Adiabatic trans-cis photoisomerization and photocyclization of 8-styrylquinoline

Article abstract of DOI:10.1134/S0018143910050103

Kinetics of the photocyclization of trans-8-styrylquinoline into 10a,10b-dihydronaphtho[1,2-h]quinoline (4-azachrysene) was studied in hexane. It was found that in addition to the expected two-step (two-quantum) route with trans-cis photoisomerization occurring in the first step with a quantum yield of Φ_tc = 0.13 with consequent photocyclization of the cis-isomer with a quantum yield of 0.23. The direct singlequantum photocyclization of the trans-isomer with a quantum yield of 0.009 is also observed. The latter observation indicates that the excited trans-isomer isomerizes without loss of excitation to the excited cis-isomer, which then undergoes cyclization, i.e., the trans-cis photoisomerization proceeds partially by adiabatic mechanism t* → c*.

Full text of DOI:10.1134/S0018143910050103

ISSN 0018ꢀ1439, High Energy Chemistry, 2010, Vol. 44, No. 5, pp. 412–417. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © M.F. Budyka, V.M. Lee, N.I. Potashova, T.N. Gavrishova, 2010, published in Khimiya Vysokikh Energii, 2010, Vol. 44, No. 5, pp. 444–449.
PHOTOCHEMISTRY
Adiabatic transꢀcis Photoisomerization and Photocyclization
of 8ꢀStyrylquinoline
M. F. Budyka, V. M. Lee, N. I. Potashova, and T. N. Gavrishova
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
pr. Akademika Semenova, 1, Chernogolovka, Moscow region, 142432 Russia
eꢀmail: budyka@icp.ac.ru
Received April 22, 2010
Abstract—Kinetics of the photocyclization of transꢀ8ꢀstyrylquinoline into 10a,10bꢀdihydronaphtho[1,2ꢀ
h]quinoline (4ꢀazachrysene) was studied in hexane. It was found that in addition to the expected twoꢀstep
(twoꢀquantum) route with transꢀcis photoisomerization occurring in the first step with a quantum yield of
ϕtc = 0.13 with consequent photocyclization of the cisꢀisomer with a quantum yield of 0.23. The direct singleꢀ
quantum photocyclization of the transꢀisomer with a quantum yield of 0.009 is also observed. The latter
observation indicates that the excited transꢀisomer isomerizes without loss of excitation to the excited cisꢀisoꢀ
mer, which then undergoes cyclization, i.e., the transꢀcis photoisomerization proceeds partially by adiabatic
mechanism t*
→
c*.
DOI: 10.1134/S0018143910050103
It was found that the transꢀisomer of 4ꢀstyꢀ
rylquinoline under irradiation isomerized into cis
ꢀ
isomer according to the adiabatic mechanism in
hexane and according to the diabatic mechanism in
alcohol [1]. It was pointed out that opposite to the
adiabatic cisꢀtrans photoisomerization, typical of
diarylethylene subjected to oneꢀway photoisomerꢀ
ization [2, 3], the adiabatic transꢀcis photoisomerꢀ
ization was relatively rare event. This is rationalized
in terms of the falling down the potential energy
surface (PES) of the excited state along the reaction
coordinate of the isomerization from the cisꢀ to
transꢀisomer, therefore the adiabatic transꢀcis
isomerization is energetically unfavorable.
N
N
ISN
4SQ
8SQ
Scheme 1.
4ꢀStyrylquinoline (4SQ) is the azaꢀsubstituted
analog of 1SN and involves azanaphthyl (4ꢀquinolyl)
fragment, in its structure. 8ꢀStyrylquinoline (8SQ) is
the other azaꢀanalog of 1SN.
Several diarylethylenes capable of adiabatic
transꢀcis photoisomerization are known: 1,2ꢀdi(1ꢀ
naphthyl)ethylene (1N1N) [4], 1ꢀ(1ꢀnaphthyl)ꢀ2ꢀ
(2ꢀnaphthyl)ethylene (1N2N) [5] and the derivative
of 1ꢀstyrylnaphthalene (1SN)–1ꢀ(1'ꢀnaphthyl)ꢀ2ꢀ
(3'ꢀhydroxyphenyl)ethylene (1N3PhOH) [6]. In all
these compounds, at least one of the aryl substituꢀ
ents is the 1ꢀnaphthyl moiety. The analog of
1N3PhOH with the 2ꢀnaphthyl moiety does not
posses this property [7].
