Cavity ring-down spectroscopy of the torsional motions of 1,4-bis(phenylethynyl)benzene

The torsional motions of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB), a prototype molecular wire, were studied using cavity ring-down spectroscopy in the first UV absorption band (316-321 nm). The torsional spectrum of 1,4-bis(phenylethynyl)-2,3,5,6-tetradeuteriobenzene was also recorded in the gas phase. Both spectra were successfully simulated using simple cosine potentials to describe the torsional motions. The ground-state barrier to rotation was estimated to be 220-235 cm(-1), which is similar to that of diphenylacetylene (tolane). Complementary DFT calculations were found to overestimate the torsional barrier.     

Metal Cluster Terminated “Molecular Wires”

The gold complexes Au(C≡CC₆H₄C≡CC₆H₄Me)(PPh₃) (3) and {Au(PPh₃)}₂(μ-C≡CC₆H₄C≡CC₆H₄C≡CC₆H₄C≡C) (6), prepared from the reaction of AuCl(PPh₃) with the corresponding terminal or trimethylsilyl protected alkynes, react readily with Ru₃(CO)₁₀(μ-dppm) to afford phenylene ethynylene derivatives featuring the Ru₃(μ-AuPPh₃)(μ-C₂R)(CO)₇ cluster “end-caps”. The hydrido cluster Ru₃(μ-H)(μ-C₂C₆H₄C≡CC₆H₄Me)(CO)₇ (4a) has also been obtained. There are significant differences in the absorption spectra of the organic precursors, the gold complexes and the clusters indicate a mixing of electronic states between the cluster and phenylene ethynylene moieties, while the presence of the Ru₃ and in particular Ru₃(μ-AuPPh₃) cluster end-caps leads to a quenching of the phenylene ethynylene centred emission. The crystallographically determined structures of 3, 4a and Ru₃(μ-AuPPh₃) (μ-C₂C₆H₄C≡CC₆H₄Me)(CO)₇ (4b) are reported.Dedicated to Professor B.F.G. Johnson, one of the pioneers of cluster chemistry, in recognition of his outstanding contributions to the field.     

Optical properties of donor-acceptor phenylene-ethynylene systems containing the 6-methylpyran-2-one group as an acceptor

Donor-acceptor phenylene ethynylene systems containing the 6-methylpyran-2-one group, synthesized via classic or microwave-assisted Sonogashira cross-coupling, exhibit pronounced solvatochromism in fluorescence suggesting a highly polar excited state; 4-[4-(4-N,N-dihexylaminophenylethynyl)phenylethynyl]-6-methylpyran-2-one has a fluorescence quantum yield >0.9 in cyclohexane.     

Opening a spiropyran ring by way of an exciplex intermediate

A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzene residue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyran group has been condensed to the central phenylene ring so as to position a weak electron donor close to the pyrene unit. Illumination of the pyrene-based chromophore leads to formation of a fluorescent exciplex in polar solvents but pyrene-like fluorescence is observed in nonpolar solvents. The exciplex has a lifetime of a few nanoseconds and undergoes intersystem crossing to the pyrene-like triplet state with low efficiency. Attaching a 4-nitrobenzene group to the open end of the spiropyran unit creates a new route for decay of the exciplex whereby the triplet state of the spiropyran is formed. Nonradiative decay of this latter species results in ring opening to form the corresponding merocyanine species. Rate constants for the various steps have been obtained from time-resolved fluorescence spectroscopy carried out over a modest temperature range. Under visible light illumination, the merocyanine form reverts to the original spiropyran geometry so that the cycle is closed. Energy transfer from the pyrene chromophore to the merocyanine unit leads to an increased rate of ring closure and serves to push the steady-state composition in favor of the spiropyran form.     

