Three-photon absorption properties of a novel symmetrical Carbazole derivative having terminal 1,10-phenanthroline rings via carbon-nitrogen (C = N) double bond

Article abstract of DOI:10.1002/poc.2937

Three-photon absorption (3PA) properties of symmetric-type carbazole derivatives show great potential for application in light-activated therapy and optical limiting. A novel symmetrical carbazole derivative (abbreviated as POCP) with end-groups of 1,10-phenanthroline rings as the donor moieties, chained via carbon-nitrogen (C = N) double bond, has been synthetized and its three photon absorption properties has been also determined by using a Q-switched Nd: YAG laser pumped with 30 ps pulses at 1064 nm in dimethylformamide. The measurement of 3PA cross-section of this compound is performed by open aperture Z-scan and σ_3PA is 481 × 10^-78 cm⁶ s²/photon² for the transition S₀ → S ₁. The influence of the molecular structure of this compound on three-photon absorption cross-sections is discussed micromechanically by Austin model 1 and Zerner's Intermediate Neglect of Differential Overlap/S method. Copyright

Full text of DOI:10.1002/poc.2937

Research Article
Received: 9 December 2011,
Revised: 13 February 2012,
Accepted: 21 February 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.2937
Three-photon absorption properties of a novel
symmetrical Carbazole derivative having terminal
1
,10-phenanthroline rings via carbon–nitrogen
(
C = N) double bond
a
b
b
a,b
c
Liang Li , Ningning Yuan , Ping Wang , Yiqun Wu *, Yinglin Song ,
Zhimin Chen and Chunying He
b
b
Three-photon absorption (3PA) properties of symmetric-type carbazole derivatives show great potential for application
in light-activated therapy and optical limiting. A novel symmetrical carbazole derivative (abbreviated as POCP) with
end-groups of 1,10-phenanthroline rings as the donor moieties, chained via carbon–nitrogen (C= N) double bond,
has been synthetized and its three photon absorption properties has been also determined by using a Q-switched
Nd: YAG laser pumped with 30ps pulses at 1064nm in dimethylformamide. The measurement of 3PA cross-section of
–
78
6
2
2
this compound is performed by open aperture Z-scan and s3PA is 481ꢀ 10 cm
•
s /photon for the transition S
0
1
! S .
The influence of the molecular structure of this compound on three-photon absorption cross-sections is discussed
micromechanically by Austin model 1 and Zerner’s Intermediate Neglect of Differential Overlap/S method. Copyright ©
2
012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: carbazole derivative; three-photon absorption; Zerner’s Intermediate Neglect of Differential Overlap/S method;
Z-scan; 1,10-phenanthroline
derivatives have never been studied up to date, especially,
nonlinear optical properties.
INTRODUCTION
Investigations of organic multiphoton absorption materials,
in particular, two-photon and three-photon absorption
In previous reports, on the basis of our theoretical and
experimental investigations on fluorene derivatives with
[
21–24]
(
2PA and 3PA), have received considerable attention recently
differently terminal structure,
we found that a major
because of their potential applications in photonics and bio
contribution to the s3PA values originates from the transition
dipole moment from the ground state to lowest charge
transfer excited state. One possible way to enhance the 3PA
cross-sections is to increase the charge-transfer dimensional-
ity by expending the p-electron conjugation length.
[
1–6]
photonics.
The limited irradiation penetration depth of
2
PA materials has hindered their medical and biological
applications because of the undesirable linear absorption
and scattering caused by the simultaneous absorption of
two red (or short wavelength near-infrared) photons. Com-
pared with the one-photon absorption process in transi-
Carbazole, as
(p type), can combined with 1,10-phenanthroline as
an electronic-transporting group (n type)
a
typical hole-transporting group
[
25,26]
[
7–9]
[27]
tion, the 3PA process observed in organic molecules
semiconductors
and
to form novel
[
10]
has longer excitation wavelength. Longer
p–n type bipolar materials efficiently balanced electron-
excitation wavelength in the near-IR and stronger spatial
confinement are better at minimizing the undesirable absorp-
tion and light scattering. Thus, greater radiation penetration
depth and less damage in these materials can be achieved.
