Abnormal Enhancement of the Photoisomerization Process in a trans

Article pubs.acs.org/JPCA

Abnormal Enhancement of the Photoisomerization Process in a trans-Nitroalkoxystilbene Dimer Sequestered in β‑Cyclodextrin Cavities Nelly Hobeika,† Jean-Pierre Malval,*,† Hélène Chaumeil,‡ Vincent Roucoules,† Fabrice Morlet-Savary,† Didier Le Nouen,‡ and Fabrice Gritti§,∥ †

Institut de Sciences des Matériaux de Mulhouse, LRC CNRS 7228, Université de Haute Alsace, 15 rue Jean Starcky, 68057 Mulhouse, France ‡ Laboratoire de Chimie Organique et Bioorganique, EA 4566, Université de Haute Alsace, ENSCMu, Institut J.-B Donnet, 3 bis rue Alfred Werner, 68093 Mulhouse, France § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States ∥ Division of Chemical Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φf), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Φt→c) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φf increases together with the decrease of Φt→c. This solventinduced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host−guest complex with β-cyclodextrins (β-CD) prevents the quenching by H2O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the trans-to-cis photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φt→c even though DPNS is sequestered in β-CD cavities.

1. INTRODUCTION

logical echoes, since modified CDs have been used for energy conversion and storage,18,22 molecular switches,23−25 and photonics.12 The photoisomerization along a carbon−carbon double bond represents one of the most studied photoreactions, and stilbene derivatives constitute the typical prototype of molecular systems undergoing this kind of reaction.13−16 Once included in the cavity of CD, it has been shown that the trans-to-cis photoconversion was strongly inhibited.17−20 This effect is assigned to the molecular restriction to rotation in CD capsules. In this paper, we show paradoxically the possibility to enhance this photoreaction even in restricted environment by combining the ability of a transnitroalkoxystilbene dimer (DPNS) to complex to β-CD in aqueous solutions with its photophysical properties which can be driven by solvent polarity. A dimer architecture (Scheme 1)

The ability of cyclodextrins (CDs), truncated cone-shaped oligosaccharides,1−3 to encapsulate organic molecules has attracted considerable attention in recent years, since it provides relevant opportunities for investigating the sizecontrolled nanoenvironment effects on the reactivity of the sequestered molecules.3−5 The confinement can, indeed, affect the chemistry of the reactive guest molecule because it may restrict or promote motions that are crucial for the reaction efficiency.6−9 This cage effect has thus opened a large field of applications such as nanoreactors, nanocapsules, nanodelivering agents, and building blocks.1,4,5 Possessing a hydrophobic central cavity suitable for inclusion of various organic analytes, cyclodextrins have been employed as receptors to understand and address the photophysical and photochemical properties of organic dyes such as fluorescence enhancement,6−8 intramolecular excimer/exciplex formation,9,10 charge and proton transfer,15−17 energy hopping,11 and photoisomerization.19−21 This fundamental approach has also potential nanotechno© 2012 American Chemical Society

Received: May 18, 2012 Revised: September 25, 2012 Published: September 27, 2012 10328

dx.doi.org/10.1021/jp304852b | J. Phys. Chem. A 2012, 116, 10328−10337

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SiMe4. Microanalyses were performed by the Service de Microanalyse du CNRS in Vernaison. High resolution MS were measured with an Agilent Technologies 6510 (Q-TOF) Spectrometer using a dual ESI source. The absorption measurements were carried out with a Perkin-Elmer Lambda 2 spectrometer. Steady-state fluorescence spectra were collected from a FluoroMax-4 spectrofluorometer. Emission spectra are spectrally corrected, and fluorescence quantum yields include the correction due to solvent refractive index and were determined relative to quinine bisulfate in 0.05 molar sulfuric acid (Φ = 0.52).21 The temperature experiments are performed using a cuvette holder provided with a temperature controller (TC 125) from Quantum Northwest. The temperature range is −5 to 95 °C. For the determination of the temperature dependence of Φf, the change in optical density with temperature was taken into account by recording corresponding absorption spectra over the same temperature range. Quantum yields of trans → cis photoisomerization were carried out under irradiation at 377 nm with a laser diode from (Cube type from Coherent). The progress of the reaction was monitored via UV−vis absorption spectra. The 1-(propoxy)-4[(E)-2-(4-nitrophenyl)vinyl]benzene (PNS) dissolved in butylacetate was used as an actinometer.22 The fluorescence lifetimes were measured using a Nano LED emitting at 372 nm as an excitation source with a nano led controller module, Fluorohub from IBH, operating at 1 MHz. The detection was based on an R928P type photomultiplier from Hamamatsu with high sensitivity photon-counting mode. The decays were fitted with the iterative reconvolution method on the basis of the Marquardt/Levenberg algorithm.23 Such a reconvolution technique allows an overall time resolution down to 0.2 ns. The quality of the exponential fits was checked using the reduced χ2 (≤1.2). The “global” analysis of a multiple set of time-resolved data for the decay-associated spectra (DAS) was performed using the GLOBALS 1.0 (Globals Unlimited) analysis program. The decomposition of the decay associated spectra (DASi(λ)) of components which contribute to the fluorescence decay was assessed:

Scheme 1. Molecular Structures of DPNS and PNS

has been preferred to a bar-shaped structure so as to promote a single mode in the dynamics of inclusion into the β-CD. The photophysical feature of this new chromophore will first be depicted in homogeneous solutions and then will be compared to that observed in the microenvironment of β-CD.

