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Martin R Bryce

Publications and source records attributed to Martin R Bryce.

At least 19 recordsLinked to original sources
PubMed2007

Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: synthesis, redox and photophysical properties, and electrophosphorescent light-emitting diodes.

Using ligands synthesized by Suzuki cross-coupling methodology, new phosphorescent homoleptic tris-cyclometalated complexes have been obtained, namely fac-[Ir(Cz-2-Fl(n)Py)(3)] (1 d-f) and fac-[Ir(Cz-3-Fl(n)Py)(3)] (2 d-f), which are solution-processible triplet emitters (Cz denotes N-hexylcarbazole, n is the number of 9,9'-dihexylfluorene (Fl) units (n=0,1,2) and Py is pyridine). In all cases, Py and Fl are substituted at the 2- and 2,7-positions, respectively, and Cz moieties are substituted by either Py or Fl at the 2- or 3-positions, in series 1 and 2, respectively. The oxidation potential of 1 d studied by cyclic voltammetry ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.14 V, versus Ag/AgNO(3), CH(2)Cl(2)) is less positive (i.e. raised HOMO level) compared to that of the isomer 2 d ({E{{{\rm ox}\hfill \atop 1/2\hfill}}}=0.30 V), where the Cz-nitrogen is meta to the Ir center. Ligand-centered oxidations occur at more positive potentials, leading to 7+ oxidation states with good chemical reversibility and electrochemical quasi-reversibility, for example, for 2 f {E{{{\rm ox}\hfill \atop {\rm pa}\hfill}}} =0.45 (1e), 0.95 (3e), 1.24 V (3e). Striking differences are seen in the solution-state photophysical data between complexes [Ir(Cz-2-Py)(3)] (1 d) and [Ir(Cz-3-Py)(3)] (2 d), in which the Cz moiety is bonded directly to the metal center: for the latter there is an 85 nm blue-shift in emission, a decrease in the luminescence lifetime and an increase in the PLQY value. Organic light emitting devices were made by spin-coating using polyspirobifluorene:bis(triphenyl)diamine (PSBF:TAD) copolymer as host and the complexes 1 d or 2 d as dopants. Turn-on voltages are low (3-4 V). With 1 d orange light is emitted at lambda(max)=590 nm with an EQE of 1.3 % (at 7.5 mA cm(-2)) and an emission intensity (luminance) of 4354 cd m(-2) (at 267 mA m(-2)). The green emission from 2 d devices (lambda(max)=500 nm) is due to the reduced electron-donating ability of the carbazole unit in 2 d. Recording the EL spectra of the 1 d device at 6 V (current density, 100 mA cm(-2)) established that the time to half brightness was about 9 h under continuous operation with no change in the spectral profile, confirming the high chemical stability of the complex.

Journal Article↗
PubMed2006

Calix[6]arene derivatives selectively functionalized at alternate sites on the smaller rim with 2-phenylpyridine and 2-fluorenylpyridine substituents to provide deep cavities.

The synthesis is described of calix[6]arene derivatives 4, 9, and 14 functionalized at alternate sites on the smaller rim with 4'-(pyrid-2' '-yl)phenylmethoxy, (6'-phenylpyrid-3'-ylmethoxy), and {6'-[2-(9,9-di-n-hexylfluorenyl)]pyrid-3'-ylmethoxy} substituents, respectively. They were obtained by 3-fold reactions of 2-[4-(bromomethyl)phenyl]pyridine (3), 5-(bromomethyl)-2-phenylpyridine (8), and 5-(bromomethyl)-2-(9,9-di-n-hexylfluorenyl)pyridine (13) with the 1,3,5-trimethylether of the t-Bu-calix[6]arene in the presence of sodium hydride in THF in 56-75% yields. Detailed analysis of the 1H NMR spectra (including variable-temperature data for 4) has established that 4, 9, and 14 exist predominantly in the C3v cone conformation with minor Cs isomers also observed. The X-ray crystal structure of 4 reveals two molecules of similar cone conformation, with all three 4'-(pyrid-2' '-yl)phenylmethoxy substituents stretched in the axial direction. Molecule I has a dimeric capsule structure with (pyrid-2' '-yl)phenylmethoxy substituents of one molecule interpenetrating those of its inversion equivalent to form a deep enclosed intermolecular cavity, which contains a CH2Cl2 guest molecule. Molecule II forms no such pair: the intramolecular cavity is filled with solvent molecules.

