However, from a practical viewpoint, it would be of greatest interest to predict the rates of slow abstraction of halide from aromatic RA because it is specifically in this case that other reactions of radical-anions such as dimerization and protonation may compete with halide abstraction. The need arises for predicting the yield of the various products and the corresponding "theoretical" values of kl.

Conclusions

A quantum chemical reactivity has been proposed which permits the prediction of the rate constant of the halide anion from aromatic halide radical-anions in a rather broad range.

LITERATURE CITED

i. K. Alwair and J. Grimshaw, J. Chem. Soc., Perkin Trans. 2, 1150 (1973). 2. J. Grimshaw and J. Trocha-Grimshaw, J. Electroanal. Chem., 56, 443 (1974). 3. G. J. Gores, C. E. Koeppe, and D. E. Bartak, J. Org. Chem., 44, 380 (1979). 4. F. A. Beland, S. O. Farwell, P. R. Callis, and R. D. Geer, J. E!ectroanal. Chem.,

78, 145 (1977). 5. V. G. Mairanovskii (Mairanovsky), J. Eiectroanal. Chem., 12>j, 231 (1981). 6. C. P. Andrieux, J. Saveant, and D. Zann, Nouv. J. Chim., ~, 107 (1984). 7. E. Candell, P. Karafiloglu, and L. Salem, J. Am. Chem. Soc., i02~ 855 (1980);

T. Clark and G. Illing, J. Chem. Sot., Chem. Commun., 529 (1985). 8. M. Tislet and V. D. Parker, Acta Chem. Scand., B36, 311 (1982). 9. G. M. Klopmmn, Reactivity and Reaction Pathways [Russian translation], Mir,

Moscow (1977). I0. K. Fukui and H. Fujimoto, Bull. Chem. Soc. Jpn., 41, 1989 (1968). ii. A. S. Mendkovich, A. I. Rusakov, and V. P. Gul'tyai, Izv. Akad. Nauk SSSR, Ser.

Khim., 225 (1986). 12. V. D. Parker, Acta Chem. Scand., B35, 655 (1981).

FORMATION OF A CIS-CISOID ISOMER AND THE NATURE OF THE

PHOTOCHEMICALLY ACTIVE ELECTRONICALLY EXCITED STATE

OF THE CLOSED FORM OF SPIROPYRANS

Ya. N. Malkin, N. A. Lysak, S. A. Tikhomirov, G. B. Tolstorozhev, V. A. Lokshin, and V. A. Kuzlmin

UDC 541.634:541. 141.7:547.816

The directed search for photochromic systems requires the establishment of the rela- tionship between the closed form of a photochromic compound and the quantum yield of bond cleavage as well as the state in which this process occurs and the chemical nature of the primary photoproducts. The cis-cisoid isomer of the photoinduced merocyanine form in the ground or excited state formed directly after cleavage of the C--O bond in the closed form, which retains the nonplanar geometry of the spiro species, is such a product in the photocoloration of photochromic spiropyrans [1-3].

The photocoloration of 6-nitrospiro[(2H-l-benzopyran)-2,2'-isobenzofuran] (SPP) in toluene and heptane was investigated in order to study the mechanism for the formation of the cis-cisoid form.

Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2129-2132, September, 1987. Original article submitted January 18, 1987.

0568-5230/87/3609-1979512.50 �9 1988 Plenum Publishing Corporation 1979

A~; tel. units

r

�9 ~ " \ O, Ol

I I I I J60 #50 550 658 750 ~,~ nm

Fig. I. Induced absorption spectra upon the photo- excitation of SPP in toluene at 295 K at different times e after excitation: i) i0, 2) 60, 3) 850 psec, 4) spectrum of triplet--triplet absorption form A after i0 nsec (obtained on a nanosecond photolysis apparatus).

