Linear free-energy correlations for the vinylheptafulvene ring closure: A probe for Hammett σ values

Article abstract of DOI:10.1002/chem.201300167

Linear free-energy relationships, like Hammett correlations, are fundamental in physical organic chemistry for the elucidation of reaction mechanisms. In this work, we show that Hammett correlations exist for the ring closure of six different model systems of vinylheptafulvenes (VHFs) to their corresponding dihydroazulenes (DHAs). These first-order reactions were easily followed by UV/Vis absorption spectroscopy on account of the significantly different absorption characteristics between VHFs and DHAs. Opposing effects displayed by substituent groups at two different positions are conveniently accounted for by simply subtracting the two Hammett σ values of each group. The linear correlations readily allow us to obtain unknown and approximate Hammett σ values for previously uninvestigated substituents. We also show that they can provide alternative values to the standard ones. We present values for a variety of substituent groups ranging from alkynes, sulfones, sulfoxides, and different heteroaromatics. The electronic effects exerted by substituent groups on VHFs are also reflected in their absorption maxima. Thus, we have established an empirical relationship between the absorption maximum of the VHF and the Hammett σ values of its substituents. This fine-tuning of electronic properties is particularly important for the ongoing efforts of using the DHA/VHF molecular switch in molecular electronics devices. The value of Hammett: The rate of vinylheptafulvene (VHF) ring closure to a dihydroazulene (DHA) obeys the Hammett correlation: electron-donating groups in the seven-membered ring enhance the ring closure, whereas such groups at the dicyanoethylene unit retard it (see figure). When substituents are present at both these positions, a Hammett correlation is obtained based on the difference in Hammett substituent values. Copyright

Full text of DOI:10.1002/chem.201300167

DOI: 10.1002/chem.201300167
Linear Free-Energy Correlations for the Vinylheptafulvene Ring Closure:
A Probe for Hammett s Values
Søren Lindbæk Broman, Martyn Jevric, and Mogens Brøndsted Nielsen*[a]
Abstract: Linear free-energy relation-
ships, like Hammett correlations, are
fundamental in physical organic chem-
istry for the elucidation of reaction
mechanisms. In this work, we show
that Hammett correlations exist for the
ring closure of six different model sys-
tems of vinylheptafulvenes (VHFs) to
their corresponding dihydroazulenes
(DHAs). These first-order reactions
were easily followed by UV/Vis ab-
sorption spectroscopy on account of
the significantly different absorption
characteristics between VHFs and
DHAs. Opposing effects displayed by
substituent groups at two different po-
sitions are conveniently accounted for
by simply subtracting the two Hammett
s values of each group. The linear cor-
relations readily allow us to obtain un-
known and approximate Hammett s
values for previously uninvestigated
substituents. We also show that they
can provide alternative values to the
standard ones. We present values for a
variety of substituent groups ranging
from alkynes, sulfones, sulfoxides, and
different heteroaromatics. The elec-
tronic effects exerted by substituent
groups on VHFs are also reflected in
their absorption maxima. Thus, we
have established an empirical relation-
ship between the absorption maximum
of the VHF and the Hammett s values
of its substituents. This fine-tuning of
electronic properties is particularly im-
portant for the ongoing efforts of using
the DHA/VHF molecular switch in
molecular electronics devices.
Keywords: heterocycles · kinetics ·
molecular electronics
· reaction
mechanisms · ring closure · UV/Vis
spectroscopy
Introduction
be known. To determine the pKa of a substituted benzoic
acid it must be soluble in water. Moreover, the use of the
pKa value in calculating the s value requires that the sub-
stituent is not more acidic than the carboxylic acid. Other
ways to estimate the s value of a given substituent are, how-
ever, known. Determination of the s values from reactivity
data or physical measurements (such as NMR spectroscopy
or intramolecular hydrogen-bonding to an imine) on the
molecule carrying the substituent presents alternative proce-
dures, and the use of redefined s values that best fit the
entire body of experimental data has also been put for-
ward.[5]
The thermally induced conversion of substituted vinylhep-
tafulvenes (VHF) to corresponding dihydroazulenes (DHA)
has recently been shown to follow a Hammett correlation,
but the data were, however, rather limited to allow for relia-
ble prediction of unknown s values.[6] DHA is photochro-
mic[7] and acts as a stable precursor for the VHF. Thus, upon
irradiation with light, it undergoes a ring-opening reaction
to form a VHF (Scheme 1). The reversible change is accom-
panied by significant changes in physical properties, such as
The Hammett equation was introduced by L. P. Hammett in
1930s,[1] and despite its empirical foundation it has been
widely used to study and interpret mechanisms of organic
reactions for decades. Hammett substituent constants, so-
called s values, were introduced as measures of the elec-
tron-withdrawing/donating character of substituents on a
benzene ring. These are defined as the difference in pKa of
a given substituted benzoic acid and benzoic acid itself.