Consequently, much effort has been undertaken recently to
develop organic molecules with large 3PA cross-sections
transporting and hole-transporting capability, which is most
*
Correspondence to: Y. Wu, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, No. 390, Qinghe Road, Jiading District, P.O.
Box 800-211, Shanghai 201800, P. R. China. E-mail: yqwu@siom.ac.cn
a L. Li, Y. Wu
(
s3PA
) because of their promising application, such as
[
1,3,11,12]
[5,13–15]
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of
Sciences, No. 390, Qinghe Road, Jiading District, Shanghai 201800, P. R. China
multiphoton fluorescence imaging,
stimulated emission,
up-converted
and light-
[
13]
optical limiting,
[
16]
activated therapy.
b N. Yuan, P. Wang, Y. Wu, Z. Chen, C. He
Recently, p-conjugated carbazole derivatives of the
general type D-p-D, D-p-A-p-D, and A-p-D-p-A, where D and
A denote electron donating and accepting groups, have been
Key Lab of Functional Inorganic Material Chemistry (Heilongjiang University)
Ministry of EducationHarbin 150080, P. R. China
c Y. Song
[
17–20]
studied in 2PA properties by many researchers.
How-
Department of Applied Physics, Harbin Institute of Technology, Harbin 150080,
P. R. China
ever, 3PA related properties of p-conjugated carbazole
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

L. LI ET AL.
extensive application in organic light-emitting diodes. In
this paper, we synthesize the novel symmetrical carbazole
derivative having terminal 1,10-phenanthroline rings via carbon–
nitrogen (C= N) double bond, 3,6-bisphenanthrolinimidazole-
N-n-octylcarbazole (D-p-D-p-D). Meanwhile, in an effort to
make clear the three-photon absorption mechanism for
D-p-D-p-D conjugated molecule, three-photon absorption
cross-section of this carbazole derivative is first investigated
in N,N-dimethylformamide (DMF) solution by Z-scan measure-
ment, pumping at 1064 nm with ps laser, and combined
with the quantum chemistry calculations, the different influence
of intramolecular charge transfer on three-photon absorption
cross-sections. Large p conjugated substituent designed the
special linkage of p chain with carbon-nitrogen (C = N) double
bond is discussed micromechanically. It is found that its s3PA
value is much larger than that of other groups in molecules
before, so this molecule has promise for irradiance limiting
applications and three-photon photodynamic therapy in cancer
treatment.
and 25 mL concentrated nitric acid were gradually added into the
flask. The reaction mixture was stirred for 2 h at 100 C. Then it was
slowly poured into 250 mL mixture with ice and water, and adjusted
its pH to neutral. After vacuum filtration, the filtrate was extracted
to get the crude product. Target product was obtained by
ꢁ
recrystallization.
ꢁ
1
,10-Phenanthroline-5,6-dione(1), yield 92.5%, m.p. 258.1–258.4 C.
Anal. calcd(%) for C12
8.47; H, 2.92; N, 13.29.
6 2 2
H N O : C, 68.57; H, 2.88; N, 13.33. Found: C,
6
Synthesis of 9-octyl-carbazole (2)
Carbazole (10 g, 0.06 mol) and NaOH (2.6 g, 0.065 mol) were added in
50 mL dimethylsulfoxide in a three-neck flask under stirring. The
ꢁ
mixture was heated to 60 C in a water bath for 40 min to dissolve
all solids and then was gradually added with 1-bromo-n-octane
ꢁ
(
10.7 g, 0.065 mol). The reaction temperature was kept at 60 C for
6
h then poured into water. The precipitation was filtered and dried
under reduced pressure to yield a rude product. Compound 1 was
obtained by recrystallization.
1
9
-Octyl-carbazole (2), yield 76.8%, HNMR(CDCl
3
): d (ppm) 0.8(t,3H,
), 4.3
), 7.1–8.2 (m, 8H, carbazole); IR(KBr): 2953, 2924,
n-octyl, CH
t, 2H, n-octyl, 1-CH
3
), 1.3(m,8H,n-octyl, 3-CH
2 2
), 1.8(m,2H, n-octyl, 2-CH
(
2
EXPERIMENTAL
–1
2
C H
20
855, 1594, 1451, 1352, 808, 749, 725 cm ; Anal. calcd (%) for
25N: C, 85.97; H, 9.02; N, 5.01. Found: C, 85.87; H, 9.06; N, 4.96.
