Part 2: L4, L5 d- and f-block lumophores

Question 1

Two key benefits of the use of spin-orbit coupled d- or f-block systems?

Answer 1

• Triplet emission; large stokes shift to evade autofluorescence
• Triplet emission; long lifetime permits time gating

Question 2

Typical features of d-block luminophores:

Answer 2

Ir(III), Re(I) and Ru(II)…

• All are triplet emitters
• Large component of 3MLCT chacacter to emission
• Charge transfer from filled d6 t2g to a conjugated pi* orbital - easily oxidisable metal, easily reducible ligand
• -> 1MLCT absorption allowed (highly coloured)

Question 3

Biotinylated structures:

Answer 3

• Avidin binds biotin with one of the strongest binding constants in nature (Kd ~ 10^-15)
• Avidin can bind 4 biotins
• e.g. Biotinylated Texas Red -> on treatment with avidin, luminescence drops. Quantum efficiency of ~80%
• e.g. Biotinylated Ir -> on treatments with avidin, both lifetime and intensity increase (up to 8x); large stokes shift; 150nm
• Self quenching mechanism!

Question 4

Attractive features of typical Re(I) lumophores:

Answer 4

• Cationic -> facilitates passive uptake
• Coordinatively saturated therefore inert -> Low toxicity
• 3MLCT excited state localised on bipyridine
• Able to tune photophysics via bipy-type ligand
• Can control biology via pyridine-R
• -> Rapid access to a library of agents

Question 5

Synthesis of Re(I) lumophores:

Answer 5

• Bought as [Re(CO)5Br], boiled with bipy, labile Br displaced by silver, solvent added in its place -> Py-R added in final step

Question 6

Ru(II) imaging agents: Tuning, key application -> issues?

Answer 6

• Simple synthesis from RuCl3
• Tuning characteristics by different bipy-like ligands (substituents) -> photophysics via X grps
• Able to do cell imaging with DA binder/sensor (emission switched off in water due to hydrogen bonding)
• Further study found [Ru(bpy)2(dppz)]2+ on/off agent to be very unspecific; switched on in any situation in vivo that lacks hydrogen bonding, despite being intended as a DNA intercalator

Question 7

Ruthenium complexes as oxygen sensors:

Answer 7

• Intensity and lifetime sensitive to [O2]
• Molecular oxygen quenches triplets and turns off emission -> O2 itself excited to singlet state
• Lifetime emission as a function of known [O2] (regardless of intensity)
• Can use hypoxia as a marker for tumour cells
• FLIM: Fluorescence lifetime image mapping -> able to illuminate oxygen

Question 8

Typical Ir(III) lumophores:

Answer 8

• Not neutral ligands; this would create tripositive systems, preventing passive diffusion across membranes
• Tris cyclometallated 2-phenylpyridine (ppy) complexes give higher luminescence intensity
• bis-cyclometallated ppy + N^N complexes allow introduction of groups to tune luminescence, biology

Question 9

Attractive features of Ir(III) agents:

Answer 9

• Very tunable via ppy unit and N^N
• Moderately cationic -> facilitating passive diffusion across membranes
• Lifetime facilitates time-gated imaging (e.g. Hoescht)

Question 10

Drawback of Ir(III) agents:

Answer 10

• Very hard to specifically target
• Will generally localise to anything lipophilic