The selective synthesis of N-arylbenzene-1,2-diamines or 1-arylbenzimidazoles by irradiating 4-methoxy-4'-substituted-azobenzenes in different solvents.

The solvent-controllable photoreaction of 4-methoxyazobenzenes to afford 1-aryl-1H-benzimidazoles or N-arylbenzene-1,2-diamines has been studied. The irradiation of 4-methoxyazobenzenes in DMF containing 0.5 M hydrochloric acid provided N 2-aryl-4-methoxybenzene-1,2-diamines as the major product, while irradiation in acetal containing 0.16 M hydrochloric acid led to 1-aryl-6-methoxy-2-methyl-1H-benzimidazoles as the major product. A possible reaction mechanism explaining the selectivity was also discussed.

Emission from charge recombination during the pulse radiolysis of 9-cyano-10-phenylethynylanthracenes with donor and acceptor substituents.

Emission from 9-cyano-10-phenylanthracene and 9-cyano-10-phenylethynylanthracenes having donor and acceptor substituents (RA = PA, PEA, OEA, NEA, and DEA) was studied with the time-resolved fluorescence measurement during the pulse radiolysis of RAs in benzene (Bz). PA and DEA showed only monomer emission, while other RAs (PEA, OEA, and NEA) showed both monomer and excimer emissions with much lower intensities. On the basis of the steady-state and transient absorption and emission measurements, the formation of RA in the singlet excited state ((1)RA*) can be attributed to the charge recombination between RA radical cation and anion (RA*+ and RA*-, respectively) which are initially generated from the radiolytic reaction in Bz. It is expected that for PA with a twisted geometry, the charge recombination between PA*+ and PA*- occurs to give (1)PA* during the pulse radiolysis in Bz. For PEA and OEA, pi-stacking interaction is possible for the formation of an encounter complex during the charge recombination between RA*+ and RA*-. For NEA, it is expected that NEA*+ and NEA*- collide neck-to-neck to generate the excimer due to the twisted geometry. For DEA, a considerably twisted structure is assumed to give (1)DEA* with strong ICT character but not (1)(DEA)2* because of the bulky donor substituent.

Stilbene analogs in Hula-twist photoisomerization.

Photoisomerization of several cis- or Z-stilbene analogs and two E-analogs in low temperature organic glasses was examined. From a mechanistic view-point, the compounds can be divided into three types: (i) those giving identical Hula-twist (HT) and one-bond-flip (OBF) products, (ii) those giving a single HT product that is different (hence distinguishable) from the OBF product and (iii) those showing two distinct HT processes but only one OBF process. Examples for all three types of analogs are provided emphasizing the most informative Type-II (stilbene analogs with identical but unsymmetrically substituted phenyl rings), including linear as well as conformationally constrained compounds. Conditions necessary for establishing HT and OBF processes are defined. Proper choice and design of model systems are essential for establishing or eliminating HT mechanism(s) of isomerization.

Emission from charge recombination during the pulse radiolysis of arylethynylpyrenes.

Emission from several 1-(arylethynyl)pyrenes with a substituent on the aryl group (REPy, R = phenyl (PEPy), 4-dimethylaminophenyl (NPEPy), 4-isopropoxyphenyl (OPEPy), 2-quinonyl (QEPy), and 9-(10-cyanoanthracenyl) (AEPy)) was studied with time-resolved fluorescence measurements during pulse radiolysis in benzene. NPEPy and AEPy showed only monomer emission, while PEPy, OPEPy, and QEPy showed both monomer and excimer emissions during pulse radiolysis. In addition, REPy's also showed long-lived emissions with very weak intensities in the absence of oxygen, which were assigned to the "P-type" delayed fluorescence derived from the triplet-triplet annihilation. The formation of REPy's in the singlet excited state (1REPy*) can be interpreted as the charge recombination between the REPy radical cation and anion (REPy*+ and REPy*-, respectively), which are initially generated from the radiolytic reaction in benzene. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PEPy are localized on the 1-pyrenyl (Py) moiety, while the HOMO of REPy's with an electron donating or withdrawing substituent on the benzene ring (R(D)EPy such as NPEPy and OPEPy or R(A)EPy such as QEPy and AEPy) is mainly localized on the donor moieties (R(D) or Py) and the LUMO on the acceptor ones (Py or R(A), respectively). Therefore, it is suggested that the one-electron oxidation and reduction of REPy's can occur from the donor and acceptor moieties, respectively. This scheme reasonably explains the relationship between the annihilation enthalpy changes (-Delta H' degrees) for the charge recombination of REPy*+ and REPy*- and the singlet excitation energies (E'(S1) of the REPy's. The results are compared with those in electrogenerated chemiluminescence.

