Resonance Raman Investigation of Transient Photoinduced Ligation

18 Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 16, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch018

Resonance Raman Investigation of Transient Photoinduced Ligation Changes in Nickel Porphyrin 1

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E. W. Findsen , M. R. Ondrias , J. A. Shelnutt , and J. M. Friedman 1

Department of Chemistry, University of New Mexico, Albuquerque, NM 87131 Sandia National Laboratories, Albuquerque, NM 87185 AT&T Bell Laboratories, Murray Hill, NJ 07974

2 3

The first picosecond and nanosecond time-resolved Raman spectra of photoexcited metalloporphyrins in coordinating and noncoordinating solvents are presented. Our data demonstrate that resonance Raman spectra can be obtained from transient nickel porphyrin species and confirm the existence of photoinduced ligation changes in nickel proto and octaethyl porphyrin species on a subnanosecond timescale. The m e t a l l o p o r p h y r i n s form a d i v e r s e c l a s s o f m o l e c u l e s exhibiting complex and v a r i e d p h o t o c h e m i s t r i e s . U n t i l recently time-resolved a b s o r p t i o n and f l u o r e s c e n c e s p e c t r o s c o p i e s were t h e o n l y methods u s e d t o s t u d y m e t a l l o p o r p h y r i n e x c i t e d s t a t e r e l a x a t i o n i n a subm i c r o s e c o n d regime. I n t h i s p a p e r we p r e s e n t t h e f i r s t p i c o s e c o n d t i m e - r e s o l v e d r e s o n a n c e Raman s p e c t r a o f e x c i t e d s t a t e m e t a l l o p o r phyrins outside of a p r o t e i n matrix. The i n h e r e n t molecular s p e c i f i c i t y o f r e s o n a n c e Raman s c a t t e r i n g p r o v i d e s f o r a d i r e c t probe o f bond strengths, geometries, and l i g a t i o n states of photoexcited metalloporphyrins. I n t h e p r e s e n t i n v e s t i g a t i o n we u s e t r a n s i e n t Raman s p e c t r o s c o p i c t e c h n i q u e s t o s t u d y t h e p i c o s e c o n d and nanosecond l i g a t i o n p r o c e s s e s i n i t i a t e d by d i r e c t e l e c t r o n i c e x c i t a t i o n o f n i c k e l o c t a e t h y l p o r p h y r i n ( N i ( O E P ) ) and n i c k e l p r o t o p o r p h y r i n IX ( N i ( P P ) ) . Similar time-resolved r e s o n a n c e Raman s t u d i e s have s u c c e s s f u l l y p r o b e d t h e dynamic p r o c e s s e s i n v o l v e d i n t h e p h o t o l y s i s and subsequent r e c o m b i n a t i o n o f exogenous l i g a n d s i n heme p r o t e i n systems s u c h as h e m o g l o b i n and m y o g l o b i n (1.2). Transient species of Ni(OEP) and N i ( P P ) formed by p h o t o e x c i t a t i o n i n c o o r d i n a t i n g and noncoordinating solvents have been probed with ~25-psec and ~10-nsec t i m e r e s o l u t i o n and t h e r e s u l t i n g s p e c t r a a r e compared t o those o f e q u i l i b r i u m species. Recently the excited state k i n e t i c s i n e a c h o f t h e s e t i m e regimes was c h a r a c t e r i z e d by s e v e r a l s t u d i e s u s i n g t r a n s i e n t absorption spectroscopy (3-5). Our r e s u l t s a r e i n g e n e r a l agreement w i t h t h e m o l e c u l a r mechanisms i n f e r r e d from t h e s e previous studies, but reveal well resolved v i b r a t i o n a l information 0097-6156/ 86/ 0321 -0266506.00/ 0 © 1986 American Chemical Society

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

FINDSEN E T A L .

18.

Transient Photoinduced Ligation Changes

about t h e e x c i t e d e l e c t r o n i c s t a t e s and l i g a t i o n b e h a v i o r transient intermediates created a f t e r photoexcitation.

