J. Phys. Chem. 1983, 87,5333-5338
5333
Dynamics of the Spiropyran-Merocyanine Conversion in Solution Y. Kalisky, T.
E. Orlowski, and D. J.
Wllliams"
Xerox Webster Research Center, Webster, New York 14580 (Received: February IO, 1983)
The photoinduced processes leading to the isomerization and complexation which preceed spontaneous aggregation are investigated for l-(~-methacryloxyethyl)-3,3-dimethyl-6'-nitrospiro(indoline-2,2'[2H] benzopyran) (SP) by picosecond and nanosecond transient absorption spectroscopy in toluene and acetonitrile. By monitoring the light (wavelength and intensity) and concentration dependences of the species produced upon photoexcitation of the ring-closed from (SP-A) a mechanism was developed which accounts for the formation of colored species in the picosecond to microsecond time domain. In toluene and methylcyclohexane colored species are formed by two routes: isomerization of a species X, in which the spiro C-0 bond is broken (but the nonplanar geometry of the parent is retained) to a ring opened form B and secondly by a bimolecular reaction of the K X * triplet of SP-A with a ground-state molecule to form a complex AB. In acetonitrile radiationless deactivation of X is favored and only one colored form AB is produced. Introduction Photoinduced spontaneous aggregation processes have been shown to occur when spiropyran molecules are irradiated in aliphatic solvents. The aggregates, which are globular in appearance, consist of submicron cores of crystalline material with an amorphous exterior and have been termed "quasicrystals".' Spectroscopic studies by Krongauz and co-workers2 indicate that the composition of the cores are A,B (n = 2,3) and the amorphous exterior AB. The most stable quasicrystals have been derived from 1-(~-methacryloxyethyl)-3,3-dimethyl-6'-nitrospiro(indoline-2,2'- [2H]benzopyran (SP-A) and its associated merocyanine form (SP-B).l It has been shown that the qua-
I
0
II c=o
c=o
I
ic=cH2
CH3
SP-A
I
/"
=cHz
CH3
SP-B
sicrystals have macroscopic electric dipoles indicating a noncentrosymmetric polar structure and that the crystalline cores are composed of J-aggregate stack^.^,^ The large molecular hyperpolarizability of the merocyanine chromophore and the highly polar environment of the quasicrystals has prompted studies of the second-order nonlinear optical properties of these material^.^ In order to gain some understanding of the primary photoprocesses and subsequent reactions leading to spontaneous ordering a laser photolysis study was made by Kronganz et al.5 on a model spiropyran SP'-A in me-
thylcyclohexane and in aqueous micelles. (A and B will be used in reference to ring-closed and fully opened trans-trans form, respectively). On the basis of these studies, a model was proposed where dimers AA were directly converted to AB upon absorption of light and triplets 3A* reacted with A2 to form A2B. The competition between A2B aggregation and incorporation of AB leads to quasicrystal formation. The transient absorption data for A in micelles and the dependence of the relative quantum yields of species attributed to A2B and AB on initial concentration of SP'-A were the basis for the proposed mechanism. A similar study of laser-induced transient absorption phenomena of SP-A in several solvents was conducted by Kalisky and Williams.6 In this study it was noted that the relative quantum yield of the species attributed to A2B and AB was independent of the concentration of A in contrast to the earlier findings for SP'-A which showed an dependence for that ratio. The assumption that AB was formed by energy transfer sensitization of the ground-state dimer A2 by the excited-singlet state 'A* was required in order that the other main features of the mechanism be consistent with the experimental data. A recent picosecond transient absorption study on a closely related system7 SP"-A indicated the formation of a cis-ciscoid intermediate and some of the final colored forms in less than 10 ps. From these experiments it was concluded that two ultrafast reactions occur from the first excited singlet state: rapid intersystem crossing to a vibrationally excited triplet and direct conversion to the ring opened form (Le., SP"-B) in less than 10 ps. The vibrationally excited triplet opens in less than 10 ps to the cis-ciscoid intermediate in competition with internal conversion to the relaxed triplet. The cis-cisoid intermediate then converts slowly to B. This explanation appears to be highly feasible but is in disagreement with the alternative m e c h a n i ~ m . ~ t ~ In an effort to further clarify the processes leading to color formation and spontaneous aggregation we have (1) V. A. Kronganz, Isr. J. C h e n . , 18, 304 (1979). (2) A. A. Parshutken and V. A. Kronganz, MOL.Photochem., 6, 437 (1974). ~~~
~I
(3) G. R. Meredith, V. A. Kronganz, and D. J. Williams, Chem. Phys. Lett., 87 289 (1982). (4) G. R. Meredith, David J. Williams, S. N. Fishman, E. S. Goldburt, and V. A. Kronganz, to be published in J.Phys. Chem., 87, 1697 (1983). (5) V. Kronganz, J. Kiwi, and M. Gratzel, J.Photochern., 13,89 (1980). ( 6 ) Yehoshua Kalisky and David J. Williams, Chem. Phys. Lett., 86,
* Current address:
Research Laboratories, Eastinan Kodak Company, Rochester, NY 14650.
