J. Phys. Chem. 1981, 85,3322-3326
3322
density of molecules within u is very similar to the density outside of u. In these terms aj must be relatively constant, and we have (B5) Again, although this relation is only true, or approximately true, for n satisfying the above condition, such values of n dominate the average as R is increased. From the point of view of eq B4,eq B5 means that the fluctuation of a. about ( a ) must be small. Furthermore, on physicai grounds, aj - ( a )should vary approximately monotonically with j - ( I ) , the fluctuation of j about ( j ) ,which, for the reasons indicated, should also be small. Thus, according to eq B4,A should be a second-order quantity in this small coordinated fluctuation and should vanish more rapidly than the fluctuation itself. ajj“
Thus, A is the fluctuation of the cross correlation of j and l/ai If these quantities are relatively uncorrelated, then A will tend to zero and A, should be a good approximation to A. Unfortunately j and l / a j are correlated. Nevertheless, it is worthwhile to examine the degree of correlation between these two quantities. Now, especially as R increases, the major contribution to the average denoted by ( ) will come from terms with j in the neighborhood of p u , i.e., from terms in which the
(a)
j=pu
New Singlets in the Phthalocyanines Tzer-Hsiang Huang,* Klaus E. Rleckhoff, and
Eva M. Voigt
Departments of Physics and Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 (Received June 9, 198 1: In Final Form: August 3, 1981)
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A heretofore unreported broad electronic excitation band was observed -1600 cm-’ above the well-known‘(*a*) arising from the alU(a) e,(n*) transition in a number of phthalocyanines, i.e., in H2Pc,MgPc, ZnPc, RuPc, e,(a*) transition and the ‘(aa*)vibronic PdPc, and PtPc, in Shpol’skiimatrices. It is attributed to the e,(NPu) transition involving egvibrations which activate the vibronic interaction between the ‘(na*) and the l(aa*) state. Intersystem crossing from ‘(nn*) to the triplet manifold at 4.2 K was found to be insignificant relative to that from ‘(a**).
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Introduction The phthalocyanines (Pc’s) are molecules of great commercial importance and of theoretical value as well, as they resemble structurally the biologically important porphyrins. Consequently, their electronic transitions have received considerable attention in the literature.’ The predominant electronic oscillator strengths of the transitions observed so far arise from the alu(n) e (T*) (visible) and aZu(a) e ( T * ) (UV) transitions? d e extended Huckel MO calculations of Schaffer, Gouterman and Davidson3predict in the vicinity of the al,(a) MO a nonbonding MO eu(NPa) which is formed mostly from the nonbonding in-plane (a) P-type AO’s of the bridging nitrogens (N) of the phthalocyanine ring. The broadness of near-UV Soret bands of the Pc’s in the gas phase are believed to be caused by the underlying n a* transition.2 It was suggested in our previous work on PtPc in a Shpol’skii matrix: based on the observation of a rather broad structure in the well-resolved vibronic progression of the aIu(r) e,(a*) transition, that the relatively broad visible transitions of the Pc’s in vapor and solution involve also the nonbonding electrons of the nitrogens. In this paper we report on the excitation spectra of H2Pc,MgPc, ZnPc, RuPc, and PdPc, along with that of PtPc (all in Shpol’skii matrices) to show that the broad structure in PtPc is actually common in many other Pc’s. Having
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*Address correspondence to this author at the following address: Xerox Research Center of Canada, Mississauga, Ontario, Canada L5L 1J9. 0022-3654/81/2085-3322$01.25/0
established this, we then go on to determine the cause of this electronic transition. Experimental Section The Pc samples obtained from commercial sources (mostly K & K) were used directly, except PtPc, which was subjected to repeated recrystallization. The RuPc used in this work was synthesized from RuCl3-3H30and diiminoisoindoline. The reactants were refluxed in dimethylformamide for 7 h, and the products extracted with excess ether. The electronic relaxation characteristics of the RuPc sample prepared in this manner resemble the diamagnetic PC’S.~ Since the open-chain normal alkanes (conventional Shpol’skii solvents) do not dissolve the Pc’s to any significant extent, spectral band sharpening was achieved by using solvent mixtures of a-chloronaphthalene (aC1N) or tetrahydrofuran (THF) and normal alkanes of proper chain lengths. On increasing the amount of the alkanes, the a a* visible bands recorded at 4.2K start to sharpen up
