Nanoscale
PAPER
Ci e his: Nanoscale, 2023, 15, 4022
Recei ed 16 h Sep embe 2022,
Accep ed 30 h Janua y 2023
DOI: 10.1039/d2n 05114a
sc.li/nanoscale
Communica ion o molecula fluo opho es wi h
o he pho oluminescence cen es in ca bon do s†
Michal Lange ,
a,b
LukášZd ažil,
a
Mi osla Med eď*
a,c
and
Michal O yepka *
a,d
The es ablishmen o s uc u e-pho oluminescence (PL) ela ionships emains an ul ima e challenge in
he field o ca bon do s (CDs). I is now commonly unde s ood ha a ious s uc u al domains may
e ol e du ing he p epa a ion o CDs; none heless, we a e s ill a om cap u ing he specific ea u es ha
de e mine he o e all PL o CDs. Al hough he co e, su ace and molecula s a es a e usually conside ed
he h ee main sou ces o PL, i is no known o which ex en hey in e ac and/o affec one ano he .
Expec edly, he communica ion be ween he diffe en PL cen es depends on he mu ual a angemen
and he ype o linking. To gain insigh s in o such a communica ion, ime-dependen densi y unc ional
heo y (TD-DFT) calcula ions we e pe o med o se e al (N-doped/O- unc ionalized) polya oma ic
hyd oca bons (PAHs) as ep esen a i e models o he co e/su aces PL s a es and he p o o ypical mole-
cula fluo opho e (MF) 5-oxo-1,2,3,5- e ahyd oimidazo-[1,2-α]-py idine-7-ca boxylic acid (IPCA), con-
side ing diffe en in e ac ion modes, namely hyd ogen bonded and s acked complexes as well as co-
alen ly bonded and used s uc u es. Ou esul s e ealed ha each o he s udied a angemen s in some
way suppo ed he communica ion be ween he PL cen es. The deac i a ion pa hways ypically in ol e
mul iple cha ge and ene gy ans e e en s ha can p omo e he o ma ion o cha ge sepa a ed s a es
and/o lead o he ac i a ion o o he PL cen es in CDs. Depending on he a angemen , he doping
pa e n and su ace unc ionaliza ion, bo h he CD co e and he MF can ac as an elec on dono o
accep o , which could help o design CDs wi h desi able hole–elec on su ace/co e cha ac e is ics.
In oduc ion
Ca bon do s (CDs) ep esen a highly a ac i e class o ze o-
dimensional ca bon nanoallo opes. Since hei disco e y by Xu
e al.,
1
he CDs ha e been p aised as low-cos , biocompa ible,
chemically s able, wa e soluble nanosys ems wi h ou s anding
op ical and elec ical p ope ies, such as in ense pho o-
luminescence (PL), high esis ance o pho obleaching, high elec-
on mobili y, and pho o-induced elec on ans e .
2,3
These p o-
pe ies endow CDs wi h applica ions anging om pho oca aly-
sis, sensing, and imaging up o ligh -emi ing diodes.
4–9
Gene ally, CDs a e de ined as quasi-sphe ical objec s wi h a
diame e below 10 nm possessing a mul ilaye g aphi e co e
and oxygen and/o ni ogen unc ional g oups on he CD
shell.
10
Al hough CDs a e o en ca ego ized in o h ee ypes,
i.e., g aphene quan um do s (GQDs), ca bon nanodo s (CNDs),
and ca bonized polyme ic do s (CPDs), hei s uc u al com-
plexi y is as . This s ems om miscellaneous syn he ic p o-
cedu es, as small changes in eac ion condi ions o p ecu so s
can lead o diffe en CDs.
11–14
To achie e be e con ol o e
he p ope ies o CDs o speci ic applica ions, he ela ion-
ships be ween he s uc u e and PL mechanisms o CDs need
o be ully unde s ood.
CDs exhibi mul i-cen e emission wi h h ee sou ces being
used o explain he PL o igins o CDs, i.e., co e s a es due o
ca bon sp
2
domains, su ace s a es s emming om su ace
chemical g oups, and molecula s a es due o he p esence o
molecula luo opho es (MFs).
15–18
Va ious s uc u al domains
wi hin CDs may con ibu e o he PL o CDs, and hey can
e en co-ope a e and/o in luence each o he .
19
Thus, many
p ocesses such as he cha ge ans e (CT), ene gy ans e
(ET), adia ionless de-exci a ions, ( e e se) in e -sys em c oss-
ing (ISC), and in ol emen o ap s a es may be expec ed o
occu upon pho oexci a ion o CDs (Fig. S5†). Ne e heless,
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI: h ps://doi.o g/
10.1039/d2n 05114a
a
Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced
Technology and Resea ch Ins i u e (CATRIN), Palacký Uni e si y Olomouc, Šlech i elů
241/27, 783 71 Olomouc, Czech Republic. E-mail: mi osla .med [email p o ec ed],
[email p o ec ed]
b
Chemical and Biological Sys ems Simula ion Lab, Cen e o New Technologies
Uni e si y o Wa saw, 2c Banacha S ee , 02-097 Wa szawa, Poland
c
Depa men o Chemis y, Facul y o Na u al Sciences, Ma ej Bel Uni e si y,
Tajo ského 40, 974 01 Banská Bys ica, Slo ak Republic
d
IT4Inno a ions, VŠB-Technical Uni e si y o Os a a, 17. lis opadu 2172/15, 708 00
Os a a-Po uba, Czech Republic
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despi e all he effo s and acqui ed da a, he clea link
be ween a pa icula s uc u al ea u e and he obse ed PL
signal has no been es ablished ye .
