Photoinduced E to Z isomerization of tetraphenylethylene derivatives within organometallic supramolecular assemblies

Isolation of E-1,2-bis(4-bromophenyl)-1,2-diphenyl-ethylene from the E/Z isomer mixture obtained by a McMurry coupling reaction and reaction of this isomer with imidazole followed by N-alkylation with nBuBr and anion exchange yielded the bisimidazolium tetraphenylethylene (TPE) derivative H2-E-1(PF6)2. The reaction of H2-E-1(PF6)2 with Ag2O yielded the dinuclear metallarectangle [Ag2(E-1)2](PF6)2 where the two bis-NHC donors E-1 bridge two silver atoms. Irradiation of [Ag2(E1)2](PF6)2 leads to E/Z isomerization of the di-NHC ligand and formation of Z-1 in the mononuclear complex [Ag(Z-1)]PF6. Demetallation of the di-NHC ligand with NH4Cl/NH4PF6 yielded bisimidazolium salt H2-Z-1(PF6)2. The unique isomerization of the E-TPE derivative into its Z-isomer via metal complex formation/irradiation/demetallation cannot be achieved by irradiation of the individual imidazolium salt. The emissive properties of the TPE complexes [Ag2(E-1)2](PF6)2 and [Ag(Z-1)]PF6 have been investigated.


Introduction
Tetraphenylethylene (TPE) and its derivatives have become the most extensively studied aggregation-induced-emission (AIE) luminogens. They have found multiple applications since the introduction of the concept of AIE by Tang and coworkers [1,2]. Many TPE derivatives exhibit strong fluorescence in the solid state or as colloidal aggregates due to restricted intramolecular rotation (RIR), whereas the observation of almost no or only weak fluorescence in dilute solution is generally ascribed to the dynamic intramolecular rotation of the phenyl groups against the ethylene stator [3][4][5]. The radiationless relaxation of excitons by the RIR mechanism has been established by a wealth of theoretical and experimental research. In addition, TPE can also undergo E/Z isomerization (EZI) under light irradiation or by heating (Scheme 1). Whether or not the EZI process leads to an emission reduction for TPE derivatives is still subject of discussion [6][7][8][9].
The photoinduced isomerization is difficult to observe for symmetrical TPE derivatives. Therefore, the EZI process has been studied with unsymmetrically disubstituted TPE derivatives [6][7][8][9][10][11][12][13][14][15][16][17]. Apart from being ideal candidates for the study of the EZI process, pure E-and Z-isomers of disubstituted TPE and the investigation of their properties has recently attracted interest based on the differences in their optical properties, host-guest chemistry and related features [18][19][20][21][22][23][24][25][26][27][28]. Disubstituted TPE derivatives are conventionally synthesized by the McMurry coupling of unsymmetrically sybstituted ketones. This protocol normally yields a mixture of E-and Z-isomers (Scheme 1) [29][30][31][32]. Separation of these isomers by column chromatography proved difficult owing to the similarity in the properties of the isomers [29][30][31][32][33]. Increasing the difference in polarity of the isomers is the usual but not the only method to achieve separation of the isomers [9,34]. The isomerization of disubstituted TPE derivatives under ultraviolet (UV) irradiation or by thermal treatment constitutes a promising method to obtain pure Eand Z-isomers. However, the quantitative conversion of one isomer into the other one has not been realized so far [8,31,34], although the isomerization rate of the E-to the Zisomer is lower than the reverse isomerization of the Z-to Eisomer due to the higher stability of the E-isomer (Scheme 1) [30,31,35]. Thus, the acquisition of pure E-and Z-isomers of disubstituted TPE derivatives remains a challenge both in terms of synthesis and separation technology.
