J. Phys. Chem. 1986, 90, 62-65
62
LASER CHEMISTRY, MOLECULAR DYNAMICS, AND ENERGY TRANSFER Photophysics, Photochemistry, Kinetics, and Mechanism of the Photochromism of 6’-Nitroindolinospiropyran Christian Lenoble and Ralph S. Becker* Department of Chemistry, University of Houston-University Park, Houston, Texas 77004 (Received: June 18, 1985)
1,3,3-Trimethyl-6’-nitrospiro[indoline-2,2’[ 2 H ] benzopyran] (6’-nitro-BIPS, A) has been examined with microsecond and nanosecond laser flash techniques in hexane and acetonitrile. New transients were found in the early time domain (1-10 ns) and the presence of oxygen was shown to be the important factor in elucidating the mechanism of photochromism of 6’-nitro-BIPS. In hexane, the triplet excited state of the spiropyran (3A) was seen to lead to the cisoid opened form in its triplet excited state (’X)in 10 ns. The transoid opened spiropyran (B) was shown to come from both the cisoid opened form (X) and ’A on the nanosecond time scale. The aggregation process was found to be the result of a bimolecular reaction between ’X and B and not between ’A and the closed spiropyran (A) in its ground state as previously believed. In acetonitrile, the cisoid opened form (X) was seen to have a shorter lifetime (< 1 ns) than in hexane. ’A had a lifetime of 32 ns and the aggregates were not stable, having a decay time of 24 f i s .
Introduction Since the first studies on the photochromism of the spiropyrans, by Fischer et al.,] a great number of studies have been done on their properties.* They have been shown to convert to merocyanine dyes upon irradiation.’-’ In the case of the 1,3,3-trimethyl-6’-nitrospiro[indoline-2,2’[2H]-benzopyran] (6’-nitroBIPS), in aliphatic solvents, photoinduced spontaneous aggregation processes were observed and the aggregates were defined as ”qua~i-crystals”.~*~ Picosecond, nanosecond, and microsecond laser flash photolysis6-8 experiments were performed to elucidate the complex photophysical and photochemical mechanism of the 6’-nitro-BIPS, to be denoted as A. Kalisky et a1.* proposed that, in a nonpolar solvent, there was formation of a cisoid ring open photoproduct, denoted X, within 8 ps which underwent isomerization to the transoid form, denoted B, in approximately 300 ps.
A
B Me
N02
Me
X (1) Bercovici, T.;Heiligman-Rim, R.; Fischer, E. Mol. Photochem. 1969, 1, 23-55.
(2) Bertelson, R.C.“Photochromism”, Brown, G. H., Ed.; Wiley-Interscience: New York, 1971;pp 45-294. (3) Tyer, N.W.;Becker, R. S. J . Am. Chem. SOC.1970,92, 1289-1302. (4) Krongauz, V.;Kiwi, J.; Gratzel, M. J . Photochem. 1980, 13, 89-97. (5) Krongauz, V.Isr. J. Chem. 1979, 18, 304-311. ( 6 ) Krysanov, S. A,; Alfimov, M . V. Chem. Phys. Lett. 1982, 91, 77-80. ( 7 ) Kalisky, Y.;Williams, D. J. Chem. Phys. Lett. 1981, 86, 100-104. (8)Kalisky, Y.;Orlowski, T. E.; Williams, D. J. J . Phys. Chem. 1983, 87, 5333-5338.
The formation of the triplet state of the spiropyran, 3A, was said to be in competition with the formation of X. Aggregation was stated to occur by a bimolecular reaction between 3A and a ground-state molecule As8 However, the latter experiments left unexplored the time range of the first few nanoseconds (1-10 ns). Our present study gives new insight into the mechanism of photochromism of the 6’-nitro-BIPS since two new transients were discovered in the early nanosecond time domain. The influence of oxygen was shown to be the important factor in elucidating the presence of the new transients. We shall propose a more complete mechanism of the process of photochromism unifying results from the early picosecond time domain to the millisecond time domain.
