Potentiometric, electronic structural, and ground- and excited-state optical properties of conjugated bis[(porphinato)zinc(II)] compounds featuring proquinoidal spacer units

Article abstract of DOI:10.1021/ja040243h

We report the synthesis, optical, electrochemical, electronic structural, and transient optical properties of conjugated (porphinato)zinc(II)-spacer- (porphinato)zinc(II) (PZn-Sp-PZn) complexes that possess intervening conjugated Sp structures having varying degrees of proquinoidal character. These supermolecular PZn-Sp-PZn compounds feature Sp moieties {(4,7-diethynylbenzo[c] [1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole) (E-BBTD-E)} that regulate frontier orbital energy levels and progressively increase the extent of the quinoidal resonance contribution to the ground and electronically excited states, augmenting the magnitude of electronic communication between terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to that evinced for a bis[(5,5′,-10,20-di(aryl)porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn). Electronic absorption spectra show significant red-shifts of the respective PZn-Sp-PZn x-polarized Q state (S₀ → S₁) transition manifold maxima (240-4810 cm^-1) relative to that observed for PZnE-EPZn. Likewise, the potentiometrically determined PZn-Sp-PZn HOMO-LUMO gaps (E_1/2^0/+ - E_1/2^-/0) display correspondingly diminished energy separations that range from 1.88 to 1.11 eV relative to that determined for PZnE-EPZn (2.01 eV). Electronic structure calculations provide insight into the origin of the observed PZn-Sp-PZn electronic and optical properties. Pump-probe transient spectral data for these PZn-Sp-PZn supermolecules demonstrate that the S₁ → S _n transition manifolds of these species span an unusually broad spectral domain of the NIR. Notably, the absorption maxima of these S ₁ → S_n manifolds can be tuned over a 1000-1600 nm spectral region, giving rise to intense excited-state transitions ～4000 cm^-1 lower in energy than that observed for the analogous excited-state absorption maximum of the PZnE-EPZn benchmark; these data highlight the unusually large quinoidal resonance contribution to the low-lying electronically excited singlet states of these PZn-Sp-PZn species. The fact that the length scales of the PZn-Sp-PZn species (～25 A) are small with respect to those of classic conducting polymers, yet possess NIR S₁ → S_n manifold absorptions lower in energy, underscore the unusual electrooptic properties of these conjugated structures.

Full text of DOI:10.1021/ja040243h

Published on Web 03/19/2005
Potentiometric, Electronic Structural, and Ground- and
Excited-State Optical Properties of Conjugated
Bis[(Porphinato)zinc(II)] Compounds Featuring Proquinoidal
Spacer Units
Kimihiro Susumu, Timothy V. Duncan, and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received October 28, 2004; E-mail: therien@sas.upenn.edu
Abstract: We report the synthesis, optical, electrochemical, electronic structural, and transient optical
properties of conjugated (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes that
possess intervening conjugated Sp structures having varying degrees of proquinoidal character. These
supermolecular PZn-Sp-PZn compounds feature Sp moieties {(4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-
BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline
(E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole) (E-BBTD-E)} that regulate frontier
orbital energy levels and progressively increase the extent of the quinoidal resonance contribution to the
ground and electronically excited states, augmenting the magnitude of electronic communication between
terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to that evinced for a bis[(5,5′,-10,20-di(aryl)-
porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn). Electronic absorption spectra show significant red-
shifts of the respective PZn-Sp-PZn x-polarized Q state (S
0
f S
1
) transition manifold maxima (240-4810
-
1
cm ) relative to that observed for PZnE-EPZn. Likewise, the potentiometrically determined PZn-Sp-PZn
HOMO-LUMO gaps (E1/20 - E1/2 ) display correspondingly diminished energy separations that range
from 1.88 to 1.11 eV relative to that determined for PZnE-EPZn (2.01 eV). Electronic structure calculations
provide insight into the origin of the observed PZn-Sp-PZn electronic and optical properties. Pump-probe
/+
-/0
transient spectral data for these PZn-Sp-PZn supermolecules demonstrate that the S
manifolds of these species span an unusually broad spectral domain of the NIR. Notably, the absorption
maxima of these S f S manifolds can be tuned over a 1000-1600 nm spectral region, giving rise to
1 n
f S transition
1
n
-1
intense excited-state transitions ∼4000 cm lower in energy than that observed for the analogous excited-
state absorption maximum of the PZnE-EPZn benchmark; these data highlight the unusually large quinoidal
resonance contribution to the low-lying electronically excited singlet states of these PZn-Sp-PZn species.
The fact that the length scales of the PZn-Sp-PZn species (∼25 Å) are small with respect to those of
1 n
classic conducting polymers, yet possess NIR S f S manifold absorptions lower in energy, underscore
the unusual electrooptic properties of these conjugated structures.
Introduction
species play crucial roles in optimizing the performance of
electronic and optical devices based on active organic com-
ponents.
Porphyrins are tetrapyrrolic conjugated macrocyclic systems
Organic π-conjugated oligomers and polymers constitute a
class of promising semiconducting materials having demon-
strated utility in device applications ranging from light-emitting
9-16
that possess modest potentiometrically determined HOMO-
1
,2
3,4
5,6
diodes, photovoltaic cells, field-effect transistors, to
nonlinear optics.7 Reducing and tuning energy gaps between
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of such π-conjugated
0
/+
-/0
LUMO gaps (Ep; E1/2
- E1/2 ) relative to those of the
,8
common monomeric aromatic building blocks used to construct
traditional electronic polymers. The electronic properties of
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10.1021/ja040243h CCC: $30.25 © 2005 American Chemical Society

Proquinoidal Bis[(Porphinato)zinc(II)] Compounds
A R T I C L E S
(porphinato)metal compounds can be modulated extensively by
Scheme 1
variation of the macrocycle peripheral meso- or â-substituents,
as well as by selection of the central metal ion; further, a variety
of modes of porphyrinoid-porphyrinoid connectivity provides
sufficiently strong interchromophore electronic interactions to
facilitate extensive electronic delocalization.1
7-42
Of these
families of multipigment ensembles that feature substantial
ground- and excited-state interchromophore electronic interac-
tions, those that feature direct ethyne-, butadiyne-, and oligoyne-
based macrocycle-to-macrocycle connectivity have evinced a
degree of cumulenic (quinoidal) character. Porphyrin-to-por-
phyrin bridging motifs involving ethynes and spacers that induce
a quinoidal structural perturbation with appropriately positioned
frontier orbital energy levels, should enhance ground- and
excited-state π-conjugation, and effect further reduction in Eop
and Ep in the corresponding oligomeric and polymeric structures.
