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Accepted Manuscript
Title: Role of tert-butyl in the linear and nonlinear optical
property of push-pull chromophores
Authors: Xiao-Chun Chi, Ran Lu, Yu-Gao, Ying-Hui Wang,
Shang-Hang Zhou, Ning Sui, Wen-Yan Wang, Mou-Cui Ni,
Yan-Qiang Yang, Han-Zhuang Zhang
PII:
DOI:
Reference:
S1010-6030(17)31009-2
https://doi.org/10.1016/j.jphotochem.2017.10.036
JPC 10961
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
12-7-2017
9-10-2017
21-10-2017
Please cite this article as: Xiao-Chun Chi, Ran Lu, Yu-Gao, Ying-Hui Wang,
Shang-Hang Zhou, Ning Sui, Wen-Yan Wang, Mou-Cui Ni, Yan-Qiang Yang, HanZhuang Zhang, Role of tert-butyl in the linear and nonlinear optical property
of push-pull chromophores, Journal of Photochemistry and Photobiology A:
Chemistry https://doi.org/10.1016/j.jphotochem.2017.10.036
This is a PDF file of an unedited manuscript that has been accepted for publication.
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Role of tert-butyl in the linear and nonlinear optical
property of push-pull Chromophores
Xiao-Chun Chi1a, Ran Lu2a, Yu-Gao2, 4, Ying-Hui Wang1*, Shang-Hang Zhou1,
Ning Sui1, Wen-Yan Wang1, Mou-Cui Ni1, Yan-Qiang Yang3 and Han-Zhuang
Zhang1*
1
Femtosecond laser Laboratory, Key Laboratory of Physics and Technology for Advanced
Batteries, College of Physics, Jilin University, Changchun, 130012, P. R. China
2 College
3
of Chemistry, Jilin University, Changchun 130012, P. R. China.
National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics,
China Academy of Engineering Physics, Mianyang 621900, Sichuan, China.
4
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun,
130012, P. R. China.
a
Xiao-Chun Chi and Ran Lu contributed equally to this work.
*Corresponding authors at: Femtosecond laser Laboratory, Key Laboratory of Physics and
Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin
University, Changchun 130012, P. R. China.
Email
addresses:
wangyinghuijlu@outlook.com
hzzhang@jlu.edu.cn (Han-zhuang Zhang).
Tel: +86-431-85167378; Fax: +86-431-85166112
Graphical abstract
(Ying-Hui
Wang)
and
Highlights




We discussed the role of tert-butyl in optical property of pull-push chromophores.
The tert-butyl unit accelerate the flurorescence relaxation rate of chromophores.
The tert-butyl unit accelerate the dynamic rate of ICT process.
The tert-butyl unit affect the two-photon optical property of chromophores.
Abstract
Through comparing the linear and nonlinear optical property of two pull-push
chromophores, which are composed of difluoroboron β-diketonate functionalized with
carbazole and tert-butyl carbazole, the role of tert-butyl in chromosphere has been
discussed in detail. The tert-butyl unit would lead to the absorption and fluorescence
peak red shift and accelerate the fluorescence relaxation rate of chromophores.
Moreover, the tert-butyl unit would affect obviously the two-photon optical property
of chromophore according to the Z-scan test. The transient absorption (TA) data
shows that these two pull-push chromopphores both own intramolecular charge
transfer (ICT) characteristics and the tert-butyl unit accelerate the dynamic rate of ICT
generation and relaxation process, which may be responsible for the variance of
fluorescence property.
Keywords: push–pull chromophores, ICT state, transient absorption, two-photon
Introduction
Organic semiconducting oligomers have exhibited a huge potential in the field of
optoelectronic field, involving light emitting diodes [1], field-effect transistors [2],
thin film transistors [3], polarity probes [4], nonlinear optics [5] and solar cells [6],
and been considered as the power alternatives to the inorganic counterparts. In
addition, people have confirmed that the oligomers composed of electron-donating (D)
and electron-accepting (A) units through π-conjugated linker (with dipole character)
own excellent optical property [7-13]. Other units like tert-butyl unit and n, n'-diethyl
(Net2) group sometimes are introduced on the chromophores so as to increase the
molecular solubility or adjust the space structure of oligomer after photexcitation
[14-15]. In addition, the number of substitution influence the inter-systerm crossing
probability through changing energy gap between the first excited singlet state (S 1)
and the lowest-lying triplet excited states (T1) [16]. Similarity, new difluoroboron
β-diketonate complexes functionalized with carbazole and tert-butyl carbazole have
been synthesized by Lu and coworkers [17] and named CBM and TCBM,
respectively, as shown in Fig. 1(A). Through linking the tert-butyl unit, the optical
properties of pull-push compound would change apparently. However, the
corresponding mechanism still remains unclear.
