Ferrocene amphiphilic D-π-A dyes: Synthesis, redox behavior and determination of band gaps

Article abstract of DOI:10.1039/c8nj00787j

We report the synthesis of a series of ferrocene amphiphilic donor-π-acceptor dyes, with the general formula (Fc-CHCH-HetNC₁₆H₃₃)⁺ X^- [where: Fc behaves as the donor group, a double bond as the π bridge, and 2-,4-pyridinium and 4-quinolinium as the potent acceptor groups (2a-b and 4, X = Br^- or BF₄^-)], in good overall yields. Together with their neutral counterparts (6a-b and 7), the photophysical and electrochemical properties of these compounds were investigated by means of UV-Vis spectroscopy and cyclic voltammetry. The optical and electrochemical band gaps of these dyes were calculated, which indicated that 4 has the lowest bandgap value. Time-dependent DFT calculations indicate that the lowest energy absorption band displayed for these compounds has mainly metal-to-ligand charge transfer character, with the HOMO-LUMO electronic transition being the main contribution.

Full text of DOI:10.1039/c8nj00787j

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Mayorga, C. Sandoval-Chávez, M. D. P. Carreon-Castro, V. M. Ugalde-Saldívar, F. Cortes-Guzman, J. G.
López-Cortés and M. C. Ortega-Alfaro, New J. Chem., 2018, DOI: 10.1039/C8NJ00787J.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Ferrocene amphiphilic D-π-A dyes: Synthesis, redox behavior and
determination of Band Gaps.
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
B. López-Mayorga, C. I. Sandoval-Chávez, P. Carreón- Castro, V. M. Ugalde-Saldívar, F. Cortés-
c
c
a
Guzmán, J. G. López-Cortés and M. C. Ortega-Alfaro*
DOI: 10.1039/x0xx00000x
We report the synthesis of a series of ferrocene amphiphilic donor-π-acceptor dyes, with general formula (Fc-CH=CH-
www.rsc.org/
+
-
HetNC16
H
33) X [where: Fc behaves as donor group, a double bond as π bridge, and 2-, 4-pyridinium and 4-quinolinium as
-
potent acceptor groups (2a-b and 4, X= Br or BF
4
-)], in good overall yields. Together with their neutral counterparts (6a-b
and 7), the photophysical and electrochemical properties of these compounds are investigated by means of UV-Vis
spectroscopy and cyclic voltammetry. The optically and electrochemically band gaps of these dyes are calculated, affording
that 4 has the lowest bandgap value. Time-dependent DFT calculations indicate that the lowest energy absorption band
displayed for these compounds has mainly metal-to-ligand charge character, being the HOMO-LUMO electronic transition
the main contribution.
The properties of D-π-A systems can be easily changed by
varying donor, spacer and acceptor moieties. We have chosen
ferrocenyl π-conjugated pyridinium salts as the key building
Introduction
Nowadays, dye-like and π-conjugated organic compounds
represent attractive targets for diverse application in advanced
1
functional materials. Organic π-systems end-capped with an
block for the next reasons: i) Literature data reveal that
ferrocene is an excellent electron-donating group and exhibits
1
8
a reversible redox behavior. ii) The ethylene group is a good
spacer that allows electronic communication between donor
electron donor (D) and an electron acceptor (A) represent a
subclass of these kinds of molecules widely known as push–
pull systems. They have predominant applications as
19
and acceptor in D-π-A system and iii) N-alkyl heterocyclic
2
0
2
cations exhibit higher electro-withdrawing character. These
facts should positively contribute to favor a good donor-
acceptor interaction in these dyes (Figure 1). Likewise, we have
incorporated a long alkyl chain to favor the solubility of these
compounds to future optoelectronic applications.
chromophores with nonlinear optical (NLO) properties,
3
electro-optic, and piezochromic materials, NLO switches,
4
5
6
photochromic, charge-based information storage and
7
8
solvatochromic probes as well as active layers in DSSCs.
9
When organometallic fragments are included as electron
donor, they can enhance the optical behavior of donor-π-
acceptor (D-π-A) systems along with other intrinsic
Thus, we describe the synthesis of six ferrocenyl π-conjugated
pyridinium salts, incorporating two different heterocycles
(pyridine and quinoline) in their cationic and neutral version.
The properties of these dyes were investigated by absorption
spectroscopy and electrochemical analysis. Using this
information, we have also determined their optical and
electrochemical band gaps. To analyze the experimental data
obtained, time dependent density functional (TD-DFT)
calculations were performed using the t-HCTHhyb functional
and the 6-311++G(d)/LANL2DZ mixed basis set.
1
0
properties.
Among the variety of organometallic compounds that have
been generated to achieve this goal, ferrocene-based
architectures bearing an unsaturated backbone have been
used for wide range of applications, like information storage
1
1
12
13
devices, molecular switches, chemical detection and so
1
on. Likewise, ferrocenyl fragments have been included as
4
1
component of D-π-A systems with NLO properties, as
5
1
sensors and other related applications.
6
17
a.
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,
Circuito Exterior, Ciudad Universitaria, Coyoacán, Cd. Mx., 04510, México.
carmen.ortega@nucleares.unam.mx.
b.
c.
Facultad de Química, Universidad Nacional Autónoma de México, Av. Universidad
3
000, Coyoacán, Cd. Mx., 04510, México.
Instituto de Química, Universidad Nacional Autónoma de México, Circuito
Exterior, Cd. Universitaria, Coyoacán, Cd. Mx., 04510, México.
Electronic Supplementary Information (ESI) available: Additional spectroscopic and
electrochemical data. See DOI: 10.1039/x0xx00000x
Figure 1. Structural design of ferrocene amphiphilic D-π-A dyes.
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.
Scheme 1. Synthesis of ferrocene amphiphilic D-π-A dyes [2a-b and 4][X], where X= Br, BF
4
and their neutral counterparts 6a-b and 7. Conditions: I) C16H33Br /110 °C; II) Piperidine,
FcC(O)H /MeOH, reflux; III) AgBF , MeCN, RT. IV) n-BuLi /-70 °C/THF, FcC(O)H; V) MsCl, NEt
4
3
/CH Cl ; VI) t-BuOK /THF, FcC(O)H.
2
2
neutral analogues 6a-b and
7 (Scheme 1). Compounds 6a-b
Results and discussion
were obtained by a tandem reaction between 2- or 4-methyl
pyridine and ferrocenecarboxaldehyde in basic conditions,
The synthesis of new amphiphilic ferrocene D-π-A dyes was
carried out using different modifications of informed
affording the corresponding alcohols intermediates 5a-b
which were subsequently dehydrated using MsCl and NEt 6a-
were obtained in excellent global yields, and the
,
2
1, 22
.
3
methods.
In a first step, the picoline cations (1) were
b
prepared by the reaction of 2- and 4-picoline with the alkyl
2
3
spectroscopic data are consistent with the literature. The
new compound was prepared in one step by a condensation
reaction between the 4-methylquinoline and
ferrocenecarboxaldehyde in basic conditions, in good yields. As
halide at 110 °C. Then, the corresponding N-
7
hexadecylpicolinium
bromide
was
reacted
with
ferrocenecarboxaldehyde, in the presence of piperidine as
base, affording 2a[Br] and 2b[Br], in excellent global yields
1
expected, the H NMR spectrum of
7 indicates that only the
(Scheme 1). For 4[Br], a similar reaction between the N-
less-hindered E isomer was formed (J = 15.9 Hz).
alkylquinolinium bromide and ferrocenecarboxaldehyde gave
this compound, in 85 % yield. (Scheme 1). The identities and
Analyzing the spectroscopic behavior of these compounds, we
observed slight differences of the chemical shift for the vinyl
and heterocyclic protons across this series. As expected, the
presence of a positive charge on nitrogen atom of compounds
purities of compounds 2a[Br], 2b[Br] and 4[Br] are confirmed
by NMR, IR-ATR, Mass Spectrometry, HRMS and CHN
elemental analyses. To evaluate the influence of the
counterion included in these compounds, we have also
-
2a[Br], 2b[Br] and
4
[Br] resulted in an upfield shift of the vinyl
[Br], the
chemical shift of H is very different in comparison with 2a[Br]
B
prepared their analogues 2a-b and
4, those containing BF
4
in
protons HA and H (Figure S-25). In the case of
4
B
good yields.
