Investigation of two-photon absorption properties in branched alkene and alkyne chromophores

Article abstract of DOI:10.1021/ja060630m

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the π-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.

Full text of DOI:10.1021/ja060630m

Published on Web 08/22/2006
Investigation of Two-Photon Absorption Properties in
Branched Alkene and Alkyne Chromophores
Ajit Bhaskar,† Guda Ramakrishna,† Zhikuan Lu,‡ Robert Twieg,‡ Joel M. Hales,§
David J. Hagan,§ Eric Van Stryland,§ and Theodore Goodson III*,†
Contribution from the Departments of Chemistry and Macromolecular Science and Engineering,
UniVersity of Michigan, Ann Arbor, Michigan 48109, Department of Chemistry, Kent State
UniVersity, Kent, Ohio 44242, and School of Optics/CREOL, UniVersity of Central Florida,
Orlando, Florida 32816
Received January 26, 2006; E-mail: tgoodson@umich.edu
Abstract: Novel alkene and alkyne branched structures have been synthesized, and their two-photon
absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the
mechanistic approach for enhancing the 2PA process which is usually dictated by the π-bridging,
delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest
that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission
measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved
measurements have explained the trends observed. It was found that, upon photoexcitation, there is an
ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of
a solvent and conformationally relaxed ICT state ′ in these branched systems.
development.16-28 Numerous dipolar chromophores have been
synthesized with varying donor-π-acceptor configurations, as
well as different π-bridging centers, and different donor-
acceptor strengths so as to probe the structure and 2PA property
relationships.1,20,22 Recent investigations indicate that increasing
the dimensionality of donor-π-acceptor molecules is a good
approach, as certain branched systems exhibit enhanced 2PA
cross sections over the linear counterparts.29,30 This is believed
to be a consequence of cooperative interaction among the
1. Introduction
The development of conjugated organic molecules with large
two-photon absorption (2PA) cross sections1 is of broad interest
in many areas of research. Much of the interest is driven by
potential applications in 3D microfabrication,2-5 photodynamic
therapy,6 two-photon microscopy,7-10 optical power limiting,11,12
and optical data storage.13-15 To realize these applications, novel
molecules with large 2PA cross sections in the visible, near-
infrared, and telecommunication wavelengths are desired.
Several design approaches have been reported for the synthesis
of organic molecules with large 2PA cross sections. Molecules
with dipolar and quadrupolar character have been the focus of
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† University of Michigan.
‡ Kent State University.
§ University of Central Florida.
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9
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J. AM. CHEM. SOC. 2006, 128, 11840-11849
10.1021/ja060630m CCC: $33.50 © 2006 American Chemical Society

2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
individual arms.30 The interest in multiply branched systems
for 2PA also stems from the fact that relatively large 2PA cross
sections may primarily be realized from low-lying excited states.
Consequently, considerable research has been carried out toward
the design and synthesis of multiply branched chromophores.
Several configurations, such as donor-π-acceptor, acceptor-
π-donor, and chromophores with different branching centers,
π-bridging units, and tri- and tetra-branched structures, have
been synthesized and studied.29-50
molecules have been synthesized with varying donor-acceptor
strength, substituents, and conjugation length.30-33
Though larger donor-acceptor strength is important for
higher 2PA cross section, the nature and length of the π-linker
do play a decisive roles in increasing the 2PA cross section
and nonlinear optical (NLO) properties. It has been shown in
dipolar chromophores that richer π-electron systems improve
the intramolecular charge-transfer character and thereby enhance
the NLO properties.51 Several π-bridging chromophores have
been considered for improving the 2PA cross section. Among
them, alkene π-bridging is most widely studied, and it has been
suggested that, in many of the linear systems, alkene π-bridging
is better than alkyne π-bridging.52,53 However, for several
reasons pertaining to intra-arm electronic coupling, charge
delocalization, and charge transfer, the consequences of π-bridg-
ing on NLO and 2PA properties in branched systems may be
different from those in the linear quadrupolar molecules.54-56
It has been theoretically predicted from ab initio calculations
that the 2PA cross sections of alkyne π-bridge-containing
quadrupolar molecules are not significantly different from those
of their alkene π-bridged analogues.56,57 However, a systematic
experimental study of 2PA in branched trimers with different
π-bridging and the underlying mechanism has not been reported.
To understand the effect of π-bridging, acceptor strength, and
conjugation length on 2PA cross sections of multiply branched
chromophores, we have synthesized new triphenylamine-core
branched molecules with pyridine as acceptor (molecules A-D,
Figure 1). Two-photon absorption cross sections of these
molecules have been measured by two-photon excited fluores-
cence and non-degenerate pump-probe techniques. Compari-
sons of 2PA cross sections were also made with N,N,N-tris[4-
{2-(4-{5-[4-(tert-butyl)phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1-
ethenyl}phenyl]amine (PRL-701) and tris[4-(3′,5′-di-tert-
butyldistyrylbenzenyl)phenyl]amine (N(DSB)3). Here, we have
varied the nature of π-bridging from alkenes (A and C) to
alkynes (B and D), conjugation length within individual branches
(A to B, C to D), and acceptor strength at the end of each branch
(B, PRL-701, and N(DSB)3). Steady-state and time-resolved
spectroscopic techniques are employed to probe the mechanism
of 2PA. We have correlated the observed 2PA cross sections
with the extent of intramolecular charge transfer probed by
femtosecond transient absorption spectroscopy. The results of
2PA measurements have shown that alkene π-bridged chro-
mophores have superior 2PA properties over the alkyne
π-bridged counterparts at low energy, and we were able to
successfully correlate the observed trend with the results of
transient absorption spectroscopy. Further, an increase in
acceptor group strength as well as an increase in conjugation
length within a branch was found to increase 2PA cross sections.
