The Synthesis and One- and Two-Photon Optical Properties of Dipolar, Quadrupolar and Octupolar Donor-Acceptor Molecules Containing Dimesitylboryl Groups

Synthesis, photophysics and two-photon absorption of imidazole-centred tripodal chromophores

Tripodal push-pull chromophores with D-(π-A)3 arrangement were synthesized using 1-methyl-2,4,5-triphenyl-1H-imidazole as a central electron donor, and their thermal, electrochemical, photophysical and non-linear optical properties were studied and corroborated with quantum-chemical calculations. Their facile synthesis involved Suzuki-Miyaura and Knoevenagel reactions, allowing the installation of various peripheral electron acceptors such as formyl, cyano, ester, trifluoromethyl and more complex moieties such as malonic/acetic acid derivatives, indan-1,3-dione and rhodanine. All phenyl rings appended at the central imidazole core were more or less twisted depending on the peripheral substitution. Although imidazole undergoes reversible one-electron oxidation, peripheral acceptors are reduced irreversibly in a multi-electron process. This behaviour is further seen as a variation of the LUMO, while the HOMO remained almost unaltered across the whole series. TD-DFT calculations revealed centrifugal charge transfer from the central imidazole to all C2, C4 and C5 branches occupied by the LUMO, LUMO+1 and LUMO+2. The HOMO-LUMO gap is tuneable within the range of 3.55-2.31 eV, while the longest-wavelength absorption/emission maxima were found within the broad range of 304-448/393-612 nm. Although the absorption spectra are solvent-independent, the emission depends strongly on the solvent polarity and the electron-withdrawing power of the peripheral acceptors. Extended chromophores with complex electron acceptors were investigated as two-photon absorbers, revealing relatively good cross-section values of up to 521 GM and a figure-of-merit (ΦF × δ2PA) of around 190 GM.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

Effects of Two Electron-Donating and/or -Withdrawing Substituents on Two-Photon Absorption for Diphenylacetylene Derivatives

Two-photon absorption for diphenylacetylene (DPA) derivatives with two substituents (-OMe and/or -NO₂) at the 4,4'-position was investigated experimentally and theoretically. The two-photon absorption spectra and the two-photon absorption cross-sections σ⁽²⁾ for DPA derivatives were obtained by optical-probing photoacoustic spectroscopy (OPPAS). The simulated two-photon absorption spectra of the DPA derivatives, obtained with the time-dependent density functional theory within the Tamm-Dancoff approximation, agreed well with the experimental ones. The mechanisms for enhancement of the σ⁽²⁾ for centrosymmetric and non-centrosymmetric DPA derivatives were found to be different. The large σ⁽²⁾ for centrosymmetric molecules (DPA-OMeOMe and DPA-NO₂NO₂) results from the magnitude of the transition dipole moment, while for non-centrosymmetric molecules (DPA-OMeNO₂), it is enhanced by the smaller detuning energy. Information on two-photon absorption properties of DPA derivatives obtained in this study will be important for the molecular design of two-photon absorption materials.

Photodissociative Modules that Control Dual-Emission Properties in Donor-π-Acceptor Organoborane Fluorophores

Donor-π-acceptor fluorophores that consist of an electron-donating amino group and an electron-accepting triarylborane moiety generally exhibit substantial solvatochromism in their fluorescence while retaining high fluorescence quantum yields even in polar media. Herein, we report a new family of this compound class, which bears ortho-P(=X)R2 -substituted phenyl groups (X=O or S) as a photodissociative module. The P=X moiety that intramolecularly coordinates to the boron atom undergoes dissociation in the excited state, giving rise to dual emission from the corresponding tetra- and tricoordinate boron species. The susceptibility of the systems to photodissociation depends on the coordination ability of the P=O and P=S moieties, whereby the latter facilitates dissociation. The intensity ratios of the dual emission bands are sensitive to environmental parameters, including temperature, solution polarity, and the viscosity of the medium. Moreover, precise tuning of the P(=X)R2 group and the electron-donating amino moiety led to single-molecule white emission in solution.

