Intersystem crossing

Intersystem Crossing (ISC) is a term from photochemistry and spectroscopy . In German it is also called inter-combination . It describes the radiationless transition from one electronic excitation state to another excitation state with changed multiplicity (the electronic ground state is included here). Usually, a vibrational level of the new state is occupied. An example of ISC is the transition from the singlet to the triplet state (S₁ → T₁ ).

Similar transitions

Radiationless transitions that run without a change in multiplicity (e.g. S ₁ → S ₀ ), on the other hand, are referred to as internal conversion .

Corresponding radiating processes are

the phosphorescence (when the multiplicity changes)
the fluorescence (while maintaining the multiplicity).

Overview:

		Multiplicity constant	Multiplicity is changed (slowly, since "forbidden")
under energy delivery	with emission of radiation	Fluorescence e.g. B. ${\ displaystyle S_ {1} \ rightarrow S_ {0}}$	Phosphorescence e.g. B. ${\ displaystyle T_ {1} \ rightarrow S_ {0}}$
under energy delivery	radiationless	Oscillation or vibronic relaxation * z. B. and ${\ displaystyle S_ {0} ^ {} \ rightarrow S_ {0}}$ ${\ displaystyle T_ {1} ^ {} \ rightarrow T_ {1}}$
Energy constant	radiationless	Inner transformation z. B. ${\ displaystyle S_ {1} \ rightarrow S_ {0} ^ {*}}$	Intersystem Crossing e.g. B. and ${\ displaystyle S_ {1} \ rightarrow T_ {1} ^ {}}$ ${\ displaystyle T_ {1} \ rightarrow S_ {0} ^ {}}$

) * from the excited oscillation state S _x * of a certain electronic state to the respective basic oscillation state S _x

The dominant mechanism for the ISC in organic molecules is the interaction between the magnetic moment of the spin and that of the associated orbital ( spin-orbit coupling ). With diradicals , direct interaction between two spins is also possible ( spin-spin coupling ). The spin-orbit coupling depends to the fourth power on the atomic numbers of the atoms involved. The ISC is thus significantly accelerated in the presence of heavy elements.

Selection rules for electronic transitions only permit transitions between states of equal multiplicity. Direct suggestions, e.g. B. from a singlet ground state to a triplet state, do not take place or only to a small extent ( prohibited transition ); Triplet states are thus predominantly occupied by an ISC, which follows the excitation into the singlet state. The triplet state can either emit its excitation energy without radiation (thermal) by changing back to the singlet ground state under ISC and vibronic relaxation , or it can relax radiantly via phosphorescence; Compared to the “allowed” fluorescence, the “forbidden” phosphorescence takes place much more slowly.

Applications

The ISC is used in practice

with lasers ("state inversion")
in the phenomenon of upconversion (ISC followed by a further electronic excitation and subsequent light emission with a short-wave shift compared to the excitation wavelengths)
more recently with organic light-emitting diodes (OLEDs), in which phosphorescent iridium complexes ensure fast ISC and - unlike usual (see above) - short time constants for phosphorescence. Due to its short time constant, the phosphorescence outweighs the thermal deactivation . Due to the fast conversion and emission rates , the phosphorescent emitters can efficiently convert both singlet and triplet excitons (formed in the OLED in a ratio of 25:75) into light.
In contrast, typical fluorescent emitters are limited to conversion efficiencies of 25%, since their triplet states are predominantly thermally deactivated.

Quantum yields

The exposure of organic compounds almost exclusively leads to the occupation of excited singlet states (S ₀ → S ₁ ). If there is no photoreaction (cf. photochemistry ) or energy transfer to another molecule, the singlet state can return to the basic state in a radiating (fluorescence) or non-radiative (internal conversion) state or change to the T ₁ state through ISC .

