J. Phys. Chem. 1993,97, 9499-9505
9499
Colored Merocyanine Aggregates: Long-Lived Crystals of Large Size (10-100 pm) and Deaggregation of Small Aggregates in Solutions Yoshiyuki Onai, Michiko Mamiya, Toshio Kiyokawa, Ken-ichi Okuwa, Makoto Kobayashi, Hisanori Shinohara, and Hiroyasu Sato’ Department of Chemistry for Materials, Faculty of Engineering, Mi’e University, Tsu 51 4, Japan Received: March 26, 1993; In Final Form: June 4, 1993”
Prolonged (>30min) UV irradiation of 6- and 8-nitro- 1’,3’,3’-trimethylspiro[2H-l-benzopyran-2,2’-indoline] in some nonpolar solvents precipitated large-sized merocyanine crystals. The sizes of crystals (10-100 pm) were much larger than those of quasicrystals reported by Krongauz et al. Colored merocyanine was long-lived (more than days) in these crystals. Raman spectra indicate a structure in which transoid merocyanine conformers are coupled to each other via the NO group in the aci-nitro structure. In the very initial stage of aggregate formation, anomalous nonexponential absorption decay at the merocyanine monomer band was observed. The anomaly was more pronounced for higher initial dye (spiropyran) concentration, for lower temperature, and for longer UV irradiation time. The anomalous absorption decay indicated partial deaggregation of small merocyanine aggregates in these solutions.
1. Introduction Spiropyrans constitute an important class of photochromic compounds.’ Their photochromism is due to reversible photoisomerization of colorless spiropyran (A) into a colored merocyanine (B), which thermally (or photochemically) reverts to the original colorless form. Formation of merocyanine aggregates has been reported by several research groups. Krongauz et al.2-5 reported the formation of “quasicrystals” (tiny globules, 0.1-0.4 Fm in diameter) upon UV irradiation (130 s) of spiropyrans in aliphatic hydrocarbons both at low temperatures (173-243 and room temperature! According to these authors, the quasicrystals are composed of an inner crystalline core with the composition A,,B (n = 2, 3) and an amorphous outer layer with the composition AB. The quasicrystals have macroscopic electric dipoles,5.6 indicating a noncentrosymmetric crystal structure for the A2B complexes. Spectral properties of the quasicrystalssuggest that the crystalline cores are composed of J-aggregate-likestacks. Nonlinear optical properties of these species also have been reported.’J The photoinduced processes leading to quasicrystal formation have been studied by several groups. Krongauz et a1.9 reported the kinetics of formation of the complexesAB and A,B. Kalisky and WilliamsloJl attributed the slow progressive red shift in the 640470-nm region to the formation of J-aggregate-like stacks (AB),, or (A,B),, (n,m = 2, 3). Aggregate formation of merocyanine in bilayer membranes,12Langmuir-Blodgett films,13-15 or polymer filmsl6 also has been reported. The present paper addresses two novel features of aggregate formation. We previously reported” that prolonged (>30min) UV irradiation of 6- and 8-nitro- 1’,3’,3’-trimethylspiro[2H-1benzopyran-2,2’-indoline](6- and 8-nitro-BIPS)in some aliphatic and other nonpolar solvents gave colored merocyanine crystals of large size (10-100 pm), which precipitated out of the solution. These crystals were much larger in size compared to the “quasicrystals”. Color of merocyanine persisted for several days in these crystals. On the other hand, in the very initial stage of aggregate formation, an anomalous nonexponential decay of the merocyanine monomer band was observed for 6-nitro-BIPS in cyclohexane.18 In the present paper, the morphology of largesized aggregates has been studied by optical and electron microscopy. Raman spectroscopic studies have been made to probe their molecular structures. The anomalousabsorption decay K)*p3
Abstract published in Advance ACS Abstracts, August 15, 1993.
has been studied in more detail, and a simulation is tried on the assumption of partial deaggregation of small merocyanine aggregates.