The chemistry of metal complexes bearing N-heterocyclic carbene (NHC) ligands has rapidly evolved over the last years with interesting applications ranging across a diverse set of fields. Recently a number of studies have focused on metallosupramolecular assemblies obtained from poly-NHC ligands and their postsynthetic modifications [36][37][38][39][40][41][42][43][44]. The synthesis of poly-NHC-Ag I assemblies from Ag 2 O and poly-NHC precursors has been most extensively investigated. This protocol benefits not only from the simplicity of the synthetic procedure but also from the lability of Ag I −C NHC bond [39], allowing the formation of the thermodynamically most stable reaction product and the transmetallation of the poly-NHC from silver to other metals [45][46][47][48].
We have demonstrated turn-on fluorescence by rigidification as an alternative to AIE in dinuclear complexes of tetra-NHC-substituted TPE of type A (Scheme 2, top) [49,50]. In complexes of type A both the RIR and the EZI mechanism for radiationless relaxation are disabled through the formation of two metallacycles involving Z-NHC donors. Inspired by the stability of the Z-arrangement of the NHC donors in A and the lability of the Ag-C NHC bond in general, we assumed that dinuclear silver complexes of di-NHC substituted TPE derivatives in E-configuration might undergo the isomerization to the Z-complexes upon UV irra-diation via the EZI process.

Results and discussion
The bisimidazolium salt H 2 -E-1(PF 6 ) 2 was obtained by the copper-catalyzed coupling reaction of E-1,2-bis(4-bromophenyl)-1,2-diphenylethene and imidazole. The resulting bis(4-imidazolphenyl)-1,2-diphenylethene was obtained as a mixture of E and Z isomers and the latter one was removed by multiple crystallization steps [51]. A subsequent double N-alkylation with n-butylbromide gave the dibromide salt which was converted into the hexafluorophosphate salt H 2 -E-1(PF 6 ) 2 by reaction with NH 4 PF 6 (Scheme S1, for experimental details see the Supporting Information online).
The bisimidazolium salt H 2 -E-1(PF 6 ) 2 was fully characterized by nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray ionization (HR ESI) mass spectrometry (see the Supporting Information online). In addition, the molecular structure of H 2 -E-1(PF 6 ) 2 was established by an X-ray diffraction study ( Figure S1, Supporting Information online).
Based on the observation of the UV radiation induced EZI process in TPE derivatives (Scheme 1), we became interested in extending this isomerization to complexes obtained from suitably disubstituted TPE derivatives. Isomerization of the di-NHC ligand E-1 in the dinuclear complex [Ag 2 (E-1) 2 )]-(PF 6 ) 2 would lead to ligand Z-1, which due to steric reasons will form a mononuclear dicarbene chelate complex [Ag(Z-1)]PF 6 similarly to the previously described tetra-NHC substituted TPE derivatives in complex A (Scheme 2) [49]. The freely less stable Z-TPE derivative is then stabilized in the chelate complex. Removal of the TPE derivative from [Ag(Z-1)]PF 6 yields H 2 -Z-1(PF 6 ) 2 (Scheme 2) and would then result in the selective transformation of the more stable E-into the less stable Z-derivative via a complex formation/ isomerization reaction sequence.
Single crystal of [Ag(Z-1)]PF 6 suitable for an X-ray diffraction analysis was grown by slow diffusion of diethyl ether into a concentrated acetonitrile solution of the compound at ambient temperature. The X-ray diffraction study ( Figure 5) confirmed the composition and geometry of [Ag-(Z-1)]PF 6 as concluded from NMR and mass spectrometric analysis. The two imidazolylidene donors attached to the TPE core are located on the same side of the olefin bond. They are linked by coordination to one silver atom to form a planar chelate ring. The C-Ag bond lengths of 2.084(7) and 2.092(7) Å and the C-Ag-C angle of 177.4(4)°fall in the typical range for silver (I) di-NHC complexes [10][11][12].
