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Photoswitchable Fluorescent Assemblies Based on Hydrophilic BODIPY-Spiropyran Conjugates† Massimiliano Tomasulo, Erhan Deniz, Robert J. Alvarado, and Françisco M. Raymo* Center for Supramolecular Science, Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Florida 33146-0431 ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: March 14, 2008

In search of strategies to design photoswitchable fluorescent probes and operate them in aqueous environments, we have envisioned the possibility of incorporating fluorescent, photochromic, and hydrophilic components within the same macromolecular construct. First, we synthesized a fluorophore-photochrome dyad, pairing a BODIPY fluorophore and a spiropyran photochrome in its molecular skeleton, and investigated the photochemical and photophysical properties of this compound in acetonitrile. Under these conditions, the photoinduced isomerization of the spiropyran causes a 56% decrease in the emission intensity of the BODIPY at the photostationary state. The photogenerated isomer has a lifetime of 2.7 × 102 s and reverts thermally to the original form, restoring the initial emission intensity. On the basis of these results, we copolymerized a similar BODIPY-spiropyran conjugate with a monomer bearing a pendant polyethylene glycol chain. The resulting polymer is soluble in aqueous environments, and its fluorescence can be modulated by operating the photochromic components with optical stimulation. Specifically, the emission intensity decreases by 40% at the photostationary state and reverts to the initial value after thermal reisomerization of the photochromic components. However, the lifetime of the photogenerated species in neutral buffer is significantly longer than that of the monomeric BODIPY-spiropyran in acetonitrile. The fluorescence of both monomeric and polymeric fluorophore-photochrome assemblies can be switched repeatedly between high and low values by alternating ultraviolet irradiation and storage in the dark. However, the fatigue resistance properties of both systems are relatively poor. In any case, our investigations demonstrate that our design is viable for the realization of hydrophilic and photoswitchable molecular assemblies. In principle, innovative fluorescent probes for biomedical applications can evolve from these studies, if methods to improve their fatigue resistance properties and optimize their reisomerization kinetics can be identified. Introduction Photochromic compounds alter their ability to absorb visible radiation in response to optical stimulation.1–8 The electronic and structural changes accompanying these photoinduced transformations can be exploited to modulate the emission of compatible fluorophores.7b,9,10 Indeed, fluorescent and photochromic components can be integrated within the same molecular skeleton, and the emission of the former can be switched by operating the latter with optical inputs. Specifically, the pronounced difference in either the absorption or redox properties of the interconvertible states of the photochrome can be exploited to photoregulate the efficiency of either an energyor electron-transfer process, respectively. Either one of these two processes can be designed to compete with the radiative deactivation of the excited fluorophore in the resulting molecular constructs. Under these conditions, the emission intensity of the fluorescent component changes with the state of the photochromic species. On the basis of these operating principles, several fluorophorephotochrome conjugates have already been synthesized, combining a diversity of fluorophores and photochromes within the same molecular assemblies.11–29 In addition, these mechanisms have been reproduced intermolecularly30–37 and extended to macromolecular constructs,38–43 nanoparticles,44–48 supramo† Part of the “Larry Dalton Festschrift”. * E-mail: [email protected].

lecular assemblies,49,50 and multilayer arrays.51 The main motivation behind these fundamental studies lies in the need to understand further the photochemical and photophysical properties of organic compounds while learning how to engineer photoresponsive materials based on molecular components. In fact, the use of these photoswitchable and fluorescent systems to fabricate optical memories for information technology20,26b,35,51 and luminescent probes for biomedical research17g,42 has already been demonstrated with prototypical examples. In particular, the potential application of switchable fluorescent probes in biomedical imaging appears to be especially promising.52 Indeed, the operation of such probes in conjunction with multiphoton excitation schemes offers, in principle at least, the opportunity to break the diffraction barrier and push the resolution of fluorescence imaging down to the nanometer level. However, the vast majority of the fluorophore-photochrome assemblies developed so far are relatively hydrophobic and have been studied almost exclusively in organic solvents.11–51 Strategies to impart hydrophilic character and biocompatibility to these functional molecular switches as well as methods to operate them efficiently in aqueous environments must be developed. Only then will these promising materials be able to evolve into valuable luminescent probes for practical applications in biology. On the basis of these considerations, we have designed hydrophilic constructs incorporating 4,4-difluoro-4-bora-3a,4adiaza-s-indacene (BODIPY) fluorophores53,54 and spiropyran photochromes.55–59 In this article, we report the synthesis and

