Organic Phototransformations in Nonhomogeneous Media - American

1 Surface Photochemistry: Temperature Effects on the Emission of Aromatic Hydrocarbons Adsorbed on Silica Gel P. D E MAYO, L. V. NATARAJAN, and W. R. WARE

Organic Phototransformations in Nonhomogeneous Media Downloaded from pubs.acs.org by 179.61.201.83 on 11/12/18. For personal use only.

Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7

The emission of aromatic hydrocarbons, in particular that of pyrene, adsorbed on silica gel serves as a surface probe. The changes in silica gel structure imposed by heat treatment in vacuum (700°C) are revealed in poorly resolved spectra, multiple exponential fluorescence decay, ground state association and shortened lifetimes for pyrene emission. Coadsorbed water or alcohols, after heat treatment, render the surface more homogeneous, as indicated by well resolved spectra, an approach to single exponential decay, longer lifetimes, diminished ground state association and even formation of dynamic excimers. For a decanol-covered surface, cooling results in the disappearance of dynamic excimers, and a single exponential decay of pyrene with a life time of 600 nsec. Quenching studies have yielded an activation energy for diffusion on the dry silica gel surface of around 4 Kcal/mol. The p h o t o c h e m i s t r y a n d photophysics of m o l e c u l e s a d s o r b e d o n solid s u b s t r a t e s have hitherto r e c e i v e d little a t t e n t i o n a n d , c o m p a r e d with their g a s phase a n d solution c o u n t e r p a r t s , a r e very poorly u n d e r stood. Many interesting questions await experimental a n d theoretical investigation. For example, what is t h e effect of a n a s y m m e t r i c a l interaction w h e r e only a portion of a m o l e c u l e interacts with a surface g r o u p a n d t h e r e m a i n d e r is essentially in t h e v a p o r p h a s e ? What is the effect of the a b s e n c e of rapidly time averaged interactions such as one has in solution? H o w fast d o m o l e c u l e s m o v e o n a s u r f a c e a n d w h a t is t h e activation e n e r g y f o r diffusion? How does the surface i n f l u e n c e the c o u r s e of a p h o t o c h e m i c a l t r a n s f o r m a t i o n , w h i c h m a y be either inter- or i n t r a m o l e c u l a r " or o c c u r via free r a d i c a l s ? ' This p a r t i a l list of q u e s t i o n s is g i v e n to i l l u s t r a t e t h e c o n s i d e r a b l e p o t e n t i a l in s u r f a c e p h o t o p h y s i c s a n d p h o t o c h e m i s t r y f o r t h e exhibition of u n i q u e phenomena, for interesting modifications of t h e behavior of excited states a s c o m p a r e d with behavior in o t h e r m e d i a , a n d for studies of molecular dynamics on the surface. 1

2

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6

T h e excited m o l e c u l e is a u n i q u e s u r f a c e p r o b e . The decay time provides a clock which c a n be used to study such d y n a m i c s . Since both singlets a n d triplets a r e potential p r o b e s , t h e time scale extends from seconds to picoseconds. In a d d i t i o n , t h e n a t u r e o f a n y p h o t o 0097-6156/85/ 0278-0001 $06.00/0 © 1985 American Chemical Society

ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

2

c h e m i c a l p r o d u c t s may give information about translational radical m o tion. T h e o c c u r r e n c e of p h o t o s e n s i t i z a t i o n a n d q u e n c h i n g m a y a l s o allow o n e to examine questions c o n c e r n i n g mobility. T h i s c o n t r i b u t i o n will d e a l m a i n l y with r e c e n t p h o t o p h y s i c a l s t u d i e s of t h e b e h a v i o r o f a r o m a t i c h y d r o c a r b o n s o n s i l i c a g e l a n d modified silica gel surfaces. T h e s u r f a c e of s i l i c a gel c o n s i s t s of s i l o x a n e b r i d g e s between tetracovalent silicon atoms. T h e p r e s e n c e of g e m i n a l , vicinal and isolated silanol groups, s o m e t i m e s together with tightly bound water, render the surface inhomogeneous and provide binding sites, via hydrogen b o n d i n g , for the 7r-electron systems or n o n - b o n d i n g electron p a i r s of p o l a r g r o u p s of a d s o r b e d m o l e c u l e s ( F i g . 1 , 2 ) . The n u m b e r of s i l a n o l g r o u p s o n the s i l i c a gel s u r f a c e d e p e n d s o n the t e m p e r a t u r e pretreatment. W h e n s i l i c a g e l is h e a t e d i n v a c u u m , a d s o r b e d w a t e r i s r e m o v e d first a n d at t e m p e r a t u r e s above 2 5 0 ° C t h e c o n c e n t r a t i o n of p u r e l y p h y s i s o r b e d w a t e r is n e g l i g i b l e . A s t h e t e m p e r a t u r e is r a i s e d a b o v e this p o i n t , t h e n u m b e r of s i l a n o l s is r e d u c e d f r o m 6 / 1 0 0 A to approximately 2 / Ί 0 0 Â at 5 0 0 ° C a n d Ί / Ί 0 0 Â at 8 0 0 ° C . In t h e temperature range of 500 to 700 ° C the surface concentration of g e m i n a l hydroxyl g r o u p s r e m a i n s roughly c o n s t a n t but the ability to regenerate silanols by exposure to water vapor decreases rather suddenly. 2

