J . Phys. Chem. 1989, 93, 4436-4441
4436
Vibrational Studies of Reactive Intermediates of Aromatic Amines. 3. Triplet (T,) State Time-Resolved Raman Spectroscopy of N,N ,N’,N’-Tetramethylbenzidine and N ,N,N’,N’-Tetraethylbenzidine V. Guichard, 0. Poizat,* Laboratoire de Spectrochimie Infrarouge et Raman, CNRS, 2 rue Henri Dunant, 94320 Thiais, France
and G. Buntinx Laboratorie de Spectrochimie Infrarouge et Raman, CNRS, USTLFA, Bit. C5, 59655 Villeneuve d’Ascq Cedex, France (Received: June 13, 1988; In Final Form: December 19, 1988)
The time-resolved resonance Raman spectra of the first triplet state are reported for various isotopic derivatives of N,N,N’,N’-tetramethylbenzidine (TMB) and of N,N,N’,N’-tetraethylbenzidine(TEB). Complete vibrational assignments are proposed and the triplet structure is discussed on the basis of previous vibrational results for the ground and radical cation states and for the triplet state of N,N,N’,N’-tetramethyl-p-phenylenediamine/N,N,N’,N’-tetraethyl-p-phenylenediamine (TMPD/TEPD). The observed bands are assigned to the Wilson ring modes 8a, 19a, 9a, and 18a, to the v(inter-ring) and uS(N-ring) stretches, and to deformations of the N(alkyl), groups. The resonance activity is consistent with a planar D2h conformation of the NC2-ring-ring-NC2 framework. The spectra are indicative of an increase of the N-ring and inter-ring bond orders and of a weak decrease of the a-bonding character on the rings, in agreement with the existence of a quinoidal distortion extended over the amino groups. The T I state of TMB is essentially aa*, as TI biphenyl, but contains a significant character of intramolecular charge transfer (na) due to partial conjugation between the amine and phenyl fragments. The analogy of the triplet and radical cation bonding configurations is well evidenced. The reducing nature of TI TMB can be understood on this basis.
Introduction
This work is a part of a general Raman investigation of photochemical reactive intermediates of aromatic amines. In two previous papers, we reported studies of N,N,N’,N’-tetramethyl-
p-phenylenediamine/N,,N,N’,N’-tetraethyl-p-phenylenediamine (TMPD/TEPD) derivatives in the radical cation (S”) and first triplet (TI) states,’ and of N,N,N’,N’-tetramethylbenzidine/N,N,N’,N’-tetraethylbenzidine (TMB/TEB) derivatives in the radical cation and dication (S2+)states,2 using resonance Raman and time-resolved resonance Raman techniques. The next paper will be devoted to the time-resolved Raman investigation of the radical cation transient of a series of N,N’-dimethylaniline/N,N’-diethylaniline (DMA/DEA) derivative^.^ The present work deals with the first triplet transient (TI) of TMB and TEB. The triplet state of aromatic amines has strongly reducing properties and undergoes photoreduction reactions in the presence of suitable electron acceptor^:^,^ As a general rule, close resem3(amine)* + A
-
amine”
+ A’-
-
blance is found between the T, TI and S’+* S” absorption As a matter of fact, significant spectra of aromatic analogies between the S’+ and T , Raman spectra have been observed in the case of TMPD,’ suggesting a certain similarity of the triplet and radical cation structures: the main common feature is probably the strengthening of the N-ring bonds, which results from a conjugation of the nitrogen n-pairs of electrons with the ring a-electrons. Such arrangement allows the two unpaired electrons in the triplet structure to stand as far apart as possible, minimizing their interaction energy, in agreement with the low value of the zero-field splitting parameter reported for TI TMPD.* ( I ) Poizat, 0.;Bourkba, A.; Buntinx, G.; Deffontaine, A,; Bridoux, M. J . Chem. Phys. 1987, 87, 379. (2) Guichard, V.; Bourkba, A.; Buntinx, G.; Poizat, O., preceding_paper .. in this issue. ( 3 ) Guichard. V.; Buntinx. G.; Poizat, 0. J . Chem. Phvs., in press. (4)Karvanos, G . .I.; Turro, N . J . Chem. Reu. 1986, 86, 401. . (5) Alkaitis, S . A.; Gratzel, M . J . Am. Chem. Soc. 1976, 98, 3549. (6) Nakato, Y.; Yamamoto, N.; Tsubomura, H. Bull. Chem. SOC.J p n . 1967, 40, 2480. (7) Cadogan, K . D.; Albrecht, A. C. J . Phys. Chem. 1969, 73, 1868. (8) Matsui, K.; Morita, H.; Nishi, N.; Kinoshita, M.; Nagakura, S. J . Chem. Phys. 1980, 73, 5514.