1
The H NMR spectra for 2a[Br] and 2b[Br] exhibit the
representative signals for a 2- and 4-pyridinium derivatives,
respectively. In both cases, we observe a large vicinal coupling
constant for the two vinylic protons supporting the (E)-
configuration for the double bond. The typical signals for a
mono substituted ferrocenyl compound appear around 4-5
ppm. Shifted at low fields (~4 ppm), we observed the
and 2b[Br]. This reveals the potent electron-withdrawing
character of the quinolinium unit that strongly affects HB,
downfield-shifts this signal in comparison with those for 2a[Br]
and 2b[Br], respectively.
Likewise, relative large upfield shifts of protons of heterocyclic
moiety for the cationic compounds 2a[Br], 2b[Br] and
comparison with their neutral analogues are observed,
revealing that in 2a[Br], 2b[Br] and [Br], the electronic
delocalization provoked by the polarization across the D-π-A
system is more effective than 6a 6b and , respectively.
Furthermore, the resonances for the protons assigned to
ferrocene moiety of 2a[Br], 2b[Br] and [Br] are offset upfield
compared with 6a 6b and (See, experimental section). This
suggests a slight contribution of a fulvene-type resonance
4[Br] in
1
3
methylene group bonded to the nitrogen atom. In the C NMR
spectra, we observed two signals between 120-125 ppm
assigned to the double bond included in these compounds.
Likewise, typical signals at 70-80 ppm are consistent with a
mono-substituted ferrocene, and shifted to higher fields, we
observe the corresponding signals of the aliphatic chain.
Similar spectroscopic data are observed for the compounds
4
,
7
4
,
7
2
a
[BF
of the ionic ferrocenyl compounds 2a-b and
effect of the electron-acceptor group, we have synthesized the
4
] and 2b[BF
4
]. To compare the spectroscopic properties
1
8g, 24
structure B as shown in Scheme 2.
-
observed for series of BF4 salts.
A similar behavior was
4
and evaluate the
2
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Optical properties
the pyridinium cation, a best π-orbital overlap is favored,
which enhances the electronic communication between the
ferrocenyl group and the N-pyridinium acceptor favoring the
The optical properties of the ferrocenyl salts [2a-b and 4][Br]
4
and [2a-b and 4][BF ] were studied to determine the suitability
mesomeric structure
B
(Scheme 2).
of these materials for possible optoelectronic applications and
to further understand the effects of the acceptor included in
this series of ferrocenyl compounds. Figure 2 shows the
On the other hand,
4
exhibits two well-defined absorption
bands around 320 and 409 nm, with ε values very similar,
2
electronic absorption spectra for 2a
3
, 2b and 4 in CHCl and
8
assigned to π-π* and n-π* transitions, respectively, and a
MLCT absorption at 600 nm in CH CN (Table S1 and Figure 2).
CH CN.
3
3
Compounds 2a-b display two characteristic absorption bands.
For 2a, the more intense band at higher energy appears at 347
We also see that the MLCT absorption band at 600 nm is red-
shifted regarding compounds 2a-b. This behavior is attributed
to the best distribution of positive charge on the additional
-1
-1
-1
-1
nm (8700 mol L cm ) in CH
in CHCl (Tables 1 and S1). According to literature for other
ferrocenyl mono-substituted compounds,
3
CN and at 355 (14100 mol L cm )
3
2
5
ring included in the quinolinium unit of 4, which decreases the
this band
energy of the MLCT band transition band, thus shifting this
absorption at low energy values, which also favors a best
corresponds to a π-π* transition, associated with an intra-
ligand charge transfer (ILCT). However, recent studies involving
density functional theory suggests that this band has a largely
1
8,24
contribution of the fulvene-type resonance.
-
In general, the
2
6
presence of BF4 as counterion does not modify the optical
properties of these compounds. Moreover, if we compare the
MLCT character. We also observe a less intense lower energy
-1
-1
band at 533 (2000 mol L cm ) in CH
3
CN and 551 nm (3800
. This band is due to metal to-ligand
charge transfer (MLCT).
Comparing the absorption spectra of 2a-b salts acquired in
CHCl and CH CN, we observe that these compounds show a
clear red-shift in both ICT bands, showing higher molar
coefficient (ε), when CHCl is used as solvent (Table S1, SI). This
-
1
-1
behavior for 6a
,
6b and
7
(Table 1 and S1), these compounds
CN and CHCl , with
mol L cm ) for CHCl
3
display similar absorption spectra in CH
3
3
similar ε values for both absorption bands, revealing a no
significant solvatochromic effect, as result of a less polarization
effect in these neutral push-pull compounds.
3
3
3
Table 1. Low Energy band displayed for ferrocenyl amphiphilic D-π-A dyes and
behavior indicates a negative solvatochromic effect favored by analogues in acetonitrile.
a polar solvent such as acetonitrile, which stabilizes the
a
b
max
2
7
Entry
Compound
λ
max (nm)
ε
charged ground state.
1
2
3
4
5
6
7
8
9
2a[Br]
533
530
552
551
598
601
461
465
476
539
2.0
1.9
3.6
4.5
3.3
3.3
1.4
1.2
1.8
8.1
40
We also observe that 2b salts exhibit a more evident red-shift
than 2a salts, this behavior can be correlated with the relative
position of the N-pyridinium moiety in the compound 2b, thus
when the yliden-ferrocenyl fragment is bearing at 4-position of
2a[BF
2b[Br]
2b[BF
4[Br]
4
]
4
]
4[BF
6a
6b
7
4
]
2
5b
1
1
0
1
6
8[PF ]
2
9
c
9[I]
478
543
29
c
Scheme 2. Fulvene-type resonance structure B.
12
10[I]
46
a
b
3
-1
-1
c
λ
max (nm), ε (10 mol L cm ), Data reported from methanol.
20
15
10
5
0
20
15
10
5
2
2
4
2
2
4
a [Br]
b [Br]
[Br]
a [Br]
b [Br]
[Br]
2
2
4
a [BF4]
b [BF4]
[BF ]
4
2a [BF4]
2b [BF
[BF4]
]
4
4
0
300
300
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 2. Right: Solid lines for UV/Vis spectra of 2a [Br] (red), 2b [Br] (bright green) and 4 [Br] (dark blue) in CH
green) and 4[Br] (dark blue) in CHCl . Left: Solid lines for UV/Vis spectra of 2a[BF ] (orange), 2b[BF ] (lime) and 4[BF
orange), 2b[BF ] (lime) and 4[BF ] (pink) in CHCl
3
CN. Dash lines for UV/Vis spectra of 2a[Br] (red), 2b[Br] (bright
3
4
4
4
] (pink) in CH CN. Dash lines for UV/Vis spectra of 2a[BF
3
4
]
(
4
4
3
.