2. Experimental Section
Reagents. 6-Nitro- and 8-nitro-BIPS, 1’,3’,3’-trimethylspiro[2H-1-benzopyran-2,2’-indoline](BIPS) (Tokyo Kasei), isopentane, hexane, heptane, l,Cdioxane, styrene monomer (Wako), cyclohexane, benzene, toluene, m- and p-xylene, diethyl ether, dibutyl ether, trichloroethylene, chlorobenzene, o-dichlorobenzene, and carbon tetrachloride (Nacalai Tesque) were used as received. Largesized Aggregate Formation. Nondegassed solutions of 6- or 8-nitro-BIPS in either of these solvents (5 X - 1 X 10-2 M) were irradiated by an Ushio 150-W xenon arc lamp at room temperature. Irradiationwascontinueduntil a substantialamount of colored aggregates precipitated (130 min). The colorless supernatant was decanted, and the precipitate was washed repeatedly with solvent until no coloration due to merocyanine was observed on irradiationof the wash solution. Then the washed precipitate was used in microscopic, absorption, and Raman studies. UV irradiation of BIPS was made under the same experimental conditions for comparison. Optical microscopic images were obtained by a microscope (Uyeno Seisakusho, Type Y),using a Canon A-1 camera and Fuji NEOPAN SS film. Electron microscope images were obtained from a Hitachi scanning electron microscope S-2300S, using Kodak Tri X Pan (IS0 400) film. The X-ray diffraction pattern was measured on a Rigaku RAD-C X-ray diffractometer. Absorption spectra were measured using a Hitachi 200-20 recording spectrophotometer. The spectra of aggregates were measured on thin (0.4 mm) KBr disks, in which the concentration of aggregate was 7 X g/g of KBr. Raman spectra were measured on KBr disks using a JASCO CT-80D double monochromator equipped with a Hamamatsu R649/C1050 photomultiplier/photon counting unit. The excitation was 0 . 1 4 . 5 W of 514.5-nm light from a Spectra Physics 165 argon ion laser. A rotating sample cell was used to avoid the deterioration of the sample dye during the measurements. AnomalousDecay Measurements. UV irradiation (160 s) was performed with the Ushio 150-W xenon lamp. The solution in a 10-mm X 10-mm quartz sample cell was kept in a thermostated water bath during the irradiation and measurements of absorption spectra. Absorption spectra and absorption decay were measured
0022-3654/93/2097-9499%04.00/0 0 1993 American Chemical Society
9500
The Journal of Physical Chemistry, Vol. 97,No. 37, 1993
TABLE I: Formation of Large-Sized Aggregates and Their Morphology In Various Solvents relative dielectric polarizabilityd/ solvent constantu 10-30 m3 aggregatesC 1.924 sc heptane 10.87 sc 2.023 cyclohexane 2.102 10.0 1,4-dioxane f 2.238 10.5 sc carbon tetrachloride 2.270 14.1 p-xylene P 10.32 2.284 benzene P 2.374 m-xylene P 12.26 2.3796 toIu en e 2.43b styrene monomer d 3.06b dibutyl ether 3.42c trichloroethylene f d 4.335 8.73 diethyl ether 5.708 12.3 chlorobenzene f 9.936 o-dichlorobenzene f a Values of 20 OC, Kagaku Benran (Chemistry Handbook), 3rd ed.; Chem. SOC.Japan, Ed.; Maruzen: Tokyo, 1984. 25 ‘C. 16 ‘C. Handbook of Chemistry andPhysics, 70th ed.;CRC Press: Baca Raton, FL, 1989. Morphology observed by optical microscope: sc = small crystals (largest dimension 1 1 0 pm), p = platelets (0.2-0.4 mm), d = dendrites (0.5-0.6 mm). /No aggregate formation.
;
on the Hitachi 200-20 double-beam spectrophotometer, using a thermostated sample cell holder. The digital output of the spectrophotometer was fed into a microcomputer in which data were stored and processed.
