metal oxide adsorbates

JournulofMoleculurStructure,175

Elsevier

Science

(1988) 117-122 B.V., Amsterdam - Printed

Publishers

NANOSECOND

TIME-RESOLVED

RELAXATION

FLUORESCENCE

OF AZAAROMATIC

S. Uhl Institut

und

f. Physikal.

117

in The Netherlands

/ METAL

and

Theoret.

STUDIES

ON THE

OXIDE

EXCITED-STATE

ADSORBATES

D. Oelkrug

Chemie

der

Universitat,

D-7400

Tubingen

ABSTRACT Fluorescence studies reveal that Brdnsted-acid as well as Lewis-acid adsorption sites are involved in the interaction of azaaromatic molecules (acridine, benzo{f}quinoline) with metal oxid catalysts (alumina, silica-alumina). Timeresolved investigations indicate that the relaxation of the initially excited (Franck-Condon)-state to the thermally equilibrated excited-state occurs on the nanosecond time-scale even at room-temperature in the case of azaaromatic molecules which are co-ordinated to Lewis-acid surface sites. The slow formation of the equilibrated excited-state2Ls assigned to restrictions of the mobility of the polar constituents (0 /OH-groups) within the adsorption complex.

INTRODUCTION The

great

studied

are

adsorbed

of these

sites

of

surface

by photophysical

es which ties

variety

oxides

paper

(acridine,

report

as well

/ metal

oxide

back

on the

on the

with

as on the

oxide

using

catalysts. nature

of the

thermally

on the

probe

moleculproper-

different

adsorption

surface

/l/.

of azaaromatic

activated

metal

dynamics

nanosecond

be sucessfully

excited-state

at the

interactions

may

aromatic

The

processes

reorganization

adsorbates

catalysts

methods

of the

as on dynamic

to report

benzo{f}quinoline)

silica-alumina) aromatic

surface

as well

we wish

on metal

photochemical

on the

molecules

on metal

In this

and

sites

oxides

of the

molecules (alumina,

excited

aza-

time-scale.

EXPERIMENTAL A SPEX Ortec

fluorolog

electronics;

constants

(non-linear

calculated 'aktiv

neutral',

vacuum

Amsterdam)

+ 10

twice -6

0022-2860/88/$03.50

mbar)

PMT

and

(450

R928)

square

in /2/.

Fa. Merck,

sublimed (2-4

spectrometer

least

as described

Akzo/Ketjen, Aldrich,

222

Hamamatsu

fit)

Thermal

Darmstadt, adsorption

under

reduced

(adsorbed

0 1988 Elsevier

and

used

and

lamp;

for

ns-flashlamp

the

of acridine

Publishers

and

have

1.0 mg

B.V.

spectra

of the metal

silica-alumina

pressure)

PRA

measurements.

time-resolved

activation

amount:

Science

W xenon

was

(25%

have

oxides A1203),

benzo1flquinoline

been

carried

/ g adsorbent).

out

510;

Decay been (alumina Fa. (Fa. under

118 RESULTS

ANU

DISCUSSION

Steady-state Spectra 1

La-band

acridine of the well

excitation

of acridine at ; < 25 000 is strongly

'Lb-band

and

emission

spectra

on alumina and silica-alumina -1 cm indicates that surface

bonded

depends

via

on the

as on the wavenumber

its

nitrogen-atom

activation

of the

are

/3,4/.

temperature

observation

shown

species

(Gem).

The

(T,) The

of acridine

1. The

formed,

in which

spectral

of the

maximum (Gem=

low-activated is found

in Fig.

are

position

adsorbent

of the

20 400

alumina

at < = 28

lLb-band

cm-')

(Ta=

100 cm -'.

This

is assigned

to acridinium

ions

(AH+)

are

which

on

100 'C)

band

protonation

as

formed

cat-

through

by Brdnsted>x acid OH-groups of the surface ). A -1 shoulder appears at G = 27 450 cm At this of the

of acridine

spectral 'Lb-band

excitation

1.

Emission

tion

(right)

acridine. = 28

CI?'

