Fingerprints of Through-Bond and Through-Space Exciton and Charge π-Electron Delocalization in Linearly Extended [2.2]Paracyclophanes

Article abstract of DOI:10.1021/jacs.6b12520

New stilbenoid and thiophenic compounds terminally functionalized with donor-donor, acceptor-acceptor, or donor-acceptor moieties and possessing a central [2.2]paracyclophane unit have been prepared, and their properties interpreted in terms of through-bo

Full text of DOI:10.1021/jacs.6b12520

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Article
Fingerprints of Through-Bond and Through-Space Exciton and Charge
-Electron Delocalization in Linearly Extended [2.2]paracyclophanes
#
Jose Luis Zafra, Agustin Molina Ontoria, Paula Mayorga Burrezo, Miriam Peña-Alvarez, Marek Samoc,
Janusz Szeremeta, Francisco Javier J. Ramírez Aguilar, Matthew D. Lovander, Christopher J. Droske, Ted
M. Pappenfus, Luis A. Echegoyen, Juan T. López Navarrete, Nazario Martín, and Juan Casado Cordón
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12520 • Publication Date (Web): 07 Feb 2017
Downloaded from http://pubs.acs.org on February 7, 2017
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Fingerprints of Through-Bond and Through-Space Exciton and Charge π−Electron
Delocalization in Linearly Extended [2.2]paracyclophanes
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José L. Zafra , Agustín Molina Ontoria , Paula Mayorga Burrezo , Miriam Peña-Alvarez , Marek Samoc , Janusz
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Szeremeta , Francisco J. Ramírez Aguilar , Matthew D. Lovander , Christopher J. Droske , Ted M. Pappenfus* , Luis
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Echegoyen , Juan T. López Navarrete* , Nazario Martín* , Juan Casado*
1
Department of Physical Chemistry, University of Málaga, Campus de Teatinos s/n, Málaga 29071, Spain
2
IMDEAꢀNanociencia, C/Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain
3
Department of Organic Chemistry, Faculty of Chemistry, University Complutense of Madrid, 28040 Madrid, Spain.
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Department of Physical Chemistry, Chemistry Faculty, University Complutense of Madrid, 28040 Madrid, Spain.
5
Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, 50370 Wroclaw, Poland
6
Division of Science and Mathematics, University of Minnesota Morris, MN 56267, USA
7
Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, USA
Corresponding authors eꢀmail addresses: teodomiro@uma.es; nazmar@ucm.es; casado@uma.es
RECEIVED DATE (automatically inserted by publisher);
Abstract. New stilbenoid and thiophenic compounds
terminally functionalized with donorꢀdonor, acceptorꢀacceptor or
donorꢀacceptor moieties and possessing central
[2.2]paracyclophane unit have been prepared and their properties
interpreted in terms of throughꢀbond and through space
π−electron delocalization (i.e., π−conjugations). Based on
photophysical data, their excited state properties have been
described with a focus on the participation of the central
through bond (TB) and through space (TS) π−electron
delocalization and conjugation in one single molecule (Scheme 1).
As a result, structureꢀproperty studies with pCp based conjugated
molecules can provide significant insights to develop novel
designs of molecules and polymers with enhanced optoelectronic
performance.
a
The coupling in π−extended pCp chromophores of the two
relevant perpendicularly disposed TB and TS electronic paths
[
2.2]paracyclophane in competition with throughꢀbond
(Scheme 1) results in the formation of complementary states
conjugation in the side arms. To this end, twoꢀphoton and oneꢀ
photon absorption and emission spectroscopy, as a function of
temperature, solvent polarity and pressure in the solid state have
been recorded. Furthermore, charge delocalization through the
which, given the nature of their orthogonal arrangement, can
compete for the stabilization of the exciton or of the charge. This
provides a scenario where the optical and electronic properties can
be tuned from paracyclophaneꢀtype to branchꢀtype. For instance,
the synergistic competition between TB and TS in donorꢀacceptor
extended pCp has been exploited in photovoltaic devices.
Thereby, direct donorꢀtoꢀacceptor electron transfer has been
shown to be effective in generating the charge separated state
[
2.2]paracyclophane in the neutral state and in the oxidized
species (radical cations, dications and radical trications) has been
investigated, allowing the elucidation of the vibrational Raman
fingerprint of through space charge delocalization. Thus, a
complementary approach to both “intermolecular” excitation and
charge delocalizations in [2.2]paracyclophane molecules is shown
which can serve as models of charge and exciton migration in
organic semiconductors.
(radical anion and cation) by action of TB plus TS connections.
However, the charge recombination kinetics is slowed down by
5
the blockage of TS conjugation. The excimerꢀlike character of
the relevant “phane” excited state has been characterized by
13
8,9,14
linear
optical
methods,
twoꢀphoton
absorption,
1
ꢀ7
fluorescence and by nanosecond and microsecond transient
5,15
absorption spectroscopies , mainly in the seminal work of
Bazan et al. The effect of charge delocalization in pCp
compounds has been much less explored.
INTRODUCTION
The soꢀcalled “phane state” of [2.2]paracyclophanes, defined
as pCp, represents an excimerꢀlike state generated by
wavefunction delocalization between the two sandwiched
benzenes, which is promoted by the coꢀfacial throughꢀspace
coupling of the two aromatic units. Thus, pCp is a wellꢀdefined
molecular model for interꢀchromophoric exciton and charge
delocalizations, a critical aspect to understand more complex
unique solid state intermolecular phenomena taking place in
crystalline and in amorphous organic semiconductors. The
1
ꢀ5
6
[
2.2]paracyclophane
unit
covalently
extended
with
and
7
ꢀ9
oligoparaphenyleneꢀvinylenes
(stilbenoids)
10ꢀ 12
oligothiophenes
has been reported as a way to combine
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Scheme 1. Top) The [2.2]paracyclophane unit, pCp, and the relevant electronic
throughꢀbond and throughꢀspace electronic effects in conjugated extended systems.
Bottom) Exciton and charge delocalization in pCp.
carried out by a twoꢀfold Heck Pdꢀcatalyzed reaction between
,16ꢀdivinyl[2.2]paracyclophane and 4ꢀiodobenzonitrile and/or 4ꢀ
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iodoꢀN,Nꢀbis(4ꢀmethoxyphenyl)aniline in good yields (67ꢀ75%).
