Photophysics

Photophysics concerns processes induced by electromagnetic radiation that do not produce an irreversible change in the covalent structure of the system. It can be contrasted with photochemistry, during which changes in covalent structure occur (as in photo-induced polymerization or decomposition).

Photophysical processes include the light-induced generation of excited states, transitions between excited states, and relaxations that return the system to its initial ground state, which are often summarized in a Jablonski diagram, G Fig. 1.4.

Excitation

• Electronic absorption usually takes a molecule from a singlet electronic ground state (S₀) to its first excited singlet electronic state (S₁). Higher singlet states exist and may be accessible with different excitation energies.
• The initial S₀ state is usually in its ground vibrational state. But the S₁ state may be produced in a vibrational excited state. Therefore S₁ can be produced with a variety of closely spaced energies, with the spacing being determined by the vibrational properties of the electronic excited state.
• The reorganization of electrons during the excitation occurs so rapidly that it can be considered to happen in a framework of fixed nuclear coordinates (Franck-Condon Principle). A typical time is on the fs scale, contrasted with perhaps 100 fs for vibration.

Transitions within (or between) excited states

• The excess vibrational energy is rapidly (10 ps) dissipated by vibrational relaxation, producing S₁ in its vibrational ground state.
• In some excited systems, S₁ can re-organize (by a process called intersystem crossing) into a vibrationally excited, electronically excited triplet state, T₁. The excess vibrational energy is rapidly dissipated by vibrational relaxation.

Relaxation to the ground state

• These processes are divided into two types: radiative process (which occur with the emission of a photon) and nonradiative processes.
  • Examples of radiative processes
    • Emission of a photon as fluorescence, during the conversion of S₁ (which is not vibrationally excited, because fluorescence is much slower than vibational relaxation), to S₀ (which may be initially produced in a vibrational excited state). If vibrational fine structure is observable in the emission, its spacing will be determined by the vibrational properties of the ground state, S₀. A typical lifetime for fluorescence is 10 ns. The energies emitted as fluorescence are lower than those involved in the initial excitation.
    • Emission of a photon as phosphorescence, during the process T₁ -> S₀. This process is much slower than fluorescence because it involves two states of different multiplicity. Often it is so slow that it cannot be observed, because other processes deactivate T₁ before phosphorescence can occur. When it is observed, phosphorescence is red-shifted from fluorescence, because T₁ is of lower energy than S₁.
  • Examples of nonradiative processes
    • Internal conversion, during which S₁ dissipates its excitation energy through nonradiative interactions with the environment, such as collisions in a condensed phase.
    • Nonradiative singlet energy transfer, in which the initially excited molecule (A*) donates its excitation to another molecule (B), by a process which can be represented as A* + B -> A + B*.
• The nonradiative processes compete with the radiative ones. In many systems, the nonradiative processes dominate completely, and the sample does not exhibit emission. In other systems, the radiative processes may dominate completely, and nearly every photon that is absorbed is responsible for emission of another photon.

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