Degenerate nonlinear absorption and optical power limiting properties of asymmetrically substituted stilbenoid chromophores

Article abstract of DOI:10.1039/b313185h

Two-photon absorption (2PA) spectra (650-1000 nm) of a series of model chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white-light continuum beam. The experimental results suggested that when either an electron-donor or an electron-acceptor was attached to a trans-stilbene at a para-position, an enhancement in molecular two-photon absorptivity was observed in both cases, particularly in the 650-800 nm region. However, the push-pull chromophores with both the donor and acceptor groups showed larger overall two-photon absorption cross-sections within the studied spectral region as compared to their mono-substituted analogues. The combined results of the solvent effect and the ¹H-NMR studies indicated that stronger acceptors produce a more efficient intramolecular charge transfer character upon excitation, leading to increased molecular two-photon responses in this model-compound set. A fairly good 2PA based optical power limiting behavior from one of the model chromophores is also demonstrated.

Full text of DOI:10.1039/b313185h

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A R T I C L E
Degenerate nonlinear absorption and optical power limiting
properties of asymmetrically substituted stilbenoid
chromophores{
Tzu-Chau Lin,a Guang S. He,a Paras N. Prasad*a and Loon-Seng Tanb
aDepartment of Chemistry, Institute for Lasers, Photonics and Biophotonics, University at
Buffalo, State University of New York, Buffalo, New York 14260-3000.
E-mail: pnprasad@acsu.buffalo.edu
bPolymer Branch, AFRL/MLBP, Materials & Manufacturing Directorate, Air Force
Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio, 45433-7750
Received 21st October 2003, Accepted 7th January 2004
First published as an Advance Article on the web 12th February 2004
Two-photon absorption (2PA) spectra (650–1000 nm) of a series of model chromophores were measured via
a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond
white-light continuum beam. The experimental results suggested that when either an electron-donor or an
electron-acceptor was attached to a trans-stilbene at a para-position, an enhancement in molecular two-photon
absorptivity was observed in both cases, particularly in the 650–800 nm region. However, the push–pull
chromophores with both the donor and acceptor groups showed larger overall two-photon absorption cross-
sections within the studied spectral region as compared to their mono-substituted analogues. The combined
1
results of the solvent effect and the H-NMR studies indicated that stronger acceptors produce a more efficient
intramolecular charge transfer character upon excitation, leading to increased molecular two-photon responses
in this model-compound set. A fairly good 2PA based optical power limiting behavior from one of the model
chromophores is also demonstrated.
organic chromophore solutions have been reported by Negres
1. Introduction
et al.11 In that work, a weaker continuum white-light (450–
750 nm) was employed as a probe beam, in conjunction with a
stronger 1210 nm laser beam as a pump beam to excite a non-
degenerate 2PA process (nonlinear absorption of two photons
with different wavelengths) in the sample medium. Another
similar pump-probe experimental set-up based on picosecond
continuum for the measurement of two-photon transient
absorption spectra has also been reported by Oulianov et al.12
Recently, an alternative method to measure degenerate 2PA
(nonlinear absorption of two photons with the same wave-
length) spectrum by utilizing a single and powerful femto-
second white-light continuum beam has been reported.13 The
major advantage of this latter approach is that since both
spectral tunability and wavelength-scanning mechanism during
the measurement are not required, the entire degenerate 2PA
spectrum for a given sample material can be recorded rapidly.
In this paper, we wish to present our studies of some structural
parameters that contribute to enhancing the molecular two-
photon absorptivities based on the selected model structures
and on the new approach to measure their degenerate 2PA
spectra. Simple nuclear magnetic resonance (NMR) techniques
were used to rank the relative electron-withdrawing strengths of
the dye acceptor groups. We also studied the solvatochromic
behavior of these organic structures, which might provide
information on the correlation of electron-acceptor strength
and the molecular 2PA behaviors of these charge-transfer dyes.
As an illustration for an application of organic two-photon
materials, the optical power limiting capability of one of the
model chromophores was demonstrated.
