Electronic structure and photochemical reaction intermediates of octakis(1,1,2-trimethylpropyl)octasilacubane

12 May 2000

Chemical Physics Letters 322 Ž2000. 33–40 www.elsevier.nlrlocatercplett

Electronic structure and photochemical reaction intermediates of octakis ž1,1,2-trimethylpropyl /octasilacubane Hiroaki Horiuchi, Yukio Nakano, Takayoshi Matsumoto, Masafumi Unno, Hideyuki Matsumoto, Hiroshi Hiratsuka) Department of Chemistry, Gunma UniÕersity, 1-5-1 Tenjin-cho, Kiryu 376-8515, Gunma, Japan Received 26 January 2000; in final form 13 March 2000

Abstract The electronic spectrum and photochemical reaction intermediates of octakisŽ1,1,2-trimethylpropyl.octasilacubane have been studied by analyzing the UV-VIS spectrum and transient absorption spectra. The first three absorption bands of the UV-VIS spectrum have been assigned to the transitions from HOMO Žt 1u symmetry. to LUMO Ža 2u ., from HOMO-1 Žt 1g . to LUMO, and from HOMO to LUMOq 1 Žt 2g . by making reference to the results obtained by INDOrS-CI calculations. The photochemical reaction intermediate has been elucidated to be the isomer whose decay rate constant is 57 " 3 sy1. The quenching rate constant of the isomer by oxygen is ca. 3 = 10 5 My1 sy1. q 2000 Elsevier Science B.V. All rights reserved.

1. Introduction Octasilacubanes were synthesized for the first time by one of the authors, H. Matsumoto and co-workers w1–4x and then by Furukawa et al. w5x, and are expected to be novel functional materials such as electric conduction materials. The structure of octasilacubane is shown in Fig. 1. OctakisŽ1,1,2-trimethylpropyl.octasilacubane Ž1a. is rather stable, and its structure, physical property and reactivity have been studied w3–7x. Kanemitsu et al. reported the luminescent property of 1a and showed that the emission intensity and lifetime depend strongly on the temper-

) Corresponding author. Fax: q81-277-30-1244; e-mail: [email protected]

ature w8,9x. Essentially no emission was observed above 80 K and the radiative lifetime was determined to be 3.1 ms below 40 K w8x. They measured ESR spectra and showed that this emitting state is assigned to the lowest triplet state of 1a. The photochemical reaction in benzene in the presence of dimethylsulfoxide was also studied by irradiation with light from a high-pressure mercury lamp w7x. Photoproducts were determined to be 9-oxaoctasilahomocubane and 5,10-dioxaoctasilabishomocubane by X-ray analysis and their yields were 47% and 37%, respectively. However, there has been no report on the electronic structure and photochemical reaction intermediates of 1a. We have studied the electronic structure of 1a by making reference to the calculated electronic spectrum obtained by the INDOrS-CI method. Photochemical reaction inter-

0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 3 5 7 - 2

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H. Horiuchi et al.r Chemical Physics Letters 322 (2000) 33–40

Fig. 1. Optimized molecular structures of octakisŽ1,1,2-trimethylpropyl.octasilacubane Ž1a., octakisŽ tert-butyl.octasilacubane Ž1b., and the isomers of 1a Ž2–5. calculated by the PM3 method.

mediates were also studied by measuring transient absorption spectra in the absence and presence of oxygen.

2. Experimental OctakisŽ1,1,2-trimethylpropyl.octasilacubane Ž1a. was synthesized according to the method reported in a previous paper w3x. 3-Methylpentane ŽACROS Organics, 99 q %. was purified by passing through a column of silica gel ŽMerck, silica gel 60. and used after distillation under an Ar atmosphere. Cyclohexane ŽWako Pure Chemical Industries, S reagent grade. was used without further purification. Steady-state photolysis was carried out by using light-output Ž253.7 nm. of a 100 W low pressure mercury lamp through vycol glass filters. Absorption spectra were recorded on a Hitachi U3300 spectrophotometer. Transient absorption spectra were measured at room temperature by using a Unisoku

