Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

Article abstract of DOI:10.1002/anie.202014780

Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of a polyene backbone. This method not only allows classification of PPAs in cis-cisoidal (ω₁<90°) or cis-transoidal structures (ω₁>90°), but also approximating ω₁. A PPA solution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPA helix by measuring the ECD spectra at different times. PPAs with a cis-cisoidal structure show a reduction of the ECD signal of at least 50 % before 30 min of irradiation, while cis-transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis-cisoidal and cis-transoidal helices require different times to decrease their ECD signal by 50 % (t_1/2), depending on the degree of compression or stretching of the helix, establishing a relationship between the secondary structure adopted by PPA (ω₁) and the time required to lose the ECD vinylic signal by light irradiation.

Full text of DOI:10.1002/anie.202014780

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Accepted Article
Title: Photochemical Electrocyclization of Poly(phenylacetylene)s:
Unwinding Helices to Elucidate their 3D-Structure in Solution
Authors: Felix Freire, Emilio Quiñoá, Ricardo Riguera, Rafael
Rodríguez, and Francisco Rey-Tarrío
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10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
Photochemical Electrocyclization of Poly(phenylacetylene)s:
Unwinding Helices to Elucidate their 3D-Structure in Solution
Francisco Rey-Tarrío, Rafael Rodríguez, Emilio Quiñoá, Ricardo Riguera and Félix Freire*
Francisco Rey-Tarrío, Dr. Rafael Rodríguez, Prof. Dr. Emilio Quiñoá, Prof. Dr. Ricardo Riguera, Prof. Dr. Félix Freire
Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica
Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
E-mail: felix.freire@usc.es
Supporting information for this article is given via a link at the end of the document
Abstract: Photochemical electrocyclizationof poly(phenylacetylene)s
(PPAs) is presented as a new tool for the structural elucidation of a
polyene backbone, where important structural information such as w1
can be obtained. This new methodology allows not only the
classification of PPAs in cis-cisoidal (w1< 90°) or cis-transoidal
structures (w 1> 90°), but also to obtain the approximate value of w1,
a crucial helical parameter. The process involves the irradiating a
dilute PPA solution with visible light and monitoring the photochemical
electrocyclization of the PPA helix by measuring the ECD spectra at
different times. Thus, PPAs with a cis-cisoidal structure show a
dihedral angle between conjugated double bonds [cisoidal (w1 <
90°); transoidal (w1 > 90°)].[24]
Pleasantly, Rh(I) catalysts allows the formation of PPAs with cis
configuration of double bonds. [45-47] A major problem emerges in
the w1 dihedral angle control by monomer design, [48] where the
balance between the steric and supramolecular interactions
involved in the structure stabilization frequently allow for a single
polymer to be constituted by an equilibrium mixture of several
helices with different pitches and/or sense. [47-50]
Moreover, an important feature of PPAs is that the
reduction of the ECD signal of at least 50% before 30 min of irradiation, macromolecular helix is described by two coaxial helices. An
while cis-transoidal helices need much longer time because the
transoidal bond must be isomerized to cisoidal before the
photochemical electrocyclization of the helix takes place. Moreover,
the different cis-cisoidal and cis-transoidal helices require different
times to decrease their ECD signal by 50% (t1/2), depending on the
degree of compression or stretching of the helix, a fact that allows
establishing a relationship between the secondary structure adopted
by the PPA (w1) and the time required to lose the ECD vinylic signal
by light irradiation. This process also permits the detection and
identification of different helical scaffolds that can coexist in some
solutions of the polymers.
internal helix determined by the conformation adopted by the
polyene backbone, and the external one described by the pendant
groups, which are arranged describing a helical structure, coaxial
to the internal one. The P/M sense of the internal helix depends
of the sign of the (w1) dihedral angle, that also influences the
external helical direction. Thus, in cis-cisoidal helices (w1< 90°),
the two helices rotate in the same sense [Pint/Pext or Mint/Mext],
while in cis-transoidal (w1> 90°) polymers, both helices rotate in
opposite directions [Pint/Mext or Mint/Pext]. [51]
Hence, to build up the 3D-structure of a PPA, it is necessary to
combine the information extracted by different techniques in
solution and in the solid state —Nuclear Magnetic Resonance
[26,52-56]
[56]
(NMR),
Differential Scanning Calorimetry (DSC),
Raman, [57] Raman Optical Activity (ROA), [58] Vibrational Circular
Dichroism (VCD), [59] Electronic Circular Dichroism (ECD), [60-62]
Atomic Force Microscopy (AFM), [50,64-72] X-Ray[73-82] as well as
Time Dependent-Density Functional Theory (TD-DFT)
calculations (Figure 1). [83-84] Nevertheless, a correct assignment
of the cis-cisoidal (c-c) or cis-transoidal (c-t) configuration of the
polyene backbone is far from solved because the DSC
Introduction
The helix is a structural motif responsible of the function of
biomolecules such as peptides, proteins, polysaccharides or DNA.