10.1021/jp8009035 CCC: $40.75  2008 American Chemical Society Published on Web 04/29/2008

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Figure 1. Photoinduced interconversion between 1a and 1b and between 3a and 3b and structures of the model fluorophore 2 and precursor 4.

photochemical and photophysical properties of these hydrophilic and fluorescent molecular switches. Results and Discussion Design, Synthesis, and Properties of a BODIPYSpiropyran Conjugate. The colorless spiropyran 1a (Figure 1) switches to the colored merocyanine 1b upon ultraviolet irradiation.60 The photoinduced transformation of 1a into 1b is accompanied by the appearance of an intense band in the visible region of the absorption spectrum (Figure 2a,b). This band is positioned in the same region of wavelengths where the BODIPY fluorophore 2 emits (Figure 2c) and disappears with the thermal reisomerization of 1b back to 1a. As a result, the excited fluorophore can transfer energy to the colored state of the photochrome, if the two components are sufficiently close to each other, but not to the colorless one. On the basis of these considerations, we designed the BODIPY-spiropyran conjugate 3a (Figure 1) and prepared this compound in one step, starting from 1a and 4, with a yield of 54%. The absorption spectrum of the fluorophore-photochrome conjugate 3a (Figure 2d) resembles the sum of those associated with the fluorophore 2 and photochrome 1a (Figure 2f). Upon ultraviolet irradiation, the photochromic component of 3a switches to its colored state and forms 3b with the concomitant appearance of the characteristic absorption in the visible region (Figure 2e). However, this band is centered at 570 nm for 3b and 563 nm for 1b.60 From the absorbance of this band, the ratio between the two isomers 3a and 3b can be estimated as 73:27 at the photostationary state,61 whereas that between 1a and 1b is 56:44 under otherwise identical conditions.60 In both instances, the photogenerated isomer reverts thermally to the original one, and the absorbance in the visible region decays monoexponentially (Figure 3). The nonlinear curve fitting of the absorbance profile indicates the lifetime of 3b to be 2.7 × 102 s in acetonitrile at 25 °C, whereas the lifetime is 1.9 × 102 s for 1b under the same conditions.60b Thus, the photochromic component can be operated effectively even when it is covalently bonded to the BODIPY fluorophore. Nonetheless, the latter has some influence on the spectroscopic signature and reisomerization kinetics of the former. The emission spectrum of the fluorophore-photochrome conjugate 3a (Figure 4a) resembles that of the model fluorophore

Figure 2. Absorption spectra of a solution of 1a (0.1 mM, MeCN, 25 °C) (a) before and (b) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 5 min) and (c) emission spectrum of 2 (0.01 mM, MeCN, 25 °C, 480 nm). Absorption spectra of a solution of 3a (0.1 mM, MeCN, 25 °C) (d) before and (e) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 5 min) and (f) sum of the absorption spectra of 1a and 2 (0.1 mM, MeCN, 25 °C).

2 (Figure 2c). Both spectra show a band at 539 nm. However, the fluorescence quantum yield of 3a is 0.8, whereas that of 2 is 1.0. Thus, the covalent attachment of the spiropyran photochrome to the BODIPY fluorophore has a depressive effect on the quantum yield. Presumably, photoinduced electron transfer from the photochrome to the excited fluorophore is responsible for the decrease in quantum yield. Indeed, the redox potentials of 1a and 2 (Table 1) suggest that the electron-transfer process

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Figure 3. Absorbance decay for a solution of 3a (0.1 mM, MeCN, 25 °C, 570 nm) maintained in the dark after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 5 min).

Figure 4. Emission spectra of a solution of 3a (0.01 mM, MeCN, 25 °C, 480 nm) (a) before and (b) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 5 min) and (c) emission spectrum of a solution of 1a (0.01 mM, MeCN, 25 °C, 560 nm) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 5 min).

