Photodecomposition of peroxides containing a 1,4-bis(phenylethynyl)benzene chromophore

The photodecomposition dynamics of 1,4-bis(2-[4-tert-butylperoxycarbonylphenyl]ethynyl)benzene (1) have been compared with those of model compounds in the picosecond and nanosecond time domains by various photophysical techniques. Ultrafast visible transient absorption spectrometry revealed the singlet excited state of 1,4-bis(4-phenylethynyl)benzene (BPB) depopulates radiatively with a rate of 1.75 x 10(9) s(-1) and 95% efficiency. Phenyl ester moieties attached to the BPB core accelerate intersystem crossing (k = 2.8 x 10(8) s(-1)) and reduce the fluorescence quantum yield (phi(FL) = 0.82). The peroxide oxygen-oxygen bond of 1 cleaves (k = 3.6 x 10(11) s(-1)) directly from the singlet excited state (60% efficiency) causing a highly reduced fluorescence yield and leading to formation of aroyloxyl radicals. The next reaction step involves decarboxylation of the aroyloxyl radicals. Transient absorption signals in the MID IR region correspond to CO2 with the formation rate (2.5 x 10(6) s(-1)) as measured by nanosecond transient IR experiments. The transient IR spectra of the excited state of BPB, as well as of the aroyloxyl radical, evidenced a red shift in the acetylene triple bond absorption indicative of a decrease in the bond order. This clearly shows that delocalization of excitation energy over the BPB chromophore induces significant structural changes. The proposed mechanism is based on the rates of photophysical and photochemical channels and involves an additional population channel of the BPB triplet excited state from the upper singlet states.     

Electrical properties of 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene and its application for organic light emitting diodes

We found that a phenylene ethynylene derivative, 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene (BPPB), provides very high photoluminescence efficiency both in solution (Phi(PL) = 95 +/- 3%) and thin films (Phi(PL) = 71 +/- 3%); further, we observed blue electroluminescence (EL) of lambda(EL(max)) approximately 470 and 510 nm with an external EL efficiency of eta(EL) approximately 0.53% and maximum luminance of approximately 70000 cd m(-2) at current density of approximately 2 A cm(-2) with BPPB as an emitter; also we identified that BPPB functions as a hole transport layer in organic light emitting diodes.     

Electronic states of 1,4-bis(phenylethynyl)benzene: A synchrotron radiation linear dichroism investigation

The electronic transitions of 1,4-bis(phenylethynyl)benzene (BPEB) were investigated by UV synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 25,000-58,000 cm⁻¹ (400-170 nm) on molecular samples aligned in stretched polyethylene. The investigation was supported by variable temperature spectroscopy and by quantum chemical calculations in the LCOAO and TD-DFT models. The molecular alignment of BPEB in stretched polyethylene was found to be remarkably efficient, leading to an orientation factor equal to 0.95 for the long molecular axis. The observed band shape depended strongly on the degree of stretching and on temperature. The combined experimental and theoretical evidence leads to the characterization of several previously unobserved transitions and supports the assumption that BPEB adopts a nearly planar conformation in stretched polyethylene.     

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.     

Performance of multi-configurational calculations for a 1,4-bis(phenylethynyl)benzene derivative conjugated molecule.

The theoretical challenge of finding a single method that quantitatively reproduces both the experimental low-lying excitation energies and the torsional barrier of a prototypical conjugated molecule, which could act as a molecular wire, has been addressed here. The results indicate that this goal can be reasonably achieved when multi-reference perturbation theory up to second order (MRMP2) based on a complete active space self-consistent field (CASSCF) wave function using large active spaces is used. The results obtained were also used to compare with less expensive Kohn-Sham (KS) density functional theory (DFT) calculations when applied to these properties. The results obtained with BLYP and B3LYP exchange-correlation functionals indicate that quantitative agreement with all the experimental data cannot be obtained with this methodology, with a clear dependence on the exchange-correlation form selected. We thus encourage a careful testing of pure and hybrid density functionals whenever KS DFT is used for the rational design of conjugated materials for charge conduits.     

