Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes.

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the ß-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH2 -rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm-1 in BODIPY versus >4170 cm-1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

Boron-based donor-spiro-acceptor compounds exhibiting thermally activated delayed fluorescence (TADF).

Four boron-based donor-spiro-acceptor compounds, composed of different donor moieties and borylated 2-phenylpyridines as the acceptor, were studied. Their intense photoluminescence in the solid state can be tuned by changing the donor and long emission lifetimes on the microsecond scale indicate thermally activated delayed fluorescence (TADF).

Synthesis, Photophysical, and Electrochemical Properties of Pyrenes Substituted with Donors or Acceptors at the 4- or 4,9-Positions.

We report herein an efficient and direct functionalization of the 4,9-positions of pyrene by Ir-catalyzed borylation. Three pinacol boronates (-Bpin), including 4-(Bpin)-2,7-di( tert-butyl)pyrene (5), 4,9- bis(Bpin)-2,7-di( tert-butyl)pyrene (6), and 4,10- bis(Bpin)-2,7-di( tert-butyl)pyrene (7), were synthesized. The structures of 6 and 7 have been confirmed by single-crystal X-ray diffraction. To demonstrate the utility of these compounds, donor (NPh2)-substituted compounds 4-diphenylamino-2,7-di( tert-butyl)pyrene (1) and 4,9- bis(diphenylamino)-2,7-di( tert-butyl)pyrene (2) have been synthesized on a gram scale. Acceptor (BMes2)-substituted compounds 4,9- bis(BMes2)pyrene (3) and 4,9- bis(BMes2)-1,2,3,6,7,8-hexahydropyrene (4) were synthesized for comparison. The photophysical and electrochemical properties of compounds 1-4 have been studied both experimentally and theoretically. The S0 → S1 transitions of the 4- or 4,9-disubstituted pyrenes, 1-3, are allowed, with moderate fluorescence quantum yields and radiative decay rates. The photophysical and electrochemical properties of 1-3 were compared with the 2,6-naphthalenylene-cored compound 4 as well as the previously reported 2,7- and 1,6- pyrenylene-cored compounds.

Triplet Energy and π-Conjugation Effects on Photoisomerization of Chiral N,C-Chelate Organoborons with PAH Substituents.

Chiral, PAH substituted N,C-chelate boron compounds are systematically investigated to establish the effect of triplet energy and substitution position on their photoreactivity. They all undergo regioselective photoisomerization, forming new dark isomers with quantum efficiencies reflecting these various factors. New PAH fused 4bH-azaborepins are obtained via thermal isomerization of the dark isomers. These results further implicate a photoactive triplet state in the photoisomerization process and its utility in achieving rare PAH-fused azaborepin-like heterocycles.

Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Complexes.

Herein, we report on the synthesis and structural characterization of a series of trigonal and tetrahedral cationic copper(I) complexes, bearing phosphine or N-heterocyclic carbene ligands as donors, with benzthiazol-2-pyridine (pybt) and benzthiazol-2-quinoline (qybt) acting as π-chromophores. The compounds are highly colored due to their 1MLCT (MLCT = metal-to-ligand charge transfer) states absorbing between ca. λabs = 400-500 nm, with 1ILCT (ILCT = intraligand charge transfer) states in the UV region. The relative shifts of the S0→S1 absorption correlate with the computed highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, the qybt complexes generally being lower in energy than the pybt ones due to the larger conjugation of the quinoline-based ligand. The compounds exhibit, for CuI complexes, rare intense long-lived near-IR emission with λmax ranging from 593 to 757 nm, quantum yields of up to Φ = 0.11, and lifetimes τ of several microseconds in the solid state as well as in poly(methyl methacrylate) films. Although a bathochromic shift of the emission is observed with λmax ranging from 639 to 812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence was found, leaving us to assign the emission to originate from a 3(Cu→pybt/qybt)MLCT state. The red to near-IR emission is a result of incorporation of the sulfur into the chromophore ligand, as related nitrogen analogues emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with density functional theory (DFT)/time-dependent DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials.

