Synthesis and characterization of the multi-photon absorption and excited-state properties of a neat liquid 4-propyl 4′-butyl diphenyl acetylene

Article abstract of DOI:10.1039/b905716a

The synthesis, characterization, and quantitative electronic structure modeling of multi-photon absorption properties of a neat liquid L34 (4-propyl 4′-butyl diphenyl acetylene) are reported. The liquid is (linearly) transparent in the visible spectrum, b

Full text of DOI:10.1039/b905716a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of the multi-photon absorption and excited-
state properties of a neat liquid 4-propyl 40-butyl diphenyl acetylene†
a
Iam Choon Khoo, Scott Webster,b Shoichi Kubo,c W. Justin Youngblood,d Justin D. Liou,a
*
Thomas E. Mallouk,e Ping Lin,f David J. Haganb and Eric W. Van Strylandb
Received 23rd March 2009, Accepted 17th June 2009
First published as an Advance Article on the web 21st July 2009
DOI: 10.1039/b905716a
The synthesis, characterization, and quantitative electronic structure modeling of multi-photon
absorption properties of a neat liquid L34 (4-propyl 40-butyl diphenyl acetylene) are reported.
The liquid is (linearly) transparent in the visible spectrum, but possesses large two-photon absorption
(2PA) and 2PA-induced singlet and triplet excited-state absorption as measured by the Z-scan
technique and non-linear transmission measurements using both picosecond and nanosecond pulses.
The most dominant contributions to the intensity-dependent non-linear absorption come from the
2PA-induced triplet excited states in the nanosecond time regime. We also present transient
absorption spectra of the liquid obtained by nanosecond laser-flash photolysis and compare these
results with electronic structure calculations. The energy of the absorption bands, both singlet
and triplet are in reasonable agreement with calculations performed with Gaussian 03.
The experimentally measured spectra and theoretical electronic structure modeling provide
information about the energy levels of the excited states of this liquid, including 2PA and 2PA-induced
process that is responsible for its non-linear optical properties.
Examples are materials that possess multi-photon absorption
properties such as reverse saturable absorption (RSA),6–9 two-
Introduction
Pulsed and continuous-wave lasers are now extensively used in
photon absorption (2PA) and excited state absorption (ESA)10–20
various settings. Direct intentional or accidental exposure to such
and non-linear scattering properties.21 These materials allow
intense radiation could lead to ‘flash blindness’ or severe perma-
maximal transmission at low input light intensity level, but
nent damage to the sensors, e.g. the eye.1 Although fixed-wave-
become increasingly ‘opaque’ via a variety of non-linear optical
length filters offer effective protection from lasers of known
processes as the incident light intensity increases, in such a manner
wavelengths, they are ineffective against agile frequency lasers,
that the energy of the transmitted laser pulse is clamped below the
the wavelengths of which can be anywhere in the UV to infrared.
damage threshold of the sensor. In general, RSA materials have
Next generation organic material based electro-optical devices,
low switching threshold, but they are (linearly) absorptive, and
e.g. tunable filters where the transmission window can be elec-
could be easily ‘bleached’ by high intensity laser. Although two-
tronically tuned away from the incident laser wavelength, and
photon absorbers are more desirable because they are linearly
various other liquid crystalline materials and devices,2–5 are
transparent, efficient 2PA materials that activate non-linear
effective against continuous-wave (CW) or long-pulse (micro-
absorption at low threshold are also susceptible to bleaching by
seconds to 10s of nanoseconds) lasers but cannot respond fast
high intensity lasers. To extend the operational dynamic range,
enough to mitigate very short laser pulses in the nano- and sub-
materials that possess simultaneously large two-photon and
nano-second time scales. For applications in the latter time scales,
excited-state absorption cross-sections, c.f. Fig. 1, have become
non-linear absorbing materials capable of very fast all-optical
switching operations have been actively investigated.2–16
Most 2PA materials are in powdery solid form; as a result of
the central focus of many material development efforts.