Mechanistic investigations aimed at understanding the factors
that influence the magnitude of the 2PA cross section in multiply
branched systems suggest that it is a complex interplay between
intra-arm coupling, electronic delocalization, and extent of
intramolecular charge transfer.37-44 Beljonne et al.29 have
suggested that, within the exciton picture, the 2PA cross section
in octupolar molecules should scale by a factor of 3 in
comparison to the dipolar counterparts by virtue of increased
charge transfer among the three arms. Cho et al.50 have shown
theoretically that, in branched molecules, the 2PA cross section
increases as the strength of the donor-acceptor interaction
increases. Among several branching centers investigated in
octupolar systems, triphenylamine systems were found to show
better enhancement of 2PA cross sections with increased
branching.29-32 Prasad and co-workers30 have shown a 7-fold
enhancement of 2PA cross section for trimer (PRL series) over
monomer in a triphenylamine-core branched molecule. Macak
et al.49 suggested that electronic coupling is nominal between
the arms and vibronic coupling between the arms is significant
in enhancing the 2PA of the PRL series of dyes. Mechanistic
investigations on the same series of dye molecules suggested
that effective electronic delocalization44 and increased intramo-
lecular charge transfer are possible reasons behind the enhanced
2PA cross section. Also, several triphenylamine-core branched
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P. N. J. Phys. Chem. B 1999, 103, 10741.
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A R T I C L E S
Bhaskar et al.
Figure 1. Chemical structures of the investigated chromophores.
Scheme 1. Synthesis of the Triply Branched Molecules A-D
(dd, J ) 4.5, 1.5 Hz, 6H), 7.53 (s, 12H), 7.45 (d, J ) 9.0 Hz, 6H),
7.37 (dd, J ) 4.5, 1.5 Hz, 6H), 7.30 (d, J ) 17.0 Hz, 3H), 7.14 (d, J
) 16.0 Hz, 3H), 7.13 (d, J ) 8.0 Hz, 6H), 7.03 (d, J ) 16.0 Hz, 3H),
7.02 (d, J ) 16.0 Hz, 3H). 13C NMR (CDCl3, 75 MHz): δ 150.3,
147.0, 144.9, 138.2, 135.3, 132.9, 132.3, 128.8, 127.8, 127.6, 126.9,
125.8, 124.5, 121.0. Anal. Calcd for C63H48N4: C, 87.87; H, 5.62; N,
6.51. Found: C, 87.48; H, 6.12; N, 6.66. HRMS (ES): m/z (M + H)
calcd for C63H48N4, 861.3957; found, 861.3964.
2. Experimental Details
2.1. Synthesis of Chromophores. H NMR (300 MHz) and 13C
1
NMR (75 MHz) spectra were recorded with a Bruker AMX 300
spectrometer in CDCl3 with tetramethylsilane as internal standard.
Melting points were measured on a TA Instruments DSC 2920
instrument operating at 10 °C/min under nitrogen or by using a
polarizing optical microscope equipped with a Mettler Toledo FP82HT
heating stage attached to a Mettler Toledo FP90 temperature controller
operating at 10 °C/min. High-resolution mass spectrometry (HRMS)
was provided by the Mass Spec & Proteomics Facility at Ohio State
University. The synthesis (Scheme 1) began with the iodination of
triphenylamine 1 to afford tris(4-iodophenyl)amine 2 as a general central
building block. The branches with alkene or alkyne conjugation and
pyridine termini were synthesized as the π-building blocks. Finally,
the triply branched molecules were synthesized by palladium-catalyzed
coupling of 2 with the corresponding π-block fragment.
Tris[p-(4-pyridylvinyl)phenyl]amine (A). A mixture of tris(4-
iodophenyl)amine (6.0 g, 9.63 mmol), 4-vinylpyridine (55.6 mmol),
Pd(OAc)2 (33 mg, 0.15 mmol), tri-o-tolylphosphine (93 mg, 0.3 mmol),
triethylamine (10 mL), and acetonitrile (70 mL) was degassed with
nitrogen. The reaction mixture was refluxed for 6 h. The mixture was
cooled and poured into water. The precipitated orange solid was filtered
off and washed with water. After drying under vacuum at 60 °C, the
product was obtained as an orange solid (5.2 g, 97% yield). Mp: 266
°C (DSC). 1H NMR (CDCl3, 300 MHz): δ 8.56 (dd, J ) 4.5, 1.5 Hz,
6H), 7.46 (d, 6H, J ) 8.5 Hz), 7.35 (dd, J ) 4.5, 1.5 Hz, 6H) 7.27 (d,
J ) 16.0 Hz, 3H), 7.14 (d, J ) 8.5, 6H), 6.93 (d, J ) 16.0, 3H). 13C
NMR (CDCl3, 75 MHz): δ 150.3, 147.5, 144.9, 132.5, 131.5, 128.3,
125.0, 124.5, 120.8. HRMS (ES): m/z (M + H) calcd for C39H30N4,
555.2470; found, 555.2505.
Tris[p-(4-pyridylethynyl)phenyl]amine (C). A mixture of tris(4-
iodophenyl)amine (1.87 g, 3.0 mmol), 4-ethynylpyridine (1.12 g, 10.8
mmol), DMF (20 mL), triethylamine (10 mL), Pd(PPh3)2Cl2 (56 mg,
0.080 mmol), PPh3 (20 mg, 0.076 mmol), and CuI (16 mg, 0.084 mmol)
was thoroughly degassed with nitrogen. The reaction mixture was stirred
at 45 °C for 4 h. All the volatiles were removed under reduced pressure,
and the residue was dissolved in 150 mL of CH2Cl2, washed with
saturated aqueous Na2CO3 solution, dried over anhydrous Na2SO4, and
filtered. The crude product was preloaded on silica gel and separated
by flash column chromatography, eluting with CH2Cl2/EtOAc (up to
1:1). The product was obtained as light yellow needle-like crystals and
was recrystallized from ethanol to give the product as yellow crystals
1
(1.05 g, 64% yield). Mp: 203 °C. H NMR (300 MHz, CDCl3): δ
8.59 (dd, J ) 4.8, 1.2 Hz, 6H), 7.47 (ddd, J ) 9.0, 2.1, 2.1 Hz, 6H),
7.35 (dd, J ) 4.8, 1.2 Hz, 6H), 7.10 (ddd, J ) 9.0, 2.1, 2.1 Hz, 6H).
