USA- November 1975 Vol. 72, No. 11, pp. 4196-4199, Chemistry
Proc. Nat. Acad. Sci.
Raman resonance of electron donor/acceptor complexes (resonance Raman spectroscopy/resonance Raman vibronic theory/electron donor-acceptor charge transfer)
KIRK H. MICHAELIAN, KLAUS E. RIECKHOFF, AND EVA-MARIA VOIGT Departments of Chemistry and Physics, Simon Fraser University, Burnaby, B.C. Canada V5A 1S6
Communicated by Joel H. Hildebrand, July 31, 1975.
ABSTRACT The resonance Raman excitation profiles of a number of charge transfer transitions in electron donor/ acceptor complexes with tetracyanoethylene as acceptor in solution at room temperature are reported and compared with absorption and fluorescence spectra of these complexes. All complexes show distinct anomalies which cannot be accounted for by existing theories unless they are extended. In particular, the excitation profiles peak at the low energy side of the absorption profiles by amounts of the order of 10002000 cm-1 and also, in the cases where two charge transfer bands are present, resonance occurs independently for the two bands. The latter observation suggests that the two bands are due to distinct species of the complex with differing geometrical configurations. The former observation is interpreted, in connection with the known asymmetry of the absorption profile and the large Stokes gap between absorp tion and fluorescence peaks, as arising from the relatively stronger contributions of the pure electronic and vibronic levels in the Stokes gap to the Raman scattering cross-section of the complex, and a frequency dependent damping of the vibronic transitions contributing to resonance. This provides important physical insights into the nature of charge transfer transitions of electron donor/acceptor complexes in general. In our discussion we also refer to similar anomalies in the work of others on the resonance Raman effect of the iodine visible absorption band in solution.
We report a study of the resonance Raman (RR) effect in electron donor/acceptor (EDA) charge transfer (CT) transitions. The complexes studied were ir-ir type complexes between tetracyanoethylene (TCNE) as acceptor and various substituted benzenes, as well as other aromatics, as donor in dichloromethane solutions. We report in this study an anomalous redshift in the RR excitation profiles vis a vis the absorption envelopes for the respective transition. We also report that in the case of two charge transfer bands RR occurs independently for both transitions. We argue that the observed effects, which fall outside the range of predictions from existing theories, are the result of the contributions to the RR effect from superimposed vibronic levels having large damping which increases with increasing energy of excitation arising from strong interactions with the environment. Most previous studies of the RR effect on a variety of stable small and large molecules follow the predictions of theories so far developed. We address ourselves to the observed anomalous features of the RR effect of charge transfer transitions in EDA complexes and provide explanations which will stimulate further experimental and theoretical work. EXPERIMENTAL We measured Raman intensities of the C=C (about 1560 cm-') and C-N (about 2230 cm-') stretching vibrations of tetracyanoethylene(TCNE) complexed in a solution of diAbbreviations: RR, resonance Raman; EDA, electron donor/acceptor; CT, charge transfer; TCNE, tetracyanoethylene.
chloromethane with the following donors: mesitylene, o-xylene, m-xylene, p-xylene, durene, isodurene, hexamethylbenzene, anisole, m-dimethoxybenzene, pyrene, and acenaphthene. Various wavelengths of an argon-ion laser were used for excitation. The experimental apparatus was the same as described previously, except that a Spex 1301 double spectrometer was used to disperse the scattered light (1, 2). Since the S/N ratio was considerably improved by using this spectrometer, even very weak Raman signals (less than 100 photons/sec) could be measured accurately. A backscattering geometry, which we previously utilized successfully, was again used for the Raman experiments. This geometry has the exciting beam incident on the cell-wall (and hence the sample surface) at an angle of approximately -30' with respect to the normal and collects scattered light in the direction of approximately 600 with respect to the normal. Thus, except under extreme concentrations, self-absorption effects are minimized since scattered light is collected only from the surface layers of the sample. Moreover, the effects of self-absorption in this particular geometry will become concentration dependent since the scattering volume looked at is essentially fixed (this would not be true were both incident and scattered light beams normal to the surface or at the same angle). No corrections were made for reabsorption of the scattered light, since profiles taken at different concentrations as well as using different reference lines (704 cm-1 and 1600 cm-1, respectively) agreed well with each other. It was found that if the concentration of the complex was about 10-2 M or less, the reabsorption effects which are evident at higher concentrations were essentially eliminated. Heating effects give rise to spurious results (spectra dependent on incident power). However, below 200 mW scattered intensity was proportional to incident intensity as it should be. At these intensities we obtained a reasonable signal to noise ratio (see error bars in all figures). We found a rotating cell arrangement less satisfactory with respect to noise. Observed intensities were corrected for the spectral response of the system, and plotted as a after dividing by v4, where v is the appropriate frequency of observation. The strong solvent (CH2Cl2) band at 704 cm-1 was used as an internal intensity standard. The donor bands at ca 1600 cm-1, which were chosen for this purpose in the preresonance Raman study of TCNE complexes, were in most cases too weak to be used as reliable standards in the present work. Some donors have sufficiently strong bands in this region. Thus, we also measured the excitation profiles for their TCNE complexes using those bands as internal standards; results similar to those obtained with the CH2Cl2 band were found. The fluorescence spectra of some of the complexes were obtained at 77 K. For these measurements the samples were placed in capillaries and the signal was collected using a conventional 900 geometry. 4196
Michaelian et al.