Polymer band-gap reduction through augmentation of π-back-
bone quinoidal character has been explored both experimen-
wide range of particularly impressive electrooptic pro-
perties.1
7-24,28-31,36,37
As increasing conjugation length dimin-
ishes significantly optical (Eop) and potentiometric (Ep) band
gaps within these families of structures, multiporphyrin com-
pounds that exploit cylindrically π-symmetric linkers define a
point of reference from which to engineer further electronic
modulation of conjugated organic materials.
An established means to further reduce the Eop and Ep gaps
of π-conjugated materials involves introducing quinoid-like
45,46
47,48
tally
and theoretically.
In this regard, benzo[1,2-c:4,5-
c′]bis([1,2,5]thiadiazole) (BBTD), exemplifies an established
conjugated unit with suitable electronic structure to induce
49-52
substantial quinoidal character in a conjugated backbone.
1
0,13,14
character into the conjugation main-chain.
Solution-phase
Because BBTD features large atomic orbital coefficients at its
bridging 4- and 8-positions in both the HOMO and LUMO,
excellent interchromophore electronic delocalization would be
17,18,21,43
spectroscopic experiments
and X-ray crystallographic
44
data obtained for bis[(5,5′,-10,20-di(aryl)porphinato)zinc(II)]-
ethyne compounds demonstrate that the bridging ethyne pos-
sesses conventional triple bond character in the ground state;
anticipated between [(5,-10,20-di(aryl)porphinato)zinc(II)]ethy-
20,21,43,53,54
nyl (PZnE) units appended at these positions;
struc-
electronic absorption,1
7,18,20,21
electroabsorption, and pump-
20
tural relaxation toward the stable form of BBTD’s two 1,2,5-
thiadiazole rings would therefore drive an increasing contribution
of the cumulenic resonance form to the ground- (S0) and low-
lying electronically excited singlet-state (S1) wave functions
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9,22
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9,60
4,9-diethynyl-6,7-dimethyl-
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J. AM. CHEM. SOC.
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VOL. 127, NO. 14, 2005 5187

A R T I C L E S
Susumu et al.
Figure 1. Structures of the bis[(porphinato)zinc(II)] derivatives PZnE-EPZn, PZnE-BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-EPZn, and PZnE-BBTD-
EPZn along with key reference compounds.
PZnE units relative to that evinced for a bis[(5,5′,-10,20-di-
aryl)porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn).
Electronic structural differences, as well as the relative mag-
nitudes of the optical (Eop) and potentiometric (Ep) band gaps
of these new conjugated PZn-Sp-PZn structures are rationalized
within the context of perturbation theory.
corresponding solvent/supporting electrolyte solution. The ferrocene/
ferrocenium redox couple was utilized as an internal potentiometric
standard.
(
Electronic Structure Calculations. All electronic structure calcula-
61
tions were carried out using the GAUSSIAN 98 programs. Geometry
optimizations and semiempirical electronic structural calculations were
performed using the PM3 method. To minimize computational effort,
the solubilizing alkoxy substituents of the PZn-Sp-PZn structures, the
PZnE trimethylsilyl group, and the methyl groups of 6,7-dimethyl-
Experimental Section
[
1,2,5]thiadiazolo[3,4-g]quinoxaline were replaced by hydrogen. Struc-
Materials. Synthetic procedures and characterization data for new
compounds [4,7-bis[(10,20-bis[2′,6′-bis(3,3-dimethyl-1-butyloxy)phen-
yl]porphinato)zinc(II)-5-ylethynyl]benzo[c][1,2,5]thiadiazole (PZnE-
BTD-EPZn), 6,13-bis[(10,20-bis[2′,6′-bis(3,3-dimethyl-1-butyloxy)-
phenyl]porphinato)zinc(II)-5-ylethynyl]pentacene (PZnE-PC-EPZn),
tural models for PZnE-EPZn, PC, BBTD, PZnE-PC-EPZn, and
PZnE-BBTD-EPZn were optimized within D2h constraints, while
optimal PZnE, BTD, TDQ, PZnE-BTD-EPZn and PZnE-TDQ-EPZn
structures were computed for C2V symmetry. Orbital contour plots were
62
visualized with the gOpenMol program.
4
,9-bis[(10,20-bis[2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)-
zinc(II)-5-ylethynyl]-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline
PZnE-TDQ-EPZn), 4,8-bis[(10,20-bis[2′,6′-bis(3,3-dimethyl-1-butyl-
Ultrafast Transient Absorption Experiments. Experimental details
are provided in the Supporting Information.
(
Results and Discussion
oxy)phenyl]porphinato)zinc(II)-5-ylethynyl]benzo[1,2-c:4,5-c′][1,2,5]-
bis([1,2,5]thiadiazole) (PZnE-BBTD-EPZn)] are given in the Sup-
porting Information.
Synthesis. Structures of the PZn-Sp-PZn supermolecules
along with related ethynylated Sp and PZn reference compounds
Instrumentation. Electronic absorption spectra were recorded on
an OLIS UV/vis/near-IR spectrophotometry system that is based on
the optics of a Cary 14 spectrophotometer. NMR spectra were recorded
on 360 MHz DMX-360 or 300 MHz DMX-300 Br u¨ ker spectrometers.
