3 research outputs found
Photoinduced Molecular Transport in Biological Environments Based on Dipole Moment Fluctuations
Ultrafast Excited-State Dynamics of Donor–Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates
The excited-state dynamics of two donor–acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck–Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from anormal to an overcritical merocyanine evidenced by steady-state spectroscopy
Ultrafast Excited-State Dynamics of Donor–Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates
The excited-state dynamics of two
donor–acceptor biaryls
that differ by the strength of the acceptor, a pyridinium or a pyrylium
moiety, have been investigated using a combination of steady-state
solvatochromic absorption, ultrafast fluorescence, as well as visible
and infrared transient absorption spectroscopies. The negative solvatochromic
behavior of pyridinium phenolate indicates that the permanent electric
dipole moment experiences a decrease upon S<sub>1</sub> ← S<sub>0</sub> excitation, implying that the ground state possesses more
zwitterionic character than the excited state. In contrast, pyrylium
phenolate exhibits a weakly positive solvatochromic behavior corresponding
to a small increase in the dipole moment upon excitation, implying
more zwitterionic character in the excited than the ground state.
Both compounds are therefore situated at different sides of the cyanine-limit
structure, which has equally polar ground and excited states. Despite
these differences, both molecules exhibit qualitatively similar excited-state
properties. They are characterized by a very short fluorescence lifetime,
increasing from about 1 to 20 ps, when varying solvent viscosity from
0.4 to 11 cP. There are, however, characteristic differences between
the two compounds: The excited-state lifetimes of the pyrylium dye
are shorter and also depend somewhat on polarity. The ensemble of
spectroscopic data can be explained with a model where the emitting
Franck–Condon excited state relaxes upon twisting around the
single bond between the aryl units to a point where the excited- and
ground-state surfaces are very close or intersect. After internal
conversion to the ground state, the distorted molecule relaxes back
to its equilibrium planar configuration, again largely dependent upon
solvent viscosity. However, in this case, the kinetics for the pyrylium
dye are slower than for the pyridinium dye and the polar solvent-induced
acceleration is significantly stronger than in the excited state.
This difference of kinetic behavior between the two compounds is a
direct consequence of the change of the electronic structure from
a <i>normal</i> to an <i>overcritical</i> merocyanine
evidenced by steady-state spectroscopy