Excitation of highly conjugated (porphinato)palladium(II) and (porphinato)platinum(II) oligomers produces long-lived, triplet states at unit quantum yield that absorb strongly over broad spectral domains of the NIR

Article abstract of DOI:10.1021/jp102901u

Transient dynamical studies of bis[(5,5a?2-10,20-bis(2,6-bis(3,3- dimethylbutoxy)phenyl)porphinato)palladium(II)] ethyne (PPd₂), 5,15-bis{ [(5a?2-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato) palladium(II)]ethynyl} ( 10,20bis(2,6-bis(

Full text of DOI:10.1021/jp102901u

14696
J. Phys. Chem. B 2010, 114, 14696–14702
Excitation of Highly Conjugated (Porphinato)palladium(II) and (Porphinato)platinum(II)
Oligomers Produces Long-Lived, Triplet States at Unit Quantum Yield That Absorb
Strongly over Broad Spectral Domains of the NIR†
Timothy V. Duncan,‡,§ Paul R. Frail,‡ Ivan R. Miloradovic,‡ and Michael J. Therien*,|
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323, and
Department of Chemistry, French Family Science Center, Duke UniVersity, Durham,
North Carolina 27708-0354
ReceiVed: March 31, 2010; ReVised Manuscript ReceiVed: May 7, 2010
Transient dynamical studies of bis[(5,5′-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II)]
ethyne (PPd2), 5,15-bis{[(5′-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II)]ethynyl}(10,20-
bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)palladium(II) (PPd3), bis[(5,5′-10,20-bis(2,6-bis(3,3-dimethyl-
butoxy)phenyl)porphinato)platinum(II)]ethyne (PPt2), and 5,15-bis{[(5′-10,20-bis(2,6-bis(3,3-dimethylbutoxy)phe-
nyl)porphinato)platinum(II)]ethynyl}(10,20-bis(2,6-bis(3,3-dimethylbutoxy)phenyl)porphinato)platinum(II) (PPt3)
show that the electronically excited triplet states of these highly conjugated supermolecular chromophores can be
produced at unit quantum yield via fast S1 f T1 intersystem crossing dynamics (τisc: 5.2-49.4 ps). These species
manifest high oscillator strength T1 f Tn transitions over broad NIR spectral windows. The facts that (i) the
electronically excited triplet lifetimes of these PPdn and PPtn chromophores are long, ranging from 5 to 50 µs,
and (ii) the ground and electronically excited absorptive manifolds of these multipigment ensembles can be
extensively modulated over broad spectral domains indicate that these structures define a new precedent for
conjugated materials featuring low-lying π-π* electronically excited states for NIR optical limiting and related
long-wavelength nonlinear optical (NLO) applications.
1. Introduction
the NIR also manifest diminished excited-state lifetimes due to
increased rates of nonradiative relaxation from their respective
low-lying excited electronic states.49-51
Organic materials that can attenuate the intensity of sudden
strong pulses of light are an important means to protect sensitive
optical devices. There is keen interest in the development of
optical power limiting (OPL) materials that function over the
750-1600 nm spectral regime of the near-infrared (NIR), as
this wavelength domain encompasses the primary emission lines
of a variety of important lasing media (Ti:sapphire, Nd:YAG,
Nd:YLF, GaAs, GaAlAs, InGaAs, krypton, HeNe, argon, Cr:
Forsterite, and Er:glass), as well as the important telecom-
munication wavelengths. While there are many physical mech-
anisms that can give rise to OPL, one of the most important is
reverse saturable absorption (RSA).1,2 Materials exhibiting RSA
possess strong excited-state absorptive cross sections and weak
ground-state absorptive intensity over the spectral window of
interest for optical limiting. The additional requirements of a
long excited-state lifetime, high resistance to degradative
photochemistry, and large magnitude nonlinear optical (NLO)
response have made identification of OPL candidates based on
RSA for the NIR difficult. In contrast, a wide variety of optical
limiting materials based upon phthalocyanine,3-11 porphyrin,12-14
fullerene,15-20 carbon nanotube,9,21-25 nanoparticle,26-34 and
other35-40 motifs have been identified for the visible spectral
region. The limited number of potential NIR OPL materials39,41-48
elucidated to date derives largely from the fact that most
compounds that possess excited states that absorb strongly in
The simultaneous requirements of a long (>µs) excited-state
lifetime, substantial photostability, and broad, high oscillator
strength excited-state absorptivity in the NIR, have led to the
development of supermolecular chromophores that manifest
coupled-oscillator photophysics and extensively delocalized low
energy excited triplet (T1) states characterized by substantial
charge-separated (CS) character.52,53 While these highly conju-
gated charge-transfer chromophores define new benchmarks for
NIR optical limiting,52,53 it remains an open question whether
conjugated materials that feature low-lying electronically excited
states that are predominantly π-π* in nature can manifest
similar T1-state lifetimes and NIR absorptive characteristics. We
report herein highly conjugated palladium(II)- (PPdn) and
platinum(II)-containing (PPtn) porphyrin arrays (Chart 1) that
exhibit near-unity S1 f T1 intersystem crossing (ISC) quantum
yields, T1-state lifetimes that exceed 1 µs, and high oscillator
strength T1 f Tn absorptive manifolds that extend over a broad
850-1350 nm spectral window.