TABLE 1: Oxidation (EOx) and Reduction (ERed) Potentials of the Model Fluorescent and Photochromic Componentsa 1a 1b 2

EOx (V)

ERed (V)

+0.72 +0.77

-1.54 -1.23 -1.52

a EOx and ERed were determined by cyclic voltammetry (Supporting Information) in a MeCN solution of Bu4NPF6 (0.1 M), using a glassy carbon working electrode, a nonaqueous Ag/Ag+ reference electrode, and a platinum counter electrode.

is exoergonic with a free energy change of -0.18 eV.62 Upon ultraviolet irradiation, 3a switches to 3b, and the emission intensity decreases to 44% of the original value at the photostationary state (Figure 4b).63 Indeed, the photoinduced interconversion of the photochromic component can activate an energy-transfer pathway and lead to a partial fluorescence quenching. Nonetheless, the emission spectrum of 3b does not show the characteristic fluorescence of the photogenerated state of the photochrome, which is positioned at 646 nm for 1b (Figure 4c). The lack of sensitized fluorescence suggests that the transfer of energy from the excited fluorophore to the colored state of the photochrome is not particularly efficient. Thus, the pronounced decrease in emission intensity with the formation

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Figure 5. Relative emission intensity of a solution of 3a (0.01 mM, MeCN, 25 °C, 480 nm) measured at 539 nm (a) before and after consecutive (g-k) ultraviolet irradiation (254 nm, 0.5 mW cm-2, 1 min) and (b-f) storage in the dark (30 min).

of 3b cannot exclusively be a consequence of energy transfer. In fact, an examination of the redox potentials of 1b and 2 (Table 1) indicates that the photoinduced transfer of one electron from the excited fluorophore to the photochrome is exoergonic with a free energy change of -0.42 eV.62 Presumably, this electrontransfer process contributes to the nonradiative deactivation of the excited fluorophore together with the anticipated energytransfer pathway. In any case, the original emission intensity of the BODIPY component is restored after the thermal reisomerization of 3b back to 3a. In fact, the fluorescence of this particular fluorophore-photochrome dyad can be switched between high (Figure 5a-f) and low (Figure 5g-k) values simply by turning on and off an ultraviolet source. However, whereas the change in emission intensity associated with each switching step remains approximately constant, the fluorescence measured before and after each step tends to decrease with the number of switching cycles. Presumably, this trend is a result of the gradual photodegradation of the BODIPY-spiropyran conjugate. Design, Synthesis, and Properties of a Polymer Incorporating BODIPY-Spiropyran Conjugates. The spectroscopic analysis of 3a demonstrates that the photoinduced interconversion of the spiropyran component can be exploited to modulate the fluorescence of the BODIPY component. However, this compound is rather hydrophobic and cannot be operated in aqueous environments. To overcome this limitation, we considered the possibility of attaching a similar BODIPY-spiropyran conjugate to a polymer backbone with pendant hydrophilic chains. Specifically, we synthesized the photochromic monomers 5a and 6a (Figure 6) and reacted them with the hydrophilic monomer 7, with the assistance of first-generation Grubbs’ catalyst,64 to generate the copolymers 8a and 9a, respectively. We confirmed the incorporation of both monomeric components within the macromolecular constructs 8a and 9a by 1H NMR spectroscopy. Indeed, the 1H NMR spectra of these copolymers (Supporting Information) show resonances for the aromatic protons of the spiropyran component between 6.2 and 8.2 ppm, together with peaks for the methylene protons of the polyethylene glycol chains between 3.4 and 3.7 ppm. From the integrals of these signals, we estimated that 8a and 9a incorporate 30 and 15 hydrophilic chains, respectively, per photochromic component. The polyethylene glycol chains appended to the main backbone of 8a impart aqueous solubility to the overall macromolecular construct. Consistently, the photochromic

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Figure 6. Synthesis of the photochromic polymers 8a and 9a.

component of this polymer can be operated in aqueous environments with optical stimulation, as observed for 1a and 3a in organic solvents. Specifically, the absorption spectra (Figure 7a,b) of a solution of 8a in sodium phosphate buffer (pH 7.0)