their low solubility and/or intermolecular interactions, the
transmittance changes induced by non-linear absorption in
solutions of 2PA are small. One technique often employed is the
engineering of chromophores with large dendrimers and
peripheral substitutions to allow for increased concentrations
while suppressing aggregation and unwanted intermolecular
interactions. Although these techniques can increase solubility,
and thereby non-linear absorption, maximum concentrations of
2PA chromophores are typically small, on the order of 10ꢀ3 to
10ꢀ2 M in solution. Neat liquids of 2PA chromophores, such as
L3416–19 or liquid crystals in their isotropic phase,20 are attractive
for their ability to maintain their linear and NLO properties at
100% concentration even when their 2PA cross sections are
aDepartment of Electrical Engineering, The Pennsylvania State University,
University Park, PA, 16802, USA. E-mail: ick1@psu.edu
bCREOL, The College of Optics and Photonics, University of Central
Florida, Orlando, FL, 32826, USA
cInstitute of Multidisciplinary Research for Advanced Materials, Tohoku
University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan
dDepartment of Chemistry, University of North Texas, 1155 Union Circle,
# 305070, Denton, TX, 76203, USA
eDepartment of Chemistry, The Pennsylvania State University, University
Park, PA, 16802, USA
fMaterials Research Institute, The Pennsylvania State University,
University Park, PA, 16802, USA
† This paper is part of a Journal of Materials Chemistry theme issue on
organic non-linear optics. Guest editor: Seth Marder.
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reaction flask was charged with 1-bromo-4-butylbenzene (25.0 g,
117 mmol), PdCl2(PPh3)2 (4.11 g, 5.86 mmol), CuI (1.12 g,
5.88 mmol), and PPh3 (3.07 g, 11.7 mmol). The flask was evac-
uated and refilled with nitrogen three times. Then a nitrogen-
sparged sample of 2-methyl-3-butyn-2-ol (14.8 g, 176 mmol) in
triethylamine (TEA) (150 mL) was transferred to the reaction flask.
The mixture was refluxed overnight with stirring. Upon cooling to
room temperature, the crude reaction mixture was diluted
with ethyl acetate (EtOAc) and washed with water, followed by
additional washings with saturated aqueous NH4Cl, water, and
brine. The organic layer was separated and dried over MgSO4,
filtered, and concentrated. The residue was chromatographed
(SiO2, hexanes/EtOAc 6:1) to obtain a colorless oil product (21.7 g,
1
86%). The compound was identified by H NMR and carried
Fig. 1 Energy level schematic for 2PA (s01(2)) with 2PA-induced ESA
from singlet (sS1n) and triplet (sT1n) excited states.
forward to the next reaction without further characterization.
To obtain 1-butyl-4-ethynylbenzene (2), a finely ground
sample of NaOH (44 g, 1.1 mol) was added to a solution of 1
(11.9 g, 55.0 mmol) in toluene (250 mL). The mixture was
refluxed with stirring for 10 h. The reaction mixture was filtered,
and then washed with water, followed by brine treatment. The
toluene layer was separated and dried over MgSO4, filtered, and
concentrated. The residue was chromatographed (SiO2, hexanes)
and fractions containing the desired product were distilled at
reduced pressure to obtain 2 as a colorless oil (7.36 g, 85%). To
obtain the final product, 4-propyl-40-butyl diphenyl acetylene
(L34), a Schlenk reaction flask was charged with 2 (7.35 g,
46.4 mmol), PdCl2(PPh3)2 (1.50 g, 2.14 mmol), CuI (406 mg,
2.13 mmol), and PPh3 (1.11 g, 4.23 mmol). The flask was evac-
uated and refilled with nitrogen gas three times. A nitrogen-
sparged sample of 1-bromo-4-propylbenzene (8.4 g, 42.2 mmol)
in TEA (150 mL) was transferred to the reaction flask. The
mixture was refluxed overnight with stirring. Upon cooling to
room temperature, the crude reaction mixture was diluted
with EtOAc and washed with water, followed by additional
washings with saturated aqueous NH4Cl, water, and brine.
The organic layer was separated and dried over MgSO4, filtered,
and concentrated. The residue was chromatographed (SiO2,
hexanes) and fractions containing the desired product were
subjected to bulb-to-bulb distillation to obtain L34 as a color-
less oil (7.40 g, 63%). GC-MS analysis indicated that the masses
of the impurities (ꢁ4%) contain the Glaser-coupled byproduct
relatively smaller. Designing new neat liquids, with additional
non-linear absorption mechanisms, will increase their effective
2PA cross sections and allow for applications requiring large
non-linear absorptions.