Experimental Section The 6’-nitro-BIPS has been synthesized as described b e f ~ r e . ~ All the solvents used were spectro-grade, dried and kept over 3A molecular sieves, and used without further purification. Absorption spectra were recorded on a Hewlett Packard 8450 A UV/visible spectrophotometer. The microsecond laser flash experiments were carried out with a q-switched Nd:YAG laser (1 1-ns pulse width). The rise time of the system is -0.4 ps. The excitation source was the 355-nm third harmonic. The kinetic absorption spectrometer used to detect OD changes (AOD) after excitation has been described previ0us1y.~ The output of the laser was 80 mJ and the energy of the beam was controlled by the use of wiremesh screens. The common chosen energies were in the range of 1-2 mJ. Typical concentration employed were -1.7 X M. The solutions were examined in a rectangular quartz cell with a 5-mm pathlength along the monitoring light and degassed by bubbling nitrogen during each experiment. Also, oxygen was used to aid in elucidating the presence of triplet excited-state transients. The nanosecond experiments used the third harmonic generated from a mode-locked Nd:YAG laser (200-ps fwhm, -10-mJ output). The kinetic absorption spectrometer (for detection on the nanosecond time scale) consisted of a continuously operating 150-W xenon arc lamp, two electromechanical shutters, quartz lenses a target cell holder, a Spex Minimate monochromator, and (9) Das, P.K.; Becker, R. S. J . A m . Chem. SOC.1979, 101, 6348-6352.
0022-3654/86/2090-0062$01.50/00 1986 American Chemical Society
Photochromism of 6’-Nitro-BIPS
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 63
... 6.4
-
U 2 0 ns Wavelength (nm)
Figure 1. Transient absorption spectrum (AOD spectrum) of 6’-nitroBIPS in hexane: (-) (a) nitrogen degassed solution, 1 ps after 355-nm laser flash; (b) nitrogen degassed solution, 13 p s after 355-nm laser flash; (-- -) oxygen-saturated solution, 1 ps after 355-nm laser flash (no further evolution in time is observed, see text).
a photomultiplier tube (Hamamatsu R928). Detector output waveforms were processed by a fast Tektronix R 7912 digitizer and computer combination (PDP 11/70). The rise time of the detection system (photomultiplier tube and digitizer) was 1 ns.
Results and Discussion Irradiation of the 6’4tro-BIPS was done at 355 nm (1 1-ns pulse, 1 mJ) in nitrogen-degassed hexane solution. At the laser excitation energies employed, the changes in optical density (AOD) were seen to be linearly dependent on the changes in intensity, reflecting single-photon excitation processes. Immediately after the pulse, we observe three new bands of absorption (+AOD) at 370, 430, and 570 nm and a shoulder at 630 nm (see Figure 1). The bands at 370 and 570 nm and the shoulder at 630 nm were rising following second-order kinetics, with a rate constant equal to 2.57 X lo6 M-’ s-l. (No further increase in intensity was observed up to >400 ps.) The band at 430 nm was decaying and also followed second-order kinetics to a positive AOD base line with an identical rate constant as given above. The rate constant was seen to be independent of the laser excitation intensities being used. When bubbling oxygen, the species maximizing at 430 nm was totally quenched and the change in optical density of the shoulder at 630 nm was substantially decreased. The two new bands at 370 and 570 nm were still present although the intensity of the bands was greater than in the presence of nitrogen after 400 ps. Also, the rise time at 370, 570, and 630 nm was now (in the presence of oxygen) less than the detection limit (400 ns). The independence of the bimolecular rate constant on the excitation intensity excludes the triplet-triplet anihilation process, as previously found by Kalisky et al.8 Nanosecond experiments were done by irradiating the solution of 6’-nitro-BIPS at 355 nm (200-ps pulse, -10 mJ) in hexane. The kinetics of photochromism were monitored in the 1-800-11s time range at 370, 430, 530, and 570 nm (see Figure 2 and 3). In nitrogen-degassed solution, we observed a rise time of 10 ns at 370, 530, and 570 nm. All kinetics were first order. At 430 nm, we observed a 10-ns rise time followed by a decay with an estimated lifetime of >2 ps within the limit of our time detection. When bubbling with oxygen, the rise time at 370, 530, and 570 nm was increased to 22 ns (from 10 ns), while at 430 nm, the rise time was decreased to 4 ns (from 10 ns) and the decay time decreased to 33 ns (from -2 ps). Before discussing our results any further, we should recall the earlier observations in the picosecond time domain. In their picosecond studies, Krysanov et a1.6 found a transient rising in 8 ps in the region from 400 to 650 nm and assigned this band to a single species, the cisoid opened form, X. Kalisky and Williams’ later found a transient in 270 ps at 530 nm and assigned this transient to be the opened transoid form B of the spiropyran with X as its precursor. However, we found still another transient
-
6)
0
8
w
50 ns Figure 2. Optical density changes observed at 570 nm after 355-nm (200-ps fwhm) laser flash of 6’4tro-BIPS in hexane: (A) nitrogen-degassed solution; (B) oxygen-saturated solution.
