J. Phys. Chem. 1988, 92, 3875-3880
3875
Electron-Transfer Fluorescence Quenching of Electron-Deficient Benzenes by Hexamethyldisilane Keith A. Horn* and Anne A. Whitenack Allied-Signal, Incorporated, Engineered Materials Sector, Morristown, New Jersey 07960 (Received: November 20, 1987; In Final Form: January 26, 1988)
The fluorescence of electron-deficient benzenes is quenched by hexamethyldisilane. The measured bimolecular quenching rate constants (k,) are consistent with values calculated for an electron-transfer (ET) process in which hexamethyldisilane is the electron donor and the fluorescent singlet state of the substituted benzene is the acceptor. Neither ground-state, charge-transfer (CT) complex absorption bands nor exciplex emissions were observed in any of the systems studied. Support for a UT* ET fluorescence quenching mechanism includes the observed decrease in the bimolecular quenching rate constants with decreased solvent polarity (isooctane vs CH,CN) and the inability of hexamethyldisilane to quench the excited singlet state of anisole. A quenching mechanism involving ET and radical ion pair formation followed by subsequent Si-Si bond cleavage is consistent with the data. The general implications of this type of ET to polysilane photophysics and photochemistry are discussed.
Introduction The involvement of excited-state complexes (exciplexes) in the fluorescence quenching of electron-deficient aromatic excited states by ground-state donors has been shown to be a general phenomenon and has been extensively reviewed.' Often charge-transfer (CT) stabilization or electron-transfer (ET) processes are involved in the subsequent deactivation of such excited-state, donor-acceptor pairs. Evidence for the involvement of either C T or E T in the fluorescence quenching of excited-singlet-state aromatics usually involves the correlation of the bimolecular rate constants for excited-state deactivation with either the ionization or oxidation potentials of a series of chemically related quenchers. An increase in the bimolecular quenching rate constants with increasing solvent polarity can also indicate the involvement of C T or ET processes. In high-polarity solvents, E T can occur directly without the intermediacy of an exciplex. While many examples of ET or CT fluorescence quenching are known where both the excited-state acceptor and the ground-state donor are benzene derivatives or polycyclic aromatic hydrocarbons, few examples of systems involving a-bonded donors are known. This is a result of the fact that most carbon-based a-bonded systems have high ionization potentials and are thus poor electron donors. The notable exceptions such as thiols: sulfide^,^ amines,' and phosphines4 all have heteroatom substituents with nonbonded electron pairs. These are all excellent electron donors because of the increased energy of their highest occupied molecular orbitals (HOMOS) and their associated low ionization potentials. The group IVA organometallics comprise a unique class of compounds that can act as electron-rich donor molecules as a consequence of their high-lying, a-bonding orbitals. Many examples of the formation of ground-state CT complexes between strong electron acceptors such as tetracyanoethylene and b e n ~ y l - ~ (1) Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, Chapter 1. Davidson, R. S. In Aduances in Physical Organic Chemistry; Gold, V., Bethel, D., Eds.; Academic: New York, 1983; Vol. 19, Chapter 1. Kebarle, P.; Chowdhury, S. Chem. Rev. 1987, 87, 811. Mattes, S. L.; Farid, S. Science 1984, 226, 917, and references therein. (2) Marakami, Y.; Aoyama, Y.; Tokunaga, K. J. Chem. SOC.,Chem. Commun.1979, 1018. (3) Brimage, D. R. G.; Davidson, R. S.; Lambeth, P. F. J . Chem. SOC.C 1971, 1241. (4) Fox, M. A. J. Am. Chem. SOC.1979, 101, 5339. Marcondes, M. E. R.; Toscano, V. G.; Weiss, R. G. Ibid. 1975, 97, 4485. (5). Sennikov, P. G.; Skobeleva, S. E.; Kuznetsov, V. A.; Egorochkin, A. N.; Rniere, P.; Satge, J.; Richelme, S. J. Organomet. Chem. 1980, 201, 213. Traven, V. F.; Eismont, M. Yu.; Redchenko, V. V.; Stepanov, B. I. Zh. Obshch. Khim. 1980, 50, 2007. Ponec, R.; Chvalovsky, V. Collect. Czech. Chem. Commun.1974, 39, 1185.
