Abstract - The photochemical ring opening reaction of unsubstituted indolino-spiropyrans in n-pentane, to the corresponding merocyanines, has been followed by transient absorption spectroscopy. The temporal resolution was better t

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@ 1990 IU PAC

The primary photochemical reaction step of unsubstituted indolino-spiropyrans

N.P. Ernsting, B. Dick, and Th. Arthen-Engeland

Abteilung Laserphysik, Max-Planck-Institut fur biophysi kal ische Chemie,

D-3400

Gottingen, Federal Republic of Germany

Abstract - The photochemical ring opening reaction of unsubstituted indolino-spiropyrans in n-pentane, to the corresponding merocyanines, has been followed by transient absorption spectroscopy. The temporal resolution was better than

0.4

ps. During the pump pulse, an ultrafast unstructured absorption covers the entire measurement range

380-680

nm.

From it emerges the structured absorption spectrum of a first merocyanine isomer, with typical time constants in the range

0.9

- 1.4 ps. The transient merocyanine spectra are compared to the spectra which are obtained when the spiropyrans are irradiated at low temperature in an argon matrix. In conjunction with semiempirical calculations, the first merocyanine isomer is assigned a trans-trans-cis structure.

INTRODUCTION

Spiropyrans consist of an (extended) pyran moiety, and a second moiety which is held orthogonal by a common spiro-carbon atom. The Tf-electron systems of both constituent halves do not interact because of their orthogonal. orientation,

so

that the absorption spectrum

o f

the compound is essentially the superimposition

of

thg-.two constituent chromophores .

Here we consider only spiropyrans in which the second moiety consists of the indoline chromophore. The parent compound 1 is shown in Fig. 1; it is commonly abbreviated BIPS for

1' ,3' ,3'

-trimethylspiro- [2H-l-benzopyran-2,2' -indo1 ine]. Upon excitation in the near

UV,

the bond between the spiro carbon atom and the oxygen atom is broken, the methine chain rearranges, and the two parts of the molecule are allowed to rotate relative to each other (2 to 5 in Fig. 1). Now ther-electrons may conjugate across the entire structure

(

which may be classed among the merocyanine dyes) resulting in intense absorption in the range

500-600

nm.

However, in many cases the merocyanine form of the molecule is thermodynamically unstable, and it reverts eventually to the original, colorless spiro form.

This "photochromism" of spiropyrans was first reported and intensely studied by

E.

Fischer and coworkers (ref.

1 - 3 ) .

Potential applications for displays, optical filters and data storage led to a surge of investigations into spiropyran photochemistry (ref. 4 gives an excellent review up to

1969).

It appeared that efficient photochromism requires a nitro substituent on the benzopyran ring for two reasons. First, the nitro group increases the quantum yield of photocoloration by introducing a triplet pathway; second, it stabilizes the zwitterionic merocyanine form of the molecule,

so

that the thermal back reaction is slowed down. However, the photochemical stability is adversely affected by nitro substitution. This problem may be alleviated by using the related spiro-oxazines, which give reasonable photocoloration yields and stability without nitro substitution (ref.

5 ) .

- 2

- 1 - 3

- 6 - 5

- 4

Fig. 1. BIPS (I), hypothetical structures for its first photochemical reaction product

X

(2. - A), and for a merocyanine isomer ( 5 ) .

1483

The photochemistry o f spiropyrans may conveniently be classed into the following areas:

Properties of the spiroform

Geometry (ref.6); electronic structure, intramolecular energy transfer between the orthogonal parts, and dependence of quantum yields on excitation wavelength (ref. 7-10), and on medium and temperature (ref. 11, 12).