On the other hand, malononitrile and cyanoacetic acid were
covalently connected to the dialdehyde Cp(PV-CHO) by a twoꢀ
2
Among the many classes of pCp´s substituted with conjugated
arms, two different concepts have been mainly studied: i) those
with “linear” shape, or pseudoꢀparaꢀderivatives, where the
branches are aligned in the same direction (these branches can be
fold Knoevenagel condensation affording Cp(PV-DCV)2 and
Cp(PV-CA) in 89% and 91% yields, respectively. The synthesis
2
1
6ꢀ18
identical or possess a donorꢀacceptor configuration
); and ii)
of asymmetrically substituted precursors bearing donor and
acceptor moieties were performed by two consecutive Heck
reactions to afford compound 6 (see Scheme S1). Finally, (DA-
PV)Cp(PV-DCV) was obtained by a Knoevenagel condensation
between malononitre and monoaldehyde 6 in 83% yield. The
aforementioned new compounds were spectroscopically
those with a “banana” shape, or pseudoꢀorthoꢀderivatives, where
1,3,8
the branches are disposed in 2D directions.
Since the
interactions between TB and TS channels govern the relevant
optical properties, we studied several cases of linearly substituted
pCp derivatives (all compiled in Scheme 2), namely: i)
phenyleneꢀvinylenes (PV, stilbene) acceptorꢀacceptor dyads
which are substituted at the terminal positions with electron
acceptor groups of variable electronꢀwithdrawing strength; ii)
phenyleneꢀvinylenes (PV, stilbene) donorꢀdonor dyads,
derivatives where the acceptors are substituted by bisphenyl
amino groups; iii) phenyleneꢀvinylenes (PV, stilbene) donor
acceptorꢀdyads where the pCp PV core is substituted with a donor
group at one side and an acceptor at the other; and iv) thiophene
donorꢀdonor dyads where the stilbenoid (without donor or
acceptor groups) branches are replaced by thiophenes.
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13
characterized by standard techniques (FTIR, H NMR, C NMR
and HRꢀMS, see Supporting Information, Section S1). As
representative spectroscopic features, all the compounds showed
the corresponding signal of the ethylene bridges as three broad
multiplets, integrating for 2:1:1, in the range of 2.8ꢀ3.6 ppm in the
1
H NMR spectra. The chemical structures of all new compounds
were ascertained by HRꢀMS.
HexO
Cp(PV-CN) : R = CN
Cp(PV-CHO) : R = CHO
OHex
OHex
CN
2
2
R
CHO
OHex
iii)
HexO
R
Cp(PV-DCV)
Cp(PV-CA)
2
I
I
OHex
OHex
2
OHex
2
3
OMe
OMe
i)
I
N
MeO
N
ii)
OMe
N
OMe
4
Cp(PV-DA)
2
1
MeO
P
C
P
-
1
ii)
NC
HexO
HexO
CN
MeO
MeO
CHO
OHex
MeO
OHex
i)
iii)
N
N
N
5
6
(DA-PV)Cp(PV-DCV)
MeO
MeO
MeO
Scheme 3 Preparation of stilbeneꢀbased donorꢀdonor, acceptorꢀacceptor and donorꢀ
acceptor materials. Reagents and conditions: i) Corresponding reagent 2 or 3,
Pd(OAc)
00 C; iii) Corresponding reagent dicyanomethane or cyanoacetic acid, AcOH,
NH Ac, CHCl
2 4 2 3 2 4 2 3
, Bu NBr, K CO , DMF, 100 °C; ii) 4, Pd(OAc) , Bu NBr, K CO , DMF,
Scheme 2 Chemical structures and abbreviation of the pCp studied in this work.
1
°
4
3
.
This study mainly focuses on the properties governed by the
phane unit with special attention on: i) excited states which
exhibit excitonic delocalization in the neutral molecules through
the pCp core and ii) oxidized species exhibiting charge
delocalization between the parallel benzenes in the ground
Previously, Collard and coworkers reported the use of a Stille
coupling reaction to produce donorꢀdonor compounds with
1
0
thiophenes in place of stillbenoid branches around the pCp core.
However, according to the authors, this coupling method provided
analytically impure products in low yield. The use of Suzuki
coupling to produce similar compounds with high purity
(according to combustion analyses) in moderate to good yield, as
shown in Scheme 4, is reported in this study. The addition of
hexylꢀcapped thienyl or bithienyl boronic esters to 4,16-
dibromo[2.2]paracyclophane produced Cp(Th) and Cp(Th )
electronic state.
A
variety of linear absorption and
photoluminescence methods have been explored in the case of the
excited state analysis, such as solvato– and thermo–chromism and
pressureꢀdependent properties and twoꢀphoton absorption
spectroscopy. To address charge delocalization in the ground
electronic state, the vibrational Raman spectra in different
oxidation states (radical cations, dications and radical trications)
were recorded. Quantum chemical calculations were conducted to
support and understand the spectroscopic data.
2
2 2
in yields of 79% and 41%, respectively. The model compound,
Ph(Th ) (Scheme 2) was prepared in a similar manner using 2-
2
bromo-p-xylene in place of the dibromoparacyclophane.
1
13
Structures of these new compounds were confirmed by H and
C
NMR (see Scheme S2ꢀ4).
Experimental and Theoretical Details.
1. Synthesis. The preparation of the symmetrically
functionalized donorꢀdonor, acceptorꢀacceptor pCp derivatives
was carried out by stepwise syntheses following wellꢀestablished
protocols under an inert atmosphere, summarized in Scheme 3.
Scheme 4. Preparation of thiopheneꢀbased donorꢀdonor materials.
Starting dialdehyde Cp(PV-CHO) was obtained as previously
2
2. Absorption and emission measurements. UVꢀvis absorption
and Fluorescence spectra were recorded in FLS920P
5,19
described.
Similarly, the syntheses of the dicyano derivative
a
Cp(PV-CN) and diphenylamine derivative Cp(PV-DA)2 were
2
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spectrofluorometer from Edinburgh Analytical Instruments
uniform scale factor of 0.96. The unrestrictedꢀB3LYP/6ꢀ31G**
approach was used for the openꢀshell radical trication and
dications. To simulate the openꢀshell groundꢀstate structures of
the dications, we used the brokenꢀsymmetry option with the key
guess=mix keyword also with the unrestricted wavefunctions at
the (U)B3LYP level.
