Novel carbazole derivatives with quinoline ring: Synthesis, electronic transition, and two-photon absorption three-dimensional optical data storage

Article abstract of DOI:10.1016/j.saa.2014.10.122

We designed carbazole unit with an extended π conjugation by employing Vilsmeier formylation reaction and Knoevenagel condensation to facilitate the functional groups of quinoline from 3- or 3,6-position of carbazole. Two compounds doped with poly(methyl methacrylate) (PMMA) films were prepared. To explore the electronic transition properties of these compounds, one-photon absorption properties were experimentally measured and theoretically calculated by using the time-dependent density functional theory. We surveyed these films by using an 800 nm Ti:sapphire 120-fs laser with two-photon absorption (TPA) fluorescence emission properties and TPA coefficients to obtain the TPA cross sections. A three-dimensional optical data storage experiment was conducted by using a TPA photoreaction with an 800 nm-fs laser on the film to obtain a seven-layer optical data storage. The experiment proves that these carbazole derivatives are well suited for two-photon 3D optical storage, thus laying the foundation for the research of multilayer high-density and ultra-high-density optical information storage materials.

Full text of DOI:10.1016/j.saa.2014.10.122

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Novel carbazole derivatives with quinoline ring: Synthesis, electronic
transition, and two-photon absorption three-dimensional optical data
storage
c
a
Liang Li a, Ping Wang b, Yanlei Hu c, Geng Lin a, Yiqun Wu a,b, , Wenhao Huang , Quanzhong Zhao
⇑
a Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, No. 390, Qinghe Road, Jiading District, Shanghai 201800, China
b Key Lab of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, Harbin 150080, China
c Department of Precision Machinery and Precision Instrument, University of Science and Technology of China, Hefei, Anhui 230026, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Novel carbazole derivatives
containing quinoline were
synthesized.
Electronic transition of carbazole
derivatives were theoretically studied
by TD-DFT.
Two-photon absorption of
compounds was measured by 120 fs
pulse at 800 nm.
7-layer data storage was realized by
two-photon 3D optical data storage
system.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 May 2014
Received in revised form 15 October 2014
Accepted 16 October 2014
Available online 24 December 2014
We designed carbazole unit with an extended
p
conjugation by employing Vilsmeier formylation reaction
and Knoevenagel condensation to facilitate the functional groups of quinoline from 3- or 3,6-position of
carbazole. Two compounds doped with poly(methyl methacrylate) (PMMA) films were prepared. To
explore the electronic transition properties of these compounds, one-photon absorption properties were
experimentally measured and theoretically calculated by using the time-dependent density functional
theory. We surveyed these films by using an 800 nm Ti:sapphire 120-fs laser with two-photon absorption
(TPA) fluorescence emission properties and TPA coefficients to obtain the TPA cross sections. A three-
dimensional optical data storage experiment was conducted by using a TPA photoreaction with an
800 nm-fs laser on the film to obtain a seven-layer optical data storage. The experiment proves that these
carbazole derivatives are well suited for two-photon 3D optical storage, thus laying the foundation for the
research of multilayer high-density and ultra-high-density optical information storage materials.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Carbazole derivatives
Electronic transition
Time-dependent density functional theory
800 nm-fs laser
Two-photon absorption
3D optical data storage
Introduction
and longer storage longevity. To date, the development of novel
three-dimensional (3D) optical storage materials has enabled opti-
The developments in information techniques require data stor-
age devices to possess ultra-high densities, ultra-fast transfer rates,
cal data storage (ODS) devices to overcome storage density disad-
vantages and physical limits. Since the groundbreaking theoretical
work of Göppert-Mayer [1], two-photon absorption (TPA), strong
penetration, and high resolution can now be applied to multilayer
ODS that have bit densities as large as 1012 bits/cm3 in theory. This
approach attains efficient storage at significantly higher densities
than that of magnetic media. Therefore, materials with enhanced
⇑
Corresponding author at: Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, No. 390, Qinghe Road, Jiading District, Shanghai
201800, China. Tel.: +86 21 69918592/69918087; fax: +86 21 69918562.
E-mail address: yqwu@siom.ac.cn (Y. Wu).
http://dx.doi.org/10.1016/j.saa.2014.10.122
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

244
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
TPA properties have gained considerable research interest during
the past years because of their potential applications, especially
in TPA 3D data storage [2]; TPA provides a means of activating
chemical or physical processes with high spatial resolutions in 3D.
