Vibrational spectra and structure of zinc phthalocyanine

Journal of Molecular Structure 475 (1999) 175–180

Vibrational spectra and structure of zinc phthalocyanine Hanming Ding, Shuyun Wang, Shiquan Xi* Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Received 27 April 1998; accepted 22 May 1998

Abstract Semi-empirical molecular orbital calculations using PM3 Hamiltonian were employed to determine qualitative assignments of the vibrational spectrum of zinc phthalocyanine (ZnPc). The assignments are from the potential energy distribution calculations in the normal coordinate analysis and optimized geometry in the PM3 calculations. The structure of the ZnPc molecule is also deduced. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: PM3 Hamiltonian; Zinc phthalocyanine; Vibrational spectra

1. Introduction The excellent stability characteristics of phthalocyanine (Pc) compounds have led to their widespread use as colorants in the chemical industry, catalysts for the electrochemical reduction of oxygen [1], organic semiconductors for gas-sensor [2], and as models for their biologically important analogues [3]. Much attention has recently been paid to Langmuir– Blodgett (LB) films of metallo-phthalocyanines (MPc) because of their utility in photovoltaic cells, gas sensors and nonlinear optical properties [4–6]. It is very important to obtain basic knowledge for elucidating the structure–function relationship of the MPc LB films used in these researches. While the structural characterization of LB films of Pc dyes is carried out mainly by surface-enhanced resonance Raman spectroscopy [7–9] and polarized visible spectroscopy [10], Fourier-transfer infrared (FTIR) spectroscopy is a more powerful tool to study * Corresponding author. Tel.: +86-431-5682801; Fax: +86-4315685653; E-mail: [email protected]

orientation, subcell packing, and the structure of hydrocarbon chains in LB films [11]. While experimental data abound in the case of vibrational spectra of MPc, the assignments of all bands are vague and incomplete because the theoretical data have not been as extensively studied. Semi-empirical methods provide useful complementary information in the assignment of vibrational wavenumbers in large molecular systems, such as MPc. In the present paper, the geometry of zinc phthalocyanine (ZnPc) is optimized using the molecular mechanics method. Then the full geometry and normal coordination analysis are calculated by means of the semi-empirical molecular orbital method.

2. Calculational method and procedure All calculations are based on the following premises: 1. the phthalocyanine skeleton of ZnPc takes square planar structure with D 4h symmetry;

0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S 00 22 - 28 6 0( 9 8) 0 05 0 5- 5

176

H. Ding et al. / Journal of Molecular Structure 475 (1999) 175–180

2. the dehydrophthalocyanine is a symmetrical conjugated system. All semi-empirical computations were performed with the MOPAC [12] program (version 6.0) in SYBYL 6.2 software (Tripos Inc.). The calculation performed here made use of the PM3 [13] Hamiltonian, which was found to be the most suitable of the computational techniques in MOPAC. This calculation was carried out on a Silicon Graphics Indigo2 workstation using the algorithms implemented in the MOPAC. Because the graphical input is employed in SYBLY, first a dehydrophthalocyanine molecule, which has a entire conjugated system, was drawn. Then a Zn atom was added to the center of the macroring (16-membered inner ring of alternating carbon and nitrogen atoms) of the dehydrophthalocyanine molecule with four unknown bonds connected to four nitrogen atoms in isoindole rings. After constructing the skeleton of ZnPc, the geometry was minimized using the molecular mechanics method to get the initial conformation. The geometry was further optimized using PM3 calculation by addition of symmetry condition (the calculated gradient norm is less than 0.02). After the structural parameters had been obtained, normal coordinate analysis was carried out. The full geometry, bond orders, Mulliken charges, and harmonic frequencies are reported here. The normal mode descriptions were obtained from matrices containing mass-weighted cartesian displacements and atomic cartesian coordinates determined by the above-mentioned semi-empirical method.

The calculated bond lengths and bond orders for the ZnPc molecule are given in Table 1, and the corresponding symbols can be seen in Fig. 1. The calculated vibrational modes of interest were analyzed and obtained from matrices containing mass-weighted cartesian displacements and atomic cartesian coordinates resulting from the PM3 geometry optimization.

