π-Stacked oligo(phenylene vinylene)s based on pseudo-geminal substituted [2.2]paracyclophanes: Impact of interchain geometry and interactions on the electronic properties

Article abstract of DOI:10.1002/anie.201205738

A clever combination: A series of phenylene vinylene oligomers, in which the conjugated segments are held in a well-defined stacked arrangement along their entire length, was studied experimentally and theoretically (see picture). The impact of the extended interchain interactions on the photophysics of the π-stacked systems is reported. Copyright

Full text of DOI:10.1002/anie.201205738

Angewandte
Chemie
DOI: 10.1002/anie.201205738
Electronic Structure
p-Stacked Oligo(phenylene vinylene)s Based on Pseudo-Geminal
Substituted [2.2]Paracyclophanes: Impact of Interchain Geometry and
Interactions on the Electronic Properties**
Sukrit Mukhopadhyay, Subodh P. Jagtap, Veaceslav Coropceanu, Jean-Luc Brꢀdas, and
David M. Collard*
The design of conjugated organic semiconducting materials
for use in field-effect transistors, light-emitting diodes, and
solar cells requires a detailed understanding of the relation-
ships between molecular geometry, interchain interactions,
and electronic properties.[1] The investigation of molecules
that consist of pairs of stacked conjugated chains held in well-
defined arrangements by a [2.2]paracyclophane (CP)[2] core is
a useful strategy to explore interactions between p systems.
For example, Bazan et al. have performed extensive spectro-
scopic and theoretical analyses of stacked phenylene vinyl-
enes (PV) based on pseudo-para (pp) analogues of [2.2]para-
cyclophane.[3] When two stilbene segments (PV2) are stacked
in this fashion, that is, pp-CP(PV2)2 in Figure 1, the fluores-
cence spectrum is dominated by a broad featureless peak that
is significantly red-shifted from the absorption maximum by
virtue of the presence of a localized “phane state”.[3] How-
ever, the fluorescence spectrum of the analogue with longer
stacked 1,4-distyrylbenzene (PV3) segments, that is, pp-CP-
(PV3)2, resembles that of the corresponding unstacked con-
jugated oligomer (Me2PV3). While the pp and pseudo-ortho
(po) analogues hold the two conjugated chains in close
proximity, only the phenylene rings within the paracyclo-
phane core are stacked. As a result, there are minimal
interactions between the two conjugated segments as soon as
their lengths are increased.
The pseudo-geminal (pg) substitution pattern of [2.2]par-
acyclophane holds substituents on each ring directly atop one
another. Such stacking of conjugated units leads to facile
intramolecular photochemical cycloadditions,[4–7] and new
electronic transitions in multi-decker stacks.[8] The opportu-
nity to prepare new materials containing the cyclophane core
has recently attracted increased attention.[9,10] Here, we
explore the absorption and fluorescence spectra of stacked
PV oligomers that are based on the pg-CP core: the stacked
stilbene pg-CP(PV2)2 and the analogoues distyrylbenzene, pg-
CP(PV3)2. We compare the spectroscopic and electronic
properties of the pg compounds to those of their pp and po
analogues and the corresponding isolated oligomers.
Conjugated arms were installed on the pg core by a Heck
reaction between 4,15-divinyl[2.2]paracyclophane[5] and 1-
iodobenzene or 1-iodo-4-styrylbenzene to afford pg-CP[PV2]2
and pg-CP[PV3]2, respectively (see Figure 2 and the Support-
ing Information for experimental details). The 1H NMR
spectra of the products are consistent with conformations in
which the arms are oriented away from the neighboring
ethano bridge, as previously elucidated for 4,15-divinyl-
[2.2]paracyclophane.[5]
The absorption maximum of pg-CP[PV2]2 (4.19 eV) is
only slightly blue-shifted relative to that of the isolated
analogue Me2PV2 (4.17 eV), see Figure 3A and Table 1.
However, a major difference between the spectra of these
compounds is the presence of a shoulder arising from
a contribution at 3.38 eV for the stacked molecule. Similar
features are observed in the absorption spectrum of the
longer stacked compound pg-CP[PV3]2 (Figure 3B).