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T h u s , h e a t i n g t h e s i l i c a g e l to t e m p e r a t u r e s a b o v e 5 0 0 ° C f o l l o w e d by rehydration changes the proportion and distribution of isolated geminal silanol sites. At higher temperatures only isolated silanol groups remain. P r i o r a d s o r p t i o n of w a t e r a n d a l c o h o l s o n t o t h e d e h y d r a t e d s u r f a c e c a n d r a s t i c a l l y c h a n g e t h e i n t e r a c t i o n of t h a t s u r f a c e with t h e a d s o r b e d s p e c i e s of i n t e r e s t . These additives generally render the surface more homogeneous and can be used to influence d y n a m i c behavior and static interactions. E x a m p l e s t a k e n f r o m r e c e n t w o r k in o u r l a b o r a t o r i e s will n o w b e given to illustrate these p h e n o m e n a . In a l l c a s e s , t h e s a m p l e s w e r e evacuated. D e t a i l s o f s a m p l e p r e p a r a t i o n a r e t o b e f o u n d in t h e references cited below. Evidence for Surface Inhomogeneity T h a t all b i n d i n g sites a r e not e q u i v a l e n t with r e s p e c t to their i n f l u e n c e o n t h e g r o u n d a n d excited state b e h a v i o r of a n adsorbed molecule is i n d i c a t e d by s e v e r a l d i f f e r e n t t y p e s o f e v i d e n c e : (a) s p e c t r a l b a n d w i d t h , ( b ) c o m p l e x i t y of t h e f l u o r e s c e n c e d e c a y , ( c ) t h e i n f l u e n c e of c o a d s o r b a t e s ( d ) t h e i n f l u e n c e of s u r f a c e t r e a t m e n t and ( e ) g r o u n d state association. T h e s e will be d i s c u s s e d in t u r n . Pyrene absorption and emission spectra exhibit much vibronic structure. If p y r e n e is a d s o r b e d o n s i l i c a g e l p r e t r e a t e d s o a s t o have only isolated silanol groups, the spectra are considerably b r o a d e n e d with loss of s t r u c t u r e as c o m p a r e d to t h e s p e c t r a o b t a i n e d w h e n t h e s u r f a c e is e i t h e r " w e t " o r c o n t a i n s p h y s i - a n d / o r c h e m i s o r b e d methanol. T h i s is i l l u s t r a t e d i n F i g . 3 . T h u s , it w o u l d s e e m t h a t t h e i s o l a t e d s i l a n o l s p r o v i d e a v a r i e t y of s i t e s w i t h d i f f e r e n t i n t e r a c t i o n s , a n e f f e c t w h i c h is m o d i f i e d by c o a d s o r b e d a l c o h o l s o r w a t e r . M o r e d r a m a t i c e v i d e n c e f o r a n i n h o m o g e n e o u s s u r f a c e is p r o v i d e d by t h e c o m m o n o b s e r v a t i o n of m u l t i e x p o n e n t i a l d e c a y . It i s , in f a c t , very unusual to observe single exponential decay when aromatic hydrocarbons are absorbed on silica gel, quite independently of 9

1.

D E MAYO E T A L .

Aromatic

Hydrocarbons

Adsorbed

OH

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on Silica

Gel

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-Si

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X

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F i g u r e 1.

T y p e s of silanol f u n c t i o n s o n t h e s u r f a c e of s i l i c a g e l . (a) lone: (b) vicinal; (c) geminal and (d) bonded water. ( R e p r o d u c e d b y p e r m i s s i o n f r o m Pure and Applied Chemistry. 54. 1623. 1982.)

F i g u r e 2.

B o n d i n g of s i l a n o l f u n c t i o n s with a r o m a t i c h y d r o carbons. ( R e p r o d u c e d b y p e r m i s s i o n f r o m Pure and Applied Chemistry. 54. Ί623. 1982.)

3


4

Figure 3.