Figure 7. Absorption spectra of a solution of 8a (4.7 mg mL-1, sodium phosphate buffer, pH 7.0, 25 °C) (a) before and (b) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 4 min) and (c) emission spectrum of 4 (0.05 mM, MeCN, 25 °C, 480 nm). Absorption spectra of a solution of 9a (1.5 mg mL-1, sodium phosphate buffer, pH 7.0, 25 °C) (d) before and (e) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 4 min) and (f) absorption spectrum of 4 (0.05 mM, sodium phosphate buffer, pH 7.0, 25 °C).

show the appearance of a band at 522 nm upon ultraviolet irradiation. This band corresponds to the colored state of the photochromic component and is indicative of the photoinduced formation of 8b. In particular, the absorbance of this band suggests that ca. 10% of the photochromes appended to the polymer chain are in the colored form at the photostationary state.65 In addition, this absorption overlaps the emission of the model fluorophore 4 (Figure 7c). Thus, the photogenerated state of the photochromic polymer can, in principle, accept the excitation energy of this particular fluorophore, if the two components are sufficiently close to each other. The polymer 9a combines BODIPY, spiropyran, and polyethylene glycol components in its macromolecular skeleton. As observed for 8a, the polyethylene glycol chains ensure solubility in aqueous environments, and the photochromic component can be operated with optical stimulation under these conditions. Indeed, the absorption spectra of 9a (Figure 7d,e) show the appearance of a band in the visible region upon ultraviolet irradiation. This band corresponds, once again, to the colored state of the photochrome and is consistent with the photoinduced transformation of 9a into 9b. Specifically, the absorbance of this band indicates that ca. 17% of the photochromes appended to the polymer chain are in the colored form at the photostationary state.66 In addition, both absorption spectra show a band for the BODIPY component that resembles the one for the model fluorophore 4 (Figure 7f). However, this band is centered at 530 nm for 9a and 522 nm for 4, whereas essentially no difference is observed between the position of the absorption of the model BODIPY 2 and the monomeric BODIPYspiropyran conjugate 3a (Figure 2d,f). Thus, the elongation of the absorption wavelength of the fluorophore with the transition from 4 to 9a is, presumably, a result of the incorporation of the BODIPY component within the polymeric construct. The photogenerated states of both polymers revert to the original forms upon storage in the dark. Consistently, the absorption band in the visible region fades with the thermal reisomerization of 8b to 8a and 9b to 9a. As observed for 1b and 3b in organic solvents (Figure 3), the absorbance of 8b and 9b decays monoexponentially (Figure 8a,b). The nonlinear

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Figure 8. Absorbance decays for solutions (sodium phosphate buffer, pH 7.0, 25 °C) of (a) 8a (4.7 mg mL-1, 526 nm) and (b) 9a (1.5 mg mL-1, 572 nm) maintained in the dark after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 4 min).

Figure 9. Emission spectra of a solution of 9a (1.5 mg mL-1, sodium phosphate buffer, pH 7.0, 25 °C, 440 nm) (a) before and (b) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 4 min) and (c) emission spectrum of a solution of 8a (4.7 mg mL-1, sodium phosphate buffer, pH 7.0, 25 °C, 440 nm) after ultraviolet irradiation (254 nm, 0.5 mW cm-2, 4 min).

curve fittings of the absorbance profiles indicate the lifetimes of 8b and 9b to be 2.0 × 104 and 0.9 × 104 s, respectively, in sodium phosphate buffer (pH 7.0) at 25 °C. These values are approximately 2 orders of magnitude greater than those of 1b and 3b in acetonitrile at the same temperature. In agreement with literature data,51a the significant enhancement in lifetime associated with the transition from acetonitrile to aqueous environments is, presumably, a result of the increase in solvent polarity. The emission spectrum of 9a in sodium phosphate buffer (pH 7.0) (Figure 9a) resembles that of the model fluorophore 4 recorded under the same conditions (Figure 7c). Both show an intense band centered at 545 nm for 9a and 539 nm for 4. However, the quantum yield for 9a is only 1.1 × 10-2, whereas that for 4 is 8.7 × 10-2. The decrease in quantum yield with the transition from the model BODIPY 4 to the polymeric BODIPY-spiropyran assembly 9a is, presumably, a consequence of photoinduced electron transfer from the photochrome to the excited fluorophore in 9a. Once again, the redox potentials in Table 1 suggest that this process is exoergonic with a free energy change of -0.18 eV.62 Upon ultraviolet irradiation, 9a switches to 9b, and the emission intensity decreases to 60% of

