Photoregulated Transmembrane Charge Separation by Linked Spiropyran−Anthraquinone Molecules

Abstract

Amide-linked spiropyran-anthraquinone (SP-AQ) conjugates were shown to mediate ZnTPPS(4-)-photosensitized transmembrane reduction of occluded Co(bpy)3(3+) within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30-35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPPS4- and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)3(3+) reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound 3ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH* or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)3(3+). Thermal one-electron reduction of Co(bpy)3(3+) by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials.

Photoreversible Fluorescence Modulation of a Rhodamine Dye by Supramolecular Complexation with Photosensitive Cyclodextrin

“…The energy of the first-excited-singlet state of RhB was estimated to be 2.18 eV from the average frequencies of the absorption and emission bands. Similar calculations, based on absorption and emission maxima, [23] gave first-excited-singlet-state energies of 3.65 eV (l max = 340 nm) for the SP-form spiropyran moieties of CDSP-2, and of 2.10 eV (l max = 552 nm, l em = 630 nm) for the McH-form moieties. These results suggest that energy transfer from RhB to the SP form of CDSP-2 is impossible, while transfer to the McH form of the compound may occur.…”

“…These results are consistent with previous reports on similar spiropyran derivatives. [23,25] Therefore, fluorescence quenching of RhB through electron quenching is unlikely; we believe that it could probably be the result of a FRET process instead.…”

Photochemical studies: Chromones, bischromones and anthraquinone derivatives

“…Anthraquinones are another category of important and well studied (Lukeman et al, 2002;Yoshihara et al, 2001;Huang et al, 2007;Perumal et al, 2009;Langhals and Saulich, 2002;Gorner, 2003;Zhu et al, 2005;Ren et al, 2009;Maurino et al, 2008;Gouloumis et al, 2006;Quereshi et al, 2011) organic compounds and absorb UV light resulting in intersystem crossing and also act as electron and hydrogen atom acceptor. …”

Pyrene-sensitized electron transport across vesicle bilayers: dependence of transport efficiency on pyrene substituents

“…The concentration of Asc − , [Asc − ], was determined as follows: the irradiated vesicle solution was aerated to oxidize MV + • , and an aqueous solution of Triton X-100 (10% (w/w), 100 lL) was added to the solution. After stirring for 5 min, a K 3 [Fe(CN) 6 ] solution (30 mM) in 1.0 M Tris-HCl buffer containing 1.0 M NaCl was added, and [Asc − ] was calculated on the basis of the consumption of [Fe(CN) 6 ] 3− determined by a decrease in the absorption of 420 nm (e = 1027 M −1 cm −1 ). In the experiment for dependence of the initial rate of MV + • formation on light intensity, the vesicle solution was divided into four portions (3 mL each), and each solution was irradiated with 366 nm light emitted by an extra-high pressure mercury lamp (Ushio, SX-UI D500HAMP) through both an optical cutoff filter (UV-35, >350 nm) and a band-path filter (UV-D36B, 360 ± 20 nm).…”