A Sustainable Preparation of Functional Perylenophanes by Domino Metathesis

A sustainable four-step synthesis of soluble perylenophanes for applications as fluorescent optical functional materials is presented and even allows upscaling because of starting with technical bulk products. Thus, terminal alkenylnitriles were alkylated reduced to amines, condensed with perylenetetracarboxylic bisanhydride and cyclised to cyclophanes by means of double cross metathesis in yields until 69% of isolated dyes. The first metathesis by means of the second-generation Hoveyda-Grubbs-catalyst brings the remaining reactive olefinic groups close together favouring the ring-closure to the cyclophanes where the locked neighboring of chromophores in a skew arrangement induce strong exciton interactions. The latter cause an increased the Stokes’ separation by means of a moderate hypsochromic shift of light absorption and a stronger bathochromic shift of fluorescence. Various applications such as for lasers, photonics, solar collectors or in analytics are discussed.


Introduction
The interaction of two or more chromophores offers many possibilities for modern optoelectronics [1] and functional materials because various optical effects can be generated where the perylene dyes [2] such as 1 [3] and other peri-arylenes [4] are suitable components because of their high chemical and photochemical stability and fluorescence quantum yields. Moreover, one single optical transition of these chromophores in the visible spectral region favors the planning, construction and operation of functional multichromophoric assemblies. The elec-Green and Sustainable Chemistry tronic interaction of two chromophores induces spectral shifts of the initial absorption to two new positions known as the Davydov splitting [5] [6]. Thus, a more bathochromic α-transition and a more hypsochromic β-transition are obtained instead of the light absorption of the individual chromophores; see Figure 1, left.
There are two fundamental arrangements of the transition moments of two or more chromophores with (i) coplanarity known as H-orientation [7] according to the initially described hypsochromically absorbing aggregates and (ii) linear shift known as J-arrangement [8]; see Figure 1, right.
The H-orientation induces an anti-synchronous electron movement according to Förster's analysis [9] because of Coulomb interactions; this is schematically indicated in Figure 1 with an arbitrary flashlight distribution of charges.
The electrostatic interactions remain still somewhat disfavorable because the comparably short distance of equal charges; thus, the transition energy of the in- A skew-type arrangement of chromophores activates both transitions so that Aggregates such as H and J arrangement of chromophores are only weakly held together by non-covalent interactions and are labile concerning dissociation such as with dilution. A higher stability can be obtained with covalently linked chromophores, preferently forming cyclophanes such as 2 [10]. However, synthesis of such cyclophanes proved to be complicated for complex chromophores and upscaling of 2 is difficult because of simultaneous and stoichiometric addition of two solids under dilution conditions. Moreover, the limited solubility of 2 means an obstacle for applications in homogeneous media. An efficient method for the preparation of such cyclophanes with increased solubility would bring about appreciable progress.

Results and Discussions
Here we applied olefin metathesis [11] [12] [13] [14] [15] for the efficient, economic and sustainable synthesis of bichromophoric perylenetetracarboxylic bisimides dyes (dyads) because of catalysed carbon-carbon linking where special optic effects are expected as a consequence of the interacting chromophores in such molecules. Firstly, we studied the linking of simple alkylene-substituted perylene carboxamides by metathesis; however, the generally very low solubility of perylenebiscarboximides means an obstacle for such bimolecular reactions.
Thus, we introduced solubility-increasing groups and firstly attached the 2,5-di-tert-butyl substituent [2] to one nitrogen atom of perylenbiscarboximide and an allyl group as the terminal olefine to the other; the nitrogen atoms form ideal positions for the ring-formation because of orbital nodes [16] in HOMO and LUMO causing an electronic decoupling of substituents at these positions.
However, metathesis with second-generation Hoveyda-Grupps-catalyst (4) [17] gave only low yields of a few percents. Obviously, the solubility of the staring material is still too low for efficient coupling.
metathesis because of higher proximity of the reacting olefinic groups.
Thus, we firstly connected in 12 two chromophores with an N-hydroxyethyl group by means of etherification in a one-pot reaction with methanesulphonic acid chloride. The nitrogen atoms at the opposite sides of the chromophores were attached to terminal olefinic alkyl groups where geminal alkyl substituents were applied for solubility increasing. Ring-closing metathesis to 13 was realised by means of second-generation Hoveyda-Grupps-catalyst (4); however, the solubility of the material was still comparably restrainingly low. Moreover, two steps with moderate yields were necessary for the preparation of a cyclophane. For further improvement, we targeted two consecutive metatheses as a Domino reaction in one pot and replaced the hydroxyethyl group in the starting material for 13 by an allyl group in 14; for structural details see Figure 2, left. However, a direct metathesis gave a complex mixture of products containing 15 and 16 both as cis/trans mixtures. As a consequence, we allowed to react the symmetrically substituted perylene dyes 17, 19 and 21 with  The yields of isolated pure material of such Domino reactions increase with the flexibility of the generated cyclophanes where 50% were obtained for the compact 18, 57% for 20 with a larger mash and even 69% for 22 with a still larger ring. A prolongation of the alkyl chains in the swallow-tail system or a replacement by iso-propyl groups appreciably decreases the yields of cyclophanes presumably caused by a less compact arrangement for the second and ring-closing metathesis as a Domino reaction. Resuming, the cyclophanes 18, 20, and 22 are preferred for special optical applications. As a consequence, a sustainable, efficient, economic and upscalable access to the starting materials 23 for metathesis would bring about further progress.
Thus, we started synthesis with the alkylation [19] of ω-akenyl nitriles with 1-iodobutane by means of LDA (lithium diisopropylamide) in a laboratory synthesis according to Figure 3; an efficient upscaling is possible applying alkyl bromides with sodium and sodium amide, respectively, in liquid ammonia according to Ref. [20] in a solvent-recycling chain of batches [21]. The alkylated nitrile was reduced with complex hydrides such as lithium aluminiumhydride or even catalytic with hydrogen to the primary amine and the latter condensed with the technical mass product perylene-3,4:9,10-tertacarboxylicbisahydride in melt imidazole [22] [23] with the addition of zinc acetate to obtain 23 with n = 1, 2 and 3. Special optic effects [9] are expected as a consequence of the interaction of chromophores [24] in the dyads (bichromophores). However, the UV/Vis spectra of the open-chain dyad 5 are nearly identic with the spectra of 1 ( Figure 4, left spectrum, red and green curves). Obviously, the long distance of the individual chromophores allows only slight exciton interactions [25]. Thus, a more tight arrangement of the chromophores was targeted for stronger interactions. Two chromophores linked with an ether group and tied close together by ring-closing