with Triphenylsulfonium Salts - American Chemical Society

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21 Fluorescence Spectroscopy and Photochemistry of Poly(4-oxystyrenes) with Triphenylsulfonium Salts Insight into the Photoinitiation of Chemically Amplified Resists N i g e l P . Hacker

1

a n d K e v i n M. W e l s h

2

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 Advanced Technology Center, IBM General Technology Division, Hopewell Junction, NY 12533 1

2

The fluorescence spectroscopy of 4-oxystyrene polymers, the quenching of the fluorescence by triphenylsulfonium salts, and the photochemistry of the triphenylsulfonium salts in these polymers were studied. Three polymers were examined: poly(4-hydroxystyrene), poly (4-methoxystyrene), and poly[4-(tert-butoxycarbonyl)oxy]styrene]; the latter polymer is employed as a photosensitive polymer for resist applications in the presence of sulfonium salts. All three polymers fluoresce in the 300-340-nm region. Triphenylsulfonium salts quench the fluorescence from the polymers both in solution and the solid state by dynamic and static quenching mechanisms respectively. Photolysis of the sulfonium salts in the polymer films gave lower than expected cage to escape ratios (C/E) for a viscous medium, which is attributed to a sensitization process by the polymer. However, in-cage products are observed, which also implies a direct photolysis mechanism. Thus it is proposed that the process for photoacid production in 4-oxystyrene polymers occurs by a dual photoinitiation pathway that involves both the excited state of the sulfonium salt and the excited state of the polymer film. Performance of the sulfonium salt-oxystyrene polymer was assessed by a low irradiation dose-high dose thickness change comparison at various sulfonium salt concentrations. This thickness 0065-2393/93/0236-0557$06.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


558

STRUCTURE-PROPERTY RELATIONS IN POLYMERS

test simulates photospeed but reduces errors introduced from the variation in dissolution characteristics caused by changing the sulfonium salt concentration. The thickness data follow the same trend as the percent excited state quenched by the static quenching mechanism with the exception that there is an additional direct photolysis component. The combined spectroscopic and photochemical studies also explain the dynamic range of photosensitivity of the resist that encompasses the UV absorption range of both the initiator and the polymer film.

TTHE

U S E O F O N I U M SALTS AS PHOTOINITIATORS f o r acid-catalyzed crossl i n k i n g reactions i n p o l y m e r chemistry has f o u n d w i d e application ( I ) . M o r e recently, poly[4-[(teri-butoxycarbonyl)oxy]styrene] ( P T B O C ) has become v i tally important i n the electronics industry as an extremely sensitive photoac tive p o l y m e r i n f o r m u l a t i o n w i t h a n o n i u m salt photoinitiator ( 2 ) . A l t h o u g h the photochemistry o f s u l f o n i u m a n d other o n i u m salts has b e e n extensively studied i n solution ( I , 3 ) a n d recent mechanistic investigations have shown the importance o f solvent in-cage reactions a n d cage-escape reactions (4-7), r e n d e r e d photosensitive b y o n i u m salts (8).

T h e initial reaction o f o n i u m salts i n the P T B O C resist is photogeneration o f Brônsted acid, w h i c h u p o n post-exposure bake catalyzes the removal (deprotection) o f the T B O C group i n the exposed areas a n d generates poly(4-hydroxystyrene) ( P H O S T ) . I f a t h i n film o f P T B O C is irradiated through a mask, t h e n b a k e d a n d developed w i t h a solvent that selectively dissolves P H O S T , a latent image can b e obtained. T h u s w i t h this process, one incident p h o t o n generates acid that catalyzes m u l t i p l e reactions and results i n a " c h e m i c a l l y a m p l i f i e d " photosensitive p o l y m e r . T h e acid-catalyzed process is very efficient, a n d chain lengths o f 1 0 - 1 0 have b e e n estimated ( 9 ) . A l t h o u g h the acid-catalyzed process is efficient, it undergoes a termination process that prevents deprotection o f the P T B O C i n the unexposed areas, 2

