Synthesis, photochemistry and photophysics of new butadiene derivatives: Influence of the fluoro, dimethylamino and nitro substituents on the molecular structure and photoinduced behavior

Article abstract of DOI:10.1016/j.jphotochem.2020.112690

Novel p-fluoro, p-dimethylamino and p-nitro substituted arylbutadiene derivatives were synthesized by Wittig reaction and their diverse photochemistry and photophysics were established and explained. Under excitation mono- and difluoro derivatives undergo

Full text of DOI:10.1016/j.jphotochem.2020.112690

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Synthesis, photochemistry and photophysics of new butadiene
derivatives: influence of the fluoro, dimethylamino and nitro substituents
on the molecular structure and photoinduced behavior
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Ana Grgicevic, Lajos Fodor, Danijela Baric, Margareta Poje, Zeljko
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Marinic, Otto Horvath, Irena Skoric
PII:
S1010-6030(20)30489-5
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112690
JPC 112690
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
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Please cite this article as: Grgicevic A, Fodor L, Baric D, Poje M, Marinic Z, Horvath O, Skoric
I, Synthesis, photochemistry and photophysics of new butadiene derivatives: influence of the
fluoro, dimethylamino and nitro substituents on the molecular structure and photoinduced
behavior, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112690
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Synthesis, photochemistry and photophysics of new butadiene derivatives:
influence of the fluoro, dimethylamino and nitro substituents on the molecular
structure and photoinduced behavior
Ana Grgičević,a§ Lajos Fodor,b§ Danijela Barić,c Margareta Poje,a Željko Marinić,d Ottó Horváth,b,*
horvath.otto@mk.uni-pannon.hu Irena Škorića,** iskoric@fkit.hr
aDepartment of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of
Zagreb, 10000 Zagreb, Croatia
bDepartment of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering,
University of Pannonia, P.O.B. 158, Veszprém H-8201, Hungary
cDivision of Physical Chemistry, Rudjer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia
dNMR Center, Rudjer Bošković Institute, Bijenička cesta 54, 10 000 Zagreb, Croatia
,
§These authors contributed equally.
Graphical abstract
1.4
1.2
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0
cis-1 EtOH
cis-1 hexane
trans-1 EtOH
trans-1 hexane
Adatsor5
0.8
0.6
0.4
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Adatsor4
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Highlights
1

.
new hexatriene systems with arylbutadiene unit(s), substituted with F, N(CH3)2 or NO2 group(s)
were synthesized
.
.
.
.
diverse photochemistry and photophysics of derivatives were established and explained
the primary photochemical step for each compound is a fast cis-trans isomerization
in some cases intramolecular cycloaddition gave photoproducts with bicyclo[3.2.1]-skeleton
computational investigation of energy profiles of Wittig reaction gave rationalization of
stereochemistry
Abstract
Novel p-fluoro, p-dimethylamino and p-nitro substituted arylbutadiene derivatives were
synthesized by Wittig reaction and their diverse photochemistry and photophysics were established
and explained. Under excitation mono- and difluoro derivatives undergo photoinduced
intramolecular cycloaddition. The parent mono-dimethylamino substituted derivative undergoes
the same reaction type giving the photocycloadduct in lower yield and with incomplete
consumption, whereas its bis(dimethylamino) analogue shows only geometrical isomerization due
to the steric effect of the substituents. Mono- and dinitro derivatives show photochemical
unreactivity. Conjugated analogues with similar structures display dramatically different
photoinduced behavior, revealing how changes of the nature of substituents impact their
photophysics and photochemistry. The computational investigation of the effects of the
conformational diversity within the steric influence of the starting geometric isomers on the
photoreaction pathway and energy profiles of the Wittig reaction is performed for the first time.
Key words: arylbutadiene, photochemistry, photophysics, substituent impact, Wittig reaction
Introduction
The synthesis and photochemistry of 1-(o-vinylphenyl)-4-phenylbutadiene derivative A and its
ω,ω’-diarylbutadiene analogue B (Figure 1, Ar = Ph), molecules that combine properties of both
butadiene and hexatriene systems, were examined and described for the first time by our group.[1]
That was the first approach to 4-substituted benzobicyclo-[3.2.1]octadienes as photoproducts from
arylbutadiene compounds. It is important to mention that the basic structure of these photoproducts
represents the unsaturated structural analogues of the bicyclo[3.2.1]octane skeleton which is found
in numerous biologically important active natural products.[2-5]
2

Our previous publications[6-13] on the photochemistry of different heteroaryl-substituted
hexatrienes showed interesting intramolecular cycloaddition reactions and the formation of
bicyclo[3.2.1]octadiene structures. By insertion of an additional double bond into the stilbene-like
moiety, we obtained the prolonged conjugated system of A and B (Figure 1) that allowed the
formation of new polycyclic structures C and D possessing the benzobicyclo[3.2.1]octadiene
skeleton with the double bond functionality for further transformations (Figure 1).[1,14-17]
Figure 1. Starting compounds A and B in E,E-configuration and their photoproducts C-F (Ar/Het
= phenyl, p-methylphenyl, p-chlorophenyl, p-methoxyphenyl, 2-furyl, 3-pyridyl, 4-pyridyl, 2-
thienyl, 3-thienyl).
The starting conjugated compounds A and B undergo intramolecular photocycloaddition reaction
to benzobicyclo-[3.2.1]octadiene C and D, respectively, giving only or preferably the endo-Ar/Het-
benzobicyclo[3.2.1]octadiene isomer due to the stereoselectivity of the photoreaction. Some of the
endo-Ar/Het-benzobicyclo[3.2.1]octadiene
derivatives
undergo
further
di--methane
rearrangement leading to endo-tricyclic structures E and F (Figure 1), or they can be used as a
suitable substrate for other transformations on the isolated double bond, easily derivatized to new
compounds with various functionalities. Depending on the aryl or heteroaryl substituent the yields
on the endo-Ar/Het-benzobicyclo[3.2.1]octadiene photoproducts like C (Figure 1) were moderate
to very good (Ar/Het = phenyl (90%), p-chlorophenyl (77%), p-methoxyphenyl (55%), 2-furyl
(57%), 3-pyridyl (44%), 4-pyridyl (51%), 2-thienyl (30%), 3-thienyl (37%).[1,15-17]
To get a deeper understanding of the photoinduced behavior of these conjugated systems, their
spectral characterization and detailed investigation of the photochemical and photophysical
behavior are examined. In these systems with prolonged conjugation, the influence of the
introduced p-substituted aryl or heteroaryl ring is examined on the possibility of the intramolecular
3