Effect of m e t h a n o l a d d i t i o n o n t h e e m i s s i o n s p e c t r a of pyrene adsorbed on silica gel. The silica gel was h e a t e d in v a c u u m to 7 0 0 C f o r 4 h o u r s . 1) Ο MeOH; 2) 3.375 χ 1 0 * ; 3) 7.5 χ 10~ : 4) 1.25 χ 1 0 ~ ; 5) 3.75 χ 10~ ; 6) 5 χ 10~ ; 7) 7.5 χ 1 0 " . M e t h a n o l c o n c e n t r a t i o n s a r e in m o l e / g S i 0 . The excitation wavelength: 332 n m . The pyrene coverage is 0 . 0 5 m g / g S i 0 ( R e p r o d u c e d by p e r m i s s i o n f r o m Canadian Journal of Chemistry. 62, 1279, 1984.) e

5

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4

4

4

2

2

1.

DE MAYO ET AL.

Aromatic

Hydrocarbons

Adsorbed

on Silica

5

Gel

w h e t h e r o r not t h e y a r e e x c i m e r - f o r m i n g s p e c i e s in s o l u t i o n . also quite c o m m o n to a c h i e v e r e a s o n a b l e fits ( a s m e a s u r e d r e d u c e d x ) with a four p a r a m e t e r m o d e l :

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a l t h o u g h t h i s is i n m a n y c a s e s o n l y a u s e f u l a p p r o x i m a t i o n to a t i m e e v o l u t i o n t h a t is m o r e c o m p l e x , a n d t h e n u m b e r o f p a r a m e t e r s is n o t a d i r e c t i n d i c a t i o n of the n u m b e r of e n v i r o n m e n t a l l y d i f f e r e n t p h o t o a c t i v e species. If t h e s u r f a c e is m o d i f i e d by c o a d s o r b e d a l c o h o l s o r w a t e r , the d e c a y t i m e s of p y r e n e a n d n a p h t h a l e n e , f o r e x a m p l e , l e n g t h e n d r a m a tically and b e c o m e essentially single e x p o n e n t i a l s . We interpret this a s a c h a n g e to a h o m o g e n e o u s s u r f a c e a l t h o u g h , a s will be d i s c u s s e d below, increased motion d u r i n g the excited state lifetime may contribute to t h e c o l l a p s e of d o u b l e e x p o n e n t i a l d e c a y to give a s i n g l e e x p o n e n t i a l decay function. T h i s p h e n o m e n o n is i l l u s t r a t e d i n F i g . 4 f o r n a p h thalene. As has been previously reported, w h e n p y r e n e is a d s o r b e d on s i l i c a g e l t h e r e is e v i d e n c e f o r g r o u n d s t a t e a s s o c i a t i o n w h i c h is n o t present in s o l u t i o n o r t h e v a p o r phase, " but w h i c h has been described as being present when pyrene is d i s s o l v e d in a plastic medium. T h i s is a l s o a m a n i f e s t a t i o n o f s u r f a c e i n h o m o g e n e i t y s o m e sites e n h a n c e the t e n d e n c y to f o r m a g r o u n d state bimolecular c o m p l e x , w h e r e a s other sites contain isolated pyrene molecules. The i n t e r a c t i o n d i f f e r e n c e s a r e sufficient to yield s i g n i f i c a n t s p e c t r a l shifts in absorption and the ground state complex emits with the characteristic pyrene excimer fluorescence. F i g . 5 s h o w s a t y p i c a l s e t of s p e c t r a illustrating this association and Fig. 6 presents evidence that this o b s e r v a t i o n is n o t d u e t o m i c r o c r y s t a l f o r m a t i o n . 9

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1

1 3

4

Evidence

for

Motion

on

the

Surface

Photophysical and photochemical studies have provided conclusive evidence for extensive motion on a m i c r o s e c o n d or even submicros e c o n d time scale for excited a n d / o r g r o u n d state molecules adsorbed on silica gel surfaces. T h r e e l i n e s of a r g u m e n t h a v e b e e n p r e s e n t e d : ( a ) A single photosensitizer molecule can sensitize many more m o l e c u l e s t h a n a r e initialy nearest n e i g h b o r s , ( b ) Both d y n a m i c and static quenching of f l u o r e s c e n c e are observed. (c) Surface modification strongly influences the dynamics on the surface and under the a p p r o priate conditions dynamic excimer f o r m a t i o n , analogous to the behavior i n s o l u t i o n , is s e e n . The evidence f r o m photosensitization derives f r o m the Rose Bengal (RB) sensitized dimerization of a c e n a p h t h y l e n e . ' It is o b s e r v e d that o n e RB m o l e c u l e c a n y i e l d , by r e p e a t e d e x c i t a t i o n , d i m e r i c m o l e cules with a high c o n v e r s i o n . This requires that d i m e r s move away f r o m the RB m o l e c u l e s a n d that m o n o m e r s m o v e to o c c u p y these v a cant places. Evidence has been p r e s e n t e d that t h e RB m o l e c u l e s a r e firmly fixed to the s u r f a c e a n d thus the a c e n a p h t h y l e n e monomers and dimers must be undergoing extensive motion on the surface d u r i n g 1