3. Results and Discussion Formation of Large-Sized Aggregates and Their Morphology. UV irradiation for 30 min was performed on 6-nitro-BIPS M) in a variety of solvents. (concentration 5 X 10-3 - 1 X Aggregate formation occurred in some of the solvents used, as shown in Table I, along with their dielectric constants and polarizability values. Aggregate formation occurred to a lesser extent for 8-nitro-BIPS and did not occur for BIPS. The morphology of aggregates (Table I) depended on solvents. Fast precipitation of a large amount of small (110 pm) crystals was observed in cyclohexane, heptane, and carbon tetrachloride. A relatively small amount of larger (0.2-0.4 mm) platelets was formed in benzene, m- andp-xylene, and toluene. Larger dendrites (0.5-0.6 mm) were formed in diethyl and dibutyl ether. Formation of aggregates in these solvents was much slower compared to three solvents mentioned above. No aggregate formation was observed for the other solvents in Table I under these experimental conditions. Thus, a little increment in solvent polarity retards the crystallization, leading to the slow formation of larger crystals. This must be due to subtle variation among the solvents in the ability to partially solubilize merocyanine monomers. Aggregates did not form in more polar solvents such as methanol or acetone. When these solvents were added onto the aggregate, it dissolved into them, giving the absorption spectra coincident with merocyanine monomers in these polar solvents. Electron microscopic images of these aggregates clarify more detailed structures. Some of them are shown in Figure 1. Minute crystals obtained fromcyclohexaneareca. 10-pmdendrites.Small platelets from m-xylene show an interesting bundle structure composed of ca. 10-pm X 100-pm plates. Star- or ribbon-like structuresof ca. 20-pm X 100-pm prisms are given by aggregates from toluene. Dendrites from diethyl ether are composed of stacks of platelets (ca. 10 pm) formed on backbone crystals of ca. 200pm length. In addition to the crystalline components, amorphous material can be observed in the background in some of the figures. X-ray diffraction patterns of the large-sized aggregates from cyclohexane and m-xylene showed sharp peaks a t 20 = 16.5O. This indicates the essentially crystalline nature of these aggregates. The peak position yields 0.53 nm as a spacing between some lattice planes.
Onai et al.
Thermal and Photochemical Stability of the Large-Sized Aggregates. Aggregates obtained by UV irradiation (30 min) of cyclohexane solution of 6-nitro-BIPS (5 X lk3M) were kept in dark at room temperature. Visible absorption, Raman, and IR spectra measured after 30 days were essentially unchanged from those immediately after preparation. The aggregates prepared in the same way were irradiated by 514.5-nm light of an argon ion laser (0.4 W) for 30 min. This did not cause any change in their Raman spectra. Theaggregateis stable tovisibleirradiation. When aggregates were heated, the dark violet color turned into yellow at about 140 OC. The yellow material was stable a t room temperature. Its UV absorption, Raman, and IR spectra were the same as those of spiropyran (uncolored form). On further heating, the yellow material melted near 177 OC (melting point of spiropyran). Thus, heating close to the melting point loosens the packing of the aggregates, and they are transformed into uncolored spiropyran. Comparison with the “Quasicrystals” of Krongauz et al. The aggregates obtained in the present study are apparently much larger in size compared to quasicrystals of Krongauz et al. Formation of aggregates of larger size must bedue to the prolonged irradiation time (>30 min). The quasicrystals were composed of a crystalline core and an amorphous outer layer. The largesized aggregate consists of large crystals and a small amount of amorphous material. The crystalline parts of the quasicrystals apparently grow much larger in size during the prolonged irradiation, following the formation of quasicrystals. Absorption Spectraof the Largesized Aggregates. Absorption spectra of the aggregates formed in various nonpolar solvents were measured in KBr disks. The essential features of the resulting spectra did not depend on the solvent used. As an example, the spectrum for the aggregate formed in cyclohexane (5 X 10-3 M, 30-min