Anthracene functionalized terpyridines – synthesis and properties

The synthesis of several symmetrically 4,4″-functionalized 2,2′:6′,2″-terpyridines is reported. In addition to the biscarboxylic acid 4,4″-tpy(CO2H)2 (3), the anthryl esters 4,4″-tpy(CO2CH2Anth)2 (5a) and 4,4″-tpy(CO2CH2CH2OAnth)2 (5b) were synthesized. Furthermore, both anthryl esters were used to synthesize symmetric iron(II)-bis(terpyridine)complexes 6a–b. Irradiation experiments were carried out with both the free ligands and an iron(II)-complex in order to investigate the photochemistry of the compounds.

Introduction 2,2′:6′,2″-Terpyridines have been of great interest over the recent years, principally because of their ability to chelate transition metals. The special (photochemical) properties of their metal complexes have led to the development of various luminescent metal compounds [1] and sensitizers for photovoltaic devices [2,3]. Ditopic terpyridyl units have been recently used to develop electrochemical sensors [4,5]. A microreview concerning the synthesis of functionalized terpyridines has also been published, since the electronic properties of the ligand are influenced by the substituents present [6].
Because of this impact of terpyridine derivatives in photochemistry, we focused our attention on the synthesis and studies of potentially photoswitchable terpyridine ligands. The synthesis of bisterpyridines linked by a diazogroup has been reported [7,8]. As well as the connection of terpyridines to spiropyran moieties [9] and diarylethenes [10,11]. There have also been reports about anthracene functionalized terpyridines in which an anthracene unit was used as a fluorescent sensor [12], spacer [13] or intercalator [14]. Herein we report the synthesis of two twofold anthracene functionalized terpyridines, their iron(II) complexes, and investigations regarding their photochemistry. Scheme 2: Synthesis of the terpyridine-4,4″-bisanthrylesters 5a and 5b. The resulting esters 5a and 5b could be isolated in good yields as pale yellow solids.

UV-vis spectra and irradiation experiments Free ligands
The free ligands 5a and 5b were investigated with respect to their UV-vis absorption spectra and their ability to undergo a [4 + 4]-cycloaddition reaction. A degassed 5·10 −5 × M solution of 5a in methylene chloride was irradiated with UV light (λ = 350 nm, Figure 1).
The absorption spectrum of 5a (solid line) shows the expected absorption pattern for a compound containing anthryl residues. Upon UV irradiation the absorption decreases which is indicative of cycloaddition of the anthryl moieties (dashed lines). There are however, no indications of any cyclo-reversion, neither thermally nor upon irradiation with visible light. Similar results were also obtained with compound 5b (Figure 2).
A degassed 4 × 10 −5 M solution of 5b in methylene chloride was irradiated and the originally observed absorption pattern due to the anthryl moieties decreased. As was the case with 5a, the absorption pattern due to the anthryl moieties could not be regenerated thermally or by irradiation.  NMR-and MALDI-TOF-measurements were of no assistance in establishing the identity of the photoproducts from 5a and 5b after UV irradiation. 1 H NMR-spectra of the irradiation products (after evaporation of solvent) indicate that there is more than one reaction pathway, the number and overlapping of peaks (especially in the aromatic region) made assignments impossible. Irradiation experiments were also carried out on the NMR scale -however, the required reagent concentration (10 −5 mol/L for intramolecular cycloadditions) made it impossible to record useful NMR spectra. MALDI-TOF meas-urements did not indicate any degradation products -the [4 + 4]-cycloadducts cannot be distinguished from the reagents, because their molecular formulas and hence their molecular masses are identical.

Irradiation of a Fe(II)-complex
To see whether the presence of iron(II) influences the photochemistry of the ligands 5a and 5b, we irradiated the iron(II) complex 6b in acetonitrile solution with near-UV light (λ = 420 nm, Figure 3). The MLCT band at λ = 585 nm does not decrease or change its position during irradiation, whilst the bands due to the anthryl moieties in the near UV region decrease. As was the case for the free ligands, regeneration of the anthryl bands was not observed.

Conclusion
Whilst the synthesis of the target molecules 5a and 5b and their iron(II)-complexes (6a-b) was successful, investigations regarding their photochemistry were unsatisfactory: Apparently, the anthryl moieties do not undergo the desired [4 + 4]-cycloaddition reaction and the photoproducts could not be identified. Further irradiation experiments with the iron(II)-complex 6b showed that the coordination properties of the terpyridine are not influenced by the photochemistry of the anthryl residues. Hence, the reported anthracene functionalized terpyridines do not appear to be suitable building blocks for photochromic supramolecular structures.
Irradiation experiments were carried out in a quartz cuvette (d = 1 cm); the solutions were thoroughly degassed before irradiation. UV-vis spectra were recorded with a Lambda 40 (Perkin-Elmer) at room temperature. NMR spectra were recorded with a Bruker DRX 500 or a Bruker Avance 600. EI mass spectra were recorded with an Autospec X (Vacuum Generators), ESI mass spectra were recorded with a Bruker Esquire 3000. High resolution mass spectra were recorded with a Bruker Apex III-FT-ICR. Measured and calculated masses are true ion masses, taking into account the mass of lost (or added) electrons.

Supporting Information
Supporting information contains 1 H NMR spectra of all compounds described in the experimental section (3, 5a-b, 6a-b) and 13 C NMR spectra of the organic precursors (3, 5a-b).

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The license is subject to the Beilstein Journal of Organic Chemistry terms and conditions: (http://www.beilstein-journals.org/bjoc) The definitive version of this article is the electronic one which can be found at: doi:10.3762/bjoc.6.54