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Figure 10. Relative emission intensity of a solution of 9a (1.5 mg mL-1, sodium phosphate buffer, pH 7.0, 25 °C, 440 nm) measured at 543 nm (a) before and after consecutive (g-k) ultraviolet irradiation (254 nm, 0.5 mW cm-2, 1 min) and (b-f) storage in the dark (12 h).

the original value at the photostationary state (Figure 9b).63 In addition to the fluorescence of the BODIPY component at 545 nm, the emission spectrum recorded after ultraviolet irradiation (Figure 9b) also shows the fluorescence of the photogenerated state of the photochromic component at 623 nm. This band is essentially identical to that observed after the ultraviolet irradiation of 8a (Figure 9c) and then excitation of the photostationary state at the very same wavelength used for 9a. Thus, this emission appears to be mainly a result of the direct excitation of the photogenerated state of the photochromic component, rather than a consequence of energy transfer from the excited BODIPY. Presumably, the pronounced decrease in the emission intensity of the BODIPY component with the transformation of 9a into 9b is mainly a result of photoinduced electron transfer. In fact, the redox potentials in Table 1 suggest that the transfer of one electron from the excited fluorophore to the photogenerated state of the photochrome is exoergonic with a free energy change of -0.42 eV.62 Upon storage in the dark, 9b switches back to 9a with a concomitant increase in the emission intensity of the BODIPY component. However, this process is significantly slower than the reisomerization of 3b back to 3a, and the original emission intensity is almost completely restored only after 12 h (Figure 10a,b). Additional switching cycles can be performed by alternating ultraviolet irradiation and storage in the dark. However, the emission intensity measured after the thermal reisomerization (Figure 10b-f) tends to decrease with the number of switching cycles. This depressive effect on the emission of the BODIPY fluorophore suggests that the photoinduced interconversion of the spiropyran photochrome is, presumably, accompanied by a significant degradation of the fluorophore-photochrome assembly. Conclusions BODIPY fluorophores can be bonded covalently to spiropyran photochromes to construct photoswitchable fluorescent assemblies. Indeed, the photoinduced transformation of the spiropyran component of the resulting constructs into the corresponding merocyanine isomer causes a decrease in the emission intensity of the BODIPY component. Specifically, the emission intensity of the fluorophore decreases to 44% of the original value at the photostationary state in acetonitrile. The photogenerated isomer reverts thermally to the original one with single-exponential kinetics and a lifetime of 2.7 × 102 s.

Photoswitchable Fluorescent Assemblies As a result, the original emission intensity is fully restored, and the alternation of ultraviolet irradiation and storage in the dark can be exploited to switch the emission intensity repeatedly between high and low values. However, the fluorophorephotochrome dyad has a modest fatigue resistance, and the emission intensity decreases by 24% after five switching cycles. A norbornene fragment can be attached to the BODIPYspiropyran conjugate, and the resulting compound can be copolymerized with a monomer incorporating a polyethylene glycol chain. The product of this reaction is a macromolecular assembly with an average of 15 hydrophilic chains per photochromic component. The polyethylene glycol appendages of this construct impart solubility in aqueous environments, and its photochromic components can be operated with optical stimulation in a neutral buffer. In particular, the photoinduced isomerization of the photochrome causes a decrease of the emission intensity of the fluorophore to 60% of the original value at the photostationary state. The original species is regenerated, once again, upon storage in the dark. However, the lifetime of the photogenerated state of the polymer in aqueous environments is approximately 2 orders of magnitude longer than that of the monomeric BODIPY-spiropyran dyad in acetonitrile. Thus, the fluorescence of this polymeric assembly can be modulated by alternating ultraviolet irradiation and storage in the dark. Nonetheless, its fatigue resistance is relatively poor, and the emission intensity drops by 30% after five switching cycles. Presumably, the transfer of either an electron or energy from the excited BODIPY to the photogenerated state of the photochrome is responsible for the decrease in the emission intensity of both the fluorophore-photochrome dyad and its polymeric counterpart. The redox potentials of the model compounds suggest that the photoinduced electron-transfer process is exoergonic with a free energy change of -0.42 eV. In both systems, the photogenerated isomer of the photochrome absorbs in the range of wavelengths where the fluorophore emits and, in principle, can accept the excitation energy of this component. However, no sensitized fluorescence of the photogenerated state of the photochrome was detected, suggesting that the energytransfer process is not particularly efficient. In summary, our studies demonstrate that water-soluble and photoswitchable fluorescent assemblies can be engineered by appending BODIPY fluorophores, spiropyran photochromes, and polyethylene glycol hydrophiles to the very same polymer backbone. In principle, our design logic and operating mechanisms can lead to the development of innovative fluorescent probes for biomedical imaging. First, however, valuable strategies to optimize their reisomerization kinetics and improve their fatigue resistances must be identified. Experimental Procedures 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 1a, 10, and 12 were prepared according to literature procedures.60,67 The experimental procedures for the synthesis of 5a, 6a, and 7 are reported 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 in a 3-nitrobenzyl alcohol matrix with a VG Mass Laboratory Trio-2 spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded with a Bruker Avance 400 spectrometer. Gel permeation chromatography (GPC) was performed with a Phenomenex Phenogel 5-µm