irradiation) is given in Figure 2. Krongauz et al.4 measured absorption spectra of quasicrystals of 6-nitro-BIPS in methylcyclohexane. The spectrum for the aggregate obtained by irradiation a t room temperature showed a very broad band covering the 500-650-nm range. The spectrum obtained by irradiation at 153 K gave a broad maximum near 510 nm, which was attributed to amorphous “dimer” AB. The differencespectrum,withamaximumnear 610nm, wasattributed to crystalline “charge-transfer complex” A,,B (n = 2, 3). The charge-transfer complex is described as corresponding to parallel dipole structures (a J-aggregate).lg Kalisky and Williamslostudied photoinduced processes leading to J-aggregate stacks of a related compound, 1’-(@-methacryloxyethyl)-3’,3’-dimethyl-6-nitrospiro [2,2’-indoline-2H- 1-benzopyran] . They assigned the rapidly formed and slowly decaying species with ,A, near 550 nm to AB and the slowly formed species in the 560-680-nm region to A2B. The slowly formed species which absorbed beyond 660 nm was associated with the formation of J-aggregate stacks (AzB),,. They remarked that A2B and stack formation occurred not only in aliphatic solvents but also in aromatic solvents such as toluene. These authors11 copolymerized the compound with methyl methacrylate and performed a similar nanosecond laser photolysis study. A gradual and slow red shift of the band in the 560-680-nm range was attributed to the formation of aggregates (AB),, or (AmB),, (n, m = 2, 3). Lenoble and BeckerZOstudied 6-nitro-BIPS in hexane and found bands at 370,430,570,and 630 nm (shoulder) immediately after excitation. While the 430-nm band decayed with time, the other three bands increased their intensity. They assigned the 430-nm band to the cisoid merocyanine monomer and the 570- and 630nm bands to the transoid merocyanine monomer and dimer, respectively. Tamaki et al.21 also assigned the bands of 6-nitroBIPS at 580 and 640 nm to merocyanine monomer and dimer. The spectral features a t 575 and 620 nm found for the largesized aggregates cannot be assigned as monomer and dimer bands,
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9501
Colored Merocyanine Aggregates
b
Figure 1. Electron microscopic images of aggregates formed in (a) cyclohexane, (b) m-xylene, (c) toluene, and (d, e) diethyl ether. A part of (d) is magnified in (e). The length of the shorter sides of figures corresponds to 25, 250, 250, 250, and 50 pm for (a), (b), (c), (d), and (e), respectively. cn c, .rl
c 3 El L
a
\ ) .
c, . , I
cn c aJ c,
c
H
700
650
600 550 500 450 W a v e l e n g t h / nm
400
Figure 2. Visible absorption spectrum of the aggregate (measured in a KBr disk).
however, since the aggregate must be composed of a large number of monomer constituents. The 620-nm band must be due to crystalline aggregates. A red shift of the band with respect to the monomer band indicates the head-to-tail structure of monomer
constituents (Le., J-type configuration) in the aggregate. However, the absorption band is not so sharp as that of the typical J-aggregate, which has been found for Langmuir-Blodgett films of some spiropyrans with long chains.13 This suggests a rather loose packing of the monomers in the aggregate. A model for quasicrystals consisting of weakly interacting molecular stacks is given by Meredith et a1.8 They contemplatedthat the difference in nearest-neighbor interaction energies between parallel and antiparallel orientations of merocyanine dipoles in the stacks is less than 0.1 eV. The 575-nm band may be due to merocyanine aggregates forming relatively loose stacks. The shoulder at 505 nm in the large-sized aggregates can be assigned to the band of amorphous materials, corresponding to the 5 10-nm band of the quasicrystals. Raman Spectra of the Large-Sized Aggregates.22 Raman spectra of the aggregates formed on UV irradiation (30 min) of cyclohexaneand diethyl ether solutionsof 6-nitro-BIPS are shown in Figure 3, a and b, respectively. Since the aggregates are mostly composed of crystalline components, the Raman spectra can be ascribed to them, although some contribution from amorphous
9502
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993
Onai et al. I
=-
1
I
A
I
I'
1900
1
I
I
1300 1700 1500 Raman S h i f t / c m - l
1100
Figure 4. Raman spectrumof uncolored 6-nitro-BIPSbefore irradiation
(in a KBr disk).