2. Experimental Section
Synthesis and Characterization. A full account of the
synthesis and characterization of all new compounds, complete
with detailed reaction schemes, is provided in the Supporting
Information.
† Part of the “Michael R. Wasielewski Festschrift”.
Instrumentation. Electronic spectra were recorded on a
Shimadzu UV-1700 spectrophotometer. Emission spectra were
recorded on a SPEX Fluorolog luminescence spectrometer that
utilized a T-channel configuration and R2658 PMT (200-900
nm)/Electro-Optical Systems, Inc. and DSS-16AO20L InGaAs
* To whom correspondence may be sent. E-mail: michael.therien@
duke.edu.
‡ University of Pennsylvania.
§ Current affiliation: U.S. Food and Drug Administration, Summit-Argo,
IL 60502.
| Duke University.
10.1021/jp102901u  2010 American Chemical Society
Published on Web 06/09/2010

(Porphinato)Pd(II) and (Porphinato)Pt(II) Oligomers
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14697
CHART 1: Structures of Bis- and
Tris(porphinato)metal(II) Arrays PZnn, PPdn, and PPtn
(n ) 2, 3)
sponding PFbn precursors via microwave-assisted synthesis
(Supporting Information).
Previous work has demonstrated that PZnn conjugated
porphyrin arrays manifest low energy Q-state derived π-π*
excited states that are polarized exclusively along the respective
long molecular axes.53,54,56,57,59,63,66,69,72,75 The lowest energy
optical transitions of these species gain in intensity and shift
progressively to the red with increasing numbers of conjugated
porphyrin units. The electronic absorption spectral data acquired
for PPd2, PPd3, PPt2, and PPt3 (Figure 1, Table 1) manifest
the general spectroscopic characteristics delineated previously
for the corresponding PZn2 and PZn3 benchmarks. Note that
all of the PPdn and PPtn species display high oscillator strength
Q-state derived low energy π-π* absorption bands, with a
molar extinction coefficient exceeding 65 000 M-1 cm-1 for
PPt3. The spectra in Figure 1 demonstrate that compared to the
analogous PZnn spectroscopic benchmarks, the Q-state derived
electronic absorption bands for the palladium and platinum
derivatives are blue-shifted; this phenomenon is common to
hypsoporphyrin species and has been attributed to efficient
mixing of the porphyrin π* and metal ndπ orbitals.82-84
Furthermore, X-ray crystallographic data (Supporting Informa-
tion) show that PPd2 and PPt2 access not only conformations
in which the adjacent porphyrin least-squares planes do not share
(800-1200 nm) detectors; these spectra were corrected using
the spectral output of a calibrated light source. Low temperature
(77 K) spectra were determined using an optical dewar. NMR
spectra were recorded on 250 MHz AMX-250 or 500 MHz
AMX-500 Bru¨ker spectrometers. Microwave reactions were
performed using a Personal Chemistry Creator (now Biotage)
utilizing custom glass reaction vessels.
Transient Absorption Spectroscopy. The femtosecond54 and
nanosecond55 transient optical systems used for the experiments
in this work have been described previously. All samples that
were interrogated via transient pump-probe spectroscopic
methods were deoxygenated via three successive freeze-pump-
thaw cycles prior to measurement.
a common plane but also those where the Cmeso-CR-ethyne
-
Cꢀ-ethyne angle deviates significantly from 180° (PPd2 ) 173.8°;
PPt2 ) 172.0°); it is well established within this class of
chromophores54,59,66 that such structural conformers augment the
magnitude of Qx-state absorption oscillator strength that is blue-
shifted with respect to that which corresponds to the maximally
conjugated conformer.