TSP601H nanosecond laser photolysis system with photolysis light of 266 nm Žoutput of fourth harmonic, pulse width of 20 ns and power of 1 mJrpulse. from a Nd 3q:YAG laser ŽQuanta-Ray GCR-130, Spectra Physics. w10x. Monitoring of the transient absorption were made with light output of a 500 W Xe short-arc lamp ŽUshio UXL-500. in the perpendicular direction to the photolysis light. A sample solution was flowed with a rate of 1 mlrmin through an optical cell with light path length of 10 mm. Sample concentrations were ca. 1–2 = 10y3 M for both steady-state and time-resolved photolyses. Semi-empirical molecular orbital calculations were performed by use of the AM1 method ŽMOPAC97. for geometry optimizations of 1a and its isomers w11x. Electronic spectra were calculated by the INDOrS-CI method ŽMOS-F ver. 4.1, Fujitsu Labs... Ionization potential and electron affinity of silicon atom were set to 17.31 and 6.94 eV for 3s orbital, and 9.19 and 2.82 eV for 3p orbital, respectively w12,13x. Several values of bonding parameter

H. Horiuchi et al.r Chemical Physics Letters 322 (2000) 33–40

Ž b 0 . were examined for silicon atom in order to reproduce the electronic spectrum of 1a, and finally b 0 was set to y9.5 eV. 3. Results and discussion The optimized molecular structure of octakisŽ1,1,2-trimethylpropyl.octasilacubane Ž1a. and numbering of the Si atoms are shown in Fig. 1. Fig. 2a shows the UV-VIS absorption spectrum of 1a in cyclohexane at room temperature. There are three absorption bands with maxima around 500, 340 and 260 nm with molar absorption coefficient Ž ´ max . of 60, 840 and 27 800 My1 cmy1 , respectively. This spectrum was first reported by Matsumoto et al. w3x but the assignment of these absorption bands has not been made yet. In order to make assignment of these bands, INDOrS-CI calculations have been carried out for 1a and octakisŽ tert-butyl.octasilacubane Ž1b.

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whose geometrical structures were optimized by the AM1 method, and the results are shown in Fig. 2c and b by the stick spectrum, respectively. The optimized structure of 1b is little deformed from Oh symmetry. The structure of 1a is slightly distorted from Oh symmetry, being consistent with the distorted structure determined by X-ray analysis w3x. The calculated results for 1a appear to be more complex than that for 1b because the slight deformation from the Oh symmetry induces a dissolution of the triply degenerate molecular orbitals, and as a result three electronic transitions appear at nearly the same energy as that of the triply degenerate electronic transition of 1a. Some forbidden bands in 1b also become allowed in 1a. However, the characteristics of the electronic spectrum of 1a is the same as that of 1b and we may compare the observed spectrum with the calculated one obtained for 1b. The first absorption band around 500 nm with ´max of 60 My1 cmy1 is attributable to the first three transitions from the triply degenerate HOMO Žt 1u symmetry. to the LUMO Ža 2u . of which energies and electron distributions are shown in Fig. 3. These electronic states are represented as follows: C I s 0.998F 114 – 117 q PPP C II s 0.998F 115 – 117 q PPP

C III s 0.998F 116 – 117 q PPP

Ž 1.

where F 114 – 117 , F 115 – 117 and F 116 – 117 are the singly-excited electronic configurations obtained by promoting an electron from the triply degenerate HOMO Ž c 114 , c 115 and c 116 . to the LUMO Ž c 117 ., respectively. The symmetry of the HOMO is t 1u and that of the LUMO is a 2u , and therefore these transitions are parity-forbidden Žt 1u = a 2u s t 2g .. This is the reason why the molar absorption coefficient of the first band is remarkably low. This may also be responsible for the non-fluorescent property of this molecule w8,9x. The second absorption band observed around 340 nm is attributable to the next three electronic states ŽIV, V and VI. of T2u symmetry, represented by the LUMO Ža 2u . HOMO-1 Žt 1g . configuration as follows: CI V s 0.980F 111 – 117 q PPP C V s 0.980F 112 – 117 q PPP

§

Fig. 2. UV-VIS absorption spectrum of octakisŽ1,1,2-trimethylpropyl.octasilacubane in cyclohexane at room temperature Ža. in comparison with the calculated electronic spectra for octakisŽ tertbutyl.octasilacubane Žb. and octakisŽ1,1,2-trimethylpropyl.octasilacubane Žc. by the INDOrS-CI method.

