Substituent Effects on the Photochromism of Bichromophoric Oxazines

Apr 17, 2009 - Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral .... The Journal of Physical Chemistry C 2016 120 (23), 12860-...
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J. Phys. Chem. C 2009, 113, 8491–8497

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Substituent Effects on the Photochromism of Bichromophoric Oxazines Erhan Deniz,† Massimiliano Tomasulo,† Salvatore Sortino,*,‡ and Franc¸isco M. Raymo*,† Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida 33146-0431 and Dipartimento di Scienze Chimiche, UniVersita` di Catania, Viale Andrea Doria 8, Catania I-95125, Italy ReceiVed: February 18, 2009; ReVised Manuscript ReceiVed: March 31, 2009

We synthesized a series of photochromic [1,3]oxazines and investigated their photochemical properties in solution and within rigid polymer matrices. These compounds share a common molecular skeleton, consisting of fused 3H-indole and nitrobenzooxazine heterocycles. They differ in the groups in the para (R1) and/or ortho (R2) positions, relative to the nitrogen atom, of the 3H-indole fragment and/or that (R3) attached to the chiral center of the [1,3]oxazine ring. Specifically, R1 can be a hydrogen atom, a methoxy group, a nitro group, or a fluorine atom, R2 can be a hydrogen or fluorine atom, and R3 can be a phenyl, 4-methoxyphenyl, 4-dimethylaminophenyl, or 2-(4-dimethylaminophenyl)ethylene group. When R1 and R2 are hydrogen atoms, the excitation of the photochrome opens the [1,3]oxazine ring in less than 6 ns, with quantum yields of 0.01-0.11 in acetonitrile at 20 °C. This process generates a zwitterionic isomer, incorporating a 3H-indolium cation and a 4-nitrophenolate anion. Consistently, the characteristic ground-state absorption of the a 4-nitrophenolate appears at 440 nm in the transient spectrum. When R3 is a 2-(4-dimethylamminophenyl)ethylene group, the spectrum reveals also an additional band at 550 nm for the extended π-system associated with the photogenerated 3H-indolium cation. When R3 is a hydrogen atom, the nature of R1 controls the photochemical behavior of these compounds. In particular, the presence of a methoxy group at R1 prevents the photoinduced ring-opening, while the introduction of a fluorine atom increases the quantum yield of the photochemical transformation to 0.29. In all instances, the transient absorptions decay monoexponentially with the reisomerization of the zwitterionic species back to the original state. Interestingly, R1 and R2 have negligible influence on the lifetime of the photogenerated isomer, which instead changes from 21 ns to 10 µs with the nature of R3. Indeed, this group dictates the stability of the 3H-indolium cation of the zwitterionic isomer and, hence, the reisomerization kinetics. Furthermore, our photochromic compounds tolerate hundreds of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, and can be operated effectively within rigid poly(methyl methacrylate) matrices. In summary, our investigations demonstrate that the color, efficiency, and speed of our photochromic [1,3]oxazines can be manipulated with the careful selection of their substituents without compromising their excellent fatigue resistances. Thus, photoresponsive materials with tunable properties can eventually emerge from our insights on the stereoelectronic factors regulating the photochromism of this particular family of heterocyclic compounds. Introduction Photochromic compounds respond to optical stimulations by altering reversibly their ability to absorb radiations in the visible region of the electromagnetic spectrum on the basis of cis/trans isomerizations, ring-opening/closing steps, or proton/electron transfer processes.1-5 These photoinduced transformations at the molecular level can occur in liquid solutions, within polymer matrices, or even in crystals and culminate with significant modifications in the macroscopic properties of the overall material. Specifically, the absorption coefficient and refractive index of these materials vary with the state of the photochromic species and, hence, can be modulated under the influence of optical inputs. Indeed, the absorptive and dispersive effects associated with photochromic compounds have already been exploited to design and implement a diversity of photoresponsive materials and devices.6-8 The promising properties of photochromic materials encouraged the development of a wealth of organic molecules able to * To whom correspondence should be addressed. E-mail: [email protected] (S.S.); [email protected] (F.M.R.). † University of Miami. ‡ Universita` di Catania.

