Cation Radical Formation with Exposure of Magnesium Porphyrin Thin

22 Cation Radical Formation with Exposure of Magnesium Porphyrin Thin Films to Light and Oxygen 1

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Robert M. Burgess, Martin Gouterman, Gamal-Eddin Khalil, and James van Zee Department of Chemistry, University of Washington, Seattle, WA 98195

When thin films of Mg tetraphenylporphin (MgTPP) are exposed to oxygen, radical cations MgTPP form in the dark; this dark reaction obeys a Langmuir absorption isotherm, is largely reversible, and is attributed to oxygen molecules on the surface. On exposure to light more MgTPP forms, some of it irreversibly and some reversibly. We attribute the irreversible reaction to oxygen in the bulk. The MgTPP is monitored in the visible adsorption, in growth of an EPR signal, in the appearance of new bands in the IR (some of which are assigned to O 2), and by reduction in the film fluor escence. Mg octaethylporphyrin shows optical absorp tion changes but no EPR. Zinc tetra(perfluorophenyl)– porphyrin shows no cation formation under these condi tions. +

+

+

-

Twenty years ago H a r r i s o n and c o w o r k e r s r e p o r t e d t h a t oxygen i n c r e a s e d t h e c o n d u c t i v i t y o f copper p h t h a l o c y a n i n e f i l m s (1). L a t e r , Assour and H a r r i s o n r e p o r t e d o b s e r v a t i o n s o f EPR r a d i c a l s i n f r e e base p h t h a l o c y a n i n e s , which i n c r e a s e d w i t h e x p o s u r e t o oxygen and d e c r e a s e d w i t h e x p o s u r e t o hydrogen (2). The e f f e c t o f oxygen on o r g a n i c s e m i c o n d u c t o r s has been w i d e l y researched s i n c e these early studies (3,4). I t has been found t h a t p h o t o c o n d u c t i v i t y i n c r e a s e s on oxygen e x p o s u r e f o r p h t h a l o c y a n i n e s (5,6), xanthine dyes ( 7 ) , and p o r p h y r i n s (8). An oxygen/water m i x t u r e was shown t o be n e c e s s a r y t o produce l a r g e band bending i n t h e p h o t o v o l t a g e s t u d i e s o f a Iuminum/phthalocyanine i n t e r f a c e s (9) and was shown t o increase the photovoltage of c e l l s based on p a l l a d i u m t e t r a p h e n y I p o r p h y r i n (PdTPP) d e p o s i t e d on η-doped SnCL e l e c t r o d e s (10). The latter study a l s o showed t h a t t h e e f f e c t o f oxygen c o u l d be partially r e v e r s e d by h e a t i n g t h e d e v i c e i n a hydrogen atmosphere. ' C u r r e n t address: Boeing Electronics C o m p a n y , P.O. B o x 3999 M I S 88-43, Seattle, W A 98124 0097-6156/ 86/ 0321 -0328506.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

22.

In another s t u d y , Bree

and

showed t h a t t h e s u r f a c e partial

Lyons

photo

p r e s s u r e o f oxygen.

photocurrent of oxygen.

329

Cation Radical Formation

BURGESS ET AL.

(11) w i t h a n t h r a c e n e t h i n

data

p r e s s u r e dependency, g r e a t e r than 10 t o r r ,

to

a

Langmuir

adsorption

" i n t e r m e d i a t e " p r e s s u r e s f o l l o w e d a Freud l i e n Recent work i n our own water was needed f o r these


be

related

exposure

devices

f a c t t h a t diamagnetic

isotherm, and t h e

isotherm.

l a b o r a t o r y (12) on p h o t o v o l t a i c d e v i c e s

[quartz/AI/A10^/MgTPP/Ag] showed t h a t

may a l s o

with

T h e i r r e s u l t s were c o n s i s t a n t w i t h t h e

change b e i n g a f u n c t i o n of a p a r t i a l monolayer coverage F o r t h e high

they f i t t h e i r

The

films

current increased non-1inearly

to

both oxygen and

t o show p h o t o v o l t a i c a c t i v i t y .

p o r p h y r i n s a r e p-type s e m i c o n d u c t o r s ( 4 ) ,

to the

d i f f u s e s into the porphyrin

requirement

providing

of

oxygen; i . e . oxygen

e l e c t r o n t r a p s , thus genera

t i n g h o l e s i n t h e v a l e n c e band. To o b t a i n more d i r e c t electrical

p r o p e r t i e s of

information porphyrin

troscopic

studies,

emission.

A b s o r p t i o n and e m i s s i o n

t h e presence

of

namely

porphyrin

FTIR,

estimate

the

ratio

of

the

EPR,

electronic

species

Absorption

cation

role

species

and data

to

measuring t h e change i n t h e n e u t r a l s p e c i e s

of

in the

we undertook some s p e c

data were mainly

cation

a d d i t i o n a l o x i d a t i o n products.

on

films,

1

a b s o r p t i o n and used t o c o n f i r m t h e absence of

was a l s o used t o

neutral

absorbance.

species

by

FTIR data

was used t o probe t h e e x i s t e n c e of any bonded oxygen, any n e u t r a l o r reduced oxygen a l o n g w i t h any p o r p h y r i n

c a t i o n r i n g modes.

