Intramolecular Charge Transfer Transition Research Articles

Intriguing Indium-salen Complexes as Multicolor Luminophores.

The series of novel salen-based indium complexes (3-tBu-5-R-salen)In-Me (3-tBu-5-R-salen = N,N'-bis(2-oxy-3-tert-butyl-5-R-salicylidene)-1,2-diaminoethane, R = H (1), tBu (2), Br (3), Ph (4), OMe (5), NMe2 (6)) and [(3-tBu-5-NMe3-salen)In-Me](OTf)2 (7; OTf = CF3SO3-) have been synthesized and fully characterized by NMR spectroscopy and elemental analysis. All indium complexes 1-7 are highly stable in air and even aqueous solutions. The solid-state structures for 3-5, which were confirmed by single-crystal X-ray analysis, exhibit square-pyramidal geometries around the indium center. Both the UV/vis absorption and PL spectra of 1-7 exhibit significant intramolecular charge transfer (ICT) transitions based on the salen moieties with systematically bathochromic shifts, which depend on the introduction of various kinds of substituents. Consequently, the emission spectra of these complexes cover almost the entire visible region (λem = 455-622 nm).

Facile Route to Quadruply Annulated Borepins.

A two-step synthesis sequence furnishes quadruply annulated borepins in high yields. The first step involves a nucleophilic substitution reaction between aryl-BF3K salts (aryl = mesityl, phenyl) and lithiated bromonapthalene derivatives LiNaphBr,R (HNaphBr,R = 8-bromonaphthalene (a), 5-bromoacenaphthene (b), 5-bromoacenaphthylene (c)). In the second step, the resulting heteroleptic triarylboranes aryl-B(NaphBr,R)2 (3a-c) are subjected to an intramolecular Ni-mediated Yamamoto reaction to close the seven-membered rings and create the borepins 4a-c. Only in the case of 3b is the Yamamoto reaction accompanied by a C-H activation reaction furnishing the 7-hydro-7-borabenzo[de]anthracene derivative 5. The product ratio 4b/5 can be influenced by control of the local Ni(0) concentration. The borepins 4a-c are benchtop stable and highly soluble even in hexane. Compounds 4a-c undergo reversible one-electron reduction; 4c is also able to accept a second electron in a reversible manner and already at moderate potential values (E1/2 = -1.49 V and -1.84 V (vs FcH/FcH+)). 4a, 4b, and 5 show photoluminescence in the blue-green region of the spectrum, while 4c is nonfluorescent, which is likely attributable to an intramolecular charge-transfer transition.

Charge-Transfer Emitting Triarylborane π-Electron Systems.

Triarylboranes have attracted significantly increasing research interest as a remarkable class of photoelectronic π-electron materials. Because of the presence of vacant p orbital on the B center, the boryl group is a very unique electron acceptor that exhibits not only electron-accepting ability through p-π* conjugation but also high Lewis acidity to coordinate with Lewis bases and steric bulk arising from the aryl substituent on the B center to get enough kinetic stability. Thus, the incorporation of a trivalent B element into π-conjugated systems is an efficient strategy to tune the electronic and stereo structures and thus the photoelectronic properties of π-electron systems. When an electron-donating group, such as amino, is present, triarylboranes would likely display intramolecular charge-transfer transitions. These kinds of molecules are often highly emissive. In addition, the geometry of the molecules has a great impact on the emission properties. In this Forum Article, we herein describe our recent progress on the charge-transfer emitting triarylborane π-electron systems with novel geometries, which include the lateral boryl-substituted π-system with amino groups at the terminal positions, the o,o'-substituted biaryl π-system with boryl and amino groups at the o,o'-positions, a triarylborane-based BODIPY system, and a B,N/S-bridged ladder-type π-system. We mainly put the emphasis on the molecular design concept, structure-property relationships, intriguing emission properties and great applications of the corresponding triarylborane π-systems.

Tuning for Visible Fluorescence and Near-Infrared Phosphorescence on a Unimolecular Mechanically Sensitive Platform via Adjustable CH-π Interaction.