3. Results and discussion 3.1. Structure of ZnPc From the bond order, it is known that the bond orders of C b –C g, C g –C d and C g –C d are similar to those of benzene (1.42). However, the bond order of C b –C b differs greatly from that of benzene. The bond order of C a –C b is 1.04, which approaches that of a single bond. This suggests that the macroring and benzene rings of phthalocyanine are essentially independent aromatic systems having a weak pelectron interaction with each other, which is consistent with Ref. [14]. The bond order of N–M suggests that there is a weak single bond between the nitrogen and zinc atoms. 3.2. Fundamental vibrational wavenumbers: Raman and infrared spectra The ZnPc molecule having D 4h point group symmetry and containing 16 hydrogen atoms, 32

Table 1 Bond lengths and bond orders for ZnPc Bond

˚ Bond length/A

Bond order

C a –N a C a –N b C a –C b C b –C b C b –C g C g –C d C d –C d C g –H a C d –H b N a –M

1.40436 1.34685 1.46757 1.42030 1.38737 1.39340 1.39533 1.09490 1.09537 2.01765

1.24 1.40 1.04 1.31 1.41 1.39 1.43 0.96 0.97 0.70 Fig. 1. Structure and atom designations for ZnPc.

177

H. Ding et al. / Journal of Molecular Structure 475 (1999) 175–180 Table 2 Calculated and observed frequencies of ZnPc molecule Sym.

Calc.

Scal.

A 1g

3079 3062 1800 1724 1526 1471 1367 1266 1151 1074 911 690 603 285 3078 3062 1794 1663 1534 1416 1370 1290 1151 1081 899 742 573 162 3072 3053 1811 1608 1557 1260 1213 1178 1106 861 597 574 222 3073 3053 1813 1630 1596 1364 1195 1134 1089 932 700 489

2771 2756 1620 1552 1373 1324 1230 1139 1036 967 820 621 543 257 2770 2756 1615 1497 1381 1274 1233 1161 1036 973 809 668 516 146 2765 2748 1630 1447 1401 1134 1092 1060 995 775 537 517 200 2766 2748 1632 1467 1436 1228 1076 1021 980 839 630 440

B 1g

A 2g

B 2g

Obs.

1590 1512 1341

1029 835 678

1617 1305 1216

748

1131

1010

483

1450 1425

947 780 591

P.E.D. (%)

Assignment

C g –H a (29.1) C d –H b (28.8) C b –C b (27.2) C a –C a (34.1) C b –C b (30.8) C a –C a (25.0) C d –C d (34.5) C b –C b (29.6) C d –H b (58.0) C g –H a (50.7) N a –M (34.0) C a –N b (41.6) C a –C a (43.3) C d –C d (123.1) C g –H a (28.4) C d –H b (32.1) C b –C b (28.4) C a –C a (32.1) C b –C b (34.9) C d –C d (27.2) C d –C d (30.8) C v –C d (31.2) C d –H b (54.7) C g –H a (32.3) C g –C d (32.4) C a –C a (49.2) C d –C d (55.6) C d –C d (183.0) C g –H a (37.3) C d –H b (47.9) C b –C g (29.9) C g –C d (26.0) C a –N b (29.8) C a –C b (24.3) C a –N a (28.3) C g –H a (25.5) C d –H b (26.5) C d –C d (47.7) C a –C b (47.6) C a –C b (49.6) C a –C b (110.0) C g –H a (37.8) C d –H b (44.4) C b –C g (29.8) C a –C a (27.5) C g –C d (23.7) C a –N a (29.0) C g –H a (25.9) C g –H a (21.3) C b –C g (24.9) C a –N b (30.6) C d –C d (57.4) C b –C b (64.5)

C–H stretching C–H stretching Isoindole stretching Macroring stretching Pyrrole stretching Isoindole stretching Isoindole stretching Benzene stretching C–H bending C–H bending M–N stretching Macroring breathing Macroring breathing Isoindole breathing C–H stretching C–H stretching Benzene stretching Macroring stretching Isoindole stretching Pyrrole stretching Isoindole deformation Isoindole deformation C–H bending C–H bending Isoindole deformation Pyrrole deformation Isoindole deformation Isoindole deformation C–H stretching C–H stretching Isoindole stretching Benzene stretching Pyrrole stretching Macroring stretching Isoindole stretching C–H bending C–H bending Isoindole deformation Pyrrole deformation Isoindole deformation Isoindole deformation C–H stretching C–H stretching Benzene stretching Macroring stretching Benzene stretching Pyrrole stretching C–H bending C–H bending Isoindole deformation Isoindole deformation Benzene deformation Benzene deformation

178

H. Ding et al. / Journal of Molecular Structure 475 (1999) 175–180

Table 2 Continued Sym.

Eg

A 2u

Eu

Calc.

Scal.

227 115 1013 968 904 788 776

Obs.

