Detection of nucleic acids in situ: novel oligonucleotide analogues for target-assembled DNA-mounted exciplexes

This research describes the effects of structural variation and medium effects for the novel split-oligonucleotide (tandem) probe systems for exciplex-based fluorescence detection of DNA. In this approach the detection system is split at a molecular level into signal-silent components, which must be assembled correctly into a specific 3-dimensional structure to ensure close proximity of the exciplex partners and the consequent exciplex fluorescence emission on excitation. The model system consists of two 8-mer oligonucleotides, complementary to adjacent sites of a 16-mer DNA target. Each probe oligonucleotide is equipped with functions able to form an exciplex on correct, contiguous hybridization. This study investigates the influence of a number of structural aspects (i.e. chemical structure and composition of exciplex partners, length and structure of linker groups, locations of exciplex partner attachment, as well as effects of media) on the performance of DNA-mounted exciplex systems. The extremely rigorous structural demands for exciplex formation and emission required careful structural design of linkers and partners for exciplex formation, which are here described. Certain organic solvents (especially trifluoroethanol) specifically favour emission of the DNA-mounted exciplexes, probably the net result of the particular duplex structure and specific solvation of the exciplex partners. The exciplexes formed emitted at approximately 480 nm with large Stokes shifts ( approximately 130-140 nm). Comparative studies with pyrene excimer systems were also carried out.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

Photophysical properties of β-(1-pyrenyl)ethyl p-cyanobenzoate in binary solvents of isooctane-ethyl acetate and ethyl acetate-acetonitrile

The effects of solvent polarity on the fluorescence spectra and fluorescence decays of β-(1-pyrenyl)ethyl p-cyanobenzoate (P2CN) were investigated in detail using binary solvents consisting of various mixing ratios of isooctane-ethyl acetate or ethyl acetate-acetonitrile (dielectric constants ()=1.94–36.2). Whereas both the intensity and wavelength maxima of an intramolecular exciplex emission (EX) are dependent on the solvent polarity, only the intensity of an emission from the locally excited pyrene (LE) is dependent on the solvents used. When monitored at 377 nm, the picosecond SPC (single photon counting) measurements reveal a slow decay (>150 ns) in addition to a fast decay (<1 ns) of the locally excited P2CN. There are also two decays for the EX which vary the intensity ratios by the monitored wavelength. The decay rate constants, k_EX1 and k_EX2, have a good linear correlation with the dielectric constants of the solvents, indicating that there exist two kinds of exciplexes. It is suggested that the decays of the locally excited-state of P2CN are so fast due to result of the efficient electron transfer that the two kinds of intramolecular exciplexes are formed from the two discrete conformers in the ground state.     

Intramolecular exciplex laser

Laser action of the intramolecular exciplex emission of p-(9′-anthryl)-N,N-dimethylaniline (ADMA) has been found in non-polar and slightly polar solvents. In moderately as well as strongly polar solvents, the laser emission is quenched in spite of the fact that the spontaneous fluorescence yields is larger in polar than in non-polar solvents.     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Excitation conversion in excimers and exciplexes of aromatic hydrocarbons

Flurorescence spectra of a number of aromatic hydrocarbons, their excimers and exciplexes with diethylaniline have been measured in n-hexane under aerobic conditions. It has been found that the value of the red shift of the structureless excimer or exciplex band in reference to the monomer fluorescence band is about 4200 or 3000 cm⁻¹, respectively. For indole—ethanol exciplexes the red shift was about 3700 cm⁻¹. It is suggested, that the shift depends on the intramolecular vibrations in the quencher accepting the vibrational energy from the electronically excited donor molecule. It has been proposed that excimer and exciplex fluorescence occurs due to the donor emission of a number of quanta from the virtual levels.     

双-β-萘甲基二胺及其衍生物的荧光光谱及分子内激发态复合物的形成

我们合成了N,N'-二甲基-二-β-萘甲基乙二胺(Ⅰ),N,N'-二甲基-二-β-萘甲基丙二胺(Ⅱ),N,N'-二-β-萘甲基哌嗪(Ⅲ)三个化合物,并测定了它们的荧光光谱。结果表明,它们既能形成分子内激基缔合物(excimer),又能形成分子内激基复合物(exciplex),与溶剂极性有关。在极性较大的溶剂如乙腈和甲醇中,由于激基复合物几乎完全解离因而不发射荧光,这时观察到的荧光峰蓝移而且强度增加,可以认为,这是激基缔合物存在的证明。这三个化合物的分子内激基缔合物形成的难易程度按Ⅱ>Ⅰ>Ⅲ的次序依次递降。     

Luminescence studies in polymers—VII. Exciplex formation in polyvinylnaphthalene and polyacenaphthylene

