Magnetooptical Spectroscopy of Zinc Octaethylporphyrin in an Argon

Cara L. McConnell and Bryce E. Williamson*. Department of ..... Gasyna et al. band. &io3 cm-'. ~ d 1 0 - 3. Md10-6B. M1/10-~ B. '4: MJB. RII pb. (14.5...
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J. Phys. Chem. 1995,99, 5865-5872

5865

Magnetooptical Spectroscopy of Zinc Octaethylporphyrin in an Argon Matrix Zbigniew Gasyna, David H. Metcalf2 and Paul N. Schatz" Chemistry Department, University of Virginia, Charlottesville, Virginia 22901

Cara L. McConnell and Bryce E. Williamson* Department of Chemistry, University of Canterbury, Christchurch I , New Zealand Received: January 19, 1995@

Magnetic circular dichroism, magnetic circularly polarized luminescence, emission, and absorption spectra have been measured over the range 12 500-50 000 cm-' for zinc octaethylporphyrin isolated in an argon matrix at -6 K. The spectra arise from transitions at a single matrix site. Values of gll for the Q, B, and N states are 7.0 f 0.7, 1.0 f 0.3, and 1.2 f 0.9, respectively. The degeneracy of the Q state (IE, in D4h symmetry) is lowered by Jahn-Teller and crystal-field effects, with the splitting of the Q(0,O) band being -30 cm-'. Analysis of the spectra in the Q-band region yields Jahn-Teller parameters YJT = 140 f 4 cm-I, AJT = 1.27 f 0.5, and EJT= 26 f 3 cm-' and suggests that the crystal-field distortion is of the same symmetry as the Jahn-Teller effect and of magnitude 30 f 4 cm-I. Qualitative indications are that Jahn-Teller effects in the B and P (lowest triplet) states are weaker than that in the Q state.

-

I. Introduction

lowest-energy spin-allowed n n* (Q) state. However, for ZnPc/Ar2 and ZnTBPlAr,4 the analysis is complicated by the In recent years, we have reported electronic spectra for presence of several inequivalent matrix sites, among which the complexes of the phthalocyanine ( P c ~ - ) ' -and ~ meso-tetrabenJT parameters can appear to vary substantially. Furthermore, zoporphyrin (TBP2-)5dianions isolated in argon matrices. Such intermolecular energy transfer results in emission that is shifted complexes have D4h symmetry, which significantly simplifies to the red of the absorption and in fact comes from minor sites, the analysis and interpretation of the spectra in comparison with whereas the absorption and MCD are strongly dominated by a lower-symmetry (free-base and/or substituted) species. In small number of major sites. addition, they are readily available from commercial suppliers, One of the main reasons for interest in metalloporphyrins is relatively stable, and easily sublimed, making sample preparation their biological importance, but in this respect, the MTBPs and simple. Our studies have been concentrated on the Zn2+ MPcs are not directly relevant. More important are the pyrrolecomplexes (ZnPc's2 and ZnTBp), since the metal ion has a filled substituted metalloporphyrins. In this regard, complexes of 3d shell so that the spectra are not complicated by the presence octaethylporphyrin (OEP) have been important model comof charge-transfer transitions and allowed transitions are not pounds and have been the subject of a number of spectroscopic complicated by spin effects. studies. Absorption spectra of ZnOEP have been measured in By using rare-gas matrices, we (and others6,') have obtained the gas phase,x polymer mat rice^,^ frozen glasses,lO~ll and spectra with very much sharper bands than can be typically various solvents.8-'8 Emission and excitation spectra have been obtained from solutions and g l a s s e ~ . ' , ~These - ~ matrices are reported in supersonic jets,I9 polymers,20liquid solvents,13s14,16 isotropic (allowing the use of polarized spectroscopic techand glasses.21-22Magnetooptical data have also been published niques) and are transparent well into the vacuum u l t r a ~ i o l e t . ~ , ~ , ~for ZnOEP: MCD in solutions?,'0 polymer^,^ and glassesI0 and Moreover, in some cases, the complex ions assume complete MCPL in poly(methy1 methacrylate).20 or partial preferential ~rientationl-~ (with their molecular planes In this paper, we report absorption, TL, MCD, and MCPL parallel with the surface of the matrix deposition window), data, over the range 200-800 nm, for ZnOEP isolated in solid which can provide a significant aid in making spectral assignAr (ZnOEPlAr) at -6 K. The spectra are dominated by ments.' transition arising at a single site and are consequently less Our major spectroscopic tools are low-temperature magnecomplicated than those of ZnPc/Ar1~2 and ZnTBP/Ar.4 More tooptical measurements using circularly polarized radiation. importantly, the fact that all data pertain to the same site presents Magnetic circular dichroism (MCD) spectroscopy, which can a situation that is much more conducive to detailed and be used to probe ground- and excited-state symmetry and consistent analysis of JT and CF effects. angular momenta, has shown that excited-state electronic degeneracies are lifted in the matrix as a result of Jahn-Teller 11. Experimental Section (JT) and crystal-field (CF) effects.1,2,4 Magnetic circularly The procedure for matrix deposition has been described polarized luminescence (MCPL) provides information concemp r e v i ~ u s l y .ZnOEP ~~ (Strem Chemicals) was sublimed from a ing emitting state^,^^^ including some that are not directly quartz Knudsen cell at about 270 "C and codeposited with a accessible in absorption, such as triplets. In conjunction with large excess of argon onto a cryogenically cooled LiF or c-cut the total luminescence (TL) and absorption, these measurements, sapphire window. in principle, allow us to quantify the JT and CF effects in the Preliminary zero-field absorption, emission, and excitation spectra were measured at -13 K using a closed-cycle helium ' Present address: Chemical Technology Division, Oak Ridge National refrigerator (CTI Cryogenics), CARY-2 145 spectrophotometer, Laboratory, Oak Ridge, TN 37831-6181. and SLM 8000C spectrofluorimeter. Abstract published in Advance ACS Abstracfs, April 1, 1995. @

