7935
J. Phys. Chem. 1995,99, 7935-7945
Band Deconvolution Analysis of the Absorption and Magnetic Circular Dichroism Spectral Data of ZnPc(-2) Recorded at Cryogenic Temperatures John Mack and Martin J. Stillman" Department of Chemistry, University of Westem Ontario, London, Ontario, Canada N6A 5B7 Received: November IO, 1994; In Final Form: February 28, 1995@
Absorption, magnetic circular dichroism (MCD), and emission spectra recorded from vitrified solutions of zinc phthalocyanine, ZnPc(-2), are reported. A spectral band deconvolution analysis was performed on the absorption and MCD data using a fitting technique in which identical band parameters are used for both sets of data by the SIMPFIT program. A detailed study of the temperature dependence of the absorption and MCD spectra of ZnPc(-2) shows that a major fraction of the spectral intensity observed at room temperature can be assigned to "hot" bands. This results in a significant discrepancy between the absorption and MCD band widths in spectral data recorded at room temperature. The first spectral band deconvolution of the optical spectra of a main group metal phthalocyanine complex recorded at cryogenic temperatures is reported. n* transition, at 604 nm, that lies just to the blue of the lowest energy n The analysis identifies an n n* transition, the phthalocyanine Q band at 671 nm. The B1 and B2 II II* transitions are assigned to overlapping bands in the 300-430 nm region. The origin of the vibrational bands associated with Q transition is resolved enabling a complete description of the optical spectrum in the UV-visible region to be made. A n* transition on the basis of an band at 509 nm in the spectrum of [ZnPc(-l)]+ is reassigned as an n earlier spectral deconvolution calculation. The results of thq spectral deconvolution calculations are compared with results reported for a number of other spectroscopic and theoretical techniques.
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Introduction Metal porphyrin complexes play a vital role in biological processes such as photosynthesis and respiration.' These complexes offer a unique chemistry that has a great many possible industrial applications. Perhaps the most commercially important group of the porphyrin class of molecules are the phthalocyanines known systematically as the tetraazatetrabenzporphyrins2 The traditional uses of metal phthalocyanine complexes have been as dyestuffs in clothes such as jeans and as coloring in ballpoint pen ink, in plastics, and on metal surfaces. Recently there has been renewed interest in metal phthalocyanine complexes in many high technology fields, including use in semiconductor device^,^ photovoltaic and solar cells: electr~photography,~ rectifying devices,6 molecular electronics? Langmuir-Blodgett films,8electrochromism in display devices? low-dimensional metals,I0 gas sensors,' liquid crystals,I2 and nonlinear optics,I3and use as photo sensitizer^'^ and electrocatalytic agents. An improved understanding of the electronic structure of metal phthalocyanine complexes would be of great assistance in the development of many of these new industrial applications. As ZnPc(-2) has a fully occupied set of d orbitals, it represents a useful model compound for studying the electronic structure of metal phthalocyanine complexes. Figure 1 shows the molecular geometry of ZnPc. Although the ring itself is planar, the zinc is displaced 48 pm from this plane with Zn-N bond lengths of 206.1 pm to form a domed shape.I6 Almost no difference is seen in the UV-visible absorption and MCD spectra of ZnPc(-2) and MgPc(-2), where the metal sits in the plane of the ring, as the electronic transitions that give rise to the W-visible spectrum are associated entirely with the phthalocyanine ring.17s18As a result, theoretical calculations
* To whom correspondence should be addressed. Fax: (519) 661 3022; Tel.: (519) 661 3821; Internet:
[email protected]. @Abstractpublished in Advance ACS Abstrucrs, April 15, 1995.
Figure 1. Molecular structure of zinc phthalocyanine showing the path of the 16-membered polyene ring used as the basis for the 4-orbital LCAO calculations of Gouterman used to account for the two lowest energy transitions in porphyrins and phthalocyanines.20 The 4-fold symmetry axis through the zinc imparts CdVsymmetry to the molecule because the metal is out of the plane; for metals that lie within the plane of the ring the symmetry is D4h.
and analyses of spectral data for ZnPc(-2) normally assume D4h rather than C4" ~ymmetry.'~-*~ A detailed understanding of the electronic structure of the phthalocyanine ring is required as the basis for the study of the more complex spectral properties of transition metal phthalocyanine complexes where the metal center, as well as the ring, can change oxidation states via redox reactions and the spectral properties are greatly complicated by the presence of charge-transfer band^.^^,^^^
Electronic Structure of MPc(-2) The electronic structure of the phthalocyanine ring is usually understood in terms of theoretical models that were originally developed to account for the spectral properties of the porphyrin^.^^-^^ In its simplest description the electronic structure of the phthalocyanine ring can be viewed as a 16-
0022-3654/95/2099-7935$09.00/0 0 1995 American Chemical Society
Mack and Stillman
7936 J. Phys. Chem., Vol. 99, No. 20, 1995
-
-
E2 E1
n-n*
e"
n - n
bi:
Q
- i t a
Polarization
- X/Y
Figure 2. Molecular orbital and state level diagrams of ZnPc(-2) showing the transitions that are predicted to give rise to absorption bands in the 280-1000 nm range. The orbital ordering on the left is based on Gouterman's model of the inner ring cyclic polyene.20 The orbitals on the right are the four aza-nitrogen lone pair orbitals. The association of orbital angular momentum (OAM) with pairs of orbitals follows from the assignment of the molecular orbitals of the aromatic inner ring in terms of the OAh4 associated with the complex wavefunctions. The Q, B 1, B2, N, and L transitions have been identified on the basis of Gouterman's through spectral deconvolution Transitions that give rise to dy-polarized bands SCMO-PPP-CI are represented with solid lines. Dashed lines are used for z-polarized E* transition is placed between the Q and B1 transitions. The n transitions on the basis of the spectral deconvolution analysis of (CN)ZnPc(-2) reported in Figure 8.