undergo photoinduced and reversible transformations. As a result, several families of photochromic compounds with diverse structural designs have emerged over the past five decades.1-5 These fundamental studies contributed precious insights on the subtle stereoelectronic factors regulating the photochemical and photophysical properties of organic compounds. In turn, this invaluable information continues to provide useful design elements to improve the fatigue resistances, quantum yields, and coloration efficiencies of photochromic compounds as well as to regulate their color and switching speeds. In particular, detailed investigations on the photoisomerization of nitrospiropyrans9-13 provided us with viable design criteria to develop a new family of photochromic compounds with excellent fatigue resistance and ultrafast switching speeds.14 Our systems are based on the photoinduced opening and thermal closing of [1,3]oxazine rings. They tolerate thousands of switching cycles, even in the presence of molecular oxygen, and switch back and forth between their two interconvertible states on nanosecond time scales. In principle, all-optical logic gates,15-19 optical limiters,20-24 photoresponsive filters,25 and photoswitchable probes26-29 can all be designed around the unique properties of our thermally reversible photochromic compounds. Each one of these ap-

10.1021/jp901494c CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009

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Deniz et al. TABLE 1: Spectroscopic Dataa for the [1,3]Oxazines 1a-8a and Their Model Compounds 16-26 1a 2a 3a 4a 5a 6a 7a 8a 16 17 18 19 20 21 22 23 24 25 26 27

λ (nm)

ε (mM-1 cm-1)

Φ

τ

316 308 346 304 309 304 310 305 302 354 292 297 281 333 373 298 295 342 438 552

11.0 13.4 15.3 11.2 12.6 11.3 12.5 30.3 3.9 9.9 2.8 3.8 3.9 12.6 6.5 10.8 13.1 12.2 15.5 60.8

0.10

25 ns

0.29 0.09 0.11 0.01 0.07

31 ns 31 ns 21 ns 10 µs 2 µs

a

The absorption wavelength (λ), molar extinction coefficient (ε) at λ, quantum yield (Φ) for the photochromic transformation, and lifetime (τ) of the photogenerated isomer were measured in MeCN at 20 °C. The data for 1a and 20 are from ref 14b. The error in the determination of Φ and τ is ca. 15%. Figure 1. Photoinduced transformation of the [1,3]oxazines 1a-8a into the zwitterions 1b-8b.

plications, however, demands stringent photochemical and photophysical requirements to be satisfied. Specifically, the excitation wavelength of the photochromic switch, the color of the photogenerated state, the efficiency of the photochemical transformation, and the time scales of the photoinduced and thermal interconversions must all be adjusted to the specific application of choice. Thus, we need to learn how to tune the photochromism of our compounds in order to develop ultimately photoresponsive materials and devices based on this promising class of molecular switches. On the basis of these considerations, we explored structural modifications of the native [1,3]oxazine skeleton. In this article, we report the synthesis of seven new members of this class of photoresponsive molecules, together with a detailed characterization of their photochemical and photophysical properties. Results and Discussion Design and Synthesis. The ultraviolet excitation of 1a (Figure 1) opens its [1,3]oxazine ring to generate 1b in less than 6 ns with a quantum yield of 0.10 (Table 1) in acetonitrile at 20 °C.14a,b The photogenerated isomer reverts thermally to the original state with first-order kinetics and a lifetime of 25 ns. The photoinduced transformation of 1a into 1b is accompanied by the appearance of a band at 440 nm in the absorption spectrum. This band corresponds to a ground-state absorption of the 4-nitrophenolate chromophore of 1b and decays monoexponentially with the thermal reisomerization of this species back to 1a. In principle, the introduction of substituents in the para (R1) and ortho (R2) positions, relative to the nitrogen atom, of the 3H-indole fragment of 1a or on its chiral center (R3) can be exploited to control the rate of the decoloration process. Indeed, electron donating groups should stabilize the 3Hindolium cation of the corresponding photogenerated isomer and delay its thermal reisomerization, while electron withdrawing groups should encourage the opposite effect. In addition, the

