J. Phys. Chem. 1995, 99, 4820-4830
4820
Magnetic Circular Dichroism and Absorption Spectra of Lutetium Bis(phtha1ocyanine) Isolated in an Argon Matrix Thomas C. VanCott,? Zbigniew Gasyna, and Paul N. Schatz* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901
Michael E. Boyle* Materials Chemistry Branch, Code 6123, Naval Research Laboratory, Washington, D.C. 20375-5342 Received: August 15, 1994; In Final Form: November 30, 1994@
The magnetic circular dichroism (MCD) and absorption spectra of lutetium bis(phtha1ocyanine) (LuPc2) are reported in an argon matrix at -7 K over the range 5 000-60 000 cm-I. The better spectral resolution of the species isolated in an argon matrix, combined with determination of excited-state symmetries by MCD, permits an assignment of the main electronic transitions. The experimental data are compared with the valence effective Hamiltonian calculations of Orti et al. and the localized orbital calculations of Ishikawa et al. The spectra support the view that LuPc2 is a delocalized mixed-valence system.
I. Introduction The lanthanide bis(phtha1ocyanine)s (LnPc2) are a unique class of molecules demonstrating potentially useful semiconducting,'-I2 third-order nonlinear ~ p t i c a l , ' ~and . ' ~ electrochromic proper tie^.'^-*^ Accompanying the recent experimental investigations of such properties have been discussions of the electronic structure of these stable free radical system^.^^-^' One of the fundamental issues being examined is the extent of ringring interactions and their influence on the optical spectrum. X-ray diffraction studies of lanthanide bis(phtha1ocyanine)s (LnPc2) show that they are sandwich-type compounds in which the lanthanide metal ion (M3+) is eightfold coordinated to the isoindole nitrogens of the phthalocyanine rings (Figure l).32-38 The rings are staggered at -45" with respect to each other. In the solvated lutetium system, LuPc2CH2C12, the phthalocyanine rings are both convex, with one ring slightly more distorted from planarity than the others3' However, a recent shows that in an unsolvated crystalline (a)form YPc2 maintains exact D4d symmetry and that YPcyCHzC12 and LuPc2CH2C12 have identical structures. It thus seems reasonable to assume, as we do throughout this paper, that LuPc2 isolated in Ar matrices also has D4d symmetry. The difference in the phthalocyanine ring distortions has been suggested to arise from the localization of the unpaired spin on the more distorted ring, and initial descriptions of the UVIvis absorption spectra of the lanthanide bis(phtha1ocyanine)s appeared consistent with a localized m ~ d e l . * ~In - *this ~ view, the broad, weak near-infrared band is assigned as an intramolecular charge transfer (intervalence band) where the dianion phthalocyanine ring acts as an electron donor and the radical monoanion phthalocyanine ring acts as an electron acceptor.28 The remaining absorption bands are then assigned as the individual spectra of a simple (Pc2-) and a radical (Pc-) phthalocyanine ring.28 However, a more recent solution MCD study26of the asymmetric (naphthalocyaninato)(phthalocyaninato)lutetium(III) suggests that these molecules should be considered delocalized systems, with the electron hole spread over both phthalocyanine
' Present address: Walter Reed Army Institute of Research, Department
of Retroviral Research, Suite 200, 13 Taft Court, Rockville, Maryland 20850. Abstract published in Advance ACS Abstracts, March 1, 1995. @
Figure 1. Schematic rendering of the LuPcz molecule based on X-ray diffraction data. The figure is adapted from ref 37. The structure of YPc2 in an unsolvated crystalline form is reported to have exact symmetry.38
D4d