Previously18,19 we have reported a neat organic liquid 4-propyl
40-butyl diphenyl acetylene (L34) that exhibits two-photon
mediated excited state absorption (2P-ESA). L34 is a neat liquid
over a wide temperature range, including room temperature, and
has a high transmittance in the visible wavelength region. In
order to further understand these 2P-ESA properties, it is
important to characterize its manifold of electronic excited
states. The lowest energy triplet state (T1) is one of the most
important states because it can be populated by intersystem
crossing from excited singlet states, typically the first excited state
(S1). Intersystem crossing can affect non-linear absorption
strongly by opening additional channels of triplet-triplet excita-
tion and non-radiative decay to the ground state (So).
Experimental
Synthesis and chemical characterization of L34 (4-propyl 40-
butyl diphenyl acetylene)
L34 was synthesized following the route depicted in Fig. 2.
Compound 1, 4-(4-butylphenyl)-2-methylbut-3-yn-2-ol, was
synthesized as described in previous reports.22–25 A Schlenk
L44 (4,40-dibutyldiphenylbutadiyne),
a longer-central-chain
Fig. 2 Synthetic scheme of L34.
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Fig. 3 Molecular structures of L34 (a) and L44 (b) and their (linear) absorption spectra.
derivative of L34, cf. Fig. 3. L44 is a known organic solid
compound. Its synthesis and liquid-crystalline properties were
first reported by Grant.26 In order to understand the nature of
L44, and also to corroborate the quantum calculations and
excited-state transient studies in the sections to follow, we have
recently performed a separate synthesis effort of L44 and
obtained its spectrum.
9010) with a pulse width of ꢁ6 ns (FWHM) measured by a fast
silicon detector and digital storage oscilloscope (Tektronix,
TDS680C, 1GHz/5Gs) at 532 nm. All measurements were per-
formed with neat-liquid samples in cell thicknesses dependent on
the measurement techniques and geometries used. Focal spot sizes
used for Z-scan and non-linear transmittance measurements were
determined by measuring and modeling the non-linear refraction
of carbon disulfide using the closed aperture Z-scan technique
detailed in ref. 12 and 28. This method serves as a calibration for
focused spot sizes and also as an irradiance calibration to double
check the independently measured energy and temporal width of
the laser pulse. Fluorescence lifetime measurements were
obtained by a time-correlated single photon counting system
(PicoQuant, PicoHarp300) with a time resolution of ꢁ80 ps using
a linear polarized frequency doubled femtosecond excitation at
390 nm and oriented at the magic angle (54.7 deg).
Fig. 3(a) shows a UV-visible absorption spectrum of L34
diluted in hexane. The liquid is transparent in the visible regime
and begins to absorb at wavelengths shorter than ꢁ300 nm. The
absorption spectrum of L34 is similar to those of diphenylace-
tylene in solutions of glycerol and cyclohexane23,27 except for
some absorption peaks located near 310nm and 325 nm. These
peaks are attributed to the presence of L44, the absorption
spectrum of which is shown in Fig. 3(b). As our ESA experiments
and electronic structure calculations in the following section will
show, the longer conjugated central linker of L44 corresponds to
lower excitation energies (longer wavelength); it also exhibits
strong excited triplet state absorption.
Laser-flash photolysis was performed using excitations of both
third (355 nm) and fourth (266 nm) harmonics of a nanosecond
Nd:YAG laser. Measurements were performed in quartz optical
cells pumped by the Nd:YAG harmonics while changes in
transmittance, probed with a Xe lamp, were monitored using
Non-linear optical characterization
a
monochromator equipped with a photomultiplier tube
Non-linear optical investigations of L34 include a broad range of
measurements facilitating a detailed study of lifetime dynamics,
2PA, and 2PA-induced ESA into both singlet and triplet states
with self-consistent results across the measured laser pulse
widths. The picosecond laser system consists of a 10 Hz mode-
locked Nd:YAG laser (EKSPLA, model PL2143) and is exter-
nally frequency doubled to produce 532 nm with a pulse width of
ꢁ23 ps (FWHM), measured by second-harmonic autocorrela-
tion. The nanosecond laser system is an externally frequency
doubled 10 Hz, seeded Nd:YAG laser (Continuum, Powerlight
detector. Measurements were averaged twenty times with a pump
pulse repetition rate of 0.25 Hz at each wavelength.
