9. Oxygen is an efficient quencher of fluorescence because it is a triplet in it

Question

9. Oxygen is an efficient quencher of fluorescence because it is a triplet in its electronic ground state. The unpaired spins of O can induce the excited state of the fluorescent molecule to undergo intersystem crossing from the singlet state to the triplet state. (a) How would you verify the system experimentally? (b) Assume that the quenching rate constant (the rate constant for the collision between O2 and fluorescent molecules is 1.0x 10 MHow many collisions per second on average does each fluorescent molecule in solution experience ifthe [03] = 3.4 x 10-M and [F] = 0.5 M, where F is the fluorescent molecule. (c) The fluorescence lifetime of pyrene, a molecule that is often used to probe biological systems, is 500 ns, while that of tryptophan is about 5 ns. Explain why under normal atmospheric conditions O2 can interfere only with the fluorescence of pyrene but not that of tryptophan. (d) A quantitative relationship of fluorescence quenching is the Stern-Volmer equation Where I, and I are the fluorescence intensities in the absence and presence of the quencher resepetively, k, is the quenching rate constant, ° is the lifetime of the fluorescent state in the absence of the quencher, and Q] is the concentration of quencher. Use this equation to support your lationshin in (c)

Explanation / Answer

a) The fact is that the unpaired spins of O2 molecule can induce the excited state of the fluorescent molecule to undergo intersystem crossing from the singlet state to the triplet state.

Experimental verification of the system: This photoinduction is an example of the photochemical process, which are generally of zero order, i.e. the rate of fluorescence depends only on the concentration of photo-sensitizer (O2). Hence it can induce the excited state of the fluorescent molecule to undergo intersystem crossing from the singlet state to the triplet state.

b) The no. of collisions that each fluorescent molecule in solution experience = kq*[Q]

= 1010 M-1s-1 * 3.4*10-4 M

= 3.4*106 collision / s

c and d) In case of pyrene, according to the Stern-Volmer equation,

I0/I = 1 + 1010 M-1s-1 * 500*10-9 s * 3.4*10-4 M

i.e. I0/I = 1 + 1.7 = 2.7 (> 1)

In case of tryptophan, according to the Stern-Volmer equation,

I0/I = 1 + 1010 M-1s-1 * 5*10-9 s * 3.4*10-4 M

i.e. I0/I = 1 + 0.017 = 1.017 (~ 1)

Hence, under normal atmospheric conditions, O2 can interfere only with the fluorescence of pyrene but not that of tryptophan.

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