Intramolecular Charge Transfer Transition Research Articles

A donor–acceptor cruciform π-system: high contrast mechanochromic properties and multicolour electrochromic behavior

A donor–acceptor (D–A) cruciform conjugated luminophore DMCS-TPA was designed and synthesized. The DMCS-TPA solid shows both aggregation induced emission (AIE) effect and high contrast mechanochromic (MC) behavior with a remarkable spectral shift of 87 nm. The obvious fluorescence switching from yellowish green to orange can be realized by pressing at only 10 MPa or simply grinding. The photophysical properties, theory calculation and XPS results demonstrate that the extension of the conjugation length and subsequent enhancement of intramolecular charge transfer (ICT) transition are responsible for the improved MC performance. In addition, DMCS-TPA is readily deposited on the ITO electrode surface by the electrochemical method to form an electrochromic (EC) film with multiple colours showing (light green at 0 V, red at 1 V, grey at 1.1 V and blue at 1.45 V) and a high optical contrast of 65% at 769 nm. The results suggest that incorporation of electroactive moieties into luminophores to constitute D–A cruciform conjugated structures is a promising design strategy for preparing dual functional materials combining MC and EC properties.

Theoretical Investigations of Tetrathiafulvalene Derivative as Electron Donor in Organic Dye for Dye-Sensitized Solar Cells

To investigate the effect of a tetrathiafulvalene (TTF) unit on the photovoltaic properties of the corresponding dye sensitizer, a TTF-carbazole-based sensitizer, Dye 2, was designed; it was based on the framework of Dye 1. The geometries, electronic structures, and optical properties of Dye 1 and Dye 2 before and after binding to (TiO2)(9) clusters were investigated using density functional theory (DFT) and time-dependent DFT. The surface morphologies of the dyes on TiO2 (101) surfaces were simulated by periodic DFT calculations using the DMol(3) program. The calculated results showed that the introduction of TTF units into dyes could help to inhibit dye aggregation on the TiO2 surface; this is conducive to intramolecular charge-transfer transitions and significantly improves the light-harvesting ability. The calculated results demonstrate that the TTF unit is a very promising electron donor for improving the photovoltaic properties of organic dye sensitizers.

Probing the performances of HISS functionals for the description of excited states of molecular systems

The performances of two range-separated functionals (HISS-A and HISS-B), recently developed by Savin and collaborators, and including a fraction of HF exchange for an intermediate range of interelectronic distance, have been tested for the prediction of vertical excited state energies over three benchmark sets of molecules mainly probing valence and intramolecular charge-transfer transitions in small- to medium-size organic compounds. The results obtained show that HISS-B outperforms HISS-A providing results that are, for singlet-to-singlet excitations, at least in line with those of more traditional long-range-separated hybrids such as LC-PBE or CAM-B3LYP possessing the correct asymptotic behavior and, in the case of larger compounds, close to those provided by global hybrids.

Theoretical study on a chemosensor for fluoride anion-based on a urea derivative

The sensing mechanism of the N-Phenyl-N′-(3-quinolinyl)urea (PQU) chemosensor for fluoride anion has been investigated by density functional theory/time-dependent density function theory. The double intermolecular hydrogen bonds are formed between the three anions (X−F−, AcO−, Cl−) and the urea fragment of PQU. In the S0 states, the HbX− hydrogen bonds are slightly stronger than the HaX− hydrogen bonds and the fluoride-induced deprotonation occurs at the NHb position rather than at the NHa position. Consequently, the absorption peaks, including an intramolecular charge transfer transition and a ππ* transition, are significantly red-shifted. Thermodynamic calculations confirm that the deprotonation in the ground state is favorable in energy only when excess fluoride anion exists. Along with the S0 → S1 transition, the HaX− hydrogen bonds strengthen and the HbX− hydrogen bonds weaken. However, the emission spectra of [PQU-Hb]−, instead of [PQU-Ha]−, are observed upon addition of fluoride anion. © 2013 Wiley Periodicals, Inc.

Anthryl-Substituted 3-Silylene-2-silaaziridine Obtained by Isomerization of Disilacyclopropanimine: An Exocyclic Silene Showing a Distinct Intramolecular Charge Transfer Transition

An anthryl-substituted exocyclic silene, 3-silylene-2-silaaziridine, was synthesized by isomerization of the corresponding disilacyclopropanimine. The UV-vis spectrum of the silene shows a distinct intramolecular charge transfer (ICT) transition from the π orbital of the Si═C double bond to the π* orbital of the anthryl moiety. The relatively high-lying π(Si═C) orbital of the 3-silylene-2-silaaziridine moiety and the low-lying π* orbital of the anthryl group would be responsible for the distinct ICT band.

