Reduction of Donor-Acceptor Distance by Skeletal Motions

Reduction of Donor-Acceptor Distance by Skeletal Motions 

The observed proton transfer times of the order of 50 fs have already been discussed in earlier work with respect to the importance of skeletal vibrations [16-18, 46[. It was proposed that a reduction in the distance between the proton donor and the acceptor results in a decrease in the energetic barrier between the enol-and the keto-form. At times when the barrier is suppressed the proton can tunnel or jump from its enol position to the keto site. In the case of HBO it was suggested that, in particular, the in-plane bending vibration modulates the donor-acceptor distance and thereby enables the proton movement [17]. This model was then applied to MS and to 2-(2 -hydroxyphenyl)-5-pheny]oxazole [18, 46]. However, due to insufficient time resolution of these experiments it was not possible to give experimental evidence for this model. 

Only when TINUVIN P was investigated with 25 fs laser pulses were oscillatory signal contributions detected [26]. They were attributed to coherently excited vibrations in two low frequency modes at 250 cm i and 470 cm . One is a translational motion of the triazole and the phenyl moieties against each other and the other a geared in-plane rotation of the two subunits. The authors interpreted the results in the framework of the model presented above, but attributed the modulation of the proton transfer barrier to a geared in-plane rotation. 

We and others found in many ESIPT molecules a strong coherent excitation of a skeletal in-plane bending mode and concluded that predominantly this motion results in the reduction of the donor-acceptor distance as it is discussed in Section 

For TINUVIN P no direct evidence for the coherent excitation of the inplane bending mode was found [26[. However, in this case the short Sj lifetime of 150 fs does not allow the observation of oscillations with a period of about 300 fs which would result from the coherent excitation of the bending mode. Therefore it can be assumed that also in this case the in-plane bending motion of the molecular skeleton provides the primary contribution to the reduction of the donor-acceptor distance. In HBT, HBO, and TINUVIN P the skeletal stretch vibration is very strongly coherently excited [26, 27, 29] and contributes probably also quite strongly to the initial motion. 

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