Preparation and characterization of novel double-decker rare-earth phthalocyanines substituted with 5-bromo-2-thienyl groups
© The Author(s) 2017
Received: 20 October 2016
Accepted: 28 March 2017
Published: 5 April 2017
A series of rare-earth bisphthalocyanines of praseodymium, samarium and gadolinium bearing 5-bromo-2-thienyl substituents were prepared for the first time.
Three bis[octakis(5-bromo-2-thienyl)] rare-earth metal(III) bisphthalocyanine complexes (Pr, Sm, Gd) were synthesized for the first time. The new compounds were characterized by UV–vis, NIR, FT-IR, mass spectroscopy and thermogravimetry as well as elementary analysis and electrochemistry. Production of singlet oxygen was also estimated using 9,10-dimethylanthracene method.
KeywordsRare-earth bisphthalocyanines UV–vis spectroscopy NIR spectroscopy Singlet oxygen production Reduction Cyclic voltammetry Acid stability Thermogravimetry
Double-decker rare-earth phthalocyanines were firstly reported by Kirin  in 1965. Since then, they found a lot of applications. Among them are colour and electrochromic displays , gas sensors , field-effect transistors  and nonlinear optical materials . Widely studied are also their magnetic  and conducting properties . For these applications, many unsubstituted and substituted derivatives were prepared and evaluated to date. Thiophene moieties as strong donors are very often adopted for tailoring electronic properties of many classes of compound studied for applications in organic electronics . Recently, a series of three thiophene-substituted rare-earth bisphthalocyanines of gadolinium, praseodymium and samarium were studied by our group . It was found that the compounds were very sensitive to the presence of an acid yielding metal-free phthalocyanines irreversibly. This unexpected instability can limit their use for organic electronics. Our working hypothesis was that the acid stability should be increased if suitable group is attached to the 2-position on the thiophene cycle. For this purpose, a bromo substituent was introduced to the phthalocyanine scaffold. The aim of this study was to evaluate the effect of this modification on their physical, photo-physical and electrochemical properties.
All starting materials were obtained from Aldrich and Penta, and were used without further purification. Unsubstituted phthalocyanines were prepared according to the literature procedure .
The ultraviolet–visible (UV–vis) spectra were measured within the range of 300–900 nm on a UNICAM UV/VISIBLE Spectrophotometer, Helios Beta. The near infra-red (NIR) spectra were measured within 800–2100 nm on a PerkinElmer Lambda 1050 UV/VIS/NIR spectrometer. FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer. Thermogravimetric analyses were performed using a Mettler Toledo TGA/DSC 1 STARe System in a 70 ll alumina crucible. A small amount of the test compound (6–7 mg) was weighed into the measuring crucible and heated using a controlled temperature program between 25 and 700 °C using a gradient of 10 °C min−1. A flow of nitrogen (about 20 ml min−1) was used as a protective gas. During the heating process weight-curves were recorded over the complete temperature range. Elemental analyses were obtained using a FISONS EA 1108 automatic analyser. Matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOF) were measured on a MALDI mass spectrometer LTQ Orbitrap XL equipped with nitrogen laser. Positive-ion and linear mode of the compounds were obtained in trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile matrix for 2 and 3 and 2,5-dihydroxybenzoic acid matrix for 4 using nitrogen laser accumulating 10 laser shots. Electrochemical measurements were carried out in 1,2-dichloroethane containing 0.1 M Bu4NPF6. Cyclic voltammetry (CV) and rotating disk voltammetry (RDV) were used in a three electrode arrangement. The working electrode was platinum disk (2 mm in diameter) for CV and RDV experiments. As the reference and auxiliary electrodes were used saturated calomel electrode (SCE) separated by a bridge filled with supporting electrolyte and a Pt wire, respectively. All potentials are given vs. SCE. Voltammetric measurements were performed using a potentiostat PGSTAT 128N (Metrohm Autolab B.V., Utrecht, The Netherlands) operated via NOVA 1.11 software.