Materials
The chemical structure of the carbazole-based molecule, 3,6-
Synthesis of 3,6-diformacy-9-octyl-carbazole (3)
bisphenanthrolinimidazole-N-n-octylcarbazole (abbreviated as POCP),
[
28,29]
was prepared referring to literature procedures,
shown in
N,N-Dimethylformamide (11 g, 0.15 mol) was added into a three-neck
ꢁ
Scheme 1. In POCP the central carbazole is symmetrically end-capped
with donor groups (1,10-phenanthroline ring) forming a D-p-D-p-D
sequence. The chemical structure of this target molecule (POCP)
had been identified by elemental analysis, mass spectrometry, IR,
and nuclear magnetic resonance, respectively (shown in supporting
information).
flask and the flask was cooled to 0 C by an ice bath. Then, phospho-
rus oxychloride (23 g, 0.15 mol) was added into the flask and it was
warmed naturally to room temperature and stirred for 1 h. Then,
4.2 g 9-octyl-carbazole (0.015 mol) dissolved in 1,2-dichloroethane
(20 mL) was gradually added in the flask. The reaction solution was
ꢁ
heated to 85 ~ 90 C under reflux and left at this temperature for
4 h. Then it was slowly poured into a mixture with ice and water,
2
and extracted with chloroform three times. The product 2 was
purified through column chromatography on silica gel using
Synthesis of 1,10-phenanthroline-5,6-dione (1)
Concentrated sulfuric acid (50 mL) was added into a three-neck flask
and the flask was cooled to below 5 C by an ice bath. Then, 4.0 g
dichloromethane–petroleum ether (1:4) as eluent.
3,6-Diformacy-9-octyl-carbazole (3), yield 72.5%, HNMR(CDCl
ꢁ
1
3
):
1,10-phenanthroline (20 mmol), 8 g potassium bromide (67 mmol),
d(ppm), 1.4(m, 8H,n-octyl, 3-CH
2
), 1.9(m, 2H, n-octyl, 2-CH
2
), 4.3
Scheme 1. The synthetic route of the target compound.
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

THREE-PHOTON ABSORPTION OF CARBAZOLE DERIVATIVE HAVING PHENANTHROLINE RINGS
(
(
t, 2H, n-octyl, 1-CH
KBr): 2952, 2922, 2850, 1687, 1594, 1471, 1352, 807, 746, 729 cm
: C, 78.77; H, 7.51; N, 4.18. Found:
2
), 7.3–8.7(m, 6H, carbazole), 10.1(s, 2H, CH = O); IR
to record the input and output energy simultaneously. The pump
energy is controlled by rotating a half-wave plate between two polar-
izers. The dye solution was placed in a 1 mm path cell and the beam
–
1
;
Anal. calcd(%) for C22H25NO
2
C, 78.68; H, 7.69; N, 4.10; LC-MS: m/z, 335.2(335.44) [M + 1].
0
waist at the focal plane w ≌ 24 mm. The Z-scan system was calibrated
with carbon disulfide as a standard and the experimental uncertainty
should be within ꢂ 10%. We also carried out the Z-scan measurements
on a quartz glass vial of thickness L = 1 mm and the pure DMF, shown
by the inset in Fig. 3, confirming that the measured nonlinear
absorption mainly originates from POCP.
Synthesis of 3,6-bisphenanthrolinimidazole-9-octyl-carbazole (4)
3,6-Diformacy-9-octyl-carbazole (0.3 g, 1 mmol), phenanthroline-5,6-
dione(0.42 g, 2 mmol), and 3.2 g ammonium acetate were added in
2
0 mL glacial acetic acid in a 150 mL three-necked flask, then the
ꢁ
reaction mixture was stirred for 3 h at 85 C. Then, it was cooled to
room temperature, poured into 40 mL water and its pH adjusted to
neutral with concentrated ammonia. It was then filtered and dried
to get the crude product. Light yellow target product was obtained
Quantum chemistry calculations
The optimized geometry of POCP and charge density distributions in
both ground-state and excited-state were calculated using Austin
model 1 (AM1) semi-empirical method in the MOPAC quantum
chemical package of the Gaussian 03 program package (Gaussion.
by column chromatography.
