8 research outputs found
Time-resolved radiation chemistry:dynamics of electron attachment to uracil following UV excitation of iodide-uracil complexes
Transient IR Spectroscopic Observation of Singlet and Triplet States of 2‑Nitrofluorene: Revisiting the Photophysics of Nitroaromatics
The dynamics of 2-nitrofluorene (2-NF)
in deuterated acetonitrile
is studied using UV pump, IR probe femtosecond transient absorption
spectroscopy. Upon excitation to the vibrationally excited S<sub>1</sub> state, the excited-state population of 2-NF branches into two different
relaxation pathways. One route leads to intersystem crossing (ISC)
to the triplet manifold within a few hundred femtoseconds and the
other to internal conversion (IC) to the ground state. The experiments
indicate that after relaxation to the energetic minimum on S<sub>1</sub>, 2-NF undergoes internal conversion to the ground state in about
15 ps. IC within the triplet manifold is also observed as the initially
populated triplet state relaxes to T<sub>1</sub> in about 6 ps. Rotational
anisotropy measurements corroborate the assignment of the transient
IR frequencies and indicate a rotational diffusion time of 2-NF in
the solvent of about 14 ps. The combined set of results provides a
unified picture of the dynamics in photoexcited 2-NF. This to our
knowledge is the first example using femtosecond vibrational spectroscopy
for the study of the fundamental photoinduced processes in nitroaromatic
compounds
Surprising Intrinsic Photostability of the Disulfide Bridge Common in Proteins
For a molecule to survive evolution and to become a key
building
block in nature, photochemical stability is essential. The photolytically
weak S–S bond does not immediately seem to possess that ability.
We mapped the real-time motion of the two sulfur radicals that result
from disulfide photolysis on the femtosecond time scale and found
the reason for the existence of the S–S bridge as a natural
building block in folded structures. The sulfur atoms will indeed
move apart on the excited state but only to oscillate around the S–S
center of mass. At long S–S distances, there is a strong coupling
to the ground state, and the oscillatory motion enables the molecules
to continuously revisit that particular region of the potential energy
surface. When a structural feature such as a ring prevents the sulfur
radicals from flying apart and thus assures a sufficient residence
time in the active region of the potential energy surface, the electronic
energy is converted into less harmful vibrational energy, thereby
restoring the S–S bond in the ground state