Compu a ional chemis y ools a e e y help ul in elucida -
ing ela ionships be ween he s uc u e and PL p ope ies o
molecula sys ems. The heo e ical s udies ocusing on he
in e play be ween a ious PL cen es in he con ex o CDs a e,
howe e , e y limi ed.
20,21
Conce ning he molecula s a es,
5-oxo-1,2,3,5- e ahyd oimidazo-[1,2-α]-py idine-7-ca boxylic
acid (IPCA) has been iden i ied as a p o o ypical MF in CDs.
22
The op ical p ope ies o IPCA monome and dime ha e been
heo e ically s udied in gas and aqueous sol en ,
23
and also as
a non-co alen ly bonded dime in he model o CD by QM/
MM.
24
None heless, nei he expe imen s no modelling eally
p o ided any di ec insigh in o he communica ion o he
molecula s a es s emming om he p esence o IPCA wi h
o he s uc u al mo i s in CDs. No ully esol ed s uc u al
o ganiza ion o MFs wi hin CDs con ibu es o his conun-
d um. Fo example, ci ic-acid based luo opho e moie ies
ha e been epo ed as a ee- loa ing by-p oduc o CDs p epa-
a ion,
25
bu also as being p esen in he CD in e io
26
as well
as CD ex e io ,
27
o e en co alen ly bonded o he CD
su ace,
28
and hese possible scena ios e y likely depend on
syn he ic condi ions.
29
He e, we desc ibe he PL p ope ies o a se ies o co e/
su ace/MF models in ol ing a p o o ypical MF IPCA in
a ious chemical and s uc u al con ex wi hin CDs. In pa icu-
la , ou models co e all plausible ypes o s uc u al a ange-
men s (H-bonded, s acked, co alen ly bonded, and used
sys ems) o IPCA and a py ene-like polya oma ic hyd oca bon
(PAH) molecule ep esen ing he small-sized co e o CDs. Ou
esul s show ha he in e play be ween hese PL cen es may
no only in oduce new peaks in he abso p ion spec a o he
complex (e.g., in co alen ly bonded sys ems), bu many new
da k s a es eme ge, which can come o play du ing de-exci-
a ion p ocesses. Also, depending on he s uc u al o ganiz-
a ion o IPCA and PAH uni s, plausible CT and ET e en s
be ween he molecula and co e s a es along he in e nal in e -
sion de-exci a ion cascade we e iden i ied, which can lead o
he emission om a PL cen e (e.g., co e) diffe en om he
one pho o-ac i a ed du ing abso p ion (e.g., IPCA).
Expe imen al
Models
Ou s udied s uc u al domains can be so ed in o ou g oups
(Fig. 1), namely H-bonded and s acked complexes, co alen ly
single-bonded sys ems, and used sys ems. All model sys ems
we e cons uc ed om one p o o ypical MF, IPCA, and one
PAH molecule, which ep esen s a CD co e (Fig. 1, Fig. S1†).
These models co e he main s uc u al ypes, which can be
p esen and co-ope a e in he o e all PL o he CDs. Le us
no e ha he used s uc u es can also be conside ed as
models gi ing ise o he su ace PL s a es.
Besides a pa en py ene molecule (model A; see Fig. S1a†),
CD co e models con aining wo inne -la ice g aphi ic ni o-
gens (g aphi ic-N-co e models B, D, and F) and wo g aphi ic-
edge ni ogens (g aphi ic-N-edge models C and G) we e con-
side ed (Fig. S1a†). As CDs usually possess oxygen-con aining
unc ional g oups on hei su ace and/o edges, some PAHs
we e unc ionalized wi h ei he wo oxo-g oups (models D and
E) o one ca boxylic g oup (models F and G). In he model E,
he g aphi ic-edge doping was eplaced by wo g aphi ic-co e
ni ogens in he same ing (g aphi ic-N-co e2), as we only con-
side ed Kekulé s uc u es. To assess he co e-size effec s, co-
alen ly bonded as well as s acked complexes con aining an
N-doped co onene moie y we e also conside ed (Fig. S15†).
Me hods
To examine he effec s o a ious binding modes on he na u e
o elec onic ansi ions in CD/MF sys ems, he lowes exci ed
s a es o coupled sys ems we e ho oughly analysed in e ms o
he e ical (de-)exci a ion ene gies and he co esponding
oscilla o s eng hs as well as in e ms o elec on densi y
diffe ence (EDD) plo s and na u al ansi ion o bi als (NTOs).
The densi y unc ional heo y (DFT) and ime-dependen
densi y unc ional heo y (TD-DFT)
30
calcula ions we e pe -
o med by applying he Coulomb-a enua ing h ee-pa ame e
Becke, Lee–Yang–Pa (CAM-B3LYP)
31,32
unc ional including
he D3 co ec ion o accoun o dispe sion in e ac ions
33
in
combina ion wi h he de 2-TZVP
34
basis se . All calcula ions
we e pe o med o hyd a ed sys ems, employing he implici
uni e sal sol a ion model based on solu e elec on densi y
(SMD),
35
and using he Gaussian 16 ( e ision B.01).
36
The
s uc u es we e ende ed in PyMOL
37
and Chemc a .
38
To simula e abso p ion spec a, he g ound s a e (GS) geo-
me y o all he models (Fig. S1†) was i s op imized wi hou
any symme y cons ain s. All he op imized geome ies we e
e i ied o be ue minima on he po en ial ene gy su ace by
he absence o imagina y equencies in he ha monic
ib a ional analysis. The elec onic e ical exci a ion ene gies
(VEEs) we e calcula ed wi hin he TD-DFT amewo k using
he linea esponse (LR) as well as co ec ed linea esponse
(cLR) app oaches,
39–43
wi h he i s hi y single s a es aken
in o conside a ion in he o me . Fu he mo e, LR-TD-DFT cal-
cula ions o en lowes single s and iple exci a ions we e pe -
o med, on he GS geome ies, o judge on he easibili y o
ISC in model s uc u es. I should be no ed ha he e alua ion
o adiaba ic single – iple ene gy gaps (ΔE
ST
) was no achie -
able o all sys ems due o con e gence p oblems (using he
applied me hod) du ing geome y op imiza ions o iple
exci ed s a e s uc u es.