1600
1400
1200
1000
Raman Shift /cm-l
Figure 3. Raman spectra (in KBr disks) of aggregates formed in (a) cyclohexaneand (b) diethyl ether, in comparison with (c) the transient Raman spectrumof 6-nitro-BIPS in acetonitrile reported by Takahashi et al. (adapted from ref 23). See text.
TABLE II: Raman Spectra of BIPS, &Nitro-BIPS (Uncolored Form), Merocyanine, and Lar e-Sized Aggregate of 6-Nitro-BIPS; Positions of Bands (cm- )
f
BIPS
6-nitroBIPS merocvanhe aggregates
1654vs 1657w
1612vs 1582s
1594 m
1597 m
1537 s
1541 s 1532 vs
benzene ring
1585x11
CTT N=O str in aggregates NO2 antisym str
1520 w 1491 m 1465 w
assignment cis-HC=CH-, C = C str CTT'
1465 m 1411 w
1466 m 1412 w
CTC
merocyanine
N-CH3, CH3 sym def
1407w 1405w 1370 m 1358 m 1335 s
1362 s TT-T (1331)b NO2 sym str
1316 m 1281 vs
N-0 str in aggregates
1281 m 1270111 1265 w 1250s
1255 sh
1227s
1225m
merocyanine
1230 m
1240 m
11701x1 lllOm
cis-CH=CH-, CH def 11901x1 aggregate (N-O str) CH3 rock 1159 m merocyanine 1126 m merocyanine 1085 w CHI rock CH3 rock CTT 950 w
1157111 1190111 1085 m 1015 w 955 w
TTC C - 0 - C antisym str
945 w
From Takahashi et al. (ref 23). Observed for aggregates formed in m- and p-xylene, diethyl ether, and dibutyl ether (see text). CTT = cis-trans-trans, with respect to the three C-C partial double bonds in the merocyanine skeleton; CTC and TTT have corresponding meanings (see text). materials also may be involved. Positions of bands are given in Table 11. The Raman spectrum of the uncolored (spiropyran) form'of 6-nitro-BIPS (Figure 4) gave bands near 1335 and 1520 cm-l which were not found for BIPS. These bands must be due to symmetric and antisymmetric stretching modes (v, and vas) of the nitro group. Raman spectra of aggregates are completely different with the v, and v,, bands not appearing. In the panel c of Figure 3, the transient Raman spectrum of merocyanine (monomer in a fluid solution) of 6-nitro-BIPS reported by Takahashi et a1.23 is shown. In cyclohexane the spectra changed with time (25 rs-2 ms) after UV excitation. In a
Figure 5. A probable structure of the aggregate of 6-nitro-BIPS. (One of resonance hybrid structures is shown.)