Two-photon absorption (2PA) was theoretically predicted by
Maria Go¨ppert-Mayer in 1931,1 the first experimental observa-
tion of this nonlinear optical phenomenon was realized about
30 years later when Kaister and Garret observed two-photon
induced upconverted fluorescence emission from a CaF2:Eu21
inorganic crystal in 1961.2 With the advent of high peak-power
laser systems, some early potential applications based on multi-
photon absorption phenomenon were demonstrated by Rentzepis
and Parthenopoulos for optical data storage3 and by Webb and
co-workers for microscopy.4 However, the relatively small two-
photon absorption cross-sections of earlier materials limited their
widespread utility. In the last decade, reports of new organic
chromophores with greatly improved two-photon absorption
cross-sections and up-converted fluorescence5 have not only led
to an increased interest in the development of highly efficient
two-photon active materials, but also opened up numerous
possibilities for new applications. These applications in photonics
and biophotonics include two-photon up-conversion lasing,6
two-photon absorption based optical power limiting and
stabilization,7 3-D optical data storage,3,8 two-photon excited
fluorescence for nondestructive evaluation and bio-imaging,9
and two-photon photodynamic cancer therapy.10
Nevertheless, the realization of the full potential of the two-
photon technology still requires continuing efforts in theory,
synthesis and optical characterization to provide further
understanding of the structure–property relationship which
can aid the development of highly active organic two-photon
chromophores for practical purposes. For this reason, it is
very helpful if the complete 2PA spectra for chromophores of
interest can be readily obtained. In fact, very recently, the
continuum-generation based 2PA spectral measurements for
2. Model compounds
The primary objective of this study is to choose a series of model
compounds and to investigate some structural parameters
which may affect molecular 2PA behaviors. These parameters
{ Electronic supplementary information (ESI) available: Experimental
details. See http://www.rsc.org/suppdata/jm/b3/b313185h/
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Fig. 1 Chemical structures of the selected stilbenoid chromophores studied in this work.
include (i) the contribution from the selected electron-donor
(D) and/or the electron-acceptor (A) (i.e. push–pull effect), (ii)
the contribution from the electron-withdrawing strength of
the selected acceptors (i.e. acceptor strength effect). Based
on this consideration, we have chosen trans-stilbene as the
p-bridge, diphenylamino functional group as the donor, and
four electron-withdrawing functional groups with various
electron-pulling strength as the acceptors to constitute D–p,
p–A, and D–p–A chromophore structures. Fig. 1 shows the
chemical structures of the chosen model chromophores studied
in this work.
coefficients vary from 2360 to 5175 M21 cm21. From Fig. 2a,
one can see that compared to the unsubstituted trans-stilbene
(TSB), both mono-substituted compounds Dor and Acc show
red-shifted maximum linear absorption wavelengths which
suggests that both the donor and the acceptor functional
groups have a positive contribution in increasing the p-electron
delocalization on the trans-stilbene backbone upon excitation.
Furthermore, the linear absorption maximum of Dor is more
red-shifted. This implies that the diphenylamino group is com-
paratively more effective in promoting p-electron delocaliza-
tion than the benzothiazole moiety in this system.
The synthetic routes for these model chromophore molecules
are briefly outlined in Schemes 1–3. All these model com-
pounds were rigorously purified prior to characterization
effort. The data for their elemental analyses and spectroscopic
characterization are provided as ESI{.
From the linear absorption spectra of compounds S101, 101,
BT101 and N101 shown in Fig. 2b, we observe further red-
shifted linear absorption maximum (compared to TSB, Dor
or Acc) with the order of S101 v 101 y BT101 v N101, which
indicates that the combination of an electron-donor and an
electron-acceptor will further enhance the p-electron delocaliza-
tion upon excitation in these structures.
3. Results and discussion
We have measured the linear transmission spectra of these
dye solutions at the same concentration (0.02 M in THF) used
for the degenerate two-photon absorption spectra measure-
ment in order to verify that no linear absorption occurs
within the spectral range of 650 nm to 1300 nm. We have found
that there is no linear absorption in the above-mentioned
spectral range for all chromophore solutions, it follows that
any intensity-dependent nonlinear absorption activities in this
spectral region can be readily recorded based on the newly
developed degenerate 2PA absorption spectral measurement
method.13 As an example, Fig. 3 shows the measured
I. Optical properties characterization
Linear (one-photon) absorption spectra measurement. Linear
absorption spectra of the studied model chromophores in
the solution phase were measured by using a Shimadzu
UV-3101PC spectrophotometer. All the sample solutions were
freshly prepared in THF at a concentration of 1 6 1025 M and
filled in 1 cm path-length quartz cuvettes for this measurement.