13C NMR (75 MHz, CDCl3): δ 149.9, 147.3, 133.4, 131.6, 125.6, 124.3,
117.2, 93.9, 87.0. HRMS (ES): m/z (M + H) calcd for C39H24N4,
549.2079; found, 549.2072.
Tris{p-[4-pyridylethynyl(4-phenylethynyl)]phenyl}amine (D). A
mixture of tris(4-iodophenyl)amine (1.25 g, 2.0 mmol), 4-(4-ethynyl-
phenylethyl)pyridine (1.46 g, 7.2 mmol), DMF (30 mL), triethylamine
(10 mL), Pd(PPh3)2Cl2 (56 mg, 0.080 mmol), PPh3 (20 mg, 0.076
mmol), and CuI (16 mg, 0.084 mmol) was thoroughly degassed with
nitrogen. The reaction mixture was stirred at 55 °C for 8 h. A blue
fluorescence was observed from the reaction mixture. All the volatiles
were removed under reduced pressure, and the residue was dissolved
in 200 mL of CH2Cl2, washed with saturated aqueous Na2CO3 solution,
and dried over anhydrous Na2SO4. This crude product was preloaded
on silica gel and separated by flash column chromatography, eluting
with CH2Cl2/EtOAc (1:1), and finally up to 5% MeOH was added to
elute the product. This product was further purified by recrystallization
from pyridine to give the product as yellow crystals (0.87 g, 51%).
Tris((E)-2-{4-[(E)-2-pyridin-4-ylvinyl]phenyl}vinylphenyl)-
amine (B). A mixture of tris(4-iodophenyl)amine (1.87 g, 3.0 mmol),
4-[4-ethenyl-(2-(E)-phenylethenyl)]pyridine (2.24 g, 10.8 mmol), Pd-
(OAc)2 (44.8 mg, 0.2 mmol), triphenylphosphine (105 mg, 0.4 mmol),
dimethylformamide (DMF) (50 mL), and triethylamine (20 mL) was
degassed with nitrogen. The reaction mixture was heated at 45-50 °C
for 24 h, and then it was cooled and poured onto 1000 mL of water.
The precipitated orange solid was filtered off and washed with water.
The orange solid was stirred in 100 mL of acetonitrile at 60 °C for 0.5
h and filtered. The resulting crude product was recrystallized from hot
pyridine. The product was obtained as red needle-like crystals (1.50 g,
58% yield). Mp: 325 °C (DSC). 1H NMR (CDCl3, 300 MHz): δ 8.57
1
Mp: 225 and 241 °C (DSC). H NMR (300 MHz, CDCl3): δ 8.61
(dd, J ) 4.5, 1.5 Hz, 6H), 7.52 (s, 12H), 7.45 (d, J ) 8.7 Hz, 6H),
7.38 (dd, J ) 4.5, 1.5 Hz, 6H), 7.09 (d, J ) 8.7 Hz, 6H). 13C NMR (75
9
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2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
MHz, CDCl3): δ 150.0, 147.0, 133.1, 132.0, 131.7, 131.4, 125.7, 124.4,
124.3, 121.8, 117.8, 93.8, 91.9, 89.1, 88.5. HRMS (ES): m/z (M + H)
calcd for C63H36N4, 849.3018; found, 849.2969.
The pump wavelength was set to 1200 nm when acquiring the
nonlinear absorption spectra to avoid any degenerate 2PA of the pump
itself. This was done for the purpose of avoiding any spurious excited-
state absorption following 2PA of the pump beam itself. The non-
degenerate spectra will exhibit larger 2PA cross sections than those
found by degenerate excitation due to pre-resonance enhancement
effects.61 The magnitude of this enhancement is determined by the
photon energies of the pump and probe and the transition energies of
the one- and two-photon allowed states. An enhancement factor (for
non-degenerate excitation) was obtained by use of eq 2 in ref 61, as
well as from the transition energies of the lowest-lying one-photon
allowed states and the energies of the higher-lying two-photon allowed
states. Although this factor did show some dispersion over the region
of interest (i.e., 380-450 nm), the effect is small and the factor was
found to be approximately 2.0 over this range. Therefore, non-
degenerate excitation will exhibit a 2 times higher strength of 2PA
compared to degenerate excitation. The values of the non-degenerate
cross sections, discussed later, have been scaled by this factor to allow
a more direct comparison with the values of the degenerate 2PA cross
sections.
2.2. Steady-State Measurements. All compounds were used as
received without any further purification. They were dissolved in
tetrahydrofuran (THF) (Sigma-Aldrich, spectrophotometric grade). The
absorption spectra of the molecules were measured using an Agilent
model 8341 spectrophotometer. To measure molar extinction coef-
ficients, the original stock solutions were diluted to 10-6 M. The
emission spectra were acquired using a Shimadzu RF-1501 instrument.
The quantum yields of the molecules were measured using a known
procedure.58 Coumarin 307 was used as the standard. The absorbance
was limited to equal to or less than 0.03. The solutions were purged
with argon for 3 min prior to measuring their emission spectra. The
following relation was then used to measure the quantum yield:
J(νj) dνj
2
∫
(Ja)S n
Ja
φF ) (φF)S
(1)
ns2
J (νj) dνj
∫
S
2.5. Fluorescence Lifetime Measurements. Time-correlated single
photon counting (TCSPC) was performed to determine the fluorescence
lifetimes of the molecules. Along with their quantum yields, their
radiative lifetimes could be determined. The laser used was the same
as for TPEF measurements. The 800 nm output was frequency-doubled
using a BBO crystal.
These measurements may have some error due to the sensitivity of the
fluorescence spectrophotometer and other environmental conditions.
