posted on 2014-10-30, 00:00authored byEric M.
M. Tan, Saeed Amirjalayer, Paul Mazzella, Bert H. Bakker, Jan H. van Maarseveen, Hans Bieraugel, Wybren J. Buma
We report on experimental high-resolution
spectroscopic studies
in combination with ab initio computational studies
that investigate the excited-state dynamics of methyl-4-hydroxycinnamate
thioester and (5-hydroxyindan-(1E)-ylidene)acetic
acid, derivatives of the photoactive yellow protein (PYP) chromophore.
These studies aim to elucidate (a) how the thioester moiety influences
the photophysics and photochemistry of the p-coumaric
acid chromophore and (b) to what extent rotation of the single bond
adjacent to the phenyl ring is involved in the decay dynamics of the
electronically excited states. The experimental studies show that
sulfur substitution leads to broad, unstructured excitation spectra
that contrast sharply with the well-resolved spectra of compounds
with an oxygen-based ester. Furthermore, internal conversion to the
lower-lying nπ* state is absent. The absence of this decay channel
is rationalized by quantum-chemical calculations that reveal that
in the nπ* state of the thio compounds the molecule exhibits
a large out-of-plane “kink” at the sulfur atom. Franck–Condon
simulations of the excitation spectra of the V(ππ*) state
highlight the activity of various vibrational modes in the neutral
chromophore and indicate that upon sulfur substitution internal conversion
to the ground state occurs at a significantly higher rate. The similarities
observed in the excitation spectra and decay dynamics of the locked
and unlocked compounds suggest that in the present experiments single-bond
torsion does not show up prominently. The conclusion that for the
isolated molecule double-bond torsion is dominating the excited-state
dynamics is tentatively confirmed by the quantum-chemical calculations.