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4
A comparison of the electronic absorption spectra of 2a[BF ], Optical band gap
2
b
[BF ] and 4[BF ] vs 6a, 6b and 7 in CH CN (Figure 3) indicates
4
4
3
UV-Vis electronic absorption spectroscopy is a well-known
technique to evaluate the optical absorption band gap of
30
that the ferrocenyl cations exhibit a red-shift in both the
higher and lower energy bands, displaying a less intense high
compounds. The band gap in molecular compounds could be
energy band than compounds 6a-b and
increasing of acceptor strength in 2a-b and
7, likewise, the
estimated as the energy difference between the HOMO and
LUMO orbitals, and can be determined by performing an
extrapolation on the absorption spectrum, finding the
4
provokes that the
lower energy bands (MLCT) gains in relative intensity. These
results confirm that the parentage of the higher energy bands
is largely π to acceptor and/or π-π*. With regards to the
position and relative intensity of the MLTC bands observed in
31
wavelength at which the material begins to absorb. The
optical band gaps for 2a, 2b and 4 salts were estimated from
the first optical absorption edge. Eg is the band gap
corresponding to a particular absorption of low photon energy
6a, 6b and 7, this also indicates a weak polar character of
these compounds.
(hν) and express the radiation energy in eV using the following
Analyzing the electronic absorption spectra for 2b and
4
with
(Table
, entry 10), displays a blue shifted LE band in comparison to 4,
equation (1).
the literature data, we observe that the ferrocenyl dye
8
Eg (eV)= hν = hc/λ………… (1)
1
Where h is Planck constant and c is speed of light in vacuum.
which implies that the incorporation of π-extended system as
spacer, possibly diminishes the π-orbital overlap arising from
twisting about the ethylene-quinolinium fragment, which in
The optical absorption spectra of all samples were acquired
from CH
to the results obtained from the CV measurements. Figure 5
shows the absorption spectrum of 2b[BF ], where the
3
CN solutions, so that they can be directly compared
2
5b
consequence weakens the electronic communication between
4
the donor-acceptor couple
Finally, comparing the optical properties of 2b and
well-known DAS analogues [I] and 10[I] (Table 1, entries 11-
2), we can see that the presence of ferrocenyl fragment
.
corresponding wavelength to the band gap energy can be
calculated from the cross point of absorption onset and
corrected base line.
4
with the
9
2
9
1
2
1
1
0
5
0
5
0
instead dimethylaminophenyl moiety provokes a red shift of
the LE band, loosing intensity. This is attributable to the MLCT
character of the LE band associated to the ferrocenyl group.
5
4
3
2
1
0
2b [BF4]
λ onset = 670nm
2
1
1
0
5
0
5
0
2
2
4
a [BF4]
b [BF4]
5
50
600
650
700
750
800
[BF4]
Wavelength (nm)
6
6
7
a
b
2
b [BF4]
3
00
400
500
600
700
800
Wavelength (nm)
3 4
Figure 5. Absorption spectrum in CH CN and optical onset band gap of 2b[BF ].
3
00
400
500
600
700
800
Table 2. Optical onset band gaps for ferrocenyl salts.
Wavelength (nm)
λ
onset
Optical Band
gap (eV)
1.87
a
Figure 3. Electronic absorption spectra of ferrocenyl compounds (cations vs neutral):
a[BF ] (orange), 2b[BF ] (lime), 4[BF ] (pink), 6a (purple), 6b (black), 7 (cyan) in CH CN.
Entry
Compound
λmax (nm)
(nm)
2
4
4
4
3
1
2
3
4
5
6
7
2a[Br]
533
530
552
551
598
601
662
657
669
670
761
758
534
2a[BF
2b[Br]
2b[BF
4[Br]
4
]
1.88
1.85
4
]
1.85
1.63
4[BF
4
2
]
1.63
9b
b
DAST
478
2.32
a
b
CH
3
CN was used as solvent in these determinations. Data reported in MeOH.
The corresponding
equivalent to 1.85 eV. The optical band gap estimated from
the onset wavelength for salts of 2a-b and are listed in Table
. As we can see, both salts of display the smaller value of
4
λonset for 2b[BF ] is 670 nm, which is
4
2
4
band gap, confirming that the quinolinium group behaves as a
potent acceptor, distributes efficiently the electron density in
Figure 4. Examples of other D-π-A dyads with similar structural features.
the push-pull system and, shifts the
λonset to lower energy
4
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values in comparison to compounds 2a and 2b. Comparing 1.92, -1.97 and -1.42, respectively (Figure 8 and S-36, SI). In the
with literature data (Table 2, entry 7), compounds exhibit three cases, this signal has its counterpart with the signal IIIap
lower optical band gaps, which denotes their potential as These results evidence the reduction of the heterocyclic cation
4
.
organic semiconductors.
in each case.
Electrochemistry
Table 3. Potential values of ferrocene moiety into ferrocenyl D-π-A dyes.
The electrochemical properties of 2a-b and
4 salts and
+
+
+
[a]
Entry
Compound
E
ap (V/Fc -Fc)
E
cp(V/Fc -Fc)
E
½
(V/Fc -Fc)
compounds 6a-b, and
7 were studied by cyclic voltammetry
1
2
3
4
5
6
7
8
9
2a[Br]
0.19
0.18
0.07
0.19
0.18
0.07
0.19
0.19
0.07
0.13
0.12
0.00
0.12
0.12
0.00
0.12
0.13
0.00
0.16
0.15
0.03
0.15
0.15
0.03
0.16
0.16
0.04
(CV). The CV data give significant information about the
4
2a[BF ]
oxidation and reduction potentials of materials. The values of
the half-wave of all the compounds are summarized in Table 3.
6a
2b[Br]
-1
Figure 6 shows typical voltammograms for 1 mmol L CH
3
CN
][Br] at 100 mV s . When the scan
potential is initiated in positive direction in these
voltammograms, two oxidation signals (Iap and IIap are
observed for all compounds, for 2a[Br] ( ap = 0.19 and 0.31,
respectively) for 2b[Br] ( ap = 0.19 and 0.35, respectively) and
for [Br] ( ap = 0.19 and 0.33, respectively) all V/Fc -Fc.
4
2b[BF ]
-1
solutions of [2a-b and
4
6b
a
4[Br]
)
4
4[BF ]
7
E
E
[a] E1/2 = (Epc + Epa)/2
+
4
E
When the potential scan was reversed, three or four reduction
signals are observed for the three bromide compounds, but
only two of them were considered to electrochemical analyses
6
4
2
0
0
0
0
60
40
20
0
IIap
Iap
(I
cp and IIIcp) at 0.13 and -1.60 for 2a[Br], 0.12 and -1.53 to
IVap
2b
[Br] and 0.12 and -1.22 for
4[Br], all in V/Fc+-Fc. Likewise,
Icp
IIcp
when switching the scan in
E
-
λ
and the potential direction is
IIIcp
-20
-20
20
IIap
positive, we observe at less three oxidation signals (Iap, IIap and
Iap
2
0
IVap). The IVap oxidation signal is observed at -0.25 for 2a[Br], -
IVap
0.79 for 2b[Br] and -0.28 V/Fc+-Fc for
4[Br].
