Spectroscopic studies of polydiacetylene solutions and glasses

Volume

106, number

4

CHEMICAL

PHYSICS

LETTERS

SPECTROSCOPIC STUDIES OF POLYDIACETYLENE S.D.D.V.

RUCHOOPUTH,

27

April 19s-i

SOLUTIONS AND GLASSES

D. PHILLIPS

Davy Faraoky Laboratory, lie Royal Inrtitutiot~ 21 Albtwmr~e Street, London IV1X 4BS. UK

of Great Britain,

and D. BLOOR and D J _AND0 Department of Physics, Queen Mary College, Mile End Road, London El 4NS, UK Received

‘7

Fcbrunry

Absorption, polymerization nitrogen

1984

Tluoresccncc excimtion end emission spcc~rs of rr soluble of the bis-(phcnyl xctore) czter of 10,12_docosadiyne-I.

temperatures

for different

xc obscrvcd on addition 2-methyl tctrahydrofuran at 77 K and well-structured

solvents.

Spectral

shifts.

similar

polydi~cctpl~ne.obtllincd by the solid-state Zl4iol. have been recorded 31 room and liquid-

IO those reported

for other

soluble

polydiacctylcncs,

of a non-solvent or on cooling the solutions. The tluorcsccnce quantum yield uas mcasurcd from solution ot room temper~rurc. The yield \ves roughly one hundred times Ixrger ior this system fluorescence

spectra were observed

l_ introduction

glass.

concentrations, mechanically deformed polymers. yray polymerized crystals and solutions, do fluoresce [S--15] _Very few detailed studies have been made of such fluorescence and the nature of the chain defects which allow radiative decay to occur is a maiter of conjecture. We have-therefore, initiated more precise studies of fluorescent emission from PDAs in order to obtGt information about esciton motion and decay. In this paper we report some initial studies of the absorption, and fluorescence excitation and emission of a soluble PDA. This is one of the class of soluble polymers obtained from the bis-esters of 10.12docosadiyne-1 , Xdiol [ 16,171 which have the general structure:

The availability of macroscopic single crystals of poiydiacetylenes (hereafter referred to as PDAs) of a high degree of perfection has led to extensive studies of their properties. Such perfect samples are obtained by the solid-state polymerization of disubstituted diacetylene monomer crystals. The solid-state reaction of the 1 and 4 acetyienic carbons can be initiated by heat, pressure, ionizing radiation (W, X- and -y-rays) and chemical radicals. The polymerization process and studies of tire polymer crystals have been extensively reviewed [l-j]. The intense optical absorption of PDAs is due to the creation of excitons. The absence of fluorescence emission from PDA single crystals has, however, prevented any studies of exciton dynamics from being made [S] . Fluorescence emission has been observed from monomers [6,7] and the efficiency of the nonradiative decay channel for the perfect PDA chains is shown by the quenching of fluorescence when the monomer is converted into polymer [6] _PDAs in which the extended chains are deformed in some way, e.g. partially polymerized crystals with high defect

0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

ior this lo\\-temprmturc

(1) The particular polymer used was the his-(phenyl acetate) ester. abbreviated 9PA, the chemical formula of which is given by: R = R’ = -(CH,),OCO

CHI a

.

This polymer has a high molecular weight and is soluble in a range of common solvents [ 16,171. B.V

247

Volume 106. number 4

27 April 1984

CHEhilCAL PHYSICS LE’ITERS

2. Experimental methods 9PA monomer was synthesized as reported in the i+amre 116]. The pmi-ty UP i-he hTtWrTrec&ii-esamiproducts were determined by 1R and mass spectrometry. The elemental analyses were in good agreement with calculated composition_ Polymerization was carried out with -y-irradiation, and the unreacted monomer was extracted using acetone to give the red polymer. Seif-supporting disks of the polymer could be obtained by the addition of acetone to the film obtained after the slow evaporation of chloroform solutions of the polymer in a crystallizing disk [ 18). Solutions of the purified polymer were prepared using spectroquality chloroform and Z-methyl tetrahydrofur~ (I-MTHF, Aldrich ChemicaIs, carefully refluxed over lithium aluminium hydride).. Optical densities (ODs) and the absorption spectra were recorded on a Perkin-Elmer model 554 IJVvisible spectrophotometer in the single-beam mode using a background recorded with pure solvent in the same cuvette. Fluorescence excitation and emission spectra were taken on a Perkin-Elmer model MPF4 spectrophotometer, suitably modified for low-temperature works. Solution spectra were recorded at room temperature in J 0 X J 0 mm? quartz cuvette; the solutions had optical densities in the range 0.02-0.07 to minimize re-absorption and re-emission. Quantum yields were determined by the common relative technique [ 19,20] using 1,8-~~~onaph~~ene sulphonate (AI%, from Eastman Kodak) in distilled water as the quantum standard [21,22]. Low-temperature spectra were recorded on the MPF4 with the sample mounted in a 3 mm diameter quartz tube in a dewar flask; all spectra taken in the ratio mode. Fluorescence lifetime measurements were made u&g a cavity-dumped argon-ion laser system and singlephoton counting detection. The apparatus and methods have been described in detail [23].