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(1)See, e.g.: (a) P. S. Vincett, E. M. Voigt, and K. E. Riechoff, J. Chem. Phys., 55,4131 (1971); (b) E.R. Menzel, K. E. Rieckhoff, and E. M. Voigt, ibid., 68, 5726 (1973); (c) T.-H. Huang, K. E. Rieckhoff, and E. M. Voight, Can. J. Phys., 64,633 (1976); (d) T.-H. Huang, K. E. Rickhoff, E. M. Voigt, and E. R. Menzel, Chem. Phys., 19, 26 (1977). (2) M. Gouterman in “The Porphyrins”, Vol. 111, D. Dolphin, Ed., Academic Press, New York, 1978, Chapter 1. (3) A. M. Schaffer, M. Gouterman, and E. R. Davidson, Theor. Chim. Acta, 30, 9 (1973). (4)T.-H. Huang, K. E. Rieckhoff, and E. M. Voigt, Chem. Phys., 36, 423 (1979). (5) Manuscript in preparation.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85,No. 22, 798 1 3323
New Singlets in the Phthalocyanines
I
,
16400
16500
17000
WAVEhJPBEER(cm- I
Figure 1. Spurious Stokes signals from the Coherent Radiation CW dye laser (rhodamine 6G) used In this work. Horizontal numbers with accompanying cm-' unk Indicate the excitation lasers; vertical numbers (in cm-I) indicate shifts of the respective bands from the excitations.
16600
,
'6800 WAVEULMBEif(cm- )
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17200
,
Figure 2. Intensity change of the 14 474-cm-' 0-0 fluorescence of H,Pc in 1 part aCiN plus 8 parts n-octane as a function of excitation energy. The vertical number, 2138, indicates the energy shift in cm-' from the origin, number in parentheses. T.4
at one part aClN (or THF) and three parts alkane. However, the RuPc used in this study was soluble in noctane, suggesting that it may be in the form of a derivative, which should not affect our qualitative interpretation. The aClN used was distilled from technical grade, whereas the THF and the alkanes were the highest purity grades available (reagent or better). That the spectra presented here are impurity free was verified by carrying out blank runs. Solution samples (concentration = 10" M or less before adding n-alkane) were contained in thin glass cells (cell gaps 1 mm or less). Effort was made to freeze the solutions slowly (-2 h) so that solute molecules occupied mostly a single site in the solvent mat rice^.^ At this freezing rate, samples still cracked, indicating the existence of thermal strain and thus birefringence. The excitation spectra reported here were taken by tuning the excitation laser while the luminescence-dispersing monochromator was set at the peak of the 0-0 fluorescence or phosphorescence, and the slits of the monochromator were set smaller than the luminescence bandwidth ( - 5 cm-l or less). Moreover, our laser bandwidth (at most 2 cm-l) was smaller than the site separation (in terms of energy)4and also smaller than the absorption width of a site.4 We are thus certain that contributions from other sites are negligible. For recording the excitation spectra, we adopted the 90° geometry. The luminescence was carefully filtered with the low-fluorescencecolor filters (Corning and Schott & Mainz), and polarization scrambled before entering the monochromators. Furthermore, the strong specular refelctions of the laser from the sample cell interfaces were directed out of the path of the luminescence signal collected by the spectrometers. The spectrometer used for the HzPc and ZnPc fluorescences was equipped with holographic gratings. Thus, we are confident that the signal emerging from the spectrometers was free from the grating ghosts, the effect of polarization preference of the gratings, and the fluorescence (if any) of the optical components used in the light path. To compensate for the intensity variation of the rhodamine 6G dye laser (which we used) as a function of
17000
MgPc excitation 0: 67128. flue
2'K
;I
16000
1620C
16400 WAVENUMBER(cm-1)
16600
16800
Figure 3. Intensity change of the 14 898-cm-' 0-0 fluorescence of MgPc in t part THF plus 50 parts n-octane as a function of the excitation energy. Vertical numbers in parantheses indicate the energy shifts from the origin at 14 940 cm-' not in resonance with any 0-0 fluorescence,' and those not in parentheses from the origin at 14898 cm-'.