Catalytic isomerization of quadricyclane using fourier transform near-infrared absorption spectroscopy: diffusion, conversion, and temperature effect.

By using Fourier transform near-infrared (NIR) absorption spectroscopy, the kinetic behaviors of quadricyclane isomerization, as catalyzed by anhydrous CuSO(4) in chloroform mixture with and without agitation, are presented. Given the acquired NIR spectra, the concentration decay of quadricyclane with the reaction time is determined with the aid of partial least-squares analysis. When the mixture is not agitated, the diffusion coefficients in chloroform are evaluated to be (3.8 +/- 0.1) x 10(-5) cm(2) s(-1) at 27 degrees C and (4.4 +/- 0.1) x 10(-5) cm(2) s(-1) at 39 degrees C. In the size-dependent experiments of the catalyst, the one-site and two-site coordinated conversion rate constants are further determined to be (8.5 +/- 5.9) x 10(-6) s(-1) A(-1) and (2.2 +/- 0.8) x 10(-8) s(-1) A(-2), respectively, at 27 degrees C and (1.3 +/- 0.8) x 10(-5) s(-1) A(-1) and (1.92 +/- 0.01) x 10(-6) s(-1) A(-2), respectively, at 39 degrees C. A denotes the total catalyst surface area per unit effective volume of solvent. Accordingly, the activation energies for one-site and two-site coordination are evaluated to be 24.8 and 286.2 kJ mol(-1), respectively. The reaction is dominated by one-site coordination (1:1 complex) between the reactant and the catalyst. Unless temperature increases, the two-site coordinated reaction may be ignored. In contrast, when analogous experiments are performed in the stirred solution, the diffusion factor is ignored but the conversion rate constants rise due to the increase of collision frequency. For instance, the one-site and two-site coordinated rate constants are increased to (1.7 +/- 1.4) x 10(-5) s(-1) A(-1) and (1.27 +/- 0.06) x 10(-5) s(-1) A(-2) at 39 degrees C. The two-site coordinated reaction rate is enhanced by a factor of 10. Thus, isomerization may proceed via both 1:1 and 1:2 coordination between the reactant and the catalyst. The Arrhenius plot yields the corresponding activation energies to be 24 +/- 3 and 275 +/- 3 kJ mol(-1). The activation energies remain constant, no matter whether the solution is agitated or not.

Practical synthesis of potential endothelin receptor antagonists of 1,4-benzodiazepine-2,5-dione derivatives bearing substituents at the C3-, N1- and N4-positions.

The expedient synthesis of various 1,4-benzodiazepine-2,5-dione compounds, particularly those having substituents at the C3-, N1- and N4-positions is achieved. The important features in these synthetic strategies include: (i) using the coupling reaction of isatoic anhydride with alpha-amino ester for direct construction of the core structure of 1,4-benzodiazepine-2,5-dione; (ii) using potassium carbonate as the base of choice for selective alkylation at the N1-site, while using lithiated 2-ethylacetanilide as the required base to furnish the N4-alkylation; and (iii) using 2-nitrobenzoyl chloride as a synthetic equivalent of anthranilic acid to facilitate the polyethylene resin-bound liquid-phase combinatorial synthesis. The prepared 1,4-benzodiazepine-2,5-dione compounds are evaluated for endothelin receptor antagonism by a functional assay that measures the inhibitory activity against the change of intramolecular calcium ion concentration induced by endothelin-1. The preliminary results indicate that 1,4-benzodiazepine-2,5-diones bearing two flanked aryl substituents at the N1- and N4-sites show better inhibitory activity than the corresponding unalkylated and N-monoalkylated compounds. A promising candidate, 1-benzyl-7-chloro-3-isopropyl-4-(3-methoxybenzyl)-1,4-benzodiazepine-2,5-dione (17b), exhibits an IC50 value in low nM range.

Structure-property relationships in conjugated donor-acceptor molecules based on cyanoanthracene: computational and experimental studies.