m
+ --rl 100 ns Figure 3. Optical density changes observed at 430 nm after 355-nm (200-ps fwhm) laser flash of 6’-nitro-BIPSin hexane: (A) nitrogen-degassed solution; (B and C) oxygen-saturated solutions.
rising in the early nanosecond time domain. Other transients have been found at those wavelengths (370 and 570 nm) in recent
64
Lenoble and Becker
The Journal of Physical Chemistry, Vol. 90, No. I , 1986
studies by Kellmann et al.”*’’ of nitrochromene and nitrospiropyran derivatives. Those transients were assigned to be the triplet state of the closed (ground state) chromene or spiropyran (decaying in the nanosecond time scale), the triplet state of the respective cisoid opened forms (decaying in the ps timescale), and the long-lived transoid opened form in the case of the nitrochromene. In the present study, we observed a transient rising in 10 ns with a maximum of absorption at 370 and 510 nm, the maximum of absorption for the opened transoid form B. We therefore assign our transient (370 and 570 nm) to B. With our assignments, the transient seen by Kalisky and Williams’ in 270 ps at 530 nm becomes the precursor of B, instead of B itself, that is the opened cisoid form X. The assignment for the transient seen by Krysanov et aL6 will be discussed later. To this point, we can describe part of the photophysical mechanism of 6’nitro-BIPS in hexane: nitrogen degassed: A oxygen degassed: A
- -- -hu
hv
‘A*
‘A*
270
PS
270 PS
X
X
10 ns
22 ns
B
B
The oxygen experiment in both microsecond and nanosecond experiments gives further insight into the mechanism of photochemistry of the 6’-nitro-BIPS. Fischer’ observed a peculiar behavior of the 6’-nitro-BIPS in his steady-state experiment using non-oxygen degassed solution whereby the AOD spectrum showed two main bands at 370 and 570 nm with greater AOD than for the degassed solution; however, no explanation has ever been given for this phenomenon. We observed the same behavior in our microsecond laser flash experiment. This suggests to us that B (with bands at 370 and 570 nm) is involved in the formation of the dimers (aggregates) which have a maximum at 630 nm. The intensity of the bands at 370 and 570 nm were less in the nitrogen experiment because the concentration of B was decreased by the formation of the dimers. Since the decay time at 430 nm and the rise time at 630 nm (dimers) match, we believe the dimers to be formed by a bimolecular reaction between the species maximizing at 430 nm (microsecond experiment) and E instead of a ground-state molecule A as proposed earlier.4,s We shall discuss now the nature of the species maximizing at 430 nm (microsecond experiment). In the nanosecond experiment, we found at 430 nm a transient rising in 10 ns and decaying in the microsecond time domain. When bubbling with oxygen, both the rise and decay were partially quenched. The transient maximizing at 430 nm (microsecond experiment) was p r e v i o ~ s l y assigned ’~~ to the triplet excited state (3A) of the 6’-nitro-BIPS. However, if this is the case, the rise time of 3A should not be shortened by the presence of oxygen. Therefore, our results infer that the species absorbing at 430 nm and decaying in the microsecond time scale is a triplet excited-state species, that is, the cisoid opened form X in its triplet excited state 3X. Finally, 3X is coming from the triplet excited state, 3A, of 6’-nitro-BIPS in 10 ns. In a recent study, Kellmann et a1.I’ found that in the case of the 6-nitrochromene (Chr) the bond opening in the triplet excited state (3Chr) led to a cisoid isomer in its triplet excited state (3X) which subsequently transformed into the colored isomer. The lifetimes of T h r and 3X were found to be about 80 and 450 ns, respectively, at 24 O C . This result completely corroborates our observation and also confirms the chromene to be a model system for studies of spiropyran photochromism.’’ . I 2 We thus have shown that the triplet excited state of the closed spiropyran 3A leads to the open cisoid isomer in the triplet excited state 3X. The latter species then reacts with the open transoid form B in a bimolecular reaction to form the dimers (maximizing at 630 nm). Moreover, we have evidence that A leads directly to B since the rise time of B is affected by oxygen (from 10 ns (10) Kellmann, A.; Lindquist, L.; Monti, S.; Tfibel, F.; Guglielmetti, R. J . Photochem. 1983, 21, 223-235. (11) Kellmann, A.; Lindquist, S.; Monti, S.; Tfibel, F.; Guglielmetti, R. J. Photochem. 1985, 28, 547-558. (12) Lenoble, C.; Becker, R. S . J . Photochem., in press.