2.3. Two-Photon Excited Fluorescence Measurements. To measure
the 2PA cross sections, we followed the two-photon excited fluores-
cence (TPEF) method.59 A 10-4 M solution of Coumarin 307 in
methanol was used as the reference. The laser used for the study was
a Kapteyn Murnane (KM) mode-locked Ti:sapphire laser. The band-
width at 800 nm was 47 nm, and the pulse duration was ∼30 fs. The
input power from the laser was varied by using a polarizer. An iris
was placed prior to the polarizer in order to ensure a circular beam.
The beam from the polarizer was focused on the sample cell (quartz
cuvette, 0.5 cm path length) using a lens with a focal length of 11.5
cm. The fluorescence was collected in a direction perpendicular to the
incident beam. A 1-in. focal length planoconvex lens was used to direct
the collected fluorescence into a monochromator. The output from the
monochromator was coupled to a photomultiplier tube. The photons
were converted into counts by a photon counting unit. A logarithmic
plot between collected fluorescence photons and input intensity gave
a slope of 2, ensuring a quadratic dependence between the same. The
intercept enabled us to calculate the 2PA cross section.
2.6. Ultrafast Transient Absorption Measurements. Femtosecond
transient absorption investigations were carried out using an ultrafast
pump-probe spectrometer detecting in the visible region. Briefly, 1
mJ, 100 fs pulses at 800 nm with a repetition rate of 1 kHz were
obtained from a Nd:YLF (Empower)-pumped Ti:sapphire regenerative
amplifier (Spitfire, Spectra Physics), with the seed pulses from a
Millennia-pumped Ti:sapphire oscillator (Tsunami, Spectra Physics).
The output of the laser beam was split to generate pump and probe
beam pulses with a beam splitter (85% and 15%). The pump beam
was produced by an optical parametric amplifier (OPA-800). The pump
beam used in the present investigation, i.e., 420 nm, was obtained from
the fourth harmonic of the idler beam and was focused onto the sample
cuvette. The probe beam was delayed with a computer-controlled
motion controller and then focused onto a 2 mm sapphire plate to
generate a white light continuum. The white light was then overlapped
with the pump beam in a 2 mm quartz cuvette containing the sample,
and the change in absorbance for the signal was collected by a CCD
detector (Ocean Optics). Data acquisition was controlled by software
from Ultrafast Systems Inc. Typical energy of the probe beam was
<0.1 µJ, while that of the pump beam was around 0.5-1 µJ per pulse.
Magic angle polarization was maintained between the pump and probe
using a wave plate. The pulse duration was obtained from fitting of
the solvent response, which was ∼130 fs. The sample was stirred by
a rotating magnetic stirrer.
2.4. Non-degenerate Two-Photon Absorption Measurements. In
addition to the degenerate 2PA cross sections taken at discrete
wavelengths described above, the full non-degenerate 2PA spectra of
the molecules were also obtained.60 The femtosecond laser system
employed was a CPA-2001 (CLARK-MXR) amplified Ti:sapphire
system operating at 775 nm with ∼0.9 mJ/pulse, 140 fs pulses at a 1
kHz repetition rate. This laser in turn pumped two widely tunable optical
parametric amplifiers (TOPAS, Light Conversion Ltd.). In this experi-
ment, a strong (high irradiance) pump beam from TOPAS1 induced
the nonlinearity in the sample and a weak probe beam from TOPAS2
monitored that nonlinearity. To be able to measure a wide spectral range
of the 2PA spectrum, the probe beam used was a very broadband white-
light continuum (WLC) pulse (400-1700 nm) produced by focusing
1-2 µJ into a 2.5 mm thick piece of calcium fluoride. The two pulses,
pump and probe, were overlapped in both space and time on the sample
under investigation. In this non-degenerate experiment, one photon from
the pump and one photon from the probe (within the broadband WLC)
were simultaneously absorbed. By monitoring the transmission of the
broadband WLC probe using a dual-fiber input spectrometer (Acton
Spectro150) that was coupled to a dual-diode array (Princeton Instru-
ments DPDA 2048), we could ascertain the strength of the 2PA versus
wavelength or equivalently the 2PA spectrum.
3. Results and Discussion
3.1. Optical Absorption and Steady-State Fluorescence
Measurements. The linear and nonlinear optical properties of
all the branched systems are summarized in Table 1. The optical
absorption and emission characteristics of compounds A-D in
THF are shown in Figure 2. One notes the expected shift in the
absorption maximum to longer wavelengths on going from A
to B and from C to D due to increases in conjugation length.
Also, the absorption maxima of alkyne π-bridged compounds
are blue-shifted relative to their alkene counterparts. This is
because, in the alkene chromophores, all the carbon atoms on
(58) Maciejewski, A.; Steer, R. P. J. Photochem. 1986, 35, 59.
(59) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481.
(60) Negres, R. A.; Hales, J. M.; Kobyakov, A.; Hagan, D. J.; Van Stryland, E.
W. IEEE J. Quantum Electron. 2002, 38, 1205.
(61) Hales, J. M.; Hagan, D. J.; Van Stryland, E. W.; Schafer, K. J.; Morales,
A. R.; Belfield, K. D.; Pacher, P.; Kwon, O.; Zojer, E.; Bredas, J. L. J.
Chem. Phys. 2004, 121, 3152.
9
J. AM. CHEM. SOC. VOL. 128, NO. 36, 2006 11843

A R T I C L E S
Bhaskar et al.
Table 1. Summary of Linear and Nonlinear Optical Properties of
the Chromophores
molecule
λ
abs (nm)
λ
em (nm)
δ
(GM)
η
τr (ns)
µ (D)
A
B
C
D
406
426
377
391
419
422
479
519
428
455
484
502
370a (187)
1037a (812)
91b (102)
280a (283)
270a
0.23
0.26
0.58
0.55
0.48
0.51
10
8
2.7
2.7
3
3.3
3.9
5.6
5.8
5.9
5.5
N(DSB)3
PRL-701
491a
2.9
a 800 nm excitation. b 770 nm excitation. Values in parentheses are from
non-degenerate pump-probe measurements. λabs and λem are absorption
and emission maxima, respectively; δ is the 2PA cross section; η is the
fluorescence quantum yield; τr is the radiative lifetime; and µ is the emission
transition dipole moment calculated from the Strickler-Berg formula.64
Figure 3. Non-degenerate 2PA spectra of the molecules in the region of
low-energy absorption band. Inset shows the entire spectrum. The lines are
shown as guides for the eye.
with same acceptors, alkene π-bridged chromophores are found
to have higher Stokes shifts than alkyne π-bridged chro-
mophores.