rings. This latter description of the molecule contrasts sharply with that used in making the spectral assignments indicated above. We and others have demonstrated that very sharp spectra can be obtained upon isolation of various phthalocyanine molecules in noble gas matrices at low t e m p e r a t ~ r e s . ~ ~ A - ~ ldetailed assignment and interpretation of such spectra is possible through the use of magnetic circular dichroism (MCD) and magnetic circularly polarized luminescence (MCPL) s p e c t r o s ~ o p y ~ ~ ~ ~ ' MCD measurements permit determinations of state symmetries and magnetic moments, information that is critical to developing an accurate description of the electronic structure and the optical properties of molecules. In this work, we report the MCD and absorption spectra of LuPc2 isolated in argon matrices at -7 K (LuPczIAr) over the range 5 000-60 000 cm-I. The main transitions are identified, and our results are compared with detailed valence effective Hamiltonian (VEH) calculations reported by Orti et al.27 and the more recent localized orbital configuration interaction calculations of Ishikawa et aL30 We argue that our data support the view that LuPc2 is a delocalized system but that the issue is not conclusively resolved, and the quantitative question of degree of delocalization is still open. 11. Experimental Section
1. Material Synthesis. Lutetium bis(phtha1ocyanine) was Lutetium synthesized by a modified procedure of Grin et
0022-3654/95/2099-4820$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 13, 1995 4821
Lutetium Bis(phtha1ocyanine) x20
2
, X I
AA ~
10.2
0
1
IVB RV Q
BV
B
LT%
N
I
I
.2
A
0 1
2
3
4
5
E I104 c" Figure 2. Survey of the absorption and MCD spectrum of LuPcdAr at -7 K from the near infrared into the ultraviolet region. An impurity band is indicated by an asterisk and a dashed line in the L band region. The absorption (A) is in optical density (absorbance) units and the MCD (AA) is in optical density units per tesla in all figures.
acetate and phthalonitrile (1:10 molar ratio) were heated in a sealed, evacuated tube at 300 "C until the mixture congealed to a solid mass (about 2-3 h). The crude product was removed and ground to a fine powder, extracted with chloroform, and evaporated to dryness. The extracted powder was then sequentially refluxed twice with 20 mL quantities of glacial acetic acid, water washed, and dried.43 This product was purified by alumina column chromatography (Wohelm, neutral, activity 1, packed in toluene) with chloroform elution and appeared as a dark green bis(phtha1ocyanine) band, which was isolated by reprecipitation in methanol. The purity of the samples was confirmed by comparisons with previously reported FTIR spectra and Q(0,O) band extinction coefficients. 2. Experimental Procedures. The procedure for matrix deposition has been described in detaiLU The LuPc:! is sublimed at -450 "C and codeposited with argon onto a cryogenically cooled ( -7 K) optical window in the bore of a superconducting magnet. A sapphire window is used for wavelengths longer than 200 nm. Simultaneous MCD and double-beam absorption spectra are obtained as described The data between 33 000 and 60 000 cm-' were collected at the Synchrotron Radiation Center of the University of Wisconsin-Madison using equipment described p r e v i o u ~ l y . ~ ~ The spectra were measured at a temperature of -7 K and field strengths between 1.0 and 3.0 T. MCD spectra were calibrated against the absorbance and natural CD of standard solutions of tris( 1,2-ethanediamine)cobalt(III) in the visible region and against camphorsulfonic acid-dlo in the UV.46 Depolarization of the matrix samples was estimated by comparing the CD of a standard solution inserted after the sample with the CD of the solution in the absence of the sample. Depolarization effects were negligible in all spectra.
111. Results In Figure 2, an overview is presented of the absorption and MCD spectra of LuPcz/Ar over the range 5000-60 000 cm-'. Because of the correlation between the spectrum of LuPcz/Ar and that of ZnPc/Ar,4° the spectral regions are denoted with the traditional labels: Q, B, N, L, C, and X. The absorption bands that are atypical of simple metallophthalocyanines (MPc's) are denoted by us as intervalence band (IVB) (1300 nm), red vibronic (RV) band (910 nm), and blue vibronic (BV) band (445 nm).