The emission spectrum of Me2PV2 presents a distinct
vibronic progression. On the other hand, the emission
spectrum of pg-CP[PV2]2 displays a broad, structureless
peak (2.86 eV) that is red-shifted by about 0.2 eV compared
to the corresponding pp and po analogues.[3a] The fluores-
cence spectrum of the longer stacked analogue, pg-CP[PV3]2,
is similar to that of the shorter analogue but with a significant
tail by virtue of a strong contribution at 2.56 eV, which is
clearly apparent in the difference spectrum shown as an inset
in Figure 3B. Importantly, this broad tail is absent from the
Figure 1. [2.2]Paracyclophane oligo(phenylene vinylene)s based on
a pseudo-para (pp), pseudo-geminal (pg) core, and the isolated model
chromophores (Me2PVn).
[*] Dr. S. Mukhopadhyay, Dr. S. P. Jagtap, Dr. V. Coropceanu,
Prof. J.-L. Brꢀdas, Prof. D. M. Collard
School of Chemistry & Biochemistry and Center for
Organic Photonics and Electronics
Georgia Institute of Technology
Atlanta, Georgia 30332-0400 (USA)
E-mail: david.collard@chemistry.gatech.edu
Prof. J.-L. Brꢀdas
Department of Chemistry, King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
[**] This work was supported by NSF awards ECCE-0437925, DMR-
0120967 and CHE-0946869.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205738.
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tures of the pg molecules and their pp and po
counterparts. To understand the origin of these
differences, we performed time-dependent density
functional theory (TD-DFT) calculations on the
stacked and isolated systems.
The TD-DFT-calculated excited-state energies
and oscillator strengths are provided in Table 2 (see
the Supporting Information for details). The wave-
function characteristics of the S0!S1 and S1!S0
transitions are illustrated in Figure 4. In Me2PV2
and Me2PV3, the lowest TD-DFT absorption corre-
sponds to the S0!S1 transition and is dominated by
Figure 2. Synthesis of the pseudo-geminal compounds, pg-CP[PV2]2 and pg-CP-
[PV3]2.
Table 1: Main characteristics of the absorption and emission spectra.
Molecule
Absorption maxima
[nm[a] (eV)[b]]
Emission maxima
[nm[a] (eV)[b]]
Me2PV2
297
345, 359, 381
(4.17)
(3.56, 3.39, 3.22)
pg-CP[PV2]2
Me2PV3
296, 358[c]
(4.19, 3.38)
435
(2.86)
352
396, 416, 440
(3.52)
(3.09, 2.92, 2.75)
pg-CP[PV3]2
350, 404[c]
404, 427, 446, 481
(3.54, 3.07)
(3.06, 2.90, 2.73, 2.56)
[a] Peaks in spectra, Figure 3. [b] Energies (eV) computed from decon-
voluted spectra (see the Supporting Information). [c] Shoulder.
a simple HOMO!LUMO excitation (HOMO/LUMO =
highest/lowest occupied/unoccupied molecular orbital). The
origins of the first absorption band in pp-CP[PV2]2 and pp-
CP[PV3]2 are different, being related to S0!S2 and S0!S1
transitions, respectively. For smaller analogues, the S2 states of
the pp compounds are primarily localized on a single segment
and may be referred to as local states.
We note that the difference in energy between the S1 and
S2 states of pp-CP[PV2]2 is small (0.1 eV, Table 2); thus, the
weak transition to the S1 state is masked by the S2 transition in
the absorption spectrum.
The picture changes markedly in the pg compounds. Here,
the absorption spectra are dominated by the S0!S2 transition
irrespective of the length of the conjugated arms and
correspond to the mixing of arm-localized and cyclophane-
core-localized (phane-state) excitations (see the Supporting
Information). The S1 state has a weak oscillator strength and
is located below the S2 state by 0.5 eV in pg-CP[PV2]2 and
0.3 eV in pg-CP[PV3]2, which is consistent with the appear-
ance of the low-energy shoulders in the absorption spectra.