1

1 5

6


Figure 4·

T h e d e c a y of n a p h t h a l e n e f l u o r e s c e n c e . Emission w a v e l e n g t h is 3 3 3 n m ( a ) s i l i c a g e l h e a t e d i n o p e n f o r 4 h o u r s at 8 0 0 ° C ; ( b ) s i l i c a gel w a s h e a t e d in v a c u u m for 4 h o u r s at 8 0 0 ° C . ( R e p r o d u c e d by p e r m i s s i o n f r o m Canadian Journal of Chemistry. 62, 1279, 1984.)

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1.

DE MAYO ET AL.

Aromatic

Hydrocarbons

Adsorbed

on Silica

Gel

W A V E L E N G T H (nm) F i g u r e 5.

Ground s t a t e a s s o c i a t i o n o f p y r e n e and p y r e n e " e x c i m e r " emission. E x c i t a t i o n and e m i s s i o n o p e c t r a o f p y r e n e on s i l i c a g e l ( ); e x c i t a t i o n ( o b s e r v e d a t 390 nmï and e m i s s i o n ( e x c i t e d w i t h 331 nm f o r a 0 = 1 s u r f a c e c o v e r a g e sample ( . · · ) ; e x c i t a t i o n (0=1, o b s e r v e d a t 480 nm) and e m i s s i o n (0=3, e x c i t e d w i t h 345 nmj. (Reproduced by p e r m i s s i o n from the J o u r n a l o f P h y s i c a l C h e m i s t r y , 8 6 , 3781, 1982.)

A Β

WAVELENGTH ( N M )

F i g u r e 6.

E x c i t a t i o n s p e c t r a of pyrene i n d i f f e r e n t s t a t e s of association. A) E x c i t a t i o n s p e c t r u m o f p y r e n e monomer ( e m i s s i o n a t 393 nm) . B) E x c i t a t i o n spectrum o f p y r e n e dimer ( e m i s s i o n a t 470 nm). C) E x c i t a t i o n s p e c t r u m o f pyrene m i c r o c r y s t a l .

1


8

t h e t i m e of i r r a d i a t i o n . T h e d i m e r i z a t i o n is a l s o o b s e r v e d u p o n d i r e c t irradiation. It i s k n o w n t h a t t h e c i s / t r a n s r a t i o i s s e n s i t i v e t o t h e r e l a t i v e i m p o r t a n c e of t h e p a r t i c i p a t i o n of t h e s i n g l e t a n d triplet in t h e dimerization. S i n c e t h e s i n g l e t , w h i c h y i e l d s t h e c i s d i m e r is p r e s u m a b l y s h o r t l i v e d ( r < 1 n s e c ) , t h e c i s y i e l d is a m e a s u r e o f n e a r e s t neighbor dimerization. The triplet yields both cis and trans dimers and it i s o b s e r v e d t h a t s u r f a c e m o d i f i c a t i o n b y t h e c o a d s o r p t i o n o f d e c a n o l strongly decreases the C / T ratio, thus suggesting enhanced triplet d i m e r i z a t i o n , a t t r i b u t a b l e to e n h a n c e d m o l e c u l a r m o b i l i t y . Here we are d e a l i n g w i t h m o t i o n d u r i n g t h e l i f e t i m e o f t h e t r i p l e t , p r e s u m e d to b e , a s is f o u n d i n s o l u t i o n , o f t h e o r d e r o f m i c r o s e c o n d s . 1

1

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Pyrene excimer formation provides pyrene and decanol are coadsorbed, one e m i s s i o n at 4 7 0 n m g r o w i n g in with the solution, ie. , -X,t I

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additional evidence. If can observe the excimer-like s a m e k i n e t i c s a s is s e e n i n 1

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T h i s is i l l u s t r a t e d i n F i g . 7 . This implies motion during the pyrene singlet lifetime, which is o f t h e order of s e v e r a l hundred nanoseconds. F u r t h e r e v i d e n c e is s e e n i n t h e q u e n c h i n g e x p e r i m e n t to be d e s c r i b e d below.

