A SPECTROSCOPIC AND SEMIEMPIRICAL QUANTUM CHEMICAL STUDY OF [610244]

A SPECTROSCOPIC AND SEMIEMPIRICAL QUANTUM CHEMICAL STUDY OF
COPPER(II) PHTHALOCYANINATE

Andrei Racu1,2*, Mihai -Cosmin Pascariu1,3, Zoltán Szabadai1,4*, Mircea Mracec5

1Renewable Energies – Photovoltaic Laboratory, National Institute of Research &
Develop ment for Electroch emistry and Condensed Matter – INCEMC Timișoara, 144 Dr.
Aurel Păunescu -Podeanu, RO -300569 Timișoara, Romania
2Institute of Applied Physics of the Academy of Sciences of Moldova, 5 Academiei , MD -2028
Chișinău, Moldova
3Faculty of Pharmacy, “Vasile Goldiș” Western University of Arad, 86 Liviu Rebreanu, RO –
310414 Arad, Romania
4Faculty of Pharmacy, “Victor Babeș” University of Medicine and Pharmacy of Timișoara, 2
Eftimie Murgu Sq., RO -300041 Timișoara, Romania
5Department of Computational Chemistry, Institut e of Chemistry Timișoara of Romanian
Academy, 24 Mihai Viteazul Av., RO -300223 Timișoara, Romania
e-mail: [anonimizat], [anonimizat]

Abstract
Copper(II) phthalocyaninate (CuPc) was studied using both the PM3 and PM7 semiempirical
molecular or bital methods, and the results were compared with its XRD, FTIR and Raman
experimental properties.

Introduction
Organic semiconductors are intensively studied for applications in electronics , optics and
spin-based information technology (spintronics ) [1]. Among these materials, t he blue pigment
copper(II) phthalocyaninate (CuPc) is a common, low -cost and chemically modifiable p-type
organic semiconductor [1,2].
CuPc (Fig. 1) exhibits a planar molecule consisting of a central metal atom bound to a ligand
with extended π conjugated system [2 ,3]. It shows good thermal and chemical stability and
can be easily deposited as a thin film [2] when its performance proves to be superior to that of
single -molecule magnets over the same temperature range [1]. It thus holds promise for
quantum information processing and medium -term storage of classical bits in all -organi c
devices on plastic substrates [1]. CuPc nanoribbons can also be fabricated using vapor phase
deposition and these were studied for p hotoluminescence , with significant differences in the
luminescent behavior being found between α-CuPc and β-CuPc nanostructures [3].

Figure 1. Molecular model (PM7) and notation of bonds for CuPc

In the past decades, the organic light -emitting diodes (OLEDs) based on CuPc as a buffer ,
hole injection or emitting layer, the organic solar cells ( OSCs ) based on CuPc as a donor
material and the organic field -effect transistors ( OFETs ) based on CuPc as an active layer
have been extensively studied due to this compound’s interesting photoelectric properties: an
optical gap ( ~1.7 eV) very suitable for visible absorption (i.e., usage in photovoltaic devices)
and a transport gap ( ~2.3 eV) fit for electronic devices [2,3]. Near-infrared (NIR)
photosensitive or ganic field -effect transistors based on CuPc/ErPc 2 heterojunction exhibit
better properties when compar ed with the ErPc 2 single -layer ones , and thus good NIR
photoresponsive layer s can be obtained [2]. Also, t he NIR light is intimately linked to
industrial applications, such as NIR p hotodetectors and night vision [2].
In this paper , we use the PM3 and PM7 semiempirical molecular orbital methods to calculate
some molecular properties of CuPc, like the bond lengths and the vibrational spectrum. We
also compare the obtained results with the experimental spectroscopic data.