[reaction: see text] Two series of pi-conjugated bipolar compounds, namely, 9-phenyl-10-anthronitriles (PAN series) and 9-phenylethynyl-10-anthronitriles (PEAN series), having inherent redox centers have been synthesized and their electronic absorption, fluorescence emission, and electrochemical behavior have been studied. Electrochemiluminescence of these molecules bearing weak, strong, and spin-polarized donors is also studied. The observed electronic properties are explained with the help of results obtained from density functional theory (DFT- B3LYP/6-31G) calculations. The structure-property relationships of all the molecules are discussed.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage.

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.

Synthesis, electronic properties, and electrochemiluminescence of donor-substituted phenylethynylanthronitriles.

A new series of pi-conjugated donor-acceptor compounds (1-6) with inherent redox centers have been prepared and studied with respect to their electronic properties. The photophysical characteristics of these compounds have been studied in relation to their structures. Cyclic voltammetry and UV-vis spectroelectrochemistry were used to probe the ground-state electronic properties of the neutral and charged species. The observed electronic absorption properties of the neutral and charged molecules are explained with the help of frontier orbital structures and electrostatic potential maps obtained from density functional theory (DFT, B3LYP/6-31G) calculations. Electrochemiluminescence (ECL) of this series of donor-substituted phenylethynylanthronitriles with different donors was also studied. The structure-property relationship of all of the compounds is discussed.

Donor-substituted phenyl-pi-chromones: electrochemiluminescence and intriguing electronic properties.

Phenylethynylchromones bearing different donor groups at the phenyl moiety have been prepared and their photophysical and electrogenerated chemiluminescence (ECL) properties have been studied with respect to their structural features. Intriguingly, the presence and variation of donor groups do not much influence the absorption spectra, which can be compared with the spectrum of unsubstituted chromone, whereas the photoluminescence (PL) spectra show pronounced changes. Density functional theory (DFT) calculations indicate enhancement of HOMO energy levels upon increasing the donor strength. The photophysical properties have also been studied in various solvents, and the PL spectra in particular show the anticipated trend. The introduction of pi-extension imparts ECL to the new molecules and the electronic coupling between the donor and the acceptor moieties through C-C triple bond influences ECL emission maxima. Weaker donors impart excimer ECL while stronger donors impart monomeric intramolecular charge transfer (ICT) ECL.

Dihydrophenanthrene-type intermediates during photoreaction of trans-4'-benzyl-5-styrylfuran.

[structure: see text] Photoreaction of trans-4'-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH2Cl2 using a Nd3+:YAG laser (30 ps, 5 mJ pulse(-1) or 5 ns, 30 mJ pulse(-1)). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1. The first photoreaction is the photoisomerization of trans-BSF, which occurs to give cis-BSF. The second photoreaction process is photocyclization of cis-BSF, which occurs to give DP1 decaying with the half lifetime (tau1/2) of 2.8-4.0 micros to produce another DHP-type intermediate (DP2) with an absorption peak at 400 nm in the absence of O2, through [1,9]-hydrogen shift. DP2 decayed with tau1/2 > 500 micros to give the product through aromatization. In O2-saturated CH2Cl2, DP1 decayed with tau1/2 = 250 ns to give a radical intermediate (X) with two peaks at 410 and 510 nm, through hydrogen abstraction of DP1 by O2. X decayed with tau1/2 = 150 micros to give the product through successive hydrogen abstraction.

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent substituted quinolines with donor-acceptor character.

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes and singlet excitation energies of PnQ. The formation of the intramolecular charge transfer state is assumed for PnQ in the singlet excited state with a strong electron donating substituent. The emission from PnQ is suggested to originate from PnQ in the singlet excited state formed from the charge recombination between the PnQ radical cation and the PnQ radical anion during the pulse radiolysis. This is strong evidence for the efficient electrogenerated chemiluminescence of PnQ.

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes.

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.

Highly fluorescent N,N-dimethylaminophenylethynylarenes: synthesis, photophysical properties, and electrochemiluminescence.