Dynamics

on

the

Surface

T h e q u e s t i o n of d y n a m i c b e h a v i o r of a d s o r b e d m o l e c u l e s will n o w be discussed in m o r e d e t a i l . In s o l u t i o n , so called Stern-Volmer b e h a v i o r is c o m m o n p l a c e , b u t it w a s s u r p r i s i n g t o o b s e r v e t h i s s a m e l i n e a r b e h a v i o r w h e n q u e n c h i n g w a s s t u d i e d in t h e a d s o r b e d state. Both q u a n t u m yield ratios and lifetime ratios have been observed to give linear plots a g a i n s t t h e s u r f a c e c o n c e n t r a t i o n of q u e n c h e r s . For example, f e r r o c e n e q u e n c h e s t h e d i m e r i z a t i o n of a c e n a p h t h y l e n e that originates from the triplet state. Fig. 8 illustrates this linear S t e r n - V o l m e r plot o b t a i n e d f r o m a study of the cis a n d trans d i m e r yield a s a f u n c t i o n of f e r r o c e n e c o n c e n t r a t i o n s . Linear Stern-Volmer plots have also b e e n o b s e r v e d f o r q u e n c h i n g of p y r e n e m o n o m e r f l u o r e s c e n c e by h a l o n a p h t h a l e n e s . Linear plots of both l p / l p and f /T have been observed, where f is t h e a v e r a g e l i f e t i m e c a l c u l a t e d f r o m eq. 1: 1

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A r g u m e n t s have b e e n a d v a n c e d e l s e w h e r e t h a t t h e q u e n c h i n g of p y r e n e o n s i l i c a g e l by 2 - h a l o n a p h t h a l e n e s is d i f f u s i o n c o n t r o l l e d . This is in c o n t r a s t t o t h e b e h a v i o r i n s o l u t i o n w h e r e t h e q u e n c h i n g is i n e f f i c i e n t . It w a s p o s t u l a t e d t h a t o n t h e s u r f a c e t h e r a t e of s e p a r a t i o n of e n c o u n t e r p a i r s is s l o w e d to t h e p o i n t w h e r e t h e r a t e of q u e n c h i n g is d e t e r m i n e d by t h e e n c o u n t e r r a t e . C e n t r a l t o t h i s a r g u m e n t is t h e 1

1

1.

D E MAYO E T AL.

F i g u r e 7·

Aromatic

Hydrocarbons

Adsorbed

on Silica

9

Gel

E m i s s i o n d e c a y p r o f i l e of p y r e n e e x c i m e r at λ = 4 7 0 n m c o a d s o r b e d with 1 - d e c a n o l ( p y r e n e 8. 1 χ 1 0 " mol/g; l-decanol, 1.8 χ 1 0 ~ m o l / g of s i l i c a g e l ) . ( R e p r o d u c e d b y p e r m i s s i o n f r o m t h e Journal of Physical Chemistry, 87, 460, 1983.) 6

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[Ferrocene] Figure 8.

S t e r n - V o l m e r plot of the q u e n c h i n g of a c e n a p h t h y l e n e c i s ( O ) a n d t r a n s ( O ) d i m e r f o r m a t i o n by ferrocene. ( R e p r o d u c e d by p e r m i s s i o n f r o m the Journal of American Chemical Society. 104, 4635, 1982).

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observation that 2 - c h l o r o , bromo and iodonapthaienes quench on the s u r f a c e with a p p r o x i m a t e l y t h e s a m e rate w h e r e a s in s o l u t i o n t h e rates are: I > B r > C I , w i t h a r a n g e of a p p r o x i m a t e l y t e n . Thus, this q u e n c h i n g r e a c t i o n is a s s u m e d to be a u s e f u l p r o b e of s u r f a c e d i f fusion. In c o n n e c t i o n w i t h m o d e l i n g t h e d y n a m i c s o n s u r f a c e s t h e r e l a t e d s o l u t i o n m o d e l is of i n t e r e s t . If o n e h a s t h e s c h e m e : hv

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k

A

6 - — >

Prod

5

+

Q +

hv

£

that

+ A e

±

x

(2)

Δ

2

- X , t E

k

- \ t * ) 0

-

1

3

e

(3)

where 2

λ

ι , 2 ±

=

k

l

[ ( ^

+

+

k

k

2

2

+

+

3

k

t

Q

1

+

k [Q]

-

3

4

k

k

4

+

-

k

5

k

5

+

k

-

k

6

6

)

2

+

4 k

3

k

4

[ Q ] ]

1

/

2

(4) If k 4 Also.

= 0. o r If In g e n e r a l

l j

'

1

+

k

q

k4

T

«

k3.

one

observes

0

single

exponential

decay.

(

5

)

i.e., one expects linear Stern-Volmer plots from intensity m e a s u r e m e n t s but not f r o m r / r plots e x c e p t in s p e c i a l c a s e s s u c h a s w h e n k3 » k4. If Q = A i n t h e s c h e m e , then one has the excimer case. It should be noted that when excimer kinetics are followed, the c o e f f i c e n t s of t h e t w o t e r m s i n I Ε a r e e q u a l a n d o p p o s i t e i n s i g n giving one just one preexponential A3, and that the difference of e x p o n e n t i a l s i m p l i e s a " g r o w i n g i n " of the e x c i m e r e m i s s i o n , provided t h a t at t = 0, [ A ^ l = 0. Otherwise, one has 0

- X , t

-X,t

1.

