Mulliken-Hush analysis of a bis(triarylamine) mixed-valence system with a N...N distance of 28.7 A

Article abstract of DOI:10.1039/b604603g

An organic mixed valence compound with a spacer length of 25 unsaturated bonds separating two amine redox centres was synthesised and the electron transfer behaviour was investigated in the context of a Mulliken-Hush analysis in order to estimate the long

Full text of DOI:10.1039/b604603g

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Mulliken–Hush analysis of a bis(triarylamine) mixed-valence system
…
˚
with a N N distance of 28.7 A{
Alexander Heckmann, Stephan Amthor and Christoph Lambert*
Received (in Cambridge, UK) 30th March 2006, Accepted 22nd May 2006
First published as an Advance Article on the web 13th June 2006
DOI: 10.1039/b604603g
bands) superpose the IV-CT band.6–8 As can be seen from Fig. 1,
An organic mixed valence compound with a spacer length of 25
unsaturated bonds separating two amine redox centres was
synthesised and the electron transfer behaviour was investigated
in the context of a Mulliken–Hush analysis in order to estimate
the longest redox centre separation for which an intervalence
charge transfer band can be observed.
this band is the more red shifted the more stabilised the hole at the
bridge is. Furthermore, the relatively weak intensity of the IV-CT
bands of these MV compounds complicates the analysis. Thus, no
˚
MV compound with a distance of more than 25.0 A between two
redox centres9 has been investigated by optical methods and the
electronic coupling evaluated, to the best of our knowledge. In
order to explore the longest separation of redox centres that will
allow the analysis of the associated IV-CT band by a Mulliken–
Hush analysis we present in this communication the synthesis of
the bis(triarylamine) MV compounds 1a and 1b in which the
Mixed valence (MV) compounds which consist of two equivalent
redox centres bridged by an unsaturated spacer play a dominant
role for the investigation of basic charge transfer processes in small
molecular model compounds.1–3 In these MV radical ions one
usually considers the so called intervalence charge transfer band
(IV-CT) which originates from an optically induced charge
transfer from one charged redox centre to the neutral other one.
This IV-CT band can be analysed by the generalised Mulliken–
Hush (GMH) theory4,5 in order to evaluate the electronic coupling
V between the two diabatic (noninteracting) states (green
potentials in Fig. 1) in which the charge is localised at either
redox centre.
˚
distance between the amine centres are 28.7 A. By using cyano-
substituted benzenes in 1b as electron deficient spacer units we
raise the energy of the local bridge state to minimise the overlap of
the bridge band and the IV-CT band (Fig. 1).
Compound 1b was synthesised starting from the terminal alkyne
210 and benzonitrile 311 by a Pd-catalysed Hagihara–Sonogashira
coupling reaction12,13 to obtain the precursor 4b which could be
directly transferred to the desired product by a new Hay coupling
reaction14 in good yield. In the same way bis(triarylamine) 1a was
synthesised starting from silane 4a16 (Scheme 1).
While the IV-CT band analysis is straightforward for bis(triaryl-
amine) radical cation MV systems exhibiting strong coupling, it is
normally very complicated for weakly coupled systems (e.g. in
cases where the redox centres are separated by a large spacer) due
to the fact that absorption bands resulting from a hole transfer
from one redox centre to the bridge unit (triarylamine to bridge
The cyclic voltammogram (CV) of 1b in 0.1 M tetrabutyl-
ammonium hexafluorophosphate (TBAH)–CH2Cl2 solution
shows four oxidation processes. The first two processes are
reversible but not resolved (digital simulations yields E1Ox = 330 mV
and E2Ox = 385 mV; for comparison, the statistical value for two
noninteracting redox centres DE = 35.6 mV) and can be assigned
to the oxidation of both triarylamine centres. The third and fourth
3=4
processes (E
# 1000 mV) are also not resolved but are
Ox
irreversible (CV and digital simulation are shown in Supporting
Information{). For a more detailed study of the ET behaviour we
performed a UV/Vis/NIR-monitored chemical oxidation of 1b in
dichloromethane by a stepwise addition of a SbCl5–CH2Cl2
solution (1.0 mM) (Fig. 2).
Fig. 1 Adiabatic (black lines) and diabatic (green lines) states within a
three level model projected on one dimension.
Institute of Organic Chemistry, University of Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany. E-mail: lambert@chemie.uni-
wuerzburg.de; Fax: +49 931 888 4606; Tel: +49 931 888 5318
{ Electronic supplementary information (ESI) available: synthesis and
spectral data of compounds 1a, 1b and 4b, CV of 1b and digital
simulation. See DOI: 10.1039/b604603g
Scheme 1 Synthesis of compounds 1a and 1b.
