J . Phys. Chem. 1990, 94, 4167-4172
+
fourteen s functions contracted to eight [I50591 (0.00026)
+
+
22629.6 (0.00200) 5223.16(0.01015) 1498.06 (0.04043) + 495.165 (0.12732)+ 180.792 (0.29872),71.1940(0.41837)+ 29.3723 (0.23705),8.68863 (l.O), 3.46382(l.O),0.811307 (l.O), 0.312555 (l.O), 0.035668 (l.O), 0.016517 (l.O)],and nine p functions contracted to four [867.259 (0.00234)+ 205.254 (0.01880)+ 65.8214 (0.08668),24.5742(0.25041)+ 9.87704 (0.42972)+ 4.11693 (0.35117), 1.55653(l.O), 0.614068(0.56842) + 0.228735 (0.32743)]. To this are added one s function [O. 11 1 (1 .O)], three p functions [0.1236(l.O), 0.06261 (l.O), 0.02281 (I.O)], and six d functions contracted to five [6.2167 (0.242602)+ 1.9493 (0.841179), 0.75929 (l.O),0.44548 (l.O), 0.13968 (l.O),0.05441 + (l.O)]. The final contracted basis set is (1 5sl3p6d/9s7p5d). (48) Wachters, A. J. H. J . Chem. Phys. 1970, 52, 1033.
4167
The oxygen basis set starts with the set of Pokier et al.:49 eleven s functions contacted to six [18045.3 (0.00041)+ 2660.12 (0.00333)+ 585.663 (0.01800)+ 160.920 (0.072860) 51.1637 (0.217960)+ 17.8966(0.424260),6.63901 (l.O), 2.09625 (l.O), 0.842082(l.O), 0.307328(l.O), 0.132539(l.O)]; seven p functions contracted to four [49.8279 (0.008940)+ 11.4887 (0.057680) + 3.60924 (0.192130)+ 1.31104 (0.355350),0.502347 (l.O), 0.195677 (l.O),0.072412 (1.0).
+
Following Bauschlicher and L a n g h ~ f fthree , ~ ~ d functions are added [3.0(l.O), 1.0 (l.O),0.3 (1.0)]. Thus the final basis set is 1 ls7p3d/6s4p3d. Registry No. K, 7440-09-7; 02, 7782-44-7; N2, 7727-37-9; He, 7440-59-7; KO,, 12030-88-5. (49) Pokier, R.; Kari, R.; Csizmadia, 1. G. Handbook of Gaussian Basis Sets: a Compendium for ab Initio Molecular Orbital Calculations; Elsevier: Amsterdam/New York, 1985.
Photochromism of Spiropyrans in Aluminosilicate Gels Deborah Preston,t Jean-Claude Pouxviel,*Thomas Novinson,s William C. Kaska,l Bruce Dunn,*,I and Jeffrey I. Zink**t Department of Chemistry and Biochemistry, and Department of Materials Science and Engineering, University of California, Los Angeles, California 90024 (Received: October 31, 1989)
Aluminosilicate sol-gels containing photochromic spiropyran molecules (6-nitro- 1’,3’,3’-trimethylspiro[2H1-benzopyran2,2’-indoline] and its derivatives) were prepared. The photochromism was studied by transient absorption spectroscopy and luminescence spectroscopy. Stable, solid photochromic gels were prepared whose photochromic properties remained constant after an initial aging period. Four stages of the gelation process were observed and proved by the rates and spectra of the encapsulated photochromic molecules. The rate of return of the excited photochromic molecule to the initial state was measured during the various stages of the gelation process.