0
0
IIcp
The half-wave potential of the ferrocene/ferrocenium redox
couple was identified around 0.1 V and estimated from the
following equation:
Icp
-20
IIIcp
-20
30
3
2
1
0
0
0
0
IIap
20
10
0
Iap
E
1/2 = (
E
ap
+
Ecp) /
2
………… (2)
IVap
Where,
Eap and Ecp are the anodic and cathodic peak
potentials, respectively. For internal reference, the half-wave
+
potential of Fc /Fc was found at 0.62 V relative to the AgCl/Ag
IIcp
-
10
I
cp
-10
-20
IIIcp
-1
-20
reference electrode. In all cases, the reduction signal (Icp) was
associated with an oxidation signal (Iap) and corresponds to
+
Fc -Fc redox couple in this series of bromide salts, with a
-2
0
1
+
E | V Fc / Fc
-1
Figure 6. Cyclic voltammograms obtained in 0.1 mmol L Bu
4
6
NPF acetonitrile solutions
difference between anodic and cathodic peak potentials of
E
ap
-
-1
-1
on glassy carbon electrode at 100 mVs using 1 mmol L solutions of 2a[Br] (red),
-1
Ecp = 0.07 V (a ferrocene 1.0 mmol L solution under the same 2b[Br] (bright green) and 4[Br] (dark blue).
shows a similar value, 0.07 V). These results are consistent
with a monoelectronic and reversible process assigned to the
ferrocene moiety. The signal IIap was assigned to the
irreversible oxidation process of bromide anion to bromine. To
confirm this assignment, we decided to study the
2a [Br]
2
0
0
electrochemical behavior of 2a-b and
4
salts, all them
as counter anion (Figures 7 and S-32 to S-34 of
SI). The CVs of [2a-b and ][BF ] reveal the absence of
oxidation peaks at high potential values associated to the
bromide anion. Comparing the CV acquired from
-
containing BF
4
4
4
-20
2
0
a
2a [BF4]
tetrabutylammonium bromide solution (TBAB), we confirmed
that the peak IIap is correctly assigned to the oxidation of
0
3
2
bromide anion (Figure S-35, SI).
We also observe a reduction signal (IIIcp) around of -1.5 V/Fc+-
-
20
2
6b, 33
-2
-1
0
1
Fc, in accord with literature,
pyridinium ion reduction. To confirm that, we have also
acquired the CVs of precursors 1a 1b and , in cathodic
at this potential value occurs
+
E / V| Fc -Fc
Figure 7. Comparative cyclic voltammograms obtained in 0.1M Bu
4
NPF
glassy carbon electrode at 100 mVs using 1 mmol L solutions of 2a [Br] (red), 2a [BF
orange).
6
solutions on
,
3
-
1
-1
4
]
direction. We observe a well-defined reduction signal around -
(
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1
1
2
0
8
6
4
2
0
2
4
6
6
E
HOMO = - [4.8 eV + Eox/onset] ………… (3)
IIap = 0.35 V
4
[Br]
ELUMO = - [4.8 eV + Ered/onset] ………… (4)
The onset potentials for the oxidation (Eox/onset) and reduction
red/onset) were determined from the intersection of the two
(
E
tangents drawn at the current rise and background charging
current of the CVs as illustrated in Figures S37-S42 (SI). For
onset oxidation, CV was acquired when the scan potential is
initiated in a positive direction. Onset reduction was calculated
from CV initiated in a negative direction. The electrochemical
band gap of the all ferrocenyl compounds were deduced by
-
-
-
IIIcp = -1.23 V
3
IIap = 0.36 V
4
2
0
2
4
6
3
7
the equation (5): EBand gap = LUMO – HOMO.
As shown in Table 4 and Figure 9a, the onset potential of
oxidation for [Br] is shifted to lower values than 2a[Br] and
[Br]. In all cases, the onset potential of oxidation
4
-
-
-
2
b
IIIcp = -1.42 V
corresponds to the oxidation process of ferrocene group.
Likewise, the onset reduction is assigned to the adding of an
electron to the LUMO, which will be based on the pyridinium
unit. Analyzing the values of reduction onset potential of [2a
and 2b][Br] (Figure 9b), they display values of -1.49 and -1.43
V, respectively. We can also see a significant difference for the
-2
-1
0
1
2
+
E / V | Fc - Fc
-
1
Figure 8. Comparative cyclic voltammograms obtained in 0.1 mmol
L
4 6
Bu NPF
-
1
-1
solutions on glassy carbon electrode at 100 mVs (negative direction), using 1 mmol L
solutions of 3 (black) and 4 [Br] (blue).
compound
4, which has a value of -1.13 V. Accordingly, the
Electrochemical band gap
quinolinium cation is a best electron-withdrawing group than
Cyclic voltammetry has been recognized as an important pyridinium cation modifying the value of LUMO energy level.
technique for measuring electrochemical band gaps, electron The lower energy gap values of salts compared with 2a and
4
affinities and ionization potentials. The oxidations process 2b may be result of the higher electron delocalization favored
corresponds to the removal of an electron from the highest in these ferrocenyl compounds.
occupied molecular orbital (HOMO) energy level, whereas the If we compare the band gaps values obtained from the optical
reduction corresponds to the electron addition into the lowest and electrochemical measurement (Table 4), we observe that
3
4
unoccupied molecular orbital (LUMO). We proceeded to electrochemical band gaps are smaller than those obtained
determine the values of band gap from cyclic voltammograms from optical approximation but remains essentially the same
of these compounds and correlate them with the optical onset tendency. This discrepancy has also been observed by other
band gaps previously obtained. It is important to notice that groups, in measuring these properties for diverse compounds,
the signal attributed to bromide anion does not affect the finding that the interaction solvent-solute can modify the band
determination of the band gap of the first series of bromide gap values, as well as the possible interactions between the
3
1, 38
salts, since the voltammograms acquired for the series [2a-b analyte and the electrode surface.
and
][BF ] exhibit the same value of Eap for the system Moreover, this discrepancy can be attributed to the
4
4
+
Fc /Fc) (Figure 7).
39
incertitude to know the real value of LUMO. A common
(
Table
4
summarizes the onset oxidation and reduction alternative to estimate the LUMO energy level is to use the
potential of the 2a, 2b and 4
. From the values of Eox/onset and information obtained from the optical band gap (Eg) obtained
Ered/onset, we can calculate HOMO and LUMO energy levels. This by UV-vis measurements and the experimental HOMO energy
conventional method has previously used to estimate the calculated from CV technique, which is associated to the
3
5
HOMO and LUMO energy levels of ferrocenyl compounds
including the reference energy level of ferrocene (4.8 eV below
ferrocene oxidation process, following the equation (6):
E
[LUMO] = E[HOMO]CV + Eg ………… (6)
3
the vacuum level) as follows:
6
4
Table 4. Electrochemical potential and estimated HOMO-LUMO and band gap of ferrocenyl D-π-A dyes [2a-b and 4][Br] and [2a-b and 4][BF ]
Electrochemical
Optical
a
a
c
Compound
E
onset ox
E
onset red
HOMO (eV)CV
LUMO (eV)CV
LUMO (eV)opt
b
Band gap (eV)
1.52
Band gap (eV)
1.87
2
a[Br]
a[BF
b[Br]
b[BF
[Br]
[BF
0.04
0.04
0.04
0.04
-1.49
-1.59
-1.43
-1.48
-1.11
-1.07
-4.84
-4.84
-4.84
-4.84
-4.77
-4.83
-3.31
-3.21
-3.37
-3.32
-3.69
-3.73
-2.97
-2.96
-2.99
-2.99
-3.14
-3.2
2
4
]
1.63
1.88
2
1.47
1.85
2
4
]
1.52
1.85
4
-0.03
0.03
1.08
1.63
4
4
]
1.10
1.63
a
+
b
c
Potential values are reported versus Fc /Fc. Electrochemical Band gap was calculated LUMO - HOMO. Energy of LUMO level calculated from E[LUMO]opt = E[HOMO] CV
+
Eg(opt).
6
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6
5
4
3
2
1
0
0
0
0
0
0
0
40
analyses. The experimental UV spectra were taken as
reference to validate the theoretical calculations. The average
difference between experimental and theoretical values is 0.2
eV, which is in average an error of less than 10%.
2
a [Br]
2b [Br]
4
[Br]
In the three molecules, the HOMO, HOMO-1, HOMO-2 and
HOMO-3 are delocalized mostly in the ferrocene group,
whereas the LUMO, LUMO+1 are observed around the
heterocyclic cation moiety. Images of the frontier molecular
orbitals, their energies and energy gaps are shown in Table 5.