0022-3654/88/2092-3875$01.50/0
or allylsilanes6 or stannanes' are known. In addition, the widespread use of silanes and stannanes in organic synthesis is based extensively on the reaction of the electron-rich metal to carbon (M-C) bonds with electrophiles.* The electron-rich character of these organometallics is also evident in their photochemical reactions. For example, allyl and benzyltrialkylstannanes have been shown to efficiently quench the excited singlet states of electron-deficient anthracenes through the intermediacy of a CT-stabilized exciple^.^ The polysilanes, polygermanes, and polystannanes have exceptionally high energy metal-metal (M-M) u bonds. This is a consequence both of the overlap of higher energy (3s, 3p) atomic orbitals than in corresponding carbon-based systems (2s, 2p) and also of u-bond conjugation effects.'O These effects are dramatically evidenced for example in the low ionization potentials" and the long-wavelength absorption spectra of polysilanes.12 The chemical consequences of this high-lying HOMO energy level are clearly evident in the ground-state reactions of polysilanes. For instance, polysilanes have been shown to form intermolecular CT complexes13 with good electron acceptors such as tetracyano(6) Egorochkin, A. N.; Skobeleva, S. E.; Lopatin, M. A.; Tumanov, An. A,; Lukevits, E. Ya.; Erchak, N. P.; Matorykina, V. F. Dokl. Akad. Nauk S S S R 1983,275,909. Zakomoldina, T. A.; Sennikov, P. G.; Kuznetsov, V. A.; Egorochkin, A. N.; Reikhsfel'd, V. 0. Zh. Obshch. Khim. 1980,50, 898. Kuznetsov, V. A.; Egorochkin, A. N.; Chernyschev, E. A.; Savushkina, V. A.; Anisimova, V. Z. Dokl. Akad. Nauk SSSR 1974, 214, 346. Ponec, R.; Chvalovsky, V.; Cernysev, E. A,; Komarenkova, N. G.; Baskirova, S. A. Ibid. 1974,39, 1177. Ponec, R.; Chvalovsky, V. Ibid. 1973,38, 3845. Traylor, T. G.; Berwin, H. J.; Jerkunica, J.; Hall, M. L. Pure Appl. Chem. 1972,30, 599. (7) Fukuzumi, S.;Mochida, K.; Kochi, J. K. J. Am. Chem. Soc. 1979,101, 5961. Reutov, 0. A,; Rozenberg, V. I.; Gavrilova, G. V.; Nikanorov, V. A. J. Organomet. Chem. 1979, 177, 101. (8) Fleming, I. In Comprehensiue Organic Chemistry. The Synthesis and Reactions of Organic Compounds;Barton, D., Ollis, W. D., Jones, D. N., Eds.; Pergamon: New York, 1979; Vol. 3, Chapter 13. (9) Eaton, D. F. J. Am. Chem. Soc. 1980, 102, 3280. Eaton, D. F. Ibid. 1981, 103, 7235. (10) Bigelow, R. W. Organometallics 1986, 5 , 1502. Herman, A,; Dreczewski, B.; Wojnowski,W. Chem. Phys. 1985,98,475. Herman, A.; Dreczewski, B.; Wojnowski, W. J. Organomet. Chem. 1983, 251, 7. Pitt, C. G. In Homoatomic Rings, Chains and Macromolecules of Main Group Elements; Rheingold, A., Ed.; Elsevier: New York, 1977; Chapter 8. (11) Loubriel, G.; Ziegler, J. Phys. Reu. B Condens. Matter 1986, 33, 4203. Bock, H.; Ensslin, W.; Feher, F.; Freund, R. J. Am. Chem. SOC.1976, 98, 668. Pitt, C. G.; Bursey, M. M.; Rogerson, P. F. Ibid. 1970, 92, 519. (12) Harrah, L. A,; Ziegler, J. Macromolecules 1987, 20, 601. Klingensmith, K. A,; Downing, J. W.; Miller, R. D.; Michl, J. J . Am. Chem. SOC. 1986,108,7438. Johnson, G. E.; McGrane, K. M. Polym. Prepr. (Am. Chem. SOC.Diu. Polym. Chem.) 1986, 27, 352. Rabolt, J. F.; Hofer, D.; Miller, R. D.; Fiches, G. N. Macromolecules 1986, 19.61 1. Sakurai, H. J. Organomet. Chem. 1980,200,261. Gilman, H.; Atwell, W. H.; Schwebke, G. L. Chem. Ind. 1964, 1063.
0 1988 American Chemical Society
3876 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Horn and Whitenack
TABLE I: Rate Constants for the Quenching of the Fluorescence of SubetiMed Benzenes by Hexamethyldisilane
red 112. d
compound
~,T,(CH:CN),~ 1O9r,(CH3CN), k,(CH,CN), mS M-I s-I
benzene a,a,a-trifluorotoluene benzonitrile hexafluorobenzene 1,4-dicyanobenzene
60.02t 0.38 f 0.01
6.3 0.55
3.8 6.9
18.76 f 0.10 18.91 f 0 . 1 g 4.94 f 0.04 190.0 f 2.3
4.2
4.5 X lo9 4.5 x 109 2.2 x 109 2.2 x 10'0
2.29 8.8
X X
k,T,(isooctane).b M-1
109r,(isooctane), k,(isooctane), S M-'
lo6 lo8
IP: eV
v vs Ag/AgNO,
9.25 9.67
-3.72 -3.18
2.23 f 0.03
5.5
4.0
X
lo8
9.70*
-2.76
1.02 i 0.01
2.19
4.8
X
lo8
9.97' 10.10*
-2.29 -1.97'
aValues of the slopes of the Stern-Volmer data of Figure 1. Error limits represent one standard deviation. bValues of the slopes of the Stern-Volmer data of Figure 3. evertical ionization potentials measured by PES. Values are from ref 26 unless otherwise noted. dReduction potentials in acetonitrile are from ref 27 unless otherwise noted. 'This is an upper limit. See text for explanation. '[Benzonitrile] = 1.57 X M. CCompare with the value of 2.0 x s for C6F6 in CC1,FCCIF2 given in ref 28. *Reference 29. 'Reference 30. iReference 31.
