Photophysical process

Jablonski diagram

Photophysical processes include the absorption and emission of light as well as the subsequent non-radiative transformations of the respective excited states (into one another). The photophysical processes find changes in the electronic states instead, not , however, changes in the chemical structure of light absorption; the latter belong instead to the photochemical processes .

Photo Physical processes can be divided into mono -molecular and bimolecular processes. The Jablonski diagram gives a clear representation of the relationships for monomolecular processes . Bimolecular processes play an important role in quenching and energy transfer processes.

A further distinction can be made between radiating and non-radiating processes.

The description given here focuses on molecular photophysical processes, but the basic principles also apply to processes on individual atoms or in solids .

Monomolecular processes

absorption

When a photon is absorbed by a molecule , an electron is lifted to a higher orbital . The excitation usually takes place from the basic oscillation state of the electronic basic state ( Boltzmann distribution : oscillation states above the basic oscillation state are generally not or only slightly occupied at room temperature ). The excitation takes place in different oscillation states of the excited state.

The electronic excitation is subject to various quantum mechanical selection rules . The probability that the interaction of a photon with the molecule leads to an electronic excitation is proportional to M ² , the square of the transition dipole  moment M. This in turn can be separated:

${\displaystyle M=\int _{Vib}\Psi _{v+}^{*}\Psi _{v}\,d^{}r\int _{Orbital}\Psi _{e+}^{*}\mu \Psi _{e}\,d^{}r\int _{Spin}\Psi _{s+}^{*}\Psi _{s}\,d^{}r}$

in

• the Franck-Condon factor , d. H. the overlap integral ${\displaystyle \int _{Vib}\Psi _{v+}^{*}\Psi _{v}\,d^{}r}$
  
  • the vibrational wave function of the excited state and ${\displaystyle \Psi _{v+}^{*}}$
  • the vibrational wave function of the ground state ${\displaystyle \Psi _{v}}$
• the contribution of the orbital overlaps ${\displaystyle \int _{Orbital}\Psi _{e+}^{*}\mu \Psi _{e}\,d^{}r}$
• the overlap integral of the spin functions. ${\displaystyle \int _{Spin}\Psi _{s+}^{*}\Psi _{s}\,d^{}r}$

The spin prohibition has the greatest influence: the spin functions of singlet and triplet states are orthogonal , the associated overlap integral disappears. The spin prohibition can be relaxed by the effect of the spin-orbit coupling - however, this is proportional to the fourth power of the atomic number and is therefore only very weak in organic molecules. Typical molar extinction coefficients for S ₀  → T ₁ transitions are therefore in the range from 0.000 01 to 1 M ⁻¹  cm ⁻¹ , and the corresponding transitions are only observed under special experimental conditions. The "orbital ban" differentiates z. B. between nπ ^* and ππ ^* transitions, the former being comparatively weak due to the small overlap (1 to 1000 M ⁻¹  cm ⁻¹ ), the latter leading to very intense bands due to the good overlap (1000 to 100,000 M ^{- 1}  cm ⁻¹ ).

Radiant processes

fluorescence

In fluorescence , the change from an electronic excited state to an energetically lower electronic state of the same multiplicity (S _{n + 1}  → S _n with n ≥ 0; T _{n + 1}  → T _n with n ≥ 1) takes place with the emission of a photon. Since the multiplicity is retained, the transition is permitted and therefore fast: fluorescence lifetimes  (τ _F ) are i. a. in the range 10 ⁻⁴ to 10 ⁻⁸  s (the lifetime is defined as 1 / k _F , where k _{F is} the unimolecular rate constant of the exponential decay of the excited states through fluorescence emission).

phosphorescence

In the case of phosphorescence , the change from an electronic excited state to an energetically lower state of different multiplicity ( usually T ₁ → S ₀ in the case of organic compounds ) takes place with the emission of a photon. Since the multiplicity changes here, the transition is forbidden and therefore slow: Phosphorescence lifetimes (τ _P ) are i. a. in the range 10 ⁻⁴ to 10 ²  s or even more (the lifetime is defined as 1 / k _P , where k _{P is} the unimolecular rate constant of the exponential decay of the excited states by phosphorus emission).

Due to the low transition probability , other deactivation processes can often successfully compete with phosphorescence. Phosphorescence of organic molecules is therefore usually not observed in solution , since deletion occurs through interaction of the excited molecule with the environment (collision processes, triplet-triplet annihilation in the course of molecule diffusion, see below bimolecular processes). These competitive processes can be suppressed at low temperatures ("freezing" at 77  K ), so in this case the phosphorescence can be observed.

