Studies on the room-temperature solution fluorescence quenching of polysilane copolymers by bromohydrocarbons

Eur. Polym. J. Vol. 32. No. 5, pp. 665-667, 1996 Copyrighl 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057/96 SIS.00 + 0.00

Pergamon

SHORT

COMMUNICATION

STUDIES ON THE ROOM-TEMPERATURE SOLUTION FLUORESCENCE QUENCHING OF POLYSILANE COPOLYMERS BY BROMOHYDROCARBONS DEBEN ‘Department

CHEN,‘*

GAOQUAN

LI,’

YIMING

MO2

and

FENGLIAN

BAI*

of Chemistry, Sichuan University, Chengdu, 610064, P. R. China; and Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China (Receiued

18 January

ZInstitute

of

1995; accepted in final form 2 June 1995)

Abstract-The room-temperature solution flourescence quenching of polysilane copolymers by bromohydrocarbons such as BrCH,CH, and BrCH,CH,Br was studied. The fluorescence quenching data were in conformity to the equation In (F,/F) = NV[Q], and almost linear In (F,/F) u [Q] straight lines were obtained. According to the fluorescence quenching theory, the fluorescence quenching here may be simply the static fluoresce&e quenching. _ -

results, completely literature [7].

INTRODUCTION because of its Si-Si D conjugation in the main chain, has distinctive optical and electric properties and can be potentially used as high-resolution photoresists [1], nonlinear optical materials [2], electroluminescent materials [3], photoconductors [4], etc. Miller et al. have succeeded in greatly improving the photodegradation rate of poly(methylpheny1 silane) film by incorporating aromatic quenchers with -Ccl, [5], which has both theoretical and practical importance in the study of polysilane microlithography. Sun et al. [6] reported the fluorescence quenching of poly(methylpropylsilane) by Ccl, at a high concentration of 2.00 M (the maximum Fe/F ranges from 20 to 60). The authors studied the room-temperature solution fluorescence quenching of two polysilane copolymers by Ccl., and CzCl, at a low concentration of 0.20 M and CHCl, at a high concentration of 5.00 M (the maximum F,/F is about 3), and processed the fluorescence quenching data in line with the competitive fluorescence quenching equation Fe/F = (1 + &[Q])exp(NV[Q]) when dynamic quenching and static quenching coexist. The conclusions were as follows. In the room-temperature solution fluorescence quenching of the polysilane copolymers by the chlorohydrocarbons, there exist both dynamic quenching and static quenching; that is, the fluorescence quenching of the polysilane copolymers by the chlorohydrocarbons is attributed to the simultaneous effects of dynamic quenching and static quenching [7]. Based on these observations the authors studied the room-temperature solution fluorescence quenching in polysilane copolymers by bromohydrocarbons such as BrCH,CH, and BrCH, CH,Br, and obtained some very interesting

different

from those

in previous

Polysilane,

*To whom all correspondence

should be addressed. 665

EXPERIMENTAL

PROCEDURES

Materials Poly(dimethylsilane-co-methylphenethylsilane) (P,,,, ) and poly(dimethyl-co-cyclohexylmethylsilane) (P(,,,) were synthesized by Wurtz-type reductive condensation of their corresponding disubstituentdichlorosilane monomers [8,9]. The copolymerization ratios (m/n) were both 1.25 and the molecular weights (MO) were 1.06 x 10’ and 2.15 x 105, respectively. Cyclohexane, BrCH,CH,, and BrCH,CH,Br, which have no absorption at the wavelength of excitation, were all analytically pure. The calculation of the concentrations of the polysilane copolymer in solutions is based upon the number of Si atoms. All concentrations are 1.00 x 10M4M and the solvent is cyclohexane. The bromohydrocarbon concentrations for all systems are listed in Table 1. Instrumentation The steady-state fluorescence spectra were recorded on a Hitachi MPF-4 Fluorescence Spectrophotometer. All the experiments were carried out at room temperature (20°C). The wavelength of excitation for all systems is located at 308 nm, near the maximum ultraviolet absorption of the polysilane copolymers.

