Photoluminescence of Phenyl- and Methylsubstituted Cyclosiloxanes

Chapter 7 Photoluminescence of Phenyl -and Methylsubstituted Cyclosiloxanes 1

2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2013 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch007

Udo Pernisz , Norbert Auner , and Michael Backer

3

1Dow Corning Corporation, Central Research and Development, Midland, M I 48686-0994 Institut für Anorganische Chemie, J . W . Goethe Universität, Marie-Curie-Strasse 11, D-60439, Frankfurt, Germany Institut für Anorganische und Allgemeine Chemie, Humboldt Universität, Berlin, Germany 2

3

The observation of strong blue photoluminescence from

several

different types o f phenylated Si-containing molecules upon excitation with UV light was investigated by measuring the emission and excitation spectra o f the luminescence as well as the time dependence of the phosphorescence.

A range o f variously substituted, S i

-containing cyclic compounds was synthesized and analyzed in order to identify the origin o f the photoluminescence and to determine the role o f the substituents in the effect. studied

were

The two classes of compounds

2,3-diphenyl-substituted

silacyclobutenes

and

stereoregularly-built phenylated cyclosiloxanes.

The excitation with U V light o f phenyl-containing S i compounds such as 2,3diphenylsilacyclobutene 1 and siloxanes of type 2 results in the emission o f strong, blue photoluminescence from the solid material. It can be assumed that the origin o f the photoluminescence is essentially associated with the Tt-electron system o f the substituents at the C and/or S i since that corresponding class o f compounds commonly shows this effect although for small molecules such as benzene or stilbene the emission usually occurs in the U V . The fact that the photoluminescence can be observed in the visible gave rise to a series of investigations aimed at understanding the role o f the S i atom when it is directly bound to such an aromatic moiety with the assumption that the S i shifts the emission from the U V into the visible part o f the spectrum. 1

The questions o f the effect that the S i atom has on the electronic structure o f aromatic molecules can be expanded to include the effect o f other substitutents on the S i , including different 7i-conjugated substituents. A second part o f this question regards the presence o f the S i - O - group or, generally, the effect o f linear or cyclic

© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

115

116

siloxane configurations with which the aromatic part interacts v i a the S i atom to which it is bound. The cyclosiloxanes are o f interest in the wider context o f the photoluminescent properties o f silica materials containing various defects, including mechanical ones (strained bond angles) and chemical ones (non-bridging oxygen defects, and S i radicals due to dangling bonds).


A series o f suitable molecules covering the range o f interest was investigated by measuring the emission and excitation spectra o f the luminescence and phosphorescence components as well as the time dependence o f the latter effect. E x p e r i m e n t a l Details The compounds investigated fall in two classes. The first one is based on a silacyclobutene subunit which is obtained through cycloaddition reaction o f dichloroneopentylsilene to tolane (Tolane CycloAdduct, T C A ) . Starting from 1,1dichloro-2,3-diphenyl-l-silacyclo-2-butene 1 ( T C A ) , a whole series o f organosubstituted derivatives is available by usual organometallic routes. Utilizing the silicon dichlorofunctionality and the outstanding chemical and thermal stability o f 1, this silacyclobutene building block is easily incorporated into cyclosiloxanes yielding silaspirocyclic compounds. The second class deals with stereoregularly-built phenylated trimethylsiloxycyclosiloxanes o f various ring sizes o f which 2 shows the cyclotetrasiloxane as an

3

SiMe

3

example. (The phenyl group on the front S i is given as Ph, the trimethylsiloxo group on the back S i was omitted for clarity.) This group o f compounds, [PhSi(OSiMe )0]„, n = 4, 6, 8, 12, was synthesized following the literature. 2

3

Tolane C y c l o a d d u c t ( T C A ) Derivatives. T C A , or l,l-dichloro-2,3-diphenyl-4neopentyl-l-silacyclo-2-butene, 1, is preparatively easily accessible by starting from equimolar amounts o f trichlorovinylsilane, tolane, and lithium-terf-butyl, forming crystalline 1 in nearly quantitative yield. The preparation o f alkyl and aryl derivatives is performed by sustituting the CI on the S i employing the usual organometallic synthetic route, e.g., by use o f Grignard reagents or L i organyls. The silanediol 3 is obtained quantitatively from 1 in a controlled hydrolysis. Thermolysis o f 3 between 80 °C and 110 °C upon reflux i n toluene solution leads to the condensation product 1,3-siloxanediol 4. This solid was characterized by x-ray analysis after recrystallization from toluene. 3



