Fluorescence Switching with a Photochromic Auxochrome

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Fluorescence Switching with a Photochromic Auxochrome Erhan Deniz,† Salvatore Sortino,*,‡ and Franc- isco M. Raymo*,† †

Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, Florida, 33146-0431, United States, and Dipartimento di Scienze Chimiche, Universit a di Catania, viale Andrea Doria 8, Catania, I-95125, Italy

‡

ABSTRACT We synthesized a photoswitchable fluorescent probe incorporating a coumarin fluorophore and an oxazine photochrome within the same molecular skeleton. The visible illumination of this fluorophore-photochrome dyad results in the excitation of the fluorescent component only if the photochromic element is activated with ultraviolet irradiation. Indeed, the photoinduced opening of the oxazine ring bathochromically shifts the absorption of the coumarin fragment sufficiently to encourage its visible excitation with concomitant fluorescence. These operating principles translate into fluorescence photoactivation with good contrast ratio and brightness as well as short fluorescence lifetime. The modular character and relative simplicity of this synthetic strategy for the assembly of photoswitchable constructs might evolve into a general design logic for the photoregulation of the electronic structure of a given chromophore with a photochromic auxochrome. SECTION Kinetics, Spectroscopy

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competitive deactivation pathways and the gradual degradation of the fluorescent and/or photochromic components with the number of switching cycles. Thus, the evolution of these molecular and supramolecular switches into functional materials for practical applications requires the identification of innovative operating principles that would ensure fluorescence photoswitching with improved contrast ratios and fatigue resistances. On the basis of these considerations, we devised a general strategy for fluorescence modulation that avoids intercomponent electron or energy transfer and is instead solely based on the photoinduced bathochromic shift of the absorption wavelength of a fluorescent chromophore with the aid of an appended photochromic auxochrome. In this article, we report the synthesis of a fluorophore-photochrome dyad, designed around this particular switching mechanism, as well as the photochemical and photophysical properties of this compound. We designed a fluorophore-photochrome dyad (1 in Figure 1), incorporating coumarin and oxazine fragments within the same covalent skeleton. We prepared this compound and its model (3 in Figure 2) in a single step from known precursors. Specifically, the condensation of a preformed coumarin (4 in Figure 2) with a preformed [1,3]oxazine (5 in Figure 2), in the presence of trifluoroacetic acid (TFA), gave the target molecule 1 in a yield of 40%. Similarly, the reaction of 4 with the iodide salt of a 3H-indolium cation (6 in

hotochromic compounds switch reversibly between colorless and colored states under the influence of optical stimulations.1-5 Their photoinduced and reversible transformations can be exploited to modulate fluorescence in molecular and supramolecular constructs.6-9 Specifically, fluorescent and photochromic components can be paired either covalently or noncovalently, and the interconversion of the latter can be invoked to regulate the emission intensity of the formers on the basis of intercomponent electron10-13 or energy14-19 transfer. Indeed, the need to understand how to regulate the excitation dynamics of inorganic and organic chromophores primarily motivates the design and investigation of these fascinating systems. Additionally, operating principles for fluorescence modulation under optical control can lead to strategies for the manipulation of information at the molecular level20-29 as well as to the design of functional assemblies of molecular components.30 The exchange of either an electron or energy between the excited state of a fluorophore and one of the two interconvertible states of a photochrome deactivates the former nonradiatively and causes a decrease in fluorescence quantum yield.6-19 The quenching efficiency, however, rarely approaches unity, and these processes generally do not result in complete fluorescence suppression. In addition, the ratio (contrast) between the emission intensity measured after the photochromic transformation and that measured before is related to the composition of the photostationary state. In most instances, both interconvertible forms are present at the photostationary state and, hence, these photoswitchable systems generally have modest contrast ratios with few notable exceptions.31-33 In addition, their excitation dynamics can encourage

r 2010 American Chemical Society

Received Date: October 30, 2010 Accepted Date: December 1, 2010 Published on Web Date: December 03, 2010

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DOI: 10.1021/jz101473w |J. Phys. Chem. Lett. 2010, 1, 3506–3509

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Figure 1. Photoinduced and reversible interconversion of 1 and 2.

Figure 3. Steady-state absorption spectra of solutions (2.5 μM, MeCN, 20 °C) of 1 (a) and the hexafluorophosphate salt of 3 (c). Time-resolved absorption spectrum of a solution (0.01 mM, MeCN, 20 °C) of 1 (b) recorded 0.03 μs after excitation at 355 nm (10 mJ, 6 ns).

