Energy Conversion and Synthesis - American Chemical Society

1 Evaluation of Heterogeneous

Photosensitizers

for Use in Energy Storage RICHARD R. HAUTALA and JAMES L. LITTLE

Downloaded by 80.82.77.83 on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch001

Department of Chemistry, University of Georgia, Athens, GA 30602 Two solid matrices, polystyrene and silica, have been functionalized with the dimethylaminobenzophenone chromophore, and the resulting immobilized photosensitizers have been quantitatively evaluated for their potential use in the norbornadiene-quadricyclene energy storage reaction. The polystyrene derivative exhibits a high limiting quantum efficiency (78%) for this sensitization process (ostensibly endothermic energy transfer), whereas the silica functionalized sensitizer is significantly less effective (24%). Homogeneous models for both systems are totally efficient (100%). The lower effectiveness of the silica derivative is attributed to a decrease in triplet yield and a low triplet energy, caused by the highly polar silica surface. Absorption spectra, emission spectra, and photostationary state isomer ratios for stilbene isomerization support this interpretation.

I

nterest i n f u n c t i o n a l i z e d i n s o l u b l e matrices f o r use as photosensitizers has s t e a d i l y i n c r e a s e d i n recent years, p a r a l l e l i n g a s i m i l a r interest i n

i m m o b i l i z e d reagents

f o r c h e m i c a l synthesis

(I).

T h e most

obvious

benefit of u s i n g i n s o l u b l e sensitizers f o r s o l u t i o n photolyses is t h e ease w i t h w h i c h t h e p h o t o p r o d u c t c a n b e i s o l a t e d f r o m t h e sensitizer. ever, there a r e n u m e r o u s o t h e r p o t e n t i a l advantages.

How

Sensitizers t h a t

tend to self-quench i n solution c o u l d , i n p r i n c i p l e , be isolated o n a matrix. T h e adverse influence of c e r t a i n solvents o n sensitizer p r o p e r t i e s c o u l d b e m i n i m i z e d b y u s i n g a m a t r i x w h e r e t h e f o r m a t i o n of a

complete

solvent c a g e w o u l d b e i m p a i r e d . A n a d s o r p t i v e affinity o f t h e h e t e r o geneous interface f o r specific substrates c o u l d result i n m o r e efficient a n d selective p h o t o c h e m i s t r y .

O t h e r p r a c t i c a l advantages p e r t i n e n t t o a solar

e n e r g y storage system h a v e b e e n r e c e n t l y d e t a i l e d ( 2 ) . 0-8412-0474-8/80/33-184-001$05.00/0 © 1980 American Chemical Society Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

2

INTERFACIAL

I n 1964, M o s e r a n d C a s s i d y (3)

PHOTOPROCESSES

r e p o r t e d the first e x a m p l e of a p o l y

meric photosensitizer used i n solution.

Soluble polyacrylophenone

f o u n d to sensitize t h e i s o m e r i z a t i o n of 1,3-pentadiene i n b e n z e n e . m a k e r s a n d James (4)

was Leer-

u s e d the p o l y m e r i n a solvent, isopentane, i n

w h i c h the p o l y m e r is t o t a l l y i n s o l u b l e . T h e y successfully d e m o n s t r a t e d the

sensitized photochemistry

of

1,3-pentadiene,

norbornadiene,

and

m y r c e n e w i t h t h e system. A significant a d v a n c e i n this area c a m e f r o m the w o r k of B l o s s e y , N e c k e r s , T h a y e r , a n d S c h a a p (5,6).

T h e y covalently

g r a f t e d rose b e n g a l onto a p r e f o r m e d p o l y s t y r e n e b e a d a n d s h o w e d t h a t this i n s o l u b l e p o l y m e r w a s a n effective sensitizer of s i n g l e t o x y g e n i n solution.


onto

a

Blossey a n d Neckers (7) Merrifield

polymer

also g r a f t e d p - b e n z o y l b e n z o i c

(chloromethylated

acid

styrene-divinylbenzene

c o p o l y m e r ) a n d d e m o n s t r a t e d t h e s e n s i t i z e d p h o t o d i m e r i z a t i o n of

cou-

m a r i n a n d i n d e n e a n d the s e n s i t i z e d c y c l o a d d i t i o n of t e t r a c h l o r o e t h y l e n e to c y c l o p e n t a d i e n e . U n f o r t u n a t e l y this p o l y m e r i c a n a l o g of b e n z o p h e n o n e r e a d i l y p h o t o d e g r a d e s , o s t e n s i b l y t h r o u g h h y d r o g e n a b s t r a c t i o n f r o m the polymer backbone.