Preparation of ordered films containing a phenylene ethynylene oligomer by the Langmuir-Blodgett technique

This paper reports the fabrication and characterization of Langmuir and Langmuir-Blodgett (LB) films incorporating an oligo(phenylene-ethynylene) (OPE) derivative, namely, 4-[4-(4-hexyloxyphenylethynyl)-phenylethynyl]-benzoic acid (HBPEB). Conditions appropriate for deposition of monolayers of HBPEB at the air-water interface have been established and the resulting Langmuir films characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. The Langmuir films are readily transferred onto solid substrates, and one-layer LB films transferred at several surface pressures onto mica substrates have been analyzed by means of atomic force microscopy, from which it can be concluded that 14 mN/m is an optimum surface pressure of transference, giving well-ordered homogeneous films without three-dimensional defects and a low surface roughness. The optical and emissive properties of the LB films have been determined with significant blue-shifted absorption spectra indicating formation of two-dimensional H aggregates and a Stokes shift illustrating the effects of the solid-like environment on the molecular chromophore.     

Scalable Synthesis for Porous Organic Cages

We describe an improved synthesis for a [4 + 6] imine-linked cage, which was shown recently to exhibit microporosity in the crystalline state and which can be converted by recrystallization to produce either porous or nonporous polymorphs. The original methodology for the synthesis of this molecule resulted in poor yields (approximately 35%). Here, we demonstrate that the cage can be prepared in almost 100% yields and can be isolated with high chemical purity.     

The synthesis and liquid crystalline behaviour of alkoxy‐substituted derivatives of 1,4‐bis(phenylethynyl)benzene

Despite the prevalence of organised 1,4‐bis(phenylethynyl)benzene derivatives in molecular electronics, the interest in the photophysics of these systems and the common occurrence of phenylethynyl moeties in molecules that exhibit liquid crystalline phases, the phase behaviour of simple alkoxy‐substituted 1,4‐bis(phenylethynyl)benzene derivatives has not yet been described. Two series of 1,4‐bis(phenylethynyl)benzene derivatives, i.e. 1‐[(4′‐alkoxy)phenylethynyl]‐4‐(phenylethynyl)benzenes (5a–5f) and methyl 4‐[(4″‐alkoxy)phenylethynyl‐4′‐(phenylethynyl)] benzoates (18a–18f) [alkoxy = n‐C₄H₉ (a), n‐C₆H₁₃ (b), n‐C₉H₁₉ (c), n‐C₁₂H₂₅ (d), n‐C₁₄H₂₉ (e), n‐C₁₆H₃₃ (f)] have been prepared and characterised. Both series have good chemical stability at temperatures up to 210°C, the derivatives featuring the methyl ester head‐group (18a–18f) offering rather higher melting points and generally stabilising a more diverse range of mesophases at higher temperatures than those found for the simpler compounds (5a–5f). Smectic phases are stabilised by the longer alkoxy substituents, whereas for short and intermediate chain lengths of the simpler system (5a–5c) nematic phases dominate. Diffraction analysis was used to identify the SmB_hex phase in (5d–5f) that is stable within a temperature range of approximately 120–140°C. The relationships between the organisation of molecules within these moderate temperature liquid crystalline phases and other self‐organised states (e.g. Langmuir‐Blodgett films) remain to be explored.     

Mixing behaviour of a conjugated molecular wire candidate and an insulating fatty acid within Langmuir–Blodgett films

The conductivity, weight loss and A.C. impedance studies were carried out to establish the influence the presence of neutral copolymer (Vinyl pyrrodlidone/Vinyl acetate copolymer) with anionic surfactant (Disodium laurethsulfosuccinate) in cyclohexane propionic acid (CHPA) on the corrosion behaviour of carbon steel. The protection efficiency (P%) of AS in the absence and presence of polymer increases with increase in AS concentration until it reaches a maximum constant value near the CMC of AS and decreases with increase in solution temperature. The protective efficiency of AS can be enhanced and its critical micelle concentration shifts to low value by adding of 0.05 g/l of polymer. The critical micelle concentration of AS in the absence and presence of polymer increases with increase in temperature. The calculated thermodynamic parameters of micellization (ΔG_mic), (ΔH_mic) and (ΔS_mic) in the presence of neutral copolymer molecules with AS show that these processes are spontaneous and exothermic in nature and indicate the association between AS and neutral copolymer molecules.