In this paper, we compare the optical properties of the two chromophores as
mentioned above through multi-spectroscopy techniques. We clarify the single-photon
and two-photon optical property of compounds and the corresponding ultrafast
photo-physical relaxation process, so as to discuss the role of tert-butyl unit in the
optical property of chromophore.
2. Experimental
The compounds of CBM and TCBM were obtained from Lu’s group in college
of chemistry in Jilin University. The two compounds were dissolved in CH2Cl2
solution with concentrations of 510-4 M for optical measurements. The samples were
placed in 1 mm and 2 mm thick quartz cuvette for linear and nonlinear optical
measurements, respectively.
Ground state absorption measurements were carried out in UV-Vis absorption
spectrometer (Purkinje, TU-1810PC). Fluorescence measurements were made with a
fiber optic spectrometer (Ocean Optics, USB4000) with an excitation wavelength of
400 nm. The femtosecond transient absorption (TA) technique is reported in previous
work [18]. The excitation spot was about 2.0 mm in diameter. The excitation
wavelength was 400 nm. The TA spectrum were carried out by a spectrometer
(AvaSpec-2048×16). The fluorescence dynamics are detected by time-correlated
single-photon counting (TCSPC) technique [19], [20]. All quantum chemistry
calculations were done with Gaussian 09 Software [21]. The ground-state geometries
were optimized with density functional theory (DFT) [22] at B3LYP/6-311G(d, p)
level [23]. All the measurements were performed at room temperature.
3. Results and discussion
F F
B
O
F F
B
O
N
N
TCBM
1.0
(B)
1.0
0.8
CBMabs
0.8
0.6
TCBMabs
0.6
0.4
CBMPL
0.4
TCBMPL
0.2
0.2
0.0
300
400
500
600
700
800
900
0.0
1000
Normalized PL/a.u.
Normalized abs/a.u.
CBM
Normalized Intensity/a.u.
(A)
O
O
/nm
1.0
CBM
TCBM
0.8
(C)
0.6
0.4
0.2
0.0
-5
0
1
10
Time/ns
Fig. 1(A) Molecular structure of CBM and TCBM; (B) Normalized ground state absorption and
steady-state fluorescence spectra of CBM (black) and TCBM (red). (C) Normalized fluorescence
decay curve of CBM (black) and TCBM (red).
Fig. 1(B) exhibits that both CBM and TCBM show two absorption bands and
one fluorescence band in visible-near UV region. After the tert-butyl unit is linked on
the chromophore, it is found that the absorption peak in the high-frequency region
shift from 320 (CBM) to 323 nm (TCBM) and the main absorption peak shifts from
412 (CBM) to 432 nm (TCBM). Meanwhile, the fluorescence peak shifts from 597 to
643 nm. Apparently, the main absorption peak and the fluorescence peak are both
much sensitive to the tert-butyl unit. And then, the fluorescence decay curve of CBM
and TCBM are given in Fig. 1(C), which are fitted with mono-exponential function.
The fitted results show that the fluorescence lifetime of CBM is 1.73 ns and that of
TCBM is 0.30 ns, indicating that the tert-butyl unit would apparently accelerate the
fluorescence relaxation rate of compounds.
0.04
0.02
(A)
PIA2
0.3 ps
PIA1
0.3 ps
(C)
0.03
1 ps
0.6 ps
0.02
20 ps
14 ps
0.01
0.00
0.00
-0.01
SE2
-0.02
SE1
-0.04
0.02
-0.03
(D)
(B)
20 ps
14 ps
1850 ps
1916 ps
0.01
T/T
T/T
-0.02
0.02
0.01
0.00
0.00
-0.01
-0.01
-0.02
-0.02
400
500
600
/nm
700
400
500
600
700
/nm
Fig. 2: Transient absorption spectra of CBM (A, B) and TCBM (C, D) in CH2Cl2. The black
arrows represent the evolution of photo-induced absorption peak and stimulated emission peak.