Carbazole/Sulfone Hybrid D-π-A-Structured Bipolar Fluorophores for High-Efficiency Blue-Violet Electroluminescence

Based on a D-π-A structural strategy incorporating carbazole as a mild electron-donor and sulfone as an electron-acceptor with a π-conjugation-breaking feature, two novel blue-violet emitting materials (CzS1 and CzS2) were successfully designed and synthesized. The two compounds exhibit high-efficiency fluorescent emissions of intramolecular charge-transfer transition type, with impressively high quantum yields in both solution and film states. CIEy below 0.06 and excellent current/power efficiencies up to 1.89 cd A–1/1.58 lm W–1 were achieved with their corresponding nondoped devices. These performances currently represent the best results for OLEDs with CIEy < 0.06. Moreover, single-carrier devices were also fabricated to demonstrate the bipolar characteristics as well as to understand the different electroluminescence performance of the two fluorophores.

Investigation on the linear and nonlinear optical properties of fluorenone-based linear conjugated oligomers: The influence of π-spacer

Abstract Effect of π-spacer on the optical properties of two novel D-π-A-π-D type fluorenone-based linear conjugated oligomers 2,7-di((E)-2-(10-octyl-10H- phenothiazin-3-yl)vinyl)-9-fluorenone (P-FO-P) and 2,7-di((E)-2-(9,9-dioctyl-7-((E)-2-(10-octyl-10H-phenothiazin-3-yl)vinyl)-9H-fluoren-2-yl)vinyl)-9-fluorenone (P-F-FO-F-P) is investigated, where fluorenone and phenothiazine units act as the electron acceptor and donor moieties, respectively. We observe that the fluorene–ethylene (FE) spacer could modulate the steady-state spectral features. Quantum chemical calculations offer complementary information regarding the frontier orbitals and molecular structures, which confirm that the intramolecular charge transfer (ICT) transition exists in the HOMO–LUMO transitions. Nonlinear optical measurements show that the FE spacer could improve the two-photon photoluminescence yield and enhance the two-photon absorption cross-section. The results of transient absorption measurements exhibit that an ICT state exists in the relaxation process of excited state, and its generation process is obviously accelerated with the introduction of π-spacer.

Synthesis, chemical–optical characterization and solvent interaction effect of novel fluorene-chromophores with D–A–D structure

Abstract The synthesis, chemical characterization and optical studies of three novel fluorene derivatives is reported. These compounds comprise a D–A–D architecture with fluorene moieties as donor groups and fluorenone or benzothiadiazole derivatives as acceptor groups. Theoretical analysis confirmed the existence and the nature of two principal electronic transitions (π → π* and intramolecular charge transfer). Spectroscopic studies in solution revealed that the intramolecular charge transfer character, and in turn the two-photon activity i.e., the fluorescence induced by two-photon absorption, is strongly affected by solvent polarity. The influence of specific solvent–solute interactions over emission properties was also studied through Lippert–Mataga plot. Evaluation of the two-photon absorption cross-sections, gave a maximum value of 105 GM (1 GM = 10−50 cm4 s) in toluene and a minimum value of 23 GM in THF solutions at 750 nm for the fluorenone derivative, a molecule with poor intramolecular charge transfer character and thus weak two-photon absorption; however the benzothiadiazole derivative, with stronger intramolecular charge transfer transition, produced a maximum value of 1000 GM in THF and a minimum value of 236 GM in methanol. Fluorescence quantum efficiency of these compounds was also affected by the medium, with fluorescence quenching in protic solvents such as methanol due to specific solvent interactions (i.e., hydrogen-bonding). Nevertheless, in a more polar medium such as water, nanoaggregates synthesized from the benzothiadiazole derivative exhibited good two-photon activity, i.e., ∼500 GM and fluorescence quantum efficiency of 0.83. Furthermore, these nanoaggregates exhibited more resistance against photodegradation processes than any of the organic solutions tested.

