Photoelectrochemistry (ch. 18)

Introduction of Luminescence Electrogenerated ChemiluminescencePhotochemistry at Semiconductors

Radiation energy electrical or chemical energye.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, 2S + 1 = 1- triplet state (T); multiplicity = 3

- sensitization & inhibition of fluorescence  applications of organic luminescent materialsfluorescent 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 luminescenceii) 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 applicationslamps, 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