Organic materials that have potential applications in two-pho-
ton 3D data storage have attracted increasing research attention
[3–6]. According to optical storage mechanisms, the design and
synthesis of organic materials are preferred for high TPA cross sec-
tion because 3D storage essentially employs a two-photon process
to store a bit of data within a volumetric medium. In other words,
the photoresponse or photochemical effect is achieved easily in the
data of volumetric materials because of the stronger ability to cap-
ture photons on a per-unit record point, thus enhancing the speed
and capacity of two-photon 3D storage. In the past few years, TPA
3D data storage was one of the most promising techniques that
triggered a variety of 3D optical storage disk trends [7–12]. Two-
photon writing and reading have been achieved by applying the
two-photon photoreaction effect of fluorene derivative materials
[13].
trometer with tetramethylsilane (TMS) as the internal standard.
Infrared (IR) spectra were recorded by using a Perkin-Elmer Fourier
transform IR System. The spectra of the solid compounds were
obtained by using KBr pellets. Mass spectrometry (MS) was con-
ducted with a Bruker-Agilent 6890-5973 matrix-assisted laser
desorption/ionization time-of-flight mass spectrometer. Elemental
analyses were performed using a PE2400 elemental analyzer. The
melting points were determined by using an X-5 micro melting
point detector.
Synthesis
Synthesis of TPA materials
The synthetic route is outlined in Scheme 1. Carbazole was hal-
ogenated by bromobutane in an alkaline condition [19]. Formyla-
tion of 9-butyl-carbazole was achieved by using the Vilsmeier
formylation reaction from [20–22]. 3-(2-Quinolin)vinyl-9-butyl-
carbazole (4-CQ) and 3,6-bis(2-[quinolin]vinyl)-9-butyl-carbazole
(5-QCQ) were synthesized by using Knoevenagel condensation.
All synthesized compounds were identified by using 1H NMR, IR,
and MS measurements, which are in agreement with the chemical
structures shown in Scheme 1.
Some research groups have reported a number of joint theoret-
ical and experimental studies on carbazole derivatives in the past
few years [14–19]. In these research processes, larger molecular
plane structures of carbazole derivatives were pursued by linking
electronic donor (D) groups, electronic acceptor (A) groups, and
p
Synthesis of 3-(2-quinolin)vinyl-9-butyl-carbazole (4-CQ)
3-Formyl-9-butyl-carbazole (2.008 g, 8 mmol) and 2-methyl-
quinoline (1.144 g, 8 mmol) were added to 50 ml acetic anhydride.
The reaction mixture was heated to 120 °C and refluxed for 36 h.
After the reaction, the mixture was cooled to room temperature.
The reaction solution was poured into water and adjusted to neu-
tral pH with an alkaline solution. Thereafter, the mixture was
extracted with dichloromethane thrice to yield a rude sample.
The target product was purified through column chromatography
on silica gel.
conjugation chains from the 3- or 3,6-position of carbazoles to
enhance the value of TPA cross sections. However, these larger
plane molecules have some disadvantages, such as low melting
points and lack of stability. These disadvantages restrict the appli-
cation of carbazole derivatives. Designing a suitable length or plane
with better thermal stability for the molecular conjugation is
important. Furthermore, only a few papers have reported the prep-
aration of thin film materials and studied the multiple-photon
properties of carbazole derivative films. The use of two-photon
photoreaction 3D multilayer optical storage on carbazole deriva-
tive films has never been researched up to now. In the current
work, a rigid carbazole unit with a donor-rigidized residue was
designed for two novel carbazole derivatives by linking quinoline
3-(2-Quinolin)vinyl-9-butyl-carbazole (4-CQ). Yield: 48.3%, m.p.
289.3 °C to 290.6 °C. 1H NMR (CDCl3): d (ppm), 1.0 (t, 3H, n-butyl,
CH3), 1.4 (m, 2H, n-butyl, 3-CH2), 1.9 (m, 2H, n-butyl, 2-CH2), 4.3 (t,
2H, n-butyl, 1-CH2), 7.2 (t, 2H, CH@CH), 7.4 to 8.4 (m, 16H, carba-
zole); IR (KBr): 2950, 2925, 1610, 1496, 1465, 1424, 1381, 1327,
1213, 966, and 820 cmꢁ1; Anal. Calcd (%) for C27H24N2: C, 86.17;
H, 6.38; N, 7.45. Found: C, 86.33; H, 6.37; N, 7.3; LC–MS: m/z,
377.2 (376.19) [M+1].
groups. The two derivatives are asymmetric-type D-
p-D and sym-
metric-type D- -D- -D. These two compounds were synthesized
p
p
by using Knoevenagel condensation. Linear absorption, fluores-
cence emission spectra, and fluorescence quantum yields were
measured. Density functional theory (DFT) and time-dependent
DFT (TD-DFT) calculations were performed to study the singlet–
singlet electronic transition of these molecules. Particular attention
Synthesis of 3,6-bis(2-(quinolin)vinyl)-9-butyl-carbazole (5-QCQ)
3,6-Diformyl-9-butyl-carbazole (1.4 g, 5 mmol) and 2-methyl-
quinoline (4.3 g, 30 mmol) were added to 125 ml acetic anhydride.
