Tuning the Circumference of Six-Porphyrin Nanorings

Abstract

Most macrocycles are made from a simple repeat unit, resulting in high symmetry. Breaking this symmetry allows fine-tuning of the circumference, providing better control of the host–guest behavior and electronic structure. Here, we present the template-directed synthesis of two unsymmetrical cyclic porphyrin hexamers with both ethyne (C2) and butadiyne (C4) links, and we compare these nanorings with the symmetrical analogues with six ethyne or six butadiyne links. Inserting two extra carbon atoms into the smaller nanoring causes a spectacular change in binding behavior: the template affinity increases by a factor of 3 × 109, to a value of ca. 1038 M–1, and the mean effective molarity is ca. 830 M. In contrast, removing two carbon atoms from the largest nanoring results in almost no change in its template-affinity. The strain in these nanorings is 90–130 kJ mol–1, as estimated both from DFT calculation of homodesmotic reactions and from comparing template affinities of linear and cyclic oligomers. Breaking the symmetry has little effect on the absorption and fluorescence behavior of the nanorings: the low radiative rates that are characteristic of a circular delocalized S1 excited state are preserved in the low-symmetry macrocycles.

Twisted-Planar-Twisted expanded porphyrinoid dimer as a rudimentary reaction-based methanol indicator

“…In recent years, considerable effort has been devoted to extending the chemistry of porphyrin analogs beyond the limits of simple macrocyclic systems to encompass, inter alia, two-/three-dimensional structures 15 , 16 , linear porphyrin tapes 17 , 18 , and annular nano-rings 19 , 20 . Directly linked porphyrin dimers have received particular attention within the context of this general paradigm because of their interesting optical and electrochemical features 21 , 22 .…”

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…Template‐free cyclooctamer 203 is strain free (∠ sp =172.4°), but alkyne moieties in 203 (with template) are more strained. The combination of Glaser coupling and Sonogashira–Hagihara coupling enables porphyrin macrocycles 204 and 205 containing monoyne or diyne moieties to be obtained upon treatment with templates 197 and 198 , respectively (Figure ) . The insertion of an ethynylene moiety into porphyrin nanoring 195 dramatically enhanced the affinity for template 198 , increasing the binding constant by a factor of 3×10 9 .…”

“…The bond angles at alkynes in cyclopentamer 194 a are estimated as 169.8 and 173.0° by DFT calculation, whereas complexation with corannulene‐based template 196 slightly warped the alkynes (∠ sp =166–170°) . The bond angles at sp carbons in the complex of 194 b with template 197 were determined as 168–171° by X‐ray crystallographic analysis (CCDC: 860774), which are in good agreement with the values obtained by calculation . Free cyclohexamer 194 b , the bond angles of which are 171.7 and 174.3° (DFT calculation), is strain free, indicating that the coordination of template molecules changes the ring structure.…”

“…[313][314][315][316] The bond angles at sp carbons in the complex of 194 b with template 197 wered etermined as 168-1718 by X-ray crystallographic analysis (CCDC:8 60774), [317] which are in good agreement with the values obtainedb yc alculation. [318] Free cyclohexamer 194 b,t he bond angles of which are 171.7 and 174.38 (DFT calculation), is strain free, indicatingt hat the coordination of template molecules changes the ring structure. Cyclododecamer 194 e could be synthesized from linear porphyrint etramer 199 and template 197 by removal of two templates from the supramolecular Vernier complex 200 (Figure 63).…”

“…The combination of Glaser coupling and Sonogashira-Hagihara coupling enables porphyrin macrocycles 204 and 205 containing monoyne or diyne moieties to be obtained upon treatment with templates 197 and 198,r espectively ( Figure 65). [318] The insertion of an ethynylenem oiety into porphyrin nanoring 195 dramatically enhanced the affinity for template 198,i ncreasingt he bindingc onstant by af actor of 3 10 9 .T he bond angles of 204 and 205 are similart ot hose of 194 b and 195, respectively.…”