Many of today’s solar energy harvesting technologies suffer from drawbacks in production cost and low efficiencies due to their low less-than-perfect ability to absorb visible light. Furthermore, many interesting photochemical reactions require high-energy photons rather than lower-energy photons in the visible range. One such extremely important photocatalytic reaction is the splitting of water into hydrogen- and oxygen gas. Both of these possible applications would benefit greatly if visible light could be effectively converted to UV or near-UV light.
This type of photon up-conversion is achievable by utilizing a phenomenon called Triplet-Triplet-Annihilation (TTA). In short, this phenomenon is based on two molecules; an acceptor and an emitter, which merge two low-energy photons to one high-energy photon.
Our initial goal is to improve the known bi-molecularly annihilating system, where the sensitizer is a Pd-octaethylporphyrin and the annihilator is 9,10-Diphenylanthracene through the use of an oligomeric annihilator. This would allow for an intramolecular TTA-process, as opposed to the commonly used diffusion controlled intermolecular process, thus making the annihilation faster, less oxygen sensitive and more efficient. This is of high interest since earlier studies have shown that the greatest room for improving the photon up-conversion through TTA lies in optimizing the annihilating part of the process.
Figure: Left: Solar radiation spectrum at sea level with marked current photon up-conversion region. Right:
Photon up-conversion in the lab, induced with a green (532 nm) laser pointer. The laser beam gives rise to a photon up-conversion region in the liquid from where blue photons (ca. 430 nm) are emitted that hold higher energy than the incoming green photons.
|This process uses an acceptor (called Sensitizer)- and an emitter (called Annihilator) molecule. The sensitizer absorbs low-energy photons and transfers corresponding triplet-energy to the annihilator through the Triplet Energi Transfer-process (TET).
When sufficient ammount of annihilators have been sensitized with triplet-energy the collision probability of annihilators with triplet-energy starts to increase. When these annihilators collide two triplet-energies can be combined in such a way that the annihilation process results in one high-energy photon.
The shematic drawing to the right illustrates one sucessfull up-conversion event when two low-energy phtons (green arrows) are absorbed and one high-energy photon (blue arrow) is emitted.