0022-365419512099-5865$09.00/0

0 1995 American Chemical Society

Gasyna et al.

5866 J. Phys. Chem., Vol. 99, No. 16, 1995

Magnet COllS

Monochromator

x8

PMT

MCPL / TL spectrometer

I PEM control

I

unil ,

1-

Lock-InAmplifier Computer

~

--J+-

Figure 1. Schematic diagram of the apparatus used to simultaneously measure TL and MCPL spectra: PEM = photoelastic modulator; PMT = photomultiplier tube. See the description in section 11.

For magnetooptical measurements, the window was mounted in the bore of a superconducting solenoid (Oxford Instruments). After deposition, the tail of the cryostat, containing the magnet and deposition window, was rotated by 90" into the optical path. The sample temperature was -6 K, and measurements were made at field strengths between 0 and 3.00 T. MCD and double-beam absorption spectra were obtained simultaneously at a resolution of 0.1 nm using a spectrometer that has been described earlier.24.25 The depolarization of circularly polarized light due to the sample was determined by comparing the natural CD of a solution of A-tris( 1,2-ethanediamine)cobalt(III)inserted after the sample with that of the same solution in the absence of the sample, and was found to be negligible. Total luminescence (TL) and MCPL were measured with an instrument that is depicted schematically in Figure 1. Excitation radiation from an argon ion laser (Coherent Innova 90-6UV) is focused to a slit image at the sample. The emitted light is collected at 180" to the incident beam and passed through a 50-kHz fused-quartz photoelastic modulator (Hinds PEM-80), a polarizer, and a 520-nm or 540-nm glass cut-off filter to the entrance slit of a 3/4-m double monochromator (Spex 1400-2). The luminescence is detected in photon-counting mode by using a cooled photomultiplier tube (S-20 response). The TL and MCPL are determined, respectively, from sum and difference signzh generated by a gated digital lock-in amplifier (designed and constructed at the University of Virginia), which is referenced to the frequency of the photoelastic modulator. A microcomputer accumulates and stores the data and controls the monochromator. The temporal resolution of this system is 20 ,us, which is determined by the period of the PEM-80. Spectral resolution of 0.3 nm was used for the data reported here. 111. Results

The MCD and MCPL per tesla, TL, and absorption spectra of ZnOEP/Ar are shown on a wavelength scale in Figure 2. They have a similar general appearance to several previously reported ~ p e c t r a , ~ - I ~ , 'but ~ . 'the ~ - bands ~ ~ are much sharper, to the extent that all transitions to the red of -350 nm show wellresolved vibrational structure. For the phosphorescence (P) and the Q transitions, this structure is particularly extensive, with overtones observed as far as -4000 cm-I from the origin in the case of the Q-band absorption. Table 1 summarizes the observed vibrational shifts out to -1650 cm-l. Two broader, structureless transitions (amplified in Figure 2) are observed to the blue of 350 nm. The one centered at

(nm)

Figure 2. Overview of the spectra of ZnOEP/Ar: Bottom left, absorbance (Abs, A); bottom right, the total luminescence (TL, I); top left, the MCD per tesla (AAIB); and top right, the MCPL per tesla (AUB). Regions of the spectra have been amplified by the indicated factors.