ethylene, and in addition, intramolecular C T interactions have been demonstrated in silyl-substituted electron-deficient benzenes.14 Also, films of polymeric polysilanes have been rendered conductive by interaction with strong electron acceptors such as AsF,.', The conduction has been demonstrated to be by hole migration along the Si-Si a-bond framework, with hole formation presumably occurring by an electron transfer from the Si-Si a-bond to AsF,. Interestingly, however, little is known about the bimolecular photophysical interactions between singlet-excited-state electron-deficient aromatics and group IVA organometallics containing M-M bonds. In addition, few examples of intramolecular excited-state C T or E T involving aromatic polysilanes are known. While the long-wavelength absorption bands of aryl-substituted polysilanes have been investigated by Pitt et a1.16 and Sakurai and co-workers," and demonstrated to involve charge transfer of the ussi -+ T * ~ X type, to our knowledge only two other such systems have been studied extensively. These are the intramolecular C T emissive states of the aryldisilanes (1)i8 and of the (phenylethyny1)disilanes (2).19 The emissive states of these two series
0
SiMe,SiMe,
R -
1
2
of compounds were originally assigned as singlet 2pa*3d?r* C T states and were postulated to involve donation of electron density from the excited singlet ?r* state of the aromatic moiety to the antibonding 3dn* energy level of the disilane. However, recent work by SakuraiZoand Hornz1 on 1 and 2, respectively, has (13) Watanabe, H.; Shimoyama, H.; Muraoka, T.; Kougo, Y.; Motohiko, K.; Nagai, Y. Bull. Chem. SOC.Jpn. 1987,60, 769. Frey, J. E.; Cole, R. D.; Kitchen, E. C.; Suprenant, L. M.; Sylwestrzak, M. S. J . Am. Chem. Soc. 1985, 107, 748. Sakurai, H.; Ichinose, M.; Kira, M.; Traylor, T. G. Chem. Lett. 1984, 1383. Traven, V. F.; Eismont, M. Yu.; Redchekno, V. V.; Stepanov, B. I. Zh. Obshch. Khim. 1980, 50, 2001. Traven, V. F.;Donyagin, V. F.; Makarov, I. G.; Kolesnikov, S.P.; Kozakova, V. M.; Stepanov, B. I. Izu. Akad. Nauk SSSR, Ser. Khim. 1977,5,4879. Traven, V. F.; West, R.; Donyagina, V. F.; Stepanov, B. I. Zh. Obshch. Khim. 1975,45,824. Sakurai, H.; Kira, M.; Uchida, T. J. Am. Chem. SOC.1973, 95, 6826. Traven, V. F.; West, R. Ibid. 1973, 95, 6824. (14) Sakurai, H.; Sugiyama, H.; Kira, M. J. Organomet. Chem. 1982, 225, 163. (15) West, R.; David, L.D.; Djurovich, P. I.; Stearley, K. L.; Srinivasan, K. S. V.; Yu, H. J. Am. Chem. SOC.1981, 103, 7352. (16) Pitt, C. G. J. Chem. SOC.,Chem. Commun. 1972, 28. Pitt, C. G.; Carey, R. N.; Toren, E. C. J. Am. Chem. SOC.1972, 94, 3806. (17) Sakurai, H.; Tasaka, S.; Kira, M. J. Am. Chem. Soc. 1972, 94, 9285. (18) Shizuka, H.; Okazaki, K.; Tanaka, M. Chem. Phys. Lett. 1985,113, 89. Hiratsuka, H.; Mori, Y.; Ishikawa, M.; Okazaki, K.; Shizuka, H. J. Chem. Soc., Faraday Trans. 2 1985, 81, 1665. Shizuka, H.; Sato, Y.; Ueki, Y . ; Ishikawa, M.; Kumada, M. J. Chem. SOC.,Faraday Trans. 1 1984,80, 341. Shizuka, H.; Obuchi, H.;Ishikawa, M.; Kumada, M. Ibid. 1984, 80, 383. (19) Shizuka, H.; Okazaki, K.; Tananka, H.; Tanaka, M.; Ishikawa, M.; Sumitani, M.; Yoshihara, K. J . Phys. Chem. 1987, 91, 2057. Ishikawa, M.; Sugisawa, H.; Fuchikami, T.; Kumada, M.; Yamabe, T.; Kawakami, H.; Fukui, K.;Ueki, Y.; Shizuka, H. J. Am. Chem. SOC.1982, 104, 2872. (20) Sakurai, H. Abstracts of Papers, Eighth International Symposium on Organosilicon Chemistry, St. Louis, MO; American Chemical Society: Washington, DC, 1987; p 2. (21) Horn,K. A.; Grossman, R. B.; Whitenack, A. A. Abstracts of Papers, Eighth International Symposium on Organosilicon Chemistry, St. Louis, MO; American Chemical Society: Washington, DC, 1987; p 156.
demonstrated that the C T states involve a asisi to T * charge donation rather than a 2 p ~ * 3 d n *C T state. In addition, it has been recently demonstrated that dodecamethylcyclohexasilanecan efficiently quench the fluorescence of dicyanoanthracene and in the presence of carbon tetrachloride results in products indicative of a full ET from the cyclic polysilane to the excited singlet state of the dicyanoanthracene.zz The photochemical cleavage of other cyclic polysilanes in the presence of good electron acceptors has also been interpreted in terms of C T complex formation.z3 In light of the apparent importance of C T and ET processes in the group IVA organometallics in general and specifically in the aromatic polysilanes 1 and 2, we have initiated an investigation of the mechanisms for the quenching of aromatic, singlet excited states by simple polysilanes. In this paper we present details of a study of the fluorescence quenching of electron-deficient benzenes by hexamethyldisilane which demonstrate clearly a a to T * ET quenching mechanism.
