• Keine Ergebnisse gefunden

An excited chromophoric group of a complex can undergo an electron t r a n s f e r to or from another part of the same complex

N/A
N/A
Protected

Academic year: 2022

Aktie "An excited chromophoric group of a complex can undergo an electron t r a n s f e r to or from another part of the same complex"

Copied!
7
0
0

Wird geladen.... (Jetzt Volltext ansehen)

Volltext

(1)

INTRAMOLECULAR EXCITED STATE ELECTRON TRANSFER FROM NAPHTHALENE TO COBALT(III)

A.H.Osman and A.Vogler

I n s t i t u t für Anorganische Chemie der Universität Regensburg, Universitä'tsstr. 31, 8400 Regensburg, FRG

Introduction

The majority of intramolecular photoredox processes of metal complexes which 1 2)

have been reported * ' takes place upon d i r e c t o p t i c a l charge t r a n s f e r (CT) e x c i t a t i o n . As an a l t e r n a t i v e intramolecular photoredox processes may occur by an excited state electron t r a n s f e r . An excited chromophoric group of a complex can undergo an electron t r a n s f e r to or from another part of the same complex. While i n intermolecular photoredox processes the s t r u c t u r a l arrangement of donor and accep- tor in the encounter p a i r i s not known intramolecular electron t r a n s f e r occurs in a better defined environment. Although these features make i t a t t r a c t i v e to study intramolecular excited state electron t r a n s f e r t h i s subject has been l a r g e l y neglected u n t i l a few years ago.

The recent i n t e r e s t i n intramolecular excited state electron t r a n s f e r i s asso- ciated with attempts to understand the primary events of photosynthesis and to design model systems f o r the natural and an a r t i f i c i a l photosynthesis. In the f i r s t step an excited state u p h i l l electron t r a n s f e r i s required in order to convert l i g h t into chemical energy. In simple systems t h i s f i r s t step i s followed by a rapid downhill charge recombination. In the photosynthesis a charge separation i s achieved by introducing a b a r r i e r f o r back electron t r a n s f e r . Recently model com- pounds have been designed to study the charge separation i n d e t a i l . A system which found much attention consists of a porphyrin as excited state electron donor which is linked covalently to a quinone as electron acceptor. In addition, a carotene may

3) be attached as a donor to accomplish charge separation over large distances .

T. J . Meyer and his research group have investigated the light-induced charge separation i n compounds which contain metal complexes as i n i t i a l l y excited chromo- phores 4)^ I n t n e s e c a s e s the charge recombination regenerated the s t a r t i n g com-

Photochemistry and Photophysics

of Coordination Compounds, Ed. by H. Yersin/A. Vogler

© Springer-Verlag Berlin • Heidelberg 1987

(2)

pounds. Under s u i t a b l e conditions another secondary reaction may be rapid enough to compete with the charge recombination. As a r e s u l t stable photoproducts can be formed.

In 1969 Adamson et a l . studied a photoreaction of t h i s type '. Upon i n t r a - ligand (IL) e x c i t a t i o n of [ C oI H( N H3)5T S C ]2 + with TSC" = trans-4-stilbene

carboxylate the excited TSC-ligand t r a n s f e r s an electron to Co(III) The Co(II) releases i t s ligands before an e f f i c i e n t charge recombination takes place. A v a r i e t y of other complexes of the type [ C oI I I( N H3)50 0 C R ]2 + with R = aromatic group

7 8) such as naphthyl shows q u a l i t a t i v e l y the same behavior as the TSC complex ' Excited state electron t r a n s f e r from aromatic molecules to Co(111) ammines takes

8 9)

place also as an intermolecular reaction * . F i r s t observations were explained by the assumption that an energy t r a n s f e r occurs to reactive CT states of the complex 9)

However, more recent investigations have shown that a l l r e s u l t s can be 6 ft)

explained best by an excited state electron t r a n s f e r mechanism

In the present study the complexes [ 2 - n a p h t h y l - C 0 N H - ( C H2)n- C 0 0 C oI H( N H3)5]2 + with n = 1 to 5 were investigated in order to learn more about the s t r u c t u r a l requirements f o r excited state electron t r a n s f e r in t h i s system.

Results and Discussion Synthesis

The free ligands were synthesized by the reaction of 2-naphthoic acid and the benzyl esters of the amino acids:

2-naphthyl-C00H + NH2-(CH2)n-C00CH2-C6H5 - 2-naphthyl-C0-NH-(CH2)n-C00-CH2-C6H5 + H20

Saponification yielded the protonated ligands which were converted by NaOH to the sodium s a l t s 2-naphthyl-C0-NH-(CH2)n-C00~Na+. The complexes [2-naphthyl- C0NH-(CH2)n-C00Co(NH3)5l2 + were obtained as Perchlorates by the reaction of [Co(NH3)^H20](C10^)3 and the sodium s a l t s of the ligands. R e c r y s t a l l i z a t i o n from acetone yielded a n a l y t i c a l l y pure compounds.