Tuning of the H-bonding ability of imidazole N–H towards the colorimetric sensing of fluoride and cyanide ions as their sodium salts in water

Imidazole functionalized receptors, 2-R-1H-naphtho[2,3-d]imidazole-4,9-dione (2a–h), containing naphthoquinone as a chromogenic signalling unit have been synthesized from the reaction of 2,3-diaminonaphthoquinone and different aldehydes. These receptors showed a color change upon addition of fluoride and cyanide ions in DMSO with a bathochromic shift of the characteristic intramolecular charge transfer (ICT) transition band. No color change was observed upon addition of other anions such as Cl−, Br−, I−, NO3−, AcO− and H2PO4−. 1H NMR and electrochemical studies revealed that these receptors sense fluoride and cyanide ion via the formation of H-bond with the imidazole N–H moiety. Electronic and spectrofluorimetric studies indicated that the binding constants of these receptors with F− and CN− ions were in the order of ∼106. The results of the spectral studies indicated that, by changing the R group in the receptor, the acidity of the imidazole N–H can be varied from δH 13.70 (for isopropyl) to 14.94 ppm (for thiophene). Theoretical calculations based on Density Functional Theory showed that the HOMO–LUMO energy gap for the ICT transition corroborate the results of the spectral studies. Receptor 2f (R = thiophene) was also able to detect fluoride and cyanide ions as their sodium salts in aqueous solution with a visual color change.

Theoretical Study of Intramolecular Charge Transfer Turn-off Switching of N-(4-Dimethylaminobenzoyl) Thiourea Induced by Anion Recognition

The development of colorimetric and fluorescence chemical sensors that have a function of in situ selective sensing of biologically important anions is one of the significant issues in host-guest chemistry, and a large number of studies have been carried out in the field. The fluorescent chemical sensor, N-(4-Dimethylaminobenzoyl)thiourea (DMABTU; Chart 1) that has been reported by Wu et al. [1], has two emission wavelengths at ca. 500 nm (stronger) and ca. 350 nm (weaker)) in the absence of anions in chloroform solution. By the addition of anions such as acetate, the emission band at 500 nm decreases while the emission band at 350 nm increases. The broad emission band at around 500 nm was assigned to an intramolecular charge transfer (ICT) transition, and the narrow emission band at around 350 nm to a locally excited (LE) transition. However, a detailed theoretical analysis has not been performed yet on the turn-off switching mechanism of ICT induced by the anion recognition. Therefore, in this work, to clarify the ICT turn-off switching mechanism, we carried out a density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on DMABTU in the ground (S0) and excited (S1) states. We obtained optimized structures of DMABTU-X−(X− = free, acetate) in S0 and S1, and also obtained potential-energy curves (PEC) with respect to a dihedral angle δ between the dimethylamino-group and phenyl-group planes. From the calculations, it was proved that the PEC of DMABTU is not quadratic but nearly flat between δ = 0∼10 degrees, and the potential minimums are located at δ = ca. 8∼9 degrees both in S0 and S1, while the PEC of DMABTU-AcO- is quadratic, and DMABTU-AcO- takes twisted structures (δ = ca. 12 degree in S0 and δ = ca. 21 degree in S1). These results suggest that ICT character of DMABTU is not a twisted ICT (TICT) but a planar ICT (PICT) state.

Strong intramolecular charge transfer emission in benzobisoxazole cruciforms: solvatochromic dyes as polarity indicators

A photophysical study was carried out on a series of nine X-shaped cross-conjugated compounds based on the benzobisoxazole nucleus. These "molecular cruciforms" were terminally substituted with electron-rich 4-(N,N-dimethylamino)phenyl, electron-neutral phenyl or electron-poor 4-pyridyl groups. Intramolecular charge transfer (ICT) transitions in molecular cruciforms were shown to be activated to a different extent by the combinations of these substituents located in different positions along vertical and/or horizontal axes of the molecules, showing strong emission throughout the visible spectrum. The high fluorescence efficiency and bathochromic shifts associated with the increasing polarity of the solvent make these cruciforms interesting candidates for use as fluorescence polarity indicators operating in a broad solvatochromic range.

Near‐Infrared Responsive Conjugated Polymers to 1.5 μm and Beyond: Synthesis and Electrochromic Switching Application

Azulene-containing conjugated polymers with near-infrared absorption up to 1.5 μm and beyond are achieved by treating with trifluoroacetic acid (TFA). Density functional theory calculations reveal that the near-infrared absorption arises from a strong intramolecular charge transfer transition on the polymer backbone, and the near-infrared absorption can be tuned by the degree of protonation. Furthermore, TFA treated polymers show a ten-fold enhancement in electrochromic contrast and significantly improved switching stability, suggesting that these polymers are promising candidates for fabrication of the first generation organic near-infrared devices.

