Photochromic Metal Complexes: Photoregulation of both theNonlinear Optical and Luminescent PropertiesLucie Ordronneau,† Hiroyuki Nitadori,‡ Isabelle Ledoux,§ Anu Singh,§ J. A. Gareth Williams,∥
Munetaka Akita,‡ Veronique Guerchais,*,† and Hubert Le Bozec*,†
†UMR CNRS 6226-Universite de Rennes 1, Sciences Chimiques de Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France,‡Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan§Laboratoire de Photonique Quantique et Moleculaire, UMR CNRS 8531, Institut d’Alembert, ENS Cachan, 61 avenue du PresidentWilson, 94235 Cachan, France∥Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom
ABSTRACT: A series of dithienylethene (DTE)-containing2,2′-bipyridine ligands and their zinc(II) diacetate, zinc(II)dichloro, rhenium(I) tricarbonyl bromo, and ruthenium(II)bis(bipyridine) complexes have been designed and synthe-sized, and their photochromic, photophysical, and quadraticnonlinear optical properties have been studied. Upon UVirradiation at 350 nm, the ligands and complexes undergo ringclosure of the DTE units, with a good to excellentphotocyclization yield. In the case of the Re(I) and Ru(II)complexes, the photocyclization of the DTE units can also betriggered using visible light, upon excitation into the metal-to-ligand charge-transfer (MLCT) bands at 400 and 490 nm,respectively. Molecular quadratic nonlinear optical (NLO) responses of the complexes have been determined by using either theelectrical field induced second harmonic generation (EFISH) or harmonic light scattering (HLS) technique at 1910 nm. Thesestudies reveal a large increase of the second-order NLO activity after UV irradiation and subsequent formation of the ring-closedisomers. This efficient enhancement clearly reflects the delocalization of the π-electron system and the formation of strong push−pull chromophores in the closed forms. The combination of the photochromic DTE-based bipyridine ligand with luminescentRe(I) and Ru(II) fragments also allows the photoregulation of the emission, leading to an efficient quenching of the ligand-based77 K luminescence and demonstrating that the photocontrol of two optical properties, linear and nonlinear, could be achieved byusing the same photochromic ligand.
■ INTRODUCTIONThe ability to switch on and off the NLO activity of a moleculeis of relevance to the development of molecular photonicdevices, where switching can be achieved by modifying one ofthe component parts of the molecule.1,2 Since π-conjugatedconnections between donor and acceptor end groups are acentral requirement for obtaining large quadratic nonlinearities,one strategy to achieve an efficient switching effect concerns thealteration of the π bridge in response to an external trigger suchas light. In this category, photochromic compounds arepromising candidates for the design of photoswitchable NLOmaterials.3−6 Diarylethene (DTE) derivatives are widely used asthe photochromic units, and changes in the π-conjugated chainof DTE derivatives can be successfully used to control donor−acceptor interactions.7 Typically, the DTE unit undergoesreversible interconversion between a nonconjugated open formand a π-conjugated closed form when irradiated in the UV andvisible spectral ranges, respectively. Recently, the combinationof transition metals and ligands featuring diarylethene units hasreceived much attention and opened up new perspectives for
the design of metal-based photoswitchable molecules.8 Forexample, versatile diarylethene-containing polyimine ligandsand their transition-metal complexes9−15 have been synthesizedand their photochromic behavior has been widely exploited forthe photomodulation of luminescence and electronic proper-ties.We have been involved for several years in the use of 4,4′-
disubstituted-2,2′-bipyridines as precursors to dipolar andoctupolar metal complexes for nonlinear optics.16,17 In orderto carry out the photoswitching of the NLO properties, werecently prepared a new type of 4,4′-bis(ethenyl)-2,2′-bipyridinel i gand func t iona l i z ed by a 4 -(d imethy l amino) -phenyldithienylethene (DTE) group and the correspondingzinc bis(acetate) complex (Scheme 1).18 The excellentphotochromic property of this complex was exploited to designthe first example of metal-containing photochromic ligandsallowing an efficient switching of the quadratic NLO properties.
Received: November 24, 2011Published: April 30, 2012
In contrast, by complexation of the same photochromicbipyridine ligand to the luminescent bis-cyclometalated Ir(III)center, an efficient photoregulation of the emission was found,whereas no significant modification of the quadratic NLOresponse could be observed accompanying photocyclization,19
due to the weaker contribution of the ILCT vs MLCT
transitions to the quadratic hyperpolarizability. These observa-tions prompted us to extend our study to another series ofchromophores combining DTE-based bipyridines La,b (a,NMe2; b, NBu2) with different metallic fragments, namelythe stronger Lewis acid ZnCl2 as well as the Ru(bipy)2
2+ andRe(CO)3Br moieties, known for their luminescent properties.
