Charge-transfer Absorption Band Research Articles

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes.

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the 3MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Anthryl-Appended Platinum(II) Schiff Base Complexes: Exceptionally Small Stokes Shift, Triplet Excited States Equilibrium, and Application in Triplet–Triplet-Annihilation Upconversion

Two anthryl platinum(II) N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine Schiff base complexes were synthesized, with the anthryl attached via its 9 position (Pt-9An) or 2 position (Pt-2An) to the platinum (Pt) Schiff base backbone. The complexes show unusually small Stokes shifts (0.23 eV), representing a very small energy loss for the photoexcitation/intersystem crossing process, which is beneficial for applications as triplet photosensitizers. Phosphorescence of the Pt(II) coordination framework (ΦP = 11.0%) is quenched in the anthryl-containing complexes (ΦP = 4.0%) and shows a biexponential decay (τP = 3.4 μs/87% and 18.2 μs/13%) compared to the single-exponential decay of the native Pt(II) Schiff base complex (τP = 3.7 μs). Femtosecond/nanosecond transient absorption spectroscopy suggests an equilibrium between triplet anthracene (3An) and triplet metal-to-ligand charge-transfer (3MLCT) states, with the dark 3An state slightly lower in energy (1.96 eV for Pt-9An and 1.90 eV for Pt-2An) than the emissive 3MLCT state (1.97 eV for Pt-9An and 1.91 eV for Pt-2An). Intramolecular triplet-triplet energy transfer (TTET) and reverse TTET take 4.8 ps/444 ps for Pt-9An and 55 ps/1.7 ns for Pt-2An, respectively. The triplet-state equilibrium extends the triplet-state lifetime of the complexes to 103 μs (Pt-2An) or 163 μs (Pt-9An), in comparison to the native Pt(II) complex, which shows a lifetime of 4.0 μs. The complexes were used for triplet-triplet-annihilation upconversion with perylene as the triplet acceptor. The upconversion quantum yield is up to 15%, and a large anti-Stokes shift (0.75 eV) is achieved by excitation into the singlet metal-to-ligand charge-transfer absorption band (589 nm) of the complexes (anti-Stokes shift is 0.92 eV with 9,10-diphenylanthracene as the acceptor).

Intriguing ‘Turn-on’ phosphorescent response in the near infrared region upon fluoride binding: Dipyrromethene chelating-based Triarylboryl‒Iridium(III) conjugates

Two novel heteroleptic cyclometalated iridium(III) complexes, namely (C^N)2Ir(BL) (Ir1 and Ir2) (C^N = 2-(2,4-difluorophenyl)pyridine (dfphpy, for Ir1) and 2-phenylpyridine (phpy, for Ir2)), chelated by the triarylboryl dipyrromethene ancillary ligand (BL) were synthesized and characterized. The molecular structures of both complexes in the solid state were fully analyzed by single-crystal X-ray diffraction, and were found to be composed of Ir(III)-centered triarylborane conjugated systems. The UV–vis absorption spectra of both complexes displayed the typical 1MLCT (metal-to-ligand charge transfer) absorption bands centered at λabs = 483 nm, in addition to borane-centered CT absorption bands at ~326 nm. The photoluminescence (PL) spectra for these complexes demonstrated phosphorescent emission in the near infrared region (λem = ~690 nm), which was mainly attributed to the 3MLCT transition (L = BL). In addition, fluoride titration experiments using these complexes showed gradually quenched absorption bands in the region of 300–350 nm, corresponding to borane-centered CT transitions. Furthermore, 1H NMR experiments in THF-d8 for both complexes in the presence of fluoride sources revealed that the fluoride was bound to the dimesitylboryl moieties. Interestingly, upon the addition of fluoride, the PL spectra exhibited a ratiometrically ‘turn-on’ and gradually blue-shifted emissive response. Computational calculations and cyclic voltammetry measurements using the neutral Ir(III)-conjugates and their fluoride adducts strongly suggested that the intriguing emissive response is the result of fluoride binding interrupting the original 3MLCT transition, reinforcing the dipyrromethene-centered π–π* transition, and further inducing the enhancement of the band-gap.

