meso-Aryl sapphyrins with heteroatoms; synthesis, characterization, spectral and electrochemical properties

Abstract

The synthesis, characterization and spectral properties of six new meso-aryl core modified sapphyrins are described. An efficient approach involving an acid catalyzed condensation of bithiophene diol 1 and modified tripyrranes 2a-2e allows preparation of the desired meso-aryl sapphyrins in 16-36% yield. The product distribution and the isolated yield were found to be dependent on the nature of the acid catalyst (Lewis acid or protic acid) and its concentration. Protic acid catalyst exclusively gave the expected sapphyrins while two additional products, an 18π tetraphenylporphyrin and a 26π modified rubyrin, were isolated under Lewis acid catalysis. An analysis of proton NMR and absorption spectral data suggests that in free base sapphyrins, the heterocyclic ring opposite to the bithiophene unit is inverted as in N-5 meso-aryl sapphyrin and the degree of inversion is dependent on the nature of the heterocyclic ring. The energy optimized structure calculated from the semi-empirical method substantiates such a conclusion. Protonation of sapphyrins generates respective mono-and dications and the heterocyclic ring retains an inverted structure in contrast to normal N-5 sapphyrins. The triplet excited lifetimes for free base and protonated derivatives are similar both under argon saturated and air equilibrated conditions, indicating that the triplet state quenching by oxygen is minimal. Cyclic voltammetric studies reveal easier reductions and harder oxidations relative to meso-aryl porphyrins and the ∆ redox observed for 3d suggests significant reduction of the HOMO-LUMO energy gap consistent with the large red shift observed for the Soret band.

Synthetic Expanded Porphyrin Chemistry

“…In two other related publications, Chandrashekar and coworkers described the syntheses of several other heterosapphyrins, [72,73] as well as a number that were published previously (see Scheme 8 and structures 51 and 59). As in previous syntheses, the same by-products (namely, heterorubyrins and heteroporphyrins) were observed under the conditions used to obtain 51 and 59, with the yields again being somewhat variable.…”

Synthesechemie expandierter Porphyrine

“…In zwei weiteren Arbeiten beschreiben Chandrashekar et al die Synthesen von einigen anderen Heterosapphyrinen und mehreren bereits bekannten Makrocyclen dieser Reihe (siehe Schema und Strukturen 51 und 59 ) 72. 73 Wie bei früheren Synthesen entstehen bei der Herstellung von 51 und 59 die gleichen Nebenprodukte (d. h. Heterorubyrine und Heteroporphyrine), und auch die Ausbeuten variieren.…”

Synthesis, Characterization, and Electrochemistry of Open‐Chain Pentapyrroles and Sapphyrins with Highly Electron‐Withdrawing meso‐Tetraaryl Substituents

“…[12] Since then, sapphyrins have remained at opic of high interest, [13][14][15][16] in part because of their potential applicationsi nt he area of photodynamic therapy and their structurals imilarity to the well-known porphyrin and corrole macrocycles, [10,11,[17][18][19][20][21] and in part because of their unique properties as ar eceptor of neutraland anionic substrates. [22][23][24][25][26][27][28][29] Al arge number of studies have characterizedd ifferent sapphyrind erivatives, including b-alkylateds apphyrins, [30][31][32][33] coremodified sapphyrins with heteroatoms (O, S, or Se), [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Nfused/N-confused sapphyrins, [51][52][53][54] and meso-tetraaryl-substituted sapphyrins. [55]…”