Experimental Section Materials. Hexamethyldisilane (Petrarch) was purified by elution through a column of activated silica gel (100-200 mesh, Aldrich) prior to use. Acetonitrile and isooctane were both Aldrich spectrophotometric gold label materials and were used without further purification. Benzene, hexafluorobenzene, a,a,a-trifluorotoluene, benzonitrile, and 1,4-dicyanobenzene were purchased from Aldrich and used as received. Instrumentation and Procedures. Fluorescence emission and excitation spectra were recorded on a Perkin-Elmer 650-40 fluorescence spectrophotometer in 1-cm fluorescence cells. The excitation and emission band-pass were 3 and 2 nm, respectively. The samples were not degassed prior to the measurements. The wavelengths used for excitation and monitoring for the static quenching experiments were as follows: benzene (260-nm excitation, 277-nm emission), hexafluorobenzene (263 nm, 369 nm), a,a,a-trifluorotoluene (265 nm, 277 nm), benzonitrile (270 nm, 289 nm), 1,4-dicyanobenzene (280 nm, 305 nm). The quenching data were analyzed by means of the Stern-Volmer equation. The fluorescence lifetimes were measured by a laser flash photolysis system. The excitation source was a Quanta Ray EXC-1 excimer laser which produces 4-10-ns-fwhm pulses of up to 50 mJ at 249 nm (KrF). The excitation energy can be varied by using suitable neutral density filters or by regulating the high-voltage discharge. The beam shape and size are controlled by a 3.81 cm diameter by 15.24 cm focal length cylindrical lens. The beam was not focused in the sample. The samples were contained in 1-cm (3-mL) fluorescence cuvettes. Sample concentrations were identical with those used in the static quenching experiments M). The fluorescence was monitored with a 1P28 photomultiplier tube (only four dynodes used) mounted on monochromator. Timing between the an Oriel Model 7240 ",I./ excitation pulse and the detection system was achieved with a Model 113 DR digital delay generator (California Avionics Lab., (22) Watanabe, H.; Kato, M.; Tabei, E.; Kuwabara, H.; Hirai, N.; Sato, T.; Nagai, Y. J. Chem. SOC.,Chem. Commun. 1986, 1662. Nakadaira, Y.; Komatsu, N.; Sakurai, H. Chem. Lett. 1985, 1781. (23) Sakurai, H.; Sakamoto, K.; Kira, M. Chem. Lett. 1984, 1213.
The Journal of Physical Chemistry. Vol. 92, No. 13, I988 3877
Quenching of Benzenes by Hexamethyldisilane 15
i
N
12
C
~
C
t
N
; I i
2
4
e
2
lo[ 9
0
0
4
0
1
2 [Me,SiSiMe,]
3
4
5
1
(M)
benzonitrile; 0,1,4-dicyanobenzene. Inc.). All transient pulses were digitized by a Tektronix 7912AD transient digitizer which is interfaced to a Hewlett-Packard 98 16 microcomputer. A minimum of 32 fluorescence decays were averaged for each lifetime measurement. Since the lifetimes were comparable to the excitation source pulse width, deconvolution of data was performed by using an iterative convolution technique. The deconvolution program was written by using the fast iterative convolution integral equation of Grinvald and S t e i r ~ b e r g . ~ ~ UV spectra were recorded on a Perkin-Elmer 320 spectrophotometer.
Results The fluorescence of a series of electron-deficient benzenes was quenched by hexamethyldisilane, using standard static quenching techniques. In each case, the quenching followed the SternV01mer~~ equation = kq~s[QI+ 1
I
/
1.6
1.9
(AgiAgN03)
Figure 1. Stern-Volmer plots for the fluorescence quenching of substituted benzenes by hexamethyldisilane in air-saturated acetonitrile at 298 K: 0, benzene; 0 , a,a,a-trifluorotoluene; A, hexafluorobenzene;A,
zo/zq
1.3
(1)
where Zo and I, are the relative fluorescent intensities in the absence and presence of quencher Q, T , is the singlet-state lifetime of the fluorophore, and k, is the bimolecular quenching rate constant. The plots of Io/Zqvs concentration of hexamethyldisilane were linear with intercepts of 1.0, as shown in Figure 1. Values of the slopes k rsare given in Table I along with gas-phase vertical ionization Jata and reduction potentials for the substituted benzenes. The kqr, value measured for benzene represents an upper limit since neat hexamethyldisilane had to be used to effect detectable quenching. Since the purified hexamethyldisilane used in these experiments had an extinction coefficient of 0.027 L mol-' cm-' at 260 nm (the wavelength used for excitation of the benzene fluorescence), some of the quenching is due to partial absorption of the excitation source by hexamethyldisilane. The use of neat quencher also constitutes a change of solvent polarity. In all other cases the concentration of hexamethyldisilane was kept below 2 M. Above this concentration hexamethyldisilane is not fully miscible with acetonitrile. No evidence was found in any case for the involvement of a ground-state interaction between these electron-deficient benzene derivatives and hexamethyldisilane. For example, an absorption M benzonitrile in acetonitrile with 0.37 spectrum of 1.57 X M added hexamethyldisilane was the simple sum of the spectra of the separate solutions taken independently. No new absorption bands or non-Beer's law dependence of optical densities was found. In addition, the emission spectra were also unaffected by the (24) Grinvald, A,; Steinberg, I. 2.Anal. Biochem. 1974, 59, 583. (25) Stem, 0.;Volmer, V. Phys. Z . 1919,20, 183. Turro, N. J. In Modern Molecular Photochemistry; Benjamin: Menlo Park, CA, 1978; p 246. (26) Turner, D. W. In Advances in Physical Organic Chemistry: Gold, V., Ed.; Academic: New York, 1966; Vol. 4, pp 31-69. (27) Mattay, J. Tetrahedron 1985, 41, 2393.