Figs. 2a and 2b show the recorded linear absorption spectra of
these chromophore solutions. The linear absorption maxima of
these dyes range from 296 nm to 432 nm and their extinction
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Scheme 1 Synthetic routes to the mono-brominated triphenylamine and intermediates with nitro and alkylsulfonyl functional groups.
Scheme 2 Synthetic routes to the key intermediates with oxadiazole and benzothiazole functional groups.
transmission and absorption spectra of N101 solution with
Clark-MXR ORC-1000). The pulse duration, wavelength, repeti-
tion rate and pulse energy of this pump beam were y140 fs,
y790 nm, 1 KHz, and 150 mJ, respectively. The generated
white-light continuum beam was then collimated and passed
though a dispersion prism made of SF10 glass. Such a spatially/
spectrally dispersed white-light beam was focused onto the
central region of a 1 cm path-length quartz cuvette via an f ~
10 cm lens. The quartz cuvette is either filled with the chromo-
phore solution or the pure solvent (here in our case, the solvent
is THF). With this particular experimental set-up, different
spectral components of the white-light continuum beam are
spatially separated from each other at the sample position and
only degenerate two-photon absorption from the same spectral
high concentration (0.02 M), as this compound possesses the
most red-shifted linear absorption maximum as compared to
the other analogues.
Degenerate two-photon absorption (2PA) spectra measurement.
Degenerate 2PA spectral measurements of these model com-
pounds were accomplished utilizing a spectrally dispersed
femtosecond white-light continuum generation from heavy
water (D2O).13 Fig. 4 illustrates the specially designed experi-
mental set-up for this purpose. The pump source for con-
tinuum generation was a focused ultra-short pulsed laser beam
from a Ti:sapphire laser oscillator/amplifier system (CPA1,
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Scheme 3 Final coupling reaction procedures for the chromophore structures by following Heck reaction.
Fig. 2 Linear absorption spectra of the chromophore solutions (a) TSB, Acc, Dor and (b) S101, 101, BT101, N101 (sample solution concentra-
tion = 1 6 1025 M; 1 cm pass-length; THF).
Fig. 3 Linear transmission and absorption spectra of N101 solution at high concentration (0.02 M in THF and 1 cm path-length).
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Fig. 4 Optical set-up for single femtosecond continuum beam based degenerate two-photon absorption spectral measurement.
components could take place within the sample solution. A
CCD array detector was used to record the spectral intensity
distribution of the transmitted continuum beam at sample
position. By comparing the obtained continuum spectra pro-
file of the sample solution with that of the pure solvent, the
attenuation of different spectral components due to degenerate
2PA of the studied chromophore can be determined. This
nonlinear attenuation of different spectral components pro-
vides the important information of relative 2PA cross-section
values as a function of wavelengths for each chromophore,
which can be finally converted into the 2PA spectrum in an
absolute scale of absorption cross-section values based on a
standard calibration process at a single laser wavelength.13
Furthermore, we have optimized the input pump power level
so that the intensity level of the output white-light continuum
beam will not create other undesirable nonlinear optical
effects12 in the sample solution, except two-photon absorption.
Fig. 5a and 5b show the measured 2PA cross-sections as a
function of wavelengths for the investigated chromophore
structures. The unit of the 2PA cross-section used in these
figures is GM, which is defined as 1GM ~ 1 6 10250 cm4 s per
photon-molecule.
Two-photon excited fluorescence measurement. Most of the
sample solutions studied manifest 2PA-induced frequency
up-converted fluorescence emission. Fig 6 presents two-photon
induced fluorescence spectra of compounds S101, 101, BT101
and N101. All the sample solutions were prepared in the same
manner at concentration of 16 1023 M in THF for this
measurement. The excitation laser pulses of y790 nm were
Fig. 5 (a) Measured two-photon absorption spectra of TSB, Acc and Dor in THF solution at 0.02 M (with experimental error y¡15%).