A more photostable

fluorinated

m e r has r e c e n t l y b e e n r e p o r t e d b y N e c k e r s

v e r s i o n of this p o l y

(1).

A t the outset of o u r w o r k , no q u a n t i t a t i v e c h a r a c t e r i z a t i o n of a n i m m o b i l i z e d p h o t o s e n s i t i z e r has b e e n m a d e .

B e c a u s e of o u r interest i n

the a p p l i c a t i o n of i m m o b i l i z e d sensitizers i n a n e n e r g y storage r e a c t i o n (2),

t h e factors of q u a n t u m efficiency a n d l o n g - r a n g e s t a b i l i t y w e r e m o r e

c r u c i a l t h a n i n s y n t h e t i c a p p l i c a t i o n s . T h i s c h a p t e r is p a r t of a c o m p r e h e n s i v e s t u d y to e v a l u a t e t h e i m m o b i l i z e d d i m e t h y l a m i n o b e n z o p h e n o n e c h r o m o p h o r e as a sensitizer f o r the c o n v e r s i o n of n o r b o r n a d i e n e to q u a d r i c y c l e n e , a p o t e n t i a l l y a t t r a c t i v e system f o r the c h e m i c a l storage solar e n e r g y

of

(8).

The Norbornadiene--Quadricy

dene Inter conversion

T h e features of the n o r b o r n a d i e n e ( N ) to q u a d r i c y c l e n e ( Q ) i n t e r -

c o n v e r s i o n t h a t m a k e i t a t t r a c t i v e as a p o t e n t i a l system for the storage of solar energy h a v e b e e n d e s c r i b e d p r e v i o u s l y (2,8,9).

T h e necessity

o f u s i n g a p h o t o s e n s i t i z e r f o r this r e a c t i o n derives f r o m the c o m p l e t e l a c k of o v e r l a p b e t w e e n the a b s o r p t i o n s p e c t r u m of N a n d the solar r a d i a n c e s p e c t r u m . N u m e r o u s sensitizers effect this t r a n s f o r m a t i o n , some of w h i c h

Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

1.

HAUTALA

AND LITTLE

Photosensitizers

a b s o r b i n or n e a r the v i s i b l e ( A >

Storage

3

spectral region

(2,8-11).

in Energy

400 n m )

A l l effective sensitizers a p p e a r to operate b y energy transfer to p o p u l a t e t h e t r i p l e t state of N , w h i c h t h e n is t r a n s f o r m e d to Q (8,10,11). t r i p l e t energy of N , a b o u t 70 k c a l m o l " (8,11,12),

The

sets a l i m i t , f o r t u n a t e l y

1

a v e r y flexible one, o n the longest w a v e l e n g t h of l i g h t t h a t c a n b e u s e d i n effecting the p h o t o c h e m i c a l r e a c t i o n . T h i s w a v e l e n g t h l i m i t is deter m i n e d b y the onset of the singlet a b s o r p t i o n b a n d of t h e sensitizer.

For

those sensitizers w i t h t r i p l e t energies e q u a l t o o r e x c e e d i n g that of N , t h e w a v e l e n g t h l i m i t w o u l d b e near 380 n m ( e q u i v a l e n t t o c a . 75 k c a l m o l " w i t h the excess e n e r g y of a p p r o x i m a t e l y 5 k c a l • m o l "

e n e r g y loss d u r i n g r e l a x a t i o n of the sensitizer f r o m t h e singlet to


triplet state).

the

A s i n d i c a t e d , this l i m i t is n o t a rigorous one, a n d s e v e r a l

v e r y efficient sensitizers w i t h t r i p l e t energies w e l l b e l o w t h a t of N known.