+
,
I
2.2
I
/
2.5
Volts
Figure 2. Plot of log k, vs E;$* for the fluorescence quenching of substituted benzenes by hexamethyldisilane in acetonitrile at 298 K. Data points: (1) benzene, (2) cu,a,a-trifluorotoluene, (3) benzonitrile, (4) hexafluorobenzene, ( 5 ) 1,4-dicyanobenzene.
addition of hexamethyldisilane. No new emission peaks attributable to formation of an exciplex were observed even at the highest concentrations of hexamethyldisilane used in the quenching experiments. The excitation spectra were also unaffected by the presence of up to 0.3 M hexamethyldisilane and were found to match the absorption spectra of the fluorophores in the absence of quencher. In contrast to the facile quenching of the fluorescence of the electron-deficient benzenes by hexamethyldisilane, electron-rich derivatives were not quenched at all. Anisole fluorescence, for example, was not quenched even in neat hexamethyldisilane. The singlet-excited-state lifetimes of the benzene derivatives were measured in air-saturated acetonitrile under the conditions of the quenching experiments by excimer laser excitation at 249 nm. The fluorescent decays were treated as single-exponential decays, and the rate constants were extracted by using an iterative convolution technique. The values of T~ are shown in Table I. In the few cases where data were available, excellent agreement was found with the reported lifetimes.28 Use of the measured fluorescent lifetimes allowed the extraction of the bimolecular quenching rate constants from the SternVolmer data. From the data in Table I it is clear that there is a general inverse relationship between the observed bimolecular quenching rate constants (k,) and the ionization or oxidation potentials of the fluorophores. This is confirmed by the observed lack of quenching of anisole fluorescence by hexamethyldisilane, the ionization potential of anisole being lower than that of hexamethyldisilane by 0.49 eV.26.32 The decrease in quenching efficiency with decreased oxidizing power of the excited singlet state is indicative of the operation of a CT or ET process in these systems. A plot of log k, vs excited-state reduction potential as measured by the sum of the ground-state reduction potentials and the singlet excitation energies is shown in Figure 2. While there is considerable variation associated with the reported values of the ground-state reduction potentials for these benzene derivatives, a distinct curvature is observed as the rate constants approach the diffusion limit. A plot of log kq vs ionization potential shows a similar curvature. The involvement of exothermic singlet energy transfer as a mechanism for the fluorescence quenching is readily ruled out on two bases. First, the ultraviolet absorption spectrum of hexamethyldisilane in either isooctane or acetonitrile has an absorption maximum at 193 nmlZand a long-wavelength tail that extends (28) Suijker, J. L. G.; Varma, A. G. 0. Chem. Phys. Lett. 1983, 97, 513. (29) Neijzen, B. J. M.; De Lange, C. A. J . Electron Spectrosc. Relat. Phenom. 1978, 14, 187. (30) DuJardin, G.; Leach, S.; Taieb, G. Chem. Phys. 1980, 46, 407. (31) Gennaro, A,; Maran, F.; Maye, A,; Vianello, E. J . Electroanal. Chem. 1985, 185, 353. (32) Boberski, W. G.; Allred, A. L. J . Organomet. Chem. 1975, 88, 6 5 .
3878 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Horn and Whitenack 10.5
9.30
-t
U
4
FI)
8.70 8.107.50-
0
6.90-
"\\,
6.30-
\
5.70 5.10 01 0
I
\ \
-
k -24.0 -16.0 -8.00 0.00 8.00
4.50
I
I
I
.4
.8
1.2
I
I
1.6
I
2
[Me,SiSiMe,] (M)
Figure 3. Stern-Volmer plots for the fluorescence quenching of benzonitrile by hexamethyldisilane at 298 K: 0, [benzonitrile]= 1.57 X M in isooctane; 0 , [benzonitrile] = 1.57 X M in acetonitrile; A, [benzonitrile] = 1.57 X M in acetonitrile.
to 250 nm (extinction coefficient 0.027 L mol-' cm-' at 260 nm, the shortest excitation wavelength used in this study). Thus, energy transfer to the hexamethyldisilane singlet state would be endothermic for the benzene derivatives investigated. Second, a plot of the observed bimolecular quenching rate constants vs the excited-singlet-state energies of the fluorophores shows no correlation. Though the position of the triplet state of hexamethyldisilane is not known, the near-diffusion-limited values of the rate constants for the quenching reaction are incompatible with a quenching mechanism involving triplet-state quenching, a spin-forbidden process. If an ET or CT mechanism for quenching is involved in these systems, a strong dependence of the magnitude of the bimolecular quenching rate constant on solvent polarity is expected. The effect of changing the solvent polarity was examined by using isooctane in lieu of acetonitrile. An example of the Stern-Volmer data for the quenching of benzonitrile fluorescence in isooctane is shown in Figure 3. Values of kqTs for both benzonitrile and hex& fluorobenzene are given in Table 1. Since the fluorescent lifetimes of benzonitrile and hexafluorobenzene are not significantly different in isooctane compared with those measured in acetonitrile, the k , values were found to decrease significantly as expected. The limited solubility of 1,Cdicyanobenzene in isooctane and the slow rate constant for quenching of benzene precluded measurement of the kqTs values for these derivatives in isooctane. In order to probe for the possible involvement of an exciplex in the quenching process, the effect of the concentration of the fluorophore was investigated. As shown in Figure 3, increasing the concentration of benzonitrile used in the static quenching experiments by 1 order of magnitude had no effect on the derived kq7, value. In addition, no new emission bands were observed.
