Interchromophore orientation scaffolding by m-terphenyl oxacyclophanes

Article abstract of DOI:10.1039/c0cc00247j

Modular oxacyclophanes featuring m-terphenyl units scaffold inter-π-system interaction in face-to-face stacked or orthogonal orientations, leading to distinct photophysical properties. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc00247j

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Interchromophore orientation scaffolding by m-terphenyl
oxacyclophanesw
Anshuman Mangalum, Brad P. Morgan, Jessica M. Hanley, Kristine M. Jecen,
Charles J. McGill, Grant A. Robertson and Rhett C. Smith*
Received 5th March 2010, Accepted 13th May 2010
First published as an Advance Article on the web 10th June 2010
DOI: 10.1039/c0cc00247j
Modular oxacyclophanes featuring m-terphenyl units scaffold
inter-p-system interaction in face-to-face stacked or orthogonal
orientations, leading to distinct photophysical properties.
Molecules featuring covalently-scaffolded interchromophore
interactions1,2 or that feature intramolecular charge-transfer
3–12
(i.e., cruciforms)
have attracted considerable interest.
Inter-p-system interactions between chromophores have
important practical consequences because the optoelectronic
performance of organic semiconducting polymers (CPs) is
dictated by intermolecular morphology.1,13–18 Developing
rational design principles for organic electronics thus requires
a deeper understanding of inter-p-system interactions, spurring a
multitude of theoretical19–22 and experimental studies focused
on scaffolded inter-p-system geometries.1,23,24 In this vein, we
have designed a modular scaffold capable of replicating
multiple types of inter-p-system orientations through the
assembly of common building blocks.
Guided by m-terphenyl-scaffolded cappedophanes and cup-
pedophanes first reported by Vinod and Hart,25,26 we devel-
oped a p-conjugated polymer (CP1, Chart 1) incorporating
cyclophane units derived from 8 (Scheme 1C) with parallel aryl
rings whose centroid-to-centroid distance (3.478 A) was
exceptionally suitable for p-stacking.27 Herein, we report the
tetrahalogenated oxacyclophanes 6 and 7 (Scheme 1B) and
their four-fold derivatization with chromophores. Compound
6 demonstrates parallel interactions between aryl rings,
whereas 7 scaffolds orthogonal interactions.
Scheme 1 (A) Routes to key precursor 5; i. (a) 10 equiv. 2-tolyl-
magnesium bromide, D; (b) I2. NBS N-bromosuccinimide,
BPO = benzoylperoxide. (B and C) Preparation of 6 and 7, OCs,
and 9; i. [Pd(PPh3)4], CuI, (4-tert-butylphenyl)acetylene, HNiPr2,
toluene, 90 1C. Details for all syntheses are provided in the ESI.w
=
Initial efforts to prepare 6 and 7 involved direct radical
bromination of the benzylic sites in 128 (Scheme 1A). This route
led to mixtures of bromomethyl- and dibromomethyl-substituted
terphenyls.29 The bis(dibromomethyl) derivative 2, however,
was cleanly produced using excess NBS, allowing access to 5
from 2 via conversion of 2 to aldehyde 3, reduction to 4, and
finally treatment with PBr3 to give 5 in 28% overall yield from
2 (Scheme 1A). Significant improvements were accomplished
by directly converting 2 to 5 by the action of diethylphosphite
and EtNiPr2,30 yielding 5 in 60% yield from 2. Tetrahalogenated
6 (55%) and 7 (10%) were readily prepared via slow addition
of a solution of 5 and 1,2-dihydroxy-3,6-diiodobenzene or
1,4-dihydroxy-2,5-diiodobenzene, respectively, to a suspension
of K2CO3 in DMF at 90 1C over 96 h (Scheme 1B).
Chart 1
Department of Chemistry and Center for Optical Materials Science
and Engineering Technologies (COMSET), Clemson University,
Clemson, SC, USA. E-mail: rhett@clemson.edu;
Fax: +1 864 656 6613
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and spectroscopic studies, 1H and 13C NMR
spectra. CCDC reference numbers 768761–768764. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc00247j
Structures for the oxacyclophanes were elucidated by a
combination of 1D and 2D NMR spectroscopic techniques.31
The tetrahalogenated cores were also characterized by single
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Fig. 1 ORTEP drawings (50% ellipsoids) showing inter-p-system
distance (A and C) and axial tilt angle as viewed looking down
through the upper aryl (B and D) for 6 (left) and 7 (right). H atoms
are excluded and all non-C atoms are labelled. Full numbering
schemes and refinement details are provided in the ESI.w
crystal X-ray diffraction. The ORTEP drawings of the two
cores (Fig. 1A and B) reveal that 6 holds upper and lower aryls
in an offset p-stacking arrangement with a centroid-to-
centroid distance of 3.623 A. The relative positions of iodo
and bromo substituents suggest that the axes of p-systems
appended at these sites would be tilted at an angle (a) of 211
with respect to one another. In 7 (Fig. 1C and D) the upper
and lower aryls are orthogonal (interplane angle = 891) to one
another with centroid-to-centroid distance of 5.577 A and a
of 691.
Chromophore incorporation into the scaffolds was readily
accomplished by four-fold Sonogashira–Hagihara type cou-
pling of the cores with 4-tert-butylphenylacetylene to afford
OC1 (50%) and OC2 (42%) (Fig. 1 and Scheme 1B). Optimized
geometries (DFT at the B3LYP/6-31G(d) level) for the
materials are shown in Fig. 2, confirming the aforementioned
inter-p-system interactions in the cores. Because the relative
orientation of the upper and lower p-systems dictates the
interchromophore interaction, we believed that the systems
supported by OC1 and the OC2 would exhibit radically
different properties.
Fig. 2 Optimized geometries (A and B, tBu groups omitted for
clarity), linebond structures (C and D, R = tBu), and calculated
LUMO (E and F) and HOMO (G and H) distributions for OC1 (left
column) and OC2 (right column) determined by DFT calculations
at the B3LYP/6-31G(d) level (Spartan 04). Unscaffolded models
for photophysical comparison are shown in I. The tBu units were