Merocyanine isomers

Several near-planar isomers of the open form are conceivable, by rotation around the three central bonds C2-C3 , ^C3-C4 , and C4-C4a . Evidence for these isomers was obtained when the photocoloration was carried out in rigid media at low temperature (ref .2, 3). A structured merocyanine absorption spectrum was thus generated, corresponding to a first isomer which was labelled B1. On heating, the spectrum changed irreversibly to a similar one which may be associated with a second, more stable isomer B2, and

so

on. The isomers interconvert thermally at room temperature,

so

that the kinetics of the thermal decoloration is determined by the isomer with the lowest activation energy for the back reaction. Several isomers have been characterized by resonance Raman spectroscopy

(ref. 13-15). The structures of open isomeric forms have been reported (ref. 16).

Dispersive kinetics of decoloration in solid media

The thermal back reaction in solid media does not generally obey first-order kinetics. Instead the kinetics may be described as a superimposition of independent first-order processes, albeit with a broad distrihution of rate constants. Each rate constant is associated with a certain free volume in the (e.g.

polymer)

matrix. Hence the distribution of rate constants reflects the glassy nature

o f

the medium, rather than the intrinsic cheinical reaction (ref.

17-14).

Aggregation of merocyanine isomers

Irradiation of nitro-substituted spiropyrans in solution, and in an electrostatic field, leads to aggregates of the merocyanine form aligned in a

"string-of-beads" structure. They precipitate in sub-micrometer sized globules which are composed of highly dipolar cores covered by amorphous envelopes (ref.

2 0 ) .

Primary photochemical reaction steps in color formation

The participation of triplet states in the photocoloration of nitro-substituted spiropyrans has been shown by triplet sensitization

(

ref. 4 and citations there; ref. 21) and by transient absorption measurements (ref. 22, 23). A triplet pathway is also responsible for aggregation of the resulting merocyanine isomers (ref. 22). By contrast, photocoloration of the unsubstituted parent compound BIPS and the spiro-naphthopyran

6

proceeds only through the singlet route (ref. 24).

Let us return for a moment to the distribution of merocyanine isomers. It has been suggested that the first isomer produced in the photochemical reaction should resemble the parent spiropyran geometry, i.e. the central C3-C4 bond should have a cisoid conformation as shown in structures 2. - 5 of Fig. 1. This primary cisoid isomer was labelled

X

(ref. 25). Ground and excited singlet and triplet states of

X

have been implicated in the photocoloration of

6-

nitro-BIPS (ref.

10,

22, 23, 26, 27).

Here we address the question whether a primary, cisoid merocyanine isomer exists, however briefly, on the picosecond time scale. We limit our investigation to the unsubstituted parent compounds BIPS (1) and the spiro-naphthopyran 6 , in order to avoid complications due to possible triplet states at early time. (For simplicity and somewhat loosely, we will call

6

naphtho-BIPS.

)

We report

))

MNDO calculations of the merocyanine isomers of BIPS in their ground state,

))

transient absorption spectra of the spiropyran

-

merocyanine conversion, with 0.35 ps time

))

spectra of a merocyanine isomer isolated in an argon matrix at 10

K.

resolution, and

M N D O C A L C U L A T I O N S OF M E R O C Y A N I N E I S O M E R S OF BIPS

The AM1 modification of the MNDO-Hamiltonian has been used for all calculations. (Program QCPE 560). All geometry optimizations were performed on the SCF level with the PRECISE option, and all geometric parameters were optimized. Configuration interaction with all singly and doubly excited configuration between the two HOMOS and the two LUMOs (C.I.=4 option) yielded only a small energy correction. These latter calculations, which also gave vertical excitation energies, have been used to construct the energy level diagram in Fig.

3 .

The spiro form of BIPS is found at

o C =

250',

13 = O n ,

and y

=

-5'. This means that the second and third angle (which are located in the pyran ring) have cis configuration, whereas the first angle is such that the two molecular halves are (only approximately) orthogonal.