The reactivities of polymers in exciplexes and CT complex formation were compared with those of model compounds for the following systems in solution: (1) exciplexes between excited diethylaniline as donor and ethylnaphthalene, acenaphthylene, poly-1-vinylnaphthalene or polyacenaphthylene; (2) exciplexes between excited dicyanoanthracene as acceptor and the same aromatics as donors: (3) CT complexes between chloranil and either poly-1-vinylnaphthalene or polyacenaphthylene. The rate constant for exciplex formation was found to be much larger for the model than for the polymer when diethylaniline is the donor and poly-1-vinylnaphthalene or ethylnaphthalene the acceptor. The equilibrium constant for CT complex formation between chloranil and the same aromatics is also higher in the case of model compounds. This difference is tentatively assigned to entropy terms arising from the lower accessibility of the aromatic groups fixed on a polymer backbone. These conclusions are extended to the other systems. Exciplexes do not form in poly-1-vinylnaphthalene or polyacenaphthylene films containing dicyanoanthracene.     

The photophysics of some intramolecular ternary complexes formed between aryl and amino groups

The fluorescence properties of (3)–(6) in solvents of varying polarity have been examined and the conclusion reached that in non-polar solvents they exhibit fluorescence (low quantum yield) from an intramolecular ternary complex. In polar solvents very little fluorescence is observable. Although each compound exhibits extensive intramolecular fluorescence quenching, the quantum yield of localised triplet naphthalene in non-polar solvents remains relatively high indicating that intersystem crossing occurs in the non-relaxed exciplex.     

Exciplex and excimer molecular probes: detection of conformational flip in a myo-inositol chair

2-O-tert-Butyldimethylsilyl-4,6-bis-O-pyrenoyl-myo-inositol-1,3,5-orthoformate (6) and 2-O-tert-butyldimethylsilyl-4-O-[4-(dimethylamino)benzoyl]-6-O-pyrenoyl-myo-inositol-1,3,5-orthoacetate (10) adopt conformationally restricted unstable chairs with five axial substituents. In the symmetrical diester 6, the two pi-stacked pyrenoyl groups are electron acceptor-donor partners, giving a strong intramolecular excimer emission. In the mixed ester 10, the pyrenoyl group is the electron acceptor and the 4-(dimethylamino)benzoyl ester is the electron donor, giving a strong intramolecular exciplex emission. The conformation of the mixed ester 10 was assessed using 1H NMR spectroscopy (1H-NOESY) and computational studies. which showed the minimum inter-centroid distance between the two aromatic systems to be approximately 3.9 A. Upon addition of acid, the orthoformate/orthoacetate trigger in 6 and 10 was cleaved, which caused a switch of the conformation of the myo-inositol ring to the more stable penta-equatorial chair, leading to separation of the aromatic ester groups and loss of excimer and exciplex fluorescence, respectively. This study provides proof of principle for the development of novel fluorescent molecular probes.     

Isomerization of dewar benzenes involving electron donor-acceptor exciplexes

Valence photoisomerization of hexamethyl (Dewar benzene) (HMDB) is sensitized by aromatic singlet photosensitizers 1,4-dicyanobenzene, 1-cyanonaphthalene, 9-cyanoanthracene, and 9,10-dicyanoanthracene with a limiting quantum efficiency of 1.0 in cyclohexane solvent. Quenching of the fluorescence of the aromatic sensitizers leads to exciplex emission which is identical to that obtained by quenching with the isomer, hexamethylbenzene (HMB). The emission is identified as HMB exciplex emission on the basis of relative lifetime and dual quenching experiments. The relative yield of HMDB-derived (“adiabatic”) emission is 20–50% depending on the excitation energy of the HMB exciplex product. Neither biacetyl singlet or triplet nor 1-cyanonaphthalene triplet photosensitization is successful in bringing about isomerization of HMDB. Dimethyl 1,4,5,6-tetramethylbicy-clo[2.20]hexa-2,5-diene-2, 3-dicarboxylate undergoes valence isomerization on quenching electron donor fluorophores, with a quantum efficiency of 0.2. The aromatic valence isomer is not produced in an excited state in this case. Factors which govern the efficiency of adiabatic and diabatic isomerization of the Dewar benzenes are discussed, including sensitizer redox properties, configuration, and multiplicity, the excitation energy and binding characteristics of exciplexes, and the Dewar benzene substituent pattern.     

Solvent effects on photoreactions of dibenzoylmethanatoboron difluoride (DBMBF2) with cyclic dienes

The pattern of the photoreactions of DBMBF₂ with a cyclic diene is highly dependent on solvent nature, giving mainly the dimers of dienes in polar solvents but cycloadduct in non-polar solvents. The quantum yields of the adduct (Φ_A) correlate smoothly with an empirical polarity constant (E_T^N) in spite of exciplex formations with aromatic solvents.     