2. EXPERIMENTAL SECTION 2.1. Materials. The solvents used for absorption and emission analysis are as follows: ethyl ether (EOE), butyl acetate (BUA), ethyl acetate (ETA), toluene (TOL), tetrahydrofuran (THF), dichloromethane (DCM), acetone (ACT), N,N-dimethylformamide (DMF), propionitrile (PPCN), propylene carbonate (PC), and acetonitrile (ACN). All the solvents employed were Aldrich or Fluka spectroscopic grade. The absorption and fluorescence of all solvents were checked for impurities and have been subtracted from the sample spectra. Reagents for synthesis were purchased from commercial suppliers and used without further purification. The β-cyclodextrin was purchased from Aldrich. 2.2. General Techniques. All melting points were taken on a Kofler bench. IR spectra (cm−1) were recorded on a Nicolet 205 FTIR spectrometer. 1H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were measured on a Bruker Avance series 400 at 295 K. Chemical shifts are reported in ppm relative to

DASi (λ) =

αiτi Fss(λ) ∑j αjτj

where αi and τi are, respectively, the pre-exponential factor and the decay time of each component. Fss(λ) corresponds to the fluorescence intensity at the detection wavelength.

Scheme 2. General Procedure for Synthesis of DPNS

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1-(Propoxy)-4-((E)-2-(4-nitrophenyl)vinyl)benzene (PNS). The synthesis and the characterization of PNS are described in ref 22.

2.3. Synthesis. Starting with 4-allyloxy-4′-nitrostilbene 1 whose synthesis was previously described,24 the preparation of thiol 3 was readily synthesized in two steps in a 66% yield, as shown in Scheme 2. In a first step, thioacetate 2 was obtained in a 66% yield by radical activation (AIBN) with thioacetic acid according to the reported procedure with slight modifications.25 No extractions are required, and only washes with hot solvents were sufficient to purify the crude thioacetate. Compound 2 was then treated under air using Zemplèn conditions26 which yields quasi-quantitatively the final compound. Thio Acetic Acid S-(3-(4-((E)-2-(4-Nitrophenyl)vinyl)-phenoxy)-propyl Ester (2). To a solution of 1-(allyloxy)-4-[(E)-2(4-nitrophenyl)vinyl]benzene (600 mg, 2.12 mmol) in a MeOH/dioxane solution (45 mL, 1/1) were added, under Ar, freshly distilled thioacetic acid (910 μL, 12.8 mmol) and AIBN (34 mg, 0.2 mmol). The resulting solution was heated at reflux for 15 h. The solution was then condensed under reduced pressure. The solid obtained was suspended in hot isopropyl ether (10 mL) and centrifuged. The supernatant organic phase was discarded. This procedure was repeated twice with isopropyl ether and twice with cyclohexane to yield 2 as yellow crystals (504 mg, 66%). Mp: 169 °C. IR (KBr): 628; 843; 1037; 1109; 1175; 1224; 1251; 1272; 1384; 1504; 1511; 1588; 1679; 2871; 2924. 1H NMR (400 MHz, CDCl3): δ: 2.09 (quint, J = 7.2 Hz, 2H), 2.35 (s, 3H), 3.07 (t, J = 7.2 Hz, 2H), 4.05 (t, J = 7 Hz, 2H), 6.91 (d, J = 8 Hz, 2H, H2, H6), 7.00 (d, J = 16.5 Hz, 1H, Hvinyl), 7.22 (d, J = 16.5 Hz, 1H, Hvinyl), 7.48 (d, J = 8 Hz, 2H, H3, H5), 7.60 (d, J = 8 Hz, 2H, H3′, H5′), 8.20 (d, J = 8 Hz, 2H, H2′, H6′). 13C NMR (100.6 MHz, CDCl3): δ: 25.8; 29.3; 30.6; 66.3; 114.8 (2C); 124.1 (2C); 126.5 (2C); 128.4 (2C); 129.1; 132.9 (2C); 144.2; 146.4; 159.4; 195.7. Anal. Calcd for C19H19NO4S (357.10): C, 63.85; H, 5.36; N, 3.92. Found: C, 63.39; H, 5.29; N, 3.95. HRMS (ESI+-Q-Tof) m/z calcd for C19H20NO4S [M + H]+ 358.1108, found 358.1120. (3-(4-((E)-2-(4-Nitrophenyl)vinyl)phenoxy)propyldisulfanyl)-propoxy)-(4-((E)-2-(4-nitrophenyl)vinyl))benzene (3). To a solution of 2 (880 mg, 2.45 mmol) in dioxane (19.5 mL) was added dropwise at room temperature a solution of sodium methoxide in methanol (0.3 M) until pH 8.5. The mixture was stirred for 2 h. H+ resin (Amberlyst 15 H+) was then added to neutralize excess NaCH3O (pH 5), and the mixture was filtered. The solution was then condensed under reduced pressure. The solid obtained was suspended in isopropanol (10 mL) at reflux. The mixture was allowed to reach room temperature and was centrifuged. The supernatant organic phase was discarded. This procedure was repeated twice with isopropanol and twice with cyclohexane to yield 3 as yellow crystals (710 mg, 97%). Mp: 143 °C. IR (KBr): 840; 1109; 1174; 1194; 1219; 1250; 1304; 1339; 1510; 1573; 1588; 1607; 2924; 3407. 1H NMR (400 MHz, CDCl3): δ: 2.20 (quint, J = 6.5 Hz, 2H), 2.90 (t, J = 6.5 Hz, 2H), 4.10 (t, J = 6.5 Hz, 2H), 6.91 (d, J = 8.4 Hz, 2H, H2, H6), 6.99 (d, J = 16.5 Hz, 1H, Hvinyl), 7.21 (d, J = 16.5 Hz, 1H, Hvinyl), 7.47 (d, J = 8.4 Hz, 2H, H3, H5), 7.58 (d, J = 8.8 Hz, 2H, H3′, H5′), 8.19 (d, J = 8.8 Hz, 2H, H2′, H6′). 13C NMR (100.6 MHz, CDCl3): δ: 29.1; 35.4; 66.3; 115.3 (2C); 124.6 (2C); 126.9 (2C); 128.8 (2C); 129.5; 132.2 (2C); 144.6; 146.5; 159.8. Anal. Calcd for C34H3N2O6S2 (628.76): C, 64.95; H, 5.13; N, 4.46. Found: C, 64.74; H, 5.27; N, 4.35. HRMS (ESI+-Q-Tof) m/z calcd for C34H36N3O6S2 [M + NH4]+ 646.2040, found 646.3034.