The reaction mixture was heated to 120 °C and refluxed for 36 h.
After the reaction, the mixture was cooled to room temperature.
The reaction solution was poured into water and adjusted to neu-
tral pH with an alkaline solution. Thereafter, the solution was
extracted thrice to yield a rude sample. The target product was
purified through column chromatography on silica gel.
was given to the effect of the conjugation chain of the
p bond sys-
tem. The highest occupied molecular orbital (HOMO), lowest unoc-
cupied molecular orbital (LUMO), energy gap, and one-photon
absorption (OPA) strength were obtained and analyzed for the
OPA properties. Moreover, the thin films of the target compound
were prepared by mixing in poly(methyl methacrylate) (PMMA).
The TPA fluorescence emission spectra of the films were then mea-
sured with various input powers. The TPA coefficient b of the
molecular films were obtained to calculate the TPA cross sections
of the film by using the Z-scan technology. Finally, we used the
TPA mechanism and method, namely, the two-photon photoreac-
tion 3D ODS technology, for writing and retrieving data on the car-
bazole derivative film, which served as a TPA ODS material stored
within an 800 nm fs-pulsed laser ODS system.
3,6-Bis(2-(quinolin)vinyl)-9-butyl-carbazole (5-QCQ). Yield: 54.3%,
m.p. 221.2 °C to 223.5 °C. 1H NMR (CDCl3): d (ppm), 0.9 (t, 3H, n-
butyl, CH3), 1.4 (m, 2H, n-butyl, 3-CH2), 1.8 (m, 2H, n-butyl, 2-
CH2), 4.3 (t, 2H, n-butyl, 1-CH2), 7.2 (t, 4H, CH@CH), 7.5 to 8.4
(m, 25H, carbazole); IR (KBr): 2968, 2926, 2854, 1619, 1594,
1457, 1487, 1314, 963, 747, and 712 cmꢁ1; Anal. Calcd (%) for
C38H31N3: C, 86.17; H, 5.90; N, 7.93. Found: C, 86.35; H, 5.877; N,
7.78; LC–MS: m/z, 529.9 (529.2) [M+1].
Experimental
Instrumentation
Measurement of one-photon optical properties
Proton nuclear magnetic resonance (1H NMR) spectra were
recorded in CDCl3 by using a Bruker AVANCE-300 MHz NMR spec-
The linear absorption spectrum was measured in a PMMA film;
the pure PMMA was not included. The linear absorption spectra of

L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
245
Scheme 1. The synthetic route of the target compounds.
Fig. 1. Setup for the two-photon writing and readout.
Uv ¼ ðAs ꢂ F ꢂ n2 ꢂ UsÞ=ðA ꢂ Fs ꢂ n2Þ;
ð1Þ
the compounds were recorded by using a Perkin-Elmer Lambda
v
v
v
s
900 UV/Vis/Near IR (San Jose, California, USA) spectrophotometer.
The one-photon fluorescence spectra including the steady-state
fluorescence spectra and time-resolved emission spectra were
obtained by using a Shimadzu RF-5301 spectrophotometer at room
where A denotes the absorbance at the excitation wavelength, F is
the area under the fluorescence curve, and n is the refraction index.
Subscripts s and
yield sample, respectively. Rhodamine
= 0.9) was made as the standard.
v
refer to the standard and the unknown quantum
temperature. Fluorescence quantum yields
according to the following formula [23]:
U was calculated
B
in ethanol at 25 °C
(U

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Measurement of two-photon absorption of compounds
controlled by rotating a half-wave plate between two polarizers.
We also performed Z-scan measurements on a quartz substance
and the pure PMMA to confirm that the measured nonlinear
absorption mainly originated from the samples.
