Kinetic Spectroscopy of Relaxation and Mobility in Synthetic Polymers

Chapter 28

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K. P. Ghiggino, S. W. Bigger, T. A. Smith, P. F. Skilton, and K. L. Tan Department of Physical Chemistry, University of Melbourne, Parkville, Victoria, Australia 3052 Time-resolved fluorescence spectroscopy and fluorescence anisotropy measurements have been applied to study (i) excimer formation and energy transfer in solutions of poly(acenaphtha1ene) (PACE) and poly(2-naphthy1 methacrylate) (P2NMA) and (ii) the conformational dynamics of poly(methacrylic acid) (PMA) and poly (acrylic acid) as a function of solution pH. For PACE and P2NMA, analysis of projections in which the spectral, temporal and intensity information are simultaneously displayed have been used to re-examine kinetic models proposed to account for the complex fluorescence decay behaviour that is observed. Time-resolved fluorescence anisotropy measurements of fluorescent probes incorporated in PMA have led to the proposal of a "connected cluster" model for the hypercoiled conformation of this polymer existing at low pH.

For many years, fluorescence techniques have proved to be extremely powerful and sensitive tools for studying the mechanisms of excitation energy dissipation in polymers and for probing macromolecular structure and dynamics (1-4). However, recent developments in time-resolved fluorescence instrumentation have led to dramatic improvements in the quality of fluorescence decay data obtainable and, when combined with computer-aided data analysis procedures, give rise to new possibilities for resolving the heterogeneous emission and complex fluorescence decay behaviour observed for many polymer systems. In this paper the application of such time-resolved fluorescence techniques is described for investigating (i) excimer formation and energy transfer in polymers containing naphthyl and anthryl chromophores, and (ii) cluster size and chain mobility in poly(methacrylic acid) (PMA) and poly(acrylic acid) (PAA) as functions of solution pH. It is now well established that the temporal decay of fluorescence from solutions of polymers containing pendant aromatic 0097-6156/87/0358-0368S06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

28. GHIGGINO ET AL.

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369

chromophores rarely follows the conventional k i n e t i c schemes that are applicable to simple molecules (2,3). The multi-exponential (or non-exponential) fluorescence decay k i n e t i c s observed i n the monomer and excimer regions of the emission spectrum have been variously attributed to more than one k i n e t i c a l l y d i s t i n c t monomer and/or excimer species (3,5) , spectrally and temporally d i s t i n c t emitting s i t e s (6,7) , configurational and conformational influences upon excimer formation (8,9), non-equilibrium d i f f u s i o n - c o n t r o l l e d excimer sampling Ç10) and time-dependent energy trapping (11) . The mechanisms and rates of the transfer of energy to incorporated chromophores which act as trap s i t e s also have attracted considerable attention due to t h e i r relevance to photodegradation and l i g h t harvesting phenomena. G u i l l e t and co-workers (12,13) have demonstrated c l e a r l y that transfer within aromatic polymer chains to chemically bound acceptor chromophores can occur with high e f f i c i e n c y and i s s i g n i f i c a n t l y influenced by polymer structure and conformation. The k i n e t i c s of these excited-state processes i n polymer systems are undoubtedly complex. However, i t i s possible to obtain an overview of the relaxation behaviour following l i g h t absorption through the recording of time/fluorescence intensity/wavelength hypersurfaces. These surfaces, when combined with fluorescence decay analyses, can provide additional insight into the number and i d e n t i t y of the emitting species present as well as the rates of energy relocation i n the polymer chain. In the present work, timeresolved fluorescence analyses on poly(acenaphthalene) (PACE) with and without anthryl end-groups have been undertaken and compared with previous studies on poly(2-naphthyl methacrylate) (P2NMA). A further application of time-resolved fluorescence measurements i s i n the study of conformational dynamics of polymer chains i n solution. Fluorescence anisotropy measurements of macromolecules incorporating suitable fluorescent probes can give d e t a i l s of chain mobility and polymer conformation (2,14). A p a r t i c u l a r example studied i n t h i s laboratory i s the conformational changes which occur i n aqueous solutions of p o l y e l e c t r o l y t e s as the solution pH i s varied (15,16). Poly(methacrylic acid) (PMA) i s known to exist i n a compact hypercoiled conformation at low pH but undergoes a t r a n s i t i o n to a more extended conformation at a degree of neutralization (a) of 0.2 to 0.3 (16). Similar conformational t r a n s i t i o n s are known to occur i n biopolymer systems and consequently there i s considerable i n t e r e s t i n understanding the nature of the structures present i n model synthetic p o l y e l e c t r o l y t e solutions. Experimental Materials. PACE and P2NMA (see Figure 1) were prepared by free r a d i c a l polymerization of the p u r i f i e d monomers i n degassed benzene using azo b i s - i s o b u t y r o n i t r i l e (AIBN) i n i t i a t o r at 7 5 ° C Anthracene end-groups were incorporated i n the polymers by polymerization i n the presence of 9-chloromethylanthracene (6% by weight) which acts as a chain transfer agent. The polymers were p u r i f i e d by multiple reprecipitations using benzene/methanol as the solvent/non-solvent u n t i l no further changes i n the fluorescence spectra were observed. Polymerization y i e l d s of approximately 15% were obtained and the


