Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

Abstract

Understanding of the aromatic properties and magnetically inducedc urrent densities of highlyc onjugated chromophores is importantw hen designing molecules with strongly delocalized electronic structure.L inear extension of the triphyrin(2.1.1) skeleton with an annelated benzo [b]heterocyclef ragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycles ubunit. Current-density calculations provide detailedi nformation about the observed pathways and their strengths.Extending the p-electron system in conjugated molecules is an important area of research, because the conjugation pathway exhibits notable influence on the opticala nd magnetic properties. [1] Linear extensions of the acene p cloud by annelation of molecular rings lead to different aromatic behavior of five-and six-membered rings. [3,4] As idef usion of af ive-membered ring to acene cores results in major differences in the electron delocalization with the bridging heteroatom separating the aromatic and antiaromaticmoieties. [5] Triphyrins(2.1.1) (Scheme 1) are triangularly shaped, contracted porphyrinoids whose structural constraints significantly affect their electronic properties. [2] The C2 bridge (1,S cheme1) makes it possible to merge an acene to the triphyrin macrocycle potentially resulting in molecular structures with novel optical andm agnetic properties as well as an unusual aromatic character. [2d,e] We have synthesized novel molecular structures with benzo[b]furan and benzo[b]thiophene annelated to triphyrin(2.1.1) (2,S cheme 1). The annelated arene ring may influence the global aromatic properties by incorporating the external ben-zoic (C6) ring into the delocalization pathway.T he synthesized macrocycles can also functiona sb oron(III)l igands forminga ntiaromatic complexes as shown in this work.Triphyrins 2a and 2b wereo btained by acid-catalyzed condensation of reagents 3a and 3b,r espectively.T hey were efficiently synthesized via Suzuki-Miyaura coupling from easily obtainable or commercially availables tarting materials (2,3-dibromobenzo[b]heterocyclea nd pyrrole-boronic acid), [6] followed by at hermolytic deprotection. [2d,e] 2a and 2b were isolated with ay ield of 10 %a nd 12 %, respectively.T he small coordination core of the macrocycle leads to av ery efficient entrapment of ap rotonm aking it possible to isolate the cationic form. As the counterion of the condensationp roduct was unknown,w ereplaced it with ac hloride anion by using ap reviously reported approach. [2d,e] Characterization and further studies were performed on the chloride derivatives. The X-ray analysis performed for 2b has confirmed the postulated pattern ( Figure S44).2a (2b)h as aU V/Vis spectrum that is typical for aromatic porphyrinoids with aS oret-like band at 415 nm (430 nm) and a set of Qb ands that reach 600 nm (620 nm). The bands are redshiftedb y4 0-50nmc ompared to triphyrins(2.1.1) such as 1b [2b] and 1c, [2f]...

Multiple Coordination in Porphyrinoid Hybrid: Changing the Delocalization within the Extended π-System

“…The antiaromatic 16 delocalization path (Scheme 2, 13) recorded for those structures drastically changes photophysical response showing a potential for redox switching between two electronic states with a drastic difference in optical response but also opening a potential for extending the delocalization over the fused fragment. 16 Thus in contrast to the PAH systems, where the incorporation of boron(III) usually required a well-defined bottom-up synthetic approach as observed, i.a. for 7 and 1 (Figure 1 and Scheme 1), the incorporation of metalloid with an employment of triphyrin(2.1.1) can be achieved after formation of the final molecule as reported previously by us 13 and others.…”

Synthesis of the β-dipyrrinyl triphyrin(2.1.1) ligand and its coordination complexes

“…Triphyrins(2.1.1) are contracted porphyrinoids containing three pyrrole rings connected via four meso carbons. 1,2 They are 14π aromatic macrocycles but contain one pyrrole less than 18π aromatic porphyrins and possess almost similar physicochemical 3 and coordination properties. 4,5 However, triphyrins (2.1.1) are mono-anionic tridentate ligands 6,7 and their coordination chemistry has not been extensively investigated, 8,9 unlike meso-tetraaryl porphyrins which are dianionic tetradentate ligands and well known to coordinate with almost every metal/metalloid/nonmetal of the periodic table.…”

B,N-doped PAHs from tridentate ‘Defects’ – a bottom-up convergent approach for π-extended systems

“…As documented, the presence of amine (Et 3 N) is crucial to neutralize the liberated acid during the formation of the final product, 24 or it was used as a reducing agent to predefine the binding environment to the central ion. 19 As postulated from the reactivity of 1a / 1c , Et 3 N does not take part in the reduction process. Thus, we have decided to test the reactivity with solely BBr 3 and the so far unreactive 1e .…”

“…S86, ESI†) present the emission maximum between λ = 478 and λ = 576 nm showing the Stokes shift of up to ∼100 nm substantially bigger compared to that of other tridentate systems. 19 The efficiency of the observed emission varies for each derivative and was recorded as 0.01 ≤ Φ ≤ 0.08, while the fluorescence lifetimes ( τ 1 ) were substantially elongated for fully m -phenylene derivative 1a ( τ 1 = 12.5 ns) while comparing to nitrogen containing derivatives ( τ 1 = 6.8 ns ( 1b ), 3.8 ns ( 1c ), 2.1 ns ( 1d ), 4.0 ns ( 1e )). It stays in a good agreement with the previously reported behaviour, where the presence of nitrogen in the coordination cavity leads to a substantial decrease in the fluorescence efficiency recorded for CH donors but lacking for N donors.…”