222 nm Photo-induced radical reactions in silazanes. A combined

laser photolysis, EPR, GC-MS and QC Studyw

Wolfgang Knolle,*a Luise Wennrich,a Sergej Naumov,a Konstanze Czihal,a

Lutz Prager,aDaniel Decker

band Michael R. Buchmeiser

c

Received 10th September 2009, Accepted 15th December 2009

First published as an Advance Article on the web 19th January 2010

DOI: 10.1039/b918814b

The initiation mechanism of the VUV-induced conversion of polyorganosilazanes into

methyl–Si–O–Si networks was studied by means of model disilazane compounds. A combined

experimental approach was chosen to determine the primary radicals and their properties

(lifetimes, spectra) as well as the major final products. It was verified that both Si–N and Si–CH3

cleavage occur in the condensed phase, the former with higher yield. The lifetime of the primary

Si- and N-centred radicals in de-oxygenated n-hexane solution is less than r10 ms. N-centred

radicals transform into amines by H abstraction, the availability of weakly bonded H as in the

case of tetramethyldisilazane accelerates the reaction considerably. In rigid matrix (frozen

solutions) �CH3, silyl radicals and methylene radicals �CH2R are trapped. In the presence

of oxygen, peroxyl radicals are formed and serve as precursors of the subsequent oxidative

conversion. Product analysis by GC-MS reveals linear R–(Si–O)n– chains rather than branched

compounds as the initial products of the oxidative conversion of tetramethyldisilazane.

It was shown that reactive silylene intermediates do not play a role in the conversion process.

Quantum chemical calculations assist in the interpretation.

Introduction

Polysilazanes are widely investigated and practically used as

precursors for SiOx gas barrier layers in packaging industry,1–3

for flexible solar cells,4,5 for applications in electronics,6 for

vacuum-insulated panels,7 and for anticorrosive protecting

coatings.8,9 Especially the inorganic oligomer perhydropoly-

silazane (PHPS, Fig. 1A) consisting of –SiH2–NH– repeat

units has been reported as a precursor for dense and stable

SiOx networks.10–14 The most conventional pathway for the

realization of such SiOx networks is the hydrolysis of the

Si–NH bonds and the subsequent condensation of the generated

silanols forming Si–O–Si bonds.15,16 At room temperature,

this process proceeds slowly. With the aim to avoid the slow

condensation process, alternatives using UV-irradiation as the

initiating step have been applied.17–20 Employing such a

technology, improved barrier properties on polymer foils as

characterized by lower oxygen and water vapour transmission

rates (OTR and WVTR, respectively) could be achieved in a

roll-to-roll process, which was also found to be suitable for

industrial applications.19 Some limitations on the application

of such coatings as the brittleness of the formed SiOx layers,

their thermoelastic properties (elongation at break, stiffness)

and the hydrophilic character of the surface can be overcome

by the use of organosilazanes, i.e. the incorporation of methyl

groups into the Si–NH–Si network.19 For these purposes,

the vacuum-UV (VUV, l o 200 nm) induced conversion

of polyorganosilazanes like poly(1,1-dimethylsilazane-co-1-

methylsilazane) (P(DMS-co-MS)) (Fig. 1B) into methyl–Si–O–Si

networks with enhanced barrier properties was studied.21

First-order kinetics were found by FTIR in the time scale of

some 10 ms for the photolytically induced decomposition

of the methyl–Si–NH–Si network, the subsequent formation

of the methyl–Si–O–Si network and the concomitant degradation

of the Si–CH3 bond. The release of ammonia and methane

accompanied the conversion process.

Thus it was concluded that the excitation of polysilazanes

leads to the direct cleavage of the Si–N bond19,21 and, in case

Fig. 1 Structures of PHPS (A), P(DMS-co-MS) (B), TMDSz (C),

HMDSz (D), and 7MDSz (E).

a Leibniz-Institut fur Oberflachenmodifizierung, Permoserstraße 15,D-04318 Leipzig, Germany. E-mail: wolfgang.knolle@iom-leipzig.de;Fax: +49 341 2352584; Tel: +49 341 2353607

bClariant Produkte (Deutschland) GmbH, Am Unisys-Park 1,D-65843 Sulzbach a. T., Germany

c Institut fur Polymerchemie, Universitat Stuttgart, Pfaffenwaldring 55,D-70550 Stuttgart, Germanyw Electronic supplementary information (ESI) available: UV-VISspectra of silazanes, EPR spectra of intermediates between 77 and130 K, quantum chemical calculations on probable radical structures:heat of formation, hfs coupling constants and UV transitions. SeeDOI: 10.1039/b918814b

2380 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

BView Online / Journal Homepage / Table of Contents for this issue

of polyorganosilazanes, also of Si–CH3 bonds.21 In contrast to

the earlier work of Naganuma,18 who reported an essential

effect on the oxidative conversion of PHPS due to oxygen

radicals O(1D) and/or ozone (O3), both formed when

oxygen in the surrounding gas absorbs high energy photons

(172 nm, [O2] up to 20%), such processes can not occur

in the present case (222 nm photons, low O2 concentration).

It should be noted that Si–N and Si–CH3 cleavages were

also concluded from product analysis after high-power laser

irradiation of gaseous tetra- and hexamethyldisilazanes.22,23

Therefore this work focuses on the deeper elucidation of the

initiation mechanism of the VUV-induced conversion by

investigating model compounds (Fig. 1). Special attention is

devoted to the determination of the primary species and their

properties (lifetimes, spectra) and to the first reaction steps

following the absorption of photons, thus verifying the

mechanism proposed in our recent paper.21

A combined experimental approach was chosen for the

direct study of fast processes (kinetic measurements by laser

flash photolysis), to investigate intermediate species by

trapping (low temperature matrix-EPR), and to identify the

final products by GC-MS following steady state photolysis.

Quantum chemical calculations support the assignment of UV

and EPR spectra and are used to distinguish between different

reaction pathways.

Experimental

Materials

1,1,1,3,3,3-Hexamethyldisilazane (HMDSz), 1,1,3,3-tetra-

methyldisilazane (TMDSz) and heptamethyldisilazane

(7MDSz) were purchased from ABCR (Karlsruhe, Germany).

Acetonitrile (Ultra gradient HPLC grade, J.T. Baker) and

n-hexane (Uvasol, Merck) were used as received. If necessary,

n-hexane was dried by distillation from sodium/benzophenone

and stored under argon. Triethylsilane (Merck) was used as

received.

Low-temperature EPR spectroscopy. Silazanes were

dissolved in acetonitrile or n-hexane at molar solute ratios

typically between about 1 : 20 to 1 : 1000 (approx.

2 � 10�2–1 mol dm�3 in acetonitrile and 7 � 10�3–4 �10�1 mol dm�3 in hexane) and carefully degassed by freeze-

thaw technique. The solutions were irradiated at 77 K in liquid

nitrogen with 222 nm light of a KrCl* excimer lamp (intensity

at sample position 10 mW cm�2) in intervals between 2 and

180 s and transferred to the EPR spectrometer. The first

spectrum was taken as soon as possible (within about 2.5 min)

after irradiation.

Some irradiation experiments were performed directly in the

cavity of the spectrometer at 100 K with a low-pressure

mercury lamp (HgLP) of low intensity (185 and 254 nm

photons). Deuterated acetonitrile ACN-d3 was used as a

solvent in order to verify that the methyl radicals formed in

dilute solution are not the result of a reaction with the solvent.

All measurements were performed using a Bruker ELEXSYS

E500 spectrometer (9.5 GHz, 100 kHz modulation) equipped

with either a finger Dewar (77 K) or a variable temperature

control unit (ER 4121 VT, at Z 95 K). Spectra were recorded

at a microwave power of 0.1 mW and a modulation amplitude

up to 0.1 mT. Isotropic and anisotropic spectra simulations

were performed using theWinSim24 and SimFonia (BRUKERr)

software, respectively.