0022-3654189 12093-4436$01 .SO ,IO I
/
However, an ambiguity remained in the interpretation of the triplet Raman spectra concerning the ring configuration: two possible structural models, planar or nonplanar (“boat-type” distortion), were discussed. The formation and reactivity of the TMB triplet state have been studied in organic solvents and in micellar solutions by laser photoly~is.~ The triplet absorption spectrum presents a strong band in the visible region (A,, = 475 nm, e 4.1 X IO4 M-’ cm-’ ) very comparable to that found for the TMB” radical cation (Figure 1). The semiquinoidal structure9 of this radical cation, schematized by the delocalized form (a), has been well charac-
-
[
.+
>N N *.](
-
a
terized by resonance Raman spectroscopy.2 No Raman spectrum of TI TMB has been reported to our knowledge. We present here a comprehensive study of the time-resolved resonance Raman spectra of the photogenerated triplet transients, obtained for the hydrogenated molecules, TMB-h and TEB-h, the ring-deuterated derivatives, TMB-d8, and TEB-d8, and the methyl-deuterated and fully deuterated species, TMB-d,, and TMB-d20. The corresponding assignments are discussed on the basis of vibrational analyses of all compounds in the ground and in the radical cation and dication states2 and are compared to those previously established for the TMPD and TEPD radical cation and triplet state.’ The structure and electronic configuration of TI TMB is debated in connection with its reducing properties and the structure of T, TMPD improved. Part of this work has been the subject of a previous communication.I2 Experimental Section A . Materials. The TMB and TEB derivatives were synthesized
according to literature methodsI3 from benzidine NH2-C6H4(9) Yakushi, K.; Ikemoto, I.; Kuroda, H. Acta Crystalfogr.1973, B29, 264. (10) Bourkba, A. Thesis, University Paris 7, 1988. ( I I ) Guichard, V . ; Bourkba, A,; Poizat, 0. Spectrochim. Acta 1989, 45A, 187. (12) Bourkba, A,; Poizat, 0.; Buntinx, G., Deffontaine, A. J . Mol. Struct. 1988, 175, 1.
0 - 1989 American Chemical Societv
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4437
Reactive Intermediates of Aromatic Amines
TABLE I: Pump and Probe Wavelengths (A), Molecular Extinctions (e), and Solution Concentrations (C) in the Two Experimental Configurations Chosen for the Reman Detection of T, TMB Pump probe C, A, t(Sl+So), A, c(T,+T,), s(S'+'+S'+), nm cm-' M-l nm cm-l M" cm-' M-' mol L-' config 1 355 300 470 40000 40000 -IO-' config 2 310 45000 532 8000 99%). All compounds were purified by column chromatography on basic alumin and sublimation in vacuo. The purity was checked by IR and R M N methods. In all cases except for TEB-d,, no imperfectly deuterated samples were discernible. In TEB-d,, deuterium labeling was better than 99% on positions a to the amino groups, but only 90% on positions a to the inter-ring bond. Accordingly the samples were a mixture of TEB-d8 (70%), TEB-d,h (23.5%), TEB-d6h2 (5.5%), and TEB-dsh3(1.2%). In other words, this means that the proportions of totally deuterated, monohydrogenated, and dihydrogenated rings were approximately 83%, 15%, and 2%. Cyclohexane, n-hexane (Prolabo), and cyclohexane-d12(CEA, >99.7%) were used as received. All solutions were deaerated by Nz bubbling prior to each measurement and studied in 1-cmz quartz cells. B. Measurements. The experimental apparatus was a classic pump-probe system using commerical laser sources and multichannel spectrometer. TI population was achieved with pump excitation in resonance with the SI So absorption, followed by TI SI intersystem crossing. Raman scattering was excited by using a probe excitation in resonance with the T,, T I absorption. Pump and probe pulses were issued from a single IO-Hz Qswitched Nd: YAG laser (Quantel, Model YG581C). Two experiments were performed, which refer to configurations 1 and 2 in the Results section. In the first case, the third harmonic (354.7 nm, 12 ns) was used as pump excitation (2 mJ) and also to drive a dye laser (Quantel, Model TDL50, Coumarin 480) and generate a probe excitation at 470 nm (8 ns, 5 mJ). In the second arrangement, the second harmonic (532 nm, 12 ns) of the YAG laser was used partly as probe excitation (5 mJ) and partly to generate a pump beam at 310 nm (7 ns, 2-3 mJ) through excitation of the dye laser (Rhodamine 640) and frequency doubling. The probe beam was retarded by ca. 35 ns with an optical delay line and spatially recombined to the pump beam in the sample. Raman emission was detected with a Dilor OMARS 89 spectrometer comprising a double subtractive foremonochromator, a onemonochromator spectrograph, and a gated intensified photodiode array cooled to -25 OC. Spectra in the 700-2000-~m-~region were analyzed in direct mode, i.e. the foremonochromator was by-passed and replaced by a long pass colored glass filter to gain maximum luminosity. Low-frequency spectra were recorded in the three-stage configuration of the spectrometer to have maximum rejection of the Rayleigh diffusion. The probe pulse and the detector gate pulse (20 ns fwhm) were synchronized so as to magnify the signal-to-noise ratio. The jitter was less than 2 ns. The spectral resolution was 8.5 and 5.8 cm-I and the analyzed spectral field about 550 and 400 cm-I for the 470- and 532-nm probe. lines, respectively. The multichannel detector was i n t e r f a d with an IBM PC AT computer. The spectra were averaged over
-
+-
-
(13) Kaplan, E. D.; Thornton, E . R. J . Am. Chem. SOC.1967,89,6644. Ho, T. L. Synrh. Commun. 1973, 3, 99.