The merocyanine form closest to this point should be the TCC form, but no local minimum could

be obtained in the vicinity of this corner of the parameter space. From the schematic

structure of the TCC form shown in Fig. 2 it is obvious that in this form strong steric

hindrance exists between the oxygen atom and the two geminal methyl groups. Interestingly, no

stable merocyanine form could be found for which

I3 = O",

i.e. for which the formal single

bond in the merocyanine chain has a cisoid configuration. All attempts to optimize the

corresponding structure finally lead to the corresponding transoid form. The only exception

is the CCT form for which a shallow minimum could be found. In this isomer

I3 =

67', i.e. the

formal "cis" bond is already strongly twisted.

c$+=J

⁰

CTT 252.4

TCC

Fig.

2 .

Structural formulas of the spiro form and the 8 possible cis/trans isomers of the merocyanine form of

BIPS.

(The isomers are drawn only in their quinoidal mesomeric form.) Within the parameter space of the three dieder angles

d , 0 ,

and

y ,

the merocyanine isomers occupy the 8 corners of a cube. The local

energy minima found with the

AM1

method are indicated by full circles. Numbers give the calculated enthalpy of formation in kJ/mole. The open circle represents the saddle point between the spiro form and the TTC isomer.

Spiro

4 0 0 1 Sl f

1

300

J

-

a

: E

²⁰⁰

Y

w

a

100

3 3

3 3

0 cm"

l300nml

I

T T C

f

^s1

Fig.

3.

Energy level diagram for the thermal reaction between the spiro form and the

TTC merocyanine isomer of

_BIPS

obtained with the

AM1

method. The optimized (on

the SCF level) geometries of the two stable ground states and the transition

state are also shown.

Thus t h e s t a b l e merocyanine isomer c l o s e s t t o t h e s p i r o f o r m o f B I P S i s found t o be t h e TTC form. The f o r m a t i o n o f t h i s isomer i n v o l v e s a simultaneous r o t a t i o n o f t h e angles IJ( and 0 by 73" and 175", r e s p e c t i v e l y . The saddle p o i n t between t h e s e two forms on t h e ground s t a t e p o t e n t i a l h y p e r s u r f a c e i s i n d i c a t e d by t h e open c i r c l e i n F i g . 2.

The energy l e v e l diagram r e l e v a n t t o t h e d i s c u s s i o n o f t h e thermal r e a c t i o n between t h e s p i r o form and t h e TTC isomer i s shown i n F i g . 3. The e n e r g i e s a r e g i v e n r e l a t i v e t o t h a t o f t h e s p i r o ground s t a t e . The c o r r e c t i o n o f t h e c o n f i g u r a t i o n i n t e r a c t i o n c a l c u l a t i o n has been i n c l u d e d s i n c e t h e s e c a l c u l a t i o n s a l s o y i e l d v a l u e s f o r t h e v e r t i c a l e x c i t a t i o n e n e r g i e s . The t h r e e b a l l - a n d - s t i c k models a r e t h e o p t i m i z e d geometries o f t h e two ground s t a t e s and t h e t r a n s i t i o n s t a t e . An a c t i v a t i o n energy o f 112.9 kJ/mole (26.9 k c a l / m o l e ) i s c a l c u l a t e d f o r t h e thermal r e a c t i o n f r o m t h e merocyanine t o t h e s p i r o form. For a s o l u t i o n i n propanol, t h e experimental a c t i v a t i o n energy was found t o be 71 kJ/mole (17 kcal/mole) ( r e f . 2 ) . The agreement i s q u i t e r e a s o n a b l e i n view o f s p e c i f i c s o l v a t i o n e f f e c t s ( r e f . 15, 16).

TRANSIENT ABSORPTION SPECTRA W I T H SUB-PS T I M E RESOLUTION

The pump-and-probe spectrometer f o r t r a n s i e n t a b s o r p t i o n i s d e s c r i b e d i n r e f . 28. An i n t e n s e pump p u l s e a t 308 nm i n i t i a t e s t h e photochemical r e a c t i o n ; i t has a d u r a t i o n o f 0.26 ps.

A f t e r a v a r i a b l e d e l a y , t h e a b s o r p t i o n spectrum o f t h e sample i s r e c o r d e d u s i n g a w h i t e - l i g h t continuum o f 0 . 1 ps d u r a t i o n .