(3)

Absorption Spectra

The e l e c t r o n i c spectra of the sodium s a l t s of the aqueous free ligands 2-naph- thyl-C0-NH-(CH2)n-C00"Na+ show two absorption bands at Xm a x = 310 nm and \m a x

= 317 nm. Both bands which are of nearly the same i n t e n s i t y (e » 1200 L mol"1 cm"1) are assigned to *n* t r a n s i t i o n s of the naphthyl group. In the complex cations C2-naphthyl-C0-NH-(CH2)n-C00Co(NH3)5]2 + these i n t r a l i g a n d (IL) bands appear

with almost the same position and i n t e n s i t y . These r e s u l t s show unambiguously that the naphthaline moiety i s an isolated- chromophoric group of these complexes since coordination does not change the absorption spectrum of the free ligands. This observation i s c e r t a i n l y not surprising because the aromatic ^-electron system i s separated by the saturated methylene groups (n = 1 to 5) from the Co^+ ion. In addition to the IL bands the f i r s t ligand f i e l d band of the complexes appears at

xm , v = 504 nm (e =85).

max

Emission Spectra

Light absorption of the free ligands (x e x c = 310 nm) i s accompanied by an intense fluorescence (x m a x = 354 nm) which originates from the lowest-energy ***

s i n g l e t of the naphthyl group. The l i f e t i m e was not measured but i s known to be approximately 10~8 s f o r related naphthaline d e r i v a t i v e s 1 0^ . This emission i s largely but not completely quenched in the complexes. The integrated fluorescence i n t e n s i t y was reduced to 2.00 % (n = 1), 1.75 % (n = 2), 1.48 % (n = 3), 1.07 % (n = 4), and 1.62 % (n = 5).

Photochemistry

Upon l i g h t absorption by the IL bands (x e x c = 333 nm) the aquepus complexes underwent a photoredox reaction. While Co(111) was reduced to C o2 + the oxidation products were not i d e n t i f i e d . In analogy to related cases 6"8^ i t i s assumed that the naphthalene ligand was oxidized. The quantum y i e l d of C o2 + formation was dependent on n: * = 0.084 (n = 1), 0.072 (n = 2), 0.034 (n = 3), 0.024 (n = 4 ) , and

(4)

-2 -3

0.041 (n = 5). In the concentration range of 10 to 10 M complex the quantum y i e l d s were constant. I t follows that under these conditions the photoredox reac- t i o n i s c e r t a i n l y an i n t r a - and not an intermolecular process.

Mechanism

Naphthalene i s oxidized at E ^2 = 1-7 2 v v s SCE 1 1 \ At an e x c i t a t i o n energy 12^

of 3.97 eV ' the *** s i n g l e t i s now strongly reducing ( E ^2 = -2.25 V). A l - though these parameters are c e r t a i n l y somewhat d i f f e r e n t from those of the ligands 2-naphthyl-C0-NH(CH2)2-C00" there i s no doubt that f o r the complexes there i s a

large d r i v i n g force f o r an electron t r a n s f e r from the excited IL 1111 * s i n g l e t to the 13)

Co(III) center. S i m i l a r Co(III) complexes are reduced at E° = +0.06 V . Fluo- rescence quenching and formation of C o2 + can then be described by the following reaction scheme (Nap = 2-naphthyl group, B = -C0-NH-(CH2)2-C00- peptide bridge, A = ammonia):

[ N a p - B - C oH IA5]2 + [ N a p * - B - C oI HA5]2 +

k

[ N a p * - B - C oI HA5]2 + - J * [ N a p - B - C oH IA5]2 + + hv

[ N a p * - B - C oi nA5]2 + - 2 * [ N a p - B - C oI HA5]2 + + heat

[ N a p * - B - C oH IA5]2 + — 2 * [ N a p+- B - C oHA5]2 + [ N a p+- B - C oHA5]2 + -X [ N a p - B - C omA5]2 +

k

[ N a p+- B - C oHA5] — ^ C o2 + + 5NH3 + oxidized Nap-B

On the basis of t h i s reaction scheme k i n e t i c equations can be derived:

= 1+ — = 1 + k~ • T

P 3 o

*Co(III) K1 + k2

£ and c0(I I I) a r e t h e fluorescence i n t e n s i t i e s of the free and coordinated

ligands. xQ i s the l i f e t i m e of the *** s i n g l e t of the free ligand which was assumed

(5)

o

to be 10" s (see above). The e f f i c i e n c y of electron t r a n s f e r (ET) from the excited IL s i n g l e t to Co(III) i s then given by:

ET

.2+

k3 + T -

The quantum y i e l d of Co formation i s not only determined by but also by rate constants of back electron t r a n s f e r ( k4) and of the decay of the Co(II) complex ( k ^ ) .