Figure 2. Synthesis of the [1,3]oxazines 2a-7a.

nature of R3 can also be exploited to regulate the absorption characteristics of the 3H-indolium cation of the photogenerated isomer. In fact, the photoinduced opening of the [1,3]oxazine ring brings R3 in conjugation with the 3H-indolium cation. Thus, an extended π-system able to absorb in the visible region, in addition to the 4-nitrophenolate fragment, can be generated with a careful choice of R3. On the basis of these considerations, we designed the seven [1,3]oxazines 2a-8a (Figure 1). In 2a-4a, R1 is a methoxy, nitro, or fluorine substituent, R2 is a hydrogen atom, and R3 is a phenyl ring. In 5a, R1 and R2 are fluorine atoms and R3 is a phenyl ring. In 6a-8a, R1 and R2 are hydrogen atoms and R3 is a 4-methoxyphenyl, 4-dimethylaminophenyl, or 2-(4-dimethylaminophenyl)ethylene group. We synthesized the [1,3]oxazines 2a-7a in one step starting from the corresponding 3H-indoles 9-14 (Figure 2). Specifically, we N-alkylated these precursors with 2-chloromethyl-4nitrophenol and isolated 2a-7a in yields ranging from 18 to 57% after the spontaneous cyclization of their [1,3]oxazine rings. Similarly, we prepared 8a in one step starting from the preformed [1,3]oxazine 15a (Figure 3) and adapting a procedure developed for the condensation of this compound to aldehydes.14d,e In particular, we reacted 15a with 4-dimethylaminobenzaldehyde in the presence of trifluoroacetic acid and isolated 8a in a yield of 47%.

Photochromism of Bichromophoric Oxazines

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Figure 3. Synthesis of the [1,3]oxazine 8a.

Figure 5. Model 3H-indoles 16-20.

Figure 4. Steady-state absorption spectra of solutions (0.1 mM, MeCN, 20 °C) of 2a before (a and f) and after the addition of either CF3CO2H (10 equiv, d) or Bu4NOH (130 equiv, g), 4-nitroanisole (b), 16 (c), the iodide salt of 21 (e), and 4-nitrophenol after the addition of Bu4NOH (4 equiv, h).

Steady-State Absorption Spectroscopy. The steady-state absorption spectra of 2a-7a (a in Figure 4 and Figures S1-S5 of the Supporting Information) show a band at 304-346 nm (Table 1) with a molar extinction coefficient of 11.2-15.3 mM-1 cm-1. In each case, this band is a result of the ground-state absorptions of the 4-nitrophenoxy and 3H-indole fragments and, hence, resembles the sum of those (b and c in Figure 4 and Figures S1-S5 of the Supporting Information) of 4-nitroanisole and the corresponding 3H-indole (16-20 in Figure 5). Similarly, the spectrum (a in Figure 6) of 8a also shows a band at 305 nm (Table 1), but the molar extinction coefficient is 30.3 mM-1 cm-1. In fact, the ground-state absorption of 4-vinyl-N,Ndimethylaniline (d in Figure 6) is positioned in the vary same range of wavelengths. Thus, the 2-(4-dimethylaminophenyl)ethylene, 4-nitrophenoxy, and 3H-indole fragments of 8a are all responsible for the band at 305 nm. The [1,3]oxazine ring of 2a and 4a-8a opens upon addition of acid to generate the cations 2c and 4c-8c (Figure 7). These transformations are accompanied by the appearance of bands at 333, 303, 307, 342, 454, and 568 nm, respectively, in the steady-state absorption spectra (d in Figures 4 and S2-S5 of the Supporting Information, e in Figure 6). These bands resemble the ground-state absorptions (e in Figures 4 and S2-S5

