Photo-switchable iron-terpyridine complexes functionalized with styrylbenzene unit

A Bichromophoric Approach to Induce Luminescence - A Blend of Experimental and Theoretical Studies

Two homoleptic terpyridyl complexes of Ru(II), 1 and Fe(II), 2 were synthesized using a ligand L1 that contained a phenyl spacer between an anthracenyl (An) and a terpyridyl (tpy) moiety. An equilibrated-bichromophoric strategy was adopted to induce photoluminescence in 1 and 2. A glimpse into the excited state photophysical properties of 1 and 2 revealed that 1 exhibited NIR emission at ~700 nm with an excited state lifetime components of 1.33 and 6.52 ns. On the other hand, 2 was found to be non-luminescent. The origin of emission in case of 1 was attributed to the effect of phenyl spacer which rendered the 3An state to be nearly isoenergetic to the emissive 3MLCT state of 1 facilitating 3MLCT-3An equilibrium. This fact was supported by experimental (photocurrent generation) and theoretical (potential energy diagram) evidences.

Control of Photoisomerization Kinetics via Multistage Switching in Bimetallic Ru(II)-Terpyridine Complexes

In this study, we carried out detailed experimental and theoretical investigation on photophysical, electrochemical, and photoisomerization behaviors of a new array of luminescent binuclear Ru(II) complexes derived from a phenylene-vinylene-substituted terpyridyl ligand possessing RT lifetimes within 60.3-410.5 ns. The complexes experienced trans-to-cis isomerization in MeCN on irradiation with visible light, accompanied by significant changes in their absorption and emission spectral profiles. The reverse cis-to-trans process is also possible with the use of ultraviolet (UV) light. On conversion from trans to cis isomers, the emission intensity increases substantially, while for the reverse process, luminescence quenching occurs. Thus, "off-on" and "on-off" emission switching is facilitated upon treatment with visible and UV light alternatively. By the use of chemical oxidants (ceric ammonium nitrate and potassium permanganate) and reductants (metallic sodium) as well as light of appropriate wavelengths, multistate switching phenomena involving reversible oxidation-reduction and trans-cis isomerization have been achieved. Interestingly, the rate of this multistate photoswitching process becomes much faster compared to only two-state trans-cis isomerization of these complexes. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations are also performed to obtain a clear picture of the electronic environment of the complexes and also for the appropriate assignment of absorption and emission spectral bands.

Light and Cation-Driven Optical Switch based on a Stilbene-Appended Terpyridine System for the Design of Molecular-Scale Logic Devices

A molecular system comprising a terpyridine moiety capable of coordinating with different cations and a photoswitchable stilbene unit has been utilized here for the fabrication of multiply configurable logic systems. Incorporation of a substituted stilbene unit into the terpyridine motif generates an intraligand charge-transfer-sensitive module whose absorption and emission spectral properties are highly sensitive to light as well as cations. On the basis of the optical response profile of the receptor in the presence of selected cations as well as light of a specific wavelength, we are able to demonstrate multiple Boolean logic functions such as INHIBIT, IMPLICATION, OR, NOR, and NAND, as well as various combinations of them. Of particular interest, we utilized the present system for the construction of security keypad locks and memory devices by maintaining a proper sequence of the stimuli and monitoring either absorption or emission spectral response at a specific wavelength as the output signal. In addition to various Boolean logic functions, the present system has also the ability to mimic fuzzy logic operations for generating an infinite-valued logic scheme depending on its emission spectral responses upon varying the concentration of cationic (Fe²⁺ and/or Zn²⁺) and anionic (CN^-) inputs.

Low-cost photo-switches based on stilbene-appended Zn(II)-terpyridine complexes

We report herein the synthesis, characterization, photophysics, and photo-isomerization behaviors of three Zn(II)-terpyridine complexes of the type [Zn(tpy-pvp-X)₂]²⁺ (X = H, Me, and NO₂) covalently tethered with stilbene moiety. The complexes exhibit absorption bands stretching up to the edge of the visible domain due to ligand → ligand charge transfer (LLCT) transitions and strong emission at room temperature in the visible due to radiative deactivation of ³LLCT state having lifetime within 1.0-3.0 ns. The stilbene motifs in the complexes undergo trans to cis isomerization upon irradiating with UV and visible light accompanied by significant alteration of their absorption, emission, and ¹H NMR spectral profiles. Apart from the variation of electron donating and electron withdrawing substituent (X), the isomerization studies were also carried out in three different solvents (DCM, MeCN, and DMSO) to further tune their kinetic and thermodynamic parameters. The rate, rate constant and quantum yield of isomerization were estimated in all the solvents. The reverse process (cis to trans) also occurs very slowly on keeping but could be accelerated upon heating. Trans to cis photoisomerization leads to quenching of emission in case of 1 and 2, whereas backward thermal cis to trans conversion leads to restoration of emission. By contrast, for the nitro-derivative (3) forward process induces emission enhancement, while backward process gives rise to emission quenching. In essence, "on-off" and "off-on" emission switching is feasible for 1 and 2, whereas "off-on" and "on-off" emission switching occurs in case of 3. Emission spectral responses upon successive action of photonic and thermal input lead to the fabrication of INHIBIT and IMPLICATION logic gates. DFT and TD-DFT computational investigations were also undertaken to visualize their electronic structures, correct assignment of the spectral bands, and mode of isomerization process.