Revisiting Indolo[3,2‐b]carbazole: Synthesis,Structures, Properties,and Applications

Indolo[3,2‐b]carbazole presents a π‐skeleton with a remarkable electronic structure and interesting potential applications. It is, however, also associated with ambiguity and controversy. Herein, new derivatives of indolo[3,2‐b]carbazole are reported and they have enabled a comprehensive study on the electronic structure of indolo[3,2‐b]carbazole and the development of a new n‐type organic semiconductor. Experimental and computational studies show that indolo[3,2‐b]carbazole has a largely localized p‐benzoquinonediimine moiety and significant antiaromaticity. When substituted with (4‐silylethynyl)phenyl groups, the indolo[3,2‐b]carbazole exhibits one‐dimensional π–π stacking and functions as an n‐type organic semiconductor in solution‐processed field effect transistors.     

An Alternating Copolymer Derived from Indolo[3,2‐b]carbazole and 4,7‐Di(thieno[3,2‐b]thien‐2‐yl)‐2,1,3‐benzothiadiazole for Photovoltaic Cells

An alternating narrow bandgap conjugated copolymer (PICZ‐DTBT, E_g = 1.83 eV) derived from 5,11‐di(9‐heptadecanyl)indolo[3,2‐b]carbazole and 4,7‐di(thieno[3,2‐b]thien‐2‐yl)‐2,1,3‐benzothiadiazole (DTBT), was prepared by the palladium‐catalyzed Suzuki coupling reaction. The resultant polymer absorbs light from 350–690 nm, exhibits two absorbance peaks at around 420 and 570 nm and has good solution processibility and thermal stability. The highest occupied molecular orbital (HOMO) energy level and lowest unoccupied molecular orbital (LUMO) level of the copolymer determined by cyclic voltammetry were about −5.18 and −3.35 eV, respectively. Prototype bulk heterojunction photovoltaic cells from solid‐state composite films based on PICZ‐DTBT and [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM), show power conversion efficiencies up to 2.4% under 80 mW · cm⁻² illumination (AM1.5) with an open‐circuit voltage of V_oc = 0.75 V, a short current density of J_sc = 6.02 mA · cm⁻², and a fill factor of 42%. This indicates that the copolymer PICZ‐DTBT is a viable electron donor material for polymeric solar cells.

     

Z‐Shaped Pyrrolo[3,2‐b]pyrroles and Their Transformation into π‐Expanded Indolo[3,2‐b]indoles

Sterically hindered 1,4‐dihydropyrrolo[3,2‐b]pyrroles possessing ortho‐(arylethynyl)phenyl substituents at positions‐2 and ‐5 were efficiently synthesized through a sila‐Sonogashira reaction. These unique Z‐shaped dyes showed relatively strong fluorescence in solution. Detailed optimization revealed that, in the presence of InCl₃, these alkynes readily undergo an intramolecular double cyclization to give hexacyclic products bearing an indolo[3,2‐b]indole skeleton in remarkable yields. Steady‐state UV–visible spectroscopy revealed that upon photoexcitation, the prepared Z‐shaped alkynes undergo mostly radiative relaxation leading to high fluorescence quantum yields. In the case of 7,14‐dihydrobenzo[g]benzo[6,7]indolo[3,2‐b]indoles, we believe that the substantial planarization of geometry in the excited state, is the underlying reason for the observed large Stokes shifts. The presence of additional electron‐withdrawing groups makes it possible to further alter the photophysical properties. The two‐photon absorption cross‐section values of both families of dyes were found to be modest and the nature of the excited state responsible for two‐photon absorption appeared to be strongly affected by the presence of peripheral groups. Serendipitous synthesis of unusual double‐Z‐shaped alkyne by Sonogashira and Glaser coupling is also reported.     

Synthesis of New tert‐Butyl‐ and Bromo‐functionalized [1,2,4]Triazino [5,6‐b]indole‐3‐thiols and Indolo[2,3‐b]quinoxalines

In the present investigation, the synthesis of a series of structurally new and interesting tert‐butyl‐ and bromo‐functionalized [1,2,4]triazino[5,6‐b ]indoles ( 6a – f ) and indolo[2,3‐b ]quinoxalines ( 8a – f ) has been achieved, involving the condensation reaction of 7‐bromo‐5‐tert‐butylisatins ( 4a – f ) with thiosemicarbazide ( 5 ) and benzene‐1,2‐diamine ( 7 ). The substrates 4a – f were prepared through bromination reaction of 5‐tert‐butylisatin ( 3 ) with NBS in PEG‐400 followed by alkylation reaction. The molecular structures of these newly synthesized compounds were elucidated on the basis of their elemental analyses and spectral data.     