Electron‐Rich EDOT Linkers in Tetracationic bis‐Triarylborane Chromophores: Influence on Water Stability, Biomacromolecule Sensing, and Photoinduced Cytotoxicity

Three novel tetracationic bis‐triarylboranes with 3,4‐ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red‐shifted absorption and emission compared to their thiophene‐containing analogues, with one of the EDOT‐derivatives emitting in the NIR region. Only the EDOT‐linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3‐methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT‐linked trixylylborane tetracation and its bis‐thiophene analogue revealed efficient photo‐induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds‐DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra‐tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well‐focused visible light.

Two-Photon Excited Fluorescent 3,3'-Diamino-5,5'-Diboryl-2,2'-Bithienyls Featuring a Quadrupolar Structure

The quest for fluorophores exhibiting large two-photon absorption cross sections and high fluorescence efficiency is an important topic. Two 2,2'-bithienyl derivatives are disclosed which contain two N,N-disubstituted amino and two dimesitylboryl groups at 3,3'- and 5,5'-positions, respectively. Despite the great steric effect of amino groups, the bithienyl skeleton still adopts a coplanar geometry. Herein, they are characterized by a quadrupolar structure and display good fluorescence efficiency and large two-photon absorption cross sections up to 473 GM.

Synthesis, photophysical, electrochemical, and non-linear optical properties of triaryl pyrazole based B-N coordinated boron compounds

We report here a set of triaryl pyrazole based B-N coordinated boron compounds ( 11 - 17 ) synthesized by electrophilic aromatic borylation strategy. All the pyrazole boron compounds were thoroughly characterized using multinuclear NMR spectroscopy, LCMS, and single crystal X-ray diffraction analysis (for 12 - 17 ). The photoluminescence measurements of 11 - 17 revealed that the emission peak maxima were tuned based on the substitution on Nphenyl. The photophysical and electrochemical properties were further supported by theoretical calculations. Z-scan based investigations at 515 nm pump wavelength showed that B-N coordination led to enhancement of nonlinear absorption (two-photon absorption (TPA)) in these compounds if an electron deficient moiety is attached. It has also been observed that an appropriate choice of moiety allows to optimally maneuver the molecular polarizability of the π-system and consequently, assists in controlling the third-order nonlinear optical response.

Synthesis and Characterization of Far-Red Emissive Boron-Based Triads Showing Large Stokes Shifts: Optical, TRANES, and Electrochemical Studies

Herein, we report the design and synthesis of far-red emissive boryl-thiophene-BODIPY triads 1-3. The π-conjugation length and electronic communication between borane and BODIPY moieties are tuned by judiciously varying the size of the oligothiophene spacer in these triads (1, terthiophene; 2, quarterthiophene; and 3, pentathiophene). Conjugates 1-3 showed intriguing triple emissions in the blue to far-red regions. Detailed optical, time-resolved decay kinetics, time-resolved area-normalized emission spectra (TRANES), fluoride binding, and computational studies suggest that the multiple emissions in these triads are due to an inefficient transfer of energy from the boryl-oligothiophene to the BODIPY unit. In addition, all of the conjugates showed a ratiometric fluorescence response to fluoride ions.

Fluorescent small molecules with alternating triarylamine-substituted selenophenothiophene and triarylborane: synthesis, photophysical properties and anion sensing studies

Two novel D-π-A fluorophores based on selenopheno[3,2-b]thiophene, possessing triphenylamine and 4,4'-dimethoxytriphenylamine units as donors and dimesitylborane as an acceptor, linked through a π-conjugated thiophene spacer (BTPAST and BOMeTPAST, respectively) were synthesized. Their photophysical properties were investigated in both solution and the state of aggregation and compared to those of their corresponding donor parts, having no dimesitylborane units (TPAST and OMeTPAST). All the compounds displayed large Stokes shifts between 100 and 140 nm with positive solvatochromism in solvents having different polarities. While BTPAST displayed both aggregation induced emission (AIE) and twisted intramolecular charge transfer (TICT) characteristics, the others preponderated with TICT effects. The sensing abilities of BTPAST and BOMeTPAST towards different anions were studied. Both exhibited chromogenic and fluorogenic responses to small anions such as fluoride and cyanide, for which the detection limits were found to be 0.12 and 2.43 ppm with BTPAST and 0.59 and 0.92 ppm with BOMeTPAST, respectively. These results provide guidance for the development of novel fused selenophenothiophene sensors in the field of anion sensing.