322 nm corresponds to the N band of Edwards and coworkers.8.26 The other, at -252 nm, has energy (-39 700 cm-I) and relative intensity close to those predicted for the L transition of the porphin d i a n i ~ n ~and ~ -is~assigned ~ accordingly. The fact that it has not been observed previously in solution or gasphase spectra of ZnOEP is probably a consequence of large bandwidths and spectral overlap with the N and M bands. Figures 3 and 4 give expanded views of the Q-band spectra on a wavenumber scale. The TL and MCPL (Figure 4) were obtained by exciting with 363.8-nm radiation, into the vibronic region of the B transition. The origin is shifted by -16 cm-* to the red of Q(0,O) in absorption, and the vibrational structure closely matches that observed in the fluorescence line-narrowed spectrum of ZnOEP in octane/2% pyridine at 5.8 Weak absorption and MCD bands are observed in the vicinity, and generally slightly to the red, of the major ZnOEP/Ar bands (asterisks in Figure 3). Excitation spectra indicate that these arise from a small amount of other species, which we believe to be either a porphyrin-like impurity or ZnOEP molecules at matrix sites distinctly different from those responsible for the dominant spectral features. Their contributions to the spectra are too weak to affect our analysis. The lifetime of the P state of ZnOEP/Ar at -6 K was determined, using the TL/MCPL spectrometer, to be 69.5 f 0.5 ms. That of the Q state (-2.5 ns in CH2C12 solution at room temperature30) is far too short to be measured with our apparatus. As reported previously for ZnOEP in benzene,13 and ZnOEFVAr shows no unlike some other metalloporphyrin~,4~~-I~ emission from the B state.

IV. Discussion Absorbance (A) and TL (Z) are respectively defined by the average absorbance and the total emission of left- and rightcircularly polarized light:

A = (AL

+ A,)/2

(1)

The MCPL (AZ) and MCD (AA) are the corresponding difference spectra obtained in the presence of an external magnetic

J. Phys. Chem., Vol. 99, No. 16, 1995 5867

Zinc Octaethylporphyrin in an Argon Matrix

TABLE 1: Vibrational Structure in the P-, Q-, and B-Band Regions of ZnOEPIAP TLMCPL Q band

Q band

B band

17 814 143 263 35 1 494 670 (664) 745 (764) 900

25 508

-770 (753)

17 798 -145 (144) -273 (-270) -357 (-350) -502 -670 (669) -765 (753)

-1 010 (1015)

-950 (938) -1 025 (1015)

modeb

P band

(O,0ld v35

14 442 -253 (-270) -342 (-350)

v9

V8

v35

+ V8

v7 VI5 v35

absorption/MCD

+ VI5

v32 v3 I v14 v5

-1 lSO(1138)

-1 140(1138) -1 210 (1211)

-1 320 (1320) -1 390 (1375)

-1 320 (1320)

VI3

CH2 twist CH2 wag v4 v35 CH2 twist v35 CH2 wag

symmetry' b2g a1g a', b,(b,@al,) ak bl, a~,(b2,Bb1,) b2, b2, bl, ak b2, a1

270 650

1010 1 130 (1131)

1150

,

1255 (1258)

+ +

1335

g

1 360 (1359) 1410

alg b2,(b2,@alg) b,(b2,@a1,) ah alp bl,

-1 460 1 500 (1520) 1 565 (1564) 1625

V3

v2

-1 570 (1584) -1 620 (1613)

VI0

-1 570 (1584) -1 625 (1613)

Energies of band origins and vibrational shifts in cm-I. Estimated uncertainties are f 8 cm-'. Values in parentheses are from Raman data for the appropriate electronic state of ZnOEP34or for the ground state of NiOPE,35with the latter in italics. Vibrational mode designations are those used in ref 35. D4,, symmetry assignments follow ref 35. The irreducible representations bl, and bzg are defined to transform as x2-y2 and xy, respectively, where x and y are along the Zn-N bonds. Energies of band origins. a

230

Q-band emission

xl0

Q-band absorption and MCD

0.10

I

I

I!

o'ffiww

A

0.04 1 lm00

I

1

1

1BMo

lso00

2woo

,

21m

-t

100

,1

Zoo0

i-

I

22wo

8 (cm" )

8(")

Figure 3. MCD per tesla (top) and absorption (bottom) of ZnOEP/Ar in the Q-band region. Negative pseudo-- 1 terms associated with V I 4 and V I S and the positive MCD feature associated with v35 are indicated by arrows. The asterisks indicate weak bands arising from either a porphyrin-like impurity or ZnOEP molecules at a minor matrix site.

field applied along the direction of propagation of the radiation:

(3)

AA=A,-A,

AI = I, - IR

(4)

MCD and MCPL are generally described as comprising Faraday terms of three types.31 JL terms are derivative-shaped features defined to have the same sign as the higher-energy lobe and are a consequence of degeneracy of the initial and/or final states. @ terms are single-signed (either positive or negative), are temperature independent, and arise from magnetic-fieldinduced mixing of states. G texms are also single-signed, but are due to degeneracy of the initial state, and show temperature dependence that follows the distribution of population among the Zeeman-split levels. The symmetry of the porphyrin skeletons of the MOEPs is very close to D4h.32,33 In this point group, the W/visible transitions of ZnOEP (which arise from n n* excitations of

-

Figure 4. MCPL per tesla (top) and TL (bottom) of ZnOEP/Ar in the Q-band region, obtained by exciting at 363.8 nm. The bands due to the JT mode, v35, are indicated by arrows.

