J. Phys. Chem. 1985,89, 1701-1705 series), electronic interaction between nonbonded atoms in the terminal rings is not restricted to Coulombic terms: electronic exchange can also take place through u-overlap of the 2 p r atomic orbitals, implying that r-r* transitions could induce changes in the helix pitch.I4 In the specific case of hexahelicene, a crude argument suggests that the changes could be small. The major configuration of triplet hexahelicene involves the highest occupied and the lowest unoccupied MOs. Thus the pairing properties of alternant M O S *imply ~ that the relative sign and amplitude of (22) Coulson, C. A.; Rushbrooke, G. S. Proc. Cambridge Philos. SOC. 1940, 36, 193.
1701
the 2pa atomic coefficients will remain unchanged on the nonbonded faced atoms, since they happen to belong to the same starring set. Indeed, the high anisotropy factor measured for the TT absorption is of the same order of magnitude as that reported for the ground state. This indicates that a high degree of helicity is retained in the lowest triplet state of hexahelicene. Acknowledgment. We are grateful to Prof. E. Gil-Av for suggesting the choice of 7,lO-dimethylhexaheliceneand for putting optically active material at our disposal. Registry No. 7,1O-Dimethylhexahelicene,9542 1-46-8.
Absorption Spectra and Photochemlcal Rearrangements of C8H,, Catlons in Solid Argon. Blcyclo[ 2.2.2]octa-2,5-dlene, Cycloocta-l,3,5-trlene, and Octatetraene Ian R. Dunkin, Lester Andrews,* Joseph T. Lurito, and Benuel J. Kelsall Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: September 17, 1984)
Matrix photoionization experiments with bicyclooctadiene and cyclooctatriene gave sharp new bands between 400 and 500 nm. Selective photolysis with visible light decreased the longer wavelength absorptions and increased a band system beginning at 447 nm; irradiation at 420-470 nm essentially restored the original spectrum. Photoionization of all-trans-octatetraene directly produced a strong 447-nm band system and a sharp weaker 756-nm band system; the latter origin and vibrational structure are in excellent agreement with the emission spectrum of the gaseous cation. Cyclooctatriene and bicyclooctadiene cations experienced a series of photochemical rearrangements, which initially gave a mixture of trans- and cis-octatetraene cations that were converted to all-trans-octatetraene cation by selective irradiation in the solid argon matrix.
Introduction Cryogenic matrices have proven to be effective media for studying the spectra and photochemically induced rearrangements of organic radial In particular the ring opening of 1,3-~yclohexadienecation to hexatriene cation and the rearrangements among hexatriene cation isomers and conformers have been studied in Freon and argon matrices!.' Similar experiments have also shown that >CH2 bridges readily migrate in bicyclic radical cation systems.2*s W e report here parallel experiments with CsHloradical cation systems, including the bicyclooctadiene, cyclooctatriene, and octatetraene isomers, which photochemically rearrange to the all-trans-octatetraene cation in solid argon.
absorptions 757 743 730 71 1
690 672 502 498.3 488.4 486.2 472.4
Experimental Section The matrix-isolation apparatus and photolysis procedures have been described in earlier reports?*10 Bicyclo[2.2.2]octa-2,5-diene was synthesized from 1,3-~yclohexadieneas recently described;" the infrared spectrum of the product was virtually identical with that reported.12 Cycloocta-l,3,5-triene was used as received from (1) (2) (3) 1788. (4)
TABLE I: Absorption Maxima (nm)Following Argon Matrix Photoionization of Bicyclooctadiene and Cyclooctatriene
Andrews, L.; Keelan, B. W. J . Am. Chem. SOC.1981, 103, 99. Kelsall, B. J.; Andrews, L. J . Am. Chem. SOC.1983, 205, 1413. Kelsall, B. J.; Andrews, L.; McGarvey, G. J. J . Phys. Chem. 1983,87,
Kelsall, B. J.; Andrews, L.; Schwarz, H.J . Phys. Chem. 1983,87, 1295. (5) Kelsall, B. J.; Andrews, L.; Trindle, C. J . Phys. Chem. 1983,87,4898. (6) Shida, T.; Kato, T.; Nosaka, Y . J . Phys. Chem. 1977, 81, 1095. (7) Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1984, 88, 2723. (8) Andrews, L.; Dunkin, I. R.; Kelsall, B. J.; Lurito, J. T. J. Phys. Chem.