On the basis of the structural analogy, we propose
that the adiabatic channel is also possible for the transꢀ
cis photoisomerization of 8SQ as it is for 4SQ. It is
documented that the photoisomerization of the cis
ꢀ
isomer 8SQ is complicated by the formation of dihyꢀ
drocyclo product, which was identified by the absorpꢀ
tion in the range of 400–450 nm [8]. It was pointed out
that 8SQ was prone to photocyclization in a higher
degree than 4SQ, but this reaction route was not studꢀ
ied in detail.
412

ADIABATIC transꢀcis PHOTOISOMERIZATION AND PHOTOCYCLIZATION
413
To reveal the mechanism of the 8SQ photoisomerꢀ and singleꢀquantum routes. The latter means that the
ization, we have used in this study the method based
on the examination of the photocyclization kinetics,
which can be applied for diarylethylenes entering in
this reaction and forming thermally stable dihydrocyꢀ
clo products. The photocyclization is the competitive
reaction relative to the photoisomerization, but the
mechanism of the photoisomerization, diabatic or
adiabatic, can be revealed from the photocyclization
kinetics [1]. This method was used for the first time for
the confirmation of the adiabatic mechanism of the
1N1N photoisomerization [4].
transꢀcis photoisomerization of 8SQ occurs at least
partially by the adiabatic mechanism (t* c*), when
the excited cisꢀisomer, which then undergoes cyclizaꢀ
tion, is formed from the excited transꢀisomer.
→
EXPERIMENTAL
8ꢀStyrylquinoline (transꢀisomer) was synthesized
according to the known procedure [8]. Electronic
absorption spectra of the transꢀisomers were measured
The method is based on the fact that the photocyꢀ on a Specord Mꢀ40 spectrophotometer in hexane. A
clization of diarylethylenes occurs in the excited sinꢀ
glet state of the cisꢀisomer [9, 10]. Therefore, the forꢀ
mation of dihydrocyclo product (DHCP) is a test on
the presence of the excited cisꢀisomer in the reaction
mixture. The formation of DHCP is readily observed
by the absorbance changes in the visible range,
because the absorption spectra of the photocyclization
products are shifted bathochromically as compared to
the spectra of the transꢀ and cisꢀisomers.
DRShꢀ500 mercury lamp was used as a source of the
UV light. The spectral line 313 nm was isolated by a set
of glass filters. Photochemical studies were carried out
at room temperature in airꢀsaturated solutions with
[8SQ] = (2–20)
optical path length of
sured by a PPꢀ1 cavity detector or a ferrioxalate actiꢀ
nometer was (3–10)
10–10 Einstein cm–2 s–1. The
×
10–5 mol/l in quartz cells with an
= 1 cm, the light intensity meaꢀ
l
×
quantum yields were calculated by digital solution of a
set of differential equations with a typical error of
20%. The following values of the extinction coeffiꢀ
cients were used: ε313 = 13600 and 5100 М–1 cm–1
When the photoisomerization occurs by adiabatic
mechanism
t +
hν → t*
→
c*
→
c,
for the transꢀ and cisꢀisomers, respectively, and ε437
=
the excited transꢀisomer (t*) isomerizes without
energy loss to the excited cisꢀisomer (c*), which then
can undergo cyclization to DHCP. If DHCP is formed
in a singleꢀquantum process directly upon excitation
of the transꢀisomer, the kinetics of DHCP formation
corresponds to the oneꢀstep reaction. This indicates
the intermediate participation of the excited cisꢀisoꢀ
mer in the reaction. This is possible only if the photoꢀ
isomerization proceeds adiabatically.
8000 М–1 cm–1 for dihydronaphthoquinoline (the
value measured for dihydrochrysene, the carbon anaꢀ
log, was used [11]).
Quantum chemical calculations of the 8SQ conꢀ
formers in the ground state (S0) were carried out by
DFT B3LYP method with a 6ꢀ31G** basis of a
GAUSSIAN program package [12]. The structures of
the compounds were calculated with complete geomꢀ
etry optimization. The correspondence of the optiꢀ
mized structure to the minimum on PES was verified
by the absence of the imaginary frequencies in the
vibrational spectrum.
