Dissymmetric Chiral Poly(diphenylacetylene)s: Secondary Structure Eluci

Chi al Polyme s Ho Pape
Dissymme ic Chi al Poly(diphenylace ylene)s: Seconda y S uc u e
Elucida ion and Dynamic Luminescence
Juan José Ta ío, Ra ael Rod íguez, Be a Fe nández, Emilio Quiñoá, and Félix F ei e*
Abs ac : The seconda y s uc u e o a dissymme ic and
chi al poly(diphenylace ylene) (PDPA) is elucida ed by
combining he da a om NMR expe imen s ( egio egu-
la head o ail s uc u e), Raman and IR s udies (E
con igu a ion o he polyene double bonds), and high-
esolu ion AFM images (helical pi ch, packing angle and
o ien a ion o he ex e nal helix). As a esul , an E-
ansoidal polyene backbone desc ibing h ee coaxial
helices is ob ained. Theo e ical elec onic ci cula di-
ch oism (ECD) s udies o he s uc u e show a good
co espondence be ween expe imen al and heo e ical
da a and allow one o deciphe ha he i s Co on
band is gene a ed by he poly(diphenylace ylene) co e
and no only by he polyene backbone. The dynamic
beha io o poly-(S)-2is also demons a ed by a helix
in e sion e ec p oduced by con o ma ional changes a
he pendan g oups when annealed in sol en s wi h
di e en dono abili ies. This phenomenon is accompa-
nied by an in e sion o he ci cula pola ized lumines-
cence o he PDPA (CPL swi ch).
In oduc ion
Molecula enginee ing allows he design o no el ma e ials
wi h a ce ain s uc u e and speci ic p ope ies ela ed o i .
Wi hin his opic, modelling new helical s uc u es based on
non-na u al building blocks such as helicenes,[1–3]
oldame s,[3–6] sup amolecula [7–9] o co alen helical
polyme s,[10–17] has a ac ed he a en ion o he scien i ic
communi y. These s udies a e inspi ed by he s uc u e/
unc ion ela ionships p esen in biomac omolecules such as
pep ides, p o eins, DNA o polysaccha ides. Among non-
na u al helical sca olds, dynamic helical polyme s cons i u e
a e y in e es ing amily due o he possibili y o uning hei
helical sense o elonga ion h ough ex e nal s imuli.[19–23] The
use o non-na u al building blocks o syn he ize hese
polyme s allows he c ea ion o sma ma e ials wi h
applica ions in di e en ields such as sensing,[24–31] asymme -
ic syn hesis,[32–36] chi al ecogni ion[37] and sepa a ion.[38–41]
Howe e , in molecula enginee ing i is manda o y o
es ablish s uc u e/ unc ion ela ionships o hese ma e ials,
a ac ha will allow o de e mine and, i possible, e ine he
s uc u al ea u es in ol ed in he unc ionali y o he
ma e ial.
Un o una ely, he s uc u al elucida ion o helical poly-
me s is no s aigh o wa d due o he p esence o monome
epea ing uni s (m. .u.) along he polyme chain, ende ing
powe ul s uc u al echniques such as nuclea magne ic
esonance (NMR) o X- ay di ac ion (XRD) useless.
Du ing he las decade a g ea e o has been made in he
s uc u al cha ac e iza ion o poly(phenylace ylene)s
(PPAs), undamen ally h ough a combina ion o echniques
(NMR,[42,43] XRD,[44–52] a omic o ce mic oscopy (AFM),[53–61]
di e en ial scanning calo ime y (DSC),[62] Raman,[63] ci cu-
la dich oism (CD),[64,65] ib a ional ci cula dich oism
(VCD),[66] Raman op ical ac i i y (ROA),[67]), compu a ional
s udies[68,69] and eac i i y expe imen s (pho ochemical elec-
ocycliza ion o PPAs[67]) ha we e used o ob ain in o ma-
ion abou di e en s uc u al pa ame e s o hese complex
mac omolecules and combined o sol e he puzzle o he
seconda y s uc u e o helical polyme s.
