The roles of molecular structure and effective optical symmetry in evolving dipolar chromophoric building blocks to potent octopolar nonlinear optical chromophores

Article abstract of DOI:10.1021/ja105004k

A series of mono-, bis-, tris-, and tetrakis(porphinato)zinc(II) (PZn)-elaborated ruthenium(II) bis(terpyridine) (Ru) complexes have been synthesized in which an ethyne unit connects the macrocycle meso carbon atom to terpyridyl (tpy) 4-, 4′-, and 4′′-pos

Full text of DOI:10.1021/ja105004k

ARTICLE
pubs.acs.org/JACS
The Roles of Molecular Structure and Effective Optical Symmetry in
Evolving Dipolar Chromophoric Building Blocks to Potent Octopolar
Nonlinear Optical Chromophores
Tomoya Ishizuka,†,‡ Louise E. Sinks,†,§ Kai Song,|| Sheng-Ting Hung,^ Animesh Nayak,†,# Koen Clays,*,^
and Michael J. Therien*,#
†Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
^Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium
#Department of Chemistry, French Family Science Center, 124 Science Drive, Duke University, Durham, North Carolina 27708-0346,
United States
S Supporting Information
b
ABSTRACT: A series of mono-, bis-, tris-, and tetrakis(porphi-
nato)zinc(II) (PZn)-elaborated ruthenium(II) bis(terpyridine)
(Ru) complexes have been synthesized in which an ethyne unit
connects the macrocycle meso carbon atom to terpyridyl (tpy) 4-,
40-, and 400-positions. These supermolecular chromophores,
based on the ruthenium(II) [5-(40-ethynyl-(2,20;60,200-
terpyridinyl))-10,20-bis(20,60-bis(3,3-dimethyl-1-butyloxy)
phenyl)porphinato]zinc(II)-(2,20;60,200-terpyridine)2þ bis-
hexafluorophosphate (RuPZn) archetype, evince strong mix-
ing of the PZn-based oscillator strength with ruthenium terpyr-
idyl charge resonance bands. Potentiometric and linear absorp-
tion spectroscopic data indicate that for structures in which multiple PZn moieties are linked via ethynes to a [Ru(tpy)2]2þ core, little
electronic coupling is manifest between PZn units, regardless of whether they are located on the same or opposite tpy ligand. Congruent
with these experiments, pump-probe transient absorption studies suggest that the individual RuPZn fragments of these structures exhibit,
at best, only modest excited-state electronic interactions that derive from factors other than the dipole-dipole interactions of these strong
oscillators; this approximate independent character of the component RuPZn oscillators enables fabrication of nonlinear optical (NLO)
multipoles with extraordinary hyperpolarizabilities.
Dynamic hyperpolarizability (βλ) values and depolarization ratios (F) were determined from hyper-Rayleigh light scattering
(HRS) measurements carried out at an incident irradiation wavelength (λinc) of 1300 nm. The depolarization ratio data provide an
experimental measure of chromophore optical symmetry; appropriate coupling of multiple charge-transfer oscillators produces
structures having enormous averaged hyperpolarizabilities (βHRS values), while evolving the effective chromophore symmetry from
purely dipolar (e.g., Ru(tpy)[4-(Zn-porphyrin)ethynyl-tpy](PF6)2, βHRS =1280 ꢀ 10-30 esu, F=3.8; Ru(tpy)[40-(Zn-porphyr-
in)ethynyl-tpy](PF6)2, βHRS = 2100 ꢀ 10-30 esu, F = 3.8) to octopolar (e.g., Ru[4,400-bis(Zn-porphyrin)ethynyl-tpy]2(PF6)2,
βHRS = 1040 ꢀ 10-30 esu, F = 1.46) via structural motifs that possess intermediate values of the depolarization ratio. The
chromophore design roadmap provided herein gives rise to octopolar supermolecules that feature by far the largest off-diagonal
octopolar first hyperpolarizability tensor components ever reported, with the effectively octopolar Ru[4,400-bis(Zn-porphyr-
in)ethynyl-tpy]2(PF6)2 possessing a βHRS value at 1300 nm more than a factor of 3 larger than that determined for any chromophore
having octopolar symmetry examined to date. Because NLO octopoles possess omnidirectional NLO responses while circum-
venting the electrostatic interactions that drive bulk-phase centrosymmetry for NLO dipoles at high chromophore concentrations,
the advent of octopolar NLO chromophores having vastly superior βHRS values at technologically important wavelengths will
motivate new experimental approaches to achieve acentric order in both bulk-phase and thin film structures.
’ INTRODUCTION
used for frequency conversion and electro-optic modulation of
signals. To engineer NLO chromophores, careful consideration
Nonlinear optical (NLO) materials are used in a variety of
applications, most notably in optical telecommunications, where
NLO properties in the near-infrared (NIR) spectral region are
Received: June 8, 2010
Published: February 15, 2011
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of molecular symmetry must be taken into account, as non-
centrosymmetry is a prerequisite for all even-order, nonlinear
effects.1-10 The molecular properties that give rise to the NLO
response are the electronic polarizability and the nature of the
transitions between electronic states. Within the context of
simple theoretical constructs, the experimental observables
that are closely correlated with the magnitude of the molecular
NLO response include (i) the frequency dependence of the
ground-state molecular linear absorption spectrum, (ii) the
absorption spectra of low-lying electronic states, and (iii) the
oscillator strengths, transition energies, and changes in transi-
tion dipole moments associated with the transitions in these
spectra.11-18
Insights derived from these theoretical frameworks have led to
the development of organic and inorganic optoelectronic chromo-
phores that manifest enhanced electronic hyperpolarizabilities
(βλ values). A classic molecular architecture that has been
developed extensively has been the dipolar, donor-bridge-
acceptor (D-Br-A) motif.5-8,19-28 Numerous examples of
dipolar D-Br-A chromophores that feature large dynamic
hyperpolarizabilities exploit a highly polarizable porphyrinic
component.18,21-25,27-33 Along these lines, it has also been
shown that strong conjugation mediated by an ethynyl unit
between multiple porphyrinic components, or between porphy-
rin moieties and other strong oscillators, creates unusually
polarizable and hyperpolarizable structures,34-43 and has en-
abled the development of a variety of compositions and materials
that possess unusual NLO properties.44-54
Porphyrin-based high βλ chromophores that possess the most
spectacular hyperpolarizabilities delineated to date feature
(porphinato)zinc(II) (PZn) and metal(II)polypyridyl (M) units
connected via an ethyne bridge. In these MPZn supermolecules,
PZn π-π*- and metal polypyridyl-based charge-resonance ab-
sorption oscillator strength are extensively mixed, and the
respective charge transfer transition dipoles of these building
blocks are aligned along the highly conjugated molecular axis.
These structures manifest significant interpigment electronic
interactions, display unusual dependences of the sign and mag-
nitude of hyperpolarizability on incident irradiation frequency,
and exhibit extraordinarily large βλ values at telecommunications-
relevant wavelengths.18,25,29-31,45,47,48
While large first hyperpolarizabilities have been realized for
several dipolar, D-Br-A chromophores over the 1300-
1500 nm spectral domain,20,25,26,55,56 the development of macro-
scopic materials that possess impressive nonlinear properties
based on these chromophores requires ordering of hyperpolariz-
able molecules in a non-centrosymmetric arrangement through-
out the medium. For dipolar molecules, ground-state electro-
static interactions drive detrimental, centrosymmetric aggregation.
Although experimental strategies exist that minimize the magni-
tude of antiparallel dipole-dipole interactions of NLO
chromophores in the condensed phase, and facilitate electric-
field poling of chromophoric guests in polymeric hosts to
produce a processable materials for device fabrication, it is
important to underscore that the net bulk-phase dipolar order
realized in such NLO materials rarely, if ever, exceeds a level
beyond that of several percent of the chromophoric dopants
present in the medium.9,56-62 Due in part to the ongoing
challenge of obtaining uniform acentric order of dipolar NLO
chromophores within a processable matrix, focus has been
placed on the development of hyperpolarizable molecules
having higher-order symmetry.
