In-situ generation and analysis of charge transfer materials using an OTTLE cell and resonance Raman scattering

Article abstract of DOI:10.1039/b402015d

Poly(aryl)amine based charge transfer materials (CTMs) are essential components in a range of present and future technologies, from the Xerox process to display devices based upon light emitting polymers (LEPs). However, there is a lack of detailed understanding regarding the electronic properties of CTMs in their various neutral and oxidized forms. This paper reports the use of an optically transparent thin layer electrochemical (OTTLE) cell in combination with a Raman microprobe system and DFT calculations to provide information on the molecular and electronic structure of the mono- and di-oxidized derivatives of the classic CTM N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1, 1′-biphenyl)-4,4′-diamine (TPD) and the closely related species N,N′-diphenyl-N,N′-bis(2,4-dimethylphenyl)(1,1′-biphenyl)-4, 4′-diamine (DMTPD). The resonance Raman scattering profile easily discriminates between the monovalent and divalent cations while DFT calculations permit correlation of the observed vibrational frequencies with localized atomic displacements. The cations are best described in terms of a symmetrical (i.e. fully delocalized) structure. The high sensitivity of the method suggests that it should be appropriate for the observation of low concentrations of the various cations generated from TPD type CTMs during device operation.

Full text of DOI:10.1039/b402015d

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R E S E A R C H P A P E R
In-situ generation and analysis of charge transfer materials using an
OTTLE cell and resonance Raman scatteringy
Rachael E. Littleford,a Michael A. J. Paterson,b Paul J. Low,b Daniel R. Tackley,c
Linda Jayes,c Geoffrey Dent,c Julian C. Cherryman,c Bev Brownc and W. Ewen Smith*a
a
University of Strathclyde, Department of Pure and Applied Chemistry, 295 Cathedral Street,
Glasgow, UK G1 1XL
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
Avecia Ltd., Hexagon House, Blackley, Manchester, UK M9 8ZS
b
c
Received10th February 2004, Accepted 23rd March 2004
F|rst published as an AdvanceArticle on the web 25th May 2004
Poly(aryl)amine based charge transfer materials (CTMs) are essential components in a range of present and
future technologies, from the Xerox process to display devices based upon light emitting polymers (LEPs).
However, there is a lack of detailed understanding regarding the electronic properties of CTMs in their various
neutral and oxidized forms. This paper reports the use of an optically transparent thin layer electrochemical
(OTTLE) cell in combination with a Raman microprobe system and DFT calculations to provide information
on the molecular and electronic structure of the mono- and di-oxidized derivatives of the classic CTM
N,N0-diphenyl-N,N0-bis(3-methylphenyl)(1,10-biphenyl)-4,40-diamine (TPD) and the closely related species
N,N0-diphenyl-N,N0-bis(2,4-dimethylphenyl)(1,10-biphenyl)-4,40-diamine (DMTPD). The resonance Raman
scattering profile easily discriminates between the monovalent and divalent cations while DFT calculations
permit correlation of the observed vibrational frequencies with localized atomic displacements. The cations are
best described in terms of a symmetrical (i.e. fully delocalized) structure. The high sensitivity of the method
suggests that it should be appropriate for the observation of low concentrations of the various cations generated
from TPD type CTMs during device operation.
interest of late. These systems often contain a characteristic
Introduction
transition in the near-infrared region,18 which in the simplest
terms, can be related to the photoinduced intramolecular elec-
tron transfer between the amine centres.12 However, since
the orbitals involved in this transition can be delocalized
and involve considerable aryl and nitrogen character, the
interpretation of this data can be challenging.18–22
Poly(aryl)amine based charge transfer materials (CTMs) are
used in established technologies, such as the Xerox process,
and also have immense potential for application in the con-
struction of light emitting polymer (LEP) based display
devices.1–7 However, the design of these compounds for both
present and future application would be significantly aided
by a detailed understanding of the electronic and physical
properties of these materials, not only in their neutral state,
but also in the ‘active’ oxidised forms which play a role in
the charge transfer process.8–10
Since charge transport through a molecular (or polymeric)
material depends upon interactions between the molecular
(or polymeric) units, compounds that form strictly amorphous
phases have been the source of the most intense investigation.
