Laser photolysis studies of spiropyran-merocyanine aggregate formation in solution

Volume 86, number 1

CHEMICAL PHYSICS LETTERS

5 February 1982

LASER PHOTOLYSIS STUDIES OF SPIROPYRAN-MEROCYANINE AGGREGATE FORMATION IN SOLUTION Yehoshua KALISKY and David J. WILLIAMS Webster Research Center, Xerox Corporation, Webster, New York 14580, USA

Received 23 October 1981

The photoinduced processes leading to formation of J-aggregatestacks of 1-~-methacryloxyethyl)-3,3-dimethyl-6'nitrospiro-(indoline-2,2'-[2H-[2H-i] benzopyran) d its associated ring opened merocyanine form B have been determined by N2-1asertransient spectroscopy. Detailed mechanisms for formation of complexes AB, A2B, and J-aggregatestacks (A2B)n in aliphatic and aromatic solvents are presented.

1. Introduction The photoinduced formation of "quasi-crystals" from spiropyrans in non-polar solvents has been investigated extensively by Krongauz and co-workers in recent years [ 1]. The quasi-crystals are formed as a result of a complex sequence of processes involving the excited states of a parent indolinobenzospiropyran and the ring opened merocyanine form. The stability of the quasi-crystals, with respect to dissolution, has been shown to vary considerably with the substituent pattern, and the most stable species observed to date have been derived from 1 -(fl-methacryloxyethyl)-3,3dimethyl-6'-nitrospiro-(indoline-2,2'- [2H-1 ] benzopyran) (A) and its associated merocyanine form (B). The quasi-crystals have been shown to be composed of an inner crystalline core with composition An B

~

NO2

0

I

0

C=O

~=0

I /c=c%

f /c=cH 2

CH 3

CH 3 A

100

NO2

B

(n = 2,3) and an amorphous outer core with composition AB. These particles can be as small as 0.1/am in diameter. It has also been shown that the quasi-crystals have macroscopic electric dipoles [2] associated with them indicating a non-centrosymmetric crystal structure for the A2B complexes. Spectral properties of the quasi-crystals suggest the crystalline cores are J-aggregate stacks. It has been shown that merocyanine dyes exhibit the largest known molecular hyperpolarizabilities (i.e. 10 -27 esu) [3-5] and conjectured that extremely large non-linear optical effects might be expected if non-centrosymmetric dipolar crystals of merocyanines could be obtained. The non-linear optical properties of these species have been measured and reported elsewhere [6]. Although the photochemistry of spiropyrans has been studied extensively in the past [7,8], a unified picture of the events leading to quasi-crystal formation has begun to emerge only recently from laser photolysis experiments on A' in aliphatic solvents [9].

~ HO

NO 2

A'

Volume 86, number 1

CHEMICAL PHYSICS LETTERS

In this paper we report laser photolysis measurements on A in a variety of solvents and suggest a mechanism leading to formation of AB, A2B, and stacks (A2B) n . Our results are compared to those obtained by Krongauz et al. [9] for A' in aliphatic solvents.

I

5 February 1982

I

~

I

I

i

I

3. Results and discussion

The transient absorption spectrum of A in p-dioxane at various delay times following laser excitation is shown in fig. 1. Oscilloscope traces showing the time dependence of the absorption signal at various wavelengths are shown in fig. 2. Similar results were obtained in MCH, hexane, and toluene. Examination of the spectra at various delay times indicates three distinct regions of spectral behavior. In the 400-500 nm region a peak which decreases with time is observed. The trace in fig. 2a indicates a two-component decay

i

I

['-"

r

I

i

I

i

I

J-\"'X

",,,./t =lOOp.SEC

i

.a in P-DIOXANE ,",ex, 337.1nm

,, ,'

,.o

Transient absorption spectra were obtained with a pulsed N2 laser and detection system. The laser was a Lumonics TE 861S excimer laser operating on a N2/ He mixture at 337.1 nm with a pulse width of 3 ns (fwhm). The maximum measured pulse energy density at the sample was 170 mJ/cm 2 and lower energies were frequently used. The detection system was similar to that reported previously [ 10,11 ]. All measurements were performed at ambient temperature and N 2 was bubbled through the sample for several minutes prior to and during the measurements. The indolinobenzospiropyran A was prepared according to the method described by Zajtseva et al. [12]. The final product A was purified on a silicic acid column and recrystallized from 1 : 10 benzenehexane. Positive identification was made through IR and NMR analysis. The colorless crystalline material exhibited a melting point which was heating-rate dependent and had a maximum value of 105°C. The solvents toluene (Burdik and Jackson High Purity), pdioxane (Baker Photrex), hexane (Baker Photrex), and methylcyclohexane (Eastman Kodak) (MCH) were used without further purification. The solutions for measurement were held in optical grade quartz cells.

