Bipyridinium‐bis(carboxylate) Radical Based Materials: X‐ray, EPR and Paramagnetic Solid‐State NMR Investigations

Abstract

The zwitterionic 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium (bp4pc) has been synthesized and crystals of its hydrated form bp4pc·2H 2 O and of its protonated reduced form H-bp4pc have been obtained. Upon heating, bp4pc·2H 2 O undergoes partial dehydration, leading to bp4pc·H 2 O at 160°C, together with a color change from yellow (room temperature) to green (140°C) and finally to brown (160-180°C). Analysis of bond lengths in the solid state reveals the expected short (d = 1.425 Å) and long (d = 1.485 Å) C-C central bond lengths in the all-radical salt H-bp4pc and bp4pc·2H 2 O, respectively, whereas the distance of 1.475 Å in bp4pc·H 2 O does not allow a conclusion to be drawn regarding the presence of radicals in this com-[a] MOLTECH

Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism

“…The green phase of 2 is very stable, and we have not observed the corresponding nonradical and expectedly yellow state. The intermediate pseudopolymorph, 2·2H 2 O , has been reported to be nonphotochromic without experimental details . We have obtained the dihydrate by different methods, and crystallographic results confirm that the structure is identical to that reported previously (Figure S15).…”

A Barrel‐Shaped Metal–Organic Blue‐Box Analogue with Photo‐/Redox‐Switchable Behavior

“…We found that irradiation of 2 results in a change of the central C−C bond length from 1.49(7) to 1.45(6) Å. Typically, a shortening of this bond is a diagnostic feature of viologen radicals owing to the delocalization of the single electron over both pyridinium rings [56, 57] . Further to this, we observed a change in the non‐coordinated C−O bond length of the carboxylate fragment upon irradiation, from 1.26(2) Å to 1.204(17) Å in 2 , and 1.237(9) to 1.211(9) Å in 1 .…”

“…There are two possible explanations for the observed photochromic behavior: (i) intermolecular electron transfer (from either Cl − or H 2 O), which changes the total charge of the ligand from 0 to −1, yielding V + . radical cations, [50, 56] or (ii) intramolecular electron transfer from the carboxylic acid to the viologen moiety, resulting in a neutral bcbp ligand with two radical centers (one on the carboxylic acid, COO . , and one on the pyridinium moiety, V + . )…”

“…We found that irradiation of 2 resultsinachange of the centralC À Cb ond length from 1.49(7)t o1 .45 (6) .T ypically,ashortening of this bond is ad iagnostic feature of viologen radicals owing to the delocalization of the singlee lectron over both pyridinium rings. [56,57] Further to this, we observedachange in the non-coordinated CÀOb ond length of the carboxylate fragment upon irradiation, from 1.26(2) to 1.204 (17) in 2,a nd 1.237(9) to 1.211 (9) significantly in 2.O verall,f rom these data, we can conclude that the structuralc hanges are too small to allow for direct structuralv isualization of reduced viologen species, yet confirm retention of the structures before and after UV irradiation.…”

“…There are two possible explanations for the observed photochromic behavior:( i) intermoleculare lectron transfer (from either Cl À or H 2 O), which changes the total chargeo ft he ligand from 0t oÀ1, yielding V + C radical cations, [50,56] or (ii)intramolecular electron transfer from the carboxylica cid to the viologen moiety,r esulting in an eutralb cbp ligand with two radicalc enters (one on the carboxylic acid, COOC,a nd one on the pyridinium moiety, V + C). [57] The former situation leads to spin density only on the pyridinium group, whereas the latter yields spin density on both the carboxylic acid and pyridinium groups,g iven the fact that two radical sites per ligand would be formed.…”