Synthesis, characterization, and modeling of naphthyl-terminated sp carbon chains: Dinaphthylpolyynes

Article abstract of DOI:10.1021/jp104863v

We report a combined study on the synthesis, spectroscopic characterization, and theoretical modeling of a series of α,ω- dinaphthylpolyynes. We synthesized this family of naphthyl-terminated sp carbon chains by reacting diiodoacetylene and 1-ethynylnaphthalene under the Cadiot-Chodkiewicz reaction conditions. By means of liquid chromatography (HPLC), we separated the products and recorded their electronic absorption spectra, which enabled us to identify the complete series of dinaphthylpolyynes Ar-C_2n-Ar (with Ar = naphthyl group and n = number of acetilenic units) with n ranging from 2 to 6. The longest wavelength transition (LWT) in the electronic spectra of the dinaphthylpolyynes red shifts linearly with n away from the LWT of the bare termination. This result is also supported by DFT-LDA simulations. Finally, we probed the stability of the dinaphthylpolyynes in a solid-state precipitate by Fourier-transform infrared spectroscopy and by differential scanning calorimetry (DSC).

Full text of DOI:10.1021/jp104863v

14834
J. Phys. Chem. B 2010, 114, 14834–14841
Synthesis, Characterization, and Modeling of Naphthyl-Terminated sp Carbon Chains:
Dinaphthylpolyynes
Franco Cataldo,*,†,‡ Luca Ravagnan,*,§,⊥ Eugenio Cinquanta,⊥,# Ivano Eligio Castelli,§,+
Nicola Manini,§,∇ Giovanni Onida,§,∇ and Paolo Milani§,⊥
Actinium Chemical Research, Via Casilina 1626/A, I-00133 Rome, Italy, Istituto Nazionale di Astrofisica.
OsserVatorio Astrofisica di Catania, Via S. Sofia 78, I-95123 Catania, Italy, Dipartimento di Fisica, UniVersita`
degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy, CIMAINA, Via Celoria 16, I-20133 Milano, Italy,
Dipartimento di Scienza dei Materiali, UniVersità degli Studi di Milano Bicocca, Via Cozzi 53, I-20125 Milano, Italy,
and European Theoretical Spectroscopy Facility (ETSF), Via Celoria 16, 20133 Milano, Italy
ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: September 21, 2010
We report a combined study on the synthesis, spectroscopic characterization, and theoretical modeling of a
series of R,ω-dinaphthylpolyynes. We synthesized this family of naphthyl-terminated sp carbon chains by
reacting diiodoacetylene and 1-ethynylnaphthalene under the Cadiot-Chodkiewicz reaction conditions. By
means of liquid chromatography (HPLC), we separated the products and recorded their electronic absorption
spectra, which enabled us to identify the complete series of dinaphthylpolyynes Ar-C2n-Ar (with Ar )
naphthyl group and n ) number of acetilenic units) with n ranging from 2 to 6. The longest wavelength
transition (LWT) in the electronic spectra of the dinaphthylpolyynes red shifts linearly with n away from the
LWT of the bare termination. This result is also supported by DFT-LDA simulations. Finally, we probed the
stability of the dinaphthylpolyynes in a solid-state precipitate by Fourier-transform infrared spectroscopy and
by differential scanning calorimetry (DSC).
1. Introduction
conductive thin film transistor structures,11 as the active
semiconductor material in light-emitting diodes in electrolumi-
nescent devices,12,13 and in organic solar cells.14
During the past years, the study of sp carbon chains (spCCs,
also known as acetylenic arrays) has attracted the interest of a
number of research group from a multitude of disciplinary areas,
ranging from organic chemistry, nonlinear optics, and solid-
state physics to astrophysics. Although bare spCCs are extremely
unstable and reactive species,1 it has been shown that their
stability is remarkably enhanced when their reactive terminations
are bound to chemical groups2-4 or to carbon nanostructures.5,6
For instance, chemists were able to synthesize spCCs which
are stable in solution or even in the solid phase by terminating
the chains with organic group.3,4 This class of compounds, often
referred to as R,ω-diarylpolyynes (DAPs), have interesting and
promising physical properties. For instance, they exhibit a
HOMO-LUMO gap strongly influenced by the number of sp
carbon atoms constituting the chain,7 and their third-order
nonlinear optical properties and their nonresonant molecular
second hyperpolarizabilities (γ) have been reported to increase
as a function of the chain length.8,9 Furthermore, it has been
shown that the fluorescence, phosphorescence, and absorption
spectra of DAPs can be modulated by applying an external
electric field.10 Finally, unusual photophysical and spectroscopic
properties have been reported for DAPs, and they are considered
promising candidates as materials for the fabrication of semi-
On the other hand, solid-state physicists have studied the
possibility to produce carbon solids containing spCCs (sp-sp2
carbon) since the 1960s.15 Although the feasibility to obtain
sp-sp2 carbon solids has been shown for some years,16,17 recent
experimental observation by HR-TEM of spCCs forming during
the irradiation of graphene planes has strongly increased the
interest on this field.18 spCCs are nowadays considered promis-
ing structures for nanoelectronic applications19,20 since they could
be used as molecular conductors bridging graphene nanogap
devices. First potential applications have been already demon-
strated for the realization of nonvolatile memories for use in
two-terminal atomic-scale switches.20 Furthermore, spCCs are
expected to have interesting rectifying performances,21 to
become spin-polarized depending to their axial strain,5 and to
be effective as spin filters and spin valves.22
Finally, spCCs are of great relevance for astrophysics in the
framework of the study of the diffuse interstellar bands (DIBs).
