Mechanism of the photodissociation of 4-diphenyl(trimethylsilyl)methyl- N,N-dimethylaniline

Article abstract of DOI:10.1021/jo000023w

On irradiation in hexane (248- and 308-nm laser light) 4- diphenyl(trimethylsilyl)methyl-N,N- dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylaminotriphenylmethyl radical, 3(·) (λ(max) at 343 and 403 nm), in very high quantum yield (Φ = 0.92). The intervention of the triplet state of 2 (λ(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3- diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3(·) is further demonstrated using EPR spectroscopy. The detection of the S₁ state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (τ(f) = 1500 ps, Φ(f)= 0.085). The S₁ state is converted almost exclusively to the T₁ state (Φ(T) = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(·)⁺, and (2) photodissociation of the C-Si bond, giving radical 3(·) as before in hexane. The formation of 2(·)⁺ occurs through a two-photon process. Radical cation 2(·)⁺ does not fragment further, as would be expected, to 3(·) via a nucleophile(MeCN)-assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2(·)⁺ with the solvent (MeCN) and transfer to it of the electrofugal group Me₃Si⁺. The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.

Full text of DOI:10.1021/jo000023w

4274
J . Org. Chem. 2000, 65, 4274-4280
Mech a n ism of th e P h otod issocia tion of
4-Dip h en yl(tr im eth ylsilyl)m eth yl-N,N-d im eth yla n ilin e
Dimitrios A. Tasis,1,† Michael G. Siskos,† Antonios K. Zarkadis,*,† Steen Steenken,‡ and
Georgios Pistolis§
Department of Chemistry, University of Ioannina, 451 10 Ioannina, Greece,
Max-Planck-Institut fu¨r Strahlenchemie, 45470 Mu¨lheim, Germany, and N.C.S.R, “Democritos”,
15310 Aghia Paraskevi, Attiki, Greece
azarkad@cc.uoi.gr
Received J anuary 10, 2000
On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,N-
dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylamino-
triphenylmethyl radical, 3• (λmax at 343 and 403 nm), in very high quantum yield (Φ ) 0.92). The
intervention of the triplet state of 2 (λmax at 515 nm) is clearly demonstrated through quenching
experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond
laser flash photolysis (LFP). The formation of 3• is further demonstrated using EPR spectroscopy.
The detection of the S1 state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was
found to be 1400 ps, in agreement with the fluorescence lifetime (τf ) 1500 ps, Φf ) 0.085). The S1
state is converted almost exclusively to the T1 state (ΦT ) 0.92). In polar solvents such as MeCN,
2 undergoes (1) photoionization to its radical cation 2•+, and (2) photodissociation of the C-Si bond,
giving radical 3• as before in hexane. The formation of 2•+ occurs through a two-photon process.
Radical cation 2•+ does not fragment further, as would be expected, to 3• via a nucleophile(MeCN)-
assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of
the triphenylmethyl group prevents interaction of 2•+ with the solvent (MeCN) and transfer to it
of the electrofugal group Me3Si+. The above results of the laser flash photolysis are supported by
pulse radiolysis, fluorescence measurements, and product analysis.
In tr od u ction
ability to undergo C-Si bond cleavage with formation of
benzyl and Me3Si• radicals. Moreover, phenyl substitution
on the benzylic carbon leads to a weakening of the C-Si
bond, which further facilitates its photocleavage, as has
been found for similar systems.7,8 Compound 1 is such a
system,7 and we showed recently that its photofragmen-
Photodissociation represents one of the most important
key steps in photochemical processes. In photopolymer-
ization reactions, for example, the photoinitiator, being
a chromophoric system, is able to undergo efficient
photodissociation giving free radicals that subsequently
trigger the polymerization reaction.2
Whereas benzyltrimethylsilane PhCH2-SiMe3 has been
described3,4 as nearly photochemically inert under direct
irradiation, introduction of chromophore groups such as
the anilino group on the benzylic carbon5 or the benzoyl
group6 on the aromatic ring considerably enhances its
tation efficiency sharply increases relative to PhCH2-
SiMe3, giving p-(benzoyl)triphenylmethyl radical with a
high quantum yield (∼0.9). Compound 1 proved to be a
good photoinitiator for the polymerization of methyl
methacrylate and styrene.9 The very reactive radical Me3-
Si•10 produced by the photodissociation of the C-Si bond
adds to the monomer double bond, thus initiating the
polymerization.9 In the past, the highly toxic Hg(SiMe3)2
* Tel: (+30 651) 98379, 98398. Fax: (+30 651) 98799. E-mail:
azarkad@cc.uoi.gr.
† University of Ioannina.
‡ Max-Planck-Institut fu¨r Strahlenchemie.
§ N.C.S.R, “Democritos”.
(1) Tasis, D. Ph.D. Thesis, University of Ioannina, in preparation.
Siskos, M. G.; Tasis, D. A.; Zarkadis, A. K.; Steenken, S. 37th IUPAC
International Chemical Congress, Berlin, 14-19 August 1999.