Fluorescence Properties of Diphenylthiazolo[4,5‐b]pyrazines Tuned by Donor‐Acceptor Substituent Effects

Fluorescence properties of 2,6‐ and 2,5‐diphenylthiazolo[4,5‐b]pyrazine (TPy) derivatives having an electron‐donating substituent (methoxy and dimethylamino) on the 6‐ and 5‐phenyl groups were studied. It was found that 2,6‐diphenyl derivatives fluoresce more efficiently than 2,5‐diphenyl derivatives. Furthermore, a 2,6‐diphenyl derivative having an additional cyano group on the 2‐phenyl ring was an excellent fluorophore showing a wide solvatochromism with great fluorescence yields. Based on the obtained spectroscopic data and mechanistic explanations concerning the substituent effects on the fluorescence properties, useful information on designing new TPy fluorophores is provided.     

Donor–acceptor‐integrated conjugated polymers based on carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole with tight π–π stacking for photovoltaics

An original strategy to construct a new donor–acceptor (D–A)‐integrated structure by directly imposing “pull” unit on the “push” moiety to form fused ring architecture has been developed, and poly{N‐alkyl‐carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole‐alt‐thiophene} (PCBTT) with D–A‐integrated structure, in which two 1,2,5‐thiadiazole rings are fixed on carbazole in 3‐, 4‐ and 5‐, 6‐position symmetrically and thiophene is used as bridge, has been synthesized. The interaction between pull and push units has fine tuned the HOMO/LUMO energy levels, and the resulting copolymer covers the solar flux from 300 to 750 nm. The interaction between pull and push units is worth noting that due to the fused five rings inducing strong intermolecular interaction, an extremely short π–π stacking distance of 0.32 nm has been achieved for PCBTT both in powder and solid states. This is the shortest π–π stacking distance reported for conjugated polymers. Additionally, an obvious intramolecular charge transfer and energy transfer from donor units to acceptor units have been detected in this D–A integration. A moderate‐to‐high open‐circuit voltage of ～0.7 V in PCBTT:[6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) (w/w = 1/2) solar cells is achieved due to the low‐lying HOMO energy level of PCBTT. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis and Structure–Property Relationships of 2,2′‐Bis(benzo[b]phosphole) and 2,2′‐Benzo[b]phosphole–Benzo[b]heterole Hybrid π Systems

The first comprehensive study of the synthesis and structure–property relationships of 2,2′‐bis(benzo[b]phosphole)s and 2,2′‐benzo[b]phosphole–benzo[b]heterole hybrid π systems is reported. 2‐Bromobenzo[b]phosphole P‐oxide underwent copper‐assisted homocoupling (Ullmann coupling) and palladium‐catalyzed cross‐coupling (Stille coupling) to give new classes of benzo[b]phosphole derivatives. The benzo[b]phosphole–benzo[b]thiophene and ‐indole derivatives were further converted to P,X‐bridged terphenylenes (X=S, N) by a palladium‐catalyzed oxidative cycloaddition reaction with 4‐octyne through the C_β? H activation. X‐ray analyses of three compounds showed that the benzo[b]phosphole‐benzo[b]heterole derivatives have coplanar π planes as a result of the effective conjugation through inter‐ring C? C bonds. The π–π* transition energies and redox potentials of the cis and trans isomers of bis(benzo[b]phosphole) P‐oxide are very close to each other, suggesting that their optical and electrochemical properties are little affected by the relative stereochemistry at the two phosphorus atoms. The optical properties of the benzo[b]phosphole–benzo[b]heterole hybrids are highly dependent on the benzo[b]heterole subunits. Steady‐state UV/Vis absorption/fluorescence spectroscopy, fluorescence lifetime measurements, and theoretical calculations of the non‐fused and acetylene‐fused benzo[b]phosphole–benzo[b]heterole π systems revealed that their emissive excited states consist of two different conformers in rapid equilibrium.     

β‐Substituted Terthiophene [2]Rotaxanes

Towards polythiophene polyrotaxanes : The β‐substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported.