DIBs include a large number of absorption lines in the
visible-near IR region of the electromagnetic spectrum that are
superposed on the interstellar extinction curve and whose
identification remains one of the oldest mysteries in stellar
spectroscopy.23 By comparing laboratory spectra of complex
molecules with the DIB lines, several molecules were identified
as constituents of the interstellar medium.23,24 Indeed, both
nonterminated spCCs and polycyclic aromatic hydrocarbons
(PAHs), such as, for example, naphthalene, have been identified
separately;25-27 however, spCCs terminated by PAHs (e.g.,
DAPs) have not been considered to date.
* To whom correspondence should be addressed. E-mail: franco.cataldo@
fastwebnet.it (F.C.); luca.ravagnan@mi.infn.it (L.R.).
† Actinium Chemical Research.
‡ Osservatorio Astrofisica di Catania.
§ Universita` degli Studi di Milano.
⊥ CIMAINA.
# Universita` di Milano Bicocca.
∇ European Theoretical Spectroscopy Facility.
+ Present address: Center for Atomic-scale Materials Design (CAMD),
Department of Physics, Technical University of Denmark, DK-2800
Kongens Lyngby, Denmark.
In the present paper, we present a combined study on the
synthesis, spectroscopic characterization, and theoretical model-
ing of a series of spCCs terminated by naphthyl groups
10.1021/jp104863v  2010 American Chemical Society
Published on Web 10/25/2010

Synthesis, Characterization, and Modeling of Dinaphthylpolyynes
J. Phys. Chem. B, Vol. 114, No. 46, 2010 14835
(dinaphthylpolyynes, in short represented as Ar-C2n-Ar, with
Ar ) naphthyl group and n ) number of acetylenic units of
the spCC). These molecules, already synthesized in the 1960s
and 1970s by the group of M. Nakagawa,28-30 represent a family
of chemically stabilized DAPs which are of high interest for
several reasons; from one side, they are potentially useful in
optoelectronic applications and are promising building blocks
for the production of sp-sp2 carbon systems for nanoelectronic
devices; from the other side, they constitute the simplest
examples of spCCs terminated by a PAH that could be identified
in the interstellar medium (naphthalene is indeed one of the
simpler and most abundant PAHs). Here, we present a new route
for the synthesis of this class of complexes which enables, by
a remarkably simpler chemical approach than that in refs 28-30,
productionof a solution of dinaphthylpolyynes Ar-C2n-Ar with
2 e n e 6 at the same time.
red precipitate) by shaking the solution with 4.0 g of CuCl
dissolved in 100 mL of aqueous NH3 30% together with 3.0 g
of NH2OH·HCl.
The resulting orange solution was then characterized by
HPLC, UV-vis, FT-IR, and DSC.
2.3. DFT Simulations. We simulated the synthesized struc-
tures using standard DFT-LSDA (density functional theory in
the local spin density approximation), based on a plane-waves
basis, and pseudopotentials.34 We relaxed all atomic positions
until the largest residual force was <8 pN. The ensuing structures
and electronic Kohn-Shan orbitals are depicted using a standard
visualization tool.35
3. Results and Discussion
3.1. Synthesis and Identification of Dinaphthylpolyynes.
Previous work32 demonstrated the great versatility of di-
iodoacetylene (C2I2) as a building block for the synthesis of
symmetrically terminated spCCs through the Cadiot-
Chodkiewicz reaction. The synthetic approach adopted previ-
ously with copper phenylacetylide and C2I2 has been extended
here to the case of the copper salt of 1-ethynylnaphthalene. As
shown in Scheme 1, the resulting spCCs are thus terminated
by the bulkier naphthalene groups.
