J. Phys. Chem. 1986,90, 569-575
569
SPECTROSCOPY AND STRUCTURE Resonance Raman Spectra of a-Copper Phthalocyanine A. J. Bovill, A. A. McConnell, J. A. Nimmo, and W. E. Smith* Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 IXL, Scotland (Received: June 12, 1985)
Raman spectra of a-copper phthalocyanine(a-CuPc) were recorded at room temperature and at 10 K with excitation wavelengths between 457 and 714 nm. Resonance enhancement was greatest for modes for which the largest displacements were on either the inner five-membered ring of the isoindole groups or the inner macrocycle and consequently assignment of the bands to modes of the entire molecule was possible by comparison with nickel octaethylporphyrin. Four out of five bands resonant in the Q band region and preresonant near the B band absorption region are totally symmetric modes. B band preresonance occurs more strongly with high-frequency modes and, due to the more delocalized nature of the transition, there is a larger contribution from six-membered ring displacements. Q band profiles are divided into two types. Those from high-frequency modes associated mainly with the isoindole ring are broad and involve a number of different vibronic levels of the excited state and those from low-frequency modes associated with the macrocycle are sharper and resonance from the lowest vibronic level of the excited state is the most intense. At low temperatures, multimode interactions are reduced and profiles were obtained which can be compared with solution profiles of porphyrins. Both Q, and Qy0-0 scattering can be identified and a helper mode is evident. A term enhancement predominates, with B,, and B,, modes enhanced because of a Jahn-Teller distortion of the excited state. The resonance studies, together with electronic absorption spectra and published theoretical studies, confirm that the Q band in a-CuPc is largely due to an allowed T-T* transition associated mainly with the macrocycle and inner five-membered rings of the isoindole groups. In addition, the study illustrates the advantages of the silver disk method of presenting samples and the advantages of low-temperature profiles in solid-state studies.
Introduction a-Copper phthalocyanine (a-CuPc) is a widely used pigment and a key component in a variety of solid-state devices. Raman spectroscopy has proved a useful method for the characterization of a-CuPc in such devices, particularly where it is present as a thin film, but the interpretation of the data is limited by a lack of understanding of the resonant or preresonant spectra obtained with excitation in the visible region. Further, the electronic spectrum in the solid state consists of a number of broad bands assigned in a manner analogous to that of heme systems to two T-T* transitions, a B or Soret band in the blue region and a Q band with associated vibronic bands in the red region.’-’ The Q band structure is very poorly resolved and consequently a method other than electronic spectroscopy is required if the details of this band system are to be elucidated. Thus, it seemed of value to study the resonance profile for a-CuPc in the Q band region. Resonance and preresonance studies with a limited range of excitation frequencies have been published for a range of phthalocyanines in the solid state and for iron and cobalt phthalocyanine in tetrahydrofuran s o l ~ t i o n . * ~Quite ~ detailed studies of films of a , p, and y polymorphs of metal-free phthalocyanines indicate that three different kinds of profile may be expected, namely those from vibrations resonant in both the Q and B band regions, those exhibiting resonance solely in the (1) (2) (3) 3926. (4) 73. (5) 503. (6) (7) (8)
B band region, and those exhibiting resonance solely in the Q band In addition, there have been studies of phthalocyanines at electrode surfaces or as thin films which have been more concerned with the structure at the electrode or metal surface than with the fundamental assignment.I2-l5 A major problem in the study of a-CuPc as a powder is that self-absorption of incident and scattered radiation severely limits the quality of the Raman spectrum obtained. This effect dominates over resonance and effectively prevents the measurement of Raman spectra with excitation frequencies in the region of the absorption maximum of the Q bands. This problem has been solved by presenting the a-CuPc as a fine powder dispersed in a compacted disk of 50-11 silver particles.16 The reasons for this improvement have been discussed elsewhere.17J8 At wavelengths where comparison has been possible with a-CuPc in other matrices, the relative intensities and energy positions of the more intense bands are the same and there is no evidence of selective enhancement such as may be present at electrode surfaces, so that it would appear that the a-CuPc spectrum is due to resonance. With the caveat that since quenching of fluorescence in particles of some dyes presented in silver disks has been observed” and there is therefore a possible weak electronic interaction between the particles and the silver powder, silver disks provide an ideal matrix (10) Aroca, R.; Dilella, D. P.; Loutfy, R. D. J . Phys. Chem. Solids 1982, 43, 707. (11) Aroca, R.; Loutfy, R. 0. J . Raman Spectrosc. 1982, 12, 262. (12) Kotz, R.; Yeager, E. J . Electroanal. Chem. 1980, 113, 113. (13) Simic-Glavaski, B.; Zecevic, S.; Yeager, E. J . Phys. Chem. 1983,87,
Edwards, L.; Gouterman, M. J. Mol. Spectrosc. 1970, 33, 292. Schechtman, D. H.; Spicer, W. E. J. Mol. Spectrosc. 1970, 33, 3328. Lee, L. K.; Sabelli, N. H.; Le Breton, P. R. J . Phys. Chem. 1982, 86,
4555. (14) Simic-Glavaski, B.; Zecevic, S.; Yeager, E. J . Raman Spectrosc. 1983, 14, 338. (15) Simic-Glavaski, B.; Zecevic, S.; Yeager, E. J . Electroanal. Chem. 1983, 150, 469. (16) Nimmo, J. A.; McConnell, A. A,; Smith, W. E. In ‘Raman Spectroscopy: Linear and Nonlinear”, Lascombe, J., Huong, P. U., Ed.; Wiley: New York, 1982; p 89. (17) Bovill, A. J.; Nimmo, J. A,; McConnell, A. A,; Smith, W. E. J.