This structure-function relationship has prompted the scientific
community to study other non-natural helical materials such as
helical polymers.[1-6] The importance of these materials lies in their
thermogram is affected by the pendant groups or even by the
[7-15]
wide applications in fields such as sensing,
asymmetric
[56]
dynamic behavior of the polymer,
rendering the technique
synthesis, [15-17] chiral recognition[18] and separation. [19-22] Among
helical polymers, poly(phenylacetylene)s constitute very
useless in many cases. Searching for a more robust technique to
distinguish cis-cisoidal (w1 < 90°) from cis-transoidal (w1 > 90°)
structures, [86-92] we turned our attention to some stimuli-triggered
transformations that linked to the polymer structure, such as those
produced by light, temperature or pressure. Thus, while high-
pressure induces a cis-to-trans isomerization of double bonds by
generation of radical species in the backbone, thermal treatment
of poly(phenylacetylene) results in an intramolecular cyclization
and subsequent chain cleavage of 1,3,5-triphenylbenzene
derivatives. The reaction can be carried out in solution or in the
solid state and stimulated with air, pressure or electric fields, but
a
interesting family due to their dynamic structure[23,25] where the
flexibility of the polyenic backbone allows to tune the elongation[26-
32] and the helical sense[33-43] of the polymer. These changes are
triggered when an external stimulus interacts with the substituents
modifying their steric and/or supramolecular interactions. [27-33,44]
In such a way that the backbone prefers to adopt a different helix
by changing the dihedral angle between conjugated double bonds.
The skeleton of Poly(phenylacetylene)s (PPAs) can adopt four
possible configurations, cis-cisoidal, cis-transoidal, trans-cisoidal
and trans-transoidal, depending on the cis/trans structure of the
double bond and the cisoidal/transoidal conformation around the
nevertheless, the reaction is not very effective and occurs partially.
[86-94]
1
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10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
PPA Chemical Structure
Main dihedral angles of a PPA
Results and Discussion
H
ω1
H
R
To perform these studies, we selected the PPA that bears the
para-ethynylanilide of the (R)-α-methoxy-α-
n
H
H
m.r.u
ω2
R
H
trifluoromethylphenylacetic acid (MTPA) [poly-(R)-1], because
this polymer adopts a cis-cisoidal scaffold in non-donor solvents
and a cis-transoidal one in donor solvents. [31] For instance, poly-
(R)-1 in chloroform, (non-donor/low polar; classification of
solvents based on a combination of donor/acceptor properties
and polarities together with their dielectric constants and
Gutmann’s values),[31] shows a compressed cis-cisoidal polyene
scaffold (3 nm helical pitch, w1 = 70° aprox.), with a P orientation
of both the internal [positive CD at 377 nm] and the external (AFM)
helices (Figure 2a). For its part, when poly-(R)-1 is dissolved in
THF, (donor/low-polar), the polyene backbone adopts a stretched
cis-transoidal structure (3.9 nm helical pitch, w1 c.a. -155°), where
the internal and the external helices are opposite oriented. Thus,
while the internal helix has a M [CD(-) at 377 nm] helical
R
X
ω3
ω4
X
Circular Dichroism (CD)
40
Computational Studies
Internal
Helix
P Internal Helix
Positive Cotton
Effect
20
0
-20
External
Helix
300
400
500
Wavelength [nm]
Atomic Force Microscopy (AFM)
Helical
Pitch
3,2 nm
orientation, the external one rotates in the opposite P direction
[31]
(AFM) (Figure 2b).
Therefore, from a single polymer, two
Orientation of
the External Part
of the Helix
different structures (c-c and c-t) can be generated and submitted
to photochemical studies.
Raman Optical Activity (ROA)
Thus, two vials containing 2.1 mg/7·mL-1 (9.00·10-4 M) of poly-(R)-
1 dissolved in chloroform (vial 1) or in THF (vial 2) under an argon
atmosphere were irradiated with visible light (l > 350 nm). 200 μL
of these two poly-(R)-1 solutions were extracted at different
irradiation times and introduced in a 0.1 cm quartz cuvette. ECD
studies show that the ECD spectra of the polymer at the vinylic
region decreases with time in both solutions, leading to a null CD
although at different rates (Figure 2b-c). Adjusting the CD signal
decay of the vinyl band —photochemical electrocyclization
process— to a first order reaction showed that the reduction of
the 377 nm band to a half height, requires for poly-(R)-1 in
chloroform, 27 min of irradiation —half-life [t1/2 (377 nm) = 27 min]—,
with a rate constant K = 2.507·10-2 min-1 (Figure 2d) , while when
poly-(R)-1 is dissolved in THF the kinetic parameters are t1/2 (377
nm)= 78 min, and K = 8.825·10-3 min-1 (Figure 2d). GPC monitoring
of both experiments shows the disappearance of the well-folded
poly-(R)-1 chain and its conversion by photochemical
electrocyclization process, which evolve after continuously
irradiation with visible light to the corresponding 1,3,5-
trisubstituted benzene —1,3,5-(R)-1— unambiguously identified
by mass spectroscopy (See figures S21 and S22). However, the
main goal of this work is not the generation of the 1,3,5-
trisubstituted benzene by a photocyclic aromatization process,
which has already been described by Aoki and coworkers. [56]
Herein, we look for the time consumed in the photochemical
electrocyclizationprocess to reach a null ECD signal in the vinylic
region, specific for each secondary structure of a PPA. Thus, to
study the photochemical electrocyclization of the polymer, we
must monitor the complete disappearance of the polyene ECD
band, which is indicative of the loss of its secondary structure. At
this stage, no rearomatization with chain cleavage takes place yet
and therefore UV-vis studies cannot be used to monitor the
photochemical electrocyclization process, due to the presence of
polyenes together with the cyclohexadiene units formed (see
Figures S20-S27).
