Laser dependent shifting of Raman bands with phthalocyanine pigmentsNadim C. Scherrer1,2
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)
Inte
nsi
ty (
arb
itrary
units
)
1543
1523 1
511
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-chloroCuPc
PG36
514 nm
1 %0.114 mW
1562 1
539
1506
1481
1446
1389
1359 1
338
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
octachloroCuPc
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)
Inte
nsi
ty (
arb
itra
ry u
nits
) 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 12
72
1203
1075
812
770 7
37
681
1500 1000 Raman Shift (cm-1)
incr
easi
ng la
ser p
ower spectral shift15
381505 1
445
1392
1339
1283
1214
1083
979
819
776
739
706
685
643
1291
1538
1505 1
445
1392
1339
1283
1214
1083
979
819
776
739
706
685
643
1291
1537
1505 1445
1388 1
337
1291
1281
1212
1082
979
817 7
76
740
706
684
643
1526
1436
1380
1330
1273
1204
1076
973
813 7
71
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 1
278
1204
1182
1076
974 8
16
771
685
642
507
1553
1534
1497
1474
1439
1381
1334 1
275
1203
1180
1075
973
814
770
684
641
506
1500 1000 500 Raman Shift (cm-1)
1562 1
539
1506
1481
1446
1389
1338
1304 1282
1214
1200
1188
1136
1083
979
818
772
685
643 51
0
514 nm633 nmPG7 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 burnt50 %5.015 mW
5 %0.605 mWpinhole in
1 %0.121 mWpinhole in
0.5 %0.0605 mWpinhole in
0.1 %0.0121 mWpinhole in
poor or noresponse on
785nm excitation
1618m1537vs /1514s tw1338vs797s
1543vs /1498vs tw1380vs1196s662s
1540s1507vs1390s686s
633nm
PB15PB15:1PB15:2
PB15:3PB15:4 PB15:6
PB16PG36PG7785nm
PG7514nm
PG7633nm
1307
121611941185
1108
953
848832
747:680ratio 1:1
w
483
288258233
174w
w
1216-
1185
w
w
w & br
1:0.5
-
w
w256w
-
-
---
w
w
--
1:0.75
716
w
-258
-
172
1529vs1341s747s680sPB15:x
1538vs1446m-s1214vs685s
α CuPc1928
β CuPc1953
ε CuPc
octachloro CuPc1936
bromo-chloroCuPc1959
metal freePc
1939
514nm excitationgood response on 785nm excitation
Phthalocyanine pigment1520-1545vs660-690s-vs
1538vs1282s776m-s
633 nm
50 %5.015 mW
785 nmpinhole out
0.5%0.0605 mW
785 nmpinhole in
10%1.21 mW
785 nmpinhole in
5%0.605 mW
514 nm
50%5.7 mW
Excessive laserpower: visible alteration on PG7
Schweizerisches Institut für KunstwissenschaftInstitut suisse pour l‘étude de l‘artInstituto svizzero di studi d‘arteSwiss Institute for Art Research
Acknowledgment for discussions:Dr. Luca Quaroni, PSI Villigen, SwitzerlandDr. Ester S.B. Ferreira, SIK-ISEA, Zurich, SwitzerlandDr. Stefan Zumbuehl, BUA, Bern, SwitzerlandProf. 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-Nm-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 pigmentsacquired with settings causing no shiftingof the main macrocycle vibration.
1500 1000 500 Raman Shift (cm-1)
Inte
nsi
ty (
arb
itrary
units
)
1618
1585
1537
1514
1451
1428
1409
1338
1314
1294
1228 1
183
1157
1140
1118
1105
1082
1025
1007
797
768
723
681
592
566
541
481 2
28
206
184
130
metal free Pc
PB16
514 nm
1 %0.114 mW
1529
1449
1428
1343
1183
1162
1143
1108
1007
952
881 8
34
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 4
83
288
258
233 1
74
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-Nm-C str.Pyrrole exp.Sym.: B1g
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.2008.11.029Scherrer, 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.
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