Fluorescence quantum yield and fluorescence lifetimes (with
time-correlated single photon counting) have been determined
for all the molecules. The alkyne π-bridged chromophores
showed approximately twice the quantum yield as their alkene
counterparts (Table 1). From the lifetimes of the excited states,
radiative lifetimes and emission transition dipole moments (µ)
have been determined and are provided in Table 1. It can be
noted from the table that the alkene π-bridged chromophores
A and B have lower µ than their alkyne counterparts C and D.
It is also noteworthy that conjugation length does not have a
significant impact on the radiative lifetime of these chro-
mophores. On the basis of the optical absorption measurements
(extinction coefficient and area under the absorption curve), the
absorption transition dipole moments (Mge) have also been
calculated. An increase in the conjugation length is found to
increase the Mge. The acceptor strength does not seem to play
a major role in determining Mge, while it does have a significant
impact on the Stokes shifts. On the basis of the absorption and
emission transition dipole moments calculated from steady-state
measurements, it appears that the alkyne π-bridged branched
chromophores would have greater 2PA cross sections than their
alkene analogues. On the other hand, the trend observed from
Stokes shift measurements shows that the alkene π-bridged
chromophores would have higher 2PA cross sections than their
alkyne counterparts. This suggests that steady-state measure-
ments alone cannot explain the trends observed in branched
chromophore systems, and time-resolved techniques should be
employed (see below).
Figure 2. (A) Absorption spectra and (B) Fluorescence spectra of
investigated branched molecules.
the branches are sp2 hybridized. However, in the case of the
alkyne chromophores, the carbon atoms are both sp and sp2
hybridized. This results in poorer π-orbital overlap and mismatch
in energies of the π-orbitals.62,63 It should also be noted from
Table 1 that, as the acceptor group strength increases (from tert-
butylbenzene to dioxazole to pyridine), the Stokes shift increases
(4206 cm-1 for the alkene system B is higher than for the two
previously reported nitrogen-centered chromophores, N(DSB)3
and PRL-701,44 where the pyridine group is replaced by tert-
butyl and oxadiazole groups, respectively). The increase in
Stokes shift can be attributed to increase in charge-transfer
character of the respective chromophores. For the molecules
3.2. Two-Photon Absorption Cross-Section Measurements.
Two-photon absorption cross-section measurements have been
carried out using two-photon excited fluorescence (TPEF) as
well as with non-degenerate 2PA setup. Figure 3 shows the 2PA
cross-section spectrum for chromophores A-D, where the cross
sections are plotted versus the transition energy expressed in
wavelength. As a comparison, the cross sections at 770 or 800
nm (at the maximum of the lowest-energy absorption band) for
the molecules are listed in Table 1 as well (shown in parentheses
are the non-degenerate 2PA cross sections at those wavelengths).
It is worth reiterating that non-degenerate 2PA has shown larger
(62) Dewar, M. J. S. J. Am. Chem. Soc. 1952, 72, 1345.
(63) Cheng, L.; Tam, W.; Marder, S. R.; Stiegman, A. E.; Rikken, G.; Spangler,
C. W. J. Phys. Chem. 1991, 95, 10643.
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2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
than 2PA at equivalent values for degenerate excitation. For
this reason, the values in Table 1 have been reduced by an
appropriate scaling factor to make a more direct comparison
with the degenerate 2PA cross sections.61
C and D are compared, suggesting the effect of conjugation
length. As the conjugation length is increased, the absorption
transition dipole moment is increased (Table 1) and the detuning
factor is decreased, thereby influencing the 2PA cross section.
It is interesting to see the trend observed in the cross section
for 2PA to high-energy states for alkene and alkyne π-bridging
branched chromophores. When we compare A and C, the alkyne
π-bridged chromophore’s (C) cross section is around 1.4 times
higher than that of the corresponding alkene analogue (A). As
mentioned above, the expected trend in transition moments
suggests that the 2PA in the alkene systems is longer. This is
contrary to what the steady-state results predicted. However,
ultrafast time-resolved measurements, which give information
about excited-state characteristics, are able to probe these
differences. It is to be noted here that, when cross sections for
2PA to higher-energy states are concerned, it involves knowl-
edge of the excited-state transition dipole moment (from the
sum-over-states formalism). Estimates of excited-state transition
dipole moment can be obtained from ultrafast transient absorp-
tion measurements.
There are several comparisons that can be made regarding
the 2PA properties observed in these systems. First, a compari-
son between the linear and nonlinear absorption spectra reveals
that all the molecules exhibit substantial 2PA into the lowest-
energy absorption band (i.e., 380-450 nm), which is both one-
and two-photon allowed.29 There also exists a stronger, higher-
energy two-photon-allowed state which lies between the two
one-photon bands (inset of Figure 3), which has been attributed
to strong electronic coupling between the branches. Furthermore,
the trends observed for both the degenerate and non-degenerate
cross sections in Table 1 are quite similar. The two alkene-
containing systems (A and B) differ only by an increase in the
conjugation length of the individual arm by one vinylene unit.
In the lowest-energy absorption band, one observes a 3-fold
increase in 2PA cross section for B in comparison to A. The
same trends can be noted for compounds C and D, which both
contain alkynes. This effect of increasing conjugation on 2PA
cross section has been well documented in the literature.64,65
An increase in conjugation reduces the detuning term (the energy
difference between the ground and first excited states and the
ground and 2PA states) in the sum-over-states expression29 and
also increases the transition dipole moment. As a result of these
two contributions, the 2PA cross section increases significantly.