Reductive Coupling of Diynes at Rhodium Gives Fluorescent Rhodacyclopentadienes or Phosphorescent Rhodium 2,2'-Biphenyl Complexes.

Reactions of [Rh(κ(2) -O,O-acac)(PMe3 )2 ] (acac=acetylacetonato) and α,ω-bis(arylbutadiynyl)alkanes afford two isomeric types of MC4 metallacycles with very different photophysical properties. As a result of a [2+2] reductive coupling at Rh, 2,5-bis(arylethynyl)rhodacyclopentadienes (A) are formed, which display intense fluorescence (Φ=0.07-0.54, τ=0.2-2.5 ns) despite the presence of the heavy metal atom. Rhodium biphenyl complexes (B), which show exceptionally long-lived (hundreds of µs) phosphorescence (Φ=0.01-0.33) at room temperature in solution, have been isolated as a second isomer originating from an unusual [4+2] cycloaddition reaction and a subsequent ß-H-shift. We attribute the different photophysical properties of isomers A and B to a higher excited state density and a less stabilized T1 state in the biphenyl complexes B, allowing for more efficient intersystem crossing S1 →Tn and T1 →S0 . Control of the isomer distribution is achieved by modification of the bis- (diyne) linker length, providing a fundamentally new route to access photoactive metal biphenyl compounds.

Cuprophilic interactions in highly luminescent dicopper(i)-NHC-picolyl complexes - fast phosphorescence or TADF?

This case study on a series of monomeric, dimeric and polymeric Cu(I) chlorido NHC-picolyl complexes shows that cuprophilic interactions can ensure strong spin-orbit coupling for fast (reverse)intersystem-crossing T1 ↔ S1 and T1 → S0, and therefore can serve as a design motif for the construction of highly efficient Cu(I)-based TADF or T1 emitters.

Electron Delocalization in Reduced Forms of 2-(BMes2)pyrene and 2,7-Bis(BMes2)pyrene.

Reduction of 2-(BMes2)pyrene (B1) and 2,7-bis(BMes2)pyrene (B2) gives rise to anions with extensive delocalization over the pyrenylene bridge and between the boron centers at the 2- and 2,7-positions, the typically unconjugated sites in the pyrene framework. One-electron reduction of B2 gives a radical anion with a centrosymmetric semiquinoidal structure, while two-electron reduction produces a quinoidal singlet dianion with biradicaloid character and a relatively large S0-T1 gap. These results have been confirmed by cyclic voltammetry, X-ray crystallography, DFT/CASSCF calculations, NMR, EPR, and UV-vis-NIR spectroscopy.

Synthesis and photophysics of a 2,7-disubstituted donor-acceptor pyrene derivative: an example of the application of sequential Ir-catalyzed C-H borylation and substitution chemistry.

We report a general and selective method to synthesize 2,7-disubstituted pyrene derivatives containing two different substituents by sequential Ir-catalyzed borylation and substitution chemistry. To demonstrate the utility of our approach, we synthesized 2-cyano-7-(N,N-diethylamino)pyrene (3), a pyrene analogue of the widely studied chromophore 4-(N,N-dimethylamino)benzonitrile (DMABN). Compound 3 and the monosubstituted compounds 2-(N,N-diethylamino)pyrene (1) and 2-cyanopyrene (2) have been structurally characterized. Their electronic and optical properties have been studied by a combination of absorption and emission spectroscopies, lifetime and quantum yield measurements, and modeling by DFT and TD-DFT. The photophysical properties of 3 are compared to those of DMABN and 2-cyano-7-(N,N-dimethylamino)-4,5,9,10-tetrahydropyrene, and we show that 2,7-disubstituted pyrene is a moderately effective π-bridge for the construction of donor-acceptor compounds. It is also shown that donor or acceptor groups are only effective at the 2,7-positions of pyrene if they are suitably strong, leading to a switch in the energetic ordering of the HOMO-1 and HOMO or the LUMO and LUMO+1 of pyrene, respectively.