117

T C A - c o n t a i n i n g Cyclosiloxanes. From co-condensation o f 4 with dichlorofunctional silanes such as dichlorodimethyl- or -diphenylsilane, such as R S i C l , R = M e , Ph, the bisilaeyelobutene-substituted cyclic trisiloxanes as exemplified by 5 are obtained. Their monocrystalline forms were characterized by x-ray analysis. 2

Me

2

Me

Based on the extraordinary chemical and thermal stability, the 1,1-dichloro2,3-diphenyl-4-neopentyl-l-silacyclobutene subunit can be easily incorporated into dimethylsiloxane chains by co-condensation reactions with a,co-oligosiloxanediols. Thus, condensation o f 1 with l,l,3,3-tetramethyl-l,3-disiloxanediol, 6, generates the discrete spirocycle l-(2',3'-diphenyl-4'-neopentyl-l-silacyclo-2'-butene)-3,3,5,5tetramethylcyclotrisiloxane, 8, while reaction with the phenyl-substituted siloxane analog, 7, yields the phenylcyclotrisiloxane 9. Co-condensation o f 4 with l,l,3,3,5,5-hexaphenyl-l,5-trisiloxanediol even generates the corresponding 8-membered cycle which was also characterized by single crystal x-ray analysis. Photoluminescence studies o f this compound w i l l be reported elsewhere. Instrumentation. The initial observations o f strong blue photoluminescence, i.e., light with wavelength X > 400 nm, in the compounds discussed were made visually by irradiating the samples with light from a pulsed nitrogen laser at a wavelength X = 337 nm and a beam energy o f ca. 200 uJ per pulse.


118

C

R

l„.Si Ph

/ HO

Ph

Si

-Si

o

R = Me

\

6

R = Ph 7

OH

1 E t 0 / NR 2

Si-

3


0 R = Me

8

\

R ^*Si

_

R The photoluminescence spectra were obtained with a S P E X Fluorolog 2 instrument (Jobin/Yvon) with excitation and emission monochromators equipped with single gratings o f 1200 lines/mm, blazed at wavelengths X = 330 nm and X = 500 nm, respectively. Both monochromators had a focal length o f / = 0.22 m; at the commonly selected slit widths o f 0.25 mm, the spectral bandwidth o f the instrument was typically 1 nm. The powdered samples were inserted into the center tube (having ca. 3 m m inner diameter) o f a quartz dewar with which measurements could be performed both at room temperature and at the temperature o f liquid nitrogen (77 K ) . Phosphorescence excitation and emission were obtained by using a flash lamp of 3 jus duration (at half maximum) for excitation o f the sample at select wavelengths and recording the emission intensity at longer wavelengths after an adjustable delay time during an appropriately set time window (photon counting). Similarly, phosphorescence intensity decay time measurements were performed by varying the delay time at fixed excitation and emission wavelengths. Results and Discussion Photoluminescence of Silacyclobutene Derivatives. The visual observation o f blue photoluminescence is confirmed by the spectral distribution o f the emitted light measured under continuous excitation at a wavelength o f X = 320 nm. Figure 1 shows the luminescence emission spectra o f the four T C A derivatives investigated: ethyl-TCA, methyl-TCA, h y d r o x y l - T C A , and phenylethynyl-TCA. Both CI are replaced with the same substituent for each compound. The measurements were made at room temperature using the samples in powder form, filled into thin quartz tubes. The four samples have very similar emission characteristics with the maximum located at around 400 nm; there is only a small effect from the substitutent on its position. This indicates that the photoluminescence is associated with the cisstilbene part o f the silacyclobutene. However, the emission intensity is strongly affected by the substituent. Most notably, the phenylethynyl group appears to quench the photoluminescence emission, an effect also observed with polysilanes. Another factor in the emission process appears to be the ring structure itself through which the Si atom is attached to the cis-stilbene group as is observed when comparing 1 to a linear analog o f comparable electronic configuration such as trimethylsilyl-c/.s'4



