Symmetric and asymmetric charge transfer process, of two-photon absorbing chromophores: bis-donor substituted stilbenes, and substituted styrylquinolinium and styrylpyridinium derivatives

Article abstract of DOI:10.1039/b009769l

Two-photon absorption properties of a series of symmetrically substituted stilbenes and asymmetrically substituted stilbene-type derivatives with the same conjugated length have been investigated. The effective two-photon absorption cross sections, δTPA,

Full text of DOI:10.1039/b009769l

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Symmetric and asymmetric charge transfer process of two-photon
absorbing chromophores: bis-donor substituted stilbenes, and
substituted styrylquinolinium and styrylpyridinium derivatives
XiaoMei Wang, Dong Wang, GuangYong Zhou, WenTao Yu, YuFang Zhou, Qi Fang and
MinHua Jiang*
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China.
E-mail: mhjiang@sdu.edu.cn; Fax: (z86)-0531-8564550; Tel: (z86)-0531-8564550
Received 6th December 2000, Accepted 27th March 2001
First published as an Advance Article on the web 30th April 2001
Two-photon absorption properties of a series of symmetrically substituted stilbenes and asymmetrically
substituted stilbene-type derivatives with the same conjugated length have been investigated. The effective two-
photon absorption cross sections, dTPA, as large as 62.0610248 cm4 s photon21 for D-p-D molecules and
48.5610248 cm4 s photon21 for D-p-A counterparts have been observed. The effect of these two types of
chromophores on the peak position of the linear absorption, one-photon fluorescence as well as two-photon
absorptivity is reported. Dipole moment change between the ground and the first excited states (Dmge), and the
transition dipole moment between the first and second excited states (Mee’) have also been calculated. It was
found that the asymmetrically substituted derivatives possess relatively large Dmge, whereas the symmetrical
counterparts show an increase in Mee’. Although a large two-photon absorption resonance is due to the
simultaneously high values of Mee’ and Dmge, correlated to intramolecular charge transfer, the former function
is larger. These results obtained have demonstrated that the magnitude and the peak position of two-photon
absorption depend not only on the amount but also on the direction of the intramolecular charge transfer.
design of molecules whereby one can systematically increase
Introduction
the TPA cross section and tune the position of the two-photon
absorption peak. The development of such a structure–
property relationship not only is greatly beneficial to the
design of new materials, but also makes these materials suitable
for the applications given.
Two-photon absorption (TPA) is a nonlinear optical process,
which can be described theoretically by either the two-state
model1 or the three-state model,2,3 that a molecule can be
excited from the ground state (S0) to the first excited singlet
state (S1) or to the second excited singlet state (S2) by
simultaneous absorption of two photons, (i.e. DES0-S1~2hn1,
or DES0-S2~2hn2). At present, the exploration of new
applications requires a better understanding of the relationship
between molecular structure and two-photon absorption
(TPA) properties. It is reported that many organic molecules
exhibit strong optical power limiting behavior resulting from
two-photon absorption (TPA).4–6 Two-photon polymerization
initiators for three-dimensional optical data storage could
improve the spatial resolution in the microfabrication.7,8
Three-dimensional fluorescence imaging,9 lithographic micro-
fabrication,10,11 laser device fabrication,12–14 two-photon
photodynamic therapy1,15 and two-photon pumped up-
conversion lasing16–18 are other promising applications for
the TPA property of organic molecules. This potential of using
two-photon absorbing molecules in applications has inevitably
stimulated research on the design, synthesis, and characteriza-
tion of new molecules with large two-photon absorptivities.
The ultimate goal is to determine the relationship between
molecular structure and the TPA property so as to optimize the
choice of application oriented molecules. In the exploration of
strong TPA compounds, Albota et al.2 have focused on
symmetric organic compounds and reported a design strategy
for symmetrically substituted conjugated molecules (D-p-D
type), while Reinhardt et al.19 have emphasized the D-p-A type
chromophores. The former emphasized the importance of
conjugation length and molecular symmetry, while the latter
laid stress on the planarity of p-center and donor–acceptor
strength. Therefore, there is an increasing need to understand
in detail about the influence of molecular symmetry on
molecular two-photon absorption properties, to guide the
We have reported previously on
a series of D-p-A
asymmetric chromophores, and investigated their two-
photon absorptivities20 and two-photon-pumped emission21
under the 1064 nm mode-locked Nd:YAG laser pulse. Experi-
mental and theoretical evidence have shown that the amount of
intramolecular charge transfer makes a positive contribution to
enhancing both the TPA cross section and the up-conversion
lasing efficiency. In this paper, a comparative study of the two-
photon absorption property for symmetrically and asymme-
trically substituted chromophores is reported for understand-
ing the relationship between the symmetric and asymmetric
charge transfer process and molecular two-photon absorption.