P Internal Helix
m-poly-2 Stretched
cis-C—Ph
C—C
PPA Chains
cis-C=C
Vibrational Circular Dichroism (VCD)
800
1000
1200
1400
1600
Wavenumber [cm-1
]
Psolvent/Pexternal
Diferential Scanning Calorimetry (DSC)
T= 198 ºC
negative VDC
solvent bands
c-c
t-t
cis-cisoidal
150
200
250
X-ray
T/ ºC
Figure 1. Chemical structure and main dihedral angles of PPAs. Combination
of structural techniques —AFM, X-ray, DSC, computational studies, DFT, ECD,
VCD, ROA— used to elucidate the 3D structures in PPAs.
In contrast, the controlled irradiation of PPAs under visible light
leads to 1,3,5-triphenyl substituted benzenes in a highly selective
intramolecular process. [56] From literature it is known that this
photocyclic aromatization process takes place in the solid state,
[56]
while in solution the reaction proceeds in small yields.
Interestingly, the reactivity of the polymer backbone depends
directly on the configuration of the starting polyene —only those
cis-cisoidal scaffolds with a short distance between double bonds
(< 4Å) were found to react—, and it was used for the formation of
self-supporting supramolecular polymeric membranes. [95,96]
Herein, we will show that visible light photochemistry can be
implemented as a unique structural tool to identify in solution not
only the presence of a cis-cisoidal or cis-transoidal skeleton of a
PPA, but also to provide an approximated dihedral angle for w1.
The process involves irradiating a dilute PPA solution with visible
light (l> 350 nm) and monitoring the CD decay of the polymer
after a short period of time (40-200 min). Moreover, this irradiation
protocol and consequent photochemical electrocyclization
the PPA helix is also useful to identify different helices coexisting
in a polymer in solution.
[78]
of
Interestingly, irradiation of samples of poly-(R)-1 with different
chain lengths produce, at the same concentration and
temperature, identical ECD decay patterns, which indicates that
the PPA photochemical electrocyclization process is independent
2
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10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
a)
of the polymer size (see Figure S30) being only dependent on the
secondary structure of the polymer.
a)
poly-(R)-1
hν (> 350 nm)
n
Cyclotrimer
H
“fast”
2. Rearomatization
with chain cleavage
hν (> 350 nm)
NH
3 nm
O
CD active
1. Photochemical
electrocyclization
O
extra time
CF3
3.9 nm
cis-cisoidal
Depends on the ω1 value
b)
Short Oligomers
not electrocyclizated
hν (> 350 nm)
CD = 0 Vinyl Region
CD = 0 Vinylic Region
“slow”
THF cis-transoidal
ω1 ca. 155º
CHCl3 cis-cisoidal
ω1 ca. 70º
b)
c)
5
110
90
5
0 min
45 min
90 min
110 min
130 min
45
30
15
0
0 min
CD active
20 min
40 min
50 min
60 min
cis-transoidal
4
3
2
1
4
70
50
150 min
170 min
3
2
1
Figure 3. Schematic illustration of the two-step photocyclic aromatization
reaction —photochemical electrocyclization (formation of cyclohexadiene units
in the main chain) + rearomatization with chain cleavage (formation of 1,3,5-
trisubstituted benzenes)— in (a) cis-cisoidal and (b) cis-transoidal polyene
scaffolds.
30
-15
-30
-45
-60
10
-10
-30
-50
-70
250 300 350 400 450 500
Wavelength [nm]
250 300 350 400 450 500
Wavelength [nm]
Comparison of the kinetic parameters obtained for poly-(R)-2 and
poly-(R)-1 show the values obtained for poly-(R)-2 are similar to
those generated for a THF solution of poly-(R)-1, and therefore
for a cis-transoidal helix (poly-(R)-1 t1/2 (THF) = 78 min; poly-(R)-2
t1/2 = 63min), which are clearly different to those obtained when
poly-(R)-1 is dissolved in chloroform and corresponds to a cis-
cisoidal helix (poly-(R)-1 t1/2 (CHCl3) = 27 min; poly-(R)-2 t1/2 = 63
min). Therefore, the photochemical electrocyclization data
suggests that poly-(R)-2 in CHCl3 has a cis-transoidal backbone
(Figure 4d), needing an extra time for the electrocyclization
reaction due to a prior transoidal to cisoidal photoinduction
reaction.
60
40
20
0
d)
CD@377nm, THF
t1/2 = 78 min.
K = 8,825 x 10-3
CD@377nm, CHCl3
t1/2 = 27 min.