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Fig. 4 Vis/NIR spectra of 1b+ and 1b2+ in dichloromethane.17
Fig. 2 Chemical oxidation of 1b (dashed black line) to the dication 1b2+
(dashed red line) in dichloromethane by stepwise addition of a SbCl5–
CH2Cl2 solution (1.0 mM). Further addition of the oxidant causes no
more change of the spectrum.
For comparison, the chemical oxidation of 1a in dichloro-
methane is shown in Fig. 3. It is obvious that an analysis of the IV-
CT band is only possible when the bridge band is blue shifted and
the overlap with the IV-CT band is minimised. The data of the
bands of 1a and 1b observed during the chemical oxidation are
summarised in Table 1.
As expected, a band A at around 23 000 cm21 which is typical
of a triarylamine system decreases with proceeding oxidation while
in the same way a new intense band at 13 000 cm21 increases
which is associated whith a p–p* transition of the dianisylphenyl-
amine radical cations.16 This band also shows a shoulder at ca.
14 000 cm21 which we assign to a hole transfer from the
triarylamine centre to the bridge unit. Owing to the electron
withdrawing nitrile substituents this band is at distinctly higher
energy than in 1a (Fig. 3) or any other analogous triarylamine
systems where the bridge is electron rich.6,8,16 Even more
interesting is the observation of a weak and broad absorption
band at about 10 000 cm21 which partially overlaps with the red
edge of the p–p* transition band. With proceeding oxidation of 1b,
this band first increases as the monocation 1b+ is mainly generated
and decreases at further oxidation to the dication 1b2+ (Fig. 4).
This behaviour proves the 10 000 cm21 band to be an IV-CT band
associated with an optically induced hole transfer from one
triarylamine to the other.1
In order to investigate the IV-CT band in the context of a
Mulliken–Hush analysis we extracted17 the spectrum of pure 1b+
from the redox titration of 1b and fitted both the p–p* transition
band and the IV-CT band with four Gaussian functions (Fig. 5).
From this fit we determined the parameters of the IV-CT band
of 1b+ (v˜max = 11 790 cm21, e = 990 M21 cm21, v˜1/2 = 4060 cm21).
mag~nmax
V12~
(1)
Dm12
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Dm12~ Dm2agz4ma2g with Dmag~e|r
(2)
For the evaluation of the electronic coupling V12 by eqn (1)
(where mag = 1.98 D is the transition dipole moment of the IV-CT
band) the diabatic dipole moment difference Dmag is needed which
we calculated by eqn (2) from the diabatic quantity Dmag which is
the diabatic dipole moment difference. In order to get a reliable
value of Dmag we performed an AM1-CI optimisation{ of 1b+
which yields Dmag = 122.8 D. The computed dipole moment also
shows that the effective ET distance r is about 90% of the often
…
used N N distance of the neutral compound.
The energy of the IV-CT band v˜max which corresponds (exactly
within a two-level approximation with parabolic potentials) to the
Marcus reorganisation energy l is significantly higher than for
corresponding bis(triarylamine) radical cations with shorter spacer
units. In fact we discovered that in such systems l passes through a
minimum when plotted against the length of the spacer, and for
MV systems with bridge units composed of more than ten bonds l
steadily increases.2 An increase of the reorganisation energy with
increasing redox centre separation is due to the dominating solvent
reorganisation contribution according to the Born model.19
Furthermore, the high energy value of v˜max in 1b+ is also
confirmed by the results of the AM1-CI calculation which yields
Fig. 3 Chemical oxidation of 1a (dashed black line) to the dication 1a2+
(dashed red line) in dichloromethane by stepwise addition of a SbCl5–
CH2Cl2 solution (1.0 mM). Further addition of the oxidant causes no
more change of the spectrum.
10 040 cm21
.
With the above mentioned data we calculated the electronic
coupling V12 = 190 cm21 by eqn (2). Current investigations about
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Table 1 Absorption maxima and extinction coefficients of the bands in the UV/Vis/NIR spectra of neutral compounds 1a and 1b, the monocation
1b+ and dications 1a2+ and 1b2+
v˜max/cm21 [e/M21 cm21
]
Band A (neutral triarylamine)
p–p*-transition
Bridge band
1b
1a
22 300 [66 700]
24 600 [73 700]
—
—
—
—
—
—
—
1b+ a
1b2+ b
1a2+ b
a
12 800 [19 900]
12 800 [44 500]
13 200 [45 000]
14 100 [10 200]
14 300 [17 900]
11 500 [22 400]
Data obtained from a fit of the spectra of the monocation 1b+ with four gaussian functions.17 b Data of the bands for the dications 1a2+ and
1b2+ were obtained from a fit of the observed bands assigned to a p–p*-transition and a hole transfer from one redox centre to the bridge unit
by three gaussian functions.18
We thank the Deutsche Forschungsgemeinschaft for financial
support and Heraeus GmbH.