In exploring the TPA properties of the target compounds, we
used a two-photon excited fluorescence method to measure the
TPA cross section by laser. The two-photon fluorescence spectrum
of the sample film was analyzed by using a 120 fs, 800 nm pulse
Ti:sapphire laser operating at a 1 kHz repetition rate. The fluores-
cence spectra were recorded by a 16-bit charged-coupled device
(Jobin Yvon, Horiba, France) mounted at the output of the mono-
chromator. TPA coefficient of films was measured by the open-
aperture Z-scan system. The incident 800 nm lasing was provided
by the above-mentioned laser. The laser beam was divided into
two by using a beam splitter. The weaker beam, which was
detected by photodiode D1, was employed to monitor the incident
laser energy. The stronger beam was focused into the sample by
using a 20 cm focal-length lens as the exciting beam. The entire
transmitted intensities through the sample were all collected by
photodiode D2. These two photodiodes were connected with a
two-channel energy meter (Molectron EPM 2000) to record the
input and output energy simultaneously. The pump energy was
ODS system
The optical system for 3D data writing and reading within the
TPA photoreaction material is illustrated in Fig. 1. For data writing,
the excitation beam was generated by a linearly polarized Ti:sap-
phire laser oscillator pumped from an Nd:YVO4 laser (Verdi-5,
Coherent) at 532 nm, generating a train of 800 nm, 80 fs pulses
at a repetition rate of 80 MHz. The fs laser was focused into the
material through a beam expender and an objective lens. A 3D
stage driven by a piezoelectric transducer and a mechanical shutter
was controlled through
a personal computer. The objective,
dichroic mirror, pinhole, photomultiplier tube (PMT), and scanning
stage constituted the confocal laser scanning fluorescence micro-
scope, which was used for data reading [24,25]. The stage and
shutter were synchronously controlled through a computer to
CQ
QCQ
Fig. 2. Optimized structures of CQ and QCQ by DFT//B3LYP/6-31G⁄.
HOMO
LUMO
CQ
QCQ
Fig. 3. HOMO and LUMO plots of CQ and QCQ by DFT//B3LYP/6-31G⁄.

L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
247
Table 1
ecules. The molecules were optimized by applying the DFT/Becke
three-parameter Lee–Yang–Parr (B3LYP)/6-31G⁄. OPA was system-
atically studied on the basis of each optimized structure by using
the TD-DFT method. Molecular data, such as HOMOs, LUMOs,
energy gaps, and oscillator strength, were obtained from the com-
puted results and subsequently compared with our experimental
data.
Negative HOMO (ꢁeHOMO) and LUMO energies (ꢁeLUMO). HOMO–LUMO gaps calcu-
lated using DFT. Lowest excited energies calculated using TD-DFT (in eV) for
Carbazole, CQ, and QCQ.
Molecule
ꢁ
eHOMO (eV)
ꢁ
eLUMO (eV)
DEH-L (eV)
Carbazole*
CQ
QCQ
5.4427
5.0392
5.0022
0.6444
1.5867
1.7221
4.798
3.453
3.280
*
Data from [27].
Results and discussion
Geometrical optimization and frontier molecular orbitals
The Gaussian 03 program package was used to optimize the
molecular geometric structures. The optimized geometries of CQ
and QCQ obtained by applying the B3LYP/6-31G⁄ are plotted in
Fig. 2. The optimized structure possessed a good planar conjugated
system, in which the quinoline ring was fused into a carbazole
moiety (Fig. 2). Planarity is an important structural property and
is directly related to the conjugation of the
p bond system. To
investigate the electronic properties of CQ and QCQ further by
means of theoretical calculations, we plotted the HOMO and LUMO
for CQ and QCQ in the gas phase (Fig. 3). The HOMO of CQ and QCQ
are located among the whole molecules, whereas the LUMO of CQ
and QCQ are located at the nodes and linkages, C@C, between the
substituent rings and the carbazole ring (Fig. 3). Distributions of
HOMO and LUMO levels were separated in all molecules, indicat-
ing that the HOMO–LUMO transition could lead to a charge trans-
fer. In addition, the lowest singlet excited state corresponds with
the orbital energy difference between the HOMO and the LUMO,
Fig. 4. Sets of one-electron energy levels of CQ and QCQ DFT//B3LYP/6-31G⁄.
namely, the HOMO–LUMO gap (DEH–L). Thus, during the transition
determine the location of each bit and the exposure time. The
recording power and exposure time were 30 mW and 100 ms,
respectively. When reading the data, the film was continuously
scanned by the laser (4.3 mW) to excite fluorescence. A 40ꢂ objec-
tive lens (numerical aperture [NA] = 0.65) was used for recording
and reading. A suitable bandpass filter was placed in front of the
PMT to select the fluorescence and prevent the laser and other
stray lights from scattering out of the detector.
from the HOMO to the LUMO by photoexcitation, the HOMO plays
an important role in the photoluminescence properties of the com-
pounds. Table 1 lists the calculated HOMO and LUMO energies and
energy gaps (DEH–L).