A fullerene-carbene adduct as a crystalline molecular rotor: remarkable behavior of a spherically-shaped rotator.

A new fullerene structure was recently obtained from the reaction of a Lewis basic N-heterocyclic carbene (NHC) and the Lewis acidic C60. The molecular features of the zwitterionic adduct can be described as a molecular rotor with the fullerene cage acting as the rotator that spins about one distinct axis given by its C-C single bond linkage with the imidazolium heterocycle stator. A detailed structural analysis of the compound by means of single-crystal X-ray diffraction (XRD) revealed significant differences in the packing motifs of solvent-free and solvent-containing crystals. Variable temperature single-crystal XRD experiments (80 K ≤ T ≤ 480 K) carried out to investigate the rotational dynamics of the fullerene group in the higher quality solvent-free structure revealed atomic displacement parameters consistent with fast rotation of the highly symmetric fullerene in the solid state, whereas the imidazolium unit remains in a fixed position and therefore represents the stator. DFT and semiempirical calculations were applied to get insight into the profile of the rotational potential of the fullerene unit, particularly considering interactions with the neighboring molecules in the crystal lattice. The results indicate that the crystal environment leads to the presence of one lowest energy minimum that is connected to seven others that are slightly higher in energy through rotational barriers of approximately 1.5-2.5 kcal mol(-1).

Design and properties of intermediate-sized narrow band-gap conjugated molecules relevant to solution-processed organic solar cells.

Increases in the molecular length of narrow band gap conjugated chromophores reveal potentially beneficial optical and electronic properties, thermal stabilities, and high power conversion efficiencies when integrated into optoelectronic devices, such as bulk heterojunction organic solar cells. With the objective of providing useful information for understanding the transition from small-sized molecules to polymers, as well as providing a general chemical design platform for extracting relationships between molecular structure and bulk properties, we set out to vary the electron affinity of the molecular backbone. Therefore, a series of donor (D)-acceptor (A) alternating narrow band gap conjugated chromophores were synthesized based on the general molecular frameworks: D(1)-A(1)-D(2)-A(2)-D(2)-A(1)-D(1) and D(1)-A(1)-D(2)-A(2)-D(2)-A(2)-D(2)-A(1)-D(1). When the central electron-accepting moiety (A(2)) was varied or modified, two classes of molecules could be compared. First, we showed that the alteration of one single electron-accepting group, while maintaining the shape of the molecular framework, can effectively impact the optical properties and energy levels of the molecules. DFT ground state structure optimizations show similar "U" shape conformations among these molecules. Second, we examined how the site-specific introduction of fluorine atom(s) modifies the thermal properties in the solid state, while maintaining relatively similar optical and electrochemical features of interest. Structure-property relationship of such molecular systems could be rationally evaluated in the aspects of thermal-responsive molecular organizations in the solid state and dipole moments both in the ground and excited states. The impact of molecular structure on charge carrier mobilities in field effect transistors and the performance of photovoltaic devices were also studied.

Aryl(hydro)boranes: versatile building blocks for boron-doped π-electron materials.

Boron-containing π-conjugated molecules offer a substantial application potential in the field of organic electronics. During the last decade, aryl(hydro)boranes have established themselves as versatile novel building blocks for sophisticated boron-doped materials. This perspective article comprehensively discusses key structural motifs and reactivity patterns of recently developed aryl(hydro)boranes and shows how these have been used for the synthesis of macromolecular organoboranes through hydroboration polymerisation, ring-opening polymerisation and condensation polymerisation protocols.

Unsymmetrically substituted 9,10-dihydro-9,10-diboraanthracenes as versatile building blocks for boron-doped π-conjugated systems.