Proc. Nat. Acad. Sci. USA 72 (1975)
Table 1. Summary of RR excitation profiles and estimation of O--O frequenciesa
Excitation profile maximum (cm-') 19,800 20,400 20,800 21,100 20,800 21,300 19,800 20,200
Fluorescence half width
Fluorescence maximum (cm-,)
VC =C VC=N VC=C
18,240 18,170 19,240 18,870 19,240 19,070 18,240
Accuracies of all frequencies are 4200 cm or better. b The Raman frequencies for vc=c and vc-N are - 1560 cm and c 00 frequencies estimated from the RR excitation profiles. d Mirror point of CT absorption and fluorescence spectra.
RESULTS AND DISCUSSION Our data are summarized in Tables 1 and 2 and Figs. 1 and 2.
For complexes whose CT transitions are in the same wavelength range as the exciting source we observe an RR effect with the following distinct characteristics: (1) The excitation profiles obtained do not match the respective absorption profiles but show maxima in the Raman intensity displaced toward lower energy when compared with the appropriate absorption maximum (Figs. 1 and 2). (2) No overtones or combination bands were observed in either of the resonance enhanced acceptor vibrations. (3) In EDA complexes with more than one CT absorption, each such band showed independent RR enhancement with the same features as described under (1) and (2) above (Figs. 1c, d, f, and 2). (4) The depolarization ratios of the two resonance enhanced acceptor vibrations (vc==c and Vc-GN) are p = 0.33 0.04 which is consistent with their totally symmetric character. This is also the case when more than one CT band is observed and indicates that the symmetry is preserved in the complexed units. (5) Vibronic transitions overlap to such an extent that neither the absorption nor the RR excitation profiles show structure. Neither does the fluorescence profile. However, from the observed Stokes-shift between absorption and fluorescence we can estimate the approximate location of the 0/0 pure electronic transition energy (Table 1). Our data show that the RR profile peaks one vibrational quantum (vc=c, vC=N) above this position. It is not entirely unreasonable that the pure electronic and vibronic transitions in the gap will contribute more to the RR excitation profile than to the absorption profile. (6) The relative RR intensity enhancements I(vc=C)/ I(vC=N) are constant within the absorption bands (except at their edges). Since in all cases, the zGcN enhancements are much less than those of vc=c (Figs. 1 and 2), the GVCN behaviour will not be discussed separately. The observed features described above under (1) to (3) while in general indicative of the RR effect are distinctly anomalous when compared with existing experimental work on normal stable molecular systems and with existing theories of the resonance Raman effect in molecules (3-7). Even Tang and Albrecht's (8) fairly sophisticated treatment
2230 cm-', respectively.
in terms of vibronic intermediate states as it stands does not predict the observed behaviour. We suggest that existing theories be extended to include the case where the absorption band is considered to arise from a superposition of a number of Lorentzian vibronic transitions with wavelength dependent damping. We support our suggestion by the following arguments. The CT absorption bands are asymmetric with the high energy side approximately 1.3 times as wide as the low energy side as measured from the absorption peak. Also, the Stokes shifts in fluorescence are abnormally large (about 6000 cm-') and the fluorescence band does not have mirror symmetry t* a' as the absorption band. These facts indicate a considerable shift between the potential energy curves and their shapes in the ground and excited states respectively. This alone implies a different frequency dependence of the matrix elements coupling the ground state (also the initial state) to the resonant intermediate state and that between the intermediate state and the final state. The square of the former enters into the absorption profile whereas the product of the former with the latter-enters into the expression for the RR excitation profile, and hence one would not expect the two profiles to be necessarily identical. In addition, if the damping factor of the vibronic states involved in the process becomes frequency dependent, with increased damping for higher frequencies, the asymmetry of the absorption envelope as well as the difference between the absorption envelope and the RR excitation profile will be enTable 2. Complexes having excitation profiles without observed maxima
durene isodurene hexamethylbenzene anisole m-dimethoxybenzene pyrene
Absorption maxima (cm-') in CH2 Cl,
20,400a 20,500a 18,500 19,600; 26,100 17,900; 22,700 13,800; 20,200; 25,600 15,300; 22,700
acenaphthene a Two strongly overlapping CT bands.
(cm.') 7100 5900 5200
5000; 5500 5200; 5800 5200;5200
Chemistry: Michaelian et al.