Sayer, P.; Gouterman, M.; Connell, C. P . Acc. Chem. Res. 1982, 15, Henriksson, A.; Poos, B.; Sundbom, M. Theor. Chim.Acta 1972, 27,
Mathur, S. C.; Singh, J. Int. J . Quantum Chem. 1972, 6, 57. Mathur, S. C.; Singh, J. Int. J . Quantum Chem. 1974, 8, 79. Alexandrov, I. V.; Bobovich, Ya. S.; Maslov, V. G.; Sidorov, A. N. Opt. Spektrosk. 1974, 37, 467. (9) Melendres, C. A.; Maroni, V. A. J . Raman Spectrosc. 1984, 5, 319.
0022-3654186 , ,12090-0569%01 SO10 I
Raman Spectrosc., accepted for publication. (18) Hayashi, S . ; Amejima, M. Surf. Sci. 1984, 137, 442.
0 1986 American Chemical Societv -
Bovill et al.
570 The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 TABLE I: Frequencies and Assignments for a-CuPc and Polarization Results from a Solution of Sulfonated Copper Phthalocyanine“
vibrn a-CuPc a-CuPc NiOEtP type assignmt symmetry
sulfonated CUPC
PE
in H 2 0
~~
1613 1594 1530 1450 1428 1380 1341 1305 1213 1 190b 1170’ 1142 1131 1108 1040 1010 955 837 775 747 738 682 590 484
1655 1603 1602 1519 (1 469) 1409 1392 1383 (1351) 1220 (1308) 1159 1128 (1095) 1025 (1022) (1016) 806 785 (754) 751 674 (536) (528)
B
D A
B
D E
y2
v3 v28 y29
AI, B2g B2n
1533 1450
P
dP W
0
C C
P
F
dP
weak
z
.=f
m
11: 0 v)
m
IR E E
dP
a
W
G G
P
H I
dP P dP
P
J K
P P
L
aP
“ A to L refer to the illustration in the text. Assignments are the phthalocyanineequivalents of porphyrin assignments.*O Values in parentheses are calculated for NiOEtP but not observed. Low-temperature observations only. ‘Polarization.
’
for the study of a-CuPc. The method has the additional advantage that good thermal conduction through the silver matrix reduces sample decomposition and provides a suitable environment for low-temperature studies.
