Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Determining Factor of the Quantum Yield of the Cyclization Reaction via Triplet States for Dye-Attached Diarylethene

The difference in the quantum yields of the cyclization reaction of two isomers of dye-attached diarylethene via the triplet state observed in the experiment [J. Phys. Chem. C 2009, 113, 11623-11627] was theoretically examined by quantum chemical calculations. By evaluating the spin-orbit couplings, we found that the ratio of the rate constants from the S₁ state to the T₂ state between two isomers agreed with that of the experimental quantum yield of the cyclization reaction. The difference in the spin-orbit couplings is due to the difference in the delocalization of the orbitals between diarylethene and dye. We further found that after the intersystem crossing took place the cyclization reaction via the triplet state occurred in the experiment due to the low energy barrier (∼10 kcal/mol) for the reaction.

Light-Triggered Metal Coordination Dynamics in Photoswitchable Dithienylethene-Ferrocene System

The C₂-symmetric photochromic molecule 3, containing dithienylethene (DTE) and ferrocene units connected by an alkyne bridge, represents a unique probe where a metal (Hg²⁺) binds with the central DTE moiety. Both photoisomerized states of 3 (open, 3o; closed, 3c) are found to interact with Hg²⁺ ion by the S atoms of the DTE core; however, the binding constants (from a UV-vis study) and DFT calculations suggest that the open isomer (3o) binds with the metal ion more strongly than that of the closed isomer (3c). Notably, the course of metal binding does not perturb the inherent photoisomerization properties of the DTE core and the photoswitchability persists even in the metal-coordinated form of 3, however, with a comparatively slower rate. The quantum yields for photocyclization (Φ_o→c) and photocycloreversion (Φ_c→o) in the free form are 0.56 and 0.007, respectively, whereas the photocyclization quantum yield in the Hg²⁺ complexed species is 0.068, 8.2 times lower than the photocyclization quantum yield (Φ_o→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by experimental means (UV-vis and electrochemical studies) as well as quantum chemical calculations.

Dinuclear ruthenium acetylide complexes with diethynylated anthrahydroquinone and anthraquinone frameworks: a multi-stimuli-responsive organometallic switch

The anthrahydroquinone (AHQ)/anthraquinone (AQ) system is of particular interest as a molecular switching system for molecule-based devices because AHQ and AQ can be interconverted by redox stimuli and their π-conjugated systems are distinct from each other. To date, however, a way to modify and/or functionalize the switching behavior based on the AHQ/AQ system is still limited. In the present contribution, the synthesis and properties of multi-responsive dinuclear molecular switches having Ru(dppe)₂ fragments bridged by the diethynylated diacetoxyanthracene (AcAHQ) and AQ units (μ-AcAHQ/AQ){C[triple bond, length as m-dash]C-Ru(R)(dppe)₂}₂ (R = Cl, C₄TMS) are presented. The terminal Ru(dppe)₂ fragments are redox-active and, therefore, the intermetallic interaction can be estimated by electrochemical as well as IVCT band analysis. As a result, the organometallic AcAHQ/AQ-Ru system turns out to be an effective bimodal molecular switch, which is triggered not only by redox stimuli but also by pH stimuli.

Excited-State Electronic Asymmetry Prevents Photoswitching in Terthiophene Compounds

The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non- C₃-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non- C₃-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3″-dimethyl-2,2':3',2″-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state.

Theoretical analysis of electrochromism under redox of bis(3-thienyl)/(2-thienyl)hexafluorocyclopentene: effects of charged and substituted systems

Electrochromism with the ring-closing or ring-opening isomerization of substituted and unsubstituted bis(3-thienyl)/(2-thienyl)hexafluorocyclopentene is discussed using the DFT method. In the neutral ground state, bond making and breaking between two reactive C atoms on thienyls are thermodynamically forbidden. Under redox conditions, the gain or loss of electrons can have a significant effect on the frontier molecular orbital distribution of both open- and closed-ring isomers, particularly in reactive sites. Corresponding structural changes show a trend toward isomerization. The reaction energy barrier shows greater reduction for dication than monocation and even becomes barrierless for dianion. During the isomerization in different states, the conjugated system switches distinctively, which is attributed to the special redistribution of molecular orbitals and spin population in each state. In monocation and monoanion, for the involvement of a single electron, isomerization is inclined to proceed sequentially between right and left thienyls, whereas it becomes synchronous in dication. The direction depends on the stabilization achieved by the formation of a global conjugated system and more average spin population on the molecule. The effect of substituents on thienyls is demonstrated in the promotion of the extent of conjugation and the determination of the spin population level on the reactive C atoms. Moreover, according to their electron-donating and withdrawing abilities, they can kinetically support or suppress the electron transfer pattern in the process from isomer to transition state, which leads to the control of reaction efficiency.

Spectroscopic and electrochemical properties of ruthenium complexes with photochromic triarylamine–dithienylethene–acetylide ligands

The charge delocalization along the molecular backbone shows a progressive increase following stepwise photocyclization in the inorganic–organic mixed-valence system.

Isolation and crystal structure of a dithiophene dication: controlling covalent connection and disconnection with temperature and phase

A dithienylethene (1o) undergoes a two-electron chemical oxidation to a singlet diradical as an open-isomer (1o(2+)) in solution, which cyclizes to a closed-form (1c(2+)) upon cooling. The latter crystallizes out and its structure is analyzed by single crystal X-ray diffraction. Equilibrium between 1o(2+) and 1c(2+) in solution is observed by NMR and UV spectroscopy at various temperatures and is further supported by reduction reactions with Zn powder.

Heteroleptic Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and bisthienylethene: structures, luminescence and photochromic properties

Bisthienylethenes L1H and L2H, and their Ir(iii) complexes have been synthesized. Their crystal structures, luminescences and photochromisms have been studied.

Two bisthienylethene–Ir(iii) complexes showing acid/base-induced structural transformation and on–off luminescence switching in solution

Compounds [Ir(dfppy)₂(pbdtiH)](PF₆)·2CHCl₃ (1-H) and [Ir(dfppy)₂(pbdti)] (1) were synthesized based on bisthienylethene pbdtiH, showing NEt₃/TFA-induced structural transformation and on–off luminescence switching.