truncated to methyl groups in E and G.
Table 1 Photophysical data for oxacyclophane materials and various
upper and lower p-system models
lp–p*/nm
log e
lemit/nm
F
OC2
OC1
9
10
12
335
335
349
335
320
330
356
4.56
4.79
4.80
4.91
5.79
5.05
5.35
429
420
390
362
366
360
395
0.52
0.012
0.007
0.58
0.58
0.40
0.50
The calculated centroid-to-centroid distance between aryls
in OC1 (3.86 A) is within the range (3.77–4.50 A) of p-stacking
interactions that lead to notable photophysical effects in
organic semiconducting polymer films.20 The OC2 is a reason-
able model for orthogonal (‘‘T-type’’) interaction between
aryls,32 albeit with longer centroid-to-centroid distance (5.39 A)
than typical (4.99–5.19 A) for such interactions. Space-filling
models33 indicate that there is likely not enough free space
between the central rings of upper and lower systems for
inclusion of solvent or other guest molecules between them
in either OC.
11
13
each OC features similar chromophores, their absorption
spectra are expected to be quite similar barring some inter-
p-system interaction in the ground state. Ground state effects
typically require quite close proximity of two p-systems, such
as that enforced by [2.2]paracyclophanes.2,34 At the distances
inferred from optimized geometries of OCs, ground state
perturbations are not likely, and NMR spectra show no
sign of notable ground state inter-p-system interaction that
would manifest as shifts in aromatic proton resonances. Each
Photophysical properties of OC2 and OC1 (Table 1) were
examined and compared to those of models (Fig. 2I) for both
upper (10 and 12) or lower p-system (9, 11 and 13). Because
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absorption spectrum exhibits one p–p* transition band with
lmax (335 nm for both) and e values (Table 1) consistent with
the chromophore units (a higher energy band attributable to
the aryls attaching the two chromophores is also observed).
11, the model for the lower p-system of OC2, has a similar
Calhoun Honors College for support through the EUREKA!
program.
Notes and references
l
max (330 nm) to that in OC2. 13, the model for the lower deck
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in OC1, however, has lmax of 356 nm. The red shift for the
transition in 13 versus the similar unit in OC1 is attributable to
the constrained nature of the OC1. In 13, methoxy units act as
strong electron donors, whereas the constrained geometry of
OC1 does not permit the planarization at oxygen necessary for
the substituents to act as resonance donors. Further support
for this explanation comes from model 9, a model for OC1
lacking the upper chromophore. Because 9 lacks the upper
chromophore, it is less constrained than OC1, but still more
constrained than 13. Predictably, the lmax for 9 lies between
values for 13 and OC1. Both the OCs have the same upper
chromophore, which we attempted to model with 10. 10 has
lmax = 335 nm, the same value as observed in the OCs.
In contrast to the absorption spectra, significant differences
should be manifest in the photoluminescence properties of
OC1 and OC2. Systems in which face-to-face inter-p-system
interactions are operative (i.e., OC1) are predisposed to
intramolecular energy/electron transfer (internal quenching
rather than fluorescence) and may be expected to have low
photoluminescence quantum yields (F). Systems in which
p-systems are orthogonal (i.e., OC2), however, are not
expected to undergo intermolecular energy/electron transfer
processes, and should display higher F values. The remarkably
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different
F values for OC2 (0.52) and OC1 (0.012)
unequivocally demonstrate the influence of the scaffolds on
the fate of absorbed energy. DFT calculations suggest that the
HOMO and LUMO are geometrically separate in OC1,
reminiscent of cruciforms. These data suggest that internal
energy transfer or internal self-quenching of fluorescence may
be possible in OC1. Even the presence of the unmodified aryl
in 9 (F = 0.007) is apparently sufficient for nonradiative
quenching via intramolecular interaction, suggesting a vibra-
tional relaxation mechanism is also viable. The exceedingly
low quantum yield of OC1 is particularly noteworthy when
compared to any of the models 11 and 13 and 10 and 12, all of
which have high quantum yields (0.40–0.58), similar to OC2.
In summary, synthetic routes have been developed to
produce synthetically versatile tetrahalogenated m-terphenyl
oxacyclophanes and to derivatize these scaffolds with up to
four p-conjugated substituents. The utility of the oxacyclo-
phanes to scaffold p-conjugated systems in discrete geometries
should have application in the rational design of materials
exhibiting energy/charge transfer or high photoluminescence.
The simple attachment strategy and modularity of upper and
lower components leads to facile elaboration of these materials
to include additional and mixed chromophore systems and
semiconducting p-conjugated polymers.35 Studies are underway
to gain further insights into other inter-p-system influences
that may be accessible with these scaffolds.
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Spectroscopy (COSY), Heteronuclear Multiple Quantum
Coherence (HMQC), Distortionless Enhancement by Polarization
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Spectra and full spectral assignments are provided in the ESIw.
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35 Stille, Heck, and Suzuki coupling to append small molecule
chromophores and polymers to these and related cores have
proven successful. Full reports on each of these themes are
currently in preparation.
This work was supported by the National Science Foundation
through a CAREER award (CHE-0847132). GAR thanks the
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This journal is The Royal Society of Chemistry 2010
5138 | Chem. Commun., 2010, 46, 5136–5138