A perfluorocyclopentene based diarylethene bearing two terpyridine moieties – synthesis, photochemical properties and influence of transition metal ions

The synthesis of a perfluorocyclopentene based diarylethene bearing two terpyridine units is reported. Furthermore studies of the free ligand’s photochromism and investigations regarding the influence of various transition metal ions on the photochromic reaction are presented. The photochromism of the central diarylethene unit is strongly dependent on the transition metal present, vice versa the photochromic reaction seems to influence the MLCT transition of a binuclear Ru(II) complex.

Introduction 2,2′:6′,2″-Terpyridines have been of great interest over the last years, mostly 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 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]. There have also been recent reports about terpyridines linked to dithienylethenes [10,11]. Herein we report the synthesis of terpyridine functionalized diarylethenes based on perfluorocyclopentene, their photochemistry and investigations regarding the influence of transition metals.
The terpyridine moieties can be attached to the diarylethene unit by Suzuki type cross coupling of the diiodo switch 4 with the boronic esters 9a and 9b under conditions similar to those described for other aryl boronic acids and their derivatives to yield the target molecules 10a and 10b (Scheme 3) [14]. Whilst 10a can be purified by column chromatography, all efforts to obtain highly pure meta-substituted 10b have, so far, been unsuccessful -MALDI-TOF-measurements nevertheless indicate the formation of 10b, and the proton-NMR spectrum displays the expected signals (see Experimental Section and Supporting Information File 1).

Photochromism of the free ligand
The obtained terpyridine substituted diarylethene 10a shows the anticipated photochromic behavior, undergoing photocyclization under irradiation with UV-light (Scheme 4).

Influence of transition metals A binuclear Ru(II)-complex
For investigations of the photochemical behavior of 10a in the coordination sphere of ruthenium(II), we synthesized the binuclear complex 12, from ruthenium(III) chloride hydrate via the monoterpyridine complex 11 (Scheme 5).
The UV-vis-spectra of the binuclear complex 12 in acetonitrile solution are shown in Figure 2 -before (solid), after UV-irradiation (dashed) and after irradiation with vis light (dotted).  The absorption of the strong MLCT band at λ = 490 nm decreases upon UV-irradiation, while absorption in the visible region of the spectrum increases. This may be regarded as indication that the photochromic reaction of the ligand not only takes place in presence of ruthenium, but also influences the MLCT transition. This supposition is supported by the difference spectra ( Figure 3, the absorption of the free ligand 10a C is shown in grey for comparison). According to studies with similar ruthenium(II) complexes of terpyridine functionalized dithienylethenes [10], the intensity of the MLCT band can be attributed to communication between the ruthenium nuclei. In this case communication is obviously reduced by the bridging photoreaction of the diarylethene [9]. On one hand this seems quite unexpected, since the extended π-system of the closed isomer 10a C should facilitate charge transfer processes between the ruthenium nuclei. On the other hand the closed isomer 10a C might act as an acceptor; a similar effect has been observed with a spiropyran moiety bridging two terpyridine units, whose (closed) merocyanine form inhibits the energy transfer between metal centres and thus acts as a T-junction relay [9].

Influence of other transition metal ions in solution
In the following photochemical investigations we added various transition metal ions to methanolic suspensions of the bisterpyridine 10a to generate terpyridine-complexes in situ [17]. Table 1 shows the results obtained with Fe(II), Co(II), Ni(II) and Zn(II). Fast reaction c a Figure 4. b See Supporting Information File 1. c Figure 5.
The UV-vis-spectra of the iron(II) complex [Fe 2+ @10a], obtained by reaction of 10a with ferric chloride tetrahydrate, are shown in Figure 4. Apart from the expected bathochromic shift of the UV-bands of the ligand [18], an MLCT band at λ = 570 nm was observed. While the UV absorption decreases upon UV-irradiation, no significant change of the absorption in the visible region occurred. This indicates inhibition of the photochromic reaction of the diarylethene by the MLCT transition, as previously reported for other iron(II) complexes of bisterpyridine thienylethenes [10].
In contrast to the iron(II) complex, the ditopic ligand seems to retain its photochromic properties in the cobalt(II) and nickel(II) complexes -synthesized from CoCl 2 and [Ni(acac) 2 ], respectively -although the photocyclisation takes much longer than with free 10a (about 5 minutes compared to ca. 30 seconds). This observation may be due to side processes involving charge transfer transitions at the transition metal centres.
The corresponding zinc(II) complex [Zn 2+ @10a] was synthesized from zinc(II) trifluoromethansulfonate and 10a in methanol. The UV-vis-spectra are shown in Figure 5. In this case neither inhibition nor any significant slowing of the photoreaction of the ligand was observed.

Conclusion
We have developed the synthesis of two highly fatigue resistant bis(terpyridinyl) diarylethenes by Suzuki cross coupling methods. The photochemical behavior of the free ligand 10a met our expectations regarding reversibility. It was shown that the presence of various transition metal ions significantly influences the photochromism of the bridging unit: While iron(II)ions completely inhibit the photochromic reaction, cobalt(II) and nickel(II) appear to slow down the photoreaction considerably. Zinc(II)-ions, on the other hand, had no influence on the photochromism of the central diarylethene unit. A rather special case is the binuclear ruthenium(II) complex 12: The diarylethene seems to undergo the expected photochromic reaction, but at the same time the absorption intensity of the MLCT band of the complex decreased, indicating a diminution in communication between the metal centres. The synthesis and investigation of asymmetrical complexes such as [L n Os(tpy-diaetpy)RuL n ] should provide further insight into the role the diarylethene unit plays in the ditopic ligand.
Irradiation experiments were carried out in a quartz cuvette (d = 1 cm) and the solutions were not degassed before irradiation. UV-spectra were recorded with a Lambda 40 (Perkin-Elmer) at room temperature.
NMR-spectra were recorded with a Bruker DRX 500. 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.