Ruthenium-Phenothiazine Electron Transfer Dyads with a Photoswitchable Dithienylethene Bridge

A molecular ensemble composed of a phenothiazine (PTZ) electron donor, a photoisomerizable dithienylethene (DTE) bridge, and a Ru(bpy)32+ (bpy = 2,2′-bipyridine) electron acceptor was synthesized and investigated by optical spectroscopic and electrochemical means. Our initial intention was to perform flash-quench transient absorption studies in which the Ru(bpy)32+ unit is excited selectively (“flash”) and its 3MLCT excited state is quenched oxidatively (“quench”) by excess methylviologen prior to intramolecular electron transfer from phenothiazine to Ru(III) across the dithienylethene bridge. However, after selective Ru(bpy)32+1MLCT excitation of the dyad with the DTE bridge in its open form, 1MLCT → 3MLCT intersystem crossing on the metal complex is followed by triplet–triplet energy transfer to a 3π–π* state localized on the DTE unit. This energy transfer process is faster than bimolecular oxidative quenching with methylviologen at the ruthenium site (Ru(III) is not observed); only the triplet-excited DTE then undergoes rapid (10 ns, instrumentally limited) bimolecular electron transfer with methylviologen. Subsequently, there is intramolecular electron transfer with PTZ. The time constant for formation of the phenothiazine radical cation via intramolecular electron transfer occurring over two p-xylene units is 41 ns. When the DTE bridge is photoisomerized to the closed form, PTZ+ cannot be observed any more. Irrespective of the wavelength at which the closed isomer is irradiated, most of the excitation energy appears to be funneled rapidly into a DTE-localized singlet excited state from which photoisomerization to the open form occurs within picoseconds

A Triarylamine-Triarylborane Dyad with a Photochromic Dithienylethene Bridge

A molecular triad composed of a triarylamine donor, a triarylborane acceptor, and a photoisomerizable dithienylethene bridge has been synthesized and explored by cyclic voltammetry, UV–vis, and luminescence spectroscopy. The effects of irradiation with UV light and fluoride addition on the electrochemical and optical spectroscopic properties of the donor–bridge–acceptor molecule were investigated. Photoisomerization of the dithienylethene bridge affects the triarylboron reduction potential, but not the triarylamine oxidation potential. UV–vis experiments reveal that the association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge. Irradiation of a THF solution of our donor–bridge–acceptor molecule with UV light, followed by F– addition, leads to a different color of the sample than UV irradiation alone or F– addition alone

Charge Delocalization in a Homologous Series of a,a’-Bis(dianisylamino)-Substituted Thiophene Monocations

A homologous series of three molecules containing thiophene, bithiophene, and terthiophene bridges between two redox-active tertiary amino groups was synthesized and explored. Charge delocalization in the one-electron-oxidized forms of these molecules was investigated by a combination of cyclic voltammetry, near-infrared optical absorption spectroscopy, and EPR spectroscopy. All three cation radicals can be described as organic mixed-valence species, and for all of them the experimental data are consistent with strong delocalization of the unpaired electron. Depending on what model is used for analysis of the optical absorption data, estimates for the electronic coupling matrix element (HAB) range from ∼5000 to ∼7000 cm–1 for the shortest member of the homologous series. According to optical absorption and EPR spectroscopy, even the terthiophene radical appears to belong either to Robin–Day class III or to a category of radicals commonly denominated as borderline class II/class III systems. The finding of such a large extent of charge delocalization over up to three adjacent thiophene units is remarkable

The GTN patch: a simple and effective new approach to cardioprotection?

This is a post-peer-review, pre-copyedit version of an article published in Basic Research in Cardiology . The final authenticated version is available online at: https://doi.org/10.1007/s00395-018-0681-2There remains a significant un-met need to reduce the extent of myocardial injury caused by ischaemia and reperfusion injury in patients experiencing an ST-elevation MI. Although nitric oxide is central to many cardioprotective strategies currently undergoing investigation, cardioprotection from the delivery of nitrates/nitrites has been inconsistently observed. The route of administration appears to be a critical variable. The glyceryl trinitrate (GTN) patch is commonly used as a simple and practical means of delivering nitric oxide to patients with ischaemic heart disease, but whether acute cardioprotection can be achieved by application of a GTN patch has not been investigated before. Here, we use a mouse model to demonstrate that a GTN patch is highly cardioprotective when applied immediately prior to 40 min occlusion of the left anterior coronary artery followed by 2 h reperfusion, reducing infarct size from 54 ± 4% in control mice, to 28 ± 4% (P < 0.001, N = 7). The degree of protection was similar to that achieved with a standard remote ischaemic preconditioning protocol. Furthermore, and of greater potential clinical relevance, a GTN patch was also protective when applied well after the initiation of ischaemia and 15 min prior to reperfusion (28 ± 4 vs 59 ± 4%; P < 0.01, N = 5). Confirmatory experiments verified the expected effect increase in plasma nitrite levels and decrease in blood pressure. The simplicity and rapidity of GTN patch application (easily applied in an ambulance or cardiac catheterization laboratory), and low cost (potentially relevant to low-income countries), make it attractive for further investigation.NIHR Biomedical Research Council (SD), British Heart Foundation PG/15/52/31598 (SD, DH) and the The Hatter Foundation