Fo he calcula ion o adia i e de-exci a ions, Kasha emis-
sions om he S
1
s a e we e conside ed. The S
1
geome y was
i s op imized wi h he de 2-SVP basis se , ollowed by calcu-
la ion o emission ene gies wi h he de 2-TZVP basis se apply-
ing he equilib ium sol a ion egime o he ES calcula ions
and nonequilib ium sol a ion o he subsequen calcula ion
o he GS.
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CT and ET analyses we e pe o med based on he NTOs
ha we e ende ed in Chemc a ( e sion 1.8) and elec on
densi y diffe ences (EDDs) displayed in GaussView ( e sion
5.0).
44
The calcula ions o o al coupling o elec onic ene gy
ans e (EET) we e pe o med using TD-DFT as implemen ed
in Gaussian 16, whe e sol en effec s we e in oduced using
he SMD ca i y o he whole sys em;
45,46
ES calcula ions we e
pe o med on each agmen , and all he couplings among all
he esul ing s a es we e compu ed; and ONIOM-like
47
link-
a om inpu in o ma ion o cap he agmen s was used o ou
models bonded h ough a single co alen bond in EET
calcula ions.
Resul s and discussion
Fi s , we desc ibe he changes in abso p ion and emission o
CDs due o he o ma ion o he PAH/IPCA complexes (Fig. 1).
In he second pa , we demons a e how a pa icula bonding
ype and/o speci ic s uc u al a angemen o IPCA and PAH
can affec he de-exci a ion p ocesses assuming he pho o-ac i-
a ion o he sys em by a 350 nm sou ce o ligh , i.e., in he
egion o he abso p ion maximum o IPCA. The analysis o
he op ical p ope ies o isola ed molecules om which we
cons uc ed ou models o es ablish s uc u e-PL ela ionships
in CDs and ace he communica ion be ween he co e/su ace
and molecula s a es is de ailed in he ESI.†
Changes in abso p ion spec a
In he H-bonded sys ems, he in e ac ion o bo h IPCA and
PAH uni s wi h ligh emains p ac ically independen o each
o he . This means ha he o ma ion o H-bonded complexes
does no gi e ise o any signi ican new peaks in he CDs
abso p ion spec a (Fig. 2a and b), e en hough he on ie
o bi al analysis (Fig. S1b and S7†) sugges s a possible CT exci-
a ion om he PAH uni o IPCA. Indeed, a CT π–π
*
ansi ion
(s a e S
4
, 401 nm) om PAH o IPCA wi h a a he small in en-
si y ( = 0.026) due o a small o e lap o he in ol ed MOs was
obse ed in ou COOH_co e model (Tables S11 and S12†). A
simila , excep being da k, CT s a e (S
3
,λ
max
= 371 nm, =
0.002) was obse ed o he COOH_edge model.
The s acking a angemen o PAHs and IPCA opens new
abso p ion channels in CDs. No ably highe ene gy o he
highes occupied molecula o bi al (HOMO) in N-doped PAH
uni s and sligh ly lowe ene gy o he lowes unoccupied mole-
cula o bi al (LUMO) in IPCA (Fig. S1†) indica e he plausibili y
o low-lying CT exci a ions om N-doped py ene o IPCA, as
demons a ed in s acked_co e and s acked_edge models
(Fig. 2c, d, S8; Tables S13 and S14†). Howe e , in he model
wi h a g aphi ic-N-edge uni (s acked_edge model), he S
0
→S
1
elec onic exci a ion (λ
max
= 558 nm) exhibi s much s onge
CT cha ac e (D index 3.48 Å) om PAH o IPCA (Table S14†)
compa ed o s acked_co e complex (D index 1.33 Å). Owing o
i s non-ze o oscilla o s eng h ( = 0.021) a new small peak in
he isible egion can be obse ed (Fig. 2d). Ne e heless, he
in ense abso p ion bands o s acked_co e and s acked_edge
(abo e 300 nm) a e due o LEs wi hin he molecula com-
ponen s (Tables S13 and S14†), which is in line wi h a good
o e lap o he sum spec a wi h hose o sepa a e molecules
albei he in ensi y o peaks sligh ly dec eases upon he
complex o ma ion (Fig. 2c and d).
In oducing O- unc ional g oups in he s acked models
(oxo_s acked1_a,oxo_s acked1_b) p ese es he shape o he
sum abso p ion spec a o componen s only abo e 380 nm,
i.e., he main abso p ion bands a e p edominan ly due o LE
ansi ions in his egion (Fig. 2e). The S
0
→S
1
ansi ion is a
b igh LE in O-PAH ( = 0.250) peaking a a ound λ
max
≈
453 nm. Below 380 nm, new abso p ion peaks, ele an o
pho oexci a ion using a ∼350 nm i adia ion sou ce, eme ge
upon he s acking. In oxo_s acked1_a, hese peaks appea due
o he hyb idized local and cha ge- ans e (HLCT) S
0
→S
3
ansi ion (λ
max
≈360 nm, = 0.049), whe e an elec on is
Fig. 1 The schema ic figu e on how CDs ( wo models in he middle), can be o med om PAHs and diffe en in e ac ion sys ems o PAH/IPCA ha
we e used as models o ou calcula ions (co esponding chemical s uc u es a e shown in Fig. S1†). Colou ing scheme: blue –ni ogen; ed –
oxygen; whi e –hyd ogen; g ey, g een, o ange, beige, cyan, iole , yellow –ca bon. Diffe en colou s o ca bons we e chosen o clea ly show hei
loca ion in he models o CDs.