3. Results and discussion

Solutions of 9PA in 2-MTHF and chloroform have a yealow colour; the absorption spectra are essentially identical with that of 4BCMU [X--26]. In addition, the 9PA soIut.ions show similar spectral shifts on addition of a non-solvent or on cooling the solutions below 248

Fig. 1. Absorption and fluorescence

emission specucl of 9PA

polymer in 2-MTHF at room temperature (yellow sofution). Emission spectrum (uncorrected): corrected for Raman

(dashed region), exited at 410 nm (5 nm bandwidth). detected with 5 nm bandwidth; OD (410 nm) = 0.060. Absorption spectrum: 2 nm slitwidth.

10°C for a few days. On addition of hexane, or cooling, the yellow solutions turn pink; heating the cooled solutions converts them back to the yellow form. Absorption and fluorescence emission spectra of 9PA in 2-MTHF are shown in fig. 1 (yeliow solution) and fig. 2 (pink solution). The quantum yield of ffuorescence for the yellow soIution of 9PA in 2-MTHF was

-

WAVELENGTH

nm

Fig. 2. Absorption and fluorescence emission spectra of 9PA polymer in 2-MTHF at rdom temperature (pink solution). Emission spectra (oneorrected): corrected for Raman (dashed region), exited at different wavelengths he (3 nm bandwidth). detected with 3 nm bandwidth. Spectra are arbitrarily offset for clarity. Absorption spectrum: 2 nm slitwidth.

Volume 106. number 4

CHEMICAL PHYSICS LETTERS

27 April 1984

FZWE [YELLOWI

z

CQQ

jr._ WARM

R.T.

FIGURE (PINKI

Fig. 3. Fluorescence emission spectra (uncorrected) of 9PA polymer in 2-MTHF at 77 K (first freezing). Fluorescence excited at different wavelengths A* (10 nm bandwidth), detected with 2 nm bandwidth. The spectra are arbitmrily ofisct for clarity.

F!!w&!s IPINKI

3

a

R .T.

FlGURE 4 #PINKI

I

77K

77K

Scheme I. Tcmpcraturc dependcncc sho\\iny rhc interrelation among rhc preferred conformations.

found to be ==:2X 10V3, compared with the absolute yield of 4 X fOB3 of ANS. Rapidly freezing the yellow 9PA/Z-MTHF solution results in a glass with a pink colour at 77 K. The emission spectra of this glass are shown in fig. 3. Cycling the glass to room temperature and requenching produces different low-temperature spectra depending on the dwell-time at room temperature. If the dwelltime at room temperature is short the solution re-

mains pink and on quenching yields rhe spectra shown in fig. 4. If the dwell-time is long. or if rhe solution is warmed to above room temperature, the solution becomes yellow and the specrra shown in fig. 3 are once again obtained on quenching. Scheme I illustrates more clearly the spectral changes caused by variation in temperature. The spectra shown in figs. 3 and 4 show welldeveloped vibronic sidebands with shifts from the

zero-phonon peaks of about 1500 and 2 100 cm- I , characteristic of the C=C and CrC stretching modes of the acetylenic polymer backbone structure. Three distinct

species

are observed

with band

origins

in

emission and absorption of approximately: (a) 5 19 and 514, (b) 595 and 535,and (c) 610 and 560 nm. The Iongest-~vav~l~n~th bands appear only for glasses recycled to room temperature with a short dwzlltime (cf. figs. 3 and 4). The band origins and distributions of emitted intensity between zero-phonon vibronic bands show thaw the shortest-wavelength

Fig. 4. FIuorescence emission spectra (uncorrected) of 9PA polymer in 2-MTHF at 77 K (second freezing)). Fluorescence excited at different wavelengths he (10 nm bandwidrh),drtected with 2 nm bandwidth. The spectra are arbitrarily offset for clarity.

and

species has a small shift of the potential minimum in the excited state while the other two species have large potential shifts in the excited state. The fluorescence spectra are more inrense at 77 K &an at room temperature; the increase in quantum effkiency is approximately one hundred fold. Very preliminary fluorescence lifetime measure249

Volume

106, number 4

CHENICAL

PHYSICS LEPERS

Table I

Preliminary fluorescence polymer

in SMTHF

lifetime measurements

on 9PA

a)

Emission wave- AShort length at

rshort (ns)

Along

550 nm (77 I<) 620 nm (77 2;)

1.84 1.89

0.001 0.06

0.57 0.57

a) Lifetimes given to thenwest

02-0.3

r10Ilg

02s)

16.13 18.10

RS.