3

T h e r e are some important aspects o f the P T B O C - s u l f o n i u m salt resist that n e e d to b e characterized. F o r example, it is w e l l k n o w n that the q u a n t u m y i e l d f o r t r i p h e n y l s u l f o n i u m salt photodecomposition decreases drastically i n viscous solvents a n d p o l y m e r films (10), yet the photoresist exhibits remarkable sensitivity at l o w doses. A l s o , the p o l y m e r has a consider able absorbance i n the deep U V that limits the amount o f incident light absorbed b y the o n i u m salt photoinitiator. T h e ideal photoresist system is where the p o l y m e r has n o absorbance at the wavelength emitted b y the exposure t o o l a n d w h e r e the photoinitiator absorbs all o f the incident light throughout the d e p t h o f the film (11). A m a x i m u m absorbance o f about 0.4 at the r e q u i r e d wavelength is thought to p e r m i t o p t i m u m light transmittance to


21.

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Poly (4-oxystyrenés) with Tnphenyhulfonium

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559

the initiator at the b o t t o m o f the film (12). D e s p i t e the fact that the P T B O C - s u l f o n i u m salt resist is nonideal (i.e., the p o l y m e r absorbs at the wavelength o f the incident light), it performs remarkably w e l l as a deep U V photoresist w i t h photospeeds o f a r o u n d 1 m j / c m at 248 n m , w h i c h is about 2 orders o f magnitude faster than o p t i m i z e d nonchemically a m p l i f i e d resists.


2

W e report here the fluorescence spectroscopy a n d photochemistry o f poly (4-oxystyrene) derivatives w i t h t r i p h e n y l s u l f o n i u m salt i n solution a n d as films. T h e goal o f this study is to understand h o w the presence o f these polymers affects the photoinitiation o f acid and h o w the sulfonium salt functions i n these polymers, a n d also to determine i f there is a relationship between the spectroscopic properties, the photochemistry, a n d the perfor mance o f the P T B O C resist.

Experimental Details Fluorescence Spectroscopy. I n a typical experiment, the samples were p r e p a r e d b y dissolving the appropriate amounts o f p o l y m e r a n d quencher i n propylene glycol m e t h y l ether acetate ( P G M Ε A ) f o l l o w e d b y filtration o f 0.45 μ ι η . T h e p o l y m e r films were f o r m e d b y spin-casting o n silicon wafers or quartz plates at 3500 r p m f o l l o w e d by a soft bake at 90 °C for 60 s. T h u s , a 2 0 - w t % solution o f p o l y m e r - o n i u m salt i n P G M E A gave films w i t h thick nesses o f 0 . 8 - 1 . 0 μηι. L u m i n e s c e n c e spectra were obtain o n a spectrofluorometer ( S h i m a d z u R F - 5 4 0 or P e r k i n - E l m e r L S 5 - B ) using an interrogation wavelength o f 290 n m , a source monochromator w i t h a slit w i d t h o f 2 m m , a n d a 2 9 0 - n m narrow bandpass filter. T h e emission was m o n i t o r e d at the wavelength of interest through a 300-nm high-pass filter a n d a monochromator w i t h a 2 - m m slit w i d t h . T h e silicon wafers (or quartz plates) coated w i t h the p o l y m e r t h i n film were p l a c e d i n a specially m o d i f i e d solid sample h o l d e r at approximately a 45° angle to the source and a shutter between the source a n d the sample was e m p l o y e d to reduce the amount o f exposure to actinic radiation. L u m i n e s cence intensity data were obtained f r o m at least five different sites o n the wafers. I n data analysis, a density o f 1.16 g / m L for the p o l y m e r i c films was used. This density was assumed to r e m a i n constant w i t h the addition o f o n i u m salt quencher.

Photochemistry. T r i p h e n y l s u l f o n i u m s a l t - p o l y m e r films w e r e pre p a r e d b y dissolving the appropriate amounts o f p o l y m e r a n d salt i n a suitable solvent ( P G M E A or dichloromethane), allowing the solvent to evaporate for 16 h , and finally drying overnight i n a v a c u u m oven at 70 °C. Pieces o f the films (0.1 g) were placed i n quartz or Pyrex tubes a n d were irradiated for 30 m i n i n a Rayonet reactor e q u i p p e d w i t h four bulbs, R P R 2 5 3 7 A or R P R 3 0 0 0 A ,


560

STRUCTUREP -ROPERTY RELATO INS N I POLYMERS


acetonitrile (4 m L ) , sonicated, q u e n c h e d w i t h b r i n e (10 m L ) , a n d extracted w i t h hexanes (1 m L ) containing n-tetradecane internal standard. T h e photoproducts were i d e n t i f i e d b y comparison o f retention times w i t h k n o w n concentrations o f authentic samples i n acetonitrile, w h i c h w e r e subjected to the same w o r k u p procedure to compensate f o r extraction efficiencies a n d response ratios (see reference 6).