(b) Measured two-photon absorption spectra of chromophores S101, 101, BT101 and N101 in THF solution at 0.02 M (with experimental error
y¡15%).
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Fig. 6 Two-photon absorption (at ~790 nm) induced fluorescence emission spectra of S101, 101, BT101 and N101 chromophore solutions at
1 6 1023 M in THF.
law, which confirms that the 2PA process is responsible for the
observed up-converted fluorescence emission.
II. Relative electron-acceptor strength studies
In view of the relative simplicity and reliability of the methods
to correlate the electron-withdrawing strength of the selected
acceptors and the molecular 2PA behavior in this model-
compound set, we have performed both 1H NMR experiments
(operating frequency: 400 MHz) to determine the trend for the
relative electron-pulling strength of these four acceptors and
solvent effect studies of these model compounds.
Proton NMR study. Table 1 presents the NMR spectral data
of the aromatic proton Ha of four chosen representative
structures for studying the strength of the deshielding effect
from these four electron-withdrawing functional groups. It
Fig. 7 Measured intensity dependence of two-photon induced fluore-
scence on the relative input intensity of ~790 nm laser pulses for
compound 101 solution at 1 6 1023 M in THF.
should be noted that the first three representative structures are
actually the key intermediates (compounds 2, 6, 9) for the
synthesis toward the model chromophores, while the fourth
one is purchased directly from Aldrich and used as received
without further purification. It is well known that the
resonance peaks of aromatic protons Ha will be shifted
toward downfield in the 1H NMR spectra when a stronger
acceptor is attached covalently to the phenyl ring as shown
in Table 1. The experimental data showed that the deshielding-
strength ordering of these electron-acceptors is as follows:
nitro w benzothiazole w 1,3,4-oxadiazole w alkylsulfonyl.
This order parallels that of their electron-withdrawing abilities
in this model set.
generated from the same Ti:sapphire laser oscillator/amplifier
system mentioned above. The average pulse energy and the
pump wavelength utilized for this measurement were y5 mJ
and y790 nm. Fluorescence spectral measurements were
accomplished by using a HoloSpec CCD-array spectrometer
in conjunction with a fiber coupler head.
The dependence of the 2PA induced fluorescence intensity on
the excitation intensity of these chromophore solutions were
also examined. As an example, Fig. 7 shows the measured
relative intensity of 2PA-induced fluorescence as a function of
the input pump intensity for chromophore 101 solution at a
concentration of 1 6 1023 M. The measured data (L) are in
good agreement with the best fitting curve following the square
Similar trend of the electron-acceptor strength has been
determined and reported before by G. A. Pagani and the
co-workers who have employed p-charge/13C NMR chemical
shift relationships to study the relative electron-withdrawing
Table 1 1H-NMR chemical shift values of aromatic proton Ha influenced by different electron-acceptors
Acceptor
–NO2
8.20
dppm(Ha)
7.78
8.01
8.05
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Table 2 The Stokes shifts data at different solvent Df of the studied
chromophores in this work
chromophore solutions based on their linear absorption–
emission spectra.16 The calculated values for the solvent-
mixture Df and Stokes shift are summarized in Table 2.
Dyv/cm21
Based on the data collected in Table 2, we prepared Lippert
plots (Dyv vs. Df ) for these six compounds, which are illustrated
in Fig. 8. No significant deviation from the linearity, predicted
by eqn. (1), was observed for any of the compounds studied
here, implying that no specific interaction between the solvent
and the solute in our system was involved. In our case, the
experimental observation might suggest that the local mole-
cular environment surrounding the studied chromophore
consists of a uniform mixture of THF and toluene molecules
i.e. the composition of the solvent cage surrounding the studied
chromophore is roughly the same as the bulk composition and
therefore, there is no significant solvent segregation occurring
in the solvent cage, which may make the commonly expected
preferential solvation phenomenon not very obvious. A similar
observation was reported for another series of two-photon
absorbing chromophores in mixed THF–hexane solvents.17
Nevertheless, further study should be pursued in order to
obtain a better understanding of the detailed solvent-solute
interaction in our system. The calculated solvatochromic slopes
from the linear fitting of these experimental data and other
related data of these chromophores are collected in Table 3.
