J . Phys. Chem. 1987, 91,2293-2298
+
metastables (by N N,). A similar mechanism cannot occur in the dissociation of HN7, since H HN7 produces NH, + N, (rather than H2 N3)26aid quenching of N;(A) by HN, p&u& N H N,.27 Since many of the reactions described by eq 9-16 above are known to occur in the analogous ClN, s y ~ t e m , ~it*seems ~ * very likely that this mechanism may also apply to chains observed in
+
+
+
(26) Jourdain, J. L.; LeBras, G.; Poulet, G.; Combourieu, J. Combust. Flame 1979, 34, 13. (27) Stedman, D. H.; Setser, D. W. Chem. Phys. Lett. 1968, 2, 542. (28) LeBras, G.; Combourieu, J. I n t . J . Chem. Kinet. 1973, 5, 559.
2293
that Because N2(A) metastables serve to carry these chains, species capable of quenching these metastables (e.g., NO) may well act to stabilize gaseous samples of the halogen azides. Acknowledgment. We are grateful to Professor J. Anderson of Harvard University for kindly loaning to us the computer program used for the calculations described above. This work was supported by the U S . Air Force Weapons Laboratory under Contract No. F29601-84-(2-0094, by the U.S.Air Force Office of Scientific Research under Grant No. AFOSR-84-003 1, and by the National Science Foundation under Grant No. CHE820533.
Aryiaminyl Radicals Studied by Laser Flash Photolysis of Di-fed-butyl Peroxide in the Presence of Arylamines Elisa Leyva, Matthew S. Platz,* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
Baohua Niu, and Jakob Wirz* Institut fur Physikalische Chemie der Universitat Basel, CH-4056 Basel, Switzerland (Received: September 11, 1986; In Final Form: December 23, 1986)
Laser flash photolysis of di-tert-butyl peroxide in the presence of arylamines (aniline, diphenylamine, carbazole, indole, 1and 2-naphthylamine, and 1-pyrenylamine) is a clean, simple, and general method to generate and observe the corresponding arylaminyl radicals. Their absorption spectra are closely and systematically related to those of the *-isoelectronic arylmethyl radicals and triplet arylnitrenes. This relationship provides a useful guideline for the identification of transient intermediates and leads to a new proposal for the structure of the diphenylaminyl radical. The absolute rates of hydrogen abstraction from arylamines by the tert-butoxy radical depend on the resonance stabilization of the resulting arylaminyl radicals and approach the diffusion-controlled limit in highly conjugated systems.
Introduction The identification of triplet arylnitrene intermediates formed by flash photolysis of aryl azides is complicated by the competitive formation of azacycloheptatetraenes and/or benzazirines and continues to be a topic of considerable controversy.'V2 Triplet aryl nitrenes are *-isoelectronic with the corresponding arylaminyl radicals and the absorption spectra of these species should be closely related. We have thus sought a general and clean method to generate and observe arylaminyl radicals by flash photolysis. Di-tert-butyl peroxide (DTBP) is an extremely useful reagent to generate free radicals in s o l ~ t i o n . ~The photolysis of DTBP yields tert-butoxy radicals which rapidly abstract hydrogen from hydrocarbons, ethers, alcohols, and alkylamines to give carboncentered radicals and from phenols and the hydrides of silicon, germanium, and tin to give the corresponding heteroatom-centered radicals. The bimolecular rates of hydrogen abstraction from oxygen in phenols (ca. lo9 M-' s-I a t room temperature) by far exceed the rates of addition to the double bond of alkenes (ca. lo6 M-' 9') or of vinylic hydrogen abstraction (not o b ~ e r v e d ) . ~ . ~ The reaction of tert-butoxy radicals with arylamines was, therefore, expected to be hydrogen abstraction from nitrogen. (1) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J . Am. Chem. SOC.1986, 108., -3783. ..-
(2) Schrock, A. K.; Schuster, G. B. J . Am. Chem. S O ~1984, . 106, 5228. 1984, 106, 5234. (3) Howard, J. A.; Scaiano, J. C. In Landolt-E8rstein, Neue Sene 11, Vol. 13d, Fischer, H., Ed.; Springer-Verlag: Berlin, 1984. (4) Wong, P. C.; Griller, D.; Scaiano, J. C. J. Am. Chem. SOC.1982, 104,
5106.