267 of the

M a t e r i a l s and Methods A l l Ni(OEP) and N i ( P P ) samples ( o b t a i n e d from P o r p h y r i n P r o d u c t s ) were p r e p a r e d (0.2-0.5 mM) i n n e a t , s p e c t r a l g r a d e s o l v e n t (used w i t h o u t f u r t h e r p u r i f i c a t i o n ) and were deoxygenated b y p u r g i n g w i t h oxygen-free N g a s . No d i f f e r e n c e s i n t h e s p e c t r a were n o t e d f o r a e r o b i c and a n a e r o b i c samples. The p i c o s e c o n d spectrum was g e n e r a t e d u s i n g a Raman s h i f t c e l l ( H , 300 p s i ) w h i c h was pumped w i t h t h e second Harmonic o u t p u t o f a Q u a n t e l a c t i v e / p a s s i v e modelocked p i c o s e c o n d l a s e r . The f i r s t a n t i s t o k e s l i n e (-436 nm) from t h e s h i f t c e l l was t h e n used t o e x c i t e t h e sample and g e n e r a t e resonance Raman s p e c t r a o f t h e s p e c i e s g e n e r a t e d w i t h i n 25 p s e c o f e x c i t a t i o n ( p u l s e w i d t h measured by a u t o c o r r e l a t i o n two p h o t o n measurements). The nanosecond s p e c t r a were o b t a i n e d u s i n g a n i t r o g e n pumped dye l a s e r ( M o l e c t r o n ) (10-nsec n o m i n a l p u l s e w i d t h ) and a s p e c t r o m e t e r system d e s c r i b e d e l s e w h e r e (6). The p i c o s e c o n d Raman d a t a was c o l l e c t e d u s i n g a 90° s a m p l i n g geometry and a s p h e r i c a l l e n s t o f o c u s t h e l a s e r i n t o t h e sample w h i l e t h e nanosecond s p e c t r a were o b t a i n e d u s i n g an ~180° b a c k s c a t t e r i n g geometry. The average power f o r t h e p i c o s e c o n d s p e c t r a was ~15 mw a t 10 Hz (1.5 m J / p u l s e ) . "Low power** nanosecond e x p e r i m e n t s at 406 and 440 nm were performed with the laser pulses (~.3 m J / p u l s e ) s l i g h t l y f o c u s e d w i t h a c y l i n d r i c a l l e n s r e s u l t i n g i n an energy d e n s i t y o f ~5 mJ/cm p u l s e . F o r " h i g h power** e x p e r i m e n t s t h e beam was t i g h t l y f o c u s e d w i t h a s p h e r i c a l l e n s . We e s t i m a t e t h e p h o t o n d e n s i t y a t t h e sample t o be a p p r o x i m a t e l y 100 t i m e s g r e a t e r i n t h e " h i g h power** t h a n i n t h e **low power** e x p e r i m e n t s . A l l t i m e - r e s o l v e d s p e c t r a were o b t a i n e d from samples p l a c e d i n a s t a t i c cuvette. The l a s e r r e p e t i t i o n r a t e i n a l l t r a n s i e n t e x p e r i ments was f i x e d a t 10 Hz. Sample t e m p e r a t u r e s i n a l l c a s e s were between 20-25°C. The e q u i l i b r i u m s p e c t r a were r e c o r d e d from samples i n a r a p i d l y r o t a t i n g c e l l u s i n g a Raman d i f f e r e n c e spectrometer (7.) and 413.1 nm o r 530.9 nm e x c i t a t i o n from a k r y p t o n i o n l a s e r o r 441.6 nm e x c i t a t i o n from a He-Cd l a s e r . A b s o r p t i o n s p e c t r a were r e c o r d e d a f t e r e a c h experiment t o m o n i t o r sample i n t e g r i t y .

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 16, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch018

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R e s u l t s and D i s c u s s i o n The s p e c t r a p r e s e n t e d i n F i g u r e s 1-3 d e m o n s t r a t e t h a t h i g h q u a l i t y , t r a n s i e n t r e s o n a n c e Raman s p e c t r a c a n be o b t a i n e d f o r Ni(OEP) and Ni(PP) s o l u t i o n s using Soret e x c i t a t i o n . These s p e c t r a c a n be i n t e r p r e t e d on a m o l e c u l a r l e v e l b y comparison w i t h t h e e x t e n s i v e t h e o r e t i c a l and e x p e r i m e n t a l d a t a b a s e t h a t e x i s t s f o r ground s t a t e n i c k e l p o r p h y r i n s p e c i e s (8-16 and r e f s . t h e r e i n ) . The c o o r d i n a t i o n s t a t e o f n i c k e l p o r p h y r i n s c a n e a s i l y be d e t e r m i n e d from t h e r e s o n ance Raman spectrum o f t h e sample (10.12). S e v e r a l modes i n t h e Raman spectrum o f p o r p h y r i n s a r e q u i t e s e n s i t i v e t o t h e s t a t e o f axial ligation (10.12). In particular, t h e marker lines ν+ ll» 2» 3» * 10 (porphyrin skeletal mode d e s i g n a t i o n s f o l l o w t h o s e o f Abe e t a l . , ( 1 2 a ) . The d e s i g n a t i o n 9

v

v

v

a n


>1586

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

268

1 1400 Raman

ι . 1500 Shift

Λ

—

S

1600

1700

(cm"" ) 1

Figure 1. Resonance Raman spectra of photoexcited and e q u i l i b r i u m Ni(OEP) i n n e a t p i p e r i d i n e o b t a i n e d w i t h 440 nm excitation. T r a c e a) d i s p l a y s the s p e c t r u m o b t a i n e d w i t h i n 25 psec of e x c i t a t i o n . T r a c e s b) and c) were o b t a i n e d w i t h 10 n s e c e x c i t a t i o n under h i g h and low power c o n d i t i o n s , r e s p e c t i v e l y (see t e x t ) . The s p e c t r u m o f the e q u i l i b r i u m sample i s shown i n t r a c e d) and was o b t a i n e d w i t h 441.6 nm e x c i t a t i o n (~40 mW) from in a l l a cw He-Cd laser. Spectra bandpass was 7-9 cases. Time-resolved s p e c t r a a r e t h e unsmoothed sum of 3-5 scans. S o l v e n t bands a r e denoted by a s t e r i s k s .