Resist Performance. Thickness changes were r e c o r d e d b y exposing a wafer to a l o w radiation dose (1 m j / c m at λ = 248 n m ) , b a k i n g at 90 °C for 60 s, a n d c o m p a r i n g the thickness r e m a i n i n g to a similarly p r e p a r e d film that was given a h i g h radiation dose (100 m j / c m ) a n d processed identically. T h e thickness o f the p o l y m e r films was measured before a n d after exposure by scratching the wafer w i t h a razor blade a n d measuring the profile across the scratch at several different points o n the wafer. 2

2

Results Fluorescence Spectroscopy. T h e three poly(4-oxystyrenes), P T B O C , P H O S T , a n d poly(4-methoxystyrene) ( P M O S T ) , a l l exhibited a n emis sion i n the 3 0 0 - 3 5 0 - n m region w i t h a b r o a d tail toward the r e d spectrum u p o n excitation at λ = 290 n m ( F i g u r e 1). P T B O C a n d P H O S T h a d maxima at λ = 304 a n d 307 n m , respectively, i n acetonitrile, whereas the m a x i m u m for P M O S T was red-shifted to 328 n m . T o determine i f the r e d tail f r o m the

300

350

400

450

Wavelength (nm) Figure 1. Comparison of PTBOC, PHOST, and PMOST fluorescence in acetonitrile solutions.


21.

HACKER & WELSH

Poly (4-oxystyrènes)

with Triphenylsulfonium

Salts

emission of each of these compounds was due to the polymer, the

561

fluores

cence spectrum o f T B O C p o l y m e r was measured as a t h i n film (1 μ π ι ) a n d i n acetonitrile solution, and was subsequently c o m p a r e d w i t h the m o d e l c o m p o u n d for the repeating unit, p - c r e s o l - B O C ( F i g u r e 2). T h e gives a

fluorescence

fluoresces

at λ = 304 n m i n solution a n d at λ = 308


addition, the

p-cresol-BOC

emission at λ = 294 n m , whereas the T B O C p o l y m e r

fluorescence

n m as a

film.

In

spectra of the T B O C polymer, as a film a n d i n

solution, b o t h exhibit the b r o a d red-shifted tail, w h i c h is not detected f r o m p - c r e s o l - B O C . T h e b r o a d red-shifted emission f r o m each of the

poly(4-

oxystyrenes) is probably due to partial o r d e r i n g of the p o l y m e r . W h e n triphenylsulfonium hexafluoroantimonate

( T P S S b ) was a d d e d to

the acetonitrile solutions o f each of the polymers, the wavelengths of the fluorescence

maxima r e m a i n e d constant, but there was a m a r k e d decrease i n

luminescence intensity o f the emissions. F i g u r e 3 shows the effect o f adding

Figure 2. Comparison of fluorescence of PTBOC in acetonitrile solution and as a film with 4-cresol-BOC fluorescence in acetonitrile solution.



562

S T R U C T U R E - P R O P E R T Y R E L A T I O N S IN P O L Y M E R S

2.0 Χ 1 0 ~ - 4 . 0 Χ 1 9 " 3

M T P S S b to the

2

fluorescence

o f P M O S T i n acetoni

trile solution using an excitation wavelength o f λ = 290 n m . T h e emission is almost completely q u e n c h e d at the higher concentrations

of TPSSb.