Multiple families of NLO chromophores having octopolar
symmetry have been interrogated.63-77 While such structures
circumvent the electrostatic interactions that drive bulk-phase
centrosymmetry at high chromophore concentrations, ap-
proaches other than electric-field-induced ordering, such as the
synthesis of high generation dendritic octopoles,71 spontaneous
organization within an appropriate liquid-crystalline phase,76 or
all-optical poling,78,79 need to be employed to organize NLO-
active species of high-order symmetry. A challenge of equal
importance lies in the fact that all NLO octopoles developed to
date exhibit only modest responses at technologically impor-
tant wavelengths relative to high βλ dipolar chromophore
benchmarks.
Evolution of NLO octopoles from dipolar chromophoric
archetypes has been demonstrated, but such examples are con-
fined largely to proof-of-principle targets that possess only
modest hyperpolarizabilities; cases in point include the synthe-
ses of 1,3,5-tridonor-2,4,6-triacceptor-aryl compounds in-
spired by the corresponding dipolar 1-donor-4-acceptor-aryl
structures.68,70,72-74,76 This fact has been emphasized recently
by Torres and co-workers, who have noted that virtually all
NLO octopoles reported to date are in fact octopolar expan-
sions of the dipolar paradigm.80 We show herein that
ruthenium(II) [5-(40-ethynyl-(2,20;60,200-terpyridinyl))-10,20-
bis(20,60-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]
zinc(II)-(2,20;60,200-terpyridine)2þ bis-hexafluorophosphate
(RuPZn), a robust, benchmark dipolar chromophore that pos-
sesses an extraordinary βλ value at a telecommunication-relevant
wavelength (β1300 = 5100 ꢀ 10-30 esu),25 can not only serve as a
template for the development of an octopolar NLO chromo-
phore (Figure 1); appropriate coupling of multiple, related
charge-transfer oscillators can be utilized to evolve the effective
chromophore optical symmetry in MPZn-based supermolecules
from purely dipolar to octopolar, and generate octopolar NLO
chromophores having impressive βHRS values at 1300 nm.
’ EXPERIMENTAL SECTION
Materials. All manipulations were carried out under argon pre-
viously passed through an O2 scrubbing tower (Schweizerhall R3-11
catalyst) and a drying tower (Linde 3 Å molecular sieves) unless
otherwise stated. Air-sensitive solids were handled in a Braun 150-M
glovebox. Standard Schlenk techniques were employed to manipulate
air-sensitive solutions. All solvents utilized in this work were obtained
from Fisher Scientific (HPLC grade). Tetrahydrofuran (THF) was
distilled from Na/benzophenone under N2. All NMR solvents and
commercially available reagents were used as received unless noted
otherwise. 2,20-Dipyrrylmethane,81,82 4-bromo-(2,20;60,200-terpyridine),83
40-bromo-(2,20;60,200-terpyridine),83 4,400-dibromo-(2,20;60,200-terpyridine),83
and (2,20;60,200-terpyridine)ruthenium(III) trichloride (RuCl3(tpy)),
along with its brominated derivatives,84-88 were prepared by literature
1
methods (see Supporting Information). Chemical shifts for H NMR
spectra are relative to the internal reference (tetramethylsilane) in
CDCl3, or solvent residual protium (acetonitrile-d3, δ = 1.93 ppm).
All J values are reported in Hertz. The number of attached protons is
found in parentheses following the chemical shift value. Chromato-
graphic purification (silica gel 60, 230-400 mesh, EM Scientific) of all
newly synthesized compounds was accomplished on the benchtop.
Standard abbreviations for metal polypyridyl compounds and terpyridyl
are used throughout the Experimental Section.
Instrumentation. Electronic spectra were recorded on a Shimadzu
UV-1700 spectrophotometry system. Cyclic voltammetric measure-
ments were carried out on an EG&G Princeton Applied Research model
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Figure 1. MM2-optimized structures of ethyne-bridged (porphinato)zinc(II)-ruthenium bis(terpyridine)-based chromophores 1-9.
273A potentiostat/galvanostat. The electrochemical cell used for these
experiments utilized a platinum disk working electrode, a platinum wire
counter electrode, and a saturated calomel reference electrode (SCE).
The reference electrode was separated from the bulk solution by a
junction bridge filled with the corresponding solvent/supporting elec-
trolyte solution. The ferrocene/ferrocenium redox couple was utilized as
an internal potentiometric standard.
than 0.5 nm. To provide shot-by-shot detection at a 1 kHz repetition rate,
the entrance slit of the image spectrometer was optically masked to shield
the upper bins of the CCD. Data points (32) were binned for each
wavelength, with the resulting spectrum transferred to the computer. Pairs
of consecutive spectra were measured with Ion(λ) and Ioff(λ) to determine
the difference spectrum, ΔA = log(Ioff(λ))/(Ion(λ)). Transient spectra in
0.9-1.8 μm NIR region were recorded using a liquid-nitrogen-cooled
InGaAs512-element linear array detector (Roper Scientific) interfaced to a
Spex-250 monochromator. All these experiments utilized a 2 mm path
length fused-silica sample cell; all transient optical studies were carried out
at 23 ( 1 ꢀC. Samples were deoxygenated by three consecutive freeze-
pump-thaw cycles prior to use. All transient spectra reported represent
averages obtained over 4-6 scans, with each scan consisting of 100-170
time points.
The delay line utilized three computer-controlled delay stages. Short
time delays were effected with Nanomover stages (Melles Griot), while
longer delay times (up to 8.5 ns) were achieved using a Compumotor-
6000 (Parker). The baseline noise level in these transient absorption
experiments corresponded to ∼0.1 mOD per second of signal accumu-
lation. The time resolution is probe-wavelength dependent; in these
experiments, the fwhm of the instrument response function (IRF) varied
between 140 and 200 fs (e.g., at 680 nm, the IRF was 150 ( 6 fs).
Nanosecond Transient Absorption Experiments. The tran-
sient absorption setup for accessing microsecond delay times, housed at
the Regional Laser and Biotechnology Laboratory at the University of
Pennsylvania (RLBL), has been described previously.89 The excitation
wavelength for all samples was 532 nm. Samples were prepared in 1 cm
quartz cells using distilled, dry acetonitrile, which was degassed by five
Ultrafast Transient Absorption Experiments. Transient ab-
sorption spectra were obtained using standard pump-probe methods.
Optical pulses (>120 fs) centered at 775 nm were generated using a Ti:
sapphire laser (Clark-MXR, CPA-2001), which consists of a regenerative
amplifier seeded by a mode-locked fiber oscillator. A frequency-doubled
near IR optical parametric amplifier (Clark-MXR) produced excitation
pulses tunable throughout the visible region. The pump beam, chopped
at half the laser repetition rate (∼500 Hz), was passed through an optical
delay line. A fraction of the output from the regenerative amplifier was
focused onto a 2 mm c-cut sapphire plate to generate a white light
continuum, which was used as the probe beam. The polarization and
attenuation of the pump and probe beams were controlled by a half-wave
plate and Rochon prism polarizer pairs. The polarization was set to
magic angle (54.7ꢀ) for these experiments. The pump beam was focused
into the sample cell with an f = 40 cm lens, while the probe beam was
focused with an f = 10 cm lens. After passing through the sample, the probe
light was focused onto the entrance slit of the computer-controlled image
spectrometer (SpectraPro-150, Acton Research Corporation). A CCD
array detector (Roper Scientific, 1024 ꢀ 128 elements), interfaced to the
spectrometer, recorded the spectrum of the probe light from the UV
(∼370 nm) to the NIR (∼1100 nm), providing spectral resolution better
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Scheme 1. General Synthetic Scheme for RuPZn-Based Chromophores
successive freeze-pump-thaw cycles. Excited-state lifetimes were
calculated by performing an exponential fit to the decay or the ground-
state bleach rise of each respective compound. Each sample was run
multiple times at various concentrations to ensure that other deactiva-
tion pathways were not present.
crystals of 2 (25.8 mg, 14.3 μmol, 70% yield) were collected by filtration.