Quantum chemical calculations
Quantum mechanical ab-initio calculations of the excited-state
properties were computed using Gaussian 0329 for several model
main-chain molecules with various end/side groups. The ground
and excited states of these molecules were investigated using
the SAC (symmetry-adapted cluster)/SAC-CI (configuration
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interaction) singles and doubles (SD)-R method as well as time-
dependent density functional theory (TD-DFT).30–32
i.e., that the phosphorescence lifetime, sPhos, is much longer than
tens of nanoseconds as described by the nanosecond laser-flash
photolysis measurements in the following section.
Shown in Fig. 4(a) are the results of the picosecond Z-scan
measurements at 532 nm modeled by Eq. 1. for determining
the 2PA coefficient and 2PA-induced singlet ESA cross-section.
For input energies ranging from 45nJ to 1.6mJ (a total
transmittance change of ꢁ60%) the 2PA coefficient b was
determined to be 0.85 ꢂ 0.15 cm/GW corresponding to
Results and discussion
Two-photon absorption and two-photon-induced excited state
absorption
In order to understand and model the non-linear absorption
processes, the population dynamics must be understood. For
both picosecond and nanosecond measurements the non-linear
absorption was modeled with the propagation and rate equations
for 2PA-induced ESA as described by:
(2)
a 2PA cross section s01 of ꢁ18 ꢂ 4 GM and a singlet
ESA cross-section of sS1n ¼ 7.2 ꢂ 1.2 ꢃ 10ꢀ18 cm2 at 532 nm,
using a neat liquid molecular density of No ¼ 1.588 ꢃ 1021/cm3.
The upper singlet excited-state lifetime, sSn1, is determined to be
ultrafast as compared to the picosecond pulsewidth. Fig. 4(b)
shows the nanosecond non-linear transmittance results also
modeled by Eq. 1. which includes 2PA and 2PA-induced ESA
from both singlet and triplet states. Laser-induced breakdown is
observed for input energies greater than 100 mJ as evidenced by
the sharp drop in transmittance. For nanosecond pulse
widths, there is sufficient time to populate the triplet state, T1,
therefore this population needs to be included. To fully
determine the singlet–triplet dynamics, both fluorescence life-
time and a long pulse-width measurement (nanosecond Z-scan
or non-linear transmittance) must be performed in order to
decouple the triplet intersystem crossing rate and yield. Using
the fluorescence lifetime measurement and the picosecond
Z-scan results of 2PA and 2PA-induced singlet ESA cross-
sections as initial fitting parameters, the nanosecond non-linear
transmittance data was fit to obtain a2 ¼ 0.75 ꢂ 0.15 cm/GW,
sS1n ¼ 6 ꢂ 1.2 ꢃ 10ꢀ18 cm2, sT1n ¼ 1.2 ꢂ 0.3 ꢃ 10ꢀ16 cm2, and
sISC ¼ 17 ꢂ 5 ns. These values are found to be in excellent
agreement with the corresponding picosecond Z-scan measure-
ments. The triplet yield FT (calculated from FT ¼ sF/(sF + sISC))
is determined to be ꢁ10 ꢂ 2%.