The EPR

data probed t h e oxygen p r e s s u r e

and

r a d i c a l f o r m a t i o n a l o n g w i t h an

attempt t o observe a reduced oxygen

r a d i c a l s p e c i e s , which has been

observed by EPR f o r oxygen adsorbed

on v a r i o u s metal s u b s t r a t e s

l i g h t dependancy of t h e c a t i o n

(13,14).

We f i r s t review t h e s p e c t r o s c o p y

of p o r p h y r i n c a t i o n s and o x y

gen a n i o n s b e f o r e p r e s e n t i n g our study of t h e i r f o r m a t i o n P o r p h y r i n C a t i o n s , Oxygen Anions:

in films.

Review

I n t h i s paper we c o n f i n e our s t u d i e s t o tetraphenyIporphyriη etioporporphyriη

(Etio)

tetraperfluorophenyIporphyriη (TFPP), Zn and MgEtio have two of complexes, 0.54 and

0.4

(TPP),

[ i . e . t e t r a m e t h y I t e t r a e t h y I p o r p h y r i n ] and and

Mg

complexes.

MgTPP

t h e lowest o x i d a t i o n p o t e n t i a l s o f p o r p h y r i n volts

v s . SCE,

r e s p e c t i v e l y (15), while

ZnTFPP has an o x i d a t i o n p o t e n t i a l of 1.3 V (16).

On o x i d a t i o n t h e s e

m o l e c u l e s form p i - c a t i o n r a d i c a l s ( i . e . an e l e c t r o n i s l o s t from t h e highest f i l l e d and MgEtio

ring orbital).

are their

solid

o r b i t a l of the c a t i o n r a d i c a l .

The major d i f f e r e n c e s between MgTPP state

structures

and t h e unoccupied

The d i f f e r e n c e s i n t h e i r s o l i d s t a t e


P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S

330 s t r u c t u r e a r i s e s by v i r t u e groups on TPP.

The

of

the s t e r i c

crystal

structure

arrangement w i t h one m o l e c u l e cell

hindrance of

of t h e phenyl

both show a t r i c l i n i c

per u n i t c e l l

(17,18).

The c r y s t a l

a n g l e s f o r both m o l e c u l e s a r e not f a r from 90°; t h e r e f o r e , t h e

crystal distances, distances.

a,

b

and

F o r t h e ZnTPP

c,

can be

complex

used

t h e phenyl

w i t h r e s p e c t t o t h e p o r p h y r i n r i n g system:

as intermoI ecu Iar

groups a r e nonplanar

a = 14.8, b = 17.2 and

c = 14.6A, and f o r N i E t i o a = 14.6, b = 12.3 and c = 12.3A. By o b s e r v i n g t h e f o r m a t i o n o f

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

MgTPP and MgOEP, t h e d i f f e r e n c e s

between

e l u c i d a t e d u s i n g EPR and

a b s o r p t i o n data

visible


t i o n spectra, obtained a t various showed t h a t t h e c a t i o n has bands absorption nm).

t h e i r ground s t a t e s were

stages

(15,19).

Absorp-

i n the o x i d a t i o n process,

t o t h e r e d and b l u e o f t h e S o r e t

(350-450 nm) and t o t h e red o f t h e v i s i b l e bands (600-900

These data

as counter

were

ions Br

a l s o be used a s

obtained e l e c t r o c h e m i c a I l y i n s o l u t i o n using

or

an

CIO^, although

oxidant.

EPR

i t was noted t h a t

solution

could

data showed t h a t t h e

c a t i o n r a d i c a l s o f MgTPP and MgOEP both e x h i b i t h y p e r f i n e s t r u c t u r e w i t h measured g - v a l u e s

of

2.0025

and 2.0031, r e s p e c t i v e l y .

exposure t o oxygen

the hyperfine

radical

to

was

shown

d i f f e r e n t p o s i t i o n s of (19),

structure

disappear.

By

the porphyrin

of MgTPP

is a

a b s o r p t i o n s were

several unique

observed

metallated IR

active

TPP ring

in the region

modes have a l s o been observed s p e c i e s adsorbed on v a r i o u s adsorption

of

spectroscopy frequency study cm

oxygen

(SERS)

o f 1053 cm

on

and OEP complexes modes (21|. These

1250-1290

complexes and 1520-1570 cm * f o r OEP complexes.

and t h a t

MO t h e o r y (20).

u

C a t i o n r a d i c a l s of

deuterating

systems, F a j e r e t . a l .

of MgOEP* i s a ^A^ , as p r e d i c t e d by s e m i e m p i r i c a I were r e p o r t e d t o have

Upon

t h e MgTPP c a t i o n

selectively

ring

have shown t h a t t h e ground s t a t e

of

cm

f o r TPP

IR and raman a c t i v e

f o r s e v e r a l d i f f e r e n t reduced oxygen metal

substrates.

silver

|22), show

by

superoxide

, and p e r o x i d e

Observations

surface

(0^ )

r e p o r t e d a d d i t i o n a l peaks were observed

enhanced

molecules^ of 697 cm

of the raman

(0 ) have a 2

T h i s same

a t 815 cm * and 1286

i f t h e oxygen exposure took p l a c e w h i l e t h e g e t t e r ion pump was

o p e r a t i n g . The 815 cm * l i n e s p e c i e s , b u t they had no

was

i d e n t i f i e d as an adsorbed ozonide

explanation

f o r t h e l i n e o b s e r v e d a t 1286

cm Exper imentaI Film preparation f o r optical/IR studies. were prepared

by vacuum

These p o r p h y r i n t h i n f i l m s

sublimation using r a d i a t i o n s h i e l d e d quartz


22.

crucibles

wrapped w i t h t u n g s t e n

by p a s s i n g an e l e c t r i c degassing

wire.

current

was a c c o m p l i s h e d

wire slowly with a

variac.