Cyano-ambivalence: Spectroscopy and photophysics of [Ru(diimine)(CN-BR3)4]2− complexes

The UV–visible absorption and luminescence spectra of [Ru(diimine)(CN)4]2− derivatives have been tuned over wide ranges through variations in solvent, substituents on the diimine ligand, and boronation of the cyanide ligands. Trifluoromethyl substitution at the 4 and 4′ positions of the diimine induces red shifts in metal-to-ligand charge-transfer (MLCT) absorption and luminescence bands. Boronation of the cyanide ligands produces substantial blue shifts in MLCT energies. The combination of diimine trifluoromethylation and cyanide boronation produces MLCT blue shifts that are about 75% as large as those produced by boronation alone.

(Invited) Ultrafast Solar Energy Conversion with Earth-Abundant Complexes

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices.[1] We have studied ultra-fast processes in a variety of DSSC and photocatalytic applications.[2,3] More recently we demonstrated the first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a CT state and show that it has a 100 ps excited state CT lifetime which is unprecedented for any iron complex.[4] This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing (ISC) in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 TM complexes of e.g. FeII and RuII. Furthermore, we have also shown [FeII(btz)3]2+ low-spin complex [5], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined, including the very latest development of an iron compound [Fe(phtmeimb)2]+ with an excited state stable enough to emit luminescence with unprecedented 2 ns lifetime and with the ability to engage in bi-molecular electron transfer [6].

Near‐IR‐Absorbing BODIPY‐5,10‐Dihydrophenazine Compact Electron Donor/Acceptor Dyads and Triads: Spin‐Orbit Charge Transfer Intersystem Crossing and Charge‐Transfer State

Abstract5,10‐dihydro‐5,10‐dimethylphenazine (PZ) and boron dipyrromethene (Bodipy) were linked to prepare compact electron donor/acceptor dyads and triads. The effect of the molecular geometry on the UV‐Vis absorption, fluorescence and the spin‐orbit charge transfer intersystem crossing (SOCT‐ISC) efficiency were studied. Interestingly, a broad near‐IR charge transfer (CT) absorption band (centred at 670 nm) was observed for the dyad/triad showing more coplanar geometry. Triplet state formation was observed in low polar solvents for the triad with orthogonal geometry (triplet state quantum yield is 30 % and lifetime is τT=210 μs). Femtosecond transient absorption spectra indicated fast charge separation (CS, 2 ps) and slow charge recombination (CR, >1.5 ns). The low‐lying 3CT state of the dyads/triads in polar solvent (1.23 eV vs. the 1.65∼1.69 eV of the 3LE state) was confirmed by intermolecular triplet photosensitizing experiments and the spin density surface analysis. These results are useful for design of new broadband, near‐IR absorbing heavy atom‐free triplet photosensitizers.

Synthesis and Characterization of Isoindigo-Based Push-Pull Chromophores.

Symmetrical and unsymmetrical chromophores of isoindigo 3-7 were designed and synthesized, in which isoindigo was used as the central unit (electron acceptor unit A), triphenylamine as the end capping unit (electron donor group D), 1,1,4,4-tetracyanobutadiene (TCBD, A') and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (A″) as the acceptor unit. The effects of multiacceptor units on photophysical, electrochemical, and computational studies were investigated. The photophysical properties of isoindigo 6 and 7 exhibit a strong intramolecular charge transfer (ICT) absorption band in the near IR region. The isoindigo 4-7 shows multi-redox waves with a low electrochemical band gap, which signifies the tuning of highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels and enhance the π-conjugation. The computational studies demonstrate that there is a good agreement with experimental data. The molecular design and synthesis of isoindigo 4-7 gives a new avenue for the development of building blocks in organic electronics.