18.

FINDSEN ET A L .


269


α

I

,

ι

1300

•

1400 Raman

•

,

1500

ι

.

1600

Shift

I

1700

(cm" ) 1

Figure 2. T r a n s i e n t Raman s p e c t r a o f N i ( P P ) (60 80 μΜ) i n pyrrolidine. Traces a) and b) were obtained with 406 nm e x c i t a t i o n a t h i g h and low power, r e s p e c t i v e l y , w h i l e t r a c e s c ) and d) were g e n e r a t e d w i t h 420 nm e x c i t a t i o n (which is a compromise f r e q u e n c y which r e s o n a n t l y enhances b o t h s p e c i e s t o some e x t e n t ) a t h i g h and low power, r e s p e c t i v e l y . F o r low power s p e c t r a t h e average l a s e r power ( a t 10 Hz) was .75-1.0 mW. The beam was only slightly focused onto t h e sample with a c y l i n d r i c a l lens. H i g h power s p e c t r a were g e n e r a t e d w i t h 5-6 mW of average l a s e r power s h a r p l y f o c u s e d a t t h e sample v i a a spherical lens. S p e c t r a a r e t h e unsmoothed sum o f 3-5 scans a t 7-9 cm" - s p e c t r a l r e s o l u t i o n . 1

I 1300

ι

I 1400

Raman

ι

I 1500

Shift

ι

I

1 1

1600 Ccm

1

1700 )

F i g u r e 3. T r a n s i e n t Raman s p e c t r a o f N i ( P P ) i n acetone/water (10% water b y volume). T r a c e s a) and b) were o b t a i n e d under h i g h and low power c o n d i t i o n s , r e s p e c t i v e l y , s i m i l a r t o t h o s e d e s c r i b e d f o r F i g u r e 2. These s p e c t r a a r e r e p r e s e n t a t i v e o f t h o s e o b t a i n e d i n n o n c o o r d i n a t i n g s o v e n t systems a t h i g h and low power.


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

270

DYNAMICS

n


\>k r e f e r s t o t h e k t h n o r m a l mode i n t h e n t h a x i a l Ligation s t a t e . ) show c o r r e l a t e d s h i f t s i n f r e q u e n c y w i t h l i g a t i o n changes (10-12). F o r example, Ni(OEP) i n piperidine at equilibrium e x h i b i t s t h e s p e c t r u m shown i n F i g u r e I d . Two s e t s o f marker l i n e s are observed i n t h e e q u i l i b r i u m p i p e r i d i n e s o l u t i o n ; the s e t with low f r e q u e n c i e s o f t h e s e Raman l i n e s ( s e e T a b l e 1) a r e i n d i c a t i v e o f a 6 c o o r d i n a t e Ni(OEP) l i g a t i o n s t a t e . These l i n e s appear 10-40 cur* higher i n f r e q u e n c y f o r Ni(OEP) i n n o n c o o r d i n a t i n g solvents (4-coordination) and a r e a l s o significantly higher than f o r

