The Hammett relationship and reactions in the excited electronic state: hemithioindigo Z/E-photoisomerization.
The photochemical reaction dynamics of a set of photochromic compounds based on thioindigo and stilbene molecular parts (hemithioindigos, HTI) are presented. Photochemical Z/E isomerization around the central double bond occurs with time constants of 216 ps (Z --> E) and 10 ps (E --> Z) for a 5-methyl-hemithioindigo. Chemical substitution on the stilbene moiety causes unusually strong changes in the reaction rate. Electron-donating substituents in the position para to the central double bond (e.g., para-methoxy) strongly accelerate the reaction, while the reaction is drastically slowed by electron-withdrawing groups in this position (e.g., para-nitrile). We correlate the experimental data of seven HTI-compounds in a quantitative manner using the Hammett equation and present a qualitative explanation for the application of ground-state Hammett constants to describe the photoisomerization reaction.
Cordes
Thorben
T
Munich Center for Integrated Protein Science, CIPSM, and Lehrstuhl für BioMolekulare Optik, Department für Physik, Ludwig-Maximilians-Universität München, Oettigenstrasse 67, Munich, Germany.
Schadendorf
Torsten
T
Priewisch
Beate
B
Rück-Braun
Karola
K
Zinth
Wolfgang
W
D004396:Coloring Agents / Q000737:chemistry*; D007203:Indigo Carmine / Q000031:analogs & derivatives* / Q000737:chemistry; D015394:Molecular Structure; D010777:Photochemistry; D013057:Spectrum Analysis; D013237:Stereoisomerism; D013267:Stilbenes / Q000737:chemistry*; D013997:Time Factors
D004396:Coloring Agents; D013267:Stilbenes; C527420:hemithioindigo; C009466:thioindigo; D007203:Indigo Carmine