1
3,6-Bisphenanthrolinimidazole-9-octyl-carbazole (4), H NMR (CDCl
3
): d
[
21]
Inc., Pittsburgh, Pennsylvania, US).
The excited states are charac-
(
ppm), 13.7(s, 2H, C = N), 7.3–9.1(m, 12H, carbazole and phenanthroline),
.5(t, 2H, n-octyl), 1.9(m, 2H, n-octyl), 1.3(m, 8H, n-octyl), 0.8(t, 3H, n-octyl).
IR (KBr): 3421, 3064, 1444, 2921, 2850, 1608, 1559, 1351, 804, 739,708.
Analysis calculated (%) for C46 : C, 77.18; H, 5.21; N, 17.61. Found:
terized, and the electronic transitions are computed by Zerner’s Inter-
4
[
22,23]
mediate Neglect of Differential Overlap/S (ZINDO/S) method
the optimized geometries.
at
37 9
H N
C, 77.11; H, 5.24; N, 17.51.LC-MS: m/z, 716.3 (716.65) [M + 1].
RESULTS AND DISCUSSION
One-photon optical properties measurements
Synthesis
The linear absorption spectrum and one-photon fluorescence spectrum
were measured by using Perkin Elmer Lambda 900 UV-VIS/NIR (San Jose,
California, US) recording spectrophotometer and Jasco RF-5301 (Jasco
Corporation, Great Dunmow, Essex, UK), respectively. This molecule solution
The general synthetic route of POCP was based on a four-
step sequence. First, the 5,6 position of 1,10-phenanthroline
was ketonized by concentrated sulfuric acid and nitric acid.
Second, the carbazole was alkylated by n-octylbromine to pro-
duce N-n-octylcarbazole (compound 2). Third, 3,6-biformyl-N-
–5
in DMF with a concentration of 3 ꢀ 10 mol/L filled in a 10mm path quartz
cuvette was used for both measurements of one-photon and three-photon,
and cell effects and solvent contributions were properly subtracted.
n-octylcarbazole (compound 3) was prepared via POCl
3
and
1
,2-dichloroethane in DMF with N-n-octylcarbazole. Finally,
Measurement of three-photon absorption coefficient
the condensation reaction of 3,6-biformyl-N-n-octylcarbazole
with 1,10-phenanthroline-5,6-dione (compound1) synthesized
the target compound 4.
[
30]
The open-aperture Z-scan method is used to obtain the real values,
which consider a laser beam with spatial and temporal Gaussian distri-
bution. It is used to estimate the three-photon absorption coefficient
(
a
3
) and the values of 3PA cross-section (s3PA) because the usual nonlin-
ear transmission method is not accurate because of the square temporal
profile of the laser beam. The measurement a of POCP/DMF solution at
Linear absorption and fluorescence
3
Figure 2 shows the linear absorption and emission spectra of POCP
in DMF solution. This molecule shows strong UV absorption in the
spectral range of 280–400nm with maximum absorption peaks
–
2
1
ꢀ 10 mol/L were conducted at 1064 nm by the well-known open-
aperture Z-scan technique. Figure 1 exhibits the experimental setup
for the open-aperture Z-scan system. The incident 1064 nm laser was
provided by a Q-switched Nd:YAG laser PY61C-10 (Continuum, Santa
Clara, USA) with pulse duration of 30 ps, repetition rate of 10 Hz. The
laser beam was divided into two beams by the beam splitter. The
(
labsmax) at 294 nm, which is assigned to the typical p ! p*, locally
excited state. In addition, the shoulder peaks can be observed at
1 2
weaker beam, detected by photodiodes D and D , was employed to
monitor the incident laser energy. The stronger beam, being focused
into the sample by a 21 cm focal-length lens, acted as the exciting
beam. The entire transmitted intensities through the sample were
totally collected to a photodiode D . These two photodiodes were
3
connected with a two-channel energy meter (Molectron EPM 2000)
Figure 1. Experimental setup for open-aperture Z-scan system. The
picosecond 1064 nm laser is provided by a Q-switched Nd:YAG
laser. 2 mm-path quartz cell, filled with the sample solution in DMF
Figure 2. One-photon absorption (solid line) and steady fluorescence
emission spectra (dotted line) of D-p-D-p-D compound POCP in DMF
solution at low concentration (3 ꢀ 10 mol/L).