Bis-borylated arylisoquinoline-derived dyes with a central aromatic core: towards efficient fluorescent singlet-oxygen photosensitizers

Polycyclic aromatic hydrocarbon chromophores that show an ideal bipartition between fluorescence and singlet oxygen production have been developed.

(Dimesityl)boron Benzodithiophenes: Synthesis, Electrochemical, Photophysical and Theoretical Characterization

Triarylboranes containing linear or angular benzodithiophene moieties and bearing one or two dimesitylboron units were synthesized. The electrochemical and optical features of these compounds were investigated by cyclic voltammetry, UV/Vis and fluorescence spectroscopy while DFT calculations were run to analyze the energetic landscape of these systems. For both linear and angular benzodithiophenes, symmetrical disubstitution leads to the highest photoluminescence yields. The linear benzodithiophene disubstituted with two dimesitylboron units proved to be the most interesting and promising molecule as an electron-transport material for organic electronics owing to its LUMO energy level of -2.84 eV which is close to those of commonly used electron transport materials like bathocuproine or bathophenantroline.

One- and two-electron reduction of triarylborane-based helical donor-acceptor compounds

One-electron chemical reduction of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthrene-4-amine (3-B(Mes)2-[4]helix-9-N(p-Tol)2) 1 and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthrene-8-amine (3-B(Mes)2-[5]helix-12-N(p-Tol)2) 2 gives rise to monoanions with extensive delocalization over the annulated helicene rings and the boron p z orbital. Two-electron chemical reduction of 1 and 2 produces open-shell biradicaloid dianions with temperature-dependent population of the triplet states due to small singlet-triplet gaps. These results have been confirmed by single-crystal X-ray diffraction, EPR and UV/vis-NIR spectroscopy, and DFT calculations.

Synthesis and Structure of an o-Carboranyl-Substituted Three-Coordinate Borane Radical Anion

Bis(1-(4-tolyl)-carboran-2-yl)-(4-tolyl)-borane [(1-(4-MeC6 H4 )-closo-1,2-C2 B10 H10 -2-)2 (4-MeC6 H4 )B] (1), a new bis(o-carboranyl)-(R)-borane was synthesised by lithiation of the o-carboranyl precursor and subsequent salt metathesis reaction with (4-tolyl)BBr2 . Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one-electron first reduction. In a selective reduction experiment the corresponding paramagnetic radical anion 1.- was isolated and characterized. Single-crystal structure analyses allow an in-depth comparison of 1, 1.- , their calculated geometries, and the S1 excited state of 1. Photophysical studies of 1 show a charge transfer (CT) emission with low quantum yield in solution but a strong increase in the solid state. TD-DFT calculations were used to identify transition-relevant orbitals.

Donor-acceptor-acceptor-type near-infrared fluorophores that contain dithienophosphole oxide and boryl groups: effect of the boryl group on the nonradiative decay

The use of donor-π-acceptor (D-π-A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating and -accepting groups, respectively, can produce dyes that exhibit high fluorescence and solvatochromism. Herein, we introduce a dithienophosphole P-oxide scaffold as an acceptor-spacer to produce a boryl- and amino-substituted donor-acceptor-acceptor (D-A-A) π-system. The thus obtained fluorophores exhibit emission in the near-infrared (NIR) region, while maintaining high fluorescence quantum yields even in polar solvents (e.g. λ _em = 704 nm and Φ _F = 0.69 in CH₃CN). A comparison of these compounds with their formyl- or cyano-substituted counterparts demonstrated the importance of the boryl group for generating intense emission. The differences among these electron-accepting substituents were examined in detail using theoretical calculations, which revealed the crucial role of the boryl group in lowering the nonradiative decay rate constant by decreasing the non-adiabatic coupling in the internal conversion process. The D-A-A framework was further fine-tuned to improve the photostability. One of these D-A-A dyes was successfully used in bioimaging to visualize the blood vessels of Japanese medaka larvae and mouse brain.

