22 Cation Radical Formation with Exposure of Magnesium Porphyrin Thin Films to Light and Oxygen 1
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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
Cation Radical Formation
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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 -
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
22.
Cation Radical Formation
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
334
PORPHYRINS: EXCITED STATES AND DYNAMICS
PDP 8/e Computer
Shutter Control
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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 .
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
22.
Cation Radical Formation
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.
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
336
PORPHYRINS: EXCITED STATES AND DYNAMICS
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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.
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Cation Radical Formation
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
338
PORPHYRINS: EXCITED STATES AND DYNAMICS
5000
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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.
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
22.
Cation Radical Formation
BURGESS ET AL.
τ
1
1
1
1
1
339
1
1
r
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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.
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
340
PORPHYRINS: EXCITED STATES AND DYNAMICS
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
(2)
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
22.
BURGESS ET A L
341
Cation Radical Formation
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.
Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
342
PORPHYRINS: EXCITED STATES AND DYNAMICS
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
Downloaded by COLUMBIA UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch022
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