For

S t e r n - V o l m e r (dynamic) q u e n c h i n g o f fluorescence,

Φ0/Φ = 1 +

where Φ

0

k i[Q]

a n d Φ are the q u a n t u m yields o f

presence o f quencher, k

q

fluorescence

i n the absence a n d

is the b i m o l e e u l a r rate constant f o r q u e n c h i n g , τ is

q

the lifetime o f the e m i t t i n g species, a n d [Q] is the quencher concentration. The

fluorescence

intensities i n the absence a n d presence o f quencher, i

I, are directly p r o p o r t i o n a l to Φ

0

versus T P S S b concentration s h o u l d be linear w i t h a gradient o f k r q

intercept o f 1 (13,

F i g u r e 4 shows the plots o f I /I

14).

0

a n d Φ , respectively, a n d thus a plot o f

and I /I 0

a n d an

for the three

0

4-oxystyrene polymers versus T P S S b concentration. A l l three plots are linear w i t h intercepts at 1. Unfortunately, lifetimes for the 4-oxystyrene polymers are u n k n o w n ; however, anisole ( T = 8.3 ns) a n d p h e n o l ( τ = 2 . 1 - 7 . 4 ns) can be considered as models for P M O S T a n d P H O S T , respectively (15). b i m o l e e u l a r constant for diffusion i n acetonitrile is 2 Χ 1 0 for anisole k r q

= 166 M "

1

Q

Q

a n d 80

M "

1

M s

a n d for p h e n o l fc T = 4 2 - 1 4 8 M "

mentally obtained values of fc T are 166 PTBOC,

1 0

M

_

1

for P H O S T . T h u s the

1

-

1

The

, a n d so

. T h e experi

for P M O S T , 90 M " Stern-Volmer

1

for

quenching

constants for P M O S T a n d P H O S T w i t h T P S S b are close to the d i f f u s i o n c o n t r o l l e d rates. It is likely that P T B O C exhibits similar behavior; that is,



21.

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Poly (4-oxystyrenes) with Triphenylsulfonium

0.01

0.02

0.03

Salts

563

0.04

TPS.Sb Concentration (M) Figure 4. Plot of I /I versus molar concentration for emission of poly (4-oxysty rene) derivatives in acetonitrile solution in the presence of triphenylsulfonium hexafluoroantimonate. 0

T h e q u e n c h i n g o f the fluorescence

o f the 4-oxystyrene

polymers b y

T P S S b as films was also measured. U s i n g an excitation wavelength o f 295 n m and m o n i t o r i n g the emission at 320 n m , the intensity is decreased b y greater than a factor o f 3 w h e n the concentration o f T P S is increased f r o m 0 to 0.12 m , w h i c h corresponds to a loading o f approximately 7 % b y weight o f T P S i n T B O C . A t this loading level, the fraction o f excited states q u e n c h e d is approximately 6 8 % .

I n each case the q u e n c h i n g o f the emission o f the

p o l y m e r films d i d not result i n linear S t e r n - V o l m e r plots. P e r r i n developed a m o d e l for solid-state (static) quenching:

b(i /0 = 0

where I

0

a n d ί are the

fluorescence

νΝ[p]

intensities i n the absence a n d presence

o f quencher, V is the v o l u m e o f the active sphere, Ν is Avogadro's n u m b e r , data presented i n F i g u r e 5 show that the observed luminescence q u e n c h i n g w e l l approximates the P e r r i n m o d e l for static quenching. T h e radius, R, the active q u e n c h i n g sphere is calculated f r o m

1/3 4TT


of

564

STRUCTURE-PROPERTY

R E L A T I O N S IN P O L Y M E R S


1.0

M

—

0.5

0

0

0.02

0.04

0.06

0.08

0.10 0.12

TPS.Sb Concentration (M) Figure 5. Plot of In I / I versus molar concentration for poly[4-[(tert-butoxycarbonyDoxy]styrene] emission at λ = 320 nm in the presence of triphenylsul fonium hexafluoroantimonate (excitation at λ — 295 nm). 0

T h e radius o f the active sphere, calculated f r o m the data i n F i g u r e 5, f o r P T B O C is f o u n d to b e approximately 16 Â. Similar treatment o f the emission q u e n c h i n g b y T P S o f the fluorescence f r o m poly(4-hydroxystyrene) ( p H O S T ) film results i n a radius o f 19 Â. These values seem to b e higher than t h e expected 8 - 1 0 - Â radius for p u r e static q u e n c h i n g (14) a n d may be the result of an additional, small dynamic component.