F i g u r e 4 shows t y p i c a l t r a n s i e n t a b s o r p t i o n s p e c t r a o f BIPS and naphtho-BIPS i n n-pentane, a f t e r some t e n s o f ps.

A l s o shown as dashed l i n e s a r e t h e s p e c t r a ( f l u o r e s c e n c e e x c i t a t i o n ) o f t h e corresponding merocyanine form, generated by i r r a d i a t i o n o f t h e s p i r o w r a n i n an arqon m a t r i x a t 10 K. The isomer s p e c t r a a t -low t e m p e r a t u r e a r e s t r u c t u r e d , ' a n d ' t h e y must be -assigned t o t h e isomer l a b e l l e d B1 i n ( r e f . 2, 3 ) .

n 0

0.00 -1

350 400 450 500 550 600 650 700

wavelength /nm

0.15 -

0.10 -

n 0

0.00 7

350 400 450 500 550 600 650 7

wavelength /nm

0

F i g . 4 . T r a n s i e n t a b s o r p t i o n s p e c t r a ( d o t s ) , f i t s by Gaussian f u n c t i o n s r e p r e s e n t i n g v i b r o n i c bands ( s o l i d l i n e s ) , and f l u o r e s c e n c e e x c i t a t i o n s p e c t r a o f merocyanine p h o t o p r o d u c t s i n an argon m a t r i x a t 10K (dashed l i n e s ) .

L e f t : BIPS; r i g h t : naphtho-BIPS; b o t h a f t e r UV e x c i t a t i o n .

I n t h e f o l l o w i n g , and f o r s i m p l i c i t y , l e t us c o n s i d e r t h e p h o t o c h e m i s t r y o f BIPS o n l y . The s p e c t r a l s t r u c t u r e f o r t h e t r a n s i e n t a b s o r p t i o n spectrum f r o m BIPS corresponds q u i t e w e l l w i t h t h a t o f t h e B1 isomer. I n f a c t , t h e t r a n s i e n t spectrum may be f i t t e d w i t h Gaussian f u n c t i o n s (on an energy s c a l e ) r e p r e s e n t i n g t h e v i b r o n i c bands which a r e c l e a r l y d i s c e r n i b l e i n t h e l o w - t e m p e r a t u r e spectrum. Comparison o f t h e f i t s g i v e s t h e s p e c t r a l s h i f t due t o d i f f e r e n c e s i n s o l v a t i o n , and s p e c t r a l broadening due t o t h e r e l a t i v e l y h i g h i n t e r n a l temperature o f t h e t r a n s i e n t species.

F i g u r e s 5 and 6 g i v e an o v e r v i e w o f t h e s p e c t r a l development up t o 8 ps. Because o f t h e low quantum y i e l d f o r p h o t o c o l o r a t i o n o f B I P S (ca. 6%, r e f . 2), i n t e n s e pump p u l s e s had t o be used. T h i s i n t r o d u c e s t r a n s i e n t a b s o r p t i o n c o i n c i d e n t w i t h t h e pump p u l s e , i n t h e range

I &

470 nm, due t o m u l t i p h o t o n a b s o r p t i o n by t h e s o l v e n t . A remnant o f t h i s a r t e f a c t can a l s o be seen i n t h e t r a n s i e n t a b s o r p t i o n o f naphtho-BIPS a t e a r l i e s t t i m e . The s p e c t r a l development f o r B I P S shows i n c i p i e n t s p e c t r a l s t r u c t u r e a l r e a d y a t 2 . 1 ps. C o o l i n g i s i n d i c a t e d by slow steepening o f t h e r e d a b s o r p t i o n edge o v e r t e n s o f ps.

K i n e t i c t r a c e s o f t h e development o f merocyanine absorbance a t t h e a b s o r p t i o n maximum a r e shown i n F i g . 7 . S i n g l e e x p o n e n t i a l r i s e i s g i v e n f o r comparison, w i t h r i s e t i m e s o f 0.9 ps f o r B I P S and 1.4 ps f o r naphtho-BIPS. D e v i a t i o n s a t e a r l y t i m e a r e s i g n i f i c a n t , and t h e y must be a t t r i b u t e d t o an u l t r a f a s t t r a n s i e n t i n t h e s p i r o p y r a n

-

merocyanine c o n v e r s i o n .