-
atom, 18 n-electron, aromatic system that runs around the inner perimeter of the ligand (Figure 1). Gouterman's model based on a 4-orbital linear combination of atomic orbitals (LCAO) has been widely used to describe the optical spectra of both metal porphyrin (MP(-2)) and metal phthalocyanine (MPc(2)) c o m p l e x e ~ . ~ ~The . ~ ~ideal - ~ ~ 16-atom cyclic polyene is distorted by the presence of the four pyrrole nitrogen atoms so that the symmetry is reduced to D4h and a slight lifting of the orbital degeneracy of the ungerade molecular orbitals takes place; see Figure 2. The HOMO has an ML value of f 4 while the LUMO has an M L value of f5. This simple scheme gives rise to an allowed B transition (AML = f l ) and a forbidden Q transition (AML = f9). In porphyrin molecules the HOMO al, and a2, orbitals lie close enough in energy that they can be viewed as being essentially degenerate. As a result the B and Q transitions retain their allowed and forbidden character. The addition of the aza-linkages and fused benzenes to form the phthalocyanine molecule breaks the accidental degeneracy of the al, and a2, orbitals. This results in significant mixing of the excited states and as a result the Q transition gains intensity to become a dominant feature of the optical spectrum.20a Gouterman's group demonstrated the validity of the 4-orbital approach through the more rigourous self-consistent molecular orbital Pariser-Parr-Pople configuration interaction (SCMOPPP-CI) This model predicted the energies of a number of higher energy transitions involving other orbitals of the n-system which are labelled as N, L, C, X I , and X2 in the case of MPc(-2) complexes. Schaffer, Gouterman, and Davidson later reported an extended Huckel calculation on Mg, Mn, Fe, Co, Ni, Cu, and Zn phthalocyanine and tetraazaporphyrin complexes.28 These calculations took into account all of the valence electrons associated with the molecule and thus
predicted the energies of both the s and p orbitals of the phthalocyanine ring. Although Gouterman' s models were developed over 20 years ago, no subsequent theoretical calculations have been reported that account for the band energies, dipole strengths, and angular momentum properties of the major transitions of MPc(-2) complexes in a more satisfactory Recent molecular orbital calculations, based, like , ~ ~all 40 C and N atoms in the that of Schaffer et ~ l . upon phthalocyanine ring, report the same basic orbital ordering for the n-system as was predicted in Gouterman's original cyclic polyene based calculations. Gouterman's original model of the electronic structure has therefore been used as the theoretical framework within which to assign the n n* transitions of a variety of MPc(-2) and MP(-2) complexes and their cation radicals to the bands calculated in spectral deconvolution studies based on UVvisible absorption and MCD spectro~copy,~~ by our research group using the program SIMPFIT.'7,'8,27b,36,37 The ground states of divalent, main group D4h MPc(-2) complexes are 'Atg, and the accessible n n* excited states will be degenerate, 'E, ("y polarization) Figure 2. The major n n* transitions which dominate the UV-visible region of the optical spectra of MPc(-2) complexes give rise to Faraday A terms in the MCD spectra. Vibronically coupled n n* states can also transform as 'Azu(z polarization), giving rise to Faraday B terms. Spectral deconvolution has indicated that there are in fact two separate A terms in the B region of the spectrum of MPc(-2) complexes. Gouterman's model has therefore been modified to include separate B1 and B2 transitions superimposed in the 350 nm r e g i ~ n . ' ~The , ~ higher ~ ~ energy N, L, and C transitions have also been identified through spectral deconvolution in the UV region of the spectrum.'s,27b136,38 Although the relative energies and intensities of the lowest energy n n*electronic transitions can be rationalized in terms of Gouterman's model, there are outstanding problems that remain before a complete description of the optical properties of main group phthalocyanines can be made. The first is the energy of the four nonbonding MO's associated with the lone pair electrons on the aza-nitrogens. Recent spectroscopic and theoretical studies have led to contradictory claims about the energies of the aza-nitrogen lone pair orbitals. According to the calculation of Schaffer et al. ,28 these orbitals lie in the same energy range as the HOMO of the p-system. In contrast, other MO calculations, such as those of Henricksson et aLZ9and the more recent studies of Orti 31.39and I ~ h i k a w a ,suggest ~ ~ . ~ ~that these orbitals lie at a significantly lower energy. The consensus amongst PES spectroscopists has been that a peak at 3 eV below the HOMO arises primarily from the aza-nitrogen lone pair orbital^.^',^^-^* In contrast, studies based upon the optical spectra of MPc(-2) complexes have suggested that the azanitrogen lone pair orbitals probably lie between the al, and az, orbitals associated with the Q and B 1 transitions to the LUMO. Fluorescence emission and excitation spectra of the Q band of H ~ P cZnPc(-2), , MgPc(-2), RuPc(-2), PdPc(-2), and PtPc(-2) measured in Shpol'skii matrices at cryogenic temperatures are not mirror images of each other as would normally be e ~ p e c t e d . 4 ~Huang 3 ~ et suggested that a transition linking an aza-nitrogen lone pair orbital with the LUMO n* level may be at least partially responsible for a band at 604 nm slightly to the blue of the Q transition that is not seen in the emission spectrum. The second problem to be resolved is the origin of the vibrational bands to the blue of the QOOband of MPc(-2) complexes. Huang et have proposed a Frank-Condon mechanism as the Qm transition gains significant intensity due to excited-state configurational interaction with the allowed B 1
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-
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~
1
2
.
1
.
~
~
~
~
3
3
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~
J. Phys. Chem., Vol. 99, No. 20, 1995 7937
Analysis of the MCD Spectrum of ZnPc(-2) transition. In contrast, VanCott et in a recent MCD study of ZnPc(-2) in an argon matrix at 4 K used the vibrational borrowing mechanism proposed by Perrin22for MP(-2) complexes to analyze the Q spectral region. In this paper, a spectral deconvolution study of the absorption and MCD spectral data recorded from vitrified solutions of ZnPc( -2) recorded at 77 K in a DMFDMA solvent mixture is reported. These spectra do not exhibit many of the spectral effects associated with solvation that can complicate the analysis of data recorded in room temperature solutions. Previous optical spectral data for ZnPc(-2) in the gas p h a ~ e ? in ~ ,thin ~ ~sublimed in Shpol'skii mat rice^:^^^ or in argon matrices at cryogenic temperature^^^ exhibit complications due either to band broadening effects or matrix site multiplicity. The deconvolution analysis provides strong spectral evidence for the nature and location of the Q vibrational and n n* bands. The absorption and MCD spectral data are the first to be reported from a dilute vitrified solution for a main group MPc(-2) complex.
!,
6.01
I
,
I
600
700
800
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Experimental Section ZnPc from Eastman Kodak, zinc tetraphenylporphyrin (ZnTPP) from Aldrich, and spectrograde dimethylformamide (DMF) from B.D.H. were used as supplied. ZnPc was analyzed by mass spectrometry and gave the expected isotope pattem. Spectrograde dimethylacetamide (DMA) from B.D.H. was purified by distillation and drying over molecular sieves. The zinc octamethyltetrabenzporphyrin(ZnOMBP) was a gift supplied by Dr. K. J. Reimer, Royal Rhodes Military College, Victoria, British Columbia. Absorption spectra were recorded with an AVIV 17DS spectrophotometer (a spectrometer based on the Cary 17 monochromator). The wavelength accuracy was tested with a holmium oxide filter and was found to be accurate to within &0.4 nm for 14 peaks between 240 and 650 nm. The photometric accuracy was tested using National Bureau of Standards Neutral Density Schott Filters and was found to be better than 0.1% at 0.3, 0.5, 1.O, 1.5, and 2.0 absorption units. Samples to be used at cryogenic temperatures were degassed at room temperature and were then quenched in liquid nitrogen in a spectroelectrochemical cell designed specifically for the Oxford Instruments Model 204 Cryo~tat.~' The MCD spectra were recorded on a Jasco J-5OOC spectrometer with a field of 5.5 T from an Oxford Instruments SM2 superconducting magnet for room temperature data and a field of 4 T from an Oxford Instruments SM4 magnet for cryogenic temperature data. MCD spectra were recorded digitally under the control of an upgraded version of the program CDSCAN51-52 running on an IBM 9000 series computer. The field strength and sign were calibrated by measuring the MCD spectrum of an aqueous solution of Cos04 at 510 nm. [ e ] was ~ calculated for this instrument to be -59.3 deg.cm2-dmol-'*T-'. The signal intensity of the CD spectrometer was also tested using ammonium camphor-d- 10sulfonate by ensuring that the Bo/A ratio for the peaks at 280 nm was 2.26.53 The (CN)ZnPc solutions that were used to record the absorption and MCD data in the spectral deconvolution and temperature dependence studies in Figures 6-9 and 11 were prepared in DMFDMA (5:2) by adding excess NaCN. This solvent mixture readily vitrifies. The CN- axial ligand has been found to significantly split the B1 and B2 transition^."^ The data were analyzed using the SIMPFIT5',54,55and TNMOMET456programs, Baselines were subtracted using SPECTRA MANAGER.54bThe fluorescence emission and excitation spectra were recorded with a PTI LS-1 spectrofluorimeter. The cryogenic temperature spectra were recorded using quartz tubes
300
400
500
h / nm Figure 3. Room temperature absorption spectra of solutions of ZnTPP(-2), ZnOMBP(-2), and ZnPc(-2) in DMF.
(i.d. 4 mm) placed inside the cold finger of a liquid nitrogen dewar sample holder.