acetonitrile and methanol the spectra did not exhibit time dependence for 200 ns-2 ms. In comparing spectra of transient species in these solvents observed a t 2 ms after UV irradiation, a good coincidence in band frequencies was observed, although relativeintensities of the band differed considerably. The transient spectrum measured in acetonitrile is reproduced here (because it compares better with our spectrum than that in cyclohexane). When one compares the spectrum of aggregates (a) with the transient spectrum (c), a general similarity is found, not only in band positions but also in their relative intensities, except for three bands shown with asterisks (1532, 1281, and 1190 cm-l) whcih are found only in the aggregate. This indicates that the colored merocyanine structure, short-lived in solutions, is stabilized in the large-sized aggregate. The three extra bands must be due to the aggregates. Takahashi et aI.23 assigned some of the bands they observed to three conformations of transoid merocyanine. The bands at 1537, 1465, and 1358 cm-l were assigned to cis-trans-trans (CTT), cis-transxis (CTC), and trans-trans-trans (TTT), respectively, with respect to the three C-C partial double bonds in the merocyanine skeleton. All of these bands are observed in the spectrum of the aggregate. This strongly suggests that the aggregate contains these three transoid merocyanine conformers. In light of the experimental finding that the aggregate is formed for 6- and g-nitro-BIPS, in lesser extent for the latter but not for BIPS, the crucial role of a nitro group in the aggregate formation is evident. The presence of a nitro group at the 6-position in the benzopyran ring gives rise to the contribution of the aci-nitro form in the resonance hybrid structure of the transient species,23 and this effect increases the resonance stabilization energy. The coordination of two merocyanine molecules via NO group is now possible. Based on these considerations, plausible molecular structures of the aggregates are those containing the coordinated aci-nitro structure in which two N-0 bonds are not equivalent. One of these conformations is shown in Figure 5. This conformation correspondsto the CTT configuration of three C-C partial double bonds in the merocyanine skeleton. The bands a t 1281 (and/or 1190) and 1532 cm-1 are most probably assigned as the N-O and N = O stretching modes of the asymmetric nitro group. However, experiments on the 15N isotopomer should be made to establish these assignments. These experiments are now underway. In Figure 3b, the band a t 1331 cm-l (shown with an arrow) is found, in addition to the bands observed in (a). The position ofthe 1331-cm-l bandisverycloseto 1335 cm-1, themostintense
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9503
Colored Merocyanine Aggregates Irradiation time (b)
u C
20s
difference
Wavelength
/ nm
Figure 6. Absorption spectra of 6-nitro-BIPS solution (1.0 X 10-3 M) after UV irradiation. Irradiation time: 10 s (a), 20 s (b), 60 s (c). Temperature: 20 O C . Difference = (c) - (a) (magnified).
band of the uncolored spiropyran. Aggregates formed in cyclohexane, heptane, and carbon tetrachloride show Raman spectra of type (a), while those formed in m- andp-xylene,diethyl ether, and dibutyl ether exhibit those of type (b). The presence of the 1331-cm-1 band may indicate that substantial amounts of uncolored spiropyran (A) remain in the aggregates formed in the latter four solvents. Ammalous Absorption Decay at the Monomer Band. Absorption spectra of cyclohexane solutions (1.0 X 10-3 M) after UV irradiation for 10,20, and 60 s are shown in Figure 6. The bands at 576 and 620 nm are monomer (B) and dimer (AB) bands of merocyanine, respectively.' The shoulder around 700 nm which appeared on prolonged UV irradiationcan be attributed to the J-aggregate, correspondingto "charge-transfer complex" (CTC) reported by Krongauz et ala3.' Absorption decays (logarithmic plots) of the monomer band (576 nm) of 6-nitro-BIPS in cyclohexane (1.0 X 10-3 M) for various conditions of temperature and irradiation time are shown in Figure 7. All the data are those for UV irradiation of 'virgin" samples, Le., for the first irradiation of fresh sample solutions. In some cases, deviations from single-exponentialdecays are found in the initial time