In order to further investigate the role of tert-butyl unit on the photo-excitation
relaxation mechanism of compounds, the transient absorption is employed to probe
the ultrafast photo-excitation process of CBM and TCBM. Fig. 2 exhibits the
time-dependent transient absorption (TA) spectra of CBM (A and B) and TCBM (C
and D). As shown in Fig. 2(A), two negative peaks around 540 and 610 nm are
attributed to the stimulated emission (SE), since they are overlapped with the steady
emission spectrum of CBM. The negative bands in short wavelength region are noted
as SE1, and that in long wavelength region are noted as SE2. The positive bands
located at about 425 nm and 500 nm are assigned to the photo-induced absorption
(PIA). The positive bands in short wavelength region and long wavelength region are
labeled PIA1 and PIA2, respectively. After photo-excitation, the amplitude of PIA1
gradually enhance with delay time, which reaches the maximum together with the
signal of SE2, suggesting that the PIA1 and SE2 should correspond to the same new
transient species after photo-excitation. Meanwhile, the intensity of SE1 and the PIA2
both weaken with delay time. The PIA2 is overlapped with SE2, therefore the spectral
feature at 500 nm gradually change from the positive region to the negative region
after 1 ps time delay. After 20 ps, there are only PIA1 and SE2 in the TA spectra and
both of them gradually weaken with delay time. The evolution of TA spectra of
TCBM is also given in Fig. 2(C) and (D). The time-dependent spectral feature and the
evolution of TCBM is much similar to those of CBM. This can be ascribed to similar
molecular structure. In comparison with CBM, the spectral feature around 500 nm of
TCBM quickly change from the positive region to the negative region after 0.6 ps.
According to Fig.2, we believed that the tert-butyl unit is able to affect the relaxation
rate of each channel in the relaxation process.
To further clarify the dynamic relaxation mechanism of compound and make
sure the role of tert-butyl unit, the kinetics recorded at significant wavelength of CBM
and TCBM are given in Fig. 3(A) and (B), and the corresponding relaxation
mechanism is summarized in Fig. 3(C). The initial relaxation lifetime (τv) recorded at
495 and 630 nm is much similar to each other in CBM and is estimated to be about
1.0 ps, which should be assigned to the vibration relaxation from the initial high
energy excited state to the low energy excited state according to their ultrafast
relaxation lifetime. In addition, the dynamic component with lifetime (τICT) of 9.1 ~
9.4 ps appears in the TA curve at 425 nm and 558 nm is attributed to the
intramolecular charge transfer (ICT) process, owing to the molecular structure of
pull-push chromophore [24], [25]. Moreover, the transition from low highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) which is
mentioned in Fig. S1 also confirms our speculation.
425 nm
495 nm
0.04
(A)
0.02
T/T
558 nm
0.00
-0.02
-0.04
630 nm
T/T
0.02
(B)
440 nm
505 nm
0.00
585 nm
656 nm
-0.02
0
5
10
100
1000
Delay time/ps
S1
(C)
ICT
kICT
kV
kr
S0
Normalized T/T
0.02
(D)
0.00
-0.02
SEICT of CBM
SEICT of TCBM
-0.04
-2 0 2 4 6 8 10
500
1000
1500
2000
Delay time/ps
Fig. 3: Kinetics recorded at significant wavelength of CBM (A) and TCBM (B). The relaxation
mechanism of compounds (C) and the kinetic curves at SE peak of CBM and TCBM (D). k
represents the relaxation rate.
The dynamic component with lifetime of 9.1 ps can be also observed at 495 and
630 nm, suggesting that the population of ICT state is originated from the initial
excited state (S1). During the ICT process, the population in ICT state gradually
increases and the intensity of fluorescence from this intermediate state also enhance at
the same time. When the ICT process finished, the intensity of fluorescence from ICT
state begins to weaken. The relaxation of TCBM is similar to that of CBM, but the
corresponding generation lifetime of ICT state in TCBM is about 6.7 ~ 6.8 ps.
Moreover, relaxation lifetime of ICT state in CBM and TCBM is 865 ~ 970 ps and
247 ~ 305 ps, respectively. The tert-butyl unit is responsible for the difference
between relaxation rates of CBM and TCBM, even though the dynamic behavior at
different wavelength of TCBM are all similar to those of CBM, as seen Table 1.
Table 1: Ultrafast dynamic relaxation fitting parameters for the significant wavelength. All the
kinetic curves are fitted with three-exponential function. τv is the lifetime of vibration relaxation.