Results and Discussion Figure 3 shows the absorption spectra of ZnTPP(-2), ZnOMBP(-2), and ZnPc(-2) recorded at room temperature in DMF. The absorption spectrum of ZnPc(-2) includes an intense Q band at 670 nm, followed by a series of vibrational components, Qv,b. In contrast, the spectrum of ZnTPP(-2), a typical porphyrin complex, consists of an intense B band and a much weaker forbidden Q band. The intensity of the Qoo band of ZnOMBP(-2) is clearly much closer to that of ZnPc(-2) than ZnTPP(-2). The OMBP ring, unlike the TPP, contains the fused benzene rings which have a significant impact on the energies and relative intensities of the major n n*transitions of MPc(-2) complexes.24 The effect of the fused benzene rings is to break the near degeneracy of the HOMO a], and a2" orbitals of the cyclic polyene. As a result, the Qoo band becomes much more intense as there is considerable mixing between the Q and B excited states. In the case of ZnPc(-2) the cmax of the Q band is significantly greater than that of the B 1/B2 bands as there is substantial broadening of the bands in the B region. Schaffer et a1.28postulated that the presence of underlying n n* excited states could account for the high degree of band broadening seen in the B region of the spectrum (Figure 3). The impact of n x* excited states on overlying and higher energy n n* excited states in the spectra of large heteroaromatic molecules has been described in detail by Hochstrasser and M a r z z a ~ c o .The ~ ~ TPP and OMBP rings do not contain the aza-linkages that could give rise to additional n n* transitions in the Q region of the optical spectrum of ZnPc(2).28 The bands in the B region are clearly significantly narrower than the corresponding bands in the ZnPc(-2) spectrum. The structure of the lowest energy excited state, the Q band, can be probed by examining the fluorescence emission and
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Mack and Stillman
J.
1600
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1
16500
18500
1200
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0
14500
12500
5 / cm-’ Figure 5. Steady-state emission and excitation spectra of the Q band of (CN)ZnPc in a DMFDMA solvent mixture at 77 K. UY
o
40A
3.0-
’
I
1
I
I
I
I
I
ZnPc (-2) Abs
4
-3000 -2000 -1000
0
1000
\
2.0-
w
1.00.0
2000 3000
V“ / cm-l Figure 4. Steady-state emission and excitation spectra of the Q bands of ZnTPP, ZnOMBP, and ZnPc in DMF at 298 K. The spectra are
plotted relative to the midpoints between the two Qm peaks which are expected to be the axis of mirror symmetry.
2
2.4
Bandwidth = I I
I
I
1.2-
-..
\ -0.0
Q
UY
0
_-. ,
-2.4
ZnPc (-2) Abs
2.4
, 4
excitation spectra (Figure 4). The emission and excitation spectra of heteroaromatic molecules are normally expected to show approximate mirror ~ymmetry.~’The spectra of ZnTPP(-2) clearly do not show mirror symmetry in terms of band intensity but the energies of the band maxima are in close agreement. In the absorption and emission spectra of ZnOMBP(-2) and ZnF’c(-2) there appear to be two vibrational bands to the blue of the main Qm transition. These are often referred to as ‘‘QO~” and ‘‘Q02)’in the literature. This is rather misleading as analysis of Shpol’skii spectral data has shown that there are in fact a much larger number of vibrational bands present which combine to form these two spectral feature^.^^.^ The Q band fluorescence spectra of ZnPc( -2) at 77 K are in agreement with the Shpol’skii matrix data of Huang et al.,43,44as the steadystate emission spectrum is clearly not a mirror image of the excitation spectrum (Figure 5 ) . In contrast with ZnF’c(-2) there is approximate mirror symmetry in the band maxima of the fluorescence emission and excitation spectra of the ZnOMBP(-2) Q band (Figure 4). As the only major structural difference is the aza-linkages, an additional n n* transition just to the blue of the Q band of ZnPc(-2) seems to be a reasonable explanation. The fact that this ‘‘402’’band is found in a number of different phthalocyanines and appears to move more or less consistently with the QOO band27bmeans that it would be unwise to assign a new transition solely on this basis. The presence of a ‘‘Q02)’band in the emission spectrum indicates that the bands in the 604 nm region of the spectrum are at least partly vibrational in origin. MCD spectroscopy has proved to be a powerful spectroscopic tool for studying the electronic structure of both MPc and MP complexes.58 Gaussian band spectral deconvolution analyses of MCD and absorption spectral data have been used in previous
-
1.6
\ Ll, 1 . 5 1
\
Bandwidth = 312 cm-’
/ \
cm-’
\/
0.0
e w lX - l . l t
580
Bandwidth = 302 600
620
640
660
680
i 700
720
X / nm Figure 6. Temperature dependence in the Q band region of the spectra of (CN)ZnPc(-2) in liquid and vitrified solutions. The Gaussian curves illustrate the bandwidth discrepancy in the room temperature data.
papers from our group, as the basis for the band assignments of a variety of MPc(-2) and MP(-2) complexes and their x-cation The band fitting procedure is based on the rigid shift approximation that in a Zeeman experiment the band shape function does not change when the magnetic field is applied.58 Holding the bandwidths of absorption and MCD spectral bands to the same values introduces a “straightjacket” on the fits calculated which insures that unambiguous fits can be produced. In the case of ZnPc( -2), however, the MCD Q band is clearly sharper than the corresponding absorption band at room temperature (Figure 6A,B). Careful reinspection of the band fitting data that have been reported for the more highly
Analysis of the MCD Spectrum of ZnPc(-2) 5.01
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J. Phys. Chem., Vol. 99, No. 20, 1995 7939 I
4.5I
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ZnPc( - 2 )
1
\ W
1
0.9 0.0
2.4
4.0t 0.0
590
-1.2 620
650
X
680
710
t
-2.4
/ nm
Figure 7. No significant temperature dependence is seen in spectra recorded in vitrified DMFDMA (5: 1) solution at cryogenic temperatures between 77 and 140 K. Temperature dependence is seen in spectra recorded in solution above the freezing point of DMF between 230 and 300 K.
symmetric D4h MgPc(-2) complex show that there is a similar bandwidth discrepancy.I8 In the spectrum of ZnPc(-2) the vibrational envelope to the blue of the main Qm band, recorded from a vitrified solution, contains a complex set of overlapping bands (Figure 6D,E). The MCD spectrum is clearly also effected by the depopulation of the higher energy solvation environments but the Qm peak to trough separation suggests that there is no significant overall sharpening of the main Qm MCD band. The broadening of the absorption bands in the Qvib region of the room temperature spectrum (Figure 6A) means that a detailed deconvolution analysis of the component bands is impossible using room temperature data. Sutherland proposed that zero field splitting of the orbitally degenerate 'E, excited state could account for an analogous band width discrepancy seen in the spectra of Zn and Mg coproporp h y r i n ~ . Sutherland ~~ concluded that the interaction between the ring and the surrounding solvent molecules must be responsible for the reduction in the symmetry of the excited state. The great multiplicity of possible solvation environments leads to a spread in the energy of the main transitions in the room temperature absorption spectrum. As MCD spectroscopy depends upon the OAM of the ground and excited states in addition to the electric dipole moment, the intensity mechanism for the spectral bands in the MCD and absorption experiments are significantly different.58 Solvation environments that lead to a significant reduction in the overall symmetry will quench the angular momentum of the excited states in the wings of the room temperature MCD envelope and result in a sharper MCD band. The Qm absorption band of ZnPc(-2) shows significant band sharpening as the higher energy solvation environments are steadily depopulated (Figure 7) when the solution is cooled from 298 to 210 K. The lack of temperature dependence in the Qm band between 77 and 180 K (Figure 7) suggests that the higher energy solvation environments are completely depopulated when
520
40
I I
K MCD
I
I
I I
560
600 h
/
640 nm
680
720
Figure 8. Deconvolution analysis of the Q spectral region of (CN)ZnPc. The parameters used are summarized in Table 1.
