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Laser dependent shifting of Raman bands with phthalocyanine pigments Nadim C. Scherrer 1,2 1 Bern University of Applied Sciences, Kunsttechnologisches Labor, Fellerstrasse 11, CH-3027 Bern (Switzerland) [email protected] 2 Swiss Institute for Art Research, SIK-ISEA, Zollikerstrasse 32, CH-8032 Zürich 1500 1000 500 Raman Shift (cm-1) Intensity (arbitrary units) 1543 1523 1511 1498 1473 1437 1380 1321 1287 1269 1196 1163 1084 1056 776 768 732 711 685 662 534 450 334 320 260 215 186 164 bromo- chloro CuPc PG36 514 nm 1 % 0.114 mW 1562 1539 1506 1481 1446 1389 1359 1338 1319 1304 1282 1214 1200 1188 1136 1083 979 818 772 685 643 510 368 348 333 234 164 147 PG7 514 nm 1 % 0.114 mW 1538 1505 1445 1424 1389 1361 1338 1317 1290 1282 1213 1188 1082 979 957 817 776 740 685 672 346 333 291 263 222 146 octachloro CuPc PG7 785 nm 0.1 % 0.0121 mW 1538 1506 1446 1396 1389 1360 1339 1317 1304 1292 1282 1214 1083 980 817 777 740 733 707 685 643 546 510 291 198 PG7 633 nm 1 % 0.148mW 1500 1000 Raman Shift (cm-1) Intensity (arbitrary units) 1534 1504 1443 1386 1336 1280 1210 1080 815 774 739 683 1538 1505 1444 1424 1389 1338 1302 1290 1282 1213 1082 817 776 740 685 1531 1333 1277 1206 1077 813 772 738 682 1526 1331 1272 1203 1075 812 770 737 681 1500 1000 Raman Shift (cm-1) increasing laser power spectral shift 1538 1505 1445 1392 1339 1283 1214 1083 979 819 776 739 706 685 643 1291 1538 1505 1445 1392 1339 1283 1214 1083 979 819 776 739 706 685 643 1291 1537 1505 1445 1388 1337 1291 1281 1212 1082 979 817 776 740 706 684 643 1526 1436 1380 1330 1273 1204 1076 973 813 771 736 700 681 640 1561 1537 1504 1479 1444 1387 1337 1303 1281 1211 1200 1184 1135 1079 977 817 802 772 739 686 643 509 1556 1536 1500 1477 1441 1383 1335 1278 1204 1182 1076 974 816 771 685 642 507 1553 1534 1497 1474 1439 1381 1334 1275 1203 1180 1075 973 814 770 684 641 506 1500 1000 500 Raman Shift (cm-1) 1562 1539 1506 1481 1446 1389 1338 1304 1282 1214 1200 1188 1136 1083 979 818 772 685 643 510 514 nm 633 nm PG7 785 nm 1 % 0.114 mW 5 % 0.57 mW 10 % 1.14 mW 50 % 5.7 mW 0.5 % 0.050 mW 1 % 0.103 mW 5 % 0.502 mW Hole burnt 50 % 5.015 mW 5 % 0.605 mW pinhole in 1 % 0.121 mW pinhole in 0.5 % 0.0605 mW pinhole in 0.1 % 0.0121 mW pinhole in poor or no response on 785nm excitation 1618m 1537vs / 1514s tw 1338vs 797s 1543vs / 1498vs tw 1380vs 1196s 662s 1540s 1507vs 1390s 686s 633nm PB15 PB15:1 PB15:2 PB15:3 PB15:4 PB15:6 PB16 PG36 PG7 785nm PG7 514nm PG7 633nm 1307 1216 1194 1185 1108 953 848 832 747:680 ratio 1:1 w 483 288 258 233 174w w 1216 - 1185 w w w & br 1:0.5 - w w 256 w - - - - - w w - - 1:0.75 716 w - 258 - 172 1529vs 1341s 747s 680s PB15:x 1538vs 1446m-s 1214vs 685s α CuPc 1928 β CuPc 1953 ε CuPc octachloro CuPc 1936 bromo- chloro CuPc 1959 metal free Pc 1939 514nm excitation good response on 785nm excitation Phthalocyanine pigment 1520-1545vs 660-690s-vs 1538vs 1282s 776m-s 633 nm 50 % 5.015 mW 785 nm pinhole out 0.5% 0.0605 mW 785 nm pinhole in 10% 1.21 mW 785 nm pinhole in 5% 0.605 mW 514 nm 50% 5.7 mW Excessive laserpower: visible alteration on PG7 Schweizerisches Institut für Kunstwissenschaft Institut suisse pour l‘étude de l‘art Instituto svizzero di studi d‘arte Swiss Institute for Art Research Acknowledgment for discussions: Dr. Luca Quaroni, PSI Villigen, Switzerland Dr. Ester S.B. Ferreira, SIK-ISEA, Zurich, Switzerland Dr. Stefan Zumbuehl, BUA, Bern, Switzerland