X-ray Crystal Structures. Dr. Patrick J. Carroll of the X-ray
diffraction facility at the University of Pennsylvania solved the
structures for compounds described in this work. PPd2 and PPt2
crystals were grown from dichloromethane solution layered with
methanol. The two solvents slowly diffused followed by slow
evaporation. X-ray quality crystals were grown after 2 weeks.
A full description of the details and results of these structure
determinations is provided in the Supporting Information.
Electrochemistry. Cyclic and square wave voltammetry were
carried out on an EG&G Princeton Applied Research model
273A potentiostat/galvanostat. The electrochemical cell utilized
a glassy carbon 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 electrolyte solution. The ferrocene/ferrocenium redox
couple was utilized as an internal potentiometric standard.
Emission spectra recorded at 77 K in glassy 2-methyltet-
rahydrofuran (MTHF) solvent for PPdn and PPtn, as well as
for the PZnn benchmarks, are depicted in the Figure 1 insets.
The palladium and platinum species manifest well-resolved
phosphorescence (T1 f S0) transitions at 77 K, approximately
3000 cm-1 red-shifted from their respective lowest-energy
ground-state singlet absorption band maxima; note that these
chromophores contrast their respective zinc analogues, which
exhibit no observable phosphorescence, even at 77 K.54,58 The
observed phosphorescence in the palladium and platinum
compounds derives in part from the efficient S1 f T1 intersystem
crossing (ISC) facilitated by the large spin-orbit coupling
constants of these heavy central metal ions.51,84 It is important
to note, however, that even at 77 K, phosphorescence for PPdn
and PPtn is weak (phosphorescence quantum yields, φp, were
estimated by comparison to fluorescent (porphinato)zinc bench-
marks to be below 5%). Note also that PPd3 and PPt3 also
exhibit residual fluorescence (S1 f S0) bands (Table 1);
excitation spectra evince that this fluorescence is authentic and
does not derive from residual free-base impurities.
3. Results and Discussion
The highly conjugated palladium(II)- (PPdn) and platinum(II)-
containing (PPtn) porphyrin arrays described in Chart 1 feature
a meso-to-meso ethyne-bridged linkage topology; a large body
of experimental data demonstrates that the strong electronic
coupling mediated by this multiporphyrin linkage motif creates
unusually polarizable and hyperpolarizable structures.53,54,56-79
The syntheses of PPd2, PPd3, PPt2, and PPt3 were accomplished
through metal-mediated cross-coupling of appropriately bromi-
nated and meso-ethyne-functionalized (porphinato)zinc(II)
precursors.57,63,80,81 Scheme 1 highlights the synthetic route to
the PPd2 and PPt2 structures, while the analogous path to the
PPd3 and PPt3 species is shown in Scheme S1 of the Supporting
Information. Following the syntheses of the parent PZn2 and
PZn3 complexes, these species were demetalated to generate
the corresponding free-base complexes (PFbn). PPdn and PPtn
complexes were synthesized in high yield from their corre-
Pump-probe femtosecond transient absorption spectroscopy
was employed to investigate the absorptive properties and
relaxation dynamics of the PPdn and PPtn excited states.
Representative transient absorption spectra recorded at various
time delays are shown in Figure 2 (PPd2, PPt2) and Figure 3
(PPd3, PPt3), along with the analogous dynamical data obtained
for the corresponding PZnn benchmarks.54,66 The early time-
delay transient absorption spectra (tdelay < 500 fs, Figures 2 and
3, black lines) of each PZnn, PPdn, and PPtn chromophore share
several common features: (a) bleaching signatures of the ground-
state Q- and B-state derived bands, (b) transient absorption in
the spectral region between these two bleaching signatures, and
(c) intense excited-state absorption manifolds that extend over

14698 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Duncan et al.
a
SCHEME 1: Synthetic Reaction Scheme for PZn2, PPd2, and PPt2
a Detailed synthetic procedures for these species, as well as for the corresponding trimeric PZn3, PPd3, and PPt3 structures, are provided in the
Supporting Information.
the low energy transient absorption manifolds observed at tdelay
< 350 fs are significantly broader than that evinced for PZn3.
In this regard, note that PPd3 and PPt3 feature prominent
transient absorption maxima evident at 984 and ∼940 nm85
(Figure 3B,C), respectively; while these highest extinction S1
f Sn absorption manifolds are considerably blue-shifted relative
to that of the PZn3 benchmark, note that both PPd3 and PPt3
also display absorption manifolds that extend beyond 1100 nm.