1

necessary for t h e

1

are

T h e e n e r g y deficiency is a p p a r e n t l y m a d e u p t h r o u g h t h e r m a l

activation or " t h e r m a l upconversion." endothermic

energy

A

more detailed discussion

transfer, a l o n g w i t h s t r o n g e x p e r i m e n t a l

f o r i t i n analogous systems, has r e c e n t l y b e e n m a d e b y Jones

Dimethylaminobenzophenone Several

features

make

evidence (8).

as a Sensitizer -dimethylamino)benzophenone

4-(N,N

a t t r a c t i v e sensitizer for s t u d y w i t h the n o r b o r n a d i e n e

system.

an

Under

c e r t a i n c o n d i t i o n s the q u a n t u m efficiency for sensitization of Q f r o m approaches

100%.

of

T h e intense a b s o r p t i o n b a n d m a x i m i z i n g

N

between

300 a n d 400 n m is sufficiently b r o a d to t a i l i n t o the v i s i b l e s p e c t r a l r e g i o n . T h e h i g h m o l a r e x t i n c t i o n coefficient

(~

25,000) of this b a n d a l l o w s for

the use of v e r y l o w sensitizer concentrations.

The photophysical,

photo

c h e m i c a l , a n d s e n s i t i z i n g p r o p e r t i e s of this c o m p o u n d are v e r y sensitive to m e d i u m p o l a r i t y . F o r e x a m p l e , the a b s o r p t i o n m a x i m u m varies f r o m 332 n m i n c y c l o h e x a n e to 355 n m i n e t h a n o l . T h e intersystems c r o s s i n g y i e l d ( t r i p l e t y i e l d ) falls f r o m 1 0 0 %

i n hexane to 6 %

p a r a l l e l i n g s i m i l a r b e h a v i o r of the p a r e n t c o m p o u n d n o n e (13). (E

T

T h e t r i p l e t energy of 4 - ( N , N

i n acetonitrile,

4-aminobenzophe-

-dimethylamino)benzophenone

< 66 k c a l m o l " ) is also m e d i u m d e p e n d e n t , b u t i n a n y case, t r i p l e t 1

energy transfer to N is c l e a r l y e n d o t h e r m i c . forward

synthetic

techniques

are

F i n a l l y , relatively straight

a v a i l a b l e to

allow

g r a f t i n g of

the

c h r o m o p h o r e onto s o l i d matrices for i m m o b i l i z a t i o n . T h e e x h i b i t e d sensi t i v i t y to m e d i u m effects a n d the h i g h e x t i n c t i o n coefficient r e l a t i v e l y l o w l o a d i n g ) m a k e this c h r o m o p h o r e

p r o b e for s t u d y i n g heterogeneous p h o t o s e n s i t i z a t i o n . corresponding

a c r o n y m s for the sensitizers e m p l o y e d

s h o w n i n F i g u r e 1.

S i m p l e d e r i v a t i v e s of

( a l l o w i n g for

a particularly interesting Structures a n d t h e i n this s t u d y are

dimethylaminobenzophenone

b e h a v e s i m i l a r l y b u t not i d e n t i c a l l y . T h u s h o m o g e n e o u s m o d e l s


more

4

INTERFACIAL

PHOTOPROCESSES


0^

MeAK Figure 1.

SiAK

Structures of the sensitizers used in this study


1.

HAUTALA AND LITTLE

Photosensitizers

in Energy

Storage

5

closely r e s e m b l i n g the i m m o b i l i z e d sensitizers w e r e p r e p a r e d a n d are i n c l u d e d f o r c o m p a r i s o n . P r e p a r a t i o n of the p o l y s t y r e n e - g r a f t e d sensitizer and general experimental procedures have been reported previously

(2).

P r e p a r a t i o n o f t h e s i l i c a - f u n c t i o n a l i z e d sensitizers w i l l b e r e p o r t e d else w h e r e , h o w e v e r a r e a c t i o n s c h e m e is i n d i c a t e d b e l o w .

BrH@)-Li ecu -O —


u

0

Si-OH

-0-

500°

Si-Cl

•0'

/

6 0 - 2 0 0 mesh s i l i c a gel

1.6 meq CI g"

n-BuLi —

:

0

—

B

0 II MeO-C

r

ether

N(CH ) 3

2

— 0 , -0-

I—o'

— 0

ho-si-

Energy Conversion and Synthesis - American Chemical Society

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