Lippert-Mataga plot

Figure 1 Stokes shift of a chromophore between absorption and emission due to reorientation of the solvent

Lippert-Mataga plots are used to determine dipole moments of molecules in electronically excited states. They are based on the application of the Lippert-Mataga equation . Solvent- dependent spectral shifts influence an emission spectrum to a far greater extent than an absorption spectrum . This is a consequence of the very different time scales of the two processes: During the fluorescence emission, which lasts a few nanoseconds, the surrounding solvent cage can structurally relax (solvent reorientations take place in about 10 to 100 picoseconds), while absorption processes are completed in a few femtoseconds, which means that no solvent reorientation takes place. The excitation of the chromophore therefore takes place with the same local environment in the ground and excited states, whereas the emission is subject to a Stokes shift , which depends on the solvent polarity (Figure 1). The resulting spectral shifts in the emission and absorption spectra are usually evaluated by plotting them in a so-called Lippert-Mattaga plot .

Solvatochromism and thermochromism

Figure 2 Solution of betaine in various solvents. The color reaction of solvatochromic dyes depends on the polarity of the solvent. E _T values ​​s. a. Dimroth et al.

The different spectral shifts of a chromophore, in different solvents, by varying the permittivity ε and the refractive index n, are called solvatochromic shifts, the corresponding shifts due to temperature changes in a solvent are called thermochromic shifts. Figure 2 shows the change in color of betaine solutions due to solvatochromic shifts in different solvents.

Lippert-Mataga equation

The Lippert-Mataga equation links the Stokes shift of the chromophore with the radius of the solvent cage a , the permittivity ε and the refractive index n of the solvent, as well as with the dipole moments in the ground and excited state ( μ _G and μ _E ) of the chromophore. ${\textstyle {\overline {\nu }}_{A}-{\overline {\nu }}_{E}}$

${\displaystyle {\overline {\nu }}_{A}-{\overline {\nu }}_{E}={\frac {2}{hc}}{\biggl (}{\frac {\epsilon -1}{2\epsilon +1}}-{\frac {n^{2}-1}{2n^{2}+1}}{\biggr )}{\frac {(\mu _{G}-\mu _{E})^{2}}{a^{3}}}+{\text{const.}}}$

Figure 3 Lippert-Mataga plot of the spectral shifts of a chromophore as a function of the refractive index and permittivity of various solvents.

If the difference is measured for different solvents and plotted as a function of ε and n of the different solvents, then the difference in the dipole moments in the case of electronic excitation can be determined from the slope of the linear regression as the slope of the straight line. This plot is called the Lippert-Mataga plot (Figure 3). ${\textstyle {\overline {\nu }}_{A}-{\overline {\nu }}_{E}}$

Approximations in the determination of dipoles using Lippert-Mataga plots

• In the Lippert-Mataga model, a spherical chromophore is assumed, which is a poor approximation for elongated molecules.
• Interactions between the chromophore and the solvent are neglected.
• The polarizability of the chromophore is neglected.
• The direction of the dipole moment vector in the ground and excited state points in the same direction.

Individual evidence

1. ^ Joseph R. Lakowicz: Principles of Fluorescence Spectroscopy . 3. Edition. Springer, 2006, ISBN 0-387-31278-1 , doi : 10.1007 / 978-0-387-46312-4 .
2. ↑ ^{a b} E. Lippert: Spectroscopic determination of the dipole moment of aromatic compounds in the first excited singlet state . In: Zeitschrift für Elektrochemie, reports of the Bunsen Society for physical chemistry . tape 61 , no. 8 , 1957, pp. 962-975 , doi : 10.1002 / bbpc.19570610819 .
3. ^ ^{A b} Noboru Mataga, Yozo Kaifu, Masao Koizumi: Solvent Effects upon Fluorescence Spectra and the Dipolemoments of Excited Molecules . In: Bulletin of the Chemical Society of Japan . tape 29 , no. 4 , April 1956, p. 465-470 , doi : 10.1246 / bcsj.29.465 .
4. ↑ Karl Dimroth, Christian Reichardt, Theodor Siepmann, Ferdinand Bohlmann: About pyridinium-N-phenol-betaine and their use to characterize the polarity of solvents . In: Justus Liebig's Annals of Chemistry . tape 661 , no. 1 , 1963, p. 1-37 , doi : 10.1002 / jlac.19636610102 .
5. ↑ A. Kawski: On the Estimation of Excited State Dipole Moments from Solvatochromic Shifts of Absorption and Fluorescence Spectra . In: Journal of Nature Research A . tape 57 , no. 5 , 2014, p. 255-262 , doi : 10.1515 / zna-2002-0509 .
6. ↑ Progress in Photochemistry and Photophysics, Volume V. (No longer available online.) In: CRC Press. Archived from the original on September 18, 2016 ; accessed on September 18, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.