Photophysics Flashcards
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Electronic levels
Rotational levels
Vibrational levels
Electronic > Vibrational > Rotational
What are the two excited states
Singlet : spin allowed florescence
Triplet: spin forbidden phosphorescence
KEY: Draw the energy level diagram
Absorption
Internal conversion
Florescence
Intersystem crossing
Phosphorescence
What are the two types of measurement for the excited state
Steady state: constant illumination to measure emission intensity
Time resolved: illuminate in short pulses
How to do steady state measuring
Do absorption spectra to find absorption wavelength
Do emission spectra at absorption wavelength to characterise the ground state
Do excitation spectra at emission maxima to characterise the the excited state
What’s the equation for time resolved measuring
Intensity (time) = intensity (0s) Exp (- time / lifetime of excited state)
What are the characteristics of the system
Stokes shift : internal conversion means absorption wavelength is lower wavelength than emission intensity
Emission only occurs from lowest vibrational S1 state
Emission spectra is mirror image of excitation state
Types of excited state deactivation
Radiative: emits photon
Non radiative: doesn’t emit photon
Equation for quantum yield
Kr / Kr + Knr
Equation for observed lifetime
1 / Kr + Knr
Equation for natural lifetime
1 / Knr or observed lifetime / quantum yield
Average lifetime of florescence state
10 nano seconds
Average lifetime of phosphorescence state
100 nanoseconds - 10 milliseconds
Equation for quantum yield and how it links to brightness
photons emitted / # photons absorbed
The brightness of emission is how many photos are emitted, poor molecular absorption coefficient means poor absorption means weak emission therefore less brightness
Factors effecting Knr
Quenching
Internal conversion
Intersystem crossing
Their total is Knr
Types of quenching
Collisional: interaction deactivates excited state
Static: molecule forms non florescent complex in ground state
Solvent: stabilises excited state
Resonance energy transfer: excited donor transfers energy to acceptor
What quenching types depend on diffusion / distance.
Collisional
Solvent
Resonance energy transfer
How to increase wavelength of emission (bathochromic shift)
Increase conjugation of ligands (pi —> pi)
Add EDG substituents ( n —> pi)
How to increase probability of triplet excited state in organic molecules
Better rate of intersystem crossing
Add halogen
n —> pi* via heteroatom
Single and triplet levels close together
Remove O2 quencher
Run at low temp
KEY: what effects the rate of excimer formation
(Diffusion limited and reversible)
Concentration dependent
Temperature
Nature of solvent
KEY: what’s the equation for the formation of excimer
M* + M <=> [MM]*
Excited + ground —> energy delocalised
Exciplex equation
Excited state complex
D* + A <=> [DA]*
Effects of EWD and EDG substituents
EDG: bathochromic shift, broad spectrum due to more pi —> pi*
EWG: more intersystem crossing, less florescence
Draw the energy level diagram for an organic molecule
Ground state
S1 excited state
Singlet excited state split into pi —> pi* and n—> pi*
Triplet state of n —> pi*
Change in n —> pi* levels depending on solvent polarity
Polar: n —> pi* then pi —> pi*
Non polar: pi —> pi* then n —> pi*
Selection rules of lanthanide absorption
No change in spin multiplicity
There must be a change in angular momentum
Characteristics of lanthanides
4f —> 4f transitions
Long lifetime as forbidden
Sharp spectra as ligands held close
How does coordination of water effect lifetime of excited state
More water ligands means the quantum yield, intensity and observed lifetime decreases as water quenches the excited state
What is sensitisation
Chromophore ligand bonded to lanthanide and absorbs energy, intersystem crossing promoted by spin orbit coupling of lanthanide means excited state is the triplet state. This feeds lanthanide excited state by energy transfer
Quantum yield of lanthanide complex
Effectiveness of sensitisation x lanthanide quantum yield
Rules for energy transfer in lanthanide complexes
Lanthanide excited state is 2 000 cm-1 less than the triplet excited state of chromophore
If bond length of chromophore to metal is too low excited state is stabilised by back energy transfer
Metal ligand charge transfer conditions
Metal: low oxidation state
Ligand: EWG / conjugated stabilises charge and decreases energy gap
Always low spin complex
Low wavenumber absorption
Broad peak
Wavelength to wavenumber
Wavenumber = 1 / wavelength
Things that effect MLCT
Heavy metal : spin orbit coupling and 100% efficiency of intersystem crossing means shorter lifetime of triplet excited state
Temp: high = visible region absorption, low = low wavelength as not stabilised by solvent
