Electron Transfer through Norbornadiene and Quadricyclane Moieties as a Model for Molecular Switching

Abstract

The bis(pentaammineruthenium(II)) complex of dicyanonorbornadiene has been prepared and characterized. Its photochemical conversion into a dicyanoquadricyclane complex by irradiation in methanol has been followed by UV−visible spectroscopy, IR spectroscopy, 1H NMR spectroscopy, and cyclic voltammetry. The dicyanonorbornadiene complex exhibits an intervalence transition in the 1000−1600 nm range when oxidized to the ruthenium(II)−ruthenium(III) state. This allows the determination of the effective coupling V ab through the dicyanonorbornadiene ligand, which is found as 0.023 eV. No such band is observed in the case of the dicyanoquadricyclane complex. These results show the possibility to modulate an intramolecular electron transfer process by using a photoisomerizable bridging ligand.

Photoswitchable mixed valence

“…9 Mixed valence compounds with molecular bridges comprised of units that can undergo reversible 45 photoisomerization reactions are well suited, simple model systems for investigating how charge delocalization can be affected with light. The first three reports on photoswitchable mixed valence appeared in 1996 and 2000, [10][11][12] but then this field of research has been dormant until 2006, when it finally 50 experienced a revival. 13 This tutorial review reports on the most significant findings from this emerging field, with particular focus on the contributions made by the groups of Launay, Akita, Nishihara, and the author's own research team.…”

“…To the best of the author's knowledge, four types of photoisomerizable linkers have been used so far as 10 bridges in photoswitchable mixed valence compounds (Scheme 1): (i) alkene linkers that can undergo E-Z isomerization; (ii) dithienylethenes which exhibit ring-closure and ring-opening reactions, (iii) norbornadiene spacers which isomerize to quadricyclane units, (iv) dimethyldihydropyrene units which 15 photoisomerize to cyclophanediene forms. In this tutorial review, a separate section will be dedicated to each of these four types of photoisomerizable units, discussing the key studies in each case.…”

“…1 In 1996, Launay, Bignozzi and coworkers reported on a molecule containing two pentaammineruthenium moieties 30 coordinated to the cyano-groups of a central 2,3-dicyanonorbornadiene unit (1n). 10 Irradiation of this molecule with UV or visible light induces photoisomerization of the bridging unit from its in initial norbornadiene (n) form to the socalled quadricyclane (q) form, resulting in molecule 1q. 35 Depending on the exact excitation wavelength, photoisomerization quantum yields () range from (8.3±0.6)•10 -4 to (2.2±0.2)•10 -2 , with the higher values obtained for UV excitation.…”

“…A splitting into two Ru(II/III) oxidation waves, as frequently 45 observed for strongly interacting di-ruthenium species, 1,3 was not reported, and the comproportionation constant (Kc) for the equilibrium Ru(II)-Ru(II) + Ru(III)-Ru(III)  2 Ru(II)-Ru(III) could be determined for neither of the two isomeric forms of molecule 1. However, direct evidence for metal-metal interaction 50 in the mixed-valence form of 1n came from optical absorption spectroscopy: 10 Upon oxidizing 1n to 1n + with tetrabutylammonium tribromide, a weak ( < 100 M -1 cm -1 ) and broad absorption band with a maximum near 1400 nm became detectable. Consistent with the assignment of this near-infrared 55 absorption to an intervalence charge transfer (IVCT) band, it disappeared again upon reducing 1n + back to 1n with zinc amalgam.…”

Steric effect studies on solar energy storage of norbornadiene-quadracyclane system: DFT calculations

“…The conversion of quadricyclane, 2, to norbornadiene, 1, leads to generate heat energy. The 1/2 system is used for solar energy storage [5][6][7], in molecular switching [8][9][10], in optoelectronic devices [11][12][13][14], as a data storage compound [15,16], as photodynamic chemosensor for metal cations [17,18], as a potential photoresponsive organic magnet [19][20][21] and as an energetic binder for solid rocket propellants [22]. Use of sensitizers and chromophores on 1/2 system are two developments to maximize the solar energy storage [23].…”