5.0 I
1
n
N
-
‘0 r
2.7
0
8
Wavelength (nm)
Figure 4. Transient absorption spectrum (AODspectrum) of 6’-nitroBIPS in nitrogen-degassed acetonitrile solution recorded (a) 1 ps after 355-nm laser flash and (b) 13 *s after 355-11s laser flash.
in nitrogen to 22 ns in oxygen). This gives proof of the direct formation of B from 3A as seen in the sensitization studies in EPA and 2MeTHF at -100 O C I 3 and in polystyrene matrix.I4 The initial concentration of 6’-nitro-BIPS was 1.7 X M. Since the quantum yield of coloration has been reported to be at least 20% in toluene at room temperature,2 a concentration of 3 x M would be expected for B. If we assume a bimolecular diffusion-controlled reaction between 3X and B, that is k lo9 M-’ s-I, we can estimate the rate constant for the decay of X to be approximately 105-106 s-I. To this point, we can summarize our result for 6’-nitro-BIPS in nitrogen degassed hexane solution in the following scheme:
-
-
where k , is the rate constant for phosphorescence. Krysanov et aL6 found a transient rising in 8 ps in the region from 400 to 650 nm and assigned this band to a single species, the open cisoid form, X. According to our result, this assignment has to be reconsidered. The spectrum of the transient seen in this study clearly shows two maxima at 450 and 580 nm. This result may infer that this spectrum is the result of the superposition of S, transition of X; (b) a T, T, two spectra: (a) a SI transition of 3A. Another possibility is that the AOD spectrum is the S, Snabsorption spectrum of the spiropyran. The latter possibility is consistent with mechanism 1 proposed above. The former possibility brings a modification to mechanism 1 and is described below in mechanism 2 for the photochromism of 6’nitro-BIPS in nitrogen-degassed aliphatic hydrocarbons:
-
-
A
h\r
‘A*
8
PS
I
+
>x*
270
PS
,x
22 ns
,B
(2)
We may also note that we did not rule out the possibility of the excited state of X (‘X*) in mechanism 2 to intersystem cross to 3X*. Also, in both schemes, the rate constant of intersystem (13) Becker, R. S.; Roy, J. K. J . Phys. Chem. 1965, 69, 1435-1436. (14) Reeves, D.; Wilkinson, F. J . Chem. SOC.,Faraday Trans. 2 1973, 69, 1381-1390.
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 65
Photochromism of 6’-Nitro-BIPS
0
8
Figure 6. Optical density changes observed at 570 nm of 6’-nitro-BIPS in acetonitrile after 355-nm (200-ps fwhm) laser flash.
H 2 0 ns Figure 5. Optical density changes observed at 440 nm of 6’-nitro-BIPS in acetonitrile after 355-nm (200-ps fwhm) laser flash; (A) nitrogen-
--
degassed solution; (B) oxygen-saturated solution.