The targeted hydrolysis of the 9,10-dihydro-9,10-diboraanthracene adduct (Me(2)S)HB(C(6)H(4))(2)BH(SMe(2)) (1) with 0.5 equiv of H(2)O leads to formation of the borinic acid anhydride [(Me(2)S)HB(C(6)H(4))(2)B](2)O (2) and thereby provides access to the field of unsymmetrically substituted 9,10-dihydro-9,10-diboraanthracenes. Compound 2 reacts with tBuC≡CH to give the corresponding vinyl derivative in an essentially quantitative conversion. Subsequent cleavage of the B-O-B bridge by LiAlH(4) with formation of hydridoborate functionalities is possible but is accompanied by partial B-C(vinyl) bond degradation. This situation changes when the related mesityl derivative [MesB(C(6)H(4))(2)B](2)O (7) is employed, which can be synthesized from BrB(C(6)H(4))(2)BBr (6) by treatment with 1 equiv of MesMgBr and subsequent hydrolysis. The reaction of 7 with LiAlH(4) in tetrahydrofuran (THF) furnishes Li[MesB(C(6)H(4))(2)BH(2)] (8); hydride elimination with Me(3)SiCl leads to formation of the THF adduct MesB(C(6)H(4))(2)BH(THF) (9·THF). Alternatively, 7 can be transformed into the bromoborane MesB(C(6)H(4))(2)BBr (10) by treatment with BBr(3). A Br/H-exchange reaction between 10 and Et(3)SiH yields the donor-free borane MesB(C(6)H(4))(2)BH (9), which forms B-H-B bridged dimers (9)(2) in the solid state. The vinyl borane MesB(C(6)H(4))(2)BC(H)=C(H)Mes (14) is accessible from MesC≡CH and either 9·THF or 9. Compared with the related compound Mes(2)BC(H)=C(H)Mes, the electronic absorption and emission spectra of 14 reveal bathochromic shifts of Δλ(abs)=17 nm and Δλ(em)=74 nm, which can be attributed to the rigid, fully delocalized π framework of the [MesB(C(6)H(4))(2)B] chromophore.

Lewis-base adducts of 9,10-dihydro-9,10-diboraanthracene: ditopic hydroboration reagents and a B-N analogue of triptycene.

The dimethyl sulfide, pyridazine, or pyrazolide adducts of 9,10-dihydro-9,10-diboraanthracene are potent hydroboration reagents for terminal alkynes; the 1,2-diazene derivatives possess a paddle-wheel structure and are stabilised towards hydrolysis.

9,10-Dihydro-9,10-diboraanthracene: supramolecular structure and use as a building block for luminescent conjugated polymers.

Building bridges: The title compound forms an unprecedented polymeric structure with bridging B-H-B three-center two-electron bonds in the solid state. This organoborane serves as an efficient precursor for the preparation of boron-doped pi-conjugated polymers by hydroboration polymerization with a functionalized 1,4-diethynylbenzene (see picture). These polymers form thin films that show intense green luminescence.

Supersilylated tetraphosphene derivatives M2[t-Bu3SiPPPPSi-t-Bu3] (M = Li, Na, Rb, Cs) and Ba[t-Bu3SiPPPPSi-t-Bu3]: reactivity and cis-trans isomerization.