     

Improved and Efficient Synthesis of Chromeno[4,3‐d]pyrazolo [3,4‐b]pyridin‐6(3H)‐ones and Their Fluorescence Properties

A series of chromeno[4,3‐d]pyrazolo[3,4‐b]pyridin‐6(3H)‐one derivatives was easily and efficiently synthesized by the reaction of 2H‐chromen‐2‐ones with pyrazol‐5‐amines catalyzed by CuCl₂?2H₂O in ethanol. This protocol has the advantages of mild reaction conditions, easy work‐up, and high yields. These compounds were showed to have high fluorescence quantum yields, which mentioned their value as luminescence or fluorescence probe.     

Synthesis of Tetrahydrothiopyrano[2,3‐b]indole [60]Fullerene Derivatives via Hetero‐Diels–Alder Reaction of C60 and α,β‐Unsaturated Indole‐2‐thiones

Hetero‐Diels–Alder reactions of [60]fullerene with α,β‐unsaturated thio‐oxindoles ( 3a , 3b , 3c ), prepared from thio‐oxindole 1 and heteroaromatic aldehydes ( 2a , 2b , 2c ), to generate tetrahydrothiopyrano[2,3‐b ]indole [60]fullerene cycloadducts ( 5a , 5b , 5c ) under thermal or microwave irradiation were described. The yields were improved, and the reaction time was decreased by conducting the reaction under microwave irradiation.     

Ni‐Catalyzed Synthesis of Fluoroarenes via [2+2+2] Cycloaddition Involving α‐Fluorine Elimination

A method for direct synthesis of tetrasubstituted fluoroarenes via nickel‐catalyzed [2+2+2] cycloaddition is presented. The reaction combines one molecule of 1,1‐difluoroethylene with two molecules of alkynes and involves sequential cleavage of the C?F and C?H bonds in difluoroethylene. The catalytic cycle is established by reduction of the intermediary Ni^II fluoride with a triethylborane‐based borate.     

Exploring Reversible Quenching of Fluorescence from a Pyrazolo[3,4‐b]quinoline Derivative by Protonation

Pyrazolo[3,4‐b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light‐emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H₂O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady‐state and transient fluorescence spectra of 1‐phenyl‐3,4‐dimethyl‐1H‐pyrazolo‐[3,4‐b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground‐state proton‐transfer complex [PAQ5 ??? H⁺] is responsible for static quenching. It changes linearly with proton concentration [H⁺] with a bimolecular association constant K_S=1.95 M ^?1 controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K_D=22.4 M ^?1 is obtained by fitting to the Stern–Volmer equation, with a bimolecular dynamic quenching rate constant k_d=1.03×10⁹ s^?1 M ^?1 under ambient conditions. A change in electron distribution is simulated and explains the experiment results.     

Synthesis of indeno[2,1‐a]pyrrolo[3,4‐c]carbazole lactam regioisomers using ethyl cis‐β‐cyanoacrylate as a dienophile and lactam precursor

Reported here is the synthesis and characterization of the indenopyrrolocarbazole ring system utilizing a Diels‐Alder reaction with 2‐indenylindole and maleimide. Clemmensen reduction of imide 10 furnished the 5‐oxo ( 16 ) and 7‐oxo ( 17 ) lactam regioisomers. A new regiospecific route to 5‐oxo 16 was developed using ethyl cis‐β‐cyanoacrylate as the dienophile. The regio and stereochemical characterization of the cycloadducts was confirmed by X‐ray crystallography.     

Low Band Gap Donor–Acceptor Conjugated Systems Based on 3‐Alkoxy or 3‐Pyrrolidino‐4‐cyanothiophene and Benzothiadiazole Units

3‐Hexyloxy‐4‐cyanothiophene, 3‐pyrrolidil‐4‐cyanothiophene, and 3,4‐ethylenedioxythiophene (EDOT) units are used with benzothiadiazole as building blocks for the development of three new conjugated donor–acceptor–donor (DAD) derivatives. The DAD molecules have the central acceptor part, which is formed by combining electron‐withdrawing cyano groups and the benzothiadiazole moiety, in common. Theoretical calculations and UV/Vis and electrochemical data reveal the key role of the end‐capped donor to tune the electronic properties of the derivatives. A study of the electropolymerization process of the three derivatives shows the strong influence of the donor parts on both the reactivity of the precursors and the electronic properties of the resulting polymers. Derivatives end‐capped with pyrrolidinocyano thiophene or EDOT units lead to films of polymers presenting low band gaps of around 0.9–1.4 eV. Upon oxidation, the two polymers present different behavior. In the presence of the pyrrolidinocyano thiophene moieties, oxidation leads to a blueshift of the absorption bands, whereas with EDOT units a classical redshift, giving high absorption in the near‐IR region, is observed for the oxidized states.     

