Vibrational studies of reactive intermediates of aromatic amines. 2

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4429

J. Phys. Chem. 1989, 93, 4429-4435 is positive if p > 1 so that the barrier is shifted toward the product limit when the reaction heat increases. In the limiting cases of very exothermic and very endothermic reactions the shift of the barrier location is slow as in these limits dBtAB/dAEo 0. The height of the barrier

Therefore,

vb

is a concave function of reaction heat because

-+

vb

= AEoB*Ag + DBCM(B*AB)

(A6)

also increases with reaction heat. (Recall that this quantity has a sign and not only a magnitude.) The Brernsted coefficient a is closely related to the location of the barrier a = dVb/dAEO = BJA#

(447)

and, according to eq A2, it is an increasing function of

AEO.

is positive at any AEo. Both if Dm increases (the reaction becomes more and more exothermic) and if it decreases (the reaction gets more endothermic), da/dAEo tends to zero; Le., the barrier height-reaction heat relation (the "free energy relation") becomes linear in the limits of very exothermic and very endothermic reactions. Registry No. H2, 1333-74-0; HF, 7664-39-3; OH, 3352-57-6; H, 12385-13-6; HCI, 7647-01-0; HBr, 10035-10-6; F, 14762-94-8; CI, 22537-15-1; Br, 10097-32-2.

Vibrational Studies of Reactive Intermediates of Aromatic Amines. 2. Free-Radical Cation and Dication Resonance Raman Spectroscopy of N ,N ,N',N'-Tetramet hylbenzidine and N ,N ,N',N'-Tetraethyibenzidine V. Guichard, A. Bourkba, 0. Poizat,* Laboratoire de Spectrochimie Infrarouge et Raman, CNRS, 2 rue Henri Dunant. 94320 Thiais. France

and G. Buntinx Laboratoire de Spectrochimie Infrarouge et Raman, CNRS, USTLFA, BLit. CS, 59655 Villeneuve d'Ascq Cedex, France (Received: June 13, 1988; In Final Form: December 19, 1988)

The resonance Raman spectra of the radical cation and dication are reported for various isotopic derivatives of the N,N,N',N'-tetramethylbenzidine (TMB) and N,N,N',N'-tetraethylbenzidine (TEB). Complete vibrational assignments are proposed. The spectra are consistent with the expected quinoidal conformation of the framework in both ions. This conformation is characterized by significant frequency increases of the v(inter-ring) and v8(N-ring) modes with respect to the ground-state spectra, and by the strong resonance enhancement of various bands due to vibrations of the N(alkyl)* groups. The radical cation is very comparable to the biphenyl radical cation concerning the ring-ring configuration, but the quinoidal distortion is extended to the amino groups by reason of the conjugation of the nitrogen n-pairs of electrons with the ring *-electrons. The Raman excitation profiles in the contour of the visible absorption have been measured for the radical cation TEB" and satisfactorily fitted by using a theoretical model based on a single Franck-Condon process.

