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

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)

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

- 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 (10-9-10-7 s) or phosphorescence (10-4-10-2 s)  excited states of molecules - singlet state (S*); antiparallel spins, multiplicity, 2S + 1 = 1 - triplet state (T); multiplicity = 3

- 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

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

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+

- 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

 emission

e.g.,

 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)

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