assignmento
absorptions
Y t - origin t + 240b Y t + 1040
462.3
r
461.8 454.6 446.6
r
n
427.5
t
+ 1420
440.0
433.6
0
422.3
P P 9
412.3
417.5
assignment"
S
t - origin t + 335 t + (2 x 335) t + (3 x 335) t + 1290 t 1560 t + 1860
+
'Bands o through t are assigned to octatetraene cation structural isomers with t denoting the all-trans isomer. Bands y are associated with cis-containing isomers. Broader band n only appeared with cyclooctatriene and is possibly due to this parent cation. bVibrational interval from origin in cm-'. Organometallics, Inc. A sample of trans,trans-l,3,5,7-octatetraene was prepared by M. F. Granville using established procedure^;'^ the sharp vibronic argon matrix UV absorption spectrum was identical with the fluorescence excitation spectrum in n-hexane14 at 4 K except for matrix shift. The precursors were degassed by cooling to 77 K and evacuating; gas samples were prepared by adding the electron trap CH2Clzto precursor vapor and diluting with argon or by simply adding argon to precursor vapor. Precursor dilutions were approximately 2001 1 for cyclooctatriene and
1985, 89, 821.
(9) Andrews, L. J. Chem. Phys. 1975, 63, 4465. (10) Kelsall, B. J.; Andrews, L. J . Chem. Phys. 1982, 76, 5005. (1 1) De Lucchi, 0.; Modena, G. J. Chem. SOC.,Chem. Commun. 1982, 914. (12) Grob, C. A.; Kry, H.; Gazneux, A. Helu. Chim. Acto 1957,40, 130.
0022-3654/85/2089- 1701$01.50/0
(13) D'Amico, K. L.; Manos, C.; Christensen, R. L. J . Am. Chem. SOC. 1980, 102, 1177. (14) Kohler, B. E.; Spiglanin, T. A.; Hemley, R. J.; Karplus, M. J . Chem. Phys. 1984, 80, 23.
0 1985 American Chemical Society
Dunkin et al.
1702 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 t
w
w 0 z a m
U
11 = . 1
I
U
0
cn
t
m
2
U
m
6 0 v1
m
U
5
1
I
I
$00
$50
500
I
I
I
650
700
750
WF1VELENGTH / NM I
I
4 50
500
WRVELENGTH / NM
Figure 1. Absorption spectra of argon/bicyclooctadiene/CHzClz sample in the blue visible region. (a) Spectrum recorded after condensation at 20 K for 5 h with argon resonance radiation; (b) spectrum after 15-min
photolysis with 310-410-nm light; (c) spectrum after 470-650-nm irradiation; and (d) spectrum after 420-470-nm photolysis. 1200/ 1 for bicyclooctadiene and octatetraene; the partial pressure of methylene chloride when added was 4 times that of the precursor. The gas mixtures were condensed on a 20 K sapphire plate at 0.5-1 mmol/h with argon passing through a 3-mm orifice discharge lampI5 at about 0.5 mmol/h for 2-6 h. After precursor condensation with simultaneous argon resonance photoionization, spectra were recorded between 300 and 800 nm on a Cary 17 spectrophotometer. The matrix samples were then exposed to high-pressure mercury arc radiation passing through Coming glass filters for 15-min periods and then more spectra were recorded. Band measurements are accurate to k0.2 nm.