1H NMR (CD3CN): δ 10.09 (s, 1H, meso-H), 9.64 (m, 2H, β-H), 9.23 (m,
2H, β-H), 9.03 (d, J = 8.0 Hz, 2H, 6-py), 8.87 (d, J = 4.5 Hz, 2H, β-H), 8.81
(m, 3H), 8.77 (d, J = 4.5 Hz, 2H, β-H), 8.54 (m, 4H), 8.47 (t, J = 8.0 Hz, 1H,
4-py), 7.96 (m, 3H), 7.79 (t, J = 8.5 Hz, 2H, -p-Ph), 7.45 (d, J = 7.5 Hz, 2H, 3
and 300-PyH for tpy), 7.44 (d, J = 8.0 Hz, 1H, 300-PyH for 4-PZn-tpy), 7.22
(m, 5H), 7.14(d,J= 8.5 Hz, 4H, -m-Ph), 3.94 (t, J= 7.0 Hz, 8H, -OCH2-),
0.74 (t, J = 7.0 Hz, 8H, -CH2CH2-), 0.13 (s, 36H, -CH3). 19F NMR
Hyper Rayleigh Light Scattering (HRS) Measurements.
Femtosecond HRS experiments were performed at 1300 nm.90,91 The
chromophores were dissolved in acetonitrile and passed through 0.2 μm
filters. Disperse red 1 (DR1, β800 = 54ꢀ10-30 esu in CHCl3)91 was
utilized as a reference chromophore. For these external references in
different solvents, standard local field correction factors were applied
(CD3CN):
δ -71.67 (d, J = 752 Hz). UV-vis (CH3CN):
λmax [nm] (ε [x 10-5 M-1cm-1]) 311 (0.58), 445 (1.18), 565 (0.14),
636 (0.35). MS (MALDI-TOF) m/z: 1513 (calcd for C88H88N10O4RuZn
(M - 2PF6)þ 1514) and 1659 (calcd for C88H88N10O4F6PRuZn
(M - PF6)þ 1659).
[((nD þ 2)/3)3, where n is the refractive index of the solvent at the
2
sodium D line]. Note that these experiments were performed at low
chromophore concentrations where the linearity of the HRS signal as a
function of chromophore concentration confirmed that no significant
self-absorption of the SHG signal occurred in these experiments.
Depolarization ratios have also been obtained, to probe the symmetry
of the nonlinear scatterer.92,93
Ru[40-(Zn-porphyrin)ethynyl-tpy]2(PF6)2 (3), Ru[4-(Zn-
porphyrin)ethynyl-tpy][40-(Zn-porphyrin)ethynyl-tpy](PF6)2
(4),Ru[4-(Zn-porphyrin)ethynyl-tpy]2(PF6)2(5), Ru(tpy)[4,400-
bis(Zn-porphyrin)ethynyl-tpy](PF6)2 (6), Ru[40-(Zn-porphyr-
in)ethynyl-tpy][4,400-bis(Zn-porphyrin)ethynyl-tpy](PF6)2 (7),
Ru[4-(Zn-porphyrin)ethynyl-tpy][4,400-bis(Zn-porphyr-
in)ethynyl-tpy](PF6)2 (8), and Ru[4,400-bis(Zn-porphyrin)
ethynyl-tpy]2(PF6)2 (9). These compounds were synthesized using
synthetic methods similar to that described for compound 2. Detailed
synthetic procedures and corresponding characterization data for chro-
mophores 3-9 can be found in the Supporting Information.
The depolarization ratio F is determined as the intensity ratio F = I /
I^ = ÆβZZZ 2æ/ÆβYZZ2æ between the HRS signal intensity (I) for parallel
(I ) and perpendicular (I^) polarization between incoming fundamental
(vertically polarized along the macroscopic laboratory Z axis) and
detected harmonic signal (polarized either along the same Z axis for
parallel and along the Y axis for perpendicular polarization). In order to
determine accurately this ratio, the analyzing polarizer in front of the
detector is rotated and the total HRS signal intensity is recorded as a
function of angle between the two polarization states. From a fitting to
this periodic pattern, an accurate depolarization ratio can be determined.
Ru(tpy)[4-(Zn-porphyrin)ethynyl-tpy](PF6)2 (2). Zinc(II)
5-ethynyl-10,20-bis[20,60-bis(3,3-dimethyl-1-butyloxy)phenyl]por-
phyrinate (19.4 mg, 0.0204 mmol) and 10 (Scheme 1, 20.2 mg,
0.0216 mmol) were placed into a 25 mL Schlenk tube with a stir bar.
Trisdibenzylideneacetone dipalladium(0) (2.9 mg, 3.2 μmol) and
triphenylarsine (3.6 mg, 0.012 mmol) were added, followed by dry
THF (2.5 mL), CH3CN (5.0 mL) and iPr2NH (0.8 mL); this mixture
was completely degassed by repeated freeze-pump-thaw cycles.
The Schlenk tube was refilled with Ar gas, and the reaction mixture
was stirred at 70 ꢀC for 24 h. The reaction mixture was cooled to rt and the
solvent evaporated. The residue was purified by column chromatography
onsilicagelelutedwithamixedsolventsystem[CH3CN:H2O:KNO3 (aq) =
90:9:1]. The second green-brown fraction was collected and evaporated.
The residual dark brown solid was dissolved in CH3CN, and excess
ammonium hexafluorophosphate and H2O were added. Dark brown
’ RESULTS AND DISCUSSION
Synthesis and Design. Figure 1 highlights a series of mono-,
bis-, tris-, and tetrakis(porphinato)zinc(II) (PZn)-elaborated
ruthenium(II) bis(terpyridine) (Ru) complexes inwhich anethyne
unit connects the macrocycle meso carbon atom to terpyridyl
(tpy) 4-, 40- and 400-positions. These species were synthesized in
high yield from appropriately ethynylated porphyrinic and
halogenated ruthenium bis(tpy) precursor molecules via metal-
catalyzed cross-coupling reactions (Schemes 1 and 2).25,34,94-100
All compounds were isolated via column chromatography on
silica gel; following counterion metathesis with ammonium hexa-
fluorophosphate, the corresponding bis(hexafluorophosphate) salts
were isolated, which were used for all spectroscopic experiments.
The Figure 1 compounds show structures that range in complexity
from the dipolar D-Br-A chromophore [PZn-(40Ru)] (1) to
the highly substituted symmetrical, yet non-centrosymmetric,
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Scheme 2. Reaction Scheme Outlining the Evolution of NLO Octopoles from Dipolar Chromophoric Building Blocks
(PZn)2-(4,400Ru4,400)-(PZn)2 (9), which possesses four
5-ethynyl-(porphinato)zinc(II) substituents arranged about a
Ru(tpy)2 core.