Factors that contribute to the more complex nature of the early
tdelay PPd3 and PPt3 transient absorption spectra likely include
the augmented degree of conformational inhomogeneity of these
species relative to that demonstrated previously for PZn3, and
Pd(II) and Pt(II) d-orbital/porphyrin π*-orbital mixing that
enables spectral contributions from transitions between states
that involve spin multiplicities higher than singlet.
Figures 2A and 3A show that for electronically excited PZn2
and PZn3, the S1 state persists for several hundred picoseconds;86
after this time, ground-state bleaching and S1 f Sn excited-
state absorption manifold intensities greatly diminish, and weak
absorption bands evolve at significantly lower energies (PZn2,
Figure 1. Electronic absorption spectra recorded for (A) PZnn,63 (B)
1017 nm; PZn3, 1196 nm) than those of the S1 f Sn peak
PPdn, and (C) PPtn in THF solvent at ambient temperature [n ) 2
maxima. The formation of these new NIR absorptive features
(blue); n ) 3 (red)]. Insets: normalized emission spectra, recorded in
is accompanied by the disappearance of the stimulated emission
contributions to the respective bleaching signals for these PZnn
species, which contribute significantly to the observed bleach
intensity at early time delays. As a result of these considerations,
the long-wavelength PZnn transient absorption bands observed
at tdelay > ∼100 ps are assigned to transitions between excited
triplet states; however, the weak intensity of these T1 f Tn
transitions, coupled with the near-total loss of the ground-state
bleaching signatures at delay times where the T1 f Tn transitions
are manifested, indicate that population of the triplet surface is
a minor deactivation pathway for 1PZnn* species. This conclu-
sion is congruent with related dynamical data that demonstrate
deoxygenated glassy MTHF at 77 K. Excitation wavelengths: (A) 460
nm (PZnn), (B) 630 nm (PPd2), 650 nm (PPd3), and (C) 600 nm (PPt2),
640 nm (PPt3).
a broad NIR energy regime. At early time delays, the NIR
transient absorption signatures (Figure 2) observed for PZn2,
PPd2, and PPt2 are dominated by respective absorption bands
centered at 980 nm (full width at half-maximum (fwhm) ) 660
cm-1), 888 nm (fwhm ) 930 cm-1), and 848 nm (fwhm ) 1580
cm-1). The early time-delay transient spectrum of PZn3 (Figure
3A) exhibits a NIR absorption band (λmax(S1 f Sn)) centered
at 1120 nm; note that this transition is considerably broader
(fwhm ) 750 cm-1) than that observed for the analogous early
time PZn2 NIR transient (fwhm ) 670 cm-1), and that the PZn3
S1 f Sn spectra also feature a higher energy NIR absorption
centered manifold at ∼980 nm. In contrast, for PPd3 and PPt3
1
that the primary deactivation modes for these PZnn* chro-
mophores are S1 f S0 internal conversion and fluorescence,
and steady-state experiments that show that these NIR fluoro-
phores manifest unusually high fluorescence quantum yields.54

(Porphinato)Pd(II) and (Porphinato)Pt(II) Oligomers
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14699
TABLE 1: Steady-State Spectroscopic Data of Zinc, Palladium, and Platinum Oligomeric Porphyrin Arrays
λmax S0 f S1 [nm]a,b
(fwhm, cm-1
εg @ S0 f S1
λmax S0 f S1 [nm]c,d
λmax T1 f S0 [nm]c,e
a
)
[M-1 cm-1
]
(Stokes shift, cm-1
)
(S0 f S1 - T1 f S0, cm-1
)
f
f
PZn2
PPd2
PPt2
PZn3
PPd3
PPt3
695 (∆ν ) 1090)
620 (∆ν ) 1330)
592 (∆ν ) ∼1030)
770 (∆ν ) 1380)
666 (∆ν ) 2170)
634 (∆ν ) 1700)
51400
23540
28510
116000
50100
65050
711(∆νStokes ) 320)
g
h
796 (∆ν ) 3570)
750 (∆ν ) 3600)
g
828 (∆ν ) 3120)
800 (∆ν ) 3260)
h
806 (∆νStokes ) 580)
747 (∆νStokes ) 1790)
∼740 (∆νStokes ) 2220)
a Electronic absorption data were recorded in THF at room temperature. b The parenthetical value is the transition manifold spectral breadth
(fwhm) in cm-1
fluorescence Stokes shift. e The parenthetical values correspond to the energy gap between the S0 f S1 absorption and phosphorescence band
maxima, in cm-1 f Spectroscopic data for PZn2 and PZn3 were taken from cited references.54,63,66 g No phosphorescence was observed. h No
.
c Emission data were recorded in glassy MTHF at 77 K. d The parenthetical value corresponds to the magnitude of the
.
fluorescence was observed.