Cyclic voltammetric measurements were carried out on an EG&G
Princeton Applied Research model 273A Potentiostat/Galvanostat. The
electrochemical cell used for these experiments utilized a platinum disk
working electrode, a platinum wire counter electrode, and a saturated
calomel reference electrode (SCE). The reference electrode was
separated from the bulk solution by a junction bridge filled with the
(
61) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
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5188 J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005

Proquinoidal Bis[(Porphinato)zinc(II)] Compounds
A R T I C L E S
are shown in Figure 1. These PZn-Sp-PZn species were
synthesized by palladium (Pd)-mediated cross-coupling reactions
involving appropriately substituted (porphinato)zinc(II) (PZn)
compounds and Sp units (see Supporting Information). The PZn-
containing structures of Figure 1 exploit 2′,6′-bis(3,3-dimethyl-
1
-butyloxy)phenyl groups as 10- and 20-meso-porphyrin sub-
stituents, which facilitate excellent solubility and straightforward
1
21,63
assignment of H NMR spectra.
4,7-Diethynylbenzo[c]-
[1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-PC-
E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinox-
aline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c′]bis-
(
[1,2,5]thiadiazole) (E-BBTD-E) were selected as proquinodal
Sp units.
The nature of the functionalized PZn and Sp moieties used
in the synthesis of the corresponding PZn-Sp-PZn complexes
varied with Sp electronic structure. For the PZn-Sp-PZn
structure featuring a E-BTD-E Sp unit (PZnE-BTD-EPZn,
Figure 1), the coupling reaction between (5-ethynyl-10,20-bis-
[2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II) and
4,7-dibromobenzo[c][1,2,5]thiadiazole gave a mixture of prod-
ucts that included not only the target molecule, but also the
butadiyne-bridged bis[(porphinato)zinc(II)] complex (PZnE-
EPZn). As these two species were difficult to separate by both
silica gel and size exclusion column chromatography under the
conditions employed, PZnE-BTD-EPZn was synthesized via
a Pd-mediated coupling reaction involving (5-iodo-10,20-bis-
[2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II) and
4,7-bis(trimethylsilylethynyl)benzo[c][1,2,5]thiadiazole in which
in situ deprotection of the trimethylsilyl (TMS) group with
K2CO3 was utilized. An identical in situ deprotection strategy
was utilized in the synthesis of PZnE-PC-EPZn, as 6,13-
diethynylpentacene (E-PC-E) possesses low solubility. Respec-
tive PZn-Sp-PZn complexes utilizing thiadiazoloquinoxaline
and benzobis(thiadiazole) moieties as Sp components (PZnE-
TDQ-EPZn and PZnE-BBTD-EPZn, Figure 1) were synthe-
sized successfully from (5-ethynyl-10,20-bis[2′,6′-bis(3,3-
dimethyl-1-butyloxy)phenyl]porphinato)zinc(II) and the cor-
responding dibromo proquinoid spacer derivatives, as trace
PZnE-EPZn contaminants could easily be separated from the
PZn-Sp-PZn product by silica gel column chromatography.
Steady-State Absorption Spectra. Electronic absorption
spectra for the PZn-Sp-PZn complexes, along with the spectrum
for the bis[(5,5′,-10,20-di(aryl)porphinato)zinc(II)]butadiyne
(
PZnE-EPZn) benchmark, are displayed in Figure 2. Analogous
Figure 2. Electronic absorption spectra of: (A) PZnE-EPZn, (B) PZnE-
BTD-EPZn, (C) PZnE-PC-EPZn, (D) PZnE-TDQ-EPZn, and (E) PZnE-
BBTD-EPZn recorded in THF solvent.
electronic spectra for trimethylsilyl-protected analogues of
proquinoidal Sp reference compounds E-BTD-E, E-PC-E,
E-TDQ-E, and E-BBTD-E, as well as comprehensive tabulated
electronic absorption spectral data, are contained in the Sup-
porting Information. As expected for the mode of porphyrin-
to-porphyrin connectivity and the nature of the conjugated
components, extensive electronic interactions are manifest
conjugation and exciton coupling, and have been analyzed in
detail for a wide-range of structurally related compounds.1
The Soret band regions of these PZn-Sp-PZn compounds
show splittings characteristic of extensive excitonic interac-
7-22
6
4
tions. Note that the PZnE-BTD-EPZn, PZnE-PC-EPZn,
PZnE-TDQ-EPZn, and PZnE-BBTD-EPZn spectra (Figure
1
7-22
between the PZn and Sp components of these structures.
The electronic absorption spectrum for PZnE-EPZn (Figure
A), as well as the linear electronic spectra of these PZn-Sp-
2
5
) display a sharp B-state absorption close in energy to that of
-ethynyl(porphinato)zinc(II) reference compound (PZnE-TMS,
2
PZn compounds (Figure 2B-E), are dominated by two absorp-
tion manifolds derived from the classic porphyrin B (Soret) (S0
f S2) and Q-band (S0 f S1) transitions. The gross spectral
features of these species evince the hallmarks of extensive π
Figure 1; see Supporting Information); interestingly, this high
energy component of the B-state manifold of these compounds
displays less structure than that observed in the PZnE-EPZn
spectrum (Figure 2A). Congruent with the body of spectroscopic
(
62) Laaksonen, L. gOpenMol, Version 2.0; Espoo, Finland, 2001.
63) Uyeda, H. T.; Zhao, Y.; Wostyn, K.; Asselberghs, I.; Clays, K.; Persoons,
A.; Therien, M. J. J. Am. Chem. Soc. 2002, 124, 13806-13813.
(
(64) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11,
371-392.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005 5189

A R T I C L E S
Susumu et al.