(2) Allen, N. S. Photopolymerization and Photoimaging Science and
Technology; Elsevier Applied Science: London, 1989. Scranton, A. B.,
Bowman, C. N., Peiffer, R., Eds.; Photopolymerization; Fundamentals
and Applications; ACS Symposium Series 673; American Chemical
Society: Washington, DC, 1997.
(6) Georgakilas, V. Ph.D. Thesis, University of Ioannina, 1998.
Zarkadis, A. K.; Siskos, M. G.; Georgakilas, V.; Steenken, S.; Karako-
stas, N.; Garas, S. K. XVIIIth International Conference on Photochem-
istry, Warsaw, August 3-8, 1997.
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J . Organomet. Chem., 1980, 201, 197.
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•
Com. 1985, 1780. (b) A very low quantum yield of PhCH2 (Φ ) 0.07)
generated by the photolysis of PhCH2SiMe3 in MeOH has been reported
in a recent paper.4c (c) Hiratsuka, H.; Kobayashi, S.; Minegishi, T.;
Hara, M.; Okutsu, T.; Murakami, S. J . Phys. Chem. A 1999, 103, 9174.
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10.1021/jo000023w CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/21/2000

Mechanism of Photodissociation
J . Org. Chem., Vol. 65, No. 14, 2000 4275
has been used as a source of Me3Si• radicals for the
photopolymerization of styrene.11 Di- and polysilanes12,13
that form silyl radicals photochemically have also been
used.
cation. The latter may subsequently transfer the elec-
trofugal group Me3Si+ to the solvent (MeCN) or to
another nucleophilic reagent present in the medium,
giving finally benzyl radicals.
In this paper we present an investigation of the
photochemical and photophysical behavior of 2 using
nanosecond and picosecond laser flash photolysis (LFP)
and explore its potential as a photoinitiator with styrene
and methyl methacrylate. The results are also supported
by pulse radiolysis, EPR spectroscopy, fluorescence, and
product analysis.
These findings prompted us to examine the photo-
chemical behavior of an analogous system 2. We es-
sentially replaced the benzophenone chromophore by the
dimethylanilino group, which possesses a higher excita-
tion energy.14 Increase of the excited state energy should
more readily lead to photodissociation15 and formation
of the stabilized radical 3• and the very reactive (against
ethylenic monomers) Me3Si• radical. The latter is a good
initiator in polymerization reactions, as we showed
earlier using 1.9
On the other hand, the dimethylanilino chromophore,
as a result of its low ionization potential, introduces an
additional complication to the system: the tendency to
photoionize in polar solvents, giving the corresponding
radical cation.16 However, radical ions are very reactive
intermediates and in general undergo very efficient
fragmentation in solution, generating a radical and a
cation.17 Especially, in the case of benzyltrimethylsilane
derivatives, numerous investigations18 focus on their
photooxidation, which affords the corresponding radical
Exp er im en ta l Section
Ma ter ia ls. The synthesis of compound 2 has been described
previously.5 Acetonitrile, hexane, cyclohexane, propanol-2
(Merck), and n-BuCl (Fluka) were spectroscopic grade and used
as received.
Meth od s. Fluorescence emission and excitation spectra
were obtained on a Edinburgh FS900 spectrofluorimeter.
Fluorescence quantum yields were measured by comparison
with the reported values for aniline (Φf ) 0.1719 and 0.1120 in
cyclohexane with excitation wavelengths λex ) 290 and 254
nm, respectively, and Φf ) 0.1521 in MeCN with excitation
wavelength λex ) 290 nm). Fluorescence lifetimes were mea-
sured using a time-correlated single-photon counter. The
samples were excited with a spark lamp (PRA 510C), filled
with hydrogen gas (0.6 bar) and operated at 6 kV.
(10) The Me3Si• radical is known to undergo very fast additions to
olefinic bonds (k ≈ 107-109 M-1s-1); see: Chatgilialoglu, C. Chem. Rev.
1995, 95, 1229. Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am.
Chem. Soc. 1983, 105, 3292. Choo, K. Y.; Gaspar, P. P. J . Am. Chem.
Soc. 1974, 96, 1248. J ackson, R. A. J . Chem. Soc., Chem. Commun.
1974, 573.
(11) Hisayoshi, I.; Yozo, M.; Masayoshi, K. Macromol. Chem. 1976,
177, 2647.
(12) Woo, H. G.; Park, S. H.; Park, J . Y.; Yang, S. Y.; Ham, H. S.;
Kim, W. G. Bull. Korean Chem. Soc. 1996, 17, 373.
(13) Alonso, A.; Peinado, C.; Lozano, A. E.; Catalina, F.; Zimmer-
mann, C.; Schnabel, W. J . Macromol. Sci., Pure Appl. Chem. 1999,
A36, 605. Miller, R. D.; Michl, J . Chem. Rev. 1989, 89, 1359. West, R.;
Wolff, A. R.; Peterson, D. J . J . Rad. Curing 1986, 13, 35. Yu¨cesan, D.;
Hostoygar, H.; Denizligil, S.; Yagci, Y. Angew. Makromol. Chem. 1994,
221, 207. Chen, H. B.; Chang, T. C.; Chiu, Y. S.; Ho, S. Y. J . Polym.
