On-surface synthesis and collective spin excitations of a triangulene-based nanostar

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On-Su ace Syn hesis Ve y Impo an Pape
On-Su ace Syn hesis and Collec i e Spin Exci a ions o
a T iangulene-Based Nanos a
Je emy Hieulle+, Sil ia Cas o+, Niklas F ied ich, Alessio Veglian e,
F ancisco Rome o La a, So a Sanz, Dulce Rey, Ma ina Co so,
Thomas F ede iksen,* Jose Ignacio Pascual,* and Diego PeÇa*
Dedica ed o P o esso Ben L. Fe inga on he occasion o his 70 h bi hday
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How o ci e: Angew. Chem. In . Ed. 2021,60, 25224–25229
In e na ional Edi ion: doi.o g/10.1002/anie.202108301
Ge man Edi ion: doi.o g/10.1002/ange.202108301
25224  2021 The Au ho s. Angewand e Chemie In e na ional Edi ion published by Wiley-VCH GmbH Angew. Chem. In . Ed. 2021,60, 25224 –25229
Open-shell nanog aphenes ha e eme ged as p i ileged
es ing g ound o s udy ca bon-based magne ism and o
explo e po en ial applica ions o o ganic molecules in spin-
onics and quan um echnologies.[1,2] Howe e , he a omi-
cally p ecise p epa a ion o such nanos uc u es is challenging
due o he high eac i i y o hese g aphene de i a i es wi h
adical cha ac e unde ambien condi ions. In his sense, on-
su ace syn hesis unde ul a-high acuum (UHV) has
become an in e es ing al e na i e o ab ica e and s udy he
po en ial unc ionali y o open-shell ca bon nanos uc-
u es.[3,4] A pa amoun example is he p epa a ion o [3] i-
angulene (Figu e 1a), a non-Kekul iangula -shaped poly-
cyclic a oma ic hyd oca bon wi h wo unpai ed pelec ons,
which was inally gene a ed and isualized on a su ace by
means o scanning p obe mic oscopy (SPM),[5] a e p e ious
unsuccess ul a emp s by solu ion chemis y me hods.[6,7] The
pa icula di adical cha ac e o [3] iangulene wi h S=1,
makes i a pe ec building block o he design o open-shell
ca bon-based nanos uc u es wi h di e en g ound s a e spin
quan um numbe s. In ac , du ing he las h ee yea s, he
de elopmen o iangulene-based nanog aphenes has been
imp essi e; no only la ge iangulenes, such as [4]-, [5]- and
[7]- iangulene,[8–10] bu o he appealing s uc u es such as
a [7] iangulene- ing,[11] an ex ended e sion o [3] iangu-
lene,[12] o [3] iangulene dime s[13] we e p epa ed by on-
su ace syn hesis. E en he iconic Cla s goble , which is
ac ually o med by wo used [3] iangulenes sha ing one
benzene ing, was gene a ed and cha ac e ized on-su ace.[14]
In e es ingly, many o hese iangulene-based nanos uc u es
exhibi a cha ac e is ic low-ene gy spec al inge p in asso-
cia ed wi h spin exci a ions induced by inelas ic unneling
elec ons, which was no obse ed in isola ed iangulene
uni s. Such inelas ic ea u es p o ide unique access o
quan i y spin in e ac ions in nanog aphenes and o es
cu en heo e ical models o desc ibing collec i e spin
s a es.
In his wo k, we p esen a new iangulene de i a i e
consis ing in six [3] iangulene moie ies a anged in a mac o-
cycle, o ming a iangulene-based nanos a (TNS) wi h
a po e size o 1.2 nm (Figu e 1b). Besides he cap i a ing
s uc u e o his ca bon-based nanosized molecula s a , we
we e pa icula ly in igued by he spin con igu a ion adop ed
by he 12 in e ac ing unpai ed elec ons in he cyclic hexame
o e a me al subs a e.