derive from conjugation expansion21,22,101 and charge resonance
character that originates from the ethyne-bridged porphyrin
meso-carbon-to-terpyridyl-carbon linkage.25,45 Note that, in or-
der to facilitate comparison of the spectral properties of these
conjugated supermolecules with those of traditional (porphi-
nato)zinc(II) and (polypyridyl)metal(II) benchmarks, MLCT
(d-π*), Soret (B)- and Q-band (π-π*) state and transition
labels are preserved throughout this report. When these terms are
used in reference to the absorption bands of these RuPZn-based
chromophores, they denote only the dominant contributor to
the oscillator strength of the transition in question; it is recog-
nized that MLCT, ligand, Soret, and Q electronic states mix
extensively in these supermolecules.102
Electronic Absorption Spectroscopy. The electronic ab-
sorption spectra (EAS) of these species (Figure 2, Table 1)
evince strong mixing of the PZn-based oscillator strength with
ruthenium terpyridyl charge resonance bands. Note that the EAS
of supermolecular chromophores 1-9 differ markedly from
those of the monomeric 5-ethynyl-PZn and [Ru(tpy)2]2þ build-
ing blocks.25,45 It is important to highlight, however, that super-
molecular chromophores 1-9 retain spectral qualities that trace
their genesis to those of classical PZn and [Ru(tpy)2]2þ oscilla-
tors. For instance, 1-9 evince (i) a strong (ε > 100,000
-1 cm-1) absorption near ∼450 nm which features significant
Examination of the Figure 2 spectra reveals that 40-tpy attach-
ment facilitates the strongest mixing of the PZn Bx (i.e., polarized
along the long molecular axis defined by the ethynyl moiety) and
metal polypyridyl charge-resonance states, giving rise to an
enhanced singlet metal-to-ligand (1MLCT) transition evident
at ∼510 nm in the spectra of compounds 1, 3, 4, and 7, relative to
analogous chromophores (2, 5, 6, 8, and 9) that feature 4 and 400
PZn-to-tpy linkage topologies. It is important to stress, how-
M
porphyrin-derived 1π-π* Soret (B) band character; (ii) a visible
band centered at ∼510 nm which exhibits significant [Ru
(tpy)2]2þ-derived singlet metal-to-ligand charge transfer (1MLCT)
character and marked contributions from porphyrin ligand
oscillator strength; and (iii) bands localized at ∼567 and ∼
650 nm which exhibit porphyrinic 1π-π* Q-state character, with
the lower energy, x-polarized transition manifesting features due
to symmetry breaking and oscillator strength redistributions that
1
ever, that these MLCT transitions centered near 510 nm for
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chromophores 1-9 all feature significant PZn-based oscillator
rise to appreciable electronic coupling between PZn units,
regardless of whether they are located on the same or opposite
terpyridine ligand (compounds 3-9). The absorption oscillator
strengths of the porphyrin-dominated bands scale roughly with
the number of macrocycles (Table 1), though the strong mixing
of MLCT, tpy ligand, Soret, and Q electronic states makes
rigorous quantification difficult. Note that as none of the hall-
marks of strongly coupled multiporphyrin systems (e.g., dramatic
transfer of oscillator strength from the B- band to the Q-band,
and strong Q-band red-shifting),34,35,37-39,41,42,44,46,48,50,52,103-107
are evident in the EAS of compounds 3-9, it is reasonable to
view the observed spectral properties of these supermolecules as
deriving from a simple composite of multiple RuPZn oscillators.
Congruent with this, note that the oxidative and reductive
electrochemical data (Table 2) indicate that the potentiometric
responses observed for 1-9 trace their origin largely to estab-
lished [Ru(tpy)2]2þ- and PZn-localized redox processes. These
key ground-state absorptive spectroscopic features and potentio-
metric characteristics make possible the evolution of NLO
octopoles from the dipolar RuPZn chromophoric archetype
(vide infra).
strength contributions (Figure 2). Other than these modest
1
spectral differences in the MLCT band, remarkable spectral
similarity is evident in the B- and Q-derived manifolds for 1-9.
The Figure 2 spectra likewise indicate that multiple PZn
moieties linked via ethynes to a [Ru(tpy)2]2þ core do not give
Excited State Dynamics. Pump-probe transient absorption
spectroscopic studies of this entire class of compounds (chromo-
phores 1-9) demonstrate that all of these species exhibit excited-
state dynamical characteristics and transient spectral features
similar to those reported for PZn-(40Ru) (1, Figure 1).45,48 In
this regard, it is important to note that the excited-state relaxation
dynamics of PZn-(40Ru), and those established for closely
related structures,45,48 exhibit marked differences from those
delineated for their component chromophoric building blocks
(PZn and [Ru(tpy)2]2þ). Monomeric (porphinato)zinc(II)
compounds generally relax through both the singlet and triplet
manifolds, leading to deactivation on both the nanosecond
(singlet) and millisecond (triplet) time scales;42 at room tem-
perature, the initially prepared [Ru(tpy)]2þ electronically ex-
cited state rapidly crosses to the low-lying metal-to-ligand charge
transfer (triplet) surface,108-110 where fast quenching dynamics
driven by thermal population of a low-lying 3MC state results in a
3MLCT lifetime of only ∼250 ps at 298 K.111 In PZn-(40Ru),45,48
Figure 2. Comparative electronic absorption spectra of: (A)
PZn-(40Ru) (1) and PZn-(4Ru) (2); (B) PZn-(40Ru40)-PZn (3),
PZn-(4Ru40)-PZn (4), PZn-(4Ru4)-PZn (5), and (PZn)2-(4,400Ru)
(6); (C) PZn-(40Ru4,400)-(PZn)2 (7), PZn-(4Ru4,400)-(PZn)2 (8), and
(PZn)2-(4,400Ru4,400)-(PZn)2 (9) in CH3CN solvent.
transient absorption spectra obtained at early time delays (tdelay
<
400 fs) demonstrate fast excited state relaxation, and manifest the
characteristics of a highly polarized T1 excited state; the com-
bined effects of rapid intersystem crossing and strong coupling to
Table 1. Electronic Spectral Data for Ru-PZn Derivatives
abs band maxima [nm] (ε ꢀ 10-5 [M-1 cm-1])
oscillator strength
Q-band regiona,c
B-banda
1MLCTa
Q-bandsa
B-band regiona,b
totald
PZn-(40Ru) (1)
428 (1.07)
445 (1.18)
426 (1.95))
445 (2.44)
446 (2.23)
447 (2.17)
445 (2.89)
446 (2.96)
446 (3.54)
506 (0.56)
567 (0.15)
639 (0.51)
636 (0.35)
648 (1.13)
642 (0.95)
639 (0.75)
644 (0.87)
646 (1.43)
644 (1.16)
647 (1.48)
2.10
1.90
3.55
4.28
3.62
3.55
5.23
4.96
6.12
0.27
0.25
0.67
0.60
0.46
0.57
0.81
0.72
0.96
2.37
2.15
4.22
4.88
4.08
4.12
6.07
5.68
7.09
PZn-(4Ru) (2)
565 (0.14)
565 (0.32)
567 (0.32)
566 (0.27)
564 (0.30)
565 (0.42)
565 (0.39)
566 (0.49)
PZn-(40Ru40)-PZn (3)
PZn-(4Ru40)-PZn (4)
PZn-(4Ru4)-PZn (5)
(PZn)2-(4,400Ru) (6)
PZn-(40Ru4,400)-(PZn)2 (7)
PZn-(4Ru4,400)-(PZn)2 (8)
(PZn)2-(4,400Ru4,400)-(PZn)2 (9)
521 (0.89)
509 (0.65)
517 (0.80)
a Transition labels denote the dominant contributor to the absorption manifold. b Oscillator strengths calculated over the 360 f 555 nm wavelength
domain. c Oscillator strengths calculated over the 555 f 850 nm wavelength domain. d Oscillator strengths calculated over the 360 f 850 nm
wavelength domain.
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Table 2. Oxidative and Reductive Cyclic Voltammetric Data for Ru-PZn Derivativesa
E1/2 values (V)d,e,f
Ru2þ/3þ
PZnþ/2þ
PZn0/þ
(tpy)-/0
PZn-/0
(tpy)-/2-
PZn-(40Ru) (1)b
PZn-(4Ru) (2)c
1.47
1.32
1.42
1.41
1.38
1.40
1.43
1.44
1.49
1.13
0.87
-1.07
-1.03
-1.07
-1.09
-1.08
-1.37
-1.22
-1.39
-1.39
-1.26
-1.37
-1.30
-1.28
-1.19
-1.58
-1.40
1.05
0.77
PZn-(40Ru40)-PZn (3)c
PZn-(4Ru40)-PZn (4)c
PZn-(4Ru4)-PZn (5)c
(PZn)2-(4,400Ru) (6)c
PZn-(40Ru4,400)-(PZn)2 (7)c
PZn-(4Ru4,400)-(PZn)2 (8)c
(PZn)2-(4,400Ru4,400)-(PZn)2 (9)c
1.10
0.75
1.24
0.78
-1.55
-1.51
-1.57
-1.53
-1.40
1.23
0.76
1.21
0.77
1.23
0.82
1.74
1.24
0.82
1.21/1.05
0.87/0.78
a Experimental conditions: [complex] = 1 mM; electrolyte = 0.1 M TBAPF6; scan rate = 100 mV/s; working electrode = Pt disk. b Solvent = CH2Cl2.