Discussion Experimentally, the characteristics of the fluorescence quenching of electron-deficient benzenes by hexamethyldisilane are clearly analogous to the results obtained by Eaton9 for the quenching of the fluorescence of substituted anthracenes by allyland benzyltrialkylstannanes. Our data are consistent with a similar quenching mechanism involving a CT-stabilized, nonfluorescent exciplex or an ET-generated radical ion pair mediated process as shown in Scheme I. Both systems involve an E T from an electron-rich (donor) organometallic a-bond (C-Sn or Si-Si) to the excited singlet state of the aromatic fluorophore (acceptor). Application of the usual steady-state approximations to the concentrations of the encounter complex and the radical ion pair for the mechanism shown in Scheme I gives eq 2 for the overall k12 k, = k21 k32k21 1+-+k23
k23k30
.IGia, (kcal/mol) Figure 4. Semilog plot of the fluorescence quenching, rate constant, k,, vs free energy, AG2,', for the electron-transfer process for the fluorescence quenching of substituted benzenes by hexamethyldisilane;see Table I and text. The solid line is that calculated by Rehm and Weller. The dashed line is the calculated best fit of the Rehm-Weller equation to the data. (1) benzene; (2) a,a,a-trifluorotoluene; (3) benzonitrile; (4) hexa-
fluorobenzene; ( 5 ) 1,4-dicyanobenzene. SCHEME I
encounter complex
ox'
Me,SiiMe,
-!%
products
radical ion pair
quenching process, where k, is the rate constant for the overall quenching reaction, and k12,k23, k32, and k30 are the rate constants for diffusion, for forward ET, for reverse ET, and for the rate of product formation from the radical ion pair, respectively. By use of a classical approach, the rate constant for E T within the encounter complex (k23) and the ratio of the rate constants for forward and reverse ET (k23/k32) are given by
kz3 = k2?e-AG23'/RT
(3)
k32/k23 = eAG231RT
(4)
Substitution of eq 3 and 4 into eq 2 gives eq 5 where the no-
k12
k, = 1 + -k21 &h'/RT
+ -&%/RT k2l
k23°
k30
(5)
menclature is chosen to be the same as that initially derived by Rehm and Weller.33 For the ET quenching of a series of excited-state acceptors by a single donor, k12,k z l , k23, and k30 are considered to be constant. If the ET is thermodynamically reversible, the free energy of activation for the ET step can be expressed as a function of the free energy of the ET (a free energy relationship). While a wide variety of free energy relationships have been proposed, the Rehm-Weller equation (eq 6)33and the hyperbolic equation derived by Marcus (eq 7)34have been ex-
(33) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834; Isr. J . Chem. 1970, 8, 259.
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3819
Quenching of Benzenes by Hexamethyldisilane tensively applied to a wide variety of ET reactions. The general characteristics of ET reactions in organic excited-state systems’ and the validity of free energy relationships have been discussed extensively elsewhere.35 Shown in Figure 4 is a semilog plot of the observed k, values for the hexamethyldisilane quenching data vs the free energies of the ET process as determined by eq 8 using AGZ3’ = 23,06[E(D/D+)
- E(A-/A) - eo2/ac-
(8)
a value of -0.06 eV for the eo2/aeterm as determined previously for acet~nitrile.~~ We have used AG23/here in both eq 8 and Figure 4 to indicate the fact that the true free energy for the ET step is not known and that the irreversible oxidation potential for hexamethyldisilane [E(D/D+) = 1.88 V vs S C E in CH3CN]32 has been used. This is discussed more fully below. The solid curve in Figure 4 represents the curve calculated via the free energy relationship of eq 6 with the empirical values determined by Rehm and Weller33for reversible ET quenching of aromatic hydrocarbon fluorescence. The dashed curve is the nonlinear least-squares best fit to the quenching data using eq 6. The best-fit line yields a value for the “intrinsic barrier”36AGz3*(0)of 2.9 kcal mol-’. A nonlinear least-squares fit using the hyperbolic function gives a value for AGz3*(0)of 2.7 kcal mol-’ and is essentially indistinguishable from the best-fit line using the Rehm-Weller equation. , kfvalues are precise as gauged by the standard While the k , ~ and deviations of the Stern-Volmer data, considerable error is introduced by the variation in reported oxidation and reduction potentials for the fluorophores. The free energy relationships of eq 6 and 7 require the use of thermodynamic values for the free energy values. The oxidation of hexamethyldisilane in acetonitrile, however, is irrever~ible~~ and results in cleavage of the Si-Si u-bond presumably by a radical ion process. The applicability of free energy relationships to irreversible ET processes has been discussed by Schuster3’ and by Balzani and S c h u ~ t e r .Several ~~ cases where photochemically induced irreversible E T leads to a-bond cleavage have been investigated in peroxide systems.38 In these cases, a plot of log k, vs the free energy values determined from the irreversible peak potentials exhibits the same features as when the true free energies are used. The only difference from the plot using the true free energy is that the true position for AG23 = 0 kcal mol-’ is not known. Since the bond cleavage involves nuclear motion, the value for the AG23*(0)is expected to be larger than that observed for thermodynamically reversible ET reactions involving little nuclear rearrangement. The curved region of the log k, vs AG23 plot is broadened by this increase in AGz3*(0)as observed in the present case. However, as seen from the nonlinear least-squares fit curves, little variation is observed from the AG23*(0)value empirically obtained by Rehm and Weller33for reversible ET reactions. The demonstration of the involvement of an ET process in the quenching of the fluorescence of electron-deficient benzenes by hexamethyldisilane suggests that ET may also play a role in the photochemical reactions of a variety of phenyl-substituted polysilanes. We suggest that u to a* C T or E T occurs in the CT emissive states of arylpentamethyldisilanes (l),in the CT emissive states of substituted (phenylethyny1)pentamethyldisilanes (2), and in the photochemical cleavage of the phenyl-substituted polysilane high polymers and can also potentially explain the origin of the observed photoconductivity of poly(phenylmethylsi1ane). The probable role of u to A* C T or E T in each of these examples will be briefly described below. Shizuka et a1.I8 have recently investigated the unique emissive singlet states from a series of phenyl-substituted pentamethyl(34) Marcus, R. A. J . Phys. Chem. 1968, 72, 891. Agmon, N.;Levine, R. D.Chem. Phys. Lett. 1977, 52, 197. (35) Scandola, F.; Balzani, V.;Schuster, G. B. J . Am. Chem. SOC.1981, 103, 2519,and references therein. (36) The intrinsic barrier includes nuclear motion and solvent reorganization necessary prior to ET. (37)Schuster, G. B. J . Am. Chem. SOC.1979, 101, 5851. (38)Schuster, G. B. Acc. Chem. Res. 1979, 22, 366. Horn, K. A,; Schuster, G. B. J. Am. Chem. SOC.1979, 101, 7097. Horn, K. A.; Schuster, G. B.; Zupancic, J. J. Ibid. 1980, 102, 5279. Horn, K.A,; Schuster, G. B. Tetrahedron 1982, 38, 1096,and references therein.