27 April 1984

mer molecules. Such interactions would lead to a chain-folded microcrystalline morphology in the pink solutions rather than a fuIly extended form; such a confomlation could occur ~dependent of side-group hydrogen-bond formation. Studies of other rigid polymers have revealed the presence of such intrachain interactions in randomly coiled chains [30]. This hypothesis will be investigated using soluble PDAs with larger aromatic pendant groups so that side-group interactions can be studied by obser-

vation of excimers. made on 2-MTHF solutions cycled between room and Iow temperatures revealed the presence of at least two excited states, as concluded from the decay curves, which required at least two exponential terms for fitting. Results are summarized in table 1. Wowever, two-component fits were not particularly good; perhaps, suggesting more than two excited states involved in the observed total fluorescence emission which could give rise to additional components in emission, which may ult~ately be revealed. Time-resolved spectroscopic studies of the above system are under investigation to confirm the observations. The chromism of n-BCMU solutions has been attributed to a random-coil to rigid-rod molecular ments

transformation, the extended form occurring when hydrogen bonds are established between the polymer

side-groups [24-271. The occurrence of extended rigid rods was deduced from light-scattering experiments, which indicated radii of gyration close to the extended chain length of chains of the mean molecular weight. This result has been questioned since light scattering is sensitive to the moiecular weight distribution and tends to emphasize the higher molecular weight components (larger particles) [ZS] _The observation of essentially identical spectral shifts for 9PA and 4BCMU polymers suggests that the phenomena are more general since hydrogen bonding does not occur between the 9PA side-groups. In addition to the similarities reported here, similar spectral shifts have been observed for 9PA and n-BCMU melts [ 161.

it seems probable #at the spectral shifts result from an order-disorder transition. The yellow solutions of PDAs contain molecules with a random-coil form [29]. It is possible that lntrachain interactions, via the side-groups, in the random-coii form are sufficiently strong to prevent full extension of the poly250

The fluorescence spectra of pink 9PA solutions are similar to those observed for disordered fiis of diynoic acid polymer [ 141. In addition, the fluorescence quantum efficiencies are comparable, suggesting that the molecular conformations are similar. The spectroscopic studies of the 9PA/2-MTI-IF glass reveal that on quenching the yellow solution two distinct molecular conformations are produced. Annealing at room temperature for a short time and then t-e-freezing to 77 K creates a third confo~ation apparently at the expense of one or both the initial conformations. This conformation cannot, however, be created directly from the yellow solution. These results are consistent with the model proposed above, i.e. on quenching, random-coil molecules are either frozen-in or partially recrysttine, full recrystallization to a ch~-folded microcrystal occurring only after feting at higher tempera~res. The lifetime measurements are also compatible with the existence of different conformations in low-temperature glasses which are retained for a considerable period at room temperature, the longer-lived species disappearing only when conversion to the random-coil form is complete. To explain the spectra of these conformations, we invoke small local deformation in the form of twisting about the sp2-sp2 hybrid orbitals of the polymer backbone in the excited state. The excited state of trans-stilbene has a minimum for a 90” rotation about the double bond [31], smaller values have been observed in similar compounds [3233] _We suggest that the short-lived blue-shifted component of the total fluorescence (see table 1) occurs as a result of negligible change of the chain ~onfo~ation in the excited state. Thus, inter- or ~~a~h~n interactions must lock this conformation in both ground and excited states. Conformations arising from a twisting of the double bond in the excited state are identified with

Volume 106, number 4

CHEhllCAL PHYSICS LEITERS

the observed long-lived red component of the total fluorescence_ Further work is in hand to elucidate the chain conformations in 9PA glasses.

Acknowledgement This work

Engineering

was supported by the Science and Research Council (SERC) and the U.S.

