| Abstract [eng] |
Investigation of Photoisomerisation Dynamics of Polymethine Dyes Featuring Dimethyl-7,8,9,10-tetrahidro-6H[1,2-a]indolium, Malononitrile and 1,3-diethyl-thiobarbituric Acid Side Groups The purpose of this graduating work of masters studies in „Laser technology“ was to examine the isomerisation properties of a new type of merocyanine dyes. The dyes were synthesized in the laboratory of prof. A. A. Ishchenko in Institute of Organic Chemistry in Kiev, Ukraine. These dyes were composed of dimethyl-7,8,9,10-tetrahidro-6H[1,2-a]indolium (further on referred as il) and either malononitrile (further on referred as cn) or 1,3-diethyl-thiobarbituric acid (further on referred as etb) side groups joined by conjugated carbon atom chains. il groups were common to all of the examined dyes. The compounds can be separated into two groups. In the first one the second heterocycle was a cn group, in the second – etb. The dyes inside the groups differed from one another by the length of their chains. The dynamics of isomerisation was examined using the transient absorption spectra of solutions of the dyes in acetone. The spectra were obtained by pump – probe spectroscopy. Two types of measurements were carried out. In the first type the same pulse from a femtosecond laser was used to both excite and probe the sample. A mechanical delay line was used. Using this type of measurements transient spectra up to 7.7 ns delay times were measured. In the second type the sample was excited by the pulse of nanosecond laser and probed with a pulse from a femtosecond one. Delay times were achieved by electronically control of the nanosecond laser. Up to 0.8 ms delay times were reached by this method. Excitation lifetimes and efficiency of isomerisation of the dyes under examination were evaluated. It was found that the compounds with the shorter chains after excitation form isomers more efficiently. This was explained by several reasons. Firstly, isomerisation may be influenced by steric effects, which should be pronounced more strongly in dyes with shorter chains. Due to steric effects parts of the same molecule may not be in the same plane, which helps form isomers. The second reason is associated with the lifetime of excited state of the molecules. It is known that the lifetime is affected by the strength of the vibrational interaction. The stronger the interaction the shorter the lifetime is. Dyes with shorter carbon atom chains have shorter excited state lifetimes which indicates stronger vibrational interaction. Isomerisation may be caused by one of the vibrational modes of the molecule, which may be strongly anharmonic. Several models were proposed, which explains the evolution of the measured transient spectra of the compounds. |