J. Phys. Chem. C, Vol. 112, No. 21, 2008 8043 MXM column (7.8 × 300 mm) operated with a Varian ProStar system, coupled to a ProStar 330 photodiode array detector, in THF at a flow rate of 1.0 mL min-1. Monodisperse polystyrene standards (2700–200000) were employed to determine the number-average molecular weights (Mn) of the polymers and their polydispersity indexes (PDIs) from the GPC traces. Absorption spectra were recorded with a Varian Cary 100 Bio spectrometer, using quartz cells with a path length of 0.5 cm. Emission spectra were recorded in aerated solutions with a Varian Cary Eclipse spectrometer. Samples were irradiated at 254 nm (0.5 mW cm-2) with a Mineralight UVGL-25 lamp. Cyclic voltammograms were recorded in degassed solutions under an atmosphere of Ar with a CH Instruments 660A apparatus and are reported in the Supporting Information. Compound 2. A solution of 4 (67 mg, 0.2 mmol), EtOH (15 mL, 0.3 mmol), and 4-dimethylaminopyridine (2 mg, 0.02 mmol) in MeCN (20 mL) was cooled in an ice bath under Ar. After the addition of N,N′-dicyclohexylcarbodiimide (36 mg, 0.2 mmol), the reaction mixture was allowed to warm to ambient temperature and was stirred under these conditions for 3 days. The solvent was distilled off under reduced pressure, and the residue was purified by column chromatography [SiO2/ CH2Cl2-hexanes 1:2 f 3:1 (v/v)] to yield 2 (27 mg, 38%) as a pink solid. FABMS: m/z ) 452 [M]+. 1H NMR (400 MHz, CDCl3): δ 0.99 (6H, t, 8 Hz), 1.30 (6H, s), 1.41 (3H, t, 7 Hz), 2.37 (4H, q, 8 Hz), 2.49 (6H, s), 4.42 (2H, q, 7 Hz), 7.49 (2H, d, 8 Hz), 8.15 (2H, d, 8 Hz). 13C NMR (100 MHz, CDCl3): δ ) 11.5, 12.2, 13.9, 14.3, 16.9, 61.6, 129.3, 130.4, 130.5, 131.6, 133.7, 139.1, 140.0, 140.4, 154.5, 166.2. Compound 3a. A solution of 1a (63 mg, 0.2 mmol), 4 (89 mg, 0.2 mmol), and 4-dimethylaminopyridine (3 mg, 0.02 mmol) in MeCN (20 mL) was cooled in an ice bath under Ar. After the addition of N,N′-dicyclohexylcarbodiimide (43 mg, 0.2 mmol), the reaction mixture was allowed to warm to ambient temperature and was stirred under these conditions for 4 days. The solvent was distilled off under reduced pressure, and the residue was purified by column chromatography [SiO2/ CH2Cl2-hexanes 1:3 f CH2Cl2/MeCO2Et 10:1 (v/v)] to yield 3a (73 mg, 54%) as a pink solid. FABMS: m/z ) 758 [M]+. 1H NMR (400 MHz, CDCl3): δ ) 0.99 (6H, t, 7 Hz), 1.20 (3H, s), 1.25 (6H, s), 1.31 (3H, s), 2.30 (4H, q, 7 Hz), 2.55 (6H, s), 3.53–3.60 (1H, m), 3.66-3.73 (1H, m), 4.54 (2H, t, 6 Hz), 5.90 (1H, d, 10 Hz), 6.78 (2H, d, 9 Hz), 6.90-6.94 (2H, m), 7.12 (1H, d, 7 Hz), 7.22 (1H, t, 8 Hz), 7.40 (2H, d, 8 Hz), 8.02-8.06 (2H, m), 8.13 (2H, d, 8 Hz). 13C NMR (100 MHz, CDCl3): δ 12.2, 15.0, 17.5, 20.3, 23.0, 26.3, 32.0, 42.9, 53.2, 63.5, 106.9, 107.2, 116.0, 118.8, 120.5, 122.1, 122.3, 123.2, 126.4, 128.3, 128.8, 129.2, 130.6, 130.7, 133.5, 136.1, 141.4, 141.6, 147.0, 154.8, 159.7, 166.2. Compound 4. A solution of 3-ethyl-2,4-dimethylpyrrole (500 mg, 4 mmol), 4-carboxybenzaldehyde (300 mg, 2 mmol), and CF3CO2H (10 µL, 0.1 mmol) in CH2Cl2 (200 mL) was stirred for 12 h at ambient temperature under Ar. After the addition of a solution of 2,3-dichloro-5,6-dicyano-p-benzoquinone (454 mg, 2 mmol) in CH2Cl2 (15 mL), the mixture was stirred for an additional 30 min. Then, Et3N (4 mL, 28 mmol) and BF3 · Et2O (4 mL, 32 mmol) were added, and the mixture was stirred for an additional 30 min, washed with H2O (3 × 100 mL), and dried over Na2SO4. The solvent was distilled off under reduced pressure, and the residue was purified by column chromatography [SiO2/MeCO2Et-hexanes 1:1 (v/v)] to yield 4 (492 mg, 58%) as a red powder. FABMS: m/z ) 424 [M]+. 1H NMR (400 MHz, CDCl3): δ ) 0.99 (6H, t, 8 Hz), 1.28 (6H, s), 2.31 (4H, q, 8 Hz), 2.55 (6H, s), 7.47 (2H, d, 8 Hz), 8.25 (2H, d, 8