region. In comparison with decays in other initialdyeconcentrations(8.0X l W , 1.0 X lWM),thedeviations were more pronounced and observable until a later time for (1) higher original dye (spiropyran) concentration, (2) lower temperature, and (3) longer irradiation time. All of these conditions are those which favor aggregate formation. Anomalous decays were observed for cyclohexane, heptane, hexane, and isopentane and were not observed for carbon tetrachloride, benzene, toluene, and p-xylene (for 6-nitro-BIPS, 1.0 X 10-3 M, 25 "C). The aliphatic hydrocarbon solvents which showed the anomaly are exactly those in which quasicrystals are formed.24 In the experiments of prolonged irradiation by Onai et al.,17 fast formation of minute crystals were observed for these aliphatic hydrocarbons and carbon tetrachloride, and slow formation of larger crystals occurred for benzene, toluene,andp-xylene. Thus, the observed anomalous decay must be associated with the fast formation of small aggregates. This phenomenon thus precedes the formation of quasicrystals reported by Krongauz et al. Although aggregate formation did occur in benzene or its analog,l'JJl it did not lead to the anomaly. Simulation of Anomalous Absorption Decay. Even when anomalous decay was observed, the later part of the decay was normal (exponential). When the absorbance due to this exponential decay (extrapolated to the initial time region)
An-,(t) = A, exp(-k,t) (1) is subtracted from the total observed absorbance A(t), we obtain a kinetic pattern such as shown in Figure 8a. Such a pattern is typical for the intermediate species (B) in the consecutivereaction A B C. Since the anomalousdecay was observed only when
--
0
150
100
50
TIME
200
/ s
Figure 7. Absorption decay (logarithmic plot) of 6-nitro-BIPS (1 -0 X 10-3 M in cyclohexane) at the merocyanine monomer band (576 nm). Temperature and irradiation time are the following: (a) 15 "C,10 s; (b) 15 OC,30 s; (c) I5 O C , 60 s; (d) 20 OC,10 s; (e) 20 OC,30 s; ( f ) 20 OC, 60 s; (8) 25 OC,10 s; (h) 25 O C , 30 s; (i) 25 OC,60 s. A small residual absorbanceat longer time (A,) issubtractedfromtheobservedabsorb. Stepwise structures in the later time region are due to degitalization of data. Data (b)-(i) are shifted downward relative to (a) for the sake of clarity.
aggregateswere formed in the solution, it is reasonable to attribute the observed kinetics to the deaggregation of the small aggregate (M,) regenerating the merocyanine monomer (M), which eventually decays to uncolored spiropyran (S): kl
kw
M,+M+S The absorbance due to M formed in this regeneration reaction (Areg)can be expressed as
= {kn(AjJo/(kl- kJl[exp(-kJ)
- ex~(-k,r)l (2)
where Anisthe absorbanceof M,. Then wecan fit the absorbance kinetics to eq 2 with k, and (An)0as adjustable parameters. (kl is known from the normal exponential decay in the later time region.) In the actual simulation, a small residual absorbance at longer time (A,) is subtracted from the observed absorbance. This residual absorbance correspondsto the accumulation of stable aggregates (see below). The result of such a simulation is shown by dotted lines in Figure 8a. The values of parameters used in the fitting are (A,)o = 1.68 X 10-1 and k, = 1.85 X 10-I s-l. Fits to the decay kinetics for several other concentrations and irradiation time are shown in Figure 8b-d. The values of (An)o and k, obtained from these fits are given in Table 111. (An)o is larger and k, is smaller for (i) higher dye concentration, (ii) lower temperature, and (iii) longer irradiation time. This can be predicted from the nature of aggregates. There must be a series of aggregates which decay stepwise as km
- - ...
kcpl
M,-Mml
M,
L a
with k,
< k,
< k,
< ...
The overall decay is dominated by the smallest k. Conditions (i)-(iii) favor both higher concentration and larger size (higher aggregation number) of the aggregates. It is thus quite natural
0.3
m
1
I
1
I
I
-em
I
'
0.3
----_sin
(a)
I
I
8
1
1
-e.rp
1
*
----- sin
(4 m
u
U
c m
c m a
5
L
L
0 v1
0.3 1
I
I
I
1
I
I
0.3I
I
I
I
1
I
I
s
Figure& Simulationof anomalousabsorption (observed absorbanceminus nonnalexponentialdecay): (-)experiment, (- - -)simulation. Concentration = 1.0 X 10-3 M. Temperature and irradiation time are the following: (a) 25 OC, 30 s; (b) 25 'C, 60 s; (c) 20 OC, 30 s; (d) 20 'C, 60 s.