Results
The time-resolved resonance Raman spectra of N2-saturated Of TMB-hv-d129 -d87 and -dm and Of TEB-h and -dB in cyc10hexane-h127 and cyc10hexane-d12 have been recorded in the 130-1800-cm-' range by using the two excitation configurations described in the Experimental Section (Table I ) . The first configuration corr&onds to a poor S1 So pumping situation (t 300 M-I cm-I) (10-1 M sample concentrations were required) and to optimum resonance conditions for probing both the T I and S" states. The spectra obtained in this way are the superposition of three types of signals of comparable intensities: the ground-state nonresonant the radical cation resonant spectrum,2 and new Raman bands due to a transient species. Observation of the ground-state compound is a consequence of its high concentration in the solution and of the weak SI So pumping. As previously r e p ~ r t e dthe , ~ formation of the radical cation arises probably via a two-photon process in the same way as for TMPD.' However, the biphotonic ionization was found much more efficient in TMB than in TMPD and could by no means be lowered. Finally the new transient spectrum was not observed in aerated solutions. We thus assign it to the lowest triplet state TI (the SI lifetime is less than 10 n ~ ) . ~ The second configuration of excitation (Table I) appears much more favorable to the triplet detection as it combines a better aptitude for populating the SI state and a better resonance condition for probing the triplet state compared to the radical state. Accordingly, only bands due to the triplet transient were observed. Figure 2 shows original spectra obtained in this configuration (Le., probed at 532 nm) in the 1300-1650-~m-~range for solutions in C6D12. Figure 3 compares the triplet spectra reconstituted for all derivatives from comparison of the Raman results in the different solvents (excitation at 532 nm). The corresponding band wavenumbers are listed in Table 11. The spectra of TI TMB recorded at 470 nm and obtained after subtraction of the ground-state and radical cation bands are shown in Figure 4 for comparison. A band observed around 1380 cm-I appears to be the strongly enhanced signal in going from 532- to 470-nm excitation. Although no excitation profile could be established, it turned out that very comparable triplet spectra (general aspect and band activity) are observed, for each derivative, with the 470- and 532-nm probe excitations. Therefore, the T, Tl visible absorption corresponds most probably to a single transition, as it was found for the related T, TI absorption of TMB" visible absorption.2 TMPDI or for the TMB'+*
-
'
+-
-
-
-
-
Assignments
-
The T, T I visible absorption band of TMB/TEB corresponds to a strongly allowed transition (emx 4.1 X 1O4 M-I cm-I ) and thus, its vibrational activity has essentially a Franck-Condon origin. Consequently, the Raman spectrum in resonance with the T I transition can be assumed to display only totally symT, metric vibrations for which the equilibrium position is displaced on going from TI to T,,.I4 Hence, the analysis of the resonance Raman activity must provide information on the symmetry of the TI state and on the nature of the chroniophor involved in the T,, TI transition. In addition, a comparison of the ground-state and triplet-state Raman frequencies can be expected to provide information on the triplet structure with respect to the ground-state one.
-
-
(14) Albrecht, A. C. J . Chem. Phys. 1961, 34, 1476.
4438
Guichard et al.
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989
U
li00
X t
I200
‘
l
I
’
800
400 c m - I
Figure 3. Time-resolved resonance Raman spectra (200-1700 cm-I), excited at 532 nm (pump at 310 nm), of the T, transient of TMB and TEB derivatives. Each spectrum is reconstituted from the spectra obtained with 5 X lo4 M solutions in n-hexane, cyclohexane, and cyclohexane-d12.Solvent bands have been subtracted. Band wavenumbers (cm-I) are reported. I
I
I
I
I
A 1650
1550 c m - l
1450
I
TMB- d 12
i 1350
Figure 2. Time-resolved resonance Raman spectra (1300-1650 cm-l) of (a) TMB-h, (b) TMB-d,,, (c) TMB-de, (d) TMB-dzo, (e) TEB-h, and ( f )TEB-d,, in the TI state, obtained at 300 K with 5 X lo4 M deaerated solutions in cyclohexane-d12.Pump excitation 310 nm, probe 532 nm
T M B -d8
A
A
TMB-d20
(delay -40 ns).