Experimental Section Silver disks were prepared by mixing 50-wm silver powder (Goodfellow metals) with finely divided a-CuPc (Ciba-Geigy UK PLC) and sodium nitrate (BDH) as an intensity standard. The mixture was pressed into a disk in a standard KBr press. Sodium nitrate was preferred to sodium sulfate because the signal was more intense and therefore was more effective as a standard in the resonance region where the absorption of scattered radiation is greatest. The dependence of the nitrate signal on wavelength was checked at points throughout the wavelength range used by preparing a disk containing nitrate, sulfate, and silver but no a-CuPc. It was found that the ratio of sulfate to nitrate peak area was linear throughout the range of excitation wavelengths and at temperatures between 300 and 10 K. Two disks were used to measure the a-CuPc spectra. One of composition 100/1/50 (Ag/a-CuPc/NaNOJ by weight was used in the region 457 to 514 nm and one of composition 10000/1/5000 was used from 563 to 714 nm in the region of the Q absorption bands. A Spectra-Physics 171-09 argon ion laser was used as the excitation source both to provide lines between 457 and 514 nm and to pump a Spectra-Physics 375 dye laser using Rhodamine 6G and DCM dyes to provide tuneable radiation between 568 and 714 nm. Samples for room temperature measurement were mounted on a spinning disk accessory and for studies at 10 K, they were mounted in a Displex closed cycle cooler and were not spun. Incident laser powers of 40-100 mW, scan rates of 1 or 2.5 cm-’ s-l, and slit widths of 2.5 or 5 cm-I were used. The spectra were both recorded directly and stored on disk using an Apple I1 computer. Using the computer, we constructed resonance profiles by comparing individual peak areas to the area of the 1068 cm-I peak of the sodium nitrate standard. The area of this peak was also checked against other nitrate bands and shown to be invariant
1
400
600 700 hhm) Figure 1. Electronic spectrum of a-CuPc compared to the spectra from soluble phthalocyanines in solution. Q, and Q, are the zero phonon transitions and are arrowed for each spectrum. 1, 2, and 3 correspond to the energy ranges of the incident laser radiation from the argon ion lines, rhodamine 6G dye, and DCM, respectively: A, a-CuPc; B, copper Phthalocyanine sulfonated in water; C, copper phthalocyanine sulfonated and tetraethylammonium bromide in ethanol; D, metal-free phthalocyanine sulfonated and tetraethylammonium chloride in water; E, metal-free phthalocyanine and tetraethylammonium chloride in ethanol. 500
within experimental error, eliminating the possibility of coincident nitrate and a-CuPc bands. To aid in the assignment, the polarized solution spectrum of sulfonated CuPc was also recorded. There was severe self-absorption, but the polarization behavior of the largest bands could be discerned and is reported in Table I.
Results and Discussion Electronic Spectrum. The Q band electronic spectrum of a-CuPc spans the energy range covered by dye laser excitation with rhodamine 6G and DCM dyes (Figure 1). Compared to heme systems, the bands are intense and broad and extend over a wider range of energies. The spectra of sulfonated copper phthalocyanine and metal-free phthalocyanine cover a similar energy region but the peaks are better resolved and the spectrum is sensitive to pH,15 solvent, and ionic strength (Figure 1). The two intense low-energy peaks are assigned as the components Q, and Qy of Qo. In solution, the phthalocyanine ring can flex about the nitrogen atoms bridging the isoindole groups and this may in part account for the variable vibronic structure. However, flexing of the phthalocyanine ring will be constrained in the a-CuPc solid, but the presence of the copper ion with its open d shell will tend to increase the likelihood of a Jahn-Teller splitting of the degenerate excited state. Q, and Qy are not readily assigned for a-CuPc from the electronic spectrum, but the assignment can be confirmed from the resonance profiles discussed later. The shoulder assigned as Q, in Figure 1 is ill-defined but has been positively identified in expanded spectra. Thus, in comparison to porphyrins, longer vibrational progressions and greater band intensities would suggest that the phthalocyanine Q bands arise from an allowed a-a* transition. A recent theoretical treatment3 of these systems confirms this and indicates that the phthalocyanine Q band is more purely from the highest-filled a,, orbital to the lowest-unfilled eg orbital than is the case for hemes, since
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 571
Resonance Raman Spectra of a-CuPc
the replacement of the ethyl side chains by a phenyl ring, and the replacement of nickel with copper.
668nm
[3ooCPS
Et
I
Et
457nm [looocps
0
V (cm-l) Figure 2. Raman spectra of a-CuPc at room temperature taken with different excitation frequencies. The spectrum with 457-nm excitation exhibits preresonance from the B band whereas those with 594- and 668-nm excitation exhibit resonance with the Q band. Nitrate standard is arrowed.