Figure 6. Steady-state absorption spectra of solutions (0.05 mM, MeCN, 20 °C) of 8a before (a and g) and after the addition of either CF3CO2H (2 equiv, e) or Bu4NOH (51 equiv, h), 4-nitroanisole (b), 20 (c), 4-vinyl-N,N-dimethylaniline (d), the hexafluorophosphate salt of 27 (f), and 4-nitrophenol after the addition of Bu4NOH (4 equiv, i).

of the Supporting Information, f in Figure 6) of the model compounds 20 and 22-26 (Figure 8) and, hence, are associated with the 3H-indolium chromophores of 2c and 4c-8c. In contrast to the behavior of 2a and 4a-8a, the addition of even a large excess of acid to 3a causes only negligible changes in the absorption spectrum (a and d in Figure S1 of the Supporting Information) under otherwise identical conditions. Presumably, the electron withdrawing character of the nitro group on the

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Figure 7. Transformation of the [1,3]oxazines 2a-8a into the cations 2c-8c or anions 2d-8d under the influence of acid or base, respectively.

Figure 8. Model 3H-indolium cations 21-27.

3H-indole fragment of 3a is responsible for preventing the opening of the [1,3]oxazine ring. The [1,3]oxazine ring of 2a-8a also opens upon addition of base to generate the anions 2d-8d (Figure 7). All these species incorporate a 4-nitrophenolate chromophore, and their formation is accompanied by the appearance of a band at 440 nm in the steady-state absorption spectrum (f and g in Figures 4 and S1-S5 of the Supporting Information, g and h in Figure 6). This band resembles the ground-state absorption of tetrabutylammonium 4-nitrophenolate (h in Figures 4 and S1-S5 of the Supporting Information, i in Figure 6) and is associated with the 4-nitrophenolate fragment of 2d-8d. Transient Absorption Spectroscopy. The local excitation of the 4-nitrophenoxy chromophore of the parent compound 1a at 355 nm opens its [1,3]oxazine ring to form the zwitterionic isomer 1b with a quantum yield of 0.10 (Table 1) in acetonitrile at 20 °C.14a,b Consistently, the ground-state absorption of the 4-nitrophenolate chromophore of 1b can be observed at 440 nm in the transient spectrum recorded several nanoseconds after excitation. The introduction of a methoxy group in the para position, relative to the nitrogen atom, of the 3H-indole fragment of 1a has a drastic effect on the photochemical behavior. Indeed, the transient spectra of 2a do not show any significant signal on a nanosecond time scale under otherwise identical conditions. By contrast, the presence of a fluorine atom in place of the methoxy group facilitates the photoinduced transformation. In fact, the excitation of 4a opens the [1,3]oxazine ring with a 3-fold enhancement in quantum yield (Table 1) relative to the parent compound 1a. Consistently, the transient spectrum (a in Figure 9) shows the ground-state absorption of the 4-nitrophenolate chromophore of 4b. Thus, the nature of the substituent in the para position, relative to the nitrogen atom, of the 3H-

Figure 9. Transient absorption spectrum (0.02 mM, MeCN, 20 °C) of 4a (a) recorded 40 ns after laser excitation (355 nm, 12 mJ). Temporal evolution of the absorbance at 430 nm (b) after excitation and the corresponding monoexponential fitting (c).

indole fragment has a pronounced influence on the efficiency of photoinduced opening of [1,3]oxazine ring. The introduction of a second fluorine atom in the ortho position, relative to the nitrogen atom, of the 3H-indole fragment has a modest influence on the photoinduced ring-opening process and the photochemical behavior of 5a is remarkably similar to that of 4a. Once again, the ground-state absorption (a in Figure S7 of the Supporting Information) of the 4-nitrophenolate chromophore of 5b can be observed upon excitation. Instead, the replacement of the fluorine atom of 4a with a nitro group alters drastically the photochemical response. Indeed, the transient spectra (a-d in Figure S6 of the Supporting Information) of 3a show a broadband similar to the one (a in Figure S10 of the Supporting Information) observed for the model 3Hindolium 22 under otherwise identical conditions. In both instances, this band decays with monoexponential kinetics (e and f in Figure S6 of the Supporting Information, b and c in Figure S10 of the Supporting Information) on a microsecond time scale and the lifetime of the transient species decreases significantly in the presence of molecular oxygen. Presumably, the nitro group on the 3H-indole fragment of 3a facilitates intersystem crossing and encourages the population of the triplet state associated with the 3H-indolium chromophore of 3b.

