Caracterización del Agua Residual Cruda

If the photochemical processes in [RuPd]²⁺ should be optimized towards an higher ef-ficiency of the charge separating ¹MLCT process, evaluation of the energetic relations between ¹MLCTs ending up at tbut-bpy or tpphz is necessary. The energies of the (tbut-bpy)←Ru^{2+ 1}MLCT and the (tpphz)←Ru^{2+ 1}MLCT can be determined by analyzing the RRS of [RuPd]²⁺ which are excited at different wavelengths covering the¹MLCT absorp-tion band. If the ν⁴ dependency of the RR-intensities is taken into account the maximal enhancement of a RR-band corresponds to the absorption maximum of the corresponding

cdppz: Dipyrido[3,2-a:2,3-c]-phenazin

5 Characterizing the initial step of photoexcitation

electronic transition. Consequently, the ¹MLCT absorption band can be deconvoluted into the (tbut-bpy)←Ru^{2+ 1}MLCT and the (tpphz)←Ru^{2+ 1}MLCT.

Since the most intense resonance Raman bands of [RuPd]²⁺ are assigned to the tbut-bpy or the tpphz ligand, the corresponding intensities at the excitation wavelength 515, 488, 477, 458, 413 nm were determined by fitting the bands in the region 1150 - 1350 cm⁻¹ and 1400 - 1650 cm⁻¹ with pseudo-Voigt profiles. Prior to the fitting, the fluorescence background was thoroughly subtracted via either pseudo-Voigt-profiles or inverse polyoma (see section 3.4).^d The intensities of the RR-bands were normalized to a solvent band and the normalized RR-intensities of the tpphz and tbut-bpy assigned bands were summed up, respectively. The summation compensates possible errors evolving from background corrections. The fitted RRS of [RuPd]²⁺, dissolved in acetonitrile, are shown in figure 5.6.

Regarding the RR-enhancement of the tbut-bpy (cyan colored) and tpphz (green colored) assigned RR-bands for increasing excitation energies in figure 5.6, a stronger amplification is present for the tbut-bpy assigned bands as compared to the tpphz-assigned RR-bands.

The summed RR-intensities dependent on the excitation energies are shown in figure 5.7. The star-shaped dots are referring to summed RR-intensities of all green labelled tp-phz located RR-bands in figure 5.6. The semi-filled dots are illustrating the RR-intensities of the cyan labelled tbut-bpy assigned RR-bands in figure 5.6. To highlight the shift be-tween the devolutions of the tpphz and tbut-bpy assigned dots, the points were fitted by means of Gauss-profiles (compare section 3.4). Subsequently, the Gauss-profiles were nor-malized, since the number of RR-bands underlying the summation is different. The shift of the Gauss-profiles corresponds to the energetic difference between the (tpphz)←Ru²⁺

MLCT and the (tbut-bpy)←Ru^{2+ 1}MLCT.

Because the solvent significantly influences the hydrogen generation, the RRS of [RuPd]²⁺

dissolved in CH3CN is compared to the corresponding CH2Cl2 solution. When using CH3CN as solvent a turn-over-number of 56 was published (generation of 56 H2 molecules by one [RuPd]²⁺ molecule), while for the respective CH2Cl2 solution no hydrogen gener-ation was observed.^{81, 243}

The data corresponding to [RuPd]²⁺ dissolved in acetonitrile are shown in the upper graph of figure 5.7. In the lower graph the data referring to the respective dichloromethane solu-tion are presented. The maxima in the wavelength dependent development ofP

I_RR(tpphz) and P

I_RR(tbut-bpy) (estimated via the Gaussian profiles) differ about 9 nm. For both

dProgram used for deconvolution: Origin7G. Fluorescence background simulation and subtraction:

points of the spectra lying on the baseline were chosen and fitted with pseudo-Voigt profiles or inverse polynoma. RRS-deconvolution via pseudo-Voigt profiles: subsequent fitting of RRS-sections contain-ing about 10 bands, followed by reassemblcontain-ing and global fittcontain-ing. Minimal χ²-values were determined using Levenberg-Marquardt and simplex-iterations.