»ET k5

k5 + k4

The rate constant k5 i s not known but i s assumed to be larger than 106 s"1 1 4^ . It follows that the rate constants k^ f o r back electron t r a n s f e r can also not be obtained. However, r e l a t i v e rates k^' were calculated assuming k^ to be constant:

4 k5 * C o2 +

Table 1.

Rate constants kg and quantum y i e l d s of excited state electron t r a n s f e r , and r e l a t i v e rate constants k^' of back electron t r a n s f e r f o r [2-naphthyl-C0-NH-

( C H2)2- C 0 0 C o ( N H3)5]2 +.

n k3 x 10~9

s-1

ET K4

1 4.9 0.980 11

2 5.6 0.982 13

3 6.6 0.985 28

4 9.2 0.989 40

5 6.0 0.983 23

(6)

In contrast to the expectation i t was found (Table 1) that the rate constant and e f f i c i e n c y of excited state electron t r a n s f e r as well as the rate of back electron t r a n s f e r drops from n = 1 t o 4. This observation suggests that the actual distance between the naphthyl group and Co(III) decreases with increasing chain length of the peptide from n = 1 t o 4. I t i s assumed that donor and acceptor come to a c l o s e r approach by an appropriate bending of the f l e x i b l e peptide linkage. This back bonding may be favored by hydrogen bonding between coordinated ammonia and the carbonyl

groups of the peptide. At n = 5 electron t r a n s f e r becomes less e f f i c i e n t (Table 1).

The donor-acceptor distance may now increase be an extension of the peptide.

1) Adamson, A. W.; Fleischauer, P. D. (Eds.) Concepts of Inorganic Photochemistry, Wiley, New York, 1975.

2) B a l z a n i , V.; C a r a s s i t i , V. Photochemistry of Coordination Compounds, Academic Press, New York, 1970.

3) Gust, D.; Moore, T. A.; L i d d e l l , P. A.; Nemeth, G. A.; Making, L. R.; Moore, A.

L.; B a r r e t t , D.; P e s s i k i , P. J . ; Bensasson, R. V.; Rougee, M.; Chachaty, C ; De Schryver, F. D.; Van der Auweraer, M.; Holzwarth, A. R.; Conolly, J . S. J . Am.

Chem. Soc. 1987, 109, 846 and r e f . c i t e d t h e r e i n .

4) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon, D.; Neveux, P. E.; Meyer, T. J . Inorg. Chem. 1987, 26, 1116 and r e f . c i t e d t h e r e i n .

5) Adamson, A. W.; Vogler, A.; Lantzke, I. ""J. Phys. Chem. 1969, 73, 4183.

6) Vogler, A.; Kern, A. Z. Naturforsch. 1979, 34b, 271.

7) Kern A. D i s s e r t a t i o n , Universität Regensburg, 1978.

8) Schaff1, S. Diplomarbeit, Universität Regensburg, 1984.

9) Scandola, M. A.; Scandola, F.; C a r a s s i t i , V. Mol. Photochem. 1969, 1, 403.

H H

References

(7)

10) Berlman, I . B. Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971.

11) Eberson, L.; Nyberg, K. J . Am. Chem. Soc. 1966, 88, 1686.

12) B i r k s , J . B. Photophysics of Aromatic Molecules,~¥iley, London, 1970.

13) Milazzo, G.; C a r o l i , S. Tables of Standard Electrode P o t e n t i a l s , Wiley, New York, 1978.

14) Simic, M.; L i l i e , J . J . Am. Chem. Soc. 1974, 96, 291.

Referenzen

ÄHNLICHE DOKUMENTE

Previous research indicates that benefits of sprouting may be negated by net DM loss from sprouting coupled with no significant improvement in nutrient concentrations or

Any combination of two out of the three pathways of electron delocalization in such systems (diagonal, lateral, or cross conjugation,; see Chart S1 of the Supporting Information)

The main features of this diode-rf combination are: a high peak gradient in the diode (up to 100 MV=m) obtained without breakdown conditioning, a cathode shape providing

Abs polymer f indigo R ð3Þ where DOD polymer is the change in the optical density (DOD) of the polymer solution with the irradiation time, DOD indigo is the change in the

cells (preferred direction on the x -axis) depends on the angular distance be- tween preferred and attended (here: upwards) direction (figure reprinted from [Martínez-Trujillo

Chlorophyll a fluorescence is a widely used probe of photosynthetic activity (specifically PSII), and therefore stress which specifically targets the

This article looks at some interesting conclusions drawn from a recent study published by Ducruet and Notteboom (2010) 1 , who examined the main features of the World

Structures, total energy differences (DE tot , with respect to the isomer yielding best agreement with experiment) and electron affinities (EA) for those isomers are given as