of H atom tunneling transfer. For the CHC, transfer of the H atom proceeds to a greater distance than for the DPC, and the height and width of the potential barrier are somewhat less. These data indicate a higher activity of the cyclohexadienone carbene in the reaction of H atom abstraction from the matrix in comparison with diphenylcarbene, possibly due to the difference in electronic structure of these carbenes.

CONCLUSIONS

i. In the photodecomposition of p-benzoquinonediazide in a frozen matrix of piperylene oligomer, the formation of triplet cyclohexadienone carbene has been registered by means of ESR, along with allyl radicals of the matrix and phenoxyl radicals in the isolated state and in the form of radical pairs.

2. When the carbene interacts with the matrix, it is inserted at C-H bonds of the macromolecules.

3. A tunneling mechanism of H atom transfer has been proposed in the reaction of the carbene with the matrix. Estimates of the height and width of the potential barrier of the reaction are given.

LITERATURE CITED

i. R. W. Murray, A. M. Trozzolo, E. Wasserman, and W. A. Yager, J. Am. Chem. Soc., 84, 3213 (1962).

2. A. P. Vorotnikov, E. Ya. Davydov, G. B. Pariiskii, andD. Ya. Toptygin, Khim. Fiz., No. 6, 818 (1983).

3. A. P. Vorotnikov, E. Ya. Davydov, and D. Ya. Toptygin, Izd. Akad. Nauk SSSR, Ser. Khim., 1499 (1983).

4. E. Wasserman and R. W. Murray, J. Am. Chem. Soc., 86, 4203 (1964). 5. B. S. Kikot', Zh. Obshch. Khim., 33, 227 (1963). 6. E. J. Land, G. Porter, and E. Strachan, Trans. Faraday Soc., 57, 1885 (1961). 7. V. I. Gol'danskii, Dokl. Akad. Nauk SSSR, 124, 1261 (1959). 8. S. I. Kuzina and A. I. Mikhailov, Dokl. Akad. Nauk SSSR, 231, 1395 (1976). 9. V. L. Klochikhin, S. Ya. Pshezhetskii, and L. I. Trakhtenberg, Vysokomol. Soedin.,

A21, 2792 (1979). i0. E. R. Lippincott and R. Schroeder, J. Chem. Phys., 23, 1131 (1955).

MECHANISM OF PHOTODISSOCIATION OF N-H BOND IN AROMATIC AMINES

Ya. N. Malkin and S. P. Makarov UDC 541.124:541.141.7:541.57:547.551

Photoexcitation of a series of aromatic amines - 1,2-dihydroquinolines (DHQ), 1,2,3,4- tetrahydroquinolines (THQ), and 7-azaindolines - leads to photodissociation of the N-H bond and the formation of aminyl radicals and hydrogen atoms [1-4]. The quantum yields in DHQ and THQ photodissociation are increased by factors of 5-12 upon photoexcitation into the short-wave band of the absorption spectrum, the quantum yield of fluorescence dropping off by a factor of 2-3 [5, 6]. Analogous changes in the quantum yields of photodissociation, photoejection of an electron, and fluorescence were observed previously for aniline and indole [7, 8] and were interpreted as photodissociation from higher electronically excited S 3 levels. In that work, the mechanisms of photodisosciation (the mode of excitation energy transfer to the N-H bond and the nature of the photodissociative state) remained obscure. In the present work, the mechanism of DHQ and THQ photodissociation is judged on the basis of a joint analysis of spectral, photochemical, and quantum-chemical data.

Calculations of the excited states of the DHQs and THQs were performed by the CNDO/S method with an account for 40 configuration interactions (TAB was calculated by the formula

Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Institute of Physical and Organic Chemistry, M. A. Suslov Rostov State University. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1282-1287, June, 1985. Original article submitted January 24, 1984.