Preparation of bis[octakis-(5-bromo-2-thienyl)phthalocyaninato] rare-earth metal(III) phthalocyanines (2–4)
Results and discussion
The synthesized complexes 2–4 were characterized by several spectroscopic techniques—UV–vis, NIR, FT-IR, MALDI-TOF, thermogravimetry and elemental analysis. Proton NMR were measured in CDCl3 or THF-d 8. No analysable signals were obtained, even by using a published trick  with oxidation with a large excess of bromine. The reduced forms (after addition of NaBH4 in THF-d 8) also showed paramagnetism.
In these sandwiches (neutral compounds), one phthalocyanine ring is the classical dianion and the second one is the radical anion with charge −1. With a trivalent rare-earth metal cation, they form a neutral compound. Generally, in solutions they exist in two forms—a neutral and a reduced form. The distribution depends (Additional file 3) on the polarity and basicity of the solvent. The exact form in solutions are discussed in respective sections of the article.
UV–vis spectral characteristics
The shape of the spectra changed completely upon reduction. The peaks characteristic for neutral form disappeared and only peaks of triethylamine at ~1400, 1700–1800 nm were observed .
The analogous bisphthalocyanines bearing thiophene moieties have shown a very limited stability in dilute acids . The next experiments were made to clarify if addition of Br as a heavy bulky substituent in 2-position on the thiophene cycle would increase acid stability. Acetic acid was chosen for stability tests due its higher compatibility with many solvents.
Similar behaviour was confirmed for 2 and 3 (Additional file 3). The difference between the series lied only in the rate of conversion from the reduced to the neutral form. While the reaction for 3 and 4 is completed within 30 min, the reaction of 2 took several hours. This behaviour corresponds well with potential of first oxidation (see Table 2).
From the comparison, it is apparent that the bromo substituent is sufficiently capable to stabilize the compounds effectively and confirmed our hypothesis mentioned in the introduction of the article.
The FT-IR spectra of 2–4 are shown in Additional file 4. In the spectra, there are many characteristic peaks which are only minimally dependent on the rare-earth metal. The huge peak appearing at 3400–3500 cm−1 is O–H vibration from residual humidity present in KBr. The peaks located at about 3095, 2923 and 2852 cm−1 are stretching C–H vibrations of thiophene substituent at the periphery. There is no sharp peak at 2250 cm−1 indicating that the prepared samples were sufficiently purified from the starting nitrile. The peak at 1610 cm−1 is typical for phthalocyanines and corresponds to the C=C vibration of the benzene ring. The peaks at 1477, 1446, 1382, 1313, 1284, 1198, 1089, 984, 967, 902, 883, 760, 749 and 693 cm−1 characterize stretching and bending vibrations of benzene, pyrrole, isoindole and thiophene. The peak at 795 cm−1 is typical for C–Br vibration and it is shifted by 20 cm−1 to longer wavenumber compared to 5-methyl-2-bromothiophene .
Singlet oxygen production
Phthalocyanines belong to a large group of the so-called photosensitizers. Photosensitizers are materials which are capable to generate singlet oxygen (1O2) from everywhere-present triplet oxygen upon illumination with the light of suitable wavelength. The ability to generate 1O2 is characterized by singlet oxygen quantum yield Φ.
The singlet oxygen quantum yield was determined according to a reported procedure using 9,10-dimethylanthracene (DMA) . The test compound was dissolved in DMF (1 mg l−1). The neutral form was prepared in situ by addition of diluted bromine. The decrease in absorbance was monitored using a UNICAM UV/VISIBLE Spectrophotometer, Helios Beta at 381 nm. The samples were irradiated with a red laser light (Maestro CCM, λmax = 661 nm) to decrease the absorbance of DMA solution to ca. 0.2–0.3. The measurements were triplicated and no degradation of phthalocyanines during irradiation was observed. The obtained reaction half-times were corrected to the unit absorbance of the sample and related to the zinc phthalocyanine (Φ = 0.56) .