Ligand-Based Charge-Transfer Luminescence in Ionic Cyclometalated Iridium(III) Complexes Bearing a Pyrene-Functionalized Bipyridine Ligand: A Joint Theoretical and Experimental Study

Two new heteroleptic iridium(III) complexes [Ir(ppy)(2)(pyr(2)bpy)][PF(6)] ([1a][PF(6)]) and [Ir(dfppy)(2)(pyr(2)bpy)][PF(6)] ([2a][PF(6)]), where Hppy = 2-phenylpyridine, Hdfppy = 2-(3,5-difluorophenyl)pyridine, and pyr(2)bpy = 5,5'-bis(pyren-1-yl)-2,2'-bipyridine, have been synthesized and fully characterized. The single-crystal structures of pyr(2)bpy and the complexes 4{[1a][PF(6)]}·2CH(2)Cl(2)·9H(2)O and [2a][PF(6)]·0.25CH(2)Cl(2)·H(2)O have been determined. The effect of the pyrene substituents on the electronic properties is investigated through a comprehensive photophysical and theoretical study on the two complexes in comparison to reference complexes without substituents on the ancillary ligand ([1][PF(6)] and [2][PF(6)]) and by making absorption and luminescence titrations of ligand pyr(2)bpy. Both theory and experiment show that the intense and broad band appearing in the 400-500 nm region of the absorption spectra of [1a][PF(6)] and [2a][PF(6)] is due to intramolecular charge-transfer (ICT) transitions from the pyrene substituents to the bipyridine ligand. [1a][PF(6)] and [2a][PF(6)] exhibit luminescence bands centered above 650 nm, attributed to a charge-transfer triplet state located on the pyr(2)bpy ligand, lying at lower energy than the strongly emitting Ir-ppy→bpy triplet states of the complexes lacking the pyrene fragments. Such luminescence, detected both at room temperature and 77 K, shows that the appendage of luminophoric moieties to luminescent Ir-based centers may further widen the emission tuneability of this exploited class of luminescent materials through purely electrostatic effects exerted on a properly designed N^N ancillary ligand.

Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)-annulated porphyrins, and their corresponding Zn(II) complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge-transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge-transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square-wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis-substituted TTF-porphyrin (both free-base and Zn(II) complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent-dependent charge-transfer transitions, it is the π-acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square-wave voltammograms. In the case of the free-base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF(⋅+) generated through one-electron oxidation, can interact with each other; this gives rise to multiple signals in the square-wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF(⋅+) and TTF(2+), are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO-LUMO gap. To probe the excited-state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF-porphyrins of this study display relatively short excited-state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge-transfer effects inferred from the ground-state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF-porphyrins and support the proposed intramolecular charge-transfer interactions and π-acceptor effects.

Methanolysis of 7,7,8,8-Tetracyanoquinodimethane: A Consequence of an Initial Charge Transfer Complexation

7,7,8,8-Tetracyanoquinodimethane (TCNQ) has several π-bonds and four nitrogen atoms having four unshared electron pairs. This unique structure enhances TCNQ to perform intramolecular charge transfer (CT) transitions within its molecular moiety and/or intermolecular CT interactions with different species having electron-donor properties. The electron affinity 1 of TCNQ is 2.8 eV which permits it to be one of the well known π-acceptors. 2-4 In addition, TCNQbased charge transfer salts are well known too. 5,6 Thus, through the interaction of TCNQ with electron donors it should be considered the simultaneous competing transitions between the intra-transitions (within TCNQ molecule) and the inter-transitions form the donors toward TCNQ. On the other hand, due to the unshared electron pairs on the oxygen atom, MeOH has been used as n-donor in several CT interactions. 2,7-9 Thus, the thing is, on considering a CT interaction between an electron donor and TCNQ (as an electron acceptor) in the presence of MeOH as a medium (solvent) for this interaction, an additional factor should be considered too: the probable occurrence of an addition reaction of MeOH molecule to TCNQ molecule in a nucleophilic attack (as a result of a probable MeOH/TCNQ CT interaction) in competition with the main electron donor/ TCNQ CT interaction. In turn, it is worthwhile to investigate the absorption spectral behavior of the sole MeOH/TCNQ interaction especially from the kinetic standpoint of view. For this purpose this piece of work has been devoted. Experimental Section

Ratiometric fluorescence sensing of fluoride ions by triarylborane–phenanthroimidazole conjugates

Abstract Triarylborane–phenanthroimidazole conjugates 1 and 2 linked by a biphenylene and phenylene group, respectively, were prepared and characterized. Both compounds are highly fluorescent ( Φ = 0.49–0.74 for 1 and Φ = 0.71–0.92 for 2 ) and show a positive solvatochromism. Upon fluoride binding to the boron center, they exhibit a ratiometric fluorescence response (Δ λ em = 84 nm for 1 and 63 nm for 2 in acetone) accompanying a vivid emission color change. Photophysical data and TD-DFT studies suggest that intramolecular charge transfer (ICT) transition from a phenanthroimidazole donor to a borane acceptor in the neutral 1 is switched to π–π* transition of phenanthroimidazole moiety after fluoride binding. Due to the greater elevation of LUMO level by fluoride complexation, the resulting fluoride adduct [ 1 F] − fluoresces at the apparently blue-shifted region.

