flask, 0.100 g of 4 (0.11 mmol, 1.1 equiv) was dissolved in 20 mL of freshly distilled chloroform and heated to 50 °C. The sodium hydride/alcohol solution was then added dropwise with stirring to the triazine solution over a period of 1 h. After 4 days of stirring, 50 mL of water was added and the solution filtered. The solid was washed with hexane and methanol until the filtrate was colorless and then dried in vacuo to afford 81.5 mg (54%) of 5 as a deep green powder.1H NMR (CDCl3) δ 2.09-2.24 (m, 2H), 3.68 (t, J ) 5.1 Hz, 2H), 4.59 (s, 2H), 4.64 (t, J ) 5.1 Hz, 2H), 6.81 (s, 10H), 7.28 (s, 5H), 8.37-8.52 (m, 16H), 9.65-9.82 (m, 16H). Anal. calcd for C89H55N19O4Si2: C, 70.77; H, 3.67; N, 17.61. Found: C, 70.85; H, 3.60; N, 17.48. Triazine Bisphthalocyanine Alcohol (6). To a solution of 5 (0.100 mg, 0.066 mmol) in 15 mL of freshly distilled chloroform was added 12.2 μL of iodotrimethylsilane (0.086 mmol, 1.3 equiv). After 30 min of stirring, 15 mL of methanol was added and the solution filtered. The solid was washed with hexane and methanol until the filtrate was colorless and then dried in vacuo to afford 99.6 mg (quant) of 6 as a deep green powder. 1H NMR (CDCl3) δ 2.07-2.28 (m, 2H), 3.67 (t, J ) 5.1 Hz, 2H), 4.68 (t, J ) 5.1 Hz, 2H), 6.81 (s, 10H), 8.37-8.52 (m, 16H), 9.65-9.82 (m, 16H). Anal. calcd for C82H49N19O4Si2: C, 69.33; H, 3.47; N, 18.72. Found: C, 69.19; H, 3.40; N, 18.60. Phthalocyanine Dendrimer (8). To a solution of 1,3,5-tribenzoic acid (48.9 mg, 0.023 mmol) in 22 mL of freshly distilled chloroform was added 13.4 mg of 1-(3dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (0.07 mmol, 3.1 equiv) and 1.4 mg of 4-(dimethylamino)pyridine (0.012 mmol, 0.5 equiv). After stirring for 10 min, 0.100 g of 6 (0.07 mmol, 3 equiv) was added to the solution. After stirring for 2 days, the reaction was quenched with water and filtered. The remaining green powder was washed with water and acetone until the filtrate was colorless. The solid was then extracted in a Soxlet extractor for 48 h using acetone as the solvent. Drying of the solid in vacuo afforded 0.254 g (81%) of 8 as a green powder. 1H NMR (CDCl3) δ 2.07-2.28 (m, 6H), 3.67 (t, J ) 5.1 Hz, 6H), 4.68 (t, J ) 5.1 Hz, 6H), 6.81 (s, 30H), 7.45 (s, 3H), 8.37-8.52 (m, 48H), 9.659.82 (m, 48H). UV (MeCN) 281, 355, 440 nm. Anal. calcd for C255H147N57O18Si6: C, 68.59; H, 3.32; N, 17.87. Found: C, 68.47; H, 3.23; N 17.70. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. The Ames Laboratory is operated by Iowa State University for the U. S. Department of Energy under Contract No. W-7405-Eng-82. JO980244Q Notes J. Org. Chem., Vol. 63, No. 21, 1998 7521