5.2 Charge localization at the ¹MLCT of [(tbut-bpy)₂Ru(tpphz)PdCl₂]²⁺

1650 1600 1550 1500 1450 1300 1250 1200 1150

ex

=413nm

Wavenumber / cm -1

ex

=458nm ex

=477nm

ResonanceRamanintensity

ex

=488nm [(tbut-bpy )

2

Ru(tpphz)PdCl 2

] 2(PF 6

)

dissolved in CH 3

CN

experimtal RRS

sum over all f itted bands

tbut-bpy located bands

tpphz located bands

ex

=515nm

Figure 5.6: Deconvolution of experimental resonance Raman spectra of [RuPd]²⁺ dis-solved in acetonitrile, excited at 515, 488, 477, 458, 413 nm (top down). Black graphs: experimental RRS; green and pale blue pseudo-Voigt-profiles: tp-phz and tbut-bpy assigned modes; dark blue graphs: sums over all respective pseudo-Voigt-profiles.

5 Characterizing the initial step of photoexcitation

400 425 450 475 500 525 550 575 600

350 400 450 500 550

448

400 425 450 475 500 525 550 575 600

[(tbut-bpy)

Figure 5.7: MLCT-absorption band of [RuPd]²⁺; dots: normalized sums of ν⁴-corrected RR-intensities, star-shaped dots: tpphz-assigned RR-bands, circle dots: tbut-bpy-assigned RR-bands, dotted and dashed lines: Gauss-fits; upper graph:

solvent = CH₃CN, lower graph: solvent = CH₂Cl₂; inset: normalized UV-vis spectra of [RuPd]²⁺dissolved in CH₃CN (solid line) and CH₂Cl₂ (dotted line).

Horizontal lines: ¹MLCT shifts at 10% absorption.

applied solvents, the P

I_RR(tpphz)-maximum is red-shifted against theP

I_RR (tbut-bpy)-maximum. Since the (tpphz)←Ru^{2+ 1}MLCT possesses a lower energy than the (tbut-bpy)←Ru^{2+ 1}MLCT, a targeted charge transfer to tpphz can be realized by excitation at the low-energetic edge of the ¹MLCT absorption band.

The energy levels of the ligand assigned ¹MLCTs are determined from the low-energy wavelength-values at 10% of the maximal intensities of the fitted Gauss-profiles, since these values are assumed to approximate the energies of electronic 0-0-transitions.^e The shifts at 10% intensity are illustrated by horizontal lines in figure 5.7. As figure 5.8

re-e0-0-transition: from the electronic and vibrational (v=0) groundstate to the vibrational groundstate (v=0) of the electronic excited state

5.2 Charge localization at the ¹MLCT of [(tbut-bpy)₂Ru(tpphz)PdCl₂]²⁺

veals, the energy difference between the ligand assigned ¹MLCT states is smaller for the CH₂Cl₂ solution than for the CH₃CN solution, even if the distances between the maxima of the ligand-assigned ¹MLCTs are minor differing between both solvents. In contrast, the maximum of the ¹MLCT absorption band of [RuPd]²⁺ dissolved in dichloromethane is red-shifted of only about 4 nm against the corresponding maximum of the acetonitrile solution. This discrepancy between the solvent dependent maxima-shifts of the particular ligand-located ¹MLCTs and the respective superimposed absorption bands in the UV-vis spectra might be explained by different intensity ratios of the underlying ligand-assigned

1MLCT bands. Therefore, it is assumed that in the case of the CH₂Cl₂ solution the

1MLCT ending up at tbut-bpy is more pronounced than the one ending up at tpphz, while this difference in the CH₃CN solution is supposed to be smaller. Hence, one can conclude that a transition to tpphz compared to tbut-bpy is of higher probability in the CH₃CN solution than in the respective CH₂Cl₂ solution. This fact is supposed to contribute to the different hydrogen-yields between CH₃CN- and CH₂Cl₂-solutions of [RuPd]²⁺.

-0,1

Figure 5.8: ¹MLCT energy differences ∆E referring to the groundstate energy (set zero).

∆E-values are estimated at 10% of the maximal amplitude of the Gauss pro-files. Left: solvent = CH₃CN, right: solvent = CH₂Cl₂

5 Characterizing the initial step of photoexcitation