K = 2,507 x 10-2
-20
-40
0
30 60 90 120 150
Time [min]
Figure 2. (a) 3D structures of poly-(R)-1 in non-donor (CHCl3, cis-cisoidal) and
donor (THF, cis-transoidal) solvents. ECD and UV-vis spectra of poly-(R)-1 after
irradiation with visible light in (b) CHCl3 and (c) THF. (d) ECD signal decay vs
time for poly-(R)-1 after irradiation with visible light. [poly-(R)-1] = 9.00·10-4
(0.3 mg·mL-1 THF or CHCl3). ECD and UV-vis studies were performed in a 0.1
cm quartz cuvette.
a)
poly-(R)-2
b)
n
H
0 min
M
120
80
40
0
45 min
90 min
110 min
130 min
150 min
170 min
O
HN
O
O
3.8 nm
3.8 nm
Mext
Hence, the different kinetic parameters obtained for the
photochemical electrocyclization of poly-(R)-1 in CHCl3 and THF
(c-c and c-t structures respectively) indicate that there is a direct
relationship between the helical scaffold —secondary structure—
adopted by the polyene skeleton and its sensitivity to light
irradiation (Figure 3).
In order to evaluate the scope of this process, the generality of
those conclusions, and its potential application as a tool to
unequivocally determine the cis-transoidal or cis-cisoidal scaffold
of PPAs, we repeated the visible light irradiation experiments with
a series of other PPAs with well-known c-c or c-t helical structure,
as described next.
Poly-(R)-2 bearing the para-ethynylbenzamide of (R)-
phenylglycine methyl ester as substituent was selected as model
compound because adopts a well-known cis-transoidal helix (3.8
nm, w1 c.a. 148°) in CHCl3, [43] with its internal helix M oriented
[CD(-) at 380 nm], and the external one describing a P helix
[determined by AFM] (Figure 4a).
Visible light (l > 350 nm) irradiation studies of [poly-(R)-2]
dissolved in CHCl3 (2.1 mg/7·mL,1.02·10-3 M) showed a ECD
decay that fits to a first order reaction with kinetic parameters: t1/2
(368 nm) = 63 min and K = 1.088.10-2 min-1— (figure 4b-c).
-40
-80
Mext
250 300 350 400 450 500
Wavelength [nm]
Pint
Cis-transoidal (ω1 ca. 148º)
d)
c)
1.0
poly-(R)-1
CD@378nm, CHCl3
t1/2 = 27 min.
60
40
20
0
CD@368nm, CHCl3
t1/2 = 63 min.
K = 1,088 x 10-2
poly-(R)-2
CD@368nm, CHCl3
t1/2 = 63 min.
0.5
0.0
poly-(R)-1
CD@377nm, THF
t1/2 = 78 min.
0
30 60 90 120 150 180
Time [min]
0
30 60 90 120 150 180
Time [min]
Figure 4. (a) Chemical structure and representative helical scaffold adopted by
poly-(R)-2. (b) CD spectra of poly-(R)-2 in CHCl3 at different irradiation times
with visible light (l > 350 nm) [poly-(R)-2] = 1.02·10-3 M. (c) First order reaction
fit of the ECD(368
signal decay vs time obtained for the photochemical
nm)
electrocyclizationdata suggests that of poly-(R)-2. (d) Comparison of the first
order reaction fit obtained for the normalized ECD signal decay vs time
corresponding to the photochemical electrocyclization data suggests that of
3
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10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
poly-(R)-1 in CHCl3 and THF and poly-(R)-2 in CHCl3. [poly-(R)-1] = 9.00·10-4
M; [poly-(R)-2] = 1.02·10-3 M. ECD studies were performed in a 0.1 cm quartz
cuvette.
In this case, the photochemical electrocyclization time needed for
poly-(S)-4 is much higher than those found for other PPAs with
cis-transoidal skeletons (poly-(S)-4 t1/2 = 1513 min vs. poly-(R)-1
t1/2
= 78 min; poly-(R)-2 t1/2 = 63 min). This is due to the
(THF)
presence of a highly stretched almost planar helix (poly-(S)-4 w1
c.a. 170° vs. poly-(R)-1 w1 c.a. 155° (THF); poly-(R)-2 w1 c.a. 148°)
and coherent with the structure previously reported (figure 6c).
These data also indicate that there is a correlation between the
elongation of the helix (w1) and the time needed for the
photochemical electrocyclization data suggests that the one
produced before the photocyclic aromatization reaction. Thus,
while poly-(R)-2 with w1 c.a. 148° needs 63 min for the
photochemical electrocyclization, poly-(R)-1 with w1 c.a. 155°
consumes 15 min. of extra time, due to the presence of a more
elongated structure (w1 poly-(R)-2 < w1 poly-(R)-1).
b)
a)
20
15
10
5
0 h
3 h
15 h
22 h
34 h
38 h
40 h
H
N
O
n
H
O
poly-(S)-4
Another example used to test the correlation between the PPA
helical structure and the kinetic parameters of the photochemical
0
13 nm
electrocyclization process was poly-(S)-3, that bears the para-
-5
cis-transoidal
(ω1 ca. 170º)
300
400
500
600
[48]
ethynylanilide of (S)-mandelic acid as substituent.
This
Wavelength [nm]
polymer adopts in CHCl3 a compressed cis-cisoidal helix (3.1 nm,
w1 c.a. 70°), where the internal and the external helices are P
oriented [CD380nm(-) at 368 nm, w1 ca. 70°] (Figure 5a). [48]
Irradiation of a chloroform solution of poly-(S)-3 (2.1 mg·/ 7 mL,
1.19·10-3 M) showed the expected evolution of the CD signal,
d)
c)
10
8
1.0
0.5
0.0
poly-(R)-1 ω1=70º
t1/2 = 27 min.
poly-(S)-3 ω1=70º
t1/2 = 26 min.