Prof. Dr. Jaap Boon, Amolf, Amsterdam, Netherlands Observations Analyses of real paint samples containing PG7 with the 785nm laser lead to the observation that phthalocyanines exhibit a peculi- ar behaviour: a) spectra of the phthalocyanine PG7 acquired with 514, 633 and 785nm excitation wavelengths differ such, that they will not be matched in a reference database, unless acquired by the same laser (Fig. 1); b) increa- sing laser intensity produces a significant shift (>10cm -1 ) of the main bands of the macrocycle (C-N bond lengths); c) band shifting is reversible upon reducti- on of the laser power, despite visible alteration at the spot of analysis upon ap- plication of excessive laser power (Fig. 2); d) sharp and distinct multiple bands are reduced to broad bands with excessive laser power (Fig. 1). Due to the thermal and chemical stability of phthalocyanines, excessive laser power may not be noticed, as a spectrum will be delivered in any case. Methods A Renishaw InVia system equipped with edge filters and 3 different laser sources was applied: 785nm (Diode-type), Renishaw HP NIR785 (300 mW); 633nm (He-Ne-type), Renishaw RL633 (17mW); 514nm (Ar-type): Spectra-Physics (24 mW). Pigment references were as stated in Scherrer et al. 2009, rolled out on an SEM Al-stub. Sequential measure- ments were run with increasing and decreasing laser power on the same spot to study the shifting behaviour of selective bands. Results and discussion PG7 is responsive to various excitati- on wavelengths, yet the result is significantly different (Fig. 1-3). Increasing laser power will cause specific bands to shift to lower wavenumbers, particu- larly the most intense band at ~1530cm -1 , assigned to the main macrocycle C-N m -C stretching vibration (Fig. 1, 3). Discussing the origin of these pheno- mena, L. Quaroni (2011, pers. comm., 28 August) made the following sugge- stions: The sensitivity to wavelength, and not just to power, suggests that the effect has a photochemical origin and is not just due to heating. Similar obser- vations were made with resonance Raman studies on porphyrins: bands above 1500cm -1 and one at ~1360cm -1 were selectively affected by increasing laser power - essentially bands known to be sensitive to the electron density on the metal. The inverse relationship of wavenumber to laser power might be due to population of Pc antibonding orbitals by exciting electrons with the laser. This would lead to a shift to lower frequency of normal modes, with a large contribution from modes of the macrocycle. Exciting these electronic states would lead to a weakening of the macrocycle. Regarding the distinction of α-CuPc and β-CuPc (PB15 vs PB15:3), the attempt by Scherrer et al. (2009) must be revised. Shaibat et al. (2010) recent- ly presented their results using 633nm excitation with criteria that are likely not distinctive on real paint samples. Using constant settings with 785nm excitati- on on all variations of PB15, it turns out that the α-, β- and ε-type modifications can indeed be differentiated, based on multiple features (Figs. 2, 3). Conclusion Interestingly, substantial shifts (>10cm -1 ) of the main fre- quency seems to have been accepted in the case of phthalocyanines in the literature published. An attempt has thus been made to deliver the ‘stable’ reference state of the spectra to allow correct identification, and to search for explanations of this specific