Spin-Orbit Charge-Transfer Intersystem Crossing (ISC) in Compact Electron Donor-Acceptor Dyads: ISC Mechanism and Application as Novel and Potent Photodynamic Therapy Reagents.

Spin-orbit charge-transfer intersystem crossing (SOCT-ISC) is useful for the preparation of heavy atom-free triplet photosensitisers (PSs). Herein, a series of perylene-Bodipy compact electron donor/acceptor dyads showing efficient SOCT-ISC is prepared. The photophysical properties of the dyads were studied with steady-state and time-resolved spectroscopies. Efficient triplet state formation (quantum yield ΦT =60 %) was observed, with a triplet state lifetime (τT =436 μs) much longer than that accessed with the conventional heavy atom effect (τT =62 μs). The SOCT-ISC mechanism was unambiguously confirmed by direct excitation of the charge transfer (CT) absorption band by using nanosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The factors affecting the SOCT-ISC efficiency include the geometry, the potential energy surface of the torsion, the spin density for the atoms of the linker, solvent polarity, and the energy matching of the 1 CT/3 LE states. Remarkably, these heavy atom-free triplet PSs were demonstrated as a new type of efficient photodynamic therapy (PDT) reagents (phototoxicity, EC50 =75 nm), with a negligible dark toxicity (EC50 =78.1 μm) compared with the conventional heavy atom PSs (dark toxicity, EC50 =6.0 μm, light toxicity, EC50 =4.0 nm). This study provides in-depth understanding of the SOCT-ISC, unveils the design principles of triplet PSs based on SOCT-ISC, and underlines their application as a new generation of potent PDT reagents.

联三吡啶铂(Ⅱ)配合物的非线性吸收和光限幅（特邀）

The reported work in 2003-2019 on the reverse saturable absorption (RSA) or two-photon absorption (TPA) and/or optical limiting (OPL) of platinum(II) terpyridine complexes was summarized in this minireview. Photophysical properties, including the ground-state absorption (GSA), excited-state absorption (ESA), excited-state lifetimes, and the quantum yields of triplet excited-state formation, RSA/OPL at 532 nm for ns laser pulses, TPA characteristics in the near-IR spectral regions, and the structure-property correlations were reviewed. This paper is composed of four sections. First, the current status of OPL materials and devices, the general requirements for reverse saturable absorbers and two-photon absorbing materials, and the different types and characteristics of square-planar platinum(II) complexes were briefly introduced. Then the photophysics and RSA/OPL of six series of Pt(Ⅱ) terpyridine-analogous complexes and the structure-property correlations were discussed. Following it the TPA of five series of Pt(Ⅱ) terpyridine complexes and the impacts of structural variations on the TPA cross sections (σ2) were reviewed. Finally, brief conclusions were drawn based on the reported studies. A general trend discovered was that the charge transfer absorption band(s) and the ESA can be readily tuned by substituents on the acetylide or the terpyridine ligand. Introducing electron-donating substituent to the acetylide or terpyridine ligand or improving the coplanarity between the aromatic substituent and the terpyridine ligand red-shifted the ground-state charge-transfer absorption band(s) at the price of decreasing/quenching the triplet ESA, which consequently reduced the RSA/OPL at 532 nm. Extending the π-conjugation of the terpyridine ligand dramatically improved the σ2 values of the Pt(Ⅱ) terpyridine complexes. Incorporation of electron-withdrawing π-conjugated aromatic substituent restrained the GSA to < 500 nm while keeping a long-lived triplet excited state with broadband ESA in the visible spectral regions and moderately strong TPA in the NIR regions. This approach could provide a solution for developing broadband OPL materials.