[2+2]-photocycloaddition followed by the formation of the benzobicyclo[3.2.1]octadiene
analogues. The possible steric hindrances probably cause deviations from planarity[18] and shift the
conformer equilibrium affecting the reaction pathways and yields. As several of the conformers
can react in the formation of intramolecular photoproducts, it is not expected that the
photoreactions take place always from the most populated conformer regardless of the
conformational distribution of the starting molecule. Although the conformers of each geometrical
isomer may display deviating photoinduced behavior, similarly to some 2-vinylbiphenyls,[19] 1,2-
dinaphthylethenes,[20] and 1-naphthyl-2-phenylethenes[21], investigation of the effects of the
conformational diversity within the steric influence of the starting geometric isomers on the
reaction pathway was not our aim in our former studies. In the study of pyridyl- and
thienylbutadiene derivatives[17] it is also aimed to determine, for the first time, the efficiency of the
intramolecular photocycloaddition reaction that takes place by utilizing chemical actinometry
measurements.
Figure 2. Structures of the investigated butadiene derivatives 1-6 in E,E-configuration.
In the continuation of the research, in this paper we describe our new study on photochemical and
especially photophysical behavior of the butadiene derivatives where p-fluorophenyl, p-
dimethylaminophenyl, and p-nitrophenyl rings were incorporated into the previously studied
framework in mono- or di-arylbutadiene analogue (Figure 2). The deep photophysical study is
performed to see the nature of photophysical processes and define competitive processes in the
excited states. We predict that various electronic and steric effects of one or two different functional
groups on the phenyl ring(s) in the molecule would provide a great influence on the molecular
structure and photoinduced behavior. Additionally, for the first time, we performed the
computational investigation of the effects of the conformational diversity within the steric influence
of the starting geometric isomers on the photoreaction pathway and energy profiles of the Wittig
reaction in the synthesis of 1-6.
4

Results and Discussion
Synthesis of butadiene derivatives 1-6
Mono-arylbutadiene derivatives 1-3 and di-arylbutadiene analogues 4-6 (Figure 2) were prepared
by the Wittig reaction from diphosphonium salt of ’-o-xylenedibromide and the corresponding
cinnamaldehydes. The synthesis of o-vinyl-butadienyl derivatives 1 and 2 was utilized for the first
time during the investigation of the benefits of flow photochemistry in comparison to the traditional
(batch) method of related conjugated systems with diversified targeted substituents and
intensification of the synthesis of benzobicyclo[3.2.l]octadiene skeleton.[22] The nitro derivative 3
was prepared at first for comparative characterization of the molecule by X-ray diffraction, analysis
of crystal packings, and determining the influence of weak interactions to the crystal structure.[23]
In both former works, the photophysical investigation of pure geometrical isomers of 1-3 was not
in the focus of the experiments, so these results here represent the first deeper photophysical study
of those mono-arylbutadienes 1-3 and additionally the new diarylbutadiene analogues 4-6 (Scheme
1).
Scheme 1. Synthesis of substituted di-arylbutadiene derivatives 4-6 by Wittig reaction.
The Wittig reaction of di-arylbutadiene derivatives 4-6 was conducted for the first time according
to the “one-pot” procedure described in our previous paper[24] and gave a mixture of isomers (4:
Z,Z : E,Z = 1:4, 5: E,Z : E,E = 1:2 and 6: Z,Z : E,Z = 1:1), in moderate to good yields (45-64%)
(Scheme 1).
The isomers of compounds 4-6 present in the reaction mixture were isolated by column and/or thin-
layer chromatography on silica gel and characterized by spectroscopic methods (see Experimental
section and SI, Figures S1-S17). It is interesting to emphasize that for all three derivatives, only
two geometrical isomers of three theoretically possible were detected and isolated. The nature of
the fluoro, dimethylamino and nitro substituent obviously influence the formation of the preferred
configuration and probably also the favourable conformation. To examine how the nature of
5

substituent present in the reactants influence the stereochemistry of Wittig’s reactions that lead to
compounds 1-6, we employed DFT calculations. Mechanism of Wittig’s reaction with
triphenylphosphonium ylide and cinnamaldehyde is shown in Scheme 2.[25]
The energy profile for this reaction, which we calculated employing Truhlar’s functional M06-
2X[26] combined with 6-31+G(d) basis set, using Gaussian09 programme package,[27] is presented
in Figure 3. The stationary points of reaction are localized at the potential energy surface (PES)
and vibrational analysis was performed to verify the minima and transition states on the PES for
all structures. The first step of the reaction, a formation of oxaphosphetane-1 (OP1), has the
activation energy of 10.4 and 12.1 kcal mol-1 (for E and Z path, respectively), which is in agreement
with previous studies.[25] This step determines the stereochemistry of the final product: transition
state (TS1) forms the complex containing four-center CCOP structure where double nucleophilic
addition (C to C and O to P) occurs, which leads to OP1. The inspection of the normal mode of
single imaginary vibration in TS1 shows the strong interaction between the C atom of the carbonyl
group and ylidic carbon, with a distance between these two atoms shortened to 2.0 and 1.9 Å for
trans and cis path, respectively. This is synchronized with the mutual approaching of oxygen and
phosphorus, with distance in TS1 being 3.2 and 3.6 Å (cis, trans). The described mode of vibration
corresponds to the formation of the four-membered ring (CCOP) in oxaphosphetane. The
frequency of imaginary vibration in TS1 is −222.3 cm−1 and −181.5 cm−1 for trans (E) and cis (Z)
path, respectively.
6

