H-chelate ring

The chapter focuses on the ultrafast ESIPT found in molecules containing an H-chelate ring (see Fig. 11.1). We will discuss the spectral features associated with the ESIPT which are observed in experiments with a time resolution down to 30 fs. They are interpreted in terms of a wavepacket motion on an adiabatic potential energy surface connecting the Franck-Condon region of the enol-form with the minimum of the electronically excited keto configuration. The reaction coordinate is reconstructed from the coherently excited vibrational product modes observed in the experiment. Strong evidence is provided that skeletal modes deter-... 

Figure 11.12 ESIPT model directly after the optical excitation the H-chelate ring contracts and the donor-acceptor distance is reduced. Then an electronic configuration change (electronic switching) occurs and the keto bonds are formed. Subsequently, the molecule is accelerated towards the keto minimum and starts to oscillate around the equilibrium geometry in coherently excited modes.

For the concerted double proton transfer both donor-acceptor distances have to be reduced simultaneously, leading to a symmetric contraction of the molecule and to the coherent excitation ofthe symmetric stretch vibration. Forthe single proton transferthe donor-acceptor distance in only one of the two H-chelate rings is compressed by an antisymmetric bending motion. 

This chapter focuses on the ultrafast ESIPT found in molecules containing an H-chelate ring (figure 4.1). We will introduce the most popular experimental techniques and discuss what kind of information can be extracted from the spectral signatures associated with the ESIPT and subsequent processes. In the remainder of the introduction, we introduce the investigated molecular systems. The subsequent experimental section describes different pump-probe techniques. Then the transient spectroscopic signatures and their interpretation and evaluation... 

For compounds with parallel or branching intramolecular reaction channels, the interesting question arises whether also coherent wavepacket dynamics can be observed there and whether it can be used to analyze the different channels. This can be studied with the ESIPT compound [2,2 -bipyridyl]-3,3 -diol (BP(OH)2) which contains two H-chelate rings (Figure 4.11) and exhibits both single and concerted double proton transfer in aprotic solvents after photoexcitation. The single proton transfer occurs within 100 fs and leads to an intermediate mono-keto isomer which subsequently transforms with a time constant of 10 ps to the final diketo form [72, 73]. In addition, a second reaction channel exists that leads to the final diketo product within less than 100 fs by a simultaneous transfer of both protons. 

For the single proton transfer, the donor-acceptor distance of only one of the two H-chelate rings has to be compressed. This is achieved by an antisymmetric bending motion. This example demonstrates that different reaction channels result in different coherent wavepacket dynamics. BP(OH)2 exhibits inversion symmetry, and direct optical excitation of the antisymmetric bending mode is not possible because of selection rules for electronic dipole transitions [74]. This proves that the observed coherent wavepacket motions result from ultrafast intramolecular reactions. 

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