Classically the Hammett correlation is the empirical rela-
tionship between the pKa of benzoic acids and the rate of a
chemical reaction (for example the rate at which ethyl ben-
zoates undergo hydrolysis). Since then, the principle has
been expanded and reasonable correlations have been
found with electrophilic, nucleophilic, and free-radical re-
agents,[2] as well as in pericyclic reactions and photochemical
E/Z isomerizations.[3] Hammett s values have also been
found to correlate with association constants of some supra-
molecular systems.[4]
Essential to the study of the mechanism of a chemical re-
action, the s values for the substituents in question have to
[a] S. L. Broman, Dr. M. Jevric, Prof. Dr. M. B. Nielsen
Department of Chemistry, University of Copenhagen
Universitetsparken 5, 2100 Copenhagen Ø (Denmark)
Fax : (+45)3532-0212
E-mail: mbn@kiku.dk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300167.
Scheme 1. Light-induced ring-opening reaction of DHA to VHF and its
thermally induced back-reaction to DHA.
9542
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 9542 – 9548

FULL PAPER
dipole moment,[8] absorption maxima[9] and single molecule
resistivity,[10] and the interconversion is strongly dependent
on solvent polarity[9,11] and the substitution pattern.[6,7,9,12]
The two latter observations suggest that the ring closure
cannot proceed through a concerted electrocyclization, but
must rather go via a zwitterionic transition state (TS), in-
volving a positively charged tropylium-like seven-membered
ring and a negatively charged malononitrile moiety.
In this paper we show that the rate of the ring-closing re-
action of substituted VHFs to DHAs is a particularly con-
venient probe for easy determination of unknown s values,
simply determined by UV/Vis spectroscopy. Thus, the
system is attractive for several reasons. From the huge com-
mercial availability of acetophenones and substituted aryl
boronic acids a large number of DHA/VHF systems can be
prepared. In this paper we present the study of more than
70 compounds.[13] The compounds were categorized in six
model systems, A–F (Figure 1), which have one functional
two s values and plotting ln(k) against this value, a straight
line was obtained. Moreover, an empirical relationship be-
tween the absorption maximum of substituted VHFs and
the difference in s values was established.
Not only does this study complement the existing methods
for determination of electronic properties of functional
groups but also uncover the ways to fine-tune the system so
that this DHA/VHF switch can be further exploited as
switches in advanced devices.
Results and Discussion
The studied DHAs had a characteristic absorption maxi-
mum in the region 353–389 nm and almost all showed a
clean light-induced conversion to VHF.[14] The VHFs exhib-
ited a characteristic absorption maximum in the region 455–
562 nm and all underwent a thermally induced back-reaction
to DHA with isosbestic points in the absorption spectra.
The back-reaction (VHF!DHA) was monitored by using
UV/Vis spectroscopy (at 258C the half-lives ranged from
21–1545 min) at a minimum of four different temperatures
by plotting the decay of VHF absorption against time pro-
viding the rate constant at each temperature (first-order ki-
netics). From Arrhenius plots, the activation energies and
pre-exponential factors were calculated. An example of a ki-
netics measurement is shown in Figure 2 and some data be-
longing to model system D are listed in Table 1, whereas all
kinetics data are listed in the Supporting Information.
Table 1. Absorption and kinetics data (258C) for VHFs belonging to
model system D.
[a]
X
sX
k25
Half-life
[min]
lmax
[ꢁ10-5 sꢀ1
]
N
[nm]
Figure 1. Model systems shown as DHA compounds (the isomeric forms
in which the compounds were prepared and isolated). In each of the
model systems A, B, D, E, and F, the substitution pattern in the seven-
membered ring is kept constant, whereas it is varied in the five-mem-
bered ring (substituent X). However, for model system C, the variation is
in the seven-membered ring (substituent Y).