Sci., Polym. Chem. Ed. 1996, 34, 679. Semenov, V. V.; Cherepennikova,
N. F.; Artmicheva, S. B.; Razuvaev, G. A. Appl. Organomet. Chem.
1990, 4, 163.
Electron paramagnetic resonance spectra were obtained on
a Varian E-109 spectrometer. The EPR tube was irradiated
with a Heraeous TNN 15/32 W low-pressure lamp and then
was transferred to the cavity of EPR. Gas chromatographic
analyses and separations were conducted on
a Hewlett-
Packard 5890, Series II, FID gas chromatograph with an OV-
1701, 15 m capillary column and a Siemens 1 mV recorder
(injector, 200 °C; detector, 300 °C; column temperature, 70-
280 °C, 8 °C/min). GC-MS analyses were performed on a SSQ
700, EI instrument with the same column as above and under
identical conditions.
P h otoch em ica l Exp er im en ts (P r od u ct An a lyses). The
samples were irradiated using a 248-nm (KF*) or a 308-nm
(XeCl*) excimer laser at 20 °C or a Philips HPK-125 W
medium-pressure mercury vapor quartz lamp in special quartz
cuvettes. The photolysis solutions were in all cases purged with
argon before irradiation.
Na n osecon d a n d P icosecon d La ser F la sh P h otolysis
Exp er im en ts. The solutions (A/cm ca. 0.8-1.6) were deoxy-
genated by bubbling with argon and photolyzed at 20 °C in a
flow system (Suprasil quartz cell) using 20-ns pulses (0.3-
100 mJ ) of 248 nm light (KrF*) from a Lambda Physik EMG
103MSC excimer laser or 308-nm light (XeCl*) from a Lambda
Physik EMG150E laser. The time-dependent optical absorp-
tion signals of the transients were digitized simultaneously
by Tektronix 7612 and 7912 transient recorders interfaced
with a DEC LSI 11/73+ computer, which also was used for
process control of the apparatus and performed on-line analy-
sis of the experimental data.
The transient picosecond spectrometer (“pump-probe” tech-
nique) was based on a mode-locked Nd:YAG laser (PY61C-10,
Continuum): λ ) 1.064 µm, pulse width of 30 ps, pulse
repetition rate of 10 Hz, single pulse energy E ) 40 mJ . As
pump beam the excitation wavelengths λexc ) 266 nm and as
a probe beam the white light continuum (λ ) 420-900 nm)
generated in the flow cell with D2O were used. The angle
between the excitation and probing white light continuum
beams was 90°. The accumulation over 100 pulses allowed for
the changes in absorbance to reach the sensitivity level of
e0.01.
(14) Benzophenone, Etriplet ) 68 kcal/mol; dimethylaniline, Esinglet
)
91.5 kcal/mol in nonpolar solvents, 89.6 kcal/mol in polar solvents and
Etriplet ) 75.8 kcal/mol in polar solvents; see: Murov, S. L.; Carmichael,
I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker:
New York, 1993.
(15) Michl, J .; Bonacic-Koutecky, V. Electronic Aspects of Organic
Photochemistry; Wiley-Interscience: New York, 1990; pp 138, 292, 374.
(16) (a) Malkin, J . Photochemical and Photophysical Properties of
Aromatic Compounds; CRC: Boca Raton, FL, 1992; pp 117, 201. (b)
Malkin, Ya. N.; Kuz′min, V. A. Russ. Chem. Rev. 1985, 54, 1041. (c)
Saito, F.; Tobita, S.; Shizuka, H. J . Photochem. Photobiol. A 1997, 106,
119. (d) Saito, F.; Tobita, S.; Shizuka, H. J . Chem. Soc., Faraday Trans.
1996, 92, 4177. (e) McKellar, J . F. Photochem. Photobiol. 1967, 6, 287.
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5931. (b) Popielarz, R.; Arnold, D. R. J . Am. Chem. Soc. 1990, 112,
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(e) Malsak, P.; Chapman, H., J r.; Vallombroso, T. M., J r.; Watson, B.
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G. Acc. Chem. Res. 1996, 29, 292. (g) Burton, R. D.; Bartberger, M. D.;
Zhang, Y.; Eyler, J . R.; Schanze, K. S. J . Am. Chem. Soc. 1996, 118,
5655. (h) Albini, A. L.W.; Siviero, E.; Mella, M.; Long, C.; Pratt, A. J .
Chem. Soc., Perkin Trans. 2 1995, 1895. (j) Su, Z.; Mariano, P. S.;
Falvey, D. E.; Yoon, U. C.; Oh, S. W. J . Am. Chem. Soc.1998, 120,
10676.
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Todd, W. R.; Mattes, S. L. J . Am. Chem. Soc., 1989, 111, 8973. (b)
Dockery, K. P.; Dinnocenzo, J . P.; Farid, S.; Goodman, J . L.; Gould, I.