P.E.D. (%)

Assignment

204 104 912 871 814 709 698

C a –N a (109.8) C d –C d (292.3) C d –H b (41.4) C g –H a (51.3) C g –H a (34.9) C d –H b (54.0) C b –C b (48.0)

697

627

C a –C a (42.8)

582

524

C a –C a (43.3)

459

413

C d –C d (52.8)

396

356

C b –C g (63.3)

270 239

243 215

116

104

C d –C d (135.9)

63

57

C a –C a (170.2)

968 788 706

871 709 635

773 728

C g –H a (54.7) C d –H b (54.5) C a –C a (30.5)

415

374

354

C b –C g (47.5)

318

286

303

C a –N b (42.5)

244

220

C g –C d (52.6)

114

103

C d –C d (141.5)

3078 3073 3062 3053 1812 1794 1693 1607 1537 1461 1433 1371 1291 1201 1165 1151 1104 1074

2770 2766 2756 2748 1631 1615 1524 1446 1383 1315 1290 1234 1162 1081 1049 1036 994 967

C g –H a (42.5) C g –H a (48.6) C d –H b (43.5) C d –H b (66.1) C b –C g (50.6) C d –C d (38.3) C a –C a (43.5) C g –C d (37.1) C b –C b (39.8) C d –C d (26.8) C a –N a (37.7) C d –C d (40.0) C g –C d (41.3) C g –H a (35.7) C d –H b (42.8) C b –C b (25.4) C g –H a (31.9) C g –H a (46.3)

Isoindole deformation Benzene deformation C–H wagging C–H wagging C–H wagging C–H wagging Isoindole OOP deformation Pyrrole OOP deformation Isoindole OOP deformation Isoindole OOP deformation Isoindole OOP deformation N–M OOP deformation Isoindole OOP deformation Isoindole OOP deformation Isoindole OOP deformation C–H wagging C–H wagging Pyrrole OOP deformation Isoindole OOP deformation Macroring OOp deformation Isoindole OOp deformation Isoindole OOP deformation C–H stretching C–H stretching C–H stretching C–H stretching Benzene stretching Benzene stretching Pyrrole stretching Benzene stretching Isoindole stretching Isoindole stretching Pyrrole stretching Pyrrole stretching Benzene stretching C–H bending C–H bending Isoindole deformation C–H bending C–H bending

232

1604 1580 1408 1284 1172 1122 1090 1060

N a –M (59.1) C g –C d (72.9)

179

H. Ding et al. / Journal of Molecular Structure 475 (1999) 175–180 Table 2 Continued Sym.

Calc.

Scal.

912 880 759 656 584 510 316 243 128

821 792 683 590 526 459 284 219 115

Obs.

258

P.E.D. (%)

Assignment

C a –N b (35.7) C d –C d (43.9) C a –N b (52.1) C d –C d (57.7) C g –C d (55.5) C a –C b (76.8) C d –C d (110.1) N a –M (104.8) C d –C d (333.4)

Isoindole deformation Isoindole deformation Isoindole deformation Isoindole deformation Isoindole deformation Isoindole deformation Isoindole deformation M–N deformation Isoindole deformation

The wavenumbers of less than 30 cm −1 are not given here. A single scale factor of 0.90 was chosen to correct all wavenumbers. The P.E.D. refers to the absolute percentage energy contribution of the pair to each mode, calculated using the formula: A:C:(A, B) = 100 × P(A, B)=Ev , where P(A,B) is the energy of the pair of atoms A and B, and E v is the total of the mode.

carbon atoms, eight nitrogen atoms and one zinc atom, has the following vibrational representation: Gvib = 14A1g (R) + 13A2g + 14B1g (R) + 14B2g (R) + 13Eg (R) + 6A1u + 8A2u (IR) + 7B1u + 7B2u + 28Eu (IR) where R represents Raman active mode and IR stands for infrared active mode. Therefore, these vibrations can be divided into two groups: one group consisting of the in-plane vibrations of symmetry A 1g, A 2g, B 1g, B 2g and E u, the other consisting of the out-of-plane modes of symmetry A 1u, A 2u, B 1u, B 2u and E g. The A 1g, B 1g, B 2g and E g modes are Raman active, while the A 2g mode can become active in resonance Raman. The infrared active modes have only two symmetry species: A 2u and E u. The other modes are not Raman or infrared active. From the above analysis, there are 55 Raman (or 68 Raman if considering A 2g modes) and 36 infrared vibrations. The vibrations are regarded as vibrations of the entire molecule, but it is possible to rationalize the rather complex motions involved into main types, namely higher-frequency vibrations due mainly to stretching of the carbon–hydrogen and isoindole rings modified by the macrocycle, and low-frequency vibrations which are primarily carbon–hydrogen bending, deformation of the macrocycle and vibrations of the metal–nitrogen bonds. The calculated and observed frequencies are given in Table 2, and the assignments are also given here.

