1,1-Dicyano-2,2-diphenyl-1,2-dihydronaphthalene: Photochromism and evidence for photochemically induced C-CN bond cleavage

Article abstract of DOI:10.1002/1521-3765(20020902)8:17<4008::AID-CHEM4008>3.0.CO;2-O

The photoreactions of the 1,1- dicyano-2,2-diphenyl-1,2-dihydronaphthalene (DHN-1) have been studied by photochemical techniques under various conditions at room temperature. A transient species, T_c, with a major maximum at 545 nm in the UV-vis

Full text of DOI:10.1002/1521-3765(20020902)8:17<4008::AID-CHEM4008>3.0.CO;2-O

FULL PAPER
1,1-Dicyano-2,2-diphenyl-1,2-dihydronaphthalene: Photochromism and
À
Evidence for Photochemically Induced C CN Bond Cleavage
Helmut Gˆrner,*[a] Thomas Mrozek,[b] and Jˆrg Daub*[b]
Abstract: The photoreactions of the 1,1-
dicyano-2,2-diphenyl-1,2-dihydronaph-
thalene (DHN-1) have been studied by
photochemical techniques under various
conditions at room temperature. A tran-
sient species, TC, with a major maximum
at 545 nm in the UV-visible spectrum,
detected in small yield in polar aprotic
solvents and in large yield in trifluoro-
ethanol (TFE) and hexafluoropropan-2-
ol (HFP), is assigned as a carbocation (1-
cyano-2,2-diphenylnaphthalenium),
which is generated photochemically by
elimination of CNÀ. The decay of TC was
found to be a first-order process in HFP,
in which the longest lifetime of 0.4 s was
observed; in TFE, the carbocation de-
cays on the ms timescale, while the
shortest lifetime of 1 ms was found in
ethanol. The yield of TC increases
strongly upon addition of water to
alcohols or acetonitrile, and remains
substantial in the presence of large
amounts of water (1 20m). On addition
of water to TFE or HFP, the lifetime of
the carbocation becomes much shorter.
This is supported by pulse-induced
charge formation due to the carbocation
and release of CNÀ; the conductivity
decay is related to the lifetime of the
carbocation under selected conditions.
In addition, a major irreversible and a
minor thermally reversible photopro-
cess were spectroscopically observed in
solvents of low as well as high polarity.
The former photoproduct, absorbing
below 300 nm, is tentatively ascribed to
benzobicyclohexenes A produced by
phenyl-vinylmethane rearrangement. In
the latter process, a longer-lived benzo-
quinodimethane derivative B, having a
maximum at 400 430 nm, is formed;
this is related to ring opening and
closure and represents a new example
of photochromic ten-electron electro-
cyclisation. The distinct differences be-
tween the photochromism of DHN and
that of dihydroazulene (DHA) stem
mainly from stereoelectronic effects.
This study, however, has also revealed
that photochemically induced bond het-
erolysis and photochemical/thermal
electrocyclisation, representing two ba-
sic processes of photochromism, may
occur in one molecular unit.
Keywords: charge separation
electrocyclization naphthalene
derivatives photochromism
stereoelectronic effects
¥
¥
¥
¥
of the DHA/VHF couple have been analyzed in great
Introduction
11]
detail.[5
In particular, we have reported on the photo-
Photochromic systems are subjects of intense investiga-
chemical behaviour of 1,8-dihydro-2-aryl-1,1-azulenedicarbo-
nitriles.[8] Their DHA forms are efficiently photoconverted
into the VHF forms on a subnanosecond timescale. Using
femtosecond-resolved transient absorption spectroscopy, the
lifetime of the excited state of cyclopentane-condensed
dihydroazulene DHA-1, corresponding to the ring-opening
reaction, was measured as about 600 fs.[10] Theory predicts a
conical intersection in the excited-state regime.[11] Recent
measurements with even shorter pulses revealed an ultrafast
two-way photochemical interconversion between DHA-2 and
s-cis-VHF-2 (Scheme 1).[12]
Another dicarbonitrile-containing biphotochromic system
is based on the photoconversion of a dithienylene, 1,8a-
dihydro-2,3-bis-3-(2,5-dimethylthienyl)azulene-1,1-dicarboni-
trile, into both the VHF and thienobenzothiophene forms, the
thermal and photochemical properties of which have recently
been studied.[9] For comparison, a diphenylethene, 1,8a-
dihydro-2,3-diphenylazulene-1,1-dicarbonitrile, was also ex-
5]
tions.[1 Ten-electron photochromism has in recent years
been shown to be a viable concept for molecular switching.[5]
In particular, the 1,8a-dihydroazulene-1,1-azulenedicarboni-
triles (DHAs) have proved to be most versatile compounds.[6]
Typically, these DHAs are photoconverted into deeply
coloured, photochemically unreactive 10,10-dicyanovinylhep-
tafulvenes (VHFs), which reconvert thermally. The properties
[a] Dr. H. Gˆrner
Max-Planck-Institut f¸r Strahlenchemie
45413 M¸lheim an der Ruhr (Germany)
E-mail: goerner@mpi-muelheim.mpg.de
[b] Prof. Dr. J. Daub, Dr. T. Mrozek
Institut f¸r Organische Chemie
Universit‰t Regensburg
Universit‰tsstrasse 31
93040 Regensburg (Germany)
E-mail: joerg.daub@chemie.uni-regensburg.de
4008
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Chem. Eur. J. 2002, 8, No. 17

4008 4016
In this work, photochemical
results concerning DHN-1 are
presented for the first time.