8044 J. Phys. Chem. C, Vol. 112, No. 21, 2008 Hz). 13C NMR (100 MHz, CDCl3): δ ) 12.2, 12.9, 14.9, 17.4, 129.3, 130.1, 130.6, 131.2, 133.5, 138.4, 138.9, 142.0, 154.8, 170.2. Compound 8a. A solution of benzylidene-bis(tricyclohexylphosphine)dichlororuthenium (2.0 mg, 0.002 mmol) in THF (1 mL) was added dropwise to a solution of 5a (9 mg, 0.01 mmol) and 7 (190 mg, 0.1 mmol) in THF (4 mL) maintained under Ar. The mixture was stirred at ambient temperature for 23 h, and then vinyl ethyl ether (1 mL, 10.4 mmol) was added. After 30 min, the solvent was distilled off under reduced pressure, and the residue was dissolved in MeOH (0.5 mL). The addition of Et2O (10 mL) caused the precipitation of a pink solid, which was isolated to afford 8a (160 mg) after centrifugation and removal of the supernatant. GPC: Mn ) 20717, PDI ) 1.12. Compound 9a. A solution of benzylidene-bis(tricyclohexylphosphine)dichlororuthenium (2.0 mg, 0.002 mmol) in THF (1 mL) was added dropwise to a solution of 6a (5 mg, 0.01 mmol) and 7 (215 mg, 0.1 mmol) in THF (4 mL) maintained under Ar. The mixture was stirred at ambient temperature for 23 h, and then vinyl ethyl ether (1 mL, 10.4 mmol) was added. After 30 min, the solvent was distilled off under reduced pressure, and the residue was dissolved in MeOH (0.5 mL). The addition of Et2O (10 mL) caused the precipitation of a pink solid, which was isolated to afford 9a (164 mg) after centrifugation and removal of the supernatant. GPC: Mn ) 26,157, PDI ) 1.03. 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: Cyclic voltammetry of 1a and 2; experimental procedures for the synthesis of 5a, 6a, and 7; and 1H NMR spectra of 8a and 9a. 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., Dürr, 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, Scotland, 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. (Special Issue “Photochromism: Memories and Switches”). (8) (a) Raymo, F. M. Angew. Chem., Int. Ed. 2006, 45, 5249–5251. (b) Raymo, F. M.; Tomasulo, M. Chem. Eur. J. 2006, 12, 3186–3193. (9) Kuz’min, M. G.; Koz’menko, M. V. In Organic Photochromes; El’tsov, A. V., Ed.; Consultants Bureau: New York, 1990; pp 245-265. (10) (a) Raymo, F. M.; Tomasulo, M. Chem. Soc. ReV. 2005, 34, 327– 336. (b) Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343– 7352. (11) Saika, T.; Iyoda, T.; Honda, K.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1992, 591–592. (12) (a) Walz, J.; Ulrich, K.; Port, H.; Wolf, H. C.; Wonner, J.; Effenberg, F. Chem. Phys. Lett. 1993, 213, 321–324. (b) Seibold, M.; Port, H.; Wolf, H. C. Mol. Cryst. Liq. Cryst. 1996, 283, 75–80. (c) Port, H.; Hennrich, M.; Seibold, M.; Wolf, H. C. Proc. Electrochem. Soc. 1998, 98, 61–70. (d) Port, H.; Hartschuh, A.; Hennrich, M.; Wolf, H. C.; Endtner, J. M.; Effenberger, F. Mol. Cryst. Liq. Cryst. 2000, 344, 145–150. (e) Ramsteiner, I. B.; Hartschuh, A.; Port, H. Chem. Phys. Lett. 2001, 343, 83–90. (13) Inada, T.; Uchida, S.; Yokoyama, Y. Chem. Lett. 1997, 321–322. (14) Tsuchiya, S. J. Am. Chem. Soc. 1999, 121, 48–53.