the solution is quenched. As the absorption and MCD Qm bandwidths recorded at cryogenic temperatures are in close agreement, the rigid shift assumption can be made and a meaningful spectral deconvolution analysis of the Qvib region can be made. A similar analysis has already been reported for [ZnPc(- 3 ) r using spectral data recorded at cryogenic temperatures.60
Spectral Deconvolution (i) Band Fitting: QM Region. The most striking feature of the fit of the Q region (see Figure 8 and Table 1) is the complexity of the Qm band. This MCD band clearly cannot be filled by a single derivative shaped A term; it can only be fitted satisfactorily if two oppositely-signed B terms, bands 1 and 3, are added to the fit centered on the lobes of the derivative shaped signal. The corresponding absorption band, however, cannot be fitted by these two bands alone. A reasonable fit can only be obtained if the band is fitted with a single Gaussian band that corresponds to the A term in the MCD signal, band 2. This band accounts for the entire intensity seen in the Qm absorption band. The bandwidth of the A term is twice that of the two B terms. Cryogenic temperature spectra have been reported for MgOEP( -2) and ZnOEP(-2) in which the Q bands also appear to show this effect.61 The distortion of the band is more pronounced in the case of ZnOEP(-2) where the metal ion is larger and sits above the plane of the ring.61 In previous deconvolution analyses by our group a single, weak B term of identical width and center to the A term has been introduced into the fit. This combination of bands accounts for the B term MCD intensity that arises from the field induced mixing of states. The two oppositely signed B terms, associated with the fit of the ZnPc(-2) Qm band, clearly arise from a different origin (Figure 8). The Jahn-Teller theory states that orbital degeneracy and stability of the nuclear configuration are incompatible unless all the atoms of a molecule lie on a straight
7940 J. Phys. Chem., Vol. 99, No. 20, 1995
Mack and Stillman
TABLE 1: Band Parameters for ZnPc(-2) from a Spectral Deconvolution Calculation Using SIMPFIT. The Q Regiona no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
band type
B A B B B B B B B B B B B B B B B B B B
B B B B B B B
B
vlcm-'
14 742 14 899 15 046 15 218 15 275 15 374 15 490 15 593 15 684 15 775 15 868 15 984 16 147 16 290 16 466 16 547 16 704 16 837 17 065 17 145 17 318 17 531 17 792 18 074 18 522 18 913 19 117 19 572
Llnm
678.3 671.2 664.6 657.1 654.7 650.4 645.6 641.3 637.6 633.9 630.2 625.6 619.3 613.9 607.3 604.3 598.7 593.9 586.0 583.3 577.4 570.4 562.1 553.3 539.9 528.7 523.1 510.9
Tlcm-' 140 279 140 156 144 122 122 130 115 114 106 172 160 168 197 258 29 1 257 124 166 208 184 344 487 593 548 472 180
(40
8650 263.2 81.4 183.4 223.6 352.2 213.8 163.1 750.8 175.3 213.2 289.2 274.2 742.3 14.2 169.3 34.9 64.1 97.8 37.6 93.7 54.4 44.9 13.1 -
Do
kM)"
26.5 0.81 0.25 0.56 0.68 1.08 0.65 0.50 2.30 0.54 0.65 0.89 0.84 2.27 0.04 0.52 0.11 0.20 0.30 0.12 0.29 0.17 0.14 0.04 -
114.7 1276 -134.4 -0.64 6.3 -0.62 -5.5 7.7 -0.92 3.1 2.6 3 .O -2.9 2.7 16.3 -3.2 0.9 0.8 0.37 0.84 0.79 0.068 0.81 0.090 0.37 0.038 0.22 0.074
A I or Bd103 752.1 8.37 881.3 -4.2 41.3 -4.1 -36.1 50.5 -6.0 20.3 17.0 19.7 - 19.0 17.7 106.8 -21.0 6.0 5.2 2.4 5.5 5.2 0.45 5.3 0.59 2.4 0.25 1.44 0.48
(~d~~)liO-~ -5.2 165 -7.3 -53.1 46.8 -9.2 40.6 7.4 36.5 -29.2 19.9 127 -9.2 150 10.0 21.8 27.5 17.3 3.8 18.3 3.5 17.1 36.0 -
no. band number from low to high energy. Qw refers to the ZnPc(-2) Q band; Y, calculated energy of the band center in wavenumbers; 1, wavelength of the band center in nanometers; bandwidth in wavenumbers; E , extinction coefficient at the band center in units of cm-' mol-] L; (&, zeroth moment of the absorption band intensity; (AEM)",zeroth moment for B terms ( n = 0); first moment for A terms ( n = 1). -Bold type is used for the Q and n n* electronic bands.
r,
-
line.62 The lowest lying singlet excited state that gives rise to the Q band is orbitally degenerate and can therefore be expected to be subject to a Jahn-Teller distortion effect. Direct evidence for a Jahn-Teller distortion can be found in the Shpol'skii ~ 3 ~ QW fluorescence matrix spectrum of Huang et ~ 2 1 . ~where peaks are seen at 14 914 and 14 885 cm-I. A separation of 29 cm-' is not a particularly significant splitting and could easily be masked in the room temperature MCD spectrum by the solvation-induced band broadening effects. A splitting of 108 cm-' has been reported for the corresponding QMIbands of zinc porphine, ZnP(-2), and as would be anticipated a much more significant deviation from A term character was seen in the MCD signal of the Qw band of this complex.63 (ii) Band Fitting: QYib Region. Clear evidence for a second electronic transition in the Q region is found in the Qvib region of the MCD spectrum of ZnPc(-2). The center of the ''Q02)' absorption band lies significantly to the blue of the corresponding maximum in the MCD spectrum (Figure 9). In the band fitting analysis, a relatively weak negative B term, band 16, lying to the high-energy side of the most intense positive B term, band 15, is required to account for this discrepancy. The band at 16 550 cm-I could easily arise from a second electronic transition as both the ground and excited states are nondegenerate in the case of the z-polarized n n* transition. Definite evidence for this assignment can be seen in the vibrational bands on either side of this band (Figure 10). The Qvib bands to the red, bands 4- 15, are a complex set of alternating positive and negative B terms clearly centered on the QOOtransition. In contrast, the bands on the high-energy side, bands 17-28, are significantly broader and weaker and are all positive B terms. We suggest that as the aza-nitrogen lone pair orbitals are orthogonal to the molecular orbitals of the n-system, a zpolarized transition is unlikely to show significant intensity. As
-.
8.0
I
I
I
1
77 K Abs 6.0 0 rl
4.0
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n-n* transition
W
2.0 0.0 1.6 m 0
0.8
rl
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a
2
0.0 f '
....".
,...
.v
. ..,
..,,-,'
..
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I
-0.8 560
580
600 620 h / nm
640
660
Figure 9. Deconvolution analysis of the Qvib spectral region of (CN)ZnPc. The parameters used are summarized in Table 1.
-
a result, the n n* excitation borrows intensity from the Qw transition via e, vibrational and is the origin for the second set of weak vibrational bands.