100 (1982). (7) S. A. Krysanov and M. V. Alfimov, Chem. Phys. Lett., 91, 77 (1982).
0022-3654/83/2087-5333$01.50/00 1983 American Chemical Society
5334
The Journal of Physical Chemistty, Vol. 87, No. 26, 1983
Kalisky et al.
I f lOmV
0.8
440nm Ions+
p
AOD
-
400
500
600
700
Nnm) Flgure 1. Differential excited-state absorption at 10 ns and 100 ~s following excitation at 337.1 nm for 1 X oscilloscope traces at various wavelengths.
performed picosecond and nanosecond transient absorption experiments on SP-A in toluene, methylcyclohexane, and acetonitrile at various concentrations and excitation intensities. A unified model consistent with the transient photoprocesses observed on the picosecond to microsecond time scale is proposed.
Experimental Section Transient sorption spectra were obtained with a pulsed N2 laser and detection system. The laser was a Lumonics TE861S excimer laser operating on an N,/He mixture at 337.1 nm with a maximum pulse energy density on the sample of 170 mJ/cm2. The detection system was similar to that reported p r e v i o ~ s l y . ~All ~ ~measurements were performed at ambient temperatures and N2 was bubbled through the solutions prior to and during measurement unless otherwise specified. A mode-locked Nd3+/glass laser system provided excitation and probe pulses for the picosecond transient absorption measurements. Single pulses (7 f 2 ps) were extracted from the train, amplified, and then passed through type-I and type-I1 KD*P crystals to generate the second (531 nm) and third (354 nm) harmonics of the laser fundamental at 1062 nm. The excitation pulse (- 100 pJ) a t 354 nm was passed through two aperatures to reduce off-axis contributions to the laser beam and focussed into an aperatured (1mm diameter) quartz cell containing the sample in methylcyclohexane solution. Pulse energy was monitored with a calibrated beamsplitter and pyroelectric energy meter. The probe pulse (- 10 pJ) at 531 nm was sent through two aperatures, an optical delay line, and then focussed into the sample cell through the aperature attached to the cell's front face. Incident and transmitted (8)C.R.Goldschmidt, M. Ottolenghi, and G. Stein, Isr. J.Chem., 8, 29 (1970). (9)U.Lachish, R.W. Anderson, and D. J. Williams, Macromolecules, 13, 1143 (1980).
M SP in toluene. Insets are
probe pulse energy was monitored with calibrated photodiodes and sample/hold circuits with a dynamic range of >500. Transient absorbance was measured as a function of probe pulse delay time, 7,and corrected for residual absorbance with the excitation pulse blocked and fluctuations in the shot-to-shot excitation pulse energy. The determination of 7 = 0 was made by replacing the sample cell with a 1-mm-thick type-I1 KD*P crystal and the 354-nm excitation pulse with the laser fundamental. With this arrangement, T = 0 corresponds to a maximum in the third harmonic intensity generated in the crystal by the frequency mixing of the laser fundamental with the 531-nm probe pulse as the optical delay line is adjusted. The spiropyran SP was prepared according to the method of Zajtseva et al.1° The final product was recrystallized from 1 : l O benzene-hexane. 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 of 105 "C. The solvents, toluene and acetonitrile, were Burdik and Jackson High Purity and used without further purification. The solutions for measurement were held in optical grade quartz cells.