Table 2
Fluorescence spectra of carbazole, CQ, and QCQ.
Compound
kabsmax/nm
kexmax/nm
kflu/nm
D
kStokes/nm
U
Carbazole*
CQ
QCQ
282
370
394
294
388
401
368
484
482
74
96
81
0.12
0.78
0.80
Computational methods
The Gaussian 03 program package (Gaussian Inc., Pittsburgh,
Pennsylvania, USA) [26] was used to calculate the considered mol-
*
Data of kabsmax, kexmax, kflu, and
D
kStokes from [27].
Fig. 5. Linear absorption of (a) CQ and (b) QCQ in the PMMA film; fluorescence intensity of (c) CQ and (d) QCQ.

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L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
Compared with carbazole (
energies (
HOMO) of CQ and QCQ increased (ꢁ5.44 eV [carba-
zole] < ꢁ5.0392 eV [CQ] < ꢁ5.0022 eV [QCQ]), whereas the LUMO
energies (
e
HOMO = ꢁ5.4427 eV), the HOMO
hole-creating and electron-accepting abilities are largely improved
when the conjugated length of the carbazole core is increased and
the different substituents of the 3-position and 3, 6-position of the
carbazole core is changed [27]. The calculated frontier orbital
energy levels (four highest occupied and four lowest unoccupied
e
e
LUMO) decreased (ꢁ0.6444 eV [carbazole] > ꢁ1.5867 eV
[CQ] > ꢁ1.7221 eV [QCQ]) (Table 1). This result indicates that the
Table 3
Electronic transition data obtained using TD-DFT (kab) for CQ and QCQ at the B3LYP/6-31G⁄ optimized geometry.
Molecule
CQ
Electronic transitions
Transitions energy (eV)
kab (nm)
exp* kab (nm)
f
Main configurations
dOPA
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
S0 ? S6
S0 ? S7
S0 ? S8
3.14
3.61
3.79
3.89
3.95
4.09
4.20
4.42
384
343
327
318
313
303
294
280
370
0.8500
0.0241
0.0051
0.3842
0.0013
0.1005
0.2510
0.2727
HOMO ? LUMO
0.7049
0.5676
0.5614
0.6194
0.6320
0.6138
0.6636
0.5076
3.1110
0.0295
0.0038
2.2882
0.0650
0.0011
0.8005
0.9034
HOMOꢁ1 ? LUMO
HOMO ? LUMO+2
HOMOꢁ1 ? LUMO+1
HOMO ? LUMO+1
HOMO ? LUMO+4
HOMOꢁ1 ? LUMO
HOMO ? LUMO+6
295
QCQ
S0 ? S1
S0 ? S2
S0 ? S3
S0 ? S4
S0 ? S5
S0 ? S6
S0 ? S7
S0 ? S8
2.90
3.13
3.44
3.46
3.58
3.78
3.89
3.94
426
395
360
358
346
328
318
314
0.1051
0.9755
0.8407
0.1398
0.1113
0.2081
0.0962
0.0000
HOMO ? LUMO
0.7027
0.6871
0.6244
0.6809
0.6077
0.6158
0.4012
0.5299
0.9599
3.1842
1.4717
0.4851
0.0942
1.1398
0.0001
0.0000
394
318
HOMOꢁ1 ? LUMO+1
HOMOꢁ1 ? LUMO
HOMO ? LUMO+1
HOMO ? LUMO+2
HOMO ? LUMO+3
HOMOꢁ2 ? LUMO
HOMOꢁ6 ? LUMO
*
exp = experimental result.
Fig. 6. Left: two-photon fluorescence for the PMMA film of (a) CQ and (c) QCQ pumped by the femtosecond laser pulse at 800 nm using different input powers; Right:
logarithm of the integrated fluorescence signal of (b) CQ and (d) QCQ versus the logarithm of the input intensity. The black squared dots are the measured fluorescence data,
and the solid line denotes a linear fit.

