Carbazolyalkyl substituted cyclosiloxanes: Synthesis and properties

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Synthetic Metals, 55-57 (1993) 890-895

CARBAZOLYLALKYL SUBSTITUTED CYCLOSILOXANES : SYNTHESIS AND PROPERTIES

J.M. MAUD*, A. VLAHOV*, D.M. GOLDIE, A.R. HEPBURN and J.M. MARSHALL Molecular Electronics Group Abertawe, Departments of Chemistry* and Materials Engineering, University College of Swansea, Singleton Park, Swansea, SA2 8PP, U.K.

ABSTRACT Carbazolylalkyl substituted oligosiloxanes undergo anodic oxidation to give electrochromic thin films which are characterised by two redox processes. These processes are well defined when the carbazole groups are separated from the oligosiloxane core by long spacer groups, a feature which has been observed previously for analogous polymers.

INTRODUCTION We have

describedpreviously [1-3] the synthesis of polymers (1) in which electron rich carbazole

groups are aUaehed to a flexible polysiloxane backbone via alkyl spacer groups. Polymers (1) are of interest for two reasons. Firstly, changes in transient photoconductivity with variation in the alkyl spacer group, and thus in the spatial disposition of the carhazole chromophores, may help to elucidate the mechanism of charge transfer in these systems. Secondly, the carbazole groups

(~H2)m

((~H2)m Me3Si+O--SiMe-~ OSiMe3 (1)

N

I

~N ~

(2)

I

(CH2)m

0379-6779/93/$6.00

© 1993- Elsevier Sequoia. All rights reserved

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undergo relatively facile electrochemical oxidation to give radical cations. Dimerisation of these radical cations serves to cause cross-linking between individual polymer chains, with the deposition of insoluble polymer films on the electrode surface. The electrochemistry of the immobilised dicarbazolyl groups (2) can be addressed, and forms the basis for a novel electrochromic display - the oxidised film is green in colour, and colourless when reduced. Although interpretation of the electrochemical features is the subject of some debate [4], we have shown that it depends upon the length of the spacer group.

N

H I Me3Si~ O--SiMe~nOSiMe3

(3)

I

(4)

Polymers (1) are obtained [5] via platinum catalysed hydrosilylation reaction between an alkenylcarbazole (3) and polymethylhydrosiloxane (4). Although polymers with short alkyl spacer groups are readily purified by precipitation from appropriate solvent mixtures (e.g., dichloromethane-hexane) this procedure is less satisfactory for materials with longer spacers because of the greater similarity between the polymer and the alkenylcarbazole precursor. In order to avoid this difficulty, we are now examining the structurally related cyclic oligomers (5; a: n=4, m=3; b: n=4, m=ll; c: n=5, m=3).. m

(~H2}m O--SiMe

'

(5) In >

C

I

Hj O--SiMe

(B)

1n >

RESULTS and DISCUSSION Synthesis Oligomers (5) were prepared via platinum catalysed hydrosilylation reaction [5] between oligocyclomethylhydrosiloxanes (6, n=4,5, Petrach) and ro-(carbazol-9-yl)alk-l-enes m refluxing toluene, and obtained in typically 50% yield following column chromatography (silica-gel, hexane). The oligomers were fully characterised by spectroscopic methods. Thus, for example, the IH nmr spectrum of (5a) exhibited the six Silvle signals groups expected for a mixture of four conformational isomers. In addition, FAB mass spectrometry revealed a peak at 1069.44 (MH+; calculated for C64H69OaN4Si4 : 1069.436).

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Electrochemistry Cross linked films containing dicarbazolyl groups were obtained during one voltammetric cycle (0-1.6-0 V vs. Ag quasi reference electrode, 100 mV s-l) at a gold wire working electrode (alummium counter) in a dichloromethane solution of oligomer (2.5 x 10-2 M ) containing NBu4PF 6 (0.1 M) as electrolyte. The gold wire was removed from the polymer solution and washed with dichloromcthane prior to cyclic voltammetry in CH2CI2-NBu4PF6 (0.1 M). Stable and reproducible voltammograms were obtained after one or more conditioning cycles. Voltammograms for the crosslinked films from (5a) and (5b) are shown in Figure 1. Both voltammograms are characterised by two reversible waves (E0 = 0.89, 1.21 V for (5a); E 0 = 0.73, 1.25 V for (5b)) which may be attributed to two consecutive redox processes (Equations 1 and 2) involving groups of k carbazole dimers (C2) ' (kC2)-

e-=

(k%) +

(kC2) + - e- -- (kC2) ++

(1) (2)