A)

(b),

both:

the

red

tail

of the

dition

to AH+

amount

of this

spectra at Ta= During

(3em= 600

thermal Al-ions

acids

/5/.

kind

species 20 400

'C, while

rated

emission

another

(Gem=

increases

cm-') AH+

dehydroxylation

in a small

the ir-n"-transitions

cm-1

(E;;

'C,

;ex=

surface

of azaaromatic

28

that

on alumina.

According

amount

(b); -1 150 cm ,

cm-'.

adsorbate

temperatures

= 18 000

silica-alumina

indicates,

only

100 'C,

5

This

is formed

is increased.

on the

T_=

(a),

20 400

prominent

formed

of adsorbed

G em=

most

at elevated are

cm-').

species

as Ta

it is the

is observed

(cus-Al-ions)

Since

18 000

of surface

600

excita-

alumina

(a),

Ta=

and

in

spectra

20 450

B) alumina

if the

is measured

(left)

170 cm-'

in ad-

The

to the

relative

excitation

on alumina for

Ta=

600

co-ordinatively which

.

the maximum

is observed

spectrum

Fig.

3

position

act

molecules

treated 'C. unsatu-

as strong are

shifted

Lewisto-

-__--___---_------------which absorbs in the same spectral region as AH+ *) Hydrogen-bonded acridine, as an adsorbate on the oxides used in this study accord/3,4/, can be excluded ing to the complete lack of its characteristic emissionand triplet-tripletabsorption bands which are observed for example on silica /6/.

119 wards

lower

27 450 ions

wavenumbers

bates

are

21 000

surface

emitting

at

spectra

and

lower

with

with

to Lewis-acid

have

obtained

thridine) The

excitation

sites

spectra

acid

is not

sites

with

- at

least

A red-shift

observed.

the

spectra

exponential

on

decay

decay

sites

law

(Fig.

benzo{flquinoline.

part

ber.

A very

the

curves

adsorbed blue

of the

spectra.

the

species

of

alumina.

the

adsor-

(? = max of the

latter

fluorescence which

are

Similar

co-

results

(benzo(flquinoline,

pretreated 'Lb-band

phenan-

spectra,

which are

at Ta=

'C

the

bonded

converted

reveal

are

the

G < 28 000

co-ordinated

however, that

are

600

to wavenumbers

of species

indicates

subsequent

undergoing

of

of the

2).

to Lewis-

red-shifted

fluorescence to cus-Al-ions.

to species

which

as

origiThus,

are

co-

to excitation:

At

adsorbates

The The

short

mean

kinetics time)

describes continuous

Decay decay

decay is the

These

excited

deviate

room-temperature

3 - 40 ns.