Synthesis, structure, and luminescence properties of arylpyridine-substituted terpyridine Zn(II) and Cd(II) complexes

Two terpyridine derivatives, 4′-(4-(pyridin-4-yl)phenyl)-[2,2′:6′,2″]terpyridine (L1) and 4′-(4′-(pyridin-4-yl)-[1,1′-biphenyl]-4-yl)-[2,2′:6′,2″]terpyridine (L2) have been synthesized by palladium-catalyzed cross-coupling reactions, and their metal complexes [Zn(L1)2](ClO4)2·2MeOH·CHCl3 (1) and [Cd(L2)2](ClO4)2·DMF·2.8H2O (2) characterized by single crystal X-ray crystallography and spectroscopic methods including electronic absorption and emission measurements. In the crystalline materials, both metal ions have the expected pseudo-octahedral N6 coordination environment and the lattice structures appear to be influenced by aromatic–aromatic interactions involving the heteroaryl tails and the pyridyl rings of the terpyridine units. In solution and in the solid state, the complexes display blue emission attributed to an intramolecular charge transfer transition.

Rare Example of TICT Based Optical Responses for the Specific Recognition of Cr3+ by a 2,2′:6′,2″-Terpyridine Derivative and Demonstration of Multiple Logic Operations

Chemosensor L showed a nonlinear fluorescence response on specific binding to Cr3+ ion in the presence of various alkali, alkaline-earth, transition, and lanthanide metal ions. A luminescence band with maxima at 512 nm for L was observed (λext = 330 nm) for a twisted intramolecular charge transfer (TICT) transition following antienergy gap law behavior. However, normal energy gap law behavior prevailed on formation of a new nonluminescent charge transfer complex, Mn+·L. For paramagnetic metal ions, paramagnetism induced luminescence quenching could have also contributed to this. A new switched on fluorescence response at λems of 356 nm (λext of 330 nm) was observed due to Franck–Condon charge transfer (FC-CT) transition only on the formation of a complex, (Cr3+)2·L. Spectral studies revealed a unique dynamic coordination behavior and migration of Cr3+ from the terpyridyl fragment to the NNMe2 center of L as a function of the varying concentration of another ion (Zn2+) and the subtle difference in the binding affinities of the terpyridyl moiety toward Cr3+ and Zn2+. Further, spectral responses of L toward Zn2+, different concentration of Cr3+, H+ and on subsequent addition of F– as different ionic inputs could be correlated well for demonstrating various basic and combinatorial circuits.

Highly selective colorimetric sensing of Cu(II) ions in aqueous solution via modulation of intramolecular charge transfer transition of aminonaphthoquinone chemosensor

An aminonaphthoquinone based colorimetric chemosensor has been developed and demonstrated for the highly selective detection of Cu(II) ions in aqueous solution. The intramolecular charge transfer (ICT) transition exits in amine moiety directly attached to the quinone ring is modulated by the d–d transition of a square planar Cu(II)-receptor complex resulting in a change of color from yellow to blue. No significant color change was observed upon addition of other selected metal ions. The sensing property has been investigated using various spectral techniques (UV–Vis, fluorescence) and product analysis (Elemental analysis, magnetic moment, UV–Vis, FT-IR, EPR).

Three Transition Metal Complexes Based on a Rigid Ligand Containing Bis(2-Benzimidazolyl) Groups: Syntheses and Crystal Structures

Three new complexes ([Cu(OBimB)2](Cl)2·(EtOH)2·(H2O)2 1, [Cu(OBimB)2](NO3)2·(EtOH)2 2, [Ni(OBimB)2](NO3)2·(MeCN)2 3 based on a rigid bis(benzimidazoles) ligand OBimB (OBimB = 1,2-bis(2-benzimidazolyl)benzene) have been synthesized and structurally characterized by X-ray single crystal diffractometry. In the structures of the three complexes each metal ion is coordinated by four N atoms from two twisted V-shaped conformational OBimB ligands in a square-planar geometry. The complex units are further linked into a 2D layer supramolecular network by hydrogen bonds and π···π stacking interactions. The electronic absorption spectra of 1, 2, 3 have also been investigated and show that the two transitions around 220 and 280 nm are mainly assigned to intramolecular charge transfer transitions of the ligand (IL).