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equipped with a pulsed xenon flashꢀlamp using the time correlated
single photon counting (TCSPC) operation mode. UVꢀVisꢀNIR
spectra were measured in a Cary 3000 spectrophotometer.
3
. Raman spectroscopy. The Raman spectra were recorded in
resonance conditions either by using the 1064 or 785 nm
excitations. The 1064 nm FT–Raman spectra were obtained with
an FT–Raman accessory kit (FRA/106–S) using a Bruker Equinox
RESULTS AND DISCUSSION
A. Acceptor-Acceptor Dyads.
5
5 FT–IR interferometer. A continuous–wave Nd–YAG laser
working at 1064 nm was employed for excitation. A germanium
detector operating at liquid nitrogen temperature was used. Raman
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A.1 Absorption spectra and orbital description. Figure 1
displays the electronic absorption spectra of the acceptorꢀacceptor
dyads in tetrahydrofuran at 298 K. The most intense absorption of
Cp(PV-CA) is at 450 nm which at the TD/DFT/6ꢀ31G** level
see Figure 1 and S1 for the theoretical spectra) corresponds to the
S0ꢀS3 excitation at 471 nm (oscillator strength, f= 2.07)
described as the sum of the Hꢀ1ꢀL and HꢀL oneꢀelectron
promotions. These orbitals in Figure 1 (Figure S2 and Table S1)
are centered in the branches with the two benzenes of the PV units
participating in the TB conjugation. No interactions between the
two benzenes of the paracyclophane unit are noticeable. There is a
medium intensity band at 396 nm in the spectrum, predicted as the
S ꢀS excitation and calculated at 396 nm (f = 0.22), arising from
scattering radiation was collected in
a
back–scattering
ꢀ
1
configuration with a standard spectral resolution of 4 cm . 1000–
3000 scans were averaged for each spectrum. Raman spectra with
the 785 nm excitation were collected using the 1×1 camera of a
Bruker Senterra Raman microscope by averaging spectra during
2
(
ꢀ
1
5
0 minutes with a resolution of 3–5 cm . A CCD camera
operating at −50 ºC was used.
4
. Thirdꢀorder nonlinear optical characterization. The thirdꢀ
order nonlinear properties were investigated using the Zꢀscan
technique. This technique allows to simultaneously measure both
the real Re(γ) and imaginary Im(γ) part of the cubic
hyperpolarizability that are responsible for nonlinear refraction
and nonlinear absorption, respectively. The nonlinear absorption
data can also be presented as the twoꢀphoton absorption cross
0
7
multiꢀelectron promotions between the Hꢀ4ꢀL and Hꢀ2ꢀL+1
orbitals. The third band at 334 nm is related to two theoretical
excitations: the first one, the S
nm (f = 0.21), due to an Hꢀ5ꢀL promotion, and the second one,
due to the S ꢀS excitation, calculated at 330 nm (f= 0.19),
arising from a Hꢀ1ꢀL+2 promotion. Interestingly, these two last
excitations involve orbitals, Hꢀ4 and Hꢀ5, where interꢀring
cofacial wavefunction delocalization within the pCp, or TS
delocalization, is feasible (Figure 1).
ꢀS11 excitation, calculated at 342
0
section σ . The measurements were performed in the wavelength
2
range from 560 nm to 1000 nm using laser pulses from a
Quantronix PalitraꢀFS optical parametric amplifier (OPA),
pumped with 130 fs, 800 nm pulses at 1 kHz repetition rate from a
Quantronix IntegraꢀC Ti:Sapphire regenerative amplifier. Open
and closedꢀaperture Zꢀscan traces were recorded simultaneously
by a pair of photodetectors, one of which was obscured by a 1 mm
aperture. A detailed description of the experimental procedure can
0
11
20
be found elsewhere. The results were analyzed using the
21
equations derived by SheikꢀBahae et al. Samples of the pCp
derivatives were prepared as 1% w/w chloroform solutions in 1
mm path length glass cuvettes.
5
. Pressure dependent measurements. High pressure emission
22
experiments were conducted in a sapphire anvil cell (SAC) with
a
diameter culet of 360 ꢀm and
a
gold gasket. The
23, 24
photoluminescence shift of the anvils
bands was used to calibrate the pressure.
sapphire R1 and R2
25
No pressure
transmitting medium and an airꢀcooled ILT argon ion laser
operated at 488.0 nm were used. Emission measurements were
conducted using an AvaSpecꢀ2048ꢀ2 spectrometer with
200l/mm grating. Raman high pressure measurements were
conducted with the same anvils configuration using the shift of
a
1
2
6
diamond chips as the pressure calibrant and performed using an
airꢀcooled argon ion laser and a SpectraꢀPhysics solid state laser
operating at 532.0 nm. The device is equipped with a 10x
Mitutoyo long working distance objective coupled to a 10x
Navitar zoom system and focused onto the slit of an ISA HR460
−1
monochromator with a grating of 600 grooves mm and a liquid
nitrogen cooled CCD detector (ISA CCD3000, 1024−256 pixels).
−1
Spectra were measured with a resolution of 2−3 cm and
calibrated with a standard neon emission lamp.
6. Theoretical calculations. Quantumꢀchemical calculations
27
were done in the framework of the density functional theory as
2
8
implemented in the Gaussian´09 package. Simulations were
2
9
performed in the gasꢀphase. The B3LYP and exchangeꢀ
3
0
correlational functional and the 6ꢀ31G** basis set were used in
all calculations. Theoretical frequencies were scaled down by a
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Figure 1. Top left: Absorption spectrum of Cp(PV-CA)
deconvolutions together with the TD/DFT theoretical excitations (as bars). Top right:
absorption spectra of the four compounds in THF at 298K: a) Cp(PV-CHO) , b)
Cp(PV-CA) , c) Cp(PV-DCV) , and d) Cp(PV-CN) Bottom) Frontier molecular
. Yellow and orange boxes highlight the main
2
with its pertinent
Emission from the TB state in coplanar donorꢀacceptor arms. D) Mixed emission
from a TS state extended partially on the arms in a TICT state. E) The case of dual
emission. F) Emission from a “compressed” excited state.
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orbitals topologies of Cp(PV-CA)
orbital π−distribution.
2
For long conjugated arms, the energy of the TB state is lower
than that of the “phane” TS state (case A in Scheme 5, ETB < ETS)
and both absorptions and emissions are dominated by the TB
delocalization in the arms. Conversely, in the case of short
conjugated arms (case B in Scheme 5, ETB > ETS), the
photophysical properties are dictated by the TS “phane” state.