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ans e ed om π-o bi al delocalized o e bo h componen s
o a π
*
o bi al localized only on IPCA (Table S16†), and S
0
→
S
4,5
(λ
max
≈330 nm, = 0.04) ansi ions (Fig. 2e, Table S16†).
In oxo_s acked1_b, a new abso p ion peak co esponding o
S
0
→S
3
CT exci a ion (PAH →IPCA) a ose a ∼350 nm
( = 0.054, Fig. 2e, Table S17†), o e lapping wi h he LE peak
o he IPCA molecule. Ano he albei less b igh CT
(IPCA →PAH) exci a ion (λ
max
≈335 nm, = 0.027) can be
de ec ed in he egion o spec um, whe e ypical exci a ion
wa eleng hs a e a ge ed o pho oexci a ion o CDs.
The change o he N-doping pa e n in he O- unc ionalized
s acked complexes can cause signi ican s uc u al dis o ions
(Tables S18 and S19, Fig. S17†). Whe eas oxo_s acked2_b
keeps a compac s acked a angemen , oxo_s acked2_a co es-
ponds o a pa ially open s acked con igu a ion s abilized by
he o ma ion o H-bonding (IPCA–N⋯O–PAH). Consequen ly,
he abso p ion spec um o oxo_s acked2_a copies o a la ge
ex en hose o he isola ed molecules. On he o he hand, he
compac a angemen o oxo_s acked2_b gi es ise o sligh
modi ica ions o he band shapes in he 350–400 nm egion
(Fig. 2 ) esul ing mainly om he S
0
→S
3
(λ
max
≈352 nm,
= 0.075) and S
0
→S
5
(λ
max
≈343 nm, = 0.041) ansi ions
exhibi ing HLCT cha ac e , whe e he elec on is ans e ed
om π-o bi al o IPCA o π
*
-o bi al delocalized o e he
complex (Table S19†).
Abso p ion spec a o models when he co e/molecula PL
cen es CDs a e connec ed wi h an es e bond ( e e ed as
es e 1_co e and es e 2_edge models) g ea ly o e lap wi h he
abso p ion spec a o he isola ed molecula componen s
(Fig. 3a and b). These s uc u al mo i s a e also i al o unde -
s anding he CDs PL as hey may occu du ing he syn hesis o
CDs be o e he ca boniza ion p ocess is comple ed. No b igh
exci a ions wi h CT cha ac e we e obse ed in hese models,
which we a ibu e o he wis ed mu ual o ien a ion o bo h
uni s (Fig. S11a–h, Tables S20 and S21†) and o he cha ac e
o he es e bond, which does no enable efficien elec on
delocaliza ion.
On he o he hand, an amide bond allows he whole
complex being quasi-plana which can signi ican ly affec he
elec onic ene gy le els o linked componen s. In he amide1_-
co e model, he peak co esponding o LE on IPCA is blue-
shi ed by 28 nm compa ed o an isola ed IPCA molecule, as
Fig. 2 Abso p ion spec a o non-co alen ly bonded complexes along wi h hose o he sepa a ed molecules which o m he complexes (see mole-
cules A–H in Fig. S1†). Fo each spec um, he line spec a (conside ing 30 lowes single s a es) we e con olu ed by a Gaussian unc ion assuming
he inhomogeneous b oadening o peaks wi h σ= 20 nm.
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he –COOH g oup o IPCA was eplaced by –CONH–g oup
(λ
max
≈315 nm, = 0.173, Fig. 3c). In amide1_edge, a small
pa o elec on densi y is ans e ed om N-doped py ene o
a linking amide g oup and o he IPCA uni upon he S
0
→S
2
exci a ion (λ
max
≈444 nm, = 0.177), which indica es efficien
in e ac ion be ween he wo PL cen es. An addi ional small
peak ( = 0.020) can be obse ed a 385 nm due o S
0
→S
3
,
which exhibi s CT cha ac e om PAH o IPCA. Due o he e-
placemen o –COOH by –CONH–, he LE on IPCA is also blue-
shi ed o 316 nm, gi ing ise o an addi ional peak in he
abso p ion spec um o amide1_edge model (Fig. 3c).
The posi ion o an amide binding g oup co alen ly linking
PL s uc u al mo i s inside CDs can also signi ican ly affec he
ene gy le els o he sys em. The obse ed changes a e
indi ec ly ela ed o he dihed al angle be ween IPCA and
py ene uni s (Fig. S11†), as he quasi-plana o ien a ion can
lead o a mo e in ense in e ac ion o molecula o bi als o he
componen s. Ou esul s co obo a e his hypo hesis, as
se e al CT exci a ions we e iden i ied in amide2 and amide3
posi ions o bo h N-doping pa e ns (Tables S22–S27†).
Models wi h a g aphi ic-co e uni ,amide2_co e and amide3_-
co e, do no abso b ligh in he spec al egion 330–400 nm,
simila ly o amide1_co e model (Fig. 3d). The elec on is ans-
e ed o he amide g oup and IPCA in S
0
→S
3
in amide2_co e
(Table S24†), causing a sligh edshi o he abso p ion
maximum. The change o he linking posi ion also in oduces
he new peaks a ound 430 nm, which can be desc ibed as
(weak) HLCT S
0
→S
4
exci a ion om PAH o IPCA. A s iking
diffe ence due o diffe en linking o –CONH–bond was no ed
o amide3_edge, in which he abso p ion peak o S
0
→S
1
HLCT ansi ion om PAH o IPCA was blue-shi ed o λ
max
≈
539 nm (Fig. 3e) and a ained a no able oscilla o s eng h ( =
0.122).