The tetraphosphenediides M2[t-Bu3SiPPPPSi-t-Bu3] (M = Li, Na, K) were accessible by the reaction of P4 with the silanides M[Si-t-Bu3] (M = Li, Na, K), whereas M2[t-Bu3SiPPPPSi-t-Bu3] (M = Rb, Cs) were obtained from the reaction of RbCl and CsF with Na2[t-Bu3SiPPPPSi-t-Bu3]. 31P NMR experiments revealed that, in tetrahydrofuran, Na2[t-Bu3SiPPPPSi-t-Bu3] adopts a cis configuration. However, treatment of Na2[t-Bu3SiPPPPSi-t-Bu3] with 18-crown-6 led to the formation of [Na(18-crown-6)(thf)2]2[t-Bu3SiPPPPSi-t-Bu3] that possesses a trans configuration in the solid state. The ion-separated tetraphosphenediide [Na(18-crown-6)(thf)2]2[t-Bu3SiPPPPSi-t-Bu3] was analyzed using X-ray crystallography (monoclinic, space group P2(1)/n). The reaction of Na2[t-Bu3SiPPPPSi-t-Bu3] with BaI2 gave, conveniently, the corresponding barium derivative Ba[t-Bu3SiPPPPSi-t-Bu3]. However, addition of AuI to the tetraphosphenediide Na2[t-Bu3SiPPPPSi-t-Bu3] yielded 1,3-diiodo-2,4-disupersilyl-cyclotetraphosphane (monoclinic, space group C2/c), which is an isomer of disupersilylated diiodotetraphosphene. A further isomeric derivative of disupersilylated tetraphosphene, the 3,5-disupersilyl-2,2-di-tert-butyl-2-stanna-bicyclo[2.1.0(1,4)]pentaphosphane, which possesses a phosphanylcyclotriphosphane structure, was obtained by the reaction of Na2[t-Bu3SiPPPPSi-t-Bu3] with t-Bu2SnCl2. Calculations revealed that the acyclic cis and trans isomers of the dianions [HPPPPH]2- and [H3SiPPPPSiH3]2- are thermodynamically more stable than the cyclic isomers with a phosphanylcyclotriphosphane or a cyclotetraphosphane structure. However, the neutral cyclic isomers of H4P4 and H2(H3Si)2P4 represent more stable structures than the cis- and trans-tetraphosphenes H2P-P=P-PH2 and (H3Si)HP-P=P-PH(SiH3), respectively. In addition, the molecular orbitals (MOs) of the silylated cis- and trans-tetraphosphene dianions of [H3SiPPPPSiH3]2-, which are comparable with those of the ion-separated supersilylated tetraphosphenediide [t-Bu3SiPPPPSi-t-Bu3]2-, show the highest occupied antibonding pi*MO (HOMO). The HOMO is represented by the (p(z)-p(z)+p(z)-p(z)) pi* MO.

Isoelectronic caesium compounds: the triphosphenide Cs[tBu3SiPPPSitBu3] and the enolate Cs[OCH=CH2].

The caesium triphosphenide Cs[tBu3SiPPPSitBu3] was accessible from the reaction of CsF with the sodium triphosphenide Na[tBu3SiPPPSitBu3] in tetrahydrofuran at room temperature. In contrast to the preparation of tetrahydrofuran-solvated silanides M[SitBu3] (M = Li, Na, K), our efforts to synthesize the caesium silanide Cs[SitBu3] as a tetrahydrofuran complex failed. When tBu3SiBr was treated with an excess of caesium metal in tetrahydrofuran at room temperature, the caesium enolate Cs[OCH=CH2] and the supersilane tBu3SiH formed rather than the silanide Cs[SitBu3]. X-Ray quality crystals of the enolate Cs[OCH=CH2] (orthorhombic, Pnma) were obtained from tetrahydrofuran at ambient temperature. In contrast to the structures of its homologues M[tBu3SiPPPSitBu3] (M = Na, K), the caesium triphosphenide Cs[tBu3SiPPPSitBu3] features a polymer in the solid state (orthorhombic, Cmcm).

A twinned triclinic polymorph of dibromidotetrakis(tetrahydrofuran-kappaO)magnesium(II).

The title compound, [MgBr(2)(C(4)H(8)O)(4)], forms crystals which appear to be monoclinic but are actually twinned triclinic. The current form is a new triclinic polymorph, with Z'= 2, in addition to the already known tetragonal polymorph. Although the geometric parameters of the two polymorphs agree well, their packing patterns are completely different.