In this paper we report absolute reaction rates for the reaction of tert-butoxy with several arylamines and the transient absorption spectra of the resulting aminyl radicals in the visible and near-UV (A > 350 nm). Open-shell PPP S C F SCI calculations provide a useful basis for the comparison with isoelectronic s-radicals. Experimental Section Benzene (spectrograde quality) was used as received. DTBP (Merck) was passed through neutral alumina. The carbazole sample (Fluka) contained a minor impurity absorbing up to 380 nm which was removed by column chromatography on alumina using a 1:9 mixture of tetrahydrofuran and petroleum ether for elution. All solid aromatic amines (Fluka; 1-aminopyrene, Aldrich) were freshly sublimed before use, except for the carcinogenic 2-aminonaphthalene (Sigma) which was used as received. Aniline was freshly distilled from KOH. Flash photolysis experiments were done with degassed solutions of the amines in mixtures of DTBP and benzene; the concentration of DTBP was adjusted to provide an absorbance of ca. 0.4 (path length 1 cm) at the excitation wavelength. A strong permanent coloration of the solutions was observed after photolysis such that a fresh solution had to be used for each flash. Consistent results were obtained with various excitation sources and monitoring systems. Samples were excited a t 353 nm with the frequency tripled output of a Nd glass laser (20 ns, 50 mJ, Basel), at 351 nm with an excimer laser operated on XeF (25 ns, 100 mJ, Basel), at 337 nm with a nitrogen laser (10 ns, 6 mJ, Columbus), or at 308 nm with an excimer laser operated on XeCl (250 mJ, 25 ns, Basel). Transient absorption spectra were obtained by using the digital spectrographic equipment (Basel) described
0022-3654187 12091-2293%01.50/0 , 0 1987 American Chemical Society ,
I
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
2294
t
Leyva et al.
A(t,400nm)
500
LOO
600
700
0.2
J
0.3
o.2
1
O.li
laser pulse
/
0.1
Y
-
0
t IKS
0
S/pm-' t-I
0 1 2 3 Figure 1. Kinetic trace and dual exponential fit with residuals of the absorbance changes measured at 400 nm upon 351-nm flash photolysis of DTBP ( 2 5 ~01%)in the presence of 1.8 X lo-* M aniline in degassed benzene solution at 20 O C .
previous1y.I The transient kinetics were determined at ambient temperature (ca. 20 "C) using a Textronix AD 7912 digitizer and were analyzed by least-squares fitting to the appropriate rate laws (see text). Open-shell PPP S C F SCI calculations were performed by the method of Zahradnik and CBrsky5using standard parameters and assuming planar, idealized structures (bond lengths 140 pm, regular polygon bond angles) throughout. The inductive effect of the aminyl nitrogen atoms was accounted for by an increased valence state ionization potential of ZN = Zc 2 eV. Up to 44 singly excited configurations ( 1 6 each of type A and B, 1 1 6 each of type C, and Cg)5were included in the CI calculations.
(5) Z+hradnk, R.;&sky, P.J . Phys. Chem. 1970, 74, 1235. &sky, P.; Zahradnik, R. Ibid. 1970, 74, 1249. (6) Stewart, L. C.; Carlsson, D. J.; Wiles, D. M.; Scaiano, J. C. J . Am. Chem. SOC.1983, 105, 3605. (7) Sample analyses using a dual exponential rate law (see, e.g., the fitted curve shown in Figure 1) gave growth rates koM which were nearly identical with those obtained by eq 1.
"
'
I
"
"
I
'
-
600
500
400
700
1.3 1
t
0.5 '.O
+
Results Laser flash photolysis of degassed mixtures of DTBP and benzene did not produce any transient absorptions in the visible region. In the presence of arylamines, the laser pulse initiated a well-resolved growth of transient absorptions on a time scale of to s, depending on the concentration of the added amines to IO4 M, cf. e.g., Figure 1). The subsequent decay of these transients obeyed second-order kinetics but was slow on the time scale of the initial growth and was not examined in detail. Most of the added arylamines did not absorb at the laser excitation wavelengths. Therefore, the delayed formation of the transient intermediates must be attributed to a reaction of the tert-butoxy radicals, formed by flash photolysis of DTBP, with the arylamines. As mentioned above, the most likely course of this reaction is hydrogen abstraction from nitrogen to yield aminyl radicals. The absorption spectra of the naphthylamines and of 1-pyrenylamine did not allow for the exclusive excitation of DTBP. In the absence of DTBP, excitation of these amines gave rise to transient triplet-triplet absorptions in the visible region ( T 2 100 ps). However, DTBP is an efficient triplet quencher: and these triplet-triplet absorptions did not interfere in the solutions containing DTBP (I 10% volume). The growth of the transient intermediates was found to obey first-order kinetics, and the observed rate constant increased linearly with the concentration of amine present in the solution. Since the slow decay of the transients was insignificant on the time scale of their f ~ r m a t i o nthe , ~ kinetic traces were analyzed by least-squares fitting to the simple single exponential relation In [ A , / ( A , - A , ) ] = kobd (1) where A , is the transient absorbance measured at the plateau
'
2.5 2.0 1.5 Figure 2. Transient absorbance of phenylaminyl in benzene/DTBP.
