Molecular engineering of intensely near-infrared absorbing excited states in highly conjugated oligo(porphinato)zinc-(polypyridyl)metal(II) supermolecules

Article abstract of DOI:10.1021/ja0707512

A new series of chromophores, MPZn_n, which combine ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) (MPZn) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZn_n) architectures, have been synthesized and characterized,

Full text of DOI:10.1021/ja0707512

Published on Web 07/13/2007
Molecular Engineering of Intensely Near-Infrared
Absorbing Excited States in Highly Conjugated
Oligo(porphinato)zinc-(Polypyridyl)metal(II) Supermolecules
Timothy V. Duncan, Tomoya Ishizuka, and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received February 2, 2007; E-mail: therien@sas.upenn.edu
Abstract: A new series of chromophores, MPZnn, which combine ethyne-bridged bis(terpyridyl)metal(II)-
(porphinato)zinc(II) (MPZn) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZnn) architectures, have
been synthesized and characterized, along with a series of derivatives bearing pyrrolidinyl electron-releasing
groups on the ancillary terpyridine units (PyrmMPZnn). Cyclic voltammetric studies, as well as NMR,
electronic absorption, fluorescence, and femtosecond pump-probe transient absorption spectroscopies,
have been employed to study the ground- and excited-state properties of these unusual chromophores.
All of these species possess intensely absorbing excited states having large spectral bandwidth that
penetrate deep in the near-infrared (NIR) energy regime. Electronic structural variation of the molecular
framework shows that the excited-state absorption maximum can be extensively modulated [λmax(T1
f
Tn)] (880 nm < λmax < 1126 nm), while concomitantly maintaining impressively large T1 f Tn absorption
manifold spectral bandwidth (full width at half-maximum, fwhm, ∼2000-2500 cm-1). Furthermore, these
studies enable correlation of supermolecular electronic structure with the magnitude of the excited-state
lifetime (τes) and demonstrate that this parameter can be modulated over 4 orders of magnitude (∼1 ns <
τes < 45 µs). Terpyridyl pyrrolidinyl substituents can be utilized to destabilize terpyridyl ligand π* energy
levels and diminish the E1/2 (M3+/2+) value of the bis(terpyridyl)metal(II) center: such perturbations determine
the relative energies of the PZnn-derived 1π-π* and bis(terpyridyl)metal(II) charge-transfer states and
establish whether the T1-state wave functions of MPZnn and PyrmMPZnn species manifest the extensive
electronic delocalization and charge-separated (CS) features characteristic of long-lived triplet states that
absorb strongly in the NIR.
nanotube,6,13-16 and other17-20 motifs have already been identi-
fied; however, examples of chromophores suitable for such NLO
Introduction
The efficiency of nonlinear optical (NLO) materials for long-
wavelength reverse saturable absorption (RSA)1-4 and optical
power limiting (OPL)4-6 applications hinges in large part upon
engineering strongly absorbing excited states having long
lifetimes. Because of this requirement, chromophores which
populate an excited triplet state after photoexcitation are often
utilized, due to the fact that ground-state recovery from excited
triplet states is usually retarded by spin-conservation selection
rules. In the visible region (400 < λ < 800 nm), OPL materials
based upon phthalocyanine,5,7-10 porphyrin,11,12 fullerene/
applications in the near-infrared (NIR) are limited. This derives
from the fact that as chromophore optical band gaps shift to
progressively lower energies, intersystem crossing (ISC) gener-
ally becomes inefficient due to the increased rates of competing
internal conversion processes.21-24 Furthermore, even in species
which possess near-unity ISC quantum yields (such as those
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A R T I C L E S
Duncan et al.
Scheme 1. Design Strategy of Highly Conjugated Hybrid
Bis(terpyridyl)metal(II)-(Porphinato)zinc(II) MPZnn Chromophores
(n ) Total Number of Conjugated PZn Units)a
mophores, PZn and (terpyridyl)metal(II) moieties possess a
characteristic ethyne-bridged porphyrin meso-carbon to terpy-
ridyl 4′-carbon linkage topology (Scheme 1), which has been
shown to facilitate efficient excited-state electronic communica-
tion between the subunits. Previous work on ethyne-bridged
PZnn oligomers that feature a meso-to-meso linkage motif
demonstrates that these compounds manifest low-energy Q-state
derived π-π* excited states that are polarized exclusively along
the long molecular axis25,26,36,37,39-41,44-46 and exhibit intensely
absorbing S1 f Sn transitions that extend deep into the NIR
energy regime [compound, ([λmax(S1 f Sn)], full width at half-
maximum, fwhm): PZn2 (980 nm, 656 cm-1),39 PZn3 (1120
nm, 750 cm-1),41 and PZn5 (1325 nm, 1980 cm-1)]41 in THF
solvent. Notably, the excited states of these species (i) relax to
their respective electronic ground states on nanosecond (ns) time
scales, (ii) display large fluorescence quantum yields, and (iii)
manifest modest-to-minimal triplet yields that decrease with
increasing conjugation length.41 In contrast, studies that probe
the relaxation dynamics of the low-lying excited states of MPZn
species show that, within 200 fs following photoexcitation, the
initially prepared excited state relaxes to a highly polarized
charge-separated (CS) T1 state43 that differs in many respects
from the MLCT state of conventional (polypyridyl)metal(II)
species;47-51 this MPZn CS triplet excited state relaxes non-
radiatively via charge recombination dynamics on timescales
ranging from 0.86 µs (OsPZn) to 44 µs (RuPZn).43 While the
primary excited-state relaxation pathway of MPZn species is
related to that for charge-recombination (CR) deactivation of
classical (polypyridyl)metal(II) charge-transfer (CT) states,
because MPZn T1 f Tn manifold transitions involve states
having substantial CS character, these absorptions feature
considerable PZn π-system oscillator strength. MPZn species
thus exhibit intense NIR excited-state absorption bands, with
large excited-state molar extinction coefficients ꢀe[λmax(T1 f
Tn)] ranging from ∼30 000 to ∼100 000 M-1 cm-1, and impres-
sive bandwidths [compound, ([λmax(T1 f Tn)], fwhm): RuPZn
(884 nm, ∼2730 cm-1); OsPZn (964 nm, >2015 cm-1).43
This work motivated the engineering of a new class of
compounds (MPZnn chromophores, Scheme 1) that exhibits the
combined properties of these two benchmark, highly conjugated
multichromophore motifs: (i) the enormous NIR T1 f Tn
transition intensities; (ii) µs excited-state relaxation dynamics
of thermally relaxed MPZn excited states, with (iii) the tunable
excited-state absorption manifolds characteristic of PZnn oli-
gomers. Chart 1 highlights RuPZn2 and OsPZn2 structures,
while Chart 2 describes related PyrmRuPZnn compounds that
feature pyrrolidinyl (Pyr) substituents that modulate terpyridyl
ligand π* energy levels and diminish the E1/2(M3+/2+) value of
the bis(terpyridyl)metal(II) center. Herein we discuss the
a R ) 2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl.
which utilize heavy atoms), excited triplet-state molar absorp-
tivities beyond 800 nm are often small,21 due to challenges
associated with engineering extensive excited-triplet wave
function delocalization in π-conjugated systems.25-35 For these
reasons, the realization of OPL chromophores which are both
optically active in the NIR and possess long-lived, strongly
absorbing excited-states remains an unmet molecular engineer-
ing goal.
This laboratory has described the design, synthesis, spectros-
copy, and excited-state dynamics of two heretofore unrelated
series of highly conjugated chromophores (Scheme 1): (i)
multimeric (porphinato)zinc(II) arrays that feature ethyne-
bridged macrocycle-to-macrocycle linkage motifs (PZnn struc-
tures);25-28,36-41 (ii) hybrid MPZn chromophores incorporating
ethyne-linked (porphinato)zinc(II) (PZn) and bis(terpyridyl)-
metal(II) (M; metal ) Ru, Os) subunits.42,43 In MPZn chro-
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9
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Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
Scheme 2. Syntheses of RuPZn, Pyr1RuPZn, Pyr3RuPZn,
Chart 1. Structures of RuPZn2 and OsPZn2 Compounds
a
RuPZn2, Pyr1RuPZn2, Pyr3RuPZn2, OsPZn, and OsPZn2
Chart 2. Structures of PyrmRuPZnand PyrmRuPZn2 Compounds
a Key: (i) Pd2(dba)3, AsPh3, 6:3:1 MeCN-THF-TEA, 60 °C, 7 h; (ii)
NH4PF6. R ) 2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl.
Luminescent lifetimes were determined by either time-correlated, single-
photon-counting (TCSPC) spectroscopy or using a streakscope-based
Hamamatsu picosecond fluorescence lifetime measurement system (see
below).
3. Time-Correlated, Single-Photon-Counting (TCSPC) Spectros-
copy. TCSPC spectroscopic measurements were performed at the
Regional Laser and Biotechnology Laboratory (RLBL) at the University
of Pennsylvania using an instrument (response function ) 25 ps fwhm)
described previously;52 these data were analyzed using Lifetime (RLBL)
and Globals Unlimited (LFD, University of Illinois) Programs.
