We report on the
motional and proton transfer dynamics of the super
photobase FR0-SB in the series of normal alcohols C1 (methanol) through
C8 (n-octanol) and ethylene glycol. Steady-state
and time-resolved fluorescence data reveal that the proton abstraction
dynamics of excited FR0-SB depend on the identity of the solvent and
that the transfer of the proton from solvent to FR0-SB*, forming FR0-HSB+*, fundamentally alters the nature of interactions between
the excited molecule and its surroundings. In its unprotonated state,
solvent interactions with FR0-SB* are consistent with slip limit behavior,
and in its protonated form, intermolecular interactions are consistent
with a much stronger interaction of FR0-HSB+* with the
deprotonated solvent RO–. We understand the excited-state
population dynamics in the context of a kinetic model involving a
transition state wherein FR0-HSB+* is still bound to the
negatively charged alkoxide, prior to solvation of the two charged
species. Data acquired in ethylene glycol confirm the hypothesis that
the rotational diffusion dynamics of FR0-SB* are largely mediated
by solvent viscosity while proton transfer dynamics are mediated by
the lifetime of the transition state. Taken collectively, our results
demonstrate that FR0-SB* extracts solvent protons efficiently and
in a predictable manner, consistent with a ca. 3-fold increase in
dipole moment upon photoexcitation as determined by ab initio calculations based on the equation-of-motion coupled-cluster theory.
Two-photon excitation is an attractive means for controlling chemistry in both space and time. Isoenergetic one-and two-photon excitations (OPE and TPE) in non-centrosymmetric molecules are often assumed to reach the same excited state and, hence, to produce similar excitedstate reactivity. We compare the solvent-to-solute excited-state proton transfer of the super photobase FR0-SB following isoenergetic OPE and TPE. We find up to 62 % increased reactivity following TPE compared to OPE. From steady-state spectroscopy, we rule out the involvement of different excited states and find that OPE and TPE spectra are identical in non-polar solvents but not in polar ones. We propose that differences in the matrix elements that contribute to the twophoton absorption cross sections lead to the observed enhanced isoenergetic reactivity, consistent with the predictions of our high-level coupled-cluster-based computational protocol. We find that polar solvent configurations favor greater dipole moment change between ground and excited states, which enters the probability for two-photon excitations as the absolute value squared. This, in turn, causes a difference in the Franck-Condon region reached via TPE compared to OPE. We conclude that a new method has been found for controlling chemical reactivity via the matrix elements that affect two-photon cross sections, which may be of great utility for spatial and temporal precision chemistry.
We
report on the changes in the dual fluorescence of two cyanine
dyes IR144 and IR140 as a function of viscosity and probe their internal
conversion dynamics from S2 to S1 via their
dependence on a femtosecond laser pulse chirp. Steady-state and time-resolved
measurements performed in methanol, ethanol, propanol, ethylene glycol,
and glycerol solutions are presented. Quantum calculations reveal
the presence of three excited states responsible for the experimental
observations. Above the first excited state, we find an excited state,
which we designate as S1′, that relaxes to the S1 minimum, and we find that the S2 state has two
stable configurations. Chirp-dependence measurements, aided by numerical
simulations, reveal how internal conversion from S2 to
S1 depends on solvent viscosity and pulse duration. By
combining solvent viscosity, transform-limited pulses, and chirped
pulses, we obtain an overall change in the S2/S1 population ratio of a factor of 86 and 55 for IR144 and IR140, respectively.
The increase in the S2/S1 ratio is explained
by a two-photon transition to a higher excited state. The ability
to maximize the population of higher excited states by delaying or
bypassing nonradiative relaxation may lead to the increased efficiency
of photochemical processes.
The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0-SB and a series...
Two-photon
excitation (TPE) proceeds via a “virtual”
pathway, which depends on the accessibility of one or more intermediate
states, and, in the case of non-centrosymmetric molecules, an additional
“dipole” pathway involving the off-resonance dipole-allowed
one-photon transitions and the change in the permanent dipole moment
between the initial and final states. Here, we control the quantum
interference between these two optical excitation pathways by using
phase-shaped femtosecond laser pulses. We find enhancements by a factor
of up to 1.75 in the two-photon-excited fluorescence of the photobase FR0-SB in methanol after taking into account the longer pulse
duration of the shaped laser pulses. Simulations taking into account
the different responses of the virtual and dipole pathways to external
fields and the effect of pulse shaping on two-photon transitions are
found to be in good agreement with our experimental measurements.
The observed quantum control of TPE in the condensed phase may lead
to an enhanced signal at a lower intensity in two-photon microscopy,
multiphoton-excited photoreagents, and novel spectroscopic techniques
that are sensitive to the magnitude of the contributions from the
virtual and dipole pathways to multiphoton excitations.
Cyanine molecules are important phototheranostic compounds given their high fluorescence yield in the near-infrared region of the spectrum. We report on the frequency and time-resolved spectroscopy of the S 2 state of IR806, which demonstrates enhanced emission upon binding to the hydrophobic pocket of human serum albumin (HSA). From excitation−emission matrix spectra and electronic structure calculations, we identify the emission as one associated with a state having the polymethine chain twisted out of plane by 103°. In addition, we find that this configuration is significantly stabilized as the concentration of HSA increases. Spectroscopic changes associated with the S 1 and S 2 states of IR806 as a function of HSA concentration, as well as anisotropy measurements, confirm the formation of HSA dimers at concentrations greater than 10 μM. These findings imply that the longer-lived S 2 state configuration can lead to more efficient phototherapy agents, and cyanine S 2 spectroscopy may be a useful tool to determine the oligomerization state of HSA.
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