Table 1. Comparative Integrated Oscillator Strengths and Absorptive Domains of the Blue and Red Spectral Regions of Conjugated
Porphyrin Dimers Relative to the PZnE-TMS Benchmarka
oscillator
strength
B-band
regiond
FWHMe
Q-band
oscillator
strength
Q-band
regionf
FWHMb
total
B-band region
[cm-1, (nm)]
region
[cm-1, (nm)]
oscillator
strengthg
compound
PZnE-TMS
696
(428)
1.059
3.643
756
57
1512
588
(562)
(603)
(566)
(628)
(678)
(689)
(823)
0.062
0.467
1.121
5.197
4
c
PZnE-EPZn
1976
(449)
(481)
c
592
536
PZnE-BTD-EPZn
PZnE-PC-EPZn
2323
1745
(426)c
(421)
2.108
2.667
1180
1992
0.719
0.737
3.519
5.456
c
1014
(493)
PZnE-TDQ-EPZn
PZnE-BBTD-EPZn
1945
1623
(423)
(429)
(479)
3.076
2.803
2510
2341
(839)
(1006)
1.144
0.972
5.515
5.167
c
2053
a
From electronic absorption spectra recorded in THF solvent. b Taken as the spectral width of the B-band region at half the height of the absorption
c
d
noted. Taken as twice value of half the spectral width of the B-band region at half the height of the absorption noted. Oscillator strengths calculated over
the following wavelength domains: PZnE-TMS (360∼470 nm); PZnE-EPZn (360∼530 nm); PZnE-BTD-EPZn (360∼510 nm); PZnE-PC-EPZn (360∼535
e
nm); PZnE-TDQ-EPZn (360∼510 nm); PZnE-BBTD-EPZn (380∼525 nm). Entries correspond to the spectral breadth of the transition envelope centered
f
at the wavelength in parentheses. Oscillator strengths calculated over the following wavelength domains: PZnE-TMS (470∼650 nm); PZnE-EPZn (530∼750
nm); PZnE-BTD-EPZn (510∼850 nm); PZnE-PC-EPZn (535∼1000 nm); PZnE-TDQ-EPZn (510∼1000 nm); PZnE-BBTD-EPZn (525∼1400 nm).
g
Oscillator strengths calculated over the following wavelength domains: PZnE-TMS (360∼650 nm); PZnE-EPZn (280∼750 nm); PZnE-BTD-EPZn
(280∼850 nm); PZnE-PC-EPZn (280∼1000 nm); PZnE-TDQ-EPZn (280∼1100 nm); PZnE-BBTD-EPZn (280∼1400 nm).
Table 2. Prominent Absorption Band Wavelength, Energies, and Extinction Coefficients of Conjugated PZn-Sp-PZn Compounds Relative to
the PZnE-TMS Benchmark in THF Solvent
UV-region
(cm-1)
B-band region
(cm-1)
Q-band region
(cm-1)
compound
λ(nm)
ν
log(
ꢀ)
λ(nm)
ν
log(
ꢀ
)
λ(nm)
ν
log(ꢀ)
PZnE-TMS
428
23,364
(5.53)
562
17,794
16,584
17,668
15,924
14,749
19,084
17,668
14,514
18,018
12,151
18,832
11,919
18,149
15,674
9,940
(4.17)
(3.92)
(4.42)
(4.60)
(4.58)
(4.55)
(4.34)
(4.81)
(4.26)
(4.78)
(4.51)
(4.80)
(4.33)
(4.16)
(4.75)
603
PZnE-EPZn
308
313
32468
31949
(4.56)
(4.39)
423
23,640
22,272
20,790
23,474
21,505
(5.19)
(5.46)
(5.21)
(5.16)
(4.79)
566
628
678
524
566
4
4
49
81
PZnE-BTD-EPZn
426
65
4
689
PZnE-PC-EPZn
326
305
308
30675
32,787
(5.05)
(4.71)
421
93
23,753
20,284
23,641
(5.32)
(4.79)
(5.38)
555
823
531
4
PZnE-TDQ-EPZn
PZnE-BBTD-EPZn
423
839
32,468
27,548
(4.56)
(4.65)
429
479
23,310
20,877
(5.39)
(4.84)
551
638
006
363
1
data obtained for PZnE-EPZn and related ethyne- and butadiyne-
driven by the proquinoidal PZn-to-PZn bridging units.20,67 This
augmentation of x-polarized Q-state absorption oscillator strength
increases concomitantly with the ability of the S0 and S1 states
of these species to delocalize charge (Tables 1 and 2, vide
bridged bis(PZn) compounds,1
7-22,28,36,65,66
this spectral signature
reflects the diminished y-polarized B-state dipolar interaction
y taken orthogonal to the highly conjugated axis of the
(
17,18,21
structure) that occurs with increasing PZn-PZn distance for
the PZnE-BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-EPZn,
and PZnE-BBTD-EPZn supermolecules, consistent with ex-
pectations based on the point-dipole approximation of the
infra),
and tracks with the magnitude of the Qx absorption
wavelength [Compound, (λmax(Qx band)): PZnE-EPZn (678
nm), PZnE-BTD-EPZn (689 nm), PZnE-PC-EPZn (823 nm),
PZnE-TDQ-EPZn (839 nm) and PZnE-BBTD-EPZn (1006
nm)]. Note that the full spectral width at half-maximum (fwhm)
of the Qx-state absorption manifold follows a similar trend with
PZnE-EPZn < PZnE-BTD-EPZn < PZnE-PC-EPZn <
PZnE-TDQ-EPZn ∼ PZnE-BBTD-EPZn (Figure 2, Table 1).
In this regard, note that the Qx manifold spectral band shapes
for PZnE-TDQ-EPZn and PZnE-BBTD-EPZn tail extensively
at low energy, with the PZnE-BBTD-EPZn spectrum display-
ing measurable oscillator strength well beyond 1200 nm (Figure
6
4
general exciton model originally developed by Kasha. In
contrast, the low energy, x-polarized B-state transitions along
the vector defined by the ethyne moieties of these PZn-Sp-
PZn compounds are markedly reduced in absolute intensity with
respect to the Bx band centered at 481 nm in the PZnE-EPZn
spectrum, and are observed to be less significant relative to their
respective prominent y-polarized B-state absorptions (Figure 2,
Table 2).
This decrease in x-polarized B-state oscillator strength
observed in the Figure 2 spectra for PZnE-BTD-EPZn, PZnE-
PC-EPZn, PZnE-TDQ-EPZn, and PZnE-BBTD-EPZn rela-
tive to that manifest for PZnE-EPZn, reflects enhanced Bx-
state intensity borrowing by the corresponding Q-state absorptions
(
65) Angiolillo, P. J.; Lin, V. S.-Y.; Vanderkooi, J. M.; Therien, M. J. J. Am.
Chem. Soc. 1995, 117, 12514-12527.
(66) O’Keefe, G. E.; Denton, G. J.; Harvey, E. J.; Phillips, R. T.; Friend, R. H.;
Anderson, H. L. J. Chem. Phys. 1996, 104, 805-811.