Calculations on the model compound consisting only of the
isolated arm (PVꢀCA unit), or donorꢀacceptor TB conjugated
compound, show that the lowest energy lying electronic transition
is due to the S ꢀS excitation, which is mostly associated with a
0
1
HOMOꢀLUMO promotion (see Figure S3 and Table S2). In this
PVꢀCA structure, the HOMO wavefunction is mainly placed on
the donor PV unit whereas the LUMO is mostly located on the
cyanoꢀcarboxylicꢀvinylene moiety. Thus, this excitation promotes
an electron density shift, or CT character. In contrast, in the
Cp(PV-CA) dyad, the lowest energy excitations (i.e., the S ꢀS
According to the description above, in the case of Cp(PV-CA) ,
2
the molecular orbitals containing the TS interaction (Hꢀ4, Hꢀ5)
contribute to higher energy excited states compared to the states
related to the TB orbitals (H, Hꢀ1, etc.). This means that, despite
the rather small stilbenoid conjugated arms (case C in Scheme 5,
0
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0
E
TB < ETS), the inclusion of acceptor moieties enables a
2
0
1
and S ꢀS ) are forbidden transitions (oscillator strength, f = 0)
substantial decrease of the ETB energy compared to ETS in these
substituted pCp.
0
2
and thus inactive in the oneꢀphoton absorption spectrum. This
means that in Cp(PV-CA) a significant blueꢀshift of the strongest
2
observed UVꢀVis band takes place, compared with that of the
individual donorꢀacceptor arm (from S ꢀS in the PVꢀCA arm to
0
1
the S ꢀS in the Cp(PV-CA) dyad).
0
3
2
For the four compounds, the strongest absorption bands move
to the red 445 ꢀ 468 and 354 ꢀ 370 nm on Cp(PV-CA) ꢀ Cp
2
(PV-DCV) , given the strengthening of the acceptor character
2
upon replacing the carboxylic group by another cyano. The main
electronic absorption bands of the UVꢀVis spectra of the cyano
and aldehyde substituted dyads [i.e., Cp(PV-CN) and Cp(PV-
2
CHO) ] blueꢀshift their bands considerably, compared to the other
2
two derivatives, in agreement with the diminished electronꢀ
withdrawing strength of these terminal groups.
A.2 Emission fluorescence spectra. The photophysical properties
reported for pCpꢀbased conjugated molecules have been
interpreted in terms of the relative energy location of the “phane”
2 2
Figure 2. a) Excitation/emission spectra of Cp(PV-CA) and Cp(PV-DCV) in
DMF at 298K. The high/lower energy excitation spectra in blue (black) are recorded
with the emission at 425 (420) / 555 (487) nm, whereas the high/lower energy
TS excimerꢀlike state (E ) relative to that of the “arms” TB state
TS
(ETB) in Scheme 5.
emission spectra are taken by exciting at 360 (350)
Excitation/emission spectra of Cp(PV-DCV) recorded in solid state (red) and in
different solvents (black). c) Excitation/emission spectra of Cp(PV-CA) as a
in solid state as a
/ 472 (420) nm. b)
2
2
function of temperature. d) Emission Spectra of Cp(PV-DCV)
2
function of applied pressure.
Figure 2 displays the excitation and emission spectra of
Cp(PV-CA)2 and Cp(PV-DCV)2 in a polar solvent, DMF.
Excitation of the strongest absorption band at 420 nm for Cp(PV-
CA)2 gives rise to a broad featureless emission at 480 nm.
However, by exciting at 350 nm, a dual emission is recorded,
which consists of a higher energy emission component with
vibronic structure between 400ꢀ430 nm, followed by a second
emission at 480 nm, identical to the one described by exciting the
lowest absorption band. According to the TDꢀDFT excited state
calculations, in the first experiment the S ꢀS3 excitation is
0
promoted, which is a TB excited state placed in the conjugated
arms. It relaxes by internal conversion to the lowest energy S1
excited state, also confined in the conjugated arms, from which it
emits light (case A in Scheme 5). This S ꢀS emission is weak
1
0
(
the S ꢀS absorption is inactive) and featureless given its charge
0 1
transfer character (HOMOꢀLUMO). By exciting the S ꢀS
0
7
“phane state” at 360 nm, another high energy vibronically
resolved band, which might be ascribed to an emission from a
mixed state involving the pCp and also, partially, the conjugated
arms (excluding the acceptor groups) (case D in Scheme 5), is
observed. Since the TB and TS states are orthogonally decoupled,
this mixed excited state can either emit (emerging the high energy
component of the emission) or be depopulated by internal
Scheme 5. Relative disposition of the relevant excited states (red: TB states on the
branches; blue: TS state in the pCp unit). A) Emission from the TB state in large
conjugated arms. B) Emission from the TS state in short conjugated arms. C)
conversion to the S charge transfer excited state (resulting in the
1
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second lowest energy emission). In order to justify why the
Cp(Th)
centered emission of Cp(Th )
2 2
Cp(Th)2 the emission is net for the TS “phane” state, for
2
while this phane effect is deactivated for the branch
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
. Consequently, whereas for
electronic contributions of the acceptor groups are removed in this
mixed excited state, the formation of TICT states can be invoked.
In conjugated donorꢀacceptor dyes, twisted intramolecular charge
transfer states (TICT) fully decouple the donorꢀacceptor
interaction and consist on conformations, as represented in case D
of Scheme 5, where the acceptor groups acquire a perpendicular
disposition relative to the stilbenoid arms. In this way the acceptor
groups are fully removed from any participation in the emission
Cp(Th ) it mainly emerges from the TB state. Additionally, in
2 2
the stilbenoid compounds the emission is complicated by the
presence of the TICT states and by the mixing of the pCp and the
phenyleneꢀvinylene spacer states.