The calculated energy values of these orbital agree with those
estimated by electrochemical and optical experiments (see
Table 4). The difference in the HOMO values is around 0.8 eV
and 0.5 eV for LUMO electrochemical estimation. The LUMO
values obtained by optical approximation is closer to
theoretical values (difference of 0.2 eV). However, the
theoretical HOMO–LUMO gaps are larger than experimental
one, mostly due to the lower HOMO energies.
-
10
-
0.4
0.0
0.4
0.8
1.2
+
E | V Fc / Fc
0
-
-
10
20
2
2
a [Br]
b [Br]
4
[Br]
-
2.4
-2.0
-1.6
-1.2
-0.8
-0.4
+
E | V Fc / Fc
Figure 9. (a) Comparison of onset potential of oxidation and (b) onset potential of
reduction of ferrocenyl compounds 2a[Br] (red), 2b[Br] (bright green) and 4[Br] (dark
blue).
Most of the electronic transitions observed within the excited
states involve these five molecular orbitals, which allow the
electron transfer from the metal complex to the
heteroaromatic ring and are related with the UV signals
around 600 and 400 nm. The features of each excited state are
presented in Table 6.
Computational studies
To gain more insight about the charge transfer properties of
the molecules studied in this work, we analyzed the excited
states of molecules 2a’, 2b’ and 4’, which are model
structures. The molecular structure predicted for 2b’ agrees
relatively well with that determined by X-ray diffraction
Table 5. Frontier molecular orbitals of structures 2a’, 2b’ and 4’. Energies are in hartrees and energy differences in eV.
Structure / Eg
HOMO
LUMO
2
a’
2.88 eV
-
-
-
0.20769 a.u., -5.65 eV
-
0.11172 a.u., -2.77 eV
2
b’
2
.76 eV
-0.10539 a.u., -2.87 eV
0.20688 a.u., -5.63 eV
4
’
2
.48 eV
0.20726 a.u., -5.64 eV
-0.11622 a.u., -3.16 eV
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Table 6. Features of selected excited states of 2a’, 2b’ and 4’.
a
b
Structure
λ
max, exp (nm)
λ
max, calc (nm)
Excited State
E
calc (eV)
ƒ
OS
Orbital Contributions
7
7
7
7
3 -> 77 -0.25597 (13.9%, HOMO-1→LUMO+2)
3 -> 78 -0.10274 (2.2%, HOMO-1→LUMO+3)
4 -> 75
0.51127 (55.4%, HOMO→LUMO)
0.12288 (3.2%, HOMO→LUMO+1)
0.10658 (2.4%, HOMO→LUMO+2)
5
33 2a[Br]
603
395
2 singlet
2.0545
3.1418
0.0871
0.7735
4 -> 76
2
a’
74 -> 77
7
4 -> 78 -0.32788 (22.8%, HOMO→LUMO+3)
7
2 -> 75
0.65690 (90.4%, HOMO-2→LUMO)
0.15351 (4.9%, HOMO-1→LUMO+2)
0.14901 (4.7%, HOMO→LUMO+1)
3
47 2a[Br]
7 singlet
73 -> 77
7
7
4 -> 76
3 -> 78 -0.24415 (12.8%, HOMO-1→LUMO+3)
5
3
52 2b[Br]
62 2b[Br]
618
404
2 singlet
7 singlet
2.0005
3.0696
0.1089
0.9265
74 -> 75
0.54490 (63.9%, HOMO→LUMO)
0.10816 (23.3%, HOMO→LUMO+2)
0.67027 (96.4%, HOMO-2→LUMO)
0.12908 (3.6%, HOMO-1→LUMO+3)
-0.18215 (7.0%, HOMO-1→LUMO+3)
0.60531 (77.4%, HOMO→LUMO)
0.17331 (6.3%, HOMO→LUMO+2)
0.20896 (9.2%, HOMO→LUMO+4)
-0.20464 (8.8%, HOMO-3→LUMO)
-0.11356 (2.7%, HOMO-3→LUMO+4)
0.62683 (82.8%, HOMO-2 – LUMO)
-0.16457 (5.7%, HOMO-1→LUMO+3)
2
b’
74 -> 77
7
7
8
8
8
8
8
8
8
8
2 -> 75
3 -> 78
6 -> 91
7 -> 88
7 -> 90
7 -> 92
4 -> 88
4 -> 92
5 -> 88
6 -> 91
5
98 4[Br]
660
448
2 singlet
6 singlet
1.8776
2.7685
0.1696
0.7363
4
’
4
06 4[Br]
a
b
Data obtained from CH
3
CN. Data obtained from the simulated absorption spectra.
2 6
Figure 10. Density difference for S and S electronic excited states of 4’ (Red = 0.001 a.u., Blue = -0.001 a.u.).
The calculated LE signal around 600 nm of the 2a’ spectrum is of the excited states are presented in the supplementary
associated to the S excited state, which is due mainly to information.
HOMO–LUMO transition but also HOMO–LUMO+3 importantly
contributes to that signal. The S excited state for 2b’ and 4’ is
2
2
Conclusions
also dominated by the HOMO–LUMO transition with
important contribution of HOMO–LUMO+2 and HOMO–
LUMO+4, respectively. Compounds 2a’ and 2b’ exhibit a
We have synthesized a series of ferrocene amphiphilic donor-
π-acceptor dyes, including N-heterocyclic cations as potent
acceptor groups. These compounds display a red-shift for both
ICT absorption bands that increase with the π-electron-
accepting ability of the heterocyclic cation. The band gaps of
calculated HE band around 400 nm, associated to a S
7
state
with a dominant contribution HOMO-2–LUMO, whereas for
compound 4’, the HE band has a S
6
state dominated by a
and S electronic
HOMO-2–LUMO transition.
[
2a-b and
cyclic voltammetry and the absorption onset show a similar
tendency. In comparison to 2a-b salts, compounds exhibit
4
4][Br] and [2a-b and 4][BF ] salts determined by
Figure 10 depicts the density difference for S
2
6
excited states of 4’, displaying the electron density reduction
around the metal complex and the increase in the π system of
the ligand, supporting the fulvene contribution in these
compounds. From the theoretical calculations it is possible to
say that the molecules studied in this work behaves as push
pull system where there is a direct charge transfer from the
metal complex to the acceptor group and both signals of the
UV spectrum can be characterized as MLCT bands as
4
both smaller optical and electrochemical band gap. These
results evidence the clear contribution of the quinolinium
acceptor included in
value, as result of the high delocalization in this ferrocenyl D-π-
A dye. Likewise, salts of exhibit a less solvent dependence as
4 for decreasing the HOMO-LUMO gap
4
revealed the UV-vis spectra, which allows a best control on the
inter-molecular arrangement.
2
6b
previously found by Coe et al. where there is a direct charge
From the theoretical calculations, we corroborated that
transfer from the metal complex to the acceptor group. Details
compounds 2a-b and 4 behave as push-pull system, revealing a
8
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4
direct charge transfer from the metal complex to the acceptor implemented in Gaussian 09. 50 states were obtained with
3
4
4
group. In three cases, the LE band is dominated by a HOMO- acetonitrile as implicit solvent modeled with SMD method.
LUMO transition, which allow us to validate the optical band This methodology is like that followed by Buckley et al. to
2
6b
gap determination.
describe Ferrocenyl helquats.
Synthesis of ferrocenyl compounds 2a[Br], 2b[Br] and 4[Br].
E)-1-hexadecyl-2-(ferrocenylvinyl)-pyridin-1-ium bromide,
[Br]: To a round-bottomed flask equipped with a condenser,
-picoline (0.002 mol) and 1-bromohexadecane (0.0024 mol)
Experimental
(
2
a
General
2
All chemicals were purchased as reagent and used without
further purification. The solvents were dried and distilled using
standard procedures. Column chromatography was performed
on 70-230 mesh silica gel. Yields are based on the pure
products isolated. All compounds synthesized were covered of
light and stored under nitrogen to avoid their decomposition.