–
3
1
ꢀ 10 mol/L was used for three-photon absorption coefficient
) measurements.
–
5
(
a
3
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

L. LI ET AL.
ꢃ 2aL
3
48nm originated from the electronic transition from the
Leff = (1 ꢃ e
)/2a is the effective sample thickness. Accord-
[
31]
ground state to the intermolecular charge transfer state.
Compared with linear absorption spectra of carbazole derivatives
ing to our linear absorption measurement, we found that
the linear absorption coefficient, a, is negligible at 1064 nm,
[
19]
with 3-styrene- substituent, it is indicated that the absorption
peaks would shift to longer wavelength when the electronic
p-conjugated structure are extended by C = N as p chain and the
large p conjugated substituents, 1,10-phenanthroline. Meanwhile,
there is no one-photon absorption for these two molecules in
the entire near-IR and most (>420 nm) of the visible spectral range.
The three-photon energy of 1064 nm radiation just falls into the
strong UV absorption, and therefore three-photon absorption in
this solution may be expected. The one-photon fluorescence
spectrum of POCP is also displayed in Fig. 2 by a dotted line. One
can find that the fluorescence band of this molecule is located at
the blue–green region with the peak wavelength of 473nm.
Fluorescence quantum yields, Φ, were measured for POCP
thus
L
eff ꢄ L = 1 mm. Using the on-axis beam irradiance
2
I
0
= 30 GW/cm , the three-photon absorption coefficient is
ꢃ20
3
2
given by g = (8.32 ꢂ 0.32) 10
cm /W . On the basis of the
known g value of the measured solution sample, the molecu-
6
2
lar absorption cross-section s3PA (in the units of cm s ) for
the given sample in solution is obtained by
[
34]
ꢄ
ꢅ
2
g
hc
s3PA ¼
(2)
ꢃ3
NAꢅd0 ꢀ 10
l
Here, N
A 0
is Avogadro constant, d is the concentration of
the sample in solution (in the units of mol/L), and h(c/l)
is the energy of an incident photon at 1064 nm. Conse-
quently, the intrinsic sample molecular s3PA values can be
(Φ = 0.27) by a standard method relative to Rhodamine B in
ꢁ
[32]
–78
6
2
2
ethanol at 25 C (Φ = 0.9).
easily estimated as 481 ꢀ 10 cm
•
s /photon for POCP.
Compared with s3PA of the reported organic molecules in our
[
21–24]
[8]
group
or other researcher groups, it is observed that the
Three-photon absorption cross-sections
s3PA value of this symmetrical carbazole derivative was larger
than the findings for symmetrical fluorene derivatives forming
a D-p-D-p-D sequence, s3PA = 20 ~ 400 GM. Also, this result
indicates that this s3PA value is strongly related to chemical
structures of this molecular, which means that this symmetrical
carbazole derivative designed special linkage of p chain with
carbon–nitrogen (C = N) double bond is different from conven-
tional modification methods because multiphoton absorption
Figure 3 shows both the experimental open-aperture Z-scan
curves given by points and the theoretical fitting curves
represented by solid lines, showing nonlinear three-photon
absorption in POCP at l =1064 nm. To fit the experimental
curves, we employed the equations for a three-photon
absorption process considering temporal and spatial Gaussian
[
33]
profiles, derived by He et al.,
which is given by
1
Z
n
o
1
=2p0
ꢂ
ꢀ
ꢁꢃ1=2
ꢀ
ꢁ
2
2
2
TðzÞ ¼ p1
ln 1 þ p0 exp ꢃ2x
þ p0 exp ꢃx
dx
ꢃ1
(1)
where z is located at the focusing plane of the tightly
focused Gaussian beam, x = z/z
0
is the relative position of
ꢂꢀ
ꢁ ꢀ
ꢁꢃ
1=2
2
0
2
2
0
2
0
the sample, p0ðzÞ ¼ 2gI Leff = 1 þ z =z
, z0 ¼ po =l is
the Rayleigh range, o is the beam waist at the focus, and
0
Figure 4. The optimized geometry of the ground state of compound.