12-24 hour

a b s o r p t i o n and

emission

When

and

an o i l r o t a r y pump f o r t h e roughing Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022

Films

s u b s t r a t e s used f o r t h e

standard

l"x3"xl/16" glass

1/4" t h i c k NaCI o p t i c a l

For f i l m d e p o s i t i o n , a low vacuum (10

torr) bell

flats.

j a r was used w i t h

s t a g e , and an o i l d i f f u s i o n pump

lowest

using

l a t e d from a b s o r p t i o n data

d e p o s i t i o n occurs.

The

were

were

with a c r y o s t a t i c trap f o r the

Sample

t h i c k n e s s e s , on t h e order of 300-

period.

s l i d e s and t h e FTIR s u b s t r a t e s

wire.

t h e c u r r e n t through t h e

f u l l y degassed, t h e temperature

point

studies

nesses f o r samples prepared

t h e tungsten

increasing

were u s u a l l y grown t o t h e i r f i n a l 500 nm, over a

Samples were heated s l o w l y

through

by

increases t o the sublimation

solution

331


BURGESS ET AL.

p r e s s u r e range.

the bell

Film thick-

j a r system were c a l c u -

assuming t h e e x t i n c t i o n c o e f f i c i e n t from

data.

Sample chamber f o r o p t i c a l and

FTIR

s t u d i e s t h e ambient atmosphere was i n vacuum t i g h t chambers

studies.

F o r o p t i c a l and FTIR

c o n t r o l l e d by p l a c i n g t h e sample

described

elsewhere (23).

These chambers

were f i t t e d w i t h o p t i c a l f l a t s

as v i e w i n g windows t o a l l o w s t r a i g h t

t h r o u g h p u t f o r a b s o r p t i o n data

a c q u i s i t i o n and p e r p e n d i c u l a r o b s e i —

v a t ion o f e m i s s i o n .

The

optical

flats

were

s e a l e d by use o f 0-

r i ngs. T r a n s f e r from t h e p r e p a r a t i o n was

a c c o m p l i s h e d by f i r s t

or argon f o r s e v e r a l m i n u t e s . sample was q u i c k l y moved

When

j a r t o t h e sample chamber t h e system w i t h d r y n i t r o g e n

t h e b e l l j a r was removed, t h e

t o t h e chamber w h i l e c o n t i n u a l l y blowing

t h e i n e r t gas a c r o s s

it.

exposure t o a i r , our

results

oxygen i n t h e dark was

bell

backfilling

While

this

show

largely

p r o c e s s does i n v o l v e b r i e f

that

t h e e f f e c t of exposure t o

reversible.

Thus, we b e l i e v e t h a t

any e f f e c t s due t o t h i s t r a n s f e r a r e s m a l l . Exposure o f t h e sample chamber t o v a r i o u s ambients was c o n t r o l led by use of a r o t a r y pump.

manometer

The lowest

system

20 mtorr (base p r e s s u r e o f s u r e b e i n g 800 t o r r

with evacuation

pressure

t h e r o t a r y pump) w i t h t h e h i g h e s t p r e s -

(maximum a l l o w a b l e on t h e manometer).

O p t i c a l and FTIR i n s t r u m e n t a t i o n . three instruments.

obtained using a

o b t a i n a b l e u s i n g t h i s system was

A

Hewlett

was

used f o r t h e v i s i b l e

region

was

computer

and

c o n t r o l led

Absorption Packard of

used

s p e c t r a were taken on

8450 UV/Vis Spectrophometer This

instrument

t o measure s t a n d a r d

t h e spectrum.

absorption

s p e c t r a a s w e l l as a b s o r p t i o n d i f f e r e n c e s p e c t r a . A Cary 14 S p e c t r o photometer was used f o r t h e near IR r e g i o n , and an

IBM IR/90 S e r i e s


PORPHYRINS: FXCITED STATES AND DYNAMICS

332 FTIR Spectrometer was

absorbance

as w e l l as

absorbance d i f f e r e n c e s p e c t r a i n t h e r e g i o n 600-4000 cm

utilized

Emission

s p e c t r a were t a k e n on a

t o measure

c o m m e r c i a l l y a v a i l a b l e P e r k i n Elmer 650-10S

FIuor imeter. The

l i g h t s o u r c e used

Watt t u n g s t e n b u l b .

t o form

Absorbance

t h e r a d i c a l c a t i o n s was a 100

d i f f e r e n c e s p e c t r a were c a l c u l a t e d

by u s i n g t h e r e l a t i o n s h i p Δ Α =

log(I /I),

I

where

Q

and I a r e t h e

q

s t o r e d absorbance s p e c t r a b e f o r e and a f t e r t h e exposure o f t h e f i l m s t o oxygen and/or

light,

respectively.