wavelength, we divided the fluorescence and phosphorescence signals by the suitably monitored intensity of the dye laser. Additionally, particular attention has been paid to avoid the spurious Raman-like signals traveling codirectional with the Coherent Radiation dye laser. They are shown in Figure 1, where the lasers are indicated in cm-l on the left side of the traces. It was determined that these signals did not arise from the sample, from the optical components used in the cryostat, or from the detection system or the Ar+ plasma lines. That their magntitudes appear to be affected by the alignment of the resonant cavity suggests that they arise from the optics in the dye laser resonant cavity (perhaps the flat threeelement-birefringent wavelength tuner), Results and Discussion The excitation spectra presented in Figures 2-7 are very complicated. Here we attempt to qualitatively account for only the broad structures around 16000 and 16 900 cm-' in H2Pc, 16510 cm-l in MgPc, 16 530 cm-' in ZnPc, 17 320 cm-l in RuPc, 16970 cm-l in PdPc, and 17220 cm-' in PtPc. The long progressions of sharper bands (partly shown) to the left of the diffuse structure in each of these figures arise from the al,(r) e,(r*) transitions, and are analyzed in a forthcoming paper.6
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The Journal of Physical Chemishy, Vol. 85, No. 22, 1981
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Huang et ai. T-42 K
T=4.2‘K
i
PtPc excitation of
9769a phos
W
I
16200
16000
16400
I
16600
16800
I
,
Flgure 4. Intensity change of the 14914-cm-’ 0-0 fluorescence of ZnPc in 1 part aClN plus 15 parts ndecane as a function of the excitation energy. The vertical number, 1531, indicates the energy shift In cm-‘ from the origln, number in parentheses.
I
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l
16800
,
l
,
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17000
,
l
17200
1
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1
17400
,
1
1
1
17f00
WAVENUMBERkm-I)
Flgure 5. Intensity change of the 9172-A 0-0 phosphorescence of RuPc in n a t a n e as a function of excitatlon energy. Vertical numbers underlined indicate energy shifts in cm-’ from the origin at 15 766 cm-’, and those not underlined from the origin at 15 672 cm-’.
I
T =l. 4‘K
I
I
I
17100
I
17300
J
WAVENUMBER(cm-I) Flgure 7. Intensity change of the 9769-A 0-0 phosphorescence of PtPc in 1 part aClN plus 5 parts n-octane. Vertical number underlined indicates the energy shift in cm-‘ from the origin at 15 526 cm-’, and those not underlined from the origln at 15 595 cm-’.
T=4.2’K
l
1
1
dicating convincingly that we are not dealing with a common Pc impurity (e.g., H2Pc) in these Pc samplesS6 The possibility that the broad bands arise from a variety of different sites (i.e., different Gaussian distributions of potential environments as seen by the solute molecules in the solvent matrix) can be excluded. The slow cooling of the solution sample favors the thermally most stable site and, additionally, narrows its Gaussian distribution (shown as the spectral bandwidth arising from inhomogeneity in the solutes’ potential environments at this particular type of site). Other sites with possibly small populations were further discriminated against by choosing small instrument widths for both excitation and luminescence detection, and also by tuning the instrument into the maximum of the predominant Gaussian distribution. Contribution of the broad phonon wings accompanying the vibronic zero-phonon lines for the single selected site to these broad structures can be ruled out. First of all, their counterparts are not observed in the emission spectra.4*6Normally, mirror images of the line wings in the absorption are expected in emission.’ Secondly, we have clearly illustrated that the phonon wings of the 0-0 So SI transition of PtPc in a Shpol’skii matrix at 7 K have insignificant intensities compared with their parent zerophonon lines. We have found that this is true for all 0-0 absorptions and for many vibronic absorptions in all Pc’s that we studied in Shpol’skii matrices at 4.2 K. One might argue that the broad structures are reminescent of the spectral feature of the Pc’s in good solvents such as aC1N and THF which interact strongly with the Pc’s.* However, this too can be ruled out, as RuPc was dissolved only in normal octane but its spectral structure looks identical. Furthermore, Jansen and Noortg have shown that Zn and Mg porphyrins Bolvated by ethanol, D20,diethyl ether, and pyridine also exhibit Shpol’skii effect in n-octane. We can also rule out a spurious effect that might arise if the sample volume excited and thus the resulting luminescence intensities depended on the excitation frequency and/or intensity which might occur in transparent samples. However, with the slow freezing rate that we used, the matrices were very scattering (Le., penetration depth of laser was very small). And that such an effect
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Flgure 6. Intensity change of the 9982-A 0-0 phosphorescence of PdPc in 1 part &IN plus 8 parts n-octane as a function of excitation energy. Vertical numbers underlined indicate energy shifts in cm-’ from the origin at 15 357 cm-’, and those not underlined from the origin at 15 287 cm-’. Excitation spectra of the 0-0 fluorescence are identical.
