7.3 Examples of homogeneous and nano-heterogeneous systems. A. Photo-oxydation of aromatic hydrocarbons by acrdinium dyes

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1 7.3 Examples of homogeneous and nano-heterogeneous systems A. Photo-oxydation of aromatic hydrocarbons by acrdinium dyes Cl N + N + N + CH 3 CH 3 SO 3 MA + CMA + APS 0 Dye τ [ns] Φf φ(s/s ) [V/ SHE] E0,0 [ev] 10-methyl-acridinium (MA + ) chloro, 10-methyl-acridinium (CMA + ) acridinium-3-propylsulfonate (APS 0 ) ket ksep + M (S + D) ( 1 S* + D) (S / D + ) S + D + S + D + M + k et 43

2 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes The quantum yield ΦM+ for the production of the radical cation monitor M + is measured by ns laser transient absorption spectroscopy (laser flash photolysis). The reaction D + + M D + M + being quantitative, ΦM+ can be expressed by: ΦM+ = Φet Φsep = ket Σ ki where Φet is the quantum yield of the forward electron transfer process, and Φsep the yield of the separation of the ion pair (S /D + ). The value of Φet is accessible experimentally by applying Stern-Volmer's method. The rate constant for back electron transfer k et can then be simply estimated from the relationship: Φet k et = ksep ( 1) ΦM+ ksep ksep + k et The separation rate constant ksep was measured separately and was found to be independent of the donor D : ksep = s 1 A 500 N + M λ / nm 700 Transient absorption spectrum of tri-p-tolylamine radical cation obtained upon ns pulsed laser irradiation of 10-methyl acridinium perchlorate M, m-xylene M, and tri-p-tolylamine M in degassed acetonitrile. Wavelengths for dye excitation and M + monitoring were λexc = 408 nm and λobs = 670 nm, respectively. 44

3 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes Ns laser flash photolysis A [M + ] ns Laser Transient Absorption Spectrometer Nd:YAG Nd:YAG laser laser Dye laser time SHG THG Transient Digitizer Monitoring Diode λ = 408 nm λ = 670 nm Xe Lamp PMT Filters Sample Monochromator 45

4 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes Donor φ(d + /D ) G 0 et [ev] [V/ SHE] MA + CMA + APS

5 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes (S/S ) N CH G 0 et = F [ϕ 0 (D/D + ) ϕ 0 (S/S )] 1.0 hυ G 0 et k et S + D + S + D (M + /M ) N G 0 et 2.5 (S * /S ) φ [V/ SHE] ket S * + D S + D + (D + /D ) 47

6 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes Donor G 0 et [ev] Φet / ΦM+ log ( k et [s 1 ] ) Toluene m-xylene p-xylene ,2,4-trimethylbenzene Durene Pentamethylbenzene Hexamethylbenzene Biphenyl Diphenylacetylene Naphthalene methylnaphthalene ,6-dimethylnaphthalene ,3,5-trimethylnaphthalene N + N + 48

7 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes log ( k et [s 1 ] ) S MA + S APS 0 D 1 ring D 2 rings G 0 et [ev] 49

8 Photo-oxydation of aromatic hydrocarbons by acrdinium dyes Non-linear fitting of the parameters of Marcus-Dogonadze's equation k et = H DA 2 1 2π λ out k B T exp λ in hν λ in hν exp ΔG 0 r + nhν + λ out n=0 n! 4 λ out k B T ( ) 2 The vibrational frequency ν is set to ῡ = constant = 1500 cm 1 S MA + APS 0 D HDA 2 [cm 1 ] 1 ring 2 rings 2 rings 1 ring λout [ev] λin [ev]

9 B. Light-induced heterogeneous ET from the conduction band of TiO2 to an acceptor in solution G e 1) Intrinsec absorption of light hυ TiO2 TiO2 (ecb... hvb + ) EC ket (A/A ) 2) Electron-hole recombination hυ 3.2 ev h + fast (D + /D) TiO2 (ecb... hvb + ) TiO2 3) Hole scavenging fast TiO2 (ecb... hvb + ) + D TiO2 (ecb ) + D + 4) Interfacial electron transfer ket TiO2 (ecb ) + A TiO2 + A Dependence of G 0 et upon the solution ph EF,n* EC = F [ φc (ph=0) V ph ] G 0 et = F φ 0 (A/A ) EF,n* G 0 et F [ φ 0 (A/A ) φc (ph=0) V ph ] where : φc (ph=0) V / SHE experimental variable 51

10 Light-induced interfacial electron transfer Aqueous suspension of TiO2 nanoparticles Particle radius ~ 5-10 nm weak Rayleigh scattering transparent colloid allows for the application of ns laser transient absorption spectroscopy technique to monitor ET kinetics Electron acceptor A H 3 C + + N N CH 3 MV 2+ (colorless) e H 3 C N N CH 3 + MV + (blue) ϕ 0 (MV 2+ /MV + ) = V / SHE 52!

11 Light-induced interfacial electron transfer Ns laser flash photolysis A exponential fit ket ns Laser Transient Absorption Spectrometer Nd:YAG Nd:YAG laser laser time SHG THG Transient Digitizer Monitoring Diode λ = 355 nm λ = 602 nm Xe Lamp PMT Filters Sample Monochromator 53

12 Light-induced interfacial electron transfer 8 7 MV 2+ C14-MV 2+ [MV 2+ ]0 = 10 3 M log ( ket [s 1 ] ) slope α = 0.8 Tafel equation: ph G 0 et [ev] 54

13 ζ potential [ mv ] Light-induced interfacial electron transfer + HOH H+ Ti O HOH + Ti O OH Ti O PZZP O- Ti O ph The surface of titanium dioxide is amphoteric. In acidic medium, terminal OH groups, as well as oxygen bridges are protonated, yielding a positive surface charge. In alkaline medium, terminal hydroxyl groups are deprotonated, resulting in a negative surface charge. An isoelectric point, corresponding to a neutral surface, is found typically for ph = 6. The zeta-potential of colloidal particles can be easily measured as a function of ph by use of electrophoresis technique. 55

14 Light-induced interfacial electron transfer Results displayed on page 51 are well fitted by Tafel linear equation with a slope α = 0.8. The latter value (α 0.5) is not compatible with the assumption of a kinetically symetric activated complex and thus with the classical Marcus theory. log ( ket [s 1 ] ) ph α obs 0.8 αobs = 0.8 αcorr = 0.5γ α = α obs - µ = Experimental results can be corrected for electrostatic interactions between the solid oxide charged surface and the acceptor cations in solution by using a semiempirical function: in which γ is an ajustment factor and µ is the ionic strength of the liquid medium. - ΔetG [ ev ] 56

15 Light-induced interfacial electron transfer Non-linear fitting of the parameters of Marcus classical relation applied to diabatic ET 10 Λ log ( ket [s 1 ] ) 8 6 HDA 2 = 7.6 cm 1 Λ = 1.4 ev G ΔetG 0 [ev] [ ev ] 57