Me
H[b]
Br
CN
NO2
ꢀ0.17
0
0.23
0.66
0.78
3.68
5.6
6.38
10.4
14.9
314
206
181
111
77
488
490
494
502
506
[a] Ref. [5a]. [b] Ref. [6b].
group varied, and one model system, G (see below), which
has two functional groups varied. These series include func-
tional groups in the para and meta position, both mesomeri-
cally electron-withdrawing and -donating groups (NO2, CN,
CO2Me, CHO, OMe, NH2), inductively donating groups
(Me), together with the both mesomerically donating and
inductively withdrawing halogens (I, Br, and F). The series
also include furans and thiophenes, alkynes, butadiynes, and
thioethers. The possibility to functionalize the system in
Arrhenius activation energies varied between 89 and
98 kJmolꢀ1 and the pre-exponential factor between 5.1ꢁ1011
and 3.4ꢁ1012 sꢀ1 in MeCN. Whereas changes in these indi-
vidual values with substituent character were less systematic,
changes in the overall rate constant showed such behavior
as described below. Although also determined at 258C, we
used the interpolated rate constants from the Arrhenius
plots at this temperature (except for one compound) for the
comparisons described below (but the single-point and inter-
polated values only differed slightly).
Like with the previously studied DHA/VHFs substituted
in the seven-membered ring,[6] a mixture of the 6- and 7-
DHAs was obtained after one cycle of light and heat. The
ratios of isomers were not determined but the presence of
the 6-isomer is clearly visible by UV/Vis spectroscopy, since
+
many ways allows the probing of the sm, sp, and sp (includ-
ing through-conjugation) values. This revealed that not only
do Hammett correlations exist when changing the functional
group at each end of the molecule, but disubstituted VHFs
can be described by two contributing s values and the rate
of ring closure (VHF!DHA) reflects the net difference be-
tween the two substituents. Thus, by simply subtracting the
Chem. Eur. J. 2013, 19, 9542 – 9548
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9543

M. B. Nielsen et al.
Figure 2. A) UV/Vis absorption spectra of pure DHA and VHF, together
with the mixture of DHAs after one light–heat cycle; model system D,
X=p-Br, in MeCN at 258C. B) UV/Vis absorption spectra showing the
gradual thermally induced conversion of VHF to DHA; model system D,
X=p-Br, in MeCN at 458C. Inset: decay of VHF absorption (lmax
=
494 nm) against time. C) Arrhenius plot (ln(k) vs. Tꢀ1). [k]=sꢀ1
.
the absorption maximum of the 6-DHA is redshifted and
more intense and the longest-wavelength absorption maxi-
mum of the resulting mixture is hence redshifted.
When placed at the dicyanoethylene unit, it is clear that
electron-withdrawing groups tend to speed up the thermal
back-reaction, whereas electron-donating groups slow it
down (Figure 3A, B, and D–F). Instead, if placed in the
seven-membered ring, electron-donating groups tend to
speed the back-reaction up, and electron-withdrawing
groups slow it down (Figure 3C). In all model systems a
linear correlation between ln
value was observed (Figure 3), but with some deviations in
series A (see below). Thus, when plotting ln(k258C) against
ACHTUNGNERTN(UNG k258C) and the appropriate s
AHCTUNGTRENNUNG
+
sm/p for model systems A, B, D, E, and F and against sp for
system C[15] the data can be described with straight lines.[16]
The slope of the Hammett plot is negative for substituent
variation in the seven-membered ring (Y), and positive for
Figure 3. Hammett correlations for VHF to DHA ring closure. The corre-
sponding model DHA is shown for each series. The point corresponding
to p-NH2 in model system A was not included in the linear regression.
[k]=sꢀ1
.
9544
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 9542 – 9548

A Probe for Hammett s Values
FULL PAPER
substituent variation at the dicyanoethylene unit (X). It is
noteworthy, that the slope is close to unity (negative or posi-
tive) for all six series as it is for the standard reaction, the
aqueous ionization of substituted benzoic acids.
We note that in the most comprehensively studied model
system A, the relatively high rate constant obtained for the
very electron-donating NH2 group deviates from that pre-
dicted from the linear correlation, and the corresponding
data point shown in Figure 3A was not included in the
linear regression. The electron-donating p-OMe substituent
also gave a faster rate of ring closure than predicted from
the straight line, but this data point was still, however, in-
cluded when fitting the data with the best line. A deviation
for strongly electron-donating substituents on the dicyano-
ethylene end of the molecule can be due to a change in
mechanism of ring closure and deserves further investiga-
tion. It seems that the correlation for model system A
should not be used for rate constants with ln(k)<ꢀ10.
The observations for all six model systems (except for
model system A with strongly electron-donating groups) can
be explained by the structure of the transition state, in
which there is a direct conjugation from Y to the positively
Figure 5. Hammett correlations for VHF to DHA ring closure. Model
systems; E (Y=OMe, top), D (Y=H, middle), and F (Y=CN, bottom).