In the following we first dwell on the authenticity of these broad structures, then on their spectral features, and finally on their interpretations. We are certain that the broad structures observed in the spectra of the Pc’s studied are not instrumental artifacts of any sort as discussed in the last section. It is also highly improbable that they are caused by certain common impurities keeping in mind the small bandwidth of the monitored 0-0fluorescence and phosphorescence, the small slit widths of the monochromator used to detect the luminescence, and also the fact that the Pc’s used to prepare the sample solutions were obtained from different sources. Most importantly, in high-resolution luminescence and excitation spectra of the Pc’s studied, there are some normal vibrations whose energies vary from Pc to Pc, in(6) Manuscript in preparation.
(7) K. K. Rebane, “Impurity Spectra of Solids”, Plenum Press, New York, 1970, Chapter 2. (8) T.-H. Huang, K. E. Rieckhoff, and E. M. Voight, Can. J. Chem., 56, 976 (1978). (9) G. Jansen and M. Noort, Spectrochim. Acta, Part A , 32, 747 (1976).
New Singlets in the Phthalocyanines
The Journal of Physical Chemistry, Vola85, No. 22, 1981 3325
TABLE I: Spectral Position (in 1 0 0 0 cm-’) of ‘(nn*) Arising from eu( NPo ) e,( n * )
16080 cm-’ (most of these cannot be assigned to the ‘ ( x x * ) vibronic transitions6 is considered here as corresponding to the lowest singlet (Sl,ax*) at 14,440 cm-l, whereas the relative to I ( nn * ) I ( n *)“ one containing the major peak a t 16 940 cm-’ and its vicalcd obsd Ac obsd obsd cinity (also cannot be accounted for as the l ( m * ) vibronic MnPc +0.3 transitions6) corresponds to the second excited singlets (E&, FePc -0.3 r a * ) at 15300 cm-l. This assignment appears reasonable CoPc +0.3 as the energy separation of these two broad structures is - 0.4 NiPc close to that between S1 and S2. We note that Fitch, CUPC -0.2 Haynam, and Levy have also observed broader structures ZnPc -0.6 +1.62 1.6 16.53 14.91 H,Pc +1.6 1.5 16.0 14.44 around 16700 cm-l in the sharp vibronic progression of +1.6 2.4 16.9 15.3 HzPc in a supersonic free jet.1° MgPc +1.62 1.6 16.51 14.89 Interpretation of these diffuse structures can conceivably RuPc +1.60 6.4 17.32 15.72 be pursued along two directions. The strong tendency of PdPc +1.65 7.0 16.97 15.32 dimerization in chhromophores in general,” and in Pc’s PtPc +1.66 7.0 17.22 15.56 in particular,12suggests that these diffuse structures and a ‘(m+ )arises from a l u ( n ) e,(.*). For H,Pc t h e the lower-lying origins can arise from the Davydov splitting spectral position (14.44) is the average for the (14.411, of the Pc dimers involving only the x x* transition. In The “calculated” values were estimated 14.474) doublet. view of the large monomer e dimer equilibrium constant, from figures in ref 3. Therefore, they are not very accurate, (2.01 f 0.49) X lo6 M-l, observed in vanadyl-4,4’,4”,4”’+ means t h a t ‘(nn*) is higher than I(..*), and - vice versa, tetraoctadecylsulfonamidophthalocyanine’zand the fact Energy difference between the excitation, i.e., ‘(nn*), and the wavelength a t which the grating for dispersing that aggregation was also observed in RuPc at higher luminescence was set. concentration,6we cannot exclude dimerization from our interpretation without a careful scrutiny. However, the need not concern us here is further borne out by the almost constant “Davydov splitting” in the Pc’s studied complete mismatch between the peak positions and widths (fourth column from the right in Table I) would be surof the broad structures observed in these Pc’s and those prising if dimers are involved. Furthermore, the diffuseof the rhodamine 6G laser intensity distribution (peak = ness of the structure in question strongly suggests the 16400 cm-’; half-width = 800 cm-’). involvement of a nonbonding electron in the electronic Since the energy differences between the broad excitatransition. As such we next turn to the following intertion bands and the corresponding luminescence bands pretation. selected by the spectrometers vary drastically among the Briefly, the enormous number of narrow bands to the Pc’s (third