RESULTS AND DISCUSSION

As we know, there exists a special kind of static quenching [lo, 111, which is due not to the complexation between fluorophores and quencher molecules, but rather to the quencher molecules being adjacent to the fluorophores at the moment of excitation. To describe this kind of static quenching, we need to introduce the concept of a sphere of action, within which the quenching after excitation is assumed to

666

Short Table

Svstems

I. Concentrations

Polvsilanes

I 2 3 4

POE,

Communcation

of the bromohvdrocarbon

BrCH,CH, BrCH,CH,Br BrCH,CH, BrCH,CH,Br

POW

=

ev(NUQI),

3

4

5

6

0.00 0.00 0.00 0.00

1.34 0.40 1.34 0.40

2.68 0.80 2.68 0.80

4.02 1.20 4.02 1.20

5.36 1.60 5.36 1.60

6.70 2.00 6.70 2.00

(1)

where N is the Avogadro constant and [Q] is the quencher concentration. If there exists both this kind of static quenching and dynamic quenching in the same system, then it can be described by the following equation: &/F

= (1 +

fGv~QI)expW’~QI),

(2)

where Ksv is the Stern-Volmer quenching constant. One factor, (1 + K,,[Q]), stands for dynamic quenching, and the other factor, exp(NV[Q]), represents static quenching. Furthermore, this equation can also be changed into the following: &/F

ev-NV[Ql)

= 1 + b[Ql,

(M)

2

be instantaneous, that is to say, the probability of the quenching is unity within the active sphere. An exponential relationship between the fluorescence intensity quenching ratio (F,/F) and the volume of the active sphere (I’) is &/F

auenchers

I

Ouenchers

(3)

where V and KS, are adjustable parameters. After measuring many groups of Fe/F - [Q] in a fluorescence quenching system by experiment, a volume of the active sphere can be found, which makes the Fe/F exp (-NI’[Q]) - [Q] relationship the most linear, and KS, can also be obtained, this being the slope of the F,/F exp( - NV [Q]) - [Q] straight line. After determining the adjustable parameters KS, and V, we can compare quantitatively the dynamic quenching factor (1 + K,,[Q]) with the static quenching factor exp(NV[Q]), and define the nature of the fluorescence quenching. From the fluorescence emission spectra, we can see that the fluorescence intensity of the polysilane copolymers is quenched step by step with the increase in the bromohydrocarbon concentrations. In addition, the maximum emission wavelength for all systems is located at 332 nm. From the fluorescence emission spectra, we calculated the fluorescence intensity quenching ratios (F,/F), Fe/F - [Q] plots. As a result, we observed that all Fe/F - [Q] plots are upward curves, concave towards the y-axis. We used the fluorescence quenching data to draw Fe/F exp(-NV[Q]) -[Q] straight lines with fine correlations according to equation (3), but we never obtained such a line. We had to draw the ln(F,/ F) - [Q] straight lines according to equation (l), as shown in Fig. 1. From Fig. 1, it can be seen that the correlation of the drawn straight line for system 4 is very satisfactory. A similar situation also exists for the other three systems. In accordance with the slopes of these straight lines, we can obtain V(A3), the volumes of the active spheres, and r(A), the radii of the active spheres for all the systems, as shown in Table 2.

From the mathematical treatment, it can be seen that the room-temperature solution fluorescence quenching of the polysilane copolymers by the above two bromohydrocarbons may simply be static quenching [l l-131, that is, as soon as the polysilane copolymer segments are excited under the ultraviolet irradiation and changed into fluorophores, the fluorophores are quenched by the bromohydrocarbon quencher molecules within the active spheres and fall down to the ground state [13]. According to Table 2, for the polysilane copolymers POE, and PO,,, the ratios of the volume of the active sphere for BrCH,CH,Br to that for BrCH,CH, are 4.60 and 3.99, respectively, which indicates that the quenching ability of BrCH,CH,Br is greater than that of BrCH,CH,, which is possibly because the former quencher possesses two Br atoms while the latter possesses only one. Moreover, we also conducted the experiment of the room-temperature solution quenching of the polysilane copolymers by ClCH,CH,Cl, which is in contrast with the former two quenchers and the results of which show that ClCH,CH,Cl at the same concentration with BrCH,CH,Br almost cannot quench the roomtemperature solution fluorescence of the same polysilane copolymers. Therefore, we can tentatively propose that the quenching ability of bromohydro-