CH-π interaction-assisted alignment of organic conjugated systems has played an important role to regulate molecular electronic and photophysical properties, whereas harnessing such a smart noncovalent interaction into the tuning of unimolecular complex emissive bands covering a wide spectral region remains a challenging research topic. Since the tuning for visible and near-infrared emissive properties in a single π-functional platform relates to its multicolor luminescent behaviors and potential superior application in analysis, bioimaging, and sensing, herein, we report a proportional control of the singlet and triplet emissions that cover visible and near-infrared spectral regions, respectively, can be straightforwardly achieved by CH-π interaction-assisted self-assembly at the unimolecular level. Employing an octathionaphthalene-based single luminophore as a prototype, we find that a strength-adjustable CH-π interaction-assisted self-assembly can be established in mixed DMF/H2O and in the film state. The hybridization of planar local excited and intramolecular charge transfer transitions occurs on the basis, allowing a competitive inhibition to the intersystem crossing process to generate a complex emission composed of visible fluorescence and near-infrared phosphorescence. Furthermore, reversible mechanochromic and mechanoluminescent conversions of the corresponding solid sample can both be observed to rely on a corresponding self-assembly alternation. These results can probably provide new visions for the development of future intelligent and multifunctional luminescent materials.

Heteroaryldisilenes: heteroaryl groups serve as electron acceptors for Si[double bond, length as m-dash]Si double bonds in intramolecular charge transfer transitions.

Although many stable disilenes (silicon-silicon doubly-bonded compounds) containing organic π-electron systems have been reported, stable disilenes with heteroaromatic groups remain rare. Herein, we report the synthesis of monodisilenyl-substituted thiophene 1, anthracene 4, acridine 5, and mesitylene 7 and bis(disilenyl)-substituted thiophene 2, 2,2'-bithiophene 3, and anthracene 6via reactions of dialkylmesityldisilenide 8 with the corresponding haloarenes. Disilenes 1-7 show absorption bands with contributions of intramolecular charge transfer (ICT) transitions from π(disilene) to π*(aryl). In the ICT transitions, (bi)thienyl groups as well as anthryl and acrydinyl groups serve as electron acceptors for the Si[double bond, length as m-dash]Si double bonds.

A Switch-On NIR Probe for Specific Detection of Hg2+ Ion in Aqueous Medium and in Mitochondria.

A new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based probe molecule (L) is synthesized for specific binding to Hg2+ ion in physiological condition with an associated luminescence ON response in the near-IR region of the spectrum. Appropriate functionalization in the 5-position of each of two pyrrole moieties with styryl functionality in a BODIPY core helped us in achieving the extended conjugation and a facile intramolecular charge transfer transition with a narrow energy gap for frontier orbitals. This accounted for a poor emission quantum yield for the probe molecule L. Binding to Hg2+ helped in interrupting the facile intramolecular charge transfer (ICT) process that was initially operational for L. This resulted in a hypsochromic shift of absorption band and a turn-on luminescence response with λMaxEms of 650 nm on specific binding to Hg2+. Observed spectral changes are rationalized based on quantum chemical calculations. Interestingly, this reagent is found to be localized preferentially in the mitochondria of the live human colon cancer (Hct116) cells. Mitochondria is one of the major targets for localization of Hg2+, which actually decreases the mitochondrial membrane potential and modifies various proteins having sulfudryl functionality(ies) to cause cell apoptosis. Considering these, ability of the present reagent to specifically recognize Hg2+ in the mitochondrial region of the live Hct116 cells has significance.

ChemInform Abstract: Triarylborane π‐Electron Systems with Intramolecular Charge‐Transfer Transitions

AbstractReview: [28 refs.

Low‐Symmetry Ω‐Shaped Zinc Phthalocyanine Sensitizers with Panchromatic Light‐Harvesting Properties for Dye‐Sensitized Solar Cells

Two low-symmetry phthalocyanines (Pcs) substituted with thiophene units at the non-peripheral (α) and peripheral (β) positions were synthesized and their optical, electronic-structure, and electrochemical properties were investigated. The substitution of thiophene units at the α positions of the phthalocyanine skeleton resulted in a red shift of the Q band and significantly modified the molecular-orbital electronic distributions just below the HOMO and just above the LUMO, with distortion of the typical Gouterman four-orbital arrangement of MOs. Two amphiphilic Ω-shaped ZnPcs (αPcS1 and αPcS2) bearing a π-conjugated side chain with an adsorption site at an α position of the Pc macrocycle were synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The absorption spectra of αPcS1 and αPcS2 showed red shifted Q bands and a broad band from 350 to 550 nm assignable to the intramolecular charge-transfer transition from the ZnPc core to the side chains. Time-dependent DFT calculations provided a clear interpretation of the effect of the thiophene conjugation on the typical phthalocyanine core π MOs. Compound αPcS1 was used as a light-harvesting dye on a TiO2 electrode for a DSSC, which showed a panchromatic response in the range 400-800 nm with a power conversion efficiency of 5.5 % under one-sun conditions.