The results we report in this paper are as follows: (1) One-
photon absorption and fluorescence emission as well as the
solvent effects for symmetrically and asymmetrically substi-
tuted chromophores are discussed. (2) An examination of how
the transition dipole moment (Mee’) and dipole moment change
(Dmge) as well as the charge density distribution vary with the
molecular symmetry is presented. (3) The effect of symmetric
molecules on the peak position and magnitude of the two-
photon absorption has been studied by comparison. The
molecular structures considered are shown in Fig. 1.
Experimental section
Materials
The molecules shown in Fig. 1 possess ‘‘D-B-D’’ and ‘‘D-B-A’’
structures with the same conjugated length, where ‘‘D’’ is a
tertiary amino electron-donating group, ‘‘A’’ is an
N-methylpyridinium iodide or a N-methylquinolinium iodide,
‘‘B’’ is a stilbene bridge for symmetric molecules and a styryl
1600
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This journal is # The Royal Society of Chemistry 2001
DOI: 10.1039/b009769l

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Fig. 1 Molecular structures of the D-p-D and D-p-A chromophores discussed in this paper.
bridge for asymmetric molecules, respectively. Here, the new
‘‘D-p-D’’ chromophores were firstly synthesized by Ti-
catalyzed reductive coupling of p-substituted aminobenzalde-
hydes and have been characterized by X-ray diffraction, mass
spectra and elemental analyses with the same methods as
reported previously.20 The syntheses of ‘‘D-p-A’’ molecules in
Fig. 1 have been reported.20
mp 172.0 ‡C. Mass spectrum, m/z 326 (Mz), 295, 249, 132.
Elemental analysis: Calcd.: C, 73.59; H, 8.03; N, 8.58. Found:
C, 73.15; H, 8.42; N, 8.91%.
Crystal structure. The molecular structures of BDPAS and
DPS and their packing diagram are shown in Figs. 2–5,
respectively. The BDPAS crystal belongs to the triclinic crystal
¯
system of centrosymmetric space group P1 with unit cell
Synthesis of symmetrically substituted stilbene derivatives.
Under anhydrous and oxygen-free conditions, a mixture of
TiCl4 (2.2 mL, 20 mmol), zinc (4.4 g, 60 mmol), sodium metal
(2.3 g, 100 mmol) and 75 mL of dry tetrahydrofuran (THF)
was stirred in a flask at room temperature. After refluxing for
one hour, a black mixture was obtained, which was then cooled
to room temperature. A solution of 4-pyrrolidin-1-ylbenzalde-
hyde21 was added slowly to the flask and further refluxed for
20 h. The mixture was then cooled, hydrolyzed by adding
dropwise 5% HCl and neutralized by using 2 M sodium
hydroxide solution, respectively. The mixture was extracted
with chloroform. The organic layer was concentrated by
evaporation and purified by column chromatography on silica
gel using appropriate solvent as eluent. After evaporation, (E)-
4,4’-di(pyrrolidinyl)stilbene, (DPS) green–yellow crystals were
obtained with a yield of 65% and mp 298.9 ‡C. Mass spectrum,
m/z 318 (Mz), 289, 275, 262, 159. Elemental analysis: Calcd.:
C, 82.97; H, 8.23; N, 8.80. Found: C, 83.36; H, 7.88; N, 8.45%.
Replacing 4-pyrrolidin-1-ylbenzaldehyde with 4-(N,N-
diphenylamino)benzaldehyde, 4-(N,N-diethylamino)benzalde-
hyde, and 4-[(N-hydroxyethyl-N-methyl)amino]benzaldehyde,
respectively, the following chromophores were obtained.