column from the right in Table I), then these red of the broad structure can be assigned to the vibronic broad bands cannot be attributed to the Raman process transitions of the alu(rr) eg(7r*)transitions6 The broad of certain vibration. e&*) transition and the structures involve the e,(NPa) That the structure in question hinges on the excitation vibronic transitions accompanying the former electronic rather than the deexcitation process is deduced from the transition. The following vibronic interaction model is following two observations. First, a broad structure similar consistent with this interpretation. to, but stronger than, the one shown in Figure 7 for PtPc The e,(NPd eg(x*)process leads to the following four was also observed previously in the direct absorption ‘Alu, ‘A%, lBlU,and lBzU.We attribute spin-orbit ~inglets:~ spectrum of the same molecule in a Shpol’skii m a t r i ~ . ~ the diffuse structure to these singlets, albeit a detailed Second, identical excitation patterns were observed when assignment of the bands in each of these broad structures fluorescence and/or phosphorescence was used to record is out of the question. These broad structures ( 1600 cm-’ the excitation spectra. It is therefore clear that the broad cannot be simply attributed to the vibronic above ‘(m*)) structure arises neither from the enhancement of intertransitions of ‘ ( x ~ * )because, in resonance Raman, system crossing (isc) in the case of phosphorescence exfluorescence, and phosphorescence spectra involving only citation nqr from the diminution of nonradiative decay ‘(m*), the vibrational bands with energies around and (including isc) in the case of fluorescence excitation when larger than 1600 cm-’ are either too weak to be detected the excitation wavelength is scanned into this spectral or very weak compared with the fundamentals of energies region. less than 1600 cm-’ (ref 4,6, and 8), but, in the excitation There is another important conclusion that can be despectra presented here, the structure in question have duced from the observations above. Namely, isc directly intensities comparable with the fundamentals. The fact from whatever state this broad structure represents to the that these structures occur 1600 cm-’ above the lowest triplet manifold at 4.2 K is not important. If it were, the excited singlet in all Pc’s studied, and also -1600 cm-l broad structure recorded in the phosphorescence excitation above the second excited singlet in H2Pc, leads us to bespectra would have been stronger than that in the lieve that the whole oscillator strength does not arise just fluorescence excitation spectra. We will now consider the from the eu(NPa) eg(r*) transition either. It is reaspectral features of these diffuse structures. sonable, though, to assume that the broad structure inWith the exception of H,Pc, the bands of interest to us volves both this transition and the vibronic transitions consist of a diffuse background superimposed with a more accompanying alu(x) eg(r*). However, vibronic bands or less regular sharp structure. Their spectral positions origin have escaped shifted by -1600 cm-l from the ‘(m*) vary from 16 500 to 17 300 cm-l (see second column detection in the PtPc resonance Raman study.* Although from the right in Table I). However, the band half-widths they have been observed in the high-resolutionfluorescence (-150 cm-l) and the spectral shifts from the respective lowest singlets ( 1600 cm-l, see fourth column from the (10)P. S. H. Fitch, C. A. Haynam, and D. H. Levy, J. Chem. Phys., right in Table I) are approximately constant. In the case 73, 1064 (1980). (11)R. W. Chambers, T. Kajiwara, and D. R. Kearns, J.Phys. Chem., of HZPc,the shifts are approximately the same as those 78,380 (1974), and references cited therein. in other Pc’s, but the structures spread over a wider range. (12) A. R. Monahan, J. A. Brado, and A. F. Deluca, J. Phys. Chem., The one containing the bands around 15 940,16 010, and 76, 1994 (1972). --f
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The Journal of Physical Chemistry, Vol. 85, No. 22, 1981