5- c o o r d i n a t e n i c k e l porphyrins ( 1 0 ) . Comparison w i t h t h e Raman spectra o f Ni(OEP) i n n o n c o o r d i n a t i n g solvents shows t h a t t h e majority of molecules i n the piperidine solution are i n the 6- c o o r d i n a t e form. I n T a b l e 1, t h e f r e q u e n c i e s o f v a r i o u s l i n e s shown t o be s e n s i t i v e t o t h e p o r p h y r i n ' s s t r u c t u r e (8,10,15), a r e g i v e n f o r N i ( O E P ) and N i ( P P ) complexes i n s e v e r a l time regimes and f o r s e v e r a l e x c i t a t i o n wavelengths. As d e s c r i b e d below, t h e g e n e r a l r e s u l t s o f t h e t i m e - r e s o l v e d s p e c t r a c a n be summarized as f o l l o w s : 1. I n c o o r d i n a t i n g solvents qualitative indications o f an increase i n f o r m a t i o n o f A- c o o r d i n a t e Ni(OEP) a r e o b s e r v e d in the behavior o f \>IQ * V3 within 20-30 p s e c o f e x c i t a t i o n a t h i g h power, 2. H i g h power n s e c p u l s e s a l s o c l e a r l y p r o d u c e an i n c r e a s e i n the amount o f A c o o r d i n a t e n i c k e l porphyrin relative to e i t h e r low power nsec p u l s e s o r an e x c i t a t i o n o f t h e ground state equilibrium as e v i d e n c e d by t h e b e h a v i o r o f v , and » , 3. I n n o n c o o r d i n a t i n g solvent systems (toluene, methylene chloride, lÔ/Acetone, CTAB) power dependent s h i f t s are observed for v^, V3, λ>2» 10» positions n e a r t h o s e o b s e r v e d f o r Ni(OEP) and N i ( P P ) i n c o o r d i n a t i n g solvents at equilibrium. a n
75%). I t was c o n c l u d e d t h a t a change o f ligation to predominantly a 6-coordinate system with solvent m o l e c u l e s as t h e a x i a l l i g a n d s o c c u r r e d a f t e r e x c i t a t i o n f r o m t h e ground state Â^g to the Blg excited state. The Âê -» " * l g e x c i t a t i o n i s a d-d t r a n s i t i o n i n v o l v i n g p r o m o t i o n o f an e l e c t r o n from t h e d o u b l y o c c u p i e d d 2 o r b i t a l t o t h e empty ^x -y orbital of the metal. The transition i s not dipole-allowed but occurs upon de e x c i t a t i o n of the allowed ΤΓ-+ΤΓ* excitation of the macrocycle (16). In the solvents p y r r o l i d i n e and p i p e r i d i n e , however, t h e o p p o s i t e s i t u a t i o n e x i s t s . The ground state species i s 6-coordinate ^ l g t and upon promotion t o t h e Â^- e x c i t e d state deligation occurs. Kim e t a l . (3.4) have e x p l a i n e d t h e e x c i t e d - s t a t e p r o c e s s e s on t h e basis o f cf-donation t o t h e metal d 2 o r b i t a l by t h e l i g a n d s . Thus an excited ^ l g state (d§2) is a ligand d i s s o c i a t i v e s t a t e because t h e a c c e p t o r o r b i t a l o f the metal i s 3

B

z

2

2

z

A


18.

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FINDSEN ET A L .

Table 1


H i g h F r e q u e n c y Resonance Raman Bands o f Ni(OEP) in Piperidine

440 nm 436 nm 25 p s e c

> 4

W

1654

1655 -1610 ( s h )

10

'\ · \

A

2

6

10 n s e c h i g h power

1586

1585

10 n s e c low power

1654 -1610 ( s h ) 1586

441.6 nm equilibrium

1655 1610 V1587

11 -1565 ( s h )

-1565 ( s h )

-1565 ( s h )

-1565 ( s h )

-1515

1518

-1515

1517

1370

1368

1368

13 70

Ni(PP) i n P y r r o l i d i n e H i g h f r e q u e n c y Raman bands (cnr *•) 406 nm Low Power H i g h Power

6

420 nm Low Power H i g h Power

V4 1368

1370

1368

1370

1480

1488

1480

1484

*»A 6

.'»

1521

1520 1565

1565

1569

1594

1594

1620

1620

1620

1565

2

4 w

. 2

6

Λ° 4 V

1620

1658

1658

10 C o n t i n u e d on n e x t page




272

Table I. Continued

Ni(PP) i n H 0/Acetone 2

406 nm e x c i t a t i o n Low Power

H i g h Power

1382 13/5 1522

1522 1494

1592

1594 1577

10

1662

1658 1628

'10


FINDSEN ET A L .

18.


273

completely f i l l e d . On t h e o t h e r hand, l i g a n d b o n d i n g i s f a v o r e d by ^-Big and lg ( z » x -y ^ excited states t h a t have a h a l f f i l l e d d 2 orbital that i s f r e e to accept charge from a f i l l e d l i g a n d o r b i t a l . Intersystem c r o s s i n g occurs r e a d i l y i n n i c k e l p o r p h y r i n systems and r a p i d r e l a x a t i o n back t o the ground e l e c t r o n i c s t a t e f o r noncoordinating solvents, 3 f i