Photochemistry. Irradiation ( λ = 254 n m ) of P T B O C films contain i n g 0.1, 1.0, a n d 10.0% T P S S b gave 2-, 3-, a n d 4-phenylthiobiphenyls a n d diphenylsulfide (Scheme I). T h e ratios o f the sum o f the three phenylthiob i p h e n y l isomers to diphenylsulfide [the cage to escape ratio ( C / E ) ] were 2.04:1 a n d 1.70:1 for 1 a n d 1 0 % salt loadings, respectively (Table I, entries 1 a n d 2). These C / E values seem remarkably l o w for a viscous m e d i u m . F o r example, irradiation o f poly(methylmethacrylate) ( P M M A ) films that contain similar loadings o f T P S S b u n d e r identical conditions gives C / E values o f 2.81-3.51:1 (Table I, entries 5 a n d 6). T o determine i f sensitization of T P S S b was the reason for the relative increase i n diphenylsulfide formation, the films were irradiated at 300 n m where the p o l y m e r absorbs, but w h e r e the salt has only a very weak absorbance (e < 1). U n d e r these conditions, at 1 a n d 1 0 % loading, t h e cage to escape ratio decreases to 0.78:1 and 0.86:1, respectively


21.

HACKER

&

WELSH

Poly (4-oxysty renés) with Triphenylsulfonium Salts

565


CAGE

1

HX

I

I

ESCAPE

Scheme I. Photoproducts from irradiation of triphenylsulfonium salts.

Table I. Photoproduct Distribution from Irradiation of Triphenylsulfonium Salts (Concentration Χ 1 0 M) 5

Run No.

TPS (%)

1. 2. 3. 4. 5. 6.

1.0 10.0 1.0 10.0 1.0 10.0

TBOC TBOC TBOC TBOC PMMA PMMA

7. 8. 9. 10.

1.0 10.0 1.0 10.0

TBOC TBOC TBOC TBOC 0.01 M + 0.1 0.01 M + 0.1

11. 12.

λ (nm)

WC Polymer

Ph S 2

Ph-PhSPh

C/E

Film 0.95 3.96 0.70 5.95 1.92 6.57

1.95 6.74 0.55 5.07 6.75 18.46

2.04 1.70 0.78 0.86 3.51 2.81

254 254 300 300

13.24 110.9 1.28 8.96

10.42 82.8

0.79 0.75

254

221.4

75.0

254 254 300 300 254 254

C H o C N Solution

CH CN M anisole CH CN M anisole

trace

—

0.37

0.04

3

0.34

3

300

14.43

0.79


0.06

566

S T R U C T U R E - P R O P E R T Y R E L A T I O N S IN P O L Y M E R S


(Table I, entries 3 a n d 4). H o w e v e r , w h e n the P T B O C - T P S S b films are dissolved i n acetonitrile a n d irradiated u n d e r identical conditions the C / E values are 0.71-0.79:1 at 254 n m a n d 0.04 at 300 n m (Table I, entries 7 - 1 0 ) . T h e n o r m a l C / E values for direct photolysis under these conditions i n acetonitrile are 1.1:1 at λ = 254 n m a n d 0.35:1 at λ = 300 n m .