Figure 1 reports the separation by elution through the HPLC
column of the products formed in the Cadiot-Chodkiewicz
reaction between diiodoacetylene and 1-ethynylnaphthalene. The
presence of very distinct peaks at different retention times
demonstrates the formation of products with different molecular
weights, in agreement with the formation of dinaphthyl-
terminated spCCs of different length (according to the Scheme
1). The formation of spCCs having more than three acetylenic
units suggests the formation of the labile asymmetric intermedi-
ate Cu-CtC-I during the coupling reaction.32 Without the
formation of this intermediate, the production of spCCs longer
than three acetylenic units would be unjustified.
By using a low flow rate for the mobile phase in the HPLC,
it was possible to observe the regular elution of all dinaphth-
ylpolyynes, although a very long retention time was observed
for the products with higher molecular weight. Figure 1 shows
the retention times of the dinaphthylpolyynes in the HPLC
column (as a function of the number of acetylenic units) in
comparison to the diphenylpolyynes ones.32 The retention times
of both series of molecules can be fitted by an exponential law
of the type (see inset of Figure 1)
2. Experimental Section
2.1. Materials and Equipment. All of the solvents and
reagents (such as, for instance, 1-ethynylnaphthalene) mentioned
in the present work were obtained from Sigma Aldrich.
Diiodoacetylene was prepared according to the method reported
in ref 31. All of the equipment and methods for analysis and
characterization of the reaction products are reported in ref 32.
We performed the HPLC analysis using an Agilent Technolo-
gies 1100 station with a C8 column (using acetonitrile/water
80/20 as the mobile phase at a flow rate of 1.0 mL/min) and
equipped with a diode array detector (used for recording the
UV-vis absorption spectra of the separated molecules). UV-vis
absorption spectra were also acquired on the solution and thin
films by a JASCO-7850 spectrophotometer. FT-IR measure-
ments were carried out on in transmittance mode on an IR300
spectrometer from Thermo-Fisher Corp. The differential scan-
ning calorimetric (DSC) measurements were performed on a
DSC-1 Star System from Mettler-Toledo (heating rate of 10
°C/min under N2 flow).
2.2. Synthesis of Dinaphthylpolyynes. We synthesized the
dinaphthylpolyynes by reacting copper(I)ethynylnaphthalide
with diiodoacetylene under the Cadiot-Chodkiewicz reaction
conditions.32,33
In a first stage, we synthesized the copper salt of ethynyl-
naphthalene by dissolving Cu(I) chloride (1.0 g) in 30 mL of
aqueous ammonia (30%) together with 0.5 g of hydroxylamine
hydrochloride (NH2OH·HCl). 1-Ethynylnaphthalene (1.5 mL)
was then added to this solution under stirring. We collected a
dark yellow precipitate of copper(I)ethynylnaphthalide by
filtration in considerable yield (1.8 g).
Rt ) AeB·n
(1)
After that, about 6.0 g of I-CtC-I (Warning! Acetylene
diiodide is an irritant and poisonous) was dissolved/suspended
in 90 mL of decalin, and it was transferred into a 500 mL round-
bottomed flask and was stirred with 100 mL of distilled water.
Copper(I)ethynylnaphthalide (1.8 g) was then added to the
reaction mixture together with 25 mL of tetrahydrofuran, 50
mL aqueous NH3 30%, and 30 mL of N,N′,N,N′-tetramethyl-
ethylenediamine (TMEDA). The mixture was stirred at room
temperature for 2 days, and after this process, the decalin layer
became deep orange. The reaction mixture was filtered with
the aid of an aspirator to separate the black residue formed.
The filtered solution consisted of two layers. The organic layer
was separated from the aqueous layer by means of a separatory
funnel. The resulting crude solution contained dinaphthyl-
polyynes dissolved in decalin, together with a significant amount
of free unreacted ethynylnaphthalene and other byproducts.
These latter were removed from the crude solution (as a dark
where Rt is the retention time in the column, n is the number of
acetylenic units of the spCC, and A and B are two parameters
whose values are reported in Table 1.
Remarkably, the longer retention times of dinaphthylpolyynes
in comparison to those of diphenylpolyynes (ascribable to their
bulkier end groups) seems to affect only the parameter A of the
exponential fit, which is almost double in the case of dinaph-
thylpolyynes in comparison to the value obtained for diphe-
nylpolyynes. In contrast, the exponents B obtained for the two
series of chains are comparable.
3.2. Electronic Absorption Spectroscopy. By using the
diode array detector interfaced to the column of the HPLC, we
could record the electronic absorption spectrum of each mo-
lecular species contained in the dinaphthylpolyynes mixture, that
is, for each peak in Figure 1. Figure 2 shows the resulting
spectra. The presence of a regular shift for the position of the

14836 J. Phys. Chem. B, Vol. 114, No. 46, 2010
Cataldo et al.