-

the ligand and are polarized in the x , y plane of the porphyrin 'E, and the corresponding MCD should be ring) are 'AI, dominated by A terms. A cursory examination of Figures 2 and 3 suggests that this is at least approximately correct, with the Q-, B-, and N-band origins showing positive Aterms. (Poor signal-to-noise prevents characterization of the L-band MCD.) However, in the case of the Q transition, closer inspection (vide infra) reveals that the excited-state degeneracy is lowered by a combination of CF and JT effects. Unlike ZnPclAr's2 and ZnTBPIAr: the bandwidths are too great for the Q-band splittings to be directly observed in the absorption and MCD of ZnOEP/Ar. However, there are a number of more subtle indications of both CF and JT activity in the excited state. Firstly, to fit Q(0,O) as a single transition in D4h symmetry, a greater bandwidth is required for the absorption than for the MCD. This is characteristic of the presence of pseudo-A terms in the MCD, features that arise from overlapping pairs of oppositely signed G7 terms and which closely resemble the parent A'terms. By numerical simulation, we estimate the widths of the individual bands to be -40 cm-' and the unresolved CF splitting to be -30 cm-I. Secondly,

Gasyna et al.

5868 J. Phys. Chem., Vol. 99, No. 16, 1995 TABLE 2: Moment Parameters and band pb

Q E' N' Lc,d

&io3 cm-' (14.5 i0.2) 18.50 f 0.05 26.0 f 0.1 30.6 f 0.4 39.1 10.4

Values for Transitions of ZnOEP/Af

~d10-3 1.9 f 0.1 17f2 1216 8 f 4

Md10-6B

454 3f4 254

M1/10-~ B 11 1 6 8 f 2 754

'4: 10.0 f 0.3 19 f 2

MJB

RII

-0.11 f 0.02 -0.63 f 0.10

7.0 f 0.7 1.0 f 0.3 1.2 f 0.9

Too weak to be observed in absorption or MCD; 2 is the emission barycenter. Emission is not a B is the magnetic field strength in tesla. observed from these states. The signal-to-noise ratio of the MCD is too poor to allow determination of MCD moments.

Figure 3 shows that some vibrational overtones in the Q-band region exhibit pseudo-d terms of the opposite sign to that of the origin. (Two such bands are indicated by arrows in Figure 3.) These are indicative of excited-state vibronic effects involving modes of big ( i = 1 or 2) ~arentage,',~ which are JT active. Thirdly, the MCD pseudo-A term associated with Q(0,O) is asymmetric, the negative lobe being significantly more intense. As we discuss later, this requires the simultaneous presence of both CF and JT effects. Quantitative spectral analysis can often be achieved by employing the method of moments. For the absorption and MCD, the relevant parameters are