Lithographic Performance. A n u m b e r o f P T B O C - T P S S b films w e r e evaluated f o r photosensitivity as a f u n c t i o n o f wavelength o f the incident light a n d the concentration o f the T P S S b photoinitiator. It was f o u n d that P T B O C - T P S S b is photosensitive over a larger dynamic range ( 2 0 0 - 3 0 0 n m ) than the absorption range o f the photoinitiator ( 2 1 0 - 2 8 0 n m ) . H o w e v e r , the photosensitivity range does seem to correlate w i t h the c o m b i n e d absorptions o f b o t h the p o l y m e r a n d the photoinitiator. T h u s the p o l y m e r participates i n the photoacid generation process. T h e photosensitivity o f the photoresist was evaluated at various concentrations o f T P S S b b y measuring the relative loss o f film thickness after a l o w dose irradiation, versus a h i g h dose o n a similarly p r e p a r e d film, f o l l o w e d b y a postexpose bake. T h i s technique eliminates the errors f r o m the changes i n dissolution characteristics o f the resist that are i n t r o d u c e d b y increasing the photoinitiator concentration. T h e loss i n film thickness is proportional to the relative amount o f deprotection a n d has a good inverse correlation w i t h lithographic photospeed. Discussion T h e c o m b i n e d spectroscopic, photoproduct, a n d lithographic performance studies indicate that the P T B O C backbone participates i n the p h o t o d e c o m p o sition o f the o n i u m salt photoinitiator. T h e p h e n y l t h i o b i p h e n y l photoproducts are f o r m e d b y an in-cage f r a g m e n t a t i o n - r e c o m b i n a t i o n reaction that also results i n p r o d u c t i o n o f Brônsted acid. H o w e v e r , the diphenylsulfide, b e n zene, a n d acetanilide are f o r m e d b y reaction o f the initially f o r m e d fragments w i t h the solvent a n d m a y b e considered as cage-escape products. T h e cage-escape reactions also generate Brônsted acid. T h e remarkable sensitiv ity o f the P T B O C - T P S S b mixtures at X = 300 n m , a n d the larger than expected amounts o f cage-escape reaction products i n b o t h solution a n d as films at b o t h photolysis wavelengths, λ = 254 a n d 300 n m , strongly indicates photosensitized decomposition o f the T P S S b initiator. T h e r e are two types o f sensitization that result i n photodecomposition o f t r i p h e n y l s u l f o n i u m salts: triplet energy transfer a n d photoinitiated electron transfer. T r i p l e t sensitization occurs i n the presence o f ketone sensitizers w i t h triplet energies > 74 k c a l / m o l to give 1 0 0 % cage-escape product ( 1 7 ) . F o r example, photolysis o f t r i p h e n y l s u l f o n i u m triflate i n acetonitrile solutions containing acetone, indanone, acetophenone, or xanthone, gives d i p h e n y l s u l fide, benzene, a n d acid. T h e absence o f the in-cage phenylthiobiphenyls


21.

Poly (4-oxysty renés) with Triphenylsulfonium

HACKER & WELSH

Salts

567


indicates that the initially f o r m e d intermediates f r o m the excited state o f the o n i u m salt react w i t h the solvent rather than recombine. T h e absence o f the cage-escape product, acetanilide, suggests that p h e n y l cation is not an intermediate, a n d the detection o f benzene suggests that p h e n y l radical is an intermediate. T h u s the excited state o f the s u l f o n i u m cleaves by homolysis to give a radical pair o f intermediates that do not recombine. T h e triplet diphenylsulfinyl radical c a t i o n - p h e n y l radical pair fit the p r e c e d i n g criteria: sensitizer —> [sensitizer]* -> [sensitizer]

—> [sensitizer]

1

[sensitizer] + P h S X ~ - » sensitizer + [ P h S 3

[Ph S X " ] 3

+

3

+

3

-> P h S

Ph X "

+

2

3

3

X~]

+

-> P h S + P h H + H 2

3

3

X"

+

A l t h o u g h the 4-oxystyrene polymers are not ketones, they can all be consid e r e d as anisole derivatives. A n i s o l e has a triplet energy o f 80.8 k c a l / m o l and so triplet energy transfer is feasible (15). E l e c t r o n transfer occurs f r o m the singlet excited state o f the sensitizer, triphenylsulfur radical-sensitizer radical cation i n the solvent cage (18). Subsequent steps p r o d u c e a c i d b y escape reactions a n d by phenylation o f the sensitizer radical cation:

A r H -> [ArH]* - » [ A r H ] [ArH]

1

+ Ph S X"-> A r H 3

+

ArH Ph S X" +

ArH

+

3

Ph P h SX 2

1

1

-> A r H

+Ph SX"

+

+

1

1

3

PhTh

2

SX"

1

-> A r P h + P h S + P h H + H 2

+

X

If the photoinitiated electron transfer reaction occurs, the conditions o f the R e h m - W e l l e r equation must be satisfied ( J 9 , 20). If anisole, w h i c h has a fluorescence q u a n t u m y i e l d o f 0.3, is used as a m o d e l for the 4-oxystyrene polymers, the requirement for electron transfer to occur is an exothermic reaction: -[£*