Analysis of the MCD Spectrum of ZnPc(-2)
J. Phys. Chem., Vol. 99, No. 20, 1995 7941 TABLE 2: Vibrational Bands Associated with the Q Excited State
Band Parameters
400
‘ O< k
-200
1
we
e
e
’
v” / cm-1 Figure 10. Scatter plot of the band widths used in Figure 12 as a function of the energy of the band centre. The symbols indicate the bands taken from the following region of the spectrum: (circles) the Qvib bands, (square) the n n* transition, (triangles) the vibrational n* transition. bands associated with the n
--
It is clear from the deconvolution analysis that the Qv,bregion of the MCD spectrum cannot be assigned to a series of overlapping positive and negative A terms as has been suggested for ZnTPP,22as there is a net excess of positive intensity. This positive intensity can only be accounted for by fitting the entire MCD spectral envelope with positive and negative B terms. A repeating pattern can be seen in the MCD signal of the “QoI” and ‘‘Q02’’ bands of the absorption spectrum. There is a +/I+/+ sequence of MCD B terms that appears to be found in each of these spectral regions. The first sequence comprises bands 5, 7, 8, and 10 while the second consists of bands 12, 13, 14, and 15; see Table 1 and Figure 9. The separation between each of the corresponding bands in these sets is ca. 690 cm-I 4 3 % . The weaker 4, 6, 9, and 11 bands may also be repeated but bands 690 cm-’ to the blue are not required to obtain a satisfactory fit. The second sequence of vibrational bands are probably overtones of the first sequence and arise from a second quantum of vibrational energy from the totally symmetric vibrational mode al,. The fourth B term of the “Q02)’ sequence is broader and more intense due to the presence of the n n* excitation. Marzzacco and Hochstra~se$~ have shown that when there is overlap of the vibrational bands of a n n* transition and an n n* electronic transition in heteroaromatic molecules, significant distortion of the vibrational bands can occur. The energy range covered by these two sequences of vibrational bands corresponds closely to the range covered by the vibrational bands in the fluorescence emission spectrum at 77 K (Figure 5 ) . The energies of these vibrational bands are in good agreement with the Shpol’skii matrix fluorescence excitation data of Huang et a1.43*44 (Table 2). There are five intense sets of bands in the “Qol” region of the Shpol’skii matrix data that closely match the bands obtained from the spectral deconvolution. Huang’s data are complicated by the presence of pseudo vibrational origins that give rise to separate sets of bands in the fluorescence excitation spectrum. In the “QoI”region the major vibrational bands of ZnPc(-2) appear as doublets separated by 62 cm-I. Analysis of the “402” region is complicated by the fact that it is difficult to distinguish the weak signal from the noise. The low signal-to-noise ratio of the Shpol’skii matrix data makes a direct comparison of our spectral deconvolution fit of this region of the spectrum (Figure 9) with Huang’s analysis impossible.
-
n
Avlcm-I
4 5 6 7 8 9
15218 15275 15 374 15490 15 593 15684
319 376 475 591 694 785
AvVcm-’ no. 10 11 12 13 14 15
477 587 676 742
n
Avlcm-I
Avilcm-’
15775 15 868 15984 16147 16290 16466
876 969 1085 1248 1391 1567
842 937
a no. band number from Table 1; n, calculated energy of the band center in wavenumbers; Av, difference in wavenumbers relative to Qm from Table 1; Av?, Av values reported by Huang et L I ~ . ~ ~ . ”
I I 1 I I -400 14500 15500 16500 17500 18500 19500
-
no.
-
The agreement in the “QoI”region does however confirm the validity of the assignment of the MCD bands to B terms rather than A terms. Two alternating series of positive and negative A terms were used in similar analyses of the Q spectral region of the metal porphyrin c o m p l e ~ e s . ~As~ a, ~result, ~ the use of B terms to fit the Qvibbands in the MCD spectrum of ZnPc(-2) is unexpected as MPc(-2) and MP(-2) complexes have similar structures and the same basic molecular symmetry. VanCott et al. recently used a similar analysis for the Q spectral region of ZnPc(2).38 It should be remembered, however, that the accidental degeneracy of the alu and a2” HOMOS in porphyrin molecules means that the Qm transition has very little electronically allowed B transition character and is essentially forbidden. As the intensity mechanism of the Faraday B term has a strong dependence upon the energy separation of the states mixed by the applied magnetic the intensity of the Qv,bMCD bands arises almost exclusively through vibrational borrowing from the B transition via al,, a2,, bl,, b2, vibrational modes. The al, and a2, modes give rise to positive A terms and the bl, and b2, modes give rise to negative A terms.22 Vibronic borrowing from the B band is less likely in the case of MPc(-2) complexes as the lifting of the orbital degeneracy of the HOMO leads to a much wider energy separation between the Q and B transitions. In the case of MPc(-2) complexes the addition of the fused benzene rings leads to significant mixing of the Q and B states and as a result the Qm band gains intensity comparable to that of a fully allowed electronic transition. The vibrational levels should therefore be populated according to the Frank-Condon factors associated with the Q transition. As a result the Faraday B term can contribute significantly to the intensity of the Qvib MCD bands. (iii) Band Fitting B1B2 Region. The complex set of vibrational bands associated with the Qm band is not seen in the B l B 2 region as the main electronic transitions are as broad as the entire Qvib range. In previous spectral deconvolution studies on room temperature data the fitting was based on the presence of two A terms arising from the B1 and B2 transition^.'^-^*^^^^ At 77 K there is a distinct band sharpening effect due to the “freezing out” of the higher energy solvent environments. When the MCD spectrum is fitted with two A terms there is considerable residual absorption intensity lying between the two transitions which cannot easily be accounted for. A better fit is obtained if it is assumed that there are in fact two oppositely signed B terms of similar intensity comprising a pseudo-A term in place of each of the A terms; see Figure 11 and Table 3. These B terms are different from those seen in the Qm band which do not contribute significantly to the absorption spectrum. Both the B1 and B2 excited states will be subject to Jahn-Teller distortion effects similar to those seen for the lowest lying singlet n n* excited state. The B1 and B2 transitions are close in energy so significant configurational interaction can be expected. This may result in a more
-
7942 J. Phys. Chem., Vol. 99, No. 20, 1995
--
I
I
I
3.U
Mack and Stillman
4.0 3.0
0 4
\
2.0
W
1.0 0.0 8.0
I
40 K MCD
_.’.
I
3.0 0 4
\ -2.0 3 W
a
t
B B B B B B
24602 25 577 26 657 28 762 30 756 32 634
1
\I
-12.0
1 2 3 4 5 6
-
-
*
-7.0
argon matrix is deposited and VanCott et therefore state that z-polarized transitions should be forbidden. As the absorption spectra recorded for ZnPc(-2) in the argon matrix are essentially identical to those obtained from our randomly oriented vitrified solutions, it should still be possible to make direct comparisons between the two data sets. A complication in the analysis of the argon matrix data is that the ZnPc(-2) molecules adopt at least seven separate orientations in the matrix. The spectral data were therefore very difficult to analyze as each of these sites exhibited a separate set of MCD signals. Our fit of the Q region is very different from the analysis of who fitted the Q v i b bands with a series of d y VanCott et polarized pseudo A terms. The results of our deconvolution analysis also differ significantly from those of VanCott et al. in the “Qoz” region.@ VanCott et ~ 1concluded . ~ ~that the n n* transition at 604 nm must have d y polarization due to vibronic borrowing from the Qm transition via eg modes as an allowed n n* transition would have z polarization. The authors then used the fact that z-polarized bands should regain intensity when the orientation of the sample is changed to assign their higher energy n n* B3 transition. The assignment of the two n JC* transitions was based on the theoretical model of H e n r i c k ~ s o n .The ~ ~ agreement between the experimentally observed and the calculated energies of the transitions was however not particularly sati~factory.~~ The presence of an n n* transition at 604 nm is more consistent with Schaffer’s modelz8 of the electronic structure. Schaffer predicts that the e, and bzg aza-nitrogen lone pair orbitals will lie slightly above the HOMO al, of ZnPc while the alg lone pair orbital will lie slightly below. Using Schaffer’s model it is easy to rationalize the fact that a transition linking the e, aza-nitrogen lone pair orbital to the LUMO eg of the n-system should be close in energy to the Q band if factors other than orbital separation are neglected. The B3 transition would have to arise from a transition linking the b2g lone pair orbital and the bl, n* orbital in the f 6 level as the transitions from the other lone pair orbitals to the f 5 level are electronically forbidden (Figure 2). (u) Photoelectron Spectroscopy. The deconvolution analysis reported in this paper can also be compared to recent papers in which photoelectron spectroscopy has been used to probe the electronic structure of MPc(-2) complexes. Tegeler et assigned eight peaks in the 0-30 eV range to CzP,zSand N2p.2s orbitals. The first peak at 5.9 eV was assigned to the HOMO molecule of the n-system. The second broad peak at 8.6 eV was assigned both to NQ contributions and to the aza-nitrogen lone pair orbitals. The assignment appears to have been based largely on the fact that there was a maximum in the nitrogen K soft X-ray emission spectrum close to that predicted by Henricksson’s calculation which places these aza-nitrogen lone pair orbitals at 10.5 eV.29 Orti and B r t d a ~have ~ ~ recently reported a valence effective Hamiltonian (VEH) method calculation based upon the earlier photoelectron and soft X-ray emission studies of Tegeler et aL4’ The 8.6 eV peak was assigned to the aza-nitrogen lone pairs. Previous spectral deconvolution studies from our group have indicated that there are at least four separate A terms in the 35 000-50 000 cm-’ region arising from the B1, B2, N, and L transitions to degenerate excited states of the cyclic polyene.I7-l9 The B1, B2, and N transitions link closely lying n-molecular orbitals with the LUMO (Figure 2). The orbitals associated with these transitions could easily account for the peak seen in the soft X-ray emission spectrum. The poorer resolution of the reported PES spectra as compared to the optical spectral data makes it difficult to derive definitive conclusions to support either Henricksson’s or Schaffer’s model of the electronic structure.
