Synthesis, Spectroscopy, Electrochemistry, and Mesomorphism of Triple-Decker Porphyrins Consisting of Two Cerium Ions and Three 5,15-Diarylporphyrins

Abstract

Triple-decker sandwich complexes, which consist of two cerium ions and three strip-like diarylporphyrins, have been synthesized and their spectroscopic and electrochemical properties and mesomorphism are reported and compared with those of the corresponding double deckers and the parent monomers. The Soret band of the triple-deckers lies at longer and shorter wavelengths than those of the double-deckers and monomers, respectively, while a sequence of small absorptions is observed in the Q band region. These are interpreted on the basis of knowledge of triple-deckers made from tetraphenylporphyrin and octaethylporphyrin. Magnetic circular dichroism (MCD) spectroscopy suggests that the Soret bands of the triple-deckers are a superimposition of at least two transitions to orbitally degenerate states. In the MCD spectra, Faraday A-terms are observed corresponding to each absorption band, suggesting that these triple-deckers take D4h or more plausibly D4d structures. Triple-deckers containing dialkoxyphenyl groups at the 5,15-positions of their parent porphyrin show a mesophase possessing both lamellar and columnar structures at lower temperatures and a columnar mesophase at higher temperatures, which differ in stacking distances. On the other hand, those containing tetraalkoxylated terphenyl groups at the 5,15-positions of their parent porphyrin show only a columnar mesophase with a single stacking distance. These differences in the mesomorphism are explained by differences of the magnitude of the steric hindrance of the side chains for rotation along the axis connecting the center of the three porphyrins.

Rational Tuning of Melting Entropies for Designing Luminescent Lanthanide‐Containing Thermotropic Liquid Crystals at Room Temperature

“…Synthesis, characterization, and properties of the ligand L11 and of its lanthanide complexes 3 ] 2 in the solid state: As previously discussed in the introduction, the increasing number of divergent flexible alkyl chains in the polycatenar ligand in going from L9 (n = 2) to L10 (n = 6) significantly reduces the melting temperature by DT m = 119 8C, a phenomenon tentatively assigned to a large and enthalpically unbalanced increase of DS m . [10] To further exploit this effect, six additional peripheral dodecyloxy chains have been connected to the central tridentate 2,6-bis-(benzimidazol-2-yl)pyridine binding unit in the target ligand L11.…”

“…[1] Pioneering research efforts indeed focussed on two well-identified classes of complexes that incorporated trivalent lanthanides, Ln III , and exhibited thermotropic liquid crystalline properties. 1) Neutral disk-like sandwich complexes [LnA C H T U N G T R E N N U N G (Li-2H) 2 ] (i = 1-5, as shown) [2] or triple-decker complexes [Ce 2 (Porphyrin) 3 ] [3] have been shown to stack on top of each other to produce hexagonal columnar mesophases (Col h ) in the 20-250 8C range, whereas; 2) lanthanide size-dependent lamellar organizations (usually smectic A, SmA) have been established to be common for the rod-like complexes [LnA C H T U N G T R E N N U N G (Li-H) 3 X 3 ] (i = 6-8; X = NO 3 , H 25 C 12 OSO 3 , Cl, b-diketonate).…”

“…) results from the programmed microsegregation induced in the crystalline phase, a phenomenon which is maintained in the associated lanthanide complexes [LnA C H T U N G T R E N N U N G (L11)A C H T U N G T R E N N U N G (NO 3 ) 3 ] and [LnA C H T U N G T R E N N U N G (L11)-A C H T U N G T R E N N U N G (CF 3 CO 2 ) 3 ] 2 . Low-temperature melting processes (À43 T m À25 8C) producing room-temperature hexagonal columnar liquid-crystalline phases thus result for these novel lanthanidomesogens.…”

“…), because of the very limited number of divergent alkoxy chains; [15] 2) the removal of liquid crystalline properties in the associated complexes [Ln(L9)A C H T U N G T R E N N U N G (NO 3 ) 3 ] and [Ln(L9)A C H T U N G T R E N N U N G (CF 3 CO 2 ) 3 ] 2 , resulting from the coordination of bulky LnX 3 units, which hamper intermolecular cohesion between the polarizable cores, thus decreasing DH c and T c , [15][16][17] and; 3) the restoration of the mesomorphism in the hexacatenar complexes [ 3 ] 2 , because the grafting of six divergent flexible alkoxy chains to the rigid core in L10 (instead of only two in L9) decreases T m to such an extent that T m < T c , despite the small values of DH c and T c . [18,19] The major criticism about our thermodynamic model results from our failure to support its predictions concerning enthalpic and entropic contributions with some quantitative determination of DS m and DH c .…”

“…The crystal structure of 18 shows the existence of centrosymmetrical doubly-bridged 3 ] 2 , together with one interstitial water molecule per dimer (Figure 1). Although the formation of CF 3 CO 2 -bridged dimeric complexes obtained with isomeric coordinated I-shaped 6,6'-disubstituted ligands in 3 ] 2 (two bridging anions, Lu···Lu = 5.067(1) ), [16] 3 ] 2 (two bridging anions, Lu···Lu = 5.7170(5) ), [19] and…”