Photochromism of Bichromophoric Oxazines

Figure 10. Transient absorption spectra (0.01 mM, MeCN, 20 °C) of 8a recorded 0.1 (a), 1.0 (b), 3.0 (c), and 10 µs (d) after laser excitation (355 nm, 12 mJ). Temporal evolution of the absorbance at 440 (e) and 550 nm (f) after excitation and the corresponding monoexponential fittings (g and h).

The introduction of a methoxy group in the para position of the phenyl ring attached to the chiral center of the parent compound 1a has negligible effects on the photochemical response. In fact, the excitation of 6a opens its [1,3]oxazine ring to generate 6b with a quantum yield of 0.11 (Table 1) and the transient spectrum (a in Figure S8 of the Supporting Information) shows the characteristic ground-state absorption of the 4-nitrophenolate chromophore. A similar band is also observed in the transient spectrum (a in Figure S9 of the Supporting Information) of 7a, in agreement with the photoinduced formation of 7b. In this instance, however, the groundstate absorption of the 3H-indolium cation [cf., 25 (e in Figure S4 of the Supporting Information)] of the photogenerated isomer is positioned in the same region of wavelengths as that associated with the 4-nitrophenolate anion. Thus, the transient band is the sum of the absorptions of the two chromophoric fragments composing 7b. Nonetheless, the photoinduced ringopening process is not particularly efficient and the quantum yield for the formation of 7b is only 0.01 (Table 1). The steady-state absorption spectrum (f in Figure 6) of the hexafluorophosphate salt of the model compound 27 shows that the attachment of a 2-(4-dimethylaminophenyl)ethylene group to the 3H-indolium cation translates into the appearance of an intense band at 552 nm. Consistently, the transient spectra (a-d in Figure 10), recorded after the excitation of 8a, reveal a band at 550 nm for the 3H-indolium cation of 8b together with that at 440 nm for the 4-nitrophenolate anion of this zwitterionic species. In agreement with this assignment, the quantum yield (Table 1) determined by monitoring the absorbance at 440 nm