I

,,,.--" I

2. Experimental

i

,,

~- ~.\,,

..

/

, ~.~\: ,,., ,.,,.

~0.8 '~

"',~

0.2

, /f <~

t.

,,

....~

,//

'~/ •

~,- ,, \'\

~ / 1 : 5 0 0 n SEC

,,' 440

360

.....

,,..... 520 X, nrn

600

680

760

Fig. 1. Transient absorption spectra of 5 X 10-4 M A in pdioxane at different delay times after the laser pulse.

with a short- and long-lived component. Bubbling with 02 significantly reduces the lifetime of the short-lived component. From 490 to 680 nm a spectrum is obtained at the earliest resolvable time (~ 10 ns). Between 560 and ~ 6 8 0 nm the absorbance increases on a time scale approximately correlating with IOmV~l'7" • I I q IIIII I LI I I IIIII i' ~"[ i "t

I T I ITI IIIII I I 149¢"~nm II ~ ]' J"i " '

-"4 FSOOnSEC

(a)

L, I ', 'L, l l l J l --,q 1"-.'-500nSEC (c)

IOrnV_.kl-'~ l~_t L .1: I L L I.±J

t 4

5 I-.*-500nSEC

(b)

!J!!!!!!J4"l -~ ~,--500nSEC (d)

l O m V ~ [ ~

le) Fig. 2. Characteristic oscillograms of the various transients formed by laser photolysis of 5 X 10-4 M A in p-dioxane.

101

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CHEMICAL PHYSICS LETTERS

the decay of the 440 nm peak. From fig. 2b at 550 nm it is seen that the rapidly formed species is stable on this time scale and that no other species contribute to the absorption in this region. At 640 nm a twocomponent spectrum is observed. The rapidly formed component is attributed to the same species observed at 550 rim. The slowly growing component appears on a time scale similar to the decay of the rapidly decaying species at 420 nm. Similar behavior is observed at 670 nm but the slow growing component is distinctly slower than the slow component at 640 nm. Beyond 680 nm a species exhibiting similar behavior to the 440 nm peak is observed. With several exceptions, these observations are in general agreement with those obtained for A', the (6-nitro-1 ',3',3'-trimethylspiro [2H-1 ] -benzo-pyran2,2'-indoline) [9]. From the results obtained above and in analogy with Krongauz et al. [9] the following general comments regarding the mechanism can be made. The rapidly (i.e. < 10 ns) formed species with absorbance between 400 and 500 nm and above 680 nm corresponds to the triplet state 3A*. The rapidly formed slowly decaying species with )tmax -~ 550 nm is assigned to the dimeric AB species. The slowly formed species in the 5 6 0 - 6 8 0 nm region is assigned to A 2 B formed from the reaction of 3 A* with A 2 . Since the "quasi-crystals" are thought to be formed from "J-aggregate like" stacks we assume that the slowly formed species beyond 660 nm is associated with stack formation from A2B trimers. The red-shift of the stacks with respect to trimers would be expected based on the exciton model [13]. The Optical absorption spectrum of A was obtained in several solvents. Typically in p-dioxane the two longest-wavelength peaks at 330 and 295 nm were found to deviate from Beer's law in the 5 × 10 - 4 5 × 10 - 6 M range; the 330 nm peak loosing relative intensity upon decreasing total concentration. We attribute this behavior to a m o n o m e r - d i m e r equilibrium with the 330 nm peak associated with the dimer. Attempts to determine the equilibrium constant were unsuccessful because of the limited accessible concentration range and interference from solvent absorption. In order to determine the concentration dependence of the yields of species A2B and AB the ratios of [A2B]t-'= and [AB] t--'= were measured from transient absorption at 620 and 550 nm, respectively. Fig. 3 shows that the ratio is independent o f concen102

o

o

l~l~ o

5 February 1982

o

A in MCH

~-o

2.0

o--o

I.£ . 0 . 2 . - - _ I

O.C

1.0

i

I

~ _

2.0

I

3.0 [A]

x

0

I

~ 8 0. m d ~

I

6 5 md • c-'m2

I

4.0 5.0 104, MOLE LITER

Fig. 3. Dependence of ~(OD)6%o/A(OD)s%o on the initial concentration of A in MCH at two laser energies.

