Near infrared dyes by combination of squaraine and ferrocene chromophores

Article abstract of DOI:10.1016/S0040-4039(00)00892-3

Squaraines represent a class of compounds which attracts a lot of attention in materials science. A synthetic sequence for the preparation of the symmetrical squaraines 12a,b, which contain ferrocene units as electron donors, is described. The compounds exhibit, in dichloromethane or chloroform, two intense absorption bands. One of them is located at 641/650 nm - a normal region for squaraines; however, the other band is strongly shifted to long wavelengths and has its maximum at 921/961 nm. Alkyl sidechains enhance the solubility of 12a,b, which represent a new type of NIR dyes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00892-3

Tetrahedron Letters 41 (2000) 5475±5478
Near infrared dyes by combination of squaraine and
ferrocene chromophores
Herbert Meier* and Ralf Petermann
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany
Received 9 May 2000; accepted 31 May 2000
Abstract
Squaraines represent a class of compounds which attracts a lot of attention in materials science. A synthetic
sequence for the preparation of the symmetrical squaraines 12a,b, which contain ferrocene units as electron
donors, is described. The compounds exhibit, in dichloromethane or chloroform, two intense absorption
bands. One of them is located at 641/650 nmÐa normal region for squaraines; however, the other band is
strongly shifted to long wavelengths and has its maximum at 921/961 nm. Alkyl sidechains enhance the
solubility of 12a,b, which represent a new type of NIR dyes. # 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: oxocarbon acids and derivatives; ferrocenes; dyes.
Squaraines, 1,3-diaryl substituted derivatives of squaric acid1 attract technical interest in
electrophotography,2 optical data storage,3 non-linear optics,4,5 conversion of solar energy6 and
as ¯uorescence markers.7 Some time ago we prepared a series of squaraines, whose chromophores
were extended by stilbene units.8 10 It turned out that the extension of the conjugation leads ®rst
to a strong bathochromic e€ect,8,9 but surprisingly the further extension of the conjugation causes
a hypsochromic e€ect.10 In order to come to NIR dyes, we replaced the dialkylamino or alkoxy
substituted stilbene units by building blocks, which contain ferrocenes as electron donors. Com-
pound 12 visualises such a target molecule. The four hydroxy groups guarantee an enhanced
stability.8,9 Moreover, alkyl side chains attached on the ferrocene moieties should increase the
solubility. The synthetic sequence was started with ferrocene (1). Alkylation with 3-bromo-
methylheptane yielded the rearranged substitution product 2 which contains the highly branched
1-ethyl-1-methylpentyl sidechain. An alternate twofold substitution of ferrocene was performed
according to the method described by Vogel et al.11 The Friedel±Crafts acylation 1!3 was fol-
lowed by a Clemmensen reduction 3!4. Both alkylated ferrocenes were subjected to a Vilsmeier
formylation which yielded the aldehydes 512 and 6.13 The resorcinol derivatives 7a,b with protected
* Corresponding author. Fax: 49 (0)6131 3925396; e-mail: hmeier@mail.uni-mainz.de
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00892-3

5476
hydroxy functions were transformed by quantitative Arbusov reactions to the phosphonates 8a,b.
Wittig±Horner reactions of 8a and 5 and 8b and 6 furnished the compounds 9a14 and 9b15 with
high trans selectivity. Deprotection of 9a was achieved by the cleavage of the methyl ethers
with lithium diphenylphosphide; the allyl ether groups in 9b were split with sodium borohydride
in the presence of tetrakis(triphenylphosphine)palladium. The methyl ether cleavage proved to be
the better and easier variant. The ®nal step consisted of the condensation of squaric acid (11)
with the resorcinols 10a,b. Although the generated water was continuously removed by azeo-
tropic distillation (n-butanol/toluene), the yields of the products 12a,b16,17 remained moderate
(Scheme 1).
Scheme 1.