CD@484nm, CHCl3
t1/2 = 1513 min.
K = 4,581 x 10-4
poly-(R)-2 ω1=148º
t1/2 = 63 min.
6
poly-(R)-1 ω1=155º
t1/2 = 78 min.
4
poly-(S)-4 ω1=175º
t1/2 = 1513 min.
which fits to a first order decay —kinetic parameters: t1/2 (386 nm)
=
2
26 min and K = 2.68·10-2 min-1— (figure 5b-c). In this case, the
half-time value obtained for poly-(S)-3 is close to the one obtained
for a chloroform solution of poly-(R)-1 (t1/2 = 27 min figure 5d) and
coherent with a similar cis-cisoidal scaffold (w1 c.a. 70°).
To further explore the t1/2 /w1 relationship, we chose as useful
example poly-(S)-4. This PPA bears the ortho-ethynylanilide of
(S)-a-methoxy-a-phenylacetic acid as pendant (Figure 6a) and
adopts a very stretched -almost flat- helical structure —w1 c.a.
170°; Pint CD484(+); Mext (AFM)— (Figure 6a). [49]
0
0
750
1500
2250
0
40
80 120 160 200 240 280
Time [min]
Time [min]
( poly-4 time x 10)
Figure 6. (a) Chemical structure and representative cis-transoidal helical
scaffold adopted by poly-(S)-4. (b) CD spectra of poly-(S)-6 at different
irradiation times under visible light. (c) ECD and (d) normalized ECD signal
decay vs time for poly-(S)-4 after irradiation with visible light. [poly-(S)-4] =
1.13·10-3 M, CHCl3. ECD studies were performed in a 0.1 cm quartz cuvette.
Irradiation of poly-(S)-4 in CHCl3 (0.3 mg·mL-1, 1.13·10-3 M)
showed a very slow CD484 signal decay that fits to a first order
reaction with a t1/2 (386 nm) = 1513 min and K = 4.581·10-4 min-1, data
that is in full agreement with the presence of an elongated cis-
transoidal helical structure(Figure 6b-c).
Next, we decided to test the merits of this correlation in more
complex systems, namely those PPAs that are constituted by two
different scaffolds in equilibrium. Two representative examples
are poly-(S)-5, that bears the meta-ethynylanilide of (S)-a-
methoxy-a-phenylacetic acid as substituent and poly-(R)-6 with
n
a)
H
10
0 min
the meta-ethynylbenzamide of (R)-phenylglycine methyl ester as
NH
20 min
[49]
O
40 min
50 min
60 min
pendant.
Poly-(S)-5 in chloroform is constituted by an
5
OH
equilibrium mixture of a compressed cis-cisoidal —w1 c.a. 70°; Pint
CD380(+); Pext (AFM)— and a streched cis-transoidal —w1 c.a.
155°; Pint CD380(+); Mext (AFM)— helices (Figure 7a), while poly-
(R)-6 in the same solvent is formed by a mixture of two cis-
transoidal helices with different elongation—helix 1: w1 c.a. 160°,
Pint CD380(+), Mext (AFM); helix 2: w1 c.a. 165°; Pint CD380(+), Mext
(AFM)— (Figure 7d).49 In both cases, poly-(S)-5 and poly-(R)-6,
the polyene band at the CD spectra is constituted by a
contribution of the two helical structures, clearly observed in poly-
(R)-6 by the presence of a CD band with two peaks. Thus,
irradiation studies with visible light (520 nm) of chloroform
solutions of poly-(S)-5 (2.1 mg/ 7 mL; 1.13·10-3 M) and poly-(R)-6
(2.1 mg/ 7 mL; 1.02·10-3 M) were followed by studying the CD
signal decay at two different wavelengths for each polymer, one
at the beginning of the Cotton band, and the second at the end —
poly-(S)-5 CD at 370 and 398 nm; poly-(R)-6 CD at 373 and 428
nm). In the case of poly-(S)-5, the ECD spectrum obtained in a
0.1 cm quartz cuvette is too weak to analyze the ECD signal
decay during the photochemical electrocyclization process. Thus,
to improve the ECD signal, a 1 cm quartz cuvette was used to
register the ECD spectrum (Figure 7b). The results showed
0
poly-(S)-3
3.1 nm
-5
-10
cis-cisoidal
ω1 ca. 70º
300 350 400 450 500
Wavelength [nm]
c)
d)
1.0
0.5
0.0
CD@386nm, CHCl3
t1/2 = 26 min.
K = 2,68 x 10-2
6
4
2
0
poly-(S)-3
CD@386nm, CHCl3
t1/2 = 26 min.
poly-(S)-1
CD@378 nm, CHCl3
t1/2 = 27 min.
0
20
40
60
0
20
40
60
Time [min]
Time [min]
Figure 5. (a) Chemical structure and 3D structure of poly-(S)-3 in THF (cis-
cisoidal). (b) ECD spectra of poly-(S)-3 after irradiation with visible light in THF.