the removal of the outer six-membered ring and the replacement of the N bridge by C-H cause a destabilization of the next highest energy filled orbital (a2") in hemes. Raman Spectra. Raman spectra taken with 457-, 5 9 4 , and 668-nm excitation indicate that with blue excitation the most intense bands lie at higher energies. In the Q band energy region there are fewer intense bands in the phthalocyanine spectra than in the porphyrin spectra1gand the low-energy vibrations are more intense relative to the higher-energy vibrations (Figure 2). The spectra are clearly enhanced by resonance, with some bands appearing strongly at all three excitation wavelengths and others enhanced either with blue excitation (B band preresonance) or red excitation (Q band resonance). Six spectra recorded with a disk held at 10 K indicate that the broad bands at about 1213 and 837 cm-' can be partially resolved (Figure 3). A number of very weak bands can be observed with 610- and 647-nm excitation, some of which may be due to breakthrough of infrared-active modes. There is a considerable increase in self-absorption a t the red end as indicated by the decrease in intensity of the standard and the dependence of individual vibrations on excitation wavelength in the resonanceenhanced Q band region is clearly different (cf. bands at 1530 and 682 cm-l). Detailed isotopic studies and normal coordinate analysis of nickel octaethylporphyrin (NiOEtP) by Abe et a1.20*2'were used as the basis for the band assignments (Table I), along with the previous literature on both phthalocyanines and porphyrins. Since the NiOEtP work is widely used in porphyrin assignments, it seemed useful to retain the numbering convention for the bands in that system so that ready comparison could be made. The main differences between a-CuPc and NiOEtP are the replacement of the methine group in the porphyrin macrocycle by a nitrogen atom, (19) Choi, S.;Spiro, T . G., J. Am. Chem. SOC.1984, 105, 3683. (20) Abe, M.; Kitagawa, T.; Kyogoku, Y . J . Chem. Phys. 1978,69,4526. (21) Kitagawa, T.; Abe, M.; Ogoshi, H. J . Chem. Phys. 1978,69, 4516.
Of these three changes one, the replacement of the ethyl groups by phenyl rings, has less effect than might be expected. The reason for this is that the bands observed in the spectrum are due to resonant modes and both the present study and previous theoretical studies attribute the electronic structure in the Q band region mainly to the macrocycle and the inner five-membered ring of the isoindole group.3 Melendres and Maroni9 have carried out a normal coordinate analysis for iron phthalocyanine and used it to assign the spectrum taken at 647 nm. Although they do not use the porphyrin notation, many of the major peaks a t higher frequencies have similar assignments, but the calculation used is applicable to nonresonant Raman scattering and consequently their assignments contain a greater weighting of outer six-membered ring modes and C-H vibrations than would be expected for strongly resonant spectra. The agreement is poorer for low-frequency modes. Since the relative intensities and polarizations are greatly affected by resonance and since, for a nonresonant spectrum, many more lines are to be expected in this frequency range, a direct application of the calculation to our resonant spectra is difficult. Thus, we prefer to relate the assignment to the more extensive work on porphyrins which also enables a discussion of the whole molecule as the basis for assignment, and to use the phthalocyanine calculation for comparison of individual bond displacements. The vibrations are regarded as vibrations of the entire molecule, but it is possible to rationalize the rather complex motions involved into two main types, namely higher-frequency vibrations due mainly to motion of the isoindole rings modified by the macrocycle and lower-frequency vibrations which are primarily vibrations of the macrocycle and of the metal-nitrogen bonds. Since all major peaks observed are those of resonant modes resulting from A-A* electronic transitions it is not surprising that modes involving movement of the main skeleton rather than C-H vibrations are the most intense. There are no coincidences between the intense bands in the infrared and Raman spectrum, except where there is evidence of breakthrough of infrared active modes in the best-resolved 10 K Raman spectrum and where there is a near coincidence at 750 cm-l. Thus, the assignments are based on an effective D4hpoint group of the molecule, although the resonance profiles indicate a significant electronic distortion from D4,,. In discussing NiOEtP, Abe et a1.20,2'point out the Occurrence pairs of closely related vibrations. A,, of Al,/Blg and AZg/BZg and B!, modes are both symmetric about the pyrole ring C2 axis, occurring with all rings in-phase (Alg) and out-of-phase in opposite pairs (Big). The magnitude of the frequency difference is related
572
The Journal of Physical Chemistry, Vol. 90, No. 4 , 1986
Bovill et al.
I
I1
~000cPs
c
[3ooocPs
6 1Onm I
---JL,!