DE MAYO ET AL.

Aromatic

Hydrocarbons

Adsorbed

on Silica

Gel

11

and one c a n observe either sign for A5. In a d d i t i o n , it i s i m p l i c i t i n this simple analysis that lp a n d l g c a n be s e p a r a t e d experimentally. W i t h t h i s b a c k g r o u n d it i s p o s s i b l e t o d i s c u s s t h e r e s u l t s o b t a i n e d w i t h pyrene on silica gel when the pyrene m o n o m e r a n d excimer emission is s t u d i e d . At all but quite low t e m p e r a t u r e s o n e o b s e r v e s decay behavior w h i c h c a n be r e p r e s e n t e d by E q . 2 . It i s i m p o s s i b l e t o separate the effects of multiple absorption sites, i.e., surface inhomogeneity, from the effect of d y n a m i c excimer formation and feedback, the latter effect b e i n g r e s p o n s i b l e for the two c o m p o n e n t d e c a y in s o l u t i o n . H o w e v e r , if t h e s u r f a c e i s d e c a n o l c o v e r e d , then o n e o b s e r v e s e m i s s i o n in t h e e x c i m e r r e g i o n that fits E q . 3 r a t h e r well at r o o m t e m p e r a t u r e s u g g e s t i n g that the d y n a m i c s a r e d o m i n a t e d by t h e g r o w t h of t h e e x c i m e r p o p u l a t i o n , s t a r t i n g w i t h [ A £ ] = 0 at t = 0. However, w h e n o n e c o m p a r e s t h e v a l u e s of λ f o r t h e d e c a y in t h e monomer and excimer regions, the λ] and λ2 values do not c o r r e s p o n d as r e q u i r e d by the s i m p l e m o d e l . The m o n o m e r - e x c i m e r behavior has been studied as a function of t e m p e r a t u r e o n b o t h d r y s i l i c a g e l a n d s i l i c a g e l to w h i c h various coadsorbates have been a d d e d . On dry ( 7 0 0 ° ) silica g e l , where we expect only isolated silanol groups to be present, the following b e h a v i o r is o b s e r v e d : (a) The lifetime lengthens on cooling but is still a double e x p o n e n t i a l a t 10 K. (b) T h e Ι470Ί390 °f emission intensities increases by l o w e r i n g of t e m p e r a t u r e . This m a y be d u e to i n c r e a s e in the concentration of ground state dimers which are r e s p o n s i b l e for t h e e x c i m e r like 4 7 0 n m e m i s s i o n . The red shift in t h e excitation s p e c t r u m c o r r e s p o n d i n g to 4 7 0 n m e m i s s i o n i n c r e a s e s with d e c r e a s e of t e m p e r a t u r e . (c) One never observes A4/A5 to be negative. (d) T h e d a t a is r e p r o d u c i b l e a s o n e g o e s u p a n d d o w n in temperature. A typical plot of f l u o r e s c e n c e intensity for t w o t e m p e r a t u r e s is s h o w n in F i g . 9. Of g r e a t e r i n t e r e s t is t h e b e h a v i o r of p y r e n e o n d e c a n o l c o v e r e d (~ m o n o l a y e r ) s i l i c a g e l a s t h e t e m p e r a t u r e i s l o w e r e d . The following observations are relevant: (a) A4/A5 * -1, that is t h e e x c i m e r g r o w t h is o b s e r v e d a n d t h e s o l u t i o n b e h a v i o r is r o u g h l y d u p l i c a t e d . (b) The observed spectral resolution of t h e p y r e n e vibronic structures suggests a quite h o m o g e n e o u s surface. (c) At low temperature the monomer emission is quite a c c u r a t e l y d e s c r i b e d by a s i n g l e e x p o n e n t i a l a n d t h e lifetime is - 6 0 0 n s e c . T h i s l o n g m o n o m e r l i f e t i m e is c o n s i s t e n t with that o b s e r v e d in low t e m p e r a t u r e glasses. ' (d) T h e λ v a l u e s o b t a i n e d by a n a l y s i s of t h e m o n o m e r and excimer decay do not c o r r e s p o n d , as required by t h e solution model. T h e d a t a relevant to t h e s e o b s e r v a t i o n s a r e p r e s e n t e d in T a b l e s 1 a n d 2. Thus, the decanol coadsorption appears to increase significantly the d y n a m i c excimer formation but t h e residual surface inhomogeneity still prevents one from observing all t h e f e a t u r e s of solution-like behavior. r

a

t

i

o

1 6

1 7

12


rch

U-C 570

I

500

I

450

I

400

λ/nm

Figure 9.

