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.     

Symmetry Breaking in Pyrrolo[3,2‐b]pyrroles: Synthesis,Solvatofluorochromism and Two‐photon Absorption

Five centrosymmetric and one dipolar pyrrolo[3,2‐b ]pyrroles, possessing either two or one strongly electron‐withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4‐nitrophenyl moiety and the electron‐rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two‐photon absorption (2PA) spectra and 2PA cross‐sections (σ_2PA) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2‐b ]pyrrole, followed the corresponding one‐photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700–900 nm. The expansion via a triple bond resulted in the largest peak value, σ_2PA=770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two‐ or even three‐fold. In the S ₀→S ₁ transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long‐wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2–7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.     

Highly Polarized Coumarin Derivatives Revisited: Solvent-Controlled Competition Between Proton-Coupled Electron Transfer and Twisted Intramolecular Charge Transfer

Linking a polarized coumarin unit with an aromatic substituent via an amide bridge results in weak electronic coupling that affects the intramolecular electron-transfer (ET) process. As a result of this, interesting solvent-dependent photophysical properties can be observed. In polar solvents, electron transfer in coumarin derivatives of this type induces a mutual twist of the electron-donating and -accepting molecular units (TICT process) that facilitates radiationless decay processes (internal conversion). In the dyad with the strongest intramolecular hydrogen bond, the planar form is stabilized, such that twisting can only occur in highly polar solvents, whereas a fast proton-coupled electron-transfer (PCET process) occurs in nonpolar n-alkanes. The k_PCET rate constant decreases linearly with the energy of the fluorescence maximum in different solvents. This observation can be explained in terms of competition between electron- and proton-transfer from a highly polarized (ca. 15 D) and fluorescent locally excited (¹LE) state to a much less polarized (ca. 4 D) charge-transfer (¹CT) state, a unique occurrence. Photophysical measurements performed for a family of related coumarin dyads, together with results of quantum-chemical computations, give insight into the mechanism of the ET process, which is followed by either a TICT or a PCET process. Our results reveal that dielectric solvation of the excited state slows down the PCET process, even in nonpolar solvents.     

How To Reach Intense Luminescence for Compounds Capable of Excited‐State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9′,10′:4,5]imidazo[1,2‐f]phenanthridine and imidazo[1,2‐f]phenanthridine skeletons, which mediate excited‐state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five‐membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady‐state and time‐resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited‐state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited‐state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT‐capable compounds originates from their enol excited states.     

How To Make Nitroaromatic Compounds Glow: Next-Generation Large X-Shaped,Centrosymmetric Diketopyrrolopyrroles

Red-emissive π-expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three-step strategy involving the preparation of diketopyrrolopyrrole followed by N-arylation and subsequent intramolecular palladium-catalyzed direct arylation. Comprehensive spectroscopic assays combined with first-principles calculations corroborated that both N-arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.     

Dipolar Dyes with a Pyrrolo[2,3‐b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis,Solvatofluorochromism, and Bioimaging

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3‐b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron‐donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two‐photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11‐carbonitrile‐2,3,4,5,6,7‐hexahydro‐1H‐3a,8,13,13b‐tetraazabenzo[b]cyclohepta[1,2,3‐jk]fluorene, was successfully utilized in two‐photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.     

Highly Emissive Luminogens Based on Imidazo[1,2‐a]pyridine for Electroluminescent Applications

A search for novel organic luminogens led us to design and synthesize some N‐fused imidazole derivatives based on imidazo[1,2‐a]pyridine as the core and arylamine and imidazole as the peripheral groups. The fluorophores were synthesized through a multicomponent cascade reaction (A³ coupling) of a heterocyclic azine with an aldehyde and alkyne, followed by Suzuki coupling and a multicomponent cyclization reaction. All of the compounds exhibited interesting photophysical responses, especially arylamine‐containing derivatives, which displayed strong positive solvatochromism in the emission spectra that indicated a more polar excited state owing to an efficient charge migration from the donor arylamine to the imidazo[1,2‐a]pyridine acceptor. The quantum yields ranged from 0.2 to 0.7 and depended on the substitution pattern, most notably that based on the donor group at the C2 position. Moreover, the influence of general and specific solvent effects on the photophysical properties of the fluorophores was discussed with four‐parameter Catalán and Kamlet–Taft solvent scales. The excellent thermal, electrochemical, and morphological stability of the compounds was explored by cyclic voltammetry, thermogravimetric analysis, and AFM methods. Furthermore, to understand the structure, bonding, and band gap of the molecules, DFT calculations were performed. The performance of the electroluminescence behavior of the imidazo[1,2‐a]pyridine derivative was investigated by fabricating a multilayer organic light‐emitting diode with a configuration of ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq₃ (20 nm)/LiF (0.5 nm)/Al(100 nm) (ITO=indium tin oxide, EML=emissive layer, BCP=2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline, Alq₃=tris(8‐hydroxyquinolinato)aluminum), which exhibited white emission with a turn‐on voltage of 8 V and a brightness of 22 cd m^?2.     