Air-Stable Spirofluorene-Containing Ladder-Type Bis(alkynyl)borane Compounds with Readily Tunable Full Color Emission Properties.

A series of air-stable spiro-fused ladder-type boron(III) compounds has been designed, synthesized, and the electrochemistry and photophysical behavior have been characterized. By simply varying the substituents on the pyridine ring and extending the π-conjugation of the spiro framework, the emission color of these compounds can be easily fine-tuned spanning the visible spectrum from blue to red. All compounds exhibit a broad and structureless emission band across the entire visible region, assigned as an intramolecular charge-transfer transition originating from the thiophene of the spiro framework to the pyridine-borane moieties. In addition, these compounds demonstrate high photoluminescence quantum yields of up to 0.81 in dichloromethane solution and 0.86 in doped thin films. Some of the compounds have also been employed as emissive materials, in which solution-processed organic light-emitting devices (OLEDs) with tunable emission colors spanning the visible spectrum from blue, green to red have been realized, demonstrating the potential applications of these boron compounds in OLEDs.

Synthesis, Aggregation Induced Emission and Mechanochromic Luminescence of New β-Diketone Derivatives Bearing Tetraphenylene Moieties.

A series of β-diketone derivatives bearing tetraphenylene (TPE) moieties were synthesized and characterized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. All compounds exhibit broad absorption bands between 300 and 450nm, which are assigned to the 1π-π* transition of the conjugated system mixed intramolecular charge-transfer (ICT) transitions. Meanwhile, the emission of these compounds in solution at room temperature (λ em=458~509nm) can be attributed to the 1π,π*/1ICT state. Introduction of freely rotatable TPE to conventional β-diketone luminophors quenches their light emissions in the solutions, but endows these molecules with aggregation-induced emission (AIE) characteristics in the condensed phase due to the restriction of intramolecular rotation. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these β-diketone derivatives can be tuned by the appended substituents, which would be useful for rational design of AIE compounds with high solid state luminescence performance. Furthermore, these AIE-active compounds exhibited distinct piezofluorochromic properties and switched reversibly upon grinding-fuming. Their photophysical properties have been investigated with the aim to provide a basis for elucidating the structure-property correlations and developing new multi-stimuli responsive luminescent materials.

Polymerization of Thienothiophenes and Dithienothiophenes via Click-Reaction for Electronic Applications

In this work, donor−acceptor (D−A) copolymers of thienothiophenes and dithienothiophenes, having 1,2,3-triazole units were prepared via a click reaction. Their optoelectronic properties were explored systematically providing the optical and electrochemical band gaps varying from 2.2 to 2.8 eV. Density functional theory (DFT) investigations of the model compounds revealed the presence of an intramolecular charge transfer transition between the donor TT/DTT units and 1,2,3-triazole acceptor group.

Insights into the effect of donor-acceptor strength modulation on physical properties of phenoxazine-based imine dyes