Ano he in e es ing amily o helical polyme s a e he
poly(diphenylace ylene)s (PDPAs)[71–93] ha esemble PPAs
in he main chain, which in bo h cases is made up o
conjuga ed double bonds. Howe e , he p esence o a
phenyl g oup on each ca bon o he double bond in PDPAs
makes he p ope ies o bo h ma e ials e y di e en . Fo
ins ance, PDPAs show be e chemical, he mal and pho o-
chemical s abili y compa ed o PPAs.[72–85] Mo eo e ,
PDPAs exhibi ano he in e es ing p ope y ha PPAs do
no ha e, i.e. pho oluminescence.[82–85,89] As a nega i e
cha ac e is ic, PDPAs show a poo dynamic beha io , being
necessa y o hea hem o 80° o induce a helical sense once
he ex e nal s imuli a e modi ied.[79–85] This deple ion o
dynamic beha io is a consequence o he in oduc ion o
he second phenyl ing in he m. .u., which conges s he
[*] J. J. Ta ío, P o . E. Quiñoá, P o . F. F ei e
Cen o Singula de In es igación en Química Biolóxica e Ma e iais
Molecula es (CiQUS) and Depa amen o de Química O gánica
Uni e sidade de San iago de Compos ela
15782 San iago de Compos ela (Spain)
E-mail: [email p o ec ed]
D . R. Rod íguez
WPI Nano Li e Science Ins i u e (WPI-NanoLSI)
Kanazawa Uni e si y
Kakuma-machi, Kanazawa 920-1192 (Japan)
P o . B. Fe nández
Depa amen o de Química Física
Uni e sidade de San iago de Compos ela
15782 San iago de Compos ela (Spain)
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published by Wiley-VCH GmbH. This is an open access a icle unde
he e ms o he C ea i e Commons A ibu ion License, which
pe mi s use, dis ibu ion and ep oduc ion in any medium, p o ided
he o iginal wo k is p ope ly ci ed.
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helical backbone and whe e s e ic epulsion does no a o
chi al ampli ica ion and helix in e sion e ec s.
PDPAs can be classi ied in wo di e en g oups,
symme ic (Scheme 1a) o asymme ic (Scheme 1b), depend-
ing on he subs i u ion o he wo phenyl ings. Mo eo e , in
bo h g oups he s e eo egula i y o he polyme depends on
he con igu a ion o he double bonds, Zo E(Scheme 1a,
b), and he dihed al angle be ween conjuga ed double
bonds, which can adop ei he cisoidal o ansoidal con ig-
u a ion, ω1<90°(cisoidal); ω1>90°( ansoidal) (Sche-
me 1c). Thus, ou di e en possible s e eo egula i ies can
be ob ained in PDPAs, namely Z-cisoidal, Z- ansoidal, E-
cisoidal and E- ansoidal, whe e he Z/Econ en depends on
he ca alys used o he polyme iza ion eac ion and he
cisoidal/ ansoidal con igu a ion is ela ed o he s e ic
hind ance be ween pendan g oups.
The complexi y o he sys em inc eases in he case o
asymme ic PDPAs whe e di e en p oduc s can be ob-
ained depending on he egio egula i y o he polyme -
iza ion eac ion. Thus, head- o- ail, head- o-head o ail- o-
ail polyme iza ions can occu (Scheme 1d), each o hem
wi h he ou possible s e eo egula i ies a he conjuga ed
polyene backbone.
This ac , in combina ion wi h he ully subs i u ion o
he double bonds, and he e o e o he polyene backbone,
makes he ex apola ion o s uc u al in o ma ion ob ained
o PPAs useless o elucida e he s uc u e o PDPAs.
Only a ew examples ela ed o chi al PDPAs[72–85] a e
ound in he li e a u e compa ed o he la ge amoun o
wo k ha has been done wi h PPAs.[12–33,37–51,94–97] This is due
o he di icul y o ob aining he o me polyme s in he
labo a o y by polyme iza ion o he co esponding dipheny-
lace ylene monome s (DPA). Thus, while PPAs bea ing a
la ge a ie y o unc ional g oups a e easily p epa ed using a
RhIca alys ,[45,51] he syn hesis o PDPAs is mo e complex
due o he low ole abili y owa ds pola g oups shown by
he ca alys s used o p epa e hese ma e ials. Fo ins ance,
Tang and co-wo ke s de eloped a syn he ic p ocedu e ha
uses a combina ion o WCl6-Ph4Sn in oluene o polyme ize
achi al diphenylace ylenes,[74–76] al hough ollowing his ap-
p oach i is no possible o polyme ize diphenylace ylenes
bea ing pola g oups due o he poisoning o he ca alys . To
p epa e pola PDPAs, i is neccessa y o i s polyme ize a
diphenylace ylene in which one o bo h o he a yl ings
ca y an ac i e pen a luo ophenyl es e g oup (PFP), which
is e icien ly ans o med in o a chi al PDPA by a pos -
unc ionaliza ion eac ion.[81] In e es ingly, all he polyme s
p epa ed in hese condi ions do no show an induced helical
s uc u e once hey a e ob ained. The mal annealing is
necessa y, i.e., 4 h a 80°C, o induce a p e e ed handedness
in he PDPA main chain.[81,82, 86,99]
Recen ly, Maeda and co-wo ke s s udied he seconda y
s uc u e o symme ic PDPAs bea ing pa a-subs i u ed
ca boxy pendan g oups.[79,80] By using a combina ion o 1H
and 13C NMR, IR, Raman, VCD and ECD spec oscopies,
hey ound ha his polyme adop s a p e e ed cis- ans-
oidal s uc u e, whe e Po M-helical senses can be induced
by he mal annealing in wa e in he p esence o chi al
amines. On he o he hand, in he case o asymme ic
PDPAs, Kwak, Aoki and co-wo ke s made g ea con ibu-
ions o he s udy o hei luminescen p ope ies and o he
achi al- o-chi al ans o ma ions o PDPAs.[78, 82, 85–89] To
pe o m hese s udies, a ca alys de eloped by Masuda
[TaCl5-n-Bu4Sn] was used.[77] Also wi h asymme ic PDPAs,
Tang and co-wo ke s epo ed polyme s p epa ed wi h he
WCl6-Ph4Sn[83,84] ca alys ha Maeda also employed o he
p epa a ion o op ically ac i e PDPAs wi h applica ion as
chi al s a iona y phases.[71] Ne e heless, al hough se e al
examples o asymme ic PDPAs a e ound in li e a u e,
hei seconda y s uc u e emains unclea . F om hese
s udies i is ound ha he helical s uc u e adop ed by a
PDPA depends on he monome s uc u e[98] and, he e o e,
o assume ha symme ic and asymme ic PDPAs ha e he
same sca old is a mis ake.