Scheme 1
Scheme 2. Stepwise Synthesis of Photochromic Bipyridyl Ligands
In particular, it was appealing to study the role of the metalcenter on the photochromic and NLO activity and, in addition,to investigate the photoregulation of the emission properties ofthe resulting system, as the use of a single photochromic metalcomplex for photocontrol of both NLO and luminescenceproperties, which could be of interest for the elaboration ofmultifunctional molecular materials and hence for optical datastorage, has not been reported yet.15 We describe herein thesynthesis, photochromic, and luminescence properties of thesecomplexes, together with the quadratic hyperpolarizability ofthe open and photocyclized forms, measured by the EFISH andHLS techniques.
■ RESULTS AND DISCUSSION
Synthesis and Characterization of Ligands andComplexes. The synthesis of the target bipyridines La,b(o)was performed according to Scheme 2. The aryl-substitutedDTE aldehydes 7a,b were first obtained by a multistepprocedure: the two thienyl fragments were preparedindependently and then successively connected to theperfluorocyclopentene ring. The 3-bromo-2-methyl-5-arylthiophene compounds 3a,b were prepared in 75−98% yieldby the Suzuki coupling of 1 with the commercially availablearylboronic acids 2a,b. The second thienyl derivative 4,3
bearing a protecting dimethylacetal group, was reacted withoctafluorocyclopentene (C5F8) using a commonly usedprocedure to afford 5 in 70% yield. Due to partial deprotectionof the carbonyl group during the chromatographic workup, thecharacterization of this synthon was made on the correspond-ing fully deprotected compound 6. Reaction of 3a,b with 5 inthe presence of n-butyllithium afforded, after subsequenttreatment with p-toluenesulfonic acid (PTSA) in wettetrahydrofuran, the desired aldehydes 7a,b in 50−55% yield.
Finally, treatment of diethylphosphonatomethyl-2,2′-bipyri-dine17d with aldehydes 7a,b under normal Wadsworth-Emmonsconditions afforded the target bipyridine ligands La,b(o) in 55−60% yield after purification by recrystallization.La,b(o) were characterized by 1H and 13C NMR, UV−visible
spectroscopy, and high-resolution mass spectrometry and gavesatisfactory elemental analysis. The 1H NMR spectra displayclassical chemical shifts for the vinyl-bipyridine protons with a3JH−H vinylic coupling constant of ca. 16 Hz typical for an Econfiguration about the double bond. The DTE moietiesexhibit two very close resonance signals for the methyl protonsat ca. 2 ppm and two singlets for the methine protons of thethiophene rings, as expected for the asymmetrical nature of thisfragment. Ligands La,b(o) show good transparencies in thevisible region, whatever the nature of the R group: the UV−visible spectra in dichloromethane show an intense absorptionat 340−350 nm which is tentatively assigned to (IL) π → π*transitions of the bipyridyl moieties with some mixing of theDTE units.The corresponding Zn(II) complexes (La(o))Zn(OAc)2 and
(La,b(o))ZnCl2 (Figure 1) were readily obtained after room-temperature treatment of La,b(o) with 1 equiv of zinc diacetatedihydrate or zinc dichloride in dichloromethane. Reaction ofLa(o) with Re(CO)5Br in refluxing anhydrous toluene affordedfac-(La(o))Re(CO)3Br as brown crystals in 82% yield. Theruthenium complex [(La(o))(dmbipy)2Ru](PF6)2 (dmbipy =4,4′-dimethyl-2,2′-bipyridine) was obtained in 60% yield fromthe corresponding cis-(dmbipy)2RuCl2 upon treatment firstwith 2 equiv of silver triflate in methanol and then reaction withligand La(o) in dichloromethane at room temperature, followedby anion exchange from triflate with hexafluorophosphate. Thecomplexes (La(o))Zn(OAc)2, (Lb(o))ZnCl2, (La(o))Re-(CO)3Br, and [(La(o))(dmbipy)2Ru](PF6)2 are soluble in
Figure 1. Chemical structures of photochromic bipyridine Zn(II), Re(I), and Ru(II) complexes.
chlorinated solvents (CH2Cl2, CHCl3) and were fullycharacterized by means of 1H and 13C NMR and UV−visiblespectroscopy and gave satisfactory microanalyses (see theExperimental Section). Conversely, the very low solubility of(La(o))ZnCl2, bearing the dimethylamino end group, pre-vented its complete spectroscopic characterization or furtherphotochromic and NLO studies. In all cases, that the Econfiguration of the double bond is retained upon complex-ation of the ligand is clearly established on the basis of the largeolefinic protons coupling constant (3JH−H ≈ 16 Hz), showingthat there is no isomerization of the CC linker.The UV−visible spectra of the zinc complexes (in CH2Cl2)
all show an intense band around 360 nm similar to that of theligands, which is slightly red-shifted by complexation (Table 1).