(E)-4,4’-Bis(diphenylamino)stilbene (BDPAS): Bright yellow
slice crystals. Yield 60% and mp 255.9 ‡C. Mass spectrum, m/z
514 (Mz), 257, 167, 77. Elemental analysis: Calcd.: C, 88.68; H,
5.88; N, 5.44. Found: C, 88.94; H, 5.58; N, 5.36%.
parameters of a~10.6231(12), b~11.3337 (10), c~
˚
12.491(3) A, a~73.503 (9), b~89.978 (10), c~79.759 (8)‡.
Least-square plane calculations show that the central stilbene
unit possesses perfect planarity, but two terminal benzene rings
deviate from both central stilbene planarity and themselves.
For example, the dihedral angles between each terminal
benzene ring and the central stilbene unit are 102.2 and
106.4‡, respectively, while that between two terminal benzene
rings is 61.8‡. The packing diagram shows that the BDPAS
molecules stack in pairs, aligning at 45‡ to the b axis and c axis.
The DPS crystal belongs to a monoclinic crystal system of
centrosymmetric space group P21/c with unit cell parameters of
˚
a~6.5813(6), b~7.7337(7), c~17.6791(16) A, a~90, b~
105.268(7), c~90‡. The packing diagram shows that the DPS
(E)-4,4’-Bis(diethylamino)stilbene (BDEAS): Yellow slice
crystals with yield 54%. Mass spectrum, m/z 322 (Mz), 307,
278, 263, 146. Elemental analysis: Calcd.: C, 81.94; H, 9.38; N,
8.69. Found: C, 81.45; H, 9.58; N, 8.36%.
(E)-4,4’-Bis(N-hydroxyethyl-N-methylamino)stilbene
(BHMAS): Green–yellow needle crystals with yield of 50% and
Fig. 2 The molecular structure of BDPAS crystal.
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Fig. 3 Packing diagram of BDPAS.
Fig. 6 Linear absorption and single-photon fluorescence spectra of
BDPAS (dotted), BDEAS (solid), BHMAS (dash dotted), and DPS
(dashed) in DMF solvent at d0~161025 M. Absorption spectra are
the left ones and fluorescence spectra are the right ones in each pair
(The fluorescence peak positions of BHMAS and DPS are red-shifted
20 nm compared with the original measured data).
recording spectrophotometer. One-photon induced fluores-
cence spectra of a Shimadzu RF5000U fluorophotometer at a
concentration of 1.061025 M in DMF or in other different
solvents were recorded. These spectra are shown in Figs. 6–8.
Nonlinear optical properties. Two-photon absorptivitives of
all the chromophores were measured by direct nonlinear
optical transmission (NLT). An optical parameter amplifier
(OPA) pumped by a mode-locked Nd:YAG laser was used as
pumping source which can be tuned in the range of 400–
2000 nm, while the input–output energy was measured by a
two-channel energy-meter. The influences of the quartz cell and
the linear absorption of the solvents have been subtracted.
Then two-photon absorption coefficient, b (in units of
cm GW21), was calculated by using measured nonlinear
transmissivity (Ti) for a given input intensity (I0) and thickness
of a sample solution (L) from eqn. (1); finally, the two-photon
absorption cross section, dTPA (in units of cm4 s photon21) can
be determined by eqn. (2),22,23
Fig. 4 The molecular structure of DPS crystal.
Ti~½ln (1zbLI0)Š=bLI0
TPA~hnb|103=NAC0
(1)
(2)
Fig. 5 Packing diagram of DPS.
d
molecules, stacking in pairs, form a layer-like structure along
the c axis. Least-square plane calculations show that the central
stilbene unit possesses perfect planarity, while the dihedral
angle between stilbene and pyrrolidinyl ring is 8.6‡, which is
different from that of BDPAS due to the variation with
terminal rings.
where NA and C0 are Avogadro’s constant and the molar
concentration of solute, respectively, and hn is the energy of the
incident photon. The calculated b and dTPA values are shown in
Table 1.
Results and discussion
CCDC 154227. See http://www.rsc.org/suppdata/jm/b0/
b009769l/ for crystallographic files in .cif format.