A re-evaluation of the photophysical properties of 1,4-bis(phenylethynyl)benzene: a model for poly(phenyleneethynylene)

Photophysical measurements, recorded in aerated cyclohexane at 283 K, indicate that 1,4-bis(phenylethynyl)benzene behaves in a conventional manner, undergoing emission from the lowest vibrational level of the first excited singlet state; there is no evidence for aggregation of this material in cyclohexane solution in the concentration range (1-250) x 10(-6) mol dm(-3). However, in highly viscous, low-temperature glasses, the material does exhibit inhomogeneous fluorescence behavior, and wavelength-dependent excitation and emission spectra, indicative of a slow rate of relaxation of conformers of the excited states compared to the rate of fluorescence.     

Ammonolysis of 1,4-bis(dimethylchlorosilyl)benzene

The ammonolysis of 1,4-bis(dimethylchlorosilyl)benzene with ammonia forms heterocyclic compounds of low molecular weight. The main reaction product is the crystalline [CH₃)₂SiC₆H₄Si(CH₃)₂NH]₄, which polymerizes on heating with the formation of polymers possessing a relative viscosity of 0.15–0.4.     

Phosphorylation of 1,4-bis(hydroxymethyl)benzene

1,4-Bis(hydroxymethyl)benzene undergoes bisphosphorylation with P(NEt₂)₃ at room temperature. The reaction produces no cyclic structures.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 440–442, February, 2005.     

Polybenzoxazole from 1,4-bis(trichloromethyl)benzene

High molecular weight poly(phenylenebenzobisoxazole) (PBO) was synthesized from 1,3-diamino-4,6-dihydroxybenzene dihydrochloride (1) and 1,4-bis(trichloromethyl)benzene (3) in polyphosphoric acid (PPA) or a mixture of PPA and methanesulfonic acid. When PPA was used as the solvent, 3 was first converted in situ to terephthalic acid before 1 was added to minimize degradation of 1. Compound 3 did not need to be sealed from atmospheric moisture because the trichloromethyl groups were not moisture sensitive. It was not necessary to use micronized 3. Adjustment of P₂O₅ content was optional for this reaction because no water was liberated from the condensation of 1 and 3. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2143–2145, 1997     

Microwave-assisted synthesis of 1,4-bis(difluoromethyl)benzene

A fast, mild, and practical microwave-assisted protocol for synthesis of 1,4-bis(difluoromethyl)benzene from 1,4-bis(dichloromethyl)benzene and KF was developed. The new protocol increased the yield and reduced the reaction time significantly in contrast to the conventional heating procedure. Also, the synergistic effect of a composite phase transfer catalyst was studied.     

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene (POPOP) molecule by electron impact in gaseous phase is studied. Potentials of appearance of some fragments of the molecule under study are determined from the experimentally measured dependences of ionization cross-section on the ionizing electron energy. A new ion with m/z = 144 [C₉H₆ON]⁺ is detected in the mass spectrum of the POPOP molecule, being complementary to the fragment with m/z = 220 [C₁₅H₁₀ON]⁺. The threshold of appearance of this ion is determined (E_ap = 9.51 eV) as well as the first ionization potential of the POPOP molecule and fragment ion appearance potentials.     

Spectroscopy and photophysics of 1,4-bis(phenylethynyl)benzene: effects of ring torsion and dark pi sigma* state

A combination of supersonic-jet laser spectroscopy and quantum chemistry calculation was applied to 1,4-bis(phenylethynyl)benzene, BPEB, to study the role of the dark pisigma* state on electronic relaxation and the effect of ring torsion on electronic spectra. The result provides evidence for fluorescence break-off in supersonic jet at high S1(pi pi*) <-- S0 excitation energies, which can be attributed to the pi pi*-pi sigma* intersection. The threshold energy for the fluorescence break-off is much larger in BPEB (approximately 4000 cm(-1)) than in diphenylacetylene (approximately 500 cm(-1)). The high-energy barrier in BPEB accounts for the very large fluorescence quantum yield of the compound (in solution) relative to diphenylacetylene. The comparison between the experimentally derived torsional barrier and frequency with those from the computation shows overall good agreement and demonstrates that the low-energy torsional motion involves the twisting of the end ring in BPEB. The torsional barrier is almost an order of magnitude greater in the pi pi* excited state than in the ground state. The finding that the twisting of the end ring in BPEB is relatively free in the ground state, but strongly hindered in the excited state, provides rationale for the well-known temperature dependence of the spectral shape of absorption and the lack of mirror symmetry relationship between the absorption and fluorescence at elevated temperatures.     

