1. If the fluorescence life time decay of the probe+metal is smaller than the free probe, will the fluorescence emission of the probe+metal be higher or lower than the probe - Whether it will be turn on(enhancing)or turn off sensor(quenching). And the same, will the quantum yield of the Probe+metal be larger than the free probe or vise versa?

Ans. If your probe is turn-on chemosensor, fluorescence lifetime of the probe+metal will be higher than the free probe.

Generally, metal+probe bonding reduces the vibrational states which causes non-radiative recombination. Due to chelation enhancement fluorescence effect (CHEF), fluorescence intensity increases in metal+probe complex and consequently, lifetime increases in complex than the probe.

In case of turn-off chemosensor, reverse situation will be observed in lifetime.

2. We proposed a PET off sensor mechanism, i.e., Probe:PET On and Probe+Metal:PET OFF, if this the case, then will the life time decay of the probe+metal be larger or lesser than the free probe.

Ans. Generally, photoinduced electron transfer (PET) on-off chemosensor possesses a fluorophore and a receptor unit.

HOMO of unbound receptor may exist between the HOMO and LUMO of excited fluorophore. Unbound receptor may transfer its one HOMO electron to HOMO of excited fluorophore hampering the recombination of electron and hole within the fluorophore and it causes the quenching of fluorescence intensity.

If the lone pair (lp) of electrons in unbound receptor participate in bonding with cations, then that lp of electrons in unbound receptor will not hamper the radiative recombination process and eventually, fluorescence will increase.

3. With help of FL life time, Can it is possible to say whether the emission band obtained for the Probe+metal complex, that the emission is because of probe+metal mixture or the probe alone. How?

Ans. If there is any change in the ligand environment, you will definitely find some change in lifetime between probe and probe+metal complex.

N.B. If you share your experimental results, I can give more input in your results.

Dear Prof. Atanu Jana

Thank you for the answer,

But still need clarification, I can't able to provide the data, but can provide a model plot of the spectrum and mechanism. Can you help me with that.

Sir, the sensor is consists of two fluorophore units linked through imine spacer. I have proposed a PET on and off sensor from imine nitrogen to one of the fluorophore say F2 as shown in scheme.

The free ligand shows a low fluorescence and the a trivalent metal ion addition shows enhanced ~ 300 a.u. difference and slight shift ~ 30 nm difference. from the fluorescence of free probe.

While, in the lifetime studies, both the ligand and the ligand-metal complex fitted with biexponential decay rate. Where I got a lesser decay lifetime of ligand-metal complex than the free ligand, but my fluorescence spectrum shows a high fluorescence for ligand-metal complex than free ligand. Is this possible. I have attached a reference paper on this, but the case in citation, is that the probe+metal complex is fitted with mono-exponential and free probe with di-exponential decay rates.

Article A novel rhodamine-based fluorescence chemosensor containing ...

Denzil Britto Now, I read the paper you shared. Upon the addition of Hg2+ to the chemosensor, the formation of a tight binding complex occurs along with the opening of the spirolactam ring. Non-radiative decay channels are increased due to ring-opening which gives more flexibility and creates additional vibrational states.

Fluorescence intensity (FI) is associated with the population of excited states. If the population of excited states is higher, then the FI will be higher if the recombination is radiative (Do not confuse with radiative recombination and radiative recombination rate). It does not mean lifetime will be always higher for higher FI fluorophores. The lifetime may increase or decrease depending on deactivation rate. Lifetime measurement is a kinetic phenomenon associated with the decay rates, radiative decay (Kr) and non-radiative decay (Knr). The radiative decay rate may be slower in case of low FI fluorophore and that will increase the lifetime.

In the case of your molecule, consider some deactivation mechanism which may increase the decay rate and consequently, reduce the lifetime of probe+metal where the population of excited states is much higher than the free probe. In your case, the higher population of excited states in probe+metal gives you higher FI than the free probe. Also, consider the HOMO-LUMO position of fluorophore and receptor. There are several types of PET mechanism depending on the HOMO-LUMO positions.