4. Picosecond Fluorescence Lifetime Measurement System (Streak-
scope). Time-resolved emission spectra were recorded using a Hamamat-
su C4780 picosecond fluorescence lifetime measurement system. This
system employs a Hamamatsu Streakscope C4334 as its photon-
counting detector; all time delays were electronically generated by a
Hamamatsu C4792-01 synchronous delay generator. The excitation light
source chosen was a Hamamatsu 405 nm diode laser. All fluorescence
data were acquired in single-photon-counting mode using Hamamatsu
HPD-TA software. The data were analyzed using the Hamamatsu fitting
module; both non-deconvoluted and deconvoluted data analyses were
performed to ascertain whether or not any emissive processes were
excitation pulse-limited.
synthesis, electrochemistry, steady-state absorption, and time-
resolved transient absorption spectroscopy of these chro-
mophoric species and correlate the structure of these supermol-
ecules with the nature of the initially prepared and thermally
relaxed excited sates. These studies show that the T1-state wave
functions of conjugated MPZnn chromophores can not only
manifest the extensive electronic delocalization and the charge-
separated (CS) features characteristic of impressive MPZn NIR
absorbers but can be engineered to give rise to excited states
that provide excellent spectral coverage and possess lifetimes
and T1 f Tn absorption maxima that are tunable respectively
over wide time and energy domains.
5. Ultrafast Transient Absorption Experiments. The transient
optical system used in this work has been discussed previously.41 All
pump-probe samples were deoxygenated via three successive freeze-
pump-thaw cycles prior to measurement.
Experimental Section
1. 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.
Results and Discussion
1. Synthesis. Scheme 2 outlines the syntheses of ethyne-
bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) com-
pounds RuPZn, Pyr1RuPZn, Pyr3RuPZn, OsPZn, RuPZn2,
Pyr1RuPZn2, Pyr3RuPZn2, and OsPZn2; synthetic procedural
details for these compounds, as well as for bis(terpyridyl)metal-
2. Instrumentation. Electronic absorption spectra were recorded on
a Shimadzu UV-1700 spectrophotometer, and emission spectra were
recorded on a SPEX Fluorolog luminescence spectrometer that utilized
a T-channel configuration and PMT/InGaAs/Extended-InGaAs detec-
tors. Emission spectra were corrected using the spectral output of a
calibrated light source supplied by the National Bureau of Standards.
Low temperature (77 K) spectra were recorded using an optical dewar.
(52) Holtom, G. R. SPIE Proc. 1990, 1204, 2-12.
9
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A R T I C L E S
Duncan et al.
(II) and (porphinato)zinc(II) precursor complexes and bench-
mark PZn-tpy and PZn2-tpy species, can be found in the
Supporting Information. The metal-mediated cross-coupling53-58
of a meso-ethyne-functionalized (porphinato)zinc(II) species
(Scheme 2) with an appropriately brominated bis(terpyridyl)-
metal(II) precursor complex yields the desired supermolecular
compound in high yield.
RuPZn and OsPZn chromophores have been synthesized by
the respective cross-coupling of the 4′-brominated ruthenium
or osmium bis(2,2′;6′,2′′-terpyridine) complex (RuBr or OsBr)
with 5-ethynyl-10,20-bis(2′,6′-bis(3,3-dimethyl-1-butyloxy)phe-
nyl)porphinato]zinc(II) (EPZn) (Scheme 2).42 RuPZn2 and
OsPZn2 were prepared in a similar fashion, utilizing a meso-
ethynylated ethyne-bridged (porphinato)zinc(II) species (EPZn2)
as the porphyrin-containing starting material (Scheme 2), while
Pyr1RuPZn, Pyr3RuPZn, Pyr1RuPZn2, and Pyr3RuPZn2
exploited appropriate pyrrolidine-bearing ruthenium bis-
(terpyridyl) bis(hexafluorophosphate) reagents (Pyr1RuBr or
Pyr3RuBr; Scheme 2). These pyrrolidine-substituted ruthenium
complexes were in turn prepared by conventional synthetic
methods using ruthenium trichloride as the starting material
(Supporting Information). Note that while Gros reported recently
the synthesis and characterization of examples of pyrrolidine-
modified ruthenium(II) polypyridyl complexes,59 because a
4,4′,4′′-pyrrolidinyl-2,2′;6′,2′′-terpyridine ligand (Pyr3-tpy) was
required and the 4′-chloro-2,2′;6′,2′′-terpyridine precursor for
4′-pyrrolidinyl-2,2′;6′,2′′-terpyridine (Pyr1-tpy) was commer-
cially available, an independent synthetic route was adopted
(Supporting Information).
Figure 1. Comparative electronic absorption spectra of (A) RuPZn (solid
blue line) and RuPZn2 (solid red line) and (B) OsPZn (solid blue line)
and OsPZn2 (solid red line), in acetonitrile solvent. The inset shows the
corresponding room-temperature emission spectra (λexc ) 700 nm) of
RuPZn2 and OsPZn2, corrected for concentration.
Information) and do not evince transitions that derive from the
simple superposition of precursor compound spectra.42 Never-
theless, it is important to highlight that RuPZn and OsPZn
absorption bands retain some spectral qualities of the transitions
characteristic of classical PZn and [M(tpy)2]2+ chromophores.
For instance, the RuPZn spectrum (Figure 1A, blue line)
possesses (i) two absorption bands in the ultraviolet (UV) at
RuPZn, OsPZn, RuPZn2, OsPZn2, Pyr1RuPZn,
Pyr3RuPZn, Pyr1RuPZn2, and Pyr3RuPZn2 were isolated via
column chromatography on silica using 90:9:1 CH3CN-H2O-
saturated aqueous KNO3 as the eluent; counterion metathesis
using ammonium hexafluorophosphate followed, providing the
corresponding bis(hexafluorophosphate) salts, which were used
in all the spectroscopic experiments. Interestingly, while MPZn
chromophoric benchmarks and PyrmRuPZn derivatives have
limited solubility in many common organic solvents with the
1
272 and 309 nm, consistent with terpyridine-localized π-π*
transitions, (ii) a very strong (ꢀ ∼ 105 000 M-1 cm-1) absorption
at 427 nm which exhibits significant porphyrin-derived 1π-π*
Soret (B) band character, (iii) a visible band centered at 506
nm which exhibits [Ru(tpy)2]2+-derived singlet metal-to-ligand
charge transfer (1MLCT) character and features contributions
from porphyrin ligand oscillator strength (vide infra), and (iv)
two bands localized at 567 and 640 nm which exhibit porphy-
rinic 1π-π* Q-state character, with the low-energy, x-polarized
transition manifesting features due to symmetry breaking and
oscillator strength redistributions that derive from conjugation
expansion60-62 and charge resonance character that originates
from the ethyne-bridged porphyrin meso-carbon-to-terpyridyl-
4′-carbon linkage.42 Note that to facilitate comparison of the
spectral properties of these conjugated supermolecules with
those of traditional (porphinato)zinc(II) and (polypyridyl)metal-
(II) benchmarks, MLCT (d-π*) and Soret- 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 MPZnn chromophores, they denote only the dominant
contributor to the oscillator strength of the transition in question;
it is recognized that MLCT, ligand, Soret, and Q electronic states
mix extensively in these supermolecules.63
exception of organonitriles and pyridines, MPZn2 and Pyrm
-
RuPZn2 species display millmolar solubilities in a wide range
of organic solvents.