3. RESULTS AND DISCUSSION Photophysical Properties of DPNS. Figure 1 shows the absorption spectra of PNS and DPNS in dichloromethane.

Figure 1. Absorption spectra of PNS and DPNS in dichloromethane.

Both chromophores clearly exhibit similar spectra; the lowenergy region of the spectra is dominated by an intensive band centered at 379 nm for PNS and 377 nm for DPNS with an εMAX value of ca. 29 300 and 58 100 M−1 cm−1. The twice higher intensity observed for the band of DPNS with respect to that of PNS confirms the absence of electronic interactions between the two molecular branches of the chromophore at ground state. Table 1 gathers spectroscopic data relative to DPNS in various solvents, and Figure 2 shows room temperature absorption and fluorescence spectra of the chromophore, respectively, in solvents of different polarity. Upon increasing solvent polarity, both absorption and fluorescence bands are red-shifted. For instance, from diethyl ether (EOE) to acetonitrile (ACN), the Stokes shift increases from a value of ca. 6650 to 10 500 cm−1. This bathochromic effect which is clearly much stronger for the emission band indicates a significant electronic and geometrical change between ground and excited states in polar solvent. If we only take into account the dipole−dipole interaction for the contribution of the solvent to the energy of the excited states, the change in dipole moment between the ground and excited states, Δμge, can be evaluated from the solvation effects on the Stokes shift27,28 according to the following relation: hc(vabs − vfluo) =

2 1 2Δμge Δf + const. 4πε0 a3

(1)

where h is the Planck constant and c is the speed of light. The Onsager radius a defined as the solvent shell around the molecule was estimated to a value of ca. 7 Å. This value corresponds to 40% of the longest axis of the 4-methoxy-4′nitrostilbene moiety, as suggested by Lippert28 for nonspherical molecules. Δf is the solvent polarity parameter defined by Δf = ((ε − 1)/(2ε + 1)) − ((n2 − 1)/(2n2 + 1)), where ε is the relative permittivity and n the refractive index of the solvent. In eq 1, we also assume that the dipole moment of the Franck− 10330

dx.doi.org/10.1021/jp304852b | J. Phys. Chem. A 2012, 116, 10328−10337

The Journal of Physical Chemistry A

Article

Table 1. Spectroscopic Data of DPNS at Room Temperature in Various Solvents

a

n°

solvent

λabsmax (nm)

λfluomax (nm)

ΔνStokes (cm−1)

Φf

τf (ns)

1 2 3 4 5 6 7 8 9 10 11

EOE TOL BUA ETA THF DCM ACT DMF PPCN PC ACN

365 373 372 368 373 377 374 382 374 376 374

482 491 514 523 525 586 577 603 595 614 615

6650 6443 7426 8053 7762 9460 9407 9594 9931 10309 10478

0.0008 0.0008 0.0113 0.0580 0.0938 0.1360 0.1360 0.1390 0.1000 0.0444 0.0488