Excited-State Dynamics of Dithienylethenes Functionalized for Self

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A: Spectroscopy, Photochemistry, and Excited States

Excited-State Dynamics of Dithienylethenes Functionalized for Self-Supramolecular Assembly Ismail Hamdi, Guy Buntinx, Olivier Poizat, Aurelie Perrier, Laura Le Bras, Stephanie Delbaere, Sophie Barrau, Maroua Louati, Michinori Takeshita, Keita Tokushige, Marumi Takao, and Stéphane Aloïse J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10767 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry

Excited-state

Dynamics

of

Dithienylethenes

Functionalized for Self-supramolecular Assembly I. Hamdi*, a,e G. Buntinx,a O. Poizat,a A. Perrier, b,c L. Le Bras,b S. Delbaere,a S. Barrau,e M. Louati,a,e M. Takeshita,d K. Tokushige,d M Takao,d and S. Aloïse*a. a

Univ. Lille, CNRS, UMR 8516 - LASIR - Laboratoire de Spectrochimie Infrarouge et Raman, F-59000 Lille, France. b

c

University Paris Diderot, Sorbonne Paris Cité, 75205 Paris Cedex 13, France;

Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris,

d

Saga University, Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, , Honjo 1, Saga 840-8502, Japan. e

Univ. Lille, CNRS, INRA, ENSCL, UMR 8207 - UMET - Unité Matériaux et Transformations, F-59000 Lille, France

E-mail : [email protected]; [email protected];

ACS Paragon Plus Environment

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KEYWORDS. Photochemistry;

diethenylethene

photochromism;

supramolecular

assembly;

ultrafast

spectroscopy.

ABSTRACT. The photoswitching and competitive processes of two photochromic dithienylethenes (DTEs) functionalized at both sides with 2-ureido-4[1H]-pyrimidone (UPy) quadruple hydrogen-bonding recognition patterns have been investigated with NMR experiments, ultrafast spectroscopy and sDFT calculations. The originality of these molecules is their ability to form large supramolecular assemblies induced by light for the closed form (CF) species while the open form (OF) species exist as small oligomers. Photochromic parameters have been determined and photochemical pathways have been rationalized with clear distinction between the antiparallel (OF-AP) and parallel (OF-P) species. A new photocyclization pathway via triplet manifold has been evidenced. The effect of the supramolecular assembly on the photochemical response is discussed. Unlike photoreversion process which is unaffected by the supramolecular assembly, rate constants of the photocyclization reaction and intersystem crossing process are sensitive to the presence of small OF oligomers.

I.

INTRODUCTION

The 2016 Nobel prize in chemistry awarding J.-P. Sauvage, Sir J. Fraser Stoddart and B. L. Feringa1 for their design and production of molecular machines is an echo for the importance of 21st century supramolecular chemistry challenges: combine and control, at the molecular level,


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different stimuli-sensitive entities in order to elaborate large assemblies with tunable macroscopic properties.2,3 More and more often, the “bottom-up” molecular organization (molecular technology) requires the use of weak and reversible non-covalent interactions to obtain defect free structures (in comparison with covalent analogs). As a matter of fact, hydrogen bonds have been extensively exploited as they offer directionality, complementarity and their strength can be adjusted.4 One of the pioneers in this domain is Meijer who implemented 2ureido-4[1H]-pyrimidone (UPy) recognition patterns, able to dimerize in solution with very high association constants (Kdim=5.7 107 M-1 in chloroform and 5.9 108 M-1 in toluene).5,6 UPy has been extensively used to form various assemblies going from discrete architectures7,8 to supramolecular polymers9,10 offering promising structural and mechanical properties.11 Among all possible external stimuli (pH, redox, temperature…),8 light is the most convenient one because (i) it is not chemically destructive and (ii) the remote control of the system can be either finely tuned (through excitation wavelength, intensity…) or triggered (ON-OFF system) via molecular switches. In this context, organic photochromes using light control are nowadays essential, with the bistable (thermally irreversible) photochromic dithienylethenes (DTEs) playing a key role.12,13 Indeed, DTEs are one of the most efficient bistable molecular switches balancing photoconversion efficiency, time response and good fatigue resistance.14-16 Hundreds of applicative examples for the use of DTEs can be found in numerous reviews.12,17 As a representative example, Guiseppone and co-workers,18 have recently reported a novel organogel that can reversibly contract and expand when exposed to light. This system relies on two mechanically-active molecular units, a motor and DTEs based modulator linked by polymer chains.