"
a1
D4b
D4d
D4b
Figure 3. MO diagram and orbital nodal patterns. The center of the figure shows the MO diagram inferred from the nodal pattern of the Pc M O S shown ~ ~ on the left and right sides (see text). It is in accord with the observed spectrum and the Orti et dZ7 calculations. Filled and open circles in the nodal pattem designate opposite signs for the 2p, lobes above (or below) the plane of the paper. The diameters of the circles are proportional to the corresponding LCAO coefficients.
The center of Figure 3 shows the MO diagram which is the basis of our subsequent discussion; the detailed MCD and absorption spectra of LuPcdAr in each region are shown in Figures 4-9. In general, the spectrum of matrix isolated LuPc2 has sharper bands which are more structured and somewhat blueshifted when compared to those in solution or thin film. In the higher energy regions, due to large inherent linewidths, the same broadened features are observed at roughly the same energies as in solution or thin film.
IV. Theoretical Analysis
1. Symmetry Considerations. As indicated earlier, we assume exact D4d symmetry for matrix-isolated LuPc2, and we use real standard basis functions in our analysis. The behavior of these functions under the group generators, SS and C i , is shown in Table 1. We approximate the MOs of LuPc2 as linear combinations of MOs of two isolated Pc rings (labeled 1 and 2 ) which are rotated by 45" with respect to each other. Using Cartesian representations, the individual phthalocyanine ring-localized MOs (D4h symmetry) are decomposed onto the DM point group using the two different ring orientations. The results are listed in Table 2. On the basis of this decomposition, a qualitative n-MO diagram for the bis(phtha1ocyanine)s is shown in the center of Figure 3. The interaction of the al, orbitals of the individual phthalocyanine rings results in a half-filled antibonding a2 HOMO and a filled bl bonding MO. The order of these MOs may be established simply by inspection of the nodal pattern of the ai,, MOs in Figure 3; a superposition of the MO pattem on the lower right and the pattem on the lower left results in overlap of 2p, lobes of predominantly the same sign. Thus, the sum of the two al, MOs forms the bonding MO, and the difference forms the antibonding MO (right column, Table 2). The relative ordering of the el * and e3* bis(phtha1ocyanine) LUMOs is similarly determined by inspection of the nodal pattem associated with the corresponding eg*x MOs illustrated in Figure 3 (along with their e,*y counterparts). The visualization is a bit more difficult in this case since the el* and e3* MOs are more complicated combinations of the two Pc ring MOs; see Table 2.) The e3*/el* order in Figure 3 is indeed required to account for the MCD pattem observed in the Q band
VanCott et al.
4822 J. Phys. Chem., Vol. 99, No. 13, 1995
TABLE 1: Behavior of Standard Basis Functions under Group Generators
region; see section V.l. Our MO diagram is also in accord with Orti et who calculate the splitting of the molecular orbitals due to the interaction between rings when passing from mono- to dimeric phthalocyanine. With respect to the calculations of Orti et it is important to note that these are done in the orthorhombic point group D2h (no degenerate irreps), while ours are done in group D4d. (It is a relatively simple matter to translate between such basis sets.) Our resulting ground state is 2A2,and the fully allowed electric dipole transitions from this state are 2A2 2 E ~(x,y-polarized) and 2A2 2B~ (z-polarized). The former can give rise to MCD A and B terms, while only B terms, are possible for the latter. While the metal ion controls the distance between the rings and therefore the interaction of their n orbitals, the electronic structure of the metal does not appear to be of primary importance in determining the electronic structure of the lanthanide bis(phtha10cyanine)s.~~ Thus, a minimal role might be expected in LuPc2, where the metal ion has all subshells filled, Lu3+(5s25p64fI4). We therefore omit metal orbitals in our discussion. Our approach also parallels that of Ishikawa et al.26,30in that molecular states are described using localized orbitals on each ring. However, these workers couple a valence-bond-like basis on each ring to form molecular states, while we form symmetryadapted MOs (Dw) from the MOs (D4h) of the two rings. 2. MCD Parameters: Fully Allowed Transitions. For fully allowed electronic transitions between Bom-Oppenheimer states, in the rigid shift approximation, the MCD (AA)and absorbance (A) are given by47
-..