d

2

d

2

2


z

•*Bi£ for strongly coordinating solvents) occurs following e x c i t a t i o n (3). A x i a l l i g a n d s added ( o r l o s t ) i n t h e e x c i t e d s t a t e are, subsequently, lost (or regained) upon return to the e q u i l i b r i u m ground s t a t e . W i t h i n -25 p s e c o f e l e c t r o n i c e x c i t a t i o n t h e r e s o n a n c e Raman s p e c t r u m o f Ni(OEP) i n p i p e r i d i n e i n d i c a t e s an i n c r e a s e i n t h e amount o f 4 - c o o r d i n a t e Ni(OEP) and l o s s o f t h e p i p e r i d i n e l i g a n d s . E v i d e n c e f o r t h i s i s found i n t h e i n c r e a s e i n i n t e n s i t y o f the *\>\0 band for 4-coordinate Ni(OEP) (-1654 cm ) relative to that of the ligated species λ>ιο (-1610 c m ) i n the pico second s p e c t r u m ( F i g u r e l a ) . Q u a n t i f i c a t i o n o f t h e e x t e n t o f l i g a n d l o s s w i t h i n 25 p s e c of e x c i t a t i o n i s c o m p l i c a t e d by two f a c t o r s : (1) a s m a l l amount o f 4 - c o o r d i n a t e N i (OEP) i s p r e s e n t i n t h e e q u i l i b r i u m s p e c t r u m ( F i g u r e s 1 and 2) s c a t t e r i n g from t h e 6 - c o o r d i n a t e species i s more r e s o n a n t l y enhanced t h a n t h a t from 4-coordinate N i ( O E P ) when 436-nm e x c i t a t i o n i s u s e d . The r e s o n a n c e enhancement effect i s quite evident i n the behavior of v (-1586 cm" ) and (-1370 cm ) in the picosecond spectrum. Both -1

6

-1

6

1

2

-1

modes appear a t v a l u e s c h a r a c t e r i s t i c o f 6 - c o o r d i n a t e Ni(OEP) and their relative intensities change d r a m a t i c a l l y compared t o the e q u i l i b r i u m Raman spectrum. The changes i n i n t e n s i t i e s o f the modes o b s e r v e d w i t h i n 25 p s e c r e l a t i v e t o t h o s e o b s e r v e d w i t h 10 n s e c p u l s e s may i n d i c a t e a d d i t i o n a l e x c i t e d s t a t e p r o c e s s e s i n the picosecond t i m e regime. D e s p i t e t h e s e c o m p l i c a t i o n s , we believe that the increase i n ^νχο (4-coordinate) i n t e n s i t y at -1654 cm" relative to îo (6-coordinate) at -1610 cm" quali t a t i v e l y d e m o n s t r a t e s an i n c r e a s e i n t h e 4 - c o o r d i n a t e s p e c i e s d u r i n g the picosecond p u l s e . 1

6

1

The spectra generated with 10-nsec h i g h power p u l s e s also i n d i c a t e a s i g n i f i c a n t amount o f p h o t o g e n e r a t e d 4 - c o o r d i n a t e for b o t h Ni(OEP) and N i ( P P ) . E v i d e n c e o f an i n c r e a s e i n t h e amount of u n l i g a n d e d Ni(OEP) r e l a t i v e t o e q u i l i b r i u m Ni(OEP) s o l u t i o n a r e i s p r o v i d e d by i n t h e i n c r e a s e d r e l a t i v e i n t e n s i t i e s o f 4-coordinate Ni(OEP) modes at -1515 ( v ) and -1655 ( yi ) and a b r o a d e n i n g t o h i g h e r f r e q u e n c y o f t h e band a t -1590 cm""-. Spectra g e n e r a t e d w i t h low power 10 n s e c p u l s e s more c l o s e l y resemble t h o s e o f t h e e q u i l i b r i u m Ni(OEP) s p e c t r a . The i n c r e a s e i n t h e amount of 4 - c o o r d i n a t e Ni(OEP) e v i d e n t i n t h e h i g h power s p e c t r a i s a p p a r e n t l y due t o t h e c r e a t i o n of a s i g n i f i c a n t c o n c e n t r a t i o n o f t h e Ââ s t a t e by t h e l o n g l i v e d ( r e l a t i v e t o d-d e x c i t e d s t a t e l i f e t i m e s ) pump/probe l a s e r p u l s e . T h i s e f f e c t i s more c l e a r l y o b s e r v e d u s i n g N i ( P P ) complexes ( F i g u r e 2)·. The Raman s p e c t r a o f N i ( P P ) i n c o o r d i n a t i n g s o l v e n t s ( F i g u r e s 2 and 3) c l e a r l y show t h a t p h o t o i n d u c e d l i g a t i o n changes o c c u r w i t h i n the 10 nsec laser pulse width. The appearance o f the modes »10» 3» and *\>2 at 1658 cm" , 1520 cm , and 1594 cm" , r e s p e c t i v e l y , i n t h e h i g h power s p e c t r a o f N i ( P P ) i n p y r r o l i d i n e i n d i c a t e s an a p p r e c i a b l e amount o f p h o t o d i s s o c i a t i o n 4