R.; Todd, W. R. J . Am. Chem. Soc. 1997, 119, 1876. (c) Freccero, M.;
Pratt, A.; Albini, A.; Long, C. J . Am. Chem. Soc. 1998, 120, 284. (d)
Mella, M.; Fagnoni, M.; Freccero, M.; Fasani, E.; Albini, A. Chem. Soc.
Rev. 1998, 27, 81. (e) Baciocchi, E.; Del Giacco, T., Rol, C.; Sebastiani,
G. V. Tetrahedron Lett. 1989 , 30, 3573. (f) Baciocchi, E.; Del Giacco,
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4276 J . Org. Chem., Vol. 65, No. 14, 2000
Tasis et al.
Sch em e 1
F igu r e 1. Absorption (Ab), fluorescence (Em), and fluores-
cence excitation (Ex) spectra of 2 in cyclohexane at 298 K, λex
at 290 nm. Inset: fluorescence decay trace and best-fitting
curve (8 × 10-6 M in cyclohexane; lower curve, lamp profile
λex ) 290 nm).
Ta ble 1. Absor p tion (νa m a x) a n d F lu or escen ce (νfm a x
)
P ea k s of th e An ilin e a n d 2, F lu or escen ce Lifetim e (τf),
a n d F lu or escen ce Qu a n tu m Yield (Φf ) in Cycloh exa n e
(CH) a n d MeCN a t 25 °C
F igu r e 2. Time-resolved absorption spectra observed on 248-
nm photolysis of an argon-saturated solution of 2 in hexane
(0.076 mM). Spectra recorded at times (O) 30 ns, (b) 125 ns,
and ()) 375 ns after the pulse. Inset: yield of triplet state as
a function of the laser power and time dependence of the
optical density at 515 and 403 nm.
Φf at λex
)
(103 cm-1
)
290 nm
max
νa
1La r 1A
1Lb r 1A
νfmax (103 ∆ν (103 τf
cm-1 cm-1) (ns) 254 nm)
(λex )
sample solvent
aniline CH
)
42.74 (234)a 31.08 (322)a 3.74 2.93 0.17 (0.11)
34.82 (287)
Getoff for aromatic amines21,24 and attributed to photo-
induced chemical transformations in the S2 state.
2. P r od u ct An a lysis. Irradiation of an argon satu-
rated solution of 2 in MeCN (0.1 mM) with the 248- and
308-nm laser light (15-100 shots) yields almost exclu-
sively the desilylation product 2a (Scheme 1) and traces
of p-phenylaniline. The products were identified by GC
and GC-MS through comparison with authentic samples.
The desilylation product 2a shows that the p-(dimethyl-
amino)-triphenylmethyl radical (3•) might be an inter-
mediate from which 2a would be formed via hydrogen
abstraction. Completely identical products were observed
in MeOH as solvent.
aniline MeCN
41.89 (239) 29.94 (334) 4.71
34.65 (289)
0.15
2
2
CH
37.88 (264) 28.65 (349) 3.75 1.5 0.085 (0.075)
32.40 (309)
MeCN
37.49 (267) 27.51 (364) 5.18
32.49 (308)
0.108
a
The values in parentheses are in nm.
P u lse Ra d iolysis Exper im en ts. A 3-MeV van der Graaf
electron accelerator was used as radiation source. The detec-
tion system was similar to that described for the laser. A 20
mm × 10 mm suprasil quartz cell was used. Dosimetry was
performed with N2O-saturated 10 mM KSCN aqueous solution
taking G(OH) ) 6.0 and ꢀ(SCN) )7600 M-1 cm-1 at 480 nm.22
2-
3. La ser F la sh P h otolysis Exp er im en ts. (a ) P h o-
t olysis of 2 w it h 248- a n d 308-n m La ser Ligh t in
Hexa n e. Photolysis of an argon saturated solution of 2
in hexane (0.076 mM) with 248-nm laser light produced
the spectrum shown in Figure 2. The transient measured
30 ns after the pulse is characterized by a broad band in
the region 420-560 nm with λmax at 515 nm and two
sharp peaks at 343 and 403 nm.
Resu lts a n d Discu ssion
1. Absor p tion a n d F lu or escen ce Sp ectr a . Figure
1 shows the absorption and fluorescence emission spectra
of 2 in cyclohexane at 298 K. The absorption spectra show
1
1
two bands, which are attributed to the Lb r 1A and La
r A transitions.23 The Stokes shifts (∆ν) are found to
1
The broad band decays exponentially with a value k
) 6.8 × 106 s-1 (τ ≈ 147 ns, see Figure 2, inset), and its
decay was accompanied by an increase of the two absorp-
tion bands at 343 and 403 nm, respectively (τrise ≈ 122
ns, see Figure 2, inset). This transient is attributed to
radical 3•, which seems to be produced via the triplet
state, since the buildup rate constant is similar to the
decay of the triplet state. The presence of an isosbestic
point (at ca. 440 nm) and the disappearance of both bands
in the presence of oxygen atmosphere support further this
assignment. The amplitude of the T-T absorption at 515-
be similar (Table 1) for the aniline and for compound 2
in cyclohexane.23 However, the larger Stokes shift in
MeCN solution for compound 2 is an indication of the
greater difference in dipole moment between the ground
and excited state due to the partial charge-transfer
character in the S1 state. The fluorescence quantum
yields and fluorescence lifetimes were measured and are
listed in Table 1. The fluorescence quantum yield of 2
decreases slightly on excitation into the S2 excited state
(λex ) 254 nm), an observation first made by Ko¨hler and
(22) J agannadham, V.; Steenken, S. J . Am. Chem. Soc. 1988, 110,
2188.
