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Resonance Raman Spectroelectrochemistry. 3 4034 Resonance Raman Spectroelectrochemistry. 3. Tunable Dye Laser Excitation Spectroscopy of the Lowest 2Blu Excited State of the Tetracyanoquinodimethane Anion Radical David L. Jeanmaire and Richard P. Van Duyne*’ Contribution from the Department of Chemistry, Northwestern University, Euanston, Illinois 60201. Received September 8, I975 Abstract: Resonance Raman (RR) excitation spectra have been measured for the ~2,u4, us, and u9 totally symmetric normal modes of the tetracyanoquinodimethane monoanion radical using the hybrid technique of resonance Raman spectroelectro- chemistry (RRSE). A CW tunable dye laser was used as the excitation source in these experiments to cover an excitation re- gion of 15 000 to 17 850 cm-’ within the 2B3, - 2B1,(1) transition of TCNQs-. These spectra reveal a substantial amount of vibronic fine structure information. An analysis of the excitation spectra has been carried out leading to frequency assignments for the electronic structure sensitive u2’ (CEN stretch) and v4’ (C=C ring stretch + C=C wing stretch) modes in the 2B~,(1) excited state. Resonance Raman (RR) spectroscopy is strikingly distin- as a function of laser excitation frequency. Mingardi, Siebrand, guished from normal Raman (NR) spectroscopy by the great Van Labeke, and Jacon15 have used the theory to calculate sensitivity of its observables to changes in the laser excitation excitation spectra for totally symmetric modes, compare them wavelength. In particular this is manifested by the following: with the corresponding electronic absorption spectrum, and (1) a subset of the normal Raman active vibrational modes compare these theoretical predictions with the available ex- exhibits intensity enhancement factors of up to lo6 when the perimental data for Mn04-. In general the theory suggests the excitation wavelength falls within an electronic absorption following rule: The excitation spectrum of a particular normal band; (2) the intensities of these resonance Raman active mode should roughly parallel the absorption spectrum. This modes are strong functions of the excitation frequency, VO, and rule is in agreement with the observation that RR scattering do not follow the yo4 dependence of NRS; and (3) inverse po- occurs only when there is absorption of the excitation light and larization (antisymmetric scattering) as well as polarization the scattering is stronger the stronger the absorption. For to- dispersion can be observed in molecules of appropriate mo- tally symmetric modes this rule will be most closely followed lecular point group. By continuously scanning the laser exci- in cases where: (1) the molecule has only one totally symmetric tation wavelength from a region of zero sample absorbance into mode or (2) the electronic absorption spectrum shows essen- and through an electronic absorption band it is possible (Le., tially no vibronic fine structure due to the merging of strongly only laser technology limited) to vary an observed Raman overlapping excited state vibrational levels. On the other hand signal from one characteristic of NR to one characteristic of for molecules having several totally symmetric modes and ei- RR passing through an intermediate preresonance (PR) state. ther partially or fully resolved vibronic structure in the ab- Considerable effort, both theoretical and experimental, has sorption spectrum, the possibility exists that RR excitation already been expended in order to develop a detailed picture spectra will show a series of resonances having similar vibra- of the dependence of Raman scattering intensity on the exci- tional spacing as the absorption spectrum but quite a different tation frequency. The vibronic theory of Raman scattering, intensity distribution. Thus in certain cases one may be able based on the application of the Kramers-Heisenberg-Dirac effectively to resolve the absorption spectrum by monitoring (KHD) dispersion equation to molecular systems,2 has been the intensity of Raman lines corresponding to different totally developed by Albre~ht,~Savin,4 Verla~~,~and to de- symmetric normal modes as a function of YO. At the present scribe the wavelength dependence of Raman intensity in the time experimental data which unambiguously demonstrate this PR region. Common to these treatments is the incorporation feature of RR exciation spectra are quite limited. This is of the adiabatic Born-Oppenheimer approximation and the largely due to the coarse wavelength spacing of data points Herzberg-Teller description of vibronic activity into the KHD imposed by the use of multiple line, fixed frequency lasers such dispersion equation. Experimental validation of these treat- as Ar+ or Kr+ for excitation profile studies. ments for the PR region has been reasonably successful.’Osll In this work we wish to report a preliminary study of reso- Recently Siebrand and Mingardil23l3 have pointed out that nance Raman excitation spectra for certain of the totally these “standard” Raman intensity vs. excitation wavelength symmetric normal modes of the electrogenerated tetracyan- treatments break down near and in resonance due to conver- cquinodimethane radical anion (TCNQ.