(c) ECD and (d) normalized ECD signal decay vs time for poly-(S)-3 after
irradiation with visible light. [poly-(S)-3] = 1.19·10-3 M. ECD studies were
performed in a 0.1 cm quartz cuvette.
4
This article is protected by copyright. All rights reserved.

10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
similar kinetic parameters (t1/2) for both quartz cuvettes (Figure
be fitted to a first order reaction, obtaining the following kinetic
parameters —t1/2 (379nm) = 24 min, K = 2.860·10-2 min-1 for helix 1,
and t1/2 (379nm) = 60 min, K = 3.462·10-2 min-1 for helix 2). The data
obtained at a different wavelength, 398 nm, provided similar
results (Figure S32). Therefore, helix 1 has a half time (25 min)
similar to those obtained for poly-(R)-1 (27 min.) and poly-(S)-3
(26 min) that correspond to a cis-cisoidal skeleton (w1 c.a. 70°),
while helix 2 has a half time (65 min), close to the one obtained
for poly-(R)-2 and that corresponds to a cis-transoidal polyene
structure (w1 c.a. 150°).
S35).
To
corroborate
these
results,
photochemical
electrocyclization studies of poly-(R)-6 were carried out in 1 cm
and 0.1 cm quartz cuvettes under the same protocols applied to
poly-(S)-5. As expected, the kinetic parameters (t1/2) obtained
were almost identical (Figure S36).
5.3 nm
n
3.2 nm
H
NH
O
O
poly-(S)-5
a)
In the case of poly-(R)-6, the photochemical electrocyclization of
helix 1 takes place between 0 and 180 min, while for helix 2 it
goes from 180 to 370 min either at 373 or 428 nm (Figure 7e).
Data fitting measured at 373 nm gave the following kinetic
parameters —t1/2 (373nm) = 188 min, K = 3.671·10-3 min-1 for helix 1,
and t1/2 (373nm) = 280 min, K = 2.475·10-3 min-1 for helix 2. Similar
data was obtained by fitting the data collected at 428 nm (Figure
S26). These results indicate that poly-(R)-6 is composed by two
cis-transoidal helices, with w1 value in a range of 150°-170° —
b)
30
0 min
10 min
15 min
20 min
25 min
30 min
35 min
20
10
0
5.3 nm
3.2 nm
40 min
45 min
50 min
55 min
-10
-20
250 300 350 400 450 500
Wavelength [nm]
Cis-transoidal (ω1 ca. 155º)
(Helix 2)
Cis-cisoidal (ω1 ca. 70º)
(Helix 1)
25
20
15
10
5
c)
25
20
15
10
5
4
poly-(R)-1 w1 c.a. 155°, t1/2 = 78 min; poly-(S)-4 w1 c.a. 170°, t1/2
=
CD@379nm, CHCl3
t1/2 = 24 min
K = 2.860 x 10-2
CD@379nm, CHCl3
t1/2 = 60 min
K = 3.462 x 10-2
Helix 1
1513 min—. The w1 value deduced from these irradiation studies
for each helical structure of poly-(S)-5 and poly-(R)-6 are
3
2
1
[49]
Helix 2
coincident with the information reported in the literature,
Helix 2
Helix 1
demonstrating again the robustness of the PPA photochemical
electrocyclization as structural tool.
0
0
0
0
20
40
60
0
10
20
30
40
0
10
20
(+40 min)
Time [min]
Time [min]
Time [min]
Finally, to illustrate the relationship between the w1 and the half-
time t1/2 of the photochemical electrocyclization, we plotted the
normalized CD signal decay vs. time for all the structures (Figures
8a) and the t1/2 versus their w1 values, taken as a stretching
indicator (Figure 8b). This graph not only shows the correlation
between t1/2 and w1 values, but also serves to illustrate the
dependence of the photochemical electrocyclization process with
the cis-cisoidal or cis-transoidal structure of the original helix,
where a transoidal to cisoidal conformational change is required
before the photocyclic reaction to takes place.
d)
n
H
O
HN
O
O
poly-(R)-6
Helix 2
λ= 428 nm
Helix 1
λ= 373 nm
100
75
50
25
0
0 min
30 min
60 min
90 min
3.8 nm
5.3 nm
120 min
150 min
180 min
210 min
240 min
270 min
300 min
330 min
-25
250 300 350 400 450 500
Wavelength [nm]
This relationship responds to equation (1)
Y=Y0 + (Plateau-Y0)*(1-exp(-K*x))
where Y is w1, Plateau is the w1 value at infinite time, K is the
photochemical electrocyclization rate and x is t1/2
Our data were fitted to equation (1), to obtain equation (2) (R
Cis-transoidal (ω1 ca. 155º)
(Helix 1)
Cis-transoidal (ω1 ca. 165º)
(Helix 2)
e)
12
8
100
100
50
0
CD@369nm, CHCl3
t1/2 = 188 min
K = 3.671 x 10-3
CD@369nm, CHCl3
t1/2 = 280 min
K = 2.475 x 10-3
.
Helix 1
Helix 2
50
square 0.993), useful to determine the approximated w1 from the
4
t1/2
.