Bis(phenylethynyl)arene Linkers in Tetracationic Bis-triarylborane Chromophores Control Fluorimetric and Raman Sensing of Various DNAs and RNAs

We report four new luminescent tetracationic bis-triarylborane DNA and RNA sensors that show high binding affinities, in several cases even in the nanomolar range. Three of the compounds contain substituted, highly emissive and structurally flexible bis(2,6-dimethylphenyl-4-ethynyl)arene linkers (3: arene=5,5'-2,2'-bithiophene; 4: arene=1,4-benzene; 5: arene=9,10-anthracene) between the two boryl moieties and serve as efficient dual Raman and fluorescence chromophores. The shorter analogue 6 employs 9,10-anthracene as the linker and demonstrates the importance of an adequate linker length with a certain level of flexibility by exhibiting generally lower binding affinities than 3-5. Pronounced aggregation-deaggregation processes are observed in fluorimetric titration experiments with DNA for compounds 3 and 5. Molecular modelling of complexes of 5 with AT-DNA, suggest the minor groove as the dominant binding site for monomeric 5, but demonstrate that dimers of 5 can also be accommodated. Strong SERS responses for 3-5 versus a very weak response for 6, particularly the strong signals from anthracene itself observed for 5 but not for 6, demonstrate the importance of triple bonds for strong Raman activity in molecules of this compound class. The energy of the characteristic stretching vibration of the C≡C bonds is significantly dependent on the aromatic moiety between the triple bonds. The insertion of aromatic moieties between two C≡C bonds thus offers an alternative design for dual Raman and fluorescence chromophores, applicable in multiplex biological Raman imaging.

Phenylpyridyl-Fused Boroles: A Unique Coordination Mode and Weak B-N Coordination-Induced Dual Fluorescence

Using 4-phenylpyridine or 2-phenylpyridine in place of biphenyl, two electron-poor phenylpyridyl-fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 adopts a unique coordination mode and forms a tetramer with a cavity in both the solid state and solution. The boron center of TipPBB2 is 4-coordinate in the solid state but the system dissociates in solution, leading to 3-coordinate borole species. Compared to its borafluorene analogues, the electron-accepting ability of TipPBB2 is largely enhanced by the pyridyl group. TipPBB2 exhibits dual fluorescence in solution due to an equilibrium between free TipPBB2 and a weak intermolecular coordination adduct with a second molecule. This equilibrium was further investigated by low-temperature NMR spectroscopy and photophysical studies. Theoretical studies indicate that the highest occupied molecular orbital (HOMO) of TipPBB2 localizes at the Tip group, in contrast to its borafluorene derivatives, wherein the HOMOs are localized on the borafluorene cores.

Syntheses, structures, and one- and two-photon excited fluorescence of dimesitylboryl-ended quadrupolar hybrid oligothiophenes

A group of dimesitylboryl-ended quadrupolar hybrid-oligothiophenes were examined aiming to optimize their TPEF properties.

Synthesis of pyrazole anchored three-coordinated organoboranes and their application in the detection of picric acid

Three-coordinated organoboron fluorophores bearing 3,5-diphenyl pyrazoles have been synthesized. The pyrazole anchored boron fluorophores show selective fluorescence quenching response to trinitrophenol (or) picric acid (PA) and have the ability to discriminate picric acid over other analytes. We investigated nonlinear optical (NLO) properties of these three-coordinated organoboron compounds (in solutions) in the presence and absence of PA. In absence of PA, the two-photon-absorption coefficient (β) of organoboron fluorophores exhibits a variation from 2 × 10-12 cm W-1 to 4 × 10-12 cm W-1. The results also reveal that the NLO characteristics of organoboron fluorophores exhibit a discernible variation with PA addition which has correlations with quenching observed in fluorescence measurements.

(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant p_z orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.

(Hetero)arene-fused boroles, ‘antiaromatic’ 2n-electron π-systems, more stable and more functionalizable than boroles, offer greater potential for a variety of applications.