(23) Sarkar, S. K.; Kastha, G. S. Spectrochim. Acta, Part A 1992,
48, 1611.
(24) Ko¨hler, G.; Getoff, N. J . Chem. Soc., Faraday Trans. 1 1980,
76, 1576. Ko¨hler, G.; Rosicky, C.; Getoff, N. In Excited States in Organic
Chemistry and Biochemistry; Pullman, B., Goldblum, N., Eds.; Reidl:
Dordrecht, 1977; p 303.

Mechanism of Photodissociation
J . Org. Chem., Vol. 65, No. 14, 2000 4277
F igu r e 3. Quenching of the triplet state of 2 at 490 nm by
2,3-dimethylbuta-1,3-diene in cyclohexane (308-nm laser light).
Inset: Stern-Volmer plot for the quenching of the radical
production at 344 and 404 nm.
F igu r e 4. Time-resolved absorption spectra observed on
picosecond 266-nm photolysis of 2 in cyclohexane. Spectra
recorded at 100 ps (full line), 1800 ps (broken line), and 5800
ps (dotted line) after the pulse. Insets: time dependence of
the optical density at 450, 500, and 720 nm, respectively.
nm was found to increase linearly with the laser pulse
power in the range 0-30 mJ (Figure 2, inset a). A linear
dependence was found also for the radical formation, a
fact that is in agreement with a radical formation via
the triplet state.
extinction coefficient of the triplet state of 2 (following
its absorption decay at 515 nm) as ꢀ515 ) 4550 M-1 cm-1
,
To examine the photochemical behavior of this com-
pound on excitation at the long wavelength absorption
band, 308-nm laser light was used; the transient absorp-
tion spectra recorded in a deoxygenated hexane solution
(0.2 mM) are identical to those obtained with 248-nm
laser. The T-T absorption has a maximum at 510 nm,
decays exponentially with a rate constant k ) 7.5 × 106
s-1 (lifetime τ ) 133 ns), and its decay is followed by the
formation of 3• (λmax at 342 and 402 nm and τrise ≈ 119
ns).
The involvement of the triplet state was further con-
firmed by quenching experiments using typical quenchers
such as 2,3-dimethylbuta-1,3-diene (Figure 3), styrene,
and methyl methacrylate (we chose 308 nm laser light
to avoid absorption from the quencher). The rate constant
kq was obtained as the slope of the plot of kobs vs [Q]
according to the simple relationship
very close to that reported for PhNMe2 (ꢀ460 ) 4000 (
500 M-1 cm-1).27 The existence of the isosbestic point at
440 nm (see Figure 2) indicates that the triplet state is
converted exclusively to radical 3•, and under this condi-
tion, the quantum yield of the radical formation is Φ3•
)
ΦT ≈ 0.92. The dissociation efficiency of 2 is very high
and similar to that of 1.
(b) P icosecon d LF P w ith 266-n m La ser Excita tion
of 2 in Cycloh exa n e. The involvement of the triplet
state in the C-Si photodissociation of 2 is further
demonstrated using 266-nm picosecond LFP. We ob-
tained thus the spectra shown in Figure 4. The transient
absorption measured at 100 ps after the exciting pulse
is characterized by a broad band in the region 400-750
nm, peaking at 720 nm and showing a shoulder at ca.
510 nm. The absorption band at 720 nm decays expo-
nentially with a rate constant k ) 7.1 × 108 s-1 (τ ) 1400
ps, see Figure 4, inset at 720 nm), while at 450 nm the
decay rate is slower (τ ≈ 3600 ps, see inset at 450 nm).
Thus, after 4 ns the above absorption spectrum is
replaced by a different absorption, having λmax ≈ 510 nm
and remaining constant at τ > 8 ns.
On the basis of the nanosecond LFP results, we assign
the remaining absorption at 510 nm to the triplet state
of 2, which is long-lived and cannot be resolved with the
ps apparatus (τ ≈ 148 ns in hexane, see above). The
transient corresponding to the 720 nm absorption and
decaying with τ )1400 ps is ascribable to the S1 state of
2 on the basis of the fluorescence lifetime reported above
(1500 ps, Table 1). The S1 state intersystem crosses to
the triplet state, a fact in accordance with the quantum
yield Φ ) 0.92 measured for the T1 formation on the ns
time scale. The presence of a quasi isosbestic point
around 500-520 nm (Figure 4 and inset at 500 nm)
kobs ) k0 + kq[Q]
where k0 is the observed first-order decay rate constant
for the triplet state in the absence of quencher and kobs
is the observed rate constant in the presence of a given
quantity of quencher [Q], Figure 3. The kq values ob-
tained in cyclohexane (the absorption maximum of the
triplet is a plateau at 490-520 nm) are 3.6 × 109 M-1
s-1 for 2,3-dimethylbuta-1,3-diene, 4.1 × 109 M-1 s-1 for
styrene, and 1.97 × 109 M-1 s-1 for methyl methacrylate,
comparable25 to reported values for aniline or benzyla-
niline triplets. Also the radical formation measured at
λmax ) 344 and 404 nm obeys a similar Stern-Volmer
relationship; see inset of Figure 3.