Research on Efficient Fast Scintillators: Evidence and X‐Ray Absorption Near Edge Spectroscopy Characterization of Ce4+ in Ce3+, Mg2+‐Co‐Doped Gd3Al2Ga3O12 Garnet Crystal

Herein, the most advanced oxide scintillator Ce3+, Mg2+‐co‐doped Gd3Al2Ga3O12 (GAGG) garnet host, grown using the Czochralski method is reported. The charge transfer absorption band in UV, such as in other Ce3+, Mg2+‐co‐doped oxides, reveals the creation of stable Ce4+ oxidation state, in addition to the usual Ce3+ one, admitted as responsible of the scintillation mechanism improvement displayed by this garnet. Strangely, the confirmation of quantitative values of Ce4+ concentration has never been done, and the goal is mainly focused on the presence of Ce4+ and the evaluation of the Ce3+/Ce4+ ratio measured by X‐ray absorption near edge spectroscopy (XANES) spectroscopy at the Ce LIII threshold of the European Synchrotron Radiation Facility (ESRF). This result is compared with those obtained for Ce3+, Mg2+‐co‐doped Lu3Al5O12 (LuAG) and, also, in garnets without any Mg2+ ions such as Ce3+‐doped GAGG, Ce3+‐doped LuAG, and Ce3+‐doped YAG. The role of Li+ ion in place of Mg2+ one is also considered in Ce3+, Li+‐co‐doped LuAG single crystal.

Green Synthesis of Cr3+ Doped CaIn₂O₄-Carbon Hybrid Nanostructure and Its Light Absorption and Emission Properties.

In this research article, we have used a novel route for synthesis of a greenish phosphor C-CaIn₂O₄ doped with Cr3+ ions using green plant (aloe vera gel) and measured its light absorption and emission properties. Our main aim was to enhance the light absorption and emission properties of undoped C-CaIn₂O₄ in order to widen its applications. The introduction of CrO₃ ions can produce Cr ion defects that favor charge balance in Cr3+:C-CaIn₂O₄ to facilitate its photoluminescence. The effect of doping in core-shell nanostructure can effectively transfer energy from charge-transfer absorption band of optical host material to characteristic transition of Cr3+ ions, utilizing more energy from host absorption for the photoluminescence of Cr3+ ions. As-prepared Cr3+:C-CaIn₂O₄ exhibits two bands at 270 nm and 370 nm, which are shifted to at 265 and 370 nm when it is annealed at higher dosage of Cr3+ ions. These are the ligand-to-metal O2- to Cr3+ charge transfer bands. The light-emission is studied in analyzing migration, transfer and recombination processes of light-induced e--h+ pairs. The results are described in correlation to a Cr3+:C-CaIn₂O₄ core-shell nanostructure.

Singlet exciton fission via an intermolecular charge transfer state in coevaporated pentacene-perfluoropentacene thin films.

Singlet exciton fission is a spin-allowed process in organic semiconductors by which one absorbed photon generates two triplet excitons. Theory predicts that singlet fission is mediated by intermolecular charge-transfer states in solid-state materials with appropriate singlet-triplet energy spacing, but direct evidence for the involvement of such states in the process has not been provided yet. Here, we report on the observation of subpicosecond singlet fission in mixed films of pentacene and perfluoropentacene. By combining transient spectroscopy measurements to nonadiabatic quantum-dynamics simulations, we show that direct excitation in the charge-transfer absorption band of the mixed films leads to the formation of triplet excitons, unambiguously proving that they act as intermediate states in the fission process.

Optical Absorption by Charge-Transfer Molecules.