-
8 = hv is the photon energy (in cm-I), B is the magnetic induction in Tesla, cl is the product of the concentration and
path length of the sample in mmol/cm2, a n d f i 4 is a normalized band-shape function. A detailed discussion of the pertinent forms and applications of the above equations to the analysis of the MCD and absorption data associated with matrix-isolated phthalocyanines has been presented previously4oand is not repeated here. 3. Theoretical Angular Momentum Matrix Elements. The theoretical determination of MCD parameters involves many-electron angular momentum matrix elements which can easily be reduced to one-electron form using standard techn i q u e ~ .To ~ ~evaluate the one-electron matrix elements, we reexpress them in terms of D4h MOs localized on the phthalocyanine rings using Table 2. The evaluation of these resulting one-electron D4h matrix elements has been discussed previously.40,45 As an example, the a2 el* excitation associated with the Q band of the bis(phtha1ocyanine)s corresponds to the fully allowed transition, 2A2(b~2a2) 2El(b12el*). Using standard method^,^' one obtains
-
-
where we are using IShMO) notation within the DMpoint group, and the bars on the left-hand side denote an average over all orientations. L, is the z component of the total orbital angular momentum operator, and x and y belong to the el irrep (Table 1). The superscripts on x and y denote spin state. Since L, = Elz is a sum of one-electron operators, eq 3 reduces to
where in the last equality we have expressed the matrix elements
Lutetium Bis(phtha1ocyanine)
J. Phys. Chem., Vol. 99, No. 13, 1995 4823
in terms of D4h Pc MOs (Table 2) and where the superscripts (1 and 2) designate ring 1 or 2, respectively. We neglect cross terms between MOs on different rings. Noting that 1, = G(') = 1i2),the above equation reduces to
2
AA 1o-2 This is exactly the same result obtained for an isolated Pc2ring, for example in ZnPc (eq 18, ref 40). When this calculation is carried out for the two transitions corresponding to the bl e3* excitation using the wave functions of Table 4,eq 5 is again obtained in each case. The evaluation of this D4h matrix element using the Huckel w technique has been discussed previously40 with the result (Table 3 of ref 40)
1
0
-
-2
.8
2 1 = 1.35
.6
Go
A
4. Vibronically Allowed Transitions. Electronically forbidden transitions can gain intensity by coupling with activating vibrations of the appropriate symmetry. In the case of the bis(phthalocyanine)s, the vibrationally induced transitions of interest are predominately 2A2 2E3 @ y . The activating vibration, y , can be of symmetry bl, b:,, or e2. Such a vibronically allowed transition can give rise to an MCD A term, which may be written
-
.4
.2
15000
-4 r= i((2E3 €3 y)2E,~lL,I(2E, €3 Y ) ~ E , ~ ) go
15200 15400
15800
16000
E /cm-'
(7)
Expanding these wave functions according to eq 9.3.12 of ref 47 and using the Wigner-Eckart theorem, we find that transitions involving bl orb:, activating vibrations will result in &:DOratios identical to that of the electronic origin, while those arising from e2 vibrations will be of the same magnitude but opposite sign.
