Bridging Heteroatoms Research Articles

Xanthene-Based Dyes for Photoacoustic Imaging and their Use as Analyte-Responsive Probes.

Developing imaging tools that can report on the presence of disease-relevant analytes in multicellular organisms can provide insight into fundamental disease mechanisms as well as provide diagnostic tools for the clinic. Photoacoustic imaging (PAI) is a light-in, sound-out imaging technique that allows for high resolution, deep-tissue imaging with applications in pre-clinical and point-of-care settings. The continued development of near-infrared (NIR) absorbing small-molecule dyes promises to improve the capabilities of this emerging imaging modality. For example, new dye scaffolds bearing chemoselective functionalities are enabling the detection and quantification of disease-relevant analytes through activity-based sensing (ABS) approaches. Recently described strategies to engineer NIR absorbing xanthenes have enabled development of analyte-responsive PAI probes using this classic dye scaffold. Herein, we present current strategies for red-shifting the spectral properties of xanthenes via bridging heteroatom or auxochrome modifications. Additionally, we explore how these strategies, coupled with chemoselective spiroring-opening approaches, have been employed to create ABS probes for in vivo detection of hypochlorous acid, nitric oxide, copper (II), human NAD(P)H: quinone oxidoreductase isozyme 1, and carbon monoxide. Given the versatility of the xanthene scaffold, we anticipate continued growth and development of analyte-responsive PAI imaging probes based on this dye class.

Highly efficient ionic thermally activated delayed fluorescence emitters with short exciton lifetimes towards High-Performance Solution-Processed OLEDs

Ionic thermally activated delayed fluorescence (iTADF) materials are newly emerging as promising candidates for constructing solution-processed electroluminescence devices. Developing iTADF materials with high emission efficiency, short exciton lifetime, and fast reverse intersystem crossing (RISC) is crucial for achieving stable and efficient organic light-emitting diodes (OLEDs), but it remains challenging. Here we report two high-performance iTADF emitters, namely DMAC-TPPO[BF4] and DMAC-TPPS[BF4], which consist of a conventional acridine as the electron-donor and newly developed oxygen/sulfur-fused arylphosphonium derivatives as the electron-acceptor. DMAC-TPPO[BF4] and DMAC-TPPS[BF4] exhibit high photoluminescence quantum yields of 77 % and 83 %, coupled with short exciton lifetimes of 1.2 and 1.1 μs, and RISC rate constants reaching up to 2.60 × 106 and 2.73 × 106 s−1, respectively. Theoretical and experimental investigations demonstrate that the imbedding of bridging heteroatoms into the phosphonium cation is pivotal for enhancing emission efficiency through molecular skeleton rigidification, as well as for accelerating the RISC process by capitalizing on the non-metal heavy-atom effect. Eventually, solution-processed OLEDs utilizing DMAC-TPPS[BF4] realize a peak external quantum efficiency (EQE) of 17.8 % and show a minor efficiency roll-off of 7.3 % (EQE = 16.5 %) even at a practical high luminance of 1000 cd/m2.

Dopant-free hole transporting polymeric materials based on pyrroloindacenodithiophene donor unit for efficient perovskite solar cells

The hole transporting materials (HTMs) have been extensively studied since the HTMs play a critical role in promoting charge extraction/transport and reducing charge recombination between the perovskite layer and the top metal electrode, thus improving the device power conversion efficiency (PCE). In this study, novel polymeric HTMs with pentacyclic pyrroloindacenodithiophene (NIDT) as an electron-rich unit, and electron-deficient benzotriazole (BTA) as an acceptor, labelled as SGT-512 and SGT-513, were designed and synthesized. To regulate the electron withdrawing ability of the acceptor part, two thiophene units were introduced to the BTA moiety as an acceptor, yielding SGT-513 with a slightly raised HOMO energy level. The alkyl side-chain was introduced to the fused BTA unit to improve the solubility and regulate the morphology and stacking behavior of HTM. Meanwhile, for the donor part, the replacement of the bridging heteroatom in indacenodithiophene with the more electron donating nitrogen atom, helped to promote molecular orbital hybridization leading to a narrower bandgap polymer. The combination of the features of the NIDT donor and BTA acceptor enabled the production of the D-A type polymeric HTM SGT-512 with a high intrinsic hole mobility of 3.8 × 10−4 cm2V−1s−1, a suitable HOMO energy level (−5.25 eV) corresponding to the valence band of the perovskite layer, and excellent film-formation ability. Therefore, the PrSCs based on dopant-free SGT-512 showed a high PCE of 15.57%, which is much higher than that of the control device based on dopant free-P3HT, with a PCE of 13.52%.

