1. Photoelectrochemistry (ch. 18)
Introduction of Luminescence
Electrogenerated Chemiluminescence
Photochemistry at Semiconductors

2. Radiation energy  electrical or chemical energy
e.g., ECL, electrochromic device, EL, sensors
1. General Concepts of luminescence
 the type of excitation
- Photoluminescence: light emission by UV or visible light
- Radioluminescence (scintillation): excited by radioactive substances
- Cathodoluminescence: excited by high velocity electron bombardment
- X-ray luminescence: by X-rays
- Chemiluminescence: by chemical reactions
Electrochemiluminescence or electrogenerated chemiluminescence: by electrochemical reactions
- Electroluminescence: by electric voltage
 Luminescent materials (or luminophors): substances which exhibit luminescence
- organic (organoluminophors)
- inorganic (phosphors)

3. 2. Organoluminophors
cf. B. M. Krasovitskii, B.M. Bolotin, Organic Luminescent Materials, VCH (1988).
 Electronic spectra
- by energy transitions between unexcited (ground) and excited states of molecules
 absorption () vs. emission (luminescence, ) spectrum
- sublevels (vibrational & rotational), 0-0 band, Stoke’s law (by nonradiative losses)
- deviation from mirror symmetry of absorption & luminescence; intra- & intermolecular processes, e.g., changes in the structure of molecules in the excited state

5. - luminescence intensity:
 quantum yield or quantum efficiency: ratio between the emitted and absorbed quanta
(occurrence of nonradiative processes lower the quantum yield)
time interval during which they emit light in the excited state; duration of light emission after excitation has stopped  fluorescence ( s) or phosphorescence ( s)
 excited states of molecules
- singlet state (S*); antiparallel spins, multiplicity, 2S + 1 = 1
- triplet state (T); multiplicity = 3

8. - sensitization & inhibition of fluorescence
 applications of organic luminescent materials
fluorescent pigments & paints. dye for plastics & fibers, optical brightening agents, organic scintillators, lasers, electrochemiluminescent or chemiluminescent compositions, analytical chemistry, biology & medicine

9. 3. Inorganic phosphors
 phosphor: a solid which converts certain types of energy into electromagnetic radiation over and above thermal radiation
 luminescence

10. e.g., Al2O3:Cr3+ (ruby, red), Y2O3:Eu3+, host lattice + luminescent center (activator)
- host lattice: hold luminescent ion tightly
- efficient luminescent materials: need to suppress nonradiative process
- if exciting radiation is not absorbed by the activator  add another ion to transfer the excitation radiation to the activator “sensitizer”
e.g., Ca5(PO4)3F:Sb3+, Mn2+

11. - luminescent molecules
e.g., bipyridine + Eu3+
i) the bipyridine cage protects Eu3+ ion against aqueous surroundings which try to quench luminescence
ii) excitation radiation  bipyridine molecule absorb & transfer it to Eu3+ ion  red luminescence
 How does a luminescent material absorb its excitation energy?
- quantum mechanics: coordination diagram, energy level diagrams of ions

12.  emission

13. e.g.,

14.  nonradiative transitions; efficiency?
 energy transfer
 applications
lamps, cathode ray, X-ray phosphor, probe, immunoassay, electroluminescence, laser
4. Electroluminescence
 luminescent material can be excited by application of an electric voltage
 applied voltage
- low field EL: light emitting diodes (LED, energy is injected into a p-n junction, a few volts), laser diodes (semiconductor lasers); normally DC
- high field EL (> 106 Vcm-1): display, thin film EL, ZnS EL; normally AC (ACEL)

15.  low field EL: LED & semiconductor lasers
- LED  band to band transition