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¡¡Our research
mainly focuses on photophysical and photochemical
reactions, especially about proton-transfer and
solvent relaxation dynamics. A typical case we
have studied is the early stage of excited state
intramolecular proton transfer (ESIPT) tautomerism
in 10-hydroxybenzoquinoline (HBQ) via femtosecond
fluorescence upconversion to get the temporal spectral
evolution spectrum. The temporal spectral evolution
at the time domain of zero to a few hundred femtoseconds
further resolves two distinct emission bands ascribed
a proton transfer tautomer S2'(570 nm) and S1'(630
nm) ? S0'transition (prime indicates the keto tautomer
form).
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¡¡The temporal
spectral evolution of HBQ in CH3CN acquired
at a delay time of (A) ¡õ: 125 fs, ¡ð: 187.5
fs, ¡÷: 250 fs, ¨Œ: 312.5 fs and ¡ó: 375 fs. (B)
¡õ:: 1 ps, and ¡ð: 3 ps. ¡÷: 5 ps, ¨Œ: 8 ps, ¡ó:
15 ps and ¡î: 20 ps The thick solid lines express
the steady-state fluorescence.
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¡¡ A schematic
diagram using a four-state approach (E*,
, and ) to depict the mechanism of ESIPT
coupled internal conversion and vibrational
relaxation dynamics in HBQ.
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| The Fluoresence Upconversion
System |
¡¡ A Ti:sapphire
laser (Spectra Physics) at 750-840 nm was
used to produce second harmonics (SH) at
375-420 nm. The resulting fluorescence
and the optical delayed remaining fundamental
pulses were collected and focused on a
BBO type-I crystal (0.5 mm) for the sum-frequency
generation. The up-converted signal was
then separated by an F/4.9 (f = 380 mm)
single monochromator (CDP2022) and detected
via a photon counting PMT (R1527P, Hamamatsu).
The cross correlation between SH and the
fundamental had a full width at half-maximum
(fwhm) of ~ 250 fs, which was chosen as
a response function of the system. For
both pico- and femtosecond time-resolved
measurements, a Berek's variable waveplate
was placed in the pump beam path to ensure
that the polarization of the pump laser
was set at the magic angle (54.7¡ã) with
respect to that of the probe laser (or
detecting system) to eliminate the fluorescence
anisotropy.
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Femtosecond
Transient Absorption
¡¡
The system consists of a femtosecond
Ti-sapphire oscillator (seed beam) coupled
to a regenerative amplifier that generates
a ~130 fs, <1 mJ light pulse (1 kHz) and laser
range is 750 ~ 840 nm which then go into Excipro
(CDP, transient absorption instrument). First,
the laser passes through a 1.0 mm BBO crystal
(type I) to produce SH. The SH is separated
from the fundamental by a dichroic mirror and
then passed through a chopper and computer-controlled
delay stage to excite the samples. The remaining
fundamental is focused to a 1-cm water to generate
a white light continuum for the probe pulse.
Typically, the energy of the excitation pulse
at the sample is <1.0 ¦ÌJ. The white light are
divided to two beams. One is probe beam, the
other is reference beam. The probe beam and
the SH are focused at the same point while
the reference is focused at another point of
the sample. The arrival time of SH can be controlled
by the length of its pathway. So that the absorbance
difference of the probe white beam can be detected.
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