Emission Switching in the Near-Infrared by Reversible Trans-Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes

Unveiling the multifaceted nature of terpyridine-based metal complexes: a synergistic study of their photophysical and electrochemical behavior with computational insights

This manuscript provides a comprehensive overview of the synthesis, characterization, and photophysical and electrochemical properties of terpyridine-based metal complexes (C1-C20). The synthesis of these terpyridine (TPY) complexes involves the coordination of TPY ligands (L1-L11) with transition metal ions, leading to a variety of novel structural and electronic configurations. The characterization of TPY ligands and their complexes is carried out using various techniques, including UV-Vis spectroscopy, NMR, FTIR and mass spectrometry. To the best of our knowledge, for the first time, we comprehensively investigate the photophysical, solvatochromic, electrochemical, and computational properties of an extensive series of TPY-based metal complexes (C1-C20) within a single framework. The solvatochromic behavior of the synthesized complexes (C1-C20) is explored, revealing their sensitivity to solvent polarity, which is a key factor influencing their photophysical properties. The TPY-based complexes (C1-C20) exhibited solvent-dependent fluorescence behavior, with distinct ILCT and MLCT mechanisms, and enhanced fluorescence in specific solvents, particularly for Zn(ii) and Cu(ii) complexes. The absorption and emission characteristics of the complexes are studied in dilute solutions to explore their structure-property relationships. Additionally, the electrochemical properties of the TPY-based metal complexes (C1-C20) are investigated, highlighting their redox activity and potential for use in energy storage and conversion applications. Density functional theory (DFT) calculations are employed to provide detailed insights into the electronic structure and reactivity of these complexes, supporting the experimental observations. The correlation of electronic band gaps with photophysical and electrochemical behaviors showed compounds as promising candidates with efficient charge transfer and strong fluorescence. The integrated analyses reveal the exceptional potential of this scaffold for advanced materials applications, highlighting its versatility and significance in cutting-edge research.

Group 8 Organometallic Photochromic Compounds: Strategies and Applications

Stimuli-responsive photochromic units have emerged as one of the key components in the development of multi-responsive switches, optoelectronics, biomedical sciences and many more. The photoswitchability of such compounds depends greatly on the molecular structure, where association of metallic species is found to produce fascinating results. This review is a comprehensive report of all such photoswitchable metal-bounded molecules with group 8 metals within a span of last six years (2018-2024). Apart from the regular photoswitching phenomenon, this review focusses on the enhanced tunability, structural flexibility and perturbation in the photophysical properties of the group 8 metal-based photoswitches. Previous reviews in this field have either focused on some specific applications or have been general with the type of metal incorporations. Herein, we have constructed the review with group 8 organometallic photochromic compounds that possess a wide range of real-life applications. Designing strategies, structure-property relationships and application-oriented approach of the photochromic organometallic compounds have been elucidated categorically for building up a comprehensive idea about this modern developing field of research.

Remarkable Increase in the Rate of Trans-Cis Photoisomerization of Os(II)-Terpyridine Complexes via Oxidation and Reduction

Luminescent homoleptic Os(II)-terpyridine complexes comprising stilbene-appended naphthalene, anthracene, and pyrene motifs are designed in this work, and their photophysical, electrochemical, and photoisomerization behaviors are extensively investigated. All complexes exhibit intense spin-allowed singlet metal-to-ligand charge transfer (1MLCT) bands in the visible (496-500 nm) and weaker spin-forbidden singlet-to-triplet 3MLCT transitions in the 600-700 nm range. They display moderate emission at room temperature with lifetimes in the range of 84.5-112.5 ns. Electrochemical studies reveal a reversible Os2+/Os3+ oxidation couple within 0.93-0.96 V, alongside multiple reversible or quasi-reversible reduction peaks associated with terpyridine units in between -1.10 and -1.85 V. The stilbene motifs facilitate reversible trans-cis photoisomerization under alternative treatment with visible and UV light, enabling the complexes to function as photomolecular switches in the near-infrared domain. Interestingly, a remarkable increase in the rate of photoisomerization has been achieved via oxidation as well as reduction of the complexes, which, in turn, induces multistep switching involving reversible oxidation-reduction and trans-cis isomerization. Computational investigations are also conducted on all three conformations {trans-trans (t-t), trans-cis (t-c), and cis-cis (c-c)} of the complexes to gain insight into their electronic structures and for accurate assignment of their absorption and emission spectral bands.