[k]=sꢀ1
.
From the Hammett correlations of model system A and
C, new Hammett s values can be derived by interpolation;
that is, for VHFs with substituent groups as listed in Table 2,
rate constants at 258C were determined and from the Ham-
mett correlations, the corresponding s values were deter-
mined. First, we have used the plots to quantify the elec-
tron-withdrawing character of alkynes (Table 2, entries 1–4).
For example, a triisopropylsilylacetylene (sp =0.20, sm =
0.06, sp+ =0.25, entry 1) is less electron-withdrawing than a
terminal acetylene (sp =0.23, sm =0.21, sp+ =0.18[5a]) when
placed in meta or para positions. However, when taking
standard deviations into account, the values are quite simi-
lar. When in conjugation with the reaction center it was
found to be slightly more electron-withdrawing. A terminal
butadiyne (sp =0.49) or trimethylsilyl-protected butadiyne
(Table 2, entries 2 and 3) were found to be more electron-
withdrawing than a terminal alkyne. These observations are
generally in accordance to the known character of al-
kynes.[17] The electron-withdrawing character of a bromo-
+
charged reaction center (thus the use of sp and the nega-
tive slope), but not a direct conjugation from X to the nega-
tively charged reaction center (thus the use of sm/p and the
positive slope). The structure of the proposed TS is shown
in Figure 4.[6b] These correlations give the opportunity to as-
certain both meta-, para-, and para+ s values for substitu-
ents with unknown s values (Table 1 and the Supporting In-
formation).
AHCTUNGTREGeNNNU thynyl (Table 2, entry 4) was found to be in-between that
of the ethynyl and 1,3-butadiyn-1-yl groups.
As expected, the StBu substituent had an insignificant in-
+
fluence when placed in the para position (sp =0.06, sp
=
0.04, Table 2, entry 5). Interestingly, the sign changed to neg-
ative when placed in the meta position (sm =ꢀ0.22), which
is opposite to that reported previously for an SMe substitu-
Figure 4. Proposed transition state of the ring closure of VHF to DHA.
The rate of ring closure (VHF!DHA) is very
Table 2. New estimated s values by interpolation from Hammett plots of model
systems A and C.
systematically influenced by the substituents at
both the dicyanoethylene unit and the seven-mem-
bered ring. In Figure 5, the Hammett-plots of
model system D, E, and F are shown, in which X is
varied, whereas Y remains fixed as either H, CN, or
OMe (for definition of X and Y, see Figure 4).
Clearly, the cyano group systematically slows down
the rate of ring closure (by parallel-displacing the
line down by 1.02 in natural logarithmic units when
proceeding from Y=H to Y=CN), whereas a me-
thoxy group speeds up the ring closure (by parallel-
displacing the plot up by 0.28 in natural logarithmic
units when proceeding from Y=H to Y=OMe)
but retaining its slope.
AHCTUNGTRENNUNG
[a]
[a]
+[b]
Entry
X or Y
sm
sp
sp
ꢁ ꢀ
1
2
3
4
5
6
7
8
9
C C Si
ꢁ ꢀ ꢁ ꢀ
G
0.06 (ꢂ0.04)
–
–
0.20 (ꢂ0.05)
0.49 (ꢂ0.07)
0.49 (ꢂ0.08)
0.36 (ꢂ0.07)
0.06 (ꢂ0.05)
0.55 (ꢂ0.08)
0.22 (ꢂ0.06)
0.19 (ꢂ0.06)
0.10 (ꢂ0.05)
0.19 (ꢂ0.06)
0.09 (ꢂ0.05)
0.25 (ꢂ0.11)
–
ꢁ ꢀ ꢁ ꢀ
C C C C SiMe3
–
–
ꢁ
C C-Br
–
StBu
ꢀ0.22 (ꢂ0.06)
0.25 (ꢂ0.06)
0.05 (ꢂ0.04)
ꢀ0.14 (ꢂ0.06)
ꢀ0.21 (ꢂ0.06)
ꢀ0.11 (ꢂ0.05)
ꢀ0.13 (ꢂ0.05)
0.04 (ꢂ0.09)
SO2tBu
SOtBu
2-thienyl
3-thienyl
2-furyl
3-furyl
0.73 (ꢂ0.16)
0.40 (ꢂ0.13)
–
–
–
–
10
11
[a] Derived from model system A. For an explanation of the s values shown in italics,
see the discussion in the main text. [b] Derived from model system C.