Laser flash photolysis experiments. The laser photolysis

set-up comprised of a 222 nm KrCl*-excimer laser (RDC-

EXC-100, Radiant Dyes, pulse width B15 ns, pulse energy up

to 10 mJ) as excitation source and a pulsed xenon short-arc

lamp (XBO 1000, Osram; power supply LP-1000, lamp pulser

MSP 05, both Muller-Elektronik, Moosinning) supplying the

analyzing light. The transient recording electronics, consisting

of a photomultiplier (R928, Hamamatsu, operated at 850 V),

a power supply (PS310, Stanford Research Systems) and a

500 MHz, 2.5 GS s�1 digitizing storage oscilloscope

(TD5034B, Tektronix) guarantee for a time resolution within

the limits set by the excitation pulse. Further details have been

published elsewhere.25,26

All experiments have been carried out in flow-through

cuvettes with 5 � 3 mm2 cross-section. The concentrations

of the solutions were adjusted to have ground state absorbances

of less than 0.3 at the excitation wavelength in order to

provide acceptable absorption conditions. Oxygen-free

solutions were prepared by purging a proper amount of

solvent for 10 min with N2 prior to adding the solute

followed by subsequent purging for 10 min. In case a certain

concentration of oxygen in the solution was to be used,

only the second purging step was performed with the

N2/O2 gas mixture. Purging was sustained during the whole

experiment.

GC-MS analysis of the irradiation products. Diluted solu-

tions of TMDSz in dry hexane (3 ml, 1–4 � 10�3 mol dm�3)

were irradiated in UV cells (Suprasil, 10 � 10 � 35 mm,

Hellma, Mullheim, Germany), modified for purging to remove

the oxygen prior to irradiation. After purging with N2

(10 min), the samples were irradiated with 222 nm photons

with an intensity of 2 mW cm�2 under continuous stirring.

The analysis of the irradiation products in diluted solution

was performed using a GC-MS system 6890N/5973N (Agilent

Technologies, Palo Alto, CA, USA) with split/splitless

injector. The GC parameters were the following: column:

HP-5 ms (Agilent, 30 m, 0.25 mm id, 0.25 mm film thickness),

injector: split mode, 250 1C, carrier gas: helium, 1 ml min�1

(constant flow), temperature program: 40 1C held for 2 min,

then with 5 1C min�1 to 45 1C, after this with 20 1C min�1 to

250 1C, and held for 2 min. The transfer line to the MS ion

source was set to 250 1C. The MS was used in the scan mode

(scan range 12 to 600 amu). In some cases a special column for

volatile compounds (CP-Volamine, Varian, 30 m, 0.32 mm id,

0.45 mm film thickness) was used.

The irradiation experiments with undiluted TMDSz were

realized in a UV cell (Suprasil, 10 � 10 � 35 mm, Hellma,

Mullheim, Germany) with septum screwing. After optional

purging of the irradiation cell with nitrogen typically 200 mlof the prepared organosilazane was injected by a syringe.

Samples were handled by standard Schlenk-techniques and

oxygen was removed from the silazane prior to injection by

3 freeze-and-thaw cycles. UV irradiations were performed with

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2381

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

the KrCl* excimer lamp (intensity 11 mW cm�2) for 1 to 5 min

from top on a horizontally oriented cell, which was gently

shaken during irradiation to exchange the surface layer

continuously. After analysing the headspace of the samples,

the liquid phase was taken up with 1 ml of dry hexane and also

measured.

In order to analyse the volatile irradiation products and to

ensure that none of such volatile compounds is masked by the

solvent peak these were enriched by inserting a solid-phase

microextraction (SPME) fibre (PDMS/DVB, Supelco, 65 mmfilm thickness) in the headspace of the irradiation cell for 1 min

at ambient temperature and by subsequent thermodesorption

of the accumulated compounds in the GC injector (splitless

mode) at 250 1C. The other GC-MS conditions were the same

as mentioned above.

Results and discussion

In order to separate unimolecular decay processes from

bimolecular reactions, experiments in dilute solution were

performed. 222 nm excitation was chosen as a compromise

between the small absorption coefficients of the silazanes

below 230 nm (vide infra), and the limiting availability of

solvents transparent at such excitation wavelengths. In our

previous paper,21 it was shown that irradiation with photons

of different wavelengths, i.e. 172, 185 and 222 nm, had no

effect on the kinetics of the photolytical degradation of

P(DMScoMS), if the dose actually absorbed in the thin layers

is properly taken into account.21 To verify that this is also true

for the model disilazanes in the present case, a number

of comparative matrix EPR experiments were performed

using both a KrCl* excimer (222 nm) and an HgLP

lamp (emission at 185 and 254 nm). Whereas 254 nm

irradiation alone, selected by a 225 nm edge filter, did not

produce any significant amount of radicals, irradiation

with 185 nm gave the same radical species stemming from

the silazanes as where produced by 222 nm photons.

If solutions of silazanes were irradiated by 185 nm photons

the EPR spectra were complicated by additional radicals

formed by direct photolysis of the solvents, which was not

the case with 222 nm light. Acetonitrile and n-hexane were

chosen as solvents, being mostly transparent at 222 nm

(absorbances of o0.01 and o0.05 at 1 cm path length vs.

water). Especially the former is considered to be inert in

photolytical reactions.

Laser flash photolysis with 222 nm light

Laser flash photolysis experiments with B10 ns time resolution

have been carried out to investigate the luminescence properties

and the photochemically induced transformations of the

different disilazanes. All three silazanes have low absorption

coefficients at the excitation wavelength (e = 16, 120 and

615 dm3 mol�1 cm�1 for TMDSz, HMDSz and 7MDSz,

respectively, the ground state absorption spectra are given in

the ESI, Fig. 1S),w thus concentrations between 1 � 10�3–4 �10�3 mol dm�3 were used in the laser flash experiments in

order to keep the absorbance of the solution across the laser

path length (3 mm) below 0.3.

Emission measurements. Luminescence spectra of the

three silazanes were similar with a maximum emission at

280–290 nm (see ESI, Fig. 2S)w and a lifetime of B25 ns.

These spectra can most likely be assigned to the triplet state,

which is quenched by oxygen in air saturated solutions. (decay

rate increases to 1 � 108 s�1, corresponding to a bimolecular

rate constant of k(T* + O2) B 2 � 1010 dm3 mol�1 s�1, no

correction for laser pulse duration was made).

HMDSz and 7MDSz. In N2-saturated hexane solutions

of HMDSz, immediately after the 222 nm laser flash the

spectrum shown in Fig. 2 (J) is observed, showing maxima

at 320 and 240 nm. A comparison of the decay characteristics

at both wavelengths (Fig. 2, traces at 320 and 240 nm in the

lower inset) revealed two different transients. One of them,

with lmax B 320 nm had a decay rate of 5 � 105 s�1, which

was independent of the solute concentration in the range

studied and suggests a unimolecular or a solvent-assisted

pseudo-unimolecular transformation. The second transient

with a maximum near 240 nm was more pronounced after

the decay of the 320 nm band (cf. Fig. 2 (&)), its decay was

somewhat slower (1.7 � 105 s�1, cf. traces at 240 vs. 320 nm,

Fig. 2, lower inset) and partially dependent on the solute

concentration, pointing to the contribution of a bimolecular

termination reaction. In case of 7MDSz, apparently only one

absorption band centred around 255 nm (Fig. 2, spectrum K)

was present, and it decayed with a similar rate constant

as the 320 nm species in HMDSz solution. But a careful

inspection of the wavelength dependence of the time profiles

revealed also the presence of a second longer-lived species

(cf. Fig. 2, 320 nm trace vs. 250 nm trace in the 7MDSz inset).