When the photoisomerization occurs by diabatic
mechanism via a perpendicular conformer (p)
t +
hν → t*
→
p*
→
p
→ αc + (1 – α)t,
the cisꢀisomer in the ground state is formed from the
excited transꢀisomer and is not able to undergo
cyclization immediately (here
α
is the partitioning
RESULTS AND DISCUSSION
factor). In this case, the absorption of an extra light
quantum is necessary for cyclization. When transꢀisoꢀ
mer is a starting compound, the kinetics of the DHCP
formation should correspond to the twoꢀstep reaction,
The possible structural transformations of 8SQ
under irradiation are shown in Scheme 2. The comꢀ
plete reaction scheme involves three steps: reversible
and the induction period will be observed, which is transꢀcis photoisomerization with consequent reversꢀ
necessary for the accumulation of some quantity of the
cisꢀisomer in the reaction mixture.
ible photocyclization into primary DHCP, 10a,10bꢀ
dihydronaphtho[1,2ꢀh]quinoline (DHNQ) in the case
under study. The latter is oxidized irreversibly by the
oxygen of the air into 4ꢀazachrysene (naphtha[1,2ꢀ
h]quinoline, NQ). This compound has a characteristic
absorption spectrum with maximums 215 (28900),
235 (18070), 265 (67350), 296 (18460), 343 (2510),
The results obtained in this work confirmed the
assumption made above about the presence of the adiꢀ
abatic transꢀcis isomerization for 8SQ. The study of
the kinetics of the 8SQ photoisomerization shows
that, when the transꢀisomer is used as starting reagent,
the photoisomerization occurs by both twoꢀquantum and 361 nm (2510 М–1 cm–1) [13].
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BUDYKA et al.
H
[O2]
h
ν
h
ν
N
N
N
N
H
tꢀ8SQ
cꢀ8SQ
DHNQ
NQ
Scheme 2.
The formation of NQ in insignificant amount with with a maximum at 437 nm are observed. By analogy
a preparative yield of 2–4% was observed earlier as a with reactions of 4SQ [1], 1ꢀstyrylnaphthalene, and
byꢀproduct in the photocyclization of 1ꢀ(1ꢀnaphthyl)ꢀ other diarylethylenes [15], we attributed the longꢀ
2ꢀ(3ꢀpyridyl)ethylene, with 2ꢀazachrysene (preparaꢀ wavelength band to DHNQ, the primary photocyꢀ
tive yield of 50–60%) being the major product [13, clization product (Scheme 2).
14]. In the photocyclization of 8SQ, 4ꢀazachrysene
(NQ) is the only reaction product.
In the course of the following irradiation, a gradual
increase in the shortꢀwavelength band with a maxiꢀ
The spectral changes occurring during irradiation mum at 267 nm is observed (spectra 4–8) correspondꢀ
of the solution of the 8SQ transꢀisomer in hexane with ing to NQ, the final oxidized photocyclization prodꢀ
the light with
λ
= 313 nm are shown in Fig. 1. In the uct. The increase in the NQ band at 267 nm continued
initial stage of the photolysis (0–400 s, spectra 1–3), a also after scission of the irradiation, with an inversely
decrease in the absorption band of the transꢀisomer in proportional decrease in the DHNQ band at 437 nm
the range of 270–380 nm caused by the transꢀcis phoꢀ being observed (Fig. 2). Both the decrease in the band
toisomerization and a simultaneous increase in a weak at 437 nm and the increase in the band at 267 nm
band in the longꢀwavelength range of 400–450 nm occurred according to the exponential law, with the
A
A
0.09
A
3
1.4
2.5
0.06
0.03
0
1
1.2
1.0
2.0
1.5
1.0
0.5
0
2
1
0
500
1000
1500
t
, s
×
10
8
9
300
400
, nm
500
λ
ꢀ5
Fig. 1. Spectral changes of the airꢀsaturated solution of transꢀ8ꢀstyrylquinoline (9.64
×
10 M) in hexane in the course of irradiꢀ
ꢀ10
ꢀ2 ꢀ1
ation by the light with
520, ( ) 700, ( ) 900, (
and dilution. Inset: kinetics of the absorbance changes at the wavelength, nm: (
λ
= 313 nm and intensity of 3.5
) 1200, ( ) 1500, and ( ) spectrum after irradiation of the reaction mixture for 30 min, storage in the dark
) 313 ( ) 267 (left axis), and ( ) 437 (right axis).
