Conformational and Electronic Consequences in Crafting Extended, π‐Conjugated, Light‐Harvesting Macrocycles

Abstract

The synthesis of a new series of free-base, Ni(II) and Zn(II) 2,3,12,13-tetra(ethynyl)-5,10,15,20-tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3-diethynyl-5,20-diphenylpiceno[10,11,12,13,14,15-jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free-base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully-cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15-fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc-pVTZ(-f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol(-1) higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin-enediynes with ethene- and H-functionality substitutions at the meso-phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol(-1) in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.

Fused porphyrinoids as promising near-infrared absorbing dyes

“…45,46 Later, the same group reported that the heating of 46 in the presence of PtCl 2 in toluene at 85-90 C gave a different fused product 47 (l max ¼ 610 nm in CH 2 Cl 2 ) in 35-48% yield (Scheme 11). 47,48 Richeter et al attempted the synthesis of N-heterocyclic carbene (NHC)-fused porphyrins by following the reaction route shown in Scheme 12. Tetrakis(4-tert-butylphenyl) Ni(II) porphyrin 51 was nitrated with LiNO 3 in a mixture of acetic acid and acetic anhydride to afford 52, which was reacted with 4-amino-4H-1,2,4-triazole to give 53.…”

Synthesis, Structures, and Near‐IR Absorption of Heterole‐Fused Earring Porphyrins

“…Porphyrins fused with aromatic units have attracted increasing attention owing to their intriguing optical and electronic properties and their applications in photodynamic therapy, two‐photon absorption, nonlinear optics, organic conducting materials, and photovoltaics . A variety of π‐expanded porphyrins have been synthesized (Scheme ), among which several multiply aromatic‐fused monomeric porphyrins such as quadruply phenylene‐fused porphyrin A , multiply phenylene‐fused porphyrin B ,– quadruply azulene‐fused porphyrin C , and quadruply anthracene‐fused porphyrin D , display remarkably red‐shifted absorption bands reaching at 1200, 750, 1300, and 1600 nm, respectively. However, repeatedly fused porphyrin in a laminated manner has remained quite rare because of synthetic difficulties.…”

Enediynes, enyne‐allenes, their reactions, and beyond

“…Recently, the spin‐state control of thermal and photochemical Bergman reaction was reported for Pt(II)‐dialkynylporphyrin 62 . The thermal Bergman cyclization reactivity of 62 reveals a surprising reduction in the temperature at which the picenoporphyrin product is formed when compared to Zn(II) derivatives or the metal‐free 62 . Hoffman and co‐workers investigated the Bergman cyclization of osmaenediynes and rhodaenediynes 65 (Figure (b)).…”