2. RuPZn2 and OsPZn2. A. Electronic Absorption Spec-
troscopy. The electronic absorption spectra (EAS) of RuPZn2
and OsPZn2 are displayed in Figure 1; for comparison, the
previously reported42 EAS of RuPZn and OsPZn are also
plotted in this figure. Electronic absorption spectral data for all
compounds are listed in Table 1. As noted previously, the EAS
of RuPZn and OsPZn exhibit strong mixing of (porphinato)-
zinc(II) (PZn) based oscillator strength with metal polypyridyl
charge-resonance bands. In this regard, note that RuPZn and
OsPZn electronic spectra differ markedly from the EAS
characteristic of benchmark monomeric building block EPZn
and [M(tpy)2]2+ chromophores (see also Figure S1, Supporting
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Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
Table 1. Ground-State Electronic Spectrala and Cyclic Voltammetricb Data for RuPZn, OsPZn, RuPZn2, OsPZn2, Pyr1RuPZn, Pyr3RuPZn,
Pyr1RuPZn2, Pyr3RuPZn2, and Selected Chromophoric Benchmarks
1
abs band max/nm (
ꢀ
/105 M-1 cm-
)
potentiometric data/V
+
compd
B bandc
1MLCTc
Q bandsc
3MLCTc
M2+/M3
ZnP0/ZnP+
RuPZn42
OsPZn42
RuPZn2
427 (1.07)
425, 440 (1.12, 1.10)
410, 437, 487 (0.75, 0.73, 1.40)
412, 434, 483 (1.05, 0.95, 1.84)
438 (0.91)
439 (1.00)
411, 435, 483 (0.69, 0.66, 1.32)
412, 441, 483 (0.70, 0.62, 1.22)
506 (0.56)
498 (0.56)
515 (0.52)
511 (0.67)
521 (0.43)
528 (0.36)
526 (0.39)
533 (0.33)
477 (0.15)
479 (0.17)
499 (0.19)
495 (0.21)
567, 639 (0.15, 0.51)
567, 642 (0.21, 0.36)
566, 740 (0.17, 0.57)
566, 740 (0.21, 0.89)
567, 640 (0.14, 0.43)
566, 640 (0.15, 0.41)
566, 737 (0.15, 0.57)
566, 737 (0.16, 0.55)
1.39
1.07
1.39
1.07
0.99
0.81e
1.01
∼0.81f
1.35
1.01
0.98
0.79
0.80
0.83
0.64
0.64
0.80
0.81e
0.64
0.64
675 (0.30)
OsPZn2
∼752d (0.84)
Pyr1RuPZn
Pyr3RuPZn
Pyr1RuPZn2
Pyr3RuPZn2
RuBr
OsBr
665 (0.04)
Pyr1RuBr
Pyr3RuBr
PZn-tpyg
PZn2-tpyg
435 (2.62)
408, 433, 481 (0.78, 0.79, 1.79)
559, 604 (0.13, 0.12)
556, 698 (0.15, 0.43)
0.84
0.58
a Spectral data were recorded in acetonitrile. “n/a” denotes transitions that were not observed. b Experimental conditions: [chromophore] ) ∼2 mM; scan
rate ) 0.1-0.2 V/s; reference electrode ) Ag wire. E1/2 values reported are relative to SCE; the ferrocene/ferrocenium couple (0.43 V vs SCE) was used
as the internal standard. Potentiometric data for these compounds were obtained in a 0.1 M (TBA)PF6/acetonitrile electrolyte/solvent system, except for
PZn-tpy and PZn2-tpy, which were interrogated in 0.1 M (TBA)PF6/CH2Cl2. All reported values correspond to one electron redox couples. c Transition
labels denote the dominant contributor to the absorption oscillator strength. d Estimated 3MLCT absorption band λmax (see text for details). e The Ru2+/Ru3+
and the ZnP0/ZnP+ redox couples are coincident or near coincident. f The redox couple centered at ∼0.825 V is associated with a pseudo-2e- process and
is a convolution of two independent 1e- Ru2+/3+ and PZn+/2+ redox couples that coincidentally occur at the same voltage in this compound. The latter
process appears at E1/2 ) 0.84 V in RuPZn2, OsPZn2, and Pyr1RuPZn2; thus, E1/2(Ru2+/3+) in Pyr3RuPZn2 is estimated to be ∼0.81 V. g Spectral data for
these compounds were obtained in chloroform solvent.
OsPZn exhibits an electronic absorption spectrum similar to
that observed for RuPZn; note that the OsPZn Qx-band
nearly isoenergetic (Figure 1), as the ancillary terpyridine ligands
(i.e., terpyridines not featuring porphyrin substitution) are
essentially electronically isolated in both compounds; likewise
the Qy transitions for RuPZn and RuPZn2 occur at nearly the
same energies, as augmented electronic delocalization afforded
by an additional PZn unit and the meso-to-meso ethyne bridge
in RuPZn2 exerts the greatest impact on x-polarized absorptions.
Note that the maximum of the RuPZn2 Qx-band transition
manifold is red-shifted by nearly 100 nm (∼2135 cm-1) with
respect to that of the RuPZn benchmark; in contrast, the
transition centered at ∼505 nm in RuPZn that manifests
manifold, however, is broader [OsPZn Qx(fwhm) ∼900 cm-1
;
RuPZn Qx(fwhm) ∼760 cm-1] and exhibits an additional peak
maximum (λmax ) 675 nm) that tails farther to the red. These
spectral features derive from the larger degree of ground-state
charge resonance character inherent to the osmium-containing
3
complexes43 and the fact that direct excitation to the MLCT
state is made possible by the larger nuclear core-charge (Z) of
osmium49 relative to that of ruthenium.
RuPZn2 and OsPZn2 possess electronic absorption spectra
(Figure 1) that differ markedly from those of the RuPZn and
OsPZn benchmarks, although the transition assignments are
largely identical. For example, the RuPZn2 EAS manifests
terpyridine 1π-π* transitions and transitions dominated by
1
extensive MLCT character red-shifts a modest 9 nm (∼345
cm-1) in RuPZn2. In RuPZn and RuPZn2, the π-π* Qx-band
transition energy is related to the energy gap between the
π-symmetric HOMO delocalized over the (PZn-ethyne)n)1,2
-
1
porphyrin-derived Soret and Q-state character, as well as a -
terpyridine framework and its corresponding LUMO having
similar spatial delocalization. This π-π* energy gap differs
substantially in RuPZn and RuPZn2 and derives in large part
from the fact that E1/2(PZn0/+) is destabilized by ∼160 mV in
RuPZn relative to RuPZn2 (Table 1). On the other hand, the
dominant contributor to the magnitude of the 1MLCT transition
energy is the energy gap between the ruthenium d-orbitals and
the (porphyrin-ethyne)n)1,2 LUMO (π*). Cyclic voltammetric
measurements (Table 1) show that E1/2(Ru2+/3+) is nearly
MLCT-derived band that is partly obscured by the S0 f S2
(Soret) electronic absorption manifold. Due to the extensive
interpigment electronic and excitonic64,65 interactions made
possible by the meso-to-meso ethyne bridge,26,36,37,39-41,44,66 the
x-polarized Q-state transition manifold increases in oscillator
strength relative to that observed for RuPZn; further, the
RuPZn2 Soret band is split in a pattern characteristic of this
conjugation topology, with the x-polarized B-bands shifted to
lower energy and the y-polarized B-bands shifted to higher
energy and broadened, reflecting the inhomogeneous distribution
of structural conformers that differ with respect to the inter-
porphyryl torsional angle.39,41,44 As expected, the respective
terpyridine 1π-π* transitions for RuPZn and RuPZn2 are
1
identical for RuPZn and RuPZn2; thus the MLCT transition
energy depends largely on the (PZn-ethyne)n)1,2-terpyridine
acceptor ligand LUMO energy in RuPZn and RuPZn2. Because
the magnitude of LUMO stabilization with increasing conjuga-
tion length is modest with respect to the extent of HOMO
destabilization in meso-to-meso ethynyl-bridged oligomeric
(porphinato)zinc(II) species (PZnn structures),40 the additional
PZn-ethyne unit of RuPZn2 exerts a large influence on the
magnitude of Qx π-π* gap but little upon the 1MLCT transition
energy, relative to the analogous optical band gaps of RuPZn
(Figure 1).
(63) For instance, the term “Qx-state” is used to refer to the lowest energy π-π*-
derived transition that is polarized along the long-molecular axis in RuPZn2,
but it is understood that MLCT (d-π*) character is mixed with this
x-polarized absorption.
(64) McRae, E. G.; Kasha, M. The Molecular Exciton Model. In Physical
Processes in Radiation Biology, Proceedings of an International Sympo-
sium; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press:
New York, 1964; pp 23-42.
(65) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11,
371-393.
As RuPZn and OsPZn EAS are similar, so are those
determined for RuPZn2 and OsPZn2 (Figure 1). As before, the
(66) Anderson, H. L. Inorg. Chem. 1994, 33, 972-981.
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9695

A R T I C L E S
Duncan et al.
only significant difference between the RuPZn2 and OsPZn2
weak compared to that of the precursor PZn2-tpy compound
(Supporting Information, Figure S4) and other, related meso-
to-meso ethyne bridged porphyrin arrays.41 These observed
relative emission intensities are consistent with fluorescence
lifetimes measured via time-correlated single-photon counting
(TCSPC), of 430 and 9 ps, for RuPZn2 and OsPZn2, respec-
tively (Table 1). While the RuPZn2 and OsPZn2 fluorescence
signatures will be discussed in more detail in the context of the
ultrafast pump-probe transient absorption spectroscopic mea-
surements below, a few points are worth noting here: (1) Given
the lifetime values, the relatively small (∼1040 cm-1) Stokes
shifts, and the spectral similarity of the appropriate transient
bleach-band signal for both compounds, this emission is assigned
to fluorescence (S1 f S0) from an excited-state dominated by
(porphyrin-ethyne)2-terpyridine 1π-π* (Qx) character. (2)
While the S1 f S0 λmax for PZnn chromophores shows modest
solvent dependence, the emission maxima for RuPZn2 and
OsPZn2 show marked solvent dependence (Supporting Informa-
tion, Figure S5), with polar solvents driving larger Stokes shifts
(e.g., for RuPZn2, λmax(acetonitrile) ) 804 nm (∆νj ) 1040
cm-1) and λmax(toluene) ) 771 nm (∆νj ) 525 cm-1)). (3)
Notably, in toluene, the RuPZn2 S1 f S0 band is significantly
narrower (fwhm ) 1130 cm-1 vs 1823 cm-1 in acetonitrile)
and nearly 1 order of magnitude more intense than in highly
polar solvents. (4) Relative to the fluorescence Stokes shifts of
electronically symmetric PZnn chromophores (∆νj ∼ 300-600
cm-1),36,37,39-41 the Stokes shifts of RuPZn2 and OsPZn2 are
significant; as the electronically asymmetric RuPZn2 and
OsPZn2 π-π* states feature electronic mixing with an MLCT
state and are thus more polarized, excited-state solvent relaxation
plays a prominent role in determining the magnitudes of Stokes
shifts and fluorescence lifetimes. In this regard, a similarly large
fluorescence Stokes shift and attenuated fluorescence lifetime
was observed for [(5,10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)-
phenyl]porphinato)zinc(II)]-[(5′,15′-ethynyl-10′,20′-bis[10,20-
bis(heptafluoropropyl)porphinato)zinc(II)]ethyne, an electroni-
cally asymmetric meso-to-meso ethyne-bridged bis[(porphinato)-
zinc(II)] complex,39 in which the Qx absorption manifold features
a significant charge resonance contribution.