1i72 0568-5230/85/3406-1172509.50 �9 1985 Plenum Publishing Corporation

~Q

y qg-~cm -~

. . . . ~3 - - ,73

qZ/ -~--- ~z

- - S I

$0 i

"i

DHQ

57{0,~'J - - 5 5 ~ raf6,z'~

fi4 "T7 5 j

. . . . 53

m - - 3

. . . . r~ . . . . . . . . . SN~ _ _ - r z

- - 5

Fig. l. Scheme of positions of lowest electronically excited levels of 1,2- dihydroquinoline and 1,2,3,4-tetrahy- droquinoline. Calculation by CNDO/S method. Continuous lines represent calculation; dashed lines represent experiment. All Sn and Tn levels of the DHQ, other than those specifically indicated in the figure, have a ~,~* character. ENH is the rupture energy of the N-H bond in the ground state.

of Mataga and Nishimoto). The molecular geometry was first calculated by the PPP method in the ~,~ parametrization of Dewar [9, i0].

The calculations of the orbital energies and the electronic structure of the Rydberg states were performed by the extended Huckel method (E~). Values of the exponentials and ionization potentials of the hydrogen atom hound to the nitrogen, selected on the basis of the Slater rules, were 0.6 and -3.399, or 0.4 and -1.51, for the 2S and 3S functions, respectively.

The procedures used in determining the quantum yields of fluorescence and photodissoci- ation and their dependence on the excitation wavelength have been described in [2, 3, 5]~

5 ~o 14 1

8 9 NI

H H DI-IQ " ( I ) - - ( V ) THQ ( v 0 - ( v l n )

R (I) (II) (III) (IV) (u (u (VII) (VIII),

R4 H H H H H H H Ph Re H Et0 OH Me H H OH H R8 H H H H Me H H H

For the unsubstituted DHQ (I) and THQ (VI), the values calculated for the Wiberg in- dexes are listed in Table i, and the charges on the atoms in Table 2. Experimental and calculated values of the energies of the lowest electronic transitions for compounds (I) and (VI) are shown in Fig. 1 (the position of the So ~ S! transition was obtained from the point of intersection of the fluorescence and absorption spectra, plotted on the wavenumber scale; the positions of the S o ~ S 2, S 0 ~ S 3, S o , ~ Sr and S 2 ~ S 4 transitions were ob- tained from the maxima of the corresponding bands in the absorption spectrum [3, ii-13~).

From Fig. I, the difference in photochemical reactivities of the di- and tetrahydro- quinolines [3, 5] now becomes understandable. Whereas the photodissociation of DHQ upon excitation into the long-wave band of the absorption spectrum takes place only from the S~ state [3], the photodissociation of THQ (also upon excitation into the long-wave band) takes place from the T~ state [3, 5]. Obviously this is related to the fact that the energy

1173

TABLE i. Wiberg Indexes in Ground and Excited States of Compounds (I) and (VI)

Bond s0 sl Tt l ~Sa T2 Sz ] T3

I~i-C 2 C2-C 3 C6-C 7 C9_C~0 N,-C 9 N'-H'

0,939 1,005 1,437 1,358 1,038 0,967

"N'-C2 I 0,968 C 2 -C 3 0,993 CG-C 7 t,438 C9-C ~~ t,384 INI-C 9 ] 1,042 Nl=-H 1 0,966

0,935 1,035 1,300 t,115 1,t33 0,96t

0,969 0,992 t,287 1,224 1,09I 0,960

DHQ (I)

0,932 I 0,930 t,036 1,030 1,3i6 1,300 1,175 t,234 t,071 1,063 0,967 �9 0,967

THQ (VI) 0,969 0,971 0,992 0,867 1,217 1,331 1,123 1,069 t,063 t,023 0,966 0,958

0,932 t , 0 t 6 11275 t,168 t ,075 01967

0,969 0,992 1,286 1,228 1,110 0,960

0,932 t ,032 1,249 1,2t7 t,2t4 0,967

0,970 0,989 t,189 t , 0 8 t 1,105

-0,966

0,931 1,023 1,328 1,242 1,!09 0,967

0,968 0,993 -1,345 1,304 1,060 0,966

TABLE 2. Charges on Atoms in Ground and Excited States of Compounds (I) and (VI)

O R atom Se S: T~ S= T2 83 T~

H i C z C 3 C r C 5 C 6 C' C s C 9 Cio

N t H t C 2 C ~ C ~ C 5 C 6 C ~ C 8 C 9 CiO

0,226 0,127

-0,t00 -0,033 -0,051 -0,028 -0,046 -0,024 -0,054

0,093 0,004

-0,309 0,035 0,005

-0,047 -0,020 -0.052 -o1047 -0,042 -0,05I

-0~18

-0,005 0,t27

-0.034 -01136 -0,050 -0,ti1 -0,003 -0,I67 -0,068

0,087 -0,062

-0,165 0,035 0,0t3

-0,025 -0,181 -0,179 -0,062 -0,07t -0,165 0,055

--0,0tt

DHQ (I)