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J. Phys. Chem. C, Vol. 112, No. 21, 2008 8045 (57) Guglielmetti, R. In Photochromism: Molecules and Systems; BouasLaurent, H., Dürr, H., Eds.; Elsevier: Amsterdam, 1990; pp 314-466 and 855-878. (58) Tamai, N.; Miyasaka, H. Chem. ReV. 2000, 100, 1875–1890. (59) Minkin, V. I. Chem. ReV. 2004, 104, 2751–2776. (60) (a) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651– 4652. (b) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2003, 68, 4158–4169. (61) The ratio between the two isomers at the photostationary state was determined from the absorbance of the photogenerated isomer in the visible region, using the molar extinction coefficient (41 mM-1 cm-1) of 1b reported in the literature (ref 60). (62) The free energy changes for the photoinduced electron-transfer processes were estimated with the Rehm-Weller equation (Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271. ), using the redox potentials in Table 1. (63) The emission spectra of the fluorophore-photochrome conjugates were recorded before and after (Figures 4a,b and 9a,b) ultraviolet irradiation, with the sample excited at an isosbestic point. Under these conditions, the fluorescent component absorbs the same amount of exciting photons before and after the interconversion of the photochromic component. (64) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH: Weinheim, 2003. (65) The concentration of spiropyrans in the polymer sample was estimated from the absorbance at 343 nm, measured before ultraviolet irradiation, using the molar extinction coefficient (5 mM-1 cm-1) of 1a determined from Figure 2a. The fraction of photochromes in the colored form at the photostationary state was estimated from the absorbance at 522 nm, using the molar extinction coefficient (41 mM-1 cm-1) of 1b reported in the literature (ref 60). (66) The concentration of spiropyrans in the polymer sample was estimated from the absorbance at 343 nm, measured before ultraviolet irradiation, using the molar extinction coefficient (9 mM-1 cm-1) of 3a determined from Figure 2d. The fraction of photochromes in the colored form at the photostationary state was estimated from the absorbance at 580 nm, using the molar extinction coefficient (17 mM-1 cm-1) of 8b determined from Figure 7b. (67) Tomasulo, M.; Kaanumal, S. L.; Sortino, S.; Raymo, F. M. J. Org. Chem. 2007, 72, 595–605.

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