Zinc Porphyrin Chromophores A qualitative introduction…

Porphyrin Porphyrin comes from Greek porphura meaning purple: all porphyrins are intensely coloured. Metalloporphyrin ring is found in many biological systems as active sites for: oxygen transfer or storage (hemoglobin, myoglobin) electron transfer (cytochrome) energy conversion (chlorophyll) Efficient sensitizers or catalysts (chemical and photochemical processes) Zn H + Diversity of uses comes from the different metals that can be inserted in the ring.

Examples Chlorophyll a Heme group Cytochrome c with heme c

Typical UV-visible absorption spectrum Absorption properties S0S0 Soret (or B band) at 400 nm S2S2 transition S0S0 S1S1 Weaker Q band at 550 nm S 1 and S 2 first and second excited states of the molecule. Fast internal conversion S 2 → S 1 B and Q bands both arise from  to  * transitions and can be explained by considering the four frontier orbitals of the porphyrin.

Goutermann Four-Orbital Model HOMOs LUMOs Orbitals Energy states Transitions between these orbitals gives rise to two 1 E u excited states. Orbital mixing splits these two states into a high energy state with a high oscillator strength and a low energy state with a low oscillator strength. Soret (B) band Q bands S0S0 S1S1 S2S2

2 ZnCl+ +2 HCl Zn-Porphyrin Easy inclusion of the Zn 2+ ion in the cycle.

Zn-Porphyrin Because the d orbitals of Zn are far enough in energy from the  and  * levels of the porphyrin, the electronic structure of the Zn-Porphyrin is close to the free Porphyrin.

Porphyrin as sensitizer TCPP exhibit a long-lived (>1ns)  * singlet excited state and only a weak singlet/triplet mixing. Appropriate HOMO/LUMO levels position in energy. Functional groups have not much influence on the TCPP electronic structure.

Zn-Porphyrin dyes Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position Jonathan Rochford, Dorothy Chu, Anders Hagfeldt, and Elena Galoppini JACS 129 (2007) 4655

UV-visible absorption spectrum Effect of the ending groups Because the phenyl group is perpendicular to the plane of the porphyrin, changing the ending group does not strongly alter the porphyrin electronic structure. In MeOH solution, very similar absorption spectra for all the modified porphyrins. Good test molecules for interaction with surface.

Effect of the ending groups Adsorbed on ZnO np, visible peak shifts. Only adsorption on a surface makes a difference between the dyes. UV-visible absorption spectrum Molecule bonding to the surface determines electronic structure.

Effect of molecular orientation on the surface Better results when the dye lies flat on the surface, but not too far

At high coverage porphyrins have an inherent tendency to aggregate, and dipole/dipole interactions at high coverage are expected to allow rapid migration of the excited state between neighboring dyes, increasing the probability of exciton annihilation. The greater the degree of mixing, the less intense the  band relative to the Soret band. Notes

Effect of rigid linker length E. Galoppini Unpublished ? Longer linker, worst efficiency.

ZnO nanorods Faster charge diffusion into the single crystal nanorods.

Looks good… What can we do…?

Zn-Porphyrin ZnP Zinc Porphyrin ZnTPP Zinc TetraPhenylPorphyrin ZnP and ZnTPP can be sublimated in UHV.

Zn-based dyes These dye are believed to adsorb perpendicular to the substrate and might form clusters of parallel molecules.

Zn-based dyes This dye is believed to adsorb flat on the substrate.

Sample preparation Films stored in air, heated 150 o C for 30 min, then cooled to 80 o C Immersed in a 0.4 mM MeOH solution of dye for 1h Rinced with MeOH to remove physisorbed dye

Possible experiments STM: adsorption geometry of the dyes on TiO 2 or ZnO surfaces STS? XPS, UPS and IPS: electronic structure of the adsorbed dyes Energy levels alignment Effect of polymerization on the electronic structure?

Useful numbers [ 1][2] [ 1][2]