Results SP in Toluene and Methylcyclohexane. The transient absorption spectrum of SP in toluene (1 X 10" M) in the 10-ns to 100-ps time domain is shown in Figure 1. Spectra at the extremes of the time interval are shown as well as oscilloscope traces at several wavelengths following excitation. At 10 ns the spectrum exhibits two bands centered at 440 and 580 nm. The oscilloscope traces indicate that the species giving rise to the signal a t these two wavelengths are formed instantaneously on the time scale of (10)E.L. Zajtseva, A. L. Prokhoda, L. H. Kurkovskoya, R. R. Shifrina, N. S. Kardash, D. R. Drapkina, and V. A. Kronganz, Khirn. Giterotsikl. Soedin., 10 1362 (1973).
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5335
Dynamics of the Spiropyran-Merocyanine Conversion
I
b
t
1 e
0
F
+
Io X
R
s!
+ 0.251,
R
is
c
0.7510
0 0.501,
&
e
t
e
w
e
0
f
e
0
1
3
0.2 0.4 0.6 0.8 1.0 1.2 1.4
i
t
I
1.0
I
I
I
2.0
3.0
40
I
l
l
I
I
1.6
1.8
PS Figure 3. Semilog plot of OD,, and [OD(m) - OD], vs time for 5 X lo-, M SP in toluene at two light intensities differing by a factor of 5.
5 0 6 0 7.0 8.0 90 tL5
Flgure 2. Semilog plot of OD,, and [OD(m) - OD], in toluene at two concentrations.
vs. time for SP
i
0,3t 01 1 -
OD,,,=0.32(1
the experiment. On a longer time scale the absorption at 440 nm decreases and grows in time at 640 nm. The 440-nm curve does not decay to zero but reaches a plateau at a value which is stable throughout longest time interval of the experiment. On the 1 Fs/division time scale the absorption at 550 nm is nearly constant. The decay rate of the 440-nm peak increases in the presence of O2 and appears to correlate with buildup in intensity at 640 nm. In Figure 2 the decay at 440 nm and the buildup at 640 nm (expressed as OD(..) - OD(t) where OD(m) is the saturation value of the buildup) at two concentrations are plotted on a semilog scale. The plots indicate concentration-dependent behavior in the decay and buildup signals with the decay curves becoming exponential at longer times. The spectral and kinetic behavior of SP are virtually identical in methylcyclohexane and toluene and we therefore assume that results obtained in either of these solvents can be used interchangeably in formulating the general mechanistic picture for ring opening described below. So that further insight into the nature of the bimolecular reaction could be obtained the decay and buildup of absorption at 440 and 640 nm were determined (Figure 3) at two excitation intensities differing by a factor of 5. If the concentration dependence (Figure 2) was due to triplet-triplet anihilation a dependence of the rates on excitation intensity would be expected. The lack of this dependence rules out triplet-triplet annihilation as the bimolecular reaction and confirms earlier observations5s6that a reaction of an excited and ground-state species is involved. Based on these observations and those reported previously we conclude that the decaying species in nonpolar solvents at 440 nm is the triplet state 3A* and the correlated buildup in the 500-700-nm region is due to a bimolecular reaction between A and 3A*. These processes are discussed in detail below. The buildup of absorption at 530 nm following excitation with a 354 nm, 7-ps pulse of a and 5 X M solution of SP in methylcyclohexane is shown in Figure 4. The
1 I
0.351
-e-''*''
0.25
0.10
OV I00
200
300 400 500 600 700 800 OPTICAL DELAY (psec)
900 1000
Figure 4. Plot of photoinduced OD, vs. probe pulse delay for SP (5 X lo-, M, 1-mm pathlength and 1 X lo-, M, 5-mm pathlength, 0) in methylcyclohexane at room temperature.
