Part 2B: Background Flashcards
Photophysical properties of triplet emitters:
- Large stokes shift
- Long lifetime
Explain the characteristics of triplet emission:
- 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
Singlet to triplet emission: When and why is it possible?
- 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
Biotin characteristics and application as a luminophore: (including key issue)
- 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)
Rhenium Imaging Agents: (basis, properties, synthesis)
- 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
Ruthenium Imaging Agents: (basis, synthesis, optics, application)
- 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]
What is FLIM?
- Fluorescence lifetime imaging mapping
- Independent of concentration
- Cell image coloured by lifetime
- Calibration curve produced based on [O2] from spectrometer measurements
Iridium Imaging Agents: (basis, optics)
- 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
Iridium Imaging Agents (2 advantages and 2 disadvantages)
- 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
f-block imaging agents: Characteristics and one issue
- 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
What is the function of luminescence sensitisation?
Grafted on antennae/chromophore absorb UV radiation and transfer it to the emissive excited state of the lanthanide ion
What is the value of bimodal imaging?
- Desirable to combine properties required for both modalities in a single agent
e.g. MRI and Fluorescence imaging -> whole body and subcellular level
3 Key issues in bimodal imaging:
- Paramagnetism often quenches fluorescence
- Dosage
- Units may localise differently if separated (necessitates compounds like SCOMPI where both modalities come from the same compound)
Ideal bimodal agents:
- Those in which the radioisotope also gives the fluorescence
- Often studied with analogues (e.g. ‘cold’ Re with ‘hot’ Tc)