5477
Diaryl squaraines exhibit, normally in dichloromethane or chloroform, a narrow but intense
absorption band with a maximum between 510 and 670 nm.2 The compounds 12a,b show a narrow
band in this region and additionally a broad absorption in the near infrared (NIR): 12a (CHCl3),
lmax/ log": 641/4.90 nm and 921/4.65 nm; 12b (CH2Cl2), lmax/ log": 650/4.92 nm and 961/4.66 nm.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for ®nancial support.
References
1. Schmidt, A. H. In Oxocarbons; West. R., Ed.; Academic Press: New York, 1980; pp. 185.
2. Law, K. Y. Chem. Rev. 1993, 93, 449 and references therein.
3. (a) Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem. 1989, 101, 1475; (b) Angew. Chem., Int. Ed.
Engl. 1989, 28, 1445.
4. Ashwell, G. J.; Je€eries, G.; Hamilton, D. G.; Lynch, D. E.; Roberts, M. P.; Bahra, G. S.; Brown, C. R. Nature
1995, 375, 385.
5. Chen, C.-T.; Marder, S. R.; Cheng, L. T. J. Am. Chem. Soc. 1994, 116, 3117.
6. Loutfy, R. O.; Hsiao, C. K.; Kazmaier, P. M. Photogr. Sci. Eng. 1983, 27, 5.
7. Ghazarossian, V.; Pease, J. S.; Ru, M. W. L.; Laney, M.; Tarnowski, T. L. Europ. Pat. Appl. EP 356173 Az, 1990;
Chem. Abstr. 1990, 113, 61319j.
8. Meier, H.; Dullweber, U. J. Org. Chem. 1997, 62, 4821.
9. Meier, H.; Petermann, R.; Dullweber, U. J. Prakt. Chem. 1998, 340, 744.
10. Meier, H.; Petermann, R.; Gerold, J. J. Chem. Soc., Chem. Commun. 1999, 977.
11. Vogel, M.; Rausch, M.; Rosenberg, H. J. Org. Chem. 1957, 22, 1016.
1
12. 5: Red oil; H NMR (CDCl3): ꢀ 0.72 (t, 3H, CH3), 0.85 (t, 3H, CH3), 1.16 (s, 3H, CH3), 1.2±1.6 (m, 8H, CH2),
4.08, 4.20 (AA0BB0, 4H, ferrocene), 4.56, 4.74, (AA0BB0, 4H, ferrocene), 9.93 (s, 1H, CHO); 13C NMR (CDCl3): ꢀ
8.5, 14.1, 25.1 (CH3), 23.4, 26.1, 32.7, 39.8 (CH2), 35.6 (Cq), 67.4, 68.3, 69.9, 73.6 (CH, ferrocene), 79.2, 103.0 (Cq,
ferrocene), 193.5 (CHO); despite the chiral centre, the molecule shows Cs symmetry; MS (FD): m/z 326 (100%),
ꢀ
[M+ ].
1
13. 6: Red oil; H NMR (CDCl3): ꢀ 0.84 (m, 6H, CH3), 1.23 (m, 36H, CH2), 1.40 (m, 2H, b-CH2), 1.50 (m, 2H, b-
CH2), 2.20 (m, 2H, a-CH2), 2.33 (m, 2H, a-CH2), 4.05 (m, 4H, ferrocene), 4.40 (s, 1H, ferrocene), 4.57 (m, 2H,
ferrocene), 9.84 (s, 1H, CHO); 13C NMR (CDCl3): ꢀ 14.0 (CH3), 22.6 (CH2), 28.8±31.8 (CH2, superimposed), 69.3,
69.4, 69.6, 69.7, 70.2, 70.2, 74.1 (CH, ferrocene), 78.8, 91.5, 95.1 (Cq, ferrocene), 193.6 (CHO); MS (FD): m/z 550
ꢀ
(100%), [M+ ].
14. 9a: Red oil; 1H NMR (CDCl3): ꢀ 0.72 (t, 3H, CH3), 0.85 (t, 3H, CH3), 1.20 (s, 3H, CH3), 1.22±1.60 (m, 8H, CH2),
3.80 (s, 6H, OCH3), 3.93, 4.04 (AA0BB0, 4H, ferrocene), 4.25, 4.41 (AA0BB0, 4H, ferrocene), 6.35 (t, 1H, benzene),
3
6.57 (d, 2H, benzene), 6.59, 6.83 (AB, J=16.1 Hz, 2H, ole®n. H); 13C NMR (CDCl3): ꢀ 8.6, 14.2, 25.3 (CH3),
23.6, 26.3, 33.0, 40.0 (CH2), 35.8 (Cq), 67.1, 67.3, 68.2, 69.7 (CH, ferrocene), 82.8, 101.5 (Cq, ferrocene), 99.1, 103.9
ꢀ
(CH, benzene), 125.8, 127.8 (ole®n. CH), 140.1, 161.0 (Cq, benzene); MS (FD): m/z 460 (100%), [M+ ].
1
15. 9b: Red oil; H NMR (CDCl3): ꢀ 0.86 (m, 6H, CH3), 1.25 (m, 36H, CH2), 1.42 (m, 2H, b-CH2), 1.48 (m, 2H, b-
CH2), 2.22 (m, 2H, a-CH2), 2.30 (m, 2H, a-CH2), 3.90 (m, 4H, ferrocene), 4.10, 4.24 (AB, 2H, ferrocene), 4.26 (s,
1H, ferrocene), 4.53 (m, 4H, OCH2), 5.29, 5.42, 6.06 (m, 6H, ole®n. H, allyloxy), 6.37 (t, 1H, benzene), 6.52, 6.74
3
(AB, J=16.1 Hz, 2H, ole®n. H), 6.58 (d, 2H, benzene); 13C NMR (CDCl3): ꢀ 14.0 (CH3), 22.6 (CH2), 28.9±31.9
(CH2), 66.7, 67.7, 68.8, 68.9, 69.3, 69.8, 70.8 (CH, ferrocene), 68.9 (OCH2), 82.4, 90.4, 91.0 (Cq, ferrocene), 100.5,
105.0 (CH, benzene), 117.5 (CH2, allyloxy), 133.4 (CH, allyloxy), 125.4, 128.0 (ole®n. CH), 140.2, 160.0 (Cq,
ꢀ
benzene); MS (FD): m/z 736 (100%), [M+ ].
16. 12a: Green, metallic crystals which start to decompose at 200ꢁC; 1H NMR (C2D2Cl4): ꢀ 0.65 (t, 6H, CH3), 0.79 (t,
6H, CH3), 1.12 (s, 6H, CH3), 1.14±1.40 (m, 16H, CH2), 3.94 (m, 4H, ferrocene), 4.05 (m, 4H, ferrocene), 4.50 (m,