One photoreaction is photoiso-
merisation. In the course of this
study, we unexpectedly also
observed the formation of a
carbocation. It appears in wet
polar solvents at room temper-
ature, especially in the presence
of 2,2,2-trifluoroethanol (TFE)
or
1,1,1,3,3,3-hexafluoropro-
pan-2-ol (HFP). These fluoroal-
cohols are solvents that are
25]
known to stabilise ions.[21
Scheme 1. Photochemical and thermal interconversions of the DHA/VHF system.
They are also able to stabilise
the zwitterionic merocyanine
amined; this compound can be photoconverted to the VHF
form, but not to dihydrophenanthrene.[9b]
form of spiropyrans and spirooxazines.[26] On addition of
water to DHN-1 in TFE or HFP, the yield of the observed
carbocation remains substantial, but its decay is strongly
accelerated.
It seemed interesting to study 1,1-dicyano-2,2-diphenyl-1,2-
dihydronaphthalene (DHN-1), representing a potential pho-
tochrome of the 1,2-dihydronaphthalene (DHN) type in
which 1,2-elimination of HCN is precluded by the presence
of two phenyl groups at C-2. From a structural point of view,
DHN represents the dicyanomethine analogue of chromene-
type photochromes.[13] Photochromism of a 2H-1-benzopyran
(BPY) derivative was reported in 1966.[14] A recent review
surveys benzo- and naphthopyrans and their potential appli-
cations.[15] DHN is related to BPYs, in which two merocyanine
isomers (tt: trans-trans and tc: trans-cis) are involved,[16 18] and
to flindersine,[19] a photochrome found in nature belonging to
this category of compounds.
It seemed of interest to extend the study to potential
photochromes, the structures of which would allow a 1,1-
dicyanomethine-assisted ten-electron reorganisation as in
DHA/VHF, while being embedded in a divergent molecular
and electronic structure. The azulene/naphthalene dichotomy,
as represented by compounds DHA and DHN (Scheme 2),
Results and Discussion
Time-resolved absorption properties: The steady-state ab-
sorption spectrum of DHN-1 in any of the solvents used has its
main band at around 200 nm and a shoulder at 250 nm.
Generally, the transient spectra (lexc ˆ 248 nm) show an
absorption increase at 300 600 nm; this occurs within the
laser pulse width or after the disappearance of a scattering
signal. In a polar solvent such as acetonitrile, excitation of
DHN-1 leads to transient absorption spectra attributable to
two species (Figure 1b). One (denoted as B) gives rise to a
Scheme 2. Structurally related photochromes.
would comply with these requirements. With regard to the
electronic configuration, ring-opening of the dihydronaph-
thalene DHN yields an ™alternating∫ ortho-quinoid species,
whereas ring-opening of the DHA results in a ™non-alternat-
ing∫ VHF. The stereoelectronic situations in the two com-
Figure 1. a) Absorption spectrum of DHN-1 in acetonitrile prior to (––)
*
*
and after (
) excitation at 248 nm, and after relaxation (...); b) transient
~
*
*
absorption spectra at 20 ns ( ), 4 ms ( ) and 20 ms ( ) after the pulse
(abscissa as in a); c) kinetics at 400 nm (left) and 545 nm (right).
À
pounds are also different. Dissociation of the C8a C1 bond in
DHA leads to substantial overlap with the cycloheptatriene p
orbitals, whereas in the case of DHN the p orbitals are
quasi-permanent absorption increase with a maximum cen-
tered at lB ˆ 430 nm (Figure 1c). The second species is a
shorter-lived transient with a maximum at lC ˆ 540 550 nm,
denoted as TC. Transient TC, which is formed within the pulse
width, is not sensitive to oxygen and its spectrum overlaps
with that of species B in the range from about 350 to 500 nm.
À
orthogonal to the s orbitals of the elongating C1 C2 bond.
À
However, elongation of the C1 CN bond would be expected
to generate an overlapping orbital situation in the latter
case.[20]
Chem. Eur. J. 2002, 8, No. 17
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H. Gˆrner, J. Daub and T. Mrozek
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The decay of TC is a first-order process in most solvents
(Table 1), with the shortest lifetime (tC ˆ 1/kobs) of 1 ms being
observed in ethanol. The presence of water in acetonitrile (up
to 30m) does not have a strong influence on tC, but the peak at
545 nm is enhanced (Figure 2). In fact, the DA545 value
the four alcohols and in their mixtures with water (1 20m),
no solvated electrons could be detected just after the pulse,
thus ruling out photoionisation as the origin of TC (see below).
A much longer half-life of TC and a higher DA545 value was
observed for DHN-1 in HFP (Figure 4a) and in TFE; both are
Figure 2. Transient absorption spectra for DHN-1 in MeCN/H2O (1:1) at
~
*
*
<0.1 ( ), 3 ( ) and 10 ms ( ) after the 248 nm pulse; inset: kinetics at
545 nm.
strongly increases with increasing water concentration (Fig-
ure 3a). The spectral changes in methanol or ethanol are
similar, whereas those in dichloromethane, tert-butanol, or
propan-2-ol reveal only (or predominantly) the formation of
the quasi-permanent component B (Table 1 and Table 2).