The electronic absorption spectra of both the rhenium andruthenium complexes also show intense IL absorptions near340 and 380 nm (shoulders). In addition, the rutheniumcomplex displays another broad band in the visible at 490 nmcorresponding to MLCT dπ(Ru) → π*(bipy) transitions,
whereas the rhenium complex shows a moderately intenseabsorption shoulder at ca. 410 nm which can be assigned as thedπ(Re) → π*(bipy) transition.
Photochromic Properties of Ligands and Complexes.For each ligand and complex, the photocyclization process wasmonitored by 1H NMR and UV−vis spectroscopy indichloromethane. We have already described the photochromicbehavior of ligand La and its zinc diacetato complex,18,20 whichundergo ring closure of both DTE units. Similarly, uponirradiation at 350 nm, the yellow solutions of Lb(o) and(Lb(o))ZnCl2 turn green, with the emergence of a new broadbands at 679 and 714 nm, respectively, attributed to theintraligand (IL) S0 → S1 transition of the closed DTE form.Their 1H NMR spectra show the characteristic downfield shiftof the methyl signals by ca. 0.2 ppm and an upfield shift by ca.0.5−0.6 ppm of the two thiophene protons. According to 1HNMR analysis, integration of the methyl groups indicates aphotocyclization yield of 95% and 85% for Lb(o) and(Lb(o))ZnCl2, respectively, showing that complexation to theZn(II) ion does not perturb the photochromic properties of Lb.We only notice a much slower conversion rate for the zinccomplex as compared to the free ligand (the irradiation time,under our experimental conditions, to achieve the PSS is 600 sfor La, 1500 s for (La)Zn(OAc)2, 1500 s for Lb, and 2500 s for(Lb)ZnCl2
20). As shown in Table 1, the absorption maxima ofthe closed-ring forms are dependent on the nature of the donorgroups and metallic fragments. Replacement of NMe2 with themore strongly donating NBu2 end group induces a small redshift (Δλ = 10 nm) of the IL band, and complexation to ZnCl2induces a much larger bathochromic shift (Δλ= 35 nm) thanfor Zn(OAc)2, in agreement with the higher Lewis acidity ofZnCl2 vs Zn(OAc)2.Photocyclizations of (La(o))Re(CO)3Br and [(La(o))-
(dmbipy)2Ru](PF6)2 are also accomplished by irradiationwith UV light at 350 nm, giving rise to lower-energyabsorptions at 705 and 716 nm, respectively, correspondingto the ring-closed isomers (Figure 2). Thus, a substantialbathochromic shift of the IL band (Δλ = 36−47 nm) is alsoobserved upon complexation of La to the Re(I) and Ru(II)organometallic fragments. The ratio of the methyl signalsbetween the closed and open DTE units (ring closing) in thephotostationary states (PSS) is lower (60−65%) than thosefound for the zinc(II) complexes according to 1H NMR.21 It is
Table 1. Electronic Absorption Data for Ligands andComplexes in the Open and Closed (PSS) forms, EmissionData in the Open Form, and Percentage Conversion atPhotostationary State
compdλabs/nm (ε/M−1
cm−1)a open
λabs/nma
closed(PSS)
λem/nm(τ/μs)b
open
ringclosing/
%c
Lad 348 (89 000) 347, 395,438, 669
95
La 346 (76 300) 339, 400,441, 679
95
(La)Zn(OAc)2d 360 (76 000) 343, 394,
450, 68790
(Lb)ZnCl2 351 (53 000) 342, 390(sh), 463,714
85
(La)Re(CO)3Br 340 (110 000),380 (sh), 410(sh)
336, 387,443, 705
652, 725(22)
65
[(La)(dmbipy)2Ru](PF6)2
338 (78 000), 377(sh), 491 (27000)
336, 387,443 (sh),716
680, 753(13)
62
aAt 298 K in CH2Cl2.bAt 77 K in diethyl ether/isopentane/ethanol
(2/2/1, v/v). cDetermined by 1H NMR spectroscopy. dFrom ref 18.