An analytical formalism is developed for the calculation of absorption of radiation by charge-transfer molecules with electronic density delocalized between the donor and acceptor parts. The theory consistently incorporates both the vibronic coupling to quantum intramolecular vibrations and electrostatic interactions with a classical polarizable medium. The formulation operates in terms of basis-invariant parameters and can be used for calculations based on both the localized diabatic states and delocalized adiabatic wave function produced by standard quantum-chemistry algorithms. The basis-invariant reorganization energy is particularly important, since it determines observable spectroscopic parameters, in contrast to the adiabatic reorganization energy based on the molecular charge distributions in the adiabatic vacuum states. Analytical formulas are derived for the charge-transfer absorption band and first two spectral moments connecting spectroscopy to properties of charge-transfer molecules. Electronic delocalization requires a new definition of the Huang-Rhys factor traditionally used to construct vibronic band shapes. Theory's performance is illustrated by application to experimental data for self-exchange charge-transfer optical absorption and for metal-to-ligand emission transitions.

2,4,5-Triaryl imidazole probes for the selective chromo-fluorogenic detection of Cu(II). Prospective use of the Cu(II) complexes for the optical recognition of biothiols

The sensing behaviour toward metal cations and biothiols of two 2,4,5-triarylimidazole probes (3a and 3b) is tested in acetonitrile and in acetonitrile–water. In acetonitrile the two probes present charge-transfer absorption bands in the 320–350 nm interval. Among all cations tested only Cu(II) is able to induce bathochromic shifts of the absorption band in the two probes, which is reflected in marked colour changes. Colour modulations are ascribed to the formation of 1:1 Cu(II)–probe complexes in which the cation interacts with the imidazole acceptor heterocycle. Besides, the two probes present intense emission bands (at 404 and 437 nm for 3a and 3b respectively) in acetonitrile that are quenched selectively by Cu(II). Probe 3a is soluble in acetonitrile–water 1:1 (v/v) and Cu(II) also induces bathochromic shifts of the absorption bands. Moreover, the emission bands of probe 3a in this mixed aqueous solutions is quenched in the presence of Cu(II). The potential use of the 1:1 complex formed between 3a and Cu(II) for the chromo-fluorogenic detection of biothiols (GSH, Cys and Hcy) in aqueous environments is also tested. At this respect, addition of GSH, Cys and Hcy to acetonitrile–water 1:1 v/v solutions of 3a–Cu(II) complex induces a hypsochromic shift of the visible band (reflected in a bleaching of the solutions) with a marked emission increase at 470 nm.

The Nature of Anomalous Red Luminescence in K2Al2B2O7 : Fe Nonlinear Optical Crystals

Spectra of anomalously intense luminescence at 1.675 eV (740 nm, FWHM = 0.173 eV) have been experimentally recorded for the first time in wide-band (180–3600 nm) K2Al2B2O7 nonlinear optical crystals upon excitation in the spectral region of 4–5 harmonics of the fundamental frequency of Nd3+ radiation. It is shown by low-temperature (7–80 K) luminescence–optical spectroscopy under selective excitation by synchrotron radiation that red luminescence arises at the intracenter 4T1(4G) → 6A1(6S) transitions in Fe3+ impurity ions with a concentration below 2 ppm. The high excitation efficiency in the exciton region is due to partial overlap of the fundamental absorption edge of the crystal, where mobile excitons are excited, and wide O–Fe charge-transfer absorption band peaking at 6.5 eV. The luminescence spectrum and temperature dependence are caused by the superposition of the luminescence centers of two types (activation energies 9 and 20 meV) based on Fe3+ impurity ions located at regular Al3+ sites of two nonequivalent Al2O7 clusters.

Phenothiazine derivatives - synthesis, characterization, and theoretical studies with an emphasis on the solvatochromic properties