Introduction Aromatic amines are efficient photoreductors which are model compounds for the investigation of photochemical electron-transfer processes.14 Extensive work on the excited-state reactivity of aromatic amines has been performed using time-resolved methods of transient analysis, such as absorption and emission spectroscopies, chemically induced dynamic nuclear polarization (CIDNP), or ESR spectroscopy. However, resonance Raman spectroscopy is certainly an unequaled technique to obtain detailed, specific information on molecular structures, and the recent development of time-resolved Raman spectroscopy makes possible the direct investigation of transient structures. The first paper of this series5 was devoted to the resonance Raman study of N,N,N',N'-tetramethyl- and N,N,N',N'-tetraethyl-p-phenylenediamine derivatives (TMPD/TEPD) in the radical cation (S") and first triplet (TI) states. Complete vibrational assignments have been achieved and revealed significant analogies between the radical and triplet structures: in both cases quinoidal-type distortions with strengthening of the N-ring bonds ~

~~~

Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86,401. (2) Cohen, S. G.; Parola, A.; Parsons, G. H. Chem. Rev. 1973, 73, 141. (3) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (4) Malkin, Y. N., Kuz'min, V. A. Russ. Chem. Rev. 1985, 54, 1041. ( 5 ) Poizat, 0.;Bourkba, A.; Buntinx, G.; Deffontaine, A.; Bridoux, M. J . Chem. Phys. 1987, 87, 6379. (1)

0022-3654189 12093-4429$01.50/0

-

-

take place, in agreement with the resemblance of the T, T, So+ absorption spectra. These results were based and So+* mainly on two types of information: the observation of isotopic effects, and the comparison with rigorous assignments previously established for the ground state. Both arguments appeared essential for interpreting the Raman results. We are now interested by the N,N,N',N'-tetramethylbenzidine (TMB) and -tetraethylbemidine (TEB) molecules. The following

TMB: X = C H 3 T E B : X =C2H5

paper6 will be concerned with the first triplet states TI T M B and TI TEB. This paper deals with the radical cation ( S o + )and the dication (S2+)of these compounds. Both species are known t o be produced by chemical oxydation,'J by electr~lysis,~ or by photolysis.l"12

Resonance Raman studies of the photolytic

(6) Guichard, V.; Buntinx, G.; Poizat, O., following paper in this issue. (7) Kratochvil, B.; Zatko, D. A. Anal. Chem. 1968, 40, 422. (8) Takemoto, K.; Matsusaka, H.; Nakayama, S.; Suzuki, K.; Ooshika, Y.

Bull. Chem. SOC.Jpn. 1968, 41, 164. (9) Fritsch, J. M.; Adams, R. N. J . Chem. Phys. 1965, 43, 1887. (10) Alkaitis, S. A.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 3549. (11) Narayana, P. A,; Li, A. S. W.; Kevan, L. J. Am. Chem. SOC.1981, 103. 3603.

0 1989 American Chemical Societv

4430 The Journal of Physical Chemistry, Vol. 93, No. 1 I . 1989 TMB" and BZ" (benzidine, NH2-C6H4-C6H4-NH2) cations in ethanol glassI3 and of the radiolytically produced BZ" in aqueous solution,14and partial Raman spectra of TMB" and TMB2+produced by photolysis in micelles,1s have been reported recently. I n all cases the absence of isotopic data makes the assignments uncertain and the vibrational analysis very incomplete. We thus present a comprehensive investigationof the monocationic and dicationic states of the fully hydrogenated (TMB-h and TEB-h), ring-deuierated (TMB-d8 and TEB-d,), methyl-deuterated (TMB-dI2),and fully deuterated (TMB-d,,) molecules. These species were produced by chemical and electrochemical oxidation, or by pulsed photolysis (radical only), and probed by using standard C W techniques in the first cases and a time-resolved instrumentation in the latter. The complete Raman assignments and resonance effects are analyzed and the vibrational structures discussed by comparison with the related ground-state specie^'^^'^ on the one hand and with the TMPD/TEPD radical cation derivativess on the other hand.

Experimental Section The TMB and TEB derivatives were synthesized from benzidine and purified as previously described.I7 The solvents, acetonitrile, methanol, cyclohexane (Prolabo), and cyclohexane-d12(CEA, >99.7%), were used as received. The TMB and TEB radical cations and dications were studied in acetonitrile and in methanol as charge-transfer bromide salts, M solution of the prepared by adding a Br, solution to a neutral compound, or as electrogenerated free species, using I 0-3 M solutions with 0.1 M KC104 as supporting electrolyte and utilizing a standard three-electrode potentiostatic apparatus with a platinum working electrode and a saturated calomel reference electrode. Absorption spectra were recorded on a Cary 17 spectrometer. Raman spectra were obtained with a Dilor RTI triple monochromator, using the excitation lines of Spectra Physics argon and krypton C W lasers (series 2000). Integrated band intensities were measured with respect to solvent bands and corrected from the spectral response of the spectrometer and according to the frequency dependence (w4) of the emission law. Excitation of the electrochemical species was performed by setting the laser beam parallel to the flat anode surface. The monocationic species were also studied as photolytic transients in cyclohexane (hlz and dI2)and acetonitrile, using a pump and probe system already described.s Briefly, this system involves a single Q-switched Nd:YAG laser (10 ns fwhm) followed by a dye laser. The laser output is tripled to 355 nm and split in two parts. One (probe pulse) is shifted to 470 nm in coumarin, and delayed from the UV beam (pump pulse) by an optical line; finally both pulses are spatially recombined in the sample. The Raman scattering was dispersed in a single-stage spectrometer and detected by a gated, intensified photodiode array. The spectra were averaged over 500-1000 pulses and studied without further signal treatment. Results and Discussion Radical Cation. TMB" and TEB" exhibit a strong electronic = 471 nm in MeOH absorption band in the visible region (A,, = 477 nm for TEB", cmax = 40000 M-' cm-I) for TMB". , , ,A (Figure The resonance Raman spectra of the TMB-h, -dI2, -d8,and -d, and TEB-h and -d8radical cations have been recorded at various excitation wavelengths throughout this absorption band. The spectra of the chemically and electrochemically produced radicals. in CH,OH and CH3CN, were quite similar. As an example, some characteristic spectra of TEB*+-h in the range 400-1 700 cm-I, and a few representative excitation profiles are 0 2 ) Saito. T.; Haida, K . ; Sano, M.; Hirata, Y . ; Mataga. N . J . Chem. Phys. 1986. 