A.3 Solvent, temperature and pressure effects on the absorption
and fluorescence spectra of Cp(PV-CA) and Cp(PV-DCV) . To
2 2
3
2
as a result of the interaction with the polar solvent. This is likely
the case in the DMF polar solvent, where dual emission emerges
due to the presence in solution of both twisted and coplanar
conformers (case E in Scheme 5). A very similar situation is
further confirm these interpretations, photophysical measurements
in an apolar solvent, dichloromethane (Figure 2, top right), where
no double emission is observed, were conducted. The weak polar
character of this solvent deactivates the TICT effect by which the
acceptors remain coupled with the conjugated PV unit in the TB
path. Furthermore, fluorescence emission spectra as a function of
the temperature are shown in Figure 2 (bottom left) for Cp(PV-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
found for Cp(PV-DCV) , where exciting the “phane” state
2
produces a vibronically resolved emission at the same wavelength
as the Cp(PV-CA) , corroborating that it comes from the common
2
pCp phenyleneꢀvinylene spacer (TICT state). In this case, the
lowest energy coplanar charge transfer emission is resolved at 555
nm, redꢀshifted relative to the analogue emission at 480 nm in
CA) . It can be observed that by increasing the temperature the
2
lowestꢀlying energy CT emission declines in intensity in favor of
the mixed TICT state. This effect can be explained on the basis of
increased conformational mobility at high temperatures, around
the single bond connecting the phenyleneꢀvinylene unit to the
acceptor, which directly promotes the TICT effect and its
vibronically resolved emission.
Cp(PV-CA) . These TICT states are analogue to the TICTOID
2
systems in which steric hindrance between the donor/acceptor
moieties and the bridge forces them to a perpendicular disposition
which, in contrast with our cases, further imparts zwitterionic
character and large NLO response. On the other hand, these TICT
states are not analogue to those described in donorꢀπꢀdonor
Solvent and temperature modify the acceptor molecular
environment, hence, in order to get further insights about the
central “phane” unit, a pressure dependent emission experiment in
33
systems.
34
the solid state was conducted, see Figure 2. The faceꢀtoꢀface
interaction in the pCp core is expected to change by slightly
reducing the interꢀbenzene distance. Owing to the large sensitivity
of the energy of the excimerꢀlike “phane” state to this interꢀ
annular distance, a modification of the emission features was
expected. In the solid state at ambient pressure, upon excitation of
the 350 nm excimerꢀlike “phane” absorption, no double emission
was observed. Furthermore, the TB emission of the branches
appears further redꢀshifted at 610 nm compared to the solution
spectrum. However, at high pressure, up to 1.95 GPa, the TB
charge transfer emission at 610 nm develops a low energy feature
at 680 nm that becomes the most intense emission component at
7.0 GPa. For all pressures analyzed at room temperature, the 610
and 680 nm components coexist, whereas the original spectrum is
recovered upon removing pressure. These results can be explained
considering that the formation of a TICT excited state is highly
impeded in the solid state due to largely restricted conformational
Figure 3. UVꢀVis absorption and emission spectra in dichloromethane of Cp(Th)
bottom left), Cp(Th (top left), Ph-Th (bottom right) and DMQtT (top right).
2
(
2
)
2
2
To further understand the excited state distribution resulting
from the pCp insertion, two new compounds, where thiophene
units replace the stilbene branches, were prepared. The two new
compounds possess one (Cp(Th) ) or two thiophenes (Cp(Th ) )
2
2 2
in each branch terminated with hexyl groups. In Figure 3 the
absorption and emission spectra of the two compounds together
freedom. The application of pressure likely results in
a
compression of the pCp unit, increasing the π−π cofacial benzene
interaction that stabilizes the excimerꢀlike “phane” state (see case
F in Scheme 5), which would appear at lower energies, partially
overlapped with the low energy TB band and, as a result,
contributing to the 680 nm component of the emission under
pressure.
with the reference (i.e., the isolated arm) compound Ph-Th are
2
shown. The absorption maximum is at 292 nm for Cp(Th) and
2
redꢀshifts to 353 nm for Cp(Th ) . These spectra are in excellent
2
2
agreement with methyl terminated homologues previously
1
0
published by Collard. The unique effect of the insertion of the
pCp, or of the TS conjugation, is seen by comparing the results
for the Ph-Th reference compound with those of Cp(Th ) and
2
2 2
with dimethylquaterthiophene, DMQtT (i.e., the parent
compound without the central pCp unit). The reference compound
displays an absorption maximum at 337 nm whereas DMQtT and
Cp(Th ) display maxima at 400 nm and 353 nm respectively.
A.4 Twoꢀphoton absorption properties. Twoꢀphoton
absorption crossꢀsections (σ (GM)) for the four stilbenoid
2
compounds were measured by means of the Zꢀscan technique,
using a tunable femtosecond laser system. Table 1 summarizes the
data from the thirdꢀorder nonlinear measurements at wavelengths
2
2
This influence is also seen in the wavelength maxima of the
emission spectra as these are detected at 383 nm for Cp(Th) , at
2
corresponding to σ peaks and Figure 4 displays the twoꢀphoton
4
20 nm for Ph-Th , at 488 nm for DMQtT, and at 441 nm in
2
2
absorption crossꢀsection spectra. In order to properly analyze the
data for compounds having different molecular masses, the merit
factor, which is the twoꢀphoton absorption crossꢀsection divided
Cp(Th ) . The most noticeable aspect of the emission spectra of
2
2
Cp(Th ) and Ph-Th is the vibronic structure, while the emission
2
2
2
spectrum of Cp(Th) is featureless. This clearly indicates the
2
participation of a pure “phane” excited state (in contrast with the
mixed character of the stilbenoid cases above) in the emission of
by the molar mass (σ /M), is also presented. This also allows for
2
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comparison between various types of nonlinear materials (small
molecules, polymers, branched dendrimers or nanoparticles).
agreement with the excitation of the higher energy TPA active
excited state (S ꢀS /S /S ) with increasing participation of
“phane” states. Figure 4 also shows the TPA excitation spectra
that corroborate the assignments of the relevant TPA absorptions
observed between the oneꢀphoton absorption bands.
3
5
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
4
5
6
The linear absorption/emission properties show that the TB
and TS excited states are affected by several environmental and
molecular changes. The existence of two identical donorꢀacceptor
branches connected through the pCp unit gives rise to an opposite
arrangement of the local dipole moments on each branch, leading
A.5 Raman spectroscopic properties. The discussion above
mainly concerns the energy position and activity of the excited
states arising from TB and TS conjugation, a critical aspect to
correctly design the channel for energy migration in molecules. In
order to complete the characterization of pCp molecules, the
conjugation properties in the ground electronic state, were
addressed by means of Raman spectroscopy, a technique that
provides the frequencies associated with vibrational normal
to the appearance of excitations which might be prone to twoꢀ
8,9,14
photon absorption activity.