370

PHOTOPHYSICS OF POLYMERS

weight average molecular weights ( i n p o l y s t y r e n e e q u i v a l e n t s ) of the t e r m i n a t e d p o l y m e r s as d e t e r m i n e d by g e l p e r m e a t i o n chromatography were 24000 (PACE) and 23300 (P2NMA). S o l u t i o n a b s o r b a n c e s o f l e s s than 0.5 a t t h e w a v e l e n g t h o f maximum a b s o r b a n c e were used i n o r d e r to a v o i d i n t e r c h a i n i n t e r a c t i o n s during the e x c i t e d - s t a t e l i f e t i m e . P o l y ( m e t h a c r y l i c a c i d ) and p o l y ( a c r y l i c a c i d ) (PAA) were p r e p a r e d by AIBN i n i t i a t e d p o l y m e r i z a t i o n o f t h e f r e s h l y d i s t i l l e d monomer i n deoxygenated m e t h y l e t h y l ketone a t 6 0 ° C The i n c o r p o r a t i o n o f 9 , 1 0 - d i m e t h y l a n t h r a c e n e (9,10-DMA) end-groups i n t h e polymer was a c h i e v e d by t h e a d d i t i o n o f the c h a i n t r a n s f e r agent (1% by weight) t o t h e p o l y m e r i z a t i o n m i x t u r e . U n r e a c t e d 9,10-DMA was s e p a r a t e d from t h e polymer by g e l p e r m e a t i o n chromatography u s i n g a column packed w i t h Sephadex LH-20 and methanol as t h e e l u e n t . A n a l y s i s o f the PMA sample by NMR i n d i c a t e s t h a t t h e polymer p r o d u c e d under t h e s e c o n d i t i o n s c o n s i s t s o f 57% s y n d i o t a c t i c , 33% h e t e r o t a c t i c and 10% i s o t a c t i c t r i a d s (15). S o l u t i o n c o n c e n t r a t i o n s were 0.02 M i n r e p e a t i n g u n i t s o f t h e polymer. A l l s o l v e n t s were f r e s h l y d i s t i l l e d b e f o r e use and s o l u t i o n s i n o r g a n i c s o l v e n t s were degassed by m u l t i p l e freeze-pump-thaw c y c l e s . Instrumentation. A schematic r e p r e s e n t a t i o n o f t h e t i m e - r e s o l v e d f l u o r e s c e n c e spectrophotometer developed i n t h i s l a b o r a t o r y i s given i n F i g u r e 2. The e x c i t a t i o n s o u r c e i s a s y n c h r o n o u s l y mode-locked and c a v i t y dumped dye l a s e r ( S p e c t r a P h y s i c s 171 mode-locked argon i o n l a s e r / 3 7 7 dye l a s e r / 3 4 4 c a v i t y dumper). The l a s e r p r o v i d e s t u n a b l e l i g h t p u l s e s o f a p p r o x i m a t e l y 10 ps d u r a t i o n a t r e p e t i t i o n r a t e s up t o 82 MHz i n t h e s p e c t r a l r e g i o n 565 t o 630 nm ( u s i n g Rhodamine 6G d y e ) . L a s e r a l i g n m e n t and p u l s e q u a l i t y a r e m o n i t o r e d w i t h a S p e c t r a P h y s i c s 409 a u t o c o r r e l a t o r and 403B h i g h speed photodiode. S t u d i e s on n a p h t h a l e n e - c o n t a i n i n g p o l y m e r s were p e r f o r m e d u s i n g u l t r a v i o l e t r a d i a t i o n a t 295 nm which i s p r o d u c e d by means o f a t e m p e r a t u r e - t u n e d second harmonic g e n e r a t i o n (SHG) ADA c r y s t a l ( J . K. L a s e r s ) . P u l s e r e p e t i t i o n r a t e s o f 4 MHz o r 800 kHz were employed. F l u o r e s c e n c e from t h e sample i s p a s s e d t h r o u g h a J o b i n - Y v o n H-20 h o l o g r a p h i c g r a t i n g s c a n n i n g monochromator b e f o r e d e t e c t i o n by a P h i l i p s XP2020Q p h o t o m u l t i p l i e r tube. The t i m e - c o r r e l a t e d s i n g l e photon c o u n t i n g e l e c t r o n i c s i n c o r p o r a t e ORTEC modules i n c l u d i n g a 454 t i m i n g f i l t e r a m p l i f i e r , 583 c o n s t a n t f r a c t i o n d i s c r i m i n a t o r , 457 t i m e - t o - a m p l i t u d e converter (TAC) and a T r a c o r - N o r t h e r n NS-710A m u l t i c h a n n e l a n a l y z e r (MCA). The optimum r e s p o n s e f u n c t i o n o f t h e p h o t o m u l t i p l i e r tube and e l e c t r o n i c s i s 400 p s FWHM. Data from t h e MCA a r e s t o r e d u s i n g a d e d i c a t e d 6802 microcomputer and a r e then t r a n s f e r r e d t o a VAX 11/780 system f o r a l l subsequent m a n i p u l a t i o n and a n a l y s e s u s i n g F o r t r a n 77 programs t h a t were d e v e l o p e d i n t h i s l a b o r a t o r y . F l u o r e s c e n c e decay p r o f i l e s a r e a n a l y s e d as a sum o f up t o t h r e e e x p o n e n t i a l s by an i t e r a t i v e r e c o n v o l u t i o n p r o c e d u r e which has been d e s c r i b e d e l s e w h e r e (17-19) and i s b a s e d on t h e Marquardt a l g o r i t h m . Goodness o f f i t i s judged by i n s p e c t i o n o f t h e w e i g h t e d r e s i d u a l s , a u t o c o r r e l a t i o n f u n c t i o n o f t h e w e i g h t e d r e s i d u a l s , reduced c h i s q u a r e v a l u e and t h e Durbin-Watson parameter. T i m e - r e s o l v e d e m i s s i o n (TRE) s p e c t r a a r e g e n e r a t e d by s e t t i n g v o l t a g e d i s c r i m i n a t o r s on t h e o u t p u t o f t h e TAC t o s e l e c t a "time window" i n t h e decay p r o f i l e . The monochromator i s then