Figure 2. UV−vis absorption spectra changes of (a) [(La)(dmbipy)2Ru](PF6)2 and (b) (La)Re(CO)3Br in dichloromethane upon excitation at 350nm.
also interesting to note that the photochromic reaction can betriggered by irradiation of the rhenium and rutheniumcomplexes into the low-energy MLCT bands at 400 and 490nm, respectively. This MLCT photosensitization has beenpreviously established for a cyclometalated iridium(III)complex containing a similar DTE-based bipyridine ligandand suggests the intermediacy of the triplet state 3IL(DTE) inthe photocyclization process.10a Finally, for all ligands andcomplexes, excitation at 650 nm, in the absorption band of theclosed forms, leads to the quantitative regeneration of the openisomers, indicating the fully reversible closed-to-open photo-isomerization.Nonlinear Optical Properties. The second-order NLO
responses of the new, neutral, dipolar zinc and rheniumcomplexes were determined in CH2Cl2 (concentration 10−3 M)by the EFISH technique, working with an incident wavelengthof 1.91 μm. The EFISH μβ1.91 values, before and after UVirradiation, are reported in Table 2, along with that previously
reported18 for (La)Zn(OAc)2. The static μβ0 values extrapo-lated at zero frequency were calculated by using the two-levelmodel.22 As evidenced in Table 2, the EFISH μβ1.91 values of(Lb(o))ZnCl2 and (La(o))Re(CO)3Br are very weak and quitesimilar to that of (La(o))Zn(OAc)2, as expected by the absenceof conjugation between the amino donor group and thebipyridyl metal acceptor moieties. A dramatic increase of thesecond-order NLO activity is observed after UV irradiation andsubsequent accumulation of the ring-closed isomers: in the caseof (Lb)ZnCl2 the second-order NLO response is increasedalmost 14-fold after only 35% of ring closing, while a 21-foldenhancement is observed after 65% of ring closing whichconsists mainly of a mixture of open−closed and closed−closedisomers in a ca. 2:1 ratio.23 Furthermore, it is also interesting tonote that this latter EFISH μβ0 value is even higher than thatpreviously reported for (La)Zn(OAc)2 in the PSS (90% of ringclosing), as expected from the higher Lewis acidity of ZnCl2 vsZn(OAc)2. A similar, but somewhat less dramatic, behavior isobserved for the rhenium(I) complex, with an increase of μβ0from 187 × 10−48 to 1148 × 10−48 esu in the PSS (65% of ringclosing). The lower efficiency of the Re(I) complex incomparison with that of the corresponding Zn(II) complexeshas already been shown in dipolar donor-substituted styrylbipyridine metal chromophores24 and could be explained by thepresence of two vectorially opposed charge-transfer transitions(MLCT and ILCT), which contribute to a lowering of the totalNLO response.The large enhancement of μβ0 after ring closure clearly
reflects the delocalization of the π-electron system in the closed
forms, and as expected, the largest NLO enhancement isobserved with the complex featuring the harder ZnCl2 Lewisacid associated with the better NBu2 donor group, in agreementwith the higher red shift of the intraligand (IL) transition whichdominates the second-order NLO response. This efficient on/off switching markedly contrasts with the absence of switchingof the NLO response observed for the cyclometalatedphenylpyridine Ir(III) complex featuring the same photo-chromic bipyridyl ligand La (Scheme 1), in which the EFISHresponse was found to be mainly controlled by a MLCT/L′LCT process from the cyclometalated phenylpyridine Ir(III)moiety to the π* orbitals of the bipyridine, and not by anintraligand transition.19
The harmonic light scattering (HLS) technique was alsoused for the molecular first-hyperpolarizability β measurementsof the dicationic ruthenium complex and for comparison of theneutral Re complex. Unlike EFISH, HLS can be used also forionic molecular species and for nondipolar molecules such asoctupolar molecules. The measurements were performed indichloromethane at a fundamental wavelength of 1.91 μm, andthe values of ⟨β1.91⟩ and static hyperpolarizabilities ⟨β0⟩
25 aregiven in Table 3. Much attention has been given to the large β
responses of bipyridine ruthenium chromophores, such asoctupolar D3 tris-chelate ruthenium complexes, which aredominated by low-lying ILCT or MLCT excitations, dependingon the electron-donating or -accepting nature of thesubstituents on the bipyridyl ligands.17,26 It turned out that[(La)(dmbipy)2Ru](PF6)2 in its open form also shows a fairlylarge ⟨β⟩ value, larger than that of the rhenium complex, whichcan be reasonably attributed to low-energy MLCT dπ(Ru) →π*(bipy) transitions. Upon photocyclization of the DTE, ⟨β0⟩is found to increase by a factor of ca. 2, a result which isconsistent with an NLO response mainly controlled by the red-shifted IL transition.