He ein, we will s udy he seconda y s uc u e o chi al
and asymme ic PDPAs ob ained om polyme iza ion o an
achi al diphenylace ylene, whe e one o he a yl ings ca ies
an ac i e pen a luo ophenyl es e (PFP) g oup, while he
o he phenyl ing emains unal e ed.[81] This app oach is one
he mos widely used in he p epa a ion o asymme ic
PDPAs. The s e eo and egio egula i y o he polyme will
depend on he syn hesis o he i s PDPA, which bea s he
PFP as pendan g oup and ha is u he ans o med in a
chi al PDPA. Thus, h ough a pos - unc ionaliza ion eac-
ion, he PDPA bea ing he 4-benzamide o alanine me hyl
es e as pendan g oup will be p epa ed and i s seconda y
s uc u e elucida ed.
Scheme 1. Gene al molecula s uc u e o a) symme ic and b) asym-
mme ic PDPAs. c) S e eo egula i y o a PDPA ob ained ia head o ail
polyme iza ion. d) Regio egula i y in a PDPA wi h an (E)- ansoidal
polyene backbone.
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Resul s and Discussion
The syn hesis o he achi al and asymme ic p ecu so ,
pe luo ophenyl 4-(phenyle hynyl)benzoa e, and i s subse-
quen polyme iza ion o ob ain poly-1using a mix u e o
WCl6-Ph4Sn as ca alys , was ca ied ou acco ding o he
me hods p e iously epo ed.[83,84]
The numbe a e age mola mass (Mn) and mola mass
dispe si y (Mw/Mn) o poly-1we e 1.4×104and 1.43
espec i ely, as de e mined by gel pe mea ion ch oma og-
aphy (GPC) using THF as eluen and polys y ene na ow
s anda ds (PSS) as calib an s. Taking ad an age o he ac i e
es e p esen as pendan g oup, poly-1was e ec i ely
ans o med in o poly-(S)-2by coupling wi h NH2-Ala-OMe
unde common pep ide coupling condi ions (Figu e 1), as
con i med by 19F NMR and IR s udies ha co obo a ed he
disappea ance o he signals co esponding o he pen a-
luo ophenol moei y (see Suppo ing In o ma ion).
Once poly-1and poly-(S)-2we e ob ained, hei s e eo-
and egio egula i y we e analyzed using IR, Raman, and
NMR echniques as s uc u al ools.
Raman spec oscopy is a powe ul s uc u al echnique
ha p o ides use ul in o ma ion ela ed o he s e eo-
egula i y o he conjuga ed double bonds.[83] I is commonly
applied o assess cis- o ans-con en in PPAs and
PDPAs.[51,99,100] Ca alys s based on ungs en (W) a e known
o p o ide ans- ich PDPAs as shown in many examples in
he li e a u e,[99,100] albei Maeda ecen ly demons a ed ha
in he case o symme ic PDPAs, polyene backbones wi h a
p edominan cis-con en o double bonds a e also
ob ained.[79,80]
He ein, we will s udy he s e eo egula i y o poly-1and
poly-(S)-2by compa ison o hei Raman spec a wi h hose
ob ained o PPAs wi h well-known seconda y s uc u e.
The e o e, as model compounds we chose poly-(3–6) (Fig-
u e 2a). Poly-3[100] and poly-4[54] we e selec ed due o hei
esemblance wi h poly-1and poly-(S)-2, while poly-5[70] and
poly-6we e chosen due o he possibili y o gene a ing a
ans-PPA (E-PPA, poly-6) by isome iza ion o a cis-PPA
(Z-PPA, poly-5) (Figu e 2a). The isome iza ion o he
polyene backbone p oceeds h ough he gene a ion o a
delocalized adical ca ion in he polyene when he anilide
g oup o he PPA in e ac s wi h Au3+(see Suppo ing
In o ma ion).