positive MCPL vibrational overtones in the Q-band region can then be associated with aIg and b,, modes, respectively. For the P band, the signal-to-noise ratio is too low to determine the signs of the MCPL overtones, but the frequency shifts obtained from the TL can be closely correlated with those in the fluorescence (Table 1). We note that Shatwell and McCaffery reported the MCPL of ZnOEP in poly(methy1 methacrylate) at -20 K to be of the opposite sign to ours.2o We are confident of the correctness of our result as can be shown by comparison with the MCD. Consider a radiative cycle from the nondegenerate ground state to a Zeeman sublevel of the 'E, excited state and back; for angular momentum to be conserved, the absorbed and emitted photons must have the same circular polarization, and hence M,, = J(AA/&)(PdP (5) the signs of the MCD (AL - AR) and MCPL (ZL - ZR) must be the same. The negative origin of the Q transition in the MCPL A,, = J(A/L)(P - @ d P (6) should therefore coincide with the negative lobe of the Q(0,O) pseudo-A term in the MCD, exactly as we observe (Figure 2). 8 is the wavenumber of the radiation, and 2is the transition The same general argument applies even when excited-state barycenter defined by degeneracy is retained, but in that case, the MCPL will show a negative G term. 2= d&A, (7) The magnetooptical data only allow vibrational symmetries to be classified as aIg or big. Fortunately, more precise The moments for an electronic transition are obtained by carrying the integrals over the full envelope of the corresponding assignments have been made for several MOEPs (including band, including all vibrational structure but excluding contribuexcited states of ZnOEP34)on the basis of Raman data and a tions from other transitions. Some of the moments for ZnOEP/ ground-state normal-coordinate analysis.35 Some of these are listed in Table 1, along with the corresponding Raman frequenAr are listed in Table 2. MOand A0 give integrated intensities, cies. while M I measures the (pseudo-)A term contributions to the MCD. Where transitions overlap, we have had to deconvolute It is notable that essentially all vibrational structure in the the bands. This is a somewhat subjective procedure, and hence, spectra of ZnOEP/Ar can be correlated with modes that transform (in D4h) as part of the symmetric square of the excitedthe large error limits for the moments of the B, N, and L bands. The uncertainty for MCD moments of the Q band is due to state electronic symmetry ([Eu2]= alg @ bl, @ bzg). (The only extreme sensitivity to small changes in the baseline, which exception is the 900-cm-' overtone in the Q-band region, which appears to be a consequence of the long tail of weak vibrational shows a positive pseudo-A term but for which there is no features. corresponding algfundamental; we assign it to an aZg combinaFor ZnOEP in noncoordinating solvents, previous w o r k e r ~ ~ . ' ~ tion of v35 and vl5.) This is the expected result for 'Alg 'E, have obtained Ml/Ao transitions that derive their intensity entirely from a fully allowed 1.5 for the band that they designate mechanism and can show both Franck-Condon (alp) and JT Q(0,O). Numerical integration over the corresponding region of the data in Figure 3 (17 700- 18 200 cm-I, which includes (big) overtones. If we assume this to be the case, then for the ~ 3 overtone) complete preferential molecular orientation, the excited-state 5 gives Ml/Ao = 3.1 & 0.2. This discrepancy is almost certainly a consequence of orientational effects. orbital angular momentum (in units of h) is given by Molecules in solution are (normally) randomly oriented, but in Ar matrices, MPcs'-~ and Z n T B p are found to adopt a (8) g,,= 2i('EuxlL,11Ey) = M,/A&u,B preferential orientation with the plane of the porphyrin ring parallel to the surface of the deposition window. In the where B is the magnetic field strength (T), ,UB is the Bohr instrumental configuration used in our experiments, the molecmagneton (cm-' T-I), i = (-l>ln, and the moments are obtained ular z axis (normal to the porphyrin plane) is then parallel with by carrying the integrals in eqs 5-7 over the full envelope the optical path and the applied magnetic field, and the ratio (including all vibrational overtones) of the transition. MdAo is increased by a factor of 2 over that for the randomly Equation 8 is independent of first-order JT and CF effects oriented case.' since the relevant moments are invariant to unitary transformations on the excited-state bask3' However, it can be invalidated The fluorescence (Q band; Figures 2 and 4) and phosphorescence (P band; Figure 2) origins show single-signed,negative by effects that mix electronic states and cause substantial @ terms in the MCPL. The absence of positive bands to the changes to transition intensities. The ultraviolet transitions of blue of these indicates that emission occurs only from the lowest ZnOEP/Ar are strongly allowed, so such effects are not likely level of the excited state, so intrastate vibrational relaxation must to contribute significantly to their intensities. Hence, the gll be fast compared with the radiative lifetime. The negative and values for the B and N states can be estimated by using eq 8

a''

SA

-

Zinc Octaethylporphyrin in an Argon Matrix

J. Phys. Chem., Vol. 99, No. 16, 1995 5869

and the moments listed in Table 2, which give gll(B) = 1.0 f where Im indicates the imaginary part of everything to the right. EK is the energy of state K, and L, is the z component of the 0.3 and gll(N) = 1.2 f 0.9. In the case of the Q band, it has been suggested p r e v i ~ u s l y ~ ~ . ~ ' orbital angular-momentum operator. (Note here that eqs 11, that a significant fraction of the intensity arises from Herzberg12, and 14 are consistent with our previous assertion that &(A Teller (HT) coupling with the B state. Several facts support * J) and AA(A J) should be of the same sign.) this view: (1) The ratio of the Q- to B-band intensities in We restrict our considerations to the origin and the v35 band, absorption (Ao(Q)/Ao(B) x 0.11) is small in comparison with since (with support from the following analysis) we believe them (for example) ZnPc/Ar (0.66),' so vibronic intensity stealing is to be free from HT contributions. ~ 3 is 5 almost certainly the apt to be relatively more important. (2) The symmetries of the principal JT-active mode of the Q state. It is responsible for vibrational modes that can contribute to such coupling are (with the most intense big overtone in both absorption and fluoresthe addition of a2,) the same as those observed to form overtones cence, and its wavenumber (-144 cm-I) is close to those for in the Q-band region. (3) The Q-band absorption and fluoresthe dominant JT modes in the Q states of ZnPc (154 cm-l)Iv2 cence show significantly different vibrational structure, both in and other metalloporphyrins (130- 180 cm-1).4,38-41 terms of the distribution of intensity and the relative importance The excited-state basis functions are chosen to be harmonic of a', modes (which dominate overtones in absorption) and b,, oscillator functions on two unperturbed potential-energy surmodes (which are more important in fluorescence). Such faces, lE,,x and ' E o , which are parallel with the 'AI, surface. differences are to be expected at low temperatures where the The effective Hamiltonian matrix is given by eqs 10 and 11 of population of the initial state resides predominantly in the zeroref 4. It allows for a single JT-active vibration with (dimenpoint level, since any significant HT coupling must then involve sionless) coordinate QJT and frequency YJT. The strength of the final state. the JT effect is measured by An. The CF-distortion components If interstate vibronic mixing is important, then the results of are V and V ' , with the former transforming in the same way as moment analysis are not easily interpreted. For that reason (and QJT. Assuming the JT mode to be v35, then (according to the also due to the large uncertainties in the experimental moments symmetry convention of Table 1) V and QJTtransform as b2,, of Table 2), we have chosen to analyze the Q transition by while V' transforms as bl,. (The treatment applies identically explicit consideration of individual vibronic bands that we if the labels bl, and b2, are interchanged.) The general form believe to be fully allowed. The method is similar to the one for the vibronic eigenstates (at B = 0) is adopted by Kielman-van Luijt et al.38but is extended to include the emission spectra and to allow for CF distortions of two symmetries. The absorption, TL, MCD, and MCPL intensities of an allowed transition between nondegenerate vibronic levels A where v is the quantum number for the JT mode, and the factors and J are3' (x,vIN) ('E,,x,vlN) are eigenvector coefficients. The excitedstate vibronic levels are labelled N = 0, 1, 2, ... in order of ascending energy. The expression for the dipole strength of a transition IAlg,v N can be obtained by substituting eq 15 into eq 13 and then invoking the Franck-Condon principle for transitions between the ground and unperturbed excited states:

-

-

&(A

-

J) = -&'K;,M@@~(A

-

+

J)fl&)

(12)

Note that we use the convention that the higher-energy state is written on the right, so A J denotes absorption and A J denotes emission. KA and KJ' are proportional to the fractional populations of the states indicated by the subscripts and include the dependence on concentration and path length of the sample and refractive index of the medium. A&) is a normalized bandshape function, and Q0 is the dipole strength: +

Superscripts z indicate that the radiation propagates along the molecular z axis, and m, and m, are molecule-fixed Cartesian is the components of the electric-dipole moment operator. go corresponding Faraday parameter describing field-induced mixing of states. If we assume that state A is well separated from all electronically excited states, then

where ('Al,lm,('E,x) is the electronic transition dipole for the unperturbed system. For Bo,we make the further approximation that the intermediate states K of eq 14 belong to the same excited-state manifold as N. This assumption should be valid; firstly, the energy differences that enter as the denominator in eq 14 will be far smaller between vibrational levels of v35 than between electronic states, and secondly, mixing with other electronic states would give a finite Mo, whereas the observed value (Table 2) is zero within experimental uncertainty. The result of substituting eq 15 into eq 14 is then

%('Alg,v

-

N) =

where

With an appropriate band-shape function, eqs 16-18 and 9- 12 will, in principle, allow the calculation of the spectra for any given set of parameters. In practice, however, the sample

5870 J. Phys. Chem., Vol. 99, No. 16, 1995

Gasyna et al.

- 0.005

0

A I B

O

AA S

-100

-4.005 -0.15

-200

m -0.10

- 0.05

1WO

I

A 0

0 17600

17800

8(”)

17800

18ooo

18200

Figure 5. Observed (full lines) and calculated (dashed lines) spectra of ZnOEP/Ar in the vicinity of the Q-band origin. For clarity, the MCPL and TL have been displaced to the left of the MCD and

absorption. The calculated spectra were obtained by the method described in section IV, using the parameters hvm = 140 cm-I, AJT = 1.27, V’ = 0 cm-I, V = 15 cm-I, and gll = 7.0. concentration and path length are unknown, and the distribution of the intensity into overtones of totally symmetric and other JT modes has been neglected. These problems can be obviated by using the eigenvectors and eigenvalues to determine moment ratios, which are independent of common factors, including the vibrational overlap factors of all modes other than v35. The calculations employ a basis set of eight vibrational levels on each excited-state surface (16 vibronic states in all). Initially, the JT and CF parameters were varied systematically over a broad range, and calculated moment ratios for the region of interest were compared with experiment in order to identify a narrower range of potentially acceptable parameters. Next, the parameters were varied manually over the smaller range, and the theoretical spectra were determined using a normalized Gaussian band shape of half-width 40 cm-I. The unknowns K A and KJ’ (eqs 9-12) were then combined with the square of the electronic transition moment and used to scale the calculated absorbance and TL intensities to experiment. (Due to the fast intrastate vibrational relaxation, all populations were set to zero, except those associated with v = 0 of the ground state or N = 0 of the excited state.) Finally, gll was chosen to simultaneously give the best match to the experimental MCD and MCPL intensities. The “best fit” in the vicinity of Q(0,O) and the v35 overtone (as judged by eye) is compared with the experimental data in Figure 5. The fact that we are able to simultaneously obtain good fits to all four spectra gives us confidence in both the model and the values of the parameters so obtained. The calculated spectra are strongly dependent on the JT parameters, which are therefore determined with relatively high precision to be hVJT = 140 f 4 cm-I and LJT = 1.27 f 0.05. The corresponding JT stabilization energy is EJT= ( & ~ ) ~ h v m= /8 26 f 3 cm-I, which amounts to -40% of the zero-point vibrational energy. These values are in excellent accord with previously determined Q-state parameters of unsubstituted zinc porphin (ZnP): hvJT FZ 175 Cm-’, ilJT e 1.2, and EJT 30 cm-1.38-41 They are also close to those obtained for the major sites of the related species ZnTBP/Ar (hvJT= 129 cm-I, LJT = 1.12, EJT= 20 Cm-’)4 and ZllPC/h (hvm = 154 Cm-’, ~ J = T 1.57, EJT= 47 cm-1).2 The sensitivity of the spectra to the choice of CF parameters is far weaker, but the best fit is clearly obtained with V’ = 0 cm-I; V = 15 f 2 cm-’ is required to reproduce the 30 f 4 cm-l CF splitting of the origin band.