τICT is the lifetime of rising or decreasing population which is corresponding to the intramolecular
charge transfer progress. τr is the relaxation lifetime of intramolecular charge transfer state.
Sample
CBM
TCBM
λ/nm
τv /ps
τICT/ps
τr /ps
425
1.3
9.1 
872
495
1.0
9.2 
974
558
1.4
9.4 
865
630
1.0
9.1 
940
440
1.0
6.7 
247
505
1.2
6.2 
305
585
1.0
6.7 
249
656
1.1
6.3 
288
In order to further discuss the difference of fluorescence behavior, the SE curve
of CBM (at 558 nm) and TCBM (at 585 nm) are given in Fig. 3(D), showing that the
relaxation rate of TCBM is rapider than that of CBM, which would be responsible for
the difference of fluorescence curves between CBM and TCBM.
After comparing the linear optical property of the two compounds, we also test
their nonlinear optical property. We found that CBM would emit fluorescence when it
is excited by 800 nm femtosecond laser. Therefore, the excitation intensity-dependent
fluorescence spectra of CBM are given in Fig. 4(A), meanwhile the integrated
fluorescence intensity vs the square of excitation intensity is also given in inset of Fig.
4(A).The integrated fluorescence intensity linearly increases with the square of
excitation intensity, suggesting that the fluorescence of CBM excited 800 nm
femtosecond laser is assigned to two-photon fluorescence. Therefore, the two-photon
absorption cross section of CBM are detected by Z-scan technique. The nonlinear
absorption coefficient (β) and the two-photon cross absorption section (σ) can be
calculated using the same method as described in our previous work [26]. The
obtained β and σ values of CBM is 6.710−2 cm∙GW−1 and 5824 GM, respectively.
Unfortunately, there is not signal of Z-Scan, when TCBM is excited by 800 nm
femtosecond laser. This variance of nonlinear optical property should be assigned to
the tert-butyl unit.
5
3
IPLI/10
3
PL intensity/10
1.6 (A)
1.4
0.470 J
0.904 J
1.2
1.104 J
1.0
1.466 J
1.807 J
0.8
0.6
0.4
0.2
0.0
400
500
2
1
0
0
1
2
3
2
2
(Pump Intensity) /J
600
700
800
900
1000
Normalized Transmittance/a.u.
/nm
1.05
(B)
1.00
0.95
0.90
0.85
0.80
CBM
0.75
-5
-4
-3
-2
-1
0
1
2
3
4
5
Z/mm
Fig. 4: (A) two-photon fluorscence of CBM; (B) the Z-scan curve of CBM. Insert: two-photon
integrated fluorescence intensity as a function of square of excitation intensity.
Fig. 5: The optimization structure of CBM in the ground state (A) and the excited state (B).
Fig. 5 shows the structure of CBM in the ground state (A) and the excited state
(B) which are calculated using Gaussian 09. HOMO and LUMO of CBM and TCBM
are provided in Fig. S1. The CBM would twist in excited state. The dihedral angle
between carbazole and difluoroboron β-diketonate (C11-N21-C22-C24) in excited
state (90.04°) is bigger than that in ground state (49.71°). Therefore, it would be
reasonable speculated that the tert-butyl unit linked on carbazole unit would affect the
transition dipole through influencing the twist behavior of TCBM originated from the
space steric effect. Therefore, the linear and nonlinear optical property of CBM and
TCBM would have a huge difference.
4. Conclusions
The linear and nonlinear optical properties of CBM and TCBM has been
compared in detail, which confirms that the introduction of tert-butyl unit would lead
to redshift of spectral feature, accelerate the fluorescence relaxation rate and weaken
the two-photon optical property of compound. The TA data confirms that the ICT state
exist in the photo-excitation relaxation process and the variance of fluorescence
relaxation trace is originated from the change of fluorescence relaxation rate of ICT.
These variance should be assigned to the space steric effect of tert-butyl unit, which
may affect the twist of molecular structure after photo-excitation. These results are
beneficial for people to understand the optical properties of push-pull oligomers and
further design the novel oligomers.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(No. 21573094, 11274142, 51502109, 11674315, and 11474131), the National Found
for Fostering Talents of Basic Science (No. J1103202), Science Challenging Program
(JCKY2016212A501). The science and technology projects in the 13th Five-Year
Plan in The Education Department of Jilin Province, No. 2016-402.
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