Luminescent Properties. None of the ligands andcomplexes display detectable luminescence in solution atroom temperature when excited in the UV or visible bands.This behavior contrasts with that of the DTE-free counterparts:the free and Zn complexed 4-(dimethylamino)styrylbipyridineligands are emissive at room temperature in fluid solutions.17d
It is likely that the luminescence is inhibited by the competitivephotocyclization pathway of the DTE unit. The rate constantfor cyclization at room temperature will greatly exceed therather slow radiative rate constant of the triplet state (typicallyaround 105 s−1 for Ru and Re complexes). In frozen glassesupon UV irradiation, we do not observe by UV−visspectroscopy any formation of closed-DTE species. As thephotocyclization is inhibited under these experimental con-ditions, this allows the phosphorescence process to competeeffectively. At 77 K (EPA), only the rhenium and ruthenium
Table 2. EFISH μβ Values Determined at λinc 1910 nm ofZn(II) and Re(I) Complexes in CH2Cl2
EFISHa open EFISHa after UV irr
compd μβ1.91a,b μβ0
a,b μβ1.91a,c μβ0
a,c
ringclosing/%d
(La)Zn(OAc)2e 200 160 4220 1800 90
(Lb)ZnCl2 113 105 3850 1460 355650 2143 65
(La)Re(CO)3Br 240 187f 2920 1148 65aIn units of 10−48 esu. bError ±20%. cError ±5%. dDetermined by 1HNMR spectroscopy. eFrom ref 18. fcalculated by using the two-levelmodel and λMLCT value
Table 3. βHLS Values of Re(I) and Ru(II) in CH2Cl2
aIn units of 10−30 esu. bError ±15%. cCalculated by using the two-level model and λMLCT values. dCalculated by using the two-levelmodel and λIL(DTE) values.
complexes emit, displaying structured luminescence spectrawith vibronic progressions of 1400−1500 cm−1, typical ofaromatic and/or CC bond vibrations. The emissioncharacteristics are summarized in Table 1, and the emissionspectra of (La(o))Re(CO)3Br and [(La(o))(dmbipy)2Ru]-(PF6)2 and their PSS are shown in Figure 3. The long lifetimes(22 and 13 μs for the Re and Ru complexes, respectively), lowenergy, and structured spectra are indicative of emission from atriplet state predominantly localized on the bpy−CC−Armoiety, rather than from the MLCT state.Following conversion of solutions of (La(o))Re(CO)3Br and
[(La(o))(dmbipy)2Ru](PF6)2 to the PSS at room temperatureand refreezing to 77 K, a substantial reduction of the 77 Kluminescence is observed (Figure 3). The quenching can beattributed to intramolecular energy transfer from the tripletemissive state to the IL state of the closed-ring DTE part of themolecule: there is extensive overlap of the emission bands ofthe Re and Ru chromophores (λmax
em 652 and 680 nm,respectively) with the low-energy absorption band of theacceptor photochromic unit in its closed form (λmax
abs 725 and753 nm). A similar quenching of the luminescence upon DTEclosure was observed in the previously mentioned cyclo-metalated iridium complex featuring the same photochromicligand.19
■ CONCLUSION
We have presented in this work a full investigation of thesynthesis, characterization, and linear and nonlinear opticalproperties of a series of Zn(II), Re(I), and Ru(II) complexeswhich incorporate the DTE photochromic unit into thebipyridine ligand. We have demonstrated that, like the freeDTE-containing bipyridyl ligands, the corresponding com-plexes undergo reversible ring-closure reactions under lightirradiation, with photocyclization yields ranging from 62 to90%. The combination of the photochromic DTE-basedbipyridine ligand L with luminescent Re and Ru organometallicfragments allows the photoregulation of the emission and NLOproperties of the resulting system. This study demonstrates forthe first time that the photocontrol of two optical properties,linear and nonlinear, could be achieved by using the samephotochromic ligand.
■ EXPERIMENTAL SECTIONGeneral Information. All manipulations were performed using
Schlenk techniques under an Ar atmosphere. All solvents were driedand purified by standard procedures. Spectroscopic grade dichloro-methane was used for all optical measurements. Compound 4 wasprepared according to a reported procedure, namely by bromination of5-methyl-thiophene-2-carboxaldehyde, and was subsequently con-verted into its dimethyl acetal 4-(OMe)2 for further use.3 NMRspectra were recorded on Bruker DPX 200, AV 300, and AV 500 MHzspectrometers. 1H and 13C chemical shifts are given versus SiMe4 andwere determined by reference to residual 1H and 13C solvent signals.Attribution of carbon atoms was based on HMBC, HMQC, andCOSY experiments. High-resolution mass spectra (HRMS) wereperformed on a MS/MS ZABSpec TOF at the CRMPO (Centre deMesures Physiques de l’Ouest) in Rennes. Elemental analyses wereperformed at the CRMPO.
