excited state

Examples of excited state in the following topics:

• Mechanistic Background

  • Overall bonding in an excited state is usually lower than in the ground state.
  • Thus, the X–X bond length is increased in the excited state.
  • The excited state may return to the ground state by emitting a photon (light blue line).
  • It is termed phosphorescence if it occurs slowly by way of other excited states.
  • Alternatively, an excited state may return to the ground state by emitting a photon (radiative decay).

• Alkene Isomerization

  • A photochemical reaction occurs when internal conversion and relaxation of an excited state leads to a ground state isomer of the initial substrate molecule, or when an excited state undergoes an intermolecular addition to another reactant molecule in the ground state.
  • This bonding is absent in the π → π* excited state (magenta curve in the diagram).
  • Molecules occupying this new excited state then relax to either DHP or cis-stilbene ground states.
  • The stilbene reactions described above have been attributed to singlet excited states.
  • In order to study the behavior of triplet excited states it is often necessary to generate them by energy transfer from a higher triplet excited state of a suitable sensitizer molecule.

• Glow of Space Shuttles

  • The glow observed as a space shuttle re-enters the atmosphere is due to excited NO2 releasing light to return to its ground state.
  • where NO2* represents the excited state of electrons in NO2.
  • It is the relaxation of these electrons from the excited state back to the ground state that produces the glow that is visible around the space shuttle (see the concept about the emission spectra for more information).
  • When atomic oxygen from the high atmosphere combines with nitric oxide on the surface of the space shuttle, the resulting excited nitrogen dioxide returns to the ground state emitting an apparent glow.
  • Recall that excited-state nitrogen dioxide is responsible for the glow observed as space shuttles re-enter Earth's atmosphere.

• Conjugated π-Orbital Functions

  • The resultant singlet excited states undergo a variety of reactions, as shown in the following diagram for 1,3,5-hexatriene and two 2,5-dialkyl derivatives.
  • Taken together with the very short lifetimes of these excited states (≤10 nsec), this suggests that photochemical products should reflect the rotamer composition of the ground state.
  • Put another way, excited state rotamers are not expected to equilibrate prior to reaction, the NEER principle (Non-Equilibration of Excited Rotamers).
  • ACP may be formed from either the tEt or cZt excited states, but VCB and BHE require the latter rotamer.
  • Here, the rapid radiationless deactivation of the excited state by OBF is impeded and a normally non-fluorescent compound becomes fluorescent.

• Aurora Borealis and the Aurora Australis

  • Auroras result from emissions of photons in the Earth's upper atmosphere (above 80 km, or 50 mi), from ionized nitrogen atoms regaining an electron, and from oxygen and nitrogen atoms returning from an excited state to ground state.
  • The excited particles' energy is lost by the emitting photon or colliding with another atom or molecule.
  • This energy serves to move the electrons in nitrogen and oxygen from their ground state up to an excited state, where they can then decay back to the ground state by emitting photons of visible light (see the concept on emission spectra for more information).
  • Collisions with other atoms or molecules can absorb the excitation energy and prevent emission.
  • Nitrogen emissions are blue if the atom regains an electron after it has been ionized and red if the atom returns to ground state from an excited state.

• Cyclohexadienone Reactions

  • These photochemical rearrangements occur by way of triplet excited states, which are conveniently depicted as diradicals.
  • Charge separation in these states may then lead to rearrangement to a stable product.
  • These transformations are often photo-sensitized, indicating they proceed by way of triplet excited states.
  • As in the previous dienones, a triplet excited state undergoes decay to polar singlets that are thought to decompose in the manner depicted in the gray-shaded area.
  • To rationalize these reactions, polar intermediates formed by demotion of triplet excited states have been invoked, as noted previously for 4,4-diphenyl-2,5-cyclohexadienone.

• Carbonyl Compounds

  • The n → π* excited states of carbonyl compounds display a rich chemistry in their own right.
  • General equations for these primary reactions, starting from aryl ketone excited states, are described in the first diagram below.
  • The n → π* excited state shown in brackets at the center of the diagram may also be described as the resonance hybrid drawn below.
  • Note that the ground state polarity of the carbonyl group has been reversed in this excited state.
  • Since the ground state triplet of oxygen reacts rapidly with triplet excited states, air must be excluded when studying these reactions.

• Photochemistry

  • The first law of photochemistry, the Grotthuss-Draper law, states that light must be absorbed by a compound in order for a photochemical reaction to take place.
  • The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
  • Thus, we may define quantum yield as "the number of moles of a stated reactant disappearing, or the number of moles of a stated product produced, per einstein of monochromatic light absorbed
  • Here the asterisk represents an electronic excited state, the nature of which will be defined later.
  • The biacetyl product, formed in the third reaction, may itself be excited by light or accept excitation energy from an excited acetone molecule, further complicating this process.

• Emission Spectrum of the Hydrogen Atom

  • The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by an atom's electrons when they are returned to a lower energy state.
  • You need to understand convergence, production of UV, vis, IR, excitation, concentric energy levels and be able to draw the line spectra.
  • Some of the most common and readily observable series have been named as shown in this image, where n1 is the ground state and n2 are excited states.

• Coloring Agents

  • In a d–d transition, an electron in a d orbital on the metal is excited by a photon to another d orbital of higher energy.
  • Conceptually, one can imagine the oxidation state of the metal increasing by one (losing on electron), while the oxidation state of the ligand decreases by one (becomes anionic).
  • The converse will also occur: excitation of an electron in a ligand-based orbital into an empty metal-based orbital (Ligand to Metal Charge Transfer or LMCT).