Introduction Sol-gel synthesis of inorganic glasses offers a low-temperature route to the microencapsulation of organic and organometallic molecules in inorganic matrices.1-12 The encapsulated molecule can be used to induce new optical properties in the material (Le., laser action)j or to probe the changes at the molecular level which occur during the polymerization, aging, and drying of the glass. A fascinating group of molecules which can function in both of the above capacities are the photochromic substituted 6-nitrospiro[ 2H-1 -benzopyran-2,2’-indolines),abbreviated as BIPS.I3 The structures of the species before and after illumination are sketched in Figure 1. These new transparent photochromic solids have a variety of potential applications. In addition, variations in the rates of the photochromic reactions probe the changes occurring during gelation as demonstrated by Avnir et al. in silicate geI-gIass.I0 The gel-glass studied here is an aluminosilicate formed by hydrolysis and condensation reactions of the precursor (diisobutoxyalumino)triethoxysilane, (OBU)~AI-O-S~(OE~),, called ASE.’”I6 Clear, transparent monoliths having dimensions of the order of 1 X 1 X 1 cm are readily prepared under neutral hydrolysis conditions. The initial sol, which is completely fluid, is transformed into a solid, brittle material. After the addition of water, small particles on the order of 10-20 A in diameter are formed as a consequence of the hydrolysis of the AI-OR groups ‘Department of Chemistry and Biochemistry. *Department of Materials Science and Engineering. *Current address: Naval Civil Engineering Laboratory, Port Hueneme, CA 93043. Current address: Department of Chemistry, University of California, Santa Barbara. CA 93106.
to AI-OH and the condensation of the AI-OH groups into -Al-O-Al- polymers. After this step, a slow aggregation takes place which forms inorganic clusters having a ramified and open structure. Subsequent reactions form -Si-0-Si- linkages.I6 Prior studies with ASE gels reported the use of luminescent molecules to probe the local chemical and structural changes which occur during the sol-gel Luminescence from pyranine (1) McKiernan, J.; Pouxviel, J. C.; Dum, B.; Zink, J. I. J . Phys. Chem. 1989, 93, 2129.
(2) Pouxviel, J. C.; Dunn, B.; Zink, J. I. J . Phys. Chem. 1989, 93, 2134. (3) Dum, B. D.; Knobbe, E.; McKiernan, J.; Pouxviel, J. C.; Zink, J . I . In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; MRS Symp. Proc. Vol. 121; Materials Research Society: Pittsburgh, 1988; pp 331-342. (4) Avnir, D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984, 88, 5956. (5) Levy, D.; Reisfeld, R.; Avnir, D. Chem. Phys. Lett. 1984, 109, 593. (6) Avnir, D.; Kaufman, V.; Reisfeld, R. J . Non-Cryst. Solids 1985, 74, 395. (7) Kaufman, V.; Avnir, D. In Better Ceramics Through Chemistry; Brinker, C. J., Clarck, D. E., Ulrich, D. R., Eds.; Mater. Res. Soc. Symp. Ser.; Materials Research Society: Pittsburgh, 1986; Vol. 73, p 145. (8) Kaufman, V.; Avnir, D. Langmuir 1986, 2, 717. (9) Kaufman, V.; Levy, D.; Avnir, D. J . Non-Cryst. Solids 1986,82, 103. (10) Levy, D.; Avnir, D. J . Phys. Chem. 1988, 92, 4734. (1 1) MacKenzie, J. D.; Pope, E. J. A. M R S Bull. 1987, 12, 29. (12) (a) Tani, T.; Namikawa, H.; Arai, K.; Makashima, A. J . Appl. Phys. 1985,58,9. (b) Makishima, A.; Tani, T. J . Am. Ceram. SOC.1986,69, C-72. (13) Bertelson, R. C. Photochromism. In Techniques of Chemistry; Brown, G . H., Ed.; Wiley-Interscience: New York, 1971; Vol. 111, Chapter VII, p 100. (14) Pouxviel, J. C.; Boilot, J. P. J . Mater Sci., in press. (15) Pouxviel, J. C.; Boilot, J. P.; Lecomte, A,; Dauger, A. J . Phys. (Puris) 1987, 48, 921. (16) Boilot, J. P.; Pouxviel, J. C.; Dauger, A,; Wright, A. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; MRS Symp. Proc. Vol. 121; Materials Research Society: Pittsburgh, 1988; pp 121-126.