Results Three isomeric C8Hlohydrocarbon precursors were studied, and the UV-visible spectra from matrix photoionization experiments are described below. Bicyclo[2.2.2]octu-2,5-diene.Six experiments were carried out with bicyclooctadiene. Four of them differed only in the photolysis sequences adopted and gave similar results. Figure 1 shows spectra recorded in a typical matrix photoionization experiment with bicyclooctadiene and methylene chloride added as an electron trap. Spectrum a was taken after sample deposition (5 mmol) with simultaneous photoionization from the argon-discharge lamp; the most prominent features were a broad CH2C12+absorption at 350 nm from photoionization of the electron trap16 (not shown) and several weak but fairly sharp bands between 400 and 500 nm, (15) Andrews, L.; Tevault, D. W.; Smardzewski, R. R. Appl. Spectrsc. 1978, 32, 157. (16) Andrews, L.; Prochaska, F. T.; Ault, B. S.J . Am. Chem. Sot. 1979, 101. 9.
Figure 2. Absorption spectra of argon/cyclooctatriene sample in the blue and red regions. (a) Spectrum recorded after condensation at 20 K for 4 h with simultaneous argon resonance photoionization; (b) scan after 590-1000-nm irradiation for 15 min; and (c) spectrum after 450-1000-
nm photolysis. which are listed in Table I. Irradiation of this matrix with 3 10-410-nm light destroyed the CH2C12+absorption and caused the bands between 400 and 500 nm to grow (Figure lb). At the same time, several very weak absorptions in the region 650-800 nm appeared. Further irradiation with light of 470-650 nm (Figure IC) greatly increased the bands at 446.7, 440.0, 443.6, 442.3, and 417.5 nm (marked t in Figure l ) , slightly increased the band at 455.5 nm (s in Figure l ) , barely changed the band at 462.3 nm (r in Figure l), but virtually eliminated the bands at 472.3 (q), 485.6,488.5 (p), and 499 nm (0).In addition, weak bands between 380 and 415 nm and at 672, 730, and 743 nm increased markedly (not shown). Final irradiation with light of 420-470 nm reversed all of these changes (Figure Id). Besides the absorptions described above, weak bands at 297 and 283 nm grew throughout the experiment; these bands are due to the neutral molecule, 1,3,5,7-octatetraene, based on comparison with the matrix UV spectrum of the authentic material. In other experiments, the absorptions between 250 and 300 nm were found to increase substantially when matrixes with spectra similar to those shown in Figure 1 were irradiated with the Pyrex-filtered (290-1000 nm) or full (220-1000 nm) Hg arc. The other two experiments were somewhat different in nature and were conducted as controls. In the first, deposition was carried out without the Ar-discharge lamp; the resulting spectrum shows no absorptions between 280 and 600 nm, and no bands appeared when the matrix was irradiated successively by light with 470-, 380-, 290-, and 220-nm short-wavelength limits. In the second control experiment, Ar-discharge photoionization was carried out during deposition, but the electron trap, CH2C12,was omitted from the matrix gas mixture. In this instance, spectra similar to those shown in Figure 1 were recorded, but the bands were 10-fold weaker. Cyclooctu-l,3,5-triene. Two experiments were conducted with cyclooctatriene, and both gave similar results. Since cyclooctatriene gave large yields of ions without the addition of CH2C12 electron traps, matrix mixtures were made up containing only
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1703
CsHloCations in Solid Ar
A = .1
I rf1
Ix t
I
I
$00
450
I
500
I
400
I
450
I
500
WRVELENGTH / NM
Figure 3. Absorption spectra of argon/octatetraene/CH2CI2 sample in the blue region. (a) Spectrum after condensation at 20 K for 2 h with
argon raonance radiation produced by 70% of full diathermy power; (b) scan after 470400-nm photolysis; (c) spectrum after 420-470-nm irradiation; (d) scan after second 470-600-nm photolysis; (e) spectrum after second 420-470-nm irradiation; and (f) scan recorded after final irradiation at 220-1000 nm. 1