times

time

of

systems

have

been

the

decay

relaxation

are

are

with

risetime

to the

of the

where

'f f ~~

of the

times

analyzed

decreasing that

for

in the wavenum-

is resolved of

thermally

the

re-

equililifetime

~~~ = fluorescence by Bakhshiev

of

found

especially

fluorescence

fluorescence

as a product

a single-

of adsorbed

ns are

characteristic

state

to that

first

from

- 6.5

observed

increases

features

decay

of 0.3

significant

close

markedly

the

times

Franck-Condon

on a time-scale

= relaxation

Bakhshiev-e:pression a system

This

species

emission.

initially

excited-state T

emission

observation

of the

excited-state The

range

important

red-edge of

The

- from

cover

lifetime,

of the

formation

silica-alumina

acridine

/lo/.

of AH+

sites,

at

to cus-Al-

of the

as superposition

red-shift

activated

Lb-band

measurements

fluorescence

brated

spectra

amount

molecules

of the

the

of AH+.

in part

to Lewis-acid

Time-resolved

laxation

on stronger

indicates

molecules

ordinated

on the

The

azaaromatic

co-ordinated

to Lewis-acid

increasing

1

the

spectrum

be regarded

bonded /9/.

are

emission

fluorescence

may

on silica-alumina

of acridine.

protonated

The

which The

/7.8/

on alumina.

on alumina

nates

AH+

to an

for other

protonation -1 which cm

compared

are

than

due

to the

They

which

Lewis-acids

molecules

respect

energies

is then

Ta

to

nitrbgen-atom.

/3,4/).

species

ordinated been

their

in H20/H2S04

of AH+

co-ordination

to acridine

via

red-shifted

cm-'

spectra

assigned

1s

cm-'

of the

upon

intensity

an electronic

/ll/.

The

I(G,t) damping

of

120 term

i(t)

and

a spectral

shift

I(;,t) or

in a more

explicable

The

cmax(t)

term

of the tion

R(t)

of the

= G,,,(m)

+ A"vmax * R(t) the

fluorescence

relaxed

Fluorescence

(Ta=

600

'C),

Gem=

(T,=

The

curve

spectral

is found

of

arrows

position

48) whereas at higher

For t = 0.

mark

at 20 000

90 ns after

Fig.

observed

view

decay

on silica-alumina

600 'C).

At this

{see

(a),

Three-dimensional

decay

A
= Gmax(0)

revealed

difference

- Gmax(m).

by the

between

at t = 0 ("v,,,(O)) and

curves,

on alumina

fluorescence

acridine

and kinetics

is the

250

decay

22 470

B) Benzo{f)quinoline -1 23 250 (c) cm .

the

maxima

150

2.

3.

ASmax

* p("v - Gmax(t))

spectral

the

t = m

shift

spectral

(?max(m),

posi-

com-

species).

Fig.

Fig.

relaxation

spectra,

SO

of the

= a'%exp(-T/rf)

describes

fluorescence

pletely

p(;,t):

'* p(;,t)

manner

I(S,t) where

term

= i(t)

Gmax

excitation it i

energies

20 OOOe;b). (Ta=

25

timelns "v = 27 900

600

cm

16 950 'C),

Gem

-1

.

(c),

75

50

A) Acridine 16 250

= 25 900

on

alumina

(d) cm-'.

(a),

25 000

(b),

121 The

fluorescence

both

time

bers

is evident,

and

observed cmax(t) cence

intensity

wavenumber

(Fig.

the

48,

shifts

The

time

time

be used

provide

emission

energy

as a function at

time

..jQ:::::*,*

a.....

.‘-•...

l‘-•-........***_ ‘b

....

i

....

dependence

p(<,t)

follow cay

great

number

sites

which

A

I

I

1

0

I

I

I

100

50

the

150

time

/ ns

150

100

time

Fig.

4.

Acridine

(Ta=

600

'C).

Half

of the

completed (nearly

spectral

within

shift

which

is observed

30 ns on silica-alumina

independent

of Ta)

(Fig.

46,

on the

and within

5).

This

A)

exponential

on

cm-'

silica-alumina

19 690

energy cm-'

cm-'

(a),

(c)

(d). as a function

excitation

(3,,,(t).

time-scale

10 ns on

that

not

/13-15/.

Emission

maxima

about

from

- are

at 22 680

nanosecond

indicates

dif-

that

/12/

(b),

after

leading

with

in solution

functions

of time

/5/

- as predicted

even

16 000

surface

However,

Debye-relation

cm-'

de-

to the

times.

be noted

B) Emission

/ ns

oxides

adsorbates

observed

and 50

be due

on thermally

relaxation

21 000

Gmax(t)

of different

functions

shift

0

may

metal

it should shift

as

exist

to different ferent

fluores-

excited-state