- 3 - 2 - 1

0

1

2

3

’ aJT

Figure 6. Schematic representation of the simultaneousaction of JahnTeller (JT) and crystal-field (CF) effects of the same symmetry on the

‘E,(Q) state of ZnOEP. The diagram is drawn to the scale of the parameters derived in section IV and used to generate the calculated spectra of Figure 5. The JT effect separates the surfaces by Am along QJTand stabilizes them by Em. The CF effect shifts the surfaces by i V along the energy axis. N labels the vibronic levels in order of ascending energy. The vibrational wave functions for the first six levels are illustrated. Finally, scaling of the calculated MCD and MCPL to the observed spectra yields gll = 7.0 with an estimated error of f lo%, which is consistent with the expectation of a large positive value for the orbital angular momentum.27 In comparison, the experimental and theoretical values for ZnP are, respectively, 4338.41 and 8.7,27and we have previously determined gll = 7.7 f 0.4 for the Q state of ZnTBP.4 With V’ = 0 cm-I, the CF and JT effects are of the same symmetry. This particularly simple case is represented in Figure 6, which is drawn to the scale of the derived parameters. The potential-energy surfaces can be obtained by shifting the basis surfaces by A&/2 along QJTand by -Em f V along the energy axis. The vibronic wave functions can be expressed just as Born-Oppenheimer products, the vibrational parts of which are shown superimposed on Figure 6 for N = 0-5. It was noted near the beginning of this section that the asymmetry of the Q(0,O) pseudo-.A term in the MCD is a reflection of the simultaneous presence of JT and CF effects. This can be understood by reference to Figure 6. Initially, consider the case where vibronic coupling and CF effects are absent, so the excited-state degeneracy is exact and vibrational overlap factors (vlv’)for all nontotally symmetric modes (both between the two excited states and with the ground state) vanish except when Y = v’. At low temperatures, only the (0,O) and alg overtone bands will be observed, and these will be associated with true MCD .A terms. As Vis increased, each transition is split into two. However, while LJT = 0, the vibrational orthogonality is retained, so the terminating states within each pair ( e g , N = 0 and N = 1 for the origin band) can be magnetically mixed only with each other. According to eq 14, the B terms associated with these pairs will be of the opposite sign but the same magnitude, so the resultant pseudo-A term will remain symmetric. Now consider the consequences of

Zinc Octaethylporphyrin in an Argon Matrix

J. Phys. Chem., Vol. 99, No. 16, 1995 5871

separating the surfaces along Qm to give the situation illustrated in Figure 6. This removes the orthogonality and allows other magnetic interactions between vibronic states on adjacent surfaces to contribute to the Q terms. For transitions terminating in N = 0 or N = 1, the energy denominators of eq 17 determine the signs of these contributions. The interaction between N = 0 and N = 1 makes a dominant, symmetric contribution to the pseudo-A term and determines its positive sign. However, the weaker interactions with states of higher N all make negative contributions; these intensify the negative lobe and weaken the positive lobe, giving the pseudo-A term its asymmetry. At this point, it is worth considering the result obtained by applying the moment-analysismethod of earlier studies2s4to the Q transitions of ZnOEP/Ar. If HT coupling is assumed to be unimportant, then

Me, are, respectively, the zeroth TL and MCPL moments, which are defined analogously to those for the absorption and MCD:

Ai and

Me,=

f(AZl4dC:

(20)

Ai= f(N4dR

(21)

Experimental values of these moments, obtained by numerical integration over the fluorescence and phosphorescence, are given in Table 2. Fixing V' = 0 and gll = 7.0 f 0.7, eq 19 and the procedure described in ref 4 yield AJT = 2.4 f 0.3 for the Q state. The difference between this value and the one obtained above (1.27) almost certainly arises from HT contributions to the transition intensities. A similar explanation may also apply to a discrepancy between earlier analyses for the Q state of ZnTBP/Ar! In that case, the JT parameters for the major site were determined from Zeeman shifts of the origin, which are independent of HT coupling and gave Am = 1.12 f 0.08. In contrast, the moment-analysis method yielded AJT = 2.3 f 0.2 for a minor site. Although the data are insufficient to allow quantitative JT analyses for the P and B states, we can make some qualitative comments. Both transitions exhibit overtones in b,, modes. However, these are very weak in comparison with the v35 overtones of the Q-band region, which suggests that any JT effects must be substantially weaker than in the Q state. Lending support to this conclusion is the fact that a good fit to the MCD of the B-band origin can be obtained by using a single, symmetric -4 term. The N and L transitions of ZnOEP/Ar are too broad and unstructured to allow even qualitative judgements concerning JT effects. Finally, we comment briefly on the relationship between earlier semiempirical calculationsfor unsubstituted and the parameters in Table 2. The calculated energies and intensities (B > N L >> Q) are in qualitative agreement with the experimental observations for ZnOEP/Ar, but the Q-B separation is too great, and the relative intensity of the Q transition is far too small. McHugh et al.27also calculated some excited-state angular momenta and obtained gll(Q) = 8.69, gll(B) = -0.06, and gll(N) = -0.60. While the value for the Q state is in reasonable agreement with Table 2 (-24% larger), those for the other states are both significantly smaller and of the opposite sign. This may be partially accounted for by