I
,
Alnm
,
!
I
,
,
,
I
,
,
,
,
(
,
2.5 2.0 1.5 Figure 3. Transient absorbance of 1-naphthylaminyl in benzene/DTBP. 1.o
400
500
600
I
I
I
700 I
Alnm
0.6
i \
0.4 0.2-
V1pm-l c--
Laser Flash Photolysis of Di-tert-butyl Peroxide
400 I."
0.8
-I
500
I
It,
600
700
.
. . .I
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2295 600
500
400
,
1.0
I
Alnm
0.8-
0.6
0.6 -
0.4
0.4-
0.2
0.2 -
700
f--
2.0
2.5
o
1.5
Figure 5. Transient absorbance of diphenylaminyl in benzene/DTBP. 1.5
400
500
600
700
I
1
1
I
1.2
0.9 0.6 0.3 0
I
,
I
"
"
2.5
I
"
"
I
2.0
'
1.5
Figur i. Transient absorbance of carbazolyl in benzene/DTBP. 0.25
400
500
600
I
I
I
700 I
v hlnm
0.2 0.15 0.1 0.05
0
I
,
I
2.5
"
"
I
2.0
"
"
I
,
1.5
Figure 7. Transient absorbance of indolyl in benzene/DTBP. of the corresponding kinetic curves, i.e. with appropriate delays in the range of 2 to 10 ~s with respect to the laser pulse. The raw spectra were converted to absorbance changes by using the spectral distribution of the monitoring light prior to the laser pulse as the reference light intensity at each wavelength. If we ignore so"minor tailing of the DTBP absorption (which does not change significantly upon photolysis by a single laser flash), the solutions were transparent prior to photolysis in the spectral regions displayed. These spectra therefore correspond to absorbances of the transients formed by the reaction of tert-butoxy with the arylamines. The shape of these spectra did not change noticeably during the period of their buildup, Le., between the laser pulse and the plateau region.
Discussion A straightforward method to produce arylaminyl radicals is the direct flash photolysis of arylamines as described by Porter
!
a
4
1
25
8
t
~
8
20
1
n
~
r
I
,
15
Figure 8. Transient absorbance of 1-pyrenylaminyl in benzene/DTBP. and co-workers.* The present method, flash excitation of DTBP in the presence of arylamines, is equally simple in practice and has several advantages; there appear to be no side reactions, the radical yields are usually much higher, and the direct monitoring of the reaction of the tert-butoxy radicals with the arylamines (Figure 1) provides evidence both for the origin of a given transient and for the reactivity of the amines. A disadvantage is the strong absorption of DTBP below 350 MI which prohibits the observation of transient absorptions beyond this wavelength. As far as the data overlap, the arylamine radical spectra obtained in this work (Figures 2-8) are in most cases in accord with previous work. For phenylaminyl (Figure 2) Land and Portersa found a strong, narrow band at 308 nm (observed earlier at 301 nm in the gas phase)8band an additional, weak, and diffuse band around 400 nm. The fluorescence emission and excitation spectra of phenylaminyl reported recently by Smirnov et aL9 have confirmed, on the basis of Kasha's rule, that the 400-nm band corresponds to the first electronic transition(s) of this species. Our spectrum of 1-naphthylaminyl (Figure 3) agrees well with that of Land and Porter? whereas the strong band at 373 nm we observe for 2-naphthylaminyl (Figure 4)is missing entirely in their spectrum. The only prominent feature of their spectrum is a broad peak at 400 nm which appears weakly in our spectrum. Transient spectra taken immediately after the laser flash show that the 400-nm peak is formed by direct excitation of 2-naphthylamine and not be the delayed reaction with the tert-butoxy radical. Thus it appears that in this case different species are formed by the two methods. Based on its mode of formation and on the comparison with the absorption spectrum of the 2-naphthylmethyl radical (vide infra) we prefer to associate the transient shown in Figure 4 with 2-naphthylaminyl. The characteristic spectra of diphenylaminyl (Figure 5 ) and of carbazolyl (Figure 6 ) are in excellent agreement with previous A stable tetra-tert-butyl derivative of the latter has been synthesized and fully ~haracterized.'