Ca e ul examina ion o he Raman spec a unambigu-
ously e eals clea di e ences o he polyme ic backbones
con aining cis o ans double bonds. While polyme s wi h a
high con en o cis (Z) double bonds [poly-(3–5)] showed an
in ense Raman band a ca. 1585 nm oge he wi h a weak
one a ca. 1550 nm, o polyme s wi h a ans (E) con ig-
u a ion [poly-(1–2), poly-6] o he double bonds, he
opposi e scena io is ob ained, a weak band a ca. 1585 nm
ollowed by a s ong band a ca. 1557 nm (Figu e 2b).
Ano he s uc u al echnique ha p o ides use ul in-
o ma ion ela ed o he Z/Econ igu a ion o double bonds
in polyene backbones is IR. In a seminal wo k, Simionescu
e al. we e able o de e mine he cis/ ans (Z/E) con igu-
a ion o he double bonds by analyzing he a io be ween
he bands a 1500 and 1450 cm1o di e en model
compounds.[101] Thus, while PPAs wi h a 1500/1450 (cm1/
cm1) a io>1 ha e a majo i y ans con igu a ion o he
double bonds, hose wi h a 1500/1450 (cm1/cm1) a io�1
Figu e 1. Syn he ic app oach o p epa e poly-1and poly-(S)-2.
Figu e 2. a) S uc u e o poly-(3–6). Pa ial b) Raman and c) IR spec a
o poly-(1–6) showing he 1300–1600 cm1 egions.
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ha e a p e alen cis con igu a ion o he double bonds.
Hence, we decided o analyze hese bands o all he
polyme s used in his s udy, PPAs and PDPAs, and
de e mine i he epo ed pa e n ound by Simionescu e al.
o PPAs can be ex ended o PDPAs. In e es ingly, in all
PDPAs and PPAs wi h a ans- (E) con en o he double
bonds, he a io be ween he 1500/1450 (cm1/cm1) bands is
g ea e han 1, while in hose cases whe e he polyene is
o med by cis- (Z) double bonds, he a io be ween hose
wo bands is �1, indica ing ha his PPA p o ocol can be
ex ended o PDPAs (Figu e 2c).
To de e mine he Z/Econ igu a ion o he double bonds,
i is ecommended o analyze bo h he Raman and IR
spec a, being necessa y o check he a io o signals a
ca.1585/1550 (cm1/cm1) o Raman s udies and ca.1500/
1450 (cm1/cm1) o IR expe imen s.
Once he ans (E) con igu a ion o he double bonds
was demons a ed in asymme ic PDPAs p epa ed wi h
WCl6/Ph4Sn ca alysis, he egio egula i y o hese polyme s
was explo ed.
As s a ed abo e, di e en egio egula i ies (head- o- ail,
head- o-head, and ail- o- ail) can be gene a ed in hese
sys ems du ing he polyme iza ion o asymme ic DPA
monome s (Scheme 1d). 1H NMR spec a o poly-(S)-2in
di e en sol en s show a single se o NMR signals
indica ing he p esence o a egio egula polyme (Figu e 3c
and Suppo ing In o ma ion). O he wise, a mix u e o
egio egula i ies should esul in mo e complica ed NMR
spec a due o he di e en aniso opic en i onmen s
expe ienced by he p o ons o he m. .u. wi hin he di e en
egiochemis ies (Figu e 3a). To disc imina e be ween he
di e en egio egula i ies, long-dis ance 2D NMR expe i-
men s such as NOESY we e ca ied ou . De ailed obse a-
ion, a e modeling s uc u es con aining he h ee possible
egio egula i ies (head- o- ail, head- o-head, and ail- o- ail)
o poly-(S)-2and ans (E) con igu a ion o he double
bonds in he polyene backbone, e eals ha in head- o-head
o ail- o- ail egio egula i ies, all he a oma ic p o ons o
he unsubs i u ed benzene ing a e in close con ac wi h he
di e en p o ons o he alanine esidue o n1 and n+1
m. .u. (Figu e 3a). This ac is due o he al e na ing
dis ibu ion o alanine esidues and a oma ic ings on he
helix idges. On he con a y, in he case o a head- o- ail
egio egula i y, he unsubs i u ed and he subs i u ed a o-
ma ic ings a e loca ed in he opposi e helix idges (Fig-
u e 3a). As a esul , he p o on a he pa a posi ion o he
unsubs i u ed benzene ing should a o d a long-dis ance
NOE c oss peak wi h some alanine p o ons in a head- o-
head o ail- o- ail sequence, whe eas his NOE should no
be obse ed in he case o a head- o- ail sequence. The e-
o e, he absence o his c oss peak in he NOESY spec um
co obo a es he head- o- ail egio egula i y du ing he
polyme iza ion o poly-1(Figu e 3c).
In e es ingly, he head- o- ail s uc u e obse ed o
poly-(S)-2 esembles he packing obse ed o a diphenyla-
ce ylene de i a ized wi h an alanine me hyl es e g oup [m-
(S)-2] by X- ay di ac ion[102] (Figu e 3b).