(67) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
London, 1978; Vol. III, p 1-165.
5190 J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005

Proquinoidal Bis[(Porphinato)zinc(II)] Compounds
A R T I C L E S
more important determinant of the extent of π-conjugation in
π-conjugated oligomers and polymers.
While the magnitudes of PZnE-BTD-EPZn and PZnE-PC-
EPZn Ep values suggest respective HOMOs and LUMOs that
feature substantial conjugative interactions between the PZn,
Sp, and ethyne units (Figure 3), the highly stabilized LUMOs
of PZnE-TDQ-EPZn and PZnE-BBTD-EPZn are unusual:
note that the PZnE-TDQ-EPZn and PZnE-BBTD-EPZn one-
electron reduction potentials resemble those obtained respec-
tively for TDQ(E-TMS)2 and BBTD(E-TMS)2, potentially
indicating that radical anion state electron density is largely
localized on the Sp fragments of these PZn-Sp-PZn supermol-
ecules. While electronic structure calculations provide further
insight into this matter, it is important to note that the optical
band gaps (Eop values) of PZnE-BTD-EPZn, PZnE-PC-EPZn,
PZnE-TDQ-EPZn, and PZnE-BBTD-EPZn track closely with
their corresponding Eps (Figures 2-3, Tables 2-3). These data,
coupled with the facts that: (i) the steady-state absorption
spectra indicate that the visible and NIR x-polarized excitations
feature extensive mixing of PZn- and Sp-derived electronic
states, and (ii) the magnitude of the LUMO-level stabilization
in the PZnE-BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-
EPZn, and PZnE-BBTD-EPZn series is extensive with respect
to the corresponding HOMO-level destabilization evinced in
these structures, suggest that the quinoidal resonance contribu-
tion to the low lying singlet electronically excited states exceeds
greatly that for the ground-state, thus giving rise to the
expectation that the excited singlet wave functions of these PZn-
Sp-PZn compounds should feature unusual degrees of electronic
delocalization (vide infra).
Figure 3. Schematic highlighting the potentiometrically determined HOMO
and LUMO energy levels of the PZn-Sp-PZn complexes relative to those
of ethyne-functionalized PZn and proquinoidal Sp moieties. Redox potentials
shown are relative to the ferrocene/ferrocenium (Fc/Fc ) redox couple,
which was used as an internal standard in these experiments.
+
2). Absorption wavelengths, energies, and extinction coefficients
for the prominent transitions of these compounds are tabulated
in Table 2.
Electrochemical Properties. Figure 3 highlights the poten-
tiometrically determined HOMO and LUMO energy levels of
the PZn-Sp-PZn complexes relative to those of ethyne-
functionalized PZn and proquinoidal Sp moieties, PZnE-EPZn,
and a related tris[(porphinato)zinc(II)] complex, (5,15-bis-
[[(5′,-10′,20′-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)-
zinc(II)]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)-
phenyl]porphinato)zinc(II) (PZnE-PZn-EPZn).1 The cyclic
voltammetric data shown in this figure highlight a number of
electronic structural features of these PZn-Sp-PZn species.
PZn-Sp-PZn E1/20 values vary modestly with Sp electronic
structure over a 0.21 eV range, with HOMO level destabilization
increasing in the order PZnE-BTD-EPZn < PZnE-EPZn ∼
PZnE-TDQ-EPZn < PZnE-BBTD-EPZn < PZnE-PC-EPZn
7,21
/+
Electronic Structure Calculations. Further insight into the
electrooptic properties of these supermolecules can be garnered
from an electronic structural analysis of PZn-Sp-PZn frontier
orbitals (FOs) as a function of the intervening proquinoidal Sp
moiety. Figures 4-8 display respectively the FOs of PZnE-
EPZn, PZnE-BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-
EPZn, and PZnE-BBTD-EPZn. In these electronic structure
calculations, geometry optimizations and semiempirical elec-
tronic structural calculations were performed using the PM3
method. Models for the PZn-Sp-PZn structures that featured
planar macrocycles and Sp units constrained to lie in a common
molecular plane, were utilized in the geometrical optimizations.
PZn-Sp-PZn FOs presented in Figures 4-8 are depicted in an
orbital correlation diagram format that shows the corresponding
FOs and energies of their respective component PZnE and Sp
building blocks. Figure 9 displays the relative calculated HOMO
and LUMO energies of all the Figures 4-8 model compounds.
(
Figure 3). In contrast, measured PZn-Sp-PZn E1/2-/0 values
span a 0.87 eV potentiometric domain, mirroring closely the
-
/0
relative changes observed for E1/2
determined for their
corresponding trimethylsilyl-elaborated Sp [Sp(E-TMS)2] struc-
tures, and evincing LUMO level stabilization that escalates in
the order PZnE-EPZn < PZnE-BTD-EPZn < PZnE-PC-
EPZn < PZnE-TDQ-EPZn < PZnE-BBTD-EPZn. This
-
/0
strong dependence of PZn-Sp-PZn E1/2
values upon the
nature of the building block ethyne-elaborated Sp moiety plays
the predominant role in determining the magnitude of the
0
/+
potentiometrically determined HOMO-LUMO gaps (Ep; E1/2
-
-
/0
E1/2 ) within this series of supermolecular bis(PZn) com-
pounds that feature proquinoidal Sp units [Compound, (Ep):
PZnE-EPZn (2.01 eV), PZnE-BTD-EPZn (1.88 eV), PZnE-
PC-EPZn (1.46 eV), PZnE-TDQ-EPZn (1.42 eV) and PZnE-
BBTD-EPZn (1.11 eV); see Figure 3, Table 3]. Note that
despite augmented PZn-to-PZn centroid-to-centroid distances,
the magnitudes of the potentiometrically evaluated HOMO-
LUMO gaps for these PZn-Sp-PZn structures are markedly
diminished relative to the PZnE-EPZn benchmark. Further,
given that PZnE-PZn-EPZn21 can be considered a bis(PZn)
complex bridged by a 5,15-diethynyl(porphinato)zinc(II) (PZnE2)
Sp moiety, the fact that Ep for this structure (Figure 3) exceeds
that determined for PZnE-TDQ-EPZn, PZnE-PC-EPZn, and
PZnE-BBTD-EPZn, demonstrates that proquinoidal Sp elec-
tronic structure, in contrast to Sp π-aromatic size, can be the
The FOs of PZnE-EPZn (Figure 4) resemble those computed
for meso-to-meso ethyne-bridged bis(PZn) (PZn-E-PZn):
20,21,36,37
(i) the HOMO exhibits substantial butadiyne-, Cmeso-,
and N-centered electron density, with the Cmeso carbons that
constitute a portion of the conjugated macrocycle-to-macrocycle
bridge displaying substantial π overlap with their respective CR
carbons; (ii) the LUMO manifests similarly comprehensive
electronic delocalization, exhibiting extensive cumulenic char-
acter along the C axis defined by the butadiyne moiety. The
2
evolution of FO electron density distributions in bis(porphyrin)
compounds that feature a meso-to-meso linkage topology and
a cylindrically π-symmetric bridge, relative to classic monomeric
porphyrin building blocks, has been discussed in detail.20 Note
J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005 5191

A R T I C L E S
Susumu et al.