Spectral and photochemical data for phthalocyanines 2–4 in DMF
Q-bands (λmax, nm, log ε)
Q-bands (λmax, nm, log ε)
0.03 ± 0.01
0.08 ± 0.01
Electrochemical data of 2–4
E1/2 (ox1) (V)a
E1/2 (ox2) (V)a
E1/2 (red1) (V)a
The first reduction potentials range from −0.74 to −0.76 V vs. ref., hence there are just small differences between the first reduction potentials within the series. Moreover, more reduction processes were observed but they almost merge into one. Again, when comparing first reduction potentials with previously published data [9, 18], there are not big differences, this means that variation in the substitution influences more oxidation than reduction centre.
Three rare-earth metal bisphthalocyanines bearing 5-bromo-2-thienyl groups were synthesized for the first time. Their purification was achieved by flash chromatography using cellulose as an adsorbent. The prepared complexes exhibit good solubility in many organic solvents such as DMF, THF, chloroform, dichloromethane and acetone. The compounds were characterized by UV–vis, NIR, MALDI, FT-IR, thermogravimetry and elemental analysis.
Two forms of studied compounds were identified in solutions. The first form is a reduced Pc which has two maxima at 660 and 720 nm. This form has no signal in NIR area. The second form is a neutral form with one maximum located at ~700 nm. There are several characteristic peaks in NIR area. The distribution of the forms is dependent on the solvent (polarity and basicity) and the central metal. The compounds were found in reduced forms in most solvents. Transformation of the reduced form to a neutral can be achieved either by addition of small amount of acid (AcOH) or an oxidant like Br2. With increased concentration of Br2, the compounds are further oxidized to Pc+ and the spectra are red shifted to about 750 nm. Our hypothesis that the attachment of Br atom on the thiophene cycle should increase the acid stability was successfully confirmed. No degradation in diluted acids was found in contrary to non-brominated analogues.
Compared to thiophene-substituted rare-earth phthalocyanines a significant decrease in quantum yield of singlet oxygen Φ was found. This is in good agreement with high degree of paramagnetism found during NMR experiments. The electrochemical investigation of studied compounds has shown that the variation of central metal does not bring significant changes in the first oxidation (reduction) and HOMO (LUMO) respectively. Anyway, in comparison to previously published electrochemical data [9, 18], the substitution influences more oxidation than reduction (more HOMO than LUMO).
near infra-red spectroscopy
Fourier transformed infra-red spectroscopy
highest occupied molecular orbital
lowest unoccupied molecular orbital
single occupied molecular orbital
nuclear magnetic resonance
matrix-assisted laser desorption/ionization time of flight mass spectrum
rotating disk voltammetry
saturated calomel electrode
- 1O2 :
quantum yield of singlet oxygen
molar absorption coefficient
- λmax :
maximum wavelength of absorption
JČ performed the synthesis and characterization (except NMR, MALDI-TOF and CV) of bisphthalocyanines and wrote the manuscript. LD and PH performed the synthesis of phthalocyanine precursors. AL measured NMR spectra. TM measured and evaluated CV of bisphthalocyanines. FB investigated the MALDI-TOF spectra. All authors read and approved the final manuscript.
