3 research outputs found
The Gas-Phase Photophysics of Eosin Y and its Maleimide Conjugate
The use of the xanthene
family of dyes as fluorescent probes in
a wide range of applications has provided impetus for the studying
of their photophysical properties. In particular, recent advances
in gas-phase techniques such as FRET that utilize such chromophores
have placed a greater importance on the characterization of these
properties in the gas phase. Additionally, the use of synthetic linker
chains to graft the chromophores in a site-specific manner to their
target system is ubiquitous. There is, however, often limited information
on how the addition of such a linker chain may affect the photophysical
properties of the chromophores, which is of fundamental importance
for interpretation of experimental data reliant on grafted chromophores.
Here, we present data on the optical spectroscopy of different protonation
states of Eosin Y, a fluorescein derivative. We compare the photophysics
of Eosin Y to its maleimide conjugate, and to the thioether product
of the reaction of this conjugate with cysteamine. Comparison of the
mass spectra following laser irradiation shows that very different
relaxation takes place upon addition of the maleimide moiety but that
the photophysics of the bare chromophore are restored upon addition
of cysteamine. This radical change in the photophysics is interpreted
in terms of charge-transfer states, whose energy relative to the S<sub>1</sub> ← S<sub>0</sub> transition of the chromophore is dependent
on the conjugation of the maleimide moiety. We also show that the
shape of the absorption band is unchanged in the gas-phase as compared
to the solution-phase, showing a maximum with a shoulder toward the
blue, and examination of isotope distributions of the isolated ions
show that this shoulder cannot be due to the presence of dimers. Consideration
of the fluorescence emission spectrum allows a tentative assignment
of the shoulder to be due to a vibrational progression with a high
Franck–Condon factor
Visible and Ultraviolet Spectroscopy of Gas Phase Rhodamine 575 Cations
The visible and ultraviolet spectroscopy
of gas phase rhodamine
575 cations has been studied experimentally by action-spectroscopy
in a modified linear ion trap between 220 and 590 nm and by time-dependent
density functional theory (TDDFT) calculations. Three bands are observed
that can be assigned to the electronic transitions S<sub>0</sub> →
S<sub>1</sub>, S<sub>0</sub> → S<sub>3</sub>, and S<sub>0</sub> → (S<sub>8</sub>,S<sub>9</sub>) according to the theoretical
prediction. While the agreement between theory and experiment is excellent
for the S<sub>3</sub> and S<sub>8</sub>/S<sub>9</sub> transitions,
a large shift in the value of the calculated S<sub>1</sub> transition
energy is observed. A theoretical analysis of thermochromism, potential
vibronic effects, and–qualitatively–electron correlation
revealed it is mainly the latter that is responsible for the failure
of TDDFT to accurately reproduce the S<sub>1</sub> transition energy,
and that a significant thermochromic shift is also present. Finally,
we investigated the nature of the excited states by analyzing the
excitations and discussed their different fragmentation behavior.
We hypothesize that different contributions of local versus charge
transfer excitations are responsible for 1-photon versus 2-photon
fragmentation observed experimentally
Tuning Structural and Optical Properties of Thiolate-Protected Silver Clusters by Formation of a Silver Core with Confined Electrons
We
present a systematic theoretical investigation of the structural and
optical properties of thiolate-protected silver clusters with the
goal to design species exhibiting strong absorption and fluorescence
in the UV–vis spectral range. We show that the optical properties
can be tuned by creating systems with different counts of confined
electrons within the cluster core. We consider liganded silver complexes
with <i>n</i> silver atoms (Ag<sub><i>n</i></sub>) and <i>x</i> ligands (L<sub><i>x</i></sub>)
in anionic complexes [Ag<sub><i>n</i></sub>L<sub><i>x</i></sub>]<sup>−</sup> with L = SCH<sub>3</sub>. Variation
of the composition ratio gives rise to systems with (i) zero confined
electrons for <i>x</i> = <i>n</i> + 1, (ii) two
confined electrons for <i>x</i> = <i>n</i> –
1, and (iii) four confined electrons for <i>x</i> = <i>n</i> – 3. We show that the number of confined electrons
within the cluster core and the geometric structure of the latter
are responsible for the spectral patterns, giving rise to intense
absorption transitions and fluorescence in the visible or even infrared
range. Our results open a perspective for the rational design of stable
ligand-protected silver cluster chromophores that might find numerous
applications in the field of biosensing