trations of A from 5 X 10 -4 to 5 × 10 -5 M. This is in contrast to the results for the A' system [9] where the ratio depended on [A'] 1/2. The following analysis provides an overall mechanism for the photochemical processes leading to formation of AB, A2B, and stacks (A2B)n : 2A.ke-~qA2 ,

(1)

hv A ~ 1 A* ,

(2)

kR+kNR

1A* - - *

A,

(3)

1A * kisc 3A * , 3A*

(4)

kR'+kNR' ~A ,

1 A* + A 2

(5)

kAB > AB + A (energy transfer),

(6)

kA2B

3A* + A 2

A2B,

nA2B k(A2B)n' (A2B)n .

(7) (8)

It is assumed in this mechanism that internal conversion from the initial state to the lowest vibronic state of A* is unity [14]. The decay o f 3A* and formation of A2B are described by: [3A* l = [3A~] exp

(--t/rT),

(9)

where r T = (k R, + kNR, + kA2 B [A2 ])-1 = kT 1 and

(10)

Volume 86, number 1

CHEMICAL PHYSICS LETTERS

d[A2B]/dt = kA2 B [3A*] [A2] .

IoC

(ll)

For low optical densities at the excitation wavelength 337.1 nm we have: l a g * ] 0 = I0q~TeA [A] ,

(12)

[IA*]0 = I 0 e g [A] ,

(13)

5 February 1982 I

I

I

-~.__~

Substituting (9) into (11) and assuming a large excess of A 2 we obtain upon integration [A2B] = kA2Br T [3A*]0 [A2] [1 - exp

(15)

kA2Br T [3A*]0 [A21

(16)

[A2B] •

~"

o

•

"

'~

,oo

~c~

k= 1.0 x 106 SEC-I

~

\(a)

"~,,~b)

&"

k=O.5xlO 6 SEC" (670 nm) • O~(c )

i(330[ ,

,ooo r

,

,,~,

,

t, ns

Fig. 4. A plot of log absorbance versus time of 5 × 10 --4 M A in p-dioxane. (a) Decay of the triplet state, (b) build up of the absorption of A2B , (c) build up of absorption of stacks (A2B)n.

[A2B] °~ - [A2B] = [A2B ] " exp(--t/zT).

where

k S = (k R + kNR + .kis c +/CAB [A2]) and

From (15) and (16) we obtain

[AB] = = kaa [A2] [1A*]0/k s , (17)

The growth of the A2B absorption can, therefore, be analyzed according to the following expression: log [OD~A2B ) -- OD(A2B)] = log [OD~A2B ) ] - kT/2.3t.

,

(-t/rT)],

where [A2B] = is defined as =

I

Xex,53Z112m

kisc (14)

,

A in P-DIOXANE

--

where [3A*]0 and [1A*]0 are the initial concentrations of the excited triplet and singlet, respectively, I 0 is the laser pulse intensity, eA is the extinction coefficient of A, and q5T the quantum yield for triplet formation is defined as

dpT =kR +kNR +kisc+kAB[A2 ] = kiscT"s .

L

(20)

where [AB] = is the concentration of AB 100/as after the excitation pulse. From (16) and (20) and using (12) and (13) for the values of [3A*]0 and [1A*]0 the ratio [A2B]** _ kA2B'rT [3A*] 0 [A 2 ] = kA2BrT~bT

(18)

A semilog plot of the decay of the triplet absorbance at 430 nm [eq. (9)] and expression (18) at 640 and 670 nm appears in fig. 4. The rate constant for the decay of the shorter component at 420 nm (triplet decay) and the build up of the slower component (A2B) at 6 4 0 n m are 1.0 X 106 and 0.9 X 106 s -1 , respectively. Thus, the decay of triplet state 3A* and formation of A 2 B are correlated within experimental error. The build up at 670 nm exhibits a rate constant of 0.5 X 106 s -1 . We attribute this to the progressive red-shift expected to ~'ccompany stack (Jaggregate) formation [ 13]. In a manner similar to the build up of the A 2 B transient from the triplet state we obtain for AB [AB] = (kAB/ks)[A2] [1A*]0 [exp ( - k s t ) - 1], (19)