Table 3. Optical HOMO-LUMO Gaps (EopS) and Potentiometrically Determined HOMO-LUMO Gaps (EpS) of the PZn-Sp-PZn Complexes
Relative to Those of Ethyne-functionalized PZn and Proquinoidal Sp Moieties
PZnE-
TMS
PZnE-
EPZn
BTD(E-
TMS)
PZnE-BTD-
EPZn
PC(E-
TMS)
PZnE-PC-
EPZn
TDQ(E-
TMS)
PZnE-TDQ-
EPZn
BBTD(E-
TMS)
PZnE-BBTD-
EPZn
2
2
2
2
Eop(max)a
2.06
2.01
2.11
1.83
1.77
2.01
3.25
2.95
d
1.80
1.69
1.88
1.94
1.89
1.80
1.51
1.41
1.46
2.46
2.31
d
1.48
1.31
1.42
2.18
2.01
d
1.23
1.05
1.11
b
Eop(edge)
c
Ep
a
b
Optical HOMO-LUMO gap determined from the lowest absorption maximum measured in THF. Optical HOMO-LUMO gap determined from the
absorption edge measured in THF. The absorption edge is defined as the wavelength on the red side of the lowest energy absorption band where the slope
changes abruptly. Potentiometrically determined HOMO-LUMO gap (E1/2 - E1/2 ) measured in CH2Cl2. d Irreversible oxidation; value not determined.
c
0/+
-/0
Figure 4. Frontier orbital correlation diagram for PZnE and PZnE-EPZn.
that both the PZnE-EPZn electron density distribution within
the frontier orbital set, and the magnitude of the energy
separation between these orbitals, are consistent with the
x-polarized nature of its low lying electronically excited
1
7-27
Figure 5. Frontier orbital correlation diagram for PZnE, benzothiadiazole
BTD), and PZnE-BTD-EPZn.
state.
The PZnE-EPZn HOMO is 0.206 eV destabilized
(
with respect to that of the PZnE benchmark, whereas its LUMO
is 0.098 eV stabilized relative to this ethyne-elaborated mono-
mer. This diminished computed HOMO-LUMO gap for PZnE-
EPZn relative to PZnE is larger than that calculated for PZn-
of PZnE, while its LUMO energy level is stabilized by 0.203
eV with respect to this benchmark.
PZnE-PC-EPZn displays FO electron density distributions
similar to that noted for PZnE-BTD-EPZn, with substantial
electronic interactions evident between PZnE and PC Sp
moieties in both the HOMO and LUMO (Figure 6). The energy
to which the PZnE-PC-EPZn HOMO is destabilized relative
to PZnE (0.499 eV) has increased with respect to that observed
for PZnE-BTD-EPZn; likewise, the PZnE-PC-EPZn LUMO
is considerably more stabilized (0.251 eV, Figure 9) than that
computed for PZnE-BTD-EPZn. Note in this regard that both
(i) large atomic orbital coefficients at the porphyrin meso- and
pentacene 6,13-carbon positions in PZnE-PC-EPZn, and (ii)
excellent energy matching within the PZnE and PC FO sets
drive these substantial HOMO and LUMO energy level
perturbations relative to that observed for the PZnE-BTD-EPZn
case.
2
0,21
E-PZn,
consistent with the expectation, and extensive
spectroscopic evidence indicating, that interporphyryl electronic
interactions diminish with increasing macrocycle-macrocycle
distances.
An analogous computational study carried out on PZnE-
BTD-EPZn evinces both a delocalized HOMO and LUMO
(Figure 5), similar to that noted for PZnE-EPZn. The PZnE-
BTD-EPZn HOMO illustrates antibonding interactions between
the PZn, ethyne, and BTD units that define the highly conjugated
axis of the supermolecule, while the LUMO features an
extensive degree of quinoidal character, with bonding interac-
tions between the PZn-Cmeso and ethyne CR-carbon atoms,
antibonding interactions between the ethyne CR and Câ atoms,
and bonding interactions between the ethyne Câ and benzothia-
diazole 4- and 7-carbon atoms. The HOMO energy level of
PZnE-BTD-EPZn is destabilized by 0.106 eV relative to that
The energy difference between the PZnE and Sp LUMOs
increases as the Sp fragment LUMO energy levels become
5192 J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005

Proquinoidal Bis[(Porphinato)zinc(II)] Compounds
A R T I C L E S
Figure 6. Frontier orbital correlation diagram for PZnE, pentacene (PC),
and PZnE-PC-EPZn.
Figure 7. Frontier orbital correlation diagram for PZnE, thiadiazoloqui-
noxaline (TDQ), and PZnE-TDQ-EPZn.
increasingly stabilized (|ELUMO(PZnE) - ELUMO(Sp)| ) 0.171
/0
-
E1/2- values, large CI guarantees orbital contributions from
(
PZnE/BTD), 0.356 (PZnE/PC), 0.407 (PZnE/TDQ), 1.232
PZnE/BBTD) eV; Figures 4-9). Given that the extent of the
multiple high-lying filled and low-lying empty levels for PZn-
Sp-PZn electronically excited states. Such CI ensures that the
low-lying singlet of these species will possess both global
delocalization and extensive quinoidal character (vide infra).