An Iterative Divergent Approach to Conjugated Starburst Borane Dendrimers

Using a new divergent approach, conjugated triarylborane dendrimers were synthesized up to the 2nd generation. The synthetic strategy consists of three steps: 1) functionalization, via iridium catalyzed C-H borylation; 2) activation, via fluorination of the generated boronate ester with K[HF2 ] or [N(nBu4 )][HF2 ]; and 3) expansion, via reaction of the trifluoroborate salts with aryl Grignard reagents. The concept was also shown to be viable for a convergent approach. All but one of the conjugated borane dendrimers exhibit multiple, distinct and reversible reduction potentials, making them potentially interesting materials for applications in molecular accumulators. Based on their photophysical properties, the 1st generation dendrimers exhibit good conjugation over the whole system. However, the conjugation does not increase further upon expansion to the 2nd generation, but the molar extinction coefficients increase linearly with the number of triarylborane subunits, suggesting a potential application as photonic antennas.

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9-Borafluorenes

Three different perfluoroalkylated borafluorenes (F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para substituents on their exo-aryl moieties, being a proton (F XylF Bf, F Xyl: 2,6-bis(trifluoromethyl)phenyl), a trifluoromethyl group (F MesF Bf, F Mes: 2,4,6-tris(trifluoromethyl)phenyl) or a dimethylamino group (p-NMe2 -F XylF Bf, p-NMe2 -F Xyl: 4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative F MesF Bf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor-substituted derivative p-NMe2 -F XylF Bf exhibits thermally activated delayed fluorescence (TADF) from a charge-transfer (CT) state, whereas the F MesF Bf and F XylF Bf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

Electronically Driven Regioselective Iridium-Catalyzed C-H Borylation of Donor-π-Acceptor Chromophores Containing Triarylboron Acceptors

We observed a surprisingly high electronically driven regioselectivity for the iridium-catalyzed C-H borylation of donor-π-acceptor (D-π-A) systems with diphenylamino (1) or carbazolyl (2) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl (B(F Xyl)2 ) as the acceptor, and 1,4-phenylene as the π-bridge. Under our conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C-C coupling, functional group transformations) the C-H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D-π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated (1-(Bpin)2 ) and unborylated (1) diphenylamino-substituted D-π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.

Computationally Guided Molecular Design to Minimize the LE/CT Gap in D-π-A Fluorinated Triarylboranes for Efficient TADF via D and π-Bridge Tuning

In this combined experimental and theoretical study, a computational protocol is reported to predict the excited states in D-π-A compounds containing the B(FXyl)2 (FXyl = 2,6-bis(trifluoromethyl)phenyl) acceptor group for the design of new thermally activated delayed fluorescence (TADF) emitters. To this end, the effect of different donor and π-bridge moieties on the energy gaps between local and charge-transfer singlet and triplet states is examined. To prove this computationally aided design concept, the D-π-B(FXyl)2 compounds 1-5 were synthesized and fully characterized. The photophysical properties of these compounds in various solvents, polymeric film, and in a frozen matrix were investigated in detail and show excellent agreement with the computationally obtained data. Furthermore, a simple structure-property relationship is presented on the basis of the molecular fragment orbitals of the donor and the π-bridge, which minimize the relevant singlet-triplet gaps to achieve efficient TADF emitters.

Tetracationic Bis-Triarylborane 1,3-Butadiyne as a Combined Fluorimetric and Raman Probe for Simultaneous and Selective Sensing of Various DNA, RNA, and Proteins

A bis-triarylborane tetracation (4-Ar2 B-3,5-Me2 C6 H2 )-C≡C-C≡C-(3,5-Me2 C6 H2 -4-BAr2 [Ar=(2,6-Me2 -4-NMe3 -C6 H2 )+ ] (24+ ) shows distinctly different behaviour in its fluorimetric response than that of our recently published bis-triarylborane 5-(4-Ar2 B-3,5-Me2 C6 H2 )-2,2'-(C4 H2 S)2 -5'-(3,5-Me2 C6 H2 -4-BAr2 ) (34+ ). Single-crystal X-ray diffraction data on the neutral bis-triarylborane precursor 2 N confirm its rod-like dumbbell structure, which is shown to be important for DNA/RNA targeting and also for BSA protein binding. Fluorimetric titrations with DNA/RNA/BSA revealed the very strong affinity of 24+ and indicated the importance of the properties of the linker connecting the two triarylboranes. Using the butadiyne rather than a bithiophene linker resulted in an opposite emission effect (quenching vs. enhancement), and 24+ bound to BSA 100 times stronger than 34+ . Moreover, 24+ interacted strongly with ss-RNA, and circular dichroism (CD) results suggest ss-RNA chain-wrapping around the rod-like bis-triarylborane dumbbell structure like a thread around a spindle, a very unusual mode of binding of ss-RNA with small molecules. Furthermore, 24+ yielded strong Raman/SERS signals, allowing DNA or protein detection at ca. 10 nm concentrations. The above observations, combined with low cytotoxicity, efficient human cell uptake and organelle-selective accumulation make such compounds intriguing novel lead structures for bio-oriented, dual fluorescence/Raman-based applications.