The quantum yield Φ _ISC now indicates the proportion of the transition to the triplet state (0 ≤ Φ _ISC ≤ 1). The table below gives an overview of the quantum yields for organic compounds. The energy difference ΔE _ST between S ₁ and T ₁ indicates the minimum amount of oscillation excitation with which the T ₁ state is formed. ΔE _ST corresponds to twice the exchange energy ( quantum mechanics ), which in turn is proportional to the overlap integrals of the two singly occupied orbitals. This explains the clear differences in ΔE _ST

for carbonyl compounds (n, π * excitation, low orbital overlap) and
for simple aromatics (π, π * excitation, significant orbital overlap).

connection	E _S [kJ mol ⁻¹ ]	E _T [kJ mol ⁻¹ ]	Δ (E _S - E _T ) [kJ mol ⁻¹ ]	Φ _ISC	Φ _F	Configuration of S ₁ / T ₁
benzene	459	353	106	0.25 (0.7)	0.2	π, π *
toluene	445	346	99	0.53 (0.7)	0.2	π, π *
acetone	372	332	40	0.9 - 1.00	0.001	n, π *
Acetophenone	330	310	20th	1.00		n, π *
Benzaldehyde	323	301	22nd	1.00		n, π *
Triphenylamine	362	291	71	0.88
Benzophenone	316	287	29	1.00	0	n, π *
Fluorene	397	282	115	0.22		π, π *
Triphenylene	349	280	69	0.86	0.1	π, π *
Biphenyl	418	274	144	0.84		π, π *
Phenanthrene	346	260	86	0.73		π, π *
Styrene	415	258	157	0.40		π, π *
naphthalene	385	253	132	0.75	0.2	π, π *
2-acetylnaphthalene	325	249	76	0.84
Biacetyl	267	236	31	1.00	0.002	n, π *
Benzil	247	223	24	0.92		n, π *
Anthracene	318	178	140	0.71	0.4	π, π *
Eosin	209	177	32	0.33		π, π *
Bengal pink	213	164	49	0.61		π, π *
Methylene blue	180	138	42	0.52		π, π *

Delayed fluorescence (delayed fluorescence)

An interesting photophysical phenomenon based on ISC is delayed fluorescence. An ISC takes place from the T ₁ state to the energetically higher S ₁ state. The prerequisite is a comparatively small energy difference between the two states so that the necessary energy can be applied thermally. Benzophenone is a prime example of the phenomenon.

Delayed fluorescence can be demonstrated by the fact that light emission occurs with the wavelength of the fluorescent light, but the emission shows the typical temperature dependence of a thermally activated process ( Boltzmann factor , Arrhenius equation ). The delayed fluorescence can thus “freeze out”, since at low temperatures the ISC for the singlet state can no longer compete with the phosphorescence or the radiationless deactivation of the triplet state.

Individual evidence

↑ J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Lee: "Upconversion Luminescent Materials: Advances and Applications". In: Chem. Rev. 2015, 115, pp. 395-465.
^ J. Mattay, A. Giesbeck (editor): Photochemical Key Steps in Organic Synthesis. VCH, Weinheim, New York, Basel, Cambridge, Tokyo 1994, ISBN 3-527-29214-4 .
↑ Quantum yields of fluorescence in solution quoted from NJ Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.7, ISBN 0-935702-71-7 .
↑ ^a ^b N. J. Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.9, ISBN 0-935702-71-7 .
^ NJ Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.13, ISBN 0-935702-71-7 .

[1] J. Zhou, Q. Liu, W. Feng, Y. Sun, F. Lee: "Upconversion Luminescent Materials: Advances and Applications". In: Chem. Rev. 2015, 115, pp. 395-465.

[Mattay-2] J. Mattay, A. Giesbeck (editor): Photochemical Key Steps in Organic Synthesis. VCH, Weinheim, New York, Basel, Cambridge, Tokyo 1994, ISBN 3-527-29214-4 .

[3] Quantum yields of fluorescence in solution quoted from NJ Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.7, ISBN 0-935702-71-7 .

[Turro-4] N. J. Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.9, ISBN 0-935702-71-7 .

[Turro2-5] NJ Turro: Modern Molecular Photochemistry. University Science Books, Sausalito 1991, Chapter 5.13, ISBN 0-935702-71-7 .