According to previous spectroscopic studies (ESR, phosphorescence) and semiempirical calculations on benzene derivatives in the triplet state (literature results have been briefly discussed in ref 1), different in-plane (quinoidal, antiquinoidal) or nonplanar distortions can be expected for the TMB triplet structure. The highest symmetry corresponds to a DZh,planar NCz-ring-ringNCz skeleton, as for the radical cation and dication, with a symmetry axis C2 passing through the nitrogen atoms, and a u,, mirror perpendicular to this axis and passing through the inter-ring bond. In this hypothesis, 11 in-plane modes below 1800 cm-I, besides the alkyl vibrations, are totally symmetric (the ring modes 8a, 19a, 9a, 18a, 1, 12, and 6a, the v(inter-ring) and vS(N-ring) stretches, and the uS(NC2) and AS(NCz) distortions). If the structure presents an in-plane distortion nonsymmetric to the above-mentionned C2axis or uh plane, the number of ag in-plane vibrations is approximately doubled. In the case of a nonplanar distortion (inter-ring twist, N(alkyl), bending, ...), a series of out-of-plane vibrations become a active. In any case, previous analyses of the ground state,l0%l8 radical cation, and dication, Raman spectra have shown that strong mixings arise among these
I
TMB- h
A
A
I
1600
I
I
1200
I
I
800
4b0 cm-1
Figure 4. Time-resolved resonance Raman spectra (200-1700 cm-’), excited at 470 nm (pump at 355 nm), of the TI transient of TMB derivatives. Each spectrum is reconstituted from the spectra obtained with IO-’ M solutions in n-hexane, cyclohexane, and cyclohexane-d,,. after subtraction of the ground-state and radical cation bands and of the solvent signals. vibrations, particularly between modes 19a, 1, 6a, v(inter-ring), vS(N-ring), and AS(NCz). Such coupling effects can also be expected in the triplet spectra. The interpretation of the triplet Raman spectra in Figure 3 is rather puzzling as they show no evident relation with the ground-state and radical-state spectra, nor with the TMPD triplet spectra. Even the analysis of the isotopic effects is not straightforward and requires a meticulous discussion. Nevertheless, a few vibrations can be confidently assigned according to their typical isotopic shifts. The most intense band, observed at 956 cm-I (-825 cm-I) in the ring-hydrogenated (ring-deu-
The Journal of Physical Chemistry, Vol. 93, No. I I , 1989 4439
Reactive Intermediates of Aromatic Amines
TABLE II: Raman Frequencies (cm-I) and Assignments for the N,N,N',N'-Tetramethylbenzidine and N,N,N',N'-TetraethylbenzidineTriplet Transient, TI (& = 532 nm)'
TMB-h
TMB-di, ..
1593 s 1570 sh 1508 s 1479 sh 1441 m 1380 s 1360 sh
1592 s 1570 sh 1504 s
1219 w 1162 w
1212
w
1132
m
1127 m
1127 m
956 vs
825 vs 947 w
841 s
957 vs 942 sh O Y
1378 s 1365 sh
TMB-ds
TMB-dm 1562 m 1545 sh
-1
1565 s 1459 s 1480 sh 1430 s 1373 m
874
1464 s
1371 m
w
876
= very; s = strong; m = medium; w = weak; sh = shoulder.
w
TEB-de 1567 s
1506 s 1476 m (1460) sh 1408 m 1364 s 1269 w 1214 m 1152 w
1464 sh 1491 m 1452
955 vs
1404 s 1265 w 874 m 1127 1072 823 896
w w s
sh
assianmentsb 8a
(8b) 19a
} G(CH,/CHI) d(N-ring) u(inter-ring) CH2 twisting 9a
} see text p(CH,) + CH2 wagging 18a V'(NC2)
= stretch; 6 = in-plane distortion; p = rocking.
