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Y. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 309 (2015) 1–7
In this paper, the ultrafast excited state dynamics and photo-
isomerization of trans-4-DEAAB, with the * excitation is
1024 ꢂ 256 pixel array) equipped with a spectrometer (Princeton,
p
!
p
SpectraPro 2500i) was used to collect the probe and reference
beams. Each of the recorded spectra is accumulated on the CCD for
typically 10000 laser shorts. The instrumental response of the
system was determined to be ꢀ0.17 ps by cross-correlation
measurements between the pump and probe pulses. The cross-
correlation measurements were also used to determine the precise
zero time-delay at each probe wavelength. In order to eliminate
any persistent offset in the optical density at long delay times, a
time constant of 10,000 ps was included. The time evolutions of
transient absorption data of trans-4-DEAAB for all three solvents
were fitted by global fit analysis with singular value decomposition
(SVD).
investigated in ethanol, acetone and ethylene glycol by femtosec-
ond transient absorption spectroscopy combined with quantum
chemical calculations. The spectra are measured until the delay
time up to 300 ps to obtain more complete information on the
molecular dynamics. The quantum chemical calculations are used
to present the energetic ordering, vertical excitation energies,
oscillator strength and static absorption spectra of the cis and trans
isomer. Following the excitation at 400 nm, ultrafast excited-state
dynamics associated with the photoisomerization of trans-4-
DEAAB are observed and analyzed in detail.
2. Experimental and computational details
All quantum chemical calculations were carried out with the
Gaussian 09 software package [26]. The molecular geometries in
the electronic ground state of both trans and cis-4-DEAAB were
optimized using ab initio density-functional theory with the B3LYP
functional and the 6-311G++(d,p) basis set. All the minima are
verified by vibrational frequency analysis at the same level of
theory. The polarizable continuum model (PCM) was used to
incorporate the bulk solvent effect [27]. The vertical transition
energies, oscillator strengths and absorption spectroscopy were
performed by the TD-DFT/B3LYP with the basis set of 6-31G in
ethanol and acetone and 6-311G in ethylene glycol, respectively.
Trans-4-DEAAB (98% purity) was purchased from Alfa Aesar,
and used for experiments without further purification. Ethanol,
acetone and ethylene glycol purchasing from Aladdin (99% purity)
were used as solvents. All of them are polar solvents with
viscosities of 1.2, 0.32 and 13.5 mPa s at 20 ꢁC, respectively [23,24].
The concentration of trans-4-DEAAB in each solvent was 1 mM at
room temperature and a fresh sample was prepared for each
experiment. The static absorption spectra were conducted in a
1-mm quartz cell by the UV–vis spectrometer (INESA, L6) and
normalized in all three solvents.
The apparatus used for ultrafast transient absorption measure-
ments is based on a regeneratively amplified femtosecond laser
system which has been already described in detail earlier [25].
Briefly, the 800 nm seed beam with repetition rate of 78 MHz is
generated by a commercial Ti:sapphire oscillator. The seed beam is
brought into the regenerative amplifier and the output used as
fundamental pulse was set at repetition rate of 1 kHz, center
wavelength of 800 nm, duration of 35 fs and the energy up to
1 mJ/pulse. The second harmonic generation of the fundamental
pulse (400 nm) was generated by a 0.5 mm thick BBO crystal as the
excitation light. The excitation intensity was set to less than
3. Results and discussion
3.1. Quantum chemical calculations
Table 1 shows the orbital transitions, configuration-interaction
(CI) coefficients, vertical excitation energies, and oscillator
strengths of the two lowest excited singlet states of trans-4-
DEAAB in ethanol, acetone and ethylene glycol, respectively. For
trans-4-DEAAB in ethanol, the first transition is from HOMO-1 to
LUMO and the oscillator strength is zero, while the second
transition from HOMO to LUMO is much more intense. The
assignments of molecular orbitals are done by visual inspection.
4 mJ/pulse for measurements. A portion of the fundamental pulse
was focused into a 1 mm sapphire to generate a white continuum
(450–690 nm). In order to correct the pulse-to-pulse intensity
fluctuations and improve the measuring sensitivity, the white light
was split into the probe and the reference beam by a metallic-
coated beamsplitter. The polarization angle between the pump and
probe beam is set at the magic angle (54.7ꢁ) for eliminating
polarization and photoselection effects. The sample was contained
in a flow cell with 0.2 mm quartz windows and 1 mm optical path
length. The pump and probe pulses with an intersection angle of
ꢀ4ꢁ overlapped in the sample spatially, and the reference beam
was focused on the sample at a different spot. The probe pulse was
temporally delayed with respect to the pump pulse through a
computer controlled translation stage. A CCD camera (PI-MAX,
The HOMO-1, HOMO and LUMO are assigned to n,
orbitals, respectively, as shown in Fig. 1. Evaluation of molecular
orbitals reveals that the first transition is of 99% n ! * character
from the CI coefficients while the second transition is of 100%
* character. In acetone and ethylene glycol, the calculated
results are the same as that in ethanol. It clearly demonstrates that
the S1 state is optically-forbidden with n * character while the S2
p and p*
p
p
!
p
p
state is optically-allowed with pp* character.
3.2. Static absorption spectrum
The static absorption spectra of trans-4-DEAAB in ethanol,
acetone and ethylene glycol were measured and shown in Fig. 2(a),
Table 1
Orbital transition, configuration-interaction (CI) coefficients, vertical excitation energy, and oscillator strength of the two lowest excited singlet states of trans-4-DEAAB in
ethanol, acetone and ethylene glycol.
State
Orbital transition
CI coefficients
Vertical excitation/eV
Oscillator strengths
Ethanol
S1
S2
HOMO-1 ! LUMO
HOMO ! LUMO
0.70531
0.70709
2.51
2.97
0.0000
1.1016
Acetone
S1
S2
HOMO-1 ! LUMO
HOMO ! LUMO
0.70531
0.70708
2.51
2.97
0.0000
1.1003
Ethylene glycol
S1
S2
HOMO-1 ! LUMO
HOMO ! LUMO
0.70500
0.70662
2.57
2.90
0.0000
1.1142