0022-3654/90/2094-4167$02.50/00 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
(thermal)
T!
hs
TRANS
Preston et al. opening one or two holes of about 0.5 mm in diameter in the top which allowed the volatile solvents to escape. These samples lost about 80% of their original weight during the drying. Other samples were left completely sealed. The spectroscopic measurements were made on at least four samples to check the reproducibility. Three BIPS derivatives were studied here. They are PNMPN-BIPS, 1’-phenylBIPS, l’-phenyl-6-nitro-8-methoxy-BIPS; 6-nitro-BIPS; and CNM-BIPS, l’-chloro-6-nitro-8-methoxy-BIPS, Transient Absorption Spectra. Transient absorption spectra were taken by using a high-intensity tungsten lamp as the probe source, a Hg lamp filtered with a Schott UG-5 filter as the exciting pump source, and a detector consisting of a 1/4-m monochromator and an EG&G 1421 photodiode array detector. The data acquisition and manipulation were performed with the OMA 111 system. Spectra were collected every 3-15 s depending on the growth and decay rates of the colored species in the sample. In a typical experiment, two exposures were taken before the pump beam was applied; the next six were taken during irradiation by the pump beam, and the remainder were taken after the pump beam was blocked. For each set of transient absorption spectra, four sets of data were taken: (1) pump probe; (2) pump only; (3) probe only; and (4) thermal background only. The percent transmittance is the transient transmittance (pump plus probe minus the pump) divided by the ground-state transmittance (probe minus the thermal background). The net transient absorption = log [(probe) - (background)] spectrum is given by Atransient log [(pump + probe) - (pump)]. Ground-state absorption spectra of each gel were also taken on the same apparatus for comparison with the transient absorption spectra. The ground-state data was taken by measuring the lamp output with and without the gel = log sample in place. The net absorbance is given by Aground [(probe without sample) - (background)] - log [(probe and sample) - (background)]. Luminescence. Luminescence was excited by using 3638 A, 0.4-mJ pulses from a XeCl excimer pumped dye laser. Timeresolved spectra were obtained by gating the diode array detector with a high-voltage pulser. The spectra were not corrected for detector response. Dynamics. The transmittance at a given wavelength as a function of time was measured by passing a beam of the desired wavelength through the sample and monitoring its intensity with a 1P28 photomultiplier tube. The pump source was a 100-W Hg lamp with a UV pass filter (UG-5, Schott Glass Co.) focused on the sample with a 100-mm glass lens. In many of the measurements a 5-mW HeNe laser was used to provide the probe beam. A shutter was used to unblock and block the pump beam which was roughly collinear with the probe beam in the sample cuvette. Scattered light from the Hg lamp was excluded from the detector by using an appropriate filter. The output from the photomultiplier tube was digitized by an A / D converter interfaced to a computer.
+
fi v
Figure 1. The structure of the ground state of the BIPS molecule and several of the ring-opened forms which have been proposed to exist after photoexcitation. The cis form is associated with the transient absorption band at 21 050 cm-’ and the trans form is associated with that at 16950 cm-I.
(8-hydroxy- 1,3,6-trisulfonated pyrene) was found to be sensitive to internal solvent chemistry and was used to monitor the changes in the propanol/water ratio during polymerization and drying of the gel.* Matrix rigidity of ASE sols and gels was investigated by luminescence shifts in the rigidochromic molecule ReCI(C0)3bpy.1 In both cases, the probe molecules became increasingly tangled and ultimately trapped in the growing gel network. The properties of BIPS in ASE gels and gel-glasses are reported in this paper. Not unexpectedly, the properties are significantly different from those in silicate glasses.I0 Aged ASE gels yield photochromic monoliths. The transient absorption spectra, transient emission spectra, and rate constants of the photochromic changes are reported and discussed.