Novel phenoxazine-based imine dyes having a donor-acceptor-donor (D-A-D) topology were designed, synthesized and thoroughly investigated with regard to their thermotropic, photo-physical and electrochemical behaviour. The hexyl-substituted phenoxazine was selected as donor moiety and diphenyl-1,3,4-oxadiazole or fluorene-containing moieties were employed as electron-acceptor groups, being connected via the imine bond. The supramolecular architecture from solution and in molten state has been investigated by wide angle X-ray diffraction, differential scanning calorimetry and polarized light microscopy. The imines formed distinctive thermotropic mesophases in close relationship with their structure. The molecular rigid rods of the synthesized dyes formed supramolecular layered structures that further combine into worm-like anisotropic primary particles in the condensed phase with a strong impact on the physical properties of the dyes. UV–Vis absorption, photoluminescence, and electrochemical measurements proved the ability of the imine bond to facilitate the intramolecular charge-transfer transition from the donor to the acceptor units. The dyes showed an inverted solvatochromic behaviour, the fluorescence emission being highly sensitive to solvent polarity, yielding blue to green and even orange light in different organic solvents. A change in the relative intensity of the photoluminescence with the excitation wavelength of those phenoxazine-based imine dyes has been detected. The fluorescence quantum yield in solution reached up to 63%, whilst in film state it significantly dropped. By incorporating segments with different electron-accepting capability, the HOMO–LUMO energy gap was modulated from 1.88 to 1.92 eV. The electrochemical features demonstrate that p- and n-type doping to the conducting state of these phenoxazine-based imine materials is achievable. As a consequence of interesting physical properties encountered in these dyes, applications in optoelectronic devices are foreseen.

Copolymers possessing dithienothiophene and boron for optoelectronic applications

In this work, copolymers of dithienothiophenes (DTT) and DTT‐4,4‐dioxide (DTT‐S,S‐O2) with mesitylboron were prepared for optoelectronic applications. Optical and electrochemical investigations were performed giving rise to the band gaps of 2.46–3.21 and 2.18–2.88 eV, respectively. Copolymers possessing DTT‐S,S‐O2 units demonstrated the lower band gaps. Density functional theory investigations of the model compounds revealed the presence of an intramolecular charge transfer transition between the donor DTT units and the boron acceptor atoms. POLYM. ENG. SCI., 56:1390–1398, 2016. © 2016 Society of Plastics Engineers

Triarylborane π -electron systems with intramolecular charge-transfer transitions

Abstract The incorporation of B element into π -conjugated system is an efficient strategy to tune the steric and electronic structure and thus optoelectronic properties of π -electron systems. The vacant p orbital on the tricoordinate B center makes it exhibit several electronic and steric features, such as electron-accepting ability through p- π * conjugation, the high Lewis acidity to coordinate with Lewis bases, as well as the steric bulk arising from the aryl substituent on the B center to get enough kinetic stability. As a result, the boryl group is a very unique electron acceptor. When an electron-donating amino group is present, the triarylboranes would display intense intramolecular charge transfer transitions, which lead to interesting optoelectronic properties and great utilities. This short review summarizes the recent progress in π -electron systems, which contain both B and N elements and thus display intramolecular charge-transfer transitions. The triarylboranes are introduced based on their structural features, including the linear π -system with boryl and amino groups at the terminal positions, the lateral boryl-substituted π -system with amino groups at the terminal positions, the biphenyl π -system with an amino and a boryl groups at o , o ′-positions, nonconjugated U- and V-shaped π -system, macrocylcic π -system with B and N embedded in the ring, B,N-bridged ladder-type π -system, as well as the polycyclic π -system with B embedded in the center.

Turmeric, naturally available colorimetric receptor for quantitative detection of fluoride and iron

Turmeric (Curcuma longa) an important food ingredient has been used from ancient times for various biological applications, but to date it has not been considered for the colorimetric detection of anions and cations. In this work, the application of turmeric solution for the colorimetric detection of F− and iron ions was demonstrated. Results showed a significant colour change from yellow to blue and yellow to orange upon addition of F− and iron ions respectively. The detection mechanism was investigated using curcumin, a major component of turmeric powder. The change in colour and fluorescence quenching was attributed to the formation of receptor complex with F− and iron ions which resulted in intramolecular charge transfer transition. The mechanism of binding has been confirmed by UV–vis, 1H NMR and fluorescence titrations. The present detection concept was further explored to develop a low cost, reusable F− detection kit, which showed its ability to detect F− ions in both aqueous and organic medium at a very low concentration such as 1ppm.