assuming that configuration interaction between the Q and B states of ZnOEP is weaker than the calculations for porphin would suggest.

V. Conclusion Magnetooptical spectra are reported for the lowest-energy triplet (P) and the four lowest-energy allowed singlet (Q, B, N, and L) transitions of ZnOEP. Matrix isolation affords wellresolved spectra that arise from a single matrix site. The P, Q, and B transitions show vibrational structure that correlates excellently with earlier vibrational data.34,35The L transition, which has not been previously reported for ZnOEP, appears at an energy close to those predicted for the porphin d i a n i ~ n ~ ~ - * ~ and observed for other MOEPsS8 The origins of the three lowest-energy singlets exhibit positive MCD (pseudo-),[terms, indicating that the excited-state orbital angular momenta are positive. The gll value for the Q state is consistent with a theoretical calculation for porphin,*' but those for the B and N states are of the opposite sign. Analysis of the Q state, performed by simultaneously modeling individual band intensities in all spectra, demonstrates a significant JT effect with a magnitude similar to those in related s p e ~ i e s . ~ , ~ Qualitative . ~ ~ - ~ ' considerations suggest that JT effects in the P and B states are substantially weaker. Our results contradict an earlier assertion, made on the basis of excitedstate resonance-Raman data, that JT distortions are weak or absent in the Q state of ZnOEP but are relatively strong in the P state.34 The magnitude of the Q-state JT effect determined by moment analysis is twice that obtained by consideration of individual band intensities. The difference almost certainly arises from the presence of interstate vibronic (HT) coupling, which invalidates the approximations inherent in eq 19. It seems likely that the apparent site dependence of the JT parameters of ZnTBP/AI' is amenable to a similar interpretation.

Acknowledgment. The work was supported by the National Science Foundation under Grants CHE8902456 and CHE9207886 to P.N.S. and Grant CHE9213473 to Prof. F. S. Richardson (the latter for support of D.H.M. and the development and operation of the CPL/emission spectrometer). References and Notes (1) VanCott, T. C.; Rose, J. L.; Williamson, B. E.; Boyle, M. E.; Misener, G. C.; Schrimpf, A. E.; Schatz, P. N. J . Phys. Chem. 1989, 93, 2999-3011. (2) Metcalf, D. H.; VanCott, T. C.; Snyder, S. W.; Schatz, P. N.; Williamson, B. E. J . Phys. Chem. 1990, 94, 2828-2832. (3) Williamson, B. E.; VanCott, T. C.; Boyle, M. E.; Misener, G. C.; Stillman, M. J.; Schatz, P. N. J . Am. Chem. SOC. 1992, 114, 2412-2419. (4) VanCott, T. C.; Koralewski, M.: Metcalf, D. H.: Schatz, P. N.; Williamson, B. E. J . Phys. Chem. 1993, 97, 7417-7426. ( 5 ) VanCott, T. C.; Gasyna, Z.: Schatz, P. N.; Boyle, M. E. J. Phys. Chem., in press. (6) Bajema, L.; Gouteman, M.; Meyer, B. J . Mol. Spectrosc. 1968, 27, 225-235. (7) Bajema, L.; Gouteman, M.; Rose, C. B. J. Mol. Spectrosc. 1971, 39, 421-431. (8) Edwards, L.; Dolphin, D. H.; Gouteman, M. J . Mol. Spectrosc. 1970, 35, 90-109. (9) Gale, R.; McCaffery, A. J.; Rowe, M. D. J. Chem. Soc., Dalton Trans. 1972, 596-604. (10) Barth, G.; Linder, R. E.; Bunnenberg, E.; Djerassi, C.; Seamans, L.; Moscowitz, A. J. Chem. Soc., Perkin Trans, 2 1974, 1706-1711. (11) Aranowitz, Y .J.; Gouteman, M. J . Mol. Spectrosc. 1976, 64, 267289. (12) Wang, R. M.-Y.; Hoffman, B. M. J . Am. Chem. SOC. 1984, 106, 4235-4240. (13) Ohno, 0.;Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1985,82, 17791787. (14) Kobayashi, H.; Kaizu, Y. Coord. Chem. Rev. 1985, 64, 53-64.

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