Effect of t e m p e r a t u r e o n t h e p y r e n e " e x c i m e r " i n t e n s i t y . Excitation wavelength: 345 n m . The pyrene c o n c e n t r a t i o n is 1 m g / g d r y S i 0 2

1.

DE MAYO ET AL.

Table I.

Aromatic

293

R

Adsorbed on Silica

Gel

A

J L

0.05

9

as a Function of Temperature

τ

χ

228

A

r

2

0.135

REMARKS

2

66

λ λ

270

0.14

263

0.09

90

250

0.13

375

0.06

152

225

0.10

575

0.04

327

200

—

540

—

-

150

586

— — — — —

-

150

— — — — —

293

0.09

80 10 80

13

Pyrene Monomer Decay Time on Decanol-Covered Si0

T

Hydrocarbons

623 620 613 588 237

-

-

0.138

Note: Pyrene coverage i s 2.5 mg/g

60

^0^-

_3

Decanol coverage i s 1 χ 10

mol/g SiO,

Ε

χ

Β 1 ί

«

331

-

393

f

nm

Single expo nential


14

Table I I . Pyrene Excimer-like Emission on Decanol-Covered Si0

Τ

293

A

l

0.43

T

2

l

104

as a Function of Temperature

A

2

-0.44

T

2

26

V

A

-1.02

270

0.45

166

-0.39

37

-0.87

250

0.26

283

-0.19

47

-0.73

225

0.022

485

0.018

150

0.013

533

80

0.016

10

124

—

0.02

61

—

550

0.023

60

—

0.014

560

0.025

54

—

250

0.31

260

-0.22

47

-0.71

293

0.49

109

-0.49

27

-1.0

Mote: Pyrene coverage i s 2.5 mg/g SiO, Decanol coverage i s 1 χ 10

mol/g SiO.

REMARKS

1

λ

ΕΧ

-

λ

ΕΜ

~

3

7

0

nm

1.

DE MAYO ET AL.

Aromatic

Hydrocarbons

Adsorbed

on Silica

Gel

15

T h e r a p i d d e c r e a s e in t h e e x c i m e r - l i k e e m i s s i o n w i t h t e m p e r a t u r e on decanol-covered silica gel, illustrated in Figs. 10, 11, is c o n s i s t e n t with a t e m p e r a t u r e c o e f f i c i e n t f o r p y r e n e d i f f u s i o n of 4 to 6 K c a l / m o l , b u t e x a c t c a l c u l a t i o n is m o d e l d e p e n d e n t a n d n o t j u s t i f i e d a t this time. N e v e r t h e l e s s , t h e q u a l i t a t i v e b e h a v i o r is r e a s o n a b l e . T h e q u e n c h i n g of p y r e n e m o n o m e r e m i s s i o n by 2 - b r o m o n a p h t h a lene on dry silica gel has also been studied as a function of temperature. Linear S t e r n - V o l m e r plots a r e obtained either with f / r o r w i t h τ^/Τι a n d τ$/τ2 v s [Q] w h e r e [ Q l is a s u r f a c e c o n c e n t r a t i o n . Fig. 12 i l l u s t r a t e s t h e f / f plot. The rate constants derived from these Stern-Volmer plots give a r e m a r k a b l y g o o d A r r h e n i u s plot as shown in F i g . 1 3 , with a n activation e n e r g y of -4 Kcal/mol. This a c t i v a t i o n e n e r g y is i n t e r p r e t e d a s t h a t a s s o c i a t e d w i t h d i f f u s i o n o f t h e two m o l e c u l e s o n the s u r f a c e . In t h i s c o n t e x t it is s i g n i f i c a n t t h a t t h e e n e r g y is o f t h e o r d e r o f h y d r o g e n b o n d e n e r g i e s . 0

0

The A r r h e n i u s plots f r o m τ and r data give activation e n e r g i e s a b o v e a n d b e l o w t h i s v a l u e o f 4 K c a l / m o l w h i c h is c o n s i d e r e d t o b e an average. T h e low t e m p e r a t u r e studies o n d e c a n o l c o v e r e d silica g e l a l s o a l l o w e d of t h e o b s e r v a t i o n of p y r e n e p h o s p h o r e s c e n c e (Fig. 1 4 ) ; t h e p h o s p h o r e s c e n c e d i s a p p e a r e d a t t e m p e r a t u r e s a b o v e 2 0 0 ° K. λ