As mentioned above, the intramolecular charge transfer
Solvent
mixture Dfa
N101
BT101
101
S101
Dor
Acc
0.0131
0.0058
0.0879
0.1337
0.1901
0.2099
3884
4227
4881
5120
5639
5817
3354
3669
3889
4397
4854
5136
3181
3527
3712
4225
4634
4889
3580
3898
4116
4544
4917
5136
3062
3645
4318
4537
4067
3328
3621
3660
3700
3739
3739
3799
a Orientational polarizabilities of the solvent mixture are calculated
h
i
e{1
2ez1
n2{1
2n2z1
by following the equation: Df ~
{
, where e is the dielec-
tric constant and n is refractive index at 589.32 nm, which is directly
obtained from ref. 18.
strength of different subsitituent groups in benzyl carbanion
systems.14
Solvent effect study. It is well known that measuring the
Stokes shift of a dye solution as a function of the orientational
polarizability of the solvent is a common method to study the
solvent effect based on the Lippert–Mataga equation:15
ꢀ
ꢀ
2
ꢀ
e{1
ꢀ
Dꢀ
1
~
Dv~
Df zConst:
(1)
(2)
2pe0hc a3
Table 3 Solvatochromic slope, sphere radius, dipole moment change
and calculated distance of charge shift data of N101, BT101, 101, S101,
Dor and Acc
ꢁ
ꢂ
n2{1
Df ~
{
2ez1 2n2z1
Solvatochromic
slope
a
c
˚
a/A
˚
dd/A
In this model, a chromophore molecule is treated as a dipole
in a sphere of radius a within a dielectric medium (n,e) and
a linear correlation between Stokes shift (Dyv/cm21), and
orientational polarizability (Df ) of this dielectric medium is
predicted. Solvatochromism analysis can provide a fairly good
estimation of the dipole-moment change (Dm) of the chromo-
phore in the solution phase upon excitation. This change in
dipole moment corresponds to a distance of intramolecular
charge shift along the p-bridge of chromophores upon
excitation,16 which can serve as an indication of the electron-
pulling effectiveness of the selected acceptors in our dye set. In
this study, we used a series of solvent-mixtures (THF–toluene)
with different Df and measured the Stokes shifts of these
Chromophore
|Dm2|
|Dm|/Db
N101
BT101
101
S101
Dor
Acc
10070
9003
8642
7830
7283
780
6.44
6.57
6.53
6.47
5.4
267
254
239
211
113
16
16.4
15.9
15.5
14.5
11
3.41
3.31
3.23
3.03
2.31
0.83
5.9
4
a This value was estimated from the optimized molecular geometry
calculated with HyperChem (PM3) parameterisation. (ref. 19); b In
the units of Debye (1D ~ 3.336 6 10230 Cm); c dd, intramolecular
charge shift. This value was calculated from |Dm|/4.8 D, where 4.8 D
˚
corresponds to a 1 A separation distance of a pair of charges e and
2e.
Fig. 8 Lippert plots [Stokes shift (cm21) vs. orientational polarity (Df) of the solvent mixture] for chromophores N101, BT101, 101, S101, Dor
and Acc.
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(dd in Table 3) of our dye molecules during the excitation can
be considered as a measure of the strength of the electron-
acceptors. In comparing the results of dd from Table 3 and dppm
from Table 1, a reasonably good agreement between these two
independent experimental results confirms that the electron-
pulling ability of these acceptors follows the same order.
Furthermore, the ‘‘push-pull’’ stilbenoid structures with both
the donors and the acceptors attached show larger dd, indicat-
ing that greater p-electron delocalization (through intramole-
cular charge shift) are occurring during excitation.
dispersion prism before interacting with the sample solution.
This particular experimental arrangement also warrants that
the two-photon excitation process observed here is a one-color,
two-photon excitation process within the studied spectral range
since there is practically no overlapping between different
components of the white-light continuum.