We en isioned he p epa a ion o TNS by he on-su ace
cyclodehyd ogena ion (CDH) o mac ocycle 1(Figu e 1b), in
pa icula by he e icien o ma ion o 12 CC bonds (in
blue) and 12 p adicals in one annealing s ep p omo ed by
Au(111) unde UHV condi ions. We selec ed 2,7-dib omo-10-
(2,6-dime hylphenyl)-an h acene (2) as iangulene building
block wi h he app op ia e b omine disposi ion o he
gene a ion o he cyclic hexame 1by su ace-assis ed
Ullmann-coupling. Compound 2was ob ained in one s ep
by eac ion o dib omoan h one 3wi h (2,6-dime hylphenyl)-
li hium, ollowed by acid-p omo ed dehyd a ion (see sup-
po ing in o ma ion (SI) o de ails). Howe e , in ou hands,
he on-su ace polyme iza ion o compound 2 esul ed in
a wide a ie y o iangulene-based oligome s, as i has been
independen ly epo ed e y ecen ly.[15] The e o e, inspi ed
by he syn hesis o “nano-sa u ns” by Toyo a and co-wo k-
e s,[16] we decided o a emp he p epa a ion o mac ocycle
1by means o in-solu ion coupling o dib omoan h acene 2.
In ac , Ni-ca alyzed Yamamo o coupling o compound 2
esul ed in a mix u e o soluble and insoluble oligome s,
which, a e pu i ica ion by il a ion and washing p ocedu es,
led o he isola ion o TNS p ecu so 1in 10% yield.
Despi e he la ge molecula mass o mac ocycle
1(C132H96, 1680 Da), i s acuum deposi ion was possible by
[*] D . S. Cas o,[+] D . D. Rey, P o . D . D. PeÇa
Cen o Singula de In es igacin en Qumica Biolxica e Ma e iais
Molecula es (CiQUS) and Depa amen o de Qumica O gnica,
Uni e sidade de San iago de Compos ela
15782-San iago de Compos ela (Spain)
E-mail: [email p o ec ed]
D . J. Hieulle,[+] N. F ied ich, A. Veglian e, F. R. La a, D . J. I. Pascual
CIC nanoGUNE-BRTA
20018 Donos ia-San Sebas in (Spain)
E-mail: [email p o ec ed]
F. R. La a, D . M. Co so
Cen o de Fsica de Ma e iales CSIC/UPV-EHU-Ma e ials Physics
Cen e , 20018 Donos ia-San Sebas in (Spain)
S. Sanz, D . M. Co so, D . T. F ede iksen
Donos ia In e na ional Physics Cen e (DIPC)
20018 Donos ia-San Sebas in (Spain)
E-mail: [email p o ec ed]
D . T. F ede iksen, D . J. I. Pascual
Ike basque, Basque Founda ion o Science
48013 Bilbao (Spain)
[
+
] These au ho s con ibu ed equally o his wo k.
Suppo ing in o ma ion and he ORCID iden i ica ion numbe (s) o
he au ho (s) o his a icle can be ound unde :
h ps://doi.o g/10.1002/anie.202108301.
 2021 The Au ho s. Angewand e Chemie In e na ional Edi ion
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 Non-Comme cial
NoDe i s License, which pe mi s use and dis ibu ion in any
medium, p o ided he o iginal wo k is p ope ly ci ed, he use is non-
comme cial and no modi ica ions o adap a ions a e made.
Abs ac : T iangulene nanog aphenes a e open-shell mole-
cules wi h p edic ed high spin s a e due o he us a ion o
hei conjuga ed ne wo k. Thei long-sough syn hesis became
ecen ly possible o e a me al su ace. He e, we p esen
a mac ocycle o med by six [3] iangulenes, which was
ob ained by combining he solu ion syn hesis o a dime hyl-
phenyl-an h acene cyclic hexame and he on-su ace cyclo-
dehyd ogena ion o his p ecu so o e a gold subs a e. The
esul ing iangulene nanos a exhibi s a collec i e spin s a e
gene a ed by he in e ac ion o i s 12 unpai ed p-elec ons
along he conjuga ed la ice, co esponding o he an i e o-
magne ic o de ing o six S=1 si es (one pe iangulene uni ).
Inelas ic elec on unneling spec oscopy esol ed h ee spin
exci a ions connec ing he single g ound s a e wi h iple
s a es. The nanos a beha es close o p edic ions om he
Heisenbe g model o an S =1 spin ing, ep esen ing a unique
sys em o es collec i e spin modes in cyclic sys ems.