SCHEME 1: Chemical Structures of the Dinaphthylpolyynes Synthesized with the Cadiot-Chodkiewicz Reaction
between Copper(I)ethynylnaphthalide and Diiodoacetylene
TABLE 1: Fitting Parameters for the Exponential Law
Describing the Evolution of Rt with n (eq 1)
absorption peaks as the molecular weight of the species increases
(i.e., the retention time increases) supports the assignment to
diphenylpolyynes of increasing length. Indeed, it is well-known
that the position of the electronic absorption peaks of spCCs is
correlated with the number of acetylene units.7,28-30 The exact
attribution of the different spectra to the corresponding dinaph-
thylpolyyne is furthermore possible by comparing the absorption
spectra with the one obtained by the group of M. Nakagawa.28-30
Remarkably, the position of the peak at the longest wavelength
(longest wavelength transition, LWT) of our spectra matches
the values in refs 28-30, corresponding to dinaphthylpolyynes
with n ) 2-6, within a shift of about 3 nm. The presence of
this shift has to be ascribed to the different solvent used in our
spCCs series
A [min]
B
dinaphthylpolyynes
diphenylpolyynes
3.0 ( 0.3
1.4 ( 0.1
0.53 ( 0.02
0.51 ( 0.02
case (decalin) with respect to the one used in the past
(tetrahydrofuran).29
Considering the different spectra in detail, they are character-
ized by a complex structure, in which two main regions can be
identified. The region at higher wavelength (labeled as Reg. A
in Figure 2a) is related to the electronic transitions between the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO); the region at lower
wavelength (labeled as Reg. B in Figure 2a) is related to
transitions between other molecular orbitals (such as, for
instance, the transitions between the HOMO and LUMO+1 or
the HOMO-1 and LUMO). In region A in particular, a
multipeak structure can be identified that can be fitted as the
superposition of several Gaussian components. The presence
of several peaks can be attributed to the Franck-Condon shifts
of the main HOMO-LUMO transition,7,28-30 that is, the
simultaneous occurrence of the electronic transition and the
excitation of vibrational modes of the molecule. In this
framework, the LWT peak of the electronic absorption spectrum
corresponds to the HOMO-LUMO transition without the
excitation of the molecule to a new vibrational level, while
the most intense transition (MIT) is the transition for which
the starting and final wave functions overlap more significantly
(thus, the transition has a higher probability to occur). As shown
in Figure 2a, dinaphthylpolyynes have MITs that are always
distinct by the LWT. This is a characteristic of spCCs terminated
by bulky groups and indeed has been observed also for
diphenylpolyynes32 and for spCCs terminated by dendridic
organic groups.3 Instead, in the case of simple polyynes, the
LWT and the MIT coincide.2
Figure 1. HPLC analysis of the dinaphthylpolyynes recorded at 350
nm. For each observed peak, the retention time and the corresponding
number of acetylenic units (n) is indicated. Inset: comparison of the
retention time of dinaphthylpolyynes and diphenylpolyynes32 as a
function of the number of acetylenic units composing the spCCs. In
both cases, the mobile phase in the C8 column was acetonitrile/water
80/20 vol/vol, and the pressure used was 105 bar.

Synthesis, Characterization, and Modeling of Dinaphthylpolyynes
J. Phys. Chem. B, Vol. 114, No. 46, 2010 14837
Figure 2. (a) The electronic absorption spectra of the dinaphthylpolyynes series (Ar-C2n-Ar, n ) 2-6) as recorded by the diode array detector
of the HPLC. For each spectrum, the longest wavelength transition (LWT, inversely related to the HOMO-LUMO gap) and the most intense
transition (MIT) are indicated. (b) The electronic absorption spectra of the dinaphthylpolyynes series (Ar-C2n-Ar, n ) 2-6) represented as a
function of the energy shift from the LWT (expressed in cm-1). The distance between the peaks in the absorption spectra is consistent with the
phonon energy of the optical C-C stretching mode of the sp carbon chain.
Figure 2b shows the same spectra as Figure 2a (limited to
Reg. A), where the abscissa is expressed as an energy shift from
the LWT. This means that the wavelength scale of each
spectrum in Figure 2a was first converted on the energy scale
(in cm-1 units), and then, the energy of the LWT peak was
subtracted from it; as a result, in Figure 2b, the LWTs of all
spectra are centered at 0 cm-1. Thanks to this representation, it
is possible to observe that the energy shift between the peaks
in region A is on the order of 1900-2000 cm-1, which is
consistent with the typical main vibrational modes of spCCs.36
Furthermore, the value of this shift decreases as the chain length
of the dinaphthylpolyyne increases; this again is consistent with
the behavior of the vibrational modes of spCCs that are known
to be softer (i.e., characterized by a lower energy) when the
chain length is larger.36 As a matter of fact, this behavior was
Figure 3. The position of the LWT as a function of the number of
also observed in ref 29, even if the Franck-Condon shifts
acetylenic units composing the carbon chain of three spCCs series,
measured in our case are slightly lower. Like for the shift of
naphthyl-terminated (dinaphthylpolyynes, circles), phenyl-terminated
the LWT, this slight deviation can be related to the different
(diphenylpolyynes, squares)32 and hydrogen-terminated (polyynes,
triangles).2 The LWTs for benzene and naphthalene molecules are
solvent.