absorption at 530 nm is due to the same species associated with the 550-nm signal in the nanosecond experiments. The buildup in intensity at the two concentrations can be fit by a single exponential with r 270 ps and no concentration dependence can be detected. We conclude that this species which is distinct from both the triplet state and the product of the bimolecular reaction results from direct isomerization of the first excited singlet state lA* to the trans transoid form B which is stable over the longest time interval measured. These observations appear to be contradictory to the results of Krysanov et al.7 where no buildup in absorption was seen at this wavelength in the 0-100-ps time interval. One possible explanation is that the improved signal-to-noise ratio and extended time range in the present experiment makes it possible to detect this buildup. Unfortunately, due to the limitations of our experimental setup we were not able to probe the 440-nm range where Krysanov et aL7 reported detection of the
-
5336
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
Kalisky et al.
0.5
0.4
OD 0.3
0.2
0.I 0 500
X (nrn)
700
600
Figure 5. Differentialexclted state absorption at 10 ns. 50 ns. and 100 jts following excitation at 337.1 pm for 5 X are oscllioscope traces at various wavelengths.
lo4
M SP in CH3CN. Insets
nonplanar cis-cisoid isomer X which they claim isomerizes to the planar trans isomer on the 100-ns time scale.
A
X
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B This species is thought to be nonplanar, having the geometry of the parent molecule, and thus the polar resonance form would predominate. Based on the weight of the present evidence we assign the 530-nm buildup to species B and will assume that X is superimposed on the triplet absorption in the 400-500-nm region. Further justification for this assignment is given below. SP in Acetonitrile. The transient absorption spectrum of SP (5 X M) in CH&N is shown in Figure 5. Several striking differences occur with respect to the behavior in the nonpolar solvents. The first is the presence of a rapidly decaying component (-50 ns) in the 400500-nm region superimposed on the otherwise normal triplet decay. A similar component was observed in a lo4 M solution and the rate of decay and relative intensity appeared to be independent of concentration. The spectral features in the 400-500-nm range are virtually identical with those observed by Krysanov et al.' at 10 ps in nonpolar solvents. After 50 ns the spectrum and its decay kinetics are consistent with 3A* observed in nonpolar solvents and support our earlier contention that the absorption spectra of A3 and X overlap. Apparently the polar CH3CN stabilizes the X form and its lifetime is extended to the nanosecond regime. Since no accompanying growth
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1.0
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2 .o
1
3.0
I
I
4.O
5 .O
1
CLS
Figure 6. Semilog plot of log O ,D , and log (OD(m) - OD], for SP in CH,CN at two concentrations.
vs. time
occurs in any other region of the spectrum we assume that radiationless deactivation of X to the ground state occurs rather than isomerization. Apparently the polar environment preserves the geometry favoring reformation of the C-0 cr bond. Another distinguishing feature of SP in CH&N is the buildup in signal a t 550 nm which correlates with the decay of the triplet state (Figure 6). The rapid drop in absorbance at 440 nm in the 100-ns time frame is evident. The decay and buildup are similar to that observed in nonpolar solvents where concentration-dependent behavior is seen at early times and the triplet decay rate approaches concentration independent decay a t longer times.