119

stilbene which does not show observable photoluminescence upon excitation with light o f 337 nm. In contrast, one observes strong blue emission from the triphenylsilyl-ds-stilbene under these conditions, confirming the previously stated role o f the S i atom in lowering the energy levels o f aromatic compounds bound to it. While there is no discernible effect of the O atom from the hydroxyl groups at the S i atom on the photoluminescence emission maximum, compared to the C o f the methyl and ethyl groups, the oxygen bridge between the S i atoms o f the b i s - T C A disiloxanediol configuration, 4, appears to shift the maximum o f the emission to longer wavelengths as shown i n Figure 2. This observation raises the question o f the effects that contribute to the shift o f the emission from the phenyl groups i n the cisstilbene moiety o f the T C A unit. Since the oligosiloxanediols as well as the T C A cyclosiloxanes can be readily prepared it is o f interest to understand the various possible contributions to the photoluminescence o f these Si-containing compounds. Photoluminescence of Cyclosiloxane. For comparison, the photoluminescence emission spectra o f the silaspirocycles 8 and 9 are shown in Figure 3; both exhibit the strong blue emission seen i n the other parent silacyclobutenes (k > 400 nm). A g a i n it is noted that the methyl-substituted siloxane ring has a higher emission intensity than the phenyl-containing ring. This is interesting because the perphenylated cyclosiloxane (commercially available) is itself also a very strong blue-luminescing material, see Figure 4, while the permethylated cyclosiloxanes do not exhibit any similar photoluminescence, a statement equally true for siloxane rings containing any substituent other than one with a it-electron system. Notably, the photoluminescence intensity increases with the number o f the phenyl substitutents as they replace the methyl groups at the silicon. Thus, we found a series of decreasing photoluminescence intensity for the cyclotrisiloxane D compounds as D > D D > D D ; D shows no photoluminescence. Another conclusion that can be drawn from the fact that non-aromatic substitution on the S i atom in a siloxane ring does not give rise to (visible) photoluminescence is that the effect - at least at the observed very high intensity - has its origin not in the siloxane ring itself but in the Si-Ph bond. P h 2

3

p h 2

2

M e 2

P h 2

M e 2

2

3

M e 2

3

The experimental findings presented in the preceding material lead to the question o f the mechanism by which the U V photoluminescence emission usually observed in the far U V with most aromatic compounds is shifted towards the blue and into the visible part o f the spectrum. A n analogous effect was observed for the organic molecule phenylazo-/er/-butyl i n which the ionization energy is lowered by 0.5 eV when the C on the azo group is replaced by S i , that is, when the tert-butyl is replaced by trimethylsilyl. This is explained by an inductive effect resulting in considerable electron donation from the S i substituent. In an attempt to analyze the role played by the Si atom that is attached to an aromatic moiety such as a phenyl group, in the conversion of the absorbed photon energy to visible luminescence, the series o f stereoregular phenylcyclosiloxanes (of which the four-membered molecule is shown as 2) o f different ring sizes was investigated. 5

Effect of Ring Size of Phenyl-substituted Cyclosiloxanes. The stereoregularly built phenylated cyclosiloxanes (all-cis and all-trans) with trimethylsiloxy groups attached opposite each phenyl group, 2, has been described i n the literature and was


120

2.5e+6 . . .

2.0e+6 -

1 Et-TCA 2 Me-TCA 3 OH-TCA 4 Ph-C=C-TCA

c

3 -Q 03

1.5e+6 -

c


0) c g

1.0e+6 -

E

cu 5.0e+5 -

0.0 350

400

450

550

500

600

wavelength [nm]

Figure 1. Photoluminescence emission spectra at room temperature o f differently silicon-substituted tolane cycloadduct ( T C A ) compounds with general fomula 1,1R -2,3-diphenyl-4-neopentyl-l-silacyclo-2-butene where R = ethyl (trace 1), R = methyl (trace 2), R = hydroxyl (trace 3), and R = phenylethynyl or C H C = C (trace 4). Excitation wavelength 320 nm, spectral bandwidth 1 nm. 2

6

5

5e+6 bis-TCA-disiloxanediol 4e+6 -Q

3e+6 (75 c 0)

-

2e+6

o CO

E 0)

1e+6

350

400

450

500

550

wavelength [nm]

Figure 2. Photoluminescence spectrum o f the bis-TCA-disiloxanediol, 4, at room temperature, as a solid. Excitation wavelength 320 nm, spectral bandwidth 2 nm.