Figure 3 shows the evolution of the LWT of the electronic
indicated at n ) 0.
absorption spectra of the dinaphthylpolyynes as a function of
the number of acetylenic units (n) in the chain, compared to
the same evolution for diphenylpolyynes32 and hydrogen-
terminated spCCs (i.e., polyynes).2 In all cases, a linear relation
emerges between the LWT and the number of acetylenic units,7
as described by the following equation
LWT0,28-30 our data are more consistent with a linear depen-
dence with n. The fitting parameters of LWT0 and R obtained
for the three series of spCCs in Figure 3 are reported in Table
2.
Remarkably, the value of LWT0 for dinaphthylpolyynes at
319.8 is very close to LWT of the B band of naphthalene at
313 nm37-39 as well as to the LWT of binaphthyl at 315 nm.37-39
Similarly, the value of LWT0 for diphenylpolyynes at 256 nm
is close to LWT of the B band of benzene at 267 nm37 and is
very close to the LWT for biphenyl at about 252 nm.37
As suggested in ref 30, these observations indicate that the
intercepts at n ) 0 correspond to the LWT absorption of the
LWT ) R·n + LWT0
(2)
where LWT0 is the extrapolated LWT for a chain of “null”
length and R is the slope. Although other groups have suggested
that LWT for dinaphthylpolyynes evolves as LWT ) R ·n1/2
+

14838 J. Phys. Chem. B, Vol. 114, No. 46, 2010
Cataldo et al.
TABLE 3: Relative Molar Concentration of the
TABLE 2: Fitting Parameters for the Linear Relation
Describing the Evolution of LWT with n (eq 2)
Dinaphthylpolyynes (with 2 e n e 6) in the Solution
spCCs series
R [nm/C2 unit]
LWT0 [nm]
n
dinaphthylpolyyne formulas
C(n)
dinaphthylpolyynes
diphenylpolyynes
polyynes
25.7 ( 0.5
35.3 ( 0.8
23.0 ( 0.5
319.8 ( 2.2
256 ( 3
2
3
4
5
6
Ar-C4-Ar
Ar-C6-Ar
Ar-C8-Ar
Ar-C10-Ar
Ar-C12-Ar
47 ( 4%
19.7 ( 0.6%
18.2 ( 2.2%
11.3 ( 1.2%
4.1 ( 0.3%
133 ( 3
end groups, while the addition of the acetylenic unit causes a
red shift in the LWT transition.
gap reduction, with a corresponding movement “off-resonance”
of the chain HOMO and LUMO levels relative to those of the
end groups.
3.3. Relative Yield of Dinaphthylpolyynes. We compute
the relative yield C(n) of dinaphthylpolyynes of different lengths
from the intensity of the most intense transition in the electronic
absorption spectra (i.e., the MIT in Figure 2). We take advantage
of the molar extinction coefficients ε(n) previously reported.29
We use the following relation
In order to better understand the observed behavior, we carried
out DFT-LSDA simulations of the geometric and electronic
structure of a free-standing naphthalene molecule and of dinaph-
thylpolyynes with different chain lengths. Naphthalene molecules
exibit a HOMO-LUMO gap larger than that of a dinaphth-
ylpolyynes of any length. Figure 4 (left panel) shows the HOMO
and LUMO wave functions obtained for Ar-C4-Ar, Ar-C8-Ar,
and Ar-C12-Ar. As can be observed, the HOMO and LUMO
orbitals of the spCCs are hybridized with those of the terminations
since their wave functions extend out from the sp chain region in
the naphthyl region. Nevertheless, as the chain length increases,
the HOMO and LUMO orbitals become more and more localized
on the sole chain backbone, indicating a diminishing hybridization.
At the same time, as the chain length increases, the computed
HOMO-LUMO gap decreases, and consequently, the theoretical
value for the absorption wavelength (LWTDFT, proportional to the
inverse of the computed HOMO-LUMO gaps) increases. Figure
4 (right panel) shows the evolution of LWTDFT with n. Although
the computed values for the LWTDFT do not match the experimental
values for the LWT in Figure 3, the overall trend is in very good
agreement. The simulations reproduce the linear relation between
LWTDFT and n observed experimentally, and the intercept at n )
0 is not far from to the simulated LWTDFT for naphthalene, given
the expected underestimation of the HOMO-LUMO gap of DFT-
LSDA.