90. 4017. (13) Hester. R. E.: Williams, K. P. J . J . Chem. SOC.,Furuduy Trans. 2 1981. 77, 541. \ 14) Tripathi. G. N R.: Schuler, R. H. Radial. Phys. Chem. 1988, 32, 25 I . ( 15) Beck. S . M.; Brus, L . E. J . Am. Chem. Sot. 1983, 105, 1 107. ( 16) Bourkba, A . Thesis, University Paris V I I , 1988. ( 1 7 ) Guichard. V.; Bourkba, A.; Lautie. M . F.: Poizat, 0. Spectrochrm. Acta 1989, 4 5 A . I87

Guichard et ai.

T

"'I

\

'I.

\\

300

I

....

500 n m

400

Figure 1. Absorption spectra of the TMB ground state (full line), radical cation (broken line) (from ref lo), and dication (dotted line) (from ref 31) in methanol at 300 K. 1

;

I d

I

-

I

I

A e x C 4 0 6 . 7 nm

x

h

Aexc4 7 6 . 5 nm

I !I

13

I

,

1$00 cm-' - - -800

400

Figure 2. Resonance Raman spectra (400-1700 cm-I) of the radical cation TEB'*-h obtained by adding a Br2 solution to a lo-' M TEB-h solution in CH'CN. Intensitiesare normalized with respect to the solvent bands, which have then been subtracted.

displayed in Figures 2 and 3, respectively. Nonresonant spectra have been obtained for excitation at 647.1 nm. Two resonance maxima are observed around 475 and 455 nm, with relative enhancement factors depending on the Raman frequency. The maximum enhancement factors in Table I1 are calculated as the ratio Imsx/1647.]n,,,. The resonance Raman spectra of all TMB" and TEB" isotopic derivatives, excited at 488.0 nm, are shown in Figure 4. The corresponding band wavenumbers are listed in Tables I and 11. The maximum enhancement factors, measured relative to the intensities at 647.1 nm, are also reported for TEB'+- h. Photoionization of TMB (TEB) has been achieved at 355 nm. I n hexane, electron ejection is known to result mainly from biphotonic ionization.I0 The corresponding transient spectra, excited at 470.0 n m , were the superposition of two types of transient spectra: the former, not observed in oxygen-saturated solutions, are assigned to the triplet state;6 the latter appear insensitive to the presence of oxygen and are not significantly different from the chemically produced radical spectra shown in Figure 4; they are in good agreement with the spectra previously published for TMB'+-h.13.1sThe complete disappearance of the triplet species, SO ns after pump excitation, in aerated solutions suggests that a process more efficient than simple diffusion-controlled quenching by O2 takes place. A possibility is that weak association between the amine and O2exists already in the ground state in such a way that very rapid triplet quenching or electron capture by O2can arise after excitation. A complete kinetic approach in the picosecond time scale would be necessary to specify this point. I n

The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4431

Reactive Intermediates of Aromatic Amines

TABLE I: Raman Frequencies (cm-I) and Assignments for the N,N,N',N'-TetramethylbenzidineRadical Cation, s",and Dication, S2+(& 488.0 nmP

TMB-h SO

1605 vs 1540w

s.+

TMB-diz .S'+

SO

s2+

1603 vs 1587vs 1603 vs 1558vs 1595sh 1535 m

1603 vs 1546s

TMB-dg S2+

1590vs 1572s

1142 w 1360 vw

S'+

1580vs 1490w

1580vs 1535s

1574sh 1578 vs 1550vs 1470m

1467 w

1520 sh 1460w

1450sh

1431 w 1380 m

1130 w 1425 sh 1402 vs 1326 w

1117 w 1344 w

1196 m (1235 vw) (1215 w) 1184 m 889m 891 s 888 m 890m

1379 m 1239s

1288 vs 1218 m

1344 s 1232s

1377 s 1236vs

1165 w

1178 w 991 s 939 m 779 vs

1182 m

1037 w 1010 w 830 w 768 w 632vw 595 vw 483 vw

1037 m 980s 830 m 764vs 605vw 582vw 470vw

1024 m

415vw 400 vw

416 w

502 vw 467vw

926 w 759s 635 w

564 vw 480 vw 452vw 412vw

405 vw 300 vw 228vw

[ ;;::$:

(800vw) 743 s 612w

[tlz:

:z

1117 w

948 vw 760 w 618vw 465 vw

475 vw

449 vw

422 vw

400 vw

423vw

assignmentsb 8a

19a + 9(N-ring)

942 m 742 m 615w 581 vw

1020 w 861 w 815 vw 745 vw 615vw

l2

1114 w

6(CH,/CDp) u'

vS(N-ring) + 19a (1236 vw) (1230 vw) u(inter-ring) 896 s 892 m 9a 1026 w 1024 vw p(CH3/CD3) 856 w 18a 810 w 825 vw v9(NCz) 742vs 725 m 1 612vw 593 w 590vw 580sh 1392 s

1200 vw 836 vw 945 w 758 vs 625vw

1416 vs

1

415 vw

418 vw

385 vw 275 vw 225 vw

235vw

1570vs 1512s 1485 vw

1357 vw (1434 vw) 1467 vs

1342 s 1232s

S2+ 1568 sh 1550vs

S'+

SO

S2+

1418 m

1260 vs 1218 m

950 w 785 w 636 vw

TMB-dZo

SO

1526 w 1495 m

=

312 vw 222 vw

skeleton distortions

'The values in parentheses correspond to uncertain assignments. v = very, s = strong, m = medium, w = weak, sh = shoulder. distortion, rocking.

p =

= stretch, 6,

TABLE 11: Raman Frequencies (cm-') and Assignments for the N,N,N',N'-Tetraethylbenzidine Radical Cation, S'+, and Dication, S2+(b,,, = 488.0 nm)'

TEB-h

TEB-dg

s.+ E

SO

1609 vs 1532 m 1450 vw 1358 vw 1293 s 1266 w 1216 m 1155 m 1130 sh 1080 w 1012 w 902 w 800 w 766 m 635 w 417 w

1600 vs 1550 m 1524 w 1442 vw

1750 440 92

1345 s

3200

1230 s 1165 w

2000

1010 vw 992 m 907 m 804 w 764 s

S2+ 1587 s 1575 s 1445 sh 1467 s 1345 m 1375 s 1230 vs 1165 m 998 w