Experimental Section Sample Preparation. Aluminosilicate sol-gels containing spyropyrans were prepared by dissolving 2 mg of the appropriate spiropyran in 10 mL of anhydrous reagent grade 2-propanol and adding 10 mL of (diisobutoxy)aluminoxytriethoxysilane, [(OBU),AI-O-S~(OE~)~] (Petrarch Chemical Co.). A solution of I O mL of 2-propanol in 5 mL of distilled water was then added dropwise to the BIPS-alcohol-aluminosilicate solution with manual stirring. Upon addition of the first few drops the solution became warmer. Further additions did not warm the solution appreciably beyond the initial warming. For some batches, 2-4 drops of reagent grade dimethyl sulfoxide was added to each cuvette. The gels prepared in this manner did not contain opaque macroscopic colloidal particles or spiropyran precipitates. The sol was poured into 10 mm X 10 mm X 40 mm polystyrene cuvettes, covered with Parafilm, and allowed to gel and polymerize for 3-6 days. The cuvettes were kept in the dark except for the brief times when they were removed to take readings. After polymerization of the samples the Parafilm was removed and the cuvettes were covered with either weighing paper sealed with tape or with aluminum foil sealed with Torrseal. In order to study the effects of drying, some of the covered cuvettes were vented by
Results Photochromism in Sols, Gels, and Dried Gels. General Observations. The three spiropyrans studied, PNM-BIPS, PN-BIPS, and CNM-BIPS, showed reversible photochromism in the gels. Of these, PNM-BIPS was the most stable in the gels and gelglasses and it was studied in the most detail. Freshly prepared gels containing PNM-BIPS were clear and highly photochromic. Aging of the gels did significantly change the photochromic properties. Photochromic transparent solids of dimensions of 1 X 1 X 1 cm were easily prepared and studied. PNM-BIPS gels that were allowed to dry and shrink exhibited four basic stages. These four stages are defined in this work by the ground-state color of the gel, the color of the transient absorption, the color of the emission present during pumping, and the fading rate of the colored species when the pump beam is blocked. Stage I gels are defined as those with up to 70%of the weight loss (which corresponded to about 80% of the volume loss measured as linear dimensional loss). At the inception of this stage the gels were clear and almost colorless. Stage I gels which has
Photochromism in Aluminosilicate Gels aged more than about 3 months turned slightly brown or pink. Permanent coloring was more pronounced in gels which had been repeatedly exposed to the UV pump beam. In stage I gels the photochromic transient causes the material to appear violet-blue. Generally, stage I gels exhibited only the blue-white luminescence which is characteristic of the intrinsic luminescence of the pure gel. However, 5-month-old gels which has been exposed to the UV pump beam for 1 h or more showed an orange luminescence. At stage 11, which occurred between 70% and 78% weight loss (80% and 85% volume loss), the gel became permanently colored. The observed color in stage I1 gels which were less than 2 months old was either brown (if no DMSO has been added) or pink (if DMSO had been added). As with the stage I gels, the permanent coloration during stage 11 was greater with gels that had been exposed to the pump beam many times. However, even when not exposed to a UV pump beam and kept in the dark during shrinkage, the latter gels always showed some coloring. In older stage I1 gels (3 months or more), some of the gels which contained no DMSO also turned pink. The pink color appeared to be more intense in gels which had been used frequently for the kinetics measurements. The color of the transient produced by pumping with the UV lamp was brownish in stage I1 gels. The transient color faded more slowly in stage I1 gels than in stage I gels. Stage 111 generally occurred when the gel had lost about 79-8 1% of its weight. At this stage the permanent coloration was more pronounced than that of the stage I1 gels. An important characteristic of stage 111 gels was that they were translucent and resembled frosted window glass. In a vented