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However, focusing on photoswitchable supramolecular self-assembling systems,19-21 examples combining hydrogen bond recognition patterns and DTEs are clearly sparse in the literature.22 Recently, Yagai and co-workers23,24 have shown that merocyanine-bismelamine excitonic interactions can be photo-triggered through conformational change of merocyanine substituted DTEs. In the same vain, monocharged bispyridinium-modified DTEs combined with cucurbit[8]uril have been reported to obtain light reversible fluorescent supramolecular system.25 Actually, in 2005, Takeshita and coworkers were the pioneers of this strategy: they elaborated two new DTEs functionalized at both sides with UPy substituents (in blue in scheme 1) bearing distinct lateral alkyl chains R (DTE-UPy1 with R=C11H23 and DTE-UPy2 with R=C17H35). These alkyl chains have been introduced for solubility purpose.26,27 The operating principle of these systems is subtle: the open form (OF) of DTE-UPy molecules is highly flexible due to thermal equilibrium between anti-parallel (AP) and parallel (P) conformers (scheme 1). This OF species is thus not suitable for intermolecular linking through the UPy recognition patterns. After UV irradiation, the solution (in chloroform typically) turns to blue, due to photocyclization process which leads to a more rigid closed form (CF). Thanks to this rigidity, the adequate mutual orientation of UPy groups is possible allowing the formation of quadruple hydrogen bond blocks and thus supramolecular assemblies of 600 nm size for DTE-UPy1 (100 µM in chloroform)26 and 180 nm size for DTE-UPy2 (200 µM in toluene).27 The overall kinetic process takes several hours at ambient temperature. It has been shown that total recovery of the starting material is feasible under visible light irradiation (CF→OP), some additional heating being eventually necessary. Note that for DTE-UPy1, the aggregation process is so efficient that a strong blue precipitate is reported for initial concentrations over 200 µM.


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In this paper, we want to investigate the photochemistry of both DTE-UPy1 and DTE-UPy2 and compare these systems with our recent study28 dedicated to the parent molecule DTE, in green in scheme 1, which is not substituted with UPy groups. For the DTE molecule and a bridged analogue (additional polyether chain between thiophene rings), we have studied the photoswitching processes distinguishing OF-AP, OF-P and CF species with the help of TD-DFT calculations and ultrafast spectroscopy supported by advanced chemometric data processing.28 The main results can be summarized as follow: for the photo-excited OF-P conformer, after ultrafast relaxation (∼200 fs) toward the S1 relaxed state, an intersystem crossing (ISC) occurs (∼55 ps) to produce a triplet state, 3P with a 2.5 µs lifetime. Concerning the OF-AP conformer, the photocyclization reaction is reported to proceed immediately (100 fs) starting from the Franck-Condon (FC) region while the relaxed singlet state is populated at the same time. For the first time, we reported that the latter state evolves through an unexpected ISC process (1 ps) leading to a second triplet state, 3AP. Furthermore, we showed that, by constraining structurally the molecule (DTE with a polyether bridge between the thiophenes), the 3AP triplet state becomes reactive to some extent. Concerning the photoreversion, apart from a two-step process including a ring opening reaction (~6 ps), we did not succeed to prove the existence of new excited state species. In this new contribution, we will focus our attention on DTE-UPy1 and DTE-UPy2 in order to determine if the UPy substitution patterns modify the main photophysical and photochemical processes reported for DTE. Additionally, we will attempt to distinguish the photochemistry arising from the DTE-UPy monomers from the one originating from oligomers or larger assemblies.


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Scheme 1. Photochromic reaction for DTE-UPy between OF-AP and CF (the OF-P conformer is not photoactive). Through UPy-UPy quadruple hydrogen bonding, the supramolecular assembly observed for the CF species is sketched.

II.

EXPERIMENTAL AND THEORETICAL METHODS

Stationary techniques. Both DTE-UPy1 and DTE-UPy2 were synthesized by Takeshita et al.29 and the preparation is detailed in the Supporting Information section (S.I., Document S. 0). Chloroform (CHCl3) solvent (spectroscopic grade, Sigma-Aldrich) was used as received. The stationary absorption spectra were measured with CARY 100bio absorption spectrometer. 1H and 19

F NMR spectra in CDCl3 were recorded on Bruker NMR spectrometer (500 MHz for 1H and

470 MHz for 19F). The photocyclization and photoreversion yields have already been published


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for DTE30 and were determined in this work for DTE-UPy1 and DTE-UPy2 according to the usual procedure.28