The o ma ion o used PAH/IPCA sys ems ( used, used_-
co e, used_edge models) in CDs would p oduce many new
peaks in hei abso p ion spec a (Fig. 3 –h), which may be
assigned o he ex ended delocaliza ion effec s and he p es-
ence o mul iple b igh CT exci a ions. Con a y o p e iously
analysed sys ems, he on ie o bi al analysis sugges s ha he
used sys ems should be conside ed mo e as co e/shell
sys ems because hei HOMOs and LUMOs a e delocalized
o e he en i e model s uc u es (Fig. S14†). In used model,
all en lowes b igh exci a ions ha e o some ex en he π–π
*
cha ac e wi h he π-o bi als delocalized o e he whole sys em
(Table S28†), e.g., he S
0
→S
2
ansi ion is a HLCT exci a ion,
whe e he elec on densi y p edominan ly ans e s om he
whole s uc u e o a PAH pa , howe e , i is less in ense ( =
0.014). Fo he used_co e model, LE on IPCA is now he S
0
→
S
4
ansi ion wi h λ
max
≈363 nm (Fig. 3g), i.e., ed-shi ed by
21 nm compa ed o IPCA, wi h an inc eased oscilla o s eng h
( = 0.392) in compa ison wi h an isola ed IPCA molecule
(Table S29†). The e a e se e al CT exci a ions om PAH o he
whole sys em o mos ly o IPCA uni o S
0
→S
3
(λ
max
≈
503 nm, = 0.079). The S
0
→S
5
ansi ion is a HLCT exci a ion
om PAH o he whole sys em. The ac ha i is he b igh
exci a ion lying in he isible pa o elec omagne ic spec um
Fig. 3 Abso p ion spec a o co alen ly bonded complexes along wi h hose o he sepa a ed molecules which o m he complexes (see molecules
A–H in Fig. S1†). Fo each spec um, he line spec a (conside ing 30 lowes single s a es) we e con olu ed by a Gaussian unc ion assuming he
inhomogeneous b oadening o peaks wi h σ= 20 nm.
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(354 nm, = 0.026) offe s he in e es ing egion o a ge he
abso p ion in CDs. Conce ning he used_edge model, he
S
0
→S
1
exci a ion (λ
max
≈592 nm, = 0.122) occu s on he
whole model s uc u e, dominan ly on he PAH uni . Again,
new exci a ions in compa ison wi h IPCA and N-doped isola ed
molecules a e p esen , which co espond o CT exci a ions
delocalized o e he whole sys em (Table S30†). These peaks
posi ioned a 350, 558 and 511 nm make he used_edge
model plausible o he a ionaliza ion o ypical pho o-
exci a ion wa eleng hs o CDs.
To add ess he impac o he co e size on he abso p ion
spec a o he s udied sys ems, ep esen a i e co alen ly
bonded as well as s acked complexes con aining an N-doped
co onene moie y we e in es iga ed (Fig. S15†). In amide-linked
models (amide1_co and amide1_co _edge), b igh CT exci-
a ions om PAH o IPCA co esponding in bo h cases o S
0
→
S
4
ansi ion a e cen e ed a ound 460 nm (Fig. S16, Tables S31
and S32†), i.e., hey a e ed-shi ed (by ca. 14 and 72 nm,
espec i ely) compa ed o py ene-like analogs (Tables S22 and
S23†). The S
0
→S
1
e ical exci a ion ene gy is e en mo e ed-
shi ed (by 171 nm and 415 nm) o amide1_co and amide1_-
co _edge, espec i ely, wi h espec o he smalle -co e
sys ems. Fo he s acked complex wi hou O- unc ionaliza ion
(s acked_co model), a b igh e CT s a e was ound o
N-co onene/IPCA in compa ison o he N-py ene/IPCA coun e -
pa , i.e.,S
0
→S
7
ansi ion (λ
max
≈320 nm, = 0.030) o
N-co onene/IPCA s. S
0
→S
6
ansi ion (λ
max
≈317 nm, =
0.004) o N-py ene/IPCA (Tables S13 and S33†). Again, he S
0
→S
1
maximum is ed-shi ed by 156 nm (λ
max
≈938 nm) wi h
espec o N-py ene/IPCA. In s acked_oxo_co model, he
HLCT exci onic ansi ion (S
0
→S
7
;λ
max
≈341 nm) has he
oscilla o s eng h wi h he alue = 0.044 (Table S34†), which
is simila as in he py ene coun e pa (see oxo_s acked1_a, b).
Simila CT s a es we e ound also in s acked_oxo2_co model
(S
0
→S
4
;S
0
→S
6
;S
0
→S
7
; Table S35†) and in he s acked
complex wi h diffe en N-doping pa e n (s acked_oxo_co 2;S
0
→S
4
;S
0
→S
5
;S
0
→S
7
; Table S36†). In e es ingly, he S
0
→S
1
exci a ion ene gy is ed-shi ed only by 38 nm (λ
max
≈491 nm)
o he N-co onene/IPCA s acked complex wi h oxo- unc ionali-
za ion (s acked_oxo_co model) in compa ison o he oxo_s-
acked1_a model (wi h py ene-sized PAH). Despi e a diffe en
N-doping pa e n imp in ed in he s acked_oxo_co 2 model,
he abso p ion peak co esponding o S
0
→S
1
is no ed-
shi ed ou side he Vis egion (λ
max
≈518 nm, = 0.051). In
gene al, ou esul s indica e ha e en in models wi h a la ge
co e han py ene, he o e lap o molecula o bi als o he com-
ponen s emains efficien , and new CT s a es a e p esen .