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371


OC=0 CH CH 1

— ( - C H - C H 4η—

2

3

P2NMA

PACE

F i g u r e 1. S t r u c t u r e s o f p o l y ( a c e n a p h t h a l e n e ) p o l y ( 2 - n a p h t h y l m e t h a c r y l a t e ) (P2NMA).

(PACE) and

mode locked Ar laser

dye laser

cavity dumper

f pump i

3 8 8 8 8 8 8

I

! dye

delay photodiode

sample

scanning monochromator / PMT detection system

TAC

H

MCA

H

data acquisition computer

FLUORESCENCE INTENSITY

F i g u r e 2. Schematic r e p r e s e n t a t i o n o f t h e t i m e - r e s o l v e d f l u o r e s c e n c e s p e c t r o m e t e r . I n s e t diagram: i n t e n s i t y / t i m e / wavelength h y p e r s u r f a c e f o r PACE i n benzene a t 25°C.



372

synchronously scanned over the desired wavelength range with the MCA operating i n multichannel scaling mode. Multiple ΤRE spectra can be displayed as either a two or three dimensional intensity/wavelength/ time representation. Three dimensional hypersurface representations are generated by non-linearly interpolating between, and connecting i n t e n s i t y values of the gated spectra f o r a given wavelength. Intensity/wavelength/time cross-sectional diagrams (or time-resolved fluorescence "contour" diagrams) are generated using a weighted non l i n e a r least squares polynomial surface procedure (20). Areanormalized TRE spectra can be used for convenient p i c t o r i a l representation, since the absolute emission intensity of individual time-resolved spectra vary substantially with time a f t e r excitation. An Applied Photophysics Model SP 2X nanosecond spectrometer incorporating an alternating p o l a r i z a t i o n rotation unit (15) was used for the time-resolved fluorescence anisotropy measurements. An excitation wavelength of 365 nm was employed for excitation of the anthracene end-groups and emission above 400 nm was isolated with a Schott GG 400 f i l t e r . Results and Discussion TRE Studies of PACE and P2NMA. An intensity/time/wavelength surface for PACE i n benzene at 25°C i s shown i n Figure 2 (inset diagram) and the corresponding contour diagram i s presented i n Figure 3. Fluorescence a r i s i n g from monomeric s i t e s with a maximum near 340 nm and an excimer emission with a maximum at 400 nm are c l e a r l y evident. I t should be noted that these surfaces are generated using areanormalized time-resolved spectra and thus the apparent rates of monomer fluorescence decay and "grow-in" of excimer emission depicted in Figures 2 and 3 are distorted. Analysis of these diagrams indicates that there are only two spectrally d i s t i n c t species present. Support f o r t h i s conclusion i s provided by the presence of one isoemissive l i n e i n each surface at 373 nm which would be most unlikely to occur i f more than two d i f f e r e n t species contribute to fluorescence. In addition, the application of P r i n c i p a l Component Analysis (21) to the TRE spectra i s able to c l e a r l y resolve two spectral components with a high degree of confidence. Analysis of the fluorescence decay p r o f i l e s indicates that a two exponential f i t , which would be expected f o r conventional monomer/ excimer k i n e t i c s (2-4) , i s inadequate to describe the decay at any wavelength of observation throughout the emission