All compounds were characterized by IR spectra recorded on a
Perkin-Elmer Spectrum 100 FT-IR equipped with ATR accessory
were heated in an oil bath at 110 °C for 4 hours. The resulting
oil was cooled, 10 mL of methanol, 0.1 mL of piperidine and
solution of ferrocenyl carboxaldehyde (0.002 mol) in methanol
were added and the mixture was refluxed under argon for 12
h. The solvent was evaporated under vacuum then, the crude
was purified by chromatography on silica gel using a mixture of
diethyl ether and methanol as eluent, to give a dark solid
1
(0.891 g 75 %). Mp 97 °C. H NMR (300 MHz, CDCl
-
1
3
): δ 9.16 (s,
and all data are expressed in wave numbers (cm ). NMR
spectra were recorded with a Bruker Avance III, at 300 MHz
1
H, Py), 8.44 (s, 2H, Py), 7.88 (d, J = 15Hz, 1H, CH=CH), 7.78 (s,
H, Py), 6.91 (d, 1H, J = 15 Hz, CH=CH), 4.86 (s, 4H, 2 CH, Cp
-N-alkyl), 4.60 (s, 2H, C-H Cp subst), 4.25 (5H, Cp), 1.93
s, 2H, CH -alkyl) 1.24 (s, 26H, CH -alkyl), 0.88 (s, 3H, CH -alkyl).
C NMR (75MHz, CDCl ): 152.2 (Cipso Py), 147.5 (C-H Py), 145.1
1
3 3 3
using CDCl and CD COCD as a solvent. Chemical shifts are in
subst CH
2
ppm (δ) relative to TMS, J values are given in Hz. The following
(
2
2
3
abbreviations are used: s = singlet, d = doublet, t = triplet and 13
3
m = multiplet. MS-EI spectra were obtained with a JEOL
(
JMSAX505-HA using 70 ev as the ionization energy and for MS-
1
C-H Py), 144.1 (CH=CH), 125.2 (C-H Py), 124.1 (C-H Py),
11.8(CH=CH), 79.1 (Cipso Cp), 72.5 (C-H Cp subst), 70.1 (Cipso Cp),
FAB a JEOL JMS-SX102a using nitrobenzyl alcohol and ethylene
+
6
2
2
9.5 (C
9.4 (-CH
849, 1603. MS (FAB ) m/z: 514 [M -Br]; 395 [M
−
H, Cp subst), 69.5 (Cp), 58.7 (-CH
2
-N ), 31.9 (CH
2
-1
-alkyl),
glycol as a matrix. Elemental analyses for carbon, hydrogen
and nitrogen atoms were performed on a Thermo Scientific
elemental analyzer, Flash 2000 using sulfanilamide as
standard.
2
)
13), 22.7 ( CH ), 14.1 (CH
−
2
−
3
). ATR-FTIR
ν
(cm ) 2918,
FeCpBr]; 224
48NFe: calculated 514.3136,
found 514.3135. Elemental analysis (%): calcd for C33 48BrFeN:
C, 66.67; H, 8.14; N, 2.36; found: C, 66.50; H, 7.94; N, 2.41.
E)-1-hexadecyl-4-(ferrocenylvinyl)-pyridin-1-ium bromide,
[Br]: The compound 2b[Br] was obtained by a similar
+
+
+
−
+
14
[C H33]. HR-MS (FAB ) m/z for C33H
H
The UV–VIS spectra were obtained on a CaryWin 100 Fast-
Scan-Varian spectrophotometer, using fresh solutions of the
(
3 3
corresponding ferrocenyl compounds in CHCl and CH CN
2b
spectrophotometric grade. The precise value of the molar
absorption of each ferrocenyl compound was determined from
1
procedure, as a dark solid (0.889 g, 75 %). Mp 93 °C. H NMR
(300 MHz, CDCl ): δ 8.89 (br s, 2H, Py), 7.96 (br s, 2H, Py),
.78(d, 1H, CH=CH, J = 15.3 Hz), 6.72 (d, 1H, CH=CH, J = 15.3),
-
3
-1
3
the corresponding calibration curve. 1. 0 x 10 mol L solution
stock of each ferrocenyl compound was prepared and the
7
+
-4
-4
solutions of lower concentrations (9.0 x 10 to 1.0 x 10 mol L
-
4.67 (br s, 4H, 2H –CH
2
-N , 2H, C-H, Cp subst), 4.58 (br s, 2H, C-H
-alkyl), 1.24 (br
1
Cp subst), 4.22 (br s, 5H, C-H, Cp), 1.97 (br s, CH
2
)
were prepared by accurate dilution.
13
s, 25 H, CH
CDCl ): 153.4 (Cipso Py), 145.1.5 (CH=CH), 143.7.1 (C-H-Py),
22.9 (C-H Py), 118.8(CH=CH), 79.4 (Cipso Cp), 72.3 (C-H, Cp
2 3
-alkyl), 0.88 (br s, 3H, CH ). C NMR (75MHz,
Cyclic voltammetry (CV) experiments were carried out with a
-
standard three-electrode configuration. NBu PF (0.1 mol L )
4 6
1
3
1
in acetonitrile was used as a supporting electrolyte, a carbon
glass disc was used as the working electrode and a platinum
+
subst), 70.1 (Cp), 69.1 (C-H, Cp subst), 69.5 (Cp), 58.7 (
31.9 (CH -alkyl), 29.4 (-CH 13), 22.3 ( CH ), 14.1 (CH
FTIR (cm ) 2949, 2915, 2849, 3095, 1643. MS (FAB ) m/z,
14 [M
m/z for C33
Elemental analysis (%): calcd for C33
N, 2.36; found: C, 66.59; H, 8.03; N, 2.40.
E)-1-hexadecyl-4-(ferrocenylvinyl)-quinolin-1-ium
[Br]: To round-bottomed flask with condenser 4-
methylquinoline (0.0019 mol) and 1-bromohexadecane
0.0022 mol) were heated in an oil bath at 110 °C for 4 h. The
−
CH
2
−N ),
-1
2
2
)
−
2
−
3
+
). ATR-
wire as the auxiliary electrode at a scan rate of 100 mVs with
an CH Instruments Model 660C Potentiostat-Galvanostat. The
-1
ν
+
−
+
+
+
5
Br], 394 [M
48NFe, calculated 514.3136 observed 514.3147.
48BrFeN: C, 66.67; H, 8.14;
16
−FeCp], 289 [M −C H33]. HR-MS (FAB )
half-wave potential of the ferrocenium/ferrocene ion couple
+
Fc /Fc) under these conditions using a AgCl/Ag reference
H
(
H
electrode, was 0.62 V and this system was used as internal
reference. The electrochemical determinations were carried
out without compensation of ohmic drop.