Ruthenium-Phenothiazine Electron Transfer Dyad with a Photoswitchable Dithienylethene Bridge: Flash-Quench Studies with Methylviologen

A molecular ensemble composed of a phenothiazine (PTZ) electron donor, a photoisomerizable dithienyl­ethene (DTE) bridge, and a Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) electron acceptor was synthesized and investigated by optical spectroscopic and electrochemical means. Our initial intention was to perform flash-quench transient absorption studies in which the Ru­(bpy)₃²⁺ unit is excited selectively (“flash”) and its ³MLCT excited state is quenched oxidatively (“quench”) by excess methylviologen prior to intramolecular electron transfer from phenothiazine to Ru­(III) across the dithienylethene bridge. However, after selective Ru­(bpy)₃²⁺¹MLCT excitation of the dyad with the DTE bridge in its open form, ¹MLCT → ³MLCT intersystem crossing on the metal complex is followed by triplet–triplet energy transfer to a ³π–π* state localized on the DTE unit. This energy transfer process is faster than bimolecular oxidative quenching with methylviologen at the ruthenium site (Ru­(III) is not observed); only the triplet-excited DTE then undergoes rapid (10 ns, instrumentally limited) bimolecular electron transfer with methylviologen. Subsequently, there is intramolecular electron transfer with PTZ. The time constant for formation of the phenothiazine radical cation via intramolecular electron transfer occurring over two <i>p</i>-xylene units is 41 ns. When the DTE bridge is photoisomerized to the closed form, PTZ⁺ cannot be observed any more. Irrespective of the wavelength at which the closed isomer is irradiated, most of the excitation energy appears to be funneled rapidly into a DTE-localized singlet excited state from which photoisomerization to the open form occurs within picoseconds

Photoswitchable Organic Mixed Valence in Dithienylcyclopentene Systems with Tertiary Amine Redox Centers

The electronic structures of the radical cations of two dithienylperfluorocyclopentene molecules with appended tertiary amine units were investigated by electrochemical and optical spectroscopic methods. The through-bond N–N distances in the photocyclized (closed) forms of the two systems are 9.3 and 17.6 Å, respectively, depending on whether the nitrogen atoms are attached directly to the two thienyl units or whether xylyl spacers are in between. In the case of the radical cation with the longer N–N distance, photocyclization of the dithienylperfluorocyclopentene core induces a changeover from class I to class II mixed valence behavior. In the case of the shorter system, the experimental data is consistent with assignment of the photocyclized form to a class III mixed valence species

A Triarylamine–Triarylborane Dyad with a Photochromic Dithienylethene Bridge

A molecular triad composed of a triarylamine donor, a triarylborane acceptor, and a photoisomerizable dithienylethene bridge has been synthesized and explored by cyclic voltammetry, UV–vis, and luminescence spectroscopy. The effects of irradiation with UV light and fluoride addition on the electrochemical and optical spectroscopic properties of the donor–bridge–acceptor molecule were investigated. Photoisomerization of the dithienylethene bridge affects the triarylboron reduction potential, but not the triarylamine oxidation potential. UV–vis experiments reveal that the association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge. Irradiation of a THF solution of our donor–bridge–acceptor molecule with UV light, followed by F^– addition, leads to a different color of the sample than UV irradiation alone or F^– addition alone

Charge Delocalization in a Homologous Series of α,α′-Bis(dianisylamino)-Substituted Thiophene Monocations

A homologous series of three molecules containing thiophene, bithiophene, and terthiophene bridges between two redox-active tertiary amino groups was synthesized and explored. Charge delocalization in the one-electron-oxidized forms of these molecules was investigated by a combination of cyclic voltammetry, near-infrared optical absorption spectroscopy, and EPR spectroscopy. All three cation radicals can be described as organic mixed-valence species, and for all of them the experimental data are consistent with strong delocalization of the unpaired electron. Depending on what model is used for analysis of the optical absorption data, estimates for the electronic coupling matrix element (<i>H</i>_<i>AB</i>) range from ∼5000 to ∼7000 cm^–1 for the shortest member of the homologous series. According to optical absorption and EPR spectroscopy, even the terthiophene radical appears to belong either to Robin–Day class III or to a category of radicals commonly denominated as borderline class II/class III systems. The finding of such a large extent of charge delocalization over up to three adjacent thiophene units is remarkable