Army European Research Office. We thank Professor R.C. Schulz and Dr. C. Plachetta of the Universjty of Mainz for an initial sample, Dr. N.A. Cade and Dr. R.N. Batchelder of Queen Mary Cclfege, Dr. S.R. Meech, Professor R.L. Christensen and G. Rumbles of the Royal Institution of Great Britain for helpful discussions and assistance, and Dr. P. Clay of the Chemical Engineering lege for r-irradiation

Department of samples.

at Imperial

Col-

References [I] G. Wegner. in: hlolecular mcrals,ed. WE. Hatfield (Plenum Press, New York, 1979) p. 209. [21 G. Wegner, Faraday Discussions Chem. Sot. 68 (1980) 494. j3] R.H. Baughman and R.R. Chance, Ann. NY Acad. Sci. 313 (1978) 705. [41 13. Btoor,in: DcvcIopmen~ in crystalline poIymcrs, Vof. 1, ed. DC. Bassett (Appl- Sci.. London, 1982) p. 151. [S]

D. Bloor, in: Quantum theory of polymers, solid state aspects, cds. J. Ladik and J_hl. Andre (Rcidcl, Dordrccht).

to be published. 161 V. Enkelmann, G. Schleicr,G. Wegner. H. Eichele and N. Schwoerer,Chem. Phys. Letters52 (1977) 314. 171 hf. Bertault, J.L. Fave and hf. Schott. Chem. Phys. Letters 62 (1979) 161. [SI D. Bloor. D.N. Batchelder and F_H_ Preston, Phys. Stat. Sol. 40a (1977) 279. 191 H. Eichete and M. Schwoerer. Phys. Stat. sot. 43n (19771465.

D. Bloor, W. Hcrsel and D.N. Batchelder, Chcm. Phys. Letters 45 (1977) III. B. Tiekc and D. Blow. hlakromol. Chsm. 180 (1979) 2275.

D. Moor. D.N. Batchelder. DJ. Ando. R.T. Rtad and R.J. Yom-g, 1. Polym. Sci. Poiym. Phys.Ed. 19 (1961) 321. H.R. Bhattachqce, X.F. Preziosi and C.N. PJtcl. J. Chcm. Phys. 73 (1980) l-176. J. Ohnstcd 111and Xl. Strand. J. Phys. Chcrn. 87 (1983) -1790. C. Bubcek, B. Ticke and G. Wegwt. Bcr. Bunscnpes. Physik. Chcm. 86 (1982) 495. C. P1achctt.r. PLO. Rau, A. Hawk and RX. Schulz. Makromol. Chem. Rapid Commun. 3 (1982) 249. C. PIachctta and R.C. Schulz. Stakromoi. Chcm. Rapid Commun. 3 (1982) 815. C. Plachettx, privdta communication. W.H. >iclhuish.J. Phys. Chcm. 65 (1961) 229. C.A. Parker. Anal. Chem. 31 (1962) 50. PJ. Sadkowski and G.R. Fleming. J. Chcm. Phgs. j=$ (1980) 79. L. Srryer. J. Mol. Biol. 13 (1963) 1S2. G. Rumbles and 5. Phtirps, in. Dcconvoturion and rcconvolution oi anttlyticiil signals - appliwrions to tluorcsccnce spectroscopy.ed. .\f. Bouchy (E.N.S.I.C.. Nancy, 1982). C.N. Pate!. R.R. Chmcc snd J.D. \Virt. J. Chcm. Phgr. 70 (1979) 4387. R.R. Chancc,G.N. Pate1 and J.D. \Vitt. J. Chsm. Phy s. 71 (1979) 206. IX. Lim, CR. Finchcr Jr. and A.J. Heeger. Phys. Rev. Lcttcrs 50 (1983) 1931. G.N. Pat+ R.R. Chance and J.D. Witt, 1. Polym. Sci. Polpm. Phys. Ed. 16 (1978) 607. [ 281 G. Wenz and G. Wrgnzr, Makromot. Chem. Rapid Commun. 3 (1982) 231. 1291 h1.L. Shand. R.R. Chzmce, M. Ic Posrelloc and 81. Schott. Phys. Rev. B25 (1982)1431. [ 3Oj Xl. Cmlcy, A. Rciser, A.J. Roberts and D. Phtiips. Mxromoleculcs 1-I (198 1) 1752. [31) R.M. Hochstrasscr, Pure Appl. Chem. 32 (19SO) 26S3. f321 CJ. Tred=ciJ and CM. Kury. J. Chcm. Phys. 43 (I 979) 307. [33] K.P. Ghigino, K. Ham. G.R. Manr, D. Phillips. ii. Sahsbury. R.P. Steer and M.D. S\\ords, J. Chtm. Soa. Perkin II (1977) 88.