spectra can be discerned in the low-energy absorption manifold;
3
a MLCT oscillator strength contribution is quasi-allowed in
OsPZn2, though this feature is obscured by the high oscillator
strength Qx-band contribution to the low-energy absorption
manifold. The RuPZn2 and OsPZn2 spectra were analyzed on
the basis of the assumption they are essentially identical, save
3
for the augmented MLCT oscillator strength in the OsPZn2
spectrum; thus, normalizing these spectra and subtracting the
RuPZn2 EAS from that of OsPZn2 yields an estimate of the
OsPZn2 3MLCT band position and extinction coefficient
(λmax(S0
f
3MLCT) ) 752 nm, ꢀ ∼ 84 000 M-1 cm-1
)
(Supporting Information, Figure S2). The energy gaps between
3
the S0 f MLCT and S0 f Qx transitions for OsPZn and
OsPZn2 are 240 and 180 cm-1, respectively;67 these data suggest
that the MLCT and Qx 1π-π* states lay closer together in
3
OsPZn2 than in OsPZn, consistent with the fact that the addition
of an extra conjugated porphyrin unit to the OsPZn framework
diminishes the Qx 1π-π* transition energy and perturbs only
slightly the energies of states possessing MLCT character,
congruent with that discussed above the analogous RuPZn and
RuPZn2 spectra. While these experiments provide no insight
into the magnitude of the RuPZn2 3MLCT transition energy,
the fact that bis(terpyridyl)ruthenium(II) complexes typically
exhibit 3MLCT energies significantly higher in energy (∼2000
cm-1 for [M(tpy)2]2+ 49
) than analogous bis(terpyridyl)osmium-
(II) complexes suggests that the RuPZn2 3MLCT state probably
lies far above the Qx-state energy; this supposition is supported
by fluorescence and transient absorption data described below.68
B. Steady-State Fluorescence Spectroscopy. Neither RuPZn
nor OsPZn exhibits observable luminescence from their respec-
tive 3MLCT states at ambient or liquid-nitrogen temperatures,42
3
despite the fact that the MLCT states possess µs lifetimes at
298 K. These spectral and dynamical features are consistent
with extensively delocalized T1-state wave functions for these
complexes characterized by substantial charge-separated char-
acter.43 Such highly polarized, charge-separated RuPZn and
OsPZn 3MLCT states would be expected to exhibit dramatically
attenuated Franck-Condon overlap relative to that manifested
typically for emissive (polypyridyl)metal(II) complexes, where
excited-state relaxation is dominated by charge recombination
dynamics (see Supporting Information for an overview of basic
[M(tpy)2]2+ excited-state dynamics).43
Contrary to what is observed for the benchmark RuPZn and
OsPZn complexes, RuPZn2 and OsPZn2 both exhibit observ-
able emission bands. As depicted in the inset of Figure 1, the
emission from RuPZn2 is much stronger than that of OsPZn2
(φF(RuPZn2)/φF(OsPZn2) ∼ 12).69 The RuPZn2 emission is
(67) The 3MLCT and Qx states for OsPZn2 are closer in energy than the values
determined from electronic absorption spectra alone would indicate. Because
of the large (∼980 cm-1) Stokes shift for fluorescence from the Qx state
(vide infra), the Qx-state energy is ∼490 cm-1 closer to the ground state
than as determined solely by absorption spectroscopy; this places the
3MLCT ∼311 cm-1 above the Qx state. These relative energies are in better
agreement with fluorescence and transient absorption measurements.
(68) As the RuPZn charge-separated triplet state is efficiently populated
following electronic excitation, it must lie either below or close to the
1π-π* (Qx) state. Given the solvent and torsional relaxation dynamics that
occur on both the 1π-π* and charge-separated triplet state surfaces, the
absolute state energies evolve with time; the relative state energies are thus
discussed in the text only at a qualitative level.
C. Femtosecond Pump-Probe Transient Absorption
Spectroscopy of RuPZn and OsPZn. The early time-delay
transient absorption spectra of RuPZn and OsPZn share several
common features. These include (i) weak transient absorption
signals in the spectral region lying between the two dominant
ground-state absorption bleaching signatures and (ii) intense NIR
transient absorption manifolds that feature extraordinary spectral
breadth and enormous absorptive extinction coefficients at λmax
-
(T1 f Tn).43 Consistent with the established spectroscopy of
[M(tpy)2]2+ and related chromophores, which have intersystem
crossing times of <200 fs,49,70-73 the intersystem crossing times
of RuPZn and OsPZn are similarly fast. As such, the early
time-delay (tdelay ∼ 360 fs) spectra already evince characteristics
of a highly polarized T1 excited state, with the hallmark NIR
absorption bands assigned as T1 f Tn. The lack of any
stimulated emission (S1 f S0) signal at even the earliest
(70) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.;
McCusker, J. K. Science 1997, 275, 54-57.
(71) Damrauer, N. H.; McCusker, J. K. J. Phys. Chem. A 1999, 103, 8440-
8446.
(72) McCusker, J. K. Acc. Chem. Res. 2003, 36, 876-887.
(73) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T J. Am. Chem.
Soc. 2002, 124, 8398-8405.
(69) While there is a large disparity in the S1 f S0 band intensities of RuPZn2
and OsPZn2 at room temperature, in a rigid butyronitrile glass at 77 K,
the OsPZn2 band is strongly intensified (see Supporting Information, Figure
S6), signifying the presence of a thermally activated quenching mechanism
in OsPZn2.
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9696 J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007

Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
observable time delays supports the conclusion that charge
separation is faster than the instrument response.43 Representa-
tive transient spectra of RuPZn and OsPZn are provided in
the Supporting Information, Figure S7.
The excited-state relaxation dynamics following photoexci-
tation of RuPZn and OsPZn are multiexponential. RuPZn*
and OsPZn* both exhibit a relaxation component with a 10-
40 ps time constant, which is associated with a substantial
increase in the intensity of the T1 f Tn NIR transient absorption
band. Processes on this time scale are typically assigned to
structural equilibration;39,41 the observed spectral changes in the
RuPZn and OsPZn excited-state spectra indicate that the
relaxed 3MLCT states of these species feature conformeric
distributions having a reduced mean torsional angle between
the porphyrin and terpyridine least-squares planes that are more
homogeneous than that for their respective initially prepared
excited states.39,41 The relaxed excited-state conformational
distributions in MPZn species augment conjugation in the
terpyridine-porphyrin π system and intensify T1 f Tn manifold
transitions (Figure S7); note that related excited-state structural
dynamics are seen in simple ruthenium polypyridyl complexes
that feature pyridyl ring aromatic substituents.71,72
Figure 2. Pump-probe transient absorption spectra recorded at several
time delays for (A) RuPZn2 and (B) OsPZn2. Experimental conditions:
solvent ) acetonitrile; λexc ) 775 nm; ambient temperature; magic angle
polarization. R ) 2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl. Scaled steady-
state absorption and emission spectra (inverted dotted lines) are displayed
for comparison.
While the torsional relaxation time constants determined in
electronically excited RuPZn and OsPZn species are nearly
identical (τ ∼ 16 ps), the respective associated amplitudes for
these processes differ greatly. Note that the 16 ps time scale
torsional relaxation process nearly doubles the T1 f Tn transient
absorption intensity for RuPZn; these dynamics intensify only
modestly this absorption manifold in electronically excited
OsPZn (Figure S7). These data suggest that OsPZn possesses
a ground-state conformational distribution having a smaller
average terpyridine-porphyrin interplanar torsional angle rela-
tive to RuPZn; this result is consistent with the facts that larger
degrees of ground-state charge-transfer character and more
metal-ligand bond covalency are manifest in osmium poly-
(pyridyl) chromophores relative to analogous ruthenium com-
plexes.49 In general, the fs-ps decay dynamics of RuPZn and
OsPZn electronically excited states do not change appreciably
as a function of excitation wavelength, indicating that higher
lying π* and CT states undergo efficient internal conversion to
the charge-separated T1 state.
state character; as such, at short time delays, the high oscillator
strength NIR absorption manifold features significant S1 f Sn
character.
Transient spectra of RuPZn2 observed at time delays >300
fs indicate that the excited-state relaxation dynamics differ
significantly relative to those evinced for RuPZn. In electroni-
cally excited RuPZn, the 3MLCT state populated on a sub 200
fs time scale persists well beyond the maximum time delay (∼7
ns) of the transient optical system utilized for these experiments.