Design of Near-Infrared Emissive Molecular Switches Based on Stilbene-Appended Cyclometalated Bimetallic Ru(II)-Terpyridine Complexes

In the present study, we have synthesized and thoroughly characterized two Ru(II) dimers with compositions [(ttpy)Ru(tpvpt')Ru(ttpy)](ClO4)3 and [(ttpy)Ru(t'pvpvpt')Ru(ttpy)](ClO4)2 incorporating phenylene-vinylene-substituted terpyridine bridging ligands capable of coordinating in both an NNN- and cyclometalated NNC-fashion. The complexes display strong absorption across the entire UV-vis spectral domain and exhibit luminescence in the NIR region (820-850 nm). The N atoms in the outer coordination sphere were employed for alteration of the photoredox behaviors of the complexes via acid-base equilibria. Decoordination of the Ru-C bonds in the presence of acid, followed by recoordination in the presence of base and heat, is also possible. Additionally, the phenylene-vinylene units in the bridging ligands facilitate trans → cis and trans-trans → trans-cis isomerization under visible light irradiation. The reverse isomerization (cis → trans and trans-cis → trans-trans) is also achieved upon UV light irradiation. Thus, "on-off" and "off-on" emission switching is facilitated through a judicious choice of external stimuli like acid, base, temperature, or light of particular wavelengths. Interestingly, the rate of photoswitching is significantly faster in the presence of acid compared to that in the absence of acid. DFT and TD-DFT calculations were also conducted to clearly visualize the electronic environment around the complex backbone and also for accurate assignment of the absorption and emission bands.

Synthesis, Electrochemistry and Density Functional Theory of Osmium(II) Containing Different 2,2':6',2″-Terpyridines

In coordination chemistry, 2,2':6',2″-terpyridine is a versatile and extensively studied tridentate ligand. Terpyridine forms stable complexes with a variety of metal ions through coordination sites provided by the three nitrogen atoms in its pyridine rings. This paper presents an electrochemical study on various bis(terpyridine)osmium(II) complexes, addressing the absence of a systematic investigation into their redox behavior. Additionally, a computational chemistry analysis was conducted on these complexes, as well as on eight previously studied osmium(II)-bipyridine and -phenanthroline complexes, to expand both the experimental and theoretical understanding. The experimental redox potentials, Hammett constants, and DFT-calculated energies show linear correlations due to the electron-donating or electron-withdrawing nature of the substituents, as described by the Hammett constants. These substituent effects cause shifts to lower or higher redox potentials, respectively.

Advancing Molecular-Scale Logic Devices through Multistage Switching in a Luminescent Bimetallic Ru(II)-Terpyridine Complex

Stimuli-responsive multistep switching phenomena of a luminescent bimetallic Ru(II) complex are employed herein to fabricate multiple configurable logic devices. The complex exhibits "off-on" and "on-off" emission switching upon alternative treatment with visible and UV light. Additionally, remarkable augmentation of the rate as well as quantum yield of photoisomerization was achieved via the use of a chemical oxidant (Ce4+) as well as a reductant (metallic sodium). Upon exploiting the emission spectral response of the complex, several advanced Boolean logic functions, including IMPLICATION as well as 2-input 2-output and 3-input 2-output complex combinational logic gates, are successfully implemented. Additionally, by utilizing the vast efficacy of Python, a novel "logic_circuit" model is devised that is capable of making accurate decisions under the influence of various input combinations. This model transcends traditional Boolean logic gates, offering flexibility and intuition to design logical functions tailored to specific chemical contexts. By integrating principles of logic circuits with chemical processes, this innovative approach enables structure determination of the chemical states based on input conditions, thereby unlocking avenues for exploring intricate interactions and reactions beyond conventional Boolean logic paradigms.

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.

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.