Impact of polycyclic aromatic hydrocarbons and heteroatomic bridges (N, S, and O) on optoelectronic properties of 1,3,5‐triazine derivatives: A computational insight

AbstractThe theoretical investigation of four series of 1,3,5‐triazine derivatives (viz.,TN,NTN,STN, andOTNseries), which are substituted with polycyclic aromatic hydrocarbons, is carried out by employing density functional theory and time dependent‐density functional theory. The triazine ring acts as electron acceptor (A) and polycyclic aromatic hydrocarbons act as electron donors (D). The aromatic hydrocarbons like benzene, naphthalene (two fused rings), anthracene (three fused rings), and pyrene (four fused rings) are substituted on 1,3,5‐triazine core by means of a bridging heteroatom N, S, and O in the case ofNTN,STN, andOTNseries of molecules, respectively, whereas polycyclic aromatic hydrocarbons are directly connected to 1,3,5‐triazine in the case ofTNseries of molecules. The present work aims at examining the impact of number of fused aromatic rings and changing the bridging unit between electron acceptor and electron donor on absorption properties, HOMO‐LUMO gap and first hyperpolarizabilities of molecules under study and provides important insights for architecture of organic molecules for applications in organic optoelectronics.

Enhanced electrocatalytic oxygen evolution by manipulation of electron transfer through cobalt-phosphorous bridging

Bridging heteroatoms on the surface of electrocatalysts, aiming to modify the surface atomic and electronic configuration, is of prime importance to improve the electrocatalytic performance. The present work establishes Co-P bridging on the surface of Co3O4 via a vapor-assisted treatment in a controlled molar ratio of P/Co (0.08–0.66). The oxygen evolution reaction (OER) activity of P/Co3O4 showed a compositional dependence, reaching the optimal performance with a low overpotential of 314 mV for achieving a current density of 10 mA cm−2. Both experimental characterization and density functional theory calculations indicate that the improved OER performance derives from the establishment of Co-P bridging. The bridging creates more Co2+ ions and oxygen vacancies by driving electrons transfer between P and Co, and thus results in the creation of more active sites, improvement of intrinsic activity, enhancement of electrical conductivity and improvement of electrochemical stability. Furthermore, the present work demonstrates that this generic synthetic approach can also be applied to improve HER and overall water splitting with higher electrochemical activity.

Heteroatom Bridged Tetrathiafulvalenes

Tetrathiafulvalene (TTF) derivatives are the most representative electroactive precursors for the preparation of crystalline conducting materials. The occurrence of mixed valence states, through electron delocalization, as well as strong intermolecular interactions, are important prerequisites to account for the high electronic conduction in the solid state and the dimensionality of the material. In this respect, the modulation of through space or through bond intramolecular electron communication can be efficiently achieved in heteroatom bridged bis(TTF) derivatives. Moreover, the bridging heteroatoms can engage, depending on their nature, in intermolecular interactions in the solid state. In this Minireview, we focus on flexible and rigid bis(TTF) derivatives formulated as X(TTF)2 and X2(TTF)2 (X = Si, Ge, Sn, P, Sb, S, Se, Te) thus containing one or two bridges in ortho position. A special attention is given to the solid‐state structures of these precursors and their oxidized species, their electrochemical behavior, reactivity, role of the heteroatom in the establishment of intramolecular mixed valence and intermolecular interactions, and the conducting properties.

How the electron-deficient cavity of heterocalixarenes recognizes anions: insights from computation

We have quantum chemically analyzed the bonding mechanism behind the affinity of various heterocalixarenes for anions with a range of geometries and net charges, using modern dispersion-corrected density functional theory (DFT-D3BJ). The purpose is to better understand the physical factors that are responsible for the computed affinities and thus to develop principles for a more rational design of anion receptors. Our model systems comprise heterocalixarenes 1-4 as hosts, which are characterized by different bridging heteroatoms (O, N, S) as well as the anionic guests Cl-, Br-, I-, BF4-, CH3CO2-, H2PO4-, HSO4-, NCS-, NO3-, PF6-, and SO42-. We use various analysis schemes (EDA, NCI, and NBO) to elucidate the interactions between the calixarene cavity and the anions to probe the importance of the different bonding modes (anion-π, lone-pair electron-π, σ-complexes, hydrogen bonds, and others) of the interactions. Electrostatic interactions appear to be dominant for heterocalixarenes with oxygen bridges whereas orbital interactions prevail in the case of nitrogen and sulfur bridges. Dispersion interactions are however in all cases non-negligible.