Compounds Cp(PV-CA)2 and
Cp(PV-DCV)2 show the highest σ (GM) TPA values, much
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
higher than those of Cp(PV-CHO) and Cp(PV-CN) given their
2
2
smaller π−spaces and electronꢀwithdrawing strengths (both
weaken the charge transfer character). It is interesting to inspect
the TPA bands in terms of TDꢀDFT calculations by assuming that
those excitations predicted with zero or vanishing oscillator
strength in the oneꢀphoton absorption spectrum are caused by
36
modes involved in the C=C/CꢀC conjugation paths. Figure 5
displays the Raman spectra of the acceptorꢀacceptor dyads. See
Figure S4 for the theoretical spectrum of Cp(PV-CA) where the
2
“dipolar symmetric transitions”, which is actually the condition
vibrational normal modes used to assign the bands are also shown.
for twoꢀphoton absorption bands to be active. Two main TPA
peaks are observed at 825 and 680 nm. The TPA band maximum
at 825 nm, which is accompanied by a peak around 880 nm, can
be associated with the S0ꢀS2 and S0ꢀS1 excitations,
respectively, theoretically predicted at 472/480 nm (944/960 nm).
There are two other less intense TPA bands around 680 nm which
might be correlated with the oneꢀphoton silent excitations, S ꢀS ,
The band at 1626 cm− corresponds to a C=C stretching
1
vibration of the internal vinylene bond [i.e., ν (C=C)] which
int
−1
connects to the pCp; the band at 1602 cm arises from a CC
stretching vibration of the PV benzene rings (i.e., νTB(CC), in the
−1
TB conjugation path), whereas the band at 1587 cm corresponds
to a CC stretching mode of the benzenes in the pCp unit [i.e.,
0
4
S ꢀS and S ꢀS , predicted theoretically at 446, 404 and 399
ν1pCp(CC)]. A second band arising from the pCp fragment is at
0
5
0
6
−1
nm, respectively (Table S1).
1552 cm , assigned to an analogue CC stretching mode [i.e.,
−1
ν2pCp(CC)]. The most intense Raman band is at 1562 cm and
Table 1. Thirdꢀorder NLO data determined with the Zꢀscan technique.
results from a C=C stretching vibration of the external double
bond [i.e., ν (C=C)], connected to the acceptors units. This
ext
λ
max
Re(γ)
Im(γ)
σ
2
2
σ /M
[GM mol/g]
Sample
36
ꢀ36
[
nm] [10 esu] [10 esu]
[GM]
assignment agrees with that reported for the pCp compound which
describes their relevant CC stretching modes in this region at
6
25
50
75
ꢀ230
62
200
160
82
0.094
−1
34
1599 and 1559 cm respectively.
Cp(PV-CHO)
2
8
6
36
0.041
ꢀ1080
ꢀ670
ꢀ670
ꢀ240
730
255
247
233
340
0.254
Cp(PV-CA)
2
825
1050
610
0.246
6
8
50
25
0.241
Cp(PV-DCV)
2
1440
0.352
600
900
ꢀ530
ꢀ72
102
74
45
15
0.052
0.017
Cp(PV-CN)
2
Figure 5. Left: FTꢀRaman spectra of the four compounds in solid state: a) Cp(PV-
CHO) , b) Cp(PV-CN) , c) Cp(PV-DCV) , d) Cp(PV-CA) . Right: Raman spectra
of Cp(PV-DCV) in solid state at a) room pressure and b) 8 GPa. Lorentzian
2
2
2
2
2
deconvolutions are shown in colors.
Based on this description of the vibrational Raman spectrum
in terms of ground electronic state frequencies, their evolution in
−
1
the series is possible. The two νpCp(CC) bands at 1592/1565 cm
−1
for Cp(PV-CHO) , 1590/1559 cm for Cp(PV-CN) , 1587/1552
cm for Cp(PV-DCV) and 1588/1554 cm for Cp(PV-CA)2
2
2
−1
−1
2
indicate changes of the pCp Raman bands due to the external
acceptor groups in the ground electronic state. Generally
speaking, a frequency downshift of the vibrational modes
associated with CC stretching of the aromatic benzenes reveals
the transformation of the benzoaromaticꢀlike structure into a
Figure 4. Left: twoꢀphoton absorption spectra recorded with the Zꢀscan techniques.
Right: twoꢀphoton induced upꢀconversion fluorescence analysis for compound
Cp(PV-DCV) in DMF.
2
To complete the TPA study in terms of the distributions of
excited states, twoꢀphoton induced upꢀconversion fluorescence
36
benzoquinoidalꢀlike one.
Comparing Cp(PV-CHO)2 and
Cp(PV-DCV)2, larger νpCp(CC) frequency downshift is
a
analysis for compound Cp(PV-DCV) , as a representative
2
detected, resulting from the stronger electronꢀwithdrawing effect
of the dicyanovinyls compared with the aldehyde derivative along
the TB path. The Raman frequency behavior of the νTB(CC)
example, was conducted. The spectra are shown in Figure 4. By
exciting at the longest wavelength (900 nm, TPA state at 450 nm)
only one fluorescence emission peak at 555 nm was observed, in
line with the excitation of the lowest energy lying TPA active
excited states of the branches (S ꢀS and S ꢀS ). By exciting at
−
1
benzene/ν (C=C) vinylene bands is: 1602/1626 cm for Cp(PV-
int
−1
−1
CHO)2, 1604/1627 cm for Cp(PV-CN) , 1602/1626 cm for
Cp(PV- DCV)2 and 1606/1626 cm for Cp(PV-CA) . These
2
0
1
0
2
−1
shorter wavelengths, between 850 and 750 nm, two fluorescence
peaks with maxima at 467 and 545 nm, are observed, in
2
show changes towards a more TB conjugated situation in the
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Journal of the American Chemical Society
cases with stronger acceptors. From these data, changes
associated with TS conjugation cannot be deduced.