Chem. Eur. J. 2013, 19, 9542 – 9548
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9545

M. B. Nielsen et al.
ent (sm =0.14).[5a] Our value was obtained from a rate con-
stant of 4.31ꢁ10ꢀ5 sꢀ1 (ln(k)=ꢀ10.05), that is, at the as-
sumed borderline of the validity of the correlation, and the
numerical value of sm should thus be taken with some care
(such borderline values are italicized in Table 2), whereas
the sign still seems reasonable. Moreover, the corresponding
SO2tBu and SOtBu groups (Table 2, entries 6 and 7) are
found to have smaller s values than those determined previ-
ously for the corresponding methyl sulfone and methyl sulf-
mett correlations at the same time, since it is put together
by model system C and model system D. The contribution
from model system C is opposite to that of model system D.
If a VHF contains a rate-enhancing group and a rate-retard-
ing group, the rate of ring closure is then influenced by both
groups and a straight line is achieved when simply plotting
ln(k) against the difference in s values for the two substitu-
ents (Figure 6).
For the VHF with the most rate-enhancing groups (X=
CN, sp =0.66, Y=NMe2, sp+ =ꢀ1.70), the difference in s
values is 2.36. Thus, the dimethylamino group is rate-en-
hancing due to its strong electron-donation into the seven-
membered ring, whereas the cyano group is rate-enhancing
on the account of its electron-withdrawing effect upon the
dicyanoethylene unit making the ring closure (for this
VHF), the fastest recorded with a half-life of only 21 min.
At the other end of the scale, for the VHF with the slowest
ring closure (X=Me, sp =ꢀ0.17, Y=CN, sp+ =0.66, differ-
ence=ꢀ0.83), the reaction is retarded by both electron-
withdrawal from the seven-membered ring and donation
into the dicyanomethylene unit, raising the half-life to
668 min. In comparison, the half-life of the “parent” com-
pound (Y, X=H) is 206 min. Thus, it is possible to tune and
easily predict the half-life of functionalized VHFs and as
shown in Figure 7, in the range of 21 to 668 min the half-life
can be fine-tuned to have any rate of back-reaction.[22] The
ability to fine-tune the switching events is particularly rele-
vant for further incorporation of the DHA/VHF system into
advanced molecular devices. Indeed we have recently shown
the potential of the system for conductance switching in a
single molecular junction.[10]
ACHTUNGTRENNUNG
11), such as furans and thiophenes, can be determined in
this manner. The s values of the meta- and para-positioned
2- and 3-furyl and thienyl substituents[19] were determined
from suitable DHA/VHF derivatives, and in contrast to pre-
viously reported values,[5a,20] the study showed that furans
and thiophenes do exert notable electronic effects (Table 2,
entries 8–11). All four heteroaromatics were found to be
weakly electron-withdrawing when placed in the para posi-
tion, but weakly electron-donating when placed in the meta
position. Yet, again here some care should be taken in
regard to the numerical values as the rate constants deter-
mined for the heteroaromatics in the meta position were at
the borderline around ln(k)=ꢀ10. The electron-withdraw-
ing effect of para-positioned furan and thiophene might at
first seem surprising; yet, their dipole moments of 0.71 and
0.52 D, respectively,[21] are both towards the heteroatoms.
This allows for a favorable delocalization of the negative
charge building up on the dicyanoethylene unit thus stabiliz-
ing the TS (Figure 4). This overall polarization of the het-
erocycles thus seems important for the ring closure of VHF,
and for related reactions, the s values presented in Table 2
may be good alternatives to the standard ones. In light of
the relatively large standard deviation relative to the size of
the s values of some of the thienyl and furyl substituents,
caution must be shown when interpreting these values, al-
though the sign of the values does not change within the un-
certainty range. Finally, we note that the data set for model
system A is significantly larger than that for model system
C, and the estimated sp and sm values are for that reason of
Figure 7. Top: Half-lives for the ring closure (VHF!DHA) for model
systems A and G (data include model systems C, D, E, and F). Bottom:
Absorption maxima of VHFs belonging to model system G (selected
data).
+
better quality than the estimated sp values.
If we define a new model system G, including two func-
tional groups (one in each end, Figure 6), it obeys two Ham-
The DHA absorption maxima is only weakly affected by
the presence of a substituent in either positions 2 or 7,
whereas the VHF absorption is weakly affected by the pres-
ence of a substituent in the five-membered ring, but strongly
affected by a substituent in the seven-membered ring
(Figure 7). For model system A, electron-withdrawal causes
a redshift and donation a blueshift in the VHF absorption.