5478
8H, ferrocene), 6.39 (s, 4H, benzene), 6.41, 7.23 (AB, 3J=16.1 Hz, 4H, ole®n. H), 10.97 (s, 4H, OH); MS (FD): m/
ꢀ
z 942 (100%), [M+ ].
17. 12b: Green, metallic crystals, m.p. 141ꢁC; 1H NMR (CDCl3): ꢀ 0.83 (m, 12H, CH3), 1.23 (m, 72H, CH2), 1.39 (m,
4H, b-CH2), 1.50 (m, 4H, b-CH2), 2.13 (m, 4H, a-CH2), 2.31 (m, 4H, a-CH2), 3.95 (m, 8H, ferrocene), 4.36 (m,
3
6H, ferrocene), 6.37, 7.15 (AB, J=16.1 Hz, 4H, ole®n. H), 6.38 (s, 4H, benzene), 11.02 (s, 4H, OH); 13C NMR
(CDCl3): ꢀ 14.1 (CH3), 22.7 (CH2), 28.8±32.0 (CH2, superimposed), 68.6, 69.1, 69.6, 70.3, 70.6, 70.8, 72.6 (CH,
ferrocene), 81.0, 91.6, 93.2, (Cq, ferrocene), 107.1 (CH, benzene), 124.0, 139.4 (ole®n. CH), 110.8, 153.1, 162.6,
ꢀ
167.4, 182.3 (Cq); MS (FD): m/z 696 (100%), [M2+], 1392 (30%), [M+ ].