SO 1585 vs 1493 m 1380 m 1362 w 1190 m 1262 m 883 m 1200 sh 1125 vw 1079 w 1010 w

I050 450

700 3150

745 s

477 w 425 vw

'v = very, s = strong, m = medium, w = weak, sh = shoulder. enhancement factors of the TEB'+-h band intensities (see text).

CH3CN, photoionization of T M B occurs monophotonically via the formation of an excited ion pair, or exciplex (TMB'+-. (CH3CN);)* which has a lifetime in the microsecond time scale.I2 The spectra recorded in aerated solutions 50 ns after the pump excitation appear essentially characteristic of the TMB" species. However, weak but reproducible frequency shifts (Av 10-15 cm-I) are observed with respect to the spectrum obtained in cyclohexane. They reflect probably the charge-transfer interactions of TMB'+ in the ion pair. This point will be studied elsewhere. In deaerated CH3CN solutions the spectrum of the triplet state is weakly observed in addition to the radical cation spectrum, as in hexane. The large Raman enhancement factors which characterize the band intensities (for example, five bands are enhanced by a factor greater than lo3 in the spectrum of TEB"; see Table 11) strongly

-

902 m 796 vw 745 w 632 w 460 vw

S'+

S2+

1570 vs 1525 s 1480 w

1558 sh 1551 s

1413 s 1350 w 1225 vw 1283 w 891 s 1200 vw 1125 vw 1070 vw 1003 vw 962 vw 835 vw 907 sh 798 m 742 s 630 vw 471 w

assignmentsb 8a

19a + uS(N-ring) 2 x UI 6(C2H5) uS(N-ring) + 19a ~(C,H,) u(inter-ring) 6(C2H5)

1450 w 1430 s 1342 m 1282 m 887 w

9a

\ 1065 w 995 w 915 m 727 m

}

~,P(CZHS)

J 18a S(NC2) P(CZH5)

I'

skeleton distortions

= stretch, 6,p = distortion, rocking. 'Values in italics are the maximum

suggest that the resonance effect occurs through a Franck-Condon mechanism, and thus involves only totally symmetric vibrations, as it was the case for TMPD'+.5 As a matter of fact, the experimental excitation profiles in the contour of the visible absorption have been rather satisfactorily fitted from the spectral band shape of this absorption, by using a theoretical modell* based on a single Franck-Condon process (Figure 3). This model results from the existence of a connection between optical absorption and Raman profile line shapes. A dependence law, known as the integral transform relations,l* permits calculation of the Raman profile line shapes directly from the absorption spectrum in the case of a multimode system. The method consists in decomposing the optical absorption into contributions corresponding to the different vibrational states and then using transform methods to convert these contributions into resonance Raman profiles. Several

Guichard et al. . I

I

I

I

I

I

1

0

9

)A

992 cm-1

-

L'0

\

J.E..- 1 I

20 000

TEB'Ld8

R I

1800 I

I

I

cm-1

I

I

25 000

Figure 3. Absorption spectrum (19000-25 500 cm-I) and calculated (full line) and observed (symbols) excitation profiles for the TEFP-h Raman bands at 764 cm-I (+), 992 cm-' (O), and 1345 cm-I ( 0 ) . Raman intensities are normalized with respect to the peak value. Arrows on the absorption spectrum indicate the position of the Raman exciting lines.

approximations are considered, namely (i) adiabatic, Condon, and harmonic conditions, (ii) single electronic excited state, (iii) linear coupling between the excited electronic state and the phonons. This model has been largely described in the The only parameters necessary for the calculation are the electronic absorption intensities and the Raman frequencies of the vibrations to be investigated. The qualitative agreement concerning the calculated and experimental profiles shape and structure and their dependence on the Raman frequency is in favor of this model. This indicates undoubtedly that the TMB'+/TEB'+ visible absorption band corresponds to a single electronic transition, the structure observed in this band resulting mainly from vibrational activity. In the assumption of a pure Franck-Condon process, the Raman enhancement factors can be interpreted in terms of displacements of the excited-state potential minima along specific normal coordinates which characterize the S" chromophore. They provide information about the rearrangements occurring during the So+* S" transition. On the other hand, the variations of band frequency in the radical cation with respect to the ground-state

-

(18) Hizhnyakov, V . V.; Tehver, I. Phys. Status Solidi 1967,21, 755. (19) Chan, C. K.; Page, J. B. J . Chem. Phys. 1983,79, 5234. (20) Stallard, B. R.; Champion, P. M.; Callis, P. R.; Albrecht, A. C. J . Chem. Phys. 1982,78, 712. (21) Page, J . B.; Tonks, D. L. J . Chem. Phys. 1981,75. 5694. (22) Tonks, D. L.; Page, J. 9. Chem. Phys. Lett. 1979,66,449.

I

1

1200

800

1 I

I

400 c m - l

Figure 4. Resonance Raman spectra (200-1700 cm-I), at 488.0 nm, of the TMB'+-h, d12, -ds, and -&,. and TEB"-h and -d, derivatives. Solvent (acetonitrile and methanol) bands have been subtracted.

TABLE 111: Totally Symmetric Vibrations Expected for the NC2-Ring-Ring-NCz Skeleton in the 1800-200-cm-' Range, Assuming a DzhSymmetry ring CC" Sa, 19a, 1, 12, 6a

u(inter-ring)

ring CH"

9a, 8a

-NC, v W G ) , hs(NC2) us( N-ring)

Wilson's notation. values are to be related to geometric distortions between states So and S'+. The Raman assignments for all TMW+ and TEB'+ derivatives are given in Tables I and 11. They rely on those previously established for the ground state16si7and are also derived from the experimental isotopic effects and from the comparison with Raman results available for similar cations, such as TMPD'+,S biphenyl" ( Bph'+),23,24 methylviologen'+ ( MV'+),25 and benzidine'+ ( B Z ~ . + ) . ' ~Ring < ' ~ vibrations have been designed by using Wilson's notation.26 According to crystallographic studies of solid charge-transfer complexes,27TMB" has a planar C2N-C6H4-C6H4-NC,skeleton with partial quinoidal character. A fully quinoidal structure is (23) Kato, C.; Hamaguchi, H.; Tasumi, M. Chem. Phys. Left. 1985,120, 183.