c Solvent = CH3CN. d E1/2 values are relative to SCE. e ΔEp values: ΔEp(Ru2þ/3þ) ∼ 100 mV; ΔEp(PZnþ/2þ) ∼ 125 mV; ΔEp(PZn0/þ) ∼ 100 mV;
under these experimental conditions, ΔEp(ferrocene/ferrocenium = 100 mV). Potentiometric data reported for (tpy)-/0, PZn-/0, and (tpy)-/2-
f
redox processes correspond to Epc values as these electrode-coupled reactions evince varying degrees of chemically irreversible character.
low-lying MLCT states gives rise to excited-state lifetimes on the
order of tens of microseconds. Interrogation of the electronically
excited states of PZn-(40Ru) and related chromophores via
pump-probe transient optical methods reveals a number of
conspicuous spectral characteristics that include (i) prominent
visible region bleaching due to ground-state depletion, (ii) a
broad, weak transient absorption in the spectral region between
the high oscillator ground-state bleaches, and (iii) an expansive,
intense T1 f Tn absorption manifold that dominates the 800-
1200 nm region of the NIR.45,48
for example the chromophore 7 transient spectra of Figure 3B). A
global exponential fit to the PZn-(4Ru) transient absorption data
acquired at multiple wavelengths produces time constants of 1.7
and 18.9 ps; while the latter component corresponds to the
torsional dynamics described above, the 1.7 ps component is
associated with the simultaneous decay of the absorption feature
observed over 700-800 nm spectral window characteristic of the
initially prepared excited state, and the growth of the NIR
absorption manifold (900-1200 nm).
The transient absorption signal observed over the 700-800
nm spectral region suggests an initially prepared excited state for
2 that possesses significantly less electronic delocalization than
its corresponding relaxed state. As evinced in the steady state
absorption spectra (Figure 2), the 40-tpy-to-5-ethynyl(porphinato)
zinc linkage topology facilitates stronger mixing of PZn Bx and
metal polypyridyl charge-resonance states relative to those that
utilize corresponding 4- and 400-connectivities. Congruent with
the suggestion that linkage-topology-dependent differences in
electronic coupling may play a role in determining excited-state
dynamics, electronic structure calculations indicate that the tpy
4- and 400-positions manifest diminished electron density relative
to the 4-carbon atom in the Ru(tpy)2 LUMO.113 A qualitative
analysis based on the Table 2 potentiometric data and the Gibbs
free energy relation suggests that reaction thermodynamics may
play a role as well in limiting the extent of electronic delocaliza-
Figure 3 highlights exemplary transient absorption spectral
data for chromophores 2 and 7 obtained over a 200 fs (fs) to 3 ns
(ns) time domain. These data typify many of the time-dependent
transient absorption spectral signatures characteristic for chromo-
phores 1-9 and highlight the multiexponential nature of their
relaxation dynamics. Supermolecules 1-9 manifest an ∼10-20
ps time constant component that leads to an increase in the
intensity of the T1 f Tn NIR transient absorption band; these
dynamics are associated with torsional motion about the ethynyl
3
bridge.40,45,48,112 The relaxed MLCT states of these species
feature conformeric distributions having a reduced mean tor-
sional angle between the porphyrin and terpyridine least-squares
planes; moreover, the distribution of these torsional angles in the
relaxed excited states of 1-9 is more homogeneous than that for
their respective initially prepared excited states. Note that these
relaxed excited triplet states thus display augmented T1 f Tn
manifold transition oscillator strengths with respect to that
tion evidenced in the PZn-(4Ru) excited state probed at tdelay
=
200 fs, relative to that manifest for chromophores 1 and 3-9 at
an identical time delay.114 Whatever the origin of these apparent
differences in 2's initially prepared excited state, picosecond time
scale spectral evolution gives rise to a high oscillator strength
T1 f Tn NIR transient absorption manifold qualitatively indis-
tinguishable from that observed for the initially prepared electro-
nically excited states of related supermolecules 1 and 3-9. Note
that, in this family of chromophores, the lifetimes of the relaxed
3MLCT states are on the order tens of microseconds.
Table 3 tabulates excited-state lifetimes and the T1 f Tn
absorption extinction coefficients for 1-9; note that these
estimated T1 f Tn extinction coefficients are calculated from
spectra acquired at 1.5 ns following excitation, and were deter-
mined by comparing the excited state absorption to the Qx-band-
derived ground state bleach (having a known extinction
coefficient), following a method detailed previously.45 As the
3
evident in the respective initially prepared MLCT states of
these structures.45,48
The excited-state dynamics elucidated for PZn-(4Ru), how-
ever, show a number of differences relative to those determined
for chromophores 1 and 3-9; some of these dissimilarities can
be recognized in the Figure 3 data. While the excited state
dynamical data acquired for PZn-(4Ru) (2, Figure 3A) bear some
resemblance with those reported for PZn-(40Ru),45,48 note that
PZn-(4Ru) does not manifest a prominent 1MLCT band in its
ground-state absorption spectrum (Figure 2). As a result, more
complex features are evident throughout the 700-800 nm
spectral domain over the initial few picoseconds following
excitation of this complex (Figure 3A), contrasting that reported
earlier for related chromophores,45,48 and that observed in the
analogous dynamical data acquired for supermolecules 3-9 (see
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Figure 3. Magic angle pump-probe transient absorption spectra of compounds 2 (A) and 7 (B), recorded at several time delays illustrating the general
transient features of chromophores 1-9. Note that the NIR absorption manifold is strongly established at a time delay of 250 fs in compound 7 (as well
as in chromophores 1, 3-6, and 8 and 9), but not in compound 2. The laser line from the pump pulse at 605 nm is observed in both spectra.
Experimental conditions: solvent = acetonitrile, ambient temperature, λexc = 605 nm.
Table 3. Excited-Triplet State Data for 1-9a
further support the notion that the individual RuPZn fragments
of chromophores 3-9 exhibit, at best, only modest excited-state
λmax
extinction coeff,
triplet
electronic interactions that derive from factors other than the
dipolar interactions of the component RuPZn oscillators. Be-
cause the nature of the chromophore electronically excited states
present at early and long delay times are reminiscent of that
previously established for PZn-(40Ru) (1),45,48 the electronically
excited states of supermolecules 3-9 can thus be approximated
as a collection of interacting RuPZn transition dipoles that to first
order behave as independent oscillators. While a detailed de-
scription would certainly need to consider excitation delocaliza-
tion via energy transfer and the impact of conformational
dynamics upon these excited states, the electronic structural data
for 1-9 that emerge from these pump-probe transient absorp-
tion experiments indicate fundamentally sound design principles
underpinning these structures: because the nature of the instan-
taneously prepared excited states determine the virtual states that
govern the NLO response, the approximate independent char-
acter of the component RuPZn oscillators for 3-9 suggest
nonlinear optical responses for these supermolecules consistent
with those defined for multipolar chromophores (vide infra).