argon and the triene. Figure 2 illustrates spectra obtained in one of the experiments. After deposition (6 mmol) with simultaneous Ar-discharge photoionization, the resulting matrix had the absorption spectra shown in Figure 2a. The product bands marked t, s, r, q, and p occurred within the jz0.2-nm error of measurement, at the same wavelengths as their counterparts in Figure 1, though with somewhat differing relative intensities. In addition, a new broad band was observed at 502 nm (n in Figure 2), unique to this precursor. Irradiation of this matrix with 590-1000-nm light reduced the intensities of the p and n bands, increased the t, s, r. and q bands, and produced weak y bands (Figure 2b). Subsequent irradiation a t 500-1000 nm had little effect on the spectrum (not shown), but irradiation at 450-1000 nm (Figure 2c) nearly doubled the intensity of the t bands, while reducing the s, r, and q bands. At the same time, weak absorptions at 673, 692, and 744 nm became distinct for the first time and are also marked t in Figure 2c. truns,trans-1,3,5,7-Octatetraene.In all, six experiments were carried out with octatetraene; spectra from two studies are illustrated in Figures 3 and 4. The spectrum shown in Figure 3a was recorded after sample deposition (1 mmol) with simultaneous argon resonance photoionization. As in the experiments with bicyclooctadiene and cyclooctatriene, absorption bands marked t, s, r, q, p, and o appeared at wavelengths within f l nm of those counterparts in the other experiments. Irradiation of the matrix with 590-1000-, 500-1000-, and 470-600-nm light caused the progressive decrease and almost complete disappearance of the q, r, and o bands, a slight decrease in the s and r bands, and an increase in the t bands (Figure 3b). Site splittings were especially obvious in the octatetraene experiments; for example, the strong t band at 447 nm exhibited splittings at 447.7, 447.0, and 445.7 nm, and other bands showed similar splittings. In contrast to the experiments with bicyclooctadiene and cyclooctatriene, however, no general increase in the intensity of all bands between 400 and 500 nm occurred during Hg-arc irradiation. Subsequent irradiation at 420-470 nm brought about a sharp decrease in the t
I
370 420
I
I
970
1
600
I
I
I
650 700 750
1
WVELENGTH / NM
Figure 4. Absorption spectra of argon/octatetraene/CH2CI2 sample in the blue and red regions. (a) Spectrum after condensation at 20 K for 4 h with argon resonance radiation powered by diathermy at 30% power; (b) spectrum after 15-min 470-600-nm photolysis; (c) spectrum after 420-470-nm irradiation for 15 min.
bands, a slight further decrease in the s and r bands, and a marked growth of the q, p, and o bands (Figure 3c). Thereafter, interconversion of the species giving rise to the t bands and those giving rise to the q, p, and o bands was repeated in both directions by further irradiation, first with 470-600-nm light (Figure 3d), then with 420-470-nm light (Figure 3e). Irradiation at 380-600 nm led again to increased t absorptions and reduced q, p, an o absorptions (not shown), while subsequent irradiation at 340-600 nm and 290-1000 nm had little further effect on the spectrum (not shown). Final irradiation with the full H g arc resulted in a general reduction in the intensity of the absorptions at 400-500 nm (Figure 3f). In another experiment, illustrated in Figure 4, matrix deposition was carried out d t h photoionization using a much lower intensity Ar discharge. Figure 4a is the spectrum recorded after deposition (2 mmol) in this manner, and it can be seen that the bands marked s, r, q, p, and o are all at much lower relative intensity compared to the t bands than in Figure 3a. Figure 4 also displays the 600-800-nm region, where prominent t bands at 677.5, 682.8, 692.2, 702.2, and 756.4 nm are visible. Irradiation of the matrix at 470-600 nm virtually eliminated the q, p, and o bands, slightly decreased the s and r bands, and increased all the t bands. Finally irradiation at 420-470 nm reduced the t bands substantially, left the s band virtually unchanged, and increased the r, q, p, and o bands. At the same time new absorptions marked y appeared in the region 600-800 nm, while those marked t decreased. Table I1 lists the prominent bands observed in octatetraene photoionization experiments.