-0,032 I -0,051 0,t27] 0,127

-0,043 I -0,044 -0,2t24 [ 0,037 -0,068 [ O,Oll -0,078 -0,I83 -0,025 -0,154 -0,t52 0,002 -0.054 -0,18t

0,085 -0,057 -0,049 0,024

~Iq (vi) -0,167 0,312

0,035 0,035 0,015 0,t30

-0,023 0,099 -0,180 0,t97 -0;089 -0,t16 -0,105 -0,248 -0,t25 -0,260 -0,078 -0,t12

0,019 0,089 -0,061 -0,137

-0,069 0,t27

-0,049 -0,013 -0,056 -0,t16 -0,t23 -0,027 -0,t36

0,002 0,046

-0,143 0,035

-0,012 -0,030 -0,185 -0,274 -0,0t4 -0,035 -0,254

0,104 0,039

-0,204 0,127

-0,042 -0,105 -0,204 -0,031 -0,090 -0,t03 -0,099

0,t10 0,092

-0,159 0,035 0,012

-0,033 -0,t82 -0,075 -0,t06 -0,147 -0,059

0,056 -0,055

-0,033 0,127

-0,039 .-0,048 -0,066

0,13I -0,092 -0,065 -0,t46

0,04I 0,032

-0,196 0,035

-0,017 -0,015 -0,173 -0,104 -0,103 -0,083 -0,t00 -0,00t -0,057

TABLE 3. Quantum Yields of Photodissociation (~dis) (Forma- tion of Aminyl Radicals), Fluorescence (~fl), and Intercom- binational Conversion (~icc) upon Excitation into Different Absorption Bands of 1,2-Dihydroquinolines and 1,2,3,4-Tetra- hydroquinolines

C0miO~un d r dis

(I) (II) (III) (IV) (V) (VI) (VII) (VIII)

*Benzene. Heptane.

0,011

0,O16 0,0t2 0,11. %

O,OlS,

2oo-~so+ ] ~icc:' ~dis "

- [ 0,02 - 0,014

0,42 0,017 0,15 0,0t2 0,t7 0,20 'd~ >074 t t

>0,22 ~I 0~33~W~ >0,25 :""

$For THQ, light with ~ 300-320 nm, *~%For THQ, light with }( < 280 nm,

5~

0,33 0,39 0,35 0,24 0,t7 0,09 0,09

%%For THQ, values are listed for the quantum yield of phosphorescence at 77~ in 3-methylpentane, representin E the lower limit of ~icc-

1174

E, eV

-10

if'

=~= ~r

-15 =~=

+-

=~= ~ =~=

_+_ -+- -+-

-Z5 =~=

i§ --H--

-Jg

DHQ

anion

Fig. 2. Energies of MOs of 1,2-dihydroquino- line and dihydroquinoline anion (calculation by extended Huckel method).

of the T~ state of the DHQ is less than the dissociation energy of the N-H bond, and photo- dissociation of the N-H bond cannot be accomplished by any such mechanism. At the same time, the energy of the S ! state of the DHQ is sufficient for rupture of this bond.

For the THQ, the energy of the T~ state is sufficient (see Fig. i) for rupture of the N-H bond; therefore, for the THQ, the photodissociation takes place from the TI stateo It should be noted that the quantum yields for the THQs (VI)-(VIII) are considerably greater than the quantum yields for the DHQs (Table 3) with excitation into the long-wave band of the absorption spectrum (even though these yields are almost identical upon excitation into the second band); the apparent reason for this difference is that the photodissociation of R-H bonds, with an appreciable yield, can take place only from a triplet state [13].