An unfortunate consequence of this is that there is little struc-
tural information in the open literature relating to the mole-
cular structure of even simple poly(aryl)amines,11,12 and
structural data relating to the oxidized forms of these materials
is extremely rare. In the absence of crystalline samples for
structural analysis, the majority of current research has utilized
computational methods to explore the geometry and electronic
structure of model CTMs and their oxidized derivatives.13–17
In addition to the intermolecular electron transfer events
between neutral molecules and their radical cation counter-
parts, which are related to charge mobility in the bulk sample,
intramolecular electron transfer events within tetra(aryl)benzi-
dene ‘‘mixed valence’’ radical cations have been a source of
Raman spectroscopy offers an effective method through
which to interrogate the structure of poly(aryl)amine based
CTMs such as N,N0-diphenyl-N,N0-bis(3-methylphenyl)(1,10-
biphenyl)-4,40-diamine (TPD, 1) and the closely related species
N, N 0-diphenyl-N,N0-bis(2,4-dimethylphenyl)(1,10-biphenyl)-4,
40-diamine (DMTPD, 2).23–27 The neutral forms of these
molecules are readily accessible, while spectroelectrochemical
techniques provide a solution to the problem of obtaining
the radical samples for study,28–35 and by using an optically
transparent thin layer electrochemical (OTTLE) cell a solution
of the radical cations may be generated in the spectrometer
beampath and analyzed in-situ. In the particular case of the
Raman experiments described here, excitation frequencies were
chosen after consideration of the absorption spectra of the
monovalent and divalent cations of species 1 and 2 (1nþ and
2nþ), which have also been obtained by spectroelectrochemical
methods.12 In combination with DFT calculations, which can
be used to correlate the Raman vibrational frequencies with
the atomic displacements associated with each vibration, the
method provides information on the structure and electronic
structure of the oxidized forms of these CTMs in-situ.
y Electronic supplementary information (ESI) available: BPW91/6-
31G(d) predicted vibrational frequencies and intensities for the neutral,
radical cation and dication species of 1 and 2. See http://www.rsc.org/
suppdata/cp/b4/b402015d/
Experimental
TPD and DMTPD were prepared by minor variation of
literature methods.36
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 2 5 7 – 3 2 6 3
3257

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Spectroelectrochemical studies were carried out in an
OTTLE cell (quartz, 1 mm pathlength) of standard design
fitted with a platinum gauze working electrode and platinum
wire counter and pseudo-reference electrodes37 using a 0.5
mM solution of the material of interest with supporting elec-
trolyte of tetrabutylammonium tetrafluoroborate (0.1 M) in
freshly distilled dichloromethane at room temperature. Poten-
tials were obtained from conventional cyclic voltammetry
experiments, using the same solvent and a similar all-platinum
electrode set, and are quoted against SCE using a ferrocene
internal standard such that the ferrocene–ferrocenium couple
falls at 0.46 V.38
Electronic spectra were recorded on a Varian Cary 5
spectrophotometer.
Raman spectra were collected using Renishaw 2000 micro-
probe systems with a Spectra-Physics model 163 argon-ion
laser providing excitation at 514.5 nm, and a Spectra-Physics
model 127 helium–neon laser providing excitation at 632.7
nm. The OTTLE cell was mounted in the beampath of the
spectrometer and the laser focused onto a pore within the
gauze working electrode using the microscope. In-situ spectra
were collected using 1 accumulation of 1 s. Raman spectra
of the solid CTM were the result of three 10-s accumulations.
Data collection and manipulation used GRAMS software.
