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Luminescence can be defined as the emission of light by a substance when its electrons return from an excited, high-energy state to a lower-energy state, releasing the energy difference as photons (i.e. particles of light). This process can occur when molecules absorb energy from external sources, enabling electrons to move from their original energy level, known as the ground state, to a higher-energy excited state.
A practical way to visualize and understand these processes is through the Jablonski diagram. Invented in 1935 by the polish physicist Alexander Jabłoński this diagram graphically illustrates various energetic states (singlet states S₀, S₁, S₂..., and triplet states T₁, T₂...) and the transitions electrons undergo between these states.
The key processes shown in a Jablonski diagram include:
The key processes shown in a Jablonski diagram include:
Absorption (A): Electrons absorb external energy (usually photons) and move from the ground state (S₀) to an excited singlet state (S₁ or higher states like S₂ or S₃).
Fluorescence (F): Electrons quickly relax from higher-energy singlet states back to the lowest excited singlet state (S₁) through internal conversion (IC) and vibrational relaxation (VR). From this state, electrons emit photons as they return to the ground state (S₀). Fluorescence typically occurs very rapidly, on the order of nanoseconds.
Phosphorescence: A slower process involving intersystem crossing (ISC), where electrons transition from a singlet state (S₁) to a triplet state (T₁). Emission of photons occurs when electrons return from T₁ to the ground state (S₀). Due to spin restrictions, phosphorescence is slower, often taking milliseconds or even seconds.
Other significant processes represented are:
Internal Conversion (IC): Non-radiative transition between electronic states of the same spin multiplicity.
Intersystem Crossing (ISC): Non-radiative transition between electronic states of different spin multiplicities (e.g. from singlet to triplet).
Vibrational Relaxation (VR): Rapid non-radiative energy dissipation through molecular vibrations, typically causing an increase in thermal energy.
Luminescence can be classified according to how the electrons are excited:
Luminescence can be classified according to how the electrons are excited:
Photoluminescence (e.g. Fluorescence, Phosphorescence): Energy absorption through photons i.e. light.
Chemiluminescence/Bioluminescence: Energy provided through chemical reactions.
Electroluminescence: Energy supplied electrically.
Triboluminescence: Mechanical excitation.
Thermoluminescence: Thermal excitation.
Sonoluminescence: Excitation by ultrasound
Radioluminescence: Excitation by radioactive processes
Crystalloluminescence: Energy provided by the crystallisation of a compound
Regardless of how electrons are initially excited, photons typically emit from the lowest excited energy state (S₁ for fluorescence or T₁ for phosphorescence). This observation is known as Kasha’s rule. Furthermore, the emitted photons have lower energy (longer wavelengths) than the absorbed energy; the relative energy difference between the absorbance and emission maximum is called the Stokes shift.
For those interested in deeper insights, examining the Frank-Condon principle and Marcus theory can further enhance understanding of the basic underlying photophysical phenomena. Additionally, modern quantum chemical methods can be employed to investigate these processes in detail, both statically and dynamically. Contemporary spectroscopy techniques not only allow the measurement of luminescence spectra but also enable the exploration of the kinetics of these processes down to the nanosecond range and even shorter time scales.
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