-). The RR scattering gence problems in the schemes used to effect the sum over the spectrum, electronic absorption spectrum, and electrochemical vibrational levels of the resonant excited state. These authors properties of TCNQm- are discussed in the preceding paper.I6 have advanced a more detailed treatment of this problem in The RR excitation spectra are all obtained by excitation into which the convergence difficulties in the excited state vibra- the region of the lowest doublet excited state (2Blu(1))of tional sum are avoided and nonadiabatic coupling terms are TCNQs- (Dz~,Y axis parallel to the long C2 axis, 2 axis retained for generality. A related approach has been formu- perpendicular to the molecular plane) with an Ar+ pumped lated independently by Van Labeke, Jacon, Berjot, and Ber- CW “jet-stream” dye laser. This laser can be tuned contin- nard.14 uously over a 2850-cm-’ region within the 2B~,-+ 2B1,(’) Experimental evidence for the validity of these formulations transition (15 000 to 17 850 cm-I). of the theory of resonance Raman scattering can be obtained most directly from the study of resonance Raman excitation Experimental Section spectra, i.e., the intensity of a particular Raman line plotted Purification of materials, sample preparation, electrochemical Journal of the American Chemical Society / 98:14 / July 7, 1976 4035 methodology, and the apparatus for carrying out resonance Raman spectroelectrochemistry (RRSE) experiments in the controlled po- tential electrolysis electrogeneration mode for the purpose of obtaining RR scattering spectra have been previously de~cribed.'~.'~ Resonance Raman excitation spectra are plots of the intensity of a particular RR line in the scattering spectrum relative to the intensity of an NR internal standard line (typically a solvent line) as a function of the laser excitation frequency, vo. This relative intensity, I,,I (uo), is independent of experiment to experiment variations in laser power and sample concentration. However, Zre1(vo) must be corrected for the effects of self-absorption and spectrometer sensitivity, which are both I 151 70 functions of VO, before it can be meaningfully related to its theoretically calculable counterpart, the molar scattering cross section, a(u0). The I I bi corrected relative intensity is given by:18,19 [ 1-IO-bCRS[~(~O)+f(~S)1] (y(vo) X [l-lO-*CRE[.('O)+f(uRS)~]] --- a0 (1) where I(VRS) is the experimentally measured intensity (Le., fully re- solved, background corrected peak height) for the resonance enhanced line whose scattered frequency is URS (URS = uo - AVRS);CRS is the molar concentration of the resonant scatterer; and S(YRS)is the spectrometer sensitivity factor at URS-P(vs), @, and So(vs)are the -15930 corresponding quantities for the nonresonance enhanced line of the internal standard whose scattered frequency is vs (us = uo - Avs). The expression in braces is the self-absorption correction derived by Shriver li and Dunn19 for the backscattering geometry used in these experi- ments. a0 is the YO independent molar scattering cross section for the NR internal standard line. YO), and ~(YRS)are the extinction i c(vs), coefficients of the resonant scatterer at vo, US, and VRS. RR excitation spectra were acquired using a Spex Model 1400-11 double monochromator equipped with a cooled RCA C31034 pho- tomultiplier tube and standard low-level threshold photon counting O' ' ' ' 1;s' ' ' li4' ' ' Ii0' ' ' 1d.G' ' ' 16.2' ' ' 15.8' ' ' 15.4' ' ' 1k' ' .G DYE LASER WAVELENGTH detection electronics. The excitation source was a Coherent Radiation ICM-I x 10-3) Laboratories (CRL) Model 490 CW tunable dye laser pumped by the Figure 1. Resonance Ramanexcitation spectrumof the u2 (C= N stretch) CRL Model CR-8 Ar+ laser. The dye laser was manually scanned in mode of electrogenerated TCNQa- at 2192 cm-I. The inset shows the 2.0, 5.0, or 20.0-8, increments over the range 5600 to 6667 8, de- 16 200 cm-' Q uo Q 17 850 cm-I excitation region with a 3X expanded pending on the excitation resolution desired. This range of excitation relative intensity scale. 1.0 mM Q [TCNQ.-] S 3.0 mM. wavelengths was achieved using sodium fluorescein (NaFI), rhoda- mine 6G (R6G), and rhodamine B (RB) dye solutions. 1,3,5,7-Cy- clooctatetraene (COT) purchased from Aldrich Chemical Co. and digitized absorption spectrum was then replotted in linear wave- used without purification was added to the NaFl and RB dye solutions. number format chosen to match that for the excitation spectra. COT acts as a quenching agent for the laser dye triplet stateZoand thereby lowers the pumping threshold, extends the tuning range of Results and Discussion the dye, and improves overall pump to dye laser conversion efficiency. 2B~g-. 2B1,(1) RR Excitation Spectra for the u2, u4, us, and NaFI would not lase at all (up to 8 W all lines pump power) without u9 Modes of TCNQ-. Resonance Raman excitation spectra the addition of COT. In the case of RB, which lases moderately well for the v2 totally symmetric CrNstretching mode at AUR~= without COT (ca.
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