×
10 Einstein cm c , photolysis time, s: (1) 0, (2) 160, (3) 320, (4)
5
6
7
8
9
1
2
3
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ADIABATIC transꢀcis PHOTOISOMERIZATION AND PHOTOCYCLIZATION
415
measured pseudoꢀfirstꢀorder rate constant being equal
to 2
10–4 s–1 and characterizing the oxidation of
A267
×
DHNQ into NQ.
The observed spectral changes agree well with a
sequence of the structural transformations of 8SQ
given in Scheme 2. The spectrum of the reaction mixꢀ
ture after longꢀterm irradiation and storage in the dark
for the completion of the thermal stage of the reaction
2.0
1.9
1.8
1.7
1.6
(Fig. 1, spectrum 9) is the additional evidence. The
final product has absorption bands, which correspond
by their position and intensity ratios to the absorption
spectrum of NQ (see above).
As assumed above, the kinetic scheme of the
DHNQ formation in addition to the twoꢀstep (twoꢀ
quantum) route (ϕtc, ϕcd) shown in Scheme 2 may
involve the reaction of the direct singleꢀquantum
cyclization of 8SQ into DHNQ with a quantum yield
of ϕtd, as shown in Scheme 3.
0
0.02
0.04
0.06
A437
0.08
0.10
Fig. 2. Proportionality between the decrease in absorbance
of DHNQ at = 437 nm and the increase in the absorꢀ
bance of NQ at = 267 nm in the oxidation of DHNQ into
ϕ
tc
λ
tꢀ8SQ
c
ꢀ8SQ
ϕ
λ
ct
NQ in the thermal study of the 8SQ photocyclization after
scission of irradiation of the reaction mixture.
ϕ
ϕ
td
cd
ϕ
dc
ko
DHNQ
NQ
The measured rate constant of the thermal stage is
2
×
10–4 s–1 corresponding to transformation halflife of
Scheme 3.
58 min. Thus, the DHNQ oxidation is relatively slow
reaction, which can be neglected in the initial stage of
the photocyclization. This is also confirmed by the
kinetic curve measured in the absorption maximum of
NQ (267 nm, Fig. 1 inset, curve 2), which shows the
absence of NQ in the reaction mixture in the initial
stage of the photolysis.
Taking into account all possible routes of the
DHNQ formation and consequent reactions, the
kinetics of the variations in the solution absorbance
(A) upon irradiation of the reaction mixture is deterꢀ
mined by a set of equations:
A = εt[tꢀ8SQ]l + εc[cꢀ8SQ]l
εDHNQ[DHNQ]l + εNQ[NQ]l,
(1)
It should be pointed out that, in the UV spectral
range, all components of the reactions mixture absorb,
and the kinetic curves measured in this spectral range
are the superposition of several curves. However,
+
A
d[tꢀ8SQ]/dt = (–ϕtc At + ϕct Ac) I
0(1–10– )/(A l), (2)
d[cꢀ8SQ]/dt = (ϕtAt – ϕctAc – ϕcdAc + ϕdcAd)
curve
1 (Fig. 1, inset) measured at the irradiation
(3)
× I0(1 – 10–A)/(A l),
wavelength (313 nm) in the initial stage of the photolꢀ
ysis characterizes mainly the kinetics of photoisomerꢀ
ization. In the visible range (437 nm), only DHNQ
d[DHNQ]/dt = (ϕcdAc – ϕdcAd + ϕtdAt)
(4)
absorbs, therefore curve
3 (Fig. 1, inset) reflects
× I0(1 – 10–A)/(A l) – ko[DHNQ],
directly the changes in the DHNQ concentration.
d
[NQ]/dt
=
k
o[DHNQ],
(5)
Thus, in the initial stage of photocyclization, which
is most interesting for the revealing the mechanism of
photoisomerization, the reaction mixture contains
three components, the transꢀ and cisꢀisomers and
DHNQ, and the set of Eqs. (1)–(5) may be restricted
by the reactions of photoisomerization and photocyꢀ
clization. Moreover, to simplify the system in the iniꢀ
tial stage of photoisomerization, the reverse photoꢀ
chemical reaction of the cycle opening can be
where Ai is the absorbance of the ith compound, εi
is its extinction coefficient (М–1 cm–1) at the irradiaꢀ
tion wavelength, I0 is the intensity of the light (milliEinꢀ
stein cm–2 s–1),
l
is the optical path length (cm) and ko
is the efficient rate constant of the DHNQ oxidation
s–1).