Radiationless processes

Vibronic relaxation

Vibronic relaxation (VR) describes the deactivation of vibrationally excited states and leads the molecules   to the vibrational ground state of the respective electronic state within 10 ⁻¹¹ to 10 ⁻⁹ . VR closes z. B. to the absorption, the internal conversion (internal conversion) and the intersystem crossing (see below).

Internal conversion

Internal conversion  (IC) is the radiationless, isoenergetic change between states of equal multiplicity, whereby a vibration-stimulated level of the new state is occupied. The IC is followed by rapid vibronic relaxation. In addition to intersystem crossing (ISC), IC is a competitive reaction to fluorescence.

Intersystem crossing

Intersystem Crossing  (ISC) is the radiationless, isoenergetic change between states of different multiplicity, whereby a vibration-excited level of the new state is occupied. The ISC is followed by rapid vibronic relaxation. In addition to internal conversion, the ISC is a competitive reaction to fluorescence. Since the ISC is a prohibited process, the associated time constants are comparatively large. The ISC rates can be favored by spin-orbit coupling (heavy atom effect).

T is the ₁ state just below the S ₁ -state, so thermally induced ISC can in the direction T ₁  → S ₁ take place extending through a time- delayed fluorescence ( Delayed Fluorescence noticeable). The phenomenon can be identified by its temperature dependence .

Bimolecular processes

Radiant energy transfer

Radiant energy transfer is understood as the trivial process of emission of fluorescent or phosphorescent light by an excited molecule with subsequent re-absorption by a second molecule. The process is thus composed of two elementary photophysical processes, each of which is subject to the principles of the regularities discussed above. The overlap integral of the emission and absorption spectra determines the efficiency of the process.

Radiationless energy transfer

Electronic excitation energy can be transferred without radiation from a donor  (D) to an acceptor  (A) by the following processes :

• Coulomb or Förster mechanism : coupling the corresponding transition dipole moments of donor and acceptor; corresponds in the classical picture to the direct coupling of a transmitter with a receiver; Effect over comparatively large distances up to 100  Å (depending on the molecular distance r to r ⁻⁶ )
• Exchange or Dexter mechanism : energy transfer through quantum mechanical exchange interaction ; corresponds to the mutual exchange of electrons in a collision process ; ^Assumes short distances of 5 - 10 Å (depending on the distance between the molecules according to e ^{−2r / L} , L being the sum of the van der Waals radii of the molecules involved).

Both Förster and Dexter transfers are subject to the spin conservation rules , which allows the following processes:

¹ D * + ¹ A → ¹ D + ¹ A *

and

³ D * + ¹ A → ¹ D + ³ A *

The first process is expressed in fluorescence quenching ( quenching ) with respect to ¹ D *, the second process is of importance in sensitized photoreactions (see FIG. Photochemistry , photosensitizer ).

Triplet-triplet annihilation

If two triplet successive -angeregte molecules, it may be excited transmission along the following lines:

³ D * + ³ A * → ¹ D + ¹ A *

The excited acceptor molecule can deactivate with delayed fluorescence .

An example of triplet-triplet annihilation that is important in photochemistry is the formation of singlet oxygen :

³ Sens * + ³ O ₂ → ¹ Sens + ¹ O ₂

with Sens = sensitizer.

literature

• M. Klessinger, J. Michl: Light absorption and photochemistry of organic molecules. VCH Verlagsgesellschaft, Weinheim, New York 1989, ISBN 3-527-26085-4 .
• G. von Bünau, T. Wolff: Photochemistry: Basics, Methods, Applications. VCH Verlagsgesellschaft, Weinheim, New York 1987, ISBN 3527265066 .
• DC Harris, MD Bertolucci: Symmetry an Spectroscopy. An Introduction to Vibrational and Electronic Spectroscopy. Dover Publications, New York 1989, ISBN 0-486-66144-X .
• NJ Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, ISBN 0-935702-71-7 .
• M. Hesse, H. Meier, B. Zeeh: Spectroscopic methods in organic chemistry. Thieme, Stuttgart, New York 1984, ISBN 3-13-576102-9 .

Individual evidence

1. ↑ Entry on photophysical processes . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.P04647 Version: 2.3.3.
2. ↑ Derivation cf. DC Harris, MD Bertolucci: Symmetry an Spectroscopy. An Introduction to Vibrational and Electronic Spectroscopy. Chapters 3.4 and 5.4, Dover Publications, New York 1989, ISBN 0-486-66144-X .
   
3. ↑ ^{a b} M. Hesse, H. Meier, B. Zeeh: Spectroscopic methods in organic chemistry. Thieme, Stuttgart, New York 1984, Chapter 3 - Chromophores, ISBN 3-13-576102-9 .