Distance Dependence of Electronic Coupling in Rigid, Cofacially Compressed, π-Stacked Organic Mixed-Valence Systems

Abstract

A series of new π-stacked compounds, 1,8-bis­(2′,5′-dimethoxybenzene-1′-yl)­naphthalene (1), 1,4-bis­(8′-(2″,5″-dimethoxybenzene-1″-yl)­naphthalen-1′-yl)­benzene (2), and 1,8-bis­(4′-(8″-(2‴,5‴-dimethoxybenzene-1‴-yl)­naphthalen-1″-yl)­benzene-1′-yl)­naphthalene (3), have been synthesized and characterized herein as precursor molecules of monocationic mixed-valence systems (MVSs). The three-dimensional geometries of these compounds were determined by X-ray crystallography. A near-orthogonal alignment of the naphthalene pillaring motif to the dimethoxybenzene redox center, or the phenylene spacer, imposes cofacial alignment of these units in a juxtaposed manner with sub-van der Waals interplanar distances. Cyclic and differential pulse voltammograms reveal that the ΔE values between two sequential oxidation potentials are 0.30, 0.11, and 0.10 V for 1, 2, and 3, respectively. MVSs derived from these compounds are recognized as class II according to the Robin and Day classification. The decay parameter β, which describes the distance dependence of the squared electronic coupling in the three mixed-valence systems, was experimentally determined via Mulliken–Hush analysis of the intervalence charge transfer band (β = 0.37 Å–1) and theoretically assessed from charge-resonance contributions derived from DFT computations (β = 0.37 Å–1). These values are extraordinarily mild, indicating that the electronic interaction between redox centers in the longitudinal direction may be comparable to that in the transverse direction, if the MVS system is appropriately designed.

Pre‐Planarized Triphenylamine‐Based Linear Mixed‐Valence Charge‐Transfer Systems

“…Molecular MV-CT systems feature two oxidizable or reducible building blocks covalently linked through molecular bridges.T he molecular bridges serve two roles:a sa n electronic coupling medium and as ad istance keeper.T hey are typically built from carbon frameworks,s uch as aromatics, [8][9][10][11] alkynes, [8,12] cyclophanes, [13][14][15] oligo-p-phenylene vinylenes, [16,17] etc., or combinations thereof. [18,19] All of these allow the electronic coupling to be fine-tuned at either short or long distances.…”

Vorplanarisierte Triphenylamin‐basierte lineare gemischtvalente Ladungstransfersysteme

“…Sie fungieren als elektronisches Koppelmedium und als Distanzhalter. Typischerweise sind sie aus Kohlenstoffgerüsten, wie Aromaten, [8–11] Alkinen, [8, 12] Cyclophanen, [13–15] Oligo‐ p ‐phenylvinylenen [16, 17] usw., oder Kombinationen davon aufgebaut [18, 19] . Diese molekularen Brücken ermöglichen eine Feinabstimmung der elektronischen Kopplung sowohl bei kurzen als auch bei langen Distanzen.…”

“…Molekulare MV-CT-Systeme weisen zwei oxidierbare oder reduzierbare Bausteine auf,w elche kovalent durch molekulare Brücken verbunden sind. Die molekularen Brücken erfüllen zwei Zwecke.S ie fungieren als elektronisches Koppelmedium und als Distanzhalter.T ypischerweise sind sie aus Kohlenstoffgerüsten, wie Aromaten, [8][9][10][11] Alkinen, [8,12] Cyclophanen, [13][14][15] Oligo-p-phenylvinylenen [16,17] usw., oder Kombinationen davon aufgebaut. [18,19] Diese molekularen Brücken ermçglichen eine Feinabstimmung der elektronischen Kopplung sowohl bei kurzen als auch bei langen Distanzen.…”

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

“…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.…”

“…In this manuscript, we report a new mixed‐valence system (MVS), [1,8‐bis(2′,5′‐dimethoxy‐4′‐methyl‐[1,1′‐biphenyl]‐4‐yl)naphthalene] • + ( 1 • + , Chart 1), in which two dimethoxytoluene redox centers, hereinafter designated as D (or D 1 and D 2 if necessary), are separated by 1,8‐diphenylnaphthalene spacer featuring a van der Waals D‐A interplanar separation (3.50–3.70 Å). In our previous works, we have shown that the charge transfer through the orthogonally attached naphthalene pillars is prohibited and the donor–acceptor electronic coupling is mediated by direct through‐space π – π interaction in 2 • + 30 . We put one more phenylene unit between redox active dimethoxytoluene and naphthalene moieties so that the overall through‐bond distance between two D units is far more separated, and thus through‐bond charge transfer between neutral and cationic D units is essentially precluded.…”