non-exponential

This

is

and

of the

as well

laws.

spectra

4A)

of the

complex

activated

,a ..=.............

(Fig.

kinetics

of

low wavenum-

fluorescence

of p(G,t)

of the

functions

in I(G,t)

of the

Plots the

measure

shift

silica-alumina rise

red-shift

window.

to analyze

a direct

on

3. The

in Fig.

dependent

observation

5) may

which

of acridine

is shown

a clear

during

relaxation.

I(<,t)

the

is

alumina

formation

of

-! 20.4+.

20,2j-.~......“‘_.

l ‘_-•~......._._ rn

20,0

~

.

~

**.....

.

4. b

l

;::-J:;

o~c;J1"308"o~'~~;n~~d

600

19.8

. .. .. .. .. . .

, . ~

C

* . .

19.6i

to 0

‘0

8

I

50

”

”

I””

I ’

150

100

time

/ ns

'C

Emission of time

(c). maxima after

as a function

excitation

(Gmax

ct12.

122 the

thermally

oxides are

than

equilibrated even

< 2 ns)

excited-states

in highly-viscous

/16,17/.

On the

a sub-nanosecond

time-scale

sites

contribute

to the

shift

of cnax(0)

to

sing

Ta

(Fig.

fluorescence complex

spectra

electronic /18/.

As

as T

due

transition

the

on

to Lewis-acid

as well as the max are observed with increa-

contribution

of these

model

site)

is located

excitation

glycerol

in AS

A schematic

of acridine

of the

on metal

for

Ati+ equilibrates

co-ordinated

increase

surface

slower

values

in solution

are

on alumina

growing

to a Lewis-acid

moment

a consequence

The

which

is increased.

a

bonded

to the

like

which

shift.

wavenumbers

distinctly

(corresponding

that

species

spectral

1 ower

5) is then

(acridine

assumption only

develops

solvents

to the

of an adsorption

is shown

in the

electron

species

in Fig.

short

density

6. The

molecular

axis

is increased

at the

//' \

Fig.

'N

1

/OH-groups,

complex

nitrogen

which

atom,

Electronic

tion

/l/ /2/ /3/ /4/ /5/ /6/ /7/ /8/ /9/ /lo/ /ll/ /12/ /13/ /14/ /15/ /16/ /17/ /18/

This

complex

which

the

are

low mobility

the

Al-ion).

within the

displacement

become

(open arrows the

circles: indicate

excited

reorganization

more

positively

0

2-

the

adsorpprocess.

charged.

that

formation of the equilibrat2of 0 -ions or OH-groups of the

at the

oxides

bonded

for

atoms

suggest

on metal

The

important

charges

ionically

complex

displacements

carbon

reasons

positive

observed of the

to be

opposite

involves the

circle:

of the

suggested

as sterical

to compensate

rates

in the

whereas

as well

ed adsorption surface

are

of an adsorption

full

directions

@ tion

Model

6.

carbon

may

atoms.

therefore

surface

find

The

slow

relaxa-

an explanation

O*-/OH-groups.

D. Oelkrug, W. Flemming, R. Fullemann, R. Gunther, W. Honnen, G. Krabichler M. Schafer and S. Uhl, Pure Appl. Chem. 58, (1986). 1207 D.V. O'Connor and D. Phillips, 'Time-correlated Single Photon Counting', Academic Press, London 1984 Z. Elektrochem. 61 (1957), 956 A. Weller, K. Kasama, K. Kikuchi, S. Yamamoto, K. Uji-ie, Y. Nishida and K. Kokobun, J. Phys. Chem. 5 (1981), 1291 H. Knozinger and P. Ratnasamy, Catal. Rev. - Sci. Eng. 17 (1978), 31 0. Oelkruq, S. Uhl. F. Wilkinson and C.J. Willsher, in preparation K. Hensen and W. Sarholz, Theoret. Chim. Acta, 12 (1968), 206 R. Snyder and A.C. Testa, J. Phys. Chem, 88 (1984), 5948 A.V. Karyakin, T.S. Sorokina and M.G. Skvortsov, Opt. Spectrosc. 52 (1982), 26 A. Gafni, R.P. DeToma, R.E. Manrow and L. Brand, Biophys. J. 17 (1977), 155 N.G. Bakhshiev. Yu.T. Mazurenko and I.V. Piterskava, Opt. Spectrosc: 21 (1966). 307 H. Frohlich, TheGy of Dielectrics, Oxford University Press, London, 1949 and G.R. Flemming, E.W. Castner, Jr., M. Maroncelli J. Chem. Phys. 86 (1987), 1090 T.-J. Kang and P.F. Barbara, V. Nagarajan, A.M Brearley, J. Chem. Phys. 86 (1987). 3183 J. Chem. Phys. 86 (1987), 6221 M. Maroncelli and G.R. Flemming. R.P. DeToma, J.H. Easter and L. Brand, J. Am. Chem. Sot. 98 (1976). 5001 E. Bismuto, D.M. Jameson and E. Gratton, J. Am. Chem. Sot. 109 (1987). 2354 J. Bendig. D. Kreysig and A. Kawski. Z. Naturforsch. 36a (1981). 30d

work

was

supported

by

the

Deutsche

Forschungsgemeinschaft.