(
bromide,
4
a
Computational Methods
(
4
1
The computational study was performed with the t-HCTHhyb
resulting oil was cooled, 10 mL of methanol, 0.1 mL of
piperidine and a solution of ferrocenyl carboxaldehyde (0.0019
mol) in methanol were added. The mixture was refluxed under
argon for 12 h. The solvent was evaporated under vacuum
then, the crude was purified by chromatography on silica gel
using a mixture of diethyl ether and methanol as eluent, to
functional and the 6-311++G(d) basis set for C, N and H atoms
and Lanl2dz for Fe. The t-HCTH-hyb functional has
demonstrated to properly reproduce excited state properties
4
2
such as absorption and emission energies. Excited states
were calculated within the time dependent DFT formalism as
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1
give a green dark solid (1.045 g, 85 %). Mp 158-160 °C. H NMR NMR (282 MHz, CDCl
3
): -152.1 (B-F). Elemental analysis (%):
(
300 MHz, CDCl
H, J = 6 Hz), 8.26 (d, 1H, quinoline, J
quinoline), 7.86 (m, 2H, CH=CH and C-H quinoline), 7.38 (d, 1H,
3
): δ 10.06 (d, 1H, quinoline, J = 6Hz), 8.54 (d, calcd for C37
4
H50BF FeN: C, 68.22; H, 7.74; N, 2.15; found: C,
1
=
6 Hz), 8.10 (m, 2H, 68.17; H, 7.78; N, 2.20.
+
CH=CH, J = 15 Hz), 5.03 (t, 2H, CH
subst), 4.62 (s, 2H, C-H Cp subst), 4.21 (s, 5H, Cp), 2.0 (br s, 2H, The compounds 6a and 6b were prepared by a modification of
2
−
N ), 4.75 (s, 2H, C-H, Cp Synthesis of ferrocenyl compounds 6a, 6b and 7.
1
3
23
CH
2
-alkyl), 1.18(m, 25H, CH
2
-alkyl), 0.88 (br s, 3H, CH
3
).
C
the procedure reported in the literature.
NMR (75MHz, CDCl
3
): 152.3 (Cipso quinoline), 148.9 (C-H
To a cooled solution of 2-picoline in 5 mL of anhydrous THF
-1
quinoline), 147.3 (CH=CH), 138.1(Cipso quinoline), 134.6 (C-H
quinoline), 128.6 (C-H quinoline), 126.4 (Cipso quinoline), 126.2
under Argon at -70 °C was added n-BuLi (1.2 mL of 2.5 mol L
in THF), and the resulting solution was reacted for 5 minutes at
70 °C, then the temperature left to reach 0 °C and the mixture
was stirred for at °C. solution of ferrocene
(C-H quinoline), 118.1 (C-H quinoline), 116.0 (C-H quinoline),
-
1
6
2
2
14.7 (CH=CH) 80.1 (Cipso Cp), 72.8 (C-H, Cp subst), 70.4 (Cp),
1
h
0
A
+
9.4 (C-H, Cp subst), 57.3 (
6.7, 22.6 ( CH ), 14.3 (CH
848, 1698, MS (FAB ) m/z, 564[M
−
CH
2
−
N ), 31.9, 30.1, 29.6 , 29.3, 29.2,
carboxaldehyde in THF (10 mL) was then added dropwise, and
the mixture was stirred for 3 hours, before quenching with
−
2
−
3
). ATR-FTIR (cm-1) 2954, 2915,
ν
+
+
+
−
Br], 444[M
33]. HR-MS (FAB ) m/z for C37 50NFe, calculated
64.3293 observed 564.3306. Elemental analysis (%): calcd for
−FeCp], 339
water and HCl 10%. The mixture was extracted with CH
2
Cl
2
(3 x
+
+
[M
−
C
16
H
H
1
5 mL), and then washed with brine. The combined organic
5
C
2 4
layer was separated, dried with anhydrous Na SO
and
.
37
H
50BrFeN H
2
O: C, 67.07; H, 7.91; N, 2.11; found: C, 67.36; H,
evaporated to dryness in vacuum. The resulted product was
used without further purification. The alcohol I or II was solved
8.04; N, 2.16.
in anhydrous CH
methanesulfonyl chloride was added dropwise and the
In a round-bottomed flask was dissolved the corresponding mixture was stirred for 1 h at 0 °C. Then, the reaction was
bromide salt in 10 mL of CH CN and then 1.1 equivalents of quenched by adding 30 mL of water. The organic layer was
AgBF were added. The mixture was stirred overnight at room washed with water and then dried over anhydrous Na SO and
2
Cl
2
(10 mL) and cooled at 0 °C,
4 4 4
Synthesis of ferrocenyl compounds 2a[BF ], 2b[BF ] and 4[BF ].
3
4
2
4
temperature, covered of light. The AgBr was separated by evaporated to dryness in vacuo. Purification by flash column
filtration over celite and the crude was purified by flash chromatography using alumina and hexane-CH Cl2 (9:1)
2
chromatography on neutral alumina using a mixture of AcOEt/ afforded 6a as bright red solid in 75 % yield. 6b was also
methanol as eluent, to give a dark solid.
E)-1-hexadecyl-2-(ferrocenylvinyl)-pyridin-1-ium
tetrafluoroborate, 2a[BF ]: NH BF was used instead AgBF
Purple solid, 51 %, Mp 90 °C. H NMR (300 MHz, CD COCD
.89 (d, 2H, J = 6 Hz, Py), 8.51 (m, 2H, Py), 8.00 (d, 1H, J = 15 Py) 7.10 (s, 1H, Py) 6.76 (d, 1H, CH=CH, J = 15 Hz), 4.52 (br s,
obtained as bright red solid in 85% yield.
(
(E)-2-ferrocenylvinylpyridine (6a): Red bright solid (75%). Mp
1
4
4
4
4
.
114-115 °C. H NMR (300 MHz, CDCl ): δ 8.55 (br s, 1H, Py),
3
1
3
3
), δ 7.62 (br s, 1H, Py), 7.40 (d, 1H, CH=CH, J = 15Hz), 7.29 (s, 1H,
8
1
3
Hz, CH=CH), 7.91 (t, 1H,Py), 7.29 (d, 1H, 15 Hz, CH=CH), 4.90 (t, 2H Cp subst), 4.32 (br s, 2H Cp subst), 4.15 (br s, 5H Cp). C NMR
+
2H, C-H, Cpsubst), 4.86 (d, 2H, –CH
2
3 ipso
-N ), 4.64 (t, 2H, C-H, Cpsubst), (75MHz, CDCl ): 156 (C Py), 149.4 (C-H Py), 136.7 (C-H Py),
4
.29 (s, 5H, C-H Cp), 1.45 (m, 5H, CH
2
-alkyl), 1.28 (br s, 22 H, 132.2 (C-H Py), 125.2 (CH=CH), 121.3 (C-H Py) 121.2 (CH=CH),
). F NMR (282 MHz, CD COCD ): - 82.0 (Cipso Cp), 69.6 (CH, Cp subst), 69.4 (Cp), 67.5 (CH, Cp subst
F). Elemental analysis (%): calcd for ATR-FTIR
FeN: C, 65.91; H, 8.04; N, 2.33; found: C, 65.83; H, m/z 290 [M +1]. Elemental analysis (%): calcd for C H FeN: C,
1
9
CH
2
-alkyl), 0.89 (t, 3H, CH
3
3
3
)
-1
+
151.6 ppm (B
−
ν (cm ) 2956, 2927, 2858, 1594, 1547. MS (DART )
+
C
33
H48BF
4
1
7 15
7
.98; N, 2.29.
E)-1-hexadecyl-4-(ferrocenylvinyl)-pyridin-1-ium
tetrafluoroborate, 2b[BF ]: Purple solid, 87 %. Mp 68 °C.
NMR (300 MHz, CD COCD
d, 2H, J = 6.0 Hz Py), 8.03 (d, 1H, CH=CH, J = 15.9 Hz), 7.08 (d, 4.49, (s, 2H, Cp subst), 4.35, (s, 2H, Cp Subst), 4.15 (s, 5H, Cp).
70.61; H, 5.23; N, 4.84; found: C, 70.65; H, 5.26; N, 4.82.