Optical Spectroscopy. UV−vis irradiations were performed eitherwith a Rayonet RPR 100 photochemical reactor equipped with 16RPR 3500 Å lamps or with a LS series Light Source of ABETTechnologies, Inc. (150 W xenon lamp), with “350FS 10-25”, “450FS20-25”, and “650FS 10-25” single-wavelength light filters. UV/visabsorption spectra were recorded using a UVIKON 9413 or BiotekInstruments XS spectrophotometer using quartz cuvettes of 1 cm pathlength (spectroscopic grade dichloromethane was used for all opticalmeasurements). Steady-state luminescence spectra were measuredusing a Jobin Yvon FluoroMax-2 spectrofluorimeter, fitted with a red-sensitive Hamamatsu R928 photomultiplier tube. The spectra shownare corrected for the wavelength dependence of the detector, and thequoted emission maxima refer to the values after correction. Lifetimeswere obtained by multichannel scaling following excitation with amicrosecond-pulsed xenon lamp and detection of the light emitted atright angles using an R928 photomultiplier tube, after passage througha monochromator.
Second-Order NLO Properties. EFISH Measurements. Themolecular quadratic hyperpolarizabilities were measured by thesolution-phase dc electric field induced second harmonic (EFISH)generation method, which can provide direct information on the
intrinsic molecular NLO properties through eq 1, 22where μβλ/5kT isthe dipolar orientational contribution and γ(−2ω;ω,ω,0), a third-orderterm corresponding to the mixing of two optical fields at ω and of theDC poling field at ω = 0, is the electronic cubic contribution to γEFISH,which is usually negligible with respect to μβλ/5kT. βλ is theprojection along the dipole moment axis of the vectorial component ofthe tensor of the quadratic hyperpolarizability, working with an
Figure 3. (left) Emission spectrum of [(La(o))(dmbipy)2Ru](PF6)2 at 77 K in diethyl ether/isopentane/ethanol (2/2/1 v/v) (blue line) uponexcitation into the lowest energy absorption band (490 nm) and the corresponding spectrum recorded for the photostationary state (red line).(right) Corresponding spectra of (La(o))Re(CO)3Br and its PSS.
incident wavelength λ. All EFISH measurements were carried out inCH2Cl2 solutions at a concentration of 1 × 10−3 M, working with anonresonant incident wavelength of 1.907 μm.HLS Measurements. The HLS technique27,28 involves the detection
of the incoherently scattered second harmonic light generated by asolution of the molecule under irradiation with a laser of wavelength λ,leading to the measurement of the mean value of the β × β tensorproduct, ⟨βHLS⟩. All HLS measurements were carried out in CH2Cl2solution at a concentration of 1 × 10−3 M, working with a low-energy,nonresonant incident radiation of 1.91 μm. The 1.91 μm fundamentalbeam was emitted by a high-pressure (30 bar), 50 cm long Raman cellpumped by a Nd3+:YAG laser operating at 1.06 μm and providing a 10Hz repetition rate, with pulses of 15 ns duration. Only thebackscattered 1.91 μm Raman emission was collected by use of adichroic mirror, in order to eliminate most of the residual 1.06 μmpump photons. Our reference sample was a concentrated (10−3 M)solution of ethyl violet, its β value being calibrated at 1.91 μm withrespect to that of the N-4-nitrophenylprolinol (NPP) reference dipolarmolecule, leading to β = 170 × 10−30 esu for ethyl violet at 1.91 μm.The HLS photons at 955 nm were focused onto the photomultipliertube using two collecting lenses (we used a Hamamatsu R632-01photomultiplier tube). The signal detected was then sampled andaveraged using a boxcar and processed by a computer. The referencebeam was collected at a 45° incidence angle by a glass plate andfocused onto a highly nonlinear NPP powder, which was used as thefrequency doubler. The variation of the scattered second harmonicintensity from the solution was recorded on the computer as afunction of the reference second harmonic signal provided by the NPPpowder, which scales as the square of the incoming fundamentalintensity. Values for β were then inferred from the slopes of theresulting lines. The dispersion-free hyperpolarizabilities β0 are inferredfrom experimental ones using a two-level dispersion model accordingto ref 22.Synthesis. 3-Bromo-2-methyl-5-(p-dimethylaminophenyl)-
thiophene (3a). In a Schlenk flask, the boronic acid 2a (1.66 g, 10mmol) and 3,5-dibromo-2-methylthiophene 1 (3.86 g, 15 mmol) wereadded to a solution of Na2CO3 (8 g, 75 mmol) and Pd(PPh3)4 (0.58 g,0.5 mmol) in H2O/THF (40 mL/40 mL). The reaction mixture wasrefluxed overnight. The resultant mixture was extracted into 2 × 25mL of chloroform. The organic layer was first washed with an aqueoussolution of NaCl and then with water (25 mL) and finally dried overmagnesium sulfate. After evaporation of the solvent, the crude productwas purified by column chromatography on silica gel (pentane/dichloromethane, 75/25) to give 3a as a white powder (2.2 g, 75%).1H NMR (200 MHz, CDCl3): δ (ppm) 7.42 (d, 3J = 8.7 Hz, 2H,C6H4), 7.15 (s, 1H, thio), 6.70 (d,
Following the procedure for 3a, the title compound 3b was obtainedas a yellow powder (2.4 g, 80%) from the boronic acid 2b (2 g, 8mmol), 3,5-dibromo-2-methylthiophene (1; 2.7 g, 10.5 mmol), andPd(PPh3)4 (0.36 g, 0.3 mmol).