[AB]-

kABr s [1 A*]0 [A2]

kgs r s

(21)

For concentrations [A0] < 10 - 4 M ([A0] = [A] + 2 [A 2 ]) and assuming a diffusion controlled bimolecular rate constant kA B ~ 109 M-1 s-1 we would exr~ 2 pect the triplet lifetime to be controlled by unimolecular radiationless decay processes and independent of concentration [9]. The concentration independence of ~-~ was observed in the concentration range 5 X 1 0 - - 5 X 10 -6 M at low excitation intensities. Substituting for ~T from (14) one obtains [A2B] °° -

-

[AB]"

-

kA2B'rT kisc

- constant.

(22)

kAB

The observed independence of this ratio on concentration demonstrates the importance of the bimolecular step (6) in the mechanism. In the case of the spiropyran A' Krongauz et al. [9] found a weak [A] 1/2 103

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CHEMICAL PHYSICS LETTERS

dependence on this ratio which served as evidence for direct photochemical conversion o f A 2 to AB. While this process is not ruled out in the present case, it does not account for the observed concentration independence o f [A2B ]**/[AB]**. Future experiments will address the details o f AB formation.

4. Conclusion A series o f photochemically induced reactions leading to the formation o f J-aggregate stacks o f A n B complexes has been proposed to explain the transients observed following laser excitation o f A. The observed transient species have been assigned to 3A*, AB, A2B , and (A2B)n stacks. Both AB and A2B were formed via bimolecular reactions involving singlet (1 A*) energy transfer to dimers (A2) and bimolecular reaction o f the triplet state (3A*) with dimers (A2) , respectively. Careful measurements in the 6 4 0 - 6 7 0 nm region indicated that the absorbance near 670 nm increased at a slower rate than triplet decay. We have attributed this slower progressive red-shift to stack formation. Although quasi-crystal formation has not been observed in aromatic solvents such as toluene [1 ] it is important to note that A2B and stack formation occur. It m a y therefore be possible to obtain stabilized stacks without the necessity o f quasi-crystal formation. Detailed measurements in other solvents and a more extensive description o f the photochemical processes will be presented elsewhere [15].

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5 February 1982

References [1] V.A. Krongauz, Israel J. Chem. 18 (1979) 304. [2] V.A. Krongauz, S.N. Fishman and E.S. Goldburt, J. Phys. Chem. 82 (1978) 2469. [3] B.F. Levine, C.G. Bethea, E. Wasserman and L. Lenders, J. Chem. Phys. 68 (1968) 5052. [4] A. Dulcic and C. Flytzanis, Opt. Commun. 25 (1978) 462. [5] A. Dulcic, Chem. Phys. 37 (1979) 57. [6] D.J. Williams, G.R. Meredith, J. VanDusen, G. Olin and V.A. Krongauz, Abstracts of the 28th IUPAC, Vancouver (1981); G.R. Meredith, V.A. Krongauz and D.J. Williams, Opt. Commun., to be published. [7] R.C. Bertelson, in: Techniques of chemistry: photochromism, Vol. 3, ed. G.M. Brown (Wiley-Interscience, New York, 1971) pp. 158-164. [8] J.B. Flannery Jr., J. Am. Chem. Soc. 90 (1968) 5660. [9] V.A. Krongauz, J. Photochem. 13 (1980) 89. [10] C.R. Goldschmidt, M. Ottolenghi and G. Stein, Israel J. Chem. 8 (1970) 29. [ 11] U. Lachish, R.W. Anderson and D.J. Williams, Macromolecules 13 (1980) 1143. [12] E.L. Zajtseva, A.L. Prokhoda, L.H. Kurkovskoya, R.R. Shffrina, N.S. Kardash, D.A. Drapkina and V.A. Krongauz, Khim. Giterotsikl. Soedin. 10 (1973) 1362. [13] M. Kasha, Radiation Res. 20 (1963) 55. [14] R.S. Becker, E. Dolan and D.E. BaRe, J. Chem. Phys. 50 (1969) 239. [15] J. Kalisky and D.J. Williams, to be published.