15600
Figure 4. Absorption (A) and MCD (AA) spectra of LuPc2 isolated in an argon matrix at -7 K in the visible region corresponding to the Q(0,O) (no-phonon) band. The heavy line indicates the sum of three Gaussians used to fit the absorption and MCD data, and the dashed lines are the individual band fits. The experimental MCD band profile is significantly different from those reported in other MCD s t ~ d i e s . ~ ~ . ~ *
V. Discussion 1. Q Band Region, Q(0,O). The Q band region of LuPc2/
Ar encompasses four major transitions, which are labeled Q(O,O), Q(l,O), Q(2,0), and Q(3,O) (Figures 4 and 5 ) . Unlike the broad featureless A term reported by Ishikawa et a1.26and Burkov et aL4*in solution MCD studies in this region, our MCD spectra clearly show a number of features in the main, no-phonon (Q(0,O))band of LuPc:,/Ar (Figure 4). The observed characteristic asymmetric shapes in absorption and MCD are reproducible under a variety of deposition conditions and following annealing. Deconvolution of the absorption and MCD using a least-squares fitting routine and Gaussian band profiles indicates the presence of at least three positive A terms (Table 3). The complex absorption and MCD are the result of the overlap of these three features with perhaps a contribution from low frequency vibronic sidebands.40 Because of the presence of A terms, we assign the absorptions in this region to 2El transitions. These observations can be explained using the MO diagram given in the center of Figure 3. In our interpretation, three transitions are expected in this region corresponding to two excitations: a2 el* and bl e3*. The latter results in two transitions which arise from the spin parentage of the 2El(blaze3*) excited state. That is, with three open shells of spin I/z, we first couple bl @ a2 (= b2) to form either a spin singlet or triplet; we then couple each of these to e3* to form two
-
-
-
16000
17000
18000
19000
E /cm-' Figure 5. Absorption (A) and MCD (AA) spectra of LuPc2 isolated in an argon matrix at -7 K in the visible region corresponding to the Q band vibronics. Because of the extensive overlap and weak signal, a quantitative analysis is not possible. However, there are indications that these bands contain a number of transitions in addition to the vibronic progression of the Q(0,O) band; see text.
linearly independent 2 E ~states. The explicit wave functions for these states are listed in Table 4. There will be a small energy difference between the final excited states due to spinpairing differences, and A terms are associated with each of these fully allowed 2A2 2 E transitions. ~ In order to account for three overlapping, fully allowed transitions in this region,
-
VanCott et al.
4824 J. Phys. Chem., Vol. 99, No. 13, 1995
TABLE 3: Data Analysis and Assignments band label
a
c'?oa/cm-l
A"/cm-'
30 229 32 135 33 441 20 712 21 591 22 324 23 431 24 281 15 654 15 612 15 525 15 573b 11 099 10 608 11 019b 10 882 10 638 10 185b 7692
1328 904 806 682 416 660 921 723 17 12 118
Gaussian fit unless otherwise indicated,
213 293 264 860
u - ~ l / ~ ~ a
4.8 5.4 0 5.2 0.3 0.7 3.6 5.0 1.4 2.1 1.3 2.6b -3.7 -2.6 -4.0b 0.9 2.4 0.9b
transition assignment
-
excitation assignment
2A2 'El 2A2 2 E ~ 2A2+2B~
-0.2 -0.02 -2.9 -1.9 -1.0 0.1 0.3 -3.7 7.4 0.6 -1.7 -0.3' 1.2 1.7 1.3b -0.5 -0.3 -0.4'
'A2
+
-
2E~
and 'A2
'E3
2A2-* *E1 2A? 'El 2A2 'El 'A2 2E~ 2A2 2E3 @e2
bl
-
e3* and a2 e l *
--
-
-.-.
- e3*
@h
a2
- 'BI
a2 e3* bl -a2
2Az 'A2
2E3
-
is the resonance frequency, and A is the Gaussian band width. By moment analysis.