For model compound C, electron-withdrawal causes a blue-
shift and donation a redshift in the VHF absorption. This
systematic behavior of the VHF absorption enables the pre-
diction of the longest-wavelength absorption maxima of all
VHFs for model compounds D to G, by adding up the con-
Figure 6. Merged Hammett correlation for VHF to DHA ring closure for
compounds belonging to model system G. [k]=sꢀ1
.
9546
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 9542 – 9548

A Probe for Hammett s Values
FULL PAPER
tribution from the substituents at the dicyanoethylene unit
and the seven-membered ring. For example a nitro group,
placed at the dicyanoethylene unit (model system A), causes
a redshift in VHF absorption of 14 nm, and a methoxy
group in the seven-membered ring (model system C), causes
a redshift of 15 nm, in comparison to each of the two parent
compounds. When incorporating both substituents, one at
each end of the VHF as shown in Figure 8 (model system G,
substituents for which standard values are not available. Al-
though we observe good correlations with the standard
Hammett s values for a wide variety of substituents, some
deviations are also observed. Thus, when it comes to the
ring closure of VHF to DHA, we find that 2- and 3-furyl
and thienyl substituents do exert electronic effects, which
would not have been predicted from the standard s values
of previous studies. We present alternative s values for
these substituents, which may be of use for related reactions
for which the standard values do not seem to describe the
electronic effects correctly. Interestingly, the VHF system
can have substituents in each end of the molecule, and a
new Hammett correlation is obtained by simply subtracting
the two s values. An empirical correlation with the VHF ab-
sorption maximum was also established. The huge commer-
cial availability of the starting materials makes the VHF-to-
DHA conversion a useful alternative to the classic determi-
nation of Hammett s values and for probing electronic
properties of substituents. A study seeking to quantify dif-
ferences in cross- and linear conjugation pathways for p-
electron delocalization by using the DHA/VHF switching
event is currently underway. In addition, this systematic be-
havior of the DHA/VHF system allows for the fine-tuning
of switching events, which is particularly relevant in the
design of molecular switches for molecular electronic devi-
ces.
Figure 8. Structure of VHF, redshifted by both a nitro group at the di-
ACHTUNGTRENNUNGcyanoethylene unit and a methoxy group in the seven-membered ring.
X=NO2, Y=CN), the longest-wavelength absorption maxi-
mum is conveniently calculated as; lmax =lparent +lredshiftpos7
+
lredshiftpos2 =490+15+14=519 nm, which exactly corresponds
to the measured value for this compound.
As shown in Figure 9, the model system G covers the
range 472–519 nm (if not including X=CN, Y=NMe2,
lmax =562 nm). This systematic behavior enables prediction
and fine-tuning of the absorption of the VHF. In fact, the
absorption maximum of the VHF does also give a straight
line when plotting lmax against the difference in s values.[23]
Experimental Section
General: All spectroscopic measurements (including photolysis) were
performed in a 1 cm path length cuvette. UV/Vis absorption spectra were
obtained by scanning the wavelength from 800 to 200 nm. The photo-
switching experiments were performed by using a 150 W xenon arc lamp
equipped with a monochromator; the DHA absorption maximum (lowest
energy absorption) for each individual species was chosen as the wave-
length of irradiation (line width ꢂ2.5 nm). The thermal back-reaction
was performed by heating the sample (cuvette) by a Peltier unit in the
UV/Vis spectrophotometer. The temperature of the solvent in the cu-
vette was measured to be accurate within ꢂ0.28C at 258C. All com-
pounds were studied by the same equipment, although the Peltier unit
was changed during the study. All compounds were prepared according
to the referenced procedures (see main text) and satisfactory elemental
analysis was obtained of all new compounds with only few exceptions.
All measurements were performed in HPLC-grade solvents. The water
content of the solvents was not determined.
Figure 9. Empirical relationship between lmax of substituted VHFs
+
(model system G) and the difference (spXꢀsp Y) (black) and (spXꢀspY
)
Example procedure: A sample of a pure photochromic DHA was dis-
solved in a given solvent. This stock solution was kept in the dark at all
times. A sample of the stock solution was diluted further until the ab-
sorption of the DHA was well within the instrument limits (concentration
between 1–3ꢁ10ꢀ5 m). Irradiation of the solution at the absorption maxi-
mum of the DHA generated the corresponding VHF. An absorption
spectrum was measured of both the DHA and the corresponding VHF,
and the absorption spectra were baseline corrected. The molar absorp-
tion coefficient was determined from the known concentration. The solu-
tion was then heated at a given temperature and an absorption spectrum
was measured at a fixed interval of time until at least six half-lives had
passed. After this light–heat cycle, the absorption spectrum of the result-
ing DHA was compared with that of the pure DHA. In the case of a 7-
substituted DHA, the UV/Vis spectra were often different (more intense
and redshifted), because of partly isomerization to the 6-substituted
DHAs. The absorption maximum (lmax) for both the DHA- and VHF-
(red).