(24) Buntinx, G.; Poizat, O., submitted for publication. (25) Poizat, 0.;Sourisseau, C.; Mathey, Y. J . Chem. Soc., Faraday Trans. I 1984,80,3257. (26) Wilson, E. B.; Decius, J . C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955; p 240. (27) Yakushi, K.; Ikemoto, I.; Kurcda, H. Acta Crystallogr. 1973,B29, 264.

Reactive Intermediates of Aromatic Amines assumed for TMB2+. Therefore, a D2h symmetry is adopted in both cases. However, the vibrations of the terminal alkyl substituents may not obey the same selection rules. Eleven totally symmetric in-plane modes are expected under DZhsymmetry below 1800 cm-', apart from the vibrations of the alkyl substituents; four are specific benzenic modes, 8a, 19a, 9a, 18a (vCc and BCH), five are strongly mixed vibrations derived from the ring modes 1, 12, and 6a and the inter-ring and N-ring stretches, and two are specific of the N ( a l k ~ 1 groups, )~ the N C 2 symmetric stretch, vs(NC2),and in-plane deformation, AS(NC2) (see Table 111). A rapid survey of the spectra (Figure 4) indicates that close analogies arise between the TMB'+-h, TMB'+-d12,and TEB'+-h, or between the TMB'+-d8, TMB'+-d20, and TEB'+-d8 spectra, while the relationship between these two groups is not straightforward; the vibrational energy distribution is largely disturbed upon ring deuteration. Such a situation is different from that encountered with TMPD", whose Raman spectra were much more sensitive to alkyl deuteration than to ring deuteration. These differences can arise from two possible reasons: (i) the activity of several additional totally symmetric ring modes (19a, 18a, 12, Y ~ ~ ~leading ~ ~ to - new ~ ~ vibrational ~ , ) mixings in TMB'+ compared to TMPD" and (ii) the fact that in the case of TMB" the chromophore may be less delocalized and thus may involve lower contributions of the N(alkyl), bonds. The estimation of the relative contributions of these two effects is certainly a determinant step for deriving structural informations from the TMB'+ vibrational data. However, several strong bands with typical shifts are assigned unambiguously to vibrations 8a (1603 and 1570 cm-I in TMB'+-h and -d8), 9a (1232 and 891 cm-I in TMB'+-h and -d8), 18a (991 and 836 cm-I in TMB'+-h and -ds) and 1 (779 cm-I in TMB'+-h). They display comparable frequency values, isotopic dependence, and resonance enhancements in the related TMB'+ and TEB" derivatives. Several weaker signals correspond to C H deformations of the alkyl substituents (see Tables I and 11) and the bands observed at ca. 940 and 480 cm-l in TMB'+-h and -d8, which undergo characteristic shifts upon methyl deuteration (830 and 416 cm-I in TMB'+-d12)or methyl substitution by ethyl groups (907 and 477 cm-' in TEB'+-h and 4) are confidently assigned to the 2(NC2) and As(NC2) vibrations, respectively. The $(NC,) frequency has comparable values in the TMB (TEB) and TMPD (TEPD) ground and monocationic states5*I7and is insensitive to the electronic perturbation resulting from ionization. The mode AS(NC2) appears more coupled. Its frequency drops from 517 cm-l in TMPD" to 480 cm-l in TMB'+ and by ca. 60 cm-' upon methyl deuteration but remains in all cases insensitive to ring deuteration. No signal can be ascribed to mode 12, as commonly encountered for para-disubstituted benzenes, nor to mode 6a, as previously noticed for TMPD'+/TEPD'+. Three main framework vibrations, 19a, vS(N-ring), and v(inter-ring), expected between 1100 and 1700 cm-', are still unassigned. Only two strong signals appear in this region, besides those already ascribed to modes 8a and 9a; the former lies at 1558 cm-I in TMB'+-h and is slightly lowered upon ring deuteration (1535 cm-I) and methyl deuteration (1 546 cm-I) or substitution by ethyl groups (I550 cm-I). It certainly does not correspond to the first harmonic of mode 1, as formerly p r o p o ~ e d , since ' ~ the isotopic shifts observed for these two signals are not correlated, and since the harmonic 2 4 is clearly identified at lower frequency in TMB'+-d12and -dzo,and in TEB'+-h and -d8(Tables I and 11). The 1558-cm-I band can be related to the 1548-cm-' one reported for Bzd". Its assignment to vS(N-ring) in seems doubtful because of its high frequency compared to the values in the p-phenylenediamine radical cation (PPD'+) (141 8 cm-1)28 and in TMPD" (1 5 12 and 1465 cm-I in TMPD'+-h and -d12).5 In fact, various experimental evidences such as bond lengths determined by X-ray diffractionz7 and hyperfine splitting parameters measured by ESR spectroscopy9 are in favor of a lower (28) Ernstbrunner, E. E.; Girling, R. B.; Grossman, W. E. L.; Mayer, E.; Williams, K. P. J.; Hester, R. E. J . Raman Spectrosc. 1981, 10, 161.

The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4433 double-bond character of the N-ring bonds in Bzd'+ and TMB'+ than in PPD" and TMPD'+. As a consequence and by analogy with the ground-state TMB/TEB spectra, it turns out that the largest contribution to the 1558-cm-I line certainly arises from the ring mode 19a. However, its higher frequency value in TMB'+ compared to the TMB ground state, with an increased dependence on methyl deuteration and a reduced sensitivity to ring deuteration, indicates that additional couplings occur in the radical cation, between 19a and one or several vibrations of the amino groups. Finally, a last band is observed at ca. 1342 cm-' in the ring hydrogenated derivatives (spectra a, b, and e in Figure 4) while a strong peak appears at 1380 cm-I in TMB'+-d8 (1413 cm-I in TEB'+-d8) (Figure 4, spectra b and f). The 1342-cm-' band is assigned to the v(inter-ring) vibration, which is found a t almost the same frequency