Intramolekulare Schwingungsenergieumverteilung in verbrückten Azulen-Anthrazen-Verbindungen (English shop)

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In this work, the intra- and intermolecular vibrational energy transfer of bridged azulene-anthracene compounds and of an azulene crown ether was investigated by UV absorption spectroscopy using a pump-probe experiment.
By means of this technique it was possible to analyse the influence of the length of the bridge and of its structure on the energy transfer. In addition, it was investigated whether complexed cations affect the energy transfer dynamics of the azulene crown ether.
The investigations were carried out in supercritical xenon, dichloromethane, 1,1,2-trichlorotrifluoroethane and acetonitrile.

By laser excitation of the azulene moiety into the S1 state followed by rapid internal conversion into the highly vibrationally excited ground state, the molecules were prepared such that their excitation energy was localised in the azulene moiety. The laser pulses used for this purpose had a central wavelength of 550 nm and a full width at half maximum of 120 fs and yielded molecules with an excess energy of 18180 cm-1. With a time-delayed second laser pulse of wavelength 305 to 312 nm, the decrease of the vibrational energy in the azulene moiety could be followed by means of the transient absorption change. The dependence of the extinction coefficient on the mean vibrational energy required for this is known.

The establishment of the intramolecular equilibrium proceeded in two phases. First, about 20% of the excitation energy left the azulene moiety due to harmonic propagation into the remainder of the molecule. This dephasing of the normal modes lasted about 300 fs. The second phase corresponded to the intramolecular vibrational energy redistribution (IVR), whose time constants ranged between 2.0 and 6.5 ps depending on the system.

In the first part of the work it could be shown that, for those compounds in which the azulene and anthracene moieties were bridged by an aliphatic molecular chain, the IVR time constants increase approximately linearly with the chain length for short chains (up to 3 CH2 units), whereas for longer chains they approach a constant value. It is therefore assumed that for longer chains the coupling of the azulene moiety to the chain determines the rate of the IVR process.
The use of chains with non-conjugated double bonds had no influence on the energy transfer. Furthermore, it was shown that the energy flow from the azulene through a cyclohexene ring is considerably slower than in the case of single chains of comparable length. This was attributed to the altered conformation, a possible change in the coupling of the azulene moiety to the ring, as well as the possible occurrence of ring-flipping processes.

In the second part of the work, the influence of dissolved salts on the energy transfer of azulene-1,3-15-crown-5 in acetonitrile was investigated. The salts used were LiClO4, NaClO4, KSCN, Mg(ClO4)2, Ca(ClO4)2 • 4 H2O and NH4SCN. No significant influence of the added salts on the intramolecular energy transfer could be detected.
Furthermore, the intermolecular energy transfer into the solvent was investigated. The addition of the salts had no significant effect on this process either. However, it was found that traces of water markedly accelerate the energy transfer.