Nex , ECD s udies we e ca ied ou o poly-(S)-2in
DMF. As expec ed, no ECD signal was ob ained un il he
sample was annealed a 80°C o 4 hou s. A e annealing,
an ECD ace wi h a i s posi i e Co on e ec a 400 nm
was ob ained (Figu e 4b). The dissymme y ac o (gabs)
ob ained o poly-(S)-2was 1.8×103, wi hin he same ange
as he alues epo ed o symme ic PDPAs o PPAs,[10,12,79]
ac ha co obo a es bo h good s e eo and egio egula i ies
wi hin poly-(S)-2wi h a sc ew sense excess.
To ob ain mo e s uc u al in o ma ion, and o e ine he
helix adop ed by poly-(S)-2, a 2D-c ys al o his polyme
was p epa ed using he p o ocol desc ibed by Yashima
e al., which consis s o spin coa ing a dilu e DMF solu ion
o poly-(S)-2on highly o ien ed py oly ic g aphi e (HOPG)
ollowed by annealing unde he co esponding sol en
a mosphe e.[59] In ou case, be o e spin coa ing he sample,
he polyme was annealed in DMF a 80°C o 4 h o induce
a sc ew sense excess in he polyene backbone. High-
esolu ion AFM images o poly-(S)-2 e eal he p esence o
2D c ys als whe e single PDPA chains a e packed in a
pa allel manne , one a e he o he , o o m a igh -handed
(clockwise) pendan disposi ion wi h he pe iodic oblique
s ipes in 50°angles and a helical pi ch o 6.35 nm (Fig-
u e 4a).
By combining he in o ma ion ob ained om he ECD
and AFM s udies o poly-(S)-2in DMF, we can s a e ha
he i s posi i e Co on band (CD400 >0) co esponds o a P
helical s uc u e desc ibed by he pendan g oups (ex e nal
helix). As wi h PPAs, PDPAs a e made up o wo coaxial
Figu e 3. a) Dime ic model s uc u es o a head- o-head, ail- o- ail and
head- o- ail egio egula i ies o poly-(S)-2, emphasizing he expec ed
NOE c oss peaks o Ha. b) X- ay s uc u e o m-(S)-2. c) NOESY
spec um o poly-(S)-2 in DMSO.
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helices, one desc ibed by he polyene backbone and he
o he desc ibed by he pendan s.[70]
In o de o s ablish a ela ionship be ween in e nal and
ex e nal helices, compu a ional s udies [TD-
DFT(CAMB3LYP)/3-21G][103–106] we e pe o med on a P
helix (n=20 m. .u.) o a PDPA wi h ans- ansoidal
skele on, ω1=165°,ω2=180°,ω3=40°(Figu e 4a, d). In
his model, he chi al subs i uen s we e emo ed o educe
he compu a ional demands. The simula ed ECD spec um
(Figu e 4c and Suppo ing In o ma ion o de ailed in o ma-
ion) is in good ag eemen wi h he expe imen al one, which
indica es ha he p oposed model is a good app oxima ion
o he s uc u e o poly-(S)-2.
The helical s uc u e desc ibed by an asymme ic PDPA
in a ans- ansoidal con igu a ion is e y di e en om he
cis- ansoidal helix desc ibed by a PPA. In a PDPA helix,
h ee di e en helices coexis : a) helix 1, which is he
classical in e nal helix desc ibed by he o ien a ion be ween
conjuga ed double bonds (ω1=165°) and which in his
example (i.e., poly-(S)-2) is Mo ien ed due o he nega i e
alue o ω1(Figu es 4a, d); b) helix 2, which is he helix
desc ibed be ween consecu i e poly(diphenylace ylene)
g oups, and which o a es in he opposi e di ec ion o ha
de ined by he dihed al angle be ween conjuga ed double
bonds, helix 1=ω1(so, in poly-(S)-2, is P; Figu e 4a); and c)
helix 3, which is p esen in PDPAs bu does no exis in
PPAs. This helix appea s when he polyene backbone g ows
because o he ans- ansoidal con igu a ion o he polyene,
esul ing in a wis o he whole sca old. The helical sense
desc ibed by helix 3 is coinciden wi h ha desc ibed by
helix 2, in his case a Phelix (Figu e 4a).
When analyzing he da a ob ained om he heo e ical
calcula ions, we could obse e ha unlike he PPAs, he
PDPAs show a i s Co on band gi en by a combina ion o
he poly(diphenylace ylene) backbone and no only by he
polyene (see Suppo ing In o ma ion). The e o e, hese
esul s indica e ha he co ela ion be ween he ECD signal
and he helical sense o he polyene (ω1) ound in cis-PPAs
canno be applied o asymme ic PDPAs. In he la e i is
ound ha , al hough he double bonds a e Mo ien ed (helix
1ω1<0), he Po ien a ion o consecu i e diphenylace ylene
m. .u. (helix 2) in combina ion wi h he Po ien a ion o he
whole sca old (helix 3), esul s in a inal posi i e Co on
band. Thus, a his poin we can s a e ha in an asymme ic
PDPA wi h a ans- ansoidal helix, a i s posi i e Co on
band a 400 nm indica es a helical s uc u e wi h a M/P/P
(helix 1/helix 2/helix 3) o ien a ion. On he con a y, i he
asymme ic PDPA shows a CD400 <0, hen he polyme
adop s a P/M/M(helix 1/helix 2/helix 3) o ien a ion.