Values for the oxidation potentials closely resemble those for related polymers. Thus those for (Sa) are extremely close [1] to those (E 0 = 0.88, 1.22 V) for the polymeric analogue of (1) in which the carbazole group is substituted with a bromine at the 3-position., while the potentials for (Sb) are identical with those [2] for a polymer analogous to (1) but in which the carbazole group is attached to the polymer backbone via a long spacer group at the C3 position. The most striking difference between the voltammograms for (5a) and (5b) resides in the very different widths of the peaks at half-height (PWHHs). In the case of (5b) the PWHHs are very close to 100 mV for all peaks, whereas for (Sa) they lie in the range 230-320 inV. Electrochemical theory [6] for a surface adsorbed redox couple (Equation 3) suggests that, at ambient temperature, the PWHH of the voltammetric peak should be close to (90/j) mV where j is the number of electrons transferred between the oxidised (OX) and reduced (RED) components of the couple. R E D - j e- = OX

(3)

Thus, for the couple related to the crossqinked film from (5b), it is seen that j is very close to unity. On the other hand, the values o f j for the polymeric film from (Sa) lie in the range 0.28-0.4. The "j-value" is clearly the reciprocal of the "k-value" (Equations 1 and 2). Thus for (51)) k is very nearly unity. In other words, the redox couples associated with (5b) involve the transfer of one electron for each dicarbazolyl group. For (Sa) however, k lies in the range 2.6-3.6.. In the case of (5a) therefore, one electron is transferred for every three (approximately) dicarbazolyl groups. This variation, in the effective stoichiometry of the redox couples involving dicarbazolyl groups, has been seen previously for related polymers [3]. The simplest explanation recoguises that oxidation of the dicatbazolyl groups involves incorporation of counterions (PF6- in this case) which must be incorporated into the polymer matrix. For systems with short spacers, such as the oligomer (5a), it can be argued that there is simply not enough "space" for the incorporation of the counterions needed to balance formal charges on every dicarbazolyl group.

893

100 g A cm

I'

r..) cJ

o

5 4,

0.0

1.6 POTENTIAL, E/V vs. "Ag" quasi reference

100 ~

cm-2

4'

o

r,.)

< L) 4,

0.0

1.6

POTENTIAL, E/V vs. "Ag" quasi reference

Fig. 1 Cyclic voltammograms o f the dicarbazolyl thin films obtained via anodic oxidation of (5a) (upper) and (5b) (lower); gold wire working electrode, scan rate = 100 mV s -1.

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The electrochemical behaviour of (5c) closely resembled that of (Sa) thus demonstrating the importance of the length of the spacer group as a reported in detail elesewhere.

determining factor. Subtle differences will be

Potential Applications The reversible electrochemistry associated with the dicarbazolyl containing films suggests several applications. The films are electrochromic - green when oxidised and colourless when reduced. Of greatest significance however is the presence of two well defmed redox couples for the film derived from (Sb). There are three regions of potential ((i) <0.5 V, (ii) ca. 1 V, and (iii) >1.5 V) where little or no current flows. Thus any one of these regions should be accessible via controlled charge injection. When placed "open-circuit" the film would then retain the corresponding potential, which might then be "read" potentiometrically. One thus has the basis for a tristate memory element. Transient Photoconductivity Variation in both (i) the size of the oligomeric siloxane core and (ii) the length of the spacer group aRachin~ the carbazole unit to the core, affects the spatial relationship between the individual carbazole units. As seen above, this has a dramatic effect on the electrochemistry of the cross-linked films produced by oxidation of the oligomers. In principle, it should also affect the moevement of (photogenerated) charge through the materials. One might hope therefore that a study of charge transport parameters with variation in oligomer structure, might lead to an understanding of the actual mechanism of charge transport. Unfortunately the oligomers (5) have relatively low glass transition temperatures - ca. 50-600 for (5a) and (5c), and even lower for (5b). This has hindered the metallisation of thin films of the oligomers (5) and prevented transient photoconductivity measurements on the virgin materials. Nevertheless, we believe that the principle is a valid one, and we are currently studying dispersions of (5) in polycarbonate In addition, we are attempting to design and synthesise analogues of (5) with higher glass transition temperatures.

CONCLUSIONS The electrochemical behaviour of small cyclo-oligosiloxancs modified with pendant carbazoles closely resembles that of the relate~ polymers. However the ease of purification of the oligomers offers distinct advantage over the polymers.

ACKNOWLEDGEMENT We thank the SERC (UK) for financial support.

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REFERE-2qCES 1 T.W. Booth, S. Evans, and J.M. Maud, J. Chem. Soc., Chem. Commun., (1989), 196-198. 2 A.R. Hepburn, J.M. Marshall and J.M. Maud, Synth. Met., 41-43, (1991), 2935-2938. 3 J.M. Maud, T.W. Booth, A.R. Hepburn and J.M. Maud, in M. Borissov et al. (eds.), Proc. Sixth International School on Condensed Matter Physics, Varna, Bulgaria, 21-29 Sept. 1990, World Scientific, Singapore, 1991, pp. 294-300. 4 B. Tieke and M.O. Chard, Polymer, 30, (1989), 1150-4. 5 P. Strohriegl, Makromol. Chem., Rapid Commun., 7 (1986), 771-775. 6 A.J. Bard and L.R. Faulkner, Electrochemical Methoc~_, (Wiley, New York, 1980), Ch. 12, p. 522.