Synthesis, Structure, and Molecular Recognition of S6‐ and (SO2)6‐Corona[6](het)arenes: Control of Macrocyclic Conformation and Properties by the Oxidation State of the Bridging Heteroatoms

We report herein the synthesis, structure, and molecular recognition of S6 - and (SO2 )6 -corona[6](het)arenes, and demonstrate a unique and efficient strategy of regulating macrocyclic conformation and properties by adjusting the oxidation state of the heteroatom linkages. The one-pot nucleophilic aromatic substitution reaction of 1,4-benzenedithiol derivatives, biphenyl-4,4'-dithiol and 9,9-dipropyl-9H-fluorene-2,7-dithiol with 3,6-dichlorotetrazine afforded S6 -corona[3]arene[3]tetrazines. These compounds underwent inverse-electron-demand Diels-Alder reaction with enamines and norbornadiene to produce S6 -corona[3]arene[3]pyridazines. Facile oxidation of sulfide linkages yielded (SO2 )6 -corona[3]arene[3]pyridazines. All corona[6](het)arenes adopted generally hexagonal macrocyclic ring structures; however, their electronic properties and conformation could be fine-tuned by altering the oxidation state of the sulfur linkages. Whereas (SO2 )6 -corona[3]arene[3]pyridazines were electron-deficient, S6 -corona[3]arene[3]pyridazines acted as electron-rich macrocyclic hosts that recognized various organic cations in both aqueous and organic solutions.

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

The effects of heteroatom substitution from a silicon atom to a germanium atom in donor‐acceptor type low band gap copolymers, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PSiBTBT) and poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl‐C70‐butyric acid methyl ester (PC70BM). It is found that the longer C‐Ge bond length, in comparison to C‐Si, modifies the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBTBT. This increase in molecular planarity leads to enhanced crystallinity and an increased preference for a face‐on backbone orientation, thus leading to higher charge carrier mobility in the diode configuration. These results provide important insight into the impact of the heavy atom substitution on the molecular packing and device performance of polymers based on the poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole) (PCPDTBT) backbone.

Evaluation of Heterocycle-Modified Pentathiophene-Based Molecular Donor Materials for Solar Cells

Two novel solution-processable acceptor-donor-acceptor (A-D-A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.

Synthesis and Properties of Bisphosphole‐Bridged Ladder Oligophenylenes

Ladder-type oligophenylenes (LOPP) with bridging heteroatoms are interesting systems as they offer novel electronic and photophysical properties on account of the rigid structural features, more efficient electron delocalization on the coplanar aromatic framework, and strong intermolecular interactions. LOPPs incorporating multiple phosphorous centers combine the excellent electronic properties of phospholes and rigidified conjugated framework of LOPPs, thus positioning themselves as an attractive class of organic semiconductors. To date, there still lacks an effective synthetic methodology towards LOPPs with multiple phosphorous bridges. Herein, we describe the synthesis and properties of a new class of bisphosphole-bridged ladder oligo(p-phenylene)s and the related phosphoxides. The synthesis of phospholes was achieved by a four-fold free-radical phosphanylation reaction of a tetrabromo p-terphenylene or biphenyl-thiophene. Sequential trapping of four highly reactive aryl radicals occurred effectively to give the desired phosphorous-containing ladder compound. The oxides of the phospholes are shown to be strong fluorophores that can be used as potential n-type building blocks for organic semiconducting materials.

Comparative Optoelectronic Study between Copolymers of Peripherally Alkylated Dithienosilole and Dithienogermole

Here we report a simple methodology for the synthesis of dithienosilole and dithienogermole monomers in which the necessary solubilizing long chain alkyl groups are incorporated into the peripheral 3,5-positions of the fused ring. We report four novel monomers in which methyl or butyl groups are attached to the bridging Si and Ge atom. Copolymers with bithiophene were synthesized by a Stille polymerization in high molecular weight. We report the optical, electrical, electrochemical and morphological properties of the resulting polymers. We find that the nature of the bridging heteroatom (Si or Ge) has only a minor influence on these properties, whereas the nature of the alkyl chain attached to the bridging atom is found to have a much larger effect.

