Part 2B: Background

Question 1

Photophysical properties of triplet emitters:

Answer 1

• Large stokes shift
• Long lifetime

Question 2

Explain the characteristics of triplet emission:

Answer 2

• Singlet excited state converts to triplet then loses lots of energy relaxing to idea triplet ground state geometry
• Triplet emission is forbidden so slow with a long lifetime

Question 3

Singlet to triplet emission: When and why is it possible?

Answer 3

• Forbidden
• Possible in heavy metal luminophores via spin orbital coupling
• e.g. d6 transition metal complexes (Re, Ru, Ir), where ligands have aromatic rings (acting as electron acceptors to facilitate MLCT) -> Lifetime of 100s of ns, stokes shift of 1-200nm

Question 4

Biotin characteristics and application as a luminophore: (including key issue)

Answer 4

• High affinity for avidin (in which the biotinylated molecules retain their photophysical properties)
• Issue: organic fluorophore conjugates get efficient self-quenching via RET (within forster distance)
• Transition metal complexes are not so prone to this due to their larger stokes shift (also has increased lifetime and intensity)

Question 5

Rhenium Imaging Agents: (basis, properties, synthesis)

Answer 5

• Re(I)
• Commonly based on fac-[Re(CO)3(bpy)]+ core, with similar Tc analogues
• Attractive photophysics and stability
• Flexibility in choice of 6th ligand confers tunability in properties
• Synthesised from parent pentacarbonyl halides in a 3 step process

Question 6

Ruthenium Imaging Agents: (basis, synthesis, optics, application)

Answer 6

• Ru(II)
• Typically based on derivatives of [Ru(bpy)]2+ -> one of bpy units replaced with more complex ligand
• Synthesised from neutral dichloride, [Ru(bpy)2Cl2]
• Complexes exist as a mixture of optical isomers -> results in a racemic mixture of products, with implications due to biological chirality
• Popular in oxygen sensing -> both intensity and lifetime of Ru emission drop proportionally to [O2]

Question 7

What is FLIM?

Answer 7

• Fluorescence lifetime imaging mapping
• Independent of concentration
• Cell image coloured by lifetime
• Calibration curve produced based on [O2] from spectrometer measurements

Question 8

Iridium Imaging Agents: (basis, optics)

Answer 8

• Ir(III)
• Commonly has two cyclometallated ligands (e.g. ppy) or similar with third chelating ligand such as bpy
• Produces monocationic complexes as a pair of optical isomers

Question 9

Iridium Imaging Agents (2 advantages and 2 disadvantages)

Answer 9

• Remarkable range of photophysical properties (variable excited state) -> widely tunable emission (blue to red); lifetime emissions from ns to ms
• Good cellular uptake (cationic, lipophilic due to Ir(III) biscyclometallated core
• Can be difficult to control uptake due to high lipophilicity
• Certain complexes show ligand-dependent cytotoxicity

Question 10

f-block imaging agents: Characteristics and one issue

Answer 10

• Great stokes shift and luminescence lifetimes (since 4f-4f transition is laporte forbidden) -> appropriate for time-resolved techniques
• Easily recognisable emission patterns (protection from environmental influence via 5s25p6 subshells)
• Very low absorption coefficient so require luminescence sensitisation
• Issues arise from coordinated water (O-H bonds quench) -> necessary to coordinatively saturate the metal centre

Question 11

What is the function of luminescence sensitisation?

Answer 11

Grafted on antennae/chromophore absorb UV radiation and transfer it to the emissive excited state of the lanthanide ion

Question 12

What is the value of bimodal imaging?

Answer 12

• Desirable to combine properties required for both modalities in a single agent
  e.g. MRI and Fluorescence imaging -> whole body and subcellular level

Question 13

3 Key issues in bimodal imaging:

Answer 13

• Paramagnetism often quenches fluorescence
• Dosage
• Units may localise differently if separated (necessitates compounds like SCOMPI where both modalities come from the same compound)

Question 14

Ideal bimodal agents:

Answer 14

• Those in which the radioisotope also gives the fluorescence
• Often studied with analogues (e.g. ‘cold’ Re with ‘hot’ Tc)