Addi ionally, he abso p ion peaks a e ed-shi ed o he
la ge co e.
To sum up his pa , in he 340–360 nm egion, whe e he
isola ed IPCA and i s dime s abso b ligh , o he mul iple
abso p ion e en s can occu , hus pho oac i a ing diffe en PL
cen es. I was also shown ha no only la ge sp
2
ca bon co e
domains bu also he in e ac ion be ween IPCA and PAH uni s
in used and amide-bonded sys ems a e likely esponsible o
he abso p ion in he isible egion. Mo eo e , i was demon-
s a ed ha he elec onic exci a ions wi h CT cha ac e om
IPCA o PAH only occu ed in CD co e models unc ionalized
wi h oxo-g oups due o he ela i e posi ions o he on ie
o bi als isola ed o IPCA and oxo-PAHs.
De-exci a ion pa hways
The analysis o de-exci a ion pa hways in he s udied models
o a ious in e ac ing PAH/IPCA s uc u al domains o CDs
p o ides aluable insigh s in o he PL deac i a ion p ocesses
occu ing in eal CD samples a e i adia ion. We chose o
desc ibe he sequen ial de-exci a ion cascades (S
n
→S
1
) a e
he pho oexci a ion wi h a 350 nm ene gy sou ce, which is he
ypical exci a ion wa eleng h p esumably a ge ing he abso p-
ion maxima o MFs such as IPCA. Along he de-exci a ion
pa hways based on he NTO analysis, we iden i ied CT/ET p o-
cesses be ween he in e ac ing CD componen s (Fig. S5†).
Despi e being da k, he CT and ET s a es can play an impo -
an ole du ing de-exci a ion dynamic p ocesses, leading o
cha ge-sepa a ed exci ons and/o ac i a ion o mul iple PL
cen es in CDs. The he e p esen ed de-exci a ion cascades ep-
esen al e na i e elaxa ion channels o local de-ac i a ion o
IPCA and i s dime s leading o he emission om molecula
s a es.
23,24
The o ma ion o H-bonded complexes o IPCA and
N-doped PAHs wi hin CDs appea s o p omo e adia ionless
channels and hus lowe s he PL QY in CDs. The eason is ha
he de-exci a ion cascade b ings he pho oac i a ed H-bonded
sys ems down o he S
1
s a e which ge s (a e elaxa ion) oo
close o he GS (Fig. 4a and b). In pa icula , in he
COOH_co e model, he 350 nm exci a ion sou ce causes popu-
la ion o S
6
and S
5
ene gy le els, which a e LEs on IPCA and
PAH, espec i ely. Sequen ial in e nal con e sions o S
5
o S
3
in ol e he elec on ans e om PAH o IPCA (S
5
→S
4
) and
e e se CT (S
4
→S
3
) acco ding o he NTO analysis
(Table S11†). The EDD plo o S
1*
→S
0*
a he elaxed S
1
geo-
me y (inse in Fig. 4a) shows he localiza ion o an exci on on
PAH, and he p oximi y o S
0
and S
1
ene gy le els in his geo-
me y sugges s he non- adia i e de-exci a ion. An analogous
pic u e was d awn o he COOH_edge model, wi h one low-
lying da k s a e missing in he deac i a ion cascade (Fig. 4b,
Table S12†).
Simila ly, he s acked complexes o non- unc ionalized
N-doped PAHs wi h IPCA endo se non- adia i e channels in
CDs, lowe ing he QY (Fig. 4c). The e a e se e al da k CT s a es
(e.g.,S
2
and S
6
in s acked_co e model, and S
6
in s acked_edge
model) which canno be iden i ied in he abso p ion spec um
bu a e impo an in he de-exci a ion p ocesses. A e local
exci a ion (S
5
) on IPCA a 345 nm in he s acked_co e model,
S
5
→S
4
ET o he PAH pa o he complex can occu (see
sec ion 10 in ESI† o he e alua ions o o al couplings o he
EET analysis) al hough he ela i ely la ge ene gy diffe ence
(∼1 eV) be ween he wo s a es sugges s a compe i i e pa hway
ia di ec emission om he S
5
s a e localized on IPCA in some
s acked a angemen s.
23,24
In he case o he S
5
→S
4
in e nal
con e sion, ET is ollowed by hyb id local cha ge- ans e
(HLCT) o IPCA (S
3
→S
2
) and HLCT o PAH (S
2
→S
1
) and
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in e nal con e sion o S
1*
. Thus, he emission akes place om
his HLCT s a e, and i is signi ican ly edshi ed o 1512 nm.
Mo eo e , hese HLCT exci a ions ha e almos ze o oscilla o
s eng hs, sugges ing low efficiency o adia i e deac i a ion.
Wi h he 350 nm sou ce used o he pho oexci a ion, he o -
ma ion o an exci on (S
1
→S
5
) dominan ly localized on PAH
pa o he s acked_edge model can be expec ed (Fig. 4d). The
ansi ion o om he S
5
s a e o CT S
1
s a e could be desc ibed
as a weak ET om PAH o IPCA o h and back, wi h se e al
NTOs in ol ed in he ansi ions. Relaxing om S
2
o S
1
, he
elec on is ans e ed o IPCA pa . He e, he TD-DFT p e-
dic ed ha he S
1
s a e in he elaxed geome y is ene ge ically
lowe han GS, which is appa en ly an a e ac o TD-DFT,
which is no app op ia e o desc ibing quasi-degene a e s a es
and sugges s he occu ence o a conical in e sec ion in his
model.