3.2.1. Raman spectra [7–9,15–17] The bands calculated in the range of 3080– 3010 cm −1 contribute to C–H stretching vibrations, which amount to eight. None of them involve CyC or CyN vibrational modes. There are other C–H vibrational modes: eight bands of C–H bending and four bands of C–H wagging, which lie in the range of 1200–800 cm −1. The bands in the range of 2000– 1200 cm −1 are CyC or CyN stretching vibrations. The bands below 1200 cm −1 include all kinds of bending and deformation vibrations. It is worth noting that with the exception of the bands of 1724, 1630 and 1364 cm −1, all remaining bands below 2000 cm −1 appear to involve coupling the CyC and CyN vibrations with C–H vibrational modes. There are only two calculated bands of 911 and 270 cm −1 corresponding to M–N vibration. Considering that the vibrational data for ZnPc were obtained from a condensed phase, and that most, if not all, of the sets of closely spaced modes of the same symmetry could be perturbed by Fermi resonance, the overall agreement between observed and calculated frequency appears to be reasonable. 3.2.2. IR spectra [9,16,18,19] The IR vibrations for the free ZnPc molecule are all A 2u and E u modes. Although the entire molecule bends out of plane, analysis of the PED suggests that the C–H wagging vibrations contribute to the normal modes of 968 and 788 cm −1 to a significant extent, which correspond to 728 and 773 cm −1 in the observed spectra, respectively. The other six bands of

180

H. Ding et al. / Journal of Molecular Structure 475 (1999) 175–180

A 2u modes are related to the out-of-plane deformation of isoindole moiety modified by the macroring, and some of them include the vibrations of M–N out-ofplane deformation at low frequencies. The other IR bands are all E u modes in plane. The bands above 3000 cm −1 are C–H stretching vibrations. The bands at 1201, 1165, 1104 and 1074 cm −1 are C–H bending motion. The band at 1151 cm −1 is a mixture of CyC and C–H deformation. The remainder are the stretching or deformation of CyC or CyN bonds and the deformation of N–M bonds.

Acknowledgements The authors are grateful for the support of the National Natural Science Foundation of China. We also thank Prof. Guangfu Zeng for his assistance and advice.

References [1] H. Jahnke, M. Schoborn, G. Zimmerman, Top. Curr. Chem. 61 (1976) 133.

[2] J. Simon, J.J. Andre, Molecular Semiconductors, Springer Verlag, New York, 1985. [3] J.E. Falk, Porphyrins and Metalloporphyrins, Elsevier, Amsterdam, 1964. [4] G.G. Roberts, M.C. Petty, I.M. Dhamadasa, IEE Proc. 128 (1981) 197. [5] G.G. Reborts, Sensors Actuators 4 (1983) 131. [6] L.M. Blinov, N.V. Dubinin, L.V. Mikhnev, S.G. Yudin, Thin Solid Films 120 (1984) 161. [7] M.K. Debe, K.K. Kam, Thin Solid Films 186 (1990) 289. [8] R. Aroca, D. Battisti, Langmuir 6 (1990) 250. [9] Jess Dowdy, J.J. Hoagland, K.W. Hipps, J. Phys. Chem. 95 (1991) 3751. [10] H. Nakahara, K. Fukuda, K. Kitahara, H. Nishi, Thin Solid Films 178 (1989) 361. [11] M. Fukui, N. Katayama, Y. Ozaki, T. Araki, K. Iriyama, Chem. Phys. Lett. 177 (3) (1991) 247. [12] QCPE program # 455. [13] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 221. [14] B.D. Berezin, in: Coordination Compounds of Porphyrins and Phthalocyanines (V.G. Vopian, Trans.), John Wiley and Sons Ltd., New York, 1981, pp. 26–31. [15] C.A. Melendres, V.A. Maroni, J. Raman Spectrosc. 15 (5) (1984) 319. [16] R. Aroca, A. Thedchanamoorthy, Chem. Mater. 7 (1995) 69. [17] A.J. Bovill, A.A. McConnel, J.A. Nimmo, W.E. Smith, J. Phys. Chem. 90 (1986) 569. [18] H.F. Shurvell, L. Pinzuti, Can. J. Chem. 44 (1966) 125. [19] T. Kobayashi, F. Kurokawa, N. Uyeda, E. Suito, Spectrochimica Acta 26A (1968) 1305.