Novel Dimeric o-Carboranyl Triarylborane: Intriguing Ratiometric Color-Tunable Sensor via Aggregation-Induced Emission by Fluoride Anions

A dimeric o-carboranyl triarylborane compound (2) with a biphenylene bridge group was prepared and characterized. Also, its solid-state structure was determined via X-ray diffraction. Treatment of 2 with an excess amount of KF in the presence of 18-crown-6 formed a dimer-type potassium salt, [2·F2][K·18-crown-6]2; its structure was fully confirmed by multinuclear NMR spectroscopy. UV–vis titration experiments carried out in THF showed that 2 binds fluoride ions with a binding constant (K) of 8.5 × 105 M–1. The linear decline of the UV/vis absorption of 2 upon titration with fluoride suggested that the triarylborane moieties acted as independent binding sites, which were not affected by each other. Contrary to a single emission (λem = 376 nm) of 2 assignable to an intramolecular charge transfer (ICT) transition at 298 K in THF, a broad low-energy emission band was additionally observed at 77 K, which is dominant in the film state. The TD-DFT calculation on the first excited singlet state (S1) of 2 shows th...

Terpyridine-based donor–acceptor metallo-supramolecular polymers with tunable band gaps: Synthesis and characterization

Three new building blocks containing the electron-donor fused-ring motifs carbazole, dithienosilole (DTS) and dithienopyrrole (DTP) and the 2,2′:6′,2″-terpyridine electron-acceptor motif were designed and synthesized. Directed by transition metal ions, the self-assembly of the building blocks triggered polymerization to form the corresponding metallo-supramolecular polymers PCzTPY, PSiTPY and PNTPY, respectively. The UV–vis absorption maxima of the building blocks occur at long wavelengths (351, 368 and 430 nm for CzTPY, SiTPY and NTPY, respectively), which arises from intramolecular charge transfer (ICT) transitions. However, the absorption maxima of their corresponding metallo-supramolecular polymers are clearly red-shifted (to 394, 431 and 509 nm for PCzTPY, PSiTPY and PNTPY, respectively), which is caused by the incorporation of the transition metal ion into the backbones of the target polymers. Based on the above strategies, the resulting metallo-polymers exhibit reduced energy gaps, which are 2.07, 1.97 and 1.56 eV for the PCzTPY, PSiTPY and PNTPY metallo-supramolecular polymers, respectively.

Unprecedented intramolecular cyclization in strongly dipolar extended merocyanine dyes: A route to novel dyes with improved transparency, nonlinear optical properties and thermal stability

Novel magenta dyes were obtained via an unprecedented base-promoted intramolecular cyclization of strongly dipolar elongated merocyanine dyes bearing a phthalimide substituted tricyanopropylidene terminal segment. These shorter diarylpolyene derivatives retain a dipolar character and show a typical intramolecular charge transfer (ICT) transition. The cyclization noticeably modifies the electronic structure inducing both onset of bond length alternation and reduction of dipole moment as well as a marked hypsochromic and hypochromic shift of the ICT absorption band compared to their merocyanine precursors. Yet the new dyes show improved quadratic hyperpolarizability and thermal stability as compared to the merocyanine dyes having the same number of conjugated double bonds in the polyenic linker. As such these derivatives hold promise as new class of dyes for NLO materials and SHG probes.

Structural effects on the photoelectrochemical properties of new push-pull dyes based on vinazene acceptor triphenylamine donor

The push-pull behavior of novel dyes, based on vinazene electron-acceptor groups linked to arrays of triphenylamine (TPA) electron-donor group, was studied by electronic absorption and emission spectroscopy, as well as by cyclic voltammetry. The most stable ground state structure and their electronic properties were modeled by density functional theory (DFT) calculations using the B3LYP functional and 6-31G++ basis set, whereas the electronic properties in the excited states were calculated by TD-DFT, under the same functional and basis set, using SCF and PCM methods. The theoretical calculations matched well with experimental data, showing that λmax of the lowest energy absorption band can be assigned to an intramolecular charge transfer transition. In fact, the HOMO and LUMO are respectively localized on the TPA donor and the dicyanomethylene acceptor moiety confirming a remarkable push-pull character. Photoelectrochemical cells parameters were correlated with dyes structural properties showing to be consistent with the anchoring through the nitrogen atoms of CN groups. The nature of the donor-acceptor groups, conformation and number of anchoring CN groups (2 seems to be the best) strongly influenced the overall efficiency of dye sensitized solar cells.

A diphenylamino-substituted quinacridone derivative: red fluorescence based on intramolecular charge-transfer transition

A red emissive quinacridone derivative has been prepared through construction of an ICT system. This resulted in the first efficient and bright red OLED based on a quinacridone derivative.