Synthesis of π-extended B ← N coordinated phenanthroimidazole dimers and their linear and nonlinear optical properties

B ← N coordinated phenanthroimidazole dimers exhibit excellent fluorescence quantum yields in solution and conjugation length dependant two-photon-absorption properties.

π-Extended Four-Coordinate Organoboron N,C-Chelates as Two-Photon Absorbing Chromophores

Four-coordinate N,C-chelate organoboron dyes with alkynyl spacers were synthesized by Heck alkynylation. These dyes are π-extended analogues of the recently reported class of four-coordinate borylated arylisoquinolines (BAI). Depending on the electron-donor substitution, they feature an intramolecular charge-transfer (ICT) character in the excited state. This translates into pronounced apparent Stokes shifts (up to 8500 cm^-1) and a solvatofluorochromic behavior. In general, the observed emission quantum yields are high in nonpolar media (Φ_F ca. 0.5-0.6). For the dye with the most pronounced ICT rather high emission quantum yields (Φ_F ca. 0.4) are observed for emissions with maxima longer than 600 nm in solvents of moderate polarity. The π-extended dyes show interesting two-photon absorption (TPA) properties, maintaining high cross sections (up to 60 GM) in the near-infrared wavelength window (>900 nm). One of the dyes was designed as dimeric chromophore, integrating the acceptor-π-acceptor (A-π-A) format. This alternative design showed no ICT behavior but led to the observation of high two-photon-absorption (TPA) cross sections (ca. 220 GM at 700 nm). All investigated dyes show pronounced photostability, providing added value to this structural and photofunctional extension of the BAI dye platform.

The Effect of Branching on the One- and Two-Photon Absorption, Cell Viability, and Localization of Cationic Triarylborane Chromophores with Dipolar versus Octupolar Charge Distributions for Cellular Imaging

Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two-photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two-photon excited fluorescence (TPEF) live-cell imaging.

N-Heterocyclic Olefins as Electron Donors in Combination with Triarylborane Acceptors: Synthesis, Optical and Electronic Properties of D-π-A Compounds

N-heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor (D)-π-A compounds 1-3. In addition, an enamine π-donor analogue (4) was synthesized for comparison. UV-visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs). Solvent-dependent emission studies indicate that 1-4 have moderate intramolecular charge-transfer (ICT) behavior. Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3, E ox 1 / 2 =-0.40 V vs. ferrocene/ferrocenium, Fc/Fc⁺ , in THF). Time-dependent (TD) DFT calculations show that the HOMOs of 1-3 are much more destabilized than that of the enamine-containing 4, which confirms the stronger donating ability of NHOs.

Synthesis, photophysical and electronic properties of tetra-donor- or acceptor-substituted ortho-perylenes displaying four reversible oxidations or reductions

Via regioselective Ir-catalyzed C-H borylation and subsequent reactions (i.e., via Br4-Per or (BF3K)4-Per intermediates), we have introduced strong π-donors and acceptors at the 2,5,8,11-positions of perylene leading to unusual properties. Thus, incorporation of four donor diphenylamine (DPA) or four acceptor Bmes2 (mes = 2,4,6-Me3C6H2) moieties yields novel compounds which can be reversibly oxidized or reduced four times, respectively, an unprecedented behavior for monomeric perylene derivatives. Spectroelectrochemical measurements show NIR absorptions up to 3000 nm for the mono-cation radical of (DPA)4-Per and a strong electronic coupling over the perylene bridge was observed indicative of fully delocalized Robin-Day Class III behavior. Both (DPA)4-Per and (Bmes2)4-Per derivatives possess unusually long intrinsic singlet lifetimes (τ 0), e.g., 94 ns for the former one. The compounds are emissive in solution, thin films, and the solid state, with apparent Stokes shifts that are exceptionally large for perylene derivatives. Transient absorption measurements on (DPA)4-Per reveal an additional excited state, with a long lifetime of 500 μs, which sensitizes singlet oxygen effectively.