cuvette the change between stage 11 and stage 111 could take place in less than 24 h. Photochromism could still be observed in these gels. However, the scattering caused by the glass was so large that the decay rates of the transient absorbance could not be measured. From visual observation, the transient in a stage Ill gel took about a minute to fade back to its original color. At stage IV, which occurred at about 82-84% weight loss (88-90% volume loss), the PNM-BIPS gels again became transparent and were permanently colored a deep maroon. The maroon compound could be leached from the gel with ethanol, DMSO, or 2-propanol. No photochromism or photobleaching of the maroon compound was apparent upon visual inspection of either the stage IV gels or the maroon extracts from the gels. No photochromism was detected in the gels or extracts when the pump-probe kinetics experiments were done (vide infra). The gels which were prepared with PN-BIPS exhibited slightly different behavior. Stage 1 gels which had been prepared with this spiropyran were clear. The fresh gels were highly photochromic with a dark brownish-purple transient color. Unlike PN M-BIPS gels, PN-BIPS gels showed reverse photochromism. Red light accelerated the fading rates. Therefore all rate measurements on this type of gel were performed using a greatly attenuated probe beam. The PN-BIPS gels lost their photochromic properties after about 30% weight loss. Thus it was impossible to define the onset of stage I1 in PN-BIPS gels. Stage 111 PN-BIPS gels were uncolored; otherwise they were similar to stage 111 PNM-BIPS gels. On the other hand, stage IV PN-BIPS gels prepared without DMSO were deep maroon. Transient Absorption Spectroscopy. Transient absorption spectra for a sealed stage I gel containing PNM-BIPS are shown in Figure 2. Both the growth of the absorption in the visible during irradiation with the UV pump beam and the decay of the absorption after the pump beam is blocked are shown. In the spectral range examined, 22 000-14 000 cm-], the spectrum is dominated by a single band growing in at 16 950 cm-l. At the high-energy end of the 16 950-cm-l band, the tail of second band can be seen. When the pump beam is unblocked (Figure 2a), the band at 16 950 cm-I grows in rapidly, reaching a maximum absorbance of about 0.2 in about 12 s. Upon blocking the pump beam (Figure 2b) the band at 16 950 cm-' disappears over several seconds to leave the net zero transient absorption seen before the pump beam was unblocked. The total disappearance of the transient absorption band shows that the photochromism is reversible in the gel. The reversibility exists in gels 3 months old.
The Journal of Ph. ical Chemistry, Vol. 94, No. 10, 1990 4169 d
0.20 0.15 0.10
0.05 W V
c
em
'
0.00
z
0.15
0.10
0.05 0.00
2.2
2.0
1.%
1.6
1.4
Energy (cm-l)/lo4 Figure 2. Transient absorption spectra for a sealed stage I gel containing PMN-BIPS. (Top) Growth of the transient absorption band at 16950 cm-' during irradiation with a UV excitation source: (a) no excitation; (d) 1 min. (Bottom) Decrease in the absorbance of the transient upon cessation of the excitation. Transient absorption spectra are shown at times (a) 0, (b) 2, (c) 6 , (d) 9, (e) 12, and (f) 45 s after the pump beam was blocked.
I
e
7
0.4
0.2
8c
em
0.0
z
9
a
0.4
.
0.2
.
0.0
. 2.2
2.0
1.8
1.6
1.4
Energy (cm-l)/lo4 Figure 3. Transient absorption spectra for a stage I1 gel containing PMN-BIPS. (Top) Growth of the transition absorption bands at 16950 and 21 050 cm-' during irradiation with a UV excitation source. (Bottom) Decrease in the absorbance of the transient bands upon cessation of the excitation. Transient absorption spectra are shown taken at times (a) 0, (b) 5, (c) 10, (d) 15, and (e) 30 s after the pump beam was
blocked. The violet color of the transient species results from the higher transmittance in the blue region and a smaller transmittance in the red region. In a gel which has been partially dried, a new band at 21 050 cm-l is seen in the transient absorption spectra. Figure 3 shows
4170
TABLE 1: Correlation of the Lifetime of T16m with the Relative Absorbance at 1695021050 em-' drying time. days re1 absorbance" lifetime, s 1