Comparing chromophores B and D, which contain similar
chromophore density, it is clearly seen that the 2PA of the alkene
branched system B is approximately 3.7 times higher than that
of its alkyne counterpart D. Similarly, the ratio of 2PA cross
sections between A and C is 4. A direct comparison between
molecular systems can be made by normalizing the cross
sections listed in Table 1 to their respective molecular weights.
Still, the alkene chromophores show a 4-fold increase in 2PA
cross section when compared to the corresponding alkynes at
the low-energy peak.
The influence of acceptor strength is also seen from the results
presented in Table 1. As the acceptor unit is changed from tert-
butylbenzene to oxadiazole in N(DSB)3 to PRL-701, respec-
tively, we see a substantial increase in 2PA cross section. This
effect is more pronounced when a stronger acceptor, namely
pyridine, is introduced in molecule A. The argument that
acceptor strength increases from tert-butylbenzene to pyridine
can be verified from their Hammett constants.67 Chromo-
phore A has 4 times higher δ than N(DSB)3. These observa-
tions can be attributed to increased acceptor strength, which
imparts a higher amount of charge-transfer character. It has been
shown theoretically by Cho et al.49 that the 2PA cross section
increases with an increase in donor or acceptor strength. The
measurements presented here show that the extent of charge-
transfer character is important in increasing the 2PA cross
section.
3.4. Transient Absorption Measurements. Among all the
investigated trimers, trimer B showed the largest 2PA cross
section in comparison to the other chromophores. By changing
the bridging unit from alkyne to alkene, there is a substantial
increase in 2PA cross section, which also increases with an
increase in acceptor strength. Thus, it is evident that the charge-
transfer character of the molecules does play an important role
in determining the 2PA cross section of the molecules. Steady-
state measurements based on absorption, emission transition
dipole moments, and Stokes shift were unable to provide a
conclusive explanation for the observed differences in the 2PA
cross section of alkene and alkyne π-bridging molecules as well
as branched molecules with increase in acceptor strength. Thus,
we evaluated the extent of charge transfer in the excited state
as well as excited-state transition dipole moments by ultrafast
pump-probe measurements. To illustrate the importance of the
extent of intramolecular charge transfer (ICT) in the excited
state immediately after photoexcitation with regard to the 2PA
cross sections, ultrafast transient absorption measurements of
molecules A-D were carried out. Shown in Figure 4A are the
transient absorption spectra at different time delays (from 150
fs to 26 ps) of trimer B in THF. At the time delay of 150 fs, a
bleach in the region of 450-540 nm and a positive transient
absorption greater than 540 nm are observed. The bleach
centered at ∼470 nm can be ascribed to the stimulated emission
from the ICT state of the trimer. The positive absorption is
attributed to the excited-state absorption (ESA) arising from the
ICT state.
The observation of stimulated emission and ESA from the
ICT state at a time delay of 150 fs suggests that the internal
conversion from the initially populated C3-symmetry state to
the charge-localized ICT state happens much faster than 100
fs, which indicates that the Franck-Condon (FC) state and the
ICT state are intimately linked by the same reaction coordinate.
However, as the time delay is increased to 26 ps, a new transient
started to grow in the region of 460-540 nm with a decay of
ESA of the ICT state. Population of this transient was found to
increase with increasing polarity of the solvents and strength
of donor-acceptors. Thus, this ESA is ascribed to the solvent
and the conformationally stabilized ICT state (ICT′ state). It is
A surprising trend is observed when considering the high-
energy 2PA band shown in the inset of Figure 3. When we
compare A and B, there is an increase in the 2PA cross section
by a factor of 2, and the increase is a factor of about 1.5 when
(64) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
(65) Rubio-Pions, O.; Luo, Y.; Agren, H. J. Chem. Phys. 2006, 124, 094310.
(66) Ray, P. C.; Leszcczynski, J. J. Phys. Chem. A 2005, 109, 6689.
(67) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
9
J. AM. CHEM. SOC. VOL. 128, NO. 36, 2006 11845

A R T I C L E S
Bhaskar et al.
Figure 5. (A) Transient absorption spectra of D in THF at different time
delays from 150 fs to 30 ps. (B) Kinetics of the transients at 590 and 450
nm.
Figure 4. (A) Transient absorption spectra of B in THF at different time
delays from 150 fs to 30 ps. (B) Kinetics at 590 and 495 nm.
to be noted here that the ICT′ state is not emissive and can be
ascribed to a twisted intramolecular charge transfer ICT state.
The kinetic traces shown in Figure 4B show that the decay of
the ICT state matches the growth of the ICT′ state, and they
constitute a predecessor-successor pair. Global fit analysis of
all the decay traces has yielded two time constants of 2.8 ps
and >1 ns. The time constant of 2.8 ps has been ascribed to
the time scale of internal conversion from the ICT state to the
ICT′ state, and a >1 ns time constant is ascribed to the excited-
state lifetime.
Transient absorption measurements have also been carried
out on trimer D dissolved in THF, and the corresponding
transient absorption spectra are shown in Figure 5A. At the
initial time delay, a bleach in the region of 450-500 nm and
a positive absorption greater than 500 nm with a maximum
around 560 nm are observed. The bleach has been ascribed to
stimulated emission from the ICT and the positive absorption
to ESA of the ICT state. Ultrafast charge localization from the
FC state to the ICT state has also been observed here, suggesting
that the FC and ICT states are linked by the same reaction
coordinate.
Similar to what has been observed in the case of trimer B,
formation of a new transient is observed as the time delay is
increased to 30 ps, although the amplitude of the observed new
transient is small. Evolutions of the transients are better viewed
by following the kinetic decay traces at 450 and 590 nm, as
shown in Figure 5B. The kinetics at 450 nm shows the presence
of the new transient state, and its growth matches well with the
decay of ESA at 590 nm. This new transient can be once again
Figure 6. Comparison of the ICT′ states for trimers D and B. It can be
observed that the population of ICT′ for B is higher than that for D.
attributed to ESA of the ICT′ state. Global fit analysis has
yielded two time constants, 2.1 ps and >1 ns, similar to those
observed in the case of B. However, it is interesting to note
that the populations of this ICT′ state are different for D and
B; the population is higher in the case of B over D in the same
solvent.
