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Photoreceptor Excitation

Introduction

Upon excitation from a laser or other light energy source, photoreceptor molecules transition from a lower energy state to a higher energy state. During this process, electrons of photoreceptor absorbs the energy, and turn into excited state therefore change photoreceptors form. Now, let's see the basic concept of excitation of electrons.

Excitation of Electron

In order to convert between stereoisomers, photons are required to excite an electron to a higher energy state. The lifetime of an excited electron ranges from a few femtoseconds to several hours. When the electron relaxes to a lower energy state, a photon is emitted equal in energy to the difference between the two states.An example of an energy diagram is shown below.

In the retina, electronic excitation converts trans-retinal to cis-retinal by breaking the π Bond of an alkene and rotating the molecule about its σ bond to change the relationship of neighboring groups from cis to trans.

Molecular Orbitals and Light-induced bond rotation

Ethene is a simple molecule that contains only one double bond, and serves as a convenient model to explain electronic excitations and subsequent changes in molecular geometry.

 

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Yellow lobes indicate σ bonding regions and purple lobes represent π bonding regions. In the molecule's lowest-energy conformation, each carbon's unhybridized p orbitals are coplanar, allowing ethene to form a stabilized π bond;  the molecule is subsequently "locked" in htis conformation. When one of the electrons in the π bonding orbital is excited to a higher energy orbital, the  bond is destabilized, allowing rotation about the carbon-carbon axis. The two figures below show this rotation from the π-stabilized conformation to the rotated, destabilized rotation.[4]

 

Molecular orbital diagrams for ethene are given in the figure below: the ground state configuration is on the left, and an excited state configuration on the right.  In the ground state, the highest occupied molecular orbital (HOMO) is the carbon-carbon π bonding orbital; the lowest unoccupied molecular orbital (LUMO) is the carbon-carbon π antibonding orbital. Upon exposure to light, an electron is excited from the fully-occupied HOMO to the LUMO as shown.

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Photoreceptor Photocycle

Upon excitation photoreceptors under go several changes in conformation, forming photocycles. Photoreceptors can be stimulated by unique wavelengths of light. For example, photoactive yellow protein (PYP) is a UV-blue light photoreceptor: ultraviolet and blue light can initiate photocycle formation in PYP.

References

  1. Is the photoactive yellow protein a UV-B/blue light photoreceptor?  Elizabeth C. Carroll, Marijke Hospes, Carmen Valladares, Klaas J. Hellingwerfb and Delmar S. Larsen,    Photochem. Photobiol. Sci., 2011, 10, 464–468
  2. Fayer, M. D. Elements of Quantum Mechanics; Oxford University Press: New York, 2001; pp 158-162.
  3. Chemical Bonding And Molecular Structure.Textbooks. NCERT, n.d. Web.
  4. W. Locke and the ICSTM Department of Chemistry 1996-97.