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A series of cationic Ir(III) substituted bipyridyl ()(N(∧)N (N(∧)N-bpy) complexes incorporating electron-donor and -acceptor substituents, [Ir(C(∧)N-ppy-R')(2)(N(∧)N-bpy-CH═CH-C(6)H(4)-R)][X] (X(-) = PF(6)(-) or C(12)H(25)SO(3)(-)), 2 (a, R = NEt(2) and R' = Me; b, R = O-Oct and R' = Me; c, R = NO(2) and R' = C(6)H(13); C(∧)N-ppy = cyclometalated 2-phenylpyridine, [Ir(C(∧)N-ppy-Me)(2)(N(∧)N-bpy-CH═CH-thienyl-Me)][PF(6)], 2d, and the dithienylethene (DTE)-containing complex 2e have been synthesized and characterized, and their absorption, luminescence, and quadratic nonlinear optical (NLO) properties are reported. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations on the complexes facilitate a detailed assignment of the excited states involved in the absorption and emission processes. All five complexes are luminescent in a rigid glass at 77 K, displaying vibronically structured spectra with long lifetimes (14-90 μs), attributed to triplet states localized on the styryl-appended bipyridines. The second-order NLO properties of 2a-d and related complexes 1a-d with 1,10-phenanthrolines have been investigated by both electric field induced second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques. They are characterized by high negative EFISH μβ values which decrease when the ion pair strength between the cation and the counterion (PF(6)(-), C(12)H(25)SO(3)(-)) increases. The EFISH response is mainly controlled by metal-to-ligand charge-transfer/ligand-to-ligand charge-transfer (MLCT/L'LCT) processes. A combination of HLS and EFISH techniques is used to evaluate both the dipolar and octupolar contributions to the total quadratic hyperpolarizability, demonstrating that the major contribution is controlled by the octupolar part. The incorporation of a photochromic DTE unit into the N(∧)N-bpy ligand (complex 2e) allows the luminescence to be switched ON or OFF. The photocyclisation of the DTE unit can be triggered by using either UV (365 nm) or visible light (430 nm), leading to an efficient quenching of the ligand-based 77 K luminescence, which can be restored upon irradiation of the closed form at 715 nm. In contrast, no significant modification of the EFISH μβ value is observed upon photocyclization, suggesting that the quadratic NLO response is dominated by the MLCT/L'LCT processes, rather than by the intraligand excited states localized on the substituted bipyridine ligand.

The photoinduced ring-closure/ring-opening reactions of a series of bis-dithienylethene derivatives, as free ligands and Zn(II)-complexes, are investigated by resorting to theoretical (time-dependent density functional theory) and kinetic analyses in solution. The originality of the system stems from the tunability of the photoreaction quantum yields and conversion yields as a function of the electronic structure. The latter could be varied by modifying the electron-donating character of the DTE-end substituents L(a-d) (o,o) (a, D = H; b, D = OMe; c, D = NMe(2); d, D = NBu(2)) and/or the Lewis character of the metal ion center L(a-d)ZnX(2) (o,o) (L(a-c), X = OAc; L(d), X = Cl). The orbital description of the doubly-open form (o,o) and half-closed form (o,c) predicts that double closure to the form (c,c) would occur using UV irradiation. Photokinetic studies on the complete series demonstrate that photocyclization proceeds following a sequential ring closure mechanism. They clearly point out distinct quantum yields for the first and second ring closures, the latter being characterized by a significantly lower value. Dramatic decrease in both the quantum yields of the ring-closure and ring-opening processes is demonstrated for the complex L(d)ZnCl(2) exhibiting the strongest charge-transfer character in the series investigated. These studies show that this series of DTE derivatives provides an efficient strategy to tune the photochromic properties through the combination of the electron-donor and electron-acceptor (D-A) groups.

Reaction of a variety of CCH bond-containing 1,6-enynes with N2CHSiMe3 in the presence of RuCl(COD)Cp* as catalyst precursor leads, at room temperature, to the general formation of alkenylbicyclo[3.1.0]hexanes with high Z-stereoselectivity of the alkenyl group and cis arrangement of the alkenyl group and an initial double-bond substituent, for an E-configuration of this double bond. The stereochemistry is established by determining the X-ray structures of three bicyclic products. The same reaction with 1,6-enynes bearing an R substituent on the C1 carbon of the triple bond results in either cyclopropanation of the double bond with bulky R groups (SiMe3, Ph) or formation of alkylidene-alkenyl five-membered heterocycles, resulting from a beta elimination process, with less bulky R groups (R = Me, CH2CH=CH2). The reaction can be applied to in situ desilylation in methanol and direct formation of vinylbicyclo[3.1.0]hexanes and to the formation of some alkenylbicyclo[4.1.0]heptanes from 1,7-enynes. The catalytic formation of alkenylbicyclo[3.1.0]hexanes also takes place with enynes and N2CHCO2Et or N2CHPh. The reaction can be understood to proceed by an initial [2+2] addition of the Ru=CHSiMe3 bond with the enyne CCH bond, successively leading to an alkenylruthenium-carbene and a key alkenyl bicyclic ruthenacyclobutane, which promotes the cyclopropanation, rather than metathesis, into bicyclo[3.1.0]hexanes. Density functional theory calculations performed starting from the model system Ru(HCCH)(CH2=CH2)Cl(C5H5) show that the transformation into a ruthenacyclobutane intermediate occurs with a temporary eta3-coordination of the cyclopentadienyl ligand. This step is followed by coordination of the alkenyl group, which leads to a mixed alkyl-allyl ligand. Because of the non-equivalence of the terminal allylic carbon atoms, their coupling favors cyclopropanation rather than the expected metathesis process. A direct comparison of the energy profiles with respect to those involving the Grubbs catalyst is presented, showing that cyclopropanation is favored with respect to enyne metathesis.