Calixarenes↗
PubMed2006

Precision control of single-molecule electrical junctions.

There is much discussion of molecules as components for future electronic devices. However, the contacts, the local environment and the temperature can all affect their electrical properties. This sensitivity, particularly at the single-molecule level, may limit the use of molecules as active electrical components, and therefore it is important to design and evaluate molecular junctions with a robust and stable electrical response over a wide range of junction configurations and temperatures. Here we report an approach to monitor the electrical properties of single-molecule junctions, which involves precise control of the contact spacing and tilt angle of the molecule. Comparison with ab initio transport calculations shows that the tilt-angle dependence of the electrical conductance is a sensitive spectroscopic probe, providing information about the position of the Fermi energy. It is also shown that the electrical properties of flexible molecules are dependent on temperature, whereas those of molecules designed for their rigidity are not.

Journal Article↗
PubMed2006

Are terminal aryl butadiynes stable? Synthesis and X-ray crystal structures of a series of aryl- and heteroaryl-butadiynes (Ar-CC-CC-H).

The synthesis and isolation are reported of a range of terminal aryl- and heteroaryl-butadiynes (ArCC-CCH) 4a-h from 2-methyl-6-(aryl/heteroaryl)hexa-3,5-diyn-2-ol precursors. The stability of 4a-h in solution is concentration dependent: many of the derivatives can be stored as dilute solutions for several days or even weeks. The X-ray crystal structures have been obtained for five ArCC-CCH derivatives [Ar = 2-(9-fluorenonyl), 4-biphenyl, 2-pyridyl, 4-pyridyl, and 2-pyrazyl].

Journal Article↗
PubMed2006

Single-molecule electrical studies on a 7 nm long molecular wire.

A self-assembled arylene-ethynylene molecular wire with a rigid 7 nm long backbone exhibits symmetrical current-voltage (I-V) characteristics and a single-molecule current of 0.35 +/- 0.05 nA at 0.3 V; these data are supported by theoretical calculations.

Electrons↗
PubMed2006

Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers.

The strong solvatochromism observed for two fluorene-dibenzothiophene-S,S-dioxide oligomers in polar solvents has been investigated using steady-state and time-resolved fluorescence techniques. A low-energy absorption band, attributed to a charge-transfer (CT) state, is identified by its red shift with increasing solvent polarity. In nonpolar solvents, the emission of these conjugated luminescent oligomers shows narrow and well-resolved features, suggesting that the emission comes from a local excited state (LE), by analogy to their conjugated fluorene-based polymer counterparts. However, in polar solvents, only a featureless broad emission is observed at longer wavelengths (CT emission). A linear correlation between the energy maximum of the fluorescence emission and the solvent orientation polarizability factor Deltaf (Lippert-Mataga equation) is observed through a large range of solvents. In ethanol, below 230 K, the emission spectra of both oligomers show dual fluorescence (LE-like and CT) with the observation of a red-edge excitation effect. The stabilization of the CT emissive state by solvent polarity is accompanied/followed by structural changes to adapt the molecular structure to the new electronic density distribution. In ethanol, above 220 K, the solvent reorganization occurs on a faster time scale (less than 10 ps at 290 K), and the structural relaxation of the molecule (CT(unrelaxed) --> CT(Relaxed)) can be followed independently. The magnitude of the forward rate constant, k(1)(20 degrees C) approximately 20 x 10(9) s(-1), and the reaction energy barrier, E(a) approximately 3.9 kcal mol(-1), close to the energy barrier for viscous flow in ethanol (3.54 kcal mol(-1)), show that large-amplitude molecular motions are present in the stabilization of the CT state.

Journal Article↗
PubMed2006

Extreme conformational constraints in pi-extended tetrathiafulvalenes: unusual topologies and redox behavior of doubly and triply bridged cyclophanes.