Figure 4. Absorbance evolution at 580 nm for a solution (0.01 mM, MeCN, 20 °C) of 1 upon excitation at 355 nm (10 mJ, 6 ns) and the corresponding monoexponential fitting. Figure 2. Synthesis of the fluorophore-photochrome dyad 1 and its model 3.

induced transformation of one component within the fluorophore-photochrome dyad alters the electronic structure of the other and causes a bathochromic shift in absorption by ca. 160 nm. The process, however, is fully reversible, and the photogenerated isomer 2 reverts to the original state with first-order kinetics in a few microseconds with the concomitant decay of its absorption band in the visible region. Nonlinear curve-fitting of the corresponding temporal absorbance profile (Figure 4) indicates the lifetime of 2 to be 0.2 μs. In addition, this fluorophore-photochrome dyad tolerates hundreds of switching cycles with no sign of degradation, even in the presence of molecular oxygen. In fact, the absorbance measured in the visible region 0.03 μs after activation at λAc does not change, even after 500 switching cycles (a in Figure 5). The emission spectrum (a in Figure 6) of 1, recorded within an excitation pulse of 6 ns at 532 nm (λEx), does not show any significant fluorescence because the coumarin fluorophore does not absorb at λEx (a in Figure 3). However, the photoinduced opening of the adjacent [1,3]oxazine ring can be exploited to shift the band of the fluorophore sufficiently to

Figure 2), followed by counterion exchange, gave the hexafluorophosphate salt of the model compound 3 in a yield of 82%. The steady-state absorption spectrum (a in Figure 3) of 1 shows a band for the coumarin fluorophore at 412 nm. Upon excitation at 355 nm (λAc), the [1,3]oxazine ring opens to generate the zwitterionic isomer 2 (Figure 1) in less than 6 ns with a quantum yield of 0.02, as observed for similar photochromic compounds.34-42 This photoinduced isomerization brings the coumarin appendage in conjugation with a 3H-indolium cation and, as a result, shifts its absorption band further into the visible region. Indeed, a band for the resulting extended π-system appears at 570 nm in the absorption spectrum (b in Figure 3) recorded 0.03 μs after excitation. Consistently, this band resembles that observed in the steadystate absorption spectrum (c in Figure 3) of the hexafluorophosphate salt of the model compound 3. Thus, the photo-


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the lifetime (cf., 0.2 μs) of the photogenerated isomer 2 and, in principle, offers the opportunity to excite a single dyad at λEx tens of times per activation step. Furthermore, the behavior of the model compound 3 suggests that the fluorescent fragment of 2 should tolerate multiple excitation cycles without decomposing. Indeed, the emission intensity of the hexafluorophosphate salt of 3 does not change even after 500 excitation pulses at λEx (b in Figure 5). In summary, we designed and implemented a mechanism for fluorescence modulation under optical control based on a photoinduced bathochromic shift in the absorption of a fluorophore connected to a photochrome. The modular character and synthetic accessibility of our design logic offers the opportunity, at least in principle, to photoregulate the electronic structure of a given chromophore with an appended photochromic auxochrome. Thus, this general strategy can ultimately lead to an entire family of molecular switches with photocontrollable electrochemical and spectroscopic signatures.

Figure 5. Evolution of the absorbance (a) measured at 580 nm 0.03 μs after activation at 355 nm (10 mJ, 6 ns) for a solution (0.01 mM, MeCN, 20 °C) of 1 with the number of excitation pulses. Evolution of the emission intensity (b) measured at 645 nm upon excitation at 574 nm for a solution of the hexafluorophosphate salt of 3 (2.5 μM, MeCN, 20 °C) with the number of excitation pulses.

SUPPORTING INFORMATION AVAILABLE Experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: ssortino@ unict.it (S.S.); [email protected] (F.M.R.).

ACKNOWLEDGMENT We thank the NSF (CAREER Award CHE0237578 and CHE-0749840) and MIUR (PRIN 2008) for financial support.

REFERENCES (1) Figure 6. Emission spectra of solutions (0.01 mM, MeCN, 20 °C) of 1 illuminated at 532 nm (30 mJ, 6 ns) without (a) and with (b) simultaneous irradiation at 355 nm (10 mJ, 6 ns) and of the hexafluorophosphate salt of 3 (c) recorded upon illumination at 532 nm only.

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permit absorption at λEx. Consistently, the simultaneous illumination of the sample at λAc and λEx results in the appearance of an intense emission at 650 nm in the corresponding spectrum (b in Figure 6). Indeed, the beam at λAc opens the [1,3]oxazine ring of 1 with the formation of 2, and that at λEx excites the coumarin fragment of the photogenerated isomer with concomitant fluorescence. In agreement with these observations, the emission spectrum (c in Figure 6) of the hexafluorophosphate salt of 3, recorded under identical experimental conditions but exciting sample only at λEx, shows essentially the same band. This particular model fluorophore has a molar extinction coefficient of 83 mM-1 cm-1 at 573 nm with a fluorescence quantum yield of 0.09, corresponding to a brightness of 8 mM-1 cm-1. In addition, the fluorescence lifetime of the hexafluorophosphate salt of 3 is only 0.3 ns. This value is 3 orders of magnitude shorter than

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Fluorescence Switching with a Photochromic Auxochrome

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