Chapter 10
Structural and Spectroscopic Studies of Polydiacetylenes D. Bloor Downloaded via STONY BROOK UNIV SUNY on July 6, 2018 at 08:26:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Physics, Queen Mary College, London E1 4NS, United Kingdom
Many aspects of the preparation and properties of polydiacetylenes are the subject of lively debate. This review presents recent results that bear on some of these controversies. First the relationship of diacetylene monomer crystal structure and solid-state reactivity is discussed. Secondly the temporal evolution of solvato-chromic transitions of soluble polydiacetylenes is displayed. Optical and Raman spectra reveal the occurrence of an intermediate form of the polymer. A model compatible with these results is described. 1
The polydiacetylenes, ^CR-CaC-CR ^, hereafter abbreviated PDAs, have been expensively studied since the late 1960s (l)· This research has been the subject of several reviews and a recent book (2). Current studies continue to bring to l i g h t interesting and unexpected phenomena and much of t h i s i s the subject of a series of l i v e l y debates. The importance of PDAs stems from the a v a i l a b i l i t y of macroscopic single c r y s t a l s with larger inter-chain separations, obtained by s o l i d - s t a t e polymerisation of diacetylene monomers. Such samples enable the fundamental properties of the quasi-one-dimensional, f u l l y extended, conjugated polymer chains to be probed free from the compl i c a t i o n s imposed by the complex morphologies usually encountered i n polymeric materials. Such d i r e c t studies are not possible with other conjugated polymers which can, at best, be obtained as aligned but disordered samples. The PDAs may be obtained i n less perfect form so that the influence of disorder on t h e i r physical properties can be properly assessed. Thus, PDAs provide us with a class of model mate r i a l s f o r the study of conjugated polymers. A basic requirement of the subject i s to obtain an understanding of the s o l i d - s t a t e r e a c t i v i t y of diacetylene monomers. This can, i n p r i n c i p l e , be used to develop a predictive capability and enable r e active monomer molecules to be 'engineered . Progress towards t h i s goal has been achieved by the c o r r e l a t i o n of l a t t i c e packing and r e a c t i v i t y . Some recent studies, providing further information on t h i s t o p i c , are reviewed i n the following section. PDAs can be disordered by a variety of processes but to date there 1
0097-6156/87/0337-0128$06.00/0 © 1987 American Chemical Society
Sandman; Crystallographically Ordered Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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has been l i t t l e detailed characterisation reported i n the l i t e r a t u r e . An exception to t h i s i s the class of PDAs which dissolve, without degradation, i n a range of common solvents. These PDAs display thermochromism and solvent composition dependent colour changes. These effects derive from order-disorder t r a n s i t i o n s but there i s not yet a concensus about the d e t a i l s of the structure of the ordered and d i s ordered phases. Obtaining a proper understanding of these effects i s complicated by the marked differences that exist i n the sidegroups of PDAs. Thus, though there i s a generality of chromic e f f e c t s , there are many differences in d e t a i l . Examples i l l u s t r a t i n g the e f f e c t of changes of sidegroups structure on the solvato-chromism w i l l be presented i n the f i n a l section. In p a r t i c u l a r , the temporal evolution of the spectra after a stepwise change i n solvent composition are described. The relevance of these r e s u l t s to the current models f o r PDA chains i n solution i s discussed. S o l i d State Reactivity The colouration of certain diacetylene compounds on prolonged storage or exposure to l i g h t has been known f o r over 100 years. Though the true nature of t h i s reaction was not shown experimentally by Wegner (l) u n t i l 1969, a number of e a r l i e r workers i d e n t i f i e d some of the salient points (3)· Most notable was the work of Strauss et a l i n 1930. They observed colour changes leading to m e t a l l i c , lustrous crystals of dihalogen-diacetylenes. This was attributed to the crys t a l packing giving r i s e to interactions