Part 2B: Background Flashcards

1
Q

Photophysical properties of triplet emitters:

A
  • Large stokes shift
  • Long lifetime
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2
Q

Explain the characteristics of triplet emission:

A
  • 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
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3
Q

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

A
  • 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
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4
Q

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

A
  • 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)
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5
Q

Rhenium Imaging Agents: (basis, properties, synthesis)

A
  • 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
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6
Q

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

A
  • 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]
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7
Q

What is FLIM?

A
  • Fluorescence lifetime imaging mapping
  • Independent of concentration
  • Cell image coloured by lifetime
  • Calibration curve produced based on [O2] from spectrometer measurements
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8
Q

Iridium Imaging Agents: (basis, optics)

A
  • 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
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9
Q

Iridium Imaging Agents (2 advantages and 2 disadvantages)

A
  • 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
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10
Q

f-block imaging agents: Characteristics and one issue

A
  • 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
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11
Q

What is the function of luminescence sensitisation?

A

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

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12
Q

What is the value of bimodal imaging?

A
  • Desirable to combine properties required for both modalities in a single agent
    e.g. MRI and Fluorescence imaging -> whole body and subcellular level
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13
Q

3 Key issues in bimodal imaging:

A
  • Paramagnetism often quenches fluorescence
  • Dosage
  • Units may localise differently if separated (necessitates compounds like SCOMPI where both modalities come from the same compound)
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14
Q

Ideal bimodal agents:

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