Again, the DA545 value in these solvents strongly increases
with [H2O] (Figure 3a). It is interesting to note that in each of
Figure 3. Plots versus [H2O] of a) the relative DA545 value (open symbols)
and b) log kobs (filled symbols) for DHN-1 in HFP (circles), TFE (squares),
~
^
ethanol ( ) and acetonitrile ( ).
polar, protic solvents in which ions or radical ions are known
25]
to be stabilised.[21
The time-resolved spectra in TFE
(Figure 5a) can be fitted by two first-order decay components
with t1/2 ˆ 1 ms and 10 ms. On addition of water (0.5 5m) to
TFE, the decay of TC becomes faster and monoexponential,
Table 1. Lifetime and relative yield of the carbocation.[a]
[b]
[c]
Solvent
ET
Additive
tC [ms]
DAC
MCH
0
< 0.02
< 0.02
< 0.05
0.1
dichloromethane
tert-butanol
acetonitrile
0.31
0.39
0.46
ꢀ 0.006
ꢀ 0.006
H2O (15m)
H2O (5m)
0.6
propan-2-ol
ethanol
0.55
0.65
< 0.05
0.08
0.3
0.05
1
0.001
0.001
0.001
1/10[d]
2
methanol
TFE
0.76
0.90
H2O (5m)
1
EtOH (2m)
MeCN (2m)
0.2
0.2
1
1
HFP
1.07
450
1
H2O (5m)
50
0.9
EtOH (10m)
0.1
0.9
[a] lexc ˆ 248 nm. [b] Polarity parameter (normalised) from ref. [32].
max
[c] Relative values, measured at lC ˆ545 nm (note: DAB ˆ0.1ÂDACmax).
Figure 4. Transient absorption spectra for DHN-1 in a) neat HFP at <1 ms
~
&
[d] Two components.
*
*
(
), 0.1 s ( ), 0.2 s ( ) and 1 s ( ) after the 248 nm pulse; b) HFP/H2O (1:1,
~
&
*
v/v) at <1 ms ( ), 1 ms ( ) and 10 ms ( ) and c) HFP/EtOH (1:1) at <1 ms
&
Table 2. Maxima and relative yields of A and B.[a]
*
*
(
), 0.1 ms ( ) and 10 ms ( ); insets: kinetics at 545 nm.
[d]
Solvent
lB/nm
FrAel[b]
FrBel[c]
DAB
indicating a single component. An example with TC as the
major species and tC ˆ 0.1 ms is seen in TFE/H2O (1:1)
(Figure 5b). The plot of log kobs shows an almost linear
dependence on [H2O] (Figure 3b). A first-order decay of TC
and the longest lifetime of tC ˆ 0.4 s were observed for DHN-
1 in HFP (Figure 4a). The yield of TC (DA545 value) does not
depend markedly on [H2O] (Figure 3a). The plot of log kobs
versus [H2O] is similar to that in the case of TFE, but is shifted
by about two orders of magnitude to smaller values (Fig-
ure 3b). In the presence of water (Figure 4b) or ethanol
MCH
400
410
430
410
400
410
< 400
< 400
0.5
0.5
1
0.5
0.5
1
0.5
0.5
1
0.6
0.6
dichloromethane
acetonitrile
tert-butanol
propan-2-ol
ethanol
0.6
0.8
0.8
0.6
0.4
< 0.2
0.5
< 0.5
TFE
HFP
[a] From light-induced changes of the absorption spectrum; lexc ˆ 248 nm.
[b] Relative values, measured at 290 nm. [c] Relative values, measured at
lB. [d] From transient measurements at 1 ms.
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Photochemically Induced Bond Cleavage
4008 4016
Scheme 3. Schematics representing ground state and excited state trans-
formations.
Figure 5. Transient absorption spectra for DHN-1 in a) neat TFE at <1 ms
~
*
*
(
), 1 ms ( ) and 0.1 s ( ) after the 248 nm pulse and b) TFE/H2O (1:1) at
~
*
*
<1 ms ( ), 3 ms ( ) and 10 ms ( ); insets: kinetics at 545 nm.
(Figure 4c), TC is the major species. The yields in neat polar
media are much lower than those in HFP or TFE (Table 1).
Time-resolved conductivity: The pulse-induced conductivity
signals of DHN-1 in acetonitrile/water (pH 7) (1:1, v/v) and
water/TFE mixtures (1:1; 1:9), as well as in HFP in the
presence of 5 and 0.5m water, are shown in Figure 6a e,
respectively. The amplitude increases within 0.1 ms and decays
Figure 7. a) Absorption spectrum of DHN-1 in MCH prior to (––) and
*
*
after (
) excitation at 248 nm and after relaxation (...); b) transient
*
*
absorption spectra at <100 ns ( ) and 0.1 ms ( ) after the pulse; c) kinetics
at 400 nm.
The spectral changes that follow 248 nm excitation of
DHN-1 in dichloromethane, tert-butanol and propan-2-ol
reveal species A and the quasi-permanent component B
(Table 2), but not TC. This is also the case in the nonpolar
MCH, in which the transient absorbs from <270 nm to about
500 nm with a maximum centered at lB ˆ 400 nm (Figure 7a)
and exhibits no further changes for a duration of up to a few
seconds (Figure 7b,c). In fact, the absorption increase is
practically the same as that recorded within 0.1 1 min
following removal of the light source. A similar increase in
absorption above 265 nm and a pronounced band with lB ˆ
430 nm was recorded in acetonitrile (Figure 1a). The presence
or absence of oxygen has no discernible effect.