As mentioned above, the population of the ICT′ state depends
on the polarity of the solvent and the strength of the acceptor.
Thus, it can be used as an indicator of the amount of charge
transfer in the excited state of the trimers. Figure 6 shows the
comparison of the populations of the ICT′ states for B and D
in THF at a time delay of 30 ps. Under similar solvent polarity,
the population of the ICT′ state can be directly ascribed to the
degree of charge-transfer character. It can be observed from
Figure 6 that the amplitude of ICT′ for the alkene branched
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11846 J. AM. CHEM. SOC. VOL. 128, NO. 36, 2006

2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
system B is much higher in comparison to that for the alkyne
branched system D in the same solvent. This result suggests
that the amount of charge transfer in the case of B is larger
than that in D. As the 2PA cross section mainly concerns the
FC state, knowledge of the amount of charge transfer concerning
the FC state is needed. The FC and ICT states are intimately
linked by the same reaction coordinate by virtue of ultrafast
charge localization from the FC to the ICT state. Hence, one
can extrapolate the extent of charge transfer observed in the
ICT state to the FC state.68,69 The results thus confirm that the
extent of CT in the alkene branched system B is higher than
that in D, and therefore B displays a higher 2PA cross section.
The result of a larger δ for B in comparison to D was somewhat
surprising, as one also notes the increase in quantum yield and
calculated transition dipole moments for the alkyne system.
However, the magnitude of δ in the alkyne π-system D is
significantly smaller than that observed for the alkene π-system
B, and this mainly is related to the increased CT character and
stronger delocalization. This conclusion is obtained from time-
resolved absorption measurements, as the steady-state measure-
ments favor a larger cross section for D than for B.
Another important aspect that has been observed in 2PA
cross-section measurements is that, as the acceptor strength is
increased, the 2PA cross section is increased. This can be mainly
ascribed to the CT of both ground and excited states. It is well
known that higher acceptor strength results in a greater degree
of charge transfer in a donor-acceptor type system, which
thereby shows higher NLO properties. It has been demonstrated
earlier in this section that the extent of CT in the excited state
can be monitored by femtosecond pump-probe spectroscopic
results. Similar transient absorption measurements have been
carried out to understand the effect of acceptor strength on 2PA
cross section. Shown in Figure 7 are the transient absorption
spectra of PRL-701 and N(DSB)3 in acetonitrile (3D representa-
tion of the ESA spectra), in which PRL-701 has shown a larger
2PA cross section by virtue of its larger acceptor strength. Upon
comparing N(DSB)3 and PRL-701, with PRL-701 having the
stronger acceptor in the oxadiazole molecule, it is clearly seen
that the ratio of ICT′:ICT population is greater in the case of
PRL-701, suggesting that the amount of CT in the case of PRL-
701 is obviously higher than that in N(DSB)3.
Figure 7. Comparison of the ICT′ states’ populations for molecules
N(DSB)3 and PRL-701 in acetonitrile. It can be observed that ICT′:ICT is
higher for PRL-701.
effective 2PA cross sections of molecules. Within the framework
of the sum-over-states model,29 the cross section for 2PA into
the lowest-energy absorption band for non-centrosymmetric
molecules can be expressed as
2
Mge′′2 ∆µge′
δ2-state
(2)
Estimates of excited-state extinction coefficients have also
been made for the chromophores B and D under actinometric
conditions by ultrafast transient absorption spectroscopy. It has
been observed from our transient measurements that the excited-
state extinction coefficient of chromophore B is about 2 times
larger than that of D. Using the excited-state extinction
coefficients and the area under the curve of the ESA spectrum,
we have determined the excited-state transition dipole moment.
It is found to be around 1.5 times higher for alkyne branched
chromophore D in comparison to alkene branched chromophore
B. The higher excited-state transition dipole moment contributes
mainly to the 2PA into the higher-energy two-photon absorption
bands.
Γ
and that for 2PA into high-lying states that are not dipole
coupled to the ground state can be written as
2
2
Mge′′ Mee′
(Ege - Ege′/2)2Γ
δ2-state
(3)
Here, Mge and Mge′ are the transition dipole moments from the
ground state to excited states e and e′, respectively; Ege and
Ege′ are the corresponding transition energies from the ground
state to the e and e′ states, respectively; and Γ is the damping
factor. Equation 2 is the dipolar term while eq 3 is the two-
photon term in the complete sum-over-states model. Under this
model, the cross section for 2PA to the low-lying excited state
of chromophores containing charge-transfer states is mainly
controlled by three important parameters: the excitation energy,
the oscillator strength, and the change in dipole moment related
to the low-lying CT state.29 For the higher-energy 2PA states,
3.5. Factors and Rationale for Influencing the Two-Photon
Absorption Cross Section in Branched Systems. As men-
tioned in the Introduction, several parameters influence the
(68) Ramakrishna, G.; Goodson, T. G., III, submitted for publication.
(69) Premvardhan, L.; Papagiannakis, E.; Hiller, R. G.; van Gorndelle, R. J.
Phys. Chem. B 2005, 109, 15589.
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A R T I C L E S
Bhaskar et al.
along with these parameters the cross section for 2PA is also
controlled by the excited-state transition dipole moment. The
present 2PA cross-section measurements of triphenylamine-core
branched molecules have displayed some interesting magnitudes
of 2PA cross section with respect to the nature of the
substituents.
the extent of charge transfer in the ICT state of B is higher
than in D. It has also been observed that there is ultrafast charge
localization from the FC state to the ICT state, indicating that
the two states are intimately connected by the same reaction
coordinate. Thus, the amount of CT in the ICT state is correlated
to the FC state, which indicates that the extent of charge transfer
is higher for alkene π-bridged chromophores in comparison to
the alkyne π-bridged chromophores. This will contribute
significantly to the net change in dipole moment of the molecule
and to the dipole term of 2PA (eq 2, 2PA into the one-photon-
allowed CT state). This explains the higher cross section
observed for alkene π-bridged chromophores over alkyne
π-bridged chromophores for the low-energy peak.
