Photoelectrochemistry (ch. 18) Introduction of Luminescence Electrogenerated Chemiluminescence

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