1704 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 TABLE 11: Most Intense Product Absorptions (nm) Following Matrix Photoionization of a//-trans-Octatetraene absorptions 778.0 756.4 744.1 739.7 721.8 718.3 702.2 692.2 688.4 682.8 673.0 498.4 486.8
assignment'
Y t - origin t + 220b Y Y Y t + 1020 t + 1230 Y t + 1220 + 210 t + 1640 0
P
absorptions 473.9 462.6 454.2 447.7 441.1 434.7 428.7 423.3 417.7 413.6 409.0 404.0 384.0
assignmenta
Dunkin et al. TABLE 111: Vibrational Fundamentals (cm-') for the all-trans -0ctatetraene Cation and Neutral Molecule in Different States cation blue absorption" 330, 1290, 1640
q r
S
t - origin t 330 t (2 X 330) t + (3 X 330) t 1290 t 1640 t (1640 + 200)
+ + + + +
X
molecule uv absorptionb 200, 346, 1228, 1628
red absorptiono 220, 1020, 1230, 1640 red emission' 230 (220): 340, 1230, 1610
'Argon matrix at 20 K, this work. bAlkane matrix at 4 K, ref 14. 'Gas phase, ref 17. dHot band at 220 cm-l.
X
t
'Bands
o through t are assigned to octatetraene cation structural isomers, with t denoting the all trans isomer. The x and y bands are probably associated with cis-containing isomers. Vibrational interval from origin in cm-I.
When matrix deposition was done without the argon resonance lamp, the resulting matrix revealed sharp absorptions due to octatetraene beginning at 297.8, 287.5, and 284.1 nm, and subsequent irradiation, even with the full Hg arc, produced no change. Deposition of a gas mixture lacking the normally added CH2C12 electron trap but with simultaneous argon resonance photoionization gave a 10-fold lower yield of the species absorbing in the 400-500-nm region.
Discussion In the following discussion, the new product absorptions are identified, band assignments for all-trans-octatetraene cation are given, and mechanisms for the photochemical rearrangements of CsHIoradical cations are proposed. Identification of Products. The major new absorptions described above are all assigned to cationic species, derived from photoionization of the parent molecules followed by photoinduced rearrangements. These bands increased by an order of magnitude in experiments where the matrices included CHzClzas an electron trap, which clearly supports their assignment to cations. The CH2C12molecules dissociatively capture electrons to give chloride ions, which prevents recombination of the parent cations and the electrons removed in the primary photoionization process. The ionization energies of all-trans-octatetraene and bicyclooctadiene - ~ ~ that of cyclooctatriene are 7.8 and 8.4 eV, r e s p e c t i ~ e l y , ' ~while is expected to be in the 8-9 eV range, like cy~loheptatriene.'~ Thus all three precursors can be ionized by the 11.6-1 1.8 eV output from the argon resonance lamp.15 Finally, there is excellent agreement between the present observations and those made by Bally et al. after X-irradiation of octatetraene in Ar matrices,20 and by Shida et al. in an investigation of the photolysis of cyclooctatriene cation formed by y-radiolysis of Freon matrices containing cyclooctatriene.2' All three precursors gave rise ultimately to the same mixture of ions with absorptions marked t, s, r, q, p, and o in the figures. Cyclooctatriene alone gave an additional band at 502 nm (marked n in Figure 2) which disappeared along with the band at 498 nm (0in Figure 2) but, unlike the latter band, did not reappear later in the photolysis sequence. Although it is possible that the 502-nm band is due to a site splitting peculiar to this one precursor, it is equally probable that it belongs to the parent cation, cyclooctatriene cation. for which Shida et al. have observed an ab(17) Jones, T. B.; Maier, J. P. Znt. J. Mass. Specrrom. Zon Phys. 1979.31, 287. (18) Demeo, D. A.; El-Sayed, M. A. J . Chem. Phys. 1970,52, 2622. (19) Bodor, N.; Dewar, M. J. S.; Worle, S. D. J . Am. Chem. SOC.1970, 92, 19. (20) Bally, T.;Nitsche, S . ; Roth, K.; Haselbach, E. J. Am. Chem. SOC. 1984,106, $927. (21) Shida, T.; Maekawa, K. "Abstracts, Annual Meeting Chemical Society of Japan, April 1977"; 3M05, p 408.