Donor–acceptor rod–coil block copolymers comprising poly[2,7‐(9,9‐dihexylfluorene)‐alt‐bithiophene] and fullerene as compatibilizers for organic photovoltaic devices

The successful synthesis is described for a donor–acceptor rod–coil block copolymer comprising blocks of poly[2,7‐(9,9‐dihexylfluorene)‐alt‐bithiophene] (F6T2) and polystyrene functionalized with fullerene (PS(C₆₀)) (F6T2‐b‐PS(C₆₀)). This new material was obtained by combining Suzuki polycondensation with radical addition fragmentation chain transfer. The block copolymer was characterized by nuclear magnetic resonance, gel permeation chromatography, and optical spectroscopy methods. Photophysical data for (F6T2‐b‐PS(C₆₀)) and a related block copolymer (F6T2‐b‐PS(PCBM)) (PCBM, phenyl‐C61‐butyric acid methyl ester) are reported and their performance as compatibilizers in bulk heterojunction organic solar cells is assessed. It is demonstrated that the addition of the rod–coil block copolymers to the active layer extends the operational stability of organic photovoltaic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 888–903     

An Alternate Synthesis of 6H‐Indolo[2,3‐b]quinoline via One‐Pot Alkylation–Dehydration–Cyclization–Aromatization Approach

A simple, straightforward and efficient synthesis of 6H‐indolo[2,3‐b]quinoline, a natural product isolated from leaves of Justicia betonica, is achieved through a pivalic acid‐assisted one‐pot alkylation–dehydration–cyclization–aromatization approach. This synthesis constitutes a formal approach toward a biologically important alkaloid neocryptolepine.     

Donor–acceptor conjugated copolymers incorporating tetrafluorobenzene as the π‐electron deficient unit

We present the synthesis and characterization of a new family of perfectly alternating conjugated polymers, obtained through different methodologies (Stille, Direct Arylation, and Horner–Wadsworth–Emmons polymerizations). The polymers comprise either 2,5‐dialkoxybenzene or benzodithiophene electron rich units, and 1,2,4,5‐tetrafluorobenzene as the electron‐deficient unit, eventually separated by a vinylene bridge, if suitable monomers and HWE polymerization procedures are used. As shown by NMR spectroscopy, the introduction of the fluorinated aromatic units brings complications in the polymer stereodefinition in the HWE polymerization, and regiodefinition in the case of the Direct Arylation. The polymers show moderate degrees of polymerization (up to 10 repeating alternating units in the backbone), which are however significant enough to unravel interesting properties such as energy HOMO–LUMO gaps and aggregation behavior in solution at room temperature. In depth calculations fully confirmed the aggregation tendency, highlighting the key role of the benzodithiophene as the donor component when in combination with the tetrafluorobenzene unit. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1601–1610     

Construction of Spirocyclic Oxindoles through Regio‐ and Stereoselective [3+2] or [3+2]/[4+2] Cascade Reaction of α,β‐Unsaturated Imines with 3‐Isothiocyanato Oxindole

On the basis of asymmetric regioselective [3+2] or [3+2]/[4+2] cascade reaction of 3‐isothiocyanato oxindoles with C=C and C=N bonds of α,β‐unsaturated methanesulfonamides, diversified S‐containing heterocyclic spirooxindole derivatives could be obtained in high yields along with good to excellent diastereo‐ and enantioselectivities under mild conditions in the presence of cinchona alkaloid‐derived organocatalysts.     

A meso–meso β‐β β‐β Triply Linked Subporphyrin Dimer

A meso–meso β‐β β‐β triply linked subporphyrin dimer 6 was synthesized by stepwise reductive elimination of β‐to‐β doubly Pt^II‐bridged subporphyrin dimer 9 . Dimer 6 was characterized by spectroscopic and electrochemical measurements, theoretical calculations, and picosecond time‐resolved transient absorption spectroscopy. X‐ray diffraction analysis reveals that 6 has a bowl‐shaped structure with a positive Gaussian curvature. Despite the curved structure, 6 exhibits a remarkably red‐shifted absorption band at 942 nm and a small electrochemical HOMO–LUMO gap (1.35 eV), indicating an effectively conjugated π‐electronic network.