Calculations were performed using the Gaussian 98W
software package.39 For each molecule the geometry was opti-
mised and frequency-intensity (including Raman) calculations
were run for the neutral, radical cation (charge ¼ 1, multi-
plicity ¼ 2) and dication (charge ¼ 2, multiplicity ¼ 1) states
at a DFT level using the BPW91 method (Becke’s 1988
exchange functional40 with Perdew and Wang’s 1991 gradi-
ent-corrected correlation functional).41 The 6-31G(d) basis
set was used for all calculations.42–46 Vibrational spectra were
simulated from the frequency-intensity point data by adding
a mixed (1:1) Gaussian : Lorentzian lineshape with a fixed
Fig. 1 Electronic spectra of the neutral species and the monovalent
and divalent cations of TPD (1) and DMTPD (2). Spectra taken at
voltages appropriate to the formation of each species.
consequently weaker resonance enhancement is to be
expected. However, for the divalent species 514.5 nm excita-
tion would be expected to give even less enhancement, conse-
quently, for the divalent species 632.7 nm should still give the
greatest enhancement.
linewidth of 6 cmꢀ1
.
The absorption profiles associated with 2nþ are similar to
those of 1nþ, and while there are small shifts in the electronic
absorption maxima, resonant enhancement of the Raman
spectra will still be achieved with 514.5 nm (2þ) or 632.7 nm
(22þ) excitation.
Results
Electrochemical response
We have previously described the electrochemical response of
1 and 2, and observed two, one electron waves associated with
the stepwise formation of the mono- (1 E1/2 ¼ 0.292 V, 2
The Raman spectra of a solid sample of 2 and 2þ, prepared
chemically,12,36 were collected using a 514.5 nm excitation
wavelength (Fig. 2(a) and (b)) in order to assess the relevance
of the solution based OTTLE spectra to the solid state. In the
case of neutral 2, the spectrum is dominated by the large bands
at 1614 and 1606 cmꢀ1, and other strong bands at 1294 and
1182 cmꢀ1. The distinctly different spectrum of 2þ in the solid
state clearly indicates the ability of Raman scattering as a ‘‘fin-
gerprint’’ technique in discriminating the CTM in these oxida-
tion states. Spectra (b) and (c) show that there is a strong
similarity between the solid state and OTTLE cell spectra, giv-
ing confidence that the solution state spectra can be accurately
assigned.
The OTTLE cell enabled the observation of the monovalent
and divalent cations of 1 and 2 in-situ. In the case of 2þ, which
is illustrated in Fig. 2(c), the solution state spectra compare
well with those obtained from the solid samples. The Raman
spectra of the electrochemically generated series 2nþ recorded
using excitation at 514.5 nm (Fig. 3(a)) and 632.7 nm (Fig.
3(b)) show the general trends, with the solvent band at ꢁ695
cmꢀ1 providing a useful internal reference for the relative
intensity of the sample bands. As well as obvious increases
in intensity in the Raman scattering due to resonance enhance-
ment as the mono- (lex ¼ 514.5 nm) and di-cations
(lex ¼ 632.7 nm) are formed, there is clear evidence of over-
tones in the region between 1600 and 3200 cmꢀ1 in Fig. 3(a).
The presence of overtones is common for resonant systems
E1/2 ¼ 0.250 V) and di-cations (1 E1/2 ¼ 0.507 V, 2 E1/2
¼
0.505 V).12 The large potential difference between the two
redox processes for 1 (215 mV) and 2 (255 mV) give rise to
significantly large comproportionation constants, KC and indi-
cate the high thermodynamic stability of the radical cations 1þ
(KC ¼ 4300) and 2þ(KC ¼ 20 500).
Resonance Raman scattering
The electronic spectra of the cations are shown in Fig. 1.12
Both the monovalent and divalent cations are coloured and
consequently resonance Raman scattering should provide a
good in-situ probe for the appearance of the ions, providing
both vibrational and electronic information.
For 1þ, excitation at 514.5 nm falls within the absorption
band envelope centred at 477 nm. As the excitation wave-
length lies towards the lower energy side of the absorption
band this will be close to the 0 ! 0 and 0 ! 1 vibronic band
frequencies, and consequently should be effective in creating
resonance enhancement. In the case of 12þ, excitation at
632.7 nm falls within the absorption envelope which has its
maximum at 732 nm, therefore resonance enhancement
would be expected. However, since the 632.7 nm excitation
wavelength is on the high-energy side of the absorption band,
higher vibronic levels associated with this will be closest;
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
3258
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 2 5 7 – 3 2 6 3

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since the selection rule preventing overtones is not applicable
for A term scattering, and is less rigorous for B term.47
A broad emission profile around 7400 cmꢀ1 (514.5 nm exci-
tation), or 3700 cmꢀ1 (632.7 nm excitation) accompanies the
Raman bands associated with 22þ. This appears to be a stan-
dard emission process with a maximum at about 826 nm,
based on absorption and emission from the transition which
gives rise to the absorption band at 742 nm (Fig. 1). In the case
of 514.5 nm excitation, there also appears to be emission from
the monocation with a maximum at approximately 2500 cmꢀ 1
lower than the excitation wavelength which is less easily
explained because of the lower energy electronic transition.