~The spectrum of indolyl (Figure 7) is similar to that reported for the tryptophanyl radical by Santus and Gros~weiner.'~ The general similarity in the absorption spectra of the benzyl radical and its wisoelectronic derivatives phenylaminyl (anilino), phenoxy, and triplet phenylnitrene has been noted by previous workers: the energies of the electronic transitions are hardly affected, but the intensity of thefirst transition, which is very (8) (a) Land, E. J.; Porter, G. Trans. Faraday SOC.1963, 59, 2027, and references therein. (b) Porter, G.; Wright, F. J. Trans. Faraday SOC.1955, 51, 1469. Cf. Porter, G.; Ward, B. J . Chim. Phys. 1964, 1517. (9) Smimov, V. A.; Brichkin, S. B.; Efimov, S . P. Khim. Vys. Energ. 1984, 18, 60. (10) Lewis, G. N.; Lipkin, D. J . Am. Chem. SOC.1942, 64, 2801. (11) Wiersma, D. A.; Kommandeur, J. Mol. Phys. 1967, 13, 241. (12) Shida, T.; Kira, A. J . Phys. Chem. 1969, 73, 4315. (13) Neugebauer, F. A.; Fischer, P. H. H. Chem. Ber. 1965, 98, 844. (14) Neugebauer, F. A,; Fischer, H.; Bamberger, S . ; Smith, H. 0. Chem. Ber. 1972, 105, 2694. (15) Santus, R.; Grossweiner, L. I. Photochem. Photobiol. 1972, 15, 101. Cf. Creed, D. Ibid. 1984, 39, 537.
l
2296 The Journal of Physical Chemistry, Vol, 91, No. 9,1987
Leyva et al.
TABLE I: Summary of Observed Absorption Bend Maxima and PPP Calculations’ of Arylaminyl Radicals and dsoelectronic Systems
first AT* transition exptl calcd X/nm (intIc A/nm (Od
exptl X/nm (int)c
calcdb X/nm (Od
283 (0.06) 281 (0.05)
8, e
482 (0.002)
316 (m) 308 (m) 314 (m)
281 (0.05)
1, 16
595 (v) 580 (w) 540 (w)
571 (0) 589 (0.01) 589 (0.01)
370 (m) 376 (m) 365 (m)
307 (0.14) 333 (0.12) 333 (0.12)
20-22 8, e 2, 16
600 (v) 640 (w)
556 (0) 603 (0.02)
384 (m)
336 (0.11) 355 (0.07)
355 (0.07)
20, 22 e 2
424 (m) 415 (m)
382 (0.44) 400 (0.33) 400 (0.33)
e 2, 23
452 (v) -400 (w) -400 (w)
benzyl
phenylaminyl phenylnitrene 1-naphthylmethyl
1-naphthylaminyl 1-naphthylnitrene 2-naphthylmeth yl
2-naphthylaminyl 2-naphthylnitrene
second AT* transition
440 (0)
482 (0.002)
603 (0.02)
1-pyrenylmethyl
770 (0)
1-pyrenylaminyl 1-pyrenylnitrene
793 (0.01)
793 (0.01)
indenyl
373 (m) 365 (m)
ref 19
indolyl
1053 (0) 1504 (0.01)
415 (w) 515 (w)
463 (0.04) 541 (0.06)
24 e, 15
fluorenyl carbazolyl
725 (0.001) 1029 (0)
500 (m) 610 (m)
478 (0.03) 570 (0.09)
25 12, 13, e
345 (m) 410 (m)
302 (0.47) 304 (0.33)’
26 10-13, 4
522 (v) 760 (m)
diphenylmethyl diphen ylaminyl
489 (0) 528 (0.05)’
Cf. vperimental Section. ’bThe first allowe AT* transition is given for alternant hydrocarbons (and their isoelectronic derivatives). cExperimental band intensities are characterized as very weak (v), weak (w), or medium (m); as extinction coefficients are usually not available. Calculated band intensities are characterized by the oscillator strength f. CThiswork. ’These calculations refer to planar diphenylaminyl. The discrepancy with the observed absorption bands is discussed in the text.
