Ultrafast photoinduced intra- und intermolecular charge transfer and solvation

Intra- and intermolecular charge transfer as well as internal
conversion processes are studied in various molecular systems. The
dynamics of these fundamental photoinduced processes are
investigated by pump-probe femtosecond spectroscopy and
steady-state fluorescence. Transient spectra are obtained using
white light continuum as probe, while time resolved measurements
are performed by probing at specific wavelengths with non
dispersive detec-tion. Noncollinearly phase matched optical
parametric amplifiers (NOPAs) are used as tun-able pump and probe
pulse sources. This technique enables to study ultrafast phenomena
with an unprecedented time resolution of 60 fs for UV-C excitation
pulses at 270 nm. Upon excitation of their structural subunits,
triphenylmethane lactones (phenolphthalein and malachite green
lactone) undergo ultrafast photoinduced electron transfer (ET) with
the formation of a radical ion pair of their structural subunits.
The phenol radical cation is formed by ET with a time constant of
50 fs and 80 fs after excitation to the S1 state of
phe-nolphthalein in acetonitrile and ethyl acetate, respectively.
In malachite green lactone both S2 S1 electronic relaxation and
ET are completed within 150 fs in aprotic (acetonitrile and ethyl
acetate) as well as in protic environment (methanol). Subsequently,
in methanol the opening of the lactone ring is detected directly by
observing the appearance of the malachite green cation absorption
band on a 2 - 4 ps time scale. The results demonstrate that the ET
in these molecules occurs faster than the time scale of inertial
solvation dynamics, while the lactone ring opening occurs on the
time scale of longitudinal dielectric relaxation. It is as-sumed
that on one side an intramolecular vibrational mode promotes the
charge transfer and on the other side diffusive solvation dynamics
is responsible for the breakage of the C-O bond in the lactone
ring. The controversial behavior of the excited biochromophore
indole in water, which fluoresces on a nanosecond time scale and
undergoes photoionization within 60 fs, is explained with an
ultrafast branching occurring immediately after excitation. The
excited indole population is divided by the ultrafast branching
into a fraction (62%) which exhibits a 1La/1Lb state re-versal and
a fraction (38%) that undergoes photoionization generating indole
radicals and solvated electrons. The electron solvation dynamics is
resolved to occur with a time constant of 350 fs. From the
comparisons with the photoionization dynamics in neat water and in
1-methylindole the origin of the solvated electron has been found
to be the intermolecular electron transfer to the solvent and not
an H-transfer to the solvent. Internal conversion (IC), an
important nonradiative process occurring in organic com-pounds upon
the UV radiation, is explored in o-hydroxybenzaldehyde (OHBA), a
molecule that exhibits excited state intramolecular proton transfer
(ESIPT). It is shown that the IC of OHBA proceeds as a thermally
activated process over an energy barrier of about 200 meV caused by
an avoided crossing between the ππ*- and πσ*-state. The IC shows
pure statistical behavior depending on the total excitation photon
energy although the preceding ESIPT is a balistic motion of a
well-defined wavepacket. Thus, the coordinates involved in the
ESIPT and in the IC are found to be orthogonal.