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single quasi-reversible two-electron oxidation wave, D0 --> D2+), cyclic voltammetry of the cyclophanes reveals two reversible, one-electron oxidation steps (D0 --> D*+ --> D2+), with differences between the half-wave potentials (E2(1/2) - E1(1/2)) of 0.22-0.26 V. The conformational changes and gain in aromaticity which drive the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy band (lambdamax = 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction. The steric constraints imposed by multiple bridging have become so extreme that the pi-framework of 15, 16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems.

Journal Article↗
PubMed2006

Remarkable interplay of redox states and conformational changes in a sterically crowded, cross-conjugated tetrathiafulvalene vinylog.

Derivatives of 9-[2-(1,3-dithiol-2-ylidene)ethylidene]thioxanthene have been synthesized using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with thioxanthen-9-ylidene-acetaldehyde (5). Further reactions lead to the sterically crowded cross-conjugated "vinylogous tetrathiafulvalene" derivative 9-[2,3-bis-(4,5-dimethyl-1,3-dithiol-2-ylidene)-propylidene]thioxanthene (10). X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry, and simultaneous electrochemistry and electron paramagnetic resonance spectroscopy, combined with theoretical calculations performed at the B3LYP/6-31G(d) level, elucidate the interplay of the electronic and structural properties in these molecules. For compound 10, multistage redox behavior is observed: the overall electrochemical process can be represented by 10-->10(.+)-->10(2+)-->10(4+) with good reversibility for the 10-->10(.+)-->10(2+) transformations. At the tetracation stage there is the maximum gain in aromaticity at the dithiolium and thioxanthenium rings. Theory predicts that for 10, 10(.+), and 10(2+) the trans isomers are more stable than the cis isomers (by ca. 2-18 kJ mol(-1)), whereas for 10(4+) the cis isomer becomes more stable than the trans isomer (by ca. 25 kJ mol(-1)) [trans and cis refer to the arrangement of the two dithiole moieties with respect to the central ==C(R)--C(H)== fragment]. These data explain the detection in cyclic voltammograms of both trans and cis isomers of 10 and 10(.+) during the reduction of 10(4+) at fast scan rates (>100 mV s(-1)) when the cis-trans isomerization is not completed within the timescale of the experiment. The X-ray structure of the charge-transfer complex (CTC) of 10 with 2,4,5,7-tetranitrofluorene-9-dicyanomethylenefluorene (DTeF) [stoichiometry: 10(.+)(DTeF)(2) (.-)2 PhCl] reveals a twisted conformation of 10(.+) (driven by the bulky thioxanthene moiety) and provides a very rare example of segregated stacking of a fluorene acceptor in a CTC.

Journal Article↗
PubMed2006

The interplay of inverted redox potentials and aromaticity in the oxidized states of new pi-electron donors: 9-(1,3-dithiol-2-ylidene)fluorene and 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives.

Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single-electron oxidations (E(1)ox < E(1)ox) were observed for 9-(1,3-dithiol-2-ylidene)fluorene systems (9). In contrast, derivatives of 9-(1,3-dithiol-2-ylidene)thioxanthene (10) display the unusual phenomenon of inverted potentials (E(1)ox > E(1)ox) resulting in a single, two-electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (10(2+)) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two-electron oxidation wave in the thioxanthene derivatives is split into two separate one-electron waves in the corresponding sulfoxide and sulfone derivatives 27-29 owing to destabilisation of the dication state.

Journal Article↗
PubMed2006

Molecular wires comprising pi-extended ethynyl- and butadiynyl-2,5-diphenyl-1,3,4-oxadiazole derivatives: synthesis, redox, structural, and optoelectronic properties.

