A Versatile Bis‐Porphyrin Tweezer Host for the Assembly of Noncovalent Photoactive Architectures: A Photophysical Characterization of the Tweezers and Their Association with Porphyrins and Other Guests

Abstract

A bis(Zn(II)-porphyrin) tweezer host with anthracene components as apex and side-arms has been synthesized. Mono- (pyridine) and bidentate (4,4'-bipyridine) guests were used as models for single and double axial coordination inside the cavity, respectively. A series of dipyridylporphyrin guests with different substitution patterns and excited-state energy levels have association constants with the tweezers that are of the order of 10(6) M(-1), which is indicative of complexation with the inside of the cavity. This complexation can only occur upon an important distortion of the cavity that opens the bite by about 30 %. This characteristic, in conjunction with their ability to reduce the bite distance by rotation around single bonds, makes these porphyrin tweezers amongst the most versatile so far reported, with tuning of the bite distance in the range of approximately 5-20 Angstroms. Energy transfer to the free-base guest within the triporphyrin complex is nearly quantitative (95-98 %) and the rates of transfer are consistent with a Förster mechanism that is characterized by a reduced orientation factor.

The cage effect on assembly of cyclic-bis-(zinc porphyrin) with 5,15-dipyridylporphyrin

“…The assembly stability can increase greatly in the presence of cage effect between cyclic bis(zinc porphyrin) host 1 and dipyridyl guest 3, although there is weak coordination stability when zinc-porphyrins bind to pyridine or pyridyl residues through an axial bond of the Zn(Ⅱ) ion [9,20,30] . 1 H NMR spectral experiments involving the titration of 1 (~2.0 × 10 −3 mol/L) against increaseing concentration of the potential nitrogenous ligands were used to follow the chemical shift changes of diagnostic protons upon complexation [19,20] .…”

“…1 H NMR spectral experiments involving the titration of 1 (~2.0 × 10 −3 mol/L) against increaseing concentration of the potential nitrogenous ligands were used to follow the chemical shift changes of diagnostic protons upon complexation [19,20] . When free-base dipyridylporphyrin guest 3 was added to the chloroform solution of host 1, the Zn-N coordination made the host molecule expand and form 'sandwich' complex (Scheme 4) [30] . The inclusion complex 1 ⊃ 3 is of good stability, and the signal of central N-proton of 3 shows a sharp peak at −3.5 ppm, 0.7 ppm upfield shift compared with the free 3 (Figure 3), which reveals that the guest lies inside the host cavity.…”

“…Derived spectral profiles, ΔA, are also used to analyze the absorption changes of host 1 due to strong disturbance of absorption of the guests to the host [30] . Assuming that no spectral changes occur in the object guests upon complexation, the ΔA profile will represent changes of the bis-(zinc porphyrin) host.…”

“…In these assemblies, noncovalently bound components would be easily induced to relative movement under external driving force, and achieve mechanical motionlike function in solution at the spectroscopic concentration [27,28] . Several bis(zinc porphyrin) tweezers have been reported [20,29,30] , but few [30] of them can accommodate larger functional guests (e.g., 5,15-dipyridylporphyrin derivatives) owing to the size-matching difficulties [20] and molecular thermal effect [22] . Our interest lies in (i) the design of a cyclic bis(zinc porphyrin) with suitable structure which allows the stable accommodation of 5,15-dipyridylporphyrin derivatives by means of the cage effect and (ii) the potential application of such assemblies in functional molecular devices [25] used as a molecular gyroscope in solution [22,[31][32][33] as well as in solid [23,24] .…”

“…The cavity size of compound 1 can be easily adjusted through rotation around single bonds of bridges to efficiently include diverse bidentate guests, such as Bpy, DABCO, and guest 3 which has approximately 16 Å dipyridyl N-N distances and has been proved to be nearly perfectly size-matching to fit within the cavity of reference compound 2 [20] . Taking into consideration the presence of flexible bridges and cage effect, guest 3 would be easily included in cavity of 1 [30] . Inclusion complex 1 ⊃ 3 would possess two characteristics: high stability as entity and easy relative movement of its components, which are two necessary conditions to construct a molecular gyroscope.…”