EPR sample p r e p a r a t i o n . F o r EPR samples, e i t h e r t h e b e l l j a r system Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022

was used o r t h e f o l l o w i n g s m a l l amount o f sample

preparation

(10-50

mg)

a t t a c h e d t o a vacuum a p p a r a t u s oil

a

current

Samples c o n s i s t e d o f p o r p h y r i n tube.

The f i l m morphology

q u i t e d i f f e r e n t from

those

A

t o r r w i t h an

oven, c o n s i s t i n g o f a q u a r t z tube

wrapped w i t h t u n g s t e n w i r e , was p l a c e d passing

was f o l l o w e d .

and evacuated t o ~10

d i f f u s i o n pump. A c y l i n d r i c a l

s u p p l i e d by

procedure

was p l a c e d i n s i d e an EPR tube,

over t h e EPR tube.

through

the wire

sublimed

o f samples prepared

Heat was

with a variac.

on t h e w a l l s o f t h e EPR prepared i n t h i s manner a r e

using

the bell

j a r . These

samples c o n s i s t o f need le-1 i k e c r y s t a l s e n t e n d i n g towards t h e c e n t e r of t h e EPR tube.

The placement

o f t h e tube

i n t h e EPR c a v i t y

i n s u r e d t h a t o n l y t h e s u b l i m e d p o r p h y r i n was observed. EPR I n s t r u m e n t a t i o n . The EPR

instrument

used

i n t h i s study was a

c o m m e r c i a l l y a v a i l a b l e V a r i a n E-3 EPR Spectrometer. used t o measure t h e l i f e t i m e

The a p p a r a t u s

of the photoassisted radical formation

i s shown, s c h e m a t i c a l l y , i n F i g u r e 1. The l i g h t s o u r c e was a waterc o o l e d 1000 watt

tungsten

lamp.

The

l i g h t was p u l s e d u s i n g an

e l e c t r o n i c s h u t t e r w i t h a r i s e t i m e o f under 2 ms. well as t h e t i m i n g

f o r t h e experiment,

"FOCAL" f u n c t i o n s a v a i l a b l e on t h e PDP a c q u i r e d by f i r s t a b s o r p t i o n maxima.

setting After

the buffer

Data was

f i e l d to the differential memory

was a c t i v a t e d , t h e

The output s i g n a l from t h e EPR

i n s t r u m e n t passed t h r o u g h a p r e a m p l i f i e r ms.

8/e computer ( 2 4 ) .

t h e magnetic

sample was p u l s e d w i t h w h i t e l i g h t .

The s h u t t e r , as

was c o n t r o l l e d by s p e c i a l

w i t h a t i m e c o n s t a n t of 10

The t i m e v a r y i n g s i g n a l then passed through a n u l l a m p l i f i e r t o

e l i m i n a t e t h e DC o f f s e t s i g n a l .

F i n a l l y , t h e data was i n t e g r a t e d by

t h e computer f o r a p r e s e l e c t e d number o f l i g h t p u l s e s . The V a r i a n E-3 was a l s o used f o r t h e study o f r a d i c a l c a t i o n f o r m a t i o n as a f u n c t i o n of oxygen p r e s s u r e i n t h e dark. Oxygen was s u p p l i e d by a h i g h p r e s s u r e c y l i n d e r , and t h e lower p r e s s u r e s were c o n t r o l l e d by a manometer assembly. The i n i t i a l s t u d i e s were p e r -


22.


BURGESS ET AL.

formed by s e t t i n g t h e magnetic maxima and

observing

l e v e l s perturbed

333

f i e l d to the d i f f e r e n t i a l

the signal

t h e sample.

as

various

Using

an

absorption

p r e s s u r e s and l i g h t

internal standard,

i t was

r e a l i z e d t h a t i n c r e a s i n g oxygen p r e s s u r e s c o u l d p e r t u r b t h e b a s e l i n e of t h e s e scans r e s u l t i n g i n a p r e s s u r e dependency.

s c a n n i n g through t h e peak

and

ofthe signal strength. is unlikely

to

DC

o f f s e t m i s t a k e n f o r a t r u e oxygen

Therefore,

form

a l l subsequent scans were made by using

t h e peak h e i g h t as a measure

Copper tetraphenyIporphyriη (CuTPP), which a

cation

i n t h e presence

o f oxygen

o x i d a t i o n i s 0.9 V vs SCE (15), was used as t h e i n t e r n a l


Source of m a t e r i a l s .

MgTPP

was

(ring

standard.

o b t a i n e d from Strem c h e m i c a l s and

was p u r i f i e d by e l u t i o n through a dry-packed column of A I ( W o e l m , A c t i v i t y Grade I , n e u t r a l ) w i t h d i c h l o r o m e t h a n e and acetone. was s y n t h e s i z e d

i n our

( 2 5 ) , f o l l o w e d by

laboratory

metal I a t i o n

by

first

using

ZnTFPP

making t h e f r e e base

Zn a c e t a t e (26).

s y n t h e s i z e d u s i n g a procedure from Fuhrhop e t a l .

M g E t i o was

(27).