CI V s 0.980F 113 – 117 q PPP

Ž 2.

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H. Horiuchi et al.r Chemical Physics Letters 322 (2000) 33–40

Fig. 3. MO diagram of octakisŽ tert-butyl.octasilacubane responsible for the low-lying electronic transitions.

The transitions to these states are forbidden in Oh symmetry because a 2u = t 1g s t 2u , but become allowed by the structural deformation and have an oscillator strength of 0.138. The near-UV band around 250 nm can be assigned to the triply degenerate electronic states ŽXXII,

XXIII and XXIV. of T1u symmetry. These states are formed by the transitions from HOMO Žt 1u . to LUMO q 1 Žt 2g ., which give rise to three symmetry-allowed T1u states together with six forbidden states of T2u , E u and A 2u symmetry Žt 1u = t 2g s t 1u q t 2u q e u q a 2u .. These T1u states are repre-

H. Horiuchi et al.r Chemical Physics Letters 322 (2000) 33–40

sented by singly-excited electronic configurations as follows.

C X X II s 0.595F 115 – 120 q 0.518F 116 – 118 q 0.373F 116 – 119 q 0.324F 115 – 119 q PPP

C X X III s 0.518F 114 – 118 q 0.440F 115 – 119 q 0.422F 114 – 119 q 0.360 F 115 – 118 q 0.273F 116 – 119 q PPP

C X X I V s 0.649F 114 – 120 q 0.430F 116 – 119 q 0.416F 116 – 118 q 0.200F 115 – 119 q PPP Ž 3. The experimental oscillator strengths of the 250-nm and 350-nm bands were estimated to be 0.65 and 0.024, respectively. The former is comparable to that of the calculated one Ž0.342 = 3 s 1.026.. These assignments of the lowest three electronic bands are summarized in Table 1. Fig. 4a shows the differential absorption spectra of 1a in 3-methylpentane at 77 K before and after the photolysis with 253.7-nm light for 10, 20, 30, 40 and 50 min. Three absorption bands are observed with maxima around 700, 470 and 340 nm. These bands disappeared when the solution was warmed up to room temperature and are attributable to the reaction intermediates. Transient absorption spectra determined for 1a at 1, 6, 15 and 40 ms after the laser flash in Ar-saturated cyclohexane at room temperature are also shown in Fig. 4b. The spectra are similar to those observed at 77 K, indicating that the same reaction intermediates are produced in both

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photolyses. The decay profiles monitored at 730 nm and at 470 nm were analyzed by a single exponential function with a rate constant of 57 " 3 sy1 , indicating that these absorption bands are attributable to the same reaction intermediate. Transient absorption spectra of 1a in O 2-saturated cyclohexane were also measured at room temperature and the decay profiles at these bands were analyzed by a single exponential function with a rate constant of k obs s 3.6 = 10 3 sy1 . The quenching rate by the oxygen was determined by k obs s k q k q w O 2 x

Ž 4.

where k is the rate constant in the absence of oxygen Ž57 " 3 sy1 . and wO 2 x is the concentration of oxygen Ž11.5 = 10y3 M. in cyclohexane at 1.0 = 10 2 kPa oxygen w14x. The value of k q was determined to be ca. 3 = 10 5 My1 sy1 . This quenching rate is much smaller than that of the excited singlet states or the lowest triplet states of aromatic molecules Ž10 7 y 10 8 My1 sy1 . w15x. Furthermore, it is pointed out that the absorption spectra observed upon the steady-state photolysis of 1a in 3-methylpentane at 77 K did not decay at least in several hours. Thus, these results indicate that transient absorption spectra are not attributable to the triplet state. It is also confirmed that the absorption spectrum observed after the photolysis at room temperature was essentially the same as that before the photolysis, indicating that the reaction intermediate easily reverts to the parent molecule and is ascribable to the photoisomers of 1a. Excitation light power dependence of the 700-nm absorp-