dI
dz
sð021ÞNS0I2
¼ ꢀ
¼ ꢀ
ꢀ sS1n
N
S1I ꢀ sT1n
NT1I
ꢀhu
dNS0
dt
sð021ÞNS0I2 NS1 NT1
þ
þ
2
sF
sPhos
S1I NSn NS1
2ðhuÞ
ꢀ
dNS1
dt
sð021ÞN0I2 NS1 sS1n
N
¼
ꢀ
ꢀ
þ ꢀ
2
sF
ꢀhu
sSn1 sISC
2ð
ꢀ
huÞ
(1)
dNSn
dt
sS1n
N
S1I NSn
¼
ꢀ
NT1I NTn NS1 NT1
ꢀhu
sT1n
sn1
dNT1
dt
¼ ꢀ
þ þ ꢀ
ꢀhu
sTn1 sISC sPhos
dNTn
dt
sT1n
N
T1I NTn
ꢀ
¼
ꢀhu
sTn1
where NS0, NS1, NSn, NT1, and NTn are the populations of each
level (Ntotal ¼ NS0 + NS1 + NSn + NT1 + NTn); s01(2), sS1n, and
sT1n are the two-photon, singlet excited, and triplet ESA cross-
sections, respectively. The fluorescence lifetime, sF, of L34 was
experimentally determined to be sF ¼ 2.1 ꢂ 0.2 ns. The highest
singlet, Sn, and triplet, Tn, excited states were assumed not to be
populated since the lifetimes sSn1 and sTn1 are ultrafast compared
to the pulse widths used in these experiments, and therefore can
also be neglected, simplifying the rate equations in Eq. 1. All of
the modeling in this section assumes that the recovery from the
triplet to the ground singlet state is slow and can be neglected,
As previously alluded to, a large 2PA cross section alone may
not necessarily lead to large non-linear absorption changes needed
for applications. A material that combines the ability for large
concentrations (neat liquids being the most ideal ones) with
Fig. 4 (a) Picosecond Z-scan results at 532 nm. (b) Nanosecond non-linear transmittance results at 532 nm. (c) Effective 2PA coefficients as a function
of irradiance for both nanosecond (red circles) and picosecond (black squares) pulse widths. Note: solid lines are linear regressions that appear non-
linear on the logarithmic scales.
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Transient excited-state absorption and quantum mechanical
several non-linear absorption mechanisms is desired. Fig. 4(c)
shows the effective 2PA coefficient a2 versus input irradiance for
both picosecond and nanosecond pulse widths. An effective 2PA
coefficient a2 is the result of modeling the total non-linear
absorption as if it were only due to 2PA, i.e., pure Im [c(3)]. The
2PA coefficient, b, is independent of irradiance while the effective
2PA coefficient includes additional non-linear absorption mech-
anisms that cause an increasing dependence when plotted versus
irradiance. An estimate for the intrinsic 2PA coefficient b can be
obtained by extrapolating the linear fit of a2 to zero input irra-
diance when the contributions from other non-linear mechanisms
are small. Clearly, the nanosecond data show larger values due to
the combined non-linearities of 2PA and 2PA-induced ESA from
both singlet and triplet absorptions whereas the picosecond data
only include contributions from 2PA and 2PA-induced singlet
excited-state absorption. Linear extrapolation to zero input irra-
diance works fairly well for the picosecond results but not for the
nanosecond results since its non-linear absorption is dominated
mostly by ESA from both singlet and triplet states. From the
modeled data in Fig. 4(a) and (b), the 2PA absorption coefficient
in L34 is determined to be b ¼ 0.8 ꢂ 0.2 cm/GW while the
nanosecond data in Fig. 4(c) show effective 2PA coefficients
a2 from 5 to 30 cm/GW corresponding to an order of magnitude
increase due to these combined non-linearities.
calculations
In view of the critical role played by triplet ESA that underlie the
superb all-optical switching performance of L34,17–19 we have
performed quantum mechanical electronic structure calculations
of the excited-state energies and dipole transition strengths and
present the findings here with corroborating transient ESA
spectra obtained by nanosecond laser-flash photolysis.
The excited-state properties were computed using Gaussian
0329 for several model main-chain molecules including DPY1
(diphenyl acetylene) and DPY2 (1,4-diphenylbutadiyne) with
various end/side groups, cf. Fig. 5. Note that in this notation,
L34 becomes DPY1-L34 and L44 becomes DPY2-L44. The
1 and 2 after ‘‘DPY’’ denotes the number of central linkage
groups. The calculated molecular orbitals (MO) for DPY1 are
composed primarily of p-type orbitals, and the highest occupied
MOs (HOMO) and lowest unoccupied MOs (LUMO) are similar
to those calculated in an earlier study.33 The ground and excited
states of these molecules were investigated using the SAC/SAC-
CI (SD)-R method as well as TD-DFT. Information about the
singlet ground and excited states obtained using TD-DFT is
presented in Tables 1 and 2, respectively. The calculated singlet
and triplet excitation energies in eV, and their corresponding
Fig. 5 Schematic of the main chain molecules DPY1 and DPY2 and some singlet–triplet energy levels used in the computation.