A new family of aryl-pi-donor-aryl molecules has been synthesized and studied with respect to their photophysical properties and electrogenerated chemiluminscence (ECL) for the first time. Anthracene, phenanthrene, naphthalene, biphenyl, and fluorene were coupled with N,N-dimethylanilino moiety via a C-C triple bond (1-7). Introduction of such a strong electron-donating moiety as N,N-dimethylanilino group through a triple bond imparts new properties to the resultant molecules that are not commonly observed for the parent arenes. All molecules show absorption in the near-visible region and emission totally in the visible region with high fluorescence quantum yields. Bright solid-state photoluminescence has also been noticed for all the compounds in the visible region. 9-Anthryl- and 1-naphthyl- derivatives exhibited blue-shifted electrochemiluminescence (ECL) relative to their photoluminescence because of aggregation. 9-Phenanthryl- and 2-naphthyl- derivatives did not show ECL. 2-Biphenyl derivative showed monomeric ECL while 4-biphenyl counterpart exhibited excimer ECL. No ECL was observed for 2-fluorenyl derivative. The observed electronic properties are discussed with regard to the structure of the molecules. The characteristics of the molecules chosen in the present study open up new prospects and promises for novel tunable organic materials, on the basis of simple extension of conjugation to promote intramolecular communication, for ECL, OLED, and other optoelectronic applications.

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylethynylcoumarins.

Two series of donor-bearing phenylethynylcoumarins have been synthesized, and their photophysical properties have been evaluated. Chemiluminescence was observed through the annihilation of their electrogenerated radical ions and was found to be only slightly affected by the presence of various donor groups on the phenyl moiety linked through the C-C triple bond. The overall properties of the two series of compounds are discussed with respect to their structures. The observed electronic absorption properties are explained with the help of computational studies.

Arylethynylacridines: electrochemiluminescence and photophysical properties.

Electrogenerated chemiluminescence (ECL) of six new ethyne-based acridine derivatives (1-6) has been studied. The new acridine derivatives were synthesized by cross-coupling of 9-chloroacridine and corresponding donor-substituted phenylethynes under modified Sonogashira conditions. The donor groups were varied in the order of increasing steric hindrance and donor strength at the donor site. The solution phase photophysical properties and ECL of these compounds were studied comparatively in acetonitrile solvent. The UV-Visible spectra of compounds 1-5 exhibit closely the same maxima. Density functional theory (DFT) has been invoked to analyze and understand the unexpected UV-Visible absorption behavior. Compounds with weak electron donors produce excimer ECL irrespective of steric hindrance at the donor site, while the compound with a stronger donor gives rise to ECL that is blue-shifted with respect to its photoluminescence spectrum. All except one of these compounds also exhibit solid state fluorescence which may be useful for solid state devices such as organic light emitting diodes (OLEDs) and as laser dyes. The observed properties are discussed with reference to the structure of the compounds synthesized.

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylquinolinylethynes and phenylisoquinolinylethynes: effect of positional isomerism.

In furtherance of our research on the design, synthesis and study of electrogenerated chemiluminescence (ECL) of new donor substituted phenylquinolinylethynes, we report here more new series with the aim of studying the effect of positional isomerism on their overall photophysical properties with a special focus on ECL. For this study we have chosen 2-, 3-, and 4-(p-substituted phenyl)ethynylquinolines, and 1- and 4-(p-substituted phenyl)ethynylisoquinolines. These ethynes were synthesized in good yields by modified Sonogashira coupling of the corresponding terminal alkyne with the respective haloquinolines. The photophysical properties and ECL were studied in acetonitrile solvent and the various results are discussed.

Excimer and intramolecular charge transfer chemiluminescence from electrogenerated ion radicals of donor-acceptor stilbenoids.

Electrongenerated chemiluminescence (ECL) of a series of intramolecular charge transfer (ICT) donor acceptor stilbenoid systems (2-9) bearing N,N-dimethylamino group as donor and pyridine, thiophene, quinoline or aryl groups as acceptors are studied. Most of the compounds (3-9) show ICT ECL through direct annihilation of the radical ions. For the weaker ICT compound (2), excimer ECL is observed instead.

First synthesis and electrogenerated chemiluminescence of novel p-substituted phenyl-2-quinolinylethynes.

A series of p-substituted phenyl-2-quinolinylethynes as blue-green emitters were synthesized using a modified Sonogashira coupling reaction and their electrogenerated chemiluminescence properties are reported.

Sonogashira coupling reaction with diminished homocoupling.

[reaction: see text] The side product from homocoupling reaction of two terminal acetylenes in the Sonogashira reaction can be reduced to about 2% using an atmosphere of hydrogen gas diluted with nitrogen or argon. Terminal arylethynes, diarylethynes, and a few new arylpyridylethynes with donor substituents have been synthesized in very good yields. Comparative control experiments suggest that the homocoupling yield is determined by concentration of both catalyst and oxygen.