Core‐Modified meso‐Aryl Hexaphyrins with an Internal Thiophene Bridge: Structure, Aromaticity, and Photodynamics

Abstract

Take the shortcut: The synthesis of core-modified meso aryl hexaphyrins with an internal thiophene bridge is reported. Introduction of the thiophene bridge alters the electronic structure as well as the π-electron circuit, resulting in increases in singlet lifetime (τ(s)) and the two-photon absorption (TPA) cross-section. Furthermore, for the sulfur derivative, the internal bridging thiophene participates in a π-electron conjugation pathway.

Phenylene‐Bridged Core‐Modified Planar Aromatic Octaphyrin: Aromaticity, Photophysical and Anion Receptor Properties

“…The free‐base form exhibited a sharp Soret band at 617 nm and Q‐bands at 731 and 915 nm, with a molar absorption coefficient ( ɛ ) for the Soret band in the order of ×10 5 m −1 cm −1 . Comparison of these data with other non‐bridged congeners ( 1 ) and bridged expanded porphyrins ( 3 and 4 ) revealed the following: 1) the non‐bridged [34 π] octaphyrin ( 1 a ) showed a broad absorption spectrum and the ɛ value was in the order of ×10 4 m −1 cm −1 , which was attributed to its non‐aromatic character; 2) the Soret and Q‐bands for compound 5 were red‐shifted compared to those of the bridged derivatives ( 3 and 4 ), which reflected an increase in the number of π electrons in the molecular framework; and 3) the sharp Soret band in compound 5 suggested a single conjugation pathway, as observed in the bridged derivatives ( 3 a , 3 b , 3 d , and 4 b ) . Overall, the Soret and Q‐bands reflected the aromaticity, planarity, and major conjugation pathway in the [34 π] octaphyrin, as observed from NMR spectroscopic analysis.…”

“…Furthermore, variable‐temperature 1 H NMR analysis of the free‐base form of compound 5 did not alter the peak position, which reflected the rigidity of the macrocycle (see the Supporting Information, Figure S5). A comparison of these results with non‐bridged [34 π] octaphyrin 1 and bridged derivatives 3 and 4 revealed that compound 1 a adopted a figure‐eight conformation in solution and was non‐aromatic, whereas compound 1 b was aromatic –. On the other hand, bridged derivatives 3 c and 4 a followed a dual‐aromatic conjugation pathway, in which the rest of the bridged derivatives followed a single aromatic conjugation pathway …”

“…For the synthesis of compound 5 , we adopted a similar synthetic methodology to that used for the synthesis of compound 4 , as outlined in Scheme . The required precursor, 1,4‐phenyl‐bis(dipyrromethane) ( 7 ), was synthesized by condensing 1,4‐diformylbenzene with pyrrole in the presence of trifluoroacetic acid (TFA) as a catalyst .…”

“…Many approaches for rigidifying these macrocycles have been reported by various research groups, such as metal coordination, temperature control, solvent variation, protonation with appropriate acids, and functional‐group modifications . Recently, fused and bridging strategies to control their topology become an interesting research topic, because they do not alter the intrinsic properties of the porphyrin . However, the bridging approach has attracted the attention of researchers owing to its significant impact on the electronic structure, conjugation pathway, aromaticity, and physical and chemical properties of the macrocyclic behavior, depending upon the bridging group (Scheme ) …”

“…Recently, we reported the synthesis of two core‐modified hexaphyrins ( 4 ) that contained an internal thiophene bridge . Depending upon the heteroatom present in the macrocyclic ring, the macrocycle followed either a dual [18 π] and [26 π] conjugation pathway ( 4 a ), or had a major contribution from the [26 π] aromatic circuit ( 4 b ).…”