(5) The cause of some small structures that have appeared on
the measured degenerate 2PA spectra of these model com-
pounds is not quite clear at this stage. More efforts on the
understanding of solvent effects such as solvent polarity on the
molecular 2PA spectral behavior and quantum-mechanical
analysis of the 2PA process in this system are needed, and are
under way currently.
III. Discussion of results
From the measured linear/nonlinear optical properties and the
results obtained via electron-acceptor strength studies of these
model chromophores, the following features can be seen:
(1) In Fig. 5, with trans-stilbene (TSB) as the reference point,
the slightly enhanced two-photon absorption in compounds
Dor and Acc suggests that either a donor or an acceptor
can contribute to the increase of the molecular nonlinear
absorptivity. The attachment of both donor (diphenylamino)
and acceptor (benzothiazolyl) to the TSB moiety has shown
further enhanced molecular two-photon absorptivity (e.g.
BT101 vs. Dor and Acc). According to the results from the
solvatochromic effect study (Table 3), the dipole moment
change (Dm) values of these chromophores follows the order of
BT101 w Dor w Acc, which implies that the dipole moment
change of these compounds upon excitation may have a certain
effect on the enhancement of molecular two-photon absorp-
tivity. The correlation between dipole moment change and
molecular two-photon absorption has been reported in sym-
metrically substituted stilbenoid systems by Marder, Perry
and co-workers5a and in that work, a concept of ‘‘charge
redistribution/p-electron delocalization’’ has been introduced
to interpret the enhanced molecular two-photon absorptivity.
In our case, the nature of the D–p–A molecular structure is
also suitable to promote the p-electron delocalization when
excited by light, which may also increase the molecular two-
photon absorptivity.
4. Two-photon absorption based optical power
limiting
It is well known that 2PA is one of the best mechanisms to
achieve optical power limiting and stabilization performance.7
For this purpose, the two-photon absorbing medium should
exhibit a larger 2PA coefficient that is proportional to the
concentration of the nonlinear absorbing molecules and their
2PA cross-section values. In our present study, a 2PA based
optical power limiting performance was demonstrated by using
a 1 cm path length solution of BT101 in THF with a con-
centration of 0.04 M. Using this chromophore for optical
power limiting performance is based on the fact that it
possesses a relatively larger 2PA at y800 nm wavelength
position compared to other model compounds. We utilized
nanosecond pulses to perform this experiment since most 2PA-
based optical power limiting applications were operated within
a nanosecond regime. In this regime of laser pulse duration, the
excited-state absorption may contribute to a much larger
effective 2PA effect that is desirable for optical power limiting
applications.7 In our experiment, as the input laser excitation
source, a linearly polarized y800 nm pulsed laser beam was
provided by a dye laser system, transversely pumped by a
frequency-doubled and Q-switched Nd:YAG laser system
(Quanta-Ray Pro 230 from Spectra-Physics). The pulse
duration, beam size, divergence angle, and repetition rate of
the y800 nm laser beam were y8 ns, y1.5 mm, y1 mrad, and
10 Hz, respectively. After passing through an f ~ 15 cm lens,
this laser beam was focused onto the center of the 1 cm cuvette
filled with the above-mentioned sample solution. The local
intensity within this sample solution can be changed by varying
the incident laser power level. The transmitted laser beam from
the sample cell was detected by an optical power (energy) meter
with a large detection area of y30 mm in diameter. Fig. 9a
shows the measured data (#) for nonlinear transmissivity, Ti,
as a function of the local intensity of the input y800 nm laser
beam. The influences from the windows of cuvette and from the
solvent were eliminated by subtracting the transmission data
from a pure THF-filled cuvette sample. One can see from
Fig. 9a that the intensity-dependent nonlinear transmission of
the sample dropped from y0.94 to y0.51 as the input intensity
increased from y54 MW cm22 to y652 MW cm22. In Fig. 9a,
the solid-line curve is the theoretical curve predicted by the basic
(2) The peak position of the two-photon absorption maxima
of the chromophores in Fig. 5b are blue-shifted (or at signifi-
cantly shorter wavelengths) compared to the wavelength
positions of twice of their linear absorption maximum (i.e.