terated) derivatives, corresponds to the ring-CH bend 18a. A weak signal at -1215 c d , lowered to -874 cm-l upon deuteration of the rings, is assigned to another ring-CH distortion, 9a. The strong line at 1593 cm-l, slightly shifted by ring deuteration (- 1565 cm-l), is readily assigned to the ring stretch Sa, and another strong band at -1506 cm-l, insensitive to methyl deuteration or substitution by ethyl and lowered to 1460 cm-' in the ring-deuterated derivatives, is likely due to the ring mode 19a. The shift observed for the later vibration on deuteration of the rings is weaker than that usually found for benzene derivatives but is comparable to that reported for the ground This effect was explained as the result of a coupling between modes 19a and S(N-ring). By analogy, such a coupling may be assumed in TI TMB as in So TMB. Nevertheless this interaction is much weaker than that encountered in the TMB" and TMB2+species where extensive normal-mode mixing has been noticed.* Finally, weak signals situated at -945 cm-' in the TMB-h and TMB-d, triplet spectra and at 896 cm-l in the TMB-d8 one are ascribed to the S(NC2) stretch by analogy with the assignments established for the ground state," the radical cation and dication states,2 and also the TMPD (TEPD) triplet transient.l It is noted that the vibrations identified up to now correspond to totally symmetric modes in the DZhsymmetry. In addition, the fact that the spectra exhibit few bands is consistent with a high molecular symmetry. This suggests that the NC2-ring-ring-NC2 framework has actually the symmetry D2h in this triplet state. Identification of the remaining bands is much less evident and the following assignments are only tentative. Consider first the general aspect of the 1600-1300-~m-~region (Figure 2). Significant spectral rearrangements take place on going from TMB-h to TMB-d12and TEB-h, or from TMB-d8 to TMB-d2,, and TEB-d,, which are indicative of an important participation of the alkyl vibrations 6(CH3/CH2/CD3)in the resonance Raman activity. In addition, the large discrepancies observed between the TMB-h and TMB-d, spectra contrast with the close analogy of the TMB-d12and TMB-d20 ones and can be accounted for by changes in the coupling between 6(CH3) and vibrations of the r-chromophore upon ring deuteration. Such coupling effects do not exist in the methyl-deuterated compounds as the 6(CD3) modes appear at lower frequencies (1050-1200 cm-l) and do not interact with ring vibrations in the 1600-1300-~m-~range. However, in spite of these spectral alterations and besides the bands already assigned to Sa and 19a, a strong line observed at 1380 cm-' in the TMB-h triplet spectrum is almost insensitive to isotopic substitution. A shoulder at 1360 and 1365 cm-' in the TMB-h and -dlzspectra has no apparent counterpart in the TMB-ds and -d20spectra. These two signals can manifestly be related to the bands at 1408 and 1364 cm-', respectively, in the TEB-h triplet spectrum as they display comparable isotopic effects. We propose to assign the former to a mode involving a large contribution from the S(N-ring) motion, and the second to a mode corresponding principally to the v(inter-ring) motion. This assignment is not absolutely without doubt but is supported by the observation of several analogies with the ground state, radical
-
TEB-h 1593 s
cation, and dication results, such as the ca. 30-cm-' increase of the S(N-ring) frequency on methyl substitution by ethyl groups, or the disappearance of the v(inter-ring) band on deuteration of the rings. The lines observed at 1479 and 1441 cm-' in the TMB-h spectrum (I480 and 1430 cm-' in the TMB-d8 spectrum)disappear on deuteration of the methyl groups and are, therefore, attributable to 6(CH3)vibrations. By analogy, the signals at 1476 and 1460 cm-I in the TEB-h spectrum (1491 and 1452 cm-' in the TEB-d8 spectrum) correspond probably to C H distortions in the C2H5 groups. These assignments are consistent with those established for the related ground state and cationic species.11 A strong enhancement of the intensity of the 6(CH3/CH2) band in the 1430-1 460-cm-' region happens for the ring-deuterated derivatives TMBITEB-d8. This effect, already noticed for the TMPD triplet derivatives,' probably due to changes in the vibrational couplings, remains not clear. Finally, the very weak shoulder observed at 1570 cm-' in the TMB-h and -ds spectra, and possibly at 1455 cm-l in the ring-deuterated compounds, may correspond either to a broadening of the band assigned to Sa, due to a large energy distribution of this vibration,15 or to the activity of another ring mode such as Sa' or Sb, indicating a partial deviation from the DZhsymmetry. The main uncertainty in these spectra is the line found in the 1120-1 160-cm-' range for all derivatives, which may have different plausible origins. A first possibility is to ascribe it to a distinct alkyl vibration in each derivative: p(CH3) for TMB-h and -d,, p(CH2) for TEB-h and -ds, and 6(CD3) for TMB-d12and -d20. In fact, such an alkyl vibration has been found systematically in this frequency range in the ground-state" and radical cationl,2 spectra of all the TMB (TEB) and TMPD (TEPD) derivatives, and also in the TMPD (TEPD) triplet spectra.l However, the frequency coincidence observed for the related signals in the TEB-d,, TMB-d,, and spectra (v = 1127 cm-l in all cases) makes their assignments to different modes rather doubtful. Another possibility is to assign indistinguishably the band in all derivatives to a strongly coupled ag component issued from the skeleton vibrations, or to a nonsymmetric mode which activity would result from a deviation from the DZh conformation. In any way, no reliable assignment can be proposed at the moment as no coherent isotopic effect is observed. Two additional weak triplet bands are noted for the ethylsubstituted derivatives, at ca. 1269 cm-' (TEB-h and -4)and at 1072 cm-' (TEB-d,). They are ascribable to ethyl deformations by analogy with the TEB ground-state, radical cation, and dication assignments. No signal is apparent below 800 cm-l. The ring breathing mode 1 is not observed, as previously noticed for the TMPD triplet spectra.' Discussion As can be seen in Table 11, the triplet Raman spectra of TMB and TEB have been assigned essentially to totally symmetric ( 1 5 ) Vergragt, P. J.; Van der Waals,