On the other hand, the RuPZn2 excited-state spectra indicate
that near-complete ground-state recovery is achieved within 7
ns after photoexcitation. In fact, in all but the longest time delays
probed (tdelay > 1 ns, cyan line, Figure 2A), the spectra are
characterized by stimulated emission, suggesting that the primary
excited-state relaxation pathways for RuPZn2 involve either
fluorescence or internal conversion (nonradiative decay) directly
to the ground-state surface from a relaxed electronically excited-
state dominated by (porphinato)metal-terpyridyl ligand singlet
character. Note that, at tdelay ∼ 1 ns, the stimulated emission
signal contributes little to the transient spectrum, and a weak
red-shifted NIR transient absorption band is evident, indicating
a low quantum yield conversion to excited states having higher
spin multiplicity. Given the similarity of the RuPZn2 excited-
state spectral evolution with that determined for meso-to-meso
ethyne bridged (porphinato)zinc(II) (PZnn) arrays,39,41 the red-
shifted, low oscillator strength NIR transient absorption manifold
evident at long time delays arises likely from ligand-localized
T1 f Tn transitions.
D. Femtosecond Pump-Probe Transient Absorption
Spectroscopy of RuPZn2 and OsPZn2. The transient absorp-
tion spectrum of RuPZn2 (Figure 2A, black line) in acetonitrile
solvent (λexc ) 775 nm; tdelay ∼ 300 fs) exhibits many of the
same general spectral features evident in the analogous spectrum
of RuPZn (Figure S7); Figure 2A thus shows a transient
absorption signal of moderate intensity in the visible region
between the two prominent bleaching signatures and an intense
NIR transient absorption feature (λmax(T1 f Tn) ) 1046 nm).
In contrast to the early time delay transient absorption spectra
acquired for RuPZn, which exhibit no stimulated emission
signals, the RuPZn2 tdelay ∼ 300 fs transient spectrum displays
a bleaching signature due to stimulated emission, as evidenced
by the negative ∆Αbs contribution arising at the steady-state
fluorescence spectrum (Figure 2A, dotted line) λmax (∼800 nm).
These data suggest that the transient absorption features that
characterize the early time-delay RuPZn2 spectra in Figure 2A
derive in large part from a low-lying electronic state that is
Quantitatively, the dynamics of the RuPZn2 NIR S1 f Sn
transition spectral evolution can be fit to a function of four
exponentials, with time constants and associated amplitudes of
τ1 ) 974 ps (A ) 9.5), τ2 ) 244 ps (A ) 55.2), τ3 ) 10.4 ps
(A ) -14.3), and τ4 ) 3.4 ps (A ) 17.7). τ1 and τ2 contribute
to ground-state recovery (τavg ∼ 351 ps) and are assigned to
the intrinsic singlet-state lifetime, in close agreement with the
1
primarily ligand localized, possessing substantial π-π* (Qx)
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9697

A R T I C L E S
Duncan et al.
Table 2. Excited-State Electronic Spectral Dataa for RuPZn, OsPZn, RuPZn2, OsPZn2, Pyr1RuPZn, Pyr3RuPZn, Pyr1RuPZn2, and
Pyr3RuPZn2
1
c
compd
λ
max(S1 f Sn)b/nm
λ
max(T1 f Tn)b/nm (ꢀe/105 M-1 cm-
)
τCSb/ps
τCR
/
d µ
s
τFe/ps
λmax(S1 f S0)/nm
RuPZn
OsPZn
RuPZn2
OsPZn2
Pyr1RuPZn
Pyr3RuPZn
Pyr1RuPZn2
Pyr3RuPZn2
n/a
n/a
1046
1040
n/a
n/a
985
987
884 (∼0.30)
964 (∼0.49)
n/a
1140 (∼0.83)
931 (∼0.38)
958 (∼0.41)
978/1122 (∼0.66)
976/1126 (∼0.73)
<0.3
<0.3
f
∼13
<0.3
<0.3
∼170g
12 ( 2
44
0.86
f
0.87
19.9
3.5
14.2
6.2
n/a
n/a
430
9
n/a
n/a
46.0
12.6
n/a
n/a
804
798
n/a
n/a
804
805
a All spectral data were acquired in acetonitrile solvent. b λmax(S1fSn), λmax(T1 f Tn), and τCS values were determined using femtosecond transient
absorption spectroscopy; (λmax(S1 f Sn) computed at tdelay ) 300 fs; λmax(T1 f Tn) computed at tdelay ) 1 ns). c Excited-state extinction coefficients (ꢀe)
were estimated using a method described previously.43 No ꢀe values were estimated for the S1 f Sn transition due to substantial stimulated emission contributions
evident at tdelay ) 300 fs.41 d Charge-recombination (3MLCT) lifetimes were determined using ns-to-µs time-domain transient absorption measurements.
e Fluorescence lifetimes were determined via time-correlated single photon counting (TCSPC) measurements. f A spectral signature for a photoinduced
charge-separated state was not observed for this compound. g The reported 170 ps charge-separation time constant for Pyr1RuPZn2 includes contributions
due to structural relaxation.
fluorescence lifetime time constant (τF ∼ 430 ps, Table 2)
determined by time-correlated single-photon counting (TCSPC).
τ3 and τ4 contribute to positive and negative time-dependent
∆Abs changes for the S1 f Sn manifold transitions as well as
a slight bathochromic shift; τ3 and τ4 are thus assigned to
structural and solvent relaxation processes, respectively. The
excited-state spectra and dynamics are relatively insensitive to
the excitation wavelength (Supporting Information, Figure S8),
although the excited-state relaxation dynamics following excita-
tion on the high-energy side of the lowest energy electronic
absorption band λmax (λexc ) 690 nm) are characterized by larger
amplitude spectral changes associated with structural relaxation
processes than those that occur following 775 nm excitation.
This derives from the fact that optical pumping at 690 nm
preferentially excites a distribution of ground-state conformers
featuring relatively large porphyrin-terpyridine torsional angles,
whereas excitation at 775 nm interrogates a conformeric
distribution characterized by a smaller mean (porphinato)zinc-
terpyridyl torsional angle, which resembles more closely the
relaxed excited-state conformational distribution. These data and
dynamics are similar to that evinced in spectral hole-burning
experiments of oligomeric PZnn species.39,41 Consistent with
the observed, fast structural relaxation dynamics, RuPZn2
steady-state emission spectra are insensitive to excitation
wavelength.
While the RuPZn and OsPZn excited-state relaxation
dynamics are very similar, the excited-state dynamics observed
for OsPZn2 offer an interesting contrast to those evinced for
RuPZn2. Representative OsPZn2 transient absorption spectra
acquired in acetonitrile solvent after photoexcitation with 775
nm light are displayed in Figure 2B. Note that the early time-
delay OsPZn2 spectrum (tdelay ∼ 300 fs, black line) is
qualitatively very similar to that of RuZPn2 (Figure 2A) and
features a stimulated emission signal nearly identical with that
evident in the analogous RuPZn2 spectrum. These observations
support the conclusion that, at early time delays following optical
excitation (tdelay ∼ 300 fs), RuPZn2* and OsPZn2* states are
similar and dominated by 1π-π* (Qx, S1) character; the intense
NIR excited-state absorption manifold distinctive of the initially
prepared OsPZn2 excited-state is thus predominantly S1 f Sn
character, as it is in RuPZn2*.
a few ps (Figure 2B) are markedly different from analogous
spectra recorded for RuPZn2 (Figure 2A). Whereas the transient
absorption data for RuPZn2 indicate that the excited-state
complex relaxes primarily by conventional singlet-state deac-
tivation channels (i.e., fluorescence and internal conversion),
data recorded for OsPZn2* demonstrate that the initially
prepared S1 state relaxes completely (τavg ∼ 13 ps, Table 2) to
a new excited state (see for example spectra recorded at tdelay
) 10.7 and 100 ps; Figure 2B, green and blue lines) with
isosbestic behavior.74 The electronic spectrum characteristic of
this excited state exhibits the same spectral features as that
evinced for the excited state probed at 300 fs time delay, except
(i) the NIR transient absorption band λmax is red-shifted by ∼100
nm (∼845 cm-1) with respect to the S1 f Sn transition λmax
and (ii) there is no observable stimulated emission signal in
the 800-900 nm spectral region. These facts support the
assignment of this new excited state as the 3MLCT state,
analogous to the charge-separated T1 state generated after
photoexcitation of the RuPZn and OsPZn benchmarks. Note
that this state persists beyond tdelay ∼ 7 ns and that the formation
of the state occurs with almost no change in the intensity of
the B-band bleaching signature (λ ∼ 480 nm), indicating that
the quantum yield for this process approaches unity. Recall that
although steady-state OsPZn2 fluorescence was observed (vide
supra), the integrated OsPZn2 fluorescence intensity was more
than 1 order of magnitude weaker than that of RuPZn2 and the
measured OsPZn2 fluorescence lifetime (τF ∼ 9 ps) is in good
agreement with the dynamics of S1 state relaxation to the charge-
separated triplet (τcs ∼ 13 ps).
E. Excited-State Lifetimes. While the 3MLCT states of many
ruthenium bis(terpyridyl) complexes are efficiently quenched
by thermal population of the low-lying metal centered (3MC)
states (Supporting Information), the RuPZn excited state is long-
lived in deoxygenated acetonitrile solution (τ ∼ 44 µs, Table
1).43 These dynamics derive from the highly polarized, charge-
separated nature of the RuPZn and OsPZn T1 states, which
causes excited-state relaxation to be determined by the charge
recombination time scale. The attenuated Franck-Condon
overlap of RuPZn and OsPZn 3MLCT states relative to those
characteristic of emissive (polypyridyl)metal(II) complexes that
drive these dynamics originates from the extensive LUMO
In contrast to the similarity of the RuPZn2 and OsPZn2
transient absorption spectra acquired at sub-ps time delays, the
OsPZn2 pump-probe spectra recorded at time delays later than
(74) The isosbestic behavior is not perfect; see spectra recorded at time delays
>10 ps. This discrepancy is likely due to structural relaxation processes
akin to those observed in RuPZn* and OsPZn* that occur concomitant
with or following the S1
f
3MLCT relaxation process.