Using naphthalene as a triplet quencher and the
known value ꢀ415 ) 24 500 M-1 cm-1 (quantum yield Φ
) 0.75) for its triplet state,26 assuming ΦF + ΦT ) 1 (ΦF
) 0.075, see Table 1), we were able to calculate the molar
strengthens this interpretation, and a value of ꢀ720
≈
15 520 M-1 cm-1 can be calculated for S1 at 720 nm on
(25) Using 2,3-dimethylbuta-1,3-diene as a quencher, the values 1.3
× 1010 M-1 s-1 for aniline triplet16d and 6.4 × 109 M-1 s-1 for
benzylaniline triplet8b were reported in hexane.
(26) Bensasson, E.; Land, E. J . Photochem. Photobiol. Rew. 1978,
3, 163.
(27) Previtali, C. M. J . Photochem. 1985, 31, 233.

4278 J . Org. Chem., Vol. 65, No. 14, 2000
Tasis et al.
F igu r e 5. Time-resolved absorption spectra observed on 248-
nm photolysis of an argon saturated solution of 2 in MeCN
(0.11 mM). Spectra recorded at (b) 70 ns, (Ã) 750 ns, and (()
3 µs after the pulse. Insets: time dependence of the optical
density at 522, 340, and 407 nm, respectively.
F igu r e 6. Time-resolved absorption spectra observed on 248-
nm photolysis of an oxygen saturated solution of 2 in MeCN
(0.1 mM). Spectra recorded at times (0) 25 ns, (b )130 ns, and
(Ã) 7 µs, after the pulse. Inset: time dependence of the optical
density at 265 and 307 nm.
the basis of ꢀ515 ) 4550 M-1 cm-1 of the T1 reported above.
Furthermore, on the basis of the quantum yield of
formation of T1 and lifetime of S1, the intersystem
crossing rate constant kisc ≈ 6.6 × 108 s-1 is calculated,
which is comparable to those obtained for other ani-
lines.16c,d
(c) P h ot olysis of 2 w it h 248- a n d 308-n m La ser
Ligh t in Aceton itr ile. Photolysis of an argon saturated
solution of 2 in MeCN (0.1 mM) produced the spectrum
shown in Figure 5. About 70 ns after the laser pulse a
very broad intense absorption band in the 350-800 nm
region with a maximum centered at ∼530 nm was
recorded. A second band was observed at 300 nm. As
these bands decayed, a new transient with maxima
centered at 340 and 407 nm was formed. As can be seen
in the inset of Figure 5, the rate constant for the decay
of the band at 522 nm closely matches the rate constant
for the growth of the new transient at 340 and 407 nm
(k ≈ 6 × 106 s-1).
It is also worth noting that under oxygen atmosphere
the intensity of the broad band became smaller by at least
3 times and its decay became slower. The shape was
changed, while the maximum was shifted from 530 to
640 nm. Consequently, under argon, the broad band
should consist of at least two transients species: one with
λmax at 530 nm, which disappears under oxygen and
therefore has a radical or triplet character, and a second
one (640 nm), which was unaffected by O2 and probably
possesses cationic character. On the basis of this behav-
ior, the detection of the triplet state of 2 in hexane at
∼515 nm, and the independent generation of the radical
cation 2•+ using pulse radiolysis (see below), we assign
these transients to the triplet state of 2 and to 2•+
,
respectively.
To study the photochemical behavior of 2 on excitation
at the long wavelength band of the absorption, 308-nm
laser light was used. Figure 7a shows the spectra
recorded on irradiation of a 0.55 mM solution of 2 in
MeCN 30, 150, and 700 ns after the pulse. They are
similar with those produced by the 248-nm laser light.
The broad band extending from 400 to 800 nm with λmax
at about 515 nm decays almost exponentially with a rate
constant of 4.2 × 106 s-1. This transient was produced
by a one-photon process, and its decay leads to the
formation of 3• with maxima at 345 and 408 nm (rate
constant of formation k ) 4.1 × 106 s-1 at both wave-
lengths). Since these two absorption maxima also follow
a monophotonic buildup and have the same kinetic profile
as the transient at 515 nm, we can confidently assume
that 3• is produced through the decay of this transient.
The existence of an isosbestic point at 440 nm strength-
ens this assumption further.
