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Langmuir 1996, 12, 5736-5738
Laser Ablation Time-of-Flight Mass Spectral Studies of Metal-Substituted Phthalocyanines R. D. George,†,‡ C.-W. Chou,‡ P. Williams,‡ V. A. Burrows,§ and P. F. McMillan*,‡ Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, and Department of Chemical, Bio, and Materials Engineering, Arizona State University, Tempe, Arizona 85287-6006 Received March 26, 1996X We have obtained time-of-flight mass spectra for metal complexes of tetrakis(cumylphenoxy)phthalocyanines, MPc(CP)4, by pulsed laser ablation from frozen benzene solutions. The Pc complexes themselves act as chromophores for the 355 and 580 nm excitation wavelengths used. Prior vapor pressure osmometry (VPO) and UV/visible absorption studies indicate an increasing degree of molecular association in solution in the order Pb < Cu < Ni < Pd < Hg. The mass spectra of all but the Hg-containing sample are dominated by the monomeric molecular ion (ML+) peak. The HgPc(CP)4 sample shows a pattern containing strong peaks due to HgL2+, Hg2L3+, Hg3L4+, and other complex molecular ions. We suggest that this is consistent with formation of a cofacial Hg-linked polymer or oligomer in solution, as proposed in a previous VPO and UV/visible spectroscopy study.
Introduction The phthalocyanines (Pc’s) have been studied extensively for their interesting electronic, optical, and chemical properties. Of particular interest are the soluble tetrakis(cumylphenoxy)phthalocyanines (abbreviated Pc(CP)4), which have been investigated as potential thin film chemiresistor materials1-5 and, more recently, for their nonlinear optical (NLO) properties.6,7 In these NLO studies, it was noted that favorable NLO responses appeared to be related to MPc(CP)4 complexes which were coordinated to large metal ions (e.g. M ) Pb2+). The structure of the common metal Pc’s consists of the M2+ ion coordinated to N atoms in the center of the Pc ring, in a square planar geometry.1 The complex with Pb2+ has been shown to contain the metal ion out of the plane of the ring, presumably due to the large size of the metal.8,9 This leads to the loss of the inversion center and the NLO behavior. UV/visible spectroscopy and vapor pressure osmometric (VPO) measurements indicate that PbPc(CP)4 is monomeric in solution.8 The other metal-substituted Pc’s show varying degrees of aggregation due to cofacial
attractive interaction, resulting in effective degrees of association up to 5-6 Pc units on average, dependent upon the concentration8. This aggregation behavior is important in determining the solubility of the Pc’s and their spreading behavior in Langmuir-Blodgett (LB) film applications.1 We have recently reported the synthesis of a Pc(CP)4 material containing Hg as the metal species.10 Elemental analysis indicated a 1:1 Hg:Pc ratio. VPO measurements indicated that this compound was highly associated in solution, with an average molecular weight corresponding to approximately 11 HgPc(CP)4 units, much greater than for the other metal-Pc materials. The UV/visible spectra showed a Q-band (π-π* transition) which was broadened and highly blue-shifted, indicating that the Pc units were in close cofacial contact with one another.1,8 It was proposed that these data indicated a cofacial polymeric structure for the HgPc to give the oligomeric compound 1. In this model, the Hg2+ ions reside out of the plane of
* Author to whom correspondence should be addressed. Fax: 602-965-2747. Phone: 602-965-6645. E-mail:
[email protected]. † Current address: Naval Command, Control, and Ocean Surveillance Center, RDT&E Division, Code 361, Building 111, 53475 Strothe Road, San Diego, CA 92152-6325. ‡ Department of Chemistry and Biochemistry. § Department of Chemical, Bio, and Materials Engineering. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Snow, A. W.; Barger, W. R. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989. (2) Grate, J. W.; Klusty, M.; Barger, W. R.; Snow, A. W. Anal. Chem. 1990, 62, 1927-1934. (3) Barger, W. R.; Wohltjen, H.; Snow, A. W. In International Conference on Solid-State Sensors and Actuators-Transducers ’85, 1985, pp 410-413. (4) Wohltjen, H.; Barger, W. R.; Snow, A. W.; Jarvis, N. L. IEEE Trans. Electron. Devices 1985, 32, 1170-1174. (5) Wohltjen, H.; Snow, A. W.; Barger, W. R.; Ballantine, D. S. IEEE Trans. Ultrason., Ferroelectrics, Frequency Control 1987, UFFC-34, 172-178. (6) Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.; Hoffman, C. A.; Zafafi, Z. K.; Snow, A. W. Appl . Phys. Lett. 1989, 55, 1287. (7) Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.; Kafafi, Z. H.; Snow, A. W. In Materials for Nonlinear Optics: Chemical Perspectives; Marder, S. R., Sohn, J. E., Stucky, G. D., Eds.; ACS Symposium Series 455; American Chemical Society: Washington, DC, 1990; pp 626-634. (8) Snow, A. W.; Jarvis, N. L. J. Am. Chem. Soc. 1984, 106, 47064711. (9) Ukei, K. Acta Crystallogr. 1973, B29, 2290-2292.