been replaced by methyls. The Raman spectrum of the neutral
model displays the two characteristic Raman bands at 1590 and
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
−1
1552 cm already discussed as exclusively resulting from this
pCp part. In the radical cation state, the theoretical Raman
MicroꢀRaman spectroscopy allows monitoring frequency
changes under pressure in the solid state. Pressure dependent
Raman experiments were conducted (shown in Figure 5) to follow
TS conjugation changes in the pCp core. The first noticeable
pressure effect in the Raman spectra is an overall spectral
broadening, which is a consequence of the increment of
intermolecular interactions (going from the absolute minimum
along the repulsive branch of the ground electronic state potential
energy surface with pressure). As a result, frequencies experience
a general upꢀshift. Based on the presence of five bands in the
spectrum of Cp(PV-DCV)2 at room pressure (RP), the broad
Raman feature at 1530ꢀ1600 cm− was deconvoluted into five
Lorentzian components (Figure 5, fit of 0.99), which emerge at:
−1
spectrum features a strong band at 1441 cm which is
−1
accompanied by medium intensity bands at 1456 and 1416 cm ,
representative of the vibrational fingerprint of the structure
(“phane state” in Scheme 1). The positive charge is delocalized
between the two benzenes and, as a result, displays an interꢀ
benzene distance smaller than in the neutral case, 3.050 ꢀ2.942
Å, according to B3LYP/6ꢀ31G* calculations.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
A diphenylamino stilbene paracyclophane derivative (Cp(PV-
DA) ) was prepared for the study of the oxidized species. Cyclic
2
1
voltammetry of Cp(PV-DA) shows two processes: one at +0.56
2
V involving two electrons and another at 1.10 V which involves
only one electron (see cyclic voltammogram in Figure 7 insert and
Figure S5 and Table S3 in Section S4). It is straightforward to
assign the first event to the formation of a bis(radical cation) (i.e.,
dication) or of a radical cation on each arm, followed by the
oxidation of the central part of the molecule or radical trication
mostly localized on the pCp center. Figure 7 shows the UVꢀVisꢀ
NIR spectra obtained upon in situ electrochemical oxidations of
1
632 cm− (i.e., 1626 cm at RP), at 1604 cm (i.e., 1602 cm
1
−1
−1
−1
−1
−1
−1
at RP), at 1588 cm (i.e., 1562 cm at RP), at 1573 cm (i.e.,
1
587 cm− at RP) and finally at 1525 cm (i.e., 1552 cm at
1
−1
−1
RP). Two clearly different behaviors are observed: i) that of the
1
1
632/1604/1588 cm− bands with small to moderate upshifts
compared with frequencies at RP; and ii) that of the 1573/1525
−1
cm bands which show downshifts. The first three bands are due
to vibrational modes on the arms and the upshifts are caused by
the increment of intermolecular interactions, however, the second
pair of bands own to the pCp units, which conversely downshift
upon application of pressure. Application of pressure compresses
the pCp structure, resulting in a decrease of the interbenzene
distance and, therefore, an increase of the TS π−delocalization.
Consequently, TS delocalization might displace π−electron
density from the benzene ring to the interꢀannular region and
result in the Raman downshifts.
Cp(PV-DA) . Two different spectra are obtained, one after the
2
disappearance of the neutral species and formation of the dication
and a second spectrum upon oxidation to the radical trication. The
spectrum of the dication consists of three main bands at 533(577),
712 and 1062 nm, a multiꢀband spectroscopic pattern typical of
3
9
openꢀshell radical cations of π−conjugated molecules. This
dicationic species can be viewed as an acceptorꢀacceptor dyad
(Scheme 5) where, in fact, the structured band at 500ꢀ570 nm can
be correlated with, for instance, that at 468 nm for Cp(PV-
DCV) . Further oxidation of this species gives rise to a clear
2
distinctive electronic absorption spectrum dominated by one band
at 755 nm that results from the radical trication.
B. Donor-Donor Dyad.
B.1 Ground electronic state properties. In the donorꢀdonor
derivatives, the vinylꢀacceptor groups have been substituted by
donor aminoꢀphenyl moieties. The insertion of the nitrogens in
diarylamino groups enhances the electron donor character of the
3
8
molecules and their ease of oxidation. Therefore, these donorꢀ
donor molecules allow the analysis of the TB/TS conjugation
properties of oxidized species through the pCp unit.
2
Figure 7. UVꢀVisꢀNIR spectroelectrochemistry of Cp(PV-DA) (left) and of (PVꢀ
DA)Cp(PV-DCV) (right). Dashed bars show the wavelength of the laser excitation
used in the Raman experiments. The inserts correspond to the cyclic voltammetry of
each compound in the anodic branch showing the formation of different polycationic
states.
Figure 8 displays the Raman spectra of Cp(PV-DA) donorꢀ
2
donor dyads in the three relevant oxidation states. The spectrum
of the neutral species features the presence of four bands due to
−
1
−1
the ν (C=C) at 1626 cm , the ν (CC) benzene at 1601 cm
int
TB
−1
and the two νpCp(CC) bands at 1591/1563 cm .
Figure 6. Top: (U)B3LYP/6ꢀ31G* theoretical Raman spectra of the dimethyl pCp
unit in the neutral (a) and radical cation (b) states. Bottom: Optimized geometries
with the interꢀbenzene average distances.
Figure 6 shows the theoretical Raman spectra of the pCp unit
alone, where the two arms in the studied model compound have
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trication of Cp(PV-DA) , and of the participation of the “phane
state” in the stabilization of the charge.
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
C. Donor-Acceptor Dyad.
The arylꢀamino donor and dicyanoꢀvinyl acceptor substituted
paracyclophane compound, (PV-DA)Cp(PV-DCV) shows a oneꢀ
electron oxidation at +0.46 V, associated with the formation of a
radical cation, followed by a second oneꢀelectron oxidation at
+1.03 V corresponding to the stabilization of a dication (see
cyclic voltammogram in Figure 7 insert and Figure S5 and Table
S3, in Section S4). The correlation between the formation of the
dication of Cp(PV-DA)2 and of the radical cation of (PV-
DA)Cp(PV-DCV) is straightforward, as both species stabilize
one positive charge in the diphenylamino arm. On the other hand,
the radical trication and dication, respectively, correspond to
charge extracted from the central pCp unit. Figure 7 shows the
spectroelectrochemical UVꢀVisꢀNIR spectra of (PV-DA)Cp(PV-
DCV) where the formation of two distinctive spectra emerging
progressively from the radical cation and dication species is
noticed. The radical cation is characterized by three bands
Figure 8. Left: Raman spectra of (a) neutral, (b) dication (1064 nm) and (c) radical
trication (785 nm) of Cp(PV-DA) . Right: Raman spectra of the (a) neutral, (b)
radical cation (1064 nm) and (c) dication (785 nm) of (PV-DA)Cp(PV-DCV). In
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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parenthesis the excitation wavelength used in the Raman experiment, (see Figure 7).