-
weak in the benzyl radical (log E < 2), increases considerably upon heteroatom substitution (log E 3).8J6 Such behavior ispredicted from theory for isoelectronic derivatives of neutral alternant hydrocarbons and is well-known and extensively documented for closed-shell systems, cf., e.g., the absorption spectra of naphthalene and 1- or 2-quinoline. Within the framework of PPP SCF CI calculations, neutral alternant hydrocarbons (including radicals) have an even 7r-charge distribution (s;= 1) in all electronic states.” Therefore, electronic transition energies should be little affected by inductive perturbation upon heteroatom substitution.I8 On the other hand, the parity rules” are lost upon inductive perturbation leading to increased intensity of symmetry-allowed, but parity-forbidden electronic transitions. The above-mentioned rules appear to hold very well for all the other alternant-hydrocarbon derived aminyl radicals studied in this work, except for diphenylaminyl which will be discussed below. In order to facilitate a comparison, the experimental wavelengths and intensities of the first “forbidden” and the first “allowed” AT* transitions of related systems have been collected in Table I,’92vs312-16,19-26 together with the results of open-shell PPP SCF SCI calculations. The calculations predict several (usually three) parity-forbidden transitions at longer wavelengths than the first allowed transition. Only the first of these is noted in Table I, since the others cannot usually be located with any confidence in low resolution absorption spectra. As expected, considerable wavelength shifts are found upon heteroatom substitution in the nonalternant hydrocarbons indenyl and fluorenyl, since here electronic excitation is accompanied by a change in the charge distribution. The observed shifts are, however, reliably predicted by explicit
PPP calculations. It is noteworthy that fluorescence emission has been observed from the indenyl radical at 415-500 nmZ4and from triplet pyrenylnitrene at 420-500 111111.~~ According to our calculations (Table I), the first electronic transitions of these species lie at much longer wavelengths. However, these violations of Kasha’s rule, if authentic, are readily explained by the large energy gap (>lOOOO cm-’) between the first and second excited states in these systems.27 We now turn to the unusual absorption spectrum of the diphenylaminyl radical, which is totally different from that of diphenylmethyl and appears to contradict the rules mentioned above for alternant systems. The first observation of diphenylaminyl dates back to the pioneering matrix photolysis work of Lewis and co-workers.lo Twenty years ago Wiersma and Kommandeur noted that the long wavelength absorption band generated by the photolysis of tetraphenylhydrazine was incompatible with semiempirical calculations for diphenylaminyl, I a conclusion which seems to be forcefully supported by the extensive set of related data available at present. They attributed the 760-nm band to an intermolecular charge-transfer band of the radical pair which is formed upon photolysis of tetraphenylhydrazine in the rigid matrix. However, Shida and Kira have since shown that the same absorption band is observed with isolated diphenylaminyl radicals,12 and this is confirmed by the present work which provides a different bona fide source for the same species.28 What is the way out of this dilemma? Obviously, the usual relationships between the spectra of isoelectronic radicals are not expected to hold, if the ground-state equilibrium geometry is changed significantly by heteroatom
(16) Reiser, A.; Bowes, G.; Horne, R. J. Trans. Faraday SOC.1966, 62, 3162. See also ref 1. (17) McLachlan, A. D. Mol. Phys. 1959, 2, 271. (18) Julg, A. J . Chim. Phys. 1955, 52, 377. (19) Huggenberger, C.; Fischer, H. Helv. Chim. Acra 1981, 64, 338. (20) Porter, G.; Strachan, E. Trans. Faraday SOC.1958, 54, 1595. (21) Watts, A. T.; Walker, S . Trans. Faraday SOC.1964, 60, 484. (22) Hilinski, E. F.; Huppert, D.; Kelley, D. F.; Milton, S. V.; Rentzepis, P. M. J . Am. Chem. SOC.1984, 106, 1951. (23) Sumitami, M.; Nagakura, S.; Yoshihara, K. Bull. Chem. SOC.Jpn. 1916, 49, 2995. (24) Izumida, T.; Inoue, K.; Noda, S.; Yoshida, H. Bull. Chem. SOC.Jpn. 1981, 54, 2517. (25) Griller, D.; Hadel, L.; Hazran, A. S.; Platz, M. S.; Wong, P. C.; Savino, T. G.; Scaiano, J. C. J . Am. Chem. SOC.1984, 106, 2227. (26) Bromberg, A.; Meisel, D. J . Phys. Chem. 1985, 89, 2507.
(27) Although the calculations for I-pyrenylaminyl predict two further parity-forbidden transitions at longer wavelengths than the first allowed transition, the calculated energies of these are very close to the latter. Thus the observed band at 424 nm may in fact correspond to a transition to the second excited state of the I-pyrenylaminyl radical. (28) A referee has suggested that the transient observed might be the radical cation of diphenylamine. Although the formation of long-lived ionic species in a relatively apolar medium seems unlikely, this hypothesis cannot be rigorously excluded on the basis of our results, since a protic cosolvent (DTBP) was present in all our experiments. However, it can be ruled out for the species with the same electronic spectra observed by the photolysis of tetraphenylhydrazine in benzene solution,’* and by the reversible thermal dissociation of tetrakis@-toly1)hydrazine in dry xylene in the temperature range of 40 to 90 OC.” The resulting his@-tolylaminyl) radical (A,, = 735 nm) has been identified by ESR spectroscopy.