Observations O p t i c a l absorbance changes w i t h oxygen v a t i o n t o be p r e s e n t e d

and l i g h t .

by oxygen i s by v i s i b l e a b s o r p t i o n s p e c t r o s c o p y . in Figure 2 a r e absorption M g E t i o and ZnTFPP b e f o r e and

light.

spectra

(b) and

obser

absorption

The data

presented

f o r t h i n f i l m samples o f MgTPP, after

Exposure t o oxygen

on t h e f i l m

The f i r s t

f o r t h e s o l i d s t a t e o x i d a t i o n of p o r p h y r i n s

(a) exposure t o both oxygen

a l o n e o r l i g h t a l o n e has no e f f e c t

spectrum

a b s o r p t i o n data shows c l e a r l y t h e

(data

n o t shown).

The MgTPP

r e d u c t i o n o f t h e v i s i b l e bands o f

t h e normal ground s t a t e and t h e r i s e o f bands a t 500 and 800 nm t h a t correspond f i l m s show

t o t h e absorbance of t h e c a t i o n r a d i c a l the

approximately

largest

percentage

of

converted

10% r e d u c t i o n i n t h e v i s i b l e

(28).

The MgTPP

neutral species,

a b s o r p t i o n bands.

The

M g E t i o and ZnTFPP f i l m s of F i g u r e 2 were t h i c k e r than t h a t o f MgTPP, and t h e i r s p e c t r a extend o n l y down t o and oxygen, t h e M g E t i o r e g i o n 700 t o 900 nm

shows

while

a

590 nm.

clear

On exposure t o l i g h t

absorbance i n c r e a s e i n t h e

t h e ZnTFPP shows e s s e n t i a l l y no change.

The MgEtio data can be compared

t o changes r e p o r t e d by F a j e r e t a l .

(28) f o r t h e c o n v e r s i o n MgOEP ·» MgOEP* i n s o l u t i o n . Q(0,0) bands a r e red s h i f t e d , ~600 cm to t h e i r s .

They observed a

1

and ~200

relatively

Our Q(1,0) and 1

cm , w i t h

respect

sharp band a t 720 nm i n t h e

c a t i o n , w h i l e we observe a very broad band w i t h a maximum of 800 nm. The

absorbance r i s e we observe i n t h e near IR i s roughly

to the intensity

loss

in the v i s i b l e ,

which

comparable

i s t h e same f o r t h e


334

PORPHYRINS: EXCITED STATES AND DYNAMICS

PDP 8/e Computer

Shutter Control


Sample 1NULL AMP| ' 1

Oxygen Vacuum Pump

Manometer! |N [Thermocouple

2

Source]

Temperature Control

F i g u r e 1 . B l o c k diagram o f EPR a p p a r a t u s used l i f e t i m e of photoexcited porphyrin cation r a d i c a l s .

500

600

700 800 WAVELENGTH nm

900

t o measure t h e

1000

F i g u r e 2 . A b s o r p t i o n s p e c t r a o b t a i n e d on MgTPP, MgEtio and ZnTFPP t h i n f i l m s (b) b e f o r e and (a) a f t e r exposure t o oxygen and l i g h t .


22.


BURGESS ET AL.

MgOEP -> MgOEP*

conversion.

Thus,

broader and more r e d s h i f t e d

335

although

than

that

our near IR band i s

reported f o r the i s o l a t e d

m o l e c u l e o x i d a t i o n i n CH^Cl^, t h e assumption t h a t we a r e o b s e r v i n g a s i m i l a r o x i d a t i o n i s reasonable. EPR

s p e c t r a produced by oxygen i n t h e dark.

was

measured on MgTPP t h i n

of oxygen i n t h e dark.

The

t h e base p r e s s u r e o b t a i n e d initial

strength pressures

oxygen p r e s s u r e v a r i e d from 20 m t o r r ,

using

a

rotary

pump, t o 760 t o r r .

The

EPR scan shows a s m a l l p o r p h y r i n c a t i o n r a d i c a l peak a t 3357

gauss, w i t h a g - v a l u e

of

2.0031

(Figure

r e s i d u a l oxygen p r e s e n t a t t h e base Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022

The EPR s i g n a l

f i l m s upon exposure t o v a r i o u s

peak, s e v e r a l o t h e r

sharp

peaks,

i n t e n s i t i e s , a r e observed over a r e p o r t e d t o be t h e r e s u l t

of

3a)

pressure. 1-2

gradually 3b).

absorbance

that

as t h e ambient p r e s -

shown

i n F i g u r e 3b i s t h e

absorbance no

levels

e l e c t r o n s ( 2 9 ) . These

disappear

Also

cation

oxygen p r e s s u r e . Note a l s o ,

and o f v a r y i n g

the coupling of t h e r o t a t i o n a l

sure

i n c r e a s e of t h e p o r p h y r i n

wide

1000 gauss range. These peaks were

oxygen peaks d i m i n i s h and (Figure

In addition t o t h i s

gauss

of gaseous oxygen t o t h e s p i n s o f t h e u n p a i r e d i s increased

p o s s i b l y due t o t h e

with

change

an i n c r e a s e i n

o c c u r s w i t h t h e CuTPP

peaks.

Room temperature s p e c t r a were taken over t h e range o f p r e s s u r e s 1-760

torr.