It is also relevant to note that in the case of polyynes, the π
and π* orbitals (i.e., the HOMO and LUMO orbitals) are
completely localized on the carbon chain axis, and therefore,
little or no hybridization of those orbitals occurs with the
termination groups. Nevertheless, the same linear relation
between LWT and n is observed. This indicates that the slope
R of the linear relations in eq 2 is not simply a measure of the
hybridization between the π electron state of the chains and
the π electron state of the end groups but reflects a gap reduction
intrinsic of the carbyne chain. The decrease in chain end-group
hybridization seems in fact a direct consequence of this intrinsic
IMIT(n)
A(n, λ)
·
ε(n)
IMIT(i)
I(n, λ)
C(n) )
(3)
6
A(i, λ)
·
∑
ε(i)
I(i, λ)
i)2
where IMIT(n) is the intensity (i.e., the absorbance) of the MIT
of the absorption spectrum for the dinaphthylpolyyne with n
acetylenic units (see Figure 2), I(n,λ) is the value of absorbance
measured in the same spectrum at the wavelength λ, and A(n,λ)
is the time-integrated intensity of the corresponding peak in the
HPLC spectrum acquired using the wavelength λ. Table 3
reports the average of the values of C(n) obtained by applying
eq 3 to two different HPLC spectra, acquired at λ ) 350 (see
Figure 1a) and 250 nm (not shown).
Figure 5 shows the relative molar concentration of the
dinaphthylpolyynes in comparison to the relative molar con-
centration of the diphenylpolyynes series produced using the
same synthetic approach.32 As can be observed, for any chain
length larger than n ) 2, the concentration of dinaphth-
ylpolyynes exceeds that of the diphenylpolyynes. Only at n )
2 is the concentration of diphenylpolyynes higher than that of
dinaphthylpolyynes. These data suggest that our synthetic
approach favors longer chains when bulkier terminations are
used as end groups (i.e., naphthyl groups) and shorter chains
Figure 4. (Left panel) The electron density of HOMO and LUMO states for three different dinaphthylpolyynes, Ar-C-Ar, Ar-C8-Ar, and
Ar-C12-Ar. As the sp carbon chain is longer, the HOMO and LUMO states become more and more localized on the chain backbone. (Right panel)
DFT-LSDA value of LWTDFT (evaluated as the inverse of the HOMO-LUMO Kohn-Sham gap) as a function of the number of acetylenic units
composing the carbon chain (n) for dinaphthylpolyynes. The evaluated LWTDFT for a single naphthalene molecule is indicated at n ) 0.

Synthesis, Characterization, and Modeling of Dinaphthylpolyynes
J. Phys. Chem. B, Vol. 114, No. 46, 2010 14839
Figure 5. Relative yield of dinaphthylpolyynes and diphenylpolyynes32
as a function of the number of acetylenic units (n). Both series of spCCs
are produced by the Cadiot-Chodkiewcz reaction.
Figure 6. The UV-vis spectra of the dinaphthylpolyynes solution (a)
and of the dinaphthylpolyynes solution dried on a CaF substrate (b).
Spectrum (a) is consistent with the weighted sum of the spectra of the
separated dinaphthylpolyynes in Figure 2.
when the lighter phenyl groups are employed. Nevertheless, in
both cases, the shortest chains are predominant, and the
abundance of the species with longer chains decreases steadily
with the increase of n, as normally expected in the synthesis of
mixtures of polyynes.2
In the calculation of the distribution of the dinaphthylpolyynes
produced by the Cadiot-Chodkiewicz synthesis, we have
neglected the presence of the residual 1-ethynylnaphthalene,
which is not detected in the chromatogram shown in Figure 1a
that was recorded at 350 nm but was present and detected in
the chromatogram recorded at 250 nm (not shown).
3.4. Dinaphthylpolyynes in the Solid Phase. As previously
discussed, spCCs are interesting candidates for the development
of a new generation of nanoelectronic devices, where they could
be used as molecular conductors or spin filters and spin
valves.19,20,22 In order to produce similar devices, it is necessary
to introduce spCCs in solid-state systems and to ensure their
stability in time. Unfortunately, spCCs are notoriously unstable
structures, and, in particular, if they are terminated simply by
hydrogen or nitrogen atoms, they can be prepared only as very
diluted solutions; if they are deposited as thin films, they undergo
fast decay.2 Furthermore, spCCs are known to be highly reactive
with oxygen,16,40 making the production of devices containing
spCCs quite challenging. Nevertheless, several groups were able
to demonstrate that spCCs terminated by bulky end groups have
remarkably higher stability,3,4 so that in some cases, even their
crystallization was achieved.4 In order to study the possibility
to use dinaphthylpolyynes in solid-state systems, we have
deposited droplets of the solution of dinaphthylpolyynes in
decalin on hard substrates (made by CaF, ZnSe, and KBr
depending on the characterization carried out), and we let the
solvent evaporate by placing the sample in a dryer. The resulting
film was then characterized by UV-vis and FT-IR spec-
troscopies and by DSC.