1
1139 1197 1966 2714 2361 1445
I
947.4 2.90 -5.721 1061.1 3.25 0.540 1868.5 5.72 3.530 3488.0 10.7 -5.782 1886.0 5.77 4.542 221.6 0.68 1.555
37.5 3.54 23.1 37.9 29.8 26.9
-12.9 -1.1 4.0 -3.5 5.2 39.6
a no. band number from low to high energy. Qm refers to the ZnPc(2) Q band; n, calculated energy of the band center in wavenumbers; r, bandwidth in wavenumbers; (&, zeroth moment of the absorption band intensity; (A€&, zeroth moment of the MCD band intensity. Bold type is used for the B1 and B2 electronic band.
significant lifting of the excited-state degeneracies.than was seen with the Qm band. The -/+ ordering from low to high energy seen in the A term of the Q band is retained in the signs of the pairs of B terms as this ordering will still be determined by the relative angular momenta of the ground and excited states.24 VanCott et al. have assigned a band to the blue of the B2 transition to a z-polarized n n* transition labelled B3.38 Although our deconvolution analysis requires the presence of an additional B terms to the blue of the B2 transition, the B3 transition is not included in our band assignment as it is impossible to distinguish whether this band is vibrational or electronic in origin (Figure 11).
-
Comparison with Recent Spectroscopic and Theoretical Studies (i) MCD Spectroscopy. VanCott et ~ 2 1 recently . ~ ~ reported the absorption and MCD spectra of ZnPc(-2) recorded in argon matrices at 4 K. The ZnPc molecules are believed to adopt a preferred orientation in the plane of the window on which the
-
-
-
J. Phys. Chem., Vol. 99, No. 20, 1995 7943
Analysis of the MCD Spectrum of ZnPc(-2)
Other PES researchers have reached conclusions markedly different from those of Orti and Bredas. Ozaki and Harada have recently assigned the 8.6 eV peak to many different n-molecular orbitals from the cyclic polyene and fused benzene n-systems on the basis of Penning ionization electron spectroscopy data.66 N n-system Guay et al.42proposed that a drop in intensity in the first two LUMO 82 b2U \ PES peaks at 5.9 and 8.6 eV upon protonation of the aza81 nitrogens suggests that the aza-nitrogen lone pair orbitals are associated with both peaks. As reduction of the ring through TI--TI protonation at the aza-linkages would result in complete disruption of the cyclic polyene molecular orbitals, a change in n - rr’ the PES signal is consistent with the assignment of these bands -n eu a-++Q primarily to n-molecular orbitals. bi: (iii) Molecular Orbital Calculations. Recent MO calculaPolarization tions have attempted to describe the electronic structure of MPc(-2) complexes starting from first principles using density X/Y functional methods.32 The density function calculations, like that of H e n r i c k ~ s o n ,predict ~~ that the molecular orbitals associated with the aza-nitrogen lone pair orbitals lie several electronvolts below the HOMO of the n-system. Unfortunately, Figure 12. Molecular orbital and state level diagrams of [ZnPc(little or no mention has been made in these papers of the state 1)]+ showing the transitions that are predicted to give rise to absorption degeneracy and band polarization information that is available bands in the 280-1000 nm range. Transitions that give rise to x/ypolarized bands are represented with solid lines. Dashed lines are used from MCD spectroscopy. For example, in the MO calculation n* transition is placed between for z-polarized transitions. The n of Liang et al.32aand the polarized specular reflectance study the Q and B1 transitions on the basis of the spectral deconvolution of Rende et al.,6’ the ‘‘Qol’’region of the spectrum was assigned analysis of (CN)ZnPc(-2) reported in Figure 8. as a second electronic transition linking orbitals labeled as 5a2, 7e,. This corresponds to the B1 transition that arises from TABLE 4: Band Assignment of [MPc(-l)]+ Gouterman’s model of the electronic structure. As this transition band band results in a degenerate excited state, an intense A term similar center polarization assignment center polarization assignment to that seen for the Qm band must appear in the MCD spectrum. 826 XlY Q 381 XlY B1 Our present MCD spectrum clearly shows that the “QoI”region 509 Z n-z* 330 XIY B2 is vibrational in origin and can not be assigned as the second 413 XlY n--n 211 XlY N electronic tran~ition.~’Rosa and B a e r e n d ~have ~ ~ ~also sugarises from a n n transition into the ai, HOMO. The authors gested that the 5a2, 7e, transition could be responsible for state that as the angular momentum of the excited state is the 604 nm band in the ‘‘Q02)’ region proposed by H ~ a n g . ~ ~ ,predicted ~ by their calculation to be small the MCD band could As the 604 nm band appears as a B term in the MCD spectrum be completely dominated by the observed B term. The excited 7e, transition. it cannot arise from the 5a2, state that arises from the transition into the half-filled M L = Ishikawa a n d - c o ~ o r k e r have s ~ ~ ~reported ~~ MO calculations &4 al, n-orbital will have OAh4 associated with the imbalance for the electronic structure of rare earth phthalocyanine dimers, in the circulation of the electrons in partially filled orbitals in M(Pc)2, and their cation radicals. As the rare earth metals are the *Aluground-state and the 2E, excited state (Figure 12). Some trivalent they are ligated by a Pc(-2) and a Pc(-1) ring. A term intensity should still therefore be present in the MCD Analogy can therefore be made to the spectrum of [MPc(envelope. Deconvolution studies on the spectrum of the [MgPc1)]+. As is the case with MPc(-2), the Q, B1, and B2 bands (- 1)]+ cation radical species found no evidence of any A term in the MCD spectrum of [MPc(- l)]+ appear as positive A terms intensity in the 509 nm region of the spectrum.36 Indeed, the (Figure 12). [MPc(-l)]+ species can show differing degrees fit with a single Gaussian was better than for other bands in the of dimerization in solution depending upon the polarity of the spectrum. ~ o l v e n t . ~Spectral ~ . ~ ~ studies of the temperature dependence of In light of the assignment developed for