Typical r e s u l t s obtained

a r e p l o t t e d i n Figure 4 along

w i t h t h e c a l c u l a t e d f i t o f t h e data p o i n t s t o a Langmuir a d s o r p t i o n i sotherm, EPR

signal

L e a s t square f i t s were

=

k P/(l k P) a

(1)

a

o b t a i n e d by t h e g r a d i e n t - e x p a n s i o n

developed by Marquardt ( 3 0 ) . m t o r r ) was s u b t r a c t e d

+

from

The each

DC

algorithm

o f f s e t ( i . e . EPR s i g n a l a t 20

data

p o i n t before performing the

l e a s t squares f i t . The i n v e r s e of

t h e f i t t i n g parameter, k , i s t h e

p r e s s u r e a t which t h e EPR

reaches h a l f maximum and,

signal

a b l y , i s r e l a t e d t o t h e number oxygen.

In these

studies,

g

of

presum-

surface s i t e s a v a i l a b l e t o bind

t h e value

of k

g

remained

relatively

c o n s t a n t a t 475 * 25 t o r r . The

spectra obtained a r e completely

t i o n times of under one

hour.

For

r e v e r s i b l e during

exposure i n t h e d a r k , t h e DC o f f s e t s i g n a l of t h e Langmuir isotherm v a l u e per hour. i f t h e p o r p h y r i n f i l m s a r e heated gun,

observa-

longer t i m e p e r i o d s of oxygen i n c r e a s e s s l o w l y , ~5-10% I t i n c r e a s e s more q u i c k l y

f o r s e v e r a l minutes w i t h a heat

w h i l e under oxygen.


336



a. P

0

= 20mtorr

g = 2.0031

b. P Q = 200 torr

F i g u r e 3. EPR s p e c t r a of a t h i n f i l m MgTPP sample showing; (a) r e s i d u a l MgTPP c a t i o n radical and gas phase oxygen a t an oxygen p r e s s u r e of 20 m t o r r and (b) a d d i t i o n a l MgTPP c a t i o n r a d i c a l formed a t an oxygen p r e s s u r e o f 200 t o r r . A l s o shown i s t h e i n t e r n a l s t a n d a r d s, CuTPP, peaks.



22.

BURGESS ET

AL.

EPR

changes w i t h oxygen

c a t i o n s i g n a l occurs

and

r e v e r s i b l e and an initial nal

light.

when

a d d i t i o n a l l y exposed t o

An

the

MgTPP

light.

This

strength to a constant

With

i n c r e a s e s t o a new

l i g h t / o x y g e n exposure shows a

A

t u r n e d o f f , t h e s i g n a l decays

a

15

to

Q

etc.

Thus, each

light

The

r a i s e s the EPR

sig

F i g u r e 5, as g i v e n by second

level

the

l i g h t exposure, the

i n F i g u r e 5.

causes a r i s e t o B^ and decays t o C^. C

as shown i n F i g u r e 5.

in

level,

radical

exposed t o oxygen a r e

dark immediately

level,

i s o t h e r m e q u a t i o n above.

i n c r e a s e i n the

films

i r r e v e r s i b l e component,

oxygen exposure i n t h e

signal

337

.

When the l i g h t i s

A second l i g h t p u l s e

A third

l i g h t pulse gives

p u l s e produces a r e v e r s i b l e

B^,

increase,B.-

C , and an i r r e v e r s i b l e i n c r e a s e . C C . W i t h i n c r e a s i n g i , the ι i+l j ' C C. 0 w h i l e t h e B.-C r i s e t o a maximum, i+l i l l F i g u r e 6 shows the r e s u l t s o b t a i n e d when a 200 ms l i g h t p u l s e


3

was

a p p l i e d t o a sample

data f o r the r i s e and

of

MgTPP

decay

u s i n g a program developed by

to

In every case

in

noise r a t i o greater

than

w i t h two e x p o n e n t i a l s .

The

obtained

u s i n g data summed

cular film. tial case,

in

returned

a

33

double

maximized.

(31).

The

The

decays

program chose

which t h e computer c a l c u l a t e d a s i g n a l 10,

the

best from

computer r e t u r n e d a b e s t f i t

signal many

ms.

to

n o i s e r a t i o , 27,

e x p e r i m e n t s u s i n g one

was

parti

i n t h i s c a s e , a double exponen

Also,

exponential

r e s u l t s a r e not presented

was

the curves u t i l i z i n g a best f i t

The computer r e t u r n e d ,

decay of 273 and

Β^ —C^

were f i t t o e x p o n e n t i a l

Provencher

t h e number of components p r e s e n t criteria.

after

curves

because

the r i s e time f i t s , best

the

f i t .

i n each

[The r i s e t i m e

l i f e t i m e determined by

this

method r e q u i r e s t h a t t h e e x a c t e x c i t a t i o n r a t e s be known.] Using the

lifetime

data

obtained

p e r a t u r e s , an A r r h e n i u s p l o t l e a s t squares f i t , the

of

energy

Inr of

from s e v e r a l d i f f e r e n t tem vs.

1/T

was

activation

was

made.

From the

c a l c u l a t e d t o be

_1

~580cm . I n f r a r e d changes.

The

r e g i o n 600 -4000 cm sure times to

white

films

were

using

various

light.