J. H.Mol. Phys. 1977, 33, 1507.
4440
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989
vibrations in the assumption of a DZhsymmetry. Interestingly, the activity is very comparable to that found for the radical cation and dication species (see Table I in ref 2). These results confirm that the NC2-ring-ring-NC2 skeleton adopts a DZhplanar configuration in the triplet state as in the ionized states. Nevertheless; the strong dissimilarity of the intensities of modes 8a, 19a, 18a, 1, and 9a in the triplet and radical spectra suggests that the visible TI and So+* S" have different electronic transitions T, natures and involve specific molecular orbitals, Le., distinct chromophoric groups. This conclusion contrasts with the clear resemblance of the visible absorption spectra in both cases (Figure 1). A comprehensive discussion of these resonance effects in a series of biphenyl derivatives will be given in a next publication. Another analogy is the surprising resonance activity of vibrations of the N ( a l k ~ l groups )~ (Le., vS(NC2),G,p(CH3,CH2)). Such activity has been also observed for the TMPD radical cation and triplet state' and for the DMA radical cation3 and has been shown to result from specific interactions between motions of the nchromophore and of the alkyl substituents. As we stated previo u ~ l y , these I ~ ~ interactions clearly characterize a tendency for the nitrogen atoms to adopt an iminium bonding configuration, >N+=. A double-bond character of the N-ring bonds in TI TMB (TEB) is thus probable, in agreement with the high frequency of the band assigned to the vS(N-ring) mode compared to the ground-state value. A last common feature between the triplet and radical Raman spectra is the increased v(inter-ring) frequency relatively to the ground state ( A v 70 cm-I; see Table 11). Actually the value in the triplet state (ca. 1360 cm-I) is intermediate between those found2 for the radical cation (ca. 1342 cm-I) and the dication (ca. 1380 cm-I). As suggested before, these values have to be considered with prudence, since strong mixings probably occur among the skeleton vibrations. Consequently, no quantitative comparison can be made between the ground-state, radical cation, and triplet-state structures. However, in the reasonable hypothesis that the main contribution of the band around 1360 cm-' originates from the inter-ring stretching vibration, its high frequency compared to the ground-state value is likely to reflect an increasing bond order in the triplet state as in the ionized states. It turns out that the different analogies observed between the TI and S" spectra of T M B agree to characterize a common trend to a quinoidal-type structural configuration. This conclusion is in agreement with literature results for polyphenyl compounds: it is well e ~ t a b l i s h e d 'that ~ ' ~ the electronic distribution in TI biphenyl ( m * ) is in favor of a DZhpara-biradical quinoidal configuration (b) with a low zero-field splitting parameter ( D 0.1 1 cm-I 17)
-
-
Guichard et al. the ground-state frequency (1284 cm-' in terphenyl and 1287 cm-l in biphenyl) is very comparable to that found for TMB. On the other hand, the frequencies of the ring modes 8a and 19a are weakly lowered (12-15 and 30 cm-', respectively) in going from the ground state to the triplet state, whereas the former was not significantly perturbed and the latter was rather shifted to higher frequencies in the radical state.2 These shifts are surprisingly weak, in contrast to what can be expected from the presence of an electron in an antibonding orbital a*. From the evidence, the n-bonding density in the rings is only very slightly lowered in the triplet state relative to the ground state. For comparison, the frequency decreases reported for mode 8a in going