Experimental
CuPc was obtained by using phthalic anhydride, copper(I) chloride, urea and ammonium
molybdate, as described in the literature [4].
The UV/Vis spectrum (250–1000 nm) was acquired using an M ‐2000 (J.A. Woollam Co.,
USA) spectroscopic ellipsome ter by diluting the sample with KBr, as pellets. The FTIR
spectrum (400–4000 cm-1) was also acquired using KBr pellets, on a Vertex 70 (Bruker,
Germany) FT -IR spectrometer. The Raman spectrum was obtained at room temperature on a
Multi Probe Imaging – Mult iView 1000 scanned probe microscopy (Nanonics Imaging,
Israel) system, which incorporates the Shamrock 500i Spectrograph (Andor, UK). A laser
wavelength of 514.5 nm was used as the excitation source, with a 20 s exposure time and a
300 l mm-1 grating. The XRD diffraction pattern was obtained on a X’Pert PRO MPD
(Philips -FEI PANalytical Company , Netherlands) diffractometer.
The PM3 [5] optimization was performed by using the HyperChem software [6]. The SCF
“Convergence limit” was set at 10-5 with an iteratio n limit of 100 and without using the
“Accelerate convergence” procedure. For geometry optimization, the “Polak -Ribière
(conjugate gradient)” algorithm was selected with an RMS gradient of 0.0 01 kcal/(Å mol) .
For PM7 [7], the MOPAC2016 software [8] was used with the following keywords:
CHARGE=0, PM7, DOUBLET, EF (or BFGS) , OPT, BONDS, AUX, GRAPHF , PDBOUT,
SCFCRT=1.D -10, PRECISE, GNORM=0.001, CYCLES=5000, LARGE . The BFGS
(Broyden -Fletcher -Goldfarb -Shanno) algorithm gave very similar results with those obtaine d
with the EF (Eigenvector Following) algorithm. Discarding the OPT keyword also gave very
similar results in all cases . The Jmol [9] and Avogadro [1 0] programs were used for
visualizing the molecular geometries.

Results and discussion

010002000300040005000
05000100001500020000
6,5 7,0 7,505000100001500020000 Obtained CuPc

PDF 00-036-1883 CuPc

PDF 00-036-1884 CuPc
2 / degree Intensity / counts

The powder XRD peaks (Fig. 2) indicate that the synthesized CuPc is a
mixture of α and β phases, as seen when compared with the reference
PDF data. The β -CuPc phase crystallize s in the monoclinic crystal system
with space group P21/n and lattice parameters a=14.64 Å, b=4. 69 Å,
c=17.31 Å, α=90.00°, β=105.49°, γ=90.00 ° [11]. α-CuPc crystallize s in
the orthorhombic crystal system with lattice parameters a =12.97 Å, b =
12.15 Å, c = 6.66 Å and α = β = γ = 90° [12]. The selected 6.5-7.5° 2 θ
domain is suitable for the identifica tion of α and β phases of CuPc.
Figure 2. Comparison between the o btained diffractogram and standard CuPc patterns

Differences between α and β phases of CuPc can also be revealed using optical absorption
spectroscopy [3,13,14]. The obtained spectrum of CuPc (Fig. 3) consists of absorption peaks
in the UV (B band) and red (Q band) spectral regions. One of the B band peaks is located at
330 nm, while the Q band has two peaks, located at 620 and 696 nm, in close agreement with
the literature [3,15]. The pea k at 620 nm in the Q band is assigned to the π –π* transition of
the CuPc molecule, while assignment of the peak from 696 nm is still under discussion : a π–
π* transition, an exciton peak, a surface state, a vibrational structure, and a Davydov splitting
are possible candidates [15]. The d ifference between α and β phases of CuPc can be observed
via the shape change of the Q band [3]. α phase shows a more intense absorption at lower
wavelengths, while a pronounced absorption at a higher wavelength is specific for the β phase
[12,14]. Taking in consideration the literature reported results, we can confirm that the
obtained absorbance spectrum is an evidence that the obtained compound is a mixture of α –
and β -CuPc [3].

Figure 3 . UV/ Vis absorption spectrum of CuPc

The obtained Raman spectrum (Fig. 4 ) confirms the CuPc compound’s formation, as seen in
Table 1. The vibrational modes of Raman bands can be attributed to vibrations of the
macrocycle, of the isoindole moieties, to C–H bend ings and to the metal –nitrogen stretch.