9
9698 J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007

Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
Scheme 3. RuPZn, OsPZn, RuPZn2, and OsPZn2 Jablonski Diagrams Illustrating the Relative Energetic Arrangement of Selected Electronic
States and Relevant Excited-State Relaxation Processesa
a State energy level separations are not to scale. Dotted green arrows represent nonradiative internal conversion and intersystem crossing processes, solid
red arrows represent radiative processes (fluorescence), and dotted red arrows represent nonradiative charge-recombination (CR) processes. Measured lifetimes
(τ) for relaxation of the lowest energy electronic states are indicated. Note that the 3MLCT label is synonymous with the long-lived charge-separated T1 state
characteristic of many relaxed MPZnn* species; we use this label for both brevity and emphasis of the connection of these MPZnn* states with that of the
[Ru(tpy)2]2+* benchmark. R ) 2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl.
stabilization afforded by the conjugated (porphinato)zinc(II)-
Scheme 3, the 3MLCT label is synonymous with the long-lived
charge-separated T1 state characteristic of many relaxed MPZnn*
species; we use this label for both brevity and to emphasize the
ethyne moiety. While conjugation augmentation that increases
3
[Ru(tpy)2]2+-type MLCT lifetimes has been demonstrated,75
it should be emphasized, however, that the room-temperature
excited-state lifetimes of such species are several orders of
magnitude smaller than those elucidated for extensively delo-
calized, charge-separated T1-states of RuPZn and OsPZn. The
charge-separated T1-state of OsPZn features a diminished
lifetime (τ ∼ 0.8 µs, Table 1) relative to RuPZn*, due to the
increased spin-orbit coupling constant of osmium.
connection of these MPZnn* states to that of the [Ru(tpy)2]2+
*
benchmark. LUMO stabilization (Scheme 3) causes RuPZnn*
to feature a diminished 3MLCT energy relative to [Ru(tpy)2]2+
*
3
(Scheme S6) while not impacting the energy level of the MC
state. It is well-established that the (polypyridyl)metal(II) 3MC-
3MLCT energy gap is a dominant factor in determining the
photophysical properties of these chromophores;47-50,75-80 in
RuPZn, because the 3MLCT state lies far below the 3MC energy
level (Scheme 3), the lifetime of the RuPZn charge-separated
T1 state remains long. Relative to RuPZn, the 3MLCT level of
OsPZn is stabilized further, while the 3MC level lies higher in
energy due in large part to osmium’s lower M2+/3+ potential
(Table 1); as the EAS and electrochemical data (Table 1) show
that the RuPZn and OsPZn1π-π* levels are energetically
Oxygen efficiently quenches the 3MLCT states of both
compounds (Supporting Information, Figures S9A and S9G),
consistent with the higher order (S ) 1) spin multiplicities of
these chromophores.43 Congruent with the formation of a charge-
separated T1 state, the OsPZn2 excited-state lifetime is also long
(τ ∼ 0.87 µs). While the triplet-state lifetime of RuPZn2* was
also measured (τT ∼ 23 µs), its low oscillator strength NIR
transient absorption manifold evident at long time delays arises
from ligand-localized T1 f Tn transitions (vide supra) and thus
is unrelated to the RuPZn and OsPZn2 3MLCT states which
were formed at high quantum yield and feature substantial
charge-separated character. The excited-state lifetimes of these
compounds are listed in Table 2, and representative spectra and
dynamics are plotted in the Supporting Information, Figures S9
and S10.
F. Relative State Energies for MPZn and MPZn2 Chro-
mophores. On the basis of these transient spectral data and the
well-established photophysical properties of [M(tpy)2]2+ com-
plexes (see Supporting Information), a clear picture of the factors
that determine whether or not a long-lived charge-separated T1
state is formed in high yield following photoexcitation of MPZn
and MPZn2 chromophores can be obtained; these insights are
summarized in the Scheme 3 Jablonski diagrams. Note that, in
3
similar, quenching of the OsPZn MLCT state by the higher
3
lying MC states is also not observed.
3
As described above, a long-lived MLCT (charge-separated
T1) state of RuPZn2 is not populated appreciably at long time
delays following photoexcitation. This observation stems from
the fact that, in RuPZn2, the low-lying1π-π* (Qx) state is
stabilized dramatically but the 3MLCT state energy is lowered
in energy by a comparatively small degree relative to the
analogous RuPZn state; this results in an inversion of the
3
energetic ordering of the MLCT and Qx excited states, with
the latter being the lower energy state in RuPZn2. As a result,
(76) Kirchhoff, J. R.; McMillin, D. R.; Marnot, P. A.; Sauvage, J.-P. J. Am.
Chem. Soc. 1985, 107, 1138-1141.
(77) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer,
T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627.
(78) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar, J.
V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755-2763.
(79) Liu, D. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Am. Chem. Soc.
1986, 108, 1749-1755.
(75) Hung, C.-Y.; Wang, T.-L.; Jang, Y. C.; Kim, W. Y.; Schmehl, R. H.;
(80) Hecker, C. R.; Gushurst, A. K. I.; McMillin, D. R. Inorg. Chem. 1991, 30
538-541.
Thummel, R. P. Inorg. Chem. 1996, 35, 5953-5956.
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9699

A R T I C L E S
Duncan et al.
the excited-state relaxation dynamics of RuPZn2 resemble those
of ethyne-bridged oligomeric (porphinato)zinc(II) PZnn arrays,
with fluorescence and internal conversion being the primary
pathways for excited singlet-state deactivation.39,41 On the other
hand, electronic excitation of OsPZn2 results in rapid excited-
state relaxation to a long-lived charge-separated T1 state (labeled
3MLCT in Scheme 3) with near-unity quantum yield, although
the process (τCS ∼ 13 ps) is much slower than that observed in
the MPZn benchmarks (τCS < 300 fs); note also that some
fluorescence from OsPZn2* (energetically identical with that
observed from RuPZn2) is still observed. As highlighted in the
3
OsPZn2 EAS (Figure 1), the MLCT and S1 states appear to
lie in close energetic proximity to one another (vide supra). Low-
temperature (77 K) emission spectroscopy (Supporting Informa-
tion) suggests that there is a slight thermal activation barrier
for S1-3MLCT intersystem crossing, which is also consistent
with the observed OsPZn2* and OsPZn* charge-separation
dynamics, underscoring that this energy gap is a primary
determinant of the fate of the initially prepared excited state in
these MPZnn chromophores.
Figure 3. Comparative electronic absorption spectra of (A) RuPZn (black),
Pyr1RuPZn (red), and Pyr3RuPZn (blue) and (B) RuPZn2 (black),
Pyr1RuPZn2 (red), and Pyr3RuPZn2 (blue) recorded in acetonitrile solvent.
The inset shows the corresponding emission spectra (λexc ) 700 nm) of
RuPZn2 (black), Pyr1RuPZn2 (red), and Pyr3RuPZn2 (blue) recorded at
room temperature and corrected for concentration. R ) 2′,6′-bis(3,3-
dimethyl-1-butyloxy)phenyl.
3. PyrmRuPZn and PyrmRuPZn2. A. Design and Poten-
tiometric Data. Given the excited-state dynamical properties
of RuPZn2 and OsPZn2, it is clear that further electronic
modification of the MPZnn motif is required to develop
chromophores that possess the intense and wavelength-tunable
NIR excited-state absorptive properties of PZnn species, with
the characteristic µs excited-state relaxation dynamics of MPZn
chromophores. For example, further extension of the MPZn2
conjugation with additional (meso-ethynylporphinato)zinc(II)
units would only enhance the S1-3MLCT energy gap and give
rise to chromophores that possess nanosecond excited-state
lifetimes, in which the initially prepared excited state relaxes
primarily via fluorescence and nonradiative internal conversion.
diminishing ruthenium oxidation potential. Note that the largest
Ru2+/3+ potentiometric perturbation in this series of chromo-
phores occurs after adding a single pyrrolidine unit, with the
additional two pyrrolidines having a comparatively smaller influ-
ence [E1/2(Ru2+/3+)(RuPZn) - E1/2(Ru2+/3+)(Pyr1RuPZn) ∼
400 mV; E1/2(Ru2+/3+)(Pyr1RuPZn) - E1/2(Ru2+/3+)(Pyr3RuPZn)
∼ 180 mV], consistent with established effects of multiple
substitution in aromatic systems.84 Note also that there is little
or no change in the measured E1/2(Ru2+/3+) value in related
derivatives featuring identical degrees of pyrrolidinyl substi-
tution [E1/2(Ru2+/3+)(Pyr1RuPZn) ) 0.99 V, E1/2(Ru2+/3+)-
(Pyr1RuPZn2) ) 1.01 V; E1/2(Ru2+/3+)(Pyr3RuPZn) )
Because the most significant difference between RuPZn2 and
OsPZn2 complexes is the oxidation potential of the terpyridine-
bound metal ion, which directly impacts the 3MLCT state
2+/3+
energy, lowering the M
E
1/2
value by an appropriate margin
without significantly altering key π-framework orbital energy
levels should drive the development of RuPZn2-derived species
E
1/2(Ru2+/3+)(Pyr3RuPZn2) ) 0.81 V], reflecting the weak
3
in which long-lived MLCT (charge-separated T1) states are
influence of the additional (meso-ethynylporphinato)zinc(II) unit
on the metal oxidation potential; additionally, while the ruthen-
ium oxidation potentials change significantly as a function of
the number of appended pyrrolidine units, the PZnn-terpyridine
oxidative85 electrochemistry [E1/2(PZn0/+)] is relatively unper-
turbed (Table 1), reflecting the fact that attenuating the metal
center oxidation potential does not significantly influence the
HOMO/LUMO ligand orbital energies.