Figure 3. Magic angle transient absorption spectra recorded at selected
delay times for (A) PZn3, (B) PPd3, and (C) PPt3. Experimental
conditions: solvent ) THF, ambient temperature, λexc ) (A) 775 nm,
(B) 620 nm, and (C) 600 nm.
Figure 2. Magic angle transient absorption spectra recorded at selected
delay times for (A) PZn2, (B) PPd2, and (C) PPt2. Experimental
conditions: solvent ) THF, ambient temperature, λexc ) (A) 684 nm,
(B) 590 nm, and (C) 560 nm.
centered at 1235 nm having a fwhm of ∼2400 cm-1 (Figures 2
and 3; Table 2). It is also noteworthy that the PPdn and PPtn
transient spectra evince ground-state bleaching signatures that
remain intense at all delay times (Figures 2-3), even after
formation of the excited triplet state is complete (tdelay ∼6 ns);
these data contrast the analogous pump-probe spectroscopic
results obtained for PZnn species, and show that T1-state
formation occurs at near-unit quantum yield.
By monitoring the intensification of the PPdn and PPtn (T1
f Tn) transient absorption bands as a function of time delay,
the ISC time scales are easily determined; the corresponding
exponential time constants (τISC) are compiled in Table 2. ISC
rates scale inversely with the nuclear charge (Z) of the central
metal, with τISC (Zn) > τISC (Pd) > τISC (Pt), consistent with the
relative magnitudes of the respective spin-orbit coupling
Transient spectra of PPdn and PPtn observed at later time
delays (tdelay > 4 ps, Figures 2B,C and 3B,C) differ markedly
from those of the PZnn benchmarks. As observed in the PZnn
transient absorption data, the excited-state spectra of PPdn and
PPtn also evolve at long time-delays to give rise to new transient
absorption features that lie 150-200 nm to the red of the S1 f
Sn excited-state absorptions. These transient absorption mani-
folds that persist through the microsecond time domain are
likewise assigned to transitions between states of high spin
multiplicity (T1 f Tn). Unlike the T1 f Tn transition manifold
observed for 3PZnn*, the T1 f Tn absorption features for both
PPdn and PPtn are intense and span a broad spectral domain:
3
for example, PPd3* displays a T1 f Tn absorption manifold

14700 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Duncan et al.
TABLE 2: Excited-State Spectroscopic Data of Zinc, Palladium, and Platinum Oligomeric Porphyrin Arrays in THF Solvent at
Room Temperature
λmax S1 f Sn [nm]a
λmax T1 f Tn [nm]a
εe @ T1 f Tn
b
(fwhm, cm-1
)
(fwhm, cm-1
)
[M-1 cm-1
]
τisc [ps]
τT [µs]
c
c
PZn2
PPd2
PPt2
PZn3
PPd3
PPt3
980 (670)
888 (930)
1017 (630)
1023 (2400)
976 (1280)
1196 (820)
1235 (1180)
1124 (1230)
n/a
∼7500
n/a
49.4
8.4
n/a
33.3
5.2
∼31000
∼71000
n/a
8.6
848 (1580)
1120 (750)
984 (>5000e)
∼0.3
>2000d
52.0
∼0.6
∼45000
906, 972 (>5000e)
∼95000
a The parenthetical value is the spectral breadth (fwhm) in cm-1 b Excited triplet-state extinction coefficients were determined from the
.
ultrafast transient absorption data at tdelay ) 1 ns by a method discussed elsewhere;52 values for PZn2 and PZn3 were not calculated due to the
facts that the T1 f Tn transition intensity is weak, and the stimulated emission signal contaminates the Q-band bleaching signal even at a 1 ns
time delay. c Spectroscopic data for PZn2 and PZn3 were taken from the cited references.58,66,72 d There is large uncertainty for τisc for PZn3 due
to overlap of the excited-state singlet and triplet absorption features and a low ISC quantum yield;54 thus a lower limit for this value is
reported. e The S1 f Sn transition manifolds of PPd3 and PPt3 encompass multiple distinct peak maxima extending over a large portion of the
measured NIR spectral window.