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2291
Laser Flash Photolysis of Di-tert-butyl Peroxide substitution. This is clearly a possibility to be considered for the diphenylmethyl-diphenylaminyl pair, where the radical centers are not part of a ring. Consider a conformation of diphenylaminyl in which the two phenyl groups are orthogonal to each other (1);
@+
H&H 1
2
(I
log (k/M-‘
5-‘
1
9.0
3
4 5 such a structure would allow for conjugative interaction of the nitrogen lone pair with one phenyl group, and of the odd electron in the nitrogen p-orbital with the other phenyl group. Diphenylaminyl radicals with structure 1 should exhibit the absorption bands of the two orthogonal chromophores related to aniline and phenylaminyl. In addition, we may expect interchromophore charge-transfer transitions, the lowest in energy corresponding to the transfer of an electron from the anilke moiety to the phenylaminyl moiety. Indeed, the well-known A 2Al-X 2B1electronic transition of parent aminyl (amidogen, 2), which is observed throughout the visible range ( 9 W is related in character (p, l b l n 3aJ. The energy difference between the ground state and the excited charge-transfer state of 1 must be very sensitive to the C-N-C angle, since the two states merge to a degenerate state when this angle approaches 180°, structure 3 (for a calculated potential energy surface for NH2 as a function of the H-N-H angle cf. e.g. Figure 4 of ref 30). Thus, from the exceptional and striking discrepancy between the spectra of the diphenylmethyl and -aminyl radicals we are led to propose that their structures are significantly different in the electronic ground state, both phenyl *-systems retaining considerable overlap with the radical center in the former but not in the latter. Some experimental support for this hypothesis comes from the observation that the electronic spectra of diphenylmethyl and 10,lldihydrodibenzo[a,~cyclohept-5-y1(4) are quite similar,% whereas those of diphenylaminyl and acridanyl (5) differ considerably.’* On the other hand, Danen and Neugebauer have argued that structures such as 1 or 3 are excluded by the observation of symmetrical spin density distributions in diphenylaminyl radicals.31 However, this fact could also be accommodated by assuming that the rotation of the phenyl groups is fast with respect to the ESR time scale. It would be difficult to obtain a reliable prediction for the lowest energy structure of diphenylaminyl from theoretical calculations. Structure 3 of DU symmetry is ruled out by the Jahn-Teller theorem. Semiempirical CNDO CI calculations kindly performed by H. Baumann, ETH Zurich, fully confirm the qualitative prediction of a long-wavelength, intramolecular charge-transfer transition (p, loa” n 22a’) in a radical of structure l.32As expected, the energy calculated for this transition is very sensitive to the C-N-C angle.
-
-
(29) Herzberg, G. Molecular Spectra and Molecular Structure, Vol. 3, ‘Electronic Spectra and Electronic Structure of Polyatomic Molecules”, Van Nostrand: New York, 1966. Jungen, Ch.; Merer, A. J. Mol. Phys. 1980,40, 1 . Jungen, Ch.; Hallin, K. E. J.; Merer, A. Ibid. 1980, 40, 25. (30) Buenker, R. J.; Peric, M.; Peyerimhoff, S. D.; Marion, R. Mol. Phys. 1981, 43, 987. (31) Danen, W. C.; Neugebauer, F. A. Angew. Chem. 1975, 87, 823. Angew. Chem., Znt. Ed. Engl. 1975, 14, 783. (32) Program DOCNDUV (1984) by H. Baumann, based on the RHF theory of R ~ t h a a n and ’ ~ using the standard parameter set of JafE.” Bond lengths were chosen as 108 (C-H), 101 (N-H), 144 (C-N), and 140 pm (C-C); all bond angles (except C - N C ) were taken as 120O. A total of 50 singly and doubly excited doublet configurations were included for CI. The first charge transfer transition was predicted to lie at 860 nm,f = 0.0004 for a C-N-C angle of 150’ and at 387 nm,f= 0.34 for 120’.
0.9 1.0 1.1 1.2 1.3 1.4 1.5 Figure 9. Logarithmic plot of the absolute rate constants k/M-’ s-’ for hydrogen abstraction from arylamines by tert-butoxy vs. structure count ratios (SCR)34for arylmethyl radical/arene.