Calculations show that for the DHQs and THQs, the lowest electronically excited states (the same as foraniline [ii]) are states of the ~,v* type (in the DHQs, there is no sign of the bands of the n,o* states that had been postulated for substituted anilines in [7]). From the molecular diagrams (see Tables 2 and 3) it follows that, in contrast to aniline with the lowest electronically excited state that mainly corresponds to the B2u state of benzene (the contribution of the charge transfer state amounts to 22%), the lowest singlet state of the DHQs is due mainly to charge transfer from the nitrogen to the benzene ring; in the higher excited states, the fraction of electron transfer is lesso This decrease

1175

I

DHQ anion

H

DHQ in 2S b a s i s §

Fig. 3. Electronic characteristics of anion of 1,2-dihydroquinoline and Rydberg state of DHQ molecule (extended Huckel method).

in the fraction of the state with charge transfer is explained by the previously observed [3, 12] shift of the first and second absorption bands of the DHQ in different directions when there is a change in the polarity of the solvent.

A sharp increase in the quantum yield of photodissociation and a drop in the quantum yield of fluorescence takes place for the DHQs in the region above 5.21 eV (42,000 cm -i) (see Fig. i and Table 3), i.e., in the region of the S 3 transition. As can be seen from the calculation, all of the electronically excited states of the DHQs with energies less than 6.37 eV are delocalized through the entire molecule by ~,~* states; therefore, the follow- ing question arises: How does the energy of the entire system lead to rupture of the N-H bond?

..... There are two different mechanisms of such a process for the DHQs [13, 14]: photo- dissociation from a bound state (nonadiabatic photodissociation) and predissociation. In the first case, the electronic energy that is delocalized through the entire molecule is converted to vibrational energy ofthe individual N-H bond; in the second case, the initially delocalized energy is converted to electronic energy of the individual bond. There is practically no specific evidence favoring one mechanism of photodissociation over another in a condensed phase (with the exception of triphenylmethane [15]). For theDHQ, itcan beseen (Fig. i) that a sharp increase in the photodissociation yield takes place in the region where the locally excited 8-I-3S state of the N-H group is located: 5.71 • 0.5 eV (46,053 cm -i) [16] (@-2p~ orbitalof the NH group).

Thus, with excitation energies below 5.21 eV, dissociation of the N-H bond apparently proceeds through dissociation from a bound state with a nonadiabatic mechanism. Since the rupture energy of the N-H bond in the ground state is 3.46 eV (in aniline), the rate of photodissociation of this bond through the nonadiabatic mechanism must be quite low (for a C-H bond with the same energy, upon excitation by light with an energy of 6.67 eV, the photodissociation rate constant is no greater than 6.3-104 sec -z [13]). When the excitation energy is greater than 5.21 eV, transition of the molecule to the repulsion term of the N-H bond becomes possible, with photodissociation through the predissociation mechanism [17].

The orbital nature of the state from which the dissociation of DHQs and THQs takes place (and in general, that of aromatic amines) with large excitation energies (greater than 4.1 eV) is not clear at the present time.

Getoff [7] suggested that upon excitation of aniline into the S 2 state, there is a transition from the Rydberg potential surface to photodissociation from the Rydberg state; for certain anilines, in the gas-phase absorption spectra, bands have actually been ob- served between the S 1 and $2(~, ~*) bands, which were attributed to transitions to the Rydberg state [18, 19]. Although Rydberg states in the gas phase are usually located at a very high level, it can be assumed that in a condensed phase their energy will be lowered to 4-5 eV [20] (the Rydberg states must be very highly solvated by the solvent).

1176

The analysis performed in the present work to determine the MO energies, the coefficients of the AOs and MOs, the charges on the atoms, the electron densities in the bonds, and the excitation energies, obtained by the extended Huckel method (Figs. 2 and 3), Showed the following: i) Both the 2S and 3S AOs of the hydrogen atom lie substantially higher than the HOMO (by 12.42 and 12.53 eV, respectively); 2) there is essentially no overlap of these AOs with other MOs of the DHQ molecules; 3) the distribution of charges and electron densi- ties is analogous to the electronic characteristics of the DHQ anion (see Fig. 3); 4) the electron density between the nitrogen and hydrogen atoms is very low and negative in sign, corresponding to strong attractive interaction of these atoms. The charge on the hydrogen atom is +i. The characteristics that we have named in this paragraph are preserved in each of the 21 transitions calculated, with energies up to 16.42 eV. Thus, on the basis of these calculations, it is extremely improbable that Rydberg states will participate in the dissocia- tion of the N-H bond. Moreover, dissociation through Rydberg states should lead to the formation of an anion-proton pair, not a pair of radicals.