hyde (6). nBuLi (2.4 M in hexane, 15.6 mmol, 6.5 mL) was addeddropwise to a stirred solution of 4 (3.00 g, 12 mmol) in 100 mL ofTHF at −78 °C under a nitrogen atmosphere. After 60 min, thereaction mixture was transferred into a Schlenk tube containing asolution of perfluorocyclopentene (4.8 mL, 36 mmol) in 20 mL ofTHF. The reaction mixture was stirred for 1 h at −78 °C and then
warmed to room temperature and stirred for an additional 16 h. Afteraddition of 200 mL of water, THF was removed under reducedpressure, and the residue was extracted with CH2Cl2 (3 × 30 mL). Thecombined organic phases were dried over MgSO4, filtered, andevaporated in vacuo. The residue was extracted with 50 mL of THF.PTSA (1.2 mmol, 228 mg) and a few drops of water were added. Thereaction mixture was stirred for 16 h at 40 °C. After addition of water(100 mL), THF was removed under reduced pressure, and the residuewas extracted with CH2Cl2 (3 × 30 mL). The combined organicphases were dried over MgSO4, filtered, and evaporated in vacuo, andthe residue was chromatographed with silica gel. Elution with a 2/3dichloromethane/pentane mixture afforded 6 as orange crystals (1.9 g,50%). 1H NMR (200 MHz, CDCl3): δ (ppm) 9.87 (s, 1H, CHO),7.76 (s, 1H, thio), 2.58 (d, 6JH−F = 3 Hz, 3H, CH3).
4-(3,3,4,4,5,5-Hexafluoro-2-(2-methyl-5-N,N-dimethylaminophe-nylthiophenyl-3)cyclopent-1-enyl)-5-methylthiophene-2-carbalde-hyde (7a). To a solution of 3-bromo-2-methyl-5-phenylthiophene 3a(2.10 g, 7.12 mmol) in 50 mL of THF, which was cooled to −78 °C,was added dropwise nBuLi (2.32 M in hexane, 4.6 mL, 10.7 mmol).After the reaction mixture was stirred at −78 °C for 1 h, a solution of 5(2.6 g, 7.12 mmol) in 50 mL of THF was added. After it was stirred at−78 °C for 1 h and at room temperature for 16 h, the reaction mixturewas hydrolyzed with water, and the solvent was removed in vacuo. Theresidue was extracted with CH2Cl2 (2 × 30 mL) and then dried overMgSO4. After evaporation of the solvent, the residual orange oil wasdissolved in 20 mL of THF, and then PTSA (85 mg, 0.44 mmol) and afew drops of water were added. After the mixture was stirred at 40 °Cfor 16 h, the solvent was removed and the oil was purified by columnchromatography (SiO2, pentane/ethyl acetate 95/5) to give yellow-green microcrystals (1.80 g, 50%). 1H NMR (500 MHz, CDCl3): δ(ppm) 9.88 (s, 1H, CHO), 7.80 (s, 1H, thio), 7.42 (d, 3J = 8.8 Hz, 2H,C6H4−), 7.07 (s, 1H, thio), 6.74 (d, 3J = 8.8 Hz, 2H, C6H4−), 3.02 (s,6H, NMe2), 2.08 (s, 3H, Me), 1.93 (s, 3H, Me). 13C{1H} NMR(125.77 MHz, CDCl3): δ (ppm) 182.16 (CHO), 152.00, 150.35,143.81, 141.64, 139.14, 138.14, 136.43, 134.09, 126.74, 126.61, 125.02,121.30, 119.45, 116.03, 112.45, 111.16, 40.41, 15.49, 14.45. Anal.Calcd for C24H19NF6OS2: C, 55.91; H, 3.71; N, 2.72; S, 12.44. Found:C, 55.41; H, 3.61; N, 2.70; S, 12.49. HRMS: m/z 515.0813 [M]+, calcdfor C24H19NF6OS2 515.0812.