This transient, however, was masked by the intense UV

absorption band.

Taking the short fluorescence lifetimes and the ‘‘prompt’’

(within the time resolution of our experiment) formation of

two different transients during the laser pulse into account, the

most likely interpretation of the experimental findings is the

fast fragmentation of the silazanes out of the (unrelaxed)

triplet state, which in turn is formed after excitation of the

Fig. 2 Transient spectra observed after laser flash photolysis of N2

saturated solutions of HMDSz and 7MDSz in n-hexane (times after

the flash as indicated). Insets: time profiles of the absorbance at

distinct wavelengths demonstrating the presence of at least two

species.

2382 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

silazanes into their excited singlet state followed by inter-

system crossing (reaction (1)).

ð1Þ

This interpretation is strongly supported by quantum chemical

calculations (vide infra). It was discussed earlier,21 that the

energy optimization of the triplet state geometry was practically

impossible, as a strong bond elongation of the Si–N bond (to

be taken as a hint towards molecular fragmentation) occurred

immediately, when triplet optimization was started from

singlet state geometry. Moreover, calculations of the electronic

absorption spectra of the fragments resulting from the Si–N

scission predict strong absorption bands at 302 nm, 275 nm

and in the 250–270 nm range for the (CH3)3SiN�(H),

(CH3)3SiN�(CH3) and (CH3)3Si

� radicals, respectively (see

ESI, Table 1S),w which is in good agreement with the experi-

mental values. Thus, the strong shorter-lived absorption bands

observed experimentally at 320 and 255 nm can be reasonably

assigned to the N-centred fragments, and the longer-lived

absorption at 240–260 is attributed to the (CH3)3Si� radical,

which is calculated to show a relatively broad band (main

transitions at 247 and 270 nm). The spectrum of the (CH3)3Si�

radical (same fragment for HMDSz and 7MDSz) strongly

overlaps with the more intense spectrum at 320 nm (HMDSz)

and is nearly completely masked by the 255 nm band in case

of 7MDSz.

In case of HMDSz, the influence of oxygen on the decay

rate was investigated. With increasing concentrations of O2 in

solution, the decay of the 320 nm transient (R–N�H, spectrum

J in Fig. 2) becomes slightly faster and a bimolecular rate

constant of B1 � 108 dm3 mol�1 s�1 was derived from a

Stern-Volmar-type plot (see ESI Fig. 3S)w taking the saturation

concentration of O2 in hexane as 1.5 � 10�2 mol dm�3.

Concomitant to the faster decay of the 320 nm band an increasing

absorption band resulting from peroxyl radicals with its

maximum around 260–270 nm was observed and nearly the

same rate constant was derived from the build-up at 270 nm

(reaction (2)). Unfortunately, the peroxyl radical band strongly

overlapped the spectrum of the primary 240 nm transient, thus

the O2-dependence of the latter could not be measured.

ð2Þ

TMDSz. Spectra observed in N2-purged TMDSz/hexane

solutions are shown in Fig. 3. At least three different transients

can be distinguished. First, a fast decay in the 320 nm region is

accompanied with a build-up below 260 nm (cf. Fig. 3, spectra

at 100 ns (K) and 250 ns (J), arrows indicate the regions of

changes). The difference spectrum (cf. Fig. 3, left inset,

spectrum E) of the spectra recorded at 100 and 250 ns

resembles much the initial 320 nm transient observed with

HMDSz, however its decay (best visible at l4 320 nm, Fig. 3,

right inset) is roughly one order of magnitude faster (time

constant of 3.5 � 106 s�1). The new band with a maximum at

260 nm formed concomitantly (cf. Fig. 3, spectrum J and

right inset, compare kinetics at 320 and 260 nm within the first

half ms) disappeared within a couple of microseconds by a

bimolecular termination reaction, as was proven by a variation

of both the TMDSz concentration and the dose at a fixed

concentration. The remaining spectrum finally measured at

20 ms after the decay of the secondary species was similar to

the 240 nm transient in case of HMDSz.

The interpretation of the transient spectra is based on the

same molecular cleavage of the Si–N bond (reaction (3)),

leading to the short-lived 320 nm and the longer-lived

o250 nm transients. Quantum chemical calculations predict

the electronic transitions of the fragments (CH3)2Si�H and

(CH3)2SiH–N�H to occur at nearly the same wavelength as

those of the corresponding radicals with the trimethylsilyl

group in case of HMDSz (see ESI, Table 1S).w The much

shorter lifetime of the N-centred radical of TMDSz in

comparison with HMDSz can be explained by a fast intra-

molecular 1,2 H-shift, leading to the H2N–Si�(CH3)2 radical

(reaction (4)). This reaction is exergonic by DG B�12 kcal mol�1, and the calculated electronic spectrum displayed

very strong transitions at 262 and 275 nm (cf. ESI, Table 1S),win good agreement with the secondary transient spectrum

observed. However, a contribution of another reaction, i.e.

H-abstraction from a ground state molecule (reaction (5)) to

the overall decay can not be excluded at concentrations much

higher than in our laser flash experiments. The latter reaction

is exergonic as well (DG B �13 kcal mol�1) and the

(CH3)2Si�–NH–SiH(CH3)2 radical formed also displays

transitions in the region of interest (270 and 283 nm, cf.

ESI Table 1S).w Such H-abstraction could also stem from

the solvent, however, as the GC-MS experiments do not reveal

Fig. 3 Transient spectra observed after laser flash photolysis of N2

saturated solutions of TMDSz in n-hexane (times after the flash as

indicated). Left inset: difference spectrum of spectra measured at

100 and 250 ns. Right inset: time profiles of the absorbance at distinct

wavelengths.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2383

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

recombination products involving hexane fragments, this can

only be a minor pathway.

ð3Þ

ð4Þ

ð5Þ

In solutions containing O2, the decay of the fast 320 nm

transient was not affected by oxygen up to a concentration

of 3 � 10�3 mol dm�3 (air-saturated solution). The transient

with the maximum at 260 nm decayed much faster with O2 and

a bimolecular rate constant of 6 � 109 dm3 mol�1 s�1 for the

reaction with O2 was estimated. The sensitivity vs. oxygen

supports our assumption of a radical species. The faster decay

at 260 nm at higher O2 concentration was accompanied by an

increase of absorbance at longer timescales (420 ms, see

Fig. 4). This effect can be easily rationalized by the formation

of a new absorption band due to long-lived peroxyl radicals

with a lower absorption coefficient. The same peroxyl radical

band was also observed in the 260–270 nm region in case of

HMDSz. Again, due to spectral overlaps, no information

could be deduced on the reaction of the primary 240 nm

transient with oxygen.

Intermediate species

One question to be answered in the present work was the

possible formation of silylene intermediates. Several silylenes

have been identified spectroscopically in the literature, e.g.

dimethyl-, diphenyl- and dimesityl silylene, all of them

showing intense transitions in the UV and in some cases also

in the visible. 27–29 Their lifetimes in oxygen-free, completely

dry solvents were measured to be in the range of 400 ns

(dimethylsilylene)27 up to 20 ms (dimesitylsilylene),29 quite well

accessible by conventional laser flash experiments.

In the present case, especially for TMDSz, it appeared to be

prudent to check for any formation of silylenes via concerted

reactions involving Si–CH3 and Si–NH bond cleavages, for

clarity denoted as two step processes (reactions (6) and (7)).