The band at lB in the steady-state absorption spectrum of
DHN-1 in several solvents after pre-irradiation (for example,
at 248 nm) is related to a ring-opened form of DHN-1, namely
the benzoquinodimethane (B) form (see below). The thermal
relaxation time (tB) was fitted by a first-order decay. Note that
a smaller portion remains or disappears over a much longer
timescale; in addition, the maximum is slightly red-shifted
with time. These phenomena might be due to (E)/(Z)-
stereoisomerism of B.[18] The values for the major initial part
of tB in several solvents at room-temperature range from 102
to 103 s. On increasing the temperature, tB becomes shorter,
but analysis failed due to the small magnitude of the effect (at
the required low conversion).
Figure 6. Traces of conductivity signals of DHN-1 in a) water (pH 7)/
acetonitrile (1:1, v/v), b) water/TFE (1:1), c) water/TFE (1:9), and in HFP
in the presence of d) 5m and e) 0.5m water after the 248 nm pulse.
according to first-order kinetics, almost reverting to the value
prior to the pulse. The lifetimes in the cases illustrated are
t'C ˆ 6 ms, 50 ms, 170 ms, 50 ms and 0.25 s, respectively.
Photoconversion and thermal relaxation: Only weak fluores-
cence below 400 nm was observed for DHN-1 in methylcy-
clohexane (MCH) or ethanol at 25 and À1968C, and virtually
no phosphorescence could be detected in glassy media. This
indicates rapid deactivation processes from the excited singlet
state (1DHN-1*). One reaction is the irreversible formation of
product A (Scheme 3), which can best be detected at around
290 nm. The relevant spectra are shown for the cases of
acetonitrile (Figure 1a) and MCH (Figure 7a) at room tem-
perature.
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H. Gˆrner, J. Daub and T. Mrozek
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Table 3. Absolute quantum yield of photoconversion.[a]
The increase at 290 nm (product A) with increasing dose of
incident radiation is initially linear and then approaches a
constant value (Figure 8b), whereas the signal at lB reaches a
Solvent
[H2O]/m
F[b]
acetonitrile
none
0.5 (0.4)[c]
0.4 (0.3)
0.4
28
none
ethanol
5
5
5
0.3
0.2
0.2
TFE
HFP
[a] From light-induced changes of the absorption spectrum, lexc ˆ 248 nm.
[b] Absolute values, measured by HPLC. [c] Values in parentheses refer to
lirr ˆ 254 nm.
1HNMR spectroscopy and FD mass spectrometry : To gain a
1
deeper insight into the photochemistry of DHN-1, H NMR
spectroscopy was used in combination with mass spectrom-
etry (FD-MS). In one experiment (a), a wet soltuion of DHN-
1 in acetonitrile (<12 mg DHN-1 in CD3CN containing 20%
D2O; ca. 0.8 mL) was placed in a quartz NMR tube and
irradiated with light of lirr ˆ 254 nm. New minor signals
appeared in the 1H NMR spectrum at around d ˆ 7.0
7.8 ppm following irradiation for 78 min. However, the photo-
product could not be positively identified (such as by 1H NMR
difference spectra), since most of the product signals were
coincident with the intense DHN-1 signals. Attempts to
accumulate the photoproduct by irradiation for an additional
360 min were unsuccessful because of decomposition. An
attempt to induce changes by thermal relaxation at room
temperature in the dark (24 h) also failed.
A mass spectrometric investigation of the photoproduct
mixture revealed compounds with m/z ˆ 305.1 (3%), 333.1
(29%) and 334.1 (6%), besides the DHN-1 signal at m/z ˆ
332.1 (100%).[27] The compound with m/z ˆ 305.1 is tenta-
tively assigned as the naphthalene derivative C-1 (Scheme 4),
which was presumably formed via the cationic intermediate
TC. Upon addition of aqueous AgNO3 solution to a photo-
product mixture, a colourless precipitate was formed, which
we attribute to AgCN. This precipitate arising from DHN-1,
which was not observed in the absence of light, supports the
proposed initial photochemical step in Scheme 4. In a second
experiment (b), DHN-1 (<7 mg) was dissolved in about
0.8 mL of a CF3CH2OD/CD3CN mixture (1:1) due to its
limited solubility in neat CF3CH2OD. Irradiation (for 15
355 min) under otherwise identical conditions as in experi-
ment (a) caused very similar effects as those discerned from
the aforementioned 1H NMR spectra.
Figure 8. Increase of absorption a) at lB (400 430 nm) and b) at 290 nm
versus the 248 nm dose for DHN-1 in MCH (triangles), acetonitrile
(circles), TFE (squares), and ethanol (diamonds).
maximum value and decreases upon further pulsing (Fig-
ure 8a). When the light is removed, the two parts of the
spectrum that are separated by a minimum at about 360 nm
behave differently. The changes to the UV absorption are
essentially retained, whereas the band at lB ˆ 400 430 nm
reverts thermally to a value close to zero (see below). Two
quantum yields were therefore measured for low conversion,
that is, from the linear parts in Figure 8: one relates to product
formation at 290 nm (FArel† and the other to reversible
photocolouration (FrBel†. These effects were observed in
virtually all organic solvents. In the presence of water, the
product formation at 290 nm is similar, but the reversible
photocolouration is small, especially when compared with the
contrasting findings regarding the DA545 value under the same
conditions. The lB values and the FA and FB values are listed
in Table 2. The yields are substantial in most organic solvents
and are largest in acetonitrile.