O
P(Ph)3
HC
CH
HC
CH
O
C
P(Ph)3
C
H
O
C
HC
CH
P(Ph)3
Reactants
H
C
H
(Cynnamalaldehyde + Ylide)
H
HC
CH
TS1
O
C
P(Ph)3
O
C
P(Ph)3
C
H
C
H
H
HC
CH
HC
CH
H
OP1
pseudorotation
TS-OP
O
P(Ph)3
O
C
P(Ph)3
C
C
C
H
H
HC
CH
H
H
HC
CH
OP2
P(Ph)3
P(Ph)3
O
C
O
C
C
H
C
HC
H
H
TS2
CH
HC
CH
H
Ph
Ph
P
O
HC
CH
CH
CH
Ph
E
Z
HC
CH
HC
Products
CH
Scheme 2. Mechanism of Wittig’s reaction. Left path leads to E-isomer, right path to Z.
In oxaphosphoetane-1, hydrogen at C of the carbonyl group and hydrogen at carbenium in ylide
are placed either at the opposing sides of CC bond – leading to E-product, or at the same side of
CC bond, which results in Z-isomer of product. Oxaphosphoetane-1 (where oxygen is in apical
position) then undergoes pseudorotation and yields oxaphosphetane-2 (OP2), with ylidic carbon
becoming apical. This step has been confirmed by the detection of oxaphosphetanes by low-
7

temperature 31P NMR during reactions of semistabilized ylides.[32] The transition state for
pseudorotation (TS-OP) has been located for both paths from Scheme 2, with the frequency of
imaginary mode being −29.3 cm−1 and −32.3 cm−1 for trans (E) and cis (Z) path, respectively. This
value is in agreement with previous computational studies of the mechanism of Wittig reaction.[25]
In both paths, the calculated barrier for the pseudorotation is very low, so it is clear that this step
does not play an important role in defining reactivity. Moreover, the mutual orientation of
hydrogens attached to carbon atoms in CCOP does not change, so this step does not affect the
stereoisomerism of the final products either. The final step is a cleavage of carbon-phosphorus
bond and formation of alkene and triphenylphosphine oxide, via transition structure TS2.
Figure 3. The free energy profile (in kcal mol-1) for the reaction presented at Scheme 2, obtained
at M06-2X/6-31+G(d) level of theory.
To predict how the presence of fluoro, dimethylamino, and nitro substituents influence the
stereoisomerism of di-arylbutadienes 4-6, we computationally investigated steps in the mechanism
which determine the stereochemistry of products (i.e. formation of OP1) for these compounds.
Each of these molecules contains two arylbutadiene units: first, mono-arylbutadiene is formed,
using ylide that was activated by deprotonation of -CH2 group (of -CH2PPh3) in the diphosphonium
salt of ’-xylenedibromide.
8

Scheme 3. Formation of oxaphosphetane-1, OP1.
After that Wittig reaction starts again by activation of the second ylide, by deprotonation of -CH2
group placed at the ortho-position relative to the first ylide. We computed the activation energy for
the formation of OP1 for the path that leads to the addition of the first arylbutadiene (Scheme 3),
to estimate the stereochemistry of mono-arylbutadienes. However, the stereochemistry of the final
products, di-arylbutadienes, depends on two factors: (i) isomerism of mono-arylbutadienes that has
been formed in the first stage, and (ii) on the energy barrier for the formation of oxaphosphetane-
1 in reaction path that leads to di-arylbutadiene (denoted here as OP1’). Therefore, the activation
energy for the formation of OP1’ is calculated, too (Scheme 4), and barriers for both processes are
compared to estimate the stereochemistry of final products.
Scheme 4. Formation of oxaphosphetane-1’, OP1’.
The results for R = F, N(CH3)2, and NO2, respectively, are presented in Figure 4. In reality, the
formation of TS1' does not follow directly after the formation of TS1; for the sake of convenience
and simplicity, these energy barriers are presented here in a way that enables their direct
comparison. Di-arylbutadiene can have E,E, E,Z, and Z,Z-configuration. E,E-product is a result of
the path that goes via TS1 which leads to E-monoarylbutadiene, which is then followed by the path
that goes via TS1' that gives E configuration of the second arylbutadiene unit. The situation is
analogous for the formation of Z,Z-isomers. However, E,Z-diarylbutadiene may be formed in two
9

ways: via TS1 that leads to E-monoarylbutadiene and followed by the path that gives Z-
configuration of the second arylbutadiene, and vice versa, via TS1 leading to Z-monoarylbutadiene,
followed by the process that involves TS1' which results in E-configuration of the second
arylbutadiene unit.
The relative barriers presented in Figure 4 show that for compound 4 (R = F) the formation of E-
monoarylbutadiene goes faster (10.5 vs. 13.1 kcal mol-1, similarly to unsubstituted aldehyde, Fig.
3), implying that diarylbutadienes possessing E,E- and E,Z-configuration are preferred as a final
products. However, the formation of E,E-isomer has the highest barrier (TS1’) of all (19.0 kcal
mol-1); therefore, in the second stage the occurence of E,Z-isomer is preferred. This prediction is
in agreement with the experimental results: among stereoisomers of 4, the molecule with E,E-
configuration is not present. Formation of isomer Z,Z-isomer that occurs within the products is due
to the lowest energy demand for the second stage, (i.e. for TS1’), however only 20% of compound
4 with Z,Z-configuration is present, due to the competition between E and Z path in the first stage,
when the formation of TS1 that belongs to E path is preferred. Analogous analysis for molecule 5
(R = N(CH3)2) shows that in the first stage of synthesis, the occurrence of E isomer is highly
preferred (12.9 vs. 17.8 kcal mol-1 for TS1 that leads to E and Z-monoarylbutadiene, respectively.
10

Figure 4. Direct comparison of calculated free energy barriers for the formation of OP1, and OP1’,
that lead to mono-arylbutadiene and diarylbutadiene, for molecules 4, 5, and 6.
Therefore, among the final products of 5, isomers E,E, and E,Z dominate, whereas the Z,Z is not
present at all despite the barrier for TS1’ being the lowest – there is no Z-monoarylbutadiene to
enable the formation of Z,Z-diarylbutadiene. In molecule 6, the barriers for E and Z are similar and
competitive (11.3 and 9.0 kcal mol-1), implying that both E and Z-monoarylbutadiene occur in the
first stage, but the formation of E,E-diarylbutadiene here is not probable, since the energy barrier
for this path is the highest (18.3 kcal mol-1). These predictions for molecules 5 and 6 are also in
agreement with experimental results.
To summarize these findings, for compounds 4 and 6, the first phase of synthesis gives both (Z and
E) stereoisomers of mono-arylbutadiene, so the stereochemistry of the final product is determined
by the second phase, during the formation of TS1’. However, for dimethylamino-substituted
compound 5, the stereochemistry of the final product is already defined in the first phase, when
path leading to E stereoisomer of mono-arylbutadiene is highly preferred.
Preparative photochemistry of compounds 1-6
The preparative irradiation experiments were performed in toluene solutions under anaerobic
conditions at 350 nm and low concentrations in the order of 10-3 M suitable for intramolecular
reactions. The systems were argon-saturated to avoid the photoinduced oxidation of the excited
molecules. Depending on the starting material and different substituents, various photochemical
routes were identified and confirmed. The formation of the photoproducts was generally
accompanied by the formation of some polymeric products which are not further investigated.
11