The s acked complexes o IPCA wi h O- unc ionalized
N-doped PAHs can lead o bo h adia i e and adia ionless de-
exci a ion pa hways in CDs depending on he doping pa e n.
The exci a ion o he S
3
HLCT s a e in oxo_s acked1_a is ol-
lowed by weak ET localizing he exci on on he PAH moie y
(Fig. 4e). The S
1
→S
0
emission is b igh ( = 0.558) wi h λ
max
≈
500 nm. I oxo_s acked1_b is pho oexci ed wi h he 350 nm
sou ce, S
4
and S
3
s a es become popula ed (Fig. 4 ). The NTO
analysis sugges s ha he S
1
ene gy le el is eached ia a
sequen ial ET p ocess ( om IPCA o PAH) h ough hole ans-
e (S
4
→S
3
) ollowed by elec on ans e (S
3
→S
2
). This non-
co alen s uc u al domain simila ly emi s he ligh o g een
colou (525 nm, = 0.464).
Al e ing he doping si es in O- unc ionalized N-doped
s acked complexes can esul in diffe en de-exci a ion cas-
cades lowe ing PL QY in CDs including possible occu ence o
ISC (Fig. 4g and h). In oxo_s acked2_a, he 350 nm sou ce
would cause he popula ing o S
3
localized on PAH, which can
in e nally con e o S
1
ia he S
2
s a e, bo h keeping he cha -
ac e o LE on PAH. The LE cha ac e is also p ese ed du ing
he elaxa ion o he S
1
s a e, despi e no able diffe ences
be ween he S
0
and S
1
s uc u es (Fig. S17†). Besides he Kasha
emission being in he IR egion (λ
max
≈976 nm, = 0.093), a
small single – iple ene gy gap ΔE
ST
= 1.1 kcal mol
−1
(Fig. S6†) sugges s he plausibili y o ISC, hus opening a adia-
ionless deac i a ion channel.
In he case o oxo_s acked2_b, he 350 nm exci a ion sou ce
would popula e HLCT S
5
,CTS
4
and HLCT S
3
s a es. The an-
si ion om S
5
o S
1
in ol es hole ans e om IPCA o PAH
and back, ollowed by ET om S
3
o S
2
and ib a ion elaxa ion
o S
1
.Asinoxo_s acked2_a, he Kasha emission occu s om
S
1
localized on PAH uni (see EDD plo in Fig. 4g) a 952 nm
( = 0.091), and he small single – iple ene gy gap ΔE
ST
=
0.4 kcal mol
−1
(Fig. S6†) again sugges s ISC as a possible adia-
ionless decay channel.
The linking o IPCA wi h N-doped PAHs ia a single
co alen bond leads ei he o local emission om IPCA,
23,24
o
o a non- adia i e de-exci a ion pa hway lowe ing he PL QY o
Fig. 4 De-exci a ion cascade o he non-co alen ly bonded complexes (see Fig. 1 o sys em labeling) exci ed by he 350 nm ene gy sou ce. LE
ep esen s local exci a ion, CT (e
−
/h
+
) cha ge (elec on/hole) ans e , HLCT hyb idized local and cha ge- ans e , ET ene gy ans e . Inse s: EDD
plo s o he S
0
→S
1
ansi ion ( ed/blue egions indica e inc ease/dec ease o he elec on densi y upon he exci a ion). No e: Fo s acked_edge
s uc u e, he S
1*
s a e was calcula ed wi h he LR app oach (see ESI† o he explana ion).
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CDs, as he Kasha emission occu ing on PAH is ed-shi ed o
he ed end o he isible ange wi h low oscilla o s eng h o
e en o he IR egion. The in ol emen o low-lying da k CT
s a es wi h a hole on PAH and an elec on on IPCA in de-exci-
a ion p ocesses in models wi h he es e bond is depic ed in
Fig. 5a and b. The S
0
→S
1
ansi ion in es e 1_co e co es-
ponding o a LE on PAH is da k and lies in he ed spec al
egion (λ
max
≈748 nm, = 0.000), while he emission is a in
he IR egion (1370 nm, = 0.001), sugges ing a non- adia i e
decay. On he o he hand, he S
1
s a e in es e 1_edge model
wi h a g aphi ic-N-edge uni is he b igh LE on py ene
peaking a 493 nm ( = 0.045), and he emission is shi ed o
he ed spec al egion (751 nm, = 0.080) and i only sligh ly
dec eases he QY in compa ison wi h he PL o isola ed IPCA
molecule. Howe e , i should be no ed ha he S
5
s a e in
es e 1_co e is ela i ely well sepa a ed om S
4
, which sugges s
ha a compe i i e pa hway ia di ec emission om he S
5
s a e localized on IPCA is plausible.
Con a y o o he s udied cases, he 350 nm ene gy sou ce
canno ac i a e PL channels o single-bonded complexes o
IPCA/N-doped PAHs wi h g aphi ic-N-co e doping pa e n as
he e a e no abso p ion peaks in his egion. A lowe ene ge ic
sou ce could s imula e a LE on PAH (Fig. 5c) o amide1_co e.
I s de-exci a ion om his S
3
s a e only includes he in e nal
con e sion o S
1
and he calcula ed e ical emission ene gy in
he IR egion (λ= 1372 nm) sugges s non- adia i e deac i a ion
(Fig. 5c). I he HLCT s a es amide2_co e and amide3_co e was
a ge ed wi h he lowe -ene gy exci a ion sou ce, simila de-
exci a ion pa hways could be ollowed in bo h hese models,
s a ing wi h elec on ans e om IPCA o PAH (S
4
→S
3
) and
sequen ial in e nal con e sions and ib a ion elaxa ions o S
1
,
whe e he exci on is localized on PAH (Fig. 5e and g). The exis -
ence o hese complexes would, again, lowe he QY o CDs
emission as he emission was calcula ed o be in he IR egion.