and phosphorescence spectra of the Pc’s,~,~ the signals were very weak and no assignment was made. It is thus of interest to understand the nature of these vibronic transitions. According to the first-order perturbation theory, the linear vibronic interaction, (dHev/dQJOQi,can lead to the interaction coefficient13 a=
(ol[n.*l(d~ev/dQi)~Q~l~~*lIl) qm*lll) - Elnn*llO)
The small denominator makes the interaction significant if the numerator is nonvanishing. One of the necessary conditions is that the normal coordinate, i.e., the Qi in (dHev/dQJo, must transform as one of the representations in the direct product of the representations of electronic states, Im*] and In7~*],which are Eu
(Ah, AZu, Blu, B2u) = eg(D4h)
Note that we use captial letters for the symmetry representations of the electronic wave functions (e.g., E, for Im*]), whereas we use small letters for the vibrational wave functions, as well as the MO’s. The vibrational integral, (O(Qill), in the numerator is nonvanishing because the normal coordinate of a molecular vibration transforms identically with the wave function of the fundatmental level, Il), and because the harmonic vibrational wave function in the ground states, IO), is totally ~ymmetrica1.l~ A DdhPc molecule has 13 e, normal modes,15 active in Raman but not in IR. Each of these eg modes will lead to a b2gmode and a b3gmode in a D2h molecule. It is easy to conceive that some or perhaps many of these vibrations activate the vibronic interaction between Im*]and (na*]. The required participation of the eg vibrations in this ‘(m*)-l(n?r*) interaction does not contradict our observation in the high-resolution vibronic spectra arising from configuration al,(r),eg(a*). Only the alg,a2,, blg, and bz normal modes are capable of vibronically coupling ‘(m*$ (13) H. Hamaguchi, J . Chern. Phys., 66, 5757 (1977). (14) E. B. Wilson, J. C. Decius, and P. C. Cross, “Molecular Vibration”, McGraw-Hill, New York, 1955, Chapter 7. (15) H. F. Shurvell and L. Pinzuti, Can. J. Chern., 44 125 (1966).
Huang et al.
in the visible, with that in the UV having E, symmetry under D4h, and only the e, modes can couple the visible l ( m * ) with the totally symmetric ground statea8 Consequently, the vibration of interest (e,) has not been observed in the PtPc resonance Raman spectrum8 or in the highresolution fluorescence and phosphorescence vibronic spectra of the Pc’s.41~Thus, the eBvibrations, which should otherwise have been silent if the above-mentioned vibronic interaction were not operative, borrow intensities from the l(n.rr*)’s,making the spectral shifts of the broad structures from the lower-lying singlets roughly constant, and add to the complexity of the spectral appearance in this region. Conclusion In this study we provide evidence for the existence of a nonbonding MO, -1600 cm-l below the well-known bonding MO involved in the strong visible absorption bands of the Pc’s. This evidence is based on excitation spectra of a number of Pc’s, ranging from HzPc to PtPc in Shpol’skii matrices, and on their subsequent vibronic analysis and is consistent with those of Davidsson16 and Corwin et al.I7 for the porphyrins. The existence of such a nonbonding MO at -500 cm-’ from the initial MO of the visible transition was predicted by Schaffer, Gouterman, and Davidson3 (see left-most column in Table I) for the first transition-metal Pc’s. Thus, our data qualitatively affirm the theoretical predictions. Quantitative comparison with the theoretical result should await detailed vibronic calculation to reveal the real spectral positions of the four l(n.lr*) states, which as observed are strongly vibronically perturbed. Acknowledgment. We acknowledge the operating and special equipment grants from the Natural Sciences and Engineering Research Council of Canada to K.E.R. and E.M.V. for this research. T.-H.H.’s acknowledgement is further extended to Xerox Research Center of Canada for all conveniences provided in the preparation of this manuscript. T.-H.H. is also grateful to Dr. Boris F. Kim for his critical reading of the manuscript. (16) A. Davidsson, Chern. Phys., 45, 409 (1980). (17) W. Corwin, A. B. Chiwis, R. W. Poor, D. G. Whitten, and E. W. Baker, J . Am. Chern. SOC.,90, 6577 (1968).