If

oxygen, i n the absence of observed.

observed by FTIR i n the s p e c t r a l p r e s s u r e s of oxygen and

the

l i g h t , no new

However, changes

were

films

s t r o n g a b s o r p t i o n bands were

observed

in

when d i f f e r e n c e a b s o r p t i o n s p e c t r a were used. shift

t h i s spectral

range

There appears a small

i n many of the p o r p h y r i n v i b r a t i o n modes. F i g u r e 7a shows t h e

absorption

cuated MgTPP f i l m , and F i g u r e 7b to

expo

were exposed t o j u s t

spectra

o b t a i n e d from an e v a

shows t h e same f i l m a f t e r exposure

750 t o r r of oxygen and

48

hours

t h a t appear a r e i n d i c a t e d

by

arrows.

of w h i t e

light.

These new

The

new

bands

bands a r e b e t t e r


338


5000


2500

800

400 Oxygen Pressure (torr)

F i g u r e 4 . EPR s i g n a l ( A ) and l e a s t s q u a r e s f i t of MgTPP c a t i o n r a d i c a l peak h e i g h t a s a f u n c t i o n o f oxygen p r e s s u r e . The upper c u r v e shows an e x t e n s i o n o f t h e l e a s t s q u a r e s f i t t o higher p r e s s u r e s . DC o f f s e t has been s u b s t r a c t e d from each p o i n t p r i o r t o l e a s t squares c a l c u l a t i o n .

3 >»

CO J5 to c σ>

oc

OL

juumnm

on

light exposure

off

UJ

TIME

F i g u r e 5. MgTPP c a t i o n radical EPR peak h e i g h t measured w h i l e t h e t h i n f i l m i s exposed t o oxygen and n i n e , 15 second, p u l s e s o f w h i t e Iight.


22.


BURGESS ET AL.

τ

1

1

1

1

1

339

1

1

r


MgTPP

I

1

0

1

0.2

ι

ι

0.4 TIME

ι

ι

ι

1

0.6 [seconds]

1

1

0.8

1.0

F i g u r e 6. EPR s i g n a l s t r e n g t h v s . t i m e f o r a t h i n f i l m MgTPP sample showing t h e l i f e t i m e o f t h e r a d i c a l formed upon exposure t o 200 ms of w h i t e l i g h t . The double exponental decay i s f i t w i t h l i f e t i m e s of 273 and 33 ms.

H 0.15

H 0.3

0.15

1600

1200 800 WAVENUMBERS cm"

600

1

F i g u r e 7. FTIR absorbance s p e c t r a , 600-1600 cm , o f MgTPP t h i n f i l m (a) i n i t i a l e v a c u a t i o n and (b) e v a c u a t e d a f t e r exposure t o 750 t o r r o f oxygen and l i g h t f o r 48 hours.


340


seen i n t h e a b s o r p t i o n d i f f e r e n c e s p e c t r a shown f o r MgTPP i n F i g u r e 8a.

New a b s o r p t i o n peaks appear a t 729, 833^ 982, 1060, 1217, 1269,

and 1537 cm

The new band

a t 1269 cm

i s a s s i g n e d t o t h e I in-

a c t i v e ring-mode f o r t h e c a t i o n s p e c i e s , as r e p o r t e d by Shimomura e t al.

(21) and i s n o t a b l y absent

in

F i g u r e 8b.

s e v e r a l o f t h e same bands a p p e a r i n g The

band expected f o r t h e r a d i c a l c a t i o n o f MgEtio s h o u l d be s i m i l a r

to that reported f o r metallated (21).

There does appear


adsorbed oxygen a l s o

OEP c a t i o n r a d i c a l s , near 1530 cm

t o be a

absorbs

small

in this

d e f i n i t e assignment cannot

be made.

p r o x i m i t y of t h e peaks f o r

t h e MgTPP

cm

T h i s spectrum shows

as t h e MgTPP sample, F i g u r e 8a.

, respectively^

oxygen (1554.7 cm

we

assign

) (32).

peak a t t h i s energy, b u t

energy

range,

However,

such t h a t a

due t o t h e c l o s e

and M g E t i o , a t 1535 and 1560

these

peaks

t o absorbed

molecular

The peaks observed a t 1060 and 1058 cm-1

f o r t h e two complexes, we a s s i g n t o s u p e r o x i d e ,

0^ (1053 cm-1 ( 2 2 ) ) .

We, t e n t a t i v e l y , a s s i g n t h e peaks a t 833 and 827 cm-1 t o a reduced o z o n i d e s p e c i e s (802-831 cm cannot be a s s i g n e d a t t h i s

( 3 3 ) . The o t h e r FTIR a b s o r p t i o n bands time.

Another a b s o r p t i o n t h a t becomes t h i n f i l m s o f MgEtio and MgTPP F i g u r e 9a and 9b.

range o f 6 0 0 - 4 0 0 0 cm V m

instrument;

w i t h exposure o f t h e

A t l e a s t t h r e e , very broad, peaks a r e observed t o

grow w i t h i n c r e a s i n g oxygen peaks j u s t above

apparent

t o oxygen and l i g h t a r e presented i n

and l i g h t exposure i n t h e IR absorbance The two s p e c t r a

and j u s t

a t h i r d broad

below

absorption

1

~2500 cm"" f o r t h e MgTPP t h i n

suggest t h a t there a r e

t h e region

film

band

spanned by t h e FTIR

i sclearly

and a t "2300

observed a t 1

cm" f o r MgEtio,

1

w i t h FWHM -800 cm" . Emission

s p e c t r a l changes.