Discussion Before discussing the experimental results it would be useful to breifly review the two models mentioned in the Introduction that have been proposed to account for the photoinduced ring opening and subsequent behavior in molecules containing the SP chromophore. Krongauz et al.6 proposed the following model to account for the transient photoprocesses in SP'. For simplicity we
Dynamics of the Spiropyran-Merocyanine Conversion
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5337
will refer to the A and B forms of SP': 2A
A2
A*(420 m)
A B (530 nm)
1
/[A21
A@ (600 nm)
A monomer-dimer equilibrium is proposed but no direct spectral evidence is given. The triplet state is formed rapidly and reacts with dimer Az or returns to the ground state. The absorption in the 400-500-nm region is attributed to 3A* and a slowly formed species in the red portion of the spectrum correlated with the decay of 3A* is attributed to A2B. The second instantaneously formed species, absorbing in the 500-550-nm region is attributed to AB,the directly excited dimer. The ratio of absorbances of species attributed to A2B and AB was found to vary as and provided self-consistencyfor the model. Kalisky and Williams6 conducted similar experiments on SP in nonpolar solvents and obtained general agreement with Krongauz's results5 with one exception. The ratio [A,B]/[AB] was found to be independent of [A]. In order for the remainder of the mechanism to be valid a bimolecular process was required to account for AB formation and it was proposed that energy transfer sensitization of Az by lA* might occur. More recently Krysanov et al.7 proposed an alternative scheme to account for the photocoloration processes. Their mechanism is based on picosecond and nanosecond transient absorption data. A kph
%
1
3A*
h
*
L
B
I
4scI F_
*IC
'A**
k3
_ I
X
k2
The rate constants in that scheme have the following values: kl = kist I 1011 s-1
kic = k z I 10" s-l kph = 10 s-1 k3 = 107 ~1 They observed photoinduced absorption at 440 nm within the duration of the 8-ps excitation pulse and attributed it to X, the nonplanar ring opened species with the geometry of A. Although no direct experimental evidence is presented a vibrationally excited photochemically active a-r* triplet state (3A*') which is about 1500 cm-' above 3A* is postulated as the precursor to X. This state can be accessed in two ways: internal conversion from a higher lying n-r* triplet state which enhances intersystem crossing from the r-r* singlet state (lA*) or by triplet sensitizersll with 1500-cm-l excess energy with respect to 3A* but insufficient energy (-5000 cm-l) required to sensitize the n-r* triplet. Since the spectral features in the 400-500-nm region were very similar on the picosecond and nanosecond timescale they assumed that absorption in this region is due solely to X. This also lead them to conclude X exists into the microsecond regime.IJ2 Since we have observed
-
(11)D.A. Reeves and F. Wilkenson, J . Chem. Soc., Faraday Trans. 2, 1381 (1973). (12) V. A. Murin, V. F. Mandikov, and V. A. Barachevsky, Opt. Spectrosc., 37,1174(1974);40,1084(1976);42,79(1977);45,411(1978).
5 t (ps)
0
IO
Flgure 7. Data points are OD,,, vs time for (a) 1 X M and (b) 2X M SP in toluene. Solid line are a fit of eq 4 with parameters indicated in the text.
two independent species in this spectral region with very similar features we believe that X exists in the 100-300-ps timeframe in nonpolar solvents and converts thermally to B on this time scale. In CH3CN its lifetime increases by an order of magnitude. The direct formation of B from lA* is also a postulated reaction and is not supported by direct experimental evidence.I Our picosecond data suggest that B is not formed instantaneously and is formed only through the route that we attribute to the isomerization of X. We propose the following mechanism to account for the experimental data presented herein as well as the available published data on transient behavior in the picosecond to microsecond time domain: 3A* I
A'
A I *
[AI
CAB1
kAB
*
A ( X or 'A*)
Intersystem crossing is assumed to be rapid so that kisc = k, < 8 ps. The intermediacy of various short-lived states such as X* (a vibrationally hot form of X) and 3A*i (an nr* state or vibrationally excited rr*)have been proposed as intermediates in the formation of 3A*, X, and B.'J5 There is evidence, however, that B is formed directly from lA* in a related systern.l6 The values of the rate constants (13)T.J. Chung and K. B. Eisenthal, J. Chem. Phys., 62,2213(1975). (14)R. M.Noyes, Progr. React. Kinet., 1 129 (1961). (15)R. C. Bertleson in "Techniques of Chemistry: Photochromism", Vol. 3, G Brown, Ed., Wiley-Interscience, New York, 1971,pp 158-64. (16)Y. Kalisky and D. J. Williams, submitted to Macromolecules.
