Decoupling optical and potentiometric band gaps in π-conjugated materials

Article abstract of DOI:10.1021/ja0203925

Syntheses, optical spectroscopy, potentiometric studies, and electronic structural calculations are reported for two classes of conjugated (porphinato)metal oligomers that feature a meso-to-meso ethyne-bridged linkage topology. One set of these systems, b

Full text of DOI:10.1021/ja0203925

Published on Web 06/28/2002
Decoupling Optical and Potentiometric Band Gaps in π-Conjugated Materials
Kimihiro Susumu and Michael J. Therien*
Department of Chemistry, UniVersity of PennsylVania
Philadelphia, PennsylVania 19104-6323
Received March 18, 2002
Low optical band gap (Eop) organic materials figure prominently
in a wealth of optoelectronic devices.1-4 The prominent, low-energy
electronic transitions characteristic of such materials typically
correlate with the presence of HOMO and LUMO levels that are
extensively destabilized and stabilized, respectively, relative to the
corresponding energy levels of the conjugated building blocks of
the organic oligomer or polymer. As such, these conjugated
structures manifest modest potentiometrically determined HOMO-
0/+
LUMO gaps (Ep; E1/2 - E1/2-/0) that correlate closely with Eop.
The measured Ep and Eop values of low optical band gap
oligomers and polymers commonly follow a predictable dependence
upon conjugation length;1-10 augmented HOMO destabilization and
LUMO stabilization track with increasing size of the conjugated
oligomer, until cation radical (positive polaron) and anion radical
(negative polaron) delocalization (diffusion) lengths limit further
0/+
perturbation of E1/2 and E1/2-/0, respectively. The magnitude of
polaron diffusion lengths depends on the nature of the conjugated
building block and rarely extends over spatial dimensions that
exceed a handful of monomer units. Strategies to further reduce
Eop values in low optical band gap materials once such delocal-
ization limits have been reached have included conjugation expan-
sion of the monomeric unit2,4 and engineering charge resonance
Figure 1. Structures of bis-, tris-, and pentakis[(porphinato)zinc(II)] com-
pounds DD, DA, DDD, DAD, DDDDD, and DADAD, along with ethyne-
elaborated (porphinato)zinc(II) building blocks E-D, E2-D, and E2-A.
interactions in the polymer backbone through the alternation of
9,11-15
donor and acceptor moieties.
While these approaches
meso-phenyl-substituted structures; furthermore, because the non-
π-conjugating, σ-electron-withdrawing perfluoroalkyl group stabi-
lizes extensively the PZn a2u orbital, such species manifest a1u-
derived HOMOs.23-25
commonly augment desired polymer optoelectronic properties such
as the polarizability, hyperpolarizability, and conductivity, it is
crucial to note that redox instability is a common shortcoming of
organic materials that possess low optical band gaps. In this report,
we outline a strategy that enables engineering of conjugated
oligomeric structures that possess highly delocalized singlet (S1)
excited states yet manifest apparent E1/2 and E1/2 values that
are essentially invariant with respect to those elucidated for their
constituent monomeric precursors.
Figure 1 displays exemplary conjugated (porphinato)metal oli-
gomers16 that feature a meso-to-meso ethyne-bridged linkage
topology, along with their corresponding ethyne-elaborated (por-
phinato)zinc(II) (PZn) building blocks (E-D, E2-D, E2-A). This
mode of macrocycle-to-macrocycle connectivity gives rise to
extensive interpigment electronic interactions.17-22 Compounds E-D,
E2-D, DD, DDD, and DDDDD constitute highly soluble analogues
of previously studied examples of this structural motif having simple
10,20-diaryl substituents,17-22 while DA, DAD, and DADAD define
related conjugated arrays in which electron-rich and electron-poor
PZn units alternate. Importantly, these A porphyrin units feature
macrocycle 10,20-bis(perfluoroalkyl) groups. Potentiometric and
electronic structural studies establish that [5,15-bis(perfluoroalkyl)-
porphinato]zinc(II) species possess HOMOs and LUMOs that are
uniformly lowered in energy by ∼0.3 eV relative to corresponding
Figure 2 displays optical spectra for these classes of highly
conjugated PZn arrays. Arrays DD and DDD display absorptive
and emissive signatures similar to those elucidated for their parent
compounds that bear unelaborated phenyl groups (Figure 1).17
Impressively, conjugation length expansion of this motif utilizing
a combination of PZn building blocks with 10- and 20-[3,5-bis(9-
methoxy-1,4,7-trioxanonyl)phenyl] and [3,5-bis(3,3-dimethyl-1-
butyloxy)phenyl] substituents gives rise to a highly soluble
pentakis(PZn) structure (Figure 1). While the electronic structural
characteristics of meso-to-meso ethyne-bridged porphyrin com-
pounds have been discussed previously, it is worth noting that
DDDDD displays a high oscillator strength Q-state-derived π-π*
absorption (λmax ) 842 nm) with an extinction coefficient exceeding
225 000 M-1 cm-1 (Figure 2A). The corresponding optical spectra
for the DA-based arrays are shown in Figure 2B. Note that while
the DA-conjugated porphyrin arrays display (i) B-state domain
spectral breadths that exceed that observed for their corresponding
DD counterparts and (ii) high oscillator strength, low-energy
Q-derived transitions, and corresponding S1-S0 fluorescence emis-
sion bands that are blue-shifted slightly with respect to the analogous
transitions of conjugated porphyrin arrays composed of all electron-
rich macrocycles (Figure 2A), these two classes of ethyne-linked
porphyrinic oligomers manifest remarkably similar optical proper-
0/+
-/0
* To whom correspondence may be sent. E-mail: therien@a.chem.upenn.edu.
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C O M M U N I C A T I O N S
Figure 3. Dependence of the potentiometrically determined HOMO and
