Page 31 - Photoreactive Organic Thin Films

J Q HERHANN RAD

differing sites of isomerizing probe molecules with different quantum yields,
An example is azobenzene in monomeric methylmethacrylate and poly-
methylmethacrylate with a linear plot according to Equation 1.2 in the
28 29 30
monomer, but nonlinear plots in polymers. ' ' In this case, Scheme I, which
for photoreactions requires equal values for E and <|> for all probe molecules is
not sufficient. Different sites may be present (see Section 1.2.2.3).

1.2.2.2 The Quantum Yield: How Effective?
One of the most important features of a photoreaction is the value of
31
the quantum yield $ of compound i, which is the quantifying answer to the
question: "How effective?" In principle, the quantum yield is the ratio of
the number of reacting molecules to the number of quanta absorbed. In praxi,
there are several definitions of the quantum yield: true (only light absorbed
by the reactant is considered) and apparent (there are other absorbers pres-
ent), differential (at the moment ) and integral (mean). In the previous rate
equation, <J) E and <|) z are the true differential yields. The monoexponential
kinetics of Equation, 1.2 or 1.4 allow one to determine the yields in systems
where the starting solution is already a mixture of E- and Z-forrns (which
can happen easily if the E-form is not prepared under strict exclusion of
light). It turns out, however, that the values of the Z —» E quantum yield are
especially sensitive to small errors in the e values.
For the determination of quantum yields in photoisomerizations without
thermal back reaction, several situations regarding the availability of data of
E- and Z-isomers are considered.
• The spectra and absorption coefficients of both E- and Z-forms are avail-
able. This is the case when both isomers can be isolated in sufficient quan-
tities to allow the determination of their absorption coefficients. In this
situation, sufficient information for the determination of <j>£ and <j>| is avail-
able via Equations 1.4 and 1.5. 32
• The spectrum and absorption coefficients of the E-form are known, but for
the Z-form only a spectrum is available. This is the case when the amount
of Z-isomer is sufficient to isolate it (e.g., by TLC) and to take a spectrum
but not enough material is available to weigh it. In this situation, there is
one piece of information lacking for the evaluation according to Equations
33 34
1.4 and 1.5. This can be obtained by using two analysis wavelengths. '
« The spectrum and absorption coefficients of the E-form are available, but
there is no spectral information for Z. This is the case where the isolation
of Z is impossible—a case often encountered with thin films. In this situa-
tion, two pieces of information are lacking: the value and the spectral
35
dependence of the absorption coefficient £ z. Fischer presented a solution
to this problem (1) by doing two isomerization experiments employing
two different irradiation wavelengths: Xj and A, 2, and (2) by assuming that
e
t le ratio
2
^Z^E^ ^z^E ' i- *» ' °f the E -> Z and Z -> E quantum yields at
the two irradiation wavelengths should be equal, which is only partly true
for azobenzene (see Section 1.3.2.1). Fisher's article is not easy to read; one
must be careful to discriminate strictly between irradiation and analysis
wavelengths.