[1000cPs
580nm j
ZJ z W IE z a
5LT
poocps
Y (cm-l) V (cm-') Figure 3. Six Raman spectra at 10 K taken with different excitation frequencies in the Q band region. Intensities are affected by absorption of scattered radiation and should be compared to the standard arrowed. TABLE 11: Comparison of the Pattern of Enhancement in the Q Band Region with That Caused bv B Band fieresonance" strongly enhanced strongly in the B band enhanced in the Q preenhanced band region resonance region in both regions %8
CoNM
B2g
VI2
C"
Bl,
y13
cocp
y30
CaNM
B2g
B2g
u31
NMCeN
y15
CaNMCa
u33
NMCaN
uZ5
complex
CeNM
VI0 v2
cpc,
Blg
u3
AI,
v4
u29
cacp
B2g
cps
CaNM CON
y6
CpNM C,NMca
vl
CBCeNM
v14
frequency shifts, the main features correspond well with those of a-CuPc, but it must be borne in mind that the spectrum is taken close to resonance and, consequently, polarization data must be interpreted with extreme caution.22 Band Assignments. The most pronounced displacements which contribute to the high-energy vibrations are caused by stretching and deformation of the five-membered ring of the isoindole groups which in some cases produces an appreciable distortion of the macrocycle. The bands at 1613 and 1530 cm-l are due to type
A,,
" For comparison, the major displacement on one or two bonds has been listed in each case. A more complete description is given in Table I and the text. C, and C, have their conventional meanings, NMis the nitrogen in the bridging position between the rings, N is the nitrogen coordinated to the metal, and S is the six-membered ring. The positions are labeled in the diagram of a-CuPc in the text. F "I3
by Abe et al. to the contribution of the methine bridge vibrations and it will clearly be influenced by the replacement of the methine C H bridge by the N of the phthalocyanines. Deuteration studies show that, in NiOEtP, the C-H stretching contribution raises the frequency markedly for some vibrations, particularly u I 3 and vZ1. Such vibrations would be expected to show a frequency drop in a-CuPc. The replacement of the methine C-H by a nitrogen atom renders the macrocycle more symmetrical and, in so far as it produces alternate C and N atoms, closer coupling and smaller frequency separations between the pairs may also be expected and, in general, are observed. One further effect of this change is that nitrogen atoms may enable more out-of-plane flexibility in phthalocyanines in solution but, in a-CuPc solid, this effect will be constrained by packing forces in the crystal. To aid the assignment, the solution spectrum of sulfonated CuPc was recorded with 628-nm excitation. The spectrum had a poor signal-to-noise ratio but it was possible to identify the main bands and record the polarization behavior. Although there were some
G "S/%
B stretching with the 1530-cm-' band (v3) due to a symmetric motion of all four isoindole groups and the band at 1613 cm-' (vlo) due to antisymmetric motion with opposite pairs in phase. Only the symmetric v2 vibration at 1594 cm-' of the possible pair u2 and uI1 due to type A stretching can be identified but it cannot firmly be distinguished from u19 since the solution polarization spectrum was not sufficiently resolved. The symmetric and antisymmetric vibrations of type C are assigned to the bands at 1341 and 1305 cm-', (v4) and ( u I 2 ) , respectively. The band at 1450 cm-I is assigned to vZ8,the highest-energy vibration of a series of bands involving a deformation of the isoindole ring system as illustrated by D ( Y ~ ~ )E, (v3,, at 1142 cm-' and v22 at 1131 cm-I), and ~ 2 at 9 1428 cm-I. The band at 1213 cm-' is broad in the room temperature spectrum and resolves into three components at low (22) Zgierski, M. Z.; Pawlikowski, H. Chem. Phys. 1982, 65, 335
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 573
Resonance Raman Spectra of a-CuPc temperature (Figures 2 and 3). The main band at 1213 cm-l is 3 involves a stretch of the isoindole ring (F) which assigned to ~ 1 and is similar to that of B but includes a displacement directly along the C - C bonds and has a different, less-symmetrical effect on the macrocycle. The subsidiary band at 1190 cm-' is probably u21 and that at 1170 cm-' is probably an infrared-active E,, mode. Weaker bands at 1108 ( ~ 1 4 ) and 1040 cm-' ( u 5 ) represent antisymmetric and symmetric motion of type G. Some of these motions involve a larger effect on the macrocycle than others, with the most intense bands generally arising from displacements which do cause significant alterations in the macrocycle (u3, u4, u12,and v28) as well as changes in the C,-C, or C,-N stretches. The intense bands in the high-energy spectrum correspond in the main to intense bands in the NiOEtP spectrum and it would appear that the assignment of A,, modes u3, u4, and u5, B,,modes uIo,~13,and ~14,and B2 modes v28, ~29,and ~ 3 is0 unambiguous. The band at 1305 cm- k is depolarized in the solution spectrum and is unlikely to be due to the Y6 combination band of NiOEtP. It is assigned here as u12,the B,, band which pairs with u4 and should therefore appear in the spectrum. The polarization data for low-frequency vibrations of sulfonated cy-CuPc are not consistent with the D4hassignment used in the solid complex. They involve a predominant contribution from the macrocycle and metal ligand bonds and are thus more sensitive to the structure of the entire complex than are high-frequency modes. Thus, the polarization data in solution may not accurately reflect the polarization behavior expected from a molecule constrained by solidstate packing. The bands at 1010 and 775 cm-l are assigned to a complex deformation of the macrocycle (~23and ~32,respectively) and the band at 955 cm-I (~31)to a more specific motion (H). The strong
+
"