ð6Þ

ð7Þ

In the first case, dimethyl silylene must be expected as an

intermediate, showing a strong transition at 465 nm.27

Reaction (6) is exergonic with respect to the primary fragments

(DG B –34 kcal mol�1, DG of +51 kcal mol�1 with respect to

ground state TMDSz). However, this intermediate could be

excluded as the corresponding spectrum was not observed in

our laser photolysis experiments. (Note, that one would expect

fast recombination upon back reaction of the silylene with the

lone pair at nitrogen.)

The second silylene :SiCH3–NH–SiH(CH3)2 has, to the best

of our knowledge, never been identified before. Therefore, its

spectrum was estimated by quantum chemical calculations.

First, a comparison of the known spectra of the silylenes from

literature27 revealed an average red-shift of B20 nm between

the experimentally found and calculated values. Based on this

average red-shift and on a calculated transition at 341 nm (see

ESI, Table 2S)w one can expect the spectrum of methyl aminyl

silylene to be observable near 320 nm. Additionally, calculations

revealed that reaction (7) would be favourable by a change of

the Gibbs free energy DG of �48.4 kcal mol�1 with respect to

the primary radical pair (DG B 21 kcal mol�1 with respect to

the ground state, cf. Fig. 11). As described above, there was a

short-lived spectrum observed in this region, however, this

spectrum was rather assigned to the �NH-R radical than to a

silylene for several reasons. The strongest argument results

from the finding that the decay of the initial 320 nm transient

is concomitant with the formation of a new species, which is

Fig. 4 Time profiles observed at 260 nm in TMDSz/n-hexane

solution in dependence of the oxygen concentration. Concentration

of O2 in the purge gas as indicated. Inset: transient spectra observed at

20 ms after the laser flash, spectrum at 3.5% O2 corresponds to peroxyl

radicals formed.

2384 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

definitely a radical due to its reactivity with oxygen. Such a

reaction (leading to a radical) would not be possible with a

silylene as precursor. Moreover, the insensitivity of the

primary 320 nm transient towards O2 also rules out silylene

as a candidate for the assignment.

It is therefore concluded that reactive silylene intermediates

are unlikely formed or should be extremely short-lived to

escape direct observation. In order to corroborate this con-

clusion, trapping experiments at high scavenger concentration

were conducted (vide infra).

Low-temperature EPR

Tetramethyldisilazane (TMDSz). Irradiation of neat

TMDSz at 77 K and at exposure times o5 s leads mainly to

the formation of a broad singlet (DHpp B 26 G) and some

features with a separation of approx. 70 G (marked with

asterisks, Fig. 5). Magnification of the spectrum (Fig. 5(a))

revealed additional broad sidebands with a separation of

B200 G of low intensity. Such sidebands stemming from the

coupling with 29Si (I = 12, natural abundance 4.7%) are

indicative of Si-centred radicals. Satellite splittings of a simple

silyl radical and methyl-substituted silyl radicals have been

reported to be in the range of 180–190 G (aiso),30 which is in

fact in good agreement with the current measurements.

Essentially the same spectra with slightly better resolution

were observed, in case solutions of TMDSz in ACN or

ACN-d3 were irradiated (see ESI Fig. 4S(a) (c) and (d));w in

the latter case, however, features with B70 G separation were

less pronounced. Keeping the sample at 77 K, these lines

disappeared with a time constant of approx. 1.7 h�1 (= 4.7 �10�4 s�1). The difference spectrum in Fig. 5(b) derived from

the spectra taken immediately and after 1.5 h (keeping the

sample at 77 K) revealed a quartet with sharp lines at binomial

intensity, a coupling constant of 22.6 G (3H) and a g-value of

2.0023. This spectrum can definitely be assigned to �CH3

radicals. The decay rate of 4.7 � 10�4 s�1 in the present case

fits well into the range of rate constants of 7 � 10�5 s�1–4 �10�3 s�1 collected for the decay of methyl radicals in a number

of organic glasses at 77 K.31,32 The decay of the �CH3

spectrum at 77 K was accompanied by a slight increase in

the central part of the spectrum, whereas the total spin

concentration (double integral) remained constant; thus it is

justified to assume a 1 : 1 transformation of the �CH3 radicals

most likely by reaction (8).

ð8Þ

Increasing the temperature to 95 K resulted in a further

decrease of the remaining side bands (marked with *) and a

concomittant increase of both the central part of the spectrum

and the 29Si satellites. At 95 K the main spectrum had a

slightly better resolution and a substructure with 6–7 G

splitting (see ESI Fig. 5S (b)) could be recognized within the

broad singlet (DHpp = 22 G, g B 2.0027). The observation of

this substructure and the presence of the 29Si satellites is

consistent with the assignment of the central ‘‘singlet’’ to silyl

radicals �Si(CH3)2–R. The H-couplings of B6 G resulting

from the two magnetically equivalent CH3 groups are

nearly unresolved in the rigid matrix due to anisotropic line

broadening effects.

Increasing the temperature from 100 to 110 K led to the

decay of the central part of the spectrum and of the 29Si

sidebands as well (see ESI Fig. 6S).w We conclude that in this

temperature range silyl-type radicals disappear mainly by

recombination as no other new lines were observed. The

remaining multiplet signal (see ESI, Fig. 6S, 110 K)w could

not be assigned yet.

Extending the irradiation time from 10 to 80 s in neat

TMDSz gave rise to a considerable change in the spectrum

recorded immediately after at the end of the irradiation (Fig. 6,

spectra (a) and (b)). The newly formed species can be identified

if the primary spectrum recorded at 10 s is substracted in

such a way that its contribution to the spectrum at 80 s is

Fig. 5 EPR spectra observed at 77 K after 222 nm irradiation of

TMDSz. The spectral changes are mainly due to the decay of methyl

radicals as evident from the difference spectrum in (b).

Fig. 6 Effect of irradiation time on the EPR spectra of TMDSz

recorded at 77 K after irradiation with 222 nm light. (a) 10 s, (b) 80 s,

(c) 80 s, corrected for contributions of the initial spectrum at 10 s, (d)

10s irradiation of a 1 : 20 solution in ACN-d3, corrected by initial

spectrum after 5 s.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2385

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

minimized. As one can see in Fig. 6(c), a nearly binomial

quartet is observed with a(3H)B 18.5 G and gB 2.0026. This

quartet is also formed in solution (Fig. 6 (d), 1 : 20 in ACN-d3)

and it seems to be already present at short irradiation times

(cf. ESI Fig. 7S (b) 1 : 99 ACN solution).w In case a neat

system was irradiated at 120 K, only one species was observed,

which is obviously again the same four line spectrum with

somewhat better resolution.

This spectrum can most reliably be assigned to a C-centered

radical with 2 a- and 1 b-proton, i.e. the �CH2–SiHCH3–R

radical. The spectral shape can be well reproduced if a small

anisotropy of the a-protons in the cryogenic matrix is taken

into account (Fig. 7(b)). Another possible assignment, i.e. the�NH–SiH(CH3)2 radical with couplings of B20 G (HN) and

an additional anisotropic splitting due to N (12 G (Niso),

�12 G, �12 G, +24 G (Nani), cf. ESI Table 1S),w has to be

ruled out because of the spectral shape, its g-value of 2.0026 and

the fact, that this species is observed only at long irradiation time

(i.e. higher UV doses). Although the �CH2–SiHCH3–R radical is

thermodynamically less favourable than the silyl radical�Si(CH3)2–R (cf. Fig. 11), the formation of the former is

rationalized in a way that in the rigid cryogenic matrix H

abstraction initiated by �CH3 or�NH-R will occur preferably

in the near vicinity of the primary radicals formed. In this way

partial depletion of Si-bonded hydrogen by reactions (5) and

(8) can increase the yield of H abstraction occurring from the

methyl groups especially at longer irradiation times. Reaction

(9) is still exergonic for both radicals �CH3 and�NHR.