B. Electronic Absorption and Fluorescence Spectroscopy
of PyrmRuPZn and PyrmRuPZn2 Chromophores. The com-
parative electronic absorption spectra of RuPZn, Pyr1RuPZn,
and Pyr3RuPZn are displayed in Figure 3A, whereas the
analogous spectra of RuPZn2, Pyr1RuPZn2, and Pyr3RuPZn2
are shown in Figure 3B; selected transitions are compared in
efficiently populated. To that end, PyrmRuPZnn derivatives
were synthesized, which feature electron-donating pyrrolidinyl
groups attached to the ancillary terpyridine ligand in the 4′-
(Pyr1RuPZn and Pyr1RuPZn2) and 4,4′,4′′- (Pyr3RuPZn and
Pyr3RuPZn2) positions (Chart 2). It should be noted that there
is literature precedent for using dialkylamino electron-donating
groups59,81-83 to lower the ruthenium oxidation potential in
(polypyridyl)ruthenium(II) complexes and, consequently, modu-
3
late MLCT transition energies.
Potentiometric data acquired for Pyr1RuPZn, Pyr1RuPZn2,
Pyr3RuPZn, Pyr3RuPZn2, RuPZn, and RuPZn2, as well as
for RuBr, Pyr1RuBr, and Pyr3RuBr building blocks, are
chronicled in Table 1 (see also Supporting Information, Figure
S11). These data show that the measured Ru2+/3+ E1/2 values
decrease as pyrrolidine units are added to the ancillary terpyridyl
ligand due to the augmented electron density on the ruthenium
center; an increasing red-shift of the RuBr (660 nm), Pyr1RuBr
(709 nm), and Pyr3RuBr (750 nm) luminescence band λmax
(solvent ) acetonitrile, T ) 77 K; see Supporting Information,
Figure S12) is a spectroscopic manifestation of the progressively
(81) Constable, E. C. New J. Chem. 1992, 16, 855.
(82) Slattery, S.; Gokaldas, N.; Mick, T.; Goldsby, K. A. Inorg. Chem. 1994,
33, 3621-3624.
(83) Hathcock, D. J.; Stone, K.; Madden, J.; Slattery, S. Inorg. Chim. Acta 1998,
282, 131-135.
(84) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.
(85) The porphyrin-terpyridine reductive electrochemistry does not appear to
change either; however, the reductive processes are in most cases
irreversible; PZn0/-1 E1/2 values are thus not listed in Table 1.
9
9700 J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007

Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
Table 1. Comparative EAS of monomeric precursors RuBr,
Pyr1RuBr, and Pyr3RuBr are provided in the Supporting
Information (Figure S13). Data highlighted in the Figure 3
spectra and Table 1 data show that, in both the PyrmRuPZn
1
and PyrmRuPZn2 complexes, the MLCT transition red-shifts
with increasing numbers of attached pyrrolidine groups, but the
PZnn-terpyridine derived Qx-state (S0 f 1π-π*) transition λmax
remains relatively unperturbed, indicating that the 3MLCT states
of Pyr1RuPZn2 and Pyr3RuPZn2 lie lower than that of
RuPZn2. Consistent with the electrochemical results, the largest
change in the 1MLCT energy occurs with the RuPZnn-to-
Pyr1RuPZnn electronic structural modification [λmax(1MLCT)-
(RuPZn) - λmax(1MLCT)(Pyr1RuPZn) ) 15 nm (∼570 cm-1);
λ
max(1MLCT)(RuPZn2) - λmax(1MLCT)(Pyr1RuPZn2) ) 11
nm (∼406 cm-1)], with the corresponding Pyr1RuPZnn-to-
Pyr3RuPZnn perturbation producing a more modest shift in
this parameter [λmax(1MLCT)(Pyr1RuPZn) - λmax(1MLCT)-
(Pyr3RuPZn) ) 7 nm (∼255 cm-1); λmax(1MLCT)(Pyr1RuPZn2)
- λmax(1MLCT)(Pyr3RuPZn2) ) 7 nm (∼250 cm-1)].
The Figure 3 inset shows the emission spectra of RuPZn2,
Pyr1RuPZn2, and Pyr3RuPZn2, normalized for their respective
absorptions at the excitation wavelength; these data demonstrate
that pyrrolidine groups effect a dramatic quenching of the
PyrmRuPZn2 emission intensity relative to the RuPZn2 bench-
mark and now resemble the corresponding normalized OsPZn2
emission spectrum (Figure 1). Insofar as the RuPZn2 emission
band was assigned to fluorescence from an excited-state
dominated by 1π-π* (Qx, S1) character due to the large
energetic barrier between this and the 3MLCT state (vide supra),
the significant drop in fluorescence intensity in Pyr1RuPZn2
and Pyr3RuPZn2 relative to this benchmark is consistent with
a significantly stabilized 3MLCT state and efficient intersystem
crossing in these pyrrolidine-derivatized chromophores.
Figure 4. Pump-probe transient absorption spectra recorded at several
time delays for (A) Pyr1RuPZn and (B) Pyr3RuPZn. Experimental
conditions: solvent ) acetonitrile; λexc ) 775 nm; ambient temperature;
magic angle polarization. Scaled steady-state absorption and emission spectra
(inverted dotted lines) are displayed for comparison. R ) 2′,6′-bis(3,3-
dimethyl-1-butyloxy)phenyl.
C. Femtosecond Pump-Probe Transient Absorption
Spectroscopy of PyrmRuPZn and PyrmRuPZn2 Species.
Analogous to the ultrafast transient absorption spectroscopic
experiments described previously for MPZn and MPZn2
chromophores, PyrmRuPZn and PyrmRuPZn2 species were
interrogated by pump-probe transient absorption spectroscopy.
Representative transient absorption spectra at selected time
delays are displayed for Pyr1RuPZn, Pyr3RuPZn, Pyr1RuPZn2,
and Pyr3RuPZn2 (Figures 4 and 5); key spectral data are
highlighted in Table 2. PyrmRuPZn and PyrmRuPZn2 transient
spectra exhibit the same absorptive and bleaching features
evident in the analogous MPZn and MPZn2 spectra. At early
time delay (tdelay ∼ 350 ps), Pyr1RuPZn and Pyr3RuPZn
transient spectra (Figure 4) exhibit the hallmarks of charge-
separated T1 states (i.e., no stimulated emission signature and
Figure 5. Pump-probe transient absorption spectra recorded at several
time delays for (A) Pyr1RuPZn2 and (B) Pyr3RuPZn2. Experimental
conditions: solvent ) acetonitrile; λexc ) 775 nm; ambient temperature;
magic angle polarization. Scaled steady-state absorption and emission spectra
(inverted dotted lines) are displayed for comparison. R ) 2′,6′-bis(3,3-
dimethyl-1-butyloxy)phenyl.
3
fast intersystem crossing to the MLCT manifold). Note that
the NIR (T1 f Tn) transition manifold λmax for Pyr1RuPZn
[λmax(T1 f Tn) ) 931 nm] and Pyr3RuPZn [λmax(T1 f Tn) )
985 nm] are red-shifted (Table 2) with respect to that observed
for RuPZn [λmax(T1 f Tn) ) 884 nm]. The initially prepared
Pyr1RuPZn and Pyr3RuPZn electronically excited states form
species. The NIR excited-state absorption bands of both
Pyr1RuPZn and Pyr3RuPZn exhibit extraordinary spectral
breadths (∼2500 cm-1) and intensities [ꢀe @ λmax(T1 f Tn) ∼
4 × 104 M-1 cm-1; tdelay ) 1 ns]. Additionally, as observed in
the excited-state spectra of RuPZn (Figure 2), during the first
50 ps following photoexcitation, a 25-30% growth in the
Pyr1RuPZn and Pyr3RuPZn T1 f Tn transient absorption band
intensity is manifest, consistent with structural relaxation
3
the corresponding MLCT states in the ultrafast time domain
(τCS < 300 fs), similar to that observed for charge-separated T1
state formation in the early time pump-probe spectra of the
unelaborated RuPZn* and OsPZn* benchmarks; these dynam-
3
ics are consistent with a MLCT state lying well below the
electronic state composed primarily of Qx character for these
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9701

A R T I C L E S
Duncan et al.