The photochemistry of o-vinyl-butadienyl derivatives 1-3 was utilized for the first time during the
investigation of flow photochemistry and targeted cycloaddition to benzobicyclo-[3.2.l]octadiene
skeleton[22] but no attempt was made to investigate the deeper photochemical or photophysical
behavior of isolated isomers and/or deactivation processes.
Utilizing the stereoselective intramolecular cycloaddition of 1-6 by flow photochemical approach,
we obtained the desired structures of the photoproducts (Figure 5) in the case of starting mono-p-
fluorophenyl-butadiene 1 (87% isolated yield of endo-7) and di-p-fluorophenyl-dibutadiene 4
(36% isolated yield of exo,endo,trans-9) both with complete consumption of starting material,
respectively, and in 24% of isolated yield for endo-8 in the case of starting substrate mono-p-
dimethylamino derivative 2, with 60% of consumption. Obviously, the tertiary amino group in the
photoproduct endo-8 (in comparison to photocycloadducts endo-7 and exo,endo,trans-9 with one
or two fluoro substituent(s)) contributes to the lower stability possibly through the lowering of the
intensity of  interactions by competitive electronic effects. Evidently, the strong electronic
effects of the p-dimethylamino in the starting substrate 2 during the photocycloaddition reaction
also bring to the incomplete consumption of the starting material. With this double effect, the result
of photochemistry of the derivative 5 with two p-dimethylamino groups in the starting structure
did not give any photoproduct. Starting compound 4 gave also one minor polycyclic photoproduct
endo,trans-10 in 18% of isolated yield (Figure 5). In the case of compounds 1 and 2, only in traces,
tricyclic structure like E (Figure 1) can be detected from the NMR spectra of the photomixture due
to characteristic signals in the aliphatic region. Nitro derivatives 3 and 6 show photochemical
unreactivity and no formation of any cycloadduct.
Figure 5. Structure of fluoro- and dimethylamino-substituted photoproducts endo-7, endo-8,
exo,endo,trans-9 and endo,trans-10 with proton assignments (A-G or A-H).
In the case of 4, intramolecular cycloaddition reaction is the favorable photochemical path giving,
besides exo,endo,trans-9, the other stereoisomer endo,endo-9 in traces. The formation of 9 is
12

explained by the same mechanism (Scheme 5) as described for related derivatives in one of our
previous papers[1,14,22] from the preferred conformation of trans,trans-4 via a 1,4-biradical 11
followed by the preferred cyclohexene ring closure to product 9.
Scheme 5. Proposed mechanism for the formation of the benzobicyclo-[3.2.1]octadiene structure
endo,endo,trans-9 from p-fluoro substituted derivative 4.
The preferred ring closure with endo-orientation of one p-fluoro-phenyl group can be ascribed to
the stabilization of the transition state by the strong attractive intramolecular  interactions of
the benzo-phenyl groups.[28-30]
The stereoselective formation of exactly the exo,endo,trans-9 with the exo-orientation of the
unreacted styryl group can be explained (Scheme 5) by preferred trans ring closure to the indane
intermediate 11, presumably because of steric reasons, which then closes to yield exo,endo,trans-
9. The formation of the endo,endo-isomer, found only in traces, was explained by cis ring closure
to the indane intermediate 11'. The proposed mechanism (Scheme 2) suggests the formation of the
intermediate 11 from the trans,trans-4, which is in accordance with the photophysics showing that
the all-trans configuration of 4 contributes the most to the relaxation via photochemical and internal
conversion processes. After the complete conversion, the exact main photoproduct exo,endo,trans-
13

9 was separated and isolated by column chromatography on silica gel, and its structure was deduced
unequivocally from spectral studies (see Experimental section and SI, Figures S18-S23). The
formation of benzotricyclic compound endo,trans-10 from the starting compound 4 at 350 nm
could be explained (Scheme 6) only by initial sixmembered-ring-closure via 12 and consequent
thermal [4+2]-cycloaddition and re-aromatisation. The preferred endo-orientation of one p-fluoro-
phenyl group and trans-configuration on the double bond are explained and confirmed as in the
case of exo,endo,trans-9.
Scheme 6. Proposed mechanism for the formation of the tricyclo structure endo,trans-10 from p-
fluoro-substituted derivative 4.
From the NMR spectra of exo,endo,trans-9, using additional techniques (COSY and NOESY) the
structure was completely assigned. It can be seen the resolved four-proton pattern in the 1H NMR
spectrum (Figure 6) between 3.2 and 4.1 ppm unmistakably points to the bicyclo[3.2.1]octadiene
structure. The exo-orientation of the styryl at the methano bridge carbon is defined on the basis of
COSY interaction between protons F and G (Figure 6).
14

HG
HD
HAr
HB
HE
HF
HA
HC
HD
HE
HAr
HF
HA
HC
HG
HB
PPM
6.2
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
Figure 6. The crucial part of the 1H NMR spectra of endo-7[22]; and exo,endo,trans-9 (CDCl3, 300
MHz).
In previously obtained bicyclo[3.2.1]octadiene derivatives[1,22] with no substituent on the methano
bridge carbon, the proton F couples only with the exo-oriented proton G. As the endo-oriented
proton does not couple with the proton F, the exo-structure was assigned. The trans-configuration
on the styryl substituent on the methano bridge is confirmed by the existence of two coupling
constants with 16.1 Hz for protons G and H (see Experimental section and SI, Figures S21-S22).
The signals in the lower field between 5.3 and 6.5 ppm are assigned to the A and B protons on the
isolated double bond. One of the aromatic protons, Har of the benzo moiety is shifted to the higher
field at 6.2 ppm due to the anisotropic effect of phenyl endo-substituent.
15