Comparable results, that is reversible colour (B) and
irreversible product formation (A), provided that the irradi-
ation time was markedly shorter than the thermal relaxation
time, were found using continuous irradiation (lirr ˆ 254 nm).
Species B could not be enhanced by prolonged irradiation at
254 nm. The absolute quantum yield of conversion upon
pulsed excitation is relatively large and does not show a
marked dependence on the solvent or the amount of added
water (Table 3). Photodecomposition of DHN-1 in various
solvents was also monitored by HPLC (reversed-phase
column) at 215 290 nm. Under conditions for which transient
TC was the major intermediate (see above), the major product
was found to elute after a retention time of 3.8 min, indicating
that the subsequent product (C) is more polar than DHN-1,
which has a retention time of 4.2 min. In other media, such as
MCH, several minor peaks with longer retention times
appeared, indicating less polar products.
Salient molecular signals can be discerned in the FD mass
spectrum of the photoproduct mixture at m/z ˆ 324.2 (100%)
and m/z ˆ 332.2 (68%), along with some weak ones, for
example, at m/z ˆ 305.1 (5%) and 405.2 (1%). Peaks at m/z ˆ
324.2 and m/z ˆ 405.2 may be assigned to compounds C-2 and
C-3, formed by trapping of the carbocation TC with residual
ODÀ and CF3CH2OÀ, respectively (Scheme 4).
The identity of transient TC: At least three photoproducts, all
of which are formed via the excited singlet state of the DHN-1
form within 20 ns (Scheme 3), have to be considered: species
A and B, absorbing at 250 300 nm and at lB ˆ 370 440 nm,
respectively, as well as transient TC. For TC, several alter-
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Chem. Eur. J. 2002, 8, No. 17

Photochemically Induced Bond Cleavage
4008 4016
concentration of 0.1mm. If electron transfer occurs intra-
molecularly, however, then a radical anion moiety should be
observable, which will subsequently react with oxygen. The
rate constant for this reaction is typically 2 Â 1010 mÀ1 sÀ1 in
ethanol or acetonitrile.[30, 31] Thus, a radical anion with a half-
life of >1 ms is excluded, since TC does not react with oxygen.
Features of the carbocation: The most plausible possibility is
that TC is a carbocation (Scheme 4), this being in agreement
with all the results. For example, TC was not detected in neat
dichloromethane, tert-butanol or propan-2-ol, and its yield
was rather low in methanol, ethanol and acetonitrile, these
being media with polarity parameters[32] below 0.8 (Table 1).
On addition of water to these solutions, the yield increases
substantially (Figure 3a). In HFP, in which the longest lifetime
was observed (Figure 4a), TC decays according to first-order
kinetics. In TFE, TC also has a rather long half-life and two
first-order decay components in the ms range (Figure 5a). On
addition of water to TFE or HFP, the yield remains substantial
or decreases only slightly (Figure 3a), but the decay becomes
much faster (Figure 3b).
The presence of water in acetonitrile or alcohols is
necessary for efficient formation of the observed carbocation.
The proposed reasons are better solvation of TC and of the
released CNÀ (Scheme 4), as well as the possible formation of
hydrogen cyanide. It is interesting to note that for the
photoheterolysis of 9-fluorenol, a related case, it has been
suggested that protonation of the molecule occurs even in the
ground state and that the driving force is separation of the two
ions by one water molecule.[26] Analogously, we propose that
the photoheterolysis of DHN-1 involves an assistance of ion
separation by a water molecule. The first-order decay kinetics
of the carbocation and the shorter lifetime in mixtures of HFP
with alcohols (ROH) or water (Figure 4a c) exclude the back
reaction with CNÀ. Instead, the decay kinetics of the
carbocation can be attributed to the addition of ROH and
deprotonation. In the presence of BrÀ in aqueous TFE, the
lifetime tC is strongly reduced, and this is ascribed to a
corresponding addition reaction (ROÀ is replaced by BrÀ).
To account for the final re-aromatisation of the naphtha-
lenium ion, a rearrangement has to occur (Scheme 4). The
major product with a retention time of 3.8 min upon HPLC,
formed under conditions under which transient TC is the
major intermediate, especially in the presence of water, is
compatible with the above assignment to 1-cyano-2,3-diphe-
nylnaphthalene (C-1) or to the dihydronaphthalene deriva-
tives C-2 and C-3. Fluorescence and a structured absorbance
would be expected for naphthalene C-1. However, no
fluorescence could be observed under conditions under which
TC is the major intermediate, indicating that re-aromatisation
to the naphthalene structure does not occur, at least not to any
spectroscopically detectable extent, under the applied con-
ditions. It is more probable that the presence of the
naphthalene derivative C-1 in the FD-MS spectrum (see
above) is merely due to its formation from C-2 or C-3 through
extrusion of ROH in the course of the FD-MS experiment.
The proposed mechanism is supported by pulse-induced ion
formation (Figure 6). The initial conductivity increase in the
presence of neutral water is due to the formation of the
Scheme 4. Assumed carbocation route of the DHN-1 photochemistry.
natives to a carbocation may be considered: a triplet state, a
radical, a biradical, a zwitterion, a radical cation and a radical
anion. The triplet state can be excluded as oxygen does not
reduce the lifetime tC. Moreover, attempts to generate a
DHN-1 triplet state under sensitised excitation, by using
acetone, acetophenone or naphthalene in argon-saturated
acetonitrile at room temperature, failed. The lack of any
reaction with oxygen is also strong evidence against the
hypotheses of TC being a radical or biradical.