band. A f i t to a sum of three exponentials leads to s i g n i f i c a n t improvements i n the f i t t i n g c r i t e r i a and examples of the results obtained at wavelengths in the monomer and excimer regions of the spectrum are presented i n Table I. Lifetimes obtained from t r i p l e exponential f i t s at 25 nm wavelength i n t e r v a l s across the emission band are reasonably consistent and give mean values of τι = 0.80 ± 0.14 ns, τ 2 = 6.8 ± 0.8 ns and T3 = 33 + 4 ns (1 standard deviation error l i m i t s ) . However, the pre-exponential factor f o r τχ becomes negative i n the excimer region while the magnitude of the contribution of each l i f e t i m e component to the t o t a l emission remains constant throughout the excimer band.


28. GHIGGINO ET AL. Table I.

373


Fluorescence Decay Data f o r PACE and P2NMA Homopolymers i n Benzene at 25°C

Data are f i t t e d to functions of the form I(t) = ΣΑ.exp(-t/τ.)

POLYMER

X

obs

( n m )

V

n

s

)

/

A

T (ns)/A

l

2

2

x (ns)/A 3

3

PACE

325 450

0.97/42359 0.74/-12474

6.1/29170 7.7/31535

38.7/5126 33.1/27775

P2NMA

340 460

3.1/37679 3.1/-9717

12.0/18930

34.1/3369 34.1/25445

For P2NMA i n benzene at 25°C, analysis of the time-resolved fluorescence surface (not shown) also indicates that only two spectrally d i s t i n c t species contribute to emission - a monomer with a fluorescence maximum at 335 nm and an excimer with an emission maximum at 395 nm. A minimum of a sum of three exponentials i s required to f i t the fluorescence decay p r o f i l e s i n the monomer spectral region whilst i n the excimer emission band a difference of two exponentials proves to be adequate (see Table I ) . The l i f e t i m e s extracted are consistent throughout the spectrum r e s u l t i n g i n the mean values of τχ = 3.00 ± 0.14 ns, T2 = 12.6 ± 1.5 ns and 1 3 = 33.88 ± 0.74 ns (1 standard deviation error l i m i t s ) . In t h i s case the τ 2 component i s s i g n i f i c a n t only i n the monomer spectral region and i t s contribution to the t o t a l emission follows the same trend as the monomer emission band (Figure 4). The pre-exponential factor associated with the component becomes negative i n the excimer band although i t should be noted that the absolute values of the preexponential factors associated with τχ and 13 do not become equal i n t h i s spectral region, contrary to the situation predicted by conventional excimer k i n e t i c schemes (2-4). The results presented above may be compared with previous studies on these polymers. I t has been suggested that more than one spectrally d i s t i n c t excimer species may exist i n some naphthalenecontaining polymers (6,22). Analyses of the spectral surfaces f o r the polymers studied i n t h i s work c l e a r l y indicate that only one spectrally d i s t i n c t monomer and one excimer species contribute to emission. P h i l l i p s and co-workers (3,23) also found that the monomer fluorescence decay from PACE requires a minimum of three exponential terms to provide adequate f i t t i n g . They explained t h e i r results on the basis of a k i n e t i c model involving two temporally d i s t i n c t excited monomers (M^*,M2*) which can form the excimer (D*) as outlined i n Scheme 1. Reverse dissociation of the excimer was also considered important. D*