However, Raman scattering uses a high-power laser for excita-
tion, and emission from higher excited states is known. This
will be helped by the large energy separation between the
monocation absorption band at 472 nm and the near infrared
absorption band at 1327 nm. This emission is broad compared
to that of the dication.
The frequencies of the Raman active modes of 2þ are simi-
lar, but not identical, to those of 22þ (Fig. 4(a) and (b)) and
there are significant intensity changes in individual bands, par-
ticularly in the 1600 and 1200 cmꢀ1 regions. The lower inten-
sity of the 22þ bands with 514.5 nm excitation wavelength may
indicate the much weaker resonance enhancement at this
excitation wavelength.
Figs. 5 and 6 show the results of an OTTLE experiment
using 1. An obvious increase in intensity in the Raman scatter-
ing occurs when the electrochemically generated species fall in
resonance with the excitation wavelength. Thus, excitation at
632.7 nm results in very little resonance enhancement from
the monocation, but greater enhancement from the dication.
It is therefore clear that resonance Raman scattering offers a
convenient method for the detection of the oxidized forms of
poly(aryl)amine based CTMs within a complex mixture. How-
ever, in order to derive structural information relating to the
various species which may be generated electrochemically
and detected by resonance Raman spectroscopy it is necessary
Fig. 2 Experimental Raman spectra of DMTPD: (a) neutral solid,
(b) monocation solid and (c) monocation in solution. Excitation at
514.5 nm.
Fig. 3 Raman spectra showing the generation of the DMTPD mono-
cation and dication in-situ inside an OTTLE cell. Excitation at (a)
514.5 nm and (b) 632.7 nm.
Fig. 4 Selected Raman spectra showing the generation of the
DMTPD monocation and dication in-situ inside an OTTLE cell.
Excitation at (a) 514.5 nm and (b) 632.7 nm.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 2 5 7 – 3 2 6 3
3259

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DFT calculations
Given the similarities of the experimental spectra of 1nþ and
2nþ, the Raman active vibrations of 2nþ were modelled using
DFT methods as described in the Experimental section. Allow-
ing for the minor errors which are inherent in the calculations
arising from the neglect of solvent effects, the calculated spec-
tral frequencies are expected to be relevant to both systems.
The solid-state Raman spectra (Fig. 2(a) and (b)) were selected
for comparison with the calculated neutral and 2þ spectra in
Fig. 7 because of the better signal to noise ratio. However,
due to the instability of the 22þ species in the solid state, the
solution phase resonance Raman spectrum obtained from gen-
eration of the dication in the OTTLE cell (Fig. 4(b)) is used.
There is close agreement between the calculated and observed
vibrational frequencies in the region 400 to 1800 cmꢀ1. In the
case of the neutral systems the largest difference between the
experimental and calculated frequencies for any clearly
assigned band was 21 cmꢀ1, and a fit of the ten most intense
bands gave an average error of 8.4 cmꢀ1. A similar comparison
of the theoretical result with spectra from the monocation
(Fig. 2(b)) reveals the largest mismatch for any band was
37 cmꢀ1, with an average error of 8.9 cmꢀ1 for the ten most
intense bands. For the dication, the largest error for any one
band is 18 cmꢀ1, with an average error of 7.1 cmꢀ1 for all
assigned bands. Thus, the frequency assignments as made by
the DFT calculations can be used with some confidence.
