UV-vis and ECD spectroelectrochemistry of atropisomeric naphthalenediimide derivative

The UV-vis and ECD spectroelectrochemistry (SEC) of a chiral binaphthalenylamine derivative of the N-butyl naphthalenediimide (NDIB-NH₂) enantiomers were applied to measure UV-vis and ECD spectra of NDIB-NH₂ radicals and dianion formed in the reduction and oxidation processes observed in cyclic voltammetry (CV). The CV curves and EPR spectroelectrochemistry enabled us to establish conditions at which a radical-anion [NDIB-NH₂]̇^.-, a dianion [NDIB-NH₂]^2-, and a radical-cation [NDIB-NH₂]̇^.+ are formed. The DFT restricted open-shell CAM-B3LYP-D3/def2TZVP/PCM calculations demonstrated that in the radical-anion [NDIB-NH₂]̇^.-, spin is spread over the NDI system while in the radical-cation [NDIB-NH₂]̇⁺ it is spread over the aminonaphthalene moiety. The UV-vis spectra of radical-anion and dianion show the most significant changes in the 400-800 nm range. In that range, the ECD spectra varied with the change of electrode potential more than the UV-vis did and enabled the identification of a new ECD band of [NDIB-NH₂]̇^.- at ca. 400 nm hidden in the background in the UV spectra at -1000 mV. A broad structured ECD pattern with a maximum at ca. 530 nm was observed for [NDIB-NH₂]̇^.- (-1000 mV), while a single smooth ECD band of [NDIB-NH₂]^2- was located at 520 nm (-1750 mV). For the first time, an isosbestic point (455 nm) was found in ECD spectroelectrochemical measurements for the radical-cation [NDIB-NH₂]̇^.+ in equilibrium with the NDIB-NH₂ neutral form. The TD-DFT CAM-B3LYP-D3/6-31G** calculations combined with the hybrid (explicit combined with implicit) solvation model fairly well reproduced the UV-vis and ECD SEC of neutral and redox forms of NDIB-NH₂ but the ECD spectrum of [NDIB-NH₂]̇^.+ above 390 nm.

On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence

The synergistic functioning of redox-active components that emerges from prototypical 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2'-di(N-methylpyrid-4-ylium)-1,1'-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Redox-active tetraaryldibenzoquinodimethanes

Quinodimethanes (QDs) are a class of non-aromatic π-conjugated compounds that are well-known to be interconvertible scaffolds in many response systems. While parent ortho- and para-QDs (o-QD and p-QD) can be easily converted to benzocyclobutenes or oligomers/polymers by the formation of C-C bonds at α-positions, the attachment of four phenyl groups to these reactive sites makes o-Ph₄QD and p-Ph₄QD long-lived. We have demonstrated that further dibenzo-annulation of such tetraaryl QD units also drastically increases their stability, and many tetraarylated dibenzoquinodimethane derivatives have been developed. This Feature Article shows our milestones in creating functional redox systems, where drastic changes in structure occur upon electron transfer (dynamic redox "dyrex" behaviour).

Molecular Wires with Controllable π-Delocalization Incorporating Redox-Triggered π-Conjugated Switching Units

A perfluorobiphenyl-2,2'-diyl dication and its corresponding dihydrophenanthrene-type electron donor are interconvertible upon two-electron transfer. Redox-triggered C-C bond-formation/cleavage caused a drastic change in the torsion angle of the biphenyl unit. Thus, π-delocalization ON/OFF switching was observed as a change in the UV absorption upon electrolysis of the linearly extended analogue with two phenylethynyl groups. A further extended π-system with a molecular length of ca. 3.5 nm, which has two switching units, was synthesized. Spectroelectrograms as well as voltammetric analyses showed that the two units act nearly simultaneously because of the very small inter-unit electrostatic repulsion in the tetracationic state. Thus, the present pair is a promising candidate as a switching unit for "molecular wires" with controllable π-delocalization, in which a higher ON/OFF ratio of delocalization could be realized by incorporating multiple switching units.

Organic Molecular Layer with High Electrochemical Bistability: Synthesis, Structure, and Properties of a Dynamic Redox System with Lipoate Units for Binding to Au(111)

Biphenyl-2,2'-diylbis(10-methyl-9-methyleneacridan)-type electron donor 1, which has two tethered cyclic disulfide units at the 6,6'-positions, was designed and synthesized as the first member of a dynamic redox (dyrex) system that can form molecular layers on a Au(111) electrode. Upon the two-electron (2 e) oxidation of 1, the persistent dicationic dye 2²⁺ was generated with the formation of a new C-C bond, which is reversibly cleaved upon 2 e reduction to regenerate 1 (dyrex behavior). Similar dyrex interconversion occurs in the molecular layer of 1 on gold. The chemical identities of 1/Au and electrochemically generated 2²⁺ /Au were unambiguously determined by in situ IR spectroscopy in the attenuated total reflection mode. In situ scanning tunneling microscopy (STM) was conducted under electrochemical conditions to examine the surface structure of 1 adsorbed on a Au(111) electrode. Although no long-range-ordered morphology was found in the STM image of 1, an in situ STM study of the potential-induced dyrex reaction of 1 to 2²⁺ showed that the grained spots in the image became slightly brighter.

Bis(diarylethenyl)-thiophenes, -bithiophenes, and -terthiophenes: a new series of electrochromic systems that exhibit a fluorescence response

2,5-Bis(diarylethenyl)thiophenes (1) and their bithiophene (2) and terthiophene (3) homologues were designed as a new series of violene/cyanine-hybrid-type electrochromic materials. Alkoxyphenyl and dialkylaminophenyl groups are used as the aryl group in the cyanine parts of 1/2/3O and 1/2/3N, respectively, which emit fluorescence in the neutral state but not in the oxidized state. Voltammetric analyses of 18 newly prepared electron donors show that they all undergo two reversible one-electron oxidations. Redox potentials, UV–vis spectral features, and fluorescence quantum yields are important parameters for characterizing the present electrochromic behavior with a fluorescence response, and can be finely tuned by changing aryl groups in the cyanine part, the number of thiophene rings in the violene part, and the alkyl-chain length of the alkoxyphenyl or diaklylaminophenyl groups (R = CH3, C8H17, C16H33), which makes this a versatile platform for the design of novel electrochromic materials.

Assembly of an Axially Chiral Dynamic Redox System with a Perfluorobiphenyl Skeleton into Dumbbell- or Tripod-type Electron Donors

The incorporation of F atoms endows a diethenylbiphenyl-based electron donor with configurational stability and SN Ar reactivity. The former enables the dynamic redox pair of (Rax)-1/(Rax ,R,R)-1(2+) to exhibit drastic UV/Vis and CD spectral changes upon electrolysis, whereas the latter makes it possible for (Rax)-1 to serve as a useful chiral synthon for the production of larger assemblies [(Rax ,Rax)-2 d,p,m and (Rax ,Rax ,Rax)-3] containing two or three dyrex units. These dyads and triad also exhibit a clean electrochiroptical response with isosbestic points owing to one-wave multi-electron transfer.

Two-way chromic interconversion of the 2,2′-biphenol-6,6′-diyl dication with 5H,10H-dioxapyrene or 9H,10H-4,5-dihydroxyphenanthrene

Two-proton or two-electron transfer of the title biphenolic dication proceeds nearly simultaneously to induce 2,6′/2′,6- or 6,6′-bond formation to give dioxapyrene or dihydrophenanthrene derivatives, respectively, with vivid changes in color.