l
2PA
max
v l1mPaAx). This implies that the two-photon states with
larger transition probabilities of these dye molecules are
energetically above their lowest one-photon states. Similar
observations have been reported for the symmetrically sub-
stituted stilbene compounds.5a
(3) In our model chromophore set, when stronger electron-
pulling functional groups are used, a larger 2PA maximum is
observed (Fig. 6), which means that the intramolecular charge
transfer (or p-electron delocalization/redistribution) is impor-
tant to enhance the 2PA of these model dye molecules upon
excitation. Also, the 2PA peak position is red-shifted as
2PA
max
the acceptor strength increased in this set (i.e. l
748 nm; (l 101) ~ 761 nm; l
(N101) ~ 840 nm).
S101) ~
2PA
(BT101) ~ 779 nm; l
2PA
max
2PA
max max
(4) In Fig. 5, non-negligible two-photon absorption at
wavelengths equal to twice their linear absorption maximum
positions shows that in these structures, one-photon transition
allowed excited states are also accessible by two-photon transi-
tion (i.e. for Dor at 733 nm, Acc at 708 nm, S101 at 773 nm, 101
at 788 nm, BT101 at 801 nm, and N101 at 863 nm). Moreover,
it can be seen that certain detectable two-photon absorption
appear at those wavelengths at which one-photon transitions
are almost forbidden. This provides the evidence that excited
states inaccessible by one-photon absorption can be reached by
two-photon absorption. We wish to reiterate that with our
experimental set-up, each component of the generated white-
light beam from heavy water cells is spatially separated by the
2PA theory with a best fitting parameter of b ~ 3.14 cm GW21
,
where b is the effective 2PA coefficient in nanosecond regime
of the sample solution. Fig. 9b shows the measured output
intensity as a function of the input intensity. The experimental
data are presented by open diamonds, and the solid-line curve
is the theoretical curve, with the same fitting parameter of
b ~ 3.14 cm GW21. The diagonal dash-line curve shows the
behavior for a medium without nonlinear absorption for
comparison. A fairly good optical power limiting behavior of
BT101 solution can be seen when the input intensity is
increased from y150 MW cm22 to y650 MW cm22
.
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Fig. 9 (a) Measured nonlinear transmission of a 1 cm path-length BT101 solution in THF (0.04 M) as a function of the input intensity of the
~800 nm laser beam. (b) Measured output intensity as a function of the input intensity based on the same sample solution. The solid curves are the
theoretical curves with a best fitting parameter of b = 3.14 cm GW21 (b is the effective 2PA coefficient).
potential to provide the important information with respect to
the choice of proper operating wavelengths for a specific
Conclusion
We have measured the degenerate 2PA spectra by using a single
and spectrally dispersed femtosecond white-light continuum as
a probe tool and investigated some molecular parameter effects
such as electron-donor and electron-acceptor effects on the
2PA behavior of a series of the chosen model chromophores.
The results suggested that whereas either a strong electron-
donor or electron-acceptor could play an important role in
enhancing the molecular two-photon absorptivity in this trans-
stilbene-based chromophore set. Asymmetric structures with
both donor and acceptor appear to have more 2PA enhance-
ment. This enhancement is increased with an increase in the
electron-withdrawing strength of the acceptor. The ordering of
electron-pulling strength among the acceptors used in this
study can be determined by simple 1H-NMR. Furthermore, in
this trans-stilbene-based chromophore set, those structures
with stronger electron-acceptors showed more efficient intra-
molecular charge shift/redistribution upon excitation, which is
considered as one of the determinants in improving the mole-
cular 2PA cross-sections. Using a femtosecond white-light
continuum to probe molecular 2PA spectra may also have the
chromophore for nonlinear optical applications such as the
optical power limiting demonstrated in this work.
Acknowledgements
This work was supported in part by the U. S. Air Force Office
of Scientific Research, Washington, DC and the Polymer
Branch, Materials & Manufacturing Directorate, U. S. Air
Force Research Laboratory, Dayton, Ohio and in part by
a grant from the Infotonic Center of Excellence, Rochester,
New York. We thank Professors Mike Detty and Frank Bright
of our Chemistry Department for helpful discussions.
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