Question

At intermediate concentrations it is found that a solution of pyrene in cyclohexane shows no excimer emission in the presence of oxygen but does in the absence of oxygen. Explain this observation as quantitatively as possible.

Answer 1

Let us consider a binary system, containing a solvent X (molar concentration c_x) and a solute Y (molar concentration c_y). When the solution is excited by u.v. radiation (intensity I_o photons sec^-1) the reactions that can take place are indicated in table I. It is assumed that both solvent and solute form excimers (D*) by interaction of an excited monomer (M*) and a ground state molecule (M), and that solvent-solute energy transfer can occur both from the excited solvent monomer (M_X*) and excimer (D_X*) to the solute monomer (M_Y). This implies that there is no stable ground state for the solute excimer. We shall consider only steady state conditions of excitation. If all the exciting ligth is absorbed by the solute alone and the solution contains no oxygen, reactions 10 to 16 are the only ones to be considered (with k_0Y = I_o). It has been shown that the intensities of monomer and excimer emission are given, respectively, by

ChY MY Cy Ipy DY DY with keary keary MY ktDy fDY DY MDY kpy kDMY

If under the same conditions of excitation the solutions contain oxygen, then the intensities of monomer and excimer emission are:

DY with aMy My MY MY

DY DY DY

Let us assume now that there is absorption by the solvent with subsequent transfer to the solute, the optical density of the solvent being much greater than that of the solute so that direct solute excitation can be neglected. The intensities of monomer and excimer solute emission can be obtained from the steady state analysis of reactions 1 to 15 (with k_0Y = I_o) for deoxygenated solutions. It is possible to obtain then

hY F. Chy cy with MDX DMX CX If then and is independent of cy

If the solutions contain oxygen, using reactions 1 to 15 and 17 to 20 from tahle t it is possible to obtain

SY XY h Y with

In all these expressions, I_MY and I_DY are the total intensities of monomer and excimer solute emissions, respectively. This means that to use the above equations it is necessary to measure the fluorescence emission, and obtain the areas under the spectra, having corrected the spectra for the relative response of the monochromator-photomultiplier system. However, as shown below, it is possible to obtain some information about the occuring processes even if the spectra are not corrected. This has the advantage of avoiding the errors introduced by the callibration curves. For a given wavelength of emission $\lambda$ the monomer and excimer emission intensities will be given by

1D (λ) . K SI) (A) 11)

Where S_M ($\lambda$) and S_D ($\lambda$) are proportional to the callibration factors (which depend on the wavelength) and I_M and I_D are given, for each particular case, by the relationships derived above.

Note: J.C. Conde. Oxygen Quenching and Energy Transfer in Pyrene Solutions. Rev. Port. Quim., 9, 13, 1967.

Table I reaction rate (sec-1) process (1) MX--> Mİ solvent excitation monomer emission (solvent) monomer radiationless de-activation (solvent) excimer formation (solvent) excimer dissociation (solvent) excimer emission (solvent) excimer radiationless de-activation (solvent) solvent-solute transfr (from solvent monomer) solvent-solute transfer (from solvent excimer) monomer emission (solute) monomer radiationless de-activition (solute) excimer formation (solute) excimer dissociation (solute) excimer emission (soluce) excimer radiationless de-activation (solute solute excitation monomer oxygen quenching (solvent) excimer oxygen quenching (solvent) monomer oxygen quenching (solute) excimer oxygen quenching (solute) MX (3) M ->MX MX DMx C kMDx kEDX DX (10) MMyh vry (11) M,-→ MY k(MY KiMY (13) (14) (15) (16) D*->M,+ MY D-→ My + MY + h VMY D->MY + MY MY-→M" DMY Cy MDY MY iMY * * kitx [o] (19) M[My (20) D, + [02]->MY MY DY L