Scheme 7. Possible conformations of E-isomers of 1-3 and E,E-isomers of 4-6.
Conformational analysis of compounds 1-6 is performed using the M06-2X/6-31G(d,p) level of
theory. Conformers of E-isomers of 1-3 and E,E-isomers of 4-6 (that further go to the productive
photochemical reactions) are shown at Scheme 7. Figure 7 shows the relative stabilities of
conformers for molecules 1-6. Relative stability G298 (in kcal mol-1) is obtained as a difference
between computed Gibbs energy of each examined conformer and the most stable one found for
each compound 1-6.
For all systems, the most stable conformation is the one denoted as g (Scheme 7), which was
expected, because it is clear that this structure has the least steric hindrance (Figure 8). The overall
conclusion is that there is no big diversity in relative stabilities of conformers within compounds
with different substituents: for both series, mono- (1-3) and di-arylbutadienes (4-6), the sequence
of first three most stable conformations is the same (g, f, c), with the difference in stability not
larger than 2 kcal mol-1. The least stable conformation in all systems is conformer denoted as a,
with G values going from 6.4 to 8.1 kcal mol-1 (see SI, Table S1). The remaining four conformers
(b, d, e, h) in all molecules have relatively close values of energy, with deviation from the most
stable conformer between 2.5 and 4.8 kcal mol-1.
Although butadiene derivatives 3 and 6, which contain nitro substituent, do not undergo
photochemical synthesis, their conformations were analyzed to see if the electron-withdrawing
16

group such as –NO2 has any effect to the relative stabilities of conformers compared to derivatives
with electron-donating substituent (dimethylamino group).
Figure 7. Stabilities of conformers of molecules 1-3 (left) and 4-6 (right), calculated as Gibbs
energies relative to the most stable conformer for each compound (in kcal mol-1
The electron-inductive effect of substituents placed in the p-position of phenyl in studied systems
did not make any significant influence on the energy distribution of conformers of isomers that are
relevant for further photochemical synthesis.
Figure 8. The structures of the most stable conformer for molecules 1-6, optimized at M06-2X/6-
31G(d,p) level of theory.
17

Absorption and emission spectra of compounds with one butadiene chain 1-3
Absorption spectra of 1-3. The UV spectra of separated cis- and trans-1, 2, and 3, respectively
(Figures 9-11), clearly indicate that the absorption characteristics considerably depends on the (cis-
trans) configuration, on the substituents in para position on the phenyl group, and (in the case of
the nitro-substituted derivatives) on the solvent as well. Interestingly, in the case of 1 (with fluoro
substituent), the spectra in ethanol well agree with those of the corresponding unsubstituted
analogues.[1] This phenomenon suggests that, despite the high electronegativity of fluorine, it does
not influence the energy levels of the electronic states involved in the UV spectra of these
compounds. It may be the result of two opposite effects counterbalancing each other. Accordingly,
mesomer (resonance) and inductive effects may result in such an extinction. Notably, the spectra
of the corresponding compounds having thiophene instead of phenyl group at the end of the
butadiene chain[17] display spectra in ethanol with characteristics very similar to those of the phenyl
derivatives.
1.4
1.2
1
800
700
600
500
400
300
200
100
0
cis-1 EtOH
cis-1 hexane
trans-1 EtOH
trans-1 hexane
Adatsor5
Adatsor4
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
500
Wavelength / nm
Figure 9. Absorption and emission spectra of the isomers of compound 1 in EtOH (red) and n-
hexane.
1.4
1.2
1
1200
1000
800
600
400
200
0
cis-2 EtOH
cis-2 hexane
trans-2 EtOH
trans-2 hexane
Adatsor5
Adatsor4
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
500
550
Wavelength / nm
Figure 10. Absorption and emission spectra of the isomers of compound 2 in EtOH (red) and n-
hexane.
18

1.6
1.4
1.2
1
7
6
5
4
3
2
1
0
cis-3 EtOH
cis-3 hexane
trans-3 EtOH
trans-3 hexane
Adatsor5
UUUU
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
500
550
Wavelength / nm
Figure 11. Absorption and emission spectra of the isomers of compound 3 in EtOH (red) and n-
hexane.
Similarly to the unsubstituted phenyl derivatives,[1] the cis isomer of 1 displays a characteristic
wide band at 306-308 nm in both ethanol and n-hexane, while the corresponding trans species
exhibits a double band with maxima at 322 and 335 nm. This deviation may be attributed to the
structural difference affecting the conjugation of the double bonds. The cis arrangement is less
favorable in this respect, hence the weaker conjugation results in a higher energy difference
between the ( and *) orbitals involved in the corresponding transitions. Besides, the double band
indicates the appearance of the vibrational fine structure of the electronic transition, which is
generally the consequence of a weaker interaction between the solvent and the dissolved
compound. Accordingly, the trans configuration of 1 decreases the interaction with both of these
solvents. In the case of the nitro-substituted derivatives, both cis- and trans-3, the appearance of
the vibrational fine structure in n-hexane suggests that nitro group results in a relatively high dipole
moment in the excited state. Hence, the much stronger interaction with a more polar solvent such
as ethanol completely vanishes the fine structure of the absorption band as also observed earlier in
the case of nitro-substituted 2,3-distyrylfurans.[31] The least influence of the cis-trans configuration
on the absorption spectrum was observed for the dimethylamino-substituted derivatives, cis- and
trans-2. Their spectra were very similar, displaying a strong band at about 364-367 nm in ethanol,
without any vibrational fine structure, which may be attributed to the strong interaction via
hydrogen bonds between EtOH and the amino groups. Only a very moderate red shift could be
detected in the case of the trans configuration compared to the cis one (360 nm vs. 364 nm). This
suggests a stronger conjugation of the double bonds in the former isomer, which is in accordance
with the observation at the fluoro derivatives (cis- and trans-1). In n-hexane, both isomers
displayed a slight blue shift of this band compared to its wavelength in ethanol. This phenomenon
19

