1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
utilization of equation (1), contemplating the maxima of a specific Q-band in tautomers A
and B.
56
ln(1/r AB )
𝑟 �� = 𝑂𝐷 ��� [𝑄 �(0,0)]
𝑂𝐷 ��� [𝑄 �(0,0)] (1)
At 240 K (-33 °C), 𝑟 �� equals 12 while at room temperature it is determined to be 4. Thus, by
lowering the temperature, tautomer A becomes more and more predominant. Based on
theoretical studies, tautomeric structure T1 (Scheme 33) is considered to be of lower energy
(thermodynamically more stable) and should hence be assigned tautomer A. To really deliver
solid proof, further investigations will have to be conducted as already mentioned.
Nevertheless, it is possible to deduce the energetic difference for the NH-tautomers ∆𝐸 �� (in
the ground state, analogous for exited states ∆𝐸 ��) from the temperature dependent
spectra in accordance to VAN’T HOFF’s theory using equation (2), in which the ratio 𝑟 �� is
referred to and 𝐶 represents a constant, 𝑘 � BOLTZMANN’s constant and 𝑇 the temperature in
K.
𝑙𝑛 � 𝑂𝐷 ��� [𝑄 �(0,0)] 1
�=𝑙𝑛� � =𝐶−
𝑂𝐷 ��� [𝑄 �(0,0)] 𝑟 ��
∆𝐸 ��
𝑘 �𝑇 (2)
The corresponding VAN’T HOFF-plot is shown in Figure 23.
-1.5
-2.0
-2.5
3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2
1/T [10-3 K-1 ]
Figure 23. Temperature dependency of 𝑟 ��of tautomers A and B of 53.
Finally, we were able to construct a potential diagram depicting possible conversions and
potential barriers being shown in Figure 24. Hence is to be concluded, that the potential
barrier for the conversion A → B is higher in the S1 state than in the ground state S0.
Therewith, a photoinduced conversion from A to B by selective excitation of A is not
possible.