121

7e+6

r

6e+6 c

5e+6

03 "ii/) c a> Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2013 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch007

c

/

4e+6

; ! i

2e+6

.

1: (D-2,2-Ph) -TCA

.

2: (D-2,2-Me) -TCA

2

2

X = 320 nm, T= 77 K

\

/

3e+6

ti

"E

CD

x \

2

1

1e+6

f 350

400

450

500

550

600

650

wavelength [nm]

Figure 3. Photoluminescence emission spectra o f functionalized silaspirocycles ( D - 2 , 2 - R ) - T C A or l-(2',3'-diphenyl-4'-neopentyl-l-silacyclo-2'-butene)-3,3,5,5tetra-R-cyclotrisiloxane, 8 and 9. Lower trace (1): R = phenyl (Ph); upper trace (2): R = methyl (Me). Measured at liquid nitrogen temperature, T = 77 K , excitation wavelength 320 nm, spectral bandwidth 1 nm. 2

2.5e+5

wavelength [nm]

Figure 4. Photoluminescence emission spectrum o f octaphenylcyclotetrasiloxane solid, at liquid nitrogen temperature.

Excitation wavelength 320 nm, spectral

bandwidth ca. 8 nm.


122

accordingly re-synthesized. The four cyclics prepared were the four-, six-, eight-, and twelve-membered rings; in the context o f this work, they w i l l be referred to as D 4 , D 6 , D 8 , and D12, using D as an abbreviation for the functionalized S i - O - group in the siloxane chain. For comparison, a commercially available small linear compound, tetraphenyldisiloxanediol,

was

also

included i n the

photoluminescence

study.

Although not a cyclic, it is referred to as D2 in the following. Since the phenyl groups in the stereoregular cyclosiloxanes chosen for this investigation all lie on the same side o f the siloxane ring, it appears possible that an


effect

of

energy

transfer

between

the

phenyl

groups

is

observed

in

the

photoluminescence emission, either by direct interaction between the 7i-electrons o f the aromatic rings, or mediated by the siloxane groups separating adjacent phenyl groups that are attached to the S i atoms.

In addition to an interaction between the

pendent groups, to the extent that they participate in the internal energy exchange and transfer from the absorption to the emission process, one could also expect an effect o f the ring size itself on the photoluminescence characteristics. Figure

5 shows the

photoluminescence

emission spectra

o f the

four

stereoregular phenylcyclosiloxanes, D 4 . . . D12, together with the linear tetraphenylsiloxanediol, D 2 . The graph demonstrates that the ring structure has a pronounced effect on the spectral characteristics o f the photoluminescence, however, it is not immediately obvious in what way the cyclosiloxane size controls each spectrum. One notes the very strong emission above 370 nm o f the D 2 ; this feature recurs i n the other molecules with decreasing intensity although there is a D 8 - D 6 reversal i n this sequence.

(The short-wavelength flank o f all spectra is due to a second-order filter

with an absorption edge at 370 nm.) The strong wide peak around 460 nm o f the D8 ring is surprising. Figure 6 which shows the luminescence excitation spectra does not clarify the picture although one can observe the same rank ordering in the emission intensity (at a wavelength o f 505 nm) as with the emission spectra shown in Figure 5 except for the D 2 molecule having the lowest value. This appears plausible considering that i f interaction effects were essentially localized between two adjacent aromatic groups the behavior o f the large ring should be very similar to that o f the short linear molecule. The other common feature o f the spectra i n Figure 6 is the sequence o f small peaks and shoulders which can be analyzed into at least two vibrational bands, one 1

having an energy separation o f 0.36 e V (2930 cm" ), and the other separated by 0.13 1

e V (1050 cm" ). T w o more line series are discernible but were not further identified and assigned. The two major series are ascribed to the C - H stretch vibration on the phenyl ring, and the S i - O - S i vibration from the siloxane ring, according to IR data in the literature.