suggests a moderate increase of the dipole moment in the excited state compared to the ground
state.
Emission spectra of 1-3. Some characteristic features of the emission spectra (also accompanied
by the excitation spectra corresponding to the absorption spectra) will be shown here. Besides,
what more important is, generally, the quantum efficiency of the emission (fluorescence) is orders
of magnitude lower than the primary photochemical quantum yield for each compound studied, in
accordance with the high efficiency of the latter one (see in Table 1). Since the preliminary
experiments indicated that EtOH as solvent displayed own emissions very disturbing those of the
butadiene derivatives, regarding both the spectral characteristics and the lifetime of the their
luminescence, only solutions in n-hexane were used for these measurements. It is was reasonable
for the comparisons with the luminescence properties of the analogous compounds with double
butadiene structure, due to their very limited solubility in EtOH. Regarding the lifetime of the
emissions displayed by all compounds studied, it did not exceed 0.1 ns, the lower limit for our
equipment. This observation, on the basis of the results regarding compounds with similar
structures, predicted rather low quantum efficiencies for the butadiene derivatives studied in this
work.
Upon excitation at 313 nm, cis-1 displayed characteristic fluorescence with the main band at 399
nm, merged with a less intense one at 384 nm and a shoulder at 363 nm (Figure 9). The emission
spectrum of the corresponding trans isomer displayed a much better resolved fine structure, in
which the shorter-wavelength shoulder in the previous case became a distinct band at 363 nm,
while the most intense peak arose at 379 nm. This phenomenon is in a good accordance with the
characteristics of the corresponding absorption spectra, due to the stronger conjugation and weaker
interaction with the solvent in the case of the trans configuration.
The resolutions of the corresponding bands in the emission spectra of the cis and trans isomers of
the dimethlyamine-substituted derivatives (2) are more similar (Figure 10) than for those with
fluoro groups (1). For cis-2, the main band arose at 437 nm, merged with a less intense one at 412
nm and a shoulder at 462 nm. The corresponding wavelengths for trans-2 are 433, 411, and 459
nm. These values indicate moderate blue shifts in each cases. This phenomenon, suggests that the
structure of the trans isomer, due to the enhanced conjugation, is more rigid as confirmed by the
moderately lower Stokes-shift compared to that of the cis configuration (337 cm-1 vs. 374 cm-1).
20

For the nitro-substituted cis-3 isomer, two separated bands, at 412 and 435 nm, can be observed in
the emission spectrum, along with a shoulder at 463 nm (Figure 11). Compared to the spectrum of
the corresponding fluoro derivative (cis-1), a significant red shift can be observed at each
wavelength, while the spectrum of the dimethlyamino analogue (cis-2) is very similar to that of
cis-3. These relations can be accounted for the electronic effects of these substituents.
Unfortunately, the emission of trans-3 in n-hexane was extremely weak, hence its spectrum could
not be reliably characterized.
Photolysis of monoarylbutadienyl derivatives 1-3
The photolysis of cis- and trans-1, 2, and 3 compounds at wavelengths corresponding to the main
absorption bands (313 nm for 1 and 365 nm for 2 and 3) generally resulted in a rather fast
isomerization as the primary photochemical reaction. In this respect, the fluoro-substituted
derivatives displayed the most characteristic spectral changes in ethanol (Figures 12a and 13a).
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.2
1
a
b
0.8
0.6
0.4
0.2
0
200
250
300
350
400
200
250
300
350
400
Wavelength / nm
Wavelength / nm
Figure 12. Spectral change during the irradiation of cis-1 in EtOH (a) after 0 (red), 105, 525, 2685,
9285, 12885 s; and in n-hexane (b) after 0 (red), 40, 80, 120, 160, 220, 520 s (λir = 313 nm, l = 1
cm).
No matter, which isomer is the starting compound of the photolysis, the resulting spectrum of the
product(s) of the primary photochemical reaction was the same, and it unambiguously differed
from those of both starting compounds. It is red-shifted compared to the main band of cis -1, while
blue-shifted in relation with that of the trans-1 isomer, displaying the maximum at about 321 nm.
Besides, its characteristics indicate a slight vibrational fine structure by the appearance of shoulders
on both sides. It suggests that its interaction with ethanol is weaker than that of the cis-isomer, but
21

stronger than that of the trans one. From the four isomers of 1, both starting compounds (cis- and
trans-1) contain trans configuration at the double bond connecting to the fluoro-substituted phenyl
group. Hence, as the other two isomers (containing cis-configuration at that double bond) could
come into consideration. Theoretically, both could be formed upon excitation, however, as it was
discussed in the interpretation of the absorption spectra, cis configuration is less favorable for the
conjugation, increasing the energy difference between the ( and *) orbitals involved in the
corresponding transition. Hence, the main band of this isomer would display a blue shift compared
to that of cis-1. Thus, the other possible isomer is a more reasonable candidate as a product of the
photoisomerization in the respects of both the position and the fine structure of the main band.
Notably, the corresponding unsubstituted analogues behaved very similarly upon irradiation in
ethanol,[1] indicating that not only the spectra but the primary photoreactions of these two types of
derivatives agree very well.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.4
1.2
1
a
b
0.8
0.6
0.4
0.2
0
200
250
300
350
400
200
250
300
350
400
Wavelength / nm
Wavelength / nm
Figure 13. Spectral change during the irradiation of trans-1 in EtOH (a) after 0 (red), 20, 80, 180,
320, 640, 1060, 4000 s; and in n-hexane (b) after 0 (red), 30, 70, 130, 210, 570 s (λir = 313 nm, l =
1 cm).
The photoisomerization took place within less than 10 minutes for cis-1 in ethanol (Figure 12a),
and its quantum yield determined from the initial rate of the reaction was rather high (0.26). It is in
accordance with the very low value of the fluorescence quantum efficiency (see above).
Nevertheless, competitive photophysical (or –chemical) processes may take place such as internal
conversion, intersystem crossing, or another transformation.
After the isomerization, the excitation of the photoisomer led to a rather fast transformation
accompanied by the disappearance of the main (longer-wavelength) band at 321 nm, indicating
22