Scheme 1

This scheme leads to a sum of three exponential terms being required to f i t the monomer and excimer fluorescence decay curves (2,23). The results obtained i n the present study are consistent with the e a r l i e r conclusions of P h i l l i p s i n so f a r as the presence of monomer emission a t a l l decay times a f t e r excitation confirms


374


Figure 4. Total fluorescence spectrum f o r P2NMA i n benzene a t 25°C. Overlaid data (large c i r c l e s ) indicate the contribution of the components with l i f e t i m e s τ^, τ 2 and τ$ to the t o t a l emission.


28.

GHIGGINO ET AL.


375

that excimer dissociation occurs. Three decay times would also a r i s e from a k i n e t i c scheme whereby two d i s t i n c t subgroups of monomer chromophores are able to form the excimer with d i f f e r e n t average rate constants (2,24)(c.f. Scheme 2). I t should be noted, however, that the r e l a t i v e magnitudes of the pre-exponentials obtained for PACE are not as expected from analysis of Schemes 1 and 2 (24). M* 2

£ D*

M* 1

Scheme 2

The results obtained with P2NMA also indicate the presence of only two spectrally d i s t i n c t emitting s i t e s . The τ2 component present i n the short wavelength region, whose spectral c h a r a c t e r i s t i c s correspond to monomer fluorescence, may be attributed to chromophores which do not p a r t i c i p a t e i n excimer formation since no corresponding decay component i s observed at wavelengths above 400 nm. The τ2 component observed i n the monomer fluorescence decay curves must a r i s e from excimer d i s s o c i a t i o n . The i d e n t i f i c a t i o n of i s o l a t e d monomer chromophores which do not form excimers i s i n agreement with the scheme proposed by G u i l l e t and co workers (25) f o r a range of naphthyl a l k y l methacrylate polymers. However, the fact that the pre-exponential terms for the τχ and τ3 components are not equal and opposite i n the excimer region suggests that the conventional monomer/excimer k i n e t i c scheme i s s t i l l not applicable to t h i s polymer. Recent t h e o r e t i c a l approaches indicate that transient d i f f u s i o n a l effects on excimer sampling (10) and time-dependent energy trapping phenomena (11) may lead to functional forms f o r fluorescence decay i n polymers that are d i f f e r e n t to the sum of exponential terms implemented here. The treatment of one-dimensional electronic energy transport as applied to v i n y l aromatic polymers (11), indicates that the sum of exponential terms i n the fluorescence decay that i s predicted from Schemes 1 and 2 might be applicable to the polymers under investigation i f the rate of energy migration i s low. A l t e r n a t i v e l y , i f e f f i c i e n t energy migration i s present then non-exponential fluorescence decay k i n e t i c s might be expected. In certain cases, i t has been shown that data described by a sum of exponentials can also be simulated by non-exponential decay functions (10). In the present work functions comprising the sums of two or three exponential terms could be used to adequately describe the data. However, the extracted pre-exponential parameters are inconsistent with monomer/excimer k i n e t i c schemes that are based on d i s t i n c t excited-state species. The unambiguous assignment of the appropriate k i n e t i c s that describe the excited-state dynamics i n these v i n y l aromatic polymers awaits further improvement i n the quality and time resolution of fluorescence decay data. A further i l l u s t r a t i o n of the application of time-resolved fluorescence surfaces i n studying energy d i s t r i b u t i o n i n polymers i s provided by the intensity/time/wavelength surface for PACE with a 9-methylanthryl end-group (see Figure 5). Although nearly a l l the excitation radiation at 295 nm i s absorbed by the acenaphthyl chromophores, there i s considerable emission that i s c h a r a c t e r i s t i c of the anthryl groups present i n the t o t a l fluorescence spectrum indicating that energy transfer to the end-group occurs. The anthracene emission very rapidly "grows i n " (

Kinetic Spectroscopy of Relaxation and Mobility in Synthetic Polymers

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