Notes and references
1 J. Bonvoisin, J.-P. Launay, W. Verbouwe, M. Van der Auweraer and
F. C. De Schryver, J. Phys. Chem., 1996, 100, 17079–17082.
2 C. Lambert and G. No¨ll, J. Am. Chem. Soc., 1999, 121, 8434–8442.
3 S. F. Nelsen, Chem.–Eur. J., 2000, 6, 581–588.
4 R. J. Cave and M. D. Newton, Chem. Phys. Lett., 1996, 249, 15–19.
5 R. J. Cave and M. D. Newton, J. Chem. Phys., 1997, 106, 9213–9226.
6 C. Lambert, G. No¨ll and J. Schelter, Nat. Mater., 2002, 1, 69–73.
7 S. Amthor and C. Lambert, J. Phys. Chem. A, 2006, 110, 1177–1189.
8 S. Barlow, C. Risko, S.-J. Chung, N. M. Tucker, V. Coropceanu,
S. C. Jones, Z. Levi, J. L. Bre´das and S. R. Marder, J. Am. Chem. Soc.,
2005, 127, 16900–16911.
…
9 The N N through space distance in compound 9 reported in the
Fig. 5 Fit of the IV-CT band and the p–p* absorption band (red) with
four Gaussian functions (green and blue).18 The blue Gaussian function
7
literature is 25.0 A.
10 C. Lambert, G. No¨ll, E. Schma¨lzlin, K. Meerholz and C. Bra¨uchle,
Chem.–Eur. J., 1998, 4, 2129–2135.
˚
represents the IV-CT band.
11 J.-J. Hwang and J. M. Tour, Tetrahedron, 2002, 58, 10387–10405.
12 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara,
Synthesis, 1980, 628–630.
13 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
50, 4467–4470.
14 K. Ikegashira, Y. Nishihara, K. Hirabayashi, A. Mori and T. Hiyama,
Chem. Commun., 1997, 1039–1040.
15 S. Amthor, B. Noller and C. Lambert, Chem. Phys., 2005, 316, 141–152.
16 C. Lambert, S. Amthor and J. Schelter, J. Phys. Chem. A, 2004, 108,
6474–6486.
…
N N distance in
the trends of V12 depending on the
bis(triarylamines) with unsaturated spacer units anticipate a
somewhat higher value for V1/2 (V1/2 # 400–500 cm21) for 1b+.
The lower value evaluated for 1b+ might be due to increasing
formation of unfavourable conformers about the triple bond
axes.20
In summary, our strategy to build up a spacer with electron
deficient units to raise the energy of the bridge state and to avoid
an overlap of the bridge band and the IV-CT band was successful.
In this way, we were able to determine the ET parameters of the
organic MV compound 1b+ with a spacer unit consisting of 25
unsaturated bonds, the longest separation of redox centres for
which an IV-CT band has ever been observed. A rather weak
electronic interaction of both triarylamine units was ascertained by
a Mulliken–Hush analysis of the IV-CT band. AM1-CI calcula-
tions are in agreement with the experimental results and also show
that the charge in 1b+ is localised on the triarylamine centres. We
anticipate that in cases where the total reorganisation energy is
even lower than in the present case, and consequently the IV-CT
band is at lower energy, one could observe an IV-CT band for a
compound with an even longer redox centre separation.
17 The Vis/NIR spectrum of pure 1b+ was obtained by a chemical
oxidation of a concentrated solution of 1b (1023 M) in CH2Cl2 by
adding dropwise a SbCl5 solution in CH2Cl2 (1.0 mM) and multiplying
the spectrum after the addition of the first drop of the oxidant solution
with a factor. This factor was chosen in a way that the extinction
coefficient of the band at ca. 13 000 cm21 which we assign to a localised
p–p*-transition of the triarylamine radical cation in 1b+ is half that of
the corresponding band of the dication 1b2+
.
18 We fitted the spectrum of the dication 1b2+ with three Gaussian
functions, one representing the bridge band, and two representing the
p–p*-transition. The spectrum of the monocation 1b+17 was fitted with
an additional Gaussian function for the IV-CT band. In this fit the
energies of the three Gaussians were kept at almost the same energy as
in 1b2+
.
19 P. Chen and T. J. Meyer, Chem. Rev., 1998, 98, 1439–1478.
20 S. B. Sachs, S. D. Dudek, R. P. Hsung, L. R. Sita, J. F. Smalley,
M. D. Newton, S. W. Feldberg and C. E. D. Chidsey, J. Am. Chem.
Soc., 1997, 119, 10563–10564.
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