5 research outputs found
Excited-State Proton Transfer in 3‑Cyano-7-azaindole: From Aqueous Solution to Ice
We
investigated the excited-state proton transfer (ESPT) reaction
for 3-cyano-7-azaindole (<b>3CAI</b>) in aqueous solution and
in ice. <b>3CAI</b> undergoes water-catalyzed ESPT in the aqueous
solution, giving normal (355 nm) and proton transfer tautomer (∼472
nm) emission bands. Detailed temperature-dependent studies showed
that the values of activation free energy (Δ<i>G</i><sup>‡</sup>) were similar between N–H and N–D
isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism
incorporating solvation equilibrium (<i>K</i><sub>eq</sub>) to form a 1:1 (molar ratio) water:<b>3CAI</b> cyclic hydrogen-bonded
complex as an intermediate, followed by perhaps proton tunneling reaction.
In sharp contrast, <b>3CAI</b> in ice undergoes entirely different
photophysical properties, in which <b>3CAI</b> self-organizes
to form a double-hydrogen-bonded dimers at the grain boundary of the
polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state
double proton transfer reaction, giving rise to solely a tautomer
emission (∼450 nm). The distinct difference in ESPT properties
between water and ice makes azaindoles feasible for the investigation
of water–ice interface property
Revisiting Dual Intramolecular Charge-Transfer Fluorescence of Phenothiazine-triphenyltriazine Derivatives
The
photophysical properties of the phenothiazine-triphenyltriazine
derivative, PTZ-TRZ, are reinvestigated. The results, in combination
with the computational approaches, lead us to draw the conclusion
that the complicated excitation behavior in toluene (ref ), in part, is due to the
UV absorption cutoff region for toluene where the <315 nm excitation
is greatly distorted by solvent absorption, i.e., the inner filter
effect, in a regular sample cuvette (1.0 cm path length). Switching
the solvent to cyclohexane with the UV cutoff wavelength at 235 nm
simplifies the results. In cyclohexane, two isomers exist for PTZ-TRZ
in the ground state and quasi-axial and quasi-equatorial conformers.
Upon electronic excitation, both quasi-axial and quasi-equatorial
conformers undergo structural relaxation to an energy minimum state
where the phenothiazine is in a planar configuration
Revisiting Dual Intramolecular Charge-Transfer Fluorescence of Phenothiazine-triphenyltriazine Derivatives
The
photophysical properties of the phenothiazine-triphenyltriazine
derivative, PTZ-TRZ, are reinvestigated. The results, in combination
with the computational approaches, lead us to draw the conclusion
that the complicated excitation behavior in toluene (ref ), in part, is due to the
UV absorption cutoff region for toluene where the <315 nm excitation
is greatly distorted by solvent absorption, i.e., the inner filter
effect, in a regular sample cuvette (1.0 cm path length). Switching
the solvent to cyclohexane with the UV cutoff wavelength at 235 nm
simplifies the results. In cyclohexane, two isomers exist for PTZ-TRZ
in the ground state and quasi-axial and quasi-equatorial conformers.
Upon electronic excitation, both quasi-axial and quasi-equatorial
conformers undergo structural relaxation to an energy minimum state
where the phenothiazine is in a planar configuration
Insight into the Amino-Type Excited-State Intramolecular Proton Transfer Cycle Using N‑Tosyl Derivatives of 2‑(2′-Aminophenyl)benzothiazole
Studies have been carried out to gain insight in to an
overall
excited-state proton transfer cycle for a series of N-tosyl derivatives
of 2-(2′-aminophenyl)Âbenzothiazole. The results indicate that
followed by ultrafast (<150 fs) excited-state intramolecular proton
transfer (ESIPT), the titled compounds undergo rotational isomerization
along the C<sub>1</sub>–C<sub>1</sub>′ bond. For the
model compound 2-(2′-tosylaminophenyl)Âbenzothiazole (PBT-NHTs)
the subsequent cis-trans isomerization process in both triplet and
ground states are probed by nanosecond transient absorption (TA) and
two-step laser-induced fluorescence (TSLIF) spectroscopy. Both TA
and TSLIF results indicate the existence of a long-lived trans-tautomer
species in the ground state with a lifetime of few microseconds. The
experimental results correlate well with the theoretical approach,
which suggests that PBT-NHTs proton transfer tautomer generated in
the excited state undergoes intramolecular C<sub>1</sub>–C<sub>1</sub>′ rotation to ∼100° between benzothiazole
and phenyl moieties in which the energetics for the S<sub>1</sub> and
T<sub>1</sub> states are nearly identical. As a result, the intersystem
crossing between S<sub>1</sub> and T<sub>1</sub> states serves as
a fast deactivation pathway for the excited-state cis-tautomer to
channel into both cis- and trans-tautomer in their respective T<sub>1</sub> states, followed by the dominant T<sub>1</sub>-S<sub>0</sub> radiationless deactivation to populate the trans-tautomer in the
ground state. The trans-tautomer species in the S<sub>0</sub> state
proceeds with intermolecular double proton transfer to regenerate
the cis-normal form. An overall proton-transfer cycle describing the
amino-type ESIPT and the subsequent isomerization processes is thus
depicted in detail
Harnessing Excited-State Intramolecular Proton-Transfer Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules
A series of new amino (NH)-type hydrogen-bonding
(H-bonding) compounds
comprising 2-(2′-aminophenyl)Âbenzothiazole and its extensive
derivatives were designed and synthesized. Unlike in the hydroxyl
(OH)-type H-bonding systems, one of the amino hydrogens can be replaced
with electron-donating/withdrawing groups. This, together with a versatile
capability for modifying the parent moiety, makes feasible the comprehensive
spectroscopy and dynamics studies of amino-type excited-state intramolecular
proton transfer (ESIPT), which was previously inaccessible in the
hydroxyl-type ESIPT systems. Empirical correlations were observed
among the hydrogen-bonding strength (the N–H bond distances
and proton acidity), ESIPT kinetics, and thermodynamics, demonstrating
a trend that the stronger N–H···N hydrogen bond
leads to a faster ESIPT, as experimentally observed, and a more exergonic
reaction thermodynamics. Accordingly, ESIPT reaction can be harnessed
for the first time from a highly endergonic type (i.e., prohibition)
toward equilibrium with a measurable ESIPT rate and then to the highly
exergonic, ultrafast ESIPT reaction within the same series of amino-type
intramolecular H-bond system