6

The appearance o f the S i - 0 vibronic structure indicates that the

siloxane ring participates in the energy relaxation processes between the absorption and emission o f photons in these compounds. While the measurements o f the photoluminescence intensity under steadystate excitation as discussed above did not reveal a systematic influence from the cyclosiloxane rings, it was possible to observe such an effect i n the characteristics o f the phosphorescence emission. T w o types o f experiments were performed in this mode, namely recording the emission intensity after a fixed delay time (At = 50 us) at a fixed emission wavelength (k

ms

= 505 nm) as a function o f excitation wavelength,


123 9.0e+6

c

7.5e+6

xi 6.0e+6 -

4.5e+6

3.0e+6

E 0

1.5e+6

wavelength

[nm]

Figure 5. Photoluminescence emission spectra o f the stereoregular phenylcyclosiloxanes, [PhSi(OSiMe )0] , n = 4, 6, 8, 12, together with the linear tetraphenyl1,3-siloxanediol, measured (in powder form) at the temperature o f liquid nitrogen, T= 11 K . Excitation wavelength 280 nm, spectral bandwidth ca. 2 nm, edge filter (370 nm). D 2 : linear; D 4 : n=4; D 6 : n=6; D 8 : rc=8; D 1 2 : n=\2. 3

M

9.0e+6 -j 7=77K (LN2) intensity, arb. unit

8.0e+6 -

c o

nissi


c g "c> /

7.0e+6 6.0e+6 5.0e+6 4.0e+6 3.0e+6 2.0e+6 -

0

1.0e+6 0.0 500 wavelength [nm]

Figure 6. Photoluminescence excitation spectra o f the stereoregular phenylcyclosiloxanes, [PhSi(OSiMe )0]„, n = 4, 6, 8, 12, together with the linear tetraphenyl1,3-siloxanediol, measured (in powder form), at the temperature o f liquid nitrogen, T = 77 K . Emission wavelength 505 nm, spectral bandwidth ca. 1 nm. D2: linear; D 4 : w=4; D 6 : n=6\ D 8 : « = 8 ; D12: « = 1 2 . 3


124

and measuring the time dependence o f the phosphorescence emission at a fixed excitation wavelength (k = 320 nm) at two emission wavelengths (k = 380 nm and X = 458 nm). xc

ms


ms

Figure 7 shows the phosphorescence excitation spectra o f the five compounds. Intensity is no longer an obvious distinction, except for the D12 molecule for which the data are scaled up by a factor o f 10, and for D 2 which drops off i n emission towards long wavelengths. However, the characteristic features in each spectrum can be sorted according to the symmetry o f the siloxane ring: most obviously, D 4 and D 8 fall into one group, and D 6 with D12 fall into another one; D 2 shares most o f the peaks common to both groups with more uniform intensity distribution. It thus appears that 3-fold and 4-fold symmetry o f the siloxane ring emphasizes different features in the (vibronic) structure o f the long-lived phosphorescence o f phenylated siloxane compounds, and that the disiloxanediol constitutes the building block which exhibits the basic interactions by which the absorbed photon energy relaxes into the emitting state from which the energy is released at long times. This model was confirmed with a measurement o f the phosphorescence time dependence which is shown i n Figure 8. T w o features can be observed. The first one is a build-up o f the delayed emission intensity for about 50 |as during which time the energy from molecular state that absorbs 3.87 e V photons (320 nm) is pumped into the emitting state at 3.26 e V (380 nm, cf. Figure 5) at a higher rate than it emits energy. The second one is the ordering of the spectra by the appearance o f a second such peak for the higher-symmetry molecules, i.e., D 2 , D 4 , and D 8 , which is missing from the traces o f the molecules with 3-fold symmetry, i.e., D 6 , and D12. It is remarkable that D 2 shares the double-peak feature with the molecules o f 4-fold symmetry; this suggests that the 3-fold symmetry o f the siloxane ring suppresses a second electronic state to which the phenyl-Si system has otherwise access, and from which long-lived delayed emission can occur. It is also noted that the D 6 molecule shows a substantial emission with much longer time constant than the comparatively rapid decay observed between 30 jis and 100 \is for the other molecules. This rapid decay which applies to all five species, is characterized by a time constant between 10 us and 20 us, somewhat dependent on the molecule, while for times t > 70 (as the D 6 molecule shows an additional exponential phosphorescence decay characterized by a time constant o f 0.25 ms. The origin o f the electronic state from which this emission occurs or the mechanism by which it is created are currently not understood. S u m m a r y a n d Conclusions. The photoluminescence emission obtained from 7 i electron systems such as stilbene or benzene is red-shifted into the blue part o f the visible spectrum i f a Si atom is attached to the aromatic moiety. Additional substitutions on the S i atom affect the spectral features o f the molecule as demonstrated with the diphenylsilacyclobutene compounds 1 or 3 and their derivatives (tolane cycloadducts or T C A ) . D i o l derivatives o f this molecule that were also investigated include the cyclosiloxanes 5, and 8 or 9 (spirocyclics). W i t h a related class o f compounds, the phenylated cyclosiloxanes, the effect o f siloxane ring size on the spectral characteristics o f the blue photoluminescence was investigated in more detail. It was shown that the cyclosiloxane symmetry is the controlling parameter for the spectral distribution o f the phosphorescence intensity as