the ceasing of the conjugation. This is in accordance with the result of the preparative
photochemistry. The quantum yield of this transition in ethanol was rather low (0.015). Notably,
in n-hexane the corresponding quantum yields were considerably higher than those in ethanol; 0.94
for the isomerization and 0.43 for the transformation. These are in accordance with the 87% yield
of the endo-7 photoproduct at complete consumption of the starting compound in the preparative
photolysis.
The spectral changes of cis- and trans-1 during the photolyses in n-hexane are shown in Figures
12b and 13b, respectively. The corresponding quantum yields are involved in Table 1. The higher
quantum yields in n-hexane indicate that the intermediate and the final product are better stabilized
in this apolar solvent than in ethanol.
Table 1. Absorption and emission parameters as well as photochemical quantum yields of
butadiene derivatives at room temperature.
compound (solvent)
λabs, nma
λF, nma
Stokes-shift
cm-1
ΦF (λexc, nm)b
Φiso (λir, nm)b
Φp (λir, nm)b
cis-1 (EtOH)
(n-hexane)
trans-1 (EtOH)
306
308
322, 335
322, 333
0.262 (313)
0.939 (313)
0.743 (313)
0.015 (313)
0.426 (313)
0.076 (313)
<0.85 (313)
363sh, 384, 399
363, 379, 399
412, 437, 462sh
411, 433, 459sh
492
248
374
337
0.028 (313)
0.111 (335)
0.076 (367)
0.103 (367)
(n-hexane)
≈0.85(313)
<1 (366)
0.914 (366)
0.885 (366)
0.916 (366)
<1 (366)
cis-2 (EtOH)
(n-hexane)
trans-2 (EtOH)
(n-hexane)
360
357
364
361
378
0.020 (366)
0.095 (366)
0.011 (366)
0.049 (366)
0.0022
cis-3 (EtOH)
(366)
0.0051
(366)
(n-hexane)
trans-3 (EtOH)
(n-hexane)
351
412, 435, 463sh
422
0.0011 (360)
<1 (366)
378
0.034 (366)
<1 (366)
0.0006
(366)
361, 376, 399sh
125
0.0001 (360)
0.0013
(366)
≈ 420 (very weak)
c
c
cis,cis-4 (n-hexane)
cis,trans-4 (n-hexane)
290, 327
355, 390d, 411sh
361sh, 385, 405, 426sh
437sh, 462
288
145
0.0063 (324)
0.0043 (353)
-
-
-
c
c
-
cis,trans-5 (n-hexane)
trans,trans-5 (n-hexane)
cis,cis-6 (n-hexane)
356, 382
354, 383
306, 340
437, 463, 490sh
435, 463
329
312
163
0.0077 (352)
0.0049 (355)
0.0018 (313)
0.890 (366)
0.736 (366)
0.125 (336)
0.0022
(366)
0.0027
(366)
0.0049
(366)
360, 376, 394
aThe main maximum is underlined. bStandard deviation ±5%. cNot measurable. dApproached by the excitation spectrum.
Also the primary photoisomerization of the dimethylamine-substituted derivatives (cis- and trans-
2) was very fast (Figure 14 and 15), the quantum yield for this step was close to unity for these
isomers in both EtOH and n-hexane. The isomerization was accompanied by a slight red shift in
23

the case of cis-2 in both solvents, while no shift of the main band occurred for the trans isomer,
leading to the formation of a common photoisomerization product from both starting compounds.
1.2
b
1
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
Wavelength / nm
Figure 14. Spectral change during the irradiation of cis-2 in EtOH (a) after 0 (red), 30, 570, 1230,
1890, 2670, 3990, 5670 s; and in n-hexane (b) after 0 (red), 30, 180, 390, 570, 930, 1350, 2370 s
(λir = 366 nm, l = 1 cm).
1.4
1.2
1
1.2
1
b
a
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
200
250
300
350
400
450
Wavelength / nm
Wavelength / nm
Figure 15. Spectral change during the irradiation of trans-2 in EtOH (a) after 0 (red), 30, 690,
1770, 3090, 4530, 9210 s; and in n-hexane (b) after 0 (red), 40, 300, 700, 1240, 2200, 4720 s (λir =
366 nm, l = 1 cm).
This, similarly to the case of the fluoro-substituted derivatives, contains cis configuration regarding
the double bond closer to the dimethylamine-substituted phenyl group. (Notably, the spectrum of
this isomer was very similar to that of cis-2, but the fast change of the absorption in the first minute
of the irradiation clearly indicated the photoisomerization.) However, the transformation of the
photoisomer was considerably slower than in the case of 1. The quantum yields in n-hexane were
in the range of 0.05-0.10. (In EtOH, similarly to the fluoro-substituted derivatives, the quantum
24

yields for the transition of the photoisomer(s) were much lower than those in n-hexane, in the range
of 0.011-0.020.) This lower reactivity of the dimethylamine-substituted derivatives may be
attributed to the enhanced stability due to the conjugation strengthened by the N(CH3)2 group, from
which the donation of the lone pair is promoted by the two alkyl groups. This diminished reactivity
is in accordance with the lower yield (36%) of the endo-8 photoproduct in the preparative
photolysis with the incomplete (60%) conversion of the starting compound.
1.2
1
1.4
1.2
1
b
a
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
200
250
300
350
400
450
500
Wavelength / nm
Wavelength / nm
Figure 16. Spectral change during the irradiation of cis-3 in EtOH (a) after 0 (red), 450, 8550 s;
and in n-hexane (b) after 0 (red), 30, 2970, 6150, 9030, 13500 s (λir = 366 nm, l = 1 cm).
1.6
1.4
1.2
1
1.2
1
a
b
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0
0
200
250
300
350
400
450
500 200
250
300
350
400
450
Wavelength / nm
Wavelength / nm
Figure 17. Spectral change during the irradiation of trans-3 in EtOH (a) after 0 (red), 570, 7230 s;
and in n-hexane (b) after 0 (red), 90, 7470 s (λir = 366 nm, l = 1 cm).
The nitro-substituted derivatives (cis- and trans-3) behaved similarly to the dimethylamine-
substituted analogues, but with an even lower reactivity after photoisomerization (Figure 16 and
17). In n-hexane, the photoisomerization is very fast for both starting isomers of 3, taking mostly
place in the first one minute (see Figure S24), with a quantum yield about unity (Table 1).
25