A series of ethynylated N-octylphenothiazine aldehydes (PTZ1–PTZ4) with terminal substituents of different electronic ability (p-methoxyphenyl, p-fluorophenyl, p-trifluoromethylphenyl and 2,5-bis(trifluoromethyl)phenyl) acting as A/D-π-D-A system were synthesized and fully characterized. The influence of these terminal groups on their photophysical, thermal, and biological properties was thoroughly examined and confronted with theoretical calculations. The novel compounds exhibit high thermal stability up to 238–318 °C. Compounds showed an intramolecular charge transfer (ICT) absorption band at about 399 nm and emission band at 535 nm with quantum yields between 40 and 63% in chloroform solution. All compounds exhibit positive solvatochromism in the full range of solvent polarities from hexane to DMSO, whereas in the methanol solution they demonstrate the hypsochromic shift of emission maximum compared to DMSO with significant fluorescence quenching. PTZ1–PTZ4 compounds possess a significant emission from blue to yellow region 488–542 nm, high quantum yields 24–91%, and lifetimes ranging from 6.03 to 8.56 ns in solvents of different polarity. The marked positive solvatochromism indicates the considerable ICT in PTZ1–PTZ4 compounds. A noticeable increase in dipole moment in the excited state calculated in various solvent causes o significant red shift in the emission spectrum of PTZ1–PTZ4. This work demonstrates the strong variation of photophysical properties with a simple alteration of the chemical structure of the phenothiazine framework by terminal groups. The toxicity of the compounds that were obtained was evaluated on cancer as well as normal fibroblast cell lines. The applicatory potential of those phenothiazines as cellular probes have been also evaluated.

Impact of Benzannulation Site at the Diimine (N^N) Ligand on the Excited-State Properties and Reverse Saturable Absorption of Biscyclometalated Iridium(III) Complexes.

Ten biscyclometalated monocationic Ir(III) complexes were synthesized and studied to elucidate the effects of extending π-conjugation of the diimine ligand (N^N = 2,2'-bipyridine in Ir1, 2-(pyridin-2-yl)quinoline in Ir2, 2-(pyridin-2-yl)[6,7]benzoquinoline in Ir3, 2-(pyridin-2-yl)-[7,8]benzoquinoline in Ir4, phenanthroline in Ir5, benzo[ f][1,10]phenanthroline in Ir6, naphtho[2,3- f][1,10]phenanthroline in Ir7, 2,2'-bisquinoline in Ir8, 3,3'-biisoquinoline in Ir9, and 1,1'-biisoquinoline in Ir10) via benzannulation at 2,2'-bipyridine on the excited-state properties and reverse saturable absorption (RSA) of these complexes. Either a bathochromic or a hypsochromic shift of the charge-transfer absorption band and emission spectrum was observed depending on the benzannulation site at the 2,2'-bipyridine ligand. Benzannulation at the 3,4-/3',4'-position or 5,6-/5',6'-position of 2,2'-bipyridine ligand or at the 6,7-position of the quinoline ring on the N^N ligand caused red-shifted charge-transfer absorption band and emission band for complexes Ir2, Ir8, Ir10 vs Ir1 and Ir3 vs Ir2, while benzannulation at the 4,5-/4',5'-position of 2,2'-bipyridine ligand or at the 7,8-position of the quinoline ring on the N^N ligand induced a blue shift of the charge-transfer absorption and emission bands for complex Ir9 vs Ir1 and Ir4 vs Ir2. However, benzannulation at the 2,2',3,3'-position of 2,2'-bipyridine or 5,6-position of phenanthroline ligand had no impact on the energy of the charge-transfer absorption band and emission band of complexes Ir5-Ir7 compared with those of Ir1. The observed phenomenon was explained by the frontier molecular orbital (FMO) symmetry analysis. Site-dependent benzannulation also impacted the spectral feature and intensity of the triplet transient absorption spectra and lifetimes drastically. Consequently, the RSA strength of these complexes varied with a trend of Ir7 > Ir5 ≈ Ir6≈Ir1 > Ir3 > Ir2 > Ir10 > Ir4 > Ir8 > Ir9 at 532 nm for 4.1 ns laser pulses.

Antibacterial Polymeric Films Fabricated by [2+2] Cycloaddition-Retroelectrocyclization and Ag+ Ion Coordination.