L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
249
(370 nm for S1 and 295 nm for S4; Fig. 5). For the CQ molecules, the
electron transitions from HOMO ? LUMO and from HOMOꢁ1 ? -
LUMO+1 are dominant in the configurations of the state transition;
for the QCQ molecule, however, the electron transitions from
HOMOꢁ1 ? LUMO+1 and from HOMO ? LUMO+3 have important
contributions to OPA (Table 3). Compared with the kab (S0 ? S1)
and kab (S0 ? S4) values of CQ, the ICT state and the OPA final state
of QCQ changes to S2 (S0 ? S2) and S6 (S0 ? S6), respectively. This
result is caused by the change in the QCQ molecule in the conju-
gated bridge from the 3,6-position of the carbazole. This calculated
result is in accordance with the experimental value of the OPA
absorption spectra.
orbitals and energy gaps between HOMO and LUMO) from the
change in the HOMO and LUMO energies are shown in Fig. 4. For
the carbazole unit type, the increase in conjugation strength
results in the decrease of
DEH–L (4.798 eV [carbazole] > 3.453 eV
[CQ] > 3.280 eV [QCQ]) (Fig. 4). This result indicates that the quin-
oline group on the 3,6-position of the unilateral or bilateral carba-
zole unit are more favorable for molecular system stability.
Moreover,
DEH–L obviously decreased more when the longer the
conjugated length was introduced (3.453 eV [CQ] > 3.280 eV
[QCQ]).
OPA, emission spectra, and singlet–singlet electronic transition
A comprehensive photophysical study was conducted to under-
stand the energetics of the electronically excited states of CQ and
QCQ compounds as well as their potential for a number of applica-
tions, such as solar cells and fluorescence materials. The PMMA,
which has high chemical resistance, advantageous optical proper-
ties, and low production cost, was employed in mixing the sample
to study the optical properties of the sample film more deeply. In
our experiment, the films consisted of the CQ and QCQ compounds
and PMMA. The films were prepared on a quartz glass substrate by
using a spin coater and were dried under ambient air at room tem-
perature. The thickness of the film was approximately 0.2 mm
under ambient air at room temperature. The UV-V is absorption
spectra of the CQ and QCQ compounds in the PMMA film are
TPA properties of the film
Considering the application of TPA materials in devices, the
materials were not only dissolved in some solvent but were also
mixed into a polymer to become a useful film. The concentrations
of the compounds in the polymer were 0.02 M. As shown in the
absorption spectra of the compound films in Fig. 5, the two com-
pounds have no linear absorption in the 450 nm to 900 nm range.
This range provides a wide window for investigating the two-pho-
ton optical properties. Before measuring the TPA properties of the
CQ and QCQ films, the effects of the pure, undoped PMMA film
sample and the quartz glass were avoided. The results demonstrate
the absence of a linear absorption at 400 nm to 900 nm. Using the
shown in Fig. 5. The maximum absorption wavelength (kabsmax
)
of the two carbazole derivatives were observed at 370 and
394 nm for CQ and QCQ, respectively (Fig. 5). The values were
assigned to the typical p–
p⁄, which was obtained during electronic
transition from the ground state to the intramolecular charge
transfer (ICT) state [28,29]. For CQ and QCQ with C@C as the con-
jugated bridge, the kabsmax of QCQ was red-shifted because of the
increase in the conjugated length of QCQ from the 3,6-position of
the carbazole by the electron donor group (quinoline ring).
Fig. 7(c) and (d) exhibit the fluorescence emission spectra of the
CQ and QCQ samples in the dimethylformamide at a concentration
of 1.0 ꢂ 10ꢁ5 mol dmꢁ3, respectively. The kexcmax and kflu of each
molecule are shown in Table 2. The kflu fluorescence wavelengths
of CQ and QCQ are at 484 and 482 nm, thus locating the blue spec-
tral range. Using Eq. (1), the fluorescence quantum yields
U of the
compounds are 0.78 (CQ) and 0.80 (QCQ) in the
dimethylformamide.
The TD-DFT/B3LYP/6-31G⁄, on the basis of the optimized geo-
metric structures, were used in calculating the OPA properties of
all investigated molecules to study the electronic transition of
the three compounds. The transition intensity for OPA (dOPA) is
described by using the oscillator strength [30–32]:
X
2
xf
3
2
dOPA
¼
h0jlajfi ;
ð2Þ
a
where xf denotes the excitation energy of the excited state |f > , la
is the electronic dipole operator, and the summation is performed
over the molecular x, y, and z axes. Table 3 lists the calculated
and experimental wavelength kab (S0–S8), transition energy, oscilla-
tor strength, and main configurations. The wavelength location and
oscillator strength are not exactly the same with the experimental
results because TD-DFT was assumed at vacuum conditions to cal-
culate the absorption spectrum. The calculated data for CQ and QCQ
have the same variation trend with the experimental results
(Table 3). For the molecular CQ, the first (S1) and fourth (S4) excited
states have large absorptions (384 nm for S1 and 318 nm for S4) and
their absorption intensities are almost the same (f = 0.8500 for S1
and f = 0.3842 for S4). The experimentally measured absorption is
positioned in almost the same wavelength and absorption intensity
Fig. 7. Open-aperture normalized Z-scan transmittance of (a) CQ and (b) QCQ films
measured using 120 fs pulses at 800 nm. The circles denote the experimental data,
and the solid line signifies the theoretical fit.