Summary

2

and

Conclusions

T h e m o s t i n h o m o g e n e o u s s u r f a c e a s j u d g e d by t h e p y r e n e p r o b e is t h a t w h i c h r e s u l t s f r o m h e a t i n g in a v a c u u m at h i g h temperature (700°C). One sees multiple exponential decay, poorly resolved s p e c t r a , bimolecular ground state association and shortened lifetimes. If t h e s u r f a c e is p r e p a r e d by c o a d s o r b i n g a l c o h o l s o r w a t e r , a m u c h m o r e h o m o g e n e o u s s u r f a c e r e s u l t s , as i n d i c a t e d by t h e a p p r o a c h of the d e c a y to o n e c o m p o n e n t , longer lifetimes, a n d d i m i n i s h e d b i m o l e cular ground state association. In a d d i t i o n , the surface containing c o a d s o r b e d a l c o h o l s o r w a t e r a l l o w s m o r e r a p i d d i f f u s i o n a n d o n e is a b l e to o b s e r v e d y n a m i c e x c i m e r f o r m a t i o n o n a t i m e s c a l e of s e v e r a l hundred nanoseconds. Q u e n c h i n g studies have yielded an activation energy for diffusion o n t h e d r y s i l i c a g e l s u r f a c e of a r o u n d 4 K c a l / m o l .

16


1.

DE MAYO ET AL.

F i g u r e 11.

Aromatic

Hydrocarbons

Adsorbed

17

on Silica Gel

E f f e c t of l o w e r i n g t h e t e m p e r a t u r e o n t h e p y r e n e e x c i m e r formation. T h e s i l i c a g e l is c o v e r e d w i t h d e c a n o l (Ί χ 10" mol/g S i 0 ) . Pyrene is 2 . 5 m g / g S i 0 . E m i s s i o n s p e c t r u m o f p y r e n e a t 2 0 0 ° K. The excitation wavelength: 345 n m . 3

2

F i g u r e 12.

e

Effect of t e m p e r a t u r e o n t h e q u e n c h i n g of p y r e n e m o n o m e r e m i s s i o n by 2 - b r o m o n a p h t h a l e n e . S t e r n - V o l m e r plots at different t e m p e r a t u r e s .

2

18


360h

lnk

q

1

χ 10 Κ ' 3

F i g u r e 13.

A r r h e n i u s plot of q u e n c h i n g by 2 - b r o m o n a p h t h a l e n e .

F i g u r e 14.

P h o s p h o r e s c e n c e e m i s s i o n of s i l i c a gel s u r f a c e at 6 0 ° K . 360 n m .

]

of

pyrene

monomer

emission

pyrene on decanol covered The excitation wavelength:

1.

DE MAYO ET AL.

Aromatic

Hydrocarbons

Adsorbed

on Silica Gel

19

L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Bauer, R.K.; Borenstein, R.; de Mayo, P.; Okada, K.; Rafalska, M.; Ware, W.R.; Wu, K.C. J. Am. Chem. Soc., 1982, 104, 4635. Nicholls, C. H.; Leermakers, P.A. Adv. Photochem., 1971, 8, 315. Frederick, B.; Johnston, L . J . ; de Mayo, P.; Wong, S.Κ., Can. J. Chem., 1984, 62, 403. Johnston, L.J.; de Mayo, P.; Wong, S.K. J. Org. Chem., 1984, 49, 20. Leffler, J.Ε.; Zupancic, J.J. J. Am. Chem. Soc., 1980, 102, 259. Leffler, J . E . ; Barbas, J. T., J. Am. Chem. Soc., 1981, 103, 7768. Fripiat, J.J.; Uytterhoeven, J . , J. Phys. Chem., 1962, 66, 800. Kiselev, A.V.; Lygin, V.I., "Infrared Spectra of Surface Com pounds", Chapters 4-7, John Wiley and Sons, New York, 1975. Bauer, R.K.; de Mayo, P.; Natarajan, L.V.; Ware, W.R., Can. J. Chem., 1984, 62, 1279. Hara, K.; de Mayo, P.; Ware, W. R.; Weedon, A.C.; Wong, G.S.K.; Wu, K.C. Chem. Phys. Lett., 1980, 69, 105. Bauer, R.K.; de Mayo, P.; Ware, W.R.; Wu, K.C. J. Phys. Chem., 1982, 86, 3781. Bauer, R.K.; de Mayo, P.; Okada, K.; Ware, W.R.; Wu, K.C. J. Phys. Chem., 1983, 87, 460. de Mayo, P.; Natarajan, L.V.; Ware, W.R. Chem. Phys. Lett., 1984, 107, 187. Avis, P.; Porter, G. Trans. Faraday. Soc., 1974, 1057. Okada, Κ., unpublished observations. Kawski, A.; Weyna, I.; Kojro, Z.; Kubicki, A. Z. Naturforsch, 1983, 38a, 1103. Barradas, I.; Ferreira, J.A. and Thomaz, M.F. Trans. Faraday Soc., 1973, 388.

RECEIVED January 10, 1985

Organic Phototransformations in Nonhomogeneous Media - American

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