Racemization Mechanisms and Electronic Circular Dichroism of [4]Heterohelicenium Dyes: A Theoretical Study

Triarylmethylium cations with the three rings linked by two bridging groups constitute a special class of [4]heterohelicenium dyes that combine high configurational stability with the optical properties of classic dye compounds. The racemization barriers and electronic circular dichroism of seven [4]heterohelicenium analogues are investigated with density functional theory to consider different design strategies. The racemization barriers are examined with the B3LYP functional utilizing the basis set 6-31+G* with respect to bridging heteroatoms in the helicenium motif and with different bulky substituents in the helix pitch. The racemization barriers of the [4]heterohelicenium are found to be highly dependent on the relative size of the bridging atom. Nucleophilic attack at the carbenium ion leads to formation of center adducts for which the racemization barriers are found to be lowered by 8-12 kJ/mol. This finding of a nucleophilic racemization catalysis may be rationalized by the loss of conjugation upon formation of an sp(3)-hybridized carbon in the center adducts. The effect of the heteroatom substitution and center adduct formation is reflected in the electronic properties of the compound calculated with the Coulomb-attenuated method CAM-B3LYP with the basis set 6-311++G**. The excitation energies are found to be highly dependent on the twisting of the helicenium framework and only weakly influenced by the electronic nature of the bridging substituent. The electronic circular dichroism is evaluated, as the rotational strength is found to be highly dependent on both the overall molecular structure and substitution pattern but with no simple correlation between structure and circular dichroism (CD) response. The calculations reveal that the magnitude of rotational strength in most cases is dominated by the angle between the electronic and magnetic transition dipole moments. Finally, it is found that computational screening of many different structures and substituents might be needed to find target structures with maximized rotational strength.

Nitrogen and Oxygen Bridged Calixaromatics: Synthesis, Structure, Functionalization, and Molecular Recognition

Pedersen, Lehn, and Cram established supramolecular chemistry through their pioneering work with crown ethers, cryptands, and spherands. Since then, the hallmark of supramolecular science has been an increasing sophistication in the design and construction of macrocyclic molecules, as manifested in cyclodextrin derivatives, calixarenes, resorcinarenes, cyclotriveratrylenes, cucurbiturils, calixpyrroles, cyclophanes, and many other examples. Indeed, macrocyclic compounds provide unique models for the study of noncovalent molecular interactions. They also constitute building blocks for constructing high-level molecular and supramolecular architectures and fabricating molecular devices and advanced materials. As a postgraduate in the Huang laboratory in the late 1980s, I became interested in the calix[n]arenes because of their unique conformational structures and versatile complexation properties. The notion of introducing heteroatoms, and particularly nitrogen, into the bridging position of conventional calixarenes was particularly intriguing. Nitrogen, unlike methylene, can adopt either an sp(2) or sp(3) electronic configuration, providing different conjugation systems with adjacent aromatic rings. Consequently, depending on the configuration and conjugation, a range of C-N bond lengths and C-N-C bond angles is possible. The conformation and cavity size in heteroatom-bridged calixarenes might thus be tuned through the bridging heteroatoms and the number of aromatic rings. Furthermore, because heteroatom linkages significantly affect the electron density and distribution on aromatic rings, the electronic features of macrocyclic cavities might be regulated by heteroatoms. Given the essentially limitless combinations possible, only synthetic hurdles would prevent access to numerous diverse heteracalixaromatics. We began a systematic study on nitrogen- and oxygen-bridged calixarenes in 2000, years later than originally envisioned. Before this study, very few heteracalixaromatics had been reported, owing to the formidable synthetic challenges involved. Apart from thiacalixarene, the synthesis of nitrogen- and oxygen-bridged calixarenes appeared very difficult. But since our first publications in 2004, we have been delighted to see the rapid and tremendous development of the supramolecular chemistry of this new generation of macrocycles. In this Account, I summarize the synthesis of N- and O-bridged calixaromatics and their regiospecific functionalization on the rims and bridging positions, focusing on the fragment coupling approach and contributions from our laboratory. I describe the construction of molecular cages based on heteracalixaromatics and discuss the effect of both bridging heteroatoms and substituents on macrocyclic conformations and cavity sizes. Molecular recognition of neutral organic molecules and charged guest species is also demonstrated. The easy accessibility, rich molecular diversity, unique conformation, and cavity tunability of heteracalixaromatics make them invaluable macrocycles for research in supramolecular chemistry. New heteracalixaromatics, with well-defined conformations and cavity properties, will provide powerful tools for probing noncovalent interactions, leading to the development of new molecular sensing and imaging systems. Multicomponent molecular self-assembly of heteracalixaromatics as functional modules with metals, metal clusters, or charge-neutral species should result in multidimensional solid and soft materials with diverse applications. The profitable incorporation of heteracalixaromatics into molecular devices can also be anticipated in the future. Moreover, the construction of enantiopure, inherently chiral heteracalixaromatics should provide important applications in chiral recognition and asymmetric catalysis.