Disentangling Multiple Effects on Excited-State Intramolecular Charge Transfer among Asymmetrical Tripartite PPI-TPA/PCz Triads

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron–hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.     

Naphthyl and Thienyl Units as Bridges for Metal-Free Dye-Sensitized Solar Cells

A series of new organic dyes, comprising a naphthyl moiety as a π-conjugated bridge, different amines as donors, and a cyanoacrylic acid group as an electron acceptor and anchoring group, have been designed and synthesized for applications in dye-sensitized solar cells (DSSCs). One of the compounds was also characterized by single-crystal X-ray structural analysis. All of the dyes exhibited maximum absorptions in the range of 371–441 nm. The short-circuit photocurrent density, open-circuit voltage, and fill factor (FF) values of the devices are in the range of 6.13–10.90 mA cm⁻², 0.62–0.69 V, and 0.62–0.67, respectively, corresponding to an overall conversion efficiency of 2.76–4.55 %. The conversion efficiency reached 38–62 % of that of a N719-based device (7.31 %) fabricated and measured under similar conditions. Steric congestion between the naphthyl and aromatic moieties jeopardizes charge transfer from the donor to the acceptor. Insertion of an alkenyl entity between the naphthyl entity and the aromatic ring alleviates steric congestion and leads to longer wavelength electronic absorption spectra.     

Extended Alkenyl and Alkynyl Benzotriazoles with Enhanced Two-Photon Absorption Properties as a Promising Alternative to Benzothiadiazoles

A series of donor–π–acceptor–π–donor (D -π-A-π-D) benzoazole dyes with 2H-benzo[d][1,2,3]triazole or BTD cores have been prepared and their photophysical properties characterized. The properties of these compounds display remarkable differences, mainly as a result of the electron-donor substituent. Dyes with the best properties have visible-light absorption over λ=400 nm, large Stokes shifts in the range of about 3500–6400 cm⁻¹, and good fluorescence emission with quantum yields of up to 0.78. The two-photon absorption properties were also studied to establish the relationship between structure and properties in the different compounds synthesized. These results provided cross sections of up to 1500 GM, with a predominance of S₂←S₀ transitions and a high charge-transfer character. Time-dependent DFT calculations supported the experimental results.     

Synthesis of Functionalized Tetrahydropyridines by SnCl4-Mediated [4+2] Cycloaddition between Donor–Acceptor Cyclobutanes and Nitriles

Cycloadditions of strained carbocycles promoted by Lewis acids are powerful methods to construct heterocyclic frameworks. In fact, the formal [3+2] cycloadditions of donor–acceptor (DA) cyclopropanes with nitriles has seen particular success in synthesis. In this work, we report on the first [4+2] cycloaddition of nitriles with DA cyclobutanes by Lewis acid activation. Tetrahydropyridine derivatives were obtained in up to 91 % yield from various aryl-activated cyclobutane diesters and aliphatic or aromatic nitriles.     

Synthesis and Reactions of 4-Oxiranylmethylfuro[3,2-b]pyrroles and Their Benzo Derivatives

Methyl 4-oxiranylmethyl-4H-furo[3,2-b]pyrrole-5-carboxylates 2a-c and methyl 1-oxiranylmethyl-1H-benzo[4,5]furo[3,2-b]pyrrole-2-carboxylate (2d) were prepared by reaction of the appropriate starting compounds 1a-d with excess chloromethyloxirane. The compounds 2a-d undergo oxirane ring opening by heterocyclic amines (morpholine, pyrrolidine, piperidine or 4-methylpiperazine) giving N-2-hydroxy-3-heteroaminopropyl-substituted compounds 3a-f or substituted 4,5-dihydrofuro[2',3':4,5]pyrrolo[2,1-c][1,4]oxazin-8-ones 4a-e.     