Fluoroalcohols are able to stabilise the zwitterionic mer-
ocyanine form of spiropyrans and spirooxazines by hydrogen
bonding.[26] The appearance of TC in polar media, with a
maximum in the visible region at lC ˆ 545 nm, might be
suggestive of it being a zwitterion. However, the indepen-
dence of the spectral bands of the polarity of the medium and
the effects of water concentration on the yield and lifetime
(Table 1) are not consistent with this hypothesis.
Excitation of aromatic molecules with 5.0 eV pulses may
well lead to photoionisation products arising from the
solvated electron and the radical cation as primary spe-
cies.[28, 29] However, TC cannot be assigned to a radical cation,
since its counterpart, the solvated electron, was not detected
under conditions (e.g., in ethanol and in mixtures with water)
under which it is normally readily detectable.[29 31] Note that in
acetonitrile the solvated electron is scavenged within 1 ns.
One could, however, argue that the solvated electron rapidly
reacts with DHN-1. If this occurs intermolecularly, the
solvated electron should be detectable within about 1 ms,
given a diffusion-controlled rate constant and a DHN-1
Chem. Eur. J. 2002, 8, No. 17
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4013

H. Gˆrner, J. Daub and T. Mrozek
FULL PAPER
carbocation and CNÀ, the latter, in terms of charges, being
rapidly replaced by OHÀ. The ensuing decrease in conductiv-
ity results from neutralisation in the course of the decay of the
carbocation. The increase in its lifetime from t'C ˆ 6 ms in
water/acetonitrile (1:1) to 0.2 s in water/HFP mixtures (1:100)
is in reasonable agreement with the findings of absorption
spectroscopy. In particular, in HFP in the presence of 0.5 and
5m water, the lifetimes from the two methods are consistent.
contribute to the efficient C8a C1 bond cleavage in this
photochrome. From Figure 9, it also follows that one of the
À
À
C CN bonds in DHN is aligned almost parallel to the
benzene p system, whereas in DHA the corresponding
interaction is less.
Conclusion
Other photoreactions: The photoreaction of DHN-1 relating
to species B (benzoquinodimethane isomers) is suggested to
be ring-opening and to involve a thermal relaxation process,
in agreement with the synthesis of DHN-1. This photochrom-
ism of DHN-1 is a minor process and may be interpreted in
terms of the following reaction mechanism. Irradiation
populates the 1DHN-1* state, and the ring-opening takes
place on the appropriate reaction coordinate in the excited
singlet moiety, since intersystem crossing does not occur at all.
The thermal relaxation process (Scheme 3) differs from
that of chromene or other spiro compounds in that the
kinetics deviate from a monoexponential decay. Interestingly,
two ring-opening reactions have also been considered for
chromene and both photoprocesses involve thermal relaxa-
tion.[17]
Photoinduced charge separation and bond cleavage, as
fundamental steps in energy conversion, underpin a wide
range of processes, extending from molecular dimensions
(light-induced solvolysis, photoinduced heterolytic bond
cleavage)[36] up to photosynthetically active membrane-inte-
grated protein-pigment complexes,[37] photovoltaic devices[38]
and materials for photolithographic applications.[39] A heter-
olytic bond cleavage involving the cyano groups is suggested
as the driving force for carbocation (TC) formation from
electronically excited 1,1-dicyano-2,2-diphenyl-1,2-dihydro-
naphthalene. The three suggested photoprocesses all occur
on a timescale of less than 10 ns. Photochromic ring opening
and closure via the benzoquinodimethane analogue B is a side
reaction under these conditions, which becomes even less
efficient in the presence of water. Water in organic solvents is
necessary for the efficient formation of the observed carbo-
cation, but the quantum yield of photoconversion (leading
also to species A, presumably through a phenyl-vinylmethane
rearrangement) is rather independent of the solvent polarity
and the amount of added water. This suggests competing
photoprocesses, decomposition of DHN-1 via A, photochro-
mic ring-opening via B, and formation of carbocation TC,
which eventually yields stable compounds. These presumably
have dihydronaphthalenoid and naphthalenoid structures
(C-1 to C-3). As already addressed in the introduction,
structural and stereoelectronic reasons may be the decisive
factors responsible for the competition of electrocyclisation
and heterolytic bond cleavage. Further investigations of 2,2-
diphenyl-1,2-dihydronaphthalene and 1,1-diphenyl-1,2-naph-
thalene derivatives with better leaving group(s) and more
efficient stabilisation of the cationic intermediate should be
helpful to shift the photoprocesses more efficiently towards
the two photochromic pathways, electrocyclisation and pho-
toinduced bond dissociation.
Species A, being only weakly characterised by an absorp-
tion increase of the irreversible part below 400 nm, is
tentatively ascribed to a 1,1a,6,6a-tetrahydrocycloprop[a]in-
dene derivative resulting from a phenylvinylmethane rear-
rangement of DHN-1 (Scheme 3).[33, 34] Photoinduced forma-
tion of A, B and C are competing processes, but the
1
vibrationally relaxed DHN-1* state must be questioned as
a common precursor for species A as well, since the formation
of A is not sufficiently suppressed when the yield of C reaches
a maximum (Tables 1 3). We therefore propose that the
pathways to B and C are competing deactivation processes of
1
the vibrationally relaxed DHN-1* state and that the forma-
tion of A takes place from the Franck Condon state or via
relaxation to the 1DHN-1* state.