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8495 is essentially identical to that calculated by probing the absorbance at 550 nm. The photogenerated isomer 1b of the parent compound 1a switches back to the original form in the dark with first-order kinetics and a lifetime of 25 ns (Table 1) in acetonitrile at 20 °C.14a,b The photogenerated species 4b-6b show essentially the same behavior. The band of their 4-nitrophenolate chromophore decays monoexponentially on a nanosecond time scale (b and c in Figures 9 and S7-S9 of the Supporting Information) and their lifetime ranges from 21 to 31 ns (Table 1). Thus, the introduction of either fluorine atoms on the 3H-indole fragment or a methoxy group on the phenyl ring attached to the chiral center has a negligible influence on the reisomerization kinetics. A similar monoexponential decay is also observed for the band associated with the 4-nitrophenolate chromophore of 7b (b and c in Figure S9 of the Supporting Information) as well as for those corresponding to the 4-nitrophenolate anion (e and g in Figure 10) and 3H-indolium cation (f and h in Figure 10) of 8b. The reisomerization of these compounds, however, occurs on a microsecond time scale and their lifetimes are 10 and 2 µs (Table 1), respectively. The relatively long lifetimes of these species are, presumably, a result of the ability of their electron donating substituents to stabilize the 3H-indolium cation and, hence, delay the reisomerization process. Thus, all five photochromic compounds can be switched back and forth between their two states, with switching rates ranging from 0.1 to 50 MHz. Furthermore, they all have excellent fatigue resistances and survive unaffected by multiple switching cycles in air. Specifically, their steady-state absorption spectra recorded before and after 300 switching cycles are essentially identical, indicating that the photodegradation of these compounds is negligible. The photochromic compounds 4a-8a can also be operated within rigid matrices. Specifically, the excitation of these compounds trapped within poly(methyl methacrylate) (PMMA) films results in the opening of their [1,3]oxazine ring with the formation of the corresponding isomers 4b-8b. Consistently, the corresponding transient absorption spectra reveal the groundstate absorption (Figures S11-S14 of the Supporting Information) of the 4-nitrophenolate chromophore associated with the photogenerated isomers. The excitation of 8a, however, is also accompanied by the appearance of the ground-state absorption (a in Figure 11) of the 3H-indolium cation in addition to that of the 4-nitrophenolate anion. In all instances, the transient bands decay on a microsecond time scale with the reisomerization of 4b-8b back to 4a-8a. For example, the absorbance at 630 nm associated with the 3H-indolium cation of 8b decreases biexponentially over the course of 20 µs (b in Figure 11). Nonlinear curve fitting (c in Figure 11) of the temporal absorbance evolution indicates the lifetimes of the two decaying components to be 1 and 33 µs. This behavior is analogous to that observed for the parent compound 1a in PMMA and, presumably, is a result of the aggregation of the photogenerated isomer within the polymer matrix.14b Conclusions We investigated the influence of three substituents on the photochromism of heterocyclic compounds fusing 3H-indole and nitrobenzooxazine fragments in their molecular skeleton. Specifically, we varied the groups in the para (R1) and ortho (R2) positions, relative to the nitrogen atom, of the 3H-indole fragment and that (R3) attached to the chiral center at the junction of the two heterocycles. Two of them (R1 and R3) affect the efficiency of the photochemical process, involving the opening of a [1,3]oxazine ring with the generation of a

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Figure 11. Transient absorption spectrum (2% w/w, PMMA, 20 °C) of 8a (a) recorded 0.1 µs after laser excitation (355 nm, 12 mJ). Temporal evolution of the absorbance at 630 nm (b) after excitation and the corresponding monoexponential fitting (c).

zwitterionic isomer. One of them (R3) also dictates the absorption properties of the photogenerated species and the reisomerization kinetics. In particular, the photochromic transformation is completely suppressed when R1 is a methoxy group, while it is greatly facilitated when R1 is a fluorine atom. Thus, the ability of R1 to donate or withdraw electrons to or from the 3H-indole heterocycle regulates the quantum yield of the photoinduced ring-opening process. Nonetheless, the nature of R1 has a negligible influence on the lifetime of the photogenerated isomer, which is instead dependent on R3. Indeed, the introduction of dimethylamino or 2-(4-dimethylaminophenyl)ethylene groups at R3 prolongs the lifetime of the photogenerated isomer from the nanosecond to the microsecond time scale. Presumably, these electron-rich substituents increase the stability of the 3Hindolium cation associated with the photogenerated isomer and increase its lifetime. Similarly, R3 also regulates the absorption properties of the 3H-indolium cation and, thus, can be exploited to impose bichromophoric character on these photochromic compounds. When R3 is a 2-(4-dimethylaminophenyl)ethylene, for example, a band at 550 nm for the 3H-indolium cation of the zwitterionic isomer develops in the transient spectra, together with one at 440 nm for the associated 4-nitrophenolate anion, upon excitation. In addition, these photochromic compounds tolerate hundreds of excitation cycles without decomposing and can also be operated within PMMA matrices. In summary, the nature of R1 and R3 has a significant influence on the photochromism of this particular class of molecular switches and, in principle, can be exploited to regulate their color, efficiency and switching speed.