2,5-Diphenyl-1,3,4-oxadiazole (OXD) derivatives with terminal ethynyl- (4a,b) and butadiynyl- (8a,b) substituents have been synthesized in high yields. 2-Methyl-3,5-hexadiyn-2-ol has not been exploited previously in the synthesis of terminal butadiynes. Crystals of 8a and 8b are remarkably stable to long-term storage under ambient conditions. The X-ray crystal structure of 8a reveals that the butadiyne moieties are spatially isolated by the aromatic moieties, which explains the high stability. Two series of derived pi-conjugated molecules, Donor-(C[triple bond]C)(n)-OXD (n = 1, 2) and OXD-(C[triple bond]C)(n)-Donor-(C[triple bond]C)(n)-OXD (n = 1) [Donor = tetrathiafulvalene (TTF), bithiophene, 9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene, and triphenylamine], have been synthesized using Sonogashira reactions and characterized by X-ray crystallography, cyclic voltammetry, and optical absorption/emission spectroscopy. The electron-withdrawing effect of the OXD units is manifested by a positive shift of the donor oxidation waves in these systems: the butadiynylene spacer (n = 2) further shifts the first oxidation waves by 40-80 mV compared to analogues n = 1. The absorption spectra of TTF-OXD hybrids 10d and 11 are blue-shifted by 80 nm compared to the bithienyl-bridged derivative 10f and are similar to the butadiynyl-OXD building-block 8a, demonstrating that conjugation is disrupted by a neutral TTF unit. Solutions of the TTF-OXD and 9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene-OXD hybrids, 10d, 10g, 11, and 13, are only very weakly fluorescent due to quenching from the electron-donor moieties. In contrast, the triphenylamine-OXD hybrids 12a, 12b, 14a, and 14b are fluorescent; the PLQYs of the butadiynylene derivatives 14a and 14b are lower than those of the ethynylene-bridged analogues 12a and 12b.

Journal Article↗
PubMed2006

Electronic interactions in a new pi-extended tetrathiafulvalene dimer.

The first pi-extended tetrathiafulvalene (exTTF) dimer in which the two exTTF units are covalently connected by 1,3-dithiole rings has been obtained in a multistep synthetic procedure involving the Ullmann cross-coupling reaction by using copper(I) thiophene-2-carboxylate (CuTC). The electronic spectrum reveals a significant electronic interaction between the exTTF units. The electrochemical study carried out by cyclic voltammetry in solution and in thin-layer conditions, and the electrochemical simulation and spectroelectrochemical (SEC) measurements confirm the electronic communication and show that the oxidation of dimer 14 occurs as two consecutive 2 e(-) processes D(0)-D(0)-->D(2+)-D(0)-->D(2+)-D(2+). Theoretical calculations, performed at the B3P86/6-31G* level, confirm the experimental findings and predict that 14(2+) exists as a delocalized D(.+)-D(.+) species in the gas phase and as a localized D(2+)-D(0) species in solution (CH(3)CN or CH(2)Cl(2)). Oxidation of 14(2+) forms the tetracation 14(4+) which is constituted by two aromatic anthracene units bearing four aromatic, almost orthogonal 1,3-dithiolium cations.

Anthracenes↗
PubMed2006

Organic rectifying junctions from an electron-accepting molecular wire and an electron-donating phthalocyanine.

Self-assembled monolayers (SAMs) of arylene-ethynylene oligomers that incorporate electron-accepting 9-fluorenone and 9-[(4-pyridyl)methylene]fluorene units in the backbone exhibit symmetrical current-voltage (I-V) characteristics, but rectifying junctions with current ratios of 20-80 at +/-1 V have been obtained by protonating these wire-like molecules and ionically coupling with anionic donors.

Journal Article↗
PubMed2006

Convergent synthesis of 10 nm aryleneethynylene molecular wires by an iterative regioselective deprotection/Sonogashira coupling protocol.

[structure: see text] The synthesis of a new series of rigid-rod aryleneethynylene derivatives of up to ca. 10 nm molecular length (compounds 16 and 17) is reported using iterative Pd-mediated Sonogashira coupling methodology combined with regioselective removal of the different protecting groups (namely, trimethylsilyl and 2-hydroxyprop-2-yl groups) from the terminal alkyne units. Additionally, the TMS-acetylene unit has been cleanly deprotected to afford a terminal alkyne in the presence of a cyanoethylsulfanyl group. Some of these molecular wires are functionalized with terminal protected thiophenol units for attachment to metal surfaces (compounds 16 and 17). Internal electron-acceptor units have been incorporated into their structures, namely, 9-[di(4-pyridyl)methylene]fluorene (compound 17) or fluorenone (compounds 19-22). Optical absorption and photoluminescence spectra reveal a red shift in the value of lambda(max) with increasing molecular length, which approaches saturation at an effective conjugation length of ca. 15-20 pi-units in the molecules, where each phenyl ring or a triple bond is counted as one pi-unit.