4

3

0

1

4

4 v

1

1


1


274


o f t h e 6 - c o o r d i n a t e ground s t a t e s p e c i e s . The e x t e n t o f p h o t o d i s s o c i a t i o n i s b o t h w a v e l e n g t h and power dependent. Photodissociation i s o b s e r v e d u s i n g e i t h e r 406 nm o r 420 nm e x c i t a t i o n . Even though t h e 4 - c o o r d i n a t e modes a r e o f s i m i l a r r e l a t i v e i n t e n s i t y a t b o t h e x c i t a t i o n wavelengths, the f r a c t i o n of 4-coordinate molecules i s a p p r e c i a b l y l e s s i n t h e 406 nm s p e c t r a . T h i s i s expected s i n c e the absorptivity, and therefore the resonance enhancement, of the 6-coordinate s p e c i e s i s c o n s i d e r a b l y lower a t 406 nm. At e i t h e r wavelength the p r o p o r t i o n of 4-coordinate species i n c r e a s e s d i r e c t l y w i t h l a s e r power d e n s i t y . Because t h e Ligand r e c o m b i n a t i o n h a l f - l i f e (>20 n s e c ) ) i s l o n g e r t h a n t h e l a s e r p u l s e , a l i n e a r i n c r e a s e i n the p r o p o r t i o n of e x c i t e d s t a t e s p e c i e s d u r i n g t h e l a s e r p u l s e , i s e x p e c t e d u s i n g 440 nm e x c i t a t i o n . T h i s wave l e n g t h p r i m a r i l y e x c i t e s the 6-coordinate s p e c i e s . E x c i t a t i o n at 406 nm p r o d u c e s a s l i g h t l y more c o m p l i c a t e d situation since the p h o t o g e n e r a t e d 4 - c o o r d i n a t e m o l e c u l e s can, i n p r i n c i p l e , be e x c i t e d by subsequent p h o t o n s i n t h e l a s e r p u l s e . The p r o p o r t i o n s o f 4- and 6 - c o o r d i n a t e m o l e c u l e s w i l l t h e n be p r e d i c a t e d upon t h e i n t e g r a t e d solution (during the l a s e r pulsewidth) of the d i f f e r e n t i a l rate e q u a t i o n s i n v o l v e d i n t h e e x c i t e d s t a t e k i n e t i c s and s h o u l d be q u i t e s e n s i t i v e to the r a t i o of the r a t e constants f o r the formation of t h e two s p e c i e s . O b v i o u s l y , o u r d a t a a l l o w s us t o p r e s e n t l y make o n l y q u a l i t a t i v e comparisons. The c l o s e s i m i l a r i t y between t h e p h o t o p h y s i c s o f N i ( P P ) and Ni(OEP) i n c o o r d i n a t i n g solvents observed in this study argues strongly that the photodissociation (or photoassociation) of l i g a n d s i s p r e d i c a t e d upon m e t a l - b a s e d e x c i t e d s t a t e s and t h a t t h e d e c a y pathways f r o m p o r p h y r i n t o m e t a l d - o r b i t a l s t a t e s a r e s i m i l a r f o r t h e two m o l e c u l e s . Thus, o u r r e s u l t s c o r r o b o r a t e t h e p r o p o s e d photocycle of Hoiten and co-workers (3.4) and suggest that the initial e x c i t a t i o n of the p o r p h y r i n i r - s y s t e m r a p i d l y (IQ to lower frequency as a function of increased photon density in noncoordinating solvents such as toluene, CH2CI2, and H2O/acetone which cannot be predicated upon changes i n m e t a l l o p o r p h y r i n ligation. Upon 10 nsec pulsed e x c i t a t i o n of Ni(PP) i n acetone/water (90:10) there is a decrease in relative intensity of (1656 cm" ), v (1593 cm* ) and v (1522 cnT ) i n d i c a t i v e of a decrease i n the p o p u l a t i o n of the 4-coordinate (ground s t a t e ) s p e c i e s . P h o t o e x c i t a t i o n a l s o p r o d u c e s a pronounced s h i f t t o lower f r e q u e n c y (-1374 cm"" ) and a b r o a d e n i n g o f V 4 . The new modes w h i c h a p p e a r i n t h e s p e c t r a o c c u r a t f r e q u e n c i e s t h a t are distinct from those observed f o r e q u i l i b r i u m 4-, 5-, and 6-coordinate Ni(PP) species. I n p a r t i c u l a r , (* i n d i c a t e s e x c i t e d state species) * 2» * 3 appear at 1

4

1

4

1

2

3

1

v

1

a

n

d

v

1

1

1627-1630 cm- , 1575-1580 cnT and 1492-1495 cm" , respectively. These frequencies are significantly higher than those observed for the ligated Βχ states of Ni(PP) in &


18.


FINDSEN ET AL.