from So to T1(nn*) are 42 and 73 cm-' in biphenyIZ2sZ3 and terphenyl,2' respectively. In summary, the triplet structure of TMB appears to be characterized by a substantial double-bond nature of the ringsubstituent bonds (N-ring and inter-ring bonds) and by a slight antibonding character on the rings. This structure can be described as a planar delocalized form (d) intermediate between the para-biradical quinoidal conformation (c), analogous to the triplet biphenyl representation (b), and the extended quinoidal conformation (e). It implicates a strong conjugation of the nonpaired
-
-
b
-
compared to this found in TI benzene ( D 0.16 cm-IZo) and particularly high spin densities on carbons-4 and -4'.19 A similar structure has been proposed for terphenyl and quaterphenyl derivative~.'~Wilbrandt et aL2' have investigated the triplet state of p-terphenyl by time-resolved Raman spectroscopy and have ascribed the inter-ring stretching vibration to a band at 1350 cm-'; recently HamaguchiZ2and Buntinx et aLZ3have reported timeresolved Raman analyses of triplet biphenyl and agree to localize this vibration at ca. 1365 cm-I; the shift upwards with respect to (16) Dewar, M. J. S.; Trinajstic, N. Collect. Czech. Chem. Commun. 1970, 35, 3 136. (17) Orloff, M. K.; Brinen, J. S. J . Chem. Phys. 1967, 47, 3999. (18) Wagner, P. J. J . Am. Chem. SOC.1967, 89,2820. (19) Hutchinson, C. A,; Kemple, M . D . J. Chem. Phys. 1981, 74, 192. (20) Smaller, 8 . J . Chem. Phys. 1962, 37, 1578. (21) Wilbrandt, R.; Jensen, N. H.; Pagsberg, P.; Sillesen, A . H.; Hansen, K. B. Nature 1978, 276, 167. (22) Hamaguchi, H. In Vibrational Spectra and Structures; Durig, J. R.; Elsevier: Amsterdam, 1987; Vol. 4, p 272. ( 2 3 ) Buntinx, G.; Benbouazza, A,; Poizat, 0.;Guichard, V. Chem. Phys. Left. 1988, 153, 279
d
\k*i(
/
(A- )
-
-
-
e
C
electrons with the n-electron pairs of the nitrogen atoms. In this regard, the mean form (d) is comparable to the allyl-type conformations assumed for TI butadiene or TI h e ~ a t r i e n where e~~~~~ conjugation of the nonpaired electrons with ?r-electron pairs takes place:
t
/ e SO
v
e ,_-.. Tl
%.-t_,.
Such delocalization is consistent with the fact that the odd electrons tend to repulse each other as far as possible.26 A very low zero-field splitting parameter D can thus be expected for TI TMB. The triplet structure (d) differs from the radical cation structure (a) only by the presence of one additional electron which is essentially nonbonding. Accordingly, ionization from the triplet +.
a
d
state does not perturb markedly the bonding configuration and must not require much energy. This is consistent with the close T I and So+* S" absorption spectra resemblance of the T, and explains the strong reducing power reported for TI TMB.6 The comparison of TI TMB (TEB) and TI TMPD (TEPD)' is very instructive. From the evidence, the main differences in the general aspect of the spectra can be explained in terms of
-
-
(24) Baird, N . C.; West, R. M . J . Am. Chem. SOC.1971, 93, 4427. (25) Ohmine, I.; Korokuma, K. J. Chem. Phys. 1980, 73, 1907. (26) Turro, N. J. In Modern Molecular Photochemistry; Benjamin/Cummings: London, 1978; p 265. (27) Vernois, M.; Friedmann, G.; Brini, M.; Federlin, P. Bull Chem. SOC. Fr. 1971, 1794.
J. Phys. Chem. 1989, 93, 4441-4447 TABLE 111: Comparative Raman Assignments for N,N,N’,N’-Tetramethylbenzidineand N,N,”,N’-Tetramethyl-p-phenylenediamine in the Ground (So) and Triplet (TI) States‘
TMB-h TI 1605 1593
TMPD-H TI 1623 1573
Sn
1540
1508
1360 1290 1218 1165
1441 1380’ 1360 1219 (1162) 957 942
Sn
1450
1479}
950
1435
1345 1217 1155
1170 1156
945
930
iii Wavenumbers in cm-I.
assianmentsb 8a 8a + d(N-ring) 19a 6(CH3) d(N-ring) u(inter-ring) 9a p(CH3) 18a d(NC2)
:y; ) 6a + As(NC2)
= stretch; 6, A = in-plane distortions; p
= rocking.