ð9Þ

Hexamethyldisilazane (HMDSz). Short-time irradiation of

HMDSz at 77 K with 222 nm UV light led to the formation of

the spectrum shown in Fig. 8(a). Obviously, several species

overlap, which can be distinguished by their decay temperature

and their kinetics. As in the case of TMDSz, a quartet

spectrum due to methyl radicals can be identified (Fig. 8(a),

stick plot), though the relative intensity is comparably low.

A triplet spectrum dominates the central part right from the

beginning. This spectrum is quite stable and survives all

other species. The small anisotropy of the side bands at low

temperatures (Fig. 8(d)) disappeared at 190–200 K just before

the complete decay of the remaining spectrum, and true

binomial intensities were observed. It is more than reasonable

to assign this triplet spectrum (a(2Ha) = 20 G at 190 K,

g B 2.0025) to the �CH2Si(CH3)2NHR radical, previously

observed by Rhodes33 in a frozen matrix, and probably formed

by different H-abstraction reactions (10) (see discussion later).

ð10Þ

Additionally to the two species readily assigned, two other

species can be identified at 77 K. The low-structured sidebands

(DH (29Si) B 175 G) shown with 10-fold magnification in

Fig. 8(a) definitely point to a Si-centered radical, the main

features (due to 28Si) of which are obscured by the intensive�CH2– triplet signal in the center of the spectrum. Note that its

g-value ofB2.007 (rough estimate) is higher than expected for

simple silyl radicals and therefore it can only stem from a

photolysis product not yet assigned. Another spectral feature,

which is roughly a broad singlet (DHpp B 15 G; cf. Fig. 8(a),

asterisk), was found at a g-value of B2.022. This species

became more pronounced at longer irradiation times (cf. ESI,

Fig. 8S)w and was observed in neat HMDSz or at higher

concentration in ACN (molar ratio41 : 100) only, pointing to

a species formed in a bimolecular reaction or in a reaction

within a complex (aggregated molecules). The g-value

Fig. 7 (a) EPR spectra observed after irradiation of TMDSz at 120 K

with a HgLP lamp. (b) Fit assuming 28/18/18 G (2 Ha) and 17G (1Hb).

Fig. 8 EPR spectra observed after UV irradiation of HMDSz

(at 77 K, 222 nm) and measured at the temperatures indicated.

Irradiation time and microwave power: (a) 2 s, 1 mW; (b) 10 s,

10 mW; (c),(d) 10 s, 1 mW.

2386 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

of B2.022 is too high for regular silyl or aminyl radicals,

however due to its simple, unresolved spectrum an assignment

was impossible up to now. Both signals decayed at 77 K within

a couple of hours, the 29Si satellites being slightly more

persistent. Concomitantly, the triplet signal increased.

Thus, the spectral changes observed prove the formation of

secondary �CH2–Si(CH3)2–R type radicals, which seem to be

the most persistent radical species in the system. After the

decay of the 29Si-sidebands and the spectrum marked with an

asterisk, another species was observable, which consisted of

quintets with a splitting of B10 G separated by 110 G and a

g-value of B2.0025 (cf. Fig. 8(c)). This signal can not stem

from another Si-centred radical, because the corresponding

sharp multiplet lines with a 20-fold intensity due to 28Si,

expected in the center of the spectrum, were completely

missing. Although the spectrum can be well simulated with

two comparably large proton couplings of B55 G (the central

part being obscured by the stable �CH2-type radicals), there is

no simple transformation conceivable, leading to an adequate

radical structure. The nature of this signal is still under

investigation and quantum chemical calculations on a possible

structure are in progress to assist in the interpretation.

In the case that air saturated solutions were irradiated, the

same initial spectra are observed at 77 K, but temperature

increase to T 4 110 K resulted in the formation of silyl

peroxyl radicals, which were stable up to 200 K in case of

HMDSz and 170 K in case of TMDSz.

Steady-state photolysis and GC-MS measurements

In order to further elucidate the mechanism of the (V)UV

initiated decomposition reaction of (poly)organosilazanes we

investigated the stable products of the model compounds that

were subject to 222 nm irradiation. Irradiation experiments

were conducted using a lockable UV cell either under a N2

atmosphere or in air. For comparison, the non-irradiated

samples were analyzed under the same conditions of sample

preparation and handling. Selected results of the experiments

with TMDSz are presented in Fig. 9. Both the hexane and

TMDSz peaks were excluded from the measurement by cut-off

for the protection of the ion source.

Without any irradiation, some impurities and products most

likely derived from the decomposition of TMDSz at room

temperature were detected (cf. Fig. 9(a)), compounds 6 and 7

(vide infra) were not identifiable using the NIST reference

library.34

Irradiation under a N2 atmosphere (Fig. 9(b)) preferably

lead to the formation of compound 11, but also to compounds

8 and 12 to a minor extent. As no reference data were available

for those compounds, plausible structures were assigned by

interpretation of the mass spectra (cf. Fig. 10(b)–(h)) in

comparison to the spectrum of the parent silazane TMDSz

(Fig. 10(a)) and available siloxane spectra. All methylsilyl

compounds show a very pronounced peak at (M-15)+ (loss

of a methyl group, often as base peak), which can be used to

unambiguously identify the low-intensity mol peak M+ and

to determine the molecular mass of the compound under

investigation.35 The (M-1)+ peak corresponding to the loss

of H is only present at reasonable intensity if the terminal

Si group carries at least one hydrogen atom, i.e. it is hardly found

in case of terminal trimethylsilyl groups. In the mass spectrum

of TMDSz (see Fig. 10(a)), additionally to the peaks at m/z =

132 (M-1)+ and 118 (M-15)+, significant peaks were observed

at m/z = 116 and 102, which were assigned to (M–1–CH4)+

(losses of H and methane) and (M–CH3–CH4)+ (losses of a

methyl group and methane), respectively. It was shown by partial

deuteration36 that methane is mainly formed with the hydrogen

bonded to the NH/ND-group. Therefore, the peaks (M-1-CH4)+

and (M-CH3–CH4)+ do not appear for siloxanes and 7HMDSz

(containing –O– and –NCH3–, respectively, instead of –NH–),

and their observation points to the existence of a silazane unit in

the molecule. Further characteristic ions were found atm/z= 45

(CH3H2Si+, methylsilyl group), 59 (CH3)2HSi+, dimethylsilyl

group), and 73 ((CH3)3Si+, trimethylsilyl group).

The mass spectra of the irradiation products 8 and 11

(see Fig. 10(b) and (c)) showed a similar fragmentation pattern

as TMDSz. In addition to the typical silazane fragment ions

((M-15)+, (M-1)+, (M-1-16)+ and (M-15-16)+), fragments

at m/z = (M-59)+, (M-75)+ and (M-89)+ corresponding

to (M-(CH3)2HSi)+, (M-1-(CH3)2HSiNH)+ and (M-CH3-

(CH3)2HSiNH)+, respectively, appear in the two spectra,

but with different intensities. The mass spectrum of compound

11 with the mol peak atm/z=264, the base peak atm/z=249

(M-CH3)+ and further typical silazane fragments points to a

‘‘dimer’’ of 1,1,3,3-tetramethyldisilazane with the structure F:

Such a dimer is expected to easily form by recombination of

two �Si(CH3)2–NH–SiH(CH3)2 radicals, e.g. formed by

reactions (5) and (8) (vide supra). Because of the same molar

mass (m/z = 264), compound 12 appears to be a structural

isomer of compound 11. In contrast to 11, the peak at

m/z= 132 (fragment (CH3)2HSi–NH+QSi(CH3)2) is the base

peak and the peak at m/z= 205 (M-59)+ = (M-(CH3)2HSi)+

Fig. 9 GC-MS (TIC) chromatograms of the products, dissolved after

irradiation in n-hexane and resulting from the irradiation of TMDSz

with UV light (222 nm) in a N2 atmosphere (trace b) and in air (c),

respectively. The lower trace (a) shows the chromatogram of the

non-irradiated sample. Product assignment: 1, dimethylsilanol; 2,

3-methylpentane; 3, 1,1,3,3-tetramethyldisiloxane; 4, 1,1,3,3,5,5-

hexamethyltrisiloxane; 5, tris(dimethylsilyl)amine; 9, 1,1,3,3,5,5,7,7,9,9-

decamethyl pentasiloxane. The mass spectra of the compounds 6, 7, 8

as well as 10–14 are shown in Fig. 10.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2387

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

has significant intensity, the latter pointing to a preferable loss

of the dimethylsilyl group. We tentatively assign the

mass spectrum 12 to the branched structure G, which is

formed either by recombination of secondary radicals�Si(CH3)2–NH–SiH(CH3)2 and SiH(CH3)2–N

�–SiH(CH3)2 or

by an intramolecular rearrangement of F to its energetically

more stable isomer G (DG B 16 kcal mol�1)

Compound 8 also shows the typical silazane fragmentation

pattern and a mol peak at m/z = 250. In comparison to 11,

low molecular fragments up to m/z = 130 appear at the same

m/z values with quite similar intensities, whereas the masses of

the higher molecular fragments at m/z = 161, 175, 191, 219,

235 and 249 are all shifted by 14 units towards lower masses.

This strongly indicates that 8 should have an analogous

structure to 11, however, with one methyl group substituted

by a hydrogen. In this case, recombination of a secondary�Si(CH3)2–NH–SiH(CH3)2 radical with a primary �SiHCH3–

NH–SiH(CH3)2 radical photolytically formed by Si–CH3

cleavage was likely to form structure H.

In the presence of oxygen (Fig. 9(c)), considerable amounts

of 6 and 7 were generated. The mass spectrum of 6 with the

base peak at m/z = 192 (M-15)+, a mol peak at m/z = 207

and typical silazane fragments points to an oxygen-containing

compound with a hybrid structure consisting of both a

silazane and a siloxane unit (structure I).

This assignment is further supported by comparison

with the spectrum of 1,1,3,3,5,5-hexamethyltrisiloxane.34 The

siloxane displays a similar fragmentation pattern as 6 with the

main difference that all significant ion peaks with m/z Z 132

are shifted by 1 unit towards higher masses, as was expected

from the replacement of the –NH– group (m/z = 15) by

oxygen (m/z = 16), if the –O–SiH(CH3)2– fragment is the

primary leaving group in both cases. In the mass spectrum

of 7, significant ions similar to 6 with m/z = (M-15)+,

(M-75)+and (M-89)+ are present, all shifted towards higher

masses by m/z = 74, corresponding to an additional

–(CH3)2Si–O– unit. Again, the spectrum is quite similar to

the analogous 1,1,3,3,5,5,7,7-octamethyltetrasiloxane34 with

the difference of 1 unit for peaks with m/z Z 132 (vide supra).

It is more than reasonable to assign 7 to a structure with one

silazane and two siloxane fragments, probably as structure J.

The peaks 13 and 14 show both similar mass spectra (cf. 13

in Fig. 10(h)) and retention times. This suggests that the

chemical structures of these isomers differ only marginally.

Comparing the spectrum of 13 with that of compound 7 one

can easily rationalize that the former differs by an additional

group with 74 mass units at an otherwise similar pattern. Thus

we assign compound 13 to structure K with an additional

–(CH3)2Si–O– unit in comparison to J.

The spectrum of 10 shows a mol peak at m/z = 280 and

typical silazane fragments ((M-15)+, (M-31)+, (M-59)+,

(M-89)+). Additionally, a distinct (M-17)+ peak was observed

with much higher relative intensity than found for TMDSz due

to the subsequent losses of H and methane (M-1-CH4)+. Also,

as the precursor ion (M-1)+ for the CH4 loss did not appear,

we concluded that a direct loss of a fragment with 17 mass

units was observed, which can be assigned to the loss of

NH3.35 This fact points to the presence of a terminal amino

group and a tentative structure L is supposable, formed by a

Fig. 10 Mass spectra of compounds resulting from the irradiation of

TMDSz. For comparison, the spectrum of TMDSz is given at the top

(a). (b)–(d) Irradiation under N2 atmosphere, (e)–(h) air atmosphere.

2388 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

recombination reaction involving a secondary radical�Si(CH3)2–NH2 (cf. reaction (4)).

In addition to the irradiation products in the liquid phase

(cf. Fig. 9), the volatile compounds in the headspace above the

liquid were analyzed. These were accumulated using an SPME

fibre, then thermo-desorbed in the hot GC injector and

subsequently analyzed by means of GC-MS. The components

detected in both the gaseous and the liquid phases were

essentially the same. Although the sensitivity using SPME is

generally higher for volatile compounds as compared to

standard GC-MS with diluted samples, one should note that

quantification in SPME is difficult because of the different

volatilities of the analytes. Peak 3 corresponding to tetra-

methyldisiloxane, which was partially clipped by the solvent

cut-off in the conventional GC-MS, is clearly present in the

SPME measurements. However, no dependency on the

irradiation was observed whether under N2 or O2/N2 conditions,

therefore TMDSO is likely to be an impurity in TMDSz.

Using SPME and a special GC column for the separation of

more volatile compounds, we could additionally observe

dimethylsilane and traces of trimethylsilane. The amount of

dimethylsilanol in the chromatograms (peak 1) varied from

sample to sample on a low level. It was confirmed that it is

formed by hydrolysis due to residual moisture introduced during

sample handling. As thermal decomposition at room temperature

is assumed to be slow,37 it is likely to occur in the injector and

transfer line of the GC-MS. If a TMDSz/hexane solution was

intentionally saturated with H2O, the yield of dimethylsilanol

strongly increased already for the non-irradiated reference sample,

however, its concentration further increased with increasing

irradiation time. This shows that thermally activated hydrolysis

occurs with an enhanced rate under photolytic conditions.

Additionally to the experiments with pure TMDSz and

with TMDSz/hexane solutions, respectively, irradiation was

performed using 1 : 1 mixtures of TMDSz and triethylsilane

with the aim to scavenge any highly reactive silylene inter-

mediates according to the literature26 (reaction (11)):

ð11Þ

The corresponding compound could not be detected. Thus, the

GC-MS results further support our hypothesis that silylenes

do not play a significant role (if any) in the entire reaction

mechanism.

Quantum chemical investigations

Recently, we reported quantum chemical calculations on the

irradiation-induced oxidative conversion of perhydropoly-

silazane19 and poly(1,1-dimethylsilazane-co-1-methylsilazane)21

into Si–O–Si or methyl–Si–O–Si networks, respectively.

Parameters such as the electronic absorption spectra, energy

levels of the excited states and the dissociation Gibbs energies

(DGdiss) were calculated on appropriate model molecules

showing structural resemblance to the silazane precursor

networks such as perhydro-, tetramethyl- and octamethyl

cyclotetrasilazanes. The calculations supported substantially

the discussion of the experimental results. Similar calculations

(see ESI, Table 1S)w were made here on TMDSz, HMDSz and

7MDSz using the DFT B3LYP method with 6-31G(d)

and 6-31+G(d,p) basis sets for geometry optimization and

calculation of the excitation energies, respectively, as implemented

in the Gaussian 03 (Rev. B.02) package.38 Frequency calculations

were done at the same level of theory to characterize the

stationary points on the potential surface and to obtain the

Gibbs free energy (G) at a standard temperature of 298.15 K

and a pressure of 1 atm using unscaled vibrations. The electronic

transition spectra were calculated using the time-dependent

(TD) B3LYP/6-31+G(d,p) method.39As an example, the

results obtained on the fragmentation of tetramethyldisilazane

(TMDSz) are summarized in Fig. 11 (the dissociation Gibbs

energy DGdiss is more reliable to characterise the probability of

Fig. 11 Quantum chemical calculations on the photolytic excitation of TMDSz and possible fragmentation pathways.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2389

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

the fragmentation and the relative order of the different path-

ways than the bond dissociation enthalpy).