125

T=77K(LN2)

450 wavelength

[nm]

Figure 7. Phosphorescence excitation spectra of the stereoregular phenylcyclosiloxanes, [PhSi(OSiMe )0]„, n = 4,6, 8, 12, together with the linear tetraphenyl1,3-siloxanediol, measured (in powder form), at the temperature o f liquid nitrogen, T = 77 K . Emission wavelength 505 nm, spectral bandwidth ca. 8 nm, edge filter (370 nm). Excitation flash duration (at half maximum) 3 p,s, delay time (after flash trigger) before sampling 50 us, sampling window size 10 ms, cumulative emission from 10 flashes per data point. 3



126

time

[ms]

Figure 8. Phosphorescence time dependence o f the emission from the stereo regular phenylcyclosiloxanes, [PhSi(OSiMe )0],„ n = 4, 6, 8, 12, together with the linear tetraphenyl- 1,3-siloxanediol, measured (in powder form), at the temperature of liquid nitrogen, T = 77 K . Excitation wavelength 320 nm, emission wavelength 380 nm, spectral bandwidth ca. 8 nm, edge fdter (370 nm). Excitation flash duration (at half maximum) 3 jus, delay time increment 5 us, sampling window size 10 jus, cumulative emission from 10 flashes per data point. 3



127

well as for the temporal behavior o f the emission. The specific features in the phosphorescence excitation spectrum and the phosphorescence emission time dependence that are modified by the size and symmetry o f the siloxane ring are also present in the linear phenylsiloxanediol which indicates that the molecular structure responsible for the observed effects consists o f two phenyl groups each attached to a Si atom in a siloxane configuration. From this observation, as well as from the fact that the S i - 0 vibronic signature is observed in the excitation spectrum o f each molecule with phenylsiloxane structure, it is concluded that the siloxane unit participates in the energy absorption, relaxation, and emission processes i n such compounds. This is remarkable since the siloxane moiety itself does not exhibit a comparable photoluminescence phenomenon as demonstrated by octamethylcyclotetrasiloxane in comparison to octaphenylcyclosiloxane. Thus the effect o f the S i atom bonded to the phenyl group on the shift o f the photoluminescence from the U V into the blue part o f the visible spectrum is further modified by the interaction with the siloxane bridges between the phenyl groups. Acknowledgements The experimental skills o f A . A . Hart and F . N . Noble who carried out most o f the photoluminescence measurements reported here are gratefully acknowledged. References 1. 2.

3. 4. 5. 6.

Jaffé, H . H . , Orchin, M. Theory and Applications of Ultraviolet Spectroscopy, John Wiley and Sons, N e w York, 1992. Shchegolikhina, O.I., Igonin, V.A., Molodtsova, Y u . A . , Pozdniakova, Y u . A . , Shdanov, A.A., Strelkova, T . V . , Lindeman, S . V . , J. Organomet. Chem. 1998, 562(1-2), 141-51. Auner, N., Seidenschwarz, C., Seewald, N., Herdtweck, E . Angew. Chem. 1991, 103, 1172; Int. E d . Engl. 1991,30, 1151. Horn, Keith A.; Grossman, Robert B . ; Thorne, Jonathan R. G . ; Whitenack, Anne A., J. Am. Chem. Soc. (1989), 111(13), 4809-21. Bock, H., Wittel, K., Veith, M., Wiberg, N., J. Am. Chem. Soc. 1976, 98, 10914. L i p p , E . D . , Smith, A.L., in: The Analytical Chemistry of Silicones ( A . L . Smith, ed.), Chemical Analysis 112; John Wiley & Sons: N e w York, 1991; Chapter 11, pp 305.


Photoluminescence of Phenyl- and Methylsubstituted Cyclosiloxanes

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