(
(E)-4-ferrocenylvinylpyridine (6b): Red bright solid. 85 %. Mp:
1
1
4
H
142 °C. H NMR (300 MHz, CDCl ) δ 8.52 (br s, 2H, Py), 7.28,
3
H
3
3
): δ 9.01 (d, 2H, J = 6.3 Hz, Py), 8.18 (br s, 2H, Py), 7.12 (d, 1H, J = 15 Hz), 6.60 (d, 1H, J = 15 Hz),
1
3
(
C
1
H, CH=CH, J = 15.9 Hz), 4.80 (br s, 2H, C-H Cp subst), 4.71 (br s, NMR (75MHz, CDCl ). 150.1 (C-H Py), 145.1 (C
ipso
Py), 132.5
3
+
2
2H, –CH -N ), 4.60 (br s, 2H, C-H Cp subst), 4.25 (br s, 5H, C-H, (CH=CH), 123.0 (C-H Py) 120.2 (CH=CH), 81.6 (Cipso Cp), 69.9 (C-
+
Cp), 2.07 (br s, 2H, CH
s, 25 H, CH -alkyl), 0.86 (t, 3H, CH
151.4 (B-F). Elemental analysis (%): calcd for C33
2
-alkyl), 1.41 (br s, 2H, CH
2
-alkyl), 1.29 (br H, Cp
), 69.4 (Cp), 67.5. (CH, Cp
-1
subst
). MS (DART ) m/z 290
subst
1
9
+
2
3
). F NMR (282 MHz, CDCl
FeN: C, Elemental analysis (%): calcd for C H FeN: C, 70.61; H, 5.23;
3
)
[M +1]. ATR-FTIR ν (cm ) 3095, 2956, 2927, 1628, 1594.
-
H48BF
4
1
7 15
6
5.91; H, 8.04; N, 2.33; found: C, 65.76; H, 7.89; N, 2.27.
N, 4.84; found: C, 70.54; H, 5.19; N, 4.80.
(E)-4-ferrocenylvinylquinoline ( ): In a 100 mL two-neck round-
bottom flask fitted equipped with a magnetic stirrer and reflux
): 9.13 (d, 1H, quinoline, J = 9Hz), 8.51 (d, condenser, 4.54 mL (0.0045 mol, potassium t-butoxide (1 mol
(
E)-1-hexadecyl-4-(ferrocenylvinyl)-quinolin-1-ium
tetrafluoroborate, [BF ]: Purple solid, 88 %. Mp 138 °C.
NMR (300 MHz, CDCl
H, quinoline, J = 9Hz), 8.21 (m, 1H, quinoline), 8.08 (br s, 2H,
quinoline), 7.93-8.88(m, 1H, quinoline), 7.92 (d, 1H, J = 15 Hz), After that, 0.0038 mol of 4-methylquinoline was added and
7
1
4
4
H
3
-1
1
L in THF) was placed in 15 mL of anhydrous THF under argon.
+
7.37 (d, 1H, J = 15 Hz), 4.79 (br s, 2H, -CH
2
-N , and 2H, C-H, Cp the mixture was refluxed. Then, a solution of 0.0038 mol of
subst), 4.71(br s, 2H, CH Cp subst), 4.29 (s, 5H, Cp), 2.02 (br s, 2H, ferrocenecarboxaldehyde in 10 mL of anhydrous THF was
1
9
CH
2
-alkyl), 1.25(br s, 25H, CH
2
-alkyl), 0.87 (br s, 3H CH
3
).
F
slowly added. The reaction mixture was refluxed overnight.
1
0 | J. Name., 2012, 00, 1-3
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Finally, 15 mL of brine was added, and the product was
extracted with CH Cl (2 x 30 mL). The combined organic layers
were dried with anhydrous Na SO , and the solvent was
removed in vacuo. Purification by column chromatography on
silica-gel with hexane-ethyl acetate (70:30, v/v) afforded as
bright red solid, 86 % yield. Mp 108-109 °C, H NMR (300 MHz,
CDCl ): δ 8.85 (br s, 1H,), 8.16 (d, 1H J = 9 Hz), 8.10 (d, 1H, J = 9
5
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2
4
6
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8
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3
Hz), 7.70 (t, 1H, J = 6 Hz), 7.60-7.54 (m, 2H), 7.39 (d, 1H, J = 15
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1
3
H). C NMR (75MHz, CDCl
5
3
): 150.2 (Cipso quinoline), 148.8
(
C
ipso quinoline), 143.3 (C-H quinoline), 134.4 (C-H quinoline),
30.0 (CH=CH), 129.3 (C-H quinoline), 126.3 (C-H quinoline),
26.1 (C-H quinoline), 123.4 (C-H quinoline), 119.5 (CH=CH),
16.2 (C-H quinoline), 82.1 (Cipso Cp), 70.0 (C-H, Cp subst), 69.4
9
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(
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D. Roberto, F. Tessore and R. Ugo, Top. Organomet. Chem.,
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(Cp), 67.6 (C-H, Cp subst). ATR-FTIR ν (cm ) 2925, 2237,1575,
+
+
1
559, 1504. DART-MS m/z, 339 [M+]. HR-MS [M +1] m/z for
18FeN, calculated 340.0788 observed 340.0789. Elemental
analysis (%): calcd for C21 17FeN: C, 74.36; H, 5.05; N, 4.13;
21
C H
H
1
found: C, 74.13; H, 5.00; N, 4.07.
2
010, 28, 1–55; (b) O. Maury and H. Le Bozec, in Molecular
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Conflicts of interest
Chichester, 2010, pp. 1–59; (c) B. J. Coe, Coord. Chem. Rev.,
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013, 257, 1438–1458; (d) P. G. Lacroix, I. Malfant, J.-A. Real
There are no conflicts to declare.
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G. Humphrey, T. Schwich, P. J. West, M. P. Cifuentes and M.
Samoc, in Comprehensive Inorganic Chemistry II: From
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Acknowledgements
The authors would like to acknowledge the technical
assistance provided by Martin Cruz Villafañe, Luis Velasco,
Javier Pérez, Héctor Rios and Ma. Paz Orta Pérez. We thank to
DGAPA IN208117 and PAIP 5000-9053 projects and CONACYT
for the Ph.D. grant extended to B. J. López-Mayorga. C. I.
Sandoval-Chavez also thanks to DGAPA-ICN for the granted
post-doctoral fellowship. The authors acknowledge to DGTIC-
UNAM (SC17-1-IG-55-MZ0) for supercomputer time.
3
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DOI: 10.1039/C8NJ00787J
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ARTICLE
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4 M. Cossi, N. Rega, G. Scalmani, and V. Barone, J. Comp.
Chem., 2003, 24, 669-681.
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DOI: 10.1039/C8NJ00787J
Ferrocene amphiphilic D-π-A dyes: Synthesis, redox behavior and
determination of Band Gaps.
1
1
1
Byron López-Mayorga, César I. Sandoval-Chávez , Pilar Carreón- Castro , Víctor M.
2
3
3
,1
Ugalde-Saldívar , F. Cortés-Guzman, José G. López-Cortes , M. Carmen Ortega-Alfaro*
1
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Circuito
Exterior, Ciudad Universitaria, Coyoacán, Cd. de México, 04510, México.
2
Facultad de Química, Universidad Nacional Autónoma de México, Edificio B. Av.
Universidad 3000, Coyoacán, Cd. de México, 04510, México.
3
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Cd.
Universitaria, Coyoacán, Cd. de México, 04510, México
“
For table of contents”
The synthesis of six ferrocenyl π-conjugated pyridinium salts, incorporating two different
heterocycles (pyridine and quinoline) in their cationic and neutral version is informed. The
properties of these dyes were investigated by absorption spectroscopy and electrochemical
analysis. We have also determined the optically and electrochemically band gaps of these dyes.
Time-dependent DFT calculations indicate that the lowest energy absorption band displayed for
these compounds has mainly metal-to-ligand charge character, being the HOMO-LUMO electronic
transition the main contribution.