However, the intensity values calculated do not fit well with
the experimental values. This is quite common with current
calculations, and in this case it must also be borne in mind that
the calculations are for normal Raman scattering in the gas
phase and the spectra obtained from the cations are due to
Fig. 5 Raman spectra showing the generation of the TPD mono-
cation and dication in-situ inside an OTTLE cell. Excitation at (a)
514.5 nm and (b) 632.7 nm.
to turn to DFT based methods to provide a correlation
between the electronic transitions responsible for the reso-
nance enhancement, the observed molecular (Raman)
vibrational frequencies and the atomic displacements.
Fig. 6 Selected Raman spectra showing the generation of the TPD
monocation and dication in-situ inside an OTTLE cell. Excitation at
514.5 nm (top) and 632.7 nm (foot).
Fig. 7 Predicted Raman spectrum of neutral, monocation and
dication DMTPD.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
3260
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 2 5 7 – 3 2 6 3

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frequencies for the neutral, radical cation and dication species
is available as ESI.y
resonance Raman scattering in the solution and solid phase.
Lists of calculated band frequencies together with the experi-
mental frequencies and the atomic displacements to which
the Raman active bands may be assigned are given in Tables
1–3.48 A full listing of all of the calculated vibrational
Discussion
The Raman spectra of 2 and 2þ in the solid state are given in
Fig. 2. On the basis of the DFT calculations, the bands
between 1550 and 1620 cmꢀ1 can be assigned to the quadrant
stretches of all the phenyl ring systems. The main differences in
the spectra lie in the relative intensity of the bands between
Table 1 Assignment of neutral DMTPD48
Experimental/ Calculateda /
cmꢀ1
cmꢀ1
1618
1609
1603
Assignment
ꢂ
þ
1614 (s)
Quadrant str. on all rings
þ C–N str. þ biphenyl str.
Quadrant str. on all rings
þ C–N str. þ biphenyl str.
Quadrant str. on all rings
except dimethyl þ C–N str.
þ biphenyl str.
Table 2 Assignment of DMTPD (Radical cation)
Experimental/ Calculateda /
cmꢀ1
cmꢀ1
1612
1599
1583
Assignment
1606 (s)
1649 (w)
1594 (s)
1577 (s)
Quadrant str. on all rings
þ C–N str. þ biphenyl str.
Quadrant str. on all rings
þ C–N str. þ biphenyl str.
Quadrant str. on all rings
except dimethyl þ C–N str.
þ biphenyl str.
1524 (m)
1488 (w)
1517
1496
Biphenyl str.
Dimethyl rings deformation
þ C–N str.
1489
Non-dimethyl rings deformation
þ C–N str.
1371 (w)
1380
1377
1320
Methyl C–H wag
Methyl C–H wag
1578
Quadrant str. on all rings
except dimethyl þ C–N str.
þ biphenyl str.
1322 (sh, w)
Biphenyl str þ biphenyl rings
breathing þ C–N str.
1562
1512
Quadrant str. on dimethyl rings
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
Dimethyl rings deformation
þ C–N str.
þ non-dimethyl rings C–H wag
Biphenyl rings C–H wag þ C–N str.
Biphenyl str. þ biphenyl rings
C–H wag þ non-dimethyl
rings C–H wag
1525 (m)
1490 (w)
1315 (sh, w)
1294 (s)
1304
1289
1485
1479
Non-dimethyl rings deformation
þ C–N str.
1281
1275
Biphenyl str. þ biphenyl rings
C–H wag þ non-dimethyl rings
C–H wag
1469
1460
Methyl C–H scissoring
All rings except dimethyl
deformation þ methyl
Biphenyl rings C–H wag
þ dimethyl rings C–H wag
C–N str. þ all rings C–H wag
Dimethyl rings deformation
þ C–H wag
C–H scissoring
1264
1228
1366 (sh, w)
1324 (s)
1381
1322
Methyl C–H wag
Biphenyl str. þ biphenyl rings
breathing þ C–N str.
1233 (m)
1182 (s)
1196
1178
1173
C–N str. þ all rings deformation
All rings deformation
Mainly non-dimethyl rings
C–H wag
þ non-dimethyl rings C–H wag
Biphenyl str. þ biphenyl rings
breathing þ C–N str.
1305
1303
þ non-dimethyl rings C–H wag
Biphenyl str. þ biphenyl rings
breathing þ C–N str.