Ano he in e es ing s uc u al ea u e obse ed in
PDPAs is ound when he ECD and he UV spec a
ob ained o a DMF solu ion o poly-(S)-2a e compa ed
(Figu e 4b). By looking a he wo spec a, we can see ha
he e is a misma ch be ween he i s Co on band in he
ECD spec um (400 nm) and he highes wa eleng h UV
band (450 nm), co esponding he ECD Co on band o he
second band o he UV-spec um (400 nm). To de e mine
and cha ac e ize he elec onic ansi ions esponsible o
hese wo UV-bands, heo e ical ECD and UV s udies we e
ca ied ou o an n=12 oligome . The heo e ical UV-
spec um ep oduced he expe imen al one, showing wo
maxima a ca. 400 and 450 nm, being he i s ansi ion he
one ha p oduces he i s Co on band.
By analyzing he elec on densi y di e ences in ol ed in
he ansi ions esponsible o hese bands, i was ound ha
while he UV band a highe wa eleng hs, he one ha is
no ECD ac i e, has an asymme ic con ibu ion o he
polyene and he a yl ings (Figu e 5), he second UV band
ha co esponds o he i s Co on in he ECD spec um
shows a mo e symme ic con ibu ion o each diphenylace y-
lene m. .u. (Figu e 5). These s udies show how he UV and
ECD Co on bands appea ing a ca. 400 and 450 nm a e
a ec ed by he o ien a ion o he diphenylace ylene m. .u.
and no jus because o he polyene.
In e es ingly, Kwak and co-wo ke s ound ha a new
ECD ac i e signal could appea in he spec a o an
asymme ic PDPA when measu ed in ilm o agg ega ed
s a e. This band appea s a ca. 450 nm, has he same sign as
he Co on band a 400 nm and was associa ed o a
longi udinal coupling be ween polyene chains.[88,89]
Thus, we explo ed di e en sol en s o y o s udy his
ECD band u he , choosing hose whe e poly-(S)-2shows
Figu e 4. a) 3D s uc u e o poly-(S)-2, AFM image o poly-(S)-2a e
annealing in DMF and CD/UV spec a o poly-(S)-2be o e and a e
annealing, c=0.5 mgmL1. b) Compa ison be ween he expe imen al
and heo e ical ECD spec um ob ained o poly-(S)-2. c) Dihed al
angles o he 12-me oligome used o compu e he heo e ical
spec a.
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poo solubili y and appea s as a dispe sed agg ega e. O he
di e en poly-(S)-2(0.5 mgmL1) solu ions es ed, only he
MeCN solu ion was ound o p oduce an ECD signal a
450 nm a e annealing a 80°C o 4 h.
In his sol en , he PDPA shows a nega i e Co on band
a 400 nm indica i e o a P/M/M(helix 1/helix 2/helix 3)
helical sca old. In e es ingly, AFM s udies o poly-(S)-2,
p epa ed in MeCN, showed he p esence o supe helices
ha ha e a helical pi ch o 14 nm and a wid h o 8 nm,
pa ame e s ha co espond o a bundle o PDPA helices
(Figu es 6a–c).
Addi ionally, con olled nanop ecipi a ion[107,108] s udies
o poly-(S)-2in a THF/H2O 1/1 / a io we e ca ied ou o
gene a e nanosphe es ha can p oduce his new ECD band
a 450 nm due o he agg ega ion o poly-(S)-2. As expec ed,
ECD s udies o hese nanosphe es dispe sed in THF/H2O
(Figu e 6d– ) show again he p esence o his new ECD
band, which he e o e can be a ibu ed o an in e ac ion
be ween di e en PDPA chains, and mo e p ecisely, o a
longi udinal coupling be ween asymme ic con ibu ions o
he polyene and he a yl ings as ex ac ed om compu a-
ional s udies (Figu e 5).
Dynamic Beha io o Asymme ic PDPAs
Once he helical s uc u e o an asymme ic PDPA (i.e.,
poly-(S)-2) was puzzled ou by he di e en s uc u al
echniques, he s imuli- esponsi e p ope ies o poly-(S)-2,
and he e o e i s dynamic beha io , we e s udied. In his
case, he a ia ion o he helical s uc u es wi h sol en s was
analyzed a e annealing a 80°C o 4 h (Figu e 7a).