TABLE IIk Typical Results of Simulation: Values of Adjustable Parameters (AJo and k, for QNitro-BIPS in Cyclohexane Solutions~ dye temp irrad kl kn concn(M) ('C) time (s) (102s-l) (10s-l) Ao (Anlo A2.35 1.47 30 0.48 60 2.35 3.75 1.85 25 30 3.75 1.29 60 2.35 1.32 8 X lo" 20 30 0.64 2.35 60 3.75 1.58 25 60 2.35 0.74 5 X lo" 20 60 aA(r) - A ( - ) - A0 exp(-klr) simulated by eq 1
X
20
0.547 0.666 0.659 0.551 0.592 0.542 0.587 0.717
0.119 0.490 0.168 0.258 0.078 0.293 0.197 0.229
0.066 0.075 0.050 0.051 0.048 0.147 0.067 0.083
2.
that these three conditions lead to larger (AJo and smaller k,. Simulation with only one M, and one kn representing all of the M,'s and k,'s is naturally an oversimplification, which works best when theextent of anomaly is small. Such a simplesimulation fails for higher/larger aggregate formation. Larger aggregates beyond some critical size have very small km's. They are practically stable. Such stable aggregates accumulate gradually during irradiation, resulting in the residual absorbance at the longer time. When UV irradiation is repeated on thesame sample, the residual absorbance accumulates and the absorbance kinetics become more complex. The accumulation and growth in size of aggregateslead eventuallyto nondeaggregating or only very slowly deaggregating aggregates. These large aggregates absorb at longer wavelengths. However, when they accumulate, their absorption bands extend to 576 nm, the peak of merocyanine monomer band. The anomalous absorption decay was observed not only at the monomer band but also for longer wavelengths (1660 nm), where merocyanine dimers and higher aggregates absorb. The anomaly ends at the earlier time for longer wavelengths. This can be interpreted by the stepwise deaggregation shown above. We believe that this is the first report of deaggregation of merocyanine in fluid solution. For H-stack in a polymer matrix, however, Eckhardt et al.24reported a similar stepwise deaggregation reaction of 5-chloro-6-nitro-BIB. They observed the shift of absorption maxima of H-aggregates during color fading, indicating that longer-livedspeciesabsorb at shorter wavelengths.
The decay at 555 nm was fitted to a double-exponential decay. Our case is distinct from theirs in that (1) the merocyanine monomer as an intermediate is clearly observed and, more significantly, in that (2) the consecutive decay of J-aggregates is observed. In the caseof Fxkhardt et al.,u J-aggregates appeared indefinitely stable and did not show the deaggregation behavior. Such an deaggregation pattern must be in common in merocyanine systems and may be important in relation to the practical use of these dyes.
4. Conclusions We report two novel aspects of merocyanine aggregate formation, one preceding and the other following the formation of "quasicrystals". (1) On prolonged (>30min) UV irradiation of 6-nitro-BIB in aliphatic and nonpolar solvents, long-lived colored merocyanine aggregates are formed. The aggregates are much larger than the quasicrystals reported by Krongauz et al.24 and are essentiallycrystalline, with a small amount of amorphous constituents, as revealed by their electron microscopic images. Resonance Raman spectra of the aggregates measured in KBr disks are very similar to those of transient merocyanine monomers in fluid solution reported by Takahashi et al. Additional bands are ascribed to N 4 and N-O stretching modes of transoid merocyanine conformers, which are coupled each other via the N-0 group in the aci-nitro structure. (2) In the very initial stage of aggregate formation in aliphatic hydrocarbon solvents, anomalous nonexponential absorption decay kinetics were observed for the merocyanine monomer band. This can be interpreted by a model involving the deaggregation of small merocyanine aggregates to regenerate monomers.
Acknowledgment. The authors are grateful to Mr. Y.Oshima for assistance. References and Notes (1) Bertelwn,R. C.Photochromism. In TechniquesofChemistry;Brown, G. H., Ed.; Wilcy-Intedencc: New York, 1971;Vol. 111, Chapter 3. ( 2 ) Krongauz, V. A,; Parshutkin, A. A. Phorochem. Phorobiol. 1972, 15,503. (3) Parshutkin, A.A.;Krongauz, V. A. Mol. Phorochem. 1974,6,437. (4) Krongauz, V. A.; Goldburt, E.S.Nclrure 1978,271,43.
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