Figure 4. Raman (left) and FTIR (right) spectr a of CuPc

Table 1. Raman lines identification and interpretation for CuPc

Our results Literature results [16,17]
α + β CuPc powder β-CuPc powder α-CuPc film β-CuPc film Interpretation Peak position (cm-1) Peak position (cm-1) Peak position (cm-1) Peak position (cm-1)
591 593 591 590 –
677 677 684 681 16 membered inner ring breathing
771 773 – – macrocycl e deformation
828 830 839 833 C-N stretching (aza groups)
841 848 – – –
1004 1008 1010 1010 isoindole in -plane bending
1037 1036 1041 1040 C-H bending -isoindole group
1102 1108 1109 1104 C-H bending out of plane
1195 1193 – – isoindole in -plane bendi ng
1337 1336 1338 1339 Cα-Cβ stretching pyrrole group
1403 1408 1414 1409 C-N stretching pyrrole group
1448 1448 – – C-N stretching
1524 1523 1527 1523 Cα-Cβ stretching pyrrole group
1586 1586 1589 1586 –

However , due to the small shift in the peak positions attributed to the α- and β -CuPc phase s,
this technique is not relevant for the identification of the CuPc phase .
The computed vibrational frequencies (PM7) and bond lengths (PM3 and PM7) for CuPc are
shown in Table 2 and, respectively, Table 3.

Table 2 . Simulated (PM7, with or without using the OPT keyword) and experimental
vibrational frequencies for CuPc ; the Raman and FTIR spectra of CuPc are shown in Fig. 4

EF BFGS Experimental
FTIR/Raman * w/ OPT w/o OPT w/ OPT w/o OPT
493.04 (1.2) 502.45 (1.0) 502.72 (1.1) 503.7 1 (1.1) 505 (w)/ 495 (w)
563.78 (2.0) 558.19 (1.8) 558.54 (1.9) 558.50 (1.8) 573 (w)/ 565 (sh)
-/591 (s)
681 (w) /677 (s)
726.59 (5.5) 727.91 (5.6) 728.04 (5.5) 727.56 (5.6) 727 (vs)/ –
745.99 (1.0) 758.70 (1.3) 759.15 (1.3) 759.08 (1.3) 754 (m)/ 743 (w)
781 (w)/ 771 (w)
819.83 (1.3) 822.18 (1.3) 820.21 (1.3) 823.04 (1.3) 800 (w)/ 828 (m)
901 (w)/ –
1032 (w, sh) /1037 (w)
1069 (m)/ –
1101.13 (1.0) 1105.76 (0.8) 1105.30 (0.8) 1106.17 (0.8) 1090 (s)/ 1100 (w)
1121 (s)/ 1124 ( w, sh)
-/1141 (m)
1165 (m )/1157 (w, sh)
1192 (w) /1195 (w)
1227.80 (1.4) 1227.36 (1.5) 1227.64 (1.4) 1227.76 (1.4)
1276.59 (1.8) 1277.30 (1.9) 1276.76 (1.8) 1277.06 (1.9) 1286 (m)/ –
1289.63 (2.4) 1291.75 (1.9) 1292.46 (2.0) 1291.97 (1.9)
1333 (s)/ 1337 (s)
1369.80 (2.5) 1373.59 (2.2) 1373.53 (2.1) 1373.93 (2.2)
1403.36 (2.5) 1403.97 (3.8) 1404.40 (3.9) 1402.99 (3.9) 1420 (m)/ 1422 (w)
-/1448 (s)
1464 (w)/ 1477 (w)
1506 (m)/ –
1518 (w) /1524 (vs)
1540.67 (2.4) 1541.27 (2. 8) 1540.49 (3.5) 1541.00 (2.9)
1559.21 (16.6) 1562.96 (15.6) 1561.26 (15.0) 1563.30 (15.5)
1587 (w) /1586 (w)
1610 (w)/ 1601 (w)
1651.88 (8.5) 1647.03 (9.0) 1647.34 (8.6) 1646.89 (8.8)
1712.49 (7.5) 1715.20 (6.7) 1714.84 (6.8) 1715.62 (6.8)
1727.13 (7.8) 1729.34 (9.7) 1728.93 (9.7) 1729.61 (9.8)
*w – weak, m – medium, s – strong, vs – very strong, sh – shoulder