Table 1. Energies (ev), the gaps (ev) between the HOMO
and LUMO for the studied molecule
Figure 3. Normalized open-aperture Z-scan data with a concentration
–
2
of 1 ꢀ 10 mol/L showing three-photon absorption at =1064 nm and
Molecules
HOMO(ev)
LUMO(ev)
LUMO–HOMO(ev)
2
0
input irradiance I =30 GW/cm . The experimental data are shown as
POCP
BVCZF*
ꢃ6.214
ꢃ7.896
ꢃ0.682
ꢃ0.750
5.532
7.146
scattered points, while the solid lines are theoretical fits. The fit (solid
curve) was obtained with three-photon absorption coefficient
ꢃ20
3
2
g = (8.32 ꢂ 0.32) 10
cm /W . The inset is nonlinear absorption experi-
[24]
*BVCZF was reported by our group.
mental data of quartz glass vial of thickness L = 1 mm and the pure DMF.
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

THREE-PHOTON ABSORPTION OF CARBAZOLE DERIVATIVE HAVING PHENANTHROLINE RINGS
Table 2. Electronic transition date obtained by the ZINOD/S semi-empirical method for POCP at the AM1-optimized geometry
[
c]
Electronic
transition
Excitation
energy
l
Transition electric dipole moments (au)
f
[
a]
[
b]
X
Y
Z
S
S
S
S
S
S
0
0
0
0
0
0
! S
! S
! S
! S
! S
! S
1
2
3
4
5
6
3.3325
3.5487
3.5872
3.6611
3.6701
3.8804
420
406
388
371
360
356
ꢃ4.6206
0.1862
ꢃ0.9625
ꢃ0.5201
1.5528
ꢃ0.0628
ꢃ0.3578
ꢃ0.4081
1.2105
0.6842
1.3367
ꢃ0.0043
0.5369
ꢃ0.0250
ꢃ0.1537
0.0317
1.1804
0.0077
0.0415
0.0672
0.1440
0.1804
ꢃ0.5634
ꢃ1.2124
a
l values are from ZINDO computations.
X is the long molecular axis.
b
c
Oscillator strength.
materials via carbon–carbon (C = C) double bond and terminal p-
conjugated substituent, 1,10-phenanthroline, is larger than
others on 3PA molecules. Consequently, the charge redistribu-
tion and the magnitude of the transition dipole moment in
the different terminal substituent and C = N as p conjugated
chain should result in being much more influenced on s3PA
value. Using AM1 semi-empirical and ZINDO/S methods, we sys-
tematically studied the charge density distributions in both
ground-state and excited-state of this molecule, and the influ-
ence of different molecular structures on 3PA cross-sections is
discussed in the following section.
1
that the first excited state, S , is the strongest 3PA
state for this molecule and that the transition dipole moment
between the ground state (S ) and final state (S ), |m|, is the
0
1
most definitive factor for 3PA, by which the 3PA cross-section
can be increased. The excitation energies, oscillator strength
(f) and transition dipole moments from the ground state to
the first excited state are reported in Table 2. It can be seen
that the oscillator strength (f) and the transition dipole
moment along the long axis of the molecule (X) of the S ! S
0
1
electronic transition are the largest for this molecule, which
indicates that the first excited state, S , is the strongest
1
one-photon absorption state. A strong one-photon absorp-
[
35]
tion state is corresponding to a strong 3PA state as well.