disilanes. These states are characterized by an emission usually observed in polar solvents which is broad and featureless and is red-shifted of the phenyl substituent monomer fluorescence. These short-lived fluorescent emissions which are often observed in organic glasses at 77 K have been assigned to a singlet 2pa*3da* state. However, recent work by SakuraiZ0suggests that the direction of CT is in fact from the disilane moiety to the 2pn* excited singlet state of the aromatic. He has suggested that the C T requires a twisting in the excited state similar to that in the well-known TICT states.39 However, one distinguishing feature involves the fact that the ET is from a a-bonding level and not from a lone pair of electrons. SakuraiZohas suggested that these systems be assigned the general name of orthogonal intramolecular charge-transfer states (OICT). In fact, if one surveys the compounds studied by Shizuka et al., the C T emission is only observed in those cases where the I P of the aryl substituent is sufficiently high that the ET from the disilane to the excited singlet state of the aromatic is exothermic. Thus, the emission is observed when the aromatic ligand is phenyl, 2-methylphenyl, and 2,s-dimethylphenyl. However, when the I P is raised by further alkyl substitution, the band is very weak and no C T is observed for the anthryl, phenanthryl, or pyrenyl substituents. This is exactly the prediction for a u to 2pa* C T state but not for the 2pa*3da* state postulated by Shizuka. Similar emissive states have also been observed for the substituted (phenylethyny1)pentamethyldisilanes(2) and have been assigned once again to 2pa*3da* C T excited singlet states.19 We have, however, recently investigated the low-temperature emission spectra of a series of meta- and para-substituted (phenylethyny1)pentamethyldisilanes(2) and have been able to show that the C T emission is only observed in those cases where the substituent increases the IP of the phenylacetylene chromophore above that of the pentamethyldisilane fragment2I (estimated by the IP of hexamethyldisilane, I P = 8.69 eV).” In the case where the arylacetylene chromophore is (p-methoxypheny1)acetylene or 2-naphthylacetylene, no C T emission is observed. We have therefore assigned these stsites as u to a* C T states.” While no direct evidence has been provided to date, these likely provide another example of the OICT states proposed by Sakurai. Much effort has also recently been extended on the determination of the mechanism of the photochemical cleavage of the polysilane high polymers.40 The investigation of these mechanisms has taken on particular importance because of the potential technological applications of these materials as photoresists and photo initiator^.^^ While the mechanism of the cleavage of alkyl-substituted polysilane high polymers has been shown to occur largely through radical and silylene extrusion processes, some anomalies occur with the phenyl-substituted polysilanes. For example, the quantum yield for photofragmentation of poly(phenylmethylsilane) is 5 times higher than that of poly(dodecylmethyl~ilane).~~This, coupled with the fact that the naphthyl-substituted polysilanes show only emission properties that can be associated with the isolated naphthyl and silane backbone chromophore^,'^^^^ suggests that an ET pathway for photofragmentation may be operating. Since the naphthyl chromophore’s IP is lower than that of the Si-Si u-bond backbone, no E T occurs when the naphthyl chromophore is excited. In the phenyl-substituted case, however, the IP of the phenyl group is higher than that of the Si-Si framework. An ET followed by subsequent fragmentation of the Si-Si bond by a radical ion process could be involved. A photochemically induced ET mechanism may also potentially explain the observed photoconductivity in phenyl-substituted (39)Rettig, W. Angew. Chem., In!. Ed. Engl. 1986, 25, 971. (40) Trefonas, P.,111; West, R.; Miller, R. D. J . Am. Chem. SOC.1985, 107, 2131. (41) West, R. L’actualite Chim. 1986, 64. Miller, R. D.;Fickes, G. N.; Hofer, D.; Scoriyakumaran, R.; Willson, C. G.; Guillet, J. E.; Moore, J. Polym. Mnter. Sci. Eng. 1986, 55, 599. Miller, R. D.;Hofer, D.; Fickes, G. N.; Willson, C. G.; Marinero, E. Polym. Eng. Sci. 1986, 26, 1129. (42) West, R.; Trefonas, P., 111; Miller, R. D.; Hofer, D. J . Polym. Sci., Polym. Lett. Ed. 1983, 21, 823. (43) Todesco, R.V.;Kamat, P. V. Macromolecules 1986, 19, 196.