However, 2PA into high-energy 2PA states involves the
summation of eqs 2 and 3. From transient absorption spectros-
copy, two important results are obtained with regard to alkene
and alkyne π-bridged chromophores. The extent of charge
transfer, which contributes mainly to the low-lying 2PA effect
band, is higher for alkene π-bridged chromophores than for the
alkyne analogues, while the estimates of excited-state transition
dipole moment favor the alkyne π-bridged chromophores over
the alkene analogues. These two terms contribute in opposite
directions, and their summation gives almost equal cross sections
for 2PA into higher-energy 2PA bands for both B and D. This
stresses very strongly the need for the use of combined
techniques (TPEF, non-degenerate pump-probe, ultrafast tran-
sient absorption spectroscopy) to evaluate the differences in
observed 2PA cross section. Actually, this is a novel approach
in which different 2PA techniques as well as steady-state and
time-resolved measurements are all used to unravel the mech-
anism of the 2PA cross section. The extent of charge transfer
in the excited state has been clearly evaluated, and one finds
that this approach is more in-depth than the measurements of
the cross section alone.
Effect of Conjugation Length. The two alkene π-bridge-
containing systems (A and B) differ in the conjugation length
of the ligand, and concurrently one observes about a 3-fold
increase in the cross section for B in comparison to that for A.
The same trend can be noted for compounds C and D, both
containing alkyne π-bridges. It can be observed from Table 1
that, as the conjugation length is increased, there is a clear red
shift in the optical absorption and emission for both alkene and
alkyne π-bridging molecules. The Stokes shift is also increased
with increase in conjugation length, indicating an increase in
effective conjugation length. All these parameters result in
increases of the oscillator strength of the absorption band66,67
and the transition dipole moment of the absorption. Also, the
shift of absorption to longer wavelengths with an increase in
conjugation decreases the detuning energy present in the two-
photon term (eq 3), thereby enhancing the 2PA cross section.
Effect of Acceptor Strength. With increasing the acceptor
strength from N(DSB)3 to PRL-701 to B, there is and enhance-
ment in the 2PA cross section. Unlike the quadrupolar mol-
ecules, where the one-photon-allowed states are not two-photon
allowed, in multiply branched chromophores, the low-lying
states are both one- and two-photon allowed. In such a situation,
one-photon photophysics and excited-state deactivation can be
correlated with the observed 2PA cross sections. Although
enhancement of the 2PA cross sections through substitution with
electron-withdrawing and electron-donating groups is well
established both experimentally and theoretically, the electronic
origin of the enhancement is not yet fully understood.29 In the
present investigation, through our transient absorption measure-
ments, we have shown that, as the acceptor strength is increased,
the extent of charge transfer in the FC state is increasing, which
cannot be understood on the basis of steady-state measurements
alone. This will result in a net change in the dipole moment
and significantly contribute to the dipole term in the 2PA cross
section, thereby increasing the 2PA cross section with increase
in acceptor strength.
4. Conclusions
In conclusion, we have synthesized a set of triphenylamine-
centered branched structures with pyridine acceptor groups to
investigate their two-photon absorption and photophysical
properties and compared them with previously reported branched
chromophores. Two-photon absorption measurements have
shown that increases in conjugation length and acceptor strength
enhance the cross section, while the alkene π-bridged branched
chromophores show significantly higher 2PA cross section than
the alkyne π-bridged chromophores, especially in the region of
the low-energy absorption band. The Stokes shift and emission
transition dipole moments determined from steady-state and
fluorescence lifetime measurements are not sufficient to predict
the trends observed in 2PA cross sections. The results indicate
that charge transfer in the excited state plays a vital role in
increasing the 2PA cross section. Ultrafast transient absorption
spectroscopy was used to measure the amount of charge transfer
in the excited state. It has been shown that alkene π-bridged
systems possess a greater amount of charge transfer by virtue
of a greater population of the ICT′ state, and this is the reason
for the increased 2PA cross section in these chromophores.
Estimates of excited-state transition dipole moments have
explained the observed differences in 2PA in high-energy 2PA
states. These trends were not predicted by steady-state measure-
ments; the combined approach allows one to probe the mech-
anism of 2PA in branched structures closely. Overall, the most
Effect of π-Bridging. It has been shown in the literature that,
in donor-bridge-acceptor molecules, the lowest-energy ICT
state is influenced not only by the donor-acceptor strength but
also by the bridge and substituents.70 In the present investigation,
a direct comparison between molecular systems can be made
by normalizing the cross sections listed in Table 1 to their
respective number of π-electrons. In this case, alkene π-bridge-
containing molecules A and B exhibit larger normalized 2PA
values (by 4.4 and 4.0 times, respectively) than their alkyne
π-bridged counterparts of C and D. One also notes from Table
1 that, within the two respective systems (alkene and alkyne),
there is no significant change in the transition dipole moment.
Transient absorption measurements carried out for alkene and
alkyne π-bridged chromophores B and D have shown a higher
population of the ICT′ state for B over D, which indicates that
(70) Ahlheim, M.; Barzoukas, M.; Bedworth, P. V.; Blanchard-Desce, M.; Fort,
A.; Hu, Z.-Y.; Marder, S. R.; Perry, J. W.; Runser, C.; Statehelin, M.;
Zysset, B. Science 1996, 271, 335.
9
11848 J. AM. CHEM. SOC. VOL. 128, NO. 36, 2006

2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
extended alkene π-bridged system showed the largest 2PA cross
section at the low-energy peak for reasons related to increased
conjugation length and larger CT character. These results suggest
that alkene and alkyne π-extended systems with acceptors such
as pyridine have better 2PA properties, and they would serve
as good building blocks for larger branched systems.
acknowledges the Army Research Labs and the National Science
Foundation for support.
Supporting Information Available: Complete refs 1a,b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. T.G. acknowledges the Army Research
Office and the National Science Foundation for support. J.M.H.
JA060630M
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