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lash-annealing a silicon wa e loaded wi h g ains o his
compound. Figu e 2a shows an o e iew STM image o he
Au(111) subs a e a e deposi ion. Molecula ings su i ing
he mal sublima ion appea in closed-packed molecula
islands, su ounded by polyme ic agmen s, p obably om
ings b oken du ing he he mal sublima ion ( o he co e age
o Figu e 2a, abou 8% o he ma e ial deposi ed co esponds
o in ac p ecu so 1). Indi idual molecules 1, ex ac ed om
he domains by la e al manipula ion wi h he STM ip, appea
composed o six p o usions co esponding o he six dime-
hylphenyl g oups a ached o he an h acenes moie ies
(Figu e 2b).
To induce he ull plana iza ion o he mac ocycle 1in o
he a ge ed TNS, we annealed he subs a e o 330
8
C. A his
empe a u e, he CDH eac ion leading o he o ma ion o p
adicals akes place wi h he emo al o 3 H a oms pe me hyl
side g oup and ing closing h ough CC bond o ma ion.
Howe e , due o he high empe a u es equi ed o ully
plana ize he whole ings and o hei adical cha ac e , he
CDH compe es wi h addi ional CC coupling and bond-
b eaking eac ions, esul ing in a ew su i ing isola ed TNS
(abou 20%) and many iangulene oligome s a ound (Fig-
u e 2c and d). Bond- esol ed scanning unneling mic oscopy
(BR-STM) images o molecula ings like in Figu e 2d,
ob ained using a CO unc ionalized STM ip, esol ed he
iangulene uni s connec ed in a closed ing s uc u e (Fig-
u e 2e), in ag eemen wi h he expec ed s uc u e o TNS
(Figu e 1b). The plana iza ion eac ion in ol es he o ma-
ion o wel e six-membe ed ings, each one a he posi ion o
he me hyl g oups o p ecu so 1. The esul ing s uc u e
accommoda es wo unpai ed pelec ons in each iangulene
uni , which a e expec ed o in e ac h oughou he conju-
ga ed p-la ice and s abilize a collec i e spin s a e.[13]
A i s inspec ion o he magne ic in e ac ions in he ing
ob ained by mean- ield Hubba d (MFH) simula ions (Fig-
u e 3a, see me hods in he SI o de ails) e eals ha in a-
iangulene pa allel magne ic coupling emains s ong in he
TNS, building a o al spin S=1 in each iangulene uni , and
domina ing o e a weake an i-pa allel coupling be ween
iangulene sub-uni s.[17,18] Such in e ac ion pa e n is
expec ed o lead o a global spin single g ound s a e in he
TNS. In e es ingly, an i e omagne ic (AFM) ings o in ege
spins ha e been widely s udied o demons a ing he
Haldane conjec u e,[19,20] s a ing ha he spin exci a ion
spec um o an in ini e Heisenbe g S=1 AFM chain emains
gapped, wi h an ene gy alue close o 0.4 J(Jbeing he
Heisenbe g exchange cons an ). Such a gapped spec um is an
anomalous beha io o in ege AFM spin chains, which
con as s wi h he gapless spin exci a ion o simila chains
made o hal -in ege spins.[20]
To expe imen ally de e mine he spin con igu a ion o he
TNS, we p obed hei exci a ion spec um by Inelas ic
Elec on Tunneling Spec oscopy (IETS).[21,22] Speci ically,
we measu ed di e en ial conduc ance (dI/dV) s. bias spec a
o e he sides o iangulene uni s o a TNS and ound bias-
symme ic s epped ea u es (Figu e 3c), which a e a cha ac-
e is ic inge p in o spin exci a ion induced by inelas ic
elec on unneling.[13,14,23] All iangulene uni s show spec a
Figu e 1. a) P e iously epo ed iangulene de i a i es. b) Syn hesis o iangulene-based nanos a (TNS) de eloped in his wo k, by combining
in-solu ion and on-su ace chemis y.