sorption at 506 nm in Freon matricesV2'All the remaining bands are assigned to isomers of octatetraene cation. The bands marked t are assigned to the all-trans isomer for three reasons. Firstly, they are the strongest absorptions observed after photoionization of all-trans-octatetraene, especially at low photoionization source intensity. Secondly, the origin of the red system at 13 220 cm-I is in very good agreement with that measured for the gas-phase emission spectrum'7 of all-trans-octatetraene cation at 13 460 cm-], provided allowance is made for the expected red shift in solid argon. Thirdly, a vibrational analysis of both the red and blue band systems (see below) provides intervals which correspond closely to those found in the photoelectron spectrum of octatetraene and gas-phase emission spectra of octatetraene cation.17 The bands marked s, r, q, p, and o all behave differently from t bands and from each other and are therefore assigned to a mixture of five different isomers of octatetraene cation, each with one or more cis bonds. It is considered unlikely that any of these bands belongs to a cyclic CsHIocation isomer, since they all appear after photoionization of the acyclic octatetraene, and cyclization of octatetraene cation seems improbable. The weak y bands are also associated with CsHlocation isomers with one or more cis bonds. Assignment of the t, s, r, q, p, and o bands to acyclic octatetraene cation isomers is further supported by the observation that these spectra are formed from cyclooctatriene and bicyclooctadiene indirectly rather than directly. Although a parent cation or intermediate was not detected after photoionization of bicyclooctadiene, the octatetraene cation absorptions were initially weak and grew markedly during photolysis with the Hg arc. The octatetraene cation isomers are therefore secondary products from both cyclooctatriene and bicyclooctadiene cations. Moreover, the proportion of cis isomers relative to the all-trans ion was much higher following photoionization of cyclooctatriene or bicyclooctadiene than in the similar photoionization of all-trans-octatetraene, consistent with their arising from cyclic precursors. The failure to detect parent or intermediate ions from bicyclooctadiene was most probably due to those cations having weak, broad absorptions. It should be noted that the cis isomers and the all-trans isomer of octatetraene cation may be readily interconverted by suitable visible irradiation. Finally, irradiation in the UV region results in ultimate formation of the neutral alltrans-octatetraene. This arises from photoneutralization of alltrans-octatetraene cations by electrons photodetached22 from chloride anion electron traps. The all-trans-octatetraene cations produced here exhibited slightly different spectra depending upon photochemical history. Direct photoionization of octatetraene followed by matrix trapping of the cation gave blue absorption bands within *1 nm of those produced by photochemical rearrangement of bicyclooctadiene cation and cyclooctatriene cation in the matrix cage; the red band system was, however, much sharper in the former case than with the latter where different matrix cages gave rise to a 13-nm displacement. Similar behavior was observed for hexatriene cation in a recent study.' Assignments. The band systems marked t are assigned to the all-trans-octatetraene cation. Agreement with the gas-phase (22) Berry R. S.; Reimann, D. W. J . Chem. Phys. 1963,38, 1540.
