Syntheses of Sterically Hindered Pyridinium Phenoxides as Model Compounds in Nonlinear Optics

Abstract

Noncentrosymmetric molecules with a π-conjugated system and, among them, push-pull molecules such as pyridinium phenoxide, are a promising new class of materials for applications in optoelectronics due to their nonlinear optical (NLO) properties. Modelling studies have indicated that an increase in the twist angle between the two aromatic rings leads to an enhancement of the NLO properties. In order to

Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component

“…propylbenzene by following the literature procedures. [11] It should be noted that the Wittig route gave a mixture of E and Z isomers, which could be separated by column chromatography (1 NO) or by preparative high-performance liquid chromatography (HPLC; 1 H and 1 OM). In contrast, the Heck route led to the E isomers only.…”

Syntheses of Sterically Hindered Zwitterionic Pyridinium Phenolates as Model Compounds in Nonlinear Optics

“…The reaction between unsubstituted pyridin-4-ylboronate 4a and 4-bromoanisole 3 certainly provided, under our standard conditions, [4] corresponding biaryl 5a in 85 % yield (Scheme 2). Nevertheless, because of steric hindrance induced by the R 4 and R 5 alkyl groups, a moderate yield may be expected for the preparation of pyridin-4-ylboronates 4b-e by borylation of the other 3,5-dialkylated bromopyridines. As a consequence, this synthetic pathway was rapidly given up.…”

“…[4] Biaryls 9a-e were recovered in good-to-excellent yield (from 71 to 89 %) (Table 1), in spite of the steric hindrance involved by the alkyl groups at the meta position of the N-oxide.…”

“…[3] In order to experimentally confirm this features, we have already published the synthesis of a first series of sterically hindered pyridinium phenolates 1a-g (Figure 1). [4] Alkyl groups of increasing size (methyl, ethyl, isopropyl) were anchored at meta positions of the phenolate functionality. Unfortunately, these zwitterions were far too insoluble to allow conventional EFISHG (electronic field-induced second harmonic generation) measurements.…”

“…[7] This reaction is one of the most valuable reactions for the synthesis of dissymmetric biaryls, and it was already the key step of the preparation of our first series of zwitterions. [4] To avoid the protonolysis of the C-B bond, which occurs especially with sterically hindered coupling partners, the best and cheaper alternative was the use, under anhydrous conditions, of aryl boronic esters in place of boronic acids. Moreover, the presence of base in the Suzuki reaction led to the necessity of phenol protection, that is, in our case, the use of anisole derivatives.…”