4-(3,3,4,4,5,5-Hexafluoro-2-(2-methyl-5-N,N-dibutylaminophe-nylthiophen-3-yl)cyclopent-1-enyl)-5-methylthiophene-2-carbalde-hyde (7b). Following the procedure for 7a, the title compound 7b wasobtained as yellow-green microcrystals (0.5 g, 64%) from 3b (0.5 g,1.31 mmol), nBuLi (1.6 M in hexane, 1 mL, 1.57 mmol), and 5 (0.48g, 1.31 mmol). 1H NMR (400 MHz, CD2Cl2): δ (ppm) 9.88 (s, 1H,CHO), 7.83 (s, 1H, thio), 7.39 (d, 3J = 9.0 Hz, 2H, C6H4−), 7.08 (s,1H, thio), 6.67 (d, 3J = 9.0 Hz, 2H, C6H4−), 3.34 (m, 4H, CH2N),2.11 (s, 3H, CH3), 1.96 (s, 3H, CH3), 1.62 (m, 4H, CH2CH2N), 1.40(sext, 3J = 7.5 Hz, 4H, −CH2CH3), 1.01 (t, 3J = 7.3 Hz, 6H,−CH2CH3).
4,4′-Bis((E)-2-(4-(3,3,4,4,5,5-hexafluoro-2-(2-methyl-5-p-N,N-di-methylaminophenylthiophen-3-yl)cyclopent-1-nyl)-5-methylthio-phen-2-yl)vinyl)-2,2′-bipyridine (La(o)). A THF solution (25 mL) of7 a ( 0 . 2 0 g , 0 . 3 7 mmo l ) w a s a d d e d t o 4 , 4 ′ - b i s -(diethylphosphonomethyl)-2,2′-bipyridine (0.076 g, 0.17 mmol) andtBuOK (0.081 g, 0.72 mmol) at 0 °C. The reaction mixture was stirredfor 2.5 h at room temperature. After addition of water, the organiclayer was washed with brine and water, dried over MgSO4, filtered, andconcentrated. Crystallization in a CH2Cl2/pentane mixture afforded La
as a green powder (0.15 g, 75%). 1H NMR (200 MHz, CDCl3): δ(ppm) 8.66 (d, 3J = 5 Hz, 2H, Py6), 8.50 (s, 2H, Py3), 7.55 (d, 3J = 16
ca lcd for C72H70N4F12S4 1346.4292. Anal . Calcd forC72H70F12N4S4·0.5CH2Cl2: C, 62.64; H, 5.15; N, 4.03; S, 9.23.Found: C, 62.38; H, 5.06; N, 4.12; S, 8.98.Spectroscopic Data of the Photocyclized Lb(c). 1H NMR (500
[(La(o))(dmbipy)2Ru](PF6)2. In a Schlenk tube, under an argonatmosphere, AgOTf (54 mg, 0.21 mmol) and Ru(4,4′-diMe-2,2′-bpy)2Cl2 (55.7 mg, 0.103 mmol) were dissolved in MeOH (8 mL) for5 h at room temperature in the dark. To the resulting red solutionwere added La (120 mg, 0.102 mmol) and CH2Cl2 (8 mL). After themixture was stirred overnight at room temperature in the dark, a largeexcess of NaPF6 was added, this mixture was stirred for 6 h, and thenthe solvent was evaporated. The residue was suspended in the mixtureof EtOH (5 mL) and H2O (50 mL), filtered, and washed with H2Oand then with diethyl ether. Crystallization in a CH2Cl2/pentanemixture afforded a dark brown powder (99.0 mg, 60% yield). 1H NMR(500 MHz, CD2Cl2): δ (ppm) 8.48 (m, 2H, Py3‑DTEbpy), 8.27 (m, 4H,Py3‑Mebpy), 7.67 (d, 3J = 16 Hz, 2H, CH), 7.62−7.53 (m, 6H,Py6‑Mebpy+bpyDTE), 7.45 (d, 3J = 8.8 Hz, 4H, C6H4−), 7.44 (m, 2H,Py5‑bpyDTE), 7.36 (s, 2H, thio), 7.27 (m, 4H, Py5‑Mebpy), 7.13 (s, 2H,thio), 6.96 (d, 3J = 16 Hz, 2H, CH), 6.76 (d, 3J = 8.8 Hz, 4H,C6H4−), 3.01 (s, 12H, NMe2), 2.62 (s, 6H, CH3
■ ACKNOWLEDGMENTSThis work has been financially supported by the ANR (BLANCCOMET) and LEA Rennes-Durham (Molecular Materials andCatalysis). L.O. thanks the Region Bretagne for a Ph.D. grant.H.N. is grateful to the ITP (Japan−Europe−US InternationalTraining Program for Young Generation in Molecular MaterialsScience for Development of Molecular Devices) and JSPS(Japan Society for Promotion of Science; research fellowshipDC2) programs for a travel grant.
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