275

coordinating solvents (10). T h e r e a r e two possible explanations f o r the t r a n s i e n t b e h a v i o r o f N i ( P P ) i n : 1) a v e r y weak a x i a l l i g a n d (water) i s p h o t o a s s o c i a t i n g subsequent t o N i ( P P ) e x c i t a t i o n , o r 2) t h e o b s e r v e d s p e c t r a a r e t h o s e o f t h e e x c i t e d s t a t e ( B ^ ) N i ( P P ) w i t h o u t bound a x i a l l i g a n d s . Water i s a p o s s i b l e a x i a l l i g a n d f o r t h e t r a n s i e n t N i ( P P ) i n t h e s e systems and has been shown t o f o r m weak complexes w i t h o t h e r n i c k e l porphyrin species (18). W h i l e we cannot u n e q u i v o c a l l y rule out weak, t r a n s i e n t l i g a t i o n , t h e o b s e r v a t i o n o f s i m i l a r t r a n s i e n t b e h a v i o r i n N i ( O E P ) and Ni(PPDME) i n n o n c o o r d i n a t i n g , nonaqueous, solvents (toluene, methylene c h l o r i d e (9, u n p u b l i s h e d results)) l e a d s us t o c o n c l u d e t h a t t h e t r a n s i e n t b e h a v i o r o f t h e N i ( P P ) i n a c e t o n e / w a t e r i s n o t p r e d i c a t e d upon l i g a n d b i n d i n g . 3


g

I t i s t h e r e f o r e l i k e l y t h a t the 10 n s e c l a s e r p u l s e s c r e a t e a significant amount o f the unligated B^g excited state. This is not unexpected since time-resolved absorption studies have demonstrated that e x c i t a t i o n to this state from an unligated * lg Bround s t a t e o c c u r s i n β and \>IQ are quite sensitive to porphyrin core-size. Their positions i n the transient spectra i n d i c a t e t h a t t h e d-d t r a n s i t i o n a l o n e engenders a c o r e e x p a n s i o n f r o m -1.95 k t o -2.02 À. T h i s i s approximately t w o - t h i r d s of the t o t a l e x p a n s i o n o b s e r v e d between e q u i l i b r i u m 4 - c o o r d i n a t e Ni(PP) and equilibrium 6-coordinate Ni(PP). This i s not unreasonable since the symmetry of the d 2_ 2 orbital allows it to i n t e r a c t d i r e c t l y w i t h t h e p o r p h y r i n sigma system i n an a n t i b o n d i n g fashion. 3

A

3

&

x

y

1

The -10 cm" shift to lower frequency of *V4 in the unligated B state further indicates a substantial redistribution of e l e c t r o n d e n s i t y i n the porphyrin o r b i t a l s . T h i s mode has been traditionally viewed as a s e n s i t i v e i n d i c a t o r of the oxidation s t a t e o f t h e c e n t r a l m e t a l presumably b e c a u s e o f i t s s e n s i t i v i t y t o t h e d e g r e e o f ττ-backbonding between t h e m e t a l and the porphyrin eg(ir*) o r b i t a l s (15). I n t h i s c a s e , however, i t a p p e a r s t h a t a substantial portion (-70%) o f the f r e q u e n c y s h i f t i n V 4 between e q u i l i b r i u m 4 - and 6-coordinate N i ( P P ) r e s u l t s from the n e t d-d transition. This sensitivity to the population of the metal d 2_y2 orbital probably arises from the increased pyrrole nitrogen-nickel bond distances of the expanded metalloporphyrin which, i n turn, leads to poorer overlap between the d^ and porphyrin eg(ir*) orbitals. It should be noted that the r e l a t i o n s h i p between the frequency of ν 4 and the frequency of t h e c o r e - s i z e marker l i n e s i s s i m i l a r t o t h a t o b s e r v e d f o r m e t a l substitution (17). Metal s u b s t i t u t i o n e f f e c t s on marker line f r e q u e n c i e s a p p e a r t o be b a s e d on d i f f e r e n c e s i n c o v a l e n c y o f t h e porphyr in-metal bond for different metals. Thus, ττ and o* orbitals in addition to eg(ir*) and metal d^ orbitals are probably involved. 1 &

