Photophysics

Question 1

Put these into size order
Electronic levels
Rotational levels
Vibrational levels

Answer 1

Electronic > Vibrational > Rotational

Question 2

What are the two excited states

Answer 2

Singlet : spin allowed florescence
Triplet: spin forbidden phosphorescence

Question 3

KEY: Draw the energy level diagram

Answer 3

Absorption
Internal conversion
Florescence
Intersystem crossing
Phosphorescence

Question 4

What are the two types of measurement for the excited state

Answer 4

Steady state: constant illumination to measure emission intensity
Time resolved: illuminate in short pulses

Question 5

How to do steady state measuring

Answer 5

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

Question 6

What’s the equation for time resolved measuring

Answer 6

Intensity (time) = intensity (0s) Exp (- time / lifetime of excited state)

Question 7

What are the characteristics of the system

Answer 7

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

Question 8

Types of excited state deactivation

Answer 8

Radiative: emits photon
Non radiative: doesn’t emit photon

Question 9

Equation for quantum yield

Answer 9

Kr / Kr + Knr

Question 10

Equation for observed lifetime

Answer 10

1 / Kr + Knr

Question 11

Equation for natural lifetime

Answer 11

1 / Knr or observed lifetime / quantum yield

Question 12

Average lifetime of florescence state

Answer 12

10 nano seconds

Question 13

Average lifetime of phosphorescence state

Answer 13

100 nanoseconds - 10 milliseconds

Question 14

Equation for quantum yield and how it links to brightness

Answer 14

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

Question 15

Factors effecting Knr

Answer 15

Quenching
Internal conversion
Intersystem crossing
Their total is Knr

Question 16

Types of quenching

Answer 16

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

Question 17

What quenching types depend on diffusion / distance.

Answer 17

Collisional
Solvent
Resonance energy transfer

Question 18

How to increase wavelength of emission (bathochromic shift)

Answer 18

Increase conjugation of ligands (pi —> pi)
Add EDG substituents ( n —> pi)

Question 19

How to increase probability of triplet excited state in organic molecules

Answer 19

Better rate of intersystem crossing
Add halogen
n —> pi* via heteroatom
Single and triplet levels close together
Remove O2 quencher
Run at low temp

Question 20

KEY: what effects the rate of excimer formation

Answer 20

(Diffusion limited and reversible)
Concentration dependent
Temperature
Nature of solvent

Question 21

KEY: what’s the equation for the formation of excimer

Answer 21

M* + M <=> [MM]*
Excited + ground —> energy delocalised

Question 22

Exciplex equation

Answer 22

Excited state complex
D* + A <=> [DA]*

Question 23

Effects of EWD and EDG substituents

Answer 23

EDG: bathochromic shift, broad spectrum due to more pi —> pi*
EWG: more intersystem crossing, less florescence

Question 24

Draw the energy level diagram for an organic molecule

Answer 24

Ground state
S1 excited state
Singlet excited state split into pi —> pi* and n—> pi*
Triplet state of n —> pi*

Question 25

Change in n —> pi* levels depending on solvent polarity

Answer 25

Polar: n —> pi* then pi —> pi* Non polar: pi —> pi* then n —> pi*

Question 26

Selection rules of lanthanide absorption

Answer 26

No change in spin multiplicity There must be a change in angular momentum

Question 27

Characteristics of lanthanides

Answer 27

4f —> 4f transitions Long lifetime as forbidden Sharp spectra as ligands held close

Question 28

How does coordination of water effect lifetime of excited state

Answer 28

More water ligands means the quantum yield, intensity and observed lifetime decreases as water quenches the excited state

Question 29

What is sensitisation

Answer 29

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

Question 30

Quantum yield of lanthanide complex

Answer 30

Effectiveness of sensitisation x lanthanide quantum yield

Question 31

Rules for energy transfer in lanthanide complexes

Answer 31

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

Question 32

Metal ligand charge transfer conditions

Answer 32

Metal: low oxidation state Ligand: EWG / conjugated stabilises charge and decreases energy gap Always low spin complex Low wavenumber absorption Broad peak

Question 33

Wavelength to wavenumber

Answer 33

Wavenumber = 1 / wavelength

Question 34

Things that effect MLCT

Answer 34

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