The amide-bonded complexes wi h g aphi ic-N-edge doping
could be pho oexci ed wi h 350 nm lase . In amide1_edge,i
would cause LE on PAH (S
0
→S
4
) and he de-exci a ion would
in ol e da k HLCT om PAH o IPCA (S
4
→S
3
) and back ans-
e (S
3
→S
2
), ollowed by ib a ion elaxa ion o S
1
(Fig. 5d).
E en ually, his adia i e channel would lead o emission a
Fig. 5 De-exci a ion cascade o he co alen ly bonded complexes exci ed by he 350 nm ene gy sou ce. LE ep esen s local exci a ion, CT (e
−
/h
+
)
cha ge (elec on/hole) ans e , HLCT hyb idized local and cha ge- ans e , ET ene gy ans e . Inse s: EDD plo s o he S
0
→S
1
ansi ion ( ed/blue
egions indica e inc ease/dec ease o he elec on densi y upon he exci a ion).
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757 nm ( = 0.081). Pho oexci a ion wi h 350 nm ene gy sou ce
would also c ea e an exci on pai on PAH in amide2_edge and
amide3_edge complexes. Simila de-exci a ion pa hways we e
iden i ied o hese wo models, whe e ansi ions be ween
b igh HLCT and CT s a es could be acked (Fig. 5 and h;
Tables S25 and S27†). The amide2_edge model exhibi ed e i-
cal emission a he end o he isible egion, while HLCT cha -
ac e o S
1
in amide3_edge esul ed in a non- adia i e de-exci-
a ion pa hway, which could be ela ed o smalle dis o ion o
he wo componen s in he S1 s a e (Fig. S18†). Mo eo e , ISC
canno be excluded om conside a ions as he closes single –
iple gap ou o all single-bonded models is 1.9 kcal mol
−1
o amide3_edge (Fig. S6†).
The o ma ion o used s uc u es in CDs can open ei he
adia i e o adia ionless channels depending on he numbe
and opology o he g aphi ic ni ogens in hei s uc u e
(Fig. 5i–k). In he used model wi h wo g aphi ic ni ogens
being emnan s o usion wi h IPCA, he calcula ed Kasha
emission was p edic ed a 441 nm ( = 0.834), which is e y
close o he IPCA emission band. I he HLCT s a e (354 nm) o
used_co e was a ge ed wi h a 350 nm exci a ion sou ce, wo
ET could be wi nessed du ing deac i a ion p ocesses o S
1
,i.e.,
in e nal con e sion om S
5
o S
4
, and a successi e ET (S
4
→S
2
)
in ol ing wo-s ep hole and elec on ans e s (Fig. 5j). The
S
0
→S
1
ansi ion is no b igh (λ
max
≈651 nm, = 0.003), and
he co esponding Kasha emission is a in IR egion a
1071 nm wi h he low oscilla o s eng h ( = 0.005), sugges ing
non- adia i e deac i a ions. Ta ge ing S
4
o used_edge wi h
he 350 nm sou ce, elec on CT om IPCA o PAH, back o
IPCA and again o PAH a e needed o each he S
1
le el. The
S
1
→S
0
emission is in he nea IR egion (λ
max
≈945 nm, =
0.023). Al hough he single – iple gap o he S
1
s a e o hese
used s uc u es is in he ange o 4.3–15.8 kcal mol
−1
, hus
discou aging he easibili y o he ISC pa hway, i should be
no ed ha he gap de e mined using he GS geome ies can
diffe om ha ob ained om he adiaba ic pic u e. The
gene al ea u es in ou models ep esen ing in e play be ween
co e/su ace and molecula s a es in possible s uc u al
domains occu ing wi hin CDs a e summa ized in Fig. 6.
Conclusions
We analysed abso p ion and emission p ope ies o compu a-
ionally easible CD co e/su ace/molecula luo opho e
sys ems, ocusing on he possible in e play be ween diffe en
PL cen es ia cha ge and ene gy ans e s ha can occu
du ing he de-exci a ion p ocesses. By including bo h non-co-
alen ly and co alen ly bonded sys ems as well as used s uc-
u es, ou models co e ed he main s uc u al domain ypes
and hus e lec ed he semi-local complexi y o CDs. Ou
esul s e ealed ha all he s udied a angemen s suppo ed,
o some ex en , he mu ual communica ion be ween he co e,
su ace, and molecula s a es. Howe e , we showed ha , in
some cases (e.g., in s acking s uc u es and co alen single-
bonded sys ems), he di ec emission om he pho oac i a ed
MF compe ed wi h an in e nal con e sion de-exci a ion
pa hway, while in o he s (e.g., H-bonded, and used sys ems),
Fig. 6 O e iew o he p ocesses occu ing in he ep esen a i e models o CD s uc u al domains a e he i adia ion wi h a 350 nm ene gy sou ce
p esumably a ge ing he abso p ion maximum o IPCA (displayed as incoming a ows). The ou coming a ows ep esen he PL (solid lines) om
co e and/o molecula s a es and dissipa i e NR channels (dashed lines). No a ion: LE –local exci a ion, CT –cha ge ans e , ET –ene gy ans e ,
ISC –in e sys em c ossing, NR –non- adia i e de-exci a ion, HB –hyd ogen bond, MF –molecula fluo opho e, PAH –polya oma ic hyd oca bon.
Pape Nanoscale
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