Figure 4 shows how the S1 states of the pg compounds, in
contrast to the pp analogues, are delocalized over the entire
molecules and correspond to a linear combination of intra-
arm localized excitations and interarm charge-transfer exci-
tations (see the Supporting Information).[11,12]
Figure 3. Normalized UV/Vis and fluorescence spectra (CHCl3, 238C)
of A) pg-CP[PV2]2 (solid line; c=1.3ꢁ10ꢀ6 m), Me2PV2 (dotted line;
c=2.5ꢁ10ꢀ6 m), and pp-CP[PV2]2 (dashed line) and B) pg-CP[PV3]2
(solid line; c=1.5ꢁ10ꢀ6 m), Me2PV3 (dotted line; c=3.0ꢁ10ꢀ6 m), and
pp-CP[PV3]2 (dashed line; abs=absorption and em=emission). The
inset in (B) depicts the difference in emission spectra between pg-
CP[PV3]2 and Me2PV3 on an energy scale. The spectra of pp compounds
are adapted from ref. [3a]; those of the po analogues are similar.
fluorescence spectra of the unstacked oligomer Me2PV3 and
the corresponding pp and po analogues (see Figure 3 and the
Supporting Information).[3a] Thus, the spectroscopic data
reveal significant differences between the electronic struc-
The results of these calculations allow us to propose the
following scenario. In pp-CP[PV2]2, absorption into the S2
state is followed by internal conversion to the S1 state, with
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Table 2: DFT estimates of vertical absorption (in the S0-state geometry)
and emission energies (in the S1-state relaxed geometry), ground-state
relaxation energies upon emission (lgs), and 0–0 emission energies. The
oscillator strengths are given in parentheses. All energies are in electron
volts.[a]
strongly red-shifted compared to the emission maxima in the
pp, po, and isolated analogues.
Turning to the longer CP[PV3]2 compounds, an even more
striking difference in the nature of the S1 state is apparent,
when comparing the pg and pp analogues. In pp-CP[PV3]2, the
S1 state is strictly confined to a single distyrylbenzene segment
(Figure 4C). In contrast, the S1 state of pg-CP[PV3]2 is
delocalized over the entire molecule (Figure 4D) and pres-
ents a similar excimer nature as the S1 state of pg-CP[PV2]2.
This is consistent with the presence of the broad shoulder in
the emission spectrum (inset of Figure 3B), unlike the
situation for the isolated pp and po analogues that display
no sign of excimer emission. Thus, the fully delocalized
excimeric nature of the S1 state is observed only when the
conjugated chromophores stack along their entire length.
To summarize, the excimeric emission of pg-stacked
oligo(phenylene vinylene)s is in contrast to emission from
a phane-state in pp-CP(PV2)2 and from a localized excited
state in pp-CP(PV3)2. The stacking of conjugated oligomers in
a well-defined manner along their entire length by virtue of
the pg-CP scaffold demonstrates the impact of extended
interchain interactions on the photophysics of p-stacked
systems.
Molecule
Excited S0 state
state
Relaxed S1 state lgs 0–0 Emission
energy
Me2PV2
S1
4.4 (0.89) 3.2 (0.82)
0.5 3.7
pp-CP[PV2]2 S1
4.1 (0.02) 3.3 (0.01)
4.2 (1.26)
0.4 3.7
0.7 2.8
S2
pg-CP[PV2]2 S1
3.7 (0.01) 2.1 (0.01)
4.2 (0.44)
S2
Me2PV3
S1
3.7 (1.81) 3.0 (1.92)
3.6 (3.48) 2.9 (2.30)
0.1 3.1
0.3 3.2
0.5 2.5
pp-CP[PV3]2 S1
pg-CP[PV3]2 S1
3.3 (0.02) 2.0 (0.01)
3.6 (1.61)
S2
[a] Computed at the wB97X-D/6-31g** level for the transoid conforma-
tion of Me2PV3 and the transoid–transoid conformations of pg-CP[PV3]2
and pp-CP[PV3]2.
Received: July 18, 2012
Revised: September 4, 2012
Published online: October 12, 2012
the S1!S0 emission dominated by a localized phane-state
contribution (see Figure 4A). In contrast, the pg isomer
undergoes a marked geometry relaxation in the S1 state that
results in a decrease of the distance between the stacked
conjugated segments. The relaxed S1 state (Figure 4B) is
delocalized over the entire molecule and has larger contribu-
tions from intersegment charge-transfer excitations than from
intrasegment localized excitations. Thus, the S1 state of the pg
compound corresponds to an excimer-like state delocalized
over the entire molecule, in contrast to the phane-localized S1
state of the pp and po analogues. The characteristics of such
an excimer state are consistent with the experimental
observation of a broad, featureless emission band that is
Keywords: conjugation · density functional calculations ·
.
photophysics · p interactions · stacking interactions
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