Factores relacionados con la convivencia escolar

After the ESIPT the molecule vibrates in specific modes which initially contain most of the vibrational energy. If they participate in the IC the coherent motion in these modes should periodically modulate the transition rate. No indications for such signal contributions have been found. Anyway, in this case the IC should be quite fast at the beginning and slow down later since internal vibrational redistribution leads to a loss of energy in these modes. A significant decrease of the S1 population in the first few picoseconds would result in contradiction to the experiment. Obviously, the IC rate is sensitive to the total amount of vi- brational energy but not to the energy content of the modes directly excited by the ESIPT. Thus, the significant coordinates for both processes are orthogonal to each other and the en- ergy flow in modes relevant for the IC is associated with vibrational redistribution. This is reflected by the statistical behavior of the IC whereas coherent wavepacket dynamics is ob- served for the ESIPT [255, 261, 264-266].

7 Summary

Fundamental photoinduced processes like intra- and intermolecular charge transfer and relaxation via internal conversion are investigated in several molecular systems by combin- ing pump-probe femtosecond spectroscopy and steady-state fluorescence studies. After photoexcitation in the deep UV-C spectral region the time resolved dynamics are observed with an unprecedented time resolution of 60 fs for excitation at 270 nm.

Triphenylmethane lactones are investigated as model systems for ultrafast intramolecular charge transfer processes. Due to the orthogonal structural configuration in the investigated lactones (phenolphthalein and malachite green lactone) the subunits are decoupled in the electronic ground state and the transient spectral features allow an unambi- guous identification of the appearing radical ions. Both lactones undergo ultrafast photoin- duced electron transfer (ET) with the formation of a radical ion pair of their structural sub- units. This observation confirms earlier predictions deduced from steady-state fluorescence spectra and from the analysis of structurally analogous compounds. The ET process was monitored directly by measuring the kinetics of the radical cation formation after excitation of phenolphthalein to the S1 state and malachite green lactone to the S2 state.

In phenolphthalein in acetonitrile the phenol radical cation, a product of the primary charge separation, appears in the transient absorption spectra with a rise of 50 fs. This is the fastest ET time constant directly observed for organic intramolecular donor-acceptor systems. The charge separation occurs on a time scale shorter than what was postulated so far for ultrafast inertial solvation dynamics and is quite likely promoted by vibronic coupling via the C-O bond in the lactone ring.

In malachite green lactone both S2→ S1 electronic relaxation and charge separation are completed within 150 fs in aprotic as well as in protic environment. Subsequently, in protic solvent like methanol opening of the lactone ring is detected directly by observing the ap- pearance of the malachite green cation absorption band. The corresponding time constant of 2 - 4 ps is related to the longitudinal dielectric relaxation of methanol and indicates that dif- fusive solvation is responsible for the breakage of the C-O bond in the lactone ring of mala- chite green lactone.

Results of the research work done on triphenylmethane lactones prove that they are ideal model systems for both ultrafast ET and solvation dynamics studies. Triphenylmethane lactones are also excellent candidates for gaining insight into the detailed mechanisms of the very first steps of electron transfer as a fundamental process of photosynthesis in biological donor-acceptor systems.

The response of indole, a common chromophore of many nitrogen heterocyclic biomolecules (such as the essential amino acid Tryptophan), to the UV radiation is investi- gated. The state reversal of the two lowest (ππ*) excited states (the 1La and 1Lb state) occur- ring in polar solvents as opposed to nonpolar solvents and gas phase is temporally resolved for the first time by observing the population transfer between the reversed excited states on a 6 - 7 ps time scale.

The controversial behavior of the excited indole in water, which fluoresces on a nanosecond time scale and undergoes photoionization within 100 fs, is explained with an ultrafast branching occurring immediately after excitation. The excited indole population is divided by the ultrafast branching into a fraction which exhibits a state reversal but stays in the optically accessible excited ππ* states and a fraction that undergoes photoionization via a crossing between the ππ*- and the πσ*-state resulting in an intermolecular electron trans- fer to the solvent.

The photoionization quantum yield of 0.38 is determined by taking into account the ab- sorption spectra of all contributing species over the complete visible range. The dynamics of solvated electron, which appears from charge transfer to solvent state, is observed. The electron solvation time of 0.35 ps is resolved for the first time allowing us to describe the mechanism of electron solvation stemming from indole in a similar way as reported for neat water.

For a long time the origin of the solvated electron was addressed controversially. The dispute is resolved by observing the initial excited state dynamics of 1-methylindole. Due to its structure 1-methylindole is able to exhibit only electron transfer. Since the same solvation dynamics is observed by both indole molecules it is clear that the photoionization in indole follows the same path and that a direct electron transfer to the solvent and not an H-transfer occurs.

Considering only the population fraction of 62%, which does not exhibit ionization and is capable of fluorescing, the fluorescence quantum yield is corrected to the new value of 0.4 for those molecules which reached the La state. From the corrected fluorescence quantum yield and the radiative lifetime calculated from the absorption and fluorescence spectra the lifetime of the La state is determined to be (3.6 ± 1.0) ns.

Internal conversion (IC) is a further nonradiative process which occurs in organic compounds upon the UV radiation. The mechanism of internal conversion is investigated in o-hydroxybenzaldehyde (OHBA). OHBA belongs to the class of molecules that exhibit excited state intramolecular proton transfer (ESIPT) and show rather fast IC. This makes them good candidates for photostabilizers, which are used to protect other light-sensitive materials from degradation caused by the UV component of the sunlight.

It is found that the IC of OHBA proceeds over an energy barrier of about 200 meV as a thermally activated process. The barrier most likely arises from an avoided crossing between the ππ*- and πσ*-state. The height of this barrier is sensitive to the energetic location of these two states with respect to each other. Comparatively small changes in a molecular con- figuration should lead to significant variations of the IC rate. This explains why the IC rate of various ESIPT molecules varies strongly whereas the ESIPT mechanism just involving the ππ*-state is in all cases very similar.

The IC shows pure statistical behavior dependent on total excitation energy although a well-defined wavepacket exists after the ESIPT showing coherent behavior. It indicates that the coordinates drastically changed by the ESIPT are orthogonal to those responsible for the IC and a significant vibrational energy redistribution has to take place before the IC. This is in agreement with the model that the O-H distance is crucial for the ESIPT whereas the IC is due to electronic coupling along coordinates corresponding to a hydrogen detachment.

On the basis of the results presented here we can conclude that in both nonradiative processes, the intermolecular charge transfer to solvent and internal conversion, a dark excited state (πσ*) promotes the ultrafast nonradiative relaxation via crossing between the light ππ* and the dark πσ* state. Presumably this mechanism can be extended to most ultrafast processes.

Appendix