1150 (sh, w)
1022 (w)
1157
Dimethyl and non-dimethyl
rings C–H wag
1156
Dimethyl and non-dimethyl
rings C–H wag
þ non-dimethyl rings C–H wag
Biphenyl rings C–H wag þ C–N str.
þ non-dimethyl rings C–H wag
Biphenyl rings C–H wag þ C–N str.
þ dimethyl and non-dimethyl
rings breathing
1293
1246
1030
1028
983
Non-dimethyl rings deformation
Methyl C–H wag
–
1004 (m)
922 (w)
Non-dimethyl rings deformation
C–H wag on biphenyl and
dimethyl rings
924
–
1229
1196
1176
1171
Mainly dimethyl rings breathing
C–N str. þ all rings C–H wag
C–N str. þ all rings C–H wag
Mainly non-dimethyl rings
C–H wag
910
Deformation on all non-dimethyl
rings þ dimethyl rings C–H wag
C–H wag on biphenyl and
non-dimethyl rings
1214 (m)
1185 (m)
827 (m)
816
810
807
–
C–H wag on biphenyl rings
C–H wag on all rings
1160
1113
Mainly non-dimethyl rings
C–H wag
772 (w)
753 (w)
1119 (w)
Biphenyl rings C–H wag and
dimethyl rings C–H wag
Non-dimethyl rings deformation
C–H wag on biphenyl and
dimethyl rings
757
Biphenyl rings deformation
þ dimethyl rings C–H wag
Dimethyl rings deformation
Mainly ring deformation and
C–H wag on dimethyl rings
Movements on all rings
993 (w)
923 (m)
985
932
721 (w)
413 (w)
711
459
912
Biphenyl rings deformation
þ C–H wag on dimethyl and
non-dimethyl rings
422
406
Deformation on biphenyl and
non-dimethyl rings
815 (w)
800
C–H wag on biphenyl rings
a
a
s ¼ Strong, m ¼ medium, w ¼ weak, sh ¼ shoulder.
The most
s ¼ Strong, m ¼ medium, w ¼ weak, sh ¼ shoulder.
The most
intense of the calculated vibrations is italicised.
intense of the calculated vibrations is italicised.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 2 5 7 – 3 2 6 3
3261

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Table 3 Assignment of DMTPD2þ (Dication, Singlet)
peripheral phenyl rings, which is entirely in agreement with our
previous assignment of this band.12
Calculateda /
cmꢀ1
Fig. 2 shows a plot of the Raman spectra of 2þ in the solid
and solution state in the region below 1800 cmꢀ1. There is
obvious agreement between the spectra, and an excellent
match in terms of the relative intensity of the various bands.
The small frequency differences may be attributed to instru-
mental error. These similarities in spectral response between
solution and solid state give further confidence in the applic-
ability of these methods to the study of active devices.
Experimental/
cmꢀ1
Assignment
1569 (m)
1594
Quadrant str. on all rings
þ C–N str. þ biphenyl str.
Quadrant str. on all rings
except dimethyl þ C–N str.
þ biphenyl str.
1575
1569
Quadrant str. on all rings except
dimethyl þ biphenyl str.
Quadrant str. on dimethyl rings
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
Methyl C–H scissoring
Methyl C–H scissoring
þ non-dimethyl rings
While the use of an excitation wavelength of 514.5 nm
results in resonance enhancement of the spectrum of the mono-
cation 2þ due to the absorption band centered at 472 nm, there
is much less resonance enhancement of the dication 22þ at
this excitation wavelength as the absorption band for this
species centered at 492 nm is much less intense. For similar
reasons, more intense Raman spectra are obtained from the
dication 22þ than the monocation 2þ with 632.7 nm excitation.
The frequencies of the Raman active modes of the dication 22þ
are similar, but not identical, to those of the monocation 2þ.
The observed 22þ band at 1569 cmꢀ1 can be correlated with
the cluster of calculated bands between 1569 and 1594 cmꢀ1
with the most intense of the predicted bands occurring at
1575 cmꢀ1 in the calculated vibrational profile (Fig. 7).