variations of band activity in the same way as for the radical cation state.2 For example, 19a and 18a are symmetry-forbidden in the case of TMPD but lead to intense signals in the TMB spectra. However, besides these discrepancies, close similarities are noticed concerning the - N ( a l k ~ l )vibrations ~ (resonance activity of various 6,p(CH3/C2H5) modes) and of d(NC2);high IP(N-ring) frequency with respect to the ground-state value; notable resemblance with the radical cation spectra, which clearly indicate that comparable iminium configurations occur in both cases. This is in favor of a planar, quinoidal structure for T I TMPD as for TI TMB. The
4441
‘io .q
/
\
T i TMPD
alternative nonplanar model proposedl for TI TMPD is thus definitely rejected. According to the discussion of the Raman assignment,’ the strong band observed around 1500 cm-’ for this species is likely to correspond to the ring mode 8a, coupled to the vs(N-ring) stretching. On this basis, the assignments of TI TMB and TI TMPD are compared in Table 111. The important frequency decrease noted for modes 8a and 9a in TMPD on going from the ground state to the triplet state (90 and 47 cm-l, respectively) contrasts with the quasi-intensitivity observed in the case of TMB. This reflects a much stronger antibonding character on the ring skeleton in the former compound, where the charge is confined in a smaller space. In conclusion, two aspects of the structure of TI TMB have been evidenced from this Raman investigation: (i) an analogy with the structure of TI biphenyl regarding the quinoidal configuration of the C6H5-C6HSframework and the significant double-bond character of the inter-ring bond, and (ii) a manifest similarity with TI TMPD concerning the iminium character of the -N(alkyl)2 groups due to a conjugation of the unpaired electrons with the nitrogen lone pairs of electrons. This double aspect of the triplet structure is remarkably comparable to the double resemblance found for the TMB‘+ structure with the biphenyl and TMPD radical cations, respectively. The analogy between the triplet and radical cation structures is still supported by various vibrational features. The reducing nature of TI TMB can be understood on this basis. Registry No. TMB, 366-29-0;TEB, 6860-63-5; D,, 7782-39-0.
Singlet Adiabatic States of Solvated PRODAN: A Semiempirical Molecular Orbital Study Predrag Ilich* and Franklyn G. Prendergast Department of Biochemistry and Molecular Biology, Mayo Foundation, Rochester, Minnesota 55905 (Received: July 5. 1988; In Final Form: December 2, 1988)
X-ray structural analysis depicts PRODAN (6-propionyl-2-(dimethylamino)naphthalene)as a planar system with four molecules stacked in a unit cell of a moncclinic crystal. AMI semiempirical calculationspredict planar conformationfor isolated PRODAN with rotational barriers 0.03 kcal/mol for the propionyl and 0.33 kcal/mol for the amino group, and AHf = 6.74kcal/mol. Adiabatic INDO/S-CI calculations suggest antiquinoidal distortion as the likely ring structural change in the lowest excited singlet state. Twisting of the dimethylaminogroup at position 2 induces, in comparison with either 1- or 3-substituted analogues, strong molecular orbital (MO) localization in the former derivative and occurrence of a highly polar excited singlet state. Decrease of the charge density on nitrogen atom, induced by specific electrostatic interactions with the surrounding medium, promotes the MO with strong N(2py) character into the HOMO level. The transition to that state, determined within the {T*,T] n(N) singlet manifold, is shifted strongly to the red, and the electric dipole moment of the lowest excited state is quadrupled. These predictions are corroborated by the strong emission shifts PRODAN and chromophores of similar structure and topology of substitution exhibit in media of different polarity and viscosity.
-
Introduction Introduced by Weber et a1.I as a sensitive fluorescent probe of the electrostatic character of its environment, PRODAN (Figure 1) has been utilized in protein and more recently in lipid research.2 Substitution of H4 by cyclohexanoic acid appears to offer a definite advantage for site selection by the label in studies of the heme pocket in ap~myoglobin.~A chemical variant, 6-acryloyl-2-(dimethylamino)naphthalene,ACRYLODAN, introduced by Prendergast et al.,4 covalently binds to protein -SH ( I ) Weber, G.; Farris, F. J. Biochemistry 1979, 18, 3075. (2) Chong, P. L . 4 . Biochemistry 1988, 27, 399. ( 3 ) Cowley, D. Nature 1986, 319, 14.
group^,^ providing a combination of the essential spectroscopic characteristics of the propionylaminonaphthalene moiety with a high specificity of labeling. Both compounds are representative of a wide class of chromophores given by the general formula Gl-R-G2, where R is a conjugated ring with [4n 21 atoms or valence electrons and are typical organic functional groups. Most commonly encountered pairs, dialkylamine and aldehyde or dialkylamine and nitrile, provide centers of different local electron density, usually
+
(4) Prendergast, F. G.; Meyers, M.; Carlson, G. L., Iida, S.;Potter, J. D. J . Biol. Chem. 1983, 258, 7541. ( 5 ) Friedman, M.; Kruhl, L. H.; Cavins, J. F. J . Biol. Chem. 1970, 245, 3868.
0022-3654/89/2093-444l$01.50/00 1989 American Chemical Society