ZnPc(-2) in this the monomer-dimer equilibrium have led to a complete assignpaper, the 509 nm band can be assigned to a transition linking ment of the spectral bands.36 The major difference seen in the the e, aza-nitrogen lone pair orbital with the e, LUMO n* spectra of MPc(-2) and monomeric [MPc(-l)]+ (see Table 4) orbital. This would account for the nondegeneracy of the is the presence of an an intense and unusually broad positive B excited state, see Figure 12. The partial occupation of the HOMO n-orbital can be expected to alter dramatically the term at 509 nm in the [MPc(-l)]+ MCD spectrum. Orti et energy of the Q transition relative to the n ~ 1 . have ~ ’ assigned x/y polarization to this band using a VEH n* transition. The n n transition can now be assigned to the A term at 413 calculation of the UV-visible absorption spectrum of LiPc(nm and the B1, B2, and N transitions to the bands at 387, 330, 1). A spectral deconvolution analysis of the MCD spectrum and 277 nm bands in the [MgPc(- l)]+ spectrum. The n n* of monomeric and dimeric [MgPc(-l)]+ using SIMPFIT has transition is perhaps more apparent in the cation radical spectrum clearly indicated that this region is dominated by a B term arising as the overall n n* intensity is only about 15% of that of from a transition linking a nondegenerate ground state and a the neutral compound. nondegenerate excited state.36 The B term was assigned to a transition from a low-lying nondegenerate n-orbital to the Conclusions partially filled HOMO.36 Ishikawa et have correctly pointed out that this would require a significant distortion of The first complete deconvolution analysis of spectral data the MPc(-2) ring which is clearly incompatible with the obtained for a main group metal phthalocyanine complex in presence of A terms in the MCD spectrum of this species. They vitrified solutions at cryogenic temperatures enables a complete assignment of the transitions that give rise to the optical instead assign this transition to a degenerate excited state that
-
~
-
-
-
-
-
-
-
-
-
7944 J. Phys. Chem., Vol. 99, No. 20, 1995
spectrum to be made. In addition to the Q (671 nm), B1 (386 nm), B2 (331 nm), and N (274 nm) transitions that were identified previously through fitting of room temperature data for (CN)ZnPc(-2), an n n* transition is identified linking the e, aza-nitrogen lone pair orbital with the eg LUMO at 604 nm. An additional n n* transition proposed by VanCott et al.38 may also be present just to the blue of the B2 transition. Band fitting also provides definitive evidence that the vibrational bands associated with the Q transition do not arise from the vibrational borrowing mechanism seen for the corresponding bands in the spectra of metal porphyrin complexes. The molecular orbital model developed by Schaffer et aL2*appears to be in closest agreement with the available spectroscopic data. Development of new molecular orbital models that can fully account for the band polarization and state degeneracy information that can be derived from the MCD spectral data of MPc( - 2 ) complexes and their anionm and ati ion'^,'^^^^^^^^ radical species, in addition to the data that has been obtained from PES s t ~ d i e s , ~ would ~ - ~ * result in a greatly improved understanding of the electronic structure of the phthalocyanine ring.
-
-
Acknowledgment. We thank the Province of Ontario and the University of Western Ontario (UWO) for a differential fee waiver (to J.M.) and NSERC for Operating and Equipment grants (to M.J.S.). We thank Dr. K. J. Reimer, Royal Rhodes Military College, Victoria, British Columbia, for the gift of the sample of ZnOMBP. M.J.S. is a member of the Centre for Chemical Physics and the Photochemistry Unit at UWO. This is publication No. 515 of the Photochemical Unit at UWO. References and Notes (1) (a) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978. (b) Ann. New York Acad. Sci. 1973, 206. (2) Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1989; Vol. I. (b) Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1993; Vol. 11. (c) Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1993; Vol. 111. (3) Simon, J.; Andre, J.-J. Molecular Semiconductors; Springer: Berlin, 1985; Chapter 3. (4) Tang, C. W. Appl. Phys. Lett. 1986, 60, 1047. ( 5 ) Loutfy, R. 0.;Hor, A. M.; Hsiao, C. K.; Baranyi, G.; Kazmaier, P. Pure Appl. Chem. 1988, 60, 1047. (6) Abe, K.; Sato, H.; Kimura, T.; Ohkatsu, Y.; Kusano, T. Makromol. Chem. 1989, 190, 2693. (7) Simic-Glavaski, B. In Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1993; Vol. 111, Chapter 3, pp 119-166. (8) (a) Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N. J. Thin Solid Films 1985,132, 113. (b) Cook, M. J.; Dunn, A. J.; Daniel, M. F.; Hart, R. C. 0.;Richardson, R. M.; Roser, S. J. J. Am. Chem. SOC. 1988, 159, 395. (c) Palacin, S.; Lesieur, P.; Stefanelli, I.; Barraud, A. J. Am. Chem. Sac. 1988, 159, 83. (9) Nicholson, M. M. In Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1993; Vol. 111, Chapter 2, pp 71-118. (10) (a) Marks, T .J. Agnew. Chem., Int. Ed. Engl. 1990, 29, 857. (b) Hanack, M.; Datz, A,; Fay, R.; Fischer, K.; Keppeler, U.; Koch, J.; Metz, J.; Mezger, M.; Schneider, 0.;Schulze, H. J. In Handbook of Conducting Polymers; Skotheim, T. A. B., Ed.; Marcel Dekker: New York, 1986; Vol. 1, Chapter 5, p 133. (11) Snow, A. W.; Barger, W. R. In Phthalocyanine. Principles and Properries; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1989; Vol. I, Chapter 3, pp 341-392. (12) (a) Van der Pol, J. F.; Neeleman, E.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W.; Aerts, J.; Visser, R.; Picken, S. J. Liq. Cryst. 1989,6, 577. (b) Simon, J.; Sirlin, C. Pure Appl. Chem. 1989, 61, 1625. (13) (a) Casstevens, M. K.; Samoc, M.; Pfleger, J.; Prasad, P. M. J. Chem. Phys. 1990, 92, 2019. (b) Simon, J.; Bassoul, P.; Norvez, S. New J. Chem. 1989, 13, 13. (14) Kato, M.; Nishioka, Y.; Kaifu, K.; Kawamura, K.; Ohno, S. Appl. Phys. Lett. 1985, 46, 196. (15) Lever, A. B. P.; Hempstead, M. R.; Leznoff, C. C.; Lin, W.; Melnik, M.; Nevin, W. A.; Seymour, P. Pure Appl. Chem. 1986, 18, 1467.