Table 1 Energies associated with singlet excitation for DPY1 (diphenyl acetylene), DPY1-L34 (4-propyl 40-butyl diphenyl acetylene), DPY2
(1,4-diphenylbutadiyne), DPY2-L44 (4,40-butyl 1,4-diphenylbutadiyne), DPY2-L33 (4,40-propyl 1,4-diphenylbutadiyne) and DPY2-L34 (4,40-butyl
1,4-diphenylbutadiyne)
DPY1
DPY1-L34
DPY2
DPY2-L44
DPY2-L33
DPY2-L34
S0(1Ag)–S3(1B1u)/eV
S0(1Ag)–S3(1B1)/nm
Osc. strength
4.0948
302.78
0.9482
3.9779
311.69
1.3049
3.6009
344.31
0.9456
3.5289
351.34
1.2897
3.5319
351.04
1.2610
3.5302
351.21
1.2744
Expt./eV (gas, ref. 35)
Expt./eV (sol., ref. 34)
4.37
4.073
3.712
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Table 2 Energies associated with triplet excitation: theoretical results from TD-DFT calculations. T1–Tn represents the strongest excitation, T1–Tn0 and
00
T1–Tn represent the weaker excitations
DPY1
DPY1-L34
DPY2
DPY2-L44
DPY2-L33
DPY2-L34
T1(3B1u)–Tn(3Ag)/eV
T1–Tn/nm
T1–Tn/Osc.strength
2.9865
415.15
1.2311
2.8037
442.22
1.3571
2.5997
476.92
0.1127
2.7712
447.40
0.0174
2.7076
457.92
1.4324
2.5710
482.24
1.8290
2.8540
434.43
0.0238
2.5781
480.91
1.7952
2.8609
433.38
0.0213
2.5745
481.58
1.8101
2.8577
433.85
0.0225
0
T1–Tn /eV
T1–Tn /nm
0
0
T1–Tn /Osc. strength
T1–Tn /eV
00
T1–Tn00/nm
T1–Tn00/Osc. strength
Of particular interest to the present studies are the excited-
state T1(3B1u) / Tn(3Ag) transitions. As listed in Table 2, the
transition energies (in eV) and wavelengths for DPY1-L34 are
2.80 eV and 443 nm, while for DPY2-L33, -L34 and -L44, they
are ꢁ2.57 eV and 482 nm, respectively. It is to be noted that these
excited-state transition energies decrease with longer central
linkage but do not vary appreciably as we alter the side groups.
These calculations are corroborated by transient absorption
spectroscopy. From the linear absorption spectrum in Fig. 3, as
well as the electronic structure calculations in the preceding
section, the 266 nm excitation will access the singlet excited states
of L34 (DPY1-L34) whereas the longer-wavelength 355 nm
excitation will access the triplet excited states of the by-product
L44 (DPY2-L44). Fig. 6(a) shows a typical transient absorption
induced by laser-flash photolysis. Very similar decay curves were
observed at all wavelengths between 370 and 520 nm after
355 nm excitation. The trace collected at 470 nm was used for
analysis. It was fit to a single exponential function (dotted line in
Fig. 6(a)), with the lifetime of 110 ns ꢂ 2 ns. From these traces,
a transient absorption spectrum was compiled 100 ns after
excitation. The results (355 nm) along with a separate study
pumped with 266 nm excitation are plotted in Fig. 6(b). In the
case of 266 nm excitation, a broad absorption band that peaks at
440 nm was obtained and is attributed to L34 (DPY1-L34),
whereas the 355 nm excitation yields a broad absorption band
that peaks at 480 nm, attributed to L44 (DPY2-L44).
Fig. 6 (a) Time evolution of the changes in absorbance at various
wavelengths following a ꢁ7 ns laser flash at 355 nm. (b) Transient
excited-state absorption spectra obtained with 266 nm (squares) and 355
nm (circles) laser excitation; the solid line is the ground-state linear
absorption spectrum for reference purposes.