This set of equations cannot be solved analytically
(
or digitally, because involves both photochemical steps
occurring only during irradiation and the thermal step
occurring continuously. However, the set of Eqs. (1)–
(5) may be reduced taking into account experimental DHNQ formation measured at
results.
neglected, because the DHNQ absorbance
The initial portion of the kinetic curve for the
= 437 nm is shown
in Fig. 3. To minimize the contribution of the thermal
Ad Ӷ At, Ac.
λ
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416
A437
BUDYKA et al.
tocyclization for 4SQ was observed in ethanol, and the
change of the character of the photocyclization rate
curves on passing from hexane to ethanol was
accounted for by the change in the photoisomerizaꢀ
tion mechanism, from adiabatic in hexane to diabatic
in ethanol [1].
0.03
3
0.02
As follows from the obtained data, the transꢀcis
photoisomerization for 8SQ also proceeds partially by
adiabatic route, because the presence of the direct
1
0.01
0
cyclization
tꢀ8SQ
→
DHNQ indicates the intermediꢀ
2
ate participation of the excited cisꢀisomer (c*), which
is generated in the adiabatic reaction (t*
→
c*) from
the excited transꢀisomer.
The twoꢀquantum route of the DHNQ formation
upon excitation of the transꢀisomer of 8SQ indicates
the intermediate participation of the cisꢀisomer in the
ground state, and the absorption of the second light
quantum is necessary for its cyclization. Similarly to
the case of 4SQ, this may be accounted for by the fact
that a portion of the excited transꢀisomer isomerizes
0
200
400
, s
600
800
t
Fig. 3. Kinetics of the absorbance change at
the irradiation of the airꢀsaturated solution of transꢀ8SQ in
hexane by the light with = 313 nm. Points, experimental
data; fitting curves, calculated with consideration of ( ) a
singleꢀquantum route, ( ) twoꢀquantum route, and ( ) the
two routes of the DHNQ formation.
λ
= 437 nm in
λ
1
2
3
by the diabatic route (t*
→
p*
→
p
→ αc + (1 – )t).
α
However, taking into account the specific structure
of 8SQ, another explanation is possible for the twoꢀ
step mechanism of photoisomerization of this comꢀ
pound. It is known that the transꢀisomer of 8ꢀSQ exists
in the form of two conformers (rotamers), A and B,
due to hindered rotation around quasiꢀsingle bond
between the ethylene group and quinoline moiety
[18].
stage, each point was obtained in a separate experiꢀ
ment so that the overall time of the photolysis and regꢀ
istration of the spectrum takes less than 16 min for the
last point. The experimental points and three variaꢀ
tions of the approximating curves calculated on the
assumption of only singleꢀquantum route of the
DHNQ formation (ϕtd), only twoꢀquantum route with
the intermediate formation of the cisꢀisomer in the
ground state (ϕtc, ϕcd), and considering both mechaꢀ
nisms (Scheme 3) are given in Fig. 3. In the fitting proꢀ
cedure, the least rootꢀmeanꢀsquare error for the deterꢀ
N
N
H
H
H
H
mination of the absorbance
A
Δ
A
= (
Σ
(Acalc –
exp)2/m)1/2, where Acalc is the absorbance calculated by
digital integration of the set of Eqs. (1)–(4), Aexp is the
measured absorbance of the reaction mixture, and m is
a number of experimental points in the kinetic curve,
was minimized, with the values of the quantum yields
being the fitting parameters [16]. One can see in
Fig. 3, that the variation considering the both mechaꢀ
nisms of the DHNQ formation is optimal with the folꢀ
lowing values of the quantum yields of photoisomerꢀ
ization ϕtc =0.13 and ϕct = 0.17 and of photocyclizaꢀ
tion ϕtd = 0.009 and ϕcd = 0.23.
A
B
In the case of 1SN and 4SN, only one conformer A
exists in the solution, because conformer B is sterically
hindered and unstable due to repulsion of the hydroꢀ
gen atoms in the periꢀposition of the naphthalene
(quinoline) moiety and at the ꢀcarbon atom of the
β
ethylene group. The steric hindrance disappears in
8SQ, because the nitrogen atom is in the periꢀposition.