J . Phys. Chem. 1988, 92, 3880-3883
3880
polysilane high polymers. Ziegler et aL4, have reported that poly(phenylmethylsi1ane) is a good photoconductor in which only holes are mobile. The electronic transition which creates the carriers was reported to be a x*-a* transition from the excited singlet state of the aromatic to the silicon backbone a-bonds. The hole was reported to move by a hopping mechanism between phenyl groups. A speculative explanation for the formation of the holes based on the current work would involve excitation of the x* singlet state of the phenyl group followed by ET from the Si-Si bond framework to the lowest energy singly occupied molecular orbital (SOMO) of the excited phenyl group. Recent provides evidence for hole migration in the silicon framework. Not only would this mechanism be in keeping with the direction of C T observed in the quenching of the excited singlet states of aromatics by hexamethyldisilane, but it would also be analogous to the mechanism for generation of semiconductor (44) Kepler, R. G.; Ziegler, J.; Harrah, L. A,; Kurtz, S. R. Bull. Am. Phys. SOC.1983, 28, 362. (45) Fujino, M. Chem. Phys. Left. 1987, 136, 451.
properties in polysilanes by doping with strong electron acceptors. In contrast, u-u* states are most likely involved in the photoconduction mechanism in alkyl-substituted p ~ l y s i l a n e s . ~ ~ Conclusion We have shown that the quenching of the excited singlet states of electron-deficient benzenes by hexamethyldisilane proceeds by an ET or C T process and can proceed at essentially the diffusion limit in cases where the exothermicity of the ET step is large and negative. This ET reaction is one member of a unique class in which the ground-state donor is an electron-rich, u-bonded species. An investigation of the photochemical reactions of polysilanes currently extant in the literature suggests that these u to T * ET processes may be widespread.
Acknowledgment. Support for this research by the AlliedSignal Corporation is gratefully acknowledged. Registry No. Polysilane, 32028-95-8;hexamethyldisilane, 1450-14-2; benzene, 7 1-43-2; cu,cu,a-trifluorotoluene,98-08-8;benzonitrile, 100-47-0; hexafluorobenzene, 392-56-3; 1,4-dicyanobenzene,623-26-7.
Resonance-Enhanced Multiphoton Ionization Spectra of the SiCl Radical between 430 and 520 nm Russell D. Johnson 111: Erti Fang,$ and Jeffrey W. Hudgens* Chemical Kinetics Division, Center f o r Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: November 23, 1987)
The resonance-enhanced multiphoton ionization spectra of the Sic1 radical observed between 430 and 520 nm are reported. The spectral structure arises from two-photon transitions to the C 211r,D *2+,and E states. Absorption of a third laser photon formed the cation. No subsequent fragmentation of cations was observed. The spectra terminating on the C 211r(v’=3-7) levels and the Si37Clspectrum of the C *I& state are reported for the first time. The Si35C1C 211rstate spectroscopic constants of w, = 682.7 iZ 3.8 cm-’ and wexe= 3.8 iZ 0.5 cm-’ are reported.
I. Introduction This paper demonstrates the first detection of Sic1 radical by resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Throughout the spectral region of this study very strong SiCl+ ion signals were observed which suggests that REMPI spectroscopy can be used to sensitively detect relatively small concentrations of Sic1 radicals. Sic1 appears as one of the intermediates in the plasma reduction of Sic], to produce semiconductor Si crystals.’ A sensitive method for detecting Sic1 radicals, which REMPI provides, may further the understanding of such Si growth mechanisms. Resonance enhancement originates from two-photon preparation of the C 211r,D 2Z+, and E states. Part of the present work confirms bands previously observed with the traditional emission and absorption spectroscopies, but the REMPI spectrum revealed many more, previously unreported vibrational bands. The sensitivity and mass selectivity of REMPI also enabled the first measurement of the band positions of the SP7C1isotopic radical. This isotopic information ascertained the vibrational numbering assignments of the REMPI bands. On the basis of this larger assigned data set, we report spectroscopic constants, u, and for the C 211, state. 11. Experimental Methods
The apparatus is similar to one previously described.2 Briefly, it consists of a flow reactor in which C1 atoms reacted with silane
* Author to whom correspondence should be addressed. + NRC/NBS
Postdoctoral Associate.
.\‘;siting Scientist.
0022-3654i58/2092-3880$01.50/0
(SiH,) to produce Sic1 radicals. A portion of the flow reactor effluent leaked through a skimmer and into the ion optics of a time-of-flight mass spectrometer. The Sic1 radicals were ionized by the focused light from an excimer laser pumped tunable dye laser. The ions were extracted into the time-of-flight mass spectrometer, and the mass-resolved m / z 63 or 65 (corresponding to Si35C1or Si37C1) ion currents were recorded. The vacuum chamber which enclosed the mass spectrometer was operated at a pressure of Torr. Chlorine atoms were produced by passing -2 Torr of a 10% cl2/90% He mixture through a microwave discharge. Downstream from the microwave discharge near the sampling skimmer, an injector tube introduced 10 mTorr of a 15% SiH4/85% H e mixture. Maximum signal was obtained when the injector was very close (-3 mm) to the skimmer. Under these very C1 atom rich conditions Sic1 was produced. Presumably, other radical species were also generated under these reaction conditions and the Sic1 radical may not have been the predominant product. As expected of such a multistep reaction, the magnitude of the Sic1 signal was very sensitive to the amount of SiH, introduced. On the basis of our calculations, which assume 100% conversion of SiH4 into SiC1, we can detect at least lo9 radicals/cm3 in the ionization region of the mass spectrometer with a single laser shot. The tunable dye laser generated 5-20-mJ pulses which were focused into the ionization region by a 250-mm lens. The spectra presented are not corrected for variations in laser power; however,
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(1) Bruno, G.; Capezzuto, P.; Cicala, G.; Cramarossa, F. Plasma Chem. Plasma Process. 1986, 6, 109. (2) Dulcey, C. S.: Hudgens, J. W. J . Phys. Chem. 1983, 87, 2296.
0 1988 American Chemical Society