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wi h he same shape (see addi ional da a in SI), composed o
h ee s eps o di e en heigh a E1=14(1) mV, E2=
42(2) mV and E3=80(2) mV. The i s wo dI/dVs eps
ha e simila heigh (each amoun ing o a ela i e inc ease o
conduc ance o 35%), while he hi d one appea s wi h
small spec al weigh (<7%). As we show in he s acked dI/
dVplo o Figu e 3d, he inelas ic signal appea s localized
o e he edges o he iangulene uni s. Such localiza ion
pa e n is clea ly pic u ed by mapping he inelas ic signal d2I/
dV2o e he whole TNS (shown in SI), con i ming ha he
exci a ion p obabili y o collec i e spin s a es coincides wi h
he la ge spin densi y o e edge si es expec ed o p- adical
s a es.
The p esence o h ee spin exci a ion s eps in he dI/dV
spec a is a ema kable ea u e ha con as s wi h he single-
s epped spec a measu ed in iangulene dime s[13] and o he
g aphene nanos uc u es.[14,23–25] Due o he DS=0, 1
selec ion ule imposed by conse a ion o angula momen-
um,[21] single IETS s eps we e a ibu ed o single - iple
ansi ions induced by he unneling elec ons. The mul i-s ep
spec um ound o he TNS un eils a complex exci a ion
pa e n o collec i e spin s a es in he iangulene ing,
accessible by inelas ic unneling elec ons.
To un a el he o igin o he inelas ic s eps, we modeled he
coupled spin sys em wi h an iso opic Heisenbe g Hamil o-
nian desc ibing he AFM exchange in e ac ion o six S=1
spins in a closed ing, as in Figu e 3e ( heo e ical de ails in
SI). The many-body g ound s a e o he TNS, ob ained by
nume ically exac diagonaliza ion o he spin Hamil onian
(see me hods in SI), has a o al spin ST=0 (single ), and
consis s o he supe posi ion o he classical solu ions o
an i e omagne ically coupled iangulene segmen s (i.e.
g ound s a e om he MFH model,[26] in Figu e 3a), and
o he combina ions o he iangulene spins wi h global spin
ST=0. Simila ly, he se o exci ed s a es o he TNS a e
collec i e spin modes wi h o al spin amoun ing om ST=0 o
ST=6, which can be desc ibed as a supe posi ion o iangu-
lene S=1 mul iple s a es.
To explain he spin exci a ion spec um, we assume ha
s eps co espond o ansi ions wi h DS=1[27] and adjus he
Heisenbe g exchange cons an J o i he lowes allowed
exci a ion wi h he i s dI/dVs ep (cen e ed a 14 mV). The
bes spec al ma ch occu s using J=18 meV. This alue is
la ge han he exchange cons an de i ed om he Heisen-
be g model in iangulene dime s.[13] Figu e 3 shows he
compu ed exci a ions wi h ene gy up o 100 meV, classi ied
acco ding o hei global spin (up o ST=3). Se e al single
and iple exci ed s a es appea wi hin he ene gy window o
he IETS spec um, including iple s a es g ouped a ound
exci a ion ene gy alues (13 meV, 45 meV and 88 meV) ha
Figu e 2. On-su ace syn hesis o iangulene-based nanos a (TNS). a,b) STM images o p ecu so 1as deposi ed on he Au(111) subs a e.
Molecula ings appea assembled in packed domains. The isola ed molecule 1in (b) was ex ac ed using he STM ip, o p o e he co alen
na u e o he mac ocycle. c) STM image o he su ace a e annealing. TNS and agmen s appea mos ly plana ized. d) STM image o a TNS
o med a e he mal annealing. e) BR-STM image o a TNS.(a:V=0.5 V, I=60 pA; b: V=0.2 V, I=40 pA; c: V=1V,I=40 pA; d: V=1V,
I=100 pA; e: V=5 mV). Da a analysis using WSxM.[30]
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ag ee wi h he h ee dI/dVs eps in ou expe imen al spec a.
Howe e , no all ansi ions a e equally accessible by unnel-
ing elec ons. To accoun o he c oss-sec ion o each spin
ansi ion and explain hei di e en dI/dVs ep heigh , we
calcula ed he co ela ion o collec i e TNS spin s a es wi h
unneling elec ons ollowing he exp ession by Te nes.[28]
This quan i y e lec s he sca e ing p obabili y o a unnel
elec on wi h e e y collec i e spin eigens a e and, hus, i is
ela ed o he inelas ic signal and, hence, o he s ep heigh .