Protonation of 1,4-bis(1-vinyl-2-pyrrolyl)benzene

In the reaction of 1,4-bis(1-vinyl-2-pyrrolyl)benzene with hydrogen chloride and hydrogen bromide the hydrogen halide adds at the vinyl groups, and -protonation takes place in both pyrrole rings. In reaction with the superacidic system HSO₃F+SbF₅ (in SO₂ClF) position 5 of one heterocycle and position 4 of the other are protonated with retention of the vinyl groups.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1041–1045, August, 1991.     

Synthesis of 1, 2-bis(b-amino-o-carboranyl)ethane and 1,4-bis(b-amino-o-carboranyl)benzene

Conclusions 1,2-Bis(B-amino-o-carboranyl)ethane and 1,4-bis(B-amino-o-carboranyl)benzene were synthesized.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1669–1670, July, 1976.     

Dependence of charge-transport parameters on static correlation and self-interaction energy: the case of a 1,4-bis(phenylethynyl)benzene derivative conjugated molecule

The current research on molecular-based devices built with highly unsaturated molecules is largely assisted by computational techniques. These modern computational tools are intended to serve (i) to understand the relation between the mechanism of charge transport and the chemical composition of the semiconductors and (ii) to perform the molecular engineering needed to design new and more efficient organic materials. We have studied the case of a rod-shaped conjugated molecule widely used in molecular electronics. The results of multireference perturbation theory up to second order (MRMP2) and complete active space self-consistent field calculations (CASSCF) are compared with the results provided by energy density functionals. Motivated by the diverse accuracy of the results depending on the theoretical method selected, we have systematically studied the physical origin of the discrepancies. We find that a subtle interplay between correlation effects and the self-interaction energy mainly governs the results, which makes it thus difficult to anticipate the quality of a method without knowing in advance its dependence on both effects. We thus encourage careful testing of computational methods for the rational design and understanding of conjugated materials for charge conduits.     

Solid-state kinetics and gas-phase prediction of 1,4-bis(trimethylsilyl)benzene

The solid-state kinetics and gas-phase predictions of the 1,4-bis(trimethylsilyl)benzene (TMSB) are visualized by utilizing thermogravimetric and mass spectral data. The statistical analyses and reduced time plots of zero order (F0) and Avrami-Erofeev (A2) nucleation and growth models provides the best fit to experimental data for isothermal evaporation process for TMSB. The activation energy for non-isothermal evaporation processes of TMSB is calculated using isoconversional methods. The molecular structure and energetics of the predicted gas phase molecules and species in chemical vapor deposition process are investigated using semi-empirical quantum chemical calculations.     

Phosphorylation of 1,4-bis(hydroxymethyl)benzene with trivalent phosphorus derivatives

Phosphorylation of 1,4-bis(hydroxymethyl)benzene with complete phosphorous acid amides and phenyl phosphorodichloridite is studied. Some phosphorus-containing linear systems, the phosphamacrocycle precursors, are synthesized. The possibility of synthesis of phosphamacrocyclic systems based on 1,4-bis(hydroxymethyl)benzene is considered; it was shown that this compound does not tend to form macrocycles.     

Synthesis of new 1,3-bis（phenylethynyl）disilazanes

Three new 1,3-bis(phenylethynyl)disilazanes were synthesized from the reaction of 1,3-dichlorodisilazanes with (phenylethy- nyl)lithium,and characterized by infrared (IR) spectra,nuclear magnetic resonance (NMR) and mass spectrometry (MS).     

Synthesis and Fluorescence of alt-Copoly[1, 4-divinylenebenzene／1, 4-bis（dimethylsilylene）benzene]

Silicon inserted n conjugated copolymer was synthesized by Heck reaction with moderate molecular weight. This silicon containing copolymer is thermally stable to 400℃, while emission of this polymer is blue-shifted compared with polyphenylenevinylene greenish emission. It shows potential application as organic emitter in OLED.