In Figure 6, we compare the UV-vis spectra of the whole
solution of diphenylpolyynes in decalin (not separated by the
HPLC column) and of the film obtained by the same solution
after the evaporation of the solvent. As it is possible to observe,
the complex structure present in the solution’s spectrum
(corresponding to the overlap of the separate spectra of Figure
2a) is radically lost in the dried film, where three broad peaks
(indicated by arrows in the figure) are still distinguishable on
top of an amorphous band. This indicates that during the
evaporation of the solvent, the dinaphthylpolyynes undergo
Figure 7. (Left) The FT-IR spectra of the black precipitate produced
during storage at room temperature (in closed flasks) of the dinaphth-
ylpolyynes solution (a), of the dried dinaphthylpolyynes solution (b),
of pure 1-ethynylnaphthalene (c), and of pure naphthalene (d) (from
ref 41). Spectrum (a) has been acquired in reflectance mode, depositing
the sample on ZnSe, while spectrum (b) has been acquired in
transmittance mode after drying the solution on a KBr substrate. (Right)
Detail of the triple-bond stretching region of the FT-IR spectra (a),
(b), and (c). A linear background has been subtracted from spectra (a)
and (c).
chemical reactions that do not completely destroy the molecules,
as indicated by the survival of spectral features in the UV-vis
spectrum.
To gather more detailed information on the structure of the
“polymerized” polyynes, we carried out FT-IR measurements
on the samples. Figure 7 shows the FT-IR spectrum of the dried
dinaphthylpolyynes film measured in ambient conditions (i.e.,
the sample has been exposed to air) as compared to the spectrum
of pure naphthalene41 and pure 1-ethynylnaphthalene. The figure
also shows the FT-IR spectrum of the dark precipitate deposited
on the bottom of the sealed flask containing the dinaphth-
ylpolyynes solution after months of the solution being stored
at room temperature.
As can be observed, the FT-IR spectra of both 1-ethynyl-
naphthalene and of the dried dinaphthylpolyynes exibit the

14840 J. Phys. Chem. B, Vol. 114, No. 46, 2010
Cataldo et al.
Figure 8. Differential scanning calorimetry (DSC), 10 °C/min under N2, of the dinaphthylpolyynes mixture. The exothermic reaction with onset
at 143 °C and the peak at 182 °C can be assigned to the thermal decomposition of the dinaphthylpolyynes.
vibrational modes typical of naphthalene (as well as of
the naphthyl group). For instance, we can identify in all of the
spectra the CH stretching band at about 3060 cm-1, the in-phase
-CH- out-of-plane bending band at about 780 cm-1 (CH
wagging) and a series of CdC stretching bands (at about 1590,
1506, and 1390 cm-1).42 1-Ethynylnaphthalene is furthermore
characterized by the triple bond tC-H stretching band at 3282
cm-1 (νtC-H), by the bending of the tC-H at about 600 cm-1
(δtC-H), and by the acetylenic stretching band at 2100 cm-1
(νCtC).43 Remarkably, both νtC-H and δtC-H are no any longer
detectable in the spectrum of the dried dinaphthylpolyynes, and
the νCtC of 1-ethynylnaphthalene at 2100 cm-1 is replaced by
a complex and relatively more intense band at 2194 cm-1 with
a series of sub-bands (at 2178, 2153, and 2125 cm-1; see left
panel in Figure 7). Indeed, the position of those bands is
consistent with the CtC stretching modes for isolated dinaph-
thylpolyynes.29
These observations indicates on one hand that the interaction
of the dinaphthylpolyynes occurring during their drying (as
indicated by the modification of the UV-vis spectrum) is not
altering significantly the dinaphthylpolyyne structure. In the FT-
IR spectrum, we can indeed identify clearly the vibrational
modes of both the naphthyl terminations and the CtC stretching
modes of the spCCs, whose peaks do not show any significant
broadening in comparison to the case of naphthalene and
1-ethynylnaphthalene.
On the other side, the complete absence in the dried
dinaphthylpolyynes spectrum of the νtC-H and δtC-H vibrational
modes can be interpreted as the indication that the Cadiot-
Chodkiewicz reaction of 1-ethynylnaphathalene with diodoacet-
ylene was complete and that the reaction products are essentially
constituted by dinaphthylpolyynes. The residual, unreacted
1-ethynylnaphthalene detected in the HPLC analysis of the
dinaphthylpolyynes (using the 250 nm wavelength) is thus a
minor component not detectable by FT-IR spectroscopy.