Journal Article↗
PubMed2005

Self-assembly and multistage redox chemistry of strong electron acceptors on metal surfaces: polynitrofluorenes on gold and platinum.

Nitrofluoren-9-one and nitrofluoren-9-dicyanomethylene electron acceptors 7, 8, and 11 functionalized with a terminal thioctic acid unit have been synthesized from 2,4,5,7-tetranitrofluorenone. The self-assembled monolayers (SAMs) of these compounds on gold, formed via gold-sulfur interaction, have been fully characterized by electrochemical, FTIR, ellipsometry, and contact angle measurements. Cyclic voltammetry of SAMs reveals two reversible single-electron reduction waves for fluorenone derivatives 7a,b and 11, and three single-electron reductions for the dicyanomethylene-fluorene 8b, providing the first observation of a radical trianion species in SAMs. The tendency of the thioctic anchor to form multilayers via disulfide links is noted.

Electrochemistry↗
PubMed2005

The first studies of a tetrathiafulvalene-sigma-acceptor molecular rectifier.

Langmuir-Blodgett monolayers of a donor-acceptor diad TTF-sigma-(trinitrofluorene) (8) with an extremely low HOMO-LUMO gap (0.3 eV) have been used to create molecular junction devices that show rectification behavior. By virtue of structural similarities and position of molecular orbitals, 8 is the closest well-studied analogue of the model Aviram-Ratner unimolecular rectifier (TTF-sigma-TCNQ). Compressing the monolayer results in aligning the molecules, and is followed by a drastic increase in the rectification ratio. The direction of rectification depends on the electrodes used and is different in n-Si/8/Ti and Au/8/C16H33S-Hg junctions. The molecular nature of such behavior was corroborated by control experiments with fatty acids and by reversing the rectification direction with changing the molecular orientation (Au/D-sigma-A versus Au/A-sigma-D).

Journal Article↗
PubMed2005

A versatile synthesis of pyrazolo[3,4-c]isoquinoline derivatives by reaction of 4-aryl-5-aminopyrazoles with aryl/heteroaryl aldehydes: the effect of the heterocycle on the reaction pathways.

The reaction of 4-(3,4-dimethoxyphenyl)-5-aminopyrazoles 7A-D with aromatic and heterocyclic aldehydes in strong acidic media (trifluoroacetic or formic acid) has been studied. The initial azomethine derivatives 8 undergo cyclization similar to the Pictet-Spengler condensation to form the intermediate 4,5-dihydroisoquinolines 9 which readily dehydrogenate giving 5-aryl(heteroaryl)-pyrazolo[3,4-c]isoquinoline derivatives 10 as the final products. Whereas for benzaldehyde and its derivatives this one-pot synthesis presents a convenient general route to 5-aryl-pyrazolo[3,4-c]isoquinolines 10, in the case of heterocyclic aldehydes the product structure varies markedly with the structure of the aldehyde used: (i) 3-pyridyl-, 3-quinolyl-, 3-thienyl-, and 1,2,3-thiadiazolyl-5-carboxaldehydes give 5-heteroarylpyrazolo[3,4-c]isoquinolines; (ii) 1-methylbenzimidazolyl-2-carboxaldehyde gives only intermediate azomethine 8Dh, which does not cyclize; (iii) 1-R-3-indolylcarboxaldehydes (R = H, CH3, CH2Ph) eliminate the heteroaryl fragment resulting in 5-unsubstituted pyrazolo[3,4-c]isoquinolines 11. Thienyl-2-carboxaldehyde reacts by both pathways (i) and (iii) depending on the reaction conditions. The single crystal X-ray structures for 10Dj, 10Cd and 11D provide confirmation of the different types of products formed in these reactions. Mechanisms which explain these transformations are presented.

Aldehydes↗