Table 2. The radii (r) and volumes (V) of the active spheres for room temperature solution static quenching of polysilane copolymers by bromohydrocarbons

Svstems

Polvsilanes

I

P IPF)

2 3 4

P ,,.Y,

Ouenchers

V (A’)

I (A)

166 763 183 731

3.41 5.67 3.52 5.59

BrCH,CH, BrCH,CH,Br BrCH,CH, BrCH;CH;Br

lCll,

1.00 . ./

0.80 -

z

0.60

-

0.40

-

0/

% S -1

0.00

./

I 0.00

0.50

I

I

I

1.00

1.50

2.00

lQl(M) Fig. 1. The fluorescence under

intensity quenching for system the conception of active sphere.

4

Short Communication

carbons is much greater than that of their corresponding chlorohydrocarbons. The primary reason why the room-temperature solution fluorescence quenching of the polysilane copolymers by the bromohydrocarbons is perhaps simply the static quenching, and the quenching ability of the bromohydrocarbons is much greater than that of their corresponding chlorohydrocarbons, may be described as follows. The number of the nuclear charges of the Br element (35) is larger than that of

the Cl element (17) by 18, the number of a whole long period. Therefore, compared with the Cl element, the Br element can better display the “outer heavy atom effect” [14], that is, the large number of the nuclear charges of the Br element leads to the very strong mutual effect of the spin angular momentum and the orbital function angular momentum of the Q conjugational electrons delocalized along the polysilane copolymer segments. Therefore, the “spin and orbital function coupling” increases the probability of processes such as absorptional transition S,+S, , the intersystem crossing S, -+T, , phosphorescence, the intersystem crossing T,+S,. In this way, because of the improvement in the probability of the intersystem crossing S,-+T,, the number of the excited state S, of the u conjugational electrons delocalized along the polysilane copolymer segments is reduced, which weakens the room-temperature solution fluorescence in the polysilane copolymers to a great degree, or even makes it disappear completely. It should be pointed out that the “outer heavy atom effect” here is not required to form an exciplex, which can be seen from the simple tails of the fluorescence quenching emission spectra or equation

667

(l), with which the fluorescence [lo-121.

quenching

data tally

REFERENCES

1. P. C. Hofer, R. D. Miller and C. G. Wilson. Proc. SPIE Adv. Resist Tech. 16, 469 (1984). 2. P. Shukla. P. M. Cotts. R. D. Miller. S. Ducharme. R. Asthana and J. Zavislan. Mol. Cryst. Liq. Cryst. 183; 241 (1990). 3. J. Kido, K. Nagai, Y. Okamoto and T. Skotheim. Chem. Lett. 7, 1267 (1991). 4. R. G. Kepler, J. M. Zeigler, L. A. Harrah and S. R. Kults. Phys. Rev. E 35, 2818 (1987). 5. R. D. Miller, P. C. Hofer, D. R. Wilson, R. West and P. Trefonas. Am. them. Sot., ACS Symp. Ser. 266,293 (1984).

6. Sun Ya-Ping, T. Karatsu, V. Balaji, R. Sooriyakumaran, R. D. Miller and J. Michl. Polym. Prep. 31, 238 (1990).

I. G.-Q. Li, D.-B. Chen, F.-L. Bai and Y.-M. MO. J. Polym. Sci., Polym. Phys. Edn,. accepted for publication. 8. X.-H. Zhang and R. West. J. Polvm. Sci., Polvm. Chem. Edn 22, 159-(1984). 9. R. D. Miller and D. Thomuson. Polvm. Preo. 32. 300 (1991). 10. J. B. Birks. Photophysics of Aroma+ Molecules. WileyInterscience, London (1970). 11. N. J. Turro. In Modern Molecular Photochemistry, Chap. 8. Benjamin/Cummings, Menlo Park. CA (1978). 12. Sun Ya-Ping, M. Gregory, R. D. Miller and J. Michl. J. Photochem. Photobiol. A: Chem. 62, 333 (1992).

13. K. Shimakawa, T. Okada and 0. Imagawa. Appf. Phys. Lett. 59, 1078 (1991).

14. G.-Z. Chen, X.-Z. Huang, Z.-Z. Zhen, J.-J. Xu, Z.-B. Wang and Y. Fenxifa. Kexue Express, Beijing, p. 92 (1990).