From Table 2, it is found that the transition dipole moment,
Ground-state geometries and electronic excitations
0
f
In an effort to support our experimental results, optimized
ground-state geometries of POCP was obtained by the AM1
method, shown in Fig. 4. The ZINDO/S semi-empirical method
was employed to obtain the energy and the transition dipole
moments of the first six singlet–singlet electronic transitions.
The lowest singlet excited state corresponds to the orbital
energy difference between the highest occupied molecular or-
bital (HOMO) and the lowest unoccupied molecular orbital
m , between the ground state and the first excited state
along the X axis were determined for this symmetric mole-
cule, showing 4.0206 (au), which is in proportion to the 3PA
cross-sections when the electronic p-conjugated structure is
extended by C = N as p chain and the large p conjugated
substituents, 1,10-phenanthroline. Therefore, compared with
other fluorene derivatives reported by our group, it can be
seen that the transition dipole moment of the S
0
! S
1
elec-
(LUMO), which is called the HOMO–LUMO gap (ΔEH–L). Thus,
tronic transition was changed larger to enhance the 3PA
cross-section of POCP via designing the special linkage of
the p chain with carbon–nitrogen (C = N) double bond and
large p conjugated terminal substituents, 1,10-phenanthro-
line. The bilateral symmetric-type carbazole derivatives are
good reference molecules for 3PA properties, which are
promising for determining the fundamental structure for
designing 3PA materials and for research on three-photon
photodynamic therapy in cancer treatment.
when the transition occurs from the HOMO to the LUMO by
photoexcitation, the HOMO plays an important role in the photo-
luminescence properties of the compound. Table 1 lists the
calculated HOMO and LUMO energies and energy gaps. Com-
pared with the results of our reported compound, it was observed
from Table 1 that the HOMO energies (eHOMO) and the LUMO ener-
gies (eLUMO) of POCP increased, –6.214 eV (POCP) > –7.896 eV
(
BVCZF, 9,9-dirthylhexyl-2,7-bis-(N,N-vinylcarbazole) filuorene)
and –0.682 eV (POCP) > –0.750 eV (BVCZF) because of the influ-
ence of the D-p style character in the different terminal substitu-
ents of carbazole derivative, which meant that p conjugation of
CONCLUSIONS
1
,10-phenanthroline is larger than that of carbazole.
In summary, a novel symmetric-type D-p-D-p-D charge transfer
carbazole-based molecule having terminal 1,10-phenanthroline
via carbon–nitrogen (C = N) double bond, POCP, was synthe-
sized. The 3PA effect of the compound was demonstrated using
a picosecond pulse at 1064 nm by Z-scan because of its great po-
tential for application in nonlinear optical materials. Also, the
Additionally, on the basis of the theoretical investigation of
[35]
Cronstrand et al. on the possible transition paths from ground
to final states, S , involved in a three-photon absorption process,
f
S ! S ! S ! S is present in symmetrical system and is also an
0
i
i
f
important contributor. The ZINDO/S semi-empirical method was
used to obtain the nature and the energy of the first six singlet–
singlet electronic transitions of the compound and we find that
–78
6
2
2
measured 3PA cross-section is 481 ꢀ 10 cm
•
s /photon for
POCP. Semi-empirical calculations of the transition dipole
moment between the ground state and the excited state were
performed using the ZINDO/S method. The experimental and
theoretical results demonstrated that the transition dipole
the first excited state, S , is the strongest one-photon absorption
state for this carbazole derivative. In our group reported papers,
the experimental and theoretical results have demonstrated
1
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

L. LI ET AL.
moment of the S ! S electronic transition of 3PA molecule
[12] I. Gryczynski, H. Malak, M. Schrader, P. Engelhardt, H. Kano, S. W.
0
1
Hell, Biophys. J. 1997, 72, 567.
was changed significantly by designing the special linkage of
the p chain with carbon–nitrogen (C = N) double bond and large
p-conjugated terminal substituents, 1,10-phenanthroline, to
enhance the 3PA cross-section of POCP. The investigation on
its applications is progressing in our laboratory, and the results
will be released in the future.
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[
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SUPPORTING INFORMATION
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Acknowledgement
This work was partially supported by the National Natural Sci-
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