Mack and Stillman (16) Kobayashi, T.; Ashida, T.; Uyeda, N.; Sumo, E.; Kakuda, M. Bull. Chem. SOC. Jpn. 1971, 44, 2095. (17) Nyokong, T.; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1987, 26, 548. (18) Ough, E. A,; Nyokong, T.; Creber, K. A. M.; Stillman, M. J. Inorg. Chem. 1988, 27, 2724. (19) Ough, E. A.; Stillman, M. J. Inorg. Chem. 1994, 33, 573. (20) (a) Gouteman, M. In The Porphyrins;Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111, Part A, pp 1-165. (b) Gouteman, M. J. Mol. Specfrosc. 1972,44, 37. (c) Gouteman, M.; Wagniere, G. H.; Snyder, L. C. J. Mol. Spectrosc. 1963, 11, 108. (21) Simpson, W. T. J. Chem. Phys. 1949, 17, 1218. (22) Perrin, M. H. J. Phys. Chem. 1973, 59, 2090. (23) (a) Moffitt, W. J. Chem. Phys. 1954, 22, 320. (b) Moffitt, W. J. Chem. Phys. 1954, 22, 1820. (24) (a) Michl, J. J. Am. Chem. SOC.1978, 100, 6801. (b) Michl, J. Pure Appl. Chem. 1980, 52, 1549. (25) Weiss, C.; Kobayashi, H.; Gouteman, M. J. Mol. Spectrosc. 1965, 16, 415. (26) McHugh, A. J.; Gouteman, M.; Weiss, C. Theor. Chim. Acta 1972, 24, 346. (27) (a) Nyokong, T.; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1987, 26, 1087. (b) Stillman, M. J.; Nyokong, T. N. In Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1989; Vol. I, Chapter 3, pp 133-289. (c) Stillman, M. J. In Phthalocyanine. Principles and Properties; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publications: New York, 1993; Vol. 111, Chapter 5 , pp 227-296. (28) Schaffer, A. M.; Gouteman, M.; Davidson, E. R. Theor. Chim. Acta 1973, 30, 9. (29) Henriksson, A.; Roos, B.; Sundbom, M. Theor. Chim. Acta 1972, 27, 303. (30) Dedieu, A,; Rohmer, M.-M.; Veillard, A. Adv. Quantum Chem. 1982, 16, 43. (31) Orti, E.; Bredas, J. L.; Clarisse, C. J. Chem. Phys. 1990,92, 1228. (32) (a) Liang, X. L.; Flores, S.; Ellis, D. E.; Hoffman, B. M.; Musselman, R. L. J . Chem. Phys. 1991, 95, 403. (b) Rosa, A,; Baerends, E. J. Inorg. Chem. 1992, 31, 4717. (c) Rosa, A,; Baerends, E. J. Inorg. Chem. 1994, 33, 584. (33) Ishikawa, N.; Ohno, 0.;Kaizu, Y.; Kobayashi, H. J. Phys. Chem. 1992, 96, 8832. (34) Ishikawa, N.; Ohno, 0.;Kaizu, Y. J. Phys. Chem. 1993, 97, 1004. (35) The MCD signal arises from the same transitions as those seen in the UV-visible absorption spectrum but the selection rules are different as the intensity mechanism depends upon the magnetic dipole moment in addition to the electric dipole moment which normally determines UVvisible absorption intensity.j8 MCD spectroscopy is therefore complementary to UV-visible absorption spectroscopy as it can provide ground- and excited-state degeneracy information essential in understanding the electronic structure of molecules of high symmetry. The specificity of the MCD technique arises from three highly characteristic spectral features, the Faraday A, B, and C terms.j8 The derivative shaped Faraday A term is temperature independent and identifies degenerate excited states, while the normally Gaussian-shaped C term is highly temperature dependent and identifies an orbitally degenerate ground state. Gaussian-shaped, temperature independent B terms arise from mixing between closely related states linked by a magnetic dipole transition moment. (36) Ough, E. A,; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1991, 30, 2301. (37) Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1990, 29, 5101. (38) VanCott, T. C; Rose, J. L.; Misener, G. C.; Williamson, B. E.; Schrimpf, A. E.; Boyle, M. E.; Schatz, P. N. J. Phys. Chem. 1989, 93, 2999. (39) Orti, E.; Bredas, J. L. J. Am. Chem. SOC.1992, 114, 8669. (40) Kawai, T.; Soma, M.; Matsumoto, Y.; Onishi, T.; Tamaru, K. Chem. Phys. Lett. 1976, 37, 378. (41) Tegeler, E.; Iwan, M.; Koch, E. E. J. Electron Spectrosc. Relat. Phenom. 1981, 22, 297. (42) Guay, D.; Tourillon, G.; Gastonguay, L.; Dodelet, J. P.; Nebesny, K. W.; Amstrong, N. R.; Garret, R. J. Phys. Chem. 1991, 95, 251. (43) Huang, T. H.; Reickhoff. K. E.; Voigt, E. M. J. Chem. Phys. 1982, 77, 3424. (44) Huang, T. H.; Reickhoff, K. E.; Voigt, E. M. J. Phys. Chem. 1981, 85, 3322. (45) Eastwood, D.; Edwards, L,; Gouteman, M.; Steinfeld, J. J. Mol. Spectrosc. 1966, 20, 381. (46) Edwards, L,; Gouterman, M. J. Mol. Spectrosc. 1970, 33, 292. (47) Sharp, J. H.; Lardon, M. J. Phys. Chem. 1968, 72, 3230. (48) Schechtman, B. H.; Spicer, W. E. J. Mol. Spectrosc. 1970, 33, 28. (49) Hollebone, B. R.; Stillman, M. J. Chem. Phys. Lett. 1974, 29, 284. (50) Hollebone, B. R.; Stillman, M. J. Chem. SOC., Faraday Trans. 2 1977, 74, 2107. (51) Mack, J. Ph.D. Thesis, University of Westem Ontario, 1994. (52) Gasyna, Z.; Browett, W. R.; Nyokong, T.; Kitchenham, R.; Stillman, M. J. Chemom. Intell. Lab. Syst. 1989, 5, 233.
J. Phys. Chem., Vol. 99, No. 20, I995 7945
Analysis of the MCD Spectrum of ZnPc(-2) (53) Chen, G. C.; Yang, J. T. Anal. Lett. 1977, IO, 1195. (54) (a) Browett, W. R.; Stillman, M. J. Comput. Chem. 1987, 11, 241. (b) Browett, W. R.; Stillman, M. J. Comput. Chem. 1987, 11, 73. (55) Kirkby, S.; Mack, J.; Stillman, M. J. Unpublished program. (56) Nyokong, T.; Stillman, M. J. Unpublished program. (57) Hochstrasser, R. M.; Marzzacco, C. J. Chem. Phys. 1968,49,971. ( 5 8 ) (a) Piepho, S. B.; Schatz, P. N. Group Theory in Spectroscopy with Applications to Magnetic Circular Dichroism; Wiley: New York, 1983. (b) Stephens, P. J. Adv. Chem. Phys. 1976, 35, 197. (59) Sutherland, J. C. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111, Part A, pp 225-248. (60) Mack, J.; Stillman, M. J. J. Am. Chem. Soc. 1994, 116, 1292. (61) Linder. R. E.: Barth. G.: Bunnenberg. E.: Dierassi, C. J. Chem. Soc., Perkin 2 1974, 1712. (62) Jahn, H. A.; Teller, E. Proc. R. Soc. (London) 1937, A161, 220. (63) Barth, G.; Linder, R. E.; Bunnenberg, E.; Djerassi, C.; Amowitz, Y.; Gouterman, M. Anal. N.Y. Acad. Sei. 1973, 206, 233. (64) We found that the MCD band associated with the n x* excitation must be a negative B term to account for the mismatch seen in the energies of the major absorption and MCD bands in this spectral region. There is an excess of positive intensity in this region of the MCD spectrum so VanCott had to assign a positive B term to the n x* transition as only negative and positive A terms could be used for the Q v r b bands. In our spectral I
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deconvolution analysis an intense positive B term is placed just to the red of the n x* excitation which accounts for the net excess of positive intensity in this region of the spectrum. (65) Henricksson places the aza-nitrogen lone pair orbitals at least 3 eV lower than the HOMO and predicts the energies of three n R* transitions in the 14 700-74 000 cm-I region of the spectrum at 22 000, 31 400, and 42 300 c ~ - I . The * ~ 22 000 cm-I band arises from a forbidden triplet excited state and therefore is not expected to give rise to significant intensity. VanCott et assign the transition in the ‘‘Q02)’region at 16 550 cm-’ to the 31 400 cm-’ band in the calculation. This band is said to arise from a transition linking the e, lone pair orbital to the LUMO e, of the n-system mixed with transitions from the other lone pair orbitals into higher energy R* orbitals. VanCott et ai. assign their B3 transition to Henricksson’s 42 300 cm-l. Henricksson predicted that this band would not show any absorption intensity as it arises from a mixing of the transitions to the singlet and triplet excited states associated with the 22 000 and 31 400 cm-I transitions. (66) Ozaki,H.; Harada, Y. J. Chem. Phys. 1990, 92, 3184. (67) Rende, D. E.; Heagy, M. D.; Heuer, W. B.; Liou, K.; Thompson, J. A,; Hoffman, B. M.; Musselman, R. L. lnorg. Chem. 1992, 31, 352. (68) Homborg, H. Z. Anal. Chem. 1983, 507, 35.
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