Photolysis

Photolysis meaning is given as, it is a chemical reaction in which molecules are broken down into smaller units by light absorption. The experimental method known as flash photolysis, which is used in the study of short-lived chemical intermediates produced in many photochemical reactions, is one of the most well-known examples of a photolytic process.

About Photolysis

This technique was developed by the English chemists in 1949 named R.G.W. Norrish and George Porter that mainly consists of subjecting either a liquid or gas phase photolysis to an intense burst of light, resulting in lasting some microseconds or milliseconds, followed by a second, which is an ordinarily less intense flash. The first flash dissociates the absorbing compound into the short-lived molecular fragments, whereas the second flash gives a means for their identification by the spectrophotometry technique. This method is a valuable tool for transient chemical intermediates identification and thus for the study of fast chemical reaction mechanisms.

Photolysis in Photosynthesis

Photolysis is defined as either a part of the light-dependent reaction or photochemical phase or light phase or Hill reaction of photosynthesis. Photosynthetic photolysis general reaction can be given as follows:

 H₂A + 2 Photons(Light) → 2e^- + 2H⁺ + A

The chemical nature of "A" is based on the organism type. In the case of purple sulfur bacteria, hydrogen sulfide (H₂S) oxidizes to sulfur (S). And, in oxygenic photosynthesis, the water (H₂O) molecule serves as a photolysis substrate resulting in the diatomic oxygen (O₂) generation. This is the process that returns oxygen to the atmosphere of the Earth. Photolysis of water takes place in the chloroplasts of green algae and plants and thylakoids of cyanobacteria.

Energy Transfer Models

The semi-classical, conventional model defines the photosynthetic energy transfer process as one, where the excitation energy hops from the light-capturing pigment molecules to the reaction center molecules stepwise down the molecular energy ladder.

The effectiveness of the photons of various wavelengths depends upon the absorption spectra of the photosynthetic pigments present in the organism. Also, chlorophylls absorb the light in the red and violet-blue parts of the spectrum, while the accessory pigments capture other wavelengths too. The red algae's phycobilins absorb blue-green light, which penetrates deeper into water than red light, allowing them to photosynthesize in deep waters.

Every absorbed photon causes the exciton formation (which is an electron, excited to a higher energy state) in the pigment molecule. The exciton energy can be transferred to a chlorophyll molecule (which is P680, where P is the pigment and 680 is the absorption, at a maximum range of 680 nm) in the photosystem's reaction center II via resonance energy transfer. Also, the P680 can directly absorb a photon at a suitable wavelength.

Photolysis during photosynthesis takes place in a series of light-driven oxidation events. The energized electron (which is called exciton) of P680 can be captured by a major electron acceptor of the photosynthetic electron transfer chain and exits photosystem II. In order to repeat this reaction, the electrons present in the reaction center need to be replenished. This happens by the oxidation of water in oxygenic photosynthesis cases. The electron-deficient reaction center of the photosystem II (which is the P680*) is given as the strongest biological oxidizing agent yet discovered that allows it to break besides molecules as stable as water.

Quantum Models

A quantum model was proposed in 2007 by Graham Fleming with his co-workers that include the possibility where the photosynthetic energy transfer might involve quantum oscillations, explaining its unusual high efficiency.

According to Fleming, there is a piece of direct evidence that the long-lived wavelike electronic quantum coherence remarkably plays a considerable part in the processes of energy transfer during the photosynthesis that explains the energy transfer's extreme efficiency because it allows the system to sample all of the potential energy pathways with minimal loss and choose the most efficient one. However, this claim has since been proven to be wrong in many publications.

Further, this specific approach has been investigated by Gregory Scholes with his team at the University of Toronto, where in early 2010 published research results that indicate a few marine algae make the most of quantum-coherent Electronic Energy Transfer - EET to enhance their energy harnessing efficiency.

Photoinduced Proton Transfer

Photoacids are the molecules that, upon the light absorption, undergo a proton transfer to form a photobase.

In these particular reactions, dissociation takes place in the electronically excited state. After the proton transfer and the relaxation to the electronic ground state, acid and proton again recombine to form the photoacid.

In ultrafast laser spectroscopy experiments, photoacids are a handy source for causing pH jumps.

Photolysis in the Atmosphere

Photolysis takes in the atmosphere as part of a reaction series, where the primary pollutants such as nitrogen oxides and hydrocarbons react to form secondary pollutants like peroxyacetyl nitrates.

The two most essential photodissociation reactions in the troposphere are firstly given as follows:

 O₃ + hv → O₂ + O(¹D)  λ < 320nm

which generates the excited oxygen atom that can react with a water molecule to give the hydroxyl radical, which is represented as follows:

O(¹D) + H₂O → 2 *OH