The use of spY gives a significantly poorer fit (R2 =0.91)
compared with that obtained by using the through-conjuga-
tion values (R2 =0.97).
Conclusion
We have established Hammett correlations for several dif-
ferent VHF/DHA model systems, which have allowed us to
obtain approximate Hammett s values (m, p, and p+) of
Chem. Eur. J. 2013, 19, 9542 – 9548
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9547

M. B. Nielsen et al.
species was determined and at their respective lmax the increase and
decay were plotted against time. The two graphs were compared to make
sure that the rate of consumption of VHF corresponds to the rate of for-
mation of DHA. The exponential decay of the VHF absorption was sub-
jected to curve fitting, from which the rate constant k was obtained. The
rate of reaction was also verified to follow first-order kinetics by plotting
ln(A) against time, and a straight line is observed, if the absorption of
the DHA species does not absorb at the absorption maximum of the
VHF species. In an Arrhenius-plot, ln(k) is plotting against Tꢀ1 and from
the slope and the intersection of the straight line, the activation energy
and pre-experimental factor were calculated. All measured rate constants
are collected in the tables provided in the Supporting Information. Only
rate constants with less than 0.1% uncertainty were included in the Ar-
rhenius plots.
[9] a) H. Gçrner, C. Fischer, S. Gierisch, J. Daub, J. Phys. Chem. 1993,
97, 4110–4117; b) H. Gçrner, C. Fischer, J. Daub, J. Photochem.
Photobiol. A 1995, 85, 217–224.
[10] a) S. Lara-Avila, A. V. Danilov, S. E. Kubatkin, S. L. Broman, C. R.
Parker, M. B. Nielsen, J. Phys. Chem. C. 2011, 115, 18372–18377;
b) S. L. Broman, S. Lara-Avila, C. L. Thisted, A. D. Bond, A. V.
Danilov, S. E. Kubatkin, M. B. Nielsen, Adv. Funct. Mater. 2012, 22,
4249–4258.
[11] S. L. Broman, S. L. Brand, C. R. Parker, M. ꢃ. Petersen, C. G. Tort-
zen, A. Kadziola, K. Kilsꢄ, M. B. Nielsen, Arkivoc 2011, IX, 51–67.
[12] a) J. Daub, C. Fischer, S. Gierisch, J. Set, Mol. Cryst. Liq. Cryst. Sci.
Technol. Sect. A 1992, 217, 177–185; b) S. Gierisch, W. Bauer, T.
Burgemeister, J. Daub, Chem. Ber. 1989, 122, 2341–2349; c) H.
Spreitzer, J. Daub, Liebigs Ann. 1995, 1637–1641; d) M. Santella, V.
Mazzanti, M. Jevric, C. R. Parker, S. L. Broman, A. D. Bond, A.
Kadziola, M. B. Nielsen, J. Org. Chem. 2012, 77, 8922–8932.
[13] From substituted acetophenones, malononitrile, and tropylium tetra-
fluoroborate, the simple DHAs (model system A) were prepared in
3–8 steps. By using a regioselective bromination–elimination proce-
dure, the 7-bromo DHAs (model system B) were prepared and by a
subsequent Suzuki coupling the disubstituted DHAs (model systems
C, D, E, F, and G) were prepared. In total 76 compounds with
Acknowledgements
We acknowledge financial support from The Lundbeck Foundation and
The Danish Council for Independent Research j Natural Sciences (No.
10–082088). Mr. A. Vlasceanu is acknowledged for providing one of the
ꢁ ꢀ
ꢁ
ꢁ ꢀ ꢁ
groups such as CN, NO2, CO2Me, CHO, C CH, C C C CH, I, Br,
SMe, StBu, H, OMe, NHAc, and NH2 were obtained. The synthesis
of new compounds will be published elsewhere. References for
known compounds: [6], [7], [10b], [11], and [12].
compounds (model system C, Y=C C SiACTHNURTGNENG(U iPr)3).