Transient absorption techniques. The nanosecond transient absorption spectroscopy setup has been described elsewhere.31 Briefly, a third harmonic pulse (355 nm) of a Nd:YAG laser with ca. 1mJ output energy and 5 ns pulse width was used as excitation light and a pulsed Xe lamp was utilized as probe light. Transient absorption spectra were obtained from the transient absorption decays recorded at various wavelengths by sampling the absorbance changes for different given delay-times. The triplet yields ϕ of DTE-UPy1 and DTE-UPy2 were determined according to the usual procedure.28 Femtosecond time-resolved experiments were performed using a pump-probe spectrometer32,33 based on Ti:sapphire laser system (Coherent Mira-900-D oscillator and Libra-S regenerative amplifier) delivering 800 nm, 1 mJ and 90 fs pulses with a repetition rate of 1 kHz. The pump pulses tuned at 320 nm were generated by frequency quadrupling the output of a Quantronix Palitra OPA pumped at 800 nm and the energy at the sample was about 2 µJ (0.2 mJ cm ). Note that, we choose to excite the molecule far from the maximum of absorption localized near 270 nm to avoid short-time signal coming from 2-photon excitation of the solvent. The probe pulses were obtained by focusing 1 µJ, 800 nm pulses into a 1 mm thick CaF2 plate to generate a white light continuum. The pump-probe polarization configuration was set at the magic angle (54.7°). Transient absorbance was obtained by comparing signal and reference spectra with and without pump pulses for different delay-times. The delay-time between the pump and probe was varied up to 0.5 ns using an optical delay line. Sample solutions (∼10 − 10 mol l, OD = 1 at the pump wavelength) were circulated in a cell equipped with 200 µm thick CaF2 entrance window and having 2 mm optical path length. The instrumental response function (IRF) of our


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setup was determined by fitting the Stimulated Raman transient pic of the solvent (acetonitrile). This method which is an alternative to the 2-photon absorption one, is nowadays widely used34 In this study, the IRF was determined to be 180 fs for a 320 nm excitation wavelength (See Figure S. 5). All the transient spectra presented in this paper are GVD (Group velocity dispersion) corrected according to the typical extrapolation method.35,36 The characteristic times deduced from kinetics are obtained by fitting the data with the result of a multiexponential function convolved with a Gaussian function mimicking the pump-probe cross-correlation function with FWHM at 180 fs. During photoclyclization experiments, the sample solution was circulated and irradiated by a Xe lamp visible light to prevent the accumulation of CF species. For photoreversion experiments, we initially irradiated the solutions with 320 nm Xe lamp UV light to turn it blue (CF).

Theoretical Calculations. All the calculations have been performed using the Gaussian 09 package37 within the density functional theory (DFT) framework. In the course of the geometry optimizations, we used the ωB97XD range-separated hybrid (RSH) functional38,39 combined with the 6-31G (d) atomic basis set. This computational scheme was used in a previous study to investigate the structural properties of the DTE parent molecule.28 The ωB97XD functional was shown to be the most accurate methodology among DFT including dispersion corrections (DFTD) methods to describe the geometrical parameters and interaction energy within hydrogen bonds.40 All the geometry optimizations have been carried out in CHCl3, applying the Polarizable Continuum Model (PCM)41 in the equilibrium limit to quantify the impact of the environment. Vibrational frequencies were computed to ensure that the geometries correspond to true minima of the potential energy surfaces.


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III. RESULTS AND DISCUSSION First of all, due to the respective orientation of the thiophene rings, it is worth reminding that after UV irradiation, only OF-AP conformers can induce C-C electrocyclization while, in the meantime, OF-P conformers can relax their energy only upon photophysical processes. So, to properly assess the photochemical/photophysical properties of the investigated systems, the proportion of OF-AP vs OF-P conformers has to be determined by NMR. At the same time, the proportion

of

dimers

(and

larger

oligomers)

may

also

influence

the

photophysics/photochemistry: the determination of the association constant as well as the monomer/dimer ratio are thus of high interest. As detailed by Li et al., a full determination of dimerization constant can be undertaken for UPy functionalized DTEs using 1H-NMR spectra for different DMSO/Chloroform mixtures.42 This group found an association constant K∼2.105 M-1. For practical reasons (large quantities required) and due to solubility issues (DTE-UPy1 is not soluble in DMSO), we have not yet succeeded to determine such a value for both DTE-UPy1 and DTE-UPy2 (see scheme1 for a representation). However, considering a range of two orders of magnitude with respect to the value determined by Li and co-workers, i.e. 104

Excited-State Dynamics of Dithienylethenes Functionalized for Self

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