Table 1 Calculated electronic transitions of octakisŽ tert-butyl.octasilacubane Calculated

Observed

Electronic transition

Energyr eV Žnm.

Oscillator strength

Symmetry

Energyr eV Žnm.

Molar absorption coefficientrMy1 cmy1

Oscillator strength

I, II, III IV, V, VI VII VIII, IX, X XI, XII XIII, XIV XV, XVI, XVII XVIII, XIX, XX XXI XXII, XXIII, XXIV

2.55 Ž486. 3.29 Ž377. 4.27 Ž290. 4.31 Ž288. 4.35 Ž285. 4.73 Ž262. 4.79 Ž259. 4.83 Ž256. 4.95 Ž250. 5.06 Ž245.

0.000 0.138 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.342

T2g T2u

2.48 Ž500. 3.65 Ž340.

60 840

– 0.02

T1u

4.77 Ž260.

27 800

0.65

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biradical species by AM1 calculations. Therefore the next three species, 2–4 shown in Fig. 1, have been examined as candidates for the isomer. The species, 2 and 3, are formed by the scission of two parallel Si–Si bonds Ž1–4 and 1X –4X bonds. accompanied with formation of two p-bonds between SiŽ1. and SiŽ1X . and between SiŽ4. and SiŽ4X .. The structure of 2 is considerably distorted from C 2 n symmetry and that of 3 is slightly deformed from C 2h symmetry. The difference in heat of formation between these species and 1a are 216 and 241 kJ moly1 , respectively. The species 4 is formed by the scission of two

Fig. 4. Differential absorption spectra of octakisŽ1,1,2-trimethylpropyl.octasilacubane in 3-methylpentane upon 253.7-nm steadystate photolysis at 77 K Ža. and transient absorption spectra observed upon nanosecond laser photolysis in cyclohexane at room temperature Žb., in comparison with the calculated electronic spectra of the isomers Žc–f.. See text in detail.

tion band was examined at 77 K and a linear relationship with a slope of ca. 0.9 was confirmed. This indicates that the formation process of the intermediates is monophotonic. We studied several possible isomers which may be produced by the monophotonic process and may readily revert to the parent molecule. The first candidate for the intermediate is the biradical species produced from 1a by the scission of the SiŽ1. –SiŽ1X . bond, but no stable structure was obtained for the

Fig. 5. MO diagram for isomer 4 of octakisŽ1,1,2trimethylpropyl.octasilacubane responsible for the low-lying electronic transitions.

H. Horiuchi et al.r Chemical Physics Letters 322 (2000) 33–40

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Table 2 Calculated electronic transitions of the isomer 4 Calculated

Observed

Electronic transition

Transition energyr eV Žnm.

Oscillator strength

Transition energyr eV Žnm.

Relative absorption intensity

I II III IV V VI VII VIII IX X

1.97 Ž630. 2.20 Ž563. 2.27 Ž545. 2.63 Ž472. 2.75 Ž450. 2.88 Ž430. 2.92 Ž424. 3.12 Ž397. 3.27 Ž379. 3.30 Ž376.

0.114 0.003 0.000 0.012 0.010 0.119 0.005 0.004 0.056 0.025

1.76 Ž700.

1.0

2.64 Ž470.