In e es ingly, i is possible o obse e he dynamic beha io
o a chi al and asymme ic PDPA. This dynamic beha io
was ound be o e in achi al PDPAs bea ing a pa a-benzoic
g oup and whe e he sense can be enhanced owa ds Po M
senses by in e ac ion wi h he wo enan iome s o a chi al
amine.[79]
VCD s udies we e ca ied ou o demons a e ha he
helix in e sion is associa ed o a con o ma ional change a
he pendan g oup. So, VCD spec a o poly-(S)-2in bo h
CDCl3and DMSO-d6show ac i i y in he ca bonyl inge -
p in egion (Figu e 7d). Mo e p ecisely, he C=O s e ching
bands o he amide and he es e g oups, 1650 and
Figu e 5. a) Compa ison o calcula ed and expe imen al ECD and UV/
Vis spec a o a 12-me oligome ic PDPA model. b) Elec on densi y
di e ences o ansi ions esponsible o UV and ECD bands a 400 and
450 nm.
Figu e 6. a) Concep ual ep esen a ion o he supe helix o ma ion.
b) AFM image and heigh p o ile o he supe helical assemblies
gene a ed by poly-(S)-2du ing i s annealing in MeCN. c) Compa ison
o he ECD spec a o poly-(S)-2in DCM (good sol en , non-agg ega ed
s a e) and a e annealing in MeCN (bad sol en , agg ega ed s a e)
displaying he new low-ene gy ECD ac i e band due o he o ma ion o
sup amolecula agg ega es. d) Compa ison o he ECD spec a o poly-
(S)-2in THF (good sol en , non-agg ega ed s a e) and THF/H2O (1/1)
(nanop ecipi a ed s a e) displaying he new low-ene gy ECD ac i e
band due o he o ma ion o nanosphe es. e) SEM image o he
nanosphe es gene a ed by nanop ecipi a ion. ) DLS aces moni o ing
he nanop ecipi a ion p ocess.
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1750 cm1, espec i ely, show posi i e bands in CDCl3
[ECD-, P/M/M(helix 1/helix 2/helix 3)] and nega i e ones in
DMSO-d6[ECD+,M/P/P(helix 1/helix 2/helix 3)] which
a e asc ibed o an i and syn con o ma ions a he pendan
g oup espec i ely, as in e ed om VCD heo e ical
calcula ions o he monome ic uni in bo h con o ma ions
(Figu es 7c, ).
One o he mos in e es ing ea u es o PDPAs is ha
a e adop ing a sc ew sense excess, hey esul in ci cula
pola ized luminescen (CPL) ac i e ma e ials.[109,110] This
unique p ope y has ound many applica ions in di e en
cu ing-edge ields such as secu i y encoding[111] o CP-
OLEDs among many o he s.[112] Consequen ly, CPL s udies
we e ca ied ou o poly-(S)-2in P, ac and Mhelical s a es
exhibi ing P, null and MCPL ac i i y espec i ely (Fig-
u e 7b). The CPL sign is, as expec ed, coinciden wi h ha
o he high ene gy band in he ECD spec a, indica ing ha
a CPL (+) is due o a M/P/P(helix 1/helix 2/helix 3)
a angemen , while a CPL () is asc ibed o a P/M/M(helix
1/helix 2/helix 3) helical o ien a ion.
Mo eo e , ecen ly expe imen al, and compu a ional
CPL s udies suppo he ans- ansoidal s uc u e o
PDPAs.[113] The poo con o ma ional lexibili y in he
polyene o PDPAs c ea es a e e sing in he exci ed ene gy
le els ha pe mi an elec onically dipole-allowed ansi ion
esul ing in emissi e ma e ials, such as is obse ed he e in
poly-(S)-2. Howe e , in he case o PPAs he emissi e o m
o he helical sca old is when he polyene backbone adop s
jus a cis-cisoidal con igu a ion.[113]
The CPL emission was gauged by he dissymme ic
ac o de ined as jglum j=2(ILIR)/(IL+IR), being ILand IR
he le - and igh -handed luminescen emissions espec-
i ely. In bo h cases a maximum glum alue o ca. +/1×103
a 520 nm was ob ained, wi h quan um yield alues o φ=26
and o φ=44 o he CHCl3and DMSO solu ions espec-
i ely. To he bes o ou knowledge, his sys em ep esen s
one o he ew examples whe e he CPL sign can be
swi ched,[114] pa ing he way o he design o new s imuli-
esponsi e CPL ac i e ma e ials wi h enhanced p ope ies
and applica ions.