Proc. Nat. Acad. Sci. USA 72 (1975)
4000 E -3000 2000 1000
X(nm) v (cm-' ) 22,000 20,000 T-
FIG. 1. Excitation profiles of the vc=c (open circles) and VCN (filled circles) TCNE vibrations for (a~mesitylene/TCNE; (b) o-xylene/ TCNE; (c) m-xylene/TCNE; (d) p-xylene/TCNE; (e) hexamethylbenzene/TCNE; (f) isodurene/TCNE. The solid lines show the absorption spectra.
hanced. A distinct redshift of the peak of the RR excitation profile vis a vis the absorption maximum is predictable under these circumstances. Physical mechanisms for such frequency dependent damping are under investigation. We postulate that they arise from strong interactions with the environment leading to a reduced lifetime for the higherlying vibronic levels in the ground and/or excited states.
It is interesting to note that the anomalies observed by us for resonance with a CT transition have also been reported for RR with the visible iodine (12) transition in complexing solvents, the anomalies being more pronounced for the more strongly complexing solvents (9). The authors attribute their observations to increasingly destructive interference with the contribution to resonance from the CT level. In iodine
Michaelian et al.
Proc. Nat. Acad. Sci. USA 72 (1975)
v,(cm-') ,v(cm-' )
FIG. 2. Excitation profiles of the vc=c (open circles) and VC--N (filled circles) TCNE vibrations for the second CT transition of (a) pyrene/TCNE; (b) acenaphthene/TCNE. The solid lines show the absorption spectra.
complexes, the CT level lies considerably above (about 14,000 cm-') the electronic level of the acceptor molecule (12). In some of our complexes the next higher level is another CT transition and the lowest lying acceptor and donor states are far above the charge transfer bands. We find that in the case of two CT transitions each resonates independently. This supports the contention that the two CT bands arise from different geometrical configurations of the complex (10). If this were not the case the intermediate state for the Raman process would contain strong contributions from both overlapping and/or separate charge transfer states. This would not lead to the clearly separated and distinct RR behaviour we observed and which is indicative of the presence of two species each with its own CT transition. The sensitivity of the visible absorption profile of I2 to strongly interacting solvents suggests also (11) an explanation in terms of the strongly frequency dependent damping factor arising from interactions with the environment invoked by us, rather than interference arising from the charge transfer state proposed by Matsuzaki and Maeda (9), particularly since the charge transfer state is so far removed from the resonant state in this case. We realize that preresonance with higher lying states will be present in all systems, but given the relative magnitude of resonance to preresonance enhancements which we observed in our systems (1), we consider it unlikely for far removed states to significantly influence the resonance Raman excitation profiles. Otherwise one would expect the anomalies described to be the rule rather than the exception in the RR effect. We expect these anomalies to be a general phenomenon for the RR effect of EDA complexes of all types and not just the specific ones reported here. Furthermore, we suspect
that large damping is also at least partially responsible for the absence of overtone or combination bands in the resonance enhanced TCNE modes. In the Iodine study referred to, the relative strength of the overtone also decreased with more strongly complexing solvents as we would have predicted. In summary, the observed anomalies in the RR effect of charge transfer transitions in EDA complexes lead us to believe: (1) the CT state observed in absorption is a superposition of vibronic states, highly damped by interactions with the environment which become stronger with increasing excitation energy; (2) existing theories of the RR effect need to be extended to take account of this situation; (3) the RR effect is a useful tool for the elucidation of details of the charge transfer interactions. The research reported here was supported by grants from the National Research Council of Canada to E.M.V. and K.E.R. 1. Michaelian, K. H., Rieckhoff, K. E. & Voigt, E. M. (1975) Chem. Phys. Lett. 30,480-484. 2. Michaelian, K. H., Rieckhoff, K. E. & Voigt, E. M. (1973) Chem. Phys. Lett. 23,5-8. 3. Verma, A. L., Mendelsohn, R. & Bernstein, H. J. (1974) J. Chem. Phys. 61, 383-390. 4. Kiefer, W. & Bernstein, H. J. (1972) Mol. Phys. 23,835-851. 5. Albrecht, A. C. (1961) J. Chem. Phys. 34, 1476-1484. 6. Sonnich Mortensen, 0. (1971) Mol. Phys. 22, 179-182. 7. Sonnich Mortensen, 0. (1975) Chem. Phys. Lett. 30,406-409. 8. Tang, J. & Albrecht, A. C. (1970) in Raman Spectroscopy, ed. Szymanski, H. A. (Plenum Press, New York), Vol. 2, chap. 2. 9. Matsuzaki, S. & Maeda, S. (1974) Chem. Phys. Lett. 28, 2730. 10. Voigt, E. M. (1964) J. Am. Chem. Soc. 96,3611-3617. 11. Voigt, E. M. (1968) 1. Phys. Chem. 72,3300-305.