constants of these metal ions. ISC rates for PPd3 and PPt3 are
slower than those for the corresponding PPd2 and PPt2 species,
indicating that S1-to-T1 conversion rate constants may continue
to diminish with increasing conjugation length. This effect has
been verified in benchmark PZnn compounds as a function of
conjugation length; for example, the measured ISC rate for PZn5
is so small relative to the corresponding radiative and internal
conversion rate constants that its excited triplet state remains
virtually unpopulated following S0 f S1 excitation.54 Because
the heavy-atom-derived ISC rates in PPdn and PPtn species are
much faster than the intrinsic59,66,87,88 porphyrin nonradiative
decay processes, ISC quantum yields approach unity, even for
the PPd3 and PPt3 derivatives; T1 f Tn excited-state absorptions
are intense in comparison to those of their PZnn analogs at 500
ps < tdelay < 50 µs. As a result, the molar absorptivities of the
PPdn and PPtn excited triplet states are universally large, as
indicated by the T1 f Tn molar extinction coefficients compiled
in Table 1. The value of εe(λ)1124 nm) ∼ 95 000 M-1 cm-1
at tdelay ) 1 ns for PPt3 (Figure 3C) indicates that this compound
is one of the most intensely absorbing excited triplet states
reported to date in the NIR energy regime.
4. Conclusion
In conclusion, we have demonstrated that coupled oscillator
photophysics can be exploited to fabricate supermolecular
chromophores that possess intense excited-state absorption bands
over broad spectral regions of the NIR. In these systems, the
large spin-orbit coupling has been utilized to drive near-
quantitative yield of the electronically excited triplet states of
PPdn and PPtn species, which possess spectacular, intense, broad
absorptivity over technologically important spectral domains of
the NIR. While intense NIR excited-state absorption features
in heavy-metal containing multimeric porphyrin complexes have
been reported before,47,52,53 it is clear that PPdn and PPtn
structures offer unique flexibility with respect to tuning the
position of maximum excited-state absorptivity while assuring
microsecond T1-state lifetimes. For sequential one-photon
absorptive processes, PPd2, PPt2, PPd3, and PPt3 chromophores
evince enormous excited-state absorptive extinction coefficients
(30 000-100 000 M-1 cm-1) at λmax values that range from 970
to 1235 nm. Given that the lifetimes of these species range from
5 to 50 µs, coupled with the fact the full widths at half-maximum
of their T1 f Tn absorptive manifolds span 1200-2400 cm-1
,
PPdn and PPtn structures define a new precedent for conjugated
materials that feature low-lying electronically excited states that
are purely π-π* in nature for NIR optical limiting and related
long-wavelength NLO applications.
Anaerobic transient absorption experiments carried out over
a nanosecond-microsecond time scale were utilized to deter-
mine the lifetimes of the PPdn and PPtn excited triplet states
(τT) described above (Table 1; spectral data are displayed in
the Supporting Information, Figures S1-S4). While the triplet
lifetimes of these compounds all exceed 5 µs, the data show
that τT (PPdn) is generally longer than the analogous τT (PPtn)
value, reflecting the trend in the τISC values noted above. While
the lifetimes of the PPd3 and PPt3 are shorter than their PPd2
and PPt2 counterparts in agreement with the energy gap law,49,89
it is important to underscore that the extent to which τT
diminishes with increasing conjugation length is modest for a
given metal ion: it is anticipated that PPdn and PPtn species
having longer conjugation lengths will afford further modulation
of the OPL window while maintaining T1 states having
microsecond lifetimes.
Acknowledgment. This work was supported by a grant from
the Department of Energy (DE-SC0001517). We thank the NSF
MRSEC (DMR0520020) and DOE EFRC (P353-0127) pro-
grams for infrastructural support. T.V.D. acknowledges a
Carolyn Hoff Lynch Graduate Fellowship, and M.J.T. is grateful
to the Francqui Foundation (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. We also thank Patrick J. Carroll at
the University of Pennsylvania X-ray Crystallography Facility
for solving the PPd2 and PPt2 structures.
Supporting Information Available: Synthetic and charac-
terization details; X-ray crystallographic data for PPd2 and PPt2;
microsecond time domain transient absorption data. This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
Carrying out similar pump-probe experiments under aerobic
conditions results in efficient excited-state quenching (complete
ground-state recovery within 1 µs), as expected for states with
high spin multiplicity.51 It is noteworthy that these PPdn and
PPtn T1 lifetimes are comparable to or exceed the triplet-state
lifetimes of a variety of metal(II) naphthalocyanines that are
currently being utilized for OPL applications in the visible
regime.7
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