Helix 1
Helix 2
w1= -147.5 +312.6*(1-exp(-4.659·10-2*t1/2)) (2)
0
0
0
100
200
300
0
50
100 150 200
Time [min]
0
50
100
150
(+180 min)
Time [min]
Time [min]
As a proof of this concept, we decided to test it with a novel PPA
designed to be folded into a compressed (helix 1) and a stretched
(helix 2) helix. To this end, we prepared poly-(R)-7 PPA that bears
a meta substituted (R)-MTPA group as pendant— (Figure 9a for
synthetic and characterization details). [97] Poly-(R)-7 is an isomer
of poly-(R)-1, which has the MTPA group in the para position.
Next, poly-(R)-7 was submitted to light irradiation in CHCl3 and
THF ([Poly-(R)-7] = 2.1 mg/ 7 mL; 9.00·10-4 M).
Again, the ECD signal obtained for poly-(R)-7 in both solvents was
weak when using the 0.1 cm quartz cuvette. Because of this, we
present the photochemical electrocyclization studies of this
polymer in a 1 cm quartz cuvette (Figure 9) (see Figures S36 and
S37 for 0.1 cm quartz cuvette studies).
Figure 7. (a) Chemical structure and representative cis-cisoidal and cis-
transoidal helical scaffolds adopted by poly-(S)-5. (b) ECD spectra of poly-(S)-
5 at different times of irradiation under visible light. (c) ECD signal decay vs time
for poly-(S)-5 after irradiation with visible light. [poly-(S)-5] = 1.13·10-3 M, CHCl3.
(d) Chemical structure and representative cis-transoidal helical scaffolds
adopted by poly-(R)-6 and CD spectra of poly-(R)-6 at different times of
irradiation under visible light. (e) ECD signal decay vs time for poly-(R)-6 after
irradiation with visible light. [poly-(R)-6] = 1.02·10-3 M, CHCl3. ECD studies were
performed in a 1 cm quartz cuvette for poly-(S)-5 and poly-(R)-6.
The CD signal decay measured at the two wavelengths showed
two different slopes for both polymers that correspond to the
photochemical electrocyclization processes followed by the
existence of two helices in solution (Figure 7 c, e and Figures S35
and S36). For instance, in poly-(S)-5 the photocyclic process
takes place between 0 and 60 min for helix 1, while for helix 2
goes from 40 to 90 min. Both curves —helix 1 and helix 2—can
In both cases, the CD signal showed in coherence with the
previous results, the expected decay of the polyene band
associated to the photochemical electrocyclization (Figure 7b-c).
Plotting the CD370 signal versus time, in both solvents, gave CD
curves with two different slopes corresponding to a mixture of two
5
This article is protected by copyright. All rights reserved.

10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
helices in solution. The following kinetic parameters were
obtained after independent first order decay fits of the data in the
two solvents.
Poly-(R)-7 dissolved in chloroform showed t1/2 (381nm) = 32 min, K
= 2.131·10-2 min-1 for helix 1, and t1/2 (381nm) = 61 min, K = 4.425·10-
2 min-1 for helix 2. Application of equation 2 to those data gives for
helix 1, w1 = 89°, which corresponds to a cis-cisoidal structure and
w1 = 146° for helix 2, that corresponds to a stretched cis-transoidal
scaffold.
The same study for poly-(R)-7 dissolved in THF, afforded t1/2 (362nm)
= 24 min, K = 2.863·10-2 min-1 for helix 1, and t1/2 (362nm) = 76 min,
K = 9.110·10-3 min-1 for helix 2. These kinetic values correspond
to a mixture of a cis-cisoidal (helix 1, w1 =60°) and a cis-transoidal
(helix 2, w1 = 156°) helices, in good agreement with the expected
w1 value for poly-(R)-7.
The results show that there is a direct relationship between the
elongation of a PPA (w1) and its t1/2. For instance, t1/2 =26 min
corresponds to a cis-cisoidal PPA with w1 c.a. 70°. such as poly-
(R)-1 (CHCl3), poly-(S)-3 or poly-(S)-5 (helix 1). For their part,
PPAs with more elongated chain (larger w1) show higher t1/2
values —poly-(R)-1 (THF) (w1=155°, t1/2=78 min.); poly-(R)-2
(w1=148°, t1/2=63 min); poly-(S)-4 (w1=170°, t1/2=1513 min)—. The
kinetic data for all the polymers used in this study, fit well into
equation 2 that therefore can be used to predict the scaffold of a
PPA (w1) from its t1/2 kinetic value. The use of the visible light for
the photochemical electrocyclization of PPAs as a tool to
determine their helical structure by using a t1/2/w1 relationship, is
exceptionally important in this field because no other techniques
can provide an approximated value for w1 in solution. This value
is necessary to build up an approximated helical structure of the
PPA, where important helical parameters such as relative helical
sense of the internal and external helices of a PPA depends on
this value —cis-cisoidal structures (w1 < 90°) both helices rotate
in the same direction, while in cis-transoidal scaffolds (w1 < 90°)
both helices rotate in opposite directions
CONCLUSIONS
In conclusion, it has been demonstrated through a good number
of PPAs with different well-known helical structures —6 polymers,
9 helical scaffolds— that visible light irradiation can be introduced
as a powerful technique to determine the secondary structure of
polyacetylenes. The basis of the method lies on the quite different
rate observed for the photochemical electrocyclization depending
on the scaffold (cis-cisoidal, cis-transoidal) of the helical polymer.
