1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

OD
0.5
0.4
0.3
0.2
0.1
0
230 K
500 600 λ [nm] 700
fluorescence
3·10 4
2·10 4
1·10 4
0
295 K
295 K
Discussion and Results 3
The origin of those three components was clarified taking into account the following facts:
• 53 appears “chemically pure”, meaning at least >98 % purity bsd. on analytic means
• component C is only detectable as it has a quite different fluorescence life-time and
as the fluorescence of the major components is a priori low
• regular metallo-porphyrins like Zn(II)-53 (which will be discussed later) only show one
major fluorophore
• variations in concentration (e.g. high dilution) do not change the ratio P(A) : P(B)
Thus, C could be assigned an impurity – maybe free base TTBPP 19 or any other synthetic
residue. Concerning A and B, it seemed to be clear, that they could not be monomer and
dimer or any other aggregate and that it has to be a specific feature of free base compound
53. One possible explanation could be the presence of distinct tautomeric structures with
differing photophysical properties, like the already presented VT-NMR studies hinted to.
3.2.3.4.2 Fluorescence Spectroscopy at Varied Temperatures 108
To verify the abovementioned assumption, temperature dependent absorption and
fluorescence spectra were recorded in solution. Changing the temperature should thereby
have a significant effect onto the equilibrium position between both tautomeric structures
resulting in varying shapes in both spectra. Some results are depicted in Figure 22.
290 K
230 K
290 K
225 K
660 720 λ [nm]
Figure 22. Temperature dependent absorption spectra (left) and fluorescence spectra (right,
excitation at 532 nm) of 53.
From those findings, several conclusions can be deduced. The ratio 𝑟 �� of tautomers
P(A):P(B) is temperature dependent whereas at lower temperatures, one tautomer (e.g.
tautomer A) becomes more predominant. A quantitative evaluation can then be achieved by
780
55