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are consistent with Krysanov et al.7 but we assume that at early times the absorption in the 400-500-nm region is due to both X and 3A* (kB > k, 4 X lo9 s-l as determined by the buildup in signal a t 530 nm (Figure 3) in toluene). In CH3CN, X is considerably longer lived (kB < k, 3 x lo7 s-l as estimated from the rapidly decaying component at 440 nm (Figures 5 and 6) and the fact that no correlated buildup is seen in the 500-600-nm region). The bimolecular nature of AB formation in toluene is evident from an analysis of buildup of absorption at 640 nm (Figure 7). If kAB[AB]> ktthe buildup in signal which is proportional to [AB] will be determined by the second-order rate constant. If k A B 1O’O M-l s-l, ie., a diffusion-controlled reaction, it would require that kT C lo5 s-l in order for second-order kinetics to dominate. The two initial concentrations of SP-A and 2.5 X 10“ M) were chosen to provide uniform excitation of the sample volume while allowing a 4X variation in concentration. The optical densities a t the excitation wavelength were 0.8 and 0.16 thus fulfilling these conditions. It has been shown13 that when an instantaneous and uniform distribution of reacting pairs is created, as with pulsed laser excitation, transient behavior due to the instantaneous flux occurs a t early times. This situation is equivalent to having a time-dependent second-order rate constant. Under these conditions the expression for buildup of products is given by
-
Kallsky et al.
5t
4
4
-
-
where [AB], is the value of AB a t long times. A simple expression for the rate constant km(t 9 has been given as14 1 . . 4apD k
L
where D is the diffusion coefficient for relative motion of the two molecules, p is the sum of the radii of the reacting molecules, and k is the rate constant if the equilibrium molecular distribution were maintained. Since we do not have good estimates of the three parameters we rewrote (2) as kA,(t) = A + B/2t-lJ2 (3) substitution of (3) into (1)followed by integration gives [OD,,]
= [OD]A,~{~ - exp[-(AT -k B!i“l”’31]
(4)
where OD is the optical density associated with AB. This expression predicts that the buildup curves should be described by a single set of parameters A and B with a scaling factor [AB], which can be experimentally determined. The solid curves in Figure 7 represent fits of (4) to the experimental data. A reasonable fit was obtained and [OD]*B- (0.23, 0.69) with A (8 X B (9 X for the two solutions. Since experimental conditions (i.e., laser power, distribution of excitation, optical alignment, etc.) can vary slightly between experiments the deviation of the values of [OD]AB~ a t the two concentrations from (4), the ratio of the concentrations does not seem partic-
‘t
-+- I IO
LOG A
Figure 8. Plot of saturation value of OD,/OD, In toluene.
vs. log [A] for SP
ularly significant. The general agreement between experiment and the model convinces us that a diffusioncontrolled bimolecular process is responsible for the buildup in absorption at 640 nm. If kAB[A]> kT as we have suggested, it is obvious from inspection of the mechanism that the ratio [AB]/[B] a t long times is independent of concentration. The saturation value of the optical densities associated with AB and B are plotted as a ratio in Figure 8 over two decades of concentration and the lack of dependence on the concentration of A is evident. Although we did not apply the same analysis to the buildup in signal a t 560 nm in CH3CN because of poor signal-to-noise ratio a definite concentration dependence to the buildup in signal suggests that the species is AB is formed. A significant blue shift in the absorption spectrum is expected in the polar CH3CN relative to nonpolar solvents due to stabilization of the polar resonance form contributing to B.
Conclusions We have used picosecond and nanosecond spectroscopic techniques to investigate the transient photoinduced processes leading to ring opening and coloration of SP in nonpolar and polar solvents. A comparison of newly obtained data with published results on this and related systems has lead to a mechanism which accounts for all of the experimentally verifiable data. The proposal of the formation of a ring-opened species X within the time of the 8-ps excitation pulse which retains the geometry of the parent spiropyran and undergoes isomerization on the 300-ps time scale in nonpolar media is an essential feature of the mechanism. The formation of triplet state 3A* is represented as being in competition with X. 3A* and X could result from the nr* triplet which is lower but close in energy to lA* but this does not appear to be experimentally verifiable a t present and does not fit the experimental observations better than the proposed mechanism. The formation of B and AB are described by unimolecular and bimolecular kinetic models and the rates of formation occur on time scales consistent with the proposed processes. In summary we feel that the current study unifies many of the concepts that have been published and discussed regarding the primary photoprocesses in these molecules. Registry No. S P A , 25952-50-5; SP-B, 87495-24-7.