Hg2+ recognition by triptycene-derived heteracalixarenes: selectivity tuned by bridging heteroatoms and macrocyclic cavity

The binding properties of triptycene-derived oxacalixarenes containing 1,8-naphthyridine subunits 1a and 1b toward metal ions were investigated in detail by spectroscopic methods. A couple of new N,O-bridged diazadioxacalixarenes 2a, 2b were synthesized and their binding properties to metal ions were also evaluated. The results showed that the oxacalixarene 1a, a cis-isomer with a boat-like 1,3-alternate conformation and a symmetrical cavity structure, exhibited a highly selective fluorescence response toward Hg(2+) ions, while 1b, 2a and 2b did not show selective response towards any specific metal ions. Such selectivity can thus be controlled by the bridging heteroatoms and cavity structures of the macrocycles. Moreover, it was found that the fluorescence of 1a was considerably quenched upon the addition of Hg(2+), and a 1 : 2 stoichiometry host-guest complex was proposed on the basis of the Job plot and (1)H NMR titrations.

$$\pi$$ -Conjugated Heterocyclic fused Bithiophene Materials

This article covers the developments on the synthesis and properties of heterocyclic fused π-conjugated bithiophene materials that are potentially applicable in molecular electronics and optoelectronics. This fairly young strategy to efficiently tuning the electronic properties generates materials with very narrow band gaps. The nature of the central bridging heteroatom has a significant impact on the electronic and luminescence properties of these materials leading to intriguing species that can be employed in organic light emitting diodes (OLEDs) or organic field effect transistors (OFETs). So far a variety of heteroelements of group 13–16 (B, Si, Ge, Sn, N, P, S) have been investigated and incorporated into molecular as well as polymeric systems. A significant number of these materials can potentially act as organic emitters, electron or hole transport materials in organic devices but further studies are needed to optimize the necessary properties for the utility of this young class of compound in molecular electronics.

A General and High Yielding Fragment Coupling Synthesis of Heteroatom-Bridged Calixarenes and the Unprecedented Examples of Calixarene Cavity Fine-Tuned by Bridging Heteroatoms

A number of aza- and/or oxo-bridged calix[2]arene[2]triazines have been synthesized through an unusually high yielding and efficient fragment coupling approach starting from cyanuric chloride and resorcinol, 3-aminophenol, m-phenylenediamine, and N,N'-dimethyl-m-phenylenediamine. These novel macrocycles, which belong to the next generation of calixarenes or cyclophanes, form a unique cavity that is resulted from two isolated benzene planes and two bis-heteroatom-conjugated triazine planes in a 1,3-alternate fashion. The nature of the bridging heteroatoms, i.e., combination of the electronic, conjugative, and steric effects of the nitrogen and oxygen atoms, strongly regulates the cavity size, generating a set of fine-tuned cavities in which the distance between two benzene rings at the upper rim ranges from 5.011 to 7.979 A. The multiple intermolecular hydrogen bond interactions among N,N'-dimethylated tetraazacalix[2]arene[2]triazines and among tetraazacalix[2]arene[2]triazines lead to the formation of infinite one-dimensional chain structure and two-dimensional zigzag layered structure, respectively, in the solid state. The ease of preparation and further chemical manipulations, and the readily tunable cavity structures render these aza- and/or oxo-bridged calix[2]arene[2]triazines the unique platforms in the study of supramolecular chemistry.

A New Approach to the Synthesis of 2‐Vinylthiophenes and Selenophenes; Competition Between Free Radical and Anionic Cycloaromatization of Bridged Di‐ and Tetrapropargylic Sulfides and Selenides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A new approach to the synthesis of 2-vinylthiophenes and selenophenes; competition between free radical and anionic cycloaromatization of bridged di- and tetrapropargylic sulfides and selenides

A series of unknown di- and tetrapropargylic sulfides and selenides have been prepared. In the presence of t-BuOK in dry THF these compounds underwent isomerization to the corresponding diallenes, followed by a tandem anionic cyclization and aromatization to 2-vinylthiophene or selenophene derivatives. Some mechanistic studies indicated competition between free radical and anionic cycloaromatization. The latter is influenced by the nature of the bridging heteroatom, substitution of the allenyl group and base concentration.

Indolopyridines with a bridging heteroatom. 9. Synthesis, structure, and thermolysis of 5-hydroxy-5-(2-pyridyl)-fluorene and 4-azafluorene

Treatment of fluorenone or 4-azafluoren-9-one with 2-pyridyllithium gives 45-hydroxy-5-(2-pyridyl)fluorene and its aza analog. The structure of the former has been studied by x-ray crystallography. It was found that, in contrast to the non-condensed diaryl-2-pyridylcarbinols, these alcohols do not undergo acid catalyzed dehydration and heterocyclization. Under pyrolytic conditions. 5-pyridylfluorenol undergoes fission to form fluorenone.