of the molecules rendering the structure unstable. Molecular r o t a t i o n within the c r y s t a l could then allow bonds to form between the i n d i v i d u a l molecules producing a polymer. The metallic appearance was presumed to derive from loosely bound electrons i n the product. The authors surmised that x-ray s t r uctural studies would provide a deeper insight into the process. This e s s e n t i a l l y correct description went unnoticed; apparently these insights were ahead of t h e i r time. Part of the lack of i n t e r est may have been due to the fact that the dihalogen-diacetylene crys t a l s exploded under impact. In any event, i t was a further 39 years before the important x-ray studies were undertaken. The l a t t i c e packing of reactive diacetylenes i s most conveniently d i s played using the monomer separation and t i l t angles (c/andîf) f i r s t introduced by Baughman (5), see Figure l ( a ) . Crystal structure data f o r both reactive and unreactive compounds was used i n a comparison of the l a t t i c e packing of diacetylene monomers and t h e i r solid-state r e a c t i v i t y . This showed that a reasonable c r i t e r i o n f o r r e a c t i v i t y i s that the reacting ( l and A ) carbon atoms must be separated by less than 0.4 nm (3). This c r i t e r i o n was o r i g i n a l l y proposed by Schmidt (6). More recent compilations have come to the same general conclusion (7). I t i s also found to hold, even when there are complex molecular re-arrangements during polymerisation (8)· The rate of polymerisation i s strongly influenced i n these cases but not the occurrence of solid-state r e a c t i v i t y . Since the close packing of monomers i s determined by the van der Waals r a d i i of the diacetylene carbons, the values of d and f o r reactive packing must l i e within the shaded area of Figure 1(b). Here the lower curve i s the l i m i t of closer packing and the upper curve i s set by Schmidt's c r i t e r i o n . Thus, f o r any given monomer separation d there i s a range of 1
Sandman; Crystallographically Ordered Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
CRYSTALLOGRAPHICALLY ORDERED POLYMERS
Figure 1. Solid-state polymerization of diacetylenes (a) Monomer array i n c r y s t a l l a t t i c e (b) Reactive packing (shaded area) shown i n terms of d and "é.
Sandman; Crystallographically Ordered Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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Structural and Spectroscopic Studies of Polydiacetylenes
values from i f . to / which allows solid-state polymerisation , min max to occur. The arrangement shown i n Figure l ( a ) presupposes an extended conform ation of the diacetylene molecule. For monomers with bulky end groups, the f l e x i b i l i t y of the monomer molecule can r e s u l t i n a com pact molecular structure i n the c r y s t a l . The sidegroups then impede the reaction, i n contrast to the situation i n reactive monomers where the sidegroup interactions bring the diacetylene moieties into the desired reactive contact. Both these general rules are i l l u s t r a t e d i n the structures of a ser ies of monomers related to 2,4-hexadiynylene bis(p-toluene sulphonate), (TS) (£). The parent compound of t h i s series i s 2,4-hexadiynylene bis(benzene sulphonate),(BS), R = -CHp-O-SO^C^H- (10). The nature of the substitution of the terminal benzene ring has marked e f f e c t s on molecular conformation and packing. Five compounds have the conformation shown i n Figure 2(a): these have terminal H (BS), F (FBS), CH (TS), OCH^ (MBS), CI (CBS) and the t r i p l y CH sub s t i t u t e d ring (mesitylene sulphonate, MS). This i s known from the structures of BS (10), TS (9), FBS ( l l ) and MS (12). CBS and MBS are polymorphic and the reactive phases are available only as small crys t a l s grown from highly supersaturated solutions (13,1^.) · From the s i m i l a r i t i e s i n polymer structure of TS and MBS (15) and the powder data which indicates a monomer separation of 0.518 nm i n MBS, a simi l a r conformation can be inferred for MBS. I t seems probable that reactive CBS adopts the same form. The d , ^min» ^max ^ values are l i s t e d i n Table I . The observeS°packing and r e a c t i v i t y correlates as expected. 3
n
Table I .
Monomer
d
L a t t i c e packing of BS related monomers
(nm)
mon
mm
BS FBS TS MS
0.528 0.515 0.515 0.5Λ7