250
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
where NA = n sinh, h is the half-angle of the collection for the lens, I0
is the intensity at the geometric focal point, and P refers to the inci-
dent power. Thus, using Eq. (3) to solve the logarithms, the linear
relationship between the logarithm of fluorescence intensity and
of input power can be attained; the slope of the line should be equal
to two to confirm whether the process has a TPA mechanism.
The films of the two compounds CQ and QCQ were measured by
using a fs laser pulse at 800 nm using different input powers. The
800 nm fs laser excited the CQ and QCQ in producing a two-photon
fluorescence (Fig. 6). Thereafter, we adjusted the input power and
recorded the intensity dependence of the fluorescence to confirm
that the collected light signal was not from the other absorption
two-photon excited fluorescence method to study the TPA prop-
erty, the intensity If of the fluorescence was induced by conducting
a two-photon excitation determined by the laser intensity I0. The
total number of molecules N is expressed by the following equa-
tion [33]:
If ¼ CI2N;
ð3Þ
0
where C is a constant. The pulsed intensities at the geometric focal
point were calculated using the following equation:
2
p
ðNAÞ
I0ðtÞ ¼
PðtÞ;
ð4Þ
k2
Fig. 8. (a) to (g) Show the seven-layer data storage and readout with a confocal fluorescence microscope. TPA photoreaction ODS system for 3D two-photon writing and
reading. Two-photon writing and readout were conducted at 800 nm (PWriting = 30 mW and PReadout = 4.3 mW), 80 fs, 100 ms exposure with an A 40ꢂ, NA = 0.65 objective lens.
The layer-by-layer separation is approximately 18 lm, and the distance between the adjacent dots in each layer is approximately 4 lm. Scale bar: 10 lm. (h) and (i) Exhibit
the 3D storage data images on the first and seventh layer using MATLAB 7.0.

L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 243–252
251
mechanisms but from the fluorescence after the TPA. The slopes of
the lines are 1.96 and 1.97 for the CQ and QCQ compounds, respec-
tively (Eq. (4)). This value confirms that the TPA is the main exci-
tation mechanism for the intense fluorescence emission at the fs
regime of the CQ and QCQ compounds. The shape and location of
the two-photon fluorescence emission spectra are almost identical
with the shape and location of the one-photon fluorescence emis-
sion spectra (Fig. 6). This result demonstrates that the two-photon
and one-photon fluorescence processes occurred from the same
final excited state.
An 800 nm fs laser was used for two-photon excitation in both
3D writing and 3D reading in a novel carbazole derivative film to
design the storage system. The novel asymmetric-type D-p-D car-
bazole derivative, CQ, with a quinoline substituent was used in our
storage work based on the photoreaction effect. This material have
the capability for high nondestructive multilayer writing and read-
ing rates of PWriting = 30 MW and PReadout = 4.3 MW, respectively.
When CQ film is exposed to an 800 nm fs laser light, the written
region absorbs the two strong photons and induces a photoreac-
tion effect on the storage material. The bleached areas do not pro-
duce fluorescence if a laser beam of the same wavelength used in
the excitation is adopted for illumination. On the contrary, the
fluorescence of the unwritten region remains strong. In the readout
data process, the strong and weak fluorescence intensities can be
detected by using a two-photon fluorescence microscope and clas-
sified as stored zero and one data bits. Fig. 8 exhibits the results for
the seven layers of data without layer-by-layer crosstalk that were
written and read by the TPA ODS system. The separation between
Measurement of TPA coefficient and TPA cross section of the films
The nonlinear absorption coefficient b plays an important role
in TPA materials. The Z-scan method [34] was applied by using a
120 fs 800 nm femtosecond laser to obtain the b value. In this
experiment, we considered a spatially and temporally Gaussian
laser beam traveling in the +z direction within the samples. The
normalized energy transmittance T(z) for the TPA is derived by
the following equation:
two adjacent layers is approximately 18
lm, and the distance
between the adjacent dots in each layer is approximately 4
lm.