On the relationship between fluorescence and molecular geometry solvent-assisted formation of luminescent intramolecular exciplexes from anthronyl-substituted anthracenes

The fluorescence efficiency of 1′,2′-dihydro-spiro[anthracene-9(10H),3′-[3H]benz[de]anthracen]-10-one in 10 solvents of varying dielectric constant has been investigated. The observed exciplex luminescence is attributed to a solvent-assisted intramolecular charge-transfer interaction between the ground-state anthrone and excited-state anthracene moietis. The geometry of the exciplex is suggested to resemble that of a σ-π complex.     

高分子链对激发态芳烃和胺相互作用的影响

本文研究了在激发状态下芳烃(受体)和脂肪胺(给体)的相互作用以及高分子链在其中的影响。结果表明,高分子胺与不带高分子链的胺相比,其对激发态芳烃的淬灭速度和形成激基复合物的能力都强得多,并且激基复合物光谱发生明显红移。     

Salt-induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl-borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.     

芳香叔胺引发丙烯腈光聚合的引发机理

芳香叔胺引发丙烯腈(AN)光聚合是通过形成激基复合物(exciPlex)进行的。紫外光谱和荧光光谱表明,芳香叔胺在基态可以和AN形成电荷转移复合物(CTC),而在激发态可和AN形成exciplc(称定域激发)。CTC经光照亦可激发(称CTC激发)。 定域激发引起光聚合速率为CH_3C_6H_4N(CH_3)_2>C_6H_5N(CH_3)_2>HOCH_2·C_6H_4N(CH_3)_2>CH_3C_6H_4N(CH_2CH_2OH)_2,与芳胺荧光被AN淬灭的Stern-Vo-lmer常数顺序一致。CTC激发引起的光聚合顺序为:CH_3C_6H_4N(CH_3)_2>CH_3C_6H_4N(CH_2CH_2OH)_2>HOCH_2C_6H_4N(CH_3)_2>C_6H_5N(CH_3)_2,与芳胺上取代基推电子能力一致。端基分析表明聚合物有芳胺端基。     

Temperature effect on the vapor-phase exciplexes of cyano-substituted anthracenes

The emission spectra and lifetimes of the vapor-phase exciplexes of four cyano-substituted anthracenes with N,N-dimethylaniline (DMA) as a donor are examined over a wide temperature range. The activation energies associated with the exciplex dissociation are calculated to be of the order of 10 kcal/mol. The entropy change in forming the exciplexes is discussed in relation to the efficiency of the exciplex formation. For various donors other than DMA, preliminary results on how they interact with excited 9,10-dicyanoanthracene are also given.     

Effects of conformation and solvent polarity on intramolecular charge transfer: a picosecond laser study

Picosecond dynamics of the intramolecular exciplex anthracene-(CH₂)₃-N,N-diniethylaniline have been measured in acetonitrile. The results indicate two processes. First, very rapid (7 ± 1 ps) electron transfer for molecules in extended conformations, producing solvated ion pairs without passing through the exciplex state. Second, folded conformers yield exciplexes within 2 ps, having a lifetime of 580 ± 30 ps.     

Activation Enthalpy and Entropy for the Formation of 9-Cyanophenanthrene Exciplexes

Exciplexes of 9-cyanophenanthrene with a series of weak electron donors with the Gibbs energy of electron transfer G _et * varying in the range –(0.02–0.09) eV were studied. The exciplexes exhibited fairly intense emission both in nonpolar and aprotic polar solvents. The kinetics of the exciplex formation was found to be controlled mainly by diffusion and reactant orientation. This is clearly manifested in the low-temperature region in which the activation enthalpy of exciplex formation is very close to the activation enthalpy of diffusion, and the activation entropy of exciplex formation does not exceed 18 J mol^–1 K^–1 in absolute value.     

Emission from aromatic hydrocarbon-diene and -olefin exciplexes

Luminescence from aromatic hydrocarbon-olefin and -diene exciplexes, providing strong evidence for their intermediary in singlet quenching processes, is reported. Solvent dependence of the emission maximum gives a value of 10.8D for the dipole moment of the 1-cyanonaphthalene-1,2-dimethylcyclopentene exciplex while the temperature dependence affords a value of 6.7 kcal/M for the heat of formation. Linear-free-energy correlations of rate constants for 1-cyanonaphthalene and naphthalene fluorescence quenching by dienes and olefins and strained hydrocarbons with the adiabatic ionization potentials of the quenchers are consistent with major contributions from charge-resonance in the exciplex formation process.