Question

Analytical Chem Tutorial: Molecular Luminescence Theory

Answer 1

\({\bf{Photoluminescence:}}\) the excitation and resulting emission caused by the absorption of a photon
- fluorescence: does not change the electron spin
- phosphorescence: does change the electron spin
- chemiluminescence: emission caused by the formation of an excited species in a chemical reaction

Advantages of luminescence: lower detection limits, larger range of concentration methods
Disadvantages: the excited states can easily be de-activated by molecular interactions
can possibly counter this by combining quantitative methods with separation techniques like chromatography and electrophoresis

Answer 2

Types of fluorescence:
- resonance radiation: absorbed radiation and emitted radiation have same frequency
- Stoke's shift: emitted radiation has longer frequency

Types of States:
- singlet states: all electrons are paired
- doublet states: contain unpaired electrons, ex: free radicals
- triplet state: contains paired electrons with parallel spin states



in the excited state, the singlet and triplet states maintain their spins. notice that a collapse back to the ground state from the triplet state would require a spin change in order to not violate the pauli exclusion principle. this decreases the probability of this transition happening, thus increasing the lifetime of this excited state.

Answer 3

\({\bf{Jablonski~Diagram:}}\)


S represents singlet states, T represents triplet states
> the blue arrows represent typical absorption for a compound, going from S0 to S1 or S2, etc.
> the smaller arrows are nonradiative transitions, or vibrational relaxations, that don't change the energy level
> the green arrows are transitions between singlet states, or fluorescence, while the red arrows are transitions between triplet and singlet states, or phosphorescence. I will go over intersystem crossing shortly.

Answer 4

\({\bf{Deactivation:}}\) the process by which a molecule can return to the ground state

- Internal conversion: a molecule transitions to the lower energy state without emitting radiation, between two states of the same multiplicity
- External conversion: a molecule transitions to the lower energy state by transferring energy to the solvent/solutes
- Intersystem crossing: conversion between energy states w/ diff. multiplicities
> increased if there is overlap between the vibrational levels of the two states
> increased by the presence of heavy atoms like iodine or bromine (heavy-atom effect) and paramagnetic species

Answer 5

Quantum Yield = (total luminescing molecules)/(total excited molecules)
represented by Ф =
\[\frac{ k_{f} }{ k_{f}+k_{i}+k_{ec}+k_{ic}+k_{pd}+k_{d} }\]
where kf is the fluorescence constant, ki is the intersystem crossing constant, kic and kec are the internal conversion and external conversion constants, kpd is the predissociation constant, and kd is the dissociation constant

Transition types:
typically pi --> pi*, low energy
compounds with many aromatic conjugated rings have higher molar absorptivities

quantum efficiency tends to decrease w/ increasing T (increases molecular collisions and deactivation)

F = K(P-P0) where P is the power of the beam at the end of the path, P0 is the incident power, K is a constant representing quantum efficiency

Answer 6

Source material is section 15A of Principals of Instrumental Analysis, 6th edition by Skoog, Douglas A., Holler, James F., Crouch, Stanley R.