The recording data that were bleached showed up as a darker area
than the surrounding fluorescent area. The 3D storage data images
of the first layer and the seventh layer were obtained via MATLAB
7.0 (Fig. 8). From the above storage result, the novel compounds
used as TPA photoreaction storage materials must have the follow-
ing characteristics: efficient photochemical process or fluorescence
emission with high quantum yield; high TPA cross section for the
accompanying writing laser; better film forming ability and ther-
mal stability to prevent the destruction of the storage device
between writing and reading.
Z
1
1
TðzÞ ¼
ln½1 þ q0 expðꢁx2Þꢃdx;
ð5Þ
p
1=2q0
ꢁ1
where q0 ¼ bI0Leff =ð1 þ z2=z20Þ; Leff ¼ ð1 ꢁ eꢁ Þ=2, z0
¼
px
20=k is the
a
L
Rayleigh range, x0 is the minimum beam waist at the focal point
(z = 0),
a is linear absorption coefficient, k is the laser free-space
wavelength, and L is the sample thickness. The TPA coefficients
were extracted by using the best fit between the above equation
and the open-aperture Z-scan curve. For the open-aperture Z-scan
in Fig. 7, the best fits result in a b value of 0.192 cm/GW and
0.254 cm/GW for the CQ and QCQ films, respectively. After the
experimental measurement of the TPA coefficient, the two-photon
cross section of the film rfilm was obtained using the following
expression:
Conclusion
In this paper, two novel carbazole derivative-linked quinoline
groups, namely, 3-(-quinolin)vinyl-9-butyl-carbazole (CQ) and
3,6-bis(2-(quinolin)vinyl)-9-butyl-carbazole (QCQ), were synthe-
sized by using the Vilsmeier formylation reaction and Knoevenagel
condensation. The substituents on the 3-position and 3,6-position
of the carbazole of the structural design allowed for the investiga-
tion of numerous factors affecting the electronic transition. The
substituents also enabled the measurement of the linear absorp-
tion, fluorescence emission spectra, and fluorescence quantum
yields as well as the employment of the DFT and TD-DFT theoret-
ical calculation. We investigated the TPA properties of the CQ and
QCQ films and obtained a larger two-photon cross section. The
two-photon photoreaction writing and reading experiment with
the confocal fluorescence microscope were first realized by using
a 3D multilayer ODS with an 800 nm fs laser in the novel carbazole
derivative-doped PMMA film. The experimental results show that
different data can be written on the seven layers of the novel car-
bazole derivative without layer-by-layer crosstalk. The novel car-
bazole derivative exhibits good optical memory, thus widening
the applications of carbazole derivatives on multilayer high-den-
sity and ultra-high-density optical information storage materials.
b ¼
r
filmN0 ¼ h
m
r0filmNAd0 ꢂ 10ꢁ3
;
ð6Þ
where b is expressed in cm/GW, h
m
is the energy of the incident
photon, NA is the Avogadro constant, d0 is the concentration of the
TPA compound in the film (mol/L), and rfilm is expressed in cm4 s/
photon. The TPA cross section of the CQ and QCQ films were
obtained by using Eq. (6), where
r
film(CQ) = 470 ꢂ 10ꢁ50 cm4 s/pho-
ton and
r
film(QCQ) = 621 ꢂ 10ꢁ50 cm4 s/photon, respectively. These
experimental results are more than the results of anthracene deriv-
atives [35] (335 ꢂ 10ꢁ50 and 308 ꢂ 10ꢁ50 cm4 s/photon) and diary-
lethenes derivatives [36] (5 ꢂ 10ꢁ50and 48 ꢂ 10ꢁ50 cm4 s/photon),
which have been applied in TPA optical storage. Based on superior
high thermal stability, film forming ability, and TPA film property
measured using the 800 nm fs laser, the CQ and QCQ compounds
are concluded to be well suited for TPA optical storage.
TPA 3D data storage
TPA 3D data storage takes advantage of the TPA process to
excite the molecules inside a volume rather than the surface.
[10] When two photons collide simultaneously at any point within
the volume of the storage media and the energy sum of the two
laser photons is equal to or greater than the energy gap of the tran-
sition, the absorption will occur only at the place of the pulse over-
lap. At the point where the two beams interact, the absorption
induces a photochemical change to the TPA materials, thus creat-
ing a new form distinct from the unexcited molecules. These two
distinct forms provide the two states necessary for the storage of
information in a binary format; the original form designates a zero
(0), whereas the new form designates a one (1).
Acknowledgement
This work is partially supported by the National Natural Science
Foundation of China (Grant Nos. 61137002 and 50872139).
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