Calculation of the energy of the first excited singlet state S1

(129 kcal mol�1) showed that photons with an energy of

E 4 5.6 eV (l o 222 nm) are required in order to excite

the molecule. The absorption thresholds of the hexa- and hepta-

methylated disilazane compounds shifted towards lower energies

with respect to TMDSz and accounted for 5.4 eV (230 nm) and

5.1eV (243 nm) for HMDSz and 7MDSz, respectively.

Fast vibrational relaxation shall lead to the S1 state, which

serves as a precursor for the subsequent intersystem crossing

(ISC) into the excited triplet state T*. The energy of the lowest

triplet state T1 could not be calculated. Attempts to perform

geometry optimization for the triplet state were unsuccessful.

Simulations running from the singlet state geometry as starting

point did not converge into a stable triplet conformation.

Instead, immediate bond elongation of the Si–N bond was

observed. Thus, the QC calculations suggest that bond scission

maybe faster than the relaxation of the excited triplet state T*

into its T1 ground state, or at least, that any triplet state is

expected to be rather short-lived. In fact, the laser flash experi-

ments reveal a short-lived luminescence band at 285 nm, being

oxygen sensitive, which was assigned to the short-lived triplet

state. Taking the lmax of 285 nm as a rough measure of the triplet

energy, this wavelength corresponds to B100 kcal mol�1.

In Fig. 11, the dissociation Gibbs energies (DGdiss) are

calculated as differences of Gibbs free energies of the corres-

ponding fragments. As can be seen, all types of bonds present in

the molecule may be subject to dissociation with respect to the

excited singlet state, however, taking the estimate for the triplet

energy of 100 kcal mol�1 as a threshold, H cleavages from the

methyl groups and from the amine group are less likely.

By comparing the HOMO and LUMO orbitals (the latter can

be taken as a first approximation of the electron distribution in

the excited state) given in Fig. 11, it is obvious that excitation

leads to a strong intramolecular electron redistribution, resulting

in a reduced Coulomb attraction between these atoms, and

therefore gives rise to weakening of the corresponding –Si–NH–

bond. Thus, the partial charges in the ground state of +0.73e

and �0.85e at Si and N, respectively, change to +0.41e (Si) and

�0.32e (N) in the excited state. From the analysis of the electron

distribution from MOs shown in Fig. 11 it can also be seen that

the n-electron from nitrogen is delocalized in the ground state

onto the Si–C s-bonds due to a favourable symmetry. After

excitation this s-electron density will be removed from the Si–C

bonds resulting in weakening the bond, facilitating the cleavage.

Thus, according to the calculations the reaction pathway may be

based on both Si–NH bond scission and on the degradation of

methyl groups (Si–CH3 scission).

Conclusion

Summarizing the time-resolved, matrix-assisted and steady-

state experiments, two main photolytically-induced pathways

are confirmed, i.e. the cleavage of the Si–N and of the Si–CH3

bonds. Unlike the gas-phase photolysis of HMDSz, where

Si–CH3 cleavage with subsequent formation of methane was

found to be the favourable reaction pathway (54% CH4 vs.

14% (CH3)3SiH due to cleavages of Si–C and Si–N bonds,

respectively),40 in the condensed phase Si–N cleavage

dominates the overall process. Si–CH3 scission in fact takes

place, as approved by the observation of CH3� radicals by

low-temperature EPR, and as earlier suggested21 by the

observation of small amounts of CH4 (gas-phase IR) and by

the decay of Si–CH3 vibrations (ATR-FTIR). Even though the

smallest dissociation Gibbs energy was calculated for the

Si–CH3 bond cleavage, Si–NH bond scission is found to be

the preferred photochemically induced reaction pathway in

condensed media, which can be rationalised by the favourable

change of electron distribution along the Si–NH–Si bonds

upon excitation. (The higher relative yield of methane

observed in the gas phase photolysis is likely due to a higher

probability of the small �CH3 fragment to escape radical pair

recombination in comparison with the larger �SiH(CH3)2 and�NH–SiH(CH3)2 fragments.)

The cleavage of the excited silazane was expected to be fast,

as the excited triplet state was calculated to be dissociative.

Indeed, only short-lived fluorescence spectra were observed.

On the basis of the current results it can not be completely

excluded that the cleavage of the silazanes may also occur

from the excited singlet state. In order to finally distinguish

between excited singlet and triplet state processes, additional

ultra-fast kinetic measurements have to be performed, which

were beyond the scope of the present paper. Nevertheless, the

origin of the primary radical pairs has no impact on the

subsequent reactions discussed here.

Aminyl radicals �NH–R could not be trapped in the matrix

EPR experiments in the present case (although silylaminyl

radicals were readily observed under stationary conditions)41–43

and there was no indication of stable products bearing the amino

group in the GC-MS experiments under oxygen-free conditions.

However, aminyl radicals were observed by laser flash photolysis

and they disappear rapidly, most probably by H-transfer

reactions leading to terminal amino groups R-NH2. The faster

decay of the aminyl radicals in case of TMDSz in comparison

with HMDSz and 7MDSz can be explained by a facile intra-

molecular 1,2 H-shift (reaction (4), DG = �12 kcal mol�1)

leading to �Si(CH3)2–NH2, and in undiluted TMDSz also by

H-abstraction from another molecule (reaction (5), DG B �13kcal mol�1) with the formation of the �Si(CH3)2–NH–SiH(CH3)2radical and dimethylsilylamine. Although the �NH-R radical

is longer-lived in case of HMDSz, it still reacts fast enough

to escape trapping in the matrix EPR. In this case

H-abstraction will occur from one of the six methyl groups

(at a lower rate than from SiH in case of TMDSz), and indeed�CH2–Si(CH3)2–NH–R radicals are observed as the dominating

species in the EPR spectra of irradiated HMDSz (Fig. 8).

Under continuous irradiation conditions, intermediate

products with terminal NH2–groups undergo further photolysis

and the cleavage of the NH2–Si bond most likely occurs with the

subsequent formation of NH3 by H abstraction. Ammonia was

found to be a major volatile decomposition product in the VUV

irradiation of PHPS19 and polyorganosilazanes.21 The present

results suggest that the rate of photolytical decomposition of

oligosilazanes should depend on the amount of labile (reactive) H

available in the matrix (i.e. H bound to Si). This conclusion

explains at least qualitatively the experimental finding that PHPS

is much faster converted into an Si–O–Si network than the

2390 | Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 This journal is �c the Owner Societies 2010

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online

corresponding (P(DMScoMS)) into a methyl–Si–O–Si network.

P(DMScoMS) contains only 33% H bound to Si in comparison

to PHPS (Fig. 1).

No silylenes were observed in the present laser flash experi-

ments, and all attempts to trap them by high concentration of

triethylsilane (as in Moisseev et al.44) also failed. It is therefore

concluded that silylenes do not play a (significant) role in the

photochemistry of silazanes.