Conclusions
The 2PA-induced ESA of the neat liquid, L34, containing some
of its longer main-chain derivatives have been investigated both
experimentally, using non-linear transmission techniques and
transient absorption spectroscopy, and theoretically using
quantum mechanical ab-initio calculations. Measurements made
with both picosecond and nanosecond pulse widths to quantify
2PA and 2PA-induced ESA from both singlet and triplet excited
states are in excellent agreement. Modeling of these data, using
a five-level model, resulted in the following parameters: 2PA
coefficient, b ¼ 0.8 ꢂ 0.2 cm/GW; singlet ESA cross section,
sS1n ¼ 6.6 ꢂ 1.5 ꢃ 10ꢀ18 cm2; triplet ESA cross section,
sT1n ¼ 1.2 ꢂ 0.3 ꢃ 10ꢀ16 cm2; singlet–triplet intersystem crossing
lifetime, sISC ¼ 17 ꢂ 5 ns; and triplet yield, 10 ꢂ 2%. The
quantum mechanical calculations for the triplet excited-state
transition are in good agreement with the experimentally
obtained ESA spectra. These results are important for
wavelengths, are in good agreement with previously reported
experimental observations.34–36 For example, SAC-CI calcula-
tions of the DPY1 molecule using the 6-311G(d,p) basis set
gave an energy of 4.54 eV for the S0(1Ag)–S3(1B1u) excitation,
in reasonable agreement with experimental values of 4.07 eV
[ref. 34, measurement in methylcyclohexane solution] and
4.37 eV (35, 248 cmꢀ1) [ref. 35, measurement in the gaseous
state]. The excitation energies of the S0(1Ag)–S1(1B2u) and
S0(1Ag)–S2(1B3g) transitions were calculated to be 4.797 and
4.804 eV, respectively, while the corresponding experimental
values are 4.346 eV (35 051 cmꢀ1 36 and 4.334 eV (34 960 cmꢀ1).35
)
The discrepancy is in part due to the differences between vertical
and adiabatic transitions.
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9 R. L. Sutherland, M. C. Brant, D. M. Brandelik, P. A. Fleitz,
D. G. McLean and T. Pottenger, Optics Letters, 1993, 18, 858–860.
10 J. Barroso, A. Costela, I. Garcia-Moreno and J. L. Saiz, Journal of
Physical Chemistry A, 1998, 102, 2527–2532.
understanding the excited-state structures of L34 and provide
insights into designing new efficient non-linear absorbers at
longer wavelengths.
11 G. S. He, L. S. Tan, Q. Zheng and P. N. Prasad, Chem. Rev., 2008,
108, 1245–1330.
Acknowledgements
12 M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan and E. W. Van
Stryland, IEEE J. Quant. Electron., 1990, 26, 760–769.
13 J. E. Ehrlich, X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. Rockel,
S. R. Marder and J. W. Perry, Optics Letters, 1997, 22, 1843–1845.
14 I. C. Khoo, M. V. Wood, B. D. Guenther, M.-Yi Shih, P. H. Chen,
Z. Chen and X. Zhang, Optics Express, 1998, 2(12), 471–482.
15 I. C. Khoo, P. H. Chen, M. V. Wood and M.-Y. Shih, Chemical
Physics, 1999, 245, 517–531.
16 I. C. Khoo, M. V. Wood, B. D. Guenther, M.-Y. Shih and
P. H. Chen, J. Opt. Soc. Am. B, 1998, 15, 1533–1540.
17 I. C. Khoo, A. Diaz, M. V. Wood and P. H. Chen, IEEE Journal on
Selected Topics in Quantum Electronics, 2001, 7, 760–768.
18 I. C. Khoo, A. Diaz and J. Ding, J. Opt. Soc. Am. B, 2004, 21,
1234–1240.
19 I. C. Khoo, IEEE J. Selected Topics in Quantum Electronics JSTQE,
2008, 14(3), 946–951.
20 G. S. He, T.-C. Lin, P. N. Prasad, C.-C. Cho and L.-J. Yu, Appl. Phys.
Letts., 2003, 82, 4717–4719.