x




276

I n summary t h e b e h a v i o r o f Ni(OEP) i n c o o r d i n a t i n g solvents o b s e r v e d w i t h i n 25 p i c o s e c o n d s o f p h o t o e x c i t a t i o n appears t o f o l l o w t h a t r e p o r t e d from t r a n s i e n t a b s o r p t i o n s t u d i e s (3,4). The Raman s p e c t r a c o n c l u s i v e l y show t h a t t h e r e i s an i n c r e a s e i n t h e amount o f 4 - c o o r d i n a t e Ni(OEP) w i t h i n ~25 p s e c o f e x c i t a t i o n . The s p e c t r a o f Ni(OEP) and N i ( P P ) i n c o o r d i n a t i n g s o l v e n t s o b s e r v e d w i t h -10 n s e c p u l s e s c l e a r l y show t h e l o s s o f n i t r o g e n o u s ligands i n the excited state. In addition to corroborating the general n i c k e l p o r p h y r i n p h o t o c h e m i c a l scheme p o s t u l a t e d by Kim e t a l . ( 3 , 4 ) , we have demonstrated t h a t t r a n s i e n t Raman s p e c t r o s c o p y i s a powerful means o f d i r e c t l y observing the molecular b a s i s o f the photo chemistry and p h o t o p h y s i c s o f e x c i t e d s t a t e m e t a l l o p o r p h y r i n s on short timescales. The power dependencies of various nickel p o r p h y r i n modes i n n o n c o o r d i n a t i n g s o l v e n t s s u g g e s t t h a t t h e e x c i t e d state formed h a s s i g n i f i c a n t B^- c h a r a c t e r . Further transient Raman s t u d i e s u s i n g a v a r i e t y of laser e x c i t a t i o n frequencies, powers, and pump-probe p r o t o c o l s w i l l be r e q u i r e d t o c o m p l e t e l y s e p a r a t e e x c i t e d s t a t e e f f e c t s from t h e e f f e c t s o f a x i a l l i g a t i o n . However, t h i s p r e l i m i n a r y s t u d y opens t h e door t o a whole r a n g e o f time-resolved r e s o n a n c e Raman s t u d i e s o f m e t a l l o p o r p h y r i n photo c h e m i s t r y , some o f w h i c h a r e c u r r e n t l y underway i n o u r l a b o r a t o r i e s .

Acknowledgments We g r a t e f u l l y acknowledge t h e s u p p o r t o f t h e NIH (GM33330), R e s e a r c h C o r p o r a t i o n , t h e donors o f t h e P e t r o l e u m R e s e a r c h Fund as a d m i n i s tered by t h e American Chemical Society ( t o MRO), The U.S. Department o f Energy Contract DE-AC04-76-DP00789 and t h e Gas R e s e a r c h I n s t i t u t e C o n t r a c t 5082-260-076 7 ( t o JAS) and t h e g r a d u a t e r e s e a r c h s c h o l a r s h i p fund o f t h e A s s o c i a t e d Western U n i v e r s i t i e s ( t o EWF).

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1.

Findsen, E. W.; Friedman, J. M.; Ondrias, M. R.; MacDonald, W. Science 1985, 229, 661. 2. Findsen, E. W.; Scott, T. W.; Chance, M. R., Friedman, J. M.; Ondrias, M. R. J. Am. Chem. Soc. 1985, 107, 3355. 3. Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305. 4. Kim, D.; Holten, D. Chem. Phys. Letters 1983, 98, 584. 5. Kobayashi, T.; Straub, K. D.; Rentzepis, P. M. Photοchem. Photobiol. 1979, 29, 925. 6. Findsen, E. W.; Ondrias, M. R. J. Am. Chem. Soc. 1984, 106, 5736. 7. Shelnutt, J. A. J. Phys. Chem. 1983, 87, 605. 8. Spaulding, L. D.; Chang, C. C.; Yu, N.-T.; Felton, R. H. J. Am. Chem. Soc. 1975, 97, 2517. 9. Cheung, L. D.; Yu, N.-T.; Felton, R. H. Chem. Phys. Letters 1978, 55, 527. 10. Shelnutt, J. Α.; Alston, K.; Ho, J.-Y.; Yamamoto, T.; Rifkind, J. M. Biochemistry, 25, 620. 11. LaMar, G. Ν.; (Jensen) Walker, F. A. In "The Porphyrins"; Dolphin, D., Ed.; Academic: New York, 1979; Vol. 4, p. 61.



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12. Shelnutt, J. Α.; Straub, K. D.; Rentzepis, P. M.; Gouterman, M.; Davidson, E. R. Biochemistry 1984, 23, 3946. 12a. Abe, M.; Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978, 69, 4526. 13. Teroaka J.; Kitagawa, T. J. Phys. Chem. 1980, 84, 1928. 14. Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, Z. J. Phys. Chem. 1975, 79, 2629. 15. Spiro, T. G. In "Iron Porphyrins, Part 2;" Lever, A.P.B., Gray, Η. Β., Eds.; Addison-Wesley: New York; 1984. 16. Ake, R. L.; Gouterman, M. Theoret. Chim. Acta (Berl.) 1970, 17, 408. 17. Shelnutt, J.; Ondrias, M. R. Inorg. Chem. 1984, 23, 1175. 18. Pasternak, R. F.; Spiro, E. G.; Teach, M. J. Inorg. Nucl. Chem., 1974, 36, 599. 19. Shelnutt, J. Α.; Dobry, M. M. J. Phys. Chem., 1983, 87, 3012. 20. Shelnutt, J. A., J. Am. Chem. Soc. 1983, 105, 774. 21. Walker, F. Α.; Hui, E.; Walker, J. M. J. Am. Chem. Soc. 1975, 97, 2390. RECEIVED April 8, 1986


Resonance Raman Investigation of Transient Photoinduced Ligation

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