in Bzd'+,13*14 and MV'+.25 As remarked for biphenyl24 and methyl~iologen,~~ this band has no counterpart in the spectra of the ring-deuterated species. The 1380-cm-' line in TMB'+-d8 cannot be ascribed to the u(inter-ring) mode as no coupling effect can account for such a frequency increase upon ring deuteration. We thus associate this signal to the vS(N-ring) vibration. Its frequency increase upon methyl deuteration (1392 cm-' in TMB'+-dzo) or substitution by ethyl (1413 cm-' in TEB'+-d8), which results from different couplings with the GS(CH3),GS(CD3),and GS(C2H5)vibrations, confirms this assignment (a strong coupling of these modes with v(inter-ring) is rather improbable). Similar coupling effects have been mentioned for TMPD'+.5 As a further support of this assignment, it is worthwhile to notice that the shift of uS(N-ring) on going from TMB'+-d8 to TEB'+-d8 is the same as in the neutral molecule (AY 35 cm-I). No Raman line corresponds to this mode in the ring-hydrogenated species. Therefore, the resonance Raman intensity of the N-ring stretching vibration in these derivatives seems to be redistributed over several high frequency modes in such a way that no particular band corresponds to this vibration. Such potential energy redistributions result from strong variations of the couplings upon ring deuteration. In particular, the line assigned to 19a contains most probably a substantial contribution from $(N-ring) in agreement with the anomalous frequency shifts observed for this signal. The coupling between modes 19a and us(N-ring) in TMB was already suggested for the ground-state molecule.17 This interaction is reinforced in the radical cation as the vS(N-ring) frequency is increased and lies closer to mode 19a. In the TMPD" species, 19a is not a totally symmetric vibration and thus cannot be mixed with uS(N-ring); as a matter of fact, the vS(N-ring) frequency was found insensitive to ring d e ~ t e r a t i o n . ~ The a, activity of 19a in TMB" and its inherent coupling with the N-ring stretching motion explain the vibrational dependence of the latter upon ring deuteration. Similarly, no Raman signal corresponds to the inter-ring stretching in the ring-deuterated species. The disappearance of this vibration on ring deuteration is a common feature in several radical cations (see above) and anion^^^,^^ of biphenyl derivatives. The vibrational mixing causing this effect has been well evidenced for the biphenyl anion (Bph'-), from a calculation of the normal coordinates and of their change on electronic e x ~ i t a t i o n .In~ ~the ring-hydrogenated derivative the inter-ring stretching is the predominant contribution to a strongly enhanced vibration at 1326 cm-l; in the ring-deuterated compound, the normal mode with the closest potential energy distribution is calculated at 1220 cm-I, but its intensity is drastically reduced, as it contains much less contribution from the v(inter-ring) motion. By analogy, we correlate the weak signals observed near 1200 cm-l in the ringdeuterated derivatives of TMB'+/TEB'+ to the inter-ring stretching. Such effects of potential energy redistribution have been noticed, to a much lesser extent, for TMB17 and benzidine13 in the ground state. The increasing perturbation noted in the

-

(29) Aleksandrov, V.; Bobovich, Y. S.; Maslov, V. G.; Sidorov, A. N. Opr. Spectrosc. 1915, 38, 381. ( 3 0 ) Yamaguchi, S.;Yoshimizu, N.; Maeda, S. J . Phys. Chem. 1978,82, 1078.

4434

The Journal of Physical Chemistry, Vol. 93, No. 11, 1989

radical state probably results from a strengthening of the interactions of the N-ring and inter-ring stretches with ring vibrations, due to the frequency increase of the former. It turns out from the preceding analysis that the main change noted in the Raman spectra of the TMB/TEB radical cation with respect to the ground state is the intensification of the interactions of the N-ring and inter-ring stretches with alkyl distortions (GS(CH3/C2Hs))and ring vibrations. These effects can be principally ascribed to a significant strengthening of the N-ring and inter-ring bonds, in agreement with the semiquinoidal character of the radical structure. These conclusions are consistent with a delocalized structure ( a ) intermediate between various resonant forms:

r,

,1*+ a

Very similar behavior has been noted for the u(inter-ring) mode (frequency, intensity, and isotopic effects) in TMB, in benzidine, and in biphenyl, as well for the ground state as for the radical cation. Therefore, the inter-ring bond strength is comparable in (b) and (c), or in (d) and (e), and is not significantly perturbed when the charge is further delocalized on the amino groups.

b

--

d

-e-

C

e

The comparison of TMB'+ and TMPD" is also instructive. The resonant activity of several C H 3 (C2H5) vibrations in both cases (even some 6(CH3) modes of the C2H5 groups in TEPD" in TEB'+ are enhanced by resonance) indicates that similar interactions occur which enable a substantial contribution of the alkyl vibrations to the vibronic rearrangement following the S'+* S" excitation. Several possible mechanic or electronic interactions have been discussed for TMPD'+.5 In order to compare the strength of the N-ring bond in TMB" and TMPD", one must estimate the values of the uncoupled u(N-ring) frequencies. In TMPD", the strong coupling between this mode and the GS(CH3)vibration leads to a pronounced splitting of the corresponding Raman bands in the methyl-hydrogenated derivatives; this coupling disappears in the methyl-deuterated ones, where uS(N-ring) is found at 1460 cm-I. In TMB" the mixing with P(CH3) is weaker but the additional mixing with 19a must be taken into account. This interaction can be very approximatively evaluated by comparing the observed frequency of 19a with its