Transient Optical Spectra of PZn-Sp-PZn Initially Pre-
pared Electronically Excited Singlet States. The combination
of steady-state electronic absorption, potentiometric, and com-
putational data, coupled with the expectation that substantial
CI drives global electronic delocalization in electronically
excited PZn-Sp-PZn species, suggests that quinoidal resonance
contributions to S1-Sn states exceed that for the corresponding
ground (S0) states. Pump-probe transient absorption spectro-
scopic experiments that interrogate the S1 f Sn transition
manifolds of these supermolecules further support this view.
Figure 10 shows transient absorption spectra recorded at 300
fs time delay for PZnE-EPZn, PZnE-BTD-EPZn, PZnE-PC-
EPZn, PZnE-TDQ-EPZn, and PZnE-BBTD-EPZn in the NIR
spectral region.
Qualitatively, these S1 f Sn spectra bear many features in
common with those previously delineated for meso-to-meso
ethyne-bridged bis[(porphinato)zinc(II)] (PZn-E-PZn) struc-
tures. Like PZn-E-PZn species, PZn-Sp-PZn supermolecules
display intense NIR S1 f Sn transitions; these absorptions
possess integrated oscillator strengths of comparable magnitude
to their corresponding B- and Q-state manifold bleaching
transitions (data not shown). While the benchmark PZnE-EPZn
transient absorption spectrum (Figure 10A) bears features closely
(
orbital interaction is inversely proportional to the energy
68
difference between the interacting orbitals, this orbital energy
mismatch attenuates the extent of LUMO delocalization in these
PZn-Sp-PZn supermolecules, causing the LUMO to become
increasingly localized on the Sp unit. Thus, while contour plots
of the PZnE-BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-
EPZn, and PZnE-BBTD-EPZn HOMOs all show global
delocalization, significant LUMO delocalization is evident only
in PZnE-BTD-EPZn and PZnE-PC-EPZn (Figures 5 and 6),
with PZnE-TDQ-EPZn and PZnE-BBTD-EPZn showing
highly Sp-localized LUMOs (Figures 7 and 8). Note that the
relative calculated HOMO and LUMO energy levels shown in
Figure 9 mirror the differences in the potentiometrically
determined PZn-Sp-PZn HOMO and LUMO energy levels
(Figure 3). While these computational and electrochemical data
underscore the cardinal role that PZnE and Sp fragment orbital
energy differences play in fixing the radical cation and anion
state energy levels in these PZn-Sp-PZn structures, it is
important to appreciate that in contrast to many simple
conjugated organic building blocks, whose low-lying excited
states are described adequately by one-electron transitions,
22
extensive configuration interaction (CI) is necessary to describe
correctly porphyrin electronically excited states.2
1,67
While
0
/+
absolute HOMO and LUMO energies largely determine E1/2
(68) Salzner, U. J. Phys. Chem. B 2002, 106, 9214-9220.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 14, 2005 5193

A R T I C L E S
Susumu et al.
Figure 10. Scaled magic angle transient absorption spectra recorded for:
(A) PZnE-EPZn, (B) PZnE-BTD-EPZn, (C) PZnE-PC-EPZn, (D) PZnE-
TDQ-EPZn, and (E) PZnE-BBTD-EPZn measured at a delay time of 300
fs. Experimental conditions: solvent ) THF; T ) 25 °C; λex ) 655 nm
(A), 662 nm (B), 775 nm (C), 775 nm (D), and 650 nm (E).
determined for PZnE-EPZn (∼1000 nm). Note that the
corresponding fwhm values for the S1 f Sn manifolds of these
PZn-Sp-PZn species [Compound, (fwhm): PZnE-BTD-EPZn
-
1
-1
(
∼2800 cm ), PZnE-PC-EPZn (>2500 cm ), PZnE-TDQ-
-1 -1
EPZn (g1160 cm ) and PZnE-BBTD-EPZn (g1150 cm )]
Figure 8. Frontier orbital correlation diagram for PZnE, benzobis-
determined at 300 fs following optical excitation, meet or exceed
that evinced for PZnE-EPZn (1100 cm ).
(thiadiazole) (BBTD), and PZnE-BBTD-EPZn.
-
1
Further, the Figure 10B-E spectra bear absorptive signatures
that indicate that the nature of the proquinoidal bridge plays a
pivotal role in determining the degree of excited-state conjuga-
tion and the extent to which nuclear relaxation dynamics69
impact the early-time evolution of the excited state. As a case
in point, note that while sharp differences exist between the S0
f S1 absorption manifolds of PZnE-BTD-EPZn and PZnE-
PC-EPZn (Figure 2), the low energy excited states for these
species present 300 fs following optical excitation are similar,
with significant NIR-absorptive spectral breadths and substantial
S1 f Sn absorptive oscillator strengths at wavelengths longer
than 1300 nm. In this regard, we underscore that there are
relatively few examples of electronically excited chromophoric
singlet states that absorb strongly in the NIR; systems that
exhibit such singlet excited-state manifold absorptive properties
are largely confined to a select group of semiconducting
Figure 9. Diagrammatic representation of PZn-Sp-PZn and Sp HOMO
and LUMO energy levels calculated using the PM3 semiempirical
method.
aligned with that reported for PZn-E-PZn chromophores (bis-
7
0-76
[(5,5′-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphi-
π-conjugated polymers.
nato)zinc(II)]ethyne and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-
butyloxy)phenyl]porphinato)zinc(II)]-[(5′,-15′-ethynyl-10′,20′-
bis(heptafluoropropyl)porphinato)zinc(II)]ethyne),2 PZnE-
BTD-EPZn, PZnE-PC-EPZn, PZnE-TDQ-EPZn, and PZnE-
BBTD-EPZn manifest S1 f Sn absorption manifolds that shift
to progressively lower energies (Figure 10B-E), tracking with
expectations based on the relative frontier orbital energy levels
of these supermolecules determined by potentiometric and
computational methods (vide supra).