The Z-TPE bridged dicarbene ligand in [Ag(Z-1)]PF 6 was liberated as the bisimidazolium salt H 2 -Z-1(PF 6 ) 2 by reaction of the complex with NH 4 Cl in a methanol and acetonitrile solvent mixture followed by anion exchange with NH 4 PF 6 in methanol (Scheme 2). Compound H 2 -Z-1(PF 6 ) 2 was characterized by multinuclear NMR spectroscopy and mass spectrometry (Figures S21-S23). The 1 H NMR spectrum exhibits the resonance for the imidazolium N-CH-N proton at δ = 8.81 ppm. The base peak in the mass spectrum was   Inspired by the irradiation induced conversion of [Ag 2 (E-1) 2 ](PF 6 ) 2 to [Ag(Z-1)]PF 6 , an equimolar mixture of the isomeric imidazolium salts H 2 -E-1(PF 6 ) 2 and H 2 -Z-1(PF 6 ) 2 was prepared from a 1:1 isomer mixture of E/Z-1,2-bis(4bromophenyl)-1,2-diphenylethene and imidazole followed by N-alkylation (Scheme 3, for experimental details see the Supporting Information online). Irradiation of this mixture of bisimidazolium salts does not yield only one, the slightly more stable E-isomer ( Figure S30). Instead, the composition of the mixture of isomeric salts does not change noticeable upon irradiation. In addition, irradiation of the single isomer H 2 -E-1(PF 6 ) 2 leads to partial isomerization and formation of a mixture of isomeric salts H 2 -E/Z-1(PF 6 ) 2 as shown by 1 H NMR spectroscopy ( Figure S30).
Contrary to these observations, reaction of an equimolar mixture of the isomeric bisimidazolium salts H 2 -E-1(PF 6  Given the rigidification-induced desirable emission properties of dinculear complexes of type A (Scheme 2) [12], we became interested in the emission properties of the less rigid complexes [Ag 2 (E-1) 2 ](PF 6 ) 2 and [Ag(Z-1)]PF 6 . The emission spectra of the complexes have been recorded in CH 3 CN/ H 2 O solution with varying water fractions. Apparently due to intramolecular phenyl rotation, the emission is weak for [Ag 2 (E-1) 2 ](PF 6 ) 2 in acetonitrile and does not change significantly as long as the water content is less than 60%. A sudden increase in emission intensity due to agglomeration accompanied by a blue shift of the emission maximum from 492 to 443 nm was observed when the water content was increased to 70% (Figure 7a). When the water content was further increased to 90%, the emission intensity increased approximately the 19 fold magnitude relative to the emission intensity in pure CH 3 CN solution (Figure 7b) and the emission colour changed from invisible green to bright blue visible to the naked eye. The fluorescence quantum yield (Φ F ) reached 47% for the aggregated state in the CH 3     shift (~19 nm) of the emission maximum and a significant increase in emission intensity (Φ F up to 26.60%) as the water content of the acetonitrile/water solution reached 90% (Figures S47-S50 and Table S10).
Finally, the fluorescence behavior of complex [Ag(Z-1)]-PF 6 at different temperatures was investigated [52,53]. No obvious influence on the emission wavelength was observed by temperature dependent fluorescence spectroscopy ( Figure  8a) aside from an approximately linear decrease in emission intensities with increasing temperatures (Figure 8b). The emission intensities recovered when the solutions were allowed to cool. Listings of all photophysical properties of complexes [Ag 2 (E-1) 2 ](PF 6 ) 2 and [Ag(Z-1)]PF 6 are presented in the Supporting Information online.

Conclusions
We have demonstrated the quantitative isomerization of E-to Z-isomers of bisimidazolium substituted TPE by a lighttrigged structural transformation via a dinuclear silver(I) molecular rectangle featuring an E-di-NHC bridge to a mononuclear silver(I) complex featuring a chelating Z-di-NHC. The E-to Z-isomerization is not possible directly with the bisimidazolium salts without their previous transformation into di-NHC complexes. The isomerization process of the complexes has been monitored by NMR and fluorescence emission spectroscopy. Significantly different fluorescence properties of the complexes bearing the E or Z di-NHC ligands have been observed. In addition, the complexes exhibit classical AIE characteristics. Our study provides a simple and efficient approach towards the controlled isomerization of Z to E disubstituted TPE derivatives through organometallic intermediates, paving the way for further studies of the EZI process in TPE derivatives.