Nonlinear Optical Properties. Chromophores 1-9 possess
large CT transition oscillator strengths (Pges) and substantial
dipole moment differences between their respective ground and
excited electronic states (Δμges). How these parameters qualitatively
impact the magnitude or the dynamic hyperpolarizability can be
seen in classic two-level formulation of β (eq 1), where β0n refers to
an electronic-state specific two-level contribution to the sum-over-
states expression for β, Eop refers to a specific electronic transition
energy and Einc is the incident irradiation energy.11,12
(T1fTn)b λmax(T1fTn) lifetimes
(nm)
[M-1 cm-1 b,c
]
(μs)
PZn-(40Ru) (1)
884
908
900
909
926
917
938
930
957
29900
18200
44400
48200
38600
26200
54000
48200
67100
44d
73
33
44
26
82
33
61
14
PZn-(4Ru) (2)
PZn-(40Ru40)-PZn (3)
PZn-(4Ru40)-PZn (4)
PZn-(4Ru4)-PZn (5)
(PZn)2-(4,400Ru) (6)
PZn-(40Ru4,400)-(PZn)2 (7)
PZn-(4Ru4,400)-(PZn)2 (8)
(PZn)2-(4,400Ru4,400)-(PZn)2 (9)
a All spectral data were acquired in acetonitrile solvent. b λmax(T1fTn)
corresponds to the T1 f Tn manifold absorption having the largest
extinction coefficient at tdelay = 1.5 ns. c Excited state extinction coeffi-
cients were estimated using a method described previously.45 d Electro-
nically excited triplet data for 1 reported previously.45
NIR absorption manifold clearly consists of many transitions, the
λmax value reported corresponds to the extinction coefficient of
the absorption maximum located in the higher-energy portion of
the T1 f Tn manifold. Note that λmax(T1fTn) increases with
increasing numbers of 5-ethynyl-(porphinato)zinc(II) substitu-
ents connected to the Ru(tpy)2 core (e.g., 1, λmax(T1fTn) =
884 nm; 4, λmax(T1fTn) = 909 nm; 7, λmax(T1fTn) = 930 nm;
9, λmax(T1fTn) = 957 nm). While this may signal a modest
degree of excited-state electronic delocalization involving multi-
ple RuPZn units, it is important to underscore that the overall
T1 f Tn absorption bandwidth and normalized intensity are
virtually identical for chromophores 1-9; along this line, note
that the extinction coefficient of λmax(T1fTn) normalized by the
number of (porphinato)zinc(II) units is roughly constant
(∼20,000 M-1 cm-1) throughout these supermolecules.
2
2
6ðPgeÞn ðΔμgeÞnðEopÞn
βn0
¼
ð1Þ
2
2
2
2
½ðEopÞn - ð2EincÞ ꢁ½ðEopÞn - Einc
ꢁ
These Ru-PZn-based species manifest complex nonlinear
responses because of the coupling among the electronic transi-
tions of the chromophoric building blocks (i.e., three-level
Congruent with insights derived from electronic spectral and
potentiometric data, these transient absorption spectral studies
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Table 4. Dynamic Hyperpolarizabilities [β1300, (ꢀ 10-30
esu)] and Depolarization Ratios (G) Determined by Hyper-
Rayleigh Experimentsa
chromophore symmetry from purely dipolar to octopolar. Be-
cause excited-state dynamical, ground-state electronic absorp-
tion, and potentiometric data all point to the approximate
independent character of the building block RuPZn oscillators
of supermolecular chromophores 3-9, for the purpose of
analyzing the depolarization ratio data, it is useful to consider
these species as assemblies of RuPZn dipoles. Insight into the
nature of excited-state electron density distributions for 3-9 is
thus gleaned through vector addition of these RuPZn dipoles.
The geometric nature of the PZn substitution pattern about the
Ru(tpy)2 core of these chromophores determines the extent to
which these dipoles add or cancel, which can be estimated by
simple symmetry arguments. It is important to note that some of
the supermolecular chromophore arrays do not, in fact, possess
high molecular symmetry, due to the fact that the tpy 4- and 40-
(400-) positions are not electronically or chemically equivalent. In
this electronic structural limit relevant to 3-9 where these strong
RuPZn oscillators exhibit significant independent character,
however, depolarization ratio measurements reveal the effective
chromophore optical symmetry. This effective optical symmetry
reflects the spatial arrangement of the RuPZn oscillators about
the Ru(tpy)2 core, that is, an approximate molecular symmetry
that would exist if all points of connectivity to this unit were
equivalent.
Depolarization ratio data compiled in Table 4 shows evolution
of F values from dipolar (F∼4, chromophores 1-3) to octo-
polar (F = 1.5, chromophore 9). Given the lower approximate
molecular symmetries of supermolecules 4-8, intermediate
values of F are manifest. Note that these F values correspond
to depolarization ratios that are neither consistent with dipolar
nor octopolar optical symmetry, but reflect the spatial arrange-
ment of the component RuPZn oscillators of these species and
the corresponding vector sums of these dipoles. Congruent with
this, the small measured difference in the depolarization ratios for
supermolecules 4 and 5, which possess a similar relative spatial
arrangement of their respective component RuPZn oscillators,
yet different 5-ethynyl(porphinato)zinc-to-tpy linkage topolo-
gies, illustrates the utility of the notion of approximate molecular
symmetry in rationalizing the relative magnitudes of F for com-
pounds 4-9. Given the electronic structural characteristics of
1-9 discussed above and the substantial RuPZn oscillator
strength, the absolute nature of PZn-tpy connectivity (4, 40, or
400) has little impact upon the determination of chromophore
effective optical symmetry.
βijk
5100d
βHRS
Fc
b
PZn-(40Ru) (1)
2100
3.8 ( 0.1
PZn-(4Ru) (2)
3100 ( 50
1920 ( 70
1280 ( 20 3.8 ( 0.1
PZn-(40Ru40)-PZn (3)
PZn-(4Ru40)-PZn (4)
PZn-(4Ru4)-PZn (5)
(PZn)2-(4,400Ru) (6)
PZn-(40Ru4,400)-(PZn)2 (7)
PZn-(4Ru4,400)-(PZn)2 (8)
800 ( 30
880 ( 30
960 ( 30
3.9 ( 0.1
2.2 ( 0.1
2.1 ( 0.1
1080 ( 40 3.4 ( 0.1
750
2.1 ( 0.2
2.1 ( 0.2
894
(PZn)2-(4,400Ru4,400)-(PZn)2 (9) 1380 ( 260118 1040 ( 200 1.5 ( 0.1
a λinc = 1300 nm. b βijk = βzzz for upper 3 entries with dipolar molecular
symmetry (F ∼ 4) and βijk = βxyz for last entry (9) having effective
octopolar symmetry (F = 1.5); βHRS2 = [ÆβZZZ 2æ þ ÆβYZZ2æ] = 6/35 βzzz
2
for dipolar compounds, and βHRS2 = [ÆβZZZ 2æ þ ÆβYZZ2æ] = 20/35 βxyz
2
for octopolar (Td, D2, or D2d) symmetries;118 c F = ÆβZZZ 2æ/ÆβYZZ2æ;
d Value taken from ref 25.
contributions arising from mixing of transitions that trace their
genesis to (porphinato)zinc B- and Q-states and metal polypyr-
idyl ligand-to-metal CT states).18,25,48 Because multiple absorp-
tion manifolds contribute to the nonlinear response and the
transitions that dominate these manifolds possess distinct transi-
tion dipole directions, chromophores 1-9 manifest complex
frequency dispersions of the hyperpolarizability. As excited state-
to-excited state transition dipoles play such a prominent role in
determining the NLO responses of Ru-PZn-based chromophores,
two-level analyses such as that described in eq 1 provide only a
qualitative understanding of the frequency dispersion of βλ. This
issue has recently been discussed in detail, and the frequency
dispersion of the hyperpolarizability for chromophores within this
family has been analyzed using a sum-over-states expression with
three-level contributions that employed measured spectroscopic
parameters and generalized Thomas-Kuhn sum rules.18
Table 4 lists dynamic hyperpolarizability (βλ) values and
depolarization ratios (F) determined from hyper-Rayleigh light
scattering (HRS) measurements carried out at an incident irra-
diation wavelength (λinc) of 1300 nm in CH3CN solvent. Briefly,
the depolarization ratio provides insight in to the nature of
contributions to the NLO response.31 For a purely octopolar
compound, the depolarization ratio is predicted to be 1.5, while
for a molecule with a single contributing major βzzz hyperpolar-
izability tensor component, this value is 5.115,116 This latter F
value is attained only in the limit of an HRS experiment carried
out using an infinitely small numerical aperture for a hypothetical
chromophore that possesses no finite off-diagonal tensor con-
tributions to βλ for the experimental molecular symmetry. In
practice, dipolar compounds exhibit a reduced depolarization
value relative to this limiting theoretical value (e.g., for disperse
red 1, a dipolar compound, F = 3.4); HRS depolarization ratios
for classic octopolar chromophores such as 1,3,5-trihydroxy-
2,4,6-trinitrobenzene and crystal violet have been determined
to be 1.5.63-77,117
Consistent with the established NLO properties of RuPZn25
and the dependence of chromophore effective optical symmetry
upon structure, supermolecules 1-9 exhibit impressive second-
order nonlinear optical responses at 1300 nm. In Table 4, note
that βHRS2 = [ÆβZZZ 2æ þ ÆβYZZ2æ] reflects the total HRS intensity
(i.e., the total magnitude of the HRS signal with polarization
parallel and perpendicular to the Z-polarized fundamental laser
beam), providing an average hyperpolarizability value that has
not been analyzed toward specific hyperpolarizability tensor
components for a specific molecular symmetry.115,116 When
considering these quantities, it is important to note the defini-
tions of the x, y, and z axes, as the reference frame convention in
the NLO community differs from that used to discuss the
spectroscopic properties of these structures. βzzz denotes a first
hyperpolarizability tensor component along the dipolar molec-
ular z axis for charge transfer; in contrast, βxxx, for example, refers
to one of the four nonzero hyperpolarizability tensor compo-
nents for a D3h-symmetric octopolar molecule in the xy plane, the
While it is well established that the depolarization ratio
provides an experimental measure of chromophore optical
symmetry, we note that this parameter provides an important
experimental validation that appropriate coupling of multiple
charge-transfer oscillators can be utilized to evolve the effective
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Journal of the American Chemical Society
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z axis again being the unique molecular axis perpendicular to
this plane.