Along with the increase in the quantum yield of the reaction as the excitation energy is increased, another matter that becomes understandable from Fig. 1 is the appearance of the Ts(o, v*) state specifically in the region of 5.8 eV~ The energy gap between the terms S s and Ts is extremely small (0.01 eV); moreover, these terms are different in nature, corresponding to ~,~* and o,v* types. Therefore, the intercombinational conversion S~T8 can proceed quite efficiently, even 'in comparison with the conversion S1~e T~ (the distance between which is 0.08 eV, and which are identical in nature). Consequently, excitation into the short-wave band of the DHQ may lead to effective occupancy of the locally excited T~(o, v*) state, from which a new channel of photodissociation (predissociation) is opened up, leading to an increase in the quantum yield, incomparison with excitation into the long- wave band. It should be noted that the quantum yields of photodissociation upon excitation into the short-wave band are approximately identical for the DHQ and THQ (see Table 3), even though the quantum yields for light with a longer wavelength differ severa!fold; this may be related to the identical nature of the dissociative state in these two classes of compounds.

CONCLUSIONS

The increase in yield in the photodissociation of 1,2-dihydroquinolines and 1,2,3,4- tetrahydroquinolines with increasing excitation energy is related to the appearance of a new channel for photodissociation -- through the Ts(o, v*) state.

LITERATURE CITED

I0 ii 12 13 14 15

16. 17. 18. 19. 20.

i. Ya. N. Malkin and T. D. Nekipelova, Kinetics of Physicochemical Reactions [in Russianj, Chernogolovka (1980), p, 26.

2. T. D. Nekipelova, Ya. N. Malkin, and V. A. Kuz'min, Izd. Akad. Nauk SSSR, Sero Khim., 80 (1980).

3. Ya. N. Malkin, N. O. Pirogov, and V. A. Kuzmin, J. Photochem., 26, 193 (1984)~ 4. Ya. N. Malkin, A. S. Dvornikov, V. A. Kuz'min, and L. A. Yakhontov, Izd. Akad. Nauk

SSSR, Ser. Khim., 2466 (1982). 5. N. O. Pirogov, Ya. N. Malkin, and V. A. Kuz'min, Dokl. Akad. Nauk SSSR, 264, 636 I1982)~ 6. Ya. N. Malkin, A. S. Dvornikov, N. O. Pirogov, and V. A. Kuz'min, Sun, aries of Pro-

ceedings of 26th Conference on Luminescence [in Russian], Khar'kov (1982), p. 62~ 7. G. Kohler and N. Getoff, J. Chem~ Soc., Faraday Trans. i, 76, 1576 (1980). 8. J. Zechner, G. Kohler, N. Getoff, and J. Tatischeff, J. Photochem., 2, 304 (1978)~ 9. V. I. Minkin, B~ Ya. Simkin, L. E. Nivorozhkin, and B. S. Luk'yanov, Khim.

Geterotsikl. Soedin., 67 (1974). V. I. Minkin and B. Ya. Simkin, Int. J. Sulf. Chem. 3_A, 312 (1973)o K. Kimura, H. Tsubomura, and S. Nagakura, Bull. Chem. Soc. Jpn., 3__77, 1336 (1964). L. P. Zalukaev and R. P. Vorob'eva, Khimo Geterotsiklo Soedin., 293 (1968). V. G. Plotnikov and A. A. Ovchinnikov, Usp. Khim., 47, 444 (1978)0 V. A. Smirnov and M. V. Alfimov, Usp. Nauchn. Fotogr., 19, 166 (1978). V. A. Smirnov, V. G. Plotnikov, Yu. A. Zav'yalov, and M. V. A!fimov, Opt. Spektrosk., 3__3, 230 (1972). H. Lami, J. Chem. Phys., 67, 3274 (1977). V. G. Plotnikov, Opt. Spektrosk., 27, 596 (1969). H. Tsubomura and T. Sakata, Chem. Phys. Lett., 21, 511 (1973). K. Fuke and S. Nagakura, J. Mol. Spectrosc., 64, 139 (1977). Y. Muto, Y. Nakato, and H. Tsubomura, Chem. Phys. Lett., 2, 597 (1971)o

1177