0.7

parallel Si–Si bonds Ž1–2 and 3X –4X . accompanied with formation of two s-bonds between SiŽ1. and SiŽ4X . and between SiŽ2. and SiŽ3X .. The optimized structure of 4 is slightly distorted from D 2d and the difference in heat of formation between 4 and 1a is 219 kJ moly1 . Isomers 2 and 4 are energetically more favorable. The next candidate is the species denoted as 5 which can be produced by the scission of four Si–Si bonds. The difference in heat of formation between 5 and 1a is 459 kJ moly1 and is energetically unfavorable. The electronic spectra of these species 2–5 were calculated by the INDOrS-CI method and the results are shown in Fig. 4b–e. It is apparent that the observed spectrum can be explained by the calculated results for 4 Žsee Table 2., although the correspondence between them is not good in the higher energy region. The 700-nm band and the 470-nm band are ascribable to the first and the sixth electronic transitions with oscillator strengths of 0.114 and 0.119, respectively. They are represented by the single main electronic configuration; LUMO Ž c 165 . HOMO Ž c 164 . and LUMOq 1 Ž c 166 . HOMO y 1 Ž c 163 ., respectively. These MO’s are shown in Fig. 5. In conclusion, the intermediate responsible for the transient absorption spectrum is attributable to the photoisomer 4. The oxidation products reported by Unno et al. are consistent with our assignment w7x. The photoproducts in benzene in the presence of dimethylsulfoxide were 9-oxaoctasilahomocubane and 5,10-dioxaoctasilabishomocubane. The latter can be pro-

§

§

duced by the attack of oxygen on photoisomer 4 because two oxygen atom may insert into the lengthened SiŽ1. –SiŽ2. and SiŽ3X . –SiŽ4X . bonds in 4. Isomer 4 is also supported by trap experiments using alcohol or diene, both of which may trap the intermediate species containing unsaturated bondŽs.. No trapped compound was detected after the photolyses, indicating that unsaturated bond is not included in the reaction intermediate species so that isomer 4 is most probable w16x. Kanemitsu et al. reported that fluorescence of 1a was observed intensely below 40 K but diminished above this temperature w8x. This indicates that nonradiative process becomes much more efficient than the radiative process. This nonradiative process may be related with the photoisomerization.

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research on Priority-Areas, ‘The Chemistry of Inter-element Linkage’ from the Ministry of Education, Science, Sports and Culture of Japan.

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w3x H. Matsumoto, K. Higuchi, S. Kyushin, M. Goto, Angew. Chem. Int. Ed. Engl. 31 Ž1992. 1354. w4x M. Unno, H. Shioyama, M. Ida, H. Matsumoto, Organometallics 14 Ž1995. 4004. w5x K. Furukawa, M. Fujino, N. Matsumoto, Appl. Phys. Lett. 60 Ž1992. 2744. w6x M. Unno, K. Higuchi, M. Ida, H. Shioyama, S. Kyushin, H. Matsumoto, M. Goto, Organometallics 13 Ž1994. 4633. w7x M. Unno, T. Yokota, H. Matsumoto, J. Organomet. Chem. 521 Ž1996. 409. w8x Y. Kanemitsu, K. Suzuki, M. Kondo, H. Matsumoto, Solid State Commun. 89 Ž1994. 619. w9x Y. Kanemitsu, K. Suzuki, M. Kondo, S. Kyushin, H. Matsumoto, Phys. Rev. B 51 Ž1995. 10666.

w10x H. Hiratsuka, S. Kobayashi, T. Minegishi, M. Hara, T. Okutsu, S. Murakami, J. Phys. Chem. A 103 Ž1999. 9174. w11x M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 Ž1985. 3902. w12x J.H. Hinze, H.H. Jaffe, J. Am. Chem. Soc. 84 Ž1962. 540. w13x H. Hiratsuka, Y. Mori, M. Ishikawa, K. Okazaki, H. Shizuka, J. Chem. Soc., Faraday Trans. 2 81 Ž1985. 1665. w14x S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, 2nd edn., Section 12, Marcel Dekker, New York, 1993, p. 290. w15x S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, 2nd edn., Sections 8 and 9, Marcel Dekker, New York, 1993. w16x M. Unno, T. Matsumoto, H. Matsumoto, unpublished results.