Linear absorption and single-photon fluorescence spectra
Figs. 6–8 present linear absorption and one-photon fluores-
cence spectra of symmetric and asymmetric chromophores,
respectively. First of all, the absorption spectra of D-p-A
compounds are noticeably red-shifted with respect to those of
their symmetric counterparts. The peak absorption for
symmetric stilbene derivatives is located at 374–387 nm (on
the left side of Fig. 6), while those of asymmetric pyridinium
derivatives are at y480 nm, and quinolinium derivatives at
y550 nm (on the left side of Fig. 7). This due to the electronic
interactions between donor and acceptor inducing a spectral
shift to lower energy with respect to those between donor and
donor. Along with the red shift of the absorptivities, one-
photon fluorescence intensity of chromophores decreases
drastically from symmetric molecules to pyridinium,
and further to quinolinium derivatives. From the right of
Calculations
Charge density distribution of the ground and the first excited
states was calculated using the PM3 semi-empirical method.
Dipole moment change between ground and first excited states
(Dmge), and transition dipole moment between first and second
excited states (Mee’
program.
) were calculated using the ZINDO
Optical properties measurement
Linear optical properties. Linear absorption spectra for all
chromophores in DMF with d0~1.061025 M have been
measured in quartz cuvettes of 1 cm path on a Hitachi U-3500
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small red shift generally with decreasing solvent polarity
parameter ET (30), e.g., the maximum shifts from 445 nm in
benzyl alcohol (ET (30), 50.8), 450 nm in DMF (ET (30), 43.8),
420 nm in chloroform (ET (30), 39.1), and to 470 nm in toluene
(ET (30), 33.9). It is interesting to note that asymmetric
molecules in chloroform give the strongest emission while
symmetric molecules in chloroform give the weakest, and in
one case even quenches fluorescence completely. These results
indicate that the asymmetric molecules are more polar in the
excited state than in the ground state,27 and an increase in the
polarity of the solvent will lower the energy level of the charge
transfer excited state. In addition, the TICT process, pre-
sumably favored in more polar solvents, will result in a
pronounced decrease in the fluorescence yield.27 The predicted
dipole moments obtained by PM3 calculations confirm that the
dipole moment of PSPI in the excited state is greater than that
in the ground state by 3.4 D. In contrast, the difference in
dipole moment (Dmge) for BDPAS is almost negligible.
Therefore, the influence of the solvent effect upon the
fluorescence behavior is different from that of the asymmetric
counterparts.
Fig. 7 Linear absorption of PSPI (solid), DEASPI (solid), HEASPI
(solid), HMASQI (solid), and PSQI (solid) in DMF solvent at
d0~161025 M, and single-photon fluorescence spectra of HEASPI
(dotted), DEASPI (dashed) and PSPI (dash dotted) in DMF solvent at
d0~161025 M.
Figs. 6–7, one can see that the relative fluorescence intensities
for symmetric chromophores are almost 100 times as strong as
those of their pyridinium counterparts, and for quinolinium
derivatives, there is no detectable one-photon fluorescence.
Since the measurement is performed under exactly the same
conditions, the change of the relative fluorescence intensity
means that the fluorescence quantum yields follow the same
sequence: symmetric chromophoreswpyridinium derivatives
wquinolinium derivatives. This is probably due to a partial
quenching24 associated with a twisted intramolecular charge
transfer (TICT) state. Since the structure of asymmetric
molecules satisfies all the criteria of the TICT process,
namely, an electron donor (disubstituted amino) and an
aromatic acceptor moiety joined by a flexible bond, there is
more tendency for asymmetric molecules to take part in a
twisted charge transfer process than their symmetric counter-
parts. To examine further the different excited state emission
behavior of the symmetric and asymmetric chromophores, the
emission spectra of BDPAS and PSPI in various solvents were
determined, and presented in Fig. 8. The molecular polarity
parameters ET (30)25 are shown in Fig. 8 for the relevant
solvents. It is interesting to note that there are quite different
solvent effects between the symmetric and asymmetric
chromophores. One can see from Fig. 8 that the asymmetric
chromophore PSPI gives a red shift with an increase in solvent
polarity parameter ET (30), e.g. the maximum shifts from
567 nm in chloroform (ET (30), 39.1), 590 nm in DMF (ET (30),
43.8) to 599 nm in acetonitrile (ET (30), 46.0), which is the same
as noted for ASPI previously.26 In contrast, the fluorescence
spectra of symmetric BDPAS in different solvents exhibit a
Symmetric, asymmetric charge transfer process and two-photon
absorption
Table 1 presents the calculated dipole moment difference
between the ground and first excited state (Dmge) and transition
dipole moment between the first and second excited states
(Mee’) for all chromophores. It is interesting that the symmetric
chromophores all have a negligible Dmge value, while the
asymmetric counterparts show a large dipole moment differ-
ence. Moreover the variation in the transition dipole moment
(Mee’) for these two types of molecules also shows a different
behavior, which we consider to originate from their intramo-
lecular charge transfer character.