[14] For systems including nitro, furyl, or thienyl group(s), a degradation
of <5% was observed. Protonation of amino groups was performed
by the addition of trifluoroacetic acid (TFA).
[15] Rate constants for model system C, X=NMe2, OMe, H, and NO2
were taken from Ref. [6b].
[1] a) L. P. Hammett, H. L. Pfluger, J. Am. Chem. Soc. 1933, 55, 4079–
4089; b) L. P. Hammett, Chem. Rev. 1935, 17, 125–136; c) L. P.
Hammett, J. Am. Chem. Soc. 1937, 59, 96–103.
[2] S. S. Kim, Pure Appl. Chem. 1995, 67, 791–795.
[3] a) A. Viola, R. J. Proverb, B. L. Yates, L. Larrahondo, J. Am. Chem.
Soc. 1973, 95, 3609–3613; b) T. Cordes, T. Schadendorf, B. Prie-
wisch, K. Rꢂck-Braun, W. Zinth, J. Phys. Chem. A 2008, 112, 581–
588.
[4] P. R. Ashton, M. C. T. Fyfe, S. K. Hickingbottom, J. F. Stoddart,
A. J. P. White, D. J. Williams, J. Chem. Soc. Perkin Trans. 2 1998,
2117–2128.
[5] a) C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165–195;
b) P. J. Bray, R. G. Barnes, J. Chem. Phys. 1957, 27, 551–560;
c) A. W. Baker, A. T. Shulgin, J. Am. Chem. Soc. 1959, 81, 1523–
1529; d) A. T. Shulgin, A. W. Baker, Nature 1958, 182, 1299.
[6] a) S. L. Broman, M. ꢃ. Petersen, C. G. Tortzen, K. Kilsꢄ, A. Kadzio-
la, M. B. Nielsen, J. Am. Chem. Soc. 2010, 132, 9165–9174; b) M. ꢃ.
Petersen, S. L. Broman, A. Kadziola, K. Kilsꢄ, M. B. Nielsen, Eur. J.
Org. Chem. 2011, 1033–1039.
[7] a) J. Daub, T. Knçchel, A. Mannschreck, Angew. Chem. Int. Ed.
Engl. 1984, 23, 960–961; b) J. Daub, S. Gierisch, U. Klement, T.
Knçchel, G. Maas, U. Seitz, Chem. Ber. 1986, 119, 2631–2646; c) T.
Mrozek, J. Daub, A. Ajayaghosh, Optoelectronic Molecular Switches
Based on Dihydroazulene-Vinylheptafulvene (DHA-VHF), in Molec-
ular Switches, 1st ed. (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001, pp. 63–106.
[16] The s values are taken from Ref. [5a].
[17] M. B. Nielsen, F. Diederich, Chem. Rev. 2005, 105, 1837–1867.
[18] C. J. Price, J. J. Hydock, J. Am. Chem. Soc. 1952, 74, 1943–1946.
[19] Reversible at 258C, but a conversion, possibly a cycloaddition reac-
tion, seems to occur at elevated temperatures.
[20] 2-Furyl: sp =0.02 and sm =0.06, 2-thienyl: sp =0.05 and sm =0.09,
3-thienyl: sp =ꢀ0.02 and sm =0.03. Ref. [5a].
[21] J. A. Joule, K. Mills, Heterocyclic Chemistry, 5th ed., Wiley, Chiches-
ter, 2010.
[22] The switching of compounds: Y=NMe2, X=CN; Y=OMe, X=
CN; Y=OMe, X=NO2; Y=OMe, and X=Br occurs irreversibly in
MeCN due to the lack of photochromism of the 6-isomer in this sol-
vent. This will be further investigated and published elsewhere.
[23] We found the best empirical correlations by using the actual wave-
lengths rather than energies of the absorptions. For other examples
of correlations with UV/Vis absorption data, correlating instead,
however, frequencies/wavenumbers, with Hammett s values, see:
ˇ´
´
ˇ ´
´
´
a) D. Z. Mijin, G. S. Uscumlic, N. U. Perisic/Janjic, N. V. Valentic,
Chem. Phys. Lett. 2006, 418, 223–229; b) R. Papadakis, A. Tsolomi-
tis, J. Phys. Org. Chem. 2009, 22, 515–521.
Received: January 16, 2013
Revised: April 12, 2013
[8] A. Plaquet, B. Champagne, F. Castet, L. Ducasse, E. Bogdan, V. Ro-
driguez, J.-L. Pozzo, New J. Chem. 2009, 33, 1349–1356.
Published online: June 6, 2013
9548
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 9542 – 9548