~ NO z

The picosecond laser spectrometer used was described in our previous work [4]. The excitation of the SPP solutions was achieved by a single pulse of the third harmonic of a neodymium laser emitted at 352 nm. The broad-band picosecond continuum was used as the probe. The recording and automated treatment of the spectra for different delay times 8 between the excitation and probe pulses were achieved using a polychromator, vidicon, and multichannel analyzer. The spectrometer has time resolution of • psec with precision of the absorption measurements equal to (2 • 5).i0 -s. The nanosecond photolysis apparatus with 6 nsec resolution was described in our previous work [5].

The photoexcitation of a solution of SPP (form A) in toluene or heptane at 352 nm leads to a shortlived intermediate with an absorption maximum at 460 nm, whose annihila- tion occurs over about 60 psec and is accompanied by the formation of a more longlived product with ~max 550 nm (Fig. I).

Nanosecond laser photolysis was used to establish that the optical density at 550 nm continues to increase up to about 25 nsec. The annihilation of the longlived product occurs in the microsecond range.

The absorption spectrum with %max 550 nm should be ascribed to a stable isomer of merocyanine form B recorded by nanosecond and microsecond photolysis. The identical nature of the spectrum in the picosecond, nanosecond, and microsecond ranges indicates that the same isomer is found in the entire time range studied.

The formation of B occurs with the annihilation of the shortlived intermediate with kmax 460 nm (Fig. i) and this intermediate is the precursor of isomer B. In the general case, isomer B may be formed from a) the $I state of the closed form, b) TI state of the closed form, and c) another isomer of the merocyanine form such as isomer X.

Nanosecond laser photolysis was used to record the spectrum of T--T absorption of the closed form of SPP (see Fig. i) whose lifetime is 250 nsec. Thus, the TI state of form A cannot be the direct precursor of isomer B.

We have also recorded the fluorescence of the closed form of SPP in heptane at 295 K and of 3-methylpentane at 77 K (Fig. 2) and the lifetime of the $I state of form

1980

,.to3n~cm -'

o o o

.r " \ : : / \

o 3 ~ / \ 3~2 / / \ ! ..~'( \

I : I I [ i , "'- ja/7 JJJ #UQ

f a

Fig. 2. Spectra for the absorption (I) and fluorescence (2) of the

as closed form of SPP in heptane at 295 K, 3) dependence of the relative fluorescence quantum yield on the excitation wavelength.

~, nllq

Scheme

A* ($1) > A (S~) >. A (T1) + iT.

A(So) '< B (So)

A was determined using a mode for the calculation of single photons (i,5 • 1.0 nsec at 77 K). Thus, the S: state of form A also cannot be a precursor of isomer B. Therefore~ the direct precursor of isomer B is another shortlived isomer of the merocyanine form. In accord with the terminology employed in the photochemistry of spiropyrans, this isomer is known as the cis-cisoid isomer (X).

The kinetics for the increase in the optical density at 460 nm shows that the forma- tion of X completely occurs in less than 6 psec (length of the excitation and probe pulses of the apparatus). The high rate of the formation of the X state in addition with the reported values for the lifetimes of the $I and T: states of the closed form of SPP indicates that its formation is possible either from higher electronically excited states S n and T n (n > i) or from a nonthermalized Franck--Condon S~ singlet state.

The detailed analysis of the nature of the transitions in the heterocyclic and chromene parts of the closed form of SPP carried out in earlier work [6, 7] showed that light absorption at 353 nm for the closed forms of all SPP leads to excitation of the So--> S: transition of the nitrochromene part. This finding indicates that the cleavage of the C--O bond cannot proceed from high electronic states. Thus, Franck--Condon sub- levels of the $I state are the only possible starting state for the formation of the cis-cisoid isomer (see Scheme).

The formation of the cis-cisoid isomer of indoline spiropyrans occurs as the result of an adiabatic process and this isomer is formed in the triplet state (3X). Quenching of this state leads to the formation of the closed form [2]. On the basis of the analo- gous spectral kinetic characteristics of the isomer formed in the photocoloration of SPP and 6-nitroindoline SPP, we propose that the cis-cisoid isomer is formed in the triplet state in this case as well (3X).