Calculated charge density distributions for the symmetric
molecule DPS, as presented in Fig. 9 show that the charge
transfer shifts from the central stilbene unit to the two terminal
substituted amino group in the S0 ground state, that is,
Q
amino~20.0255e and Qcentr~z0.0510e, whereas in the S1
state the charge transfer process is reversed, shifting from the
two terminal groups to the central stilbene moiety, that is,
Qamino$z0.031e and Qcentr$20.062e. Although such a
symmetric charge transfer behavior results in a negligible
Dmge value, it can greatly affect the quadrupole moment, which
is beneficial in enhancing the corresponding transition dipole
2
moment (Mee’
asymmetric quinolinium or pyridinium derivatives there is a
)
(seen in Table 1). In contrast, for the
Fig. 8 One-photon induced fluorescence spectra for symmetric BDPAS
(on the left) and symmetric PSPI (on the right) in various solvents with
the same concentration of d0~1.061025 M. Curve (d) for PSPI is
obtained by reducing the original measured data 10 times in the
intensity scale.
Fig. 9 PM3-optimized charge density (e) for DPS in the S0 (a) and S1
(b) states. The individual charge densities on the molecular backbone
represent the summations of the charge densities on each carbon atom
and the hydrogen atom(s) attached. D and B provide the total charge
on the amino group and the stilbene bridge, respectively.
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Table 1 Nonlinear optical properties and calculated structural parameters for symmetric and asymmetric chromophores
Chromophores
Dmge/debye
Mee’/debye
b
TPA/cm GW21
dTPA (610248) cm4 s photon21
lmaxTPA/nm (solvent, d0)
D-p-D
D-p-A
DPS
0.02
0.03
0.04
0.01
3.4
8.2
8.1
10.0
10.6
13.6
13.0
14.4
12.2
8.5
8.6
8.2
7.3
8.7
1.19
0.81
1.37
0.68
4.04
3.38
4.47
0.07
0.15
57.7
44.5
62.0
39.4
28.5
24.0
38.8
26.8
48.5
600 (toluene, 0.0005 M)
590 (DMF, 0.005 M)
730 (toluene, 0.005 M)
730 (toluene, 0.005 M)
930 (DMF, 0.05 M)
930 (DMF, 0.05 M)
930 (DMF, 0.05 M)
970 (DMF, 0.001 M)
1010 (DMF, 0.001 M)
BHMAS
BDEAS
BDPAS
PSPI
HEASPI
DEASPI
PSQI
HMASQI
major asymmetry of charge density distribution. For example,
the charge distribution of PSPI, presented in Fig. 10, shows
that there exists a strong dipolar character, directing from the
substituted amino group to the acceptor group in the S0 state,
that is, QD~z0.2133e and QA~20.2611e. Further charge
separation occurs in the excited state, that is, QD~z0.6670e
and QA~20.8365e. Similar results were also obtained for all
the other asymmetric molecules. Clearly, such a charge transfer
behavior results in a large difference in dipole moment (Dmge)
between the ground and first excited state (see in Table 1).
It is accepted that the molecular TPA cross section, dTPA, is
proportional to the imaginary part of the third-order polariz-
ability,2 and if the frequency (v) is neglected, the third-order
polarizability, c, can be expressed as eqn. (3),28,29
"
#
Mg2e Dm2
Eg2e Ege
Mg2e
2
X
Mee’
Eg2e’
dTPA!cxxxx~24
ge z
{
(3)
Ege
e’
Therefore, the dTPA value depends on the ground state to
first excited state dipole moment change (Dmge), the transition
dipole moment between the first and second excited state (Mee’
and the energy differences between the ground and the excited
states (Ege, or Ege’), etc.
)
It is understandable that the difference of symmetric or
asymmetric charge transfer for structural parameters may
result in the variation in molecular two-photon absorptivity.