The enhanced bands at 1343 and 1395 cmꢀ1 in 22þ are also
of interest as these bands seem to be specific to the dication
only. The observed band at 1343 cmꢀ1 is assigned to a cluster
of calculated bands between 1303 and 1334 cmꢀ1 with the most
intense predicted band being that at 1334 cmꢀ1. These bands
all involve combinations of C–C and C–N stretches. The dis-
placements are similar in nature to those involved in the neu-
tral band at 1294 cmꢀ1 and those in the monocation band at
1324 cmꢀ1. The observed band at 1395 cmꢀ1 is assigned to a
single calculated band at 1391 cmꢀ1. This mode involves only
the methyl groups and is similar in nature to those involved in
the neutral band at 1371 cmꢀ1 and those in the monocation
1526 (w)
1484 (w)
1538
1516
1515
1481
1478
C–H wag þ C–N str.
1473
All rings C–H wag
þ dimethyl C–H scissoring
Methyl C–H wag
1395 (m)
1343 (m)
1391
1334
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
þ non-dimethyl rings C–H wag
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
þ non-dimethyl rings C–H wag
Biphenyl str. þ biphenyl rings
deformation þ C–N str.
þ non-dimethyl rings C–H wag
Biphenyl rings C–H wag
þ C–N str. þ non-dimethyl
rings C–H wag
1318
1314
1303
1244
–
Biphenyl rings C–H wag
þ C–N str. þ dimethyl and
non-dimethyl rings breathing
Mainly dimethyl rings breathing
Biphenyl str. þ C–N str.
þ all rings C–H wag
band at 1366 cmꢀ1
.
–
1200 (s)
1233
1204
Conclusions
The OTTLE cell combined with Raman spectroscopy provides
an ideal way in which to generate and monitor the formation
of radical species of charge transfer materials. In conjunction
with DFT calculations it is possible to predict that the
nature of the radical cations are best described in terms of
a symmetrical, fully delocalized, radical structure.
Raman scattering from the two radical species is clearly dif-
ferent, allowing discrimination between the mono- and di-
cations. The signal to noise ratios from the radicals in solution
under resonance conditions are good, affording high levels of
sensitivity. This suggests it may be possible to detect the radi-
cals in-situ in a device at low concentrations. With a tuneable
laser it should be possible to obtain resonance profiles for the
monocation and dication, providing an effective way of
probing structure.
1182
1176
Biphenyl str. þ C–N str.
þ all rings C–H wag
1177 (sh, m)
Dimethyl and non-dimethyl
rings C–H wag
All rings C–H wag
1172
1118
Biphenyl rings C–H wag and
dimethyl rings C–H wag
Dimethyl rings deformation
Biphenyl rings deformation
þ C–H wag on non-dimethyl
rings
942 (w)
904 (w)
935
901
881 (w)
881
C–H wag on dimethyl and
non-dimethyl rings
a
s ¼ Strong, m ¼ medium, w ¼ weak, sh ¼ shoulder.
The most
intense of the calculated vibrations is italicised.
Acknowledgements
1350 and 1200 cmꢀ1. Both the relatively intense band at 1294
cmꢀ1 in 2 and the relatively intense band at 1324 cmꢀ1 in 2þ
arise from a combination of C–N and C–C stretches. The main
difference between the 1294 cmꢀ1 mode (2) and the 1324 cmꢀ1
mode (2þ) is that the 2þ mode has a larger contribution from
C–N displacements and more involvement from the peripheral
rings. A similar picture is obtained for the other pair of vibra-
tions at 1182 cmꢀ1 (2) and 1214 cmꢀ1 (2þ). Thus the resonant
enhancement in the monocation is favored by contributions
from C–N and C–C stretches mainly associated with the outer
phenyl rings. This indicates that the electronic absorption band
being used to provide the resonant enhancement (lmax 472 nm,
n ! p*) involves a significant change in electron density on the
R. E. L. thanks the RSC and EPSRC for an analytical student-
ship, and Avecia Ltd. for a CASE award. M. A. J. P. was the
recipient of an EPSRC/Avecia CASE for new academic
appointees awarded to P. J. L. Other financial support to this
project from EPSRC and Avecia (W. E. S.) is gratefully
acknowledged.
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