Materials and Methods. Chemicals were purchased from commercial sources and used as received with the exception of MeCN and CH2Cl2, which were distilled over CaH2, and THF, which was distilled over Na and Ph2CO. Compounds 9, 15a, 20, and 1,2,3,3-tetramethyl-3H-indolium hexafluorophosphate were prepared according to literature protocols.14b,30,31 Compounds 2a-8a, 10-14, 16-19, and 21-27 were prepared as described in the Supporting Information. All reactions were monitored by thin-layer chromatography using aluminum sheets coated with silica (60, F254). Fast atom bombardment mass spectra (FABMS) were recorded with a VG Mass Laboratory Trio-2 spectrometer in a 3-nitrobenzyl alcohol matrix. High resolution electrospray ionization mass spectra (HRESIMS) were recorded with an Agilent LCTOF spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded with Bruker Avance 400 or 500 spectrometers. Steady-state absorption spectra were recorded with a Varian Cary 100 Bio spectrometer using quartz cells with a path length of 0.5 cm. Steady-state emission spectra were recorded with a Varian Cary Eclipse spectrometer in aerated solutions. Time-resolved absorption spectra were recorded with a Luzchem Research mLFP-111 spectrometer after excitation with a Continuum Surelite II-10 Nd:YAG laser [pulse width ) 6 ns (fwhm), wavelength ) 355 nm]. The photochromic films were prepared by spin-coating CH2Cl2 solution of 4a-8a (1.0 mg mL-1) and PMMA (52 mg mL-1) on a glass plate at 420 rpm for 9 s. The thicknesses (6 µm) of the resulting films was measured with a Tencor Instruments 1000090 surface profilometer. Acknowledgment. We thank the National Science Foundation (CAREER award CHE-0237578 and CHE-0749840) and the University of Miami for financial support. Supporting Information Available: Synthetic procedures; steady-state absorption spectra of 3a-7a; determination of the quantum yields for the photochromic transformations; transient absorption spectra of 3a-7a and 22. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Dorion, G. H.; Wiebe, A. F., Eds. Photochromism; Focal Press: New York, 1970. (2) Brown, G. H., Ed. Photochromism; Wiley: New York, 1971. (3) El’tsov, A. V., Ed. Organic Photochromes; Consultants Bureau: New York, 1990. (4) Bouas-Laurent, H.; Du¨rr, H., Eds. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 1990. (5) Crano, J. C.; Guglielmetti R., Eds. Organic Photochromic and Thermochromic Compounds; Plenum Press: New York, 1999. (6) McArdle, C. B., Ed. Applied Photochromic Polymer Systems; Blackie: Glasgow, 1992. (7) (a) Irie, M., Ed. Photo-ReactiVe Materials for Ultrahigh Density Optical Memory; Elsevier: Amsterdam, 1994. (b) Irie, M. Chem. ReV. 2000, 100, 1683-1890. (8) (a) Raymo, F. M.; Tomasulo, M. Chem.sEur. J. 2006, 12, 3186– 3193. (b) Raymo, F. M. Angew. Chem., Int. Ed. 2006, 45, 5249–5251. (9) (a) Bertelson, R. C. In Photochromism; Dorion, G. H.; Wiebe, A. F., Eds.; Focal Press: New York, 1970; pp 45-431. (b) Bertelson, R. C. In Organic Photochromic and Thermochromic Compounds; Crano, J. C.; Guglielmetti R., Eds.; Plenum Press: New York, 1999; Vol. 1, pp 11-83. (10) Kholmanskii, A. S.; Dyumanev, K. M. Russ. Chem. ReV. 1987, 56, 136–151. (11) Guglielmetti, R. In Photochromism: Molecules and Systems; BouasLaurent, H.; Du¨rr, H., Eds.; Elsevier: Amsterdam, 1990; pp 314-466 and 855-878. (12) Tamai, N.; Miyasaka, H. Chem. ReV. 2000, 100, 1875-1890. (13) Minkin, V. I. Chem. ReV. 2004, 104, 2751–2776.

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