As shown in Table 1, the symmetric structural chromophores
exhibit TPA cross sections almost twice as large as those of
their asymmetric counterparts. For example, the dTPA values
vary from 62.0610248 cm4 s photon21 for BDEAS, to
38.8610248 cm4 s photon21 for DEASPI. When the donor
group is pyrrolidinyl, the dTPA values vary from 57.76
10248 cm4 s photon21 for DPS, to 28.5610248 cm4 s photon21
for PSPI, and 26.8610248 cm4 s photon21 for PSQI. When the
donor group is N-hydroxyethyl-N-alkylamino, the dTPA values
vary from 44.5610248 cm4 s photon21 for BHMAS, to
24.0610248 cm4 s photon21 for HEASPI. Though the sym-
metric chromophores have a negligible dipole moment change
(Dmge), they exhibit larger dTPA values and we can assume that
the transition dipole moment (Mee’) function plays a much
more important role in enhancement of the dTPA value. With an
increase of Mee’ value, the symmetric molecules possess the
same sequence in their dTPA values: BDEASwDPSw
BHMASwBDPAS, which means that the dTPA value is
proportional to the amount of intramolecular charge transfer.
In the case of their asymmetric counterparts, any simultaneous
increase of Mee’ and Dmge serves to increase their dTPA values. It
is for this reason that HMASQI and DEASPI exhibit relatively
large dTPA values. As for PSPI and PSQI, the large dTPA value
for PSPI is due to its larger Mee’ value. As for HEASPI, its
relatively large Ege (seen in Fig. 7) can contribute to making its
dTPA value smaller.
Fig. 10 PM3-optimized charge density (e) for PSPI in the S0 (a) and S1
(b) states. The individual charge densities on the molecular backbone
represent the summations of the charge densities on each carbon atom
and the hydrogen atom(s) attached. D, B, and A provide the total
charge on the amino group, the styryl bridge and the acceptor,
respectively.
It is interesting to compare the effect of the type of symmetry
on the TPA peak positions, shown in Fig. 11 and Table 1. The
peak positions for symmetric molecules are located at 590–
730 nm, while those for the asymmetric counterparts are
located at 930–1010 nm, that is, at nearly twice the wavelength
value of their corresponding one-photon absorption peak.
Comparing DPS, BHMAS and BDEAS, as shown in Fig. 6 and
Fig. 11, one can see that although their linear absorption peaks
(l(1)max) are all at y374 nm, the two-photon absorption peaks
(l(2)max) are observed at 730 nm for BDEAS, and 600 nm for
DPS, and 590 nm for BHMAS. Thus BDEAS exhibits a
significant red shift of its two-photon absorptivity. A similar
red shift also occurs in the asymmetric quinolinium derivatives.
For example, the l(1)
of HMASQI and PSQI lie at
is at 1010 nm for HMASQI and
max
max
Fig. 11 The TPA cross sections (dTPA) for parts of chromophores in
DMF (d0~0.05 M) for PSPI, DEASPI and HEASPI; and in DMF
(d0~0.005 M) for BHMAS, in toluene (d0~0.005 M) for BDEAS and
BDPAS, in toluene (d0~0.0005 M) for DPS.
y550 nm, but the l(2)
970 nm for PSQI (seen in Fig. 7 and Table 1). Such a
bathochromic shift of the two-photon absorptivity for
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3
4
5
Conclusions
A
comparative study of electronically symmetrical and
electronically asymmetrical charge-transfer chromophores of
the D-p-D and D-p-A types respectively is reported. It was
interesting to note that the molecular structural symmetry
exerts a markedly different behavior on the linear and
nonlinear optical properties of the chromophores.
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Solvent effect studies have shown that the asymmetric
chromophores are more polar in the excited state than in the
ground state while symmetric counterparts behave differently.
There is a trend for the asymmetric chromophores to take part
in a twisted charge transfer process more readily than their
symmetric counterparts, so the one-photon fluorescence
intensity for the asymmetric molecules is dramatically
decreased, in comparison with their symmetric counterparts.
The influence of the direction of the intramolecular charge
transfer process, i.e., symmetric and asymmetric, on the two-
photon absorption cross section has been theoretically and
experimentally demonstrated. We have shown that asymmetric
chromophores exhibit relatively larger ground state–first
excited state dipole moment change (Dmge), whilst their
symmetric counterparts exhibit relatively larger transition
dipole moments between the first and second excited states
(Mee’). Although a high TPA cross section is provided by the
simultaneously high values of Mee’ and Dmge, the former
parameter is more important. As a consequence, the symmetric
molecules exhibit larger dTPA values compared to their
asymmetric counterparts.
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