The spectrum recorded in the presence of oxygen
(Figure 7b) consists of a broad band in the range 400-
800 nm with maximum at ∼640 nm (as in the case of
248-nm laser irradiation). This transient was produced
by a two-photon process (see inset, Figure 7b) and was
attributed to the corresponding radical cation.
4. P u lse Ra d iolysis Exp er im en ts. As already men-
tioned, the radical cation 2•+ is produced via photoion-
ization of the parent compound in the polar solvent
When oxygen was admitted to the solution ([O2] ) 8.2
mM), a different transient spectrum was observed, see
Figure 6; it consists of two absorption bands, one at 307
nm and a second broad band in the region of 400-800
nm. The latter has a maximum at ∼640 nm and a
shoulder at ∼450 nm. The intense negative signal at 265
nm is attributed to the absorption band of the parent
compound. A second very weak negative signal detected
in the region 310-380 nm (λmax ∼350 nm) within 25 ns
after the pulse can be assigned to the fluorescence
emission of the parent compound (see fluorescence data
in Table 1 and Figure 1). The decay of the absorption at
307, 450 and 640 nm does not follow first-order kinetics
but occurs on the same time scale (1.3 µs), which means
that the bands belong to the same transient. Almost the
same value was calculated for the recovery of the nega-
tive signal at 265 nm (see inset in Figure 6), a fact
indicating regeneration of the parent compound as a
result of the decay of the transient. The transient with
the two bands that appeared under argon at 340 and 407
nm was eliminated when oxygen was present and thus
should belong to radical 3•, an assignment supported by
the detection of 3• in hexane reported above.

Mechanism of Photodissociation
J . Org. Chem., Vol. 65, No. 14, 2000 4279
F igu r e 8. Transient absorption spectra observed on pulse
radiolysis of an oxygenated (b) and a deoxygenated (Ã) 1.6
mM solution of 2 in n-BuCl, recorded at 12.6 µs after the pulse.
F igu r e 7. (a) Time-resolved absorption spectra observed on
308-nm photolysis of an argon saturated solution of 2 in MeCN
(0.55 mM). Spectra recorded at times (b) 30 ns, (O) 150 ns,
and (() 700 ns after the pulse. (b) Absorption spectra observed
from the oxygen saturated solution (b)300 ns and (O)4 µs after
the pulse. Inset: photonity of the transient at 640 nm
immediately after the laser pulse.
Sch em e 2
F igu r e 9. Transient absorption spectra observed on pulse
radiolysis of an oxygenated 2.5 mM solution of 4 in n-BuCl.
Spectra recorded at (b) 600 ns, (Ã) 4.4 µs, and (() 14 µs after
the pulse. The inset shows the decay of the transient at 490
nm.
MeCN. To generate 2•+ independently and to examine
its fate, pulse radiolysis was used. This is a convenient
method for generating radical cations. For this purpose
n-BuCl was used as an appropriate solvent because of
its ability to scavenge irreversibly the electrons produced
by ionizing radiation, Scheme 2.28
band appearing at ∼405 nm that slightly increased as
the radical cation 2•+ decayed. This band is attributed to
radical 3•, produced after desilylation of 2•+
.
The red shift of the long wavelength absorption of 2•+
(∼650 nm) relative to PhN•+Me2 (490 nm) is attributed
to the hyperconjugative interaction of the benzylic Si-C
bond with the delocalized system. Figure 9 shows the
absorption spectrum obtained upon radiolysis of 4 in
The time-resolved absorption spectrum observed upon
radiolysis of a 1.6 mM solution of 2 in n-BuCl in oxygen
atmosphere is given in Figure 8. Two main absorption
bands at 450 and ∼650 nm and a shoulder at <340 nm
were observed 12.6 µs after the pulse; their similar
lifetime (∼32 µs) implies that they belong to the same
transient species. The similarity of this spectrum with
the absorption spectra produced after photolysis of 2 (see
Figures 6 and 7b) in oxygen atmosphere supports the
assignment to 2•+. By conducting the experiment under
argon the spectrum remains the same as expected for a
radical cation, apart from a small additional absorption
n-BuCl in oxygen atmosphere; 4•+ thus produced shows
a maximum at ∼490 nm. Obviously, after replacing Si
by carbon the bathochromic shift observed for 2•+ is
eliminated.
5. EP R Sp ectr oscop y. Irradiation of a solution of 2
in various solvents (hexane, diethyl ether, dioxane,
benzene) with a low-pressure lamp (Heraeus TNN-15W,
(28) Sprinks, J . W. T.; Woods, R. J . An Introduction to Radiation
Chemistry; Wiley-Interscience: New York, 1976; p 396. Adam, W.;
Kammel, T.; Toubartz, M.; Steenken, S. J . Am. Chem. Soc. 1997, 119,
10673.

4280 J . Org. Chem., Vol. 65, No. 14, 2000
Tasis et al.
Sch em e 3
emission at 254 nm) produces a very stable and highly
resolved EPR spectrum with g value 2.0027, character-
istic of carbon centered radicals.29 A complete analysis
of the spectrum was not performed.