The [2+2] cycloaddition-retroelectrocyclization (CA-RE) between N,N-dialkylaniline-substituted alkynes and 7,7,8,8-tetracyanoquinodimethane (TCNQ) is employed to fabricate functional cross-linked polymer films containing the intramolecular charge-transfer (CT) chromophores at the cross-linking points. Polystyrene bearing N,N-dialkylaniline-substituted alkynes (P1) and TCNQ polyester (P2) are mixed in tetrahydrofuran (THF), then this solution is spray-coated onto an indium tin oxide or glass plate. Heating to 100°C initiates the [2+2] CA-RE reaction, resulting in the formation of cross-linked polymer films. The reaction progress and completion are evaluated by monitoring the CT absorption band and cyano vibration peaks. The resulting cross-linked polymer films show reversible cathodic electrochromism between the neutral and anion radical states. In addition, they also display the visual detection behavior of protic acids and Lewis acids, such as Ag+ ions. Accordingly, the Ag+ ion-loaded polymer films are prepared, and their antibacterial activities are studied. As more Ag+ ions are loaded, the CT band more bathochromically shifts and more potent antibacterial activities are obtained. Therefore, the antibacterial activity of the polymer films can be visually recognized by the film colors. Furthermore, the loaded Ag+ ions can be released from the polymer films by application of an electrochemical potential.

Spin–Orbit Charge Recombination Intersystem Crossing in Phenothiazine–Anthracene Compact Dyads: Effect of Molecular Conformation on Electronic Coupling, Electronic Transitions, and Electron Spin Polarizations of the Triplet States

Phenothiazine (PTZ)–anthracene (An) compact electron donor/acceptor dyads were synthesized. The molecular conformation was constrained by rotation restriction to achieve an orthogonal geometry between the electron donor (PTZ) and the electron acceptor (An), with the aim to enhance the spin–orbit charge-transfer intersystem crossing (SOCT–ISC). The substitution positions on the PTZ and An moieties were varied to attain dyads with different mutual orientations of the donor/acceptor as well as different rotation-steric hindrances. The electronic coupling strengths between the electron donor and the acceptor were quantified with the matrix elements (VDA, 0.04–0.18 eV); the smallest value was observed for the dyad with orthogonal geometry. Charge-transfer absorption and fluorescence emission bands were observed for the dyads, for which the intensity varied, manifested by the VDA values. The fluorescence of the An moiety was significantly quenched in the dyads, efficient ISC, and the formation of the triplet st...

Effects of Pyrazine Derivatives and Substituted Positions on the Photoelectric Properties and Electromemory Performance of D⁻A⁻D Series Compounds.

Pyrazine derivatives quinoxaline and pyridopyrazine were selected as the acceptors, and benzocarbazole was used as the donor to synthesize four different D–A–D compounds. The results showed that 2,3-bis(decyloxy)pyridine[3,4-b]pyrazine (DPP) exhibited stronger electron-withdrawing ability than that of 2,3-bis(decyloxy)quinoxaline (DPx), because DPP possesses one more nitrogen (N) atom, resulting in a red-shift of the intramolecular charge transfer (ICT) absorption bands and fluorescent emission spectra for compounds with DPP as the acceptor compared with those that use DPx as the acceptor. The band-gap energy (Eg) of the four D–A–D compounds were 2.82 eV, 2.70 eV, 2.48 eV, and 2.62 eV, respectively, for BPC-2DPx, BPC-3DPx, BPC-2DPP, and BPC-3DPP. The solvatochromic effect was insignificant when the four compounds were in the ground state, which became significant in an excited state. With increasing solvent polarity, a 30–43 nm red shift was observed in the emissive spectra of the compounds. The thermal decomposition temperatures of the four compounds between 436 and 453 °C had very high thermal stability. Resistor-type memory devices based on BPC-2DPx and BPC-2DPP were fabricated in a simple sandwich configuration, Al/BPC-2DPx/ITO or Al/BPC-2DPP/ITO. The two devices showed a binary non-volatile flash memory, with lower threshold voltages and better repeatability.