TABLE 4: Wave Functions of Q(0,O) Excited States IShM0)s.v
L%d~~a/10-2 cm
MO
the e3*/el* MO ordering given in Figure 3 must be used, as we stated previously. If the ordering of these two MOs were inverted, the 2El(bla2e3*) excited state would shift to a much higher energy, resulting in a single Q band at lower energy and a pair of Q-like transitions at higher energy. Our excitation assignments are in accord with the VEH calculations of Orti et ~ l . , ~who ' use an orthorhombic (D2h)point group description of LuPc2 based on earlier X-ray data.37 Within this point group, they predict four excitations in the Q band region, which are nearly degenerate, two by two, and centered at 1.71 and 2.02 eV (13 793 and 16 293 ~ m - l ) .These ~~ arise from the half-filled HOMO and the next lower energy MO, respectively. Translating these assignments back to an assumed molecular geometry of D4d, we obtain the two excitations, labeled Q in Figure 3. We assert that our three observed transitions correspond to these two excitations with spin-pairing effects splitting one excitation into two transitions. The localized orbital configuration interaction calculations reported by Ishikawa et aL30 indeed predict that two transitions to degenerate excited states occur in the Q band region. However, these workers claim that the Q band is due to a single fully allowed transition and that the weaker transition (2El in the Ishikawa nomenclature) should be assigned to the absorption at -11 000 cm-I (910 nm), the RV band in our nomenclature (see section V.7), despite their calculation. This assignment is based on their observations that for solutions the line width and spectral shape of the Q band of the dimer radical are nearly equal to that of the monomer, that the vibronic structure found for the monomer is substantially conserved in the dimer, and that a simple A term is observed in the solution MCD. That is, there is no evidence, in the solution spectra, of a distorted Q band profile which would result from overlapping transitions.
In the argon matrix, vibrational and solvent effects are greatly minimized as compared to solutions. Our matrix isolation data clearly indicate a significant, reproducible asymmetry in the Q(0,O) absorption that is not seen in the solution data, demonstrating the advantage of this spectroscopic technique. On the basis of the asymmetric absorption profile and the corresponding complex MCD that we observe, the Ishikawa argument fails, and we suggest that their 2El state is present in the Q band region as their calculation predicts. (We show in section V.7 that in any case the RV band does not have the characteristics of a fully allowed transition.) Quantitative examination of the MCD and absorption data demonstrates that significant configuration interaction occurs among the Q(0,O) states. Thus, calculating the dipole strengths associated with each of the Q(0,O) transitions using the wave functions in Table 4, we find
-.
1
L30(2A2 2El(b,2e,*))= -I(b,llme,lle3*)12 3
-
(8)
where (blllm,llle3*) is the reduced electric dipole matrix element corresponding to the bl e3* excitation. So we expect the ratios of the intensities of the transitions to be 1:2:3. (Strictly, the experimental absorbances are proportional to &!&, but the bands differ so little in energy that we can simply use C& values.) Band fitting (Figure 4) gives the ratio 1:14:18. While the band fittings are subject t o substantial uncertainties (see below), the pronounced disagreement with eqs 8- 10 demonstrates that the three excited states mix in accord with the overall conclusions of Ishikawa et aL30 The close proximity of the excited states responsible for the Q band raises the possibility of large B term contributions. These would take the form of pseudo A terms arising from the interaction of each pair of overlapping 2A2 *E1 transition^.^^ We address this and related questions next. An altemate and much superior method of analyzing the Q region is to employ the method of moments. Such an analysis has the great virtue of depending only on numerical integration of the experimental data. It is entirely independent of band fittings which necessarily involve significant subjective con-
-
Lutetium Bis(phtha1ocyanine)
J. Phys. Chem., Vol. 99, No. 13, 1995 4825
siderations. We demonstrate now, by moment analysis, the following two important points regarding the Q(0,O) band of LUPCZ:(1) there are no B term contributions arising from the mixing of nearby excited states; (2) the ratio of first MCD to zeroth absorption moment (MIIAo),which measures the excitedstate angular momentum, has the theoretical value calculated for an isolated Pc2- ring. Putting it another way, Ml/Ao for the Q(0,O) band of LuPcz should be the same, to first order, as for an isolated monophthalocyanine ring, as in, for example, ZnPc. For the case of excited-state near degenera~y:~assuming that out-of-state mixing (Le. mixing with states outside the Q band manifold) and ground-state spin-orbit coupling are negligible, the pertinent moment expressions for our overlapping 2Az 2E~ bands are (per Tesla):
-
M,
+
J(AA/&)(d' - a d d ' = 1 5 2 . 5 c Z [ ~ A I ( J i ) i