To perform these studies, vials containing a dilute solution of a
PPA [(poly-(1-6)] = 2.1 mg/ 7 mL; c.a. 1·10-3 M] was irradiated with
visible light (l > 350 nm) and the CD signal decay monitored
versus time to afford the photochemical electrocyclizationrate and
the t1/2 as the relevant parameters.
60
40
20
0
40
30
20
10
0
c)
0 min
a)
b)
0 min
CHCl3
10 min
30 min
50 min
70 min
80 min
90 min
100 min
110 min
120 min
130 min
140 min
150 min
160 min
170 min
THF
7.5 min
15 min
22.5 min
30 min
37.5 min
45 min
52.5 min
60 min
67.5 min
75 min
80 min
85 min
n
H
NH
O
CF3
O
-10
poly-(R)-7
-20
-20
250 300 350 400 450 500
Wavelength [nm]
250 300 350 400 450 500
Wavelength [nm]
8
40
30
20
10
0
40
30
20
10
0
d)
e)
CHCl3
Helix 1
CD@381nm
CD@381nm
t1/2 = 32 min.
K = 2.131 x 10-2
t1/2 = 61 min.
K = 4.425 x 10-2
6
4
PPA
ω1
t1/2
a)
Helix 2
1.0
0.5
0.0
poly-(R)-1
poly-(S)-3
poly-(S)-5 (1)
Helix 1
poly-(R)-1
poly-(R)-1
poly-(R)-2
poly-(S)-3
poly-(S)-4
72º
27 min
2
Helix 2
155º 78 min
148º 63 min
poly-(R)-2
0
poly-(R)-1
0
20 40 60 80
Time [min]
0
10
20
30
40
0
10 20 30 40 50
70º
175º 1513 min
24 min
26 min
Time [min]
poly-(S)-5 (2)
poly-(R)-6 (1)
poly-(R)-6 (2)
poly-(S)-4
Time [min]
60
g)
60
25
20
15
10
5
f)
poly-(S)-5 (1) 68º
THF
CD@362nm
t1/2 = 24 min.
K = 2.863 x 10-2
CD@362nm
t1/2 = 76 min.
K = 9.110 x 10-3
Helix 1
poly-(S)-5 (2) 150º 60 min
poly-(R)-6 (1) 160º 188 min
poly-(R)-6 (2) 165º 280 min
(time x 5)
40
20
0
40
20
0
Helix 2
0
100
200
Time [min]
300
400
500
Helix 2
Helix 1
ω1 ca. 175º
t1/2 = 1513 min.
ω1 ca. 160-165º
t1/2 = 185-280 min
ω1 ca. 148-155º
t1/2 = 60-78 min
ω1 ca. 70º
t1/2 = 25-27 min
0
0
30 60 90 120 150 180
Time [min]
0
20
40
60
80
0
20 40 60 80 100
Time [min]
Time [min]
ω
1
(+40 min)
t1/2
-
+
poly-(S)-4
poly-(S)-5
poly-(R)-1
poly-(S)-3
poly-(R)-2
poly-(R)-1
poly-(S)-5
Figure 9. (a) Structure of poly-(R)-7. (b) CD spectra of poly-(R)-7 in CHCl3, at
different times of irradiation under visible light (c) CD spectra of poly-(R)-7, in
THF, at different times of irradiation under visible light. (d) ECD signal decay vs
time for poly-(R)-7 after irradiation with visible light. [poly-(R)-7] = 9.00·10-4 M,
CHCl3. (e) ECD signal decay vs time for poly-(R)-7 after irradiation with visible
light in CHCl3. (f) ECD signal decay vs time for poly-(R)-7 after irradiation with
visible light. [poly-(R)-7] = 9.00·10-4 M, THF. (g) normalized ECD signal decay
vs time for poly-(R)-7 after irradiation with visible light. ECD studies were
performed in a 0.1 cm quartz cuvette.
b)
200
100
0
ω1= -147.5 +312.6*(1-exp(-4.659·10-2*t1/2))
200
100
0
0
100
200
300
Although the studies in this work are limited to PPAs substituted
by amide groups (anilide, benzamide), no doubt similar
investigations can be carried out with other PPAs that bear other
connecting groups (ester, ether...). To do this, it will be essential
to have a solid base of structural information on these polymers,
including AFM images and theoretical calculations, in order to
know if the results would be comparable to those of the
anilide/benzamide PPAs.
t1/2 [min]
0
500
1000
1500
t1/2 [min]
Figure 8. a) Normalized CD signal decay of poly-(1-6) during photochemical
electrocyclizationof the different PPA backbones. b) Variation of the PPA w1 vs
t1/2 of the backbone photochemical electrocyclization process and the
corresponding fit to an exponential equation.
6
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10.1002/anie.202014780
Angewandte Chemie International Edition
RESEARCH ARTICLE
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8
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Entry for the Table of Contents
The photochemical electrocyclization of poly(phenylacetylene)s in solution constitutes a novel methodology for obtaining important
structural information that allows determine their helical structures —cis-cisoidal (w1 < 90°) or cis-transoidal (w1 > 90°)— while it also
provides an estimated value of w1. In addition, when applied to mixtures, it allows the identification of the different helical scaffolds that
coexist in solution.
9
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