The i s exci a ion a 13 meV has he la ge spec al weigh
(60%, as shown by a ows in Figu e 3 ), while se e al
s a es bundled a a ound E2=45 meV con ibu e o mos o
he emaining weigh (39%). Highe s a es a e seemingly
weake in e ms o spec al weigh and p oduce a ain s ep in
he spec um. A simula ed spin exci a ion spec um shown in
Figu e 3c ( ed plo ) ep oduces wi h good ag eemen he
posi ion and ela i e in ensi y o he h ee s eps obse ed in
he expe imen al plo . The e a e, howe e , small di e ences
in s ep posi ion and heigh ha can be accoun ed o by
including a mo e p ecise desc ip ion o spin in e ac ions. Fo
example, including a ini e in a- iangulene coupling can
co ec ly ep oduce he equal heigh o i s and second s eps
(see analysis o wel e 1/2 spins in he SI). O he e ec s such
as a eno maliza ion o spin in e ac ions in he TNS induced
by he me al subs a e,[29] o highe o de co ec ion o he
Heisenbe g coupling be ween spins,[15,18] can be addi ionally
inco po a ed in o he model o quan i a i ely ep oduce he
expe imen al spec a.
In summa y, we ha e demons a ed he success ul ab i-
ca ion o a mac ocycle o med by he co alen a achmen o
six [3] iangulenes, h ough a combina ion o solu ion syn-
hesis coupling and on-su ace plana iza ion. The design-
d i en s a egy o p ecu so molecules is hus a possible
me hod o p oduce monodispe sed ensembles o complex
open-shell nanos uc u es. Fo he p esen ed case, he ian-
gulene cyclic hexame accumula ed 12 unpai ed elec ons.
Figu e 3. Collec i e spin exci a ions in a iangulene-based nanos a . a) Spin densi y map o a TNS ob ained om mean- ield Hubba d simula ions
(on-si e Coulomb po en ial U=3.5 eV). b) BR-STM image indica ing he TNS si e whe e spec a in (c) and line spec a in (d) we e aken. c) dI/dV
spec um on a iangulene uni o a TNS, showing inelas ic s eps a E1=14 mV, E2=42 mV and E3=80 mV. The ed line shows he
simula ed spec al unc ion om he Heisenbe g model wi h exchange cons an J=18 meV. d) dI/dVspec al line measu ed ac oss he single
iangulene uni indica ed in (b). The exci a ion s eps appea a ound he bo de s o he TNS. Se -poin : V=100 mV, I=0.1 nA; indi idual spec a
we e o se ed o appea wi h he same ze o-bias conduc ance. e) Model o a six-membe ed ing o spin-1, simula ed using he Spin Hamil onian
shown in he panel. ) Exci a ions o he collec i e spin s a es ob ained om a Heisenbe g model wi h J=18 meV (only s a es up o ST=3 a e
shown). Thin/ hick lines indica e singly/doubly degene a e o al spin s a es. A ows ma k he h ee dominan ansi ions induced by unneling
elec ons (see SI), which coincide in ene gy and weigh wi h he h ee s eps in he expe imen al plo in (c).
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Tunnel spec oscopy e ealed ha he unique spin s a es
expec ed o such in e ac ing sys ems su i e on a me al
su ace. We de ec ed h ee collec i e spin exci a ions om he
many-body single g ound s a e o iple s a es o he spin
ing. These indings p o ed ha he spin s a es o he nanos a
a e compa ible wi h an an i e omagne ic S=1 spin ing.
Acknowledgemen s
We acknowledge unding om Agencia Es a al de In es iga-
cin (PID2019-107338RB, FIS2017-83780-P, MDM-2016-
0618), om he Xun a de Galicia (Cen o singula de
in es igacin de Galicia, acc edi a ion 2019–2022, ED431G
2019/03), he Basque Go e nmen (G an IT1255-19), he
Eu opean Union H2020 p og am (FET Open p ojec
SPRING #863098), he Eu opean Regional De elopmen
Fund and he Basque Depa amen o de Educacin o he
PhD schola ship no. PRE_2020_2_0049 (S.S.) The au ho s
hank D. P ez and E. Gui in o ui ul discussions.
Con lic o In e es
The au ho s decla e no con lic o in e es .
Keywo ds: bond- esol ed STM · on-su ace syn hesis ·
open-shell · spin exci a ion · iangulene
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