Triarylborane-Based Helical Donor-Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (3-B(Mes)₂ -[4]helix-9-N(p-Tol)₂ 1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (3-B(Mes)₂ -[5]helix-12-N(p-Tol)₂ 2) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φ_f =0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (Φ_f =0.18 for [4]helicene; Φ_f <0.05 for [n]helicenes (n≥5)). In the solid state, the Φ_f values are higher (Φ_f =1.00 and 0.55 for 1 and 2, respectively) than those in solution, most likely due to the restrictions of molecular motions. The S₁ ←S₀ transitions of 1 and 2 are predominately HOMO→LUMO transitions. Upon excitation with UV light, the interplanar angle between the two terminal aryl rings of the [5]helix core of 2 decreases (S₁ state compared with S₀ state), which is similar to placing a spring under an external force.

Push-pull isomers of indolizino[6,5,4,3-def]phenanthridine decorated with a triarylboron moiety

1,3-Dipolar cycloaddition reactions between a new azomethine ylide and three BPhMes₂-functionalized internal alkynes produced three pairs of fluorescent push-pull regioisomers, which show distinct electronic and photophysical properties. All the six compounds are found to exhibit charge-transfer (CT) fluorescence, and some of which show rare and interesting temperature "turn-on" fluorescence.

Tuning the π-bridge of quadrupolar triarylborane chromophores for one- and two-photon excited fluorescence imaging of lysosomes in live cells

A series of tetracationic quadrupolar chromophores containing three-coordinate boron π-acceptors linked by different π-bridges, namely 4,4'-biphenyl, 2,7-pyrene, 2,7-fluorene, 3,6-carbazole and 5,5'-di(thien-2-yl)-3,6-diketopyrrolopyrrole, were synthesized. While their neutral precursors 1-5 displayed highly solvatochromic fluorescence, the water-soluble tetracationic target molecules 1M-5M, did not, but their emission colour could be tuned from blue to pink by changing the π-bridge. Compound 5M, containing the diketopyrrolopyrrole bridge, exhibits the most red-shifted absorption and emission maxima and the largest two-photon absorption cross-section (4560 GM at 740 nm in MeCN). Confocal laser scanning fluorescence microscopy studies in live cells confirm localization of the dye at the lysosome. Moreover, the low cytotoxicity, and high photostability of 5M combined with two-photon excited fluorescence imaging studies demonstrate its excellent potential for lysosomal imaging in live cells.

Reversible Photoisomerization from Borepin to Boratanorcaradiene and Double Aryl Migration from Boron to Carbon

B(npy)Ar₂ (npy=2-(naphthalen-1-yl)pyridine) compounds bearing various nonbulky aryl groups undergo a clean and sequential two-step photoisomerization in which two aryl substituents on boron migrate to a carbon atom of the naphthyl moiety. The second isomerization step is the first example of a reversible photoisoermization between a borepin and a borirane. Both steric and electronic factors have been found to have a great impact on this photoreactivity. Furthermore, the borirane isomer reacts with oxygen, forming a rare oxaborepin dimer.

Near-Infrared Quadrupolar Chromophores Combining Three-Coordinate Boron-Based Superdonor and Superacceptor Units

Herein, two new quadrupolar acceptor-π-donor-π-acceptor (A-π-D-π-A) chromophores have been prepared featuring a strongly electron-donating diborene core and strongly electron-accepting dimesitylboryl (BMes₂ ) and bis(2,4,6-tris(trifluoromethyl)phenyl)boryl (B^F Mes₂ ) end groups. Analysis of the compounds by NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV/Vis-NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π-systems spanning their B₄ C₈ cores. The combination of exceptionally potent π-donor (diborene) and π-acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO-LUMO gaps, resulting in strong absorption in the near-IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m^-1  cm^-1 . Both molecules also display weak near-IR fluorescence with small Stokes shifts.