The authors acknowledge the financial support of the Czech Science Foundation (Grant No. 14-10279S). We also appreciate the help of Michal Novotný from Institute of Physics of the Czech Academy of Sciences for the co-operation with measurement of NIR spectra.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
- Kirin IS, Moskalev PN, Makashev YA (1965) Formation of phthalocyanines of rare-earth elements. Zh Neorg Khim 10:1951–1953Google Scholar
- Simic-Glavaski B (1993) Phthalocyanine-based molecular electronic devices. Phthalocyanines 3:119–166Google Scholar
- Trometer M, Even R, Simon J, Dubon A, Laval JY, Germain JP, Pauly A, Robert H (1992) Lutetium bisphthalocyanine thin films for gas detection. Sensor Actuat B Chem 8:129–135View ArticleGoogle Scholar
- Hatano M, Konami H (1991) Structures and properties of multi-layered lanthanide phthalocyanine complexes. Senryo to Yakuhin 36:63–75Google Scholar
- Shirk JS, Lindle JR, Bartolli FJ, Boyle ME (1992) Third-order optical nonlinearities of bis(phthalocyanines). J Phys Chem 96:5847–5852View ArticleGoogle Scholar
- Ishikawa N, Sugita M, Ishikawa T, Koshihara S-Y, Kaizu Y (2003) Lanthanide double-decker complexes functioning as magnets at the single-molecular level. J Am Chem Soc 125:8694–8695View ArticleGoogle Scholar
- Souto J, Aroca R, DeSaja JA (1994) Gas adsorption and electrical conductivity of Langmuir–Blodgett films of terbium bisphthalocyanine. J Phys Chem 98:8998–9001View ArticleGoogle Scholar
- Lind SJ, Gordon KC, Gambhir S, Officer DL (2009) A spectroscopic and DFT study of thiophene-substituted metalloporphyrins as dye-sensitized solar cell dyes. Phys Chem Chem Phys 11:5598–5607View ArticleGoogle Scholar
- Černý J, Dokládalová L, Lyčka A, Mikysek T, Bureš F (2016) Preparation, characterization and investigation of photo-physical properties of thiophene-substituted rare-earth bisphthalocyanines. J Porphyr Phthalocyanines 20:1–6Google Scholar
- Gürek AG, Ahsen V, Luneau D, Pécaut J (2001) Synthesis, structure, spectroscopic properties, and magnetic properties of an octakis(Alkylthio)-substituted lutetium(III) bisphthalocyanine. J Inorg Chem 40:4793–4797View ArticleGoogle Scholar
- Gürol I, Durmuş M, Ahsen V (2012) Investigation of photophysical and photochemical properties of octa-substituted double-decker rare-earth metallophthalocyanine complexes. J Porphyr Phthalocyanines 16:907–916View ArticleGoogle Scholar
- Ayhan MM, Singh A, Jeanneau E, Ahsen V, Zyss J, Ledoux-Rak I, Gürek AG, Hirel C, Bretonnière Y, Andraud C (2014) ABAB homoleptic bis(phthalocyaninato)lanthanide(III) complexes: original octupolar design leading to giant quadratic hyperpolarizability. Inorg Chem 53:4359–4370View ArticleGoogle Scholar
- Oliveira JIS, Pires DC, Diniz MF, Siqueira JL, Mattos EC, Rezende LC, Iha K, Dutra RCL (2014) Determination of primary amine content in bonding agent in composite solid propellants. Propell Explos Pyrot 39:538–544View ArticleGoogle Scholar
- Kamigata N, Suzuki T, Yoshida M (1990) Novel halogenation of thiophenes with benzeneseleninyl chloride and aluminium halide. Phosphorus Sulfur 53:29–35View ArticleGoogle Scholar
- Černý J, Karásková M, Rakušan J, Nešpůrek S (2010) Reactive oxygen species produced by irradiation of some phthalocyanine derivatives. J Photochem Photobiol A Chem 210:82–88View ArticleGoogle Scholar
- Lee PPS, Lo PC, Chan EYM, Fong WP, Ko WH, Ng DK (2005) Synthesis and in vitro photodynamic activity of novel galactose-containing phthalocyanines. Tetrahedron Lett 46:1551–1554View ArticleGoogle Scholar
- Venediktov EA (2004) Deactivation of O2 (1Δg) by diphthalocyanines of rare-earth metals. Zh Fiz Khim 78:575–576Google Scholar
- Orman EF, Koca A, Özkaya AR, Gürol I, Durmuş M, Ahsen V (2014) Electrochemical, spectroelectrochemical, and electrochromic properties of lanthanide bis-phthalocyanines. J Electrochem Soc 161:H422–H429View ArticleGoogle Scholar