Formation of the dication from neutral Cp(PV-DA) gives
2
rise to a Raman spectrum that exhibits a downshift of the
frequencies of the four neutral bands in Figure 8, as a
consequence of the weakening of the vinylene bond and of the
quinoidization of the benzenes. Further oxidation of this dication
gives way to the radical trication, which, in contrast with the
frequency behavior of the neutralꢀdication, displays a general
frequency upshift. In addition, two new medium intensity bands
appear in the Raman spectrum of the radical trication at
−1
1497/1472 cm . The Raman spectra of the dication and radical
trication were recorded in resonance conditions with the
characteristic electronic absorptions of each species, as shown in
Figure 7. The dication was recorded with the 1064 nm Raman
excitation line in resonance with the absorption band with
maximum at 1062 nm. On the other hand, the radical trication
species was obtained with the 785 nm Raman excitation line in
resonance with the absorption band with maximum at 755 nm.
The radical cation and dication of (PV-DA)Cp(PV-DCV) were
similarly generated and studied, as shown in Figure 7 and
discussed in the next section.
5
34/585, 710 and 1047 nm [i.e., correlate with the bands of the
bis(radical cation) of Cp(PV-DA) ]. Oxidation of the radical
2
cation of (PV-DA)Cp(PV-DCV) leads to the formation of the
dication species exhibiting one main band at 757 nm [i.e.,
correlated with the band at 755 nm assigned in the radical
trication of Cp(PV-DA)2].
Figure 8 also displays the Raman spectra of the oxidized
species of (PV-DA)Cp(PV-DCV). The Raman spectrum of the
neutral contains the Raman bands associated with the
−1
−1
ν (C=C)/ν (C=C) at 1625/1560 cm , the 1602 cm band of the
int
ext
−1
benzene νTB(CC) and the νpCp(CC) features at 1588/1549 cm .
Oxidation, such as in the case of Cp(PV-DA) , produces an
Scheme 6 displays the main resonant forms associated with
2
the different oxidized species of Cp(PV-DA) , that might justify
2
overall frequency downshift which is ascribed to the formation of
the radical cation placed and delocalized on the bis(aryl)amino
stilbene unit, producing the quinoidization of the benzene and the
transformation of the vinylene. Further oxidation of this radical
cation to the dication leads to a similar behavior as that found for
the Raman frequency changes. In the dication, the structural
transformation spreads over the whole branch, forming quinoidal
structures in the benzenes and conferring the vinylene a more
single bond character. For the trication the charge is extracted
mainly from the central pCp, while the existing positive charges
of the dication are fully pushed towards the outermost nitrogens
for which the benzenes are forced to change their quinoidal
structure towards a more aromatic shape such as shown in
Scheme 6. The new Raman bands of the radical trication at
the radical trication of Cp(PV-DA) , consistent with an overall
2
frequency upshift of these bands due to the mitigation of the
quinoidal structures. The presence of an acceptor branch on one
side, and of a radical cation on the other, forces the second
electron extraction from the pCp unit as shown in Scheme 6. The
−1
−1
1
497/1472 cm , therefore, might be related to the formation of a
new bands for the dication at 1487/1470 cm are, therefore, the
characteristic vibrational signature of the “phane state”, as
described for the radical trication of Cp(PV-DA).
charged “phane” state with charge delocalization within the two
coꢀfacial benzenes.
NC
OHex
CN
Interestingly, the Raman spectrum of neutral Cp(PV-DCV)2
OHex
Ar
N
as a function of pressure, discussed in section A.5, shows a
OHex
N
OHex Ar
OHex
N
OHex
Ar
Ar
Ar
Ar
significant frequency downshift for the pCp bands (1587 ꢀ1573
OHex
OHex
−1
−1
2 e-
-1 e-
cm and 1552 ꢀ1525 cm ). This is also consistent with the
-
NC
−1
−1
OHex
CN
strong downshifts from ≈1550 cm to 1470 cm for the new
bands observed for the radical trication and dication of Cp(PV-
DA)2 and (PV-DA)Cp(PV-DCV), respectively. In the former
case, the charge delocalization between the two sandwiched
benzenes is induced by pressure and in the oxidized species the
driving force is the stabilization of the positive charge by
delocalization. Obviously, the latter effect results in a much
stronger alteration of the interꢀbenzene distance and, therefore, it
leads to a larger impact on the vibrational frequencies.
OHex
Ar
N
OHex
N
OHex Ar
OHex
OHex
Ar
Ar
Ar
Ar
N
+
OHex
OHex
-1 e-
-
1 e-
NC
OHex
Ar
OHex
CN
N
OHex
OHex Ar
OHex
OHex
Ar
Ar
Ar
Ar
N
N
OHex
OHex
Scheme 6. Resonant forms stabilizing the charged species of Cp(PV-DA)
2
( left) and
of (PV-DA)Cp(PV-DCV) (right).
These Raman bands are in good agreement with the Raman
spectrum predicted for the radical cation of the dimethyl pCp unit,
which supports the presence of charge delocalization in the radical
CONCLUSIONS
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Intermolecular exciton and charge delocalization is an
0033, and the US National Science Foundation, grants DMRꢀ
1205302 and CHEꢀ1408865, for generous financial support. MS
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important topic in organic electronics as it is at the origin of the
energy and charge transport of semiconductor substrates. In order
to better understand these complex processes, molecules with
wellꢀdefined and precise structures and conformations are
required to establish reliable structureꢀproperties relationships.
acknowledges
the
National
Science
Centre
DECꢀ
2013/10/A/ST4/00114 grant.
Supporting Information. Synthetic procedures, chemical
characterizations, details of the quantum chemical calculations.
This information is available free of charge via the Internet at
http://pubs.acs.org
Here, the pCp unit, which has
a faceꢀtoꢀface blocked
conformation of two benzenes separated by ≈3 Å, that mimics the
effect of throughꢀspace delocalization, has been chosen. Several
new pCp derivatives were synthesized and studied, possesing
stilbene moieties, further substituted with donors and acceptors in
several structures, and also containing thiophene derivatives
replacing the stilbenoid bridges.
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the sandwiched structure to be involved in the emissions. The
excited states were analyzed by using oneꢀ and twoꢀphoton
absorption properties which allow to characterize the bright and
dark excited states with and without pCp participation.
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