Therefore, it was assumed that conformers A and B
should be almost isoenergetic [18]. To estimate the relꢀ
ative stability of the two conformers, we carried out the
quantumꢀchemical calculations by the B3LYP/6ꢀ
31G** method.
The value ϕtc = 0.13 (hexane) corresponds to the
value ϕtc = 0.12 (cyclohexane) measured before [8].
The value ϕct = 0.20 (ethanol [8]) is known for the
back photoisomerization. This value is comparable
with that obtained in this study. The values ϕtc = 0.15
and ϕcd = 0.28 [17] are known for 1SN, the carbon
analog of 8SQ.
The calculation shows that conformer A is more
stable than conformer B by 1.01 kcal mol–1, and, if this
Thus, in comparison to 4SQ, for which ϕtd = 0.013 relationship is the same in solution, the content of the
[1], the reaction of the direct singleꢀquantum photoꢀ conformers in the equilibrium mixture is 85 and 15%,
cyclization of the transꢀisomer occurs with lower effiꢀ respectively. The calculation also shows that both conꢀ
ciency (ϕtd = 0.009), and the twoꢀquantum formation formers have planar structure and are stabilized with
of DHNQ contributes mainly. The twoꢀquantum phoꢀ the intramolecular hydrogen bond. The distance
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ADIABATIC transꢀcis PHOTOISOMERIZATION AND PHOTOCYCLIZATION
417
between the atoms of nitrogen and hydrogen at the
α
ꢀ
state was flattened with the absence of high barriers
and deep minimums at the t* c* path, and the
introduction of the naphthyl (quinolinyl) substituent
into the diarylethylene molecule assist this. To reveal
the peculiarities of the PES structure in the S1 states of
these compounds, the quantumꢀchemical calculaꢀ
tions of the photoisomerization paths are necessary.
→
carbon atom of the ethylene group in the fiveꢀmemꢀ
bered pseudoꢀcycle is 2.33 Å in conformer A, and the
distance between the atoms of nitrogen and hydrogen
at the ꢀcarbon atom of the ethylene group in the sixꢀ
β
membered pseudoꢀcycle is slightly smaller and equal
to 2.25 Å in conformer B. These two values are lower
than the sum of the van der Waals radii, which is equal
to 2.65 Å for the nitrogen and hydrogen atoms [19].
REFERENCES
Only conformer A from the two conformers of the
transꢀisomer can undergo cyclization into DHNQ
after photoisomerization into the cisꢀisomer, whereas
the cisꢀisomer of conformer B should first converts to
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)
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ꢀ
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isomer of conformer B occurs by the adiabatic mechꢀ
anism, the conversion of the cisꢀisomer of conformer
B into conformer A is necessary for the cyclization,
which is possible only after the loss of the excitation
and the transition into the S0 state. Then the cisꢀisoꢀ
mer of conformer A absorbs an addition light quantum
and undergoes cyclization to DHNQ. Hence, only a
fraction of the 8SQ transꢀisomer, which is in the form
of conformer A in the solution, is able to undergo the
cyclization into DHNQ after absorption of a single
light quantum. For the cyclization of conformer B, the
absorption of two light quanta is necessary in any case,
in diabatic and adiabatic photoisomerization.
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Thus, the study confirmed the assumption about
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tion of 8SQ made in the Introduction Section. When
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clization occurs by both twoꢀquantum and singleꢀ
quantum route. The latter indicates that the transꢀcis
photoisomerization of 8SQ at least partially occurs by
the adiabatic mechanism, when the excited transꢀisoꢀ
mer affords the cisꢀisomer also in the excited state.
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As was mentioned in the Introduction Section, the
back adiabatic transꢀcis isomerization is a relatively
rare event in comparison with the adiabatic cisꢀtrans
isomerization, and all known diarylethylenes entering
this reaction have 1ꢀnaphthyl moiety or its azaꢀanalog,
quinolinyl moiety, in their structure.
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The results obtained in this study confirm this
empiric regularity. It is clear that, for the occurrence of
the adiabatic transꢀcis photoisomerization, it is necesꢀ
sary that the potential energy surface of the excited
19. Steiner, T., Angew. Chem., Int. Ed. Engl., 2002, vol. 41,
p. 48.
20. Mazzucato, U. and Momicchioli, F., Chem. Rev., 1991,
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HIGH ENERGY CHEMISTRY
Vol. 44
No. 5
2010