Molecular Luminescence Spectroscopy Flashcards
Forms of Luminescence (8)
Radio
Electro
Thermo
Tribo
Sono
Photo
Chemi
Bio
Jablonski Diagram Components (6)
Vibrational Relaxation
Internal Conversion
Fluorescence
Phosphorescence
Intersystem Crossing
External Crossing
a molecule of analyte absorbs a photon and excites a species in a process called photo-excitation
photoluminescence
no excitation radiation source; molecules of analyte are excited via chemical reaction to give a species whose emission spectrum provides information for quali/quanti analyses
chemiluminescence
Nonradiative deactivation in Jablonski Diagram
Vibrational Relaxation
Internal Conversion
Intersystem Crossing
External Conversion
Radiative Deactivation in Jablonski Diagram
Fluorescence
Phosphorescence
collisions of excited state analyte molecules with other molecules lead to loss of excess vibrational energy
vibrational relaxation
molecule passes to a lower energy state, vibrational energy levels of two electronic states overlap and molecules pass from one electronic state to other
Internal conversion
Spin of electron is reversed leading to change from singlet to triplet state
Jntersystem Crossing
Interaction and energy transfer between the excited molecules and the solvent or other molecules
External Conversion
emission of a photon via a singlet to singlet transition
fluorescence
emission of a photon via a triplet to singlet transition
Phosphorescence
all electron spins are paired; no energy level splitting occurs when the molecule is exposed to EMF
singlet
unpaired and parallel electron spins; excited state is less energetic than the corresponding state
Triplet
no net magnetic field due to spin pairing; electrons are repelled by permanent magnetic fields
Diamagnetic
magnetic moment and attracted to a magnetic field
paramagnetic
Types of fluorescence (2)
Resonance Fluorescence
Non-Resonance Fluorescence
absorbed radiation is re-emitted without a change in frequency and wavelength
Resonance Fluorescence
molecular fluorescence bands are shifted to wavelengths that are longer than the resonance line
non-resonance fluorescence
Factors affecting luminescence (7)
Quantum Yield
Molecular Structure
Structural rigidity
Dissolved oxygen
temperature
solvent
pH
Quantum yield formula
total no. luminescing molecules/ total no. of excited molecules
occurs when a molecule changes from higher electronic stage to an upper vibrational level of a lower electronic state in which the vibrational energy is strong enough to rupture the bond
Predissociation
occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break
Dissociation
compounds that do not fluoresce have structures that ____
permit rapid relaxation by nonradiative processes
what increases fluorescence intensity? (3)
increasing benzene substituents,
decreasing molar mass of substituents, increasing substitution in oxygenated species
Flexibility increases, quantum yield?
decreases
____ in a molecule causes an enhanced internal conversion rate and a consequent increase in likelihood for nonradiative deactivation
lack of rigidity
Luminescence favored in molecules with ___ structures
rigid
Presence of dissolved oxygen, ___ fluorescence
reduces
Luminescence favored ____ temperatures
lower
___ solvent is preferred in luminescence
polar
luminescence preferred more ____ solvents
viscous
Fluorescence and Phosphorescence relationship
inverse
Fluorescence ____ if solvents contain heavy atom
decreases
two thoriated tungsten made electrodes are placed face to face with a small gap in an airtight transparent envelope of fused silica filled with xenon gas in very high pressure
xenon arc lamp
LASERS
Light Amplification by Stimulated Emission of Radiation
LIF
Laser Induced Fluorescence
determined by scanning the excitation monochromator in a range less than the preset wavelength of the emission monochromator
excitation spectrum
excitation monochromator is adjusted at the determined lambda ex and emission spectrum is scanned
emission spectrum
Usually the detector is set at ___ to the incident beam
90°
PMT
photomultiplier tube
CCD
charge-coupled devices
Techniques in Molecular Luminescence Spectroscopy (3)
Molecular fluorescence spectroscopy
Molecular phosphorescence spectroscopy
Chemiluminescence spectroscopy
Fluorometer: Hg Lam and Interference filter
Spectrofluorometer:
xenon lamp and diffraction grating monochromator
excited molecules lose their energies by collisions with other molecules of solvent
self-quenching
when an emission ban overlaps woth an excitation, emitted photons excite other molecules in the ground state which results in no net emission
self-absorption
sample tubes that are placed in liquid nitrogen held in a quartz Dewar flask
phosphoroscope
produced when a chemical reaction yields an electronically excited species, which emits light as it returns to its ground states
chemiluminescence