-
crossing ]A* 3A* is very large, 10’o-1012 s-I. To get still further insight in the mechanism of photochromism of the 6’-nitro-BIPS, we performed microsecond and nanosecond laser flash experiment in acetonitrile. In the microsecond experiment, the AOD spectrum of a degassed solution of the 6’nitro-BIPS gave noticeable differences with respect to the AOD spectrum in hexane (see Figure 4). We observed three main bands of absorption at 400, 440, and 560 nm and a shoulder in the 590-670-nm region. The transient with maxima at 400 and 560 nm had a rise time of 24 ps. The transient with a shoulder at 620 nm decayed with a lifetime of 24 ps while the transient at 440 nm decayed with a lifetime of 20 pus. All kinetics were of first order. When oxygen was bubbled through the solution, the species at 440 nm and the shoulder in the 590-670 nm region were quenched. The bands at 370 and 560 nm were still present and equal in magnitude to the band observed in the nitrogen experiment after complete evolution of the spectrum to 400 ps. All rise times were less than the detection limit (400ns) in this case and no further change in intensities were observed up to >400 @S.
In the nanosecond experiment (A,, = 355 nm, 200 ps, 10 mJ), we monitored the kinetics of photochromism at 440 and 570 nm. For the nitrogen-degassed solution of 6’-nitro-BIPS in acetonitrile, we found a rise time of less than 1 ns for the transient at 570 nm (see Figure 6) and a biexponential decay (see Figure 5) for the transient at 440 nm; the short component having a lifetime of 32 ns and the long component having a lifetime in the microsecond time scale. When the solution was bubbled with oxygen, the two decays at 440 nm were shortened to 11 and 70 ns respectively. The rise time at 570 nm remained unchanged in the
presence of oxygen (vs. nitrogen). Since the decay of the dimers in the 590-670-nm region and the rise of the open transoid form (B) maximizing at 560 nm matched, we believe that the dimers are not stable in acetonitrile and return to the opened transoid form B. We again saw the decay of the triplet state at 440 nm of the closed spiropyran in the nanosecond range (7 = 32 ns) which is the precursor of the excited triplet state 3X as described in the hexane solution case. We may note that the short component of the decay of the species at 430 nm had already been seen in previous studies for the 6’-nitro-BIPS in acetonitrile and for the homopolymer of 1-(/3-(methacryloxy)ethy1)-3,3-dimethyl-6’nitrospiro[indolene-2,2’-[2H]benzopyran] and methyl methacrylate in toluene by Kalisky and W i l l i a m ~ . ~ However, J~ since no oxygen quenching experiment were done in those s t ~ d i e s , ~ J ~ this decay was assigned as originating from the open cisoid form X stabilized in acetonitrile with respect to the hexane solution. We find, however, that the X transient is less stabilized in acetonitrile since the rise time of B is less than 1 ns. Therefore, the lifetime of X is in the subnanosecond time range. Finally, the experiment in acetonitrile reinforces our mechanism of the photophysics and photochemistry of the 6’-nitrospiropyran.
Conclusion We have studied the photochromism of 6’4tro-BIPS in polar and nonpolar solvents in the 1-ns-400-pstime domain. The early nanosecond (1-10 ns) time domain has been shown to be determining in clarifying the complex photophysical mechanism of 6’4tro-BIPS. We found the interesting result that the triplet excited state of the spiropyran (3A) leads to the opened cisoid form in its triplet excited state (3X) in 10 ns. Similar behavior was found for the 6’-nitrochromene.Io The 3A was also shown to go directly to the opened transoid form (B) in 10 ns. Moreover, we found that the aggregation process is the result of a bimolecular reaction between 3X and B and not between 3A and the closed ground state molecule (A, closed spiropyran) as previously believed. Using the published result on the picosecond studies and our results, we can determine the complete mechanism of the photophysical and photochemical path of 6’-nitro-BIPS. Acknowledgment. The laser-flash experiments were performed at the Center for Fast Kinetics Research at the University of Texas at Austin, which is supported by N I H Grant RR-00886, the Biotechnology Branch of the Division of Research Resources, and the University of Texas. (15) Kalisky,
Y.;Williams, D.J. Macromolecules
1984, 1 7 , 292-296.