LUMO energy levels upon the S0-S1 optical band gap (Eop) for (A) DD
and (B) DA porphyrin arrays as a function of conjugation length. Oxidation
and reduction potentials are denoted respectively by filled and open circles.
All these redox potentials are relative to the ferrocene/ferrocenium (Fc/
Fc+) redox couple, which was used as an internal standard in these
experiments. Solvent: for DD systems, CH2Cl2; for DA systems, THF.
Tabulated potentiometric data are available in the Supporting Information.
Figure 2. Room-temperature electronic absorption spectra of bis-, tris-,
and pentakis[(porphinato)zinc(II)] arrays based on the DD and DA structural
motifs in THF solvent: (A) DD (broken line), DDD (dotted line), and
DDDDD (solid line); (B) DA (broken line), DAD (dotted line), and DADAD
(solid line). Corrected emission spectra, with labeled emission wavelength
maxima, are shown in the insets. Experimental details are available in the
Supporting Information.
ties, displaying both the hallmarks of extensive π conjugation and
exciton coupling.
Examination of the dependence of both Ep and Eop on increasing
conjugation length, however, shows marked divergence between
the conjugated DD and DA PZn arrays (Figure 3). The DD systems
manifest (i) potentiometric responses indicating that the radical
anion and radical cation states of these species become increasingly
stabilized and destabilized, respectively, with increasing conjugation
length and (ii) E(LUMO) - E(HOMO) potential differences (Ep’s)
that track with Eop; such characteristics epitomize the key properties
of highly conjugated oligomers. In stark contrast, while optical data
(Figure 2B) show that the DA oligomers manifest globally
delocalized singlet-excited states, cyclic voltammetric data (Figure
3B) indicate that both the radical cation- and anion-state energy
levels of these species vary little with increasing conjugation length.
Note that the experimentally determined E1/20/+ and E1/2-/0 potentials
0/+
-/0
for these DA-conjugated oligomers resemble E1/2 and E1/2
values measured for benchmark ethyne-elaborated PZn mono-
mers: Figure 3B underscores this uncommon redox behavior,
highlighting that DA, DAD, and DADAD one-electron reduction
Figure 4. Frontier molecular orbitals for the tris[(porphinato)zinc(II)]
species DDD and DAD determined from PM3 calculations. Computational
details, as well as a more expansive set of frontier orbitals for these species,
are available in the Supporting Information.
-/0
potentials track the E1/2 value established for E2-A, while their
corresponding one-electron oxidation potentials conform to the
0/+
magnitude of E1/2 determined for E-D (E2-D).
similarly comprehensive electronic delocalization, exhibiting ex-
tensive cumulenic character along the C2 axis defined by the ethyne
moieties.
The nature of the DAD FOs differ markedly from those
elucidated for DDD (Figure 4); these differences derive from the
fact that substantial energy gaps separate the D- and A-localized
PZn fragment orbitals of equivalent symmetry (Supporting Informa-
tion). Two effects of this are evident in the Figure 4 FOs. First,
while the DAD HOMO bears an electron density distribution similar
to that determined for the analogous DDD orbital, note that it lies
Insight into this unusual electrooptic behavior can be gleaned
from electronic structural analysis of the frontier orbitals (FOs) of
the DD- and DA-conjugated oligomers. Figure 4 displays the
HOMO-1, HOMO, LUMO, and LUMO+1 for the DDD and DAD
supermolecular species. As expected,21 the DDD HOMO exhibits
substantial ethyne-, 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; the DDD LUMO manifests
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C O M M U N I C A T I O N S
0.4 eV lower in energy; this energy difference is reflected in the
magnitudes of the respective E1/2 values determined for these
Acknowledgment. This work was supported through the
MRSEC Program of the National Science Foundation (DMR00-
79909) and the Office of Naval Research (N00014-98-1-0725).
M.J.T. and K.S. thank respectively the Camille and Henry Dreyfus
Foundation and the Japanese Society for the Promotion of Science
for research fellowships.
0/+
species in the anodic electrochemistry. Two σ-electron-withdrawing
perfluoroalkyl groups thus stabilize the FOs of meso-to-meso
ethyne-bridged tris[P(Zn)] compounds to an extent similar to which
these two substituents stabilize A-type PZn monomers relative to
their meso-aryl counterparts (Supporting Information);23-25 this
substituent-derived electronic stabilization counterbalances the
magnitude of HOMO destabilization that occurs concomitant with
meso-to-meso ethynyl conjugation of three PZn monomer units in
DDD. Second, in contrast to the case for DDD, note that the DAD
LUMO is localized exclusively upon the central [10,20-bis-
(perfluoroalkyl)porphinato]zinc(II) unit, with electron density dis-
tributed primarily upon the pyrrole CR, Câ, and N atoms, and the
meso carbon positions lying orthogonal to the highly conjugated
supermolecular axis. Figure 4 hence underscores the cardinal role
that D and A PZn fragment orbital energy differences play in fixing
the potentiometrically determined, conjugation-length-independent
radical cation- and anion-state energy levels evinced in Figure 3B
for the DA PZn oligomers. 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, extensive configuration interaction (CI) is necessary to
describe correctly porphyrin electronically excited states.26 While
absolute HOMO and LUMO energies and electron density spatial
Supporting Information Available: Detailed syntheses and char-
acterization data, tabulated optical and electrochemical data, along with
the frontier molecular orbitals determined for the DDD and DAD arrays
and a selection of benchmark (porphinato)zinc(II) species (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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