behaviour of phthalocyanines. Introduction Phthalocyanines (Pc) have been an important class of dyes and pigments since their discovery. Their chemical and physi- cal properties have attracted a wide range of research fields exploring their potential far beyond their colour. Technological applications based on their far reaching chemical and physical properties are as diverse as e.g. semi- conducting sensors, catalysts, optoelectronic devices, photodynamic the- rapy against cancer and many more (Liu et al. 2007, Shaibat et al. 2010). The widespread use of phthalocyanine pigments in artist’s paints [PG7 (1936), PG36 (1957), PB15 (1928), PB16 (1939)] has also attracted the interest to identify these on painted artwork for authentication purposes. Raman spectroscopy is an attractive technique for detection with mini- mal impact. Popular excitation wavelengths are 488nm, 514nm, 532nm, 633nm and more recently 785nm. Apart from PG36 and PB16, they all give a good response to the 785nm laser. PB15 exists in different crystal forms (α, β, γ, δ, ε) and the distinction of the alpha (1928) versus the beta (1953) modification is of particular interest as a time line. Scherrer et al. 2009 made an attempt to distinguish the modifications of PB15 that was based on the main macrocycle stretching vibration around 1529cm -1 . Strong variation of this main frequency lead to a revision of the earlier attempt. Fig. 1 Excitation wavelength and laser power dependent behaviour of octachloro Cu-phthalocyanine PG7 Fig. 2 Identification of different Pc pigments Fig. 3 Reference spectra of Pc pigments acquired with settings causing no shifting of the main macrocycle vibration. 1500 1000 500 Raman Shift (cm-1) Intensity (arbitrary units) 1618 1585 1537 1514 1451 1428 1409 1338 1314 1294 1228 1183 1157 1140 1118 1105 1082 1025 1007 797 768 723 681 592 566 541 481 228 206 184 130 metal free Pc PB16 514 nm 1 % 0.114 mW 1529 1449 1428 1343 1183 1162 1143 1108 1007 952 881 834 777 748 716 698 680 592 495 482 425 257 172 126 ε-CuPc PB15:6 785 nm 0.1 % 0.0121 mW 1610 1529 1451 1429 1371 1341 1307 1217 1194 1158 1143 1108 1007 953 848 832 781 747 719 694 680 641 594 492 483 421 288 258 233 174 128 β-CuPc PB15:4 785 nm 0.1 % 0.0121 mW 1610 1529 1451 1429 1341 1307 1216 1195 1158 1143 1108 1007 954 848 832 781 747 718 681 641 594 492 483 289 258 234 175 128 β-CuPc PB15:3 785 nm 0.1 % 0.0121 mW 1529 1451 1431 1341 1305 1213 1185 1159 1142 1108 1007 953 835 778 747 681 592 484 289 256 155 131 α-CuPc PB15:2 785 nm 0.1 % 0.0121 mW 1610 1529 1451 1429 1341 1306 1216 1194 1185 1158 1143 1108 1007 953 847 832 780 747 718 681 641 593 492 483 288 258 233 174 160 127 α-CuPc PB15:1 785 nm 0.1 % 0.0121 mW 1529 1451 1431 1340 1306 1184 1159 1142 1107 1007 952 779 747 680 592 484 256 α-CuPc PB15 785 nm 0.1 % 0.0121 mW CuPc PB15 C-N m -C str. Pyrrole exp. Sym.: B 1g Publication: Scherrer N.C., Zumbuehl S., Delavy F., Fritsch A. and Kuehnen R. (2009). Synthetic organic pigments of the 20th and 21st century relevant to artist's paints: Raman spectra reference collection. Spectrochimica Acta Part a-Molecular and Biomolecular Spectroscopy 73 (3) 505-524. doi:10.1016/j.saa.2008.11.029 Scherrer, N.C. (2011). „Laser dependent shifting of Raman bands with phthalocyanine pigments“ presented at RAA2011, 6th international Congress on the Application of Raman Spectroscopy in Art and Archaeology, Parma, Italy, Book of abstracts, 203-204.