S0743-7463(96)00293-4 CCC: $12.00
the Pc rings, like the metal ions in PbPc compounds. However, unlike the PbPc materials, which are monomeric in solution, the Hg2+ ions bind symmetrically to two Pc (10) George, R. D.; Snow, A. W.; McMillan, P. F.; Burrows, V. A. J. Am. Chem. Soc. 1992, 114, 8286-8287.
© 1996 American Chemical Society
Metal-Substituted Phthalocyanines
rings, resulting in a cofacial polymer or oligomer. Consistent with this model, 1H and 13C NMR spectra show highly broadened lines, indicating large, slowly tumbling molecules in solution which remain intact on the NMR time scale. Attempts to disaggregate the HgPc(Cp)4 system by extreme dilution, by heat, or by addition of known disaggregating solvents were all unsuccessful.10 These observations suggest strong chemical binding of the adjacent Pc units through the Hg2+ ions. In the present study, we have used laser ablation time-of-flight mass spectrometry to further examine the structure and binding of this compound, in comparison to those of other metalsubstituted Pc’s. Mass spectra of Pc(CP)4 compounds have been previously collected by fast atom bombardment mass spectrometry (FABMS).11 In that investigation, protonated molecules and molecular ions were observed with little or no fragmentation, but dimers, trimers, and higher mass aggregates were not observed. Until now, FABMS has been the only means by which structurally meaningful mass spectral information for these compounds could be obtained. Laser ablation of analytes from dilute solid solution in a variety of matrices has recently been shown to generate intact molecular ions of large molecules such as proteins and oligonucleotides.12,13 Of relevance to the present study is the finding that intact molecular ions can be generated by pulsed laser ablation (LA) of frozen (aqueous) solutions.14 The Pc(CP)4 complexes are minimally soluble or insoluble in water but can be dissolved in organic solvents such as benzene or toluene. In this study, the metal Pc(CP)4 complexes were ablated from frozen benzene solutions for time-of-flight mass spectrometry (TOFMS). Experimental Details All Pc(CP)4 compounds (metal-free, Ni, Cu, Pb, Hg and Pd) were synthesized according to established methods.8,10 In all cases, purification of the crude product was performed by filtration on neutral alumina, followed by precipitation of the purified product (dissolved in a minimum amount of chloroform) into rapidly stirred methanol. The ratio of chloroform to methanol was optimized in each preparation for particle size control. The precipitate was typically recovered by centrifugation and allowed to dry in air overnight. Laser Ablation Sample Preparation. All Pc(CP)4 samples were dissolved in benzene to give a final concentration of 1-3 mg mLl-1 (∼10-3 M). The Pc(CP)4 solution (20 µL) was spread over a 0.5-1 cm2 area on a copper sample block partially submersed in liquid N2. Once the solution had frozen, the entire block was submerged and then rapidly inserted into a liquidnitrogen-cooled sample mount inside the TOF sample chamber, which was continuously flushed with dry N2 gas. The chamber was sealed and pumped down, and the sample was kept at 77 K throughout the experiment. When sample transfer was sufficiently rapid, no frost was visible on the sample surface. Care was taken to ensure that the frozen film was sufficiently thin (