Figure 9 shows DHA and DHN in their ground states. This
À
clearly demonstrates that the C1 C2 s bond in DHN-1 is
essentially orthogonal to both the p system of the benzene
Experimental Section
Synthesis: The photochromic chromene-like 6p-electron system DHN-1
was synthesised according to Scheme 5.[27] o-Iodobenzyl alcohol (1) was
first converted to o-iodobenzyl bromide (2) with HBr according to ref. [40];
this was then converted to the corresponding phosphonium salt (2-
iodobenzyl)triphenylphosphonium bromide (3) with triphenylphosphine
according to ref. [41]. Both reactions proceeded in near quantitative yield.
Wittig reaction[42] of 3 with diphenylacetic aldehyde gave cis/trans-3,3-
diphenyl-1-(2-iodophenyl)propene and 1,1-diphenyl-3-(2-iodophenyl)pro-
pene (4), the latter being a tautomeric product formed by base-induced
rearrangement. Palladium-catalysed coupling of 4 and malonodinitrile with
tBuOK as base in dimethoxyethane[43] yielded a mixture of isomeric
dicyano compounds (5), with one isomer predominating, namely the
Figure 9. Structures of DHA (left) and DHN-1 (right). After geometry
optimisation using force field methods, the calculations were carried out
with Spartan[35] on a semiempirical level using AM1.
À
unit and the C3 C4 p bond. In contrast, for DHA an
overlapping orbital situation between the lengthening
tautomeric form 1,1-diphenyl-3-(2-dicyanomethylphenyl)propene. In
a
À
C8a C1 s bond and the cycloheptatriene p system may
final one-pot dehydrogenation reaction, the benzoquinodimethane deriv-
4014
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Chem. Eur. J. 2002, 8, No. 17

Photochemically Induced Bond Cleavage
4008 4016
6.11 (t, 1H, Hb of tautomeric form, J ˆ
7.43 Hz), 7.82 7.00 ppm (m, aromatic
H of all three isomers); MS (70 eV,
‡
EI): m/z (%): 334 (55) [M ].
1,1-Dicyano-2,2-diphenyl-1,2-dihydro-
naphthalene (DHN-1): Under N2,
compound
5 (mixture of isomers;
0.39 g, 1.2 mmol), NBS (0.21 g,
1.2 mmol), and AIBN (catalytic
amount) were heated in CCl4 at
1208C for 24 h. The progress of the
reaction was monitored by TLC
(CH2Cl2/PE, 1:1), irradiating the TLC
plate with 254 nm light (UV lamp),
while cooling it in liquid N2. The
product could be identified from a
red spot, which immediately faded
when the plate was warmed to room
temperature. After cooling to room
temperature, the reaction mixture was
filtered, and the residue was washed
twice with CCl4. Evaporation of the
solvent in a rotary evaporator, fol-
Scheme 5. Synthesis of DHN-1.
lowed by column chromatography of
the residue on silica gel eluting with CH2Cl2/PE (1:1) (Rf ˆ 0.50) afforded a
still impure product. Further purification by washing several times with n-
pentane/diethyl ether yielded 54 mg (0.16 mmol, 13%) of a colourless
solid. M.p. 1888C; IR (KBr): nÄ ˆ 3079, 3034, 2243, 1791, 1600, 1487, 754,
701 cmÀ1; 1H NMR (400 MHz, CDCl3): d ˆ 3.95 (m, 2H, 3- and 4-H), 7.54
7.26 (m, 13H, phenyl- and benzo-H), 7.61 7.57 ppm (m, 1H, benzo-H); MS
atives 6 were produced by allylic and benzylic mono- or dibromination of 5
with NBS/AIBN in a first step, followed by thermolytic dehydrobromina-
tion and debromination, respectively.[44] Under the applied reaction
conditions, the (E)/(Z)-benzoquinodimethane isomers
rearranged to DHN-1.
6 immediately
(E/Z)-4 (and tautomeric form): Under N2,
3 (2.0 g, 3.6 mmol) was
‡
(70 eV, EI): m/z (%): 332 (100) [M ]; HRMS calcd for C24H16N2: 332.1313;
suspended in THF (25 mL). tBuOK (0.40 g, 3.6 mmol) was then added
portionwise under ice-cooling, whereupon the reaction mixture immedi-
ately became orange. Stirring was continued for a further 20 min, and then
diphenylacetic aldehyde (0.64 mL, 3.5 mmol) was slowly added. The
mixture was stirred for 5 h at room temperature, while the colour faded
to a yellowish hue. The reaction mixture was then diluted with diethyl ether
and 0.1m aqueous HCl. After separation of the layers, the aqueous phase
was extracted a further four times with Et2O. The combined organic layers
were washed with water, dried over anhydrous Na2SO4 and filtered. After
evaporation of the solvent in a rotary evaporator, column chromatography
of the residue on silica gel eluting with CH2Cl2/PE (1:5) afforded 0.87 g
(2.2 mmol, 63%) of a yellowish resin. IR (KBr): nÄ ˆ 3078, 3059, 3028, 2926,
found: 332.1310.
