wave initiation in the ferroin /ferriin- catalysed bz reaction with visible light

Wave Initiation in the Ferroin/Ferriin-Catalysed BZ Reaction with Visible LightRita Tóth and Vilmos GáspárInstitute of Physical ChemistryUniversity of Debrecen4010 Debrecen, P.O. Box 7, Hungary

Andrew Belmonte and Megan C. O'ConnellDepartment of MathematicsPennsylvania State University,University Park, PA 16802 USA

Annette Taylor and Stephen K. ScottSchool of ChemistryUniversity of Leeds,Leeds LS2 9JT, UK

The illumination is provided in a region of membranesufficiently long after the passage of an oxidation front

red spot6 s after laser off increasing blue spot

t = 18 and 32 s

laser on for 5 s

The illumination is provided closer to the oxidation front

red spotimmediatelyafter laser off

increasing blue spiralt = 10 and 14 s sustained spiral

t = 62 s

laser on for 3 s

red spot6 s after laser off

increasing blue spiralt = 16 and 46 s

sustained spiralt = 190 s

Illumination in the resting or recovered states does not lead to initiation of oxidation structures

b) high loading

Mechanism proposal

This band extends to the wavelength of the light (632.8 nm) used here. The more likely absorber is ferriin.

Ferroin (red, reduced form of the catalyst):max = 510 nm, max = 11100 mol-1 dm3 cm-1

Ferriin (blue, oxidized form of the catalyst):max = 590 nm, max = 600 mol-1 dm3 cm-1

Photoreduction of Fe(phen)33+ proceeds at this wavelength with a

quantum efficiency of ca. 2 10 2 in 0.05 M H2SO4 in the presence oforganic reducing species. [6]

The reduction is believed to proceed through the formation of a ligand-metal charge transfer (LMCT) excited state with electron transfer fromthe solvent.

Ferriin + h FerroinFerriin + h Ferroin

Introduction Belousov-Zhabotinsky (BZ) reaction: a chemical excitable medium

Effects of light on the BZ system:• UV illumination suppresses oscillations in the cerium-catalysed system [1]• UV illumination initiates waves in the ferroin-catalysed system [2]• Ferroin- and Ru(bpy)3

2+ catalysed systems are sensitive to visible light, cerium-catalysed system is unaffected [3]• Visible light inhibits spirals and changes their behavior [4].

Experiments• Visible light of wavelength = 632,8 nm, He-Ne (red) laser• circular polysulfone membrane (Gelman) pore size 0.45 m, 47 mm diameter loaded with bathoferroin

• BZ reaction mixture without catalyst: [malonic acid] = 0.213 M, [NaBrO3] = 0.213 M, [H2SO4] = 1.596 M, [NaBr] = 0.162 M, and [(NH4)2SO4] = 0.64 M.

The membrane is in the ‘resting’ steady state

laser on for 5 s (illuminated area ca. 2 mm2 )

red spot 6 s after laser off

increasing blue spott = 16 and 26 s

Illuminating a small region of the membranea) low loading

x: autocatalyst (HBrO2)z: oxidised form of the catalyst (ferriin): dimensionless rate coefficient for the photoreduction step : light intensity

Explanation of the effect of visible lightModified Tyson-Fife model

zxdtdz

qxqxfxxx

dtdx )](1[,

)()()1(

x

zIncreasing

Changing the intensityof illumination changes the position of z-nullcline (dz/dt = 0) defined as:

)(1

xz

laser off laser on laser off

x

z

x

z

x

z

steady-statelow value of z (red)low value of x

new steady-statelower value of z (more red)slightly increased value of xbut less than threshold

The system returns tothe original steady state.No excitation in the centreof illumination.

> 0

laser off laser on laser off

•

x

z

x

z

x

z

steady-statelow value of z (red)low value of x

new steady-statelower value of z (more red)higher value of xover the threshold

The system returns tothe original steady state.An excitation occurs in the centre of illumination,and a chemical wave is initiated.

>> 0

[1] V. A. Vavilin, A. M. Zhabotinsky, and A. N. Zaikin, Russ. J. Phys. Chem., 1968, 42, 1649. [2] H. Busse and B. Hess, Nature, 1973, 244, 203. [3] V. Gáspár, G. Bazsa, and M. T. Beck, Z. Phys. Chem. (Leipzig), 1983, 264, 43. [4] O. Steinbock, V. Zykov, and S. C. Müller, Nature, 1993, 366, 322. [5] A. Belmonte and J.-M. Flesselles, Phys. Rev. Lett., 1996, 77, 1174. [6] V. Balzani and V. Carassiti, Photochemistry of Coordination Compounds, Academic Press, London, 1970.

References