Conformation‐Determined Through‐Bond versus Through‐Space Electronic Communication in Mixed‐Valence Systems with a Cross‐Conjugated Urea Bridge

Abstract

Bis-triarylamine 2 and cyclometalated diruthenium 6(PF6)2 with a linear trans,trans-urea bridge have been prepared, together with the bis-triarylamine 3 and cyclometalated diruthenium 8(PF6)2 with a folded cis,cis-N,N-dimethylurea bridge. The linear or folded conformations of these molecules are supported by single-crystal X-ray structures of 2, 3, and other related compounds. These compounds display two consecutive anodic redox waves (N(·+/0) or Ru(III/II) processes) with a potential separation of 110-170 mV. This suggests that an efficient electronic coupling is present between two redox termini through the cross-conjugated urea bridge. The degree of electronic coupling has been investigated by using spectroelectrochemical measurements. Distinct intervalence charge-transfer (IVCT) transitions have been observed for mixed-valent (MV) compounds with a linear conformation. The IVCT transitions can also be identified for the folded MV compounds, albeit with a much weaker intensity. DFT results support that the electronic communication occurs by a through-bond and through-space pathway for the linear and folded compounds, respectively. The IVCT transitions of the MV compounds have been reproduced by TDDFT calculations. For the purpose of comparison, a bistriarylamine and a diruthenium complex with an imidazolidin-2-one bridge and a urea-containing mono-triarylamine and monoruthenium complex have been synthesized and studied.

Insight into the strong aggregation-induced emission of low-conjugated racemic C6-unsubstituted tetrahydropyrimidines through crystal-structure–property relationship of polymorphs

“…The inter-crossed excited states are not only proved to lead to a red shift in absorption/emission but also to favor fluorescence efficiency enhancement. 15 In addition, the lone pair of electrons on N1 and the π-electrons conjugate not only through bonds but also through space 16 (marked with red circles), which can enhance electronic delocalization and restrict intramolecular motion, leading to a fluorescence efficiency enhancement and red-shifted absorption/emission to some extent. Besides the through-space conjugations marked with red circles, through-space conjugation exists between the dicarboxylates of 1c (marked with a yellow circle), which should be one of the reasons why 1c shows a longer wavelength emission than 1b .…”

Evaluation of Through‐Space Electronic Coupling in the Cofacially Aligned π‐Stacked Organic Mixed‐Valence System

“…This feature is clearly different from that of 2 • + in our previous work in which the spectrum was optically transparent below 10000 cm −1 30 . Nevertheless, this new absorption band is not strange in light of many precedents observed for the similar type of MVS with π – π stacked aromatic units, and was regarded as IVCT band a priori 14,24,27,31,57 . The spectroelectrochemical data were analyzed by multipeak Gaussian deconvolution (Figure 7(b)).…”

“…A number of representative structural motifs used for through‐space electronic interactions have been reported: cyclophanes, 13–17 1,2‐phenylene derivatives, 18–21 bicycloalkanes, 16,22,23 various kinds of sp 3 hydrocarbons, 24–29 1,8‐naphthalene, 30–33 and metal–organic frameworks 34,35 . The magnitudes of H 12 range from tens to thousands of wavenumbers depending on the bridge systems.…”

Small anion-assisted electrochemical potential splitting in a new series of bistriarylamine derivatives: organic mixed valency across a urea bridge and zwitterionization

“…In contrast to the vast majority of oxidation-active ureas, those having two redox centers at both ends have not received attention except for 1,3-bis(ferrocenyl)urea FcFc [41–42], a cyclometalated diruthenium complex [43], and bis(NAr 3 ) counterparts, including 1a (Fig. 1) [44]. The electrochemically control of H-bonding would have a dramatic impact on the field of mixed valency, which is striving to utilize charge delocalization for molecular devices.…”