AG=

+E

r e d

]

w h e r e £ * is the excited state oxidation potential o f the d o n o r ( P T B O C ) a n d £ is the reduction potential for the acceptor ( T P S S b ) . T h e excited state oxidation potential is estimated f r o m the energy of the excited state and ground-state oxidation potential o f the donor; that is, E* = S — E for the singlet excited state and £ * = T — E for the triplet excited state, where x

r e d

x

Y

l

ox

ox


568

STRUCTURE-PROPERTY

R E L A T I O N S IN P O L Y M E R S

S a n d T aie the respective singlet a n d triplet excited state energies. F o r TPSSb, F = —1.2 V versus saturated c a l o m e l electrode ( S C E ) (21); for anisole, the m o d e l m o n o m e r for P T B O C , E = 1.35 V vs. S C E ( 2 2 ) a n d S = 4.47 e V a n d Γ, = 3.50 e V (15). Substituting these values i n the p r e c e d i n g equation yields A G = —1.92 e V ( — 44 k c a l / m o l ) for S a n d A G = —0.95 e V ( —22 k c a l / m o l ) for T T h u s electron transfer is energeti cally favorable f r o m b o t h singlet a n d triplet excited states. x

x

r e d

o x

x

x


v

F r o m the foregoing arguments it appears that sensitization can occur b y three processes: triplet energy transfer, electron transfer f r o m the singlet excited state, a n d electron transfer f r o m the triplet excited state o f the 4-oxystyrene polymers are all energetically favorable. T h e fluorescence spec troscopic studies s h o u l d differentiate between the three possible modes o f sensitization. T h e detection o f an emission o f the 4-oxystyrene polymers f r o m films a n d solutions, the observation o f emission quenching, a n d its insensitivity to oxygen (a triplet excited state quencher), implicate the activity o f the singlet excited state o f the polymers. A l t h o u g h it c o u l d be argued that the emission f r o m the p o l y m e r films s h o u l d be insensitive to oxygen because o f l i m i t e d diffusion, the emission spectra o f the films are measured close to the surface o f the film where oxygen diffusion can occur. A l s o , for the triplet excited state to be responsible for the observed emission quenching, there w o u l d have to be an e q u i l i b r i u m between the singlet a n d triplet excited states o f the 4-oxystyrene polymers. F o r the m o d e l c o m p o u n d , anisole, the difference between S a n d T (22 k c a l / m o l ) is too large for this to occur a n d renders participation o f the triplet excited state an u n l i k e l y mechanism for acid generation. T h e m o d e o f the emission q u e n c h i n g is also an interesting aspect o f the sensitization process. Solutions o f the polymers a n d T P S S b apparently behave as m o n o m e l i c species a n d the fluorescence q u e n c h i n g gives n o r m a l S t e r n - V o l m e r plots, w h i c h implies a dynamic q u e n c h i n g mechanism that occurs at close to diffusion c o n t r o l l e d rates. H o w e v e r , the emission q u e n c h i n g o f the films of the 4-oxystyrene polymers appears to occur b y a static q u e n c h i n g mechanism. A l t h o u g h dynamic q u e n c h i n g c o u l d occur b y an exciton migration mechanism, it appears that q u e n c h i n g data better fit the P e r r i n formulation for static quenching. T h e radii o f the active sphere for the films are 1 5 - 2 0 Â, w h i c h seems higher than the n o r m a l values o f 8 - 1 0 Â that are expected for pure static quenching. T h u s there may be an additional, 1 - 3 units o f the p o l y m e r . T h i s hypothesis is supported b y the observation o f the red-shifted emission tails i n the polymers, w h i c h indicate a partial o r d e r i n g o f the m o n o m e r units i n the polymers that may p e r m i t short-range energy migration. x

x

T h e mechanism for the photolysis o f P T B O C - T P S S b mixtures is shown i n Scheme II. T h e incident light is absorbed b y b o t h the p o l y m e r a n d the


21.

Poly (4-oxystyrènes)

HACKER & WELSH

p + Ph S X~ 3

+

[Ph S X~]* 3

+

[P]* + Ph S X" 3


P

+

+ Ph S' + X"

+

with Triphenylsulfonium Salts - American Chemical Society

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