1. Chapter 3. Light emitting diod

2. Electron configuration

3. Covalent bond

4. Organic semiconductors:
extensively conjugated molecular systems
such as fully conjugated polymers or cata-condensed polyacenes (PAH, e.g., anthracene)

6. p-electron clound in anthracene
Electron density sections through central molecular plane, with each contour line representing an increase of 500 electrons/nm3

7. Molecular orbitals

8. types of electronic transitions in polyatomic molecules

11. The total spin quantum number S = Σ si
The multiplicities M = 2S + 1

12. Spin of electrons

13. Statistical states
Ms:Magnetic quantum number

14. Energetics of spin states
Externally applied H ~ 1 to 10 kG
Energetics of spin states
Zero-field splitting EZF ~ 0.1 cm1 ~ 1 kG
Spin interaction
Coulomb repulsion

15. Electronic excitation and de-excitation in organic molecules

16. Jabloński Diagram

19. probability of transition. The Beer-Lambert Law. Oscillator strength

20. The molecules whose absorption transition moments are parallel to the
electric vector of a linearly polarized incident light are preferentially excited.
The probability of excitation is proportional to the square of the scalar product of the transition moment and the electric vector. This probability is thus maximum when the two vectors are parallel and zero when they are perpendicular.

21. Frank-Condon principle
Electronic transition ~ s
Vibration ~ – s
Vertical transition
Bathochromic: red-shift
Hyposochromic: blue-shift

22. Effects of domain overlap & corresponding wave amplitudes
Mirror image rule

24. Mirror image rule

25. Rigorous relatioship between absorption & fluorescence
Radiative & non-radiative processes
Kasha’s rule

26. Internal conversion Fluorescence
Internal conversion is a non-radiative transition between two electronic states of the same spin multiplicity.
From S1, internal conversion to S0 is possible but is less efficient than conversion from S2 to S1, because of the much larger energy gap between S1 and S0.
Fluorescence
Emission of photons accompanying the S1  S0 relaxation is called fluorescence.
In most cases, the absorption spectrum partly overlaps the fluorescence spectrum. At r. t., a small fraction of molecules is in a vibrational level higher than 0 in the ground state as well as in the excited state. At low temp, this departure from the Stokes Law should disappear.
It should be noted that emission of a photon is as fast as absorption of a photon (~1015 s). However, excited molecules stay in the S1 state for a certain time (a few tens of picoseconds to a few hundreds of nanoseconds, depending on the type of molecule and the medium) before emitting a photon or undergoing other de-excitation processes (internal conversion, intersystem crossing).

27. Experimental schematic for detection of the Stokes’ shift
The energy of the emission is typically less than that of absorption. Hence, fluorescence typically occurs at lower energies or longer wavelengths.

28. After the 1850s, Stokes became involved in administrative matters, and his scientific productivity decreased.
Some things never change.

29. Stokes shift

30. Intersystem crossing and subsequent processes
is a non-radiative transition between two isoenergetic vibrational levels belonging to electronic states of different multiplicities.
Delayed fluorescence
T1S1 can occur when the energy difference between
S1 & T1 is small and when the lifetime of T1 is long enough.
Triplet-triplet annihilation
In concentrated solutions, a collision between two molecules
in the T1 state can provide enough energy to allow one of
them to return to the S1 state leads to a delayed
fluorescence emission.
Triplet-triplet transitions
Once a molecule is excited and reaches triplet state T1,
it can absorb a different wavelength because triplet-triplet
transitions are spin allowed.

31. Energy transfer

33. Distinction between radiative and non-radiative transfer

34. hopping

35. Excitation energy transfer
heterotransfer
homotransfer

38. Radiative energy transfer

39. (rate constant)
(lifetime of excited state)

40. Non-radiative energy transfer
EET: excitation energy transfer or electronic energy transfer
RET: resonance energy transfer
FRET: fluorescence resonance energy transfer

42. Energy transfer can result from different interaction mechanisms
Energy transfer can result from different interaction mechanisms. The interactions may be Coulombic and/or due to intermolecular orbital overlap.
* * The Coulombic interactions consist of long-range dipole-dipole interactions (Forster’s mechanism) and short-range multi-polar interactions.
* * The interactions due to intermolecular orbital overlap, which include electron exchange (Dexter’s mechanism) and charge resonance interactions, are of course only short range.
It should be noted that for singlet-singlet energy transfer, all types of interactions are involved, whereas triplet-triplet energy transfer is due only to orbital overlap.

44. CI: Coulombic interaction
EE: electron exchange
The total interaction energy can be expressed as a sum of two terms: a Columbic term Uc and an exchange term Uex. The Coulombic term corresponds to the energy transfer process in which the initially excited electron on the donor D returns to the ground state orbital on D, while simultaneously an electron on the acceptor A is promoted to the excited state. The exchange term corresponds to an energy transfer process associated with an exchange of two electrons between D and A.

45. Formation of excimers and exciplexes

46. Excimers
An excimer is located at wavelengths higher than that of the monomer and does not show vibronic bands.

48. P3HT PL heating-1

50. Exciplexes

52. Lifetimes and quantum yields
Excited-state lifetimes
The rate constants for the various processes will be denoted as follows:

54. A very short pulse of light at time 0 brings a certain number of molecules A to the S1 excited state by absorption of photons. These excited molecules then return to So.
The rate of disappearance of excited molecules is expressed by the following differential equation:

55. iF(t), the d-pulse response of the system, decrease according to a single expontial.

56. Quantum yields

57. τr = 1/krs
When the fluorescence quantum yield and the excited-state lifetime of a fluorophore are measured under the same conditions, the non-radiative and radivative rate can be easily calculated by means of the following relations:

59. quenching
The sum kM of the rate constants for de-excitation processes is equal to the reciprocal excited-state lifetime t0:
Kq : the observed rate constant for the bimolecular process

60. The fluorescence characteristics (decay time and/or fluorescence quantum yield) of M* are affected by the presence of Q as a result of competition between the intrinsic de-excitation and these intermolecular processes.

62. Overview of the intermolecular de-excitation of excited molecules leading to fluorescence quenching
4.2.1 Phenomenological approach
(static quenching)

63. Dynamic quenching
Stern-Volmer kinetics (ignore the transient effects)

64. i(t) = i(0) exp (-t/t)

65. KSV : Stern-Volmer constant
I0/I vs. [Q] Stern-Volmer plot

66. Static quenching
The term static quenching implies either the existence of a sphere of effective quenching or the formation of a ground-state non-fluorescent complex (Figure 4.1)

67. Sphere of effective quenching

70. Formation of a ground-state non-fluorescent complex

71. I ～ [M]

72. Simultaneous dynamic and static quenching
non-fluorescent complex

73. static
Sphere of effective quenching