6. Con clu sion s. We have shown that on irradiation
in MeCN 2 undergoes photodissociation and photoion-
ization to 3• and 2•+, respectively (Figure 5-7 and Scheme
3). Very probably the formation of 3• takes place from
the triplet state, which is produced monophotonically,
while that of 2•+ is a two-photon process. In a subsequent
step, 2•+ is reduced back to the parent compound 2
(Figure 6), probably through return electron transfer with
superoxide anion O2-‚, as has been also found previously
for the benzyltriisopropylsilane radical cation:18b while
•+
PhCH2SiMe3 undergoes desilylation via nucleophile-
F igu r e 10. Time-resolved absorption spectra observed on 248-
nm photolysis of a saturated with oxygen 0.1 mM solution of
4 in MeCN. Spectra recorded at times (b)125, (Ã) 700, and
(() 7.5 µs after the pulse. Insets: time dependence of the optical
density at 265, 285, and 515 nm, respectively.
assisted C-Si bond cleavage (with MeCN acting as a
nucleophile) and gives PhCH2 radical, PhCH2Sii-Pr3
being more sterically hindered, prevents a similar nu-
cleophilic attack and leads through return electron
transfer with O2-‚ to the regeneration of the parent
PhCH2Sii-Pr3.18a,b,g
•
•+
,
τf )1500 ps, and is converted almost exclusively to the
T1 state (ΦT ) 0.92). The T1 state is persistent on the ps
time scale. The whole sequence of events 2 f S1 f T1 f
3• is thus clearly elucidated in nonpolar solvents such as
hexane or cyclohexane.
EPR spectroscopy also supports the above conclusions.
Photolyzing 2, we recorded a highly resolved EPR spec-
trum that corresponds to a carbon-centered radical of the
trityl type.
In the case of 2•+ the even more demanding steric con-
straints of the triphenylmethyl group (relative to Sii-Pr3)
prevent an analogous nucleophilic attack by the solvent
(MeCN) and thus lead to a regeneration of 2 (Scheme 3)
and not to 3•. This is more surprising since the C-Si bond
strength in 2•+ must be much lower than that in PhCH2-
SiMe3+•. The above conclusion appears to be in contradic-
tion to the pulse radiolysis results, where 2•+ gives to
some extent 3•; this fact is, however, not unexpected if
we consider the presence of a better and smaller nucleo-
phile Cl- produced by the pulse radiolysis experiment.
A further support for the above findings stems from
the behavior of 4•+; here a nucleophile-assisted C-C bond
cleavage with subsequent formation of 3• is unlikely.17e
Radical cation 4•+, produced in MeCN through 248-nm
photolysis (Figure 10), show decay rate constant at 285
and 515 nm (early decay time 1.1 × 106 s-1; inset b,c)
similar to the rate of regeneration of 4 at 265 nm (1.3 ×
106 s-1; inset a). When the experiment is conducted under
argon, no formation of 3• from 4•+ has been observed.
Similar results we obtained also by the 248-nm photolysis
of 4 in i-PrOH/H2O (1/1).
In hexane where the photoionization pathway is absent
(Figure 2), the involvement of the triplet state in the
formation of 3• is clearly demonstrated. The triplet state
decays with the same rate constant as the formation of
3• (isosbestic point at 440 nm), and both follow a linear
Stern-Volmer plot (Figure 3) with 2,3-dimethylbuta-1,3-
diene as a quencher. Furthermore, using picosecond LFP
(Figure 4) we were able to detect the S1 state of 2, which
was found to absorb at λmax ≈ 720 nm. The latter shows
a lifetime τ )1400 ps, close to the fluorescence lifetime
Su m m a r y
In polar media like MeCN, 2 undergoes photoionization
to the radical cation 2•+ in a two-photon process and in
part dissociates to the radical 3• via the triplet state. The
bulkiness of the triphenylmethyl group obviously pre-
vents the dissociation of 2•+ via a nucleophile-assisted
C-Si cleavage to 3•. In nonpolar solvents such as hexane
or cyclohexane, only the second process takes place,
leading to 3• in high quantum yield (∼0.92); the complete
sequence 2 f S1 f T1 f 3• is elucidated using ns and ps
LFP. Despite the high dissociation quantum yield, the
high quenching rate constants of the triplet state of 2 by
styrene (kq ) 4.1 × 109 M-1 s-1) and methyl methacrylate
(1.97 × 109 M-1 s-1) compared to the photodissociation
rate constant 6.8 × 106 s-1 renders questionable its
potential as a photoinitiator with these two monomers.
Work is in progress hereupon.
Ack n ow led gm en t. We thank Bundesministerium
fu¨r Forschung und Technologie (Germany) and General
Secretariat for Research and Technology (Greece) for a
grant (6BOA1A). We thank also Dr. G. Gurzadyan for
the ps measurements and Dr. J . Leitich and Dr. M.
Canle for reading the draft. A.K.Z. thanks the Deutscher
Akademischer Austauschdienst for a fellowship (1998).
(29) Neumann, W. P.; Uzick, W.; Zarkadis, A. K. J . Am. Chem. Soc.
1986, 108, 3762.
J O000023W