21 K. Mansour, M. J. Soileau and E. W. VanStryland, J. Opt. Soc. Am.,
1992, B9, 1100–1109.
22 J.-H. B. Meng, L. D. Dalton and S.-T. Wu, Mol. Cryst. Liq. Cryst.,
1994, 250, 303–314.
This work was supported by Air Force Office of Scientific
Research, Defense Advanced Research Projects Agency,
National Science Foundation Materials Research Science, and
Engineering Center (MRSEC) at Penn. State under Grant
DMR-0213623. S.K. was also supported by a research fellowship
of the Japan Society for the Promotion of Science for Young
Scientists. We are indebted to P. Fleitz and Joy Rogers of US Air
Force Research Laboratory at Wright Patterson AFB for tran-
sient absorption data of L34 and A. Diaz for technical assistance
in the non-linear transmission modeling. CREOL authors
gratefully acknowledge the support of the National Science
Foundation ECS 0524533, the US Army Research Laboratory
W911NF0420012, the US Army Research Laboratory and the
US Army Research Office under Contract/Grant Number 50372-
CHMUR, the Office of Naval Research MORPH N00014-06-1-
0897, Dr Kevin Belfield’s group in the Department of Chemistry
at the University of Central Florida for their help with the
fluorescence lifetime measurements and Davorin Peceli and
Lazaro A. Padilha for technical assistance.
23 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483.
24 C. Sekine, K. Iwakura, N. Konya, M. Minai and K. Fujisawa, Liq.
Cryst., 2001, 28, 1375–1387.
25 Y. Fujita, Y. Misumi, M. Tabata and T. Masuda, J. Polym. Sci. Pol.
Chem., 1998, 36, 3157–3163.
26 B. Grant, Mol. Cryst. Liq. Cryst., 1978, vol. 48, 175–182.
27 Y. Hirata, T. Okada and T. Nomoto, Chem. Phys. Lett., 1998, 293,
371–377.
28 M. Sheik-Bahae, A. A. Said and E. W. Van Stryland, Opt. Lett., 1989,
14, 955–957.
References
1 ANSI Standard Z136.1. American National Standard for the Safe Use
of Lasers. American National Standards Institute, Inc., New York
2000.
2 A. Urbas, J. Klosterman, V. Tondiglia, L. Natarajan, R. Sutherland,
S. Tsutsumi, T. Ikeda and T. Bunning, Adv. Mat., 2004, 16,
1453–1456.
3 I. C. Khoo, Phys. Report, 2009, 471, 221–267.
4 I. C. Khoo, J. H. Park and J. D. Liou, Appl. Phys. Letts., 2007, 90,
Art. # 151107.
5 I. C. Khoo, J. H. Park and J. D. Liou, J. Opt. Soc. Am., 2008, B25,
1931–1937.
6 J. W. Perry, K. Mansour, S. R. Marder, K. J. Perry, D. Alvarez and
I. Choong, Opt. Letts., 1994, 19, 625–627.
29 M. J. Frisch et al., Gaussian 03; Gaussian, Inc.: Wallingford CT,
(2004).
30 H. Nakatsuji, Chem. Phys. Lett., 1979, 67, 329–333.
31 H. Nakatsuji, Chem. Phys. Lett., 1979, 67, 334–342.
32 N. Nakatsuji, Chem. Phys. Lett., 1978, 59, 362–364.
33 Y. Amatatsu and M. Hosokawa, J. Phys. Chem. A, 2004, 108,
10238–10244.
34 Y. Nagano, T. Ikoma, K. Akiyama and S. Tero-Kubota, JACS, 2003,
125, 14103–14112.
35 K. Okuyama, M. C. R. Cockett and K. Kimura, J. Chem. Phys., 1992,
97, 1649–1654.
7 C. W. Spangler, J. Mater. Chem., 1999, 9, 2013–2020.
8 J. S. Shirk, G. S. Pong, F. J. Bartoli and A. W. Snow, Appl. Phys.
Letts., 1993, 63, 1880–1882.
36 K. Okuyama, T. Hasegawa, M. Ito and N. Mikami, J. Phys. Chem.,
1984, 88, 1711–1716.
This journal is ª The Royal Society of Chemistry 2009
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