400
500
600
700
r
A Inm
0.4 -
0,3: 0.2
\
Figure 10. Transient absorbance of triplet 1-naphthylnitrene observed with a delay of 5 ps after direct excitation of 1-naphthylazide (5 X M) in hexane solution by a laser flash at 308 nm3.2138
The absolute rates of NH hydrogen abstraction by the tertbutoxy radical from the arylamines are close to those for OH hydrogen abstraction from the corresponding phenols,3sand about two orders of magnitude above those for CH hydrogen abstraction from the corresponding arylmethyl compound^.)^ The rates increase with increasing resonance stabilization of the resulting arylaminyl radicals and approach the diffusion-controlled limit in highly conjugated systems. A logarithmic plot of reaction rates vs. the difference in resonance energy as estimated by Herndon’s structure count ratios (SCR)37is shown in Figure 9. The present study was initiated in the course of our work on triplet arylnitrene intermediates. Since the assignment of, e.g., triplet phenylnitrene as a transient intermediate has been a matter of some controversy,1*2we hoped to obtain some independent guidelines to predict the positions and intensities of the absorption bands of triplet arylnitrenes from a comparison with authentic, *-isoelectronic arylaminyl radicals. It turns out that such a close relationship indeed exists in many cases, cf. e.g. the absorption spectra of 1-naphthylaminyl (Figure 3) and of 1-naphthylnitrene (Figure 10). However, there are two possible complications which must be kept in mind. First, the relationship pertains only to m* transitions. Thus the similarity in shape, intensity, and position of the two absorption bands near 300 and 400 nm observed both (33) Roothaan, C. C. J. Rev. Mod. Phys. 1960, 32, 179. (34) Ellis, R. L.; Kahnlenz, G.; Jaffi, H. H. Theoret. Chim. Acta 1972, 26. _ ,131. . _
~
(35) Das, P. K.; Encinas, M. V.; Stecnken, S.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 4162. (36) Lissi, E. A.; Collados, J.; Olea, A. Znt. J . Chem. Kiner. 1985, 17,265. (37) Herndon, W. C. J . Org. Chem. 1981, 46, 2119. (38) Reiser, A.; Bowes, G.; Horne, R. J. Trans. Faraday SOC.1966, 62, 3 162.
2298
J . Phys. Chem. 1987, 91, 2298-2303
with phenylaminyl and with triplet phenylnitrene lends support to our previous proposal that the additional weak bands of the latter in the range of 400 to 500 nm are due to n ?r* and/or ?r n transitions.’ Second, the relationship is expected to hold only as long as the ground-state geometries of two isoelectronic species do not differ markedly (cf. the case of diphenylmethyl and diphenylaminyl discussed above).
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Acknowledgment. This work is part of Project 2.21 3-0.84 of the Swiss National Science Foundation. Financial support was
received from Ciba-Geigy SA,Hoffmann-La Roche SA, Sandoz SA, and the Ciba-Stiftung. M.S.P. is indebted to the International Division of the US NSF for a travel grant. Registry No. H2, 1333-74-0; tert-butoxy, 3141-58-0;phenylaminyl, 2348-49-4; 1-naphthylaminyl, 93439-96-4; 2-naphthylaminyl, 9343997-5; diphenylaminyl, 2143-67-1; carbazolyl, 83045-62-9; indolyl, 50614-06-7; 1-pyrenylaminyl, 107099-16-1; aniline, 62-53-3; 1naphthylamine, 134-32-7; 2-naphthylamine, 91-59-8; diphenylamine, 122-39-4;carbazole, 86-74-8; indole, 120-72-9; 1-pyrenylamine,160667-3.
Charactedzatlon of the Exclted States of Photoreactive Dlhydrophenazine Doped in Molecular Crystals B. Prass, C. von Borczyskowski,* P. Steidl, and D. Stehlik Fachbereich Physik, Freie Universitat Berlin, D-1000 Berlin 33, West Germany (Received: October 10, 1986)
Chemically unstable dihydrophenazine can be stabilized by substitution into molecular crystals. The structure of ground and excited states of dihydrophenazine doped in fluorene crystals has been investigated by optical and EPR spectroscopy. The lowest excited singlet and triplet states reveal a planar structure whereas the ground state is nonplanar.