Since the photoreaction involving the cleavage of the C-O bond and formation of the merocyanine form proceed from nonzero vibrational sublevels of the S: state, the photo- chemical reaction in this case should compete with vibrational relaxation (thermaliza- tion) of the $I state. Hence, the fluorescence quantum yield in the first absorption band should depend on the energy of the exciting quantum. Indeed, the fluorescence quantum yield of this form at 77 K decreases in the region shorter than 360 nm~ which is due to competition of the photochemical reaction with vibrational relaxation (the change in the quantum yield in the region shorter than 320 nm is apparently related to absorption by the heterocyclic part of the molecule). We should note that dependence of the fluor- escence quantum yield on wavelength was observed previously for chromenes, which also may be related to a reaction from nonrelaxed states [8].

Preliminary studies showed that the formation of cis-cisoid form X from a vibration- ally nonrelaxed singlet state is observed for unsubstituted SPP (not containing a nitro

1981

group), for unsubstituted and 6-nitroindoline spiropyrans, and for pyrans studied in detail by picosecond laser photolysis methods [3].

Conclus ions

Photoexcitation of a solution of 6-nitrospiro(2H-l-benzopyran)-2,2-isobenzofuran under picosecond laser photolysis conditions leads to cleavage of a C--O bond in a Franck--Condon state and formation of a shortlived cis-cisoid isomer of the merocyanine form of the spiropyran.

LITERATURE CITED

i. A. Kellman, L. Lindqwist, S. Monti, et el., J. Photochem., 28, 547 (1985). 2. A. S. Kholminskii, Ya. N. Malkin, and V. A. Kuz'min, Izv. Akad. Nauk SSSR, Ser.

Khim., 2124 (1984). 3. S.A. Krysanov and M. V. Alfimov, Dokl. Akad. Nauk SSSR, 272 , 406 (1983). 4. N. A. Lysak~ S. V. Mel'nichuk, S. A. Tikhomirov, and G. V. Tolstorozhev, Zh. Prikl.

Spektrosk., 57, 136 (1987). 5. P. P. Levin and V. A. Kuz'min, Izv. Akad. Nauk SSSR, Ser. Khim., 464 (1986). 6. A. S. Dvornikov, Ya. N. Malkin, and V. A. Kuz'min, Izv. Akad. Nauk SSSR, Ser. Khim.,

1520 (1982). 7. A. S. Kholmanskii, Zh. Fiz. Khim., 57, 689 (1983). 8. R. S. Becker, E. Dolan, and D. E. Balke, J. Chem. Phys., 50, 239 (1969).

QUANTUM CHEMICAL ASPECTS OF THE 1,3-HYDROGEN MIGRATION

IN RADICAL TELOMERIZATION

D. A. Bochvar, I. V. Stankevich, A. L. Chistyakov, A. B. Terent'ev, and R. Kh. Freidlina*

UDC 530.145:541.515:66. 095.2:547.313.2

A study of the radical telomerization of ethylene and propylene by carboxylic acid derivatives showed a significan t difference in the behavior of the first growing radicals [i]. The reaction with propylene features 20-30% 1,3-hydrogen migration (HM) in radical ( i ) .

XCH2CH2r 1,a-H X@HCH~CH2CHa (I) (II)

X= CO2CHa

In the reaction with ethylene, 1,3-HM in radical (III) is not observed although the differ- ence in the stability of rearranged (IV) and starting radical (III) is much greater than for the case of propylene [i].

XCH2CH~H~ -- • -+ XCHCH~CHa (In) (IV)

In the present work, quantum chemical methods were used to analyze the factors re- sponsible for this telomerization behavior.

In the radical telomerization of monomer M by telogen XCH3, the growing radical, which contains only one monomer unit, may participate in three competing reactions: isomerization (i), chain transfer (2), and chain propagation (3).

*Deceased.

A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2132-2134, Septemberj 1987. Original article submitted January 19, 1987.

1982 0568-5230/87/3609-1982512.50 �9 1988 Plenum Publishing Corporation