octopolar symmetries (6/35 vs 8/21 or 20/35)118 results in a
seemingly small octopolar response (βHRS value) for compound
9 compared to that determined for dipolar chromophores 1 and
2. Calculating the tensor elements for an average βHRS = 1040 ꢀ
10-30 esu for structures of different symmetries, it is easy to
appreciate the extraordinary response of octopolar chromophore
9. For a dipolar compound, a βzzz value of 2500 ꢀ 10-30 esu would
be needed to provide a corresponding βHRS = 1040 ꢀ 10-30 esu;
For the simple dipolar compounds, note that the β1300 value
determined for PZn-(40Ru) (1, βHRS = 2100 ꢀ 10-30 esu)
exceeds that of PZn-(4Ru) (2, βHRS = 1280 ꢀ 10-30 esu); this
result is consistent with the fact that 40-tpy attachment facilitates
the strongest mixing of the PZn Bx and metal polypyridyl charge-
resonance states. In supermolecules 3-9, which feature multiple
RuPZn oscillators, the magnitude of βHRS does not reflect such a
dramatic difference between 40 or 4 (400) points of connectivity
for the 5-ethynyl(porphinato)zinc units; this is likely due in part
to symmetry dependent incomplete cancellation of dipolar
contributions to the HRS response. In the more complex 3-9
superstructures, chromophore effective optical symmetry plays a
critical role in determining the NLO response as underscored by
the measured F values, and the relative impact of topological
connectivity becomes less obvious.
similarly, an octopolar, but D3h, molecule would require βxxx
=
1700 ꢀ 10-30 esu to give rise to a βHRS value exceeding 1000 ꢀ
10-30 esu.115 This analysis provides a yardstick by which to
compare molecules of different optical symmetries benchmarked
to a specific second order response. Note that octopolar
(PZn)2-(4,400Ru4,400)-(PZn)2 gives rise to a βHRS value approxi-
mately equivalent to that for dipolar PZn-(4Ru) (βzzz = 3100 ꢀ
10-30 esu), showing that it is possible to retain large NLO
responses as the nature of the optical symmetry is modulated.
These considerations are important in the development of
nonlinear optical materials based upon hyperpolarizable chromo-
phores. The omnidirectional averaged response for molecules
with high symmetry (yet non-centrosymmetry) results in a large
nonlinear effect, as is observed for the 6 nonzero βijk, i ¼ j ¼ k
tensor components that contribute to the total average HRS
signal observed for 9 (βHRS = 1040ꢀ10-30 esu). A large off-
diagonal βxyz hyperpolarizability tensor component translates
into an omnidirectional NLO response in a bulk materials and
thin films, resulting in polarization insensitive devices that can be
utilized as frequency-converters or electro-optic modulators.
It is important to underscore that the magnitude of the off-
diagonal hyperpolarizability tensor component βxyz for octopolar
chromophore 9 (1380 ꢀ 10-30 esu)118 was determined for a
technologically important incident irradiation wavelength of
1300 nm. Pump-probe transient optical data evince that 1-9
are nonemissive; furthermore, the molecular hyperpolarizabilites
determined for these species were acquired under experimen-
tal conditions that utilized classic fluorescence demodulation
methods.91,119 It is thus certain that the large βHRS values
reported herein are uncontaminated by multiphoton fluores-
cence contributions to the HRS signal. The largest octopolar
β1300 value reported to date corresponds to structural analog of
With respect to the magnitude of the measured hyperpolariz-
ability, note that the coupling of multiple RuPZn oscillators in the
manner highlighted by the 3-9 structures not only evolves
chromophore symmetry from dipolar to octopolar; Table 4
highlights that this mode of chromophore design gives rise to
octopolar supermolecules that possess by far the largest off-
diagonal octopolar first hyperpolarizability tensor components
ever reported. (PZn)2-(4,400Ru4,400)-(PZn)2 (9), which features
a 5-ethynyl(porphinato)zinc substitution pattern compatible
with D2d symmetry, shows complete cancellation of the vectorial
dipolar contributions to the NLO response associated with its 4
component RuPZn oscillator fragments, and manifests a F value
of 1.5, as expected for an ideal octopole. Note that chromophores
3-8, which exhibit substantial variation in the magnitude of the
measured depolarization ratio, display averaged hyperpolariz-
abilities (βHRS values) that remain extremely large, independent
of structure. These values range from 750 to 1080 ꢀ 10-30 esu,
similar in magnitude to that determined for the octopolar
supermolecule 9 (βHRS = 1040 ꢀ 10-30 esu).
Because variations in molecular structure determine chromo-
phore effective optical symmetry and lead to different nonzero
hyperpolarizability tensor components contributing to the over-
all HRS response, the simplest comparative metric is the
averaged hyperpolarizability, as discussed above. However, in
order to contrast the HRS responses of these systems with key
literature chromophoric benchmarks, the various tensor ele-
ments that contribute to the HRS response must be evaluated;
such tensor-element-specific HRS responses for molecules of
different symmetries can be compared directly. This approach is
the [Ru(bpy)3]2þ 120,121
As HRS measurements for this chromo-
.
phore at this irradiation wavelength are known to be heavily
contaminated by multiphoton fluorescence, in order to provide a
more accurate benchmark with which to compare to 9, we note
that βxxx for this particular [Ru(bpy)3]2þ derivative determined
at λinc = 1900 nm corresponds to 320 ꢀ 10-30 esu.71 While
multifluorescence contamination is always an issue when deter-
mining the hyperpolarizabilities of emissive chromophores, these
concerns are somewhat mitigated at long irradiation wavelength.