Figure

from a t h i n f i l m sample o f MgTPP t o oxygen. and 690 nm.

The f i l m e m i s s i o n

10 shows t h e e m i s s i o n

spectrum

and from t h e same f i l m on exposure

consists

o f t h r e e bands a t 618, 666,

The bands a t 618 and 666 nm a r e t h e e x p e c t e d Q(0,0) and

Q(0,1) e m i s s i o n bands and t h e band a t phyrin f i l m s quite generally

show

690 nm i s an i m p u r i t y .

impurity

emissions

(34).

PorI t is

presumed t h a t on e x c i t a t i o n o f t h e f i l m s , t h e e x c i t e d s t a t e can move through t h e s o l i d

and t r a n s f e r

which then e m i t s a s a t r a p .

i t s energy

t o an i m p u r i t y

F i g u r e 10 shows t h a t t h e e m i s s i o n

guest, drops

on exposure t o oxygen, w i t h t h e i m p u r i t y e m i s s i o n d r o p p i n g more than t h e e m i s s i o n from t h e MgTPP.

This

i s c o n s i s t e n t w i t h a quenching

mechanism t h a t we a s c r i b e t o , MgTPP* + MgTPP* — • MgTPP* + MgTPP


(2)


22.

BURGESS ET A L

341


Figure 8 ( l e f t ) . FTIR absorbance difference spectra, 600-2000 cm" , of evacuated MgTPP and MgEtio thin f i l m s , previously exposed to oxygen and white l i g h t . See text f o r peak assignment. Figure 9 (right). FTIR absorbance difference spectra, 600-4000 cm" , of evacuated MgTPP and MgEtio thin f i l m s , previously exposed to oxygen and white l i g h t 1

0

600

675 WAVELENGTH nm

750

Figure 10. Emission spectra, 600-750 nm, obtained from MgTPP t h i n f i l m (a) evacuated and 00 exposed to 150 torr of oxygen. E x c i tation wavelength was 400 nm.


342


Here MgTPP* i s t h e e x c i t e d s t a t e s of porphyrin agents.

singlet.

a r e both

The c a t i o n

oxidizing

i s , of c o u r s e ,

t h e r e f o r e , quenching

a

good

of t h e e x c i t e d

excited electron t o the cation Eq.

I t i s known t h a t t h e e x c i t e d

good

and good

reducing

o x i d i z i n g agent.

state

by

So

t r a n s f e r of t h e

i s q u i t e reasonable.

Quenching by

(2) would presumably lower t h e g u e s t e m i s s i o n more than t h e host

e m i s s i o n because t h e p r o b a b i l i t y of t h e e x c i t a t i o n r e a c h i n g would be decreased w i t h s h o r t e r

a guest

l i f e t i m e o f MgTPP*.

Piscussion


MgTPP.

The most d a t a were t a k e n on MgTPP, and t h e r e s u l t s a r e q u i t e

unambiguous.

On exposure t o oxygen

changes i n a b s o r p t i o n ,

emission,

w i t h t h e change MgTPP

and

FTIR

MgTPP .

l i g h t there are spectral

and EPR t h a t a r e c o n s i s t e n t

S i n c e , presumably, t h i s

reaction

depends on t h e p r e s e n c e o f oxygen, i t i s u s e f u l t o c o n s i d e r

the free

energy changes f o r t h e s e two r e a c t i o n s : dark:

Ρ + 0^

light:

P* • 0

2

+

+

P

• 0~, AF = 1.2 eV

+

P* • Q~, AF = -0.8 eV

Here, Ρ s t a n d s f o r t h e MgTPP state.

The

AF

values

hydrogen o f +0.75

V

ground

a r e based

f o r MgTPP

more n e g a t i v e ,

cell

(II)

state on

and P* f o r i t s e x c i t e d

a half cell

oxidation

p o t e n t i a l of -0.43 V f o r 0^ r e d u c t i o n excited state, the half

(I)

(35).

potential

potential vs.

(15) and a h a l f c e l l We assume t h a t i n i t s

o f MgTPP* s h o u l d be "2 eV

due t o t h e e x c i t a t i o n energy. Thus, dark r e a c t i o n ( I )

i s n o t thermodynamicaI I y a l l o w e d However, we would e x p e c t

while

the light

the light reaction

reaction

(II)

p r o d u c t s of

is.

(II) t o

r e v e r t t o n e u t r a l s p e c i e s by r e v e r s i n g r e a c t i o n ( I ) . Our

observations

on MgTPP

f o l l o w i n g reactions occur;

with

(la), a

t h a t f o l l o w s a Langmuir a d s o r p t i o n a slow

irreversible

dark

oxygen fast

and

isotherm with

reaction;

light

(lib),

that

(lb),

an i r r e v e r s i b l e

( l i a ) , a s t h e i r r e v e r s i b l e l i g h t r e a c t i o n goes o f MgTPP

reaction

pressure;

d r i v e n r e a c t i o n t h a t b u i l d s up MgTPP* t o some s a t u r a t i o n is a r e v e r s i b l e production

show t h e

r e v e r s i b l e dark

light

l e v e l ; and

to saturation,

there

r e v e r t s t o MgTPP w i t h a

I i f e t i m e of

Cation Radical Formation with Exposure of Magnesium Porphyrin Thin

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