expected noncoupled value, derived from the neutral molecule. On this basis, the frequency of the pure uS(N-ring) vibration is found in the range 1420-1460 cm-I. The N-ring bond strength in TMB" is thus comparable to that i n TMPD" (I460 cm-I) or possibly slightly weaker, in agreement with the fact that the charge is delocalized over a larger number of ring bonds. The analogy of the Raman spectra of TMB" and TMPD'+ concerning the activity of the alkyl vibrations and the N-ring stretching frequency is indicative of comparable electronic configurations around the nitrogen atoms in both compounds. Therefore, it turns out that the large discrepancies observed between the Raman spectra of the two compounds result primarily from changes in the mechanical couplings, due to the activity in TMB'+ of several ring vibrations which are symmetry forbidden in TMPD*+, and to a much lesser extent to electronic effects. Finally, the two coupled modes 1 (ring breathing) and u(inter-ring) show the highest enhancement factors (-3 X IO3) while the ring vibrations 8a and 9a are slightly less enhanced (-2 X IO3). Thcse vibrations are those which match best the symmetry

-

Guichard et al. of the quinoidal distortion of the framework. Their enhancement means that the quinoidal structure, and particularly the central bond, are strongly perturbed by excitation of the radical. Modes 19a and 18a also display high enhancement factors, probably because the corresponding atomic displacements fit the symmetry of the u(inter-ring) vibration and thus may undergo distortions in favor of the Sa+* S" transition. The other Raman modes seem to be much less involved in this transition. Their resonance activity probably comes from their coupling with the preceding enhanced vibrations. In conclusion of this Raman investigation, a reliable assignment of the observed vibrations has been proposed. On this basis, the vibrational structure of the TMB/TEB radical cation has been precisely correlated to that established for the ground ~ t a t e . ~ ~ ? ~ ' The quinoidal character of the framework is well evidenced and can be characterized by two dominant aspects: (i) a notable similarity with the ionized biphenyl, Bph", concerning the C6H5-C6H5skeleton, and (ii) a close analogy with the TMPD*+ compound as far as we consider the C=N(alkyl), groups. This result seems consistent with a DZh,planar conformation of the radical cation. In fact, the planarity of TMB" in solid chargetransfer complexes has been evidenced from crystallographic studies.27 However, several very weak Raman signals, particularly In the low-frequency region, are not due to a, vibrations under DZhsymmetry. This activity could result from slight rupture of the selection rules, and would then be indicative of small departure from the radical planarity in solution. However, the activity of nonsymmetric modes may also result from a weak non-Condon, vibronic scattering process which may superpose to the intense scattering of CGndon origin. Moreover, some of these weak Raman signals do not present coherent isotopic effects and their activity appears somewhat uncertain. In any way, it thus seems hazardous to conclude to the nonplanarity of the TMB" radical structure from the resonance Raman data. Dication. TMB2+decomposes quickly in neutral organic solutions and can be partly stabilized in acid solutions (pH l , 8). Its visible absorption spectrum is well characterized3) and consists of an intense, structureless band at nearly the same energy as the TMB" one (Figure 1). The resonance Raman spectra of the TMB2+ and TEB2+ derivatives have been obtained for various excitations within the contour of this absorption band. The chemically and electrochemically generated species show comparable spectra. Figure 5 gives the spectra excited at 647.1 nm for all compounds. The corresponding band frequencies are reported in Tables I and 11. The TMB2+-h spectrum is similar to this previously reported.l5 No excitation profiles could be established due to the low stability of the dicationic species and thus the intensity enhancement factors could not be determined. However, the general aspect of the spectra obtained for the different excitations remains nearly unchanged, indicating that the visible absorption band corresponds to a single transition. The coincidence in energy and intensity of this band with that observed for the monocationic state reflects the common trend to a quinoidal Dzh structure in both cases. It is thus reasonable to assume a same Franck-Condon origin for the resonance Raman effect in TMB2+and in TMB" and comparable activity of the totally symmetric vibrations can be expected (Table 111). We shall now discuss the assignments (Tables I and 11). Several modes such as 9a, 1, V"(NC2),GS/pS(C2H5/CH3/CD3) are straightforwardly characterized by direct comparison with the ground-state and radical-state spectra. In the low-frequency region, a band found in the TMB2+derivatives around 600 cm-', equally shifted upon ring and methyl deuteration, must correspond to an in-plane distortion of the skeleton. However, there is no equivalent signal in the ground-state and radical cation spectra nor in the TEB2+spectra. Strong variations of the low-frequency couplings probably appear upon ionization. Finally, no band corresponds to the C H deformation 18a, as observed for the ground-state molecule. I n the 1300-1 600-cm-' region, strong

-

(31) Hasegawa, H. J . Phys. Chem. 1961, 65, 292

The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4435

Reactive Intermediates of Aromatic Amines

r

I

I

I

I

I

I

TABLE IV: Comparison of the 19% u'(N-ring), and u(inter-ring) Frequencies (cm-I) for Various Ions of Biphenyl (Bph), Tetramethylbenzidine (TMB), and Tetraethylbenzidine (TEB)

I

19a