Projecting this β1900 value back to 1300 nm using the conven-
tional dispersion relation gives βxxx = 500 ꢀ 10-30 esu, and a
corresponding averaged nonlinear response βHRS = 310 ꢀ
10-30 esu. The supermolecular chromophore (PZn)2-
(4,400Ru4,400)-(PZn)2 thus possesses a βHRS value at 1300 nm
that is more than a factor of 3 larger than that determined for any
chromophore having octopolar symmetry examined to date.
exemplified for (PZn)2-(4,400Ru4,400)-(PZn)2 (9), where βHRS
=
1040 ꢀ 10-30 esu and βxyz = 1380 ꢀ 10-30 esu for the 6
contributing nonzero off-diagonal hyperpolarizability tensor
components.118 Supermolecule 9 exhibits the largest octopolar
response reported, irrespective of symmetry or dispersion
(frequency dependence of the response) considerations. Analyz-
ing compound 9 NLO data within the context of effective D2d
symmetry, there are 6 nonzero tensor hyperpolarizability tensor
elements βijk, i ¼ j ¼ k that contribute to the total signal. With
respect to comparisons of tensor-element-specific HRS re-
sponses, note that the relations between the observables are
given by the following equations: βHRS2 = [ÆβZZZ2æ þ ÆβYZZ2æ] =
6/35 βzzz2 for dipolar compounds, βHRS2 = [ÆβZZZ2æ þ ÆβYZZ2æ]
= 8/21 βxxx2 for octopolar D3h symmetry, and βHRS2 = [ÆβZZZ2æ
’ CONCLUSION
A new series of mono-, bis-, tris-, and tetrakis(porphinato)zinc-
(II) (PZn)-elaborated ruthenium(II) bis(terpyridine) (Ru) com-
plexes has been synthesized in which an ethyne unit connects
2
þ ÆβYZZ2æ] = 20/35 βxyz for octopolar Td, D2, or D2d
symmetries.118 This difference in prefactors for dipolar and
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Journal of the American Chemical Society
ARTICLE
the macrocycle meso carbon atom to terpyridyl (tpy) 4-, 40- and
400-positions. These RuPZn-based species were synthesized in
high yield from appropriately ethynylated porphyrinic and
halogenated ruthenium bis(tpy) precursor molecules via metal-
catalyzed cross-coupling reactions. While these supermolecular
chromophores evince strong mixing of the PZn-based oscillator
strength with ruthenium terpyridyl charge resonance bands,
potentiometric and linear absorption spectroscopic data indicate
that for structures in which multiple PZn moieties are linked via
ethynes to a [Ru(tpy)2]2þ core, little electronic coupling is mani-
fest between PZn units, regardless of whether they are located on
the same or opposite tpy ligand. Congruent with these experi-
ments, pump-probe transient absorption studies suggest that
the individual RuPZn fragments of these structures exhibit, at
best, only modest excited-state electronic interactions that derive
from factors other than the dipole-dipole interactions of these
strong oscillators. Because the nature of the instantaneously
prepared excited states determines the virtual states that govern
the nonlinear optical (NLO) response, the approximate inde-
pendent character of the component RuPZn oscillators suggests
that the design strategy described herein is ideal for designing
NLO multipoles based on the potent dipolar hyperpolarizable
RuPZn archetype.
related charge-transfer oscillators can be utilized to evolve the
effective chromophore symmetry in RuPZn-based supermole-
cules from purely dipolar to octopolar, and generate octopolar
NLO chromophores having impressive βHRS values at 1300 nm
(βHRS > 1000 ꢀ 10-30 esu). Because NLO octopoles possess
omnidirectional NLO responses while circumventing the elec-
trostatic interactions that drive bulk-phase centrosymmetry for
NLO dipoles at high chromophore concentrations, the advent of
octopolar NLO chromophores having vastly superior βHRS
values at technologically important energies will motivate new
experimental approaches to achieve acentric order in both bulk-
phase and thin film structures.
’ ASSOCIATED CONTENT
S
Supporting Information. Syntheses and characterization
b
data for precursor compounds, along with benchmark optical
spectra for the ethyne-elaborated bis(terpyridyl)metal and
(porphinato)zinc(II) building blocks. Complete author lists are
provided for refs 58 and 59 This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Dynamic hyperpolarizability (βλ) values and depolarization
ratios (F) were determined from hyper-Rayleigh light scattering
(HRS) measurements carried out at an incident irradiation
wavelength (λinc) of 1300 nm. The depolarization ratio data pro-
vide an experimental measure of chromophore optical symmetry,
and underscore that appropriate coupling of multiple charge-
transfer oscillators evolves the effective chromophore symmetry
from purely dipolar to octopolar. These effective chromophore
optical symmetries were shown to depend solely upon the geo-
metric nature of the PZn substitution pattern about the Ru(tpy)2
core, highlighting that the NLO response derives from the
collective properties of the component dipolar RuPZn super-
molecular oscillators.122 These chromophore design insights give
rise to octopolar supermolecules that possess by far the largest
off-diagonal octopolar first hyperpolarizability tensor compo-
nents ever reported. The averaged hyperpolarizabilities (βHRS
values) for these species are extremely large, regardless of structure,
ranging from 750 to 1080 ꢀ 10-30 esu. (PZn)2-(4,400Ru4,400)-
(PZn)2, which possesses four 5-ethynyl-(porphinato)zinc(II)
substituents arranged about a Ru(tpy)2 core and rigorous
octopolar symmetry, possesses the largest octopolar response
ever reported, irrespective of symmetry or dispersion (frequency
dependence of the response) considerations; importantly, its
βHRS value at 1300 nm (1040 ꢀ 10-30 esu) is more than a factor
of 3 larger than that determined for any chromophore having
octopolar symmetry examined to date. Because octopolar (PZn)2-
(4,400Ru4,400)-(PZn)2 possesses six large off-diagonal βxyz hyper-
polarizability tensor components, large omnidirectional NLO
responses can in principle be realized in both bulk materials and
thin films, resulting in polarization insensitive devices that can be
utilized as frequency-converters or electro-optic modulators.
This work demonstrates that ruthenium(II) [5-(40-ethynyl-
(2,20;60,200-terpyridinyl))-10,20-bis(20,60-bis(3,3-dimethyl-1-
butyloxy)phenyl)porphinato]zinc(II)-(2,20;60,200-terpyridine)2þ
bis-hexafluorophosphate (RuPZn), a robust, benchmark dipolar
chromophore that possesses an extraordinary βzzz value at a tele-
communication relevant wavelength (β1300 = 5100 ꢀ 10-30 esu),
can not only serve as a template for the development of an
octopolar NLO chromophore; appropriate coupling of multiple,
Corresponding Author
koen.clays@fys.kuleuven.be; michael.therien@duke.edu
Present Addresses
‡Department of Chemistry, Graduate School of Pure and
Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba 305-8571, Japan.
§Department of Biochemistry and Biophysics, University of
Pennsylvania, School of Medicine, 316 Anatomy Chemistry
Building, Philadelphia, PA 19104-6059.
’ ACKNOWLEDGMENT
This work was supported by a grant from the Department of
Energy Biomolecular Materials program DE-FG02-04ER46156
(T.I., L.E.S, A.N., M.J.T.); K.S., S.-T.H., and K.C. are grateful to
the Flemish Fund for Scientific Research (G.0312.08) and the
University of Leuven (GOA/2006/03). The authors thank the
MRSEC (DMR-00-79909) Program of the National Science
Foundation, RLBL (NIH P41RR001348), and UNC EFRC -
Solar Fuels and Next Generation Photovoltaics, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
Number DE-SC0001011, for infrastructural support. T.I. thanks
the Japan Society for the Promotion of Science (JSPS) for a
postdoctoral fellowship. M.J.T. is grateful to the Francqui Foun-
dation (Belgium) and VLAC (Vlaams Academisch Centrum),
Centre for Advanced Studies of the Royal Flemish Academy
of Belgium for Science and the Arts, for research fellowships.
The authors are indebted to Dr. Timothy V. Duncan for many
helpful discussions.
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value for the xyz component is identical for all 6 permutations. Note that
this does not mean that the xzy, yxz, yzx, zxy and zyx components are
zero and do not contribute; ref116 clearly indicates the necessity to sum
over i, j, and k for i ¼ j ¼ k to determine the βijk tensor components. This
convention then leads to the expression βHRS2 = [ÆβZZZ2æ þ ÆβYZZ2æ] =
2
120/35 βxyz for octopolar Td, D2, or D2d symmetries. Note that this
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convention is also widely accepted, and results in an interpretation of the
experimentally obtained βHRS2 values in terms of 6 nonzero βijk tensor
components that are a factor of 61/2 smaller in magnitude than that
arrived at via the convention used in the compilation of the Table 4 data.
As a case in point, a previously reported value of 440 ꢀ 10-30 esu for the
βxyz value of the compound PZnE2PZn, derived according to the former
convention,31 transforms into 1080 ꢀ 10-30 esu when the βxyz
computational method of this paper is followed. It is important for the
reader to appreciate that, whichever of these conventions are used for
computing and reporting βxyz values, the overall response for an
octopolar NLO chromophore remains directly related to βHRS2, the
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