-0.05

F i g . 5. T r a n s i e n t a b s o r p t i o n o f B I P S i n n-pentane.

0.10

c1 0.05 0

0.00

-0.05

F i g . 6. T r a n s i e n t

Pa -.z

a b s o r p t i o n o f naphtho-BIPS i n n-pentane.

F i g . 7. K i n e t i c t r a c e s f o r t h e development o f merocyanine absorbance, f o r s o l u t i o n s o f B I P S ( l e f t ) and naphtho-BIPS ( r i g h t ) i n n-pentane.

DISCUSSION

L e t us r e t u r n t o t h e scheme i n F i g . 2. A p r e v i o u s t h e o r e t i c a l s t u d y o f t h e r i n g opening r e a c t i o n i n chromene ( r e f . 29) had p r e d i c t e d t h a t t h e photochemical, f o r w a r d r e a c t i o n i n v o l v e s a change i n

8 .

We b e l i e v e t h a t t h e r e a c t i o n p a t h on t h e S 1 p o t e n t i a l energy s u r f a c e l i k e l y proceeds f r o m t h e s p i r o f o r m i n t h e d i r e c t i o n o f t h e CCT isomer f o r s t e r i c reasons.

The photochemical r e a c t i o n proceeds even a t 10K i n an argon m a t r i x . T h e r e f o r e t h e S 1 s u r f a c e can o n l y have a v e r y small

-

i f any

-

a c t i v a t i o n b a r r i e r , and i t s s l o p e a l o n g t h e p a t h must be small even a t a s i g n i f i c a n t d i s t a n c e from t h e s p i r o form. Then t h e f o l l o w i n g p i c t u r e would be c o n s i s t e n t w i t h o u r o b s e r v a t i o n s . The p a t h s k i r t s t h e saddle p o i n t o f t h e ground s t a t e s u r f a c e , and i n t e r n a l c o n v e r s i o n occurs. A l a r g e f r a c t i o n r e t u r n s t o ground s t a t e s p i r o p y r a n . T h i s e x p l a i n s a l o w quantum y i e l d o f p h o t o c h e m i s t r y ( r e f . 4) and luminescence even a t low temperature ( r e f . 8 ) . The remainder u n f o l d s t o t h e TTC c o n f o r m a t i o n w i t h a t i m e c o n s t a n t o f 0.9 o r 1.4 ps. C a l c u l a t i o n s p r e d i c t t h a t t h i s isomer should r e a r r a n g e i n i t s S 1 e x c i t e d s t a t e ( r e f . 30); i t s h o u l d t h e r e f o r e have a l o w quantum y i e l d f o r f l u o r e s c e n c e . T h i s is a l s o c o n s i s t e n t w i t h o b s e r v a t i o n s ( r e f . 3 ) . I n t h i s c o n t e x t , t h e broad, u l t r a f a s t t r a n s i e n t a b s o r p t i o n c o i n c i d e n t w i t h t h e pump p u l s e must be assigned t o e x c i t e d s t a t e a b s o r p t i o n from t h e u n f o l d i n g s p i r o p y r a n .

I n c o n c l u s i o n : we f i n d no evidence f o r a r e a s o n a b l y s t a b l e merocyanine isomer X w i t h a c i s o i d geometry. I n s t e a d , t h e p r i m a r y photochemical r e a c t i o n s t e p o f t h e u n s u b s t i t u t e d s p i r o p y r a n B I P S and s i m i l a r s p i r o p y r a n s s h o u l d l e a d d i r e c t l y t o a TTC isomer w i t h i n 2 ps a t room temperature.

1. E 2. T 3.

c

4. R 5.

N

6. S 7. N 8.

N

9. I

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