Photoactivated cell-killing involving a low molecular weight, donor-acceptor diphenylacetylene

Drug-like, donor–acceptor diphenylacetylenes cause efficient cell death upon photoactivation and hence have potential phototherapeutic applications.

Photoactivation of photosensitisers can be utilised to elicit the production of ROS, for potential therapeutic applications, including the destruction of diseased tissues and tumours. A novel class of photosensitiser, exemplified by DC324, has been designed possessing a modular, low molecular weight and ‘drug-like’ structure which is bioavailable and can be photoactivated by UV-A/405 nm or corresponding two-photon absorption of near-IR (800 nm) light, resulting in powerful cytotoxic activity, ostensibly through the production of ROS in a cellular environment. A variety of in vitro cellular assays confirmed ROS formation and in vivo cytotoxic activity was exemplified via irradiation and subsequent targeted destruction of specific areas of a zebrafish embryo.

Preparation and Characterization of a π-Conjugated Donor-Acceptor System Containing the Strongly Electron-Accepting Tetraphenylborolyl Unit

A novel thiophene-bridged donor-acceptor system was synthesized with a carbazole as donor and a borole as acceptor unit. The borole group was successfully installed via the tin-boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole. The effect of the borole on the optoelectronic properties of the donor-acceptor system was explored by spectroscopic (UV/Vis and fluorescence spectroscopy), electrochemical (cyclic voltammetry) and theoretical (TD-DFT) methods as well as by modifying its structure. The corresponding donor-acceptor compound bearing the widely employed dimesitylboryl acceptor group was also synthesized for comparison.

Donor-π-Acceptor Type Unsymmetrical Triarylborane-Based Fluorophores: Synthesis, Fluorescence Properties, and Photostability

A two-step synthesis to prepare tricoordinate organoboron compounds bearing three different aryl groups has been developed. After the first aryl substitution to an aryl boronic ester took place, the intermediate species, that is, bis(diarylborinate) species, was isolated as an air- and moisture-stable solid, which allowed the second aryl substitution to carry out in a selective manner. Subsequently, a series of unsymmetrical triarylboranes possessing a sterically bulky aryl group, triarylamine moiety, and para-functionalized phenyl ring was synthesized. Not only did these triarylboranes exhibit remarkable solvent-dependent fluorescence as expected for donor-π-acceptor (D-π-A) systems, they were also accompanied by profound persistence against photoirradiation especially for that bearing a 1,3,5-tri- tert-butylphenyl ring. This survey exemplifies that sufficient electronic and steric modification is key to construct photostable D-π-A type triarylborane-based fluorophores.

Diphenylamino-substituted tristyryl vs. triphenyl isocyanurates: improved conjugation has minimal impact on two-photon absorption

Planarization and extension of the central π-system does not increase two-photon absorption cross-section.

Pyrene Molecular Orbital Shuffle-Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals

We show that by judicious choice of substituents at the 2- and 7-positions of pyrene, the frontier orbital order of pyrene can be modified, giving enhanced control over the nature and properties of the photoexcited states and the redox potentials. Specifically, we introduced a julolidine-like moiety and Bmes₂ (mes=2,4,6-Me₃ C₆ H₂ ) as very strong donor (D) and acceptor (A), respectively, giving 2,7-D-π-D- and unsymmetric 2,7-D-π-A-pyrene derivatives, in which the donor destabilizes the HOMO-1 and the acceptor stabilizes the LUMO+1 of the pyrene core. Consequently, for 2,7-substituted pyrene derivatives, unusual properties are obtained. For example, very large bathochromic shifts were observed for all of our compounds, and unprecedented green light emission occurs for the D/D system. In addition, very high radiative rate constants in solution and in the solid state were recorded for the D-π-D- and D-π-A-substituted compounds. All compounds show reversible one-electron oxidations, and Jul₂ Pyr exhibits a second oxidation, with the largest potential splitting (ΔE=440 mV) thus far reported for 2,7-substituted pyrenes. Spectroelectrochemical measurements confirm an unexpectedly strong coupling between the 2,7-substituents in our pyrene derivatives.