Scheme 4. RuPZn2, Pyr1RuPZn2, and Pyr3RuPZn2 Jablonski Diagrams Illustrating the Relative Energetic Arrangement of Selected
Electronic States and Relevant Excited-State Relaxation Processesa
a State energy level separations are not to scale. Dotted green arrows represent nonradiative internal conversion and intersystem crossing processes, solid
red arrows represent radiative processes (fluorescence), and dotted red arrows represent nonradiative charge-recombination (CR) processes. Measured lifetimes
(τ) for relaxation of the lowest energy electronic states are indicated, as well as the respective observed emissive properties. R ) 2′,6′-bis(3,3-dimethyl-1-
butyloxy)phenyl.
processes that occur along the charge-separated T1-state
(3MLCT) surface.
2; Supporting Information, Figure S9). Note that PyrmRuPZn
and PyrmRuPZn2 species exhibit room-temperature charge-
separated T1-state lifetimes ranging from 3.5 to 20 µs, which
are significantly longer than that exhibited by corresponding
OsPZn and OsPZn2 chromophores (Table 2). Consistent with
the high-spin multiplicities of the relaxed excited states of these
species, appreciably reduced lifetimes are manifest in the
presence of dioxygen.
While the excited-state relaxation dynamics for
RuPZn, Pyr1RuPZn, and Pyr3RuPZn are similar, the excited-
state relaxation dynamics of Pyr1RuPZn2 (Figure 5A) and
Pyr3RuPZn2 (Figure 5B) contrast those exhibited by RuPZn2.
As observed in the analogous spectrum of RuPZn2, the
Pyr1RuPZn2 and Pyr3RuPZn2 early time (tdelay ∼ 350 fs)
transient absorption spectra exhibit (i) strong stimulated emission
signals that are coincident with their respective steady-state
fluorescence spectra and (ii) intense NIR transitions centered
at ∼986 nm that resemble that manifest by electronically excited
D. Relative State Energies for PyrmRuPZn and Pyrm
-
RuPZn2 Chromophores. From potentiometric data and the
electronic absorption, fluorescence, and transient absorption
spectroscopic experiments described above, comparative quali-
tative Jablonski diagrams illustrating the excited-state and
relaxation dynamics of RuPZn2, Pyr1RuPZn2, and Pyr3RuPZn2
were constructed (Scheme 4). As illustrated by the analogous
Jablonski diagrams for RuPZn2 and OsPZn2 displayed in
Scheme 3, the dominant factor in determining the nature of the
lowest lying excited state is the S1-3MLCT energy gap. In the
RuPZn2 benchmark, the ruthenium center lies at high potential
[E1/2(Ru2+/3+) ) 1.39 V], leading to a relatively high-energy
3MLCT state. The energy gap between the low-lying excited
39
PZn2 and, thus, likely feature extensive singlet character.
While the nanosecond excited-state decay time scale dynamics
for RuPZn2 featured ground-state recovery via fluorescence or
direct internal conversion to the S0 surface (Figure 2), the S1
f Sn transient absorption feature characteristic of the initially
prepared Pyr1RuPZn2 and Pyr3RuPZn2 electronically excited
states decays on faster time scales (τavg ∼170 and 12 ps,
respectively); the decay of these transients is accompanied by
the concurrent growth of a new excited-state absorptive manifold
for these complexes [λmax(T1 f Tn)(Pyr1RuPZn2) ) 1122 nm;
3
state composed predominantly of Qx (S1) character and the -
λ
max(T1 f Tn)(Pyr3RuPZn2) ) 1126 nm] and the simultaneous
MLCT level is large with respect to kT at ambient temperature,
and RuPZn2* displays dynamics similar to that manifested by
conventional meso-to-meso ethyne-bridged (porphinato)zinc(II)
(PZnn) arrays (high quantum yield S1 f S0 fluorescence, S1 f
S0 internal conversion, and low quantum yield S1 f T1
intersystem crossing).39,41 In contrast, Pyr3RuPZn2 possesses
loss of the stimulated emission signal centered at ∼825 nm.
The time scale for depopulation of the initially prepared excited
state that features substantial singlet character is more than 1
order of magnitude faster in Pyr3RuPZn2* than in Pyr1RuPZn2*,
consistent with (i) the expectation that the magnitude of this
time constant should track inversely with the Qx-3MLCT state
energy gap, (ii) the fluorescence spectra determined for these
complexes (Figure 3), and (iii) TCSPC spectroscopic data
(Table 2).
a much lower potential metal terpyridyl center (E1/2(Ru2+/3+
)
3
) 0.81 V), which diminishes its corresponding MLCT state
energy. Due to potentiometric and spectroscopic properties
described previously, the Qx states of RuPZn2 and Pyr3RuPZn2
are nearly isoenergetic; Pyr3RuPZn2 thus possesses a low-
energy 3MLCT state, making possible the light-driven produc-
The 3MLCT lifetimes of all of these species were determined
from ns-µs time scale transient absorption spectroscopy (Table
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Highly Conjugated Zinc/Metal(II) Supermolecules
A R T I C L E S
tion of a charge-separated T1 state in which the time scale for
ground-state recovery lies in the microsecond regime and is
determined by charge-recombination dynamics. Note that
Pyr1RuPZn2 exhibits relaxation dynamics midway between
those discussed for RuPZn2 and Pyr3RuPZn2. Because the
Pyr1RuPZn2 ruthenium center possesses an intermediate po-
tential [E1/2(Ru2+/3+) ) 0.98 V], its S1-3MLCT gap is dimin-
ished with respect to that for Pyr1RuPZn2; due to this modest
energy gap, the Pyr1RuPZn2 initially prepared excited state
manifests a lower driving force for formation of a charge-
separated T1 state and thus exhibits steady-state fluorescence
intensity intermediate between that observed for RuPZn2 and
Pyr3RuPZn2 (Figure 3).
scale) excited states that feature intense absorption bands which
extend deep into the near-infrared energy regime, (ii) terpyridyl
pyrrolidinyl substituents can be utilized to diminish the
E
1/2(M3+/2+) value of the bis(terpyridyl)metal(II) center (such
perturbations determine the relative energies of the PZnn-derived
1π-π* and bis(terpyridyl)metal(II) charge-transfer states and
establish whether the T1-state wave functions of MPZnn and
PyrmMPZnn species manifest the extensive electronic delocal-
ization and charge-separated features characteristic of long-lived
triplet states that absorb strongly in the NIR), (iii) while the
excited-state dynamics and lifetimes of these complexes are
unusually sensitive to the magnitude of the S1-3MLCT energy
gap that is controlled by the pyrrolidinyl substitution, Pyrm
-
MPZnn compounds possess ground-state absorptive properties
that are nearly identical with their respective MPZnn bench-
marks, and (iv) because this new class of chromophores possess
many properties (µs excited-state lifetimes, solubility in a wide
range of organic solvents, highly polarized low-energy excited
states, and extremely large ground-state and excited-state molar
absorptivities over large, technologically important wavelength
windows) that have heretofore been without precedent in single
chromophoric entities, PyrmMPZnn compounds and related
structures may find unique utility in a wide range of optoelec-
tronic applications.
Conclusions
A new series of chromophores, MPZnn, which combine
ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II)
(MPZn) and oligomeric, ethyne-bridged (porphinato)zinc(II)
(PZnn) architectures, have been synthesized and characterized,
along with a series of derivatives bearing pyrrolidinyl
electron-releasing groups on the ancillary terpyridine units
(PyrmMPZnn). Cyclic voltammetric studies as well as NMR,
electronic absorption, fluorescence, and femtosecond pump-
probe transient absorption spectroscopies have been employed
to study the ground- and excited-state properties of these unusual
chromophores. All of these species possess intensely absorbing
excited states having large spectral bandwidth that penetrate deep
in the near-infrared (NIR) energy regime. Electronic structural
variation of the molecular framework shows that the excited-
Acknowledgment. This work was supported by a grant from
the Department of Energy (DE-FG02-04ER46156). T.V.D
respectfully acknowledges a Carolyn Hoff Lynch Graduate
Fellowship, and T.I. gratefully thanks the Japan Society for the
Promotion of Science (JSPS) for a postdoctoral fellowship. We
acknowledge the MRSEC and NSEC programs of the National
Science Foundation for infrastructural support and express their
gratitude to Dr. Thomas Troxler, Dr. Louise E. Sinks, and Mr.
Ian Stanton at the University of Pennsylvania for fluorescence
lifetime measurements.
state absorption maximum can be extensively modulated [λmax
-
(T1 f Tn)] (880 nm < λmax < 1126 nm), while concomitantly
maintaining impressively large T1 f Tn absorption manifold
spectral bandwidth [full width at half-maximum, fwhm, ∼2000-
2500 cm-1]. These studies further correlate supermolecular
electronic structure with the magnitude of the excited-state
lifetime (τes) and demonstrate that this parameter can be
modulated extraordinarily over 4 orders of magnitude (∼1 ns
< τes < 45 µs).
This work highlights that (i) coupled oscillator photophysics
and metal-mediated cross-coupling reactions can be exploited
to create highly conjugated (polypyridyl)metal(II)-(porphinato)-
zinc(II) supermolecules which exhibit long-lived (ns-µs time
Supporting Information Available: Synthetic details and
characterization data for precursor compounds, a review of
metal(II)-bis(terpyridyl) photophysics, and supporting figures
and schemes. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0707512
9
J. AM. CHEM. SOC. VOL. 129, NO. 31, 2007 9703