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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)
N
NH N
HN
Zn2++
N
N N
N
Zn + 2H+
•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
S0
Soret (or B band) at 400 nm
S2 transition
S0 S1 transition
Weaker Q band at 550 nm
•S1 and S2 first and second excited states of the molecule.
•Fast internal conversion S2 → S1
•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 1Eu 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
S0
S1
S2
N
NH N
HN
2 ZnCl+
N
N N
N
Zn + 2 HCl
Zn-Porphyrin
Easy inclusion of the Zn2+ 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 PositionJonathan Rochford, Dorothy Chu, Anders Hagfeldt, and Elena GaloppiniJACS 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. GaloppiniUnpublished ?
Longer linker, worst efficiency.
ZnO nanorods
Faster charge diffusion into the single crystal nanorods.
Looks good…What can we do…?
Zn-Porphyrin
N
N N
N
Zn
ZnPZinc Porphyrin
ZnTPPZinc TetraPhenylPorphyrin
N
N N
N
Zn
ZnP and ZnTPP can be sublimated in UHV.
N
N N
N
Zn
O
HO
O
OH
N
N N
N
Zn
O
HO
O
OH
Zn-based dyes
These dye are believed to adsorb perpendicular to the substrate and might form clusters of parallel molecules.
Zn-based dyes
N
N N
N
Zn
O
O
O
O
O
O
O
O
H
N+
Et EtEt
H
N+
Et EtEt
H
N+
Et EtEt
H
N+
Et EtEt
This dye is believed to adsorb flat on the substrate.
Sample preparation
•Films stored in air, heated 150oC for 30 min, then cooled to 80oC
•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 TiO2 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]