Subsequently, the transition of the photo-isomer(s) was extremely slow, with quantum yields in the
range of 0.001-0.006 (one order of magnitude lower than those for the dimethylamine-substituted
derivatives). In EtOH the photoinduced behavior of cis-3 well agreed with that in n-hexane, with
about the same quantum yields. Interestingly, in the case of trans-3 the primary photoisomerization
step was rather slow (with a quantum yield of 0.034), hence no breaking point could be observed
in the absorption change at the transition from the isomerization to transformation (Figure S25).
The very low quantum yield of the latter reaction for the nitro derivatives is in good accordance
with the lack of any photocycloadduct in the preparative photolysis. This unreactivity may be
attributed to the strong electron-withdrawing effect of the NO2 group. This substituent in para-
position considerably decreases the electron density on the conjugated double bond system,
reducing the chance for the photocycloaddition, the determining reaction in the preparative
photolysis. A similar phenomenon was observed for the nitro-substituted 2,3-distyrylfurans.[29]
Absorption and emission spectra of compounds with two butadiene chains 4-6
As to the compounds with two butadiene chains, their solubilities in n-hexane, and especially in
EtOH are rather limited. Hence, they were only studied in the less polar solvent. Even so, one of
the six compounds proved to be insoluble in n-hexane, and two others could only be dissolved in
a very low concentration, which made luminescence measurements possible. These concentrations
were not enough for reliable photolysis experiments (with quantum yield determinations), but the
characteristic spectral changes upon irradiations could be detected.
Absorption spectra of 4-6
Unfortunately, the fluoro derivatives of the compounds with two butadiene chains (cis,cis- and
cis,trans-4) could only be dissolved in very low concentrations. Hence, the absorption spectrum of
the cis,cis-configuration is a bit uncertain, while in the case of the cis,trans isomer-only the
excitation spectrum can be used for the approach of the absorption characteristics. Nevertheless,
also these possibilities could be used for the detection of the effects of the substituent and
configuration.
The absorption spectrum of the cis,cis-4 isomer displays a strong band at 290 nm, partly merged
with another band at about 327 nm (Figure 18). The excitation spectrum of the cis,trans-isomer
also displays two bands in the 250-400-nm range (at 359 and 397 nm), but more separated than in
26

the case of the cis,cis-configuration. These values clearly indicate that the energies for the
corresponding transitions in the trans-configuration are significantly lower than those in the cis
one. A similar phenomenon was observed for the derivatives with one butadiene chain and can be
attributed to the stronger conjugation in the trans conformation, leading also to a more planar
structure.
In the case of the dimethylamine-substituted derivatives (5), the cis,trans- and trans,trans-isomers
were investigated. Similarly to the analogues with one butadiene chain, the configuration hardly
affects the absorption characteristics. Both isomers display two bands; at about 354-356 nm and
382-383 nm (Figure 19). This phenomenon indicates that the dimethylamine substituents, due to
their efficient lone pair donating feature, so strongly stabilize the electronic structure of the
conjugated system that not even the configurational change can significantly influence the energy
differences of the states involved the corresponding transitions.
This is in accordance with the low reactivity for photo-cycloaddition (see later regarding the
photolysis). Compared to the corresponding absorption bands of the unsubstituted analogs,[14] those
of both the cis,trans- and the trans,trans-isomers display a significant red shift. This phenomenon
is in accordance with the enhanced conjugation, due to the lone pair donating effect of the
dimethylamine substituents.
Deviating from the cis,trans-isomer of the nitro substituted derivatives (6), which could not be
dissolved in n-hexane at all, the solubility of the cis,cis-configuration was suitable enough for both
spectral analysis and photolysis. Its absorption spectrum displays two distinct bands; at 306 and
340 nm (Figure 20). These indicate a moderate blue shifts, compared to the corresponding peaks
of the unsubstituted analogue.[14] This phenomenon may be attributed to the electron-withdrawing
effect of the nitro groups, decreasing the conjugation and, thus, increasing the energy differences
between the states involved in the corresponding transition.
27

0.4
0.35
0.3
20
18
16
14
12
10
8
cis,cis-4
cis,trans-4
Adatsor5
Adatsor4
0.25
0.2
0.15
0.1
6
4
0.05
0
2
0
250
300
350
400
450
500
550
600
Wavelength / nm
Figure 18. Absorption and emission spectra of the isomers of compound 4 in n-hexane. (Due to
the low solubility, the absorption spectrum of cis,trans-4 is approached by its excitation spectrum.)
1.2
25
20
15
10
5
cis,trans-5
trans,trans-5
Adatsor5
1
Adatsor4
0.8
0.6
0.4
0.2
0
0
250
300
350
400
450
500
550
600
Wavelength / nm
Figure 19. Absorption and emission spectra of the isomers of compound 5 in n-hexane.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
6
5
4
3
2
1
0
cis,cis-6
Adatsor5
250
300
350
400
450
500
Wavelength / nm
Figure 20. Absorption and emission spectra of cis,cis-6 in n-hexane.
28

Emission spectra of 4-6
Generally, the compounds with double butadiene structure displayed less intense emissions than
the one-chain analogues. This phenomenon can be attributed to the higher probability of the
relaxation of the excited molecules via vibrational routes.
Interestingly, deviating from the analog with one butadiene chain (cis-1), the fluoro-substituted
derivative (cis,cis-4) displayed separated bands with 385 and 405 nm wavelength (Figure 18). (The
intensity of the latter one was a bit higher.) The emission spectrum of the cis,trans-4 isomer,
however, is characterized by a single band at 462 nm, along with a shoulder at 437 nm). These
energies clearly indicate, that the trans configuration results in red shifts compared to the bands of
the corresponding cis isomer, probably due to the stronger conjugation. Besides, the structure of
the trans configuration is more rigid as indicated by its much lower Stokes-shift (2147 cm-1 vs.
4940 cm-1).
The cis,trans-isomer of the dimethylamine-substituted derivative (5) showed a strong band at 463
nm, with a less intense one at 437 nm and a shoulder at 490 nm (Figure 19). The fluorescence of
the trans,trans-isomer was much weaker with a less resolved spectrum having the main band at
463 nm, strongly merged with another one at 435 nm. In accordance with the fluoro derivatives,
the trans configuration (in the case of 4, the all-trans one) resulted in, even if a slight, red shift of
the corresponding bands, due to the stronger conjugation and more rigid structure (the Stokes-shifts
are 3311 cm-1 and 3211 cm-1 for the cis,trans-and trans,trans-isomer, respectively).
The cis,cis-6 isomer, the only nitro derivative soluble in n-hexane, displayed a well-resolved
emission spectrum with three separate bands at 360, 376 (main), and 394 nm (Figure 20). These
bands are slightly blueshifted, compared to those of the corresponding unsubstituted parent
compound.[14] This observation is in accordance with that regarding the absorption bands and may
be attributed to the electron-withdrawing effect of the nitro groups as discussed there.
Photolysis of diarylbutadienyl derivatives 4-6
The photolysis of the isomers of the 4, 5, and 6 compounds which could be dissolved in appropriate
concentrations in n-hexane (at wavelengths corresponding to their main absorption bands; 313 nm
for 4 and 366 nm for 5 and 6) led to a relatively fast isomerization as the primary photochemical
reaction, similarly to the analogous compounds with one butadiene chain.
29