Conclusion
2
Conjugated (porphinato)zinc(II)-spacer-(porphinato)zinc(II)
(PZn-Sp-PZn) complexes that possess intervening conjugated
(
69) Duncan, T. V.; Susumu, K.; Therien, M. J., manuscript in preparation.
70) Moraes, F.; Schaffer, H.; Kobayashi, M.; Heeger, A. J.; Wudl, F. Phys.
ReV. B 1984, 30, 2948-2950.
(
(
71) Woo, H. S.; Graham, S. C.; Halliday, D. A.; Bradley, D. D. C.; Friend, R.
H.; Burn, P. L.; Holmes, A. B. Phys. ReV. B 1992, 46, 7379-7389.
72) Lanzani, G.; Nisoli, M.; De Silvestri, S.; Barbarella, G.; Zambianchi, M.;
Tubino, R. Phys. ReV. B 1996, 53, 4453-4457.
(
The wavelength of the most intense transition within the NIR
absorption manifold [λmax(S1 f Sn)] for PZnE-BTD-EPZn,
PZnE-PC-EPZn, PZnE-TDQ-EPZn, and PZnE-BBTD-EPZn
occurs at 1207, ∼1187, 1270, and 1610 nm, respectively, 1700-
(73) Frolov, S. V.; Liess, M.; Lane, P. A.; Gellermann, W.; Vardeny, Z. V.;
Ozaki, M.; Yoshino, K. Phys. ReV. Lett. 1997, 78, 4285-4288.
(
74) Kraabel, B.; Klimov, V. I.; Kohlman, R.; Xu, S.; Wang, H.-L.; McBranch,
D. W. Phys. ReV. B 2000, 61, 8501-8515.
(
(
75) Kraabel, B.; McBranch, D. W. Chem. Phys. Lett. 2000, 330, 403-409.
76) Jiang, X. M.; O¨ sterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.;
Janssen, R. A. J.; Vardeny, Z. V. AdV. Funct. Mater. 2002, 12, 587-597.
-
1
3
800 cm red-shifted from the analogous λmax(S1 f Sn)
5194 J. AM. CHEM. SOC.
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VOL. 127, NO. 14, 2005

Proquinoidal Bis[(Porphinato)zinc(II)] Compounds
A R T I C L E S
Sp structures with varying degrees of proquinoidal character
have been synthesized by palladium-catalyzed cross coupling
reactions. These supermolecular PZn-Sp-PZn compounds
feature Sp moieties {(4,7-diethynylbenzo[c][1,2,5]thiadiazole
nm spectral region, giving rise to intense excited-state transitions
-
1
∼4000 cm lower in energy than that observed for the
analogous excited-state absorption maximum of the PZnE-
EPZn benchmark; these data highlight the unusually large
quinoidal resonance contribution to the low-lying electronically
excited singlet states of these PZn-Sp-PZn species. In this
regard, we underscore that there are relatively few examples of
electronically excited chromophoric singlet states that absorb
strongly in the NIR; systems that possess such singlet excited-
(E-BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-
6
,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline (E-TDQ-E),
and 4,8-diethynylbenzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole) (E-
BBTD-E)} that regulate frontier orbital energy levels and
progressively increase the extent of the quinoidal resonance
contribution to the ground and electronically excited states,
augmenting the magnitude of electronic communication between
terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to
that evinced for a bis[(5,5′,-10,20-di(aryl)porphinato)zinc(II)]-
butadiyne benchmark (PZnE-EPZn). Electronic absorption
spectra show significant red-shifts of the respective PZn-Sp-
PZn x-polarized Q state (S0 f S1) transition manifold maxima
state manifold absorptive properties are largely confined to a
select group of semiconducting π-conjugated polymers.7
0-76
It
is important to emphasize with respect to this comparison,
however, that the classic intrachain S1 f Sn absorption energies
of highly conjugated phenylene- and thiophene-based polymers
7
0-76
exceed 1 eV (λabs < 1250 nm).
The length scale spanned
by the PZn-Sp-PZn structures (∼25 Å), and the fact that the
NIR manifold absorptions chronicled in the Figure 10 data do
not correspond to T1 f Tn absorptions, or transitions related to
-
1
(
240-4810 cm ) relative to that observed for PZnE-EPZn.
Likewise, the potentiometrically determined PZn-Sp-PZn HO-
MO-LUMO gaps (E1/20 - E1/2 ) display correspondingly
diminished energy separations that range from 1.88 to 1.11 eV
relative to that determined for PZnE-EPZn (2.01 eV). Elec-
tronic structure calculations show that while all PZn-Sp-PZn
structures possess delocalized HOMOs, structures having ben-
zothiadiazole- and pentacene-based spacers exhibit delocalized
LUMOs, while analogous compounds based on thiadiazoloqui-
noxaline and benzobis(thiadiazole)-derived bridging moieties
evince highly stabilized LUMO energy levels that feature Sp-
localized electron density. Collectively, steady-state optical,
potentiometric, and computational data point to the existence
of PZn-Sp-PZn electronically excited singlet states that display
augmented quinoidal character relative to the S0 state.
/+
-/0
the polaron absorption bands of semiconducting polymers
70-76
derived from charge-doping or intra- and interchain charge
separated states, highlight the tremendous potential of these
PZn-Sp-PZn species and their higher molecular weight ana-
logues as electrooptic materials.
Acknowledgment. This work was supported through the
National Cancer Institute (NO1-CO-29008), and the MRSEC
Program of the National Science Foundation (DMR00-79909).
Supporting Information Available: Synthetic details, char-
acterization data, synthetic schemes, details concerning ultrafast
transient absorption experiments, electronic absorption spectra
(PZnE-TMS, BTD(E-TMS)2, PC(E-TMS)2, TDQ(E-TMS)2,
Pump-probe transient spectral data for these PZn-Sp-PZn
supermolecules are remarkable, and demonstrate that the S1 f
Sn transition manifolds of these species span an unusually broad
spectral domain of the NIR. Notably, the absorption maxima
of these S1 f Sn manifolds can be tuned over a 1000-1600
and BBTD(E-TMS)2), tabulated optical and electrochemical
data, electronic structure computational data. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA040243H
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