Methods: Melting points: uncorrected, B¸chi SMP 20 and Reichert
Thermovar. IR: Bio-Rad FTS 155. EI-MS: Varian CH-5. FD-MS:
MAT 95. NMR: Bruker AC250 (248C) and ARX400 (218C) spectrom-
eters at 250/400 MHz for 1H and 101 MHz for 13C (ARX400). Chemical
shifts d are reported with respect to TMS as an internal standard. Solvents
and reagents for synthesis were used as purchased without further
purification unless stated otherwise (CCl4 was dried over P2O5; THF and
DME were dried over Na/K). The progress of reactions was monitored by
TLC. In all cases, solvent impurities were removed in vacuo by using oil-
diffusion pumps. Further methods were as described elsewhere.[9, 27]
2860, 1643, 1598, 1560, 1012, 968, 747, 700 cmÀ1
;
1H NMR (250 MHz,
The molar absorption coefficient of DHN-1 in methylcyclohexane was
e248 ˆ 1.0  104 mÀ1 cmÀ1. The solvents used for spectroscopic investigations
(Merck) were of analytical (TFE, tert-butanol, propan-2-ol, methanol) or
Uvasol grade (acetonitrile). MCH was purified by passage through a basic
alumina column (Woelm); HFP (Aldrich) and ethanol were distilled; water
was from a milli-Q system. Steady-state measurements were performed by
using a diode-array spectrophotometer (Hewlett Packard 8453) and a
CDCl3, mixture of three isomers): d ˆ 3.55 (d, 2H, J ˆ 7.53 Hz; -CH2 of
tautomeric form), 4.98 4.88 (m, 2H; -CPh2H of cis and trans forms), 7.40
6.15 (m, 43H; aromatic and olefinic H), 7.48 (dd, 1H, J1 ˆ 7.93 Hz, J2 ˆ
1.59 Hz; HA or HD), 7.87, 7.81, 7.79 ppm (3 dd, 1H per isomer, J1 ˆ 7.93 Hz,
J2 ˆ 1.19 Hz for all isomers; HA or HD); 13C NMR (62.9 MHz, CDCl3):[45]
d ˆ 41.24 (CH2 of tautomeric form), 54.20, 49.09 (-CPh2H of cis and trans
forms), 100.72, 100.01, 99.73 ppm (-CI of all three isomers); MS (70 eV, EI):
spectrofluorimeter (Perkin Elmer LS-5). Continuous irradiation at lirr
ˆ
‡
‡
‡
m/z (%): 396 (70) [M ], 318 (23) [M À Ph], 269 (27) [M À I]; elemental
analysis calcd (%) for C21H17I (396.27): C 63.65, H 4.32; found C 63.71, H
4.42.
254 nm was achieved by using a low-pressure Hg lamp. The relative
quantum yields were based on absorption increases as a function of the
irradiation time or laser dose. Uridine in aqueous solution at pH 7 was used
as an actinometry standard.[46] To determine the absolute quantum yield,
the decomposition of DHN-1 was monitored by HPLC using a reversed-
phase Nucleosil 3-C-18 column (125 Â 4.6 mm, 0.8 mLminÀ1), detection at
215 290 nm and with methanol/water (5:1) as eluent. Laser flash
photolysis with a time resolution of 20 ns was carried out using radiation
of lexc ˆ 248 nm from an excimer laser.[8, 9] The relative yields were obtained
from the linear part of the absorption increase at 290 nm and the long
wavelength maximum (lB) versus the laser intensity using optically
matched solutions. Time-resolved conductometry was carried out as
described elsewhere.[31] Unless otherwise indicated, measurements refer
to 258C.
5 (stereoisomeric and tautomeric forms): Under N2, [Pd(PPh3)4] (23 mg,
20 mmol), CH2(CN)2 (66 mg, 1.0 mmol) and tBuOK (130 mg, 1.2 mmol)
were suspended in DME (10 mL), and the mixture was stirred for 10 min at
room temperature. Then, compound
4 (200 mg, 0.50 mmol, isomeric
mixture) was added, and the reaction mixture was refluxed for 24 h. At
least three new spots were detected by TLC (CH2Cl2/PE, 1:1, Rf ˆ 0.30
0.52), indicative of product formation. After the addition of further
portions of [Pd(PPh3)4] (23 mg, 20mmol) and tBuOK (130 mg, 1.2 mmol),
refluxing was continued for another two days. The mixture was then cooled
to room temperature, diluted with 0.1m HCl and stirred for 10 min. The
layers were allowed to separate, and the aqueous phase was extracted
another four times with Et2O. The combined organic layers were dried over
anhydrous Na2SO4 and filtered. After evaporation of the solvent in a rotary
evaporator, column chromatography of the residue on silica gel eluting
with CH2Cl2/PE (1:1) (Rf ˆ 0.30 0.52) afforded 66 mg (0.20 mmol, 39%)
Acknowledgements
of a brown resin. IR (KBr): nÄ ˆ 3061, 3030, 2924, 2257, 2226, 1600, 761,
We thank Professors K. Schaffner and W. Lubitz for their support and Mrs.
M. Lichtenauer, Mr. O. Kushnir, L. J. Currell and H. Selbach for technical
assistance. T.M. was a recipient of the ™Stipendium zur Fˆrderung des
701 cmÀ1
;
1H NMR (400 MHz, CDCl3): d ˆ 3.56 (d, 2H, -CH2 of
a
tautomeric form, J ˆ 7.43 Hz), 4.76 (s, 1H, -C(CN)2H of tautomeric form),
Chem. Eur. J. 2002, 8, No. 17
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4015

H. Gˆrner, J. Daub and T. Mrozek
FULL PAPER
wissenschaftlichen und k¸nstlerischen Nachwuchses∫. This work was
supported by the German Science Foundation (Graduiertenkolleg ™Sen-
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Received: February 4, 2002 [F3847]
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