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1705
C8HI0Cations in Solid Ar SCHEME I
n l+*
n
7'
8' - m1+' h
-+*
- -
emission spectrum" indicates assignment of the red system to the 2B transition, which involves a simple hole promotion 2A, (ul2u22 u3l u4 B 2) ( ~ 1 ~ ~ 2 * ~ 3 % 4 Although ~ ) . the photoelectron spectrum predicts another transition near 422 nm, this one is not dipole allowed. Following hexatriene cation: the blue transition probably involves electron promotion to an antibonding molecular orbital, (u12u&732us1)(7r127r227r&r41), although both upper states are probably mixed by configuration interaction. The vibronic intervals observed in the red and blue absorption bands for all-trans-octatetraene cation in solid argon are compared in Table 111with gas-phase emission and alkane matrix excitation spectra.I4.I7 The four different cation and molecule states represented have modes in close agreement, and matrix absorption spectra of octatetraene give intervals near 1200 and 1620 cm-', which are due to "single and double" bond stretching modes. Thus, removal or excitation of one electron makes little difference in the vibrational potential function. The 330-cm-' interval due to the symmetric skeletal deformation mode is intermediate between 1,3,5-heptatriene cation (360 an-') and decapentaene cation (275 cm-*) value^.^^* Finally, the gas-phase emission spectrum shows a 220 f 10-cm-' hot band, which corresponds exactly with the first interval in the red matrix absorption spectrum. Photochemical Rearrangements. The ring opening of cyclooctatriene cation is a straightforward electrocyclic process, and irrespective of whether it occurs in a conrotatory or disrotatory it should lead to the all-cis isomer of octatetraene cation, as shown in Scheme I. Subsequent photoinduced rearrangement gives ultimately the more stable all-trans-octatetraene cation. The formation of acyclic octatetraene cation isomers from bicyclooctadiene cation is of greater interest. At least two plausible mechanisms may be written. The first (step (i) in the scheme) is an intramolecular 2+2-cycloaddition to yield a tetracyclooctane
-
(23) Dunkin, I. R.; Andrews, L. Spectrochim. Acta, submitted for publi-
cation.
cation, which may undergo scission of three u-bonds to give cyclooctatriene cation. Alternatively cyclooctatriene cation may be formed by a 1,Zshift of the ethano bridge (step (ii) in the scheme), yielding the isomeric bicyclo[4.2.0]octa-2,4-dienecation followed by electrocyclic opening of the cyclohexadiene cation system. Both these processes are allowed, according to simple frontier orbital arguments,23 but the latter is preferred since it involves the least-strained intermediate and because it is analogous to the previously reported rearrangement of norbornadiene cation.* In either case, the involvement of cyclooctatriene cation accounts for the virtually identical octatetraene cation isomer ratio observed after photoionization of both bicyclooctadiene and cyclooctatriene. Irradiation of octatetraene cation in the red absorption system does not provide sufficient internal energy to promote rearrangement among octatetraene cation isomers, but absorption of blue light by a particular isomer (Figure 3) activates that isomer. This allows dynamic equilibria with other structural isomers to be established, and those isomers not absorbing the activating radiation will be deactivated by the matrix and preserved for spectroscopic study. Similar rearrangements have been observed among hexatriene cation and decapentaene cation isomers in solid argon mat rice^.^^^
Conclusions Matrix photoionization experiments have shown that the C8Hlo isomers, bicyclooctadiene, cyclooctatriene, and all-trans-octatetraene, give rise to radical cations which can undergo subsequent photoisomerizations. Cyclooctatriene cation yields a mixture of all-trans- and cis-octatetraene cations by an electrocyclic ring opening, with cis isomers predominating initially. A similar mixture of all-trans- and cis-octatetraene cations is formed from bicyclooctadiene cation by a more extensive photorearrangement, probably beginning with a 1,3-shift of the ethano bridge, followed by two successive electrocyclic ring openings. Photoionization of all-trans-octatetraene yields a mixture of all-trans and cis cations, with the all-trans isomer predominating especially at low photoionization source intensity. In all instances, the structural isomers of octatetraene cation were interconverted by selective visible irradiation into the higher energy 400-500-nm absorptions of these species, but not by irradiation into the lower energy 650-800-nm absorptions. The role of the matrix in quenching internal energy to minimize dissociation and sustain rearrangement photochemical processes for organic radical cations is again demonstrated. Finally, the red matrix absorption spectrum of the all-trans-octatetraene cation agrees well in both the position of the origin and in the vibrational spacings with the gas-phase emission spectrum of the cation. Acknowledgment. We gratefully acknowledge financial support from National Science Foundation Grant C H E 82-17749, the gift of octatetraene from M. F. Granville, and helpful discussions with T. Shida and T. Bally prior to publication of results. I.R.D. acknowledges financial support from the University of Strathclyde, while on leave of absence as Visiting Scholar at the University of Virginia.
