Ferrocene‐Donor and 4,5‐Dicyanoimidazole‐Acceptor Moieties in Charge‐Transfer Chromophores with π Linkers Tailored for Second‐Order Nonlinear Optics

Abstract

A series of new nonlinear optical chromophores (1-15) that were comprised of ferrocene-donor and 4,5-dicyanoimidazole-acceptor moieties and various π linkers of different length were synthesized. Support for the presence of significant DA interactions in these NLO-phores was obtained from the evaluation of the quinoid character of the 1,4-phenylene moieties and their electronic absorption spectra, which featured intense high-energy (HE) bands that were accompanied by less-intense low-energy (LE) bands. The redox behavior of these compounds was investigated by cyclic voltammetry (CV) and by rotating-disc voltammetry (RDV); their electrochemical gaps decreased steadily from 2.64 to 2.09 V. In addition to the experimentally obtained data, DFT calculations of their absorption spectra, HOMO/LUMO levels, and second-order polarizabilities (β) (-2ω,ω,ω) were performed. A structure-property relationship study that was performed by systematically altering the π linker revealed that the intramolecular charge-transfer and nonlinear optical properties of these inorganic-organic hybrid D-π-A systems (1-15) were primarily affected by: 1) The presence of olefinic/acetylenic subunits; 2) the length of the π linker; and 3) the spatial arrangement (planarity) of the π linker.

1,4‐Phenylene and 2,5‐Thienylene π‐Linkers in Charge‐Transfer Chromophores

“…In contrast to common organic push-pull chromophores, ferrocene-functionalized organic/ inorganic hybrid systems feature two low-energy transitions. According to Marder and co-workers [21] and our recent results, [15] the low energy (LE) absorption can be attributed to the HOMO-LUMO transition (Fe!NO 2 ), whereas the higher energy transition results from the HOMO-3-LUMO interaction (Cp!NO 2 ). Hence, generally two more or less distinguishable CT bands are present for ferrocene-substituted D-p-A systems.…”

“…General Ferrocene-functionalized precursors 16-23, [15] 25, [16] 29, [17] 30, [18] and 31 [19] were synthesized according to literature procedures. The synthesis and spectral characterization of intermediates 28, 32, and 33 are given in the Supporting Information.…”

“…The synthesis involves Suzuki-Miyaura and Sonogashira cross-coupling reactions with ferrocene-functionalized precursors, such as iodoferrrocene (16), [15] 4-bromophenylferrocene (17), ferroceneboronic acid (18), ethynylferrocene (19), 4-ethynylphenylferrocene (20), [15] and boronic acid pinacol esters 21-23. [15] Target compounds 1-6 in series a were obtained by the cross-coupling reactions of 16-18 with compounds 24-28 with isolated yields of 31 (1), 67 (2), 82 (3), 51 (4), 19 (5), and 61 % Abstract in Czech: Bylo syntetizovµno patnµct modelový ch chromoforů s vnitřním přenosem nµboje, kterØ obsahují ferrocen donor, NO 2 skupinu jako akceptor a 1,4-fenylenovØ a 2,5-thienylenovØ p-mů stky. Tyto sloučeniny byly dµle zkoumµny pomocí rentgenostrukturní analý zy, elektrochemie, absorpčních spekter a DFT vý počty.…”

“…One or both 1,4-phenylene subunits of the p-linker in 1-6 (series a, Scheme 2) were systematically replaced by one or two 2,5-thienylene units as in 7-12 (series b) and 13-15 (series c), respectively. [15] Herein, we report the design and synthetic approaches to chromophores 1-15, full spectral characterization, and further investigation of the efficiency of ICT by X-ray analysis, absorption spectroscopy, electrochemistry, and DFT calculations. The obtained results were further compared to previous series of ferrocene-and imidazole-4,5-dicarbonitrile-derived chromophores.…”

“…The obtained results were further compared to previous series of ferrocene-and imidazole-4,5-dicarbonitrile-derived chromophores. [15] Results and Discussion…”