Finally, it is significant to consider the FT-IR spectrum of
the dark precipitate in Figure 7. The spectrum exibits four main
broad bands, whose positions are still consistent with the main
vibrational contributions observed for the dried dinaphth-
ylpolyynes. We can distinguish two broad bands at about 3000
and 750-800 cm-1, consistent with the CH stretching and CH
wagging of the dinaphthyl terminations, respectively,42 an
intense band in the 1300-1600 cm-1 region that can be
attributed to CdC stretching modes, and a band in the
2000-2200 cm-1 region, related to CtC stretching modes. This
indicates that the dark precipitate is the result of the polymer-
ization of the dinaphthylpolyynes in the solution. Remarkably
anyway, the polymerization process is only capable of inducing
a broadening of the vibrational bands, but it does not cause a
significant depletion of the spCCs. Furthermore, the obtained
spectrum is very similar to the one measured on the black
precipitate obtained from the solution of diphenylpolyynes in
ref 32. The product of the polymerization of spCCs terminated
by aromatic groups can then be considered the intermediate step
between the isolate spCCs molecules and a solid made by
sp-sp2 carbon.5,16 Remarkably anyway, both the dried dinaph-
thylpolyynes and the precipitate do not show any decay
evolution during their exposure to air (i.e., to oxygen). This
indicates that the high reactivity of the spCCs to oxygen
observed in pure sp-sp2 carbon systems16,40 is strongly inhibited
by the presence of the hydrogen atoms in the aromatic terminal
groups.
To gather further information on the stability of dried
dinaphthylpolyynes, we repeated the process of vacuum drying
of the solution at room temperature, and we recovered the dried
dinaphthylpolyynes in the form of a dark orange powder. This
powder was then characterized in DSC, using a heating rate of
10 °C/min under a N2 flow. Figure 8 shows the resulting DSC
trace. At 103.5 °C can be observed a weak endothermic peak,
indicating the onset of melting, probably for the shorter-chain
oligomers. The peak is then followed by a broad exothermal
transition with an onset temperature at 143 °C and a peak at
182 °C. The exotherm peak is quite broad, and the released
energy is 442 J/g. This suggests the occurrence of a decomposi-
tion reaction for the dinaphthylpolyynes, probably initiated by
the longer spCCs (that are expected to be more unstable than
the shorter ones5,44) and then propagating to the shorter ones.

Synthesis, Characterization, and Modeling of Dinaphthylpolyynes
J. Phys. Chem. B, Vol. 114, No. 46, 2010 14841
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4. Conclusions
In conclusion, we have shown how the Cadiot-Chodkiewicz
reaction can be successfully applied to the synthesis of dinaph-
thylpolyynes. The key innovation in this synthetic approach is
the use of diiodoacetylene, which simplifies the synthetic route
to long polyyne chains with taylor-made end caps, thus
indicating its general applicability in the synthesis of R,ω-
diarylpolyynes.
HPLC analysis in conjunction with electronic absorption
spectroscopy enabled us to identify the complete series of
dinaphthylpolyynes with 2-6 acetylenic units. The length-
resolved electronic spectroscopy of the dinaphthylpolyynes
indicates, together with the well-known reduction of the
HOMO-LUMO gap for increasing chain length, a nontrivial
relation to the HOMO-LUMO gap of the end groups. This
opens interesting perspectives in the interpretation of DIBs since
spCCs terminated by PAHs could give rise (if present in the
interstellar medium) to peculiar absorption lines depending both
on the chain length of the spCC and on the structure of the
PAHs.
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By exploiting the observed independence of the molar
exctinction coefficient of the sp2-terminated spCCs by the mass
of the terminating organic groups, it was possible to estimate
the relative molar concentration of the dinaphthylpolyynes
produced. The obtained results indicate that the Cadiot-
Chodkiewicz reaction favors the synthesis of longer chains when
bulkier terminations are used.
UV-vis and FT-IR spectroscopy show that by evaporating
the decalin solvent of the dinaphthylpolyyne solution, a solid
film can be deposited where a relevant fraction of spCCs is
still present. Although a partial polymerization of the dinaph-
thylpolyynes has been observed, the obtained film is stable at
room temperature even when it is exposed to air (i.e., to oxygen).
By DSC, we checked the thermal stability of the dried
dinaphthylpolyynes, observing the onset of their decomposition
under nitrogen for temperatures higher than about 140 °C. This
moderately high thermal stability, together with the lack of
reactivity at room temperature with oxygen, makes dinaphth-
ylpolyynes promising building blocks for the integration of
spCCs in all-carbon electronic systems.
Acknowledgment. Franco Cataldo thanks the Italian Space
Agency for the support of part of this research (Contract Number
I/015/07/0 - Studi di Esplorazione del Sistema Solare). This
work was supported by the European Union through the ETSF-
I3 project (Grant Agreement No. 211956, user project 164).
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