Flip the NLO switch! A straightforward access to hexadithienylethene-based photochromes by using a coordination-chemistry approach through the combination of bis(dithienylethene)bipyridyl ligands and metal ions (M = Zn, Fe) is reported. The photostability of the isomeric forms of the iron(II) complexes allows the photoswitching of second-order nonlinear optical (NLO) responses (see figure), and the results are rationalized with the help of time-dependent density functional theory.

An original design of a fluorescent dithienylethene (DTE)-based bipyridine, where donor (D) and acceptor (A) groups are located on the same thiophene ring of the DTE unit, is reported; in non-polar solvents, UV or visible excitation triggers a photochromic reaction, disrupting the conjugation and quenching the fluorescence.

The synthesis and characterization of a series of fluorescent bis-dithienylethene (DTE)-based bipyridines, where the donor (D) and acceptor (A) groups are located on the same thiophene ring of the DTE unit, and their zinc(II) and rhenium(I) complexes are reported. Their photochromic properties have been investigated by UV-visible and (1)H NMR spectroscopy. These studies reveal that in non-polar solvents it is possible to modulate the photoreactivity, single vs. double ring-closure, by changing the nature of the donor group. The solvent effect, as well as the influence of the organometallic moieties on the photochromic behavior of these molecules, is also discussed. Finally, upon photoconversion to the photostationary state (PSS), a quenching of fluorescence is observed for the bipyridine ligands, due to disruption of the conjugation upon ring-closing.

A current challenge is the development of efficient strategies for the design of switchable nonlinear optical (NLO) materials.[1] As most molecules with large first hyperpolarizability values b comprise p systems that are unsymmetrically endcapped with donor and acceptor moieties, various strategies have been explored to alter the electron-donor (or acceptor) capacity of the end groups using external stimuli such as redox methods [2] and protonation/deprotonation reactions. [3] Another elegant approach to the reversible switching of NLO properties is the use of photochromic compounds. [1b, 4] Among them, dithienylethene (DTE) derivatives are the most promising because of their good fatigue resistance, the remarkable thermal stability of both isomers, and the rapid response time, which are prerequisite conditions for practical applications.[5] Typically, DTE derivatives undergo reversible interconversion between an unconjugated open form and a pconjugated closed form when irradiated in the UV and visible spectral ranges, respectively.In recent years, considerable effort has been devoted to the development of transition-metal complexes as NLO chromophores, and their large second-order nonlinearities have been demonstrated.[6] The photoswitching of their NLO properties has not been reported to date, although recent studies have highlighted the potential of metal-containing DTE ligands for luminescence or electron-transfer photoswitching. [7] We previously described the use of 4,4'-bis(dialkylaminostyryl)-2,2'-bipyridine compounds for the molecular engineering of noncentrosymmetric dipolar [8] (Scheme 1, type I) or octupolar metal complexes. [9] With the aim of photoswitching the NLO properties, we designed a new type of 4,4'-bis(ethenyl)-2,2'-bipyridine ligand functionalized by phenyland dimethylaminophenyl DTE groups. These ligands allowed us to prepare the corresponding photochromic dipolar (bipyridyl)zinc(II) complexes (Scheme 1, type II). Herein, we report the synthesis and photochromic properties of these new compounds, as well as the efficiently phototriggered enhancement of the NLO activity of dipolar complexes. The target bipyridine derivatives 1 a(o) (D = H, (o) indicates open form) and 1 b(o) (D = NMe 2 ) were readily prepared in 55-60 % yield by a double Horner-WadsworthEmmons condensation between the bisphosphonate bipyridine and the aryl-substituted DTE aldehydes 6 (Scheme 2). The latter compounds were obtained by a multistep procedure. The two thienyl fragments were prepared independently and were successively connected to C 5 F 8 (see the Supporting Information for details).For both ligands, the photocyclization process was confirmed by 1 H NMR spectroscopy experiments. The colorless solutions of 1 a(o) and 1 b(o) in CD 2 Cl 2 turn blue and green, respectively, upon UV irradiation. The 1 H NMR spectra of the closed forms 1 a(c) and 1 b(c) show the characteristic upfield shift of the two thiophene protons, while the methyl proton signals are shifted downfield. Integration of the methyl-group reso...