Table 3 . Computed and experimental bond lengths (in Å) for CuPc

Bond
(see Fig. 1) PM3 (ΔHf = 132.641 kcal/mol) PM7 (ΔHf = 240.3 03 kcal/mol) Experimental
a 1.899 -1.900 1.982 1.950 -1.953
b 1.401 -1.487 1.367 -1.423 1.379 -1.389
c 1.331 -1.353 1.326 -1.341 1.344 -1.371
d 1.443 -1.464 1.469 -1.485 1.441 -1.490
e 1.417 -1.420 1.425 1.407
f 1.387 -1.397 1.377 -1.381 1.377 -1.399
g 1.394 -1.399 1.403 -1.405 1.372 -1.401
h 1.398 -1.403 1.393 1.399 -1.412
C-H 1.094 -1.096 1.089 -1.091 –

Conclusion
CuPc was synthesized and spectroscopically analyzed. The XRD, Raman , FTIR and UV/Vis
spectra confirmed the compound’s identity. Both PM3 and PM7 gave good results regarding
the molecular geometry. The vibrational spectra obtained with the PM7 method was only
partially confirmed by the experimental FTIR and Raman spectra.

Acknowledgements
This work was supported by the Romanian National Authority for Scient ific Research
(CNCS -UEFISCDI) through project PN 16 14 03 -10. We are gratefully acknowledging the
generous support of J.J.P. Stewart for providing an academic l icense for the MOPAC2016 .

References
[1] M. Warner, S . Din, I .S. Tupitsyn, G .W. Morley, A.M . Stoneham, J .A. Gardener, Z . Wu,
A.J. Fisher, S . Heutz, C .W.M. Kay, G . Aeppli, Nature 503 (2013) 504.
[2] J. Zhang, Y . Li, Y . Tang, X . Luo, L . Sun, F . Zhao, J . Zhong, Y . Peng, Synth. Met. 218
(2016 ) 27.
[3] W.Y. Tong, H.Y. Chen, A.B. Djurišić, A.M.C. Ng, H. W ang, S. Gwo, W.K. Chan, Opt.
Mater. (Amsterdam, Neth.) 32 (2010 ) 924.
[4] F. H. Moser, A. L. Thomas, Phthalocyanine Compounds, Reinhold, New York , 1963.
[5] J.J.P. Stewart , PM3, in: Encyclopedia of Computational Chemistr y, Wiley, 2002 .
[6] HyperChem™ Profe ssional, Hypercube, Inc., 1115 NW 4th Street, Gainesville, Florida
32601, USA, version 8.0.10 for Windows .
[7] J.J.P. Stewart, J. Mol. Model. 19 (2013) 1-32.
[8] J.J.P. Stewart: MOPAC2016 (Version: 16.035W), Stewart Computational Chemistry,
Colorado Spring s, CO, USA, http://OpenMOPAC.net/ .
[9] Jmol: an open -source Java viewer for chemical structures in 3D, http://www.jmol.org/ .
[10] (a) Avogadro: an open -source molecular builder and visualization tool. Version 1.1.0,
http://avogadro.cc/; (b) M .D. Hanwell, D .E. Curtis, D .C. Lonie, T . Vandermeersch, E . Zurek ,
G.R. Hutchison , J. Cheminf. 4 (2012 ) 17.
[11] L. Ruiz -Ramirez, A. Martinez, J. Mendieta, J.L. Brianso, E. Estop , J.S. Chi nchon,
Afinidad 44 (1987) 45.
[12] R. Prabakaran, R. Kesavamoorthy, G.L.N. Reddy, F .P. Xavier, Phys. Status Solidi B 229
(2002) 1175 .
[13] W.Y. Tong, A.B. Djurišić, M.H. Xie, A.C.M. Ng, K.Y. Cheung, W.K. Chan, Y.H.
Leung, H.W. Lin, S. Gwo, J. Phys. Chem. B 110 (2006) 17406.
[14] E.A. Lucia, F.D. Verderame, J. Chem. Phys. 48 (1968) 2674.
[15] A.T. Davidson, J. Chem. Phys. 77 (1982) 168.
[16] R. Prabakaran, R. Kesavamoorthy, G.L.N. Reddy, F.P. Xavier, Phys. Status Solidi B 229
(2002) 1175 .
[17] W. R. Scheidt , W. Dow, J. Am. Chem. Soc. 99 (1977) 1101 .