ring-h ring-d 1507 1502

1412 1421

;;;:

1510 1540 1532 1558 1550

1416 1490 1493 1535 1525

TMB2+

1587

1574

TEB2+

1575

1558

IB$'(:i rTMB I TEB

{

1

uYN-rina) dinter-ring) ring-d ring-h ring-d -I

ring-h

1285 1343

1360 (1420-1460)" 1442

(1418 1495 1465

1188

1344 1380 1380 1413

1326 (1220)b 1290 1196 1293 1190 1342 (1200)' 1342 (1200)e

1402

1379 (1215)'

1430

1373

" Estimated value (see text). bCalculated value.30 CTentativeas-

signment. ation5). However, weak frequency increases of vS(N-ring) from TMBZ+-d8to TMBz+-dzo(Av = 14 cm-I) or to TEB2+-ds (Av = 28 cm-I) still indicate the presence of small interactions with the corresponding alkyl deformations (GS(CH3),GS(CD3),or P(CH2), respectively). Comparable frequency shifts take place in the related radical cation derivatives (Av = 12 and 33 cm-l, respectively). (ii) The N-ring stretching frequency in TEB2+-hor in TMBZ+-dlz(1467 cm-l) is higher than the value estimated for the monocationic state (1420-1460 cm-I). A ca. 25-cm-' shift is also noticed for the ring-deuterated derivatives. This effect underlines once again the increasing quinoidal distortion of the structure on going from the monocationic to the dicationic states. (iii) Finally, the large frequency decrease of mode $(N-ring) upon ring deuteration results from the above-mentioned coupling of this mode with 19a. L

1600

I

1200

I

800

I

Conclusion

400 cr

Figure 5. Resonance Raman spectra (200-1700 cm-I), at 647.1 nm,of the TMB*+-h, -dI2,-ds, and -d2,,. and TEBZ+-hand -ds derivatives.

Solvent (acetonitrile) bands have been subtracted. perturbations seem to occur with respect to the radical cation. However, careful examination of the spectra reveals the presence of a regular, coherent spectral evolution between the ground state, the radical cation, and the dicationic species. Two overlapping signals are observed in the 1550-1 590-cm-' range. A strong band at 1587 cm-I (1550 cm-l) in the ring-hydrogenated (ring-deuterated) compounds corresponds to mode 8a, while a shoulder weakly dependent on ring and methyl deuterations can be ascribed to a mixed 19a/d(N-ring) vibration by analogy with the radical cation assignment. The strong band situated near 1377 cm-I in the ring-hydrogenated derivatives, insensitive to methyl deuteration, is assigned to the inter-ring stretch. N o related signal is found in the ring-deuterated compounds, apart from a very weak and doubtful line around 1200 cm-I. Therefore comparable redistribution of the vibrational potential energy takes place in the monocationic and dicationic species (see discussion in the preceding chapter). The high frequency of this mode compared to its value in the radical cation (6v 35 cm-I) is consistent with the expected strengthening of the quinoidal character in the dication. Finally, one strong line remains unassigned in this region: it is found at 1467 cm-' in TEB2+-hand TMBZ+-d12,at 1430, 1402, and 1416 cm-' in TEB2+-d8,TMBZ+-d8,and TMB2+-d20,respectively, and is split in two signals at 1495 and 1418 cm-' in TMB2+-h. We assign it to the vS(N-ring) vibration in reason of its triple frequency dependence on ring deuteration, methyl deuteration, and methyl substitution by ethyl. Several remarks concern this vibration. (i) The band splitting noted in TMBz+-h arises necessarily from a strong coupling with a GS(CH3)mode. A very similar coupling has been observed for the TMPD radical cation5 This interaction is strongly reduced in TEB2+-h or in TMB2+-d8as one of the coupled components is frequency shifted (the coupling effect was identical in TMPD'+-d4 and in TMPD'+-h since, in this case, vS(N-ring) was not shifted upon ring deuter-

-

"

Table IV summarizes the main Raman perturbations observed between the ground state and the monocationic and dicationic states of TMB and TEB. A comparison with the biphenyl ground state,32radical c a t i ~ n ,and ~ ~radical , ~ ~ anion29v30is also given. As expected, the extension of the quinoidal distortion in the radical cation and, to a greater extent, in the dication structures is well characterized by an increase of the N-ring and inter-ring stretching frequencies. These frequency shifts are followed by important variations of the couplings among the aBvibrations and of the normal-mode potential energy distributions. These interactions are present in the ground-state TMB (TEB) but become more pronounced in the radical cation and in the dication. They appear to account for most of the spectral perturbations noticed between the ground state and the ionized states. In particular, as can be seen in Table IV, a strong coupling between vS(N-ring) and the ring mode 19a is evidenced; in the biphenyl ground state and semiquinoidal radical states, mode 19a is found around 1500 cm-' and is shifted by ca. 100 cm-' upon ring deuteration, as is usual in benzene derivatives. In considering the sequence BP-TMB-TMB'+-TMB2+ in Table IV, one observes a continuous progression of the 19a frequency on going from BP to TMBZ+,while the dependence of this frequency on ring deuteration is simultaneously attenuated. This evolution follows the frequency increase of mode vS(N-ring) and reflects the intensifying coupling between these modes as their frequencies become closer. Another fundamental feature of the TMB" and TMB2+ Raman spectra, already pointed out for TMPD'+,S is the pronounced resonance activity of many modes of the N(alkyl), groups. This activity, which results from interactions between the alkyl substituents and the a-chromophore, is characteristic of the iminium character, > N + = , of the bonding configuration around the nitrogen atoms. Registry No. TMB", 21 296-82-2; TMB2+, 2655-65-4; TEB", 21 296-83-3; TEB2+,68820-52-0; Dz,7782-39-0. (32) Zerbi, G.; Sandroni, S . Spectrochim. Acta 1968, 2 4 4 483.