Benzo[1,2‐d:4,5‐d′]bisimidazoles as a Convenient Platform Towards Dyes that are Capable of Excited‐State Intramolecular Proton Transfer and of Two‐Photon Absorption

New, strongly fluorescent benzo[1,2‐d:4,5‐d′]bisimidazoles have been prepared by the reaction of Bandrowski′s base with various aldehydes. Their structures were carefully designed to achieve efficient excited‐state intramolecular proton transfer and good two‐photon‐absorption (2PA) cross‐sections. Functional dyes that possessed both high fluorescence quantum yields and large Stokes shifts were prepared. A π‐expanded D‐A‐D derivative that possessed Φ_fl=50 % and σ₂=230 GM in the spectroscopic area of interest for biological imaging is an excellent candidate as a fluorescent probe. Thanks to the presence of two reactive amino groups, such compounds can be easily transformed into probes for bioconjugation. All of these benzo[1,2‐d:4,5‐d′]bisimidazoles were also strongly fluorescent in the solid state.     

Thermally Activated Delayed Fluorescence from d10-Metal Carbene Complexes through Intermolecular Charge Transfer and Multicolor Emission with a Monomer–Dimer Equilibrium

A series of two-coordinate Au^I and Cu^I complexes ( 3 a , 3 b and 5 a , 5 b ) are reported as new organometallic thermally activated delayed fluorescence (TADF) emitters, which are based on the carbene–metal–carbazole model with a pyridine-fused 1,2,3-triazolylidene (PyTz) ligand. PyTz features low steric hindrance and a low-energy LUMO (LUMO=−1.47 eV) located over the π* orbitals of the whole ligand, which facilitates intermolecular charge transfer between a donor (carbazole) and an accepter (PyTz). These compounds exhibit efficient TADF with microsecond lifetimes. Temperature-dependent photoluminescence kinetics of 3 a supports a rather small energy gap between S₁ and T₁ (ΔE =60 meV). Further experiments reveal that there are dual-emission properties from a monomer–dimer equilibrium in solution, exhibiting single-component multicolor emission from blue to orange, including white-light emission.     

New Dyes Based on Extended Fulvene Motifs: Synthesis through Redox Reactions of Naphthoquinones with Donor–Acceptor Cyclopropanes and Their Spectroelectrochemical Behavior

Novel dyes based on extended fulvene motifs are reported. The carbon skeleton was generated by a catalyzed addition of donor–acceptor cyclopropanes to naphthoquinone. The hydroxy group at the central ring of the tricyclic fulvene motif was converted into the triflate, which reacted efficiently with a wide range of nucleophiles, resulting in substitution and thereby providing new derivatives. The synthetic versatility allowed us to investigate the absorption, electrochemical, and UV/Vis-NIR spectroelectrochemical properties of these dyes as a function of the substituents. The dyes were shown to participate in reductive electrochemistry, the reversibility of which can be improved by appropriate selection of the substituents. Additionally, first signs of NIR electrochromism are presented, opening new avenues for the future investigations of such dyes.     

Imidazole‐Annulated Tetrathiafulvalenes Exhibiting pH‐Tuneable Intramolecular Charge Transfer and Redox Properties

CT or not CT : Three imidazole‐annulated TTF derivatives 1 – 3 have been prepared and fully characterized. The influence of the TTF unit on the pK_a values of the acceptor units as determined by photometric titration has been discussed. The results reported here are part of an initial exploratory study to generate a range of well‐defined coordination networks.

     

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.     

Synthesis of pyrrolo[3,4-b]pyrroles and perhydrothiazolo[3′,4′-2,3]pyrrolo[4,5-c]pyrroles

The 1,3-dipolar cycloaddition reactions of various N-tethered alkenyl aldehydes with some cyclic and acyclic amino acids have been studied. Some key sulfonamides having strategically positioned aldehyde and olefinic tether have been synthesized and effectively subjected to intramolecular azomethine ylide cycloaddition reaction resulting in a series of pyrrolo[3,4-b]pyrrole and its N-1-C-2 derivatives, and a series of novel heterotricyclic compounds, perhydrothiazolo[3′,4′-2,3]pyrrolo[4,5-c]pyrroles, in good yields. The intramolecular cycloaddition reaction was found to be highly stereoselective to form only cis-fused cycloadducts in all cases.