The main product identified by GC-MS after photolysis of

TMDSz is a ‘‘dimer’’ of the silazane, most probably formed by

recombination of two Si–N–Si� fragments. This dimer is also

formed in the presence of oxygen, and this clearly shows that

such radical recombination is fast enough to compete with the

reaction of O2 with the Si–N–Si� radical. In the presence of O2,

siloxazane compounds are the main irradiation products, formed

after several reaction steps from the secondary silylperoxyl

species. The identification of oligomers with subsequently

increasing number of –Si(CH3)2–O– units strongly points to a

reaction mechanism for the conversion based on building of

linear R–(Si–O)n– chains, which in turn may later crosslink by

further cleavage of Si–CH3 bonds along the backbone.

If traces of moisture are present in the system, a photo-

lytically-assisted hydrolysis leads to the enhanced formation of

silanol groups as byproducts.

The present work shows that the reactivity of organo-

silazanes does not depend only on the quantum yield of radical

pair formation, but also on structural parameters such as the

availability of reactive hydrogen atoms. Thus, investigations

of the influence of different substituents at the Si atom like

vinyl or acrylate groups will be performed in the future.

Acknowledgements

These studies have been supported by the Bundesministerium

fur Bildung und Forschung, BMBF, contract no. 01RI06007,

the Federal State of Germany and the Free State of Saxony.

The authors are grateful to Mrs Ingrid Reinhardt for the

measurement of the UV-VIS spectra of silazanes.

References

1 H. Chatham, Surf. Coat. Technol., 1996, 78, 1–9.2 J. Madocks, J. Rewhinkle and L. Barton, Mater. Sci. Eng., B,2005, 119, 268–273.

3 Y. Leterrier, Prog. Mater. Sci., 2003, 48, 1–55.4 F. Kessler, D. Herrmann and M. Powalla, Thin Solid Films, 2005,480–481, 491–498.

5 D. Pech, P. Steyer, A. S. Loir, J. C. Sanchez-Lopez and J. P. Millet,Surf. Coat. Technol., 2006, 201, 347–352.

6 T. N. Chen, D. S. Wuu, C. C. Wu, C. C. Chiang, Y. P. Chen andR. H. Horng, J. Electrochem. Soc., 2006, 153, F244–F248.

7 J. Fricke, H. Schwab and U. Heinemann, Int. J. Thermophys.,2006, 27, 1123–1139.

8 F. Fracassi, R. d’Agostino, F. Palumbo, E. Angelini, S. Grassiniand F. Rosalbino, Surf. Coat. Technol., 2003, 174–175, 107–111.

9 E. Angelini, S. Grassini, F. Rosalbino, F. Fracassi, S. Laera andF. Palumbo, Surf. Interface Anal., 2006, 38, 248–251.

10 K. Reichelt and X. Jiang, Thin Solid Films, 1990, 191, 91–126.11 A. Gruniger and P. Rudolf von Rohr, Thin Solid Films, 2004, 459,

308–312.12 A. G. Erlat, B. C. Wang, R. J. Spontak, Y. Tropsha, K. D. Mar,

D. B.Montgomery andE.A.Vogler, J.Mater. Res., 1999, 15, 704–717.13 S. Iwamori, Y. Gotoh and K. Moorthi, Surf. Coat. Technol., 2003,

166, 24–30.

14 T. P. Chou, C. Chandrasekaran and G. Z. Cao, J. Sol–Gel Sci.Technol., 2003, 26, 321–327.

15 H. Kriegsmann and G. Engelhardt, Z. Anorg. Allg. Chem., 1960,310, 100–109.

16 K. Kamiya, T. Tange, T. Hashimoto, H. Nasu and Y. Shimizu,Res. Rep. Fac. Eng. Mie Univ., 2001, 26, 23–31.

17 C. Kato, S. Tanaka, Y. Naganuma and T. Shindo, J. Photopolym.Sci. Tec., 2003, 16, 163–164.

18 Y. Naganuma, S. Tanaka, C. Kato and T. Shindo, J. Ceram. Soc.Jpn., 2004, 112, 599–603.

19 L. Prager, A. Dierdorf, H. Liebe, S. Naumov, S. Stojanovic,R. Heller, L. Wennrich and M. R. Buchmeiser, Chem.–Eur. J.,2007, 13, 8522–8529.

20 Eur. Pat. EP0745974B1, 1996; Jap. Pat. JP11092666AA, 1999; Jap.Pat. JP10279362AA, 1998.

21 L. Prager, L. Wennrich, R. Heller, W. Knolle, S. Naumov,A. Prager, D. Decker, H. Liebe and M. R. Buchmeiser,Chem.–Eur. J., 2009, 15, 675–683.

22 J. Pola, A. Galikova, Z. Bastl, J. Subrt, K. Vacek, J. Brus andA. Ouchi, Appl. Organomet. Chem., 2006, 20, 648–655.

23 J. Pola, A. Galikova, A. Galik, V. Blechta, Z. Bastl, J. Subrt andA. Ouchi, Chem. Mater., 2002, 14, 144–153.

24 D. R. Duling, J. Magn. Reson., Ser. B, 1994, 104, 105–110.25 J. von Sonntag and W. Knolle, J. Photochem. Photobiol., A, 2000,

132, 25–27.26 J. von Sonntag, J. Photochem. Photobiol., A, 1999, 126, 1–5.27 G. Levin, P. K. Das, C. Bilgrien and C. L. Lee, Organometallics,

1989, 8, 1206–1211.28 A.G.Moiseev andW. J. Leigh,Organometallics, 2007, 26, 6268–6276.29 R. T. Conlin, J. C. Nettoferreira, S. Zhang and J. C. Scaiano,

Organometallics, 1990, 9, 1332–1334.30 C. Chatgilialoglu, Chem. Rev., 1995, 95, 1229–1251.31 T. Doba, K. U. Ingold, W. Siebrand and T. A. Wildman, Faraday

Discuss. Chem. Soc., 1984, 175–191.32 W. G. French and J. E. Willard, J. Phys. Chem., 1968, 72, 4604.33 C. J. Rhodes, J. Chem. Soc., Perkin Trans. 2, 1992, 235–241.34 NIST Standard Reference Database 1A (Data Version: NIST 02),

National Institute of Standards and Technology, Gaithersburg,MD, 2002.

35 J. Silbiger, C. Lifshitz, J. Fuchs and A. Mandelba, J. Am. Chem.Soc., 1967, 89, 4308.

36 J. Tamas, M. Mak and S. Makleit, Org. Mass Spectrom., 1974, 9,847–853.

37 F. Bauer, U. Decker, A. Dierdorf, H. Ernst, R. Heller, H. Liebeand R. Mehnert, Prog. Org. Coat., 2005, 53, 183–190.

38 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,M. A. Robb, J. R. Cheeseman, J. Montgomery, J. A., T. Vreven,K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,W. Chen,M.W.Wong, C. Gonzalez and J. A. Pople,GAUSSIAN 03(Revision C.02), Gaussian, Inc., Wallingford, CT, 2004.

39 R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996, 256,454–464.

40 J. Pola and R. Taylor, J. Organomet. Chem., 1993, 446, 131–134.41 J. C. Brand, M. D. Cook and B. P. Roberts, J. Chem. Soc., Perkin

Trans. 2, 1984, 1187–1196.42 J. C. Brand, B. P. Roberts and J. N. Winter, J. Chem. Soc., Perkin

Trans. 2, 1983, 261–270.43 M. D. Cook, B. P. Roberts and K. Singh, J. Chem. Soc., Perkin

Trans. 2, 1983, 635–643.44 A. G. Moiseev and W. J. Leigh, Organometallics, 2007, 26,

6277–6289.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2380–2391 | 2391

Dow

nloa

ded

by U

NIV

ER

SIT

Y O

F SO

UT

H A

UST

RA

LIA

on

17 S

epte

mbe

r 20

12Pu

blis

hed

on 1

9 Ja

nuar

y 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/B91

8814

B

View Online