1. Introduction A substantial amount of literature is available concerning the physical and photochemical properties of various 5,lO-dihydrophenazine (PH2) derivatives in solution and glass matrices.’” They are known for the ease with which they transfer an electron and have been used as electron donors for the studies of electron-transfer mechanisms: Particularly the photoinduced ionization found to be the most active photochemical channel has been widely investigated. Moreover, the oxidation-reduction characteristics of PH2 derivatives are of biological importance, as they play a role in biological electron transport systems.’-I0 Ideally, the investigations should start with the simplest molecule in the series of PHI derivatives, i.e., PH2itself. However, PH2 has been found very difficult to handle due to its extreme instability in air, where it oxidizes very rapidly to phenazine. Consequently, only very few data exist on the photochemical properties and even less on the spectroscopic properties of PH, itself. To our knowledge, only Bailey et al.” and Wheaton et a1.12 have published UV absorption and IR spectra of PH2. In some solid-state matrices, however, we found that PH2 is effectively stabilized in the ground state. This offers the possibility to investigate PHI and elementary photochemical reactions of PH2, which presumably start with an electron transfer as the primary reaction steps6 Moreover, due to the fixed geometry of reactants (1) Rubasowska, W.; Grabowski, Z. R. J. Chem. SOC.,Perkin Trans. 2 1975,417.
(2) BriIblmann, U.; Huber, J. R. J . Phys. Chem. 1977,82, 386. ( 3 ) Ruaseger, P.;Moms, J. V.; Huber, J. R. Chem. Phys. 1980, 46, 1. (4) Nelson, R. F.; Leedy, D. W.; Seo, E.T.; Adams, R. N. Fresenius’ Z . Anal. Chem. 1966, 224, 184. ( 5 ) Chalvet, 0.;Jaffe, H.H.; Rayez, J. C. Photochem. Phorobioi. 1977, 26, 353. (6) Keller, H. J.; Soos, 2.G. In Topics in Current Chemistry; SpringerVerlag: Wcat Berlin, 1985; Vol. 127, p 169. (7) Halaka, F.G.; Barnes, Z. K.; Babcock, G. T.; Dye, J. L.Biochemistry 1%4, 23, 2005. (8) Low, A.; Vallin, H.; Vallin, I. Biochem. Blophys. Acto 1%3,69, 361. (9) Michaelis, K.; Schubert,L.; Schubert, M.P. Chem.Rev.1938,22,253. (10) Zaugg, W. S . J. Biol. Chem. 1964, 239, 3964. (11) Bailey, D. N.; Roe, D. K.; Hercules, D. M. Appl. Spectrosc. 1968, 22, 185. (12) Wheaton, G. A.; Stoel, L. J.; Stevens, N. B.; Frank, C. W. Appi. Spectrosc. 1970, 24, 339.
and products, PH2-doped molecular crystals can be regarded as ideal model systems for such reactions. The primary intention of this publication is to give the first detailed report on the optical excitation and emission spectra together with the excited-state lifetimes as well as EPR results for the excited triplet state of PHI doped in fluorene single crystals. Although not in the same detail, spectroscopic data of PH2 in the host crystals biphenyl, dibenzofuran, carbazole, and dibenzothiophene will be presented here. One question of major importance is the geometry of PH2 in the ground and excited states since it is expected to have a strong effect on the photochemical and spectroscopic properties of PH2. If PH2 is planar, the nitrogen “lone pair” orbitals would still participate in the conjugation of the whole ring system. On the other hand, if PH2 is bent along the N-N axis, the conjugation is interrupted a t the N positions and the molecule would be diphenylamine-like.’3 Photoinduced chemical reactions originating from PH2 could be observed in all host crystals which we have investigated. Depending on the host, these reactions can probably be described as a sequence of electron-, proton-, and/or hydrogen-transfer processes. The corresponding data will be published in detail separately.I4J5 2. Materials The structure of PH2 as well as the molecular axes notation used is shown in Figure 1. For easier identification the short notation for phenazine (P) is supplemented with the substituents hydrogen (H) or deuterium (D) in capital letters for the 5- and/or 10-positions of PH2, while the outer ring substituents are given in small letters if required. Selectively deuteriated fluorene host material was prepared as described earlier.I6 Single crystals were grown from the melt, which was doped with 2000 ppm of the guest molecule. PH2 was (13) Adams, J. E.;Mantulin, W. W.; Huber,J. R. J. Am. Chem. Soc. 1973, 95, 5417. (14) Prass, B.;von Borczyskowski, C.; Stehlik, D., to be published. (1 5 ) Steidl, P.; von Borczyskowski, C.; Fujara, F.; Prass, B.; Stehlik, D., submitted for publication in J . Chem. Phys. (16) Furrer, R.; Heinrich, M.;Stehlik, D.; Zimmermann, H.Chem. Phys. 1979, 36, 27.
0022-3654/87/2091-2298$01.50/00 1987 American Chemical Society