Conclusion
In summa y, he combina ion o da a collec ed om di e -
en s uc u al echniques allowed us o build an app oxi-
ma ed s uc u e o an asymme ic PDPA p epa ed by
polyme iza ion wi h WCl6/Ph4Sn o a diphenyl ace ylene
monome subs i u ed wi h an ac i e pen a luo ophenyl es e
(PFP) g oup. This polyme was u he ans o med in o a
chi al polyme by a pos - unc ionaliza ion eac ion ha
inco po a es L-alanine me hyl es e in o he pendan s. To
elucida e he seconda y s uc u e o he polyme , Raman
and IR spec oscopies we e used o disce n be ween Zand
Econ igu a ions o he double bonds. In his case, he
polyme adop s mainly a ans con igu a ion o he double
bonds. 1D and 2D NMR expe imen s helped us o elucida e
he egio egula i y o he polyme , indica ing a p e e ed
head- o- ail polyme iza ion eac ion. AFM s udies p o ided
use ul in o ma ion ela ed o di e en helical s uc u al
pa ame e s such as helical pi ch, packing angle and he
o ien a ion o he ex e nal helix desc ibed by he pendan
g oups. The use o molecula modelling, oge he wi h he
cons ains p o ided by he esul s o IR, Raman, NMR and
AFM s udies allowed us o build an app oxima e seconda y
s uc u e o his polyme . Thus, poly-(S)-2adop s a
p e e ed ans- ansoidal helical sca old ha is o med by
h ee di e en helices. Helix 1, desc ibed by he o ien a ion
Figu e 7. a) Pho og aphs o poly-1and poly-(S)-2unde oom ligh and
UV-ligh . b) ECD and UV spec a o poly-(S)-2in di e en sol en s
(0.5 mgmL1, 1 mm pa h). c) CPL spec a o poly-(S)-2(λExc =365 nm,
0.3 mgmL1). d) Schema ic illus a ion o he sol en - igge ed con-
o ma ional change in he pendan g oup ha p omo es helix in e sion.
e) Expe imen al VCD spec a o poly-(S)-2in CDCl3and DMSO-d6.
) Calcula ed VCD spec a o he monome ic uni s wi h an i and syn
con o ma ions. g) Op imized s uc u es o he monome ic DPA uni s
wi h an i and syn con o ma ions employed o ob ain he heo e ical
spec a depic ed in ( ).
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be ween conjuga ed double bonds; helix 2, desc ibed by he
pendan g oups; and helix 3, desc ibed by a wis ing deg ee
o he whole polyme chain and obse ed when he polyme
g ows. Compu a ional s udies on he model compound
esul in a heo e ical ECD spec um ha esembles he
expe imen al one and ha allows us o demons a e ha in
an asymme ic PDPA, he sign o he i s Co on band is
uled by helices 2 and 3, and no jus by he o ien a ion o
he polyene backbone. Mo eo e , he p esen s udy also
made i possible o explain why in di e en agg ega ion
s a es, such as supe helices o nanosphe es, a new ECD
band appea s because o a longi udinal coupling be ween
polyme chains. Addi ionally, his wo k e eals he dynamic
beha io o poly-(S)-2and shows how Po Mhelical senses
can be induced by con o ma ional changes a he pendan
g oups using ex e nal s imuli such as sol en dono abili y,
ac co obo a ed by a combina ion o VCD expe imen s
and heo e ical calcula ions. Finally, associa ed o his
dynamic helical beha io , a dynamic ci cula pola ized
luminescence swi ch wi h a jglum j= +/1×103was ob-
ained o poly-(S)-2, whe e a CPL (+) is p oduced by a M/
P/P(helix 1/helix 2/helix 3) a angemen gene a ed in dono
sol en s, while a CPL () is asc ibed o a P/M/M(helix 1/
helix 2/helix 3) helical o ien a ion p oduced when poly-(S)-2
is annealed in non-dono sol en s.
The elucida ion o he h ee-dimensional s uc u e o a
PDPA opens a new ho izon in he de elopmen o unc-
ional ma e ials due o he possibili y o es ablishing
s uc u e/ unc ion ela ionships ha can esul in an im-
p o emen o he unc ionali y o he polyme h ough a
di ec ed uning o i s seconda y s uc u e.
Acknowledgemen s
We hank Se icio de Mic oscopía Elec ónica (RIAIDT,
USC). Financial suppo om AEI (PID2019-109733GB-
I00), Minis e io de Ciencia e Inno ación (PID2019-
107307RB-100 and PID2020-117605GB-100). Xun a de
Galicia (ED431C 2018/30, ED431C 2021/40, Cen o Singula
de In es igación de Galicia ac edi ación 2019–2022,
ED431G 2019/03, Beca Leona do a In es igado es y
C eado es Cul u ales 2020 de la Fundación BBVA and he
Eu opean Regional De elopmen Fund (ERDF) and is
g a e ully acknowledged. J.J.T. hanks MICINN o a FPU
con ac . We acknowledge CESGA o compu a ional ime
and we also hank Se icio de Nano ecnología y Análisis de
Supe icies (CACTI-CINBIO, UVigo).
Con lic o In e es
The au ho s decla e no con lic o in e es .
Da a A ailabili y S a emen
The da a ha suppo he indings o his s udy a e a ailable
in he Suppo ing In o ma ion o his a icle.
Keywo ds: AFM images ·Ci cula Pola ized Luminescence
Swi ch ·Poly(diphenylace ylene)s ·Seconda y S uc u e ·
ans- ansoid
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